Quinn, A.E.

Quinn, A.E.

Wednesday, 30 March 2011 02:23

Synthetic Fibres

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.

 

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Saturday, 19 February 2011 01:08

Safe Handling and Storage of Chemicals

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

Before a new hazardous substance is received for storage, information concerning its correct handling should be provided to all users. Planning and maintaining of storage areas are necessary to avoid material losses, accidents and disasters. Good housekeeping is essential, and special attention should be paid to incompatible substances, suitable location of products and climatic conditions.

Written instructions of storage practices should be provided, and the chemicals’ material safety data sheets (MSDSs) should be available in storage areas. Locations of the different classes of chemicals should be illustrated in a storage map and in a chemical register. The register should contain the maximum allowed quantity of all chemical products and the maximum allowed quantity of all chemical products per class. All substances should be received at a central location for distribution to the storerooms, stockrooms and laboratories. A central receiving area is also helpful in monitoring substances that may eventually enter the waste-disposal system. An inventory of substances contained in the storerooms and stockrooms will give an indication of the quantity and nature of substances targeted for future disposal.

Stored chemicals should be examined periodically, at least annually. Chemicals with expired shelf lives and deteriorated or leaking containers should be disposed of safely. A “first in, first out” system of keeping stock should be used.

The storage of dangerous substances should be supervised by a competent, trained person. All workers required to enter storage areas should be fully trained in appropriate safe work practices, and a periodic inspection of all storage areas should be carried out by a safety officer. A fire alarm should be situated in or near the outside of the storage premises. It is recommended that persons should not work alone in a storage area containing toxic substances. Chemical storage areas should be located away from process areas, occupied buildings and other storage areas. In addition, they should not be in proximity of fixed sources of ignition.

Labelling and Relabelling Requirements

The label is the key to organizing chemical products for storage. Tanks and containers should be identified with signs indicating the name of the chemical product. No containers or cylinders of compressed gases should be accepted without the following identifying labels:

  • identification of contents
  • description of principal hazard (e.g., flammable liquid)
  • precautions to minimize hazards and prevent accidents
  • correct first aid procedures
  • correct procedures for cleaning up spills
  • special instructions to medical personnel in case of an accident.

 

The label may also offer precautions for correct storage, such as “Keep in a cool place” or “Keep container dry”. When certain dangerous products are delivered in tankers, barrels or bags and repackaged at the workplace, each new container should be relabelled so that the user will be able to identify the chemical and recognize the risks immediately.

Explosive Substances

Explosive substances include all chemicals, pyrotechnics and matches which are explosives per se and also those substances such as sensitive metallic salts which, by themselves or in certain mixtures or when subject to certain conditions of temperature, shock, friction or chemical action, may transform and undergo an explosive reaction. In the case of explosives, most countries have stringent regulations regarding safe storage requirements and precautions to be taken in order to prevent theft for use in criminal activities.

The storage places should be situated far away from other buildings and structures so as to minimize damage in case of an explosion. Manufacturers of explosives issue instructions as to the most suitable type of storage. The storerooms should be of solid construction and kept securely locked when not in use. No store should be near a building containing oil, grease, waste combustible material or flammable material, open fire or flame.

In some countries there is a legal requirement that magazines should be situated at least 60 m from any power plant, tunnel, mine shaft, dam, highway or building. Advantage should be taken of any protection offered by natural features such as hills, hollows, dense woods or forests. Artificial barriers of earth or stone walls are sometimes placed around such storage places.

The storage place should be well ventilated and free from dampness. Natural lighting or portable electric lamps should be used, or lighting provided from outside the storehouse. Floors should be constructed of wood or other non-sparking material. The area surrounding the storage place should be kept free of dry grass, rubbish or any other material likely to burn. Black powder and explosives should be stored in separate storehouses, and no detonators, tools or other materials should be kept in an explosive store. Non-ferrous tools should be used for opening cases of explosives.

Oxidizing Substances

Oxidizing substances provide sources of oxygen, and thus are capable of supporting combustion and intensifying the violence of any fire. Some of these oxygen suppliers give off oxygen at storage-room temperature, but others require the application of heat. If containers of oxidizing materials are damaged, the contents may mix with other combustible materials and start a fire. This risk can be avoided by storing oxidizing materials in a separate storage place. However, this practice may not always be available, as, for example, in dock warehouses for goods in transit.

It is dangerous to store powerful oxidizing substances near liquids that even have a low flash point or even slightly flammable materials. It is safer to keep all flammable materials away from a place where oxidizing substances are stored. The storage area should be cool, well ventilated and of fire-resisting construction.

Flammable Substances

A gas is deemed to be flammable if it burns in the presence of air or oxygen. Hydrogen, propane, butane, ethylene, acetylene, hydrogen sulphide and coal gas are among the most common flammable gases. Some gases such as hydrogen cyanide and cyanogen are both flammable and poisonous. Flammable materials should be stored in places which are cool enough to prevent accidental ignition if the vapours mix with the air.

Vapours of flammable solvents may be heavier than air and may move along the floor to a distant ignition source. Flammable vapours from spilled chemicals have been known to descend into stairwells and elevator shafts and ignite at a lower storey. It is therefore essential that smoking and open flames be prohibited where these solvents are handled or stored.

Portable, approved safety cans are the safest vessels for storing flammables. Quantities of flammable liquids greater than 1 litre should be stored in metal containers. Two-hundred-litre drums are commonly used to ship flammables, but are not intended as long-term storage containers. The stopper should be removed carefully and replaced by an approved pressure-relief vent to avoid increased internal pressure from heat, fire or exposure to sunlight. When transferring flammables from metal equipment, the worker should use an enclosed transfer system or have adequate exhaust ventilation.

The storage area should be situated away from any source of heat or fire hazard. Highly flammable substances should be kept apart from powerful oxidizing agents or from materials which are susceptible to spontaneous combustion. When highly volatile liquids are stored, any electric light fittings or apparatus should be of certified flameproof construction, and no open flames should be permitted in or near the storage place. Fire extinguishers and absorbent inert materials, such as dry sand and earth, should be available for emergency situations.

The walls, ceilings and floors of the storage room should consist of materials with at least a 2-hour fire resistance. The room should be fitted with self-closing fire doors. The storage-room installations should be electrically grounded and periodically inspected, or equipped with automatic smoke- or fire-detection devices. Control valves on storage vessels containing flammable liquids should be clearly labelled, and pipelines should be painted with distinctive safety colours to indicate the type of liquid and the direction of flow. Tanks containing flammable substances should be situated on ground sloping away from the main buildings and plant installations. If they are on level ground, protection against fire spread can be obtained by adequate spacing and the provision of dykes. The dyke capacity should preferably be 1.5 times that of the storage tank, as a flammable liquid may be likely to boil over. Provision should be made for venting facilities and flame arrestors on such storage tanks. Adequate fire extinguishers, either automatic or manual, should be available. No smoking should be allowed.

Toxic Substances

Toxic chemicals should be stored in cool, well ventilated areas out of contact with heat, acids, moisture and oxidizing substances. Volatile compounds should be stored in spark-free freezers (–20 °C) to avoid evaporation. Because containers may develop leaks, storerooms should be equipped with exhaust hoods or equivalent local ventilation devices. Open containers should be closed with tape or other sealant before being returned to the storeroom. Substances which can react chemically with each other should be kept in separate stores.

Corrosive Substances

Corrosive substances include strong acids, alkalis and other substances which will cause burns or irritation of the skin, mucous membranes or eyes, or which will damage most materials. Typical examples of these substances include hydrofluoric acid, hydrochloric acid, sulphuric acid, nitric acid, formic acid and perchloric acid. Such materials may cause damage to their containers and leak into the atmosphere of the storage area; some are volatile and others react violently with moisture, organic matter or other chemicals. Acid mists or fumes may corrode structural materials and equipment and have a toxic action on personnel. Such materials should be kept cool but well above their freezing point, since a substance such as acetic acid may freeze at a relatively high temperature, rupture its container and then escape when the temperature rises again above its freezing point.

Some corrosive substances also have other dangerous properties; for example, perchloric acid, in addition to being highly corrosive, is also a powerful oxidizing agent which can cause fire and explosions. Aqua regia has three dangerous properties: (1) it displays the corrosive properties of its two components, hydrochloric acid and nitric acid; (2) it is a very powerful oxidizing agent; and (3) application of only a small amount of heat will result in the formation of nitrosyl chloride, a highly toxic gas.

Storage areas for corrosive substances should be isolated from the rest of the plant or warehouses by impervious walls and floor, with provision for the safe disposal of spillage. The floors should be made of cinder blocks, concrete that has been treated to reduce its solubility, or other resistant material. The storage area should be well ventilated. No store should be used for the simultaneous storage of nitric acid mixtures and sulphuric acid mixtures. Sometimes it is necessary to store corrosive and poisonous liquids in special types of containers; for example, hydrofluoric acid should be kept in leaden, gutta percha or ceresin bottles. Since hydrofluoric acid interacts with glass, it should not be stored near glass or earthenware carboys containing other acids.

Carboys containing corrosive acids should be packed with kieselguhr (infusorial earth) or other effective inorganic insulating material. Any necessary first-aid equipment such as emergency showers and eyewash bottles should be provided in or immediately close to the storage place.

Water-reactive Chemicals

Some chemicals, such as sodium and potassium metals, react with water to produce heat and flammable or explosive gases. Certain polymerization catalysts, such as alkyl aluminium compounds, react and burn violently on contact with water. Storage facilities for water-reactive chemicals should not have water in the storage area. Non-water automatic sprinkler systems should be employed.

Legislation

Detailed legislation has been drawn up in many countries to regulate the manner in which various dangerous substances may be stored; this legislation includes the following specifications:

  • type of building, its location, the maximum amounts of various substances that may be stored in one place
  • type of ventilation required
  • precautions to be taken against fire, explosion and the release of dangerous substances
  • type of lighting (e.g., flameproof electrical equipment and light fixtures when explosive or flammable materials are stored)
  • number and location of fire exits
  • security measures against entry by unauthorized persons and against theft
  • labelling and marking of storage vessels and pipelines
  • warning notices to workers as to the precautions to be observed.

 

In many countries there is no central authority concerned with the supervision of the safety precautions for the storage of all dangerous substances, but a number of separate authorities exist. Examples include mine and factory inspectorates, dock authorities, transport authorities, police, fire services, national boards and local authorities, each of which deals with a limited range of dangerous substances under various legislative powers. It is usually necessary to obtain a licence or permit from one of these authorities for the storage of certain types of dangerous substances such as petroleum, explosives, cellulose and cellulose solutions. The licensure procedures require that storage facilities comply with specified safety standards.

 

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Contents

Preface
Part I. The Body
Part II. Health Care
Part III. Management & Policy
Part IV. Tools and Approaches
Part V. Psychosocial and Organizational Factors
Part VI. General Hazards
Part VII. The Environment
Part VIII. Accidents and Safety Management
Part IX. Chemicals
Part X. Industries Based on Biological Resources
Part XI. Industries Based on Natural Resources
Part XII. Chemical Industries
Part XIII. Manufacturing Industries
Part XIV. Textile and Apparel Industries
Part XV. Transport Industries
Part XVI. Construction
Part XVII. Services and Trade
Part XVIII. Guides