Mechanism of Dyeing -
When cotton is immersed in a solution of a direct dye, the following mechanism takes place -
(1) Fibre Swelling in the liquor.
(3) Diffusion of dye molecules into the interior of the fibre.
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| Mechanism and effects of Dyeing |
The phenomenon of adsorption and Diffusion can be illustrated by a Microscopic view of the different stages of dyeing cotton.
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| Diffusion of dye into the fibre with time |
The affinity of direct dyeing is the result of -
(2) Coplanar dye structure
(3) Hydrogen bonds between-
(a) hydroxy (OH) of cellulose as a proton donor
(b) Amine (NH2) group of dye as a proton acceptor
(4) Van der Waals forces
Direct dyes' affinity for cotton cellulose is thanks to hydrogen bond-like connections formed between the dye's delocalize pi(Ï€) electron system and also the hydroxylic groups of cellulose (protein donors) (proton acceptor).
Additionally, it's believed that a system of alternate single and double bonds is connected to the affinity (conjugated double bonds). Van der Waals forces and other interactions also contribute.
The effects of various factors on Direct dyes are the following :
(A) Effect of salt
(B) Effect of temperature
(C) Liquor ratio
Explanation :
(A) Effect of Salt (Electrolyte) -
(i) It has been noted that adding salt while using direct dyes tends to form people feel exhausted. But the impact varies greatly from one dyestuff to a different one.
(ii) Cellulose fibres develop a little, low electrical phenomenon referred to as zeta potential when submerged in water.
(iii) The anionic dyestuff in an aqueous bath containing both fibre and dyestuff is going to be interested in the negative surface charge on the fibre. At the same time, the previous are interested in it.
Therefore, little tiredness will happen.
(iv) Now, when a binary substance like an electrolyte is put into the bath, it ionises into sodium cations and chloride anions.
(v) The Sodium cations neutralise or reduce the charge on the fibre surface and so the dye anions within the tub, repelled by the chloride anions move to the fibre where they're absorbed.
(vi) The dye anions are much larger than the chloride anions but they have a wonderful Substantivity for the Cellulose fibre and are therefore quickly absorbed by the nearly neutral fibre surface.
(vii) Once adsorption of dye has occurred, the other steps of dyeing namely Diffusion and fixation occur subsequently Thus, in the dyeing of cellulose fibres electrolytes enhance the Exhaustion of direct dyes to a considerable extent,
However, the Exhaustion affected by electrolytes varies from dye to dye as stated above.
(viii) The graphical representation in the below fig. indicates that dye B exhaustion to a greater extent on the addition of salt where because the effect on Dye A isn't considerable.
(ix) The existence of the acid groups within the dye molecule has been associated with the range within the extent of Exhaustion of various dyestuffs within the presence of salt.
(x) The quantity of acid groups directly affects how well salt promotes tiredness. Therefore, the action of the electrolyte is stronger the more acid groups a colour contains.
The figure below illustrates the repellent forces between dye and fibre in the absence of salt.
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| Variation in the Exhaustion of direct dyes due to salt |
(B) Effect of temperature -
The amount of dye preoccupied with the fibre (while dyeing a cellulosic fibre with direct dye) depends on the temperature of dyeing.
The rate of dying increases with the rise in temperature i.e dyeing takes place slowly at lower temperatures and also the dyestuff rushes on the fibre with the increase in temperature.
It is therefore imperative for the results to be uneven when dyeing is applied at higher temperatures. In light of the above facts, it's concluded that dyeing should start at temperature then the temperature is raised gradually to the foremost dyeing temperature.
As the temperature rises the speed at which equilibrium is attained until it reaches the utmost. Affinity however decreases with a further increase in temperature.
This can be seen clearly below in fig. showing the influence of temperature on the exhaustion of dye.
It is seen within the figure that the foremost exhaustion takes place at a particular temperature which varies from dye to dye and knowledge of this is of utmost importance from the sensible dyeing point of view, particularly while producing a mixture shade.(C) Effect of liquor ratio -
Below fig. the variation of dye uptake (g/kg of fibre)with changing dye bath concentration.
It is often seen that because the initial concentration of the dye within the dye bath increases (or because the fabric to Liquor ratio decreases) the dye uptake also increases.
Therefore it follows that a deeper shade is obtained by dyeing the tenth shade at a material to Liquor ratio of 1:10 than by dyeing the tenth shade at a ratio of 1:40.
Therefore deeper shades are obtained by dyeing giving shade Percentage at low MLR.
It is, therefore, essential to remain this ratio constant throughout the dyeing and also while dyeing many identical shades.
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| Applied direct dye |
Questions -
What is the diffusion of dye into the fibre with time?
What is the effect of Salt (Electrolyte) on direct dyes?
What is the effect of temperature on direct dyes?
What is the effect of the liquor ratio on direct dyes?
References
Chakrabarti, R., & Mehta, N. (2008, November 8). Quick Level Dyeing of Direct Dyes. Fibre2fashion.com; Fibre2Fashion. https://www.fibre2fashion.com/industry-article/3742/quick-and-level-dyeing-of-direct-dyes-dyrite-process
Clark, M. (2011). Handbook of textile and industrial dyeing: Principles, processes and types of dyes (Matthew Clark, Ed.). Woodhead Publishing.
Hasin, S. (2020, July 16). All about direct dyes. Textile Property. https://www.textileproperty.com/direct-dyes/
Sayed, A. (n.d.). Direct dye: An overview [A to Z]. Blogspot.com. from https://textileapex.blogspot.com/2015/01/direct-dye.html
The Editors of Encyclopedia Britannica. (1998). direct dye. In Encyclopedia Britannica.
Trivedi, Y. (2020, November 18). Dyeing of cotton fabric with direct dyes. Textilesphere.com. https://www.textilesphere.com/2020/11/dyeing-of-cotton-with-direct-dyes.html
Further read,
Writer - Rushikesh Patil (Textile Engineer)
