DYEING, PRINTING AND FINISHING
J. M. Strother, A. K. Niyogi *
* The section on dyeing is adapted from A.K. Niyogi’s contribution to the 3rd edition of the Encyclopaedia of Occupational Health and Safety.
Dyeing involves a chemical combination or a powerful physical affinity between the dye and the fibre of the fabric. An extensive variety of dyes and processes is used, depending on the type of fabric and the end-product desired.
Classes of dyes
Acid or basic dyes are used in a weak acid bath for wool, silk or cotton. Some acid dyes are used after mordanting the fibres with metallic oxide, tannic acid or dichromates. Direct dyes, which are not fast, are used for the dyeing of wool, rayon and cotton; they are dyed at the boil. For dyeing cotton fabrics with sulphur dyes, the dyebath is prepared by pasting the dye with soda ash and sodium sulphide and hot water. This dyeing is also carried out at the boil. For dyeing cotton with azo dyes, naphthol is dissolved in aqueous caustic soda. The cotton is impregnated with the solution of the sodium naphthoxide that is formed, and it is then treated with a solution of a diazo compound to develop the dye in the material. Vat dyes are made into leuco-compounds with sodium hydroxide and sodium hydrosulphite; this dyeing is done at 30 to 60 °C. Disperse dyes are used for the dyeing of all synthetic fibres which are hydrophobic. Swelling agents or carriers which are phenolic in nature must be used to enable the disperse dyes to act. Mineral dyes are inorganic pigments which are salts of iron and chromium. After impregnation, they are precipitated by addition of hot alkaline solution. Reactive dyes for cotton are used in a hot or a cold bath of soda ash and common salt.
Preparing fabrics for dyeing
The preparatory processes before dyeing cotton fabrics consist of the following sequence of steps: The cloth is passed through a shearing machine to cut the loosely adhering fibres and then, to complete the trimming process, it is passed rapidly over a row of gas flames and the sparks are extinguished by passing the material through a water box. Desizing is carried out by passing the cloth through a diastase solution which removes the size completely. To remove other impurities, it is scoured in a kier with dilute sodium hydroxide, sodium carbonate or turkey red oil for 8 to 12 hours at high temperature and pressure.
For coloured woven material, an open kier is used and sodium hydroxide is avoided. The natural colouring in the cloth is removed by hypochlorite solution in the bleaching pits, after which the cloth is aired, washed, dechlorinated by means of a sodium bisulphite solution, washed again and scoured with dilute hydrochloric or sulphuric acid. After a final, thorough washing, the cloth is ready for the dyeing or printing process.
Dyeing is carried out in a jig or padding machine, in which the cloth is moved through a stationary dye solution prepared by dissolving the dyestuff powder in a suitable chemical and then diluting with water. After dyeing, the cloth is subjected to a finishing process.
The preparation of polyamide (nylon) fibres for dyeing involves scouring, some form of setting treatment and, in some cases, bleaching. The treatment adopted for the scouring of woven polyamide fabrics depends mainly on the composition of the size used. Water-soluble sizes based on polyvinyl alcohol or polyacrylic acid can be removed by scouring in a liquor containing soap and ammonia or Lissapol N or similar detergent and soda ash. After scouring, the material is rinsed thoroughly and is then ready for dyeing or printing, usually in a jigger or winch dyeing machine.
The raw wool is first scoured by the emulsification process, in which soap and a soda ash solution are used. The operation is carried out in a washing machine which consists of a long trough provided with rakes, a false bottom and, at the exit, wringers. After thorough washing, the wool is bleached with hydrogen peroxide or with sulphur dioxide. If the latter is used, the damp goods are left exposed to the sulphur dioxide gas overnight. The acid gas is neutralized by passing the fabric through a sodium carbonate bath, and then it is thoroughly washed. After dyeing, the goods are rinsed, hydroextracted and dried.
Hazards in Dyeing and Their Prevention
Fire and explosion
The fire hazards found in a dye works are the flammable solvents used in the processes and certain flammable dyestuffs. Safe storage facilities should be provided for both: properly designed storerooms constructed of fire-resisting materials with a raised and ramped sill at the doorway so that escaping liquid is contained within the room and prevented from flowing to a place where it may be ignited. It is preferable that stores of this nature be located outside the main factory building. If large quantities of flammable liquids are kept in tanks outside the building, the tank area should be mounded to contain escaping liquid.
Similar arrangements should be made when the gaseous fuel used on the singeing machines is obtained from a light petroleum fraction. The gas-making plant and the storage facilities for the volatile petroleum spirit should preferably be outside the building.
Many factories use hypochlorite solution for bleaching; in others, the bleaching agent is gaseous chlorine or bleaching powder which releases chlorine when it is charged into the tank. In either case, workers may be exposed to dangerous levels of chlorine, a skin and eye irritant and a dangerous pulmonary tissue irritant causing delayed lung oedema. To limit the escape of chlorine into the workers’ atmosphere, bleaching vats should be designed as closed vessels provided with vents that limit the escape of chlorine so that the relevant recommended maximum exposure levels are not exceeded. Atmospheric chlorine levels should be checked periodically to ensure that the exposure limit is not being exceeded.
The valves and other controls of the tank from which the liquid chlorine is supplied to the dyeworks should be controlled by a competent operator, since the possibilities of an uncontrolled leak could well be disastrous. When a vessel that has contained chlorine or any other dangerous gas or vapour has to be entered, all of the precautions advised for work in confined places should be taken.
The use of corrosive alkalis and acids and the treatment of cloth with boiling liquor expose the workers to the risk of burns and scalds. Both hydrochloric acid and sulphuric acid are used extensively in dyeing processes. Caustic soda is used in bleaching, mercerizing and dyeing. Chips from the solid material fly and create hazards for the workers. Sulphur dioxide, which is used in bleaching, and carbon disulphide, which is used as a solvent in the viscose process, can also pollute the workroom. Aromatic hydrocarbons such as benzol, toluol and xylol, solvent naphthas and aromatic amines such as aniline dyes are dangerous chemicals to which workers are likely to be exposed. Dichlorobenzene is emulsified with water with the help of an emulsifying agent, and is used for dyeing of polyester fibres. LEV is essential.
Many dyestuffs are skin irritants that cause dermatitis; in addition, workers are tempted to use harmful mixtures of abrasive, alkali and bleaching agents to remove dye stains from their hands.
Organic solvents used in the processes and for the cleaning of machines may themselves cause dermatitis or render the skin vulnerable to the irritant action of the other harmful substances that are used. Furthermore, they may be the cause of peripheral neuropathy-for example, methyl butyl ketone (MBK). Certain dyes, such as rhodamine B, magenta, b-naphthylamine and certain bases such as dianisidine, have been found to be carcinogenic. The use of b-naphthylamine has generally been abandoned in dyestuffs, which are discussed more fully elsewhere in this Encyclopaedia.
In addition to the fibre materials and their contaminants, allergy may be caused by the sizing and even by the enzymes used to remove the sizing.
Suitable PPE, including eye-protective equipment, should be provided to prevent contact with these hazards. In certain circumstances when barrier creams have to be used, care should be taken to ensure that they are effective for the purpose and that they can be removed by washing. At best, however, the protection they provide is rarely as reliable as that afforded by properly designed gloves. Protective clothing should be cleaned at regular intervals, and when splashed or contaminated by dyestuffs, it should be replaced by clean clothing at the earliest opportunity. Sanitary facilities for washing, bathing and changing should be provided, and the workers should be encouraged to use them; personal hygiene is particularly important for dye workers. Unfortunately, even when all protective measures have been taken, some workers are found to be so sensitive to the effects of these substances that transfer to other work is the only alternative.
Serious scalding accidents have occurred when hot liquor has been accidentally admitted to a kier in which a worker has been arranging the cloth to be treated. This can occur when a valve is accidentally opened or when hot liquor is discharged into a common discharge duct from another kier on the range and enters the occupied kier through an open outlet. When a worker is inside a kier for any purpose, the inlet and outlet should be closed, isolating that kier from the other kiers on the range. If the locking device is operated by a key, it should be retained by the worker who might be injured by an accidental admission of hot liquid until he or she leaves the vessel.
Printing is carried out on a roller printing machine. The dye or pigment is thickened with starch or made into emulsion which, in the case of pigment colours, is prepared with an organic solvent. This paste or emulsion is taken up by the engraved rollers which print the material, and the colour is subsequently fixed in the ager or curing machine. The printed cloth then receives the appropriate finishing treatment.
Wet printing is performed with dyeing systems similar to those used in dyeing, such as vat printing and fibre-reactive printing. These printing methods are used only for 100% cotton fabric and for rayon. The health hazards associated with this type of printing are the same as those discussed above.
Solvent-based pigment printing
Solvent-based printing systems use large amounts of solvents such as mineral spirits in the thickening system. The major hazards are:
· Flammability. The thickening systems contain up to 40% solvents and are highly flammable. They should be stored with extreme caution in properly ventilated and electrically grounded areas. Care should also be taken in transferring these products to avoid creating a spark from static electricity.
· Air emissions. Solvents in this print system will be flashed off from the oven during drying and curing. Local environmental regulations will dictate the permissible levels of volatile organic compound (VOC) emissions that can be tolerated.
· Sludge. Since this print system is solvent based, the print paste cannot be allowed to enter the wastewater treatment system. It must be disposed of as a solid waste. Sites where sludge piles are used can have environmental problems with ground and groundwater contamination. These sludge storage areas should be equipped with waterproof linings to prevent this from occurring.
Aqueous-based pigment printing
None of the health hazards for solvent-based pigment printing apply to the aqueous-based printing systems. Although some solvents are used, the amounts are so small that they are not significant. The primary health hazard is the presence of formaldehyde.
Pigment printing requires the use of a cross-linker to assist in the bonding of the pigments to the fabric. These cross-linkers exist as stand-alone products (e.g., melamine) or as part of other chemicals such as binders, antiwicks, and even in the pigments themselves. Formaldehyde plays a necessary role in the function of the cross-linkers.
Formaldehyde is a sensitizer and an irritant that may produce reactions, sometimes violent, in workers who are exposed to it either by inhaling the air around the printing machine as it is operating or by coming into contact with the printed fabric. These reactions may range from simple eye irritation to welts on the skin and severe difficulty with breathing. Formaldehyde has been found to be carcinogenic in mice but it has not yet been conclusively associated with cancer in humans. It is classified as a Group 2A Carcinogen, “Probably Carcinogenic to Humans”, by the International Agency for Research on Cancer (IARC).
To protect the local environment, emissions from the plant have to be monitored to ensure that levels of formaldehyde do not exceed those stipulated by applicable regulations.
Another potential hazard is ammonia. Since the print paste is pH (acidity) sensitive, ammonia is often used as a print-paste thickener. Care should be taken to handle ammonia in a well-ventilated area and to wear respiratory protection if necessary.
Since all dyes and pigments used in printing are usually in a liquid form, dust exposure is not a hazard in printing as it is in dyeing.
Finishing is a term applied to a very broad range of treatments that are usually performed during the last manufacturing process before fabrication. Some finishing can also be performed after fabrication.
This type of finishing involves processes that change the texture or appearance of a fabric without the use of chemicals. They include:
· Sanforizing. This is a process where a fabric is overfed between a rubber belt and a heated cylinder and then fed between a heated cylinder and an endless blanket to control shrinkage and create a soft hand.
· Calendering. This is a process where fabric is fed between large steel rollers under pressures that range up to 100 tonnes. These rolls can be heated with either steam or gas to temperatures up to 232 °C. This process is used to change the hand and appearance of the fabric.
· Sanding. In this process, fabric is fed over rolls which are covered with sand to change the surface of the fabric and give a softer hand.
· Embossing. This is a process where fabric is fed between heated steel rollers which have been engraved with a pattern which is permanently transferred to the fabric.
· Heat-setting. This is a process where synthetic fabric, usually polyester, is run through either a tenter frame or a semi-contact heat-set machine at temperatures that are high enough to begin the molecular melting of the fabric. This is done to stabilize the fabric for shrinkage.
· Brushing. This is a process where fabric is run across brushes revolving at high speeds to change the surface appearance and the hand of the fabric.
· Sueding. In this process, fabric is run between a small steel roller and a larger roller that is covered with sandpaper to change the appearance and the hand of the fabric.
The principal hazards are the presence of heat, the very high temperatures being applied and nip points in the moving machine parts. Care should be taken to properly guard the machinery to prevent accidents and physical injury.
Chemical finishing is performed on a variety of types of equipment (e.g., pads, jigs, jet dye machines, becks, spray bars, kiers, paddle machines, kiss roll applicators and foamers).
One type of chemical finishing does not involve a chemical reaction: the application of a softener or a hand builder to modify the feel and texture of the fabric, or to improve its sewability. This presents no significant hazards except for the possibility of irritation from skin and eye contact, which can be prevented by the use of proper gloves and eye protection.
The other type of chemical finishing involves a chemical reaction: resin finishing of cotton fabric to produce desired physical properties in the fabric such as low shrinkage and a good smoothness appearance. For cotton fabric, for example, a dimethyldihydroxyethylene urea (DMDHEU) resin is catalysed and bonds with the cotton molecules of the fabric to create a permanent change in the fabric. The primary hazard associated with this type of finishing is that most resins release formaldehyde as part of their reaction.
As in the rest of the textile industry, dyeing, printing and finishing operations present a mixture of old, generally small establishments in which worker safety, health and welfare are given little if any attention, and newer, larger establishments with ever-improving technology in which, to the extent possible, hazard control is built into the design of the machinery. In addition to the specific hazards outlined above, such problems as substandard lighting, noise, incompletely guarded machinery, lifting and carrying of heavy and/or bulky objects, poor housekeeping and so on remain ubiquitous. Therefore, a well-formulated and implemented safety and health programme that includes the training and effective supervision of workers is a necessity.