Polyethylene terephtalate is a thermoplastic polyester (PET) that features great mechanical and thermal properties. It’s also one of the versatile engineering plastics that is used for manufacturing bottles, packaging, as well as textiles, video, audio tapes, as well as photographic films.
Glycolysis is involved in the chemical recycling of PET, as well as other processes such as:
Additionally, PET can be depolymerised by glycolysis and results are:
- oligomeric diols
- oligomeric polyols
- a mixture of its oligomers
One of the most interesting and studied glycolysis reaction of PET is with an excess of 2,2’-oxydiethanol (diethylene glycol, DEG) or with 1,2-ethanediol (ethylene glycol, EG). The result of this reaction consists of polyols of high crystalinity.
The crystallisation of polyols has been studied by G. Colomines and his co-workers. Their studies have shown that the polyols obtained through glycolysis of PET using a mixture of DEG were less amorphous than those obtained using oligoesters.
When polyester polyols are prepared, two processes take place:
- depolymerization of PET and the result is a glycolysed product (GP) such as a mixture of BHET, oligomers and unreacted glycols.
- GP is reacted with dicarboxylic acids, glycols and other additives, the result being polyester polyols.
There are recent studies that try to observe what happens after recycling of industrial PET waste by glycolysis with EG directed to BHET and glycolysis in the presence of glycerol and adipic acid, which are directed to polyester polyols with an increased stability against crystalization.
Industrial PET was analyzed from various points of view such as:
- intrinsic viscosity
- melting point
- acetaldehyde (AA), EG
- the content of water
- end-carboxylic groups
- acid number (AN)
Glycolysis is one of the cost-effective viable process when chemically recycling PET waste. During glycolysis, a lot of things take place such as:
- ester linkages are broken and replaced by hydroxyl terminals
- waste is converted into BHET and its oligomers: dimmer and trimmer
Additionally, the extent of depolymerization of PET depends on the excess of EG taken for glycolysis.
Studies have shown that the products of recycling of industrial PET waste through glycolysis with EG contain an increased amount of bis(hydroxyethyl) terephthalate (BHET), this process being very complex and important.