How to dye and procedure of dyeing for textile: Dyeing

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6/6/13

Pretreatment Processes of dyeing

Dyeing Pretreatment :

Pretreatment processes of dyeing consist of cleaning operations to rid the fabric of all soil and additives that have accumulated during the weaving or knitting process. These processes are usually the first treatments a fabric undergoes after leaving the loom or knitting machine, and are required before any dyeing, printing, or finishing can be accomplished. In the strict definition of finishing, pretreatment processes do not quality as textile finishes because they are performed prior to dyeing or printing

Grey goods contain warp starches or other sizing (to add stiffness and strength to warp yarns during weaving), as well as oils, waxes, and other lubricants, plus floor dirt or other soils picked up during processing. Complete removal of all these impurities is necessary before any dyeing; printing, or finishing can be done. The processes for cleaning are varied depending on the fiber, the impurities present, and the fabric construction. In cotton, cotton-blend, silk, and manufactured-fiber fabrics, the processes are generally known as the boil-off. In woolens and worsteds, it is called a scour or scouring. Boil-off or scouring is similar to ordinary laundering. Fabrics are treated with soaps or detergents, rinsed, and then dried. If warp starches are present, the fabric is treated with an additional process known as desizing. Enzyme solutions are used to dissolve the starch.

Woolen and worsted fabrics may undergo an additional pretreatment process known as carbonizing. This process removes leaf particles and bits of grass and other cellulosic impurities that become embedded in the wool white sheep are grazing. The treatment consists of steeping the wool fabric in sulfuric acid, which destroys the cellulose but leaves the wool unharmed.

Another dyeing pretreatment process is an operation called singeing, which involves burning off projecting fibers or filament splinters from the surface of a fabric. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics. . Singeing is accomplished by passing gray goods rapidly over gas flames, usually two burners to a side, at a speed of 100 to 250 yards per minute. Improper singeing or elimination of this operation results in unclear print patterns, mottled fabric surface, or premature pilling of fabrics.

Dyeing pretreatment processes may also include bleaching. Fabrics to be dyed in light to medium shades, as well as most prints, are first bleached. Bleaches are required to obtain pure whites because natural fibers are rarely pure white in their natural state; they are usually slightly yellowish or grayish. Bleaches are chemical agents that react with the color compounds in the fiber and render them colorless. Our eyes see the fabric as pure white.

The effectiveness of optical brighteners after dyeing is dependent on an ultraviolet source of light. Natural sunlight is such a source. Fluorescent lighting is also, but to a small degree. Ultraviolet lamps (also called black-light lamps) are another source. Incandescent lighting (regular household bulbs) contains no ultraviolet, so optical brighteners have no effect on fabrics used under this lighting.

The Dyeing Process

The science of dyeing is highly complex, and the mechanisms of dyeing are not completely understood. This discussion about dyeing is necessarily superficial and does not provide an in-depth exploration of the subject. The medium most often used to dissolve or disperse dyes for application to textiles is water. The dye solution, called the dye liquor, is agitated or circulated to increase the migration of the dye to the fiber surface. The attraction or affinity of the dye for a particular fiber is influenced by several factors. Different dye types are attracted chemically or physically to specific fibers.

Fibers often undergo swelling in aqueous dyeing processes, increasing dye absorption. For hydrophobic fibers that do not swell in water, organic solvents can be used as swelling agents or “carriers” for the dye. Mordants can be added to increase the acceptance of dyes. Dyebath additives such as these are classed as dye auxiliaries. Surface features such as the scales on wool or waxes or finishes on fibers can inhibit attraction of the dye to the fiber. Careful preparation of fabrics for dyeing minimizes this effect by removing waxes and sizes. Further, application of aesthetic and functional finishes usually follows the dyeing process, so that the dyeing finishes do not compete for dye sites on the fiber.

Once dyes are attracted to the fiber, they should diffuse through it rather than remaining adsorbed on the surface. Dyes are absorbed into the fiber predominantly in the amorphous areas, and making these areas larger or more accessible enhances the dyeing process. Mercerization of cotton, which moves the polymer chains farther apart, allows more dye in. Higher temperatures, around the glass transition phase of thermoplastic fibers, cause some of the crystalline areas to move and separate more, increasing dye accessibility. Nylon fibers have a low glass transition temperature and can often pick up dyes from other fibers.

Exhaustion is the amount of transfer of dye from the dye bath to the fiber, either by adsorption or absorption of dyes. It is possible for all the dye to be removed from the dyeing bath and for the dye bath to be clear at the end of the dyeing process. Some dye classes exhaust better than others. It is not always prudent or even necessary, to dye to complete exhaustion of the bath.

A concern in dyeing is how evenly distributed the dye is in the fiber, a characteristic known as levelness. Low leveling will result in a streaked appearance of the fabric.

Dyes may be only adsorbed on the fiber surface, or dyes that have been absorbed by fibers may migrate back to the surface when the textile is dried, resulting in uneven dyeing. Leveling agents and retarders are dye-bath auxiliaries than can promote even dye absorption.

Once the dye has entered the amorphous regions of fiber, it must be retained within the fiber not only during the dyeing but also when fabrics are laundered or dry cleaned during use. Several factors make for dye retention. The nature of the dyeing chemical bonds and physical attraction between fiber and dye is a crucial element. Dyes may be physically bound to fibers by forces such as hydrogen bonding or ionic forces. Ionic forces are the attraction between positively and negatively charged ions, one on the dye, and the other on the fiber. The chemical structure of dye and fiber may allow the formation of covalent chemical bonds between fiber and dye. Dyes that are absorbed into swollen fibers can be physically trapped when the fibers are cooled and dried. Dyes that are soluble in water, which may also swell some fibers, can be removed when the textile is wetted during dyeing.

5/8/13

State the problems occurred in dyeing due to water hardness

Problem occurring in dyeing due to hardness:

Potential dangers of textile items:
a) Potential dangers:

1) Reduced depth of shade: Dyeing shade is a vital point of industrial coloration process. Shade of a color is depend upon the choromophore or auxochoromophore groups of dyes. The material reserved in hard water can change the choromophore complex ions. In that case it is not possible to get desire shade percentage.

2) Dulling of shade: This is a big problem in dyeing. Water hardness change the activities of dyes chemical that’s why dyeing machine could not properly dull the shade.

3) Unleveled dyeing: It is one of the common and major problem in dye house. It means the variation of color shade in same amount of fabric or other textile material which are dyed in same dyeing bath and batch. Water hardness is one of the main issue of uneven dyeing.

4) Poor shade reproducibility: Shade reproducibility is not a usual operation on dyeing industry. It used to fix the shade according to buyer requirement if it is not up to slandered. It means refit the dyed fabric according to the required shade due to achieve the proper shade. Usually uneven dyed fabric need to shade reproduce. To reduce the shade stripping is required but increase the shade, usually need to re-treatment with dyes and chemicals.

5) Damaged fiber: Degree of hydrogen (PH) is important issue in dye liquor. Hard water contains organic salt that’s why it can change the acidity or alkality of dye and auxiliary solution. On the other hand every fibre have a tolerant limit of acid alkali. As the hard water change the PH of dye solution so the solution could be harmed the fibre. I we rework with dyed material due to dull or unleveled shade, it could easily damage the textile fibre due to more chemical expose.

b) Problem in boiler:
-Scale formation: Hard water makes scales inside the wall and tubes of boiler. It create a layer of organic salt and it is difficult remove the scale inside the boiler.

-Heat loss: It also the cause of heat loss inside boiler.

c) Wastage of soap: Hard water contains the organic salt and oxides ions. Those ions react with soap and reduce the working capability soap and need more soap to maintain the exact ratio during scouring and bleaching.

d) Problem in machine: It makes corrosion of boiler and looses its life time.

e) Problem in processing:
Desizing, Scouring, Dyeing, Printing, Finishing

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12/4/12

Silk Fibre Dyeing

Dyeing of silk fiber

Like wool, silk is a protein fibre and can therefore be dyed using the same methods as for wool dyeing (Õ Dyeing of wool). As silk is considerably more resistant to alkali than wool, dyeing processes which are carried out under weakly alkaline conditions are also used. In this regard, it must also be borne in mind that under alkaline conditions, high temperatures and conditions of high mechanical stress at the same time, individual fibres (fibrils) can split away from the silk filaments giving the material a fluffy and rough appearance. Compared to wool, silk also has a distinctly lower affinity because of its weaker acid binding capacity. Additional factors influencing the affinity of silk are its origin, extent of degumming, the yarn quality (tram silk, organize) and whether the silk has been weighted or not. In general, it can be said that under the same dyeing machine conditions, level dyeing are less problematic to produce on silk than on wool. The dyeing temperature should not exceed a maximum of 80°C since temperatures at the boil have a negative influence on the luster and handle of silk.

Silk is dyed in all its commercial forms but is best dyed in the form of hanks (yarn) and piece goods. In principle, all the well-known types of dyeing machine can be used if they have been suitably modified to take into account the sensitivity of silk to tension and mechanical stress.

The most important types of dyeing machine for silk fibre dyeing include:

Hank dyeing machines: no hank deformation (with double stick systems), no filament splitting, level dyeings obtainable provided the rate of liquor circulation is adequate;

Spray type dyeing machines for hank yarn: good liquor circulation and therefore hardly any leveling problems, hank deformation and filament splitting is possible; particularly suitable for high-twist and folded yarns;

Package dyeing machines for cross-wound packages: short liquor ratio, no levelling problems with adequate liquor circulation, no filament splitting, uniform winding of packages important because of the risk of channeling;

Star frame dyeing machine: suitable for all types of woven silk fabrics, no chafe marks, low productivity, high energy and water consumption (liquor ratio 100–300:1);

Winch dyeing machine: good levelness is possible, high productivity, average liquor ratio, high risk of chafe marks,

Jigger dyeing machine: high productivity possible, extremely short Dyeing of polyester/polyester copolymer liquor ratio, risk of moiré formation, good leveling resp. slow-exhausting dyes are an advantage here;

Jet-dyeing machines: good levelness, high productivity, short to medium liquor ratio, especially suitable for jersey and crêpe fabrics (due to the virtual absence of fabric tension), possibility of chafe marks;

Beam dyeing machines: good to excellent levelness, short liquor ratio 10–20 : 1, no chafe marks, especially suitable for heavy and smooth silk qualities, risk of moiré formation.

Tab.: Fields of application, advantages and disadvantages and fastness properties of various dye classes suitable for dyeing silk.

Practically all classes of dye can be used for colouring silk (see Table). There is hardly any other textile fibre for which this is possible. Nowadays, silk fibre is mainly dyed with direct, acid, metal-complex or reactive dyes.

Metal-complex and acid dyes for silk: from the dye classes listed in the Table the 1: 2 metal-complex and acid dyes are the most important in practice. Apart from a few exceptions in the acid dye range, they can all be applied from a weakly acidic bath (Fig. 1) and exhibit good levelling properties. By careful selection, all fashion shades, including those specific to silk, can be dyed although, from time to time, the colour fastness has to take second place. Thus, for the brilliant shades, red, blue, turquoise and green, often only moderate light fastness ratings and, in deep shades, only moderate wet fastness ratings are achieved (e.g. colour fastness to water severe, alkaline perspiration and washing at 40°C).

Fig. 1: Method and dyeing recipes for dyeing silk with metal complex and acid dyes (Ciba-Geigy). - - pale shades; dark shades

Reactive dyes for silk: These are being used to an increasing extent (Fig. 2). They are characterised by their brilliance, high degree of fixation, high light fastness and outstanding wet fastness properties. In general, they are not very substantive although differences exist between the individual types. Consequently, the aim should be to use the shortest possible liquor ratios for these dyes. Dyeing is carried out first of all in a neutral bath with the addition of sodium sulphate at tempera tures of 60–70°C, with the exception of black dyeing which are dyed at 90°C. Only in the last 15–20 min. at the end temperature is an addition of sodium carbonate made to adjust the pH of the dyebath to 8–8.5 for dye fixation. After rinsing, the dyeing must still be soaped in order to remove the unfixed dye which is not covalently bound to the substrate (source: Flensberg and Hammers).

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Polyester and natural fibre dyeing | Blebds Dye

Dyeing of polyester fibres

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Polyester fibres are hydrophobic, have a very low swelling capacity and, with the exception of copolymer fibres, have no reactive groups and no affinity for water-soluble dyes. Dye accessibility must be achieved by the addition of carriers or the use of high temperatures (Õ High-temperature dyeing). The heat-setting of polyester before dyeing has a considerable influence on dye affinity, especially in exhaust dyeing with carriers. Oligomers also have an effect on the dyeing results. Exhaust processes are of importance for disperse dyes and naphthol-based diazo combinations. In continuous dyeing processes, pigments are used for pastel shades whilst vat and vat leuco ester dyes are likewise only used for pale shades. In general, disperse dyes are applied by the heat-set thermofixation process. For dyeing polyester loose fibre and card sliver, HT circulating liquor dyeing machines with the material press-packed into cages are used; card sliver can also be dyed by continuous methods. Smooth yarns in the form of cross wound packages and textured yarns in the form of cross-wound muffs loaded on to spindles are likewise dyed in HT circulating liquor dyeing machines. For woven and knitted fabrics, HT winches or HT jet and HT beam dyeing machines are available. Thermosol plants are used for the continuous dyeing of woven fabrics. Polyester copolymer fibres (modified polyester) contain anionic groups and can therefore be dyed with cationic dyes as well. Optimal batchwise polyester dyeing processes are characterised by coordinated temperature control and liquor circulation. In this connection, it should be noted that an increased rate of liquor circulation leads to improved levelness. As a result, the critical rate of dyeing increases in proprtion to the rate of liquor circulation. However, an increased rate of liquor circulation through the textile material only leads to optimum levelness up to a certain limit. Moreover, with increased liquor throughput, increasingly unlevel results are observed. The conditions for this minimum unlevelness are to a large extent independent of the rate of heating.

Dyeing of acrylic polyester blends Single-bath and two-bath exhaust processes are used with disperse dyes for the polyester and cationic dyes for the acrylic fibres.

Dyeing of cotton or regenerated cellulose and polyester blends

These mixtures occupy a very large share of the market, and the blend ratio is nearly always 67% polyester and 33% cotton; with regenerated cellulose, the ratio is often 70 : 30 in many cases.

Disperse dyes are almost exclusively used for the polyester component and reactive, vat, vat leuco ester or sulphur dyes for the cellulosic component. Direct dyes are used only occasionally. In most cases, a tone-in-tone dyeing of both types of fibre is required, for which special ranges of pre-mixed dyes are available from different manufacturers. The formulation of these mixed dyes has been fine-tuned so that both fibre components can be dyed to the same shade by exhaust as well as continuous dyeing methods.

I. Exhaust method: for the dyeing of yarns this process is only of secondary importance. Dyeing is carried out on cross-wound yarn packages or warp beams by the single-bath single-stage or single-bath two-stage process. Direct dyes can be used for cotton in the single-bath single-stage process provided they have sufficient resistance to carriers. With the single-bath two stage process, the cellulosic component is dyed first in the case of reactive dyes whereas the polyester component is dyed first in the case of vat dyes. In two-bath processes, the polyester is likewise dyed first followed by the cellulose with appropriate dyes for each fibre.

Where the cellulosic fibres are too heavily stained with the disperse dyes, an intermediate reduction clear with dithionite must be given (as well as for processes involving the use of carriers in order to remove carrier residues). The exhaust method is also used for light weight woven fabrics as well as knitgoods. Dyeing equipment as for 100% polyester fabrics.

II. Continuous method: this process is used for all other qualities and offers numerous possibilities. Intermediate drying is critical as a certain amount of dye migration can take place at this stage. Most of the disperse dye on the cellulose is transferred by diffusion on to the polyester component during thermofixation.

Polyester natural silk blend Dye

The polyester component is dyed first with disperse dyes in a single-bath two-stage or two-bath process, followed by an intermediate reduction clear if necessary. Finally, the silk is dyed with acid dyes.

Polyester or polyester copolymer (cationic dyeable) Dyeing

This mixture is only seldom encountered. Disperse dyes are used for the polyester and cationic dyes for the modified polyester fibres in a single-bath two-stage or a two-bath process. For continuous dyeings, the modified polyester fibres are pre-dyed with cationic dyes on the pad-steam range, then disperse and vat dyes are applied on the padder followed by thermofixation and development on the pad-steam range. In the knitgoods sector, two-colour effects are produced as well as solid shades. Acid-modified polyester with a high rate of dyeing: carrier-free dyeing at the boil with suitable disperse and cationic dyes.

Dyeing of wool and polyester blends

The blend ratio is mainly 55% polyester and 45% wool. For the highest quality it is recommended to dye each fibre separately.

Optimum fibre protection is not possible. As there is nothing to be gained by two-bath dyeing, it has been substituted by single-bath processes. A major disadvantage in dyeing this fibre blend is the fact that disperse dyes result in a more or less pronounced staining of the wool component and this staining has no colour fastness to speak of. A strict selection of those dyes which exhibit the least possible staining on wool is therefore necessary. Dyeing is carried out by the exhaust process at 105–107°C if possible, and with reduced amounts of carrier. Disperse dyes are used for the polyester and 1 : 2 metal-complex dyes for the wool. Pre-mixed dyes are also available from a few manufacturers.

Dyeing of polyvinylchloride (PVC)

fibres Polyvinylchlorid fibres are mainly dyed with disperse dyes. Cationic or metal-complex dyes are also frequently used. Pigments are seldom and naphthol dyes only occasionally used. The dyes must be carefully selected and, depending on the type of fibre, are dyed with or without the addition of a swelling agent (carrier) or dyeing is carried out under HT conditions, whereby polyvinylchloride filament yarns have a considerably lower dye affinity than polyvinylchloride staple fibres.

Dyeing of regenerated cellulose fibres

(Viscose, cupro fibres, high wet modulus /polynosic fibres). In principle, all dyes used for dyeing cotton are suitable for dyeing regenerated cellulose fibres (Õ Dyeing of cotton). The most important dyes are substantive and reactive dyes, with preference being given to hot dyeing types. The affinities of fibres produced by various manufacturers differ and can even vary from batch to batch from the same manufacturer. Textiles made from these materials are sensitive to pressure, friction and tension and this must be taken into account during the dyeing process. A well-known fault is the streaky appearance of woven or knitted fabrics due to slight variations in fibre manufacture. This streakiness can be avoided to some extent by selection of suitable dyes. The pronounced swelling properties of these fibres must be taken into account from the dyeing kinetics point of view (exothermic swelling) so that a fibre specific processing sequence is necessary.

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Dyeing of jute, flex, modacrylic, polyamide fibres and their blends

Dyeing of hemp fibre

This is of no significance as the products manufactured from hemp fibre are almost exclusively used for technical textile purposes (fire-engine hoses and belts, etc.).

Dyeing of hat trimmings of straw, panama and Sisal for hemp

1. For dyeing straw: acid and 1: 1 metal-complex dyes

2. For dyeing panama: disperse and cationic dyes

3. For dyeing sisal: direct, acid, and 1: 1 and 1: 2 metal complex dyes.

Iridescent effects by piece-dyeing

A prerequisite is that the warp and the weft have different affinities – usually acetate/triacetate or triacetate mixtures. Single-bath dyeing can be carried out with direct and disperse dyes, usually giving contrasting shades.

Jute fibre Dyeing

The jute fibre dyeing is similar like cotton fibre dyeing. The high proportion of non-cellulosic material gives jute an affinity for cationic dyes (mordant unnecessary) and acid dyes. Unlike cotton, jute is sensitive to alkalis. The light fastness of jute dyeings is poorer than on cotton.

Dyeing of leather(goods) Because of the wool like chemical nature of leather, appropriately modified dyeing methods are used depending on the type of tanning process. The usual treatment with aqueous dye solutions given to new articles (by brushing on or dipping) has only a limited application for second-hand goods (garment dyeing). For expediency, dyeing is carried out using tried and tested commercially available

Dyeing of linen (flax) fibre

In principle, dyeing is carried out using all the dye classes that are suitable for cotton (Õ Dyeing of cotton). Dye penetration is generally more difficult, especially in the case of vat dyes. The best penetration and levelness is achieved with reactive dyes.

Loose material (loose stock, loose fibre) Dyeing,
Advantages: Any unlevelness in dyeing is evened out during the subsequent spinning process, large quantities can be produced to one shade by blending several batches and a low liquor ratio is possible in dyeing.

Disadvantages: satisfactory processing in spinning isusually only achieved through the application of spinning oils and dressings. The spinning of coloured yarns demands total separation of different shades in spinning and laborious cleaning is necessary when changing colours. Unavoidable co-processing of short fibres and dyeable impurities. Continuous dyeing can be carried out by a pad steam process (e.g. the Smith Piston Pad Dyeing Machine).

Dyeing of modacrylic (acid dyeable) and wool fibre blends

This modacrylic and woo fibre blend can only be dyed in solid shades with dyeing difficulty because of the considerably higher affinity of the acid dyes for the wool fibre component. The best possible solid shade dyeings are achieved with the Chem Acryl process and other process variants (manufacturer: Chemstrand Corp., Decatur, Alabama/USA).

An addition of the following dyeing auxiliaries is necessary: cationic and non-ionic dyeing assistants which retard the exhaustion of selected acid dyes and bring about a satisfactory balance between the dye on both fibre components in the sulphuric acid bath.

Modacrylic fibre dye

Dyeing of ombré or shadow effects, shadow dyeing on one and the same hank, e.g. by hand using square or U-section rods. Firstly, the palest colour is dyed from a full vat dye then, after dropping a little of the dyebath, a dye addition is made and a somewhat darker shade is dyed – this procedure is repeated with the increase in depth of shade becoming less and less as ever smaller additions of dye are made. In this way, light to dark shadow effects of the same colour, i.e. so-called tonal gradations, are obtained. Ombré variations are produced by using different dyes. Excellent ombré dyeing are obtained on thoroughly pretreated dry goods by employing a material-specific dye selection (hot and cold dyeing types and no pre-mixed dyes if possible) as well as the absence of wetting agent (undesired upward migration of dye) with, if necessary, controlled additions of salt and temperature regulation. The most efficient results are obtained on the hank dyeing machine where good dye penetration is achieved by raising and lowering the entire yarn carrier assembly. With increasing depth of shade, the hanks are lowered to a lesser and lesser extent into the dye liquor which is correspondingly increased in strength with additions of dye. By dropping the liquor level approx. 5 cm for each individual shade a reduced depth of immersion is achieved.

After reaching half the original bath volume, the yarn carrier assembly is lowered to a reduced depth. For piece goods, dyeing on the star frame is very practical (star dyeing machine) by continually raising and lowering the frame with the aid of a beam assembly (a cable running over a boom-arm); this method has proved particularly suitable for sensitive qualities (e.g. pure silk).

Polyamide fibre dye

Various dye classes of dye are suitable for dyeing polyamide fibres, and dye selection is based on the form of the material, the fastness requirements and the dyeing properties.

I. Disperse dyes: these dyes have good affinity and levelling properties. Affinity differences in the fibre are easily levelled out. The wet fastness properties of medium to deep dyeings are unsatisfactory.

II. Acid dyes: the most widely used class of dyes for dyeing polyamide fibres. They offer a wide colour gamut and good fastness properties, especially following an aftertreatment with products to improve the wet fastness. For combination dyeings it is essential to use dyes with similar rates of exhaustion. The selected dyes must not exhibit any blocking action. In order to cover up affinity differences in the material, anionic leveling agents, which possess an affinity for the fibre, are used in the dyebath.

III. Metal-complex dyes: these dyes have good light and wet fastness properties. Their use is limited (except for black) since differences in the polyamide material cannot be levelled out in most cases. Fastness properties can be improved with a tannin/tartar emetic (potassium antimony(III) tartrate) aftertreatment. IV. Metal-containing disperse dyes: the fastnesses properties are the same as those of the metal-complex dyes. Material-dependent affinity differences are largely covered in dark shades. Improved fastness properties are possible with a tannin/(tartar emetic) (potassium antimony(III) tartrate) aftertreatment.

V. Reactive disperse dyes: dyeable as disperse dyes; chemical bonding with the fibre takes place following an alkaline aftertreatment thereby giving good wet fastness properties.

VI. Direct dyes: the selection is limited: used chiefly for reasons of shade and price.

VII. Naphthol dyes: of no importance.

VIII. Chrome dyes: of interest only for black shades (applied by the so-called Metachrome process in which dye and mordant are applied simultaneously from a single bath).

IX. Reactive dyes: the light fastness of these dyes varies appreciably on polyamide. Wet fastness properties are good. Any affinity differences that may arise are not covered in all cases. Combination dyeings are only possible to a limited extent. The build-up is also limited.

X. Logwood black: This dye is of interest for reasons of shade and fastness but, the method of dyeing is very involved.

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12/3/12

Glass fibre or filament dyeing

Glass fibre is the most important invention in the modern textile age. Glass fibre is special types of fibre that used for special purpose. This type of sophisticated fibres are used for communication, data transfer, or decorative purpose. Glass fibres able to transfer light, ray and radiation inside its subtle hole. Dyeing of glass fibre filaments, fabrics or fabric is not so easy. In this case, it must be taken into consideration that, under normal circumstances, there is no affinity between glass fibre and dye and that no penetration of the dye into the glass fibre is possible, only surface dyeing or coloration is possible. The following overview includes patented processes of glass fibre dyeing or coloration which must be regarded as being legally protected:

I. Fused mass coloration or dyeing: using coloured metal oxides yielding weak dyes only as the colour intensity inevitably decreases with the cross section of the filament.

II. Dyeing of activated surfaces of glass fibre: predominantly older processes of relatively little practical importance and usually yielding weak coloration, deficient in crocking fastness and, almost without exception, having a tendency to damage the fibres surface.

III. Direct dyeing methods of glass fibre: without pre-treatment, using conventional dyeing techniques and, in this case, yielding surface coloration or dyeing which is barely acceptable. Methods involving the deposition of water-insoluble dyes are exceptions to this but usually only weak coloration can be achieved, e.g., when using sulphur or vat dyes.

IV. Pre-mordanting dyeing methods: a) metal-salt process gives strong colours in some cases but almost always with insufficient crocking fastness, b) cation active process gives very strong colorations which, with the right choice of dye, can have good fastness properties against water and light but only moderate crocking fastness.

V. Bonding-agent dyeing methods: coating with dyeable or dyed films: a) pigment dyeing methods are the most elegant and simplest to use. Pale to medium shades are possible with maximum light, water and crocking fastness (with the right choice of dye) while the properties of the dark colours (marine and black) are frequently much worse. The principle consists of fixing the pigment using synthetic resins. b) Other bonding-agent dyeing methods are to some extent still very inconvenient and unsatisfactory (especially with regard to crocking fastness). Exception: Õ Corona discharge process with good overall fastness but crocking fastness and grease resistance are not completely satisfactory. No dark full shades obtainable.

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12/2/12

Dyeing of cotton, acetate, regenerated cellulose, polyamide fibre and their blands

Dyeing of coir
Method as for jute – dyeing is usually carried out using direct or acid dyes under mildly acidic conditions (cationic dyes have inadequate light fastness). Coir is also dyed before spinning using oilsoluble dyes in the spinning oil.

Cotton fibre dyeing
The affinity of cotton for dye varies according to its history. The adsorption capabilities of cotton are increased by caustic solutions or mercerization.
There is a huge selection of dyestuffs available for cotton: substantive dyes, vat dyes, sulphur dyes, leuco-ester vat dyes, reactive dyes, naphthol dyes and phthalocyanine dyes. Of less importance are pigment dyes and chrome dyes. Cationic, aniline black and mineral khaki dyes do not have any significant application.
There is a clear trend towards dye groups with high fastness properties. A continual problem for the finisher is the proportion of immature or dead fibres (dead cotton) which give rise to the appearance of bright spots in coloured fabrics. These faults are significantly reduced by treating the fibre with caustic solution or by mercerization.

Dyeing of cotton and acetate or cotton and triacetate fibre blends,
I. Acetate or triacetate white, cotton coloured: dyeing is usually carried out with selected substantive dyes which reserve the acetate and triacetate. The dyeing temperature must not exceed a maximum of 80°C with acetate since staining of the fibre occurs at the boil.
Acetate which has been scoured at too high a temperature and which is partially saponified will no longer be reserved. Even for acetate and triacetate which have been delustered in spinning a good reserve is more difficult to obtain.
II. Cotton white, acetate or triacetate coloured: this is a considerably less favourable combination as disperse dyes stain the cotton to a greater or lesser extent. An afterclear treatment is necessary with sodium dithionite, sodium hypochlorite or potassium permanganate.
III. Cotton/acetate or triacetate dyed tone-in-tone: usually carried out by single-bath dyeing with substantive and disperse dyes. Two-bath method : a) the acetate or triacetate is dyed with diazotizable disperse dyes; b) then using a fresh bath for the acetate or triacetate, the cotton is filled in with substantive dyes. All the above variants are carried out by exhaust dyeing; pad dyeing is only used in exceptional cases.

Ccotton and regenerated cellulose fibre blend dyeing
Cotton/regenerated cellulose blends are dyed using the same classes of dyes as for cotton. The achievement of tone-in-tone uniformity between cotton and regenerated fibres is problematic due to the greater dye affinity of the latter (greatest with cupro fibres and weakest with high-wet-modulus fibres). The dyeing method, especially for substantive dyes, must be appropriately adapted with regard to salt additions and temperature, i.e. dyeing must be carried out with less salt or no salt at all and at lower temperatures. If necessary, dyeing is started at the boil with a portion of the dye, and the rest is added to the bath after cooling down.
Padding methods are more suitable than exhaust methods for vat, leuco-vat ester and naphthol dyes. Causticizing before dyeing improves the affinity of the cotton. However, despite all these measures, tone-in-tone dyeing is not achievable with dark shades.

Cotton and regenerated cellulose with polyamide blends
Tone-in tone dyeings with a single class of dyes are only achieved with great difficulty. In most cases, one or other of the fibre components will have to be filled in with specifically appropriate dyes.

Exhaust methods: selected substantive and vat dyes (for which the semi-pigmentation process is particularly suitable) are capable of producing tone-in-tone dyeing in pale shades by using appropriate dyeing methods. A single-bath method with substantive and metal-containing disperse dyes can be used here. For dark shades, the polyamide must be dyed first with metal-containing disperse dyes followed by the cellulose in the same bath with substantive dyes. A single bath two-stage method may also be used with selected reactive dyes. In this case, either the cellulose is dyed first under alkaline conditions followed, after acidifying the bath, by the polyamide or, the polyamide is dyed first at pH 5 and followed, after neutralization, by the cellulose. Two-bath two-stage method: metal-containing disperse or 1 : 2 metal complex and reactive or vat dyes. Pad dyeing: pigments may be used for pastel shades. Where high fastness is required, each fibre component must be dyed separately by the methods customarily used for it. For the polyamide component, metal-containing disperse dyes are by far the most important, and vat, sulphur or reactive dyes for the cellulosic components. Selected acid dyes may be applied together with vat dyes by a single-stage method, e.g.: both classes of dye are padded together, followed by acid steaming, drying, thermofixation, padding with caustic/sodium dithionite, then steaming, oxidizing and washing. Phthalocyanine dyes may be employed for turquoise shades.

Dyeing of flammé effects
This process is mainly carried out on hank yarns. This is effectively done by tying off the hanks individually at the sites where they are to be dyed in one particular colour first. Dyeing is then carried out in the customary way with the liquor just reaching the ties. The hanks are withdrawn and thoroughly rinsed by immediately spraying them with cold water in an upward direction. The ties are undone (after an intermediate drying step if necessary), rehung and tied again for the next dyeing with another dye, as already described, and rinsed down again etc. until the whole hank is dyed in the various colours desired. Two, three, four or more different colours may be involved and these may be interspersed with non-dyed areas or the colours may be adjacent to each other or even overlapping.

Instead of obtaining flame effects on non-dyed yarn, hanks can be pre-dyed in a certain shade and then dyed in certain places as described above. Instead of using the technique described above, dyeing is also carried out using a pulley lifting system without tying off the hanks. Õ Dyeing of ombré or shaded effects.

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Acetate, acrylic fibre and their blend dyeing

Dyeing of acetate fibres

(Dyeing of triacetate fibres). Disperse dyes are predominantly used both for acetate and triacetate fibres and are mainly used in exhaustion processes. As is the case for polyesters, triacetate must be dyed either with the addition of carriers or under HT conditions. In the case of acetate, selected cationic and naphthol-based dyestuffs can also be considered. There are only limited possibilities for using the padding process as the thermosol dyeing process cannot be used for acetate fibres and can only be used for triacetate fibres when lighter shades are being applied.

Acetate, wool, cotton fibre union dye:
I. Acetate white, union coloured: single-bath dyeing with suitable neutral-dyeing acid, direct or union (mixed fibre) dyes. Two-bath method, this is possible by pre-dyeing the cellulosic fibres with direct dyes at approx. 70°C, rinsing and filling in the wool with acid dyes from an acidic bath.
II. Acetate coloured, union white: it is questionable whether a satisfactory union reserve is possible as even the most suitable acetate dyes cause staining of the wool.
III. Acetate and union in two colours: single-bath method using a combination of dyes according to I. +
II. (if necessary II. + union dyes) in a neutral sodium sulphate bath (Glauber’s salt). Two-bath method by pre-dyeing the acetate, filling in the union and rinsing.
IV. Acetate, union in three colours: single-bath method using dyes which reserve the other fibres completely by dyeing according to dyeing methods I.–II. In a neutral bath. Two-bath method as single-bath with filling in of the vegetable fibre.
V. All fibres dyed the same colour: dye with a combination of direct-dyeing union dyes and acetate dyes.

Dyeing of acrylic fibres
These fibres can exhibit very different dye affinities. Acrylic staple fibres are more easily dyed and in greater depths than filaments. The dyeing of acrylic fibres has proved difficult to the extent that whilst polyacrylonitrile fibres only begin to exhibit adequate affinity at relatively high temperatures of 70–85°C (depending on origin), the rate of dyeing is virtually doubled with an increase in temperature of around 3–4°C. Moreover, only a very low degree of migration takes place at the usual dyeing temperatures, i.e. as a rule, uneven dyeings cannot be levelled out adequately.
I. Cationic dyes: suitable for pale to dark shades; pale to medium depth shades are usually dyed using cationic or anionic retarders at the boil. The HT process is only suitable for acrylic filaments and fibres of low affinity. Continuous methods are used for loose stock, tow and combed sliver as well as, to some extent, acrylic/ cellulosic blends (fixation medium = saturated steam). The thermosol process is of practically no significance (levelling problems, unsatisfactory reproducibility and effect on handle).
II. Disperse dyes: the material is introduced into the dye bath at 70°C, brought to the boil quickly and dyed at the boil for 1 h. The thermosol and pad-steam processes are of secondary importance.

Acrylic and wool fibre blend dye
Dyeing methods: single-bath single-stage, single-bath two-stage or a two-bath process. Cationic dyes are used to dye the acrylic component and acid, 1 : 2 metal-complex and reactive dyes are used to dye the wool component.
When dyeing acrylic fibres and wool, the cationic dyes stain the wool component first. As the boil is approached during dyeing, they transfer slowly from the wool on to the acrylic fibre and if the proportion of wool is at least 30%, retarders are not necessary. The transfer of cationic dyes from the wool is dependent on the quantity of dye, dyeing time, dyeing temperature and the pH. In the case of deep shades, a dyeing time of 45–60 min. at the boil is necessary to achieve a good wool reserve. With falling pH, the wool reserve increases. A good wool reserve is a prerequisite for the achievement of optimum wet and light fastness properties.

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Dyeing machines

Dyeing machine is Device in which the textile material which is to be dyed (as distinct from Dyeing machine, circulating) is agitated in more-or-less stationary liquor – for example: padder, rope-winch vat, jigger, continuous dyeing machine, paddle dyeing machine, star frame and, among others, modern processors such as the pad-jigger, pad-roll, pad-steam, pad-wet and pad-winch processes. The dyeing machines which are in use today for batch dyeing consist of the following basic elements:

– The vat as the stationary zone,

– The pump for circulating the liquor,

– The mechanical elements for controlling the liquor(pipes and valves),

– The heat exchanger (heating unit),

– The mechanical elements for transporting the material and circulating the liquor, such as guide bars, reversing winches, transporter winches, material carriers and immersion wetting-out apparatus.

Fig. 1: Types of flow between fabric and liquor

Dye movement in the solution is brought about by convection whilst liquor movement is brought about by agitation or circulation pumps. High temperatures and liquor exchange accelerate this process so that the overall dyeing speed is not adversely affected by diffusion.

Diffusion is the dynamic dispersion of dye particles caused by the system’s tendency towards a state of equal concentration of particles throughout the liquor irrespective of the liquor movement. Optimum flow at the interface and through the textile material to produce the highest possible degree of levelness depends on the liquor-exchange system of the dyeing machine in question.

High flow rates accelerate the dye absorption, the various machine designs produce different types of Different machines or apparatus are used according to the material being dyed. There are dyeing machines in which the liquor is stationary and the material is moving and dyeing machines in which the liquor is moving and the material is stationary (Fig. 2).

Dyeing is carried out according to the makeup of the material to be dyed:

Dyeing is carried out according to the makeup of the material to be dyed:

I. Dope-dyed synthetic fibres: In synthetic fibre production the dye pigment (insoluble but finely divided dyestuff) is added to the spinning solution so that a coloured fibre is produced when extruded through the spinneret.

II. Loose stock dyeing: Loose stocks can be dyed by batch (non-continuous or continuous methods). In the case of batch dyeing, the fibre material is packed in a perforated vessel through which the dye liquor is forced. Coloration is not always evenly distributed but this is immaterial as the loose fibres are subsequently dispersed in the spinning mill and evenly mixed so that the colour ultimately appears to be uniform. In the case of continuous dyeing, dye solution is continuously applied to the stock on a padder and the dye fixed in a steam ager. The material is then washed and dried. Stock of different colours can be used for colour blending.

III. Sliver dyeing (slubbing dyeing): top or combed sliver consists of a continuous sliver from which the short fibres have been combed out on a combing machine. This sliver can be, for example, dyed in ball form.

There are several areas where slubbing dyed material can be used in coloured wool yarn production.

a) Sold shade yarn: the essential advantage of dyeing combed sliver is the high fastness achieved. However, it is only economically viable where large batches are involved.

b) Melange yarn: in a melange yarn specified percentages of combed slivers are dyed in different colours. These different-coloured slivers are then mixed together at the drawing stage and spun.

c) Vigoureux yarns: In this case, printing paste is applied at different intervals by means of a printing roller and fixed. When the combed sliver is drafted during the spinning cycle, the coloured and non-coloured areas intermix so as to create the characteristic melange yarn. In the vigoureux printing process, the individual fibres are striped zebra fashion. The patterning in the finished product resulting from this process is so small that the eye can only perceive a mixed colour. For example, from a mixture of black and white stripes the resulting colour is grey. In melange products, on the other hand, the eye is still able to identify the individual black and white fibres.

IV. Yarn dyeing: The following different machines are used for dyeing yarns:

a) Hank dyeing machines: the yarn is wound into hanks, the individual hanks laid on a roller conveyor and the hanks passed through the dyebath on the rollers.

b) Spray-dyeing: in this case, hanks are laid over a perforated tube out of which the dye liquor flows. The hanks are placed at regular intervals so that dyeing is uniform. The amount of dye liquor required for this machine is significantly less than for a).

c) Pack cage: the hanks are placed in a cage and dyed in a similar way to the loose stock dyeing process.

d) Assembling columns of cross-wound packages and subjecting these to a stream of dye liquor in an autoclave.

V. Piece dyeing: there is a whole series of machines which can be used for dyeing fabrics. Modern dyeing machines operate via mechanical systems using program controllers or microprocessors, the whole dyeing process being controlled by a program.

a) Winch vat: these machines are suitable for qualities which are not subject to running cracks. The cloth is passed through the dye liquor at boiling point as a continuous rope with a rotating winch providing the traction. Temperatures can be as high as 100°C (Fig. 3).

Fig. 4: “Krantz” overflow machine for fabrics in rope form.

1 = dwell chamber; 2 = liquor-exchange pipe; 4 = circulation pump; 5 = liquor extraction; 6 = heat exchanger; 7 = liquor injection; 8 = wetting-out device; 9 = wetted-out fabric; 10 = return winch.

b) Jet dyeing machine or Overflow machine: the overflow machine has been especially designed for dyeing fabric which is sensitive to tensile stress. These fabrics can be dyed using this apparatus with the minimum possible tensile loading. In this type of machine (Fig. 4), both the fabric and the liquor are in motion. The construction can be either vertical or horizontal. In jet dyeing machines, the liquor is caused to move by jets from nozzles, thus causing the fabric to move at the same time. In most cases, the fabric is also driven by means of a winch.

Dyeing machines, circulating liquor type:

These are differentiated into package systems, Hussong type systems and spindle-and-separator systems. The principles differ according to working methods: stationary liquor - moving material (dye machines: padder, winch vat, jigger, roller box and hank-dyeing machine); stationary material - moving liquor (less significance, particularly common for loose stock and bulk dyeing in, for example, garment dyeing and hat dyeing; garment dyeing machines and paddle-dyeing machines). Examples of these are:

a) Cheese dyeing machine: the packages are on perforated plastic tubes which are stacked on perforated spindles and then put into a sealed vessel (Fig. 5). In this vessel the dye liquor can be forced through the packages under pressure. Dyeing temperatures > 100°C are possible.

Fig. 5: Cross-wound package yarn dyeing machine.

b) HT Beam dyeing machine: In this case, dyeing is carried out at temperatures above 100°C. The fabric is wound on to a perforated cylinder and inserted into a sealed vessel (Fig. 6). The dye liquor can be pumped in an inward or outward direction through the rolled fabric.

Fig. 6: Operational cross-section of a dyeing machine for open-width piece dyeing.

1 = coupled dye beams with outward circulation; 2 = stock reservoirs with separate pump; 3 = main pump; 4 = heat exchanger.

Dyeing methods for dyeing machine

These have been the subject of continual development over the years especially because of the need to rationalize or for reasons related to color levelness and dye compatibility or because of new problems appearing with new substrates. Thus, there is a whole series of possibilities for variation such as short-liquor dyeing, foam applications, thermosol processes, jet dyeing (jet-dye machines), overflow dyeing, (overflow dyeing machines), HT dyeing processes, pad-steam dyeing processes and continuous wet-steam processes among others. In order to optimize the dyeing process, the dyeing procedure is controlled according to a program and process control systems are used o control the operations. This means that a set of measurement instruments of maximum reliability continually records certain parameters and issues appropriate commands to the valves etc. Actuators, for example, are used for speed/time control and other operational control aspects for individual forms of apparatus, machine or automatic process management and control systems. Development in the area of textile dyeing and dyestuffs and in relation to technical applications is very dynamic.

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