Plastic recycling

Plastic recycling is the process of recovering scrap or waste plastic and reprocessing the material into useful products. Since the majority of plastic is non-biodegradable, recycling is a part of global efforts to reduce plastic in the waste stream, especially the approximately 8 million metric tonnes of waste plastic that enters the Earth's ocean every year.[1][2]

Plastic recycling bin in Poland

Compared with lucrative recycling of metal, and similar to the low value of glass recycling, plastic polymers recycling is often more challenging because of low density and low value. There are also numerous technical hurdles to overcome when recycling plastic. Materials recovery facilities are responsible for sorting and processing plastics. As of 2019, due to limitations in their economic viability, these facilities have struggled to make a meaningful contribution to the plastic supply chain.[3]

When different types of plastics are melted together, they tend to phase-separate, like oil and water, and set in these layers. The phase boundaries cause structural weakness in the resulting material, meaning that polymer blends are useful in only limited applications. The two most widely manufactured plastics, polypropylene and polyethylene, behave this way, which limits their utility for recycling. Each time plastic is recycled, additional virgin materials must be added to help improve the integrity of the material. So, even recycled plastic has new plastic material added in. The same piece of plastic can only be recycled about 2–3 times before its quality decreases to the point where it can no longer be used.[4]

Recently, the use of block copolymers as "molecular stitches"[5] or "macromolecular welding flux" has been proposed[6] to overcome the difficulties associated with phase separation during recycling.[7] Certain bioplastics, such as PLA, recycled by breaking down plastic polymers into their chemical building blocks, can be recycled hundreds of times.[8]

The use of biodegradable plastics or plastics which can be organically recycled or can be composted in industrial composting is increasing for certain short-lived packaging applications.[9]

The percentage of plastic that can be fully recycled, rather than downcycled or go to waste, can be increased when manufacturers of packaged goods minimize mixing of packaging materials and eliminate contaminants. The Association of Plastics Recyclers has issued a "Design Guide for Recyclability".[10]

Methods

Broadly, there are two major ways to recycle plastic:[11] (1) mechanical recycling ("chop and wash"[12]), where the plastic is washed, ground into powders and melted, and (2) chemical recycling, where the plastic is broken down into basic components.

Before recycling, most plastics are sorted according to their resin type. In the past, plastic reclaimers used the resin identification code (RIC), a method of categorization of polymer types, which was developed by the Society of the Plastics Industry in 1988. Polyethylene terephthalate, commonly referred to as PET, for instance, has a resin code of 1. Most plastic reclaimers do not rely on the RIC now; they use various sorting systems to identify the resin, ranging from manual sorting and picking of plastic materials to mechanized automation processes that involve shredding, sieving, separation by rates of density i.e. air, liquid, or magnetic, and complex spectrophotometric distribution technologies e.g. UV/VIS, NIR, Laser, etc.[13][Dead Link] Some plastic products are also separated by color before they are recycled.

After sorting, for mechanical recycling the plastic recyclables are then shredded. These shredded fragments then undergo processes to eliminate impurities like paper labels. This material is melted and often extruded into the form of pellets which are then used to manufacture other products. The highest quality purification may be referred to as "regeneration".[14]

Thermal depolymerization
Main articles: Depolymerization and Thermal depolymerization

Scientists have estimated that the potential commodity value of waste plastic may be in excess of $300 per ton when used in process pathways yielding high-value chemical products or to produce electricity in efficient IGCC (Integrated Gasification Combined Cycle) processes.[15]

Waste plastic pyrolysis to fuel oil

Plastic pyrolysis can convert petroleum-based waste streams such as plastics into fuels and carbons.[16][17][18][19][20]

Given below is the list of suitable plastic raw materials for pyrolysis:

  • Mixed plastic (HDPE, LDPE, PE, PP, Nylon, Teflon, PS, ABS, FRP etc.)
  • Mixed-waste plastic from waste paper mill
  • Multi-layered plastic.

Heat compression

Heat compression takes all unsorted, cleaned plastic in all forms, from soft plastic bags to hard industrial waste, and mixes the load in tumblers (large rotating drums resembling giant clothes dryers). The most obvious benefit to this method is that all plastic is recyclable, not just matching forms. However, criticism rises from the energy costs of rotating the drums, and heating the post-melt pipes.[21]

Distributed recycling

Distributed recycling of plastics using additive manufacturing (or DRAM) can include mechanical grinding to make granules for 1) fused granular fabrication, 2) heated syringe printing, 3) 3-D printed molds coupled to injection molding and 4) filament production in a recyclebot to fused filament fabrication.[22] For some waste plastics, technical devices called recyclebots[23] enable a form of distributed recycling by making 3-D printing filament. Preliminary life-cycle analysis (LCA) indicates that such distributed recycling of HDPE to make filament of 3D printers in rural regions is energetically favorable to either using virgin resin or conventional recycling processes because of reductions in transportation energy.[24][25]

Chemical recycling

For some polymers, it is possible to convert them back into monomers, for example, PET can be treated with an alcohol and a catalyst to form a dialkyl terephthalate. The terephthalate diester can be used with ethylene glycol to form a new polyester polymer, thus making it possible to use the pure polymer again.

An estimated 60 companies are pursuing chemical recycling as of 2019.[12]

In 2019, Eastman Chemical Company announced initiatives for methanolysis of polyesters and polymer gasification to syngas designed to handle a greater variety of used material.[26]

In 2019, Brightmark Energy in the United States began building a facility to convert 100,000 tons of mixed plastic per into diesel, naphtha blend stocks, and wax;[27] the company plans to expand into building another plant which can process an additional 800,000 tons of plastic per year.[3] The company has said that the economics have a significant margin of safety from price declines.[28]

Other processes
Plastic or other polymer compatibilization

A process has also 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.[29] There are also possibilities for better recycling of mixed plastics, avoiding the need for expensive/inefficient separation of the plastic waste stream. One such method is called compatibilization which uses special chemical bridging agents called compatibilizers to maintain the quality of mixed polymers.[30]

Applications

PET
Main article: PET bottle recycling

Post-consumer polyethylene terephthalate (PET or PETE) containers are sorted into different color fractions and baled for onward sale. PET recyclers further sort the baled bottles and they are washed and flaked (or flaked and then washed). Non-PET fractions such as caps and labels are removed during this process. The clean flake is dried. Further treatment can take place e.g. melt filtering and pelletizing or various treatments to produce food-contact-approved recycled PET (RPET). This sorted post-consumer PET waste is crushed, chopped into flakes, pressed into bales, and offered for sale.[31]

One use for this recycled PET is to create fabrics to be used in the clothing industry.[32] The fabrics are created by spinning the PET flakes into thread and yarn.[31] This is done just as easily as creating polyester from brand new PET.[33] The recycled PET thread or yarn can be used either alone or together with other fibers to create a very wide variety of fabrics. Traditionally these fabrics are used to create strong, durable, rough products, such as jackets, coats, shoes, bags, hats, and accessories since they are usually too rough for direct skin contact and can cause irritation.[34] However, these types of fabrics have become more popular as a result of the public's growing awareness of environmental issues. Numerous fabric and clothing manufacturers have capitalized on this trend.

Other major outlets for RPET are new containers (food-contact or non-food-contact) produced either by (injection stretch blow) moulding into bottles and jars or by thermoforming APET sheet to produce clamshells, blister packs and collation trays. These applications used 46% of all RPET produced in Europe in 2010. Other applications, such as strapping tape, injection-moulded engineering components and building materials, account for 13% of the 2010 RPET production.

In the United States, the recycling rate for PET packaging was 31% in 2013, according to a report from The National Association for PET Container Resources (NAPCOR) and The Association of Postconsumer Plastic Recyclers (APR). A total of 1.798 billion pounds was collected and 475 million pounds of recycled PET used out of a total of 5.764 billion pounds of PET bottles.[35]

HDPE

Plastic #2, high-density polyethylene (HDPE) is a commonly recycled plastic. HDPE's highly crystalline structure makes it a strong, high density, moderately stiff plastic. HDPE Thermoplastic materials become liquid at their melting point—around 130 °C. A major benefit of thermoplastics is that they can be heated to melting point, cooled, and reheated again without significant degradation. Instead of burning, thermoplastics like PE (Polyethylene) liquefy, allowing them to be easily extruded or injection molded and turned into brand new HDPE pipe. Often it is typically downcycled into plastic lumber, tables, roadside curbs, benches, truck cargo liners, trash receptacles, stationery (e.g. rulers) and other durable plastic products and is usually in demand.[36]

PS
The resin identification code symbol for polystyrene

Most polystyrene products are not recycled due to the lack of incentive to invest in the compactors and logistical systems required. As a result, manufacturers cannot obtain sufficient scrap. Expanded polystyrene (EPS) scrap can easily be added to products such as EPS insulation sheets and other EPS materials for construction applications. When it is not used to make more EPS, foam scrap can be turned into clothes hangers, park benches, flower pots, toys, rulers, stapler bodies, seedling containers, picture frames, and architectural molding from recycled PS.[37]

Recycled EPS is also used in many metal casting operations. Rastra is made from EPS that is combined with cement to be used as an insulating amendment in the making of concrete foundations and walls. Since 1993, American manufacturers have produced insulating concrete forms made with approximately 80% recycled EPS.

Other plastics

The white plastic polystyrene foam peanuts used as packing material are often accepted by shipping stores for reuse.[38]

Successful trials in Israel have shown that plastic films recovered from mixed municipal waste streams can be recycled into useful household products such as buckets.[39]

Similarly, agricultural plastics such as mulch film, drip tape and silage bags are being diverted from the waste stream and successfully recycled[40] into much larger products for industrial applications such as plastic composite railroad ties.[41] Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.[42]

CNN reports that Dr. S. Madhu of the Kerala Highway Research Institute, India, has formulated a road surface that includes recycled plastic: aggregate, bitumen (asphalt) with plastic that has been shredded and melted at a temperature below 220 °C (428 °F) to avoid pollution. This road surface is claimed to be very durable and monsoon rain resistant. The plastic is sorted by hand, which is economical in India. The test road used 60 kg of plastic for an approximately 500-meter-long, 8-meter-wide, two-lane road. The process chops thin-film road-waste into a light fluff of tiny flakes that hot-mix plants can uniformly introduce into viscous bitumen with a customized dosing machine. Tests at both Bangalore and the Indian Road Research Centre indicate that roads built using this 'KK process' will have longer useful lives and better resistance to cold, heat, cracking, and rutting, by a factor of 3.[43]

Some new innovations propose plastics much easier recycled, like 2019 polydiketoenamines. (PDK)[44]

Equipment vendors

Major plastic recycling equipment companies include Tomra.[45] Equipment such as shredders and granulators may be sold by a variety of companies.[46]

In 2016, startup Precious Plastic created a marketplace called Bazar for selling machines and products targeted to DIY designers to recycle plastic.[47]

Recycling rates

The quantity of post-consumer plastics recycled has increased every year since at least 1990, but rates lag far behind those of other items, such as newspaper (about 80%) and corrugated fiberboard (about 70%).[48] Overall, U.S. post-consumer plastic waste for 2008 was estimated at 33.6 million tons; 2.2 million tons (6.5%) were recycled and 2.6 million tons (8%) were burned for energy; 28.9 million tons, or 86%, were discarded in landfills.[49]

As of 2015, approximately 6.3 billion tons of plastic waste had been generated, around 9% of which had been recycled, 12% was incinerated, and 79% was accumulated in landfills or the natural environment.[50] In 2016 only 14% of plastic waste was recycled globally.[51] According to the EPA, the recycling rate for plastics overall was 9.1% in 2015. Certain products have higher rates, such as PET bottles and jars at 29.9%, and HDPE natural bottles at 30.3%. These rates are lower than certain other materials, like steel cans, that had an estimated recycling rate of 71.3% in 2015.[52][53]

Japan's plastic waste utilization rate stood at 39% in 1996, increasing to 73% in 2006, 77% in 2011,[54] 83% in 2014[55] and 86% in 2017, according to the nation's Plastic Waste Management Institute.[56]

Economic and energy potential

In 2008, the price of PET dropped from $370/ton in the US to $20 in November.[57] PET prices had returned to their long-term averages by May 2009.[58]

Plastic identification code
Main article: Resin identification code

Seven groups of plastic polymers,[59] each with specific properties, are used worldwide for packaging applications (see table below). Each group of plastic polymer can be identified by its plastic identification code (PIC), usually a number or a letter abbreviation. For instance, low-density polyethylene can be identified by the number "4" or the letters "LDPE". The PIC appears inside a three-chasing-arrow recycling symbol. The symbol is used to indicate whether the plastic can be recycled into new products.

The PIC was introduced by the Society of the Plastics Industry, Inc., to provide a uniform system for the identification of various polymer types and to help recycling companies separate various plastics for reprocessing. Manufacturers of plastic products are required to use PIC labels in some countries/regions and can voluntarily mark their products with the PIC where there are no requirements.[60] Consumers can identify the plastic types based on the codes usually found at the base or at the side of the plastic products, including food/chemical packaging and containers. The PIC is usually not present on packaging films, since it is not practical to collect and recycle most of this type of waste.

Plastic identification code Type of plastic polymer Properties Common packaging applications Melting- and glass transition temperatures (°C) Young's modulus (GPa)
 Polyethylene terephthalate (PET, PETE) 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 consumer electronics Tm = 250;[61] Tg = 76[61] 2–2.7[62]
 High-density polyethylene (HDPE) Stiffness, strength, toughness, resistance to moisture, permeability to gas Water pipes, Gas & Fire Pipelines, Electrical & Communications conduit,[63] hula hoop rings, five gallon buckets, milk, juice and water bottles; grocery bags, some shampoo/toiletry bottles Tm = 130;[64] Tg = −125[65] 0.8[62]
 Polyvinyl chloride (PVC) Versatility, ease of blending, strength, toughness. Blister packaging for non-food items; cling films for non-food use. May be used for food packaging with the addition of the plasticisers needed to make natively rigid PVC flexible. Non-packaging uses are electrical cable insulation; rigid piping; vinyl records. Tm = 240;[66] Tg = 85[66] 2.4–4.1[67]
 Low-density polyethylene (LDPE) Ease of processing, strength, toughness, flexibility, ease of sealing, barrier to moisture. Frozen food bags; squeezable bottles, e.g. honey, mustard; cling films; flexible container lids Tm = 120;[68] Tg = −125[69] 0.17–0.28[67]
 Polypropylene (PP) Strength, toughness, resistance to heat, chemicals, grease and oil, versatile, barrier to moisture. Reusable microwaveable ware; kitchenware; yogurt containers; margarine tubs; microwaveable disposable take-away containers; disposable cups; soft drink bottle caps; plates. Tm = 173;[70] Tg = −10[70] 1.5–2[62]
 Polystyrene (PS) Versatility, clarity, easily formed Egg cartons; packing peanuts; disposable cups, plates, trays and cutlery; disposable take-away containers Tm = 240 (only isotactic);[65] Tg = 100 (atactic and isotactic)[65] 3–3.5[62]
 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), prescription glasses, automotive headlamps, riot shields, instrument panels.[71] Polycarbonate: Tg = 145;[72] Tm = 225[73] Polycarbonate: 2.6;[62] ABS plastics: 2.3[62]

Asia and Africa

The Ocean Conservancy reported that China, Indonesia, Philippines, Thailand, and Vietnam dump more plastic in the sea than all other countries combined.[74] Scientific American reported that China dumps 30% of all plastics in the ocean, followed by Indonesia, the Philippines, Vietnam, Sri Lanka, Thailand, Egypt, Malaysia, Nigeria and Bangladesh.[75]

United States

In 2015, the United States produced 34.5 million tons of plastic, which was about 13% of total waste.[76] About 9% of that was recycled. Most of the waste stream is biodegradable but plastic though only 13% of the waste stream is persistent and accumulates.[76]

Low national plastic recycling rates have been due to the complexity of sorting and processing, unfavorable economics, and consumer confusion about which plastics can actually be recycled.[77] Part of the confusion has been due to the use of the resin identification code,[78] which is only found on a subset of plastic products,[79] and which includes the recycling symbol as part of its design. The resin identification code is stamped or printed on the bottom of containers and surrounded by a triangle of arrows. (See the table in Plastic.) The intent of these symbols was to make it easier to identify the type of plastics used to make a particular container and to indicate that the plastic is potentially recyclable. The question that remains is which types of plastics can be recycled by local recycling centers. In many communities, not all types of plastics are accepted for sidewalk recycling collection programs due to the high processing costs and complexity of the equipment required to recycle certain materials. There is also sometimes a seemingly low demand for the recycled product depending on a recycling center's proximity to entities seeking recycled materials. Another major barrier is that the cost to recycle certain materials and the corresponding market price for those materials sometimes does not present any opportunity for profit. The best example of this is polystyrene (commonly called styrofoam), although some communities, like Brookline, Massachusetts, are moving toward banning the distribution of polystyrene containers by local food and coffee businesses.[80][81]

See also

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