2. Определите, какие из данных утверждений истинны, а какие ложны:

1. Steel is less elastic than rubber.

2. Copper and zinc are widely used ductile materials.

3. Toughness is desirable in parts subjected to shock and impact loads.

4. Steel is more easily machined than brass.

5. Creep is considered in designing shafts and gears.

6. Hardness embraces many other properties such as deformation and machinability.

3. Ответьте на вопросы к тексту:

1. How can the properties of metals be grouped?

2. What are the mechanical properties of metals associated with?

3. What is elasticity?

4. What are the commonly used malleable materials?

5. How is resilience measured?

Задание VI

1. Выполните письменный перевод текста.

2. Выполните устный пересказ текста, используя необходимые фразы.

Unit 7

Задание I

1. Подберите русские эквиваленты к следующим словам, содержащим интернациональные корни:

Commercial, attack, dielectric, medical, polymerization, chloride, thermoplastic, container

2. Прочитайте следующий текст:

PLASTICS

Plastics are large and varied group of materials consisting of combinations of carbon and oxygen, hydrogen, nitrogen, and other organic and inorganic elements. While solid in its finished state, a plastic is at some stage in its manufacture, liquid and capable of being formed into various shapes. Forming is usually done through the application either singly or together, of heat and pressure. There are over 40 different families of plastics in commercial use today, and each may have dozens of subtypes and variations.

A successful design in plastics is always a compromise among highest performance, attractive appearance, efficient production, and lowest cost. Achieving the best compromise requires satisfying the mechanical requirements of the part, utilizing the most economical resin or compound that will perform satisfactorily, and choosing a manufacturing process compatible with the part design and material choice.

Most people have now outgrown the impression that plastics are low cost substitute materials. Those that still view plastics as cheap and unreliable have not kept up with developments in polymer technology for the past ten years.

Many plastics did indeed evolve as replacements for natural products such as rubber, ivory or wood, which became unavailable or on short supply. But the new materials did not necessarily replace the older ones permanently nor made them obsolete. In many cases, they met an increased demand that could not be met by the natural product alone.

Today’s engineering resins and compounds serve in the most demanding environments. Their toughness, lightness, strength, and corrosion resistance have won many significant applications for these materials in transportation, industrial and consumer products. The engineering plastics are now challenging the domains traditionally held by metals: truly load bearing, structural parts.

Types of Plastics

Plastics are non-metallic, synthetic, carbon-based materials. They can be molded, shaped, or extruded into flexible sheets, films, or fibers. Plastics are synthetic polymers. Polymers consist of long-chain molecules made of large numbers of identical small molecules (monomers). The chemical nature of a plastic is defined by the monomer (repeating unit) that makes up the chain of the polymer.

Most plastics are synthesized from organic chemicals or from natural gas or coal. Plastics are lightweight compared to metals and are good electrical insulators. The best insulators now are epoxy resins and teflon.

Plastics can be classified into several broad types.

Thermoplastics soften on heating, and then harden again when cooled. Thermoplastics molecules are also coiled and because of this they are flexible and easily stretched.

Typical example of thermoplastics is polystyrene. Polystyrene resins are characterized by high resistance to chemical and mechanical stresses at low temperatures and by very low absorption of water. These properties make the polystyrenes especially suitable for radio-frequency insulation and for parts used at low temperatures in refrigerators and in airplanes. PET (polythene terephthalate) is a transparent thermoplastic used for soft-drinks bottles. Thermoplastics are also viscoelastic, that is, they flow (creep) under stress. Examples are polythene, polystyrene and PVC.

PVC is a thermoplastic polymer made from vinyl chloride, a colourless solid with outstanding resistance to water, alcohols, and concentrated acids and alkalis. It is obtainable as granules, solutions, lattices, and pastes. When compounded with plasticizers, it yields a flexible material more durable than rubber. It is widely used for cable and wire insulation, in chemical plants, and in the manufacture of protective garments. Blow moulding of unplasticized PVC produces clear, tough bottles, which do not affect the flavour of their contents. PVC is also used for production of tubes or pipes.

Polystyrene is a thermoplastic produced by the polymerization of styrene. The electrical insulating properties of polystyrene are outstandingly good and it is relatively unaffected by water. Typical applications include light fixtures, toys, bottles, lenses, capacitor dielectrics, medical syringes, and light-duty industrial components. Extruded sheets of polystyrene are widely used for packaging, envelope windows, and photographic film. Its resistance to impact can be improved by the addition of rubber modifiers. Polystyrene can be readily foamed; the resulting foamed polystyrene is used extensively for packaging.

Polythene is a plastic made from ethane. It is one of the most widely used important thermoplastic polymers. Polythene is a white waxy solid with very low density, reasonable strength and toughness, but low stiffness. It is easily moulded and has a wide range of uses in containers, packaging, pipes, coatings, and insulation.

Thermosetting plastics (thermosets) do not soften when heated, and with strong heating they decompose. In most thermosets final cross-linking, which fixes the molecules, takes place after the plastic has already been formed.

Thermosetting plastics have a higher density than thermoplastics. They are less flexible, more difficult to stretch, and are less subjected to creep. Examples of thermosetting plastics include epoxy resins, most polyesters, and phenolic polymers such as phenol-formaldehyde resin.

Epoxy resin is a thermoset plastic containing epoxy groups. Epoxy resin hardens when it is mixed with solidifier and plasticizer. Plasticizers make a polymer more flexible.

Epoxy resins have outstanding adhesion, toughness, and resistance to attack from chemicals. They form strong bonds and have excellent electrical insulation properties. Large, complex, void-free castings can be made from them. They are also used as

adhesives, and in composites for boat building and sports equipment.

Elastomers are similar to thermoplastics but have sufficient cross-linking between molecules to prevent stretching and creep.