AEPA
United States
Environmental Protection
Agency
Environmental Sciences Research
Laboratory
Research Triangle Park NC 2771 1
Research and Development
EPA-600/S2-81-048 Aug. 1981
Project Summary
Monitoring Techniques for
Carbon Fiber Emissions:
Evaluation A
William D. Conner
An investigation was carried out of
methods and techniques applicable to
the detection and monitoring of carbon
fibers as they are emitted in processes
involving their manufacture or use.
The specific activities of these pro-
grams were: (1) to perform a detailed
literature search on relevant informa-
tion about candidate measurement
methods, (2) to determine the typical
effluent conditions under which carbon
fibers are emitted to the atmosphere,
(3) to evaluate the various applicable
candidate monitoring techniques, (4)
to perform a comparison of these
methods, and (5) to select a preferred
monitoring technique. The following
conclusions were reached: (a) routine
carbon fiber emissions to the atmo-
sphere are, at present, negligible; (b)
no extant instrument is capable of
selective detection and measurement
of carbon fiber aerosols; and (c) tech-
niques can be developed to provide a
practical instrumental solution to
carbon fiber monitoring.
This Project Summary was devel-
oped by EPA's Environmental Sciences
Research Laboratory, Research Tri-
angle Park, NC, to announce key find-
ings of the research project that is fully
documented in a separate report of the
same title (see Project Report ordering
information at back).
Introduction
Recent years have seen the rapidly
growing importance of carbon and
graphite composites in their application
to high-strength materials in aircraft,
automobiles, military hardware and
other uses. The promise of drastic
reductions in weight, and other signifi-
cant advantages, as results of the
replacement of steel and aluminum by
these composites has stimulated their
development and optimization at a
rapidly accelerating pace. A significant
drawback of this material, however, has
been recently identified: inadvertent
combustion of such a composite can
result in large-scale aerosolization of
the carbon or graphite fibers embedded
in the composite binder The release
and airborne transport of these fibers,
whose length may reach 20 mm, has
been found to cause serious effects on
electrical and electronic equipment as a
result of the relatively low electrical
resistivity of these fibers. Spark-over
shorting, degradation of insulation,
circuit impedance alteration, and sec-
ondary effects accompanying such
primary ones have created an under-
standable concern about potential cata-
strophic results of any massive release
of this type (i.e., aircraft crash, etc.)
affecting a wide urban and/or industrial
area.
Carbon Fibers - Properties and
Release Mechanisms
In the period 1963-1965, it was dis-
covered that very high strength filaments
could be obtained by subjecting a pre-
cursor fiber to a rigidly controlled tensile
-------
stress during high temperature pyroliza-
tion. Technically, the term "carbon
fiber" applies to fibers which have been
pyrolized at temperatures of 1100°C to
1200°C, and the term "graphite fiber"
applies to those carbon fibers which
have been heat treated at temperatures
on the order of 2200°C to 2700°C. In
practice, however, the two terms are
often used interchangeably to describe
the high-stiffness carbon-based fibers.
Carbon Fiber Properties
Individual fibers are about 8 /um in
diameter and are produced in the form
of yarns, each strand containing thou-
sands of individual fibers. The chemical
and physical properties that produce
strength and stiffness characteristics
also result in very high electrical con-
ductivity for the fiber. The very high
temperatures at which fibers are formed
ensure their virtual indestructibility
under most conditions.
The singular properties of carbon/
graphite fibers become of practical
interest when they are translated into a
useful form through consolidation with
a matrix (binder), into a composite
material. Studies based on experience
gained from R&D programs, and from
production of advanced composite aero-
space structures, indicate that utilization
of graphite fiber composites in aircraft
can add strength and reduce weight
providing significant cost and perform-
ance benefits.
The high electrical conductivity of the
carbon/graphite fibers is the prime
factor in their negative effects on elec-
trical equipment; however, other
properties such as small fiber diameter,
generally short length and low density
are also important contributing factors.
These latter fiber characteristics permit
any small movement of air to cause free
fibers to become airborne and to be
transported over relatively long distances
by normal atmospheric motion. Because
of their high conductivity, carbon/
graphite fibers which settle on or across
electrical contacts or circuits can cause
effects which could damage equipment
or cause it to malfunction. They can
cause: (1) resistive loading; (2) temporary
shorts; or (3) electrical arcing
A summary of the most important
properties and their typical values, or
range of values, are shown in Table 1.
Emissions from Carbon Fiber
Production
Basic Process
Carbon and graphite fibers are manu-
factured from precursor fibers, most
commonly polyacryliomtrile (PAN), but
pitch, tar and rayon fibers are also used
as precursors. Pitch and tar can be
transformed into a suitable fiber by
pyrolysis in a nitrogen atmosphere with
subsequent extrusion. Bundles or tows
of precursor fibers are wound around
frames to maintain a tensile stress
during the initial heat treatment step.
PAN fibers are heated to 220°C in an
oxidizing atmosphere with various
degrees of stretching to improve Young's
modulus
The next step in the process is to
carbonize the oxidized fibers in an inert
Table 1. Typical Properties of Carbon Fibers
Diamagnetic susceptibility
Index of refraction:
Real part
Imaginary part
Tensile strength
Tensile modulus
amorphous carbon
Density
graphite
Electrical resistivity
Diameter
Typical length range
Specific heat
Melting point
Boiling point
Ignition temperature in air
Carbon assay
PH
5 x 10~6
1.8 to 2.7
0.7 to 1.6
1.4 x 10* pascal
2.4 x 10" pascal
1.8 x 103to2.1 x 103 kg m~3
1.9 x 103 to 2.3 x ro3 kg m~3
1 2 x 10s to 1.4 x W~5 O/7?
5 to 10 /jm
100 /jm to 20 mm
711 joule kg~" °/T1
3823°K (graphite sublimes at 3640°K)
5100°K
673°K
92 to 99 percent
6
atmosphere at temperatures of up to
1500°C. Rayon fibers are also stretched
during this stage (or held in tension to
prevent shrinkage) to improve tensile
strength and stiffness. A final heat
treatment step at temperatures of up to
3000°C may be included.
Technically, carbon fibers pyrolized at
temperatures between 1100° and
1500°C, consist of an amorphous
carbon network and exhibit a higher
electrical resistivity. Graphite fibers are
pyrolyzed at temperatures between 2000°
and 3000°C, consist of a crystalline
fiber structure, and exhibit a very low
electrical resistivity.
Uses
There are two major uses for carbon
and graphite fibers (1) carbon fiber
reinforced plastics (CFRP) and (2) carbon
fiber reinforced carbon (CFRC). The
reinforced plastic may be produced from
either resin impregnated carbon-base
molding composites or preimpregnated
laminates. In either case the plastic
parts are produced in molds at tempera-
tures usually less than 165°C and at
pressures of about 21-35 kg/cm2 (300-
500 psi). The CFRC is produced by heat-
ing carbon fibers in a bulk carbon matrix
to 2700°C at ambient pressure in nitro-
gen, argon and other inert atmospheres.
Carbon fibers may be found in the
exhaust gases of the CFRC process.
Current Emissions
The result of a survey undertaker
within this program indicate atthistimi
that routine emission by manufacturim
operations, of significant amounts o
carbon fibers into the atmosphere i
rather unlikely. It appears quite probabh
that the only environmentally detrimen
tal releases of such fibers are to b>
associated with large scale, high tem
perature, possibly explosive, open mcin
eration of carbon fiber composite mate
rials, such as those studied by NASA.
Future drastic increases in the vol
umes of production of both fibers an
their composites may, however, chang
this picture, as different methods c
production are applied and as economi
considerations may affect the degre
and effectiveness of emission contn
measures At this time and in the vie\
of manufacturers of these material
routine incineration of scrap composite
and/or fibers is unusual because of th
high cost of these materials whic
dictates minimization of waste and r
disposal.
-------
ethod of Carbon Fiber
Detection
A wide variety of potentially applicable
methods of carbon fiber detection and
assessment can be identified. However,
very few methods, if any, are available
at present for the unequivocal identifi-
cation and sizing of such airborne
particles, and even less so, for their
automated monitoring. Most of the
techniques used heretofore are either
cumbersome, or nonspecific to carbon
fibers, or both. Table 2 is a comprehen-
sive summary of the state-of-the-art in
carbon graphite-fiber detection and
measurement technology. The detection
specificity of each of these techniques is
graded by its ability to discriminate
carbon particles from those of predomi-
nantly noncarbon composition, by its
specificity to fibrous shaped aerosols,
and its combined selectivity to fibers
composed mainly by carbon Table 2
grades each method by itscompatability
with automated, continuous or continual
monitoring, i.e., without requiring in-
tensive human intervention. Those
methods that are potentially more com-
patible with source monitoring applica-
tions are so marked Estimates of cost
for development and commercialization
of the methods are given For the devel-
opment category, the three categories
have the following approximate equiva-
lence
(a) LOW—The technique has already
been developed and tested. It may
require a relatively small addi-
tional effort to finalize a practical
design. This additional develop-
ment cost wou Id be on the order of
$50,000 or less
(b) MED—The method has been re-
searched, and applied to other or
at best similar types of measure-
ments. It requires additional efforts
to evolve a practical system ap-
plicable to carbon fiber monitoring.
Further development costs are on
the order of $50,000 to $ 150,000.
(c) HIGH—This technique has not
been explored sufficiently for this
application, or its overall practi-
cability has not been demonstrated
empirically. A dedicated develop-
ment effort is required whose cost
equals or exceeds $150,000.
The approximate commercial cost hier-
archy is defined as follows:
(a) LOW—The cost of the presently
available device or instrument or
of an instrument eventually de-
veloped, is equal to or less than
about $2,000
(b) MED—Instrument cost, as defined
in (a), between $2,000 and
$10,000.
(c) HIGH—Instrument cost, as defined
in (a), exceeding $10,000.
Conclusions
Several important conclusions were
reached within this program These
conclusions relate to the various areas
investigated as part of this project- (a)
the review of candidate monitoring
methods, (b) the determination of the
conditions and magnitude of carbon
fiber emissions, and (c) the relative
merits of the potentially applicable
monitoring techniques
One of the central corollaries derived
from the information research performed
within this program is that routing
emission of carbon fibers from manu-
facturing operations are, in general, of
negligible importance; i.e., the emission
rate of carbon fibers into the atmosphere
resulting from the normal production
activities does not warrant, at the
present time, an intensive monitoring
program. Incidental and uncontrolled
carbon fiber releases, however, remain
a matter of concern.
Future drastic intensification of the
industrial volume of production of carbon
fibers and related products may, how-
ever, modify this situation sufficiently to
warrant a careful reassessment of the
above presented conclusions. It appears,
at this time, that instrumentation for m-
plant monitoring as well as ambient
monitoring of carbon fibers may be
required in order to reduce or prevent
electrical equipment failure within
industrial environments, as well as to
provide adequate means to assess the
potential damaging effects of open and
uncontrolled combustion of carbon-
fiber containing materials.
The second major conclusion, reached
as a result of the study under considera-
tion, is that no airborne carbon fiber
detection and monitoring instrument is
presently available capable of unambig-
uous identification and measurement of
such fibers, in the concomitant presence
of other aerosols.
The third important inference derived
from this study is that there are sensing
and detection techniques which, if
properly developed for the specific
objective under consideration, can
provide unequivocal and selective meth-
odology for the continuous automated
monitoring of airborne carbon fibers, in
the presence of other contaminating
particles. It appears feasible that such a
technique, or combination of techniques,
may be applicable to in-plant, emission
testing, and ambient monitoring appli-
cations. The most promising of these
techniques is: a photo-thermal-electric
alignment method, combined with light
scattering characterization.
-------
Table 2. Summary Tabulation of Potentially Applicable Techniques to the Detection and Measurement of Carbon Fibers,
Including a Qualitative Cost Analysis
Detection Specificity
Method
1 High Volt Spark
2 Brass Ball
3. Low Volt. Grid
4. Com Optical Counter
5 Lidar
6. Microwave
7. Sticky Tape
8 Filter Screen
9. Spectrophone
10. Micro-Raman
11 Optical Absorption
12. Angular Light Scat
13. Light Polarization
14. Differential Conductivity
15. Differential Elect.
Mobility
16. Electr. Alignment
17. Magnetic Alignment
18 Aerodyn. Alignment
19. Ultrasonic Effects
20. Video-Microscopy
21 Spark Spectrometry
22. Laser-Spark Spectrometry
23. Scintillation Analysis
24 X-Ray
25. Differential Light Scat.
26. LISMEFA
Carbon
Med
Low
Low
Low
Low
Low
Low
Low
High
High
High
High
Low
Low
Low
Low
Med
Low
Low
Low
High
High
High
High
High
Low
Fiber
Med
Med
Med
Low
Low
Med
High
High
Low
Low
Low
High
High
Med
Med
High
High
High
Med
High
Low
Low
Low
Low
Low
High
Carbon-Fiber
Med
Low
Low
Low
Low
Low
Med
Med
Low
Low
Low
High
Low
Low
Low
Low
Med
Low
Low
Med
Med
Low
Low
Low
Low
Low
Compatibility
with
Autom Monit.
High"
High"
Med
High*
High
High
Low
Low
Low*
Low
Med
Low
High
Med
High'
High*
Med*
High
High
Low
High*
High*
High
Low
High*
High*
Develop.
Low"
Low*
Low*
Low"
Low*
Highc
Low*
Low"
Med*
High"
Med*
Med*
Medc
High*
Med*
Low"
Med*
Med*
High*
Med*
Medc
Medh
Med*
Med*
Medc
Med*
Cost
Commercial
Med
Med
Low
Low
High"
High
Low"
Low"
High
High
Med
High"
Med
Med"
Med
Med"
Med
Med
Med
High"
Med
High
High3
High"
Med
Med*
Observations
Limited to fibers longer than 1mm.
Limited to fibers longer than 2mm
Unpredictable operation, low
collection efficiency.
Nonspecific to C-fibers, insensitive.
Nonspecific to C-fibers.
Requires microscopy of collected
sample.
Requires microscopy.
See Photo-thermal detection as
preferred technique.
Required collection of particles.
Limited usefulness except for
LISMEFA (see No. 26).
Not useful for individual fiber
detection
Coulter-counting may be incom-
patible with conductive fibers.
This technique must be used in
combination with other detection
methods
Same as above.
This technique must be used in
combination with other detection
methods.
Requires fiber alignment.
Requires collection on a medium.
Requires other techniques for fiber
identification.
Same as above.
Same as above.
Same as above.
Same as above.
Applied in GCA-FAM. Useful in
27. Photo-thermal detection High High
and Electric Alignment
^Development largely completed.
^Partially developed.
°To be developed.
^Commercially available.
"Potentially adaptable to source monitoring.
High
High*
High"
Med
combination with carbon-specific
techniques.
Highly specific to carbon fibers.
As applicable to the detection of carbon fibers.
-------
This Project Summary was authored by William D. Conner, who is also the EPA
Project Officer (see below).
The complete report, entitled "Monitoring Techniques for Carbon Fiber Emis-
sions: Evaluation A," (Order No. PB 81-205 932; 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
* US GOVERNMENT PRINTING OFFICE 1981-757-012/7281
-------
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
j "/
,
u " S ]j )
u i;P'/i ^h
rtfr'/ ff
------- |