What is meant by degradation of recycled polymers: biological, oxidative, photo-degradation and thermal?

Among the advantages of the plastic materials that can be underlined, we find the lightness, the ease of processing, the possibility of coloring and the low production cost.

In fact, in those years we concentrated on the undisputed advantages of plastics without investigating the issues that determined their degradation .

Today, with the great experience that users and manufacturers of plastics have acquired, we can balance the advantages and disadvantages of such an innovative material.

We can classify the disadvantages between internal and external:

Internal disadvantages

• chemical and physical modification
• polymer production process
• chemical reactivity of additives

External Disadvantages

• thermo-hygrometric variations
• UV exposure
• pollutants
• heat
• microorganisms
• oxygen
• accidental causes

Furthermore, degradation can be physical or chemical . In physical deterioration, an increase in crystallinity and consequently in density can be noted, with the emergence of internal tensions, cracks and deformations.

The chemical one, which occurs at the molecular level, based on the degrading agent, affects the polymer chains with a loss of cohesion and a decrease in molecular weight.

OXIDATIVE DETERIORATION

Although the degradation of organic and inorganic polymers under the effect of oxygen is very slow, this causes the release of chemical substances that lead to the self-catalyzation of the polymer itself, that is, the chemical agents resulting from degradation in turn attack the polymer chain, activating a self-destructive process.

Furthermore, if this phase is affected by the formation of free radicals by the action of heat or light , then the reaction between the polymer and oxygen increases the rate of cleavage of the chains, which leads to crosslinking and the formation of volatile elements.

This process is called photo-oxidation or thermo-oxidation, depending on whether the trigger was light or heat. The direct consequences on the quality of the polymer can be seen through the reduction of the mechanical properties, especially with regard to elasticity and breaking strength.

ORGANIC DECAY

By biological degradation is meant the attack by fungi and bacteria on some polymers , especially those of natural derivation. These are subject to the phenomenon of hydrolysis , which can expose the polymer, in the presence of a high rate of humidity, to the breaking of the chains.

To stop degradation, you can opt for storage in an oxygen-free environment, but you need to know the origin of the polymer well as it is not a universally valid treatment.

THERMAL DETERIORATION

The phenomenon of thermal degradation is caused by the presence of mobile hydrogens in the chain or by the radical activity that are triggered by heat, causing the chain to break with the formation of breaks and the production of volatile elements.

The lack of oxygen leads to the depolymerization of the chain which occurs in three dissociative phases: initiation, molecular transfer and propagation.

To increase the chemical resistance of the polymers to thermal degradation, the best solution is to add additives during production.

PHOTO-DEGRADATION

The photo-degradation phenomenon occurs when the polymer is subject to the influence of UV rays in the wavelength range between 290 and 400 nm.

At the atomic level, we know that light radiation functions as a flow of particles, specifically photons , which, coming into contact with the molecules of materials and, under certain conditions, can interact passing from a state of low energy to one with high energetic excitation. .

These particular flows and movements are defined as photo-physical and / or photo-chemical . In the first case, there are no chemical modifications between the polymer molecules, while for the photochemical process, there are possibilities that the molecules alter their chemical characteristic by virtue of the presence of abundant energy.

In some synthetic macromolecules, the energy of the photons contained in UV radiation have the power to cause breakage of the covalent bonds.

Automatic translation. We apologize for any inaccuracies. Original article in Italian.