Research proposes maleimide phosphite PIN FR, achieving UL 94 V-1 (3 mm) at 2.5% loading in epoxy (0.12% phosphorus). The phosphorus and nitrogen containing molecule was synthesised by reacting phosphorus with N-(4-hydroxyphenyl)maleimide (ref. 2 below) to generate the maleimide phosphite TDPPI (tris[4-(2,5-dioxopyrrol-1-yl)phenyl] phosphite). This was reacted into curing of an epoxy resin, diglycidyl ether of bisphenol-A. Inclusion of TDPPI into the epoxy resulted in moderate transparency, improved thermal stability and improved mechanical parameters (tensile strength, Young’s modulus, flexural strength, Izod strength), increased LOI from 25% to 31% and UL 94 V-1 rating (3 mm, no dripping). The authors consider the fire protective effect to result from charring and synergy between the phosphorus and maleimide.
“Novel self curable phosphorus- and spiro phosphorus-based maleimides: synthesis, characterization, cure behavior and thermal properties”, A. Ranjith et al., Phosphorus Sulfur Silicon Relat Elem. 2023;198(7):583-590, https://doi.org/10.1080/10426507.2023.2176499
“Revolutionizing epoxy performance: A new flame retardant with phosphorus and maleimide for enhanced cure behavior, thermal stability, flame retardancy, and mechanical properties”, A. Ranjith et al., Polym Eng Sci. 2023;63:3963–3974, https://doi.org/10.1002/pen.26497