MEASUREMENT OF RUBBER PROPERTIES

PROPERTY

METHOD OF MEASUREMENT

ASTM Reference

BS Reference

ISO Reference

Hardness

Measured in degrees and based on the penetration into the rubber of a definined indentor under a set load. Three scales are commonly used: IRHD (International Rubber Hardness Degrees), Shore A and Shore D for hard materials over 90o Shore A. IRHD is preferred for for most specifications but Shore A is also in widespread use.

D2240
D1415

BS 903 Part A26

ISO48*

ISO1400*

ISO1818*

Tensile Strength

This is measured in various units and is expressed as a force per unit area. A standard dumbbell type test piece of known cross sectional area is stretched until it breaks. The force required to do so is recorded and expressed as force per unit area.

D412

BS903 Part A2

ISO37*

Elongation

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%.

D412

BS903 Part A2

ISO37*

Modulus

Modulus is measured as the force per unit area required to extend a rubber to a stated percentage of its original length e.g. 100%, 200% or 300% (modulus in rubber is non-linear). It is often written as M100 = 3.0MPa (i.e. modulus at 100% = 3.0MPa)

D412

BS903 Part A2

ISO37*

Compression Set

A cylindrical button of rubber of known thickness 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 thickness is measured. Compression set is the height that is not recovered expressed as a percentage of the amount by which it was compressed.

D395

BS903 Part A6

ISO815*

Permanent Set (Tensile Set)

A standard test piece of known length is stretched by a stated percentage for a period of time and is then released. After recovery the length is measured and the change in length (i.e. unrecovered length) is expressed as a percentage of the original length

D412

BS903 Part A5

ISO/R2285*

Density (Specific Gravity)

Density is defined as the mass per uit volume and is measured by weighing the sample in air and water SG = weight in air / (weight in air - weight in water)

BS903 Part A1

ISO2781**

Resilience

Resilience is measured on standard test equipment (of which there are several types e.g. Dunlop Tripsometer, Lupke Rebound). Standard test pieces are struck with a 'hammer' and the 'bounce back' of the hammer is measured. This is expressed as a percentage of the flight path of the striking hammer.

D1054

D2632

BS903 Part A8

ISO/R1767*

Chemical Resistance

This is usually expressed as the change in properties (such as hardness, strength and elongation at break) caused by the presence of chemicals under defined conditions (including concentration, time and temperature).

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, -10% indicates the volume reduced as a result of exposure). 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.

Fluid resistance may also be defined in the same way as for chemical resistance (above) i.e. by change in properties of the rubber

D471

BS903 Part A16

ISO1817**

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, geography, angle and direction of exposure relative to the sun etc. The properties of the test pieces are measured before and after exposure and are expressed as a percentage change (with the exception of hardness changes which are usually 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%.

D1171

BS903 PartA53 PartA54 PartA55

Ozone Resistance

Ozone causes cracking in rubber. Test pieces are usually placed under a small degree of tension e.g. by bending round a mandril or stretching by 5%. The sample is exposed under static conditions to a controlled atmosphere containing ozone (typically 50 pphm).

These cracks are graded by standard photographs, by measurements or by description, e.g. visible under "10 x 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.

D1149

BS903 PartA43

ISO1431/1**

Electrical Properties

Anti-static and conductive properties of rubbers are defined by measuring their electrical resistances. 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 is limited to 3W max.) The surface should be cleaned before measuring. Patterned surfaces should be wetted with a conductive solution or paint. Results should be recorded in W, defining the conditions of testing.

D991

D257

BS2050

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.

D624

BS903 Part A3

ISO34**

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 Du Pont. 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.

D394

BS903 Part A9

Electric Strength

Electric strength is measured by placing a disc of rubber of known thickness between two electrodes and increasing the voltage gradually or stepwise until electrical breakdown occurs. It is expressed in Volts per unit thickness (e.g. KV per mm or V per 0.001").

D149

BS903 Part C2

BS903 Part C4

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. More sophisticated tests include analysis on the pyrolitic products for toxicity etc.

BS2011

Low Temperature

A variety of methods are used for determining the low temperature characteristics of rubbers. These fall into two groups a) measuring brittleness at low temperatures by impact testing 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 modulus 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 mandril 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 rubbery phase into a "leathery" phase and then into it’s "glassy" phase as it is cooled. Different methods can therefore give rise to different values according to their sensitivity to this "leathery" phase.

D2137

D1053

D1329

BS903 Part A25

BS903 Part A29

BS5294

ISO/R812*

ISO2921**

Staining

The staining of organic finishes is 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 discolouration. Results are recorded as staining or non-staining. It is normal to give a description of any staining that is observed.

D925

BS903 Part A33

ISO3865**

Accelerated Ageing

Heat ageing is widely used as a method of evaluating long term ageing properties. Hardness and dumbbell type tensile test pieces are placed in an air circulating oven for a specific period of time at a given temperature (e.g. 7 days at 70o c). 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).

D573

BS903 Part A19

ISO188**