Thermal properties and combustion behaviors of flame-retarded glass fiber-reinforced polyamide 6 with piperazine pyrophosphate and aluminum hypophosphite

Flame-retardant glass fiber-reinforced PA6 (GFPA6) composites were prepared by incorporating the mixture of piperazine pyrophosphate (PAPP) and aluminum hypophosphite (AHP) into GFPA6. From the results of UL-94 test, limiting oxygen index (LOI) and microscale combustion calorimetry (MCC) tests, the as-obtained composites exhibited significantly enhanced flame retardancy including higher LOI values, better UL94 rating (V-0 for 18.0 mass% PAPP/AHP mixture), and decreased peak heat release rate and total heat release. And PA6-4 with a 4/1 mass ratio of PAPP to AHP reached the highest LOI value. The influence of PAPP/AHP on the decomposition pathway of GFPA6 was discussed based on TG and TG-FTIR analysis. The interaction between PAPP/AHP and GFPA6 altered the decomposition pathway of GFPA6 resulting in the formation of compact char layer, high thermal stability within high-temperature region, and weak intensities of a variety of pyrolysis gas products. Meanwhile, the thermomechanical property data revealed the storage modulus of samples were increased with increasing PAPP/AHP content, whereas the glass transition temperatures were reduced gradually. Moreover, the results of scanning electron microscope illustrated that the introduction of PAPP/AHP mixture can promote the formation of compact char layer and change the surface element composition of the char.