Thermosets

Thermosets, thermosetting plastic or thermosetting resins cure to a solid state when they reach a certain temperature (as opposed to melting like many materials), that is, their parts and properties become set. They create an infusible, insoluble network via a chemical reaction in which the whole entire heated finished object becomes in effect a single large molecule. These materials cannot be melted back down they just burn or char, so cannot be reshaped or restored to their original properties.

These thermosets have covalent intermolecular cross-links, and consist of major resin classes such as isocyanate, unsaturated polyester, formaldehyde, epoxies, and alkyds, and because of the cross-links they are extremely strong, whilst often also being lightweight. It is the strength in the cross-links that also make these products difficult to recycle.

Whilst thermoplastics have the characteristic of being able to be moulded, remoulded or recycled repeatedly, thermosets don't - although some plastics such as PVC (Polyvinyl Chloride) and PE (Polyethylene) can be processed as thermoplastics or as thermosets.

Other well known thermoset materials include:

Polyester
Silicone
Melamine
Epoxy
Urea formaldehyde

The main advantages of thermosets are that they are very hard and rigid, very stable and resistant to high temperatures. These types of plastics can also be reinforced for extra tensile strength with materials such as Kevlar, carbon, and fibreglass. Of the total global production of plastic, thermosets account for around 11%, whilst 75% are thermosplastics that can be recycled, although it is estimated that only about 5-9% of global plastic is recycled.

Whilst most commercial plastic, both thermoset and thermoplastic, is made from fossil-based resources, both types of plastic can also be made from renewable sources, often referred to as bio-plastics. Monomers are extracted or synthesised from biomass compounds (such as sugar) and then polymerised to make bioplastics in a similar way to standard plastics.