SPECIALITY MATRIX RESINS 5 David A. Shimp

5.1 INTRODUCTION

Bismaleimide (BMI) and cyanate ester (CE) resins were first commercialized in the 1970s as 250-300°C Tg class laminating resins for circuit board substrates. In the early 1980s structural prepregs were introduced to an aircraft indus- try searching for primary structure composites with higher service temperatures and improved damage tolerance relative to multi- functional epoxy based composites. Both BMI and CE resins have since evolved as easy-to- process thermosetting resins qualified for 177°C (350°F) hot-wet service. Toughening technologies provide compression-after-impact ratings approaching or matching the damage tolerance of thermoplastic resin composites.

Bismaleimides, with higher modulus values and established higher thermal ratings, earned a strong position in military aircraft primary structures with recent selection for the F-22 fighter. Cyanate esters, with superior dielectric loss properties and lower moisture absorption, are strong contenders for radomes, skins cover- ing phase-array antennae, advanced Stealth composites and space structures.

5.2 RESIN CHEMISTRY

Cyanate ester monomers are prepared by react- ing bisphenols or polyphenols with cyanogen chloride in the presence of an organic base (Rottloff, 1977). Crystalline monomers are ther- mally advanced to amorphous prepolymer

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

resins by resin suppliers accomplishing from 1550% of the cyclotrimerization curing reac- tion using closely controlled reactor processing. Figure 5.1 illustrates formation of the s-triazine ring (cyanurate trimer) by the cycloaddition of three cyanate groups. Formulators catalyze amorphous monomers or prepolymer resins with latent catalysts which promote full con- version to the thermoset polycyanurate with subsequent heating to 177-250°C. Cyanates also serve to cure epoxy resins, forming cost- effective hybrids retaining an anomalously high fraction of CE homopolymer properties (Shimp, 1992).

Bismaleimide monomers are prepared by the reaction of aromatic diamines with maleic anhydride in the presence of dehydrating agents (Stenzenberger, 1990). Homopolymers of BMI monomers are excessively brittle and in practice are co-reacted with chain-extending diamines, diallyl bisphenols or dipropenyl phenoxides to develop toughness via reduced cross link density. Figure 5.2 illustrates chain extension with aromatic diamine (Bargain, 1971) to form longer linear segments which ultimately crosslink by homopolymerization of maleic double bonds. Kerimid@ resins and Fiberite PI molding compounds are examples of commercial BMIs using aromatic amine modification.

Figure 5.3 depicts a series of reactions whereby o-allyl-phenols add across the maleic double bond via the 'ene' reaction and a second maleimide enters into a Diels-Alder ring-form- ing reaction with the now conjugated propenyl residual double bond. The proposed reaction

100 Speciality matrix resins

0 0

I V I

MICHAEL ADDITION

I HOMOPOLYMERIZATION

V I Prepolymer resin

0 0 0 0

Curing via cyclotrimerization

0 R

i Thermoset plastic (polycyanurate)

Fig. 5.1 Dicyanates cure by forming triazine rings on heating, advancing to prepolymers (up to 50% conversion) and to thermoset plastics at -O-C=N conversions >60-64%.

Y Y

X = alkylidene Y = alkyl or H

Fig. 5.2 Sequence of chemical reactions for advanc- ing BMI monomers with aromatic diamines to resin adducts by chain extension and ultimately to toughened thermoset plastics.

mechanism includes isomerization to form the aromatic ring. Crossllnking can occur by a con- tinuation of these reactions (difunctional components) or via residual maleic double bond homopolymerization. The chemistry by which o,o'-diallyl bisphenol A coreacts with and toughens 4,4'-bismaleimido-diphenylmethane (BMI-DAB) is described by Zahir (1978) and

Matrixformulation 101

King (1984). Allyl functional phenoxy com- pounds follow the same reaction path while propenyl functional phenoxides eliminate the 'ene' reaction step (Stenzenberger, 1990).

r! R i I +

@- OH I "ENE" REACTION

Ri I

bv2b OH

1 yy Ri DIELS-ALDER I

REACTION

b % d HOMOPOLY-

e.9 MERIZATION - I OH

AROMATIZATION 1

5.3 COMMERCIAL RESINS

The chemical structure of seven commercial di(po1y)cyanate ester monomers is shown in Table 5.1 along with supplier information, physical state and key homopolymer proper- ties. The three crystalline monomers are usually supplied only as amorphous prepoly- mers in semisolid, hard resin, or ketone laminating solution form. CE homopolymer properties are not affected by prepolymer advancement, which is only an interruption of the ring-forming curing reaction to alter phys- ical state and rheological properties. Monomer asymmetry, e.g. AroCy@ L-10, can yield low RT viscosity. CEs have a low toxicity profile and storage stability comparable to epoxies.

Table 5.2 describes several BMI monomers, advanced resins, reactive tougheners with allyl and propenyl functionality and RTM resins. Most BMI monomers have a crystalline physical state. Eutectic blends of monomers are available as resolidified melts of lower melt point. Resins prepared by prereacting a molar excess of BMI with tougheners have a powder or hard resin physical state. Allyl and propenyl functional reactive tougheners are usually viscous liquids which serve to dissolve crystalline BMIs at temperatures below 110°C to offer convenient melt processibility.

5.4 MATRIX FORMULATION

5.4.1 MEETING RHEOLOGICAL HoMoPoLY- REQUIREMENTS MERIZATION - Both resin classes offer a wide selection of

monomers and prepolymers enabling the fomulator to satisfy the rheological properties of fluid RTM compounds, tack and cohesive integrity of compliant prepreg and the short flow of compression molding compounds. Figure 5.4 illustrates the limiting direct rela- tionship in families of thermosetting resins between fluid monomer (150 mPa s viscosity) temperature and Tg On curing. High temperature polyimides locate Off-sCale in the

Fig. 5.3 Sequence Of reactions between Phe- nols and BMI monomers involves grafting via the 'ene' reaction and fused ring formation via Diels-Alder. Crosslinking with di(po1y) functional components involves completion of these reactions and/or maleic double bond homopolymerization.

102 Speciality matrix resins

Polycyanate monomer structure/ arecursor

Trade name/ Homopolymer property supplier/ T.2 wt.% Dk G, physical state O e H,O MHz

~~

AroCy B e c - ~ c ~ ~ + c E N Ciba Specialty Chem.*

2.91 289 2.5 CH3 BT-2000 Mitsubishi GC

Bisphenol A Crystal

9 4 3 p 3 AroCy M N E c - o o { *)N Ciba Specialty Chem. 252 1.4 2.75

$ Crystal dH3 CH3

Tetramethylbisphenol F

J m-2

140

1 75

AroCy F ,oOcP*ZN Ciba Specialty Chem. 270 1.8 2.66 140

CF3 Crystal

Hexafluorobisphenol A

Bisphenol E

Bisphenol M

AroCy L-10 Ciba Specialty Chem. 258 2.4 2.98 190

Liquid

XU-366 Ciba Specialty Chem. 192 0.7 2.64 210

Semisolid

Novolac resin

Primaset PT 270 Lonza, Inc.

to 3.8 3.08 60 XU-371 Ciba Specialty Chem. >350 Semisolid

XU-71787 *CZN Dow Chemical 244 1.4 2.80 125

Dicyclopentadienyl bisphenol Semisolid ~

* The complete name of the Ciba company supplying AroCy cyanate resins is Ciba Specialty Chemicals Corp., Performance Polymers Group.

Matrix formulation 103

Table 5.2 Commercial BMI monomers, resins (adducts), reactive tougheners and compounds

~

Supplier Trade name/structure

Ciba

Ciba

Inspec

Inspec

Inspec

Inspec

Inspec

Inspec

Inspec

Description

0 0

Matrimidm 5292A

Matrimid 5292B

Cornpimidem MDAB

Compimide 353 Eutectic monomer blend

Compimide 796 Proprietary BMI adduct

Compimide TM 121

a r' 0

Compimide TM 123

Compimide 15 MRK

Compimide 65 FRW

Basic BMI monomer Crystalline powder m.p. 150-160°C

o,o'-Diallyl Bisphenol A Reactive toughener 12 000-20 000 mPa s at 25°C

Basic BMI monomer

Resolidified BMI melt 400-1400 mPa s at 110°C

Resolidified BMI resin melt 10004500 mPa s at 110°C

Bisallyl polyphenoxide Reactive toughener 120-250 mPa s at 71°C

Bispropenyl phenoxy benzophenone Reactive toughener 1000-1600 mPa s at 71°C

Powder for molding compounds

Resolidified melt for filament winding and RTM

Ciba designates Ciba Specialty Chemicals Corp., Performance Polymers. Inspec designates Inspec Fine Chemicals Co.

Table 5.1 (on facing page) Commercial cyanate ester monomers, suppliers, physical states and homopoly- mer properties. Water absorption is wt.% at saturation. D, = dielectric constant

104 Speciality matrix resins

400

300

0

d,

z 200 I-

v) w [r

O w a 2 100

I BM' I 0

AROCY R T ~ T m - EPOXIDE

L-10 366 n U -

DlEPOXlDE

VINYL

- POLYESTER

0 0 50 100 150

MONOMER TEMP. ("C) at 150 MPA.S

Fig. 5.4 Relationship between fluid monomer tem- perature and cured T g in families of thermosetting resins. Higher service temperature is normally asso- ciated with increasing processing difficulties.

upper right quadrant. AroCy L-10, derived from an asymmetric bisphenol, breaks the pat- tern and can be used as a 250°C T g resin or as a reactive diluent of 120mPas viscosity (Fig. 5.5).

5.4.2 CURE CATALYSTS

Catalysts are not required to cure BMI resins at temperatures above 200"C, but several types provide effective cure acceleration. Tertiary amines, imidazoles and free radical generators are noted by Zahir (1978). Boyd (1987) describes the preferred latency of tri- phenylphosphine and its phosphonium halide derivatives as prepreg catalysts.

Cyanate esters require catalysis to cure at practical rates. Copper (most active at low temperatures) and cobalt (latent) acetylaceto- nates provide 295% conversion within 2-6 h at post cure temperatures in the range of 200-250°C. Metal coordination catalysts in general are difficult to solubilize in neat resins,

1 02- 0 2 0 40 60 80 1

AROCY L-10, WEIGHT %

10

Fig. 5.5 The asymmetric structure of AroCy L-10 dis- rupts crystallinity, permitting optional use of this ring-forming resin as a reactive diluent. AroCy num- bers are the % cyanate conversion of prepolymers.

but predissolving in 2-6 phr (parts per hun- dred resin) alkyl phenol, e.g. nonyl or dinonyl phenol, forms stable liquid packages which are readily miscible (Shimp, 1988). The alkyl phenol provides the active hydrogen co-cata- lyst and can serve as a monofunctional reactant to increase conversion and resistance to boiling water at marginal cure temperatures (Fig. 5.6). Extension of this principle to AroCy XU-366 enables this monomer to convert satis- factorily at 121°C for use with high modulus polyethylene fibers (Shimp, 1994a) and with composite tools.

5.4.3 TOUGHENING TECHNIQUES

Concentrated effort over the last decade has produced composite toughening techniques which satisfy damage tolerance requirements

Matrix and composite properties 105

L I ' I ". 0 2 4 6

NONYLPHENOL CONC. (phr) I I I 0 10 2 0 30 40

Meq OHIOCN

Fig. 5.6 Increasing concentrations of alkyl phenols in cyanate ester homopolymers increase conversion (numbers at right) for a given cure temperature and increase resistance of 3 mm thick castings to hydrol- ysis in boiling water. Cure temperature: a: 250°C; A,: 210°C; 0: 177°C. (for AroCy B)

of primary aircraft structures and microcrack resistance in earth orbit and cryogenic service. BMI resins earlier required the development of allyl, propenyl and amine functional reactants to achieve >2% tensile elongation-at-break, a minimum requirement for efficient secondary toughening with thermoplastic polyimides. Boyd (1990) describes the development of improved chain extending reactants by cou- pling 2- or 4-propenyl phenol with diepoxides. Incorporation of thermoplastic polyimide fine particles of 5-15 pm diameter, described by Boyd (1991a), increased the compression-after- impact (CAI) performance of BMI composites to 2276 MPa (240 ksi). Further toughening to

the CAI performance level of 245 ksi was demonstrated (Boyd, 1993a) by combining in situ epoxy extension of o,o'-diallyl bisphenol A with thermoplastic polyimide particles.

CE monomers and prepolymers dissolve powdered amorphous thermoplastics (Tps) of the polysulfone, polyethersulfone, polyether- imide, polyphenylene oxide and copolyester families, then subsequently phase separate these thermoplastics during cure. Co-continuous morphologies are developed at Tp concentra- tions 215% which increase GI, values in a non- linear response to concentration (Shimp, 1994a). Lee (1991) describes the development of a CE matrix formulation toughened with polyoxazolidinones, polyethersulfone and co- polyester Tp resins. Table 5.3 lists a number of Tp resins used to toughen both CE and BMI resins as well as reactive rubbers used to elim- inate microcracking in orbital service.

5.4.4 EPOXY RESIN MODIFIERS

Epoxy resins derived from epichlorohydrin co-react with CE resins at equivalent ratios of up to 1.2 epoxides per monomer cyanate. Hybrids with typical epoxy weight fractions of 50-70'/0 develop Tg values in the 180-200°C range and retain dielectric constants 13.1 with loss tangents generally below 0.010 (Shimp, 1992). The use of diepoxides to toughen BMI resins via chain extension of alkenyl (bis)phe- nols was described in the previous section. Epoxides can also react with the secondary amine formed by the Michael addition of aro- matic diamine to the BMI maleic double bond.

5.5 MATRIX AND COMPOSITE PROPERTIES

The following acronyms will be used to denote BMI resin system properties in figures used thoughout this section:

BMI-MDA = the reaction product of 4,4'-bis- maleimidodiphenyl methane (molar excess) with methylene dianiline.

BMI-DAB = the equimolar reaction product of

106 Speciality matrix resins

Table 5.3 Thermoplastic and reactive rubber tougheners

Classification Product

Soluble T," Polyethersulfone Polysulfone Polye therimide Polyphenyleneoxide Poly imide

Elastomeric T, Copolyester

Reactive rubbers Solublea (OH) Soluble" (Epoxy) Preformed Core/Shellb

Epoxy functional Maleimide functional

Polysiloxanes

Particulate T," Polyimide Poly imide Polyamide

Victrex 5003P Udel P-1700 Ultem lOOO(P) PPO Matrimid 5218

Vitel PE-307

ATX-013 Hycar ETBN CRS (exp.)

Experimental PAP Series

P-84 Matrimid 5218 1002 D NAT

203 175 215 202 300

14

<25 <25 <25

<25 <25

290 300 85

a Initially soluble but phase separate during cure. Small particles swell but do not completely dissolve with cure.

Note: Most of this toughening technology is described in patents.

4,4'-bismaleimido diphenylmethane with o,o'-diallylbisphenol A (Matrimid 5292).

5.5.1 MECHANICAL PROPERTIES

Table 5.4 compares properties of representa- tive CE and BMI castings. Significant differences are the higher room temperature modulus values of BMI matrices (superior stress transfer to fiber) and higher CE elonga- tion-at-break values.

5.5.2 THERMAL PROPERTIES

T, values average about 20°C higher for BMI matrices (Table 5.5). CTE values below T, are comparable while CE resins retain higher char yields as a result of increased aromaticity.

Used with

Supplier

CE CE CE CE CE

CE

CE CE CE

CE CE

BMI BMI CE

ICI/Mitsui Amoco General Electric General Electric Ciba

Bostik

Echo Resius B.F. Goodrich Dow Chemical

Proprietary National Starch

Lenzing AG Ciba Atochem Corp.

Although CE resins demonstrate higher tem- perature onsets of rapid thermal degradation in TGA tests, available long term isothermal aging tests in air indicate superior BMI perfor- mance. Boyd (1993b) classifies 1-year CAI retention life of BMI composites as >177 but <205"C. Stenzenberger (1991) rates 2000 h life of BMI castings as >2OO0C but <250"C based on retention of shear and flexure strengths. CE/E-glass laminates are rated at 162-180°C for 25 000 h retention of flexure strength at 50% of the unaged values (Shimp, 1989).

5.5.3 DIELECTRIC PROPERTIES

Figure 5.7 ranks the dielectric constant (D,) and dissipation factor (D,) or loss tangent of thermoset matrix resin castings compared with reference thermoplastics. Effects of

RESIN DIELECTRIC PROPERTIES 25 O C

DK Df

BMI-DAB AroCy 6, L

AroCy B AroCy M ,F AroCy M AroCy F F XU - 366 POLYETHYLENE

2 1 PTFE

1 1 AIR I oe5 I Fig. 5.7 Thermosetting and thermoplastic resins are ranked for dielectric constant (D,) and dissipation factor (D,) at 1 MHz frequency.

moisture absorption, test temperature and fre- quency are summarized by Shimp (1994b). Low D, values of CE homopolymers and CE/epoxy hybrids are attributed to the sym- metrical arrangement of electronegative oxygen and nitrogen atoms around a central electropositive carbon atom in these struc- tures, resulting in weak dipoles. Dielectric constants of CE composites compared with fiber type and loading are plotted in Fig. 5.8. D, and loss tangent values of quartz reinforced BMI, CE and epoxy composites are compared at four radar bandwidths in Fig. 5.9 (Speak, 1991).

5.5.4 MOISTURE ABSORPTION AND EFFECTS

CE homopolymers absorb less moisture than BMI and TGMDA/DDS epoxy matrix sys-

Matrix and composite properties

DK 'I,

HOMOPOLYMER

0 20 40 60 80 11 RESIN CONTENT, Volume %

107

0

Fig. 5.8 Effect of reinforcement and concentration on dielectric constant of AroCy M composites. Test data at 25°C and 1 MHz.

tems. Moisture locates primarily in the CE free volume fraction, resulting in less swelling (Fig. 5. 10) than is caused by association with strong dipoles. Plasticization of moisture-con- ditioned matrix castings, compared as a function of flexural modulus retention at ele- vated test temperatures, is minimized by the low absorption of AroCy M o-methylated CE resin (Fig. 5.11). Hydrolysis of AroCy M cya- nurate linkages in 121°C steam requires >600 h exposure (Fig. 5.12).

5.5.5 PROPERTIES OF UNIDIRECTIONAL COMPOSITES

Properties of intermediate modulus carbon fiber reinforced BMI composites (Table 5.6) and CE composites (Table 5.7) indicate good translation of fiber strength for both classes. Damage tolerance ratings based on CAI results at 6.7 KJ m-l impact energy fall in the 200-345 MPa (30-50 ksi) class, approaching or equaling the damage resistance of thermoplastic

108 Speciality matrix resins

Table 5.4 Mechanical properties of CE and BMI resins

AroCy B

AvoCy M

Composition (PBW) AroCy B-30 AroCy M-20 AroCy L-10 Matrimid 5292A Matrimid 5292B Nonylphenol Cobalt acetylacetonate

Property of casting"

Tensile strength, MPa ksi

Tensile elongation, YO Flexure strength, MPa

ksi

Young's modulus

25"C, GPa msi

149"C, GPa msi

163"C, GPa msi

204"C, GPa msi

G,,, J m-2 in lb in-2

100 -

-

- -

2 0.13

88 12.7 3.2 174 25.2

Flexure

3.17 0.46 -

-

2.55 0.37 - -

140 0.80

-

100 -

- - 2

0.13

76 11.0 2.7 159 23.0

Flexure

2.97 0.43 - -

2.35 0.34 - -

175 1.00

AroCy L

-

- 100 -

-

2 0.13

87 12.6 3.8 187 27.1

Flexure

3.24 0.47 -

-

2.28 0.33 -

-

190 1.08

Matrimid 5292

-

-

-

100 85 -

-

82 11.9 2.3 167 24.2

Tensile

4.28 0.62 2.42 0.35 -

-

2.00 0.29 170 0.97

a Step-cure with post cure of 2 h at 250°C for CE; 6 h at 250°C for BMI. Data courtesy of Ciba Specialty Chemicals Corp., Performance Poiymers Group.

composites. BMI composites have demon- strated hot-wet performance in 177°C rated aircraft. CE composites may attain that goal with AroCy M resin, but insufficient 177°C hot-wet compression data has been published for commercial materials of this class.

5.6 DESIGN CONSIDERATIONS

5.6.1 SELECTION OF ARAMID FIBER AND CORE

Aramid fiber and core reinforcements for CE composites should be selected from second generation materials wluch absorb <2% mois- ture in the workplace. Kevlar@ aramid fiber and

KorexTM aramid/phenolic honeycomb core (DuPont) are recommended for use with CE resins catalyzed with copper or cobalt acety- lacetonates to eliminate blistering associated with post cures >190°C. (Shimp, 1993; 1994a).

5.6.2 GALVANIC CORROSION

Both BMI and CE carbon fiber composites have been reported to undergo resin degrada- tion in accelerated galvanic cell tests producing strongly alkaline conditions at cathodic sites (Boyd, 1991b; Olesen, 1991). Figure 5.13 compares the onsets and rates of alkaline hydrolysis (etching) for CE and BMI matrix castings. (See also Boyd, 1991b).

Suppliers of prepreg and other formulated products 109

Table 5.5 Thermal properties of CE and BMI resins

AroCy AroCy AroCy Matrimid B M L 5292

Composition (PBW) - - AroCy B-30 100 -

AroCy M-20 - 100 - - AroCy L-10 - - 100 -

Nonylphenol 2 2 2 Cobalt acetylacetonate 0.13 0.13 0.13 -

- 100 85

Matrimid 5292A - - Matrimid 5292B - - -

-

Property of casting"

HDT, "C Dry 254 252 249 273 Wet 197 226 183 217

T,' "C by DMA 289 267 270 295 by TMA 257 255 259 273

CTE by TMA, ppm/"C

TGA at 10"C/min

40 to 200°C 64 66 64 63

Onset in air, "C 411 406 408 371 Char in N,, Yo 41 46 43 29

Specific gravity at 25°C 1.201 1.151 1.228 1.232

a Step-cure with post cure of 2 h at 250°C for CE: 6 h at 250°C for BMI. Data courtesy of Ciba Specialty Chemicals Corp., Performance Polymers Group.

Effective design practices for susceptible com- posites are use of titanium rather than aluminum rivets, placement of a fiberglass reinforced insulating ply and/or modification of CE resin with 55-70% epoxy resin.

5.6.3 MICROWAVE TRANSPARENT COMPOSITES

Composite design for radomes, antennas and advanced stealth structures should utilize low dielectric loss materials (Speak, 1991; Shimp, 1994b; Stonier, 1991a,b). Figure 5.14 summa- rizes microwave interactions with a radome wall. Reflection weakens returning signals and overheats emitter sources; refraction distorts signal quality; absorption decreases signal

strength and generates destructive heat, limit- ing power and range. CE composites curing at 121°C (250°F), e.g. Bryte Technologies' EX-1515, are thermally compatible with high modulus polyethylene reinforcement. Such composites are characterized by D, values as low as 2.6 and D, values as low as 0.004 when measured at 10 GHz.

5.7 SUPPLIERS OF PREPREG AND OTHER FORMULATED PRODUCTS

Table 5.8 lists suppliers of BMI and/or CE prepreg, adhesive, syntactic foam, RTM/fila- ment winding systems and chopped fiber reinforced molding compounds formable by compression, injection or transfer processes.

110 Speciality matrix resins

4.0 I 1 IIELECTRIC I I CONSTANT

3.5

3.0 XBAND KaBAND UBAND WBANC

GHz GHz GHz GHz

J

8-12 26-40 40-60 75-100

0.030

TANGENT 0.020

0.0 10

CE t 0.000 ' I

XBAND KaBAND UBAND W BAND

Fig. 5.9 Comparison of typical quartz reinforced radome composites for dielectric loss properties measured at four radar bandwidths. Redrawn from Speak, S.C., Sitt, H and Fuse, R.H.. 1991. Novel cyanate ester based products for high performance radome applications. Int. S A M P E Symp., 36 pp. 336-347.

i P I +

10' 1 o2 1 o3 1 o4 Hours at 25°C & >95% RH

Fig. 5.10 Changes in 3 mm thick bar volumes during water immersion for a period of one year indicate swelling rates and limits of thermoset resins. The ratio of volume increase to total volume of water absorbed (numbers on right) indicates the fraction of water associated with dipoles. A: BMI-MDA; X: BMI-DAB; 0: TGMDA-DDS; 0: AroCy B; 0 :XU-366

Suppliers of prepreg and other formulated products 111

Table 5.6 Properties of BMI/IM-7 unidirectional composites

Mechanical st rengt k Rigidite 5250-4"

Rigidite 5260b

0" Tensile, MPa (ksi)

0" Compression, MPa (ksi)

25°C

25°C Dry 105°C Wet 149°C Wet 177°C Dry 177°C Wet

0" Compressive modulus

Open hole compression, MPa (ksi)

25"C, GPa (msi)

25°C Dry 177°C Dry 191°C Wet

Compression after impact At 4.5 kJ m-l, MPa (ksi) At 6.7 kJ m-l, MPa (ksi)

2618 (380) 2691 (390)

1820 (235) - - - -

1310 (190) 966 (140)

1746 (253) 1346 (195) 1276 (185)

158 (23) 152 (22)

420 (61) 351 (51) 303 (44)

352 (51) 269 (39) 221 (32)

248 (36) 214 (31)

380 (55) 345 (50)

Edge delamination, MPa (ksi)

a Data courtesy of Cytec. Post cure 6 h at 227°C; 60% fiber vol. Data courtesy of Cytec. Post cure 6 h at 215°C; 60% fiber vol.

241 (35) ~~

25°C 358 (52)

Table 5.7 Properties of CE unidirectional composites

Cytec Fiberite Hexcel 5245C 954-2 HX-1562

Reinforcementhre Carbon fiber Max. cure temp., "C

Mechanical strength

0" Tensile, MPa (ksi)

0" Compression, MPa (ksi) 25"C, Dry 121"C, Wet 132"C, Wet 149"C, Wet

CAI, MPa (ksi) At 6.7 kJ m-I

Edge delamination, MPa (ksi)

IM-6 210

IM-7 232

IM-7 177

2439 (356) 2814 (408) 2610 (378)

1690 (245) 1350 (196) 1310 (190) 987 (143)

1573 (228) 1331 (193)

1290 (187) - -

1700 (246)

1140 (165) - -

- -

214 (31) 262 (38)

262 (38)

317 (46)

269 (39)

112 Speciality matrix resins

0 ° 1 1 O0 79

74

61

~

AroCy AroCy BMI/

Fig. 5.11 (left) Moisture plasticization of cast matrix systems is inversely related to the percent- age of dry room temperature flexural modulus retained at elevated test temperatures.

: at 149°C wet; : at 177°C wet.

Fig. 5.12 (below) Hydrolysis of unsubstituted CE (bisphenol A dicyanate) homopolymer begins to reduce mechanical properties after 200 h exposure to 121°C steam autoclave at 15 psig. Ortho-methy- lation is an effective technique for increasing hydrolytic stability of cured CE resins in aggressive environments. 0: AroCy B; 0: AroCy M.

% WEIGHT GAIN 121

91 AROCY B

0 200 400 600 TIME, HOURS

Table 5.8 Sources of formulated/compounded CE and BMI products

Supplier Prepreg Adhesive Syntactic RTM Compression molding foam compound compound

Bryte Technologies CE CE CE CE - Cytec BMI, CE BMI, CE BMI, CE BMI -

- - - Hexcel BMI, CE BMI Fiberite, Inc. CE - - - BM1,CE YLA CE CE CE CE -

AppIications 113

+1.0 AROCYBIEKXY

AROCYM w H.5 P I 3 I-

0 10 20 30 40 50 60

DAYS IMMERSION IN 20% NaOH AT 50°C

Fig. 5.13 Cured CE and BMI resins hydrolyze (etch) in strongly alkaline solutions, as indicated by the onset of weight loss. Ortho-methylated CE resin and blends with epoxy resin (50/50 blend shown) increase resistance to alkaline environments gener- ated in galvanic cells.

5.8 APPLICATIONS

Toughened BMI/carbon fiber composites have been specified as the principal composite mate- rial for F-22 fighter primary and secondary structures (Fig. 5.15). BMI service temperatures are sufficiently high for cowlings, nacelles and thrust reversers of jet engines. CE composites

t Y

TRANSMISSION Fig. 5.14 Interactions of microwaves with a radome wall.

were used to construct EFA (Eurofighter) pro- totypes and are used in construction of the Dassault Rafale. Both materials are candidates for HSCT (High speed civil transport) use.

Principal applications for CE composites (McConnell, 1992) include radomes for mili- tary aircraft, fighter aircraft retrofitted with improved tracking systems, skins over phase array radar, weather tracking aircraft radar and missile nose cones. CE prepreg reinforced with high modulus pitch-based carbon fibers are preferred materials for earth orbit service,

Fig. 5.15 F-22 fighter constructed with BMI composites. Photograph courtesy of Lockheed.

114 Speciality matrix resins

demonstrating low outgassing, microcrack resistance and resistance to lo9 rads of ionizing radiation (Willis, 1991). Applications in space include communication satellites, solar arrays, parabolic antennas, optical benches and preci- sion segmented reflectors.

BMI film adhesives are employed in jet engine or high speed aircraft sandwich panels where hot-wet service up to 190°C is required. CE film and paste adhesives are used together with syntactic foams in the construction of radomes. BMI molding compounds reinforced with up to 65 wt.% of chopped reinforcements are used to mold ducts, drive sprockets for heated rolls in copy machines, helicopter gear boxes and missile strongback mounting sup- ports.

REFERENCES

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