From building blocks to plastics, fibres and detergents: Synthetic fibres

4.1 From paraxylene to polyester takes three plants

Polyester (PET) is made from terephthalic acid and ethylene glycol. The difficult step is the acid. Para-xylene oxidises easily at its first methyl group, but the carboxyl group that forms pulls electrons from the ring and makes the second methyl group hard to attack. The Amoco Mid-Century process solves this. Oxidation runs in acetic acid with cobalt and manganese salts and a bromide promoter, which turns the stubborn methyl group into a free radical that oxygen then attacks readily. Terephthalic acid is much less soluble in acetic acid than the intermediates, so it drops out as fairly pure crystals.

The main impurity is 4-carboxybenzaldehyde, a half-oxidised molecule that would stop polymer chains growing and discolour them. The crude acid is dissolved in water and treated with hydrogen over palladium at about 250 C and 36 bar, which converts that impurity to p-toluic acid, removed by crystallisation. The product is purified terephthalic acid (PTA). By 2002, 70% of US and West European capacity made PTA and all new plants did.

Polymerisation is a step-growth reaction in two stages. PTA and glycol are esterified to a short molecule with glycol at both ends, bis-hydroxyethyl terephthalate, which links into a low-molecular-weight prepolymer. Then, at about 275 C under high vacuum, glycol is pulled off and chains lengthen to the melt viscosity required.

In 2002 about 70% of PET went into fibre, 25% into bottles and 5% into film. World polyester fibre output rose from 17.6 billion lb (8.0 Mt) in 1990 to 39.4 billion lb (17.9 Mt) in 2000. The IEA reports that polyester has overtaken cotton and now makes up about 60% of world fibre production.

• Wittcoff (2004), Ch 9 p-Xylene and terephthalic acid; PET · pdf pp. 346-348, 355-362
• Kent (2012), Ch 12 Polyesters · pdf pp. 451-452
• Shreve (1984), Ch 35 Man-made fiber industries · pdf p. 692
• IEA (2018), Ch 1; Ch 4 · pdf pp. 20, 74

4.2 Melt spinning turns molten polymer into fibre at kilometres a minute

Nylon 66 would not dissolve in any cheap, low-boiling solvent, so its developers needed a new method: melt spinning, now used for nylon, polyester and polypropylene. A fibre line has a fixed sequence. Molten polymer arrives either straight from the polymerisation reactor or from dried chips melted in a heated screw extruder. A gear pump meters the melt at constant rate and high pressure through a filter of sand, metal screens or sintered metal, then through the spinneret, a thick plate with many fine holes, about 230 microns across for polyester. The whole pack is held 20-30 C above the polymer's melting point. The emerging filaments are cooled by a cross-flow of air in a quench chimney, a lubricating finish is applied, and the yarn is wound up.

As-spun fibre is weak. Drawing stretches it several times its length; at one point along the filament the diameter suddenly narrows, called necking, and the chains line up along the fibre axis and crystallise. That orientation gives strength. Nylon is drawn about fourfold; PET, which is almost amorphous as spun, is drawn 300-400% above its glass transition of about 80 C. Industrial polyester yarns use higher-molecular-weight polymer, slower spinning at 500-1,000 m/min and drawing to several times their length over very hot rolls.

Speed is where the money is. Modern polyester feed yarn, called partially oriented yarn, is spun at 3,000-4,000 m/min and later drawn and textured at 800-1,200 m/min. Spinning and drawing can be combined, giving fully oriented yarn with winding at about 4,000 m/min; Kent says this spin-draw step represents major cost savings, and nylon yarn is packaged at speeds of 6,000 m/min. For staple, a spinning machine carries 10-30 packs of 1,000-3,000 holes each; filaments are combined into a tow, drawn, crimped, dried, cut and baled.

• Kent (2012), Ch 12 Melt spinning, drawing, polyesters · pdf pp. 432-434, 448-455
• Shreve (1984), Ch 35 Man-made fiber industries · pdf pp. 687-689

4.3 Nylon 66 and nylon 6 both start from benzene but differ in by-products

Both main nylons begin with benzene hydrogenated to cyclohexane over nickel or platinum, which needs benzene with under one part per million of sulphur. Cyclohexane is then oxidised with air over cobalt salts at about 150 C and 10-15 bar to a mixture of cyclohexanol and cyclohexanone called KA oil. Conversion is held to 10-12% per pass, because pushing further breaks rings into shorter acids, with selectivity of at most 85%.

For nylon 66, KA oil is oxidised with nitric acid over copper and vanadium to adipic acid at 93-95% yield. No way round nitric acid has been found, and each tonne of adipic acid releases about 0.32 t of nitrous oxide, a potent greenhouse gas. The other monomer, hexamethylenediamine, comes from adiponitrile, made most cheaply by adding hydrogen cyanide to butadiene. The two monomers are mixed one to one in water to precipitate nylon salt, which is purified by crystallisation, because the 1:1 balance must be exact for long chains. The salt is heated to about 280 C for 2-3 hours and finished at 300 C, with a little acetic acid to cap chain length. Nylon 66 melts at about 264 C.

For nylon 6, cyclohexanone reacts with hydroxylamine sulphate to an oxime, which sulphuric acid rearranges into caprolactam (the Beckmann rearrangement). Heating caprolactam with a little water opens the ring and links it into chains, leaving about 10% monomer in equilibrium to be washed out. Nylon 6 melts at about 220 C.

Kent says about 90% of nylon becomes fibre: carpets, apparel, parachutes and tyre cord. World nylon production doubled from 3.8 billion lb (1.7 Mt) in 1970 to about 7.5 billion lb (3.4 Mt) in 1990, about 24% of synthetic fibre.

• Wittcoff (2004), Ch 7 Cyclohexane, adipic acid, caprolactam · pdf pp. 311-320
• Wittcoff (2004), Ch 5 Hexamethylenediamine · pdf pp. 257-260
• Kent (2012), Ch 12 Nylon · pdf pp. 446-450
• Kent (2012), Ch 15 Nylon · pdf p. 601
• Wittcoff (2004), Ch 15 Step polymerisation of caprolactam · pdf p. 491

4.4 Acrylic and viscose are spun from solution, which costs more

Some fibre polymers cannot be melted. Polyacrylonitrile softens, discolours and decomposes before it melts, so it must be dissolved and spun from solution. An acrylic fibre must contain at least 85% acrylonitrile; up to 15% of comonomers such as vinyl acetate or methyl acrylate are added to make it dyeable and more soluble. It is polymerised as a slurry in 2-3 parts of water, which carries the heat away. The dried polymer is then dissolved in dimethylformamide, dimethylacetamide or strong salt solutions, filtered, and either wet spun into a coagulating bath or dry spun into hot air that evaporates the solvent. The US market for acrylic fibre shrank from 671 million lb (304 kt) in 1984 to 339 million lb (154 kt) in 2000, which Wittcoff puts down to fashion; Kent adds that world output has fallen because of the environmental concerns of solution spinning.

Viscose, sold as rayon, is a regenerated fibre: natural cellulose dissolved and reformed. Dissolving pulp from wood is steeped for about an hour in 17-19% caustic soda, pressed, shredded and aged in air for up to two or three days, which shortens the cellulose chains to a spinnable length. Carbon disulphide, about 30% of the cellulose weight for regular rayon, converts it to sodium cellulose xanthate, which dissolves in dilute caustic to a honey-coloured, viscous liquid, hence the name. After ripening, filtering and removing air, it is extruded into an acid bath of 7-10% sulphuric acid, 1-5% zinc sulphate and 15-22% sodium sulphate at 35-60 C, where the xanthate decomposes back to cellulose. Spinning runs at about 100 m/min, against several thousand for melt-spun fibres. Kent notes that filament rayon capacity was shut down where the capital needed to meet environmental rules could not be justified.

• Kent (2012), Ch 12 Rayon; Acrylics · pdf pp. 433-438, 456-459
• Wittcoff (2004), Ch 4 Uses of acrylonitrile · pdf pp. 209-210
• Shreve (1984), Ch 35 Man-made fiber industries · pdf pp. 684, 687

See it in the table

Check your understanding

What does the palladium hydrogenation step in PTA manufacture achieve?

Why must acrylic fibre be spun from solution rather than melt spun?

Which statement about drawing synthetic fibres is correct?