Hardness testing

One of the most important parameters in rubber technology is its hardness. For all materials used at Freudenberg Process Seals, the nominal hardness is given in the material name before the base elastomer (Example: 70 EPDM 291). The hardness is determined using various standardized procedures. These procedures always measure the distance that a specified test piece subject to a defined force can penetrate the rubber material.

• Hardness testing according to Shore A and DIN 53 505 or DIN EN ISO 868 respectively on elastomers
• Hardness testing according to IRHD and DIN ISO 48 on seals

On this basis, Freudenberg Process Seals provides you with material and sealing solutions with the degree of hardness optimally suited to your application.

Volume change measurement

Seals come into contact with all sorts of media – liquids and gases. All of these media influence the elastomer materials in different ways. A distinction is made between chemically active and physically active media. Chemically active media react with the elastomer mixtures and irreversibly change their properties which means the material loses strength, it hardens or softens.

Physically active media trigger two processes at the same time:

• a. absorption of the medium by the elastomer and
• b. extraction of soluble mixture components (e.g. softeners) from the elastomer.

These swelling processes are generally reversible. The size of the volume change thereby depends on the following factors:

• Type of reacting medium
• Composition of elastomer compound
• Temperature
• Geometric shape (thickness) of seal
• Stress condition of the seal

All aging processes under the influence of media are connected to changes of the physical properties such as volume, hardness, elasticity, tensile strength and elongation at break. This is why Freudenberg Process Seals thoroughly tests its materials in accordance with DIN ISO 1817, to ensure that we are giving you reliable statements on the stability of elastomer compounds in the media used.

Tear resistance

Tear resistance is a measure of the sensitivity of elastomers against propagation of cut and tear damage. The most common test to determine the tear resistance is the strip in accordance with DIN ISO 34-1 (DIN 53 507).

Another test method is the angle test in accordance with DIN ISO 34-1 (DIN 53 515). The values measures do not have to correlate with the elongation value determined in the tensile test. As the results of tear resistance depend greatly on the special test conditions and especially on the shape of the sample, Freudenberg Process Seals always states the test method and sample shape when listing the testing results.

Compression set measurement/elasticity

The compression set is an important parameter for evaluating the properties of a sealing material. This parameter describes the ability of a material to generate sufficient restoring force on the sealing surface, even after lengthy operating time in contact with the respective media, thus ensuring the tightness of the seal.

In our company-internal testing facilities, the compression set of our materials is tested comprehensively in accordance with DIN ISO 815 and is identified as a meaningful parameter for your material selection.

Please note:

Compression set values are only comparable if the following parameters are the same:

• Compression (standard 25%)
• Duration of compression (standard 24 or 72 hours)
• Temperature during compression (depending on the elastomer being tested)
• Medium which test is carried out in (standard: air)
• Form of test object

Density measurement

The density of vulcanized rubber is a specific parameter. With a simple measurement of the density that is easy to carry out, initial conclusions can be drawn on the compound used.

When producing a compound, the density measurement can be used as evidence of whether all components of the mixture have been added correctly, with the aim of ensuring our material solutions are of consistently high quality standards.

As a general rule, the density is measured with the buoyancy method. According to the „Archimedean Principle“ a body will exhibit a buoyancy force when submerged in a liquid. From this, by recording the volume of the test object, its specific weight can be calculated.

Tensile test

The tensile test is used to determine the tensile strength, ultimate elongation or elongation at break and the elongation values of elastomer materials. The test objects to be used for this are preferably standard bars. However, it is also possible to used standardized rings with a square cross section or O-rings. As a general rule, it is not possible to carry out tensile tests on other seals, as the shape factor has too greater influence on the measurement results.

In every tensile test, 3 values are determined:

The tensile strength is the force applied at the moment the test object fails with reference to the initial cross section of the test object before the test

The modulus at 100% elongation is defined as the force required for a certain elongation with reference to the initial cross section

The elongation at break is the percentage ratio between the elongation attained at the time of breaking and the initial length

The three parameters tensile strength, modulus at 100% elongation and elongation at break are given on all Freudenberg Process Seals data sheets for all materials. Values are affected by the degree of vulcanization, the specific compound mixture, filler materials (carbon-black, softening agents, anti-aging chemicals, etc.), storage conditions and product age.