Plastic

Summary

Household items made of various types of plastic

Plastics are a wide range of synthetic or semi-synthetic materials that use polymers as a main ingredient. Their plasticity makes it possible for plastics to be moulded, extruded or pressed into solid objects of various shapes. This adaptability, plus a wide range of other properties, such as being lightweight, durable, flexible, and inexpensive to produce, has led to its widespread use. Plastics typically are made through human industrial systems. Most modern plastics are derived from fossil fuel-based chemicals like natural gas or petroleum; however, recent industrial methods use variants made from renewable materials, such as corn or cotton derivatives.[1]

In developed economies, about a third of plastic is used in packaging and roughly the same in buildings in applications such as piping, plumbing or vinyl siding.[2] Other uses include automobiles (up to 20% plastic [2]), furniture, and toys.[2] In the developing world, the applications of plastic may differ; 42% of India's consumption is used in packaging.[2] In the medical field, polymer implants and other medical devices are derived at least partially from plastic. Worldwide, about 50 kg of plastic is produced annually per person, with production doubling every ten years.

The world's first fully synthetic plastic was Bakelite, invented in New York in 1907, by Leo Baekeland,[3] who coined the term "plastics".[4] Dozens of different types of plastics are produced today, such as polyethylene, which is widely used in product packaging, and polyvinyl chloride, used in construction and pipes because of its strength and durability. Many chemists have contributed to the materials science of plastics, including Nobel laureate Hermann Staudinger, who has been called "the father of polymer chemistry" and Herman Mark, known as "the father of polymer physics".[5]

The success and dominance of plastics starting in the early 20th century has caused widespread environmental problems, due to their slow decomposition rate in natural ecosystems. Toward the end of the 20th century, the plastics industry promoted recycling in order to ease environmental concerns while continuing to produce virgin plastic. The main companies producing plastics doubted the economic viability of recycling at the time, and this is reflected in contemporary plastic collection. Plastic collection and recycling is largely ineffective because of failures of contemporary decision-making mechanisms that resulted in a complexity and amount of cleaning and sorting post-consumer plastics too high for contemporary methods. Most plastic produced has not been reused, either being captured in landfills or persisting in the environment as plastic pollution. Plastic pollution can be found in all the world's major water bodies, for example, creating garbage patches in all of the world's oceans and contaminating terrestrial ecosystems.

Etymology

The word plastic derives from the Greek πλαστικός (plastikos) meaning "capable of being shaped or molded," and in turn from πλαστός (plastos) meaning "molded."[6] As a noun the word most commonly refers to the solid products of petrochemical-derived manufacturing.[7]

The noun plasticity refers specifically here to the deformability of the materials used in the manufacture of plastics. Plasticity allows molding, extrusion or compression into a variety of shapes: films, fibers, plates, tubes, bottles and boxes, among many others. Plasticity also has a technical definition in materials science outside the scope of this article referring to the non-reversible change in form of solid substances.

Structure

Most plastics contain organic polymers.[8] The vast majority of these polymers are formed from chains of carbon atoms, with or without the attachment of oxygen, nitrogen or sulfur atoms. These chains comprise many repeating units formed from monomers. Each polymer chain consists of several thousand repeating units. The backbone is the part of the chain that is on the main path, linking together a large number of repeat units. To customize the properties of a plastic, different molecular groups called side chains hang from this backbone; they are usually hung from the monomers before the monomers themselves are linked together to form the polymer chain. The structure of these side chains influences the properties of the polymer.

Properties and classifications

Plastics are usually classified by the chemical structure of the polymer's backbone and side chains. Important groups classified in this way include the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Plastics can be classified by the chemical process used in their synthesis, such as condensation, polyaddition, and cross-linking.[9] They can also be classified by their physical properties, including hardness, density, tensile strength, thermal resistance, and glass transition temperature. Plastics can additionally be classified by their resistance and reactions to various substances and processes, such as exposure to organic solvents, oxidation, and ionizing radiation.[10] Other classifications of plastics are based on qualities relevant to manufacturing or product design for a particular purpose. Examples include thermoplastics, thermosets, conductive polymers, biodegradable plastics, engineering plastics and elastomers.

Thermoplastics and thermosetting polymers

The plastic handle of a kitchen utensil has been deformed by heat and partially melted

One important classification of plastics is the degree to which the chemical processes used to make them are reversible or not.

Thermoplastics do not undergo chemical change in their composition when heated and thus can be molded repeatedly. Examples include polyethylene (PE), polypropylene (PP), polystyrene (PS), and polyvinyl chloride (PVC).[11]

Thermosets, or thermosetting polymers, can melt and take shape only once: after they have solidified, they stay solid.[12] If reheated, thermosets decompose rather than melt. In the thermosetting process, an irreversible chemical reaction occurs. The vulcanization of rubber is an example of this process. Before heating in the presence of sulfur, natural rubber (polyisoprene) is a sticky, slightly runny material; after vulcanization, the product is dry and rigid.

Amorphous plastics and crystalline plastics

Many plastics are completely amorphous (without a highly ordered molecular structure),[13] including thermosets, polystyrene, and methyl methacrylate (PMMA). Crystalline plastics exhibit a pattern of more regularly spaced atoms, such as high-density polyethylene (HDPE), polybutylene terephthalate (PBT), and polyether ether ketone (PEEK). However, some plastics are partially amorphous and partially crystalline in molecular structure, giving them both a melting point and one or more glass transitions (the temperature above which the extent of localized molecular flexibility is substantially increased). These so-called semi-crystalline plastics include polyethylene, polypropylene, polyvinyl chloride, polyamides (nylons), polyesters and some polyurethanes.

Conductive polymers

Intrinsically Conducting Polymers (ICP) are organic polymers that conduct electricity. While a conductivity of up to 80 kS/cm in stretch-oriented polyacetylene,[14] has been achieved, it does not approach that of most metals. For example, copper has a conductivity of several hundred kS/cm.[15]

Biodegradable plastics and bioplastics

Biodegradable plastics

Biodegradable plastics are plastics that degrade (break down) upon exposure to sunlight or ultra-violet radiation; water or dampness; bacteria; enzymes; or wind abrasion. Attack by insects, such as waxworms and mealworms, can also be considered as forms of biodegradation. Aerobic degradation requires that the plastic be exposed at the surface, whereas anaerobic degradation would be effective in landfill or composting systems. Some companies produce biodegradable additives to enhance biodegradation. Although starch powder can be added as a filler to allow some plastics to degrade more easily, such treatment does not lead to complete breakdown. Some researchers have genetically engineered bacteria to synthesize completely biodegradable plastics, such as polyhydroxybutyrate (PHB); however, these are relatively costly as of 2021.[16]

Bioplastics

While most plastics are produced from petrochemicals, bioplastics are made substantially from renewable plant materials like cellulose and starch.[17] Due both to the finite limits of fossil fuel reserves and to rising levels of greenhouse gases caused primarily by the burning of those fuels, the development of bioplastics is a growing field.[18] [19] Global production capacity for bio-based plastics is estimated at 327,000 tonnes per year. In contrast, global production of polyethylene (PE) and polypropylene (PP), the world's leading petrochemical-derived polyolefins, was estimated at over 150 million tonnes in 2015.[20]

Plastic industry

The plastic industry includes the global production, compounding, conversion and sale of plastic products. Although the Middle East and Russia produce most of the required petrochemical raw materials; the production of plastic is concentrated in the global East and West. The plastic industry comprises a huge number of companies and can be dived into several sectors:

Production

Since the birth of the plastic industry in the 1950s global production has increases enormously, reaching some 381 million metric tonnes in 2015 (excluding additives). The total amount of plastic generated in that time is estimated to be 8.3 billion tonnes.[21]

Polypropylene plants
A SOCAR Polymer polypropylene plant in Sumgayit, Azerbaijan
A SOCAR Polymer polypropylene plant in Sumgayit, Azerbaijan
Annual global plastic production 1950–2015.[21] Vertical lines denote the 1973–1975 recession and the financial crisis of 2007–2008 which caused brief reductions in plastic production.

Plastics are produced in chemical plants by the polymerization of their starting materials (monomers); which are almost always petrochemical in nature. Such facilities are normally large and are visually similar to oil refineries, with sprawling pipework running throughout. The large size of these plants allows them to exploit economies of scale. Despite this, plastic production is not particularly monopolized, with about 100 companies accounting for 90% of global production.[22] This includes a mixture of private and state-owned enterprises. Roughly half of all production takes place in East Asia, with China being the largest single producer. Major international producers include:

Global plastic production (2020)[23]
Region Global production
NAFTA 19%
Europe 16%
Latin America 4%
Middle East & Africa 7%
CIS 3%
China 31%
Japan 3%
Rest of Asia 17%

Compounding

Plastic compounding scheme for a thermosoftening material

Plastic is not sold as a pure, unadulterated material but is instead mixed with various other chemicals and materials, which are collectively known as additives. These are added during the compounding stage and include substances such as stabilizers, plasticizers and dyes, which are intended to improve the lifespan, workability or aesthetics of the final item. In some cased this can involve mixing different types of plastic together to form a polymer blend, such as high impact polystyrene. Large companies may do their own compounding prior to production but some producers may have it done by a third-party. Companies which specialize in this work are known as Compounders.

The compounding of thermosetting plastic is relatively straightforward; as it remains liquid until it is cured into it's final form. For thermosoftening materials it is necessary to melt the plastic, which involves heating it to anywhere between 150–320 °C (300–610 °F). Molten plastic is viscous and exhibits laminar flow, leading to poor mixing. Compounding is therefore done using extrusion equipment, which is able to supply the necessary heat and mixing to give a properly dispersed product.

Additive concentrations are usually quite low, however high levels can be added to create Masterbatch products. The additives in these are concentrated but still properly dispersed in the host resin. Masterbatch granules can be mixed with cheaper bulk polymer and will release their additives during processing to give a homogeneous final product. This can be cheaper than working with a fully compounded material.

Converting

Short video on injection molding (9 min 37 s)
See caption
Blow molding a plastic drinks bottle

Companies than produce finished goods are known as converters (sometimes processors). The vast majority of plastics produced worldwide are thermosoftening and must be heated until molten in order to be molded. Various sorts of extrusion equipment exist which can then form the plastic into almost any shape.

For thermosetting materials the process is slightly different, as the plastics are liquid to begin with and but must be cured to give solid products, but much of the equipment is broadly similar.

Types of plastics

Commodity plastics

Chemical structures and uses of some common plastics

Around 70% of global production is concentrated in six major polymer types, the so called commodity plastics. Unlike most other plastics these can often be identified by their resin identification code (RIC):

Symbol Resin Code 01 PET.svg Polyethylene terephthalate (PET or PETE)
Symbol Resin Code 02 PE-HD.svg High-density polyethylene (HDPE or PE-HD)
Symbol Resin Code 03 PVC.svg Polyvinyl chloride (PVC or V)
Symbol Resin Code 04 PE-LD.svg Low-density polyethylene (LDPE or PE-LD),
Symbol Resin Code 05 PP.svg Polypropylene (PP)
Symbol Resin Code 06 PS.svg Polystyrene (PS)

Polyurethanes (PUR) and PP&A fibres[24] are often also included as major commodity classes, although they usually lack RICs, as they are chemically quite diverse groups. These materials are inexpensive, versatile and easy to work with, making them the preferred choice for the mass production everyday objects. Their biggest single application is in packaging, with some 146 million tonnes being used this way in 2015, equivalent to 36% of global production. Due to their dominance; many of the properties and problems commonly associated with plastics, such as pollution stemming from their poor biodegradability, are ultimately attributable to commodity plastics.

A huge number of plastic exist beyond the commodity plastics, with many having exceptional properties





Global plastic production by polymer type (2015)[21]
Polymer Production (Mt) Percentage of all plastics Polymer type Thermal character
Low-density polyethylene (LDPE) 64 15.7% Polyolefin Thermoplastic
High-density polyethylene (HDPE) 52 12.8% Polyolefin Thermoplastic
polypropylene (PP) 68 16.7% Polyolefin Thermoplastic
Polystyrene (PS) 25 6.1% Unsaturated polyolefin Thermoplastic
Polyvinyl chloride (PVC) 38 9.3% Halogenated Thermoplastic
Polyethylene terephthalate (PET) 33 8.1% Condensation Thermoplastic
Polyurethane (PUR) 27 6.6% Condensation Thermoset[25]
PP&A Fibers[24] 59 14.5% Condensation Thermoplastic
All Others 16 3.9% Various Varies
Additives 25 6.1% - -
Total 407 100% - -

Engineering plastics

Engineering plastics are used to make more robust products such as car parts, building and construction materials, and some machine parts. In some cases they are polymer blend

iPhone 5c, a smartphone with a polycarbonate unibody shell
  • Polycarbonate (PC): compact discs, eyeglasses, riot shields, security windows, traffic lights, and lenses
  • Polycarbonate + acrylonitrile butadiene styrene (PC + ABS): a blend of PC and ABS that creates a stronger plastic used in car interior and exterior parts, and in mobile phone bodies
  • Polyethylene + acrylonitrile butadiene styrene (PE + ABS): a slippery blend of PE and ABS used in low-duty dry bearings
10-meter (33-foot) deep Monterey Bay Aquarium tank has acrylic windows up to 33 centimeters (13 inches) thick to withstand the water pressure
  • Polymethyl methacrylate (PMMA) (acrylic): contact lenses (of the original "hard" variety), glazing (best known in this form by its various trade names around the world; e.g. Perspex, Plexiglas, and Oroglas), fluorescent-light diffusers, and rear light covers for vehicles. It also forms the basis of artistic and commercial acrylic paints, when suspended in water with the use of other agents.
  • Silicones (polysiloxanes): heat-resistant resins used mainly as sealants but also used for high-temperature cooking utensils and as a base resin for industrial paints
  • Urea-formaldehyde (UF): one of the aminoplasts used as a multi-colorable alternative to phenolics: used as a wood adhesive (for plywood, chipboard, hardboard) and electrical switch housings

High-performance plastics

High-performance plastics usually expensive, with their use limited to specialised applications which make use of their superior properties.

  • Aramids: a class of heat-resistant and strong synthetic fibers used in aerospace and military applications, includes Kevlar and Nomex, and Twaron.
  • Polyetheretherketone (PEEK): strong, chemical- and heat-resistant thermoplastic; its biocompatibility allows for use in medical implant applications and aerospace moldings. It is one of the most expensive commercial polymers.
  • Polyetherimide (PEI) (Ultem): a high-temperature, chemically stable polymer that does not crystallize
  • Polyimide: a high-temperature plastic used in materials such as Kapton tape
  • Polysulfone: high-temperature melt-processable resin used in membranes, filtration media, water heater dip tubes and other high-temperature applications
  • Polytetrafluoroethylene (PTFE), or Teflon: heat-resistant, low-friction coatings used in non-stick surfaces for frying pans, plumber's tape and water slides
  • Polyamide-imide (PAI): High-performance engineering plastic extensively used in high performance gears, switches, transmission and other automotive components, and aerospace parts.[26]

Applications

The largest application for plastics is as packaging materials, but they are used in a wide range of other sectors, including: construction (pipes, gutters, door and windows), textiles (stretchable fabrics, fleece), consumer goods (toys, tableware, toothbrushes), transportation (headlights, bumpers, body panels, wing mirrors), electronics (phones, computers, televisions) and as machine parts.[21]



Representative polymers

Bakelite

The first plastic based on a synthetic polymer was invented in 1907, by Leo Hendrik Baekeland, a Belgian-born American living in New York State. He had been looking for an insulating shellac to coat wires in electric motors and generators. He discovered that combining phenol (C6H5OH) and formaldehyde (HCOH) formed a sticky mass and that the material could be mixed with wood flour, asbestos, or slate dust to create strong and fire-resistant "composite" materials. The new material tended to foam during synthesis, requiring that Baekeland build pressure vessels to force out the bubbles and provide a smooth, uniform product.[27] Bakelite, named for himself and patented in 1909, was originally used for electrical and mechanical parts; it came into widespread use in general goods and jewelry in the 1920s. A purely synthetic material, Bakelite was also an early thermosetting plastic.

Nylon

DuPont Corporation began a secret development project in 1927 designated Fiber66 under the direction of Harvard chemist Wallace Carothers and chemistry department director Elmer Keiser Bolton. Carothers's work led to the discovery of synthetic nylon fiber, which was very strong and flexible. The first application was for toothbrush bristles. Carothers and his team synthesized a number of different polyamides including polyamide 6.6 and 4.6, as well as polyesters.[28]

Nylon was the first commercially successful synthetic thermoplastic polymer. The first women's nylon stockings (nylons) were introduced by DuPont at the 1939 World's Fair in New York City. It had taken 12 years and US$27 million to refine nylon and develop the industrial processes for its bulk manufacture. In 1940, 64 million pairs of nylons were sold.

When the US entered World War II, the capacity DuPont had developed to produce nylons shifted to manufacturing vast numbers of parachutes for fliers and paratroopers. After the war ended, DuPont resumed selling nylon to the public, engaging in a 1946 promotional campaign that brought on the so-called nylon riots.

Subsequently, polyamides 6, 10, 11, and 12 have been developed based on monomers that are ring compounds, such as caprolactam. Nylon 66 is a material manufactured by condensation polymerization. Nylon of different types remains an important plastic, and in its bulk form is very wear-resistant, particularly if oil-impregnated. It is used to build gears, plain bearings, valve seats, and seals; and because of good heat-resistance, increasingly for vehicular transportation applications, as well as for other mechanical parts.

Poly(methyl methacrylate)

Poly(methyl methacrylate) (PMMA), also known as acrylic or acrylic glass as well as by the trade names Plexiglas, Acrylite, Lucite, and Perspex, is a transparent thermoplastic often used in sheet form as a lightweight or shatter-resistant alternative to glass. PMMA can also be utilized as a casting resin, in inks and coatings, and has many other uses.

Polystyrene

Unplasticised polystyrene is a rigid, brittle, inexpensive plastic that has been used to make plastic model kits and similar knick-knacks. It also is the basis for some of the most popular "foamed" plastics, under the name styrene foam or Styrofoam. Like most other foam plastics, foamed polystyrene can be manufactured in an "open cell" form, in which the foam bubbles are interconnected, as in an absorbent sponge, and "closed cell", in which all the bubbles are distinct, like tiny balloons, as in gas-filled foam insulation and flotation devices. In the late 1950s, high impact styrene was introduced, which was not brittle. It finds much current use as the substance of toy figurines and novelties.

Polyvinyl chloride

PVC is used extensively in sewage pipes due to its low cost, chemical resistance and ease of jointing

Polyvinyl chloride (PVC, commonly called "vinyl") [29] incorporates chlorine atoms. C-Cl bonds in the backbone are hydrophobic and resist oxidation (and burning). PVC is stiff, strong, heat and weather resistant, properties that make it suitable for use in devices for plumbing, gutters, house siding, enclosures for computers and other electronics gear. PVC can also be softened with chemical processing, and in this form it is now used for shrink-wrap, food packaging, and rain gear.

All PVC polymers are degraded by heat and light. When this happens, hydrogen chloride is released into the atmosphere and oxidation of the compound occurs.[30] Because hydrogen chloride readily combines with water vapor in the air to form hydrochloric acid,[31] polyvinyl chloride is not recommended for long-term archival storage of silver, photographic film or paper (mylar is preferable).[32]

Rubber

Natural rubber is an elastomer (an elastic hydrocarbon polymer) that originally was derived from latex, a milky colloidal suspension found in specialised vessels in some plants. It is useful directly in this form (indeed, the first appearance of rubber in Europe was cloth waterproofed with unvulcanized latex from Brazil). However, in 1839, Charles Goodyear invented vulcanized rubber: a form of natural rubber heated with sulfur (and a few other chemicals), forming cross-links between polymer chains (vulcanization), improving elasticity and durability. In 1851, Nelson Goodyear added fillers to natural rubber materials to form ebonite.[33]

Synthetic rubber

The first fully synthetic rubber was synthesized by Sergei Lebedev in 1910. In World War II, supply blockades of natural rubber from South East Asia caused a boom in development of synthetic rubber, notably styrene-butadiene rubber. In 1941, annual production of synthetic rubber in the US was only 231 tonnes which increased to 840,000 tonnes in 1945. In the space race and nuclear arms race, Caltech researchers experimented with using synthetic rubbers for solid fuel for rockets. Ultimately, all large military rockets and missiles would use synthetic rubber based solid fuels, and they would also play a significant part in the civilian space effort.

Additives

Additives consists of various organic or inorganic compounds which are blended into plastics to enhance performance functionality.[34] The amounts added can vary significantly; for instance as much as 70% of the weight of PVC can be plasticisers whereas pigments may account for less than 1%.[34] Many controversies associated with plastics actually relate to the additives.[35][36][34][further explanation needed]

Typical additives include:

Colorants

Plastic colorants are chemical compounds used to color plastic. Those compounds come in a form of dyes and pigments. The type of a colorant is chosen based on the type of a polymeric resin, that needs to be colored. [citation needed] Dyes are usually used with polycarbonates, polystyrene and acrylic polymers. Pigments are better suited for use with polyolefins.[37] [38]

The colorant must satisfy various constraints, for example, the compound must be[39] chemically compatible with the base resin, be a suitable match with a color standard (see e.g. International Color Consortium), be chemically stable, which in this case means being able to survive the stresses and processing temperature (heat stability) in the fabrication process and be durable enough to match the life duration of the product.

The parameters of the compound vary with a desired effect, which may include the final product being pearlescent, metallic, fluorescent, phosphorescent, thermochromic or photochromic.[40]

The exact chemical formula will furthermore depend on the type of application: general purpose, food contact item, toy, package subject to CONEG,[41] etc.[40]

Different methods for delivering colorants in molding plastics include masterbatches (concentrates), a method which involves a concentrate being separated into resin, cube blends ("salt & pepper mixes" - dry blending) which are natural polymers, already sprayed into natural polymers, surface coating, and precolored resins, which involve using precolored materials to make manufacturing cheaper.[42][43]

Fillers and Reinforcements

Despite appearing similar these additives serve different purposes. Fillers are inert low-cost materials added to the polymer to reduce cost and weight.[44] Examples include chalk, starch, cellulose, wood flour, and zinc oxide. Reinforcements may be added to strengthen the polymer against mechanical damage. Examples include adding carbon fibre to form fibre-reinforced plastic.[33]

Flame retardants

Flame retardant
The term flame retardants subsumes a diverse group of chemicals which are added to manufactured materials, such as plastics and textiles, and surface finishes and coatings. Flame retardants are activated by the presence of an ignition source and are intended to prevent or slow the further development of ignition by a variety of different physical and chemical methods. They may be added as a copolymer during the polymerisation process, or later added to the polymer at a moulding or extrusion process or (particularly for textiles) applied as a topical finish.[45] Mineral flame retardants are typically additive while organohalogen and organophosphorus compounds can be either reactive or additive.

Plasticizers

Plasticizers are used to improve the flexibility and rheology of plastics and are important in making films and cable. By mass they are often the most abundant additives, [36] although this varies significantly between polymers. Around 80–90% of global production is used in PVC,[46] which may itself consist of up to 70% plasticiser by mass. Cellulose plastics, such as cellophane, also use significant amounts of plasticizers.[34] By comparison, little or no plasticizer is present in polyethylene terephthalate (PET). Phthalates remain the most common class of plasticisers, despite public concern over their potential health effects as endocrine disruptors.

Stabilizers

Polymer stabilizers are important during the forming and casting of molten plastic but also prolong the life of the polymers by suppressing polymer degradation that results from UV-light, oxidation, and other forces. Typical stabilizers thus absorb UV light or function as antioxidants.

Other classes

Release agents

Release agents are used during the production of plastic items to prevent them sticking to the mold, for instance in injection moulding. Slip Additives are similarly used to prevent polyolefin films from adhering to metal surfaces during processing. Erucamide and oleamide are common examples.

Biocides

Biocides are added to prevent the growth of organisms of the plastic surface. This is usually aimed at making the plastic antibacterial. The majority of biocides are added to soft PVC and foamed polyurethanes. Compounds include isothiazolinones, triclosan, arsenic and organotin compounds.

Toxicity

Pure plastics have low toxicity due to their insolubility in water, and because they have a large molecular weight, they are biochemically inert. Plastic products contain a variety of additives, however, some of which can be toxic.[47] For example, plasticizers like adipates and phthalates are often added to brittle plastics like PVC to make them pliable enough for use in food packaging, toys, and many other items. Traces of these compounds can leach out of the product. Owing to concerns over the effects of such leachates, the EU has restricted the use of DEHP (di-2-ethylhexyl phthalate) and other phthalates in some applications, and the US has limited the use of DEHP, DPB, BBP, DINP, DIDP, and DnOP in children's toys and child-care articles through the Consumer Product Safety Improvement Act. Some compounds leaching from polystyrene food containers have been proposed to interfere with hormone functions and are suspected human carcinogens (cancer-causing substances).[48] Other chemicals of potential concern include alkylphenols.[36]

While a finished plastic may be non-toxic, the monomers used in the manufacture of its parent polymers may be toxic. In some cases, small amounts of those chemicals can remain trapped in the product unless suitable processing is employed. For example, the World Health Organization's International Agency for Research on Cancer (IARC) has recognized vinyl chloride, the precursor to PVC, as a human carcinogen.[48]

Bisphenol A (BPA)

Some plastic products degrade to chemicals with estrogenic activity.[49] The primary building block of polycarbonates, bisphenol A (BPA), is an estrogen-like endocrine disruptor that may leach into food.[48] Research in Environmental Health Perspectives finds that BPA leached from the lining of tin cans, dental sealants and polycarbonate bottles can increase the body weight of lab animals' offspring.[50] A more recent animal study suggests that even low-level exposure to BPA results in insulin resistance, which can lead to inflammation and heart disease.[51] As of January 2010, the Los Angeles Times reported that the US Food and Drug Administration (FDA) is spending $30 million to investigate indications of BPA's link to cancer.[52] Bis(2-ethylhexyl) adipate, present in plastic wrap based on PVC, is also of concern, as are the volatile organic compounds present in new car smell. The EU has a permanent ban on the use of phthalates in toys. In 2009, the US government banned certain types of phthalates commonly used in plastic.[53]

History

The development of plastics has evolved from the use of naturally plastic materials (e.g., gums and shellac) to the use of the chemical modification of those materials (e.g., natural rubber, cellulose, collagen, and milk proteins), and finally to completely synthetic plastics (e.g., bakelite, epoxy, and PVC). Early plastics were bio-derived materials such as egg and blood proteins, which are organic polymers. In around 1600 BC, Mesoamericans used natural rubber for balls, bands, and figurines.[2] Treated cattle horns were used as windows for lanterns in the Middle Ages. Materials that mimicked the properties of horns were developed by treating milk proteins with lye. In the nineteenth century, as chemistry developed during the Industrial Revolution, many materials were reported. The development of plastics accelerated with Charles Goodyear's 1839 discovery of vulcanization to harden natural rubber.

Plaque commemorating Parkes at the Birmingham Science Museum

Parkesine, invented by Alexander Parkes in 1855 and patented the following year,[54] is considered the first man-made plastic. It was manufactured from cellulose (the major component of plant cell walls) treated with nitric acid as a solvent. The output of the process (commonly known as cellulose nitrate or pyroxilin) could be dissolved in alcohol and hardened into a transparent and elastic material that could be molded when heated.[55] By incorporating pigments into the product, it could be made to resemble ivory. Parkesine was unveiled at the 1862 International Exhibition in London and garnered for Parkes the bronze medal.[56]

In 1893, French chemist Auguste Trillat discovered the means to insolubilize casein (milk proteins) by immersion in formaldehyde, producing material marketed as galalith.[57] In 1897, mass-printing press owner Wilhelm Krische of Hanover, Germany, was commissioned to develop an alternative to blackboards.[57] The resultant horn-like plastic made from casein was developed in cooperation with the Austrian chemist (Friedrich) Adolph Spitteler (1846–1940). Although unsuitable for the intended purpose, other uses would be discovered.[57]

The world's first fully synthetic plastic was Bakelite, invented in New York in 1907 by Leo Baekeland,[3] who coined the term plastics.[4] Many chemists have contributed to the materials science of plastics, including Nobel laureate Hermann Staudinger, who has been called "the father of polymer chemistry," and Herman Mark, known as "the father of polymer physics."[5]

After World War I, improvements in chemistry led to an explosion of new forms of plastics, with mass production beginning in the 1940s and 1950s.[58] Among the earliest examples in the wave of new polymers were polystyrene (first produced by BASF in the 1930s)[2] and polyvinyl chloride (first created in 1872 but commercially produced in the late 1920s).[2] In 1923, Durite Plastics, Inc., was the first manufacturer of phenol-furfural resins.[59] In 1933, polyethylene was discovered by Imperial Chemical Industries (ICI) researchers Reginald Gibson and Eric Fawcett.[2]

The discovery of polyethylene terephthalate is credited to employees of the Calico Printers' Association in the UK in 1941; it was licensed to DuPont for the US and ICI otherwise, and as one of the few plastics appropriate as a replacement for glass in many circumstances, resulting in widespread use for bottles in Europe.[2] In 1954 polypropylene was discovered by Giulio Natta and began to be manufactured in 1957.[2] Also in 1954 expanded polystyrene (used for building insulation, packaging, and cups) was invented by Dow Chemical.[2]

Environmental effects

A communication campaign infographic showing that there will be more plastic in the oceans than fish by 2050

Because the chemical structure of most plastics renders them durable, they are resistant to many natural degradation processes. Much of this material may persist for centuries or longer, given the demonstrated persistence of structurally similar natural materials such as amber.

There are differing estimates of how much plastic waste has been produced in the last century. By one estimate, one billion tons of plastic waste have been discarded since the 1950s.[60] Others estimate a cumulative human production of 8.3 billion tons of plastic, of which 6.3 billion tons is waste, with only 9% getting recycled.[61]

The Ocean Conservancy reported that China, Indonesia, Philippines, Thailand, and Vietnam dump more plastic into the sea than all other countries combined.[62] The rivers Yangtze, Indus, Yellow, Hai, Nile, Ganges, Pearl, Amur, Niger, and Mekong "transport 88% to 95% of the global [plastics] load into the sea."[63][64][verify quote punctuation]

The presence of plastics, particularly microplastics, within the food chain is increasing. In the 1960s microplastics were observed in the guts of seabirds, and since then have been found in increasing concentrations. [65] The long-term effects of plastics in the food chain are poorly understood. In 2009 it was estimated that 10% of modern waste was plastic,[58] although estimates vary according to region.[65] Meanwhile, 50% to 80% of debris in marine areas is plastic.[65]

Prior to the Montreal Protocol, CFCs had been commonly used in the manufacture of the plastic polystyrene, the production of which had contributed to depletion of the ozone layer.

Efforts to reduce environmental effects of plastics may include reduction of plastics production and use, waste- and recycling-policies, and the proactive development and deployment of alternatives to plastics such as for sustainable packaging.

Microplastics

Microplastics in sediments from four rivers in Germany. Note the diverse shapes indicated by white arrowheads. (The white bars represent 1 mm for scale.)

Microplastics are fragments of any type of plastic[66] less than 5 mm (0.20 in) in length, according to the U.S. National Oceanic and Atmospheric Administration (NOAA)[67][68] and the European Chemicals Agency.[69] They cause pollution by entering natural ecosystems from a variety of sources, including cosmetics, clothing, and industrial processes.

Two classifications of microplastics are currently recognized. Primary microplastics include any plastic fragments or particles that are already 5.0 mm in size or less before entering the environment. These include microfibers from clothing, microbeads, and plastic pellets (also known as nurdles).[70][71][72] Secondary microplastics arise from the degradation (breakdown) of larger plastic products through natural weathering processes after entering the environment. Such sources of secondary microplastics include water and soda bottles, fishing nets, plastic bags, microwave containers, tea bags and tire wear.[73][72][74][75] Both types are recognized to persist in the environment at high levels, particularly in aquatic and marine ecosystems, where they cause water pollution.[76] 35% of all ocean microplastics come from textiles/clothing, primarily due to the erosion of polyester, acrylic, or nylon-based clothing, often during the washing process. [77] However, microplastics also accumulate in the air and terrestrial ecosystems. The term macroplastics is used to differentiate microplastics from larger plastic waste, such as plastic bottles.

Because plastics degrade slowly (often over hundreds to thousands of years),[78][79] microplastics have a high probability of ingestion, incorporation into, and accumulation in the bodies and tissues of many organisms. The toxic chemicals that come from both the ocean and runoff can also biomagnify up the food chain.[80][81] In terrestrial ecosystems, microplastics have been demonstrated to reduce the viability of soil ecosystems and reduce weight of earthworms.[82][83] The cycle and movement of microplastics in the environment are not fully known, but research is currently underway to investigate the phenomenon. Deep layer ocean sediment surveys in China (2020) show the presence of plastics in deposition layers far older than the invention of plastics, leading to suspected underestimation of microplastics in surface sample ocean surveys.[84]

Decomposition of plastics

Plastics degrade by a variety of processes, the most significant of which is usually photo-oxidation. Their chemical structure determines their fate. Polymers' marine degradation takes much longer as a result of the saline environment and cooling effect of the sea, contributing to the persistence of plastic debris in certain environments.[65] Recent studies have shown, however, that plastics in the ocean decompose faster than had been previously thought, due to exposure to the sun, rain, and other environmental conditions, resulting in the release of toxic chemicals such as bisphenol A. However, due to the increased volume of plastics in the ocean, decomposition has slowed down.[85] The Marine Conservancy has predicted the decomposition rates of several plastic products: It is estimated that a foam plastic cup will take 50 years, a plastic beverage holder will take 400 years, a disposable diaper will take 450 years, and fishing line will take 600 years to degrade.[86]

Microbial species capable of degrading plastics are known to science, some of which are potentially useful for disposal of certain classes of plastic waste.

  • In 1975, a team of Japanese scientists studying ponds containing waste water from a nylon factory discovered a strain of Flavobacterium that digests certain byproducts of nylon 6 manufacture, such as the linear dimer of 6-aminohexanoate.[87] Nylon 4 (polybutyrolactam) can be degraded by the ND-10 and ND-11 strands of Pseudomonas sp. found in sludge, resulting in GABA (γ-aminobutyric acid) as a byproduct.[88]
  • Several species of soil fungi can consume polyurethane,[89] including two species of the Ecuadorian fungus Pestalotiopsis. They can consume polyurethane both aerobically and anaerobically (such as at the bottom of landfills).[90]
  • Methanogenic microbial consortia degrade styrene, using it as a carbon source.[91] Pseudomonas putida can convert styrene oil into various biodegradable plastic|biodegradable polyhydroxyalkanoates.[92][93]
  • Microbial communities isolated from soil samples mixed with starch have been shown to be capable of degrading polypropylene.[94]
  • The fungus Aspergillus fumigatus effectively degrades plasticized PVC.[95]: 45–46  Phanerochaete chrysosporium has been grown on PVC in a mineral salt agar.[95]: 76 </ref> P. chrysosporium, Lentinus tigrinus, A. niger, and A. sydowii can also effectively degrade PVC.[95]: 122 
  • Phenol-formaldehyde, commonly known as Bakelite, is degraded by the white rot fungus P. chrysosporium.[96]
  • Acinetobacter has been found to partially degrade low-molecular-weight polyethylene oligomers.[88] When used in combination, Pseudomonas fluorescens and Sphingomonas can degrade over 40% of the weight of plastic bags in less than three months.[97] The thermophilic bacterium Brevibacillus borstelensis (strain 707) was isolated from a soil sample and found capable of using low-density polyethylene as a sole carbon source when incubated at 50°C. Pre-exposure of the plastic to ultraviolet radiation broke chemical bonds and aided biodegradation; the longer the period of UV exposure, the greater the promotion of the degradation.[98]
  • Hazardous molds have been found aboard space stations that degrade rubber into a digestible form.[99]
  • Several species of yeasts, bacteria, algae and lichens have been found growing on synthetic polymer artifacts in museums and at archaeological sites.[100]
  • In the plastic-polluted waters of the Sargasso Sea, bacteria have been found that consume various types of plastic; however, it is unknown to what extent these bacteria effectively clean up poisons rather than simply release them into the marine microbial ecosystem.
  • Plastic-eating microbes also have been found in landfills.[101]
  • Nocardia can degrade PET with an esterase enzyme.[citation needed]
  • The fungus Geotrichum candidum, found in Belize, has been found to consume the polycarbonate plastic found in CDs.[102][103]
  • Futuro houses are made of fiberglass-reinforced polyesters, polyester-polyurethane, and PMMA. One such house was found to be harmfully degraded by Cyanobacteria and Archaea.[104][105]
Manual material triage for recycling.

Recycling

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 and useful products. When performed correctly, this can reduce dependence on landfill, conserve resources and protect the environment from plastic pollution and greenhouse gas emissions.[106][107][108][109] Although recycling rates are increasing, they lag behind those of other recoverable materials, such as aluminium, glass and paper. The global recycling rate in 2015 was 19.5%, while 25.5% was incinerated and the remaining 55% disposed of to landfill.[110] 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.[110]

Recycling is necessary because almost all plastic is non-biodegradable and thus builds-up in the environment, 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.[111][112]

Presently, almost all recycling is perform 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, which is unappealing to industry.[113]

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 has been advocated since the early 1970s,[114] 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 while industry research showed that most plastic could not be economically recycled; all the while increasing the amount of virgin plastic being produced.[115][116]

Climate change

According to one report, plastic contributed greenhouse gases in the equivalent of 850 million tons of carbon dioxide (CO2) to the atmosphere in 2019. Emissions could grow to 1.34 billion tons by 2030. By 2050, plastic could emit 56 billion tons of greenhouse gas emissions, as much as 14% of the earth's remaining carbon budget.[117]

The effect of plastics on global warming is mixed. Plastics are generally made from petroleum, thus the production of plastics creates further emissions. However, due to the lightness and durability of plastic versus glass or metal, plastic may reduce energy consumption. For example, packaging beverages in PET plastic rather than glass or metal is estimated to save 52% in transportation energy.[2]

Production of plastics

Production of plastics from crude oil requires 7.9 to 13.7 kWh/lb (taking into account the average efficiency of US utility stations of 35%). Producing silicon and semiconductors for modern electronic equipment is even more energy consuming: 29.2 to 29.8 kWh/lb for silicon, and about 381 kWh/lb for semiconductors.[118] This is much higher than the energy needed to produce many other materials. For example, to produce iron (from iron ore) requires 2.5-3.2 kWh/lb of energy; glass (from sand, etc.) 2.3–4.4 kWh/lb; steel (from iron) 2.5–6.4 kWh/lb; and paper (from timber) 3.2–6.4 kWh/lb.[119]

Incineration of plastics

Controlled high-temperature incineration, above 850°C for two seconds,[citation needed] performed with selective additional heating, breaks down toxic dioxins and furans from burning plastic, and is widely used in municipal solid waste incineration. Municipal solid waste incinerators also normally include flue gas treatments to reduce pollutants further. This is needed because uncontrolled incineration of plastic produces polychlorinated dibenzo-p-dioxins, a carcinogen (cancer causing chemical). The problem occurs because the heat content of the waste stream varies.[120] Open-air burning of plastic occurs at lower temperatures, and normally releases such toxic fumes.

Pyrolytic disposal

Plastics can be pyrolyzed into hydrocarbon fuels, since plastics include hydrogen and carbon. One kilogram of waste plastic produces roughly a liter of hydrocarbon.[121]

See also

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External links

  • "J. Harry Dubois Collection on the History of Plastics, ca. 1900–1975". Archives Center, National Museum of American History, Smithsonian Institution. Archived from the original on 2006-02-12.
  • "Material Properties of Plastics – Mechanical, Thermal & Electrical Properties". Plastics International. Archived from the original on 2017-03-24.
  • "Plastics Historical Society".
  • "History of plastics, Society of the Plastics Industry". plasticsindustry.org. Archived from the original on 2009-07-06.
  • Knight L (17 May 2014). "A brief history of plastics, natural and synthetic". BBC Magazine.
  • "Timeline of important milestone of plastic injection moulding and plastics". Tangram Technology Ltd. 27 June 2014.