This article is being published in towo part. This is the first part

Introduction

The launch of the first fibre-reactive dyes for cellulose in 1956, a century after the development of the first synthetic dye, enabled new and high fastness properties to be obtained. Reactive dyes have the unique property that they are designed to bond covalently with the substrate on application. They are principally used for dyeing and printing of cellulose, have been used to a lesser extent on polyamides but are used for important outlets in the coloration of wool and silk. Their brilliance of shade and high fastness to washing makes this class of dye ideal for materials that are subjected to frequent wash-wear cycles, which has become increasingly important with changes in fashion and washing practices. It is the aim of this paper to discuss the history, development and exploitation of reactive dyes together with the benefits that they have achieved, with a clear emphasis on practical coloration techniques.  It is not intended to cover the chemistry of reactive dyes in detail since this is extensively discussed elsewhere. The early patent literature and dye chemistry has been discussed 1 and more recent books covering dye chemistry have been published 2-5 whilst a number of the references in this paper are also relevant.

Historically, the development of reactive dyes can be considered to have started in the 1880s 5-7 but consisted of isolated investigations and none of these early investigations can be classed as failures since they did not start with the objective of producing fibre-reactive dyes. Naturally occurring dyes and the early products of the synthetic-dye industry were largely unsuitable for cellulosic coloration without the use of mordants. The investigations of Böttiger into the properties required for dyeing cellulose directly resulted in the so-called 'direct cotton dyes' in the 1880s, which had substantivity for cotton and slow diffusion properties. Following Böttiger's success with direct dyes, other methods were developed for the production of dyes together with related application processes for the successful dyeing of cellulosics. However, most of these commercial dye ranges had some limitation in the colour gamut possible and most of these dye types, including direct, vat and azoics, were developed particularly for small batch, low-productivity exhaust dyeing methods 6. Reactive dyes gave a much enlarged colour gamut and properties that were ideal for continuous application methods. This resulted in major developments in continuous dyeing in the thirty years following the launch of reactive dyes. Rattee 7 compared the requirements of dyes for exhaust and continuous dyeing of cellulose, as shown in Table 1. Methods of application for reactive dyes to cellulosics by exhaust and continuous methods were developed 5-7.

Reactive dye development and marketing

The successful investigation started by Rattee and Stephen in 1953, which resulted in a viable industrial method in which a covalent link was formed between the dye and the fibre, was another isolated investigation started without reference to earlier work. It was only the success of this latter investigation that placed the earlier studies in context. The characteristic features of a typical reactive dye molecule include those shown in Table 2 8.

All the important classes of chromogen have been included in reactive dye structures. The sulphated ester of the vinylsulphone reactive group contributes significantly to aqueous solubility. The nature of the bridging links, especially in dyes of the haloheterocyclic type, generally influences the reactivity and other dyeing characteristics of such dyes. The structure of the reactive grouping and substituents attached to it is decisive with regard to the chemical stability of the dye-fibre bond that is formed.

In 1949, Hoechst patented the first precursors of vinylsulphone dyes and two of these were introduced as Remalan dyes for wool in 1952. A trichromatic combination of Procion, dichlorotriazine

dyes, was marketed by ICI in 1956. The less reactive monochlorotriazine derivatives were introduced as Procion H (and Cibacron - by Ciba) dyes a year later. Table 3 shows the activity of the major dye-makers in marketing reactive dye systems between 1952 and 1971 9. These were mainly intended for reaction with cellulose and were of two general types: either precursor variations of unsaturated systems such as vinylsulphone and acrylamide, or nitrogenous heterocyclic rings bearing active chloro substituents. During the 1970s, several companies introduced difluoropyrimidine dyes and ICI developed Procion H-E dyes, the first bifunctional reactive dyes.

Numerous factors, other than the chemistry of the dye, must be considered in designing reactive dyes of commercial interest, some of the more important being listed in Table 4 6, 8.

In effect, only relatively few reactive systems or speculative dye ranges have met these requirements to become commercially established in a significant segment of the reactive dye market, about seven reactive systems and nine commercial dye ranges accounting for a large proportion of the market 8.

Many of these were early entries into the market and have remained successful. Despite continued R& D and high levels of patent activity, few new reactive dye ranges became commercially successful. The relative reactivity of the market leaders has been defined in relation to the temperature required for exhaust dyeing (Figure 1).

Reactive dye developments from 1970  A major development from the 1970s onwards was bifunctional reactive dyes. A highly successful range of high-fixation, bifunctional dyes containing two monochlorotriazine groups per molecule were introduced by ICI in the 1970s, raising fixation for a one reactive group analogue from typically 60% to 80% for the bifunctional dyes. Bifunctional reactive systems containing two different reactive groups also became important for exhaust dyeing, since fixation is relatively insensitive to variations in dyeing temperature. In view of the increasing and remaining importance of polyester/cellulosic blends, an important contribution from the Procion H-E dyes was the New Select process, a one-bath, two-stage reactive/disperse dyeing process 10.

Other advantages of mixed bifunctional dyes include application over a wider temperature range with improved reproducibility, easier washing-off and improved fastness. The Sumifix Supra (Sumitomo) dyes are mixed bifunctional dyes, containing two dissimilar reactive groups 10.  Cibacron C (Ciba-Geigy - now Huntsman) are mixed bifunctional dyes, particularly for padding applications, based on MFT and vinylsulphone chemistry. By 2000 11, rationalisation in the dye-making and dye-using industries worldwide had commenced and this continued apace as has been extensively documented 12. Although patent activity still remained high, few new products reached commercial exploitation. Manufacturers followed the trend to evaluate heterobifunctional structures containing two dissimilar reactive groups. Structures containing three reactive groups appeared and homobifunctional products were introduced. Specific research targets included improvement in environmental performance, fastness to light or detergent resistance, improving dyehouse efficiency and right-first-time (RFT) performance. Some improvement in dye fixation was obtained by the five Procion XL+ dyes launched by BASF (now DyStar) in 1999, when applied at 90°C, these being bihomofunctional dyes based on monochlorotriazine. The Cibacron LS (Ciba-Geigy) range, which requires less salt for application, includes bis-monofluorotriazine dyes. Procion H-EXL dyes are based on bis-monochlorotriazine. A novel trichromatic combination of vinylsulphone dyes, Remazol Yellow RR, Red RR and Blue RR, was introduced by DyStar, supplied as low-dusting granules to improve safety in handling and suitability for automatic dosing. These dyes give a high degree of reproducibility at 60°C with high fixation and consistent washing-off behaviour together with a high level of light fastness. About 50% of leisurewear colours can be achieved with this combination 13 and it is also suitable for dyeing polyester/cellulosic blends 14.

The various ranges of commercially important reactive dyes have been discussed in some detail 8 and the relatively few reactive systems that have reached the stage of commercial success are summarised in Table 5 8. Since the bis-monochlorotriazine (Procion H-E) and vinylsulphone dyes were amongst early developments, they also became patent-free at an earlier stage and were thus copied by many other manufacturers, resulting in price erosion. However, some other commercially important ranges of reactive dyes still in use were developed during this period.

Despite the availability of suitable and successful ranges of reactive dyes plus the contraction of the industry, R& D on reactive dyes continues, albeit on a reduced scale. Recent developments have been aimed at achieving high levels of fixation and sustainable technology 15. These include the Avitera SE tri-reactive dyes (Huntsman) 16 which, when combined with the clearing additive, Eriopon ET, can be applied at 60°C, with a saving of 50% in water and energy. This is assisted by the high solubility of the dyes, allowing low liquor ratios to be employed.

Physical form, storage and handling

Environment concerns, initially associated with the inhalation of reactive dye powders 17, led to the development of physical forms of dyes to minimise dusting. Low-dusting granular and powder forms containing dedusting agents became available. Dusting problems can be eliminated entirely if liquid brands are selected. Cold-dissolving granules, the Drimarene CDG dyes, are supplied by Clariant. The supply of dyes in the long supply chain from non-traditional suppliers (NTS) with the associated long distances involved in transport means that the effects of dedusting agents are lost due to settling or adsorption onto the dye particles. Retreatmenr near the point of use has often been found necessary.

Storage of dye powders in a moist atmosphere can lead to caking but variations in the moisture content of the dye has more serious implications for the accuracy of weighing and the ability to achieve consistent results and right-first-time (RFT) production 18. Modern dyestores are air-conditioned with double-door entry systems to maintain a suitable atmosphere. The storage and handling of dyes and chemicals has been reviewed 19, 20.

All reactive dyes tend to hydrolyse in the presence of moisture, especially the high-reactivity ranges, and they may deteriorate unless carefully handled and stored. Cool, dry conditions are essential and the lids of containers must be replaced firmly after use. Since reactive dyes in powder form may release dust when disturbed, it is always possible for respiratory allergies to arise with some workers who handle them. Suitable dust-excluding respirators should be used and weighing or dissolving procedures should be carried out in ventilated enclosures. Conventional reactive dye powders are usually dissolved either by:

- pasting with cold water followed by the speedy addition, with stirring, of the required amount of water at the correct temperature or

- sprinkling of a steady stream of dye powder into the vortex formed by rotary agitation of the required amount of water at the correct temperature

Although Remazol (DyStar) vinylsulphone dye powders are dissolved in boiling water followed by passing immediately through a fine sieve into the required amount of cold water, few ranges of reactive dyes require boiling water. Highly reactive dyes, including DCQ, DCT or DFP types, should be dissolved at a temperature no higher than 50°C. Most dyes of lower reactivity, such as the bis-MCT, MCT or TCP ranges, are usually dissolved at 80°C. For continuous dyeing or printing, especially where automated metering equipment is installed, liquids are particularly convenient.

A useful classification of reactive dye systems is based on three of the important controlling parameters in the reactive dyeing process 21:

- Group 1: Alkali-controllable reactive dyes

These dyes have optimal temperatures of fixation between 40 and 60°C. They are characterised by relatively low exhaustion in neutral salt solution before alkali is added for fixation. They have high reactivity and care in alkali addition is necessary to achieve level dyeing, preferably at a controlled dosing rate. Typical examples of dyes in this group include DCQ, DCT, DFP and VS reactive systems.

- Group 2: Salt-controllable reactive dyes

Dyes in this group show optimal fixation at a temperature between 80 and 100°C. Such dyes exhibit relatively high exhaustion at a neutral pH, so it is important to add the salt carefully to ensure level dyeing. Salt addition is often made portionwise or preferably at a controlled dosage rate. Dyes with these properties typically have low reactivity systems such as MCT, bis-MCT or TCP. The MFT and bis-MFT ranges of Huntsman have high substantivity and thus should thus be regarded as salt-controllable but they are sufficiently reactive for fixation at 60°C by batchwise application.

- Group 3: Temperature-controllable reactive dyes

This group is represented only by the Kayacelon React (KYK) range of bis-nicotinotriazine dyes. These react with cellulose at temperatures above the boil in the absence of alkali, although, if desired, they can be applied conventionally with alkali at 80°C. Dyes in this group have self-levelling characteristics and satisfactory results can be achieved by simply controlling the rate of temperature rise.

The importance of RFT processing 18 has already been mentioned and the Reactive Dye Compatibility Matrix (RCM) 8, 22- 23 allows the optimum dyeing profile and dye selection to be defined for the exhaust application of reactive dyes to cellulose in terms of five key criteria. The first dyes to be developed using this approach were the Procion H-EXL (DyStar) range in the early 1980s 24 with target and actual ranges of values for the five parameters being listed. A further useful measure of the degree of compatibility of a trichromatic combination of reactive dyes is provided by the so-called Robustness Index (RI) 8. This is a statistical indicator of the differences in chroma (  C) and hue angle (  H) produced by deliberate variations in liquor ratio, time and temperature of fixation from a control dyeing using standard conditions.