The rheology, processing, and chemistry of newly developed N-phenylnadimide modified PMR (PMR-PN) polyimide resins are reviewed. The 371 C performance of their composites reinforced with Celion 6000 graphite fibers is also reviewed, along with the state of the art Celion 6000/PMR-15 composite. The effects of the 371 C exposure in air for up to 300 hr on the composite glass transition temperature, weight loss characteristics, and dimensional stability are presented. The changes in the composite 371 C interlaminar shear and flexural properties are also presented. In addition, composite interfacial degradation at a function of exposure time at 371 C was followed by scanning electron microscopy. The results suggest that the composite materials can be used at 371 C for at least 100 hr

Pater, R. H.

English

NASA Technical Reports Server

N86-I1278
RHEOLOGICAL, PROCESSING, AND 371 °C MECHANICAL PROPERTIES
OF CELION 6000/N-PHENYLNADIMIDE MODIFIED PMR COMPOSITES
Ruth H. Pater
National Aeronautics and Space Administration
Lewis Research Center
The rheology, processlng, and chemistry of newly developed N-phenylnadlmlde
modified PMR (PMR-PN) polylmlde resins are reviewed. The 371 °C performance of
their composites reinforced with Cellon 6000 graphite fibers is also reviewed,
along with the state-of-the-art Cellon 6000/PMR-15 composite. The effects of the
371 °C exposure in air for up to 300 hr on the composlte glass transition tempera-
tures, weight loss characteristics, and dimensional stability are presented. The
changes in the composite 371 °C Interlamlnar shear and flexural properties are also
presented. In addition, composite Interfaclal degradation as a function of expo-
sure time at 371 °C was followed by scanning electron microscopy. The results
suggest that the composite materials can be used at 371 °C for at least lO0 hr.
INTRODUCTION
High-performance polymer matrix composites constitute a class of engineering
materials that have significant potential for application in advanced aircraft
engines. In the past lO years considerable effort has been made toward the deslgn
and fabrication of a variety of advanced composite components which operate at a
temperature range of 260 to 318 eC (ref. l). A significant improvement in engine
efficiency could be achieved by extending the use temperature of advanced compos-
ites up to 371 °C. Thus far, most high-temperature polymer matrix composite stud-
ies have been conducted at temperatures of 316 °C and below. Composite performance
above 316 "C, however, has not been extensively investigated (refs. 2 and 3).
Because of this lack of property data at temperatures above 318 °C, composite
applications at higher temperatures, such as 371 °C, cannot be assessed realis-
tically at the present time.
In 1981, a new series of N-phenylnadlmlde (PN) modified PMR polylmldes
(ref. 4) was reported. These are designated as PMR-PI, -P2, etc., where Pl, P2,
etc., refer to a formulation containing a given quantity of PN. Collectively, they
are designated as PMR-PN. Initial studies revealed that the composites prepared
from these matrices and Cellon 8000 graphite fibers exhibited excellent process-
ability and thermo-oxldatlve stability a_ 316 °C in air for up to 1500 hr (ref. 4).
Composite dynamic mechanical properties were also investigated from the viewpoint
of clarifying the resin chemistry (ref. 5).
This paper reviews the rheology, processing, chemistry, and 371 °C properties
of PMR-PN resins and their composites reinforced with Cellon 6000 graphite fibers.
Data are presented on the physical, thermal, and mechanical properties of the com-
posites aged at 371 °C in air.
243
https://ntrs.nasa.gov/search.jsp?R=19860001811 2020-03-23T06:25:09+00:00Z
RESULTS AND DISCUSSION
Rheologlcal and Processing Characteristics
Resin rheoloqy. - The modified resins are prepared from In-sltu thermal poly-
merlzatlon of four monomer reactants: the monomethyl ester of 5-norbornene-
2,3-dlcarboxyllc acld (NE), 4,4'-methylenedlanlllne (MDA), the dlmethyl ester of
3,3,4,4'-benzophenonetetracarboxyllc acld (BTDE), and N-phenylnadlmlde (PN). Their
chemical structures along with the idealized cross-llnked polylmlde structure are
shown in figure I. Table I gives the chemical compositions of the modified
(PMR-PN) and unmodified (PMR-15) PMR polylmldes investigated. This table shows
that the PN contents In the four PMR-PN formulations are relatively low compared to
the amounts of the other three monomers.
Despite relatively low quantities of PN employed, PN exerts a significant
Influence on the rheologlcal properties of PMR-PN resins and composites as
explained below. Further, the flow properties of the modified resins exhibited a
strong dependence on the stereochemlstry of PN which exists in two isomeric forms:
endo and exo. As seen In figure 2, the use of endo-N-phenylnadlmlde In PMR-PI
formulation (curve c) causes a decrease while the use of exo-N-phenylnadlmlde In
the same formulation (curve a) causes an increase in the minimum viscosity of
PMR-15 (curve by. Thls behavior can be attributed to the meltlng-polnt difference
between these two isomers as shown in figure 3. The endo-lsomer shows a sharp
endotherm at 145 °C due to melting, while the exo-lsomer shows a considerably
higher melting temperature (195 °C). The exotherms at 360 and 367 °C for exo- and
endo-PN, respectively, are due to their polymerization reaction.
Therefore, to obtain improved flow, the use of the endo-form of PN, rather
than the exo-lsomer, is recommended. For thls reason, PMR-PN resins reported here-
after use the endo-form of PN to improve their flow characteristics.
Figure 4 shows the influence of endo-PN concentration on the dynamic shear
viscosity of the Imldlzed molding powders (ref. 4). In comparing curves b through
9 wlth curve a, It can be seen that the PMR-PN samples exhibited lower shear vis-
cosities than PMR-15 over the temperature range of 238 to 303 °C. The minimum vis-
cosities for the PMR-PN samples ranged from 1769 dyne/cm 2 for PMR-PI to 884 dyne/cm 2
for PMR-P5, compared with a value of 2940 dyne/cm _ for the control PMR-15. This
represents a decrease of 40 to 70 percent in the minimum viscosity of PMR-15.
Figure 5 shows that the addition of the first 4 mole percent of PN (PMR-PI) to
PMR-15 causes a significant and disproportionate reduction In the minimum shear
viscosity. A linear correlation between the minimum viscosity and PN concentration
appears to exist in the PN concentration range of 4 to 20 mole percent.
Processing characteristics. - The resin flow characteristics in composites are
known to have significant effects on composite processing and properties. A plot
of the resin flow In Cellon 6000/PMR-PN composites as a function of the PN concen-
tration Is shown In figure 6. Note that, In order to provide a valid basis for
comparing resin flow behavior, all of the composites were molded using the same
cure cycle (ref. 4). From figure 6, It Is evident that the PMR-PN composites, llke
the PMR-PN molding powders discussed previously, exhibited improved resin flow com-
pared to PMR-15 composite. These results indicate that PN is an effective flow-
modifying agent. Thls can be attributed to the fact that PN exists in the liquid
244
state up to the cure temperature (316 °C). These results also suggest that the
flow properties of PRR-PN resins and composites can be tailored to meet specific
processing requirements by adding small quantities of PN.
Studies reported In reference 4 showed that resin flow tn compression-molded
PMR-PN composites could also be controlled by varying the pressurization tempera-
ture In the cure cycle. For example, increasing the pressurization temperature
from 232 °C for PRR-15 to 282, 288, 293, and 299 °C for PRR-P1, -P2, -P3, and -P4
composite systems, respectively, reduced the composite resin flow to about 3 per-
cent, which ts equivalent to the resin flow of PMR-15 composite.
Postulated Structure of PNR-PN Polylmldes
As mentioned previously, figure 1 shows the chemtcal structures of four mono-
mer reactants, PN, NE, MDA, and BTDE, used in the synthesis of PHR-PN polylmldes.
It Is postulated that the three monomers, NE, MDA, and BTDE, first react to form a
PMR-15 lmtde prepolymer through an amtc acid intermediate. It ts further postu-
lated that the lmlde prepolymer can undergo a crossllnktng reaction with ttself and
with PN to give a crossllnked homopolymer or copolymer system. The presence of a
copolymer as an integral part of the polymer system ts supported by the dynamic
mechanical properties reported previously (ref. 5).
The incorporation of PN into the PRR-15 polymer network could result tn a
change In the basic structure of PRR-15 which, In turn, could alter the chemtcal
properties of PMR-15. The idealized structures of PHR-15 and PHR-PN polytmldes are
shown In figures ? and 8, respectively. In PRR-15 the molecular structure between
the crossllnklng sites consists of several aromatic and tmlde functionalities. A
molecular model (not shown) of the Idealized structure of PHR-15 demonstrated that
there Is conslderable sterlc crowding and interaction between neighboring chains.
This stertc crowding can be substantially reduced, when some of the PRR-15 chain
groups are replaced with the pendant phenyl group from PN. With such a replace-
ment, not only would the sterlc crowding be reduced, the crosslinklng density would
also be reduced because there are only two crossllnktng sites in each PN, while
there are four In each PMR-15 structural unit. Thus, the replacement would lead to
a copolymer having increased flexlblltty due to decreased crossltnklng density and
posslbly increased free volume. This hypothesis is consistent wlth the phystcal
and mechanical properties determined In the previous studies (refs. 4 and 5).
Effects of 371 °C Postcure and Aging on Physical, Thermal, and
Mechanical Properties of Celton 6000/PHR-PN Composites
Physical properties. - The effects of 16- and 30-hr postcures at 371 "C on
composite weight loss and dimensional changes of the composite systems are listed
In table II. Approximately 2 percent welght loss was experienced by a11 composites
after 30 hr of postcure. A11 composites exhibited 1 percent shrinkage In the
transverse direction to the fibers, and as expected, nondetectable dimensional
change In the longitudinal direction for either 16- or 30-hr postcure. Weight
losses at 371 °C for exposure times up to 300 hr are shown in ftgure 9. After
300 hr, PMR-P1 and PMR-P2 exhibited the lowest weight losses (~5.7) compared to the
control system PHR-15 (7.2).
245
Thermal properties (Tgl. - Figure 10 shows the effects of 371 °C exposure in
alr on the glass transition temperature (Tg) of the composites. The lower lnl-
ttal Tg for PMR-PN systems compared to the Ta of PMR-15 system may be explained
on the-basts of a lower crossllnk density foF PMR-PN relative to PMR-15 as dis-
cussed previously. The attainment of maximum Tg for the PMR-PN and PNR-15 com-
posites follows a stepwlse process and occurs In two distinct stages: (a) stage
one occurs within the first 30 hr of postcure In which a rapid increase in Tg
occurs. The rate constant (k 1) for thts step varied from 2.0 °C/hr for the con-
trol (PMR-15) to 1.5 °C/hr for the P-4 specimen; (b) stage two occurs between 30 to
-160 hr; the rate constant k 2 was found to be 0.11 °C/hr, which ts approximately
14 times slower than k1. Thts may be explained by a decrease In concentration of
crossllnkable groups wtth time and/or by a quenching effect due to molecular
immobility. The dependence of Tg on the exposure time can be expressed by the
following equation:
Tg = ki In t + C (I)
where kI Is the rate constant for either stage one or stage two, t is time,
and C Is a constant. Normally, a postcure time of 10 hr at 316 °C is employed
for PMR composite materials intended for use at 316 °C. The results presented In
figure 10 and equation (1) suggest that longer postcure times at 371 °C are
required, If the materials are to be used at 371 °C. The composites attained a
Tg of 371 °C after a postcure time of 50 hr or longer at 371 °C.
Mechanical properties. - Figures 11 and 12 show the 371 °C mechanical
properties of the composites, after a 30-hr 371 °C postcure, followed by exposure
up to 100 hr In air at 371 °C. Even though testing temperature (371 °C) was above
or close to the glass transition temperatures, most of the composite materials
exhibited good mechanical properties at 371 °C during the first 100 hr of
exposure. Thus, a 30-hr postcure time appears to be adequate for attaining a
useful level of mechanical properties at 371 °C. Prior to exposure at 371 °C,
PHR-PN composites exhibited lower flexural strengths (fig. 11), but somewhat higher
lnterlamlnar shear strengths (fig. 12), compared to the control. After the lO0-hr
exposure, all PMR-PN composites exhibited higher flexural and lnterlamtnar shear
strengths than the control specimen PMR-15, particularly the PHR-P1 and PHR-P2
composite systems. The increase In the 371 °C flexural and lnterlamlnar shear
properties during the 50-hr exposure tnterval from 50 to 100 hr indicates that the
useful )lfetlme of PMR-PN composite materials may be greater than 100 hr. This is
supported by the thermogravtmetrlc analysis (TGA) data of PMR-P1 resin shown In
figure 13 which shows that PMR-P1 resin aged for 100 hr at 371 °C In air Is more
thermo-oxtdatlvely stable than both the unaged and 371 °C, 200-hr aged samples.
In addition to the mechanical properties presented previously, the trend for
the higher thermo-oxldattve stability of PMR-P1 and PMR-P2 composite systems at
371 °C Is further supported by the weight loss data presented tn figure 9 and the
fiber retention comparison Illustrated tn figure 14. Thus, the thermo-oxldatlve
stability of the PMR-PN composite systems follows the order (from ref. 4)
PMR-PI ~ PMR-P2 > PMR-P3 ~ PMR-I5 > PMR-P4
Scola reported that the thermo-oxldatlve degradation of Cellon 6000/PMR-15 at
335 °C occurs only at exposed composite s_rfaces and that oxidative degradation and
pyrolysis do not occur within the bulk of'the composite system (ref. 3). Figure 15
shows the SEM mlcrographs of PMR-PI composltes aged at 371 °C for tlme periods up
to 200 hr. The mlcrographs shown were taken near the exposed surfaces of the
246
composites. No matrix or Interfaclal degradation was observed as a result of the
30-hr postcure at 371 °C in air (fig. 15(a)). However, some matrix degradation
occurred between 0 and 120 hr at 371 °C in air (fig. 15(b)). Continued exposure to
200 hr resulted in extensive matrix and Interfaclal degradation (figs. 15(c) and
(d)). The other PMR-PN composite systems exhibited similar Interfaclal character-
istics as that of PMR-PI during the thermo-oxldatlve treatment.
The minimal degradation as assessed by SEM analysis and the hlqh level of
mechanical properties of the composites after lO0 hr at 371 °C kin air suggest that
the 371 °C useful life of the PMR-PN composites is at least lO0 hr.
CONCLUSIONS
PMR-PN resins exhibit higher resin flow characteristics than the unmodified
PMR-15 system. Addition of PN does not compromise the elevated temperature proper-
ties of PMR-15; in fact, the results indicate that the 371 °C shear and flexural
properties of the PMR-PI and PMR-P2 modified systems after lOO hr aging at 371 °C
are superior to those of PMR-15. The addition of PN appears to be a technically
sound approach for attaining PMR-polylmlde formulations with improved processing
characteristics without sacrificing the thermo--oxldatlve stability of the resin and
composite systems.
.
.
.
.
.
REFERENCES
Seraflnl, Tito T.; Delvlgs, Peter; and Alston, William B.: PMR Polylmldes -
Review and Update. NASA TM-82821, 1982.
Pike, R.A.; and DeCrescente, M.A.: Elevated Temperature Characteristics of
Borslc and Graphite Fiber/Polyphenylqulnoxallne Resin Composites. The Wide
World of Reinforced Plastics. Society of the Plastics Industry, New York,
1974, Section IBF.
Scola, Daniel A.: A Study of the Thermo-Oxldatlve Process and Stability of
Graphite and Glass/PMR Polylmlde Composites. 27th National SAMPE Symposium
and Exhibition, SAMPE, Azusa, CA, 1982, pp. 923-939.
Pater, Ruth H.: Novel Improved PMR Polylmldes. Technology Transfer, SAMPE,
Azusa, CA, 1981, pp. 38-52.
Pater, Ruth H.: Dynamic Mechanical Properties of N-Phenylnadlmlde Modified
PMR Polylmlde Composites. NASA TM-83051, 1983.
247
TABLE I. - COMPOSITIONS OF PMR-PN POLYIMIDES
Resin
system
_MR-I5
(control)
PMR--PI
iPMR-P2
PMR-P3
PMR- P4
Monomer (mole)
NE MDA BTDE
2 3.087 2.087
r 1
PN
0.0
.041
.099
.136
.177
PN mole
percent a
0.00
0.57
1.36
1.87
2.41
apN mole percent = moles of PN/moles of
(NE+MDA+BTDE+PN).
TABLE II. - EFFECTS OF 317 °C POSTCURE ON PHYSICAL PROPERTIES
OF CELION 6000/PMR-PN POLYIMIOE COMPOSITES
Composite
property
Weight loss,
percent
Transverse
shrinkage,
percent
Postcure
time, hr
16
30
16
30
PMR- 15
.(control )
1.6
2.1
0.9
0.9
Resin system
PMR-PI PMR-P2
1.6 1.6
2.0 2.1
o.g 1.0
l.0 l.l
PMR- P3
1.6
2.2
1.0
1.1
PMR-P4
1.7
2.3
l.O
l.l
248
0 0 O O| H 0 II
_C"NII -_ + '_I_C- OM_ H _'_ " _'_NH MeO_C..,.,__C..,_C_OMel
"_'c_ "=" tlJ..c__ _u'_u_2u_- 2__cA,_J u_..___| | |
0 0 0 0
PN NE MOA gTOE
(1) 204"C, 1 hr
(2) 316"C. 2 hr
34.5 x IOS Nhn 2
O O O 2r0_? O_
; PMR-PN POLYIMIDE
i\ .o
O
Figure 1. - Synthetic reaction for PMR-PN polylmldes.
e_
3E
t_J
a_
:to-
O
a
:D
im
u')
=D
LJJ
U_
0
-J
6
5
4
3
240 320
Figure 2. - Log shear viscosity as
function of temperature for molding
powders.(a) PMR-PI using exo-PN.
(b) PMR-15.(c) PMR-PI using endo-PN.
Temperature, eC
EXO
PMR-I5(Control)
-NDO
24g
ENDO-N-phenylna dimide
! -x',
EiO EXO-N-phenylnadimide
_0
0 100 200 300 4oo
Tempefalul'o, *¢
Figure 3.- Differential scanning calorimetry scans
of endo- and exo-PN.
_z
_J
_wJ
Z
C:
:D
(X
m
238
Figure 4. - Log shear vlscoslty as function of
temperature for PMR-PN molding powders.
250
X 10 3
3
a
2
IE
Z
0 I I I I
0 4 8 12 16 20
MOLE PERCENT OF N-PHENYLNADIMIDE (PN)
Ftgure 5. - Hlnlmum shear viscosity as function of PN concentration
for PRR-PN molding powders.
2O
15
UJ
O.
O_ 10
Ix
Z
!
O0
UJ
oc 5
0
O
O
I I I I I
0 2 4 6 8 10
MOLE PERCENT OF N-PHENYLNADIMIDE
Figure 6. - Resin flow as functton of PN concentration for
Celton 6000/PHR-PN composites.
2
251
_+,,+' F l + o 7
I!' ° F ILl + + + 7 J''
Figure 7. - Ideallzed structure for PRR-15.
I
o r'o o o 7
I II _ I I
o .+
+?.
Ftgure 8. - Idealized structure for PNR-PN polytmlde.
252
10
8
F--
L_
_J
p,-
W
"_ 4
k--
L=J
2
371 "C AIR EXPOSURE UP TO 300 hr
3OO
hr
• --300 ' -300
; hr hr
2OO
- 200 - 200
-125 - 125
-100 -1001
ii )o) .so,"25 - 25
0
(CONTROL)
- 300 - 300hr
II i
hr i
- 200 - 200
-150 -150
-125 -125
-100 -100
-50 -50
-25 -25
PMR-15 PMR-P1 PMR-P2 PMR-P3 PMR-P4
RESIN SYSTEM
Figure 9. - gelght loss of postcured Cellon 6000/PMR-PN composites.
(.)
O
=;
r,-
p-
¢¢
W
W
p-
Z
O
p-
4OO
¢/3
Z
¢/3
¢/3
'=C
.J
¢-q
375
35O
325
300
0 400
• PMR-lS (CONTROL)
O PMR-P1
PMR-P2
PMR-P3
PMR-P4
AFTER 371 *C AIR EXPOSURE
I I I
100 200 300
EXPOSURE TIME, hours
Figure 10. - Glass transition temperature as function
of exposure time for as fabricated Celton 6000/PMR-PN
composites.
253
IL_
So.S_
254
A CELION 60001PMR-P1 U 
1 ooop 
U 
1 ooop 
B. CONTROL CELION 60001PMR-15 
Figure 14. - Comparlson of fiber retentlon characteristics 
for Celion 6000/PMR-P1 and control PMR-15 composites after 
a 30 hr postcure followed by a 200 hr exposure in air at 371 "C. 
255  
v) a 
3 
0 
I 
0 
v) 
7 
a 
a 
m 
W 
k 
L 
f 
256 


Rheological, processing, and 371 deg C mechanical properties of Celion 6000/N-phenylnadimide modified PMR composites

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