Basic/Cationic Dyes | Dyeing of Acrylic with Basic Dyes | Stripping of Basic Dyes

Last Updated on 08/09/2021

What is Basic / Cationic Dyes?
Basic dyes are also called cationic dyes because in solution the basic dye molecule ionizes, causing its colored component to become an action of positively charged radical. This a class of synthetic dyes, that act as bases and when made soluble in water, they form a colored cationic salt, which can react with the anionic sites on the surface of the substrate. The basic dyes produce bright shades with high tinctorial values, on textile materials. The major application of basic dyes is in dyeing of acrylic fibers that have been copolymerised in combination with anionic copolymers.

Dyeing of Acrylic with Basic Dyes

The first coal-tar dye was a so-called basic dye. It was developed to give many bright shades for silk and wool. The chemical agent that binds the dye to a fiber, which otherwise has little or no affinity for the dye, is known as a mordant. Basic dyes like Methyl violet, Auramine, Chrysodine, Methylene Blue, Magenta, Malachite Green and Rhodamine are used for dyeing Acrylic fibers in Hank form.

Basic dyes are mainly applied the following type of fibers:

  1. Acidic types of acrylic fibers
  2. Mordanted cotton
  3. Cationic dyeable polyester fiber (CDP)

Though basic dyes produce attractive, bright and highly intensive dyes, but their fastness to light and wet on mordanted cotton, protein fibers and acrylic fibers are very low.

Dyeing of Acrylic Fiber with Basic Dyes:
Acrylic fiber has become the third largest produced synthetic fiber in the world. Regular acrylic fibers are produced from polymers containing 85% or more of acrylonitrile monomer, the other co-monomers usually being methacrylic acid, methylmethacrylate, vinyl acetate or a similar vinyl compound which are incorporated in order to improve dyeability and mechanical properties.

Initially, acrylic fibers were very difficult to dye and, until acidic groups were incorporated as dye sites in the fiber, basic dyestuffs were of little interest. Cationic dyes are currently used in large quantities to dye acrylics (Orlon, Acrilan, Creslan, Zefran) and modified acrylics (Verel). Subsequent developments led to the introduction of acidic groups to polyester and polyamide fibers, further increasing the market for these dyes.

Basic dyes are water-soluble and dissociate into anions and coloured cations. The cations have a strong affinity for the acidic group (sulfonic or carboxylic) and form salts. Because of these strong bonds, washfastness is usually outstanding, and lightfastness varies considerably, depending on the dyestuff. Basic dyes are usually applied in a batch process with skein, stock, and package dyeing more prevalent than piece dyeing on becks or jet dyeing machine.

The fundamental steps in the acrylic dying mechanism are: absorption of the basic dyestuff at the fiber surface, which occurs only when the glass transition point of the fiber is exceeded; diffusion of the dye into the fiber as the fiber molecules acquire enough energy to move; formation of the dye-fiber bond; and migration of dyestuff from the dyesite to another or from within the fiber to the surface. The degree of migration varies considerably, depending on the dyestuff, but generally basic dyes do not migrate well.

Leveling of basic dyestuffs can be a major problem which often can be traced partially to differing exhaustion rates of individual dyestuffs which may occur in combination shades. Every basic dyeable fiber has a saturation value, i.e, it has given number of acidic dyesites, which limit the quantity of basic dyestuff that can be fixed. The dyestuff also has a saturation factor, which is a measure of the relative molecular weight per caution in the dye. These two values determine the depth of shade obtainable with a given dye and fiber.

Acrylic fibers also vary considerably in their rate of dyeing, depending on whether they are of a dry-spun or a wet-spun fiber. Wetspun fibers dye at a higher rate than dry spun fibers.

To overcome the high affinity of some basic dyestuffs and to prevent unlevel dyeings, often a cautionic retarder is used. Cationicdyeable polyester has achieved a moderat3e degree of success because it lends itself to two-or three colour effects in blends with regular dispensedyeable polyester, cellulosic fibers, or wool. Typical basic-dyeable polyester fibers are Dacron T-64 and T-92, Fortrel 402, and Trevira 640.

The dyeing rate of cationic-dyeable polyester is much lower than that of acrylic; however, an even greater difference exists in the diffusibility associated with each, which is estimated for the cationhicdyeable polyester to be only 10% of that for acrylics. This has to be overcome by dyeing at higher temperatures (110-130o C) and by using a carrier to improve penetration. Another use for the carrier is the prevention of cross-staining the disperse-dyeable protion of the blend by the basic dyestuffs.

Nonionic products must be present to act as antiprecipitants or to suspend particles resulting from the reaction between the cationic dyes and anionic dyestuffs when blends of basic-dyeable and disperse-dyeable polyester are dyed.

The most common anionic group attached to acrylic polymers is the sulphonate group, -SO3-, closely followed by the carboxylate group, -CO2-. These are either introduced as a result of co-polymerisation, or as the residues of anionic polymerisation inhibitors. It is this anionic property which makes acrylics suitable for dyeing with cationic dyes, since there will be a strong ionic interaction between dye and polymer (in effect, the opposite of the acid dye-protein fiber interaction).

Dyeings on polyacrylic fibers can be obtained with high color depth, high brilliance and good fastness. The cationic dyes can be applied in exhaust processes as well as in continuous dyeing processes. In exhaust dyeing careful temperature control in the range of high dyestuff absorption rate is required.

A scheme of a polyacrylic dyeing is shown in below Figure.

Functional scheme of a dyeing with cationic dyes and retarders
Figure: Functional scheme of a dyeing with cationic dyes and retarders.

Retarders can be added to the dyebath to support levelling of a dyeing. Anionic retarders reversibly form soluble adducts with the dye in solution. Thus, the effective concentration of dyestuff cs in the concentration gradient for dyestuff adsorption is lowered and the rate of dyestuff sorption is reduced. The strength of the adduct must be lower than the binding strength between fiber and dyestuff; otherwise, the dye will not be released completely from the adduct and substantially lighter dyeings will be obtained. Condensation products between naphthalene sulphonic acid and formaldehyde are chemical representatives for anionic retarders.

Cationic retarders (e.g. quaternary ammonium compounds) compete for the adsorption and ionic binding at the fiber. Apparently, the number of binding sites on the fiber SF is reduced and the rate of dyestuff sorption is lowered. The cationic retarder thus acts like a colourless dye, which also blocks a share of the available binding sites. In the calculations of fiber saturation through dyestuff binding, the concentration of cationic retarders used has to be considered. In case the amount of retarder has been chosen to high, the dyestuff sorption will remain incomplete.

As an example, the dyeing can be started at 60 °C and heated up to 70–75 °C rapidly. The actual interval of temperature is dependent on the fiber type. Then very slow and well-controlled increase in temperature with a gradient of 0.25–0.5 °C/min is required to maintain rate of dyestuff sorption low. At the boil, the dyestuff absorption is completed within 20–60 min, based on concentration of dye and exact bath temperature.

Stripping of Basic Dyes:
Stripping and leveling: A degree of leveling can be achieved by treatment with a carefully calculated amount of cationic retarder at the boil for 1–2 hours. For some stripping up to 10% owg of a suitable anionic retarder can be used also at the boil. For severe or chemical stripping, the goods should be treated with sodium hypochlorite acidified to pH ca. 6 with acetic acid for 30 minutes at the boil. Sodium nitrate (1 to 4 g/L) is required to passivate the stainless steel of the vessel.

References:

  1. Textile Dyes by N. N. Mahapatra
  2. Handbook of Textile and Industrial Dyeing Volume 2: Applications of Dyes Edited by M. Clark
  3. Textile Dyes By Mansoor Iqbal
  4. Physico-chemical Aspects of Textile Coloration by Stephen M. Burkinshaw
  5. Textile Chemistry by Thomas Bechtold, Tung Pham
  6. Textile Engineering-An Introduction Edited by Yasir Nawab
  7. An Introduction to Textile Coloration: Principles and Practice By Roger H. Wardman

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