Elastomer mechanical properties and how they are measured

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30.07.2018

 

The previous blog entry (How do you create a rubber component? A blog around and about the world of elastomers), discussed which factors require consideration when selecting a raw rubber for a specific use. For many applications the mechanical properties of the rubber component play a critical role.

In this post, we are taking a look at the different characteristics of raw rubbers and investigating the test methods.

If you are going to compare different materials and their properties, the testing method has to be administered and scored in the same, consistent and ‘standard’ manner.  The standards that are internationally recognized and most often applied are those of the German Institute of Standardization (DIN) and the American ASTM. These tests are usually carried out on standard test pieces at room temperature so that the results derived can be compared.

Hardness

An important property of a rubber molding is its hardness. Hardness is defined as the resistance of a (rubber) specimen to the penetration of another harder body. Hardness testing equipment is designed so that a specialized needle is pressed by means of a spring into the surface of an elastomer for a specified amount of time. The further the needle penetrates the softer the rubber. The hardness scale ranges from 0 to 100.    

Shore A and IRHD (International Rubber Hardness Degree) are the standard test methods used to determine the hardness of a rubber. The methods differ in the design of the test equipment, the contours of the indentation needle and the type of test pieces that can be investigated.  

The test method Shore A is the classic method.  Hardness of rubber materials is often expressed in Shore A units (ShA). The test is performed on a standard test piece. The test piece has to be smooth and have a thickness of 6 mm and a diameter of a minimum of 35 mm. The Shore A method is used for measuring materials with a hardness ranging from 10 to 90 with an indentation time of 3 seconds. Harder materials are tested with the Shore D test. The test methodology is detailed in the standards DIN 53505, ISO 71619 and ASTM D 2240.

If you want to determine hardness by applying the IRHD method, there are four different procedures: IRHD N (normal hardness), IRHD H (high hardness) IRHD L (low hardness) and IRHD M (micro hardness). Typically, IRHD M is used, which is basically a version of the IRHD N method, but allows for the testing of thinner, smaller pieces.  The standard test piece must be between 1.5 mm and 2.5 mm thick. The advantage of the IRHD M process is that even finished parts can be tested. These are permitted be outside the range of 1.5 mm 2.5 mm, but due to the design of the test needle, cannot not be thinner than 1 mm. Please note that when testing finished parts, it is usually the case that the results are not the same as the test results of standard test pieces. The hardness range that can be measured with the IRHD M method is between 30 and 90. One measurement takes 30 seconds. The relevant standards are DIN ISO 48 and ASTM D 1415.

Because the procedures for Shore A and IRHD M hardness measurements are different, the results are not the same, nor are they correlated in relationship that would allow for conversion. Generally, however, the measurement results are relatively conform. At the harder end of the range, IRHD M tends to deliver higher values.

 

Comparison of Shore A and IRHD-M

 

Today, systems and devices have been developed that allow a Shore hardness measurement of finished parts. This has the advantage, among other things, that soft, finished, rubber parts below the IRHD M limit of 30 can also be measured.   

Tensile strength at break

In many applications the elasticity of the rubber parts is critical. A certain level of force is required to tear an elastomeric molded part. The force required to tear a standard test piece is known as ‘tensile strength at break’ or ‘tensile stress at yield’. The force is measured by tests performed on a tensile testing machine. Tensile strength at break (tensile stress at yield) is measured in N/mm² (= MPa) units. The procedure for the testing is detailed in the standards ISO 37, the almost identical DIN 53504, as well as the American ASTM D412.

Elongation at break

Elongation at break is the linear expansion of a test piece to its breaking point. It is described as the percentage increase in length before the original test piece breaks. An elongation at break of a material by 300 percent means that the test specimen was elongated to 300% of its length when it snapped. The standards relevant for tensile strength are ISO 37, DIN 53504 and ASTM D 412.

Modulus (Stress values)

In order to stretch a test piece, a force measured in N/mm² (MPa) is applied.  This force is called the modulus. The modulus is measured at different percentages of stretching, for example a 100% modulus or 200% modulus. Stress testing is regulated by the same standard as tensile strength at break and elongation at break testing.  

Tear strength

Tear strength describes the force opposing the further tearing of an already torn rubber piece. This is important, for example, for the elastomer membrane of a membrane pump. 

Determination of tear strength is always carried out using a tensile testing machine. There are two alternative test methods available differing only in terms of the geometry of the test pieces.

For the trouser test (DIN 53507 or ISO 34-1-method A) an elastomeric strip is nicked following the method given by the standard and tested to find the force required to propagate the tear. The result is recorded in N/mm. 

For the angle test (DIN 53515 or ISO 34-1 – method B), an angle elastomer test piece is nicked in the angle, pulled and then its tear resistance measured. The unit is also N/mm.

There are also tests on non-nicked angle test pieces and crescent-shaped test pieces. The relevant American standard is ASTM D 624.  

The results can only be compared if the tests are conducted following the same standard. The values ​​obtained from testing the test pieces then allow a comparison of a range of rubber compounds. However, it is important to note that no conclusions can be drawn as to how a rubber molded part would perform under operating conditions in in terms of its tear strength in comparison to the standard test piece.

Compression set

Permanent deformation is called compression set, i.e. the depth a rubber part or elastomer piece is compressed by after prolonged pressure compared to its original thickness. If a rubber part or a test piece is squeezed for a period of time, it will not rebound completely to its initial state once the pressure is removed. Permanent deformation (compression) results. Compression is given as a percentage. The result shows us the percentage of the elastomer specimen height lost by compression, i.e. does not return to the original state after the compression force is removed. Compression set is particularly important for seals. The thickness of the test piece is important, the compressed distance, the duration of the load and the prevailing temperature.

There are standards in place for the standardized testing process for compression set allowing the comparison of different raw rubbers and elastomer compounds. These uniform test methods are detailed, for example, in ISO 815, DIN 53517 and ASTM D 395.

As a rule, during testing a cylindrical specimen of height H0 is clamped and deformed by 25% of its height between two smooth-polished steel plates over an extended period of time. The height of the piece under pressure is referred to as H1. After the pressure is removed, the remaining height is given as H2.        

The formula for calculating the compression set is:

How to calculate the compression set

The prevailing temperature and duration of the test duration may differ from test to test. So, for instance, the test could take place at:

100 °C/24 hr.

  70 °C/24 hr.

175 °C/22 hr.

175 °C/24 hr.

200 °C/70 hr.

The test conditions are always specified in the material’s technical information. Comparisons can only be made for the results of rubber compounds from tests conducted under the same conditions.

Rebound resilience

The elasticity behavior of elastomers is measured as their rebound resilience. For this, a swinging pendulum hammer is dropped from an angle of 90°. The pendulum bounces onto the elastomer test piece. The extent of pendulum return after impact provides the basis for calculating the rebound resilience of the rubber.

The rebound resilience is expressed as a percentage. The higher the percentage, the more energy is returned by the rubber piece and the more the pendulum swings back. Low rebound resilience can also be used to evaluate the damping characteristics of a rubber compound.

The exact test procedures are detailed in the standards ISO 4662 or the virtually identical DIN 53512 as well as the American ASTM D 1054.

Elastomers with good mechanical properties

The different rubbers or elastomers available differ significantly in the mechanical characteristics described above. Even within a single class of rubber, compounds can be manufactured that produce significantly different values.

In addition, the mechanical properties of an elastomer are very dependent on the ambient temperature. When the ambient temperature rises, the mechanical properties of many types of rubber decrease significantly. At the same time, in addition to mechanical properties, chemical resistance is often a key factor when choosing a suitable elastomer. It is often the case that a material cannot be used despite its excellent mechanical properties, because it does not have the desired chemical resistance.

In other words, it is not possible to formulate general rules about rubbers. Nevertheless, we would like to provide some orientation in the following section.

 

Tensile strength at break: Materials with high tensile strength are natural rubber (NR), styrene-butadiene rubber (SBR), chloroprene rubber (CR) and nitrile rubber (NBR) or its heat-resistant variant HNBR (hydrogenated nitrile rubber). Polynorbornene rubber (PNR, better known under the trade name Norsorex ®) also produces good values.

 

Elongation at break: Butyl (IIR) and natural rubber (NR) have good elongation at break strength. Similarly to tensile strength at break, very good materials with regard to elongation are polynorbornene rubber (PNR), chloroprene rubber (CR), styrene-butadiene rubber (SBR) and nitrile rubber (NBR) or HNBR.

 

Tear strength: Natural rubber (NR) and chloroprene (CR) have good resistance to tearing propagation of an already torn rubber piece.

 

Compression set: Silicone (VMQ or liquid silicone LSR) and fluorine silicone (FVQM) have excellent compression sets. The same is true for fluoroelastomer FKM (Viton®) or FFKM (Kalrez®), as long as the temperature is not too low. Butyl (IRR) and especially EPDM (with peroxide crosslinking) have low and therefore good compression sets and are often used as materials for seals.

 

Rebound resilience: The two materials ethylene propylene diene rubber (EPDM) and natural rubber (NR) have a high rebound resilience. FKM (Viton®), butyl (IRR), fluorine silicone (FVQM), as well as PNR (Norsorex®) exhibit low rebound resilience. Thus, these materials are very interesting for applications in which good damping properties are required.

As indicated above, factors such as the usage temperature have a great influence on the mechanical properties of a rubber component or material. Bearing this in mind, natural rubber (NR) has very good values ​​with regard to tear resistance, elongation at break and tear propagation resistance, but can only resist heat to about 70 °C. Silicone (VMQ) or liquid silicone (LSR), however, keep their mechanical properties over a wide temperature range and therefore are widely used. EPDM is also a commonly used all-rounder material.

It is therefore useful to consult with specialists on the (pre) selection of an appropriate material for a specific application. It may be necessary to adapt the compound to your requirements. Before it is put to use in the final application, the finished part should first be tested to assess the suitability of the material.

If you have any questions or comments on this blog, or in future would like us to focus on a specific aspect of elastomers, feel free to contact us by email at info@hepako.de  

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