During the long-term service life of rubber products such as tires, seals, and industrial hoses, thermal-oxidative aging, dynamic fatigue aging, and photo-oxidative/ozone aging are the primary causes of the continuous deterioration of a compound’s mechanical properties. This deterioration manifests primarily as hardening and embrittlement of the compound, surface cracking and chalking, and a significant decline in tensile strength and elasticity, ultimately leading to premature failure and scrapping of the products. In rubber vulcanization formulations, zinc oxide is no longer limited to its traditional role as a vulcanization activator; it has become a core functional filler that enhances the compound’s heat resistance, aging resistance, and weather stability. Through a synergistic mechanism combining multiple physical protective measures and chemical reactions, it suppresses polymer chain degradation and cross-link network breakdown at the source, significantly extending the service life of rubber products.
Rubber is a high-molecular-weight elastic polymer. Under complex operating conditions—such as high temperatures, oxygen-rich environments, dynamic alternating loads, ultraviolet radiation, and ozone—it undergoes irreversible, autocatalytic free-radical chain degradation reactions, which constitute the fundamental mechanism of rubber aging. The cross-linked network of conventional sulfur-cured rubber consists primarily of polysulfide and disulfide bonds. Among these, polysulfide bonds have low bond energy and poor thermal stability, constituting a structural weakness that makes the vulcanized network prone to breakdown at high temperatures.
Under high-temperature, oxygen-rich conditions, the rubber molecular backbone triggers a free-radical chain reaction, continuously causing molecular chain breakage and disordered restructuring of cross-links, thereby compromising the integrity of the network structure; At the same time, the vulcanization process and long-term aging continuously generate acidic byproducts such as hydrogen sulfide, carboxylic acids, and trace amounts of hydrogen chloride. These acidic substances act as strong catalysts, accelerating the hydrolytic breakdown of cross-links and further damaging the rubber’s three-dimensional elastic network. This manifests as soft rubber compounds softening when heated and experiencing increased permanent deformation, while hard rubber compounds become hardened and brittle, with significant declines in tear resistance and abrasion resistance. Furthermore, under outdoor conditions, ultraviolet (UV) radiation and ozone synergize with thermal-oxidative effects to continuously erode the polymer chains on the rubber’s surface, leading to surface cracking and chalking. This results in multi-factor coupled aging and degradation, which drastically shortens the service life of the product.
Thanks to its excellent chemical reactivity, thermal conductivity, and optical shielding properties, zinc oxide can comprehensively block multiple pathways of rubber aging and degradation through five core mechanisms: passivating aging radicals, stabilizing the vulcanized cross-linked network, neutralizing acidic catalytic byproducts, conducting heat to reduce thermal accumulation, and shielding against UV and ozone erosion. This establishes a stable, long-lasting anti-aging system for rubber compounds.
Overall, zinc oxide comprehensively improves rubber’s anti-aging performance and blocks multi-path degradation and failure through multiple synergistic mechanisms. In terms of chemical protection, zinc oxide utilizes its surface active sites to efficiently quench alkyl and peroxy radicals generated by thermal-oxidative aging, thereby terminating polymer chain degradation reactions. Simultaneously, it neutralizes acidic catalytic byproducts—such as hydrogen sulfide and carboxylic acids—generated during rubber vulcanization and aging, eliminating the problem of accelerated hydrolysis of cross-links caused by acidic media; Furthermore, it works synergistically with stearic acid to optimize the vulcanization process, increasing the proportion of highly stable single-sulfide and disulfide bonds within the system, thereby reconstructing and stabilizing a highly thermally stable cross-linked network and inhibiting the collapse and damage of the vulcanized network at high temperatures. In terms of physical protection, highly thermally conductive zinc oxide fillers create continuous thermal conduction pathways within the rubber compound, rapidly dissipating heat accumulated during dynamic service and reducing accelerated aging damage caused by heat buildup; At the same time, it possesses excellent UV absorption and ozone barrier capabilities, shielding against outdoor photo-oxidative erosion and suppressing surface cracking and chalking. It effectively adapts to complex operating conditions such as high temperatures, heavy loads, and prolonged outdoor exposure, ultimately achieving a comprehensive improvement in the compound’s heat resistance, weather resistance, and fatigue resistance.
The extent to which zinc oxide enhances a rubber compound’s anti-aging performance is determined by the product category, the addition ratio in the formulation, and the mixing and processing techniques. Scientific formulation ratios and processing methods are key to maximizing its anti-aging benefits. Regarding dosage, the standard amount of zinc oxide in conventional sulfur-cured rubber systems is 3–5 phr. Excessive amounts can lead to increased rubber hardness and reduced toughness and elongation, as well as issues such as powder agglomeration and surface blooming in the finished product. Conversely, too low a dosage fails to establish a complete protective system for acid absorption, free radical scavenging, and cross-link stabilization, resulting in incomplete vulcanization and a significant decline in thermal stability and anti-aging performance. In contrast, active zinc oxide and nano-zinc oxide can be used as substitutes at reduced levels of 2–3 phr. This reduces the zinc content in the compound while maintaining or even enhancing anti-aging performance, aligning with the industry’s trend toward low-carbon and environmentally friendly development. In terms of processing technology, powder agglomeration is the core issue that undermines the anti-aging efficacy of zinc oxide. During production, the mixing and feeding sequence must be optimized: zinc oxide should be pre-mixed with stearic acid to form active zinc soaps before being added to the main compound. Additionally, appropriate mixing temperatures, speeds, and times must be selected to ensure uniform dispersion of the powder within the rubber matrix, thereby achieving uniform protection across the entire cross-linked network. Furthermore, while zinc oxide alone has certain performance limitations, its scientific blending with organic antioxidants and carbon black can create