April 12, 2008

One bath dyeing of cellulosic blends

A number of one bath methods have been developed for cellulosic blend dyeing. Disperse/direct dyeing of cotton/nylon or polyester is well known although of little practiced use because of the poor wet fastness, except in the pale shades.

Methods based on hot dyeing reactive dyes in which the disperse and reactive dyes are added to the bath with 5 gram/liter of Resist salt L (m-nitro-sodium-benzene-sulphuric acid) which prevents hydrolysis of reactive group. Dyeing of polyester is first carried out at 120°C. Then the bath is cooled to 80°C. Electrolyte is added and dyeing of the cotton proceeds in the usual way.

Disperse and vat (pigment) dyes are added to the bath plus a large quantity of dispersing agent. Dyeing at 100° to 130°C proceeds. Then the bath is cooled to 80°C and caustic soda and hydros are added and dyeing of the cotton carried out. There are several Union dyes (Cottestren) on the market using this principle. These commercial blends have to be formulated for a predetermined fibre mix and may turn out to be uneconomic for a customer's particular end use and further more may not give solid shade dyeing under adverse conditions of applications.

A Hoechst patent for single bath application reactive/metal complex dyes has the following method. A dyestuff and alkali to give a pH of 8 to 12 and raise temperature to 80°C to dye the cotton. The pH is then reduced to 6.5 to 7 by the addition acid. The temperature is raised to 95°C and the nylon portion is dyed. Acid dyes will precipitate under these conditions, metal complex dyes will not.

Azoic Dyeing of cotton yarn

Naphthol dyestuff have traditionally been applied by the multi stage route of impregnation, hydro extraction/rinsing and development.
German and Swiss manufacturers have now developed a one bath application process which offers a real rationalization of the dyeing process. This one bath process is cotton in hank in open becks and spray dyeing units and cotton piece on the winch.
The basis of the method is to retain the bath after impregnation and maintain the color pigment which is formed during development in highly dispersed form, by means of a special auxiliary. The one bath method consist of

  1. impregnation for about 20 minutes. at 20 to 30°C.
  2. addition of acid and dizao solution without letting off the bath.
  3. coupling the dyestuff of about 30 minutes
  4. cleansing after treatment.

A fairly wide selection of naphthol/base combinations are suitable for this process.