Plastics

The first synthetic plastic was made in the 1860s. Before that, nat­ural materials such as ivory and amber were widely used. Many of these are polymers - from the Greek word poly, meaning 'many', and mer, meaning 'part'. Polymers are composed of giant molecules, made up of large numbers of a small molecule strung together in long chains. This small molecule is called a monomer (mono means 'one'). The search for synthetic materials started over a hundred years ago to replace materials like ivory, which were becoming scarce, and to make materi­als that could be moulded or extruded as fibres. The first plastics were semi-synthetic polymers and relied on modifying cellulose, the natural polymer in cotton. Later, completely synthetic plastics, such as Bakclite, were made.

Synthetic materials resembling ivory were widespread by 1900. They were used for all kinds of products, from knife handles, collars, and culls, to evening handbags. These plastics could be moulded when hot inlo shapes which became rigid on cooling. Ivoride, like other plastics, could be easily moulded to resemble intricately carved ivory. The early synthetic plastics were modelled to resemble the natural polymers.

Alexander Parkes (1813-1890) introduced a mouldable material made from cellulose nitrate. He dissolved cotton fibres in nitric acid, added a plasticizer such as camphor, and evaporated off the solvent. The material called Parkesine was used to make all kinds of domestic goods like hair slides. Parkes exhibited his first successful plastic in London in 1862.

By 1870 John Wesley Hyatt (1837-1920) was manufacturing cellu­loid, an ivory substitute, from cellulose nitrate. It was widely used for billiard balls and all kinds of decorative products, such as evening bags made in 1900.

When sulphur-containing compounds are heated with rubber, the rubber absorbs them, forming crosslinks between the chains of mole­cules. Large amounts of sulphur lead to hard, chemically resistant ma­terials, such as the vulcanite used to make fountain pens.

Bakelite, or phenolic resin (a synthetic plastic developed by Teo Baekeland), was used for domestic items such as clocks and electrical fittings. It is resistant to heat and has good insulating properties. The phenolic resins are always dark in colour. They are easy to mould and are strengthened using fillers such as textiles.

Film made of cellulose nitrate was introduced for motion pictures in 1887 and for still pictures in the following year. Cellulose nitrate is notoriously inflammable, so modern film is made from the safer plas­tic, cellulose triacetate.

In the 1920s the search for a light-coloured plastic with similar prop­erties to Bakelite, which was always black or reddish-brown, led to the ureaformaldehyde plastics. Using cellulose filters and suitable colour­ing materials, both white and coloured articles could be manufactured.

Some plastics can be extruded to form fibres. Textiles used to be derived from natural fibres. Regenerated cellulose from a viscose so­lution was introduced in 1892. This plastic material could be pumped through fine holes into acid to produce an artificial thread for tex­tiles. Large-scale production was possible with the introduction of a spinning box in 1900. The box collected the filaments without tan­gling them.

By the 1950s many different plastics had been developed. They were used in industry and throughout the house, especially the kitchen. Strong polyvinyl chloride (PVC) is used as a floor covering. Melanine formal­dehyde plastics, which have a good resistance to heat, water, and de­tergents, were introduced in the mid 1930-s. They are laminated by sand­wiching alternate layers of plastic and paper or cloth and pressing them to make formica for work surfaces. Other plastics, such as polystyrene, are used for buckets, blows, and jars. Unfortunately most plastics are not biodegradable; they do not rot away, and they may emit poisonous fumes when burned.

New plastics are being introduced all the time. The first of the ther­moplastic materials was polyvinyl chloride (PVC). Observed in the 1870s, it was not successfully produced until the 1930s. Waterproof and weather resistant, it has a wide ran.ge of uses: rigid when thick, it makes up guttering, toys, and curtain rails; flexible when thin, it covers electric cables, makes baby pants, and upholstery. Polyacrylic plastics include perspexs, a plastic of exceptioixally high transparency. Its re­sistance to shattering makes it invaluable for aircraft canopies. Poly­thene, or polyethylene, was first discovered in 1933. Like many plas­tics, it was some time before successful! commercial production was achieved in the late 1930s, when its valuable insulating properties were immediately pressed into service for wartime radar equipment. Rigid polyethylene was not produced until trie introduction of a catalyst in the 1950s. In the USA the Du Pont company successfully launched nylon-66, which revolutionized the textiles industry. It was strong, stretchy, and non-absorbent. Today it is difficult to imagine a world without plastics. Many new processes and products depend upon them. Wallace H. Carrothers (1896-19373 joined Du Pont in 1928. He used two chemicals in solutions (an acid and a diamine) to prepare nylon-66. Where the two solutions met, the liquid could be pushed out (extruded) into threads that were stronger than natural fibres. The dis­covery gave a huge boost to the textile industry and led to a revolution in fabrics.

Two alternating monomers are used to make nylon-66: adipic acid and hexamethylenediamine. Each has reactive groups of atoms at both ends of a straight molecule - acid groups in the former, and aniline in the latter. Each acid group combines with an amine to form long chains of alternating monomers. The diamine is dissolved in water forming the lower layer in the beaker, while the upper layer is a solution of the acid in hexane. Nylon is formed where the two reactances meet and can be pulled out of the beaker and wound around the rod.

Polyethylene is a polymer of ethylene monomers. Discovered in 1933, it did not become commercially available until 1939 after the problems of attaining high temperatures and pressures on a large scale were solved. Pressure vessel was used to prepare the first polyethylene samples. While examining the effects of very high pressures on ethylene gas, solid, white particles were observed. Fortuitously, a hole in the equipment had al­lowed oxygen from air to enter the chamber, which initiated (started) the reaction. This low-density polyethylene is soft, flexible, and clear. In the 1950s a catalyst was introduced, and rigid, high-density polyethylene was obtained.

All plastics fall into one of two categories depending on how they act when heated. Thermoplastic materials (polyethylene) soften each time they are heated. Thermosetting plastics will not soften again once they have been heated and cooled down (Bakelite). On first heating, these molecules form cross linkages which lead to a permanent rigid structure. They make components such as electric plugs.

Plastics increasingly find applications in replacement surgery because they do not react with their surroundings. Unlike transplant material from human donors, they do not prompt the body to reject them as foreign material.

Plastic tubing is being made from pellets of polyethylene. Waste pol­yethylene has been softened with hot water and ground up to produce the pellets. The plastic tubing is blown out using hot air, which shapes and dries the tubing. Recycled plastic is used in the construction indus­try, where impurities in it are not so important. Only thermoplastics can be recycled in this way.

Ответьте на вопросы по тексту:

1.When was the first synthetic plastic made?

2.What are polymers composed of?

3.Who introduced a mouldable material made fromcellulose nitrate?

4.Where and when did Parkes exhibit his first successful plastic?

5.Why was large-scale production of plastics possible in 1900?

6.Where are many different plastics used?

7.Why do plastics find applications in replacement surgery?