Aging and Degradation of Recycled Polymers

We often talk about the degradation of recycled polymers due to factors concerning the transformation and recycling phases of raw materials, with negative consequences on the final product.

Less is said about the aging phenomena of polymers that concern the amorphous ones and the amorphous part of the semicrystallines, below the glass transition temperature.

While for aging the temperature conditions can affect or recover an ideal state of the polymer, degradation is, in itself, a more complex issue because it concerns, not only the components of the recycled polymer recipe, but also the processes of use. of the polymers themselves.

Not being able to analyze the aging and degradation behaviors of all the compositions of recycled plastics, especially as regards those that come from post-consumption, we limit ourselves to illustrating the main causes that can determine the factors described above.

The temperature

If we take the conventional temperature at 20 ° we can say that the variations, positive or negative of the same, generate in the polymer significant changes in the mechanical characteristics and their behavior, which can change from ductile to brittle.

In particular, the thermal degradation can be chemical, with the determination of the breakage of the links of the chains, or physical, with behavioral variations with respect to the status at the glass transition temperature.

While physical degradation is always reversible, for the chemical one we always speak of the irreversibility of the phenomenon.

Thermal degradation can be expressed visually with the yellowing of the product or in the partial loss of color.

Fire

Most plastics are combustible and when they come into contact with high temperatures they can burn and develop harmful gases.

When making the products, the fire behavior must be taken into consideration which, in addition to influencing the stability of the product, can create dangerous phenomena of toxicity.

In some materials, however, the combustion is delayed or even inhibited thanks to the presence in them of significant quantities of chlorine (as in PC) or fluorine (as in PTFE or ETFE).

Quick Cooling

As we have seen previously, a sudden change in temperature can cause aging in polymers. For example, a too fast cooling in the production phase of the product can create a phase of imbalance in the molecules with respect to the starting neutral state.

However, with time, the macromolecules tend to move towards a condition of equilibrium, however, causing a slight decrease in volume, an increase in rigidity and thickening of the material.

The Solvents

Polymers such as PE, PVC, PTFE or ETFE, do not corrode electrochemically like metals, normally offering good resistance to acids on an inorganic basis, but can react with organic solvents (e.g. acetone) and sometimes with water (for example nylon).

In this situation we can find as negative effects the breaking of the intermolecular bonds, the decrease of the elastic modulus and the swelling of the materials.

Oxidation

The reduction of mechanical properties can also be determined by oxidation: free radicals from the breaking of chemical bonds of the chains fix oxygen. Particularly sensitive to this type of degradation is polypropylene.

Ultraviolet Rays

The action of ultraviolet rays is harmful in the long run, because it not only deteriorates the appearance of the material by bleaching or browning it, but also reduces its mechanical properties.

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