Elastomer Properties
Temperature
Each elastomer type has a specific working temperature range, however this will be influenced by many factors: media compatibility, dynamic or static operation and seal design being several of these.
Many factors affect the service temperature of elastomers. All dynamic and shock loads should be avoided at temperatures below the minus limit of a given compound. However, elastomers stored in static conditions, below the low temperature flexible range, will recover full physical properties during the warm-up period. At elevated temperatures, consideration must be given to the long term running limit and the short term peak limit of each elastomer. Elastomers exposed to the extreme limit can suffer an accelerated loss of flexibility, resulting in excessive compression set. This can dramatically shorten effective sealing life. Also there is an influence in volume swell and age hardening at high temperatures, dependent on compound type.
Hardness
The term hardness is the measure of a material’s resistance to a set deforming force exerted by a given standard indentation implement over a defined length of time. Hardness is measured in degree units of IRHD (International Rubber Hardness Degrees). It is generally the unit of measure on standard dimensioned test pieces. It is also the unit used for measuring the finished component (e.g. o-ring cross section) by the Micro Hardness Test. Standard compounds are nominally 70 IRHD. Superior can, however, provide compounds from 30-90 IRHD range, depending upon material type. Selection of hardness is dependent upon specific application requirements. For example:
Softer compounds
- deform more readily under load (e.g. cover/housing assembly) conform to surface irregularities
- lower stick/slip effect
- higher running friction.
Harder compounds
- higher extrusion resistance
- lower running friction
- higher stick/slip effect.
The hardness is usually expressed and controlled as a nominal figure with ± 5 points tolerance.
Compression set
This is the measurement of a compound’s loss of elastic memory. A standard cylindrical test piece of rubber is subjected to a defined pre-load at given temperature and time parameters (e.g. 24hrs/100°C). The test deformation is usually 25% of the original height.
The measured recovery of the cross-section is carried out at ambient temperature. The end result is recorded as the height not recovered, expressed as a percentage of the amount by which the part was compressed. Usually it can be stated that the better the elastomeric memory, the lower the compression set. This is regarded as an important feature of any compound, as leakage will occur if high set (and therefore loss of memory) occurs.
Tensile Strength
This is the force necessary to rupture a standard test piece at a given rate of elongation and expressed as force per unit area.
In practical terms, this property result does not assist the end user to select a compound, because a correctly assembled o-ring does not rely on its tensile strength to achieve effective sealing.
Elongation at break
Elongation at break is measured at the moment of rupture of a test piece under tensile load, expressed as a percentage.
This is a useful indication of a compound’s suitability as a large percentage of stretch may be necessary during assembly (e.g. piston seal).
Tear strength
Resistance to tear propagation from a point of initial damage, sustained for example, during assembly.
Gas permeability
Elastomers allow gas to enter into the structure. They will diffuse or permeate through and escape via the low pressure side.
The rate of permeation is governed by temperature, pressure, gas type and elastomer type. This may be critical for vacuum and gas containment.
To reduce permeability:
- use larger o-ring sections
- apply more compression
- optimise surface finish
- select high-density elastomers.
Abrasion
Abrasion resistance is a general term indicating wear resistance.
It can be noted, generally, that HNBR compounds perform best.
NBR and EPDM have relatively good abrasion resistance. FKM has a lower abrasion resistance.
Abrasion resistance improves with hardness (up to 80 IRHD). However, Silicone and Fluorosilicone have poor properties and should only be applied to static environments.
Colour
Rubbers are usually formulated with the addition of carbon black fillers. Hence, the majority of compounds are black in colour. Rubbers gain much of their strength and heat resistance from the addition of carbon black fillers.
However, at Superior we have the expertise to formulate and produce colour pigmented compounds for most of the elastomer range. Colouring is mainly used as a means of identification, allowing differentiation of compound grades in safety-critical applications. In addition, colour can be used to separate similar sizes on a customer’s production line.
Please contact our Material Science department on +44 (0)1202 854300 for application and colour range availability.
For a choice of compounds, you can download our compounds brochure here
Ageing
Heat ageing tests are widely used to record changes in property of an elastomer.
Usually hardness, tensile strength and elongation are measured and compared to original properties.
Air and relevant fluid ageing over standard time/temperature durations are meaningful to compare life expectancies of rubbers. Standard test conditions are:
- NBR: 24/70 hrs @ 100°C
- EPDM: 24/70 hrs @ 120°C
- FKM: 24/70 hrs @ 200°C
- VMQ: 24/70 hrs @ 200°C
If elastomers are pushed beyond their ageing resistance parameters, they will suffer from cracking, splitting and/or hardening.
Low temperature flexibility
The TR test (low temperature retraction) provides a measure of the rate of recovery of an elastomeric material after it has been subjected to low temperature.
The test, which is described in ISO 2921, consists of stretching a test piece with an effective length of 50 or 100mm and placing it in a bath at -70°C.
The test piece is allowed to retract freely whilst the temperature is raised at the rate of 1°C per minute.
The percentage retraction of the test piece is plotted against temperature.
Retraction values are calculated automatically with TR10, TR30, TR50 and TR70 values being most commonly recorded.
TR10 and TR70 values are of particular interest.
