Prediction of Natural Ageing

Extrapolation of the accelerated results to longer times at lower temperatures was attempted by two approaches—the Arrhenius relation and the WLF equation—and compression set results analysed using a dose rate equation. These techniques are outlined below and are discussed in more detail in a guide to the assessment of the useful life of rubbers [8]. 

In this report, the predictions are made for change at 23 °C and 40 °C to equate to long-term natural ageing under temperate and hot dry conditions. For some applications, particularly with the more heat resistant polymers, it may be of greater interest to extrapolate to more elevated temperatures that are representative of service conditions. 

The reaction rate/temperature relationship can often be represented by the Arrhenius equation  

To enable Arrhenius plots to be constructed, a single measure to represent change with time at each temperature is derived from the graphs. Ideally, the single measure would describe the 

For the remaining compounds, the times to reach a convenient, but arbitrary, percentage change were used, which varied from compound to compound and from property to property. 

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