plastic

plastic

[plas-tik]
plastic, any organic material with the ability to flow into a desired shape when heat and pressure are applied to it and to retain the shape when they are withdrawn.

Composition and Types of Plastic

A plastic is made up principally of a binder together with plasticizers, fillers, pigments, and other additives. The binder gives a plastic its main characteristics and usually its name. Thus, polyvinyl chloride is both the name of a binder and the name of a plastic into which it is made. Binders may be natural materials, e.g., cellulose derivatives, casein, or milk protein, but are more commonly synthetic resins. In either case, the binder materials consist of very long chainlike molecules called polymers. Cellulose derivatives are made from cellulose, a naturally occurring polymer; casein is also a naturally occurring polymer. Synthetic resins are polymerized, or built up, from small simple molecules called monomers. Plasticizers are added to a binder to increase flexibility and toughness. Fillers are added to improve particular properties, e.g., hardness or resistance to shock. Pigments are used to impart various colors. Virtually any desired color or shape and many combinations of the properties of hardness, durability, elasticity, and resistance to heat, cold, and acid can be obtained in a plastic.

There are two basic types of plastic: thermosetting, which cannot be resoftened after being subjected to heat and pressure; and thermoplastic, which can be repeatedly softened and remolded by heat and pressure. When heat and pressure are applied to a thermoplastic binder, the chainlike polymers slide past each other, giving the material "plasticity." However, when heat and pressure are initially applied to a thermosetting binder, the molecular chains become cross-linked, thus preventing any slippage if heat and pressure are reapplied.

See epoxy resins; polyacrylics; polycarbonates; polyethylene; polyolefins; polypropylene; polystyrene; polyurethanes; polyvinyl chloride; vinyl plastics.

Molding of Plastic

Plastics are available in the form of bars, tubes, sheets, coils, and blocks, and these can be fabricated to specification. However, plastic articles are commonly manufactured from plastic powders in which desired shapes are fashioned by compression, transfer, injection, or extrusion molding. In compression molding, materials are generally placed immediately in mold cavities, where the application of heat and pressure makes them first plastic, then hard. The transfer method, in which the compound is plasticized by outside heating and then poured into a mold to harden, is used for designs with intricate shapes and great variations in wall thickness. Injection-molding machinery dissolves the plastic powder in a heating chamber and by plunger action forces it into cold molds, where the product sets. The operations take place at rigidly controlled temperatures and intervals. Extrusion molding employs a heating cylinder, pressure, and an extrusion die through which the molten plastic is sent and from which it exits in continuous form to be cut in lengths or coiled.

Environmental Considerations

Plastics are so durable that they will not rot or decay as do natural products such as those made of wood. As a result great amounts of discarded plastic products accumulate in the environment as waste. It has been suggested that plastics could be made to decompose slowly when exposed to sunlight by adding certain chemicals to them. Plastics present the additional problem of being difficult to burn. When placed in an incinerator, they tend to melt quickly and flow downward, clogging the incinerator's grate. They also emit harmful fumes; e.g., burning polyvinyl chloride gives off hydrogen chloride gas.

Development of Plastics

The first important plastic, celluloid, was discovered (c.1869) by the American inventor John W. Hyatt and manufactured by him in 1872; it is a mixture of cellulose nitrate, camphor, and alcohol and is thermoplastic. However, plastics did not come into modern industrial use until after the production (1909) of Bakelite by the American chemist L. H. Baekeland. Bakelite, made by the polymerization of phenol and formaldehyde, is thermosetting. New uses for plastics are continually being discovered. Following World War II optical lenses, artificial eyes, and dentures of acrylic plastics, splints that X rays may pierce, nylon fibers, machine gears, fabric coatings, wall surfacing, and plastic lamination were developed. More recently a hydrophilic, or water-attracting, plastic suitable for use in non-irritating contact lenses has been developed. Among the trade names by which many plastic products are widely known are Plexiglas, Lucite, Polaroid, Cellophane, Vinylite, and Koroseal. Plastics reinforced with fiberglass are used for boats, automobile bodies, furniture, and building panels.

Bibliography

See L. K. Arnold, Introduction to Plastics (1968); J. H. DuBois, Plastics History, U.S.A. (1972); H. D. Junge, Dictionary of Plastics Technology (1987); A. W. Birley et al., Plastics Materials: Properties and Applications (1988).

Surgery to correct disfigurement, restore function, or improve appearance. It may involve reshaping or moving tissues to fill a depression, cover a wound, or improve appearance. Cosmetic surgery solely to improve appearance is not the main focus of plastic surgery. It is utilized after disfigurement by burns or tumour removal or for reconstructive work, and it may involve hiding incisions in skin folds or using buried sutures to hold wounds closed. Reconstructive plastic surgery corrects severe functional impairments, fixes physical abnormalities, and compensates for tissue lost to trauma or surgery. Microsurgery and computerized diagnostic imaging techniques have revolutionized the field.

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Polymers that can be molded or shaped, usually by heat and pressure. Most are lightweight, transparent, tough organic compounds that do not conduct electricity well. They fall into two classes: Thermoplastics (e.g., polyethylene, polystyrene) can be melted and formed again and again; thermosetting plastics, or thermosets (e.g., polyurethane, epoxy), once formed, are destroyed rather than melted by heating. Few plastics contain only the polymer resin; many also contain plasticizers (to change the melting point and make them softer), colorants, reinforcements, and fillers (to improve mechanical properties such as stiffness), and stabilizers and antioxidants (to protect against aging, light, or biological agents). Traditional plastics are not biodegradable (see biodegradability); recycling of plastics, especially thermoplastics, has become an important industry, and the development of low-cost biodegradable plastics and plastic substitutes is a significant pursuit of industrial research. Major commercial uses of plastics include cars, buildings, packaging, textiles, paints, adhesives, pipes, electrical and electronic components, prostheses, toys, brushes, and furniture. Common plastics include polyethylene terephthalate, or PET (beverage bottles), PVC (pipes and hoses), foamed polystyrene, or Styrofoam (insulated food containers), and Lucite (shatterproof windows). Seealso Leo Baekeland.

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Plastic is the general common term for a wide range of synthetic or semisynthetic organic solid materials suitable for the manufacture of industrial products. Plastics are typically polymers of high molecular weight, and may contain other substances to improve performance or reduce costs.

The word derives from the Greek πλαστικός (plastikos), "fit for molding", from πλαστός (plastos) "molded" . It refers to their malleability, or plasticity during manufacture, that allows them to be cast, pressed, or extruded into an enormous variety of shapes—such as films, fibers, plates, tubes, bottles, boxes, and much more. The word is also commonly used an adjective with the sense of "made of plastic" (e.g. "plastic cup", "plastic tubing").

The common word "plastic" should not be confused with the technical adjective "plastic", which is applied to any material which undergoes a permanent change of shape (a "plastic deformation") when strained beyond a certain point. Aluminum, for instance, is "plastic" in this sense, but not "a plastic" in the common sense; while some plastics, in their finished forms, will break before deforming—and therefore are not "plastic" in the technical sense.

Overview

Plastics can be classified by their chemical structure, namely the molecular units that make up the polymer's backbone and side chains. Some important groups in these classifications are the acrylics, polyesters, silicones, polyurethanes, and halogenated plastics. Plastics can also be classified by the chemical process used in their synthesis, e.g. as condensation, polyaddition, cross-linking, etc.

Other classifications are based on qualities that are relevant for manufacturing or product design. Examples of such classes are the thermoplastic and thermoset, elastomer, structural, biodegradable, electrically conductive, etc. Plastics can also be ranked by various physical properties, such as density, tensile strength, glass transition temperature, resistance to various chemical products, etc.

Due to their relatively low cost, ease of manufacture, versatility, and imperviousness to water, plastics are used in an enormous and expanding range of products, from paper clips to spaceships. They have already displaced many traditional materials—such as wood, stone, horn and bone, leather, paper, metal, glass and ceramic—in most of their former uses.

The use of plastics is constrained chiefly by their organic chemistry, which seriously limits their hardness, density, and their ability to resist heat, organic solvents, oxidation, and ionizing radiation. In particular, most plastics will melt or decompose when heated to a few hundred celsius. While plastics can be made electrically conductive to some extent, they are still no match for metals like copper or aluminum. Plastics are still too expensive to replace wood, concrete and ceramic in bulky items like ordinary buildings, bridges, dams, pavement, railroad ties, etc.

Chemical structure

Common thermoplastics range from 20,000 to 500,000 in molecular mass, while thermosets are assumed to have infinite molecular weight. These chains are made up of many repeating molecular units, known as "repeat units", derived from "monomers"; each polymer chain will have several thousand repeat units. The vast majority of plastics are composed of polymers of carbon and hydrogen alone or with oxygen, nitrogen, chlorine or sulfur in the backbone. (Some of commercial interest are silicon based.) The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To vary the properties of plastics, both the repeat unit with different molecular groups "hanging" or "pendant" from the backbone, (usually they are "hung" as part of the monomers before linking monomers together to form the polymer chain). This customization by repeat unit's molecular structure has allowed plastics to become such an indispensable part of twenty first-century life by fine tuning the properties of the polymer.

Some plastics are partially crystalline and partially amorphous in molecular structure, giving them both a melting point (the temperature at which the attractive intermolecular forces are overcome) and one or more glass transitions (temperatures above which the extent of localized molecular flexibility is substantially increased). So-called semi-crystalline plastics include polyethylene, polypropylene, poly (vinyl chloride), polyamides (nylons), polyesters and some polyurethanes. Many plastics are completely amorphous, such as polystyrene and its copolymers, poly (methyl methacrylate), and all thermosets.

History/Types of Plastics

The development of plastics has come from the use of natural plastic materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., rubber, nitrocellulose, collagen, galalite) and finally to completely synthetic molecules (e.g., bakelite, epoxy, polyvinyl chloride, polyethylene).

Rubber

Rubber is an elastic material obtained by "curdling" the milky sap (latex) of certain plants. Natives in Central America and Mexico used rubber before Columbus. In 1839, Charles Goodyear invented vulcanized rubber, a form of natural rubber modified by cross-linking (vulcanization).

Cellulose-based plastics

In 1855, an Englishman from Birmingham named Alexander Parkes developed a "synthetic ivory" which he marketed under the trade name "Parkesine", and which won a bronze medal at the 1862 World's fair in London. Parkesine was made from cellulose (the major component of plant cell walls) treated with nitric acid and 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. By incorporating pigments into the product, it could be made to resemble ivorycamphor{

Bakelite

The first plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods invented in 1909 by Leo Hendrik Baekeland, a Belgian-born American living in New York state. Baekeland was searching for an insulating shellac to coat wires in electric motors and generators. He found that mixtures of phenol (C6H5OH) and formaldehyde (HCOH) formed a sticky mass when mixed together and heated, and the mass became extremely hard if allowed to cool. He continued his investigations and found that the material could be mixed with wood flour, asbestos, or slate dust to create "composite" materials with different properties. Most of these compositions were strong and fire resistant. The only problem was that the material tended to foam during synthesis, and the resulting product was of unacceptable quality.

Baekeland built pressure vessels to force out the bubbles and provide a smooth, uniform product. He publicly announced his discovery in 1912, naming it bakelite. It was originally used for electrical and mechanical parts, finally coming into widespread use in consumer goods in the 1920s. When the Bakelite patent expired in 1930, the Catalin Corporation acquired the patent and began manufacturing Catalin plastic using a different process that allowed a wider range of coloring.

Bakelite was the first true plastic. It was a purely synthetic material, not based on any material or even molecule found in nature. It was also the first thermosetting plastic. Conventional thermoplastics can be molded and then melted again, but thermoset plastics form bonds between polymers strands when cured, creating a tangled matrix that cannot be undone without destroying the plastic. Thermoset plastics are tough and temperature resistant.

Bakelite was cheap, strong, and durable. It was molded into thousands of forms, such as radios, telephones, clocks, and billiard balls. The U.S. government even considered making one-cent coins out of it when World War II caused a copper shortage.

Phenolic plastics have been largely replaced by cheaper and less brittle plastics, but they are still used in applications requiring its insulating and heat-resistant properties. For example, some electronic circuit boards are made of sheets of paper or cloth impregnated with phenolic resin.

Phenolic sheets, rods and tubes are produced in a wide variety of grades under various brand names. The most common grades of industrial phenolic are Canvas, Linen and Paper.

Polystyrene and PVC

After the First World War, improvements in chemical technology led to an explosion in new forms of plastics. Among the earliest examples in the wave of new plastics were "polystyrene" (PS) and "polyvinyl chloride" (PVC), developed by IG Farben of Germany.

Polystyrene is a rigid, brittle, inexpensive plastic that has been used to make plastic model kits and similar knickknacks. It would also be the basis for one of the most popular "foamed" plastics, under the name "styrene foam" or "Styrofoam". Foam plastics can be synthesized 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.

PVC has side chains incorporating chlorine atoms, which form strong bonds. PVC in its normal form is stiff, strong, heat and weather resistant, and is now used for making 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 raingear.

Nylon

The real star of the plastics industry in the 1930s was "polyamide" (PA), far better known by its trade name nylon. Nylon was the first purely synthetic fiber, introduced by DuPont Corporation at the 1939 World's Fair in New York City.

In 1927, DuPont had begun a secret development project designated "Fiber66", under the direction of Harvard chemist Wallace Carothers and chemistry department director Elmer Keiser Bolton. Carothers had been hired to perform pure research, and he worked to understand the new materials' molecular structure and physical properties. He took some of the first steps in the molecular design of the materials.

His work led to the discovery of synthetic nylon fiber, which was very strong but also very flexible. The first application was for bristles for toothbrushes. However, Du Pont's real target was silk, particularly silk stockings. Carothers and his team synthesized a number of different polyamides including polyamide 6.6 and 4.6, as well as polyesters.

It took DuPont twelve years and US$27 million to refine nylon, and to synthesize and develop the industrial processes for bulk manufacture. With such a major investment, it was no surprise that Du Pont spared little expense to promote nylon after its introduction, creating a public sensation, or "nylon mania".

Nylon mania came to an abrupt stop at the end of 1941 when the USA entered World War II. The production capacity that had been built up to produce nylon stockings, or just "nylons", for American women was taken over to manufacture vast numbers of parachutes for fliers and paratroopers. After the war ended, DuPont went back to selling nylon to the public, engaging in another promotional campaign in 1946 that resulted in an even bigger craze, triggering the so called "nylon riots".

Subsequently polyamides 6, 10, 11, and 12 have been developed based on monomers which are ring compounds, e.g. caprolactam.nylon 66 is a material manufactured by condensation polymerisation

Nylons still remain important plastics, and not just for use in fabrics. In its bulk form it is very wear resistant, particularly if oil-impregnated, and so is used to build gears, bearings, bushings, and because of good heat-resistance, increasingly for under-the-hood applications in cars, and other mechanical parts.

Synthetic rubber

A polymer that was critical to the war effort was "synthetic rubber", which was produced in a variety of forms. Synthetic rubbers are not plastics. Synthetic rubbers are elastic materials.

The first synthetic rubber polymer was obtained by Lebedev in 1910. Practical synthetic rubber grew out of studies published in 1930 written independently by American Wallace Carothers, Russian scientist Lebedev and the German scientist Hermann Staudinger. These studies led in 1931 to one of the first successful synthetic rubbers, known as "neoprene", which was developed at DuPont under the direction of E.K. Bolton. Neoprene is highly resistant to heat and chemicals such as oil and gasoline, and is used in fuel hoses and as an insulating material in machinery.

In 1935, German chemists synthesized the first of a series of synthetic rubbers known as "Buna rubbers". These were "copolymers", meaning that their polymers were made up from not one but two monomers, in alternating sequence. One such Buna rubber, known as "GR-S" (Government Rubber Styrene), is a copolymer of butadiene and styrene, became the basis for U.S. synthetic rubber production during World War II.

Worldwide natural rubber supplies were limited and by mid-1942 most of the rubber-producing regions were under Japanese control. Military trucks needed rubber for tires, and rubber was used in almost every other war machine. The U.S. government launched a major (and largely secret) effort to develop and refine synthetic rubber. A principal scientist involved with the effort was Edward Robbins.

By 1944 a total of 50 factories were manufacturing it, pouring out a volume of the material twice that of the world's natural rubber production before the beginning of the war.

After the war, natural rubber plantations no longer had a stranglehold on rubber supplies, particularly after chemists learned to synthesize isoprene. GR-S remains the primary synthetic rubber for the manufacture of tires.

Synthetic rubber would also play an important part in the space race and nuclear arms race. Solid rockets used during World War II used nitrocellulose explosives for propellants, but it was impractical and dangerous to make such rockets very big.

During the war, California Institute of Technology (Caltech) researchers came up with a new solid fuel, based on asphalt fuel mixed with an oxidizer, such as potassium or ammonium perchlorate, plus aluminium powder, which burns very hot. This new solid fuel burned more slowly and evenly than nitrocellulose explosives, and was much less dangerous to store and use, though it tended to flow slowly out of the rocket in storage and the rockets using it had to be stockpiled nose down.

After the war, the Caltech researchers began to investigate the use of synthetic rubbers instead of asphalt as the fuel in the mixture. By the mid-1950s, large missiles were being built using solid fuels based on synthetic rubber, mixed with ammonium perchlorate and high proportions of aluminium powder. Such solid fuels could be cast into large, uniform blocks that had no cracks or other defects that would cause nonuniform burning. 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.

Plastics explosion: acrylic, polyethylene, etc.

Other plastics emerged in the prewar period, though some would not come into widespread use until after the war.

By 1936, American, British, and German companies were producing Polymethyl methacrylate (PMMA), better known as acrylic glass. Although acrylics are now well known for their use in paints and synthetic fibers, such as fake furs, in their bulk form they are actually very hard and more transparent than glass, and are sold as glass replacements under trade names such as "Perspex", "Plexiglas" and "Lucite". These were used to build aircraft canopies during the war, and its main application now is large illuminated signs such as are used in shop fronts or inside large stores, and for the manufacture of vacuum-formed bath-tubs.

Another important plastic, Polyethylene (PE), sometimes known as polythene, was discovered in 1933 by Reginald Gibson and Eric Fawcett at the British industrial giant Imperial Chemical Industries (ICI). This material evolved into two forms, low density polyethylene (LDPE), and high density polyethylene (HDPE).

PEs are cheap, flexible, durable, and chemically resistant. LDPE is used to make films and packaging materials, while HDPE is used for containers, plumbing, and automotive fittings. While PE has low resistance to chemical attack, it was found later that a PE container could be made much more robust by exposing it to fluorine gas, which modified the surface layer of the container into the much tougher polyfluoroethylene.

Polyethylene would lead after the war to an improved material, Polypropylene (PP), which was discovered in the early 1950s by Giulio Natta. It is common in modern science and technology that the growth of the general body of knowledge can lead to the same inventions in different places at about the same time, but polypropylene was an extreme case of this phenomenon, being separately invented about nine times. The ensuing litigation was not resolved until 1989.

Polypropylene managed to survive the legal process and two American chemists working for Phillips Petroleum, J. Paul Hogan and Robert Banks, are now generally credited as the "official" inventors of the material. Polypropylene is similar to its ancestor, polyethylene, and shares polyethylene's low cost, but it is much more robust. It is used in everything from plastic bottles to carpets to plastic furniture, and is very heavily used in automobiles.

Polyurethane (PU) was invented by Friedrich Bayer & Company in 1937, and would come into use after the war, in blown form for mattresses, furniture padding, and thermal insulation. It is also one of the components (in non-blown form) of the fiber spandex.

In 1939, IG Farben filed a patent for polyepoxide or epoxy. Epoxies are a class of thermoset plastic that form cross-links and cure when a catalyzing agent, or hardener, is added. After the war they would come into wide use for coatings, adhesives, and composite materials.

Composites using epoxy as a matrix include glass-reinforced plastic, where the structural element is glass fiber, and carbon-epoxy composites, in which the structural element is carbon fiber. Fiberglass is now often used to build sport boats, and carbon-epoxy composites are an increasingly important structural element in aircraft, as they are lightweight, strong, and heat resistant.

Two chemists named Rex Whinfield and James Dickson, working at a small English company with the quaint name of the "Calico Printer's Association" in Manchester, developed polyethylene terephthalate (PET or PETE) in 1941, and it would be used for synthetic fibers in the postwar era, with names such as polyester, dacron, and "Terylene".

PET is less gas-permeable than other low-cost plastics and so is a popular material for making bottles for Coca-Cola and other carbonated drinks, since carbonation tends to attack other plastics, and for acidic drinks such as fruit or vegetable juices. PET is also strong and abrasion resistant, and is used for making mechanical parts, food trays, and other items that have to endure abuse. PET films are used as a base for recording tape.

One of the most impressive plastics used in the war, and a top secret, was polytetrafluoroethylene (PTFE), better known as Teflon, which could be deposited on metal surfaces as a scratch-proof and corrosion-resistant, low-friction protective coating. The polyfluoroethylene surface layer created by exposing a polyethylene container to fluorine gas is very similar to Teflon.

A Du Pont chemist named Roy Plunkett discovered Teflon by accident in 1938. During the war, it was used in gaseous-diffusion processes to refine uranium for the atomic bomb, as the process was highly corrosive. By the early 1960s, Teflon adhesion-resistant frying pans were in demand.

Teflon was later used to synthesize the breathable fabric Gore-Tex, which can be used to manufacture wet weather clothing that is able to "breathe". Its structure allows water vapour molecules to pass, while not permitting water as liquid to enter. Gore-Tex is also used for surgical applications such as garments and implants; Teflon strand is used to make dental floss; and Teflon mixed with fluorine compounds is used to make decoy flares dropped by aircraft to distract heat-seeking missiles.

After the war, the new plastics that had been developed entered the consumer mainstream in a flood. New manufacturing techniques were developed, using various forming, molding, casting, and extrusion processes, to churn out plastic products in vast quantities. American consumers enthusiastically adopted the endless range of colorful, cheap, and durable plastic gimmicks being produced for new suburban home life.

One of the most visible parts of this plastics invasion was Earl Tupper's Tupperware, a complete line of sealable polyethylene food containers that Tupper cleverly promoted through a network of housewives who sold Tupperware as a means of bringing in some money. The Tupperware line of products was well thought out and highly effective, greatly reducing spoilage of foods in storage. Thin-film plastic wrap that could be purchased in rolls also helped keep food fresh.

Another prominent element in 1950s homes was Formica, a plastic laminate that was used to surface furniture and cabinetry. Formica was durable and attractive. It was particularly useful in kitchens, as it did not absorb, and could be easily cleaned of stains from food preparation, such as blood or grease. With Formica, a very attractive and well-built table could be built using low-cost and lightweight plywood with Formica covering, rather than expensive and heavy hardwoods like oak or mahogany.

Composite materials like fiberglass came into use for building boats and, in some cases, cars. Polyurethane foam was used to fill mattresses, and Styrofoam was used to line ice coolers and make float toys.

Plastics continue to be improved. General Electric introduced Lexan, a high-impact polycarbonate plastic, in the 1970s. Du Pont developed Kevlar, an extremely strong synthetic fiber that was best known for its use in ballistic rated clothing and combat helmets. Kevlar was so impressive that its manufacturer, DuPont, deemed it necessary to release an official statement denying alien involvement.

Toxicity

Due to their insolubility in water and relative chemical inertness, pure plastics generally have low toxicity in their finished state, and will pass through the digestive system with no ill effect (other than mechanical damage or obstruction).

However, plastics often contain a variety of toxic additives. For example, plasticizers like adipates and phthalates are often added to brittle plastics like polyvinyl chloride (PVC) to make them pliable enough for use in food packaging, children's toys and teethers, tubing, shower curtains and other items. Traces of these chemicals can leach out of the plastic when it comes into contact with food. Out of these concerns, the European Union has banned the use of DEHP (di-2-ethylhexyl phthalate), the most widely used plasticizer in PVC. Some compounds leaching from polystyrene food containers have been found to interfere with hormone functions and are suspected human carcinogens.

Moreover, while the finished plastic may be non-toxic, the monomers used in its manufacture may be toxic; and small amounts of those chemical may remain trapped in the product. The World Health Organization's International Agency for Research on Cancer (IARC) has recognized the chemical used to make PVC, vinyl chloride, as a known human carcinogen. Some polymers may also decompose into the monomers or other toxic substances when heated.

The primary building block of polycarbonates, bisphenol A (BPA), is an estrogen-like hormone disrupter that may leach into food.. Research in Environmental Health Perspectives finds that BPA leached from the lining of tin cans, dental sealants and polycarbonate bottles can increase body weight of lab animals' offspring. 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.

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. Toxic chemicals allegedly released by the reuse of water bottles have been the subject of urban legend.

Environmental issues

Plastics are durable and degrade very slowly. In some cases, burning plastic can release toxic fumes. Also, the manufacturing of plastics often creates large quantities of chemical pollutants.

Prior to the ban on the use of CFCs in extrusion of polystyrene (and general use, except in life-critical fire suppression systems; see Montreal Protocol), the production of polystyrene contributed to the depletion of the ozone layer; however, non-CFCs are currently used in the extrusion process.

By 1995, plastic recycling programs were common in the United States and elsewhere. Thermoplastics can be remelted and reused, and thermoset plastics can be ground up and used as filler, though the purity of the material tends to degrade with each reuse cycle. There are methods by which plastics can be broken back down to a feedstock state.

To assist recycling of disposable items, the Plastic Bottle Institute of the Society of the Plastics Industry devised a now-familiar scheme to mark plastic bottles by plastic type. A plastic container using this scheme is marked with a triangle of three "chasing arrows", which encloses a number giving the plastic type:

Plastics type marks: the Resin identification code

  1. PET (PETE), polyethylene terephthalate: Commonly found on 2-liter soft drink bottles, cooking oil bottles, peanut butter jars.
  2. HDPE, high-density polyethylene: Commonly found on detergent bottles, milk jugs.
  3. PVC, polyvinyl chloride: Commonly found on plastic pipes, outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging.
  4. LDPE, low-density polyethylene: Commonly found on dry-cleaning bags, produce bags, trash can liners, food storage containers.
  5. PP, polypropylene: Commonly found on bottle caps, drinking straws, yogurt containers.
  6. PS, polystyrene: Commonly found on "packing peanuts", cups, plastic tableware, meat trays, take-away food clamshell containers
  7. OTHER, other: This plastic category, as its name of "other" implies, is any plastic other than the named #1–#6, Commonly found on certain kinds of food containers, Tupperware, and Nalgene bottles.

Unfortunately, recycling plastics has proven difficult. The biggest problem with plastic recycling is that it is difficult to automate the sorting of plastic waste, and so it is labor intensive. Typically, workers sort the plastic by looking at the resin identification code, though common containers like soda bottles can be sorted from memory. Other recyclable materials, such as metals, are easier to process mechanically. However, new mechanical sorting processes are being utilized to increase plastic recycling capacity and efficiency.

While containers are usually made from a single type and color of plastic, making them relatively easy to sort out, a consumer product like a cellular phone may have many small parts consisting of over a dozen different types and colors of plastics. In a case like this, the resources it would take to separate the plastics far exceed their value and the item is discarded. However, developments are taking place in the field of Active Disassembly, which may result in more consumer product components being re-used or recycled. Recycling certain types of plastics can be unprofitable, as well. For example, polystyrene is rarely recycled because it is usually not cost effective. These unrecyclable wastes can be disposed of in landfills, incinerated or used to produce electricity at waste-to-energy plants.

Biodegradable plastics

Research has been done on biodegradable plastics that break down with exposure to sunlight (e.g. ultra-violet radiation), water or dampness, bacteria, enzymes, wind abrasion and some instances rodent pest or insect attack are also included as forms of biodegradation or environmental degradation. It is clear some of these modes of degradation will only work if the plastic is exposed at the surface, while other modes will only be effective if certain conditions are found in landfill or composting systems. Starch powder has been mixed with plastic as a filler to allow it to degrade more easily, but it still does not lead to complete breakdown of the plastic. Some researchers have actually genetically engineered bacteria that synthesize a completely biodegradable plastic, but this material, such as Biopol, is expensive at present. The German chemical company BASF makes Ecoflex, a fully biodegradable polyester for food packaging applications.

A potential disadvantage of biodegradable plastics is that the carbon that is locked up in them is released into the atmosphere as a greenhouse gas carbon dioxide when they degrade, though if they are made from natural materials, such as vegetable crop derivatives or animal products, there is no net gain in carbon dioxide emissions, although concern will be for a worse greenhouse gas, methane release. Of course, incinerating non-biodegradable plastics will release carbon dioxide as well, while disposing of it in landfills will release methane when the plastic does eventually break down.

So far, these plastics have proven too costly and limited for general use, and critics have pointed out that the only real problem they address is roadside litter, which is regarded as a secondary issue. When such plastic materials are dumped into landfills, they can become "mummified" and persist for decades even if they are supposed to be biodegradable.

There have been some success stories. The Courtauld concern, the original producer of rayon, came up with a revised process for the material in the mid-1980s to produce "Tencel". Tencel has many superior properties over rayon, but is still produced from "biomass" feedstocks, and its manufacture is extraordinarily clean by the standards of plastic production.

Researchers at the University of Illinois at Urbana have been working on developing biodegradable resins, sheets and films made with zein (corn protein).

Recently, however, a new type of biodegradable resin has made its debut in the United States, called Plastarch Material (PSM). It is heat, water, and oil resistant and sees a 70% degradation in 90 days. Biodegradable plastics based on polylactic acid (once derived from dairy products, now from cereal crops such as maize) have entered the marketplace, for instance as polylactates as disposable sandwich packs.

An alternative to starch-based resins are additives such as Bio-Batch an additive that allows the manufacturers to make PE, PS, PP, PET, and PVC totally biodegradable in landfills where 94.8% of most plastics end up, according to the EPA's latest MSW report located under "Municipal Solid Waste in the United States": 2003 Data Tables.

It is also possible that bacteria will eventually develop the ability to degrade plastics. This has already happened with nylon: two types of nylon eating bacteria, Flavobacteria and Pseudomonas, were found in 1975 to possess enzymes (nylonase) capable of breaking down nylon. While not a solution to the disposal problem, it is likely that bacteria will evolve the ability to use other synthetic plastics as well. In 2008, a 16-year-old boy reportedly isolated two plastic-consuming bacteria.

The latter possibility was in fact the subject of a cautionary novel by Kit Pedler and Gerry Davis (screenwriter), the creators of the Cybermen, re-using the plot of the first episode of their Doomwatch series. The novel, "Mutant 59: The Plastic Eater", written in 1971, is the story of what could happen if a bacterium were to evolve—or be artificially cultured—to eat plastics, and be let loose in a major city.

Bioplastics

Some plastics can be obtained from biomass, including:

Price, environment, and the future

The biggest threat to the conventional plastics industry is most likely to be environmental concerns, including the release of toxic pollutants, greenhouse gas, litter, biodegradable and non-biodegrable landfill impact as a result of the production and disposal of petroleum and petroleum-based plastics. Of particular concern has been the recent accumulation of enormous quantities of plastic trash in ocean gyres, particularly the North Pacific Gyre, now known informally as the Great Pacific Garbage Patch or the Pacific Trash Vortex.

For decades one of the great appeals of plastics has been their low price. Yet in recent years the cost of plastics has been rising dramatically. A major cause is the sharply rising cost of petroleum, the raw material that is chemically altered to form commercial plastics.

With some observers suggesting that future oil reserves are uncertain, the price of petroleum may increase further. Therefore, alternatives are being sought. Oil shale and tar oil are alternatives for plastic production but are expensive. Scientists are seeking cheaper and better alternatives to petroleum-based plastics, and many candidates are in laboratories all over the world. One promising alternative may be fructose .

Common plastics and uses

Polypropylene (PP) :Food containers, appliances, car fenders (bumpers). Polystyrene (PS) :Packaging foam, food containers, disposable cups, plates, cutlery, CD and cassette boxes. High impact polystyrene (HIPS) : fridge liners, food packaging, vending cups. Acrylonitrile butadiene styrene (ABS) :Electronic equipment cases (e.g., computer monitors, printers, keyboards), drainage pipe. Polyethylene terephthalate (PET) :carbonated drinks bottles, jars, plastic film, microwavable packaging. Polyester (PES) :Fibers, textiles. Polyamides (PA) (Nylons) :Fibers, toothbrush bristles, fishing line, under-the-hood car engine mouldings. Poly(vinyl chloride) (PVC) :Plumbing pipes and guttering, shower curtains, window frames, flooring. Polyurethanes (PU) : cushioning foams, thermal insulation foams, surface coatings, printing rollers. (Currently 6th or 7th most commonly used plastic material, for instance the most commonly used plastic found in cars). Polycarbonate (PC) :Compact discs, eyeglasses, riot shields, security windows, traffic lights, lenses. Polyvinylidene chloride (PVDC) (Saran) :Food packaging. Polyethylene (PE) :Wide range of inexpensive uses including supermarket bags, plastic bottles. Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) :A blend of PC and ABS that creates a stronger plastic. :Car Interior and exterior parts

Special-purpose plastics

Polymethyl methacrylate (PMMA) : contact lenses, glazing (best known in this form by its various trade names around the world, e.g., Perspex, Oroglas, Plexiglas), aglets, fluorescent light diffusers, rear light covers for vehicles. Polytetrafluoroethylene (PTFE) (trade name Teflon) :Heat-resistant, low-friction coatings, used in things like non-stick surfaces for frying pans, plumber's tape and water slides. Polyetheretherketone (PEEK) (Polyetherketone):Strong, chemical- and heat-resistant thermoplastic, biocompatibility allows for use in medical implant applications, aerospace mouldings. One of the most expensive commercial polymers. Polyetherimide (PEI) (Ultem) :A high temperature, chemically stable polymer that does not crystallize. Phenolics (PF) or (phenol formaldehydes) : high modulus, relatively heat resistant, and excellent fire resistant polymer. Used for insulating parts in electrical fixtures, paper laminated products (e.g. "Formica"), thermally insulation foams. It is a thermosetting plastic, with the familiar trade name Bakelite, that can be moulded by heat and pressure when mixed with a filler-like wood flour or can be cast in its unfilled liquid form or cast as foam, e.g. "Oasis". Problems include the probability of mouldings naturally being dark colours (red, green, brown), and as thermoset difficult to recycle. Urea-formaldehyde (UF) : one of the aminoplasts and used as multi-colorable alternative to Phenolics. Used as a wood adhesive (for plywood, chipboard, hardboard) and electrical switch housings. Melamine formaldehyde (MF) : one of the aminoplasts, and used a multi-colorable alternative to phenolics, for instance in mouldings (e.g. break-resistance alternatives to ceramic cups, plates and bowls for children) and the decorated top surface layer of the paper laminates (e.g. "Formica"). Polylactic acid : a biodegradable, thermoplastic, found converted into a variety of aliphatic polyesters derived from lactic acid which in turn can be made by fermentation of various agricultural products such as corn starch, once made from diary products. Plastarch material : biodegradable and heat resistant, thermoplastic composed of modified corn starch.

See also

References

External links

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