No Soap Dyeing

 

Question:
Is soaping necessary after Reactive dyeing of cellulosic materials?

Answer:

Omitting the use of detergents in after washing of cotton dyed with reactive dyes:
Description

Both international literature and practical experience in textile mills show that detergents do not improve removal of hydrolysed reactive dyestuffs from the fabric.

On the contrary, high temperatures do have an affect on rinsing effectiveness. Tests carried out with rinsing at 90 - 95 °C have shown that rinsing is more effective and faster at high temperatures. About 30 % more unfixed hydrolysed reactive dyestuff is rinsed out after 10 minutes of rinsing at 95 °C than at 75 °C.

Many dye houses already carry out hot rinsing and omit the use of detergents in rinsing after reactive dyeing. The product quality is not negatively affected. On the contrary, most often the fastness of the goods are better after the hot rinsing than after the traditional rinsing with detergents, complexing agents and neutralisation in the first rinse.

Energy should be recovered when using large volumes of hot process water. Energy reclamation can be done either by heat exchange between the hot outgoing process water and the cold incoming clean water or by reclamation of the hot water and re-use of both energy and water.

Main achieved environmental benefits

The main benefit is the reduction in consumption of detergents and pollution load discharged to the waste water. Obviously, the potential for reduction will vary according to the existing dyeing procedure at the company.

The experience of two dye houses (one mainly dyeing knitted fabrics and the other dyeing garments) shows that the average potential load reduction can be in the order of 1 kg detergent, 1 kg complexing agent and 1 kg acetic acid per 100 kg of textile.

Additional advantages are the savings achievable in the amount of chemicals consumed to destroy reactive dyes by free radical treatment processes. In the Fenton reaction for example since the OH* radicals react very fast not only with the dyestuffs but also with many detergents, a large amount of expensive H2O2 can be saved by omitting the use of detergents.

Operational data

The high degree of fixation and the excellent wash-off properties typical of some new low-salt, polyfunctional reactive dyes are important factors that help obtain sufficient wash fastness with hot rinsing without the need for detergents.

It has been reported that difficulties might arise with accidental stops of the machinery. In such conditions the high temperature of the rinsing water could cause irreversible cleavage of the bond between the reactive groups of the dye and the hydroxyl groups of cotton or viscose.

Applicability

A Danish textile mill has during the last 5 years totally omitted the use of detergents in the rinsing process after reactive dyeing. The company treats knitted and woven goods made of cotton or cotton/PES and dyes them in all colours and shades. The application of this technique may involve a change in the type of dyes employed. The referenced company works with bifunctional reactive dyes as Cibachron C or Bezaktiv S. Soft water is used.

Another textile mill dyeing garments of knitted and woven fabric has not used detergents during the last 5 - 6 years, apart from a few exceptions (i.e. red, dark red or bordeaux colours).

Reference literature

«Environmentally friendly method in reactive dyeing of cotton». Water Science and Technology Vol. 33, No.6, pp.17-27, 1996»

«Reclamation and re-use of process water from reactive dyeing of cotton». Desalination 106 (1996) 195-20»