Are Reliable and Emerging Technologies Available for Plastic Recycling in a Circular Economy?

Abstract

A spectrum of plastics has been produced in the last 70 years, and plastic production has increased faster than any other manufactured material. Current recycling of all plastic materials is pegged at 10% or less. The social value that plastics enjoys is reflected in its myriad uses for engineered durability to single-use applications. Disposable or single-use plastic items have become a significant problem. Plastic debris has become ubiquitous to the landscape and aquatic resources, leading to human health, ecological concerns, and sustainability issues. Past disposal practices relied on waste plastic flows to certain countries for disposal, but these have been summarily curtailed, needing alternatives as productive and environmentally conscious recycling technology. Waste plastics can be repurposed using purification, decomposition, or conversion processes that are based on established and emerging mechanical and chemical technologies. Plastic recycling technologies, such as thermal, chemical, and biological depolymerization processes, including pyrolytic technologies using plastics-to-fuel strategies, are under development ranging from bench-scale demonstrations to full-scale implementation. The ideal of closed supply chain constraints offers optimal solutions to plastic recycling. Evaluation of new processes requires performance assessment to understand better how plastics recycling technologies contribute to the environment and the sustainable reuse of plastic materials.

1. Introduction

Plastics are chemical success stories of the past 100 years that have touched every aspect of modern life. However, their success has a huge downside. Today more than 40 kg of plastic waste per person is produced each year globally. In 2018, the total amount of municipal solid waste (MSW) was 292.4 million tons, which is 2.2 kg/per capita per day (Figure 1a). This is a 40% increase from the 1990 waste generation. Although 23% of this waste is recycled, there is considerable variability in the type of waste [1]. Solid waste management conducted by local municipalities is a global challenge requiring aggressive attention. Poorly managed waste flows from terrestrial and air-born sources contaminate oceans and other water bodies. Flooding from clogged drains, air pollution emissions from burning, and contaminated urban areas provide significant input to this global dispersal of plastic waste. The complexity of solid waste management is a combined effect of social, environmental, and economic factors contributing to a multi-dimensional challenge demanding solutions. Inadequate solid waste management practices contribute to environmental problems in treating ecosystems, impacts of human health, and undesirable socioeconomics.

Figure 1.

(a) Total municipal solid waste generation by material in 2018 (total 262 million tons) (b) generation trend of plastic waste in the United States between 1960 and 2018, showing (c) recycling and composting rates as a percentage of generation (Source: US EPA, 2018).

2. Current global plastics problem

Plastic-related waste has increased by two orders of magnitude in the past half-century [2]. Paper and paperboard recycling is currently 66.5%, whereas plastic recycling was estimated at 4.4% in 2018. Some 5 million tons of plastic particles derived from the 300 million tons of annual global virgin plastic production are transported by rivers and deposited in the oceans. Mechanical degradation of macro size plastics leads to the formation of a large volume of microplastics that when combined with manufactured microbeads from personal care products lead to harmful conditions in aquatic ecosystems. The raw material for these virgin plastic resins is derived from petroleum. Its short use-life and high durability in the environment translates into an environmental lifetime exceeding hundreds of years underscoring plastic disposal/reuse as a significant environmental concern [3, 4, 5, 6]. Although almost all commodity plastics carry the recycling symbol (Table 1), plastic recycling in the United States is crude, energy-intensive with recycled materials characterized by lower qualities than virgin plastic.

Table 1.

Types of plastic resin and resin identification codes (RIC).

Source for recycling rates ref. [1].

A large portion of plastic products is used to make short-lived products such as packaging discarded after a single use [7]. European reports indicate that 70% of marine plastic litter is caused by single-use plastics [8]. Recycling is one of the sustainable ways to reduce the impacts of plastics and product reuse and energy recovery as fuel.

3. Plastic economy

Traditional plastic production is based on fossil feedstocks, where over 90% of currently manufactured plastics is derived from virgin fossil feedstocks with a huge carbon impact. Plastic production has been growing at an average of 40% in the past 50 years, surging from 15 M tons in 1964 to over 320 M tons in 2018, as plastic use increased in every application. Plastic packaging is one of the main applications of plastic use, representing 26% of the total volume of plastic use. Plastic packaging has many economic benefits ranging from protecting products during shipping, preserving and reducing food waste, and reducing fuel consumption for transport due to lightweight. However, 95% of plastic packaging and 90% of total plastics are not recycled, because of which an estimated over 400 billion is lost to the economy annually [9].

Although universal recycling symbols were introduced 45 years ago, only a small fraction of plastics are recycled. Successful recycling requires reliable and efficient post-consumer collections for similar products, consistent consumer demand, and economical remanufacturing. These factors are interdependent, since the economy of scale requires effective collocation of used materials that are not too contaminated, located close to recycling sites, and require a community that is committed to sorting properly based on Resin Identification Codes (RIC; Table 1). However, there are too many chemical variations of plastic products distributed among too few categories. Packaging represents 26% of the total volume of plastics used and out of the single-use plastic, mainly packaging materials, which have short first-use cycle one third escape collection system [9]. Of the different resins listed, polyethylene terephthalate (PET) and high-density polyethylene (HDPE), codes 1 and 2, are the more easily recycled plastics. The cost of plastic waste on oceans, clogging urban infrastructure, in addition to greenhouse gas emission during manufacturing, was estimated to exceed USD 40 billion [10].

Sustainable solid waste management requires improved collection infrastructure that includes the separation at the source, policies that encourage reuse/recycling, and supporting recycling [11]. The cost of recycling depends on the market, volume, required sorting, and decontamination of the feedstock. The flow of recycled plastic materials makes it uneconomical to justify separating low-volume streams. Many promising technologies emerging in the market offer opportunities to recover the value of some low-volume recycle streams. Optical and robotic sorting techniques along with digital watermarks are expected to increase the efficiency of sorting. Further investigative research is required to determine how to retrofit existing facilities to include new sorting technologies optimally.

4. Current plastic linear value chain economy

Plastics have been used in many applications, and their uses follow a linear economy, take-make-waste model. The fraction of global recycled plastics is low; 30% in Europe, 25% in China, and 9% in the United States. The current plastic economy is best understood as a “linear” model of value creation where the product lifecycle begins with extraction and ends with landfill disposal or release to the environment. About 4% of the world’s petroleum product is used as plastic feedstock and a similar amount is used for processing products and transporting them. The raw materials are extracted and transformed into products. Generally, at the end of their use phase, the items are environmentally mismanaged in the disposal. Value is created in this economic system by producing and selling as many products derived from virgin polymeric materials as possible. The linear economic model has been in practice since the industrial revolution, has achieved economic growth, and has improved the standard of living. The model does not consider the natural depletion of resources or the disposition of the products after use, which often is landfilling, incineration, or other means of release to the environment, nor the value of the disposed of material for recycling or reuse (Figure 2). The linear plastic economy has proven unsustainable for its resource consumption and environmental and human health impacts.

Figure 2.

Schematic representation of linear economy of plastic.

The negative influences of the linear economic model of plastics, including the depletion of nonrenewable resources, climate change, and severe ecological impact, leave a significant footprint [1, 9]. The high reliance on an extracted virgin resource subject to high price fluctuations, the need for an increase in trade, and the geopolitical interconnectedness for raw material, and end-of-life disposal are major challenges in a linear economy. As the population and welfare of emerging economies have grown over the past few decades, the number of middle-class consumers has also been increasing. This, in turn, resulted in an increased demand for newer products and a shortened product lifespan that accelerated the disposal of more plastic waste. The move on to a sustainable economy can be achieved through the concept of the circular economic (CE) model that incorporates strategies for continuously reuse of material and resources [11]. Whereas recycling techniques like energy recovery through incineration generate air pollution, CE of plastics economy involves waste recovery using a range of recycling techniques to minimize plastic waste disposal by converting it to valuable products.

5. Plastics and the circular economy

The global recognition of a fundamental shift in the design, use, and reuse of plastics is implicit to his new economic order. A CE advances a closed-loop system where materials are kept in the product lifecycle and not disposed of to reduce raw material usage and energy demand. The concept of a circular economy was proposed over 30 years ago as the impacts of the traditional open-ended economy with no recycling and the environment was being treated as a waste sink became obvious. Previously, the policies of many governments have been focused on encouraging pollution reduction, promoting closed-loop waste management by increasing recycling and dematerialization. For example, the Pollution Prevention Act of 1990 in the United States encouraged reducing pollution at the source and the three R-strategies: reduce, reuse, and recycle. Similarly, the 1996 German law Closed Substance Cycle and Waste Management and the 2002 Japanese law for Establishing A Recycling-based society [12, 13, 14].

A shift to a CE could create a disruptive change that is restorative and regenerative by intention, including green by design that generates no waste, keeps products and materials in use, and regenerates natural systems (Figure 3). The plastic industry CE deals with environmental impacts, resource scarcity, disruption in traditional end-of-life management options, and simultaneous economic benefits. To create a successful CE of plastics, various approaches should be followed, including eliminating problematic plastics, innovating to ensure that unused plastics are reusable and recyclable, and circulating the plastic in the economy. The complete lifecycle of plastics should be analyzed with reference to their ability to circulate back for reuse to the economic system to achieve the maximum value.

Figure 3.

Schematic of the circular economy of plastic.

One of the cornerstones of CE is the creation of an effective after-use plastics economy requiring more material value and higher resource productivity. This requires designing products for recyclability, including designing nonrecyclable packages to become recyclable, which can be achieved by using a mono-material design modifying protective coatings, introducing components that will increase compatibility with processing, developing film-orientation technology, and simplifying the product design for ease of recycling. Many of the most common plastics are recycled at meager rates in the United States. When recycled, they are often recycled into lower-value products due to the degradation of polymer properties resulting from mechanical recycling. To support increased recycling, new plastic recycling protocols should ensure that the monomer stream generated after depolymerization is free from contaminants and colors, a common problem with the chemical recycling of plastics.

6. Reliable and emerging technologies for plastic recycling

Existing recycling offers a spectrum of applications to single polymer compositions and to mixed plastic compositions (Figure 4).

Figure 4.

Major Plastic waste recycling technologies (primary or closed-loop, and secondary recycling – mechanical recycling, tertiary recyling – chemical recycling, AND recycling energy recovery and valorization, quaternary – incineration).

Significant different processing strategies are required to accommodate the complexity of waste feedstock components. Changes to existing and emerging recycling technology are anticipated to convert feedstock to desired products optimally. Conventional and emerging mechanical and chemical technologies ranging from bench-scale demonstrations to full-scale implementation are becoming investment targets across the globe. Notable examples involve thermal, chemical, and biological depolymerization processes (Table 2).

6.1 Plastic recycling strata

The recycling process for plastics has been subdivided into four components: primary recycling, secondary recycling, tertiary recycling, and quaternary recycling [16, 17, 18]. Recovery of production scraps as a pure polymer stream or the extrusion of preconsumer polymer has been identified as the operational component of primary recycling. Secondary recycling involves sorting of waste polymer feedstock, waste polymer size reduction, cleaning the waste polymer flow, drying and extrusion to form desired recycled polymer products [19, 20, 21, 22, 23, 24]. Multiple cycles of recycling at primary and secondary stages can be used to recover many polymers while maintaining expected performance characteristics.

Conversion of the polymer feedstock to its constituent monomers can be selected when the first two recycling stages do not offer the desired recycling flexibility, economic opportunity, or product quality [25]. This tertiary stage recycling can complement the earlier, more traditional recycling technology strata (Figure 4). A selective approach to converting the feedstock polymer to its monomeric components can retain significant feedstock value with the ability to convert the recycle stream to a spectrum of polymer applications. This depolymerized feedstock can be utilized in a spectrum of pathways leading to chemical production syntheses other than reforming recycled polyethylene terephthalate (rPET) as an upcycling endeavor. Chemical recycling utilizes reactions with the chemical structure of the polymer to convert the polymeric mass to monomers and related chemicals [15, 26, 27, 28, 29, 30]. The chemical recycling process permits the removal of material consigned to downcycling recycling technology such as colorants, process additives, and other contaminants. This removal provides a material that can be used as a feedstock for opportunities of upcycling plastic waste to access value-added chemicals, thereby enhancing economic performance (Figure 5).

Figure 5.

Tertiary plastic recycling strata.

The nonselective technologies found useful at the last recycling stage are generally thermolytic such as pyrolysis and hydrocracking technologies that convert the polymer feedstock into monomer and pyrolytic oils [31, 32]. At a quaternary stage, incineration is employed to recover the energy content of the waste polymer feedstock [33, 34, 35]. The relegation of polymer feedstock to quaternary recycling can be attributed to feedstock complexity, processing economics, waste unsuitable to other forms of recycling, market forces, and waste polymer treatment demands and capabilities [24, 36, 37]. This stage of recycling recovers little value of the feedstock and may contribute to greenhouse gas emissions having other unintended environmental consequences. Selective chemical recycling of polymers has become more attractive to engage significant process funding recently, since chemical depolymerization is designed to form the original monomers of the recycled polymer in successive recycling to approach continuous recyclability [38, 39, 40].

6.2 Chemical recycling example—Polyethylene terephthalate (PET)

PET is the most common thermoplastic polymer of the polyester family of organic polymers [41, 42]. It is a naturally transparent and semi-crystalline plastic, which is used for packaging, for manufacturing stretch-blown bottles, sheet, and thermoforms, and for producing fibers for textile products [43]. The important characteristics of PET include resistance to water, high strength to weight ratio, and wide availability as an economic and recyclable plastic [44].

As condensation polymers, polyesters are constructed from the reaction of multifunctional carboxylic acids and polyols in which water or alcohol is eliminated to form a growing polymeric chain. The condensation polymer construction of PET provides a unique chemistry leading to its formation through the elimination of water or alcohol depending on the composition of monomers. PET as a condensation polymer has unique chemistry controlling its assembly or disassembly. It is important to understand the synthesis of PET to enable the depolymerization option to recycle PET optimally.

Exhibiting significant mechanical properties, PET’s rigid polymer chains and high melting point contribute resistance to fatigue and plastic deformation as desirable features. Resistance to solvents, chemicals, and hydrolysis at normal application temperatures identify PET as a highly desirable component to a host of applications. PET’s manufacture is affected with the unattractive disadvantage of a relatively slow rate of crystallization leading to increased processing time of a slow cooling cycle necessitating nucleating agents. As a crystallizable polymer, PET exhibits properties of different desirable states of physical order ranging from amorphous (transparent) to crystallized that are transparent and opaque [45].

6.2.1 PET synthetic technology

PET synthesis is configured in two steps [46]. An esterification of terephthalic acid (TPA) or dimethyl terephthalate (DMT) reaction with ethylene glycol (EG) can be used to form a prepolymer mixture product composed of bis(2-hydroxyethyl) terephthalate (BHET) and short-chain oligomers (Figure 6). The condensation reaction requires the removal of water or methyl alcohol to force the reaction to completion. The growing polymer formed by equilibrium esterification is susceptible to depolymerization by hydrolysis or methanolysis, so the removal of any solvolytic agent is important to build molar mass.

Figure 6.

Polyethylene terephthalate (PET) synthesis.

A second step is transesterification in melt phase leading to the polycondensation of short chains to form the molar weight range and intrinsic viscosity required for an intended application [47]. The prepolymer formation step can be conducted with DMT or TPA as one monomer with EG or another polyol as a monomeric partner. Even at the prepolymer step, the oligomeric mixture is difficult to purify and this emphasizes the need for high purity starting materials. DMT can be purified by distillation or crystallization from a melt. Heating TPA beyond its boiling point leads to decomposition, and purified TPA can be produced by recrystallization at process production scale. Currently, TPA is the monomer of choice for more than 70% of the PET production across the globe [48]. Where DMT is employed, purification is done by distillation to remove higher boiling constituents and light molecular weight esters. High-purity DMT is achieved by recrystallization. The quality and color of high-purity DMT are required to produce high molecular weight PET.

Industrially, PET is produced by the reaction of EG with the DMT or TPA in the presence of a catalyst (Figure 7) [48]. High conversions to condensed polymer in the first stage are established from a stoichiometric balance of reacting groups. Excess EG is employed in industrial processes, which is removed and recycled as part of the process. Early processes synthesized PET at industrial scale through the two-step polymerization reaction between DMT and a 30–50% excess of EG in the presence of a catalyst involving transesterification and polycondensation [49]. In the first step, transesterification of DMT with EG leads to the initial formation of BHET and a small number of assorted oligomers. This reaction is conducted at atmospheric pressure at a temperature range from 150 to 210°C under an inert atmosphere to minimize oxidative side reactions. Methanol is removed to force the reaction to completion with the formation of BHET and hydroxyethyl–terminated oligomers [49].

Figure 7.

rPET synthesis from PET waste using depolymerization.

Polycondensation is the chemistry of the second step, which involves increasing the temperature to 270–280^°C, exceeding the melting temperature of PET, and initiating the application of high vacuum (10–50 Pa). During this stage, EG is formed and removed as a by-product. The product PET has a degree of polymerization of approximately 100. The equilibrium reaction of polycondensation requires the removal of excess EG through high vacuum and intensive mixing of the molten PET in the reactor. At the polycondensation stage, the intrinsic viscosity of the reaction mixture increases as the molar weight of the polymer increases. Removal of volatiles from the molten phase becomes a rate limiting process. Thermal degradation of the polymer occurs as heating more than 300^°C is applied due to PET’s limited thermal stability.

Judicious selection of polymerization catalyst and temperature control can be used to reduce the reaction time of the two steps. Significant investigation of the tertiary strata options for PET recycling has been conducted (Figure 8) leading to commercial opportunities for the recycle industry. Ammonia can be used to recycle PET to different materials using similar chemistry [50].

Figure 8.

Sequence of PET recycling process components.

6.2.2 PET recycle

Globally, some 245 million tons of plastic are estimated to be in use annually [51]. The largest component is fibers with a quarter of this polymeric composition attributed to packaging [52]. The second largest component of the plastic mass is composed of PET bottles whose contribution expands annually. The oxygen content of PET identifies it to be a poor candidate for thermal recycling.

Secondary recycling based on mechanical technology enables the recycling of granulated plastic feedstock that has been processed from common waste plastic materials [15, 16, 53, 54]. The heated process of extrusion in a rotating screw configuration offers conditions that induce thermal softening of the plastic mass and plasticization. This process provides blending of the plastic melt, which permits the recycling of single, mixed, or blended plastic compositions. An example of the desired properties of vPET and rPET is displayed in (Table 3) [55]. The characteristics of rPET can vary widely with different sourced feedstocks. The components of a mechanical recycling process are shown in (Figure 8) targeting waste PET to provide the cleanest feedstock to the extrusion process. Thermal conditions of the extruder can lead to oxidative degradation of the plastic mass, and shear induced in the melt can result in undesirable polymer chain breaking, branching, and cross-linking. Polymer chain length degradation leads to reduced mechanical properties and lessens the features required for polymer processing into forms such as bottles, which require a higher molar weight polymeric mass.

Reactor type	Advantage	Disadvantage	References
Traditional methods
Batch reactor	High conversion efficiency; Low quality product	Lower quality products (e.g., formation of PAHs) Higher energy consumption	[1, 2]
Fixed bed reactor	Simplicity in design	Difficulty with high viscous low thermal conductivity Small catalyst surface area Used in tandem with a noncatalytic stage	[3, 4]
Fluidized bed reactor	Yield of liquid products is more than 90 wt % Low level formation of gas coke	Complex operation Difficulty in separating catalyst particles from the from exhaust gas Reactor erosion	[5, 6]
Conical spouted bed reactor	Allows process flexibility Can treat large particles with density disparity		[7, 8]
Free-fall under vacuum	Produces important liquid chemicals such BETX, and naphthalene		[9, 10]
Alternative methods
Catalytic pyrolysis	Lowers pyrolysis temperature Reduces the heat energy requirement Favors scale-up to larger scale Produces commercial type final products		[11, 12]
Solvent-assisted pyrolysis	High heat and mass transfer rate Reduce operating temperature Higher liquid yields		[13, 14]
Microwave-assisted pyrolysis	Extremely rapid heating Higher production rate Low production cost Energy saving	Economics of process scale-up; Large-scale heterogenous; Hotspots on heat adsorber due to low MW adsorption and conductivity of plastic
Supercritical fluid	Efficient heating and fast dissolution of plastic homogeneous enhanced depolymerization	Challenge in scale-up
Plasma reactor	High monomer recovery due to efficient heating	Low technology readiness	[15]

Table 2.

Plastic recycling process.

Descriptor	vPET	rPET
Crystallinity
Crystalline region (%)	37.2	28.5
Amorphous region (%)	62.8	71.5
Thermal properties
Glass transition temperature (°C)	75.35	87.27
Melting point	242.72	244.15
Degradation temperature (°C)	420	419.6
Residue
Strength properties
Tensile strength (kg/cm^2)	75.4	87
Breaking strength (kg/cm^2)	82	42
Elongation strength (kg/cm^2)	7	5.0
Young’s modulus (kg/cm^2)	5690	10,500
General
Molar mass (g/mol)	19,342	15,812
Shrinkage % (150 denier)	9.31	6.18

Table 3.

Virgin vs. recycled polyester characteristic properties (Reconstructed after Vardicherla) [27].

The process conditions can be optimized to avoid some of these problematic issues. The chemical composition of the plastic mass in an extrusion process is important to the mechanical effects placed on the melt. The fluid mechanics of the extrusion is adjusted to allow the melt to begin to flow. The intrinsic viscosity of the melt rises with increased polymeric molecular weight. Heated extrusion processing is the primary technology employed in the mechanical recycling of plastics to form a granulated feedstock without the use of solvents.

Atmospheric oxygen can react with shear-induced radicals to form peroxy radicals, and this leads to radical-induced decomposition. Plastics with high oxygen permeability have been observed to exhibit enhanced thermo-oxidation rates within the melt. The chemical and physical forces accompanying extrusion leads to undesired changes in tensile strength, elongation, and many other properties for rPET. Inventive use of semi-closed or open-loop recycling involving the addition of vPET during the recycling process is employed to alleviate material properties decline experienced during extrusion.

6.2.3 Contamination effects

Contamination in the recycling feedstock occurs as a leftover from previous processing or desired modifications to the original feedstock but without proper treatment before recycling generally exerts significant negative influence on the quality and variability of the recycled plastic [30, 56, 57]. A list of contaminants (Table 4) found in PET feedstock have been identified for their significant contribution to quality decrease and performance variability increase of the regenerated polymer [57]. PET is hygroscopic and must be dried to remove water to reduce any chain length reduction through hydrolysis at the melt stage. Super cleaning techniques are used in some situations to ensure the optional flake quality for the later polymerization steps [58]. The label adhesives can release acetic acid, which will catalyze polymer chain reduction through hydrolysis, specifically during heat-related segments of the extrusion process. In the United States, the majority of virgin and rPET infrastructure are in the southeast and midwest (Table 5).

Metal ion	Concentration (ppm)	Source
Antimony	240–260	Polycondensation
Cobalt	50–100	Polycondensation
Manganese	20–60	Transesterification
Titanium	0–80	Polycondensation
Iron	0–6	Washing
Sodium, magnesium, silicon	—	Food contamination

Table 4.

Metal contamination found in rPET.

Location	Throughput	Process
Tennessee	50 million lbs./yr. (2020)	Methanolysis of PET and glycolysis of PET
South Carolina	46 million pounds/yr. 1st yr. 88 million pounds/yr. (2020)	Low heat, pressure-less process
Illinois	Pilot plant	Glycolysis of highly colored recyclate
France	Lab scale/pilot plant	Depolymerization
France	Reactor development/pilot stage	Enzymatic Hydrolysis
Italy	Lab stage	Glycolysis
Switzerland	Lab stage	Microwave hydrolysis
Netherlands	Proof of concept	Glycolysis
Virginia	Startup stage	Subcritical water hydrolysis

Table 5.

Applications of chemical recycling with PET feedstock.

EG degradation and recombination products have been identified as the source of discoloration and clarity losses in the polymer product. Carry-over degradation products from thermal and oxidative conditions lead to yellowing and reduction of PET mechanical properties. Polyvinyl chloride (PVC) is used in bottle cap liners commonly connected with PET bottles and presents a major problem in the PET recycling process. Under thermal treatment conditions, PVC degrades to form hydrochloric acid, which in PET melt conditions leads to the reduction of the polymer chain length by hydrolysis reducing the value of the rPET [59, 60]. This problem is intensified, since PET and PVC have the same density and are difficult to separate.

Catalyst residues and processing additives such as trace metals (antimony, cobalt, and manganese) from the previous processing of consumer PET wastes promote transesterification and polycondensation reactions in the recycled PET as part of the heated extrusion process (Table 6). The recycled PET formed by the action of these trace metal has a chemically heterogeneous composition and may be affected by changes to the viscosity of the melt leading to batch–batch variability [60, 61]. Melt degradation can also be caused by the presence of pigments that are used to color plastics. Colored feedstock leads to the formation of an unattractive and lower commercial value gray color rPET product.

Contaminants	Use/cause	Effects
Paper	Labels & adhesives	Incompatible with melt formation and releases minor amounts of acid that will catalyze polymer hydrolysis
Moisture	Residual water not removed by drying	Reduces the polymer molecular weight through hydrolysis
Degradation products	Products of thermal oxidation during extrusion	Reduces the PET mechanical properties and leads to yellowing
Dissimilar plastics	Poor feedstock sorting	Incompatibilities in the melt stage
PVC	Bottle cap incomplete removal	Releases HCl when treated thermally which catalyzes polymer chain hydrolysis
Colored feedstock	Inks and plastic dyes	Colored bodies exhibit undesirable gray color in product and may catalyze decomposition
Trace metals	Polymerization catalysts and processing additives	May catalyze transesterification or polycondensation reactions leading to batch–batch quality issues
Acetaldehyde	Thermal release of ethylene glycol and elimination of water	Easily oxidized to acetic acid which can catalyze polymer linkage hydrolysis

Table 6.

Contaminants to the PET recycling process and their effects.

The mechanical recycling of complex and contaminated PET feedstock is difficult [62, 63, 64]. Mechanical recycled rPET is commonly characterized by intrinsic viscosities that are relatively low and heterogeneous. These properties have excluded rPET from direct incorporation in the production of bottle-grade PET, high-quality industrial fibers and films. Carpets, textile fibers for wearing apparel, and plastic containers designed for non-food applications have employed lower quality rPET. Contaminant removal has been accomplished through a thorough cleaning step, which removes the spectrum of contaminants encountered. There is a significant difference in the performance of the cleaning step depending on the relative level of contamination. Heavily contaminated PET feedstock is much more difficult to recycle to high value products. An array of chemicals employed as necessary adjuvants to plastic production may exert significant roles to the recycling process and may be emitted throughout the plastic’s life cycle [64, 65].

6.2.4 Market for PET recycling

Plastic recycling has become a crucial practice for the recovery of the fossil fuel resources embedded in the waste along with society’s desire to control plastic use to avoid the detrimental littering of much of the annual plastic mass [61, 65]. A recent survey of U.S plastics recyclability paints a rather negative picture for the currently available technology [66, 67]. The recycling of plastics is at a significant growth stage begging for significant expansion. For certain segments of this expansion, an infrastructure development is necessary requiring considerable investment to a market that counts margins in pennies per pound.

A recognition of the resource value of this plastic mass continues to underscore the waste’s value to create products of significant economic value [68]. Evaluations of the importance of this reutilization of plastic waste have been estimated in billions of dollars and have a transformative effect on the U.S. chemical industry and has led to an unforeseen boost to the economy [69, 70].

PET offers remarkable opportunities for recycling. At the mechanical recycling level, recycled material can be reprocessed multiple times. Feedstock is available in large quantities as a high PET content material. Dedicated collection systems for the recycling of PET bottles or separation from more complex feedstock can provide the required composition needed for recycling. Waste collection logistics is assisting the plastic recycling effort through the introduction of collection rate mandates, disposal bans, and quotas for reuse and reprocessing [71, 72]. Collection rates and plastic consumption in all markets continue to increase. Increased rPET content has been found to be highly desirable in the production of new bottles; this has been observed with the willingness of drink brands to pay slightly more for the rPET of the vPET. Furthermore, drink brands have agreed to increased rPET composition over the next few years.

6.2.5 rPET material flow

The collected feedstock for recycle is the result of manual or automated sorting to concentrate the PET fraction of a waste stream [61]. Post-consumer PET must be separated to reclaim the desired feed from contaminating non-PET. Items such as caps, lids, labels, adhesives, dirt, foreign plastics, and residual content reduce the quality of the final product and may significantly change the molar mass of the newly former polymer [61, 73]. Upon completion of the polymerization process, conversion to final product continues to the consumer and a new collection for recycle or disposal.

6.3 Pyrolytic and thermal processing

Pyrolysis is a chemical process using heat and pressure to depolymerize long-chain polymer molecules to form smaller, less complex molecules. Pyrolysis requires oxygen-free conditions for fast thermal degradation of plastic waste, which can be accomplished by using noncatalytic or catalytic conditions with zeolite, spent Fluid Catalytic Cracking catalyst, and MfO catalyst at temperatures varying from 300 to 800°C. Major products formed from the pyrolytic treatment of waste plastic feedstock are gas, oil, and char [23, 24]. Pyrolysis yields on average 45–50% of oil, 35–40% of gases, and 10–20% of tar. This conversion process can achieve up to 80% liquid oil at a moderate temperature of 500°C [27] and reaching 88 wt% liquid yields at 580°C. Plastics offer higher energy content mass than biofuels such as wood and coal.

A facility in Woodlands, Texas, USA initially was used to produce coal-derived products such as benzene, toluene, and xylene (BTX). It currently uses catalytic pyrolysis to convert post-consumer waste plastics into BTX. Similarly, a plastic-to-fuel facility in Virginia converts scrap plastic into fuel or chemical products [23]. The commonly used catalysts for pyrolysis conversion involve cracking, oligomerization, cyclization, aromatization, and isomerization reactions. Catalysts used in these processes include ZSM-5, zeolite, Y-zeolite, FCC, and MCM-41 [15, 28]. Light oil recovered from pyrolysis can be used as a chemical feedstock to produce new plastics or chemicals, whereas medium and heavy oils are commonly used as diesel fuel and heavy oils are used for power generation. For example, a large plant in Sapporo, Japan has a novel flexible system that receives not only PET, and PP, but also PVC, which is considered contaminants for PET recycling (Figure 9) [29].

Figure 9.

Schematic diagram showing process steps for pyrolysis and other thermo-catalytic processing of plastic waste.

7. Controlling elements for plastic recycling

Achieving a transition to a circular economy of plastics depends on the cost-effective and efficient reuse and recycling of plastics at large scales. A combination of end-of-life management approaches for plastics can reduce use, reuse, and repair, leading to a decreased energy and materials [74]. Recycling is a vital part of industrial ecology that can reduce environmental impacts and resource depletion. PET and HDPE are widely recycled when used in specific categories such as plastic bottles. However, this is an exception and is not seen for other plastic categories (Table 1). This could be due to the cost structure of recycled plastics, the challenges of sorting mixed plastics, and the variability of the cost of virgin plastics. Many post-consumer plastics may have to be converted to other products, and they are recycling a limited number of times. The controlling factors for recycling plastics include the effectiveness of collection and segregation of mixed materials, collection and storage of recyclables, geographical location of recycling facilities concerning the processing markets, and volume of the recyclable materials collected for the region.

The incompatible nature of most plastics due to their inherent immiscibility at a molecular level is one of the significant challenges for producing high-quality resins. For example, a small quantity of PVC in PET recycling degrades the quality of the recycled material. Hence, mixing recycled plastics with virgin plastics could reduce some of the attributes of the virgin materials, and hence, the recycled plastics are mainly used for non-critical applications. Hence, post-consumer recycling involves multiple steps: collection, sorting, washing, size reduction, and separation to minimize contamination by incompatible plastics [7]. Plastic additives such as plasticizers, color, and flame retardants used in manufacturing some plastics complicate the recycling process and present risks to human health and ecology.

8. Infrastructure development and access

The successful implementation of the plastic circular economy will require sufficient infrastructure to process-collected material. Due to the current lack of processing capacity, there is a need to modify or modernize existing recycling infrastructure [75].

8.1 Availability of suitable feedstock

The effective implementation of a CE for plastics is dependent upon the availability of feedstock, adequate recycling infrastructure, and technologies that can convert nonrecycled plastics to fuel or chemical feedstock. In 2016, only 16% of the inflow of polymers was collected for recycling, 40% were sent to landfills, and 25% incinerated. 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 what was recycled in 2006 [55].

The degree to which plastic recycling can be increased is limited by the availability of suitable feedstock. The ability to increase plastic recycling rates is inextricably linked to the collection rates, quality of the collected plastic, and the availability of recycling technologies to manage the post-use waste stream. Mechanical, or closed-loop processes are limited by the types of plastics that can be processed, resulting in sub-optimal yields. Plastic bottles are a highly desirable product for recyclers, but just about a third find their way into a recycling bin [76].

Chemical recycling enables the processing of mixed waste streams. However, just like mechanical recycling, advanced recycling facilities require a dependable flow of materials (feedstock) that fit the technology process [10]. Some chemical recyclers currently in operation rely on high-quality material, such as post-consumer bottles or recycled PET (rPET) polyethylene terephthalate flake as a feedstock to get a better quality of output. However, there is a concern in the recycling community that the new technologies are in danger of stripping the mechanical recycling industry of its much needed and increasingly scarce feedstock of high-quality post-consumer plastic waste. Because of the cost and challenge of sorting and separating, most chemical recyclers are not targeting low-quality waste but rather the same waste streams that would typically be used in mechanical recycling [10].

8.2 Availability of recycling facilities

The vision of a circular economy can only be realized by deeming used plastic as a resource and not as waste, and through employing advanced recycling technologies that “keep the molecule in play” and maintain materials at an economic value. Advanced recycling technologies have been used to turn plastics into fuels for decades; however, to meet the growing market demand for high-quality plastic recyclate, there is a need to improve domestic and global capabilities of waste management and recycling systems. (The future of plastics Sustainability: advanced recycling). A new era of advanced recycling for plastics, also referred to as “chemical recycling,” has the potential to help the nation achieve global sustainability goals and a climate-neutral, circular economy. As much as half of all global plastics packaging could be recycled by 2040 if chemical recycling technologies were widely adopted [10].

Over the past 10 years, the capacity for processing post-consumer (rPET) has had its ups and downs. The industry recently lost 400 mm lbs. of capacity with the closure of some facilities. Capacity was expected to return to roughly 2 billion lbs./year by 2018, with at least 350 million lbs. of new PET-processing capacity becoming online. As of 2018, facilities that reprocess rPET were operating at 75% capacity [61, 73]. Brand container users are aggressively pursuing rPET for new container construction. Recently, the recycling market was unable to fill these demands for rPET.

The metropolitan area comprising Houston, Texas, USA, has the largest concentration of petrochemical manufacturing in the world. Significant tonnage quantities of BTX, synthetic rubber, insecticides, and fertilizers form the basis of this robust chemical industry. Chemical and thermal-based methods of breaking down plastics required for plastic recycling are currently available in the Houston area’s chemical and petrochemical facilities to aid the utilization of a feedstock derived from plastic recycling. The metropolitan area comprising Houston, Texas, USA has the largest concentration of petrochemical manufacturing in the world. Significant tonnage quantities of BTX, synthetic rubber, insecticides, and fertilizers form the basis of this robust chemical industry.

Chemical and thermal-based methods of breaking down plastics required for plastic recycling are currently available in the Houston area’s chemical and petrochemical facilities to aid the utilization of a feedstock derived from plastic recycling. This alternative based on the pyro-catalytic conversion of mixed waste polyolefins form pyrolysis oil, which can be inserted in the material flow stream of an olefin unit to be processed by a catalytic cracker to form a mixture of small molecules. This mixture contains a variety of small molecules including olefins, which can be separated and repolymerized into a recycled virgin version of the polymers. Tank cars and wagons can provide the transportation necessary to deliver the oil from a small pyrolytic oil plant to the refinery. Apart from the physical and technical assets of the Houston area, other contributions are evident in the intellectual capital of the energy industry value chain, which can provide leadership opportunities for chemical recycling advancements in the global economic, energy, and sustainability arenas [10]. The effort has been estimated to remove 10 million metric tons of CO₂, while supporting 100 advanced recycling facilities by 2020, each capable of processing 25,000 tons per year [76, 77]. Such recycling technology using facilities in Houston, TX area provides an example of how the circular plastic economy could be supported [78].

8.3 Availability of conversion facilities

The alternate approach under investigation would substitute plastics as feedstock material for oil. Even mixed plastic waste and difficult-to-recycle polymers can be used to make new, high-quality fuels and plastics—for the most demanding applications like food contact [79]. Conversion, or pyrolysis and depolymerization technologies, can convert waste plastic, which cannot be treated by mechanical recycling, into oil and gases. The compatibility of pyrolytic oil composition is important to the success of this technology. Avoidance of certain pyrolytic oil compositions will be critical to this recycling technology, since they can be detrimental to the material flow within the refinery. This is an attractive option for plastic products that are difficult to recycle mechanically due to their low-quality, composite nature, or low economic value. These monomers can be used as virgin material alternatives in manufacturing new polymers [80].

Although successful use of this technology has been demonstrated at PTF facilities worldwide, no commercial-scale systems have yet been developed in North America. There are several U.S.- and Canadian-based technology manufacturers that have operational pilot facilities. Several other global technology manufacturers have also emerged. Many of these companies have pilot-scale facilities that tend to be about one-fifth of the smallest recommended capacity for a commercial-scale facility [81].

9. Conclusion

The long-term solution for sustainable management of plastic requires stakeholder engagement from design to product across the value chain to achieve a circular economy. Although clean-up of the environment from plastic contamination is necessary for the short-term, sustainable solution demands a fundamental shift from the current practice of design, use, and recycling of plastics where plastic would not become a waste; instead, it contributes to the circular economy. The need for reliable plastic technology is imminent. Mechanical recycling offers technology that can be economically engaged to meet the clamoring requirements for highly increased plastic recycling in large quantities. The dispersion of plastic recycling technology across the U.S. is not responsive to the recycling needs of many population centers. Take-back and collection programs require attention to optimize the economics controlling the collection process. Incentive programs to enhance plastic waste collection efforts could provide a necessary support.

Competent technologies capable of converting plastic waste into useful materials have been developed to pilot scale or beyond. Performance data that is sufficiently transparent, describing the efficiency and environmental footprint for many of the technologies have not generally been available to assist the evaluation of a given technology. To the public, transparency of process operations is critical to the acceptance of the technology. This data may not be made available for technological and economic reasons in a very competitive market. The technology developer must offer the transparency of information regarding the performance of a conversion process to ensure public acceptance of new technology. Recent reviews have delineated that some advanced technologies in the thermal processing sector are not sustainable and should be avoided [82, 83]. Perhaps, this argument discards a viable technology without a replacement. Reviewers’ expectations have shown an understanding of recycling process development and the time scales required for full-scale implementation. News of new plastic recycling technology often builds unachievable expectations in the minds of the public. Technologies described at a concept level may require a long and tortuous research path to a recycling technology that can be operated optimally at full-scale. The time scale for converting a recycling process at a concept level to a full-scale optimized process can be highly variable. A promising technology of chemical recycling still needs to be evaluated at full-scale to understand its performance and environmental footprint. New technologies will appear as developers can harness processing technology that is adaptable to the slight economic margins of operation currently found with plastic recycling.

The current availability and quality of waste plastic feedstock can be significant determining factors. The basic economics of plastic recycling impedes the desired expansion of recycling centers across the United States. The demonstration of plastic waste depolymerization processes has begun and requires optimization to ensure continued use against severe economic constraints. The success of these operations will be important to the expansion of the technology across the United States. The reduction of single-use plastics required innovative business models to develop novel packaging designs. There is also an urgent need to reduce the leaking of plastic waste into natural systems to become environmental waste. Increasing collection, sorting, and recycling rates are required to support circular economy to provide sustainable solutions to up-recycling, reducing plastic waste, and valorizing used plastics.