According to Petcore Europe, Polyethylene Terephthalate (PET) was first developed for use in synthetic fibres by British Calico Printers in 1941. Although it was originally produced for this purpose, it was soon after that, in the mid-1960s, that it began to be used for packaging film, after which, in the early 1970s, the biaxially oriented blow-moulding technique for bottles was marketed. One of the main characteristics of this material is its environmental aspect, as it is fully recyclable. It was in 1977 when the first PET bottle was recycled and turned into a glass. Today, the fibre market is still the main consumer of rPET. But what are the different processes this material must go through before being recycled for a new use?
The European Union (EU) directive on packaging and waste (2004/12/EC) stipulates that Member States must set up collection systems to recover used packaging. However, it is up to each Member State to decide the most appropriate collection method. The following collection systems for post-consumer waste are present in different countries:
- Street collection: Citizens must separate recyclable materials from the rest of the rubbish by putting them in specific bags which are then collected. Usually, between 40% and 60% of selected materials are returned through this type of collection.
- Collection centres: citizens must dispose of recyclable materials in specific places. About 10-15% is recovered through this method.
- Landfills and deposits: this approach is quite common in countries such as Denmark, Sweden, Norway, Finland, the Netherlands and Germany. This system implies that bottles are sold with refundable deposits. These can be redeemed when the bottle is returned via vending machines, for example. These deposit schemes achieve remarkably high rates of return of PET, approximately 90%, and have low contamination levels.
Classification of plastics
The plastics collected are first sorted by material (PET, HDPE, etc.), then packaged to reduce the volume and ensure the profitability of the transport, and finally, sold to the reclaimers. After sorting, they are compacted into bales.
To ensure the treatment of plastic waste and its recycling, the EU has established a code (Resin Identification Code or RIC, integrated within the European Waste Catalogue) which, by means of a numerical sequence from 1 to 7 and a series of acronyms, identifies the waste according to the type of plastic polymer it is made of:
- PET or PETE (Polyethylene Terephthalate): Packaging and water bottles, soft drinks, etc.
- PE-HD or HDPE (High Density Polyethylene): containers for juices, perfumes, liquid detergents, etc.
- PVC (Polyvinyl chloride): toxic plastic used for packaging non-food products such as cables, pipes, etc.
- LDPE (Low Density Polyethylene): easily mouldable elements such as squeezable cans, flexible lids, or rubbish bags.
- PP (Polypropylene): common in the automotive and construction industry for the manufacture of engine parts, batteries, funnels, etc.
- PS (Polystyrene): many disposable elements used in the food industry such as cutlery, plates, trays, etc. are made out of the material.
- Others: those classified with the number 7 and the letter 0 are considered highly polluting and are the only ones that cannot be recycled.
The waste collected through the deposit system is sorted by the retailers. If it is done through the street collection systems, it is taken to local sorting centres. In both cases, this can be done manually or using machines manufactured for this purpose. Petcore mentions that an investigation carried out by the US Environmental Protection Agency reveals that automated systems result in a reduction of costs of approximately a 25%.
There are two main types of PET recycling: mechanical and chemical. The most widely used is the mechanical recycling, which consists of obtaining clean PET flakes that are used directly or mixed with virgin polymer in the subsequent transformation process to obtain other end products. The degree of purity of the recovered material is truly relevant in this process.
This system is currently used in Spain to recover PET. With this process, flakes of an optimum quality are obtained to manufacture sheets, fibres, or strips with the distribution of rPET on the market for each of these categories being a 65%, 30% and 3% respectively.
Chemical recycling is another process used to recover post-consumer waste. It consists of depolymerizing PET using chemical agents, which could release toxic substances and harm the environment. There are different processes that are determined based on the quality of the material to be treated and the demand for intermediate products. These are:
- Glycolysis: PET is dissolved in excess of ethylene glycol at high temperatures and the condensation reaction is reversed.
- Methanolysis: PET depolymerisation is carried out by treating the polymer with high amounts of methanol in the presence of a catalyst at high pressure and temperature (180-280ºC).
- Hydrolysis: PET is hydrolysed by a water and acid treatment to produce terephthalic acid (TA) and ethylene glycol. These require purification before reuse. Commercial hydrolysis is less common than glycolysis and methanolysis.
Advantages of the recycling of plastics
Recycling plastic materials has more advantages than disadvantages and that is where the trend is heading. Amongst others, the main benefits are:
- A reduction of the waste in landfills.
- Savings in the use of non-renewable fuels: plastics are manufactured from oil and by using pellets recovered from plastic waste the need for this energy resource is reduced.
- Plastics do not lose their energy content during use, so at the end of their life they can be used as fuel.
- Less water is consumed in the recycling process than in primary production, so it reduces the use of natural resources.
- In the recycling process, a lower amount of chemicals needs to be added to improve the properties of plastic products.