ReviewPolyethylene terephthalate degradation under natural and accelerated weathering conditions
Graphical abstract
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.
Section snippets
Field studies in the natural environment
The magnitude and impact of plastic waste pollution on our environment has been made abundantly clear in several excellent scientific studies, as well as through popular science media such as the BBC’s Blue Planet series [22], [23], [24], [25], [26]. These studies have looked at the sources, the scale and the biological impact of fugitive plastics, and the conclusions have invariably argued that if more is not done further ‘up’ in the life-cycle of plastics, their final destination is often in
Photolytic degradation of PET
Studies on the photolytic degradation of PET have been carried out since the 1960 s. Chain scission occurs when PET is exposed to radiation near the ultraviolet (UV) region and the mechanisms by which these reactions occur are due to Norrish type I and type II reactions which involve radicals formed as a result of chain scissions (see Fig. 3). Norrish type I mainly involves formation of radicals during ester bond cleavage, whereas Norrish type II are intramolecular reactions whereby a hydrogen
Accelerated weathering
Accelerated weathering techniques combine UV radiation, heat and moisture as these are the main stimuli in the natural environment responsible for polymer degradation. Accelerated weathering techniques simulate the damaging effects to the polymer caused by outdoor exposure in a more compressed time, allowing for the evaluation of the suitability of materials for outdoor applications. Polymer degradation can be measured under various accelerated conditions and is generally thought to be a more
Biodegradable copolymers of PET
In order to make PET biodegradable, the most common method employed is to copolymerize PET (or PBT) with a biodegradable aliphatic polyester such as polylactic acid (PLA), polyglycolide (PGA), polyethylene glycol (PEG) or polycaprolactone (PCL). Alternatively, co-polymers are based on polyesters from different dicarboxylic acids, such as adipic acid or succinic acid. The degradation of these copolymers in water or buffer solutions with various pH and at different temperatures has been studied
Depolymerization of PET
Chemical depolymerization of PET has been heavily investigated during the last two decades. It is generally considered to be the alternative method for recycling PET by recovering the monomer so that it can be used as a feedstock for new PET [5]. The most common depolymerization method for PET is glycolysis, performed in an excess of ethylene glycol and a suitable catalyst at high temperatures (150–300 °C) for 2–10 h, resulting in the formation of bis-hydroxyethyl terephthalate (BHET, one of
Enzymatic degradation of PET
A final word should be devoted to enzymatic PET degradation. PET materials are generally resistant to microbial attack. However, Yoshida et al. have identified a PET-degrading enzyme recently, aptly named PET-ase, which is found in sediment dwelling bacteria, Ideonella sakaiensi in Japan [99]. The degradation product was identified as mono(2-hydroxyethyl) terephthalic acid. The mechanism of PET-ase degrading PET has been investigated by several research groups and has been the subject of
Conclusions and outlook
This review of experimental studies on PET degradation through hydrolysis and photolysis in the natural environment and under accelerated weathering conditions has shown that the degradation of PET is highly dependent on reaction conditions, in particular temperature and time. Both hydrolysis and photolysis in the natural environment predominantly cause changes to the surface chemistry rather than to the bulk of the polymer. To affect the core of the polymer, temperatures closer to the Tg of
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We thank Innovate UK for funding (Grant 104608).
References (108)
- et al.
Competitive sorption of persistent organic pollutants onto microplastics in the marine environment
Mar. Pollut. Bull.
(2012) - et al.
Challenges associated with plastic waste disposal and allied microbial routes for its effective degradation: A comprehensive review
J. Cleaner Prod.
(2019) - et al.
Chemical recycling of post-consumer PET wastes by glycolysis in the presence of metal salts
Polym. Degrad. Stab.
(2010) - et al.
Deeper insight into hydrolysis mechanisms of polyester/cotton blended fabrics for separation by explicit solvent models
Int. J. Biol. Macromol.
(2020) - et al.
Thermal degradation and mechanical properties of PET blends
Polym. Degrad. Stab.
(2005) - et al.
Aspects of poly(ethylene terephthalate) degradation for archival life and environmental degradation
Polym. Degrad. Stab.
(1991) - et al.
Hydrolytic degradation of poly(ethylene terephthalate): Importance of chain scission versus crystallinity
Eur. Polym. J.
(1991) - et al.
An FT–IR study of the effect of hydrolytic degradation on the structure of thin PET films
Polym. Degrad. Stab.
(2000) - et al.
Recycling of PET
Eur. Polym. J.
(2005) - et al.
Impact of hydrolytic degradation on mechanical properties of PET - Towards an understanding of microplastics formation
Polym. Degrad. Stab.
(2019)
Improvement in gas permeability of biaxially stretched PET films blended with high barrier polymers: The role of chemistry and processing conditions
Eur. Polym. J.
Hydrolysis kinetics of condensation polymers under humidity aging conditions
Polym. Degrad. Stab.
Surface characterization of photodegraded poly(ethylene terephthalate). The effect of ultraviolet absorbers
Polymer
Impact of photooxidative degradation on the oxygen permeability of poly(ethyleneterephthalate)
Polym. Degrad. Stab.
Vacuum-ultraviolet photolysis of polymers
Surf. Coat. Technol.
Orientation and structure of drawn poly(ethylene terephthalate)
Polymer
Crystallization and melting behaviour of photodegraded polypropylene — II. Re-crystallization of degraded molecules
Polymer
Oxo-biodegradable carbon backbone polymers – Oxidative degradation of polyethylene under accelerated test conditions
Polym. Degrad. Stab.
Oxidation and biodegradation of polyethylene films containing pro-oxidant additives: Synergistic effects of sunlight exposure, thermal aging and fungal biodegradation
Polym. Degrad. Stab.
Characterizing the weathering induced degradation of Poly(ethylene-terephthalate) using PARAFAC modeling of fluorescence spectra
Polym. Degrad. Stab.
Depth profiling of degradation of multilayer photovoltaic backsheets after accelerated laboratory weathering: Cross-sectional Raman imaging
Sol. Energy Mater. Sol. Cells
Comparison of the UV-degradation chemistry of polypropylene, polyethylene, polyamide 6 and polybutylene terephthalate
Polym. Degrad. Stab.
Nanoscale analysis of the photodegradation of polyester fibers by AFM-IR
J. Photochem. Photobiol., A
Synthesis and in vitro degradation study of poly(ethylene terephthalate)/poly(ethylene glycol) (PET/PEG) multiblock copolymer
Polym. Degrad. Stab.
Atmospheric and soil degradation of aliphatic–aromatic polyester films
Polym. Degrad. Stab.
Manganese-, cobalt-, and zinc-based mixed-oxide spinels as novel catalysts for the chemical recycling of poly(ethylene terephthalate) via glycolysis
Polym. Degrad. Stab.
Organocatalysis for depolymerisation
Polym. Chem.
Poly (ethylene terephthalate) recycling for high value added textiles
Fashion Text.
Solid heterogeneous catalysts for transesterification of triglycerides with methanol: A review
Appl. Catal. A
Sustainable development: efficiency and recycling in chemicals manufacturing
Green Chem.
New motivation for the depolymerization products derived from poly(ethylene terephthalate) (PET) waste: a review
Macromol. Mater. Eng.
Chemical recycling of waste plastics for new materials production
Nat. Rev. Chem.
Pet waste management by chemical recycling: a review
J. Polym. Environ.
Lost at sea: where is all the plastic?
Science
Pathways for degradation of plastic polymers floating in the marine environment
Environ. Sci. Processes Impacts
Marine litter distribution and density in European seas, from the shelves to deep basins
PLoS ONE
Degradation rates of plastics in the environment
ACS Sustainable Chem. Eng.
New advances in poly(ethylene terephthalate) polymerization and degradation
Polym. Int.
Plastic degradation and its environmental implications with special reference to poly(ethylene terephthalate)
Polymers
Degradation behaviors, thermostability and mechanical properties of poly (ethylene terephthalate)/polylactic acid blends
J. Central South Univ.
Evaluation of biodegradability of poly(ε-caprolactone)/poly(ethylene terephthalate) blends
J. Environ. Polym. Degrad.
Cleaning up plastic pollution in Africa
Science
Plastic waste inputs from land into the ocean
Science
Future scenarios of global plastic waste generation and disposal
Palgrave Commun.
Beyond plastic waste
Science
Strategies to reduce the global carbon footprint of plastics
Nat. Clim. Change
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