Research On Microstructure Evolution And Properties Of Polypropylene Via Equal Channel Angular Extrusion

Equal channel angular extrusion (ECAE) is a novel process to realize shear deformation on polymer in solid state. In this respect, ECAE appears as a promising technique for monitoring microstructure, resulting in enhanced mechanical properties of polymer. Polypropylene was deformed by ECAE under melting point temperature. The need for establishing correlations between ECAE mechanics, microstructural evolution, deformation mechanisms, and the corresponding physical and mechanical property enhancement due to the ECAE process constitutes the broad focus of this work. These offered the basic data for the application of ECAE on polymer.In the first part of this paper, Isotactic Polypropylene (iPP) was processed in solid state by self developed ECAE device. The homogeneity of deformation is a fundamental condition to ensure homogeneity of microstructure and mechanical properties. The workability of iPP at the different conditions, including extrusion temperature (45-125℃,25mm/min) and extrusion velocity (25-500mm/min,125℃) was investigated. The microstructural evolution of iPP from all dimensional levels was discussed by reflected optical microscopy (ROM), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and dynamic mechanical analysis (DMA).The results demonstrate that a complex inhomogeneous strain field occurs on iPP at25℃, otherwise, the workability of iPP can be improved by increasing the extrusion temperature above45℃. Under evaluated extrusion temperatures, the uniform shear strain about1.43is introduced to iPP specimen. It was found that the original spherulites are continuously deformed into macrofibrils along the shear direction at45℃. The lamellar evolution under different extrusion conditions was elucidated. Two preferred crystal orientation have been found to be evolved from the original crystalline lamellae, furthermore, the chain slip is activated by increasing the extrusion temperature, followed by fast rotation of crystallites toward the shear direction. The molecular chains in the amorphous regions are highly stretched after45and65℃process. Property characterization indicates that the ECAE-induced fibril structure under45and65℃deformation is responsible for the improved mechanical properties, including the high impact strength about160.9J/m of iPP after45℃ECAE. A unique advantage of ECAE is that the specimen has the same cross-sectional area as the original one after one pass extrusion, which makes different controlled structure possible by multi-pass extrusion. The iPP specimen were processed for up to two pass in route A (processed in the same direction) and route C (with the specimen rotate180°around loading axis after the previous pass). The macroscopic deformation behavior, structure and the mechanical behavior of iPP as well as the plastic deformation mechanism of the crystalline region were investigated. The results demonstrate that the shear strain is superimposed to iPP about2.75after extruded twice in route A. At the continuous shear deformation, the main mechanisms occurred in iPP are chain slip and the amorphization because of the special parent-daughter structure of the material, which lead to the high degree crystal orientation and a decrease in crystallinity in iPP, thus contributing to the impact strength of iPP improved to490.5J/m. This research has also clarified that the large shear strain obtained via route A played a dominant role with respect to the orientation structure formation as well as the impact property enhancement of iPP.ECAE was further utilized to study the influence of shear deformation of random co-polypropylene (PPR) on its fracture toughness. The experiments find out that the crack propagation direction of the shear deformed specimen accords with the orientation direction of spherulites. The energy needed in unit area newly fractured surface of the specimens deformed at45℃reaches3.7times of that of un-deformed reference ones. The structure orientation induced by shear deformation explains the major reason for the increment in energy needed in crack propagation as inferred from the morphology of the fracture surfaces.The second part of this paper is focused on the numerical investigation on ECAE of polymers. In order to optimize the process conditions, finite element simulation was performed on the detailed effects of tooling design, polymer behavior, friction conditions and back pressure. The results indicated that the channel angle Φ has more influence on the equivalent plastic strain than the outer corner angle Ψ. The maximum equivalent plastic strain is achieved when the angles Φ and Ψ are respectively equal to90°and0°. In practice, most of polymers are strain hardening, which led to the strain homogeneity reduction and the warping of the extrudate material. Furthermore, suitable back pressure is benefit for changing the stress distribution on the polypropylene, thus avoiding damage and to attain stable shear deformation.