Acid dyes are so called because they are usually applied under acidic conditions. These are the sodium salts of sulphonic acids. Dyes are classified as acid if they can be applied satisfactorily to protein fibres.
Suitability to textiles:
Acidification of basic dyes led to creation of acid dyes that are used mostly on wool and silk.
They are also being more widely used for dyeing man-made, synthetic, nylon fibres and natural protein fibres.
These anionic dyes have also been found useful in printing chlorinated wool and silk.
Used for printing on chlorinated wool, silk and acetate.
Method of application:
The application of acid dyes to protein fibres results in an ionic or salt link between the dye molecule and the fibre polymer. The point of the fibre polymer at which the dye is attached is termed as dye site. In wool the dye sites are the many amino groups of the fibre. Under dyeing the amino group becomes positively charged and attracts the negatively charged dye anion.
Satisfactory dye uptake can be ensured only after heating the acid dye liquor. Acetic acid, HCL, or sulphuric acid is used in dyeing. While dyeing blends of wool with cotton Phosphorous acid is preferable. No transfer of dye molecules from the dye liquor to the fibre polymer system can be observed below 40 0c. The rate of dyeing increases steadily as the temperature increases. Increasing the temperature of the dye liquor provides the energy required by the dye molecule to leave the dye liquor and enter the polymer system of the fibre.
For printing, steaming is necessary for the fabrics printed with acid dyes.
Fastness properties:
Fabrics which are dyed or printed with acid dyes have good light fastness rating from 4-5.
Wash fastness ratings for acid dyed or printed fabrics are about 2-3 with good leveling characteristics, 3-4 for those with average leveling characteristics, and 4-5 for those with poor leveling characteristics.
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