The role of nucleating agents in high-pressure-induced gamma crystallization in isotactic polypropylene

Nucleation of the gamma-formin isotactic polypropylene (PP) under high pressure was investigated. Three nucleating agents were used to nucleate crystallization of PP under atmospheric pressure: commercial Hyperform HPN-20E from Milliken Chemical, poly(tetrafluoroethylene) particles nucleating the alpha-form, and calcium pimelate nucleating the beta-form. Crystallization of neat PP and PP with addition of 0.2 wt% of the nucleating agents was studied. Specimens were either kept at 200 degrees C under pressure of 200 MPa for time ranging from 2 min to 4 h or for 15 min under pressure ranging from 1.3 to 300 MPa. After cooling to ambient temperature and releasing the pressure, the specimens were analyzed by DSC, WAXD, and PLM to have an insight into the structure and to determine a crystallinity level and contents of crystallographic forms. Both alpha-nucleating agents strongly nucleated crystallization of PP under high pressure in the gamma-form, whereas the beta-nucleating agent had only a slight effect. The results show the possibility to use nucleating agents to nucleate the gamma-form of PP under high pressure.