Dynamic mechanical behavior of oriented semicrystalline polyethylene terephthalate

Microstructure and Mechanical Properties of Severely Deformed Polypropylene in ECAE (Equal Channel Angular Extrusion) via Routes A and C

Equal channel angular extrusion (ECAE) is a solid-state extrusion process for modifying microstructures via severe plastic deformation without modifying the specimen cross section. In this study, changes in the microstructure and mechanical properties of polypropylene resulting from extrusion orientation route A (no rotation between extrusions) and extrusion orientation route C (a rotation of 180° between extrusions) are investigated using a 90° die-angle tooling outfitted with back pressure. Important differences are reported for the ECAE-induced deformation behavior between the two processing routes. A focus is made on the occurrence of heterogeneous plastic deformations (periodic shear banding and warping) for both routes and the control and inhibition of the plastic instabilities via regulated back pressure and ram velocity. Wide-angle X-ray scattering is carried out to characterize the structural evolution as a function of the processing conditions including route, extrusion velocity and BP application. The mechanical properties of the specimens machined from the ECAE pieces are examined under different loading paths including uniaxial tension/compression and simple shear. Full-field displacements converted to volumetric strains revealed the profound impacts of the processing route on the deformation mechanisms during tensile deformation.

Solid-state extrusion of polymers using simple shear deformation

The review considers the possibilities of new methods of solid-state extrusion of polymers based on the use of deformation schemes that include simple shear - equal-channel angular extrusion, equal-channel multi-angle extrusion and combined extrusion. Information on the evolution of the physico-mechanical properties of glassy, semi-crystalline polymers, polymer blends and composites is given.

Biocomposites Produced from Hardwood Particles by Equal Channel Angular Pressing Without Additives

Equal channel angular pressing (ECAP) has been shown to be a promising method for producing biocomposites from wood particles. However, severe plastic deformation during ECAP would cause considerable cracking when consolidation is carried out without a binder. In this study, the processing conditions were investigated for ECAP of hardwood particles into bulk biocomposites without any additives. Crack formation and wood cell deformation were examined in conjunction with thermal stability and crystallinity of the biocomposites. In comparison with hot pressing without severe shearing, a decrease in crystallinity and severe deformation of wood cells occurred during ECAP. Improved processability and homogeneous deformation would occur at high ECAP temperature (e.g., 210 °C) or low ECAP speed (e.g., 10 mm/min), leading to reduced crack formation in the ECAP-produced biocomposites. Despite its tendency to cause periodic cracking, effective plastic deformation in the regions between cracks was shown to improve interparticle binding. Ongoing research points to the potential achievement of crack-free hardwood (HW) consolidated without a binder, leading to significantly enhanced strength.

Deformation of high density polyethylene by dynamic equal-channel-angular pressing

The effect of high strain rate and large shear deformation on the orientation of crystallites in high density polyethylene (HDPE) was investigated with dynamic equal-channel-angular pressing (D-ECAP). The HDPE samples were processed by two loading routes, route A and route C. Grid lines were used to obtain macroscopic strain distributions, which were substantiated by finite element modeling. Owing to the strain rate effect, the number of D-ECAP processing passes has a minor effect on the shear strain accumulations compared to ECAP. D-ECAP leads to a decrease in the thickness of the crystalline stem, and the crystallinity. After route-A or route-C D-ECAP processing, a new monoclinic phase emerges, and two types of crystallographic c-axis orientations appear: the crystallographic c-axis is approximately parallel to the flow direction (FD), or is tilted at approximately 55° clockwise away from FD. However, only one type of crystallographic c-axis orientation is detected after 2 passes of route-C D-ECAP. It is viable to utilize D-ECAP to control the structure and orientation of crystalline polymers, as a complement to ECAP and other processing techniques.

Free-volume hole relaxation in molecularly oriented glassy polymers

The free-volume hole relaxation in polycarbonate and poly(methyl methacrylate) with different levels of molecular orientation was studied by positron annihilation lifetime spectroscopy at variable pressures. The molecular orientation was achieved through a simple shear process performed at different temperatures and extrusion rates. It has been demonstrated that the β relaxation is largely responsible for the free-volume hole anisotropy after simple shear orientation. Upon the removal of mechanical force, the deformation of the free volume is mostly reversible at temperatures much lower than the glass transition. No strong correlation between macroscopic deformation and the free-volume hole deformation was found regardless of molecular orientation.