Rubber Compression Seal Expansion Joint Aging and Service Life Prediction

Rubber aging is the primary factor limiting the service life of compression seal expansion joints. Understanding the aging mechanisms and their rates allows engineers to predict service life and plan replacement schedules. Thermal aging is the most important aging mechanism for rubber compression seals. At elevated temperatures, the polymer chains in the rubber undergo oxidation and crosslink scission, causing the rubber to become harder and more brittle. The rate of thermal aging follows the Arrhenius relationship, approximately doubling for every 10 degrees C increase in temperature. Ozone attack causes surface cracking in rubber under tension. Ozone in the atmosphere reacts with the carbon-carbon double bonds in the rubber polymer, breaking the polymer chains and creating cracks perpendicular to the direction of stress. EPDM rubber has no carbon-carbon double bonds in the main polymer chain, making it highly resistant to ozone attack. This is a major advantage of EPDM over natural rubber and neoprene for outdoor applications. UV radiation degrades rubber by initiating photochemical reactions that break polymer chains and crosslinks. Carbon black pigment in the rubber absorbs UV radiation and protects the underlying polymer from degradation. EPDM rubber with 40-50 phr carbon black content has excellent UV resistance. Light-colored rubber compounds with less carbon black have lower UV resistance and shorter outdoor service life. Service life prediction uses accelerated aging tests to estimate the service life under actual service conditions. The rubber is aged at elevated temperature in an oven for a specified period, and the change in mechanical properties is measured. The Arrhenius relationship is used to extrapolate the accelerated aging results to actual service conditions. Typical service life for EPDM compression seals in temperate climates is 20-30 years, depending on the temperature range and UV exposure. In hot climates with high UV radiation, service life may be reduced to 15-20 years. In cold climates with low UV radiation, service life may be extended to 25-35 years.