Preparation And Study Of Diatomite/silicone Rubber Ceramizable Composites

Polymer materials is often used as thermal protection material such as cable coating for fire protection in the aerospace industry because of lightness, easily molding, low thermal conductivity, good electrical insulation.However, the product of polymer material at high temperatures is mostly powdery combution residues with low strength and toxic fumes are released in the combustion process to pollute the environment. Ceramizable polymer matrix composites is prepared with active ceramic filler and polymer matrix.As a new type of thermal protection material, it is low smoky, non-toxic, non-melt dripping and can maintain shape and size.Combustion residues with a complete self-supporting structure can hinder the matter convection and heat transfer. Ceramizable polymer matrix composites,preventing the matter inside and outside exchanging and the internal from the heat fom the environment,is a new direction of development in the field of thermal protection. Silicone rubber matrix which is easily proceed has excellent heat resistance and cold resistance, does not release toxic fumes during the process of combustion,producing mainly SiO2 which is friendly to the environment, it is one of perfect polymer matrix of ceramizable composite materials. Ceramizable silicone rubber as one of ceramizable polymer composites has been extensively studied.In this thesis, Composite are prepared with diatomaceous as a ceramic filler, methyl vinyl silicone rubber as polymer matrix, Sb2O3, MgO, Bi2O3 as fluxing agent with different melting points, dicumyl peroxide(DCP) as vulcanizing agent.First, the surface of diatomaceous is modified with silane coupling agent KH570. The optimum conditions for surface modification is determined with orthogonal experiment by using the mechanical properties test and field emission scanning electron microscopy morphology analysis to characterize the effect of modification :3wt% coupling agent for the amount of diatomaceous, reaction time of 4h, the heat treatment temperature of 120℃. Secondly,Composites are prepared with surface-modified diatomite and silicon rubber, TG and DTG curves of composites were obtained by thermal gravimetric analysis,the impact of surface modification and the amount of diatomite added on thermal stability are studied, The result show that by surface modification, composite initial thermal decomposition temperature has increased by 17 ℃, the peak decomposition temperature increased by 9.2 ℃. Composite thermal decomposition initiation temperature, the residual rate of composites increases with the amount of diatomaceous increases, when the amount of diatomaceous is 100wt% of the silicone rubber, initial thermal decomposition temperature and the rate of pyrolysis residue of the composite material are the highest, respectively 466.9 ℃ and 60.03wt%, but the change of peak decomposition temperature with the amount of diatomaceous is not obvious. The phase of ablation residue was analyzed by XRD and the morphology of cross-section and surface was analyzed by field emission scanning electron microscopy(FESEM)respectively, Changes in phase and microstructure of the of the residues with evaluating temperature is investigated,the ceramizable mechanism of composites is discussed.It show that with increasing temperature, a liquid phase is generated from the reaction between diatomaceous and decomposition product of silicone rubber at 1100℃, liquid phase diffuse and penetrate to interface between these filler particles, the filler is firmly bonded by liquid phase to generate a strong ceramic structure when it is cooled.Composites are prepared with methyl vinyl silicone rubber as matrix,surface organically modified diatomaceous(optimal modification conditions component) as active filler, Sb2O3, MgO, Bi2O3 and their mixture as fluxing agent system.The effect of different fluxing agent system on thermal stability of composites is investigated, eventually it is found that a single-component system makes the thermal stability of the composites decrease. The initial thermal decomposition temperature of Sb2O3, MgO, Bi2O3 system decreased by 43℃, 35℃, 47.1℃ respectly, the impact of double component system on the thermal stability is not as obvious as the single component systerm. the initial decomposition temperature of Sb2O3-MgO system, Bi2O3-MgO system, Sb2O3-MgO system increased by-9.5 ℃, 13.0 ℃, 3.2 ℃ respectly. In addition,bending strength of diffrent fluxing agent systems ablation sample at different temperatures is tested to determine the effect of the temperature and fluxing agent system on the strength of ablation residue. It was eventually found that flexural strength improved with increasing temperature and at the same temperature, bending strength of composite with fluxing agent are stronger than those without fluxing angent;At the same temperature bending strength of multi-component system is stronger than that of double component system, bending strength of double component system is stronger than that of the single component system. bending strength of Multi-component system reaches 5.2MPa, increase nearly 2.1 times than that of residues of composites without adding a flux. The phase of ablation residue was analyzed by XRD and the morphology of cross-section and surface was analyzed by field emission scanning electron microscopy(FESEM)respectively, According to the Changes in phase and microstructure of the residues with evaluating temperature, the ceramizable mechanism of composites with different fluxing agent systerm is discussed.It is found that the ceramiazable mechanism of single component systerm is similar to that of double component system and muli- component systerm, that is at the first fluxing agent of the composite melt at high temperatures to generated phase, with increasing temperature, eutectic reaction between fluxing agent and diatomaceous、silicone rubber decomposition products occurs to generate more liquid phase, the liquid phase diffuse and penetrate into the interface between fillers and bridge them and eventually form strong ceramic phase when the temperature decreases, The difference is that the tempreture at which the system starts to generate liquid phase, liquid phase start to be observed at 900 ℃ in the system of Sb2O3, liquid phase start to be observed at 1100 ℃ in the system of Bi2O3,liquid phase is not observed in MgO system in all temperature.