Text A: «PLASTICS»

Plastics are non-metallic, synthetic, carbon-based materials. They can be moulded, shaped, or extruded into flexible sheets, films, or fibres. 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. Polyethene is a polyolefin; its monomer unit is ethene (formerly called ethylene). Other catego­ries are acrylics (such as polymethylmethacrylate), styrenes (such as polystyrene), vinys (such as polyvinyl chloride (PVC)), polyes­ters, polyurethanes, polyamides (such as nylons), polyethers, acetals, phenolics, cellulosics, and amino resins. The molecules can be either natural — like cellulose, wax, and natural rubber — or synthetic — in polyethene and nylon. In co-polymers, more than one monomer is used.

The giant molecules of which polymers consist may be linear, branched, or cross-linked, depending on the plastic. Linear and branched molecules are thermoplas­tic (soften when heated), whereas cross-linked molecules are thermosetting (harden when heated).

Most plastics are synthesized from organic chemicals or from natural gas or coal. Plastics are light-weight com­pared to metals and are good electrical insulators. The best insulators now are epoxy resins and teflon. Teflon or polytetrafluoroethene (PTFE) was first made in 1938 and was produced commercially in 1950.

Plastics can be classified into several broad types.

1. Thermoplastics soften on heating, then harden again when cooled. Thermoplastic 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 (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 — 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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