Highly enhanced mechanical properties of polypropylene-long carbon fiber composites by a combined method of coupling agent and surface modification of long carbon fiber

The aim of this study was to enhance the mechanical strengths of polypropylene/long carbon fiber thermoplastic (PP/LCFT) composite by increasing the adhesion between the PP matrix and the long carbon fiber (LCF). Bi-functional group grafted polypropylene (BFPP) was used as a coupling agent with surface modified LCF (SMLCF) in a long fiber thermoplastic (LFT) melt impregnation system to increase the interaction between the PP matrix and the LCF. The BFPP was produced by melt compounding of maleic anhydride grafted polypropylene (MAPP) and polyether amine (PEA). The surface modified LCF (SMLCF) was produced by dipping LCF into a sizing bath with 3-methacryloxypropyltrimethoxysilane (MPTS) to obtain oxygen functional groups. The composites were produced with a LFT melt impregnation system with PP, BFPP, and SMLCF. Tensile and flexural tests and scanning electron microscopy (SEM) results of the PP/BFPP/SMLCFT composite showed significantly enhanced mechanical strengths, compared with those of the common PP/LCF composite with a conventional maleic anhydride grafted polypropylene (MAPP) coupling agent. These improvements in mechanical properties are attributed to better fiber/matrix interfacial adhesion, as confirmed by micro droplet specimen tests and SEM micrographs of the fracture surface after inter laminar shear testing. The PP/SMLCFT composite with BFPP 5 wt% as coupling agent showed the highest tensile strength and flexural strength, which increased by 1.5 times and 1.7 times respectively, compared with PP/LCFT with a conventional MAPP coupling agent. The composite produced by this effective combination method of a coupling agent and surface modification of long carbon fiber can potentially be applied to automobile materials, leading to the replacement of metal parts and car weight reduction.