Rubber is definitively a type of elastomer. The term elastomer is a shortened form of “elastic polymer,” which describes a material capable of significant deformation when a force is applied and then rapidly returning to its original shape once that force is removed.
What Defines an Elastomer?
Elastomers are distinguished by their exceptional ability to recover their initial dimensions after being subjected to substantial stress. This characteristic is a reversible process known as high elasticity. Materials in this category can often be stretched to 500% or more of their original length without breaking or suffering permanent deformation.
A macroscopic measure of this flexibility is the material’s low Young’s Modulus, which indicates that only a small amount of stress is required to cause a large amount of strain. Elastomers also possess a glass transition temperature that is significantly below room temperature, ensuring they maintain their pliable, rubber-like properties in typical conditions. This combination of properties allows the material to absorb and release considerable mechanical energy, making it ideal for applications requiring shock absorption or sealing.
The Molecular Basis of Rubber’s Elasticity
Rubber’s unique elastic properties stem from its structure as a polymer, which consists of long, chain-like molecules. In its relaxed state, natural rubber, a polymer called polyisoprene, exists as a disorganized mass of highly coiled and tangled chains. These chains are in constant motion, randomly shifting between countless coiled configurations, a state that maximizes the material’s entropy, or molecular disorder.
When a piece of rubber is stretched, the applied tension causes the individual polymer chains to temporarily straighten out and align themselves in the direction of the pull. This alignment forces the chains into a less random, lower-entropy state. The thermodynamic preference of the material is to return to the higher-entropy, coiled configuration, which creates a powerful retractive force that pulls the material back to its original shape when the external tension is released.
This mechanism is only effective because of a process called vulcanization, which introduces chemical cross-links between the chains. Cross-links are stable covalent bonds that act as fixed anchors connecting the long polymer chains at various points throughout the material. These connections prevent the chains from slipping past one another and flowing irreversibly, which would cause permanent deformation, or plastic flow. The cross-links ensure that when the tension is removed, the chains are guided back to their initial, high-entropy, tangled network, providing the material with its lasting resilience.
Classifications of Elastomers
The category of elastomers encompasses a broad range of materials, traditionally divided into natural and synthetic types. Natural rubber is primarily polyisoprene harvested as latex from the rubber tree and is recognized for its excellent tensile strength and resistance to tearing. However, natural rubber exhibits poor resistance to oils, solvents, and environmental factors like ozone.
The limitations of the natural version led to the development of numerous synthetic elastomers, which are polymers synthesized from petroleum-based raw materials. These engineered materials share the fundamental cross-linked molecular network but are chemically modified to achieve specialized performance characteristics.
Examples of Synthetic Elastomers
- Styrene-Butadiene Rubber (SBR) is widely used in tires for its abrasion resistance.
- Nitrile Rubber (NBR) is valued for its superior resistance to oils and fuels in automotive seals and hoses.
- Neoprene, a polychloroprene polymer, is known for its resistance to weathering and chemicals.
- Silicone excels due to its ability to maintain flexibility and stability across extreme temperature ranges.