We often hear about antioxidants when we come into contact with plastics or other polymer materials. Maybe I also understand that the purpose of adding antioxidants is to resist oxidation and aging, but most of my friends may not understand the mechanism. Today, let's learn together about the causes, mechanisms, and solutions of polymer material aging.
So, first of all, we need to understand what oxidation is?
Professional explanation: Oxidation, in a narrow sense, refers to the chemical reaction between oxygen and other material elements, and is also an important chemical unit process. Broadly speaking, oxidation refers to the process of a substance losing electrons (increasing oxidation number).
Simply put:phenomena such as yellowing, aging, hardening, brittleness, softening, dullness, rusting, etc. caused by exposure to wind, sun, rain, oxygen in the air, water, and wind erosion can be roughly classified as oxidation.
The best example is the phenomenon of a sliced apple gradually turning yellow after being exposed to air for a period of time.
In fact, not only apples will oxidize, but our skin will also oxidize. Plastic products and high molecular polymers will also oxidize and age!
Aging is a universal process in nature, and the most common one around us is the process of life from birth, growth, and aging, which is the most typical aging process.
The rusting of metal materials is also a form of aging, from shiny objects to rust stains, until they lose their practical value.
Similarly, in the processing and use of polymer materials, due to the comprehensive effects of environmental factors such as heat, oxygen, water, light, microorganisms, and chemical media, the chemical composition and structure of polymer materials will undergo a series of changes, and their physical properties will also deteriorate accordingly, such as hardening, sticking, brittleness, discoloration, and loss of strength. These changes and phenomena are called aging.
1, What are the disadvantages of molecular polymer aging?
1. Reduce mechanical performance
2. Fading
3. Reduce transparency
4. Yellowing and aging
5. Cracking
6. Odor (alcohols, aldehydes, ketones)
2, What factors are causing polymer aging?
1. The structure of polymers: structures with unstable double bonds and benzene rings can easily lead to polymer decomposition and yellowing
2. Residual impurities in polymer synthesis: peroxides, metal ions leading to polymer degradation, yellowing
3. Oxygen (air): The presence of oxygen and ozone causes the decomposition of polymers
4. Energy: mechanical stress, heat, ultraviolet radiation
5. Air pollution: NOx, SOx.
6. Water and humidity: may cause extraction, dissolution, and precipitation. Organisms: degradation caused by biological enzymes
3, Tracing the root cause and delving into the mechanism of polymer aging:
Why does it age? (Aging mechanism)
Under the action of heat or light, polymer molecules will form excited states. When the energy is high enough, the molecular chains will break and form free radicals. Free radicals can form chain reactions inside the polymer, continue to cause degradation, and may also cause cross-linking.
Main culprit: free radical R·
If there is oxygen or ozone in the environment, it can also induce a series of oxidation reactions, forming hydrogen peroxide (ROOH) and further decomposing into carbonyl groups.
If there are residual catalyst metal ions in the polymer, or if metal ions such as copper, iron, manganese, cobalt are introduced during processing or use, it will accelerate the oxidative degradation reaction of the polymer.
So, in order to prevent polymer aging, it is important to add antioxidants to isolate light and heat!
