April 24, 2008

Fibre-Reactive Dyes - A Practical dyer's Guide to reactive Dyeing - Part-2

What are Fibre-Reactive Dyes?
A fibre-reactive dye will form a covalent bond with the appropriate textile functionality. This is of great interest, since, once attached, they are very difficult to remove.
Early fibre-reactive dyes
The first fibre-reactive dyes were designed for cellulose fibres, and they are still used mostly in this way. There are also commercially available fibre-reactive dyes for protein and polyamide fibres. In theory, fibre-reactive dyes have been developed for other fibres, but these are not yet practical commercially.
Although fibre-reactive dyes have been a goal for quite some time, the breakthrough came fairly late, in 1954. Prior to then, attempts to react the dye and fibres involved harsh conditions that often resulted in degradation of the textile.
The first fibre-reactive dyes contained the 1,3-5-triazinyl group, and were shown by Rattee and Stephen to react with cellulose in mild alkali solution. No significant fibre degradation occurred. ICI launched a range of dyes based on this chemistry, called the Procion dyes. This new range was superior in every way to vat and direct dyes, having excellent wash fastness and a wide range of brilliant colours. Procion dyes could also be applied in batches, or continuously.
The general structure of a fibre-reactive dye is shown below:

Note the four different components of the dye
The chromogen is as mentioned before (azo, carbonyl or phthalocyanine class).
The water solubilising group (ionic groups, often sulphonate salts), which has the expected effect of improving the solubility, since reactive dyes must be in solution for application to fibres. This means that reactive dyes are not unlike acid dyes in nature.
The bridging group links the chromogen and the fibre-reactive group. Frequently the bridging group is an amino, -NH-, group. This is usually for convenience rather than for any specific purpose.
The fibre-reactive group is the only part of the molecule able to react with the fibre. The different types of fibre-reactive group will be discussed below.
A cellulose polymer has hydroxy functional groups, and it is these that the reactive dyes utilise as nucleophiles. Under alkali conditions, the cellulose-OH groups are encouraged to deprotonate to give cellulose-O- groups. These can then attack electron-poor regions of the fibre-reactive group, and perform either aromatic nucleophilic substitution to aromatics or nucleophilic addition to alkenes.
Nucleophilic substitution
Aromatic rings are electronically very stable, and will attempt to retain this. This means that instead of the nucleophilic addition that occurs with alkenes, they undergo nucleophilic substitution, and keep the favorable p-electron system. However, nucleophilic substitutions are not very common on aromatics, given their already high electron density. To encourage nucleophilic substitution, groups can be added to the aromatic ring which will decrease the electron density at a position and facilitate attack.

For example:
But this requires harsh conditions. To improve the rate under mild conditions, powerful electron-withdrawing groups such as -NO2 may be added.

However, this will only work if there is a good leaving group, such as -Cl or -N2.
The major fibre-reactive group which reacts this way contains six-membered, heterocyclic, aromatic rings, with halogen substituents. For example, the Procion dye2: (This is the same as the chime molecule at the top of the page)

Where X = Cl, NHR, OR. Nucleophilic substitution is facilitated by the electron withdrawing properties of the aromatic nitrogens, and the chlorine, and the anionic intermediate is resonance stabilized as well. This resonance means that the negative charge is delocalised onto the electronegative nitrogens:

One problem is that instead of reacting with the -OH groups on the cellulose, the fibre-reactive group may react with the HO- ions in the alkali solution and become hydrolyzed. The two reactions compete, and this unfavourable because the hydrolyzed dye cannot react further. This must be washed out of the fabric before use, to prevent any leakage of dye, and not only increases the cost of the textile, but adds to possible environmental damage from contaminated water.
Nucleophilic addition
Alkenes are quite reactive due to the electron-rich p-bond. They normally undergo electrophilic addition reactions. Again, nucleophilic additions are less favored generally, because of the repulsion between the Nu- and the electron-rich p-bond. However, they will occur if there are sufficient electron withdrawing groups are attached to the alkene, much as before, with aromatic substitution. In this case, the process is known as Michael addition or Conjugate addition.
For this reaction type, the most important dye class is the Remazol reactive dye. This dye type reacts in the presence of a base such as HO-. The mechanism for the reaction of one of these dyes is shown below:
As before, the intermediate is resonance stabilized, but this has not been shown.

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