Elastomer is defined as a macromolecular material (natural or synthetic polymer) that when subjected to high deformation (stretching with elongation even up to 5 or 10 times the initial length) within a certain temperature range, after the removal of the applied stress returns to the initial shape and size; this is possible thanks to their particular molecular structure, composed of large mesh networks (tangle of molecular chains) that even if stretched, as soon as the external force stops acting on the polymer, they quickly return to their initial state.
Elastomers, such as natural rubber or styrene butadiene rubber, are made of polymers that when above the glass transition temperature can be deformed by traction or compression, but then return to their original shape. This behavior comes from the fact that their chains are wound like springs and when a moderate tensile force is applied they can stretch, but some points of the chain remain adhered and allow the material to return to its original shape. Elastic characteristics can be improved by chemically creating points of attachment between the chains that allow the material to withstand greater deformations without yielding. However, this operation makes the material thermoset. If the links between the elastomer chains are physical, instead of chemical, then thermoplastic elastomers can be obtained such as SBS styrene-butadiene-styrene block rubber in which the links between the chains are not covalent but are made by Van der Waals bonds between the benzene rings of styrene.
In relation to their mode of production they can be included in the group of thermosetting or thermoplastic polymers. Thermoplastic elastomers entered the market in the late seventies, they are produced by molding and extrusion and they can be melted again after processing. Thermosetting elastomers, on the other hand, undergo chain welding through vulcanization, and consequently it is no longer possible to melt them after molding.
The main fields of use and application of these particular plastics are: tires, footwear, adhesives, building components, pipes, electrical cables, automotive components and gaskets, household appliances; plastic and bitumen modifiers, additives for lubricating oils (solid elastomers); carpet backing, paper coating, molded foam (synthetic latex).
Elastomers are usually processed in the plastic state before crosslinking by adding a curing agent or crosslinking accelerator.
The main chemical characteristics of the elastomers are:
- consist of linear molecules with a high molecular weight;
- presence along the polymeric molecule of reactive groups that allow cross-linking (equivalent to vulcanization);
- ability to accept reinforcing charges and to add satisfactory mechanical properties after cross-linking.
Elastomers, depending on their nature, may possess different mechanical characteristics such as:
- flexibility at room temperature;
- negligible permanent deformation;
- high breaking load;
- abrasion resistance;
- resistance to high temperatures;
- tear resistance;
- resistance to oxidation and ozone;
- resistance to aging and light;
- resistance to solvents and oil.
Thermoplastic elastomers (TPE), sometimes referred to as thermoplastic rubbers, are a class of copolymers or a physical mix of polymers (usually a plastic and a rubber) which, above its melt temperature, exhibits a thermoplastic character that enables it to be shaped into a fabricated article and which, within it’s designed temperature range, possesses elastomeric behaviour without cross-linking during fabrication.
This process is reversible, and the products can be reprocessed and remolded.