Graphite fabric/PMR-15 laminates develop matrix cracks during long-term exposure in air at temperatures in the range of 500 to 600 F. This study was performed to demonstrate the effectiveness of incorporating graphite mat surface plies as a means of reducing the developing of matrix cracks. Celion 3000 graphite fabric/PMR-15 laminates were fabricated with graphite or graphite mat/325-mesh boron powder surface plies. Laminates without mat surface plies were also fabricated for control purposes. Composite flexural strength, flexural modulus, and interlaminar shear strength were determined at 288 C before and after long-term exposure (up to 1500 hr) in air at 316 C. The results of this study showed that the incorporation of graphite mat surface plies reduces matrix cracking and improves the elevated temperature mechanical property retention characteristics of the composites

Hanson, M. P.

Serafini, T. T.

English

NASA Technical Reports Server

SURFACE PROTECTION OF GRAPHITE FABRIC/PMR-15
COMPOSITES SUBJECTED TO THERMAL OXIDATION
M. P. Hanson and T. T. Seraflnl
National Aeronautics and Space Administration
Lewis Research Center
Graphite fabrlc/PMR-15 laminates develop matrix cracks during long-term expo-
sure In air at temperatures in the range of 500 to 600 °F. Thls study was per-
formed to demonstrate the effectiveness of incorporating graphite mat surface plies
as a means of reducing the development of matrix cracks. Cellon 3000 graphite
fabrlc/PMR-15 laminates were fabricated with graphite or graphite mat/-325-mesh
boron powder surface plies. Laminates without mat surface plies were also fabri-
cated for control purposes. Composite flexural strength, flexural modulus, and
Interlamlnar shear strength were determined at 288 °C before and after long-term
exposure (up to 1500 hr) in alr at 316 °C. The results of this study showed that
the incorporation of graphite mat surface plies reduces matrix cracking and
improves the elevated temperature mechanical property retentlun characteristics of
the composites.
INTRODUCTION
In many instances graphite fabric is being selected as the fiber reinforcement
material in structural composites intended for hlgh-performance aerospace applica-
tions. For example, a graphite fabrlc/PMR-15 polylmlde composite material has been
selected for replacement of the current bill-of-materlals titanium outer duct used
on the General Electric F404 engine (ref. l). The unique bidirectional properties
of graphite fabric laminates enable designers to design composite structures having
simplified laminate configurations which not only meet design requirements but are
also less costly to fabricate than structural components made from unidirectional
tape.
All polymer matrix composites subjected to long-term exposure in air at ele-
vated temperatures, even those employing the so-called hlgh-temperature polymers,
exhibit weight loss and degradation of mechanical properties. Composlte weight
loss and concomitant property degradation have been attributed to diffusion-
controlled oxidative degradation of the matrix (ref. 2). However, a recent study
(ref. 3) on the effects of long-term, elevated-temperature exposure on the proper-
ties of T3OO/PMR-15 fabric laminates also identified matrix cracking as a contrib-
uting factor to weight loss and property degradation. Crack initiation is likely
due to stresses in the matrix resulting from the local, nonuniform fiber distribu-
tion in woven fabrics.
Recently developed graphite mat (ref. 4) has potential for providing a more
uniform fiber distribution. Thus, it was postulated that incorporation of a ply of
graphite mat on the surface of fabric laminates would minimize cracking and thereby
improve the property retention characteristics of graphite fabric laminates exposed
to an elevated temperature oxidative environment.
The results of a study to improve the char-formlng characteristics of matrix
resins (ref. 5) suggested that boron powder might be an effective antloxldant for
hlgh-temperature resins. It was further postulated that a surface layer consisting
,'  cwaNG PAG-7Precedingpageblank 287
https://ntrs.nasa.gov/search.jsp?R=19860001814 2020-03-23T06:24:40+00:00Z
of both graphite mat and boron powder would significantly improve the elevated-
temperature performance of graphite fabrlc/PMR-1S laminates.
The purpose of this investigation was to determine the effectiveness of sur-
face plies of either graphite mat or graphite mat wtth boron powder as a means of
protecting graphite fabrlc/PMR-15 laminates exposed to thermo-oxtdatlve environ-
ments. Cellon 3000 graphite fabric laminates wtth and without mat or mat/boron
powder surface plies were fabricated using PMR-15 polytmide. Composite flexural
strength, flexural modulus, and tnterlamlnar shear strength were determined at
288 °C before and after long-term exposure (up to 1500 hr) in air at 316 °C. Com-
posite weight loss at 316 °C was also determined.
EXPERIMENTAL PROCEDURE
Materials
Style Wl133 8 harness satin-woven fabric from Cellon 3000 graphite yarn was
used as the primary reinforcing material. The yarn was sized with an epoxy compat-
Ible sizing. The mat material was a commercial product consisting of 1.9-cm-long
Cellon carbon fibers wtth a polyester binder. The mat weight was 112 gm/m2. The
boron additive used was an amorphous -325-mesh powder.
The polylmlde resin used tn this investigation was the high-temperature poly-
lmtde designated as PRR-15. The monomers used to formulate PMR-15 are shown In
table 1. The monomethyl ester of 5-norbornene-2,3-dtcarboxyltc acid (NE) and
4,4'-methylenedlantllne (RDA) were obtained from conrnercial sources. The dlmethyl
ester of 3,3', 4,4'-benzophenonetetracarboxyltc acid (BTDE) was prepared as a
50 weight percent solution by refluxlng a suspension of the corresponding dtan-
hydride in anhydrous methanol for -2.75 hr. The monomer stolchtometry for the
PNR-15 solution was 2 NE/3.087 NDA/2.087 BTDE. The PMR-15 solutton was prepared by
dissolving the monomers In a calculated amount of anhydrous methanol to yield a
50 weight percent solution.
Composite Fabrication and Specimen Preparation
The woven fabric was impregnated wtth a predetermihed quantity of the PMR-15
resin solution to provide cured laminates having a fiber content of ~55 volume
percent. The prepregs were air dried to reduce the volatile content to ~lO percent
prior to cutting Into plies.
Prior to impregnating the mat material, the polyester binder was burned-off by
placing the as-received mat between perforated steel sheets and heating the steel/
mat sandwich at 450 °C in an air-convection oven for 2 hr. The binder-free mat was
allowed to cool to room temperature, placed between 0.625 cm mesh screens, and then
impregnated with the PNR-15 resin solution. The mat containing the boron additive
was impregnated with a PMR-15 solution to which 5-percent-by-weight boron powder
had been added. Both mat preparations were air dried to reduce the volatile con-
tent to ~lO percent prior to cutting Into plies. The impregnated fabric and mat
materials were cut Into 7.6 by 20.3 cm plies. The warp yarns of the fabric plies
were parallel to the 20.3 cm dimension. The basic laminate consisted of a 6-ply
layup of the woven fabric; laminates wtth mat surfaces consisted of the basic lami-
nate with I ply of mat on each side. All of the fabric plies were stacked wtth
their warp yarns in the 0° direction. Each layup was then stacked between a porous
release fabric in a preformlng mold and tmldtzed at 204 °C for ] hr under a
pressure of ~0.07 N/cm2. Compression molding was accomplished by placing the
288
staged layup lnto a matched metal die and heating to 232 °C at essentially zero
pressure. A pressure of 345 N/cm 2 was applied and the mold temperature was
increased to 316 °C at a rate of 5.6 °C/min. Pressure and temperature were main-
tatned for 2 hr followed by cooling to 204 °C before releasing the pressure and
removing the laminate for the mold. The cured laminates were postcured in an air-
circulating oven In which the temperature was raised from ambient temperature to
316 °C at a rate of 2.2 °C/min and then held at 316 °C for 16 hr.
Composite Environmental Exposure
Coupons (approximately half of a 7.6 by 20.3 cm laminate) were subjected to
thermo-oxldatlve exposure. All of the coupons were cut from essentially vold-free
laminates as assessed by ultrasonic C-scan. The thermo-oxldatlve environments were
provided by an alr circulating oven. Alr was metered Into the oven at a rate of
lOO cm3/mln. Coupons were periodically removed from the oven and allowed to cool
to room temperature In a desiccator before rewelghlng to determine weight loss.
Composite Testlnq
Flexural tests conformed essentially to the ASTM standard method D?90. Tests
were made on a 3-polnt loading fixture wlth a variable span. Tests were performed
using a span-to-thlckness ratio of -32. The rate of center loading for flexural
testing was 0.127 cm/mln. Interlamlnar shear strength tests were conducted In
accordance to ASTM D2344 using a constant span-to-thlckness ratio of 4. For the
elevated temperature tests, the load was applied to the specimens after the chamber
had equlllbrlated at the test temperature for IO mln.
RESULTS AND DISCUSSION
Figure l shows the variation of 316 °C weight loss wlth exposure tlme In air
at 316 °C for all of the Cellon 3000 graphite fabrlc/PMR-15 composites investigated
In thls study. Also shown In the figure Is the 316 °C weight loss of unidirec-
tional Cellon 6000 (unslzed)/PMR-15 composites (ref. 6). As can be seen In the
figure, all of the composites exhibited about the same level of thermo-oxldatlve
stability. These results indicate the surface plies of mat or mat/boron powder had
neither a beneficial or adverse effect on composite thermo-oxldatlve stability
(TOS) as assessed by welght-loss measurements. The data also show that the pres-
ence of the epoxy sizing, used In weaving the fabric, dld not cause increased
degradation of the fabric composites. While an improvement In thermo-oxidatlve
stability would have been desirable, the absence of a detrimental effect encouraged
further investigation of the concept of using surface plies of mat material to
reduce internal cracking.
Retention of composite mechanical properties after prolonged exposure at ele-
vated temperature Is a more meaningful assessment of composite TOS than composite
weight loss. Figure 2 shows the 288 °C flexural property retention characteristics
of the various laminates exposed at 316 °C as a function of exposure time In air at
316 °C. It can be seen that the property retention of the all-fabrlc laminates Is
lower than the property retention of laminates surface protected wlth mat or mat/
boron. After 15DO hr of exposure at 316 °C, the unprotected fabric composite flex-
ural strength retention was 56 percent, compared to -74 percent retention of flex-
ural strength for both the mat and mat/boron protected materials. The retention of
modulus was 66 percent for unprotected fabric composites as compared to 88 and
IO4 percent retention for mat and mat/boron protected fabrlc/PMR-15 composites,
289
respectively. These results show that the surface protection is effective and that
the Improved retention of modulus Is partlcularly significant.
The foregolng flexural strength comparisons were based on the ultimate proper-
tles of the laminates tested. However, the load-deflection characteristics of the
Isothermally-exposed, unprotected fabrtc/PRR-15 laminates revealed severely degraded
structural Integrity. Figure 3 shows typical flexural load-deflection curves for
unprotected laminates tested after postcure (fig. 3(a)) and after 624 hr of 1so-
thermal exposure at 316 °C (fig. 3(b)). It can be seen that the postcured, but not
exposed, lamlnate (fig. 3(a)) exhibited a load-deflection curve that Is essentially
linear to the ultimate load. In contrast, exposed, unprotected composites showed a
quasi-yield point, or discontinuity, In the load-deflection curve (fig. 3(b)) that
occurred at a relatively low load depending upon the duration of exposure. The
average retention level as a function of time at which the yield occurred Is Indi-
cated by the talled symbols In figure 2. In marked contrast, the load-deflection
characteristics of either graphite mat or graphite mat/boron surface plies had
load-deflection characteristics similar to that of the postcured, but not exposed,
fabric laminates. Using the discontinuity in the load-deflection curve Instead of
ultimate properties as the basis for flexural strength retention, the useful flex-
ural strength retention is 45 percent after 624 hr and 22 percent after 1500 hr.
Figure 4 shows the 288 °C tnterlamlnar shear strength retention of the lami-
nates as a function of exposure time In air at 316 °C. It can be seen that the
shear strength retention after long-term exposure has been improved by the mat and
mat/boron surface plles. After 624 hr of exposure the shear strength retention
varied ~5 percent among the three lamlnate constructions. After 1500 hr of expo-
sure the lnterlaminar shear strength retention was 69 and 73 percent for the
all-fabric laminates and mat-protected laminates, respectively. The mat/boron
surface-modified laminates exhibited 87 percent retention of lnterlamtnar shear
strength after 1500 hr exposure In air at 316 °C. As Is known, lnterlaminar shear
specimens fall close to the specimen neutral plane. Thus, the higher tnterlamtnar
shear strength retention level for the mat/boron protected laminates suggests that
the addition of boron to the mat increased the effectiveness of the mat In
preventing interior cracking.
To ascertain the changes in structural features which might have resulted from
elevated temperature exposure, cross sections of as-fabricated (postcured) lami-
nates and exposed laminates were examined metallographically. Figure 5 shows
representative photomicrographs of specimens from graphite fabrlc/PMR-15 as-
fabricated laminates and from laminates that had been exposed for 624 hr at 316 °C.
Figure 5(a) shows that the as-fabricated laminates were defect free. In figure 5(b)
It can be seen that numerous through-the-surface-ply cracks had developed after
624 hr of exposure. Because specimens subjected to flexural testing fall at one of
the surfaces by either a tensile or compressive failure mode, the presence of
numerous surface cracks in the exposed composites accounts for the severe reduction
In the flexural strength retention of the unprotected laminates (fig. 3).
Figure 6 shows representative photomicrographs of graphite fabric laminates
with graphite mat surface pltes. As can be seen in figure 6(a), the as-fabricated
laminate was essentially defect free. Figure 6(b) shows the same laminate after
624 hr at 316 °C. It can be seen that the thermal-oxidative exposure has caused
some tnternal cracking. Also, the surface cracking of the protected laminate was
negligible compared to the surface cracking of the unprotected laminate (compare
ftgs. 5(b) and 6(b)).
290
Figure 7 shows representative photomicrographs of graphite fabric laminates
with graphite mat impregnated with PNR-15 and boron powder. It can be seen that
the as-fabricated lamlnate (fig. 7(a)) as well as the exposed laminate (fig. 7(b))
were essentially free of surface and interior cracking. The absence of interior
cracks In the mat/boron protected laminates accounts for their higher lnterlamlnar
shear strength retention compared to the other two laminate constructions. The
important point to be noted, however, ts that the incorporation of a surface ply of
mat material was essential for the achievement of improved properties after ele-
vated temperature exposure.
Although no attempt was made In this investigation to optimize the physical
performance of the mat as a surface protection for graphite fabrlc/PMR-15 compos-
ites, variables such as mat thickness and density undoubtedly Influence proper-
ties. If optimized, these variables would lead to significantly enhanced
performance.
CONCLUSIONS
Based on the results obtained In this investigation, the following conclusions
can be drawn:
(1) Isothermally exposed Cellon-3000 graphite fabrlc/PMR-15 composites wlth
graphite mat surface protection have greater flexural strength, flexural modulus,
and Interlamlnar shear strength retention than fabric laminates without mat
protection.
(2) Isothermally exposed graphite composites wlth mat surface protection
exhibit essentially a linear flexural Ioad-deflectlon relationship to failure
whereas composites without mat protection exhibit a pronounced quasl-yleld signifi-
cantly below the failure load.
(3) Adding boron powder to the mat had essentially no effect on the flexural
strength retention. However, flexural modulus and Interlamlnar shear strength
retention were significantly improved.
(4) Mat protected composites did not exhibit improved thermo-oxldatlve
stability in isothermal weight loss as compared to unprotected graphite fabric
composites.
REFERENCES
I . Seraflnl, Tlto T.: PMR Polylmlde Composites for Aerospace Applications. NASA
TM-83047, 1982.
2. Alston, William B.: Characterization of PMR-15 Polylmlde Resin Composition In
Thermo-Oxldatlvely Exposed Graphite Fiber Composites. NASA TM-B1565, IgBO.
. Seraflnl, T. T. and Hanson, M. P.: Environmental Effects on Graphite Fiber
Reinforced PMR-15 Polylmlde. Composites for Extreme Environments. ASTM STP
76B, N. P. Adslt, ed., American Society for Testing and Materials, IgB2, pp.
5-19.
. New Nonwoven Fiber Mats Finding Multiple Uses In Plastics. Plastics Design
Forum, Sept./Oct. 1980, pp. 82, 84, B6.
291
oe
Gluyas, R. E. and Bowles, K. 3.: Improved Fiber Retention by the Use of
Ftllers tn Graphlte F_ber/Res_n Matrlx Composites. NASA TR-79288, 1980.
Vannuccl, Raymond O.: Properties of PMR Polytmtde Composites Hade with
Improved Htgh Strength 6raphlte Flbers. NASA TM-81557, 1980.
TABLE 1. - HONORERS USED FOR PMR 15 POLYIMIDE
STRUCTURE NAME ABBREVIATION
o
C -OMt,
C-OH
|
o
0 o
I |
o o
H |ti._ CH_)-NH|
MONOMETHYL ESTER OF 5-NORBORNENE-
2, 3-DICARBOO(YUC ACID
DIMETHYL ESTER OF 3.3'.4,4'-
BENZOPHENONETLrlRACARBOXYUC ACID
4.4'-METHYLENEDIANIUNE
NE
BTDE
MDA
C$-71603
292
O4
WEIGHT
LOSS,
PERCENT
REINFORCEMENT
_
__ 0 FABRIC
-- -I_ -_l (3 FABRIC+ MAT
_,_l _ FABRIC+ MAT+ BORON
-- _ -_.jq m m CELION PMR-15
O Z_ _ _ UNIDIRECTIONAL
__ _ coMeosm
!12
O 500 1000 1500
TIME,hr
Figure 1. - Welght loss of Cellon 3000 graphlte fabrlc/PMR-I5
lamlnates exposed In alr at 316"*C.
120!
PROPERTY
RL_ENTICN.
1_(2NT
60--
O FABRIC
O FABRIC÷ MAT
40 --Z_ FABRIC+MAT*BORON
OP£N-FLEXURALSAt£NGIH
CLOSED-REXURALMODULUS
TALLIED-RJEXURALYIELD
20 I
0 5e0 1000 1500
TIMF.,hr
Figure 2. - F1exural propertles of Cellon 3000 graphILe fabrIc/PMR-15
laminates exposed In atr at 316 °C and tested at 288 °C.
293
LOAD
DEFLECTION
Figure 3. - Typical flexural load - deflection curves of Cellon 3000 graphtte
fabrlc/PMR-15 laminates; (a) tested at 288 °C after postcure (b) tested at
288 °C after 624 hr exposure In atr at 316 °C.
I00
90
PROPERTY
RETENTION,80
PERCENT
7O
6o I I
0 500 I00O 15oo
TIME,hr
Figure 4. - Shear property retention of Cellon 3000 graphtte fabrlc/PHR-15
laminates exposed In air at 316 °C and tested at 288 °C.
294
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Surface protection of graphite fabric/PMR-15 composites subjected to thermal oxidation

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