Investigation of shear-induced migration of filler particles in molding flows

This study investigated migration of filler particles during processing of filled polymer compounds. The effects of filler particle migration on electrical conductivity and mechanical properties of injection and compression molded articles were evaluated.; The extent of particle migration in various processing flows was evaluated using micrometer size glass beads as filler particles, and a shear induced migration model was used to interpret the effects of pertinent variables on shear-induced migration. The model predictions were compared with experimental observations, and good agreements were found. While most mechanical properties were insensitive to migration effects, a small improvement in tensile elongation was observed apparently due to the existence of particle lean region near the surface.; A more dramatic effect of particle migration was found when conductive compounds with filler concentration around the percolation threshold were injection molded---both surface and volume conductivity reduced drastically. Such reduction in conductivity was due to shear-induced migration, as was verified by gradual ablation of materials from the surface layers. It was found that an article, originally turned insulator due to shear-induced migration, became conductive with the removal of surface layers by laser ablation. The thickness of the surface layer removed before the articles became conductive again, was found to be a strong function of polymer type, shear rate of injection, and the nature of the conductive filler particles.; In view of reduction of conductivity during injection molding, several multi-layer processing schemes were studied. In this research, two-layer articles with one conductive layer and one non-conductive layer were prepared to obtain good conductivity from the surface layer and mechanical strength from the bulk part. The interfacial strength of adhesion between conductive layer and non-conductive layers was evaluated as a validation of multi-layer molding methods.