Comparative investigation of the structural evolution of zinc stearate and calcium stearate in a polypropylene random copolymer upon heating and cooling

Abstract

Zinc stearate (ZnSt2) and calcium stearate (CaSt2) are two kinds of extensively used additives in polymer industry. However, the selection principle of the two soaps in different polyolefins is still unclear. To establish the action mechanism of these soaps in polyolefins application, the understanding of the exact structures when they play roles is prerequisite. In this work, considering that these additives play their roles during the melt processing of polyolefin, in which the structure may be different with their as-received states, the structures and evolution behavior of the two soaps in a polyolefin melt upon heating and cooling were systematically explored. It was found that the two soaps exhibited different dispersed structures and evolution behavior in the polypropylene random copolymer (PPR). The ZnSt2 in PPR presented microcrystal at low temperatures and dissociated into ionic clusters during heating, and then gradually aggregated and recrystallized upon cooling; while the CaSt2 in PPR exhibited vitreous agglomerates with lamella ordered structure at low temperatures and transformed to diverse colloidal liquid crystal aggregates during heating, and then gradually reassembled to vitreous agglomerates in cooling process. At concentration closed to practical application (about 1000 ppm or less), ZnSt2 in molten PPR dispersed in form of ionic clusters like closed small molecules, whereas CaSt2 in PPR melt presented weakly ordered aggregates. These findings provided an important foundation for building up the well-defined relationship between the structure and performance of these soaps in polyolefins.