Modulus enhancement of polypropylene by sorbitol nucleating agent in flow field

The rigidity of an injection-molded isotactic polypropylene (PP) containing 1,3:2,4-bis-o-(4-methylbenzylidene)-D-sorbitol (MDBS) as a crystal nucleating agent can be enhanced by selection of appropriate processing conditions. A new method by differential scanning calorimetry measurements showed that the dissolution temperature of 0.4 wt.% MDBS in molten PP is around 210°C. When injection molding was performed below the dissolution temperature, for example, 190°C, the molecular orientation of PP was greatly enhanced. Although this is an anomalous condition for the system to expect good transparency, the product obtained showed a high flexural modulus. In contrast, with high-temperature processing at 240°C, that is, conventional condition, the modulus decreased because of poor molecular orientation. Transmission electron microscopy observation revealed that MDBS fibrils strongly orient to the flow direction under the low-temperature processing, in which network structure of MDBS fibers is not detected. The oriented MDBS fibers led to a well-developed shish-kebab structure, which is responsible for the pronounced rigidity.