Reactive dyes have better property in terms of fastness. Dyeing of cellulosic substrates takes place with reactive dyes under alkaline conditions. But this alkaline conditions also facilitates reaction of reactive group with the dye liquor, resulting in deactivation or hydrolysis of the dye. This hydrolyzed dye adhere onto the substrate and keep on getting removed during washing treatments causing poor wash fastness. This hydrolyzed dye must be removed by rinsing and using an appropriate Washing-off agent in order retain the fastness properties. Approximately 3/4th of the dye gets fixed while remaining 1/4th gets hydrolyzed. Due care must be taken to reduce the hydrolysis as much as possible.
Following factors must be controlled in order to reduce hydrolysis :
pH : It is necessary to control the pH 10-11 as rise in pH leads to more hydrolysis
Temperature : It is also necessary to control Temperature
Dye concentration : Increase in concentration of dye promotes its aggregation and the rate of hydrolysis decreases.
Electrolyte concentration : More the amount of the electrolyte present in the dye-bath, more will be the hydrolysis.
Mechanism of Washing-off agents :
The washing-off efficiency of dyes would depend upon the amount of unfixed dye and ease of removal of unfixed dye. The washing–off agent should facilitate the diffusion of unfixed dye molecules from the fiber into the water. It must eliminate the negative effect of hard forming ions on the solubility of unfixed hydrolyzed dye.
Normally, soaping off includes a first rinse step, a soaping step, a second rinse step, and a drying step. Typically, soaps, such as phosphate-based detergents, are added during the soaping step to assist in removal of unreacted reactive dyes. However, the liquid alkalis used during fixing have not been considered for use during soaping since they would not be expected to assist in excess dye removal. In addition, if the reaction mixture is too "hot" or alkaline, such as is seen with pure sodium hydroxide, the sensitive-type reactive dyes will hydrolyze with the water in the rinse bath and form a nonreactive pigment that has no effect on the fabric colour. Furthermore, phosphate & silicate based soaping agents are not accepted by European countries due to their biodegradability issues.
Soaps have been replaced by surfactants. But surfactants lacks chelating properties. Thus surfactants have been replaced by polymeric washing-off agents. Together with low foaming surfactants, they exhibit excellent washing-off properties. The dispersing properties in the washing-off agent helps to keep the dyes, particularly, disperse, naphthols in the disperse condition, thus not allowing it to re-deposit on the fabric. In case of high degree ethoxylated products, good dispersing properties are obtained. In order to know good techniques for washing-off, one must understand the reasons for poor washing-off as under :
i) Too high substantivity of dyes
ii) Low diffusion coefficient dyes
iii) Short liquor ratio of washing bath
iv) High electrolyte concentration
v) Inadequate washing time
vi) Inadequate chemicals
Washing of reactive dyed fabrics is more complex. It is very difficult to remove the unreacted/unfixed dye from the fabric due to the presence of electrolytes. Thus it is advisable to rinse cold before treating with washing-off agents. It should be well noted that the fabric has to undergo softening treatment further. So it is advisable to use washing-off agents in the pH range 5.0-6.0. One such product has been developed by Matex after extensive R&D work, named Matwash DSP conc. Due to its protective colloidal property, it does not allow the hydrolyzed/ unfixed dye to re-deposit on the fabric. Moreover it also imparts slight acidic nature to the fabric which is required for cationic softeners to readily exhaust onto the fabric.
In the dyeing of knits of polyester and blends, staining on the unwanted portion of the fiber blend has become pronounced. This is the due to the use of high speed spinning machines that results in oligomers. The redox potential which forms in the presence of caustic soda is sufficient to destroy most disperse dyes but not adequate to attack vat, naphthol and many reactive and direct dyes. There has been a great demand for washing-off agents that are capable of dealing with staining of the substrate and the machine. Matwash RC develops a high dispersing power at all temperatures so that unfixed disperse dye attached to the fiber surface is destroyed and the degradation products are finely distributed in the washing liquor.
During washing-off dyeings and prints with acid dyes, the wet-fastness and rubbing fastness properties must be retained or improved. Matwash SAN liquid accelerates migration of hydrolyzed dye from the textile fiber into the dye bath. It also prevents staining of the ground in case of printed goods. Matwash SAN liquid is an excellent poly-condensate that has amphoteric properties to satisfy these needs.
In case of vat dyes, the washing process aims at removing insoluble pigment substances from the fiber surface by means of wetting or dissolving agents. Thus the purpose of soaping after oxidation is to remove any dye that is not diffused into the fibers and to stabilize the final shade. This results in improved fastness properties and resistance to any shade change caused by further treatments.
Thus the factors governing the washing-off process are :
Type of washing-off agent
Concentration of washing-off agent
It is worth specifying that the above mentioned discussion is not at all exhaustive, in particular it is not a text relating to the treatment and elimination of severe stains which cannot be treated with standard cleansing processes. Readers are recommended to consult specific literature available on this subject.