Research On Structure Evolution Of High-density Polyethylene By Dynamic Equal Channel Angular Pressing

Dynamic equal channel angular pressing（D-ECAP）is a novel process to realize shear deformation on materials in solid state,which can induce the severe plastic deformation of solid material at a high strain rate(with strain rates of 10^-4-10^-5 s^-1).So far,the research of D-ECAP is still limited and the technology has not been applied in the field of polymer material research.In this paper,the D-ECAP technology is adopted to the processing of high-density polyethylene,mainly through the route-A（the load direction remains the same）and the route-C（180°from the loading direction）.The macroscopic shear deformations of the samples after pressing were analyzed by visual plastic method and izod impact test.The thermal properties and microstructure of the samples were analyzed by differential scanning calorimetry（DSC）and X-ray diffraction（XRD）.A comparative analysis of the samples pressed from different routes is performed after that.The visual plastic method demonstrates that the D-ECAP processing causes large shear plastic deformation of the samples,and the distribution of shear strain on the top and bottom surface of the samples is not uniform,however,the shear strain occurring in the middle region is relatively homogeneous.The largest shear strain is about 1.A shear strain recovery of the samples occurs after even-number of processing along route C.Due to strain rate effect,the D-ECAP processing times has little effect on the shear strain of the samples compared with ECAP processes.Through izod impact test,the crack of undeformed specimen was observed developing along the direction of impact force until it fracture.After pressing,the crack of the samples developed approximately along the shear direction.After even-numbered pressing through the route C,the deformation of the samples did not recover completely,because of the existence of residual stress,and there was still a tilt angle between the crack and the flow direction.The DSC analysis was used to study the thermal properties and crystallization of HDPE samples.It was found that the melting temperature and heat flow of the samples decreased after pressing,and the D-ECAP process destroyed the original structure of the sample,which reduced the crystallinity and the thickness of the crystalline stem.The XRD results show that after route-A or route-C D-ECAP processes a new diffraction peak of the（010）crystal plane belonging to the monoclinic system emerges,and the diffraction peak intensity of（010）planes is enhanced with the increase of the processing times and the diffraction peak intensity of other crystal planes have also obvious changes.The XRD texture analysis shows that after D-ECAP path C is loaded for odd number of times,two different orientations of the crystal planes of the specimens are produced,but after the second extrusion along path C,only one orientation of the crystal planes exists.Meanwhile,the crystal planes of the sample after loading through the path A show similar bi-orientation characteristics to the crystal planes of the sample after the odd number of loadings of the path C,and with the increase of loading times,the orientation of the crystal planes is enhanced but the orientation direction remains the same.The azimuthal scan curve of the sample after loading is consistent with the results of the texture analysis.