What Causes Rubber Seals To Harden?

As a polymer elastic material, rubber may harden during its use, resulting in a decrease in performance and affecting the service life of the product. There are many reasons why rubber seals harden, including physical, chemical, environmental, and technological factors. The following will be an in-depth analysis of many aspects such as cross-linking structure changes, aging mechanisms, formula influences, environmental factors, and processing technology.

The mechanism of hardening of rubber seals

The essential reasons for the increase in the hardness of rubber seals mainly include:

1. Increased degree of cross-linking - the vulcanization continues, and the cross-linking density increases, which restricts the movement of the rubber chain segment and hardens the material.

2. Loss of plasticity - the migration or volatilization of low-molecular substances such as plasticizers and softeners makes the rubber lose its flexibility.

3. Molecular chain degradation - the main chain of rubber molecules is broken, resulting in damage to the elastic network structure, which is manifested as a change in hardness.

4. Filler aggregation or phase separation – Fillers (e.g., carbon black, silica) aggregate or settle in the rubber matrix, increasing local hardness.

The main influencing factor of the stiffening of rubber seals

Changes in cross-linking structure

The degree of vulcanization crosslinking of rubber has a significant effect on its hardness:

· Overcuring: The vulcanization time or temperature is too high, resulting in a crosslinking density exceeding the optimal value, causing the rubber to become hard, brittle, and lose its flexibility.

· Secondary vulcanization (Post-curing) In a high-temperature environment, rubber continues to undergo cross-linking reactions to increase hardness, such as some silicone rubber and fluorine rubber will show a hardening phenomenon at long-term high temperatures.

· Excessive vulcanizing agent In the vulcanizing system, excessive vulcanizing agents (such as sulfur, peroxide, and resin) will form too many cross-linked bonds, resulting in hard rubber.

· Vulcanization systems (such as monosulfide bonds, disulfide bonds, and polysulfide bonds) with different types of cross-linked bonds have different effects on the hardness of rubber, and disulfide bonds and monosulfide bonds are harder than polysulfide bonds.

Aging mechanism

Aging is one of the main causes of hardening of rubber seals, and common aging factors include:

(1) Thermal-oxidative aging

· High temperature will promote the oxidation reaction between oxygen and rubber molecules, causing the molecular chain to break or increase the crosslinking density, resulting in the stiffness of the rubber seal.

· Typical thermal-oxidative aging reactions: R−H+O2→R−O−O−H (peroxide) R-H + O_2 → R-O-O-H (peroxide) peroxide further decomposes, which promotes oxidative cross-linking and hardens the rubber seal.

(2) Ozone aging

· Ozone can break the unsaturated double bonds of rubber, causing it to form oxidative cross-links and improve hardness. For example, rubber with double bonds, such as NR, SBR, BR, etc., is susceptible to ozone.

(3) Ultraviolet aging

UV radiation triggers a free radical reaction that accelerates oxidative cross-linking and hardens rubber seals. In particular, rubber products used outdoors, such as seals and tires, will become hard and brittle when exposed to sunlight for a long time.

(4) Hydrolytic aging

· Rubber containing ester group (-COO-) or amide group (-CONH-), such as polyurethane rubber (PU) and neoprene rubber (CR), is prone to hydrolysis in a humid and hot environment, making the rubber hard or pulverized.

(5) Radiation aging

High-energy rays such as nuclear radiation and X-rays can cause the breakage or cross-linking of rubber molecular chains, thus changing their hardness. For example, rubber products in the aerospace and nuclear industries need to be specially considered for radiation protection formulations.

Formulation factors

The different components of the rubber formulation have an important impact on the hardness variation:

Effect of fillers

· Carbon black High-structure carbon black (e.g., N220, N330) can increase hardness, while low-structure carbon black (e.g., N550, N660) has less effect on hardness.

· SiO₂ filler has a strong reinforcement effect, which may lead to hard rubber, especially the cross-linking effect of the silicon hydroxyl group under wet conditions.

· Inorganic fillers such as calcium carbonate, talc, clay, etc., too much will increase the rigidity of the rubber and increase the hardness.

Effect of plasticizers/softeners

· Plasticizers volatilize or migrate such as paraffin oil, naphthenic oil, DOP (dioctyl phthalate), etc., which may migrate or volatilize after long-term use, causing the rubber to lose its softness and become hard.

· Softeners degrade certain ester plasticizers that decompose under high temperature or hydrolytic conditions, hardening rubber.

Effects of antioxidants

· Antioxidant types such as TMQ, 6PPD, etc., can effectively delay thermal and oxidative aging and prevent the rubber from hardening.

· Antioxidant ConsumptionDuring long-term use, the antioxidant is gradually depleted, and the aging rate accelerates, increasing hardness.

The impact of the cross-linking system

· Vulcanization system selection

The peroxide vulcanization system is harder than the sulfur vulcanization system;

The resin vulcanization system is easy to cause the rubber to harden;

The use of a vulcanized system with high cross-linking density will accelerate hardening.

· Accelerator dosage

Too much accelerator may lead to per vulcanization, hardening of the rubber;

Insufficient accelerator will lead to insufficient vulcanization and low hardness but may continue to be cross-linked and hardened in the later stage.

Environmental factors

· Temperature affects the glass transition temperature (Tg) of rubber in a low-temperature environment, resulting in hard and even brittle cracking of materials, such as the increase of the rigidity of NR and SBR at low temperatures.

· Humidity affects the hydrolysis or cross-linking of certain rubbers such as PU, CR, EVA, etc. in a humid environment, so the hardness increases.

· Chemical contact with chemicals such as acids, alkalis, oils, solvents, etc., will cause changes in the chemical structure of the rubber, increasing its hardness.

Technological factors

· Uneven mixing of fillers and vulcanizing agents may lead to uneven cross-linking density in local areas, which is manifested as abnormal hardness.

· Vulcanization time control Vulcanization time too long may lead to overvulcanization, which increases the hardness of the rubber.

· Improper storageExposure to high temperatures, oxygen, and ozone for a long time may cause cross-linking or aging of the rubber, resulting in stiffness.