Polymers research at the NASA Lewis Research Center has produced high-temperature, easily processable resin systems, such as PMR-15. In addition, the Polymers Branch has investigated ways to improve the mechanical properties of polymers and the microcracking resistance of polymer matrix composites in response to industry need for new and improved aeropropulsion materials. Current and future research in the Polymers Branch is aimed at advancing the upper use temperature of polymer matrix composites to 700 F and beyond by developing new resins, by examining the use of fiber reinforcements other than graphite, and by developing coatings for polymer matrix composites to increase their oxidation resistance

Meador, Michael A.

English

NASA Technical Reports Server

N92-22513
HIGH-TEMPERATURE POLYMER MATRIX COMPOSITES
Michael A. Meador
An increasing emphasis on high-performance aircraft and high thrust-
to-weight systems has driven the need to develop and use new, lightweight,
hlgh-specific-strength materials in aeropropulsion. These newer materials (in
particular, composites) must not only offer a weight savings over more conven-
tional materials, but should also be amenable to fabrication into complex
shapes and designs, be resistant to the threats of their use environment (ther-
mal and thermal oxidative degradation, corrosion, etc.), have good mechanical
properties, and be cost effective.
Current state-of-the-art polymer matrix composites will withstand extended
use at temperatures as high as 600 to 650 °F in oxidizing environments. While
this may limit their application as aeropropulsion materials, polymer matrix
composites can be shaped or molded easily during processing, are corrosion
resistant, and can be chemically modified to provide a variety of specific
properties, for example, increased toughness, and increased electrical or ther-
mal conductivity (fig. I). Where appropriate, the use of polymer matrix com-
posites in aeropropulsion and other material applications can offer savings
not only in weight, but in production costs as well.
The first polymer matrix composites to find use in aircraft engines were
fiberglass/epoxy systems (fig. 2). The earliest uses of polymer matrix compos-
ites were in noncritical applications, and were also severely limited by the
upper use temperatures of epoxies (250 to 300 °F). Since that time, further
research has led to polymer matrix composites with higher and higher use tem-
peratures (e.g., 600 =F for graphite/polyimides). In addition, time has seen
the acceptance of these materials move from noncritical to critical structural
applications. Current research in the Polymers Branch at the NASA Lewis
Research Center is directed at advanced polymers and polymer matrix composites
for extended use at 700 °F and beyond.
A major concern in the processing of polymer matrix composites is the pro-
duction of voids. These voids can arise from a number of sources: the evolu-
tion of volatile byproducts during processing, contaminants, degradation, or
poor resin flow. Regardless of their origin, voids are not only potential
sources for mechanical failure but can also lead to enhanced thermal oxidative
degradation in the composite. Therefore, one of the major goals of polymer
matrix composite processing is the minimization of void content. One approach
to this (fig. 3) involves a two-step cure procedure. Generally, the first
step (condensation reaction) of this procedure involves the formation of a
low-molecular-weight prepolymer. This first step also produces some quantity
of volatile byproducts which escape during this step. The prepolymer formed
in this step is endcapped with a group which undergoes a cross-linking reac-
tion at a higher temperature (addition reaction) to form a more thermally
45
https://ntrs.nasa.gov/search.jsp?R=19920013270 2020-03-17T12:00:15+00:00Z
stable polymer network. Unlike the first step, the cross-linking addition
reaction proceeds with no formation of byproducts. The result is a highly pro-
cessable, void-free composite.
In the early 1970's researchers at the NASA Lewis Research Center deve-
loped an addition-curing polyimide resin system known as PMR (polymerization
of monomer reactants) polyimides (ref. I). These polyimides (in particular,
PMR-15) afford exceptional thermal oxidative stability coupled with good pro-
cessability by way of a number of conventional techniques. The useful life-
times of PMR-15 in air (60 psia) are I0 000 hr at 550 °F and 1500 hr at 600 °F
(fig. 4).
PMR-15 represents the state of the art in 550 °F matrix resins and has
found a wide range of use in both aeropropulsion and nonpropulsion applica-
tions. A study prepared by General Electric (ref. 2) under contract to the
Navy concluded that the inclusion of graphite/PMR-15 composites in a number of
military aircraft engines, such as the F-404, F-If0, and F-101, could result
in considerable savings in weight and manufacturing costs (fig. 5). As a
result of this study and a joint Navy-NASA Lewis funded program, General Elec-
tric is currently producing a graphite/PMR-15 duct for their F-404 engine
(fig. 6). Originally made out of titanium, this duct was manufactured by form-
ing and machining titanium plates followed by chem-milling to reduce the final
weight. The graphite/PMR-15 replacement duct, on the other hand, is autoclave
molded in two pieces. In initial tests, the graphite/PMR-15 replacement duct
successfully completed over i000 engine test cycles for a total engine exposure
time of 700 hr. Replacement of the original titanium duct by the
graphite/PMR-15 duct has resulted in a total weight savings of 7 pounds per
engine, as well as in a substantial reduction both in production costs and in
the use of titanium.
Offering the combination of good processability and thermal/oxidative sta-
bility, PMR-15 is a recognized leader in high-temperature polymer matrix res-
ins. It is one of the most widely used resins for 550 °F applications and is
available from a number of suppliers (fig. 7).
Since the development of PMR polyimides, the Polymers Branch at NASA Lewis
has responded to industry needs for improvements in these resins, for under-
standing the mechanisms for their thermal/oxidative degradation (ref. 3), and
for developing quality control procedures (ref. 4). (See fig. 8.) For
example, modification of the original PMR-15 formulation has produced a toug-
hened polyimide, designated PMR-T (ref. 5), with an increase in failure shear
strain and failure impact energy (fig. 9).
One concern that has arisen with graphite/PMR-15 and other polymer matrix
composites is that they experience some microcracking during thermal cycling,
which can affect mechanical properties. The NASA Lewis Polymers Branch has
approached this problem from two angles - one a chemical solution, the other
an engineering solution. The use of PMR-T in place of PMR-15 resulted in a
50-percent reduction in the composite transply crack density after i000 thermal
cycles (0 to 450 °F). Use of a random fiber mat finish on a graphite/PMR-15
composite virtually eliminated microcracking during the same thermal cycling
test (fig. i0).
A major thrust of the Polymers Branch at Lewis has been in developing PMR
polyimides with improved thermal oxidative stability over PMR-15. One such
46
improvement is PMR-II (ref. 6), a 1977 I-R i00 Award winner. Increased ther-
mal oxidative stability was achieved in this system by substituting more ther-
mally stable monomers into the original PMR-15 formulation. The result is a
lower weight loss and better retention of mechanical properties at 600 °F in
air than PMR-15 (fig. ii). A higher molecular weight formulation of PMR-II,
PMR-II-700, shows promise as a 700 °F resin candidate (ref. 7). This resin
experiences only a slight loss of mechanical properties after 200 hr in air
(60 psia) at 700 °F and has a weight loss roughly a third of that of PMR-15
under these conditions (fig. 12).
Mechanistic studies have revealed that the end cap is responsible for
much of the thermal oxidative instability of PMR polyimides at high tempera-
tures (above 600 °F). The polyimide PMR-II-700 represents one approach to
counteracting the deleterious effects of these end caps. By increasing the
formulated molecular weight of the polymer, such as is done in PMR-II-700, the
overall weight percent of endcap is reduced. The result is improved thermal
oxidative stability with some sacrifice in processability (higher molecular
weight polymers do not flow as well as their lower molecular weight analogs).
However, it is clear that to achieve higher and higher use temperatures
in polymer matrix composites, alternatives to PMR polyimides must be deve-
loped. These alternatives may be anything from simply changing to a more ther-
mal, oxidatively stable end cap to developing entirely new polymer systems. A
recent effort in the Polymers Branch has been focused on finding new addition-
curing polymers which offer improved thermal oxidative stability over PMR poly-
imides, while at the same time affording good processability. The polyimide
MARVIMIDE (ref. 8) is one product of this effort (fig. 13). This linear lad-
der polyimide combines thermoplastic processing behavior with high softening
temperatures (glass transition temperatures (Tg's) as high as 790 °F) and good
thermal oxidative stability. Figure 14 shows a comparison of the short-term
weight loss (obtained by thermal gravimetric analysis (TGA)) of samples of
MARVIMIDE and PMR-15. The MARVIMIDE has a (131 °F, 55 °C) higher onset of
decomposition than the PMR-15. More recently prepared formulations of
MARVIMIDE exhibit onsets of decomposition approaching 1150 °F (620 °C).
Graphite/MARVIMIDE composites are currently being prepared, and their long-
term thermal oxidative behavior is under investigation.
Polymers research at the NASA Lewis Research Center has produced high-
temperature, easily processable resin systems, such as PMR-15. In addition,
the Polymers Branch has investigated ways to improve the mechanical properties
of polymers and the microcracking resistance of polymer matrix composites in
response to industry need for new and improved aeropropulsion materials. Cur-
rent and future research in the Polymers Branch is aimed at advancing the upper
use temperature of polymer matrix composites to 700 °F and beyond by developing
new resins, by examining the use of fiber reinforcements other than graphite,
and by developing coatings for polymer matrix composites to increase their oxi-
dation resistance.
REFERENCES
I. Serafini, T.T.; Delvigs, P.; and Lightsey, G.R.: Thermally Stable Poly-
imides From Solutions of Monomerlc Reactants. J. Appl. Polym. Sci., vol.
16, NO. 4, Apr. 1972, pp. 905-915.
_7
2. Stotler, C.L.: Development Program for a Graphite/PMR-15 Polyimide Duct
for the F404 Engine. The 1980's - Payoff Decade for Advanced Materials,
SAMPE, 1980, pp. 176-187.
3. Alston, W.B.: Structure-to-Property Relationships in Addition Cured Poly-
mers. I. Thermooxidative Stability of Norbornenyl-Cured Polyimide Resins.
Polym. Prepr., vol. 27, no. 2, Sept. 1986, pp. 410-411.
4. Roberts, G.E.; and Lauver, R.W.: Quantitative Analysis of PMR-15 Poly-
imide Resin by HPLC. J. Appl. Polym. Sci., vol. 33, no. 8, June 1987, pp.
2893-2913.
5. Vannucci, R.D.; and Bowles, K.J.: Graphite/PMR Polyimide Composites With
Improved Toughness. 17th National SAMPE Technical Conference, SAMPE, 1985,
pp. 352-363. (NASA TM-87081).
6. Serafini, T.T.; Vannucci, R.D.; and Alston, W.B.: Second Generation PMR
Polyimides. NASA TM X-71894, 1976.
. Vannucci, R.D.: PMR Polyimide Compositions for Improved Performance at
371 °C. Advanced Materials Technology '87, R. Carson et al., eds., SAMPE,
1987, pp. 602-612. (NASA TM-88942).
8. Meador, M.A.B.: Addition Polymers From 1,4,5,8-Tetrahydro-l,4,5,8-
diepoxyanthracene and Bis-dienes: Processable Resins for High Temperature
Applications. Polym. Prepr., vol. 28, no.2, Aug. 1987, pp. 375-376. (NASA
TM-89838).
• CAN BE SHAPED OR FORMED EASILY
• LIGHTWEIGHT
• CORROSION RESISTANT
• CAN BE TAILORED TO SPECIFIC NEEDS
CD-87-29452
Figure i. - Properties of polymer matrix composites.
_8
USE
TEMPERATURE,
°F
800
700
60O
5OO
4OO
3OO
2O0
100
1960
ADVANCEDPOLYMERS, _ _-*
J
/ f
/ I
GRAPHITE/POLYIMIDES
_ GLASS/EPOXY
___AL-____._.L ........ STRUCTURAL--__' I I II
19"70 1980 1990 2000 2010
INTRODUCTION INTO ENGINE USE
CD-87-29453
Figure 2. -Trends for polymer composites in engines.
<>
MONOMER
REACTANTS
350 to 400 °F
CONDENSATION
POLYMERIZATION
E ]
)
E )
LATENT
REACTIVE ENDCAPS
ENCAPPED
PREPOLYMER
550 to 600 °F
ADDITION
REACTION
(NO VOIDS)
CROSS-LINKED
NETWORK
CD-87-29454
Figure 3. - Reaction scheme for addition polyimides.
49
EXCELLENT PROCESSABILITY
COMPRESSION
MOLDING
AUTOCLAVE
MOLDING
FILAMENT
WINDING
OUTSTANDING ELEVATED
TEMPERATURE RESISTANCE
USEFUL
LIFETIME
IN AIR
10 000 hr
1500 hr
550 OF 600 OF
USE TEMPERATURE
CD-87-29455
Figure 4. - Lewis-developed PMR-15 polyimide technology•
Figure 5. - Navy manufacturing technology.
50
WEIGHT
SAVINGS 7 POUNDS/ENGINE
PRODUCTION QUALITY DUCT
CD-87-24957
Figure 6. - Graphite/PMR-15 titanium replacement F-404 duct (Navy/NASA
Lewis/GE joint development).
• PMR RESINS DEVELOPED IN RESPONSE TO NEED FOR A PROCESSABLE, HIGH
TEMPERATURE MATERIAL
• EIGHT COMMERCIAL LICENSES FOR PMR RESINS. MAJOR SUPPLIERS: AMERICAN
CYANAMID, CIBA-GEIGY, FERRO, FIBERITE, HEXCEL, U.S. POLYMERIC,
HYSOL/GRAFIL, TRIBON.
• ONE OF THE MOST WIDELY USED MATRIX RESINS FOR 550 "F APPLICATIONS.
Figure 7. - PMR-15:
resins.
CD-87-29458
recognized leader in high-temperature polymer matrix
51
ORIGINAL PACE
BLACK AND WHITE. PHOTOGRAPH
• IMPROVEDTACK/EASIERHANDLING PMR
• LOWER CURE TEMPERATUREMATERIALS
• MECHANISTICSTUDIES
• UNDERSTANDINGOFSTRUCTURE/PROPERTYRELATIONSHIPS
• DEGRADATIONSTUDIES
• IMPROVEDTOUGHNESS
• IMPROVED THERMALSTABILITY
• QUALITY CONTROLPROCEDURES
THISWORKHASLEDTO OVERSEVENTYSCIENTIFICPUBLICATIONS,TWELVEPATENTS
ANDSEVENPATENTSPENDING.
CD-87-29459
Figure 8. - Lewis Polymers Branch response to industry needs for improvements
and further understanding of polyimides.
0 0 0 0 0
F/ N F/N N
0 CF3 0
H2N_NH 2 - H2N _[}_-!_>-O-<(_-NH 2
CF3
__
4--
FAILURE
SHEAR 3--
STRAIN, 2--[--]
PERCENT
1
0
12
FAILURE 10
IMPACT
ENERGY,
fl-lb
m
PMR-I§ PMR-T PMR-15 PMR-T
COMPARISON OF COMPOSITE PROPERTIES
WHICH AFFECT COMPOSITE TOUGHNESS
CD-87-2G480
Figure 9. - PMR polylmldes with improved toughness.
52
..
APPROACHES TO ELIMINATE IN-SERVICE MICROCRACKING:
1) TOUGHER POLYMER MATRIX RESIN
2) REPLACING SOME UNIDIRECTIONAL PLIES WITH RANDOM
FIBER MAT
60-- --
CD-87-29461
CRACK
DENSITY,
CRACKS/sq.in.
30
_] I"-'--'1
PMR-15/ PMR-T/ PMR-15/
GRAPHITE GRAPHITE GRAPHITE
CONTROL (RANDOM
FIBERMAT)
Figure i0. - Eliminating microcracking during in-service thermocycling.
0 0 0 0 CF3I 0
REPLACE HO sO H WITH HO.,_I_ CF3L_sOH ,
0 0 0 0
REPLACE H2N_CH2--__NH 2 WITH NH2-__NH 2
160 x 103
FLEXURAL140_STRENGTH,psi 120
i J 1 I I
1000 400 800 1200 1600 2000
TIME, hr
COMPARISONOF FLEXURALSTRENGTHOF PMR-151ORAPHITEFIBERCOMPOSITE
WITH PMR-]]/GRAPHITEFIBERCOMPOSITEEXPOSEDAND TESTEDAT 600 "F IN AIR
CD-87-29462
Figure ii. - PMR-II: better retention of properties through use of more
stable monomers.
53
INTERLAMINAR
SHEAR
STRENGTH,
ksl
oTESTEDAT700OF6
_ WEIGHT 10
4 I--_'_ PMR_II'.700- LOSS,L\ PERCENT20 700
21 I PM_-15-I I I 30l J I i J J
0 50 100 150 200 250 0 50 100 150 200 250
TIME, hr TIME, hr
PROPERTIES OF GRAPHITE FIBER/PMR RESIN COMPOSITES
AFTER EXPOSURE TO AIR AT 700 OF AND 60 PSIA
CD-87-29463
Figure 12. - PMR-II-700: entry into 700 °F applications.
Figure 13. -
structure.
HEAT
• PROCESSABLE(THERMOPLASTIC?)
• HIGH SOFTENINGTEMPERATURES
• STABLELADDERSTRUCTURES CD-87-29464
MARVIMIDES: linear addition copolymers with partial ladder
lO0
SHORT 98
TEwTRM90
LOSS,
% 85
80
CD-87-29485
MARVIMIDEJ /'_% _,
/ %
PMR-18J %%
%
\
-- 5,5°C DIFFERENCEIN ONSETOFDECOMPOSITIONIN AIR
I I I J I
200 300 400 500 600
TEMPERATURE,•C
Figure 14. - Comparison of short-term weight loss of samples of MARVIMIDE and
PMR-15.
54


High-temperature polymer matrix composites

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