40207_05SPECIALITY MATRIX RESINS - SPECIALITY MATRIX RESINS...

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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
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40207_05SPECIALITY MATRIX RESINS - SPECIALITY MATRIX RESINS...

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