Improvement in fire-retardant properties of polypropylene filled with intumescent flame retardants, using flower-like nickel cobaltate as synergist

In order to enhance the flame retardancy of intumescent flame retardant (IFR) in polypropylene (PP) using synergistic effects, a three-dimensional flower-like nickel cobaltate (NiCo2O4) was prepared and first introduced into the PP/IFR composite. The combustion behavior, thermal stability, mechanical properties and hydrophobicity of PP/IFR containing NiCo2O4 are studied in detail. When the contents of NiCo2O4 and IFR are 1.5 wt% and 18.5 wt%, respectively, the limit oxygen index value of the PP composite reaches 34.5 and passes the UL-94V-0 level. The cone calorimetry test also reveals that NiCo2O4 plays an excellent synergistic role with IFR, resulting in a significant improvement in the flame retardancy of the PP composite. Compared with pure PP, the presence of 1.5 wt% of NiCo2O4 delays combustion, decreases the peak heat release rate and total heat release by 64% and 40%, respectively, as well as exhibits a significant smoke suppression effect. At high temperature, the residual char of the PP composite is increased, and the degree of graphitization of corresponding char is also improved, both of which contribute to forming a continuous dense stable char layer and acquire a better shielding effect. The presence of NiCo2O4 only has a slight negative effect on the tensile strength of the PP composite. Interestingly, the hydrophobicity of the PP composite at this ratio is improved to some extent compared with that of the composite containing only 20 wt% IFR; thus, the synergistic flame-retardant effect is also ensured in the water resistance test within a certain time.