Crystallization Effects on Foamability of Polyethylenes - Relating crystallization kinetics to processing of low density foams

摘要

Low Density Polyethylene (LDPE) has traditionally been one of the materials of choice for making low density foams in the range of 1 - 4 pounds per cubic foot (pcf), or 0.016 - 0.064 g/cm~3, that are used for applications like protective packaging, sporting goods, medical applications and others. Ease of processability, good flexibility, good cushionability and impact resistance, low compression set, abrasion resistance and chemical resistance are key properties offered. The material competes with thermoplastic polyurethane, polystyrene, flexible PVC and more recently with semi - crystalline materials such as polypropylene and PET. Relative to the latter two materials and to LLDPE and HDPE, LDPE is usually the easiest to process. This is due in part to long chain branching contributions to melt strength and in part to "more desirable" crystallization behavior. A "wider" foam processing window is noted for the "lower crystallinity" LDPE. Crystallization kinetics are presented, as a function of degree of supercooling, for LDPE, LLDPE, and HDPE. "Branched" HDPE and HDPE modified with small amounts of polystyrene (PS) and polystyrene/ethylene propylene rubber (Kraton~R) are also examined, to measure effects on crystallinity and crystallization rate. Lab foam extrusion trials were conducted to verify findings.