SYNTHETIC FIBRES

 

A. E. Quinn, R. Mattiusi *

 

 

* Adapted from 3rd edition, Encyclopaedia of Occupational Health and Safety.

 

Synthetic fibres are made from polymers that have been synthetically produced from chemical elements or compounds developed by the petrochemical industry. Unlike natural fibres (wool, cotton and silk), which date back to antiquity, synthetic fibres have a relatively short history dating back to the perfection of the viscose process in 1891 by Cross and Bevan, two British scientists. A few years later, rayon production started on a limited basis, and by the early 1900s, it was being produced commercially. Since then, a large variety of synthetic fibres has been developed, each designed with special characteristics that make it suitable for a particular kind of fabric, either alone or in combination with other fibres. Keeping track of them is made difficult by the fact that the same fibre may have different trade names in different countries.

 

The fibres are made by forcing liquid polymers through the holes of a spinneret to produce a continuous filament. The filament can be directly woven into cloth or, to give it the characteristics of natural fibres, it can, for example, be textured to add bulkiness, or it can be chopped into staple and spun.

 

Classes of Synthetic Fibres

The main classes of synthetic fibres used commercially include:

 

· Polyamides (nylons). The names of the long-chain polymeric amides are distinguished by a number which indicates the number of carbon atoms in their chemical constituents, the diamine being considered first. Thus, the original nylon produced from hexamethylene diamine and adipic acid is known in the United States and the United Kingdom as nylon 66 or 6.6, since both the diamine and the dibasic acid contain 6 carbon atoms. In Germany, it is marketed as Perlon T, in Italy as Nailon, in Switzerland as Mylsuisse, in Spain as Anid and in the Argentine as Ducilo.

· Polyesters. First introduced in 1941, polyesters are made by reacting ethylene glycol with terephthalic acid to form a plastic material made of long chains of molecules, which is pumped in molten form from spinnerets, allowing the filament to harden in cold air. A drawing or stretching process follows. Polyesters are known, for example, as Terylene in the UK, Dacron in the United States, Tergal in France, Terital and Wistel in Italy, Lavsan in the Russian Federation, and Tetoran in Japan.

· Polyvinyls. Polyacrylonitrile or acrylic fibre, first produced in 1948, is the most important member of this group. It is known under a variety of trade names: Acrilan and Orlon in the United States, Crylor in France, Leacril and Velicren in Italy, Amanian in Poland, Courtelle in the UK and so on.

· Polyolefins. The most common fibre in this group, known as Courlene in the UK, is made by a process similar to that for nylon. The molten polymer at 300 °C is forced through spinnerets and cooled in either air or water to form the filament. It is then drawn or stretched.

· Polypropylenes. This polymer, known as Hostalen in Germany, Meraklon in Italy and Ulstron in the UK, is melt spun, stretched or drawn, and then annealed.

· Polyurethanes. First produced in 1943 as Perlon D by the reaction of 1,4 butanediol with hexamethylene diisocyanate, the polyurethanes have become the basis of a new type of highly elastic fibre called spandex. These fibres are sometimes called snap-back or elastomeric on account of their rubber-like elasticity. They are manufactured from a linear polyurethane gum, which is cured by heating at very high temperatures and pressures to produce a “vulcanized” cross-linked polyurethane which is extruded as a monofil. The thread, which is widely used in garments requiring elasticity, can be covered by rayon or nylon to improve its appearance while the inner thread provides the “stretch”. Spandex yarns are known, for example, as Lycra, Vyrene and Glospan in the United States and Spandrell in the UK.

 

Special Processes

 

Stapling

Silk is the only natural fibre that comes in a continuous filament; other natural fibres come in short lengths or “staples”. Cotton has a staple of about 2.6 cm, wool of 6 to 10 cm and flax from 30 to 50 cm. The continuous synthetic filaments are sometimes passed through a cutting or stapling machine to produce short staples like the natural fibres. They can then be re-spun on cotton or wool spinning machines in order to produce a finish free of the glassy appearance of some synthetic fibres. During the spinning, combinations of synthetic and natural fibres or mixtures of synthetic fibres may be made.

 

Crimping

To give synthetic fibres the look and feel of wool, the twisted and tangled cut or stapled fibres are crimped by one of a number of methods. They may be passed through a crimping machine, in which hot, fluted rollers impart a permanent crimp. Crimping can also be done chemically, by controlling the coagulation of the filament so as to produce a fibre with an asymmetrical cross section (i.e., one side being thick-skinned and the other thin). When this fibre is wet, the thick side tends to curl, producing a crimp. To make crinkled yarn, known in the United States as non-torque yarn, the synthetic yarn is knitted into a fabric, set and then wound from the fabric by back-winding. The newest method passes two nylon threads through a heater, which raises their temperature to 180 °C and then passes them through a high-speed revolving spindle to impart the crimp. The spindles in the first machine ran at 60,000 revolutions per minute (rpm), but newer models have speeds of the order of 1.5 million rpm.

 

Synthetic Fibres for Work Clothes

The chemical resistance of polyester cloth makes the fabric particularly suitable for protective clothing for acid-handling operations. Polyolefin fabrics are suitable for protection against long exposures to both acids and alkalis. High-temperature-resistant nylon is well adapted for clothing to protect against fire and heat; it has good resistance at room temperature to solvents such as benzene, acetone, trichlorethylene and carbon tetrachloride. The resistance of certain propylene fabrics to a wide range of corrosive substances makes them suitable for work and laboratory clothing.

 

The light weight of these synthetic fabrics makes them preferable to the heavy rubberized or plastic-coated fabrics that would otherwise be required for comparable protection. They are also much more comfortable to wear in hot and humid atmospheres. In selecting protective clothing made from synthetic fibres, care should be taken to determine the generic name of the fibre and to verify such properties as shrinkage; sensitivity to light, dry-cleaning agents and detergents; resistance to oil, corrosive chemicals and common solvents; resistance to heat; and susceptibility to electrostatic charging.

 

Hazards and Their Prevention

 

Accidents

In addition to good housekeeping, which means keeping floors and passageways clean and dry to minimize slips and falls (vats must be leak proof and, where possible, have baffles to contain splashes), machines, drive belts, pulleys and shaftings must be properly guarded. Machines for spinning, carding, winding and warping operations should be fenced to keep materials and parts from flying out and to prevent workers’ hands from entering the dangerous zones. Lockout devices must be in place to prevent restart of machines while they are being cleaned or serviced.

 

Fire and explosion

The synthetic-fibres industry uses large amounts of toxic and flammable materials. Storage facilities for flammable substances should be out in the open or in a special fire-resistant structure, and they should be enclosed in bunds or dykes to localize spills. Automation of the delivery of toxic, flammable substances by a well-maintained system of pumps and pipes will reduce the hazard of moving and emptying containers. Appropriate fire-fighting equipment and clothing should be readily available and workers trained in their use through periodic drills, preferably conducted in concert with or under the observation of local fire-fighting authorities.

 

As the filaments emerge from the spinnerets to be dried in air or by means of spinning, large amounts of solvent vapours are released. These constitute a considerable toxic and explosion hazard and must be removed by LEV. Their concentration must be monitored to be sure that it remains below the solvent’s explosive limits. The exhausted vapours may be distilled and recovered for further use or they may be burned off; on no account should they be released into the general environmental atmosphere.

 

Where flammable solvents are used, smoking should be prohibited and open lights, flames and sparks eliminated. Electrical equipment should be of certified flameproof construction, and machines should be earthed (grounded) to prevent the build-up of static electricity, which might lead to catastrophic sparks.

 

Toxic hazards

Exposures to potentially toxic solvents and chemicals should be maintained below the relevant maximum allowable concentrations by adequate LEV. Respiratory protective equipment should be available for use by maintenance and repair crews and by workers charged with responding to emergencies caused by leaks, spillage and/or fire.