Thermoplastics soften on heating, thenharden again when cooled. Thermoplastic molecules are alsocoiled and because of this they are flexible and easilystretched.

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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 andPVC.

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 lesssubjected 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 topreventstretching and creep.

Vocabulary:

carbon — углерод

flexible— гибкий

fibre— волокно, нить

chain— цепь

identical— одинаковый, идентичный

molecule— молекула

branch — разветвленный

to synthesize — синтезировать

chemicals — химические вещества

to soften — смягчать

cellulose — клетчатка, целлюлоза

wax — воск

thermosetting plastics — термореактивные пласт­массы

to harden — делать твердым

coil — спираль

stretched — растянутый

transparent — прозрачный

rubber — резина, каучук

to decompose — разлагаться

soft-drink — безалкогольный напиток

to subject — подвергать

polyurethane — полиуретан

resin — смола

similar — сходный, подобный

sufficient — достаточный

to prevent — предотвращать

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