Synthetic rubbers are important engineering materials with properties that cross-over with silicones, polyurethane resin (PUR) and natural rubber (NR). They are thermosetting, and once cured cannot be re-melted. This gives them some advantages over melt-processable thermoplastic elastomers (TPE), such as thermoplastic polyurethane (TPU). They have superior resilience, toughness and resistance to chemicals, weathering and ozone. However, on the flip side, it means they can be difficult, if not impossible, to recycle.

The polymer chains are cross-linked by vulcanisation (curing). This is an absolutely necessary part of the process, because before cross-linking synthetic rubbers are not particularly strong, elastic or durable. There are different methods of cross-linking, such as with sulphur, metal oxide or peroxide. Sulphur is one of the oldest and the original technique used for curing natural rubber. Accelerators and activators are mixed in with the rubber to make the process more rapid and efficient. Devulcanisation has been explored as a potential route for recycling, but the end result is not as high quality as the original material and so this is not yet a viable commercial option.

Synthetic rubber also typically contains carbon black and is why it is opaque black in application. For example, car tyres consume a huge amount of carbon black each year, as do seals, gaskets and liners. Extracted from oil, carbon black is essential for strengthening rubber and providing UV resistance. A wide range of other ingredients are added to enhance processing (such as accelerating curing) and performance in application.

End of life options for synthetic rubbers is pretty bleak. The majority is incinerated for energy recovery. This is a problem, because the process releases many harmful and toxic gases, including dioxins and furans. An alternative approach is to grind the rubber into a crumb that can be used as a filler, additive or flooring material (sports floor, athletic tracks, playgrounds and artificial turf). The concern with crumb rubber is that it is essential a mass-produced microplastic that contaminates the surroundings. Add to that the increased surface area and thus greater potential for toxic substances to leach and evaporate from the rubber. Safe alternatives include coir, cork and other plant-based materials.