Cooperative Effect of Ni-Decorated Monolayer WS, NiO, and AC on Improving the Flame Retardancy and Mechanical Property of Polypropylene Blends.

Improving the residual char of polypropylene (PP) is difficult due to the preferential complete combustion. Here, we designed a combination catalyst that not only provides physical barrier effects, but also dramatically promotes catalytic charring activity. We successfully synthesized WS monolayer sheets decorated with isolated Ni atoms that bond covalently to sulfur vacancies on the basal planes via thiourea. Subsequently, PP blends composed of 8 wt.% Ni-decorated WS, NiO, and activated carbon (AC) were obtained (Ni-WS-AC-PP). Combining the physical barrier effects of WS monolayer sheets with the excellent catalytic carbonization ability of the Ni-WS-AC combination catalyst, the PP blends showed a remarkable improvement in flame retardancy, with the yield of residual char reaching as high as 41.6 wt.%. According to scanning electron microscopy (SEM) and transmission electron microscopy (TEM) observations, it was revealed that the microstructure of residual char contained a large number of carbon nanotubes. The production of a large amount of residual char not only reduced the release of pyrolytic products, but also formed a thermal shield preventing oxygen and heat transport. Compared to pure PP, the peak heat release rate (pHRR) and total heat release rate (THR) of Ni-WS-AC-PP were reduced by 46.32% and 26.03%, respectively. Furthermore, benefiting from the highly dispersed WS, the tensile strength and Young's modulus of Ni-WS-AC-PP showed similar values to pure PP, without sacrificing the toughness.