Full definition
Antioxidants are essential additives utilized in rubber formulations to enhance durability and longevity by mitigating the effects of thermo-oxidation. Thermo-oxidation occurs when rubber is exposed to oxygen and heat, leading to the degradation of polymer chains and cross-linking, which subsequently causes phenomena such as hardening, cracking, and loss of elasticity. Incorporating antioxidants into rubber compounds typically ranges from 1 to 3 parts per hundred rubber (phr). There are two primary families of antioxidants: amines and phenolics. Amines, which include paraphenylenediamines (like 6PPD and IPPD) and diphenylamines (such as TMQ or polymerized TMQ), offer superior protection against oxidative degradation but can lead to staining and discoloration, making them suitable primarily for black rubber products. On the other hand, phenolic antioxidants, particularly hindered phenols like BHT and Irganox 1010/1076, provide less robust protection but are non-staining, rendering them more suitable for applications involving lighter-colored or food-grade materials. Selection of the appropriate antioxidant depends on various factors such as the expected service temperature, required color stability, and anticipated service life of the rubber product. ASTM D4817 provides a classification standard for these materials, ensuring that users can choose the right antioxidant based on their specific requirements. In practice, antioxidants are often used in conjunction with antiozonants to offer comprehensive protection against both ozone and oxidative degradation, which is critical in rubber compounds like natural rubber (NR) and styrene-butadiene rubber (SBR), while being less critical for inherently stable materials such as ethylene propylene diene monomer (EPDM) and silicone.