Nylon 6/Graphite Fiber Composites by in Situ Polymerization

Abstract

The in situ polymerization of caprolactam to Nylon 6 was investigated as a method for making graphite fiber composites. It was necessary to find a co-catalyst (using sodium caprolactam as catalyst) which could be mixed at about 100°C and where the final mixture had a long enough pot life to allow for mixing and application to the fibers. The temperature could then be raised to 200°C to initiate rapid polymerization.

Co-catalysts tested were all esters as these had earlier been shown to have appropriate lifetimes. They were phenyl acetate, benzyl acetate, benzyl benzoate, t-butyl acetate and ∈-caprolactone. Benzyl derivatives gave brittle polymerizates, while the others gave tough materials. Phenyl was excluded as potentially reacting too rapidly. t-Butyl gave good polymers at 190°C but expanded into a foam when heated to 210° due to volatilization of the t-butanol. ∈-Caprolactone initiated well and produced no residues, so it was chosen as the major co-catalyst.

Jigs were designed on which graphitic fiber could be wound at various tensions. These were put in tubes containing precatalyzed monomer at 100°C. The whole was degassed several times and then heated to 200°C for polymerization.

Several samples of composites were made with fiber densities of 25 to 40 volume % using Thornel 50 S and 75 S fibers. The samples still had some voids due to lack of complete penetration of monomer. The samples varied widely in their tensile strengths and moduli due mainly to variations in fiber density, but also to some voids. The best samples achieved about 70% transfer of properties. This is a respectable figure when one considers that these are the first samples made at these fiber densities.

Scanning electron microscope photographs show the best samples had excellent adhesion of matrix to fiber. This resulted in essentially no pull-out, and the areas examined showed multiple fracture of fibers at the fracture surface with extensive deformation of the matrix. Such composites should have high impact strength.