Foaming Behavior,Combustion Behavior, and Flame-RetardantMechanism of PP/IFR/SiO₂ Microcellular Flame-RetardantFoams

The properties of polypropylene (PP) foam materials are closely related to the cell structure (cell size and density). However, the foaming behavior causes a degradation in the flame-retardant properties of PP foams compared with PP composites. In this work, ammonium phosphate (APP) and pentaerythritol (PER) were used as intumescent flame retardants (IFRs), and nanosilicon dioxide (SiO₂) was used as a synergistic agent to prepare PP/IFR/SiO₂ microcellular flame-retardant foams by using chemical injection foaming technology. The influence of IFR and SiO₂ on the foaming behavior and combustion behavior of PP/IFR/SiO₂ microcellular foams and their flame-retardant mechanism were investigated. The IFR particles had a weak “cavity nucleation” effect, and SiO₂, a typical heterogeneous nucleating agent, improved the dispersion of the IFR in the matrix. Therefore, compared to PP foams and PP/IFR foams, PP/IFR/SiO₂ microcellular flame retardant foams exhibited more refined cell structures. In addition, the introduction of 0.5% SiO₂ improved the flame retardant properties of PP/IFR/SiO₂ microcellular flame retardant foams, which even exhibited higher LOI values than those of PP/IFR composites. This was attributed to the deposition of SiO₂ on the surface of the carbon layer to form a physical insulation layer, enhancing the strength of the foam carbon layer. The experimental results provided a novel approach for preparing PP microcellular foams with excellent flame retardant properties.