Review
`
`Mechanical Recycling of Packaging Plastics: A Review
`
`Zoé O. G. Schyns and Michael P. Shaver*
`
`The current global plastics economy is highly linear, with the exceptional
`performance and low carbon footprint of polymeric materials at odds with
`dramatic increases in plastic waste. Transitioning to a circular economy that
`retains plastic in its highest value condition is essential to reduce environ-
`mental impacts, promoting reduction, reuse, and recycling. Mechanical recy-
`cling is an essential tool in an environmentally and economically sustainable
`economy of plastics, but current mechanical recycling processes are limited
`by cost, degradation of mechanical properties, and inconsistent quality
`products. This review covers the current methods and challenges for the
`mechanical recycling of the five main packaging plastics: poly(ethylene tere-
`phthalate), polyethylene, polypropylene, polystyrene, and poly(vinyl chloride)
`through the lens of a circular economy. Their reprocessing induced degrada-
`tion mechanisms are introduced and strategies to improve their recycling are
`discussed. Additionally, this review briefly examines approaches to improve
`polymer blending in mixed plastic waste streams and applications of lower
`quality recyclate.
`
`1. Introduction
`
`The global demand for plastics continues to rise. The amount
`of plastics in circulation is projected to increase from 236 to 417
`million ton per year by 2030.[1] Recycling or reuse of plastics in
`circulation is essential to prevent increased accidental or pur-
`poseful release of polymeric materials into the environment,
`and thus curb environmental pollution. In 2016, only 16% of
`polymers in flow were collected for recycling while 40% were
`sent to landfill and 25% were incinerated (Figure 1).[1] Recently,
`European countries have increased efforts to improve recycling
`rates. In 2018, 29.1 million tons of post-consumer plastic waste
`were collected in Europe. While less than a third of this was
`recycled, it represented a doubling of the quantity recycled
`and reduced plastic waste exports outside the European Union
`(EU) by 39% compared to 2006 levels. Much of this plastic flow
`(39.9%) was for packaging.[2]
`
`Z. O. G. Schyns, Prof. M. P. Shaver
`Department of Materials
`The University of Manchester
`Manchester M1 7DN, UK
`E-mail: michael.shaver@manchester.ac.uk
`The ORCID identification number(s) for the author(s) of this article
`can be found under https://doi.org/10.1002/marc.202000415.
`© 2020 The Authors. Published by Wiley-VCH GmbH. This is an open
`access article under the terms of the Creative Commons Attribution
`License, which permits use, distribution and reproduction in any
`medium, provided the original work is properly cited.
`
`DOI: 10.1002/marc.202000415
`
`Packaging recycling is often more eco-
`nomically feasible than other sectors of
`the plastic market due to high turnover
`rates of the collected post-consumer waste
`in Europe, 42% is recycled, 40% is sent
`for energy recovery and 19% is sent to
`landfill.[2] The stability of plastics, a key
`performance feature that has promoted
`their use, also reduces their ability to
`degrade. As a result landfill sites become
`saturated and excess waste is disposed of
`into the environment.[3] The ubiquity of
`the material and variability of its disposal
`has also led to physical fragmentation,
`introducing micro and nanoplastics into
`bodies of water, urban environments,
`conservation areas, and our food chain.[3]
`The EU Waste Directives imposed landfill
`taxes that have stemmed some of this tide,
`increasing recycling rates, although much
`of landfill avoidance is through question-
`able energy-from-waste strategies.[4,5] The
`severity of tax imposed depends on the country in question:
`24 out of 27 EU countries have landfill taxes in place while 18
`have landfill bans implemented.[2,4,5] In addition to this, the
`use of non-recyclable packaging will be taxted according to
`the European Council conclusion dated the 17–21 July 2020.[6]
`The UK currently charges £94.15 ton−1 for land-filling of plastic
`waste, a 1345% increase to the landfill tax in 1996.[7] According
`to a report published by The Waste and Resources Action Pro-
`gramme (WRAP) in 2018, the United Kingdom collects 47% of
`its plastic packaging waste for recycling, although only 43% is
`converted into valuable feedstock.[2,8]
`Using the UK as an exemplar, 40% of the waste collected
`in 2015–16 was poly(ethylene terephthalate), (PET), followed
`by 22% polyethylene, (PE), 10.2% polypropylene, (PP), with
`poly(vinyl chloride), (PVC), and polystyrene, (PS), making up
`2% (Figure  1).[9] These five polymers are those primarily used
`in packaging (Table 1). High-density polyethylene (HDPE) and
`PET are used to produce bottles to package toiletries, food and
`household cleaning products. Packaging films are primarily
`made out of linear low-density polyethylene (LLDPE), low-den-
`sity polyethylene (LDPE), and PVC.[10] Plastic beverage bottles
`are made out of PET, HDPE, and PVC, although the latter is
`under legislative pressure to ban its use.[10] Single use plastic
`bags are usually made out of LDPE and LLDPE.[10] In its solid
`or expanded form, PS is primarily used for packaging purposes
`in the food and consumer goods industries.
`The global plastics economy is largely linear. Plastics are pro-
`duced, used and more than half of them are disposed with no
`recovery.[8] With this disposal necessitating more production,
`
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`Figure 1. The main packaging polymers: poly(ethylene terephthalate) (PET), polystyrene (PS), polyethylene (PE), polypropylene (PP), and poly(vinyl
`chloride) (PVC) and current global and EU plastic waste management rates.[1,2]
`
`the dependence on petroleum feedstock and resultant pollution
`of the planet grows. To preserve the environment while
`meeting consumption demands, a global effort to shift the
`linear economy into a circular model must be made.
`Much of the focus on sustainable polymers has focused on
`the development of new feedstocks for the plastics industry,
`although many of these new polymers struggle to meet the
`challenging requirements of low cost, production at scale, and
`exceptional properties. A circular economy model suggests
`judicious use of the resources we have, including petroleum
`feedstocks, as it promotes re-valorizing plastics already in cir-
`culation. While reduction and reuse economies must be pro-
`moted, and biosourced feedstocks that avoid impacting our
`agricultural industry will continue to grow, the recycling of
`plastics is a lynchpin to reducing plastic waste.[11–13] With zero
`land-filling of collected waste as a target for full circularity, recy-
`cling must improve.[2]
`
`Table 1. The five main packaging polymers by collection proportion and
`their main uses.[9]
`
`Polymer
`
`PET
`
`HDPE and LDPE
`
`PP
`
`PVC
`
`PS
`
`Proportion of total waste
`collected from kerbside [%]
`
`Applications in packaging
`
`40
`
`22
`
`10.2
`<2
`<2
`
`Beverage bottles, trays, jam jars
`
`Bottles, bags, bin liners, food
`wrapping material, squeeze
`bottles
`
`Bottles, straws, bottle caps
`
`Films, trays
`
`Fast-food packaging, food
`packaging, disposable cutlery,
`consumer goods
`
`The COVID-19 pandemic has highlighted the need for single
`use plastics. Potential health risks and societal fears concerning
`virus-contaminated products increase plastic consumption,
`introduce consumer fears of reuse and decrease recycling
`rates.[14] Personal protective equipment (PPE), previously con-
`trolled through dedicated medical waste, is now appearing in
`municipal and institutional waste streams.[15,16] Increases in
`PPE waste are often unavoidable with mask use either pro-
`moted or enforced, creating new challenges for recycling and
`plastic production. The increased prevalence of PPE, paired
`with near record low crude oil prices, favors virgin plastic over
`more costly recyclate.[17] Uncertainties surrounding second
`spikes and long term behavior change complicates predic-
`tions on the lasting impact COVID-19 will have on our plastics
`economy. Nevertheless, these socio-material challenges neces-
`sitate a systems approach to plastic waste management. It
`is imperative to maintain the polymers in their highest value
`state, ensuring the materials we depend upon can stay in circu-
`lation. Thus, contamination of plastics, sorting and degradation
`remain the major barriers to efficient recycling.[2,18]
`There are four main types of recycling process: primary recy-
`cling, secondary recycling, tertiary recycling and quaternary
`recycling (Table 2).[19] Primary recycling involves extruding pre-
`consumer polymer or pure polymer streams. Secondary recy-
`cling requires sorting of polymer waste streams, reduction of
`polymer waste size, followed by extrusion. With proper con-
`trol over processing conditions, many polymers can undergo
`several cycles of primary and secondary mechanical recycling
`without concern for loss of performance (Sections 3 and 4.1.1).
`Tertiary recycling is used on polymers no longer suitable for
`these straightforward mechanical recycling methods. This
`chemical recycling is often complementary to traditional recy-
`cling methods, and can retain significant value if this process
`
`Macromol. Rapid Commun. 2021, 42, 2000415
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`© 2020 The Authors. Published by Wiley-VCH GmbH
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`Table 2. Common definitions of plastic recycling.
`
`ASTM D7209 definitions
`(withdrawn 2015)[22]
`
`ISO 15270:2008 standard
`definitions[23]
`
`Example
`
`Primary recycling
`
`Mechanical recycling
`
`Secondary recycling
`
`Mechanical recycling
`
`Tertiary recycling
`
`Chemical recycling
`
`Bottle to bottle closed
`loop recycling
`
`Recycling into lower
`value plastic
`
`Depolymerization
`of polyesters
`
`Quaternary recycling
`
`Energy recovery
`
`Pyrolysis
`
`As a foundation for future efforts, this review explores
`common mechanical recycling challenges and solutions for
`the main packaging polymers. Their degradation mechanisms
`will be discussed alongside details of current and past research
`efforts to improve their recycling, both from a process perspec-
`tive and through compatibilising polymer blends and incorpo-
`rating fillers. Efforts to understand the reprocessability of indi-
`vidual packaging polymers explores these themes with more
`specificity. This review will demonstrate that mechanical recy-
`cling is key in improving our plastics use by highlighting the
`incredible progress made in the last 30 years.
`
`is selective (by returning the polymer to its monomeric feed-
`stocks) instead of non-selective (as in pyrolytic or hydrocracking
`strategies). Quaternary recycling is applied to plastics that are
`unsuitable for any other type of recycling and are used for
`energy recovery via pyrolysis.[20] Quaternary recycling, while
`retaining little value, may also have unintended consequences
`from societal consequences and greenhouse gas production.[21]
`The need for improved plastic circularity is clear, and chem-
`ists, materials scientists and engineers have been responding
`to this challenge for several decades. The most concerted effort
`to improve the sustainability of plastics is evidenced by the
`growth of biodegradable and bio-based plastics.[24] Biodegrad-
`able plastics aim to degrade due to natural processes (enzy-
`matic or hydrolytic degradation) while bio-based plastics are
`often drop-in replacements produced using renewable carbon
`sources.[24–26] Selective chemical recycling of polymers has more
`recently gained popularity in recent years, as depolymerisation
`to form the original monomers offers the potential, if not the
`reality, of infinite recyclability.[27,28] Biological recycling has also
`grown, using fermentation and enzymatic degradation to pro-
`duce downcycle feedstocks.[29–32] Although both chemical and
`biological recycling are regarded as “green” recycling methods,
`full and objective life-cycle assessments are needed to evaluate
`their sustainability. Our rudiementary analysis suggests that
`mechanical recycling will remain the most effective method to
`recycle plastics – in terms of time, economic cost, carbon foot-
`print and environmental impact.
`Reprocessing of polymers has been further improved with
`innovation in extrusion technologies. Extruders can be built
`to include sections to degas, soften, dry and filter extrudate
`in order to improve polymer melt quality.[33,34] Degassing sec-
`tions are vacuumed or open vents from the barrel which allow
`release of a number of volatile compounds within polymer
`melts. Removal of such volatiles minimizes hydrolysis, acid-
`olysis, and improves polymer melt odor to increase the value
`of recyclate.[33,34,35] Polymer melts can also be filtered to remove
`larger, non-volatile, contaminants such as dust or gel parti-
`cles and improve blend homogeneity, mechanical and optical
`properties.[33,36,37] Melt filters are chosen according to specific
`extrusion contamination and can include screen-changers
`such as slide plates, woven screens, or filter cartridges.[38,39]
`Lengthening extruders must be balanced against the increase
`in system dwell times that can exacerbate chain scission. Recy-
`cling systems must be designed with consideration to specific
`degradation mechanisms.[39]
`
`2. Plastic Waste Recycling
`2.1. Melt Blending
`
`Extrusion is the foremost method used in mechanical recycling
`industries to produce regranulated material from the common
`waste plastics. It is cheap, large-scale, solvent-free, and appli-
`cable to many polymers.[21] An extruder uses heat and rotating
`screws to induce thermal softening or plasticization,[40] after
`which it is fed through temperature-controlled barrel sections
`to produce fixed cross-section extrudate (Figure 2).[40–43]
`The thermal conduction and viscous shearing applied to pol-
`ymers within an extruder leads to thermo-oxidative and shear-
`induced chain scission, chain branching or crosslinking of
`the material.[44–46] This chain degradation reduces the polymer
`chain length and in turn lowers its mechanical properties and
`processability.[44] The impact of the extrusion process depends
`on the chemical characteristics of the polymer and the chosen
`extrusion conditions. The main degradation mechanism is the
`formation of radicals along the polymer chain due to oxygen-
`induced peroxy radical and thermally induced abstraction
`of hydrogen atoms (Scheme  1).[46] These radicals can cause
`β-scissions of chains, exacerbated by the shear forces applied,
`decreasing chain length, and as a result viscosity.
`Degradation can be controlled to some degree by choice
`of extrusion conditions. Temperature and screw speed
`have direct impacts on the process stability as well as the
`product quality.[43] Extruding at excessive temperatures and
`screw speeds accelerates chain scission and forms unpro-
`cessable polymers.[44,47] Polymer chain lengths also impact
`
`Figure 2. Schematic representation of a single screw extruder. Repro-
`duced with permission.[43] Copyright 2009, Elsevier.
`
`Macromol. Rapid Commun. 2021, 42, 2000415
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`Scheme 1. Common radicals produced during extrusion-induced hydrogen abstraction in absence of oxygen.[46] Consequent reactions are detailed in
`Schemes 2,5,7,8
`
`degradation behavior: Liu et al. investigated the processing of
`starch through an extruder to model typical polymer behavior
`inside the machine.[48] Starch exists in two forms: high
`branch density amylopectin and linear-style amylose, with
`the two used to model LDPE and HDPE respectively.[48,49]
`Starch is chemically inert during extrusion and undergoes
`simple shear scission, which allows the study of thermo-
`mechanical chain scission in isolation. The authors suggest
`that the susceptibility of a polymer to shear-induced degrada-
`tion is directly proportional to its chain length and degree
`of chain branching, a phenomenon confirmed by Gooenie
`et al. and La Mantia et al. for PET.[47,48,50] Thus, the length of
`polymer chain controls degradation kinetics (suggesting that
`it can autoaccelerate through repeated extrusion) whereas
`thermo-oxidative processes are dictated by the both struc-
`ture of and oxygen diffusion through the polymer matrix.[51]
`Environmental oxygen reacts with shear-induced radicals and
`subsequent reactions produce peroxy radicals which propa-
`gate radical decomposition.[46] Thus, high oxygen perme-
`ability leads to increased thermo-oxidation rates within the
`material.
`Due to the chemical and physical forces at play during
`extrusion, mechanical recycling often decreases the tensile
`strength and elongation at break of rPP, tensile strength for
`rHDPE, elongation at break for rLLDPE, impact strength of
`rPP, and a multitude of issues for rPET.[46,52,53] To combat the
`degradation of material properties, many industrial recycling
`plants opt for an “open-loop or semi-closed-loop” recycling
`system where virgin polymer (v-polymer) is fed in during the
`recycling process. For example, in PET bottle recycling, the
`virgin to recyclate ratio is often 70/30 by weight.[52] While in-
`extruder degradation reactions can decrease recyclate quality,
`
`this is more acutely impacted by the lack of proper polymer
`sorting. Contamination of recycled material contributes to the
`decrease in quality and increase in variability of the regener-
`ated polymer (Section 2.2).[52] While often thought of as being
`extraneous polymers, these contaminants are often associated
`with the polymers themselves. Pigments used to color plas-
`tics can accelerate degradation reactions within extruders.
`Printing inks and plastic or paper labels can introduce vola-
`tile ink components within the final recyclate pellet. Fatty-
`acid based plastic lubricants, often used to facilitate the easy
`opening of plastic bags in shops, can be oxidized to produce
`unwanted odors in the recyclate.[54,55,56] Extraneous plastics
`from incorrect sorting can exacerbate these issues or even
`lead to process failure. Trace amounts of PVC in PET streams
`induces hydrodechlorination at PET processing temperatures.
`The resultant release of HCl in turn accelerates PET degra-
`dation (Figure  3) and damages processing equipment.[57,55]
`
`Figure 3. Processing ranges for the six most common packaging
`polymers.[60]
`
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`The presence of polyamides can also catalyze the aminolysis
`of PET which increases chain scission.[27] If a polymer blends
`is produced, due to accidental processing of mixed polymer
`waste streams, both the food-grade safety standards and
`mechanical properties are compromised., such as polyolefin
`and PET blends.[58] Effective plastic sorting is key to efficient
`recycling.[59]
`
`2.2. Waste Sorting for Recycling
`
`Plastics are currently sorted using a combination of automated
`and manual processes. Near infrared (NIR) technologies are
`used to determine the polymer type, with optical color recog-
`nition sorting plastics into clear and colored fractions.[19,60]
`There are numerous other complementary sorting technologies
`including X-rays, density, electrostatics, melting point, hydro-
`cyclons, selective dissolution, and manual sorting.[19,60–62] Plas-
`tics may then be flaked by grinding. These flakes can then be
`further separated using sink/float methods, air elutriation and
`heat discoloration for further optical separation.[19,28]
`Each of these methods depends upon the chemical nature of
`the bulk polymer. This has limitations on the value of mechan-
`ical recycling, as sorting methods are not yet available at scale
`to differentiate food-grade plastics, which command higher
`prices, from other recyclates.[63] Accurate polymer marking sys-
`tems would allow waste sorting facilities to retain value in food
`versus non-food plastics and aid with sorting of multi-layered
`materials.[48] While potential general marking systems have
`been reported in patents,[64–66] they remain commercially elu-
`sive. Several patents describe the use of fluorescent dye systems
`containing rare-earth and organic dyes to separate classes of
`polymer using spectroscopic techniques.[64–66] Maris et  al. and
`Bezati et al. report the use of other rare earth based compounds
`for marking uses.[67,68] The former report that their tracing
`methods function at low concentrations and allow detection
`in the presence of carbon black, a common filler preventing
`polymer detection by Fourier transform infrared spectroscopy
`(FT-IR).[67] Papers discussing the use of heavy metallic elements
`for fluorescent detection do not mention potential migration of
`markers from polymers into their surrounding environments
`nor their potential role as catalysts during extrusion. The use of
`perylene esters, perylene carboxylic bisimides, and terylene car-
`boxylic bisimides as fluorescent tracers is reported by Langhals
`et al. but no mention of the method of tracer incorporation is
`made.[69] Using a multidisciplinary approach, Lussini et  al.[70]
`and Micallef et  al.[71] synthesized profluorescent nitroxides to
`monitor photodegradation in cyclic olefin copolymers and PP
`respectively during natural aging. The profluorescent nitroxides
`are composed of a stable nitroxide free radical linked to a fluo-
`rophore that fluoresces when radicals are released during the
`degradation process of the polymer and detected using fluores-
`cence measurements and UV-vis spectroscopy (Figure  4).[70,71]
`Although research has not focused on using these reactive
`molecules in extrusion, these marking methods have the poten-
`tial to act as molecular tags due to the susceptibility of poly-
`mers to radical attack during processing. While the incorpora-
`tion of dyes is a key opportunity to improve sorting, questions
`surround the viability of these dyes in melt extrusion remain
`
`Figure 4. Profluorescent nitroxide compounds used by 1) Lussini
`et al. and 2) Micallef et al. to monitor radical based aging in polymeric
`materials.[70,71]
`
`unanswered. While they hold the potential to improve sorting
`accuracy, and thus minimize contamination, a marking and
`tracing system with secondary anti-degradation (Section  2.3)
`effects would be most beneficial in realising a circular economy
`of plastics.
`
`2.3. Stabilizer Use in Plastic Recycling
`
`Free radical reactions, including those occurring during extru-
`sion, can be inhibited through both thermal and light stabi-
`lizers.[72] For this reason, polymers are generally extruded with
`stabilizers, such as antioxidants, to prevent oxidation during
`both mechanical recycling and product use. Antioxidants can
`be classified as primary or secondary. Primary antioxidant stabi-
`lizers act as radical scavengers and form stable peroxy radicals
`with oxygen and protect chains during the polymers’ lifetime,
`acting as “chain breaking” antioxidants.[73] Secondary antioxi-
`dants, usually sulfur- or phosphorus-based chemicals, protect
`chains during melt processing of polymers by decomposing
`hydroperoxide accelerants into alcohols, acting as “preventa-
`tive” antioxidants.[74] Stabilizers may also absorb and dissipate
`energy from light to protect chains from UV based degrada-
`tion.[73] Polymers are generally only stabilized for their first life-
`cycle as packaging is designed to be short lived, highlighting
`the need for polymer design to incorporate consideration of
`end-of-life recycling.[75,76] Some of the main antioxidant types
`and corresponding compounds are detailed in Table  3 and
`Figure 5.
`UV vulnerability can be triggered by both the chemical com-
`position of the polymer or the additives incorporated during
`processing. For example, HDPE and PP are more affected by
`
`Macromol. Rapid Commun. 2021, 42, 2000415
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`Table 3. Common antioxidants and their corresponding stabilization mechanisms.
`
`Antioxidant type
`
`Hindered amines (HALS)
`
`Phenol based compounds
`
`Phosphorous based compounds
`
`Carbon black
`
`Chemical example
`
`Stabilization mechanism
`
`Tetramethylpiperidine, Tinuvin 770
`
`Irganox 1010, Curcumin, Vitamin E
`
`Irgafos, PEP 36
`
`–
`
`Continuous cyclisation[50,51,77] (primary)
`Hydrogen donors[78–80] (primary)
`Decomposing hydroperoxides[79,80] (secondary)
`Decomposes hydroperoxides and acts as reinforcing filler[81]
`
`UV exposure than LDPE and high impact polystyrene (HIPS)
`due to additives incorporated to promote polymer matrix sta-
`bilization.[51] PET is less susceptible to UV and thus is usually
`processed without these protective additives.[76]
`While additives offer benefits by mitigating an immediate
`challenge in reprocessing, they further complicate the recycling
`process. Antioxidants cause issues in waste sorting, migration
`and aesthetics. Carbon black is widely used to color polymers,
`
`doubling as a reinforcing filler and a UV protector.[80] Again,
`the benefits for polymer use creates issues with waste sorting
`due to high absorption rates of IR radiation. Carbon black also
`poses aesthetic issues, discoloring, and devaluing recyclate.
`Recent developments in stabilizing systems for food products
`include the incorporation of active antioxidants in packaging.
`The active antioxidants stabilize the polymer matrix and have a
`secondary antioxidant function as they migrate into food. This
`
`Figure 5. Structures of common polymer antioxidants including 1) phenolic-based compounds, 2) phosphate-based compounds, and 3) hindered
`amine systems.
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`eliminates the need for antioxidant incorporation inside food
`products.[81] Stabilizers can also migrate to phase boundaries
`present in contaminated polymeric materials and cause areas
`of degradation where the stabilizer is not properly dispersed.[81]
`Most, if not all, of these additive mitigations understand their
`response to continuous recycling, and the potential unintended
`consequences of thermal and oxidative degradation products.
`Antioxidants primarily protect polymers for their first pro-
`cessing cycle and research rarely explores antioxidant stability
`during repeated extrusion cycles.
`
`2.4. Polymer Blends in Recycling
`
`If sorting challenges are not overcome in the plastics industry,
`value must be found from this impure feedstock. Mixed
`polymer streams, though prevalent in mechanical recycling,
`usually form weakened materials due to the immiscibility of
`the polymer phases. These melt incompatibilities create frac-
`ture points in the extrudate.[82,83] Flory-Huggins theory predicts
`that, when large enough, the Gibbs free energy of mixing for
`two polymers will disfavor their blending and lead to phase
`separation.[81,84] Demixed polymers can form an array of phase
`separated morphologies, including, spheres, cylinder, lamellae
`and continuous segments.[84–86] Phase separation in blends
`results in poor mechanical properties due to ineffective stress
`and strain transfer across phase boundaries. Stress transfer
`across boundaries can be improved by increasing the number
`of interactions between phases.[87,88,21] Improved compatibi-
`lization of polymer blends would increase value of recyclate
`through better processability, plant flexibility, product tailoring,
`and upgraded mixed recyclate performance.[89] Compatibilizers
`prevent severe demixing of two polymer phases and promote
`stability of blends when the dispersed phase is larger than
`10% through reducing the interfacial tension between the two
`phases to promote dispersion, stabilize the morphology of
`the two phases and enhance phase adhesion. Enhancing this
`adhesion enables proper stress and strain transfer within the
`blend.[89] Compatibilizers can also prevent damage from con-
`taminants migrating to interphases during the extrusion pro-
`cess. This aids blending of household wastes which have higher
`levels of contamination.
`Polymer blending usually requires very similar chemical
`polarities due to energetic compatibilities. Copolymer com-
`patibilizers are used to aid in blending polymers of dissim-
`ilar chemical polarities.[90,91] These copolymers usually com-
`prise a non-polar backbone with polar functionalities spaced
`along the chain to promote interactions between both polar
`and non-polar chains. Common stabilizers include styrenes
`grafted with maleic anhydride or different polyolefins grafted
`with maleic anhydride.[90,91] These common stabilizers are
`known to be expensive and so used in small proportions to
`minimize costs.[90] Copolymers used as compatibilizers show
`varying degrees of efficiency which are largely based on their
`structural features. Di-block copolymers are much more effi-
`cient at reducing the interfacial tension between polymers
`than their random counterparts. However, longer, random
`chains outperform shorter di-block polymers in terms of sur-
`factant behavior.[21] Moreover, in multi-block compatibiliser
`
`systems, the number of entanglements between phases
`increases and can then interlock. Increased interlocks across
`the blend interphases improves toughness and adhesion
`within blends.[92]
`Reactive extrusion processes, which combine chemical reac-
`tions such as polymer modifications with melt reprocessing,
`can also be used to improve the miscibility of immiscible
`polymer blends. There are four different reactive processes to
`produce copolymers or blends in extruders:[21,93] Redistribu-
`tion reactions, in which the end group of one polymer attacks
`the chain of the other to produce graft or block copolymers;
`crosslinking polymers by reactions between pendant groups
`present in both polymer chains; ionic bond formation through
`ionic linking agents or by protonation of a basic polymer by an
`acidic polymer; dynamic vulcanization which occurs when one
`phase is immobilized in situ dispersed in a mobile thermo-
`plastic phase.[21,93]
`Examples of successful blends of recycled and virgin poly-
`mers will be discussed in further detail in subsequent sec-
`tions of this review article and a more thorough discussion
`of polymer blending in recycling processes was published by
`Maris et  al.[21] Present blending research and development
`predominantly focuses on the initial production of blends,
`without consideration for their applicability in continuous
`recycling. We suggest it is essential to consider the conse-
`quences of blending strategies on future reprocessing life-
`cycles. Incorporating an end-of-life recycling as a design prin-
`ciple for blends, where due consideration of the impact of fur-
`ther extrusion on polymer melt temperature, degradation and
`stabilization is key.
`
`3. Mechanical Recycling of Poly(ethylene
`terephthalate)
`Virgin PET (vPET) has excellent mechanical properties, pro-
`cessability and barrier properties.[42] PET is thus widely used for
`packaging, with a large proportion of this being food grade.[46]
`Virgin PET is ductile and boasts high elongation at break values
`of >80% which have been found to rapidly reduce by a factor of
`4 when mechanically recycled (Figure  6).[28] The sharp reduc-
`tion in the properties of the material is due to deterioration
`thermo-oxidative and thermo-mechanical degradation of the
`chains as well as hydrolytic scission.[47,53]
`
`3.1. Degradation Mechanisms of Poly(ethylene terephthalate)
`
`Chain scission lowers polymer molecular weights and forms
`potential side products including carbon dioxide, water and car-
`boxylic acid or aldehyde end groups.[94] Shorter chain lengths
`reduce polymer elasticity, embrittle the polymer, and decrease
`viscosity. Melt viscosity reductions thus require recalibration
`of processing equipment. Moreover, contaminants introduced
`during the life-cycle of the PET promote chain scission reac-
`tions in post-consumer polymer melts. In particular, traces
`of poly(vinyl alcohol) (PVA), or poly(lactic acid) (PLA), and
`PVC can leach acids and