Milestone Vat Dyes are insoluble in water and have a low affinity. For the dyeing process, dyestuffs must be converted by alkali and reducing agent into water soluble form. The usual reducing agent is Sodium Hydrosulphite. After application the dye must be converted by oxidation back into the insoluble form.
› Dyeing Process
1. Exhaust Dyeing Process
Yarn can be dyed with pigmentation, semi-pigmentation, Leuco (dyeing from the vat), high temperature process. In pigmentation process, the dye is vatted at optimum temperature. To improve the bath exhaustion, the dyeing temperature is reduced towards the end of the dyeing process. Semi-pigmentation process is mainly used to obtain medium to dark shades on pretreated yarn. Leuco process is employed mainly for dark shades. High temperature dyeing improves dye penetration, better dyeing of crossover points and good levelness.
2. Padding Process
Pad-steam & Wet-steam processes are continuous dyeing process. In Pad-steam, the dye is padded on in pigment form together with a wetting agent and antimigrant. It is the dried, the fixing agents are padded on, and the dye is developed in a steamer.
In Wet-steam process is continuous process without intermediate drying and useful for qualities with high possible liquor pick-up. It reduces migration and enhances dye penetration.
› Rinsing, Oxidation and Soaping
After dyeing, various methods of oxidation and soaping are used as per the dyeing process employed.
› Special Note
Milestone Black BB requires hypochlorite bleaching after treatment to develop black shade.