Pigment-induced crystallization in colored plastics based on partially crystalline polymers

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Mold shrinkage of the colored plastics based on partially crystalline polymers is a well-known phenomenon and is assumed to be caused by pigment-induced crystallization of polymers. A systematic investigation has been carried out for a variety of organic pigments in polypropylene (PP) and polyethylene (HDPE) systems in order to clarify the correlation between the nucleation effect due to pigments and the mold shrinkage. Special attention was paid to the onset temperature of crystallization for both PP and HDPE systems as monitored by DSC (differential scanning colorimeter). In parallel, the crystallization rate was also studied in the isothermal process by depolarization method for PP while by dilatometry for HDPE systems. As a result, the mold shrinkage is found to become more significant as the onset temperature is higher and the crystallization rate is faster. This tendency is well correlated with the formation of small spherulites (diameter: less than 1 mum). These results lead us to conclude that the organic pigments in colored plastics serve as a nucleation a-ent for partially crystalline polymers to form small spherulites, causing the mold shrinkage to occur. (C) 2003 Elsevier Ltd. All rights reserved.

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