Вопрос № 223794 - Английский язык

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The raw materials for plastics are coal and 1_______, and many plastic materials today are produced from oil. Plastics can be divided into two main 2_______. The first group, called thermoplastics, includes well-known plastics such as PVC, nylon and polythene. These can be treated (heated, softened and cooled) several times, and can be formed into combs, pipes and bags, in addition to many other similar 3_______.
Thermosetting 4_______ include melamine, polyurethane and phenolic resin, and can be made into tableware, foams and plugs respectively. However, this group differs from thermoplastics because they can be treated only once and harden when they are heated.

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Plastic Materials
1. Plastic materials are not found in nature. Plastic is formed by extrusion or injection molding under very high pressure. It can be molded into any desired shape. Organic plastics are divided into two general groups: thermosetting and thermoplastic. The thermosetting group becomes rigid through a chemical change that occurs when heat is applied. These plastics cannot be remolded. The thermoplastic group remains soft at high temperatures and must be cooled before becoming rigid. This group is not used generally as a structural material.
2. Plastics are rapidly becoming important construction materials because of their great variety, strength, durability and lightness. Plastics are light. The benefits of light weight coupled with good strength and absence of corrosion offer tremendous potential as alternatives to traditional building materials. A given volume of polythene weights less than one-eighth of an equal volume of iron and less than half of the same volume of aluminum.
3. Plastics are used in the industry and in the household: from rockets and electronics to toys and house ware. Plastic products offer many advantages over the materials they replace, such as ease of handling, lower maintenance costs and rapidity of assembly. The insulation and dielectric properties of plastics led to their early use in the electrical engineering industry, which was followed by special application in mechanical engineering.
4. Using of plastics as materials for a construction in the form of sheets, rods or tubes is substituting the conventional metals. Plastics offer a lot of properties for the designs. Plastics have now been developed to such an extent that they can be applied to almost every branch of building, from the laying of foundations to the final coat of paint.
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Why do plastics replace traditional building materials?

Прочитайте текст и заполните пропуски подходящими по смыслу словами 1_______ metals are based on iron. They are usually 2_______ with carbon because it increases the strength of iron and makes it much more useful to the engineer. Slight variations in carbon content can make very great differences in the properties of the 3_______. Wrought iron is a ferrous metal with carbon content of less than 0,05 %. Chains and 4_______ can be produced from this alloy.

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Riveting is a 1_______ of joining metals permanently. The process consists of drilling or punching the 2_______ plates, inserting the rivet and then 3_______ it by compression force. There are several standard types of rivet heads with 4_______ uses.

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Heat causes some 1_______ to expand more than others. For example, copper expands more than 2_______. This effect is used in the bimetal strip, which consists of two dissimilar strips of metal 3_______ together. The strips are usually made of brass (an alloy of copper and zinc) and invar, a ferrous alloy which expands very little when heated.
An increase in 4_______ causes the brass to expand more than the invar. This expansion causes the bimetal strip to bend, and consequently a switch or a valve is activated.

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Heat causes liquids to expand. This effect is used in the 1_______. An increase in air temperature 2_______ the liquid to expand in the heat sensor. This expansion causes the valve to close and consequently the 3_______ of hot water is reduced.
After a time, the temperature will go down and the 4_______ will cool and contract. The spring load will now be greater than the liquid pressure and the valve will open. Once again, the hot water flow is increased.