Polymers, Plastics and Fibres: Monomers and Uses

The correct answer is B, silk. Silk is the protein fibroin, spun by the silkworm, so it is a polymer made by a living organism and is therefore natural, like cellulose, starch, wool and natural rubber. A, bakelite, is wrong because it is a fully synthetic thermosetting plastic made in a factory by condensing phenol with formaldehyde, and it was in fact the first wholly synthetic plastic. C, Kevlar, is wrong because it is a synthetic aramid polymer, made from an aromatic diamine and an acid chloride, and used in bullet-resistant vests and helmets. D, Lexan, is wrong because it is the trade name of a synthetic polycarbonate used for unbreakable sheets, spectacle lenses and helmet visors. Remember the test: if the polymer is made by a plant or an animal it is natural, and if it is built in a reactor it is synthetic.

The correct answer is B, ethene. Polythene, also written polyethene, is formed by the addition polymerisation of ethene, in which the double bond of each molecule opens and the units join into a long chain with nothing eliminated. A, ethyne, is wrong because ethyne, the old acetylene, carries a triple bond and is used in welding and in making other chemicals, not in making polythene. C, propene, is wrong because propene is the monomer of polypropylene, the plastic of ropes, bottle caps and furniture, and it is the commonest distractor here. D, vinyl chloride, is wrong because it is the monomer of polyvinyl chloride, the plastic of pipes, raincoats and cable covering. The same monomer, ethene, gives both low density and high density polythene; the difference lies in the pressure and the catalyst used.

The correct answer is D, bakelite. Bakelite is made by condensing phenol with formaldehyde, and on moulding its chains join to one another by strong covalent cross-links into a single rigid network. Once set it cannot be softened by heating again, which is the definition of a thermosetting plastic and the reason bakelite is used for electrical switches, plug tops and the handles of pans. A, bioplastic, is wrong because the word describes any plastic made from a biological raw material or able to be broken down by organisms, and it names no single thermosetting material. B, bagasse, is wrong because bagasse is the fibrous residue left after sugarcane is crushed; it is used as fuel, for paper and for moulded tableware, but it is not a plastic. C, mycotecture, is wrong because it refers to building material grown from fungal mycelium, which again is not a thermosetting plastic.

The correct answer is A, tetrafluoroethene. Teflon is polytetrafluoroethene, made by the addition polymerisation of tetrafluoroethene. The fluorine atoms shield the carbon chain so completely that the surface is chemically inert, water repellent and almost frictionless, and it holds its properties to high temperature, which is why it coats frying pans and is used for gaskets and seals. B, chloroprene, is wrong because it is the monomer of neoprene, a synthetic rubber used for oil seals and hoses. C, styrene, is wrong because it is the monomer of polystyrene, the material of disposable cups and, as expanded foam, of thermocol. D, caprolactam, is wrong because it is the monomer of nylon-6, used for fabric, fishing nets and machine parts. A useful memory link is that Teflon was found by accident in 1938, when a cylinder of tetrafluoroethene was found to hold a waxy white solid instead of the gas.

The correct answer is B, hexamethylenediamine and adipic acid. Nylon-6,6 is a polyamide formed when hexamethylenediamine condenses with adipic acid, with a molecule of water eliminated at each linkage; the name records that each monomer contributes a chain of six carbon atoms. It is used for ropes, tyre cord, brush bristles and parachute fabric. A is wrong because caprolactam alone gives nylon-6, a related but different polyamide. C is wrong because ethylene glycol with terephthalic acid gives the polyester terylene, also called dacron, and in bottle form polyethylene terephthalate. D is wrong because phenol with formaldehyde gives bakelite, a thermosetting plastic and not a fibre at all. The naming rule is worth carrying into the hall: a nylon with two numbers is made from two monomers, one supplying the diamine and the other the diacid, while a nylon with one number comes from a single monomer, so nylon-6,6 must be the pair and nylon-6 the single one.

The correct answer is B, sulphur. Vulcanisation is the heating of raw rubber with sulphur, which forms short cross-links between the long isoprene chains. Raw rubber is soft, sticky when warm and brittle when cold; the vulcanised product is strong, far more elastic and stable over a wide range of temperature. Tyre rubber carries a few per cent of sulphur and very hard ebonite much more. A, carbon, is a careful distractor: carbon black is indeed added to tyre rubber as a filler to resist wear, but adding it is not vulcanisation and it forms no cross-links. C, phosphorus, is wrong because it has no role in rubber processing. D, silicon, is wrong; silicone rubbers are a separate class of synthetic polymer and are not made by vulcanising natural rubber. Note too that vulcanisation was discovered by Charles Goodyear, which is a fact examiners sometimes ask alongside the role of sulphur.

The correct answer is C, polyvinyl chloride. Polyvinyl chloride has linear chains held to one another only by weak forces, so it softens when heated and hardens when cooled and can be melted and moulded again and again; that reversibility is what makes a plastic thermoplastic, and it is also what allows such plastics to be recycled by melting. A, bakelite, is wrong because its chains are tied together by covalent cross-links into one network, so it is thermosetting and only chars on strong heating. B, melamine-formaldehyde, is wrong for the same reason, and that is exactly why melamine crockery keeps its shape in hot water. D, urea-formaldehyde, is wrong because it too is a cross-linked thermosetting resin, used in adhesives and laminates. The quick test in the examination hall is to ask whether the plastic can be melted again: if it can, it is a thermoplastic.

The correct answer is B, a semi-synthetic fibre made from cellulose. Rayon is made by dissolving the cellulose of wood pulp, forcing the solution through fine holes and regenerating the cellulose as a thread. The raw material is natural and the processing is chemical, so rayon is semi-synthetic, and because it takes dye well and feels like silk it is called artificial silk. A is wrong because the natural protein fibres are wool, which is keratin, and silk, which is fibroin. C is wrong because the synthetic polyamide fibre is nylon, built entirely from petrochemical monomers. D is wrong because rayon is a fibre, not a plastic, and nothing about it is cross-linked or thermosetting; cellulose acetate and vulcanised rubber are the other standard examples of semi-synthetic polymers. Remember the three-way split of source as natural, semi-synthetic and synthetic, and rayon will always fall in the middle.

The correct answer is A, butadiene and styrene. Buna-S, also called styrene-butadiene rubber, is made by copolymerising 1,3-butadiene with styrene; the S in the name stands for styrene. It wears better than natural rubber and is the commonest rubber in tyres and shoe soles. B, butadiene and acrylonitrile, is the main trap: that combination gives Buna-N, whose resistance to petrol and oil makes it the choice for fuel hoses and oil seals, and the N stands for the nitrile group. C is wrong because isoprene is the monomer of natural rubber and chloroprene of neoprene, and the two are not copolymerised together in this way. D is wrong because ethene and propene give polyolefin plastics and elastomers, not Buna-S. So read the letter at the end of the name: S for styrene and N for nitrile, and the question answers itself.

The correct answer is A, expanded polystyrene. Thermocol is polystyrene expanded with a blowing agent so that the solid becomes a rigid foam of tiny closed cells. The trapped air makes it very light and a poor conductor of heat, so it is used for packing fragile goods, for disposable food boxes and for insulating walls and cold rooms. B is wrong because the foam made from polythene is a softer, more flexible packing material and is not thermocol. C is wrong because bakelite is a hard cross-linked thermosetting resin that cannot be expanded into such a foam and is used for switches and handles. D is wrong because foamed polyvinyl chloride exists as a board for signs and cladding, but it is denser and is not the material called thermocol.

The correct answer is A, phenol. Bakelite is a phenol-formaldehyde resin, prepared by condensing phenol with formaldehyde; on curing the chains cross-link into a rigid network, giving a hard, heat-resistant and electrically insulating solid used for switches, plug tops, pan handles and distributor caps. It was the first wholly synthetic plastic. B, melamine, is wrong because melamine with formaldehyde gives melamine-formaldehyde resin, the material of unbreakable crockery. C, urea, is wrong because urea with formaldehyde gives urea-formaldehyde resin, used in adhesives, laminates and foam insulation. All three are thermosetting resins of formaldehyde, which is why examiners group them, so remember the pairing of each one with its partner. D, aniline, is wrong; it is used in making dyes and drugs, not bakelite. So learn the three formaldehyde resins as a set of pairs and the question becomes a single step.

The correct answer is C, wool. Wool is the fleece of sheep and other animals and is made of the protein keratin, so it is a protein fibre; silk, which is the protein fibroin, is the other natural fibre of this kind, and both come from animals. A, cotton, is wrong because the cotton fibre is almost pure cellulose, a carbohydrate polymer obtained from the seed hair of the cotton plant. B, jute, is wrong because jute is a bast fibre from the stem of the jute plant and is also cellulose, with some lignin. D, flax, is wrong for the same reason, being the cellulose stem fibre from which linen is woven. The rule to carry into the examination hall is simple: fibres of animal origin are protein and fibres of plant origin are cellulose.