40207_04

HIGH TEMPERATURE RESINS 4 Hugh H. Gibbs

4.1 INTRODUCTION

The high temperature resins discussed in this chapter are defined as a family of aromatic polyimides having glass transition temperatures (TJ greater than 316°C (600°F). Other resin systems such as the bis-maleimides and various aromatic thermoplastics (including lower Tg thermoplastic polyimides) are discussed in Chapters 5 and 6 respectively.

Over the years it has been found that the key to achieving outstanding high temperature mechanical properties and thermal-oxidative stability is to have a polymer made with aromatic heterocyclic repeat units where there is a minimum aliphatic content (e.g. aliphatic C-H and C=C groups). Such groups can contribute to thermal-oxidative instability. Although many types of aromatic heterocyclic polymers are possible, one type, polyimides, has turned out to be the most commercially successful. The highly aromatic character achievable in such polymers is the reason behind their thermal-oxidative stability. Also, provided that flexibilizing linkages in the monomers are kept to a minimum, the mherent rigidity of the repeat units results in the high T which is essential if an adequate level of hi& temperature mechanical property retention is to be achieved.

Over the past 25 years various strategies have been developed to introduce processibility into aromatic polyimides without detracting

Handbook of Composites. Edited by S.T. Peters. Published in 1998 by Chapman & Hall, London. ISBN 0 412 54020 7

too much from their high temperature mechanical properties and thermal-oxidative stability. As a result of a extensive work on the part of polymer scientists, mostly within the USA, a wide variety of products has been developed possessing various trade-offs between their processibility and properties. In writing this chapter the author has attempted to explain the evolution of high temperature polyimides as matrix resins for advanced composites together with a description of their processing characteristics (where available), physical properties and long term high temperature performance.

4.2 CONDENSATION POLYIMIDE CHEMISTRY

4.2.1 GENERAL COMMENTS ON CONDENSATION POLYMERIZATION

Studies carried out in DuPont in the 1950s and 1960s established that polyimides can be prepared by the reaction of an aromatic dianhydride and an aromatic diamine in a polar solvent such as dimethyl acetamide or N- methyl-2-pyrrolidone (NMP). This is illustrated in Fig. 4.1 for the polyimide based on pyromellitic dianhydride (PMDA) and 4,4'-oxydianiline (ODA). The intermediate polyamide acid solution is the basis for DuPont's Pyre ML@ wire enamel. During the second step, the imidization of the polyamide acid, 2 moles of water are eliminated per repeat unit. Two other DuPont products, Kapton@ polyimide film and Vespel@ SP polyimide pre- cision parts are also based on this chemistry.

76 High temperature resins

4.4-Oxydianiline ti ti Pyromellitic Dianhydnde i Heat (PMDA) (ODA)

(-2H,O)

0 0 PMDAiODA Polyamide Acid n

(-solvent)

+ N : ~ ~ ; N ~ o + / C'

0 0 n II

PMDAiODA Polyimide

Fig. 4.1 Typical reaction sequence for a polyimide from a dianhydride and a diamine.

It turns out that the particular polyimide shown in Fig. 4.1. is unsatisfactory for the pur- pose of making prepregs for high temperature composite parts. For one thing the PMDA/ODA polyimide is too intractable, having no detectable T, or melting point below its decomposition temperature which is well in excess of 500°C (932°F). In addition, the polyamide acid solutions which are immedi- ately generated by dissolving the diamine in a suitable solvent and then adding the reactive dianhydride, are unsuitable for prepregging. Ideally monomeric solutions are preferred that possess modest viscosities even when the solids contents are in the 50-60 wt.% range. This desired combination of properties cannot be achieved in polymeric solutions such as polyamide acids. Instead, the solutions rapidly become unacceptably viscous when solids contents in excess of 15-20% range are reached thus making them too difficult for prepregging using commercially available equipment.

In order to prepare polyimide binder solutions (or polyimide precursor solutions as they are sometimes called) it is necessary to have the aromatic dianhydride in either one of the two possible open ring forms (tetraacid or diester diacid). If the tetraacid form is commercially available then the binder solution can be made directly because there is essentially no reaction with the diamine until

temperatures in excess of 100°C (212°F) are reached. Thus high solids monomeric solutions are possible which are ideal for prepregging. If, on the other hand, the dianhydride is the commercially available starting material then it must be converted in situ to the open ring diester diacid form by prereact- ing with an alcohol such as ethanol or methanol.

4.2.2 CHEMISTRY OF SKYBOND'

Skybond from Monsanto is a product which has been available commercially since the mid-1960s. The chemistry of Skybond is illustrated in Fig. 4.2. The relatively low cost dianhydride, 3,3',4,4'-benzophenonetetracarboxylic dianhydride (BTDA), is first prereacted with ethanol using NMP as the solvent. Then, if m-phenylenediamine (MPD) is added to the solution, Skybond 700 results. If 4,4'-methylenedianiline (MDA) is employed, then Skybond 703 is produced. During cure the application of heat causes the elimination of the solvent along with 2 moles of water and 2 moles of ethanol per repeat unit to produce the polyimide. The molecular weight initially achieved will depend on the monomer imbalance employed. It has long been speculated that during the cure process branching can occur by the reaction of amine end-groups with the bridging carbonyl group of the BTDA

Condensation polyimide chemistry 77

3,3' ,4,4'-benzophenonetetracarboxylic dianhydride

BTDA I Diethyl ester of 3,3',4,4'-benzophenone tetracarboxylic acid

BTDE

m-phenylene diamine 1 Skybond700

4,4'-methylene dianiline + Skybond703

Fig. 4.2 Chemistry of SkybondO binder solutions.

moiety leading to branching and intractability reactive and present in stoicluometric propor- of the matrix resin. This is probably one of the tions, extremely high molecular weight reasons why it is difficult to fabricate low void polyimide is ultimately produced. Also, since composites using this type of chemistry. the bridging hexafluoroisopropylidene group

is inert under normal cure conditions, the poly-

4.2.3 CHEMISTRY OF NR-150 imide produced is essentially linear and & that sense thermoplastic.

The formation of DuPont's monomeric binder solution NR-150 (used to make Avimid@ N prepreg) is illustrated in Fig. 4.3. The 2,2-bis(3',4'-dicarboxyphenyl) hexafluoro- propane (6F tetraacid) is dissolved in a suitable solvent (e.g. ethanol, diglyme or NMP) along with a 95/5 mixture of p-phenylenediamine/rn-phenylenediamine to form a low viscosity, stable monomeric solution' suitable for prepregging. During the cure both solvent and 2 moles of by-product water per repeat unit are eliminated to initially form the transient intermediate polyamide acid. According to the studies of Sonnett et aL2, although a low concentration of amide acid persists for a relatively long time period during the early stages of cure, most is rapidly converted to the imide form by the thermally induced elimination of water (2 moles per repeat unit). Since the tetraacid and diamines are very pure, highly

4.2.4 CHEMISTRY OF 3F/36F POLYIMIDES

One of the keys to success in producing an essentially linear condensation polyimide is to have a chemically inert flexibilizing linkage in the dianhydride moiety. As indicated in Section 4.2.3 one of the ways of doing t h s is to employ a hexafluoroisopropylidene bridging group. Since the phenyl group is also very inert another approach is to replace one of the CF, groups with a phenyl group3. This results in the so-called 3F dianhydride. When the 3F monomer is used along with PPD the polyimide produced is called 3F-PPD polyimide. If, on the other hand, a mixture of the 3F and 6F monomers are employed along with the same diamine the copolyimide is designated 36F-PPD.


0 CF3 NHZ NH2

HO-C H O - ! D ~ F D ~ - O H / / C-OH + 0 / + 0 / NH, + Solvent

0 NH2 II 0

956 mixture PPDA4PD

Monomeric binder solution

f

J Intermediate transient polyamide acid

(-2H20)

r 1

NR-150 Polyimide L

Fig. 4.3 NR-150 polymerization chemistry.

4.3 ADDITION POLYIMIDE CHEMISTRY

4.3.1 OVERALL CHEMISTRY OF ADDITION POLYIMIDE PRECURSOR SOLUTIONS

difficult for these volatiles to escape. Therefore, ideally one would want to eliminate all of the volatiles prior to pressurization. In order to accomplish this in a conventional condensation

One of the important concerns in the cure of conventional condensation polyimides is the proper management of the evolution of volatile by-products. During the early stages prior to pressurization the composite is generally some- what porous and volatiles can readily diffuse out. However, once pressure has been applied and a low void state is achieved the diffusivity dramatically decreases making it much more

polyimide it is necessary to have a high enough monomer imbalance so that the polymer molecular weight and melt viscosity will be low enough to allow for complete consolidation. The problem is that this molecular weight is generally so low that the matrix resin properties are adversely affected. Properties such as strength, toughness and T, will be much lower than desired.

Addition polyimide chemist y 79

A common way of making a low void composite is to produce in situ an imide oligomer having a low enough molecular weight so that it has good melt flow and readily consolidates when pressure is applied and then, with further heating, can go on to produce much higher molecular weight polymer without the further evolution of volatile by-products. This can be accomplished through the use of reactive end- groups which are capable of reacting with one another in various ways without the evolution of volatiles. However, it must be pointed out that such improvements in processibility are only achieved with some sacrifices in other properties. Because of the presence of cross- links the toughness can be adversely affected. Also, the kinds of linkages produced during these addition polymerization reactions are generally aliphatic in nature thus making them much more prone to thermal-oxidative attack. Many approaches, which are now discussed, have been taken in order to achieve various trade-offs in processibility against properties.

4.3.2 CHEMISTRY OF PMR-15

The first approach to a commercially successful addition polyimide was PMR-15. The basic PMR (Polymerization of Monomeric Reactants) chemistry was originally invented and developed at NASA Lewis Research CenteF. As illustrated in Fig. 4.4 a monomeric solution is first prepared consisting of the dimethyl ester of BTDA (BTDE), the diamine, MDA and the monomethyl ester of nadic anhydride (NE), the reactive end-capping agent. The monomer ratios are n:n+l:2 respectively. Because the high solids solution is monomeric it is ideally suited to prepregging. As a result of the application of heat, solvent is eliminated along with the water and alcohol of imidization to produce the intermediate imide oligomer having a formulated molecular weight of about 1500. This normally occurs between 121°C (250°F) and 232°C (450°F). At this point essentially all of the volatile by-products have been eliminated. The stage is now set for pressurization. At some-

v

2 Moles NE 2 Moles MDA 1 Mole BTDE PMR-15 Binder Solution

Crosslinked PMR-15 Fig. 4.4 PMR-15 polymerization chemistry.


what higher temperatures the imide oligomer undergoes melting and, if pressure is applied, consolidation to form a low void structure occurs. At temperatures in the 275-316°C (527400°F) range ring opening of the nadic end-groups takes place in a reverse Diels-Alder type of reaction first discovered by Lubowitz9Jo in 1970 and a complex series of reactions takes place involving these end-groups leading to high molecular weight polymer without the further evolution of volatile by-products. Hence the low void state originally achieved during the initial consolidation is maintained and a hgh quality laminate usually results.

4.3.3 CHEMISTRYOF PMR-I1

During the early 1970s it became clear that although composites based on PMR-15 pos- sessed good enough properties to make them a commercial success there was still room for improvement. For instance, the relatively high aliphatic content which came from the nadic end-groups and the methylene group of the MDA contributed to thermal-oxidative instability. Also, the relatively high cross-link density in a polyimide with a formulated molecular weight of 1500 resulted in a rather brittle matrix resin. In order to overcome these deficiencies PMR-I1 was invented”-14 (Fig 4.5). For PMR-I1 polyimides the 6F dianhydride is used in place of the BTDA and is converted in situ to the diethyl ester diacid derivative

1 0

PMR-I1 imide prepolymer -l

V-CAP imide prepolymer L

$$ CH, -CH,

CYCAP imide prepolymer

L AFR700B imide prepolymer

Fig. 4.5 Structures of 6F based addition polyimide prepolymers.

Addition polyimide chemistry 81

(6FDE) by pre-reacting with ethanol. The sin- 4.3.6 CHEMISTRY OF AFR700B gle ring diamine, PPDJis employed in place of the MDA. The end-capping agent is still nadic anhydride. The formulated molecular weight was increased from 1500, which is what it is in PMR-15, to the 3000-5000 range (yields PMR-11-30 and PMR-11-50 respectively). While the polymerization is basically the same in both PMR-I1 and PMR-15, since the aliphatic content has been greatly reduced, significant improvements in thermal-oxidative stability and toughness can be realized.

4.3.4 CHEMISTRY OF V-CAP

Assuming that the 6F/PPD backbone has near optimal thermal-oxidative stability and formulated molecular weights in the 3000-5000 range are about right from the stand-point of melt flow, if further improvements in stability are to be realized then one approach would be to make changes in the end-group chemistry. One effective method is the replacement of nadic anhydride as the end-capping agent with p - aminostyrene The imide prepolymer that is produced is illustrated in Fig. 4.5. In this case there is only one weak bond per end-cap compared with eight for the nadic end-cap. During the addition polymerization phase of the cure free radical polymerization of the vinyl (-CH=CH,) end-groups occurs leading to high molecular weight polymer.

4.3.5 CHEMISTRY OF CYCAP

Another approach to a more stable polymer has been to replace the nadic end-capping agent with 2-amino-p-cyclophane (APC)19,20. The structure of the imide prepolymer con- taining the CYCAP end-groups that is initially produced is illustrated in Fig. 4.5. This type of end-capping agent has only two weak bonds per end-cap. Thermolytic cleavage of the -CH,-CH,- groups during the final stages of cure produces bi-radicals (-CH; CH,-) which undergo coupling with other bi-radicals to build up the molecular weight.

With any of the reactive end-groups discussed thus far in this chapter the concentration of end-capping agent has always been such that there are end-caps on both chain ends. However, by adding just enough end-capping agent to cap one chain end only and assuming that high molecular weight polymer can still somehow be obtained under reasonable processing conditions, a further significant improvement in thermal-oxidative stability should be possible simply because the concentration of unstable groups will have been cut in half. This is exactly what has been done with AFR700B based on the work of Serafini et al. at TRWz1-23.

In AFR700B the ratio of 6FDE:PPD:NE is 8:9:1 and the formulated molecular weight is about 4400 (see Fig. 4.5 for its s t r~c tu re )~~ . This means that the nadic end-group component is only 3.7% of the overall weight compared with 22% for PMR-15 and 6.5% for PMR-11-50). During cure of AFR700B composites it is necessary not only to achieve the normal imidization and nadic end-group coupling reactions but also other unspecified reactions involving the amine end-groups undoubtedly occur leading to the formation of the desired strong, stiff, tough, high Tg polyimide.

4.3.7 CHEMISTRY OF TRW-R-8XX

The most recent addition to the growing family of commercially available polyimides comes from TRW and has been designated TRW-R-8XXZ5. Although the chemical structure of this polyimide has not yet been disclosed it is reported to be a condensation/addition polyimide based on relatively low cost monomers, making its cost compara- ble to that of PMR-15. It is reported to be free of the carcinogen, MDA.


4.4 COMMERCIAL AVAILABILITY OF BINDER SOLUTIONS

Most of the binder solutions described in this chapter are not commercially available but rather are prepared by the prepregger on an as-needed basis just prior to prepregging. One notable exception is Monsanto's Skybond. The monomeric solutions (Skybond 700 and 703) have solids contents in the 45-52% range and solution viscosities of 3000-7000 poisez6.

4.5 COMMERCIAL AVAILABILITY OF PREPREGS

At the time of writing of this chapter all of the different polymide prepreg systems were commercially available from one prepregger or another. However, the reader should appreci- ate the fact that as time goes by some prepreg types will disappear from the market place and others with an improved balance of processing characteristics, properties and economics will come along to take their place. Also, some companies will go out of business or will be bought out by other companies as consolidation in the industry occurs. Therefore, if a given type of prepreg is required the reader should contact their favorite prepregger. If that particular company does not offer the product the reader requires then advice should be obtained as to where such prepreg could be commercially pur- chased, if at all. Another fruitful source of information would be to search the Internet.

4.6 NEAT RESIN PROPERTIES

The neat resin mechanical properties for cured NR-150 and PMR-15 are summarized in Tables 4.1 and 4.2 respectively. One of the important differences between these two resin systems is their toughness. Cured NR-150 has been found to be dramatically tougher (2000 Jm-' fracture toughness) compared with a value of 500 Jm-' for the cured PMR-15. Another significant difference is thermal-oxidative stability.

Table 4.1 Typical properties of neat cured NR-150 '' Property Units Value

Density Coefficient of thermal

Char yield Tensile strength Elongation, RT 316°C (600°F)

Fracture toughness Rochwell hardness

expansion

(E scale)

"C 350-370 "F 662-700

g cm-? 1.43-1.45 o c - I 5.6 x 10-5 OF-' 3.1 x % 60

MPa (ksi) 110 (16) % 6 - 65

J m-2 (in lb in") 2000 (11.4)

70

Table 4.2 Typical properties of neat cured PMR-15

Property Units Value Reference

"C (OF) 335 (635) 28 g ~ r n - ~ 1.30-1.32 28

Coefficient of oc-1 16x1O4 35 thermal O F - 1 28X 10" 35 expansion

Tensile strength MPa (ksi) 55 (8.0) 35 Tensile modulus MPa (ksi) 3200 (470) 35 Elongation "/o 1.5 36 Compressive MPa (ksi) 110 (16) 35

Compressive MPa (ksi) 186 (27) 35

Density T,

yield strength

strength

moisture absorption

toughness (in lb i r2)

Equilibrium 96 4.2 28

Fracture J m-2 500 (2.86) 37

In neat resin thermal-oxidative stability studies carried out by Sco1az7 it was found that after 24 h at 316°C (600°F) the NR-150 resin had lost 9% of its weight compared with about 76% for the PMR-15. Neat AFR700B, which has been post-cured under nitrogen, has been reported to have a room temperature tensile strength of 93.8 MPa (13.6 ksi) with 18% retention of this value at 371°C (700°F). No properties were available for Skybond, PMR-11, V-CAP and CYCAP. Very little information was available for the 3F-PPD and 36F-PPD

Processing characteristics 83

polyimides except for neat resin densities (1.35 and 1 . 4 2 g ~ m - ~ for the 3F and 36F resins respectively) and Tg of 365-370°C (689-698°F) for both systems in the as-molded state and 405-410°C (761-770°F) for the post-cured state3. TRW-R-8XX has been reported25 to yield polyimides having Tg in the 400-426°C (750-800°F) range and composite weight loss characteristics at 371°C (700°F) up to 10 times better than PMR-15. All of the resin systems described in this chapter appear to possess good strength and stiffness. Thus, provided that complete cure is achieved during processing and low void composites are produced possessing good fiber/matrix adhesion, high levels of composite mechanical properties should be obtained with good retention (at least 50%) of these properties to just below their Tg.

4.7 PROCESSING CHARACTERISTICS

4.7.1 GENERAL COMMENTS

One of the features that can clearly differenti- ate one resin system from another is the ease with which a fully cured low void composite can be produced having a specified fiber volume. Also, although all polyimides can have their Tg increased from post-cure, some systems respond much more readily than others. In all cases there is a problem of properly man- aging the release of a significant amount of volatiles (normally 10-15% of the weight of the prepreg). The ways in which this can be accomplished can vary significantly from one resin to another depending on the chemistry involved. Factors such as the techniques employed during lay-up of the vacuum bag assembly, pressure, heat-up rate, maximum cure temperature, vacuum application and intermediate holds all must be carefully con- trolled and optimized for each system. Unfortunately the story on the processing of the various systems covered in this chapter is very incomplete. In a majority of cases information on processing was simply not available

because such data was either considered pro- prietary or classified as secret by the Government Laboratories. In other cases, where some processing information was available in the open literature, very little was usually said about the quality of the part produced by a given cycle so it is difficult to compare the processibility of one resin system with another since the quality of the laminates produced is generally unknown. To make mat- ters worse there are also the issues of the processing characteristics of thick compared with thin sections and how processing can be handled, if at all, when both thin and thick sections are present simultaneously in a given part. It is, therefore, of paramount importance for workers in this field to have as clear an understanding as possible of the chemistry involved at every stage of the cure so that they can quickly and efficiently develop the opti- mum cure cycle for a given part. The cure cycles presented below will give the reader an approximate idea of the kinds of conditions that have been employed to produce a part.

4.7.2 SKYBOND PROCESSING CONDITIONS

The following autoclave cure cycle has been recommended by MonsantoZ6 for 12 ply (3.2 mm, 0.125 in thick) 181 style E-glass (soft A-1100 finish) fabric/Skybond 700 laminates:

0 apply full vacuum; 0 heat to 177°C (350°F) at 1.7-2.8"C/min

0 hold 5 min at 177°C (350°F); 0 apply 0.69 MPa (100 psi); 0 hold 30 min; 0 cool under pressure and vacuum.

In order to maximize high temperature properties it is recommended that a post-cure should be carried out up to and including the expected use-temperature. For laminates of the type described above a post-cure is suggested in which the part is heated to 200°C (392"F), 225°C (437"F), 250°C (482"F), 300°C (572"F), 325°C (617"F), 350°C (662°F) and

(3-5°F /min);

84 High temperattive resins

371°C (700°F) and held for 2 h at each temperature. It is recommended that if thicker laminates are involved the post-cure cycle should be extended.

For the same type of laminate based on Skybond 703 a similar autoclave cure cycle can be employed. The only significant differences are a slower heat-up rate (l.l-1.7"C/min, 2-3"F/min) with the pressure being applied at 121°C (250°F) on the way to the final cure temperature of 177°C (350°F). Also a similar post- cure cycle is suggested in order to achieve maximum heat resistance.

While there is very little definitive information on the void content of Skybond based laminates it is believed that they are generally in the 5-20% range. A most important feature of this particular polyimide system is that the maximum autoclave processing temperature is only 177°C (350°F). No other resin system described in this chapter can make that claim.

4.7.3 PMR-15 PROCESSING CONDITIONS

The following represents a typical autoclave cure cyclez8 cited for PMR-15:

apply 7-21 kPa (1-3 psi) vacuum; raise autoclave temperature to 227°C (440°F) at 0.83-1.l0C/min (1.5-2.O0F/min); at 163-177°C (325-350°F) apply full vacuum; dwell at 227°C (440°F) for 1 4 h depending on part thickness (up to 2.8 mm, 0.11 in = 1 h, 2.8-6.4mm, 0.11-0.25 in = 2 h, 6.4-12.8 mm (0.25-0.50 in) = 3 h); raise temperature to 238°C (460°F) at 1.1 "C / min ( 2.0°F/ min); hold at 238°C (460°F) while 1.38MPa (200 psi) autoclave pressure is applied.

noted that pressurization does not occur until a temperature of about 238°C (460°F) is reached. At that temperature essentially all of the solvent and imidization volatiles have been eliminated. Normally PMR-15 laminates are subjected to an oven post-cure:

0 heat from room temperature to 204°C (400°F) at 5.6"C/min (10"F/min);

0 heat from 204°C to 288°C (400°F to 550°F) at 1.1"C /min (2"F/min);

0 dwell at 288°C (550°F) for 1 h; 0 heat from 288°C to 316°C (550°F to 600°F) at

1.1 "C / min (2"F/min); 0 dwell at 316°C (600°F) for 10-16 h; 0 cool to room temperature at 2.8"C/min

(5"F/min) maximum.

4.7.4 PMR-I1 AND V-CAP PROCESSING CONDITIONS

The autoclave cure cycles for PMR-I1 and V-CAP based composites are similar to that of PMR-15. The main difference is that the maximum processing temperature has been increased from 316°C to 371°C (600°F to 700°F). The following typical autoclave cure and post-cure cycles for graphite reinforced PMR-I1 composites has been rep~rted'~?

0 apply full vacuum at room temperature; 0 heat to 149°C (300°F) at 3.9"C/min

(7.O"F / min); 0 hold for 30min at 149"C, then apply

172 kPa (25 psi); 0 heat to 288°C (550°F) at 3.9"C/min

(7.O0F/min) with the pressure being increased to 344 kPa (50 psi) at 177°C (350°F) and then to 1.38 MPa (200psi) at 232°C (450°F);

0 heat from 232°C to 371°C (450°F to 700°F) at Do not hold longer than 15 minutes while pressure is being applied

2.8"C/min (5"F/min); 0 cool under full pressure and vacuum to - _ _

232°C (450°F) slowly; cool from 232°C to rapidly.

raise temperature to 316°C (600°F) at 2.2-3.3"C / min (46°F /min); dwell at 316°C (600°F) for 3 h;

temperature

cool to room temperature. Vent autoclave pressure below 204°C (400°F) It should be

As with PMR-15, a post-cure is normally carried out in a circulating air oven:

Mechanical properties before and after air aging 85

heat from room temperature to 260°C (500°F) at 20"C/min (36"F/min)

0 heat from 260°C to 385°C (700°F to 725°F) at l"C/min (1.8"F/min) with 2 h holds at 316°C (600"F), 343°C (650°F) and 20 h hold at 385°C.

4.7.5 PROCESSING CONDITIONS FOR OTHER RESIN SYSTEMS

At the time of the writing of this chapter no unclassified processing information was available for CYCAP, AFR700B, TRW-R-8XX or the 3F/36F polyimides. The compression molding conditions used by DuPont to make the laminates whose properties are described in Table 4.4 were not disclosed. However, details concerning the autoclave processing of Avimid N have been previously discussedz9.

4.8 MECHANICAL PROPERTIES BEFORE AND AFTER AIR AGING

4.8.1 SKYBOND

The most common type of reinforcement that has been employed with Skybond binders is E-glass fabric. The mechanical properties of as-molded and air aged laminates based on Skybond 700 are summarized in Table 4.3. This particular resin system has been tailored for extended exposures at temperatures up to 371°C (700°F). For 343°C (650°F) applications Skybond 703 is recommended by the manu- facturer. It is interesting to note that in spite of the relatively high porosity levels (5-20%) Skybond binders are still in demand for cer- tain specialty applications.

4.8.2 AVIMID N

Some mechanical properties for compression molded/post-cured Celion G30-500 miweave/ Avimid N laminates are tabulated in Table 4.4. All laminates had a very low void content (4'30) and, therefore, high levels of mechanical properties at room temperature. Because of

their high T, (377418"C, 710-785°F) the retention of properties was excellent out to temperatures as high as 360°C (680°F).

At the writing of this chapter the long term air aging characteristics of laminates of this particular type had not been completed. However, because of its all aromatic character, ultra-high molecular weight and the complete absence of any aliphatic character from reactive end-capping agents it should air age well and the matrix in Avimid N-150 has been previously shown to possess outstanding thermal-oxidative stability30.

4.8.3 3F/36F POLYIMIDES

Although this family of all aromatic polyimides is relatively new, preliminary data indicates that high quality laminates possessing good mechanical properties and excellent long term thermal-oxidative stability can be produced. According to the work of Scola3 both the 3F-PPD and the 36F-PPD systems resulted in G40-600 laminates having the expected good room temperature mechanical properties (flex strength and short beam shear strength) with excellent retention of these 'dry' properties out to at least 371°C (700°F). Also, after 100 h in air at 371°C (700°F) these laminates retained at least 100% of these properties with weight loss values ranging from 1.4% for the 36F-PPD resin and 2.4% for the 3F-PPD polyimide. Surprisingly neat resin studies carried out by Scola have similarly shown that the 36F-PPD polyimide appeared to be some- what more stable than the 3F-PPD polymer. For instance, after 100 h exposure at 371°C (700"F), the 36F-PPD copolyimide had lost about 2% of its weight compared with 3.2% for the 3F-PPD polymer. Much more work needs to be carried out on polyimides based on the 3F monomer before any final decision can be made as to its long term viability in the market place and how it will ultimately compete with the 6F based polyimides such as Avimid N, PMR-11, V-CAP, CYCAP and AFR700B.


Table 4.3 Mechanical properties of Skybond@ 700/181 style E-glass laminates (Soft A-1 100 Finish) 26

Property High temperature, high pressure

Vacuum bag

Flex strength, MPa (ksi) 24 "C (75 OF)

371°C (700°F) after 0.5 h at 371°C 371°C (700°F) after 100 h at 371°C

316°C (600°F) after 500 h at 316°C 316°C (600°F) after 860 h at 316°C 316°C (600°F) after 1850 h at 316°C


Flex modulus, GPa (msi) 24°C (75°F)

299°C (570°F) after 335 h at 299°C

371°C (700°F) after 100 h at 371°C

316°C (600°F) after 500 h at 316°C 316°C (600°F) after 860 h at 316°C 316°C (600°F) after1850 h at 316°C


Ultimate tensile strength, MPa (ksi) 24°C (75°F) 299°C (570°F) after 335 h at 299°C 24°C (75°F) after 100 h at 250°C (482°F) 24°C (75°F) after 100 h at 300°C (572°F)

Elongation, YO 24°C (75°F) 299°C (570°F) after 335 h at 299°C 24°C (75°F) after 100 h at 250°C (482°F) 24°C (75°F) after 100 h at 300°C (572°F)

Weight loss, YO After 100 h at 371°C (700°F) After 500 h at 316°C (600°F) After 860 h at 316°C (600°F) After 1850 h at 316°C (600°F) After 2300 h at 288°C (550°F) After 4500 h at 288°C (550°F) After 9000 h at 288°C (550°F)

517-586 (75-85)

310414 (45-60)

200 (29) 138 (20) 76 (11)

283 (41) 221 (32) 103 (15)

138-241 (20-35)

22 (3.1)

22 (3.1) -

18 (2.6) 18 (2.6) 14 (2.1)

18 (2.6) 20 (3.0) 14 (2.0)

393 (57) 290 (42)

-

-

1.9 1.4 - -

3.0 2.2 3.4 7.9 3.6 5.0 12.0

524-576 (76-84)

152-221 (22-32) 138-166 (20-24)

19 (2.8) -

12 (1.8)

347 (50.1)

336 (49) 332 (48.2)

-

2.0

1.7 2.0

-

Mechanical properties b$ore and after air aging 87

Table 4.4 Mechanical properties of compression molded Celion@ G30-500 Uniweave/Avimid@ N laminates 39

No. ofplies Orientation Tf "C ( O F ) Tempera ture, "C ( O F )

16

16

16

16

10

10

10

10

16

0" 418 (785)

0" 418 (785)

0" 418 (785)

(k 45") 418 (785)

0" 377 (710)

0" 377 (710)

0" 377 (710)

0" 377 (710)

0,90,i45" 377 (710)

16

16

16

16

16

16

16

0,90,+45" 377 (710)

0,90,45" 377 (710)

0,90,i45" 377 (710)

0,90,i45" 377 (710)

0,90,+45" 377 (710)

0" 377 (710)

0" 418 (785)

24 (75) 218 (425) 316 (600) 360 (680)

24 (75) 316 (600) 360 (680)

24 (75) 316 (600) 360 (680)

24 (75) 371 (700)

24 (75) 416 (780)

24 (75) 416 (780)

24 (75) 416 (780)

24 (75) 416 (780)

24 (75) 360 (680) 416 (780)

24 (75) 360 (680) 416 (780)

24 (75) 360 (680) 416 (780)

24 (75) 360 (680) 416 (780)

24 (75)

24 (75) 360 (680) 416 (780)

24 (75) 316 (600) 360 (680)

24 (75) 316 (600) 343 (650)

Property

Short beam shear strength, MPa (ksi) 98.6 (14.3) 61.4 (8.9) 46.2 (6.7) 38.6 (5.6)

Flex strength, MPa (ksi) 1344 (195) 731 (106) 565 (82)

Flex modulus, GPa (msi) 126 (18.0) 123 (17.9) 117 (17.0)

In-plane shear strength, MPa ( h i ) 174 (25.2) 104 (15.1)

Tensile strength, MPa (ksi) 1261 (183) 1027 (149)

Tensile modulus, GPa ( m i ) 124 (18.0) 104 (15.1)

Open hole tensile strength, MPa (ksi) 1027 (153) 854 (124)

Open hole tensile modulus, GPa (msi) 132 (19.1) 114 (16.5)

Tensile strength, MPa (ksi) 460 (66.7) 314 (45.5) 236 (34.3)

Tensile modulus, GPa (msi) 56.5 (8.2) 46.8 (6.8) 30.3 (4.4)

Open hole tensile strength, MPa (ksi) 389 (56.5) 250 (36.3) 177 (25.5)

Open hole tensile modulus, GPa (msi) 54.4 (7.9) 46.8 (6.8) 38.3 (5.7)

Compressive strength, MPa (ksi) 389 (56.5)

Compressive strength, MPa ( h i ) 458 (66.4) 312 (45.4) 270 (39.2)

Compressive strength, MPa (ksi) 868 (126) 448 (65.1) 409 (59.4)

Interlaminar fracture toughness J m-2 (in Ib in-')

630 (3.6) 682 (3.9) 718 (4.1)


Table 4.5 Mechanical properties of fiber reinforced PMR-15 laminates 35

Reinforcement

Property High strength High strength

7781 Style E-glass fabric 1>3447 MPa (500 ksi)] (50-55 Vol. %fibers) Standard modulus Standard modulus

/228 GPa (33 msi)] class Carbon fiber unidirectional tape

1>3447 MPa (500 ksi)]

1228 GPa (33 msi)] class Carbon fiber 8-harness

(55-60 Vol. %fibers) (57-63 Vol. %fibers) satin fabric

Compressive strength MPa (ksi)

23°C (73°F) 288°C (550°F)

Compressive modulus GPa (msi)

23°C (73°F) 288°C (550°F)

Flex strength MPa (ksi)

23°C (73°F) 288°C (550°F) 316°C (600°F)

Flex modulus GPa (msi)

23°C (73°F) 288°C (550°F) 316°C (600°F)

Tensile strength MPa (ksi)

23°C (73°F) 288°C (550°F)

Tensile modulus GPa (msi)

23°C (73°F) 288°C (550°F)

517-586 (75-85) 827-965 (120-140) 758-896 (110-130)

28-34 (4-5)

483421 (70-90)

414-552(60-80)

21-34 (3-5)

21-34 (3-5)

Interlaminar shear strength

MPa (ksi)

288°C (550°F) 23°C (73°F) 62-76 (9-11)

316°C (600°F) 34-48 (5-7)

97-117 (14-17) 83-110 (12-16)

552-689 (80-100) 414-552 (60-80)

62-76 (9-11) 48-62 (7-9)

965-1103 (140-160) 689-896 (100-130)

5 5 4 9 (8-10) 5549 (8-10)

1241-1448 (180-210) 689-896 (100-130) 1241-1448 (180-210) 758-965 (110-140)

117-138 (17-20) 62-76 (9-11) 103-124 (15-18) 62-76 (9-11)

5549 (8-10) 34-48 (5-7)


4.8.4 PMR-15

From the data cited in Table 4.5 it can be seen that PMR-15 based composites reinforced with either E-glass or graphite fibers are character- ized by good room temperature strength properties with excellent retention of these properties at temperatures in the 288-316°C (550-600°F) range.

Over the past 20 years a great deal of effort has gone into the study of the weight loss characteristics of PMR-15 composites at various temperatures and air pressures and the effects of these exposure conditions on the laminate mechanical properties. One of the significant new property features to be uncov- ered has been the effects of temperature cycling on the development of microcracks and the effects of these microcracks on the strength and stiffness properties. In accelerated thermal cycling studies3I carried out at General Electric - Aircraft Engines in which the cycling was carried out between room tem-

perature and either 232°C (450°F) or 288°C (550°F) at heat-up and cool-down rates of 278°C (532"F)/min (achieved by employing a heated fluidized sand bed) significant microcracking was detected at 5000 thermal cycles and beyond. The data plotted in Fig. 4.6 clearly indicate that cycling to the higher temperature resulted in a much higher concentration of microcracks. According to the results plotted in Fig. 4.7 the development of microcracks was found to have a very deleterious effect on the matrix-dominated compressive strength of the graphite fabric laminates. However, GE also showed that the fiber-dominated tensile strength was not affected by the presence of the microcracks. There is every reason to believe that composites based on other resin systems will also undergo microcracking from temperature cycling and can be expected to exhibit improved microcracking performance to the extent to which they possess improved toughness compared with PMR-15.

35 c I

A

A A A A

15 1 A

c .- u)

n Y

0 2 5

0

RT - 288 "C (550 'F) Data

RT - 232 "C (450 OF) Data u

1-

0 5000 10000 15000 20000 25000 Number of Cycles

Fig. 4.6 Crack density compared with accelerated thermal cycles for graphite fabric/PMR-15 laminates. 31


100 1 I 1 I I

A

t A

A A A

0

P P

0 P

RT - 288 "0 (550 O F ) Data

O A I I I I I

0 5000 10000 15000 20000 25000

Number of Thermal Cycles

Fig. 4.7 Effect of accelerated thermal cycling on the compressive strength of PMR-l5/graphite fabric lam- i n a t e ~ . ~ ~

4.8.5 PMR-I1

The greatly improved weight loss performance of PMR-I1 based composites compared with PMR-15 as a result of aging in air at elevated temperatures (343"C, 650°F) is illustrated in Fig. 4.814,15. Note that very little difference in weight loss was found between PMR-11-30 and PMR-11-50. Although a stability improvement would have been expected with the higher formulated molecular weight it usually is not detected because of the difficulty in making low void laminates with the higher viscosity resin. Porosity itself, of course, can contribute to thermal-oxidative instability.

As a general rule, for any of the polyimides discussed in this chapter, the high temperature mechanical properties tend to increase as a result of the air aging process. This is presumably because the oxidative crosslinking that occurs during the aging process tends to increase the T,. In Fig. 4.9I4J5 it can be seen that

the 343°C (650°F) interlaminar shear strength increased for the first 200-400 h of aging. Even after 600 hours aging the interlaminar shear strengths were about the same as they were at the beginning. However, the reader must be aware that there has been a considerable weight loss (about 8%) with the resulting shrinkage and induced stresses. Also, the matrix resin has presumably become embrit- tled. Therefore, in assessing the useful lifetime of a composite at elevated temperatures it is important to not only consider the changes in various strength and stiffness properties but also the effects that weight loss itself can have on dimensional stability and matrix resin toughness.

Carbon fiber selection can also be an important factor in determining the overall high temperature performance of a composite. As illustrated in Fig. 4.1032 PMR-11-50 laminates reinforced with Celion@ G40-600 or ThomeP 650-35 retained a higher level of flex strength


I I I I I I I

12

10 I A PMR-15

0 PMR-11-30

0 PMR-i1-5(i

0 100 200 300 400 500 600 700

Time, Hours

Fig. 4.8 Weight loss of Celiona 6KJPMR laminates at 343°C (65O0F).l4Js

6 0 I I I I I I I 1 8.70

0

5

p 4 0 r; !? 3i

0 G

0 OA

0

.- 7.25 y"

I 4.35 !!

I I I I I I 0 '

0 100 200 300 400 500 600 700

Time, How

Fig. 4.9 Interlaminar shear strength of Celiona 6K/PMR laminates after aging in air at 343°C (650°F).14,15


2 00 180

160 160

.- u)

Q Y

120 5-

!?? z * O x

tl, C

120 !k s' 0 C

cy) 8 0 X 8 ii

!!! - @ u, -

40 40

0 0

Initial After 500 Hours at 371 OC (700 O F )

Fig. 4.10 Effect of fiber selection on the flex strength of PMR-11-50 laminates air aged at 371°C (700°F).32

100 r I I

1 I 1 800 Hours' Exposure I

T40R G4U-600 G40-700 G40-800 T650-42 T650-35 Fiber

Fig. 4.11 Thermal-oxidative stability of various graphite fibers exposed to 371°C (700 O F ) air. 32


after 500 h at 371°C (700°F) than laminates reinforced with other fibers such as Thornel 40R, Thornel 650-42, Celion G40-700 and Celion G40-800. It is of interest to note that according to the data in Fig. 4.1132 there was not a one-to- one correlation with the basic carbon fiber stability. While Celion G40-600 had one of the lowest weight losses in 371°C (700°F) air aging the Thornel 650-35 had one of the highest.

4.8.6 V-CAP AND CYCAP

performance in comparison with PMR-11. These same researchers have also shown that V-CAP/graphite composites possess improved thermal-oxidative stability compared with PMR-11-50. However, with the advent of AFR700B where significant improvements in thermal-oxidative stability have been realized by simply capping one end only with the low cost nadic anhydride, it is not clear whether either the V-CAP or CYCAP polyimides with their more expensive end-capping agents will be successful in the market place.

As a result of the reduced aliphatic character of the reactive end-groups in CYCAP, Meador et a l l 9 have reported improved weight loss

Table 4.6 Mechanical properties of unidirectional AFR700B/S2 glass tape laminates 24

Material Fiber VOl. % Test temperature, "C ( O F )

condit ion or ien f a f ion fibers 23 (73) 316 (600) 371 (700)

As-molded Air aged 100 h at 371°C (700°F) (1 atm)


Cycled 100 x from RT to 371°C (700°F) in air 50 h total at 371°C


Flex strength, MPa (ksi)

0" 55 1123 (163) 634 (92) 496 (70) 0" 55 958 (139) 517 (75)

90" Flex strength, MPa (ksi)

0" 55 48 (6.9) 29 (4.2) 27 (3.9) 0" 55 23 (3.3) 13 (1.9) 23 (1.7)

Flex strength, MPa (ksi) ~~~

0" 55 972 (141) 421 (61)

In-plane shear strength, MPa (ksi) ~~~

k 45 59 81 (11.8) 53 (7.7) 41 (5.9) f 45 59 29 (4.2) 21 (3.0) 26 (3.7)

Tensile strength, MPa (ksi)

54 1103 (160) 917 (133) 834 (121) As-molded 0" Air aged 100 h 0" 55 814 (118) 703 (102) 676 (98) at 371°C (700°F) (1 atm)

As-molded 0" 58 662 (96) 386 (56) 303 (44) Air aged 100 h 0" 58 621 (90) 352 (51) 331 (48) at 371°C (700°F) (1 atm)

0" Compression strength*, MPa (ksi) .~

* Non-standard test; open hole compression fixture used with no-hole specimen. For comparison the room temperature IITRI compression strength (D3410) was 1152 MPa (167 ksi)


2 1 I I I I I I I I I

% 0

E 0 .- 2"

1.6

1.2

0.8

0.4

0

0 1000 2000 3000 4000 5000 6000 7000 8000 Time, Hours

Fig. 4.12 Effect of air aging at 260°C (500°F) on the weight loss of AFR700B composites.24

4.8.7 AFR700B

One of the most promising new resin systems that has come along in the last several years is AFR700B. One indication of this is the excellent properties tabulated in Table 4.624 which had been obtained on unidirectional S2 (933 finish) glass tape laminates, both as- molded as well as after 100 h in air at 371°C (700°F).

In long term isothermal air aging studies24 carried out at 260°C (500°F) the weight loss performance of AFR700B laminates reinforced with AstroquartzO I11 fibers was superior to that exhibited by either S2-glass or Thornel 650-42 laminates (Fig. 4.12). The retention of the 260°C (500°F) flex strength of S2-glass laminates is illustrated in Fig. 4.13. Similar results were obtained with Astroquartz I11 and Thornel 650-42. Overall, composites based on AFR700B offer the promise of reasonable processability, cost and excellent mechanical properties in the dry-as-molded state out to about 371°C (700°F). Based on its chemistry

one can also expect significant improvements in both matrix resin toughness and thermal- oxidative stability compared with PMR-15.

4.8.8 TRW-R-8XX

Although very little unclassified property data was available at the time of the writing of this chapter, according to TRWZ5, the room temperature mechanical properties of composites based on TRW-R-8XX have been found to be equivalent to those based on PMR-15 but with superior toughness, higher Tg (371°C to > 449"C, 700°F to >840"F), superior retention (>70% compared with <20% for PMR-15) of properties at 371°C (700 OF) and up to 10 times better weight loss characteristics after 100 h at 371°C (700 O F ) compared with PMR-15. Based on these results and assuming reasonable processibility, TRW-R-8XX could turn out to be a serious contender in the world of high temperature polyimides, especially in view of the projected low cost.

Electrical properties 95

I?

€

!!? 3 v ii

I

UJ C

90

80

70

60

5 0

40

t I n E \

I I I 1 I 9 0

80

.- ua Y

7 0 i UJ C !! 3

6 o x ii 8

5 0

40

0 1000 2000 3000 4000 5000 6000 7000 Time, Hours

Fig. 4.13 Effect of air aging at 260°C (500°F) on the flex strength of AFR700B/S2-glass laminates."

4.9 ELECTRICAL PROPERTIES reported to be 3.2 and 3.7 respectively. Also, in

One of the useful properties of aromatic polyimides is their good all around electrical properties. Low dielectric constants and dissipation factors have been measured in quartz fabric reinforced Avimid N and Skybond based composite^^^. For instance, the room temperature dielectric constants for Avimid N and Skybond Astroquartz laminates have been

the same types of laminates the dissipation factors have been measured to be 0.001 for Avimid N and 0.015 for the Skybond. The par- tial fluorocarbon character of the 6F monomer is undoubtedly the reason behind the improved properties of Avimid N compared with Skybond. Some other miscellaneous electrical properties26 of E-glass reinforced Skybond based composites are tabulated in

Table 4.7 Electrical properties of Skybond@ 700/181 style E-glass fabric laminates 26

Property Units Value Dielectric strength short time parallel to laminate step by step parallel to laminate short time stepwise

Dielectric constant (1 MC) Dissipation factor (1 MC) Insulation resistance Volume resistivity Surface resistivity

volt volt

volts/pm volts/pm

megohm ohm-cm

Ohm

55 000 38 000

179 140 4.1

0.0045 1.9 x 107

2.47 x 1015 3.35 x 1014


Table 4.7. Skybond 700/E-glass fabric laminates have been shown to have a dielectric constant and dissipation factor measured at X-band (8.5 KMC) frequency at room temperature of 3.74 and 0.016 respectively. When measured at 300°C (572°F) there was essentially no change.

4.10 HYGROTHERMAL PROPERTIES

All of the mechanical properties discussed thus far in this chapter, either before or after air aging, have been determined on 'dry' spec- imens. Although there are many reports in the literature on the absorption of water by a wide variety of polymers and the effects that this water can have on the Tg, strangely there have been relatively few reports of the effects of moisture on polyimide composites. Hot/wet properties are normally reported for epoxy and bismaleimide composites, but usually not those based on polyimides. Unfortunately for polyimide composites water absorption is to be expected since the equilibrium water content of the neat resins being normally in the 2 4 % range.

One of first references to moisture effects occurred in 1976% in which studies at DuPont on Avimid N composites indicated that low void (<lo/,) E-glass fabric laminates under- went an almost 100°C (212°F) reduction in the Tg when that property was measured on a water saturated laminate by thermal mechanical analysis at a heat-up rate of 50°C (90"F)/min. The original 'dry' Tg of 342°C (648°F) might have indicated good mechanical property retention out to at least 316°C (600°F). However, the 'wet' Tg of 245°C (473°F) indicated a dramatically lower projected end- use temperature (<<245"C). It was also reported in the same reference that at the slower heat-up rate of 5°C (9°F) per minute a much higher apparent 'wet' Tg was possible (305"C, 581°F). Thus, if a part can be heated up slowly enough, it can dry out as it is heated and the deleterious affects of absorbed moisture can be greatly reduced. In this same paper

there were also definite indications that voids in the composite could serve to facilitate the drying out process, thus resulting in a higher apparent 'wet' T . This means that although voids are well known to adversely affect mechanical properties their presence could conceivably result in significant improvements in a composite's hot/wet properties to the point where an overall better balance of properties might be possible.

In the design of high temperature polyimide parts it is strongly suggested that moisture effects be fully taken into account. It seems apparent that the full potential of polyimide composites will not be realized until effective ways are found to reduce the adverse effects of moisture at elevated temperatures without seriously affecting the other important properties such as strength, toughness and thermal-oxidative stability.

4.11 END-USE APPLICATIONS

In spite of their recognized limitations (e.g. microcracking and hygrothermal problems) polyimide composites have been successfully employed i i ~ a wide variety of applications. For instance, autoclave molded PMR-15 graphite fabric composite has been employed in the manufacture of ducts for the F-404 engine used in the United States Navy's F-18 fighter (Fig. 4.14). Other successful applications for PMR-15 include a fire wall for the GE-90

Fig. 4.14 PMR-15 /graphite duct for the F-404 jet engine.

End-use applications 97

Fig. 4.15 AvimidO N/graphite jet engine variable stator vane bushings.

engine as well as various splitters and fairings for the F-110 engine.

Compression molded Avimid N/graphite variable stator vane bushings (Fig. 4.15) and washers, now available from DuPont/Tribon Composites, have been extensively employed since the early 1980s in conjunction with the

variable stator vanes in a variety of military and commercial jet engines. Parts of this type have also been made using PMR-15 and PMR-11. Another interesting non-aerospace application for Avimid N from DuPont has been the product, NO-CHX@ takeout jaws (Fig. 4.16), which have been used in the glass bottle industry to grab the hot blow-molded glass bottles to transport them during fabrication without cracking or checking.

Other applications for polyimide composites have been radomes, missile fins, jet engine nozzle flaps, fairings, cowls and inlet guide vanes, gear cases for helicopters and heat shields.

1

Fig. 4.16 NO-CHX@ Take out jaws based on Avimid N/graphite.


REFERENCES

1. Gibbs, H.H. 20th Natl. SAMPE Symp. Exhib., April 1975.

2. Sonnett, J.M., McCullough, R.L., Beeler, A.J. and Gannett, T.P. 24th Intern SAMPE Tech. Conf., October 1992, p. T735.

3. Scola, D.A. United Technologies, private communication.

4. Serafini, T.T., Delvigs, P. and Lightsey, G.R. 1. App. Polym. Sci., 1972,16, 905.

5. Serafini, T.T., Delvigs, P. and Lightsey, G.R. US Patent 3 745 149 (July 1973).

6. Serafini, T.T. and Delvigs, P. Appl. Polym. Symp., 1973, 89, (22).

7. Serafini, T.T. Proc. 1975 Intern. Conf. Composite Materials, AIME, New York, 1976,1,202.

8. Serafini, T.T., Delvigs, P. and Lightsey, G.R.

9. Burns, E.A., Lubowitz, H.R. and Jones, J.F. NASA TN D-6877 (1972).

NASA CR-72460 (1968). 10. Lubowitz, H.R. US Patent 3 528 950 (1970). 11. Serafini, T.T., Vannucci, R.D. and Alston, W.B.

12. Vannucci, R.D. 32nd Intern. SAMPE Symp. Exhib., April 1987.

13. Vannucci, R.D. and Cifani, D. NASA TM-100923 (1988).

14. Vannucci, R.D. and Cifani, D. 20th Intern. SAMPE Tech. Conf., Sept. 1988.

15. Serafini, T.T., Delvigs, P. and Vannucci, R.D. 36th Ann. Tech. Conf. SPI Reinforced Plastics/Composites Inst., Feb. 1981.

16. Vannucci, R.D., Malarik, D.C., Papadapoulos D.S. and Waters J.F. NASA TM 103233.

17. Vannucci, R.D., Malarik, D.C., Papadapoulos, D.S. and Waters, J.F. 22nd Intern. SAMPE Tech. Conf., Nov. 1990.

18. Malarik, D.C. and Vannucci, R.D. NASA CP-

NASA TMX-71984 (1976).

10039,15-1 (1989).

19. Meador, M.A., Cavano, P.J. and Malarik, D.C. Proc. Sixth Ann. ASM/ESD Adv. Comp. Conf., Oct. (1990).

20. Sutter, J.K. et al., NASA CP-10039,12-1 (1989). 21. Serafini, T.T. et al., US Patent 5 091 505 (Feb.

22. Serafini, T.T. et al., US Patent 5 149 760 (Sept.

23. Serafini, T.T. et al., US Patent 5 149 772 (Sept.

24. Johnson, K.M. Air Force Materials Laboratory,

25. Serafini, T.T. TRW-Redondo Beach, private com-

26. Monsanto trade literature. 27. Scola, D.A. 34th Intern. SAMPE Symp., p. 246

28. Hexcel trade literature. 29. Gibbs, H.H. 10th National SAMPE Tech. Conf,

1978, p. 21. 30. Gibbs, H.H. 1. Appl. Poly. Sci., Appl . Poly. Symp.,

1979, 35,207. 31. Ward, D. General Electric -Aircraft Engines, pri-

vate communication. 32. Meador, M.A. NASA Lewis Research Center,

private communication. 33. Tanikella, M.S. Mosco, J.A. and Rafalski, T.J.,

24th Intern. SAMPE Tech. Conf., 1992, p. 687. 34. Gibbs, H.H. 21st Natl. SAMPE Symp. Exhib.,

1976, p. 607. 35. Fiberite trade literature. 36. Engineered Materials Handbook - Composites,

37. Wilson, D. High Performance Polymers, 1991,3(2). 38. DuPont trade literature. 39. Iuliano-Picho, D. DuPont Composites, private

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