Hardness

Measured in degrees (points) and based on the penetration into the rubber of a defined indenter under a set load. Three scales are commonly used: IRHD (International Rubber Hardness Degrees), Shore A, and Shore D for hard materials over 90° Shore A. IRHD is preferred for most specifications; however, Shore A is also in widespread use.
• ASTM Reference D2240 & D1415
• BS Reference BS 903 Part A26 & BS 903 Part A57
• ISO Reference ISO 48, ISO 1400 & ISO 1818
• DIN Reference DIN 53505

Tensile Strength

Measured in various force units and is expressed as a force per unit area. A standard dumbell type test piece of known cross sectional area is used which is stretched until it breaks. The force required to do so is then recorded and expressed as force per unit area.
• ASTM Reference D412
• BS Reference BS 903 Part A2
• ISO Reference ISO 37
• DIN Reference DIN 53504

Elongation to Break

Measured in percentage (%). A standard dumbell type test piece of known cross sectional area is used which is stretched until it breaks. Elongation is defined as the length at breaking point expressed as a percentage of its original length (i.e. length at rest) e.g. if a rubber reaches twice its length before breaking its elongation is 100%.
• ASTM Reference D412
• BS Reference BS 903 Part A2
• ISO Reference ISO 37
• DIN Reference DIN 53504

Tear Strength

Tear strength is measured as the force required to tear a standard test piece. The standard test pieces are designed to produce weak points where a tear is initiated.
• ASTM Reference D624
• BS Reference BS 903 Part A3
• ISO Reference ISO 34-1 & 2
• DIN Reference DIN 53506

Compression Set

A cylindrical test button of rubber of known diameter and height is compressed to a fixed height (typically 70% or 75% of its original height) at a defined temperature for a specified period of time. The button is then released, allowed to recover (typically for 30 mins) and the height is measured. Compression Set is the height that is not recovered, expressed as a percentage of the amount by which it was compressed.
% Comp. Set = original height – recovered height original height – compressed height x 100
• ASTM Reference D395
• BS Reference BS 903 Part A6
• ISO Reference ISO 815
• DIN Reference DIN ISO 815

Modulus

Measured as the force per unit area required to extend a rubber to a stated percentage of its original length e.g. to 100%, 200% or 300%. It is often written as M100=3.0MPa (i.e. modulus at 100% =3.0MPa).
• ASTM Reference D412
• BS Reference BS 903 Part A2
• ISO Reference ISO 37
• DIN Reference DIN 53504

Bonding Strength

Measuring static adhesion strength of rubber to a rigid substrate, according to the following various methods.
Method A: the rubber part is assembled between two parallel metal plates,
Method B: 90° stripping test – the rubber part is assembled to one metal plate
Method C: measuring adhesion of rubber to metal with conical specimen,
Method D: post vulcanization (PV) bonding of rubber to metal.
Method E: 90° stripping test, rubber tank lining assembled to one metal plate.
Note” Methods B and E are also known as Peel Test.
• ASTM Reference D429
• ISO Reference ISO 813
• BS Reference BS 903 Part A21

Density (Specific Gravity)

Density is defined as the mass per unit volume and is measured by weighing the rubber sample in air and water. S.G = weight in air – weight in water
• ASTM Reference D297
• BS Reference BS 903 Part A1
• ISO Reference ISO 2781

Fluid Resistance
Fluid resistance is commonly measured by the effect of the fluid on the volume of the rubber expressed as a volume change, e.g. +100% indicates that the volume of the rubber has doubled as a result of exposure to the fluid, and -10% indicates that the volume of rubber had decreased as a result of exposure to the fluid. Volume change is measured by determining the weight of a sample in air and water before and after exposure to the fluid under defined conditions. The sample is normally totally immersed in the fluid.

Volume Change (%) = (W3 -W4) – (W1 -W2) (W1 – W2) x 100
Fluid resistance may also be defined in the same way as chemical resistance (above) i.e. by change in properties of the rubber. W1 Wt in air (initial) W2 Wt in water (initial) W3 Wt in air (swollen) W4 Wt in water (swollen)
• ASTM Reference D471
• BS Reference BS 903 Part A16
• ISO Reference ISO 1817
• DIN Reference DIN ISO 1817

Weather Resistance
Weather resistance is a fairly subjective test and it is necessary to state clearly the conditions under which the exposure took place i.e. dates, geographic location, angle and direction of exposure relative to the sun etc’. The properties of test pieces are measured before and after exposure and expressed as percentage changes (with the exception of hardness changes which are recorded in degrees). The change in any property may be measured but the most common are hardness, tensile strength, elongation at break and modulus at 100% and 300%.

• ASTM Reference D1171, D750, D518 & D1148a
• BS Reference BS 903 Part A53, BS903 Part A54 &BS 903 Part A55
• ISO Reference ISO 4665-1, 2, & 3

Low Temperature

A variety of methods are used for determining the low temperature characteristics of rubber. These fall into two groups – (a) measuring brittleness at low temperatures by impact test and (b) measuring the modulus at low temperatures. Different test jigs are required for each method, as described in the relevant specifications. ASTM D2137 measures brittleness. ASTM D1053 and BS 903 Pt A13 measure the modules characteristics. Two simple tests are a ‘bend’ test and a ‘retraction’ test. In the bend test, a sample 25mm wide x 100mm long is bent around a mandrel which is usually of a diameter 10 x the thickness of the test piece (typically 25mm dia.). If the sample bends without cracking it is deemed to be ‘flexible at this temperature’. In the ‘retraction’ test, the sample is stretched and frozen in this position below its glass transition temperature (i.e. the temperature at which the rubber becomes rigid). The temperature is then raised gradually or in steps and the temperature at which the rubber retracts is recorded. It should be noted that rubber passes from a rubber phase into a ‘leathery’ phase and then into its ‘glassy’ phase as it is cooled. Different methods can therefore give rise to different values according to their sensitivity to this ‘leathery’ phase.

• ASTM Reference D2137, D1053 & D1329
• BS Reference BS 903 Part A13, BS 903 Part A25, BS 903 Part A29 & BS 903 Part A63
• ISO Reference ISO 812, ISO 2921, ISO 1422 & ISO 3387
• DIN Reference DIN 53545

Ozone Resistance

Ozone causes cracking in rubber. Test pieces are usually placed under a small degree of tension e.g. by bending round a mandrel or stretching by 5%. The sample is exposed under static conditions to a controlled atmosphere containing ozone (typically 50pphm). The cracks are graded by standard photographs, by measurements or by description, e.g. ‘visible under 10x magnification’ ‘visible to the unaided eye’ etc. Results may be recorded as the time taken to reach a particular grade of cracking, or by the grade of cracking apparent after a fixed period of time.

• ASTM Reference D1149
• BS Reference BS 903 Part A43, BS 903 Part A44 &BS903 Part A45
• ISO Reference ISO 1431 /1 & 2
• DIN Reference DIN 53509

Flame Resistance

Flame resistance testing requires a standard burner and fuel which gives a flame of known characteristics. Rubber samples are then placed at a certain position in the flame and removed after an ignition period. The time taken for the rubber to self-extinguish is recorded with notes regarding afterglow and any hot particles emitted by the sample at any stage.

• BS Reference BS 2782

Accelerated Ageing

Heat ageing is widely used as a method of evaluating long term ageing properties. Hardness and dumbell type tensile pieces are placed in an air circulating oven for a specific period of time at a given temperature (e.g. 7 days at 70ºc). The properties of the rubber are then tested and compared with the properties before ageing. The percentage retained for each property is recorded (change in hardness is recorded in degrees).

• ASTM Reference D573
• BS Reference BS 903 Part A19
• ISO Reference ISO 188
• DIN Reference DIN 53508

Resilience

Resilience is measured in percentage (%) on standard test equipment of which there are several types – e.g. Dunlop, Tripsometer, Lupke, Rebound etc’. Standard test pieces are struck by a ‘hammer’ and the ‘bounce back’ of the hammer is measured. This is expressed as a percentage of the flight path of the hammer.

• ASTM Reference D1054, D2632 & D945
• BS Reference BS 903 Part A8
• ISO Reference ISO 4662
• DIN Reference DIN 53512

Abrasion Resistance

Abrasion properties of rubber are difficult to define. Many different abrasion machines have been designed but they do not always give similar results. The most common abraders are AKRON, Dunlop and DIN. Results are recorded as volume loss per standard test piece, or as the difference in volume loss when compared with a standard material of known abrasion value. Indices of relative abrasion are also used.
• ASTM Reference D1630, D5963 & D2228
• BS Reference BS 903 Part A9
• ISO Reference ISO 4649
• DIN Reference DIN ISO 4649

Staining

The staining of organic finishes of measured by placing a sample of the rubber against a clean sample of the organic finish and applying pressure for a period of time at a given temperature. The sample is then removed, and the organic finish examined for evidence of staining or discoloration. Results are recorded as staining or non-staining. It is normal to give a description of any staining that is observed.
• ASTM Reference D925
• BS Reference BS 903 Part A33
• ISO Reference ISO 3865

Basic Electrical Properties

Anti-static and conductive properties of rubbers are defined by measuring their electrical resistance. Typical measurements are obtained through the thickness of a sample by using 25mm sq electrodes, under a pressure of 45N and applying a voltage of 500V DC @ approx 6mA. (Energy dissipated into the test piece if limited to 3W max). The surfaces should be cleaned before measuring. Patterned surfaces should be wetted with a conductive solution paint. Results should be recorded in, defining the conditions of testing.
• ASTM Reference D991 & D257
• BS Reference BS 2050 & BS 2044

Electric Strength

Electric strength is measured by placing a disc of rubber of known thickness between two electrodes gradually or stepwise until electrical breakdown occurs. It is expressed in Volts per unit thickness (e.g. KV per mm).
• ASTM Reference D149
• BS Reference BS 903 Part C2 & BS 903 Part C4

Permanent Set (Tensile Set)

Measured in percentage (%). A standard test piece of known length is stretched by a stated percentage for a period of time and is then released. After recovery time the length is measured and the change in length (i.e. the un-recovered length) is expressed as a percentage of the extended length.
• ASTM Reference D412
• BS Reference BS 903 Part A5
• ISO Reference ISO 2285
• DIN Reference DIN ISO 2285

Permeability

Measuring the permeability of rubber to various types of gases.
• ISO Reference ISO 2782
• BS Reference BS 903 Part A7 &BS 903 Part A30

Fatigue

Flex cracking and crack growth test.
• ISO Reference ISO 132
• BS Reference BS 903 Part A10, BS 903 Part A11, BS 903 Part A49 & BS 903 Part A50

Creep

Measurement of creep in compression or shear.
• ISO Reference ISO 8013
• BS Reference BS 903 Part A15

Stress Relaxation

Measurement of stress relaxation at ambient or elevated temperatures.
• ISO Reference ISO 3384
• BS Reference BS 903 Part A42

Frictional Properties

Determining frictional properties.
• ISO Reference ISO 15113
• BS Reference BS 903 Part A61