Repeated equal-channel angular extrusion of polypropylene: Processing routes and back-pressure influence

Multi-pass equal channel angular extrusion (ECAE) process of polypropylene was investigated at room temperature using a 90 degrees die-angle tooling outfitted with a back-pressure. The process was repeated until eight times using two different extrusion orientation routes: route A where the sample orientation is kept constant between each extrusion and route C where the sample is rotated by 180 degrees. The crystalline textures were examined by WAXS measurements and the changes on the mechanical response were observed by full-field strain measurements during tensile tests. The use of a back-pressure turns the deformation behavior, along the flow direction of the ECAE products, from highly heterogeneous (characterized by a peculiar periodic shear-banding behavior) to roughly homogeneous and leads to completely different plastic processes in the crystalline phase. The textural differences subsequent to the multi-pass processing and the use of a back-pressure are the source of significant differences in tensile response along the flow direction. The observations show that the polypropylene processed via route C reaches more easily a saturation state than that obtained via route A with a strong beneficial influence of a back-pressure.