After treatment of Direct Dyed Goods -
The reasons for the popularity of direct dyes within the dyeing of cellulosic fibres are the low cost of dyeing and also the easy dyeing procedure.
These dyes don't possess adequate washing fastness properties and most of them fade in the light.
The use of direct dyes, therefore, becomes undesirable for material which can subsequently be washed frequently or continuously exposed to light.
(A) Increasing mass of fabric
(B) Decreasing solubility of water after dyeing
Several methods have been devised to spice up the fastness properties of Direct dyes.
These after treatment are supported by principles like increasing the mass of dye and thus decreasing their solubility in water dyeing.
All the direct dyes, however, are not capable of such after treatment since in many cases, the colour of the last word product changes i.e it tends to become duller and sometimes the hue is altered.
Some after treatments that are in use are summarised below:
(a) Treatment with copper Salts
(b) Treatment with chromium compounds
(2) Treatment with formaldehyde
(3) Treatment with cationic dye fixing agents
(4) Topping with basic dyes
(5) Diazotization
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| Direct dyed fabric |
Explanation -
(4) Topping with basic dyes -
Basic dyes don't adhere to cellulosic fibres, but unique techniques are developed to use them with cellulosic material to provide vivid hues.
Certain mordants, such as phenol, were employed in the procedure because they need an affinity for both cellulose and basic dyes and function as a link between the dye and therefore the fibre.
When a base dye solution is applied to a fabric that has been directly dyed, the direct dyes function because the mordant and make a posh.
Such Topping with basic dyes refers to the method of treating direct-dyed material after treatment with basic dyes.
Any dye may theoretically be further processed with basic dyes, but because the topping is finished in an acidic media, direct dyes that are acid-sensitive (such as Congo red) should not be processed in this way.
The direct dyed items are treated with 2 to 4 per cent phenol in an exceedingly hot bath for 20 to half-hour, followed by squeezing and treatment with 1 to 2 per cent mordant during a cold bath for 20 to half-hour, to urge very quick colouring. Cotton that has undergone tanning is extremely receptive to basic colours.
A dyestuff with hues just like the direct dyes is often used for topping.
To achieve beautiful hues on cotton dyed using direct dyes, only an awfully little amount of dye, say of the order of 0.1 to 0.3 per cent, is required.
The process of topping with basic dyes is finished during a cold bath that contains a selected amount (half or perhaps a quarter) of the dye solution.
The temperature is then gradually raised to 60°C, followed by the addition of the remaining dye portion at regular intervals.
To urge homogeneous hues, a little amount of ethanoic acid (0.5 to 2 per cent) is added to the dye bath.
In any case of the fundamental dye has been added, the treatment is maintained for a further 45 to an hour at the identical temperature.
This is followed by rinsing and drying then, the topping is finished as previously described.
(5) Diazotization and development of direct dyed goods -
As was previously said, by making the dye molecules of water-soluble dyes larger, the wet resistance of these dyes is improved.
A chemical process with acid can diazotize a number of the direct dyes because they include free amino,(-NH2) groups (HNO2).
After that, a coupling agent is applied to the resultant diazonium salt, which causes the assembly of an azo(-N=N-) group.
The substantive dye molecules are now significantly larger and can exhibit good resistance to wet treatments.
- The chemical reactions are shown below-
The colour of the material used to create diazotized and produced direct dyes will change since the azo group itself is a chromophore.
Thus, utilising various coupling components, different colours can be created.
In actuality, the direct-dyed material is treated in a bath containing: for 30 minutes in the cold.
Recipe | Quantity |
NaNO2 | 1 to 3%(owf) |
HCl | 5 to 10%(owf) |
The goods are rinsed and developed with a solution of the coupling component for 15 to 20 minutes. The material is then washed, soaped and dried.
Stripping of Direct Dyes -
The dyes may need to be stripped if the colouring is uneven, spotty, or when the hue is unacceptable and deeper than expected.
Nearly all direct dyes can be removed using a hot, dilute solution of caustic soda and sodium hydrosulphite; however, not all dyes can be completely removed to reveal white, but the procedure may be effective enough to repair incorrect dyeing.
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| After treatment of the direct dyed fabric |
Questions -
- Which two principles are used after the treatment of Direct Dyed Goods?
- Describe treatment with topping with basic dyes.
- Describe Diazotization and development of direct dyed goods.
- Explain the stripping of 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)
