Distribution of Oriented Lamellar Structures in Injection-Molded High-Density Polyethylene Visualized via the Small Angle X-ray Scattering-Computed Tomography Method

Abstract

Using the SAXS-CT method, which observed the spatial distribution of nanometer-scale structures on a micrometer scale, we investigated the orientation distribution of the lamellar structure of injection-molded HDPE. We reconstructed CT images in the plane perpendicular (X–Y) and the plane parallel to the injection direction (X–Z) from the SAXS intensities corresponding to the period of the lamellar structures. The CT image at a flow speed of 5 mm/s in the X–Y plane showed that the lamellar structures oriented in the injection direction mainly consisted of three layers: the skin layer, the subskin layer, and the core layer, based on their differences in the distribution states. The CT images in the X–Z plane showed that the distribution of the lamellar structures was oriented in the thickness direction in the subskin layer and part of the core layer. Furthermore, we observed the distribution of the lamellar structure oriented in the injection direction with increasing injection speed, v, to evaluate the size change in each layer. The size of the skin layer thickness decreased heterogeneously in the CT images in the X–Y plane from v = 5 up to 40 mm/s. However, the distributions and thickness variations in the subskin and core layers were different from those in the skin layer due to the balance of the elongation, relaxation, and fountain flow effects.