Study On The Nucleation And Crosslinking Modification Of Polypropylene

This paper mainly discussed the nucleation and crosslinking modification of polypropylene (PP). These two methods both change the macroscopic properties of PP by change its microstructure. The former method put emphasis on the change of spherulite morphology, but the latter method stresses the formation of crosslinking network. Also, the control of shrinkage of PP was studied.During the study of nucleation modification, the influence of two different nucleating agents on the mechanical properties of PP was studied, which corresponded to the change of crystallization behavior reveled by differential scanning calorimetry (DSC) -, polarized optical microscopy (POM) and wide angle X-ray diffraction (WAXD). Compared with pure PP, nucleated PP showed a higher crystallization temperature and a faster crystallization rate. With the addition of a nucleating agent, the spherulites became so small and even that the crystalline degree of nucleated PP increased, which resulted in higher rigidity and lower impact strength than those of pure PP. While the addition of β nucleating agent, induced a large part of a crystalline form to converted to β crystalline form. The special sheaf-like structure of β crystalline form accounted for its high toughness. Craze strip was formed when external force was exerted on β nucleated PP, which resulted in decrease of tensile strength and tensile modulus of β nucleated PP, but an increase of toughness. When 0.6% β nucleating agent was added, the relative β crystalline fraction was the highest, so the Charpy notched impact strength and the elongation at break came up to the highest level, being more than two times of that of pure PP.The influence of thermal treatment on the properties of β nucleated PP was studied. The results showed that with the increase of the thermal treatment temperature, the crystalline degree increased and the initial β crystalline form converted to a crystalline form gradually. When the material was totally composed of a crystalline form at 142癈, the tensile strength and flexural strength were both very high, and the elongation at break and the notched impact strength were the lowest, which meant the tough material had turned to be brittle.During the study of crosslinking modification, copolymerization PP was modified by peroxide initiator and multifunctional assistant crosslinker. The influence of the machining technology , initiator and assistant crosslinker on the mechanical properties of modified PP was studied. The results showed that with increasing initiator and assistant crosslinker concentration, the crosslinking degree increased, followed by increased strength and toughness. When 0.05% initiator A1 and 1.0% assistant crosslinker B1 were added, high-impact PP was prepared by twin-screw extruder. The modified PP had good mechanical properties. Its Izod notched impact strength was particularly high to 360.73J/m, as over three times as that of pure PP.The study of the shrinkage of PP showed that the control of crystalline degree was the key factor. During the molding technology, the increase of the molding temperature would increase the shrinkage, and the shrinkage could be decreased by increasing the molding pressure or prolonging the holding time. Nucleated PP has higher shrinkage because of its higher crystalline degree.