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1, Supercritical drying method of silicon dioxide aerogels

The organic solvent or water in the alcohol gel is heated and pressurized above the critical temperature and above the critical pressure. The gas-liquid interface in the system will disappear and the capillary pressure in the gel will cease to exist. (Ie, critical pressure and temperature), avoid or reduce the drying process due to the presence of the solvent surface tension caused by the substantial shrinkage and cracking, so as to maintain the original shape and structure of the air gel gel. 

2, Non-supercritical drying method of silicon dioxide aerogels

Non-supercritical drying techniques include atmospheric drying, subcritical drying, freeze drying and the like. Subcritical drying is similar to supercritical drying, but the temperature and pressure below the critical point, its mechanism is also less. Freeze-drying is to rely on low temperature liquid-gas interface into solid-gas interface, through the sublimation to remove the solvent, the same can avoid the adverse effects of surface tension. 

The basic principle of an atmospheric drying process is to first replace the pore solution in the wet gel with one or more low surface tension solvents and to hydrophobize the surface of the gel by modification to prevent excessive shrinkage deformation and structure during the drying process damage. The results show that the method of network enhancement and surface modification can reduce or eliminate the fragmentation degree of aerogels. The properties of silicon dioxide aerogels obtained by reasonable atmospheric pressure drying are basically the same as those obtained by supercritical drying process. Atmospheric drying process is the key to effective treatment of wet gel before drying, generally by the following measures: (1) increase the gel network skeleton strength, the use of a small surface tension of the solvent replacement to reduce the gel drying (2) increasing the pore size of the gel and making it uniform size, in the sol-gel process by adding controlled drying of chemical additives, such as formamide, acetamide, dimethyl phthalide (3) the surface modification of silicon dioxide particles can effectively prevent the irreversible polycondensation of the hydroxyl groups on the surface of the matrix particles caused by the drying; (4) the surface of the silicon dioxide particles is modified, to take the organic cross-linked polymer to strengthen and enhance the strength of the skeletal structure.

 

 

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