United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S2-81-145 Oct. 1981
Project Summary
Monitoring Techniques for
Carbon Fiber Emissions:
Evaluation B
Edward T. Peters, Kenneth T. Menzies, Edward J. Cook, and Michael Rossetti
Carbon fibers released from manu-
factijring, application and waste dis-
posal operations are light in weight
and can be dispersed over wide areas.
Because of their -high electrical
conductivity, they can damage any
electronic. apparatus and electrical
equipment they contact. The impact
of respirable fibers on health is not
known. The EPA has the responsibility
to evaluate and develop instrumenta-
tion for continuously monitoring the
number and mass of carbon fibers
emitted from source operations. The
current program was conducted to
evaluate available measurement
methods in light of source emission
characteristics.
Carbon fibers released during man-
ufacturing and application are gener-
ally well controlled by exhaust hoods
and filters. Major emission points
include tow rewind, chopping, textile
weaving and machining operations.
The range of fiber concentration and
length distribution is large; other
particulate matter, including other
types of fibers, is frequently present.
A total of 11 candidate monitoring
methods based on contact (electrical),
locally sensing (optical, microwave)
and remote sensing (microwave,
radar) were identified. Each method
was rated on the basis of measure-
ment (sampling), detection and instru-
mental parameters, and their fit with
fiber concentration and length ranges
produced by three emission scenarios
representing textile weaving, machin-
ing and waste incineration. Five
methods have merit for certain condi-
tions and are recommended for
further study: for moderate to high
concentrations and lengths > 1 mm,
microwave-OSGEF and electrical
.grid-arc methods; for moderate to
high concentrations and lengths; <1
mm, optical scattering-rotating lens
and fiber aerosol monitor (FAM)
methods; and for very high concentra-
tions in absence of other particulate
matter (i.e., process upset), the
optical-LED method. Microwave-
OSGEF is the only method that is
specific to carbon fibers. The electric
grid-arc method can be arranged to
sample a major portion of the air
stream, providing representative
sampling. These two methods are
recommended as having highest pri-
ority for further development.
This Project Summary was develop-
ed by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park. NC, to announce key
findings of the research project that is
fully documented in a separate report
of the same title (see Project Report
ordering information at back).
Introduction
Carbon fiber-epoxy composites are
extremely stiff and strong relative to
.their weight. They have been used
during the past decade as a structural
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reinforcement material by the aero-
space industry and, more recently, for.
recreational equipment. A 25 to 40
percent annual increase in use is
expected over the next decade, and,
depending on their future use in auto-
mobiles, the growth rate courd be
considerably more. Because of their
high electrical conductivity, carbon
fibers released into the atmosphere can
damage nearby electronic and electrical
equipment. The fibers are small and
lightweight and can disperse over fairly
long distances.
The relatively few manufacturers of
carbon fibers and composite materials
are aware of this hazard; they use in-
plant pollution control procedures and
dispose of waste materials in landfill.
Nevertheless, there have been acci-
dental releases to the atmosphere.
EPA has the responsibility to evaluate
and develop instrumentation for contin-
uously monitoring the number and
mass of carbon fibers emitted from
operations such as the manufacture,
processing and disposal of carbon fibers
or fiber containing materials. The first
phase of work which is reported here
relates to an evaluation of currently
available instruments for monitoring
carbon fiber emissions. In this context,
carbon fibers are defined as having at
least a 5 to 1 length to diameter ratio.
Manufactured carbon fibers are taken to
be >5 (jrr\ in diameter, whereas incom-
pletely incinerated carbon fibers can
have diameters <5 /urn. The program
requirements are as follows:
• The evaluation of instruments
must determine their applicability
to stacks or ducting to stacks (i.e.,
confined emissions) and their
ability to continuously measure
mass and number of carbon fibers
in the confined emissionsfrom the
manufacture, processing, and
disposal of such fibers or fiber-
containing material.
• Evaluation of performance capa-
bility shall address the following
operating parameters: accuracy,
range, reproducibility, response
time, sensitivity, specificity, stabil-
ity. Test procedures to examine
these parameters shall be pre-
sented.
• A work plan shall be prepared and
delivered to the EPA describing
the recommended monitoring
system. The work plan shall in-
clude whatever modifications and
new developments are needed to
provide an optimized prototype
system for field evaluation.
The approach to establishing the re-
quired information base was as follows:
• Develop a data base on carbon
fiber manufacturing and applica-
tion with consideration to
- Unit processes
- Carbon fiber emission points
- Control methods
- Available sampling data.
• From this data base, describe the
characteristics of typical emission
scenarios for carbon fiber manu-
facturing, use and disposal.
• Identify instrumental methods
potentially useful for monitoring
carbon fiber emissions on the
basis of literature review and
interviews with government
agencies and contractors.
• Rank candidate monitor methods
on the basis of performance cri-
teria according to the selected
emission scenarios.
• Based on the above, recommend
modifications arid new develop-
ment efforts required for a moni-
toring' method to meet the EPA
needs.
Initially, the effort was directed
toward evaluating presently available
instrumentation; however, based upon
certain shortcomings of all candidate
monitor instruments, an additional task
was added to the program to carry out a
laboratory evaluation of an Arthur D.
Little, Inc., monitoring concept. The new
concept is based on an optical signal
resulting from interaction of carbon
fibers with a high frequency electric
field, referred to herein as the OSGEF
method.
Manufacture of Carbon
Fiber Composites
Process Description
. Carbon fibers are made from precur-
sors such as resins, hydrocarbon
pitches, lignin pitches, rayon, acrylic
polymers, etc. Regardless of the precur-
sor used, processing carbon fibers
involves a series of heat treating steps
to temperatures which for some fibers
may reach 30000C. A process flow
sheet is given in Figure 1.
Carbon Fiber Emission
Scenarios
To permit a ranking of monitoring
devices and concepts on the basis of
their ability to detect and quantify
carbon fibers, it is necessary to estimate
the characteristics of emission streams
containing typical carbon fiber. On the
basis of the review undertaken in this
contract and previous studies of the
emissions from municipal incinerators,
three typical carbon fiber emission
scenarios have been developed. They
are summarized in Table 1.
For scenario A, a carbon fiber textile
weaving process conducted in an iso-
lated room equipped with a HEPA filter
air cleaning system, the monitoring
point could be anywhere between the
emission point and the HEPA filter.
In scenario B, a carbon fiber compos-
ite machining process, such as grinding,
much higher carbon fiber concentra-
tions may be present. Other particulates
may also be present at a level about
equal to the number of carbon fibers.
Finally, in scenario C, the ultimate
disposal of finished products, only a
small fraction of the waste material
combusted contains carbon fiber
composite; in a cubic meter of effluent,
only about 103-105 carbon fibers may be
emitted in the presence of about 109 or
greater particles.
Evaluation of Monitoring
Methods
Candidate Methods
Most of the instruments potentially
useful for continuous measurement of
carbon fiber mass and number were
developed in classified government
programs to determine fiber release
rates in simulated fires and explosions
of aircraft. In addition, several methods,
developed for other types of fibers, such
as asbestos, may be adapted to measure
carbon fibers. Fibers from a manufac-
turing facility differ from those from
explosive combustion sources in terms.
of emission duration, fiber concentrajfl
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PAN Precursor Purchase
As
Yarn = < 10,000 Continuous Filaments
Roving = 10,000-40.000 Continuous Filaments
Tow = > 40,0001 Continuous Filaments
Creel
mm0
Precision Winder,
Stabilized Tow
Single or Multiple Packages
Eyelet
30%
(5%)
Textile Processing or Pulp
Stabilization Furnace
Spools, Packages
Carbonization
Woven Fabrics, Yarns,
Roving, Paper
1000-1400°C
Vacuum, Argon
or Nitrogen
Minutes
Carbonization
Furnace
70%
1600-
\2200°C
Inert
Graphitizing
Furnacr
Sizing
Spray
Chopper
Container
Carbonized Tow (90%)
Graphitization (10%)
OO]OO
Textiles, Paper
Graphitization
Furnace
Precision
Winders.
Spools,
Packages
Sizing
Spray
Precision Winders
Spools, Packages
(6-8%)
Figure 1. Carbon fiber manufacturing flow sheet.
(92-94%)
tion and length of fibers. It is possible,
however, to extrapolate performance
data from previous studies to require-
ments for a continuous monitor for
carbon fibers in the manufacturing
environment.
Instrumental methods used for
carbon fiber measurement or that have
iential as a measurement method are
ed in Figure 2. These may be grouped
into categories for contacting, locally
sensing and remote sensing.
Evaluation Criteria
Various instrumental concepts/de-
vices that have been used or that have
potential for carbon fiber measurement
were reviewed. Several of these devices
are in the conceptual stage only. Some
devices, which are commercially avail-
able, are applicable to the selective
measurement of fibers of any type while
other devices permit measurement of
total paniculate matter which includes
fibers as a fraction of the total.
Each device/concept can be evalua-
ted on the basis of several factors which
define (1) the analytical adequacy of the
instrument, (2) the appropriateness of
the device to continuously monitor
carbon fibers in a manufacturing envi-
ronment, (3) the physical practicability
of the device and (4) the requirement for
improvements to achieve adequate
operation. Specific evaluation param-
eters are documented in Figure 3. Each
parameter has been assigned a weight-
ing factor (WF) on the basis of its per-
ceived importance to carbon fiber
measurement.
Carbon fiber concentrations may vary
markedly depending on intermittent
manufacturing processes. Therefore,
measuring the concentration over a
wide range is very important. The work-
ing concentration range, from the
detection limit to the saturation point,
should be as large as possible. The
desirable range, of course, depends on
the nature of the environment to be
sampled.
The size and shape of the particlesare
critical. Detecting carbon fibers to the
exclusion of all other particles requires
that certain size and aspect ratio values
be met. Specifically, carbon fibers have
lengths ranging from 5 um to >10 mm
and diameters from <1 to 10 um. The
aspect ratio for such fibers may range
from 3:1 to 1000:1. It appears that fibers
of 1 to 10 mm in length are most hazard-
ous to electrical equipment while
smaller fibers are most hazardous to the
human respiratory system.
Measurement Parameters
Three possible operating schemes are
available. First, a sample may be ex-
tracted from the effluent stream and
measured in a separate location. This
scheme requires equipment (e.g.,
pumps) to extract a sample and may
alter the condition of the sample be-
tween effluent duct and measurement
point. Second, a device may be installed
in the sample line and measure the
Concentration of carbon fibers as they
flow by. This scheme may require
periodic cleaning of the sensing device
to maintain accuracy. Third, the carbon
fibers may be monitored remotely, thus
-------
Table 1. Carbon fiber Emission Scenarios*
Parameter
Contacting
Scenario
Ball Gauge
Electric Grid - Arc
Process
Control Procedure
A
Textile
Weaving
Room
Filtration
B
CF Composite
Machining
Hood/
Baghouse
C
Waste Disposal
by Incineration
?
Electric Grit
Locally Sensing
Optical
Air Movement
Gas Volume (m3/min)
Duct cross section(m )
Linear velocity (m/sec)
Temperature (°C)
Moisture (%)
Total Paniculate
Mass (mg/m3)
Corrosive Gases
Maximum CF Concentration
(number/m3)
Average Length (mm)
Length Range (mm)
-100
«-.o
0.2
ambient
1
no
107-10*
5
0.5-25
5-20
0.01-0.1
1-2
ambient
5-10
no
;os-/o10
0.5
0.1-2
200-300
10-15
5-10
100-150
10-20
. 200-500
yes
103-10S
1
0.1-10
Presence of
other paniculate
other fibers
other conducting fibers
no
no
no
yes
maybe
no
yes
probably
maybe
Source: Arthur D. Little. Inc.. estimates.
'[Added Note: A reviewer of this final report brought to our attention the following measured data obtained on
EPA Contract No. 68-02-3229.]
B
Total Paniculate Mass
(mg/m3)
CF Concentration Range
(number/m3)
Average Length (mm)
0.1-10
10*-5x10A
1.5
0.1
precluding any interaction of the instru-
ment with a corrosive medium. The last
scheme is ranked as most useful al-
though it is acknowledged that it may be
the least suitable on other grounds.
The sample volume is important for
two reasons. A larger sample potentially
provides a lower detection level and a
more representative sample. A samp-
ling interval of small duration is prefer-
red since it provides maximum
protection against short term release of
environmentally hazardous fibers.
Continuous monitoring provides such
protection and can yield time-weighted
averages of fiber concentration.
Scattering
Fiber Aerosol Monitor (FAM)
Near Forward Scattering
Rotating Lens
Microwave
Interception
OSGEF
Remote Sensing
Radar
Infrared
Figure 2. Candidate monitor meth
ods.
Detection Parameters
Selectivity and sensitivity are of
critical importance for environments
containing carbon fibers, non-conduc-
tive fibers and other particles. This may
not be necessary in some environ-
ments. As noted above, the morphology
of carbon fibers, i.e., clumps, bundles,
single fibers, is important because of
their aerodynamic behavior. Single
fibers are generally of most concern. In
many cases, fibers must have a specific
orientation to be detected. For example,
a fiber may pass undetected through an
electric grid if it is oriented perpendic-
ular to the plane of the grid or a carbon
fiber may not be differentiated from a
non-conductive fiber in a light scatter-
ing device if it does not rotate in the
sensing field. If fibers need to be
oriented, more complex instrumenta-
tion is required. The simplest devices do
not require orientation.
Frequent failure of the instrument to
detect carbon fibers increases hazards;
frequent false positives increase costs.
Instrument Parameters
The instrument parameters in Figure
3 are serf-explanatory. Some param-
eters, such as size and power require^
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Detection Range
Fiber Concentration
(fibers/cm3)
Fiber Length (cm)
Fiber Aspect Ratio
Evaluation Parameters
1. Measurement Method
Sampling
Volume
Time
2. Detection
Selectivity
Basis
Morphology
Sensitivity
Need for fiber orientation
Detection time
Frequency of error
3. Instrument Parameter
Physical size (weight)
Power requirements
Ruggedness
Maintenance
Calibration
Unit cost
Development cost
w*
w-3
3
1
1
;cr2
5 10
1
JO'1
20
102
;
50
10*
10
100
— Increasing Usefulness —•
WF
1
2
1
•3
2
1
2
1
1
2
*
/
1
1
p
i
Value
12345
Extract - In-Line — Remote
Small Fraction - Large Fraction
Long — Short — Continuous
Vae B\> AAathnrl A//)
Mm Sec. Cont
Dft&n ------ .._ ---_ Ca//yrt/n
Often Seldom
None Primary
Score
4. Disadvantages (Negative Scores)
Concept only - no experimental verification
Based on limited laboratory experiments
Other:
i.e.. High false positive count in
moist stream
Concentration if function of velocity
Subtotal
-10)
5 )
-3 )
-3 )
Less
Total Score
Figure 3. Evaluation criteria.
ments, are dependent on sampling
environments. In other cases, (e.g.,
ruggedness; unit cost) the parameter
relates to the usefulness of the device.
Other Considerations
The evaluation criteria include a sub-
active ranking of deficiencies in sensi-
tivity or selectivity of these methods or
concepts for carbon fibers.
Ranking of Methods
Weighting factors for individual pa-
rameters were chosen on the expecta-
tion that detection parameters are most
important followed by measurement
parameters and instrument parameters.
However, since the weighting factor is
critically related to a specific sampling
environment, final ranking (based on
numerically-weighted scores) of all
methods is given on the basis of the
three sampling scenarios described
above.
Comparison of Methods
Evaluation sheets were prepared for
each of the candidate methods listed in
Figure 2, following the model presented
in Figure 3. Notethatthe estimated fiber
concentration (number) and length
ranges for these methods tend to fall
into one of three regions:
1. Very high concentrations, long
length
• Optical-opacity
• Microwave interception
• Radar
• Infrared
2. Moderate concentration, long
length
• Ball gauge
• Electric grid - arc
• Electric grid - resistance
• Microwave - OSGEF
3. Moderate concentration, short
length
• Optical scattering
These regions, which do not overlap,
are shown in Figure 4 together with the
location of the three emission scenarios
given in Table 2. Scenarios A and C fall
near boundaries of the .Region 2
methods. Carbon fibers produced in B
would be detected by most of the Region
1 methods; however, these methods
measure total particulate and are not
specific to carbon fibers, which limit
their usefulness to specific applications.
Region 2 methods would detect the
presence of Scenario B carbon fibers
but would underestimate their concen-
tration. Such devices could still be
useful for indicating an excursion in
carbon fiber concentration above some
acceptable level.
The Region 3 optical scattering
methods are expected to cover a wide
range of concentration but are limited to
relatively short fibers. These methods
would be the only useful approach for
-------
ro4
ior
10*
10
10'
10'
Region 1
Region 3
©
Optical Scattering
FAM
— Forward Scat.
Rotating Lens
^^ xx
r
k-
Electric Grid:
Arc
Resistance
i
w
w'
70"1
Length - cm
ro1
Figure 4. Detection range of candidate carbon fiber monitor methods. (See
Figure 1 for description of scenarios A, B and C.)
monitoring respirable fiber sizes and
could be used in conjunction with one of
the Region 2 methods for complete
coverage of any length of carbon fiber
over three to four decades of concentra-
tion.
The methods which are felt to be most
promising for application as a carbon
fiber monitor are as follows:
1. Fibers: 1 mm and longer in
presence of background panicu-
late.
• Microwave OSGEF - The major
advantage of the method is the
highly specific identification of
carbon fibers in the presence of
other matter, including conduc-
tive particles and fibers. The
method could be applied over a
large dynamic range of fiber
concentration. Further develop-
ment studies are required to
establish detection limits with
respect to measurement time,
flow rate (volume), minimum
length and maximum concen-
tration, the last being influ-
enced mainly by particle coinci-
dence. The detection section of
the instrument is simple, and
data processing and display can
be remote.
• Electric Grid - Arc - The device
may be constructed to intercept
a major portion of a flowing air-
stream, thereby providing good
representativeness. Detection
is based on particle interception
between two or more
electrodes. Fiber counts can be
obtained over a large dynamic
range. The method is only
approximately length specific,
depending on electrode
spacing, and fibers suspended
parallel to the airstream may be
missed. False positive counts
are possible depending on the
nature (conductivity, size) of
other paniculate matter. Data
processing and display can be
remote. Data processing for
length and mass concentration
may be complex. The instru-
ment may require frequent
cleaning and calibration,
depending on the nature of
other particulate and entrained
.moisture.
4
<
-------
2. Very high concentration of fibers
in absence of other particulate
matter.
• Optical-Opacity • The light-
emitting diode (LED) is simple
and inexpensive; it is suitable
for detecting a very high emis-
sion of carbon fibers in the
absence of other particulate
matter. For example, this device
could trigger an alarm in the
event of a process upset where
a very large number of fibers
were emitted and drawn
through an exhaust hood duct.
The device is limited to a meas-
ure of total particulate matter,
giving no information about
carbon fiber size, number or
mass concentration.
3. Respirable fibers.
• Optical Scattering - Fiber
Aerosol Monitor (FAM) - The
usefulness of the FAM instru-
ment has been demonstrated
for the measurement of asbes-
tos fibers. With some modifica-
tions to increase sampling
volume and the length of fibers
detected, the instrument could
be usefully applied to measure
the number and size distribu-
tion of carbon fibers. The instru-
ment is complex, expensive and
cannot distinguish between
fiber type. Based on results for
measuring asbestos, the
measurement is accurate and
precise.
• Optical Scattering - Rotating
Lens - This method is at the con-
cept stage. Further design and
laboratory evaluation is
required. The method offers the
possibility for measuring a large
range of fiber lengths, limited
only by particle (or fiber) coinci-
dence. The "sampling" of the
stream is remote and can be
arranged to traverse across the
duct. As with other optical
scattering methods, fibers of all
types are measured.
Conclusions
Relatively few opportunities for
jelease of carbon fibers occur during the
laking of carbon fiber epoxy com-
posites (fiber chopping, tow rewinding
and textile weaving) and during the final
shaping (grinding, sawing and drilling)
of products to which the composite has
been applied. Local emissions are con-
trolled by exhaust hoods and water
sprays. Laboratory simulations of
finishing operations show that the
majority of fibers released are less than
0.1 mm in length. Three emission
scenarios were developed to represent
the range of conditions that may be
encountered in the manufacturing,
application and disposal of carbon
fibers. These were textile weaving,
carbon fiber composite machining and
incineration.
Eleven measurement methods or
concepts were identified as candidates
for the continuous measurement of
carbon fiber emissions. These candi-
dates were scored according to param-
eters concerned with measurement
(sampling), detection, instrumentation
and with detectable ranges of fiber
concentration and length compared to
the three emission scenarios. No single
monitoring instrument is suitable for all
possible types of emission. Fiber length
and concentration ranges must be
specified to permit selection of the
appropriate instrument.
Monitor methods recommended for
further study are:
• Moderate to high concentration,
length > 1 mm
- Microwave OSGEF (Arthur D.
Little, Inc.)
- Electric grid-arc (Bionetics, JPL).
• Moderate to high concentration,
length < 1 mm
- Optical Scattering-rotating lens
(Epsilon Laboratories, Inc.)
- Optical Scattering-FAM (GCA,
Inc.).
• Very high concentration, only
carbon fibers
- Optical-LED (commerically
available).
Microwave OSGEF is the only method
that is specific to carbon fibers. The
optical methods detect all fibers, includ-
ing glass and polymer fibers. The
electric grid-arc method measures all
conductive fibers (and particles which
cross several electrodes) and may be
adversely affected by moisture content
in the air stream.
U S GOVERNMENT PRINTING OFFICE, 1981 — 559-017/7371
Edward T. Peters, Kenneth T. Mamies, Edward J. Cook, and Michael Rossetti
are with Arthur D. Little, Inc.. Cambridge, MA 02140.
William Conner is the EPA Project Officer (see below).
The complete report, entitled "Monitoring Techniques for Carbon Fiber
Emissions: Evaluation B," (Order No. PB 81-247413; Cost: $9.50, subject to
change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Environmental Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
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Agency
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Penalty for Private Use $300
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