Exercise 9.11. Match the following:
1) You access the information through one interface or tool called a...
2) People connected to the WWW through the local... have access to a variety of information.
3) The user doesn't need to know where the site is, the... follows the...
4) In 1996 there were more than 20 million users of the...
5) Each... provides a graphical interface.
6) Local... charge money for their services to access... resources.
Words to match with:
1) web browser, providers, link, WWW,
Questions for group discussion:
1) Some people think that Internet is very harmful, especially for young people, because it carries a lot of information about sex, drugs, violence and terrorism. Do you think that some kind of censorship is necessary on the WWW?
2) World famous authors and publishers say that the Internet violates their copyright because Web-programmers put all kinds of books, pictures, music, films and programs free on the Internet and this reduces their sales and profits.
3) Has anyone in your group experience working on the Internet? Ask them 1) about the difficulties they had; 2) useful information retrieved; 3) fun they got? Why so few people have experience working on the Internet?
FAMOUS PEOPLE OF SCIENCE AND ENGINEERING
Bill Gates
William Henry Gates was born in Seattle, Washington, in 1955.
He is an American business executive, chairman and chief executive officer of the Microsoft Corporation. Gates was the founder of Microsoft in 1975 together with Paul Alien, his partner in computer language development. While attending Harvard in 1975, Gates together with Alien developed a version of the BASIC computer programming language for the first personal computer.
In the early 1980s. Gates led Microsoft's evolution from the developer of computer programming languages to a large computer software company. This transition began with the introduction of MS-DOS, the operating system for the new IBM Personal Computer in 1981. Gates also led Microsoft towards the introduction of application software such as the Microsoft Word processor.
Much of Gates' success is based on his ability to translate technical visions into market strategy. Although Gates has accumulated great wealth from his holdings of Microsoft stock, he has been known as a tough competitor who seems to value winning in a competitive environment over money. Gates still continues to work personally in product development at Microsoft.
Bronze 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.
The 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
Condensation polymerisation and addition polymerisation are the two main processes in plastics production. The manufacture of plastics depends upon the building of chains and networks during polymerisation.
A condensation polymer is formed by a synthesis that involves the gradual reaction of reactive molecules with one another, with the elimination of small molecules such as water. The reaction gradually slows down as polymers are built up.
An addition polymer forms chains by the linking of small identical units without elimination of small molecules.
The most important concept in condensation polymers is that of «functionality», i.e., the number of reactive groups in each molecule participating in the chain buildup. Each molecule must have at least two reactive groups, of which hydroxyl (-OH), acidic endings (-COOH), and amine endings (-NH) are the simplest.
Hydroxyl is characteristic of alcohol endings, combining with an acid ending to give an ester, the polymer being known as a polyester. Examples are polyethylene terephthalate obtained by reaction of ethylene glycol containing hydroxyl groups at each end and terephthalic acid containing two acidic groups and polycarbonate resins.
Alcohols are a particular class of oxygen-containing chemical compounds with a structure analogous to ethyl alcohol (C-HOH). Amines are various compounds derived from ammonia by replacement of hydrogen by one or more hydrocarbon radicals (molecular groups that act as a unit). Esters are compounds formed by the reaction between an acid and an alcohol or phenol with the elimination of water.
Bulk addition polymerization of pure monomers is mainly confined to styrene and methyl methacrylate The process is highly exothermic, or heat producing. The dissipation of heat (necessary to maintain chain length) is achieved in the case of styrene by intensive stirring of the viscous, partially polymerized mixture, which is then passed down a tower through zones of increasing temperature. Alternatively, polymerization may be completed in containers that are small enough to avoid an excessive temperature rise as a result of the heat released during polymerization.
Methyl methacrylate is also partially polymerized before being poured into molds consisting of between sheets of plate glass, to produce clear acrylic sheet.
Ethylene is polymerized in tubular reactors about 30 metres long and less than 25 millimetres in diameter at pressures of 600-3,000 to give 10-20 percent conversion to low-density polyethylene. Residual gas is recycled.
Polymerization of monomers in solution allows easy temperature control, but the molecular weight of polymers formed is reduced because of chain transfer reactions
Solvent removal from such a solution may also be very difficult. The process can be applied advantageously to vinyl acetate and acrylic esters.
Suspension polymerization producing beads of plastic is extensively applied to styrene, methyl methacrylate, vinyl chloride, and vinyl acetate. The monomer, in which the initiator or catalyst must be soluble, is maintained in droplet form suspended in water by agitation in the presence of a stabilizer such as gelatin, each droplet of monomer undergoing bulk polymerization.
In emulsion polymerization the monomer is dispersed in water by means of a surface-active agent (a substance slightly soluble in water that reduces the surface tension of a liquid), its bulk aggregating into tiny particles held in suspension. The monomer enters the hydrocarbon part of the surface-active micelles and is polymerized there by a water-soluble catalyst.
This process is particularly useful for the preparation of very high molecular weight polymers.
Exposure of certain substances to X-ray or ultraviolet radiation initiates chain reactions that can be used for manufacture of such thermoplastics as polyethylene and polyvinyl chloride.
4. RESINS
Resins that cannot be softened by heating include the phenolics, furan resins, aminoplastics, alkyds, allyls, epoxy resins, polyurethanes, some polyesters, and silicones.
Phenolics or phenol-aldehydes
The important commercial phenolic resin Bakelite is based on phenol and formaldehyde. The two processes in general use are the one-step process producing resol resins (the first stage in the formation of a phenolic resin) that are either liquid or brittle, soluble, fusible solids, from more than one molecule of formaldehyde per phenol molecule; and the two-step process, using an excess of phenol to produce novolacs, resins that have no reactive methylol groups and must be mixed with an aldehyde to undergo further reaction.
Resol resins thermoset on heating and are used for adhesives. Novolacs require a further source of formaldehyde in the form of hexamethylenetetramine to produce molding powders. Both resins are run out from the reaction vessel, after removal of water by distillation, and ground up, then compounded on heated rolls with fillers that vary from wood flour to mica; for strength and heat resistance fibrous asbestos is used as a filler (hexamethylenetetramine is also added at this stage in the case of the two-step resin). Final grinding produces the molding powders, which on further heat treatment will yield the typical thermoset resin.
Phenolic moldings are resistant to heat, chemicals, and moisture and are preferred for wet-dry applications as in washing machines. Their stability to heat and low heat conductivity suit them for use in appliance parts, and their electrical insulation qualities qualify them for electric fittings such as switches, plugs, and distributor caps; resistance to hydraulic fluids has led to their use in automotive parts. All these applications have been made more economical by the development of injection molding and extrusion methods. Complex phenols are used in manufacture of brake linings.
Furan resins
Furfural is a five-membered ring compound (i.e., the basic molecule has a ring shape and contains five atoms) of four carbon atoms and one oxygen atom, carrying the aldehyde group, — CHO; it reacts like formaldehyde with phenols in the presence of an acid catalyst to give a rigid polymer with high chemical resistance, used for coatings in industry. It can be prepared in semiliquid form with a low viscosity and remarkable penetrating power when applied to porous forms such as foundry sand cores or graphite blocks, being in this respect superior to other liquid resins.
Aminoplastics
Urea resins are made by the condensation in aqueous solution of formaldehyde and urea in the presence of ammonia as an alkaline catalyst, giving a colourless solution to which cellulose filler is added to yield a molding powder upon drying, which when heated in a mold gives a water-white (transparent) molding unless previously coloured by pigment.
The filler confers considerable strength, so that thin sections such as in cups and tumblers can be molded. Very large quantities of urea-formaldehyde resin are used in kitchen and bathroom hardware details, and electric appliance housings and fittings.
Melamine behaves in the same way as urea, but the product is more moisture resistant, harder and stronger, leading to wide use for plates and food containers. Melamine moldings are glossy and harder than any other plastic and retain a dust-free surface. Solutions of the thermoplastic forms of urea-formaldehyde resins are widely used as bonding agents for plywood and wood-fibre products.