High thermal insulation and compressive strength polypropylene foams fabricated by high-pressure foam injection molding and mold opening of nano-fibrillar composites

Polypropylene (PP) foams with a thermal conductivity of as low as 36.5 mW m^{− 1} K^{− 1} are fabricated by high-pressure foam injection molding followed by mold-opening with CO₂ as a blowing agent. Regular PPs are not suitable for foaming due to their poor melt strength. To improve melt strength, the in-situ fibrillated blends of PP/polytetrafluoroethylene (PTFE) are prepared using a regular co-rotating twin-screw extruder. The micromorphology characterized by SEM shows that fibrillated nanoscale PTFE fibers disperse very well in PP matrix. The DSC, dynamic shear rheology, and extensional rheology measurements demonstrate that the in-situ fibrillated PTFE fibers can significantly improve crystallization, visco-elastic performance, and strain-hardening behaviors, respectively. All these factors confirm that PTFE fibers are very effective to improve melt strength and thus foaming ability of PP. The foam injection molding results show that the PP foam's cell size reduces by nearly one order of magnitude while its expansion ratio increases by approximately three times in presence of PTFE fibers. Compared to PP foams, PP/PTFE foams show significantly improved thermal insulation performance due to the increased expansion ratio, as well as unique cell wall structures with micro-holes and/or nano-fibrils. Moreover, it demonstrates that smaller cell size leads to improved compressive strength.