Textile Knowledge

Pigments

Pigments:

Pigment is a substance in particulate form which is insoluble in water but which can be mechanically dispersed in this medium to modify its color and light scattering properties. They are organic or inorganic colouring materials.

Pigments have no direct affinity to textile fibers. They are fixed on textile materials (fabric/yarn) with the help of a binding agent in form of a this invisible coating.

In pigments auxochrome groups are absent. A number of organic pigments are prepared from dyes by combining the dyes with a dye precipitant.

Pigment are abundant in nature. The cause of color is not only the light absorption of molecules but also the purely physical effect of optical diffraction at grating systems. Pigments are applied at thickened pigment coating from oil-in-water or water-in-oil emulsion systems.

Properties of pigments:

i. It is insoluble in liquids but can be mechanically dispersed in the medium.

ii. Pigments have no direct affinity towards fiber, so binding is used as a binding agent in case of pigment colouration which sticks the pigment particle on the fiber surface.

iii. Pigment are applicable to all kinds of fibers.

iv. No aftertreatment is required after coloration process, only drying and curing are required.

v. Only rubbing fastness of pigment colouration is poor, but water wash and light fastness are very good.

vi. A wide range of bright shade can be produced by pigments.

vii. In pigments auxochrome groups are absent.

Differences between dyes and pigments:

The difference between dye and pigment is not a clear one. Most organic pigments are closely related to dyes with respect to their chemical structure. There are dyes which become pigments after application. Vat dye is a dye when used in dyeing, when vat dyes are oxidized and made insoluble in fabric(after vatting) that is called pigment form of vat dyes. Vat dyes act like pigment when used in printing. Now some differences are mentioned below:

1 ) The basic difference between dye and pigment is that, dye has three parts-

i. Chromophore, which is responsible for color.

ii. Auxochrome, which is responsible for dye properties

iii. Chromogen, which holds the above two together.

But pigment has two part:-

i. Chromophore, i.e. color bearing part.

ii. Chromogen.

2 ) Pigments are insoluble in the materials that they are used to color, whereas most dyes are soluble in them.

3 ) pigments are water insoluble, but dyes are more or less water soluble.

4 ) Pigments are used as colorants in the physical for in which they are manufactured. But dyes are first dissolved in water to form a dyebath, then they diffuse as single molecules from these solutions into textile fibers to colour them.

5 ) Pigments have no direct affinity towards textile fibers but most dyes have that.

6 ) Pigments are metallic compounds may be organic or inorganic. But dyes are mostly organic compounds, few are inorganic.

Classification of pigments:-

There are mainly two types of pigments, namely,

1. Organic pigments and

2. Inorganic pigments

Organic pigments are further classified as below:-

I. Dyes converted into pigments.

It has two types,

1) Dyes made insoluble by precipitating on substrates.

2) Vat dyes converted into pigments

II. ISO indolinone pigments

III. Azo pigments:

It has two types

1) Metal free{Water insoluble azo compounds (group1)}

2)Metal containing

there are two types: chelates(group-3) and

Water soluble azo compounds(group-2)

–low soluble, make heavy metal salts(group-2a)

–high solubility precipitate heavy metal salts or basic substrates(group-2b)

IV. Quinacridone pigments

V. Phthalocyanine pigments

Inorganic pigments are classified by two types

I. White pigments and

II. Coloured pigments

There are three types of coloured pigments:

1. Ultramarine

2. Cadmium pigments and

3. Iron oxide pigments.

Binder:

Binders are film forming polymers which play important roles in achieving optimum properties. Pigments have no affinity towards textile materials. So actually pigment particles are sticked on the surface of fabric. Binders are those adhesive type coating forming polymeric materials which sticks pigment particles on fiber/fabric surface. Binders are white, milk-like liquid. If forms a very thin invisible film on fabric surface during curing. Under this film pigment particles are remain sticked. So in pigment dyeing and printing binder is a very important and essential ingredient.

Binders with a particle size range of 0.05 to 2 microns are usually available. These contain about 6% by weight of reactive groups for crosslinking of the binder to textile. The reaction involved is as below:

Binder-CH₂-OR + HO-textile = Binder-O-textile + HOR

[where Ris H or CH₃ ]

Crosslinking increases the rubbing washing and drycleaning fastness properties, but it affects the handle of fabric.

Types of binder:

Binder can be classified according to many aspects, Some are mentioned below:

1. According to origin: According to origin there are two types of binder,

I. Natural binder- glue, gelatin etc.

II. Synthetic binder- TROL, SLN, IG, Acramin binders etc.

2. According to chemical groups: According to chemical group present in binder there are two types of binder, those are:-

I. IG Binders: Emulsion copolymer of vinyl acetate and butyl acrylate with a modified urea formaldehyde.

II. Ag Binders: They are made by emulsion copolymerization of olifinic unsaturated monomers in an aqueous medium. These monomers include:-

a) Acrylic acid ester(butyl or ethyl acrylate)

b) Styrene

c) Acrylonitrile

d) Vinyl chloride

e) Asymmetric dichloroethane

f) Vinyl acetate

g) Diolifine such as butadiene

Required properties of a binder:

The binder must have certain qualities. For example:

i. It should not get coagulated due to shear forces.

ii. The binder film must be clear/transparent.

iii. The binder film must be of even thickness, smooth, neither too hard nor too soft.

iv. It should be elastic in nature and should have good adhesion to the substrate without being tacky.

v. It should have good resistance to chemical and mechanical stress.

vi. It should be easily removable from equipments.

Emulsion

Emulsion:

Emulsion is the mixture of two immiscible liquids like oil and water where mechanical energy is used to break up one component of the mixture into small droplet that are dispersed in the other component with the help of a surface active emulsifier to bring about stability. The most well known emulsion is milk, which liquid will be dispersed in the other is determined by two factors:

i. The nature of the emulsifier(mainly its HLB value)

ii. Ratio of volume of two immiscible liquids

Thus an emulsion has two distinct phases – disperse phase and outer continuous phase.

Emulsion in which oil is dispersed in water are described as oil-in-water (o/w) emulsion whilst those in which water forms the disperse phase are water-in-oil (w/o) emulsions.

Where disperse phase forms 70% of total volume, the emulsion will have significant viscosity. Thus in a oil-in-water emulsion there is more than 70% of total volume and in a water-in-oil emulsion there is more than 70% of the total volume.

Here is an example of water-in-oil emulsion:

Oil------74%

Water-----25%

Emulsion---1%

Total---- 100%

As emulsifier we use detergent, soap and other surface active agent.

The particle size of an emulsion is greater than 0.1μm since otherwise a colloidal or true solution would exist. A distinction is made between macro and micro emulsion according to their particle size and optical appearance.

Binder

Binder:

Binder-CH₂-OR + HO-textile = Binder-O-textile + HOR

[where Ris H or CH₃ ]

Crosslinking increases the rubbing washing and drycleaning fastness properties, but it affects the handle of fabric.

Types of binder:

Binder can be classified according to many aspects, Some are mentioned below:

1. According to origin: According to origin there are two types of binder,

I. Natural binder- glue, gelatin etc.

II. Synthetic binder- TROL, SLN, IG, Acramin binders etc.

2. According to chemical groups: According to chemical group present in binder there are two types of binder, those are:-

I. IG Binders: Emulsion copolymer of vinyl acetate and butyl acrylate with a modified urea formaldehyde.

II. Ag Binders: They are made by emulsion copolymerization of olifinic unsaturated monomers in an aqueous medium. These monomers include:-

a) Acrylic acid ester(butyl or ethyl acrylate)

b) Styrene

c) Acrylonitrile

d) Vinyl chloride

e) Asymmetric dichloroethane

f) Vinyl acetate

g) Diolifine such as butadiene

Required properties of a binder:

The binder must have certain qualities. For example:

i. It should not get coagulated due to shear forces.

ii. The binder film must be clear/transparent.

iii. The binder film must be of even thickness, smooth, neither too hard nor too soft.

iv. It should be elastic in nature and should have good adhesion to the substrate without being tacky.

v. It should have good resistance to chemical and mechanical stress.

vi. It should be easily removable from equipments.

Sulfur dye

Sulfur dyes are complex heterocyclic molecule’s or mixtures formed by melting or organic compounds containing amino or nitro groups with na-poly sulfide and/or sulfur. Sulfur dyes are so called as they all contain sulfur linkage within their molecules.

Sulfur dyes are highly colored, water insoluble compound and have to be converted into water soluble substantive forms before application to textile materials. This conversion is carried out by a treatment with a reducing agent like dilute aqueous na₂s. Since this leuco form of sulfur dye is substantive to cellulosic material they are absorbed on the fibre surface. Then they are reconverted to original water insoluble form of dye by oxidation. This oxidizing is carried out by ‘airing’ or by using an oxidizing agent like Na-dichromate (Na2Cr₂O₇).

The reducing agent converts the ‘S’ in dye into-SH group and break the sulfur linkages. Then inside the material the thiols containing –SH group are oxidized and thus reconverted to original form of dye.

This is shown as below :-

Dye-S-S-Dye + 2[H] ^{reducing agent} Dye-SH + HS-Dye

water insoluble water soluble

Dye-SH + HS-Dye + [O] ^{oxidizing agent} Dye-S-S-Dye + H₂O

water soluble water insoluble

Sulfur dyes gives best result(bright tone) when they are used to produce block, blue and brown shades. But ‘red’ shade can’t be Obtain by sulfur dyes.

Properties of sulfur dyes :

1) Sulfur dyes are water insoluble dyes, some are partially soluble

2) They have sulfur linkages within their molecules

3) They have no direct affinity toward cellulosic fibre.

4) They have good light fastness.

5) These dyes have excellent wash fastness about 3-4

6) Highly amorphous and few crystalline.

7) Suitable for heavy and durable shades

8) Unhygienic for environment.

9) Cheap and easy manufacture

Trade name	Name of manufacturer	Country of origin
1. Calcogene	Dyes dept. American Cyanamid co.	U.S.A
2. Pyrogene	Ciba	Switzerland
3. Thional	Sandoz	Switzerland
4. Solfo	ACNA	Italy
5. Sulfogene	DU pont	U.S.A

Classification of sulfur dye :

Sulfur dyes are classified in many ways. But according to their solubility, there are three types :

1) Conventional or water insoluble dyes

2) Leuco sulfur dyes (partially soluble)

3) Solubilised sulfur dyes

Reducing step of sulfur dyes :

Reducing step is the most important in the application of sulfur dyes. Unless the dyes are converted into the completely soluble form, the full color value can’t be achieved. The solubility of reduced sulfur dyes varies appreciably from dye to dyes. For example yellow sulfur dyes are the least soluble dyes(i.e. Most insoluble dyes).

The reducing agents used for sulfur dyes are:

1) Na-sulfide

2) Na-hydrosulfide

3) Thio glycolic acid

But Na-Sulfide is most widely used reducing agent for dissolving sulfur dyes. They may be replaced by Na-hydrosulfide in some cases. But the latter is a more powerful reducing agent. So over reduction of the dye may take place, leading to a product having lower affinity for cellulosic fibre. Hence lower color yields are obtain along with wastage of dye stuff. In some cases lower wash fastness results if Na-hydrosulfide is used. It also reduce the lifetime of vessel.

For dissolving the sulfur dyes, reducing agent is taken according to the weight of dye. In practice, they are taken at same weight. For example, 10 gm dye is mixed with 10 gm reducing agent. Then the mixture is boiled after adding necessary amount of water to get a good solution. For this wooden vessel or enameled iron vessels are used but not iron vessels. Because iron reacts with sulfur.

Oxidation step of sulfur dyes :

After dyeing the reduced, water-soluble form of the dye has to be converted into original water insoluble form by oxidation. the commonly used oxidizing agents are:

1) Potassium dichromate (K₂Cr₂O₇₎ and acetic acid (CH₃-COOH )

2) Na-per carbonate

3) Na-peroxide

The method of oxidizing selected plays an important roll in the development of correct shades and their optimum fastness properties. Rinsing off the dye should be done rapidly to remove as much Na-Sulfide as possible. if even after washing some Na sulfide or soda-ash stays on fabric. We used CH₃-COOH to neutralize them. Using of different oxidizing agents may give following Result :

1. Use of carbonate or per carbonate in presence of acetic acid gives bright shade.

2. Treating dyed material with dichromate without rinsing reduces color losses but causes dull shade.

The chemistry of oxidization is as below :

D-S-S-D + 2[H] ^reduction D-SH + HS-D

original molecule Thiols

2D-SH + [O] ^oxidation D-S-S-D+ H₂O

original dye

Dyeing of cellulosic fibres with sulfur dyes :

The sequence of dyeing with sulfur dyes is as follows :

Goods preparation (fabric preparation)

Dye solution preparation or reducing step

Dyeing

Oxidation

After treatment

Dyes goods

Chemistry of dyeing of sulfur dyes :

The sulfur dyes contain sulfur linkage within their molecules. They are insoluble in water but can be made soluble in water by treat them with reducing agents. This also make them substantive towards cellulose fibres. Na₂s acts as reducing agent that breaks sulfur linkage and break down the longer molecules into simple components which can penetrate the material (fibre/fabric) surface easily.

Dye-S-S-Dye + 2[H] ^{reducing agent} Dye-SH + HS-Dye

water insoluble water soluble

The thiols containing the –SH group are reading oxidize by the action of atmospheric O₂ or any other oxidizing agent. This reconverts the water soluble leuco form of sulfur dye into previous water insoluble form which have a very good wash fastness property.

Dye-SH + HS-Dye + [O] ^{oxidizing
agent} Dye-S-S-Dye + H₂O

water soluble water insoluble

Sulfur dyes are negatively ionized. No van der-waals force effect on them. Addition of salt improves efficiency of dyeing by increasing physical force.

Sulfur dyeing steps :

Goods preparation (fabric) :

Only normal pretreated fabric is required. Mercerized cotton goods cause an increased color yield of 30-40% . but goods bleached by Na hypo chloride should not be dyed by sulfur dyes.

Dye solution preparation:

Required amount of dye, soda ash and Na₂S are taken in a breaker and a little amount of cold water is added to makes a paste. in a separate vessel to it. Then the mixture is boiled and the paste is added to it. Then the mixture is boiled for 5 minutes to make the dye completely required.

Dyeing:

At first dye solution is taken in a dye bath and fabric is immersrd in that solution. Then it is heated for 10 – 15 minutes. Then half of the total NaCl solution os added and temperature is raised to boil and then the rest amount of NaCl is added. Dyeing at this temperature is continued for 30 minutes. Then the steaming is turned off and the application continued in cooling liquor for a further half hour. This steaming helps to diffuse the dye in to the fabric before fixing. Retained dye liquor should be rinsed out immediately from the surface of fabric.

Oxidation:

After dyeing the reduced water soluble form of dye should be make insoluble bu oxidation. tox it on the fabric permanently. for this oxidation we may use K₂Cr₂O₇, CH₃-COOH , Na-per borate, Na₂O etc.

After treatment of sulfur dyeing :

I. The dyes substance is soaped which makes the color bright and makes the shade permanent.

II. The light fastness properties of sulfur dyes, which is generally good, can be improve by after treatment with certain metallic salts. Thus a treatment with CuSO₄ and CH₃-COOH in presence of K₂Cr₂O₃ improves the light fastness. When the dichromate is also present the washing is slightly is also present the washing fastness is slightly improved.

Vat dye

Vat dye:

The vat dyes are found amongst the oldest natural coloring matters used for textile. The vat dyes are naturally obtained coloring matter from the ancient time and kept into wooden vat and make solubilised in vat by the process of fermentation, so it is called vat dyes. They water insoluble color compounds. They can’t be directly applied on cotton or other fibres as direct dyes are dyed. They have to be converted into their water soluble forms. During dyeinghey are soluble form of the dyes that are applied on cotton, followed be reconverted of the soluble form into the original insoluble form.They are excellent wash fastness and treatment.

Properties of vat dyes:

i. Vat dyes are insoluble in water

ii. Mainly use for cellulose fibre but protein fibre dyeing p^H should be controlled.

iii. Very costly

iv. Poor rubbing fastness

v. Good wash fastness

vi. Difficult to dyeing

Trade name of vat dyes:

Trade Name	Manufacturer	Country
Cibanone	Ciba-geigy A.G.	Switzerland
Sandothrene	Sandoz Ltd	Switzerland
Caledon	I.C.I.	U.K.
Artex vat	Rainbow Texdyes Corp.	India
Solanthrene	Francolor	France

Cotton dyeng with vat dyes:

During dyeing process the chemical and other specimen:

1) Dyestuff

2) Caustic soda

3) T.R. oil

4) Sodium hydro sulfite

5) Vetting temperature-70°C

6) Water

Dung Oxidation:

1) Hydrogen peroxide

2) Sodium peroxide

3) Sodium perborate

4) Hydroze

During Soaping:

1) Detergent

2) Soda ash

3) Water

4) Temoerature 95-100°C

Working procedure:

Ø This dye is insoluble in water. Ti is Dissolving in water and converted into leuco compound. If vat dye is in liquid form, can directly be converted but if it obtain powder form, then Y.R. oil is required and hot water are mixed. Then the solution should be stirred gently and forms a paste.

Ø The dye bath is set at 40°C at required water level with auxiliaries.

Ø Then sodium hydrosulfite and soda are added and leuco compound will be formed and vat dye dissolves in water.

Ø In the bath rest of water is mixed with the solution perfectly and closed the dye bath.

Ø Temperature raised 60°C and run time 10-15 minutes

Ø Temperature raised 70°C and run time 40-50 minutes

Ø Then dye bath drain and rinse

Oxidation:-

After dyeing the exhausted dyes are then oxidized to convert it to insoluble form into the fibre.

The oxidation process can be done in contact of air or other oxidizing agent

Soaping:

The dyed material is tt in a bath by detergent, soda ash.

Then rinse with cold water.

Then perform hot wash.

Again cold wash

Then acetic acid applied for neutralization

Then cold wash and dried.