A modification to the addition polyimide, LaRC-160, was prepared to improve tack and drape and increase prepeg out-time. The essentially solventless, high viscosity laminating resin is synthesized from low cost liquid monomers. The modified version takes advantage of a reactive, liquid plasticizer which is used in place of solvent and helps solve a major problem of maintaining good prepeg tack and drape, or the ability of the prepeg to adhere to adjacent plies and conform to a desired shape during the lay up process. This alternate solventless approach allows both longer life of the polymer prepeg and the processing of low void laminates. This approach appears to be applicable to all addition polyimide systems

Butler, J. M.

St.clair, T. L.

English

NASA Technical Reports Server

,_/,_Az-,,_-F,',PZ'_
3 1176 00162 8156
NASA Technical Memorandum 81883
NASA-TM-81883 19800025062
. TACKIFIERFORADDITIONPOLYIMIDES
John M, Butler and Terry L, St, Clair
OCTOBER1980
OCT2 8 "_'_,,GU
EA_iL4.._-: "_'L_;E:-',_i:: i .'3,EHTE__7
LH3P,/,RY, i'_ASA
NationalAeronauticsand
Space Administration
LangleyResearchContor
Hampton,Virginia23665
https://ntrs.nasa.gov/search.jsp?R=19800025062 2020-03-21T15:42:58+00:00Z

INTRODUCTION
The use of polyimide resin matrices for advanced composite
materials has shown a continuous increase in the aerospace
industry. The ubiquitous problem associated with this type
of material is the lack of "tack and drape" after prepregging
solvent is lost. The loss of solvent in high molecular weight
systems which use the polyamic acid approach to polymerization
leaves a tough "boardy" material that is difficult to fabricate.
For low molecular weight addition systems which use the monomeric
approach, some solvent must be retained to prevent a grainy de-
posit of monomeric solids from forming on the reinforcement.
The aromatic amine monomer is usually responsible for the granu-
lar texture.
A new polyimide formulation, LaRC-160BI, has been developed
to minimize these problems. This polyimide resin is a modifica-
tion of LaRC-160 (Ref. i), which was developed at NASA Langley
Research Center. LaRC-160BI neat resin is a liquid at room
temperature made without the use of solvent. The resulting pre-
preg made from LaRC-160BI is not "boardy" and has longer retention
of tack and drape because of the incorporation of a high-boiling-
Doint, liquid reactive plasticizer. The compound used to accom-
plish this was monoethylphthalate, an endcap which replaces ten
percent of the conventional endcapDinq system. The preparation
and physical DroDerties of LaRC-160Bl/graphite prepreg and the
resulting laminates will be described.
1
EXPERIMENTAL
Monomers
3,3',4,4'-Benzophenonetetracarboxylic acid dianhvdride
(BTDA), 5-norbornene-2,3-dicarboxylic: anhydride (nadic anhydride),
and phthalic anhydride were obtained commercially and used as
received. The diamine, Jeffamine AP-22, was obtained commer-
cially from Jefferson Chemical Co., Inc., Houston, Texas.
Resin Preparation
Stoichiometric amounts of diethylester of BTDA (BTDE) 0.67
mole, and the monoethylesters of nadic anhydride (NE) 1.10 mole,
and phthalic anhydride (PE) 0.12 mole were prepared by refluxing
at 351K (172°F) the corresponding anhydrides with an excess of
ethanol (2.69 mole). The esterification reaction was carried
out in a three-neck round bottom flask equipped with a mechanical
stirring device. When all the monomers had gone into solution,
the refluxing was continued for 1/2 hour longer. The solution
was allowed to cool to 310K-320K (OF) and then mixed with Jeffamine
AP-22 (2.56 molar equivalents of amine) which had previously
been warmed to 310K-320K.
The solution was cooled, bottled and refrigerated since
the ester-acids tend to disproportionate on standing; the re-
frigeration helps extend solution stability.
The original version of LaRC-160 contained BTDE (0.67 mole),
NE (1.22 mole), ethanol (2.69 mole) and Jeffamine AP-22 (2.56 mole).
*Use of trade names or names of manufacturers in this report does
not constitute an official endorsement of such products or manu-
facturers, either expressed or implied, by the National Aeronautics
and Space Administration.
2
The process for preparation was the same as previously described(Ref, i).
The amount of ethanol and Jeffamine AP-22 remained the same. For
LaRC-160 and LaRC-160BI the mixtures stayed liquid and did not
turn granular after the solvent was evaporated.
Prepregging
A resin of LaRC-160BI was prepared at a ratio of 50% solids
by weight in ethanol. It was brushed onto Hercules HT-S graphite
fiber which had been drum wound at a spacing of 5 1/2 tows per
inch. The resulting prepreg had a 50% fiber content by weight.
The solvent content was reduced to about 10% by allowing it to
evaporate for 8-16 hours after being coated. The resulting pre-
preg with good tack and drape was then removed from the drum and
could be easily cut and formed into shaped lay-ups.
Laminate Preparation
The prepreg was cut into 7.6 cm by 17.8 cm (3 in by 7 in)
sections and stacked into 17-ply unidirectional preforms. The
preformed billet was subjected to the first stage in the curing
process. This required the preform to be vacuum bagged at 7-14
kPa, gauge (2-4 in Hg, gauge). The billet was then heated from
ambient at a rate of 2.8K (5°F) per minute to 436K (325°F) and
held at that temperature for one hour, then cooled under vacuum
to room temperature. This step is a consolidation
and imidization step which results in about a 25% weight loss
and a well-formed billet averaging 36% resin by weight.
3
Molding of Laminates
The molding procedure was done using a flat matched-metal
mold which had open ends. The consolidated billet was trimmed to
fit the mold with the fiber in the 0° direction of the open ends,
thus allowing observation of the resin flow during molding. The
platens of the press were preheated to 478K (400°F), the mold
containing the billet was introduced and contact pressure applied.
A heating rate was established at 2.8K (5°F) per minute. When
a temperature of 547 (525°F) was reached, the pressure was increased
to 1.4 MPa (200 psig) over a 1 minute span, held for 1 minute,
decreased to zero (gauge) for 15 seconds (a bumping action) and then
reapplied at 1.4 MPa (200 psig). A maximum curing temperature of
603K (625°F) was then attained and held for two hours. The panel
was cooled at 2.8K (5°F) per minute under full pressure and removed
at a temperature below 366K (200°F).
Thermooxidative Aging
The laminate panels were trimmed and cut into specimens for
both short beam shear (SBS) and flexural strength measurements.
The press-molded SBS samples were aged at a temperature of 589K
(600°F). Each test group consisted of four SBS samples that were
randomly selected and weighed as a unit before and after aging.
The flexural samples were tested without aging.
4
Air Aging
The groups of specimens were isothermally aged at 589K
(600°F) and removed for testing after 250 and 500 hours of ex-
posure. Forced air ovens with an air-change rate of 75 cc/min
(4.6 in3/min) were used. The temperature of the specimens was
monitored by prepositioned thermocouples and also by the separate
oven controls.
Testing
The mechanical testing data were obtained using an Instron
Testing Machine Model TT-6, equipped with an Instron Environmental
Chamber for the elevated temperature tests. Flexural strength and
i
moduli were determined according to ASTM D790. Short beam shear
strengths were determined using a span to thickness ratio of 4.
Each specimen was held 15 minutes at test temperature prior to
testing. Ultrasonic "C" scans were performed prior to testing
with a Nano-scope Model 414 instrument.
Results and Discussion
Processing Conditions
The procedure used in making the prepreg and laminates
can be considered a successful technique, based the fact
that the polyimide resin can be either applied to the fibers by
brush coating or immersion in a resin bath with good results (Ref. 2).
Both methods leave a flexible and tacky prepreg following evap-
oration of the solvent. The laminates can be either autoclave
5
processed or molded with matched metal dies (Ref. i). Both the
prepreg and laminates have been commercially prepared using the
original formulation of LaRC-160. The mechanical properties of
graphite laminates prepared from these prepregs were also equal
to or better than those prepared from lab scale runs (Ref. 2).
The LaRC-160BI resin system press molds very well with adequate
flow. Like LaRC-160, the LaRC-160BI is processable under low
pressure and at a moderate heating rate. The graphite-reinforced
panels were post cured at 589K (600°F) 16 hours in air with no
blistering and a weight loss of less than 0.1%. A typical cure
cycle for LaRC-160 or LaRC-160BI is shown in Figure i.
Weight Loss i
Figure 2 shows the weight loss curves for the short beam
shear (SBS) specimens (8-sample average per data point) aged in
air at 589K (600°F). There was no discoloration of the LaRC-
o,.
160BI specimens, even the ones aged up to 500 hours, thus indi-
cating this resin system to be of comparable thermooxidative
stability to LaRC-160. Approximately 2 percent laminate weight
loss after 500 hours was recorded for each of these exposure
conditions which translates into a significant resin loss of
6 percent (fiber oxidation considered minimal).
Short Beam Shear Tests
The short beam shear (SBS) strengths (8-sample average per
data point) are shown in Figure 3 for LaRC-160BI. Aging was
performed at 589K (600°F) and the testing done at various indicated
temperatures. The initial increase in strength during the first
200 hours shows a definite postcuring effect. The percent
retention of the SBS with 589K (°F) aging is shown in Figure 4.
There was no significant difference in weight loss for these
resin formulations.
Flexural Test
The flexural data (4-sample average) is presented in Table I;
the specimens were not aged but tested at room temperature and
561K (550°F). The results show that LaRC-160 and the modified
version LaRC-160BI had almost identical values.
7
Conclusions
The tack and drape characteristics of LaRC-160 polyimide/
graphite prepreg has been significantly improved by the addition
of a non-toxic, reactive, high-boiling-point phthalic ester
tackifier. The modified resin designated LaRC-160BI has an
out-time of more than six months compared to about two days for
LaRC-160. Laminates made from the LaRC-160BI prepreg had excellent
strength retention at temperatures up to 589K (600°F) and showed
slightly improved oxidative stability over laminates made from
LaRC-160 for up to 500 hours aging at 589K (600°F). The ester
additive does not add to the cost of the resin system and the
prepreg retains the good fabricability of the base LaRC-160
system. The use of an ester additive that is liquid at room
temperature is a concept that may have application to other
addition polyimide systems.
References
i. T. L. St. Clair and R. A. Jewell: LaRC-160: A New 550°F
Polyimide Laminating Resin. Preprints, 8th National, SAMPE,
Technical Conference, vol. 8, p. 82, Seattle, WA, October 12-
14, 1976.
2. T. L. St. Clair and R. A. Jewell: Solventless LaRC-160
Polyimide Matrix Resin. 23rd National SAMPE Symposium,
Anaheim, California, May 2-4, 1978.
, 8
Table I. Laminate flexural data for LaRC-160BI
FLEXSTRENGTHFLEXMODULUS
MPa(ksi) GPa
RT TEST
LARC-160 1900(276) 124
LARC-160B1 1760 (259) 124
561KTEST
LARC-160 1200(175) 124
LARC-160BI 1095(161) 124
.i
600- _k
\
550 - \\
APPLY1.4MPa(200psi) \500 - \
\
\
450 - \
TEMPERATURE, . \
K 400- " RELEASEPRESSUREX
• ANDVACUUM---,_\
350
300 PLY7-14kPa(2-5in.)VACUUM
250 -
I I I I I I I I
0 1 2 3 4 5 6 7
TIME,hr
Figure i. Cure cycle for LaRC-160 and LaRC-160BI
BO LaRC-160BI
FlLaRC-160
2
WEIGHT
LOSS,
_/o
1
0
I I I I
0. 125 250 375 500
AGING,hr
Figure 2. Comparativepercent weight loss for LaRC-160 systems aged
in air at 589K
100-
75
SHORT
BEAM
SHEAR 50 _ _ 561KSTRENGTH, ZI
MPa ]- 589K
25-
I I I I
0 125 250 375 500
AGING, hr
Figure 3. Short beam shear strengths as a function of ageing time at
589K at three test temperatures for LaRC-160BI
140-
120-
I00 '_
RETENTION
SBS RT80-
60-
'K_ I I I I
0 125 250 375 500
AGING,hr
Figure 4. Percent retention of short beam shear strengths as a function
of aging at 589K for LaRC-160BI

1. Report No. 2. GovernmentAccessionNo. 3. Recipient'sCatalogNo.
NASATM-81883
-4. Title and Subtitle 5. Report Date
October 1980
TACKIFIER FORADDITIONPOLYIMIDES 6. PerformingOrganizationCode
7. Author(s) 8. Performing Organization Report No.
John M. Butler and Terry L. St. Clair
10. Work Unit No.
9. PerformingOrganizationName andAddre= 505-33-33-02
NASALangley Research Center 11 Contract or Grant No.
Hampton, VA 23665
13. Type of Report and Period Covered
12. Sponsoring Agency Name and Address Technical Memorandum
National Aeronautics and Space Administration 14.SponsoringA encyCode
Washington, DC 20546
15. Supplementary Notes
John M. Butler was an employee of Virginia Polytechnic Institute and State
University assigned to NASALangley Research Center on NASAGrant NSG-1428.
16. Abstract
A modification to the addition polyimide, LaRC-160 has been prepared to improve
tack and drape and increase prepeg out-time. The essentially solventless,
high viscosity laminating resin is synthesized from low-cost liquid monomers.
The modified version takes advantage of a reactive, liquid plasticizer which
is used in place of solvent and helps solve a major problem of maintaining
good prepeg tack and drape, or the ability of the prepeg to adhere to adjacent
plies and conform to a desired shape during the lay-up process. This alternate
solventless approach allows bath longer life of the polymer prepeg and the
processing of low-void laminates. This approach appears to be applicable to
all addition polyimide systems.
i
17. Key Words (Suggested by Author(s)) 18. Distribution Statement
Tackifier
Polyimide Unclassified - Unlimited
Composi te
Matrix Resin Subject Category 27
19. Security Classif.(of thisreportl 20. SecurityClassif.(of thispage) 21. No. of Pages 22. Price"
Unclassified Unclassified 13 A02
N-30s ForsalebytheNationalTechnicalInformationService,Springfield,Virginia2216!





Tackifier for addition polyimides

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server