Research On Preparation And Thermal Properties Of Silicone-phenolic Composite Gel Monolith

Resorcinol-formaldehyde?RF?gel monoliths obtained by sol-gel reaction have the advantages of light weight,high porosity,excellent thermal stability,etc.The poor hydrophobicity and heat resistance are the urgent problems to be solved.Polysiloxane resin is polymer with both inorganic and organic structures,and its excellent heat resistance,hydrophobicity and structural controllability make it widely used in the preparation of heat-resistant and hydrophobic materials.In this dissertation,polysiloxane-RF composite gel monoliths were prepared for combing the advantages of silicone and RF.The microstructure was controlled by adjusting reaction parameters.The chemical structure,morphology structure,hydrophobicity and thermal properties were analyzed.Furthermore the thermal conversion mechanism of polysiloxane-RF composites was discussed in the study.In this study,methylphenyl silicone prepolymer?SP?was prepared by hydrolysis-polycondensation recation of methyltriethoxysilane?MTES?,dimethyldiethoxysilane?DMDES?,and phenyltriethoxysilane?PTES?.SP solution and RF solution were mixed in certain ratio,and the sol-gel reaction was carried out under catalysis of ammonia.The obtained wet gel was dried and cured under ambinet pressure.The structure and properties of gel materials were tailored by four factors:the volume ratio of SP and RF solution;the mass concentration of SP and RF mixed solution;aging time of sol-gel reaction and the phenyl content of SP.The analysis result showed that macroscale phase separation of SP from RF could be resisted by adjusting reaction conditions.The prepared SP-RF xerogels were light weight with bulk density in the range of 0.234 to 0.289 g/cm³.Compared with RF,the hydrophobicity of SP-RF was enhanced and water contact angle of SP-RF was about 130°.Scanning electron microscope?SEM?test reveals that the particle and pore size of gel reduced with increase of mass concentration of mixed solution.Additionally,it was shown that SP formed shell structure coating on RF skeleton.Thermogravimetric analysis?TGA?results in argon and air atmosphere indicated that the mass retention of SP-RF composites was more than that of RF about 12%and 41%,respectively.X-ray photoelectron spectroscopy?XPS?analysis of the sample with heat treatment in N₂atmosphere at 500 and 800°C showed that polysiloxane transformed into Si-O-Si and partial Si-C reserved.Raman analysis showed amorphous and graphitic carbon structure appeard after heat treatment.TG-MS coupling analysis revealed the thermal transformation mechanism of SP-RF by releasing formaldehyde,water,methane,benzene and hexamethylcyclotrisiloxane during heating of SP-RF.It was deduced that polysilxoane coating on RF formed SiO₂ layer protected the RF skeleton structure.Thermal conductivity of gel monolith is 0.152 W/?m·K?,indicating the potential application in thermal insulatoin.