11 - Bismaleimide Resins

Abstract

Bismaleimide resin systems are noted for their high-strength, high-temperature performance, particularly as matrix resins in fiber-reinforced prepregs and composites. They are bridging the gap between relatively low temperature-resistant epoxy systems and very high temperature-resistant polyimides. The most important monomer is 4,4′-bis(maleimido)diphenylmethane (BMI). BMI has a melting temperature of 155–156°C and it polymerizes radically above the melting point. Networks resulting from BMI are very brittle. BMI can be used together with 2,2′-Diallyl bisphenol A (DBA). DBA copolymerizes with BMI. Imides are often substantially insoluble in ordinary organic solvents and are soluble only in high boiling aprotic polar solvents, such as N-methyl-2-pyrrolidone, N,N-dimethylacetamide, etc. This is a drawback when impregated varnishes are prepared by dissolving imides in these solvents. High temperature is required for removing the solvents, and the solvents are liable to remain in the prepregs formed from the varnishes, causing foaming in the laminates and considerably lowering the quality of flexible printed circuits (FPC). The toughness of bismaleimide resins is a major problem that is limiting the field of application. This toughness can be improved by adding reactive components that reduce the cross-linking density.