The effect of unidirectional shear flow-induced orientation on foaming properties of polypropylene

Abstract

Abstract When the semi-crystalline polymers undergo shearing during flow, the orientation of the molecular chains are induced. In order to study the effect of polypropylene orientation on foaming, we designed an injection mold with a long flow ratio. The samples were prepared by different process conditions and characterized by polarized Fourier transform infrared spectroscopy (FTIR). The degree of orientation was calculated by infrared dichroism. It was found that the orientation of the amorphous region has the greatest influence, and the subsurface layer in the cross section along the flow direction had the largest degree of orientation. The samples were foaming in a self-made sealed cavity under the same condition. The foamed samples were observed by scanning electron microscopy, and it was found that the shish-kebab crystal structure was induced in the surface layer and the subsurface layer under strong shear force. The shish-kebab crystal structure restricted the space for bubble nucleation and growth, and a large number of sub-micron and nano-scale cells appeared in the space of nucleation and growth. Along the flow orientation direction, the longitudinal and transverse sections of each foamed sample with the degree of orientation decreases, the density of cells and the average diameter of cells increases gradually, and the expansion of the foams have an advantage in the direction of flow orientation. The degree of orientation corresponding to the amorphous ribbon (1153 cm−1) of all the samples were arranged from small to large, and it was found that the expansion ratio decreased when the degree of orientation increased. The effect of orientation on foaming properties of polypropylene provides a new strategy for designing other polymer foams.