Ageing Elastomers

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Ageing elastomers

Ageing and ageing resistance of elastomers

Ageing is defined as the totality of chemical and physical changes that lead to the alteration of mechanical properties over time [1, 2]. This reduces the applicability of the products over time. These property changes can lead to the point where the component or moulded part fails completely and can no longer fulfil its function [3]. Ageing processes can occur at any time during manufacture and use [4].
The ageing resistance can be characterized within the framework of durability tests of elastomers. Here, the materials are subjected to artificial ageing under conditions that are as close as possible to those encountered in practice. Indirect or direct testing and analysis methods are used for the investigations in order to quantify the effect of ageing on the structure and properties. Ageing resistance, like all physical and chemical properties, is strongly dependent on the structure of the material. However, the variety of stress conditions encountered in practice makes a uniform approach almost impossible.

Influence of ageing processes on the material value level

The various stresses encountered in practice can lead to irreversible physical and chemical processes at the molecular level, such as polymer chain scission, cross-linking or the breaking and reforming of covalent bonds (recombination). While chain scission causes a decrease in viscosity, (re)crosslinking of the material leads to an increase in stiffness. A combination of both mechanisms can then eventually lead to the formation of microcracks, which limit the service life of the elastomer product [5]. Materials in which the process of chain scission occurs tend to form cracks on the surface. In the case of simultaneous mechanical loading, relaxation processes may also occur. The extent of the ageing-related changes depends essentially on the temperature and other ageing conditions (medium, time, etc.). An increase in crosslink density caused by thermal-oxidative ageing is, in addition to the temperature, essentially co-determined by the material-dependent oxygen absorption capacity, the diffusion rate and the oxygen transmission. The age-related changes in the molecular network structure or their occurrence depend on the crosslinking system used. Often, thermal-oxidative ageing also results in the formation of a property profile depending on the distance to the surface. This often embrittled and hardened outer layer can be regarded as a "crack initiation zone". The oxidative ageing process of polydienes such as natural rubber ( Plastics – Symbols and abbreviated terms: NR), butadiene rubber ( Plastics – Symbols and abbreviated terms: BR) or styrene – butadiene ( Plastics – Symbols and abbreviated terms: SBR) essentially takes place at the double bonds of the main chain. Accordingly, the concentration of double bonds plays a decisive role for the thermal oxidative ageing resistance [6].

See also

• Cross-linking elastomers
• Elastomers
• Degree of cross-linking elastomers
• Elastomer dispersion filler
• Durability elastomers

References

[1]	DIN 50035 (2012-09): Terms and Definitions on Ageing of Materials – Polymeric Materials
[2]	Ehrenstein, Gottfried W., Pongratz, S.: Beständigkeit von Kunststoffen, Carl Hanser Munich Vienna (2007), ISBN 978-3-446-21851-2; see AMK-Library under G 31)
[3]	Azura, A. R., Ghazali, S., Mariatti, M.: Effects of the Filler Loading and Ageing Time on the Mechanical and Electrical Conductivity Properties of Carbon Black Filled Natural Rubber. J. Appl. Polym. Sci. 110 (2008) 747–752, DOI: https://onlinelibrary.wiley.com/doi/10.1002/app.28517
[4]	Claessen, O., Mang, T., Dikland, H. G., van Duin, M.: Helle Fensterprofilmaterialien: Alterungsverhalten auf Basis von peroxidisch vernetztem EPDM. KGK Kautschuk Gummi Kunststoffe 63 (2010) 350–360
[5]	Oßwald, K., Reincke, K., Döhler, S., Heuert, U., Langer, B., Grellmann, W.: Aspekte der Alterung elastomerer Werkstoffe. KGK Kautschuk Gummi Kunststoffe 70 (2017) 8, S. 498–506, Download as pdf (Source: www.gupta-verlag.de)
[6]	Katrin Reincke, Langer, B., Grellmann, W., Döhler. S., Heuert, U.: Alterung und Beständigkeitsuntersuchungen von Elastomerwerkstoffen. KGK Kautschuk Gummi Kunststoffe 67 (2014) 10, 60–67, Download as pdf

Additional Literature References on the Ageing of Elastomers:

• Oßwald, K., Reincke, K.: Effects of Antioxidants on the Aging Behavior of NR and SBR Materials. In: Heinrich, G., Kipscholl, R., Stoćek, R. (Eds.): Degradation of Elastomers in Practice, Experiments and Modelling. Book Series Advanced in Polymer Science Vol. 289, Springer (2022) pp. 167–183, ISBN 978-3-031-15163-7
• Rahmann, M. M., Oßwald, K., Langer B., Reincke, K.: Influenze of Plasticizers Basing on Renewable Sources on the Deformation and Fracture Behaviour of Elastomers. In: Heinrich, G., Kipscholl, R., Stoćek, R. (Eds.): Crack Growth in Rubber Materials – Experiments an Modelling. Book Series Advances in Polymer Science Vol. 286, Springer (2021) pp. 331–346, ISBN 978-3-030-68922-3