The effect of high temperature annealing process on crystallization process of polypropylene, mechanical properties, and surface quality of plastic parts

This work studied the effects and action mechanism of high-temperature annealing process parameters, such as annealing temperature, annealing duration and cooling speed, on the microstructural evolution of polypropylene (PP) on different thickness layers, the surface quality, and mechanical properties of PP plastic parts. The results show that when the PP plastic parts are annealed at slightly higher than 100°C, the resin on the surface and internal layers of plastic parts just generates the relaxation and rearrangement at the molecular level. Only at an enough high annealing temperature, the secondary crystallization and phase transformation process can be observed. The crystallinity of all annealed samples is higher than that of unannealed samples, but the crystallinity is decreased with the increase of cooling speed after annealing duration, and the annealing duration exceeding 60 min almost has no effect on the crystallinity. The microstructural change of PP on the internal layer of plastic parts is weaker than that on the surface layer. The surface hardness of the plastic parts mainly depends on the crystallinity of the surface layer, whereas the surface roughness of the plastic parts depends on not only the crystallinity, but also the space conformation of molecular chains and the residual stress. With the change of annealing process parameters, the tensile and impact strengths of plastic parts show a non-monotonic change law. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42773.