Polymer materials are now widely used in many fields such as high-end manufacturing, electronic information, transportation, building energy saving, aerospace, national defence and military industry because of their excellent properties such as light weight, high strength, temperature resistance and corrosion resistance. This not only provides a broad market space for the new polymer material industry, but also puts forward higher requirements on its quality performance, reliability level and guarantee capability.
Therefore, how to maximize the function of polymer material products in line with the principles of energy saving, low carbon and ecological development is of increasing concern. Ageing is an important factor in the reliability and durability of polymer materials.
Next we will look at what polymer ageing is, the types of ageing, the factors that cause it, the main methods of anti-ageing and the anti-ageing of the five most commonplastics.
The structural characteristics and physical state of polymer materials and their external factors such as heat, light, thermal oxygen, ozone, water, acid, alkali, bacteria and enzymes in the process of use make them subject to performance degradation or loss in the process of application.
Not only does this lead to a waste of resources and even greater accidents due to their functional failure, but the decomposition of materials caused by their ageing may also cause pollution to the environment. The ageing of polymer materials during use has the potential to cause huge disasters and irreparable damage.
Therefore, the anti-ageing of polymer materials has become an issue that the polymer industry has to address.
II. Types of polymer ageing
Due to the different varieties of polymers and different conditions of use, there are different aging phenomena and characteristics. In general, the ageing of polymer materials can be categorised into the following four types of changes.
01 Changes in appearance
Stains, spots, silvering, cracks, frosting, chalking, stickiness, warping, fish eyes, wrinkling, shrinkage, scorching, optical distortion and changes in optical colour.
02 Changes in physical properties
Changes in solubility, swelling, rheological properties, cold resistance, heat resistance, water permeability, air permeability, etc.
03 Changes in mechanical properties
Changes in tensile strength, bending strength, shear strength, impact strength, relative elongation, stress relaxation, etc.
04 Changes in electrical properties
Such as changes in surface resistance, volume resistance, dielectric constant, electric breakdown strength, etc.
III. Microscopic analysis of aging of polymeric materials
Polymers form excited molecules under the action of heat or light. When the energy is high enough, the molecular chains break to form free radicals, which can form chain reactions within the polymer and continue to trigger degradation, and may also cause cross-linking.
If oxygen or ozone is present in the environment, a series of oxidation reactions are also induced, forming hydroperoxides (ROOH) and further decomposition into carbonyl groups.
If there are residual catalyst metal ions in the polymer, or if metal ions such as copper, iron, manganese and cobalt are brought in during processing or use, this will accelerate the oxidative degradation of the polymer.
IV. Main methods to improve anti-aging performance
At present, the main methods to improve and enhance the anti-aging performance of polymer materials are the following four.
01 Physical protection (thickening, painting, outer layer compound, etc.)
The ageing of polymer materials, especially photo-oxidative ageing, first starts from the surface of the material or product, manifesting as discolouration, chalking, cracking, loss of gloss, etc., and then gradually goes deeper into the interior. Thin products are more likely to fail early than thick products, so the life of the product can be extended by thickening the product.
For products that are prone to ageing, a weather-resistant coating can be applied to the surface, or a weather-resistant material can be laminated to the outer layer of the product, thus attaching a protective layer to the surface of the product, thereby delaying the ageing process.
02 Improved processing
Many materials also suffer from ageing problems during the synthesis or preparation process. For example, the influence of heat during polymerisation, thermal and oxygen ageing during processing, etc. Then, accordingly, the influence of oxygen can be mitigated by adding deaerating devices or evacuation devices during polymerisation or processing.
However, this method can only guarantee the performance of the material at the factory, and it can only be implemented from the source of preparation of the material and cannot solve the problem of its ageing during reprocessing and use.
03 Structural design or modification of materials
Many polymer materials have groups in their molecular structure that are prone to ageing, so the design of the material’s molecular structure to replace the ageing-prone groups with non-ageing groups can often have a good effect.
04 Adding anti-ageing additives
At present, an effective and common way to improve the ageing resistance of polymer materials is to add anti-ageing additives, which are widely used due to their low cost and the fact that they do not require changes to existing production processes. These anti-ageing additives can be added in two main ways.
The anti-ageing additive (powder or liquid) is mixed directly with the resin and other raw materials and then extruded and granulated or injected, etc. This is a simple and easy method of addition, which is widely used by pelletizing and injection moulding plants.