7 Reasons To Let You Know About The Aging Phenomenon Of Rubber Seals

It is a common phenomenon for rubber products to rise in hardness after being left or used for some time, especially when they are exposed to air or subjected to external pressure and temperature changes. This process is mainly due to changes in the physical and chemical properties of the rubber, which can be attributed to the following aspects: oxidation, increased cross-linking, limited segment movement, migration of additives, and the influence of environmental factors. Next, I will analyze the reasons in detail from these aspects.

Oxidation reaction

Rubber seals are subject to oxidation reactions when exposed to air, especially those containing oxygen and ozone. Oxidation is one of the main causes of rubber aging, during which the molecular structure of rubber changes, increasing hardness. The main mechanisms of oxidation reactions include the following:

- Free radical chain reaction: Rubber seals are prone to generate free radicals under the action of oxygen, and free radicals combine with oxygen to form peroxide radicals, which in turn leads to chain breakage and cross-linking reactions, resulting in changes in rubber structure. As cross-linking increases, the movement of the molecular chains is restricted, and the rubber exhibits higher hardness.

- Effect of Ozone: Ozone has a stronger damaging effect on rubber seals, causing not only chain breakage, but also oxidation products directly in the rubber molecules. These oxidation products increase the brittleness of the rubber, which exhibits a higher hardness.

Increased cross-linking

Cross-linking refers to the formation of a network structure between rubber molecules through chemical bonds, which can enhance the mechanical properties of rubber and make it have higher hardness. Rubber products are usually cross-linked during the manufacturing process through processes such as vulcanization, but the degree of cross-linking will further increase during use. This is mainly due to the following factors:

- Photoaction: UV light promotes the formation of free radicals in rubber molecules, which in turn promote cross-linking between rubber molecules. Rubber products that have been exposed to sunlight for a long time will have increased hardness due to increased cross-linking.

- Thermal aging: Rising temperatures accelerate the movement of rubber molecules, increasing the frequency of collisions between molecules, and making it easier to form new chemical bonds. Thermal action not only accelerates the oxidation reaction but also promotes the cross-linking reaction, which ultimately leads to an increase in the hardness of the rubber.

- Oxidative cross-linking: As mentioned earlier, oxidation reactions can also lead to increased cross-linking of rubber molecules, especially in the presence of oxygen and high temperatures, where cross-linking bonds are more likely to form between rubber molecules.

The increase in cross-linking significantly increases the hardness of the rubber, as cross-linking restricts the movement of rubber molecules, making the rubber material less flexible. That's why we find that rubber products become hard and brittle after a period of use.

Limited segment movement

The softness of rubber materials is mainly derived from the free movement of its molecular chains, but after a period of use, this segment movement may be limited for the following reasons:

- Cross-linking limitations: The increase in cross-linking directly limits the free movement of the rubber molecular chains, and the cross-linked rubber structure is closer to a solid structure, and the degree of freedom of the chain segments decreases, increasing hardness.

- Hygroscopic and drying: Some rubber materials may absorb moisture in a humid environment and lose it in a dry environment. The change in moisture causes a change in the movement of the chain segments inside the rubber material, which affects its softness. For example, when the moisture decreases, the segments of the rubber material are more closely aligned, which manifests as an increase in hardness.

- Freezing: At low temperatures, the movement of the rubber molecular chains weakens, which manifests as a hardening of the material. In some special applications, temperature changes may repeatedly lead to changes in rubber hardness.

Migration of additives

Additives such as plasticizers and antioxidants are often added during the manufacturing process of rubber products to improve their performance. However, during use, these additives may gradually migrate or evaporate, increasing the hardness of the rubber.

- Migration or volatilization of plasticizers: The role of plasticizers is to improve the softness of the rubber, making it more elastic. However, plasticizers may gradually migrate to surfaces or volatilize into the air during use, especially at high temperatures, where this migration or evaporation will be faster. When the plasticizer is reduced, the flexibility of the rubber decreases, which is manifested as an increase in hardness.

- Consumption of antioxidants: The role of antioxidants is to inhibit the aging process of rubber, but antioxidants will gradually decompose and fail under high temperature, light, and other conditions. Once the antioxidant is depleted, the rubber's ability to resist aging decreases, and oxidation and cross-linking intensify, increasing hardness.

Influence of environmental factors

The hardness of rubber is also affected by environmental factors, including temperature, humidity, light, chemicals, etc., which can have a significant impact on the physical and chemical properties of rubber.

- Temperature: High temperatures accelerate the oxidation and crosslinking of rubber seals, while low temperatures slow down the movement of rubber molecular chains, making rubber stiffer. In the actual use process, rubber products are often exposed to a large temperature change environment, which will have an impact on the hardness of rubber.

- Humidity: Humidity can affect the softness of rubber seals, especially some more hydrophilic rubber materials that will absorb moisture in a humid environment and lose moisture in a dry environment. This change in moisture affects the hardness of the rubber.

- Chemicals: Rubber seals may be exposed to chemicals such as oils, acids, and alkalis during use, which can trigger a degradation reaction in the rubber, causing its hardness to change. For example, some rubber products will swell under the action of oil, which is manifested by increased softness; In acid-alkaline environments, rubber may be chemically degraded or hardened.

Crystallization

Some rubber materials will crystallize under long-term standing or low temperatures, especially natural rubber and butadiene rubber. This crystallization causes the hardness of the rubber material to increase, making as brittle, and harder.

- Crystallization caused by standing: When a rubber seal is left standstill, an orderly structure gradually forms between the molecular chains, and partial crystallization occurs, causing the material to harden. This is especially evident in natural rubber, where rubber products that have not been used for a long time can harden and even appear brittle.

- Low-temperature crystallization: Some rubber materials crystallize at low temperatures, and the molecular chains are arranged in a more orderly manner at low temperatures, increasing material hardness. This crystallization gradually recovers as the temperature increases, but if it is left at a low temperature for a long time, the crystallization phenomenon may become irreversible.

Fatigue effects

Rubber will have a fatigue effect under repeated stress, and the internal structure will gradually deteriorate, resulting in a change in hardness. The fatigue effect is mainly due to the breaking or rearrangement of the internal molecular chains of the rubber material in repeated stretching and compression, which is manifested as an increase in hardness.

- Microcrack formation: Repeated stress can create microscopic cracks inside the rubber, and the propagation of the cracks will affect the overall structure of the material, causing it to harden gradually.

- Stress-induced cross-linking: Under stress, a chemical reaction occurs between rubber molecules, creating new cross-linking points, resulting in an increase in material hardness.

There are many reasons for the increase in hardness of rubber products after a while of use, the main factors include oxidation reactions, increased cross-linking, limited segment movement, migration of additives, environmental factors, crystallization, and fatigue effects. The combination of these factors leads to a change in the molecular structure of the rubber, which limits the movement of the molecular chains, which ultimately manifests itself as an increase in hardness. To slow down this increase in hardness, more effective antioxidants, antioxidants can be added to rubber products, and they can be exposed to high temperatures, ultraviolet rays, and strong oxidizing environments.