Ageing

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Ageing

Definition

Ageing is understood to be the totality of all chemical and physical processes irreversibly occurring in a material over the course of time. Ageing takes place under natural environmental conditions. In special cases, however, ageing may have special characteristics, such as elevated temperatures, chemical attack, and mechanical stress. For testing purposes, increased influence to a factor may be desirable to achieve time acceleration (see also: stepped isothermal method, macro indentation method). However, this time-lapse does not normally allow direct extrapolation to long-term behaviour. It is possible, however, to evaluate relatively simple processes, such as creep behaviour or thermal ageing. However, since in most cases the natural ageing conditions do not act individually and cannot be shortened in time, the statement for long-term application based on short-term tests is always problematic.

Internal and external ageing

In general, a distinction is made between internal and external ageing. Internal ageing is due to thermodynamically unstable states of the polymeric material, e.g. the breakdown of residual stresses, post-crystallization, phase separation in multi-component systems, plasticizer migration or swelling. External ageing, such as stress cracking, fatigue cracks, thermo-oxidative degradation, swelling or similar phenomena, is caused by physical or chemical effects of the environment on the polymer material. The distinction between chemical and physical ageing processes is not always unambiguous, since there are usually complex effects.

Influence of ageing on the material value level

If ageing is understood to mean only the time-dependent, irreversible processes, then it should correctly also be understood to mean an improvement in the service value of the material, if, for example, post-condensation, post-crystallization or radiation crosslinking leads to an improvement in certain desired properties.
For example, for a medially stressed (washing solution, 95 °C) particle-filled (e.g. talc) polypropylene (abbreviation: PP), depending on the ageing time (up to 2,000 h), the modulus of elasticity E_t can decrease significantly, the tensile strength σ_M can increase slightly, the nominal tensile strain at break ε_tB and the technically more significant nominal strain at tensile strength ε_tM show no changes, and the Charpy impact strength of notched specimens a_cN can increase noticeably. These structure-dependent property changes of the mechanical values as a function of the ageing time can also be quite different in other examples. For this reason, a multi-parameter property evaluation is particularly important in the formulation optimization of complex polymer material systems, e.g. polymer blends.
Stabilizers have even been developed against a number of particularly critical environmental influences, which delay the undesirable changes in material and service properties during processing or use. As a result, the plastic component retains its required properties during the service life. The most important stabilizers are processing stabilizers, heat and light stabilizers, UV absorbers, antioxidants and hydrolysis protection agents.

See also

• Ageing elastomers
• Durability elastomers
• Stepped isothermal method, Macro indentation method
• Stepped isothermal method, Tensile stress

References

• Ehrenstein, G. W.: Polymerwerkstoffe. Struktur und mechanisches Verhalten. Grundlage für das technische Konstruieren mit Kunststoffen. Carl Hanser, Munich Vienna (1978) p. 173 (ISBN 3-446-12478-0; see AMK-Library under G 28)
• Lechner, M. D., Gehrke, K., Nordmeier, E. M.: Makromolekulare Chemie – Ein Lehrbuch für Chemiker, Physiker, Materialwissenschaftler. Birkhäuser, Basel Boston Berlin (2010) 4. revised and expanded edition, pp. 488 ff, (ISBN 978-3-7643-8890-4; see AMK-Library under N 11)
• Grellmann, W., Schoßig, M., Reincke, K., Kirbs, J.: Bewertung der Alterungsbeständigkeit von Folien durch künstiche Bewitterung. In: Grellmann, W. (Eds.): Neue Entwicklungen in der Werkstoffprüfung – Herausforderung an die Kennwertermittlung. Tagung "Werkstoffprüfung 2011", 1. und 2. Dezember 2011, Berlin, Proceedings pp. 173–178 (ISBN 978-3-9814516-1-0; see AMK-Library under A 13)
• Langer, B., Schoßig, M., Reincke, K., Grellmann, W.: Charakterisierung des Alterungsverhaltens von Polymerwerkstoffen. In: Borsutzki, M. and Moginger, G. (Eds.): Fortschritte in der Werkstoffprüfung für Forschung und Praxis. Tagung Werkstoffprüfung 2012, 06. – 07. Dezember 2012, Bad Neuenahr, Proceedings pp. 145–152 (ISBN 978-3-514-00794-9; see AMK-Library under M 24) Download as pdf