When choosing rubber products, it is important to be aware of both physical and chemical properties of the component, therefore it is valuable to know rubber testing methods used in the industry, and their values. The properties determined by these tests are often indicated on the products or their data sheets, although we may not know the meaning of the number and letter combinations.
To help you understand these parameters, in the following we are introducing the most common rubber testing methods and interpretations of their results:
- Shore hardness test
- Abrasion test
- Discoloration Metering
- Vertical Rebound Resilience
- Burst Strength test
- Flex Cracking test
- Ozone resistance
- Cold resistance
- Mooney viscosity measurement
- Oscillating-disc rheometer
Testing methods for ready-made rubber products
Shore hardness test
Rubber hardness testing is usually done using an indentation hardness measurement method, also known as a durometer. It was developed by Albert F. Shore in the 1920s, for measuring the hardness of polymers, elastomers and rubber products, and has several scales depending on the shape of the indenter used to apply pressure to the object under test. It is denoted by 00, 0, A, B, C, or D after the word Shore, that goes from extra soft to extra hard. There is an overlap between the scales from 0 to 100, thus materials can be on more than one scale but with different values.
Rubber products are mainly on the Shore A scale, for example, the tread of a car tire is typically 70, 70 Shore A.
Different types of rubber and plastic on the Shore scale
The drawback of the test is that it cannot be used on small, less than 2 mm thick materials, and there is a potential for operator error when testing without a stand.
Two international standards are currently in use for wear testing. These are EN ISO 14890 (H, D and L) and DIN 22102 (Y, W and X). Products that comply with H (ISO) and X (DIN) are resistant not only to abrasion but to cutting, punching and chipping as well.
Proper testing is also standardized in ISO 4649 and DIN 53516. During the testing process, an abrasive surface is fixed to a rotating cylinder, the test object is moved on it and the reduction in the volume of the test object is measured. The result is the volume lost, the higher the number, i.e. the volume lost, the less resistant the material is to abrasion.
Wear resistance is an essential indicator of durability. Often, two or three cheaper products will run out, while a costly but more wear-resistant one can still be used. Therefore, when choosing a product, it is worth knowing both the standard category and the test value, as there may be variations within a category.
Testing according to ASTM D1148-13(2018) standard measures the effect of UV light on white or light-colored vulcanized rubber products such as sports shoes. The light source, temperature and humidity are controlled during the test, therefore a sealed unit is required that complies with the standard.
After the test, a colorimeter is used to compare the color of the piece exposed to UV light during the test with the new condition. The test results in a color difference, the higher the value, the less resistant the tested material was to the effects of sunlight.
Vertical Rebound Resilience
Flexible elastomer and rubber products react to an external force by applying a counterforce. The ASTM D2632 test measures this force, deducing the elasticity of the rubber product.
The vertical rebound resilience test is performed by dropping a standard plunger vertically from a specified height onto a horizontally placed object six times in succession. The heights of the fourth, fifth and sixth rebounds are visually recorded and averaged. The result given in percentage indicates the elasticity of the tested rubber product.
Testing methods for rubber materials
Burst Strength test
Puncture testing is a form of rubber tensile strength test, also used on leather, paper, textiles and similar products. A machine applies pressure to the surface of the sample between 0-40 kg/cm2 to measure when the material breaks.
A higher value means better resistance, more tensile strength. This value is especially important for packaging materials and products subject to unexpected impacts.
Flex Cracking test
Rubbers are necessarily tested whether they can withstand repeated flexing without cracking. This type of fatigue is usually tested with a DeMattia machine according to ISO 132:2017. It subjects the test material to bending at the speed, degree and density specified in the standard.
An intact material is tested to see if it cracks under bending loads repeated at high frequency. The measurement is coded according to ASTM D-430. The result indicates the number of bends required to crack, so a higher value indicates better resistance. In the other part of the test, a notch is made in the material to test for further cracking. The measurement is coded according to ASTM D-813, which also records the length of the crack and the number of bending cycles.
Rubber testing samples made to standard sizes
There is another measurement method, the Ross method. However, this only performs 100 cycles per minute, compared to the 300 cycles per minute load of the DeMattia system. A third test method subjects the rubber product to a 90° deflection on a 10 mm diameter rod after notching. The crack size is then measured after a predetermined number of bends. It is particularly suitable for testing shoe soles.
Exposure to ozone can cause cracks in several types of rubber, especially natural rubber and products made from polybutadiene, styrene-butadiene or nitrile rubber.
Ozone tolerance can be tested according to several standards. For example, ASTM D1149 indoor, ASTM D1171 outdoor or EN/ISO 1431. The latter rubber testing method requires
- a temperature of 40°C,
- an ozone concentration of 50 ppm and
- a 20% load on the sample, e.g. stretching.
Under these conditions, the sample is left for 96 hours, its condition is checked every two hours and any lesions or cracks are recorded. The smaller the lesion and the fewer the cracks, the more resistant the material.
For some products, it is worthwhile to carry out general tests, such as durometer, even at deep below-freezing temperatures. But three test standards specifically look at how materials behave in very cold conditions. These are crucial tests for rubber, leather, leatherette and plastic materials to ensure that products from them perform well in cold climates.
The ASTM D1329 testing method tests how well a material retains its elasticity in a refrigerated environment. This is tested by first stretching, then clamping and cooling, and finally unclamping and measuring the rate at which the material contracts.
The resulting percentage is expressed as a ratio of temperature. The two most commonly used indicators are TR10 and TR70, i.e. what temperature the material was when it contracted at 10% and 70% respectively. The lower the measured temperature, the better the cold tolerance of the material.
The ASTM D746 and ASTM D2137 standards test the brittleness point. They indicate the temperature at which a rubber or other elastomeric material loses its elasticity to the point where it will break on physical impact.
Testing methods to support product development
Mooney viscosity measurement
The Mooney method measures the viscosity of mixed or unmixed natural rubber, synthetic rubber and regenerated rubber. It also shows how these materials react to the vulcanization process.
The machine heats the raw rubber material at a pre-programmed temperature and for a pre-set time, while continuously testing its viscosity. The results are easily displayed graphically.
The method tests the behavior of the material before, during and immediately after vulcanization. This process is used to determine the vulcanization process best suited to the raw material and the intended finished product.
The device measures the shear force required to overcome the actual viscosity of the material, and through this the viscosity of the material being tested.
During the process, a standard disc is embedded into the sample of raw rubber, which is placed into the machine, which then exerts rotation momentum on the disc.. The first measurement records the minimum torque required for rotation. Then the vulcanization process takes place, while the shear force required to overcome the viscosity of the rubber is measured at regular intervals.
With this method, it is possible to accurately determine the vulcanisation process that achieves the best viscosity from a given raw material.
High-quality rubber products from SIC
At SIC, we ensure that every product is made from the best material for the intended application, thus developing the product for the challenge at hand. Testing is always part of the engineering process, to ensure that the material is suitable for the intended application. If you need a rubber component that meets your specific requirements, don’t hesitate to contact us.