Pigment

Pigments implied general insolubility and complete insolubility in water.

Difference between dye and pigment

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 and there are dyes which become pigments after application. Vat dye is a dye when used in dyeing but a pigment when used in printing.

	Dye	Pigment
Solubility in water	All dye must be soluble during process	Almost insoluble
Affinity	Possess a specific affinity towards fiber	Have no affinity but used as coating
Chemical nature	Organic and few are metallic	Most are metallic or organometallic.
Application	Through water medium	Through adhesive or binder

Uses of Pigment

0. Pigments are used for coloration of a very broad and diverse number of materials

1. Surface coating for interior, exterior, automotive and other application

2. Paints based on olegoresinous liquid and water emulsion

3. Printing ink for papers (lithographic, rotogravure and flexographic systems (and for other materials such as metal plates, foils, artists and writing material)

4. Coloration of plastics and rubber

5. Textile printing

6. Coloration of manmade fibers by mass pigmentation before fiber formation (dope dyeing) etc.

Required Properties of pigments

0. They should have covering power which is influenced by particle size

1. Should be inert, stable and have long life

2. Should have capability of mixing

3. Good wet fastness, light fastness and abrasion resistance

4. Good resistance to acid, base, perspiration, chlorine, peroxide and gas fading

5. Good solvent resistance (insoluble in water, CCl₄, Cl₂C=CHCl)

6. Suitable brilliance, hardness and stability

7. Suitable characteristics for good dispersion including particle size and distribution, electrical charge (most are negatively charged particle), specific gravity, purity and crystalline structure, conditions of precipitation of the pigments

8. Should be applicable to all fibers.

Physical/Chemical Properties of Pigments

Chemical Structure	Inorganic oxide, salts, organometallic toners, organic insoluble azo pigments, phthalocyanine metal complexes
Physical state	Very important, decreasing particle size increase color value but decreasing hiding power
Particle size	5-7 micron
Density	Sp gravity range from 1.17- 1.37 for most cases
Melting points	Usual range 110 -175°C
Boiling point	Decompose at 195- 345°C. phthalocyanine pigments sublimes at 500°C
Water solubility	Insoluble for all practical purposes.
Other solubility	Inorganic pigments are insoluble in most solvent
Spectra	Very strong and high, though not comparatively sharp peaks

Application Properties of Pigments

Fabric dyed	Any fiber can be dyed by selecting a suitable binder, quality greatly depends on binder used to affix the pigment
Fabrics printed	Any fiber by suitable binder even hard to print polyester blends and glass fibers
Disposable fabrics	Well suited for non woven fabrics
Dischargeability	Some pigments are suitable for discharge printing
Alkali fastness	Poor for organometallic azo toners, good for insoluble azo
Heat resistance	Extremely varied. Some are stable up to 200°C and some up to 300°C. optimum for inorganic pigments
Light fastness	Generally very good. Optimum for inorganic pigments
Wash fastness	Generally good to very good
Useful colors	Diarylide yellows and oranges, Hasna yellow, azoic reds, phthalocyanine blues and greens, carbon black, TiO2 white, violet and browns.
Processes used	Padding for dyeing
Aftertreatment	None required

Classification of pigments

• According to origin

1. Natural/Mineral: Iron ores, clays, chalk etc

2. Synthetic/chemical: white lead, ZnO, TiO₂ and large number of inorganic and organic color

• According to Reactivity

1. Reactive pigment: some pigments on account of the chemical character react with oil, fatty acids and soaps. These are called reactive pigments e.g. ZnO, red lead

2. Inert pigment: TiO₂

• According to Chemical Nature

1. Organic pigment: appx 25% (by wt.) of the world production of organic colorant is accounted for organic pigments. They account for only 4% of total pigment production. Of the total organic pigments production yellow, red and blue tones accounted for 89%.

Most organic pigments exhibit a small solubility, typically in polar solvent. All the organic pigments are soluble in one or more of the four chemical: Chloroform (CHCl₃), Methyl alcohol (CH₃OH), Dimethyl formamide (DMF) and concentrated H₂SO₄. Organic pigment consists of:

1. Azo pigment:

• Strong tinctorial strength

• Good alkali resistance

• Excellent brightness

• Cover a wide range with regard to other application properties

• Poor alkali resistance of certain organometallic pigments make them unsuitable for printing

2. Diarylide orange and yellows:

• Extremely bright color

• Inferior light fastness

3. Phthalocyanine

• Blue, greens are dominant shade especially in plastic coloration

• Offer low migration

• Good temperature stability

• Excellent light fastness

• Good heat resistance

• Excellent alkali resistance

• Good solvent resistance

• Used extremely in printing, pad dyeing and dope dyeing

4. Hasna yellow

• Good light fastness

• Have migration tendency

2. Inorganic pigment:

• They account for 96% (by wt.) of total production. More than half of their production volume is accounted for a single production, TiO₂, the most important white pigment

• H₂SO₄ is a good solvent for many inorganic pigments

• They are opaque

• Less expensive

• More weather resistant

• More chemical resistant

• Insoluble in most organic solvents

• Highest degree of light fastness

• Excellent heat resistance

They consist of

1. Salts: Sulfates, carbonates, silicates and chromates of many metal elements like, Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.

2. Oxides of Ti, Zn, Ba, Pb, Sb, Zr, Ca, Al, Mg, Cd, Fe, Mo, Cr etc.

3. Metal Complexes: Naturally occurring oxides and silicates

Difference between organic and inorganic pigment

	Organic	Inorganic
Solubility	Soluble in organic solvent	Soluble in inorganic solvent
Tinctorial strength	Higher	Lower
Brightness	Higher	Lower
Purity	Higher	Lower
Transparency	Opaque	Transparent
Weather resistance	Less	More
Chemical resistance	Less	More
Fastness	Good	Excellent
Cost	Expensive	Cheap