My Book on "Pretreatment of Textile material for dyeing and Printing" released today by Textile Commissioner of India during 56th Joint Technological Conference, Coimbatore.

Decatising:

       Decatising or decatizing, also known as crabbing and blowing, is a finishing process. In this process wool or its blends are finished in such a way that that it does not shrink during garment making. The word comes from the French décatir, which means to remove the cati or finish of the wool. Though used mainly for wool, the term is also applied to processes performed on fabrics of other fibers, such as cotton, linen or polyeste. 

       A finishing process in which the fabric is wound tightly onto a perforated roller and either immersed in hot water, which is also circulated through the fabric (wet decatising) or has steam blown through it (drydecatising). Decatized wool fabric is interleaved with a cotton, polyester/cotton or polyester fabric and rolled up onto a perforated decatizing drum under controlled tension. The fabric is steamed for up to ten minutes and then cooled down by drawing ambient air through the fabric roll. The piece is then reversed and steamed again in order to ensure that an even treatment is achieved. There are several quite different types of wool decatizing machines including batch decatizing machines, continuous decatizing machines, wet decatising machines and dry decatizing machines.

How does wool scouring differ from cotton scouring?

First point to be noted that, wool is having very high amount of waxy matter than cotton. The wax matter of cotton is removed by treating cotton with sodium hydroxide solution at boiling temperature. But sodium hydroxide solution can not be used for scouring of wool as wool is very sensitive to alkali. Therefore wool scouring is carried out with detergent or mild alkali like sodium carbonate solution (pH around 8 to 10). 

There are three methods for wool scouring:

i) Emulsion scouring: In this method the raw wool is scoured in a tank filled with detergent having pH 8-10. In the process temperature is maintained around 55-60 degree C and time duration 30-45 minutes. After that wool is washed and dried.

ii) Solvent scouring: In this type of scouring raw wool is treated with solvents like Trichloroethylene, perchloroethylene or carbon tetra chloride (now it is banned in India). These solvent remove wax from the wool. This wax and solvent is recovered. The recovered wax are used various application.

iii) Freezing: As its name suggest, wool is subjected to very low temperature about -30 degree C. Due to this wool wax becomes hard. At same low temperature is is mechanically crushed followed by shaking. Wax is removed.

Scouring and cleaning of raw wool:

Wool is composed with Keratine (approx. 33%), Dirt (Aprrox. 26%), Suint (Dry perspiration: 28%), Fat/grease (12%), Burr (Vegetable matter: 2-10%), Mineral matter (aprox. 1%) and Colouring matter (0.5 to 1%).

1. Wool grease can be removed by scouring. These Grease/waxes are comprised of a variety of monocarboxylic, dicarboxylic and hydrocarboxylic acids as well as steroidal alcohols. It has been determined that unscoured wool contains an unoxidized fraction of wool grease and other contaminants that is easily removed and readily recoverable 

2. Suint is usually considered to be a variable composition of water-soluble materials that is readily removed by scouring. 

3. The dirt that is removed from the scoured wool consist of both inorganic and organic materials.

4. Dirt: It is held by adhesive action of suint and wool fat. It removed during the scouring and washing processes.

5. Burrs: It is vegetable fragments consisting of dried grass, straw, sticks etc. These vegetable fragment come on the body of sheep during grazing and scratching the body against bush or tree to relive itching. It is remove using Carbonizing process. Following is the process of removal of Burrs:

i) Wool is resistant to acid. Burr being a vegetable matter is not resistant to acid.

ii) This property is used for the removal of burr.

iii) In this case wool is treated with 5-7% Sulphuric acid for 2 hours followed by hydro-extraction and then drying at 80-90oC for 20-30 minutes. 

iv) Carbonized burr is removed by washing with mechanical agitation.

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