Improving recycled carbon fiber by single walled carbon nanotubes in reinforcement of polyamide composites

Recycled carbon fibers (rCF) represent an inexpensive alternative to virgin carbon fibers (CF) but the mechanical improvement brought by rCF to polymer composites is noticeably lower than that from CF. In this article we evaluate two methods of coating rCF by single walled carbon nanotubes (SWCNTs), wet and dry, and demonstrate that extremely low concentrations of SWCNTs, ∼0.03%, are sufficient to measurably improve the strength (by 16%), modulus (by 38%), and Izod impact strength (by 52%) of thermoplastic crystalline polyamide polyxylylene adipamide (MXD6) with 30% rCF. The dry method with grinding of rCF and SWCNTs works better than the wet (dip coating) method and the rCF with residual epoxy is better than purified by pyrolysis rCF. We link this improvement of the mechanical properties to the enhanced crystallization rate caused by SWCNTs, which is observed in differential scanning calorimetry (DSC).