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- RESULTS AND DISCUSSION
Ever since the first industrial scale production of synthetic polymers took place in the 1940s, the production, consumption, and waste generation rate of the plastic solid waste has increased considerably. Its recycling has been a focus of many researchers in the past few decades. Such research is also driven by the changes in the regulatory issues concerned with the environmental protection.1
Poly(ethylene terephthalate) (PET) is one of the versatile engineering plastics showing excellent thermal and mechanical properties. It is a nontoxic, semi-crystalline, thermoplastic polyester with high strength and crystal clear transparency. Although its main application is in the apparel sector, large quantities are also consumed in the manufacture of X-ray films, food packaging, and especially of soft-drink bottles. The post-consumer PET product waste does not create a direct hazard; however, its substantial volume fraction in solid waste streams coupled with its high resistance to the atmospheric and biological degradation is the main threat to the environment.2 As it is not appropriate to dispose off waste PET on land-fill sites, alternate methods of recycling, including physical and chemical ones, have been developed.
Chemical recycling appears to be the only alternate that is sustainable.3 PET possesses ester groups, which can be cleaved by some reagents, such as water (hydrolysis),2–9 alcohols (alcoholysis),8,9 glycols (glycolysis),10–15 and amines (aminolysis).16–24 Great attention has been paid to chemical recycling of PET waste leading to recovery of the monomeric products that can be further used in the synthesis of chemicals or intermediates of interest.17,19 The aminolysis of PET waste is one such approach with the advantage over other chemical recycling processes since the end products of aminolysis needs less separation stages to purify them.4 Amines are organic bases and cause polyester depolymerization faster than alcohols.25 Compared to other methods of depolymerization, the aminolysis is still a little explored method. The solvolytic activity of amines at boil, which cleave the ester bond in the PET chain, is connected with their basicity.
Zahn and Pfeifer21 carried out aminolysis of PET with solutions of benzyl amine, ethylene diamine, hexamethylene diamine, piperidine, and aniline and obtained different reaction products as the diamides of terephthalic acid, which do not possess any potential for further chemical reactions. During aminolysis of PET with methylamine, the methyl terephthalamide is obtained, which is not enough reactive for its recycling into any useful product through further reactions.22 Earlier, in our laboratory, we have carried out aminolysis of PET waste using ethanol amine to get pure bis (2-hydroxy ethylene) terephthalamide16 which has been utilized in synthesis of polyurethanes,26,27 unsaturated polyesters,28 epoxy hardners,26 and nonionic polymeric surfactants.29
In addition to application of conventional heating, a growing interest has been shown in the last few years in the use of microwave irradiation in organic synthesis.30,31 Our previous work on microwave assisted recycling of post consumer PET waste with ethanolamine and ethylene glycol gave comparable results to conventional heating with advantages of time saving.13,32
Bis-oxazolines and bis-oxazines have been used as chain extenders and crosslinkers in polymers33–35 and are reported to be prepared from nitriles and amines using zinc-2-ethyl hexanoate as catalyst36 or halo amides using strong base such as potassium fluoride on solid support.37
The present communication deals with the results on the use of 3-amino-1-propanol for the aminolytic depolymerization of PET bottle waste in the presence of simple chemicals such as sodium acetate or potassium sulfate as catalysts under conventional and microwave source of heating. The depolymerization product after purification was found to be bis-(3-hydroxy propyl) terephthalamide (BHPTA) as characterized by melting point, IR spectroscopy, nuclear magnetic resonance (NMR) and differential scanning calorimeter (DSC). It was further used for synthesis of 1,4-bis (5,6-dihydro-4H-1,3-oxazin-2-yl) benzene which is used as chain extender and crosslinker.