Reinforcement of PP with polymer fibers: Effect of matrix characteristics, fiber type and interfacial adhesion

Two polypropylene polymers with considerably differing properties were modified with poly(ethylene terephthalate) (PET) and poly(vinyl alcohol) (PVA) fibers. Fiber content changed from 0 to 60 vol% and interfacial adhesion was modified with the addition of a maleated PP coupling agent. Mechanical properties were characterized by tensile and impact testing, while structure was evaluated by acoustic emission measurements and scanning electron microscopy. Attention was focused on the balance of stiffness and impact resistance and on the relationship of local deformation processes to macroscopic properties. The results showed that different inherent matrix properties result in a dissimilar combination of properties. The strength of interfacial adhesion together with matrix properties determines the extent of reinforcement and the direction in the change of properties as a function of composition. Adhesion influences local deformation processes very strongly and these processes determine the macroscopic properties of the composites. Shear yielding induced by the debonding of fibers consumes much energy, while fiber fracture is less efficient in increasing impact resistance. Fiber characteristics are important for modulus and strength, but less significant for impact resistance. A good combination of stiffness and impact resistance can be obtained in PP homopolymers modified with PVA fibers, while this type of modification is less efficient and/or advantageous in elastomer modified PP.