Repolymerization, PET

Chemical recycling of PET. Chemical recycling of PET depends on chemical reactions which break down the PET into small molecules, which can then be used as chemical feedstocks, either for repolymerizing PET or for manufacturing related polymers. Two procedures, glycolysis and methanolysis, are in commercial use. Both can be used to produce PET which is essentially chemically identical to virgin polymer, and which have been approved for food-contact use. - ... 

The major PET manufacturers are depolymerizing scrap PET with glycols (glycolysis) or methanol (methanolysis) to form low-molecular-weight polyester diols (and BHET) and dimethyl terephthalate.3 The purified products are then used to make new products. Goodyear uses glycolysis to make REPETE, a new product which contains 10-20% recycled PET. Hoechst Celanese used methanolysis to produce DMT for repolymerization. Eastman Chemicals uses depolymerization of PET to recover used X-ray scrap. 

The principal solvolysis reactions for PET are methanolysis with dimethyl terephthalate and ethylene glycol as products, glycolysis with a mixture of polyols and BHET as products, and hydrolysis to form terephthalic acid and ethylene glycol. The preferred route is methanolysis because the DMT is easily purified by distillation for subsequent repolymerization. However, because PET bottles are copolyesters, the products of the methanolysis of postconsumer PET are often a mixture of glycols, alcohols, and phthalate derivatives. The separation and purification of the various products make methanolysis a cosdy process. In addition to the major product DMT, methanol, ethylene glycol, diethylene glycol, and 1,4-cyclohexane dimethanol have to be recovered to make the process economical.1... 

Other than for simple glycolysis, a substantial capital investment must be made to conduct commercial depolymerization of PET to regain PET monomers for repolymerization of PET. As the capital costs rise at roughly the 0.6 power of the relative volume [68], larger facilities are more economically attractive than smaller facilities. Besides the availability of capital to build very large depolymerization facilities, the limiting criterion has been and is likely to continue to be the sure supply of adequate PET feedstock at acceptable prices. 

Treatment of PET with methanol yields DMT, which can also be repolymerized into PET. Compared to PET glycolysis into BHET, DMT is obtained with a higher purity by methanolysis, although it is a very capital-intensive process. 

Most polymers such as PE, PP, PS, HOPE and PET (for non-food applications) are recycled after purification while others such as PET (for food applications) are first depolymerized, then purified and repolymerized. Other polymers such as ABS and PVC are rarely recycled. Currently, most recycled polymers end up as the same polymer, but because the recycled polymer is often less pure, there can be issues with degradation of physical properties and with meeting regulatory requirements for food packaging. Therefore, there is potential for finding alternative, higher value uses for these waste materials. 

The first of these processes to receive a letter of nonobjection from the U.S. FDA was Goodyear s glycolysis process in 1991 (later sold to Shell). Later that same year Eastman Chemical and Hoechst-Celanese received approval for their methanolysis processes. The glycolysis processes typically produce partial depolymerization, which is followed by purification and repolymerization. Methanolysis processes provide full depolymerization, followed by purification by crystallization and then repolymerization. Glycolysis cannot remove colorants and certain impurities which can be removed by methanolysis. DuPont also operated a methanolysis facility for recycling PET, but indicated recently that it is discontinuing the operation for economic reasons. 

Recovered PET can be chemically broken down into small molecular species, purified, and then repolymerized. The two major processes for tertiary recychng of PET are glycolysis and methanolysis. Both result in PET that is essentially chemically identical to virgin resin and has been approved by the U.S. Food and Drug Administration for food contact applications. However, PET produced by these processes is more costly than virgin resin, which significantly limits its use. 

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