Colour Chronicle - April 2012 - Clariant

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12 textile Some Important Definitions Diffusion The molecules adsorbed onto the surface of the fi ber immediately begin to diffuse into the fi ber. Hence further sites become available on the cellulose surface for adsorption, and these are immediately occupied by new dye molecules exhausted from the bath. The uptake phase thus consists of a series of states of equilibrium, the fi nal objective of which is uniform distribution of the dye over the accessible areas of the fi ber. It is self-evident that diffusion is interrupted by reaction with fi ber. It is above all the diffusion rate of day, and not as often claimed, its substantivity that decides its rate of exhaustion. Substantivity is only one of the numerous parameters affecting diffusion rate. Of all the parameters available to the dyers, temperature has by far the greatest effect on diffusion (and hence on migration, wash-off etc…) The activation energy of diffusion is usually between 10-20 kcal/kg which is equivalent to an increase of the diffusion rate by a factor of 3 for a temperature increase of 10°C. High affi nity reactive dyes, which exhaust best from the bath, diffuse, migrate and level fairly slowly and many times poorly. Dye reactivity The reactivity of commercial dye has greatest infl uencing factor of Reactive Group itself, Nucleophilic substitution SN2; the leaving group decides the Reactivity. With every 1 unit pH increase, the concentration of secondary alcohol position of cellulose ionizes to increase by a factor of 10. Concentration of – OH in the dyebath will decide the concentration of cell – O which is crucial for reaction. Meaning pH 10-12 the reaction rate will be 1000 - 100,000 times higher than at neutral pH (for a dye). It is not suffi cient to adjust the pH of fi xation to a suitable level and relax as the reaction consumes alkali and therefore, without a proper buffer capacity, the pH therefore promptly falls to a level at which the reaction is too slow. Standard Affinity Standard affi nity, like substantivity, is a measure for the distribution of a dye between fi ber and the dyebath and thus for the difference between the dyes affi nity colour chronicle 2 | 2012 for water (hydrophilic properties) and for cellulose. Unlike substantivity, affi nity is always determined in equilibrium but under standard dyeing conditions. This may require many many hours and even days. Factors like ‘V’ volume of cellulose has to be worked out before we derive – K cal affi nity value for one dye under one set of defi ned conditions including V factor of the cellulosic sustrate. Negative K cal as the dyeing is an exothermic reaction. It is a true dye–specifi c characteristics. Factors like Number of coplanar chromophores. High number of conjugated double bonds Every discountinuity in the planarity of the chromophore that is every decrease in the size of the delocalized pi electron system Polarity Substituents Van der waal forces and hydrophobic forces Number of solubilising groups like – SO 3 in the chromophores. Substantivity Substantivity of the Dye i.e. the Quotient of the dye concentration in the fi ber and the Dye concentration in the bath in the equilibrium state. Equlibrium = [ F ] bath > < [F ] cellulose It is thus a measure for distribution of a dye between fi ber and aqueous dyebath under the prevailing dyeing conditions. Substantivity = f { [F] (cell) / [F] (bath) } It is thus only a state, which is dependant on numerous dye, dyebath and fi ber-specifi c parameters, such as: Dye concentration, dye affi nity, electrolyte concentration, liquor ratio, temperature, pH and type of fi ber. Moreover, equilibrium is by no means achieved in most bulk-working dyeing methods, so that it would often be more accurate to use the term ‘apparent substantivity’. In exhaust dyeing, dyes with no substantivity will naturally remain in the dyebath and hydrolyse, i.e. react with water. Even in Pad method, in which the dry fabric instantaneously absorbs dye liquor by capillary action until completely saturated, minimum dye substantivity is nonetheless required. A completely non-substantive dye that is of necessity in a capillary of the cellulose will, however, only rarely come into contact with the capillary wall.The probability of hydroxyl groups reacting with cellulose is thus small. Dye-fiber reaction Two reaction mechanisms give rise to a covalent bond between cellulose and the dye which in terms of degree of fi xation, rate of fi xation and stability of the dye/fi ber bond complies with industrial requirements: Nucleophilic SN2 substitution and nucleophilic addition. In nucleophilic SN2 substitution a mobile halogen atom of the dye is substituted by the ionized nucleophilic – O group of the cellulose. Chloro triazenes and groups like pyrimidine etc. reactive groups of the reactive system follow this route. In nucleophilic addition a portion of the ionized – O group of the cellulose are added to the active group of the dye. Vinyl sulphone reactive groups of the dye follow this route. In cases of Bi reactive dyes, where more than one group of the above two types or one of each type are present, they follow the respective routes. This is the reason of popularity of these type of dyes as the probability of dye-fi ber bonding is higher and even if one of the reactive groups get hydrolysed with water reaction, the second may be still available for dye-fi ber reaction. By combining two different groups in a dye, the specifi c weaknesses of each can be largely eliminated. Incorporation of the second reactive group often makes the molecule bulkier, and this can reduce the diffusion of the dye in the fi ber and for example impair washing–off properties (though extent of wasteful hydrolysed dye is lesser than mono functional dye) or fi ber levelness. The second reactive group can impair affi nity (mostly reducing it). In the exhaust method, a change such as this can impair levelness. Therefore skillful selection of the reactive groups and their positioning is important to control bulkiness of the molecule, affi nity, diffusion, wash-off and levelness.
® Archroma Color Management is a registered trademark of Clariant. Which shade will make my creation a fashion statement? Inspiration has a new name: Archroma ® Color Management. In fashion, nothing could be more important than color. Get it right, and see sales soar. Get it wrong, and your collection litters the clearance rack. Archroma’s aim is to get it right, from the design studio all the way through the supply Clariant Vertrieb (Deutschland) GmbH & Co. KG, c/o Clariant Service (Schweiz) AG, Kundendienst Papier Mitteleuropa, Rothausstrasse chain to the end 61, customer. 4132 Muttenz, We combine Switzerland, vast Phone: expertise +41 61 in 469 dyestuffs, 7638, www.paper.clariant.com textiles and customized color standards with state-of-the-art, web-based tools that keep color on track. No matter how far-flung your operations are, Clariant is close to you. Any time. Any place. So visit us today at www.archroma.com, and let your inspiration take color. Clariant Chemicals (India) Ltd., Textile Business, Kolshet Office, P.O. Sandoz Baug, Kolshet, Thane (W) 400 607, India. www.clariant.in What do you need?
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Colour Chronicle - April 2012 - Clariant

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