The correlation of the molecular structure of polyolefins with environmental stress cracking resistance
This study concerns the phenomenon of environmental stress cracking resistance (ESCR) in three impact polypropylene copolymers (IPPCs). The main purpose was to correlate the ESCR with their properties such as microstructure, molecular weight (MW), molecular weight distribution (MWD), crystallinity and morphology. Initially the selection of a suitable test method and an active stress cracking agent (SCA) were the preliminary concerns. The Bell telephone test was used to evaluate SCAs, while a published procedure for determining ESCR of ethylene based plastics was adapted for the purpose of this study. Isopropanol was selected as SCA. Polymers were fully characterized by FTIR, 13C NMR, DSC and high temperature GPC. Optical microscopy was used to investigate craze formation and crack growth, and scanning electron microscopy (SEM) was used to study the morphology of the polymers. Since IPPCs are known to have multi-fraction copolymeric structures and each of these fractions has significantly different average properties, fractions were selectively removed from the materials, either by solvent extraction at room temperature, or by TREF fractionation. The effect of removing these fractions on the ESCR was determined. The effect of the molecular composition of the three IPPCs on the ESCR of these materials, as well as the effect of the removal of the selected molecular fractions on the ESCR, morphology and molecular characteristics are discussed and compared. Conclusions are drawn as to the factors controlling ESCR in these materials.