Accelerated Ageing Tests

The correlation of accelerated tests with long age storage has been discussed by Thompson (32). 

The most common test used to study the oxidation resistance of mbber compounds involves the accelerated aging of tensile dumbbell samples in an oxygen-containing atmosphere. Brown et al. [22] recently reviewed long-term and accelerated aging test procedures. The ASTM practices (D 454 (09.01) D 865 (09.01) D 2000 (09.01, 09.02) D3137 (09.01) D 572 (09.01) D 3676 (09.02) D 380 (09.02)) for these tests clearly state that they are accelerated tests and should be used for relative comparisons of various compounds and that the tests may not correlate to actual long-term... 

The practical conclusion is that if comparisons are to be made between accelerated and natural ageing results or attempts made to predict degradation at lower temperatures from accelerated tests, the existence of degradation gradients is likely to have significant effect. Also, predictions made from tests on thin test pieces may be misleading if applied to thick products. 

Heat ageing tests are carried out for two distinct purposes. They can be intended to measure changes in the plastic at the elevated service temperature, or else as an accelerated test to estimate the degree of change which would take place over longer times at lower temperatures. Here, we are concerned with the second purpose. 

Tear strength is only applicable to flexible materials and is very little used to monitor ageing simply because tensile strength will serve perfectly well. There are circumstances where compression stress-strain properties would be relevant but the relatively bulky test pieces will be subject to the limitation of oxygen diffusion in any accelerated tests and changes can probably be estimated from tensile measurements. Similarly, shear stress-strain is very rarely used for monitoring ageing. 

Accelerated tests usually require the application of a higher electrical stress (voltage) coupled sometimes with an increase in temperature as a second accelerating factor. As in all accelerated testing, care must be taken to ensure that this does not introduce ageing mechanisms different to those that occur in service. The most common relation used to predict insulation life in the presence of partial discharges is the power law ... 

Finally, the rate of change in the critical property must be measured relative to expected environments of different severity and time intervals. If measurement cannot be made at the service temperature in the time that is available (as is generally the case), then accelerated tests may be used at elevated temperature or increased frequency. However, it is extremely important that care be taken to match the accelerated test conditions to the service conditions in as realistic a way as possible. For example, if accelerated aging by elevated temperatures is being used, the temperature must not be so high as to begin a degradation mechanism that would not normally be seen in service. 

Environmental Tests. It is desirable to know the rate at which an adhesive bond will lose strength due to environmental factors in service. Strength values determined by short-term tests do not give an adequate indication of an adhesive s performance during continuous environmental exposure. Laboratory-controlled aging tests seldom last longer than a few thousand hours. To predict the permanence of an adhesive over a 20-year product life requires accelerated test procedures and extrapolation of data. Such extrapolations are extremely risky because the causes of adhesive bond deterioration are complex (see Sec. 15.2.2). Unfortunately no universal method has yet been established to estimate bond life accurately from short-term aging data. 

The long-term behaviour of polymers is fundamentally different from the short time behaviour the first cannot be explained from the latter, if the ageing time is neglected. So the value of "accelerated tests" is dubious. Proposition 10 is illustrated by Fig. 13.48. 

The series of observations outlined above have a number of implications for any program directed at preserving the quality of cellulosic materials. These can be categorized broadly as relating to three types of objectives (1) prevention of further deterioration, (2) recovery of some of the properties already lost through aging, and (3) the interpretation of accelerated testing procedures. 

Accelerated Aging and Testing of Paper. Test samples were subjected to accelerated aging under dry (100 °C) as well as humid (90 °C, 50 rh) conditions. At the end of the aging period, the samples were conditioned at 72 °F and 50 rh... 

Two different approaches for lifetime prediction are presented. The underlying lifetime limiting processes have been identified in two cases. Mathematical expressions of chemical/physical relevance were used for the lifetime predictions for PE hot-water pipes and cables insulated with plasticized PVC. Accelerated testing, extrapolation and validation of the extrapolation by assessment of the remaining lifetime of objects aged during service conditions for 25 years were successfully applied to cables insulated with chlorosulfonated polyethylene. Polyolefin pipes exposed to chlorinated water showed a very complex deterioration scenario and it was only possible to find a method suitable for predicting the time for the depletion of the stabilizer system. 

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