Technology Type

Through Depolymerization, PET Plastic and Polyester Fiber Feedstocks are broken down into their base Building Blocks. Different Building Blocks are formed depending on the Process used. These Building Blocks can be used to create new, virgin-quality PET. The benefit over Mechanical Recycling is the possibility to take any type of Feedstock Waste such as higher IV Fiber and Trays but also Coloured Flakes since the Dyes are removed in the PET Depolymerisation Process.

Aminolysis of PET is an area that has not been widely exploited, likely because this Process requires an Amine (which is often toxic or expensive) to depolymerize PET, yielding Diamides of Terephthalic Acid (TPA) that are not usable for re-synthetizing PET.^[1]

PET Hydrolysis is a Recycling Method that involves a reaction of PET with Water in an acid, alkaline or neutral environment, leading to total Depolymerization into its monomers, TA (or a Terephthalate Salt) and Ethyleneglycol (MEG). Major drawbacks of the Acidic Hydrolysis are the Separation of MEG from the highly acidic Solution and the amount of Acid needed to industrialize the Process. Notable drawbacks of Alkaline Hydrolysis are longer Reaction times and higher temperatures compared with Acid Hydrolysis. Neutral Hydrolysis requires high temperatures (220-300°C) and elevated pressure (1-4 MPa) in the presence of Catalysts but still have relatively slow rates of Reaction and produce low purity Monomers.^[1] Hydrolysis is slow compared to Methanolysis and Glycolysis, because among the three Depolymerizing Agents (i.e. Water, Methanol, Ethylene Glycol), Water is the weakest Nucleophile. Another disadvantage of Hydrolysis is the difficulty of recovery of the TPA Monomer, which requires numerous Steps in order to reach the required purity.^[2] There is not any known example of a commercial PET Hydrolysis Recycling Process.

PET Methanolysis yields the Dimethylester of Terephthalic Acid (DMTP). Methanol reacts with PET at high temperatures (180–280°C) and pressures (20–40 atm) in the presence of a Catalyst, most commonly Zinc Acetate. This produces the two monomers of PET, Ethane-1,2-Diol (MEG) and the Dimethylester of TPA (DMTP), which are subsequently purified by Distillation. A major drawback of this method, outside of the high temperatures and pressures, is again the Purification Process. The Crude Product contains not only DMTP and MEG but also other Alcohols and Phthalate Derivatives.^[1] One of the major problem with the Continuous Process is the difficulty of introducing the Solid Polyester Wastes into the Reactor working under high pressure. In addition some impurities in the produced DMTP produced cannot be completely removed, which may cause poor color.^[3] Methanolysis was previously used at commercial scale but today it is not used for PET Recycling anymore since the lack of usefulness of recovering DMTP rendered the Methanolysis of PET to become obsolete.^[2]

PET Glycolysis involves Ethylene Glycol insertion into PET Chains at temperatures in the range of 180 to 240°C to give Bis-Hydroxyethyl-Terephthalate (BHET) - which is a Substrate for PET Synthesis - and higher Oligomers with high purity BHET is more desirable for the production of PET than TPA as it removes an intermediate Process Step. Different Glycolysis Methods have been explored: solvent-assisted, microwave-assisted, supercritical glycolysis, and catalysed glycolysis. Glycolysis is the oldest and simplest method of PET Depolymerisation with a long history of development by renowned companies such as DuPont, Goodyear, Shell Polyester, Zimmer, Eastman, Kodak, etc…  but to date there is no example of any commercial glycolysis recycling process by any of these companies.

References

1. Timmy Thiounn, Rhett C. Smith, 15 May 2020, Advances and approaches for chemical recycling of plastic waste, Journal of Polymer Science, Volume58, Issue10, May 15, 2020, Pages 1347-1364
2. Bartolome, Leian & Imran, Muhammad & Cho, Bong & Al-Masry, Waheed & Kim, Do. (2012). Recent Developments in the Chemical Recycling of PET. 10.5772/33800. 
3. Crippa, Maurizio & Morico, Barbara. (2019). PET depolymerization: a novel process for plastic waste chemical recycling. 10.1016/B978-0-444-64337-7.00012-4.