Ееее. I. Гласные звуки i, i. Ii. Text A About myself
 Скачать 0.96 Mb.
|
1. ALLOYSBronze and brass, the first alloys in the history of metallurgy, were probably obtained by man accidentally when melting mixed metal ores. Much later alloys of iron were obtained. Steel was made in small quantities in early times until the mid-19th century when it was manufactured on a large scale in the iron and steel industry. The commercial production of pure aluminium in about 1890 began a new range of alloys and among them duralumin, an alloy of about 94 per cent aluminium, with small quantities of copper, manganese, magnesium, and silicon. Most of aluminium alloys are both light and strong. Nickel is often mixed with other metals for special purposes: permalloy is a nickel-iron alloy that is magnetically soft. The polarity of its magnetic field can be easily changed and it is used for transformer cores. Monel metals contain about two parts nickel to one part copper, plus other elements. They are stronger than nickel and extremely corrosion-resistant. These properties make them useful in chemical production. Electrum is a natural or artificial alloy of gold and silver containing 15-45 per cent of silver. It was used in the ancient world for coinage. Bismuth is frequently used as a part of alloys with low melting-points. Today alloys can be designed for particular applications with certain properties. 2. MANUFACTURING OF PLASTICSThe 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 moulding 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), 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 relatively widely available and inexpensive. 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 the 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 colour. Among other additives are flame retardants and antistatics. Many plastics are manufactured as composites. This involves a system where reinforcing material (usually fibres made of glass or carbon) is 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 moulding and injection moulding. In extrusion blow moulding, the extruder fills the mould with a tube, which is then cut off and clamped to form a hollow shape called a parison. The hot, molten parison is then blown like a balloon and forced against the walls of the mould to form the desired shape. In injection moulding, one or more extruders are used with reciprocating screws that move forwards to inject the melt and then retract to take on new molten material to continue the process. In injection blow moulding, which is used in making bottles for carbonated drinks, the parison is first injection moulded and then reheated and blown. In compression moulding, pressure forces the plastic into a given shape. Another process, transfer moulding, is a hybrid of injection and compression moulding: the molten plastic is forced by a ram into a mould. 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, polytetrafluoroethene (Teflon) 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 a leading user of plastics. Much LDPE (low-density polyethene) is marketed in rolls of clear-plastic wrap. High-density polyethene (HPDE) is used for some thicker plastic films, such as those used for plastic waste bags and containers. Other packaging plastics include polypropene, polystyrene, polyvinyl chloride (PVC), 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, polypropene is an effective barrier against water vapour. Polypropene is also often used in housewares and as a fibre for carpeting and rope. Construction The building industry is a major consumer of plastics, including many of the packaging plastics mentioned above. HDPE is used for pipes, as is PVC. PVC is also used in sheets for building materials and similar items. 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, mouldings, and hardware. Other Uses Many other industries, especially motor manufacturing, also depend on plastics. Tough engineering plastics are found in vehicle components like fuel lines, fuel pumps, and electronic devices. Plastics are also used for interior panelling, seats, and trim. Many car bodies are made of fibreglass-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 |