Synergistic Effect between Aluminum Hypophosphite and Alkyl-Substituted Phosphinate in Flame-Retarded Polyamide 6

A novel binary flame-retardant system was formed by introducing aluminum hypophosphite (AP) and aluminum isobutylphosphinate (APBu) together for PA6. The optimum flame retardant formulation was 1:1 (AP:APBu, 15 wt % in total), and the resulting flame-retardant PA6 could achieve a LOI value of 28.3 vol % and UL-94 V-0 rating. Cone calorimeter testing showed the samples containing binary flame retardants became less flammable with lower peak heat release rate (PHRR, 259 kW/m2). Dynamic oscillatory rheology and simultaneous thermogravimetry–differential scanning calorimetry results proved that cross-linking reactions existed in the samples containing AP. Morphology of the char layers was analyzed via SEM and laser Raman spectroscopy, and the results demonstrated the samples containing both AP and APBu formed more effective char. These results revealed that flame-retardant synergism existed between AP and APBu when they were combined to flame retard PA6. Consequently, a brief synergistic mechanism in this system was proposed.