Addition polyimides cure with no evolution of gaseous by-products at relatively low temperatures and may be cured at low pressures to yield composites with excellent hot-wet strength retention. These properaties have made them excellent candidates as matrix resins for advanced composites. However, commercially available bismaleimides are solids and difficult to handle in preimpregnated form. V-378A is an addition polyimide composed of a mixture of bismaleimides and other reactive ingredients formulated to provide good prepreg properties and handling, facile cure and excellent composite mechanical properties. Several curing mechanisms are utilized to provide the characteristics exhibited by V-378A. Part of the mechanism is free radial and takes place at ambient temperature and above. Other mechanisms are principally Diels-Alder in nature. V-378A prepregs are tacky at ambient temperature, but do not have long tacky outlife similar to some epoxies. V-378A yields composites which exhibit hot-wet strength retention which is superior to that provided by epoxy resin systems

Street, S. W.

English

NASA Technical Reports Server

N86-11287
V-378A, A MODIFIED BISMALEIMIDE FOR ADVANCED COMPOSITES
S. W. Street
HITCO Materials Division
Addition polyimides cure with no evolution of gaseous by-products at
relatively low temperatures and may be cured at low pressures to yield
composites with excellent hot-wet strength retention. These properaties have
made them excellent candidates as matrix resins for advanced composites.
However, commercially available bismaleimides are solids and difficult to
handle in preimpregnated form.
V-378A is an addition polyimide composed of a mixture of bismaleimides
and other reactive ingredients formulated to provide good prepreg properties
and handling, facile cure and excellent composite mechanical properties.
Several curing mechanisms are utilized to provide the characteristics exhibited
by V-378A. Part of the mechanism is free radical and takes place at ambient
temperature and above. Other mechanisms are principally Diels-Alder in nature.
V-378A prepregs are tacky at ambient temperature but do not have long tacky
outlife similar to some epoxies. V-378A yields composites which exhibit hot-wet
strength retention which is superior to that provided by epoxy resin systems.
Epoxy resins, the dominant matrix system for high modulus graphite (HMG)
do provide very good composite properties and have been extensively used in the
manufacture of sporting goods and aircraft parts including ailerons, rudders,
flaps, vertical and horizontal stabilizers, and complete wing sections for
commercial and military aircraft. Epoxy resin prepregs are commonly supplied
at relatively high resin contents requiring "bleeding" to achieve the desired
resin content.
Use of this technique (bleeding) results in non-uniformity of resin
content in the composite since more resin is removed next to the bleeder. This
non-uniformity of resin content induces stresses and often results in warped
parts. Use of a "net" resin concept tends to overcome these problems. V-378A
exhibits excellent fiber wet out during the initial prepregging step and can
thus be provided as a net resin prepreg.
The 175oC curing epoxy resin systems exhibit long gel times at 175°C and
very low "watery" viscosities which tend to make it difficult to achieve low
void composites. Typically, 175°C curing epoxies require a "hold" at 150 -
175°C to build molecular weight (and viscosity) prior to full application of
pressure. V-378A, in contrast, gels fairly rapidly at 125°C and exhibits
moderate to low viscosity during heat up making it much easier to achieve very
low void content composite structures. Pressure application is done at ambient
temperature without the necessity of a hold.
359
https://ntrs.nasa.gov/search.jsp?R=19860001820 2020-03-23T06:23:16+00:00Z
LAY-UP AND CURE
V-378A exhibits light to medium tack at a prepreg net resin content of
30_ which is significantly lower than epoxies typically supplied at 42_. The
net resin concept is particularly useful for preparation of composites
having both thick and thin sections. Due to the low temperature gel and
relatively low exotherm, no difficulty has been encountered in curing
virtually void-free laminates up to four inches in thickness.
Standard nylon vacuum bags and sealant tapes (same as used for 175°C
epoxy) may be used versus expensive polyimide film bag and silicone sealant
tape required for high temperature curing polyimides.
To cure the V-378A prepreg lay-up, 85 psi autoclave pressure and vacuum
are applied at the beginning of the cycle, and the part heated to 60 - 90°C
at a rate of 2 - 5°C/minute. The vacuum is released, the bag vented to the
atmosphere, pressure increased to 100 psi and heat up continued to 175°C. The
part is cured four hours at 175°C, cooled to ambient temperature, removed and
post cured, unrestrained, from room temperature to 246°C for four hours. The
four hour post cure at 246°C is required to develop good mechanical properties.
Table 1 illustrates a study of • 45 ° tensile ultimate and strain using more
extensive post cures and indicates the four hour post cure at 246°C to be
satisfactory.
Use of a one to two hour hold at 60 - 90°C for very thick parts allows
more time before gelation in order for air, trapped between plies during lay-
up, to be released.
Figure 1 shows the effect on viscosity of heating the resin to 67 °, 70 °
and 90°C with holds of 5 hours, 3-1/2 hours and 2-1/2 hours respectively. A
heating rate of l°C/minute was used.
Figure 2 illustrates the effect of heating rate with lower minimum
viscosities obtained at faster heating rates.
EFFECTS OF MOISTURE ON PREPREG
The 175°C curing epoxy matrix resins have exhibited much variability in
processing due to moisture pickup of the uncured prepreg. This affinity for
moisture results in variations in gel time, foaming during cure and porosity
in the cured composite.
In contrast, V-378A prepreg exposed to 100_ RH and 52°C for one hour,
then laid up and cured, exhibited no deleterious effects. Figures 3 and 4
exhibit the effect of a sixteen hour exposure at 90_ RH, 24°C on uncured films
of V-378A. No porosity or foaming was noted during the RDS viscosity deter-
mination.
360
MECHANICAL AND PHYSICAL PROPERTIES
Figure 5 exhibits a Tg of V-378A (via Rheometric Dynamic Spectroscopy)
in excess of 370°C. Translation of properties on high modulus graphite are
excellent. Table 2 lists mechanical properties of V-378A/T-300.6K tape
composites. Retention of dry flexure at 310°C is approximately 40% of the
ambient temperature value. Wet flexure retention (98% RH, 71°C, 30 days) at
ambient and 175°C of 23 ksi/22,500 _ in./in, and 15.7 ksi/29,000 _ in./in.,
respectively are also impressive. It is of interest to note the increase in
composite strain at 175°C rather than a decrease usually noted in 175°C
epoxies. No significant degradation in ambient or 175°C ultimate or strain
values of the • 45 ° tensile are exhibited after wet conditioning. Epoxies
exhibit a significant drop in 175°C wet properties.
All wet elevated temperature testing was conducted using a five minute
"soak" time, for the specimen to get to temperature in the preheated test
chamber, in order to minimize drying of the specimen. V-378A composites lose
moisture (and regain) at a more rapid rate than 175°C epoxies.
Transverse or 90 ° tensile ultimate and strain values of V-378A at
ambient of 9.2 ksi/7,700 _ in./in, and 6.1 ksi/6,400 K in./in, at 175°C are
also higher than most 175°C epoxy systems.
EFFECTS OF PREPREG AGING AT 75°F
Table 3 exhibits flexure and shear tested at ambient and 177°C from
prepreg which was laid up fresh and aged at 0, 7, 14 and 21 days intervals
ambient temperature prior to cure. Ambient temperature tack retention,
however, is much shorter than for typical 177°C curing epoxies. A vacuum
bag debulk step is suggested at 2 - 3 hour intervals for large parts. Fresh
prepreg should be laid up promptly and excess prepreg stored at -18°C. Storage
stability of the prepreg is in excess of six months at -18°C.
GENERAL MECHANICAL PROPERTIES
Extensive mechanical properties on V-378A/T-300.6K unitape composites
were determined by the University of Dayton Research Institute and reported
under AF contract F 33615-78-C5172. This data includes 0 °, _ 45 ° and 90 °
tensile, 0 ° and 90 ° compressive, 0 ° and 90 ° flexure and shear at ,55°C, 22°C,
177°C and 232°C as well as static and creep testing.
Two major aircraft companies have found V-378A to exhibit open hole
tensile values 50 - 70% higher than 175°C curing epoxies.
Table 4 contains properties of V-378A/T-300.6K, 5 HS woven graphite
fabric composites with flexure and shear tests up to 232°C. This is a
relatively new style using the heavier weight 6000 filament yarn and may
replace the 3000 filament yarn 8HS fabric in many applications due to the
lower cost of the yarn.
361
Properties of V-378A/7781 E-glass fabric are listed in Table 5 and also
exhibit excellent strength retention at temperatures up to 371°C.
Table 6 lists properties of V-378A/6781 S-glass and illustrates the
higher strengths attainable with S-glass.
Some properties of NI-TEX graphite fiber, HITCO's recent entry in the
high strength, high modulus field on V-378A are listed in Table 7.
ELEVATED TEMPERATURE STABILITY OF CURED COMPOSITES
Flexure and shear of .080" thick V-378A/T-300 tape composites after
aging six months at 177°C and nine months at 232°C in circulating air ovens
are listed in Table 8. Composite weight loss after six months at 177°C is
less than 0.6_. Flexure at ambient and 177°C appear to have increased
slightly. Six months aging at 232°C exhibited a weight loss of 2.3_ and good
retention of flexure. Nine month aging produced composite weight loss of
3.5_. Retention of shear and flexure was quite high both at ambient and
232°C. Degradation appeared to be greatest on the surface.
Two new sets of panels were prepared and one set coated with 1 mil of
Skybond 703, a condensation type polyimide, as a protective coating. The
control panel exhibited a weight loss of 4_ while the Skybond 703 coated panel
lost 2.3_ weight after aging one year at 232°C. Test of flexure and shear
indicated about 15_ better retention of 232°C flexure and slightly better
shear on the coated panel after the one year at 232°C.
SMOKE DENSITY
Figure 6 illustrates the very low smoke density exhibited by V-378A
composites. After a 20 minute burn, the smoke density (NBS Smoke Chamber)
is about 1.5.
APPLICATIONS
V-378A is being evaluated in a number of applications for commercial
and military aircraft as well as industrial applications. One of the most
impressive of these is for manufacture of the complete wing skins and ribs
for the new F-16XL cranked arrow fighter plane. The first prototype of this
new concept aircraft flew on July 3, 1982 and is under intensive evaluation by
General Dynamics.
362
Figure 7 is a view of a V-378A/T-300.6K composite wing skin during lay-up.
Figure 8 shows several skins in various stages of lay-up. Figure 9 is a
completed skin removed from the tool and after post cure. This part had less
than 0.030" warp after post cure. Extensive C-scans indicated essentially no
voids. Figure 10 illustrates the completed wing structure with internal ribs
mechanically fastened to the upper and lower wing skins. The root thickness
of each skin is approximately 0.4" tapering to approximately 0.60" at the tip.
The completed wing attached to the fuselage is shown in Figure ii. Figure 12
exhibits both wings attached to the fuselage and finally the finished cranked
arrow, delta wing F-16XL is shown in Figure 13.
Proposed specifications for the F-16XL versus the current F-16A manu-
factured by General Dynamics, Ft. Worth, are listed in Table 9. Initial
results from flight tests indicate performance is close to analytical
predictions.
Figure 14 shows a wing flap from another prototype aircraft with
integrally cocured V-378A/T'300 ribs bonded to a wing skin.
V-378A/T-300.6K is also being used to produce a firewall for an advanced
concept helicopter and in numerous other applications.
SUMMARY
V-378A, a modified bismaleimide resin has been developed for composite
applications requiring greater hot-wet strength retention than currently
available with 175°C curing epoxy resins.
The new resin also exhibits good prepreg parameters, facile, epoxy-like,
curing characteristics and appears useful for applications at temperatures of
232°C for extended periods of time and in areas where low smoke density is
required.
Assistance from the U. S. Polymeric analytical, physical testing
laboratories and from Lee McKague and Clarence Hart of General Dynamics for
their help in providing photographs and technical assistance is gratefully
acknowledged.
363
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TABLE 9
F-16XL SPECIFICATIONS
WINGSPAN
WING ROOT CHORD
WING AREA
OVERALL LENGTH
EMPTY WEIGHT
MAXIMUM TAKEOFF GROSS WEIGHT
FUEL CAPACITY
TAKEOFF ROLL (AIR-AIR COMBAT)
TAKEOFF ROLL (AIR-GROUND SUPPORT)
LANDING DISTANCE (AIR-AIR COMBAT)
LANDING DISTANCE (AIR-GROUND SUPPORT)
MAXIMUM SPEED
MAXIMUM CRUISE SPEED
F-16XL SCAMP
32.4 FT
499 IN.
646.4 SQ FT
52.4 FT
17,402 LB
37,500 LB
12,750 LB
1,640 FT
1,980 FT
1,990 FTt
2_.30 FT °e
MACH 2.5
MACH 2.2
*WITH WINGTIP MISSILES. WITHOUT MISSILES. SPAN IS 31 FT
F-16A
32.8 FT*
195 IN.
300 SO FT
47.6 FT
15,137 LB
35,400 LB
6,972 LB
2,425 FT
3,030 FT
2,480 FT
2,830 FT
MACH 2.0
MACH 0.93"
tUSING BRAKES ONLY. USING DRAG PARACHUTE, LANDING OISTANCE ESTIMATED
AT 1,180 FT
"USING BRAKES ONLY. USING DRAG PARACHUTE, LANOING DISTANCE ESTIMATED
AT 1,360 FT
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368
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TEMPERATURE (°C)
Figure 3. - USP V-378A exposed to 50 percent RH
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_ 369
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TEMPERATURE (°C)
Ftgure 4. - USP V-378A exposed to 90 percent RH
at 24 °C (for 16 hr).
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Figure 5. - Tg shear modulus by rheometrlc
dynamic spectroscopy.
370
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Figure 7. - V-378A/T-300-6K composite wlng skin during lay-up. 
371 
F i g u r e  8.  - Wjng skins i n  var ious stages o f  l a y - u p .  
Figure 9. - Completed s k i n  removed from tool after post cure. 
373 
Flgure  10. - Completed wing s t r u c t u r e  w i t h  i n t e r n a l  r i b s  mechanlca l ly  fastened t o  
the  upper and lower wing sklns.  
374 
Figure  11.  - Completed wing at tached t o  fuse lage .  
37 5 
c 
F igure  1 2 .  - Fuselage w i t h  both wings a t t a c h e d .  
31 6 
F i g u r e  13 .  - Fin lshed  cranked arrow, de 
377 
t a  wing F-16XL f f g h t e r  p lane .  
ORiGlNAL PAC? (," 
OF POOR QUALITY 
F i g u r e  1 4 .  - Wing f l a p  from another pro to type  a i r c r a f t  w i t h  i n t e g r a l l y  cocured 
V-378A/T-300 r i b s  bonded t o  a wing s k i n .  
378 


V-378A: A modified bismaleimide for advanced composites

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