Impact modification of PP with short PET fibers: Effect of heat setting on fiber characteristics and composite properties

Polypropylene (PP) composites containing poly(ethylene terephthalate) (PET) fibers were prepared by injection molding in the temperature range of 190–260 °C. Increasing processing temperature led to the shrinkage of the PET fibers caused by the relaxation of orientation and recrystallization. Structural changes in the fibers resulted in the deterioration of composite properties. Decreased fiber length and modified local stress distribution restricted the plastic deformation of the matrix resulting in the very poor impact resistance of about 1 kJ/m2 at 260 °C processing temperature. The heat setting of PET fibers resulted in more stable crystalline structure and thus decreased shrinkage and improved composite properties even at high processing temperatures. PP composites impact modified with PET fibers are suitable structural materials for the automotive sector with a sufficiently large stiffness of around 2 GPa and an impact resistance larger than 20 kJ/m2 if processing temperature is kept as low as possible.