Depolymerization and Recycling

Georgia Institute of Technology, School of Textile and Fiber Engineering, Atlanta, Georgia 30332 

Poly(ethylene terephthalate) (PET) is one of the most extensively recycled polymeric materials. In 1995, 3.5 x 104 tons of PET were recycled in Europe.1 The main reason for the widespread recycling of PET is its extensive use in plastic packaging applications, especially in the beverage industry as plastic bottles. The consistency in terms of volume and availability of postconsumer bottles from sorting facilities and its high material scrap value create an excellent economic environment for PET recycling. 

PET is the polyester of terephthalic acid and ethylene glycol. Polyesters are prepared by either direct esterification or transesterification reactions. In the direct esterification process, terephthalic acid is reacted with ethylene glycol to produce PET and water as a by-product. Transesterification involves the reaction of dimethyl terephthalate (DMT) with ethylene glycol in the presence of a catalyst (usually a metal carboxylate) to form bis(hydroxyethyl)terephthalate (BHET) and methyl alcohol as a by-product. In the second step of transesterification, BHET 

Synthetic Methods in Step-Growth Polymers. Edited by Martin E. Rogers and Timothy E. Long 2003 John Wiley Sons, Inc. ISBN 0-471-38769-X 

In 1930, DuPont launched the synthetic fiber industry with the discovery of nylon-6,6.2 In 1938, a pilot plant for nylon-6,6 production was put into operation, and in 1939, production was commenced at a large-scale plant in Seaford, Delaware. The classical method for the synthesis of nylon-6,6 involves a two-step process. In the first step, hexamethylene diamine (HMDA) is reacted with adipic acid (AA) to form a nylon salt. Polymerization of the aqueous salt solution is carried out at temperatures in the range of about 210-275°C at a steam pressure of about 1.7 MPa. When 275°C is reached, the pressure is reduced to atmospheric pressure and heating is continued to drive the reaction to completion. 

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Polk M B (2003) Depolymerization and recycling, in Synthetic Methods in Step-Growth Polymers (Eds. Roger M E and Long T E) J. Wiley Sons, New Jersey, pp.527-574. Meluch W C, Campbell G A (1976) Recovering amines by the hydrolytic decomposition of polyurethanes, US Patent 3,978,128. 

Both metathesis degradation (a well documented phenomenon ) and metathesis depolymerization offer promise in issues related to chemically recycling discarded unsaturated polymers. However, it should be noted that ADMET depolymerization chemistry is distinguished from metathesis degradation chemistry by virtue of the fact that very specific products result from depolymerization and, in... 

Pyrolysis treatments are interesting regarding the aforementioned plastic refuse makeup. Other successful treatments for feedstock recycling of condensation polymers (PET, ABS, etc.), that allows for the depolymerization and recovery of their constituent monomers (e.g. hydrolysis, alcoholysis, methanolysis, etc.), cannot be applied for polyolefin plastics recycling. In contrast, pyrolysis of polyolefins yields valuable hydrocarbon mixtures of... 

The term reactive processing is used to describe a polymer processing that involves chemical reactions. In principle, any processing operation can be conducted as a reactive process, viz. reactive injection molding (RIM). However, most often the term refers to reactive extrusion, and in particular, to the reactive compatibilization of immiscible polymer blends, usually conducted in a TSE. During the last 50 years, the latter machines have been used as chemical reactors for the polymerization, depolymerization (chemical recycling), polymer modification and compatibilization [Brown, 1992, Xanthos, 1992 Utracki, 1989, 1991, 1994, 1997]. 

Recycling of nylon 6 is not limited to carpets and automobile parts. Toray, a Japanese company, in 1995 began recycling apparel made from nylon 6 by depolymerization and repolymerization.Used U.S. Postal Service nylon mailbags which are not repairable, are being recycled into pellets used for, among other applications, automobile parts. 1 ... 

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