Preparation And Characterization Of Microcellular Silicone Rubber Foam By Supercritical CO₂

Silicone rubber foam is a porous elastic polymer material exhibits excellent comprehensive performance and is widely used in areas of electronics industry, chemical industry, aerospace, etc. But silicone foam, foamed by traditional foaming methods, has uneven cells with large diameter and the common organic blowing agents pollute the environment. In order to solve the above problems, the supercritical CO₂ is used as the blowing agent. High temperature vulcanized silicone rubber/CO₂ system provides us, for the first time, the capability to create microcellular structure where the cell diameter can decrease to 10 ?m. The main findings are sumarrized as follows:Firstly, the effect of precuring time on foaming properties of silicone rubber is studied. The apparent density of silicone foam increases with the precuring time increasing from 5 min to 7 min. The longer the precuring time is, the smaller the cell diameter is and the higher the cell density is, but the cells become discontinuous with each other. When the precuring time is 6 min, the cell structure become uniform and tends to connect with each other.Secondly, study is made on the effect of silicone oil content, phenyl and type of silica on the foaming properties of silicone rubber. Silicone oil can help to improve the dispersion of silica in the matrix and increase the nucleation, which causes the increase of the cell density. When the silicone oil content is 40 phr, the silicone rubber has well-defined cell structure. Compared with methyl vinyl silicone rubber?VMQ?, the dependence on precuring time of cell morphology of methyl vinyl phenyl silicone rubber?PVMQ? is not very obvious because of large free volume, and high gas saturation concentration. The silicone rubber filled with precipitated silica T36-5 has more heterogeneous nucleation than that filled with fumed silica A200, leading to small cell diameter and high cell density.Lastly, the effect of foaming temperature and foaming pressure on foaming properties of silicone rubber is studied. The apparent density of silicone foam increases with the increase of foaming temperature and foaming pressure. As the saturation temperature increases from 40 ? to 70 ?, the coalescence and collapse of cells are limited, and cells tend to connect with each other because of the increases of gas solubility in the matrix. As the saturation temperature increases from 5 MPa to 8 MPa, the cell diameter of the microcellular silicone rubber foams decreases from 23.1 ?m to 9.8 ?m, and the cell density increases from 5.01×107 cells/cm3 to 1.07×108 cells/cm3. At 40 ?, with increasing saturation pressure from 2 MPa to 5 MPa, the amount of gas in the unit volume of high pressure reaction kettle is increased, leading to the gas saturation concentration increases from 2.45 wt % to 3.24 wt %, the gas diffusivity increases from 1.62×10-5 cm2/s to 7.83×10-5 cm2/s, respectively.