Polyethylene terephthalate (PET) is one of the most common polymers in its polyester family. Its global market size was estimated to be worth US$37.25 billion in 2021. Polyethylene terephthalate is used in several applications such as; textile fibres, bottles, rigid/flexible packaging, and electronics. However, it accounts for 12% in global solid waste. This is why bottle recycling is highly encouraged and has reached its highest level in decades (33% in 2023). In 2023, the US collected 1,962 million pounds of bottles for recycling. Compared to glass bottles, the PET bottle is lightweight and has a lower carbon footprint in production and transportation. Recycling further reduces emissions. The recycled material can be put back into bottles, fibres, film, thermoformed packaging and strapping.
After collecting the bottles from landfills, they are sorted, cleaned and grinded. This grinded material is "bottle flake", which is then processed by either: "Basic" or "physical" recycling. Bottle flake is melted into its new shape directly with basic changes in its physical properties. "Chemical" or "advanced" recycle. Bottle flake is partially or totally depolymerized then enabling purification. The resulting oligomers or monomers are repolymerized to PET polymer, which is then processed in the same way as virgin polymer. In either case, the resulting feedstock is known as "r-PET" or "rPET" ("recycled PET"). This recycled PET's carbon footprint is 79% lower than virgin PET's, namely 0.45 kg CO2 per kg instead of 2.5 kg C02 per kg.
Bottle manufacturing PET and rPET are both used in the creation of bottled water (still and carbonated). Bottled water companies have been voluntarily using rPET in the production and many companies are producing bottles using 100% rPET nowadays. Such companies include Dasani, Fiji and Nestlé Pure Life. Other goods bottled in PET include oil, vinegar, milk, and shampoo. These bottles are closed with polyolefin screw closure with antitamper ring, and have a label which may be printed on paper or plastic and may be glued on. The resin may be colourless or tinted blue, green or brown, or pigmented white.
When manufacturing these water bottles, the energy used to mold the resin into its shape varies based on the bottle shape and its thickness. Some bottles have complex shapes such as Dasani while other bottles have a very simple form such as Smartwater. Therefore, the energy used to create these bottles can range from 8.33 - 20 MJ/kg where the units "MJ" represent megajoules, a unit for energy. The emissions associated with the production range from 0.034 – 0.046 kg C02-eq per 500mL.
Collection and sorting
The empty PET packaging is discarded by the consumer after its use and becomes PET waste. In the recycling industry, this is referred to as "post-consumer PET". All types of PET packaging, including bottles are usually marked with the recycle symbol 1. The bottles are sent to trash centers (materials recovery facilities) and get sorted out from the other disposable items. There are times when the recyclables are taken to a transfer station first. At this facility, the materials are stored, sorted, and compacted before being transported to a true recycling facility where they are further processed. This step is especially common in areas where MRFs are located far from collection points Here the PET bottles are sorted and separated from other objects and bottles made of other materials
Collection and sorting process in Switzerland Source: recyclables collected from bins and sent to facility metal collection ballistic sorting (items that fall slower or faster in air such as dust, films, glass bottles and stones are removed here) metal separation again spectral sorting: sensors detect polymer type and colour sorting on a conveyor belt (manual) baling: the flattened bottles are compressed into bales for shipment to the processing centre
Types of sorting at MRFs
Air classifiers: Stream of air directed towards conveyor belt that lifts away lighter materials (paper and light plastics) Eddy current / magnetic separators: Use magnets to separate magnetized and non-magnetized metals Manual separation: Workers stationed along conveyor belt that sort out items and contaminants (usually assisting the machines and catching what they missed) Optical sorters: A camera or laser sorts items based on color, shape, and other properties (still a new technology) Screens: Rotating screens that separate materials based on size (like a colander)
The sorted post-consumer PET bottless are flattened, pressed into bales and offered for sale to recycling companies. Colourless/light blue post-consumer PET attract higher sales prices than the darker blue and green fractions. The mixed color fraction is the least valuable due simply to the fact unlike aluminium, there are few standards when it comes to the coloration of PET. Unlike clear varieties, PET with unique color characteristics are only useful to the particular manufacturer that uses that color. For material recovery facilities, colored PET bottles are therefore a cause for concern as they can impact the financial viability of recycling such materials. The Plastics Recyclers Europe (PRE, Brussels, Belgium), that an upsurge in a variety of PET colors would be a problem because no market exists for them in the current recycling climate. From there, there are two types of recycling; chemical or mechanical.
Types of collections Deposit: some countries have legislated a deposit for packaging including PET bottles. In the EU, deposit schemes average an 86% recovery rate. Collect: waste collectors pick up PET bottles mixed into some other stream (54% recovery in EU). Bring: consumers take PET bottles and place them into a container (43% recovery in EU).
Different countries have opted for different systems.
France: public voluntarily puts PET bottles into containers for plastic bottles and metal packaging. The stream in which PET bottles are collected comprises metallic packaging, plastic bottles, and unwanted contaminants. Germany: PET bottles carry a deposit, so PET bottles are collected by retailers. The collected stream consists almost entirely of PET bottles. Singapore: plastic bottles are collected with glass bottles. The stream in which PET bottles are collected comprises PET bottles, other plastic bottles and glass bottles, and contamination. Switzerland: Retailers contribute a fee to a national operator (PRS) who manages collection bins, sorting and production of rPET flake. The stream in which PET bottles are collected is intended to be PET bottles, but contains other PET packaging and other contamination. UK: Plastic producers pay a fee, and collection is devolved to municipalities. The stream in which PET bottles are collected varies by municipality, but always require further sorting. United States: curbside recycling to which most consumers have access. The waste hauler brings the recycled material to a material recovery facilities (MRFs) where it is further separated. The PET is then baled and sent on to a PET reclaimer. The PET reclaimer processes the bale, grinding the PET into flakes. Some do additional processing to make ready for food grade packaging.
Chemical recycling This method of recycling is not very common anymore since it is a more intensive and expensive process. Chemical recycling involves breaking down the plastic into its monomers which can be used as building blocks for new materials. Because of the use of more energy for the chemical reactions to take place, chemical recycling produces more emissions than mechanical recycling. This process is also known as "Tertiary" or "Advanced" recycling. Polyethylene terephthalate can be depolymerized partially or completely to yield the constituent oligomers or the monomers, MEG and PTA or DMT. The main processes are glycolysis, methanolysis or hydrolysis. After purification, the oligomers or monomers can be used to prepare new recycled polyethylene terephthalate ("r-PET"). The ester bonds in polyethylene terephthalate may be cleaved by hydrolysis, or by transesterification. The reactions are simply the reverse of those used in production.
Partial glycolysis Partial glycolysis (transesterification with ethylene glycol) converts the rigid polymer into short-chained oligomers that can be melt-filtered at low temperature. Once freed of the impurities, the oligomers can be fed back into the production process for polymerization.
The task consists in feeding 10–25% bottle flakes while maintaining the quality of the bottle pellets that are manufactured on the line. This aim is solved by degrading the PET bottle flakes—already during their first plasticization, which can be carried out in a single- or multi-screw extruder—to an intrinsic viscosity of about 0.30 dℓ/g by adding small quantities of ethylene glycol and by subjecting the low-viscosity melt stream to an efficient filtration directly after plasticization. Furthermore, temperature is brought to the lowest possible limit. In addition, with this way of processing, the possibility of a chemical decomposition of the hydro peroxides is possible by adding a corresponding P-stabilizer directly when plasticizing. The destruction of the hydro peroxide groups is, with other processes, already carried out during the last step of flake treatment for instance by adding H3PO3.Boos, Frank and Thiele, Ulrich "Reprocessing pulverised polyester waste without yellowing", German Patent DE19503055, Publication date: 8 August 1996 The partially glycolyzed and finely filtered recycled material is continuously fed to the esterification or prepolycondensation reactor, the dosing quantities of the raw materials are being adjusted accordingly.
Total glycolysis The treatment of polyester waste through total glycolysis to fully convert the polyester to bis(2-hydroxyethyl) terephthalate (C6H4(CO2CH2CH2OH)2). This compound is purified by vacuum distillation, and is one of the intermediates used in polyester manufacture (see production). The reaction involved is as follows:
[(CO)C6H4(CO2CH2CH2O)]n + n HOCH2CH2OH → n C6H4(CO2CH2CH2OH)2
This recycling route has been executed on an industrial scale in Japan as experimental production.
Methanolysis
converts the polyester to dimethyl terephthalate(DMT), which can be filtered and vacuum distilled:
[(CO)C6H4(CO2CH2CH2O)]n + 2n CH3OH → n C6H4(CO2CH3)2
Even though polyester production based on dimethyl terephthalate(DMT) is limited to legacy plants,Fakirov, Stoyko (ed.) (2002) Handbook of Thermoplastic Polyesters, Wiley-VCH, Weinheim, pp. 1223 ff, investments were announced in 2021 and 2022 into methanolysis plants.
Hydrolysis Hydrolysis can be done in a neutral, alkaline or acidic environment.
Neutral hydrolysis Polyethylene terephthalate can be hydrolyzed to terephthalic acid and ethylene glycol under high temperature (200-300 °C) and pressure. The resultant crude terephthalic acid can be purified by recrystallization to yield material suitable for re-polymerization:
[(CO)C6H4(CO2CH2CH2O)]n + 2n H2O → n C6H4(CO2H)2 + n HOCH2CH2OH
Avoiding a neutralization step consumes less resource than alkaline or acidic hydrolysis, but there is no opportunity to filter a solution, so mechanical impurities remain with the terephthalic acid. This method does not appear to have been commercialized as of 2022.
Alkaline hydrolysis
Alkaline hydrolysis is done in an aqueous solution of potassium hydroxide or sodium hydroxide. The reaction yields ethylene glycol and the terephalate salt, in aqueous solution. After separation and filtration, in a second step, the salt is neutralized with strong mineral acid to precipitate the terephthalic acid.
[(CO)C6H4(CO2CH2CH2O)]n + 2n MOH → n C6H4(CO2M)2 + n HOCH2CH2OH
C6H4(CO2M)2 + 2HCl → C6H4(CO2H)2 + 2MCl