This page explains the key terms used to define the properties of elastomers.
All types of elastomers are susceptible to chemical attack of varying degrees. The compatibility of a rubber in a particular environment is a function of both the polymer structure, and the way in which it has been formulated. Chemical incompatibility can have various effects on the rubber compound, depending on the exact form of chemical attack, with the effects being exaggerated or accelerated at elevated temperatures. The physical effects can be seen as:
- Embrittlement and hardening
- Softening and becoming tacky
- Volume loss
The most common effect is swelling – either due to a solubility effect, or chemical attack, resulting in a change in the elastomer’s polarity. Though the volume increase may be reversible, the effects on the polymer may not be.
Embrittlement and hardening are indicative of additional cross-linking, and softening is indicative of degradation of the polymer/crosslink network. Volume loss is most usually linked to extraction of plasticisers and process aids, resulting in a material that is less flexible.
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.
- 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 stress relaxation - Elastomers are viscoelastic materials which essentially behave both as an elastic solid and as a viscous liquid. Constant deformation of an elastomer can lead to internal structural changes, which in turn can alter the stress-strain characteristics of the material under load. When compressed, energy is both stored and dissipated by the material i.e. both both elastic and viscous effects occur. Therefore, as an elastomer is compressed, it will generate a reaction force (or ‘sealing force’). However, over time, the stored energy will decrease, reducing the initial sealing force. This decrease in sealing force is known as ‘Compressive Stress Relaxation’ (CSR). CSR is sometimes referred to as the retained sealing force in the seals and gaskets sector. The phenomenon can be accelerated by exposure to chemicals and/or elevated temperatures which attack the polymer backbone or cross-linking system. CSR is measured by compressing a standard test piece to a constant strain and measuring the force exerted by the test piece at specific intervals under specified conditions. The decaying force is expressed as a percentage of the initial counterforce.
- 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-tobulge’;
it is noted that a sample with a high surface area, but a low area free-to-bulge will show rapid increases in modulus.
Permeation of a gas into an elastomer under high pressure may not result in any long term effect provided the pressure is released gradually, allowing the gas to permeate out of the elastomer. However, if the pressure is released rapidly, the pressurised gas can expand suddenly, rupturing the elastomer in a catastrophic manner. Specific elastomer compounds are required to eliminate this effect, with the formulated materials tending to be very hard. See page 8 for information on Explosive Decompression testing.
Friction, Wear and Abrasion
The resistance to wear of a rubber when its surface is subjected to mechanical action. It is usually expressed as an abrasion resistance index referred to a standard rubber and is applicable to all methods.
- Friction - Friction can be defined as the resistance to sliding of one material over another. Testing of friction with relation to elastomers is mainly associated with the tyre industry, and can be split into two separate categories: static and dynamic. Testing usually is performed using a ‘sled’ of material loaded with additional weights to produce a normal force. This sled is then slid over a surface of a known surface-finish. The resultant resistant force can be related back to the coefficient of friction (μ) using the formula F = μR, where F is the resistant force and R is the normal force. There are numerous drawbacks to this method, as elastomers do not fully comply with this equation, but data can be comparable.
- Abrasion resistance index - The expression of abrasion resistance, being the ratio of the volume loss of a standard rubber to the volume loss of a rubber under test, determined under the same specified conditions, expressed as a percentage. The test involves the removal of rubber using an abrasive cloth on a rotating cylinder. The volume loss of the test rubber is calculated from the same test conditions to remove 200mg of the appropriate standard rubber.
The resistance of a material surface to penetration by an indentor of specified dimensions under specified load. The hardness property is quoted against two common systems (which do not necessarily correlate):
- 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`. 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.
- 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. The hardness scales are quoted in
degrees from 0 (infinitely soft) to 100 (bone hard).
Heat resistance and accelerated ageing
The controlled deterioration by air at elevated temperatures and atmospheric pressure after which physical properties are measured and compared with un-aged test pieces. Typical properties measured are changes in hardness, elongation at break, ultimate tensile stress, and stress at various strains. Heat resistance and accelerated ageing is also linked to ‘outgassing’, where an elastomer, on initial exposure to elevated temperatures, may lose some process oils or low molecular weight fragments of the polymer. In some applications, this loss of polymeric species may interfere with critical components or processes. Typically, this weight loss can be measured using thermogravimetric analysis (TGA), often coupled to further equipment if the specific constituents of the weight loss need to be identified.
Low temperature resistance
The low temperature resistance of any elastomer is dependent upon the material’s glass transition temperature (Tg). This is the temperature at which the elastomer changes from a rubber-like material to a brittle material. As an elastomer approaches its glass transition temperature, it would generally be expected that the tensile strength, hardness, modulus and compression set would all increase. The Tg for any given elastomer is mainly a function of the polymer structure, but can be altered slightly by the use of oils. Numerous methods exist for determining the cold temperature performance of an elastomer. Differential Scanning Calorimetry (DSC) is the most common method of evaluating the glass transition region itself. This method uses accurate measurements of enthalpy changes of materials over a given temperature range. Analysis of this data allows precise measurements of the glass transition. Other tests focus on more physical parameters. For example the temperature retraction (TR) test involves immersing a stretched, standard test piece in a bath at -70°C until it becomes rigid, then, allowing the sample to retract freely and raising the temperature at 1°C/min. The temperature at which the test piece has retracted 10% of the original stretch is referred to as ‘TR10’. A value appended to TR10 defines the initial stretch, eg. TR10/50 test will stretch the sample by 50%.
An alternative method of describing the low temperature stiffness of an elastomer is to adopt the Gehman test (ISO1432 : 1998). This test method measures the torsional modulus of a standard test piece at a range of temperatures. The relative modulus values at the measured temperatures are determined (relative modulus at a given temperature is the ratio of the torsional modulus at that temperature to the torsional modulus at 23°C). The temperature at which the relative modulus is 10 is reported as T10 or 5 as T5, etc.
The release of gas from a material, or low molecular portions of the material. These gases, together with gases upon the surface
of the solid, can be released into a vacuum environment to form a perceived leak. At ambient conditions the predominant outgased substances are water vapour and hydrocarbons. The rate of out-gasing increases at higher temperatures, which increases permeation rate and can initiate chemical reactions within the elastomer that release other gases. Out-gased substances can condense onto surfaces or react with process chemicals which may inhibit performance.
The permeation of gases or vapours 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, plasticisers, etc). The degree of permeation generally reduces from silicone elastomers (the highest), followed by NR, EPDM, SBR, CR, NBR, FKM and FFKM, ECO and IIR.
Peroxide or sulphur cure
Several cross-linking (curing) mechanisms can be employed for the different elastomer types, common amongst these are sulphur-cured systems and peroxide-cured systems. In general sulphur-cured systems offer better original mechanical properties but worst heat ageing properties and peroxide systems vice-versa.
The amount of deformation applied to the O-ring or seal expressed as a percentage of the original cross-section. Deforming the O-ring between mating hardware creates an effective seal. It is the elasticity of rubber that provides the sealing force “push back” against the mating surfaces. Increasing the squeeze increases the compressive load force generated by the seal, this can potenitally damage or deform the mating hardware. Compression set will reduce the amount of sealing force exerted by the O-ring over time.
To provide an effective seal, the inside diameter (ID) of the O-ring must be smaller than the piston groove diameter. This ensures the O-ring is slightly stretched and therefore fits securely in the groove. The ideal amount of stretch is between 1% and 5%, 2% is considered optimum in most applications. Stretch greater than 5% is not recommended as this will reduce the cross section and may result in increased stress on the O-ring, which will cause accelerated ageing.
Simple testing of elastomer samples in tension is commonly used to define the strength of the materials. The most common representation is in the form of an engineering stress-strain curve; engineering stress-strain uses the original dimensions for calculations, as opposed to the actual dimensions during testing (the cross-section may not necessarily remain constant during testing). A typical engineering stress-strain curve from a tensile test is shown opposite.
s = Engineering stress.
e = Engineering strain or elongation reported as a percentage of the original gauge length.
- Tensile Stress (S or s)
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.
- 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.
- Tensile Strength (TS) - The maximum tensile stress recorded in extending the test piece to breaking point. Also described as ‘ultimate tensile stress’.
- Tensile Strength at break (TSb) - The tensile stress recorded at the moment of rupture/sample failure.
Note: the values of TS and TSb may be different if after yield, the elongation continues and is accompanied by a drop in stress, resulting in TSb being lower than TS.
- Elongation at break (Eb) - 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.
- 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.
- Strain Energy Density (W) - Defined as the ‘work done’ for a given strain, i.e. the area under the stress-strain curve after a specified elongation.
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 precut 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.
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.
O-ring tolerances for each standard size can be found in the O-ring size tables.
We recommend the use of ISO3302-1 Class M2, X2 in the design of custom elastomer components.
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 cross-link structure to tighten causing a decrease in volume. Measurements of volume before and after exposure are expressed as a percent change.
Weathering, Ozone and UV resistance
Exposure of elastomers to weathering can result in deterioration of the product due mainly to the effects of sunlight, in particular the UV (ultra-violet) component of the light spectrum. UV light has the effect of scission of the main polymer chain, leading to rapid degradation of the elastomer. This degradation manifests itself in the form of surface cracks often referred to as crazing and can allow ingress of water to leach out soluble components as well as leading to failure of the seal. This form of attack is most notable in materials that have unsaturation (carbon-carbon double-bonds) in the backbone of the polymer, and is accelerated by stressing the material (stress-induced chemical attack). To combat this, chemical UV stabilisers can be introduced into the formulation and carbon black is generally recognised as one of the most effective UV protection systems for use with elastomers. Similarly, Ozone (a powerful oxidising agent) can degrade elastomeric components in a similar manner to UV. Use of anti-ozonants and the careful choice of elastomers (saturated polymers) can significantly reduce/eliminate this problem.