ISO 11346:2014 pdf download – Rubber, vulcanized or thermoplastic—Estimation of life-time and maximum temperature of use
ISO 11346:2014 pdf download – Rubber, vulcanized or thermoplastic—Estimation of life-time and maximum temperature of use.
It might be necessary, In addition, to carry out trial runs to determine the exposure tern peratures and the number of test points required at each temperature.
NOTE While It Is possible to reduce the number of test pieces for each test from that given In the test method standard to economize on testing, extrapolation of the results requires the best possible precision and in some circumstances It might be idvlsable to Increase the number of test pieces.
8 Exposure temperatures
Selection of the exposure temperatures involves knowing beforehand the approximate ageing characteristics of the material under test. With no previous knowledge olthe material, exploratory tests will have to be carried out. This Information will assist in selecting the exposure temperatures best suited for the evaluation of the material.
Test pieces shall be aged at not fewer than three temperatures covering a range adequate to establish the life-time estimation by extrapolation with the required degree of accuracy. The lowest exposure temperature shall be chosen so that the time taken to reach the threshold value is at least 1 000 h. Likewise, the highest temperature shall be chosen so that the time taken to reach the threshold value is not shorter than 100 h. The temperatures used should preferably be standard test temperatures taken from ISO 23529.
NOTE To obtain an estimate otihe life-time with adequate precision will often require more than three test temperatures.
9 Exposure times
The properties chosen to measure the reaction rate shall be tested alter each of at least five different exposure times at each temperature, but more exposure times will normally be needed ii the shape of the property/time curve is to be established.
The exposure times shall be such as to enable the property chosen to measure the reaction rate to be characterized adequately. For thermo-oxidative ageing. a linear progression will be satisfactory in many cases. For physical relaxation, a logarithmic progression would be more appropriate. The requirements concerning minimum exposure times in C1use U shall be respected.
Measure the selected properties using unaged sets of test pieces conditioned as required by the relevant test method standards.
Place the required number of test pieces in each of the ovens maintained at the selected temperatures.
At the end of each exposure time, condition the test pieces to be examined as required by the relevant test method standard and measure the selected properties.
Continue this procedure until the required number of sets of test pieces have been tested.
There will be different reaction rates, K(T), corresponding to different temperatures, 7’.
For each exposure temperature, plot the results for each property against time. For each plot, determine the reaction rate.
Often, a convenient measure of the reaction rate Is the time for the property to reach the threshold value, determined by interpolation as illustrated in hgurei.
NOTE The Ideal measure of the reaction rate would be the slope of the property-rime plot, but this Is rarely linear li.e. F(t) is rarely linearj. It might be possible to fit a convenient relationship to the data or to obtain a convenient plot by plotting on a logarithmic scale.
Plot the logarithm of the reaction rate (e.g. time to threshold value) against the reciprocal of the absolute temperature and construct a best-fit straight line through the points by established statistical methods. An example is given In Figure 2. If the data deviates from a straight line, this indicates that different reactions are taking place at the different temperatures and extrapolation of the data are invalid.
The life-tIme at a given temperature or service temperature (25 °C In FIgure 2) is estimated by extrapolation of the line to that temperature. The reaction rate at that temperature will give the time to reach the threshold value.
The maximum temperature of use is estimated by extrapolation of the line to a specified reaction rate or time to reach the threshold value. A time of 20 000 h is commonly used when establishing a general maximum temperature of use.
The activation energy is obtained by dividing the slope of the line by R, the gas constant.