Concepts and Definitions
Before reviewing the available elastomers and their general properties, it is necessary to fully understand the terms “polymer,” “rubber,” “elastomer” and “compound” as they are used in this ehandbook.
A polymer is a compound composed of many smaller molecules which are linked together. Both plastics and elastomers are classified as polymers. In this handbook, polymers generally refer to a basic class of elastomers, members of which have similar chemical and physical properties. O-rings are made from many polymers, but a few polymers account for the majority of O-rings produced, namely nitrile (nbr), ethylene propylene (epdm), and polychloroprene (CR).
The term rubber can refer to two different types: natural rubber and synthetic rubber. Originally all rubber was manufactured from latex extracted from rubber trees. During the early 20th century, the need arose for other rubber-like materials and synthetic rubber was born. Synthetic rubber is composed of polymers which are derived from chemical processes. Since then, usage in the industry has broadened the meaning of the term “rubber” to include both natural as well as synthetic materials having rubber-like qualities. This handbook uses the broader meaning of the word “rubber.”
Though “elastomer” is synonymous with “rubber,” it is formally defined as a “high molecular weight polymer that can be, or has been modified, to a state exhibiting little plastic flow and rapid, nearly complete recovery from an extending or compressing force.” In most instances we call such material before modification “uncured” or “unprocessed” rubber or polymer. When the basic high molecular weight polymer, without the addition of plasticizers or other dilutents, is converted by appropriate means to an essentially non-plastic state and tested at room temperature, it usually meets the following requirements in order to be called an elastomer:
A. It must not break when stretched approximately 100%.
B. After being held for five minutes at 100% stretch, it must retract to within 10% of its original length within five minutes of release.
Note: Extremely high hardness/modulus materials generally do not exhibit these properties even though they are still considered elastomers.
The American Society for Testing and Materials (ASTM) uses these criteria to define the term “elastomer.”
A compound is a mixture of base polymer and other chemicals that form a finished rubber material. More precisely, compound refers to a specific blend of chemical ingredients tailored for particular required characteristics to optimize performance in some specific service.
A Parker O-Ring Division rubber chemist begins compound development by selecting a polymer type. There may be a dozen or more different polymer types to choose from depending on the properties required. The rubber chemist may then add various reinforcing agents such as carbon black, curing or vulcanizing agents (such as sulfur or peroxide, activators, plasticizers, accelerators, antioxidants, or antiozonants) to the elastomer mixture to tailor it into a seal compound with its own distinct physical properties. Since Parker chemists have thousands of compounding ingredients at their disposal, it seems reasonable to visualize two, three, or even one hundred-plus compounds having the same base elastomer, yet exhibiting marked performance differences in the O-ring seal.
The terms “compound” and “elastomer” are often used interchangeably in a more general sense. This usage usually references a particular type or class of materials such as “nitrile compounds” or “butyl elastomers.” Please remember that when one specific compound is under discussion in this handbook, it is a blend of various compounding ingredients (including one or more base elastomers) with its own individual characteristics and identification in the form of a unique compound number, For example, N0674-70 or V1164-75.