Foaming Behavior, Combustion Behavior, and Flame-Retardant Mechanism of PP/IFR/SiO₂ Microcellular Flame-Retardant Foams

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.