THERMAL-OXIDATIVE DEGRADATION OF RUBBER

15 Jul 2021 | Posted by Andrew Onions

Most elastomers will undergo significant changes over time when exposed to heat, light, or oxygen (ozone). These changes can have a dramatic effect on the service life and properties of the elastomer and can only be prevented or slowed down by the addition of UV stabilizers, anti-ozonates, and antioxidants.

Depending on the microstructure of the diene elastomer, oxidative degradation will either cause hardening or softening. For example, polybutadiene usually undergoes oxidative hardening whereas polyisoprene softens when exposed to heat and oxygen. 

Hardening is much more common because free radicals produced when rubber is exposed to heat, oxygen and/or light rapidly combine, and in this process form new crosslinks. This drastically reduces the flexibility of the rubber. 

Polymers with pendent bulky side groups, on the other hand, will undergo strain softening because radical recombination reactions are less likely to occur due to steric hindrance. Instead, these polymers degrade by chain scission caused by disproportionation and hydrogen abstraction. 

The aging of a rubber due to oxidation and heat is greatly accelerated by stress, typical in gasket applications, and exposure to other reactive gases like ozone. Apart from embrittlement (chain hardening) or softening (chain scission) other visible changes such as cracking, charring, and colour fading may be observed. 

Although the general mechanism of autooxidation is well understood, the actual chain scission and crosslinking steps are often unknown. They depend on the composition of the rubber including concentration of accelerators, activators, and fillers as well as on the temperature and composition of the atmosphere. Two possible mechanisms of thermal oxidation with subsequent chain scission or crosslinking are shown below. The process is very complex and involves several intermediates and side reactions. 

In general, the type of degradation (chain hardening or softening) depends on the chemical composition of the polymer. For example, crosslinking dominates in polybutadiene and its copolymers such as Polybutadiene, Styrene Butadiene, and Nitrile and in many diene rubbers with less active double bonds due to electron-withdrawing groups such as halogen like Neoprene. Whereas elastomers with bulky and/or electron donating side groups (-CH₃) attached to the carbon atom adjacent to the double bonds are vulnerable to chain scission. This includes Natural rubber (NR), Polyisoprene (IR), Butyl (IIR ), and any other unsaturated polymer with electron donating groups. Some other polymers such as SBR, EPM, and EPDM undergo both chain scission and crosslinking. However, often crosslinking reactions dominate so that these rubbers harden over time.

The resistance to oxidative degradation depends on many factors, including chemical composition, molecular weight, crosslink density, and type of cross-links. Diene elastomers that have electron-donating groups attached to the diene are usually the least stable rubbers Natural (NR), Polyisoprene (IR), and they exhibit poor heat, ozone and UV resistance, whereas elastomers with a low number of double bonds Hydrogenated Nitrile (HNBR), Butyl (IIR), and Ethylene Propylene (EPDM) have good or even excellent heat resistance.

The stability of an elastomer is also affected by other ingredients in the rubber compounding formulation. Under certain conditions, residual unused cross-linkers and accelerators confined in the elastomer can decrease the thermal stability of the material because they easily undergo thermal decomposition at elevated temperature producing radicals that are capable of accelerating thermo-oxidative degradation of the molecular chains. Soluble fatty acid salts of metal ions such as Cu, Mn, Ni, Co, and Fe act as catalysts for oxidation, and can significantly accelerate the thermo-oxidative decomposition of rubber.

To prevent or slow down degradation, antioxidants and UV stabilizers are often added. The two main classes of antioxidants are free-radical scavengers and peroxide scavengers. The free-radical scavengers are sometimes called primary antioxidants or radical chain terminators whereas peroxide scavengers are often called secondary antioxidants or hydroperoxide decomposers.

The most widely used primary antioxidants are sterically hindered phenols. They are very effective radical scavengers during both processing and long-term thermal aging and are generally non-discolouring. Many also are Food Quality compliant with FDA approval or equivalent.  

The most effective primary antioxidants are secondary aromatic amines. However, they cause noticeable discolouration and can only be used if discolouration is not a problem, like carbon filled rubber products. They also function as antiozonants and metal ion deactivators.