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 (polyethene 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.
2. 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 urea-formaldehyde or polyurethane and epoxy resins, most polyesters, and phenolic polymers such as phenol-formaldehyde resin.
3. Elastomers are similar to thermoplastics but have sufficient cross-linking between molecules to prevent stretching and creep.
Vocabulary:
carbon
flexible
fibre ,
chain
identical ,
molecule
branch
to synthesize
chemicals
to soften
cellulose ,
wax
thermosetting plastics
to harden
coil
stretched
transparent
rubber ,
to decompose
soft-drink
to subject
polyurethane
resin
similar ,
sufficient
to prevent
General understanding
1. What is the definition of plastics?
2. What is the basic chemical element in plastics formula?
3. What do polymers consist of?
4. What are long-chain molecules made of?
5. What are the main types of polymers?
6. Give examples of plastics belonging to these types.
7. What plastics are the best electrical insulators?
8. Describe the difference between thermoplastics and thermosets.
9. What are the main types of structures of polymers?
10. What are the most important properties of plastics?
11. Give the examples of various uses of plastics because of their characteristic properties.
Exercise 5.1. Find English equivalents in the text:
1.
2.
3.
4.
5.
6.
7.
8.
9.
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10.
11.
12.
Exercise 5.2. Translate into English:
1. .
2. .
3. , , .
4. , .
5. , .
6. , , .
7. .
Text : TYPES OF PLASTICS
Epoxy 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.
2. PVC (polyvinyl chloride)
PVC (polyvinyl chloride) is a thermoplastic polymer made from vinyl chloride is 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.
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.
4. Polythene (polyethene, polyethylene)
Polythene (polyethene, polyethylene) is a plastic made from ethane. It is one of the most widely used important thermoplastic polymers. It was first developed by the polymerization of ethane at a pressure of 2,000 bar at 200C. This produced low-density polythene (LDPE). A relatively high-density form (HDPE) was synthesized in the 1950s using a complex catalyst. 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.
Vocabulary:
adhesion
adhesive
bond ,
insulation
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casting
void
solid ,
acid
alkali
to obtain ,
granule
solution
lattices
paste
yield
durable
rubber ,
garment
lens
capacitor .
syringe
light-duty
envelope .
impact
improved
modifiers
addition
readily ,
foam
catalyst
wax
reasonable ,
coating ,
General understanding:
1. What are the types of plastics?
2. What are the features of the epoxy resin?
3. What is epoxy resin used for?
4. What is PVC usually used for?
5. What are the typical applications of polystyrene?
6. When was polyethylen synthesized?
7. Under what conditions is polyethylen synthesized?
8. What sorts of polyethylen can be synthesized?
Exercise 5.3. Translate into Russian:
1. Polythene is a plastic made from ethane.
2. Epoxy resins have outstanding adhesion, toughness and resistance to attack from chemicals.
3. PVC is a colourless solid with outstanding resistance to water, alcohols, and concentrated acids and alkalis.
4. Polystyrene is a thermoplastic produced by the polymerization of styrene.
5. Polythene is a white waxy solid with very low density, reasonable strength and toughness but low stiffness.
Exercise 5.4. Translate into English:
1. .
2. , .
3. , , .
4. .
5. .
6. .
7. .
Text : COMPOSITE MATERIALS
The combinations of two or more different materials are called composite materials. They usually have unique mechanical and physical properties because they combine the best properties of different materials. For example, a fibre-glass reinforced plastic combines the high strength of thin glass fibres with the ductility and chemical resistance of plastic. Nowadays composites are being used for structures such as bridges, boat-building etc.
Composite materials usually consist of synthetic fibres within a matrix, a material that surrounds and is tightly bound to the fibres. The most widely used type of composite material is polymer matrix composites (PMCs). PMCs consist of fibres made of a ceramic material such as carbon or glass embedded in a plastic matrix. Usually the fibres make up about 60 per cent by volume. Composites with metal matrices or ceramic matrices are called metal matrix composites (MMCs) and ceramic matrix composites (CMCs), respectively.
Continuous-fibre composites are generally required for structural applications. The specific strength (strength-to-density ratio) and specific stiffness (elastic modulus-to-density ratio) of continuous carbon fibre PMCs, for example, can be better than metal alloys have. Composites can also have other attractive properties, such as high thermal or electrical conductivity and a low coefficient of thermal expansion.
Although composite materials have certain advantages over conventional materials, composites also have some disadvantages. For example, PMCs and other composite materials tend to be highly anisotropic that is, their strength, stiffness, and other engineering properties are different depending on the orientation of the composite material. For example, if a PMC is fabricated so that all the fibres are lined up parallel to one another, then the PMC will be very stiff in the direction parallel to the fibres, but not stiff in the perpendicular direction. The designer who uses composite materials in structures subjected to multidirectional forces, must take these anisotropic properties into account. Also, forming strong connections between separate composite material components is difficult.
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The advanced composites have high manufacturing costs. Fabricating composite materials is a complex process. However, new manufacturing techniques are developed. It will become possible to produce composite materials at higher volumes and at a lower cost than is now possible, accelerating the wider exploitation of these materials.
Vocabulary:
fibreglass
fibre ,
reinforced
expansion
matrix
ceramic
specific strength
specific stiffness
anisotropic
General understanding:
1. What is called composite materials?
2. What are the best properties of fibre-glass?
3. What do composite material usually consist of?
4. What is used as matrix in composites?
5. What is used as filler or fibers in composites?
6. How are the composite materials with ceramic and metal matrices called?
7. What are the advantages of composites?
8. What are the disadvantages of composites?
9. Why anisotropic properties of composites should be taken into account?
Exercise 5.5. Find equivalents in the text:
1.
2.
3.
4. 60%
5.
6.
7. ,
Exercise 5.6. Translate into Russian:
1. PMC is fabricated so that all the fibres are lined up parallel to one another.
2. Forming strong connections between separate composite material components is difficult.
3. Fabricating composite materials is a complex process.
4. Composite materials have certain advantages over conventional materials
5. Nowadays, composites are being used for structures such as bridges, boat-building etc.
6. Continuous-fibre composites are generally required for structural applications.
FAMOUS INVENTORS
Alfred Bernhard Nobel was a famous Swedish chemist and inventor. He was born in Stockholm in 1833. After receiving an education in St. Petersburg, Russia, and then in the United States, where he studied mechanical engineering, he returned to St. Petersburg to work with his father in Russia. They were developing mines, torpedoes, and other explosives.
In a family-owned factory in Heleneborg, Sweden, he developed a safe way to handle nitroglycerine, after a factory explosion in 1864 killed his younger brother and four other people. In 1867 Nobel achieved his goal: he produced what he called dynamite . later produced one of the first smokeless powders (). At the time of his death he controlled factories for the manufacture of explosives ( ) in many parts of the world. In his will he wanted that the major portion of his money left became a fund for yearly prizes in his name. The prizes were to be given for merits () in physics, chemistry, medicine and physiology, literature, and world peace. A prize in economics has been awarded since 1969.
UNIT 6
WELDING
I. Text A: Welding, Text : Other types of welding
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