Synergistic effect of Ni-based bimetallic catalyst with intumescent flame retardant on flame retardancy and thermal stability of polypropylene

Ni-based bimetallic catalysts were prepared and used as catalysts and synergistic agents to improve the flame retardancy of intumescent flame retardants (IFR) systems based on ammonium polyphosphate (APP) and pentaerythritol (PER) in polypropylene (PP). The synergistic effects of Ni-based bimetallic catalysts were evaluated by limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CCT), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), fourier transform infrared (FTIR), energy dispersive spectrometer (EDS). It was found that the addition of Ni-based bimetallic catalysts could dramatically enhance LOI value and improve UL-94 rating. Among all the bimetallic catalysts, NiMg catalyst exhibited the best synergistic effect. Only 2 wt% of NiMg catalyst could promote the LOI value of PP/IFR composite from 29.0% to 38.1%. What's more, the introduction of Ni-based catalysts led to great reduction in heat release rate (HRR), total heat release (THR), rate of smoke release (RSR), total smoke release (TSR), with a simultaneous increase in residual char. The TGA curves showed that the presence of Ni-based catalysts could also improve the stability of the char in high temperature. The FTIR curves and EDS analysis indicated that Ni-based bimetallic catalysts could promote the formation of POP and POC, leaving more P, N and O in the condensed phase, and thus giving rise to more crosslinks of the char. SEM observations further confirmed Ni-based bimetallic catalyst could help to form a more compact and homogeneous char layer and effectively reduce the heat and oxygen transfer, resulting in better flame retardancy of PP/IFR composites.