Acid dyes | Textile dyes | An introduction

ACID DYES

Introduction:

         Textile dyes are classified according to the textile material going to be dyed or colored.Say, cotton fiber,the garment or the fabric or the yarn made of cotton are dyed with Direct dyes or Reactive dyes.Polyester materials dyed with Dispersed dyes.Nylon materials are dyed with Acid dyes.Like that,textile materials made of different substances are required different chemical bondable or chemical structure dyes.The textile dyes selection are based on the textile fibers to be dyed.

          In this article,we tried to brief on acid dyes.What are acid dyes ?.Acid dyes are used to dye colour the textile material contain protein or polyamide.Examples of protein natural fibers are wool and silk.Wool contains protein substance called Keratin.Silk contain protein fiber Fibroin .And synthetic fiber nylon contains chemical substance Polyamide.

          Wool dyeing or nylon dyeing takesplace based on the affinity mechanism between acid dye and protein substance of wool fiber or polyamide substance of nylon fiber.Basically,acid dyes are of anionic electro charging in nature.They have the chemical sites of anion.As we studied in your previous academy,specially in high school level or 11th standard or 12th standard in chemistry.Anion and cation are the charged electronic sites.They are charged negative and positive respectively.These anion and cation are attracted to each other like north and south poles of the magnets.How the two opposite poles if two magnets attracts,like that anions and cations are attracted to each other.

          Anions of acid dyes are attracted towards the cationic sites of the protein fiber wool(or silk) or polyamide fiber nylon.The poly molecules of the wool is having cationic sites in its polymer structure.That is morphology, whatever the polymer chain is there,that is containing cationic sites attracted by the anionic sites of the acidic dyes.When the wool fiber is dyed with or the nylon is dyed with the acid dye in acidic concentration,then both the affinity that attraction takesplace between the anion or cations and form an electrovalent bond.

          The base of the acid dyes is the mechanism involved in acid dyes.That is mode of attachment of acid dyes to protein or polyamide fibers.

Types of Acid dyes:

          Acid dyes are divided into two groups.1.Molecularly split acid dyes and 2.Aggregated acid dyes.

Molecularly split acid dyes:

          When these dyes are dissolved in water, individual molecules of the dyestuff are sepersted,followed by ionisation.

Aggregated acid dyes:

          These dyes do not split into individual molecules when dissolved in water.Many molecules cluster together into bigger particles.Some of the molecules ionise,giving electrically charged aggregates.

         Other method of classification of acid dyes are divided as below.And will be briefed in separate articles.

1.Levelling dyes

2.Milling dyes

3.Super Milling dyes

          The dyes used for the particular textile material,it depends upon the class of dyes,which are selected depends upon the efficiency of the fastness rating.What fastness is required ? What efficiency is required ? Main mechanism involved basically,the acid dyes are called salt called sodium salt  produce coloured negative ions anions when dissolved in water.They soluble in water.

Properties:

          Everybody expects good fastness from applied dyes or shades. Rubbing fastness,Light fastness, Washing fastness are the three major fastness considered to judge quality of dyes.

          Shade creations that is possibility of shades produced by dyes.Those are Light shade,Medium shade and Dark shade.

         The properties of acid dyes will be briefed in separate articles.

Source:

1.Ref: Chemistry of Dyes and Principles of Dyeing by V A Shenai.

2.Educational qualification and industrial experience of the author/blogger.

Basic dyes | Textile dyes | Textile chemistry

Introduction: 

          The dyes are classified as water-soluble dyes and water insoluble dyes. Other types of classdification of dyes are synthetic dyes and natural dyes.The dyes available in the market,those are in the form of ready made, in the powder, some cases in paste form,used for dyeing of the fabrics or printing of the fabrics. Even dyes are used for paper printings.

Basic dyes: 

          Basic dyes are water insoluble dyes. These are not soluble in water directly when the powder form or paste form of the dye is dissolved in water, like reactive dyes, acid dyes, etc,.But the direct dyes are dissolved in water.Basic dyes are water insoluble,then how to make it dissolve in water ?.Without dissolution in water,the dye solution cannot be prepared.If the solution is not prepared,how the exhaustion takes place ?.That is the absorption of colour by fibers due to the affinity between the dye particle and the textile fibers.

          So to prepare the dye solution, the basic dye must dissolve in water. It will require conversion of water insoluble dyes into soluble dyes. After dissolving only, it will be absorbed by the textile material.Textile material may be a yarn or fabric.

Protein fibers:

          Basic dyes are used for dyeing of textile materials made from protein fibers. Acid dyes are also used to dye protein fibers. What are those protein fibers ?. Fibers, which are naturally available are silk and wool.

          Silk is available in filament form. The substance present in the protein fiber silk is Fibroin.

          Wool is made of the protein substance keratin extracted from skin of sheep.The fibers are naturally in the staple form.

          And basic dyes are used to dye the synthetic fibers made from synthesized proteins. That is protein fiber made synthetically

          These protein fibers, whichever fiber contain inside,the poly molecular structure, that is polymerization or polymerized structure.

Cellulosic fibers:

          In previous days or even nowadays, the cellulosic fibers like natural cotton and regenerated viscose rayon are dyed by the basic dyes. But the fastness of a dye used for cellulosic fibers is very very poor, especially washing fastness.

Mordants:

          The cellulosic fibers are dyed by basic dyes by the help of mordants.Mordants have affinity towards both dyes and textile fibers. Mordants are the chemical agents,also called as dyeing agents. Mordants will help to make the cellulosic material absorb the basic dyes. First the cellulosic material that may be yarn or fabric is mordanted. Mordanted means, the fabric or the yarn is treated with mordants. So that,it will make the cellulosic material to absorb the dye particles.Example of natural mordant is Tannic acid.

Mechanism of Dyeing:

         What is the mechanism involved in the dying of cellulosic fibers and protien fiberd by the basic dyes ?. That is the basic dyes, when dissolved in water, even after treated with mordants,ionisation takes place in the dye bath.

          The dye bath is neutralised with acetic acid or hydrochloric acid before the material is dipped for dyeing. If the dye bath is neutralised with hydrochloric acid, the chloride of the basic dyes are creates which are colourless in nature but negatively charged.Other coloured parts creates are the positively charged and are responsible for dyeing.These coloured part is called cation.

          Cations are used to fix on the anion part or site of the macromolecules of the cellulosic fiber or protein fibers.The cation of the basic dye is a coloured part of the dye. And anion is a colourless part of the dye. For dyeing is required coloured part. That is cation is required.

          If basic dyes are dyed with the cellulosic fibers, mordants are used.For protein fibers, no the mordant is required.

          Whatever the free base dye having positively charged coloured part cation is making electrovalent bond with the anion part of the of the site of the macromolecules of the fiber. So that,the fixing of the color takes place.

Chemistry of Dyeing:

          Basic Dyes have amino group in their molecular structure like -NH2, -N(CH3)2, -N(C2H2)2,etc,..are takes active part in the dyeing process.

          Cellulosic fibers contain -OH (Hydroxy)groups in their molecular structure are takes active part in the dyeing process.

          -COOH (Corboxy)group prefer in the polymer chains of protein fibers takes active part in the dyeing process.

After treatment:

          Even after dying, the after treatments are required to improve the fastness of the day or to improve the shade of the dye.

Note: This article is going to be update time to time and as and when required.

Direct dyes | Textile dyes | Textile chemistry

Introduction: 

Direct dyes. The name itself indicating direct dyes means, the dyes are applied directly on the textile fabrics. What are those textile fabrics? Fabrics made from Cellulosic fibers. Examples of cellulose containing fibers are cotton, viscose, rayon, cuprammonium, acetate,.

Dyebath requirement:

          The solution required for direct dye is water. The simple water solution is enough to dye direct dyes. The direct dyes are water soluble dyes. There is no need of converting from water insoluble into water soluble to prepare dye solutions like Vat dyes or Acid dyes. In case of acid dyes and vat dyes, they are water insoluble dyes and are converted into water soluble while preparing dye solution. Again they are reconverted into water soluble dyes, when they are absorbed by the fibers into their structure. But in the case of direct dyes, because of they are water soluble, the affinity between the cotton fiber or viscose fiber or rayon fiber or cuprammonium fiber, whichever the cellulosic fibers will be more. Because of this high affinity, the dye takes place.

          Another important point here is to consider that, the direct dyes are more affinity towards cellulosic fibers than affinity towards the water. Because of that, the dye go and penetrate into the surface of the cotton fibers.

Dyeing Mechanism:

          What mechanism is there in dyeing of cellulosic material with direct dyes?.Direct dyes,when dissolved in water,it ionizes.How? like sodium chloride.Dyes ionizes as anions.Sodium ionizes as cations.After ionization, the affinity takes place between the dye and the fiber.This is the principle of dyeing of direct dyes involved.This leads to the manufacturing of the sulphonic acids.Sulphonic acid is manufactured by the dyers.These sulphonic acids are generally converted into the solutions,dye solutions.Sulphonic acids are less soluble than their sodium salts.And also,that have less affinity to towards the cellulosic fiber than sodium salts.Sulphonic acids are produced as a direct dyes by the dyers.

          These sulphonic dyes when involved in the application of textile materials,that is cellulosic materials,they ionizes as sodium salts,their corresponding sodium salts.

Dye exhaustion:

          when the complete dyeing takes place ?,that is the total amount of dyes penetrate into the fabric completely.The amount means material to liquor ratio for dyeable textile material weight.How much amount of dye is taken by the textile material.This is the question.Many times the dyes are not completely absorbed by the fibers or penetrate into the fibers.That is,the dyeing is not taken place properly due to the less affinity between dye and material.It depends upon the different sulphonic acids used to dye different fiber materials.They produce their corresponding sodium salts in dye solutions.To increase the affinity,that is to increase the dyeability of the dyes,soda ash is added into the solution.Soda ash means sodium carbonate is added.This sodium carbonate will increase the affinity between the fibers and the dyes.That is increase in the dye absorption.

          If the exhaustion of the dyeing of fiber is not completely takes place, what will happen? Most of the dye remaining's will be left in the dye bath as residues. Due to this, incomplete dyeing will take place, shade variation will occur on the dyed material. And also waste of the dye leads to the loss in dye cost. To avoid these drawbacks, sodium carbonate is added into the solution to complete the dyeing performance of the direct dyes.

 

Note: This article will be updated as and when required.