The dyeing process may be optimised by varying the parameters to obtain a top quality dye at the lowest possible cost.
There are numerous dyeing systems and they include everything from consignment processes (batches by a defined weight and length) to semicontinuous processes (open-width or in rope form) to continuous processes (open-width or in rope form).
Depending on the fibres and dyes used, dyeing is carried out at between 20º and 135ºC in hightemperature systems.
Types of fibre
Depending on their origin, fibres can be classified into:
• Natural fibres: these fibres are of vegetable or animal origin such as cotton, wool and silk.
• Chemical fibres of natural polymers: they are so denominated because they are fibres that are artificially obtained from a natural polymer such as cellulose. Rayon, cellulose acetate, etc. are artificial fibres. Later in this document, they shall be referred to as cellulosic fibres.
• Chemical fibres of synthetic polymers: these are obtained by the organic synthesis of petrochemical derivatives. They have a polymeric structure and among them, polyester, polyamide, acrylic fibres, polyolefin and spandex fibres are notable.
Textiles products
The dyeing of textiles fibres can be done on intermediate products or end products.
Below the most significant products are described:
• Cable: the parallel union of a high number of filaments, generally to be converted into cut fibre
• Flock: fibres of a length of 2 to 30 cm
• Flock: fibres of a length of less than 1 mm
• Rovings and slivers: the union of fibres which come from napping, combing or from the roving frames
• Parallel multifilament / textured multifilament
• Yarn: generally from the stretching with twist of an appropriate sliver. It is presented in hank or bobbin form
• Warp beam: parallel disposition in beam form of all yarn necessary for the manufacture of fabric of predetermined width
• Woven fabric: laminar textile structure generally formed by the orthoganal interweaving of warp thread, with weft threads
• Knitwear: laminar textile structure formed by the interweaving of one or more threads on the base mesh structure
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ENNOBLING: DYEING, PRINTING & FINISHING
4.2. DYEING AND FINISHING PROCESSES
Here we present the basics of the most common dyeing and finishing processes. For each process, the most frequent unitary and auxiliary operations are analysed and the commonly used raw materials and chemical additives are identified.
Most operations are performed “wet” and take place in a receptacle or vat filled with liquid (usually water), into which the raw materials and additives have been dissolved or in which they are suspended. The textile material is submerged in this liquid. Immediately following this, this material is pressed in order to remove the excess liquid, which is returned to the receptacle for reuse.
Next, the washing operations are carried out in order to eliminate the remains of additives whose permanence in the material is not desired, so as not to hinder subsequent following operations.
Many operations such as colour scaling, the cleaning and preparation of the facilities, emptying, draining and rinsing the machinery, which is usually done before the processes, are not dealt with in this chapter.
The machinery used depends on the type of operation and the form of presentation of the textile material.
4.2.1. Preparation
Preparation includes all operations prior to dyeing, whose aim is to ensure the physical and chemical properties of both the finished textiles, and, in some situations, of the intermediate products, favouring the later reactions that take place in dyeing.
For this reason, some of these operations may be considered similar to the finishing operations and, in fact, they are not very different at all.
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DYEING
DYEING
Flock
Yarn Fabric
Knitwear
Fabric
Knitwear
DYEING
PRINTING
DYEING
PRINTING
F I N I S H
The aim, therefore, of preparation operations is to clean the textile materials of the impurities that are present in them or to provide them with special qualities and characteristics.
Among preparation operations, the following are noteworthy:
• Mercerising • Scouring • Degreasing • Carbonisation • Fulling • Singeing • Quenching • Heat setting • Chemical washing • Solvent washing • Chemical bleaching • Optical bleaching
4.2.2. Description of the dyeing process
Aimed at modifying the colour of a textile element, in any form, through the application of dye material, for both continuous procedures and batch dyeing. In either of the two cases, the aim is to achieve the bath exhaustion and fixation of as much dye as possible to the fabric or textile element, to limit dye losses in the later washing stages, and during its use.
The application of any dye can be described according to the following stages:
• First stage: transfer of the dye from the dye bath to the fibre surface.
• Second stage: diffusion or migration of the dye molecules from the surface of the fibre to the interior of the material to be dyed.
• Third stage: fixing of the dye on the reactive points of the fibre’s molecular structure.
Batch or discontinuous dyeing
In the case of discontinuous dyeing, which is also called exhaustion, the procedure consists of immersing a weight of fabric or yarn, normally between 100 and 1,000 kg, into the dye bath, which contains the dye solution of the auxiliary and chemical products. Given the affinity of the dyes for fibres, the molecules present in the solution are incorporated by the fibres, in a transfer that may take anything from a few minutes to hours.
The use of chemical auxiliary additives, as well as the control of the bath environment (physical variables, basically the temperature) can accelerate this operation and optimise it. Once the dye is fixed in the fibre, the fibre goes through a washing process in which both the dye which has not become fixed and the auxiliary products used to help the fixation process are eliminated.
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The theory of bath exhaustion
The maximum exhaustion that a given dye can reach is related to the affinity of the dye with the fibre and the bath ratio worked with, leading to the following equation: E= K K+L Where:
E= exhaustion K= dye affinity L= bath ratio
Dye affinity is defined as: K= Cf/Cs
Cf= concentration of the dye in the fibre Cs= concentration of the dye in the solution
Both values obtained in the equilibrium stage at constant temperature.
The value of K may vary between 50 and 1,000 for different combinations of fibre and dyes.
The practical values of L oscillate between 5 and 30 depending on the way the bath is applied and the machinery used. (From 1 kg textile cloth/5 l bath, to 1/30).
With these values, the coefficient E for exhaustion is obtained between 0.5 and 1, that is to say, between 50% and 100%.
In accordance with this approach, if the bath ratio is increased, exhaustion is reduced and, therefore, the waste concentration of the exhausted bath is increased.
This effect is greater in the dyes which have a low affinity. Hence the importance of knowing the affinity value and a correct operation.
That is to say, in order to reduce the dye content in wastewater, high-affinity dyes should be selected or the bath ratio must be reduced if the affinity is low.
The same dye can present different affinities for one or another fibre and, consequently, the generalisation of the exhaustion associated to each dye is most difficult, and requires systematic testing in the laboratories of each industry.
Machinery used
Cabinets: the textile material (yarn in hank form) is static on a support, the bath in motion is driven by a pump and the cabinet is at atmospheric pressure.
Autoclave: as in the previous case, the textile material remains static and the dye bath is in motion. It consists of a horizontal or vertical cylindrical recipient with some supports onto which the
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different textile material, spun thread, flock or fabric are placed. The bath passes through the material driven by a circulation pump. The receptacle is closed and work is done under pressure.
Continuous dyeing
In the case of continuous dyeing, the textiles materials are fed continuously into a dyeing apparatus, at a speed of between 50 and 250 m/min. The apparatus consists of a first stage of incorporating the dye, followed by the addition of the chemical auxiliaries, the application of heat to aid fixation and, later, the washing of the surplus, as in the case of discontinuous dyeing, though in this case, in continuous washing facilities.
Fixation in continuous processes is far faster than in batch dyeing, but it requires processing at least 10,000 metres. Nevertheless, today, machines may be found on the market that are capable of dyeing, in continuous form, lengths of material of only 2,000 metres.
Machinery used
Winch: this is used for fabric dyeing in rope form. The denomination of rope form refers to the passage of the fabric through a ring, joining the ends. The fabric is in motion whereas the bath is static. It is composed of a cylinder of trapezoidal section, a driving element which performs the shifting of the fabric, and some bars to separate the rope in order to avoid malformations and jams. Currently this machine is substituted by Jets and Overflows.
“Jigger”: this machine which is used for the “open-width” dyeing of fabric by means of rollers that roll it up and unroll it, passing it through the bath, while the latter is static. They may be atmospheric in order to work at 100ºC, while those that work under pressurised conditions may reach 145ºC.
“Jet”: is a rope form dyeing machine. The fabric is set in motion by the action of a nozzle (hence the denomination jet), through which the bath passes, where both the bath and the fabric are in simultaneous movement. The high speed produced by the injection in the bath causes turbulence, which facilitates the penetration of the dye solution towards the interior of the fabric and good equalisation of the dye, in a shorter space of time, and with lower water consumption than in the old winches.
“Airflow”: is similar to a “jet” but with the impulse of a mixture of air and dye solution, which allows a more delicate treatment of the fabric. Water consumption is greatly reduced since only the necessary amount of dye is added, eliminating the concept of bath accumulation.
“Overflow”: the fabric and the bath are in motion. As in the case of the “jets”, the bath acts on the fabric but in this case, the fabric is dragged by a winder and not just by the action of the nozzle. It is usually used for the dyeing of many types of fabric in rope form, from the most resistant to delicate fabrics.
“Foulard”: this universal machine is used to impregnate the textile material with any liquid. It is described in this chapter in order to present the “pad-steam” process.
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“Pad-steam”: this machine applies a steaming to a dye impregnation in a “foulard” machine. In this way, the dye is fixed on the fibre in a short period of time. It is often used in the dyeing of cellulosic fibres.
The dyeing process may be applied to any stage of the state of the textile element and to any type of material.
Below is a table of all possible combinations of dye applications.
Table 6: Combination of possible dye operations
TYPE OF TEXTILES ELABORATION FIBRE CLASS Flock and yarn Cotton and blends Wool and blends Cellulosic and blends Synthetic fibres and blends Fabric Cotton and blends Wool and blends Cellulosic and blends Synthetic fibres and blends Knitwear Cotton and blends Wool and blends Cellulosic and blends Synthetic fibres and blends Garment Cotton and blends Wool and blends Cellulosic and blends Synthetic fibres and blends Bulk polymer Synthetic fibres
4.2.3. Dyes used in the dyeing process
The families of dyes used for the dyeing of yarn, fabric and knitwear are the following:
Direct dyes
The dyeing operation with a direct dye consists of bringing the fibre into contact with the dye dissolved in water and heating to boiling point. In order to aid the operation, a neutral electrolyte is often added, such as sodium chloride or sulphate, and surfactant-type products (wetting agents, levelling agents, etc.). The direct dyes belong to several families of chemical compounds, and are characterised by being aromatic organic compounds containing sulphonic groups that act as solubilisers.
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Chemically, direct dyes belong to the following types:
• Azo dyes
• of diphenyl amines, such as benzidine, stilbene, aryl diamines, ureics, amides
• Thiazolic dyes
Insoluble azo dyes
The basis of dyeing with insoluble azo dyes lies in the formation of coloured pigment on the fibre, which is achieved when treating the textile, generally in two baths, with the two components that form the dye. The first component, the so-called developer, is a naphthenic derivative that contains amino and hydroxyl groups.
Nowadays, mainly hydroxylated derivatives are used as developers, and so this dyeing is also known as dyeing with naphtholes.
The textile material impregnated with the developer is put into a second bath in a diaze solution, which, when it reacts with the developer produces the insoluble azo dye on the fibre. This dyeing procedure gives extraordinary wash fastness, far higher than that offered by direct dyes themselves, though with far higher production costs.
Sulphur dyes
The chemical make-up of these dyes is not easy to define. They are given this name because they contain sulphur, generally forming a chain (Ar-S-S-Ar’ or Ar-S-S-S-Ar’). The sulphur may be easily oxidised to become sulphuric acid. The traditional dyes, which are generally low in cost, contain a high concentration of impurities such as salts, sulphides and polysulphides. In an alkaline medium and in the presence of reductive agents, they are transformed into soluble leuco derivatives which are easily absorbed by the fibres.
The dyeing operation using these dyes consists of the following stages:
• Dissolving the dye using a reductive agent: sodium sulphide, sodium bisulphide, ammonium sulphide, sodium hydrosulphite or glucose.
• Dyeing with the addition of a neutral electrolyte, such as sodium chloride and wetting agents.
• Oxidation of the dye absorbed in the fibre with oxidising systems based on bromates, iodates, chlorites, potassium dichromate (practically out of use), peroxides or oxygen.
• Later treatment with metallic salts, detergent, sodium acetate or with sodium dichromate and acetic acid to increase the wash fastness of the colours against light, washing, rubbing, etc.
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Soluble sulphur dyes
This type of dye is a variant of the previous ones, synthesised with thiosulphate groups. The formation of insoluble pigment is done by a reaction with sodium polysulphide in a second bath.
These dyes are suitable for application in continuous mode following the sequence:
• Padding of the dye • Drying of fabric • Padding with sodium polysulphide • Washing • Soaping
Vat dyes
Of different chemical constitution (that may derive from indigo or from anthraquinone), they are insoluble in water, and are transformed by reduction in an alkaline medium into hydrosoluble leuco derivatives with substantiveness for textile fibres, on which they develop the prime colour by ulterior oxidation.
The dyeing operation with these dyes consists of the following stages:
• Reduction of the dye with sodium hydrosulphite, formaldehyde or acetaldehyde sulphoxylate, using caustic soda as an alkali.
• Dyeing with the addition of electrolyte (common salt or sodium sulphate), wetting agents and levelling agents.
• Oxidation by washing with cold water or treatment with oxygenated water or potassium dichromate and sulphuric acid.
• Later washing and soaping treatments.
Reactive dyes
Reactive dyes are one of the most used families of dyes for the dyeing of cotton, rayon and linen fabrics. Due to their inherent chemical characteristics, only a part of the dye which is added to the dye bath reacts chemically with the fibre by means of a covalent bond. The rest of the dye reacts with the water and is known as hydrolysed dye. Part of the latter remains in the dye wastewater and the rest remains inside the fibre but does not have good fastness properties and so must be eliminated in successive soaping and hot rinsing operations.
Reactive dyes include the families of dichlorotriazines, monochlorotriazines, trichloropyridines, difluorochloropyrimidines, vinylsulphonics, etc.
The dye operation using these dyes consists of the following stages:
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• Absorption, analogous to the dyeing with direct dyes.
• Reaction, in which the dye chemically combines with the fibre by means of a covalent bond.
• Later treatment to eliminate the hydrolysed dye.
The application of these dyes can be done either in continuous mode or in batches, which, in the case of yarn is usually done by packing in an autoclave.
The use of any of these systems with reactive dyes implies the consumption of certain chemicals, such as salt. In some cases, in continuous mode processes, urea is used due to its hygroscopic nature.
Specifically for wool, the operations are:
• Exhaustion dyeing, which may be used for flock, combing, hank and fabric spinning.
• Dyeing through padding-cold rest, only applicable for knitwear.
Acid dyes
These dyes colour the wool and proteic fibres in an acid or basic solution. They may be classified into five large groups:
• Azo dyes • Anthraquinonic dyes • Derivatives of triphenylmethane • Azinic type • Xanthene type
The last two are frequently used in obtaining certain shades.
In the dyeing process with these dyes, several auxiliary agents are used such as:
• Levelling agents, which may be anionic compounds which are similar to the fibre or cationic or pseudocationic compounds, which are similar to the dyes, such as, for example, sulphated castor oils, oleic and sulphated polycastor acids or alkylaryl sulphonates.
• Acetic or formic acid to exhaust the dye onto the fibre
• Sodium sulphate
• Ammonium sulphate
Premetallised dyes
These dyes are composed of a metal atom to which one or two molecules, generally of acid dye, are added, forming a co-ordination complex with affinity for the proteic and polyamidic fibres. The metal is usually chrome, although others may be used such as copper, nickel, cobalt, etc.
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The types of premetallised dyes developed are:
• Premetallised dyes 1:1, which dye in a strongly acid bath, formed of chrome and azoic-type dyes.
• Premetallised dyes 1:2, which are in turn divided into: - Premetallised dyes 1:2, which dye in a neutral bath and do not contain ionic solubiliser groups, and for whose application ammonic salts are used to maintain the pH. - Premetallised dyes 1:2, which contain ionic solubiliser groups and, in addition to the acetate or ammonium sulphate buffer, require an equaliser and pH adjustment with acetic acid.
Chrome dyes
The dyes pertaining to this type, which are also called chromable acid dyes, need the aid of a chrome salt to be able to fix perfectly onto the fibre, and they may be classified into the following chemical groups:
• Azo dyes • Anthraquinonic dyes • Triphenylmethanes • Others, such as derivatives of thiazine, of the oxazines and of xanthene
The most commonly used chromium salts are: anhydrous potassium dichromate, sodium dichromate and potassium chromate.
The procedure depends on the dyes used and the type of material dyed. The dichromate may be applied to the wool before dyeing (pre-chroming procedure), with the dye in the same bath (simultaneous chroming procedure) or afterdyeing (afterchroming procedure). These procedures have fallen into disuse and are only used in some very specific cases in place of the “low chrome” procedure.
Disperse dyes
These are non-ionic organic compounds, almost insoluble in water, which are applied in aqueous dispersion, responding to the following structures:
• Dyes with azo groups, principally mono- and some di- azoderivatives, which encompass a broad range of shades.
• Nitrodiphenylamine dyes for yellows and oranges.
• Anthraquinonic dyes for oranges, greens and blues.
The dispersing agents (surfactants) used in the preparation and application of disperse dyes are:
• Esters of sulphuric acid, such as alkylsulphates in chains of 12-13 carbon atoms, sulphated oils, sulphated esters and amides.
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• Sulphonic derivatives, in which the radical chain may be alkyl, alkylaryl, amides, esters or lignins. Among the most commonly used are the derivatives of ß-naphthelene sulphonic acid and its products of condensation with formaldehyde.
• Oxyethylene derivatives, for example, alkylaryl oxyethylenes and alkylamine oxyethylenes.
The application methods depend on the way that the textile materials are found. Dyeing can be done by exhaustion at high temperature or with a carrier at temperatures of 100ºC. (The latter case is tending to fall into disuse).
Traditionally, in the case of polyester, following dyeing at 130ºC, a reductive bath is necessary, which is performed at a lower temperature.
Cationic dyes
Cationic dyes are highly numerous organic based salts with a wide variety of chemical structures, among which the following are included:
• Derivatives of di- and triphenyl methane.
• Derivatives of diphenyl-amine which includes a series of dyes of a simple structure which belong to the family of azines, oxazines, tiazines, indamines, rhodamines, gallocyanines, etc.
• Azoic or anthraquinonic-type dyes.
• Dyes with a heterocyclic structure containing quaternary nitrogen.
Table 7: Combination of the types of dye used in different applications
DYE APPLICATIONS TYPES Cotton Wool Cellulosic Synthetic FI FA K FI FA K FI FA K FI FA K Direct X X X - - - - X X - - Insoluble azo dyes X X X - -------Sulphur dyes X X X - - - X X X - - Sulphur dyes (soluble type) - X - - ----X--Vat X X X - - - - X X - - Co Reactive (cotton) X X X X - X - - Wo Reactive (wool) X X X Acids - X X X X X - X - X X X Premetallised - X - X X X - X - - - Chrome - - - X X X - - - - - Disperse - X X - - - - - - X X X Cationic - X X - X X - X - X X X
FI: Fibre FA: Fabric K: Knitwear
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4.2.4. Processes of colour correction
The quality requirements regarding textile products are currently highly demanding. Among the market requirements are the exactness of colour and the uniformity of aspect. Though the industrial laboratories develop processes with the aim of getting things “Right first time”, when this aim is not reached, corrective processes must be introduced:
• Added correction • Reoperation correction
Added correction
This consists of adding dye to the bath during the dyeing operation or totally or partially emptying the bath to replace it with a new one.
Reoperation correction
This consists of repeating the whole dyeing operation after its total or partial chemical dismantling when the necessary quality has not been achieved. Normally, before repeating the operation, a prior drying process is carried out.
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