Introduction To Textile Dyeing
A compound appears colored if it selectively absorbs light in the visible region and reflects the light of the wavelength in the rest of the visible region. The amount of light energy absorbed in the visible spectrum is the only responsible factor for the shade of the colour. The main function of absorbed energy is to raise the molecule from the ground state energy to the excited state.
If the electrons of a molecule are tightly bound as in saturated compound no light of visible will be absorbed and hence the compound appears colorless.
If the electrons of a molecule are loosely bound as in saturated compound absorbance occurs in the visible region and hence the compound appears coloured.
Types and Definition Dyes
Colouring materials are mainly of three types. Dyes, pigments, and lakes (ingrain dyes). A dye has three parts in its structure chromophore. chromogen and Auxochromopores and is soluble in a specific medium under certain conditions.
Chromopores : Chromopores are group of atoms, the II electrons of which may get transfer from ground state to excited state by absorption of radiation, thus producing the hue.
Auxochromopores : Auxochromopores are group of atoms which tend to increase resonance by interacting the unshared pair of electrons on nitrogen or oxygen atoms of the auxochromes, with II electrons of aromatic ring.auxochrome is a substituted acidic or basic group in dye structure to intensify depth of shade, e.g. -OH, COOH, SO.H, -NH,,-NH(CH.). etc.. Further addition of substituents to dye structure deepens the shade and extent of deepening varies with increase in molecular weight of dye.
Chromogen retains chromophore and plays a crucial role to determine the final hue and its affinity for fibre, fastness, stability, etc.
Substantivity / Affinity: The Substantivity of a dye for a fibre can be defined as an attraction between the fibre and the dye under given dyeing conditions, where by the dye is selectively extracted from the an application medium by the fibre. the term affinity is used as it is more clearly defined and can be given numerical value (usually in joules per mole). It is defined as the difference between the chemical potential of the dye in its standard state in the fibre and the corresponding potential in the dye. In simple terms substantivity or affinity indicates the ability of a dye to go from the solution phase to the fibre.
This is a measure of the proportion of the dye absorbed by the fibre relation to that remaining in the dye bath .Thus, it indicates the amount of dye that has moved from the solution into the fibre under given dyeing conditions. It is also a measure of the substantivity of the dye for the fibre.
Exhaustion is defined as the mass of dye taken up by the material divided by the total initial mass of dye in the bath, but for a bath of constant volume.
For example, if the exhaustion of a dye bath is 75%, it means that 75% of the dye in the dye bath has moved from the dye solution or dye liquor into the fibre. The term exhaustion is mainly applicable to batch –wise dyeing which is also called exhaust dyeing .Textile yarn and fabric are often dyed by the exhaust dyeing technique.
Aggregation numbers – The average number of dye molecules in a micelle – had been determined from diffusion coefficients, electrical conductivity measurements, osmotic pressures, membrane filtration, and light scattering. Aggregation increases with increasing dye concentration and decreases with increasing temperature.
Exact aggregation numbers and the number of incorporated counter ions, which determine the actual overall electric charge of the micelle, are imprecisely known. It is usually assumed that a rapid exchange occurs between free dye molecules in the solution and dye micelles of various sizes.
Material to Liquor Ratio
This expression refers to the weight volume relationship between the fibre to be dyed and the total volume of dye bath .It is normally abbreviated as MLR and sometimes written as M:L ratio. An M:L ratio of 1: 10 means that a dye bath volume of 10 litres is required to dye 1kg of dry fibre.
The material to liquor ratio is also referred to as an inverse ratio and called the liquor to goods ratio or simply the liquor ratio and this ratio is given by the following expression.
Total volume of dye liquor in ml
Liquor to goods ratio = —————————————————–
Dry weight of material dyed in grams.
Thus 5 gms of material dyed at a liquor to goods ratio of 5:1 would use 5×5 =25ml of dye liquor .Alternatively ,3gms of material dyed in dye bath of 60ml has a liquor / goods ratio of 60/3 or 20 / 1 or 20 :1.
Shade percentage refers to the quantity of dye taken for a dyeing expressed as a percentage of the dry weight of the fibre to be dyed .For example, If 1g of a dye is taken fordyeing100gof textile material, the shade percentage is referred to as 1 % Shade. If a kilogram of fibre ie required to be dyed to a3.5 5shade, the amount of dye to be taken would be (1000x 3.5 ) /100 = 35 grams.
Formula for calculation
Ml of dye solution required or added =Percentage of depth of shade x weight of fibre to be dyed in gms
Concentration of stock dye solution in g/100ml
For example .if 3% shade is required to be dyed on 4gof cotton yarn using a solution of one gram of dye in200ml of water (ie 0.5g in 100ml water ) , the ml of the stock dye solution required would be, Ml of dye solution required or added = 3×4 / 0.5 =24ml.
THE GENERAL THEORY OF DYEING
Dyeing is the process of colouring textile materials by immersing them in an aqueous solution of dye, called dye liquor .Normally the dye liquor consists of dye, water and an auxiliary. To improve the effectiveness of dyeing, heat is usually applied to the dye liquor.
The general theory of dyeing explains the interaction between dye, fibre, water and dye auxiliary. More specifically, it explains:
- Force of repulsion which are developed between the dye molecules and water .
- Force of attraction which are developed between the dye molecules and fibres.
These forces are responsible for the dye molecules leaving the aqueous dye liquor entering and attaching themselves to the polymers of the fibres.
The Dye Molecule
Dye molecules are organic molecules, which can be classified as:
- Anionic – in which the colour is caused by the anionic part of the dye molecule;
- Cationic – in which the colour is caused by the cationic part of the dye molecule;
- Disperse- in which the colour is caused by the whole molecule. The first two dye molecule types are applied from an aqueous solution. The third is applied from an aqueous dispersion.
A dye stuff is a substance which is capable of colouring a textile material in such a manner that it associate closely with the fiber , that it is not removable by simple physical means (eg : rubbing or mild deterging ). It must be soluble in water, are capable of going into solution by chemical means, whereby a highly dispersed condition may be regarded as a form of solution.
An essential feature of the dyeing process is that the dye molecule must be capable of entering the fibre structure, the path for the dye molecules is provided by the intermolecular spaces in the fibre and once the dye has entered the fibre structure it becomes firmly attached to the surface of the molecules either by purely physical forces (Secondary Valences) or by chemical combination.
The former mode of attachment is believed to be prevalent in the dyeing of cellulosic fibres, the latter mode in the dyeing of protein fibres. Acetate rayon and synthetic fibres resist penetration by the dye molecules, but certain dyes are capable of forming a solid solution with the fibrous molecule; for dyeing with other dyes, the synthetic fibres may be swollen with suitable agents.Swelling of the fibers appears to play a large part in dyeing of all fibres, and is principally affected by water (or solvents in the case of synthetics) and by raising the temperature of the dye bath.
The dyeing process can thus be considered as taking place in three phases
- Attachment of the dye molecule to the surface of the fibre.
- Penetration into the intermolecular spaces as well as diffusion through the fibre,
- Orientation (and fixation) along the long chain molecules.
Dyeing is governed by three factors, the dye, the fibre and the dye liquor. All the three lead an independent assistance which influences the technique of dyeing. A dye must be water soluble in order to dye textile materials.
It may be soluble by nature of its chemical interference. The solution of the dye from which it is applied is called the ‘dye bath’. A dye may have direct ‘affinity’ for a fibre (or vice versa) i.e., it is held by the fibre either physically (adsorption) or chemically (combination) as soon as the fibre is immersed in the dye bath .Accumulation of the dye in the fibre is a gradual processes, the rate of such building up being referred to as the ‘rate of dyeing’.
This rate of dyeing is governed by the condition of the dye bath , namely concentration of dye , temperature , and presence of electrolytes ; .It is proportional to all three factors .The rate of dyeing I is also influenced by the ‘ Material to liquor’ which is expressed by a fraction, e.g. 1:20, which means one part (by weight) of the textile material dyed in twenty times its weight of dye bath. The rate off dyeing decreases with increasing ratio of goods to liquor.
Dyeing is carried out to produce a certain ‘Shade’ by which is meant a certain colour, difference in shade being due to different ‘Hue’. A blue shade may, for instance, have a greenish or a reddish hue. The amount of dye needed for the production of a certain depth of shade is expressed as a percentage of the weight of the material.
A 1 % dyeing represents a shade produced by the colouring of 100 lbs. of material with one lb. of (commercial) dye, under well defined dyeing conditions. It is necessary to define these conditions because of their influence on the ‘exhaustion’ of the dye bath.
FASTNESS PROPERTIES OF DYES
When a dye is present on a fabric it is expected to have certain properties. Thus when a dyed or printed fabric (garments, curtain materials, furnishing fabric etc.) is exposed to sun light during its use, the dye should not fade or change its colour, i.e., it should have high light fastness.
The dye should posse’s good washing fastness if the cloth dyed with it is used for making garments. Otherwise staining of garments which strip dyestuffs occurs during the washing of many garments. These dyes should also have good perspiration fastness. Perspiration of certain people is acidic in nature and of theirs, alkaline. When people wearing colored garments perspire a part of the dye coming into contact with the perspiration may be strip and stain the undergarment or the skin of the wearer.
A single dye, which dyes all the known major textile fibres is not made yet. Some of fibres are dyed with certain classes of dyes and other fibres with other classes. Thus cotton, mercerised cotton, linen, viscose rayon, cupramonium rayon etc. (cellulosic fibers) are dyed with vat, reactive, direct, azoic (naphthol and base), sulphur, oxidation colour, mineral colour and basic dye (after moderating).
Wool and silk (protein fibres) are dyed with acid dyes, basic dyes (without the use of mordant), acid mordant dyes (special acid dyes which can combine with metals like chromium dyeing and produce wash fast dyeing .Example metal-complex dyes (dyes containing the metal atom in their molecular structure so that the metal atom need not be incorporated into the dye during dyeing) .
Acetate and triacetate and polyester fibres are dyed with disperse dyes. Polyamide fibres are dyed with acid dyes, metal-complex dyes, disperse dyes. Acrylic fibres are dyed with basic (cationic) dyes.
There are numerous factors involved in the selection of dyes for colouring a fabric in a particular shade. Some of these are:
- The types of fibres present.
- The form of the textile material and the degree of levelness required – level dyeing is less critical for loose fibres, whichare subsequently blended, than it is for fabric.
- The fastness properties required for any subsequent manufacturing processes and for the particular end-use.
- The dyeing method to be used, the overall cost, and the machinery available.
- The actual colour requested by the customer.