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.

April 20, 2008

A Practical Dyer's Guide to Reactive Dyeing of Cotton - Part-1

Reactive dyes are probably the most popular class of dyes to produce 'fast dyestings' on piece goods. These were first introduced a little over 40 years based on a principle which has not been used before. These dyes react with fibre forming a direct chemical linkage which os not easily broken.
Their low cost, ease of application, the bright shades produced by them coupled with good wash fastness make them very popular with piece good dyers. Even in thereads these classes are gaining in popularity for cotton sewings.

Chemistry of reactive dyes:
Reactive dyes differ from other colouring matters in that they enter in to chemical reaction with the fibre during the dyeing process and so become a part of the fibre substances.
A reactive dye may be represented as
R - B - X
where R - Chromogen
B - Bridging group
X - Reactive system
When this reacts with the fibre, F, it forms
R - B - X - F
The wet fastness of the dyesings produced depends on the stability of the true covalent bond X-F.

Reactive Systems:
Some of the popupar reactive systems is use today:
Reactive dyes are based on Cyanuryl chloride. The cold brand dyes (M brand) are based on dichloro triazinyl derivatives whereas the "H" brands are monochloro triazinyle derivatives.
The reactivity of the chlorine atoms decreases greatly as they are successively substituted. Thus the dichloride derivative (M) is more reactive than the mono chloro reactive (H) dyes. This is shown by the fact that "M" dyes will react readily with cellulose at room temperatire in the presence of mild alkalies such as sodium carbonate, whwere as "H" dyes need to be heated at least to 60°C and require more strongly alkalines before reaction will take place at a reasonable rate.
The other popular systems are based on Vinyl suplhones (Remazols) and trichloro pyrimidyl. The Remazols are very popular for discharge printing and can given excellant white discharge from a dark base.
Reference: Textile Processing Guide

Color Fastness - What does it mean?

In order to clarify what fastness means, we shall examine various fastness properties that a thread or yarn may be required to have. We shall study how grades of fastness are established and what they signify and finally we should take a look at what fastness you can expect from various classes of dyestuff on the major substrates of interest to us.

Fastness Properties:

The most desirable fastness properties in any thread or yarn is arguably of wash fastness.

In general, the dyers used to carry out two main wash fastness tests, viz.,

Test-1: A length of coloured thread is plaited with a white partner and treated at 95°C in an alkaline soap solution for 30 minutes. The degree of staining on to the adjacent white thread ( which can be of one or more white substrates) is assessed as in the change of shade of the original colour. The test is commonly known as ISO4.

Test-2: As in Test-1 above but treatment is only at 60°C. The test is called ISO3.

Another popular fastness demand is to rubbing both wet and dry with the sample being hand or machine rubbed. Only the staining on the white calico test fabric is recorded.

Fastness to light is either carried out in sunlight ( a low method) or in an artificial Xenon lamp tester (much faster). Along with the test sample are eight blues of known light fastness which fades to the same degree as the sample gives the light fastness grading of the sample, only change of the shade is recorded.

Fastness to bleach, either peroxide or hypochlorite are severe tests where normally only change of shade need be recorded.

A less severe bleach type test is fastness to chlorinated water which is meant to represent the effect of swimming pool waters on textiles; usually for swim and beach wear garments.

Fastness to hot pressing at a wide range of temperatures with both change of shade and staining being relevant. Disperse dyes on polyester can sublime ( literally evaporate) on some severe permanent pleating processes and even at low iron heats many classes of dye will stain ( but may not do so on ISO3 wash tests) white fabric on contact.

There are some exotic fastness requirements like fastness to vulcanizing , a process used to cure rubber footwear or fastness to stone washing, a fickle process used to fade cotton denim jeans.

Fastness Grades:

Nearly all fastness properties are assessed on a scale of 1 to 5 with 5 being the best rating and 1 the worst. The Grey Scale I can be used to assess the change in shade. Staining scales are slightly different but the same usage principle applies.

Exceptionally, light fastness is measured on a scale of 1 to 8 with 8 being the best and 1 the worst.


Textile testing procedures