Cooperative Effect of Ni-Decorated Monolayer WS2, 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 WS2 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 WS2, NiO, and activated carbon (AC) were obtained (Ni-E-(WS2)-W-S-AC-PP). Combining the physical barrier effects of WS2 monolayer sheets with the excellent catalytic carbonization ability of the Ni-E-(WS2)-W-S-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-E-(WS2)-W-S-AC-PP were reduced by 46.32% and 26.03%, respectively. Furthermore, benefiting from the highly dispersed WS2, the tensile strength and Young's modulus of Ni-E-(WS2)-W-S-AC-PP showed similar values to pure PP, without sacrificing the toughness.

Automated summary

A combination catalyst was designed to improve the residual char of polypropylene (PP), which provides physical barrier effects and promotes catalytic charring activity. The PP blends with this combination catalyst showed remarkable improvement in flame retardancy and achieved a high yield of residual char. The production of a large amount of residual char reduced the release of pyrolytic products and formed a thermal shield, resulting in reduced heat release rates. Additionally, the tensile strength and Young's modulus of the PP blends were maintained without sacrificing toughness, thanks to the highly dispersed WS2.