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Silicone
3.2-13 usd/kg
Silicone rubber is inorganic and unique among the elastomers. Made up of polysiloxane (silicon-oxygen backbone), it is very stable, more so than conventional hydrocarbon polymers (carbon-carbon backbone). It has exceptional resistance to chemicals (except strong acids), weathering and extreme temperatures, superior to neoprene. Only fluoropolymer elastomers outperform silicones in terms of chemical and weather resistance.
However, it is not particular strong and has low resistance to wear and abrasion. Even so, its impressive temperature resistance means it outperforms some other elastomers at extreme temperatures. At 150 degC there is almost no change in properties and some grades are capable of short periods at 300 degC. Similarly, at very low temperatures silicone retains its elasticity for longer than other synthetic rubbers. The intermolecular forces acting between the polymer chains is very low, resulting in high elasticity and elastic recovery.
Silicone is resistant to burning, but once ignited, it will continue to decompose. It is possible to add flame retardant additives to ensure the material self-extinguishes once the heat source is removed.
It is produced from silica (SiO2), water and methanol. The silica is refined, such as from quartz sand, into silicon and synthesised through a series of chemical reactions to produce silicone. As a thermosetting material, permanent cross-links are formed during curing, which means it cannot be easily recycled.
Silicone is a high performance elastomer that is naturally transparent, and so available in a range of saturated colours. From rigid plastic to soft gel, it is found in almost every product sector. In consumer electronics, it is used to seal components and provide protection from impact (bumpers and cases), as keypads and buttons, and conductive grades are used in keyboard contacts. It is used as the seal, gasket and drive belt in cooking appliances. Automotive applications include seals, o-rings, hoses and waterproof electrical connectors. Food applications include milking machines, nipples, nozzles, buttons and gaskets. And in sports, it is used for snorkels, goggles, mouthpieces, covers and sleeves. It is biocompatible and hypoallergenic, making it suitable for medical applications (tools, equipment and implants), baby products and food contact applications.
Liquid silicone rubber (LSR) is suitable for injection moulding and extrusion. It is cured with heat to produce an elastomer with exceptional properties over a wide temperature range. LSR is suitable for 3D printing by liquid deposition modelling (LDM).
Room temperature vulcanising (RTV) silicone is suitable for casting, pouring, potting, encapsulation and moulding. It comes in two-parts, which are mixed together to initiate the curing process. Silicone potting compounds range from soft to hard rubbers. They are used to protect delicate electronic assemblies in demanding applications, including automotive and cryogenic.
Silicone films (and coatings) have very good gas permeability, which means they are breathable. Applications include transparent gaskets and seals, food applications, keypad protectors and moisture barriers on handheld electronics. Silicone coated fabrics provide an alternative to polyurethane resin (PUR) and polyvinyl chloride (PVC) coatings in particularly demand applications. Examples include outdoor furniture, sports goods and medical equipment.
It is mixed with fillers and additives to impart specific properties, such as conductivity, enhance flame resistance, tear strength and steam resistance. While silicone itself is considered non-toxic and safe for food contact and medical applications, the additives may not be. There is a risk that some ingredients can leach out, or off-gas as volatile organic compounds (VOC).
