&EPA
United St.-ttt'S
EtwitonmPiita1
Agency
Industrial Environmental
Research Laboratory
Cincinnati OH 45268
EPA 600/2 78-004)
March 1978
Research and Development
Source Assessment:
Rubber Processing,
State of the Art
Environmental Protection
Technology Series
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into nine series. These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The nine series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
6. Scientific and Technical Assessment Reports (STAR)
7. Interagency Energy-Environment Research and Development
8. "Special" Reports
9. Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series. This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution. This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards.
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161.
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EPA-600/2-78-004J
March 1978
SOURCE ASSESSMENT:
RUBBER PROCESSING
State of the Art
by
C. T. Chi, T. W. Hughes, T. E. Ctvrtnicek,
D. A. Horn, and R. W. Serth
Monsanto Research Corporation
Dayton, Ohio 45407
Contract No. 68-02-1874
Project Officer
Ronald J. Turner
Industrial Pollution Control Division
Industrial'Environmental Research Laboratory
Cincinnati, Ohio 45268
INDUSTRIAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Industrial Environmental
Research Laboratory-Cincinnati, U.S. Environmental Protection
Agency, and approved for publication. Approval does not signify
that the contents necessarily reflect the views and policies of
the U.S. Environmental Protection Agency, nor does mention of
trade names or commercial products constitute endorsement or
recommendation for use.
11
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FOREWORD
When energy and material resources are extracted, processed,
converted, and used, the related pollutional impacts on our
environment and even on our health often require that new and
increasingly more efficient pollution control methods be used.
The Industrial Environmental Research Laboratory - Cincinnati
(lERL-Ci) assists in developing and demonstrating new and
improved methodologies that will meet these needs both effici-
ently and economically.
This report contains an assessment of air emissions from the rub-
ber processing industry. This study was conducted to provide a
better understanding of the distribution and characteristics of
emissions from rubber processing operations. Further information
on this subject may be obtained from the Organic Chemicals and
Products Branch, Industrial Pollution Control Division.
David G. Stephan
Director
Industrial Environmental Research Laboratory
Cincinnati
m
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PREFACE
The Industrial Environmental Research Laboratory (IERL) of the
U.S. Environmental Protection Agency (EPA) has the responsibility
for insuring that pollution control technology is available for
stationary sources to meet the requirements of the Clean Air Act,
the Federal Water Pollution Control Act, and solid waste legisla-
tion. If control technology is unavailable, inadequate, or un-
economical, then financial support is provided for the develop-
ment of the needed control techniques for industrial and
extractive process industries. Approaches considered include:
process modifications, feedstock modifications, add-on control
devices, and complete process substitution. The scale of the
control technology programs ranges from bench- to full-scale
demonstration plants.
IERL has the responsibility for developing control technology for
a large number of operations (more than 500) in the chemical and
related industries. As in any technical program, the first step
is to identify the unsolved problems. Each of the industries is
to be examined in detail to determine if there is sufficient
potential environmental risk to justify the development of con-
trol technology by IERL.
Monsanto Research Corporation (MRC) has contracted with EPA to
investigate the environmental impact of various industries that
represent sources of pollutants in accordance with EPA's respon-
sibility, as outlined above. Dr. Robert C. Binning serves as MRC
Program Manager in this overall program, entitled "Source Assess-
ment," which includes the investigation of sources in each of
•four categories: combustion, organic materials, inorganic mater-
ials, and open sources. Dr. Dale A. Denny of the Industrial
Processes Division at Research Triangle Park serves as EPA Pro-
ject Officer for this series. Reports prepared in this program
are of two types: Source Assessment Documents, and State-of-the-
Art Reports.
Source Assessment Documents contain data on pollutants from
specific industries. Such data are gathered from the literature,
government agencies, and cooperating companies. Sampling and
analysis are also performed by the contractor when the available
information does not adequately characterize the source pollu-
tants. These documents contain all of the information necessary
for IERL to decide whether a need exists to develop additional
control technology for specific industries.
iv
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State-of-the-Art Reports include data on emissions from specific
industries which are also gathered from the literature, govern-
ment agencies, and cooperating companies. However, no extensive
sampling is conducted by the contractor for such industries.
Results from such studies are published as State-of-the-Art
Reports for potential utility by the government, industry, and
others having specific needs and interests.
This study was undertaken to provide information on air emissions
from rubber processing. It was initiated by IERL-Research
Triangle Park in December 1974; Mr. Kenneth Baker served as EPA
Project Leader. The project was transferred to the Industrial
Pollution Control Division, lERL-Cincinnati, in October 1975;
Mr. Ronald J. Turner of the Organic Chemicals and Products Branch
served as EPA Project Leader from that time through completion of
the study.
v
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ABSTRACT
This report reviews the state of the art of air emissions from
production of vulcanized elastomers (rubbers) and fabrication
of rubber products. Nine industries are included: styrene-
butadiene rubber (SBR) production; rubber reclaiming; tires and
inner tubes; rubber footwear; rubber hose and belting; fabri-
cated rubber products not elsewhere classified; gaskets, packing,
and sealing devices; rubber wire-insulating; and tire retreading.
Hydrocarbons and particulates are emitted from various operations
in the rubber processing industries. Hydrocarbon emissions con-
sist of monomers, rubber chemicals, and solvents which are vola-
tilized during the processing. Particulate emissions consist of
carbon black, soapstone, zinc oxide, etc., which are emitted from
compounding, grinding, and talc dusting operations. Particulates
are also emitted as mists and solid particles which are formed by
condensation of hydrocarbons that are volatilized from rubber
material due to the high temperatures involved in curing, mold-
ing, and drying operations.
To assess the severity of emissions from rubber processing indus-
tries, a representative plant was defined for each industry
except SBR production, where separate representative plants were
defined for the emulsion polymerization and solution polymeriza-
tion processes. Source severity was defined as the ratio of the
time-averaged maximum ground level concentration of a pollutant
emitted from a representative plant to the ambient air quality
standard (for criteria pollutants) or to a reduced threshold
limit value (for noncriteria pollutants). The following opera-
tions have source severities greater than or equal to one: the
butadiene absorption vent in emulsion SBR production, the drying
operation in solution SBR production, green tire spraying and
curing operations in the tire industry and rubber cementing in
the rubber footwear industry.
Mass emissions from the nine industries contribute 0.26% and
0.074% respectively to the national totals of hydrocarbons and
particulates from all sources. Due to the open nature of most
emission points, control of emissions from rubber processing
operations includes collection of the contaminated gas and
removal of the pollutants from the gas. For control of hydro-
carbon emissions, carbon adsorption with solvent recovery and
incineration with heat recovery have been used. Particulate
control devices used in the industry include wet scrubbers,
cyclones, and baghouses.
vi
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This report was submitted in partial fulfillment of Contract No.
68-02-1874 by Monsanto Research Corporation under the sponsorship
of the U.S. Environmental Protection Agency. This report covers
the period December 1974 to July 1977, and work was completed as
of July 1977.
vii
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CONTENTS
Foreword ..... iii
Preface iv
Abstract vi
Figures x
Tables xi
Abbreviations and Symbols xiii
Conversion Factors and Metric Prefixes xiv
1. Introduction 1
2. Summary 2
3. Source Description 8
Source definition 8
Process description 12
Geographical distribution 62
4. Emissions 65
Locations and selected pollutants 65
Emission factors 73
Environmental effects 74
5. Control Technology 93
State of the art 93
Future considerations ..97
6. Growth and Nature of the Industry 99
Present technology 99
Emerging technology 100
Marketing strengths and weaknesses 102
References 107
Appendices
A. Development of source severity equations Ill
B. Mass emissions of hydrocarbons and particulates by
state and by SIC 113
Glossary 116
IX
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FIGURES
Number Pag(
1 Schematic flow diagram for crumb rubber production
by emulsion polymerization 15
2 Schematic flow diagram for latex rubber production
by emulsion polymerization 17
3 Schematic flow diagram for crumb rubber production
by solution polymerization 19
4 Generalized schematic flow diagram for reclaiming
rubber. 22
5 Cross section of a Banbury internal mixer mounted
over a rubber mill 38
6 Diagram of the calendering process 39
7 Extrusion processes . 40
8 Cross section of a tire 43
9 Variations of tire construction 43
10 Tire plant process flow diagram 44
11 Schematic flow diagram for the production of
typical canvas footwear items 49
12 Belting flowsheet ..... .... 51
13 Ply hose flowsheet 53
14 Retreading flowsheet 61
15 Geographic distribution of rubber processing plants
in the United States 64
16 Domestic market estimates and forecasts for molded,
extruded, and lathe cut products. 104
17 Market potential for rubber hose and belting. . . . 104
18 Total new rubber consumption, synthetic vs natural
source. 106
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TABLES
Number Page
1 Production Rate, Emission Factors, and Mass Emis-
sions for Rubber Processing Industries 2
2 Parameters Used for Representative Plants 4
3 Source Severities for Representative Elastomer Plants 5
4 Source Severities for Representative Rubber Product
Plants 6
5 Affected Population by Industry 7
6 1975 U.S. Production of Synthetic Elastomers 11
7 Production of Rubber Products in 1975 12
8 Classification of Rubbers 27
9 Commercial Antioxidants 30
10 Pigments Used in Rubber Compounding 31
11 Typical Softeners and 'Plasticizers Used in Rubber
Compounding 32
12 Commercial Accelerators 34
13 Commercial Antiozonants 35
14 Blowing Agents Which Release Nitrogen . 35
15 Organic Activators 36
16 Commonly Used Retarders 36
17 Typical Compound Compositions for Tire Parts 45
18 Typical Tire Cord Dip Solution 46
19 Preparation of a Dispersion of Aminox Suitable for
Latex Compounding 56
20 Preparation of a Dispersion of Methazate Suitable for
Latex Compounding 56
21 Preparation of a Naugawhite Emulsion Suitable for
Latex Compounding 57
22 Preparation of an Oil Emulsion Suitable for Latex
Compounding 57
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TABLES (continued)
Number
23 Geographical Distribution of Rubber Processing
Plants 63
24 Melting Points of Common Antioxidants 69
25 Melting Points of Common Accelerators . . 69
26 Emission Factors for SBR Production by Emulsion
Polymerization (SIC 2822) 75
27 Emission Factors for SBR Production by Solution
Polymerization (SIC 2822) . . . . 76
28 Emission Factors for Rubber Reclaiming (SIC 3031) . . 77
29 Emission Factors for Tires and Inner Tubes (SIC 3011) 77
30 Emission Factors for Rubber Footwear (SIC 3021) ... 78
31 Emission Factors for Rubber Hose and Belting (SIC 3041) 78
32 Emission Factors for Fabricated Rubber Products,
N.E.C. (SIC 3069) 79
33 Emission Factors for Gaskets/ Packing, and Sealing
Devices (SIC 3293) 79
34 Emission Factors for Rubber Wire-Insulating (SIC 3357) 80
35 Emission Factors for Tire Retreading (SIC 7534) ... 80
36 Parameters Used to Define the Representative Plants . 81
37 Primary Ambient Air Quality Standards and Threshold
Limit Values for Pollutants Considered 84
38 Source Severities for Representative Elastomer Plants 85
39 Source Severities for Representative Rubber Product
Plants 86
40 Affected Population by Representative Rubber Processing
Plants 88
41 Nationwide Emissions of Criteria Pollutants from
Rubber Processing Industries 89
42 Percent Contribution of Hydrocarbon Emissions from
Rubber Processing to Total State Emissions 90
43 Percent Contribution of Particulate Emissions from
Rubber Processing to Total State Emissions 91
44 Best Control Techniques and Their Control Efficiencies
for Elastomers Industry 94
45 Best Control Techniques and Their Control Efficiencies
for Rubber Products Industry 94
46 Rubber Consumption Forecast for 1980 105
xii
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ABBREVIATIONS AND SYMBOLS
A — affected area
AAQS — ambient air quality standard
C — generally achieved control efficiency
c — amount of volatile materials lost during vulcanization
c — initial weight percent of component
D — composite population density
D.^ — population density for state i
e — 2.72
EPR — ethylene propylene rubber
E — representative emission factor
E — uncontrolled emission factor
F — hazard factor, equal to the primary ambient air
quality standard for criteria pollutants or to a
reduced TLV for other pollutants
H — effective emission height
m — a constant I
Q — mass emission rat'e
R — thickness of rubber stock
S — source severity
SBR — styrene-butadiene rubber
t — averaging time
t1 — time
TLV — threshold limit value
t — short-term averaging time, 3 min
U — utilization factor
u — average wind speed
x — downwind distance from source
0 — vertical dispersion coefficient
£»
X"(x) — annual mean ground level concentration
v — instantaneous maximum ground level concentration
max
Y — time-averaged maximum ground level concentration
Amax ^
xm
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CONVERSION FACTORS AND METRIC PREFIXES
CONVERSION FACTORS
To convert from
Degree Celsius (°C)
Gram/kilogram (g/kg)
Kilogram (kg)
Kilogram (kg)
to
Meter
Meter
Meter
Meter
Meter2
Meter3
Meter3
Meter3
Metric
Metric
(m)
(m)
(m)
(m)
(m2)
(m3)
(m3)
(m3)
ton
ton
-Pascal (Pa)
Radian (rad)
Degree Fahrenheit
Pound/ton
Pound-mass (avoirdupois)
Ton (short, 2,000 pound
mass)
Angstrom
Foot
Micron
Mile
Foot2
Barrels (42 gallon)
Foot3
Gallon (U.S. liquid)
Kilogram
Ton (short, 2,000 pound
mass)
Pound-force/inch2 (psi)
Degrees (°)
Multiply by
t° =
T1
1.8 t° + 32
\*
2.000
2.204
1.102 x 10 3
1.000 x 1010
3.281
1.000 x 106
6.215 x 10"1*
1.076 x 101
6.293
3.531 x 101
2.642 x 102
1.000 x 103
1.102
1.450 x 10"^
5.730 x 101
METRIC PREFIXES
Prefix Symbol Multiplication factor
Kilo
Mega
Milli
Micro
Nano
k
M
m
y
n
10 3
106
10~3
10"6
Example
5 x 103 grams
5 x 106 meters
5 x 10"3 meter
5 x 10"6 meter
5 x 10"9 meter
kg
Mm
mm
Vim
nm
ANSI/ASTM Designation:
American Society for Te;
Materials, Philadelphia, Pennsylvania, February 1976. 37 pp.
Standard for Metric Practice.
E 380-76e, IEEE Std 268-1976, American Society for Testing and
xiv
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SECTION 1
INTRODUCTION
The rubber processing source type considered in this report
includes the major industries involved in either production of
synthetic and reclaimed rubber or fabrication of rubber products
from natural, synthetic, and reclaimed rubber. The operation of
these rubber processing plants constitutes a source of air pollu-
tion. The objective of this work was to review the state of the
art of air emissions from rubber processing industries and to
assess the environmental impact of these emissions. Emission
data used in preparation of this report were obtained from lit-
erature and government sources.
The major results of this study are summarized in Section 2. The
detailed description of the source type in Section 3 includes a
general industry description, the manufacturing plant geograph-
ical distribution, and an outline of the processes involved.
Atmospheric emissions from rubber processing plants are discussed
in Section 4. In this section, the emission points, species of
emissions, and emission factors from each rubber processing in-
dustry are identified and quantified, and environmental effects
resulting from these emissions are presented. Present and future
aspects of pollution control technology in the rubber processing
industries are considered in Section 5. The projected industry
growth and anticipated technological developments are discussed
in Section 6.
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SECTION 2
SUMMARY
Rubber processing is defined in this assessment as either produc-
tion of vulcanizable elastomers (rubbers) or fabrication of rub-
ber products from natural, synthetic, and reclaimed rubber. The
industries included can be categorized into nine Standard Indus-
trial Classification (SIC) codes: 1) SIC 2822—styrene-butadiene
rubber (SBR) production; 2) SIC 3031—rubber reclaiming; 3) SIC
3011—tires and inner tubes; 4) SIC 3021—rubber footwear, 5) SIC
3041—rubber hose and belting; 6) 3069—fabricated rubber pro-
ducts N.B.C. ;a 7) SIC 3293—gaskets, packing, and sealing devices;
8) SIC 3357—wire insulating; and 9) SIC 7534—tire retreading.
The 1975 production rates of the above nine industries are shown
in Table 1.
TABLE 1. PRODUCTION RATE, EMISSION FACTORS, AND MASS
EMISSIONS FOR RUBBER PROCESSING INDUSTRIES
Industry
SBR production
(SIC 2822)
Rubber reclaiming
(SIC 3031)
Tires and inner tubes
(SIC 3011)
Rubber footwear
(SIC 3021)
Hose and belting
(SIC 3041)
Fabricated products N.E.C.
(SIC 3069)
Gaskets, packing, and
sealing devices
(SIC 3293)
Wire insulating
(SIC 3357)
Tire retreading
(SIC 7534)
TOTAL
1975
production,
10* metric ton
1,179
83
2,038C»d
140
400
p
997
160
516
475C
N.A/
Emission factors,
g/kg product
Hydro-
carbons
a
S.I3
3.0
16.1
99.3
6.6
A
6.2e
8.3
3.56
4.2
N.A.
Partic-
ulates
a
0.35
1.1
3.6
2.9
1.1
p
3.1
3.1
l.l6
2.0
N.A.
Mass emissions for
1975, metric tons/yr
Hydro-
carbons
K
6,000
250
33,000
14,000
2,600
6,200
1,300
180
2,000
65,000
Partic-
ulates
4.0
91
7,300
400
440
3,100
500
56
950
13,200
Percent contribution
to national
total emissions
Hydro-
carbons
0.024
0.0010
0.13
0.056
0.010
0.025
0.0052
0.0007
0.0080
0.26
Partic-
ulates
0.0023
0.0005
0.041
0.0022
0.0025
0.017
0.0028
0.0003
0.0053
0.074
Represents the composite ^mission factor for emulsion and solution polymerization.
63% of this is emitted from emission polymerization; the remaining 37% is from solution polymerization.
Average weight of a tire is 10.9 kg.
Inner tubes and other tire materials, which constitute 3% of the industry economy.
Based on amount of rubber compound consumed.
Not applicable.
Not elsewhere classified.
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There are approximately 1,700 rubber processing plants in 43
states.3 About 25% of these plants are located in Ohio and Cali-
fornia. Another 25% are located in Illinois, New York, New
Jersey, and Massachusetts. The remaining 50% of the plants are
distributed among the other 37 states.
SBR is produced by either emulsion or solution polymerization,
with the former process constituting 90% of present production
and the latter representing the remaining 10%. Operations in
rubber reclaiming include size reduction, fiber separation, de-
polymerization, drying, and finishing. Fabrication of rubber
products involves a number of steps such as compounding, milling,
calendering, extrusion, fabric cementing, rubber cementing, mold-
ing, and curing.
Hydrocarbons and particulates are emitted from various operations
in the rubber processing industries. Hydrocarbon emissions con-
sist of monomers, rubber chemicals, and solvents which are vola-
tilized during the processing. Particulates consist of carbon
black, soapstone, zinc oxide, etc., which are emitted during com-
pounding, grinding, and talc dusting operations. Particulates
are also emitted as mists and solid particles which are formed by
condensation of hydrocarbons that are volatilized from rubber
material due to temperatures involved in the processing.
Emission factors and mass emissions from rubber processing indus-
tries are summarized in Table 1. Mass emissions from the nine
industries constitute 0.26% and 0.074%, respectively, of the
national totals of hydrocarbons and particulates emitted from all
sources. On the individual state basis, New Hampshire is the
only state which has emissions of at least one criteria pollu-
tant from rubber processing that exceeds 1% of the state total
emissions of that pollutant.
To quantify the hazard potential of emissions from each emission
source, a source severity, S, was defined as:
_ Xmax
S - -^
t
where Xmax is the time-averaged maximum ground level concentra-
tion of each pollutant emitted from a representative rubber pro-
cessing plant. F is the primary ambient air quality standard for
9This represents the total number of plants in each of the nine
industries except for SIC 7534, where only the plants that could
be identified were included.
Criteria pollutants in this study are hydrocarbons, particu-
lates, carbon monoxide (CO), sulfur oxides (SOX), and nitrogen
oxides (NOX), all of which have national ambient air quality
standards established.
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criteria pollutants and is the "reduced" threshold limit value
(TLV® • 8/24 • 1/100) for other pollutants.
One representative plant was defined for each type of rubber
processing, except for SBR production. For the latter, two rep-
resentative plants were defined, one for emulsion polymerization,
the other for solution polymerization. Factors considered in
defining these representative plants are summarized in Table 2.
TABLE 2. PARAMETERS USED FOR REPRESENTATIVE PLANTS
Population density around the plant: 103 persons/km2
Wind velocity around the plant: 4.5 m/s
Annual production, Emission height,
Industry metric tons/yr m
SBR by emulsion
(SIC 2822)
SBR by solution
(SIC 2822)
Rubber reclaiming
(SIC 3031)
Tires and inner tubes
(SIC 3011)
Rubber footwear
(SIC 3021)
Hose and belting
(SIC 3041)
Fabricated products N.E.C.
(SIC 3069)
Gaskets, packing, and sealing devices
(SIC 3293)
Wire insulating
(SIC 3357)
Tire retreading
(SIC 7534)
41,000
41,000
14,000
20,000
2,700
6,500
1,700
1,700
3,000
450
20
20
20
15
15
15
15
15
15
15
Using Gaussian plume dispersion theory together with the emission
factors and parameters for representative plants, source sever-
ities were calculated for each emission source in each of the
nine industries. These source severities are summarized in
Tables 3 and 4 for elastomer plants and rubber products plants,
respectively.
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TABLE 3. SOURCE SEVERITIES FOR REPRESENTATIVE ELASTOMER PLANTS*
Emission source
Styrene storage
(breathing)
Hexane storage
(breathing)
Storage area
(fugitive)
Reactor area
(fugitive)
Butadiene absorption
Monomer recovery area
(fugitive)
Desolvent area
(surge vent)
Desolvent area
(fugitive)
Purification area
(fugitive)
Carbon black operation
Size reduction
Depolymerization
Drying
Baling
SBR emulsion polymerization SBR solution polymerization Rubber reclaiming
(SIC 2822) (SIC 2822) (SIC 3031)
Criteria pollutants Chemical substances Criteria pollutants chemical substances Criteria pollutants
Hydrocarbons Particulates Styrene Butadiene Hydrocarbons Particulates Styrene Butadiene Hexane Hydrocarbons Particulates
•
0.01 0.001 0.01 0.001
0.03 0.002
0.02 0.002 0.02 _b _b _b
0.2 b b 0.2 b b b
1 _b 0.001
o.os _b _b
0.7 b b b
o.i _b _b _b
0>1 -b .b _b
0.07 0.07
0.08
0.5
0.3 0.005 0.02 b 9 0.005 0.8
0.007 0-007 0.008
Blanks indicate no emissions from unit operations. Not calculated due to lack of data.
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TABLE 4. SOURCE SEVERITIES FOR REPRESENTATIVE RUBBER PRODUCT PLANTS3
Emission source
Compounding
Milling
Calendering
Fabric cementing
Extrusion
Undertrade cementing
Green tire spraying
Buffing
Rubber cementing
Latex dipping and drying
Bonding of extruded parts
Adhesive spraying
Molding
Curing
Finish painting
SIC 3011
Hydro- Partic-
carbons ulates
0.1 0.2
0.09
0.09
0.4
0.09
0.4
4
2 0.5
SIC 3021
Hydro- Partic-
carbons ulates
0.02 0.03
0.01
0.01
6
0.01
0.1 0.03
0.1 0.02
SIC 3041
Hydro- Partic-
carbons ulates
0.04 0.07
0.03
0.03
0.3
0.003
0.1
0.4
SIC 3069
Hydro- Partic-
carbons ulates
0.01 0.02
0.006
0.006
0.001
0.005
0.008
0.07
0.06 0.03
0.07
SIC 3293
Hydro- Partic-
carbons ulates
0.01 0.02
0.008
0.008
0.1
0.2 0.03
SIC 3357 SIC 7534
Hydro- Partic- Hydro- Partic-
carbons ulates carbons ulates
0.02 0.03
0.01
0.002
0.01
0.03
0.2 0.008
0.004
Blanks indicate no emissions from unit operations.
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The number of persons exposed to an annual average ground level
concentration (x) of a pollutant from a representative plant for
which x/F is greater than 0.1 and x/F is greater than 1 was esti-
mated and designated as "affected population. " The calculation
was made for each pollutant and for each operation with a source
severity greater than 0.1. The largest number of persons
affected by any operation in each industry is given in Table 5
for both x/F is greater than 0.1 and x/F is greater than 1.
TABLE 5. AFFECTED POPULATION BY INDUSTRY
Number o^ persons Number of_persons
Industry code _ wherex/F>0.1 where x/F>l
SIC
SIC
SIC
SIC
SIC
SIC
SIC
SIC
SIC
SIC
2822 (emulsion)
2822 (solution)
3031
3011
3021
3041
3069
3293
3357
7534
500
4,000
200
800
1,000
80
0
20
20
0
1
20
300
0
60
100
0
0
0
0
0
I
The consumption of rubber in rubber products fabrication is
expected to increase at an average (simple) annual rate of 3%
between 1975 and 1980. Assuming that the same level of control
exists in 1980 as in 1975, emissions from rubber processing will
increase by 15% over that period.
Because most of the rubber processing operations resulting in air
emissions are not enclosed, the control of emissions from these
sources involves collection of the contaminated gas and removal
of the pollutants from the gas. Most rubber processing plants
have some types of particulate control devices. Only a few
operations have hydrocarbon control equipment .installed. Control
devices used in the industry for particulate control include wet
scrubbers, cyclones, and baghouses. For hydrocarbons, carbon
adsorption with solvent recovery and incineration with heat
recovery have been used. Overall control efficiency for particu-
lates ranges from 70% to 90%. For hydrocarbons it ranges from
40% to 90%, largely dependent on the gas collection efficiency.
-------
SECTION 3
SOURCE DESCRIPTION
SOURCE DEFINITION
This source type includes the major industries involved in either
production of synthetic and reclaimed rubber (vulcanizable elasto-
mers) or fabrication of rubber products from natural, synthetic,
and reclaimed rubber. Natural rubber production is not included
because no natural rubber is produced in the United States. The
industries in this source type can be categorized into nine Stand-
ard Industrial Classification (SIC) codes, defined by the U.S.
Government as follows (1):
• Synthetic Rubber (Vulcanizable Elastomers) (SIC 2822)
This industry "comprises establishments primarily engaged
in the manufacture of synthetic rubber by polymerization
or copolymerization. An elastomer, for the purpose of
this classification, is a rubberlike material capable of
vulcanization, such as copolymers of butadiene and sty-
rene or butadiene and acrylonitrile, polybutadienes,
chloroprene rubbers, and isobutylehe-isoprene copolymers."
(Only the production of styrene-butadiene rubber is con-
sidered in this assessment.
• Tires and Inner Tubes (SIC 3011)
This industry "includes establishments primarily engaged
in manufacturing pneumatic casings, inner tubes, and
solid and cushion tires for all types of vehicles, air-
planes, farm equipment, and children's vehicles; tiring;
and camelback and tire repair and retreading materials."
'• Rubber and Plastics Footwear (SIC 3021)
This industry "includes establishments primarily engaged
in manufacturing all rubber and plastics footwear. . .
having rubber or plastic soles vulcanized to the uppers."
(Processes specific to the utilization of plastics within
this industry are excluded from further consideration in
the assessment of the rubber processing source.)
(1) Standard Industrial Classification Manual, 1972. Executive
Office of the President, Office of Management and Budget,
Washington, D.C., 1972. 649 pp.
-------
• Reclaimed Rubber (SIC 3031)
This industry "includes establishments primarily engaged
in reclaiming rubber from scrap rubber tires, tubes, and
miscellaneous waste rubber articles by processes which
result in devulcanized, depolymerized, or regenerated
replasticized products containing added.ingredients.
These products are sold for use as a raw material in the
manufacture of rubber goods with or without admixture
with crude rubber or synthetic rubber."
• Rubber and Plastics Hose and Belting (SIC 3041)
This industry "includes establishments primarily engaged
in manufacturing rubber and plastics hose and belting,
including garden hose." (Processes specific to the util-
ization of plastics within this industry are excluded
from further consideration in the assessment of the rub-
ber processing source.)
• Fabricated Rubber Products N.E.C. (SIC 3069)
This industry "includes establishments primarily engaged y
in manufacturing industrial and mechanical rubber goods,
rubberized fabrics and vulcanized rubber clothing, and
miscellaneous rubber specialties and sundries."
• Gaskets, Packing, and Sealing Devices (SIC 3293)
This industry "includes establishments primarily engaged
in manufacturing gaskets, gasketing materials, compres-
sion packing, molded packings, oil seals, and mechanical
seals. Included are gaskets, packing, and sealing
devices made of leather, rubber, metal, asbestos, and
plastics." (Only the segment of this industry which
utilizes rubber is considered in the assessment of the
rubber processing source.)
• Nonferrous Wiredrawing and Insulating (SIC 3357)
This industry "includes establishments primarily engaged
in drawing and insulating, and insulating wire and cable
of nonferrous metals from purchased wire bars, rods, or
wire." (Only the segment of this industry which util-
f izes rubber is considered in this assessment.)
• Tire Retreading and Repair Shops (SIC 7534)
This industry "includes establishments primarily engaged
in repairing and retreading automotive tires. Establish-
ments classified here may either retread customers' tires
or retread tires for sale or exchange to the user or the
trade."
The nine industries defined above can be separated into two indus-
try categories: the elastomer industries and the rubber products
industries. The former produce rubber materials; the latter are
concerned with consumption of rubber. This classification is
illustrated as follows:
-------
Elastomer industries Rubber products industries
Synthetic rubber Tire and inner tubes (SIC 3011)
(SIC 2822) Rubber footwear (SIC 3021)
Reclaimed rubber Rubber hose and belting (SIC 3041)
(SIC 3031) Fabricated rubber products N.E.C. (SIC 3069)
Rubber gaskets, packing, and sealing devices (SIC 3293)
Rubber wire-insulating (SIC 3357)
Tire retreading and repairing (SIC 7534)
Following is a general description of these two industry
categories:
Elastomers Industry
Synthetic Rubber (SIC 2822)—
The synthetic rubber (elastomer) industry produces high polymers
with special, unique properties. Elastomers are considered apart
from other polymeric materials because of these unusual proper-
ties and because they generally do not lend themselves to plas-
tics uses. By definition, the synthetic elastomer activities
start with a monomer, other active chemicals, or with natural
elastomeric polymers, and terminate with the formation of a
marketable, rubberlike material.
The major raw materials are active monomer, certain chemicals
with active end groups, or natural elastomers which are com-
pounded or modified. Many of the same monomers are used in the
synthetic elastomer industry as are used in plastics and fibers.
Table 6 shows the 1975 production of synthetic elastomers (2).
Natural elastomers were not included because they are not pro-
duced in the United States. For the past few years (1970 to
1975) , approximately 78% of the new elastomers consumed in the
U.S. were synthetic; consumption of natural elastomers amounted
to 22% (3).
The chemical composition of an elastomer depends solely on the
monomers, active chemicals, or natural materials used. The raw
materials, or feedstocks, also determine the type and properties
of the product produced. The properties of the products are, in
turn, usually determined by their end use. The structure, molecu-
lar weight, and various properties of elastomers are also deter-
mined by the polymerization process, as well as by the. catalysts,
shortstops, antioxidants, and other ingredients used.
Only the production of styrene-butadiene rubber is covered in
this assessment.
(2) Facts and Figures for Chemical Industry. Chemical and Engi-
neering News, 55(23):39-79, 1977.
(3) Year of Recovery for Rubber Suppliers. Rubber World,
175(4):35-37, 1977.
10
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TABLE 6. 1975 U.S. PRODUCTION OF SYNTHETIC ELASTOMERS
1975 Production,
Product type 10 3 metric tons
Styrene-butadiene rubbera 1,179
Butyl rubber 80
Neoprene 144
Nitrile rubber 55
Polybutadiene 290
Polyisoprene 61
Ethylene-propylene 84
Others5 47
TOTAL 1,940
Excludes high styrene latex.
Includes polyacrylate, polyalkylene sulfide,
chlorosulfonated polyethylene, polyisobutyl-
ene, fluorocarbon silicone, and polyurethajie
elastomers. Polyurethane foam is excluded
because it is a plastic material which is
considered in SIC 2821.
As shown in Table 6, among the various synthetic rubbers, styrene-
butadiene has by far the largest production figure, representing
61% of total synthetic rubber production. It is this segment of
the synthetic rubber industry that is considered in the present
study.
Reclaimed Rubber (SIC 3031)—
Reclaimed rubber is the product resulting from the treatment of
ground scrap tires, tubes, and miscellaneous waste rubber arti-
cles with heat and chemical agents to facilitate devulcanization
or regeneration of the rubber compound to its original plastic
state. It can be used as a partial or complete replacement for
new rubber in many fabricated rubber products.
It has been reported (4) that the 1975 production of reclaimed
rubber in the United States amounted to 83,000 metric tons.
al metric ton equals 106 grams; conversion factors and metric
system prefixes are presented in the prefatory material.
(4) Hoogheem, T. J., C. T. Chi, G. M. Rinaldi, R. J. McCormick,
and T. W. Hughes. Identification and Control of Hydrocarbon
Emissions from Rubber Processing Operations. Contract
68-02-1411, Task 17, U.S. Environmental Protection Agency,
Research. Triangle Park, North Carolina. (Final report
submitted to the EPA by Monsanto Research Corporation, July
1977.) 383 pp.
11
-------
Rubber Products Industry
Consumption of new and reclaimed rubber by the industry can be
reported in three parts: 1) tires and tire products, including
pneumatic and solid tires, inner tubes, and tire retread and
repair materials; 2) other products, including footwear, belts,
hose, mechanical goods, foam sponge, and sundries; and 3) wire
and cable. This breakdown permits observation of trends in total
new rubber consumption. It also illustrates the dominant posi-
tion of tires and tire products, which consistently use 62% to
66% of all new rubber each year (5). Wire and cable use a small
part of the total, which has remained constant in absolute terms
but has declined from 3% to 1% over the years from 1958 to 1972
(5). The other products consume the remainder (about one-third)
of total new rubber production in manufacturing a great variety
of items (5). :
The reported production figures for the rubber products industry
in the seven SIC codes are presented in Table 7 for 1975 (4).
TABLE 7. PRODUCTION OF RUBBER PRODUCTS IN 1975 (4)
SIC
code
3011
3021
3041
3069
3293
3357
7534
1975 Production,
Industry 10 3 metric tons*
Tires and inner tubes
Rubber footwear
Rubber hose and belting
Fabricated rubber products N.E.C.
Gaskets, packing, and sealing devices
Nonferrous wire drawing and insulating
Tire retreading and repair
2,038b'C
140
400d
997°
160d
51b
475°
Based on product weight except otherwise noted.
Average weight of a tire is 10.9 kg.
Inner tubes and other tire materials, which constitute 7% of
the industry economy, are not included here.
Based on amount of rubber compound consumed.
PROCESS DESCRIPTION
Elastomer Production
Styrene-Butadiene Rubber—
Styrene-butadiene rubber (SBR) is a copolymer of styrene (or vinyl
(5) Richardson, J., and M. Herbert. Forecasting in the Rubber
Industry. Presented at the Joint Meeting of the Chemical
Marketing Research Association and the Commercial Develop-
ment Association, New York, New York, May 1974.
12
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benzene, CH2=CH-C6H5) and butadiene (CH2=CII-CH=CH2) • With the
exception of some special grades, the styrene content is 23.5
wt %; i.e., a molecular proportion in the chains of one styrene
unit to about six butadiene units (6). It is produced by two
different processes. The first, emulsion polymerization,
accounts for 90% of the total SBR production. Solution polymeri-
zation, the newer of the two, accounts for the other 10% of
production (7). Butadiene and styrene monomers are the chief raw
materials required to manufacture SBR. Others required in
smaller amounts are the various emulsifiers, modifiers (e.g.,
thiols), catalysts, shortstops, coagulating agents, antioxidants,
and antiozonants (8).
Emulsion Polymerization—Emulsion polymerization is basically the
polymerization of monomer droplets suspended in dilute aqueous
solution and stabilized by an emulsifier. In this process, the
polymerization reaction is initiated by free radicals generated
in the water phase. After the emulsifier forms spherical aggre-
gates or molecules called "micelles," monomer swells the mi-
celles, free radicals initiate polymerization, and a new phase is
formed; namely, latex particles. Monomer droplets in the aqueous
phase decrease in number and completely disappear at about 60%
conversion (4). Monomer in the latex particles can be reacted to
completion, but the polymerization rate decreases gradually with
conversion.
SBR is produced by emulsion polymerization as either rubber latex
or rubber crumb. The processes for each of these two types of
rubber are discussed below (4, 6-8).
Crumb rubber—A schematic flow diagram for crumb rubber pro-
duction by emulsion polymerization is presented in Figure 1.
Some monomers have inhibitors added to prevent premature polymeri-
zation during shipment and storage. The inhibitor is removed
before polymerization by passing the monomer through a caustic
scrubber in which a 20% NaOH solution is circulated.
Soap solution, catalyst, activator, and modifier are added to the
mixture of monomers before polymerization. The soap solution is
used to emulsify the monomers in an aqueous medium. The ingredi-
ents of this solution are generally a rosin acid soap and a fatty
(6) Allen, P. W. Natural Rubber and the Synthetics. John
Wiley & Sons, Inc., New York, New York, 1972. 255 pp.
(7) Development Document for Effluent Limitation Guidelines and
New Source Performance Standards for the Tire and Synthetic
Segment of the Rubber Processing Point Source Category.
EPA-440/l-74/013-a, U.S. Environmental Protection Agency,
Washington, D.C., February 1974. pp. 31-35.
(8) Morton, M. Rubber Technology, Second Edition. Van Nostrand
Reinhold Company, New York, New York, 1973. 603 pp.
13
-------
acid soap. The catalyst, usually a hydroperoxide or a peroxy-
sulfate, is a free-radical initiator. The activator facilitates
the generation of free radicals more rapidly and at lower tempera-
tures than thermal decomposition alone. The modifier adjusts the
chain length and molecular weight distribution of the polymeric
rubber during its formation.
Polymerization proceeds stepwise in a series of reactors. The
reactor train can produce either "cold" (4°C to 7°C, 0 kPa to
200 kPa) or "hot" (50°C, 380 kPa to 520 kPa) SBR. For "cold"
polymerization, the monomer/additives emulsion is cooled prior to
reaction, generally using an ammonia or methanol refrigerant
cooling medium. Each reactor has its own set of cooling coils
(to remove the heat of reaction) and each is agitated by a mixer.
The residence time in each vessel is approximately 1 hr. Conver-
sion of monomer to rubber is ordinarily carried out to 60% or
less. The reaction mixture is a milky white emulsion called
latex.
Shortstop solution is added to the latex exiting the reactors to
terminate polymerization at the desired conversion. Two common
shortstops are sodium dimethyl dithiocarbamate [(CHs)2NCSSNa] and
hydroquinone (1,4-dihydroxybenzene). The "stopped" latex is held
in blowdown tanks which serve as flow-regulating holding tanks.
•- —
Economics of synthetic rubber production require recovery and
purification of unreacted monomers which may comprise 10% to 40%
of the rubber latex solution. Butadiene is first stripped from
the latex in a vacuum flash tank at about 20°C to 30°C. The
butadiene vapors are absorbed or adsorbed and condensed, and
recycled to the feed area for mixing with fresh monomer. Styrene
is recovered from the latex in perforated plate-stripping columns
which operate with steam injection at 60°C. The steam-styrene
vapor mixture is condensed, followed by decanting the styrene and
water. The top styrene layer is recycled.
An antioxidant is added to the stripped latex in a blend tank to
protect the polymer from oxidation. Different batches, recipes,
or dilutions of the stabilized latex can now be mixed in the
blend tanks.
The latex is transferred from the blend tank to the coagulator
where dilute sulfuric acid (pH 4.0 to 4.5) and sodium chloride
solution are added. This acid-brine mixture, called the "coagula-
tion liquor," causes the rubber to precipitate out of the latex.
Carbon black and/or extender oils can be added to the rubber
latex during coagulation; carbon black is added as an aqueous
slurry (approximately 5 wt %), and the oil in an aqueous emulsion.
The precipitated crumb is separated from the coagulation liquor
on a shaker screen. The screened crumb is washed with water in a
reslurry tank to remove extraneous compounds, particularly
14
-------
Ul
LATEX
-STORAGE
-^
' |
COAGULATION
AND
SCREENING
CRUMB RINSING
ANDDEWATERING
DRYING
BALING
LIVE J STEAM.
STEAM
PRODUCT
SHIPMENTS
Figure 1. Schematic flow diagram for crumb rubber production
by emulsion polymerization (8).
-------
residual coagulation liquor. The crumb rubber slurry is next
dewatered using vacuum filtration. Coagulation liquor blowdown
and crumb slurry water overflows are usually passed through
separators called crumb pits to trap the floatable crumb rubber.
The rinsed, filtered rubber solids are finally dried with hot air
(50°C to 120°C) in a continuous belt or screen dryer. After dry-
ing, the rubber is weighed and pressed into bales. Normally,
bales of synthetic rubber weigh 34 kg and are wrapped in poly-
ethylene film.
Latex rubber-'VLatex rubber production includes the same
processing steps as emulsion crumb production with the exception
of latex coagulation and crumb rinsing, drying, and balingjf In
some instances, the latex polymerization reaction is taken to
completion (98% to 99% conversion) as opposed to 60% conversion
for emulsion crumb rubber. Therefore, in these instances, the
recovery of unreacted monomers is not economical.
Monomer inhibitors are removed by scrubbing with caustic soda.
Soap solution, catalysts, and modifiers are added to the mono-
mer (s) prior to feeding the reactors. Fewer reactors are gener-
ally used than for emulsion crumb production. Most latexes "are
made by the "cold" process with the polymerization temperature
kept at about 4°C to 7°C. After polymerization, the latex is
sent to a blowdown tank for holding. At this point, stabilizers
are added.
Latex passes from the storage tanks to a vacuum stripper for
removal of unreacted butadiene. Excess styrene is separated from
the latex in a steam stripper, condensed, containerized, and sent
to disposal.
The stripped latex is passed through a series of screen filters
to remove undesirable large solids. The latex is then stored in
blending tanks for mixing with other ingredients of the final
product such as antioxidants.
A schematic flow diagram for latex rubber production by emulsion
polymerization is shown in Figure 2.
Solution Polymerization—Solution polymerization is the newer
process for the-production of synthetic crumb rubber in the
United States. Solution polymerization systems permit the use of
stereospecific Ziegler-Natta or alkyllithium catalysts, which
allow polymerization of monomers, in an appropriate organic
solvent to obtain the cis structure characteristic of the natural
rubber molecule.
In contrast to emulsion polymerization, where approximately 60%
conversion of monomer to polymer is achieved, solution polymeriza-
tion systems typically proceed to conversion levels in excess of
16
-------
VACUUM SOURCE
( STEAM JET OR
VACUUM PUMP I
WITHOUT CONDENSER
EJECTOR
I STEAM JET OR
VACUUM PUMP )
WITH CONDENSER
MONOMER AND
STEAM CONDENSATE
UNIHIBITED
LIQUID
WASTE
LIGHT MONOMER
—VACUUM
DISTILLATION
HEAVY MONOMER
STEAM STRIPPING
LIVE STEAM
LATEX BLENDING
AND BULK
STORAGE
PRODUCT
SHIPMENTS
Figure 2. Schematic flow diagram for latex rubber production
by emulsion polymerization (4).
-------
r
90%. The solution polymerization reaction is also more rapid; it
is usually complete in 1 hr to 2 hr. The copolymers produced by
this process are like -emulsion SBR but with several improved
properties. They are reported to have better abrasion resistance,
better flex, higher resilience, and lower heat build-up than the
emulsion rubbers. However, they tend to be thermoplastic and are
not recommended for tire use (8).
Figure 3 is a generalized materials flow diagram for the produc-
tion of crumb SBR by a solution polymerization system (4).
Monomers as received, containing inhibitors, are first stripped
of these compounds by passage through a caustic soda (NaOH)
scrubber. The monomers are then freed of extraneous water, using
either fractionating towers or molecular sieves. Fresh and
recycled solvents are also passed through a drying column to
remove water and unwanted light and heavy components which form
as byproducts during polymerization. Drying is crucial since
ionic solution polymerizations using Ziegler-Natta coordination
catalysts are extremely sensitive to polar compounds such as
water, oxygen, and certain oxygenated organic species. A few ppm
of water are a necessary and controllable maximum in any of the
feed streams to the polymerization reactor. Similarly, active
hydrogen compounds and certain hydrocarbons (acetylenes, cyclo-
pentadiene, cyclopentene) must be excluded.
The purified solvent (usually hexane) and monomers are next
blended to form the "mixed feed." This mixture can be further
dried to remove any remaining traces of water using a desiccant
^column.
The dried mixed feed of solvent plus monomers is then ready for
polymerization. Catalysts can be added to the mixed feed just
prior to polymerization or they can be fed directly to the reac-
tor. In some cases, catalyst solutions may be premixed with a
portion of the monomers under vigorous agitation to enhance
activity and to ensure uniform distribution in the reactor.
_^
The blend of solvent, monomer, and catalyst is polymerized in a
series of vessels. The exothermic heat of reaction is continu-
ously removed through the use of chilled reactor jackets or
internal cooling coils, the latter employing an ammonia refriger-
ant, chilled brine, or glycol solutions. Temperature control is
important to ensure the desired average molecular weight and
molecular weight distribution.
At a rubber solids concentration of 8% to 10%, the solution
viscosity is at a level beyond which further conversion of mono-
mer to polymer is inadvisable. Thus, the mixture exits the
reactor train in the form of a rubber cement. Polymerization is
halted by adding a shortstop solution. The stabilized cement is
then pumped to storage tanks prior to further processing.
18
-------
RECYCLE SOLVENT
TO MONOMER
RECOVERY
ORGANIC
DECANT LAYER
BOTTOM
DECANT LAYER
STEAM,
SOLVENT AND
MONOMER VAPORS
PRODUCT
SHIPMENTS
NOTE
EXTENDER OIL AND
CARBON BLACK ARE
NOT ADDED TO
NONEXTENDED RUBBER
TYPES
Figure 3. Schematic flow diagram for crumb rubber production
by solution polymerization (4).
-------
I Excess residues of coordination catalysts are detrimental to the
\ aging stability of polymeric rubbers. Therefore, the undesirable
\ residues are removed as soluble salts in a washing and decanting
\operation, sometimes using an alcohol or an alcohol/water
jsolution.
At this point, other chemicals and ingredients are added. An
antioxidant is added to the viscous rubber solution to prevent
deterioration of the polymer. A metered flow of a suitable oil
is also added here if the product is to be "oil-extended." Oil-
extending reduces the melt viscosity of the rubber to that re-
quired for compounding in subsequent applications. The oil is
usually blended with the cement at some point between the storage
tanks and the steam-stripping operation.
Inert fillers, such as clay, whiting, or barytes, are sometimes
added to certain solution-polymerized rubbers to facilitate hand-
ling the rubber mixture. In these cases, reinforcing fillers
such as carbon black are added, in a process known as "master-
batching," to improve unsatisfactory properties of the rubber.
The rubber cement is pumped from storage to the coagulator where
rubber is precipitated in crumb form with hot water under violent
agitation. Surfactants may be added to control crumb size and to
prevent reagglomeration. In addition to coagulation, this opera-
tion partially vaporizes the solvent and the unreacted monomer;
these vapors pass overhead.
In the area collectively known as the desolvent area, the result-
ant crumb slurry passes to steam strippers to drive off the
remaining solvent and monomer. The equipment generally consists
of either a flash tank or an agitated kettle stripper. Steam,
solvent, and monomer vapors pass overhead to a condenser and
decanter for recovery. The bottom decant layer, saturated in
solvent and monomer, is discharged. The organic layer is sent to
a multistage fractionator. Light fractions are removed in the
first column. These generally consist of unreacted light monomer;
"e.g., butadiene. This is usually reclaimed at the monomer supply
plant. The second column produces purified solvent, a heavy
monomer-water fraction, and other heavy components.
The heavy monomer (i.e., styrene) is condensed, decanted, and
recycled. The bottom water layer is discharged. The purified
solvent is dried before reuse. The extraneous heavy components
stream is waste which can be either decanted before disposal or
incinerated as a slop oil.
The stripped rubber crumb slurry is separated and washed with
water on vibrating screens. Part of the slurry rinse water is
recycled to the coagulator with water or steam makeup. The
remaining portion is discharged as overflow. The screened rubber
is passed through an extruder-dryer for further dewatering and
20
-------
drying. As the rubber is extruded through a perforated die
plate, the mechanical action of the screw heats the material in
the barrel to about 143°C. Dewatering and drying could also be
accomplished using a rotary filter and a hot-air oven dryer. The
dried rubber, usually in the form of pellets, is pressed into 34-
kg bales and usually wrapped in polyethylene for storage and
shipment. ^-
Reclaimed Rubber (4)—
There currently three different process technologies used by the
rubber reclaiming industry in the United States: the digester
process, the pan (or heater) process, and the mechanical process.
The most common reclaiming technique is the pan process, which
has almost replaced the digester process, the oldest of the
three. The mechanical process is the least conventional one, and
as such, it is not widely practiced. All three processes use
similar methods of rubber scrap separation and size reduction.
The differences show up in the depolymerization and final process-
ing. Figure 4 is a generalized schematic flow diagram for rubber
reclaiming.
Metal Removal, Size Reduction, and Fiber Separation—Scrap rubber
received at a reclaiming plant is first sorted to remove steel-
belted or studded tires, which can be either sent to special
processing facilities or discarded as waste. Brass and steel
valve stems and valve seats aire manually removed from the remain-
ing tires. The bead wire, which serves to secure the tire to the
wheel rim, may also be cut out of the tire at this time.
Next the scrap rubber is size reduced using either crackers or
hammer mills. The cracker is a two-roll machine, having working
roll lengths of 0.76 m to 1.07 m and diameters of 0.46 m to
0.81 m. Each roll is axially corrugated, and the two rotate in
opposite directions at different speeds. As the rubber is drop-
ped into the cracker, the slower roll corrugations momentarily
"hold" the waste while the faster roll corrugations shear, slice,
crush, and abrade the waste. This process is repeated until all
the material passes through a screen of some predetermined mesh
size. Some reclaimers undertake futher size reduction down to
less than 1.7-mm size (10-mesh) using secondary and tertiary
crackers.
A hammer mill is essentially a high-speed rotating drum which
hammers the scrap rubber with pivoting "T" or "I" bars or with
knives located on the frame within which the drum revolves, with
or without a perforated plate or screen that retains the scrap
until it is sufficiently size reduced to pass through. The
machine containing drum knives may have a special feeding device
to control the input of the rubber waste.
Wastes containing reinforcing fiber materials, such as cotton,
rayon, nylon, polyesters, fiberglass, and metal, require either
21
-------
RUBBER SCRAP
RECEIVING
AND SORTING
CHEMICALS-
AND OILS
VALVE STEMS
AND VALVE SEATS
REMOVAL
SIZE REDUCTION
FIBER SEPARATION
FURTHER SIZE
REDUCTION
SCREENING
REUSEOR
"DISPOSAL
DEPOLYMERIZATION
DRYING
FILLERS
AND LIQUIDS
MIXING
REFINING
STRAINING
RECLAIMED
RUBBER
Figure 4. Generalized schematic flow diagram
for reclaiming rubber (4).
22
-------
mechanical fiber separation or chemical fiber degradation. The
ground rubber-and-fiber mixture is first separated into streams
of different particle size by a screener. These streams are
conveyed to separation tables which effectively separate loose
fiber from clean rubber by vibration and air flotation. This is
a continuous operation with recycle and with free scrap being
added at all times.
The fiber and rubber-fiber portions are next fed into hammer
mills for hammering or scraping. After the material has been
sufficiently size reduced to pass through a peripheral screen, it
is fed to sifters or beaters. In these machines, loose rubber
particles separate from the fiber and pass through a retaining
screen, while the fiber is conveyed for recycle, either to the
screener or to another set of hammer mills.
The final operation of the fiber separation process is baling the
waste fiber. This baled fiber is made up of small strands, less
than 38 mm long, and contains a small amount of entrapped rubber.
This fiber is discarded unless there is a market for its reuse.
Fiber-separated rubber is next subjected to fine grinding.
Crackers, similar to those used for primary size reduction, grind
the rubber to 550 ym (30-mesh) or smaller. Hammer mills can be
used for fine grinding but are not as efficient as crackers. The
finely ground rubber is then screened. Particles that pass
through the screens are ready for depolymerization, while the
remaining material is recycled for further size reduction.
Depolymerization—
Digester process—Digestion is a wet process using rubber
scrap that has been ground to thicknesses between 6.3 mm and
9.5 mm. The fine, fiber-free rubber particles are mixed with
water and reclaiming agents and fed to a jacketed autoclave.
These digesters can accommodate about 2,300 kg to 2,700 kg of
scrap, water, and chemicals in each reclaim batch. The digester
is agitated by a series of paddles on a shaft which is continu-
ously driven at a slow speed to maintain the charge in motion for
uniform heat penetration. The digestion liquor is heated by the
injection of steam, at pressures generally around 1.38 MPa for a
residence time of 8 hr to 12 hr. Reclaiming agents are fed to
the digester with the scrap rubber to accelerate depolymerization
and to impart desirable processing properties to the rubber.
Rubber scrap which has not been mechanically defibered requires
chemical degradation during digestion. In such cases, defibering
agents and plasticizing oils are added to complete the charge.
When the digestion is complete, the resultant slurry is blown
down under internal pressure into a blowdown tank. From there,
the rubber slurry is pumped to a holding tank where additional
water is added for dilution and washing. After agitation, the
23
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mixture is discharged onto vibrating screens where a series of
spray nozzles wash the rubber free from the digestion liquor and
hydrolyzed fiber. The washed scrap is then passed through a
dewatering press. A small amount of residual moisture is neces-
sary to prevent excessive buildup of heat during subsequent
refining.
Reclaiming agents that are used in the digester process include
petroleum- and coal-tar-based oils and resins as well as various
chemical softeners such as di- and trialkylphenol sulfides and
disulfides, mercaptans, and amine compounds. Preferred amines
include aliphatic long-chain (Ci0to GI^) amines and primary
amines. Reclaiming agents generally function by catalyzing the
oxidative breakdown of polymer chains and sulfur crosslinks. It
should be noted that natural rubber can be reclaimed without
using reclaiming chemicals.
Sodium hydroxide or calcium chloride and zinc chloride are used
as defibering agents in the digester process. The presence of
synthetic rubber, such as SBR, necessitates the use of metallic
chlorides instead of sodium hydroxide since the latter produces a
thermosetting effect with SBR.
Pan (or heater) process—Fiber-separated, fine-ground scrap
is reduced to an even smaller particle size by grinding on smooth
steel rolls. The rubber is next blended with reclaiming oils in
an open mixer and then placed in stacked shallow pans. The depth
of treated scrap in these pans may be 150 mm to 200 mm. The
stacked pans are placed on a carriage that can be wheeled into a
large horizontal heater, which is a single-shell pressure vessel.
In this method of depolymerization, live steam at 1.38 MPa to
1.55 MPa is introduced to the heater to directly contact the rub-
ber scrap. After this treatment, the heater is opened, and the
reclaimed scrap is unloaded and cooled. No drying is required
because the small amount of water remaining will assist in refin-
ing.
Mechanical process—Unlike the other two processes, mechani-
cal reclaiming is continuous. Fiber-separated, fine-ground
rubber scrap is fed into a high-temperature, high-shear machine.
The machine is a horizontal cylinder in which a screw forces
material along the chamber wall in the presence of reclaiming
agents and depolymerization catalysts. Temperatures generated
are in the range of 177°C to 204°C with time requirements between
1 min and 4 min. The discharged reclaimed rubber needs no drying.
Mixing, Refining, Straining, and Packaging—Reinforcing materials
such as clay, carbon black, and softeners are most commonly mixed
into the rubber using a horizontal ribbon mixer. This is an
enclosed rectangular box with a rounded bottom in which mixing is
accomplished by a horizontally driven continuous ribbon, by
24
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paddles, or by a combination of the two. The mixed rubber and
filler compounds are next intimately blended in a Banbury inter-
nal mixer. It usually takes between 1 min and 3 min to blend the
material in a single batch. Since extruders permit continuous
processing, more reclaimers are converting to that method of
blending.
The reclaimed material ("reclaim") next undergoes preliminary
refining on a short two-roll mill called a breaker refiner. The
smooth rolls are of different diameters and rotate at different
speeds so that there is a high friction ratio which tends to form
the stock into a smooth clean sheet, approximately 0.3 mm thick.
The temperature of the rolls is controlled by water cooling.
The sheet is dropped into a screw conveyor which carries the
reclaim to a strainer. The strainer is a heavy-duty extruder
which contains a wire screen with 1.7 mm to 370 ym (10-mesh to
40-mesh) openings held between two perforated steel plates in the
head of the machine. Straining removes such foreign materials as
glass, metal, wood, or sand from the rubber. After straining,
the rubber goes on to a second refiner called a finisher, which
is the same type of machine as the breaker. The final thickness
of the clean reclaim is between 0.05 mm and 0.25 mm.
Each reclaimer may complete his operations by sending his product
to the customer in the form of slabs, stacked on pallets, or in
bales. Slabs are made by allowing the thin sheet of reclaim to •,
wrap around a windup roll until the proper thickness is obtained, j
The wrapped layers are then cut off the roll, forming a solid *
slab of a certain length, width, and weight. Each slab, weighing
approximately 14 kg to 16 kg, is dusted with talc to prevent
sticking. After quality control approval, the slabs are piled on
pallets until the total weight is 680 kg to 910 kg, ready for
shipment. As an alternative to the slab process, the reclaim
sheet can be air conveyed to a baler, where the rubber is com-
pacted to form a bale of controlled weight. The bales are dusted,
bagged, stacked on pallets, tested, and shipped.
Rubber Products Fabrication
Common Feed Materials—
Common feed materials for rubber products fabrication include
rubber and rubber latex (including natural, synthetic, and
reclaimed material), and various rubber chemicals. Feed materi-
als specific to individual rubber product industries are dis-
cussed in the process description for each industry.
Rubber and Rubber Latex—
Natural rubber—Natural rubber is obtained by tapping the
tree Hevea brasiliensis and collecting latex from which the rubber
is separated by a process known as coagulation. Coagulation
occurs when various acids or salts are added. The rubber
25
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separates from the rubber serum as a white, doughlike mass, which
is then milled and sheeted to remove contaminants and to enable
drying. This rubber is known as natural rubber. Chemically, it
is built of approximately 5,000 isoprene units per molecule'in a
c-Ls (designated herein as cis) configuration. Besides the dry
natural rubber, the original latex is also concentrated and
transported for use in the production of foam, latex-dipped
goods, adhesives, etc.
Natural rubber is mainly used in the United States for truck
tires because of its heat-buildup resistance. Other reasons for
using natural rubber are its excellent properties and also
because rubber making machinery was designed to handle this
material. A 1974 total natural rubber consumption of 710,000
metric tons has been reported (9).
Synthetic rubber—The major types of synthetic rubber used
in fabricating rubber products and their chemical formulations,
properties, and preferred uses are summarized in Table 8 (10-12).
Several other elastomers are available. They are considered
specialty rubbers and are mostly limited by their cost to use in
areas where specific properties are desired. Examples of these
elastomers are listed below.
Thiokol (T), polysulfide rubber, has outstanding oil and
solvent resistance. However, its other properties are poor.
Silicone rubbers have excellent high and low temperature
resistance, good mechanical properties at high temperature,
low compression set, and fair oil resistance. Their cost,
however, restricts use mainly to aircraft and outer space
equipment. Due to their inertness and non-toxicity, the
silicone rubbers are also used for some food and surgical
applications.
EPR (EPM) is ethylene propylene rubber with good aging,
abrasion, and heat resistance. It exhibits excellent resist-
ance to oxygen, ozone, acids, alkalis, and other chemicals
over a wide range of temperatures. It is not oil resistant,
and its full utilization potential is not fully defined.
(9) Outlook 1974 - Part II: Status Report on Elastomeric Mater-
ials. Rubber World, 169 (5):38-46, 1974.
(10) fihr*»vp, N. R. Chemical Process Industries, Third Edition.
McGraw-Hill Book Company, New York, New York, 1967. 905 pp.
(11) Kent, J. A* Riegel's Handbook of Industrial Chemistry,
Seventh Edition. Van Nostrand Reinhold Company, New York,
New York, 1974. 902 pp.
(12) Rosnto, D. V. Injection Molding of Rubber. Rubber World,
166(6):45-61, 1972.
26
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TABLE 8. CLASSIFICATION OF RUBBERS (10-12)
ASTM D 1418 Common or
designation trade name
Chemical designation and formula
General properties and use
Natural
-CH? CHy-CHa CH2-
None
/ /
CH3 CHj
cis-1,4-Polyisoprene
IR
CR
BR
to
Polyisoprene
Neoprene
Butadiene
SBR (GR-S)
-CH2-C-CH-CH2-
Cl
Chloroprene
-CH2 CH2-CH2 CH2-
cis-1,4-Polybutadiene
-CH2-CH-CH-CH2-CH2-CH-CH2-CH.CH-CH2-
Buna N
Chloro-Butyl
Butadiene-styrene
-CH2-CH-CH-CH2-CH2-CH-
CN
Butadiene-acrylonitrile
CH3 CH3 CH3
I I I
-C-CH2-CH2-CHC1-C-CH-C-CH2-
CH3 CH3
Chloro-isobutylene-isoprene
CH2-0—C-CH2
CH3 H
Isoprene
CH2-CC1-CH-CH2
Chloroprene
t
CH2-CHCH-«I2
Butadiene
CH2-CHCH-CH2
Butadiene
CH2-CH
C5H5-
Styrene
CH2-CHCH-CH2
Butadiene
CH2-CHCN
Acrylonitrile
CH2-C—C-CH2
CH3 H
Isoprene
CH3
C=CH2
CH3
Excellent physical properties; good resistance to cutting, gouging,
and abrasion) low heat, ozone, and oil resistance; poor resist-
ance to petroleum-base fluids. Its use is still preferred in
applications that demand elasticity, resilience, tackiness, and
low heat buildup. It is indispensable for the treads of tires
for buses, trucks, and racing cars. Resilience properties are
utilized in engine mounts and suspension units of automobiles.
unique applications are in building foundations and bridge
bearings.
Same properties as natural rubber; requires less mastication than
natural rubber. The best replacement for natural rubber.
Excellent oxygen, ozone, heat, tearing, and weathering resistance;
good oil resistance; excellent flame resistance, high tensile
strength. Wire and cable industries, hose, extruded automobile
parts, low-voltage insulation, and protective clothing and
linings.
Excellent abrasion resistance, resistance to flex cracking, and
high resilience; used principally as a blend in other rubbers.
Used in tire treads, foams, and footwear.
Good physical properties; excellent abrasion and crack resistance;
poor strength, low resilience, low tear strength, poor tacki not
oil, ozone, or weather resistant; general purpose rubber used in
different proportions with natural rubber for tire treads. It
is used for tire carcasses, molded goods, shoe soles, flooring,
and insulation.
Excellent resistance to vegetable, animal, and petroleum oils;
poor low temperature resistance. Seals, gaskets, rubber rolls,
and hoses.
Excellent weathering resistance) low permeability to gases! good
resistance to ozone, acids, alkalis, and aging; low tensile
strength and resilience; incompatible with natural rubber.
Excellent air retention makes it suitable for inner tubes and
inner liners of tubeless tires. Also used for many automobile
components such as window strips. In its resistance to heat, it
plays an indispensable part in tire manufacture forming the hot
container for the hot water or steam required to vulcanize the
inside of tires. Its good electrical properties and low gas
permeability make it suitable for wire and cable insulation,
adhesives, coating compositions, and tank lining.
-------
Polyurethane rubber (AU) is a polyurethane diisocyanate with
exceptional abrasion, cut and tear resistance, high modulus,
and high hardness. It is not suited for normal tire service
because abrasion resistance decreases rapidly with increas-
ing temperature. The material is used in some small solid
tires, but its main applications are in foams and surface
coatings.
Hypalon (CSM) is chlorosulfonated polyethylene with excel-
lent resistance to ozone and strong chemicals like nitric
acid, sulfuric acid, chromic acid, hydrogen peroxide, and
strong bleaching agents. It has good heat resistance and
mechanical properties, limited colorability, fair oil resist-
ance, and poor low temperature resistance. Uses include
conveyor belts, steam hose tubes, 0-rings and gaskets in
ozone generators, miscellaneous molded goods, and coated
fabrics for outdoor use.
Fluoroelastomers (FDH) are fluorinated hydrocarbons with
excellent high temperature resistance, particularly in air
and oil. They are of limited use for cooking utensils.
.Reclaimed rubber—The third important feed material in the
rubber processing industry is reclaimed rubber, or vulcanized
rubber reworked to render it suitable as raw material. Reclaimed
rubber is obtained from rubber scrap, natural or synthetic in
origin, which is segregated into separate and compatible rubbers
and then graded according to quality and intended use. It is not
profitable to use reclaimed rubber unless it costs less than half
as much as virgin rubber. Its utilization therefore fluctuates
depending on the costs of virgin rubbers.
Rubber Chemicals—The commercial application of either raw
natural dry rubber or raw synthetic rubber is very limited. For
the great majority of users, the rubber must be modified, usually
by the addition of chemical agents having specific functions.
Exceptions include such uses as crepe rubber shoe soles; cement,
as in the familiar rubber adhesives; and adhesives in masking
tape.
Raw rubbers and rubber chemicals in prescribed proportions are
blended to obtain rubber having the required qualities. The
desirable properties achieved by rubber compounding are plastic-
ity, elasticity, toughness, softness, hardness, impermeability,
resistance to abrasion, etc. The variety of chemicals added in
the compounding step depends on the type of processing that will
follow and on final product use. The following is an example of
a rubber compound formulation.
28
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_ Ingredient Parts by weight
Rubber (such as SBR) 100
Sulfur ' 2
Zinc oxide 5
Stearic acid 3
Accelerator 1.5
Loading or filling pigment 50
Reclaim, softeners, extenders, colors, blowing
agents, antioxidants, antiozonants, odorants,
etc. As required
To identify the materials that are used in the fabrication of
rubber products, the following sections present the individual
compounds and their functions in rubber processing.
Antioxidants and stabilizers—Antioxidants and stabilizers
are needed to protect the rubber during its handling and shipment.
Generally, stabilizers are used to protect polymers during their
isolation and storage. The antioxidants protect the rubbers both
during processing and in the finished product. Most antioxidants
give good protection as stabilizers, but not all stabilizers give
satisfactory antioxidant activity. Natural rubber needs anti-
oxidants only, but the synthetic polymers require both. Table 9
(13) summarizes the commercially important rubber antioxidants
and stabilizers according to the three principal! groups: aryl-
amines, phenols, and phenolphosphides. Trade names of these
compounds are also given for easier compound identification.
Concentration levels of the stabilizers range from 0.5 to 1.25
parts of stabilizer per 100 parts of rubber.
Pigments—Any solid material that is mixed into rubber,
except for vulcanizing agents, may be referred to as a pigment.
Dry pigments can be classified as either reinforcing agents or
filling materials. The reinforcing agents improve the properties
of the vulcanizates; the filling agents serve as diluents.
Commonly used pigments and their average particle sizes are given
in Table 10. For example, every part of rubber used in tire
treads may contain 0.1 part of carbon black; tubes require even
more, and carcasses require only slightly less.
In the prepration of colored stocks, a sufficient quantity of a
background pigment with high hiding power (e.g., titanium pig-
ments) and organic dye are added to give the desired color. For
preparation of less bright shades, inorganic pigments such as
iron oxide, antimony sulfide, chromiun sulfide, chromium oxide,
cadmium selenide, and ultramarine blue are used. Basic require-
ments for colored pigments depend on their stability during
(13) Kirk-Othmer Encyclopedia of Chemical Technology, Second Edi-
tion, Vol. 17. John Wiley & Sons, Inc., New York, New York,
1968. 884 pp.
29
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TABLE 9. COMMERCIAL ANTIOXIDANTS (13)
Chemical name
Trade names or
Aldehyde-amine type
Aldol-1-naphthylamine
Butyraldehyde-aniline product
Acetaldehyde-aniline product
Aldol-aniline product
P_»P_* -Diaminodiphenylme thane
AgeRite Resin, Ace to AN
Antox
Crylene
Resistox
Tonox
Ketone-amine type
1,2-Dihydro-2,2,4-trijnethylquinoline resin
1,2-Dihydro-2,2,4-trimethyl-6-ethoxyquinoline
1,2-Dihydro-2,2,4-tr ime thy 1-6-phenylquinoline
1,2-Dihydro-2,2,4-trimethyl-6-dodecylquinoline
AgeRite Resin D, Flectol H, Aceto POD
Santoflex AW, Polyflex
Santoflex B
Santoflex DD
Diaryldiamine type
N_,!4' -diphenyl-p_^phenylenediaraine
Nyl^'di-fS-naphthyl-D-phenylenediamine
N_,N_f-dialkylphenyl-p_-phenylenediamine
AgeRite DPPD, JZF
AgeRite White, Aceto DIPP
Wingstay 100, Wingstay 200
Diarylannne type
Phenyl-1-naphthylamine
Phenyl-2-naphthylamine
Alkylated diphenylamine
Neozone A, Aceto PAN
Neozone D Special, AgeRite Powder, PEN, Aceto PEN
AgeRite Stalite, Octamine, Pennox A, Wytox ADP, Polylite
Ketone-diarylamine type
Diphenylamine-acetone, high-temperature product
Diphenylamine-acetone, low-temperature product
Phenyl-2-naphthylamine-acetone, low-temperature product
Diphenylamine-acetone-aldehyde product
AgeRite Superflex, BLE-25, Neozone L, Cyanoflex 100
Aminox
Betanox Special
BXA
Substituted phenol type
2,6-Di-*-butyl-4-methylphenol
Butylated hydroxyanisole
2-a-Methylcyclohexyl-4,6-dimethylphenol
Styrenated phenol
Hindered phenol
Butylated styrenated m,jD-cresol
CAO-1, DBPC, Tenamene 3, lonol, Amoco 533, Dalpac 4,
Deenax, Tenox BHT, CAO-3
Tenox BHA, Sustane BHA
Nonox WSL
AgeRite Spar, Wingstay S, Styphen 1
Wingstay T, Nevastain A, Cyanox LF, Santowhite 54
Wingstay V
Bisphenol type
4,4 'bis (2,6-t-Butylphenol)
2,2' -Methylenebis (4-methyl-6-t-buty Iphenol)
2,2' -Methylenebis <4-ethyl-6-t-buty Iphenol)
4,4'-Methylenebis(6-t-butyl-2-methylphenol)
4,4' -Methylenebis (2,6-di-t-buty Iphenol)
4,4' -Butylidenebis (6- t-butyl-3-methy Iphenol)
2,2' -Thiobis (4-methyl-6-t-buty Iphenol)
4,4'-Thiobis(6-t-butyl-2-methylphenol)
4,4' -Thiobis (6-t-butyl-3-methy Iphenol)
4,4'-Thiobis(3,6-di-sec-amyIphenol)
Bindered bisphenol
4,4' -Dioxydiphenyl
Alkylated polyphenol
Ethyl 712
Plastanox 2246, CAO-5
Plastonox 425
Ethyl 720
Binox M, Ethyl 702,'lonox 220
Santowhite powder
CAO-4
Ethyl 736
Santowhite Crystals
Santowhite L
AgeRite Superlite, Naugawhite, Pennox 0
Antioxidant DOD
Wingstay L
Hydroquinone type
Hydroquinone
Monobenzyl ether of hydroquinone
2,5-Di-t-amylhydroquinone
Tecquinol
AgeRite Alba
Santovar A
Aminophenols
N-butyl-p-aminophenol
fj-lauroyl-pj-aminophenol
2 , 6-Di-t-butyl-a-dimethy lamino-4-me thy Iphenol
4-Isopropoxy diphenylamine
Tenamene 1
Suconox 12
Ethyl 703
AgeRite Iso
Phosphite type
Modified high-molecular-weight hindered phenol phosphite AgeRite Geltrol
Tri(nonylphenyl)phosphite Polygard
2-Ethylhexyl octylphenylphosphite VC-1
30
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product cure and the requirements of the final product itself.
Other pigments that may be used for specific purposes include:
fibrous asbestos, for its stiffening effect and heat resistance;
cotton or other textile fibers, for the same purpose, but with
less heat resistance; graphite, to produce a lower friction
coefficient; ground cork, for compounds needing low density;
glue, as a stiffener; litharge or other lead pigments, where high
density is required for opacity to x-rays; and stiffening resins,
such as poly vinyl chloride, pheriolformaldehyde resins, poly-
styrene, or high-styrene/low-butadiene copolymer resins.
TABLE 10. PIGMENTS USED IN RUBBER COMPOUNDING (13)
Pigment
Carbon black
Whiting
Clay
Silica
Calcium silicate
Grade or trademark and company
CC
S30KMPC)
S300(EPC)
N440(FF)
N60KHMF)
N770(SRF)
N880 (FT)
N990 (MT)
Acetylene
Witco AA (Witco Chemical Co., Inc.)
Micronized (The Glidden Co.)
Witcarb R-12 (Witco Chemical Co., Inc.)
Witcarb R (Witco Chemical Co., Inc.)
Purecal V (Wyandotte Chemicals Corp.)
Purecal M (Wyandotte Chemicals Corp . )
Atomite (Thompson, Weinman)
Calcene TM (PPG Industries)
Catalpo (Freeport Kaolin)
Dixie (R. T. Vanderbilt Co., Inc.)
Hi-Sil (PPG Industries)
Silene EF (PPG Industries)
Average particle
diameter, nm
10 to 20
25 to 30
30 to 33
36
50 to 60
70 to 90
150 to 200
250 to 500
.43
3,900
i 1,500
' 145
50
40
1,500
1,500
100
800
1,000
25
30
Softeners, extenders, and plasticizers—A wide variety of
oils, tars, resins, pitches, and synthetic organic materials are
used as softeners in rubber compounding. These compounds do not
necessarily have any relation to the softness of the compounded
material. The softeners are used to decrease the material vis-
cosity for improved workability, reduce mixing temperature,
increase tack and stickiness, aid in dispersion of pigments,
reduce shrinkage, provide lubrication, and improve extrusion or
molding characteristics and the like. The term extended is
applied to materials that replace a portion of the rubber,
31
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usually with some processing advantage.
can also be used as diluents.
Both of these materials
Plasticizers are primarily used to lower the viscosity of the
uncured stock rubber during milling. They should not affect the
rate of vulcanization or properties of the cured rubber. The
ease of plasticization corresponds with the ease of oxidation,
which is in the following order: natural rubber > polyisoprene >
polybutadiene > polystyrene > polychloroprene > nitrile rubber.
The concentration of plasticizers applied to natural and syn-
thetic rubbers may range from 0.25 part to 1.5 parts per 100
parts of rubber material. The plasticizers are effective in
natural rubber, polyisoprene, and SBR. The other synthetic
rubbers are less affected by the presence of a plasticizer.
The best softeners are those which are good solvents for the rub-
ber. Table 11 lists some softeners and plasticizers used in the
processing of natural and synthetic rubber.
TABLE 11. TYPICAL SOFTENERS AND PLASTICIZERS
USED IN RUBBER COMPOUNDING (13)
Rubber type
Softener/plasticizer
Natural rubber (SBR)
Neoprene (CR)
Nitriles (Buna N)
Butyl rubber (IIR)
All petroleum fractions
Pine tars and resins
Coal tar fractions
Pentachlorothiophenol (RPA-6, Renacit VI) and its activated zinc salt (Endor)
Thioxylenols (Pitt-Consol 640)
2,2'-Dibenzamidodiphenyldisulfide (Pepton 22)
Zinc 2-benzamidothiophenoxide (Pepton 65)
Naphthenic petroleum fraction
Coal tar fractions
Esters
Dioctyl sebacate
Butyl oleate
Monomeric polyether
Triethylene glycol caprylatecaprate
Trioctyl phosphate
Coal tar fractions
Monomeric esters
Adipates
Sebacates
Tributoxyethyl phosphate monomeric fatty acid ester (Synthetics L-l)
Di(butoxyethoxyethyl)adipate (TP-95)
Triglycol ester of vegetable oil fatty acid (Plasticizer SC)
Coumarone - indene resins
Rosins
Modified phenolics
Tetrahydronaphthalene
Dibutyl phthalate
Dibutyl sebacate
Mineral oils
Paraffin wax
Petrolatum
Paraffinic and naphthenic oils
32
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Vulcanization and acceleration agents—When rubber is mixed
with sulfur and heated, vulcanization (cure) occurs. The terms
cure and vulcanization are interchangeable and may be defined as
the chemical reaction which combines the polymer molecules of
rubber by crosslinking into larger molecules, restricting their
further movement. Vulcanization changes the rubber to a strong
elastic substance which is tack free, abrasion resistant, and not
readily soluble in common solvents. Sulfur is the vulcanization
agent that has been used during the whole period of rubber's
existence. Regardless of how little or how much sulfur is used
in vulcanizing, some sulfur remains uncombined; it is known as
free sulfur. High sulfur materials that liberate sulfur at
vulcanizing temperatures, such as organic polysulfides, may be
substituted for sulfur. Examples of these compounds are tetra-
methylthiuram disulfide (Methyl Tuads), tetraethylthiuram disul-
fide (Ethyl Tuads), dipentamethylenethiuram tetrasulfide
(Tetrone A), 4,4'-dithiodimorpholine (Sulfasan R), selenium
diethyldithiocarbonate (Selenac), aliphatic polysulfide polymer
(Thiokol VA-7), and alkylphenol disulfides (Vultac 2,3).
Because some rubbers contain no unsaturation, they must be vul-
canized using different chemicals and techniques, such as perox-
ides or radiation. Another class of curing agents is found among
the organic peroxides, such as di-tept-(designated herein as
tert) butyl and dicumyl peroxides for SBR and silicone rubbers.
Terpolymers containing a known nonconjugated diene have been
developed and can use sulfur for vulcanization. Neoprene rubber
is vulcanized using zinc oxide and magnesium oxide. Butyl rubber
may be vulcanized using alkylphenol formaldehyde resins. Oxides
of certain metals such as lead and zinc are used to accelerate
the vulcanization.
Use of elemental sulfur as the vulcanizing agent requires the
addition of auxiliary materials to supply the desired properties.
The organic accelerator is the most important of these materials.
The accelerator has a strong influence on processing safety, the
rate of vulcanization, and the physical properties of sulfur-
vulcanized rubber. Accelerators are listed in Table 12.
Antiozonants—As their name suggests, antiozonants are used
to protect rubber from the effects of ozone. Ozone can cause se-
vere cracking in rubber articles, particularly under stress. For
example, rubber insulation used around electrical equipment, UV
lamps, and neon lights must contain antiozonants because of the
high ozone concentrations present. As a result of ozone attack
on rubber, there is a loss of double bonds. Consequently, highly
unsaturated rubbers (natural and styrene-butadiene) are most eas-
ily attacked. The antiozonants appear to work by forming a
protective film between the rubber and the ozone atmosphere.
Commercial antiozonants used for rubber protection are listed in
Table 13.
33
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TABLE 12. COMMERCIAL ACCELERATORS (13)
Ch**""cal na
Trade names or trademarked names
Aldehyde-amine reaction products
Acetaldehyde/ammonia
Formaldehyde/ethyl chloride/ammonia
Butyraldehyde/butylamine
Butyr aldehyde/aniline
Butyraldehyde/acetaldehyde/aniline
Formaldehyde/p-toluidine
Acetaldehyde/aniline
Heptaldehyde/aniline
2-Ethyl-2-hexenal/aniline
Hexamethylenetetraralne
Acetaldehyde Ammonia, Aldehyde Ammonia
Trimene Base
Accelerator 833
Accelerator 808, A-32, Beutene, Goodrite Pullman
A-100
Accelerator 8
Ethylidene Aniline
Hepteen Base
Fhenex
Aceto HMT, Methenamine NF
Arylquanidines
Diphenylguanidine
Di-0-tolylguanidine
Triphenylguanidine
Mixed diarylguanidines
Diphenylguanidine phthalate
Di-0-tolylguanidine salt of dicatechol borate
DPG
DOTG
Triphenylguanidine
Accelerator 49
Guantal
Permalux
Dithiocarbamates
Copper dimethyl-
Lead dimethyl-
Bismuth dimethyl-
Zinc dimethyl-
Selenium dimethyl-
Zinc diethyl-
Zinc dibutyl-
Zinc dibenzyl-
Selenium diethyl-
Tellurium diethyl-
Piperidinium pentamethylene-
Potassium pentamethylene-
Zinc pentamethylene-
Cadmium diethyl-
Sodium dibutyl-
Cumate
Ledate
Bismate
Methyl Zimate, Methyl Ziram, Methazate, Accelerator L, Eptac 1, Aceto ZDMD,
Vulcacure ZM
Methyl Selenac
Ethyl Zimate, Aceto ZDED, Cyzate E, Ethazate, Ethyl Ziram
Butyl Zimate, Butazate, Butyl Ziram, Cyzate B, Aceto ZDBD
Arazate
Ethyl Selenac, Ethyl Seleram
Tellurac
Accelerator 552
Accelerator 89
ZPD-Henley
Ethyl Cadmate
Butyl Namate, Pennac SDB, Tepidone, Vulcacure NB
Thiuram suitides
Tetramethylthiuram monosulfide
Tetrabutylthiuram monosulfide
Tetramethylthiurain disulfide
Tetraethylthiuram disulfide
Dipentamethylenethiuram tetrasulfide
Dimethyldiphenylthiuram disulfide
Thionex, Aceto TMTM, Cyuram MS, Unads, Monex, Mono Thiurad, TMTM-Henley
Pentex
Aceto TMTD, Cyuram DS, Methyl Thiram, Methyl Tuads, Thiurad, Thiuram M,
Tuex, Vulcacure TMD, Royal TMTD
Aceto TETD, Ethyl Thiram, Ethyl Thiurad, Ethyl Tuads, Ethyl Tuex, Thiuram E
Tetrone A, Sulfads
Accelerator J
Thiazoles
2'Mercaptobenzothiazole
Zinc benzothiazolyl mercaptide
2,2'-Dithiobis(benzothiazole)
2-Benzothiazyl-N,N-diethylthiocarbamyl sulfide
MET, Cap tax, Rotax, Mertax, Royal MBT, Thiotax, Akron MBT
Zetax, ZMBT, Pennac ZT, Vulcacure ZT, O-X-A-F, Bant ex, Zenite
MBTS, Altax, Thiofide, Royal MBTS, Akron MBTS
Ethylac •
Sulfenamides
N-t-Butyl-2-benzothiazole-
N-Cyclohexyl-2-benzothiazole-
N,N/ -Diisopropyl-2-benzothazole-
N-Oxydiethylene-2-benzothiazole
N- (2,6-dimethylmorpholine) -2-benzothiazole-
N-Diethyl-2-benzothiazole-
Santocure NS
Cydac, Comae S, Santocure, Delac S, Durax, Royal CBTS
DIBS, Dipac
AMAX, NOBS Special, Santocure MOR
Santocure 26
Accelerator AZ
Miscellaneous
Trimethylthiourea
Trialkylthiourea
1,3-Diethylthiourea
1,3-Bis (2-benzothiazolylmercaptomethyl) urea
2-Mercaptothiazoline
Thiate E
Thiate G
Pennzone E
El-Sixty
2-MT
34
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TABLE 13. COMMERCIAL ANTIOZONANTS (13)
Chemical name
Trade names or trademarked names
Symmetrical diamines
N,N'-di-B«c-butyl-p-phenylenediamine Eastozone 2, Gasoline AO-22
Jf,s'-dimethyl-NyN"-bis(1-methylpropyl)-p-phenylenediamine Eastozone 32
N,if1-bis(1-ethyl-e-raethylpentyl)-p-phenylenediamine Eastozone 31, UOP 88, Antozite 2, Santoflex 17
jl.jf1-bis(1-methylheptyl)-p-phenylenediamines Eastozone 30 OOP 288, Santoflex 217, Antozite 1
MiiTture of dialkylaryl-p-phenylenediamines Wingstay 100, wingstay 200
N,N'-bis(1,4-dtaethylpentyl)-p-phenylenediamine Eastozone 33, Antozite MPD, Santoflex 77
Unsymmetrical diamines
N-isopropyl-tl'-phenyl-p-phenylenediamine
Ii-phenyl-N.'-cyclohexyl-p-phenylenediamine
iJ-phenyl-ti'-sec-butyl-p-phenylenediamine
14-phenyl-N1 - (1,3-dimethylbutyl) -p-phenylenediamijie
tj-phenyl-N"-se£-octyl -p-phenylenediamine
Flexzone 3-C. Santoflex 36, Cyzone IP, Eastozone 34, Nonox ZA, A.O. 4010 HA
Flexzone 6-H, Santoflex 66, A.O. 4010
Plexzone 5-L
Antozite 67, Flexzone 7-L, Santoflex 13, UOP 588, Wingstay 300
UOP 688
Other types
1,2-Dihydro-2,2,4-trimethyl-6-ethoxyqu incline
Nickel dibutyldithiocarhamate
Nickel isopropylxanthate
Waxes
Santoflex AW, Polyflex
NBC
KPNI
Other rubber chemicals—
Blowing agents—Blowing agents are used to produce cel-
lular rubber (foam). They must be finely dispersed and of fine
size to give uniform pore-size product. The cellular structure
is formed by gases which are either generated within the compound
during vulcanization or dissolved in a compound under pressure.
Examples of blowing agents include-sodium bicarbonate, sodium
carbonate, ammonium bicarbonate, and ammonium carbonate. Some
organic materials which release nitrogen are: also in use; they
are listed in Table 14.
TABLE 14. BLOWING AGENTS WHICH RELEASE NITROGEN (13)
Chemicalname
Trade name
Azodicarbonamide
Azoisobutyronitrile
Diazoaminobenzene
Aiocyclohexylnitrile
K ,N' -dinitrosopentamethylenetetr araine
N ,N' dimethyl-N ,N' -dinitrosoterephthalamide
Benzenesulfonyl hydrazide
Benzene-l,3-disulfonyl hydrazide
p,p'-Oxybis(benzenesulfonyl hydrazide)
Diphenylsulfon-3,3' disulfonyl hydrazide
4,4'-Diphenyldisulfonyl azide
Celogen AZ, Genitron AC, Kempore R-125, Porofor K-1074
Genitron AZDN, Porofor N, Aceto AZIB, Warecel 70
CH3 CHj
NCC-N=N-CCN
I I
CH3 CH3
CgHgNHN^NCgHs Porofor DB, diazoaminobenzene
C6Hio(CN)N=N(CN)C6Hjo Genitron CHDN
H2 Onicel NO, DNPT, Opex, Vulacel
CH
CH2 — N - CH2
ONN CH2 NNO
III
CH2-N - CHz
CsHi,(CON(NO)C83)2
C6H5S02NHNH2
CeH^ tso2inniH2) 2
0(06^3021011012)2
S02 (C6Hi,SO2NHNH2) 2
Nitrosan
Genitron BSH, Porofor BSH
Porofor B-13
Celogen, Genitron OB, Porofor DO-44
Porofor D-33
Nitropore CL-100
35
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Organic activators—In some cases, even the addition of
an accelerator results in a slow rate of rubber cure. This rate
can be increased by incorporation of organic activators. Examp-
les of these compounds are given in Table 15.
Retarders—Prevention of premature cure during the
processing of rubber stock is important if fast accelerators are
used to prevent rubber scorching. Some of the retarding agents
are listed in Table 16.
TABLE 15. ORGANIC ACTIVATORS (13)
Composition Trade name
Primary fatty amines Alamine 7,46
Mono- and dibenzylamines DBA
Diphenylguanidine phthalate Guantal
Zinc salts of a mixture of fatty acids Laurex
Mixture of organic and inorganic acetates MODX
Dibutyl ammonium oleate Barak
Normal lead salicylate Normasal
Fatty acids and metal soaps
TABLE 16. COMMONLY USED RETARDERS (13)
Chemical name Trade name
Phthalic anhydride Retarder E-S-E-N
Benzoic acid
Salicylic acid
Maleic acid
Maleic anhydride
Terpene-resin acid blend Turgum S
N-nitrosodiphenylamine Goodrite Vultrol, Retarder J, Redax
Common Operations—
The large number of products made of rubber and rubber latex
required the development of many specific approaches to shape the
product (hose, belt, molded goods), combine it with other mate-
rials (fabric, wire), or produce proper rubber consistency (hard
rubber, foam, sponge). These approaches vary, based on product
specifications. In general, however, all rubber product fabrica-
tion processes consist of 1) preparation of a rubber or latex
compound; 2) forming the compound into the desired shape (calen-
dering, molding, extrusion, dipping), and 3) product vulcaniza-
tion or curing.
36
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Compounding—One of the most important stages in rubber proc-
isliing is compounding .(mixing). It governs the quality of the
final product since all the process steps that follow depend on
an adequate and uniform mix. Mixing must provide 1) a uniform
blend of all the constituents of the mix, 2) an adequate disper-7
sion of the pigments, and 3) uniformity in consecutive batches"'
for smooth further processing. Mixing can be carried ^Oxit'on a
two-roll mixer or an internal mixer such as the Banbury mixers.
All of these mixers are designed for batch operation.
The jbatch size processed on a mill can very depending on mixing
equipment capacity. Mills are available with roll sizes ranging
from 0.35 m to 1.07 m. The smaller sizes are more popular due to
the better batch control they provide. Mixing equipment capacity
ranges from 68 kg to 136 kg for a 2.3-m mill and 454 kg or more
for the largest internal mixers.
The roll mill consists of two parallel horizontal rolls rotating
in opposite directions at slightly different speeds. The rubber
is worked by being pulled through the nip. The temperature in
the roll mill is controlled by passing cold or hot water, steam,
or hot oil through the hollow rolls. The nip width is adjust-
able. Rubber comes out of the roll mill as a sheet, which is
cut to proper size before further use.
The internal mixer such as the Banbury is a more effective device
for rubber compounding. It consists of a completely enclosed
mixing chamber in which two spiral-shaped rotors operate,
illustrated in Figure 5 (14).
Rubber is fed through a hopper. The two rotors rotate in oppo-
site directions at slightly different speeds and are hollow to
allow circulation of water or steam for temperature control. The
product mix from the internal mixer is discharged into a two-roll
mill, producing rubber sheets. To obtain good mixing, carefully
selected individual ingredients must be added in a specific order
because some materials mix with rubber better than others do.
After the raw rubber has been passed between the heated mill
rolls a few times, it becomes sufficiently soft to adhere to the
front, slower moving roll. The distance between rolls is then
adjusted so that there is a "bank" of rubber in the "bite" of the
rolls. When the rubber is sufficiently soft, additional com-
pounding ingredients are spread on the rubber on the bank. The
rubber is cut and covered over to aid in dispersing the indi-
vidual materials throughout the batch.
(14) McPherson, A. T., and A. Klemin. Engineering Uses of Rub-
ber. Reinhold Publishing Corporation, New York, New York,
and Chapman & Hall, Ltd., London, United Kingdom, 1956.
490 pp.
37
-------
CONNECTION FOR
EXHAUST FAN TO
REMOVE DUST
FEED HOPPER
DOOR
\*—FLOATING WEIGHT
SLIDING DOOR
MILL
Figure 5. Cross section of a Banbury internal
mixer mounted over a rubber mill (14).
Internal mixers can handle large batches in relatively short
periods of time. However, they are not suitable for the addition
of sulfur because their high operating temperature could cause
premature vulcanization or scorching. Consequently, even though
most of the compound ingredients are added to the internal mixer,
the sulfur is added in a subsequent operation on a roll mill.
Forming—The rubber slabs obtained from the mixing mills may be
immediately cut into disks or rectangular pieces suitable for
charging into a mold. The consistency of the compound often
determines how the rubber will be processed and what equipment
can be used for building or making up rubber articles. Most of
the mixed rubber must be processed into a form suitable for
further fabrication. Processes utilized here include calender-
ing, extrusion, frictioning, spreading, slabbing, and cutting.
Calendering—A calender usually consists of three hollow
revolving rolls placed one above the other in such a way that the
spacing between them can be accurately adjusted. The temperature
on the rolls can be controlled by circulation of steam or cold or
hot water through the hollow rolls. The rolls can be driven
38
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either at the same or different speeds. A typical calendering
process is schematically shown in Figure 6.
CALENDER COOLING DRUMS
CONVEYOR
TO CUTTER
Figure 6. Diagram of the calendering process (14).
The purpose of calendering is to form smooth sheets of rubber
compound of accurate thickness; it can also be used to coat or
impregnate fabric. Coating operations are performed in either
three- or four-roll calenders. The three-roll calender applies a
coating to one side of the fabric; the four-roll calender coats
both sides of the fabric. ,
When using fabric is required for reinforcement, as in hose and
belting, fabric-inserted diaphragms, and tires and footwear, the
fabric is usually rubberized by passing it through a friction
calender along with the rubber compound. In fabric frictioning,
the center roll of the calender is run hotter and faster than the
top and bottom rolls. This forces the rubber into the mesh of
the fabric.
Fabric is sometimes rubberized by spreading on the fabric surface
a heavy dough prepared by blending a suitable rubber compound
with gasoline or other solvent. The fabric is stretched and the
dough is applied in a thin, uniform layer by means of a knife
mounted perpendicular to the fabric. When the spreading is
completed, the fabric is passed slowly over heating coils to
evaporate the solvent. The spreading process is applicable to
cases in which either the fabric or the compound is not adaptable
to the friction process.
Rubber compound obtained from calendering may be used in a vari-
ety of applications in many different shapes. Calendered rubber
may be automatically cut into strips as it comes from the cooling
drums, die-cut to any desired shape by means of a clicking
machine, or cut to desired lengths by means of a water-lubricated
circular cutter.
39
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Extrusion—The process of extrusion involves forcing the
rubber compound through an extrusion machine. These machines can
operate with either a cold or a warm rubber feed. Cold feed
extruders are longer than the warm feed type in order to permit
sufficient breakdown of the rubber compounds for smooth extru-
sion. Basically, a power driven screw forces the rubber through
a cylinder to the front of the machine where it is forced through
a die. The extrusion cylinder as well as the screw may be
equipped with cooling water or steam for temperature control.
Any number of dies are available to provide the desired extruded
shapes. Since the rubber expands after being pushed through the
die, the die must be smaller in size than the desired resulting
extruded article. The extruder may be fed by hand or by a force-
feed system consisting of two feed rollers. Newer extruders
operate under vacuum to eliminate trapped air and moisture.
Extrusion is a very economical and widely used method of process-
ing rubber, both for making blanks for molding and for forming
rods, tubes, strips, channels, and gaskets in a wide variety of
sizes and shapes. The operation sequence in the extrusion pro-
cess is shown in Figure 7 (14).
Figure 7. Extrusion processes (14)
When it is intended to employ a compound as insulation or jacket
on a wire, or as a cover on a previously prepared hose carcass, a
side delivery head is used on an extrusion machine. In this
case, a wire or a hose carcass is fed through the head in a
direction perpendicular to the axis of the extruder screw. The
head is designed so that the compound is deflected 1.57 rad (90°)
and completely surrounds the wire or hose carcass.
40
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Some rubber articles may be produced directly by cutting the
milled rubber stock; e.g., if large pieces of heavy gauge rubber
stock are needed as blanks for molded rubber articles, they are
cut from mixing mill stock (made into a slab of the proper thick-
ness) by means of a knife and a template. Similarly, tubed or
extruded compound is cut as needed using cutting machines which
may be synchronized with the extruder.
Vulcanization (curing)—Vulcanization of rubber products is done
at elevated temperatures (100°C to 200°C for 10 min to 30 min)
and can be carried out under numerous conditions. Some articles
are cured during the manufacturing step if sufficient heat is
generated in the process (e.g., molded products); other articles
require a separate curing step (e.g., latex products and tires).
Mold Curing—Molded rubber parts are formed and vulcanized
in a single operation by the simultaneous application of pressure
and heat. Compression is the oldest type of molding and consists
of placing preshaped rubber into a mold and closing the mold
under pressure; this causes the rubber to fill out into the mold
cavity. The heat from the heated platens of the press is con-
ducted through the mold and vulcanizes the rubber. The platens
are usually heated by circulating steam through holes drilled in
them. Occasionally, electricity or gas burners are used for this
purpose.
The rubber overflow or flash must be removed from the article.
This operation is labor intensive and thus expensive because it
requires hand labor. If possible the rubber parts are dipped in
dry ice, causing the thin rubber flash to become brittle and
easily broken off. This method can be used only if the main body
of the part is large enough not to become cool and inflexible and
if the rubber is not freeze resistant.
In transfer molding, the uncured rubber stock is transferred from
one place to another within the mold, allowing the manufacture of
complex shapes and articles containing metal inserts. Transfer
molding permits closer dimensional control and generally reduces
flash. Normally the rubber is placed in a transfer cavity which
is fitted with a ram or piston. The force applied to the ram or
piston and the heat from the mold cause the rubber to be softened
and spread in the molding cavity and cured at the same time.
Injection molding is similar to transfer molding except that the
soft rubber compound is injected into the molds. A screw mecha-
nism is utilized to force unvulcanized rubber into a tightly
closed mold. Forcing the rubber through small passages under
high pressure increases the temperature of the injected compound
and cures the rubber. In order to make injection molding profit-
able, very short cycles are required, generally in the 45-s to
90-s range. These short times require a curing temperature of
204°C.
41
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In molding thick articles, long curing times are needed because
of the low thermal conductivity of the rubber. This problem is
partially overcome by dielectric heating of the blank before it
is placed in the mold. This heating also improves the flow of
the compound in the mold.
An example of more complicated molding is that of the pneumatic
tire in which a steel mold shapes the exterior surface of the
tire from bead to bead, and the pressure during cure is supplied
from a flexible bag acting as a diaphragm that forces the uncured
tire against the mold surface. The diaphragm, an integral part
of the press, is made of a resin-cured butyl stock which has
extremely good heat resistance. Steam or hot circulating water
is introduced to the inside of the diaphragm to cure the tire
from the outside. (Tire vulcanization is further described in
this section.)
Curing of other rubber articles—Extruded articles and some
molded articles may require additional curing. The most common
method of vulcanizing these articles is to place them in pans
that are set on a truck and rolled into a large steam chamber or
heater for vulcanization. Varnish or lacquer may be applied
before vulcanization to produce a smooth, glossy product finish.
If curing at elevated pressure is desired, water is used in place
of steam. Rubber-lined vessels are steam cured unless they are
too large to fit in a steam autoclave. Boiling water is used in
such cases.
Air is sometimes preferred over steam in the vulcanization step,
especially when moisture must be avoided, or staining or water
spotting must be prevented. Hot air at either atmospheric or
elevated pressure (103.4 kPa to 275.8 kPa) is usually used. The
air is circulated at a rapid rate to provide even heating of the
article and avoid bad spots in the vulcanizates.
Tires and Inner Tubes—
The manufacturing of inner tubes involves compounding, extrusion,
and -curing. These are common operations in rubber products
fabrication and have been discussed earlier in this section.
This segment of the industry represents only 3% of the value of
product shipments in the total tire and inner tube industry. In
the following sections, the operations specific to tire manu-
facturing are discussed.
Tires are built from several parts as illustrated in Figure 8.
There are three variations in tire construction: conventional,
belted bias, and radial ply tires, as shown in Figure 9. Differ-
ent rubber compounds are used in making the several tire parts
because each part performs a different function. The carcass
(made of body fabric or cord plies), the impact plies (which are
placed between the body plies and the tread to provide extra
42
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Figure 8. Cross section of a tire (14)
CONVENTIONAL TIRE
(2 or 4 plies)
Body ply
cords rim at bias angle
BELTED BIAS TIRE
Tread
'stabilizer
belts
Body ply cords
run at bias angle
RADIAL
PLY
TIRE
Tread
stabilizer
belts
Figure 9.
Body ply cords
run at radial angle
Variations of tire construction (11)
43
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impact resistance), the bead assembly, the tread, and the tire
wall are all made from different rubber compounds. Tire manufac-
turers use both synthetic and natural rubber, the latter mainly
for steel belted and large size tires. The basic recipes for
rubber compounds are generally very similar except that synthetic
compounds require different carbon black loadings, somewhat more
softener, less sulfur, and more accelerator.
The basic steps involved in tire manufacturing are schematically
shown in Figure 10. Recipes for each specific part of the tire
are selected, and the compounds are prepared using roll mills and
Banbury mixers. Table 17 lists typical compositions of the
rubber compounds for different tire parts. All ingredients
except sulfur and the accelerator are added to the rubber in a
Banbury mixer. The batch is then dumped on a roll mill, shown
previously in Figure 5, located below the mixer for addition of
the curing ingredients. Compounded rubber is made into standard
sheets which are then used to manufacture the individual tire
parts.
RUBBER,
CARBON BLACK,
OILS, CHEMICALS,
PIGMENTS
J_
TREAD & SIDEWALL
EXTRUSION
SOLVENT
-*~ REJECTS
Figure 10.
FINISHED PRODUCT
Tire plant process flow diagram.
Carcass plies are made of cord fabric insulated with rubber com-
pounds. A variety of carcass materials are available to the tire
manufacturer: cotton, rayon, nylon, polyester, steel wire, and
glass fiber. The last two materials are used in radial tires.
Today, very little cotton cord is used in pneumatic tires.
Cotton has been replaced by rayon and more recently by nylon.
The increasing popularity of radial tires has increased the use
of steel wire and glass fiber in tire manufacture. Selection of
the cord fabric depends primarily on cost because tire cords
represent a large portion of the cost of tires.
44
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TABLE 17. TYPICAL,COMPOUND COMPOSITIONS FOR TIRE PARTS (14)
Tire part
Inner carcass or body
plies (truck tires)
Outer carcass or body
plies (truck tires)
Impact plies
Beads
Treads
Inner tubes
Component
Natural rubber
SRF black
Zinc oxide
Stearic acid
Softener
Antioxidant
Sulfur
Primary accelerator
Secondary accelerator
Natural rubber
SRF black
EPC black
Zinc oxide
Stearic acid
Softener
Antioxidant
Sulfur
Primary accelerator
Secondary accelerator
Natural rubber
EPC black
Zinc oxide
Stearic acid
Softener
Antioxidant
Sulfur \
Primary accelerator
Secondary accelerator
Natural rubber
SRF black
Zinc oxide
Precipitated whiting
Softener
Stearic acid
Sulfur
Accelerator
Natural rubber
EPC black
Zinc oxide
Stearic acid
Softener
Antioxidant
Sulfur
Accelerator
Natural rubber smoked sheets
peptizer
Zinc oxide
Fine thermal carbon black
Antioxidant
, paraffin
Sulfur
primary accelerator
Secondary accelerator
Parts by weight
100
25
3
2
5
1
2.8
0.75
0.15
100
15
20
3
2
5
1
2.8
0.75
0.15
100
40
3
2
5
1
2.80
0.80
0.20
100
120
8
15
11
5
3
1.5
100
45
3
2
3
1.50
2.75
0.90
100
1
4
40
2
1
1.5
1.4
0.2
45
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Rubber compound used to manufacture tire plies must adhere to the'
cord fabric and have enough tack to hold together while the greerv
(unvulcanized) tire is being assembled and cured. Impact plies i
are built somewhat tougher than inner plies since they must
remain intact to divert road shocks and bind the rigid carcass of
the tire to the tire tread. Both sides of cord plies for the
tire carcass are coated at once on a four-roll calender.
Its relatively rough surface texture allowed natural rubber
stocks to be applied directly to cotton cord. This is not feasi-
ble with rayoa and nylon cords, which must be coated with an
adhesive before the cord fabric can be coated with rubber com-
pound in the calender. Medium styrene-butadiene and butadiene-
styrene vinyl pyridine latexes are usually used in this appli-
cation. Vinyl pyridine latexes are universally used for nylon
tire cord. A typical tire cord dip solution is given in Table 18
TABLE 18. TYPICAL TIRE CORD DIP SOLUTION (13)
Material Dry/ parts Wet, parts
SBR type 2000 latex at 40% 80 200
Vinylpyridine latex at 47% 20 43
Stabilizer (20% Dresinate 731) 1 5
Water to 20% solids 78
Resin solution (6.5%) 17.3 266
TOTAL 118.3 592
Resin solution formula
Water to 6.5%
NaOH
Resorcinol
Formaldehyde (37%)
TOTAL
0.3
11.0
6.0
17.3
238.5
0.3
11.0
16.2
266
In the normal sequence of operations, ply fabric is passed
through the adhesive dip solution, the excess dip is removed, the
coated fabric is dried to a moisture level less than 1%, the
rubber compound is calendered on both sides of the cord fabric,
and rubber cement is applied to the carcass. The last step is
necessary only for carcass plies with a high percentage of syn-
thetic rubber compounds because the tack of synthetic rubber is
insufficient for it to adhere properly to the vulcanized rubber.
Finally, the fabric is cut to a specific angle and the required
width on a bias cutting machine.
46
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Wire bead, made of several strands of high carbon steel, is used
to keep the tire on the rim. Each strand is coated with rubber
compound while passing through an extruder. Several strands are
passed simultaneously through the die of the extruder, then
rolled together to make the bead. The bead is wrapped with
rubberized square woven fabric, then rewrapped with the same
fabric, the edges of which extend upward into the sidewall where
they can be anchored into the lower sidewall of the tire.
The tread and sidewall of the tire are formed by extrusion
through dies. The extruded profile is designed to provide suf-
ficient rubber to fill in the tread and sidewall pattern in the
mold.
Tire tread is made of two sections: the cap, which contacts the
road; and the base, the section next to the carcass. Since the
two sections are made of different rubber compounds, dual extrud-
ing units have been developed. Good adhesion between the cap and
the base is important, and in dual extrusion, these two parts are
plied together hot. Some extruding machines produce the cap and
base already joined. Passage through a water bath cools and
shrinks the continuous tread slab, which is then cut to the cor-
rect length for tire assembly.
Tires are assembled on rotating drums having a diameter slightly
larger than that of the tire. Individual tire parts are supplied
to the builder in a form that allows the fastest assembly of the
tire. Carcass plies are cut to the proper angle, width, and
length and may be delivered in rolls that allow unreeling of the
fabric without strain (to avoid angle distortion), or in bands of
two to four plies. The treads and sidewalls are also delivered
precut to length. Synthetic rubber tread is delivered with crude
rubber cement on its underside and ends to ensure proper adhesion
to the tire carcass.
Four to eight cord plies are applied to the drum without stretch-
ing; each is tied under and over the bead in a manner which
securely locks the bead. Natural rubber plies usually have
enough tackiness to adhere to themselves. Synthetic rubber plies
are coated with a rubber cement to provide sufficient tackiness.
If impact plies are used, they are added next, followed by the
sidewall and tread sections. At this point, the assembled tire
is cylindrical in shape.
Usually the whole tire is assembled on the drum by one man, but
machines have been developed that automatically rotate the drum
through several stations for addition of the successive parts.
The drum is then collapsed to release the tire, which gains its
final shape during vulcanization in the mold. The inside contour
of the tire is formed by a curing bag placed inside the tire.
The bag fulfills two functions: it gives the tire the proper
shape, and it provides a container into which heat and pressure
47
-------
can be applied to vulcanize the inside of the tire. Heat and
pressure are supplied by various combinations of steam, air, and
water.
Tire shaping and curing equipment have undergone several develop-
ments. The curing bladder is an integral part of a new curing
press. This combines the forming and curing operations in a sin-
gle machine and eliminates the labor of inserting and removing
the curing bag. Because the bladder is a part of the press and
also is thinner than the separate bags, more effective use of
internal heat in curing the tire and a significant reduction in
curing time are achieved. Tires are vulcanized at 100°C to 200°C
for 20 min to 60 min. Longer times are required to cure large
truck tires.
Rubber Footwear—
The process description presented here (4) pertains to the pro-
duction of canvas footwear, which constitutes the major product
type within the Rubber and Plastics Footwear Industry, SIC 3021.
Canvas shoes are the product of a number of processing opera-
tions. These include compounding of rubber stocks, molding of
the soles, cutting and fabricating of canvas parts, extrusion of
other rubber components, construction of the final product from
all these items, and curing of the final product. A flow sche-
matic is shown in Figure 11.
The various rubber stocks received at a canvas footwear plant are
compounded with other processing chemicals in Banbury mixers or
roll mills and then sheeted out. The compounded, sheeted stock
is next cooled. Water spraying or immersion in a cooling water
tank are the preferred techniques. After cooling, the sheeted
rubber is dipped in an antitack solution to prevent sticking
during storage.
A canvas shoe is built from four major components: soles, inner
soles, canvas uppers, and boxing. Each of these pieces is made
separately by different processes before being brought together
in the shoe-building operation.
The soles are either cut from uncured rubber sheets or, more gen-
erally, formed using injection, compression, or transfer molding
techniques. The technology employed depends on the final pro-
duct. Compression molding is now more common but requires more
manual labor and produces more molding waste than automated
injection techniques. The molded soles are deflashed, usually in
a buffing machine. A coat of latex adhesive is applied to the
soles before they are dried in an oven, which may be electric.
Production of the inner soles begins with the preparation of
flat, cellular rubber sheets by extruding or calendering a spe-
cial rubber stock. The extruded sheet can be continuously cured
by passing through heated presses. Blowing agents, such as
48
-------
IO
LI
PRODUCT
SHIPMENTS
COOLING
WATER
Figure 11. Schematic flow diagram for the production
of typical canvas footwear items (4).
-------
sodium bicarbonate (NaHCO3) or azodicarbonamide (H2NCON=N-CONH2),
which are mixed into the rubber stock during compounding, decom-
pose and release gases which blow the extruded sheet into cellu-
lar sponge. The inner soles are die-cut from the cellular sheet.
Canvas uppers for footwear are made from two- or three-ply
fabric. The canvas material is received at a plant as single
sheets. These individual plies are coated with latex, pulled
together, and passed over a steam-heated drum. The sheets are
stacked and then cut to the proper dimensions using a die and a
press. The different canvas components making up the footwear
uppers are stitched together on sewing machines.
The boxing, or edging, which protects the joint between the sole
and the canvas uppers, is extruded as a long strip from rubber
stock.
The shoe is fabricated from its four basic components on a form
called a last. The canvas upper is cemented at its edges and
placed over the last. The inner sole is attached to the bottom
of the last. The bottom of the inner sole and canvas combination
is dipped in a latex-adhesive solution which will serve to hold
the entire shoe together. Next, the outer sole, the boxing, and
the toe and heel pieces are attached to the shoe.
The finished shoes are inspected and placed on racks in an air-
heated autoclave for curing. Anhydrous ammonia is injected into
the autoclave to complete the cure, the amount required ranging
from 0.9 kg to 2.3 kg of ammonia for every thousand pairs of
shoes cured. The purpose of using anhydrous ammonia is to reduce
the tackiness of the product. The curing cycle lasts about 1 hr,
at the end of which the ammonia-air mixture is vented to the
atmosphere.
Some shoes are cured without ammonia. This is done when the
tackiness of the product is not very important or when the com-
pounding recipe can be modified to eliminate the tackiness often
associated with conventional air curing. Steam is not used for
curing because it would stain the canvas parts of the shoe in
many cases. In addition, curing is not necessary in some new
methods of shoe production.
Rubber Hose and Belting—
Rubber Belting—Operations involved in the production of rubber
belting are compounding, forming, building, and curing (4). A
flow schematic is shown in Figure 12.
Compounding and mixing are usually carried out in Banbury mixers,
although compounding mills may be used in some facilities. After
mixing, the rubber stock is sheeted out on a sheeting mill and
dipped in a soapstone slurry to reduce its tack. The rubber
i
50
-------
CEMENT
DIPPING
DRYING
FRICTION
CALENDERING
COMPOUNDING
(BANBURY )
CARCASS
CONSTRUCTION
COOLING
I WATER SPRAY TANK)
SOAPSTONE
DIPPING
DRYING
(AIR VENTS I
CALENDERING
CURING
( PRESS, ROTOCURE,
HOT-AIR OVEN)
INSPECTION
CUTTING TO LENGTH
STORAGE
)
Figure 12. Belting flowsheet.
51
-------
leaves the rolling mill in a ribbon up to 2 m wide and approxi-
mately 25 mm thick. Both the frictioning and sheeting stocks are
worked on warmup mills prior to subsequent forming operations.
In the forming operations, the hot sheeting stock passes from the
warmup mill through an extruder-calender machine where its dimen-
sions are fixed. Wire reinforcement may be extruded with the
rubber stock during this operation to increase the strength of
the belting. After calendering, the sheet rubber is cooled in a
water spray tank, dried via passage over hot air vents, and
rolled up for storage.
The frictioning compound passes from the warmup mill to a fric-
tion calender where it is impregnated into the fabric used to
build the carcass of the belt. This fabric, usually rayon or
nylon, is pretreated by dipping in latex and/or cement and drying
to a moisture content of less than 1%. Drying is carried out
immediately prior to frictioning by passing the dipped fabric
over steam-heated cylinders or platens kept at 115°C or in other
types of ovens.
In the building operation, the rubberized, single-ply fabric
leaving the calender is used to build belt carcasses of multiple-
ply thickness. A variety of techniques are employed in this
operation, depending on the specifications of the final product.
Once built, the carcass is sandwiched between two layers of
rubber sheeting by a calendering operation.
Belt vulcanization (curing) is performed in presses, rotocures,
or hot-air curing ovens. A rotocure employs a combination of
steam, cooling water, and electric heaters to continuously vul--
canize the belting as it passes around the curing drum. Press
curing is effected by two heated belts which hold the belting
between them under pressure, turn, and drag the belting through
the press. Unlike the rotocure, the press-curing technique is a
batch operation. Vulcanization requires about 30 min at 140°C.
After curing, the belting is inspected, cut to length, and stored
before shipment.
Rubber Hose—There are four types of rubber hoses: machine-
wrapped ply hose, hand-built hose, braided hose, and spiralled
hose. Similar operations are involved in the manufacture of
these four types of hoses. The process description presented
here is specific to the production of machine-wrapped ply hose.
Machine-wrapped ply hose consists of three components: the tube
(lining), the reinforcement, and the outer cover. The reinforce-
ment is constructed from rubber-impregnated fabric, while the
tube and cover are made entirely from rubber. A schematic flow
diagram is shown in Figure 13.
52
-------
COOLING
WRAP REMOVAL
MANDREL REMOVAL
TESTING
STORAGE
EQUIPMENT
Figure 13. Ply hose flowsheet.
53
-------
Tube is formed by continuous extrusion. Reinforcement is made by
impregnating the fabric with rubber on both sides by friction-
calendering. The other cover is formed by calendering a thin
sheet of rubber stock to the required thickness.
The formed tube is taken to the building area where it is tempor-
arily enlarged via air pressure and mounted on a rigid mandrel.
Lubricants are injected into the tube to prevent it from sticking
to itself or to the mandrel.
The actual hose building is carried out on a special purpose
"making machine" which consists of three long steel rolls. Two
of the rolls are fixed parallel to each other in the same hori-
zontal plane, while the top roll is mounted on lever arms so it
can be raised and lowered. One or more of the rolls are power
driven. Rubber cement is applied to the reinforcement before the
building operation.
The green hose is cured in an open steam autoclave at some prede-
termined temperature and pressure. The necessary pressure is
maintained by cotton or nylon wraps.
When vulcanization is complete, the autoclave is vented, the hose
is removed and cooled, and the cloth wrap is stripped away. The
hose is then removed from the mandrel with compressed air or
water and hydraulically tested before final storage and shipment.
Fabricated Rubber Goods N.E.C.—
This industry can be separated into general molded products, gen-
eral extruded products, and rubber goods from latex. The process
description for these rubber products is given below.
General Molded Products—This category includes items such as
battery parts, rubber rolls, rubber heels and soles, water bot-
tles, fountain syringes, nipples, pacifiers, rubber bands, finger
cots, erasers, brushes, combs, mouth pieces, and a wide variety
of mechanical goods.
Rubber molding typically consists of the following operations:
1) compounding of rubber stock, 2) preparation of the mold pre-
forms or blanks (milling, calendering, and cutting), 3) molding,
and 4) deflashing.
Metal-bonded items, which consist of a molded rubber component
bonded to the metal part, are manufactured in a manner similar to
that for other molded rubber products. Additional operations are
metal degreasing and subsequent adhesive spraying on the metal
surface to provide good adhesion between metal and rubber.
Various molding techniques have been mentioned earlier in this
section. Vulcanization is accomplished in the molding operation.
54
-------
General Extruded Products—General extruded products include
rods, tubes, strips, channels, mats and matting, floor and wall
covering, and stair treads. Operations involved are compounding,
milling, calendering,.extrusion, curing, and bonding of extruded
parts.
In the curing operation, rubber articles that would sag or flat-
ten under their own weight before they could completely set up
must be supported. In most cases, such articles are embedded in
talc or powdered soapstone. However, rubber tubing is placed on
a mandrel and wrapped with fabric to insure proper curing.
Vulcanization usually requires about 30 min at 140°C to 150°C
(4).
In the bonding of extruded and cured rubber parts, two or more
parts to be connected are inserted into a mold where uncured rub-
ber material is applied to the joint and is vulcanized in the
press. Solvent is used for lubrication in the insertion of
rubber parts and for tackifying rubber parts.
Fabrication of Rubber Goods from Latex—The first requirement in
production of rubber articles from latex is to bring the rubber
latex and all the compounding ingredients into solution or dis-
persion form. Solution is used when all of the ingredients are
water soluble. Frequently, the ingredients are not water sol-
uble, and it is necessary to emulsify the liquid ingredients and
disperse the solid materials in water.
Dispersions are generally prepared from a coarse slurry of powder
with water containing small amounts of dispersing agents and sta-
bilizer. The slurry is then ground on a suitable mill to give
the desired particle size. The function of the dispersing agent
is to keep the particles suspended. Typical dispersing agents
are sodium 2-naphthylene sulfonate and formaldehyde, and an alkyl
metal salt of sulfonated lignin. The amounts of dispersing
agents must be determined experimentally. A wetting agent is
usually used, in concentrations less than 1% by weight, to pro-
duce a satisfactory dispersion.
Dispersions are prepared using grinding equipment such as colloid
mills which break aggregates but do not change the particle size.
Colloid mills are used for clay, precipitated whiting, zinc
oxide, etc. Grinding equipment that reduces ultimate size and
breaks agglomerates is used for solids such as sulfur, anti-
oxidants, and accelerators. Ball and pebble mills, ultrasonic
mills, and attrition mills are. used for this purpose. Typical
recipes and directions for preparing dispersions of antioxidants
such as Aminox and of ultraaccelerators such as zinc dimethyl-
dithiocarbonate (Methazate) are given in Tables 19 and 20.
55
-------
TABLE 19. PREPARATION OF A DISPERSION OF AMINOX
SUITABLE FOR LATEX COMPOUNDING (13)
A.
B.
C.
Material
Water
Water
Ammonia (28% NHs)
Blancol u
Dowicide A
Casein
KWK bentonite
D . Aminox
TOTALC
Weight
68
22.8
1
4
0.2
2
2
100
200
Procedure
Add A to ball mill
Make up B separately and add to mill
Add C and D to mill
Ball mill 4 days — keep cooling water on to avoid sintering Aminox
Trademark of GAF Corporation.
Trademark of Dow Chemical Company.
Sotal solids, 54.2%; active solids, 50%.
TABLE 20. PREPARATION OF A DISPERSION OF METHAZATE
SUITABLE FOR LATEX COMPOUNDING (13)
A.
B.
C.
Material
Water
Ammonia (28% NH3)
Blancol
Dowicide A
Casein
Water
Methazate
TOTAL3
Parts
by weight
70
1
4
2
2
22.8
100
200
Procedure
Add A to ball mill
Make up B and add to ball mill
Add C to ball mill
Ball mill 48 hr
Total solids, 53%; active solids, 50%.
Emulsions are prepared by first making a coarse suspension of
liquid ingredient droplets in water and then exposing this mix-
ture to an intense shearing in a colloid or ultrasonic mill or a
homogenizer (a machine that forces the emulsion through a fine
orifice under high pressure). Emulsions can also be simply pre-
pared by adding the material to a soap solution. Soap can be
prepared quickly in the machine by adding fatty acid or anionic
parts such as a stearic, oleic, or rosin acid to a solution of
potassium hydroxide or an amine in water. Examples of emulsion
recipes are presented in Tables 21 and 22.
56
-------
TABLE 21. PREPARATION OF A NAUGAWHITE EMULSION
SUITABLE FOR LATEX COMPOUNDING (13)
Material
Water (hot)
Nopco 1444B
Naugawhite
Dry
parts
5.4
75
Wet
parts
19
6.0
75
Procedure
Add Nopco 1444B to hot water
speed stirring.
Add Naugawhite slowly, allowii
with high-
nq a few
TOTAL
80.4
minutes between additions.
100 After all the Naugawhite has been stirred
in, continue stirring for 15 min.
Nopco 1444B is a highly sulfonated castor oil produced by Nopco
Chemical Company.
TABLE 22. PREPARATION OF AN OIL EMULSION SUITABLE FOR
LATEX COMPOUNDING (13)
Material
Parts
Procedure
A. Mineral oil
Oleic acid
B. Potassium hydroxide
Water
TOTAL
70 Add A to B using an agitator such as the Eppenbach
Homo-mixer.
1.5
1.5 Put emulsion through a homogenizer to obtain a very
small particle size and a high emulsion stability.
27
100
The preparation of the latex compound is a very simple operation.
It consists of weighing and mixing the proper amounts of various
solutions, emulsions, and dispersions. This is done in a large
tank with a mechanical agitator.
Rubber articles can be fabricated using compounded latex by a
variety of methods. One of the simplest techniques is to dip a
form into the latex and dry the thin film formed on the form at
room temperature or in warm air at 49°C to 60°C while rotating
the form to ensure a uniform film thickness. Thicker films are
made by multiple dipping.
Another technique for fabricating rubber articles uses porous
forms, or porous molds, made of plaster of paris or unglazed
porcelain with smaller pore size than the smallest rubber latex
particles. The rubber particles are filtered out by this mater-
ial and latex coagulates to form a film due to the presence of
calcium ions in the plaster. The molds are dried in an oven at
57
-------
60°C for one hour. This can be repeated for 30 min after the
articles are removed from the mold. For example, dolls and
squeeze toys are manufactured using this technique.
Since the rubber particles in latex are negatively charged,
electrodeposition has been used to coagulate rubber and make
rubber articles. However, evolution of oxygen on the anode
produced oxidation of the product and caused porosity in the
article. Electrodeposition was therefore abandoned. Essentially
the same degree of coagulation can be attained by using chemical
coagulants.
A thin layer of a chemical coagulant is produced by dipping the
form in the coagulant solution and evaporating the solvent,
preferably alcohol. The thin layer of coagulant can be produced
either directly on a clean form or on a form that is coated with
a very thin layer of the latex. The form is then dipped in the
latex. When the film attains the desired thickness, it is washed
in hot water at 60°C to 71 °C for about an hour to remove the
coagulant and all other water-soluble ingredients. The film is
then dried in air at room temperature, and the article is cured
in a 66°C oven.
Typical coagulants are calcium chloride or calcium nitrate in a
solution of denatured ethyl alcohol. They are mixed with a non-
ionic surfactant and a release agent (a fine, insoluble powder
such as talc, clay, or diatomaceous earth) which is suspended in
the coagulant. The surfactant and release agent are used to aid
in wetting the form and releasing the article from the form,
respectively.
Another variety of this process uses a gelling agent (electrolyte
with a weak coagulating effect such as ammonium salts and sodium
fluorosilicate) in metal molds. This method offers the advantage
that latex sets to the gel with no change in volume and without
distortion.
Some rubber products may be made by extrusion of the latex. For
example, latex thread is produced by extrusion of the latex com-
pound through fine orifices into a coagulant bath which gels the
thread. The thread is then toughened, washed, dried, and cured.
Dilute acetic acid is usually used as the coagulant bath.
The broadest application for both latexes, natural and synthetic,
is foam sponge. There are two basic processes available, the
Dunlop and the Talalay process, applied in different variations.
In the Dunlop process, which is the most commonly used, the latex
is whipped to a froth by the mechanical incorporation of air into
the latex. The Talalay process produces the froth by chemical
rather than mechanical means. Hydrogen peroxide and an enzymic
decomposition catalyst are used for this purpose. Oxygen pro-
duced by the decomposition of the peroxide foams the latex mix.
58
-------
The foam is chilled and C02 is introduced to gel the latex.
Further treatment is the same as in the Dunlop process.
The frothed structure must be set using a coagulant or a gelling
agent. Sodium silicofluoride (Na2SiF6) is widely used in this
application. Zinc oxide is also believed to take an active part
in the process. Sodium silicofluoride decomposes and forms
sodium fluoride (NaF), silicon tetrahydroxide [Si(OH),], and
hydrofluoric acid (HP). zinc apparently reacts with the fatty
acid latex stabilizers forming a soluble soap. This destabilizes
the latex particles, causing them to coalesce and form a gel.
The Si(OH)^ may also form very fine particles which could adsorb
stabilizer and further enhance gelation. In very stable latexes,
some secondary gelling agents may be utilized to induce gelation.
Cationic soaps, other salts, and amines are used for this
purpose.
Whipping can be done either continuously or in a batch process.
After the gelling agents are added, the foam is poured into molds
and cured. Additional curing is done after the product is
removed from the mold.
Ammonium acetate or ammonium sulfate, in combination with zinc
oxide, are employed as the gelling agents in the production of
foam backings for various fabrics such as carpets, scatter mats,
and upholstery fabrics I Ammonium hydroxide is the product of the
reaction. Once gelation occurs, the foam is spread directly on
the fabric or it is spread on a belt and transferred wet to the
fabric. The gelling is carried out at elevated temperatures,
usually by means of infrared lamps.
Gaskets, Packing, and Sealing Devices—
The principal method of manufacturing rubber gaskets, packing,
and sealing devices is molding. Operations involved are the same
as those for general molded products in the Fabricated Rubber
Goods N.E.C., which is covered in the preceding subsection. The
three common molding techniques (compression, transfer, and
injection molding) are described later in this section for common
operations in manufacturing of rubber products.
Rubber Wire-Insulating (4)—
Extrusion is the preferred method of applying a rubber compound
to wire or cable as an insulating and/or protective covering.
When a suitably modified extruder is used, plastics as well as
rubber may be employed as insulation.
A wire to be covered is passed through a right-angle or side-
delivery head. In this operation, the wire is fed through the
head in a direction perpendicular to the axis of the extruder
screw. The head is designed so that the rubber compound is
deflected 1.57'rad (90°) and completely surrounds the wire.
59
-------
The covered cable is pulled through the machine by a variable- .^
speed hauloff. A satisfactorily uniform coating is ensured by
regulation of the drawing speed.
Insulated wire is continuously vulcanized by extrusion directly
into a suitable curing device. This is usually just a tube fixed
to the nozzle of the extruder and filled with steam at pressures
from 1.38 MPa to 1.72 MPa. Such tubes may be 30.5 m to 61 m in
length. Residence time for the insulated wire is approximately
15 s. Glands through which the cable exits the tube prevent -
leakage of steam. Large cables are usually processed in vertical
units, but horizontal or catenary-shaped tubes are also
available.
The exterior of insulated wire or cable must be protected against
mechanical and sometimes chemical deterioration. The type of
protective covering applied will depend on the ultimate end use
of the cable. Small wires are covered with a braid, normally of
cotton but sometimes of rayon or fine metallic wire. Another
means of protection, tough rubber sheathing (TRS), can be applied
to the insulated wire using an extruder with a side-delivery head
as described previously. The sheathing may consist of neoprene
(polychloroprene) or another oil-resistant rubber. Lastly, some
insulated wires and cables may be covered by an extruded lead
sheath applied earlier as a means of support during vulcanization.
Tire Retreading (4)—
The tire retreading process consists of a series of eight unit
operations through which worn tires are rendered servicable and
fit for resale. With the exception of studded snow tires, nearly
every tire size and design is utilized by the industry. The
majority of retreaders receive their tires from scrap dealers,
but turn-ins are also a popular source of supply.
Raw camelback is nearly always purchased from an outside sup-
plier. Very few retreaders compound their own stock. A sche-
matic flow diagram for tire retreading is presented in Figure 14.
On arrival, the tires are first inspected to determine whether or
not the casing and carcass are in good condition. There should
be no cuts or visible deterioration of the reinforcing fabric.
Hidden ply separations, the major cause of tire failure, are
detected by injecting air into the tire shoulders. Since trapped
air itself may cause ply separation, the tire is vented in the
bead area so the air can escape during molding or on highway
flexing. Tires unfit for retreading are usually passed on to the
reclaiming industry.
After sorting, the tires are sent to the buffing area where the
remaining tread is ground off with a grinding wheel. The surface
of each newly buffed tire is rendered dust free with a stiff wire
brush.
60
-------
The clean tire is measured in order to select the correct curing
rim and to assure a tight fit in the matrix. Tires can grow up
to 7% of their original width from road use, so both the width
and wall thickness must be measured. Once measured, the tires
are taken to the spray area where they are coated with vulcani-
zable rubber cement.
After the surface of the tire is coated with cement, strips of
tread rubber are wound circumferentially around it and cut to
length. Some retreaders "program" the tread on. In this opera-
tion, the machinist selects a profile to build, and the machine
automatically wraps the thin strand of tread until the exact con-
tour is obtained. The tread-winding process typically requires
about 4.53 kg of camelback per passenger-car tire and 15.85 kg
per truck tire.
RUBBER
CEMENT
RUBBER
CEMENT
SPRAYING.
TREAD
RUBBER
TREAO
WINDING
/INSPECTION
UNO SHIPPING;
Figure 14. Retreading flowsheet (4)
61
-------
After tread winding, the tire goes into a mold for curing at some
specified temperature for some predetermined length of time.
Most curing molds are steam heated, but some older ones are
electrical. After curing, the rubber flash is buffed off, and
the finished product is inspected and shipped.
GEOGRAPHICAL DISTRIBUTION
Rubber processing plants are distributed among 43 states in the
United States (4, 15). Table 23 gives the number of plants in
each of these states for industries in the nine SIC's.a Also
given in this table are percentages by state of U.S. total number
of plants, the population density in each state, and the compos-
ite state_population density. The composite state population
density, D, was calculated by the following formula:
43
_
D= £
Di
P.
TOO"
(1)
where D. = population density in the ith state
P. = state percentage of U.S. total number of plants for
ith state
Two states, Ohio and California, contain about 25% of the 1,687
rubber processing plants in the United States. Roughly another
25% of the plants are located in Illinois, New York, New Jersey,
and Massachusetts. The remaining 50% of the plants are distri-
buted among the remaining 37 states. Figure 15 is a graphic
representation of the total number of plants on a state-by-state
basis.
For the tire retreading industry (SIC 7534) , only a partial
listing is given due to unavailability of complete' data.
(15) 1972 Census of Manufacturers, Volume II: Industry Series,
Part 2: SIC Major Groups 27-34. U.S. Department of Com-
merce, Washington, D.C., August 1976.
62
-------
TABLE 23
GEOGRAPHICAL DISTRIBUTION OF RUBBER
PROCESSING PLANTS (4, 15)
Number of plants in each SIC
State
Alabama
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Nebraska
Nevada
New Hampshire
New Jersey
New York
North Carolina
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
TOTAL
2822
2
1
1
2
1
2
3
2
1
2
2
2
2
6
29
3011
10
2
5
22
3
1
1
2
9
10
5
5
2
3
1
2
6
6
1
3
4
3
3
4
7
27
7
3
14
2
13
11
5
2
2
206
3021
1
4
1
2
2
1
2
3
2
6
2
4
2
5
2
3
5
3
1
1
52
3031 3041
1
7
1
2 1
3
1 5
2
3
2
1
1
1
2
12
1 4
1
1 10
3
1
1
1
6 62
3069
11
7
6
144
3
46
2
22
31
61
56
5
4
10
64
57
24
11
13
5
2
70
68
18
174
7
11
49
16
9
1
13
40
5
1
13
5
8
24
1,116
3293
3
29
2
6
1
22
5
1
2
9
12
4
5
1
11
17
3
18
3
12
2
16
2
1
2
5
194
(f
State
3357 7534 totals
24
9
13
2 210
9
2 60
7
27
1 45
1 102
11 72
10
2
1 11
3
4
16
2 92
1 79
30
15
25
5
2
10
1 99
2 101
11 35
2 1 238
17
14
11 87
16
12
1
34
73
8
1
19
7
11
32
b
17 5 1,687
Percent
of U.S.
total
1.4
0.5
0.8
12.5
0.5
3.6
0.4
1.6
2.7
6.0
4.3
0.6
0.1
0.6
0.2
0.2
1.0
5.5
4.7
1.8
0.9
1.5
0.3
0.1
0.6
5.9
6.0
2.0
14.1
1.0
0.8
5.2
1.0
0.7
a
2.0
4.3
0.5
a
1.1
0.4
0.6
1.9
99.9
Population
density,
persons/km
26
5
14
49
8
240
107
49
31
77
56
20
11
31
81
12
153
280
60
19
18
26
7
2
32
369
147
40
100
14
8
101
366
33
3
37
17
5
18
45
20
28
31
103
Less than 0.1%.
This includes the tire retreading shops that have been identified and represents only a portion of the plants
in SIC 7534.
63
-------
NUMBER OF PLANTS PER STATE
OT010
10 TO 50
50 TO 90
OVER 90
Figure 15. Geographic distribution of rubber
processing plants in the United States
64
-------
SECTION 4
EMISSIONS
LOCATIONS AND SELECTED POLLUTANTS
Emissions from rubber processing plants are a function of the
unxt operations performed and the chemical substances used during
processing. The materials emitted include particulates (carbon
black, zinc oxide, soapstone, oil mists, etc.) and hydrocarbons
(monomers, volatilized rubber chemicals, rubber impurities, sol-
vents, etc.). The locations of these emissions and the specific
pollutants selected for assessment of environmental impact are
given below for each industry.
SBR Production
Emulsion Polymerization—
Emission points for the emulsion polymerization process are 1)
the tank farm or monomer storage area, 2) the polymerization or
reactor section, 3) the recovery area, 4) the carbon black opera-
tion, and 5) the finishing area.
The tank farm emissions result from breathing losses in styrene
storage. Butadiene is stored in pressurized vessels, sometimes
located underground or underwater, and thus would not be expected
to have breathing losses. Fugitive losses due to leaks in pump
seals and valves also exist in the tank farm area, as do losses
from the storage of other liquid organics or solvents such as
reactor coolant and gasoline.
Polymerization or reactor section emissions result from fugitive
losses, due again to pump seal and valve leaks. The specific
substances emitted have not been identified and quantified. How-
ever, total hydrocarbon emissions have been reported and are con-
sidered in this report (4). As the reactor itself is pressur-
ized, no emissions would be expected from it.
Hydrocarbon emissions from the recovery area are also due to
fugitive losses from compressor seals, pump seals, and pipeline
valves and seals (4). In addition, emissions of unrecovered
butadiene occur after adsorption, absorption, or condensation
(4).
More than two-thirds of all SBR crumb are "extended" by addition
of oil and/or carbon black. The addition of carbon black prior
65
-------
to coagulation results in emissions of carbon black particles
(16). Emissions from the finishing area in crumb rubber produc-
tion result from drying and baling operations. During the drying
operation unreacted and unrecovered styrene is emitted. Also,
fine particles of SBR are contained in the exhaust gas from the
dryer. In the baling operation, talc is applied to the exterior
of the bale to prevent the rubber from sticking to polyethylene
film used to wrap the bale. The dusting is accomplished by air-
blowing the talc onto the bales as they pass through a "dust
chamber" on a conveyor belt. This is a source of particulates
(talc) emissions. When latex rubber is produced, the above
emissions do not exist.
Solution Polymerization—
Emission points for the solution polymerization process are 1)
the tank farm area, 2) the reactor area, 3) the carbon black
application, 4) the desolvent area, 5) the monomer and solvent
purification area, and 6) the finishing area.
Emissions from tank farm area, reactor area, and carbon black
application are similar to those mentioned for emulsion polymeri-
zation. The only difference is the additional hydrocarbon emis-
sion from solvent storage (usually hexane). The desolvent area
has emissions primarily of hexane resulting from the slurried
crumb being transferred to the finishing area. The slurry is
held in surge tanks, which are vented to the atmosphere. In
addition, fugitive losses in this area and in the solvent and
monomer purification area due to pump seal and valve leaks are
present (4). Emissions from the drying operation in the finish-
ing area are almost entirely hexane (solvent which is held
tightly within the crumb even after steam stripping and dewater-
ing) (4). The particulate emissions from talc dusting in baling
operations are similar to those of the emulsion polymerization
process.
Rubber Reclaiming
The depolymerization operation is the primary emission point of
hydrocarbons in rubber reclaiming. The pan, mechanical, or
digestion processes emit vapors and mists resulting from the
addition of aliphatic and aromatic oils and solvents during
digestion or reclaiming. In addition, rubber particles are
emitted from size reduction of scrap rubber before charging into
the depolymerization equipment. In the baling operation, talc is
(16) Pervier, J. W., R. C. Barley, D. E. Field, B. M. Friedman,
R. B. Morris, and W. A. Schwartz. Survey Reports on Atmos-
pheric Emission from the Petrochemical Industry, Volume 4:
Styrene Butadiene Rubber via Emulsion Polymerization.
EPA-450/3-73-006d, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina, June 1974. 39 pp.
66
-------
applied to the reclaimed rubber to prevent sticking. As in the
baling operation in crumb SBR production, this is a source of
particulate emissions.
Tires and Inner Tubes
Emission sources in tire manufacturing include 1) compounding,
2) milling and calendering, 3) fabric cementing, 4) extrusion,
5} undertread cementing, 6) green tire spraying, and 7) curing.
Emissions from compounding consist of particulates and hydrocar-
bons. The particulates are solids (carbon black, zinc oxide,
soapstone, etc.) and liquid aerosols (organic additives) (17).
The hydrocarbon vapors originate from impurities in the rubber
and from the organic additives. They occur as a result of heat
generated during mechanical mixing of the batch. Particulate
emissions occur when the additives are introduced into the batch.
In general, these particles are in a finely divided form and
smaller than 15 ym (16).
Compounding units are equipped with exhaust hoods that remove the
heat generated by the mixing action. They also remove particu-
late and hydrocarbon emissions from the work area. Bag filters
are employed to recover the solid particulates for recycle within
the plant (17).
During the milling and calendering operations, heat is also gen-
erated from the mechanical working of the rubber. Hydrocarbon
vapors are therefore present and emitted to the atmosphere. For
the same reason, extrusion is another source of hydrocarbon emis-
sions. In most cases the operating temperature in the extrusion
operation is relatively low (below 50°C), resulting in a smaller
quantity of hydrocarbons emitted. Hydrocarbons from milling,
calendering, and extrusion operations are usually emitted to the
general work area and vented through the plant ventilation
system (4).
In the fabric cementing operation, ply fabric is cemented or
latex-dipped and dried before calendering of the rubber and
fabric. Large quantities of solvent hydrocarbons are emitted
particularly in the drying step of the operation. In undertread
cementing, solvent-based cement is used to tackify the tread
before it is sent to the tire building operation. Generally,
naphtha-based solvents are used, and they evaporate rapidly after
being applied.
(17) Air Pollution Engineering Manual, Second Edition. J. A.
Danielson, ed. Publication No. AP-40, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina,
May 1973. 987 pp.
67
-------
Green tire ^praying, which is one of the building operations,
utilizes two distinct solvent-based sprays (one internally and
one externally) to act as mold release agents and rubber flow
promoters during the curing operation (18). The solvents used in
this operation evaporate both inside and outside of the spray
booth used.
In the curing operation, vulcanization temperatures (100°C to
200°C) result in the emission of organic materials from the rub-
ber stock. Theoretically, these emissions can occur via two
distinct mechanisms: 1) the volatilization of species present in
the stock and 2) the formation of new compounds.
The available literature indicates that emissions occur primarily
among ingredients which are either liquids at room temperature or
solids with melting points at or below curing temperatures. On
this basis, the possible species emitted can be determined as
indicated below (19).
Polymer or Blend Volatiles—General purpose polymers do not
decompose until pyrolysis temperatures (300°C to 400°C) are
reached. Depolymerization reactions have been noted only upon
continued heating at 175°C to 225°C for several hours. Curing
operations of much shorter duration result in little or no break-
down. Hence, polymer emissions will be the result of residual
monomer and impurities from the manufacturing process and should
represent less than 1% of the total polymer by weight.
Monomers however, are sufficiently volatile that appreciable
amounts may be lost in precuring operations such as milling and
calendering. Typical boiling points are 145°C for styrene, 78°C
for acrylonitrile, and 54.9°C for chloroprene.
Antioxidants and Antiozonants—In most cases, emissions of phe-
nolic compounds are higher than those of amines. Total emissions
are greater in black stocks than in gum. The total emissions
from curing molds may range as high as 5% to 20% for thin stocks
and the more volatile antidegradants. However, normal vaporiza-
tion losses amount to only 0.5% to 1.0% by weight of the anti-
degradant present in the stock. The melting points of the common
antioxidants are given in Table 24.
(18) Van Lierop, G., and P. W. Kalika. Measurement of Hydrocar-
bon Emissions and Process Ventilation Requirements at a Tire
Plant. Presented at the 68th Annual Meeting of the Air
Pollution Control Association, Boston, Massachusetts,
June 15-20, 1975. 23 pp.
(19) Rappaport, S. M. The Identification of Effluents from
Rubber Vulcanization. Ph.D. Thesis, University of
North Carolina, Chapel Hill, North Carolina, 1974.
68
-------
TABLE 24. MELTING POINTS OF COMMON ANTIOXIDANTS (19)
Antioxidant
type
Compound
Melting
point, °C
Phenol
Amine
2, 6-Di-t-butyl-4-methylphenol
2,4-Di-t-amylphenol
3-t-Butyl-4-hydroxyanisole
2,2'-Methylene-bis(4-methy1-
6-t-butyl phenol)
Phenyl-3-naphthylamina
N-N'-diphenyl-p-phenylenediamine
N-N1-diphenylethylenediamine
69 to 70
Liquid
Liquid
125 to 130
105 to 106
144 to 152
60 to 65
Accelerators—As with the antidegradants, the common accelerators
have melting points between 70°C and 200°C. Hence, emissions of
these components are to be expected at normal curing tempera-
tures. Average total emissions of 0.5% to 1.0% by weight of
accelerator present can be anticipated (20). The melting points
of the common accelerators are given in Table 25.
TABLE 25. MELTING POINTS OF COMMON ACCELERATORS (19)
Accelerator
Compound
Melting
point, °C
Dithiocarbamate
Thiuram
Sulfenamide
Thiazole
Guanidine
Zinc diethyldithiocarbamate 171 to 180
Zinc dibutyldithiocarbamate 98 to 108
Sodium dibutyldithiocarbamate Liquid
Selenium dimethyldithiocarbamate 140 to 172
Tetramethylthiuram monosulfide 103 to 108
Tetramethylthiuram disulfide 140 to 148
Tetraethylthiuram disulfide 62 to 75
N,N-diethyl-2-benzothiazylsulfenamide Liquid
N-cyclohexyl-2-benzothiazylsulfenamide 93 to 108
N-oxydiethylene-2-benzothiazylsulfenamide 70 to 90
2-Mercaptobenzothiazole 164 to 176
Benzothiazyl disulfide 160 to 176
2-Benzothiazyl-NyN-diethylthiocarbamylsulfide 69
Diphenylguanidine 145 to 147
Di-o-tolylguanidine 167 to 173
(20) Angert, I. G., A. I. Zenchenki, and A. S. Kuminski. Vola-
tilization of Phenyl-2-Naphthylamine from Rubber. Rubber
Chemistry and Technology, 34(3):807, 1961.
69
-------
Processing Aids and Diluents—Processing aids are generally in
the form of oils (usually paraffinic) and function as lubricants,
plasticizers, and softeners. Diluents are primarily aromatic
extender oils used to improve the overall performance of synthe-
tic rubbers. Volatilization from these mixtures is expected to
vary considerably depending on their composition. Available data
show that the total emissions in 3 hours at 167°C range from
0.05% to 1.0% by weight (21).
Miscellaneous Compounding Ingredients—The materials, in this
category which are most likely to be volatilized are the vulcani-
zing agents and retarders. These substances include amines,
esters, and organic acids, most of which are either liquids at
room temperature or solids with melting points between 70°C and
200°C. Emissions of the order of 1% by weight can be expected
(19).
In nearly all cases, the materials used in rubber blends are of
technical grade. Hence, the purity of the principal component is
low (60% to 95%), and some of the impurities will be sufficiently
volatile to be emitted during curing. The wide melting point
ranges of many of the compounds given above are indicative of
high impurity levels. Gas chromatographic analysis of commercial
antioxidants has confirmed the high impurity levels in these com-
pounds (22). As a result, there are hundreds of compounds which
may be emitted in trace amounts during the curing operation.
The volatilization of components from rubber stock during cure
has been shown to follow the theoretical equation (20):
/ , -mt'/R
c = co(l - e
where c = amount of component lost in time, t, percent
by weight of rubber
co = initial concentration of component, weight percent
m = a constant which depends on the diffusion
coefficient of the species at the curing
temperature
R = thickness of rubber stock
t' = time
Thus, physical losses of particular ingredients are related ex-
ponentially to the temperature and duration of cure, stock thick
ness, and individual diffusion coefficients.
(21) Taft, W. K., M. Felton, J. Duke, R. W. Laundrie, and
D. C. Prem. Oil Types in the Program for Oil Extended Rub-
ber Industrial and Engineering Chemistry, 47(5):1077, 1955.
(22) Gaeta, L. J., et al. Antioxidant Analysis. Rubber Age,
101(6):47, 1967.
70
-------
Upon mixing with general plant air, some of the vaporized organic
materials are condensed into either oil mists or solid particles,
resulting in particulate emissions.
Rubber Footwear
Sources of emission in a rubber footwear plant are the 1) com-
pounding, 2) milling, 3) calendering, 4) molding, 5) rubber
cementing, 6) latex dipping and drying, and 7) curing operations.
Emissions from compounding, milling, and calendering operations
have been mentioned earlier in this section for tire manufactur-
ing. The only difference is that the compounding operation in
footwear plants involves lower temperatures^than those used_in
'tire-plants. This results^rTTower hydrocarbon emissions. Mold-
ing of the soles has hydrocarbon emissions similar to those from
tire curing due to the heat applied to the rubber material. In
addition, particulates are formed and emitted as a result of con-
densation of hydrocarbon vapor from high-temperature molding
operations.
Rubber cementing operations are performed for various purposes,
including combination of fabric sheets, molded outsole cementing,
sole lining, basket sole cementing, etc. Cementing is usually
accomplished by hand application, and solvents contained in the
cement mixture are evaporated to the general work area and jvented
to_jthe outside atmosphere by the ventilation system. """
After the various components have been attached together, the
shoes are partially or entirely dipped in latex and dried, either
by air or in an oven. The finished shoes are then cured in an
air-heated autoclave, which is vented to the plant exterior.
These are sources of hydrocarbon emissions. The possible vola-
tilized chemical substances and particulate emissions from the
curing operation have been described earlier in this section.
Rubber Hose and Belting
Sources of emissions from manufacturing of rubber hose and belt-
ing are the 1) compounding, 2) milling, 3) calendering, 4) ex-
trusion, 5) fabric cementing, 6) rubber cementing, and
7) curing operations.
Particulate and hydrocarbon emissions from compounding, milling,
calendering, and extrusion operations are similar to those men-
tioned earlier for the tire industry. Emissions from fabric
cementing and rubber cementing are from evaporation of solvents
which are used as the vehicle for application of the adhesive
materials.
In most of the curing operations in this industry a batch steam
autoclave is used and some hydrocarbons vaporized during
71
-------
vulcanization are condensed with the steam prior to venting to
the plant exterior. This reduces hydrocarbon—andn?ar£iculate
emissions but generates a water pollution problem. ^~
Fabricated Rubber Products, N.E.C. "~"
Emission sources in this industry include the 1) compounding,
2) milling, 3) calendering, 4) extrusion, 5) bonding of extruded
parts, 6) latex dipping and drying, 7) adhesive spraying,
8) molding, and 9) curing operations.
Emissions from compounding, milling, calendering, extrusion, and
latex dipping and drying have been mentioned previously. Emis-
sions from the bonding of extruded parts are hydrocarbons from
evaporation of solvents which are used for lubrication and for
tackifying the rubber parts. Solvent hydrocarbons are also
emitted during the spraying of adhesive which is applied to metal
surfaces before the molding operation in the production of metal-
bonded items. Dry metal screen filters are usually used for the
exhaust from adhesive spraying booths to catch the adhesive
aerosols. Evaporated solvents are then vented to the atmosphere.
Particulate and hydrocarbon emissions from the molding operation
are similar to those from the curing operation in tire manufac-
turing. The curing of extruded products is usually performed in
a batch autoclave. Some oil mists, most of the solid particles,
and steam are condensed before venting to the atmosphere.
Metal degreasing operations are another source of hydrocarbon
emissions. These operations are performed to prepare the metal
surfaces before adhesive spraying. This source is not considered
in this assessment because it is covered by another report
entitled "Source Assessment: Solvent Evaporation - Degreasing"
(23).
Gaskets, Packing, and Sealing Devices
Emission sources in this industry include 1) compounding, 2) mil-
ling, 3) calendering, 4) adhesive spraying, and 5) molding. The
materials emitted from these operations are similar to those from
the production of general molded products as discussed in the
preceding subsection.
(23) Marn, P. J., T. J. Hoogheem, D. A. Horn, and T. W. Hughes.
Source Assessment: Solvent Evaporation - Degreasing. Con-
tract 68-02-1874, U.S. Environmental Protection Agency,
Cincinnati, Ohio. (Final document submitted to EPA by
Monsanto Research Corporation, January 1977.) 180 pp.
72
-------
Rubber Wire-Insulating
Sources of emissions in this industry include: 1) compounding,
2) milling, 3) extrusion, and 4) curing. Materials emitted from
compounding, milling, and extrusion are similar to those from
other rubber product industries.
In the continuous curing of insulated wire, since the curing
device is enclosed, volatilized hydrocarbons are condensed with
steam and contained in the condensate (4). Fugitive emissions
from the curing operation itself occur from the curing tube (4).
When the insulated wire exits from the curing tube, the rubber
material is still hot, and it is depressurized. This results in
another source of hydrocarbon emission. In this assessment, the
above two sources are considered as emissions from the curing
operation.
Tire Retreading
Emission sources in tire retreading include 1) buffing, 2) rubber
cementing, 3) curing, and 4) finish painting.
The buffing operation is performed by using a grinding wheel to
remove the remaining tread and is a source of particulate emis-
sions. Rubber cementing is usually done in a spray booth.
Hydrocarbons are; emitted from evaporation of solvents in the
cement mixture, j Emissions of hydrocarbons and particulates from
curing operations in retreading are substantially less than those
in new tire manufacturing, because only the new tread is "green,"
or unvulcanized. In the finish painting operations, both water-
based and solvent-based coating solutions are used. Hydrocarbons
are emitted only when solvent-based solutions are applied.
EMISSION FACTORS
The quantities of materials emitted per unit of production are
reported here for uncontrolled emissions and for emissions from
average plants. The uncontrolled emission factors were obtained
from, or derived from information contained in literature sources
and Government reports. The representative emission factors (for
average plants) were derived so that they can be used to calcu-
late mass emissions and ambient pollutant concentrations by
multiplying with the production figure.
In the derivation of representative emission factors, Er, the
following were considered: 1) uncontrolled emission factors for
each unit operation in the industry, Eu; 2) the utilization fac-
tor (extent of utilization) of the unit operation in the whole
industry, U (in percent); and 3) the generally achieved control
efficiency for the unit operation representing current control
practices in the industry, C (in percent). The representative
emission factor was thus calculated by the following formula:
73
-------
E = E
r u o T0
Emission factors are given in Tables 26 through 35 for 1) SBR
production by emulsion polymerization; 2) SBR production by
solution polymerization; 3) reclaimed rubber production; 4) tires
and inner tubes; 5) rubber footwear; 6) rubber hose and belting;
7) fabricated rubber products, N.E.C.; 8) gaskets, packing, and
sealing devices; 9) rubber wire-insulating; and 10) tire retread-
ing. Blanks in the tables indicate no emissions of particular
pollutants from those unit operations. Sources of information
are indicated by the reference numbers in parentheses following
the corresponding quantities presented. The derivation of the
process utilization factor for each industry is explained by
footnotes in the respective tables. A detailed description of
control technologies and control efficiencies used in these
tables are given in Section 5.
ENVIRONMENTAL EFFECTS
Definition of Representative Plants
For the purpose of assessing the source severity for the rubber
processing industries, a representative plant was defined for
each of the 9 SIC's, except SIC 2822. In SIC 2822, it was found
that two representative plants are necessary because the two pro-
cesses used, emulsion polymerization and solution polymerization,
have different emission characteristics. The factors considered
in defining these representative plants are annual production,
emission factors, emission heights, population density, and wind
velocity around the plant.
For SIC's 2822, 3021, 3031, 3041, and 3357, the annual production
for representative plants was obtained by dividing the 1975 total
national production (as presented in Tables 6 and 7) by the total
number of plants (as shown in Table 23) . For SIC 3011 (Tires and
Inner Tubes) , the representative plant was defined for tire
production only, since this segment of the industry represents
93% of the product shipments in the SIC, with inner tubes, tread
rubber, tire sundries, and repair materials constituting only 7%
of the industry economy. The average (representative) annual
production was obtained by dividing the 1975 total production by
the number of tire plants (24). For SIC's 3069 and 3293, about
50% of total plants are small ones, with less than 20 employes
producing less than 5% of goods in the respective industries
(15). Therefore, only those plants having more than 20 employes
were considered in obtaining the average annual production. For
SIC 7534 (Tire Retreading) , owing to lack of data on the total
(24) A Look at the Tire Industry. Rubber World, 175 (4):42-46,
1977.
74
-------
TABLE 26. EMISSION FACTORS FOR SBR PRODUCTION BY EMULSION POLYMERIZATION (SIC 2822)
ui
Uncontrolled emission factors, Process
g/kg product utili-
Criteria pollutants Chemical substances zationL
Emission source
Styrene storage
(breathing)
Styrene storage^
(fugitive) ,
Reactor section
(fugitive)
Butadiene absorption
Monomer recovery
area (fugitive)
Carbon black
application
Drying ^
Baling-
TOTAL
Hydro-
carbons
0.02 (4)
0.04 (4)
0.4 (4)
2.5 (4)
0.1 (4)
0.6 (4)
N.A.9
Particu- factor , "
lates Styrene ' Butadiene %
~~ --.
C 0.02 (16).^ 100
•'- -
(^ 0.04 (16) - 100
-d - 10°
0.1 (16) 100
d d
- - 100
^ f
1.0 (16) /-" . 85f
0.02 (16)' 0.6 (16) , - 85f
0.1 (16) ^ — - 85
N.A. N.A. N.A. N.A.
Control
efficiency,
%
0
0
0
0
0
70
0
70
N.A.
Representative emission factors,
9/kg product
Criteria pollutants Chemical
c Hydro-
carbons
0.02
0.04
0.4
2.5
0.1
0.5
3.6
Particu-
lates Styrene
0.02
0.04
e
Q
0.3
0.02 0.5
0.03
0.35 -6
substances
Butadiene
_e
0.1
Q
Q
_e
Blanks indicate no emissions from unit operations. Numbers in parentheses indicate sources of data (references).
This represents the extent of utilization of a particular unit operation in the whole industry.
CThis is the control efficiency for the emissions from the unit operation which represents current control practices in the
industry. See Section IV for details.
Data for specific emissions were not available.
Not calculated due to lack of data.
This is the percentage for crumb rubber production and represents the portion of the industry that utilizes the corresponding
unit operations.
"Not applicable.
-------
TABLE 27. EMISSION FACTORS FOR SBR PRODUCTION BY SOLUTION POLYMERIZATION (SIC 2822)
Uncontrolled emission
g/kg product
Emission source
Styrene storage
(breathing)
Hexane storage
(breathing)
Storage area
(fugitive)
Reactor area
(fugitive)
Carbon black
application
De sol vent area
(surge vent)
Desolvent area
(fugitive)
Purification area
(fugitive)
Drying
Baling
TOTAL
Criteria
Hydro-
carbons
0.02 (4)
0.05 (4)
0.04 (4)
i
0.4 (4)
2.7 (4)
0.2 (4)
0.2 (4)
17.1 (4)
N.A.f
factors,
pollutants Chemical substances
Particu-
lates Styrene
i " • ^ '
\J).02 (4)
j
-
d
1.0 (16)
d
d
d
0.02 (16)
0.1 (16)
N.A. N.A.
Butadiene Hexane
0.05
d d
d d
d d
d d
d d
15.3 (4)
N.A. N.A.
Process
utili-
zation
factor,"
100
100
100
i
100
100
100
100
100
100
100
N.A.
Control
efficiency,c
0
0
0
0
70
50
0
0
0
70
N.A.
Representative
g/kg
Criteria pollutants
Hydro- Particu-
carbons lates
0.02
0.05
0.04
0.4
0.3
1.4
0.2
0.2
17.1 0.02
0.03
19.1 0.35
emission factors,
product
Chemical substances
Styrene Butadiene Hexane
0.02
0.
6 6
6 e
G G
6 G
15.
-6 ^
05
Q
G
e
3
_G
3Blanka indicate no emissions from unit operations. Numbers in parentheses indicate sources of data (references).
Represents the extent of utilization of a particular unit operation in the whole industry.
CControl efficiency for emissions from the unit operation which represents current control practices in the industry. See Section IV for details.
Data for specific emissions not available.
Not calculated due to lack of data.
Not applicable.
-------
TABLE 28. EMISSION FACTORS FOR RUBBER RECLAIMING (SIC 3031)
Uncontrolled Process
emission factors, utili- Control
g/kg product zatioa
Representative
emission factors,
Emission source
Size reduction
Depolymerization
Baling
TOTAL
Hydro-
carbons
30 (4)
f
N.A.
Particu-
lates
5.0d
oae
N.A.
f actor, b
100
100
100
N.A.
80
90
0
N.A.
•n/ *vj f
Hydro-
carbons
3.0
3.0
Particu-
lates
1.0
0.1
1.1
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (references).
Represents the extent of utilization of a particular unit operation in
the whole industry.
Control efficiency for emissions from the unit operation which represents
current control practices in the industry. See Section IV for details.
Estimated from engineering experience.
g
Assumed to be the same as in SBR production.
Not applicable.
TABLE 29. EMISSION FACTORS FOR TIRES AND INNER TUBES (SIC 3011)
1
Uncontrolled
1 emission factors,
1
Emission source
Compounding
Milling
Calendering
Fabric cementing
Extrusion
Undertread cementing
Green tire spraying
Curing
TOTAL
g/kg
Hydro-
carbons
0.3 (4)
0.2 (4)
0.2 (4)
5.0 (4)
0.2 (4)
2.8 (4)
14 (4)
5.0 (4)
N.A.^
product
Particu-
lates
11 (25)
f
2.5T
N.A.
Process
utili-
zation
factor,"
%
100
100
100
100
100
100
100
100
N.A.
Representative
Control
effi-
ciency,0
%
Od, 906
0
0
85
0
65
40
0
N.A.
emission factors,
g/kg
Hydro-
carbons
0.3
0.2
0.2
0.8
0.2
1.0
8.4
5.0
16.1
product
Particu-
lates
1.1
2.5
3.6
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (references).
Represents the extent of utilization of a particular unit operation in the
whole industry.
Control efficiency for emissions from the unit operation which represents
current control practices in the industry. See section IV for details.
d
For hydrocarbons.
g
For particulates.
Assume 50% of emitted hydrocarbons is condensed into oil mists or solid
organic particles.
q
JNot applicable.
77
-------
TABLE 30. EMISSION FACTORS FOR RUBBER FOOTWEAR (SIC 3021)
Uncontrolled
emission factors.
Emission source
Compounding
Milling
Calendering
Molding
Rubber cementing
Latex dipping and
drying
Curing
TOTAL
gAg
Hydro-
carbons
0.3 (4)
0.2 (4)
0.2 (4)
4.0 (4)
95 (4)
0.4 (4)
3.0 (4)
N.A. ^
product
Particu-
lates
11 (25)
f
2.0
f
1.5T
N.A.
Process
utili-
zation.
factor,"
%
100
100
100
50
100
25
50
N.A.
Representative
Control
effi-
ciency, c
%
Od, 906
0
0
0
0
0
0
N.A.
emission factors.
gAg
Hydro-
carbons
0.3
0.2
0.2
2.0
95
0.1
1.5
99.3
product
Particu-
lates
1.1
1.0
0.8
2.9
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (reference).
Represents the extent of utilization of a particular unit operation in the
whole industry.
Control efficiency for emissions from- the unit operation which represents
current control practices in the industry. See Section IV for details.
d
For hydrocarbons.
e
For particulates.
Assume 50% of emitted hydrocarbons is condensed into oil mists or solid
organic particles.
q
3Not applicable.
TABLE 31. EMISSION FACTORS FOR RUBBER
HOSE AND BELTING (SIC 3041)
Emission source
Compounding
Milling
Calendering
Extrusion
Fabric cementing
Rubber cementing
Curing
TOTAL
Uncontrolled
emission factors.
gAg product
Hydro- Particu-
carbons lates
0.3 (4) 11 (25)
0.2 (4)
0.2 (4)
0.03 (4)
25 (4)
1.0 (4)
3.0 (4)
N.A. N.A.
Process
utili-
zation^
factor,"
%
100
100
100
50
50
100
100
N.A.
Representative
Control
effi-
ciency,
%
Od, 906
0
0
0
85
0
0
N.A.
emission factors.
gAg
Hydro-
carbons
0.3
0.2
0.2
0.02
1.9
1.0
3.0
6.6
product
Particu-
lates
1.1
1.1
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (reference).
Represents the extent of utilization of a particular unit operation in the
whole industry.
CControl efficiency for emissions from the unit operation which represents
current control practices in the industry. See Section IV for details.
For hydrocarbons.
For particulates.
Not applicable.
78
-------
TABLE 32. EMISSION FACTORS FOR FABRICATED RUBBER
PRODUCTS, N.E.C. (SIC 3069)a
Emission source
Compounding
Milling
Calendering
Extrusion
Bonding of extruded
parts
Latex dipping and
drying
Adhesive spraying
Molding
Curing
TOTAL
Uncontrolled
emission factors.
9/kq product
Hydro- Particu-
carbons lates
0-3 (4) 11 (25)
0.2 (4)
0.2 (4)
0.06 (4)
2.0 (4)
0.5 (4)
1-8 (4)
4.0 (4) 2.0
3.0 (4)
N.A.9 N.A.
Process
utili-
zation
factor,"
%
100
75
75
50
10
25
100
40
60
N.A.
Control
effi-
ciency,''
%
Od, 90C
0
0
0
0
0
0
0
0
N.A.
Representative
emission factors.
gAg
Hydro-
carbons
0.3
0.15
0.15
0.03
0.2
0.13
1.8
1.6
1.8
6.2
product
Particu-
lates
1.1
2.0
3.1
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (reference).
Represents the extent of utilization of a particular unit operation in the
whole industry.
Control efficiency for emissions from the unit operation which represents
current control practices in the industry. See Section TV for details.
For hydrocarbons.
For particulates. ,
Assume 50% of emitted hydrocarbons is condensed into oil mists or solid
organic particles.
^Not applicable.
TABLE 33. EMISSION FACTORS FOR GASKETS, PACKING,
AND SEALING DEVICES (SIC 32.93)a
Uncontrolled
emission factors,
Emission source
Compounding
Milling
Calendering
Adhesive spray
Molding
TOTAL
gAg
Hydro-
carbons
0.3 (4)
0.2 (4)
0.2 (4)
3.6 (4)
4.0 (4)
N.A.9
product
Particu-
lates •
11 (25)
f
2.0T
N.A.
Process
utili-
zation
f actor, b
%
100
100
100
100
100
N.A.
Representative
Control
effi-
ciency, c
%
Od, 906
0
0
0
0
N.A.
emission factors.
g/kg
Hydro-
carbons
0.3
0.2
0.2
3.6
4.0
8.3
product
Particu-
lates
1.1
2.0
3.1
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (reference).
Represents the extent of utilization of a particular unit operation in the
whole industry.
Control efficiency for emissions from the unit operation which represents
current control practices in the industry. See Section IV for details.
d
For hydrocarbons.
9
For particulates.
Assume 50% of emitted hydrocarbons is condensed into oil mists or solid
organic particles.
^Not applicable.
79
-------
TABLE 34. EMISSION FACTORS FOR RUBBER WIRE-INSULATING (SIC 3357)'
Uncontrolled
emission factors,
g/kg product
Process
utili- Control
Representative
emission factors,
g/kg product
Emission source
Compounding
Milling
Extrusion
Curing
TOTAL
Hydro-
carbons
0.3 (4)
0.2 (4)
0.03 (4)
3.0 (4)
f
N.A.
Particu-
lates
11 (25)
N.A.
factor,
100
100
100
100
N.A.
ciency.
Od, 906
0
0
0
N.A.
Hydro-
carbons
0.3
0.2
0.03
3.0
3.5
Particu-
lates
1.1
1.1
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (reference).
b
Represents the extent of utilization of a particular unit operation in the
whole industry.
r*
"Control efficiency for emissions from the unit operation which represents
current control practices in the industry. See Section IV for details.
For hydrocarbons.
a
"For particulates.
Not applicable.
TABLE 35. EMISSION FACTORS FOR TIRE RETREADING (SIC 7534)
Emission source
Buffing
Rubber cementing
Curing
Finish painting
t
TOTAL
Uncontrolled
emission factors.
g/kg product
Hydro- Particu-
carbons lates
20 (4)
3.0 (4)
0.8 (4)
0.8 (4)
N.A. N.A.
Process
utili-
zation
factor ,
%
100
100
100
50
N.A.
Control
effi-
ciency,
%
90
0
0
0
N.A.
Representative
emission factors.
g/kg product
Hydro- Particu-
carbons lates
2.0
3.0
0.8
0.4
4.2 2.0
Blanks indicate no emissions from unit operation. Numbers in parentheses
indicate sources of data (reference).
Represents the extent of utilization of a particular unit operation in the
whole industry.
C
Control efficiency for emissions from the unit operation which represents
current control practices in the industry. See Section IV for details.
Not applicable.
80
-------
number of Plants and the plant size distribution, an annual
production of 40,000 tires (450 metric tons/yr of product) was
assumed and used for the representative plant. The annual pro-
duction for representative plants is summarized in Table 36.
TABLE 36. PARAMETERS USED TO DEFINE THE
REPRESENTATIVE PLANTS
Population density around the plant = 103 persons/km2
Wind velocity around the plant = 4.5 m/s
_ , . . Annual production. Emission height,
Industries ....
SBR by emulsion
(SIC 2822)
SBR by solution
(SIC 2822)
Rubber reclaiming
(SIC 3031)
Tires and inner tubes
(SIC 3011)
Rubber footwear
(SIC 3021)
Hose and belting
(SIC 3041)
Fabricated products,
N.E.C.
(SIC 3069)
Gaskets, packing, and
sealing: devices
(SIC 3293)
Wire insulating
(SIC 3357)
Tire retreading
(SIC 7534)
41,000
41,000
14,000
20,000
2,700
6,500
1,700
1,700
3,000
450
20
20
20
15
15
15
15
15
15
15
The representative emission factors as defined earlier in this
section and presented in Tables 26 through 35 were used for the
representative plants. The process utilization factor and the
generally achieved control efficiency for each operation which
are also given in the above tables, are applicable to the defi-
nition of representative plants.
Assumed emission heights of 20 m and 15 m were based on a NEDS
Point Source Listing" and engineering judgment for elastomer pro-
duction and rubber products fabrication, respectively. All
aPoint Source Listings are provided by EPA from the National
Emissions Data System (NEDS) via AEROS (26).
(26) Aerometric and Emissions Reporting System (AEROS), U.S.
Environmental Protection Agency. National Air Data Branch,
Research Triangle Park, North Carolina 27711.
81
-------
emissions in a plant were assumed to occur at the same height,
based on the assumption that fugitive emissions do not escape
through doors, windows, etc., but eventually travel through the
plant ventilation system to be discharged through the representa-
tive stack.
The population density around a representative plant was assumed
to be 103 persons/km2. This is the composite state population
density derived and given in Section 3. For the wind velocity,
the national average of 4.5 m/s was used.
Source Severity
In order to obtain a quantitative measure of the hazard potential
of rubber processing, the source severity, S, is defined as:
S = (4)
where Xmax is the time-averaged maximum ground level concentra-
tion of each pollutant emitted from a representative plant, and
F is defined as a primary ambient air quality standard for cri-
teria pollutants (particulate and hydrocarbons in this case) ,
while for noncriteria pollutants:
F = TLV • 8/24 • 0.01, g/m3 (5)
The factor 8/24 adjusts the TLV® (27) for continuous rather than
workday exposure, and the factor of 0.01 accounts for the fact
that the general population is a higher risk group than healthy
workers.
Thus, the source severity represents the ratio of the maximum
mean ground level exposure to the hazard level of exposure for a
given pollutant.
The maximum ground level concentration, Xmax, is calculated
according to Gaussian plume dispersion theory (28) :
X = -^- (6)
max TTH'eu
(27) TLVs® Threshold Limit Values for Chemical Substances and
Physical Agents in the Workroom Environment with Intended
Changes for 1976. American Conference of Governmental
Industrial Hygienists, Cincinnati, Ohio, 1976. 94 pp.
(28) Turner, D. B. Workbook of Atmospheric Dispersion Estimates.
Public Health Service Publication No. 999-AP-26, U.S.
Department of Health, Education, and Welfare, Cincinnati,
Ohio, May 1970.- 84 pp.
82
-------
where Q = mass emission rate, g/s
u = average wind speed, m/s
H = effective emission height, m
e = 2.72
Equation 6 yields a value for a short-term averaging time during
which the Gaussian plume dispersion equation is valid. The
short-term averaging time was found to be three minutes in a
study of published data on lateral and vertical diffusion (29).
For a continuously emitting source, the maximum mean ground level
concentration for time intervals between three minutes and 24
hours can be estimated from the relation (28):
(7)
where to = short-term averaging time (3 min)
t = averaging time
For noncriteria pollutants, the averaging time, t, is 24 hours.
For criteria pollutants, the averaging times are those used in
the definition of the primary ambient air quality standards.
Insertion of the national average wind speed of 4.5 m/s and the
primary air quality standards with corresponding averaging times
for hydrocarbons and particulates leads to the following severity
equations: (
i
i
for hydrocarbons, S = 162.5 QH~2 (8)
for particulates, S = 70 QH~2 (9)
where Q and H are expressed in the units of g/s and m, respect-
ively, a detailed derivation of the above two equations is given
in Appendix A.
For noncriteria pollutants (chemical substances), insertion of
Equations 5, 6, and 7 into Equation 4, using 24 hours as the
averaging time, gives the following:
S = 5.5 QH~2(TLV)~1 (10)
where TLV (threshold limit value) is in the units of g/m3 and Q
and H have the same units as those for Equations 8 and 9.
The primary ambient air quality standards and TLV's used in the
calculation of source severities are given in Table 37.
(29) Nonhebel, G. Recommendations on Heights for New Industrial
Chimneys. Journal of the Institute of Fuel. 33:479-511,
July 1960.
83
-------
TABLE 37. PRIMARY AMBIENT AIR QUALITY STANDARDS AND THRESHOLD
LIMIT VALUES FOR POLLUTANTS CONSIDERED
AAQS,TLV,
Pollutant species mg/m3 (30) mg/m3 (27)
Hydrocarbons
Particulates
Styrene
Butadiene
Hexane
0.16
0.26
N.A.
N.A.
N.A.
N.A.3
N.A.
420
2,200
360
Not applicable.
Using the parameters given in the definition of representative
plants and the above equations, source severities were calcula-
ted for emission points within the representative plants. These
source severities are presented in Table 38 for industries in
elastomer production, and in Table 39 for industries in rubber
products fabrication.
Except for SIC 3011 (Tires and Inner Tubes), particulate emis-
sions result in source severities of less than 0.1 from all
industries. SIC's 3031, 3041, 3293, and 3357 have at least one
hydrocarbon emission point which has a source severity between
0.1 and 1.0. SIC's 2822 (both emulsion and solution polymeriza-
tion), 3011, and 3021 have at least one emission point with
hydrocarbon source severity greater than 1.0. All the hydrocar-
bon emission points in SIC's 3069 and 7534 have source severities
less than 0.1. In addition, except for hexane from solution
polymerization known emissions of all noncriteria pollutants all
have source severities less than 0.1.
Affected Population
A measure of the population which is exposed to a high contam-
inant concentration due to emissions from a representative rubber
processing plant can be obtained as follows. The values of x,
downwind distance from the source, for which
= 0.1 or 1.0 (11)
are determined by iteration. The value of Y(x), the annual mean
ground level concentration, is computed from the equation (28):
(30) Code of Federal Regulations, Title 42 - Public Health,
Chapter IV - Environmental Protection Agency, Part 410 -
National Primary and Secondary Ambient Air Quality
Standards, April 28, 1971. 16 pp.
84
-------
TABLE 38. SOURCE SEVERITIES FOR REPRESENTATIVE ELASTOMER PLANTS
00
Emission source
Styrene storage
(breathing)
Hexane storage
(breathing)
Storage area
(fugitive)
Reactor area
(fugitive)
Butadiene absorption
Monomer recovery area
(fugitive)
Desolvent area
(surge vent)
Desolvent area
(fugitive)
Purification area
(fugitive)
Carbon black
application
Size reduction
Depolymerization
Drying
Baling
SBR Emulsion polymerization
(SIC 2822)
Criteria Chemical
pollutants substances
Hydro- Particu-
carbons lates Styrene Butadiene
0.01 0.001
0.02 0.002
h h
0.2 -J -b
1 - 0.001
b b
0.05 - -
0.07
b
0.3 0.005 0.02
0.007
SBR Solution polymerization
(SIC 2822)
Criteria
pollutants
Hydro- Particu-
~carbons lates
0.01
0.03
0.02
0.2
0.7
0.1
0.1
0.07
9 0.005
0.007
Chemical
substances
Styrene Butadiene Hexane
0.001
0.002
bh h
bh h
b b b
b b b
b b b
0.8
Rubber reclaiming
(SIC 3031)
Criteria
pollutants
Hydro- Particu-
carbons lates
0.08
0.5
0.008
a
Blanks indicate no emissions £ro» unit operations.
Not calculated due to lack of data.
-------
TABLE 39. SOURCE SEVERITIES FOR REPRESENTATIVE RUBBER PRODUCT PLANTS
SIC 3011
Hydro- Particu-
Emission source carbons lates
Compounding 0.1 0.2
Milling 0.09
Calendering 0.09
Fabric cementing 0.4
Extrusion 0.09
Undertrade
cementing 0.4
Green tire
spraying 4
Buffing
Rubber cementing
Latex dipping and
drying
Bonding of
extruded parts
Adhesive spraying
Molding
Curing 2 0.5
Finish painting
SIC 3021 SIC 3041
Hydro- Particu- Hydro- Particu-
carbons lates carbons lates
0.02 0.03 0.04 0.07
0.01 0.03
0.01 0.03
0.3
0.003
6 0.1
0.01
0.1 0.03
0.1 0.02 0.4
SIC 3069 SIC 3293 SIC 3357 SIC 7534
Hydro- Particu- Hydro- Particu- Hydro- Particu- Hydro- Particu-
carbons lates carbons lates carbons lates carbons lates
0.01 0.02 0.01 0.02 0.02 0.03
0.006 0.008 0.01
o.ooe o.oos
0.001 0.002
0.01
0.03
0.005
0.008
0.07 0.1
0.06 0.03 0.2 0.03
0.07 0.2 0.008
0.004
a
Blanks indicate no emissions from unit operations.
-------
x(x) = 2-03 Q
a^exp|-±(^) (12)
where Q = emission rate, g/s
H = effective emission height, m
x = downwind distance from source, m
u = average wind speed (4.5 m/s)
az ~ vertlcal dispersion coefficient, m
For atmospheric stability class C (neutral conditions), a is
given by (31): '' z
az = 0.113(x°*911) (13)
The affected area is then computed as
A = Tr(x22 - X!2), km2 (14)
where x\ and x2 are the two roots of Equation 11.
The product of affected area (A) and a composite population den-
sity (U, defined and derived in Section 3) thus give the
"affected population. "
The affected population was computed for each pollutant and each
emission point for which the source severity, S, exceeds 0.1.
The results are presented in Table 40. SIC's 3069 and 7534 were
not included because they do not have emissions with source
severity greater than 0.1. The largest population affected is
4,000 persons exposed to the value of x~(x)/F greater than 0.1,
by the drying operation from solution polymerization segment of
SIC 2822.
Contribution to Total Air Emissions
The contribution of rubber processing to statewide and nation-
wide air emissions was measured by the ratio of mass emissions
from this source to the total emissions from all sources.
The mass emissions of hydrocarbons and particulates resulting
from elastomer production and rubber products fabrication were
calculated using the representative emission factors from
Tables 26 through 35 and the annual production data given in
(31) Eimutis, E. C. , and M. G. Konicek. Derivations of Contin-
uous Functions of the Lateral and Vertical Atmospheric
Dispersion Coefficients. Atmospheric Environment, 6(11):
859-863, 1972.
87
-------
TABLE 40. AFFECTED POPULATION BY REPRESENTATIVE
RUBBER PROCESSING PLANTS3
(number of persons)
SIC
Emulsion
Hydrocarbons
Reactor area
(fugitive)
Butadiene absorption
where
X/F>1
0
20
where
X/F>0.1
50
500
2822
Solution SIC 3031
Hydrocarbons
where
X/F>1
0
where
X/F>0.1
50
Hexane Hydrocarbons
where where where where
X/F>1 X/F>0.1 X/F>1 X/F>0.1
Desolvent area
(surge vent)
Depolymerization
Drying
Compounding
Fabric cementing
Under tread cementing
Green tire spraying
Rubber cementing
Adhesive spraying
Holding
Curing
70
200
300 4,000
400
200
SIC 3011
Hydrocarbons Particulates
where where where where
7/F>l X/F>0.1 X/F>1 X/F>0.1
SIC 3021
Hydrocarbons
where _where
X/F>1 x/F>0.1
Sic 3041
Hydrocarbons
where where
X/F>1 x/F>0.1
SIC 3293
Hydrocarbons
where where
X/F>1 X/F>0.1
SIC 3357
Hydrocarbons
where _where
X/F>1 x/F>0.1
Reactor area
(fugitive)
Butadiene absorption
Desolvent area
(surge vent)
Depolymerization
Drying
Compounding
Fabric cementing
Under tread cementing
Green tire spraying
Rubber cementing
60
10
60
SO
800
0 60
0 40
100 1,000 0 10
Adhesive spraying
Molding
Curing
0
0
30 - 500 0 100 0 7 0 80
10
20
0 20
Blanks indicate no emissions from a specific operation.
88
-------
The national mass emissions were
appropriate emission factors by the total
™«- S emissi°ns were obtained by distributing
of n»n ^lsslon? among the states according to the number
of plants in the applicable states.
The nationwide mass emissions and percent contribution of hydro-
carbons and particulates for each SIC are presented in Table 41.
The mass emissions from each state are given in Appendix B. The
percent contributions of hydrocarbons and particulates from rub-
ber processing to the corresponding total state emissions are
shown in Tables 42 and 43. The total pollutant emissions nation-
wide and for each state from all sources which were used for
above calculations were obtained from the 1972 National Emissions
Report (32).
TABLE 41. NATIONWIDE EMISSIONS OP CRITERIA POLLUTANTS
FROM RUBBER PROCESSING INDUSTRIES
Industries
SBR production
(SIC 2822)
Rubber reclaiming
(SIC 3031)
Tires and inner tubes
(SIC 3011)
Rubber footwear
(SIC 3021)
Hose and belting
(SIC 3041)
Mass emission,
metric tons/yr
Hydrocarbons
6,000a
250
33,000
14,000
2,600
Fabricated Products, H.E.C. 6,200
(SIC 3069)
Gaskets, packing, and sealing devices 1,300
(SIC 3293)
Wire insulating
(SIC 3357)
Tire retreading
(SIC 7534)
TOTAL
180
2,000
65,500
Particulates
410b
91
7,300
400
440
3,100
500
56
950
13,200
Percent contribution
Hydrocarbons
0.024
0.0010
0.13
0,056
0.010
0.025
0.0052
0.0007
0.0080
0.26
Particulates
0.0023
0.0005
0.041
0.0022
0.0025
0.017
0.0028
0.0003
0.0053
0.074
a63% of this is emitted from emulsion
polymerization.
b90% of this is emitted from emulsion
polymerization.
polymerization; the remaining 37% is from solution
polymerization; the remaining 10% is from solution
(32) 1972 National Emissions Report. EPA-450/2-74-012, U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, June 1974. 422 pp.
89
-------
TABLE 42. PERCENT CONTRIBUTION OF HYDROCARBON EMISSIONS FROM
RUBBER PROCESSING TO TOTAL STATE EMISSIONS
State
Alabama
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Nebraska
Nevada
New Hampshire
New Jersey
New York
North Carolina
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
SIC 2822
0.019
0.096
0.33
0.090
0.012
0.12
0.032
0.093
0.029
0.092
0.036
0.046
0.11
0.054
SIC 3031 SIC
0.
0.
0.
0.
0.
0.038 0.
0.
0.
0.
0.0023 0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.021 0.
0.
0.
0.
0.0033 0.
0.
0.0036 0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
3011
25
17
41
16
25
073
25
052
30
088
13
25
10
15
13
11
22
13
039
24
15
54
059
051
25
37
32
20
25
035
58
081
22
093
061
SIC 3021
0.14
0.051
0.12
0.087
0.12
0.015
0.090
0.66
0.18
0.36
0.13
1.2
0.066
0.10
0.12
0.070
0.14
0.22
0.23
0.052
SIC 3041
0.022
0.013
0.022
0.019
0.19
0.012
0.014
0.027
0.012
0.010
0.010
0.16
0.061
0.014
0.0094
0.036
0.014
0.0046
0.012
0.043
SIC 3069
0.0095
0.020
0.017
0.037
0.0088
0.12
0.017
0.019
0.037
0.019
0.052
0.0088
0.0067
0.02
0.082
0.045
0.032
0.031
0.017
0.022
0.012
0.048
0.030
0.022
0.084
0.011
0.026
0.030
0.14
0.0055
0.0066
0.020
0.0099
0.028
0.014
0.020
0.0081
0.038
0.025
SIC 3293
0.0031
0.0088
0.0067
0.018
0.0011
0.0082
0.0057
0.0021
0.0044
0.014
0.011
0.0066
0.0082
0.0079
0.0090
0.0074
0.0045
0.010
0.0059
0.0090
0.0036
0.0050
0.013
0.0019
0.011
0.0065
SIC 3357
0.0010
0.0096
0.0022
0.0005
0.0017
0.0031
0.0048
0.0012
0.0017
0.0022
0.0018
0.0011
SIC
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
7534a
0044
0053
0082
012
0052
033
012
0052
012
0066
014
0038
0006
0037
0002
0033
0068
025
013
0088
0092
0073
0047
0037
014
015
0095
0089
024
0059
0068
Oil
030
0015
Oil
0039
010
0060
0023
010
0073
State
Total
0.26
0.19
0.56
0.30
0.29
0.50
0.80
0.16
0.57
0.16
0.32
0.26
0.10
0.29
0.032
0.79
0.32
0.82
0.24
0.097
0.31
0.31
0.027
0.16
1.8
0.26
0.22
0.49
0.64
0.35
0.24
0.50
0.17
0.047
0.0066
0.96
0.15
0.095
0.014
0.24
0.10
0.29
0.15
aBlanks indicate no emissions from the industry.
bThere is a lack of* data available for complete geographical distribution of tire retreading shops. In this
calculation, the percentage distribution of plants among states in this SIC was assumed to be the same as tnac
given in Table 23 for the toal of the other eight industries.
-------
TABLE 43. PERCENT CONTRIBUTION OF PARTICULATE EMISSIONS
FROM RUBBER PROCESSING TO TOTAL STATE EMISSIONS
State
Alabama
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Nebraska
Nevada
New Hampshire
New Jersey
New York
North Carolina
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
SIC
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
0.
2822
0028
035
038
0069
001
0051
Oil
029
0020
0058
0016
0015
0068
016
SIC 3031 SIC 3011
0
0
0
0
0
0.075 0
0
0
0
0.0013 0
0
0
0
0
0
0
0
0
0
0.0089 0
0
0
0
0.0094 0
0
0.0008 0
0
0
0
0
0
0
0
0
0
.030
.098
.13
.078
.055
.088
.095
.031
.079
.031
.024
.083
.020
.020
.071
.014
.22
.030
.013
.065
.069
.74
.072
.088
.052
.054
.27
.065
.028
.036
.11
.071
.038
.044
.017
SIC 3021
0.0058
0.0031
0.020
0.0071
0.004
0.0007
0.0021
0.047
0.0032
0.048
0.0079
0.21
0.010
0.024
0.0033
0.0013
0.0021
0.0056
0.0037
0.0019
SIC 3041
0.0051
0.0050
0.0035
0.018
0.057
0.0031
0.0019
0.022
0.0020
0.0026
0.0035
0.015
0.056
0.018
0.0015
0.0040
0.0012
0.0035
0.0017
0.0098
SIC 3069
0.0025
0.026
0.012
0.040
0.0040
0.32
0.016
0.027
0.021
0.015
0.021
0.0065
0.002
0.0057
0.19
0.023
0.025
0.018
0.018
0.015
0.034
0.12
0.012
0.010
0.027
0.020
0.018
0.0077
0.34
0.013
0.0057
0.0088
0.020
0.020
0.021
0.0075
0.0086
0.010
0.016
SIC 3293
0.0007
0.0075
0.0025
0.038
0.0009
0.0050
0.0017
0.0004
0.0010
0.024
0.0044
0.0038
0.0064
0.013
0.019
0.028
0.0017
0.0026
0.0086
0.007
0.0012
0.0075
0.0070
0.0004
0.0023
0.0032
SIC 3357
0.0006
0.015
0.0070
0.003
0.0004
0.0005
0.0062
0.002
0.0038
0.0006
0.0003
0.0002
SIC 7534a
0.011
0.0069
0.0058
0.012
0.0025 '
0.085
0.011
0.0066
0.0064
0.0050
0.0055
0.0028
0.0003
0.0011
0.0005
0.0041
0.0020
0.054
0.0064
0.0064
0.0048
0.0069
0.0031
0.0011
0.040
0.037
0.036
0.0040
0.0074
0.011
0.0047
0.0027
0.077
0.0035
0.0046
0.0075
0.0070
0.0021
0.0025
0.0028
0.0044
State
Total
0.034
0.13
0.16
0.15
0.070
0.70
0.22
0.071
0.12
0.062
0.058
0.10
0/020
0.029
0.012
0.12
0.026
0.59
0.067
0.053
0.095
0.11
0.018
0.016
1.0
0.32
0.32
0.079
0.10
0.31
0.088
0.045
0.42
0.055
0.0057
0.14
0.12
0.043
0.021
0.048
0.056
0.019
0.044
aBlanks indicate no emissions from the industry.
bThere is a lack of data available for complete geographical distribution of tire retreading shops. In this
calculation, the percentage distribution of plants among states in this SIC was assumed to be the same as that
given in Table 23 for the total of the other eight industries.
-------
From Table 41, particulate emissions from each and even from all
nine industries do not exceed 0.1% of the national emissions of
the said pollutant, from all sources. The Tire and Inner Tubes
industry has highest contribution of hydrocarbon emissions (over
0.1%) to the nationwide total. On a state-by-state basis, hydro-
carbons from SIC 3021 (rubber footwear) exceed 1% of total emis-
sions in New Hampshire. For the same state, total particulate
emissions from the nine industries constitute 1% of the state
emissions from all sources. Emissions of both pollutants, in
other states contribute less than 1% of the state totals.
Growth Factor
The consumption of rubber in rubber products fabrication is
expected to increase at an average simple annual rate of 3%
between 1975 and 1980 (Section 6). Assuming that the production
of rubber increases at the same rate and that the level of con-
trol remains the same during this period, emissions from rubber
processing will increase by 15% over this period.
92
-------
SECTION 5
CONTROL TECHNOLOGY
Emissions from rubber processing industries consist of hydrocar-
bons and particulates. Because most of the operations resulting
in air emissions are not enclosed, the control of emissions from
these sources involves collection of the contaminated gas and re-
moval of the pollutants from the gas. The overall control effec-
tiveness becomes the product of the efficiencies of the control
equipment and the ventilation system.
Most rubber processing plants have some type of particulate
control devices, but there are only a few operations which have
hydrocarbon control equipment installed.
The best control technologies for hydrocarbons from various emis-
sion sources in the 9 rubber processing industries have been
identified in Reference 4 after an extensive plant survey and
engineering study. These identified best technologies for hydro-
carbons and the control technologies for particulates obtained
from other references are summarized, together with their effi-
ciencies, in Tables 44 and 45 for the elastomers industry and the
rubber products industry. It should be noted that the control
efficiences given are for the best designed control systems, most
of which are not presently used in the industry.
The state of the art (for existing controls) and future consid-
erations for control technologies are discussed below.
STATE OF THE ART
Carbon Black Applications
The carbon black operation is used in SBR production to impart
various desirable characteristics to the rubber. At the present
time, scrubbers using water as the scrubbing liquid have been
used for control of particulate emissions from this operation. A
control efficiency of 95% has been reported for the scrubber it-
self (16). With an estimated contaminated air collection effi-
ciency of 75%, the overall control efficiency for this source is
about 70%.
93
-------
TABLE 44. BEST CONTROL TECHNIQUES AND THEIR CONTROL
EFFICIENCIES FOR ELASTOMERS INDUSTRY (4)a
Hydrocarbons Particulates
Emission
source
Styrene storage
(breathing)
Hexane storage
(breathing)
Storage area
(fugitive)
Reactor area
(fugitive)
Butadiene absorption
Monomer recovery
area (fugitive)
Desolvent area
(surge vent)
Desolvent area
(fugitive)
Carbon black
application
Size reduction
Depolymerization
Drying
Baling
Best control
Floating roof
Floating roof
Housekeeping
Housekeeping
Incineration
Housekeeping
Improved steam
stripping
Housekeeping
Condenser and
scrubber
Incineration
Efficiency," Efficiency,"
% Best control %
80
80
50 to 80
50 to 80
90
50 to 80
50
50 to 80
High energy
scrubber
Cyclone
90
90 _C
Cyclone
75
80
c
70
Blanks indicate no emissions from operations.
The control efficiency given here is the product of the gas collection effi-
ciency and the pollutant removal efficiency.
No control is needed due to very low emission factors and source severity.
TABLE 45. BEST CONTROL TECHNIQUES AND THEIR CONTROL
EFFICIENCIES FOR RUBBER PRODUCTS INDUSTRY9
Hydrocarbons Particulates
Emission
source
Compounding
Milling
Calendering
Fabric cementing
Extrusion
Under tread
cementing
Green tire
Spraying
Buffing
Rubber cementing
Latex dipping
and drying
Bonding of
extruded parts
Adhesive
spraying
Holding
Curing
Finish painting
Best control
Incineration
Incineration
Incineration
Incineration
Process change
(vented extruder)
Carbon adsorption
Water-base spraying
Incineration
Process change
(water-based latex)
_C
Incineration
Incineration
Incineration
Process change
(detergent wash)
Efficiency, D Efficiency, 0
% Best control %
90 Fabric filtration 90
60
55
85
80
90
90
Cyclone and 90
fabric filtration
36
90
_C
70
60 C _C
60 ~C ~C
90
Blanks indicate no emissions from operations.
The control efficiency given here is the product of the gas collection efficiency and
the pollutant removal efficiency.
Due to difficulty in control, no control technique is identified.
94
-------
H ^ ,particle size of the carbon black used in
r, h industry (around 0.2 ym) (16), high-energy type scrub-
bers such as yenturi and flooded disc types Ire required for high
control efficiencies.
Baling Operations
±? ?e^formed in baling operations in SBR production
and rubber reclaiming to prevent the rubber product from sticking
to the inside of the bag. Cyclones have been used to minimize
talc losses and to control particulate emissions. Because of the
large particle sizes of talc (1 ym to 20 ym) , standard low resist-
ance cyclones could be expected to achieve an overall control
efficiency of 70% (16).
Desolvent Area
In the desolvent area in crumb SBR production by solution poly-
merization, unrecovered or unstripped butadiene, styrene, and
hexane are emitted. The control option deemed most applicable is
improving the efficiency of the steam stripping step of the
process. The increased stripping efficiency obtained by increas-
ing the steam- to-hexane ratio results in a 50% decrease in subse-
quent emissions of hydrocarbons from this source (4).
Depolymerization
~~ I
In the depolymeribation operation of rubber reclaiming, oily
mists, solvent vapors, and other organic vapors are emitted. The
emissions are vented to the atmosphere by a stack and are con-
sidered as essentially 100% collectable. Water scrubbing has
been used for control of this source and can achieve 90% control
efficiency for hydrocarbons (4). This control results, however,
in the generation of wastewater to be treated.
Compounding
In general, emissions from Banbury mixers and rubber mills are in
a finely divided form and smaller than 15 ym. Inertial separa-
tors are not, therefore, effective control devices for this
service. The most common control device employed is the bag-
house; a well-designed baghouse can be operated with 98% to 99.5%
efficiency (17). Standard cotton sateen bags are adequate at a
filtering velocity of 0.9 m/min. In some cases, scrubbers have
also proved satisfactory and advantageous in scrubbing out some
oil vapors and oil mists that may be present in some blends with
the contaminated gas collection efficiency considered, the over-
all state-of-the-art control efficiency for this source was
estimated at 90%.
95
-------
Fabric Cementing
In the fabric cementing operation, the fabric is oven dried to ,
drive off the carrier solvent. In a small-diameter, braided-hose
plant, thermal incineration is used to reduce by 95% the hydro-
carbon vapors resulting from hose-cementing operations (4). The
incinerator operates at 760°C and has heat recovery to the oven
itself. In another plant, solvent vapors from a fabric cementer
drying oven are vented to a catalytic incinerator. The incin-
erator operates at about 260°C and is approximately 90% efficient
(4).
In addition, carbon adsorption has been reported for at least one
fabric cementer in the rubber industry. Reduction was reported
to be 85%, with losses mainly attributable to solvent handling
and less than 100% collection efficiency (4).
Undertread Cementing
This operation is a tackifying step used in tire manufacture
where the tread is dipped in rubber cement. It is one of the few
emission points where hydrocarbon control equipment is presently
installed. In one plant, the total control system consists of a
ventilation enclosure, which is designed to capture evaporated
solvent from the cementing tank and the coated tread, and a dual-
unit carbon adsorber (4).
The system has been tested and observed to have an overall
control efficiency of about 94%. The design features of the
ventilation system include 1) adequate dilution of the volatile
vapors, 2) sufficient residence time of tread on the enclosed
conveyor to ensure the capture of solvent during drying, and
3) operator accessibility to areas within the hood, especially
during tread die changes (startup) and periods of scheduled
maintenance. The total ventilation flow is ducted to the dual
adsorber before being vented to the atmosphere. The carbon unit
itself consists of two carbon beds operated on an alternating
cycle of adsorbing and steam stripping. Ninety-five percent of
the collected solvent is recovered by the steam stripping, con-
densation, and decantation steps. The recovered solvent is
reused within the plant both in undertread and other cementing
operations (4).
The above control system represents the best control with re-
design of the existing enclosure and ventilation system. For
existing plants, the vapor collection efficiency ranges from 65%
to 73%, with an overall control efficiency of about 65% (18).
Green Tire Spraying
In the green tire operation, green tires are removed from a
storage rack and placed in the spray booth where the spraying
96
-------
booS^s iLrthaf^s11^00^11311^- ^tention time in the
anotLi riJk ^££, The tire is then removed and placed on
?hncT Lnn?"^? ^qUSnt evaP°ration is to general room exhaust.
overall ^??^?in^i0n uV^ booth itself is only 44% (18). The
sonr^ il ^ciency for hydrocarbon emission control from this
source is thus estimated to be about 40%.
FUTURE CONSIDERATIONS
i rT4.iK the green tire spraying operation is substi-
tuting the solvent-based sprays with water-based sprays. If this
is widely practiced, hydrocarbon emissions from this operation
could be reduced by 90%. The inside spray, primarily needed as a
release agent during curing, is currently known to be water based
in several Plants. The outside spray is also needed as a release
agent; in addition, it helps produce an aesthetically pleasing
finished product.
Development and wide use of a water-based material for the out-
side spray which can achieve the same effect is the key to total
elimination of hydrocarbon emissions from this operation. At
present, controls for hydrocarbon emissions from adhesive spray-
ing rubber cementing, and curing (including molding) operations
have not been reported. Owing to the quantities of hydrocarbons
emitted and possible tighter government regulations, industries
will soon have to cope with these emissions.
Adhesive spraying is used to apply adhesive to metal surfaces for
metal-bonded rubber items. Control of this hydrocarbon source
presents less of la problem because most of the spraying is per-
formed in a spraying booth, and exhaust from it can be vented to
a control device. However, because of the presence of solids in
the excess spray aerosol, carbon bed or catalytic incinerators
cannot be used unless a solid collection device is used before
ether of the control devices. This is because the solid content
of the spray .aerosol will generate a coating on the surface of
activated carbon and catalyst, and regeneration for removal of
this coating will be almost impossible.
Rubber cementing is typically used in tackifying a rubber inner
sole before it is placed on the outer sole, a sheet of rubber
stock before wrapping it into a belt, and a rubber hose before
another layer of rubber is applied. This operation is generally
performed manually, and cement is used widely throughout a plant
manufacturing area. The application may be by a knife, a brush,
a roller, or even by hand. Control of this source is not feasi-
ble unless there is a major process change to eliminate the
manual operations and to locate all such operations in a confined
area. To complicate the matter, most plants have had to increase
the ventilation in work areas to meet OSHAd requirements. The -
Occupational Safety and Health Administration.
97
-------
resultant large volume of air will make the control even more
difficult both technically and economically.
In the tire curing and general molding operations, presses are
located over a large open area which is ventilated or exhausted
by large plant fans. Emissions from these operations contain
vaporized hydrocarbons and condensed oil mists and solid organic
particles. To collect the contaminated air for treatment, a hood
for each press is needed to minimize the amount of air flow. In
addition, because of the presence of oil mists and organic parti-
cles, only two alternatives are feasible for treatment of the
gas - thermal incineration and wet scrubbing. Wet scrubbing will
generate wastewater which is difficult to treat.
The drying operation in the solution polymerization process for
production of SBR is the most significant hydrocarbon emission
point in terms of emission factor and source severity. There is
now no control of this source in the industry (4). Since most of
the hydrocarbon emission is in the form of hexane solvent (4),
carbon adsorption appears to be the most feasible control tech-
nique because it recovers the solvent. However, this is compli-
cated by the presence of the small amount of extender oil which
is added to the cement before coagulation for producing "ex-
tended" SBR products. The vaporized extender oil (in the dryer)
will foul the carbon bed unless condensation and mist elimination
are performed before the contaminated gas enters the carbon bed.
Otherwise, incineration will be the next choice for controlling
this source.
98
-------
SECTION 6
GROWTH AND NATURE OF THE INDUSTRY
PRESENT TECHNOLOGY
SBR Production
At present emulsion polymerization is used to produce 90% of the
SBR made in the U.S. The remaining 10% of SBR is produced by
solution polymerization (7). Emulsion SBR is categorized into
different types; the specifications for these are related to
features such as polymerization method, chemical structure, pro-
portion of bound styrene, masterbatch type and so on, all of
which information is .codified using a system operated by the
International Institute of Synthetic Rubber Producers (33).
In the early days of production of SBR, especially during World
War II, the catalysts used were certain organic persulfates.
These catalysts generated free radicals which initiate polymeri-
zation when heated to temperatures around 60°C. The SBR so
obtained - "hot" SBR - contained a proportion of highly branched
polymer molecules and its quality was deficient in some respects.
A critical postwar development was activated catalysts, which
bring about polymerization at much lower temperatures, around
5°C. SBR obtained in this way - "cold" SBR - has far fewer
branched molecules and is distinctly superior to hot SBR which it
has largely replaced (33).
Emulsion polymerization has a number of obvious advantages over
solution polymerization. These advantages include lower viscos-
ity, better heat transfer for removal of reaction heat, no need
for recovering expensive solvent, no problems with possibly toxic
solvents, easy recovery of unreacted monomer, and direct produc-
tion of synthetic latexes.
Despite its apparent engineering disadvantages and complexities,
solution polymerization has realized a steady build-up of capac-
ity because of better properties possessed by solution poly-
merized SBR (33). It combines the best features of emulsion-
polymerized SBR and polybutadiene in one rubber. There are
(33) Blow, C. M. Rubber Technology and Manufacture. CRC Press,
Cleveland, Ohio 1971.
99
-------
several commercial types of solution SBR, most of which have a
random arrangement of the styrene and butadiene elements along
each polymer chain (31).
Rubber Products Fabrication
The five basic steps involved in rubber product fabrication are:
compounding, mixing, forming, building, and vulcanization.
Compounding is the process of determining the proper ingredients
and proportions to be used in the rubber recipe in order to
obtain the required properties of the end product. The main
objectives of the mixing operation are to obtain a uniform blend
of the ingredients and to achieve consistent properties from
batch to batch. Mixing is presently carried out as a batch
process using either a two-roll mixer or an internal (Banbury)
mixer. Batch size varies according to mixing equipment capacity,
which is typically from 68 kg to 136 kg for a 2.13 m mill and
454 kg or more for the largest internal mixers.
Forming operations usually consist of calendering or extrusion.
Calendering involves forming the rubber compound into thin
sheets, coating it on a fabric, or wiping it into a fabric by
means of a series of rollers. Thin sheets of rubber are built up
to make the final thickness desired, e.g., 8 to 10 sheets may be
used to make a final sheet 1.6 mm thick. Extrusion is accomp-
lished by a power driven screw in a stationary cylinder which
forces the heated rubber compound through a die to give the
desired shape. Other forming operations used in rubber proces-
sing include casting, blow molding, and injection molding.
Building operations vary widely according to the product being
manufactured. For example, in tire manufacture, the calendered
cord plies are applied to the assembly drum one at a time to
build up a two-, four-, six-, or eight-ply tire.
Vulcanization, which imparts elastic characteristics to rubber,
can be carried out using molds heated to 138°C for 10 min to
90 min as in tire manufacturing. Alternatively, rubber products
may be cured in an autoclave with steam or water depending on the
required temperature and pressure. Heated air, either at atmos-
pheric or elevated pressure, can also be used to vulcanize pro-
ducts that are adversely affected by moisture. Various combina-
tions of these cures are also used in order to achieve the
desired properties in the product.
EMERGING TECHNOLOGY
SBR Production
As has been mentioned in the previous subsection, solution-
polymerized SBR has several properties superior to those of
100
-------
emulsion-polymerized SBR. According to one reference (33),
hniS 0riS°iymeriZed SBR WiU °vertlke emulsion SBR as the work
horse rubber in terms of consumption by 1980.
^ SBR Producti°n is introduction of thermo-
«« elastomer is produced by block copolymeriza-
of ™ H Styren? Wlth butadiene along the polymer chains, instead
of random copolymerization by the solution polymerization pro-
cess. This plastic rubber has all the properties of a normal
sulfur vulcanizate at ambient working temperatures and it pro-
cesses as easily as polyethylene and polypropylene at higher
temperatures (33). y
Rubber Products Fabrication
During the 1950 's and the 1960 's, the rubber industry experienced
a slow rate of technological advancement. However, recent years
have witnessed an accelerated pace, and many new innovations are
now beginning to alter the industry. For example, many plants
now employ tanks and silos for bulk storage and handling of raw
materials such as fillers or reinforcers. The use of large
preb lending systems to provide more uniform quality of raw mate-
rials is being explored. In this vein, the Farrel Company is
reportedly developing technology for blending chopped or crumb
rubber to even out batch-to-batch variations (34). In addition,
some large production facilities now employ fully automated,
computer controlled charger-mixer systems.
An improvement in! the curing process is the use of cure rate
integrators that employ a special sensor to accurately monitor
the temperature. These devices have reportedly reduced curing
times by 8% (34). Another example of the trend toward increasing
automation is its use in radial tire plants. The last 2 minutes
of the 5-minute tire assembly operation are now said to be auto-
mated (34).
An important advance in blending operations, that of continuous
mixing, is being actively developed. The combination of an in-
ternal mixer with some type of screw mixing will permit increased
mixing capacity and reduced mixing times. At present, however,
this technology is considered to be several years away (34). The
increasing demand for exterior automotive components made of
dent-proof rubber and the steeply rising cost of energy are ex-
pected to further accelerate the development of new manufacturing
processes in the rubber industry.
The new manufacturing techniques should hasten the further devel-
opment of new forms of rubber and their acceptance and use by
(34) Survey Results on Machinery, Equipment. Rubber World,
170(4):57, 1974.
101
-------
fabricators. The new forms of rubber include powdered rubber for
continuous mixing, thermoplastic types which allow the vulcaniza-
tion stage to be eliminated, and liquid polymers (especially
polyurethanes) for use in casting and injection molding pro-
cesses. One source (9) estimates that within the next 5 years,
liquid and powdered rubbers will account for 20% of the total
rubber material used in fabrication of rubber products in the
United States.
MARKETING STRENGTHS AND WEAKNESSES
Tires
The future growth of the rubber industry is closely related to
the automotive industry, since about two-thirds of all new rubber
produced goes into automotive tires. Of this amount, about 85%
on a unit basis (60% on a weight basis) goes into passenger car
tires. Hence, the demand for rubber will be greatly affected by
the total passenger vehicle miles driven and by tire design,
which affects tread life. Average passenger car mileage for the
past 10 years has increased steadily from about 15 Mm to 16 Mm
annually. However, this figure is expected to remain nearly
constant or even decline somewhat during the next several years
due to increased fuel costs.
Tire tread life is expected to continue to increase due to the
shifts to belted bias and radial tires and to small, lighter-
weight cars. From 1968 to 1970, new car manufacturers switched
almost completely from bias ply to belted bias tires, which offer
about 25% better mileage. The switchover in the replacement tire
market is proceeding at a much slower rate and is expected to
stop at 35% to 40%, because the owner of an older car is less
inclined to buy expensive, long-wearing tires (5).
Another factor that may adversely affect the tire market is the
trend to only four tires per car. Development work toward this
objective is under way at all companies (35). In addition to
safety and convenience, the incentives to "eliminate the spare"
include reduced car weight, more trunk space, and reduced new-car
cost.
The above considerations lead to a projected increase in consump-
tion of rubber for automotive tires from 1.95 x 106 metric tons
in 1974 to 2.14 x 106 metric tons in 1980. The tire industry's
percentage of total rubber consumption is expected to decrease
from 68% in 1974 to 59% in 1980 (5, 35).
(35) Rubber Products: 1974-1975. Rubber World, 171(4):27,
1975.
102
-------
Molded and Extruded Products
Weaknesses °f the molded and extruded rubber
h TaS Wlth the Variety of P^ucts falling in
this category. In the automotive products area (especially those
such as bumpers, seals, electrical wiring, etc., which are not
normally replaced during the car's lifetime), the new emphasis on
weight reduction of automobiles to improve gasoline mileage
should result in the use of many more rubber and plastic parts.
Another area which could show substantial gains is the replace-
ment of PVC products by rubber products. Over the past 8 years,
vinyl resin products have replaced rubber in such products as
wire and cable, garden hose, footwear, weather stripping, seal-
ants, toys and auto mats. However, a trend back to rubber is
developing due to rising costs of vinyl resins and the lower
processing costs associated with thermoplastic elastomers (36).
This trend could be accelerated because of the health problems
recently associated with vinyl chloride monomer.
Rubber parts used by the oil industry in wells, platforms, refin-
eries, and transportation of oil also have a good outlook for the
immediate and long-term future due to the renewed emphasis on
drilling in the United States as well as other areas of the world
(37).
t
The Rubber Manufacturers Association's 1977 prediction for the
molded, extruded, and lathe-cut sectors of the rubber business,
shown in Figure 16, indicates a significant increase in dollar
volume in each of the three areas (37).
The long-term strength in the molded and extruded products sector
can also be inferred from the data in Reference 36 on specialty
elastomers. These data suggest strong growth for all but a few
specialty materials over the next 4 years . Since the use of
specialty rubbers is heavy in the molded and extruded fields , the
increases should be reflected in these areas.
Hose and Belting
The major strengths of the hose and belting sector of the rubber
industry are in equipment for the oil and mining industries, and
in automotive replacement parts.
The long-term strength of the hose and belting sector is indi-
cated by the RMA estimates of the market potential for these pro-
ducts over the next 4 years as shown in Figure 17 (37).
(36) Dworkin, D. Changing Markets and Technology for Specialty
Elastomers. Rubber World, 171(5):43, 1975.
(37) Industrial Products to Grow at 7%. Rubber World, 175(4):
38-39, 1977.
103
-------
1/1
o-
-------
New Rubber Production and Consumption
From 1960 to 1973, total new rubber consumption in the United
States increased at an average annual rate of 5.4% (35). Through
1980, consumption is -expected to increase at a more moderate
rate, 2.4% to 3.8% (5, 39), primarily due to the effects of
energy conservation programs and the socioeconomic trends in the
transportation industry discussed earlier. Table 46 gives a
breakdown of estimated new rubber consumption for 1980. The
recent history of new rubber consumption is depicted graphically
in Figure 18.
TABLE 46. RUBBER CONSUMPTION FORECAST FOR 1980 (5)
(106 metric tons)
~ Rubber type Tires Nontire Total
Styrene-butadiene rubber 0.846 0.353 1.199
Polybutadiene rubber 0.324 0.036 0.360
Isoprenic rubber 0.882 0.310 1.192
EPDM rubber 0.045 0.250 0.295
Butyl or chlorobutyl rubber 0.045 0.100 0.145
Nitrile rubber - 0.086 0.086
All other elastomers - Q.354 0.354
TOTAL 2.142 1.489 3.631
Styrene butadiene rubber (SBR), with its major position in tire
markets, currently accounts for about 60% of all synthetic rub-
ber produced and used. Although SBR production will increase in
quantity during the next few years, its percentage of total syn-
thetic rubber is expected to decline to 57% in 1980 and 56% in
1985 (37).
Reclaimed Rubber
Most reclaimed rubber is used in automobile tires. The next area
of high use is in automobile mats and automobile mechanical
goods. At one time one of the prime reasons for the use of
reclaimed rubber was to reduce the raw material costs of rubber
compounds. This is no longer true at the present cost level of
natural rubber and SBR. Although the sale of reclaimed rubber is
continuing because of advantages in the area of processing (6),
(38) Rubber Demand Faces Lower Growth Rate. Chemical and
Engineering News, 52(20):12, 1974.
(39) Rubber Consumption to Increase. Rubber World, 172(2):83,
1975.
105
-------
the production of reclaimed rubber has been declining in the past
few years. It has been predicted (4) that this decline will
continue at an annual rate of 4.5%.
3.5
3.0
2.5
2.0
o
1.5
1.0
.5
0.0
1 TOTAL NEW RUBBER
SYNTHETIC RUBBER
NATURAL RUBBER
1967 68 69 70 71 72 73 74 75 76 1977
YEAR
Figure 18. Total new rubber consumption,
synthetic vs natural source (3)
106
-------
REFERENCES
Standard Industrial Classification Manual, 1972. Executive
Office of the President, Office of Management and Budget,
Washington, D.C., 1972. 649 pp.
Facts and Figures for Chemical Industry. Chemical and Engi-
neering News, 55 (23):39-79, 1977.
Year of Recovery for Rubber Suppliers. Rubber World,
175(4):35-37, 1977.
Hoogheem, T. J., C. T. Chi, G. M. Rinaldi, R. J. McCormick,
and T. W. Hughes. Identification and Control of Hydrocarbon
Emissions from Rubber Processing Operations. Contract 68-
02-1411, Task 17, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina. (Final report
submitted to the EPA by Monsanto Research Corporation, July
1977.) 383 pp.
5. Richardson, J., and M. Herbert. Forecasting in the Rubber
Industry. Presented1, at the Joint Meeting of the Chemical
Marketing Research Association and the Commercial Develop-
ment Association, New York, New York, May 1974.
6. Allen, P. W. Natural Rubber and the Synthetics. John
Wiley & Sons, Inc., New York, New York, 1972. 255 pp.
7. Development Document for Effluent Limitation Guidelines and
New Source Performance Standards for the Tire and Synthetic
Segment of the Rubber Processing Point Source Category.
EPA-440/l-74/013-a,-U.S. Environmental Protection Agency,
Washington, D.C., February ,1974. pp. 31-35.
8. Morton, M. Rubber Technology, Second Edition. Van Nostrand
Reinhold Company, New York, New York, 1973. 603 pp.
9. Outlook 1974 - Part II: Status Report on Elastomeric Mater-
•ials. Rubber World, 169(5):38-46, 1974.
10. Shreve, N. R. Chemical Process Industries, Third Edition.
McGraw-Hill Book Company, New York, New York, 1967. 905 pp.
11. Kent, J. A. Riegel's Handbook of Industrial Chemistry,
Seventh Edition. Van Nostrand Reinhold Company, New York,
New York, 1974. 902 pp.
107
-------
12. Rosnto, D. V. Injection Molding of Rubber. Rubber World,
166(6);:45-61, 1972.
13. Kirk-Othmer Encyclopedia of Chemical Technology, Second
Edition, Vol. 17. John Wiley & Sons, Inc., New York,
New York, 1968. 884 pp.
14. McPherson, A. T., and A. Klemin. Engineering Uses of Rub-
ber. Reinhold Publishing Corporation, New York, New York,
and Chapman & Hill, Ltd., London, United Kingdom, 1956.
490 pp.
15. 1972 Census of Manufactures, Volume II: Industry Series,
Part 2: SIC Major Groups 27-34. U.S. Department of Com-
merce, Washington, D.C., August 1976.
16. Pervier, J. W., R. C. Barley, D. E. Field, B. M. Friedman,
R. B. Morris, and W. A. Schwartz. Survey Reports on Atmos-
pheric Emission from the Petrochemical Industry, Volume 4:
Styrene Butadiene Rubber via Emulsion Polymerization.
EPA-450/3-73-006d, U.S. Environmental Protection Agency,
Research Triangle Park, North Carolina, June 1974. 39 pp.
17. Air Pollution Engineering Manual, Second Edition. J. A.
Danielson, ed. Publication No. AP-40, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina,
May 1973. 987 pp.
18. Van Lierop, G., and P. W. Kalika. Measurement of Hydrocar-
bon Emissions and Process Ventilation Requirements at a
Tire Plant. Presented at the 68th Annual Meeting of the
Air Pollution Control Association, Boston, Massachusetts,
June 15-20, 1975. 23 pp.
19. Rappaport, S. M. The Identification of Effluents from
Rubber Vulcanization. Ph.D. Thesis, University of North
Carolina, Chapel Hill, North Carolina, 1974.
20. Angert, I. G., A. I. Zenchenko, and A. S. Kuzminski. Vol-
atilization of Phenyl-2-Naphthylamine from Rubber. Rubber
- Chemistry and Technology, 34(3):807, 1961.
21. Taft, W. K., M. Felton, J. Duke, R. W. Laundrie, and
D. C. Prem. Oil Types in the Program for Oil Extended Rub-
ber. Industrial and Engineering Chemistry, 47 (5):1077, 1955.
22. Gaeta, L. J., et al. Antioxidant Analysis. Rubber Age,
101(6) :47, 1967.
23. Marn, P. J., T. J. Hoogheem, D. A. Horn, and T. W. Hughes.
Source Assessment: Solvent Evaporation - Decreasing. Con-
tract 68-02-1874, U.S. Environmental Protection Agency,
Cincinnati, Ohio. (Final document submitted to EPA by
Monsanto Research Corporation, January 1977.) 180 pp.
108
-------
24. Assessment of Industrial Hazardous Waste Practices—Rubber
and Plastics Industry. (Prepared by Foster D. Snell, Inc.,
Florham Park, New Jersey, under EPA Contract 68-01-3194, for
presentation to the Environmental Committee of the Rubber
Manufacturers Association, Cleveland, Ohio, October 22,
1975.)
25. A Look at the Tire Industry. Rubber World, 175 (4):42-46,
1977.
26. Aerometric and Emissions Reporting System (AEROS), U.S.
Environmental Protection Agency. National Air Data Branch,
Research Triangle Park, North Carolina 27711.
27. TLV's® Threshold Limit Values for Chemical Substances and
Physical Agents in the Workroom Environment with Intended
Changes for 1976. American Conference of Governmental
Industrial Hygienists, Cincinnati, Ohio, 1976. 94 pp.
28. Turner, D. B. Workbook of Atmospheric Dispesion Estimates.
Public Health Service Publication No. 999-AP-26, U.S.
Department of Health, Education, and Welfare, Cincinnati,
Ohio, May 1970. 84 pp.
29. Nonhebel, G. Recommendations on Heights for New Industrial
Chimneys. Journal of the Institute of Fuel. 33:479-511,
July 1960. ,
30. Code of Federal Regulations, Title 42 - Public Health,
Chapter IV - Environmental Protection Agency, Part 410 -
National Primary and Secondary Ambient Air Quality Stand-
ards, April 28, 1971. 16 pp.
31. Eimutis, E. C., and M. G. Konicek. Derivations of Contin-
uous Functions of the Lateral and Vertical Atmospheric
Dispersion Coefficients. Atmospheric Environment, 6(11):
859-863, 1972.
32. 1972 National Emissions Report. EPA-450/2-74-012, U.S.
Environmental Protection Agency, Research Triangle Park,
North Carolina, June 1974. 422 pp.
33. Blow, C. M. Rubber Technology and Manufacture. CRC Press,
Cleveland, Ohio, 1971.
34,. Survey Results on Machinery, Equipment. Rubber World,
170(4):57, 1974.
35. Rubber Products: 1974-1975. Rubber World, 171(4):27, 1975.
36. Dworkin, D. Changing Markets and Technology for Specialty
Elastomers. Rubber World, 171(5):43, 1975.
109
-------
37. Industrial Products to Grow at 7%. Rubber World, 175(4):
38-39, 1977.
38. Rubber Demand Faces Lower Growth Rate. Chemical and Engi-
neering News, 52(20):-12, 1974.
39. Rubber Consumption to Increase. Rubber World, 172(2):83,
1975.
110
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APPENDIX A
DEVELOPMENT OF SOURCE SEVERITY EQUATIONS
Source severity, S, has been defined as follows for criteria
pollutants:
s _ Xmax
AAQS (
where Xmax = time-averaged maximum ground level concentration
AAQS = ambient air quality standard
Values of Xmax are found from the following equation:
^max Xmax
where to is the "instantaneous" (i.e., 3-min) averaging time and
t is the averaging time used for the ambient air quality standard
as shown in Table A-l for particulates and hydrocarbons.
TABLE A-l. SUMMARY OF NATIONAL AMBIENT AIR QUALITY STANDARDS
FOR PARTICULATES AND HYDROCARBONS (29)
Averaging Primary Secondary
Pollutant time standards standards
Particulate Annual
(geometric mean) 75 yg/m3 60 yg/m3
24 hrb 260 yg/m3 150 yg/m3
Hydrocarbons
(nonmethane) 3 hr 160 yg/m (Same as
(6 to 9 a.m.) (0.24 ppm) primary)
The secondary annual standard (60 yg/m3) is a guide for
assessing implementation plans to achieve the 24-hr
secondary standard.
Not to be exceeded more than once per year.
Ill
-------
HYDROCARBON SEVERITY
The primary standard for hydrocarbon is reported for a 3-hr aver-
aging time. Therefore, t = 180 min. Hence, from Equation A-2 :
. x0.17
xmax = xmax \180"J = 0>5xmax (A~3)
Substituting for x from Equation 6 (Section 4) yields:
in 3.x
- _ (0.5) (0.052)0 _ 0.026 Q ,, ,,
•- — '
For hydrocarbons, the AAQS = 1.6 x lO"4 g/m3 . Therefore
xmax _ 0.026 Q (A-5)
AAQS 1.6 x 10-tf H2
S = 162'5 Q (A-6)
H2
PARTICULATE SEVERITY
The primary standard for particulate is reported for a 24-hr
averaging time. Therefore, t = 1,440 min. Hence, for Equation
A-2:
/ \0.17
Xmax = Xmax I 1,440 j (A~7)
Substituting for x from Equation 6 (Section 4) yields:
luciX
- = 0.052 Q = 0.0182 Q
Amax H2 H2
For particulates, AAQS = 2.6 x 10" ** g/m3. Therefore
_ xmax _ 0.0182 Q
AAQS 2.6 x 10"1* H2
(A_9)
or
S =
H2
112
-------
APPENDIX B
MASS EMISSIONS OF HYDROCARBONS AND PARTICUTATES
BY STATE AND BY SIC
The state-by-state listing of emissions from rubber processing
industries was obtained by distributing the national emissions
(as shown in Table 40) among the states according to the number
of plants in the applicable states (as shown in Table 23).
Tables B-l and B-2 show the calculated mass emissions for hydro-
carbons and particulates, respectively, in each state for each
of the 9 industries. These state-by-state mass emissions were
used in the calculation of percent contribution of hydrocarbon
and particulate emission from rubber processing to the corres-
ponding total state emissions presented in Tables 41 and 42.
113
-------
TABLE B-l.
MASS EMISSIONS OF HYDROCARBONS FROM
RUBBER PROCESSING BY SICa
(metric tons/yr)
State
Alabama
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Nebraska
Nevada
New Hampshire
New Jersey
New York
North Carolina
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virg-inia
Wisconsin
Nationwide
SIC 2822 SIC 3031 SIC 3011
1,600
320
800
410 ;,500
480
210 83 160
210 160
320
410 1,400
210 42 1,600
800
800
320
410 480
620
160
320
410 960
210 960
160
42 480
640
480
480
42 640
410 1,100
410 42 4,300
1,100
480
410 2,200
320
410 2,100
1,200 1,800
800
320
320
6,000 250 33,000
SIC 3021
270
1,100
270
540
540
270
540
810
540
1,600
540
1,100
540
1,300
540
810
1,300
810
270
270
14,000
SIC 3041
42
290
42
42
120
210
84
120
84
42
42
84
500
170
42
420
120
42
42
42
2,600
SIC 3069
61
39
33
800
17
260
11
120
170
340
310
28
22
60
360
320
130
61
72
28
11
390
380
100
970
39
61
270
89
50
6
72
220
28
6 .
72
28
44
130
6,200
SIC 3293
20
190
13
40
7
150
34
7
13
60
80
27
34
7
74
94
20
120
20
80
13
110
13
7
13
34
1,300
SIC 3357
21
21
10
10
10
10
21
10
21
10
21
10
180
SIC 7534
28
10
16
250
10
72
8
32
54
120
86
12
2
12
4
4
20
110
94
36
18
30
6
2
12
120
120
40
280
20
16
100
20
14
40
86
10
22
8
12
38
2,000
State
total
1,700
370
1,100
6,500
560
1,100
510
1,000
2,600
2,900
1,900
840
320
940
620
970
950
3,600
1,700
400
600
1,300
34
86
1,600
2,100
2,800
2,200
7,400
1,200
560
4,500
110
430
6
3,500
3,400
93
6
900
360
340
790
65,500
Blanks indicate no emissions from the industry.
There is a lack of data available for geographical distribution of tire retreading shops. In this calculation,
the percentage distribution of plants among state was assumed to be the same as that given in Table 23 for the
total of the other eight industries.
-114
-------
TABLE B-2.
MASS EMISSIONS OF PARTICULATES FROM
RUBBER PROCESSING BY SIC9
(metric tons/yr)
State
Alabama
Arizona
Arkansas
California
Colorado
Connecticut
Delaware
Florida
Georgia
Illinois
Indiana
Iowa
Kansas
Kentucky
Louisiana
Maine
Maryland
Massachusetts
Michigan
Minnesota
Mississippi
Missouri
Nebraska
Nevada
New Hampshire
New Jersey
New York
North Carolina
Ohio
Oklahoma
Oregon
Pennsylvania
Rhode Island
South Carolina
South Dakota
Tennessee
Texas
Utah
Vermont
Virginia
Washington
West Virginia
Wisconsin
Nationwide'
• — • '
SIC 2822 SIC 3031 SIC 3011
350
71
180
28 780
110
14 30 35
14 35
71
28 320
14 15 350
180
180
71
28 110
42
35
71
28 210
14 210
35
15 110
140
110 |
110 |
15 140
28 250
28 15 960
250
110
28 500
71
28 460
85 390
180
71
71
410 91 7,300
SIC 3021
8
31
8
16
16
8
16
23
16
46
16
31
16
38
16
23
38
23
8
8
400
SIC 3041
7
50
7
7
21
35
14
21
14
7
7
14
85
28
7
70
21
7
7
7
440
SIC 3069
30
19
17
400
8
130
6
61
86
170
160
14
11
28
180
160
67
30
36
14
5
190
190
50
480
19
30
140
44
25
3
36
110
14
3
36
14
22
67
3,100
SIC 3293
8
75
5
15
2
57
13
2
5
23
31
10
13
2
28
44
8
46
8
31
5
41
5
2
5
13
500
b
SIC 3357 SIC 7534
13
5
8
6 120
5
6 34
4
15
3 26
3 57
3 41
6
1
3 6
2
2
10
6 52
45
17
8
14
3
1
6
3 56
6 57
3 19
6 130
10
8
3 49
10
7
19
41
5
10
4
6
18
56 950
State
total
400
95
220
1,500
140
280
80
160
480
710
430
220
72
160
44
60
130
570
470
140
160
230
17
15
150
490
520
380
1,800
290
150
810
54
110
3
580
670
31
3
230
90
41
180
13,200
Blanks indicate no emissions from the industry.
There is a lack of data available for geographical distribution of tire retreading shops. In this calculation,
the percentage distribution of plants among state was assumed to be the same as that given in Table 23 for the
total of the other eight industries.
115
-------
GLOSSARY
accelerator: Compound which reduces the time required to vulcan-
ize (cure) synthetic or natural rubber.
activator: Metallic oxide that makes possible the crosslinking
of sulfur in rubber vulcanization (curing).
adhesive spraying: Operation by which adhesive material is
sprayed onto the metal surface for subsequent fabrication of
metal-bonded rubber goods.
affected population: Number of people around a representative
plant who are exposed to high concentrations of pollutants.
antioxidant: Organic compound added to rubber to retard oxida-
tion
or deterioration.
atmospheric stability class: Class used to designate degree of
turbulent mixing in the atmosphere.
banbury mixer: Trade name for a common type of internal mixer
manufactured by Parrel Corporation; used in compounding and
mixing of rubber stock.
calendering: Operation by which rubber stock is pressed between
rolls to make smooth or to thin into sheets.
camel back: Tire tread used in the retreading of tire carcasses.
cementing: Application of a material consisting of polymeric
rubber solids dissolved in solvent to rubber surface or fab-
ric for good adhesion.
coagulation: Combination or aggregation of previously emulsified
particles into a clot or mass.
criteria pollutants: Particulates, sulfur dioxide, hydrocarbons,
carbon monoxide, and nitrogen oxides, for which national
ambient air quality standards have been established.
crumb: Small coagulated particles of synthetic rubber.
116
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curing agents: Substances which bring about the rubber cross-
linking in curing process.
devulcanization (depolymerization): Softening of cured rubber by
heat and chemical additives during reclaiming.
emission factor: Weight of material emitted to the atmosphere
per unit weight of product or raw material consumed.
emulsion: Stable mixture of two or more immiscible liquids held
in suspension by small percentage of substances called
emulsifiers.
extender: Low specific gravity substance used in rubber formu-
lations chiefly to reduce unit product costs.
extrude: Shape by forcing a material through a die.
filler: High specific gravity compound used in rubber mixture to
provide a certain degree of stiffness and hardness and to
decrease costs.
flash: Overflow of cured rubber from a mold.
fugitive emissions: Gaseous and particulate emissions that
result from industrial related operations, but which are
not emitted through a primary exhaust system, such as a
stack, flue, or control system.
hazard factor: Value equal to the primary ambient air quality
standard for criteria pollutants or to a reduced TLV for
noncriteria pollutants.
latex: Suspension of rubber particles in a water solution.
monomer: Compound of a relatively low molecular weight which is
capable of conversion into polymers.
noncriteria pollutant: Emission species for which no ambient air
quality standard has been established.
pigment: Any substance that imparts color to the rubber.
plastic: Capable of being shaped or molded with or without the
application of heat.
reclaimed rubber: Depolymerized (devulcanized) scrap rubber,
either natural or synthetic.
reinforcing agents: Fine powders, such as carbon black, zinc
oxide, and hydrated silicas, which are used to increase the
strength, hardness, and abrasion resistance of rubber.
117
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SBR: Styrene butadiene rubber, a synthetic rubber made by
either emulsion or solution polymerization of styrene and
butadiene.
soapstone: Substance used to prevent rubber stocks from sticking
together during periods of storage.
source severity: Ratio of time-averaged maximum ground level
concentration of each emission species to its corresponding
ambient air quality standard (for criteria pollutants) or to
a reduced TLV (for noncriteria emissions).
tire bead: Coated wires inserted in the pneumatic tire at the
point where the tire meets the steel rim on which it is
mounted.
tire cord: Woven synthetic or natural fabrics impregnated with
rubber which form the body of the tire and supply it with
most of its strength.
tire tread: Riding surface of the tire.
vulcanization (curing): Process by which plastic rubber is con-
verted into the elastic rubber or hard rubber state.
118
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tn,atf «-^TECHNICAL REPORT DATA
frteate read Instructions on the revent before completing!
EPA-600/2-78-053
2. —
4. TIT Lt AND SUBTITLE
SOURCE ASSESSMENT: RUBBER PROCESSING
State of the Art
7. AUTHORIS) •"
C. T. Chi, T. W. Hughes, T. E. Ctvrtnicek,
DJ A. Horn, and R. W. Serth
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Monsanto Research Corporation
1515 Nicholas Road
Dayton, Ohio 45407
12. SPONSORING AGENCY NAME AND ADDRESS
Industrial Environmental Research Laboratory-Cin. , OH
Office of Research and Development
U.S. Environmental Protection Agency
rinn'nnatia flhin dR?fift
3. RECIPIENT'S ACCESSION NO.
6. REPORT DATE
March 1978
issuing date
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
MRC-DA-730
1O. PROGRAM ELEMENT NO.
1AB604
M.CbNTRACtASRANT NO.
68-02-1874
13. TYPE OF REPORT AND PERIOD COVERED
Task Final
14. SPONSORING AGENCY CODE
EPA/600/12
IS. SUPPLEMENTARY NOTES
IERL-Ci project leader for this report is R. J. Turner, 513-684-4481
16. ABSTRACT
This report summarizes data on air emissions from .the production of vulcanized elastomers
(rubbers) and fabrication of rubber products. Hydrocarbons and particulates are emitted
from various operations. Hydrocarbon emissions consist of monomers, rubber chemicals,
and solvents which are volatilized during processing. Particulate emissions consist
primarily of carbon black, soapstone, zinc oxide, and other materials emitted from com-
pounding, grinding, and talc dusting operations. To assess the environmental impact of
this industry, source severity was defined as the ratio of the time-averaged maximum
ground level concentration of a pollutant emitted from a representative plant to the
ambient air quality standard (for criteria pollutants) or to a reduced threshold limit
value (for noncriteria pollutants) . Source severities were greater than or equal to 1
for the following operations: the butadiene absorption vent in emulsion SBR production,
the drying operation in solution SBR production, green tire spraying and curing opera-
tions in the tire industry, and rubber cementing in the rubber footwear industry. Emis-
sions from rubber processing are expected to increase 15% over the period 1975 to 1980.
J7.
a. DESCRIPTORS
Air Pollution
Assessments
Slastomers
18. DISTRIBUTION STATEMENT
Release to Public
KEY WORDS AND DOCUMENT ANALYSIS
b.lDENTIFIERS/OPEN ENDED TERMS
Air Pollution Control
Source Assessment
Unclassified
2O SECURITY CLASS fThlt pagtl
Unclassified
c. cos ATI Field/Group
68A
21. NO. OF PAGES
133
22. PRICE
EPA Form 2220-1 (»-7J)
119
* U.S. GWERKMT PRHTOG OfnCfc 1971—260-880/60
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