| Silicone rubber-based thermal protection coatings have a wide range of applications in the field of thermal protection of spacecrafts.The simple coating process and excellent thermal protection performance are the advantages of these materials.As a result,it is the preferred option of thermal protection materials,especially for the components with complex aerodynamic structures.Organic solvents are usually used as diluent in the existing coating methods,which would pose threats to the human health and safety,damage the environment,and waste energy and resources.To solve this problem,it is necessary to develop water-based coating techniques.Firstly,a stable silicone rubber emulsion was prepared by emulsion inversion point(EIP)method.The phase transition mechanism of the emulsion was analyzed and its curing was studied.The emulsification effect of different kinds of emulsifiers was investigated and evaluated based on the static stability,centrifugal stability,and freeze-thaw stability.The hydrodynamic radius and uniformity of the silicone rubber emulsion were characterized by dynamic light scattering(DLS)in order to optimize the amount of emulsifier.The phase transition mechanism and the formation of silicone rubber droplets were studied by conductivity of the silicone rubber emulsion during phase transition.The composition of the silicone rubber emulsion curing system and the amount of each component were determined by the curing state,tack-free time,tensile stress and strain properties,and cross-linking density of the water-based silicone rubber matrix.The curing mechanism of silicone rubber emulsion was studied by the combination of theoretical calculation and experimental observation.Secondly,the surface modification of alkaline silica sol and its application on stabilizing silicone rubber emulsion and reinforcing water-based silicone rubber matrix were studied.The modification degree of silica sol was characterized by thermogravimetry(TG)and Zeta potential measurements when γ-glycidoxypropyltrimethoxysilane(GTMS)was used as the modifier.The stability of the modified silica sol was evaluated by morphology-and-size analysis of transmission electron microscopy(TEM),freeze-thaw stability,and pH stability.The modification mechanism was studied by characterizing the structures of GTMS before and after hydrolysis,monitoring the pH of the sol system,and analyzing the influence of the amount of GTMS used in the state of modified silica sol.The optimal amount of silica sol was chosen based on the stability of the silicone rubber/silica sol composite emulsion and the mechanical properties of the silicone rubber/silica sol composite film.The mechanism of silica sol stabilized silicone rubber emulsion was clarified and the composition of siliconerubber/silica sol composite emulsion system was determined.Finally,the formula of water-based silicone rubber thermal protection coating was designed and optimized.The thermal decomposition mechanism of water-based silicone rubber matrix and ablation mechanism of water-based silicone rubber thermal protection coating were also studied.The aramid pulp was further fibrillated in water,and the fibrillation time was determined by the dispersion state of the pulp in water and by its morphology observed with a scanning electron microscopy(SEM).The formula of water-based silicone rubber thermal protection coating was determined by the thermal physical properties,mechanical properties,and thermal stability.The thermal decomposition process of water-based silicone rubber matrix was characterized by TG and thermogravimetry-mass spectrometry(TG-MS).Its thermal decomposition mechanism was studied.After oxyacetylene ablation,the ablation structure and ablation mechanism of the water-based silicone rubber thermal protection coating was characterized by SEM,energy spectrum(EDS)and infrared spectrum(IR).The thermal protection performance of water-based silicone rubber thermal protection coating was evaluated under the typical heat flow environment. |
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