High-strength, impact-resistant PP/PTFE composite foam with enhanced surface appearance achieved through mold-opening microcellular injection molding

Poor surface appearance and decreased mechanical performance are critical factors limiting the broad application of Microcellular injection molding (MIM) in foamed polymer products. Herein, in-situ polytetrafluoroethylene (PTFE) nanofibrils reinforced polypropylene (PP) foams with high strength, impact resistance, and enhanced surface quality were prepared by mold opening microcellular injection molding (MOMIM). PTFE nanofibers with different diameters were introduced into the PP matrix via twin screw extrusion and significantly enhanced the crystallinity and viscoelasticity of the PP matrix with the enhancement being more pronounced for the finer PTFE fibers. High-density oriented cellular structures were formed in the MOMIM PP/PTFE foams owing to the heterogeneous nucleation of PTFE and the stretching effect of the mold opening process. The optimum MOMIM PP/PTFE foam fabricated possesses a high cell orientation angle close to 90° and a large cell aspect ratio of 5.3, which reached a 34.37 % increase in tensile strength and a 73.08 % increase in impact strength owing to the synergetic effects of the PTFE network and the highly oriented fine cell structure, which effectively dissipated the tensile stress and impact stress by cell wall twisting and folding deformation. Furthermore, the MOMIM PP/PTFE foam showed a significantly enhanced surface quality compared to the MIM foam due to the reduced dragging flow in MOMIM, which greatly hindered cell rupture on the foam surface. Therefore, this work provides insights into the MOMIM of polymer composites containing fibrous fillers and the enhancement of tensile properties, impact resistance, and surface quality of foam products.