A characterization methodology has been developed on which to base quality assurance procedures for U.S. Polymeric V378A bismaleimide matrix resin. Chemical composition is established by partition reverse phase and size exclusion liquid chromatography. Cure rheology behavior is quantitatively characterized by dynamic viscoelastic analysis using the parallel plate technique. The overall cure process is characterized by differential scanning calorimetry. The sensitivity of the procedures is evaluated by studying the effects of ambient out time on the chemical end behaviorial properties of the resin

Dynes, P. J.

Hamermesh, C. L.

English

NASA Technical Reports Server

NSS- 12  
dR. +_.
QUALITY ASSURANCE PROCEDURES FOR V378A MATRIX RESIN*
Charles L. Hamermesh and Paul J. Dynes
Rockwell International Science Center
A characterization methodology has been developed on which to base quality
assurance procedures for U.S. Polymeric V378A bismaleimide matrix resin. Chemical
composition was established by partition reverse-phase and size exclusion liquid
chromatography. Cure rheology behavior was quantitatively characterized by dynamic
viscoelastic analysis using the parallel-plate technique. The overall cure process
was characterized by differential scanning calorimetry. The sensitivity of the
procedures was evaluated by studying the effects of ambient out-time on the chemical
and behavorial properties of the resin.
INTRODUCTION
The use of composites in aerospace applications has grown substantially in
recent years and it appears that this upsurge will continue into the foreseeable
future. Composites, however, are only useful if their performance is both reliable
and reproducible. With the state-of-the-art composite materials, the epoxy resins,
a considerable effort was undertaken a number of years ago to define the chemical
nature and behavior of such materlal so that quality of composites could be assured.
Bismaleimides, although not new, are being reexamined as potential candidates
for the next generation of matrix resins for intermediate temperature usage in the
450-505 K (350-450°F) range. As a class, bismaleimides show generally higher cured
glass transition temperatures and less environmental sensitivity than state-of-the-
art epoxies. They have suffered, however, from too much brittleness and difficult
processibility.
These problems have been alleviated to a large degree in V378A, a modified
bismaleimide developed by U.S. Polymeric (ref. 1). This system possesses the easy
processibility characteristics of epoxies but shows improved retention of mechanical
properties at 450 K (3500F) under moisture saturation conditions. Its rapid devel-
opment and testing in primary aircraft structures have prompted the need for a
characterization methodology upon which to base quality assurance procedures.
V378A, like most matrix resins, with the exceptions of the PMR type systems devel-
oped by the Government owned laboratories, is a proprietary formulation. Thus, the
specific composition and exact nature of the ingredients which make up the system
are not likely to be divulged by its owners beyond such general descriptions as "a
formulation of a modified bismaleimide and at least two volatile components." As
*This work was performed under the auspices of the Materials Laboratory,
Air Force Wright Aeronautical Laboratories, Wright-Patterson AFB, OH 45433.
171
https://ntrs.nasa.gov/search.jsp?R=19860001807 2020-03-23T06:25:21+00:00Z
this paper shows, this need not be a deterrent to the development of the character-
ization procedures for quality assurance of the material. With a good understanding
of the underlying chemistry of bismaleimide resins and the inferences which can be
drawn as to the general nature and purpose of the volatile components, a strong base
can be established for the characterization of such a system despite its proprietary
nature and without jeopardizing the rights of its owner.
SYMBOLS
G _
G'
G I,
q*
Ea
dH/dT
AH
¢
TOS
TIp
TEXO
dynamic shear modulus, N/m2 (dynes/cm 2)
storage shear modulus, N/m 2 (dynes/cm 2)
loss shear modulus, N/m 2 (dynes/cm 2)
oscillating shear frequency rad/s
dynamic viscosity, N.s/m 2 (poise)
activation energy, J/mole (kcal/mole)
differential heat input, J/s (mcal/s)
heat of polymerization, J/g (cal/g)
heating rate, K/mln
DSC exotherm onset temperature, K
DSC exotherm inflection temperature, K
DSC exotherm peak temperature, K
HIGH PERFORMANCE LIQUID CHROMATOGRAPHY
The primary goal of the liquid chromatographic study was to develop a quan-
titative procedure capable of separating the primary ingredients present in V378A
resin. These included the major resin, which is composed of several fractions, and
the two volatile components In the formulation.
The liquid chromatography method chosen was a partition reverse-phase separa-
tion utilizing a water/acetonitrlle solvent gradient. This type of analysis has
proven to be the most successful for prepolymer resins containing a broad range of
component polarities and oligomer molecular weights. Detection of components was
made by ultraviolet spectrophotometry. A detection wavelength of 200 nm was chosen
to enhance the analysis of Volatile Component 2 in the resin. In order to com-
pletely dissolve the resin for analysis a minimum of 25% dimethylformamide in tetra-
hydrofuran was required. A summary of the chromatographic conditions used for the
analysis of V378A resin is given in table I.
172
The separation of V378A resin using this procedure is shown in figure 1. A
total of approximately eighteen components are resolved. Eight of these have been
identified as belonging to one of the three major ingredients of V378A resin. Iden-
tification was based on peak elutton times verified by spiking of V378A resin with
individual ingredients. The liquid chromatographic separations of the three major
components of V378A resin are shown tn figure 2.
Although a large number of components are separated by the partition liquid
chromatographic technique, it was found that the acetonttrlle or tetrahydrofuran
insoluble fractions of V378A resin did not elute during the water/acetonttrtle
solvent gradient. Moreover, the fractions soluble in tetrahydrofuran or aceto-
nttrtle gave the same chromatographic peaks and peak area distibuttons as did
totally solubtlized samples. The V378A resin system thus appears to be comprised of
two fractions. One is of relatively low molecular weight consisting of the two
volatile components and btsmaletmide resin oltgomers. The insoluble fraction is
presumably composed of products formed by reactions amon; these starting
ingredients.
Characterization of the Insoluble fraction of V378A resin was made by size
exclusion liquid chromatography. With tetrahydrofuran as solvent, the once solu-
bilized resin remains in solution during size exclusion analysis. The experimental
details of this technique are given in table 2. The size exclusion separation of
V378A resin is shown in figure 3. The first and therefore highest molecular weight
component to elute was determined to correspond to the insoluble fraction of the
resin not detected by partition liquid chromatography. The remaining ingredients,
however, are not resolved as well as with the partition method. Partition and size
exclusion liquid chromatography thus complement each other and together provide the
means for a comprehensive analysis of the chemical constituents of V378A resin.
CURE RHEOLOGY
The rheologlcal behavior of V378A neat resin during cure was studied with the
Rheometrics Visco-Elastlc Tester. The experimental conditions used to analyze the
resin are given in table 3. The instrument calculates the dynamic shear modulus G*
and its two components, the dynamic storage or elastic modulus G' and the dynamic
loss or viscous modulus G". A dynamic viscosity is defined from the dynamic modulus
as, n* = G*/_, where _ is the oscillatory shear frequency. The variation in the
dynamic viscoelastic properties of V378A resin during a 2 K/min heating cycle is
plotted in flgure 4.
Several factors critical to a matrix resin's processibility can be derived from
these data. One is the initial value of the dynamic viscosity. V378A has a room
tempertaure value of ~ 6 x 102 N.s/m 2 (6 x 103 poise) which imparts satisfactory
drape in the prepreg. Resin dynamic viscosity decreases with increasing temperature "
until around 383 K where it reaches a minimum of ~ 5 x 10-I N.s/m 2 (5 poise). Above
this temperature, polymerization reactions bring about a rapid rise in dynamic
viscosity. At a specific temperature the shear storage and loss modull are equal.
This midway point between viscous llquid behavior and solid state elastic response
has been found to correlate with resin gelation as measured by standards methods
(ref. 2).
173
The dynamic viscosity profiles for several cure cycle heating rates are shown
in figure 5. Faster heating rates tend to reduce the minimum dynamic viscosity
reached during cure. This effect can be of value for controlling the amount of flow
desired during processing. The temperatures at which gelation occurs for four
heating rates are given in table 4. By analogy to differential scanning calo-
rimetry, the dependence of the temperature at gelation on heating rate can be
plotted in a modified Arrhenius equation (ref. 3). The data are plotted in figure 6
where a good fit to the Arrhenius equation is found. An activation energy to
gelatton of 89.5 J/mole (21.4 kcal/mole) is calculated from the slope of the curve.
The rheological behavior of V378A resin was also explored under isothemal cure
conditions. The dyn_mtc viscoelastic properties of V378A resin at 393 K are shown
in figure 7. Initially, the resin displays a very low viscosity with almost no
elastic character (G' - 0). At longer times G' increases rapidly and surpasses
G". Again gelation is associated wih the crossover of the two dynamic moduli. The
gel times determined by this method are given in table 5 for several temperatures.
These data are plotted in figure 8 according to the Arrhenius equation. A linear
relationship is found and from the slope of the curve an activation energy of
89.7 J/mole (21.4 kcal/mole) to gelation is derived.
DIFFERENTIAL SCANNING CALORIMETRY
The differential scanning calorimetry (DSC) behavior of V378A resin is shown in
figure 9 for a heating rate of 1.25 K/min. Curing is characterized by two exo-
therms. The major exotherm is centered at 415 K (142°C) while a second much smaller
one occurs about 70 K higher. The major exotherm is related to reactions involving
Volatile Component 1. The exotherm at higher temperature appears to result
primarily from additional polymerizatlon of bismaleimide groups.
Several DSC parameters were evaluated for quality assurance purposes. They
include the reaction onset temperature TOS, the peak inflection temperature Tie, the
peak exotherm temperature TEX and the heats of polymerization AH associated wlth
the two reaction exotherms. _ summary of such data for four heating rate experi-
ments is given in table 6.
Heats of polymerization were not sufficiently reproducible to use as quality
assurance parameters. The characteristic thermogram temperatures, however, can be
determined more precisely. The peak exotherm temperature TEX n, in particular, is
very reproducible. The inflection point temperature TTD was also nvestlgated, as
it has been show to coincide with gelatlon in epoxy reign systems (ref. 3). The
heating rate dependence of TTp and TEX n are plotted in figure 10 according to the
modified Arrhenius relation a_scussea-_reviously. Linear relationships are obtained
which define activation energies of 78.9 J/mole (18.7 kcal/mole), 69.1 J/mole
(16.5 kcal/mole) from T X and TIF, respectively. The inflection point temperature
data in table 6 are in _aVrly good agreement with the rheologically determined gel
temperatures given in table 4. Experimentally, gel times determined from
rheological data are more accurate and reproducible than can be derived from DSC
measurements.
174
RESINOUT-TIME VALUATION
It was of interest to examine the effects of ambient aging on the properties of
V378A neat resin. Film samples of resin were exposed to 296 K and 30-60% RH
conditions for periods up to three weeks. At ambient the resin loses its tack
within one day. It has, however, a twenty-one day recommended out-tlme in terms of
processibility.
Partition liquid chromatographic analysis of resin aging revealed only one
significant change to be taking place. The concentration of Volatile Component 1
decreases as shown in figure 11. The decrease in Volatile Component 1 occurs as a
result of both B-staging and evaporation. Weight loss measurements indicate
approximately one-third loss due to evaporation and the remainder cause by resin
advancement. Evidence of resin advancement is shown by size exclusion chroma-
tography in figure 12 where the concentratlon of early elutlng hlghr molecular
weight species increases with resin aging.
Two rheological tests were used to characterize the effects of ambient aging on
V378A resin. They were a linear 2 K/min heating rate cure and a 398 K isothermal
gel time test. The results of the 2 K/min cure cycle experiments are shown in
figure 13. The dynamic viscosity profiles show a significant increase with ambient
aging time. A plot of the gel times determined from the isothermal rheology data
are plotted in figure 14. For short aging times the gel time increases and then
drops markedly around the fourth day of aging.
DSC is a sensitive tool for analyzlng the effect of aging on V378A resin.
Thermograms for resin aged up to 20 days are plotted in figure 15. The primary
result of out-time is a reduction in the major DSC exotherm which is associated with
the reactions of Volatile Component 1. The smaller high temperature exotherm
appears to be unaffected by ambient aging. The major exotherm has a slight shoulder
located at 370-390 K. After about 1 week of aging, this shoulder begins to resolve
itself into a new exotherm peak centered near 395 K. These changes are indicative
of a resin in which more than one path of cure is available. In such cases, the
cure temperature and kinetic parameters of the various reaction paths control which
path predominates.
REFERENCES
I.
Q
.
McKague, L: V378A Polymide Resin - A New Composite Matrix for the 80's.
Proceedings of ASTM Symposium on Composites for Extreme Environments, Bal
Harbour, FL, November 11, 1980.
Tung, M.C.; Dynes, P.J: Relationship Between Viscoelastic Properties and
Gelation in Thermosetting Systems. J. App1. Polym. Scl., 2__7.7,569 (1982).
Carpenter, J.F: Instrumental Techniques for Oeveloplng Epoxy Cure Cycles.
Proceedings of 21st National SAMPE Symposium and Exhibition, Los Angeles, CA,
April 6-8, 1976.
175
Table 1
Experimental Conditions for Separation of
U.S. Polymeric V378A Resin by Partition
Liquid Chromatography
Col umn:
Solvent:
Gradi ent:
Flow:
Sample:
Spectra-Physics Sperisorb 10 ODS
Baker Liquid Chromatographic Water
Burdick and Jackson Acetonitrile
10-80% CH3CH/H20, Linear in 50 min
Ultraviolet at 200 nm
10 ul of a 1.5 mg/ml solution in
25/75 DMF/THF
Table 2
Experimental Conditions for Separation of
U.S. Polymeric V378A Resin by Size
Exclusion Chromatography
Column:
Solvent:
FIow:
Detec tion:
Sample:
Waters Associates u-Styragel
1 (500A) + 2 (IOOA)
Burdick and Jackson Tetrahydrofuran
(Unstabilized UV Grade
1 ml/mln
Ultraviolet at 254 nm
100 ul of a 2 mg/ml solution in
25/75 DMF/THF
Tabl e 3
Rheometrics Visco-Elastic Tester
Experim_-.tal Parameters
Geometry
Frequency
Gap
Percent Strain
Environment
25 mm Parallel plates
10 rad/s
0.5 mm
< 10%
Dry nitrogen
I76
Table 4
Variation in Temperature at
Gelation According to G'=G"
Criterion of U.S. Polymeric
V378A Resin
¢ T (G' =G")
K/rain K
2 401
3 408
4 412
5 415
Table 5
Variation in Time to Gelation
According to G'=G" Criterion
of U.S. Polymeric V378A Resin
Temperature Gel Time
K min
388 18.6
393 11.3
398 8.2
413 3.2
Table 6
Differential Scanning Calorimetry Analysis
of U.S. Polymeric V378A Resin
¢ TOS Tip TEX 0 AHz AH2
K/mln K J/g
1.25 363 388 402 235 18
2.50 345 403 415 214 15
5.00 366 418 429 264 10
10.0 367 429 442 205 10
177
It- i
<
M,I
¢/)
z
¢¢
o
i-
I,-
I
Figure 2.
DMF
MAJOR
RESIN
VOLATILE
2
\
21- __
4 3_
e_
7
8 9 ,
$C82o171_ha.
, VOLATILE 1
14
_1312
11
10
ELUTION TIME
Figure I.
Partition liquid chromatographic
analysis of U.S. Polymeric V378A
resin.
Z
o
o
h-
<
uJ
(/)
Z
2
(/)
¢u
m
m
0
0
&M
.m
a
MAJOR
RESIN
ELUTION TIME
e,.
m
O
I-
¢J
IM
 -ruM,I
O
o
r'-,
VOLATILE1
ELUTION TIME
VOLATILE2
ELUTION TIME
Partition liquid chromatographic analysis of U.S. Polymeric V378A resln
Ingredlents.
178
Zu_
_q
uJ
o0
Z
O
¢/)
uJ
mr
mr
O
F-
uJ
r_
VOLATILE
COMPONENT 1 --_
HIGH
MOLECULAR
WEIGHT
COMPONENT
;C82-17147
I
_MFI
)
I
!I
i
i
Figure 3.
ELUTION TIME
Size exclusion liquid chromatographic analysis of U.S. Polymeric V378A
resin,
3
32
o
.J
5
I I I
0 ¸
293
G'(N/m 2)
13'=13"'
333 373 413
TEMPERATURE (°K)
SC81-14054
453
Figure 4.
Dynamic viscoelastic behavior of
U.S. Polymeric V378A resin at a
2 K/min heating rate.
179
4E
z 2
%
0
..J
1
SC81-14055
i I i
5 °K/min
-1 I I l
293 333 373 413 452
TEMPERATURE (°K)
Figure 5.
Effect of heating rate on the cure
rheolo_ of U.S. Polymeric V378A
restn.
ICIl.140S0
I I I I I
1.6
1.2
0,4
1
2.40
m
I I I I I
2.42 2.44 2.46 2.48 2.50
I
T--G' = G" [°K-l] x 103
Figure 6.
Dependence of the dynamic modu14
crossover temperature wtth heating
rate for the cure of U.S. Polymeric
V378A restn.
180
3_"2
¢3
¢3
O 1
--I
0-
-1
0
G'(N/m 2)
?'t*(N • s/m 2)
G"(N/m 2)
I I
4 8
I I I
12 16 20
TIME (min)
SC81-1405R
24
Figure 7.
Dynamic viscoelastic
behavior of U.S. Polymeric
V378A resin at 393 K.
II
m
,0 ''
2.8 "
: 2.2-
r3
1.8-
1.4-
1.0 |
2.2 2.3
1 m I I IO
89.7 J/mole
I I I I
2.4 2.5 2.6 2.7
1
_-" [°K-l] x 103
SCII1-14051
2.8
Figure 8.
Dependence of the dynamic
modull crossover time wlth
isothermal cure temperature
for U.S. Polymeric V378A
resin.
j IBl
O.OI _ I I I I I
TExo
o.oc / \
-- TIp_/ _
0.04- / \
O_ I I I I 1 I
340 370 400 430 460 490 520
TEMPERATURE (°K)
sc82-17143
550
Figure 9. DSC thermogram for the curing of U.S. Polymeric V378A resin.
e-
3
2
0
SC81-14048
I I I I
TEXO
2.20
TIp
J/MOLE
I I "1 -"1
2.30 2.40 2.50 2.60
I
_" [°K-l] x 103
Figure 10.
Dependence of DSC exotherm
peak and inflection point
temperature with heating
rate for U.S. Polymeric
V378A resin.
182 "
1.0_
1.2 I [ z _ I "
m
VOLATILE COMPONENT 1
o 1 I [ I I
0 4 8 12 16 20
0.8Z
O
p-
<_
oc
_. 0.6
Z
uJ
_3
Z
o
0.4
0.2
AMBIENT EXPOSURE (DAYS)
Figure 11.
Effect of ambient aging on the
concentration of Volatile
Component 1 in U.S. Polymeric
V378A resin.
UNAGED
20DAYS
ELUTION TIME
Figure 12.
Effect of ambient aging on the size
exclusion liquid chromatographic
separation of U.S. Polymeric V378A
resin.
183
5 m
4-
2
I I
0-
293
I ! I
I I I
333 373 413
TEMPERATURE (OK)
SC81-15287
453
Figure 13.
Effect of ambient aging on the
cure rheology of U.S. Polymeric
V378A resin.
-_=
E
v
uJ
R
k-
.J
MJ
12
10F
8-
6-
4-
2-
0
0
I I
I I I I
4 8 12 16
AMBIENT EXPOSURE (days)
ISC81-152nlI "
o---
I
20
Figure 14.
Effect of ambient aging on the
tlme of gelatlon at 398K for
U.S. Polymeric V378A resin.
24
184
SC81-15289
I I 1 I I I
UNAGED
:¢
'ID
320
Figure 15.
I I I I
360 400 440 480 520 560 600
TEMPERATURE (°K)
Ambient agtng effect on the DSC behavior
of U.S. Polymeric V378A restn.
185


Quality assurance procedures for V378A matrix resin

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