Elastomers - Fundamental Properties
Elastomers are based on polymers, which have the property of elasticity. They are made up of long chains of atoms, mainly carbon, hydrogen and oxygen, which have a degree of cross-linking with their neighbouring chains. It is these crosslinking bonds that pull the elastomer back into shape when the deforming force is removed.
The chains can typically consist of 300,000 or more monomer units. They can be composed of repeated units of the same monomer, or made up of two or more different monomers. Polymers made up of two types of monomer are known as copolymers or dipolymers, while those made from three are called terpolymers.
Elastomers are arguably the most versatile of engineering materials. They behave very differently from plastics and metals, particularly in the way they deform and recover under load. They are complex materials that exhibit unique combinations of useful properties, the most important being elasticity and resilience. All elastomers have the ability to deform substantially by stretching, compression or torsion and then return almost to their original shape after removal of the force causing the deformation.
Their resilience enables them to return quickly to their original shape, enabling for example dynamic seals to follow variations in the sealing surface.
Elasticity
Elasticity is the ability of a material to return to its original shape and size after being stretched, compressed, twisted or bent. Elastic deformation (change of shape or size) lasts only as long as a deforming force is applied, and disappears once the force is removed.
The elasticity of elastomers arises from the ability of their long polymer chains to reconfigure themselves under an applied stress. The cross-linkages between the chains ensure that the elastomer returns to its original configuration when the stress is removed. As a result of this extreme flexibility, elastomers can reversibly extend by approximately 200 – 1000%, depending on the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the applied stress would result in a permanent deformation.
Resilience
Resilience as applied to elastomers is essentially their ability to return quickly to their original shape after temporary deflection. In other words, it indicates the speed of recovery, unlike compression set, which indicates the degree of recovery.
When an elastomer is deformed, an energy input is involved, part of which is not returned when it regains its original shape. That part of the energy which is not returned is dissipated as heat in the elastomer. The ratio of energy returned to energy applied to produce the deformation is defined as the material’s resilience.
Most elastomers possess a number of other useful properties, such as:
- Low permeability to air, gases, water and steam
- Good electrical and thermal insulation
- Good mechanical properties
- The ability to adhere to various fibres, metals and rigid plastics.
Also, by proper selection of compounding ingredients, products with improved or specific properties can be designed to meet a wide variety of service conditions.
This remarkable combination of properties is the reason elastomers serve a vast number of engineering needs in fields dealing with sealing, shock absorbing, vibration damping, and electrical and thermal insulation.
Most types of elastomers are thermosets, which gain most of their strength after vulcanisation – an irreversible crosslinking of their polymer chains that occurs when the compound is subjected to pressure and heat. Thermoplastic elastomers, on the other hand, have weaker cross-linking and can be moulded, extruded and reused like plastic materials, while still having the typical elastic properties of elastomers.
