Text B. Many new materials are familiar to us over centuries

Many new materials are familiar to us over centuries. We are pretty well acquainted with the advantages and disadvantages of wood. We know that glass is transparent, but in some ways it is rather brittle. Glass has the advantages of cheapness, rigidity and chemical inertness. We are aware that most metals can stand severe handling, but some of them rust. But such materials as plastic materials are not found in nature. Some years ago plastics were little more than laboratory curiosities. Today plastics are conceived in the laboratory of the chemical plant. But 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 plastic 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. Plastics are rapidly becoming important construction materials because of its great variety, strength, durability and lightness. The high strength to weight ratio of some plastic offers big field in the coming age of space travels and rockets. Plastics are light. The same benefits of light weight coupled with good strength and absence of corrosion offer tremendous potential as alternative to traditional building materials. A given volume of polythene weighs less than one-eighth of an equal volume of iron and less than half of the same volume of aluminum. The following of plastics are usually shared by all plastics lightweight, corrosion resistance, electrical and thermal insulation.