Preparation And Properties Of Polyborosiloxane And Moistureproof Coating On SiO_2f/SiO₂

SiO_2f/SiO₂ composite materials have been used for the radome due to those excellent thermal shock resistance and microwave transmission.However,the porous and hydrophilic characteristics lead to the absorption of moisture,resulting in unstability of dielectric properties.Coating is a simple and effective means to improve the moisture absorption problem of SiO_2f/SiO₂ composites.Polyborosiloxanes as an important branch of silicone resinhas introduce the-Si-O-B-into-Si-O-Si-to improve the heat resistance and adhesion.In this paper,polyborosiloxane resin was prepared by using methyltriethoxysilane?MTES?and boric acid?B?OH?3?as raw materials through the dewatering or alcohol condensation reaction.The gel precursor was dried and B were successfully incorporationed as Si-O-B.The boron doping promoted the residual mass from 34.91% to 75% and the decomposition temperature by 100?.Polyborosiloxane coatings dispersed in the alcohol were prepared by dipping.The resins impregnated into the composite material filled with the internal and surface holes partially or completely,which is beneficial to reducing the physical adsorption of the moisture.In addition,hydroxyl groups of composites were consume with the resins and hydrophobic methyl groups were introduced,reducing the chemical adsorption of moisture.As a result,moistureproof performance improved.Furthermore,the introduction of B enhanced the physical cross-linking of the coating with the composite material and improved the adhesion of the coating to the substrate.Experimental results shows that the optimal comprehensive performance of coating is obtained when B?OH?3/MTES was 0.4 and heat treatment temperature was 200?.The moisture absorption reduced from 1.15% to 0.33% with the adhesive force in 1 grade.Therefore,raw material ratio of 0.4 was chosen for subsequent experiments.The organic-inorganic composite hybrid coating was modified by using etraethyl orthosilicate?TEOS?as inorganic precursor and MTES as organic precursor with the presence of boric acid.TEOS got hydrophilic nanoparticles by selfcondensation,and methyl groups modified to get hydrophobic particles further.With the ratio of TEOS/MTES increased,the flexibility of the gel precursor decreased.B was incorporated into the network structure in the form of boric acid or B-OH.When TEOS/MTES was 3:7,the residual mass was highest to 83.3%.The presence of H2O added for the hydrolysis of TEOS suppressed the formation of Si-O-B,leading to lower decomposition temperature.To form organic-inorganic hybrid coating,TEOS was used to construct microrough structure,which was profitable for hydrophobic surface.Nevertheless,the content of –CH3 decreased and the content of –OH increased,which was harmful to moistureproof.In general,with the increase of TEOS,moisture-proof performance decreased.Three kinds of impregnation treatment methods were designed with M10B4 as the coating to study the effect of evaporation degree of ethanol and water on the coating during the impregnation process.It was found that the more evaporation degree of ethanol and water and the moisture resistance was better.However,the composite materials sustain the erosion of hygrothermal environment and boric acid during dipping and heat treatment processing,leading to the decrease of the bending strength.Therefore,ethanol and water evaporates slowly and diping times is less than 3.The bending strength is above 30 MPa and the water absorption is reduced to 0.33%.