Reactive dyes

The term “reactive” dye means that the dye chemically reacts with the fiber. These dyes are chemically bonded to the fibers unlike other classes of dyes (direct, sulphur, vat) where dye is physically trapped in the fiber pores. This chemical bonding being very strong, the dye cannot be easily washed off. Hence reactive dyed fabrics possess excellent wash fastness properties.

•      In general these dyes are represented by

D-B-R-S

                             where D is colour bearing substance known as    chromogen, R is reactive group and B is the bridging     group which connects chromogen and reactive    group. The water soluble group, S, makes the dye        easily water soluble.

Depending up on nature of the reactive group these dyes are classified into three categories. They are triazinyl dyes, vinyl sulphone dyes and bi functional dyes.

i) Triazinyl Class

The triazinyl class of dyes also popularly known as Procion dyes    (A brand name of ICI) are further classified into two groups:

Monochloro triazinyl (MCT) dyes and Dichloro triazinyl dyes (DCT) dyes.

Cynauric Chloride (1,3,5 Triazine)

Dichlorotriazine

In this class of dyes the reactive group is chlorine. In alkaline medium, this chlorine atom reacts with hydroxyl group present in cellulose (cotton) thereby linking the dye with the fiber through a chemical bond (covalent bond). 

The reactivity of DCT dyes is high; hence they can dye (react with) cotton in the presence of an alkali at room temperature. They are commonly known as “cold brand” or “M brand” dyes.

On the other hand the reactivity of MCT dyes is low. They require high temperature (80^oC) and high alkaline condition to dye cotton. They are commonly known as “Hot brand” or “H brand” dyes.

The structures of MTC and DCT dyes and formation of covalent bond with hydroxyl groups present in cotton (Cell-OH).

                        

ii)Vinyl Sulphone class

The second category of reactive dyes are known as vinyl sulphone dyes (VS).  These dyes are popularly known as Remazol dyes a brand name of Hoechst.  They have general structure as shown below. These dyes are applied on cotton in the presence of alkali at 60^oC.

iii) Bi functional dyes

The third category of reactive dyes is known as bi functional (BF) dyes. They have higher exhaustion when compared to the other classes of dyes. They are marketed as “HE” or “H-EXL” dyes. These dyes can be further divided into homo bi functional and hetero bi functional dyes.

The homo BF dyes can be either two triazinyl groups each having one reactive chlorine group or two VS groups; where as hetero BF dyes have one VS and one MCT group.

They are marketed as “ME” and “Sumifix Supra  Brand” dyes (Amino chlorotriazine sulphatoethylsulphone system).  These dyes are also applied on cotton in the presence of alkali at 60^oC. They have very good light fastness with rating about 6. Dyes have very stable electron arrangement and can protect the degrading effect of ultra-violet ray.

Dyeing with reactive dyes:

Textile materials dyed with reactive dyes have very good wash fastness, brighter shades and have moderate rubbing fastness.

Cellulose can be dyed using reactive dyes in following methods:

1. Batch wise dyeing
2. Continuous
3. Semi continuous

DIRECT DYES

Direct dyes are one of the most versatile classes of dyestuff applicable to cellulose, wool, silk, nylon fibers. This dyes have inherent substantivity for cotton, and for other cellulosic fibres. Their aqueous solutions dye cotton usually in the presence of an electrolyte such as NaCl or Na₂SO₄ (sodium sulfate also known as Glauber's salt.

The direct dyes, also known as substantive colors, differ from other classes of dyes because they are strongly substantive towards cellulose fibers such as cotton and viscose. Many of them also dye wool and silk. In the year 1884, Bottigner discovered first direct dye popularly known as Congo Red, a derivative of benzidine, which was found to be carcinogenic and is now one of the banned amines. Prior to this discovery, cotton was dyed with vegetable dyes using mordants.

Direct dyes provide a simple and relatively inexpensive way of dyeing cotton. Complete range of colors is available in this class of dyes. However they are not as bright as reactive dyes. They can dye cotton without the use of chemicals unlike other classes of dyes; hence they are called direct dyes. They are anionic in nature (sodium salts of sulphonic acid) and are soluble in water. Majority of them are di azo or tri azo derivatives. They are represented by RSO3Na, where RSO3 is color bearing dye anion.

The light fastness of dyeings with direct dyes on cellulosic fibers varies from poor to fairly good, although some copper complex direct dyes have very good light fastness. As usual, the deeper the color of the dyeing, the lower the fastness to wet treatments, and the higher the fastness to light. Various after treatments of the dyeings improve the fastness to washing. In some cases, however, such after treatments decrease the light fastness. They also invariably cause a change in hue that makes shade correction and color matching more difficult. 

Mechanism of dyeing:

It is generally accepted that when direct dyes are dissolved in water the dye molecules disassociate and are present in water either as single molecule or in small aggregates.

During dyeing these small aggregates diffuse into fiber and form bigger aggregates which get deposited in fiber pores.

They do not get chemically bonded to the fiber as in the case of reactive dyes (it is generally believed that very weak bonding such as Van der Wall forces exists between the dye molecules and fiber). Therefore during washing by the end user, these bigger dye aggregates disassociate in water and come out of the garment, resulting in color bleeding.

The addition of electrolyte (salt) to dye bath tends to promote exhaustion of direct dyes. This is because when cotton is immersed in water, its surface acquires slightly a negative charge, known as zeta potential.

The dye being anionic in nature it also has negative charge. As both cotton and the dye possess identical charges, they repel. Therefore the dye will not have tendency to reach fiber surface and diffuse into fiber core.

The addition of salt neutralizes a part of negative charge developed on the fiber surface thus enabling the dye molecules to migrate towards the fiber surface and hence diffuse into fiber core. Addition of salt to the dye bath reduces the solubility of dye in water thus inducing the dye to migrate from water to the fiber core.

Therefore both suppression of fiber surface charge and reduction of dye solubility in water help in achieving high exhaustion of dye. Sodium chloride (common salt) and sodium sulphate (Glauber salt) are the most common agents used as exhausting agents. The quantity of salt to be added to the dye bath depends on depth of shade and material to liquor ratio. A large amount of salt is required for dark shades and high material to liquor ratios.

Classification of Direct Dyes:

    

The behavior of dyes in dye bath is influenced by salt concentration and dyeing temperature. Accordingly they are divided into three groups.

•      Group A or Self leveling dyes: In general this group of dyes does not pose problems while dyeing. They dye uniformly as they possess high leveling property. The unevenness produced during the initial phase of dyeing can be corrected by prolonging dyeing time.

•      Group B or Salt controllable dyes: These dyes have poor leveling property. Their leveling property is influenced by the addition of salt. If the required quantity of salt is added in one installment at the beginning of dyeing, the dye transfer to the fiber will be rapid and will result in uneven dyeing. It would be very difficult to level the shade if unlevelness occurs with this group of dyes. Therefore to dye this group of dyes, it is recommended to add salt in small portions over a span of time.

•   Group C or Temperature controllable dyes: These dyes are sensitive to temperature. Their leveling property can be controlled by controlling rate of heating. While dyeing with this group of dyes, the dye bath temperature must be slowly increased. Rapid increase in temperature will result in uneven dyeing. It is recommended that addition of salt should also be added in portions.

Dyeing Procedure

q  Set the dye bath with substrate at room temperature

q  Add dye solution with other auxiliaries ad raise the temperature at 90⁰c

q  Run the bath for 15-20 min and add salt step according to dye bath concentration

q  This is better to add salt after reaching the temperature to build point since at this period the maximum penetration is achieved all over the substrate.

q  Run the dye bath for 30 to 50 minutes at 90-95^oc for complete the dyeing cycle

q  Cold down the bath temperature to 60-70^oc

q  Drop the bath and rinse

q  Carry on the after treatment process to improve wet fastness

Dye- Fibres interaction bonds:

For dye-fibre interaction it is needed that dye has to be substantive (attraction) to the fibre to be dyed. This affinity will enable the dye to be absorbed by the fibre.There are quite a few factors that affect the substantivity of a dye. Among them are the ability of the dye to form strong or weak bonds (e.g ionic and/or hydrogen bonds and /or Van der Waals forces) with the fibre, its polarity, and its size and shape. The rule of thumb that states: ‘Like dissolve like’,  can give us some indication of the expected substantivity.

Attraction forces between dye and fibre: Following are the four attraction forces:

• Van Der Waals forces
• Hydrogen Bond
• Electrostatic attraction
• Co-valent Bond

Van der Waals Forces:

The van der Waals force, named after Dutch scientist Johannes Diderik van der Waals, is the sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds, or the electrostatic interaction of ions with one another, with neutral molecules, or with charged molecules. The term includes:

• force between two permanent dipoles 
• force between a permanent dipole and a corresponding induced dipole
• force between two instantaneously induced dipoles (London dispersion force).

Hydrogen Bond:

A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge.

Covalent Bond:

A covalent bond is a chemical bond that involves the sharing of electron pairs       between atoms. The stable balance of attractive and repulsive forces between         atoms when they share electrons is known as covalent bonding.

Electrostatic attraction/repulsion

Electrostatic attraction is the phenomenon where a negatively charged atom or       molecule is attracted to a positively charged atom or molecule. Electrostatic           repulsion occurs between two atoms of the same charge.

                    

Dye class	Force of attraction
Direct, Vat, Sulphur, Solublized Vat (with Cellulose fibres)	Vander Wall, H-Bond
Reactive with cellulose fibres	Co-Valent Bond
Direct, Acid, Metal Complex with Wool, Silk and Nylon	Ionic or electrostatic
Reactive with Wool, Silk and Nylon	Ionic/Covalent
Disperse with polyester	VanderWaal, H-bond
Cationic with Acrylic	Ionic or Electrostatic

Semi continuous process:

Pad-Batch dyeing is one of the best example of semi continuous process. Pad-batch dyeing is a cold method used for dyeing cellulosics (mainly 100-percent cotton and polyester/cotton blends) material.

With this process one can achieve large reductions in pollution, energy requirements, and costs.

The basic technique is to saturate the prepared fabric with a premixed dye liquor and pass it through a padder (Set of Squeeze rollers), which forces the dyestuff inside the fabric for greater penetration while removing excess dye solution. The fabric is then stored, or batched, on rolls or in boxes for 4 to 12 hours. Typically, the batches are covered with a plastic film to prevent carbon dioxide absorption and water evaporation. While in batching, the dyestuff reacts with and penetrates the fabric, resulting in even, consistent color. After the reaction is complete, the fabric is washed.

Pad-batch dyeing offers several advantages over conventional dyeing processes:

• No salt or chemical specialty agents are needed. Eliminating these chemicals reduces waste as well as chemical and wastewater treatment costs.

• More efficient use of dye leaves less color in the wastewater and reduces water and energy consumption. 

 • Dye quality is more consistent. Compared with rope dyeing, pad-batch dyeing can attain more even color absorbency, greater colorfastness, and much lower defect levels (when the fabric is correctly prepared). High-reactivity dyes used in pad-batch dyeing have rapid fixation and stability, resulting in shade reliability and repeatability. With highly reactive dyes, cleanup is easy and frequent shade changes present little problem.

• Pad-batch dyeing can be used on wovens or knits in many constructions. 

• Pad-batch dyeing requires a low capital investment and offers overall cost savings, in dyes, chemicals, labor, water, and other areas. 

Continuous process of dyeing:

When there is a need of high production of one shade of fabric especially for uniform cloth for military, paramilitary, school etc, continuous dyeing process is very useful. In the continuous dyeing, fabric is dyed in open width. In the continuous dyeing process numbers of machines are placed in sequence so the dyed fabric can be obtained in one go. The textile substrates are allowed to enter continuously into a dye range. The speeds can vary between 10 to 100 meters per minute. Continuous dyeing process is very useful for polyester-cotton blends. In this process dye utilization is excellent i.e low wastage of dyes. Even the energy utilization is lower than batch wise process.

Importance of blending Polyester and cotton:

Blends of polyester and cotton fibres have become very important to the textile industry. Cotton gives the aesthetic and comfort properties demanded by consumers, while the polyester component adds to performance properties.

How to dye p/c blend in continuous dyeing process?

As both the fibres are having different dyeing behavior, both the fibres  of the two fibres, dyeing blends of these two fibres is fairly straightforward. Each fibre may be dyed the same colour, or they may be dyed different hues. Polyester has no affinity for most of the classes of dyes used to colour cotton, the cellulose being only stained by disperse dyes. The “Thermosol” process for dyeing polyester was developed by the DuPont company for the continuous dyeing of polyester fabrics. This single development allowed the rapid growth of polyester fabrics in the early to mid-fifties.

The process involves padding on the disperse dye together with auxiliaries that minimize migration, drying, For cotton/polyester, the fabric must be absolutely clean, since residual oils or dirt will be set into the fabric during the Thermosol treatment. The fabric must also wet out instantaneously and uniformly to insure adequate absorption during the padding operation.

The thermosol Continuous dyeing of polyester process typically consists the following:

Dye application: The dye solution for padding should be homogeneous. It should be thick. To make solution thick, a thickener, usually, sodium alginate at 0.15-0.3 gm/l is used. The viscous/thicker dye solution improves wet pick up and minimize migration during drying, thus eliminating side to side and two sided dyeings (i.e. when the back of the fabric is a different depth compared to the face) affects. Wetting agent may be added for rapid wetting of fabric. In this solution, fabric is padded by passing in to saturated dye solution and the excess solution is squeezed by passing between two squeeze rollers using padding mangle. The expression may be maintained around 80% (wet pick up). After that fabric is dried before fixation of dye at higher temperature.

The cotton dye may be applied from the same bath as the disperse dye. Vat dyes, Sulphur dyes or a Reactive dye can be used depending on the hue and fastness required. In this experiment, a reactive dye will be applied to the cotton fibers in the union fabric, while a disperse dye will colour the polyester fibres. Fibre reactive dyes exhibit excellent wash fastness because they are covalently bonded to the cotton fibre.

Dye fixation with heat or chemicals and finally: Then fixing the dye in the polyester by dry heating to a high temperatures about 190° – 205°C for 60-90 seconds to promote complete fixation, penetration, and heat-setting. During this process the fibre molecular chains open up at these elevated temperatures and the dispersed dyes vaporize and diffuse into the polymer. On cooling, the dyes are trapped within the fibre yielding coloured fibres that have good fastness properties.

If reactive dyes are to be fixed by dry heat, as in the Thermosol process, 50 - 200 g/l urea should be added to the pad-bath.

Washing. After dyeing dyed fabric should be washed properly to wash down unfixed dye.

Methods of dyeing

Dyeing can be carried out using any one of the following ways:

• Batch-wise dyeing process
• Semi continuous process
• Continuous process

Batch wise Dyeing:

Batch wise dyeing is oldest process of dyeing. It is still very popular as it can be related to changing fashion where colour is one of the predominant factors.  Batch wise process is very useful when small size of lot to be dyed as per the requirement of buyers.  It is some time also referred as exhaust dyeing as the dyeing is exhausted directly on to the fabric using salts. This is because in this process, the dye gets slowly transferred or exhuasted from a comparatively large volume dyebath to the substrate or material that is to be dyed. In the batch wise process any form of materials i.e fibre, yarn , fabric or garments can be dyed.T here are three general types of batch dyeing machines. These are

First type: Textile material is in circulation while dye liquor in dye bath remains stationary.

Second type: In this type, dye liquor in the dyebath gets circulated while material that is being dyed remains stationary, and

Third type:  Here both, liquor in the bath and material to be dyed gets circulated.

Examples of dyeing machines that utilizes batch dyeing process are Winch or Beck, Jet, Jigs, Beam Package dyeing machines etc.

Important processing parameters for dyeing:

Important processing parameters for batch wise dyeing:  Following are the important processing parameters for batch wise dyeing:

Material to liquor ration (MLR): MLR of shall be taken as per the type of machine. For example, in winch dyeing MLR may be 1:20 or more. Means 1 Kg of material you need at least 20 litre of water. If it is a jet dyeing machine then MLR may be 1:10.

Dyeing Temperature: Dyeing temperature plays a crucial role while dyeing. The shade card/colour card should be followed while selecting temperature. For example for polyester dyeing temperature may be 130^oC.

Time Duration of dyeing: Here also shade card should be followed.

Single bath dyeing or double bath: If material is 100% cotton or 100 Polyester and any other fibre then single bath dyeing is carried out. However if there is a blend of fibres which can not be dyed by single class of dye then double bath dyeing is carried. For example if polyester-cotton (p/c) blend to be dyed then first polyester is dyed using disperse dye at 130^oC and then cotton is dyed in the second bath. It is to be noted that in the p/c blend, polyester has to be dyed first and then cotton. If cotton dyed first then during dyeing of polyester at 130^oC, cotton dye would bleed. 

Nature of Dyestuff, auxiliaries: The selection of dye according to the fibre to be dyed is most important. If the dye is not selected according to the fibre, there will be no dyeing and all efforts will go waste. So for cotton, one has to select cotton dyes like reactive dye, vat, sulphur or direct. If polyester then disperse dye should be selected.

The auxiliaries plays an important role while dyeing. For example salts (Sodium chloride of sodium sulhate) are needed to exhaust dye bath in the case of reactive dyeing. If salt is not added, reactive dyes will not exhaust at the optimum. Similarly sodium hydroxide is used as dye fixing agent.

After wash/rinsing: After dyeing it is necessary to wash dyed material other wise colour will bleed during uses of textile material.

Information on colouring materials

Dye manufacturers provide dyers with detailed descriptions of their dyes, regarding their application and behaviour in normal use, in the so called “Color Card or Shade Card”. The most comprehensive reference on dyes and colouring material is the colour Index, first published by the Society of Dyers and Colorists (SDC) of England in 1924. The third edition published jointly with American Association of Textile Chemists and Colourist (AATCC). The information includes physical properties, chemical constitution, method of application, fastness properties, and list of potential fibres to which they can apply. 

Classification of Dyes

Before understanding classification of dyes, we must know the difference in between dyestuff and pigment. Beside this why we do dyeing and characteristic of dyes should also be understood well. Let us discuss these points first.

Dyestuff is organic or inorganic substances which can absorb light and reflect some lights to show color.  Actually, the dyestuff is water soluble substances. In textiles, it is used to dye textile materials. They absorbed or adsorbed into the fibre

Pigment is a substance that can absorb light and reflect some lights to show color. Not water soluble and possess no specific attraction for any particular fibre type. They usually adhere to the surface of the fibre Normally it is used for printing (with the presence of binder) or mass-coloration of the synthetic fibers. They do not belong to dyes.

Why we do dyeing/printing?

•         Because Colored products are attractive and gives pleasure/ comfort

•         In textile dyeing terms: The main objectives of dyeing are:

•         Colour the fibre uniformly

•          Achieve acceptable durability of the     colour to further treatments in production and normal use    ( colour fastness properties)

Characteristics of dyes: Following are the important poits related to characteristics of dyes:

•         Intense colour

•         Solubility in water

•         Substantivity to the fibre in question

•         Durability to wet treatment

•         Durability to further treatments in  production and normal use

•         Safe, easy to handle, and reasonably priced

Intense colour

•         The dyes have intense colours.

•         Due to intense colour small amount of dyes are sufficient to obtain desired shades.

•         In practice, the increase in weight of fibrs by dyeing is between 0.1% to 5%

Solubility in water

•         Dyes are soluble or dispersible in water

•         Water is the vehicle of exhaust dyeing; it carries the dyes to the fibres.

•         Disperse dyes are sparingly soluble in water, the require dispersing agent.

Substantivity to the fibre

•         Dyes have to be substantive to the fibre in question.

•         The affinity that a dye has particular fibre is referred to by the dyers as ‘Substantivity’

•         The substantivity of a dye to a fibre must be much stronger than affinity to water

Durability to wet treatment

•         Dye must possess the ability to become durable to wet treatment

•         A high substantivity dyes some time provide sufficient durability of the colour to a limited number of mild washing

•         So after treatment step may be taken to improve upon durability

Durability to further treatment in production and normal use

•         Dyes should have fastness to further treatment in production and normal use.

•         Example: To apply durable press finish, the dyes should withstand the low pH and high temperature

•         In addition dyes should have good fastness properties

•          

Safe, easy to handle, and reasonably priced

•         The dyes should be safe, easy to handle and reasonably priced