Rubber Testing & control Durability testing
Durability testing
The resistance of rubber materials is affected by a variety of environmental factors combined with the mechanical stresses arising from the application itself.
Factors such as heat, sunlight, oxygen, and moisture generally accelerate the degradation of rubber. Mechanical stress, erosion, contaminants, microorganisms and other specific influences also occur depending on the application.
The lifespan of rubber materials is affected not only by chemical reactions but also by mechanical influences, such as abrasion and dynamic fatigue.
Residual strain
If a rubber sample is held stretched for a period of time and then released, it does not return completely to its original length. This is called permanent set.
Under constant stretching, the test bodies are stretched and stored in a stretched state, which usually occurs at elevated temperatures for 1 day to several weeks. After the samples are removed from the oven, they are allowed to cool in a stretched state, after which they are unloaded and allowed to recover before measurement.
The standard for testing permanent set of rubber is ISO 2285.
Heat aging
Heat aging occurs in cell furnaces or special aging cabinets with constant temperature, low air speed, and an air exchange rate of 3-10 times per hour.
Typically, the change in hardness, tensile strength, elongation at break, and potential tear strength is measured by storing test specimens at elevated temperatures. The results are given as a percentage change. The test duration is often one or more weeks.
The standard for testing heat aging of rubber is ISO 188.
Compression set
Settlement is the permanent deformation that occurs when rubber is compressed and then unloaded and allowed to recover. The test is normally performed on standardized cylindrical test specimens. The test specimens are typically compressed in a testing rig with smooth, polished surfaces. The set testing rig with the specimens is stored in a heating cabinet at an elevated temperature for a period ranging from 1 day to several weeks. The rig is removed from the cabinet, and the specimens are unloaded and allowed to recover before measurement.
The standard for testing compression set in rubber is ISO 815.
Relaxation
Relaxation is the decrease in back pressure that can be measured in a rubber sample subjected to a constant deformation. The deformation can be either compression or stretching.
Relaxation measurements can be performed in two different ways. Firstly, the measurement of the initial force and the force after a certain time can take place at the test temperature. Secondly, the force measurements can take place at room temperature, while the samples are stored at an elevated temperature in between. In some cases, the relaxation measurement is also carried out in a liquid, e.g., oil.
Standards for testing rubber relaxation are ISO 3384 and 6914.
Lifespan
ISO 11346
Most often, one wants to get some form of lifespan as a result of an aging test.
A well-proven method for estimating the lifespan of a material is to use an Arrhenius diagram. In order to draw an Arrhenius diagram, the lifespan of a material must first be determined at at least three temperatures. The properties used to determine the lifespan are often elongation at break, tensile strength, settlement, and relaxation.
The property or properties selected, as well as the temperature at which the material’s performance ceases, depend on the intended application. Often, the test is conducted until the property has declined to 50% of its original level. Test durations can be lengthy, especially at the lowest temperature; a duration of up to one year is not uncommon.
The standard for assessing the service life of rubber is ISO 11346.
Liquid resistance
ISO 1817
During testing for liquid resistance, the effects of various liquids on rubber materials are determined. The most common liquids are:
Oils
• Fuels
Water
Chemicals
The test is carried out by immersing rubber samples in liquid, usually at an elevated temperature for 72 hours. In some cases, the test is carried out for longer periods, corresponding to several weeks.
The properties usually tested are weight and volume change, and hardness change. Sometimes change in strength is also tested. The result is stated as a percentage change.
Weather simulation
ISO 4665
Weather simulation is the term for testing when it occurs indoors and is performed in special weather aging apparatuses. Here, sun, temperature, and rain are simulated. A common test cycle has a light intensity of 1,000 W/m2, a black panel temperature of 55°C, and a rain cycle of 18 minutes of rain and 102 minutes of dry.
Properties often examined are color changes and changes in fracture strength and elongation at break.
Abrasion test
ISO 4649
The most common method for testing abrasion on a rubber material is to slide a loaded test piece against an emery cloth-covered roller. After an abrasion distance of 40 m, the test piece's weight loss is determined and converted into a volume loss.
Ozone resistance
ISO 1431
Tests of rubber materials’ ozone resistance are conducted in special ozone chambers. Since rubber is more susceptible to ozone attack when stretched, the tests are performed in a testing rig with the test specimens stretched from 5 to 80% of their original length.
The specimen is checked at certain time intervals, and the time to the first crack at each elongation is recorded.