Plastics

The word "plastic" was originally an adjective describing a material that

could flow. Its first use as a noun came with the introduction of Bakelite and
Celluloid in the early 1900s. The word is now used to mean a substance
which can be pressed into definite shape and retains its shape when the
pressure is removed.
Plastics are long-chain molecules known as polymers. The starting materials
are called monomers. Proteins, starches, and resins such as frankincense and
myrrh are natural polymers. Synthetic polymers are used in fibers and in
molded and extruded solids which are commonly called plastics or resins.
Hermann Staudinger, who was awarded the Nobel Prize in chemistry in
1953, introduced the term macromolecule in the 1920s. One of his first
compounds was polystyrene, with a molecular weight of approximately
600,000, formed by a chain of 5,700 styrene units.
In terms of the chemistry involved, the two main types of polymers are
formed by (1) addition or (2) condensation. Addition polymerization uses a
catalyst to open the double bond of an alkene monomer forming fragments
with single electrons (free radicals). Thousands or hundreds of thousands or
even millions of these monomer fragments then add to each other in a chain
reaction to form polymers. More than one compound can be used as the
monomer, producing copolymers or terpolymers. Depending on the catalyst,
it is possible to have two monomers link alternately or randomly.
Condensation polymers are usually formed between dibasic organic acids
and dihydroxyalcohols or diamines. Water is eliminated from the two
functional groups allowing the chain to grow.
Crosslinking is possible with either type of polymer. For a condensation
polymer, the presence of more than two functional groups on one or both
monomers allows the chain to grow laterally as well as linearly. With
addition polymers, a free radical chain fragment can attack the growing
chain to produce side-chains.
Choice of monomer(s), chain length, and extent of crosslinking, molding
conditions, and additives results in plastics that can be tailored to an
incredible number of types of material.
In terms of manufacturing, plastics are categorized as thermosetting or
thermoplastic. The chemical structure of thermosetting plastics is altered by
heat, which causes crosslinking to form one large molecule. Objects are
formed using compression molding and cannot be re softened. In addition to
molded objects, thermosetting resins find use in laminates and foams.
Thermoplastic resins remain stable when heated, and can be injection
molded--squirted under pressure into cooler molds, or extruded--pushed out
of a shaped orifice to form sheets, rods, or tubes.
Plastics based on cellulose (primarily wood pulp) include cellulose esters
such as cellulose acetate, diacetate, and triacetate, and mixed esters such as
cellulose acetate butyrate. The first cellulose resin, cellulose nitrate or
celluloid, was invented by John Wesley Hyatt, who was trying to find a
substitute for ivory in billiard balls in order to win a contest. He didn't win,
but he received a patent for his invention in 1870. Cellulose nitrate was
commonly used in World War I to seal the fabric wings of fighter planes, but
cellulose nitrate is extremely flammable, and the planes were soon
nicknamed "flaming coffins." This problem spurred the production of the
less flammable cellulose diacetate. At the end of the war, Camille and Henri
Dreyfus found themselves with a large factory and a product that now had
no use. They developed a method for spinning their cellulose diacetate into
fiber; they also sponsored the research that resulted in new textile dyes.
Cellulose acetate is used as a thermoplastic for photographic and movie film
and for moldings. It has good impact strength, is easy to color, and has fair
outdoor stability. The mixed ester, cellulose acetate butyrate, has better
weatherabilitly and finds use as pipe and telephone housings. Cellulose
xanthate is produced from wood cellulose in reaction with sodium
hydroxide and carbon disulfide. It is usually spun into fibers as rayon, but it
is also cast in films as cellophane. Cellulose nitrate, the original plastic in its
class, is cheap and durable, takes a high polish and good color, and can be
rolled or molded into specific shapes. As celluloid, it is used today in combs
and brushes. It is also used as artificial leather for auto seat covers and
furniture.
The first completely synthetic plastic, Bakelite, is a phenol-formaldehyde
resin. The condensation reaction between phenol and formaldehyde,
resulting in resinous products, had been known since 1870. Dr. Leo
Baekeland discovered in 1909 that moldings could be made if the reaction
was carried out under heat and pressure. Bakelite is still used today because
of its good electrical properties, high heat and acid resistance, and slow
burning rate. It does not color well, so is used for moldings for electrical
parts and as a backing material for laminates, where it is usually colored
black. Its primary use today is as an adhesive in the production of plywood
and particle board.

Nylon was the first synthetic fiber not based on natural materials such as
cellulose. W. H. Carothers of E. I. DuPont de Nemours first produced
nylon in 1931 from hexa-methylene-diamine and adipic acid, a six-carbon
amine and a six-carbon acid. The resulting product is known as nylon 66.
Other amines and acids result in nylons with different properties. Although
we think of nylon as a material that is spun into fibers, the plastic has other
uses as well. Nylon gears and other moving parts have low coefficients of
friction and excellent wear-resistance and impact and tensile strengths. They
are self-extinguishing in case of fire. Nylons are thermoplastic, and so can
be extruded into various shapes.
Polyesters were first produced in England in 1941 by condensing ethylene
glycol with terephthalic acid. This first polyester was called Terylene in
England and Dacron (by DuPont) in the United States. We think of
polyesters as fibers, but they can be used as thermosetting plastics. They find
use in laminates in aircraft, boats, and furniture. Dacron is used as tubing to
replace diseased blood vessels. Polyesters impregnated into glass cloth or
fibers forms a composite with strength approaching that of steel. Polyesters
can also be cast into clear sheets. Polyethylene terephthalate is used to make
soft drink bottles and can also be made into thin films known as Mylar, used
for packaging frozen foods, as a skin substitute for burn victims, and for
recording tape. A new type of polyester, trade named Olestra, is marketed to
snack-food producers as a fat substitute that is not absorbed by the body.
Melamine-formaldehyde resins exhibit excellent resistance to heat, flames,
abrasion, and stains. These thermosetting resins are easy to color and find
use in laminates. Urea-formaldehyde resins are similar but with somewhat
inferior properties. They are used in adhesives. Both of these formaldehyde-
based resins are thermosetting. Epoxy resins have outstanding adhesion to
metals and are widely used in surface coatings and adhesives.
Polyurethanes, another of the thermosetting plastics, are widely used as
flexible foams.
Two thermoplastic condensation polymers are Mylar (ethylene
glycol/terephthalic acid) and the polycarbonates. Mylar is known for its
toughness and tear-strength in films. Polycarbonates have excellent impact
strength at high temperatures and are self-extinguishing. The polycarbonate
trademarked as Lexan is used in "bulletproof" glass and in astronaut's
helmets. A sheet with a thickness of one inch can stop a 0.38-caliber bullet
fired from twelve feet away.
Polyethylene, a thermoplastic, is the most important plastic on the market.
The monomer, ethene or ethylene, is obtained in large quantities from the
cracking of petroleum. Polyethylene was developed shortly before the start
of World War II and was used for insulating cables in radar. Today high
density polyethylene, with its linear chains, is used for radio and television
cabinets, toys, and large-diameter pipes. Low density polyethylene, a cross-
linked polymer, is used for plastic bags, refrigerator dishes, electrical
insulation, films and sheeting, coatings, pond liners, and moisture barriers.
Vinyl monomers can be represented as CH2=CHX and vinylidene
monomers as CH2=CY2. The monomers can be thought of as substituted
ethenes, and the polymerization process proceeds in a similar way.
Polystyrene is a thermoplastic which is easily colored and fabricated. It is
molded into appliance housing and parts. Polystyrene is most familiar in its
foamed form, trade-named Styrofoam, used as insulation and cushioning.
Styrofoam is formed by expansion molding. Beads of polystyrene containing
four to seven percent by weight of a low-boiling liquid are heated with steam
or hot air. The pellets puff and are molded into the shape of the mold cavity.
Expanded polystyrene is also used for egg cartons, meat trays, coffee cups,
and packaging material. A terpolymer of styrene with acrylonitrile and
butadiene (ABS resin) is a synthetic rubber.
Polymethylmethacrylate, with trade names of Lucite and Plexiglas, is well
known as a glass substitute, especially in uses such as skylights where its
impact strength and resistance to shatter is important. It is also used in signs,
lenses, automotive parts such as taillights, and bowling balls.
Polyvinyl chloride, or PVC, is familiar as pipe. Waldo Simon, who died in
1999 at age 100, discovered vinyl in the 1920s while attempting to turn
polyvinyl chloride --then considered worthless--into glue. At his wife's
request, he first used it to waterproof a shower curtain.
Polyvinylidene chloride is known as Saran wrap. Polytetrafluoroethylene is
Teflon, used for electrical insulation as well as on pots and pans as a
nonstick coating. Polyvinyl acetate is important in latex paints and
adhesives. Polyacrylonitrile is known as Orlon.
Synthetic Rubbers
Natural rubber is a polymer of isoprene. As World War II approached and
the supply of natural rubber was cut off, the search began for synthetic
rubbers. By the beginning of the war, Germany was producing
butadiene/styrene copolymers that were acceptable as rubber. The procedure
was duplicated by the United States. Butadiene/acrylonitrile copolymers and
acrylonitrile/butadiene/styrene terpolymers are also used as synthetic
rubbers.
Polychloroprene is trademarked as Neoprene, and is a polymer or copolymer
of 2-chloro-1,3-butadiene. Neoprene makes excellent tires but is not price-
competitive. Its oil and gasoline-resistance makes it particularly suitable to
hoses. It is heat- and flame-resistant, so finds uses in wire insulation.
Hypalon is a chlorosulfonated polyethylene. Polyethylene is treated with
chlorine and sulfur dioxide; sulfonyl chloride radicals are formed, which
become part of the crosslinking of the polymer. Hypalon is so chemical-
resistant that it can be used for handling concentrated sulfuric acid. It is also
flame-resistant.
Silicones, with their backbones of silicon and oxygen atoms, do not
decompose at high temperatures and so are used as automobile brake fluids,
lubricants, and electrical insulators. They are also stable at low temperatures,
so are used as sealants in space vehicles. Because they are water-repellent,
they are used as insulating varnish on electric motors and for high
temperature greases and lubricants. Silicone rubber is easily molded,
serving as a replacement for body parts. Liquid silicone in urethane bags was
used extensively for breast implants until reports of leakage in 1992.
Recycling of plastics lags behind that for aluminum cans, paper, and glass,
but should increase as new uses are developed. PET (polyethylene
terephthalate) from soft drink bottles finds new life as fiberfill and carpet
fibers. HDPE (high density polyethylene) becomes one-gallon milk jugs and
pipe. Recycled polystyrene is used to make plastic "lumber." Polyvinyl
chloride can be converted into sewer drainpipes.