A phosphite derivate of melamine was successfully tested, with SiO2, as a PIN FR solution for polypropylene (PP). Melamine compounds are recognised N-based PIN flame retardants, and their effectiveness is improved by synergy with phosphorus. MPHP melamine phenyl hypophosphite) was here synthesised from melamine and phenyl hypophosphorus acid. It was tested in PP in combination with silicon dioxide (SiO2). PP with 30% loading of MPHP, UL 94 V-0 (3.2 mm, no dripping) was achieved, and this fire performance was maintained if up to 5% of the MPHP was replaced by silicon dioxide, but not at 20% MPHP / 10% SiO2. Substitution of part of the 30% MPHP by SiO2 improved some aspects of fire performance but deteriorated others (lower heat release rate but also lower LOI), and also improved tensile strength but deteriorated elongation at break. The authors conclude that the phosphorus-containing melamine derivate MPHP acts as a PIN FR by releasing inert gases, by P radicals in the gas phase and by char formation, and that SiO2 may be a useful catalyst by forming a mineral protective layer in combination with charring.
“Synthesis of melamine phenyl hypophosphite and its synergistic flame retardance with SiO2 on polypropylene”, L. Ai et al., J. Thermal Analysis and Calorimetry 2021 https://doi.org/10.1007/s10973-021-10953-6