2008/9 Schools Wikipedia Selection. Related subjects: Materials science
Plastic is the general term for a wide range of synthetic or semisynthetic polymerization products. They are composed of organic condensation or addition polymers and may contain other substances to improve performance or reduce costs. There are many natural polymers generally considered to be "plastics". Plastics can be formed into objects or films or fibers. Their name is derived from the malleability, or plasticity, of many of them.
Plastic can be classified in many ways, but most commonly by their polymer backbone ( polyvinyl chloride, polyethylene, polymethyl methacrylate, and other acrylics, silicones, polyurethanes, etc.). Other classifications include thermoplastic, thermoset, elastomer, engineering plastic, addition or condensation or polyaddition (depending on polymerization method used), and glass transition temperature or Tg.
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.
Plastics are polymers: long chains of atoms bonded to one another. Common thermoplastics range from 20,000 to 500,000 in molecular weight, 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 1000's of 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.
People experimented with plastics based on natural polymers for centuries. In the nineteenth century a plastic material based on chemically modified natural polymers was discovered: Charles Goodyear discovered vulcanization of rubber (1839) and Alexander Parkes, English inventor (1813—1890) created the earliest form of plastic in 1855. He mixed pyroxylin, a partially nitrated form of cellulose (cellulose is the major component of plant cell walls), with alcohol and camphor. This produced a hard but flexible transparent material, which he called "Parkesine." The first plastic based on a synthetic polymer was made from phenol and formaldehyde, with the first viable and cheap synthesis methods invented by Leo Hendrik Baekeland in 1909, the product being known as Bakelite. Subsequently poly (vinyl chloride), polystyrene, polyethylene (polyethene), polypropylene (polypropene), polyamides (nylons), polyesters, acrylics, silicones, polyurethanes were amongst the many varieties of plastics developed and have great commercial success.
The development of plastics has come from the use of natural materials (e.g., chewing gum, shellac) to the use of chemically modified natural materials (e.g., natural rubber, nitrocellulose, collagen) and finally to completely synthetic molecules (e.g., epoxy, polyvinyl chloride, polyethylene).
In 1959, Koppers Company in Pittsburgh, PA had a team that developed the expandable polystyrene (EPS) foam. On this team was Edward J. Stoves who made the first commercial foam cup. The experimental cups were made of puffed rice glued together to form a cup to show how it would feel and look. The chemistry was then developed to make the cups commercial. Today, the cup is used throughout the world in countries desiring fast food, such as the United States, Japan, Australia, and New Zealand. Freon was never used in the cups. As Stoves said, "We didn't know freon was bad for the ozone, but we knew it was not good for people so the cup never used freon to expand the beads."
The foam cup can be buried, and it is as stable as concrete and brick. No plastic film is required to protect the air and underground water. If it is properly incinerated at high temperatures, the only chemicals generated are water, carbon dioxide and carbon ash. If burned without enough oxygen or at lower temperatures (as in a campfire or household fireplace) it can produce carbon black and carbon dioxide. EPS can be recycled to make park benches, flower pots and toys.
Cellulose-based plastics: celluloid and rayon
All Goodyear had done with vulcanization was improve the properties of a natural polymer. The next logical step was to use a natural polymer, cellulose, as the basis for a new material.
Inventors were particularly interested in developing synthetic substitutes for those natural materials that were expensive and in short supply, since that meant a profitable market to exploit. Ivory was a particularly attractive target for a synthetic replacement.
An Englishman from Birmingham named Alexander Parkes developed a "synthetic ivory" named "pyroxlin", 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 treated with nitric acid and a solvent. The output of the process hardened into a hard, ivory-like material that could be molded when heated. However, Parkes was not able to scale up the process reliably, and products made from Parkesine quickly warped and cracked after a short period of use.
Englishmen Daniel Spill and the American John Wesley Hyatt both took up where Parkes left off. Parkes had failed for lack of a proper softener, but they independently discovered that camphor would work well. Spill launched his product as Xylonite in 1869, while Hyatt patented his " Celluloid" in 1870, naming it after cellulose. Rivalry between Spill's British Xylonite Company and Hyatt's American Celluloid Company led to an expensive decade-long court battle, with neither company being awarded rights, as ultimately Parkes was credited with the product's invention. As a result, both companies operated in parallel on both sides of the Atlantic.
Celluloid/Xylonite proved extremely versatile in its field of application, providing a cheap and attractive replacement for ivory, tortoiseshell, and bone, and traditional products such as billiard balls and combs were much easier to fabricate with plastics. Some of the items made with cellulose in the nineteenth century were beautifully designed and implemented. For example, celluloid combs made to tie up the long tresses of hair fashionable at the time are now highly-collectable jewel-like museum pieces. Such pretty trinkets were no longer only for the rich.
Hyatt was something of an industrial genius who understood what could be done with such a shapeable, or "plastic", material, and proceeded to design much of the basic industrial machinery needed to produce good-quality plastic materials in quantity. Some of Hyatt's first products were dental pieces, and sets of false teeth built around celluloid proved cheaper than existing rubber dentures. However, celluloid dentures tended to soften when hot, making tea drinking tricky, and the camphor taste tended to be difficult to suppress.
Celluloid's real breakthrough products were waterproof shirt collars, cuffs, and the false shirtfronts known as " dickies", whose unmanageable nature later became a stock joke in silent-movie comedies. They did not wilt and did not stain easily, and Hyatt sold them by trainloads. Corsets made with celluloid stays also proved popular, since perspiration did not rust the stays, as it would if they had been made of metal.
Celluloid could also be used in entirely new applications. Hyatt figured out how to fabricate the material in a strip format for movie film. By the year 1900, movie film was a major market for celluloid.
However, celluloid still tended to yellow and crack over time, and it had another more dangerous defect: it burned very easily and spectacularly, unsurprising given that mixtures of nitric acid and cellulose are also used to synthesize smokeless powder.
Ping-pong balls, one of the few products still made with celluloid, sizzle and burn if set on fire, and Hyatt liked to tell stories about celluloid billiard balls exploding when struck very hard. These stories might have had a basis in fact, since the billiard balls were often celluloid covered with paints based on another, even more flammable, nitrocellulose product known as " collodion". If the balls had been imperfectly manufactured, the paints might have acted as primer to set the rest of the ball off with a bang.
Cellulose was also used to produce cloth. While the men who developed celluloid were interested in replacing ivory, those who developed the new fibers were interested in replacing another expensive material, silk.
In 1884, a French chemist, the Comte de Chardonnay, introduced a cellulose-based fabric that became known as "Chardonnay silk". It was an attractive cloth, but like celluloid it was very flammable, a property completely unacceptable in clothing. After some ghastly accidents, Chardonnay silk was taken off the market.
In 1894, three British inventors, Charles Cross, Edward Bevan, and Clayton Beadle, patented a new "artificial silk" or "art silk" that was much safer. The three men sold the rights for the new fabric to the French Courtauld company, a major manufacturer of silk, which put it into production in 1905, using cellulose from wood pulp as the "feedstock" material.
Art silk, technically known as Cellulose Acetate, became well known under the trade name " rayon", and was produced in great quantities through the 1930s, when it was supplanted by better artificial fabrics. It still remains in production today, often in blends with other natural and artificial fibers. It is cheap and feels smooth on the skin, though it is weak when wet and creases easily. It could also be produced in a transparent sheet form known as " cellophane". Cellulose Acetate became the standard substrate for movie and camera film, instead of its very flammable predecessor.
The limitations of cellulose led to the next major advance, known as "phenolic" or "phenol-formaldehyde" plastics. A chemist named Leo Hendrik Baekeland, a Belgian-born American living in New York state, was searching for an insulating shellac to coat wires in electric motors and generators. Baekeland 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, of course, 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 floatation 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.
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 fibre, introduced by Du Pont Corporation at the 1939 World's Fair in New York City.
In 1927, Du Pont 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 fibre, 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 polyamide6.6 and 4.6, as well as polyesters.
It took Du Pont 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, Du Pont 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.
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 Plexiglas and Lucite. Plexiglas was used to build aircraft canopies during the war, and it is also now used as a marble replacement for countertops.
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 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 fibre 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 fibre, and carbon-epoxy composites, in which the structural element is carbon fibre. Fibreglass 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 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 fibreglass 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 fibre 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.
Negative health effects
Some plastics have been associated with negative health effects.
Polyvinyl chloride (PVC) contains numerous toxic chemicals called adipates and phthalates ("plasticizers"), which are used to soften brittle PVC into a more flexible form. PVC is commonly used to package foods and liquids, ubiquitous in children's toys and teethers, plumbing and building materials, and in everything from cosmetics to shower curtains. Traces of these chemicals can leach out of PVC when it comes into contact with food. 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. The European Union has banned the use of DEHP (di-2-ethylhexyl phthalate), the most widely used plasticizer in PVC, and in children's toys.
Polystyrene (PS) is one of the toxins the USEPA (United States Environmental Protection Agency) monitors in America's drinking water. Its production also pollutes the atmosphere, destroying the ozone layer. Some compounds leaching from Styrofoam food containers interfere with hormone functions. It's a possible human carcinogen.
Polycarbonates are a particular group of thermoplastic polymers, whose primary building block is bisphenol A (BPA), a hormone disrupter that releases into food and liquid and acts like estrogen. 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, as well as impact hormone levels. 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.
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.
By the 1990s, 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 recyclable plastic container using this scheme is marked with a triangle of three "chasing arrows", which encloses a number giving the plastic type:
- PET (PETE), polyethylene terephthalate: Commonly found on 2-liter soft drink bottles, cooking oil bottles, peanut butter jars.
- HDPE, high-density polyethylene: Commonly found on detergent bottles, milk jugs.
- PVC, polyvinyl chloride: Commonly found on plastic pipes, outdoor furniture, shrink-wrap, water bottles, salad dressing and liquid detergent containers.
- LDPE, low-density polyethylene: Commonly found on dry-cleaning bags, produce bags, trash can liners, food storage containers.
- PP, polypropylene: Commonly found on bottle caps, drinking straws, yogurt containers.
- PS, polystyrene: Commonly found on "Styrofoam peanuts", cups, plastic tableware, meat trays, take-away food clamshell containers
- 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 colour 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.
Bioplastics and 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 is expensive at present e.g. BP's Biopol. BASF make 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). PDF (96.7 KiB)
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.
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.
Some plastics can be obtained from biomass, including:
- from pea starch film with trigger biodegradation properties for agricultural applications (TRIGGER).
- from biopetroleum .
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 their 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).
- 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.
- Bayblend (PC/ABS)
- A blend of PC and ABS that creates a stronger plastic. :Car Interior and exterior parts
- Polymethyl methacrylate (PMMA)
- contact lenses, glazing (best known in this form by its various trade names around the world, e.g., Perspex, Oroglas, Plexiglas) 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) (Polyketone)
- 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 General Electric product, similar to PEEK.
- 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.