Composition

Composition

Base The base, usually a metallic oxide, is the principal constituent of the paint. It makes the paint film opaque and possesses binding properties which reduce the shrinkage cracks in the film on drying. Some of the examples of base are white lead, red lead, zinc white, aluminium powder, iron oxide, etc. Their detailed description is given in Table Lead based paints are in general affected by atmosphere and are not recommended for final coats. Zinc white is weather resistant. For inferior works Lithophone (barium sulphate chemically combined with zinc sulphide) is used for inside work. Aluminium powder is used as base for all aluminium paints.

Vehicle Also known as binder, vehicle is an oil to which the base is mixed. It holds the constituents of paint in suspension and helps spread it over the surface to be painted, imparts durability, toughness and water proofness to the paint film and resistance to weathering and gloss to the painted surface and forms the body of the paint. The examples are natural drying oils such as linseed oil, nut oil, poppy oil and tung oil (Table ); animal, paint, artificial and synthetic glues in glue paints and air slaking lime and polymer in lime water colours and polymer paints respectively.

The natural drying oils (glycerides of the unsaturated fatty acids) harden in this layers to form strong and elastic surface coats. These are available in oxidized and polymerized varieties. The former being obtained by blowing air through linseed oil heated to about 160 o C and by introducing a manganese-lead-cobalt drier the latter is obtained by polymerising linseed oil by heating it to about 275 o C and introducing a manganese-lead-cobalt siccative

S.No  Name          Description

1.       White lead : This is a carbonate of lead and forms the base of lead paints. It is dense, permanent and  water-proof.  It  is  not  suitable  for  delicate  works  as  lead becomes discoloured when exposed to sulphur vapours.  It  is most  suitable for wood surfaces; Since it does not afford protection against rusting, it is not suitable for iron surfaces.

2.       Red lead : This is an oxide of lead and forms the base of lead paints. It is most suitable for painting iron surfaces and for providing a priming coat to wood surfaces. It          solidifies in a short time with linseed oil and hence, it is used as a drier also.

3.       Zinc white : This is an oxide of zinc and forms the base of all zinc paints. It is smooth transparent and non-poisonous. It is not discoloured when exposed to sulphur vapours. It is less durable and is difficult to work.

4.       Oxide of iron : This is an oxide of iron and forms the base of all iron paints. The tint of paint varies from yellowish brown to black. It mixes easily with the vehicle. It is effective in preventing rusting of iron surfaces and is cheap and durable. It is generally used for priming coat of iron surfaces.

5.       Titanium white : This material possesses intense opacity. It is non-poisonous and provides a thin transparent film. It is used for receiving the coat of an enamel.

6.       Antimony white : This is almost similar to titanium white.

7.       Aluminium powder : This forms the bulk of aluminium paints. It keeps moisture content of wood surfaces  practically  the  same  and  also  prevents  cracking  and  warping of wood. It is generally used for a priming coat to new wood work.             

8.       Lithophone : This  is  a mixture  of  zinc  sulphide  and  barytes.  It  is  similar  in appearance to oxide of  zinc.  It  is  cheap  and  can  easily  be  applied  on  the  surface.  However, when  exposed to daylight, it changes colour, hence used for interior works only.

.Linseed oil is the most widely used vehicle. It contains acid which reacts readily with oxygen and hardens by forming a thin film known as linoxyn. A priming coat of pure linseed oil induces corrosion which is greatly retarded by the presence of pigments. For this reason priming coat should contain little oil. Raw linseed oil has slow drying rate as such pale boiled linseed oil having better drying properties is used. The best results are obtained by using double boiled linseed oil.

Pigments are used to hide the surface imperfections and to impart the desired colour. They protect the paint film by reflecting the destructive ultra violet light, which acts as a catalytic agent for the destructive oxidation of the film. They also improve the impermeability of the paint film and enhance its resistance to weathering, affect the flow characteristics making it possible to paint vertical and uneven surfaces smoothly. Pigments are finely ground mineral, organic substances or metal powders; their size in organic coatings ranges from 0.1 to 5.0 microns in diameter. Their general properties are covering power, colouring capacity, fineness, fire resistance, chemical stability and weather resistance. The fine particles of the pigments have a reinforcing effect on the paint film.

The common pigments are classified as natural and artificial. The former used for preparing limestone and glue paints, putties and coloured building mortars, include ground natural white chalk, mastics, grey graphite, dry yellow ochre (a clay containing over15 per cent of iron oxide), etc. Artificial mineral pigments, obtained by chemical processing of raw mineral materials, include titanium dioxide, zinc white, lead white (2 PbCO₃.Pb (OH)₂), lithophone (BaSO₄ + ZnS), chrome oxide, red lead, gas black soot, etc. metal powders such as aluminium powder, metallic powders, gold dust, etc. synthetic substances of organic origin, possessing high dyeing capacity. Some of the examples of pigments used to produce the desired colours are lamp black and ivory black (Black), Prussian blue, indigo (Blue), chrome yellow, yellow orchre (yellow), burnt umber, burnt siena (Brown), vermilion, red lead (Red) and copper sulphate (Green).

Solvents are the oils used to thin the paints, increase the spread, and are also known as thinners. They make the paint of workable consistency and evaporate during drying of the film. The common thinning agents used are petroleum, spirit, naptha and turpentine oil- a mixture of the various terpens, obtained from the steam distillation of the resinous exudations of the pine tree, leaving resin as a by-product. Turpentine is used extensively because of high solvent power, excellent flattening properties and ideal rate of evaporation.

Driers also known as plasticizers, are chemicals added to paint for specific purposes, e.g., as catalyst (accelerate the drying of the vehicle) for the oxidation, Polymerisation and condensation of the vehicle in paint. The quantity of drier is limited to 8 per cent, excess of it affects the elasticity of paint leading to flaking failure. Some of the examples of driers are letharge (oxidized lead, PbO), lead acetate, red lead (Pb₃O₄), manganese dioxide and cobalt, zinc and lead chromate. Red lead is the best for primary coat over steel and metal work; it produces an extremely hard and tough film, almost impervious to air and moisture, adheres firmly to the metal and is extremely effective in protecting steel from corrosion. The cost of zinc and lead chromates is high.

S.No. Name          Description

1.       Linseed oil : This  is  most  common  vehicle  extracted  from  flax  seeds. Linseed oil prepared from  fine  full-grown  ripe  seeds  is  clear,  transparent,  pale, sweet to the taste and practically odourless. It is used in the following grades:

          (a) Raw linseed oil : Raw linseed oil is thin and pale. It requires more time for drying and is used for interior works of delicate nature.

          (b) Boiled linseed oil : This oil is thicker and dark coloured than that of the raw oil. It dries quickly and  is  prepared  by  adding  some  quantity  of  drier  such  as litharge or red lead to the raw oil. It is used for exterior surfaces only.                  

          (c) Pale boiled linseed oil  : This is similar to boiled linseed oil except that it does not posses a dark colour. It is more suitable for painting plastered surfaces.

          (d) Double boiled  linseed       Oil     :  This oil dries very quickly and is suitable for external works. It, however, requires a thinning agent like turpentine.     

          (e) Stand oil : Formerly this oil was prepared by exposing raw linseed oil to sun till it thickened like honey. However, at present, heat treatment is used for this purpose. This oil dries slowly and provides a durable, clear and shining finish.

2.       Tung oil : This oil is far superior to linseed oil and is used for preparing paints of superior quality.

3.       Poppy oil : This oil is prepared from poppy seeds. It dries slowly and its colours are long lasting. It is used for making paints of delicate colours.

4.       Nut oil : This  oil  is  extracted  from  ordinary  walnuts.  It  is  nearly colourless and dries rapidly. It does not provide a durable finish and is used for ordinary work.

Adultrants bring down the overall cost, reduce the weight and increase the durability. Adultrants also help to reduce cracking of dry paint and sometimes help to keep the pigment in suspension. Barium sulphate, calcium carbonate, magnesium silicate and silica are but a few examples. The best adultrant is barium sulphate. Silica is used only in the undercoats so as to take the advantage of its roughness in development of bond with the next coat.