Improved Heat Resistance of Polypropylene with Added Sorbitol Derivatives

The influence of injection-molding temperature on the mechanical and thermal properties of isotactic polypropylene (PP) containing 0.4 wt.% of the nucleating agent 1, 3:2, 4-bis-O -(4-methylbenzylidene)-D -sorbitol (MDBS) was investigated. Isothermal crystallization measurements performed by differential scanning calorimetry revealed that MDBS at a concentration of 0.4wt. % remains undissolved in molten PP at190 ℃, whereas complete dissolution occurs at 240 ℃. Injection molding conducted at 190 ℃, where MDBS is present in a fibrous form, resulted in a pronounced enhancement of PP molecular orientation. The molded specimens also exhibited superior heat resistance. At this processing temperature, the MDBS fibers were highly aligned along the flow direction, leading to the formation of a well-developed shish-kebab structure, which is considered the primary origin of the remarkable improvement in heat resistance. In contrast, specimens molded at 240 ℃ displayed a lower elastic modulus and inferior heat resistance. Under these conditions, the MDBS fibers formed a randomly oriented network structure, resulting in a reduced degree of molecular orientation of the PP chains.