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Plastics, term applied
to organic polymeric materials (those consisting of giant organic
molecules) that are plastic-that is, they can be formed into desired
shapes through extrusion, molding, casting, or spinning. The molecules
can be either natural-including cellulose, wax, and natural rubber-or
synthetic-including polyethylene and nylon. The starting materials
are resins in the form of pellets, powders, or solutions; from these
are formed the finished plastics.
Plastics are characterized by high strength-to-density ratios, excellent
thermal and electrical insulation properties, and good resistance
to acids, alkalies, and solvents. The giant molecules of which they
consist may be linear, branched, or cross-linked, depending on the
plastic. Linear and branched molecules are thermoplastic (soften when
heated), whereas cross-linked molecules are thermosetting (harden
when heated).
History
The development of plastics began about 1860, after Phelan and Collander,
a United States firm manufacturing billiard and pool balls, offered
a prize of $10,000 for a satisfactory substitute for natural ivory.
One of those who tried to win this prize was U.S. inventor John Wesley
Hyatt. Hyatt developed a method of pressure-working pyroxylin, a cellulose
nitrate of low nitration (see Cellulose) that had been plasticized
with camphor and a minimum of alcohol solvent. Although Hyatt did
not win the prize, his product, patented under the trademark Celluloid,
was used in the manufacture of objects ranging from dental plates
to men's collars. Despite its flammability and liability to deterioration
when exposed to light, Celluloid achieved a notable commercial success.
Other plastics were introduced gradually over the next few decades.
Among them were the first totally synthetic plastics: the family of
phenol-formaldehyde resins developed by the Belgian-American chemist
Leo Hendrik Baekeland about 1906 and sold under the trademark Bakelite.
Other plastics introduced during this period include modified natural
polymers (see Polymer) such as rayon, made from cellulose products.
Breakthrough in Plastics Chemistry
In 1920 an event occurred that set the stage for the future rapid
development of plastic materials. The German chemist Hermann Staudinger
hypothesized that plastics were truly giant molecules. His subsequent
efforts to prove this claim initiated an outburst of scientific investigation
that resulted in major breakthroughs in the chemistry of plastics.
Throughout the 1920s and 1930s large numbers of new products were
introduced, including cellulose acetate, used in molding resins and
fibers; polyvinyl chloride, used in plastic pipe, vinyl coatings,
and wire insulation; urea-formaldehyde resins, used in tableware and
electrical applications; and acrylic resin, developed as a binder
for laminated glass.
One of the most familiar plastics developed in this period is polymerized
methyl methacrylate, which, beginning in 1937, was marketed as Lucite
and Plexiglas. This material has excellent optical properties and
is suitable for eyeglass and camera lenses and for producing special
effects in highway and advertising illumination. Polystyrene resins,
also first produced commercially about 1937, are characterized by
high resistance to chemical and mechanical alteration at low temperatures
and by very low absorption of water. These properties make the polystyrenes
especially suitable for radio-frequency insulation and for accessories
used in low-temperature situations, as in refrigeration installations
and in airplanes designed for high-altitude flight. Polytetrafluoroethylene,
first made in 1938, was eventually produced commercially as Teflon
in 1950. Another key development during the 1930s was the synthesis
of nylon, the first high-performance engineering plastic.
World War II
During World War II (1939-1945), both the Allies and the Axis powers
were faced with severe shortages of natural raw materials. The plastics
industry proved to be a rich source of acceptable substitutes. Germany,
for example, which was cut off early from sources of natural latex,
initiated a major program that led to the development of a practical
synthetic rubber (see Rubber). Similarly for the United States, Japan's
entry into the war eliminated most Far Eastern sources of natural
rubber, silk, and many metals. The U.S. response was to accelerate
the development and production of plastics. Nylon became a major souce
of textile fibers, polyesters were used in fabricating armor and other
war materials, and various type of synthetic rubber were produced
in quantity.
The Postwar Boom
The scientific and technological momentum in the plastics industry
carried over into the postwar years. Of particular interest were the
advances in such engineering plastics as polycarbonates, acetals,
and polyamides; other synthetics were used in place of metal in machinery,
safety helmets, high-temperature devices and many other products used
in environmentally demanding settings. In 1953 the German chemist
Karl Ziegler developed polyethylene, and in 1954 the Italian chemist
Giulio Natta developed polypropylene-two of today's most important
plastics. A decade later, these two men shared the 1963 Nobel Prize
in Chemistry for their studies of polymers.
Kinds of Plastics
Three of the ways in which plastics can be categorized are by the
polymerization process that forms them, by their processibility, and
by their chemical nature.
Polymerization
The two basic polymerization processes for producing resins are condensation
and addition reactions. Condensation produces a variety of polymer
chain lengths, whereas addition reactions produce only specific lengths.
Furthermore, condensation polymerizations produce small by-product
molecules such as water, ammonia, and glycol, whereas no by-products
are generated in addition reactions. Typical condensation polymers
are nylons, polyurethanes, and polyesters. Addition polymers include
polyethylene, polypropylene, polyvinyl chloride, and polystyrene.
The average molecular weights for the addition polymers are generally
orders of magnitude larger than those of condensation polymers.
Processibility
The processibility of a plastic depends on whether it is thermoplastic
or thermosetting. Thermoplastics, which are made up of linear or
branched polymers, are fusible: They soften when heated and harden
when cooled. This is also true of thermosets that are lightly cross-linked.
Most thermosets, however, harden when heated. This final cross-linking,
which fixes the true thermosets, takes place after the plastic has
already been formed.
Chemical Nature
The chemical nature of a plastic is defined by the monomer (repeating
unit) that makes up the chain of the polymer. For example, polyolefins
are made up of monomer units of olefins, which are open-chain hydrocarbons
with at least one double bond. Polyethylene is a polyolefin; its
monomer unit is ethylene. Other categories are acrylics (such as
polymethylmethacrylate), styrenes (such as polystyrene), vinyl halides
(such as polyvinyl chloride), polyesters, polyurethanes, polyamides
(such as nylons), polyethers, acetals, phenolics, cellulosics, and
amino resins.
Manufacture
The manufacture of plastic and plastic products involves procuring
the raw materials, synthesizing the basic polymer, compounding the
polymer into a material useful for fabrication, and molding or shaping
the plastic into its final form.
Raw Materials
Originally, most plastics were made from resins derived from vegetable
matter, such as cellulose (from cotton), furfural (from oat hulls),
oils (from seeds), starch derivatives, or coal. Casein (from milk)
was among the nonvegetable materials used. Although the production
of nylon was originally based on coal, air, and water, and nylon
11 is still based on oil from castor beans, most plastics today
are derived from petrochemicals. These oil-based raw materials are
more widely available and less expensive than other raw materials.
However, because the world supply of oil is limited, other sources
of raw materials, such as coal gasification, are being explored.
Synthesizing the Polymer
The first stage in manufacturing plastic is polymerization. As noted,
the two basic polymerization methods are condensation and addition
reactions. These methods may be carried out in various ways. In
bulk polymerization, the pure monomer alone is polymerized, generally
either in gaseous or liquid phase, although a few solid-state polymerizations
are also used. In solution polymerization, an emulsion is formed
and then coagulated. In interfacial polymerization, the monomers
are dissolved in two immiscible liquids, and the polymerization
occurs at the interface of the two liquids.
Additives
Chemical additives are often used in plastics to produce some desired
characteristic. For instance, antioxidants protect a polymer from
chemical degradation by oxygen or ozone; similarly, ultraviolet
stabilizers protect against weathering. Plasticizers make a polymer
more flexible, lubricants reduce problems with friction, and pigments
add color. Among other additives are flame retardants and antistatics.
Many plastics are manufactured as composites. This involves a system
where reinforcements (usually fibers made of glass or carbon) are
added to a plastic resin matrix. Composites have strength and stability
comparable to that of metals but generally with less weight. Plastic
foams, which are composites of plastic and gas, offer bulk with
low weight.
Shaping and Finishing
The techniques used for shaping and finishing plastics depend on
three factors: time, temperature, and flow (also known as deformation).
Many of the processes are cyclic in nature, although some fall into
the categories of continuous or semicontinuous operation.
One of the most widely used operations is that of extrusion. An
extruder is a device that pumps a plastic through a desired die
or shape. Extrusion products, such as pipes, have a regularly shaped
cross section. The extruder itself also serves as the means to carry
out other operations, such as blow molding and injection molding.
In extrusion blow molding, the extruder fills the mold with a tube,
which is then cut off and clamped to form a hollow shape called
a parison. This hot, molten parison is then blown like a balloon
and forced against the walls of the mold to form the desired shape.
In injection molding, one or more extruders are used with reciprocating
screws that move forward to inject the melt and then retract to
take on new molten material to continue the process. In injection
blow molding, which is used in making bottles for carbonated beverages,
the parison is first injection molded and then reheated and blown.
Compression molding uses pressure to force the plastic into a given
shape. Another process, transfer molding, is a hybrid of injection
and compression molding: The molten plastic is forced by a ram into
a mold. Other finishing processes include calendering, in which
plastic sheets are formed, and sheet forming, in which the plastic
sheets are formed into a desired shape. Some plastics, particularly
those with very high temperature resistance, require special fabrication
procedures. For example, polytetrafluoroethylene has such a high
melt viscosity that it is first pressed into shape and then sintered-exposed
to extremely high temperatures that bond it into a cohesive mass
without melting it. Some polyamides are produced by a similar process.
Uses Plastics have an ever-widening range of uses in both the industrial
and consumer sectors.
Packaging
The packaging industry is the leading user of plastics, accounting
for about one-third of total United States production. In the early
1990s, U.S. sales of low-density polyethylene (LDPE) exceeded 10
billion pounds per year. Most of the LDPE produced is marketed in
rolls of clear-plastic wrap. High-density polyethylene (HPDE) is
used for some thicker plastic films, such as those used for plastic
trash bags and containers. Other packaging plastics include polypropylene,
polystyrene, polyvinyl chloride, and polyvinylidene chloride. Polyvinylidene
chloride is used primarily for its barrier properties, which can
keep gases such as oxygen from passing into or out of a package.
Similarly, polypropylene is an effective barrier against water vapor.
Polypropylene also is often used in housewares and as a fiber for
carpeting and rope.
Construction
The United States building industry is the second largest consumer
of plastics, including many of the packaging plastics mentioned
above. HDPE is used for pipes, as is polyvinyl chloride (PVC); PVC
is also used in sheets for siding and similar components. Many plastics
are used to insulate cables and wires, and polystyrene in the form
of foam serves as insulation for walls, roofs, and other areas.
Other plastic products are roofing, door and window frames, moldings,
and hardware.
Other Uses
Many other industries, especially automobile and truck manufacturing,
also depend on plastics. Tough engineering plastics are found in
components like air-intake manifolds, fuel lines, emission canisters,
fuel pumps, and electronic devices. Plastics are also used for interior
paneling, seats, and trim. Many automobile bodies are made of fiberglass-reinforced
plastic.
Among the other uses of plastic are housings for business machines,
electronic devices, small appliances, and tools. Consumer goods
range from sports equipment to luggage and toys.
Health and Environmental Hazards
Because plastics are relatively inert, they do not normally present
health hazards to the maker or user. However, some monomers used
in the manufacture of plastics have been shown to cause cancer.
Similarly, benzene, which is an important raw material for the synthesis
of nylon, is a carcinogen. The problems involved in the manufacture
of plastics parallel those of the chemical industry in general.
Most synthetic plastics are not environmentally degradable; unlike
wood, paper, natural fibers, or even metal and glass, they do not
rot or otherwise break down over time. (Some degradable plastics
have been developed, but none has proven compatible with the conditions
required for most sanitary landfills.) Thus, there is an environmental
problem associated with the disposal of plastics. Recycling has
emerged as the most practical method to deal with this problem,
especially with products such as the polyethylene terephlalate bottles
used for carbonated beverages, where the process of recycling is
fairly straightforward. More complex solutions are being developed
for handling the commingled plastic scrap that constitutes a highly
visible, albeit relatively small, part of the problem of solid waste
disposal.
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