Study on depolymerization of waste polyethylene terephthalate into monomer of bis(2-hydroxyethyl terephthalate)

Abstract

The monomer bis(2-hydroxyethyl terephthalate) (BHET) was obtained with high purity and significant yield by depolymerization of waste polyethylene terephthalate (PET). The glycolysis reaction was carried out at 196 °C with a weight ratio of ethylene glycol (EG) to PET from 0.5 to 6; weight ratio of catalyst to PET from 0 to 1.5% and time of 1–5 h in the presence of zinc acetate as a transesterification catalyst. The optimal conditions are reaction time of 3 h, weight ratio (EG to PET) of 5, weight ratio (catalyst to PET) of 1% and the yield of BHET is 85.6% under these conditions. The glycolysed products were separated into monomer and oligomers and identified by IR, DTA (differential thermal analysis),¹H NMR and¹³C NMR.

Introduction

Polyethylene terephthalate (PET) is one of the versatile plastics which is widely used in the manufacture of various kinds of packaging, especially for soft drink bottles, fibres and films. The overall annual world consumption of PET amounts to about 13 million tons [1]. With such a large consumption, the effective use of PET wastes is a significant subject for environmental protection researchers. Although several methods have been proposed for recycling waste PET, it is suggested that the most attractive method is chemical glycolysis into the corresponding monomers or raw chemicals that could be reused for the production of plastics or other advanced materials [2]. The chemical glycolysis method makes it possible to employ very low amounts of reactants as well as application of lower temperatures and pressures in contrast with other methods such as supercritical methanol and thermal degradation [3], [4], [5], [6], [7], while hydrolysis under acidic or basic conditions may cause corrosion and pollution problems [8], [9].

Glycolysis of waste PET with ethylene glycol (EG) can lead to the production of bis(2-hydroxyethyl terephthalate) (BHET) monomer. BHET has been widely used in the synthesis of a number of polymeric materials, ranging from unsaturated polyester resins and polyethylene terephthalate (PET) to novel biocompatible systems. BHET has also been used as a plasticiser in unsaturated polyester resins and rigid or flexible polyurethanes [10], [11], [12].

In a previous research, zinc acetate was an efficient catalyst [13], [14] and the glycolysed products as a whole were characterized by acid value and hydroxyl value determinations. However, because of some problems in the separation and purification of the product, little effort has been made to investigate the effect of degradation parameters on the yield of monomer. Baliga and Wong [14] confirmed that after extraction by boiling water, the water soluble crystallisable fraction was composed of mainly BHET, and the water insoluble fraction was mainly the dimer. Chen et al. [15] found that the glycolysis conversion is almost 100% at 190 °C, 1.5 h glycolysis time and 0.002 mol catalyst (cobalt acetate). But the glycolysis conversion was calculated by depolymerized PET, and the depolymerization reaction into monomer was incomplete. Simultaneous glycolysis and hydrolysis of PET was carried out at 170 and 190 °C with constant amount of EG and increasing amount of water in the presence of xylene by Gamze et al. [16]. The aim of their recycling procedures was to obtain monohydroxyethyl terephthalate (MHT) and BHET monomers. Most of those studies related to industrial production were proprietary and were only revealed in a few patents [17], [18], [19], [20].

In the present study, a simple glycolysis system in the presence of zinc acetate as a catalyst that could obtain about 85.6% yield of BHET was set up. Moreover, new findings to the influences of glycolysis conditions (time, weight ratio of EG to PET and amount of catalyst) on the yield of BHET in the process of recycling of PET were obtained. The chemical structure analyses of glycolysed products are also presented.