Course : B.E & B. Tech Subject: Applied Chemistry Unit: I

PROTECTIVE COATINGS

Syllabus:Protective Coatings – IntroductionConstituents, Functions and Uses of,

PaintsVarnishesLacquers

Dr. K. SIVAKUMARDepartment of Chemistry

SCSVMV University

More than You Know !

PAINTS

Paint: Paint is a mechanical dispersion of one or more pigments in a vehicle.

Vehicle: Vehicle is a liquid consisting of non-volatile film forming material, drying oil and a highly volatile solvent thinner.

When paint is applied on a metal surface, the thinner evaporates while the drying oil slowly oxidizes forming a dry pigmented film.

PAINTS

Requisites (Characteristics) of a good paint:A good paint should possess the following characteristics,

It should have a high covering power, corrosion and water resistance.

It should not get cracked on drying.

It should form a tough, uniform, adherent and impervious film.

It should give a glossy film, decent and stable colour on the metal surface.

It should be fluid enough to spread easily on the metal surface.

PAINTS

Constituents of paint and their functions:

Pigments: Pigments are solid, colour producing substances. It comprises 60-80% of the weight of paint film.

Different types of pigments

Brown umbreBrown

Chromium oxideGreen

Prussian blueBlue

Chrome redRed

Carbon blackBlack

White leadWhite

ExampleColour of pigments

PAINTSFunctions of pigments:

It provides colour and opacity to the film.

It provides strength to the film.

It provides resistance to paint against abrasion/wear.

It protects the film by reflecting harmful UV rays.

It gives impermeability of paint film to moisture.

A good pigment should be, chemically inert, non-toxic, free mixable with vehicle and cheap.

PAINTSDrying oil or Vehicle Drying oil is the film forming, non-volatile constituents of paint. These are glyceryl esters of high molecular weight fatty acids present in animal and vegetable oils.

Examples: Linseed oil, Dehydrated castor oil etc.,

PAINTS

Functions of drying oil or vehicle

Drying oil form a protective film by the oxidation and polymerization of the vegetable oil.

They hold the pigment particles together on the metal surface.

They impart water repellency, toughness, durability and water-proofness to the film.

PAINTS

Thinners (solvents)

Thinners are solvents that are added to paint to reduce viscosity of paints, so that they can be easily applied on metal surface.

It is used to dissolve oil, pigments etc.,

It reduces the viscosity of the paint.

It increases the elasticity of the paint film.

It increases penetrating power of the vehicle.

It evaporates readily and helps drying of film.

Functions of thinners

Examples: Turpentine, benzene, kerosene, petroleum spirit, naphtha, etc.,

PAINTS

Driers

Driers are oxygen-carrier catalysts. They accelerate the process of drying of oil film by oxidation, polymerization and condensation.

Examples: Resinates, Tungstates and Naphthenates of Co, Pb, Mn, and Zn.

Functions of driers

Driers provide oxygen, which is essential for the oxidation, polymerization and hydrogenation of drying oil.

PAINTS

Fillers or Extenders

Fillers are white or colorless and finely divided solids having low refractive indices.

Functions of fillers

It reduces the cost of the paint.

It prevents shrinkage and cracking of the film.

It increases the durability of paint.

It retards the settling of pigment.

It modifies the shades of the pigment

talc

clay

gypsum

Examples: Talc, CaCO3, China clay and Gypsum

PAINTS

Plasticizers

Plasticizers are added to the paint to give elasticity and to prevent cracking of the film.

It reduces the brittleness and increases the adherences of film on the surface of metal.

Examples: Triphenyl phosphate, tributyl phthalate.

PAINTS

Anti skinning agents

Anti skinning agents are added to the paint which prevent gelling and skinning of the paint film.

Example: Polyhydroxy phenols.

PAINTS

Plasticizers

Plasticizers are added to the paint to give elasticity and to prevent cracking of the film.

It reduces the brittleness and increases the adherences of film on the surface of metal.

Examples: Triphenyl phosphate, tributyl phthalate.

VARNISHES

Varnish is a homogeneous colloidal solution of natural or synthetic resin in thinner oil or both.

When Varnish is applied on a metal or wood surface, the it dries by evaporation, oxidation and polymerization giving a transparent, durable and glossy film on surface.

VARNISHES

Requisites (Characteristics) of a good varnish: A good varnish should possess the following characteristics,

It should be soft and tender.

It should produce a glossy and shinning film on drying.

It should adopt itself to contraction and expansion due to temperature variations.

It should dry quickly.

It should not shrink / crack after drying.

VARNISHES

Types of Varnishes:1. Spirit Varnish

2. Oil varnish

VARNISHES

Oil varnish: Oil varnish is produced by dissolving natural or synthetic resins in a drying oil and volatile solvent.

Oil varnish dries up by evaporation of volatile solvent followed by oxidation and polymerization of drying oil. It is used for exterior as well as interior works.

Example: Copal varnish (copal + linseed oil + Turpentine).

VARNISHES

Spirit Varnish: Spirit varnish is produced by dissolving natural or synthetic resins in a completely volatile solvent. Spirit varnish dries up by evaporation of solvent. It is used for polishing wooden furniture.

Example: Shellac in alcohol

VARNISHES

Constituents of varnish and their functions:

Resins: Natural or synthetic resins can be used. Resins give elasticity, good adhesion, colour and glossiness to varnish film.

Example: Phenol-formaldehyde and vinyl resins.

Drying oils: They dry by oxidation and polymerization.

Example: Linseed oil, Fish oil and Soybeans oil.

Solvents / Thinners: Solvents are used for dissolving various constituents of varnish. They reduce viscosity and help in drying process.

Example: Turpentine, Ethyl alcohol, Kerosene.

VARNISHES

Constituents of varnish and their functions:…..

Driers: Driers are added to enhance the drying rate of varnish film.

Example: Co, and Pb slat of linoleates, resinates and naphthenates.

Anti skinning agents: They prevent the skinning and peeling off of varnish.

Example: Tertiary amyl alcohol.

VARNISHES

Uses of varnishes:

To protect articles against corrosion.

Used as a brightening coat to painted surface.

Used for improving appearance of wood surface.

LACQUERS

Lacquers are colloidal dispersion of cellulose derivative, resins and plasticizers in solvent or in diluents or both.

Lacquers dries quickly in air by evaporation of solvent and diluents forming a transparent hard glossy and waterproof film.

LACQUERS

Constituents, examples and functions of lacquers

Constituents Example Functions

Resins Alkyd, CopalProvide adhesion and to increase thickness of film.

Cellulose derivative

Cellulose nitrate

Provide hardness and to increase durability to film.

PlasticizersDibutyl phthalate

To reduce brittleness and to increase elasticity of film.

Solvents Ethyl acetate To dissolve film forming constituents.

Diluents Benzol, Toluol To reduce viscosity of medium

LACQUERS

Uses of lacquers

Used for interior decorations like painting woodwork and furniture.

Used in the preparations of artificial leather goods.

Used is giving finishing coat to automobile bodies.

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Good Luck!

Dr. K. SIVAKUMARDepartment of Chemistry

SCSVMV Universitychemshiva@gmail.com

+91 98423 61378

The End