Fast for sure

Abstract

Achieving the wash fastness specified by any retail organisation is not a straightforward exercise in the case of applying reactive dyestuffs to Cellulosic substrates.

Some of the problems involved are discussed in the full text.

Evidence, in the form of a practical working example, is presented to illustrate the value of understanding the basic principles involved.

Full Article

2.1 Background

The consumer requires the assurance of all clothing and home textile purchases that they keep their appearance and fastness through domestic laundry, drying and ironing.

The dyer and finisher require the assurance of all dyes that he uses that they meet the fastness demands of the customer he is serving.

These fastness assurances are provided by the dyestuff supplier in their published literature.

With the majority of dyestuff classes, the claims of the dye supplier are relatively easy to achieve in practice.

The exception to the rule occurs in the case of the application of reactive dyes to Cellulosic fibres.

Firstly, not all the reactive dye which is applied exhausts on to the fibre.

Some remains in the bath.

Secondly, not all the dye which exhausts onto the fibre is chemically fixed to the fibre.

There is therefore, a significant amount of unfixed dye which must be washed off the fibre before the optimum fastness properties of the dyed fibre can be realised.

The fastness properties claimed by the reactive dye supplier are obtained under ideal conditions in which all the hydrolysed dye has been washed off the fabric.

This will be on small scale dyeings; and it is far easier to achieve a perfect wash-off on this scale than it is in bulk production.

Furthermore, the degree of fixation achieved in bulk production will not necessarily match that achieved on smaller scale dyeings.

The degree of fixation achieved in practice can even vary from one bulk dyelot to another; even on the same dye recipe on the same substrate.

The difficulties in achieving the optimum fastness properties are compounded even further when the substrate is varied.

A wash-off cycle which is perfectly adequate for a given dye recipe applied to Cotton, for example, may not be quite so efficient when applied to Cotton / Lycra.

Even more uncertainty enters the equation when it is realised that the degree of fixation achieved by a given reactive dye recipe, and on a given substrate, can vary from day to day as the bicarbonate content of the softened Dyehouse water varies.

Let us refer to the dye supplier claims as Theoretical Fastness and the properties achieved in bulk production as Practical Fastness then the complex situation can be summarised as follows.

Light fastness (Theoretical):

* determined by the distribution of electron within the dye molecule, and therefore the molecular structure.

* varies from one dye to another within a given manufacturers range of reactive dyes.

* Usually requires the inclusion of a metal complex with the molecular structure to achieve the highest levels.

Wash Fastness (Practical):

* depends on the reactive dye range chosen.

* can vary from one dye to another within any given range of reactive dyes.

* depends on the efficiency of fixation in the dyeing process.

* depends on the efficiency of the wash-off process.

The Efficiency of the Dyeing Process:

* the design of the dyeing process / alkali system.

* the pH obtained by a given amount of alkali depends on the quality of salt used in exhaustion and the level of bicarbonate impurity in softened Dyehouse water. (see Figs 1 - inc).

* the amount of bicarbonate in softened water can vary from day to day, and even during the day see Table 1.

The Efficiency of the Wash-off process :

* will never equal that of the process used to achieve Theoretical Fastness.

* can vary from machine to machine.

* can vary from one substrate construction to another even on the same machine.

2.2 Alkaline Fixation pH and the influence of bicarbonate impurity in Dyehouse water.

The pH obtained by a given addition of alkali depends on the level of bicarbonate impurity.

Fig 1 : Effect of Bicarbonate on the pH of Soda Ash in 0 g/l Salt.

Fig 2 : Effect of Bicarbonate on the pH of Soda Ash in 40 g/l Salt.

Fig 3. Effect of Bicarbonate on pH of Soda Ash in 100 g/l Salt.

2.3 Variation of Bicarbonate in Softened Dyehouse water

Table 1. Dyehouse Bicarbonate Variation

• High Quality Operation in Europe

• Highest Commission Charges in its Market

• Wide Variation ( 170 ppm 525 ppm)

• Unpredictable

2.4 Achieving Theory in Practice

This is best summarised and illustrated by a Case Study.

In order to preserve confidentiality the name of our customer is withheld.

Customer Profile

The results of the audit by Dyehouse solutions International are shown in Table 2.

Table 2. Revised Processes with same dye selection.

2.5 Concluding Remarks

* Achieving Theoretical Fastness with reactive dyes on cellulose is not straightforward.

* The dye supplier provides the theory.

• The Dyer and Finisher must understand the principles involved, fully monitor local conditions and pay due diligence in order to achieve Theory in Practice.

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