Elastomer Terminology


Coefficient of thermal expansion
All materials (except for a very small number of exceptions) expand with increasing temperature. The degree to which any material expands is characteristic of that material. The value is expressed in terms of the amount of linear or volumetric expansion that occurs with every unit of length or volume for every degree of temperature increase.

The action of introducing various additives to the raw polymer material to effect its physical properties. Additives can include c urative, accelerator, secondary accelerator, retarder, plasticizer, softening agent, release aid, anti-oxidant, anti-ozonant, acid acceptors, flame-retardant, filler, pigment etc.

Compression set 
A measure of a material’s elasticity after prolonged action of compression, either under ambient conditions, or whilst being exposured to elevated temperatures. Compression set is often used as a measure of the state of cure or strength of crosslinking; it is quoted extensively for sealing applications as an attempt to relate material characteristics to leakage prevention, where recovery of the seal’s shape is required after distortion. 
Physical and chemical changes that can occur to an elastomer at elevated temperatures, may prevent the elastomer from fully recovering its original shape on removal of the applied compressive strain – the result is known as a ‘set, and is quantified as a percentage loss in shape compared to the original dimensions.
ASTM D395 defines two different test methods (A and B). Method A is not often stated, but relates to a constant load. Method B is the most common method where a sample of specific dimensions is compressed to a fixed deflection, after exposure to elevated temperatures, the sample is removed from the fixture and allowed to rest under ambient conditions before the final dimensions are measured. Another less commonly used variant of this test is found within ISO 815, where the samples can be allowed to cool while still under compression.

Compressive Modulus
Compressive modulus is an important physical property of elastomers and determines the amount of stress a material will exhibit for a given amount of compressive strain. The testing is sometimes referred to as ‘load deflection’ testing. The results of the test are highly dependent upon sample dimensions, due to the ‘shape-factor’ effect when testing elastomers. The ‘shape-factor’ is a ratio of the area of the test sample, compared to the area of the sample that is ‘free-to-bulge’; it is noted that a sample with a high surface area, but a low area free-to-bulge will show fast increases in modulus.

Elongation (E or e ) 
The extension expressed as a percentage of the original test length, produced on the test piece by a tensile stress, this is known as percent strain.

Elongation at break (E b)
The elongation (expressed as a percentage of the original length) at breaking point.

Elongation at a given stress (ES) 
The tensile strain in the test length when the test piece is subjected to a given tensile stress.

FFKM - also see 'Perfluoroelastomer'
A type of rubber designated perfluoro rubber of polymethylene type (generically known as a perfluoroelastomer) by ASTM International (an international standards organization originally known as the American Society for Testing and Materials).

Flourosilicone elastomers (also known as FVMQ) are modified silicone rubbers, which have many of the properties associated with silicone rubber but show great improvements in oil and fuel resistance. They have a very wide service temperature range and low chemical reactivity. They do however have low tensile strength, poor tear and abrasion resistance and high gas permeability. Typical uses include sealing systems requiring wide temperature exposure and resistance to aerospace fuels and oils.

The resistance of a material surface to penetration by an indentor of specified dimensions under specified load. Hardness is typically tested on flat buttons, for curved surfaces the ‘Apparent Hardness’ is often quoted, as IRHD and Shore A values tend to be more variable when measured across small curved surfaces, as is the case with O-rings. 
The hardness scales are quoted in degrees from 0 (infinitely soft) to 100 (bone hard).
The hardness property of an elastomer is quoted against two common systems (which do not necessarily correlate): 
1) IRHD – International Rubber Hardness Degrees based on measuring the penetration of a specified rigid ball in a test specimen under a specified dead load. A scaled down version of this dead load instrument is available for conducting measurements on small cross sections and thicknesses IRHD `M`. 
2) Shore hardness degrees – the testing equipment used to measure hardness are often referred to as durometers (type A or D), both utilise a calibrated spring to act on a specified indentor to penetrate the test specimen

IRHD - see 'Hardness'

Modulus or “Stress at a given elongation” (SE) 
The tensile stress in the test length at a given elongation. This definition is widely referred to with the term ‘modulus’, and care should be taken to avoid confusion with other uses of the term modulus, such as Young’s Modulus, which denotes the slope of a linear stress-strain curve. Neither Young’s Modulus or secant modulus are applicable to non-linear materials and are therefore not used when referring to elastomers.

The release of gas from a material, or low molecular portions of the material. These gasses, together with gasses upon the surface of the solid, can be released into a vacuum environment to form a perceived leak. At ambient conditions the predominant out-gassed substances are water vapour and hydrocarbons. The rate of out-gassing increases at higher temperatures, which increases permeation rate and can initiate chemical reactions within the elastomer that release other gasses. Out-gassed substances can condense onto surfaces or react with process chemicals which may inhibit performance.

Perfluoroelastomers are also known as FFKMs, they exhibit outstanding high temperature properties and are the most chemically resistant elastomer available; effectively a rubber form of PTFE. They are superior to FKM elastomers, showing continuous dry-heat resistance to 260°C, with extended performance to 330°C for high temperature grades. They are extremely inert chemically and show excellent resistance to the majority of chemicals that attack other elastomers. Other notable properties include excellent resistance to oil-well sour gases, high temperature steam, low out-gassing under vacuum and good long-term high temperature compression set resistance. Typical applications are sealing systems for oil refineries, pharmaceutical plant, aerospace, chemical plant and the semiconductor industry. 
PPE's range of perfluoroelastomer materials are branded as Perlast®

The rate of which gases or vapours travel through the elastomer. This is an important property if the elastomer is being used to prevent leakage of gases/vapours from chambers, etc. The permeation rate is governed by the type of elastomer used and the composition of the final compound (filler type, plasticizers, etc). The degree of permeation generally reduces from silicone elastomers (the highest), followed by NR, EPDM, SBR, CR, NBR, FKM, ECO and IIR .

Shore A - see 'Hardness'

Squeeze (compression)
The amount of compressive force/compression applied to an elastomer to form the initial seal.

Strain Energy Density
Defined as the ‘work done’ for a given strain, i.e. the area under the stress-strain curve after a specified elongation.

The amount of permanent stretch applied to an O-ring following installation. Stretch is expressed as a percentage of the original size.

Tear strength 
The median force required to propagate a cut in a specified trouser-shaped test piece by tearing, divided by the thickness of the test piece. This term indicates the resistance to propagation of small pre-cut in an elastomer. The most common test uses the “Trouser Tear” method (ASTM D624, Die T), with the test sample having the shape described in the title.

Tensile Stress (S or σ )
The stress applied to extend the test piece, calculated as force per unit area of the original cross-section of the test length. Results are normally reported in MPa.

Tensile Strength (TS)
The maximum tensile stress recorded in extending the test piece to breaking point. Also described as ‘ultimate tensile stress’.

The amount of variation permitted on dimensions or surfaces during the manufacturing process. The tolerance is equal to the difference between the maximum and minimum limits of any specified dimension. As metals are hard and interference may prevent assembly they are usually toleranced as a fit. Bores are defined as a dimension plus an allowable variation, and shafts are defined as a dimension minus an allowable variation. As elastomer parts, such as O-rings are flexible and are typically designed to operate in interference (compression or tension), they are typically given a ± tolerance for diameter and cross section.
Perlast recommends the use of standard ISO3302-1 Class M2, X2 in the design of custom elastomer components. 

Rubber is best described as a ‘viscoelastic’ material, by this, it is meant that the rubber shows both ‘viscous’ (fluid-like) and ‘elastic’ properties. It is common to find a ‘spring-dashpot’ model used to aid in the explanation of this material property.

Volume change
The degree to which a material expands or contracts during exposure to operating environments is an important factor to consider in any sealing application. Operating fluids can be absorbed into a material causing it to swell. Operating fluids can also wash out ingredients within the material causing it to decrease in volume, sometimes both can happen; an initial swelling followed by shrinkage. Some high temperature and chemical environments can cause the crosslink structure to tighten causing a decrease in volume. Measurements of volume before and after exposure are expressed as a percent change.

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