Rubber materials are widely used in industry, transportation, healthcare, and daily life due to their excellent elasticity, sealing properties, and damping characteristics. However, during long-term use or storage, rubber products undergo performance degradation—a phenomenon known as “aging”—caused by the combined effects of internal and external factors. Aging leads to material hardening, embrittlement, loss of strength, and surface cracking, severely impacting service life and safety. The mechanisms of rubber aging are complex but can generally be categorized into two main domains: Environmental Aging (initiated by environmental energy and substances such as heat, light, and oxygen) and Media-Induced Aging (initiated by contact with specific oils and chemicals). This article will systematically organize and provide a deeper analysis of the key influencing factors within these two categories.
Part 1: Environmental Aging (Initiated by External Natural Energy and Substances)
1. Oxidative Aging: The Dominant Role of Free Radical Chain Reactions
Oxidation is the most common and fundamental chemical process in rubber aging. At room temperature, the reaction between rubber and oxygen is very slow. However, under the influence of heat, light, or mechanical stress, oxygen molecules attack unsaturated bonds or active hydrogen atoms on the rubber polymer chains, initiating a free radical chain reaction. This autocatalytic process consists of three stages: chain initiation, chain propagation, and chain termination, ultimately leading to chain scission (which softens and tackifies the rubber) or crosslinking (which hardens and embrittles it).
Unsaturated rubbers (e.g., Natural Rubber NR, Styrene-Butadiene Rubber SBR), due to the high reactivity of their double bonds, are more susceptible to oxidative attack and typically have lower maximum service temperatures than saturated rubbers (e.g., Ethylene Propylene Diene Monomer EPDM, Hydrogenated Nitrile Butadiene Rubber HNBR). Protection primarily relies on the addition of antioxidants, including:
- Primary Antioxidants (Radical Scavengers): Such as hindered phenols and aromatic amines, which terminate radical chain propagation.
- Secondary Antioxidants (Hydroperoxide Decomposers): Such as phosphites and thioethers, which convert hydroperoxides into stable substances.
In practical formulations, these are often used in combination to produce a synergistic effect. Furthermore, metal-ion catalyzed oxidation can be mitigated by adding metal deactivators or chelating agents.
2. Photo-Oxidative Aging: The Catalytic Effect of Ultraviolet Light
Ultraviolet (UV) light from sunlight (wavelength 290–400 nm) possesses high enough energy to directly break chemical bonds in rubber chains (e.g., C-C, C-H), generating free radicals and thereby accelerating the oxidation process. Rubbers containing carbonyl groups or aromatic rings (e.g., polyurethane, SBS) are particularly sensitive to UV radiation.
Protection strategies mainly include:
- Light Screens: Such as carbon black and titanium dioxide, which block radiation by reflecting or absorbing UV light.
- UV Absorbers: Such as benzophenones and benzotriazoles, which convert light energy into heat.
- Quenchers: Such as nickel complexes, which deactivate excited molecules through energy transfer.
Carbon black, due to its strong absorption and cost-effectiveness, is extensively used in black rubber compounds.
3. Ozone Attack: Selective Attack and Crack Growth
Ozone present in the atmosphere (concentration ~0.01–0.1 ppm) is highly reactive and readily attacks the double bonds of unsaturated rubbers, forming ozonides and causing chain scission. A key characteristic of ozone aging is that it only occurs when the material is under tensile strain, forming surface cracks perpendicular to the applied stress direction. Cracks do not readily form when the strain is below a “critical strain value” (typically 5–10%).
Protection methods include:
- Physical Protection: Using waxes that migrate to the surface to form a barrier layer.
- Chemical Antiozonants: Such as p-phenylenediamines (PPDs), which react preferentially with ozone or terminate crack growth.
- Material Modification: Using saturated rubbers or blending with ozone-resistant rubbers.
4. Hydrolysis: Chain Scission Initiated by Water Molecules
For rubbers whose molecular chains contain hydrolyzable groups like ester, ether, or amide linkages (e.g., polyurethane, EVA, some acrylic rubbers), water molecules—especially under warm/hot conditions—can cause chemical bond cleavage. Polyester-based polyurethanes are more susceptible to hydrolysis than polyether-based types, and the reaction rate accelerates in acidic or alkaline environments.
Hydrolytic Stabilizers (e.g., polycarbodiimides) can capture carboxylic acids produced by hydrolysis, inhibiting the autocatalytic reaction. Silicone rubber is also prone to hydrolysis under harsh conditions, particularly if residual acidic peroxide catalysts remain from insufficient post-curing. Although amine crosslinks in fluoroelastomers can be degraded by steam, this is typically considered a reversible devulcanization process not involving main chain scission.
5. Mechanical Fatigue and Flex Cracking
Under cyclic stress or strain, micro-cracks gradually initiate and propagate on the surface and within the rubber, ultimately leading to fracture. This process is influenced not only by mechanical factors but is also closely related to the involvement of oxygen and ozone—the freshly exposed crack surfaces provide fresh interfaces for oxidation, accelerating chemical degradation.
Improving flex cracking resistance requires a multi-angle approach:
- Optimizing the curing system to form a network with an appropriate crosslink density.
- Selecting reinforcing fillers (e.g., silica, carbon black) to enhance mechanical strength.
- Adding anti-fatigue agents; certain antioxidants and antiozonants can also provide benefits.
6. Thermal Aging: The Overall Acceleration of Aging Processes by Temperature
Increasing temperature significantly accelerates the rates of all chemical aging reactions. As a rule of thumb, reaction rates (like oxidation) approximately double for every 10°C rise in temperature. High temperatures can also cause volatilization of plasticizers, migration of components, and may trigger thermal decomposition (e.g., HCl release from neoprene).
Heat-resistant formulation design must consider:
- Selecting rubbers with high saturation or strong bond energy (e.g., silicone rubber, fluoroelastomer).
- Using high-efficiency, extraction-resistant antioxidants.
- Avoiding additives prone to volatilization or migration.
Part 2: Media-Induced Aging (Initiated by Contact with Specific Chemical Substances)
Media-induced aging refers to the performance degradation of rubber when in contact with oils, solvents, acids, alkalis, and other chemicals. Its mechanism is fundamentally different from environmental aging described above, centering on swelling and direct chemical attack by the chemical medium.
A. Swelling (Physically Dominated)
Mechanism: When rubber contacts oils, solvents, etc., small molecules of the medium penetrate the rubber network structure. According to the “like dissolves like” principle, media with similar polarity to the rubber have high mutual solubility, leading to swelling—the medium molecules push the rubber chains apart, relaxing the network. Macroscopically, this appears as volume expansion, loss of strength and hardness, and reduced elasticity.
Typical Manifestation: Non-polar media like gasoline and mineral oils cause severe swelling and failure in non-polar rubbers like NR and SBR. Conversely, polar rubbers like Nitrile Rubber (NBR) and Fluoroelastomers (FKM) exhibit good resistance to non-polar oils.
Consequences: Excessive swelling can cause permanent deformation, accelerate oxygen ingress (triggering synergistic oxidation), and lead to internal stress and cracking upon medium evaporation/removal due to shrinkage.
B. Direct Chemical Attack (Chemically Dominated)
Mechanism: Strong chemical media directly react with specific functional groups on the rubber polymer chains.
- Strong Acids/Bases: Can hydrolyze ester groups (e.g., in PU), catalyze oxidation, or directly damage molecular chains.
- Strong Oxidizing Agents: Such as nitric acid or hydrogen peroxide, whose destructive power far exceeds atmospheric oxygen.
- Specific Chemicals: Such as amines which can attack crosslinks or main chains in certain rubbers.
Distinction from Oxidative Aging: This involves direct chemical reactions between the medium and the rubber, not relying on oxygen or free radical chain reactions. The rate and severity of damage are often greater.
C. Core Strategies for Enhancing Media Resistance
Resistance to oils and chemicals is achieved through fundamental means, not by relying on general-purpose antioxidants:
- Material Selection (Most Critical): Match the rubber polarity to that of the medium.
- For non-polar oils/fuels: Nitrile Rubber (NBR) is the primary choice (higher acrylonitrile content offers better oil resistance); the top-tier choice is Fluoroelastomer (FKM), especially perfluoroelastomers.
- For polar solvents/chemicals: Fluoroelastomer (FKM), EPDM, and Epichlorohydrin Rubber (ECO) perform well.
- For strong acids/bases: Fluoroelastomer (FKM), Perfluoroelastomer (FFKM), and certain EPDM types.
- Increase Crosslink Density: A tighter three-dimensional network physically hinders the penetration and diffusion of medium molecules, reducing swelling.
- Formulation and Process Optimization: Use fillers and additives that are not easily extracted by the medium, and ensure adequate vulcanization.
In practical applications of rubber products, they often face multiple aging factors simultaneously, including environmental and media-related influences. During the product selection stage, it is essential to fully identify both the environmental conditions and the chemical media involved. And choose a rubber material accordingly that is compatible with these conditions.
For some applications, additional protective measures for the rubber product can also be considered, such as adopting product forms with a PTFE coating or PTFE-encapsulated. Regarding rubber material selection, we welcome you to contact us to discuss and determine the most suitable material solution together.
