Silicone rubber

Silicone's backbone is alternating silicon and oxygen, with organic groups hanging off the silicon. That backbone is the whole explanation for its behaviour: the Si–O bond is stronger than C–C and is not attacked by oxygen or ultraviolet light, and the chain is unusually free to rotate.

So it stays rubbery over a range no organic elastomer matches — roughly −60 °C to 200 °C in continuous service — and it does not perish, harden or crack in sunlight the way natural rubber and most synthetics do. It is also physiologically inert, water repellent, and a good electrical insulator, and it transmits gases readily.

What it is not is strong. Even reinforced, its tensile strength and tear resistance are poor next to any structural elastomer, and it abrades easily. It is chosen where the environment is the problem and the loads are small.

Supplied as a high-consistency gum that is milled and compression or injection moulded, as a liquid two-part system injection moulded at high volume, or as a one-part sealant that cures on contact with atmospheric moisture — which is what a tube of bathroom sealant is, and why it skins over from the outside in.

Curing is by peroxide or by platinum-catalysed addition. The platinum route leaves no by-products, which is why it is used for food-contact and medical grades, and it is famously poisoned by traces of sulfur, tin and amines — so a silicone moulding shop keeps latex gloves and certain modelling clays away from the material.

Seals and gaskets at temperature extremes, including engine and oven applications. Bakeware and kitchen utensils. Medical tubing, catheters, implants and prosthetics, where inertness and biocompatibility decide. Baby bottle teats and soothers. Electrical insulation, and the high-voltage line insulators that replaced ceramic on the strength of their surface behaviour in pollution and rain.

Bathroom and construction sealants — the largest use most people encounter. Mould-making, where its release properties and detail reproduction are hard to beat. Keypads and soft-touch buttons. And in aerospace, where the temperature range is the requirement.

Silicone chemistry begins with Frederick Kipping's work from the 1890s, who coined the name on a mistaken analogy with ketones and concluded the compounds were of little use. Industrial development came in the 1940s through Eugene Rochow's direct process at General Electric, which made the necessary chlorosilanes cheaply enough for a real industry.

Wartime demand was the trigger: aircraft engines needed a gasket material that survived temperatures organic rubbers did not.

A thermoset elastomer and not meltable, so it is not mechanically recycled. It can be depolymerised back to siloxane feedstock chemically, and this is done at small scale for clean industrial scrap, but there is no consumer collection route.

Its feedstock is silica rather than oil, which is genuinely abundant — though the direct process that converts it runs at high temperature and the energy cost is substantial. Cured silicone is inert and biologically inactive; the environmental questions attach to certain low-molecular-weight cyclic siloxanes used in personal care products, which are persistent and are restricted in the EU, and not to the cured rubber.