The main endeavour of any business enterprise is to produce acceptable quality product at competitive price. This is to be achieved by way of quality management, process control, process optimisation, reduced energy cost and water conservation etc. Processing of textile is a complex exercise with numerous variables of inputs like fibres, dyes and chemicals, coupled with the process conditions and techniques of application.

Application of reactive dyes for cellulose is a three phase process.

a. Exhaustion (It involves the exhaustion of the dye onto the fibre in the presence of electrolyte).
b. Fixation (Addition of alkali to provide the chemical reaction between dye and cellulose).
c. Washing (Last but not the least, removal of unfixed & hydrolysed dye from the material in order to achieve the desired results).
These major factors are considered in order to optimise the process, which need to be controlled and thoroughly monitored when aiming for reproducibility.
· Quality of water supply
· Preparation of substrate
· Dyeability of substrate
· Weight of substrate
· Weighing of dyes and chemicals
· Selection of dyes (strength / hue)
· Moisture content of substrate
· Dye bath additives
· Liquor ratio
· pH of the dye bath
· Machine flow and sequence
· Time and temperature profile

The aforementioned parameters play an important role for Right First Time dyeing concept.
Usually, at first, test dyeing for colour matching is conducted in a laboratory, and the data thus gained is important for reproducibility in practical dyeing because it is used to decide the practical dyeing conditions. It is difficult to perfectly reproduce the dyeing conditions of the laboratory in practice, but it is important to try to get them as close as possible.

Firstly, the following points should be addressed.

1. Due to mechanical restrictions, matching the liquor ratios and the agitation speeds of the dye baths can be difficult, but the liquor ratios should be made as close as possible.
2. The material to be dyed should be the same as that dyed in the laboratory.
3. It is advisible that the test in the laboratory should be conducted with same lot of dyes as what is being used in production, particularly the light shades.
4. Good housekeeping in laboratory for dyes storage in order to prevent them from extreme conditions of heat and moisture is also crucial.
5. The dyeing auxiliaries that are used should be consistent and same as what being used in bulk.
6. The quality of water should be same for both parameters.
7. The temperature control of the dyeing machines should be accurate in laboratory

Colour matching equipments make real ease towards the above concept. The results of the laboratory dyeing and the practical dyeing can be measured and a coefficient for the degree of difference each time the dye is used can be calculated. If this coefficient is not constant, the reproducibility of results just within the laboratory or in the practical environment should be verified before the reproducibility of results between the laboratory and the factory is considered. If this coefficient shows a constant trend for all of the dyestuff, it can be used as a correction coefficient and crossed with the laboratory results in order to decide the practical dyeing conditions.

In the case of certain dyes that show large deviation, a dye may have bad compatibility, and a change of dye class should be considered.

Most of the exhaust dyers prefer the bifunctional and vinyl sulphone type of dyes. However, HE (High Exhaust) dyes have recently made their entrance due to various advantages over the conventional dyeing system. Nevertheless, hetro-bifunctional and vinyl sulphone still dominates the cellulosic dyeing in exhaust system.

General characteristics of reactive vinyl sulphone dyes

The dyes were first introduced by Hoechst, based on ß-sulphato ethyl sulphone as a reactive group. The range made the successful breakthrough due to comprehensive shade range, versatile application, ability to be dyed at 60°C and good compatibility with other bifunctional dyes. Being a compact structure, it offers ease in wash off for removal of unfixed dyes at the end of dyeing during washing. The dye fibre bond offers good acid stability. With the limitations to above, there may be problems with reproducibility associated with it, which is not accepted especially for pale shades. There is the possibility of unlevel dyeing with a small variation in the process due to its lower molecular weight.
General Characteristics of reactive hetro bifunctional dyes

Hetro bifunctional dyes made a get through in early 80's. The dyes were based on a quite simple but beautiful chemistry. They had filled a big vacuum of various hues of bright shades, which were not possible with vinyl sulphone type. The dyes were conventional Dichlorotriazine type, with the end condensation of ß-sulphato ethyl sulphone to have different type of reactive group in a single molecule. Being a dual reactive, the primary mode of fixation is from vinyl sulphone group and the dyeing temperature is same as vinyl-sulphone, which enables good intercompatibility in between the two classes.

They offer enhanced reproducibility especially for medium to pale shades. Due to triazine ring system, they offer level dyeing performance for even non critical substrates. In addition, they have high fastness profile especially the washing characteristics. Feasibility of price and application had made this class dominant for cellulosic dyeing in exhaust, especially for the knitted goods.
Fixation temperature sub optimises the fixation of both vinyl-sulphone and monochlorotriaze reactive groups. Moreover, there is a problem of rapid of fixation. 50% fixation takes place within 3 minutes of the alkali addition. Their medium exhaustive characteristic relates to long wash off sequence for heavy shades.

General Characteristics of reactive HE dyes
This class was specifically engineered for exhaust dyeing system. Two individual reactive group of monochlorotriazine in a single dye molecule enhance the exhaustion characteristic. In particular the HE dyes are most preferred choice for yarn dyers in package dyeing system. In general it offers greater benefits in application over the previously discussed classes. The only shady deal associated with this class is, its heavier molecular size. Though it offers greater exhaustion, at the same time it is very intricate to remove the unfixed and the hydrolysed dye at the end of dyeing system. Nevertheless, its advantages overweigh to look into for the selection criteria of process optimisation.

Most of the HE dyes offer unique dyeing profiles like,
· Controlled rate of primary exhaustion in neutral alkali.
· High migration and diffusion properties.
· Controlled rates of secondary exhaustion after addition of alkali.

Conclusion

Processing of textiles in today's world is getting more and more complex with increasing quality standards and finishing requirements. Since dyeing is a major component in the manufacturing of value added textiles, it has to keep pace with the modern technology and process optimisation. Tremendous research has been done in this area to develop a novel dyeing technique for cellulosic goods in exhaust and is still an ongoing progress. Various pros and cons factors are discussed above for three different classes of reactive dyes, presently being used in exhaust dyeing.

Theoretically, the vinyl sulphone dyes are superb to use for dark to extra dark shades. Hetro bifunctional are ideally suitable when considering medium to pale shades. Whereas HE and HE Supra (modified HE) are best suited for pale and some typical shades like Olive and Khaki in order to be consistent in production. However; its entirely depends on the dye-house to dye-house for selection criteria of above classes and should be considered according to the process optimisation in order to reduce the cost of processing while maximising the productivity in effect to energy and water savings.

References

1. D. Farrinton, JSDC, 105 (1989).
2. S.Glover, P.S. Collin Shaw & R.F. Hyde JSDC, 107 (1991).
3. M. Bradbury, P Collishaw & S.Moorhouse, JSDC 111 (1995).
4. V.R Kanetkar, Colourage, (Dec 2000).
5. Textile Processing, Colourage, (Aug 2002).