Accelerated Aging of Polymers
Rubber gaskets are usually expected to perform over the life of the appliance, machine, device, panel, or window in which they are installed. In most cases, however, it is impossible (or, at least, highly impractical) to test the performance over the years of service that is expected. For this reason, tests have been developed to stress the elastomer in a way that simulates accelerated aging. This brief paper describes how this is often done and its limitations.
Because elastomers are required to perform a wide variety of functions, many raw materials suppliers, when reporting on accelerated aging performance, will most often only reference in changes to three properties that are generic to elastomers: tensile strength (force needed to rupture a sample under tension), elongation (elasticity), and hardness. It is often assumed, for convenience, that the extent these properties change over time is a good indicator of other properties that have not specifically been measured. Some of these other properties might be dimensional stability, tear strength, and flexibility.
The most common method used to simulate accelerated aging is heat aging. ASTM D573 is one such test, but it does not prescribe any given temperature or time, let alone attempt to extrapolate what these tests say about long-term performance. Nonetheless, heat aging tests do a pretty good job of simulating long-term changes in a material. What is harder to predict is how many hours at which temperature will simulate years of aging under expected field conditions!
Another common test is performed in a weatherometer, in which the sample is exposed to intense UV for several months. ISO 4892 and ASTM G155 are some test methods. These tests are used when the gasket will be exposed to sunlight and, in addition to noting changes in physical specifications, color fading or discoloration is often included.
When a seal is expected to be exposed to certain fluids, like oils or acids, then the change in volume is commonly measured, along with changes to the fundamental physical properties mentioned above.
In most cases, these accelerated aging tests only give a general indication of suitability, and are best used to compare one material to another. There are many reasons for this. Accelerated aging tests are commonly performed on a lab sample that is not under stress, vibration, or compression, and so it does not precisely simulate field conditions. Also, many seals are subject to broad changes in temperature and pressure during their lifetime, which is not typically simulated in accelerated aging tests.
A final cautionary note, when selecting a material for your application. Be careful of generic descriptive terms, such as EPDM, PVC, or Neoprene. These materials are made from recipes that includes oils, plasticizers, fillers, etc, and the ratios that are used can have a dramatic impact on short- and long-term performance. That is, just because one 70 durometer neoprene appears to meet your needs, this doesn’t mean that all 70 durometer neoprenes will perform as well. You are better off specifying a brand name and compound number, so that your product will be made exactly the same every time.
