Recycling, depolymerization applications

Generalized recycling routes applicable to all thermoplastic polymers (both addition and condensation) are illustrated schematically in Figure 0.8. This may be described as a crude, rough depolymerization as opposed to the precise surgical depolymerization achieved by the hydrolysis of a condensation polymer. Taking the procedure from the start ... 

In 1977, Tokiwa and Suzuki reported that some lipases, which are extracellular enzymes that usually cleave esters in oils and fats, are also able to attack ester bonds in some aliphatic polyesters and can depolymerize such materials [38]. Aliphatic polyesters, however, exhibit only limited useful properties for many applications. Aromatic polyesters, such as PET and PBT, which are widely applied because of their excellent properties, are not attacked by hydrolytic enzymes. This led to the development of aliphatic-aromatic polyesters as biodegradable plastics that present a compromise between biodegradability and material properties [39]. Recently, however, Muller et al. [40] have isolated a hydrolase (TfH) from Thermofibida fusca which is able to depolymerize the aromatic polyester PET at a high rate in contrast to other hydrolases such as lipases. They have demonstrated for the first time that commercial PET can be effectively hydrolyzed by an enzyme at a rate that does not exclude a biological recycling of PET. The effective depolymerization of PET with the enzyme TfH will result in water... 

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. 

Applications of Dopolymerized Nylon 6 Chemical recycling of nylon 6 carpet face fibers has been developed into a closed-loop recycling process for waste nylon carpet [25, 30-32]. The recovered nylon 6 face fibers are sent to a depolymerization reactor and teeated with superheated steam in the presence of a catalyst to produce a distillate containing caprolactam. The crude caprolactam is distilled and repolymerized to form nylon 6. The caprolactam obtained is comparable to virgin caprolactam in purity. The repolymerized nylon 6 is converted into yam and tufted into carpet. The carpets obtained from this process are very similar in physical properties to those obtained from virgin caprolactam. 

Adherent Technologies Inc. [8] has developed a process for the reclamation of carbon fibers from carbon/epoxy composites. It has studied the depolymerization of thermoset carbon fiber reinforced epoxy matrix composites using a low temperature (20 min at 325°Q catalytic tertiary recycling reclamation process and has been able to obtain a product with 99.8% carbon and 0.2% residual resin, with only a loss of about 8.6% in fiber tensile strength. The process can be economically viable, provided sufficient scrap feedstock is available. Possible applications for the recovered fiber include thermoplastic and thermoset molding compounds. 

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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