The thermo-oxidative actions to which recycled plastic materials are subjected require an improvement in the mixtures of antioxidants

I recycled polymers, especially if we are talking about post-consumer plastics, are raw materials that more than others, due to their history of melting and cooling, life cycles subject to environmental conditions and due to mechanical wear conditions, they often undergo component degradation.

Indeed, under the effect of heat, solar radiation, mechanical stress, such as shear stresses and many others factors, free radicals can be created in the plastic material which cause oxidative degradation.

Furthermore, in the presence of oxygen, free radicals generate peroxy radicals, which steal atoms from the chain polymeric. The hydrogen peroxides formed in this way decompose forming other radicals, creating a chain reaction which leads to the degradation of plastic materials.

For these reasons, the use of antioxidants in polymer blends during the melting phases is considered essential to maintain the mechanical, rheological, optical and durability properties .

What are antioxidants for recycled polymers

First of all we can say that antioxidants are additives that are used as masterbaches, in order to improve characteristics of the final plastic product. The families can be classified between primary and secondary antioxidants, according to their final function in the dough.

Primary antioxidants feature reactive H2 atoms that react to free radicals, such as sterically inhibited phenols, aromatic amines, and inhibited amines sterically.

Secondary antioxidants have the function of breaking down hydrogen peroxides, thus preventing the branching of the chain. Phosphites and thioesters belong to this group.

Interesting studies are underway about the combined use of two types of antioxidants, especially in the field of PE, where it has been noted that the synergistic action of two elements can lead to a better result than the use of the single components used separately.

In fact, as we know, recycled materials, compared to virgin ones, come from cycles of wear and greater thermal suffering, therefore the use of antioxidants does not only becomes advisable, but the study of their blends can lead to interesting results.

As we said, the recycled material may have undergone oxidation cycles given by the mechanical and thermal conditions during its life, but we must also consider what is called the thermo-oxidative process started with the contact of the plastic with oxygen.

The subsequent melting of plastic waste to create the new recycled polymer can definitely aggravate the future quality of the product, as an already stressed raw material will be reused.

To avoid the thermo-oxidative degradation of a polymer, it is advisable to use one or more antioxidants, mixed together, to prolong the useful life of the plastic material by preventing its degradation.

It is rare to notice how a single antioxidant, whether it belongs to the primary or secondary type, can cover all the specificities, in terms of oxidative degradation, that plastic can present, therefore it may be necessary to combine two types of protective agents that can improve the result, only if combined, obtaining a better synergistic effect than the use of the two distinct antioxidants.

A good solution for stabilization, during processing, is the so-called phenol-phosphite, which is the combination of a hindered phenol with an organic phosphite, thus presenting an excellent synergistic effect that improves the properties compared to the effect of each of them used separately.

The stability provided by the mixture depends on its concentration. In the processing of recycled plastic material, the phosphite reacts by deactivating the hydroperoxides which are formed during the auto-oxidation of the polyolefins, while the phenol acts by trapping the free radicals which are formed.

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