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A plastic material is any of a wide range of synthetic or semi-synthetic organic solids that are moldable. Plastics are typically organic polymers of high molecular mass, but they often contain other substances. They are usually synthetic, most commonly derived from petrochemicals, but many are partially natural.
Most plastics contain organic polymers. The vast majority of these polymers are based on chains of carbon atoms alone or with oxygen, sulfur, or nitrogen as well. The backbone is that part of the chain on the main "path" linking a large number of repeat units together. To customize the properties of a plastic, different molecular groups "hang" from the backbone (usually they are "hung" as part of the monomers before the monomers are linked together to form the polymer chain). The structure of these "side chains" influence the properties of the polymer. This fine tuning of the properties of the polymer by repeating unit's molecular structure has allowed plastics to become an indispensable part of the twenty-first century world.
Most plastics contain other organic or inorganic compounds blended in. The amount of additives ranges from zero percentage for polymers used to wrap foods to more than 50% for certain electronic applications. The average content of additives is 20% by weight of the polymer. Fillers improve performance and/or reduce production costs. Stabilizing additives include fire retardants to lower the flammability of the material. Many plastics contain fillers, relatively inert and inexpensive materials that make the product cheaper by weight. Typically fillers are mineral in origin, e.g., chalk. Some fillers are more chemically active and are called reinforcing agents. Since many organic polymers are too rigid for particular applications, they are blended with plasticizers (the largest group of additives), oily compounds that confer improved rheology. Colorants are common additives, although their weight contribution is small. Many of the controversies associated with plastics are associated with the additives. Organotin compounds are particularly toxic.
Plastics are usually classified by their chemical structure of 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, such as condensation, polyaddition, and cross-linking.
There are two types of plastics: thermoplastics and thermosetting polymers. Thermoplastics are the plastics that do not undergo chemical change in their composition when heated and can be molded again and again. Examples include polyethylene, polypropylene, polystyrene and polyvinyl chloride. Common thermoplastics range from 20,000 to 500,000 amu, 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 repeating units.
Thermosets can melt and take shape once; after they have solidified, they stay solid. In the thermosetting process, a chemical reaction occurs that is irreversible. The vulcanization of rubber is a thermosetting process. Before heating with sulfur, the polyisoprene is a tacky, slightly runny material, but after vulcanization the product is rigid and non-tacky.
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, and electrically conductive. Plastics can also be classified by various physical properties, such as density, tensile strength, glass transition temperature, and resistance to various chemical products.
Biodegradable plastics break down (degrade) upon exposure to sunlight (e.g., ultra-violet radiation), water or dampness, bacteria, enzymes, wind abrasion, and in some instances, rodent, pest, or insect attack are also included as forms of biodegradation or environmental degradation. Some modes of degradation require that the plastic be exposed at the surface, whereas other modes will only be effective if certain conditions exist 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. Companies have made biodegradable additives to enhance the biodegradation of plastics.
Most plastics are produced from petrochemicals. Motivated by the finiteness of petrochemical reserves and threat of global warming, bioplastics are being developed. Bioplastics are made substantially from renewable plant materials such as cellulose and starch.
In comparison to the global consumption of all flexible packaging, estimated at 12.3 million tonnes/year, estimates put global production capacity at 327,000 tonnes/year for related bio-derived materials.
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). The 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.
In 1600 BC, Mesoamericans used natural rubber for balls, bands, and figurines. Early plastics were bio-derived materials such as egg and blood proteins, which are organic polymers. 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 (casein) with lye. In the 1800s, the development of plastics accelerated with Charles Goodyear's discovery of vulcanization as a route to thermoset materials derived from natural rubber. Many storied materials were reported as industrial chemistry was developed in the 1800s. In the early 1900s, Bakelite, the first fully synthetic thermoset was reported by Belgian chemist Leo Baekeland. In 1933, polyethylene was discovered by Imperial Chemical Industries (ICI) researchers Reginald Gibson and Eric Fawcett. After the First World War, improvements in chemical technology led to an explosion in new forms of plastics; mass production began around the 1940s and 1950s. Polypropylene was found in 1954 by Giulio Natta and began to be manufactured in 1957. Among the earliest examples in the wave of new polymers were polystyrene (PS), first produced by BASF in the 1930s, and polyvinyl chloride (PVC), first created in 1872 but commercially produced in the late 1920s. In 1954, expanded polystyrene (used for building insulation, packaging, and cups) was invented by Dow Chemical. Polyethylene terephthalate (PET)'s discovery is credited to employees of the Calico Printers' Association in the UK in 1941; it was licensed to DuPont for the USA 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. 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).
The plastic material parkesine was patented by Alexander Parkes, In Birmingham, UK in 1856. It was unveiled at the 1862 Great International Exhibition in London. Parkesine 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 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. By incorporating pigments into the product, it could be made to resemble ivory.
The first plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods invented in 1907, by Leo Hendrik Baekeland, a Belgian-born American living in New York state. Baekeland was looking for an insulating shellac to coat wires in electric motors and generators. He found that combining phenol (C6H5OH) and formaldehyde (HCOH) formed a sticky mass and later found 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, as he announced in 1909, in a meeting of the American Chemical Society. Bakelite was originally used for electrical and mechanical parts, coming into widespread use in consumer goods and jewelry in the 1920s. Bakelite was a purely synthetic material, not derived from living matter. It was also an early thermosetting plastic.
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, commonly called "vinyl") incorporates chlorine atoms. The C-Cl bonds in the backbone are hydrophobic and resist oxidation (and burning). PVC is stiff, strong, heat and weather resistant, properties that recommend its 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. Because hydrogen chloride readily combines with water vapor in the air to form hydrochloric acid, polyvinyl chloride is not recommended for long-term archival storage of silver, photographic film or paper (mylar is preferable).
The plastics industry was revolutionized in the 1930s with the announcement of 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.
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, plain bearings, valve seats, seals and because of good heat-resistance, increasingly for under-the-hood applications in cars, and other mechanical parts.
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.
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 U.S. 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.
The properties of plastics are defined chiefly by the organic chemistry of the polymer such as hardness, density, and resistance to heat, organic solvents, oxidation, and ionizing radiation. In particular, most plastics will melt upon heating to a few hundred degrees celsius. While plastics can be made electrically conductive, with the conductivity of up to 80 kS/cm in stretch-oriented polyacetylene, they are still no match for most metals like copper which have conductivities of several hundreds kS/cm.
Many properties of plastics are determined by tests as specified by Underwriters Laboratories, such as:
Many properties of plastics are determined by standards as specified by ISO, such as:
Pure plastics have low toxicity due to their insolubility in water and because they are biochemically inert, due to a large molecular weight. Plastic products contain a variety of additives, some of which can be toxic. For example, plasticizers like adipates and phthalates are often added to brittle plastics like polyvinyl chloride 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 European Union has restricted the use of DEHP (di-2-ethylhexyl phthalate) and other phthalates in some applications. Some compounds leaching from polystyrene food containers have been proposed to interfere with hormone functions and are suspected human carcinogens. Other chemicals of potential concern include alkylphenols.
Whereas the finished plastic may be non-toxic, the monomers used in the manufacture of the 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.
Some polymers may also decompose into the monomers or other toxic substances when heated. In 2011, it was reported that "almost all plastic products" sampled released chemicals with estrogenic activity, although the researchers identified plastics which did not leach chemicals with estrogenic activity.
The primary building block of polycarbonates, bisphenol A (BPA), is an estrogen-like endocrine disruptor 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.
As of January 2010, the LA Times newspaper reports that the United States FDA is spending $30 million to investigate indications of BPA being linked to cancer.
Most plastics are durable and degrade very slowly; the very chemical bonds that make them so durable tend to make them resistant to most natural processes of degradation. However, microbial species and communities capable of degrading plastics are discovered from time to time, and some show promise as being useful for bioremediating certain classes of plastic waste.
Since the 1950s, one billion tons of plastic have been discarded and some of that material might persist for centuries or much longer, as is demonstrated by the persistence of natural materials such as amber.
Serious environmental threats from plastic have been suggested in the light of the increasing presence of microplastics in the marine food chain along with many highly toxic chemical pollutants that accumulate in plastics. They also accumulate in larger fragmented pieces of plastic called nurdles. In the 1960s the latter were observed in the guts of seabirds, and since then have been found in increasing concentration. In 2009, it was estimated that 10% of modern waste was plastics, although estimates vary according to region. Meanwhile, 50-80% of debris in marine areas is plastic.
Before the ban on the use of CFCs in extrusion of polystyrene (and in general use, except in life-critical fire suppression systems; see Montreal Protocol), the production of polystyrene contributed to the depletion of the ozone layer, but current extrusion processes use non-CFCs.
The effect of plastics on global warming is mixed. Plastics are generally made from petroleum. If the plastic is incinerated, it increases carbon emissions; if it is placed in a landfill, it becomes a carbon sink although biodegradable plastics have caused methane emissions. Due to the lightness 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.
Production of plastics from crude oil requires 62 to 108 MJ of energy per kilogram (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: 230 to 235 MJ per 1 kilogram of silicon, and about 3,000 MJ per kilogram of semiconductors. This is much higher, compared to many other materials, e.g. production of iron from iron ore requires 20-25 MJ of energy, glass (from sand, etc.) - 18-35 MJ, steel (from iron) - 20-50 MJ, paper (from timber) - 25-50 MJ per kilogram.
Controlled high-temperature incineration, above 850C for two seconds, 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 as the heat content of the waste stream varies. Open-air burning of plastic occurs at lower temperatures, and normally releases such toxic fumes.
Thermoplastics can be remelted and reused, and thermoset plastics can be ground up and used as filler, although 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.
The greatest challenge to the recycling of plastics is the difficulty of automating the sorting of plastic wastes, making it labor-intensive. Typically, workers sort the plastic by looking at the resin identification code, although common containers like soda bottles can be sorted from memory. Typically, the caps for PETE bottles are made from a different kind of plastic which is not recyclable, which presents additional problems to the automated sorting process. Other recyclable materials such as metals are easier to process mechanically. However, new processes of mechanical sorting are being developed to increase capacity and efficiency of plastic recycling.
While containers are usually made from a single type and color of plastic, making them relatively easy to be sorted, a consumer product like a cellular phone may have many small parts consisting of over a dozen different types and colors of plastics. In such cases, 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 unrecycled wastes are typically disposed of in landfills, incinerated or used to produce electricity at waste-to-energy plants.
A first success in recycling of plastics is Vinyloop, a recycling process and an approach of the industry to separate PVC from other materials through a process of dissolution, filtration and separation of contaminations. A solvent is used in a closed loop to elute PVC from the waste. This makes it possible to recycle composite structure PVC waste which normally is being incinerated or put in a landfill. Vinyloop-based recycled PVC's primary energy demand is 46 percent lower than conventional produced PVC. The global warming potential is 39 percent lower. This is why the use of recycled material leads to a significant better ecological footprint. This process was used after the Olympic Games in London 2012. Parts of temporary Buildings like the Water Polo Arena or the Royal Artillery Barracks were recycled. This way, the PVC Policy could be fulfilled which says that no PVC waste should be left after the games.
In 1988, 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:
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.
In developed countries, about a third of plastic is used in packaging and another third in buildings such as piping used in plumbing or vinyl siding. Other uses include automobiles (up to 20% plastic), furniture, and toys. In the developing world, the ratios may be different - for example, reportedly 42% of India's consumption is used in packaging.
The word plastic is derived from the Greek πλαστικός (plastikos) meaning "capable of being shaped or molded", from πλαστός (plastos) meaning "molded". It refers to their malleability, or plasticity during manufacture, that allows them to be cast, pressed, or extruded into a variety of shapes—such as films, fibers, plates, tubes, bottles, boxes, and much more.
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 (plastic deformation) when strained beyond a certain point. Aluminum which is stamped or forged, for instance, exhibits plasticity in this sense, but is not plastic in the common sense; in contrast, in their finished forms, some plastics will break before deforming and therefore are not plastic in the technical sense.
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