school energy&environment

Plastic

If we look around, we will see that many of the objects that we use daily all around us are made of an extremely versatile, light and economical material, plastic. The toothbrush, the cover of the mobile phone, pens and felt pens, the computer, the television, all these objects and many others contain at least some plastic. However there isn’t only one type of plastic. Items made of this material are of many different types, and it is sufficient to compare the plastic supermarket bag and the bottle that contains a detergent to immediately see a number of differences.
The term plastic in fact is usually used to classify different families of polymers, i.e. long chains of molecules with a high molecular weight consisting of a large number of molecular groups, derived from petroleum refining and containing carbon, hydrogen, oxygen and chlorine. Each type of plastic corresponds to a different material, with specific. physical, chemical and mechanical characteristics. This heterogeneousness implies different recycling processes, depending on the polymer or the family of polymers that are treated, so that in this case it is not possible to speak of plastic recycling in general, because actually there are many plastics. The most common and most widespread in daily consumption may be subdivided into two large groups: thermoplastic material, that softens in the presence of heat and becomes hard whencooled, and thermohardening material that solidifies irreversibly when heated. Thermoplastic resins are the easiest to recycle and among these categories the most common in our daily use are:

• PE, polyethylene, generally bags, bottles and film are made of polyethylene – depending on the type of processing it is subjected to;
• PP, polypropylene, used for a large number of different items from food trays to garden furniture;
• PVC, polyvinylchloride, for trays, film, pipes;
• PET, polyethylene terephthalate used for bottles for soft drinks and mineral water, synthetic fibres;
• PS, polystyrene, better known as thermocole used mainly for corks, plates, cutlery and trays for foodstuffs.

The recycling procedure can be mechanical (more common), or chemical.
Firstly, in case of the mechanical recycling procedure, the material collected through separate waste collection must be selected so that any foreign bodies are identified and eliminated, and the different types of packaging are sorted according to the type of polymer and colour wherever possible. In order to guarantee high yields, selection of the different plastic materials is fundamental. The sorted material is then sent to the recycling line were it is crushed, washed, ground, dried and finally granulated. In the final phase granules or flakes, that can be used in transformation plants, are obtained.
Chemical recycling instead is applied on an industrial scale, and it is aimed at breaking the polymer macromolecule into its more simple individual units (monomers), to be used as new raw material. The granules and flakes can be used for different purposes depending on the initial polymer: for example PET bottles are used to produce fibres and textiles (such as pile blankets), PE is used for bottles and containers, PVC is used for pipes and sewage plumbing, and electrical materials. In Italy, separate waste collection is only carried out in the case of plastic packaging materials, for which the percentage of recovered product is however high. According to ISPRA, recycling of packaging material amounts to 84.4%. Furthermore, unlike paper and glass, for plastic, also energy recovery may be foreseen (remember, plastic is obtained from petroleum) as its lower heating power, i.e. the amount of heat that is freed during combustion, is sufficient to justify this option (30-35 MJ/kg), an option that regards approximately 33% of the recovered packaging in Italy. How much do we save by correctly recycling plastic? Energy saving is high, from 40% to 90%, with an average of 50%, while the saving of material is 100%! If we avoid sending plastic to the dumps and we recycle it correctly, we avoid emissions of 1.39 kg CO_2eq per kilo of plastic, and therefore contribute to contrasting the increase in carbon dioxide emissions in the atmosphere.