Durability testing

The durability of rubber materials is affected by a number of environmental factors in combination with the mechanical stresses that the application itself gives rise to.

Factors such as heat, sunlight, atmospheric oxygen, and moisture generally accelerate the degradation of rubber. Mechanical load, erosion, pollution, microorganisms and other special influences also occur depending on the application.

In addition to chemical reactions, the life of rubber materials is also affected by mechanical influences, such as abrasion and dynamic fatigue.

Thermal aging - ISO 188

Thermal aging takes place in cell ovens or special aging cabinets with a constant temperature, low air speed and an air change of 3-10 times per hour.

Usually, the change in hardness, breaking point, elongation at break and possible tear strength is measured by storing test pieces at an elevated temperature. The results are given in percentage change. The trial period is often one or more weeks.

Compression set - ISO 815

Settling is the residual deformation that occurs when rubber has been compressed and is then relieved and allowed to recover. The test is normally performed on standardized cylindrical specimens. The specimens are typically compressed 25% in a rig with smooth polished surfaces. The settling rig with samples is stored in a warming cabinet at an elevated temperature from 1 day to several weeks. The rig is removed from the cabinet and the samples are unloaded and allowed to recover before measurement.

Relaxation - ISO 3384, ISO 6914

Relaxation is the reduction in back pressure that can be measured in a rubber sample subjected to a constant deformation. The deformation can be either compression or elongation.

The relaxation measurement can be performed in two different ways. On the one hand, the measurement of initial force and force after different times can take place at testing temperature, on the other hand, the force measurements can take place at room temperature, while the samples in between are stored at an elevated temperature. In some cases, the relaxation measurement is also done in liquid, eg oil.

Lifespan assessment - ISO 11346

Oftast vill man få någon form av livslängd som resultat av ett åldringsprov. 

En väl beprövad metod för att uppskatta livslängden för ett material är att använda ett Arrheniusdiagram. För att kunna rita ett Arrheniusdiagram, behöver man först bestämma livslängden för ett material vid minst tre temperaturer. De egenskaper man använder för att bestämma livslängden är ofta brottöjning, brottgräns, sättning och relaxation.

Den eller de egenskaper man väljer, samt vid vilken nivå materialets funktion tar slut beror på i vilken tillämpning materialet skall användas. Ofta används tiden tills egenskapen har gått ned till 50% av ursprunglig nivå. Provningstiderna kan bli långa, särskilt vid den lägsta temperaturen, upp till ett år är inte ovanligt.

Liquid resistance - ISO 1817

When testing for liquid resistance, the effect of different liquids on rubber materials is determined. The most common liquids are:
• Oils
• The fuels
• Water
• Chemicals

The testing takes place by immersing rubber samples in liquid, usually at an elevated temperature for 72 hours. In some cases, the testing takes place over longer periods of time, equivalent to several weeks.

The properties tested are usually weight and volume change as well as hardness change. Sometimes a change in strength is also tested. The result is given as a percentage change.

Vädersimulering - ISO 4665

Weather simulation is the name of the test when it takes place indoors and it is carried out in special weather aging devices. Sun, temperature and rain are simulated here. A typical test cycle has a light intensity of 1000 W/m2, 55°C black plate temperature and a rain cycle of 18 min rain and 102 min dry.

Properties that are often investigated are color changes and changes in breaking point and breaking elongation.

Abrasion test - ISO 4649

The most common method of testing abrasion on a rubber material is to let a loaded test piece slide against an emery cloth covered roller. After an abrasion distance of 40 m, the test piece's weight loss is determined, which is converted into a volume loss.

Ozone test - ISO 1431

Testing of rubber material's ozone resistance is carried out in special ozone cabinets. As rubber is more easily attacked by ozone when it is stretched, the test takes place in a rig with the test specimens stretched from 5 to 80%.

The samples are checked at certain time intervals and the time to the first crack at each elongation is noted.

Dynamic fatigue - ISO 132, ISO 4666, ISO 6943

Fatigue tests on materials are often performed as bending or tensile fatigue in a De Mattia machine. In this test, specimens are bent or stretched until failure occurs.