An Introduction to Silicone Material
Silicone is a highly active adsorbent material, amorphous in structure, containing polysiloxane, silicone oil, fumed silica, coupling agents and fillers. Its main component is silicon dioxide, with the chemical formula mSiO₂·nH₂O. It does not dissolve in water or any solvent, is non-toxic and odourless, and is chemically stable — it reacts with nothing except strong alkalis and hydrofluoric acid.
Different grades of silicone form different microporous structures depending on the manufacturing method. Its chemical composition and physical structure give it properties that competing materials find hard to replace: high adsorption capacity, good thermal stability, chemical inertness and relatively high mechanical strength.
Classification
By curing behaviour, silicone rubber divides into heat-cured and room-temperature-vulcanised types. By performance and use, it divides into general-purpose, ultra-low-temperature, ultra-high-temperature, high-strength, oil-resistant and medical grades. By monomer, it divides into methyl vinyl silicone rubber, methyl phenyl vinyl silicone rubber, fluorosilicone rubber and nitrile silicone rubber.
Compounding and forming
Because the intermolecular cohesion of silicone gum is low, high-reinforcement and heat-resistant fumed silica becomes the most important compounding ingredient. Other additives must also be chosen for heat resistance: they must not volatilise, decompose, char or discolour under service conditions. To preserve the gum's heat resistance and prevent siloxane bonds from being attacked and broken by acids or alkalis, compounding must both exclude external acids and alkalis and promptly remove the acidic species produced when the peroxide curing agent decomposes.
- Place the compounded gum and other ingredients in a vacuum kneader. The initial temperature is low; friction raises it during mixing to 155–160 °C, which is then held for about half an hour.
- Pass the cooked compound through an open mill to flatten and roll it, taking care not to contaminate it, with 2–3 passes.
- Cool the flattened sheet for about 3–4 hours.
- Filter the compound through a screen to remove solid contaminants.