Plastic recycling

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Plastic recycling
Clockwise from top left:
  • Sorting plastic waste at a single-stream recycling centre
  • Baled colour-sorted used bottles
  • Recovered HDPE ready for recycling
  • A watering can made from recycled bottles

Plastic recycling is the reprocessing of plastic waste into new products.[1][2][3] When performed correctly, this can reduce dependence on landfill, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions.[4][5] Although recycling rates are increasing, they lag behind those of other recoverable materials, such as aluminium, glass and paper. Since the beginning of plastic production in the 20th century, until 2015, the world has produced some 6.3 billion tonnes of plastic waste, only 9% of which has been recycled, and only ~1% has been recycled more than once.[6] Additionally, 12% was incinerated and the remaining 79% disposed of to landfill or to the environment including the sea.[6]

Recycling is necessary because almost all plastic is non-biodegradable and thus builds-up in the environment,[7][8] where it can cause harm. For example, approximately 8 million tons of waste plastic enter the Earth's oceans every year, causing damage to the aquatic ecosystem and forming large ocean garbage patches.[9]

Presently, almost all recycling is performed by remelting and reforming used plastic into new items; so-called mechanical recycling. This can cause polymer degradation at a chemical level, and also requires that waste be sorted by both colour and polymer type before being reprocessed, which is complicated and expensive. Failures in this can lead to material with inconsistent properties, rendering it unappealing to industry.[10] In an alternative approach known as feedstock recycling, waste plastic is converted back into its starting chemicals, which can then be reprocessed back into fresh plastic. This offers the hope of greater recycling but suffers from higher energy and capital costs. Waste plastic can also be burnt in place of fossil fuels as part of energy recovery. This is a controversial practice, but is nonetheless performed on a large scale. In some countries, it is the dominant form of plastic waste disposal, particularly where landfill diversion policies are in place.

Plastic recycling sits quite low in the waste hierarchy as a means of reducing plastic waste. It has been advocated since the early 1970s,[11] but due to severe economic and technical challenges, did not impact plastic waste to any significant extent until the late 1980s. The plastics industry has been criticised for lobbying for the expansion of recycling programs, even while industry research showed that most plastic could not be economically recycled and simultaneously increasing the amount of virgin plastic, or plastic that has not been recycled, being produced.[12][13]

History

Although plastics were known before the 20th century, large-scale production was not realised until WWII. With metal supplies allocated towards military use and an increased demand for high-performance materials, these hitherto untested synthetic alternatives became appealing. Nylon replaced silk in parachutes, while Perspex was a light-weight alternative to glass in aeroplanes. After the war these processes were commercialised rapidly, with the plastic age beginning from around 1950, greatly aided by the post-war economic boom.

Global environmental movements in the 1960s and 1970s led to the formation of environmental agencies in many jurisdictions, including the U.S. (EPA, 1970), EU (DG ENV, 1973) Australia (EPA, 1971) and Japan (JEA 1971). In this atmosphere of environmental awareness, plastic waste came under scrutiny.[11] The earliest major development to abate plastic pollution was arguably the 1973 and 1978 MARPOL agreement, whose Annex V completely banned the dumping of plastics into the ocean.

Plastic industry lobbying

Photograph of Girl Scouts picking up discarded trash in 1970.
Girl Scouts on a Keep America Beautiful cleanup in 1970. The Keep American Beautiful campaign was a greenwashing campaign by the plastics and other polluting industries founded in the 1970s to try to displace responsibility of plastic pollution and other disposable packing trash onto consumers as "littering".[14]

As the threat of more regulation from the environmental movement grew, the plastics industry responded with lobbying to preserve their business interests. In the U.S., the Resource Recovery Act, passed in 1970, directed the nation towards recycling and energy recovery.[11] By 1976, there had been more than a thousand attempts to pass legislation to ban or tax packaging, including plastics.[15] The plastics industry responded by lobbying for plastic to be recycled. This involved a $50 million per year campaign through organisations such as Keep America Beautiful with the message that plastic could and would be recycled,[12][13] as well as lobbying for the establishment of curbside recycling collections.[16]

However, petrochemical industry leaders understood plastic could not be economically recycled using the technology of the time. For example, an April 1973 report written by industry scientists for industry executive states that, "There is no recovery from obsolete products" and that, "A degradation of resin properties and performance occurs during the initial fabrication, through aging, and in any reclamation process." The report concluded that sorting the plastic is "infeasible". The scientific community also knew this, with contemporary reports highlighting numerous technical barriers.[17][18][19][20][21]

Globally, plastic waste was almost entirely disposed of via landfill until the start of the 1980s when rates of incineration increased. Although better technology was known,[22] these early incinerators often lacked advanced combustors or emission-control systems, leading to the release of dioxins and dioxin-like compounds.[23] The replacement or upgrading of these facilities to cleaner ones with waste-to-energy recovery has been gradual.

It was not until the late 1980s that plastic recycling began in earnest. In 1988 the U.S. Society of the Plastics Industry created the Council for Solid Waste Solutions as a trade association to sell the idea of plastic recycling to the public.[24] The association lobbied American municipalities to launch or expand plastic waste collection programs and to lobby U.S. states to require the labelling of plastic containers and products with recycling symbols.[12][13] This coincided with their introduction of resin identification codes in 1988, which provided a standard system for the identification of various polymer types at materials recovery facilities, where plastic sorting was still largely performed by hand.

Global recycling trade

Increasing globalisation during the 1990's allowed the export of plastic waste from advanced economies to developing and middle-income ones, where it could be sorted and recycled more inexpensively. This formed part of a growing global waste trade, which saw the annual trade in plastic waste increase rapidly from 1993 onwards.[25]

Many governments count items as recycled if they have been exported for that purpose, however the practice has been accused of being environmental dumping, as environmental laws and their enforcement are generally weaker in less developed economies and the exported plastic waste can be mishandled, allowing it to enter the environment as plastic pollution.[26][27] By 2016 about 14 Mt of all plastic waste intended for recycling was exported, with China taking around half of it (7.35 million tonnes).[25] However, much of this was low quality mixed plastic which was hard to sort and recycle and ended up accumulating in landfills and at recyclers, or being dumped. Recycled plastic has been used extensively in manufacturing in China, and imported plastic waste was predominantly processed in an informal sector that provided low-technology processing services. High-income countries such as Germany, Japan, the United Kingdom and the United States were the top plastic waste exporters.[28]

In 2017, China began restricting waste plastics imports in Operation National Sword. Europe and North America suffered from extreme waste stream backlogs, and waste plastic ended up being exported to other countries mostly in South East Asia like Vietnam and Malaysia, but also to places like Turkey and India with less stringent environmental regulations.[29][30] Governments including those of Indonesia, Malaysia and Thailand reacted swiftly to curtail illegal plastic waste imports by reinforcing border controls. With increased control of imports, repatriation of illegal containers is occurring although this remains a long and challenging process. Consequently, plastic waste containers accumulated in ports in Southeast Asia.[28]

With global trading in waste becoming more difficult, attention has returned to solutions at a local level. Extended producer responsibility schemes have been proposed which would tax plastic producers in order to subsidise recyclers.[31]

In 2019, international trade in plastic waste became regulated under the Basel Convention. Under the Convention, any Party can decide to prohibit imports of hazardous plastic waste and, since 1 January 2021, of some mixed plastic wastes. Parties to the Convention are required to make arrangements to ensure environmentally sound management of their wastes either through alternative importers or by increasing their own capacity.[28]

The COVID-19 pandemic temporarily shrunk global trade in plastic waste, due among other reasons to reduced activity at waste management facilities, interruptions of shipping routes, and low oil prices which have reduced the cost of virgin plastic and made recycling plastics less profitable.[28]

Production and recycling rates

see caption
Global quantities of plastic produced and disposed of annually (1950–2015), showing the estimated amounts disposed of via landfill, recycling and incineration.

The total amount of plastic ever produced worldwide, until 2015, is estimated to be 8.3 billion tonnes.[6] Approximately 6.3 billion tonnes of this has been discarded as waste, of which around 79% has accumulated in landfills or the natural environment, 12% was incinerated, and 9% has been recycled, although only ~1% of all plastic has ever been recycled more than once.[6]

By 2015 global production had reached some 381 Mt per year,[6] greater than the combined weight of everyone on Earth.[32] The recycling rate in that year was 19.5%, while 25.5% was incinerated and the remaining 55% disposed of, largely to landfill. These rates lag far behind those of other recyclables, such as paper, metal and glass. Although the percentage of material being recycled or incinerated is increasing each year, the tonnage of waste left-over also continues to rise. This is because global plastic production is still increasing year-on-year. Left unchecked, production could reach ~800 Mt per year by 2040, although implementing all feasible interventions could reduce plastic pollution by 40% from 2016 rates.[33]

A focus on global averages can disguise the fact that recycling rates also vary between types of plastic. Several types are in common use, each having distinct chemical and physical properties. This leads to differences in the ease with which they can be sorted and reprocessed; which effects the value and market size for recovered materials.[34] PET and HDPE have the highest recycling rates, whereas polystyrene and polyurethane are often barely recycled at all.[35]

One of the reasons for low levels of plastic recycling is weak demand from manufactures, who fear that recycled plastics will have poor or inconsistent mechanical properties.[10] The percentage of plastic that can be fully recycled, rather than downcycled or go to waste, can be increased when manufacturers of packaged goods minimise mixing of packaging materials and eliminate contaminants. The Association of Plastics Recyclers has issued a "Design Guide for Recyclability".[36]

The most commonly produced plastic consumer products include packaging made from LDPE (e.g. bags, containers, food packaging film), containers made from HDPE (e.g. milk bottles, shampoo bottles, ice cream tubs), and PET (e.g. bottles for water and other drinks). Together these products account for around 36% of plastics use in the world. Most of them (e.g. disposable cups, plates, cutlery, takeaway containers, carrier bags) are used for only a short period, many for less than a day. The use of plastics in building and construction, textiles, transportation and electrical equipment also accounts for a substantial share of the plastics market. Plastic items used for such purposes generally have longer life spans than, for example, plastic packaging. They may be in use for periods ranging from around five years (e.g. textiles and electrical equipment) to more than 20 years (e.g. construction materials, industrial machinery).[37]

Regional data

Plastic consumption differs among countries and communities, with some form of plastic having made its way into most people’s lives. North American countries (NAFTA) account for 21% of global plastic consumption, closely followed by China (20%) and Western Europe (18%). In North America and Europe there is high per capita plastic consumption (94 kg and 85 kg/capita/year, respectively). In China there is lower per capita consumption (58 kg/capita/year), but high consumption nationally because of its large population.[37]

In 2012, 25.2 Mt of post-consumer plastic waste was collected in the European Union. Of this, more than 60% (15.6 Mt) was recovered and 40% (9.6 Mt) was disposed of with municipal solid waste (MSW). Of the 15.6 Mt of recovered plastic waste, about 6.6 Mt was actually recycled, while the remainder was likely used as refuse-derived fuel (RDF) or incinerated in MSW incinerators with energy recovery (about 9 Mt). While Europe can be considered a leader in plastics recycling, only about 26% of plastic waste is recycled.[38]

The recycling activities of the largest produces of plastic waste have the greatest effect on global averages. These are a mix of advanced economies and large developing nations, however, not all of these publish official statistics on their plastic recycling rates. Others may release partial data, usually limited to population centres. This makes it difficult to draw accurate comparisons, especially as the published recycling rates vary hugely between counties.

12 largest producers of plastic waste (+EU) and their recycling rates in 2010
Country Plastic waste per year (Mt)[39] Waste per person per day (Kg)[39] Recycled Incinerated (with energy recovery) Landfill (and incineration without energy recovery) Comments
China 59.08 0.12 - - - No official statistics
United States[40] 37.83 0.34 8% 14% 78% Source: EPA
EU total*[41] 24.7 0.15 24% 34% 42%
Germany[41] 14.48 0.48 33% 65% 2%
Brazil 11.85 0.17 - - - No official statistics
Japan[42] 7.99 0.17 27% 49% 24%
Pakistan 6.41 0.10 - - - No official statistics
Nigeria 5.96 0.10 12% 0% 88% Estimated values
Russia 5.84 0.11 6% 0% 94% World bank estimates (2013)[43]
Turkey 5.60 0.21 5% 0% 95% Estimated values
Egypt 5.46 0.18 - - - No official statistics
Indonesia 5.05 0.06 19% 0% 81% Estimated values
United Kingdom[41] 4.93 0.21 23% 8% 69%
Spain[41] 4.71 0.28 23% 17% 60%
France[41] 4.56 0.19 18% 40% 42%
India 4.49 0.01 42% 18% 40% Estimated values
Rest of World 60.76 - - - - No official statistics
World Total[6] 245.00 0.10 16% 22% 62%

* Although not formally a country, legislation affecting recycling is often made at the EU level

Identification codes

See caption
Global plastic waste generation by polymer type. Colours indicate recyclability:
  • Blue is widely recycled
  • Yellow is sometimes recycled
  • Red is usually not recycled
Main article: Resin identification code

Many plastic items bear symbols identifying the type of polymer from which they are made. These resin identification codes, often abbreviated RICs, are used internationally,[44] and were originally developed in 1988 by the Society of the Plastics Industry (now the Plastics Industry Association) in the United States, but since 2008 have been administered by ASTM International, a standards organisation.[44]

RICs are not mandatory in all countries, but many producers voluntarily mark their products. More than half of U.S. states have enacted laws that require plastic products be identifiable.[45] There are seven codes in all, six for the most common commodity plastics and one as a catch-all for everything else. The EU maintains a similar nine-code list which also includes ABS and polyamides.[46] RICs are clearly based on the recycling symbol and have drawn criticism for causing consumer confusion, as it implies the item will always be recyclable when this is not necessarily the case.[47]

RICs are not particularly important for single-stream recycling, as these operations are increasingly automated. However, in some countries citizens are required to separate their plastic waste according to polymer type before refuse collection and for this RICs are very useful. For instance, in Japan PET bottles are collected separately for recycling.

Plastic identification code Type of plastic polymer Properties Common applications Melting- and glass transition temperatures (°C) Young's modulus (GPa)
Polyethylene terephthalate (PET) Clarity, strength, toughness, barrier to gas and moisture Soft drink, water and salad dressing bottles; peanut butter and jam jars; ice cream cone lids; small non-industrial electronics Tm = 250;[48]
Tg = 76[48] 		2–2.7[49]
High-density polyethylene (HDPE) Stiffness, strength, toughness, barrier to gas and moisture Water pipes, gas and fire pipelines, electrical and communications conduits, five gallon buckets, milk, juice and water bottles, grocery bags, some toiletry bottles Tm = 130;[50]
Tg = −125[51] 		0.8[49]
Polyvinyl chloride (PVC) Versatility, ease of blending, strength, toughness. Stretch wrap for non-food items, sometimes blister packaging. Non-packaging uses include electrical cable insulation, rigid piping and vinyl records. Tm = 240;[52]
Tg = 85[52] 		2.4–4.1[53]
Low-density polyethylene (LDPE) Ease of processing; strength; flexibility; ease of sealing; moisture barrier. Frozen food bags; squeezable bottles, e.g. honey, mustard; cling films; flexible container lids Tm = 120;[54]
Tg = −125[55] 		0.17–0.28[53]
Polypropylene (PP) Strength; resistance to heat, chemicals, grease and oil; moisture barrier. Reusable microwaveable ware or take-away containers; kitchenware; yogurt or margerine containers; disposable cups and plates; soft drink bottle caps. Tm = 173;[56]
Tg = −10[56] 		1.5–2[49]
Polystyrene (PS) Versatility, clarity, easily formed, easily foamed Egg cartons; disposable cups, plates, trays and cutlery; foam food containers; packing peanuts and package cushioning; Tm = 240 (only isotactic);[51]
Tg = 100 (atactic and isotactic)[51] 		3–3.5[49]
Other (often polycarbonate or ABS) Dependent on polymers or combination of polymers Beverage bottles, baby milk bottles. Non-packaging uses for polycarbonate: compact discs, "unbreakable" glazing, electronic apparatus housing, lenses (including sunglasses), instrument panels.[57] Polycarbonate:
Tm = 225[58]
Tg = 145;[59] 		Polycarbonate: 2.6;[49] ABS plastics: 2.3[49]

Plastic waste composition

Plastic waste consists of various polymer types, its exact composition will vary, but the estimated global average is shown below.[6][60] Polyolefins make up nearly 50% of all plastic waste and more than 90% of waste is made of thermosoftening polymers, which can be remelted.

Global plastic waste by polymer type (2018)[6][60]
Polymer Waste production (Mt) Percentage of all plastic waste Polymer type Thermal character
High-density polyethylene (HDPE) 64 19.8% Polyolefin Thermoplastic
Low-density polyethylene (LDPE) 45 13.9% Polyolefin Thermoplastic
polypropylene (PP) 62 19.1% Polyolefin Thermoplastic
Polystyrene (PS) 19 5.9% Unsaturated polyolefin Thermoplastic
Polyvinyl chloride (PVC) 17 5.3% Halogenated Thermoplastic
Polyethylene terephthalate (PET) 35 10.8% Condensation Thermoplastic
Polyurethane (PUR) 18 5.6% Condensation Thermoset[61]
PP&A fibers[62] 51 15.7% Condensation Thermoplastic
All Others 12 3.7% Various Varies
Total (excludes additives) 324 100% - -

Collecting and sorting

See caption
Bales of colour sorted PET bottles (blues, clear and greens) Olomouc, the Czech Republic.
Manual separation of co-mingled waste (2 min).

Recycling begins with the collection and sorting of waste. Curbside collection operates in many counties, with the collections being sent to a materials recovery facility or MBT plant where the plastic is separated, cleaned and sorted for sale. Anything not deemed suitable for recycling will then be sent for landfill or incineration. These operations account for a large proportion of the financial and energy costs associated with recycling.

Sorting plastic is more complicated than any other recyclable material because it comes in a greater range of forms. For example, glass is separated into three streams (clear, green and amber), metals are usually either steel or aluminum and can be separated using magnets or eddy current separators, and paper is usually sorted into a single stream. By comparison, about six types of commodity polymer account for about 75% of plastics waste, with the remaining 25% consisting of a myriad of polymer types, including polyurethanes and synthetic fibers which can have a range of chemical structures. Different polymers are generally incompatible with each other when recycled, but even items made from the same type of polymer may be incompatible depending on what additives they contain. Additives are compounds blended into plastics to enhance performance and include stabilisers, fillers and, most significantly, dyes.[63] Clear plastics hold the highest value as they may yet be dyed, while black or strongly coloured plastic is much less valuable, as their inclusion can give discoloured products. Thus, plastic normally needs to be sorted by both polymer type and colour to give a material suitable for recycling.

Various approaches and technologies have been developed to sort plastic,[1] which can be combined in different ways.[64] As different polymer types can be incompatible with one another, accurate sorting is essential, although in practice no approach is 100% efficient.[65][66][64] The accuracy of sorting therefore varies between recyclers, producing a market where products are often not well standardised. This inconsistency in quality can act as a barrier to recycling. Bioplastics and biodegradable plastics currently account for only a small share of household waste but their increasing popularity may yet further complicate waste plastic sorting.

Manual separation

Sorting through waste by hand is the oldest and simplest method of separating plastic. In developing countries this may be done by waste pickers, while in a recycling center workers pick items off a conveyor-belt. It requires low levels of technology and investment, but can have high relative operating costs due to the need for a large workforce. Although many plastic items have identification codes workers rarely have time to look for them, so there are problems of inefficiency and inconsistency in the sorting process. Regardless, even advanced facilities retain manual pickers to troubleshoot and correct sorting errors by equipment.[64] Globally, the process focuses on those materials which are most valuable, such as clear PET bottles, with a significant amount of the waste continuing on to landfill. Working conditions can be unsanitary.[67]

Density separation

Plastic densities[68]
Plastic Type Density (g/cm3)
Polyvinyl chloride 1.38-1.41
Polyethylene terephthalate 1.38-1.41
Polystyrene 1.04-1.08
High-density polyethylene 0.94-0.98
Low-density polyethylene 0.89–0.93
Polypropylene 0.85-0.92
Polystyrene foam 0.01-0.04

Plastics can be separated by exploiting differences in their densities. In this approach the plastic is first ground into flakes of a similar size, washed and subjected to gravity separation.[69] This can be achieved using either an air classifier or hydrocyclone, or via wet float-sink method.[70] These approaches only allow partial sorting, as some polymers have similar density ranges.[69] Polypropylene (PP) and polyethylene (PE) will remain together as will polyethylene terephthalate (PET), polystyrene (PS), and PVC. In addition, if the plastic contains a high percentage of fillers, this may affect its density.[71] The lighter PP and PE fraction is known as mixed polyolefin (MPO) and can be sold as a low-value product,[72] the heavier mixed plastics fraction is usually unrecyclable.

Electrostatic separation

Main article: electrostatic separator

In electrostatic separators, the triboelectric effect is used to charge plastic particles electrically; with different polymers being charged to different extents. They are then blown through an applied electric field, which deflects them depending on their charge, directing them into appropriate collectors. As with density separation, the particles need to be dry, have a close size distribution and be uniform in shape.[73] Electrostatic separation can be complementary to density separation, allowing full separation of polymers,[74] however, these will still be of mixed colours.

Sensor based separation

Photograph of the interior workings of a recycling plant
An advanced recycling plant using optical separation
Main article: Sensor-based sorting

This approach can be highly automated and involves various types of sensors linked to a computer, which analyses items and directs them into appropriate chutes or belts.[75] Near-infrared spectroscopy can be used to distinguish between polymer types,[76] although it can struggle with black or strongly coloured plastics, as well as composite materials like plastic-coated paper and multilayered packaging, which can give misleading readings. Optical sorting such as colour sorters or hyperspectral imaging can then further organise the plastics by colour. Sensor based separation is more expensive to install but has the best recovery rates and produces more high-quality products.[64]

Plastic scrap

Plastic waste can be broadly divided into two categories; industrial scrap (sometimes referred to as post industrial resin) and post-consumer waste. Scrap is generated during the production of plastic items and is usually handled completely differently to post-consumer waste.[77] It can include flashings, trimmings, sprues and rejects. As it is collected at the point of manufacture it will be clean, and of a known type and grade of material, and is usually of high quality and value. As scrap is mostly traded company-to-company rather than via municipal facilities, it is often not included in official statistics.[77]

Mechanical recycling

Diagram showing plastic or other polymer compatibilisation.
Polymer compatibilisation

The majority of plastic waste is made of thermosoftening polymers, which can be re-melted and reformed into new items in a practice known as mechanical recycling. Globally, this is by far the most common form of recycling and in many countries it is effectively the only type practised. This predominance is due to it being the simplest and most economical form of recycling, in addition to it having a lower carbon footprint than most other processes.[78] However, several factors can lead to the quality of the polymer being reduced when it is recycled this way, which limits its applicability and effectiveness.[78] Thus, the limits of mechanical recycling are in practise often the current limits of plastic recycling overall.

Plastics are reprocessed at anywhere between 150–320 °C (300–610 °F), depending on the polymer type,[69] and this is sufficient to cause unwanted chemical reactions which result in polymer degradation.[79] This reduces the physical properties and overall quality of the plastic and can produce volatile, low-molecular weight compounds, which may impart undesirable taste or odour, as well as causing thermal discolouration. Additives present within the plastic can accelerate this degradation. For instance, oxo-biodegradable additives, intended to improve the biodegradability of plastic, also increase the degree of thermal degradation.[80][81] Similarly, flame retardants can have unwanted effects.[82] The quality of the product also depends strongly on how well the plastic was sorted. Many polymers are immiscible with one another when molten and will phase separate (like oil and water) during reprocessing. Products made from such blends contain many boundaries between the different polymer types and cohesion across these boundaries is weak, leading to poor mechanical properties. In more extreme cases the polymers may degrade one another, this is often the case with PVC, as it can generate hydrogen chloride which strongly affects condensation polymers such as PET.[83]

Many of these problems have technological solutions, though they bear a financial cost. Advanced polymer stabilisers and can be used to protect plastics from the rigours of thermal reprocessing.[84][85] Volatile degradation products can be removed by a range of devolatilisation techniques. Flame retardants can be removed by chemical treatment,[86] while damaging metallic additives can be rendered inert with deactivators. Finally, the properties of mixed plastics can be improved by using compatibilisers.[87][88] These are compounds which improve miscibility between polymer types to give a more homogeneous product, with better internal cohesion and improved mechanical properties. They act at the boundary between different polymers and are small-molecules possessing two different chemical regions, each of which is compatible with a certain polymer. This allows them to act like molecular-nails or screws, anchoring the areas of different polymer to one another. As a result, compatibilisers are normally limited to systems dominated by two particular types of plastic and are not a cost-effective option for unsorted mixtures of various polymer types. There is no one-size-fits-all compatibiliser for all plastic combinations. Even with these technologies, it is particularly challenging to recycle plastic so that it can meet food contact standards.

Closed-loop recycling

In closed-loop, or primary recycling, used plastic is endlessly recycled back into new items of the same quality and sort. For instance, turning drinks bottles back into drinks bottles. It can be considered an example of a circular economy. The continual mechanical recycling of plastic without reduction in quality is very challenging due to cumulative polymer degradation,[89] and risk of contaminant build-up. In 2013 only 2% of plastic packaging was recycled in a closed loop.[90] Although closed-loop recycling has been investigated for many polymers,[89] to-date the only industrial successes have been with PET bottle recycling.[91] The reason for this is that polymer degradation in PET is often repairable. PET's polymer chains tend to cleave at their ester groups and the alcohol and carboxyl groups left by this can be joined back together by the use of chemical agents called chain extenders.[92] Pyromellitic dianhydride is one such compound.

Open-loop recycling

Photograph of a re-usable carrier bag made from recycled plastic bottles processed using open-loop recycling.
This re-usable carrier bag has been made from recycled plastic bottles. It is an example of open-loop recycling

In open-loop recycling, also known as secondary recycling, or downcycling, the quality of the plastic is reduced each time it is recycled, so that the material is not recycled indefinitely and eventually becomes waste. It is the most common type of plastic recycling.[90] The recycling of PET bottles into fleece or other fibres is a common example, and accounts for the majority of PET recycling.[93] Although this approach only delays material from heading to landfill or incineration, life-cycle assessment shows it to be of ecological benefit.[94][3][93] Environmentally successful recycling displaces demand for fresh plastic production and if open-loop recycling achieves this then its benefits are indistinguishable from closed-loop recycling.[95] If instead, it is used to produce new cheap and low-quality items which would not otherwise have been made, then it is not displacing current production and is of little or no benefit to the environment.

The reduction in polymer quality can be offset by mixing recycled plastic with virgin material when making a new product. Compatibilised plastics can be used as a replacement for virgin material, as it is possible to produce them with the right melt flow index needed for good processing.[96] Low quality mixed plastics can also be recycled in an open-loop, although there is limited demand for such products, as in addition to poor mechanical properties, incompletely sorted waste often contains a wide range of dyes and colourants. When these are mixed during reprocessing the result is usually a dark-brown product which is unappealing for many applications. These blends find use as outdoor furniture or plastic lumber. As the material is weak, but of low cost it is produced in thick planks so as to be sturdy.

Thermosets

Although thermoset polymers do not melt, technologies have been developed for their mechanical recycling. This usually involves breaking the material down to a crumb, which can then be mixed with some sort of binding agent to form a new composite material. For instance, polyurethanes can be recycled as reconstituted crumb foam.[97][98] Tire recycling similarly produces crumb rubber, which can be used as aggregate. Various devulcanisation technologies have also been developed to allow the recycling of rubber wastes,[99] though few of these are commercially important.

Feedstock recycling

In feedstock recycling, also called chemical recycling or tertiary recycling, polymers are reduced to their chemical building-blocks (monomers), which can then be polymerised back into fresh plastics.[100][101][102] In theory, this allows for near infinite recycling; as impurities, additives, dyes and chemical defects are completely removed with each cycle.[103][104] In practice, chemical recycling is far less common than mechanical recycling. Implementation is limited because technologies do not yet exist to depolymerise all polymers reliably on an industrial scale and also because the equipment and operating costs are much higher. In 2018 Japan had one of the highest rates in the world at ~4%, compared to 23% mechanical recycling,[105] in the same period Germany, another major recycler, reported a feedstock recycling rate of 0.2%.[106] Depolymerising, purifying and re-polymerising the plastic can also be energy intensive, leading to the carbon footprint of feedstock recycling normally being higher than that of mechanical recycling.[78] PET, PU and PS are depolymerised commercially to varying extents,[103] but the feedstock recycling of polyolefins, which make-up nearly half of all plastics, is much more limited.[104]

Thermal depolymerisation

Certain polymers like PTFE, polystyrene, nylon 6, and polymethylmethacrylate (PMMA) undergo thermal depolymerisation when heated to sufficiently high temperatures.[107] The reactions are sensitive to impurities and require clean and well sorted waste to produce a good product. Even then, not all depolymerisation reactions are completely efficient and some competitive pyrolysis is often observed; the monomers, therefore, require purification before reuse. The feedstock recycling of polystyrene has been commercialised,[104] but global capacity remains fairly limited.

Chemical depolymerisation

Condensation polymers bearing cleavable groups such as esters and amides can be completely depolymerised by hydrolysis or solvolysis. This can be a purely chemical process but may also be promoted by enzymes, like PETase, which is able to breakdown PET.[108][109] Such technologies have lower energy costs than thermal depolymerisation but are more limited in terms of the polymers they can be applied to. Thus far polyethylene terephthalate has been the most heavily studied polymer,[110] with commercial scale feedstock recycling being performed by several companies.[103]

Energy recovery

Photograph of piles of trash including large amounts of plastic at an incinerator
Piles of trash including large amounts of plastic at an incinerator in Ko Tao, Thailand. Well regulated incinerators reduce harmful toxins released during the burning process, but not all plastic is burned in proper facilities.

Energy recovery, also called energy recycling or quaternary recycling, involves burning waste plastic in place of fossil fuels for energy production.[111][112] Its inclusion as a type of recycling can be controversial, but it is nonetheless included in the recycling rates reported by many countries,[113][114] although it is not considered recycling by the EU.[115] Care should be taken not to conflate it with incineration without energy recovery, which is historically more common, but which does not offset either future plastic production or fossil fuel use.

Energy recovery is often the default waste management method of last resort, a position previously held by landfill. In urban areas a lack of suitable sites for new landfills can drive this,[116] but it is also a result of regulation, such as the EU's Landfill Directive or other landfill diversion policies. Compared to the other recycling options its appeal is largely economic. If the correct technologies are used then the plastics do not need to be separated from one another, or from other forms of municipal solid waste (garbage), which greatly reduces costs. Compared to the sometimes variable market for recyclates, demand for electricity is universal and better understood, reducing the perceived financial risk of operations. As a means of waste management, it is highly effective, reducing the volume of waste by about 90%, with the residues sent to landfill or used to make cinder block. Although its CO2 emissions are obviously high, comparing its overall ecological desirability to other recycling technologies is difficult.[3] For instance, while recycling greatly reduces greenhouse gas emissions compared to incineration, it is an expensive way of achieving these reductions when compared to investing in renewable energy.[117]

Plastic waste may be simply burnt as refuse-derived fuel (RDF) in a waste-to-energy process, or it may be chemically converted to a synthetic fuel first. In either approach PVC must be excluded or compensated for by installing dichlorination technologies, as it generates large amounts of hydrogen chloride (HCl) when burnt. This can corrode equipment and cause undesirable chlorination of the fuel products.[118] The burning of plastics has long been associated with the release of harmful dioxins and dioxin-like compounds, however these hazards can be abated by the use of advanced combustors and emission control systems. Incineration with energy recovery remains the most common method, with more advanced waste-to-fuel technologies, such as pyrolysis, being hindered by technical and cost hurdles.[116][119]

Waste-to-fuel

See also: Waste-to-energy

Mixed plastic waste can be depolymerised to give a synthetic fuel. This has a higher heating value than the starting plastic and can be burnt more efficiently, although it remains less efficient than fossil fuels.[120] Various conversion technologies have been investigated, of which pyrolysis is the most common.[121][122] Conversion can take place as part of incineration in an IGC cycle, but often the aim is to collect the fuel so that it may be sold. Pyrolysis of mixed plastics can give a fairly broad mix of chemical products (between about 1 and 15 carbon atoms) including gases and aromatic liquids.[123][124][125] Catalysts can give a better defined product with a higher value.[126][127][128] The liquid products can be used as synthetic diesel fuel,[129] with some commercial production taking place in several countries.[130] Life-cycle analysis shows that plastic-to-fuel can displace the production of fossil fuels and result in lower net greenhouse gas emissions (~15% reduction).[131]

Compared to the widespread use of incineration, plastic-to-fuel technologies have historically struggled to be economically viable because of the costs of collecting and sorting the plastic and the relatively low value of the fuel produced.[121][132] Large plants are seen as being more economical than smaller ones but require more investment to build.[133][134]

Other processes

Millions of tonnes of plastic waste are generated annually, and this has led to numerous solutions being developed, many of which operate at a considerable scale. A process has been developed in which many kinds of plastic can be used as a carbon source (in place of coke) in the recycling of scrap steel,[135] with roughly 200,000 tons of waste plastics being processed this way each year in Japan.[136]

Construction and concrete applications

The use of recovered plastics in engineering materials is being researched and is gaining ground.[137] Ground plastic may be used as a construction aggregate or filler material in certain applications.[138] While it is generally unsuitable in structural concrete, its inclusion in asphalt concrete, (forming rubberised asphalt), subbase and recycled insulation can be beneficial.[139] An example of this is the construction of plastic roads. These are rarely made entirely of plastic but can incorporate significant amounts of plastic waste in their design. The practice is popular in India, which by 2021 had constructed some 700 km (435 miles) of highways.[140] However, the practise has also come in for criticism, in-part over uncertainties over the leaching of plastic additives into the environment.[141] Research is ongoing to use plastics in various forms in cementitious materials such as concrete. Densifying plastic materials such as PET and plastic bags and then using them to partially replace aggregate and depolymerizing PET to use as a polymeric binder to enhance concrete are actively being studied.[142][143][144]

See also

Wikimedia Commons has media related to Plastic recycling.

Sources

 This article incorporates text from a free content work. Licensed under Cc BY-SA 3.0 IGO (license statement/permission). Text taken from Drowning in Plastics – Marine Litter and Plastic Waste Vital Graphics​, United Nations Environment Programme.

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External links

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