Relationship between in-plane shear resistance and co-cure processing parameters of joints in self-reinforced polyethylene composite with reprocessed matrix

This study investigates the optimization of in-plane shear strength of co-cured overlap joints of unidirectional self-reinforced thermoplastic composite tapes made with recycled low-density polyethylene and reinforced by ultra-high molecular weight polyethylene through the relationship of this resistance with the hot-press processing parameters for the conformation of the joint (pressure, temperature, time and length). The matrix had its chemical structure analyzed to check for potential degradation due to its recycled origin. Matrix and reinforcement were thermally characterized to define the joint processing temperature window to be studied. The elaboration of the curing conditions of the specimens was done according to the Design of Experiment approach of Response Surface methodology, and the relationship between the shear strength of the joints and the respective cure parameters was obtained through a regression equation generated by the method of Ordinary Least Squares. The tensile characterization of the material was also conducted both micro and macromechanically. The chemical analysis of the matrix did not demonstrate the presence of carboxylic groups that could account for degradation by chain-scission and cross-linking resultant from the recycling of the material. The proposed test methodologies have proven to be effective and may serve in the future as a basis for the constitution of new technical standards. It has been shown that it is possible to obtain joints with optimum in-plane shear strength of 6.88 MPa when processed at 1 bar, 115°C, 5 min and 12 mm. The fracture analysis revealed that the shear rupture of the joints was preceded by multiple longitudinal cracks induced by successive debondings, both inside and outside the joint, due to the accumulated transverse tension, proportional to its length. Temperature proved to be the most relevant processing parameter for the performance of the joint, which is little affected by variations in pressure and curing time.