The simplified process of silicone rubber synthesis is as follows: Sand or silica is reduced to monomeric silicon → at 300% temperature, with copper as a catalyst, silicon interacts with methyl chloride → a mixture of methylsilicon chloride (mono-, di-, or ternary) is formed → dimethylsilicon chloride is separated by distillation → dimethylsilicon chloride hydrolyzes to silanes, which are then rapidly synthesized into linear or cyclic siloxanes → the linear siloxanes, with the aid of potassium hydroxide (KOH), form a quaternary dimethyl cyclic compound (D4) → in the presence of KOH, D4 polymerizes, and chain termination completes the process.
The formulation process of silica sol inorganic polymer coatings is not significantly different from other coatings, but the silica sol should be added slowly; otherwise, the coating will undergo a qualitative change. The following preparation process can be referenced: Add film-forming aids (commonly used ones include zinc oxide, butyl silicate, butyl acetate, sodium methyl silicate, etc.) to the reaction vessel, add a measured amount of water and stir. Then, add surfactants (sodium benzenesulfonate), thickeners (hydroxyethyl cellulose, CMC, etc.), dispersants (sodium hexametaphosphate), defoamers (tributyl phosphate), and other aids in sequence. Slowly add silica sol, extender pigments, and color pastes. Grind finely using a sand mill or colloid mill. Coarse particles can be added as needed. The pH value of the coating should be between 8.5 and 10.
Formulation design of silica sol inorganic polymer coatings: Silica sol shrinks significantly during film formation, making the coating prone to cracking. To overcome this drawback, in addition to adding fibrous fillers to the inorganic coating, a water-soluble emulsion is also added as an auxiliary film-forming agent. This allows the organic matter to fill the "-Si-O-Si-" network structure, reducing film shrinkage and expansion/contraction caused by temperature differences. This results in an organic composite coating that combines the excellent weather resistance of inorganic coatings with the superior decorative properties, stability, and workability of organic coatings. Generally, it can be considered as follows: the film-forming material comprises 50%–90% inorganic matter and 50%–10% organic matter. For internal use, polyvinyl acetate emulsion is added; for external use, styrene-acrylic emulsion is added. The titanium dioxide is initially rutile type with a volume concentration of 5%, and a large amount of extender pigments can be added, bringing the total volume concentration to 60%–70%.
The concentration stage in silica sol production is an energy-intensive and time-consuming process. Traditional concentration methods generally employ dehydration under reduced atmospheric pressure to obtain the desired concentration of silica sol.
Based on the different pH values exhibited, silica sols are further classified into alkaline silica sols and acidic silica sols. These are all important fine chemical products with different important applications: alkaline silica sol is used in precision casting and exterior wall coatings; acidic silica sol is used in color picture tubes, colloidal lead-acid batteries, and electrostatic flocking technology imported from abroad.
Ultrafiltration membrane concentration has lower energy consumption per unit output, shorter cycle time, and less equipment investment, making it an effective way to reduce product costs and improve product quality.
Ultrafiltration is a relatively advanced method for preparing silica sol. Ultrafiltration involves concentration using an ultrafilter. Unlike a regular filter, an ultrafilter membrane only allows water and soluble salts to pass through, not sol particles. Therefore, this method is quite effective; it can remove not only water from dilute sols but also small amounts of ions or easily soluble substances. However, filter cake or precipitate formation on the ultrafiltration membrane is not allowed, so ultrafiltration must be carried out with continuous stirring. The key to successful ultrafiltration is having a suitable ultrafiltration membrane.