Use and Misuse of Accelerated Aging Tests
Authors: Buist J. M. , Welding G. N. 
Published: February 2011

Abstract
1. The general term, aging, covers many different changes. When carrying out both natural and accelerated aging tests, it is essential to distinguish clearly between these changes and to test them separately if confusion is to be avoided. The chief agents responsible for aging are light, ozone, and oxygen, with or without heat. Each one of these can cause several types of degradation, the predominant type depending on the rubber and the conditions. 2. It is important to appreciate the difference between crazing and atmospheric cracking. Crazing is caused by the action of oxygen stimulated by light, whereas atmospheric cracking is due to attack by ozone on stretched samples. 3. It is suggested that crazing is produced in Williams' inelastic skin, and that the light-catalyzed oxidation which causes these effects is the same as that studied by Morgan and Naunton. 4. The only types of aging which require light are skin formation, crazing, and discoloration, and it is misleading to use light to test other types of aging. In the present state of knowledge the routine use of ultraviolet lamps should be abandoned until further work is done on the effect of different wave lengths. Instead, an enclosed arc should be used with a spectrum as close as possible to that of sunlight. The results of many light tests have been made useless by the omission of certain obvious precautions. For example, radiation of too great an intensity should be avoided, and the samples should not be stretched. 5. It is futile trying to use ovens, bombs or ultraviolet lamps as accelerated tests for atmospheric cracking, and it is impossible to correlate these tests with outdoor aging of stretched samples. The only accelerated test, at present, is the exposure of stretched samples to ozone. 6. When evaluating atmospheric cracking, exposure to ozone should be in darkness so as to reduce skin formation and crazing ; similarly, in outdoor exposure tests the stretched samples should be shielded from direct light. 7. In oven and bomb tests at high temperatures, the absorption of oxygen is so rapid in relation to the rate of diffusion that it can penetrate only to a small depth before being absorbed. Samples are, therefore, heterogeneous and there are several resulting deficiencies in the measurements of physical properties. Ovens and especially bombs, operated at lower temperatures, should give more accurate information. 8. Oxygen bomb tests, apart from requiring less time than those in an oven at the same temperature, are preferable as the samples are less heterogeneous at a given stage of degradation. 9. In oven or bomb tests, comparisons should be made only between samples of similar shape, and standard dimensions should be specified for general use. 10. When assessing the value of antioxidants, care should be taken to avoid mutual contamination by using special individual containers. 11. To follow oven or bomb aging at least two physical properties should be measured. 12. As a general criterion of aging, elongation at break, in the absence of reversion, is most suitable to follow oven or bomb aging. Modulus should be checked in case reversion occurs, and if so, tensile strength is to be preferred to elongation at break. 13. When heat aging tests are used as service tests, the physical properties should be measured at the temperature of aging. 14. To be able to coördinate results from different laboratories in this country, a detailed study of the accelerated aging of a single compound should be undertaken. Such a program requires the coöperation of all the main laboratories in this country.