Influence of surface-modification of polypropylene fibers on material properties of PP-VE composites

Polymer-polymer composites comprised of vinyl ester matrices and small diameter polypropylene fiber meshes were fabricated and tested in this investigation. The general goal is to develop toughened polymeric systems capable of absorbing and dissipating energy upon impact while maintaining structural attributes in advanced composite applications. Such composites provide several means for tailoring energy absorption capacity including interfacial surface area, fiber yielding, and melting transitions. Commercial vinyl ester (VE) resins were used and commercial small diameter (∼1 μm) polypropylene (PP) fibers were selected as the modifying phases. Composites were prepared using vacuum assisted resin transfer molding technique and characterization analysis included determination of fracture toughness, flexural properties, thermomechanical behavior, fiber surface morphology, and composite fracture surface morphology. It was found that PP fibers greatly enhance fracture toughness. However, strength of the VE was significantly reduced as voids were observed at the interface of the PP and VE materials. A two-step surface modification, oxygen-plasma treatment followed by grafting silane coupling agents, was conducted on PP fibers in an effort to improve interfacial strength. Interfacial discontinuities of composites were improved after silane-grafting on PP fiber surface. Flexural strength was enhanced but fracture toughness was reduced due to the diminishment of voids.