Rubber Field Info

Rubber Field Info

Degradation of Rubber

Elastomer oxidation is influenced by various factors, such as high temperatures, contamination by heavy metals, sulfur, light exposure, moisture, exposure to oil and solvents, dynamic fatigue, oxygen, and ozone. To resist degradation, three key variables in the compound formulation can be optimized: polymer type, cure system, and antidegradant system.

The vulcanization system plays a significant role in determining thermo-oxidative stability. Peroxide vulcanization or cure systems, which utilize carbon-carbon crosslinks and eliminate sulfur, tend to exhibit excellent resistance to reversion. Efficient vulcanization (EV) systems, characterized by low sulfur levels (0.0–0.3 phr), high acceleration levels, and the use of a sulfur donor, also demonstrate good heat stability and oxidation resistance. However, these systems may have lower fatigue resistance due to the prevalence of monosulfidic crosslinks. Conventional cure systems with high sulfur levels and low accelerator concentrations generally show poor heat and oxidation resistance because polysulfidic crosslinks are thermally unstable and prone to oxidation. Nevertheless, such vulcanization systems tend to offer better fatigue resistance. Semi-EV cure systems, which fall between EV and conventional systems, strike a balance between oxidation resistance and the required fatigue performance of the product.

Oxidation progresses through two fundamental mechanisms:

  1. Crosslinking: A network predominantly composed of di- or polysulfidic crosslinks breaks down into monosulfidic crosslinks. This leads to an increase in compound hardness, a decrease in fatigue resistance, and a significant stiffening of the compound. Elastomers like SBR, EPDM, NBR, and polychloroprene often exhibit this behavior.

  2. Chain scission: Polymer chains break, resulting in a softening of the compound and reduced abrasion resistance. Natural rubber is known to undergo this type of degradation.

The degradation of rubber is accelerated by an increase in service temperature, with the rate of reaction with oxygen following the Arrhenius equation.
Exposure to ultraviolet (UV) light triggers the initiation of free radical oxidation on the surface of elastomeric products, resulting in the formation of an oxidized rubber layer.. Subsequently, heat, moisture, or high humidity can cause surface crazing, which can be further abraded off. This degradation is more pronounced in nonblack compounds compared to black compounds. Nonblack formulations, such as white tire sidewalls, require higher levels of nonstaining antioxidants than carbon black-loaded formulations.

Compounds like oleates and stearates containing manganese or copper readily dissolve in rubber, facilitating rapid polymer oxidation. To mitigate the activity of these metal ions, para-phenylenediamine antidegradants are used.

Surface crack development is a significant cause of failure in rubber products. The growth of such cracks during cyclic deformation leads to fatigue failure. Fatigue-related cracks typically initiate in high-stress zones. Ozone attack can trigger crack initiation at the surface, which then propagates due to flexing. Ozone-induced cracking often appears as crazing on the sidewalls of aged tires. Ozone reacts readily with the carbon-carbon double bonds present in unsaturated elastomers, forming ozonides. Under strain, ozonides decompose easily, causing chain cleavage and a decrease in polymer molecular weight. This reduction in molecular weight becomes evident as surface crazing and cracking.

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