Presentation on: Mordant & Mordant dye

Prepared By:Tanvir Ahammed Rana

Id. 201200040003819th Batch, Textile Department.

SouthEast University.

Mordant

A mordant is a substance used to set dyes on fabrics or tissue sections by forming a coordination complex with the dye which then attaches to the fabric or tissue. It may be used for dyeing fabrics, or for intensifying stains in cell or tissue preparations. The term mordant comes from the present participle of French mordre, "to bite". In the past, it was thought that a mordant helped the dye bite onto the fiber so that it would hold fast during washing. A mordant is often a polyvalent metal ion. The resulting coordination complex of dye and ion is colloidal and can be either acidic or alkaline.

Common dye Mordants

Mordants include tannic acid, alum, urine, chrome alum, sodium chloride, and certain salts of aluminum, chromium, copper, iron, iodine, potassium, sodium, and tin.

Iodine is often referred to as a mordant in Gram stains, but is in fact a trapping agent.

Dyeing methods

The three methods used for mordanting are:i. Pre-mordanting (onchrome): The substrate is treated with the mordant and then dyed.ii. Meta-mordanting (metachrome): The mordant is added in the dye bath itself.iii. Post-mordanting (afterchrome): The dyed material is treated with a mordant.

The type of mordant used changes the shade obtained after dyeing and also affects the fastness property of the dye. The application of mordant, either pre-mordant, meta-mordant or post-mordant methods, is influenced by:i. The action of the mordant on the substrate: if the mordant and dye methods are harsh (for example, an acidic mordant with an acidic dye), pre-mordanting or post-mordanting limits the potential for damage to the substrate.ii. The stability of the mordant or dye lake or both: the formation of a stable dye lake means that the mordant can be added in the dye without risk of losing the dye properties (meta-mordanting).

Mordant dyeA dyestuff that requires the use of a mordant. There are very few synthetic dyestuffs currently in use that require a separate mordant, except for some dyes for wool, where mordant dyes are still quite popular. Since chromium is almost exclusively used as the mordant on wool, chrome dye has become essentially synonymous with mordant dye. Many natural dyes require a mordant. The mordant used can significantly influence the hue produced with a particular dyestuff. These are special acid dyes in which certain metal atom can be introduced during dyeing. These are water soluble dyes and affinity for silk, wool and polyamides. Mordant dyes requires a mordant in their application and these dyes upon combination with the mordant deposit on the fiber in the form of insoluble color. Most commonly dyes have hydroxyl or carboxyl groups and are negatively charged in nature.

Properties of Mordant dyes1. These dyes are economical dyes and are generally used to produce dark shades such as dark greens, dark blues and blacks.2. These dyes have good leveling and color fastness properties.3. The interaction between fiber and dye is established through very strong ionic bonds, which are formed between the anionic groups of the colorant and ammonium cations on the fiber. Chromium or the metal ion acts as bridge between the dye and fiber, which gives rise to a very strong linkage, resulting into excellent fastness properties.However there are disadvantages of the chrome dyes also such as longer dyeing cycles, difficulties in shading, risk of chemical damage to the fiber and the potential release of chromium in the waste water.

Mechanism of dyeing

Since these are a special class of acid dyes , which are soluble in water and applied to the fiber from an acidic bath. When a solution of an acid mordant dye is mixed with a solution of potassium dichromate in the presence of sulfuric acid , chromium ion from dichromate forms a complex with the dyes , this complex is insoluble in water ,and hence precipitates on the fiber.

There are three different methods of application of chrome dyes on the fiber:1. Chrome – Mordant method2. After chrome method3. Meta chrome method

Application of Mordant dyes

1. Chrome-Mordant ProcessIn chrome mordant process, the fiber is first treated with potassium dichromate in the neutral bath or in the presence of either sulfuric or formic or oxalic acid. When sufficient amount of chromium is taken up by fiber , it is taken out , squeezed and entered in the dye bath containing acid mordant dye. The dye form an insoluble complex with chrome present on the fiber .2. After Chrome MethodIn this method the substrate is first treated with the dye , the dye is exhausted by the addition of an acid , and after complete exhaustion the material is taken out squeezed and then run in a solution containing potassium dichromate and an acid. Metal dye complex is formed on the fiber , which is insoluble.

3. Meta Chrome ProcessThis is a single bath process , in which the material is treated with in a bath containing acid mordant dye , potassium dichromate and ammonium Sulphate.The dye along with potassium dichromate and ammonium Sulphate got absorbed by the fiber and evenly distributed but no complex is formed because the pH is not suitable for the chemical reaction to take place.In the second step of the meta chrome process, when the dye bath is heated , ammonium sulphate is converts into ammonia and Sulphuric acid , which makes the bath strongly acidic and potassium dichromate in the presence of strong acid now react with the dye molecule forming an insoluble complex on the fiber.

A dyeing cycle for dyeing of wool with chrome dyes

Dyeing cycle of Wool

Here, A= Glauber salt+Acetic acid+Leveling agentB= DyeC= Potassium DichromateD=Acid