EPA-600/R-96-066
June 1996
CHARACTERIZATION OF MANUFACTURING
PROCESSES AND EMISSIONS AND
POLLUTION PREVENTION OPTIONS FOR THE
COMPOSITE WOOD INDUSTRY
By:
Cybele Martin and Coleen Northeim
Research Triangle Institute
Center for Environmental Analysis
P.O. Box 12194
Research Triangle Park, NC 27709
EPA Contract 68-D1-0118, Task 93
EPA Project Officer: Elizabeth Howard
National Risk Management Research Laboratory
Research Triangle Park, NC 27709
Prepared for:
U.S. Environmental Protection Agency
Office of Research and Development
Washington, DC 20460
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FOREWORD
The U.S. Environmental Protection Agency is charged by Congress with pro-
tecting the Nation's land, air, and water resources, tinder a mandate of national
environmental laws, the Agency strives to formulate and implement actions lead-
ing to a compatible balance between human activities and the ability of natural
systems to support and nurture life. To meet this mandate, EPA's research
program is providing data and technical support for solving environmental pro-
blems today and building a science knowledge base necessary to manage our eco-
logical resources wisely, understand how pollutants affect our health, and pre-
vent or reduce environmental risks in the future.
The National Risk Management Research Laboratory is the Agency's center for
investigation of technological and management approaches for reducing risks
from threats to human health and the environment. The focus of the Laboratory's
research program is on methods for the prevention and control of pollution to air,
land, water, and subsurface resources; protection of water quality in public water
systems; remediation of contaminated sites and groundwater; and prevention and
control of indoor air pollution. The goal of this research effort is to catalyze
development and implementation of innovative, cost-effective environmental
technologies; develop scientific and engineering information needed by EPA to
support regulatory and policy decisions; and provide technical support and infor-
mation transfer to ensure effective implementation of environmental regulations
and strategies.
This publication has been produced as part of the Laboratory's strategic long-
term research plan. It is published and made available by EPA's Office of Re-
search and Development to assist the user community and to link researchers
with their clients.
E. Timothy Oppelt, Director
National Risk Management Research Laboratory
EPA REVIEW NOTICE
This report has been peer and administratively reviewed by the U.S. Environmental
Protection Agency, and approved for publication. Mention of trade names or
commercial products does not constitute endorsement or recommendation for use.
This document is available to the public through the National Technical Information
Service, Springfield, Virginia 22161.
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ABSTRACT
The composite wood panel manufacturing industry was included in the EPA's initial list of air
toxics source categories under section 112 of the Clean Air Act Amendments of 1990.1 The
industry was defined by the EPA as "...any facility engaged in the manufacturing of plywood
and/or particleboard, including, but not limited to, manufacturing of chip waferboard,
strandboard, hardboard/cellulosic fiberboard, oriented strandboard, hardwood plywood,
medium density fiberboard, softwood plywood, or any other wood composite product
manufactured using binder (EPA, 1992)." The EPA's Office of Air Quality Planning and
Standards (OAQPS) will be writing maximum achievable control technology (MACT)
standards for Hazardous Air Pollutants (HAPs) as they apply to the composite wood panel
manufacturing industry; the MACT regulations for this industry are scheduled to be proposed
in 1999.
The Pollution Prevention Act of 1990 requires the EPA to review regulations of the Agency
prior to their proposal to determine the effect of regulations on source reduction. In response
to this charge, the EPA has established the Source Reduction Review Project (SRRP). The
goals of the SRRP are to ensure that source reduction measures and multi-media issues are
considered during the earliest stages of development of regulations under the Clean Air Act,
Clean Water Act, and Resource Conservation and Recovery Act. The SRRP is focused on 17
industrial categories which will be affected by the above regulations; the composite wood
panel manufacturing industry was selected as one of the 17 industrial categories.2
The EPA's National Risk Management Research Laboratory (NRMRL)/Air Pollution
Prevention and Control Division (APPCD) worked in conjunction with OAQPS on the
implementation of the SRRP for the composite wood panel manufacturing industry. As part of
this effort, the Research Triangle Institute was contracted to characterize emissions from
manufacturing processes and to identify potential pollution prevention opportunities for
reducing them.
This report summarizes information gathered on emissions from the composite wood industry
and potential pollution prevention options. Information was gathered through a literature
search of trade association publications, journal articles, symposium presentations, university
research, etc.
xIn the EPA's initial list of air toxics, the composite wood industry was called the
plywood and particleboard industry.
2The composite wood panel manufacturing industry was listed in the SRRP as the
plywood and particleboard industry.
ii
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Little information exists in the literature pertaining to pollution prevention. Most of the
available literature focuses on ways to reduce raw material consumption and improve
manufacturing processes. However, in many instances, these reductions and improvements
lead to pollution prevention benefits. Some of these potential pollution prevention options are
presented in this report and include: conveyor belt drying; low temperature drying; high
moisture bonding adhesives; foam extrusion; and variable glue application rate. Other
pollution prevention options presented in this report include alternative fiber sources such as
agricultural fiber and recycled wood waste and naturally derived adhesives. These options are
presented as resources that are abundant and renewable. Little emissions data exist in the
literature to include with these options.
iii
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Table of Contents
Section Page
Abstract ii
List of Figures viii
List of Tables ix
Abbreviations and Symbols x
Acknowledgments xii
1.0 Composite Wood Product Classifications and Industry Statistics 1
1.1 Plywood Panels 2
1.1.1 Structural Plywood 2
1.1.1.1 Industry Outlook 2
1.1.2 Hardwood Plywood 5
1.1.2.1 Wall Paneling 5
1.1.2.1.1 Industry Outlook 5
1.1.2.2 Industrial Hardwood Plywood 8
1.2 Engineered Lumber 8
1.2.1 Industry Outlook 10
1.3 Reconstituted Wood Panels 10
1.3.1 Particleboard 11
1.3.1.1 Industry Outlook 11
1.3.2 Oriented Strandboard 11
1.3.2.1 Industry Outlook 13
1.3.3 Hardboard 13
1.3.4 Medium Density Fiberboard 15
1.3.4.1 Industry Outlook 15
1.3.5 Cellulosic Fiberboard 15
2.0 Composite Wood Manufacturing Process Descriptions 17
2.1 Plywood Manufacture 17
2.1.1 Debarking 17
2.1.2 Heating the Blocks 17
2.1.3 Cutting Veneer 18
2.1.4 Veneer Storage and Clipping 18
2.1.5 Veneer Drying 18
2.1.6 Layup and Pressing 22
2.1.7 Finishing 23
2.1.7.1 Structural Plywood and Industrial
Hardwood Plywood 23
2.1.7.2 Hardwood Plywood Wall Paneling 23
2.2 Reconstituted Panel Manufacture 24
IV
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Table of Contents (continued)
Section Page
2.2.1 Wood Reduction 24
2.2.1.1 Oriented Strandboard 24
2.2.1.2 Particleboard 24
2.2.1.3 Fiberboard (Cellulosic Fiberboard, MDF, and Hardboard) 24
2.2.2 Drying 26
2.2.2.1 Screening and Air-Classifying 28
2.2.3 Adhesive Application 28
2.2.3.1 Particleboard and Oriented Strandboard 29
2.2.3.2 Medium Density Fiberboard and Dry Process Hardboard . 29
2.2.4 Mat Forming 29
2.2.4.1 Wet Forming 30
2.2.4.2 Dry Forming 30
2.2.5 Hot Pressing 30
2.2.6 Finishing 35
3.0 Process Emissions and Wastes 35
3.1 Solid Wastes 35
3.2 Adhesive Wastes 35
3.3 Water Wastes 37
3.4 Air Emissions 38
3.4.1 Reconstituted Panel Dryers 38
3.4.1.1 Emissions Stream Characteristics 38
3.4.1.2 Variables Affecting Emissions from Reconstituted
Panel Dryers 39
3.4.1.2.1 Effects of Dryer Inlet Temperature
On TGNMO Emissions 40
3.4.1.2.2 Effects of Dryer Inlet Temperature on
Formaldehyde Emissions 40
3.4.1.2.3 Effects of Wood Species on Formaldehyde
Emissions 40
3.4.2 Veneer Dryers 43
3.4.2.1 Emissions Stream Characteristics 43
3.4.2.2 Variables Affecting Veneer Emissions 44
3.4.2.2.1 Factors Affecting Noncondensable Organics ... 44
3.4.2.2.2 Factors Affecting Particulated and Condensable
Organics 45
3.5 Press Emissions 45
3.5.1 Wood Related Emissions 45
v
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Table of Contents (continued)
Section Page
3.5.2 Adhesive Related Emissions 45
3.5.2.1 Press Emissions from Curing UF Resins 45
3.5.2.2 Press Emissions from Curing PF Resins 47
3.5.2.3 Press Emissions from Curing MDI Resins 48
4.0 Pollution Prevention 49
5.0 Alternative Fiber Sources 49
5.1 Recycled Wood Waste 50
5.2 Agricultural Fiber 51
5.2.1 Product Quality 51
5.2.2 Bulk Density 53
5.2.3 Price 53
5.2.4 Fiber Availability 55
5.3 Recycled Textile Fibers 55
6.0 Alternative Adhesives 56
6.1 Background 56
6.1.1 Product End Use 56
6.1.2 Manufacturing Issues 56
6.1.3 Consumer Safety 59
6.2 High Moisture Bonding Adhesives 59
6.3 Reformulated Urea-Formaldehyde Resins 60
6.4 Naturally Derived Adhesives 63
6.4.1 FAREZ Resin 64
6.4.1.1 Environmental Effects 64
6.4.1.2 Availability 64
6.4.2 Methyl Glucoside 64
6.4.2.1 Future Availability 65
6.4.3 Lignin Adhesives 66
6.4.3.1 Adhesive Utilization 66
6.4.3.2 Availability 67
6.4.4 Tannin 67
6.4.4.1 Adhesive Utilization 67
7.0 Reducing Adhesive Consumption 68
7.1 Foam Extrusion 68
7.2 Variable Glue Spread for Veneer Layup 70
vi
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Table of Contents (continued)
Section Page
8.0 Process Modifications 71
8.1 Low Temperature Drying 71
8.1.1 Recirculation of Dryer Exhaust 71
8.2 Steam Injection Single Opening Press 71
8.3 Shelter for Raw Materials 73
8.4 Conveyor Belt Drying 74
8.5 Three-Pass High Velocity Rotary Drum Dryer 75
References 79
Appendix A Metric Conversions for Cited Text A-l
vii
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List of Figures
Figure Nq. Page
1-1 Various types of plywood construction 3
1-2 U.S. shipments of hardwood plywood and structural plywood 4
1-3 U.S. shipments of structural plywood 1987 to 1993 6
1-4 U.S. production of natural hardwood and decorative hardwood wall paneling 7
1-5 U.S. shipments of industrial hardwood plywood 9
1-6 U.S. industry shipments of particleboard 1978 to 1993 12
1-7 U.S. structural panel market 14
1-8 U.S. shipments of medium density fiberboard 16
2-1 Cutting action on a lathe and slicer 19
2-2 Longitudinal-flow dryer 20
2-3 Cross section of a steam-heated jet dryer 21
2-4 Reconstituted wood panel process flow 25
2-5 Schematic of conventional triple-pass drum dryer 27
2-6 MDF blowline blending 29
2-7 Two types of mat forming machines 31
2-8 Various types of mat construction 32
2-9 Schematic of multiopening board press 33
2-10 Continuous press 34
3-1 Concentration of condensable organics vs. dryer inlet temperature 41
3-2 Emission rate of condensable organics vs. dryer inlet temperature 41
3-3 Noncondensable portion of TGNMO vs. dryer inlet temperature 42
3-4 Concentration of formaldehyde in dryer exhaust as a function of the dryer
inlet temperature . 42
3-5 Formaldehyde emissions associated with drying different wood species 43
3-6 Urea-formaldehyde resin mole ratios 46
5-1 Properties of composite panels made with agricultural fibers compared to
properties required for particleboard panels 52
5-2 Properties of composite panels made with agricultural fibers compared to properties
required for hardboard panels 54
6-1 Dryer inlet temperature versus furnish moisture content 60
6-2 Resin mole ratio versus board emissions (ppm) and press emissions 61
6-3 Effect of urea scavenger on formaldehyde emissions when used with various
mole ratio UF resins 62
6-4 Formaldehyde emissions results from panels made during a production trial
of scavenger resin 62
7-1 Flow diagram of foam extrusion apparatus 69
8-1 Dryer inlet temperatures as a function of the water removed per pound dry product 72
8-2 Comparison of air flows through a conventional rotary dram dryer and a
three pass high velocity rotary dram dryer 77
viii
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List of Tables
Table No. Page
1-1 Composite Wood Products 1
1-2 U.S. Production Estimates of Engineered Lumber 10
1-3 U.S. Shipments of Reconstituted Wood Panel 1985 to 1993 10
3-1 Reported Releases of SARA Section 313 Chemicals from Reconstituted
Wood Panel Plants for 1991 36
3-2 Reported Releases of SARA Section 313 Chemicals from Plywood Plants for 1991 . 37
3-3 Turpentine Content of Wood Species 44
3-4 Press Emissions from Particleboard Mills using UF Resins 47
3-5 Press Emissions from OSB Mills Using PF Resins 48
3-6 Press Emissions from MDI Resins 48
5-1 Estimated Prices of Wood and Agricultural Fibers 55
6-1 Adhesives Commonly Used in Manufacturing Composite Wood 57
6-2 Exposure Limits and Health Hazards Described in MSDS for MDI, UF, and PF
Wood Adhesives 58
7-1 Typical Foam Adhesive Mix for Gluing Plywood 68
ix
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ABBREVIATIONS AND SYMBOLS
ACGTH
American Conference of Governmental Industrial Hygienists
APA
American Plywood Association
APPCD
Air Pollution Prevention and Control Division
BACT
Best achievable control technology
Btu
British thermal unit
CARS
Constant application rate strategy
F:P
Formaldehyde to phenol
F:U
Formaldehyde to urea
FPL
Forest Products Laboratory
HAP
Hazardous air pollutants
HPVA
Hardwood Plywood & Veneer Association
HUD
Housing and Urban Development
HW
Hardwood
LVL
Laminated veneer lumber
MACT
Maxium achievable control technology
MDF
Medium density fiberboard
MDI
Methylenediphenyl diisocyanate
MeG
Methyl glucoside
MSM
Thousand square meters
msm19
Thousand square meters on a 0.19 mm basis
MTBE
Methyl tertiary butyl ether
NCASI
National Council for Air & Stream Improvement
NIOSH
National Institute for Occupational Safety and Health
NPA
National Particleboard Association
NRMRL
National Risk Management Research Laboratory
OAQPS
Office of Air Quality Planning and Standards
OSB
Oriented strandboard
OSHA
Occupational Safety and Health Administration
PB
Particleboard
PEL
Permissible Exposure Limit
PF
Phenol-formaldehyde
ppm
Parts per million
psi
Pounds per square inch
REL
Recommended Exposure Limit
RTI
Research Triangle Institute
SARA
Superfund Amendments and Reauthorization Act
SHW
Soft hardwood
SRRP
Source Reduction Review Project
STEL
Short-term Exposure Limit
SW
Softwood
x
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TGNMO
Total gaseous nonmethane organics
TLV
Threshold Limit Value
TWA
Time-weighted Average
UF
U rea-formaldehyde
USDA
U.S. Department of Agriculture
VARS
Variable application rate strategy
VOC
Volatile organic compound
3PHV
Three-pass high velocity
xi
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ACKNOWLEDGMENTS
The authors would like to acknowledge the following technical advisors who took the
time to read and comment on the draft of this report. Their input was very valuable.
Ed Price - Georgia-Pacific
Steven Correll - Georgia-Pacific
Michael Hoag - National Particleboard Association
Kurt Bigbee - American Plywood Association
Louis Wagner - American Hardboard Association
Paul Coleman - ICI Polyurethanes Group
John Galbraith - ICI Polyurethanes Group
Xll
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1.0 COMPOSITE WOOD PRODUCT CLASSIFICATIONS AND INDUSTRY
STATISTICS
Composite wood products are distinct from solid wood in that they are composed of wooden
elements of varying sizes held together by an adhesive bond (Table 1-1). In general, the
manufacturing process involves some type of wood size reduction, followed by drying (except
for wet process boards), adhesive application, and pressing at elevated temperatures.
Table 1-1. Composite Wood Products
Product Type
Wood Form After
Size Reduction
Primary
Adhesive(s)
Manufacturing
Process
Plywood panels
- Structural plywood
- Hardwood plywood
veneer
veneer
PFa
UFb
Dry
Dry
Engineered lumber
veneer and lumber
PF
Dry
Reconstituted wood panels
- Oriented strandboard
wood strands of
uniform size
PF, MDIC
Dry
- Particleboard
finely ground wood
particles of various
sizes (fluffy, dust-
like texture)
UF
Dry
- Medium density fiberboard
wood fibers of
uniform size (fluffy,
dust- like texture).
UF
Dry
- Cellulosic fiberboard
wood fibers of
uniform size (fluffy,
dust-like texture).
Starch or
asphalt
Wet
- Hardboard
wood fibers of
uniform size (fluffy,
dust-like texture).
PF
Dry, Wet, or Wet/Dry
aPF = Phenol-formaldehyde
bUF = Urea-formaldehyde
CMDI = Methylenediphenyl diisocyanate
1
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1.1 Plywood Panels
Composite wood panels made with wood veneers are classified as plywood. Veneer is produced
by cutting or peeling thin sheets of wood from a log. In plywood manufacture, veneers are
bonded together with a synthetic adhesive resin to form a laminate (a laminate is any object built
up of thin layers). The number of veneers or plies in a panel varies by product. The outside plies
of a plywood panel are called the face and back. The center ply or plies are called cores. The
center plies are layered with the grain of each sheet perpendicular to the previous one. Plywood
may be made entirely of veneer, or the core materials may be lumber, particleboard (PB), plastic,
metal, or other materials (Figure 1-1).
Plywood panels are classified into two groups according to end use: structural plywood and
hardwood plywood. Structural plywood panels are used primarily as a sheathing product in the
construction of residential homes and nonresidential buildings. Other applications include
concrete forming, and construction of transportation equipment, furniture and fixtures, and
materials handling. Hardwood plywood is used for decorative applications such as wall paneling
and industrial applications such as furniture manufacture. The structural plywood industry is
significantly larger than the hardwood plywood industry (Figure 1-2).
1.1.1 Structural Plywood
Structural plywood panels are made primarily from softwoods. Softwoods are coniferous or
needleleaved trees (pine, fir, spruce, hemlock), as opposed to hardwoods which are deciduous or
broadleaved trees (oak, ash, maple, walnut). The term "softwood" has only a general reference to
actual wood hardness. Structural panels may use either variety, but are more commonly
manufactured of softwoods (APA, 1993).
Structural plywood panels are constructed entirely of veneers (typically 2.54 millimeters (mm)
thick) that are bonded with glue containing phenol-formaldehyde (PF) adhesive resins. PF
adhesive resins are waterproof allowing structural panels to be used in exterior applications.
Panels are typically manufactured into 1.22 meters (m) by 2.44 m sheets which may be sanded.
No finishes (i.e., liquid coatings, paper coatings, etc.) are applied to the panels.
1.1.1.1 Industry Outlook
Since the mid 1980s, timber harvests from publicly owned lands have declined by more than 50
percent (Carliner, 1994). The structural panel industry attributes this decline to new land
management policies enacted by the federal government which have reduced the amount of land
available for harvesting. Heavy restrictions have been placed on national forests in the Pacific
Northwest which contain twice as much timber as all other national forests combined (Evergreen
Magazine, 1994).
2
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Various types of plywood
construction
All veneer
construction
3-layer
3 ply
3-layer
4 ply
5-layer
Sply
Alternative constructions
Com-ply Lumber core plywood
Partlcleboard
3-layer
S*f«y«r CfOMtwnd
Figure 1-1. Various types of plywood construction (Haygreen and Bowyer, 1989).
Reprinted with permission.
3
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25
Hardwood Plywood
Structural Plywood
Figure 1-2. U.S. shipments of hardwood plywood and structural plywood
(Source: USDC, 1988; Adair, 1993).
Notes
1. Data unavailable on shipments of hardwood plywood after 1988.
2. Shipments of hardwood plywood are reported by the USDC as sq. ft of surface measure, irrespective of
panel thickness. The present mix of hardwood plywood panels ranges in thickness from 0.313 in to 0.375 in
(7.950 mm to 9.525 mm) (Groah, 1994). An average thickness of 0.344 in (8.738 mm) was used to convert
surface measure to m3.
3. Shipments of structural plywood are reported by Adair as sq. ft on a 0.375 in (9.525 mm) basis; the 0.375 in
basis was used to convert sq. ft to m3.
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Harvesting restrictions have negatively impacted the structural plywood industry. The shortage
of timber has increased log prices, resulting in increased manufacturing costs for plywood
production. Rising lumber prices throughout the 1990s have also resulted in higher log costs.
Lumber and plywood operations now compete for the same log. While plywood prices are held
in check by substitution of oriented strandboard (OSB) (discussed in Section 1.3.2), lumber
prices are more free to rise because few substitutes exist for lumber. The replacement of
structural lumber is only at the beginning stages with products such as laminated veneer lumber
(LVL) and glulam beams (discussed in Section 1.2). Plywood is, therefore, less able to compete
with lumber for a common log resource (Roberts, 1994).
The combination of log shortages, rising manufacturing costs, and competition from OSB have
resulted in plywood production curtailments and mill closures, particularly in the West. There
have been 112 plywood and veneer plant closures in Washington, Oregon, and California
between 1980 and the end of 1993 (Wood Technology, 1994). United States (U.S.) shipments of
structural plywood from the West have decreased 53 percent since 1987 (Figure 1-3). Total U.S.
shipments of structural plywood have decreased 15 percent since 1987.
1.1.2 Hardwood Plywood
Hardwood plywood is categorized into two types of products: prefinished wall paneling and
industrial hardwood plywood. Both types of products are bonded with glue containing urea-
formaldehyde (UF) adhesive resins which are non-waterproof (HPVA, 1991).
1.1.2.1 Wall Paneling
Hardwood plywood wall panels are primarily 3-ply and 2.82 mm to 6.35 mm thick. All wall
panels are prefinished. There are two types of prefinished wall paneling: (1) naturally finished
wall paneling and (2) decoratively finished wall paneling. Plywood used for naturally finished
wall paneling is constructed in the U.S. from species such as oak, birch, walnut, elm, cherry, and
pecan, and is finished to retain its natural look. Plywood panels used for decoratively finished
wall paneling are imported from Indonesia. The imported panels are unfinished and are
decorated (i.e., painted and laminated) in the U.S. (HPVA, 1991).
1.1.2.1.1 Industry Outlook
There has been a substantial decline in the use of prefinished wall paneling since the late 1970s.
Total U.S. production for prefinished wall paneling has declined from around 3.11 million cubic
meters (m3) in 1978 to 0.99 million m3 in 1992 (Figure 1-4). According to the Hardwood
Plywood and Veneer Association (HPVA), the loss of market share by hardwood plywood wall
paneling is due to the following factors:
5
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25
as
T3
o
o
*T3
O
a,
a,
IS
C/3
20
15
10
l North & South ;
i
] West !
I Inland
¦ Total
Notes:
Figure 1-3. U.S. shipments of structural plywood 1987 to 1993 (Source:
Adair, 1993).
1. Shipments from the North are very small and were combined with shipments from the South to avoid
disclosure.
2. Shipments of structural plywood are reported by Adair as sq. ft on a 0.375 in (9.525 mm) basis; the 0.375
in basis was used to convert sq. ft to mJ.
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£
\D
O
•a
o
o
>,
T3
O
o.
a.
•
X!
t/5
Natural finished wall paneling
Decorative finished wall paneling
Total wall paneling
oo
r~-
Os
—
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• Customer preference for products such as gypsum wallboard that can be repainted
(in different colors, if desired) so that the appearance of a room can be more easily
changed from time to time (HPVA, 1991).
• The establishment of an Indonesian cartel which has resulted in significant
increases in the price of imported hardwood plywood blanks during the late 1970s
and 1980s. Currently, Japan and China are the largest markets for Indonesian
plywood with the U.S. a distant third (HPVA, 1991).
• A decline in promotion and advertising by some of the major manufacturers,
accompanied in some cases by their withdrawal from the hardwood plywood
paneling market (HPVA, 1991).
• Formaldehyde release from wall paneling, either real or perceived (HPVA, 1991).
1.1.2.2 Industrial Hardwood Plywood
Industrial hardwood plywood panels are commonly made using 3, 5, or 7 plies. The panels vary
in thickness; 12.70 mm and 19.05 mm thick panels are common. Industrial hardwood plywood
panels are unfinished, i.e., coatings and laminates are not applied (HPVA, 1991). The unfinished
panels are used in the manufacture of furniture, cabinets and specialty panels. Some hardwood
plywood industrial panels are made with a PB core (15 percent of the market) or a medium
density fiberboard (MDF) core (15 percent of the market). Veneer core (65 percent of the
market) is the predominant type used in industrial panels (HPVA, 1991). Unlike wall panels,
production of industrial hardwood plywood panels has remained fairly constant over the years
(Figure 1-5).
1.2 Engineered Lumber
As mentioned in Section 1.1.1.1, glulam beams and LVL are emerging as substitutes to lumber.
Glulam is short for glued-laminated structural timber ~ large beams fabricated by bonding layers
of specially selected lumber with glue containing PF adhesives. End and edge jointing permit
production of longer and wider structural wood members than are naturally available. Glulam
timbers are used with structural wood panels for many types of heavy timber construction.
LVL is constructed of veneers that are bonded together with a PF adhesive resin to form a
laminate; the veneers are layered with the wood grain along the long axis of the beam. The
thickness of the veneers varies from 35 mm to 38 mm. LVL is manufactured to typical lumber
sizes: 0.61 m by 1.22 m (2 ft by 4 ft), 0.61 m by 1.83 m (2 ft by 6 ft), etc. The length of the
beams can be manufactured up to 9.14 m long using end joints or finger joints. Another
application of LVL is in the construction of wood I-joists (an I-joist is a small beam that
resembles the letter "I"). LVL is used to construct the top and bottom of the joist and OSB is
used to construct the center.
8
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a
O
T3
O
O
&
T3
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1.2.1 Industry Outlook
As seen in Table 1-2, the production of glulam beams, I-joists, and LVL is increasing rapidly and
is expected to continue. By 2003, the North American output of LVL is expected to reach 2.78
million m3 (Blackman, 1994).
Table 1-2. U.S. Production Estimates of Engineered Lumber"
1992
1993
1994
1995
1996
1997
Glulam beams (million board meters)
83
88
94
107
119
119
I-joists (million lineal meters)
69
107
117
134
151
137
Laminated veneer lumber (million cubic
0.51
0.71
0.82
0.93
1.10
1.05
meters)
Source: Adair, 1993
"Production figures for 1993 to 1997 are estimated
1.3 Reconstituted Wood Panels
Composite wood panels made with wood strands, particles, and fibers are classified as
reconstituted wood panels. Reconstituted wood panels include PB, OSB, cellulosic fiberboard,
MDF, and hardboard. Table 1-3 lists shipments of reconstituted wood panels by U.S.
manufacturers on a volume basis (m3 of board) from 1985 to 1993. PB, OSB, and MDF
represented 84 percent of reconstituted panel shipments by U.S. manufacturers in 1993.
Table 1-3. U.S. Shipments of Reconstituted Wood Panel (million m3) 1985 to 1993
PB
OSB
MDF
Hardboard
Cellulosic Fiberboard
1983
5.33
1.19
1.07
naa
na
1984
5.66
1.81
1.12
na
na
1985
5.89
2.36
1.21
1.86
na
1986
6.38
3.11
1.38
1.72
na
1987
6.56
3.61
1.59
1.61
1.39
1988
6.78
4.07
1.66
1.53
1.40
1989
6.83
4.52
1.72
1.48
1.41
1990
6.74
4.79
1.68
1.44
1.45
1991
6.68
4.97
1.70
1.56
1.32
1992
7.04
5.89
1.89
1.55
1.53
1993
7.51
6.20
2.85
1.55
1.52
Source: (NPA, 1994b); (Wagner, 1994)
ana = Not available
10
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1.3.1 Particleboard
PB is a panel product made from finely ground wood particles of various sizes that are bonded
together with a synthetic adhesive resin under heat and pressure in a hot press. Other
materials may also be added during manufacturing to improve certain properties such as fire
resistance and dimensional stability. The wood particles come primarily from planer shavings,
sawdust, plywood trimmings, and other wood process residuals. Most PB in the U.S. is bonded
with UF adhesive resins. PB panels bonded with UF adhesive resins are used for interior
applications that do not require water resistance.
PB panels are manufactured in a variety of sizes and densities, depending on the specific end use
application. Panel sizes range from 0.91 m to 2.74 m in width; 6.35 mm to 51 mm in thickness;
and almost any length that is transportable. PB panels are used for industrial applications, floor
underlayment, mobile home decking, door core, shelving, and stair treads. The most common
use of PB panels is for industrial applications. Industrial grade panels represented 80 percent of
PB shipments by U.S. manufacturers in 1993. In comparison, PB used for floor underlayment
was 7 percent of the 1993 shipments, mobile home decking (4 percent), door core (3 percent),
shelving (2 percent), and stair treads (0.3 percent). A large volume of industrial grade PB is used
as a core stock material for furniture (domestic, institutional, office); kitchen and vanity cabinets
(sides, backs, drawers, doors); doors (solid core flush doors, bifolds, sliding doors); games (table
tennis, pool tables) and other goods. Industrial grade PB is also used as a substrate for laminated
panel construction such as countertops, desktops, wall paneling, and shelving. High-pressure
laminates, thermofused, resin-saturated papers, vinyl films, hot transfer films, decorative papers,
and wood veneers comprise the types of overlay materials most commonly applied to PB
substrates (NPA, 1994a).
1.3.1.1 Industry Outlook
Shipments of industrial PB by U.S. manufacturers have been increasing steadily since the early
1980s and in 1993 were at record levels (Figure 1-6). Statistics for PB panels manufactured in
the U.S. are compiled by the National Particleboard Association (NPA). The NPA also compiles
statistics for MDF panels manufactured in the U.S. Statistics are compiled from member
companies who represent more than 85 percent of the total U.S. manufacturing capacity for U.S.
PB and MDF. Most of the PB manufactured by NPA members is unfinished, i.e., coatings,
laminates, etc., are not applied to the board at the plant; of the 7.50 million m3 of PB shipped in
1993, 3 percent was laminated and 1 percent was coated (NPA, 1994b). Most PB is finished by
end users such as furniture and cabinet manufacturers or by companies that sell finished PB to
end users.
1.3.2 Oriented Strandboard
OSB is made from strands of wood bonded together with a synthetic adhesive resin under heat
and pressure in a hot press. The strands of wood are sliced from small diameter, fast growing
trees; tree species used are generally aspen in the northern U.S. and pine and soft hardwoods in
11
-------
•Total
¦ Industrial
Underlayment
Manufactured Home Decking
K>
S
"o
7
6
c3
O
4
o
tS
CL 3
T3
-------
the southern U.S. Strand dimensions vary depending on the slicing machinery and wood species.
Typical strand dimensions are 76 mm long, 4.52 mm to 51 mm wide, and 0.51 mm to 0.71 mm
thick (about the thickness of a business card). The strands of wood in the surface layers are
aligned in the long panel direction, while the inner layers are randomly or cross aligned. The
OSB process developed in the early 1980s as an improvement to the waferboard process in
which strands of wood were randomly distributed throughout the panel. Two-thirds of the U.S.
production of OSB is made with waterproof PF adhesive resins; the remaining third is made with
methylenediphenyl diisocyanate (MDI) adhesive resins which are also waterproof.
OSB is used in structural applications as a replacement for sheathing grade plywood. The most
common applications are for wall and roof sheathing and floor decking in wood frame
construction. Other applications include materials handling (crates and pallets), web material for
wood I-joists, siding, and other specialty products. The strength of OSB comes from the
uninterrupted wood fiber, interleaving of the long strands, and degree of orientation of the
strands in the surface layers. The waterproof adhesives combined with the strands provide
internal strength, rigidity, and moisture resistance.
1.3.2.1 Industry Outlook
As illustrated in Figure 1-7, OSB has been rapidly increasing its share of the U.S. structural panel
market. By late 1996, the capacity of OSB is expected to increase by 45 percent in the U.S. and
by 130 percent in Canada — total North America capacity is expected to rise by 70 percent
(Roberts, 1994). A key factor in the growing OSB demand is the continuing decline of structural
plywood production. The substitution of OSB for structural plywood has been relatively easy
since it competes on price at equal or nearly equal performance (Roberts, 1994). In the future,
the falling price of OSB due to new capacity will also play a key role in encouraging its use.
1.3.3 Hardboard
Hardboard is made with wood fibers bonded with a synthetic adhesive resin under heat and
pressure in a hot press. PF adhesive resins are used to bind hardboard in the U.S. Hardboard is
manufactured by a wet, dry, or wet/dry process (discussed in Section 2.2.4). Raw materials used
to generate wood fibers in the dry hardboard process are primarily dry planer shavings from
lumber operations. Green chips are used to generate wood fibers for wet and wet/dry hardboard
processes (green means that the wood has never been dried). Most green chips are chipped from
forest harvesting residues, such as branches and tops, and lumber slabs. (In the lumber process,
logs are peeled and then cut to length with squared sides; lumber slabs are the other part of the
tree which remains after the lumber has been cut.) A small amount of green chips are chipped
from roundwood.
Hardboard has a density ranging from 641 to 1,121 kilograms per cubic meter (kg/m3) and is
categorized into three product groups: basic hardboard, hardboard siding, and prefinished wall
paneling. Basic hardboard and siding make up the largest volume of hardboard products. Basic
hardboard is used in the manufacture of floor underlayment, furniture, case goods, truck and head
13
-------
25
£
"o
T3
s-c
vi
O
OQ
T3
O
a.
a.
'J3
C/3
Oriented Strandboard
Structural Plywood
10
o
(N
\o
00
o
(N
00
CO
OO
00
00
On
ON
On
ON
On
On
ON
On
On
T—
i—i
—
1—1
•—i
T—
Figure 1-7. U.S. structural panel market (Adair, 1993).
Notes
1. Shipments of structural plywood and oriented strandboard are reported by Adair as sq. ft on a 0.375 in
(9.525 mm) basis; the 0.375 in basis was used to convert sq. ft to m '.
-------
liners, and door skins and faces; door skins and faces are the fastest growing market for basic
hardboard (Wagner, 1994).
1.3.4 Medium Density Fibcrboard
MDF is made with wood fibers bonded together with a synthetic adhesive resin under heat and
pressure in a hot press. UF adhesive resins are the primary type of resin used to manufacture
MDF in the U.S. Raw materials used to generate the fibers come from dry planer shavings,
plywood trim, and sawdust. The density of MDF ranges from 641 to 961 kg/m3.
MDF is used in the manufacture of furniture, cabinets, and general millwork applications. It is
increasingly being used as a substitute for kiln-dried dressed lumber in applications such as
window frames, door jambs, and decorative moldings. Due to the fine texture and homogeneous
nature of MDF, it machines cleanly and is easily painted to produce high quality finishes. Since
MDF is made and sold as a panel, many furniture and cabinet components can be made from a
single piece. The same components made from natural wood require labor intensive jointing and
assembly operation. While nearly all MDF is made with non-waterproof UF adhesives aimed at
interior applications, interest is growing in the development of PF bonded MDF panels for
exterior applications (Roberts, 1994).
1.3.4.1 Industry Outlook
The MDF industry is growing rapidly; U.S. shipments of MDF were at record levels in 1993
(Figure 1-8). The U.S. currently accounts for about 90 percent of North America's MDF
capacity (Roberts, 1994). By late 1996, U.S. capacity is expected to increase by 30 percent,
while Canadian capacity is expected to increase to over five times its current level. Total North
American capacity is expected to rise by 75 percent (Roberts, 1994).
1.3.5 Cellulosic Fiberboard
Cellulosic fiberboard is composed of wood fibers bonded together with either starch or asphalt.
The raw materials used to generate the wood fibers are primarily green chips. A few plants in the
U.S. manufacture cellulosic fiberboard from bagasse and newsprint. Cellulosic fiberboard is a
low density board ranging from 160 to 481 kg/m3. Ninety percent of the 1993 shipments of
cellulosic fiberboard were comprised of exterior wall sheathing, roofing or other specialized
exterior panels; the remaining 10 percent were used in a variety of industrial applications
(Wagner, 1994).
15
-------
Figure 1-8. U.S. shipments of medium density fiberboard (NPA, 1994b).
Notes
1. Shipments of medium density fiberboard are reported by NPA as sq. fit on a 0.75 in (19.05 mm) basis; the
0.75 in basis was used to convert sq. fit to m3.
-------
2.0 COMPOSITE WOOD MANUFACTURING PROCESS DESCRIPTIONS
2.1 Plywood Manufacture1
The following sections describe the manufacturing processes involved in making plywood. The
major steps include the following:
1.
Debarking
2.
Heating the blocks
3.
Cutting veneer
4.
Veneer storage and clipping
5.
Veneer drying
6.
Lay-up and pressing
7.
Finishing
2.1.1 Debarking
The most widely used type of debarker in medium to large mills is the ring debarker. In this
machine the log passes through a rotating ring that holds a number of pressure bars. These
press against the log and tear off the bark. Large units of this type can debark logs at speeds
of up to 200 lineal ft/min.
A rossing head type of debarker is sometimes used in mills where high production rates are
not needed. The rossing head is a rotating cutterhead, similar to the head on a lumber planer,
that rides along the log and cuts off the bark as the log is rotated. This debarker is also suited
to situations where crooked or stubby logs must be debarked.
2.1.2 Heating the Blocks
When logs are cut to the length required for rotary veneer cutting, they are called blocks.
Almost all hardwood and many softwood blocks are heated prior to cutting the veneer.
Heating softens the wood and knots, making it easier to cut. It also improves surface quality,
reducing roughness. Some of the dense hardwoods must be heated to produce satisfactory
veneer. Softwood veneer from some species can be produced from cold logs because of their
lower density and because the roughness limitations are less critical than for high-quality
hardwoods. However, even in the softwood industry the advantages of heating the blocks
generally outweigh the cost of the process. Baldwin (1975) lists four advantages of heating
softwood logs:
1. Higher yields of veneer can be obtained from the logs. The reduction of cutting
imperfections increases the yield an average of 3 - 5 percent.
2. The grade of the veneer is improved. Studies by Lutz (1960), Grantham and
Atherton (1959), and the American Plywood Association have found that grades
of veneer are upgraded from 4 to 25 percent.
'Sections 2.1.1 through 2.1.6 were reprinted with permission from Haygreen and Bowyer, 1989.
17
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3. Labor costs are reduced. Veneer from heated peeler blocks tends to hang together in
a more continuous ribbon as it comes from the lathe. This reduces handling.
4. The amount of adhesive used can be reduced. Glue spreads can be lighter because of
the improved surface.
A variety of methods are used to heat logs. Steaming, soaking in hot water, spraying with hot
water, or combinations of these methods are all suitable to some situation. Dense hardwoods
are usually heated by soaking at temperatures up to 200°F (93 °C). Regardless of the heating
medium, the objective is to heat the log to a suitable temperature as deeply into the log as
veneer will be cut.
2.1.3 Cutting Veneer
Two major methods for producing veneer are slicing and peeling. Most veneer is produced
by peeling (rotary cutting), which is accomplished on a veneer lathe. Slicing is used for
producing decorative veneers from high-quality hardwood and is seldom used with
softwoods. The cutting action on a lathe and on a slicer are very similar and are illustrated in
Figure 2-1. In either case, the wood is forced under a pressure bar that slightly compresses
the wood as it hits the cutting edge of the knife.
On the veneer slicer a cant of wood called the flitch is rigidly dogged, i.e., clamped, to a
carriage that oscillates, cutting on the down stroke. Before each cutting stroke the knife and
pressure bar move forward the thickness of the veneer to be cut. In a rotary lathe they move
forward continuously as the block rotates.
2.1.4 Veneer Storage and Clipping
In modern mills the green veneer must be handled gently and rapidly as it comes from the
lathe. The veneer is peeled at from 300 to 800 lineal fit/min. A series of trays is used in many
softwood plywood plants to handle these long ribbons of wood. Trays are often about 120 ft
long, long enough to handle the veneer that comes from a typical 15-in. block.
Clippers are high-speed knives that chop the veneer ribbons to usable widths. In hardwood
veneer mills, clipping may be done manually to obtain the maximum amount of clear material
from the flitch. In softwood mills and in some hardwood mills, clipping is often done
automatically at speeds of up to 1500 lineal fit/min. The clipper will cut the veneer to about 54
in. (the panel width plus an allowance for shrinkage and panel trimming) if possible.
However, if open defects are present, the veneer may be clipped to less than full panel width.
Automatic clippers detect open defects with scanners that can be overridden by the operator
when it is desirable to do so.
2.1.5 Veneer Drying
Veneer driers consist of a means of conveying the veneer through a heated chamber where
temperatures range from 150 to 260°C. In older roller driers, air is circulated in a manner
similar to that in a dry kiln. This type of drier is still in wide use for hardwood veneer (Figure
2-2). Most plants built in recent years utilize jet driers. These are also called impingement
driers since a curtain of air at velocities of 2000-4000 fpm is directed against the surface of
veneer (Figure 2-3). The high velocity produces turbulent air on the surface of the veneer.
18
-------
r~
Two means of cutting
veneer
SLICER
Flitch
Nosebar
Veneer
Ro er bar
LATHE
Chuck
Veneer block
Vertical
Fibers pressure
compressed
Roller
Veneer
Knife
A = loose side
B = tight side
Horizontal pressure
Veneer
Log
rotation
Figure 2-1. Cutting action on a lathe and slicer (Haygreen and Bowyer, 1989).
Reprinted with permission.
19
-------
Direction of
Veneer Flow
Air Flow
Figure 2-2. Longitudinal-flow dryer (NCASI, 1983). Reprinted with permission.
-------
K>
mm
\ v~
cm
STACK
MOTOR
BANK OF
FINNED
STEAM
COILS
CENTRIFUGAL FAN-
AIR FLOW
T
-1
JET TUBES
J
J
J
J
Figure 2-3. Cross section of a steam-heated jet dryer (NCASI, 1983). Reprinted with permission.
-------
This eliminates the laminar boundary that slows down heat and moisture transfer under
ordinary drying conditions.
Developments such as the use of microwave energy, use of high temperature preheaters, and
increased drying temperature (up to 800°F, or 427°C) may find use for some types of veneer
drying. In most softwood veneers, however, temperatures over about 400°F (204°C) have
adverse effects on glueability.
2.1.6 Lay-up and Pressing
Almost all adhesives used in the plywood industry in the United States today are
thermosetting (cured by heat) synthetic resins. These have almost completely replaced the
blood and soybean flour protein glues that were used in the past for interior (nonwaterproof)
grades of plywood. The two most important types of resins used are phenol-formaldehyde,
which is used for interior and exterior grades of softwood plywood and for exterior grades of
hardwood plywood, and urea-formaldehyde, which is used to manufacture interior grades of
hardwood plywood. The basic components of these resins are formaldehyde, which is
derived from methanol, urea, and phenol.
Only rarely are pure or "neat" resins used as adhesives for plywood. In most cases they are
mixed with fillers or extenders such as Furafil and fine flour produced from wood, bark, or
nutshells. Furafil is a chemical lignocellulose by-product of furfuryl alcohol production that
can be produced from corn cobs, rice hulls, and oat hulls. Starch and animal blood are also
used as extenders to modify the viscosity, control the penetration into the wood, and control
other characteristics of the adhesive mix such as the tack (stickiness).
The process of applying adhesives to the veneers, assembling veneers into a panel, and
moving the panels in and out of the press are often the most labor-intensive steps in
manufacture. Veneer is highly variable in width, length, and quality, which makes it a
difficult material to handle with automated systems. Yet major advances have been made to
increase automation in this stage of manufacture.
One advancement has been in the application of adhesive to the veneer. The old method is to
pass veneer through rubber-faced grooved rollers that apply glue by contact to the top and
bottom surfaces. One person is required to feed the roller glue spreader while an offbearer
places the veneer onto the panel being laid up. If veneer is extremely rough, the glue spread
will not be uniform and skips may occur.
Newer means of glue application, spray and curtain-coaters, have distinct advantages in terms
of uniformity of the glue spread and are suited to automated lay-up systems. These methods
overcome the problem of poor glue spread on rough stock. In these systems the veneers travel
on a belt conveyor under the spray or curtain. A curtain-coater consists essentially of a box
with a slot in the bottom through which the adhesive flows in a continuous sheet or curtain.
Glue not deposited on a piece of veneer passing through the curtain is pumped back up into
the box.
Recently, two new methods of adhesive application, liquid extrusion and foamed resin
extrusion, have been used successfully. These systems lay down continuous beads of resin
on the veneer.
22
-------
The actual assembly of veneer into plywood panels can also be mechanized — at least in
larger plants producing standard-size panels. Although equipment has been developed to do
this almost automatically, most mills use systems that are partially manual and partially
mechanized. For example, the full-size 4 x 8-ft veneers for the two faces may be handled by
machine, but the narrower strips of veneer used in the core may be assembled manually.
Most softwood plywood plants prepress the loads of laid-up panels prior to final pressing in
the hot-press. This is done in a cold press at lower pressure. The purpose is to allow the wet
adhesive to "tack" the veneer together. This permits easier loading of the hot-press and helps
prevent shifting of the veneers during loading.
Pressing of the panels is usually done in the multiopening presses. Such presses can produce
20 - 40 4 x 8-ft panels at each pressing cycle, which may take 2-7 minutes. The purpose of
the press is twofold: to bring the veneers into close contact so that the glue line is very thin
and to heat the resin to the temperature required for the glue to polymerize. Adhesives made
from phenol-formaldehyde resins typically require temperatures of 240°F (115.5°C) in the
innermost glue line for approximately 90 seconds to cure properly. Resin systems must be
carefully tailored to the specific conditions in a plant. Press time and temperature can be
modified; i.e., a shorter press time may be possible if press temperature is increased.
2.1.7 Finishing
2.1.7.1 Structural Plywood and Industrial Hardwood Plywood
Stationary circular saws trim up to 1 in from each side of the pressed plywood, producing even-
edged sheets. About 20 percent of annual plywood production is sanded. Sanding depth varies
with product type. Those that are sanded may be sanded on both sides or only one. Sheets move
through enclosed automatic sanders while pneumatic collectors above and below the plywood
continuously remove the sanderdust. Sawdust in trimming operations is also removed by
pneumatic collectors. The plywood trim and sawdust are burned as fuel or sold to reconstituted
panel plants.
2.1.7.2 Hardwood Plywood Wall Paneling
As discussed in Section 1.1.2.1, hardwood plywood used for wall paneling is a finished product,
i.e, liquid coatings and paper coatings are applied. Process descriptions of these finishing
techniques are beyond the scope of this report. However, the following is noteworthy regarding
decorative wall paneling made from unfinished plywood imported from Indonesia. The imported
plywood is often treated with a formaldehyde scavenger (frequently a urea solution) and dried
before further finishes, such as paper laminates, and veneers are added (Semeniuk, 1994). This
scavenging step is often necessary for the panel product to meet U.S. Department of Housing and
Urban Development (HUD) standards for formaldehyde (Semeniuk, 1994).
23
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2.2 Reconstituted Panel Manufacture
The following sections describe the major process steps involved in manufacturing reconstituted
wood panels. Figure 2-4 is a schematic of the overall process which includes:
1. Wood reduction
2. Drying
3. Adhesive application
4. Mat forming
5. Hot pressing
6. Finishing
The drying and pressing steps are not required by all products.
2.2.1 Wood Reduction
The wooden elements used to manufacture reconstituted panels include strands, particles, and
fibers. Below is a description of how each of these wooden elements are processed.
2.2.1.1 Oriented Strandboard
The strands used to manufacture OSB are specially produced from green roundwood at the plant.
Logs entering OSB plants are cut to 2.54 m lengths by a slasher saw. The logs are debarked and
carried to stationary slasher saws, where they are cut into 0.84 m lengths called blocks. The
blocks are then sent to a waferizer which slices them into strands approximately 0.71 mm thick
(Vaught, 1990). The strands are then conveyed to a storage bin to await processing through the
dryers.
2.2.1.2 Particleboard
The wood particles used to manufacture PB are processed from residues of green or dry wood
operations (green refers to wood that has not been dried). Residues from green operations
include planer shavings from surfacing green lumber, sawdust from cutting green logs, and green
veneer wastes such as clippings, edgings, and trimmings. Dry process residues include shavings
from planing of kiln-dried lumber, mill ends from kiln-dried lumber, sawdust, sanderdust, and
plywood trim. The wood residues are ground into particles of varying sizes using mechanical
refiners and hammermills.
2.2.1.3 Fiberboard (Cellulosic Fiberboard, MDF, and Hardboard)
The wood fibers used to manufacture fiberboards are also processed from residues of green and
dry wood operations. Fibers are generated by first cooking the wood residues in a moderate
pressure steam vessel (digester). During this step, the wood changes both chemically and
24
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WOOD REDUCTION
QSB
Logs are debarked
and cut into strands
PB
Wood residues are
ground into fine particles
of varying sizes
ADHESIVE APPLICATION^
(for MDF)
ADHESIVE
APPLICATION_
(for OSB and PB)
DRYER
DRY
FORMING
PRESSING
FINISHING
MDF
Fiberboards
Wood residues are digested in a
steam cooker and mechanically
separated into fibers
Hardboard Hardboard Cellulosic
Dry Wet & Wet/Dry Fiberboard
WET
FORMING
(Adhesive Applied)
/
Hardboard Hardboard Cellulosic
Wet Wet/Dry Fiberboard
OVEN
Hardboard
Wet/Dry
PRESSING
FINISHING
Figure 2-4. Reconstituted wood panel process flow.
25
-------
physically; becoming less susceptible to the influences of moisture and less brittle as lignin
softens. This semi-plastic wood is then "rubbed" apart into fiber bundles instead of being
mechanically "broken" apart as in the PB process. The fibers are all the same size, therefore,
they need no screening.
2.2.2 Drying
The wooden elements used to manufacture a reconstituted panel are referred to as the "furnish."
In the manufacture of cellulosic fiberboard and wet and wet/dry process hardboard, the furnish is
not dried because the forming process uses water (described in Section 2.2.4). In the
manufacture of OSB, PB, MDF, and dry process hardboard, the furnish is dried to a very low
moisture content to allow for moisture gained by adding resins and other additives. The furnish
is not dried further after blending, except that some evaporation may occur from the heat in the
furnish as it comes from the dryer and from air exposure when conveyed from the blender to the
forming station. Furnishes are generally no warmer than 311 Kelvin (K) when blended to avoid
precuring and drying out the resin (Maloney, 1977).
Most dryers currently in operation use high volumes of air to convey material of varied size
through one or more passes within the dryer. Rotating drum dryers requiring one to three passes
of the furnish are most common. The use of triple-pass dryers predominates in the U.S.
Figure 2-5 shows a conventional triple-pass drum dryer. Dryer inlet temperatures may be as high
as 1,144 K with a wet furnish. However, dry planer shavings require that dryer inlet
temperatures be no higher than 533 K because the ignition point of dry wood is 503 K (Haygreen
and Boyer, 1989; Maloney, 1977).
Many dryers are heated by wet or dry fuel suspension burners. Others are heated by burning oil
or natural gas. The dry fuel suspension burner is the most common type of burner used to heat
dryers. Cyclonic and register-style suspension burners are the two classes of dry fuel suspension
burners presently in use (O'Quinn, 1991).
Cyclonic suspension burners require a fuel moisture content of less than 15 percent. Fuel must
be finely divided wood particles of 3.175 mm in any dimension or smaller (usually fines and
shredded edge trim). These burners require an auxiliary fuel source, such as natural gas, for
warm up and startup (O'Quinn, 1991).
Register-style suspension burners are fueled by wood particles that must be less than 1.600 mm
in any direction, with 90 percent of the particles being less than 0.787 mm in any dimension
(usually fines, shredded panel trim, and sanderdust). Wood fuel moisture content must be less
than 10 percent. Register-style burners require an auxiliary fuel source such as natural gas for
warm up and startup. They also require a continuous sustaining flame of 5 to 10 percent of the
rated burner output during normal operation.
26
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HAICMAL IMC1
mi 61S£S
HtURIU (XI!
to
Figure 2-5. Schematic of conventional triple-pass drum dryer (Vaught, 1990).
-------
Wet fuel burners are designed to burn high-moisture fuels such as bark and sawdust. They are
used in reconstituted panel mills that use green roundwood as a raw material. These burners can
burn both wet fuel and a limited amount of dry fuel, but the dry fuel must be mixed with the wet
fuel at a constant rate. Some plants mix wastewater with dry fuel and burn it as a method of
liquid waste disposal.
The vertical pile burner is the most common wet fuel burner used to heat rotary drum dryers in
the U.S. Another wet fuel burner, the reciprocating grate burner, has been used for dryers in
Europe and for thermal oil heaters in the U.S.
Fuel moisture content for both the vertical pile burner and the reciprocating grate burner must be
less than 50 percent. Fuel particles for each should be no larger than 76 mm in any dimension,
with typical particles being about 25 mm in any dimension (usually green shredded bark).
Auxiliary fuels are not necessary but can be provided.
2.2.2.1 Screening and Air-Classifying
In PB manufacture, screening removes the fines (which would absorb too much resin if not
removed) from the dryer exhaust and classifies particles by size for face and core layers
(Haygreen and Boyer, 1989). The PB industry commonly uses inclined or horizontal vibrating
screens or gyratory screens (Moslemi, 1974).
Air classifiers, which separate particles by particle surface area and weight, may be used alone or
in conjunction with screening equipment. Air classifiers perform best if the feed is limited to
particles with uniform widths and lengths. The classifier can then efficiently separate particles of
different thicknesses due to the weight difference among particles of approximately equal surface
area (Maloney, 1977). Undesired material is sent to a fuel preparation system for the dryer
burner. The screened particles are stored in dry bins until they are conveyed to the blender
(Vaught, 1990).
2.2.3 Adhesive Application
After drying, the furnish is blended with an adhesive resin, wax, and other additives added via
spray nozzles, simple tubes, or atomizers. Waxes are added to the furnish to retard the
adsorption of water into the board. The wax, a mixture of petroleum hydrocarbons, is typically
added to the furnish as an emulsion (wax emulsions are dispersions of very small wax particles in
water). The dispersion is stabilized with various chemicals known as emulsifiers. The amount
of wax added to the furnish ranges from 0.25 to 1 percent or more of the dry board weight. Resin
may be added to the furnish as received (usually an aqueous solution); mixed with water, wax
emulsion, catalyst, or other additives; or added as a spray-dried or finely divided powder
(Maloney, 1977). The amount of resin added to the furnish varies anywhere from 2 to 14
percent, depending on the type of product, and its specific end use.
28
-------
2.2.3.1 Particleboard and Oriented Strandboard
Particles are blended in short retention time (i.e., seconds) blenders. The blenders consist of a
small horizontal drum with high-speed, high shear impellers and tangential glue injection tubes.
As the wood furnish enters the drum, resin is injected, and the impellers hurl the furnish at high
speeds which effectively mixes it with the resin.
Strands are blended in long retention time (i.e., minutes) blenders. The blenders are very large
rotating drums that are tilted on their axes. As the strands are fed into the drums, they are
sprayed with resin. The tumbling action of the strands through the drums serves to blend them
with the resin.
2.2.3.2 Medium Density Fiberboard and Dry Process Hardboard
After refining, the fibers are discharged through a blowvalve into a blowline. In the blowline,
the fibers are sprayed with a resin. The resin can be injected immediately after the blow valve or
anywhere along the blowline (Figure 2-6).
Figure 2-6. MDF blowline blending (Frashour, 1990). Reprinted with permission.
2.2.4 Mat Forming
Mat forming is the spreading of the furnish particles into a uniform mat. Mat formation may be a
wet or dry process.
f
INJECTION
RLFINER
fURNACC
BLOW VALVE (TYP)
29
-------
2.2.4.1 Wet Forming
Wet and wet/dry process hardboard and cellulosic fiberboard are formed by a wet process. In
the wet forming process, fibers are mixed with water and adhesive and this water-fiber mixture is
then metered onto a wire screen. Water is drained away with the aid of suction applied to the
underside of the wire, and the fiber mat along with the supporting wire is moved to a prepress
where excess water is squeezed out.
2.2.4.2 Dry Forming
PB, OSB, MDF, and dry process hardboard are formed by a dry process. The dry forming
process uses air to distribute the furnish onto a moving caul (tray), belt, or screen (Figure 2-7).
PB mats are often formed of layers of different sized particles, with the larger particles in the
core, and the finer particles on the outside of the board (Figure 2-8). In PB and fiberboard
manufacture, the particles and fibers are distributed in a random orientation. OSB is produced by
deliberate mechanical or electrostatic orientation of the strands. In mechanical orientation
processes, mats are produced by dropping long slender flakes between parallel plates or disks. In
electrostatic orientation, particles align with an electrical field when dropped between charged
plates (Haygreen and Boyer, 1989).
2.2.5 Hot Pressing
All reconstituted wood panels, except for cellulosic fiberboard, are hot pressed to increase their
density and to cure the resin. For cellulosic fiberboard, the mat is simply brought to desired
thickness using a press roll and then dried in an oven. Wet/dry process hardboard is also dried in
an oven before being hot pressed.
Most plants use multi-opening platen presses (Figure 2-9). Typical multi-opening presses have
14 to 18 openings (Maloney, 1977). The last 10 years has seen the introduction of the
continuous press (Figure 2-10). Though more popular in Europe, the continuous press is
currently being used in two PB and two MDF plants in the U.S.
Steam generated by a boiler that burns plant residuals runs through a platen passageway to
provide the heat in most hot presses. Hot oil and hot water can also be used to heat the platens.
Direct heating by gas flames has also been used (Maloney, 1977).
The press temperature required to cure the adhesive resin varies, depending on adhesive type.
Press temperatures are typically 478 ± 266 K for PF adhesives, and 450 ± 266 K for UF
adhesives (Price, 1995). The press time needed to compress the mat varies depending on the
final mat thickness, platen temperature, mat moisture content, and adhesive type. Press times are
typically 15 to 20 seconds for every 1.600 mm in thickness for PF adhesives and 10 to 25
seconds for every 1.600 mm in thickness for UF adhesives (Price, 1995).
30
-------
-\
Two types
of formers
Furnish
•%v.
i
±m
Air (tow
ifea&CL.
*{*'*¦
Part c e mat >"*
m
<2L Moving caul or belt
A. Former oses air (Icm* to deposit firver particles on the
faces of tha mat
Furnish
-V.' '••¦• .^V•
Bait
a
Roller discharge head
yv' -.v\
i^r—Casting roller
r>\ M" >tf
v \
• v< t 11
's tetaMMMMMWHrawwaw
(^3 Movlnfl bait ^
B. Several forming heads can be used In series
to produce a 3- or 5-layer mat
Figure 2-7. Two types of mat forming machines (Haygreen and Bowyer, 1989).
Reprinted with permission.
31
-------
A. SINGLE LAYER
rcTp-5V3uvvra:J."
1SJiV i^\vrd&
rCTTT
B. 3-LAYER
OfCfii ft
C. 5-LAYER
0. GRADUATED
Figure 2-8. Various types of mat construction (Moslemi, 1974).
Reprinted with permission.
32
-------
SIMULTANEOUS
CLOSING
OEVICE
COLUMNS
rna
TOP PLATEN
OR CROWN
platens
OR PLATES
MOVING
TABLE OR
PLATEN
RAMS
Figure 2-9. Schematic of multiopening board press (Suchsland and Woodson, 1987).
33
-------
34
-------
2.2.6 Finishing
Primary finishing steps include cooling or hot stacking, grading, trimming/cutting, sanding, and
shipping. Cooling is important for UF-resin-cured boards since the resin degrades at high
temperatures after curing. Boards bonded using PF resins may be hot-stacked to give additional
curing time (Maloney, 1977). Secondary finishing steps include filling, painting, laminating, and
edge finishing (Maloney, 1977). However, the vast majority of reconstituted panel
manufacturers do not apply secondary finishes to their panels. Panels are primarily finished by
end users such as cabinet and furniture manufacturers. Panels are also finished by laminators
who sell finished panels to furniture and cabinet manufacturers.
3.0 PROCESS EMISSIONS AND WASTES
In accordance with Section 313 of Title III in the Superfund Amendments and Reauthorization
Act of 1986 (SARA), many industrial manufacturing facilities are required to annually report
releases to land, water and air, and off-site transfers for treatment, storage or disposal of over 300
individual chemicals. Reconstituted and plywood panel plants are among the facilities required
to report their releases. Tables 3-1 and 3-2 are reported releases of SARA section 313 chemicals
from plywood and reconstituted panel plants for 1991. At both plants, air releases were much
greater than releases to water or land. At reconstituted panel plants, formaldehyde releases to the
air exceeded the releases of all other compounds combined.
3.1 Solid Wastes
Solid wastes at plywood plants consist of wastes from wood preparation, such as bark, log
trimmings and sawdust and wastes from panel operations such as edge trimmings, off-spec
panels, and sanderdust. Most of the wood preparation wastes are burned as fuel at the plant,
although some green sawdust may be sold to reconstituted panel plants. Edge trimmings and off-
spec panels are ground and sold to reconstituted panel plants (Price, 1995).
Solid wastes at reconstituted panel plants consist of wastes from panel operations such as edge
trimmings, off-spec panels, and sanderdust. Edge trimmings and off-spec panels are ground and
reused in the process. Sanderdust is burned as fuel at the plant (Price, 1995).
3.2 Adhesive Wastes
Adhesive wastes at plywood and reconstituted panels plants are the result of adhesive spills or
leaks in the glue line. Although adhesive overspray is generated by plywood glue lines that use
spray applicators, the overspray is collected in a bin beneath the line and reused. In general, if
adhesive waste is generated during adhesive application (plywood or reconstituted panel
manufacture), it is collected and reused. Adhesive waste is collected and reused because
adhesives are a significant portion of the panel manufacturing cost (Price, 1995).
35
-------
Table 3-1. Reported Releases of SARA Section 313 Chemicals from Reconstituted
Wood Panel Plants for 1991"
Total Releases (103 kg)
Total Transfers (103 kg)
Compound
Air
Water
Land
POTW"
Other Off-Site
acetone
129
0
0
0
1
ammonia
35
45
23
29
<0.5
ammonium nitrate solution
<0.5
0
0
0
0
ammonium sulfate solution
0
<0.5
0.5
0
2
asbestos
0
0
8
0
0
barium compounds
<0.5
0
1
0
0
chlorine
0
<0.5
0
0
0
ethyl benzene
26
0
0
0
7
ethyl glycol
12
0
0
0.5
0
formaldehyde
1590
<0.5
23
<0,5
2
glycol ethers
18
0
0
0
5
lead
0
0
<0.5
0
0
manganese compounds
0
0
0
22
0
methanol
282
0
0
0
0
methylenediphenyl diisocyanate
2
0
<0.5
0
0
methyl ethyl ketone
167
0
0
0
18
methyl isobutyl ketone
53
0
0
0
13
n-butyl alcohol
50
0
0
0
1
nitric acid
<0.5
0
0
0
0
phenol
83
<0.5
1
0
0
polychlorinated biphenyls
0
0
0
0
10
styrene
0
0
<0.5
0
1
sulfuric acid
13
0
0
0
0
tetrachloroethylene
<0.5
0
0
0
0
toluene
362
0
0
0
10
1,1,1-trichloroethane
0
0
0
0
1
1,2,4-trimethy lbenzene
6
0
0
0
0.5
xylenes
195
0
0
0
34
zinc compounds
0
0
<0.5
0
0
Source: NCASI, 1993. Reprinted with permission.
aSARA = Superfund Amendments and Reauthorization Act
bPOTW = Publicly owned treatment works
36
-------
Table 3-2. Reported Releases of SARA Section 313 Chemicals from Plywood
Plants for 1991"
Total Releases (103 kg) Total Transfers (103 kg)
Compound
Air
Water
Land
POTWb
Other Off-
acetone
171
0
0
0
2
ammonium sulfate solution
0
0
0
0
0
barium compounds
<0.5
0
2
0
0
dichlormethane
0
0
0
0
1
formaldehyde
39
0
1
<0.5
0
glycol ethers
11
0
0
0
0
methanol
164
0
0
0
<0.5
diphenylmethane diisocyanate
5
0
0
0
0
methyl ethyl ketone
36
0
0
0
1
methyl isobutyl ketone
6
0
0
0
7
phenol
15
0
2
<0.5
0
sulfuric acid
<0.5
0
0
0
0
toluene
46
0
0
0
2
xylenes
34
0
0
0
3
Source: NCASI, 1993. Reprinted with permission.
aSARA = Superfund Amendments and Reauthorization Act
bPOTW = Publicly owned treatment works
3.3 Water Wastes
As seen in Tables 3-1 and 3-2, water releases from plywood plants and reconstituted panel plants
are minimal. Water wastes are generated at these plants, however, the water is reused, instead of
discharged. Below is a brief description of water usages at reconstituted panel and plywood
panel plants.
Although some hardboard plants use water in the forming process, the majority of reconstituted
panel plants are dry processes. Water is primarily used at reconstituted panel plants for rinsing
equipment such as blenders and glue lines. The spent water is kept in a holding tank and added
back into the adhesive (Price, 1995).
37
-------
Plywood plants use water in a variety of applications. During the summer, southern plywood
plants spray water on log piles to prevent blue stain. Blue stain is a discoloration caused by a
certain type of fungus; spraying the logs with water prevents the growth of the fungus. The
water that is sprayed on the logs drains into a pond; this water is then reused to spray the logs.
A second use of water at plywood plants is for soaking logs. Most plants soak logs in heated
water (339 to 355 K) prior to peeling the veneer. The logs are soaked in either vats or ponds.
The soaking water contains natural constituents from the wood, and sodium hydroxide and
fungicide which are added by the plant. Sodium hydroxide is added to the water to maintain a
PH of 7. This prevents corrosion of the vats and allows maximum penetration of water into the
wood (Price, 1995).
A third use of water at plywood plants is for rinsing glue applicators. This is typically done on a
daily or weekly basis, depending on a plant's schedule. The spent water is collected in a tank and
is added back into the adhesive formulation at the plant or sent to a resin plant which uses the
spent water in formulating adhesive (Price, 1995).
3.4 Air Emissions
At reconstituted panel and plywood panel plants, air emissions are generated from dryers and
presses. The following describes the emissions from these sources.
3.4.1 Reconstituted Panel Dryers
3.4.1.1 Emissions Stream Characteristics
Reconstituted panel dryers process strands, fine particles, and fibers. As mentioned in Section
2.2.2, the most common type of furnish dryer is a three-pass rotary drum dryer that is direct
heated by dry fuel suspension burners.2
Compounds in wood dryer emissions can be classified into several categories: (1) terpenes
and isoprenes, (2) pitch, (3) wood pyrolysis products, and (4) energy source combustion
products. Terpenes are natural constituents of softwoods. Isoprenes are found in hardwoods.
The amount of terpenes and isoprenes present in the wood will vary with wood species and
previous drying history of the furnish. Green wood contains the most terpenes or isoprenes.
The amount of these materials in the furnish will decrease as the wood dries. Wood residues
obtained from other wood products operations that dry the raw material will have low terpene
content because volatile materials are lost in the first drying process. Storage of raw wood
will also result in loss of volatile wood components. Terpenes and isoprenes evaporate from
wood or chips at low temperatures; nearly all that is present in the wood entering a dryer is
2The remaining portion of 3.4.1.1 is reprinted with permission from Dallons, 1991 and Emery, 1991 .
38
-------
probably released and emitted. Terpenes and isoprenes will remain in a vapor state in the
atmosphere. Hence they contribute only to VOC [volatile organic compounds] emissions
from dryers.
Resin and fatty acids, more commonly referred to as pitch, are also natural constituents of
wood. They can be vaporized from the wood during drying. Vaporization rates vary with
drying temperature, airflow through the dryers, and the pitch content of the wood. The
amount of pitch in the wood varies with species. When dryer emissions enter the atmosphere
and cool, these materials condense to form an aerosol that is responsible for the blue haze
associated with wood product dryer's emissions.
If wood is heated to sufficiently high temperatures, it decomposes. This process is called
pyrolysis. The exact temperatures at which various degradation products are formed are not
known with certainty; however, it is generally accepted that wood will begin to decompose
somewhere between 212 and 392 °F, giving off acetic acid, formic acid, and possibly some
carbon dioxide. Decomposition continues with increases in temperature, and wood will burst
into flames somewhere around 523 °F in the presence of oxygen. The process of pyrolysis is
literally an explosive breaking apart of the chemical structure of the wood. Hundreds, perhaps
thousands, of chemical compounds are formed during pyrolysis. In the absence of sufficient
oxygen to ensure complete combustion, this decomposition is manifested by the formation of
smoke.
One important chemical formed during pyrolysis is formaldehyde. Formaldehyde appears to
be much more prevalent for hardwoods than for softwoods. Other VOCs, as well as
particulates (smoke), are also formed during pyrolysis, and the amounts and types depend
upon drying temperature, wood species, size and residence times of the wood particles, and
possibly other variables.
Combustion products from direct fired burners can contribute VOC emissions. The burners
used for direct firing dryers are efficient units and little volatile organics are expected to be
released by them. Thus, VOCs from combustion are expected to be a minor portion of the
total VOCs emitted from dryers.
Ash from combustion of sanderdust or other wood residue fuels will contrube to the
particualte emissions. Much of the ash is small in size and contributes to PMI0.
3.4.1.2 Variables Affecting Emissions from Reconstituted Panel Dryers
As discussed above, two variables that affect the composition of dryer emissions are dryer
temperature and wood species. The effects of these variables on specific types of dryer
emissions are presented below. The discussion is based entirely on various emissions test reports
published by the National Council of the Paper Industry for Air and Stream Improvement
(NCASI). The reports summarize emissions data measured at various reconstituted panel plants.
39
-------
Dryer emissions are described in terms of total gaseous nonmethane organics (TGNMO) and
formaldehyde3.
3.4.1.2.1 Effects of Dryer Inlet Temperature on TGNMO Emissions
NCASI reported that at dryer inlet gas temperatures greater than 588 K, the concentration and
emissions rate of the condensable portion of TGNMO increased as functions of temperature
(Figures 3-1 and 3-2) (NCASI, 1986c). Little correlation was seen between emissions of the
noncondensable portion of TGNMO and the inlet dryer temperature (Figure 3-3) (NCASI,
1989). The emissions in Figure 3-3 are from drying green material only (Dallons, 1991).
Materials that had been pre-dried in other processes, such as planer shavings or sawdust from
kiln dried lumber, had lower emissions of noncondensables than drying green furnish (Dallons,
1991). When pre-dried material was processed, VOC emissions ranged between 0.36 and 0.54
gram per kilogram (g/kg) of product. However, because the raw material had been partially dried
in other processes, it contained fewer terpenes. The material also required less heat to dry which
resulted in lower dryer temperatures (Dallons, 1991).
3.4.1.2.2 Effects of Dryer Inlet Temperature on Formaldehyde Emissions
NCASI reported that the concentration of formaldehyde in the dryer exhaust was a function of
the dryer inlet temperature (Figure 3-4) (NCASI, 1986c). The formaldehyde concentration at
dryer inlet temperatures below 810 K was less than 20 parts per million (ppm). At inlet
temperatures above 810 K, the formaldehyde emission rates were as high as 110 ppm.
3.4.1.2.3 Effects of Wood Species on Formaldehyde Emissions
Formaldehyde emissions from drying different wood species are shown in Figure 3-5 (NCASI,
1989). Based on the data shown in Figure 3-5, NCASI concluded that dryers processing
hardwood or a mixture of hardwood and softwood species had a moderate to dramatic increase in
formaldehyde emissions at dryer inlet gas temperatures greater than 699 K, but dryers processing
softwood species had only a slight increase in formaldehyde emissions with increasing
temperatures (NCASI, 1989).
3Measurement of TGNMO is not recognized by EPA as a measurement of VOC.
40
-------
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a
s
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Vi
£
T3
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&
JU
Id
93
in
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11.35 -r
6.81 •-
4.54 •-
0.00
•
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L. .p. ;
•
~
•
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: o :
o
• o*
—4
o
>
JS
« 60
"es
£ 40
©
0
255 366 477 588 699 810 921 1032
Inlet temperature, K
Figure 3-4. Concentration of formaldehyde in dryer exhaust as a function of the dryer inlet
temperature (NCASI, 1986c) (Symbols refer to different mills). Reprinted with permission.
] [
i : ¦
j ¦„..[* "...
*
¦
t :
00—
0 i
o i •
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42
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>••••••
!••••••
' ' P
A
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: oQ
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JO
if;
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366
¦ft j"°
¦V
¦+-
•*»<
699
810
921
1032
1143
¦ Softwood
o Mixed
• Hardwood
Inlet temperature, K
Figure 3-5. Formaldehyde emissions associated with drying different wood species
(NCASI, 1989). Reprinted with permission.
3.4.2 Veneer Dryers
3.4.2.1 Emissions Stream Characteristics
Uncontrolled veneer dryer emissions at stack gas temperatures above 89 K contain particulates
and gaseous organics (NCASI, 1983). Particulate emissions from steam-heated dryers consist of
wood fibers from the veneer. Particulate emissions from wood-residue fired direct-heated dryers
contain wood fibers as well as ash from the combustion process. Gaseous organics released from
veneer when it dries consist primarily of non-structural wood components that are commonly
termed "extractives." Extractives are wood constituents which can be removed with "neutral"
solvents, i.e., solvents which do not significantly affect the strength of the wood, such as water,
alcohol, benzene, and ether (Emery, 1991). Most extractives are of low molecular weight and
therefore, volatilize from the veneer as it dries. Upon entering the atmosphere, some of the
gaseous extractives remain VOC while others condense to form aerosols.
43
-------
3.4.2.2 Variables Affecting Veneer Emissions
The effects of dryer type, dryer temperature, wood species, and other variables on veneer
emissions are presented below. The discussion is based entirely on an emission test report
published by NCASI. The report summarizes emissions data measured at several plywood plants
on the West Coast and in the South.
3.4.2.2.1 Factors Affecting Noncondensable Organics
The NCASI report concluded that emissions of noncondensable organic compounds were related
to the turpentine content of the veneer dried and to the dryer type. Average noncondensable
organic compound emissions from steam heated dryers were 10.7 kg per thousand square meters
(MSM) for lodgepole pine, 3.9 kg/MSM for Douglas fir, and 3.4 kg/MSM for hemlock. Average
noncondensable organic compound emissions from wood-residue-fired dircct-heated dryers were
11.7 kg/MSM for Douglas fir and 4.4 kg/MSM for hemlock (lodgepole pine was not tested).
The report's conclusion that noncondensable organic compounds were related to the turpentine
content of the veneer to be dried does not coincide with a table in the report that listed the
turpentine content of various wood species (part of this table is reproduced in Table 3-3). As
seen in Table 3-3, the turpentine content of all three species of hemlock is nondetectablc yet
noncondensable emissions of hemlock from steam-heated dryers were almost the same as
noncondensable emissions of Douglas fir. The study does not indicate which species of Douglas
fir were tested; however, lodgepole pine emissions from steam-heated dryers were higher than
Douglas fir emissions from these dryers. The latter suggests that the Washington specie of
Douglas fir was tested (since it has a lower turpentine content than the lodgepole pine).
Table 3-3. Turpentine Content of Wood Species
Specie
Turpentine (mVmillion kg of dry wood)
Douglas Fir (Washington)
Douglas Fir (Canada)
Hemlock, Eastern
Hemlock, Mountain
Hemlock, Western
Lodgepole pine
nondetectable
nondetectable
nondetectable
2.01
3.91
0.71
Source: NCASI, 1983. Repinted with permission.
44
-------
3.4.2.2.2 Factors Affecting Participated and Condensable Organics
Particulate and condensable organic emissions from steam-heated dryers were found to be a
function of the veneer specie, dryer temperature, and the amount of air passed through the dryer.
Particulate and condensable emissions from steam-heated dryers ranged between 0.3 and 9.9
kg/MSM. Wood-residue fired direct-heated dryers were found to have particulate and
condensable organic emissions ranging between 2.4 and 7.8 kg/MSM; according to the report,
these emissions appeared to be independent of wood specie due to the greater influence of ash in
the exhaust gas from the combustion of wood (NCASI, 1983).
3.5 Press Emissions
Press emissions from plywood and reconstituted panels consist of VOCs and particulates. VOCs
are generated from the wood and the adhesive that binds the wood. Particulates are primarily
wood dust that is generated in the manipulation of the panels and unpressed mats (O'Quinn,
1991).
3.5.1 Wood Related Emissions
At elevated press temperatures, various extracts may be driven from wood as VOCs (O'Quinn,
1991). VOC formation is higher in woods with higher extract content; i.e., softwoods. However,
hardwoods tend to release more secondary formaldehyde from the pyrolysis of extracts
(O'Quinn, 1991).
3.5.2 Adhesive Related Emissions
Both UF and PF adhesives involve polymerization with formaldehyde. The high press
temperatures required to promote the reaction drive off part of the unreacted formaldehyde
(O'Quinn, 1991). The cyanates contained in MDI are of such low volatility that polymerization
is complete, resulting in no detectible emissions (O'Quinn, 1991). The following sections
describe in more detail, press emissions associated with UF, PF, and MDI resins.
3.5.2.1 Press Emissions from Curing UF Resins
The formaldehyde to urea (F:U) mole ratio is the ratio of the number of moles of formaldehyde
to number of moles of urea in UF adhesive resins. For example, a F:U mole ratio of 1.15 has
1.15 moles of formaldehyde for each mole of urea. As seen in Figure 3-6, a wide range of mole
ratios are used in UF bonded products: for PB, when a single resin is used throughout the board,
the F:U mole ratio can fall anywhere within the range set by the face/core systems; MDF
products use resins with F:U mole ratios higher than PB resins; hardwood plywood products use
the highest F:U mole ratios. The nature of the product and process dictates the F:U mole ratio of
45
-------
F:U Mole Ratio
1.4
1.2
Fact Car*
Partlcltboud
MOr HOWD Plywood
Figure 3-6. Urea-formaldehyde resin mole ratios (Rammon, 1990).
Reprinted with permission.
resin used. The mole ratio directly impacts the ultimate strength the resin will produce in the
board, i.e., certain products require higher mole ratio resins to attain an adequate level of bond
strength (Rammon, 1990).
The F:U mole ratio in UF resins is related to formaldehdye board and press emissions; the higher
the mole ratio, the higher are formaldehdye emissions and vice versa (Gollob, 1990). Table 3-4
presents data from a NCASI study which measured press emissions at four PB mills where UF
adhesives were used. According to the study, a direct relationship between F:U ratios and press
emissions was observed at these mills, in particular, Mill B. At Mill B, two types of PB products
were produced: commercial grade and door core. The amount of resin used in each product was
the same (8.8 percent), however, resins with different F:U mole ratios were used to make each
product. A F:U mole ratio of 1.25 was used to make the commercial grade PB and a F:U mole
ratio of 1.5 was used to make the door core. For the commercial grade PB, the formaldehyde
emission rate was 3.86 kg/MSM on a 19 mm thickness basis (MSM,9); for the door core PB, the
formaldehyde was 5.86 kg/MSM. According to the study, the formaldehyde emission rate
increased in proportion to the increase in the F:U mole ratio of the resin. The study also reported
that data from Mill A illustrated a relationship between increasing formaldehyde emission rates
with increasing resin usage and that data from Mill D illustrated a relationship between
increasing formaldehyde emissions rates with increasing press temperatures. The data in Table
3-4 do not seem to be sufficient to draw these conclusions. At most, they suggest a trend
between these variables and press emissions. The effect of variables such as furnish moisture
46
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Table 3-4. Press Emissions from Particleboard Mills using UF Resins
Percent
CH2Oa
Press
Resin
F:U
kg/MSM
Temp.,
in
Mole
19 mm
TGNMO
Mill
Product
Wood Species
K
Board
Ratio
Basis
kg/MSM
A
Commercial
Douglas fir
433
5.8
_ _b
1.76
4.20
A
Industrial
Douglas fir
433
7.1
2.15
—
B
Commerce
Aspen/pine
455
8.4
1.25
3.66
6.35
B
Door Core
Aspen/pine
466
8.4
1.50
5.86
9.28
C
Commercial
Douglas fir/pine
433
8.8
1.51
9.77
C
Industrial
Douglas fir/pine
433
10
1.76
11.23
D
Commercial
Pine
430
6.9
1.46
—
D
Commercial
Pine
469
6.9
_ _
1.76
__
Source: NCASI, 1986b. Reprinted with permission.
aCH20 = chemical formula of formaldehyde
hNot reported
c TGNMO = total gaseous nonmethane organics
d MSM = thousand square meters
c UF = urea-formaldehyde
f F:U = formaldehyde to urea
content and wood species on press emissions was not examined. The study also failed to
mention the use of formaldehyde scavengers or the influence that scavengers would have on
emissions (see section 6.3).
3.5.2.2 Press Emissions from Curing Phenol-Formaldehyde Resins
The data in Table 3-5 are from a NCASI study that measured press emissions at two OSB mills
(NCASI, 1986a). Based on this data, the study concluded that emissions of formaldehyde and
phenol were not related to any of the operating parameters monitored, but were instead affected
by different resin compositions. At Mill A, three types of liquid PF resins were used along with
an unspecified type of wax. The resin application rate was 5.0 percent in the core, and 4.75
percent in the surface material. At Mill B, a powdered PF resin and an unspecified type of wax
were used. The resin application rate was 2.0 percent in both the core and surface material. The
study stated that the different resin compositions used at Mill A were unknown because it was
kept confidential by the manufacturer of the resin. Consequently, the lack of information about
the resin composition precluded development of relationships to resin properties (NCASI, 1986a).
47
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Table 3-5. Press Emissions From OSB" Mills Using PFb Resins
Press Temp.
Percent Resin
Formaldehyde
Phenol
Mill
Wood Species
K
in Board
kg/MSMc,d
kg/MS M
A
Aspen
478
4.8
1.61
>0.10
A
Aspen
478
4.8
2.73
>0.68
A
Aspen
478
4.8
2.59
>0.78
B
Lodgepole pine
478
4.2
0.83
>0.25
Source: NCASI, 1986a. Reprinted with permission.
° OSB = oriented strandboard
b PF = phenol-formaldehyde
c MSM = thousand square meters
dStudy did not indicate thickness basis
3.5.2.3 Press Emissions from Curing MDI Resins
NCASI compiled press vent emissions measured at four mills using MDI resins. Table 3-6
summarizes the measured emissions. The average formaldehyde emissions rate from the three
mills using MDI ranged between 0.05 and 0.34 kg/MSM19. One mill used a combination of MDI
and PF resins to bind the board; press vent emissions of formaldehyde were the highest at this
mill.
Table 3-6.
Press Emissions from MDIa
Resins
Formaldehyde
MDI
Resin
kg/MSMbl9
kg/MSM19
MDI
0.05
0.049
MDI
0.10
0.103
MDI
0.34
0.0000
MDI (core) & PF° (face)
0.54
Source: NCASI, 1989. Reprinted with permission.
a MDI = methylene diphenyl diisocyanate
h MSM = thousand square meters
c PF = phenol-formaldehyde
48
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4.0 POLLUTION PREVENTION
Sections 5.0 through 8.0 present potential pollution prevention options for reducing
manufacturing emissions from composite wood manufacturing processes. These options were
found through a literature search of journal articles, symposium publications, company
brochures, phone inquires, etc. Most of the available literature focused on reducing costs and
improving manufacturing processes. As a result, this report contains little information on
quantitative emissions reduction. The intent of this report is to provide the reader with potential
pollution prevention options. The inclusion of any particular technique or technology does not
imply best achievable control technology (BACT), MACT, or any other regulatory required
technology. In addition, life-cycle assessments on potential pollution prevention options were
beyond the scope of work for this report. Consequently, whether or not the options presented in
this report are pollution prevention will require further investigation.
5.0 ALTERNATIVE FIBER SOURCES
As discussed in Section 1.1.1, harvesting restrictions from publicly owned lands have curtailed
production of plywood and lumber, particularly in the West. Production of PB and MDF has
also been affected by the harvesting restrictions. Over 80 percent of the wood used to
manufacture PB and MDF are residuals from lumber or plywood manufacturing such as chips
and shavings (McCredie, 1993). Curtailment of the production of plywood and lumber affects
the supply of chips and shavings available for the production of PB and MDF. Western board
manufacturers have been especially hard hit by the drop in timber supply. Oregon alone has 11
PB/MDF plants that account for more than 25 percent of the U.S. production of these products.
The shut-down of plywood and lumber mills has put a premium on the remaining supply of
wood residuals (McCredie, 1993). PB and MDF manufacturers that were historically able to
satisfy their wood supply requirements within a 161-kilometer (km) radius of the plant now
routinely go out 322 km to 483 km (McCredie, 1993). This results in increased competition for
available supplies and increased transportation costs (McCredie, 1993). While no Oregon PB or
MDF mills have been permanently shut down for lack of wood, some production curtailments
have occurred (McCredie, 1993).
Two potential alternative fiber sources for composite panels are recycled wood waste and
agricultural fibers. In an interview regarding the shortages of wood for manufacturing composite
panels and the use of recycled wood waste as an alternative raw material source, Rich Margosian,
executive director of the NPA, predicted that in the next five to ten years more manufacturers
will be using alternate fiber sources. "I think we're at the beginning of this," he stated. "Board
producers are having to do research in two different directions—looking for alternate resources
such as recycled wood waste and economizing on material needed to make board" (Plantz,
1994).
49
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5.1 Recycled Wood Waste
In 1993, Willamette Industries initiated a program to recycle wood waste into chips for PB
production (Plantz, 1994). This wood waste includes construction site debris, discarded
household items, crates, and pallets that had been burned or put into landfills. Willamette's
Eugene PB and Duraflake mills now use up to 15 percent recycled material, and its Korpine mill
is developing sources and starting to use recycled material.
Medite Corporation is another board producer that is using waste wood at its New Mexico MDF
mill. Recycling was something that in past years was not economically feasible for the
company; however, increased prices for wood have now made it possible.
Recycling wood waste requires a different mind set for raw material acquisition — instead of
looking to the forests, a company has to look to urban areas. That can work to a mill's
advantage. Medite has made arrangements with trucking companies to develop equipment that
can haul finished board to the user and bring raw material back from the city to the mill. Medite
is currently hauling material from Oklahoma City, Denver, and Albuquerque. Willamette has
also invested time, training, and money to develop their suppliers. An example is Wood
Recycling, Inc., in Eugene, Oregon. Willamette purchased the operation's equipment and leases
it back to Wood Recycling, which sells all its product to Willamette.
Beyond finding sources, extensive training and research is required on what materials will and
will not work, careful control of the quality of recycled materials, and cleaning of the material to
remove any foreign matter. The mills must make sure the mix of material and resins is correct.
There are many hurdles in cleaning the material, especially with being able to adjust the process
to run different kinds of material, while still maintaining a quality product. It is not simply a
matter of taking discarded wood and grinding it up. From a product standpoint, if the wood
waste is clean it makes good PB (Plantz, 1994).
By late 1995, CanFibre hopes to start up its first plant to produce MDF using 100 percent post
consumer waste and PF adhesives. The plant (the first of its kind in North America) will be
located near Toronto, Ontario. Approximately 34,000 m3 per year of structural MDF will be
produced from recycled urban waste such as waste wood, cardboard, drink containers, and
newspaper (Wood Technology, 1994). The plant will have two significant cost advantages over
conventional MDF plants: (1) the cost of post-consumer waste is currently negative and (2)
savings in freight costs due to the plant's location near an urban site (most existing MDF plants
are remotely located and the cost of hauling wood waste back to these mills is high). The net
mill cost for the process used by CanFibre's Toronto plant is estimated to be $893/MSM versus
$1,113/MSM for a conventional plant (Roberts, 1994). The company plans to build a total of
nine plants in North America: six in the U.S. and three in Canada. All plants will use 100
percent post consumer waste and PF adhesives.
50
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5.2 Agricultural Fiber
Agricultural fiber comes from two main sources: agricultural crops grown for fiber (e.g., kenaf)
and residues of crops grown for other purposes (e.g., wheat, cotton). Agricultural fiber is used in
many countries to manufacture composite panel products such as cellulosic fiberboard, PB,
MDF, and hardboard. A worldwide literature search conducted at the U.S. Department of
Agriculture (USDA) Forest Service's Forest Products Laboratory (FPL) in Madison, Wisconsin,
found 1,039 citations on the use of agricultural fibers for manufacturing composite panels
(Youngquist et al., 1993). Many of these applications are in developing countries where there is
not enough wood to cover the needs for fiielwood, industrial wood, sawn wood, and wood-based
composite panels (Youngquist et al., 1993). Recently, a plant was built in North Dakota which
manufactures particleboard from wheat straw and MDI resins (Galbraith, 1995).
Large volumes of agricultural fiber are generated each year in the U.S. Sources of this fiber
include bagasse, cereal straw, corn stalks and cobs, cotton stalks, kenaf, rice husks, rice straw,
and sunflower hulls and stalks (Youngquist et al., 1993). The volume of agricultural fiber
generated in the U.S. is so great that if 75 reconstituted wood panel plants were to switch entirely
to agricultural fiber and, on average, each plant required 135 billion kg of fiber annually, more
than 30 times as much agricultural fiber would be available as would be consumed (Youngquist
et al., 1993). The above calculation is based only on agricultural residues, except for bagasse,
and does not account for agricultural fibers from nonresidue sources like kenaf. Thus, in terms
of potential availability, the amount of residues generated by U.S. agriculture far exceeds present
and future composite panel fiber requirements (Youngquist et al., 1993).
Although there is more than enough agricultural fiber in the U.S. to supplant wood in composite
panel manufacture, the feasibility of such a substitution depends on many factors such as product
quality, cost, current uses of agricultural residues, and others. Whether this substitution is
pollution prevention requires a comparison of the total environmental impacts (air, water, and
soil) associated with manufacturing composite panels using agricultural fibers versus wood. An
abundance of information is available on the feasibility of using agricultural fiber for composite
panel manufacture (as stated above, the FPL literature search found 1,039 citations). However,
information on the environmental impacts associated with agricultural fiber use was not found
during the literature search for this report. Despite not knowing if agricultural fiber is a pollution
prevention option for wood composite panels, it is included in this report as a raw material
substitute for wood; the discussion, however, is limited to feasibility issues regarding its use in
composite panel manufacture.
5.2.1 Product Quality
In North America, available agricultural fiber for composite panel manufacture includes bagasse,
cereal straw, corn stalk, cotton stalks, kenaf, rice husks, rice straw, and sunflower hulls and stalks
(Youngquist et al., 1993). Many of these fibers have been used to construct composite panels
which meet or exceed standards for PB (Figure 5-1). Fewer of these composite panels meet the
51
-------
c3
-a
c
c3
CQ
cu
c—
O
-------
standards for hardboard (Figure 5-2 - note that thickness swell and water absorption values below
the dotted line are desirable).
Although some composite panels made from agricultural fibers possess properties adequate for
PB and hardboard use, they may not be adequate to meet the structural requirements of OSB
panels. Whereas PB and hardboard panels are composed of finely ground wood residues, OSB
panels are constructed of specially cut wood strands. The uninterrupted wood fiber of the
strands, interleaving of the strands, and degree of orientation of the strands in the panel imparts
structural properties to OSB panels. The composite panels in Figures 5-1 and 5-2 are composed
of ground up agricultural residues that may not provide the necessary strength required for OSB
panels. While adequate structural properties may be developed by increasing panel thickness,
this will increase both the amount of fiber needed and the manufacturing cost of the product
(Bigbee, 1994). Further investigations are required to determine the suitability of agricultural
fibers for OSB (Bigbee, 1994).
5.2.2 Bulk Density
A major difference between wood and non-wood fibers is bulk density. One obstacle to
agricultural fiber utilization is low bulk density. Low bulk density can increase transportation
costs significantly. A standard cord of wood has a bulk density (dry basis) of 240 to 320 kg/m3.
Processing and transportation costs limit the practical procurement of cords to a radius of about
64 km (Vaagen, 1991). In contrast, annual fiber stems of plants such as kenaf or straw cannot be
compacted much beyond 135 kg/m3. The procurement range for these fibers is about 24 to 32
km (Sandwell and Associates, 1991).
5.2.3 Price
Price estimates for various types of wood species and agricultural fiber are presented in Table 5-1
(Youngquist et al., 1993). As shown, pulpwood prices range from $41 to $60/1000 kg and
agricultural fiber prices range from $25 to $90/1000 kg. Among the agricultural fibers, kenaf is
generally the highest in estimated price because all cultivation and harvest costs are born by the
fiber component of the output (Youngquist et al., 1993). Straw and corn generally cost less than
do crops grown specifically for their fiber content because the grain portion of the output bears
the expense (Youngquist et al., 1993). An exception to this is where the fiber has value for other
uses, such as animal bedding. In those cases, straw prices can be almost twice as high as
pulpwood and not currently within economic reach of PB producers. For example, in Wisconsin
and Pennsylvania where animal bedding needs are high, baled straw is priced from $50 to
$90/1000 kg (Youngquist et al., 1993). In North Dakota, straw is generally left on the ground;
the small amount of straw that is baled markets for only $25 to $35/1000 kg (Youngquist et al.,
1993).
53
-------
T3
c3
TJ
G
w
200
oo
-g
cd
150
o
-a
Ui
cd
100
X
<_
o
50
0
o
-------
Table 5-1. Estimated Prices of Wood and Agricultural Fibers (5/1000 kg)
Pulpwood & Agricultural Fiber Stumpage3 Harvested and Delivered Total
Southern pineb
16
25
41
Southern hardwoodb
8
31
39
Aspenc
13
34
47
Hybrid poplard
40
20
60
Kenaf
36
19
55
Cereal strawf
5 to 70
20
25 to 90
Corn stalksf
5
20
25
Source: Youngquist et al., 1993. Reprinted with permission.
aStumpage is growing cost plus return to land and farmer; for kenaf, straw, and corn, cost of
harvest is included in stumpage
Trices from Timber Mart South (1992)
cAspen prices from Wisconsin and Minnesota state forestry officials
dHybrid poplar based on Turhollow (1991)
eKenaf based on Sandwell and Associates (1991)
'Based on partial survey of state agricultural extension economists
5.2.4 Fiber Availability
Not all the gross potential supply of agricultural fiber in the U.S. is freely available. For
instance, to participate in Federal farm programs, all farms must have an approved conservation
plan by 1995 (Youngquist et al., 1993). In some cases, this entails leaving some portion of the
residue mass on the ground as cover for soil protection. In other instances, fiber by-products are
used for animal bedding. In addition, fibers are available only on a seasonal basis and only in
certain geographical areas, therefore shipping and storage issues must be considered as well.
5.3 Recycled Textile Fibers
Professor Chris Pastore at North Carolina State University has developed a process that converts
wasted textile fibers into composite panels. The process uses thermoplastic fibers such as nylon
and polyester and some glass fiber. The fibers are formed into a mat and then cured to melt the
thermoplastic fibers (adhesives are not necessary to bind the fibers). After curing, the fibers
solidify into a mat. A veneer laminate can also be applied to the mat prior to curing; adhesive is
not required to attach the laminate since the fibers melt onto the veneer.
55
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The textile composite can be used in traditional MDF and OSB applications; it actually out
performs OSB structurally (Pastore, 1995). Currently, a small business in Raleigh, N.C is
purchasing the material instead of OSB because it is cheaper (Pastore, 1995). The pollution
prevention benefits of this product are: (1) it reuses a wasted material; (2) it reduces the use of
green roundwood (OSB manufacture); (3) manufacturing emissions associated with adhesives
are nonexistent; and (4) indoor air emissions may be reduced since adhesives are not contained in
the product.
6.0 ALTERNATIVE ADHESIVES
6.1 Background
In the U.S., composite wood panels are manufactured primarily with UF, PF, and MD1 adhesive
resins (Table 6-1). The use of these adhesives depends on many factors including product end
use, manufacturing issues such as process emissions, worker safety, process adaptability, and
consumer safety (associated with product emissions).
6.1.1 Product End Use
The selection of an adhesive is dictated by whether the product is used for interior or exterior
applications. Composite wood used for exterior applications requires a waterproof adhesive such
as PF or MDI. Composite wood used for interior applications requires an adhesive that is
moisture resistant, but not necessarily waterproof. Most composite wood used in interior
applications is bonded with UF resins; UF resins are moisture resistant, however, they will not
withstand continuous cycles of wetting and drying and will begin to degrade at about 333 K and
60 percent relative humidity. Wood moisture content of 15 to 20 percent accelerates UF resin
degradation at temperatures lower than 333 K (Sellers et al., 1988).
6.1.2 Manufacturing Issues
Many manufacturing issues exist regarding the selection of an adhesive. The first issue is cost.
As seen in Table 6-1, UF resins are lowest in cost among the three resins, however, these
adhesives are limited to interior applications. For exterior purpose resins, the cost of MDI is
twice as high as that for liquid PF resins and almost one and a half times as high as that for
powdered PF resins on a solids basis. Based on price alone, manufacturers have no incentive to
select MDI adhesives. However, since the early eighties, one third of the OSB industry has
switched from using PF adhesives to MDI adhesives. According to the U.S. manufacturer of
MDI adhesives (ICI Americas), manufacturers have switched to MDI resins because of several
pollution prevention benefits associated with using them. The first advantage is reduced VOC
emissions associated with low temperature drying. As will be discussed in detail in Section 6.2,
MDI adhesives are capable of bonding wood flakes with a moisture content as high as 12
percent. Conventional PF adhesives are only capable of bonding wood flakes with a moisture
content of around seven percent. More dryer energy at a higher temperature is required to dry
56
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Table 6-1. Adhesives Commonly Used in Manufacturing Composite Wood
Adhesive resin
Characteristics
Typical
applications
Price solid
basis'
S per 1000 kg
Spread rangeb
Urea-formaldehyde
Phenol-
formaldehyde
MDI*
Hot setting and cold setting;
acid curing with heat and/or
catalyst accelerated fast
cure; cold-water resistant;
colorless; may emit
formaldehyde in use
Hot setting; normally cured
above 220°F (105°C);
usually highly alkaline for
rapid cure; waterproof; dark
in color
Hot setting; water and heat
accelerate cure, waterproof
under severe conditions;
neutral in color (press
release agent tans wood
surfaces).
Hardwood
plywood, PBd,
MDFe
Structural
plywood, and
OSBf
OSB
400
and
800 (liquid)
1100
(powder)
1600
220-317
(PB 5-8%)c
171-244
(plywood)
(OSB 3.5-6%,
liquid PF)
(OSB 2-3%,
spray-dried PF)
(OSB 2-4.5%,
PB 4-5 %)
Source: Sellers et al., 1988, and Sellers, 1994.
"March 1994; prices can vary by ± 25 % depending on type and quantity ordered.
bSpreads are shown on a weight-per-area basis (kg/1000 m2 of joint area) merely for comparison purposes.
CPB 5-8% denotes 5 to 8 percent resin solids applied to board on a weight basis
dPB = particleboard
eMDF = medium density fiberboard
fOSB = oriented strandboard
SMDI = methylene diphenyl diisocyanate
wood to seven percent versus 12 percent. Thus, because less dryer energy is required to dry
flakes suitable for MDI bonding, fewer VOC emissions may be generated (ICI, 1993).
Additional advantages to using MDI adhesives are lower press temperatures and shorter press
cycles, both of which may lead to reduced press emissions (ICI, 1993). In the past, a
manufacturing disadvantage in the use of MDI has been its characteristic of sticking to the
pressing equipment. For this reason, the development of release agents to prevent costly buildup
of the adhesive on the equipment has been an important factor in the increased use of MDI. Two
types of release agents can be used: those which are sprayed onto the surfaces of the pressing
equipment and those which are added to the resins (either at the resin manufacturing facility, or
at the composite wood plant).
57
-------
Another manufacturing issue which influences the selection of an adhesive is occupational
safety. MDI adhesives are an example of how occupational safety concerns factor into adhesive
selection. Although MDI adhesives may offer pollution prevention benefits in terms of
manufacturing emissions, some companies are opposed to using them for reasons such as worker
toxics exposure, potential acute impacts of possible spills, and inconsistency with toxic use
reduction objectives (Correl, 1994). Table 6-2 is a comparison of exposure limits and health
Table 6-2. Exposure Limits and Health Hazards Described in MSDS for MDI,
UF, and PF Wood Adhesives
Exposure Methylenediphenyl diisocyanate Formaldehyde
limits (hazard contained in MDI adhesives) (hazard contained in PF and UF
adhesives)
OSHA
PEL
ACGIH
TLV
NIOSH
REL
Health
Hazards
0.02 ppm ceiling
0.005 ppm TWA
0.005 ppm TWA;
0.020 ppm ceiling (10-min)
Irritating to eyes, respiratory system
and skin. Risk of serious damage to
respiratory system. May cause
sensitization by inhalation and skin
contact. Repeated inhalation of
aerosol at levels above the
occupational exposure limit could
cause respiratory sensitization. The
onset of respiratory symptoms may be
delayed for several hours after
exposure. A hyper-reactive response
to even minimal concentration of MDI
may develop in sensitized persons.
0.75 ppm TWA;
2 ppm STEL (15 min)
0.3 ppm ceiling; A2 (suspected human
carcinogen)
Carcinogen; 0.016 ppm TWA;
0.1 ppm ceiling
Skin absorption: no hazards to be
known to Borden (manufacturer);
Ingestion: no hazards to be known to
Borden; Inhalation: not expected to be
harmful under normal conditions of
use. However, if allowed to become
airborne, may cause irritation of nose,
throat and lungs; Skin: may cause
irritation on prolonged or repeated
contact; Chronic exposure: potential
carcinogen.
Source: ICI, 1995 and Borden, 1995.
Abbreviations used in this table are: OSHA = Occupational Safety and Health Administration; PEL = Permissible
Exposure Limit; ACGIH = American Conference of Governmental Industrial Hygienists; TLV = Threshold Limit
Value; NIOSH = National Institute for Occupational Safety and Health; REL = Recommended Exposure Limit;
TWA = Time-weighted Average; STEL = Short-term Exposure Limit; MSDS = Material Safety Data Sheet; PF =
Phenol-formaldehdye; UF = Urea-formaldehyde; MDI = Methylenediphenyl diisocyanate
58
-------
hazards associated with MDI and formaldehyde (formaldehyde is the chemical hazard associated
with use of PF and UF adhesives resins). As seen in Table 6-2, exposure limits to MDI are much
lower than exposure limits to formaldehyde. Manufacturers of MDI state that safe exposure
levels are obtainable through good engineering controls which include making sure that blenders
are sealed well and that the blending and forming areas are well ventilated (Galbraith, 1995).
6.1.3 Consumer Safety
Product emissions are another issue regarding adhesive selection. The most frequently measured
compound emitted from composite wood products is formaldehyde. One source of
formaldehyde is the UF adhesives that bond composite wood products such as PB and MDF.
Formaldehyde emissions from composite wood bonded with PF and MDI adhesives are
minimal (Graves, 1993). In 1984, HUD passed a standard requiring that all PB used in
manufactured homes emit less than 0.3 ppm formaldehyde under standard conditions. To meet
this standard, manufacturers of UF bonded composite wood switched to reformulated UF
adhesives and/or formaldehyde scavenging systems. Today, PB and MDF manufactures have
a voluntary standard of 0.2 ppm for all products. As progress is made to reduce formaldehyde
emissions associated with wood adhesives, research is focussing on other board emissions
associated with adhesives such as VOCs.
6.2 High Moisture Bonding Adhesives
Phenol-formaldehyde and UF adhesives cure by a condensation reaction, i.e., water is released.
Consequently, furnish that is bonded with conventional PF and UF adhesives must be dried to a
very low moisture content (a few percent) for these resins to cure properly. Switching to an
adhesive that is capable of bonding a high moisture furnish eliminates the need to dry wood to a
very low moisture content. Dryer energy and temperature can be reduced because less water
must be removed from the wood. Press temperatures can also be lowered since heat transfer is
more efficient in high moisture furnish (Phillips et al., 1991).
The pollution prevention advantage to using a high moisture bonding adhesive is the potential to
reduce VOC emissions from both dryers and presses. Figure 6-1 shows drying temperatures
required for specific furnish moisture contents. Significantly lower temperatures are adequate to
dry furnish to a 12 percent moisture content instead of a six percent moisture content. Because
VOC emissions increase with increasing dryer temperature, these emissions should be reduced
by drying at lower temperatures (Chelak and Newman, 1991).
Many OSB plants are switching to high moisture bonding adhesives with the primary goals of
reducing dryer emissions and possibly reducing wood drying costs (Phillips et al., 1991). Efforts
have been made to improve phenolic resin technology to allow better bonding in the presence of
water. Highly pre-reacted (advanced) PF adhesives and catalyzed PF resins are capable of
bonding OSB furnish with six or seven percent moisture content (Chelak and Newman, 1991).
An even better adhesive for bonding high moisture furnish is MDI. MDI adhesives do not
59
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1032
921
*
6 7 8 9 10 11 12 13 14
Furnish Moisture Content (%)
Figure 6-1. Dryer inlet temperature versus furnish moisture
content (Chelek and Newman, 1991). Reprinted with permission.
produce water when they cure so they are not inhibited if water is present. They can utilize the
moisture in wood to cure. As a result, MDI adhesives are capable of bonding OSB furnish with
moisture contents as high as 12 percent (Chelak and Newman, 1991).
The gluing of high moisture content wood has become an established practice in manufacturing
plywood (Phillips et al., 1991). The primary incentive for bonding high moisture veneer is a
reduction in adhesive consumption. As discussed in Section 7.3, gluing high moisture veneer
requires less adhesive than gluing low moisture veneer because there is less chance of dryout. In
the southern plywood industry where dryout is a problem, a dramatic reduction in glue
application rate has been achieved by gluing high moisture veneer; reduced dryer emissions and
savings in dryer cost have also been achieved as a result of gluing high mosiutre veneer (Phillips
et al., 1991).
6.3 Reformulated Urea-Formaldehdye Resins
As mentioned in Section 3.5.2.1, formaldehyde emissions are related to the F:U mole ratio in UF
resins; the higher the mole ratio, the higher are formaldehyde emissions and vice versa (Gollob,
1990). Switching to a low mole ratio UF resin is one way panel manufacturers can lower
formaldehyde emissions from the press and board (Figure 6-2). Often, however, the use of a low
mole ratio UF resin results in significant losses of resin efficiency or productivity. To avoid
these problems, many plants use formaldehyde scavengers in conjunction with their regular UF
60
-------
a.
Ou
c
o
C/5
E
u
•o
c3
o
PQ
2.S -
2 -
1.5 1-
0 5
Press emissions
Board emissions
1.45 1 35 1.25
21.52
cs
fS
16-14 3.
C/5*
C/5
o*
5
C/5
3
10.76
i 5.38
0.00
vo
3
3
cr
Resin Mole Ratio
Figure 6-2. Resin mole ratio vs. board emissions and press emissions (Outman, 1991).
Reprinted with permission.
resins to reduce emissions. Two common types of formaldehyde scavengers are urea scavengers
and scavenger resins. The basic principle behind their uses is essentially the same; both allow
the use of a high mole ratio UF resin with all its attendant benefits while achieving acceptable
emissions by scavenging the excess formaldehyde (Graves, 1993). Urea scavengers have been
widely used by North American panel manufacturers for a number of years. The urea is used as
a solution or as a dry chemical and is added to the raw material before pressing. Application
rates generally are 0.25 to 1.0 percent urea based on dry wood weight. These levels usually
result in formaldehyde reductions of 15 to 50 percent depending on many plant characteristics.
Figure 6-3 shows the effect on formaldehyde emissions of various levels of urea scavenger with
three different mole ratio resins; for the same level of urea scavenger, the emission reductions
were more dramatic with the higher resin mole ratios (Graves, 1990).
The use of scavenger resins, commonly called combi-blending, has recently become popular
among panel manufacturers as a way to reduce board and press emissions of formaldehyde.
With combi-blending, a normal UF resin is used in conjunction with an ultra-low mole ratio UF
resin to yield a composite mole ratio low enough to yield acceptable emissions. One advantage
to using combi-blending versus a urea scavenger is that, unlike the urea, the scavenger resin has
adhesive properties and contributes to bonding. Figure 6-4 shows the results from a plant trial
where resin substitution levels up to 30 percent were employed; the data show that formaldehyde
concentrations were reduced almost 50 percent at the 30 percent substitution level (Graves,
1993).
61
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4>
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"cS ^
|s
o
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exi
F/U Ratio
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Scavenger level
Figure 6-3. Effect of urea scavenger on formaldehdye emisions when used with various
mole ratio UF resins (Graves, 1990). Reprinted with permission.
J3
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to 3.
Percent substitution
Figure 6-4. Formaldehdye emissions results from panels made during a production trial of
scavenger resin (Graves, 1993). Reprinted with permission.
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6.4 Naturally Derived Adhesives
The three major adhesives used in the wood panel industry (UF, PF, and MDI) are all
synthesized from petroleum-derived chemicals. The wood panel industry consumes about
25 percent of the total U.S. adhesives production (Koch et al., 1987). Over the past decade, the
price of the raw materials used in these adhesives has been growing rapidly. The price of phenol,
for example, has more than doubled since 1985 (in 1990 it was $0.99/kg compared to $0.42/kg in
1985). The price of methanol (the basic raw material used to produce formaldehyde) jumped
from $0.18/liter (1) in June of 1994, to $0.34/1 in October 1994 (Wellons, 1994). The rapid price
increases are a result of increased competition for these raw materials.
The major competition for phenol has come from demand for bisphenol A and caprolactam.
Bisphenol A is the feedstock component for polycarbonates which are used to produce
automotive plastics, compact discs, and computer discs. Caprolactam is a basic raw material for
the production of Nylon 6™ which is used in the fast growing stain resistant carpet market.
Phenol demand for both bisphenol A and caprolactam is expected to exceed PF resin by 1995
(White, 1990). In addition to causing higher prices for PF adhesives, the demand for phenol is
reaching world capacity. In 1988, the consumption of phenol equaled 96 percent of the world
capacity (White, 1990).
The increase in demand for methanol has been fueled both by general economic improvement in
the U.S. and abroad, and most importantly, by the growth of methyl tertiary butyl ether (MTBE)
as an oxygenate and octane booster in gasoline (Wellons, 1994). The MTBE market is
forecasted to grow from the 1993 level of 4400 million kg to over 8000 million kg in 1997
(Wellons, 1994). Other major methanol markets, formaldehyde and acetic acid, are forecasted to
grow equal to or above the rate of the gross national product (Wellons, 1994).
The significant price increases and potential scarcity of raw material supplies for UF, PF, and
MDI adhesives have created interest in renewable raw material sources. Research and
development funds are being expended by chemical raw material suppliers, forest products
companies and wood adhesive/binder suppliers to search for renewable raw material sources to
replace entirely, or least partially, petroleum-derived chemicals in the manufacture of wood
adhesives. Some of the current research is presented below. Naturally derived adhesives are
included in this report as pollution prevention because of their potential to use renewable
resources, which in many cases are by-products of other processes. An effort was made to
collect information on availability, cost, properties, composition, environmental effects, and
stage of product development for each product. Any information found on these topics is
included with each adhesive. However, for most of the adhesives, the above information was not
found.
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6.4.1 FAREZ® Resin
FAREZ® resin is manufactured from furfuryl alcohol, a derivative of corn. The resin is being
evaluated at the University of New Brunswick, in Canada, as an alternative low VOC binder to
substitute for PF resins used for glass fiber insulation products and wood products (Rude, 1994).
6.4.1.1 Environmental Effects
Unlike PF resins, the FAREZ® resin is storage stable at ambient temperatures, without
refrigeration. As delivered, the FAREZ® resin contains very low amounts of volatile
components. Upon curing, especially heat curing, the FAREZ® resin liberates 80 to 90 percent
less total VOC emissions and 80 to 90 percent less emissions of materials on the HAP list (Rude,
1994). The FAREZ® system offers the same relative speed of cure as the PF resin systems.
Various acidic catalyzation options are available that allow either ambient temperature or heat
curing capability.
6.4.1.2 Availability
The FAREZ® resin is manufactured by the Quaker Oats Company and is currently in the
experimental stage of development. The insulation and wood products industries have shown
little interest in the resin because of its high cost; the cost of the resin is twice that of a PF resin.
However, cost analyses have been performed for the insulation industry which show that using
the resin to meet future HAP standards would be cheaper than purchasing and operating control
devices such as scrubbers. The same is likely to hold true for the wood products industry (Rude,
1994).
6.4.2 Methyl Glucoside
Plywood glue contains PF adhesive resins. Conventional PF resins typically have formaldehyde
to phenol (F:P) ratios ranging from 1.7 to 2.1. Lower F:P ratios are not used because the cure
rate is too slow. Higher F:P ratios are not used because the cure rate is too fast which causes the
glue to dry out before the veneer is pressed together. Higher F:P ratios also lead to high levels of
residual formaldehyde that can be emitted during curing as well from the product during use
(Drury et al., 1990).
PF resins are synthesized in regional plants and shipped to individual plywood mills. At the
mills, the PF resins are combined with proteinaceous extenders, lignocellulosic fillers, and
caustic to make a glue mixture. The extenders, fillers, and caustic are added to modify the
viscosity of the adhesive to be compatible with the method of glue application (curtain, roll,
spray, foam); allow better distribution of the adhesives; faster cure; and lower costs. A typical
proteinaceous extender is soft wheat flour; a typical lignocellulosic filler is pecan flour (Sellers et
al., 1988).
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Researchers at the University of Illinois have been investigating the use of methyl glucoside
(MeG), a corn-derived monosaccharide, in the manufacture of plywood adhesives. Researchers
have incorporated MeG into plywood adhesives as both an additive and as a partial replacement
for phenol in PF resins used in plywood adhesives.
MeG can be incorporated into plywood adhesives either by adding it to the glue mix used at a
mill or cooking it into the PF resins manufactured at regional plants. At the mill, MeG can be
added to the glue mix as an additional ingredient or it can replace part of the PF resin in glue
mix. (Details about the incorporation of MeG into glue mixes is published in Sellers and
Bomball [1990].) Laboratory studies and large-scale mill trials indicate several positive
properties from the incorporation of MeG into glue mixes including lower adhesive cost, better
dry-out resistance, and better flow properties of the glue. The improved dry-out resistance
allows spread reductions of up to 12 percent, resulting in significant cost savings (Drury et al.,
1990).
A more direct and easier way to incorporate MeG into plywood adhesives is to cook it into the
PF resins which are manufactured at regional distribution plants. The modified PF resins can
then be distributed to individual plywood mills, as opposed to attempting to optimize MeG in
every plywood mill's glue mix. Laboratory studies have shown that MeG can replace up to 50
percent of the phenol in a base 1.7 to 2.1 F:P mole ratio resin, resulting in a resin with a F:P mole
ratio of up to 4.2. Wood failure and tensile strengths comparable or superior to conventional
resins were achieved with no increases in free formaldehyde or loss in storage stability (Drury et
al., 1990).
One pollution prevention benefit of adding MeG to PF resins is that the modified resin can bond
high moisture veneer (veneer with 9 percent moisture was bonded in the above studies). As
discussed in Section 8.4, the pollution prevention advantage to using an adhesive that is capable
of bonding high moisture wood is the potential to reduce VOC emissions from both dryers and
presses. Another pollution prevention benefit of adding MeG to the PF resin is that phenol
consumption is reduced, which in turn may reduce phenol emissions at the press (emissions data
were not found in the literature to confirm this).
6.4.2.1 Future Availability
MeG is made from corn. As the drought of the summer of 1988 demonstrated, even when yields
of corn are almost halved, there remain corn surpluses sufficient for all current needs (Drury et
al., 1990). In normal years, American farmers, produce nearly twice what is needed in the U.S.
or can be exported (Drury et al., 1990).
The incorporation of MeG into PF resins (the easier and more direct way of incorporating MeG
into plywood adhesives) has yet to be scaled-up to resin producers/plywood mills. But if the
modified resins can be demonstrated on a plant scale, incorporation of MeG into plywood
adhesives will proceed at a much faster pace than at present. The long term goal after successful
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commercialization of this technology in the plywood industry is to extend this approach to other
structural panels such as OSB. OSB panels use a much higher amount of PF resin adhesive
because of the higher surface area of the wood strands being glued compared to plywood. The
use of MeG in the OSB industry could lead to greater cost savings and pollution prevention
benefits through reduction of adhesives use.
6.4.3 Lignin Adhesives
Lignin is an aromatic polymer made up from phenylpropane units. Lignin makes up one of the
three major components of wood, along with cellulose and hemicellulose. In chemical paper
processes, such as kraft pulping, lignin is transformed into a soluble form and removed. In 1980,
an estimated 20 billion kg of lignin (in the form of spent liquor) were produced from kraft
pulping in the U.S. Only 40 million kg (0.20 percent) of this lignin was recovered from the spent
liquor and used in manufacture of by-products (Zhao et al., 1994).
6.4.3.1 Adhesive Utilization
Lignin can be used as a feedstock for aromatic-based chemicals. Consequently, the abundance of
lignin as a waste product in pulp mills has made it a desirable raw material alternative to
nonrenewable petroleum-derived chemicals in the production of wood adhesives. Research into
utilizing lignin for adhesive production has been ongoing for years. A large portion of the
research has focused on the substitution of lignins for PF adhesives. While 100 percent lignin
substitution has yet to be achieved, there has been industrial success with partial substitution of
lignins into PF adhesives. Substitution of 20 to 30 percent of kraft lignin into PF resins is
common in plywood applications. Until recently, no more than 20 to 30 percent of lignin could
be substituted into PF resins because cure times increased as the amount of lignin increased.
Another drawback has been that lignin adhesives have low cross-linking and strength. However,
recently, a new approach has been developed that can substitute large amounts of kraft lignin for
PF adhesives, while actually increasing the cure speed, as well as board strength (Stephanou and
Pizzi, 1993). According to research published by Stephanou and Pizzi (1993), a different, and up
to now, unknown chemical reaction mechanism has been developed to give "truly competitive
kraft lignin-based wood adhesives of good exterior grade performance." The new approach,
which utilizes MDI resins, has been demonstrated on a lab scale level; exterior grade PB panels
with excellent strength properties were prepared from adhesives containing 46 to 63 percent kraft
lignin resin solids, at press times much faster than those used for traditional synthetic PF resins
(Stephanou and Pizzi, 1993). The results of this research "...clearly indicate that excellent lignin-
based adhesives for exterior grade panel products can be manufactured. These adhesives have
none of the drawbacks which have been the main stumbling blocks in the effective industrial
utilization of lignin for panel binders, namely low cross-linking and strength, and particularly,
far too slow pressing times. The adhesives produced gave panels of excellent strength, good
durability and very fast press times" (Stephanou and Pizzi, 1993).
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6.4.3.2 Availability
Currently, Westvaco is the only company in the U.S. that operates a commercial lignin extraction
facility. In early 1990, the selling price of kraft lignin was approximately $0.71/kg to $0.75/kg,
and the selling price of phenol was approximately $0.99/kg (White, 1990). Although an
adhesive manufacturer can purchase kraft lignin at a cheaper cost than phenol, additional
processing is required to make the lignin appropriate for adhesive production; the additional
processing results in the cost of lignin being equal to that of phenol (White, 1990). However, if
the price of phenol continues to rise and exceed the total manufacturing cost of lignin, and
assuming sufficient quantities of extracted commercial lignin could be made available, lignin
could compete as a partial raw material coreactant with phenol in PF wood adhesives (White,
1990). For there to be sufficient quantities of extracted lignin, additional lignin extraction
facilities must be built. The capital cost of a new commercial lignin extraction facility compared
to the capital cost of a new phenol plant is estimated to be almost equal per kg of product
produced.
6.4.4 Tannin
Tannin refers to a widely occurring group of substances derived from plants. In their most
common usage, these substances are capable of rendering raw hides into leather. Common
tannin or tannic acid is found in a variety of nuts, tea, sumac, and bark. It is a complex, dark
polyhydroxy PF compound.
As a bark product, tannins are readily available. Large quantities of bark are available at mill
sites where bark is a waste product of sawmilling and other panel making processes. On average,
bark contains 15 to 30 percent polyphenols, 20 to 30 percent lignin, 30 to 45 percent
carbohydrates and 1 to 3 percent fats and waxes (Koch et al., 1987). These values are dependent
upon wood species, location, age, and exposure.
6.4.4.1 Adhesive Utilization
The use of tannin adhesives for the manufacture of exterior grade weatherproof PB has been
gaining increasing industrial and technical acceptance during the last twenty years (Pizzi, 1983;
Pizzi, 1989). Until recently, all industrial formulations have been based on tannins such as
wattle (mimosa extract of commerce) and quebracho (Pizzi 1983; Pizzi, 1989). In countries
where these tannins are produced, wattle and quebracho tannins have been progressively
displacing synthetic PF adhesives for manufacturing exterior PB, due to their lower cost and
excellent performance (Pizzi 1983; Pizzi, 1989). However, the total worldwide production of
these two tannins is only 150 million kg per year. This is coupled with the fact that the tannin
producers do not make more than 20 to 30 percent of their total production available for
adhesives application (the rest being reserved for their traditional market, leather). For these
reasons these materials, although industrially and commercially very successful, have had little
chance to influence the PF adhesives market.
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A tannin that can easily be extracted and rendered available in most countries is pine tannin,
extracted from the bark of the most diffuse worldwide forestry crop, pine trees (Pizzi et al.,
1993). Significant amounts, well into the billions of kg, of this tannin could be rendered
available if a successful wood adhesive system based on it could be proven industrially (Pizzi et
al., 1993). Recently in Chile, a small industrial producer of native and sulphated pine tannins
has developed an adhesive consisting of MDI, tannin, and formaldehyde for exterior grade PB.
Industrial results of extended plant production runs of this adhesive system showed "very
encouraging results" (Pizzi, 1993).
7.0 REDUCING ADHESIVE CONSUMPTION
In the plywood industry, spray-line layup systems are commonly used to apply adhesives. These
systems generate a lot of overspray. Although most of the overspray is collected and reused,
some waste is generated. Applying adhesives more efficiently and/or reducing the amount of
adhesive required in the panel can reduce this waste. As discussed throughout Section 3.5.2,
reducing the amount of resin in the panel may also reduce press emissions.
7.1 Foam Extrusion
An efficient way of applying adhesive to veneer is by foam extrusion (a typical foam adhesive
mix for gluing plywood is shown in Table 7-1). In the extrusion process, strands of glue are
deposited precisely on the panel thus eliminating over application (Figure 7-1). The layup line
consists of multiple extrusion stations similar to the one in Figure 7-1. At each station, mixed
glue is pumped from a central storage tank (not shown in Figure 7-1), into a holding tank. From
the holding tank, the glue is pumped at a controlled rate to the foaming unit. The foamed
adhesive is then forced under pressure to the extrusion head. The extrusion head converts the
large incoming stream of foamed glue into 160 small round streams through a series of branches.
The extrusion head consists of three plastic plates: an intake plate, a center plate which contains
Table 7-1. Typical Foam Adhesive Mix for
Gluing Plywood
Water
792 (kg)
Blood
82
Wheat Flour
204
Caustic Soda
61
Resin (43% solids)
1588
Surfactant
4
Total
2731
Source: Nylund, 1985.
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unfoamed
LIQUID
GLUE
extrusion
HEAD
FOAMER
METER
Figure 7-1. Flow diagram of foam extrusion apparatus (Cone, 1969).
Reprinted with permission.
the 160 extrusion orifices spaced about 21 mm apart, and a defoam plate. If the line stops, the
glue is diverted to the defoam plate. Inside the defoam plate are branching channels through
which the glue flows into a central stream to a defoamer. The defoamed glue is sent back to the
holding tank where it mixes with fresh incoming glue. From there, it will be refoamed and sent
back to the extruder when the line restarts. Sheets of veneer are laid up either by hand or by
vacuum system. After the last sheet is placed on the panel, the panel passes through a mashout
press.
The mashout press consists of two rolls that apply pressure to the laid up panel to flatten out the
glue strands, providing glue coverage between the strands and intimate contact between the glue
and both veneer surfaces. The panel then travels to the prepress and hot press as in a
conventional lay up line.
In addition to reducing adhesive waste, foam extrusion also reduces the chance of dryout. When
wet glue comes into contact with dry veneer, the water in the glue migrates into the veneer,
which causes a loss of fluidity in the glue. The drier the veneer, the faster is the migration of the
water. If too much water migrates into the wood, the glue film no longer has sufficient fluidity
to make intimate contact under pressure with the opposing veneer surface, resulting in poor
bonding. This condition is referred to as dryout. In spray-line applications, dryout is prevented
by increasing the amount of adhesive applied to the panel. Applying excess adhesive is not
necessary in the foam extrusion process. Foaming the glue and applying it in the form of rods,
slows down the migration of water, thereby impeding dryout. Foaming, by increasing the
thickness of the glue layer, forces the water contained in the glue to travel farther in order to
reach the wood. Depositing the foamed glue onto the veneer surface in the form of rods that are
spaced apart reduces the area where glue and wood make contact and therefore, reduces the
opportunity for water to migrate.
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The combination of less waste and lower glue application rates when using foam extruders can
reduce adhesive costs by 20 to 31 percent, depending on the type of equipment being compared
to the foam extruder (Nylund, 1985). In 1985, seven plywood plants were using the foam
extrusion process. Foam extrusion appears to be the future of adhesive application in the
plywood industry (Nylund, 1985).
7.2 Variable Glue Spread for Veneer Layup
The amount of adhesive/glue required to bond veneer varies with moisture content. For example,
high moisture veneer requires less glue than low moisture veneer because there is less migration
of water from the glue into the veneer. Although the moisture content of veneer varies at a
plywood mill, glue is applied at a constant rate to prevent dryout of low moisture veneer. As a
result, an excess amount of adhesive is applied to high moisture veneer. When this happens, the
glueline contains too much moisture. As a result, the glue penetrates too deeply into the veneers
when they are pressed together and a poor bond forms because not enough glue remains between
the veneers. This condition is referred to as washout.
In a study by Faust and Borders (1992), variable and constant glue rates were used to bond
plywood and bond quality was compared. The variable application rate strategy (VARS)
adjusted the glue application rate for each individual plywood panel according to its moisture
content. For instance, when conditions for dryout were present, the glue application rate was
increased to provide additional moisture to the glueline and increase glue penetration during hot-
pressing. When conditions for washout were present, a lower glue application rate was used to
reduce the moisture in the glueline and retard glue penetration during hot-pressing. Essentially,
the application rate was adjusted to control the rate of penetration of glue into the wood
substrate. With constant rate application strategy (CARS), the glue application rate was held
constant for all veneer moisture contents.
The Faust and Borders study investigated the effect of VARS on bond quality and glue mix
consumption in manufacturing southern pine plywood. When compared to CARS, VARS
improved overall wood failure by 8.3 percent while reducing glue consumption by more than
13.1 percent (see footnote for definition of wood failure).4 The broad range of application rates
used by VARS was effective in reducing dryout problems. VARS performed extremely well in
conditions that normally induce dryout and in extreme conditions of washout.
Process-sensing and control technology has been developed for the practical application of
VARS. Sensor technology is currently available for on-line measurement of veneer moisture
4 Wood failure is a measure of how well glue bonds to veneer. To measure wood failure, two pieces of bonded
veneer are pulled apart. The bond is good when the separation damages the surfaces of the veneers, i.e., the separation
causes wood failure; the bond is poor when the two pieces of veneer pull apart without any damage to their surfaces.
The higher the damage to the veneers when pulled apart, the higher is the wood failure rating, which is reported as a
percentage. The APA sets a minimum of 70 percent wood failure for plywood.
70
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content and temperature. The accurate on-line adjustment and measurement of application rate
has also been investigated and has been commercially implemented for spray applicator systems.
From a manufacturing standpoint, the greatest benefit of implementing VARS is to compensate
for problem bonding conditions that occur unexpectedly during production, i.e., when there is a
shortage of dry veneer and hot veneer is used directly on the layup line causing dryout problems;
when high moisture content veneer is used in a veneer shortage situation causing washout or
steam blows; when there is a shortage of properly conditioned peeler block and rough veneer
results; when crews are unaware of problem bonding conditions due to lack of experience; and
when an unexpected shutdown of the hot-press results in long assembly times. From a pollution
prevention standpoint, the greatest benefit of VARS is a reduction in adhesive consumption, and
consequently, a reduction in plant emissions.
8.0 PROCESS MODIFICATIONS
8.1 Low Temperature Drying
8.1.1 Recirculation of Dryer Exhaust
As discussed throughout Section 3.4, high dryer emissions of TGNMO and formaldehyde are
associated with high drying temperatures. Process modifications that will lower the required
dryer temperature should lower these emissions. As discussed in Section 6.2, the use of high
moisture bonding adhesives is one way to lower dryer emissions. Another option for lowering
dryer temperatures is to recirculate the dryer exhaust. Figure 8-1 shows the relationship between
the dryer inlet temperature and the amount of water removed from dryers at four reconstituted
panel mills (NCASI, 1986c). At high water removal rates (greater than 0.80 kg of water per
pound of product), the dryers represented by the solid triangles and open squares operated at
lower inlet temperatures than the dryers represented by the solid squares. The dryers represented
by the solid triangles operated at low inlet temperatures by diluting the inlet gas stream with air.
The dryers represented by the open squares operated at low inlet temperatures by recirculating
exhaust gas to the burner outlet to cool the gases entering the dryer. Although both methods
lowered the inlet temperature, the dryers which used dilution air required a thermal input ranging
from 3,527 to 4,630 British thermal units per kilogram of water removed (Btu/kg H20). The
dryers that recirculated their exhaust stream required about 2,866 Btu/kg H20. Thus,
recirculation of exhaust gas is the most efficient way to lower inlet dryer temperatures.
8.2 Steam Injection Single Opening Press
As mentioned in Section 5.1, the CanFibre company is planning to build nine MDF plants in
North America that will use 100 percent post-consumer waste and PF adhesives. CanFibre's
plant process will be similar to traditional MDF processes, in that wood will be refined, dried,
combined with adhesive, and pressed. However, there will also be unique components of the
process that have potential pollution prevention benefits.
71
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Figure 8-1. Dryer inlet temperatures as a function of the water removed per pound dry
product (NCASI 1986c). Reprinted with permission.
A typical CanFibre plant will receive pre-sorted wood waste that will be free of most brick,
concrete, stucco, and gypsum. A primary crusher will break down the wood into 251 mm square
chips. The chips will be fed onto a vibrating feeder and go through a series of metal detectors
and rock traps. A secondary crusher will break the wood chips into chips approximately 25 mm
square by 6.35 mm thick. The chips will be once more screened by magnets before being
washed to remove any remaining grit (Duncanson Investment Research Inc., 1993).
After cleaning, the wood chips will be processed through a high pressure steam digester and a
double disc refiner (similar to traditional MDF plants). The refined fiber will be dried to
approximately 4 to 6 percent moisture content. The fiber will be blended with PF resins in a high
pressure steam conditioning blender. Combining the resin in this manner will result in 50
percent less adhesive being added to the board than is typically required in a standard MDF
process (Duncanson Investment Research Inc., 1993). As a result, the resin content in
CanFibre's MDF panel will be less than 6 percent by weight as compared to an industry average
of 10 percent (Duncanson Investment Research Inc., 1993).
In standard MDF presses, adhesive coated fiber mats are "sandwiched" between hot plates called
platens; the heat from the platens transfers gradually from the panel surface towards the center.
The combination of heat and pressure sets off the curing of the adhesive. The heat transfer takes
approximately 5 minutes for a 19.05 mm panel. The CanFibre process will use a type of press
that heats panels by injecting live steam into them. The CanFibre press, recently developed by
Forintek Canada Corporation, consists of a pressure bar mounted to the upper press platen, which
compresses and seals the edges of the panel. Orifices located on the underside of the upper
72
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platen intermittently inject steam into the panels and then vacuum the steam out. As the steam
permeates through the panels, it drives off the moisture in the wood and adhesive. Because the
curing process of the CanFibre press involves both mass and heat transfer (the curing process of
a hot platen press involves only heat transfer), cure time using the CanFibre press is half that of a
standard MDF press (Duncanson Investment Research Inc., 1993). Also, because the heat
transfer using the Forientek press works from the interior of the panel to the surfaces, the cure is
more uniform. (A major disadvantage with a hot platen press is surface overcuring, especially
with thicker panels.)
In the U.S., the majority of PB and MDF are bonded with UF adhesives. UF adhesives are used
to bond these panels instead of PF or MDI adhesives for various reasons. As discussed earlier,
most PB and MDF are used for interior purposes, thus waterproof adhesives such as PF and MDI
are not necessary. Although PF and MDI adhesives could be used to bond PB and MDF panels,
UF adhesives are substantially cheaper than these adhesives (see Table 6-1) and they also cure
faster and at lower temperatures than PF adhesives. Because the Forintek steam press reduces
panel curing time by more than 50 percent, the CanFibre process will be able to use slower
curing PF adhesives at a cost comparable to using UF adhesives (Duncanson Investment
Research Inc., 1993). Manufacturing MDF panels with PF adhesives will also allow the panels
to be used in exterior applications.
Production costs for the proposed CanFibre mills are projected to be 35 percent lower than those
of Medite Corporation - one of the leading MDF producers in the U.S. (Duncanson Investment
Research Inc., 1993). Significant cost savings will be from a negative wood cost, i.e., waste
companies will pay CanFibre to dispose of construction wastes, waste paper, and other dry
wastes. Significant savings will also be from low transportation costs. Most of the existing
MDF plants are remotely located and the cost of hauling wood waste back to these mills is high.
The proposed CanFibre mills will be located in the "urban" forest - close to post consumer waste
and close to end customers. On a delivered basis, CanFibre's costs are projected to be 45-50
percent lower than those of its competitors (Duncanson Investment Research Inc., 1993).
Many pollution prevention benefits are associated with the future CanFibre process.
Manufacturing emissions may be reduced (compared to a traditional MDF plant) due to: (1) the
use of PF resins, (2) lower resin usage, and (3) lower energy usage. Emissions may also be
reduced from consumer products (versus products made with UF-bonded MDF) (Duncanson
Investment Research Inc., 1993). In terms of recycling, the entire wood supply for the CanFibre
process will consist of construction wastes, waste paper, and other dry wastes which would
otherwise be landfilled.
8.3 Shelter for Raw Materials
Dry wood residues such as planer shavings from kiln-dried lumber, plywood trim, etc., require
little heat to reduce their moisture content to that required for panel manufacture. The energy
savings are lost, however, if the dry residues are stored outside and get wet. Consequently,
73
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shelters should be used to prevent the wood from getting wet. Even green residues should be
kept under a shelter to allow air drying. It has been shown in the asphalt industry that air drying
the aggregate to reduce the moisture content from 6 to 4 percent can reduce fuel consumption by
19 percent and increase production capacity by 31 percent (Foster, 1987). Protecting the
aggregate from getting wet has much the same effect (Foster, 1987).
8.4 Conveyor Belt Drying
Rotary drum dryers are used in the OSB industry to dry strands. Rotary dryers are typically
characterized by high drying air temperatures, short retention times, and aggressive handling of
strands (Teal, 1994). The adverse effects of these characteristics include the generation and
emissions of various airborne pollutants, and strand damage. Consequently, attempts have been
made to utilize low-temperature air in rotary dryers. However, one limitation to this change has
been the fact that as temperature is reduced, retention time and/or air flow must be increased to
maintain the same evaporative load. In a rotary dryer, the air flow serves two purposes: to
remove the evaporated water, and to move the product through the drum. Therefore, it has been
difficult to increase retention time in order to compensate for reduced drying air temperatures
(Teal, 1994). Although mechanical devices have been used in attempts to retard the flow of
strands through the drum, these too have met with limited success, and often contribute to strand
breakage (Teal, 1994).
Conveyor dryers offer an alternative method to drying OSB strands as well as other reconstituted
wood materials. A conveyor dryer is typically constructed of sections of perforated plate, linked
by continuous hinges, and attached to carrier chains. The bed moves continuously through the
dryer and may be driven at varying speed, thus allowing for precise control of retention time of
product within the dryer.
The drying air may be heated by various means. Natural gas or oil may be used to fire a burner,
over which air is passed. Steam or thermal oil coils also frequently serve as a source of heat.
Exhaust gases from a furnace may also be used and blended with ambient air to control drying
air temperature.
Fans pull or push the drying air through the dryer conveyor and the product on it. As the heated
air passes through the wet product, it picks up moisture, and cools. The "spent" drying air may
be exhausted entirely, or partly recirculated to conserve thermal energy. Dampers are typically
used to determine the portion of air recirculated.
According to Benny Teal of TSI, conveyor drying provides several pollution prevention benefits
when compared to rotary drying.5 The first pollution prevention benefit is lower exhaust volume.
5TSI is located at 115 Second Ave. N., Edmonds, WA 98020/206-771-1190. TSI works with conveyor dryer
manufacturer Proctor & Schwartz, which is headquartered in Horsham, PA. and has a manufacturing facility in
Lexington, NC.
74
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The volume of air exhausted from a conveyor dryer using thermal oil as a heating medium is
generally one-half to one-third of the volume exhausted by a conventional three-pass rotary dryer
operating at the same capacity. The lower exhaust volume reduces the capacity of any
equipment which might be required to treat the spent air prior to release to the atmosphere.
Another benefit to using a conveyor dryer is its ability to adjust to variations in flow rate, wood
species and moisture content (Teal, 1994). With rotary dryers, temperatures and air flows are
high and retention times are short, offering little opportunity for variation and therefore
adaptability to changes in demands on the dryer (Teal, 1994). Rotary dryers are slow to respond
when mass flow rates vary or when evaporative load changes quickly and significantly (Teal,
1994). In contrast, conveyor dryers can independently vary temperature, retention time, and air
flow. With a conveyor dryer, it is feasible to stop the dryer with partially-dried product inside,
then restart several minutes, even hours later.
A third benefit associated with conveyor dryers is that they are capable of operating at
temperatures below 478 K. According to Teal, the low operating temperature of the conveyor
dryers reduces VOC emissions (Teal, 1994). An article by Teal presents emissions data from
recent VOC tests conducted on full-size and lab-size conveyor dryers. Measurements were made
at several operating temperatures. Based on the data, VOC concentrations decrease as
temperatures decrease, however, the changes are not linear, implying a diminishing reduction in
VOC as the temperature drops to very low levels (Teal, 1994). Additionally, lower drying
temperatures result in longer drying times, so that VOC concentrations alone are not directly
indicative of the comparative benefits of changes in drying temperature (Teal, 1994). The VOC
data in the article were not compared to VOC concentrations from rotary dryers. Consequently,
the magnitude of reduction in VOC emissions from conveyor dryers (compared to rotary dryers)
could not be determined.
Proctor & Swartz, a manufacturer of conveyor dryers, was contacted for a rough cost estimate of
a conveyor dryer for the OSB industry. The contact at Proctor & Swartz was unable to estimate
the cost of a conveyor dryer or give any indication of the relative cost between a conveyor dryer
and a rotary drum dryer. The contact stated that estimating the cost of a conveyor dryer required
very specific information such as the inlet and outlet moisture content of the strands and strand
throughput.
8.5 Three-Pass High Velocity Rotary Drum Dryer
The following is an excerpt from an EPA report (Vaught, 1990). The excerpt describes the three
pass high velocity dryer (3PHV) manufactured by Productization, Inc., located in Independence,
Kansas. The 3PHV is a new type of OSB dryer which may generate fewer emissions than
conventional OSB dryers.
Another critical cause of blue haze is overloading a dryer by attempting to remove too much
moisture within a given time. Overloading results in the introduction of green material to a
high-temperature flame or gas stream causing a thermal shock that results in rapid and
75
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excessive volatizing of hydrocarbons that condense upon release to ambient air, causing the
characteristic blue haze.
The conventional three-pass dryers is a rotating cylindrical drum that consists of three,
concentric interlocked cylinders. Hot gases enter the innermost cylinder with the wet wood
chips and progress through the intermediate and then the outer drum shells in a serpentine
flow path while pneumatically conveying the wood chips through the dryer.
In a conventional three-pass dryer, the velocity of the air slows through the second and third
passes, allowing larger particles to settle out and smaller particles to pass through; however,
this is not the case at high material flow rates. Larger settled particles will interrupt the flow
of the smaller, faster moving particles with the result being that all particles are traveling at a
rate determined only by the forward velocity of the larger particles. When smaller particles
are held at these slower velocities in the second and third passes for a prolonged period of
time, volatilization of their surfaces occurs, which results in the formation of hydrocarbon
and carbon monoxide emissions. Should plugging occur in the second or third passes due to
the material dropping out of suspension, elevation of particle surface temperature to their
flash points will result in combustion.
The conventional three-pass dryer is a rotating cylindrical drum that consists of three,
concentric, interlocked cylinders. Hot gases enter the innermost cylinder with the wet wood
chips and progress through the intermediate and then the outer drum shells in a serpentine
flow path while pneumatically conveying the wood chips through the dryer.
The 3PHV rotary dryer, shown in figure 8-2, like the conventional three-pass dryer, is a
rotating cylindrical drum consisting of three, concentric, interlocking cylinders. In the 3PHV
dryer, hot gases enter the outermost cylinder with the wood chips and progress through the
intermediate and then the inner drum shells in a serpentine flow path. This flow path
direction is the opposite of that in the conventional three-pass dryer (see Figure 8-2). The
reason the 3PHV should reduce emissions is described below.
A determinant in establishing a saltation (conveying) velocity for a particle is the relationship
between the density of the particles to the density of the supporting gases. As wet particles
dry, their density decreases, and as gases cool (due to the transfer of heat), their density
increases. When the density of the particle decreases sufficiently due to its drying, and the
density of the cooling gases increases sufficiently, the aforementioned relationship
determining an individual particle's saltation velocity dictates that the saltation velocity will
decrease. If the gas velocities are greater than the particle's saltation velocity, the particle
will be pneumatically conveyed. Gas velocities in the primary and secondary passes of the
3PHV are not capable of supporting (pneumatically conveying) any but the least dense
(driest) particles. The denser (still moisture laden) particles will undergo a showering action
in these passes while being propelled not at conveying velocities, but at velocities determined
by a combination of drag and gravity forces.
76
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Material
Inlet
Hot
Gases
=0
Material
Inlet
Hot A~~V
Gases
\\
>1
\
(/
— J
\
'
) -
\
X
J
1
/
Material
Exit
¦=>>
Productization, Inc. 3PHV drum dryer
\
U
Material
Exit
<=0
Conventional triple pass drum dryer
Figure 8-2. Flow comparison of conventional triple pass dryer and 3PHV drum dryer.
(Vaught, 1990)
Because small particles have a higher surface area in proportion to their mass, moisture is
more rapidly evaporated from their surfaces. Also, heat and, therefore, temperature gradients
traverse to the center of smaller particles more rapidly than larger ones. As the moisture is
evaporated from the particle, the particle density is reduced. Once the saltation velocity of
the particle is reduced below the prevailing gas velocity, the particle is picked up in the gas
stream and conveyed through the remaining drums.
Larger particles are retained in the outer drum where they undergo showering action and are
subjected to turbulent airflow. Once the moisture of these larger particles is reduced to the
desired moisture content, particle densities are likewise reduced, which allows them to reach
their own saltation velocities. Large particles will reach their respective saltation velocities in
either the outer (first) pass or intermediate (second) pass depending upon their size, weight,
and moisture content.
In the first pass, the 3PHV dryer allows smaller, dried particles to pass through the slower
moving mass of larger, wetter particles in an area bounded by the outer and intermediate
drum cylinders, which is much larger than the area of the inner drum of conventional triple-
pass dryers. As the larger particles are dried, they will "catch up" with the smaller faster
77
-------
moving particles in an area bounded by the intermediate (second pass) drum cylinder. Here,
air flow velocities become high enough to convey the entire mass of particles out of the
drying portion of the drum and into the inner (third pass) drum cylinder where they will be
conveyed out of the dryer.
In summary, as particles dry, they approach their saltation velocity. As they reach saltation
velocity, it is important to provide a gas velocity sufficient to pick up the particle and
pneumatically convey it out of the drying environment. This action prevents the product
from reaching temperatures in excess of the wet bulb temperature, thus reducing carbon
monoxide and hydrocarbon emissions associated with pyrolysis and combustion of the wood
chips.
78
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Appendix A. Metric Conversions for Cited Text
Velocity
Temperature
Length
Pressure
feet/
minute
meters/
second
Fahrenheit
Kelvin
feet
meters
inches
milli-
meter
pounds per
square inch
kilo-Pascal
Sections 2.1.1 through 2.1.5
300
2
200
366
120
37
15
37
300
2068
800
4
302
423
10
3
54
1372
200
1379
1500
8
500
533
4
1.22
I
25
110
758
4000
20
8
2.44
Section 3.4.1.1
212
373
392
473
523
546
A-l
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before comp
1. REPORT NO. 2.
EPA-600/R-96-066
/
4. TITLE AND SUBTITLE
Characterization of Manufacturing Processes and
Emissions and Pollution Prevention Options for the
Composite Wood Industry
5. REPORT DATE
June 1966
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Cybele Martin and Coleen Northeim
fl. PERFORMING ORGANIZATION REPORT NO.
94U-5807-00
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Research Triangle Institute
P.O. Box 12194
Research Triangle Park, North Carolina 27709
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-D1-0118, Task 93
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
^rsPkE ^iRnEaT;Rr^-P§?'§g C°VERE°
14. SPONSORING AGENCY COOE
EPA/600/13
is.supplementary notes ^PPCD project officer is Elizabeth Howard, Mail Drop 54, 919/
541-7915.
I6'abstractrep0rj. summarizes information gathered on emissions from the compo-
site wood industry (also called the Plywood and particleboard industry) and potential
pollution prevention options. Information was gathered during a literature search that
included trade association publications, journal articles, symposium presentations,
and university research. Little information exists in the literature pertaining to pol-
lution prevention. Most available literature focuses on ways to reduce raw material
consumption and improve manufacturing processes. However, in many instances,
these reductions and improvements lead to pollution prevention benefits. Some of
these potential pollution prevention options presented in the report include: conveyor
belt drying, low temperature drying, light moisture bonding adhesives, foam extru-
sion, and variable glue application rate. Other pollution prevention options presented
in the report include alternative fiber sources (e.g., agricultural fiber and recycled
wood waste) and naturally derived adhesives. These options are presented as resour-
ces that are abundant and renewable. Little emissions data exist in the literature to
include with these options.
17. KEY WOROS AND DOCUMENT ANALYSIS
a. DESCRIPTORS
b.lOENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
Pollution Adhesives
Wood Particle Board Wood
Composite Materials Wastes
Emission Foaming Agents
Manufacturing
Drying
Pollution Control
Stationary Sources
13 B 11A
11L
11D
14 G 11G
05C
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
99
20. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
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