Which plastics are recyclable?
To answer the question which plastics are recycled, we first need to understand the types of plastics we can find and how they behave at high temperatures. According to this criterion, we can classify plastics into two main groups: thermoplastics y thermosets. A very simple way to understand what is a thermoplastic polymer and what is a thermosetting polymer is to compare them with some foods.
Thermoplasticpolymers are those that behave like butter. When we have a jar of butter in the fridge, it is in a solid state, but if we heat it in a frying pan, it turns into a liquid state. But in this case the change of state is completely reversible, because if we let the butter cool down again, it will return to its solid state. Moreover, if we let it cool in a mould, it will take the shape of a mould. So we can say that butter can be melted, moulded and solidified several times without losing its mechanical properties.
The same applies to thermoplastic polymers. This type of polymer can be melted and moulded several times without losing its mechanical properties. This is why thermoplastic polymers are considered recyclable plastics, because they can be mechanically recycled by melting and moulding over several cycles.
Within this group there are different types of plastics, some of the best known are:
LDPE y HDPE
All these plastics can be recycled and there are now collection and segregation schemes and companies that recycle and sell recycled materials.
On the other hand, we have the thermosetting polymers which are those that behave like an omelette. When we cook an omelette, we start with beaten eggs in a liquid state that we mould into shape with heat in a frying pan. And it can only be moulded once, because if once the omelette has been cooked and cooled we heat it again, the only thing we achieve is to burn it. The same happens with thermosetting polymers. Once melted and moulded, if we apply heat again we will only burn them but we will not be able to transform them again.
We can say that this group of polymers are mechanically non-recyclable plastics, as we cannot re-melt them to use them again as raw material for the same application they were originally used for.
One type of thermosetting polymer that is widely used is polyurethane (PUR). This polymer is widely used in the manufacture of sponges, building insulation, shoe insoles, mattresses, etc. Currently the only recycling route for PUR is chemical recycling, whereby PUR is converted into polyols which are then used to re-manufacture PUR.
Although mechanical recycling of PUR is not possible, PUR waste is often reused as filler or filler in PUR products. For example, black shavings can be seen in some shoe insoles. These shavings correspond to reused PUR fillers. This reduces production costs and also the consumption of virgin raw materials.
In summary, thermoplastic polymers can be recycled both mechanically and chemically while thermoset polymers can only be chemically recycled. This implies a lower recyclability of this type of plastics, as currently more than 90 % of the recycling industry is dedicated only to mechanical recycling of plastics and the supply of chemical recycling is rather scarce and expensive.