Do you want to know the estimated lifetime of your rubber materials, or compare the aging properties between different rubber materials? An effective way is to run tests with stress relaxation according to ISO 3384.
How does stress relaxation tests work?
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In stress relaxation testing, a sample is compressed between two plates, usually 25 %. In the test rig, the force and temperature are measured, which are registered continuously. The test usually takes place in air at a suitable aging temperature for the material. It can also be performed in elongation and in liquid.
When compressing a rubber material, you measure the force and how it decreases over time. In the beginning, there is usually a rapid decline in force and then it turns into a slower decline. The initial decrease in force comes from physical effects when the molecular chains and fillers find new resting positions during compression. The slower decline in force over longer periods is caused by breaks in the molecular chains and is a measure of aging.
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The decrease in force during stress relaxation testing correlates well with the reduction of elongation at break in an aged tensile test. But with stress relaxation testing, you can continuously follow the decrease in force.
How do you do an estimation of lifetime?
An estimation of lifetime of a rubber material is performed according to ISO 11346.
The standard states that one should test an important property, at least at 3 temperatures, until the property has decreased, so much so that the function of a material is over, at the respective temperature. The limit is usually set at 50 % decline. In this way you get a time for each temperature. Measuring aging through stress relaxation testing is a practical way to accurately obtain the time-temperature points.
These points are plotted in an Arrhenius plot, which is named after our first Swedish Nobel laureate, who received the chemistry prize in 1903. He has formulated an equation of how the rate of a chemical reaction depends on temperature.
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The Arrhenius plot has the time in log scale on the Y-axis and the temperature as 1/K, the temperature as 1 through the temperature in degrees Kelvin. In such a plot one can make a linear extrapolation to lower temperatures. As a rule of thumb, it is usually said that a chemical reaction goes twice as fast at a temperature increase of 10 °C = factor 2. But by making a lifetime estimation according to the ISO 11346 standard, one finds that this factor varies greatly.
Elastocon performs contract testing, and estimation of lifetime is one of our specialities,.
