The standard “ISO 11346 Rubber, vulcanized or thermoplastic - Estimation of service life and maximum temperature” published by the International Organization for Standardization (ISO) describes the principles and procedures for estimating the thermal resistance of rubbers resulting from exposure to high temperatures for long periods.
Two approaches are specified in this standard: — one using the Arrhenius relationship; — the other using the WLF equation. In this International Standard, the prediction of thermal resistance is based solely on the change in selected properties resulting from periods of exposure to elevated temperatures. The various properties of rubbers change at different rates during thermal ageing, so comparisons between different rubbers can only be made using the same properties.
The rate of a chemical reaction normally increases with increasing temperature. By exposing test pieces to a series of high temperatures, the relationship between the rate of reaction of the degradation mechanisms and temperature can be deduced. Estimates can then be made by extrapolating the degree of degradation after a certain time at a certain temperature or the time taken to reach a certain degree of degradation at a certain temperature.
The reaction rate and temperature relationship can often be expressed by the Arrhenius equation. The reaction rate at a given temperature is obtained from the change in the value of a selected property with the exposure time at that temperature. The reaction rate can be expressed by the time it takes to reach a certain degree of degradation (the threshold value), and this may be the only measure to use if the property/temperature relationship is complex. The Arrhenius approximation is only suitable for chemical degradation reactions and may give incorrect results in tests where physical (viscoelastic) changes cannot be easily separated from chemical changes.
An alternative approach for rubbers is to use the Williams Landel Ferry equation. This equation performs a time/temperature transformation and makes no assumptions about the form of the property/time relationship at any given temperature. Therefore, in principle, it can be applied to any physical property, including clustering and relaxation, or to situations where the property/time relationship is complex.
This standard is based on ISO 2578 and IEC 60216 standards.
For the purposes of this standard, the following terms and definitions apply:
In principle, at a selected test temperature, the changes in the numerical value of a selected property, for example a mechanical or viscoelastic property, are determined as a function of time. Testing is continued until the relevant threshold value of the property in question is exceeded. Further testing is carried out at at least two other temperatures. For the Arrhenius procedure, the measurements of the reaction rates obtained are plotted logarithmically as a function of the inverse of temperature and the resulting straight line is extrapolated or interpolated back to the temperature of use. For the Williams Landel Ferry procedure, shift constants are calculated and used to transfer the property/time relationship to the temperature of use.
The tests selected should preferably relate to properties that are likely to be of practical importance. Wherever possible, test methods specified in international standards should be used. Hardness and tensile stress-strain properties are commonly used for general assessments, but stress relaxation or compression stabilization are required, for example, for sealing applications. For ageing of test pieces, a circulating air oven meeting the requirements of the standard “ISO 188 Rubber, vulcanized or thermoplastics - Accelerated ageing and heat resistance tests” should be used.
The threshold value should be selected as the maximum acceptable degree of deterioration for the property and end use being tested. Typically, 502 percent of the initial value of the property is selected. Testing should normally be continued for a sufficiently long period until the threshold value is reached. Although it is possible to extrapolate to higher degrees of deterioration, this is not recommended.
The dimensions and preparation method of the test pieces shall be in accordance with the relevant test method standard. The minimum total number of test pieces required for a destructive test method depends on:
The minimum number of test pieces required is given in the formula.
If problems occur after several weeks, months or years of aging, or if an additional exposure temperature is needed to increase sensitivity, it is recommended that additional test pieces be aged at each temperature. The minimum number of test pieces required for a non-destructive test method is normally given in the formula.
When measuring compression set, tension set and relaxation, tests are preferably performed on the same test pieces at different times to reduce the number of test pieces required. This also reduces variation in test results. In addition, trial runs may be required to determine the exposure temperatures and number of test points required at each temperature. Although it is possible to reduce the number of test pieces given in the test method standard to save testing, extrapolation of results requires the best possible precision and in some cases it may be advisable to increase the number of test pieces.
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