Composition of oil paints

Learn :Composition of oil paints: Base: White lead, Red lead, Zinc oxide (or zinc white), Iron Oxide, and metallic powders such as Aluminum, Copper and Bronze etc. are the commonly used bases,Vehicle: Oils most commonly used as vehicles are : Linseed oil, Poppy oil. Nut oil and Tung oil. Colouring pigments,Solvent or thinner,Drier,Inert filler

Composition of oil paints

Oil paints consist essentially of – (i) a base, (ii) a vehicle (always an oil, generally raw or boiled linseed oil), and (iii) one or more colouring pigments. It may also contain one or more of (iv) a solvent or thinner, (v) a drier, and (vi) an inert filler.

By suitable variation of the type and proportion of the various constituents the paints can be made dry, glossy or flat as desired. Other properties such as permeability to water could also be varied accordingly. All the possible constituents of paints are described in details below :

 Base.

It provides body to the paint and on it depends the nature of paints to a great extent :

(i) It is the slid matter forming the main body of the paint.

(ii) It makes the paint film harder and more resistant to abrasion.

(iii) It forms an opaque layer to obscure the surface of material to be painted.

(iv) It reduces shrinkage cracks formed on drying.

White lead, Red lead, Zinc oxide (or zinc white), Iron Oxide, and metallic powders such as Aluminum, Copper and Bronze etc. are the commonly used bases. Paints are very often named after the bases used e.g. Lead paints, Zinc paints, Aluminum paints etc., etc.

Commonly used bases

Characteristics of the more commonly used bases are discussed below:

White lead :

(i) It is the cheapest and the most commonly used base for ordinary building works.

(ii) It has greater covering power than any other base.

(iii) It is dense and has a good body to obscure the painted surface.

(iv) It weathers well.

(v) Not suitable for delicate work as it gets discoloured on exposure. It may be used for under coat and zinc white for the finishing coat.

(vi) Not suitable for painting iron work as it does not stop rusting.

Red lead.

(i) With oil it is considered best for first coat or iron and in priming coats for wood work as it sticks well and gives protection against rust.

(ii) It is a good drier for linseed oil.

Lead paints are poisonous and as such necessary precautions should be taken while painting with spray machine or while scrapping old paint. These should not be used fresh.

Zinc oxide or Zinc white.

(i) It is not affected by weather.

(ii) It takes a fine polish and is used for decoration work.

(iii) It is not poisonous.

(iv) It is less durable and more costly than lead bases. 

 Lead drier should not be used with zinc paints.

Oxide of Iron.

(i) Used primarily in the finishing coat for painting iron work.

(ii) It prevents rust formation.

(iii) It is comparatively cheaper.

(iv) Tints vary from yellowish brown to black. 

Vehicle.

(i) It is an oily liquid in which the base and pigment are soluble.

(ii) It facilitates the paint to be conveniently spread evenly over the surface by means of a brush.

(iii) It acts as a binder for the base and cause it to stick to the surface.

(iv) On drying it forms a tough and an clastic film.

Commonly used as vehicles

Oils most commonly used as vehicles are : Linseed oil,Poppy oil. Nut oil and Tung oil.

Linseed oil.

It is the most widely used vehicle for all ordinary painting works. It is used either raw or boiled.

Raw linseed oil.

(i) It is thin, pale in colour and transparent.

(ii) It has sweet taste and no smell.

(iii) It becomes hard and stiff on being exposed to air.

(iv) When spread in a thin film it looks like varnish.

(v) It dries very slowly but by adding a little white lead and allowing it to settle for a week its drying properties could be improved.

It is used for painting delicate and interior work or wood-work should not be used for work exposed to weather.

Bolied linseed oil.

(i) It is thicker and darker in colour.

(ii) It dries quicker.

(iii) Its colour varies from deep amber to rich brown.

(iv) On drying it leaves a hard glossy and more durable surface.

(v) It has more covering capacity.It is used for external work.

Double boiled linseed oil

(i) It is as light in colour as the raw linseed oil but smells differently.

(ii) It dries quicker and gives better results.

(iii) It requires a thinning agent like turpentine oil.

It is better suited for painting metals and plastered surfaces.

Tung oil.

It is far superior to linseed oil and is used for preparing superior paints.

Poppy oil.

Though its drying qualities are inferior to those of linseed oil still it is used for very delicate colours which last longer.

Nut oil.

It is almost colourless; dries quicker and is not durable. It is cheap.

Colouring pigments.

These are finely ground colouring matters. Their main function is to give colour and opacity to the paint. Pigments are liable to fade because of the bleaching action of sun rays. These are also subjected to change of colour under the influence of moisture, heat or hydrogen sulphide. Commonly used pigments are :

Blacks :

Lamp black, vegetable black, ivory black

Blues :

Indigo, prussian blue.

Yellow :

Chrome yellow, raw siena, yellow ochre.

Greens :

Copper sulphate.

Browns :

Raw umber, burnt umber.

Red :

Red lead, vermillion, carmine.

Solvent or thinner.

A liquid thinner is added to the prepared paints to increase their fluidity to the desired consistency so as to make them work more smoothly and also to help penetration of porous surfaces. Oil of turpentine is the most commonly used thinner. An excessive use of thinner dulls the colours and the gloss of linseed oil. It reduces the protective value of coating. As such a thinner is generally not used in the finishing coat on exposed surfaces. It is better to use raw linseed oil as thinner except in white paint for which raw linseed oil mixed with turnpentine oil is used. For getting a good finish sometimes Copal varnish is also used as a thinner. 

Drier.

Driers are added to paints to quicken the drying of vehicles. Linseed oil dries by absorbing oxygen and it could be expedited by adding substances rich in oxygen and it could be expedited by adding substances rich in oxygen. Some of the commonly used driers are : Litharge, Red lead, lead acetate, Managanese dioxide and Zinc sulphate.

Litharge.

It is the most commonly used drier (its proportion being 25 gms per litre of vehicle). It is especially used for lead paints but is not used for finishing coats.

Red lead.

It is less powerful than litharge and is used only when it does not affect the tint.

Lead acetate.

When ground in oil it is used for lighter tints.

Manganese dioxide.

It gives quick effect but can be used only for deep tints because of its darker colour.

Zinc sulphate.

It is more costly and is never used in paints with lead base.

Driers should not be used unnecessarily as they have a tendency to destroy the elasticity and cause flaking of the paint. Drier should not be used in a paint that dries well.

Not more than one drier should be used at a time and it should be added to the paint just before the paint is to be used.

Inert filler.

It is an adulterant mixed to replace the base in part and thus reducing the cost of paint. Commonly used fillers are silica, charcoal, powdered chalk, aluminium silicate and barium sulphate etc.