Non-isothermal crystallization of polypropylene with sorbitol-type nucleating agents at cooling rates used in processing

Sorbitol-type nucleating agents used as clarifiers for polypropylene (PP) show a complex interplay of phase and crystallization behavior. To study the crystallization behavior, cooling measurements were performed by fast scanning calorimetry using rates between 10 and 4000 K s−1. These rates correspond to cooling rates used in processing. These measurements were combined with conventional differential scanning calorimetry (DSC) measurements at 10 Kmin−1.The optical properties were characterized by haze measurements. For this investigation the commercially available clarifiers1,3:2,4-bis(3,4-dimethylbenzylidene)sorbitol and 1,2,3-tridesoxy-4,6:5,7-bis-O-[(4-propylphenyl)methylene]nonitol were added to PP in various amounts up to 0.6 wt%. At relatively slow cooling rates only a single crystallization process was observed. In contrast, fast cooling leads to a complex crystallization behavior with up to four different crystallization processes. It was found that the temperature of the main crystallization process during fast cooling correlates with the optical properties from haze measurements. Finally, the cooling rate dependence of the different crystallization processes is discussed in terms of nucleating efficiency of the clarifiers. We conclude that the results of conventional DSC measurements cannot be extrapolated in a simple way to describe the nucleation activity of nucleating agents at cooling conditions relevant during processing.