Influence of zinc borate on the flame retardancy and thermal stability of intumescent flame retardant polypropylene composites

•The flame retardancy and thermal stability of PP/IFR/ZB has been studied.•ZB could promote to remain more P and O, and improve the thermal stability of the PP/IFR.•B could participate the reaction, forming more continuous and compact char layer.

The influence of zinc borate (ZB) on the flame retardancy and thermal stability of intumescent flame retardant polypropylene composites (PP/IFR) containing ammonium polyphosphate (APP) and charring-foaming agent (CNCA-DA) were characterized by limiting oxygen index (LOI), UL-94 measurement, cone calorimeter test (CCT), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS) and thermogravimetry analysis (TGA). The results revealed that a small amount of ZB could effectively improve the LOI value, UL rating of the PP/IFR systems, and reduce the combustion performance of PP/IFR systems from CCT test, including heat release rate (HRR), total heat release (THR), smoke production rate (SPR) and total smoke production (TSP). The catalytic effectivity (CAT-EFF) results showed that when 1% ZB was added to PP/IFR, it had the highest CAT-EFF, and could enhance the LOI value from 27.1 to 30.7. The morphological structures observed by digital photos and SEM indicated that ZB could promote to remain more P and O, and B could participate the connecting reaction to form the more continuous and more compact intumescent char layer on the char surface to hinder heat diffusion and oxygen transmission effectively. The TGA data revealed that ZB could change the degradation behavior of the IFR and PP/IFR, improve the thermal stability of the PP/IFR systems at high temperature and increase the char residue.