Ageing processes in plastics
How do plastics age and how can their service life be extended? Causes, consequences and effective measures against material ageing.
The ageing resistance of polymers describes the ability of a plastic to retain its mechanical, chemical and physical properties over a long period under real environmental conditions. It significantly determines the service life of technical plastic components in industry, mechanical engineering and outdoor applications.
In technical applications, plastics are often used for many years. During this time, various environmental conditions act on the material. Temperature, UV radiation, oxygen, moisture or mechanical loads change the structure of a polymer over the long term. The ageing resistance of polymers therefore describes the ability of a plastic to remain stable in its properties over a long period.
For design, material selection and product development, this property is of central importance. Materials with high ageing resistance enable durable components, reduce maintenance costs and increase the operational safety of technical systems.
What does ageing resistance mean for polymers?
Ageing resistance describes the ability of a polymer to retain its mechanical, chemical and optical properties over a long period under various environmental conditions. During use, factors such as temperature, UV radiation, oxygen, moisture or mechanical load act on the material. These influences can trigger chemical reactions in the polymer, for example chain degradation, oxidation or structural changes within the molecular structure.
In the course of these processes, typical material properties change. Plastics can become brittle, lose their strength or develop visible changes such as yellowing or surface cracks. Dimensional stability and electrical properties can also change. The ageing resistance of a material therefore describes how pronounced these changes are and how long a plastic retains its original properties.
Particularly in technical applications, this property often determines the actual service life of a component. Materials with high ageing resistance remain functionally stable over a long period and enable reliable products.
Typical signs of ageing in plastics
With increasing duration of use, many plastics show typical signs of ageing. These include embrittlement of the material, yellowing due to UV influence, surface cracks, chalking or a gradual loss of mechanical strength. Changes in dimensional stability or electrical properties can also occur. These effects arise from structural changes in the polymer chains and are a key reason why the ageing resistance of polymers plays an important role in material selection.
Chemical ageing mechanisms in polymers
The ageing of plastics is based on various physical and chemical mechanisms. One of the most important processes is thermo-oxidative degradation. In this process, polymer chains react with oxygen, causing molecular chains to be shortened or altered. This frequently leads to a reduction in mechanical strength.
Another central mechanism is photochemical degradation. UV radiation can directly break polymer chains and generate radicals in the material. These continue to react with oxygen and lead to progressive material damage.
Hydrolysis can also play an important role. In this process, water or moisture reacts with certain polymer bonds. Polyesters and some engineering plastics in particular are sensitive to this process.
In addition, stress cracking can occur. Mechanical stresses in combination with chemical influences lead to microscopic cracks that spread over time.
Factors influencing the ageing resistance of polymers
| Influencing factor | Effect on the polymer |
|---|---|
| Temperature | accelerates chemical reactions and molecular chain degradation |
| UV radiation | photochemical degradation of polymer chains |
| Oxygen | oxidative ageing and molecular chain shortening |
| Moisture | hydrolysis in sensitive polymers |
| Mechanical load | stress cracks and material fatigue |
The ageing resistance of polymers is determined by a large number of physical and chemical influences. One of the most important factors is temperature. Elevated temperatures considerably accelerate chemical reactions in the polymer. As a result, processes such as thermal oxidation or molecular chain degradation proceed much faster, which can lead to a loss of strength and structural stability over the long term.
Another central influencing factor is UV radiation. Plastics outdoors in particular are permanently exposed to sunlight. UV rays can break polymer chains and degrade additives in the material. Typical consequences are embrittlement, surface cracks or colour changes.
Oxygen also influences the ageing of plastics. Oxidative reactions can alter or destroy polymer chains. This process often occurs together with thermal load and accelerates as temperatures rise.
In certain materials, moisture can trigger chemical reactions known as hydrolysis. Polyesters such as PET or PBT as well as some polyamides in particular react sensitively to long-term exposure to moisture.
In addition to chemical influences, mechanical loads also play an important role. Permanent stresses or repeated loads can promote material fatigue or stress cracks. In combination with chemical or thermal influences, such loads considerably accelerate the ageing process.
Test methods for assessing ageing resistance
- Climate testing: Materials are exposed to alternating temperature and humidity conditions in order to simulate long-term environmental influences and realistic operating conditions.
- UV ageing tests: In test devices, sunlight is simulated using xenon or UV fluorescent lamps in order to investigate the effects of UV radiation on polymer chains, colour and surface structure.
- Thermal ageing tests: Plastics are exposed to elevated temperatures over a longer period, after which mechanical properties such as tensile strength, impact strength or modulus of elasticity are analysed.
- Chemical resistance testing: Materials are brought into contact with chemicals such as acids, alkalis, solvents or oils in order to assess changes in the material structure and the mechanical properties.
- Standard-based test procedures: Many tests are based on established standards such as ISO 4892 (artificial weathering), ISO 2578 (thermal ageing) as well as ASTM G154 and ASTM G155 for UV ageing tests.
Service life estimation and accelerated ageing
In materials testing, accelerated ageing tests are frequently used. In these, materials are exposed to higher temperatures or more intensive UV radiation in order to make ageing processes proceed faster.
An important concept for service life prediction is the Arrhenius model. It describes the relationship between temperature and reaction rate. Through tests at higher temperatures, conclusions can thus be drawn about the long-term stability of a material under normal operating conditions.
This method is frequently used to estimate the service lives of technical plastic components and to carry out material comparisons.
Material examples: ageing resistance of various polymers
In addition to their fundamental ageing resistance, plastics also differ greatly with regard to their typical service life and stability under certain operating conditions. Particularly in outdoor applications, chemical exposure or high temperatures, the choice of material plays a central role.
| Plastic | UV resistance | Temperature resistance | Ageing resistance | Typical applications |
|---|---|---|---|---|
| PEEK | very high | very high | very high | High-temperature components, mechanical engineering, aviation |
| PTFE | extremely high | very high | extremely high | Seals, chemical plants |
| PVDF | very high | high | very high | Pipe systems, outdoor applications |
| POM | good | moderate | good | Precision components, bearings |
| PA | moderate | moderate | moderate | Mechanical engineering components |
| PET | moderate | moderate | moderate | Technical components, packaging |
High-performance plastics generally have a significantly higher ageing resistance than standard polymers, particularly at high temperatures, under chemical exposure or with long-term UV exposure.
Influence of additives on ageing resistance
Additives play an important role in the long-term stability of plastics. UV stabilisers absorb harmful UV radiation and protect the polymer chains from photochemical degradation. Antioxidants prevent or slow down oxidative reactions in the material.
Carbon black is also frequently used to stabilise plastics against UV radiation. Glass fibre reinforcements can additionally improve mechanical stability and thus indirectly increase the service life of a component.
The targeted combination of suitable additives makes it possible to adapt plastics to specific operating conditions.
Practical relevance: importance of ageing resistance in industry
- Mechanical engineering: Technical plastic components must remain mechanically loadable over a long period so that systems function reliably and maintenance intervals are reduced.
- Outdoor applications: Plastics in façades, covers or technical constructions are permanently exposed to UV radiation, moisture and temperature fluctuations.
- Chemical industry: Materials must remain stable over the long term even in contact with aggressive media in order to ensure safety and process stability.
- Medical technology: Materials must retain stable properties over many years, as components often remain in use for the long term.
How the ageing resistance of polymers can be improved
The ageing resistance of polymers can be increased in a targeted manner through various measures. Additives such as UV stabilisers or antioxidants, which are incorporated into the material during production, play an important role. These substances slow down chemical degradation processes and protect polymer chains from harmful reactions.
The selection of the right material is also crucial for the service life of a component. High-performance plastics such as PEEK, PTFE or PVDF inherently possess a very high resistance to temperature, chemicals and environmental influences.
In addition, the design of a component can also influence its service life. Protection from direct UV radiation, suitable surface coatings or a reduction in mechanical stresses can considerably slow down ageing processes.
Why the ageing resistance of polymers is crucial
The ageing of plastics directly influences the safety, cost-effectiveness and service life of technical systems. Embrittlement, crack formation or loss of strength can lead to component failures and thus cause maintenance or replacement costs. Particularly in industrial applications with long-term periods of use, the selection of an ageing-resistant material is therefore a central part of the design.
Summary
The ageing resistance of polymers describes the ability of a plastic to retain its properties over a long period under environmental loads such as temperature, UV radiation, oxygen or moisture.
The ageing resistance of polymers significantly determines the service life of technical plastic components. Temperature, UV radiation, oxygen, moisture and mechanical loads change the material structure over time.
Through suitable test methods, targeted material selection and the use of stabilising additives, ageing processes can be controlled and the service life of plastic components considerably extended.
FAQ
Why do plastics age at all?
Plastics age through chemical reactions within the polymer structure. Factors such as UV radiation, temperature, oxygen or moisture alter the molecular chains and lead to changes in the material properties over the long term.
What does ageing resistance mean for plastics?
Ageing resistance describes how stably a plastic retains its properties over a long period under various environmental conditions.
Which factors accelerate the ageing of polymers?
The most important influencing factors include UV radiation, temperature, oxygen, moisture and mechanical load.
Which plastics have a particularly high ageing resistance?
High-performance plastics such as PEEK, PTFE or PVDF have a very high long-term stability against temperature, UV radiation and chemical influences.
How is the ageing resistance of plastics tested?
Typical test procedures are UV ageing tests, climate tests, thermal ageing tests and chemical resistance tests.
