Polybenzimidazole Prepreg: Improved Elevated Temperature Properties with Autoclave Processability

Abstract

Polybenzimidazoles (PBI) are heterocyclic polymers which are known for their outstanding thermal and chemical stability. Hoechst Celanese has recently developed PBI (2,2'-(m-phenylene)-5,5'-bibenzimidazole) as an engineering plastic with the trade name CelazoleTM. The polymer has excellent compressive strength, high tensile strength and modulus, and a glass transition temperature of 425°C. Low molecular weight, oligomeric forms of PBI have been used in the past as a composite matrix material. The prepregs produced were stiff and board-like at ambient temperature. In addition, in situ polymerization resulted in large amounts of condensation by-products (phenol and water) which meant that high pressure curing conditions were needed to minimize voids. More recently, medium molecular weight PBI (12 000-20 000 g molweight average) has been used in combination with NN-dimethylacetamide (DMAc) solvent to produce prepregs with excellent tack and drape. These prepregs have significantly reduced condensation by-products, resulting in improved processability and properties. Reported in this article are polymer modifications which enhance the ability to cure PBI prepreg at standard autoclave pressures and have the added benefit of improved high temperature composite properties.