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Karina This article was first published on 11 June 2021 and last updated on 29 July 2021.
Polyethylene terephthalate (PET) is a thermoplastic from the polyester family, i.e. its monomers contain the ester functional group. The chemical formula for PET is (C10H8O4)n. PET was patented in 1941.
Production (in theory)
The base molecules are p-xylene (para isomer of xylene) and ethylene. Dimethyl terephthalate (DMT) and ethylene glycol produced from these base molecules react to produce PET, often using terephthalic acid as an intermediate molecule. Polymerization is achieved through a polycondensation reaction with water as a byproduct; this results in amorphous PET.
Amorphous PET can be transformed into bottle grade PET through solid state polymerisation and into PET film through film extrusion.
PET is clear and tough. It can be semi-rigid or rigid. Its density is about 1.4 g/cm3. PET softens at 80℃; its melting point is above 250℃, its boiling point is at 350℃ (then it starts to decompose).
PET is used to produce synthetic fibres (60% of PET is used in this way) like polyester, polycotton and fleece, drinks bottles (30% of PET is used in this way), food containers, ovenable and microwavable food trays, carpets, films, etc.
More than half of the world’s synthetic fiber demand is fulfilled by PET. PET mixes well with cellulose, like cotton fibers, which results in textiles that are stronger and resistant to creasing.
More than half of the world’s plastic bottle demand is fulfilled by PET. This visualisation by Visual Capitalist shows how the amount of plastic bottle waste generated daily compares to the size of the Eiffel Tower.
Reusable PET bottles are typically washed with a strong caustic soda solution (sodium hydroxide, NaOH) and sterilized before being refilled. The main disadvantage of PET bottles is that they are not completely gas tight. This can be fixed by applying a silicon oxide (SiOx) barrier coating, but this coating is not resistant to caustic soda. IKV and KHS Corpoplast GmbH have developed the PECVD gas barrier coating that is resistant to caustic soda, and allows PET bottles to be reused up to 20 times.
The content of additives in PET bottles is less than 1%.
Polyethylene terephthalate glycol-modified (PET-G) is a type of PET that is more resistant to heat, impact and solvents. It is made by replacing the ethylene glycol in the molecular chain with the cyclohexane dimethanol, a larger monomer.
LEGO bricks, traditionally made from ABS, are moulded incredibly precisely compared to most consumer products - the tolerance is 1-2 microns, less than the thickness of a human hair. This precision is essential to ensure that two joined bricks stick together exactly as intended. While PET is easy to recycle, PET is less stiff, less hard and less precise than ABS. Testing more than 250 variations of PET materials over three years resulted in the first recyclable LEGO brick in July 2021. It is made from a new grade of PET with “secret sauce” (containing impact modifiers), that performs like ABS.
Biobased PET, for example synthesized from sugarcane, is identical to oil-based one and can be recycled in the same stream.
Effect on climate
PET doesn’t biodegrade, but if it was biodegradable it would release ethylene, a powerful greenhouse gas.
Production (in practice)
According to Plastics Europe, 40 million tonnes of PET are produced every year. The majority of PET production capacity and consumption is located in Asia. The top 5 producers account for more than 30% of the total production of PET globally. These include Indorama Ventures, M.G. Chemicals, Zhejiang Yisheng Petrochemical (PetroChina Group), Sang Fang Xiang and DAK America.
More than 50% of PET consumption is in China. In North America and Western Europe PET consumption has stopped increasing rapidly since 2015, mainly due to increasing imports of polyester products from other regions.
PET is the most widely recycled plastic. Its recycling rates are 97% in Norway, 85% in Japan and 84% in Sweden.
Clear PET waste has especially high recycling value.
Recycled PET (rPET) is in high demand. This demand is in part driven by policies, such as the EU’s Directive on single-use plastics that requires all PET bottles to contain at least 30% recycled plastic from 2030.
PET and PC in a molten state forms a homogeneous mixture suitable for production of high quality recycled material. PET is not compatible with PP, PE, PS, PVC, PA, ABS, PMMA or SAN - they don’t form a homogeneous mixture in a molten state.
PET bottles can be recycled into textiles, but textiles made from PET can not be recycled into bottles.
Different recycling methods are available for PET: mechanical recycling, solvent-based recycling, depolymerization, feedstock recycling (including biological decomposition). The DMT has to be pure enough to be used in polymerization reactions, which is the main barrier for chemical recycling of PET.