Plastic, polymeric material that has the capability of being molded or shaped, usually by the application of heat and pressure. This property of plasticity, often found in combination with other special properties such as low density, low electrical conductivity, transparency, and toughness, allows plastics to be made into a great variety of products. These include tough and lightweight beverage bottles made of polyethylene terephthalate (PET), flexible garden hoses made of polyvinyl chloride(PVC), insulating food containers made of foamed polystyrene, and shatterproof windows made of polymethyl methacrylate.
In this article a brief review of the essential properties of plastics is provided, followed by a more detailed description of their processing into useful products and subsequent recycling. For a fuller understanding of the materials from which plastics are made, see chemistry of industrial polymers.
The Composition, Structure, And Properties Of Plastics
Many of the chemical names of the polymers employed as plastics have become familiar to consumers, although some are better known by their abbreviations or trade names. Thus, polyethylene terephthalate and polyvinyl chloride are commonly referred to as PET and PVC, while foamed polystyrene and polymethyl methacrylate are known by their trademarked names, Styrofoam and Plexiglas (or Perspex).
Industrial fabricators of plastic products tend to think of plastics as either “commodity” resins or “specialty” resins. (The term resin dates from the early years of the plastics industry; it originally referred to naturally occurring amorphous solids such as shellac and rosin.) Commodity resins are plastics that are produced at high volume and low cost for the most common disposable items and durable goods. They are represented chiefly by polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Specialty resins are plastics whose properties are tailored to specific applications and that are produced at low volume and higher cost. Among this group are the so-called engineering plastics, or engineering resins, which are plastics that can compete with die-cast metals in plumbing, hardware, and automotive applications. Important engineering plastics, less familiar to consumers than the commodity plastics listed above, are polyacetal, polyamide (particularly those known by the trade name nylon), polytetrafluoroethylene (trademark Teflon), polycarbonate, polyphenylene sulfide, epoxy, and polyetheretherketone. Another member of the specialty resins is thermoplastic elastomers, polymers that have the elastic properties of rubber yet can be molded repeatedly upon heating. Thermoplastic elastomers are described in the article elastomer.
Plastics also can be divided into two distinct categories on the basis of their chemical composition. One category is plastics that are made up of polymers having only aliphatic (linear) carbon atoms in their backbone chains. All the commodity plastics listed above fall into this category. The structure of polypropylene can serve as an example; here attached to every other carbon atom is a pendant methyl group (CH3):
The other category of plastics is made up of heterochain polymers. These compounds contain atoms such as oxygen, nitrogen, or sulfur in their backbone chains, in addition to carbon. Most of the engineering plastics listed above are composed of heterochain polymers. An example would be polycarbonate, whose molecules contain two aromatic (benzene) rings:
The distinction between carbon-chain and heterochain polymers is reflected in the table, in which selected properties and applications of the most important carbon-chain and heterochain plastics are shown and from which links are provided directly to entries that describe these materials in greater detail. It is important to note that for each polymer type listed in the table there can be many subtypes, since any of a dozen industrial producers of any polymer can offer 20 or 30 different variations for use in specific applications. For this reason the properties indicated in the table must be taken as approximations.
|Properties and applications of commercially important plastics|
|*All values shown are for glass-fibre-reinforced samples (except for polyurethane).|
|polymer family and type||density
at 1.8 MPa
|high-density polyethylene (HDPE)||0.95–0.97||high||–120||137||—|
|low-density polyethylene (LDPE)||0.92–0.93||moderate||−120||110||—|
|polyvinyl chloride, unplasticized (PVC)||1.3–1.6||nil||85||—||—|
|polymethyl methacrylate (PMMA)||1.2||nil||115||—||—|
|polyethylene terephthalate (PET)||1.3–1.4||moderate||69||265||—|
|polyphenylene sulfide (PPS)||1.35||moderate||88||288||—|
|polycaprolactam (nylon 6)||1.1–1.2||moderate||50||210–220||—|
|urea and melamine formaldehyde||1.5–2.0||nil||—||—||190–200|
|polymer family and type||tensile
|typical products and applications|
|high-density polyethylene (HDPE)||20–30||10–1,000||1–1.5||milk bottles, wire and cable insulation, toys|
|low-density polyethylene (LDPE)||8–30||100–650||0.25–0.35||packaging film, grocery bags, agricultural mulch|
|polypropylene (PP)||30–40||100–600||1.2–1.7||bottles, food containers, toys|
|polystyrene (PS)||35–50||1–2||2.6–3.4||eating utensils, foamed food containers|
|acrylonitrile-butadiene-styrene (ABS)||15–55||30–100||0.9–3.0||appliance housings, helmets, pipe fittings|
|polyvinyl chloride, unplasticized (PVC)||40–50||2–80||2.1–3.4||pipe, conduit, home siding, window frames|
|polymethyl methacrylate (PMMA)||50–75||2–10||2.2–3.2||impact-resistant windows, skylights, canopies|
|polytetrafluoroethylene (PTFE)||20–35||200–400||0.5||self-lubricated bearings, nonstick cookware|
|polyethylene terephthalate (PET)||50–75||50–300||2.4–3.1||transparent bottles, recording tape|
|polycarbonate (PC)||65–75||110–120||2.3–2.4||compact discs, safety glasses, sporting goods|
|polyacetal||70||25–75||2.6–3.4||bearings, gears, shower heads, zippers|
|polyetheretherketone (PEEK)||70–105||30–150||3.9||machine, automotive, and aerospace parts|
|polyphenylene sulfide (PPS)||50–90||1–10||3.8–4.5||machine parts, appliances, electrical equipment|
|cellulose diacetate||15–65||6–70||1.5||photographic film|
|polycaprolactam (nylon 6)||40–170||30–300||1.0–2.8||bearings, pulleys, gears|
|polyester (unsaturated)||20–70||<3||7–14||boat hulls, automobile panels|
|epoxies||35–140||<4||14–30||laminated circuit boards, flooring, aircraft parts|
|phenol formaldehyde||50–125||<1||8–23||electrical connectors, appliance handles|
|urea and melamine formaldehyde||35–75||<1||7.5||countertops, dinnerware|
|polyurethane||70||3–6||4||flexible and rigid foams for upholstery, insulation|
For the purposes of this article, plastics are primarily defined not on the basis of their chemical composition but on the basis of their engineering behaviour. More specifically, they are defined as either thermoplastic resins or thermosetting resins.
Polymers are chemical compounds whose molecules are very large, often resembling long chains made up of a seemingly endless series of interconnected links. The size of these molecules, as is explained in chemistry of industrial polymers, is extraordinary, ranging in the thousands and even millions of atomic mass units (as opposed to the tens of atomic mass units commonly found in other chemical compounds). The size of the molecules, together with their physical state and the structures that they adopt, are the principal causes of the unique properties associated with plastics—including the ability to be molded and shaped.
Thermoplastic and thermosetting
As mentioned above, polymers that are classified as plastics can be divided into two major categories: thermoplastics and thermosets. Thermoplastics such as polyethylene and polystyrene are capable of being molded and remolded repeatedly. Thus, a foamed-polystyrene cup can be heated and reshaped into a new form—for instance, a dish. The polymer structure associated with thermoplastics is that of individual molecules that are separate from one another and flow past one another. The molecules may have low or extremely high molecular weight, and they may be branched or linear in structure, but the essential feature is that of separability and consequent mobility.
Thermosets, on the other hand, cannot be reprocessed upon reheating. During their initial processing, thermosetting resins undergo a chemical reaction that results in an infusible, insoluble network. Essentially, the entire heated, finished article becomes one large molecule. For example, the epoxy polymer used in making a fibre-reinforced laminate for a golf club undergoes a cross-linking reaction when it is molded at a high temperature. Subsequent application of heat does not soften the material to the point where it can be reworked and indeed may serve only to break it down.
Physical states and molecular morphologies
figureThe plastic behaviour of polymers is also influenced by their morphology, or arrangement of molecules on a large scale. Stated simply, polymer morphologies are either amorphous or crystalline. Amorphous molecules are arranged randomly and are intertwined, whereas crystalline molecules are arranged closely and in a discernible order. Most thermosets are amorphous, while thermoplastics may be amorphous or semicrystalline. Semicrystalline materials display crystalline regions, called crystallites, within an amorphous matrix. In addition, the relative crystallinities of the major plastics are indicated in the table of properties and applications.
By definition, thermoplastic materials retain their molded shapes up to a certain temperature, which is set by the glass transition temperature or the melting temperature of the particular polymer. Below a certain temperature, known as the glass transition temperature (Tg), the molecules of a polymer material are frozen in what is known as the glassy state; there is little or no movement of molecules past one another, and the material is stiff and even brittle. Above Tg the amorphous parts of the polymer enter the rubbery state, in which the molecules display increased mobility and the material becomes plastic and even elastic (that is, able to be stretched). In the case of noncrystalline polymers such as polystyrene, raising the temperature further leads directly to the liquid state. On the other hand, for partly crystalline polymers such as low-density polyethylene or polyethylene terephthalate, the liquid state is not reached until the melting temperature (Tm) is passed. Beyond this point the crystalline regions are no longer stable, and the rubbery or liquid polymers can be molded or extruded. Thermosets, which do not melt upon reheating, can be dimensionally stable up to a temperature at which chemical degradation begins.
The physical state and morphology of a polymer have a strong influence on its mechanical properties. A simple measure of the differences produced in mechanical behaviour is the elongation that occurs when a plastic is loaded (stressed) in tension. A glassy polymer such as polystyrene is quite stiff, showing a high ratio of initial stress to initial elongation. On the other hand, polyethylene and polypropylene, two highly crystalline plastics, are usable as films and molded objects because at room temperature their amorphous regions are well above their glass transition temperatures. The leathery toughness of these polymers above Tg results from the crystalline regions that exist in an amorphous, rubbery matrix. Elongations of 100 to 1,000 percent are possible with these plastics. In PET, another semicrystalline plastic, the crystalline portions exist in a glassy matrix because the Tg of PET is above room temperature. This gives the material a stiffness and high dimensional stability under stress that are of great importance in beverage bottles and recording tape.
Almost all plastics exhibit some elongation on being stressed that is not recovered when the stress is removed. This behaviour, known as “creep,” may be very small for a plastic that is well below its Tg, but it can be significant for a partly crystalline plastic that is above Tg.
The most commonly specified mechanical properties of polymers include stiffness and breaking stress, quantified in the table of properties and applications as flexural modulus and tensile strength. Another important property is toughness, which is the energy absorbed by a polymer before failure—often as the result of a sudden impact. Repeated applications of stress well below the tensile strength of a plastic may result in fatigue failure.
Most plastics are poor conductors of heat; conductivity can be reduced even further by incorporating a gas (usually air) into the material. For instance, foamed polystyrene used in cups for hot beverages has a thermal conductivity about one-quarter that of the unfoamed polymer. Plastics also are electrical insulators unless especially designed for conductivity. Besides conductivity, important electrical properties include dielectric strength (resistance to breakdown at high voltages) and dielectric loss (a measure of the energy dissipated as heat when an alternating current is applied).
In many plastic products, the polymer is only one constituent. In order to arrive at a set of properties appropriate to the product, the polymer is almost always combined with other ingredients, or additives, which are mixed in during processing and fabrication. Among these additives are plasticizers, colorants, reinforcements, and stabilizers. These are described in turn below.
Plasticizers are used to change the Tg of a polymer. Polyvinyl chloride (PVC), for instance, is often mixed with nonvolatile liquids for this reason. Vinyl siding used on homes requires an unplasticized, rigid PVC with a Tg of 85 to 90 °C (185 to 195 °F). A PVC garden hose, on the other hand, should remain flexible even at 0 °C (32 °F). A mixture of 30 parts di(2-ethylhexyl) phthalate (also called dioctyl phthalate, or DOP) with 70 parts PVC will have a Tg of about −10 °C (15 °F), making it suitable for use as a garden hose.
Although other polymers can be plasticized, PVC is unique in accepting and retaining plasticizers of widely varying chemical composition and molecular size. The plasticizer may also change the flammability, odour, biodegradability, and cost of the finished product.
For most consumer applications, plastics are coloured. The ease with which colour is incorporated throughout a molded article is an advantage of plastics over metals and ceramics, which depend on coatings for colour. Popular pigments for colouring plastics include titanium dioxide and zinc oxide (white), carbon (black), and various other inorganic oxides such as iron and chromium. Organic compounds can be used to add colour either as pigments (insoluble) or as dyes (soluble).
Reinforcements, as the name suggests, are used to enhance the mechanical properties of a plastic. Finely divided silica, carbon black, talc, mica, and calcium carbonate, as well as short fibres of a variety of materials, can be incorporated as particulate fillers. (The use of long or even continuous fibres as reinforcement, especially with thermosets, is described below in Fibre reinforcement.) Incorporating large amounts of particulate filler during the making of plastics such as polypropylene and polyethylene can increase their stiffness. The effect is less dramatic when temperature is below the polymer’s Tg.
In order for a plastic to have a long and useful life in any application, the properties of that plastic should change as little as possible with time. Stabilizers are added, usually in small quantities, to counter the effects of aging.
Because all carbon-based polymers are subject to oxidation, the most common stabilizers are antioxidants. Hindered phenols and tertiary amines are used in plastics in concentrations as low as a few parts per million. For example, butylated hydroxytoluene (BHT) is used in polyolefin packaging films for foods and pharmaceuticals. PVC requires the addition of heat stabilizers in order to reduce dehydrohalogenation (loss of hydrogen chloride [HCl]) at processing temperatures. Zinc and calcium soaps, organotin mercaptides, and organic phosphites are among the many additives found to be effective. Other stabilizers are designed specifically to reduce degradation by sunlight, ozone, and biological agents.
The Processing And Fabrication Of Plastics
The processing of raw materials into usable forms is termed fabrication or conversion. An example from the plastics industry would be the conversion of plastic pellets into films or the conversion of films into food containers. In this section the mixing, forming, finishing, and fibre reinforcing of plastics are described in turn.
The first step in most plastic fabrication procedures is compounding, the mixing together of various raw materials in proportions according to a specific recipe. Most often the plastic resins are supplied to the fabricator as cylindrical pellets (several millimetres in diameter and length) or as flakes and powders. Other forms include viscous liquids, solutions, and suspensions.
Mixing liquids with other ingredients may be done in conventional stirred tanks, but certain operations demand special machinery. Dry blending refers to the mixing of dry ingredients prior to further use, as in mixtures of pigments, stabilizers, or reinforcements. However, polyvinyl chloride(PVC) as a porous powder can be combined with a liquid plasticizer in an agitated trough called a ribbon blender or in a tumbling container. This process also is called dry blending, because the liquid penetrates the pores of the resin, and the final mixture, containing as much as 50 percent plasticizer, is still a free-flowing powder that appears to be dry.
The workhorse mixer of the plastics and rubber industries is the internal mixer, in which heat and pressure are applied simultaneously. The Banbury mixer resembles a robust dough mixer in that two interrupted spiral rotors move in opposite directions at 30 to 40 rotations per minute. The shearing action is intense, and the power input can be as high as 1,200 kilowatts for a 250-kg (550-pound) batch of molten resin with finely divided pigment.