Elastomers

Elastomers (rubbers) are special polymers that are very elastic. They are lightly cross-linked and amorphous with a glass transition temperature well below room temperature. They can be envisaged as one very large molecule of macroscopic size. The intermolecular forces between the polymer chains are rather weak. The crosslinks completely suppress irreversible flow but the chains are very flexible at temperatures above the glass transition, and a small force leads to a large deformation (see also rubber elasticity).1 Thus, elastomers have a low Young's modulus and very high elongation at break when compared with other polymers. The term elastomer is often used interchangeably with the term rubber, although the latter is preferred when referring to vulcanized rubbers.

Elastomers can be classified into three broad groups: diene, non-diene, and thermoplastic elastomers. Diene elastomers are polymerized from monomers containing two sequential double bonds. Typical examples are polyisoprene, polybutadiene, and polychloroprene. Nondiene elastomers include, butyl rubber (polyisobutylene), polysiloxanes (silicone rubber), polyurethane (spandex), and fluoro-elastomers. Non-diene elastomers have no double bonds in the structure, and thus, crosslinking requires other methods than vulcanization such as addition of trifunctional monomers (condensation polymers), or addition of divinyl monomers (free radical polymerization), or copolymerization with small amounts of diene monomers like butadiene. Thermoplastic elastomers such as SIS and SBS block copolymers and certain urethanes are thermoplastic and contain rigid (hard) and soft (rubbery) repeat units. When cooled from the melt state to a temperature below the glass transition temperature, the hard blocks phase separate to form rigid domains that act as physical crosslinks for the elastomeric blocks.

Manufacturing elastomeric parts is achieved in one of three ways: injection molding, transfer molding, or compression molding. The choice of the molding process depends on various factors, including the shape and size of the parts, the required tolerance, as well as the quantity, type of elastomer, and raw material cost.

As with almost any material, selecting the right elastomeric product for the application requires consideration of many factors, including mechanical and physical service requirements, exposure to chemicals, operating temperature, service life, manufacturability of the parts, and raw material and manufacturing cost.

 

Heat Resistance

Elastomer performance becomes less predictable and reliable when an elastomer is used near the limits of its service temperature range. If, for example, the temperature drops, elastomers become harder and less flexible and when the temperature reaches the glass transition temperature, they loose their rubber-like properties entirely. At even lower temperatures, i.e. at the brittle point, they may crack. Changes in elastomer properties due to low temperature are usually physical, and fully reversible unless the elastomeric part is exposed to large tensions which can cause damage below the brittle or glass transition temperature. The opposite is true when an elastomer is exposed to high temperatures, that is to temperatures near or above the service temperature limit. At these temperatures, elastomers often undergo irreversible chemical changes. For example, the polymer backbone may undergo chain scission or the polymer molecules may crosslink, causing the elastomeric part to become either (much) softer or more rigid, which, in turn, reduces their resistance to compression set.

The maximal service temperature can greatly vary from elastomer to elastomer. The highest continuous service temperatures do have silicone and fluorocarbon elastomers which can exceed 400°F (230°C)2, followed by polyacrylic and hydrogenated nitrile elastomers with a maximal service temperatures between 320 and 350°F (160 - 180°C), whereas more ordinary elastomers such as Neoprene and Nitrile have a maximal operating temperature between 210 to 250°F (100 - 120°C).

 

Fluid Compatibility

Strong swelling and rapid deterioration or complete breakdown of an elastomeric part may occur if the elastomer is not compatible with the fluid it is exposed to. Factors such as chemical concentration, operating temperature and pressure affect the stability / compatibility with the chemicals. When in doubt, the elastomer should be evaluated in functional tests prior use.

Because many applications involve hydrocarbon oils, elastomeric parts such as seals are classified according to their heat and oil resistance. For example, in the ASTM D2000 system, elastomers are ranked by heat resistance (type) and by oil resistance (class). Fluorosilicone and fluorocarbon elastomers have excellent oil resistance at elevated temperatures (> 200°C). Other elastomers with good oil but only medium heat resistance include NBR, ACM and HNBR. In the case of ACM and HNBR, the operating temperature in hydrocarbon oils should not exceed 150°C and in the case of NBR 100°C.  Silicone and Neoprene elastomers have only medium oil resitance. However, silicone elastomers can be operated at much higher temperatures than Neoprene. Poor oil resistance can be expected for EPDM, SBR, butyl (IIR, CIIR, BIIR) and natural rubber based elastomers (NR, IR).

Abrasion and Tear Resistance

Abrasion resistance is generally an important selection criteria for dynamic seal and tire applications of elastomers whereas good tear resistance may be important for other mechanical applications where the elastomers have to resist nicking, cutting and tearing. Elastomers such as hydrogenated nitrile (HNBR), polyester (AU) and polyether urethanes (EU), isoprene rubber (NR/IR), styrene butadiene rubber (SBR) and tetrafluoroethylene propylene copolymers have inherent abrasion resistance, whereas silicone (VMQ,), butyl (IIR), and perfluoro elastomers (FFKM) have poor abrasion resistance. In many cases, the abrasion and tear resistance can be enhanced by compounding with internal lubricants such as Teflon® or molybdenum disulfide. Nitrile and and acrylic elastomers have fair abrasion resistance. However, carboxylated nitrile (XNBR) offers noticeably better abrasion resistance.

Most elastomers with good abrasion resistance have also good tear resistance and elastomers with poor abrasion resistance have usually poor tear resistance. For example silicone and fluorosilicone are only suitable for static applications due to their poor tear and abrasion resistance.

 

Price

Cost is one of the most important selection criteria. Assuming that more than one elastomer meets all other requirements for a given application, price will usually dictate which elastomer will be chosen. The prices of elastomers may vary widely due to differences in raw material, compounding and processing costs. Inexpensive elastomers are styrene-butadiene (SBR) < natural rubber (NR) < isoprene (IR) < neoprene (CR) < nitrile (NBR) rubbers, whereas EPDM < urethane < silicone < polyacrylate (ACM) < butyl (IIR) < hydrogenated nitrile (HNBR) are somewhat more expensive but often still an economical choice. Expensive elastomers are fluorocarbons (FKM) (copolymers) < perfluorocarbons (FFKM) < fluorosilicones (FVMQ). These elastomers are usually only chosen if no other elastomer can meet the requirements.

Notes
  1. Elastomers are often described as viscoelastic materials (see for example Wikipedia). However, viscose flow is undesired, that is, a "true" elastomer should always return to its undeformed dimensions after an apllied force is removed, that is, no noticeable permanent deformation should be observed when an elastomer is stretched below its elastic limit and then relaxed.
  2. Some grades maybe suitable for continuous use at even higher temperatures  (>600°F or  > 315°C).