A test method was developed to determine relative fiber loss from pyrolyzed composites with different resins and fiber construction. Eleven composites consisting of woven and unwoven carbon fiber reinforcement and different resins were subjected to the burn and impact test device. The composites made with undirectional tape had higher fiber loss than those with woven fabric. Also, the fiber loss was inversely proportional to the char yield of the resin

Fish, R. H.

Gilwee, W. J., Jr.

English

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NASA Technical Memorandum 81179
2	 .
A Small-Scale Test for Fiber Release
from Carbon Composites
W. J. Gilwee, Jr. and R. H. Fish
(NASA —TA-81179) A SULL— SCALE TEST FOR	 H80-18105	 j
FIBER RELEASE FROM CARBON COHQUSITES (NASA)
9 p HC A02/11F A01	 CSCL 11D
Unclas
G3/24 47267
February 1980
1
ARSA
National Aeronautics and
Space Administration
	 I
NASA Technical Memorandum 81179
A Small-Scale Test for Fiber Release
from Carbon Composites
W. J. Gilwee, Jr.
R. H. Fish, Ames Research Center, Moffett Field, California
N/GA
National Aeronautics and
Space Administration
Ames Research Center
Moffett Field, California 94035
A SMALL-SCALE TEST FOR FIBER RELEASE FROM CARBON COMPOSITES*
W. J. Gilwee, Jr. and V. H. Fish
Ames Research Center
SUMMARY
The release of electrically conductive fibers from resin/carbon fiber composites presents a
potential problem to electronic equipment. A test method was developed to determine fiber loss
from pyrolyzed composites. Eleven composites consisting of woven and unwoven carbon fiber
reinforcement and different resins were subjected to the burn and impact test device. The com-
posites made with unidirectional tape had higher fiber loss than those with woven fabric. Also, the
fiber loss was inversely proportional to the char yield of the resin. This technique provides a method
to determine relative fiber loss with different resins and fiber construction.
INTRODUCTION
Composites consisting of carbon fibers and a matrix resin have been used in a wide variety
of applications from sports equipment to aircraft parts. The high strength, stiffness, and low weight
of carbon composites provide sufficient advances so that it is reasonable to assume that greater use
of this material will be seen in the future.
Early in 1978 it was reported (ref. 1) that the release of free carbon or graphite fibers into
the environment was a potential problem. The high electrical conductivity of the fibers and the ease
of mobility of the free fibers present problems when they settle across electrical contacts. This
could result in failure of equipment. An accidental fire on an aircraft could result in significant
numbers of free fibers of carbon being released and, if the fibers reach the electronics, a serious
malfunction could occur.
We have been studying fire-resistant resins for aircraft applications for a number of years
(ref. 2). It has been reported (ref. 3) that the higher the char yield the more resistant the resins
are to ignition and burning. Therefore, if these high-char-yielding resins are used as the resin matrix
in carbon composites, less fibers would be released into the atmosphere.
To evaluate the fiber release from composite materials, we have designed a new burn/impact
test apparatus as a screening device to determi-m the amount of fiber release after burn and impact.
We have also calculated the theoretical char binder content of the composites when exposed to the
burner.
*Presented at The Conference on Advanced Composites —Special Topics, December 4-6.1979, El Segundo, California.
DISCUSSION
The equipment used to evaluate the burn/impact properties of carbon composites is shown
in figure 1. The test specimen consists of carbon composites with various resin matrices which were
THERMOCOUPLE	 machined to dimensions of 133 mm (5.25 in.)EXHAUST
PORT
	
long and 12.7 mm (0.5 in.) wide and a thick-
ness as received (2.3 to 3.5 mm) but prefer-
ACRYLIC _	 ably 3.1 mm (0.125 in.). The test specimens
7
were weighed and mounted in the holder and
a tensile load of 444.8 N (100 lb) was applied
Lcm	 - 	to the test specimen by-^mp:essing the
spring. The burner was ,.,,plied with a
mixture of air (13.1 liters/min) and propane
IMPACTOR	 gas (0.55 liters/min) and ignited. These
SAMPLE	 BURNER	 values for air and propane are equivalent to
LOAD SPRING
	
5 mol of oxygen per mol of propane which
would make a nonoxidizing flame. The burner
Figure 1.— Ames bum-impact unit.	 was allowed to heat up for 3 min and the
temperature in the warmup position with the
door open was 927°-100 0 C (17000-1850° F). A shop vacuum was attached to the exhaust port
which was covered with "bridal veil" net coated with a tacky polyvinyl chloride adhesive (Rhoplex
N619). The bumer was rotated info position and the sample was burned for 3 min. The burner was
then shut off and the specimen was allowed to cool 5 min. The specimen was then weighed and
placed back in the fixture and the load of 444.8 N was applied. The specimen was then impacted at
an impact velocity of 12.2 m/sec with the piston from the air cylinder. The load on the air cylinder
was 0.648 MPa (94 psi). After the specimen was impacted it was removed and weighed.
The percent of fiber weight loss from impart was calculated as:
% Impact loss = Weight after burn — weight after Impact X 100Orig^nal weight (6017o)
The calculation of the char binder of the composite was made 'based on the temperature of
the test specimen and the char yield of the resin as detennined by thermogravimetric analysis. In
the burn-impact procedure, the 50-mm (2-in.) diam heater was mounted at a distance of 20.0 mm
(0.8125 in.) from the test specimen. Because of conductive heat flow through the test specimen,
there is a resulting thermal profile across the specimen (see fig. 2). From the center to a point
12.7 mm (0.5 in.) on either side of the centerline, the maximum temperature was 750° C (1385° F).
Nineteen mm (0.75 in.) on either side of the centerline, the maximum temperature was 649° C
(1200° F). At a distance of 25.4 mm (1.0 in.) on either side of the centerline, the temperature
was 520° C (968° F). At 38.1 mm 0.5 in.)on either side of the centerline, the maximum tempera-
ture was 427° C (800° F). Thus, if the temperature gradient, resin content (% wt), and char yield
are known, the percent char binder can be calculated.
2
1„.
I	 : 4 13:2i	 1	 !2i3i 4:	 I
20%,7500
Max.
OZONE 2
10%,649' max.
ZONE 
10%, 520° max.
ZONE 4
20%,4271 max.
Figure 2.— Thermal profile of burn test specimen.
The theoretical char binder of the composite is calculated as:
R'Calculated char binder = Specimen weight X 607o
where:
R'- R(0.2)(Yc750° C) + R(0.1)(Yc649° C) + R(0.1)(Y c520° C) + R(0.2)(Yc427° C)
R = Specimen weight X resin content (% weight)
In this equation the weight fraction of each temperature zone is multiplied by the char yield
(table 1). The sum of the char yield for each zone is divided by 605o of the weight of the specimen
to give the percent char binder. The original specimen weight is normalized to 60°Io because this
is the portion of the specimen that is heated to a temperature above 280° C (536° F). Below
that temperature, the weight loss is insignificant (5% or less).
An example of the calculated char binder for an epoxy/carbon composite with a resin content
of 33.2% (wt) and the char yield at 750° C of 36%, at 649° C of 37%, at 520° C of 41%, and at
427° C of 56`no, with a sample weight of 8.02 g, is 14.5%.
TABLE 1.— RESIN CHAR YIELD — THERMOGRAVI-
METRIC ANALYSIS
RESIN TYPE 7WC
PERCENT CHAR YIEI O
649°C	 520°C 427°C
POLYSTRYL PYRIDINE 68 72 80 96
EPDXY 36 37 41 56
PHENOLIC - A 58 60 72 64
PHENOLIC - B 54 55 68 92
PHENOLIC - C 61 64 76 88
POLYIMIDE 55 58 72 Be
3
^r
TEST RESULTS
Eleven test composites were prepared using the processing conditions given in table 2. Five
specimens of each were subjected to the burn/impact procedure. The resin content, resin type,
fiber type, calculated char binder, and percent weight loss after burn and impact are summarized
in table 3. The resins used included epoxy, phenolic, polystryl pyridine, and polyimide.
The carbon reinforcements used include unidirectional tape, satin weave, and plain weave.
The samples listed in table 3 are grouped according to the type of reinforcement. The first two
samples were made with unidirectional tape in a "quasi-isotropic" layup (0, ±45, 90); the next
seven samples were made with a satin weave (T-300, 24X24 yarn count, 11 oz/yd 2 ) cloth; and the
last two samples were made with plain weave (T-300, 12X12 yam count, 5.5 oz /yd') cloth. The
quasi-isotropic samples had higher fiber loss (as determined by weight loss after burn and impact)
than the woven samples. For example, the epoxy resin quasi -isotropic composite had fiber release
of 12.6%
 with calculated char binder of 13.1 %; the same resin on satin weave cloth had fiber release
of 1.02% with char binder (cal.) of 14.5 %; on a plain weave cloth sample, the fiber release was
0.9% with calculated char binder of 11.4 %.
Because the majority of the samples tested was made with satin weave cloth, a comparison
can be made between the different resin systems and in two cases comparing two different resin
contents with the same resin type. The test results listed in table 3 are ranked according to the
calculated char binder. The values for the calculated char binder are listed with the highest values
at the top. The values for percent impact loss and calculated char binder are also presented in
figure 3. It can be seen that the higher t1 a Aar yield the lower the impact loss. Samples with the
same type of resin (phenolic-2), such as S-2 and S4, show that with higher resin content there is
necessarily a higher char binder and less fiber release after bum and impact. The same results held
true for the two epoxy samples (S-6 and S-7). The upper values for achieving improved fiber reten-
tion is not known, but within the resin contents employed there is a correlation between resin
content and fiber release.
TABLE 2.— COMPOSITE PROCESSING
RESIN (SAMPLE CURE POST CURE
NUMBER) PRESSURE,
TIME, hr TEMP., "C MPs TIME, hr	 TEMP.,' C
POLYSTRYL PYRIDINE 4 200 1.01 NONE
IT 1, S-7)
2 250 1.01
EPDXY I7-2, S-6, 9-7,
P-2) J 177 0.69 NONE
PHENOLIC - A(S-1) 4 170 0.69 4	 121
PHENOLIC - B(S-2, S-4) 4 202 1.78 17	 250
PHENOLIC -- CIS-S) 4 149 0.69 NONE
POLYIMIDE IP-11 4 177 0.69 is	 210
4
61
1.2
1.0
.6
.6
4
.2
r
TABLE 3.- TEST RESULTS - BURNAMPACT TEST
RESIN %
SAMPLE RESIN	 STYLE FIBER CONTENT CHAR %IMPACT
TYPE %/wt- BINDER LOU
T-1 POLYSTRYL UNIDIRECTIONAL 20.0 24.0 26
PYRIDINE	 TAPE
T-2 E►JXY	 UNIDIRECTIONAL 20.0 12.1 12.6
' .PE
5-1 PHENOLIC A SATIN WEAVE 22.0 22.2 0.22
9-2 PHENOLIC 9 SATIN WEAVE 20.0 20.7 0.40
9-2 POLYSTRYL SATIN WEAVE 24.0 19.2 0.51
PYRIDINE
S-4 PHENOLIC 9 SATIN WEAVE 21.5 19.1 0.62
S-5 PHENOLICC SATIN WEAVE 21.0 15.2 0.77
S 5 EPDXY	 SATIN WEAVE 22.2 14.5 1.02
6-7 EPDXY	 SATIN WEAVE 29.0 11.4 1.20
P-1 POLYIMIDE	 PLAIN WEAVE 44.5 20.6 0.22
P-2 EPDXY	 PLAIN WEAVE 26.0 11.4 0.90
1.4
10	 12	 14	 /6	 16	 20	 22	 24
%CHAR BINDER
Figure 3.- Char vs impact loss after burn of satin weave
composites.
5
	
1
CONCLUSIONS
The burn/impact chamber provides a small-scale test apparatus for determining a relative
fiber-release value for burned carbon composites. The test results indicate a difference in carbon
fiber release depending on the type of reinforcement used in the composite material. There was
more fiber release with the quasi-isotropic composite made with unidirectional tape than with the
woven fabric reinforcement. The amount of fiber release in the impact chamber after burning is
coincident with the calculated char binder present.
One goal for our research effort is to reduce or eliminate the release of conductive fibers.
The use of the burn/impact test chamber will assist in the evaluation of new resins and fibers.
REFERENCES
1. A Report of Observed Effects on Electrical Systems of Airborne Carbon/Graphite Fibers. NASA TM 78653,
Jan. 1978.
2. Parker, J. A.; Kourtides, D. A.; Fish, R. H.; and Gilwee, W. J. Jr.: "Fire Dynamics of Modern Aircraft From a
Materials Point of View," J. Fire and Flammability, vol. 6, 1975, pp. 534.553.
3. Gilwee, W. J. Jr.; Parker, John A.; and Kourtides, Demetrius A.: "Oxygen Index Tests of Thermosetting Resins,"
36th SPE ANTEC, Tech. Papers, 1978, p. 430.
6
1. Report No.
NASA TM-81179
2. OovwnnNnt Aseenion No. 3, Recipient's Catalog No.
4. Title and Subtitle 6. Report Late
A SMALL-SCALE TEST FOR FIBER RELEASE FROM
6. Performing organization co*CARBON COMPOSITES
7. Authorls) g. Performing organization Report No,
W. J. Gilwee, Jr. and R. H. Fish A-7962
10. Work Unit No.
534-03-119. Performing organisation Nam. and Address
Ames Research Center, NASA
Moffett Field, California 94035
11. contract or Grant No.
13. Type of Report and Period Covered
Technical Memorandum12. sponsoring Agency Name end Address
National Aeronautics and Space Administration 14. Sponsoring Agency Code
Washington, D.C. 20546
16. Suppltmentery Notes
16. Abstract
The release of electrically conductive fibers from resin /carbon fiber composites presents a
potential problem to electronic equipment. A test method was developed to determine fiber
loss from pyrolyzed composites. Eleven composites consisting of woven and unwoven carbon
fiber reinforcement and different resins were subjected to the burn and impact test device.
The composites made with unidirectional tape had higher fiber loss than those with woven
fabric. Also, the fiber loss was inversely proportional to the char yield of the resin. This
technique provides a method to determine relative fiber loss with different resins and fiber
construction.
17. Key Words (suggested by Author(Q)
	
18. Distribution statement
Carbon composites	 Unlimited
Carbon fiber release
Impact test
Epoxy/carbon fiber	 f	 STAR Category -- 24
19. Secur ity aaeslf. (of this report) 20. Security Cleri f. (of this page ► 21. No. of Pages 22.
$^7001Unclassified Unclassified 9
'For "by the National Teehimb! Information Service, Springfield, Virginia 22161


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