A plastic material is any of a wide range of synthetic or semi-synthetic organic solids used in the manufacture of industrial products. Plastics are typically polymers of high molecular mass, and may contain other substances to improve performance and/or reduce production costs. Monomers of plastic are either natural or synthetic organic compounds.
The word 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.
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 moulded again and again; examples are polyethylene, polypropylene, polystyrene, polyvinyl chloride and polytetrafluoroethylene (PTFE). Thermosets can melt and take shape once; after they have solidified, they stay solid.
The raw materials needed to make most plastics come from petroleum and natural gas.
Overview
Plastics can be classified by
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, such as
condensation,
polyaddition, and
cross-linking.
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.
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 properties, such as hardness, density,heat resistance, organic solvents, oxidation, and ionizing radiation. In particular, most plastics will melt or decompose when heated 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. Plastics are still too expensive to replace wood, concrete and ceramic in bulky items like ordinary buildings, bridges, dams, pavement, and railroad ties.
Chemical structure
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. 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 interests are
silicon based.) 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 linking monomers together to form the polymer chain). This fine tuning of the properties of the polymer by repeating unit's molecular structure has allowed plastics to become an indispensable part of twenty first-century world.
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.
Families
History
The first human-made plastic, called parkesine, was patented by Alexander Parkes in 1856. It was unveiled at the 1862 Great International Exhibition in London. 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).
In 1866, Parkes formed the Parkesine Company to mass produce the material. The company, however, failed due to poor product quality as Parkes tried to reduce costs. Parkesine's successors were Xylonite, produced by Daniel Spill (an associate of Parkes), and Celluloid from John Wesley Hyatt. Parkesine was made from cellulose treated with nitric acid and a solvent. The generic name of Parkesine is pyroxylin, or Celluloid. Parkesine is often synthetic ivory. The Parkesine company ceased trading in 1868. Pictures of Parkesine are held by the Plastics Historical Society of London. There is a plaque on the wall of the site of the Parkesine Works.
Fossil-based plastics
Bakelite
The first so called 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 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 cases for radios, telephones and clocks, and billiard balls.
Phenol-based ("Phenolic") plastics have been largely replaced by cheaper and less brittle plastics, but they are still used in applications requiring their insulating and heat-resistant properties. For example, some electronic circuit boards are made of sheets of paper or cloth impregnated with phenolic resin.
Polystyrene and Polyvinyl Chloride
and firestops being installed in Ontario. Certain plastic pipes can be used in some non-combustible buildings, provided they are firestopped properly and that the flame spread ratings comply with the local building code.]]
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).
Polystyrene is a rigid, brittle, inexpensive plastic that has been used to make plastic model kits and similar knick-knacks. 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.
Polyvinyl Chloride (PVC, commonly called "vinyl") 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 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).
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 polymerization.
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, and because of good heat-resistance, increasingly for under-the-hood applications in cars, and other mechanical parts.
Rubber
Natural rubber is an elastomer (an elastic hydrocarbon polymer) that was originally derived from
latex, a milky
colloidal suspension found in the sap of some plants. It is useful directly in this form (indeed, the first appearance of rubber in Europe is cloth waterproofed with unvulcanized latex from Brazil) but, later, in 1839,
Charles Goodyear invented vulcanized rubber; this a form of natural rubber heated with, mostly, sulfur forming cross-links between polymer chains (
vulcanization), improving elasticity and durability.
Synthetic rubber
The first fully synthetic rubber was synthesized by Sergei Lebedev in 1910. In World War II, supply blockades of natural rubber from South East Asia caused a boom in development of synthetic rubber, notably styrene-butadiene rubber. In 1941, annual production of synthetic rubber in the 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.
Bioplastics
Cellulose-based plastics Parkes developed a synthetic replacement for 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 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.
Bois Durci is a plastic molding material based on cellulose. It was patented in Paris by Lepage in 1855. It is made from finely ground wood flour mixed with a binder, either egg or blood albumen, or gelatine. The wood is probably either ebony or rose wood, which gives a black or brown resin. The mixture is dried and ground into a fine powder. The powder is placed in a steel mold and compressed in a powerful hydraulic press while being heated by steam. The final product has a highly polished finish imparted by the surface of the steel mold.
High oil prices have seen a resurgence in interest in bioplastics made from renewable cellulose and starch., including:
Pea starch film with trigger biodegradation properties for agricultural applications (TRIGGER). Biopetroleum.
Because of the fragmentation in the market and ambiguous definitions it is difficult to describe the total market size for bioplastics, but estimates put global production capacity at 327,000 tonnes. In contrast, global consumption of all flexible packaging is estimated at around 12.3 million tonnes.
Biodegradable (compostable) 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 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. The German chemical company BASF makes Ecoflex, a fully biodegradable polyester for food packaging applications.
Oxo-biodegradable
Oxo-biodegradable (OBD) plastic is polyolefin plastic to which has been added very small (catalytic) amounts of metal salts. As long as the plastic has access to oxygen (as in a littered state), these additives catalyze the natural degradation process to speed it up so that the OBD plastic will degrade when subject to environmental conditions. Once degraded to a small enough particle they can interact with biological processes to produce to water, carbon dioxide and biomass. The process is shortened from hundreds of years to months for degradation and thereafter biodegradation depends on the micro-organisms in the environment. Typically this process is not fast enough to meet ASTM D6400 standards for definition as compostable plastics.
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,
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. 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. 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 suspicious indications of BPA being linked to cancer.
Bis(2-ethylhexyl) adipate, present in plastic wrap based on PVC, is also of concern, as are the volatile organic compounds present in new car smell.
The European Union has a permanent ban on the use of phthalates in toys. In 2009, the United States government banned certain types of phthalates commonly used in plastic.
Environmental issues
Plastics are durable and
degrade very slowly; the molecular bonds that make plastic so durable make it equally resistant to natural processes of degradation. Since the 1950s, one billion tons of plastic have been discarded and may persist for hundreds or even thousands of years. In some cases, burning plastic can release
toxic fumes. Burning the plastic polyvinyl chloride (PVC) may create
dioxin. 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.
Plastic can be converted as a fuel. Plastic is made from crude oil, so it can be broken down to liquid hydrocarbon. One kilogram of waste plastic produces a liter of hydrocarbon. Plastic wastes are used in cement plants as a fuel.
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:
# PET (PETE), polyethylene terephthalate # HDPE, high-density polyethylene # PVC, polyvinyl chloride # LDPE, low-density polyethylene, # PP, polypropylene # PS, polystyrene # Other types of plastics (see list, below)
Unfortunately, recycling of plastics has proven to be a difficult process. The biggest problem is that it is difficult to automate 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.
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-biodegradable
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.
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
Polyester (PES) - Fibers, textiles. Polyethylene terephthalate (PET) - Carbonated drinks bottles, peanut butter jars, plastic film, microwavable packaging. Polyethylene (PE) - Wide range of inexpensive uses including supermarket bags, plastic bottles. High-density polyethylene - Detergent bottles and milk jugs. Polyvinyl chloride (PVC) - Plumbing pipes and guttering, shower curtains, window frames, flooring. Polyvinylidene chloride (PVDC) (Saran) - Food packaging. Low-density polyethylene (LDPE) - Outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging. Polypropylene (PP) - Bottle caps, drinking straws, yogurt containers, appliances, car fenders (bumpers), plastic pressure pipe systems. Polystyrene (PS) - Packaging foam/"peanuts", food containers, plastic tableware, disposable cups, plates, cutlery, CD and cassette boxes. High impact polystyrene (HIPS) -: Refrigerator liners, food packaging, vending cups. Polyamides (PA) (Nylons) - Fibers, toothbrush bristles, fishing line, under-the-hood car engine moldings. Acrylonitrile butadiene styrene (ABS) - Electronic equipment cases (e.g., computer monitors, printers, keyboards), drainage pipe. Polycarbonate (PC) - Compact discs, eyeglasses, riot shields, security windows, traffic lights, lenses. Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) - A blend of PC and ABS that creates a stronger plastic. Used in car interior and exterior parts, and mobile phone bodies. 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).
Special purpose plastics
Melamine formaldehyde (MF) - One of the aminoplasts, and used as a multi-colorable alternative to phenolics, for instance in moldings (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). Plastarch material - Biodegradable and heat resistant, thermoplastic composed of modified corn starch. 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 molded 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 moldings naturally being dark colors (red, green, brown), and as thermoset it is difficult to recycle. Polyetheretherketone (PEEK) - Strong, chemical- and heat-resistant thermoplastic, biocompatibility allows for use in medical implant applications, aerospace moldings. One of the most expensive commercial polymers. Polyetherimide (PEI) (Ultem) - A high temperature, chemically stable polymer that does not crystallize. Polylactic acid (PLA) - 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 dairy products. 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. It forms the basis of artistic and commercial acrylic paints when suspended in water with the use of other agents. Polytetrafluoroethylene (PTFE) - Heat-resistant, low-friction coatings, used in things like non-stick surfaces for frying pans, plumber's tape and water slides. It is more commonly known as Teflon. Urea-formaldehyde (UF) - One of the aminoplasts and used as a multi-colorable alternative to phenolics. Used as a wood adhesive (for plywood, chipboard, hardboard) and electrical switch housings.
See also
List Of Plastics, Their Chemical Codes, Uses And Applications Conductive polymer Corn construction Molding (process) * Flexible mold * Injection molding Films Light activated resin Nurdle Organic light emitting diode Plastics engineering Plastics extrusion Plastic recycling Plasticulture Progressive bag alliance Roll-to-roll processing Self-healing plastic Thermoforming Timeline of materials technology
References
Substantial parts of this text originated from An Introduction To Plastics v1.0 / 1 March 2001 / greg goebel / public domain.
External links
J. Harry Dubois Collection on the History of Plastics, ca. 1900–1975 Archives Center, National Museum of American History, Smithsonian Institution. Material Properties of Plastics – Mechanical, Thermal & Electrical Properties List of over 600 plastics Plastics Historical Society History of plastics, Society of the Plastics Industry What Are Plastics Made Of? January 1943 Popular Science
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