Residual compressive deformation in elastomers: characteristic value, testing and significance for component design
Elastomers They can absorb large deformations and return to their original shape approximately after relief has been applied. However, if an elastomer is compressed over a prolonged period, a residual deformation remains after the load is removed. The pressure deformation residual or Compression Set in English describes the extent of this permanent deformation under specified test conditions.
The residual compressive deformation (shortly DVR) is primarily used to assess materials for structural components subjected to compressive stress. This includes: O-rings, Flat seals, Membranes, Elastic supports and vibration elements. For technical evaluation, it is important to know that the residual compressive deformation is not a universal material value, but depends on temperature, load duration, geometry of the specimen body, and recovery conditions.
1. What is a pressure deformation remnant?
The residual compressive deformation is a characteristic for describing elastomers that indicates the remaining deformation after a load compared to the original size and is expressed in percent. This residual deformation depends, in particular, on the duration of the load and the temperature. DVR values (short for residual compressive deformation) are therefore always given in combination with temperature and time.
The residual compressive deformation results from two overlapping effects that are attributable to the structure of the polymer chains and their bonding. Elastomers They consist of long, highly interconnected polymer chains that exist as a bundle. Under short-term loading and room temperature, viscoelastic effects predominate, i.e., partial elastic, reversible deformations caused by stretching the polymer chains (elastic component) and a sliding of the chains, combined with a redistribution of additives and fillers (viscous component). With increasing temperature and longer pressure loading, chemical processes become more important. These include re-crosslinking, chain splitting, oxidation, and changes at the crosslinking sites.
In addition to the compressive deformation residual, there are other variables that describe a different process in each case:
- Crawling refers to the deformation that increases over time under constant tension.
- Stress relaxation refers to the decreasing tension or force over time at a constant deformation state.
- Compression set refers to the shape change remaining after relief, relative to the previously imprinted deformation.
In particular, the stress relaxation is relevant for the design and material selection of static seals, since these are usually pressed under constant load. The stress relaxation is the measure of the time-dependent loss of the contact pressure and thus the sealing effect. In contrast, the pressure deformation residual measures the extent of the remaining deformation. Therefore, both values describe the same effect, but measure different quantities (force and deformation).
Three values are required for the assessment of the residual deformation in the printed part:
- h0: original height of the unladen test body
- h1Height in the compressed state, usually specified by a spacer piece
- h2: Height of the test specimen after loading and recovery
A value of 0 % corresponds to a complete return to the starting height h0. Conversely, a DVR of 100 would be % a completely irreversible deformation of the test specimen after the relief. Thus, a lower value under identical test conditions indicates a stronger geometric shift.
Figure 1: Example of a test for pressure deformation
Example: The cylindrical test specimen has a height of 10.0 mm before testing. During the loading phase, the test device limits its height to 7.5 mm. After the load is released and the required recovery time has elapsed, the measured height is 9.5 mm.
Of the deformation under load of 2.5 mm, 0.5 mm remains as residual deformation. The determined residual compressive deformation is 20 %. Since this value also depends on the test conditions, it is only meaningful in conjunction with temperature and time of application, e.g. 20 % after 24 hours at 70 °C.
2nd test of the pressure deformation residue
A cylindrical test specimen is compressed between two parallel pressure plates, and the distance between the plates is limited by metallic spacers. If the plates are attached to the spacers, the test height is reached.
In the DIN ISO 815-1:2022-04 Disc-shaped, cylindrical test specimens are prescribed in two different sizes:
| Sample body type | Diameter | Starting height |
| Type A | 29.0 ± 0.5 mm | 12.5 ± 0.5 mm |
| Type B | 13.0 ± 0.5 mm | 6.3 ± 0.3 mm |
The test specimens should have flat faces and be positioned parallel to each other, as they would otherwise be subjected to uneven loading when the device is closed. The test specimens are either manufactured in a specially designed tool or cut from an existing component or test plate. For thin test specimens or complex shaped parts where the production of a standard cylinder is not possible, the test can also be performed using different test specimens. However, these tests should be labeled as component tests and cannot be easily compared with the material data sheet.
Figure 2: Testing of the pressure deformation residue according to DIN ISO 815-1 – 2022-04
The practical process can be divided into six steps:
1. Conditioning the test specimens: The test specimens are stored under specified conditions after manufacture and, if necessary, after tempering. This limits the influence of an immediately preceding thermal or mechanical load.
Determine the 2. starting height: The height (h0) is measured before loading with a thickness gauge and uniquely assigned to each test specimen.
3. Tightening device: Test specimens and spacers are arranged between the pressure plates. Depending on the hardness of the test specimen, a different distance is selected to avoid overloading particularly hard materials:
- Below 80 IRHD: 25 % deformation
- 80-89 IRHD: 15 % Deformation
- Over 90 IRHD: 10 % deformation
The plates are applied evenly until they completely rest on the spacer pieces and the height h1 has been achieved.
4. Store at a fixed temperature: The closed device remains in place for the required duration in ambient or elevated temperatures. Often, short-term tests of over 24 hours are performed. Longer tests more closely involve aging processes.
5. Discharge: After the testing time has expired, the device is opened according to the selected procedure. The time and temperature of the relief are set because the elastomer immediately starts resetting after opening.
6. Rest and measurement: After the prescribed recovery period, the h value is again set to its original value.2 using the same measurement method as h0 determined and calculated from the three determined heights of the pressure deformation residual.
3. Influencing factors on the residual compressive deformation
The residual compressive deformation is influenced by several factors such as the selection of the base polymer, additives, ambient temperatures, load duration, ambient media, and the geometry of the component. Therefore, measurements from laboratories (at a defined temperature and load duration) can only be applied to real components to a limited extent.
Polymer structure
The chemical structure of the base polymer (e.g. EPDM, NBR, NR) affects chain mobility, temperature resistance, aging behavior, chemical resistance, and also how far the network can recover after prolonged compression. Since two mixtures of the same base polymer contain completely different fillers, crosslinkers, additives, etc., the DVR can also vary significantly. Therefore, the polymer family only allows for a limited statement regarding the DVR.
Networking system and density
The crosslinking system determines the type of bond that forms between the polymer chains. Sulfur-crosslinked and peroxidically crosslinked compounds differ, among other things, in their thermal stability and the mobility of the crosslinking sites. An increasing crosslinking density can increase the restoring force, but at the same time it can alter the elasticity and the dynamic behavior. Therefore, it is not a matter of having the crosslinking density as high as possible, but of tailoring the crosslinking to the intended application, the temperature range, and the type of load.
Additives
Fillers, such as carbon black or silica, affect the stiffness, strength, heat development, and the mobility of the polymer chains. Softeners reduce the hardness of the elastomer and change the low-temperature behavior; however, they can migrate from the component upon exposure to heat or contact with media, resulting in a change in volume, a change in hardness, and a change in return behavior. Other substances such as anti-aging agents, processing aids, and resin additives also affect the properties of the compound and thus the DVR.
Temperature and duration of stress
As the temperature increases, molecular rearrangements and chemical reactions occur more quickly. Over longer periods of loading, these effects also have a longer effect. Therefore, the residual compressive stress often increases with temperature and test duration, and is often very different for different mixtures.
Media and chemical contact
Oxygen and heat can, depending on the compound, polymer chains and cross-linking sites, alter and thus weaken them. Ozone primarily attacks elastomers with double bonds in the main chain, especially when the surface is under tensile stress. With low media resistance, liquid or gaseous media can penetrate the material, extract components, react chemically, and cause swelling. Therefore, tests of the DVR in an application-specific environment are required for rubber molded parts in contact with media.
Geometry
Since elastomers deflect laterally under pressure, the ratio of diameter to height, the size of the free lateral surfaces, and the friction on the pressure plates affect the deformation. Therefore, different specimen geometries can yield different DVR values despite identical materials and the same nominal degree of deformation.
4. Importance for component design
The residual compressive deformation describes the permanent change in height of an elastomer after a defined pressure load. Since it is known what factor a component is expected to be compressed by, a geometric return reserve is used. The remaining compressive strength does not result directly from this, because the contact pressure decreases additionally during the installation period due to stress relaxation.
From DVR to remaining pressing
The initial compression of a sealing element with the free height (h_0) and the installation height (h_1) is:
After relief and the prescribed recovery period have been applied, the height of the measured residual compressive deformation (DVR) results as follows:
If this subtracted height is mentally placed back into the same installation space, the geometric overlap remains:
A sealing element with a 5.0 mm initial height is installed at 4.0 mm. The initial compression is therefore 1.0 mm or 20 %. With a DVR of 25, the material % returns to 4.75 mm after testing. Compared to the unchanged installation gap, there remains a calculated overlap of 0.75 mm or 15 % of the original sealing height.
This calculation exclusively evaluates the geometry based on the normalized recovery time. It does not provide remaining contact pressure. The relationship between the path of compression and the force is not linear and changes with temperature, aging, medium, and stress relaxation.
Determination of the pressure deformation residue in 5 steps
When designing the DVR, the installation situation plays a crucial role. Therefore, the ambient conditions, the geometry of the adjacent components, and the requirements for possible assembly determine the required DVR value:
Defining the operating environment 1. Chemical contact, pressure differences, the highest and lowest operating temperatures, the duration of the load, and relative motion all determine the boundary conditions for the base polymer.
2. Calculate the compression over the tolerance chain: The minimum compression consists of the smallest seal height and the largest installation gap. Analogously, the largest compression results from the largest seal height and the smallest installation gap. The minimum compression forms the basis for calculating the seal. The maximum compression determines the mounting force, material stress, and the required free space.
3. DVR under suitable conditions: The testing temperature, testing duration, degree of deformation, test specimen and relief method must meet the operational conditions. A DVR at room temperature does not provide an indication of the long-term stress at 150 °C. Likewise, a 24-hour test does not replace an indication of an operating life of several years.
4. Determine the geometric reserve: The remaining overlap is calculated using the DVR and the smallest initial press fit. This makes it clear how greatly dimensional tolerances and remaining set-up together reduce the geometric reserve.
5. Optional: Determine the reserve power and perform component testing: The contact force is measured using a force-deformation curve, from which the stress relaxation is also determined. If the component in question is a component critical to failure, a further long-term test can be performed under serial conditions and in the intended operational environment.
