Unit:8 Paints and Varnishes ( Engineering materials Second year first parts)

8.1 Introduction 

Paints 

Paints are applied on the surfaces of timber, metals and plastered surfaces as a protective layer and at the same time to get pleasant appearance. Paints are applied in liquid form and after sometime the volatile constituent evaporates and hardened coating acts as a protective layer. 

Properties of an Ideal Paint 

1.It should be possible to apply easily and freely. 

2.It should dry in reasonable time. 

3.It should form hard and durable surface. 

4.It should not be harmful to the health of workers. 

5.It should not be easily affected by atmosphere. 

6.It should possess attractive and pleasing appearance. 

7.It should form a thin film of uniform nature i.e., it should not crack. 

8.It should possess good spreading power. 

9.It should be cheap. 

Varnish 

Varnish is the solution of resinous substances like amber, copal, shellac, gum resin etc. in solvents like oil, turpentile, alcohol etc. Depending upon the solvents used varnishes are classified as, oil varnishes, turpentile varnishes, spirit varnishes and water varnishes. 

The desirable properties of an ideal varnish are:

1.It should give glossy surface. 

2.Should be durable. 

3.It should dry rapidly after application. 

4.It should not develop cracks after drying. 

5.It is commonly used on wooden surfaces. 

8.2 Uses of Paints and Varnishes 

Paint is a substance used as the final finish to all surfaces and as a coating to protect or decorate the surface. Paint is a pigmented opaque material that completely covers and hides the surface to which it is applied. Paint is available in oil-based and water-based formulae. 

It is used as a protective coating and is normally sprayed/brushed on. Paint prevents corrosion. It is a combination of pigments with suitable thinners or oils to provide decorative and protective coatings. 

Painting protects a surface from weathering effects and also prevents corrosion of metals. Paint consists of two ingredients: 

• A base of solid matter which helps to obscure the surface. 
• A liquid vehicle, which carries the solid matter. 

8.3 Composition of various types of Paints: Oil paint, Water Paint, Cement paints and Acrylic paint Composition / Constituents of Paint 

The essential composition / constituents of paints are: 

1.Base  2.Vehicle  3.Pigment  4.Drier and  5.Thinner. 

1.Bases :

It is a principal constituent of paint. It also possesses the binding properties. It forms an opaque coating. Commonly used bases for paints are white lead, red lead, zinc oxide, iron oxide, titanium white, aluminium powder and lithophone. A lead paint is suitable for painting iron and steel works, as it sticks to them well. However it is affected by atmosphere action and hence should not be used as final coat. While zinc forms good base but is costly. Lithophone, which is a mixture of zinc sulphate and barytes, is cheap. It gives good appearance but is affected by day light. Hence it is used for interior works only. 

2.Vehicles :

The vehicles are the liquid substances which hold the ingredients of a paint in liquid suspension and allow them to be applied on the surface to be painted. Linseed oil, Tung oil and Nut oil are used as vehicles in paints. Of the above four oils, linseed oil is very commonly used vehicles. Boiling makes the oil thicker and darker. Linseed oil reacts with oxygen and hardens by forming a thin film. 

3.Pigments :

Pigments give required colour for paints. They are fine particles and have a reinforcing effect on thin film of the paint. The common pigments for different colours are: 

Black - Lamp black, suit and charcoal black. 

Red - venedion red, red lead and Indian red. 

 Brown - burned timber, raw and burned sienna Green - chrome green, copper sulphate. 

Blue—prussian blue and ultra marine Yellow—ochre and chrome yellow. 

4.Drier :

These are the compounds of metal like lead, manganese, cobalt. The function of a drier is to absorb oxygen from the air and supply it to the vehicle for hardening. The drier should not be added until the paint is about to be used. The excess drier is harmful because it destroys elasticity and causes flaking. 

5.Thiner :

It is known as solvent also. It makes paint thinner and hence increases the coverage. It helps in spreading paint uniformly over the surface Terpentine and neptha are commonly used thinners. After paint applied, thinner evaporates and paint dries. 

Types of Paints 

Depending upon their constituents there are various types of paints. A brief description of some of them which are commonly used are given below: 

1.Oil Paint: 

These paints are applied in three coats-primer, undercoat and finishing coat. The presence of dampness while applying the primer adversely affect the life of oil paint. This paint is cheap and easy to apply. 

2.Enamel Paint: 

It contains white lead, oil, petroleum spirit and resinous material. The surface provided by it resists acids, alkalis and water very well. It is desirable to apply a coat of titanium white before the coat of enamel is applied. It can be used both for external and internal walls. 

3.Cement Paint:

It is available in powder form. It consists of white cement, pigment and other additives. It is durable and exhibits excellent decorative appearance. It should be applied on rough surfaces rather than on smooth surfaces. It is applied in two coats. First coat is applied on wet surface but free from excess water and allowed to dry for 24 hours. The second coat is then applied which gives good appearance. 

4.Acrylic paints 

Acrylic paints are one of the most popular paints for professionals, artists and crafters for everyday projects like painting, crafting, ceramics and art classes. Acrylic paint is much like any other paint in that it is made from a simple combination of ground pigment and binder. They are water-based and easily come out of brushes and most art materials. 

 There are three main components in any acrylic paint. 

 1.Pigment 2.Binder and 3.Vehicle: 

1.Pigment - 

Pigments are granular solids which give paint its color. They are milled to a tiny particle size and do not dissolve, but remain suspended in the paint. Pigments can be organic, inorganic, natural and synthetic. They have little or no affinity for the surface to which they are applied.

2.Binder - 

A binder is the substance that keeps pigment in place after the paint dries. Acrylic paint has acrylic polymer as its binder and this forms a film after the water has evaporated.

3.Vehicle - 

This refers to the part of the paint that carries the pigment and binder. Water is the vehicle for water-based acrylic and when combined with the binder, it creates a polymer emulsion. Once the water leaves the system via evaporation or absorption, the paint dries, creating a stable clear polymer film full of trapped colored pigment particles.

8.4 Methods of application of various paints 

Preparation of surface for application of paint is the most important part in painting. The surface to be painted should not be oily and it should be from flakes of the old paint. Cracks in the surface should be filled with putty and then with sand paper. Then primer is applied. Painting work should be carried out in dry weather. The under coats and first coats must be allowed to dry before final coat is applied.