Investigations Of The Preparation And Properties Of Halogen-Free Flame Retardant Liquid Silicone Rubber Composites

Different flame retardant (FR) inorganic fillers including wollastonite with different particle size and aspect ratio, quartz, aluminum trihydroxide (ATH), low melt point glass powder (LMPGP), apatite with fluorin (F-apatite) or hydroxide (OH-apatite) and heat resistant additives including ferric oxide (Fe2O3), ferric oxide hydrate (FeOOH) and cerium hydroxide (Ce(OH)4) were used to prepare FR liquid silicone rubber composites in this study. Effects of FR fillers and heat resistant additives on the rheological behavior, vulcanization characterization, mechanical properties, FR properties, heat decomposition behavior and the surface morphology after combustion of the silicone rubber composites were investigated. Through the FR test result analysis, char structure observation and FR mechanism discussion, possible FR synergetic combination systems based on wollastonite were designed, and halogen-free FR liquid silicone rubber composites that can meet UL94V-0standard were finally prepared.Firstly, the influence of different FR inorganic fillers on the properties of silicone rubber composites was investigated. The rheological results reveal that all the silicone composites incorporated with different FR fillers show typical shear thinning behaviors. The viscosity of the composites increases after the addition of wollastonite, especially the wollastonite with smaller particle size and high aspect ratio. Among the composites with other FR fillers, the viscosity is relatively lower for those with quartz and LMPGP, and is highest for that with OH-apatite. The vulcanization characteristics results reveal that little impact of particle size and aspect ratio of wollastonite on the scorch and optimal cure time of the composites can be found, while relatively significant impact on optimal cure time can be found for the composites with other FR fillers, among which the composites filled with F-apatite, OH-apatite and LMPGP show shorter optimal cure time (less than1minute). Mechanical property measurements reveal that the wollastonite with smaller particle size and higher aspect ratio is more helpful for tensile strength improvement, and the wollastonite with smaller particle size and lower aspect ratio is more effective for high elongation. Among the composites with other FR fillers, the composite filled with ATH shows greatest tensile strength and elongation but far lower tensile strength than that with wollastonite, and the composites filled with quartz and ATH show high tear strength. FR test results reveal that bigger particle size or higher aspect ratio is not beneficial for FR property. Among the composites with other fillers, the composites filled with quartz and ATH show better FR property, and then followed by that with LMPGP. After aging, the FR property changes less for the composite filled with LMPGP, but becomes much worse for the composites with other fillers remarkably. The heat decomposition behavior reveal that the addition of wollastonite with smaller particle size and low aspect ratio helps to enhance thermal stability and FR property, and the composites filled with quartz, F-apatite and LMPGP show higher thermal stability among the composites with other fillers. Through the observation of the surface morphology of the composites after combustion, it was found that the wollastonite with smaller particle size and low aspect ratio is helpful to form compact and integrated char structure which is beneficial for FR property improvement, and the surface morphology of composite filled with LMPGP is similar with that filled with wollastonite, but its char structure is more compact, integrated with less cracks with almost no sample deformation.In this paper, the effect of NYAD M400content on the properties of the silicon rubber composites was investigated in detail, because wollastonite NYAD M400possesses low cost, good FR property and less influence on the composite viscosity. FR property test results reveal that NYAD M400can improve the FR property of silicon rubber, but can not make the composites meet UL94V-0requirements, and certain level of FR effects can be achieved only when the content of NYAD M400is higher than40%in Part A. In order to enhance the FR property of the silicon rubber composites, the content of NYAD M400is fixed at47%with the addition of3%heat resistant additives, and the effect of heat resistant additives on the properties of the these composites was investigated. In order to investigate the synergetic FR effect between different FR fillers and heat resistant additives, wollanstonite was partly replaced by LMPGP that was anticipated to help to generate more compact and integrated char structures. The experimental results reveal that before aging the composite filled with FeOOH is best in flame retardancy, and after aging the composite filled with Ce(OH)4is best in flame retardancy. According to the analysis of FR property and FR mechanism, several possible formulations with FR synergetic effect were designed. The experimental results show that there are synergetic effects in combination of wollanstonite and Ce(OH)4and the combination of wollanstonite, Fe2O3and LMPGP, and the silicon rubber composites which meet UL94V-0requirements can be prepared.