Role of anti-oxidants in the performance of polymers

By Mehmil Khalid Kunwar

Nowadays, plastics make an important contribution to our society. They possess remarkable innate properties that make them ideal for various applications. But, there are some properties that need to be altered in order to enhance the efficiency of the end product. Now, you might be wondering how it happens?

The change in properties is achieved by way of using additives. These additives play their role during the polymer processing, giving the desired effects and changing the overall properties. They come in variety of types exempli gratia antistatic, anti fog, UV absorber, heat stabilizer, pigments, anti slip etc. Each one of them has a specific purpose to offer to the polymer and transfiguring it into a more convenient form depending upon the requirement in the end usage.

Here our discussion would revolve around additives commonly known as anti-oxidants and the role plays by them in the processing of polymer. Ever wondered what would happen to the polymer if it loses its strength due to bond cleavage as result of oxidative process it experiences? Obviously, you would notice that it ceases to possess the required mechanical properties or undergo discoloration that is needed for the end product and ultimately make the polymer to decompose. To avoid these undesirable effects and prolong the life of polymer and hence the product, anti-oxidants are used. The anti-oxidant functions in a manner that not only inhibits the degradation of plastic but also maintains the strength and stiffness of the polymer. When exposed to high temperatures, ozone and oxygen during processing or in service of the polymers, they tend to die due to the thermo-oxidative process that interferes with the strength of the polymer. Anti-oxidants help in protecting the plastics against thermally induced oxidation by reacting with and decomposing the species that are responsible for oxidation such as compounds having free radicals and hydro peroxide intermediates.

Anti-oxidants agents come under two major categories i.e. primary anti-oxidant and secondary anti-oxidant. Primary anti-oxidant, also known as free radical scavenger, acts by suppressing the formation of free radicals. The most commonly used type is sterically hindered phenols which are effective over a wide range of temperature, used both during processing and long-term thermal aging. 

Secondary anti-oxidants are known as peroxide scavengers and as the name suggests they decompose hydroperoxides into non reactive products and normally used in a combination with primary oxidants to achieve a synergistic inhibition effect. The most commonly used secondary anti oxidants are phosphites and thioethers, the former being used at high temperatures during melt operations and the latter used as solid phase at long term use temperatures.

Nevertheless, it depends upon the temperature range under which the polymer is being processed to choose the most effective form of anti oxidant. The best anti-oxidant should protect the polymer and preserve its properties during its processing under high melt temperatures and also works effectively when it is opted for upper service temperatures.

In near future, due to increase in innovative products around the world and safer use of plastic as an alternative to metal and steel, we might expect more advancement in the polymer additives, especially, the anti-oxidants, tailoring the properties of polymers as per product needs.