Full definition
Aging refers to the progressive and irreversible degradation of elastomer physical properties over time, primarily due to exposure to environmental factors. This phenomenon affects various elastomers differently, with some being more resilient than others. The main aging mechanisms include thermal oxidation, ozone attack, UV photodegradation, and chemical attack. Thermal oxidation occurs when heat and oxygen interact with polymer chains, leading to hardening and cracking. For instance, elastomers subjected to high temperatures in industrial settings may experience accelerated aging, compromising their performance and lifespan. Ozone attack, particularly harmful to natural rubber (NR) and styrene-butadiene rubber (SBR), can cause surface cracking even at low concentrations of ozone (as low as 25 parts per billion). This degradation is often observed in outdoor applications where rubber components are exposed to environmental conditions. UV photodegradation breaks down polymer bonds under sunlight exposure, resulting in surface chalking and cracking, which are detrimental to both aesthetics and functionality.
Chemical attacks from solvents, acids, and bases can lead to swelling, softening, or embrittlement of elastomers. For example, neoprene shows good resistance to certain chemicals but can deteriorate in the presence of strong acids. Evaluating aging resistance is crucial for selecting the right elastomer for specific applications. Accelerated aging tests such as ASTM D573 assess changes in hardness, tensile strength, and elongation after exposure to elevated temperatures for 70-168 hours. ASTM D1149 measures ozone resistance, while ASTM D750 evaluates rubber deterioration through cut growth. The aging resistance of various elastomers ranks from best to worst: EPDM and silicone exhibit excellent ozone and UV resistance, followed by neoprene with good ozone resistance, NBR with moderate resistance, and NR and SBR, which require antioxidant and antiozonant protection to mitigate aging effects. ISO 188 provides a standardized methodology for heat aging assessments, ensuring consistent evaluation across different materials and applications.