Harboro Engineering in Excellence

Method of Measurement

Elastomeric (i.e rubbery or resilient) thermoset.

• 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

Method of Measurement

Measured in degrees and based on the penetration into the rubber of a defined indentor 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 but Shore A is also in widespread use.

• ASTM Reference D412
• BS Reference BS 903 Part A2
• ISO Reference ISO 37
• DIN Reference DIN 53504

Method of Measurement

This is measured in various 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

Method of Measurement

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

Method of Measurement

Modulus is 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 D395
• BS Reference BS 903 Part A6
• ISO Reference ISO 815
• DIN Reference DIN ISO 815

Method of Measurement

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.

• ASTM Reference D412
• BS Reference BS 903 Part A5
• ISO Reference ISO 2285
• DIN Reference DIN ISO 2285

Method of Measurement

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. unrecoverd length) is expressed as a percentage of the extended length.

• ASTM Reference D297
• BS Reference BS 903 Part A1
• ISO Reference ISO 2781

Method of Measurement

Density is defined as the mass per unit volume and is measured by weighing the rubber sample in air and water.

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

Method of Measurement

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

Method of Measurement

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).

• ASTM Reference D471
• BS Reference BS 903 Part A16
• ISO Reference ISO 1817
• DIN Reference DIN ISO 1817

Method of Measurement

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 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.

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

Method of Measurement

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 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%.

• 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

Method of Measurement

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 D991 & D257
• BS Reference BS 2050 & BS 2044

Method of Measurement

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 Ω, defining the conditions of testing.

• ASTM Reference D624
• BS Reference BS 903 Part A3
• ISO Reference ISO 34-1 & 2
• DIN Reference DIN 53506

Method of Measurement

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 D1630, D5963 & D2228
• BS Reference BS 903 Part A9
• ISO Reference ISO 4649
• DIN Reference DIN ISO 4649

Method of Measurement

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 D149
• BS Reference BS 903 Part C2 & BS 903 Part C4

Method of Measurement

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).

• BS Reference BS 2782

Method of Measurement

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.

• 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

Method of Measurement

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 D925
• BS Reference BS 903 Part A33
• ISO Reference ISO 3865

Method of Measurement

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

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

Method of Measurement

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).

• BS Reference BS 903 Part A10, BS 903 Part A11, BS 903 Part A49 & BS 903 Part A50
• ISO Reference ISO 132

Method of Measurement

Flex cracking and crack growth test.

• BS Reference BS 903 Part A15
• ISO Reference ISO 8013

Method of Measurement

Measurement of creep in compression or shear.

• BS Reference BS 903 Part A42
• ISO Reference ISO 3384

Method of Measurement

Measurement of stress relaxation at ambient or elevated temperatures.

• BS Reference BS 903 Part A61
• ISO Reference ISO 15113

Method of Measurement

Determining frictional properties.

• BS Reference BS 903 Part A21
• ISO Reference ISO 813

Method of Measurement

Measuring adhesion to a rigid substrate.

• BS Reference BS 903 Part A7 &BS 903 Part A30
• ISO Reference ISO 2782

Method of Measurement

Measuring the permeability of rubber to gases.