Abstract :
Elastomeric materials are natural or synthetic rubber products that are used in a variety of equipment related applications such as seals (O-rings and gaskets), hoses, diaphragms, belts and valve seats. When these materials are placed in storage, they are generally assigned a shelf life. The shelf life defines the time period an item can be stored and maintain physical properties which are considered acceptable for its intended application. Shelf life limitations are usually obtained from the supplier of the elastomer or component or established by the User. Shelf life is typically based on military specification MIL-STD-1 523A or MIL-HDBK-695C. These specifications address most of the commonly used elastomers and provide conservative shelf life restrictions. It is not unusual for utilities and equipment manufacturers to add further conservatism when establishing their shelf life limits. This results in inconsistent shelf life limits that are typically very conservative. In 1956, the Rubber Manufacturer´s Association (RMA) sponsored an elastomer natural aging study which lasted ten years. The program concluded that while the physical properties of the elastomers did change, the O-rings were still acceptable. The RMA discontinued the study at this point. Recent test data has been obtained which when combined with the original RMA study results provides information on how physical properties of various elastomers changed due to natural aging over a 31 year period. The elastomers tested included neoprene, viton, nitrile, SBR, ethylene propylene, urethane, butyl, and acrylate. A review of the material property changes resulted in the following conclusions: 1.
