The Effect of Composition and Processing Conditions on the Structure Development in Injection Molded Dynamically Vulcanized PP/EPDM Blends

The effect of composition and processing conditions on the spatial structural variation in dynamically vulcanized injection molded poly(propylene)/ethylene—propylene—diene rubber (PP/EPDM) was investigated using matrixing microbeam X-ray system and transmission optical microscopy techniques. The structure gradient in the thickness direction in these samples is composed of very thick, highly oriented, skin regions followed by core regions of lower preferential chain orientation. In these samples, the shear-crystallized layers are observed to be much thicker than the comparably processed pure PP. The nucleation densities in the PP phase were too high to allow for observation of individual crystallites in most of the regions except near the very core, where sparsely distributed PP crystallites, that appear bright under cross polars, were observed. The wide-angle X-ray scattering (WAXS) patterns taken at different distances from the skin indicate that chain axes are mostly oriented in the flow direction, and distinct bimodal c-axis and a* axis oriented dual population of orientation is observed in the PP phase. c-axis orientation factors, fc, start at intermediate values at the skin and increase steadily and after showing a maximum roughly in the middle of the shear crystallized region, decrease towards the core, and in most cases never achieve a state of isotropy. In the blends that contain very small polypropylene fractions (ca ∼ 15%), unusually high orientation levels were observed. This was attributed to the “shear amplification” phenomena that dominates the thin PP regions between the rubber particles and causes significant orientation levels in the thin layers of PP coating, the rubber particles with the relative motion of the particles in the shear flow field.