Studies in Wool Dyeing: Comparison of Metheds for Applying Chrome Dyes

Abstract

When a wool staple is dyed with certain chrome dyes the tip portions are heavily dyed and the body portions are undyed or only lightly dyed. Cross‐sectional examination shows that the tip portions of such a staple are heavily dyed, the body portions are undyed and the intermediate portions are “ring” dyed, the thickness of the “rings” being greater toward the tip portions. This variation in dye penetration is caused by the condition of the epidermis of the wool fibre, and if this epidermal layer is broken or “damaged” the dye will penetrate the fibre more readily. When the dyed staple is carded or when wool is dyed after carding, the cross‐sections show unlevel distribution of dye from fibre to fibre. This fibre unlevelness causes poor colour values, skittery or heathery effects, and entails an inefficient use of dye. Exhaustion curves show that there are variations in wools from different regions and after physical and chemical processing. These variations increase the possibility of fibre unlevelness when various wools are blended together.Wool exhibits reactivity in aqueous solutions and induo.09 changes in pH both above and below the isoelectric zone, viz. pH 4–5. When a wool dyebath is made alkaline with ammonia at the start of the dyeing operation there is a gradual change in pH during the dyeing process, which influences the chromation, the decomposition of dye by oxidation and the exhaustion.Wool can be successfully dyed with chromable dyes which are not usually applied by the metachrome methed. This can be accomplished by dyeing in a bath which initially has a pH greater than 7, by using an optimum low percentage of bichromate, by having magnesium sulphate present, and by employing a surfaceactive agent as a dyeing assistant. This procedure has been called the Calcomet process. The addition of magnesium sulphate to the alkaline dyebath results in the formation of a magnesium‐dye complex without appreciably affecting the pH. The magnesium complex, however, appears to be unstable below a pH of about 7. The use of the specially designed surface‐active agent as a dyeing assistant promotes dye exhaustion from the alkaline bath and also causes the exhaustion of any chromium‐dye complex that is formed in the bath. Consequently, good penetration and levelness of shade are obtained.The afterchrome, chrome mordant, metachrome and Calcomet metheds were compared in the laboratory using several chrome dyes, and a detailed comparison of the various metheds was made with Calcochrome lied ECB in regard to rate of dyeing, rate and extent of chromation, pH and the completeness of exhaustion.