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  Pigments, Chemistry tutorial

Introduction

Both dyes and pigments appear to be coloured since they absorb several wavelengths of light preferentially. In contrast through a dye, a pigment usually is insoluble, and has no affinity for the substrate. Several dyes can be precipitated through an inert salt to generate a lake pigment, and depend on the salt utilized they could be aluminum lake, calcium lake or barium lake pigments.

Definition of Pigments

Pigments are usually coloured, organic or inorganic solid powder, and generally are insoluble. They aren't affected physically or chemically in the substrate in which they are incorporated. Pigments can provide a full range of colours.

Features of Pigments

The pigments are versatile colouring agents that come through all round features to provide credence to their suitability in a variety of mediums. Several of the striking characteristics are: Excellent light and weather fastness:  

A good baking stability that makes them suitable for automotive and other industrial paints

High tinting strength--

Good over spray fastness whenever applied in paints

Provides heat stability of around 300° C in the case of Polyolefins

Plastics

Outstanding solvent resistance properties

Easily dispersible

Consistency and uniqueness of shades.

Types of Pigments

a. Inorganic Pigments:  Those pigments that are made up of mineral compounds are identified to be Inorganic Pigments. Such minerals are mostly oxides, sulphides of one or more metals. To impart colours in different compounds, inorganic pigments are applied.

b. Organic Pigments: In organic pigments, the molecules are made of carbon atoms along through hydrogen, nitrogen or oxygen atoms. Organic pigments can be sub separated into 2 viz. Natural organic pigments and Synthetic organic pigments.

c. Metallic Pigments: Obtained from the ores of metal

Natural and Synthetic Organic Pigments

i. Natural  Organic  Pigments - Pigments  of  this  category  are derived  from   animal  and  plant  products.  Utilize of such pigments is rare due to their poor light fastness property.

ii. Synthetic Organic Pigments - Pigments of this category are carbon based and are often made from petroleum products. Under intense pressure or heat, carbon base molecules are created from petroleum, acids, and other chemicals and synthetic organic pigments have been formulated from such molecule. Most of the synthetic organic pigments except carbon black aren't stable and they will wear away at the time of use as a pigment.

Table: Comparison between Natural and Synthetic Organic Pigments

Natural

Organic

Pigments

Synthetic Organic Pigments

Miscellane

ous: e.g.

Rose-

Madder,

Bone

Black,

Carbon

Black

Quinacr

id-

ones

Phthalo

c-

yanines

Peryl

e-nes

Pyrrol

es

Ary-

lamid

es

Metal

Complex

es: e.g.

Transpar

ent

Yellow,

New

Gambog

e

Miscellane

ous: e.g.

Dioxazsine-

in

Danthrone.

Differences among Types of Pigments

 Table: Property of various Kinds of Pigments 

Property

Behaviour

Inorganic

Pigments

Classical

Organic

Pigments

Specialty

Organic

Pigments

Organic

Dyestuffs

Opacity

Usually high

        Translucent to Transport

Very

Transparent

Colour Strength

Low  to

moderate

Considerably stronger than

Inorganic Pigments

Strongest

Dispersability

Usually

Good: Often

Abrasive

Adequate

Poor to good

Not

required;

Soluble

Heat Resistant

Usually 5000F; Some 2000C

1500 C-  3000 C

2000 C- 3000 C

2500 C- 3500 C

Migration

resistance

Excellent

Moderate -

Good

Good -

Outstanding

Very Poor -

good

Light Fastness

(on a Blue scale

6 to 8

2 to 6

6 to 8

2 to 7

Weather

resistance

Outstanding

for selection

Insufficient

Excellent for

Selection

Good for

selection

Natural pigments

Different kinds of natural pigments are originate in different types of natural sources. Several of such pigments are attained from insects/animals and others are acquired from plants. Other pigments belong to cosmetics and contain affinity to all these foods and drugs that need colour additives. Natural pigments are good for utilize in the shower gel, bath bombs, bath salts, shampoo, soap, lotion and in many more other products. Such pigments have good quality of bleed protectiveness in soap and these are as well water dispersible. But, such pigments don't have high intensity in light.

Kinds of Natural Pigments

Annatto natural pigments

Annatto natural pigments contain reddish-orange colourant. This colourant is obtained from seeds of Achiote trees. Annatto pigments are given 'annatto' as a name since such pigments are derived from Annato shrubs. The concentration of annatto pigments in the annatto shrubs, fluctuate from place to place. Whenever the foodstuffs need hues of yellow to orange, annatto based pigments are utilized. The chief food items in that annatto dyes are utilized are: in making cheese (around 50%), fish processing (around 20%), and confectionery (around 10%), and dairy products except cheese (around 20%). Bixin is the main carotenoid of annatto Bixin

Plant pigments

Chlorophyll is responsible for the green colour of flowering plants. Acetone is needed for the extraction of plant pigments. Calcium carbonate must be present for stimulating the extraction process. Any other mild alkali can as well be utilized for the extraction of plant pigments.

The alkali is utilized during the extraction procedure to neutralize the acid that releases from the plant tissues.

37_Chlorophyll.jpg

Fig: Chlorophyll

Carotenoid pigments

Carotenoid pigments for instance β-carotene, have colour range from yellow to red. Mixture of acetone and hexane in the 1:3 ratios is used for the extraction of carotenoid pigments. The acetone fraction enclosing the extracted carotenoid pigments is eliminated through water. To eliminate the hexane residue, the mixture is treated by activated MgO2 in diatomaceous earth column.

630_chemical structure of carotene series.jpg

Fig: chemical structure of carotene series.

Betalain pigments

Betalains are another type of colour pigments which are also derived from plants. These pigments are present in two forms. The first is - cyanin having purple-red colour and are present in high concentration. The second is -xanthine that is yellow in colours that are obtained in low concentration. Such pigments can easily be extracted from plant tissues by water as they are highly soluble in water. The extracted water is then blended smoothly through ethyl alcohol in 1:1 ratio. The utilize of ethanol in the mixture is to diminish the enzymatic action; otherwise the pigments will be degraded.

2197_Betalain Pigments.jpg

Fig: Betalain Pigments  

Anthocyanin pigments

Anthocyanin pigments are coloured pigments and such are found in abundance in plant kingdom. The colours imparted via such pigments are blue, red and purple. The colour of fruits and flowers are as well due to the colour impartation of such pigments. Anthocyanins are soluble in water and their extraction from the plant parts is as well easy. Slightly acid mixed water is needed for the extraction of such pigments.

2029_Anthocyanin Pigments.jpg

Fig: Anthocyanin Pigments

Limitations of Natural Pigments

1. Natural pigments are generated through traditional process.

2. Such pigments have lower intensity in comparison to synthetic pigments.

3. To attain the similar depth like synthetic pigments, natural pigments need huge quantities of raw materials.

4. Such pigments aren't efficient for colouring of the synthetic clothes.

5. To ensure adequate wash and light fastness, such pigments need application through different metallic mordants.

6. Application through few mordants like alumina and iron has adverse consequences on the environment.

7. The liquid waste of natural pigments encloses heavy metal that is much more than the desired limit.

Factors affecting the selection of Pigments

Factors affecting the choice of a pigment based on application. The subsequent broad features can be taken as a guide:

Hiding efficiency

Colour pH value

Bulking value

Density

Refractive index

Hardness

Oil absorption

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