Sub- and supercritical glycolysis of polyethylene terephthalate (PET) into the monomer bis(2-hydroxyethyl) terephthalate (BHET) Muhammad Imran a , Bo-Kyung Kim b , Myungwan Han b , Bong Gyoo Cho c , Do Hyun Kim a, * a Department of Chemical and Biomolecular Engineering and Center for Ultramicrochemical Process Systems, Korea Advanced Institute of Science and Technology (KAIST), 373-1, Guseong-dong, Yuseong-gu, Daejeon 305-701, Republic of Korea b Department of Chemical Engineering, Chungnam National University, 220 Gung-dong, Yuseong-gu, Daejeon 305-764, Republic of Korea c Mineral Processing Department, Korea Institute of Geoscience and Mineral Resources, Gwahang-no 92, Yuseong-gu, Daejeon 305-350, Republic of Korea article info Article history: Received 7 October 2009 Received in revised form 14 May 2010 Accepted 20 May 2010 Available online 26 May 2010 Keywords: PET Supercritical Subcritical Glycolysis Plastics recycling abstract Sub- and supercritical glycolysis of polyethylene terephthalate (PET) with ethylene glycol (EG) to bis(2- hydroxyethyl) terephthalate (BHET) was investigated for the purpose of developing a PET recycling process. Supercritical glycolysis was carried out at 450  C and 15.3 MPa while subcritical glycolysis was carried out at 350  C and 2.49 MPa or at 300  C and 1.1 MPa. High yields (gt; 90%) of the monomer BHET were obtained in both super- and subcritical cases. For the same PET/EG weight ratio of about 0.06, the optimum reaction time was 30 min for supercritical glycolysis and 75 and 120 min for two cases of subcritical glycolysis. GPC, RP-HPLC, 1 H NMR and 13 C NMR, and DSC were used to characterize the polymer and reaction products. Supercritical glycolysis will be suitable to a process requiring a high throughput due to its short reaction time. Ó 2010 Elsevier Ltd. All rights reserved. 1. Introduction Polyethylene terephthalate (PET), a thermoplastic linear poly- ester synthesized by the esterification of terephthalic acid (TPA) and ethylene glycol (EG) or by the transesterification of dimethyl terephthalate (DMT) and EG, is one of the most versatile polymers in use today. It exhibits both an amorphous and semi-crystalline structure and is used extensively for the fabrication of polyester fiber, soft drink bottles, and photographic films, as well as audio and video tapes. It has shown a tremendous increase in yearly growth of use. From 2000 to 2010, the world PET demand is expected to increase from 27.6 million tons to 56.0 million tons [1]. This proliferation of PET poses serious environmental and economic threats, which necessitates recycling of PET. Mechanical, chemical, and thermal recycling are the three main processes to recycle a polymer. Among these, the process consistent with the principles of sustainable development is chemical recy- cling, because it leads to the production of monomers from which the polymer is made [2]. Several chemical depolymerization processes using solvolysis such as methanolysis [3], hydrolysis [4], glycolysis [5], aminolysis [6], or amonolysis [7] have been reported, depending upon the depolymerization agents and reaction condi- tions. Methanolysis, hydrolysis, and glycolysis are well-known commercialized processes, while aminolysis and amonolysis have been studied on a lab-scale and a little work has been done on the commercialization of these processes. Solvolysis processes are further classified based on the use of catalyst and supercritical fluids. Catalytic [8,9] and supercritical [10e13] methanolysis produce DMT and EG. Catalytic [14], neutral, alkaline and acidic [15e18], and supercritical [19] hydrolysis produce TPA and EG. Glycolysis has significant advantages over methanolysis or hydrolysis, primarily because BHET can be used as a raw material for either DMT-based or TPA-based PET production units, while DMT and TPA cannot be used interchangeably. Another significant advantage of glycolysis is that the removal of glycol from the depolymerization solvent is not necessary; an analogous removal is compulsory, however, in the case of methanolysis or hydrolysis [20]. BHET may also be used to make textile softener [21] and unsaturated polyester resins [22,23], which are used in foams, polyurethanes, and polyester-polyol copolymer production. Cata- lytic glycolysis [24e26] depolymerizes PET to bis(2-hydroxyethyl) terephthalate (BHET). As far as we know, the use of supercritical glycolysis in PET depolymerization has not been reported to date. Most catalytic processes have disadvantages, such as longer reaction * Corresponding author. Tel.: þ82 42 350 3929; fax: þ82 42 350 3910. E-mail address: DoHyun.Kim@kaist.edu (D.H. Kim). Contents lists available at ScienceDirect Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.05.026 Polymer Degradation and Stability 95 (2010) 1686e1693