Molecules of Basic dyes are invariably organic cations and they are preferably called cationic dyes. They usually have brilliant colors and high tinctorial strength; some are even fluorescent.
Many basic dyes are now obsolete because of their very poor light fastness on natural fibers but a few are still used for dyeing paper and leather and for making inks.
Cationic dyes will dye fibers with anionic sites by a process of ion exchange. This is usually a simple direct dyeing process.
Anionic auxiliary products must be avoided as they may precipitate cationic dyes in the form of an organic salt. In dyeing protein fibers with cationic dyes, acids retard dye absorption by suppressing the dissociation of the anionic carboxylate groups in the fibers, thus making the fiber more cationic and inhibiting adsorption of dye cations.
Cationic dyes have very low substantivity for cotton unless excessive oxidation has generated anionic carboxylate groups. For dyeing cotton with cationic dyes, the cotton was usually mordanted with tannic acid fixed with tartar emetic. The insoluble, anionic tannin attracts colored dye cations, just as it repels dye anions when on a nylon surface after back-tanning.
“Basic dyes” – Why so called?
Basic dyes contain chromophore which is basic or cationic in nature. When they are made soluble in water, they act like base that forms colored cationic salt and react with the anionic site of the substrate.
As they contain basic or cationic chromophores in their structure, they are called Basic dyes.
These are cationic dyes
These dyes can produce bright shade
They have high tinctorial value
They don’t have any affinity for cotton
But cotton can be printed or dyed only by mordanting before the coloration process.
They are suitable for acrylic, nylon, modified polyester or jute materials.
In the presence of glacial acetic acid, their solubility is very good in water.
They have high color strike, therefore exhibits poor levelling properties.
They exhibits good wet fastness
Light fastness is poor to moderate
Dyeing With Basic Dye
Today, the major use of cationic dyes is in dyeing anionic acrylic and modacrylic fibres and, to a much lesser extent, modified polyester and nylon.
Dye adsorption by acrylic fibers involves interaction between anionic sulphonate and sulphate polymer end groups and the cationic dye molecules.
This results from the co-polymerization or the residues of anionic polymerization inhibitors.
This anionic property makes the acrylic fiber suitable to dye with basic dye by strong ionic bond.
Chemical Structure of Basic dye
Cationic dyes belong to a variety of different chemical classes. Many of the older basic dyes were di- and triphenylmethane, heterocyclic azine, oxazine or polymethine, or aminoazo compounds.
In many cases, the cationic charge is extensively delocalized over the entire dye molecule. Typical anionic counter ions include chloride, oxalate (C2O4 2–) and tetrachlorozincate (ZnCl4 2–) ions.
Since the introduction of acrylic fibers in the 1940s, new cationic dyes have been developed specifically for dyeing these fibers. Many of these have quaternary ammonium groups with a localized cationic charge that is not part of the chromophore.
Dyeing of Acrylic with Basic Dye
Nowadays, basic dye is mainly used for dyeing acrylic and mod acrylic. Jute, modified polyester, nylon is also dyed with basic dyes in a little extent.
Preparation of materials
The preparation of acrylic fiber materials may involve desizing of woven materials, scouring and bleaching.
Combined desizing and scouring are often possible since relatively soluble sizing materials such as modified starch or polyvinyl alcohol are normally used.
Scouring with weakly alkaline solutions of ammonia or trisodium pyrophosphate (Na3HP2O7) is carried out commonly.
A non-ionic detergent is essential. Cationic auxiliary products may have substantivity for the anionic groups in the fibers and block dyeing sites whereas residues of anionic products will interact with and even precipitate the cationic dyes in the bath.
Acrylic materials sometimes have a slight yellow cast, usually a sign that drying was too severe. Bleaching is possible with sodium chlorite (NaClO2) and formic acid or brightening with a fluorescent whitening agent. Some fluorescent whitening agents can be used in the presence of sodium chlorite.
Some cationic dyes are very sensitive to traces of chlorine and will rapidly fade giving poor color yields, particularly when dyeing pale shades. An anti-chloro treatment of fabric bleached with sodium chlorite may be necessary and small additions of sodium bisulphite or thiosulphate to the dyebath will avoid problems with cationic dyes sensitive to traces of chlorine in municipal water.
Dye Bath Preparation
The dye powder is usually pasted with acetic acid and then mixed with boiling water.
Cationic dyes with delocalized cationic charges are intensely colored and it is essential to avoid dust escaping from the powders. Concentrated liquid dyes avoid this problem.
Solid forms of these dyes are often not easy to dissolve because of their tendency to form gummy material.
Preparation of a paste with methanol and addition of warm or hot water is sometimes a useful alternative.
Acidic pH is maintained for dye bath preparation.
Why choose acidic medium for dye bath preparation?
Some cationic dyes are not stable in boiling water.
Many react with alkali to give colorless products such as the free amine from neutralization of an ammonium ion group, or a carbinol by reaction of the cationic group with hydroxide ion.
Dyeing with cationic dyes therefore invariably takes place in weakly acidic solution to avoid these problems.
Acrylic fibers may contain a variety of different anionic groups. These include a limited number of terminal sulphate and sulphonate groups arising from the persulphate polymerisation initiator.
In other types, there may be appreciable numbers of carboxylate groups from acrylic acid or similar co-monomers added to the acrylonitrile before polymerization. These anionic groups are responsible for the substantivity of cationic dyes for such fibers.
Dyeing acrylic fibers with cationic dyes is carried out in weakly acidic solution containing acetic acid and sodium acetate and a nonionic wetting agent and dispersant.
A small amount of sequestrant ensures that heavy metals do not interfere with chemicals in the solution
The dye solution is often prepared by pasting with acetic acid, and a stable pH of 4.5 to 5.5 can be obtained by addition of sodium acetate to buffer the solution.
Level dyeing requires strict control of the pH and dyeing temperature, as well as the use of cationic retarding agents. The latter initially block the anionic sites in the fiber and are gradually replaced by the more substantive dye cations.
An addition of up to 2.5 g/l of anhydrous sodium sulphate helps to offset the negative surface charge and sodium ions weakly block anionic sites in the fiber.
If dye additions are needed to give the correct shade, the bath temperature is first slowly reduced to below 80 °C.
As Acrylic materials are quite thermoplastic, they easily form crack marks and creases and texturized acrylic filaments also readily lose their characteristic bulk.
After dyeing is complete, the bath is slowly cooled to 50–60 °C to avoid these problems. Rapid cooling by addition of cold water to the dyebath can be disastrous as it causes immediate setting of creases in the goods.
The material is finally rinsed, and possibly given a mild scour with a non-ionic detergent and a little acetic acid plus a softening agent.
Basic dye is a good cheap alternative while dyeing acrylic or mod acrylic as Disperse dye don’t build up well on acrylic. They are used only for pale shades.
Salma Hasin Shila, the author of this site completed her BSc. in Textile Engineering (Wet Processing Engineering) from Bangladesh University of Textiles www.butex.edu.bd (BUTEX). She has a passion for textile technology and love to write about it. She wants to share her knowledge to help the students in their studies and businessman & entrepreneurs in their business in making wise decisions fast.