Concentrated leuco-indigo solution, especially for dyeing cotton warp yarn for blue denim, contains leuco-indigo in the form of a mixture of salts of at least two alkali metal hydroxides

摘要

Concentrated leuco-indigo solutions consisting essentially of 25-55 wt. % leuco-indigo in the form of a salt of a mixture of at least two alkali metal hydroxides containing not more than 70 mol. % of any one metal hydroxide, in the stoichiometric amount required for conversion of the leuco-indigo into the neutral indigo salt, together with water. Independent claims are also included for a process for the production of leuco-indigo solutions by catalytic hydrogenation of a 20-30 wt. % aqueous indigo suspension containing 4-10 wt. % alkali metal hydroxide mixture as defined above, after which the leuco-indigo content is adjusted to the required level by: (a) adding more indigo and optionally more alkali and continuing the hydrogenation; or (b) evaporation of the solution.