Research Of POSS In Silicone Thermal Protective Coating And Primer

Silicone resin is a kind of thermoset polysiloxane with a highly cross-linked structure that can be further solidified to form polymers due to the active groups.As a typical organic-inorganic hybrid material,it endows materials with different properties according to different branched functional groups:it provides thermal stability and compatibility with organic resin in the presence of phenyl groups;vinyl groups can control the degree of curing and cross-linking of the resin;long chain alkyl can further enhance the hydrophobicity of resin.Due to the dual properties of organic and inorganic materials,silicone resin is particularly applied in the thermal protection coatings of the new generation of aircrafts with high temperature requirements.However,the low mechanical strength and poor shear resistance at room temperature limit the application of silicone resin.In addition,the response parts of spacecraft are mostly polymers or metals,and there are problems such as insufficient adhesion of silicone resin to the high-temperature interface on the surface of polar substrates.To solve the above issues in this area,we conducted a series of experimental studies while were listed as follows:(1)In this thesis,octahydrosilsesquioxane(H-POSS)was used as a precursor to prepare octaepoxylsilsesquioxane(OG-POSS).A mixed silane solution in collaboration with OG-POSS was sprayed on the surface of stainless steel substrate to prepare a primer with good adhesive property and environmental adaptability.The interfacial hydrolysis curing behavior and macroscopic properties of primers were investigated.It showed that OG-POSS was a good film-forming agent and could facilitate the formation of a homogeneous and dense film structure on the substrate surface from the silane solution of the primer.At the same time,OG-POSS significantly improved the interfacial adhesion strength(from 1.1 MPa to 1.42 MPa)and thermal stability(initial decomposition temperature increased from 204.6℃ to 342.9℃,and maximum thermal decomposition temperature increased from 356.0℃to 437.2 ℃)of the primers.In addition,the primer with an OG-POSS 15 g/L concentration exhibited good corrosion resistance:after 10 hours of exposure to the salt spray,the sheet remained undamaged by corrosion.Back temperature tests showed that OG-POSS was able to improve the thermal protection of the coating to a certain extent,maintaining a back temperature of 208℃ after 20 minutes of ablation at 1000℃.(2)Using octavinylsilsesquioxane(Vi-POSS)as a precursor,different grafting amounts of ethoxy silsesquioxane(collectively referred to as EtO-POSS)were prepared.A mixed solution of primer consisting of orthosilicate,titanate and EtO-POSS was prepared and further applied to the surface of stainless steel substrate.The results of salt water spray test and scanning electron microscope showed that EtO-POSS could effectively improve the corrosion resistance of primer to metal substrate,it provided hydrophobicity for the substrate surface in the presence of-Si-Vi in the primer,and-OEt groups could promote the formation of micro nano structures.In addition,by employing Dow Corning DC 1-2577 silicone elastomer as the matrix,the effects of different fillers on the mechanical properties and ablation resistance of the matrix were investigated and the basic formulation of silicone thermal protection coating was explored.The results showed that EtO-POSS could be chemically grafted into the DC 1-2577 structure through alkoxy sol-gel reaction,thus improving the hardness of the silicone thermal protective coating(from 49 HA to 68 HA at room temperature,and from 0 HA to 80 HA at 400 ℃),interlaminar shear strength(up to 2.35 MPa,which was 172.8%of that before modification),and mechanical properties(tensile strength reached up to 2.56 MPa,1.77 times before modification).In addition,the results of thermal ablation showed that the ablated coating was mainly divided into three layers:the original layer,the pyrolysis layer,the silica layer.In an ablation environment,the protective coating surface could form a dense silica layer,reducing oxygen diffusion to the internal material and further enhancing the anti-ablation effect.This work serves as a reference for studying ablation resistance and modification of mechanical properties of other composites.