Plastic recycling

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Plastic recycling is the process of recovering scrap or waste plastic and reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down soft drink bottles and then casting them as plastic chairs and tables. Typically a plastic is not recycled into the same type of plastic, and products made from recycled plastics are often not recyclable.[citation needed]

Contents

Challenges

When compared to other materials like glass and metal, plastic polymers require greater processing (heat treating, thermal depolymerization and monomer recycling) to be recycled.[citation needed] Due to the high molecular weight of their large polymer chains, plastics have a low entropy of mixing. A macro molecule interacts with its environment along its entire length, so total energy involved in mixing it is large compared to that of an organic molecule with a similar structure (see enthalpy). Heating alone is not enough to dissolve such a large molecule, so plastics must often be of nearly identical composition to mix efficiently.

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.

Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too viscous to economically remove fillers, and would be damaged by many of the processes that could cheaply remove the added dyes. Additives are less widely used in beverage containers and plastic bags, allowing them to be recycled more often. Yet another barrier to removing large quantities of plastic from the waste stream and landfills is the fact that many common but small plastic items lack the universal triangle recycling symbol and accompanying number. A perfect example is the billions of plastic utensils commonly distributed at fast food restaurants or sold for use at picnics.[citation needed]

The use of biodegradable plastics is increasing.[1] If some of these get mixed in with the other plastics for recycling, the reclaimed plastic is not recyclable because of the variance in properties and melt temperatures.[citation needed]

Processes

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 automatic sort systems to identify the resin, such as near infrared (NIR) technology. Some plastic products are also separated by color before they are recycled. 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.

Monomer recycling

Many recycling challenges can be resolved by using a more elaborate monomer recycling process, in which a condensation polymer essentially undergoes the inverse of the polymerization reaction used to manufacture it. This yields the same mix of chemicals that formed the original polymer, which can be purified and used to synthesize new polymer chains of the same type. Du Pont opened a pilot plant of this type in Cape Fear, North Carolina, USA, to recycle PET by a process of methanolysis, but it closed the plant due to economic pressures.[2][3]

Thermal depolymerization

Another process involves the conversion of assorted polymers into petroleum by a much less precise thermal depolymerization process. Such a process would be able to accept almost any polymer or mix of polymers, including thermoset materials such as vulcanized rubber tires and the biopolymers in feathers and other agricultural waste. Like natural petroleum, the chemicals produced can be made into fuels as well as polymers. A pilot plant of this type exists in Carthage, Missouri, USA, using turkey waste as input material. Gasification is a similar process, but is not technically recycling since polymers are not likely to become the result.

Heat compression

Yet another process that is gaining ground with startup companies (especially in Australia, United States and Japan) is heat compression.[citation needed] The heat compression process 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 the fact 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.

Other processes

A process has also been developed in which many kinds of plastic can be used as a carbon source in the recycling of scrap steel.[4]

Applications

PET

Bales of PET bottles to be recycled.

Post-consumer polyethylene terephthalate (PET or PETE) containers are sorted into different colour 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 pelletising or various treatments to produce food-contact-approved recycled PET (RPET).

RPET has been widely used to produce polyester fibres.[5] This sorted post-consumer PET waste is crushed, chopped into flakes, pressed into bales, and offered for sale.[6]

One use for this recycled PET that has recently started to become popular is to create fabrics to be used in the clothing industry.[7] The fabrics are created by spinning the PET flakes into thread and yarn.[6] This is done just as easily as creating polyester from brand new PET.[8] 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 were used to create strong, durable, rough, products, such as jackets, coat, shoes, bags, hats, and accessories. However, these fabrics are usually too rough on the skin and could cause irritation. Therefore, they usually are not used on any clothing that may irritate the skin, or where comfort is required.[9] But in today's new eco-conscious world there has been more of a demand for “green” products. As a result, many clothing companies have started looking for ways to take advantage of this new market and innovations in the use of recycled PET fabric are beginning to develop. These innovations included different ways to process the fabric,[6] to use the fabric, or blend the fabric with other materials.[10] Some of the fabrics that are leading the industry in these innovations include Billabong's Eco-Supreme Suede,[10] Livity's Rip-Tide III,[11] Wellman Inc's Eco-fi(formerly known as EcoSpun),[5] and Reware's Rewoven.[8] Some additional companies that take pride in using recycled PET in their products are Crazy Shirts[12] and Playback.[7]

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 clam shells, 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 even building materials account for 13% of the 2010 RPET production.

PVC

PVC SPI code, for recycling (Society of the Plastics Industry). The Unicode character for this symbol is U+2675.

In Europe, developments in PVC waste management are monitored by Vinyl 2010,[13] a legal entity established in 2000. In the waste management area their commitment is to

  1. Support integrated waste management approaches, using raw materials as efficiently as possible;
  2. Work with the stakeholders to research, develop, and implement recycling of 200,000 tonnes per year of PVC postconsumer waste in 2010 in addition to waste already recycled in 2000, or regulated by the PPW, ELV and E&E Waste Directives;
  3. Recycle collectable, available PVC postconsumer waste from pipes, window profiles, and roofing membranes.

Vinyl 2010 has a Monitoring Committee and publishes annual reviews.[14][15] In 2011, it reported that 260,842 tonnes of post-consumer PVC waste was recycled in 2010, i.e. an increase of 220,000 tonnes over the 1999 volumes, exceeding the 10-year target of 200,000 tonnes.

Collection and recycling schemes for the PVC waste stream are managed through Recovinyl[16] which reported the recycled tonnage as follows: profile 107,000 tonnes; flexible cables 79,000 tonnes; pipe 25,000 tonnes; rigid film 6,000 tonnes; and mixed flexible 38,000 tonnes. Recovinyl states that of the recycled material, 75% is for floors, 15% for foils, 5% for traffic cones, 3% for hoses end 2% for other applications.

One of the recycling processes is the Vinyloop Texyloop[17] used for solvent-based mechanical recycling. It involves recovering PVC plastic from composite materials through dissolution and precipitation, and is a closed-loop system, recycling the solvent and regenerating PVC. It offers a major ecological benefit, as Vinyloop-based recycled PVC's primary energy demand is 46 percent lower than conventional produced PVC. The global warming potential is 39 percent lower. This is why the use of recycled material leads to a significant better ecological footprint.[18]

HDPE

The most-often recycled plastic[citation needed], HDPE (high-density polyethylene) or number 2, is 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.

PS

The resin identification code symbol for polystyrene

Most polystyrene products are currently 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 scrap can be easily 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.[19]

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.[20]

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.[21]

Similarly, agricultural plastics such as mulch film, drip tape and silage bags are being diverted from the waste stream and successfully recycled[22] into much larger products for industrial applications such as plastic composite railroad ties.[23] Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.[24]

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 degrees 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 500m-long, 8m-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 three.[25]

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%).[26] 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 (7.7%) were burned for energy; 28.9 million tons, or 85.5%, were discarded in landfills.[27]

Economic and energy potential

In 2008, the price of PET dropped from $370/ton in the US to $20 in November.[28] PET prices had returned to their long term averages by May 2009.[29]

Consumer education

United States

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.[30] Part of the confusion has been due to the recycling symbol that is usually on all plastic items[citation needed]. This symbol is called a resin identification code. It 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 arrows was to make it easier to identify plastics for recycling. The recycling symbol doesn’t necessarily mean that the item will be accepted by residential recycling programs.[31]

United Kingdom

In the UK, the amount of post-consumer plastic being recycled is relatively low,[32] due in part to a lack of recycling facilities.

The Plastics 2020 Challenge was founded in 2009 by the plastics industry with the aim of engaging the British public in a nationwide debate about the use, reuse and disposal of plastics, and hosts a series of online debates on its website framed around the waste hierarchy.

Plastic identification code

Five groups of plastic polymers,[33] 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.[citation needed]

The PIC was introduced by the Society of the Plastics Industry, Inc., to provide a uniform system for the identification of different polymer types and to help recycling companies separate different 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.[34] 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, as it is not practical to collect and recycle most of this type of waste.[clarification needed][citation needed]

Plastic Identification CodeType of plastic polymerPropertiesCommon Packaging ApplicationsGlass Transition and Melting Temperatures (°C)Young's Modulus (GPa)
Plastic-recyc-01.svg
Polyethylene terephthalate (PET, PETE)Clarity, strength, toughness, barrier to gas and moisture.Soft drink, water and salad dressing bottles; peanut butter and jam jarsTm = 250;[35] Tg = 76[35]2-2.7[36]
Plastic-recyc-02.svg
High-density polyethylene (HDPE)Stiffness, strength, toughness, resistance to moisture, permeability to gas.Water pipes, hula hoop rings, five gallon buckets, milk, juice and water bottles; grocery bags, some shampoo / toiletry bottlesTm = 130;[37] Tg = -125[38]0.8[36]
Plastic-recyc-03.svg
Polyvinyl chloride (PVC)Versatility, ease of blending, strength, toughness.Blister packaging for non-food items; cling films for non-food use. Not used for food packaging as the plasticisers needed to make natively rigid PVC flexible are usually toxic. Non-packaging uses are electrical cable insulation; rigid piping; vinyl records.Tm = 240;[39] Tg = 85[39]2.4-4.1[40]
Plastic-recyc-04.svg
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;[41] Tg = -125[42]0.17-0.28[40]
Plastic-recyc-05.svg
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; plates.Tm = 173;[43] Tg = -10[43]1.5-2[36]
Plastic-recyc-06.svg
Polystyrene (PS)Versatility, clarity, easily formedEgg cartons; packing peanuts; disposable cups, plates, trays and cutlery; disposable take-away containers;Tm = 240 (only isotactic);[38] Tg = 100 (atactic and isotactic)[38]3-3.5[36]
Plastic-recyc-07.svg
Other (often polycarbonate or ABS)Dependent on polymers or combination of polymersBeverage bottles; baby milk bottles. Non-packaging uses for polycarbonate: compact discs; "unbreakable" glazing; electronic apparatus housings; lenses including sunglasses, prescription glasses, automotive headlamps, riot shields, instrument panels;[44]Polycarbonate: Tg = 145;[45] Tm = 225[46]Polycarbonate: 2.6;[36] ABS plastics: 2.3[36]

See also

References

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External links