Polyethylene terephthalate degradation under natural and accelerated weathering conditions

Abstract

This review presents an overview of the state of the art in understanding of the chemistries behind polyethylene terephthalate (PET) degradation under hydrolytic and photolytic conditions, which are the main degradation processes that operate in the natural environment. Laboratory studies, carried out under so-called accelerated weathering conditions, have been reviewed and are compared to findings obtained from field studies through hydrolysis and photolysis from natural exposure. The review concludes by correlating the techniques used to date and our current understanding of the mechanisms and key intermediates involved in PET degradation.

Introduction

Polyethylene terephthalate (PET) is one of the most commonly used thermoplastic polymers in the packaging and textile industries. In 2014, approximately 41 million tonnes of PET were produced worldwide, predominantly through the polycondensation of bis(hydroxyethyl)terephthalate (see Fig. 1). Global production has since increased to 56 million tonnes in 2016 and it has been forecasted that in 2020 more than 70 million tonnes we be produced annually [1]. In addition to this increase in PET production, more than 10 million tonnes of PET contribute to plastic waste every year [2], [3], [4]. PET bottles have more than 50% recycling efficiency within the European Union, which makes them the most recyclable plastic product to date [1]. Other PET-based products such as films and fibres do not achieve such high recycling efficiencies as PET bottles. Regrettably, a significant proportion escapes from waste management systems and eventually ends up as pollution in the ocean. To avoid the widespread and growing accumulation of plastic waste in the environment, more efforts are invested into recycling technologies and the EU has stipulated that by 2025, plastic bottles should contain 25% recycled material and all plastic packaging waste should be recyclable by 2030. The excessive use of PET, especially in single use packaging, combined with an inefficient waste management infrastructure in the majority of developing countries, makes PET-based fugitive plastics a major contributing factor to the global problem of plastic pollution.

Reuse and depolymerisation are generally viewed as the most environmentally sound solutions for dealing with PET waste [5]. Notwithstanding, large amounts of PET waste are recycled through mechanical means to produce recycled PET (rPET). This takes place in two ways: primary or secondary recycling. Primary recycling involves mixing rPET with virgin PET to produce predominantly PET bottles with a percentage of rPET. This is sometimes referred to as ‘closed loop’ recycling and is generally believed to be the most efficient recycling process. If rPET from waste PET bottles is used to make PET fibres, which have a lower market value, then this is considered secondary recycling, also known as down-cycling [6], [7], [8]. Both primary and secondary recycling involve mechanical processes such as grinding and extruding. These processes can introduce varying degrees of PET degradation due to the exposure to high temperatures and the presence of moisture, thereby reducing important mechanical properties such as strength and physical characteristics such as transparency. This is the main reason why virgin PET is generally added to any recycled PET product to offset these deficiencies. More recently, tertiary recycling of PET includes chemical depolymerisation such as glycolysis (the reverse of the reaction in Fig. 1) has gathered greater commercial interest. The aim in this case is to convert waste PET back into monomers and/or oligomers that can be used as feedstock to make ‘new’ (pseudo-virgin) PET. The technique has shown promising yields of monomers and oligomers in small scale productions, but the relatively high cost of production compared to petroleum-derived virgin monomers limits its applications [9].

In the natural environment, PET has shown extremely slow degradation rates, resulting in bio-accumulation for decades if it escapes waste management systems or is littered. Once in the natural environment, the exposure of the PET products to natural weathering conditions causes fragmentation by both mechanical and chemical means, leading to the generation of microplastics of PET (≤5 mm), which are even more difficult to track and collect [10], [11]. While numerous studies have highlighted the damaging effect that microplastics can cause in a marine environment [12], the magnitude and awareness of terrestrial-based plastic pollution, both macro and microplastics remains largely unstudied [13].

Although PET is labelled as non-biodegradable, chemical and physical changes to the polymer structure do occur under natural weathering conditions. In addition, it is well known that used or degraded PET reduces the efficiency of the recycling process, to the point where virgin PET needs to be added to maintain the desired physical properties of the final product. In order to further improve the recyclability of PET, in such a way that less virgin PET needs to be added, as well as to prevent the release of microplastics in the natural environment, it is imperative to understand the degradation processes that take place in PET at the molecular level. A range of degradation products of PET obtained under various experimental conditions have been reported and these will be reviewed here [14], [15], [16], [17]. Studies on the degradation and stability of PET blends with other biodegradable components are beyond the scope of this review, for example PET blended with cotton [18], polylactic acid (PLA) [19], polycaprolactone [20], or starch and wood flour [21] and the reader is referred to the references given. In general, degradation studies on these blended materials have indicated that as the biodegradable content degrades, spaces and wounds are created on the blends, which facilitate attack by water, oxygen and microorganisms on PET segments. No results have indicated so far any beneficial effect played by the biodegradable segments on the degradability of PET itself.

The information summarised in this review regarding the processes that occur in nature, mainly hydrolysis and photolysis, will generate insights into the mechanisms and the key intermediates involved in PET degradation. This review will focus specifically on PET degradation through hydrolysis and photolysis in the natural environment and under simulated laboratory conditions. These so-called accelerated weathering techniques that have been designed to combine the two processes and thereby mimic the natural environment for PET degradation. Chemical recycling and biological depolymerisation will also be summarised, but not described in detail.