A versatile, highly effective intumescent flame-retardant synergist for polypropylene and polyamide 6 composites

A novel phosphorus-based macromolecular flame-retardant synergist named DPS was designed and synthesized via three steps from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), dihydroxybenzophenone (DHBP) and pentaerythritol (PER). The prepared synergist was used in combination with a base flame-retardant (melamine-coated ammonium polyphosphate (APP-M), or aluminum diethylphosphinate (ADP)) to create novel binary intumescent flame retardant (IFR) systems for polypropylene (PP) and polyamide 6 (PA6) matrices. The structure of DPS was characterized by fourier transform infrared (FTIR), phosphorus nuclear magnetic resonance spectroscopy (31P NMR) and gel permeation chromatography (GPC) analysis. The thermal stability and combustion behaviors of the flame-retardant polymer composites were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), UL-94 vertical burning tests. It was found that with a low fraction of DPS (20–25 wt%), the developed binary IFR systems significantly enhanced the flame-retardant performance of both PP and PA6. The results indicate that owing to the rich phosphorus content and abundant aromatic structures, DPS has synergistic actions with the base flame retardants to scavenge free radicals and promote char formation in the gaseous and condensed phases, respectively. Additionally, the mechanical properties of the PP composites were also evaluated, revealing the mechanical reinforcement effect of the binary IFR, i.e., simultaneous improvement of tensile modulus, ultimate tensile strength and impact toughness. This further demonstrates the effectiveness of the developed IFR formulations in terms of well-balanced flame-retardant performance and mechanical properties.