Deterioration in an Air Oven
It is necessary to determine the influence of elevated temperatures on the physical properties of vulcanized rubber. Since testing takes place in heated ovens, effects are dependent on the laboratory environment and conditions, and should be analyzed for comparison purposes.
One of the concerns of a product design engineer is the expected life of a component. Properties of vulcanized rubber change during service, so elastomers and rubber products must resist the deterioration of physical properties over time caused by oxidative and thermal aging.
Chemically similar specimens are suspended in ovens designed to circulate heated air at atmospheric pressure. An oven design that divides the available capacity into a number of aging cells is recommended to segregate the test pieces. These tests are performed such that there is an automatic recording of temperature over time. The heating of the air is done externally at its source, and not in the oven itself.
The sample size of the test material needs to be sufficient to allow for the determination of the original properties on three specimens, and also on three or more specimens for each exposure period of the test. At least 24 hours must elapse after completion of the vulcanization of the samples, and the start of the aging test.
Results of the heat aging test for each aging period are expressed as a percent change in each physical property (tensile strength, ultimate elongation, tensile stress), and are calculated as follows:
P = (A – O)/O X 100
- P = change in property, %
- = original value
- A = value after aging
Using the above formula, results are reported as percent change where attention is made to include repeatability and reproducibility. These inclusions are important when results on various types of specimens are listed along with results from other testing laboratories.