The basic core polymer of an elastomeric compound is called a rubber, produced either as natural gum rubber or manufactured synthetically by the chemical industry. Today, more than 32 synthetic rubbers are known, the most important ones are listed here.
Modern elastomeric sealing compounds generally contain 50 to 60% base polymer and are often described simply as “rubber.” The remaining balance of an elastomeric compound consists of various fillers, vulcanizing agents, accelerators, aging retardants and other chemical additives which modify and improve the basic physical properties of the base polymer to meet the particular requirements of a specific application.
Elastomers used in producing seals, and particularly those used in O-rings, will usually provide reliable, leak-free function if fundamental design requirements are observed.
What exactly makes a rubber compound behave the way it does? The magic is in "cross-linking". Bridges tie together the polymer chains forming bonds during the vulcanization process, as depicted in the image below. Cross-linking of the molecules changes the rubber from a plastic-like material to an elastic material.
Elasticity is what allows a rubber compound to function as a seal. When a rubber compound is compressed, the "cross-linked" polymer chains are pushed close together. These cross linked bonds want to extend to their original state acting as springs, pushing the rubber compound outwards. The rubber conforms to the gland surfaces and creates a barrier, preventing fluid from crossing, and thus creating a good seal.
Elastomer with no cross-links
Elastomer with cross-links
After vulcanization, including any required “post-cure,” an elastomer compound attains the physical properties required for a good sealing material. As with all chemical reactions, temperature is responsible for the speed of reaction. Only when the ideal process temperature is constant during the entire vulcanization time, will the optimum degree of curing be reached. For this reason, the conditions of vulcanization are closely controlled and recorded as part of the Parker quality assurance process.
Concepts and Definitions
An introduction to the terminology and meanings of polymer, rubber, elastomer, and compound.
Physical and Chemical Characteristics
It is important to know the physical and chemical properties and they interact and affect seal material selection.
Material Selection Guide
This section provides a brief review of the various elastomers currently available for use in O-rings.