Plastic recycling is the process of recovering scrap or waste plastics and reprocessing the material into useful products, sometimes completely different in form from their original state. For instance, this could mean melting down
soft drinkA soft drink is a drink that does not contain alcohol . Soft drinks are often carbonated and commonly consumed while cold. The most common soft drinks are colas, flavored water, sparkling water, iced tea, sweet tea, lemonade, squash and fruit punch....
bottles then casting them as plastic chairs and tables.
Before recycling, plastics are sorted according to their
resin identification codeThe SPI resin identification coding system is a set of symbols placed on plastics to identify the polymer type. It was developed by the Society of the Plastics Industry in 1988, and used internationally....
, a method of categorization of polymer types, which was developed by the
Society of the Plastics IndustryFounded in 1937, The Society of the Plastics Industry, Inc. is the trade association representing one of the largest manufacturing industries in the United States. SPIs members represent the entire plastics industry supply chain, including processors, machinery and equipment manufacturers and raw...
in 1988.
Polyethylene terephthalatePolyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P), is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
, commonly referred to as PET, for instance, has a resin code of 1.
When compared to other materials like
glassIn general Glass refers to a solid, brittle, transparent material, commonly used for windows, bottles, or eyewear. Examples of glassy materials include, but are not limited to, soda-lime glass, borosilicate glass, acrylic glass, sugar glass, Muscovy-glass, or aluminium oxynitride. The term glass...
and
metalA metal is a chemical element that is a good conductor of both electricity and heat, forms cations and ionic bonds with non-metals. In chemistry, a metal is an element, compound, or alloy characterized by high electrical conductivity. In a metal, atoms readily lose electrons to form positive ions...
materials, plastic polymers require greater processing to be recycled. Plastics have a low
entropyEntropy is a concept of information maintaining great importance in physics, chemistry, and information theory...
of mixing, which is due to the high molecular weight of their large polymer chains. A
macromoleculeA macromolecule is a very large molecule most often created by some form of polymerization. In the context of biochemistry, the term may be applied to the four conventional biopolymers , as well as non-polymeric molecules with large molecular mass such as macrocycles...
interacts with its environment along its entire length, so its
enthalpyIn thermodynamics and molecular chemistry, the enthalpy is a thermodynamic property of a thermodynamic system. It can be used to calculate the heat transfer during a quasistatic process taking place in a closed thermodynamic system under constant pressure...
of mixing is large compared to that of an organic molecule with a similar structure. Heating alone is not enough to dissolve such a large molecule; because of this, plastics must often be of nearly identical composition in order 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 blendA polymer blend, polymer alloy, or polymer mixture is a member of a class of materials analogous to metal alloys, in which two or more polymers are blended together to create a new material with different physical properties...
s are only useful in limited applications.
Another barrier to recycling is the widespread use of dyes, fillers, and other additives in plastics. The polymer is generally too
viscousViscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms , viscosity is "thickness." Thus, water is "thin," having a lower viscosity, while honey is "thick," having a higher viscosity...
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 bagA plastic bag or pouch is a type of flexible packaging made of thin, flexible, plastics film. Plastic bags are used for containing and transporting goods such as foods, produce, powders, ice, chemicals and waste....
s, allowing them to be recycled more frequently.
The use of
biodegradable plasticBiodegradable plastics are plastics that will decompose in natural aerobic and anaerobic environments. Biodegradation of plastics can be achieved by enabling microorganisms in the environment to metabolize the molecular structure of plastic films to produce an inert humus-like material that is...
s is increasing. If some of these get mixed in the other plastics for recycling, the reclaimed plastic is not recyclable because the variance in properties and melt temperatures.
Many such problems can be resolved by using a more elaborate
monomerA monomer is a small molecule that may become chemically bonded to other monomers to form a polymer...
recycling process, in which a
condensation polymerCondensation polymers are any kind of polymers formed through a condensation reaction, releasing small molecules as by-products such as water or methanol, as opposed to addition polymers which involve the reaction of unsaturated monomers...
essentially undergoes the inverse of the
polymerizationIn polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
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
methanolysisIn organic chemistry, transesterification is the process of exchanging the organic group R" of an ester with the organic group R' of an alcohol. These reactions are often catalyzed by the addition of an acid or base....
, but it closed the plant due to economic pressures.
Another potential option is the conversion of assorted polymers into
petroleumPetroleum or crude oil is a naturally occurring, flammable liquid found in rock formations in the Earth consisting of a complex mixture of hydrocarbons of various molecular weights, plus other organic compounds.The term "petroleum" was first used in the treatise De Natura Fossilium, published in...
by a much less precise
thermal depolymerizationThermal depolymerization is a process using hydrous pyrolysis for the reduction of complex organic materials into light crude oil. It mimics the natural geological processes thought to be involved in the production of fossil fuels...
process. Such a process would be able to accept almost any polymer or mix of polymers, including thermoset materials such as vulcanized
rubberNatural rubber is an elastomer that was originally derived from a milky colloidal suspension, or latex, found in the sap of some plants. The purified form of natural rubber is the chemical polyisoprene, which can also be produced synthetically...
tireA tire is a ring-shaped covering that fits around a wheel to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...
s and the biopolymers in
featherFeathers are one of the epidermal growths that form the distinctive outer covering, or plumage, on birds. They are considered the most complex integumentary structures found in vertebrates. They are among the outstanding characteristics that distinguish the extant Aves from other living groups....
s 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, MissouriCarthage is a city in Jasper County, Missouri, United States. The population was 12,668 at the 2000 census. It is the county seat of Jasper County and is nicknamed "America's Maple Leaf City."...
, USA, using turkey waste as input material.
See the main article on thermal depolymerizationThermal depolymerization is a process using hydrous pyrolysis for the reduction of complex organic materials into light crude oil. It mimics the natural geological processes thought to be involved in the production of fossil fuels...
. GasificationGasification is a process that converts carbonaceous materials, such as coal, petroleum, biofuel, or biomass, into carbon monoxide and hydrogen by reacting the raw material, such as house waste, or compost at high temperatures with a controlled amount of oxygen and/or steam. The resulting gas...
is a similar process, but is not technically recycling since polymers are not likely to become the result.
Recently, a process has also been developed in which many kinds of plastic can be used as a
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
source in the recycling of scrap
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
.
Yet another process that is gaining ground with startup companies (especially in
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the continental mainland , the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans...
,
United StatesThe United States of America is a federal constitutional republic comprising fifty states and a federal district...
and
Japanis an island country in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, People's Republic of China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
) is heat compression. 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
tumblersIn the design of the Xanadu computer system, a tumbler is an address of any range of content or link or a set of ranges or links. Tumblers were proposed by Ted Nelson in his book Literary Machines...
(large rotating drums resembling giant
clothes dryerA clothes dryer or tumble dryer is a household appliance that is used to remove moisture from a load of clothing and other textiles, generally shortly after they are cleaned in a washing machine....
s). 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.
Applications
Post-consumer PET (number 1) is often sorted into different color fractions. This sorted post-consumer PET waste is crushed, pressed into bales and offered for sale to recycling companies. PET flakes are used as the raw material for a range of products that would otherwise be made of
polyesterPolyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
.
PVCPolyvinyl chloride, commonly abbreviated PVC, is the third most widely used thermoplastic polymer after polyethylene and polypropylene. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in...
- or
VinylA vinyl compound is any organic compound that contains a vinyl group , −CH=CH2. These are derivatives of ethene, CH2=CH2, with one hydrogen atom replaced with some other group...
Recycling has historically been difficult to perfect on the industrial scale. But within the last decade several viable methods for recycling or upcycling PVC
plasticPlastic is the general common term for a wide range of synthetic or semisynthetic organic amorphous solid materials suitable for the manufacture of industrial products...
have been developed.
The most-often recycled plastic, HDPE or number 2, is
downcycledDowncycling is the recycling of a material into a material of lesser quality or re-use of a product with crippled functionality for alternative purposes. A clear example is plastic recycling, which turns the material into lower grade plastics...
into
plastic lumberPlastic lumber is a 100% recyclable material lumber or timber made of recycled plastic.There are also applications in the market, which utilize only virgin raw materials...
, tables, roadside curbs, benches, truck cargo liners, trash receptacles, stationery (e.g. rulers) and other durable plastic products and is usually in demand.
The white plastic
foam peanutFoam peanuts, also known as packing peanuts, are a common loose-fill packaging and cushioning material used to prevent damage to fragile objects during shipping. They are shaped to interlock when compressed and free flow when not compressed...
s used as packing material are often accepted by shipping stores for reuse.
In
IsraelIsrael officially the State of Israel , is a developed state in Western Asia located on the eastern shore of the Mediterranean Sea. It borders Lebanon in the north, Syria in the northeast, Jordan in the east, and Egypt on the southwest, and contains geographically diverse features within its...
successful trials have shown that plastic films recovered from mixed municipal waste streams can be recycled into useful household products such as buckets.
Similarly,
agricultural plasticsThe term plasticulture refers to the practice of using plastic materials in agricultural applications.The plastic materials themselves are often and broadly referred to as "ag plastics." Plasticulture ag plastics include soil fumigation film, irrigation drip tape/tubing, nursery pots and silage...
such as
mulch filmPlastic mulch is a product used, in a similar fashion to mulch, to suppress weeds and conserve water in crop production and landscaping. Certain plastic mulches also act as a barrier to keep methyl bromide, both a powerful fumigant and ozone depleter, in the soil. Crops grow through slits or...
,
drip tapeDrip tape is a type of thin walled dripperline used in drip irrigation. The first drip tape, a product known as "Dew Hose", was developed in Watertown, New York, in the 1960's by Richard D. Chapin of Chapin Watermatics, USA....
and
silageSilage is fermented, high-moisture fodder that can be fed to ruminants or used as a biofuel feedstock for anaerobic digesters. It is fermented and stored in a process called ensilage or silaging, and usually made from grass crops, including corn or sorghum or other cereals, using the entire green...
bags are being diverted from the waste stream and successfully recycled into much larger products for industrial applications such as plastic composite railroad ties. Historically, these agricultural plastics have primarily been either landfilled or burned on-site in the fields of individual farms.
CNN reports that Indian Dr. S. Madhu of the Kerala Highway Research Institute 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 approx. 500m long, 8m wide, two-lane road.
Financial justification
In 2008, the price of PETE dropped from $370/ton in the US to $20 in November.
United States
Plastic recycling rates lag far behind those of other items, such as newspaper (about 80%) and corrugated fiberboard (about 70%). 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. Part of the confusion has been due to the
recycling symbolThe universal recycling symbol is an internationally-recognized symbol used to designate recyclable materials. It is composed of three chasing arrows that form a Möbius strip or unending loop....
that is usually on all plastic items.
This symbol is called a
resin identification codeThe SPI resin identification coding system is a set of symbols placed on plastics to identify the polymer type. It was developed by the Society of the Plastics Industry in 1988, and used internationally....
. It is stamped or printed on the bottom of containers and surrounded by a triangle of arrows. (See the table in
PlasticPlastic is the general common term for a wide range of synthetic or semisynthetic organic amorphous solid materials suitable for the manufacture of industrial products...
.) 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. They just indicate the plastic resin content.
United Kingdom
In the UK, the amount of post-consumer plastic being recycled is relatively low, due in part to a lack of recycling facilities.
Plastic Identification Code
Seven groups of plastic polymers, 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 and/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. which provides a uniform system for the identification of different polymer types and helps recycling companies to 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. 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.
| Plastic Identification Code |
Type of plastic polymer |
Properties |
Common Packaging Applications |
 |
Polyethylene Terephthalate Polyethylene terephthalate , commonly abbreviated PET, PETE, or the obsolete PETP or PET-P), is a thermoplastic polymer resin of the polyester family and is used in synthetic fibers; beverage, food and other liquid containers; thermoforming applications; and engineering resins often in combination...
(PET, PETE) |
Clarity, strength, toughness, barrier to gas and moisture. |
Soft drink, water and salad dressing bottles; peanut butter and jam jars |
 |
High Density Polyethylene (HDPE) |
Stiffness, strength, toughness, resistance to moisture, permeability to gas. |
Water pipes, Hula-Hoop (children's game) rings, Milk, juice and water bottles; the occasional shampoo / toiletry bottle |
 |
Polyvinyl ChloridePolyvinyl chloride, commonly abbreviated PVC, is the third most widely used thermoplastic polymer after polyethylene and polypropylene. In terms of revenue generated, it is one of the most valuable products of the chemical industry. Around the world, over 50% of PVC manufactured is used in...
(PVC) |
Versatility, clarity, ease of blending, strength, toughness. |
Juice bottles; cling films; PVC piping |
 |
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. |
 |
PolypropylenePolypropylene or polypropene is a thermoplastic polymer, made by the chemical industry and used in a wide variety of applications, including packaging, textiles , stationery, plastic parts and reusable containers of various types, laboratory equipment, loudspeakers, automotive components, and...
(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 and plates. |
 |
PolystyrenePolystyrene , sometimes abbreviated PS, is an aromatic polymer made from the aromatic monomer styrene, a liquid hydrocarbon that is commercially manufactured from petroleum by the chemical industry...
(PS) |
Versatility, clarity, easily formed |
Egg cartons; packing peanuts; disposable cups, plates, trays and cutlery; disposable take-away containers; |
 |
Other (often polycarbonatePolycarbonates are a particular group of thermoplastic polymers. They are easily worked, moulded, and thermoformed; as such, these plastics are very widely used in the modern chemical industry. Their interesting features position them between commodity plastics and engineering plastics...
or ABSAcrylonitrile butadiene styrene is a common thermoplastic used to make light, rigid, molded products such as piping , musical instruments , golf...
) |
Dependent on polymers or combination of polymers |
Beverage bottles; baby milk bottles; electronic casing. |
The quantity of post-consumer plastics recycled has increased every year since at least 1990. In 2006 the amount of plastic bottles recycled reached a record high of 2.2 trillion pounds.
The amount of PET bottles recycled in 2006 increased more than 102 million pounds compared to 2005.
HDPE bottle recycling increased in 2005 to 928 million pounds.
All plastic bottles were recycled at a rate of 24 percent in 2005.
External links
- Plastic Packaging Resin Codes data from the American Chemistry Council
The American Chemistry Council , formerly known as the Chemical Manufacturers' Association, is an industry trade association for American chemical companies, based in Arlington County, Virginia.-History:...