Development of high thermal insulation and compressive strength BPP foams using mold-opening foam injection molding with in-situ fibrillated PTFE fibers

Polymer foam has become an important thermal insulation material due to its outstanding features, while its fabrication is still very challenging by foaming injection molding technology. Herein, we reported the successful fabrication of low-density branched polypropylene (BPP) foam with an expansion ratio of up to 25-fold and fine cellular structures using mold-opening foam injection molding (MOFIM) technology. To improve foaming ability, polytetrafluoroethylene (PTFE) nanofiber reinforced BPP composite was prepared in an in-situ way through a simple twin-screw blending technology. The DSC results show that PTFE nanofibers can promote crystallization of BPP effectively, while the rheological measurements demonstrate that PTFE fibers can improve the viscoelastic behavior of BPP. Furthermore, the MOFIM experimental results show that the cell size is reduced by one order of magnitude while the cell density is increased by four orders of magnitude. Interestingly, PTFE nanofibers endow BPP/PTFE foams a unique cell wall structure with plenty of micro-holes and micro/nano fibrils. Moreover, BPP/PTFE foams show markedly improved thermal insulation and compressive mechanical performance with a thermal conductivity of as low as 32.4 mW·m−1·K−1. Thus, this unique low-density BPP/PTFE foam shows a promising future in high-performance thermal insulation applications, particularly considering the used scalable, versatile, facile and cost-saving processing technology.