Preparation of functionalized zirconium phosphate and its effect on the flame retardancy of silicone rubber

Abstract

How to effectively enhance the flame retardancy of silicone rubber is an important but challenging task. In this contribution, functionalized zirconium phosphate (F-ZrP) was prepared by the intercalation of 1,2,2,6,6-pentamethyl-4-(vinyldiethoxysiloxy) piperidine (PMVP) into α-zirconium phosphate (α-ZrP), and was employed to enhance the flame retardancy of addition-cured liquid silicone rubber (ALSR). Results showed that ALSR had no rating in the UL-94 test with the LOI value of 28.0%. F-ZrP could significantly improve the flame retardancy of ALSR. When only 4.0 phr F-ZrP was added, the limiting oxygen index (LOI) value reached 31.0% and the vertical burning test (UL-94) passed the V-0 rating. Moreover, compared to ALSR, the peak heat release rate (PHRR) and the total heat release (THR) of ALSR/F-ZrP decreased by 54.2% and 47.5%, respectively. The probable flame retardant mechanism of F-ZrP was mainly attributed to the synergism of the catalyzing carbonization and free-radical quenching effects. On the one hand, PMVP suppressed the thermal degradation of silicone chains in the condensed phase and quenched the active free-radicals in the gas phase by generating nitroxyl radicals. On the other hand, ZrP promoted silicone chain self-crosslinking and the formation of the graphitized carbons, resulting in the formation of the compact and thermostable ceramic layer, which was good barrier to protect the underlying matrix and inhibit the exchange of degradation products and oxygen.