plastics

materials that contain polymers. While they are being manufactured, plastic articles are in a state of viscous flow or in a highly elastic state, but upon completion of processing they assume a glass-like or crystalline state. Plastics can either be thermosetting materials or thermoplastic materials, depending on which processes are used in their synthesis. Thermosetting materials undergo hardening during processing, a chemical reaction in which a cross-linked polymer is formed. As hardening proceeds, the plastic irreversibly loses its capacity to assume the state of viscous flow, either as a solution or as a melt. Hardening does not occur during the processing of thermoplastics, and an object that is made from a thermoplastic retains its capacity to reenter the state of viscous flow.

Plastics usually consist of several compatible or incompatible components. In addition to the polymer, a plastic may contain several additives, for example, plasticizers, which lower the flow temperature and the viscosity of the polymer; stabilizers, which retard the aging of the polymer; colorants; and fillers. Plastics may be homogeneous, that is, consisting of a single phase, or heterogeneous, that is, consisting of several phases. In homogeneous plastics, the polymer is the principal component that determines the properties of the material. The remaining components are dissolved in the polymer, and these additives also account for some of the plastic’s properties. The polymer in heterogeneous plastics functions as the dispersion medium and binder for the components that are dispersed in it; these components exist as separate phases. In order that an external force be evenly distributed over all the components of a heterogeneous plastic, firm cohesion is required at the contact boundary between the binder and particles of filler. This cohesion is achieved by adsorption or chemical reaction between the binder and the surface of the filler.

Filled plastic. The filler in a plastic material can be in a gas or condensed phase. When it is in a condensed phase, its modulus of elasticity can be either lower or higher than the modulus of elasticity of the binder; fillers of the first kind are called low-modulus fillers, while those of the second kind are called high-modulus fillers. Gas-filled plastics include expanded, or foamed, plastics, which are the lightest of all plastics and which usually have an apparent density of 0.02–0.8 g/cm³.

Low-modulus fillers, which do not lower the polymer’s hardness or thermal stability, are elastomers that give the plastic increased stability toward direct impact and stresses of alternating sign (see Table 1); these fillers also prevent the occurrence of microfissures in the binder. However, the coefficient of thermal expansion for plastics with low-modulus fillers is higher, and the resistance to deformation is lower, than with solid binders. The low-modulus filler is dispersed in the binder in the form of particles that range in size from 0.2 to 10 microns (μ). The proper dispersion results from polymerization of the monomer on the surfaces of synthetic latex particles, from hardening of the oligomer in which the elastomer has been dispersed, and from grinding the mixture of hard polymer and elastomer. Filling should be accompanied by the formation of a copolymer at the phase boundary between the particles of low-modulus filler and the binder. The copolymer permits the binder and low-modulus filler to react in unison to any external factor that may arise under actual conditions of use. The higher the filler’s modulus of elasticity and the more the plastic is filled, the more stable the plastic is to deformation. Nevertheless, introduction of high-modulus fillers usually generates residual stresses in the binder and, consequently, reduces the strength and density of the polymer phase.

The properties of plastics with solid fillers are determined by several factors, including the degree of filling, the type of filler and binder, the strength of adhesion at the contact boundary between phases, the thickness of the boundary layer, and the shape and relative position of the particles of filler. The properties of plastics that contain small filler particles in uniform distribution are isotropic; the most desirable properties result when the plastic is filled to the extent that all of the binder is adsorbed onto the surface of the filler particles. An increase in temperature and pressure results in partial desorption of the binder from the surface of the filler; thus it is possible to mold the plastic into intricately shaped objects that contain brittle reinforcing elements. Depending on their nature, small filler particles can increase, within certain limits, the plastic’s modulus of elasticity as well as its hardness and strength; the particles can also be chosen so as to impart certain frictional, antifrictional, thermal-insulating, thermal-conducting, or electrical-conducting properties. Fillers whose particles are hollow are used to prepare low-density plastics. Such materials are good insulators against heat and sound.

The use of such fillers as natural and synthetic organic fibers as well as inorganic fibers of glass, quartz, carbon, boron, or asbestos limits the methods by which objects are molded and complicates production of articles with complex shapes; however, it also greatly increases the strength of the material. Fibers 2–4 mm in length are of sufficient size to strengthen fiber-reinforced materials in which the synthetic fibers are made from organic compounds, carbon, or glass (see VOLOKNIT). Increasing the length of fibers strengthens the plastic because long fibers can intertwine easily. Furthermore, with high-modulus fillers the stresses that are distributed throughout the binder at the ends of fibers are reduced by the presence of longer fibers, and this stress reduction also strengthens the plastic.

When applicable, fibers can be combined into yarn and fabrics of various weaves. Plastics that are reinforced with fabrics are examples of laminate plastics, a class of substances distinguished by anisotropic properties. In particular, laminate plastics are able to resist stresses that are applied along the length of the fibers in a layer but do not resist stresses that are applied at right angles to the fibers. This disadvantageous aspect of the anisotropy of laminate plastics is partially overcome in three-dimensionally woven plastics, in which the separate fabric layers are interwoven. The binder fills the voids in the interwoven structure and, after hardening, fixes the shape that has been imparted to the object by filler.

In products with simple shapes and, in particular, in hollow rotating bodies, the fibers that are used as fillers are oriented in the direction of application of external forces. The strength of such plastics along the given axis is basically determined by the strength of the fibers; the binder merely determines the shape of the article and uniformly distributes stresses over all the fibers. The modulus of elasticity and the tensile strength of the article in the direction in which the fibers are oriented attain very high values (see Table 1). These properties depend on the degree of filling. Panel structures may be conveniently produced from laminate plastics with fillers that consist of wood ply or paper, including paper made from synthetic fibers. Considerable weight reductions can be achieved, and stiffness can be retained by using a three-layer, or sandwich, structure in which the intermediate layer consists of foamed or cellular plastic.

Thermoplastics. The most widely used thermoplastics are polyethylene, polyvinyl chloride, and polystyrene, which are manufactured as homogeneous materials or which can contain low-modulus fillers; less frequently, thermoplastics are filled with gases, mineral powders, or organic synthetic fibers.

Plastics that are based on polyethylene are readily molded and welded to produce intricately shaped articles. Such articles are resistant to impact and vibration loads, are chemically stable, and are distinguished by low density and outstanding electrical-insulating properties (their dielectric constant ranges from 2.1 to 2.3). Articles with increased strength and thermal stability are produced from polyethylene that is filled with short glass fibers up to 3 mm in length. Filling a plastic to 20 percent increases the tensile strength by a factor of 2.5, bending strength by a factor of 2, impact viscosity by a factor of 4, and thermal stability by a factor of 2.2.

Rigid plastics that are based on polyvinyl chloride are called vinyl plastics. They include impact-resistant types, which contain low-modulus fillers. These types are molded with considerably greater difficulty than the polyethylene plastics, but they are much stronger under static loads, their creep is lower, and they are harder. Plasticized polyvinyl chloride is more widely used than rigid polyvinyl-chloride plastic. It is easily molded and reliably welded, while the required combination of thermal resistance and resistance to deformation is achieved by selecting the proper ratio of plasticizer and solid filler.

Plastics that are derived from polystyrene are processed much more easily than those that are derived from polyvinyl chloride. They resemble polyethylene plastics in their dielectric properties, are transparent, and are not much less resistant to static loads than polyvinyl-chloride plastics. However, they are more brittle and less solvent-resistant, and they are also flammable. Low impact viscosity and failure owing to rapid propagation of microfissures are particularly characteristic of polystyrene plastics. These undesirable properties can be eliminated by filling with elastomers, that is, polymers or copolymers with glass-transition temperatures below — 40°C. Impact-resistant polystyrene of the highest quality, which is filled with a low-modulus filler, is produced by polymerizing styrene on particles of butadiene-styrene latex or butadiene-nitrile latex. The plastic that is designated ABS contains about 15 percent gel fraction, which consists of block and graft copolymers of polystyrene with the above-mentioned copolymers of butadiene; this gel fraction forms the boundary layer and binds particles of elastomer to the polystyrene matrix. The lower limit to thermal resistance is a function of the elastomer’s glass-transition temperature, while the upper limit is a function of polystyrene’s glass-transition temperature.

The thermal resistance of thermoplastics ranges from 60° to 80°C, and the coefficient of thermal expansion is high (1 × 10^–4). The properties of these plastics undergo sharp changes with small temperature changes, and the resistance to deformation under stress is low. These disadvantages are partially avoided in thermoplastics that are called ionomers, such as the copolymers of ethylene, propylene, or styrene with monomers that contain ionic groups, usually unsaturated carboxylic acids or salts thereof. The interaction of the ionic groups between macromolecules below the glass-transition temperature generates strong physical bonds that are broken as the polymer softens. Ionomers successfully combine the increased rigidity and resistance to deformation that are characteristic of cross-linked polymers with the capacity of thermoplastics to be easily molded. However, the presence of ionic groups in the polymer lowers its dielectric properties and resistance to moisture.

Plastics that are based on polypropylene, polyformaldehyde, polycarbonates, polyacrylates, and polyamides (especially aromatic polyamides) have a higher thermal resistance (100°-130°C) and exhibit a less abrupt change in properties with increasing temperature than plastics that are produced from polyethylene, polyvinyl chloride, and polystyrene. The variety of articles that are being produced from polycarbonate plastics, especially glass-fiber filled plastics, is rapidly increasing. Parts that have to withstand friction are frequently made from plastics that are based on aliphatic polyamides and are filled with a thermal-conducting substance, for example, graphite.

Plastics that are derived from polytetrafluoroethylene polymers and tetrafluoroethylene copolymers exhibit especially high chemical stability and impact resistance as well as very desirable dielectric properties. Polyurethane plastics successfully combine resistance to wear with resistance to low temperatures and long-term durability under stresses of alternating sign. Polymethyl methacrylate plastics are used to make transparent, weatherproof materials. The production of polymethyl methacrylate plastics and thermoplastics with increased thermal resistance accounts for about 10 percent of the total production volume of all plastics.

Since cross-linkage does not occur during the formation of thermoplastics, it is possible to intensify processing to certain limits. The basic methods for shaping articles that are made from thermoplastics are injection molding, extrusion molding, vacuum shaping, and pneumatic molding. Since melts of high-molecular polymers are very viscous, thermoplastics are shaped in molding machines or extruders at pressures that range from 30 to 130 meganewtons per square meter (MN/m²), or 300 to 1,300 kilograms-force per square centimeter (kgf/cm²). Research on the production of thermoplastics is aimed at producing new combinations of properties with the old polymers through the addition of low-modulus fillers as well as powdered fillers and fillers that consist of short fibers.

Thermosetting plastics. In finished thermosetting-plastic products, the polymer phase assumes a three-dimensional network structure. As a result, thermosetting plastics exceed thermoplastic materials in hardness, modulus of elasticity, thermal resistance, and endurance limit, while their coefficient of thermal expansion is lower than that of thermoplastics. At the same time, the properties of hardened thermosetting plastics are not as dependent on temperature. However, one disadvantage is that cross-linked thermosetting plastics cannot enter a state of viscous flow, and thus the polymer must be synthesized in several stages.

The first stage ends with the formation of oligomers, which are polymers with a molecular weight that ranges from 500 to 1,000; according to certain usages, the oligomers are called resins. Because of the low viscosity of their solutions or melts, resins can be easily spread over the surface of the particles of filler, even if the degree of filling is as high as 80–85 percent by weight. After all components are mixed, a thermosetting plastic is sufficiently fluid for articles to be shaped by casting, contact molding, or winding. Thermosetting plastics are called premixes if their filler consists of fine particles and prepregs if the filler consists of continuous fibers, fabric, or paper. Equipment for shaping articles from premixes and prepregs is simple, and the energy expenditures are small, but the process includes the extra step of holding the material in individual molds so as to allow the binder to harden. If the resin is cross-linked by polycondensation, molding is accompanied by extensive shrinking, which gives rise to significant residual stresses in the material; at the same time, the solidity, density, and strength of the material falls far short of the limiting values (with the exception of articles that are produced by winding under tension). To avoid these disadvantages, resins that are cross-linked by polycondensation reactions are preliminarily hardened by rolling or drying after their components are mixed. This reduces the length of time that the material must be held in molds and improves the quality of products, but the molds must be filled at increased pressures that range from 25 to 60 MN/m² (250–600 kgf/cm²).

The resin in thermosetting plastics undergoes hardening spontaneously at a rate that is proportional to temperature; hardening is also accomplished by using polyfunctional low-molecular materials that are called hardeners. Thermosetting plastics— regardless of the filler added—can be prepared by using phenol-aldehyde resins as binders; these plastics are called phenoplasts. The binders are frequently made elastic with polyvinyl butyral, butadiene-nitrile elastomers, polyamides, or polyvinyl chloride; epoxide-resin binders are also used, in which case phenol- or aniline-formaldehyde resins or hardening oligoesters are sometimes added.

High-strength plastics with a thermal resistance of up to 200°C are produced by combining glass fibers or fabrics with hardening oligoesters, phenol-formaldehyde resins, or epoxide resins. Articles that are designed to function for prolonged periods at 300°C are made from glass- or asbestos-fiber-reinforced plastics with organosilicon binders. Polyimides in combination with silica, asbestos, or carbon fibers are manufactured into articles that are designed to perform at temperatures between 300° and 340°C. Phenoplasts and plastics that are based on polyimides can be filled with carbon fibers and subjected to graphitization after molding, the resultant plastic is suitable for use between 250° and 500°C in air and 2000° and 2500°C in inert mediums.

High-modulus plastics, whose modulus of elasticity ranges from 250 to 350 GN/m² (25,000–35,000 kgf/cm²), are produced by combining epoxide resins with fibers of carbon or boron or with monocrystalline fibers. Both solid and lightweight plastics that resist impact and vibrational stress can be produced by combining epoxide, polyester, or melamine-formaldehyde resins with synthetic fibers or fabrics as well as with paper that consists of synthetic fibers or fabrics. Such thermosetting plastics are water resistant and retain their dielectric properties and impermeability under complex stresses. Plastics that are produced from quartz fibers and a polyester or organosilicon binder characteristically have outstanding dielectric properties, for example, a dielectric constant that ranges from 3.5 to 4.0. Laminated wood-plastic materials are widely used in shipbuilding and in the building-materials industry.

Production volume and uses. Plastics that are derived from such natural resins as rosin, shellac, and bitumen have been known since ancient times. Celluloid, which was first produced in 1872, in the USA, was the first plastic ever prepared from a synthetic polymer—cellulose nitrate. Between 1906 and 1910 the first thermosetting plastics were produced in Russia and Germany on an experimental basis; the polymers used were phenol-formaldehyde resins. The production of thermoplastic materials, for example, polyvinyl chloride, polymethyl methacrylate, polyamides, and polystyrene, was first organized during the 1930’s in several industrially developed nations, including the USSR, the USA, and Germany. However, it was not until after World War II that the plastics industry began to develop at a rapid pace. Polyethylene, whose production volume is the largest of all plastics, was first produced in many countries during the 1950’s.

The establishment of the production of plastics as an independent branch of industry in the USSR dates back to the period of the prewar five year plans. The production of plastics totaled 24,000 tons in 1940, 75,000 in 1950, 312,000 in 1960, 1,673,000 in 1970, and about 2,300,000 in 1973. Eighty-four percent of the USSR’s entire production of plastics takes place in the European USSR. The plants that are located there include the Karbolit Works in Orekhovo-Zuevo, the Kazan Organic Synthesis Works, the Sverdlovsk Plastics Works, the Polotsk Chemical Combine, the Vladimir Chemical Works, the Gorlovka Chemical Combine, and the Moscow Refinery. About 30 percent of the production of plastics will take place in the eastern part of the country once the Tomsk and Tobol’sk Petrochemical Complexes, which exploit the Tiumen’ oil deposits, are completed and once the Omsk Petrochemical Complex and affiliated factories are further developed. At present, the principal enterprises in these areas are the Karbolit Works in Kemerovo and the Tiumen’ Plastics Works.

In 1973, the production volume of plastics in some of the industrially developed capitalist countries was 13.2 million tons in the USA, 6.5 million in Japan and the Federal Republic of Germany, 2.5 million in France, 2.3 million in Italy, and 1.9 million in Great Britain. The production of plastics is developing much faster than that of such traditional construction materials as aluminum and cast iron (see Table 2).

The worldwide production volume of polymers for plastics attained about 43 million tons in 1973. Thermoplastics made up about 75 percent of this figure; specifically, 25 percent of the worldwide volume was accounted for by polyethylene, 20 percent by polyvinyl chloride, 14 percent by polystyrene and its derivatives, and 16 percent by other polymers. A tendency has appeared toward a further increase in the share of thermoplastics, mainly polyethylene, in the overall production of plastics.

Although thermosetting resins account for only 25 percent of the total production of polymers for plastics, the actual production volume of thermosetting plastics (polymer plus filler) is higher than that of thermoplastic materials, since the degree of filling in thermosetting plastics is only 60 to 80 percent. Table 3 shows the distribution of applications of plastics in various areas of technology.

The consumption of plastics in the construction industry is steadily increasing. While the worldwide production of plastics increased between 1960 and 1970 about fourfold, the use of plastics in construction increased eightfold. This was due to the unique physicomechanical and valuable architectonic properties of polymers. The principal advantages of plastics over other structural materials are lightness and a relatively high specific strength; these properties provide a solution to one of the most important problems in modern industrial construction, namely, that of reducing the weight of structures. Plastics, mostly in the form of rolls or tiles, are used most often as flooring and other finishing materials; they are also often used as sealants, moisture insulators, and thermal insulators as well as in the production of pipes and sanitary equipment. They are also used to manufacture other items, including partitions, stationary panels, roofing materials (including translucent roofing materials), window sash, doors, air-supported architectural structures, tourist’s kiosks, and summer pavilions.

Plastics play a leading role among construction materials that are used in machine building. The volume of their consumption in this area is approaching that of steel, primarily because of their low cost. The use of plastics also improves the most important technical and economic specifications of machines: mass is reduced, while other parameters, for example, durability and reliability, improve. Plastics are used to make many parts of machines, including toothed and worm gears, pulleys, bearings, slide guides for lathes, and rollers, as well as piping, bolts, nuts, and a wide assortment of industrial fittings.

Light weight and the possibility for extensive manipulation of technical properties are the principal advantages that have resulted in the wide use of plastics in aviation construction. The number of parts in an airplane that are produced from plastics has increased from 25 in 1940 to 10,000 in 1970. The most progress in the use of polymers has been made in the construction of light planes and helicopters. Plastics have been increasingly used in rockets and spacecraft, which can be 50 percent plastic by weight. Thermosetting plastics are used in aeronautics in the production of many parts, including rocket jet engines and such reinforced elements as the tail group, wings, and fuselage of airplanes and the shells of rockets. Furthermore, they are used for bearings and chassis supports, the rotors of helicopters, thermal-insulating elements, and suspended fuel tanks. Applications of thermoplastic materials include the production of glazing compounds and antenna shrouds as well as the interior-decorative finishing of aircraft, while foamed and cellular plastics are used as fillers in three-layer structures that must withstand heavy loads.

Today, plastics are used in many ways in shipbuilding, and the future possible uses of these materials are practically unlimited. Hulls and hull structures are usually made from glass-reinforced plastics. Machinery parts as well as the interior finish and thermal, sound, and moisture insulation of ships’ quarters are made from other plastics.

Plastics are particularly promising in their applications to the motor-vehicle industry, where they can be used to make interiors and bodies. The body accounts for about one-half of an automobile’s entire weight, and about 40 percent of the cost. Plastic bodies are more reliable and durable than metal bodies, and they are simpler and less costly to repair. However, plastics have not yet achieved wide acceptance as construction materials for the large parts of automobiles mainly owing to inadequate stiffness and relatively low resistance to atmospheric conditions. They are most widely used today for finishing auto interiors, and they are also used to make parts for motors, transmissions, and chassis.

Plastics have become very important in electrical technology, where they are either a fundamental or necessary component of all insulating elements in electrical machinery, electrical instruments, and cables. Plastics are frequently used to protect insulation from mechanical wear and from the action of aggressive mediums. Other applications of plastics to electrical technology exist, among them, the making of structural parts of electrical ware.

A characteristic trend in agriculturally developed nations is the ever wider consumption of plastics, particularly plastic films, which are used, for example, in machinery parts, in soil mulching, as coatings for seeds, and to protect and store agricultural produce. In land reclamation, and irrigation, sheets of plastic are used as screens to prevent unfiltered water from escaping from irrigation channels and reservoirs. Plastics are also used in agriculture to make various kinds of pipes and aquicultural structures.

Plastics in medicine have made it possible to mass-produce instruments, special dishes, and various types of packaging for medicines. Surgery makes use of plastic heart valves, prosthetic limbs, orthopedic inserts, braces, stomatological prostheses, and crystalline lenses of the eye.