Thermal-Oxidative Degradation of Rubber

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 elastomers and can only be prevented or slowed down by the addition of UV stabilizers, antiozonates, 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, and exposure to other reactive gases like ozone. Besides embrittlement (chain hardening) or softening (chain scission) other visible changes such as cracking, charring, and color fading is 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.1-3

 

General mechanism of thermal degradation

Inhibition and Retardation

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 BR, SBS, NBR and in many diene rubbers with less active double bonds due to electron-withdrawing groups such as halogen (e.g. CR) 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), isobutylene isoprene rubber (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.

 

Rubber Chemical Structure Type of Degradation

Natural Rubber, NR

Chain Scission
(Softens)

Polyisoprene, IR

Chain Scission
(Softens)

Polychloroprene (CR)

Cross-linking & Chain Scission (Hardens)

Polybutadiene (BR)

Cross-linking
(Hardens)

Styrene Butadiene
(SBR)

Cross-linking & Chain Scission (Hardens)

Acrylonitrile Butadiene (NBR)

Cross-linking
(Hardens)

Isobutylene Isoprene
(IIR)

Chain Scission
(Softens)

 

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 (NR, IR), i.e. they have poor heat, ozone and UV resistance, whereas elastomers with a low number of double bonds (HNBR, IIR, EPDM) have good or even excellent heat resistance.

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

The composition of the atmosphere plays an equally important role. For example, ozone, even when present in very small concentration, will cause extensive cracking perpendicular to the stress in the rubber.

References
  1. Y.S. Duh, T.C. Ho, J.R. Chen, and C.S. Kao, Polymers 2, 174-187 (2010)

  2. F.O. Aguele, J.A. Idiaghe, T.U. Apugo-Nwosu, J. Mat. Sci. & Chem. Eng., 3, 7-12 (2015)

  3. A.J. Boon, J. nat. Rubb. Res., 3(2), pp. 90 - 106 (1988)

  • Summary

    Degradation of Elastomers

    The deterioration of the thermo-physical and mechanical properties of vulcanized diene rubbers during service is mainly caused by oxidative degradation.

  • The absorption of no more than 1 to 2% by weight of oxygen can result in loss of all useful elastomeric properties. 

  • The degradation of a polymer can be induced by light, heat, and oxygen and can be greatly accelerated by stress and exposure to  other chemicals.

  • Oxidative degradation is usually initiated when polymer chains form radicals, either by hydrogen abstration or by homolytic cleavage of carbon-carbon bonds.

  • Thermal-oxidative degradation leads to chain scission or cross-linking resulting in the loss of mechanical properties.

  • Chain scission prevails in polymers which have electron donating groups such as -CH3 attached to a carbon of the double bond (NR, IR, IIR).

  • Cross-linking dominates in butadiene and its copolymers and in polymers that have less reactive double bonds due to electron-withdrawing groups such as halogens attached to a carbon of the double bond (BR, NBR, SBR, CR).

  • The resistance of an elastomer to oxidative degradation depends on its chemical composition, molecular weight, crosslink density, type of cross-links, as well as on the concentration of other ingredients such as residual accelerator and activator; filler and pigments.

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