Organic Coatings

ORGANIC COATINGS

An organic coating is a thin protective (or) decorative layer made from carbon based materials like polymers and resins. They are applied to surfaces for protection against corrosion, wear and other environmental factors.

Examples:

1. Paints

2. Varnishes

3. Enamels

4. Lacquers.

1. Paints

Paint is a mechanical dispersion of one or more finely divided pigments in a medium (thinner + vehicle). When a paint is applied to a metal surface, the thinner evaporates, while the vehicle undergoes slow oxidation forming a pigmented film.

Requisites or characteristics of a good paint

(i) It should spread easily on the metal surface.

(ii) It should have high hiding (covering) power.

(iii) It should not crack on drying.

(iv) It should adhere well to the surface.

(v) The colour of the paint should be stable.

(vi) It should be a corrosion and water resistant.

 (vii) It should give a glossy film.

Constituents and their functions of a paint

1. Pigments

Pigments are solid and colour producing substances in the paint.

Functions

(i) It gives colour and opacity to the film.

(ii) It also provides strength to the film.

(iii) It protects the film by reflecting the destructive UV rays.

Examples

(i) White pigments

(ii) Black pigments

(iii) Red pigments

 (iv) Blue pigments

(v) Green pigments

2. Vehicle or drying oil

This is a non‒volatile portion of a medium. This is the film forming constituent of the paint. These are high molecular weight fatty acids present in vegetable and animal oils.

Functions

(i) They form a protective film by the oxidation and polymerisation of the oil.

(ii) They hold the pigment particles together on in the metal surface.

(iii) They impart water repellency, toughness and durability to the film.

Examples: Linseed oil, dehydrated castor oil.

3. Thinners (or) solvents

This is a volatile portion of a medium. It easily evaporates after application of the paint.

Functions

(i) It reduces the viscosity of the paint, so that it can be easily applied on the surface.

(ii) Its dissolves the oil, pigments, etc. and produces a homogeneous mixture.

(iii) It increases the elasticity of the film.

(iv) It increases the penetrating power of the vehicle.

4. Extenders or fillers

These are white (or) colourless pigments.

Functions

(i) It reduces the cost of the paint.

(ii) It retards the settling of the pigment in all paints.

(iii) It modifies the shades of the pigments.

(iv) It prevents shrinkage and cracking.

Examples: Talc, gypsum, china clay, etc.

5. Driers

These are the substances, used to accelerate the process of drying.

Functions

(i) They act as oxygen‒carriers (or) catalysts.

(ii) They provide oxygen, which is essential for oxidation, polymerisation of drying oil.

Examples: Metallic soaps, linoleates and resinates of Co, Mn and Pb.

6. Plasticisers

These are chemicals added to the paint to provide elasticity to the film and to prevent cracking of the film.

Examples: Triphenyl phosphate, tricresyl phosphate, etc.

7. Anti‒skinning agents

These are chemicals added to the paint to prevent gelling and skinning of the paint.

Example: Polyhydroxy phenol.

Pigment Volume Concentration (P.V.C)

It is an important property of a paint. The following equation is used to calculate the P.V.C.

P.V.C = Volume of pigment in the paint / [ Volume of pigment in the paint + Volume of non‒volatile vehicle in the paint ]

Higher the volume of P.V.C, lower will be the durability, adhesion, consistency of the paint.

Mechanism of drying of drying oils

Drying oils are the film forming constituents of the paint. These are glyceryl esters of high molecular weight fatty acids.

Structure of an oil

The mechanism involves oxidation, polymerisation condensation reactions. The oil containing conjugated double bonds dry very faster than the non‒conjugated oils.

Mechanism for conjugated oil

The various steps involved in the mechanism of drying of conjugated oil are given below.

Step ‒ 1

The oil absorbs oxygen and gives di‒radical

Step 2

The di‒radical again reacts with oxygen and fatty group to give peroxy and poly peroxy radicals

Step ‒ 3

Polyperoxides are decomposed by heat and light to give alkoxy radicals 

Step ‒ 4

These alkoxy radicals react with another fatty groups, form ether linkages.

Thus, the various oil molecules are linked through ether linkages and forms highly cross‒linked macromolecular film. The structure of macromolecular film is shown below.

Failure of a paint

A paint may fail due to the following reasons.

1. Chalking

It is the progressive powdering of the paint film on the painted surface. This occurs due to improper dispersion of pigment in vehicle.

2. Cracking

Cracking of paint film is due to the unequal expansion or contraction of paint coats.

3. Erosion

It is very quick chalking.

4. Blistering

This is due to improper surface exposure of paint to strong sunshine

2. Varnishes

Varnish is a homogeneous colloidal solution of natural or synthetic resin in spirit or oil. Varnish is used for both protection and decoration of metal surfaces.

Types of varnishes

Depending upon the type of solvent used, varnishes are classified into two types.

1. Spirit varnish

2. Oil varnish.

1. Spirit varnish

Spirit varnish is prepared by dissolving natural or synthetic resins in a volatile solvent (spirit). This type of varnish dries just by the evaporation of the solvent.

The film formed is brittle and not strong. It is used for polishing wooden surfaces.

2. Oil varnish

Oil varnish is prepared by dissolving natural or synthetic resins in a drying oil and volatile solvent. This type of varnish dries by the evaporation of the solvent followed by oxidation and polymerisation of the drying oil.

Oil varnishes dry slowly, but the film is hard. It is used for interior and exterior works.

Characteristics of a good varnish

(i) It should be soft.

(ii) It should produce a shining and glossy film on drying

(iii) It should dry quickly.

(iv) It should not shrink or crack after drying.

Uses of Varnish

1. It is used to protect the materials against corrosion.

2. It is a coating prior to paint coating.

3. It is also used for improving the appearance of wooden surfaces.

3. Enamels

Enamels are pigmented varnish, ie., a colloidal dispersion of pigments and varnish. Enamels dry slowly but form hard, lustrous and glossy film.

Drying occurs in two ways

(i) Brushing enamels

The drying of enamel painted surface may occur at normal temperature (sun light) in presence of air. This type of enamels are known as "brushing enamels."

(ii) Baking enamels

The drying of enamel painted surface may also occur at elevated temperature either in the presence or absence of air. This type of enamels are known as "baking enamels."

Table 4.5 Constituents of enamels

Japans

When coloured pigments are used, such enamels are called japans. If black pigments are used, the enamels are called black japans.

Black japans are prepared by the homogeneous mixing of black asphalt dye, linseed oil, turpentine and solvent and baked at 210°C for 2 to 3 hours and then cooled.

Uses of Enamels

1. Enamels are used to protect the materials against corrosion.

2. Japans are used for painting bicycles, automobiles, etc.,

4. Lacquersb

Lacquer is a colloidal dispersion of cellulose derivatives, resins and plasticisers in solvents and diluents. Lacquers dry by the evaporation of the solvent and diluents. It is used for interior decoration.

Table 4.6 Constituents of lacquers

Uses of lacquers

(i) It is used for interior decoration of wood work and furniture.

(ii) In the preparation of artificial leather goods.

(iii) It is also used for giving a finishing coat to automobile bodies.