How to dye and procedure of dyeing for textile: Dyes

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5/20/13

Dyeing methods of Direct dyes

Direct dye is populae and conventional dye. There are some popular dyeing method of direct dyeing with suitable dyeing machine those are describeb bellow:

I. Batch method: With batch dyeing, the dyeing method to be selected in each case depends on the type of dyeing equipment (loading system, winch vat, jig, paddle or jet), the nature of the material to be dyed, as well as the solubility and the affinity of the dye. Before the actual dyeing process, the material is pre-treated with a wetting agent. The dye is mixed into a paste with some warm water, and is then diluted with more water, boiled up, filtered, and then added to the dyebath.

Whilst the bath, with the fabric, heats up to the optimum temperature (usually 80–90°C), the electrolyte is added, gradually if necessary. Dyeing takes 30–60 minutes. After the dyebath has been run off, the dyed material is briefly rinsed with cold water and, in general, subjected to after-treatment. The dyeing process is divided into 3 phases: a) uptake (adsorption) through substantivity; b) penetration of the dye into the fibre (diffusion); c) bonding to the fibre (immobilisation) through van der Waals interactions. Assurance of level take-up through progessive addition of salt, temperature control, sufficiently long dyeing time, and use of levelling agents.

II. High-temperature dyeing method: With suitable dyes, one can work in a closed system at temperatures over 100°C (up to approx. 130°). Due to the rapid diffusion rate, particularly level dyeings are achieved with short dyeing times, even with fabric with difficult dye penetration. After the high-temperature phase, the dyebath is cooled down to 80–90°C, with dye pick-up continuing, and the result is the same depth of colour as in the normal dyeing method at 80–90°C.

III. Continuous and semi-continuous method: With these so-called pad methods, the dyeing material, mostly in the form of woven fabric, is first of all steeped in a concentrated dye solution, passed full-width through a trough filled with the dye solution, and subsequently the excess liquor is removed between the rubber rollers. With high fabric speed, temperatures as low as possible (30–40°C for light shades, 60–80°C for medium to dark shades), and with the minimum amount of pad liquor, the situation can be avoided where the dye already picks up substantively in the padding process. Distinction may be made between the following methods:

1. Pad-jig method: Pad, salt bath develop on jig or winch vat.

2. Pad-roll method: Pad, heat up in an IR zone, roll up and rotate in a dwell chamber for a considerable time under fixation temperature/moisture conditions.

3. Pad-salt method: Pad, pass through salt solution at boil in continuous piece-dyeing machine (light shades only; for darker shades an intermediate steam process is necessary).

4. Pad-steam method: Pad, continuous steam, optional final salt bath.

5. HT-steam method: Pad, HT-steam.

After-treatment: The wetfastness properties of direct dyeings are not adequate for the demands of everyday use, particularly in medium and deep shades.

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Direct dyes

Direct dyes are very conventional dyes. It is not used any more for industrial dyeing. Direct dye is one kind of anionic dye and having substantively for all types of cellulosic fibre like cotton, jute, hemp. They have strong affinity to the fibre which can be applied directly on the fibre surface and do not need any assistance.

Direct dye Also known as direct, substantive, benzidine or salt dyes. Group classification for dye which exhaust substantively on cellulosic fibres, cotton, viscose, cupro and high wet modulus fibres, especially in the presence of salt. Chemically they are polyazo or sulphonated disazo dye.It contains sulphonic acid group that’s why is has good solubility in water. The dye molecule contains a complex bond metal atom that’s why they have high light fastness properties. The color fastness is poor but it is easy to improve by dyeing after treatment with metal salts (Õ Aftercoppering dyes).

Benefit of Direct dye:
Direct dyes represent the main class of dyes for cellulosic fibres in terms of usage:
- Specially selected direct dyes are available for streaky dyeing viscose qualities.
- They are simple to apply.
- Economical or cost effective than any other dyes.
- It has generally good leveling properties
- A wide range of color variation and shade.
- Special types contain better wet fastness. It is possible to couple with readily soluble diazo compound by after treatment.
-Direct dyeing on cellulosic fibres exhibit considerable differences in colour fastness to light which ranges between 1 and 7–8.

Different direct dyes can therefore be used for articles which are only required to have low light fastness as well as goods with high light fastness. In general, direct dyes have poor wet fastness properties and dyeing must be after treated to improve their serviceability.

Properties of direct dye:
- Direct dye has sodium salt of sulphonic acid or carboxylic acid group. So it easily dissolve in water.
- Cheap comparative
- It has strong affinity to cellulose fibre. It is possible to dye protein fibre.
- Easily disperse in water and diffusible in to fibre.
- Wash fastness is not good. The range vary from 2 to 3
- It is possible to use in neutral and alkaline medium
- The dye is common in the practical point of view.
- The tin tropical power of this dye is very good.

Some trade and brand name of direct dyes
Benzo, Benzoform       - F.Bayer (Germany)
Coprantine                  - Ciba-geigy (Switzerland)
Solphenyl    - Ciba-geigy (Switzerland)
Chlorazol Durazol    - Impartial Chemical Industry (ICI)
Solar                          - Sandoz A.G (Switzerland)

There are some related posts to get deep knowledge of direct dye:

# Classification of Direct dye
# How to dye with direct dyes
# Dyeing aftertreatment of direct dye
# Chemistry of dyeing related to direct dye
# Dyeing auxiliaries required for direct dye

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12/1/12

Dye class identification on fibres,

Iidentification of Dye class on fibres

Dye class identification is very important test to select a relevant dye for particular fibre or fabric. All kinds dyes are not suitable for all textile fibres. For example Reactive dyes are confidently identified for natural cellulose fibres, because it is possible to associate the reactive dyes structure with the cellulose fibre structure. If the fibre material is unknown, Fibre identification should be carried out first.

Dye class identification for various fibres

The most well-known dye class identification systems on fibres are:

I. Krefeld method:

a) For cellulose fibres (cotton, silk, jute etc): dissolve fibre sample in suitable solvent, sulphur test (for sulphur-based dyes), blind vat (oxidation and cationic dyes), stripping and staining reactions (for mordant, acid, direct and diazo dyes), dimethyl formamide test (for reactive dyes), glacial acetic acid test (for vat, naphthol and phthalocyanine dyes).

b) For animal fibres (wool): paraffin test, blind vat and reoxidation (for vat and naphthol dyes), detection of metals (metal complex and chrome dyes), cold glacial acetic acid (cationic dyes), suitable solvent (reactive, acid and direct dyes).

c) For synthetic fibres: a caprolactam melt is prepared or stripping by means of glacial acetic acid carried out. The melt and the solution are mixed with ether and processed further. Dye classes which can be identified are disperse, metal complex, chrome, acid, cationic and naphthol dyes.

II. Reutlinger method:

a) For cellulosic fibres: water, wash, paraffin, pyridine, glacial acetic acid, hydrogen sulphide, chlorine, benzene tests and blind vat.

b) For animal fibres: water, wash, paraffin, glacial ace tic acid, ammonia, pyridine, benzene tests and blind vat.

c) For synthetic fibres: Acetate, Polyamide and Acrylic dyes.

III. In accordance with the AATCC guidelines for cellulosic, animal and synthetic fibres.

Dye classification:

A distinction is made between the following methods of classification:

I. Scientific classification of dyes based on their chemical structure, e.g. anthraquinone, azo dyes, etc.

II. Technical classification of dyes based on their dyeing properties, e.g. direct, acid, wool dyes, etc.

III. Commercial classification of dyes based on various aspects according to the manufacturer, e.g. according to fastness properties such as Sirius Light, Indanthren, etc. or according to the method of dyeing under collective names, e.g. Remazol = reactive dyes (Dystar), Palanil = disperse dyes (BASF), etc.

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Dye affinity

Dye affinity is an important dyeing factor for all dyes and dyeing process. Most of the properties of a dye depend on the dye affinity and dye reactivity. In simple terms, dye affinity (also neutral affinity) is the dye absorbing capacity (of textile fibres, yarn, fabrics). It varies between wide limits depending on the state of dyebath equilibrium between the fibre and the dye in solution. A number of factors play an important role in this process, e.g. dyeing liquor ratio, dye physical and chemical characteristics (it could be organic or inorganic), additions to the dyebath (such as dye acid, organic or inorganic salts and dyeing auxiliaries), dyeing time (time from start to end of dyeing), dyeing temperature (temperature vary according to dyes and dyeing property), and the degree of purity (purity of all dyes, chemicals, acids, salts and auxiliaries), extent of drawing and degree of crystallinity (rate of crystalline region and amorphous region) of the fibre being dyed, as well as the number of dye bonding groups available in the fibre (it means the reactivity of dyes). For dye affinity- controlled dyeing processes, a temperature is selected at which diffusion proceeds at a relatively fast rate inside the dye bath . If it is assumed, as a model, that a dyebath exhaustion equilibrium is established momentarily at any one time, then the kinetics of exhaustion are given by the change in chemical equilibrium. For synthetic polyamide fibres and natureal wool fibre, the chemical equilibrium between the dye in the textile fibre (concentration CF) and the dye in the liquor (CL) can be approached by a superimposed Langmuir and Nernst distribution with the pH-dependent distribution coefficients kL and kN, and the saturation value SL. The pH dependencies are both dye and fibre-specific.

Dye affinity of wool, treatments to improve,

a) Pretreat for 1 h at the boil in a 5% inorganic potassium thiocyanate solution, rinse, dye in the presence of 20% sodium sulphate and 5% acetic acid 30%;

b) Bring the yarn to the boil in a solution of 2,5–4% solution of sodium thiosulphate or neutral sodium sulphite or sodium tetraborate, boil for 1 h, hydroextract and dry;

c) Treat the natural wool fibre for 20–30 min. in a liquor containing 1–1,5% active chlorine under weakly acidic conditions, treat in a fresh bath with 1–2 g/l sodium dithionite as an antichlor and bleach.

Dye ager is a horizontal continuous-dyeing machine for all conventional textile fabrics (including difficult qualities such as cotton velvet and lining fabrics) with all classes of dye, by the pad-steam process and the wet-in-wet process of vat dyeing voluminous fabrics in which dye is applied on a preceding padder and the reducing agent padded on directly before the ager entry using a special applicator unit. A high degree of reproducibility is achieved by the ager.

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Dye acid

Most of the dyes are complex structure organic or inorganic chemical compound. Consistence of acid inside the dye is not so important for all dyes. But some of the dyes are show different attitude when associate with the organic or inorganic acid solution inside dyebath of dyeing machine. These are chemical organic acid consisting of the hydrophobic dye residue and the hydrophilic auxochrome groups of dyes. The latter are mainly organic sulphonic acid (–SO3H) and/or carboxyl (–COOH) or hydroxyl groups (–OH) which, with their acid character and negative charge, make the dye residue function as a dye acid and become a dye ion inside the dye solution. Free dye acids exist, for example, in the acid dye range, either in the commercial form itself or they are formed by the addition of acid in an acidic dyebath:

chemical construction of dye acid

he dye acid formed during the vatting of vat dyes are known as Vat acids. Due to their ionic behavior in aqueous solution, formation of the dye acid and their alkali salts ( Dye salts) occurs in contrast to Õ Colour bases, i.e. the main group contained in cationic dyes ( Dye ions).

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11/29/12

Dyeability of dyestuff

Dyes dyeability

Dyeability As a prerequisite for the use of textile fibres. One of their most important properties is dyeabilily. Consequently, the range of application of some synthetic fibres has been, and still is limited either because they have poor dyeability or the problem of dyeing these fibres has not yet been satisfactorily solved. For example, when 21/2 acetate fibres were first introduced. they were not successful for a long time because they could not be dyed with the known dye classes at that time, Indeed, it was only after the discovery of disperse dyes that acetate fibres gained wide acceptance. The same problem applies to polypropylene fibres although in this case, appropriate modifications have provided a partial solution. Comparable examples are the attempts to develop polyester fibres with a higher dye uptake as well as the efforts to produce a polyamide fibre with diff erential dyeing characteristics (— Differential dyeing) which has had a favourable influence on their range of application. Some textile fibres may be dyed with several technologically different dye classes capable of achieving relatively good color fastness: others. however, can be dyed with one particular dye class only (and. even then. not without difficulty in some cases).

In practice the dyeability of a textile fibre is determined by the rate of dyeing and the degree of saturation which can be achieved. For example the different dyeing properties of mercerized cotton compared to non- mercerized cotton can be characterized by results from the time of half dyeing (i.e. the reciprocal of the rate of dyeing) and the saturation concentration of the substantive dye.

Dyeability is dependent on the following factors:
a) Chemical composition of the fibre: fibres of native and regenerated cellulose differ from each othei for example in their physic 0-mechanical structure but. despite this. they can be dyed with the same classes of dye. The same situation applies to protein fibres (a gram equivalent of wool is 1200 g and that of silk is 4200 g for reactions as bases) as well as for polyam ide and polyester fibres.

b) The coloristic dye class resp: the entire dyeing syst em including additions to the dyebath: certain dye classes are only suitable for one particular type of fibre. i.e. either the dye is not capable of dyeing certain fibres or can only dye them very slightly resp stain the fibre this can however, be caused by tin- suitable dyeing conditions. e.g. by using an inappropriate dyebath pH.

c) The geometrical form of the fibre: the fibre dimens ions as well as the morphological and histological structure of the fibre (the cuticle in cotton. the scale layer of wool and the skiii effect in viscose fibres) and the fine, or supra molecular structure of the fibre, whereby each change in fine structure results in a change in dye uptake: e.g. mercerized or non- mercerized cotton different regenerated cellulose fibres, drawn or heat-set synthetic fibres.

Those changes which, for example. involve changes in the ratios of crystalline. oriented and amorphous yeg ions due to the ageing of cotton or changes in the pore size, can be attributed to changes in the accessibility of the fibre to certain reagents. The relationship between fibre accessibility A% and the degree of crystallinity K% can be derived as follows:

A=(lOO-K)+aK

a is a constant which is related to the crystalline part of the fibre. It can have values from 0—1. An ideal subs tance with hundred percent accessibility would be represented by a = 1. The first part of the formula repres ents the amorphous part of the fibre p. and the second part represents the accessible surface of the crystallites (if a = 0. this surface would be inaccessible):

A=p+ctK
In the case of hiah tenacity viscose fibres K can vary, for example between the limits of 33—49%. a = 0.09—0.67. The accessibility A represents an appropria te numerical criterion of dyeability. It is an absolute parameter which represents a previously given property of the fibre and in general gives expression to the req uirement for its dyeability in achieving dark shades or black within an acceptable dyeing time provided the dye has adequately high build up properties. The fact that in isothermal dyeing dyeability decreases with the time of dyeing and that a fibre has a greater affinity for the dye at the beginning of the dyeing process than at the end is well known to every dyer. It is also possible to assess dyeability from the standpoint of the momentary quantity and rate of dye uptake by the fibre at a given time. The relative dyeability is always a kinetic parameter which is related to a specific dyeing system. If it is assumed that at time t. a dye concentration c1 exists in the fibre then tile reciprocal of this parameter Ct’ is the dilution of tile dye in tile fibre at tune t. At the start of dyeing the dilution reaches a value which inc reases beyond all limits (apart from the dye adsorption on the fibre surface at time t = 0: the dilution has a value of c1).

Tile dilution of solutions behave in an analogous manlier; before a substance dissolves in a solvent, tile dilution is infinite (since tile reciprocal value. i.e. the concentration, is equal to 0): with increasing concentrat ion tile dilution decreases (not. however, to 0 but to a certain value which corresponds to the dilution of the substance ill a saturated solution). Tile rate of dyeing at any time of dyeing t is proportional to this dye dilution:
Dc=K
dt= Ct

Tile dyeability of the fibre at time t is the dye dilution related to a unit of time. It is infinite at tile beginning (if the dye adsorption at time t = 0 is disregarded) and decreases with the time of dyeing (usually very quickly) and is equal to 0 at equilibrium. According to experience, the fibre is now no longer cap able of absorbing any dye from the dyebath (only all interchange of dye particles between the fibre and the dyebath takes place as a result of the dynamic equilibrium which exists in reversible processes).

Dye absorption index (fibre affinity index SF), fibre characteristic which gives information on the max dyestuff affinity of an acrylic fibre for cationic dye stuffs.

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7/23/12

Disperse dyes for a polyester fabric

Disperse dyes for a compound shade on polyester can have quite incompatible dyeing properties. The SDC classification of disperse dyes is based on migration ability during exhaust dyeing, colour build-up, sensitivity to changes in temperature and the rate of dyeing. This type of dye is often classified on the basis of dyeing rate and sublimation fastness, particularly for polyester dyeing.

Classification of disperse dyes for polyester

These two properties are a function of molecular weight and the number of polar groups in the dye molecule. Table shows the most common classification. It applies to the dyeing of acetate, of polyester with or without carrier, and of polyester/cotton, but is somewhat arbitrary.

Most dyeing and fastness properties change gradually with increase in molecular size. Small dye molecules with low polarity are levelling, rapid dyeing dyes with poor heat resistance. These are called low energy disperse dyes. More polar, higher molecular weight dyes have low dyeing rates, poor migration during dyeing but good heat and sublimation fastness. These constitute the high energy disperse dyes. The development of disperse dyes of improved sublimation fastness required dye molecules with relatively polar and hydrophilic substituents to reduce their vapour pressure at high temperatures. This promotes somewhat higher solubility in water but the increase in molecular size reduces the dyeing rate at a given temperature. The high energy disperse dyes are those requiring a higher Thermosol temperature. The light fastness does not depend on the molecular size. Dyes in a mixture are usually selected from the same energy class. Build-up of the colour on shade requires that the dyes all have about the same dyeing rate. Testing of dye recipes is essential because many disperse dyes, even dyes of the same dyeing group, are incompatible in mixtures. This is true even though they may have the same dyeing rates and build-up properties when tested separately. The dye manufacturers provide considerable information assisting the dyer to select appropriate dyes for a given application.

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7/20/12

High temperature pressure dyeing of polyester with disperse dyes

As we have seen, the dyeing of polyester with disperse dyes at the boil is slow because of the low rate of diffusion of the dyes into the fibre. The activation energy for diffusion is quite high and raising the dyeing temperature from 100 to 130 °C considerably increases the rate of dye diffusion. Dyeing at this higher temperature under pressure, without a carrier, considerably increases the rate of dyeing and gives better coverage of filament irregularities because of the improved migration of the dyes. Dyeing is then also possible using higher molecular weight dyes, whose rates of diffusion at 100 °C are unacceptable. This permits production of dyeings with better fastness to light and to sublimation during permanent pleating. For those fabrics and yarns that lose bulk when dyed at 130 °C, dyeing at a lower temperature (110–120 °C) in the presence of some carrier is preferred.

The dyebath is usually set at pH 4.5–5.5 using either ammonium sulphate plus formic or acetic acid, or acetic acid alone. The weakly acidic dyebath ensures neutralisation of any residual alkali from scouring, which readily catalyses hydrolysis of the polyester, decreasing its strength. Reduction of some azo disperse dyes can occur during dyeing at high temperatures, while others undergo hydrolysis. These effects are minimal when dyeing in weakly acidic solution. The concentrated dye dispersion is added to the bath at 50–60 °C. The bath may already contain a small amount of dispersant (0.5 g l–1), if required.

Lubricants in the dyebath avoid possible crack and crease marks in dyeing fabric in jet machines. The temperature of the bath is then slowly raised to 130 °C. A typical heating rate is about at 1.5 °C min–1. Dyeing continues at the maximum temperature for about 60 min.

Each particular dyeing will have an optimum temperature/time profile, depending upon the type of goods, the machine being used and the dyes in the formula. A set of generalised dyeing conditions is used, however, provided that the dyebath exhaustion, the colour uniformity, and the shade reproducibility from batch to batch are acceptable. Dyeing times can be kept to a minimum by temperature control of the rate of exhaustion that gives uniform dye absorption. In this way, long leveling times at the maximum dyeing temperature are not needed. The dyeing time should be long enough for the dyes with the lowest dyeing rate to approach equilibrium.

Disperse dyes do not generally interfere with each other and prevent their mutual absorption but they do have different dyeing rates. The dyeing rate is always higher at low dye concentrations in the bath. Some disperse dyes are deliberate mixtures of dyes of the same or different hue and about the same dyeing rate. They give fairly rapid dyeing because each dye is only present at low concentration.

PET fibre contains 1–4% of oligomers, mainly a cyclic trimer of ethylene terephthalate. It has a high melting point and is soluble enough in hot water during pressure dyeing to be extracted from the fibre. The oligomers also migrate to the PET fibre surface during steam heat setting, and to a lesser extent on dry setting. The oligomer can often be seen as a white dusty powder on the surface of the goods, or on the dyeing machine walls. Hydrolysis of oligomer deposits on machine surfaces by heating with an alkaline solution under pressure provides effective cleaning. Precipitated oligomer can cause nucleation of disperse dye crystal formation leading to coloured specks on the goods. In addition, oligomer particles reduce the rate of liquor flow through yarn packages and cause filament friction in spinning. The oligomer is much less soluble at temperatures below the boil. To avoid its precipitation once dyeing is concluded, the dyebath is drained at as high a temperature as possible, even above 100 °C. This can lead to problems in dyeing woven goods in rope form in jet machines since creases and crack marks can form while the polymer is still somewhat plastic. In these cases, draining at a lower temperature is necessary and the dyer must depend to a greater extent upon the subsequent rinsing and reduction clearing process to remove oligomer residues.

During dyeing, particularly of deep shades, there will invariably be some dye particles that adhere to the fibre surfaces, or are retained by yarns without penetration into the fibre. These mechanically held particles result in decreased fastness to washing, rubbing, sublimation and dry cleaning. Their presence also tends to dull the shade. Superficial dye particles can be detected by rinsing a dyed sample with a little cold acetone. This will dissolve the surface particles and produces a coloured solution but it does not remove any dye from within the PET fibres. For pale shades, scouring removes deposits of surface dye. Deep dyeing with disperse dyes on PET fibres will invariably require treatment by reduction clearing to give satisfactory crocking fastness. This process involves treatment with alkaline hydros (2 g l–1 NaOH, 2 g l–1 Na2S2O4.2H2O) and a surfactant ( 1 g l–1) for 20 min at 70 °C. The reduction clearing temperature is well below the glass transition temperature of the polyester. The ionic compounds do not therefore penetrate into the fibres and only reduce the dye on the fibre surface. The reduction of azo disperse dyes is relatively easy but anthraquinone derivatives are more difficult to remove. The latter must be reduced and washed off the surface before re-oxidation occurs. The less soluble oxidised form is then held in suspension by the surfactant in the bath.

Some disperse dyes, originally from ICI (now available through DyStar), allow easy clearing of surface deposits. These are methyl esters of carboxylic acids that readily hydrolyse under alkaline conditions. The free carboxylic acids formed by hydrolysis are soluble in alkaline solution. This allows clearing without a reducing agent. Since the alkali does not penetrate into the PET fibre at the clearing temperature, the dye within the fibres is unaffected.

Carrier dyeing of polyester with disperse dyes

There are obvious advantages to dyeing polyester fibres with disperse dyes at the boil, within a reasonable time, particularly for medium to deep shades. Unfortunately, this is only feasible with the most simple disperse dyes of low molecular weight. The more complex disperse dyes, which have the required fastness to heat setting and hot pressing and pleating, only diffuse extremely slowly into polyester fibres at 100 °C. One solution to this problem that avoids dyeing under pressure at temperatures above 100 °C is dyeing in the presence of a carrier. A carrier is an organic compound, dissolved or emulsified in the dyebath, which increases the rate of dyeing. Carriers allow dyeing of even deep shades at the boil within a reasonable dyeing time. Common polyester dyeing carriers include butyl benzoate, methylnaphthalene, dichlorobenzene, diphenyl and o-phenylphenol, the latter two being the most popular. These are all aromatic compounds of low water solubility, so they are present in the dyebath as an emulsion. Typical commercial carriers therefore usually already contain anionic emulsifying agents.

A typical carrier dyeing procedure involves running the goods in the bath 60 °C and adding dilute dispersing agent, emulsified carrier and lastly the dispersed dyes. The temperature is then gradually raised to the boil and dyeing continued at this temperature. The sodium salt of o-phenylphenol is soluble in water and acidification liberates the insoluble phenol once dyeing has started. This ensures a fine emulsion. The usual effect of the carrier is to increase both the rate of dyeing and the dyebath exhaustion, but not in all cases. Benzoic acid, for example, decreases the exhaustion at equilibrium but increases the dyeing rate. Its effect is probably simply to increase the water solubility of the dye in the bath.

Methylnaphthalene gives the best colour yield with many dyes at the lowest cost. During dyeing in certain machines, such as winches and jigs, a steam-volatile carrier may condense as a concentrated emulsion on colder internal surfaces. Drops of this condensed emulsion that fall onto the goods produce darker dyed spots. This can also occur if the carrier emulsion is not stable during dyeing and drops deposit on the fabric.

The actual mechanism by which a carrier accelerates dyeing has been widely debated and probably depends upon the carrier used. The polyester fibres absorb the carrier and swell. This swelling can impede liquor flow in packages causing unlevelness. The overall effect seems to be a lowering of the polymer glass transition temperature (Tg), thus promoting polymer chain movements and creating free volume. This speeds up the diffusion of the dye into the fibres. Alternatively, the carrier may form a liquid film around the surface of the fibre in which the dye is very soluble, thus increasing the rate of transfer into the fibre. Incorporation of other monomers into the polyester also decreases the Tg value. Comonomers such as suberic acid (1,8-octanedioic acid) increase the polymer chain flexibility and give polyester fibres that can be dyed at 100 °C without a carrier. However, a polyester fibre, dyeable at the boil with disperse dyes of good heat fastness, without use of a carrier, and without any modification of the properties of regular PET, remains somewhat elusive. The new polytrimethylene terephthalate fibre (Corterra) is a step in response to this problem.

After dyeing, scouring of the goods removes most of the carrier. Any carrier remaining in the fibres invariably decreases the light fastness of the dyeing. Residual amounts of carrier vaporise during subsequent drying of the scoured fabric. Some carriers are quite volatile, have unpleasant odours and are toxic. Polyester dyeing carriers pose a serious environmental threat if present in the effluent or exhausted air. One of the easiest ways to eliminate o-phenylphenol is by mild alkaline washing, which dissolves this weakly acidic phenol. Carrier dyeing has steadily declined since the development of suitable machines for dyeing polyester under pressure at temperatures around 130 °C. Carriers are still used in some garment and small commission dyehouses where high temperature pressurised dyeing machines are not available. The quantity of carrier required in dyeing decreases with increase in the dyeing temperature. The use of a small amount of carrier is useful for dyeing at 110–120 °C. Dyeing at this lower temperature leaches less oligomer from the polymer and better preserves the fibre bulk and elasticity. Carriers are also useful for dyeing wool/polyester blends when there is a risk of damaging the wool at dyeing temperatures above 100 °C. In this case, the carrier also helps to prevent cross-staining of the wool by the disperse dye.

Partial stripping of the colour of PET materials dyed with disperse dyes is usually possible by treatment with a solution of dyeing carrier or retarding agent at high temperature under pressure. Oxidative and reductive stripping are also possible but are likely to involve some undesirable effects upon the fabric handle or appearance. Prolonged treatment of polyester materials with alkaline solutions causes surface hydrolysis of ester groups and loss of weight. Once the surface has been degraded it is difficult to obtain the originally anticipated appearance.

Preparation for batch dyeing of polyester with disperse dyes

Loose PET fibre is usually dyed directly without pretreatment because emulsification of the small amount of superficial processing chemicals is easy. This is not the case for knitted goods, that may contain additional oil or wax, or for woven goods with sized warp yarns. Typical preparation involves scouring with 2 g l–1 each of soda ash (sodium carbonate) and an anionic detergent at 50 °C. Addition of an organic solvent may be useful if wax or much knitting oil is present. Because the dispersants present in the dyes or added to the dyebath are usually anionic, removal of any cationic auxiliary chemicals in the spin finish is necessary before dyeing.

When fabrics of PET are heated in water at the boil there is often considerable shrinkage as the tensions in the filaments relax. The shrinkage may be even greater at higher temperatures. Fabrics of PET can be dry heat set at 200–225 °C for 30–60 s. Alternatively, steam heat setting at 130–140 °C for several minutes is also possible but can cause a loss of strength due to some hydrolysis of the polyester. Steam setting provides dimensional stability in boiling water but, for stability to ironing, higher setting temperatures must be used.

After heat setting in air under conditions of free shrinkage, the dye exhaustion first decreases and then increases with increasing setting temperature. The minimum exhaustion occurs after setting at around 160–190 °C. If applied tension prevents fabric shrinkage during heat setting, the dye uptake/ temperature profile is similar to that under conditions of free shrinkage, but with higher uptake values. Heat setting changes the morphology of the polyester fibres. The effects on the dyeing rate and the extent of dyeing are variable depending upon the particular dye, the setting temperature and heating time, and the tension imposed.


Influence of hot air setting temperature on dye uptake of polyester at dyeing temperatures of 100 and 130 °C

The problem of dyeing polyester with disperse dyes

Polyester fibres are essentially undyeable below 70–80 °C, leaving only a 20– 30 °C range for increasing the dyeing rate before reaching the boiling temperature. At any temperature, the rate of dyeing of polyester with a given disperse dye is very much lower than for cellulose acetate or nylon fibres. The rate of diffusion of disperse dyes into the polyester below 100 °C is so low that dyeing at the boil does not give reasonable exhaustion. The rate of dyeing is higher for dyes of small molecular size that have higher diffusion coefficients. Dyeing is faster when using fibre swelling agents called carriers to improve the fibre accessibility, or when dyeing at higher temperatures above 100 °C to increase the dye diffusion rate.

Fibres of the most common polyester, polyethylene terephthalate (PET or PES), are quite crystalline and very hydrophobic. Hot water does not swell them and large dye molecules do not easily penetrate into the fibre interior. Polyesters have no ionic groups and are dyed almost exclusively with disperse dyes. The better diffusion at the boil of low molecular weight dyes results in moderate migration during dyeing but then the washing fastness is only fair. Many of the more recent disperse dyes are specifically for dyeing polyester. These are of higher molecular weight to provide adequate fastness to sublimation during heat treatments. Some of these produce a reasonable depth of shade by dyeing at the boil. Most, however, require higher dyeing temperatures or carriers for satisfactory results. Dyeings of polyester with disperse dyes have good light fastness. This does not always correlate with the light fastness on other fibres such as cellulose diacetate. The disperse dyes provide a full range of colours with adequate to good build-up on PET fibres. Uneven filament texturising or heat setting can lead to barré but higher dyeing temperatures, or addition of some carrier, will promote migration to minimise this. Again, a full black requires aftertreatment of the dyeing by diazotisation of an amino disperse dye and coupling with a suitable component, often BON acid. Concurrent dyeing with a mixture of the amino disperse dye and dispersed BON acid, followed by treatment with sodium nitrite and hydrochloric acid, is a common procedure. Some blacks are mixtures of dull yellow, red and blue dyes.

Application of disperse dyes to nylon

The disperse dye is pasted in warm water and the dispersion slowly diluted. Hot water and concentrated dispersant favour the formation of large dye particles. The concentrated dispersion is then strained into the dyebath that usually also contains additional dispersing agent. The bath is gradually heated and dyeing continued at the boil. The disperse dyes used for nylon are usually level dyeing.

The exhaustion rates of individual disperse dyes on nylon are not overly high. They do vary from dye to dye so that selection of compatible dyes is necessary. Although some dyes have good migration and build up well, deep shades are rarely dyed with disperse dyes because of their inferior washing fastness. Many of the simple disperse dyes developed for dyeing acetate at 85 °C are not particularly fast to heat and can sublime from the nylon during processes such as boarding. This is a form of heat setting used to stabilise the shape of ladies’ hosiery after dyeing. Dyes of higher fastness to sublimation are invariably of greater molecular size and therefore have lower rates of dyeing. The usual temperature for rapid dyeing disperse dyes on nylon is 85–100 °C. If slow dyeing heat fast dyes are used, dyeing under pressure at up to 120 °C may be useful. The disperse dyes used for dyeing nylon will also colour spandex (segmented polyurethane) filaments in stretch hose but the washing fastness is only fair. As for cellulose acetates, blacks are produced by diazotisation of a disperse dye containing a primary amino group and coupling of the generated diazonium ion with a suitable coupling component. Simple dyeing tests evaluate the migration, temperature range characteristics and dyeing rates of disperse dyes on nylon [3]. With rapid dyeing dyes, the dyeing rate increases with increasing temperature but the equilibrium exhaustion decreases. The more rapid dyeing dyes also migrate better and tend to be less temperature sensitive so that dyeings at different temperatures are close in shade. Nylon 6 is more amorphous and has a lower melting point than nylon 6.6. Disperse dyes dye nylon 6 using the same method as for nylon 6.6. Dyeing is usually faster than for nylon 6.6 under the same conditions and the dyes will usually show better migration. This usually means that the washing fastness is somewhat lower on nylon 6. One advantage of nylon 6 is that heat setting using hot air or steam is at lower temperatures than for nylon 6.6.

Preparation of nylon for dyeing with disperse dyes

The preparation of nylon goods for dyeing usually involves scouring with a detergent and soda ash (sodium carbonate) solution at 70 °C. This removes any soluble sizing material, lubricants, and spin finishes that might hinder access of the dye solution to the fibre surface.

Heat setting of nylon fabrics has already been discussed. This process may be performed before or after dyeing, preferably the latter. It causes variations in dye substantivity and may be non-uniform, leading to unlevel dyeings. Dry heat setting in hot air decreases the rate of dyeing of nylon 6.6 and 6 with both acid and disperse dyes, but setting in steam increases their dyeing rates. Steam setting, however, decreases the wet fastness of dyeings with disperse dyes, particularly if setting is carried out after dyeing. The more open fibre structure resulting from heat setting in steam allows easier dye desorption during washing of the dyed material. If it is necessary to bleach nylon that has become yellow from over-vigorous heat setting in dry air, peracetic acid or sodium chlorite solution can be used

DYEING NYLON WITH DISPERSE DYES

The use of acid dyes on nylon to produce dyeings of good washing fastness invariably involves the risk of barré because dyes of poor migration do not evenly dye filaments with chemical and physical variations. This risk is almost absent when using disperse dyes. The consequence of their good migration during dyeing, however, is poor to moderate wet fastness, especially in heavy shades. The dyeing of nylon with disperse dyes is therefore limited mainly to pale shades for lingerie fabrics and sheer hose that do not require repeated or severe washing. Disperse dyes on nylon are also more sensitive to fading by ozone and nitrogen dioxide. They are, however, economical and easy to apply.

Most nylon filaments are oriented by drawing but both undrawn and partially oriented yarns can be dyed with disperse dyes. With increasing draw ratio, the increased polymer chain orientation decreases the rate of dyeing (decreased fibre accessibility) but not the extent of dye absorption at equilibrium (unchanged fibre availability). It is only at the very high draw ratios typical of strong industrial yarns that the equilibrium dye absorption decreases. Uniform drawing of filaments is essential. Accessibility differences in dyeing can be minimised provided that the selected dyes and conditions are conducive to levelling. Disperse dyes on nylon are much better in this respect than acid dyes.

Dyeing of nylon involve with two steps

Preparation of nylon for dyeing

Application of disperse dyes to nylon

DYEING CELLULOSE ACETATE FIBRES WITH DISPERSE DYES

The disperse dyeing of cellulose acetate materials is a simple direct dyeing process. The dispersion of disperse dyes in warm water is sieved into the bath, possibly already containing additional dispersant. Boiling water and concentrated solutions of dispersing agents must be avoided as they can adversely affect the dye particle dispersion. Cellulose diacetate is dyed at temperatures not exceeding 85 °C, because of the risk of acetyl group hydrolysis on the fibre surface, which causes considerable dulling of the attractive lustre of the bright filaments. Because this thermoplastic material readily forms permanent creases at the usual dyeing temperature of 80–85 °C, dyeing of the full width fabric on a roller is necessary. A typical jig dyeing procedure of disperse dyeing involves two ends at 40–50 °C, followed by two ends at each higher bath temperature, up to the final dyeing temperature of 80–85 °C. At the higher temperatures, the lengthways tension must be as low as possible, to avoid elongation of the fabric. Beam dyeing is possible provided that the material allows good liquor flow through the roll at a pressure low enough to avoid deforming the plastic filaments.

Disperse dyes for cellulose acetate varies widely in their rates of exhaustion and levelling ability. Dyeing with mixtures of compatible dyes is essential. The SDC gives testing procedures for dyeing cellulose diacetate with disperse dyes. These tests establish the migration ability of the dye, the influence of temperature on dye uptake (temperature range test), the rate of dyeing and the colour build-up with increasing dye concentration relative to standard dyes of known properties.

The results of the temperature range test provide classic examples of the influence of temperature on dyeing kinetics and equilibrium. For dyes that adsorb rapidly at 50–60 °C, the amount of dye absorbed after dyeing for an hour will decrease as the dyeing temperature increases. This is the expected effect of temperature on an exothermic dyeing process that has reached or come close to equilibrium. The exhaustion (equilibrium constant) decreases with increasing temperature. For slow dyeing dyes, the amount of dye absorbed in one hour increases steadily with increasing dyeing temperature because this increases the rate of diffusion of dye into the fibre. After dyeing for one hour, the dyeing may be sufficiently far from equilibrium that the expected decrease of the exhaustion with increasing temperature does not occur. Some dyes may show a temperature of maximum dye exhaustion, showing the effects of temperature on dyeing rate at lower temperatures and on exhaustion at higher values. Slow dyeing dyes with poor temperature range properties will likely cause ending and listing when dyeing on a jig because the fabric ends and selvages tend to be cooler than the bulk of the material.

Blacks can be obtained in one of two ways. The simplest involves the use of a mixture of dull red, blue and yellow or orange disperse dyes at relatively high total concentrations. With appropriate combinations, this is quite successful. The second method is by a diazotisation and coupling aftertreatment. This involves diazotisation of a primary aromatic amino group in the disperse dye in the fibre and subsequent reaction of the diazonium ion with a suitable coupling component such as 3-hydroxy-2-naphthoic acid (BON acid after beta-oxy-naphthoic acid). Coupling in alkaline solution, as in the aftertreatment of direct dyes on cotton is less suitable for cellulose acetate because of the risk of surface hydrolysis of acetate groups. The amino disperse dye, for example CI Disperse Black is applied by conventional dyeing at 80 °C. After rinsing the orange fabric, the amino groups of the dye in the cellulose acetate are diazotised by reaction with a solution of sodium nitrite and hydrochloric acid at room temperature. After rinsing again, the fabric is treated with a dispersion of BON acid. This is prepared by precipitation of the free acid from a solution of its sodium salt in the presence of a dispersing agent. It is absorbed by the fibres at pH 4.5 exactly like a disperse dye. It reacts with the diazonium ion to form the dark navy pigment.

CI Disperse Black; dark navy pigment formed after the disperse dye is diazotised and then treated with a dispersion of BON acid

Other sequences for dye application, diazotisation and coupling, are possible. The coupling component can be applied to the fibre after dyeing, as above, or even concurrently along with the amino disperse dye followed by its diazotisation. Once the colour has fully developed, scouring the material in soap or detergent solution at relatively low temperature removes pigment from the fibre surface and biproducts from the diazotisation and coupling sequence. If this is not done, inferior fastness properties result, particularly poor fastness to washing and rubbing.

Cellulose diacetate fabrics must be handled and dyed with care to avoid forming crease marks and stretching. Even at a dyeing temperature of 85 °C, the material is quite plastic and easily deformed. It is therefore preferable to dye such fabrics in open width using a jig machine. This is, however, not as simple as it might seem outlines some of the problems inherent in jig dyeing. With many disperse dyes, ending and listing effects are all too common, and are particularly noticeable when using less compatible combinations of dyes.

Cellulose triacetate is considerably more hydrophobic than diacetate and dyeing it with disperse dyes requires higher temperatures, but carries less risk of surface hydrolysis. The more compact internal structure gives lower dye diffusion rates in this fibre. It is normally dyed with disperse dyes at the boil. Dyeing temperatures up to 130 °C are possible and give improved washing and crocking fastness because of the better penetration of the dyes into the fibres. This is beneficial when dyeing heavy shades. It also allows use of dyes that are absorbed too slowly at 100 °C, thus increasing the range of available dyes. For dyeing deep shades, dyeing at the boil using a carrier such as diethyl phthalate is possible. This acts as a fibre swelling agent and thus accelerates dye absorption by increasing the diffusion rate. For a typical black, the amino disperse dye and coupling component are applied sequentially, or simultaneously. The black is developed by aftertreatment with a solution of sodium nitrite and hydrochloric acid that causes diazotisation of the dye and immediate coupling of the generated diazonium ion. Soaping removes surface colour, but usually a process called reduction clearing is preferred. In this, the dyed material is treated with a weakly alkaline solution of sodium hydrosulphite (hydros, Na2S2O4.2H2O), which reduces and eliminates the azo pigment on the fibre surface. Each combination of dye and coupling component requires its own particular dyeing and aftertreatment conditions so the dye supplier’s recommendations should be consulted. As for nylon, dry heat setting of cellulose triacetate fabrics improves their dimensional stability but reduces the dyeing rate. If heat setting or texturising has not been uniform, barré effects may be evident on fabrics made of filament yarns. Dyeing under pressure at above 100 °C increases the rate of dye migration and minimises barré effects. For heat pleating of cellulose triacetate materials after dyeing, it is essential to use disperse dyes that do not readily sublime from the heated fibre.

FASTNESS PROPERTIES OF DISPERSE DYES

The fastness to washing and light of dyeings with disperse dyes on synthetic and acetate fibres is usually moderate to good. The washing fastness on nylon, however, is only fair, particularly for deep shades. The results of washing fastness tests on deep polyester dyeings often depend upon how well residual disperse dye particles on the fibre surface have been cleared after dyeing. When disperse dyes have migrated from inside the polyester fibre to the surface during thermal treatments such as heat setting or drying, the dyeings may have reduced fastness to washing, dry cleaning and rubbing (crocking). This effect is enhanced when the dyes are soluble in hydrophobic surface finishes such as softeners. The fastness to wet treatments of dyeings on secondary cellulose diacetate is inferior to that on the more hydrophobic triacetate. Dyeings of artificially-made fibres with disperse dyes generally have good fastness properties. For any fibre, however, a particular fastness property will vary considerably from dye to dye.

Typical fastness properties of disperse dyes on all synthetic fibres

Fastness property	Acetate	Triacetate	Nylon	Polyester	Acrylic
Washing	Moderate	Good	Poor to fair	Good	Very good
Light	Good	Good	Moderate to good	Good	Good
Crocking	Good	Good	Good	Moderate to good	Moderate
Gas fume fading	Fair	Moderate	Fair	Moderate	Good

The light fastness of disperse dyes may be very good in standard shades but is less so for pale shades, and lower still for tests conducted using a carbon arc light source. Non-ionic UV absorbers increase the light fastness for dyed fabrics such as those used for automobile upholstery. Certain blue and violet anthraquinone disperse dyes with basic amino groups are very sensitive to fading by nitrogen dioxide in polluted air. High temperature combustion processes produce low concentrations of nitrogen oxides. They are most abundant in industrial or city environments. Their effect on dyeings is called gas fume fading. This type of fading is usually worst for dyeings with sensitive dyes on cellulose acetate fibres. It is less severe on nylon and polyester fibres, but still poses a problem when the highest fastness is required. Nitrogen dioxide will nitrosate a relatively nucleophilic primary amino group of the dye, converting it into a hydroxyl group. This reaction usually reddens the shade. Colourless fading inhibitors protect sensitive dyes. These are readily nitrosated amines that preferentially react with the nitrogen dioxide and thus protect the dye. The fading inhibitor is added to the dyebath towards the end of dyeing, or is applied in an aftertreatment. Dyes that are more resistant to nitrogen dioxide fading have less nucleophilic phenylamino groups.

Ozone is a major air pollutant in metropolitan centres. It causes oxidation of many types of dyes, close to the fibre surface. Ozone fading of dyed nylon carpets and fabrics in automobile interiors is particularly serious for pale shades when the dye has not adequately penetrated into the fibres. Aftertreatment of the dyeing with amine or phenol anti-oxidants, similar to gas fume fading inhibitors, improves the fastness to ozone fading of sensitive blue anthraquinone disperse dyes.