In the twentieth century, the term plastic has come to refer to a class of materials that, under suitable conditions, can be deformed by some kind of shaping or molding process to produce an end product that retains its shape. When used as an adjective, the term plastic (from Greek plastikos meaning to mold or form) describes a material that can be shaped or molded with or without the application of heat. With few exceptions, plastics do not flow freely like liquids, but retain their shapes like solids even when flowing.

When used in a chemical sense, the term plastic usually refers to a synthetic high molecular weight chain molecule, or polymer, that may have been combined with other ingredients to modify its physical properties. Most plastics are based on carbon, being derived from materials that have some relationship to living, or organic, materials, although, although some plastics, like acetal resins and silicones, contain oxygen or silicon atoms in their chains.

As plastics are heated to moderate temperatures, the polymer chains are able to flow past each other. Because of the organic nature of most plastics, they usually cannot withstand high temperatures and begin to decompose at temperatures around 392^°F (200^°C).

The oldest known examples of plastic materials are soft waxes, asphalts, and moist clays. These materials are capable of flowing like synthetic plastics, but because they are not polymeric, they are usually not referred to as plastics.

History

The history of synthetic plastics goes back over 100 years to the use of cellulose nitrate (celluloid) for billiard balls, men’s collars, and shirt cuffs. Before plastics were commercialized, most household goods and industrial products were made of metals, wood, glass, paper, leather, and vulcanized (sulfurized) natural rubber.

The first truly synthetic polymer was Bakelite, a densely cross-linked material based on the reaction of phenol and formaldehyde. It has been used for many applications, including electrical appliances and phonograph records. Among the first plastics developed that could be reformed under heat (thermoplastics) were polyvinyl chloride, polystyrene, and nylon 66.

The first polymers used by man were actually natural products such as cotton, starch, proteins, or wool. Certain proteins that are in fact natural polymers once had commercial importance as industrial plastics, but they have played a diminishing role in the field of plastics production in recent years.

Chemistry

There are more than 100 different chemical atoms, known as elements. They are represented by the chemist by the use of simple symbols such as “H” for hydrogen, “O” for oxygen, “C” for carbon, “N” for nitrogen, “Cl” for chlorine, and so on; these atoms have atomic weights of 1, 16, 12, 14, and 17 atomic units, respectively.

A chemical reaction between two or more atoms forms a molecule. Each molecule is characterized by its elemental constitution and its molecular weight. For example, when carbon is burned in oxygen, one atom of carbon (C) reacts with two atoms of oxygen (O₂; equivalent to one molecule of molecular oxygen) to form carbon dioxide (CO₂). The chemist represents this reaction by a chemical equation, i.e.,

C + O₂ = CO₂

Similarly, when four atoms of hydrogen (2H₂; equivalent to two molecules of molecular hydrogen) and two atoms of oxygen (O₂; equivalent to one molecule of oxygen) react to form two molecules of water (2H₂O), the chemist writes

2H₂ +O₂ = 2H₂O

Note that one molecule of oxygen combines with two molecules of hydrogen, and one atom of carbon combines with one molecule of hydrogen. This is because different elements have different combining capacities. Thus hydrogen forms one bond, oxygen two bonds, and carbon four bonds. These bonding capacities, or valences, are taken for granted when writing a chemical formula like H₂O.

In the case of methane, or CH₄, the carbon is bonded to four hydrogen atoms. But carbon can also form double bonds, as in ethylene (C₂H₄) where two CH₂molecules share a double bond. The chemist could also describe the ethylene molecule by the formula CH₂ = CH₂, where the double bond is represented by an equal sign.

Plastic materials consist of many repeating groups of atoms or molecules (called monomers) in long chains, and hence are also known as polymers or macromolecules. Elements present in a polymer chain typically include oxygen, hydrogen, nitrogen, carbon, silicon, fluorine, chlorine, or sulfur. The way the polymer chains are linked together and the lengths of the chains determine the mechanical and physical properties of the plastic.

Molecular Weight

Polymers exist on a continuum that extends from simple gases to molecules of very high molecular weights. A relatively simple polymer has the structure

H - (CH₂)_n-H

where the number (n) of monomers (CH₂ groups, in this case) in the chain may extend up to several thousand. Table 1 shows how the physical properties and uses of the polymer change with the number of repeating monomer units in the chain.

Polymerization

Most commercial plastics are synthesized from simpler molecules, or monomers. The simple chemicals from which monomers, and ultimately polymers, are derived are usually obtained from crude oil or natural gas, but may also come from coal, sand, salt, or air.

For example, the molecules used to form polystyrene, a widely used plastic, are benzene and ethylene. These two molecules are reacted to form ethyl benzene, which is further reacted to give a styrene monomer. With the aid of a catalyst, styrene monomers may form a chain of linked, bonded styrene units. This method of constructing a polymer molecule is known as addition polymerization, and characterizes the way most plastics-including polystyrenes, acrylics, vinyls, fluoroplastics-are formed.

When two different molecules are combined to form a chain in such a way that a small molecule such as water is produced as a by-product, the method of building the molecule is known as condensation polymerization. This type of polymerization characterizes a second class of plastics. Nylons are examples of condensation polymers.

Manufacture And Processing

When polymers are produced, they are shipped in pelletized, granulated, powdered, or liquid form to plastics processors. When the polymer is still in its raw material form, it is referred to as a resin. This term antedates the understanding of the chemistry of polymer molecules and originally referred to the resemblance of polymer liquids to the pitch on trees.

Plastics can be formed or molded under pressure and heat, and many can be machined to high degrees of tolerance in their hardened states. Thermoplastics are plastics that can be heated and reshaped; thermo-sets are plastics that cannot.

Table 1. Change in Molecular Properties with Molecular Chain(Thomson Gale)
Change in molecular properties with molecular chain length
Number of CH₂ units in chain	Appearance at room temperature	Uses
1 to 4	simple gas	cooking gas
5 to 11	simple liquid	gasoline
9 to 16	medium viscosity liquid	kerosene
16 to 25	high viscosity liquid	oil and grease
25 to 50	simple solid	paraffin wax candles
1,000 to 3,000	tough plastic solid	polyethylene bottle and containers

Thermoplastics

Thermoplastics are plastics that become soft and malleable when heated, and then become hard and solid again when cooled. Examples of thermoplastics include acetal, acrylic, cellulose acetate, nylon, polyethylene, polystyrene, vinyl, and nylon. When thermoplastic materials are heated, the molecular chains are able to move past one another, allowing the mass to flow into new shapes. Cooling prevents further flow. Thermo-plastic elastomers are flexible plastics that can be stretched up to twice their length at room temperature and then return to their original length when released.

The state of a thermoplastic depends on the temperature and the time allowed to measure its physical properties. At low enough temperatures, amorphous, or noncrystalline, thermoplastics are stiff and glassy. This is the glassy state, sometimes referred to as the vitreous state. On warming up, thermoplastics soften in a characteristic temperature range known as the glass transition temperature region. In the case of amorphous thermoplastics, the glass transition temperature is the single-most important factor determining the physical properties of the plastic.

Crystalline And Noncrystalline Thermoplastics

Thermoplastics may be classified by the structure of the polymer chains that comprise them.

In the liquid state, polymer molecules undergo entanglements that prevent them from forming regularly arranged domains. This state of disorder is preserved in the amorphous state. Thus, amorphous plastics, which include polycarbonate, polystyrene,

Thermoplastics
Type	Chemical basis	Uses
ABS plastics	Derived from acrylonitrile, butadiene, and styrene	Elecroplated plastic parts; automotive components; business and telecommunication applications such as personal computers, terminals, keyboards, and floppy disks; medical disposables; toys; recreational applications; cosmetics packaging; luggage; housewares
Acetals	Consist of repeating CH2 O units in a polymer backbone	Rollers, bearings and other industrial products; also used in automotive, appliance, plumbing and electronics applications
Acrylics	Based on polymethyl methacrylate	Automobile lenses, fluorescent street lights, outdoor signs, and boat windshields; applications requiring high resistance to discoloration and good light transmission properties
Cellulosics	Derived from purified cotton or special grades	Insulation, packaging, toothbrushes
Fluoroplastics	Consist of carbon, fluorine, and or hydrogen atoms in a repeating polymer backbone	Applications requiring optimal electrical and thermal properties, almost complete moisture resistance, chemical inertness; non-stick applications
Nylons	Derived from the reaction of diamines and dibasic acids; characterized by the number of carbon atoms in the repeating polymeric unit	Electrical and electronic components; industrial applications requiring excellent resistance to repeated impact; consumer products such as ski boots and bicycle wheels; appliances and power tool housings; food packaging; wire and cable jacketing; sheets,rods, and tubes; and filaments for brush bristles, fishing line, and sewing thread
Polyarylates	Aromatic polyesters	Automotive appliance, and electrical applications requiring low shrinkage, resistance to hydrolysis, and precision void-free molding
Polyarylsulfones	Consist of phenyl and biphenyl groups linked by thermally stable ether and sulfone groups	Electrical and electronic applications requiring thermal stability including circuit boards, connectors, lamp housings, and motor parts
Polybutylenses	Polymers based on poly(1–butene)	Cold- and hot-water pipes; hotmetal adhesives and sealants
Polybutylene terephthalate (PBT)	Produced by reaction of dimethyl terephthalate with butanediol	Automotive applications such as exterior auto parts; electronic switches; and household applications such as parts for vacuum cleaners and coffee makers
Polycarbonates	Derived from the reaction of bisphenol A and phosgene	Applications requiring toughness, rigidity, and dimensional stability; high heat resistance; good electrical properties; transparency; exceptional impact strength. Used for molded products, solution–cast or extruded films, tubes and pipes, prosthetic devices, nonbreakable windows, street lights, household applicances; compact discs; optical memory disks; and for various applications in fields related to transportation, electronics sporting goods, medical equipment, and food processing
Polyesters	Produced by reacting dicarboxylic acids with dihydroxy alcohols	Reinforced plastics, automotive parts, foams, electrical encapsulation, structural applications, low–pressure laminates, magnetic tapes, pipes, bottles. Liquid crystal polyesters are used as replacements for metals in such applications chemical pumps, electronic components, medical components, and automotive components
Polyetherimides	Consist of repeating aromatic imide and ether units	Temperture sensors; electrical/electronic, medical (surgical instrument parts), industrial; applicance, packaging, and specialty applications
Polyetherketones	Polymerized aromatic ketones	Fine monofilaments, films, engine parts, aerospace composites, and wire and cables, and other applications requiring chmical resistance; exceptional toughness, strength, and rigidity; good radiation resistance; and good fire-safety characteristics

Polyethersulfones	Consist of diaryl sulfone groups with ether linkages	Electrical applications including multipin connectors, integrated circuit sockets, edge and round multipin connectors, terminal blocks, printed circuit boards
Polyethylenes, polypropylenes, and polyallomers	Polyethylenes consist of chains of repeated ethylene units; polypropylenes consist of chains of repeated propylene units; polyallomers are copolymers of propylene and ethylene	Low density polyethylene is used for packaging films, liners for shipping containers, wire and cable coatings, toys plastic bags, electrical insulation. High density polyethylene is used for blow-molded items, films and sheets, containers for petroleum products. Low molecular weight polyethylenes are used as mold release agents, coatings, polishes, and textile finishing agents. Polypropylenes are used as packaging films, molded parts, bottles, artificial turf, surgical casts, nonwoven disposable filters. Polyallomers are used as vacuum formed, injection molded, and extruded products, films, sheets, and wire cables
Polyethylene terephthalate	Prepared from ethylene glycol and either terephthalic acid or an ester of terephthalic acid	Food packaging including bottles, microwave/conventional oven-proof trays; x-ray and other photographic films; magnetic tape
Polyimides and polyamideimides	Polyimides contain imide (–CONHCO–) groups in the polymer chain; polyamide–imides also contain amide (–CONH–) groups	Polyimides are used as high temperature coatings,laminates, and composites for the aerospace industry; ablative materials; oil sealants; adhesive; semi- conductors; bearings; cable insulation; printed circuits; magnetic tapes; flame-resistant fibers. Polyamideimides have been used as replacements for metal parts in the aerospace industry, and as mechanical parts for business machines
Polymethylpentene	Polymerized 4–methylpentene-1	Laboratory ware (beakers, graduates, etc.); electronic and hospital equipment; food packaging; light reflectors
Polyphenylene ethers, modified	Consist of oxidatively coupled phenols and polystyrene	Automobile instrument panels, computer keyboard bases
Polyphenylene sulfides	Para-substituted benzene rings with sulful links	Microwave oven components, precision molded assemblies for disk drives
Polystyrenes	Polymerized ethylene and styrene	Packaging, refrigerator doors, household wares, electrical equipment; toys, cabinets; also used as foams for thermal insulations, light construction, fillers in shipping containers, furniture construction.
Polysulfones	Consist of complicated chains of phenylene units linked with isopropylidene, ether, and suflfone units	Power tool housings, electrical equipment, extruded pipes and sheets, automobile components, electronic parts, appliances, computer components; medical instrumentation and trays to hold instruments during sterilization; food processing equipment; chemical processing equipment, water purification devices
Vinyls	Polymerized vinyl monomers such as polyvinyl chloride and polyvinylidene chloride	Crystal-clear food packaging, water pipes, monolayer films

Table 2. Thermoplastics (cont_’d). (Thomson Gale.)

acrylonitrile-butadiene-styrene (ABS), and polyvinyl chloride, are made up of polymer chains that form randomly organized structures.

These polymer chains may themselves have attached side chains, and the side chains may also be quite long. When the side chains are particularly bulky, molecular branching prevents the molecules from forming ordered regions, and an amorphous plastic will almost certainly result.

Under suitable conditions, however, the entangled polymer chains can disentangle themselves and pack into orderly crystals in the solid state where the chains are symmetrically packed together; these materials are known as crystalline polymers.

Crystalline thermoplastics consist of molecular chains packed together in regular, organized domains that are joined by regions of disordered, amorphous chains. Examples of crystalline thermoplastics include

Table 3. Thermosetting Plastics. (Thomson Gale.)
Thermosetting plastics
Type	Chemical basis	Uses
Alkyd polyesters	Polyesters derived from the reaction of acids with two acid groups, and alcohols with three alcoholic groups per molecule	Moldings, finishes; applications requiring high durability, excellent pigment dispersion, toughness, good adhesion, and good flowing properties
Allyls	Polyesters derived from the reaction of esters of allyl alcohol with dibasic acid	Electrical insulation, applications requiring high resistance to heat, humidity, and corrosive chemicals
Bismaleimides	Generally prepared by the reaction of a diamine with maleic anhydride	Printed wire boards; high performance structural composites
Epoxies	Derived from the reaction of epichlorohydrin with hydroxyl-containing compounds	Encapsulation, electrical insulations, laminates, glass-reinforced plastics, floorings, coatings, adhesives
Melamines	Derived from the reaction of formaldehyde and amino compounds containing NH2	Molded plates, dishes, and other food containers
Phenolics	Derived from the reaction of phenols and formaldehydes	Cements, adhesives
Polybutadienes	Consist of polyethylene with a cross-link at every other carbon in the main chain	Moldings, laminating resins, coatings, cast-liquid and formed-sheet products; applications requiring outstanding electrical properties and thermal stability
Polyesters (thermosetting)	Derived from reactions of dicarboxylic acides with dihydroxy alcohols	Moldings, laminated or reinforced structures, surface gel coatings, liquid castings, furniture products, structures
Polyurethanes	Derived from reactions of polysiocyanates and polyols	Rigid, semi-flexible, and flexible foams; elastomers
Silicones	Consist of alternating silicon and oxygen atoms in a polymer backbone, usually with organic side groups attached to the chain	Applications requiring uniform properties over a wide temperature range; low surface tension; high degree of lubricity; excellent release properties; extreme water repellency; excellent electrical properties over a wide range of temperatures and frequency; inertness and compatibility; chemical inertness; or weather resistance
Ureas	Derived from the reaction of formaldehyde and amino compounds containing NH2 groups	Dinnerware, interior plywood, foams, insulation

acetals, nylons, polyethylenes, polypropylenes, and polyesters.

Liquid crystalline plastics are polymers that form highly ordered, rodlike structures. They have good mechanical properties and are chemically unreactive, and they have melting temperatures comparable to those of crystalline plastics. But unlike crystalline and amorphous plastics, liquid crystalline plastics retain molecular ordering even as liquids. Consequently, they exhibit the lowest shrinkage and warpage of any of the thermoplastics.

Thermosets

Thermosetting plastics, or thermosets, include amino, epoxy, phenolic, and unsaturated polyesters. These materials undergo a chemical change during processing and become hard solids. Unlike the linear molecules in a thermoplastic, adjacent molecules in a thermosetting plastic become cross-linked during processing, resulting in the production of complex networks that restrain the movement of chains past each other at any temperature.

Typical thermosets are phenolics, urea-formaldehyde resins, epoxies, cross-linked polyesters, and most polyurethanes. Elastomers may also be thermosetting. Examples include both natural and synthetic rubbers.

Manufacturing Methods

At some stage in their processing, both thermoplastics and thermosetting plastics are sufficiently fluid to be molded and formed. The manufacture of most plastics is determined by their final shape.

Many cylindrical plastic objects are made by a process called extrusion. The extrusion of thermoplastics consists of melting and compressing plastic granules by rotating them in a screw conveyor in a long barrel, to which heat may be applied if necessary. The screw forces the plastic to the end of the barrel where it

is pushed through a screen on its way to the nozzle. The nozzle determines the final shape of the extruded form. Thermosets may also be extruded if the screw in the conventional extruder is replaced with a plunger-type hydraulic pump.

Plastic powders are directly converted into finished articles by molding. Two types of molding processes are compression molding and injection molding. In compression molding, which is used with thermosetting materials, steam is first circulated through the mold to raise it to the desired temperature; then a plastic powder or tablets are introduced into the mold; and the mold is closed under high pressure and the plastic is liquefied so that it flows throughout the mold. When the mold is re-opened, the solid molded unit is ejected. Injection molding differs from compression molding in that plastic material is rendered fluid outside the mold, and is transferred by pressure into the cooled mold. Injection molding can be used with practically every plastic material, including rubbers.

Sheets, blocks, and rods may be made in a casting process that in effect involves in situ, or in-place, polymerization. In the case of acrylics, sheets are cast in glass cells by filling cells with a polymer solution. The polymer solution solidifies and the sheet is released by separating the glass plates after chilling the assembly in cold water. Blocks can be made in the same way using a demountable container; and rods can be made by polymerizing a polymer syrup under pressure in a cylindrical metal tube.

Plastic foams are produced by compounding a polymer resin with a foaming agent or by injecting air or a volatile fluid into the liquid polymer while it is being processed into a finished product. This results in a finished product with a network of gas spaces or cells that makes it less dense than the solid polymer. Such foams are light and strong, and the rigid type can be machined.

Fillers And Other Modifications

Very few plastics are used in their commercially pure state. Additives currently used include the following: Finely divided rubbers added to more brittle plastics to add toughness; glass, carbon, boron, or metal fibers added to make composite materials with good stress-strain properties and high strength; carbon black or silica added to improve resistance to tearing and to improve stress-strain properties; plasticizers added to soften a plastic by lowering its glass transition temperature or reducing its degree of crystallinity; silanes or other bonding agents added to improve bonding between the plastic and other solid phases; and fillers such as fire retardants, heat or light stabilizers, lubricants, or colorants.

Filled or reinforced plastics are usually referred to as composites. However, some composites includes neither fillers nor reinforcement. Examples are laminates such as plastic sheets or films adhered to non-plastic products such as aluminum foil, cloth, paper or plywood for use in packaging and manufacturing. Plastics may also be metal plated.

Plastics, both glassy and rubbery, may be cross-linked to improve their elastic behavior and to control swelling. Polymers may also be combined to form blends or alloys.

Applications

Plastics have been important in many applications to be listed here. Table 2, “Thermoplastics,” and Table 3, “Thermosetting Plastics,” list hundreds of commercial applications that have been found for specific plastics.

Engineering plastics are tough plastics that can withstand high loads or stresses. They can be machined and remain dimensionally stable. They are typically used in the construction of machine parts and automobile components. Important examples of this class of plastics include nylons, acetals, polycarbonates, ABS resins, and polybutylene terephthalate. The structure of their giant chains makes these plastics highly resistant to shock, and gives them a characteristic toughness.

KEY TERMS

Amorphous— Noncrystalline; lacking a definite crystal structure and a well-defined melting point. Casting—Formation of a product either by filling an open mold with liquid monomer and allowing it to polymerize in place, or by pouring the liquid onto a flat, moving surface.

Composite— A mixture or mechanical combination (on a macroscopic level) of materials that are solid in their finished state, that are mutually insoluble, and that have different chemistries.

Crystalline— Having a regular arrangement of atoms or molecules; the normal state of solid matter.

Extrusion— An operation in which material is forced through a metal forming die, followed by cooling or chemical hardening.

Glass— An amorphous, highly viscous liquid having all of the appearances of a solid.

Inorganic— Not containing compounds of carbon.

Molding— Forming a plastic or rubber article in a desired shape by applying heat and pressure.

Monomer— A substance composed of molecules that are capable of reacting together to form a polymer. Also known as a mer.

Organic— Containing carbon atoms, when used in the conventional chemical sense. Originally, the term was used to describe materials of living origin.

Plastic— Materials, usually organic, that under suitable application of heat and pressure, can be caused to flow and to assume a desired shape that is retained when the pressure and temperature conditions are withdrawn.

Polymer— A substance, usually organic, composed of very large molecular chains that consist of recurring structural units.

Synthetic— Referring to a substance that either reproduces a natural product or that is a unique material not found in nature, and which is produced by means of chemical reactions.

Thermoplastic— A high molecular weight polymer that softens when heated and that returns to its original condition when cooled to ordinary temperatures.

Thermoset— A high molecular weight polymer that solidifies irreversibly when heated.

Plastics are almost always electrically insulating, and for this reason they have found use as essential components of electrical and electronic equipment (including implants in the human body).

Major applications have been found for plastics in the aerospace, adhesives, coatings, construction, electrical, electronic, medical, packaging, textile, and automotive industries.