MANUFACTURE OF PAINT

Paint manufacturing is a small but very important sector of Australia's manufacturing industry. While some sectors, such as general chemicals, are particularly sensitive to terms of trade, the paint industry has proved itself a steady performer in boom or recession, and with or without tariff protection.

The Constituents of all Paints are Summarised Below:

PIGMENT VEHICLE ADDITIVES BINDER SOLVENT

Pigment
Pigment is the most obvious part of the paint and is normally a solid material composed of very fine particles which are insoluble in the vehicle. The pigment provides the colour of the paint film. Its presence affects the flow, gloss and durability of the paint and other properties such as corrosion resistance, fire resistance, antifouling etc.
It must remain suspended in the paint and not settle to form a hard sediment. Pigments can be mixed with substances called extenders. Although similar to pigments they do not provide colour or obliteration. They are used to improve the strength of the paint film, the adhesion to the surface to be painted, the ease of sand papering, and to reduce the cost of the product.

The most common white pigment is use is titanium dioxide but also used are zinc oxide, lithopone (a mixture of zinc sulfide and barium sulphate) and antimony oxide. The common coloured pigments used include inorganic varieties such as iron oxide, chromium oxide, prussion blue, aluminium metal etc. Common extenders are calcium carbonate, barium, sulphate and various silica compounds e.g. talc and china clay.

Vehicle
Vehicle is the name given to the liquid portion of the paint, because it "carries" the pigment. It is composed of a binder which is either dissolved in liquid solvents or dispersed in diluents which do not dissolve it.
Binder
The binder is the portion of the dry paint film which binds the pigment particles together and to the painted surface. The binder gives the film many necessary properties such as gloss, adhesion, hardness, toughness, flexibility, durability and speed of drying.

Solvent
The solvent or dilutant is the material with which the binder and pigment are mixed in order that the paint may be of the correct consistency to be applied to the surface by brush, roller, spray, dipping and other methods.

Binder
The binder is the portion of the dry paint film which binds the pigment particles together and to the painted surface. The binder gives the film many necessary properties such as gloss, adhesion, hardness, toughness, flexibility, durability and speed of drying.

Additives
Additives are materials which are added to the paint, usually in small quantities to help it dry more quickly or flow out evenly to remove brush marks and to stop skinning in the can. One example is silicone, this makes the surface of the paint film more resistant to marking and scratching.
During paint manufacture the pigment and vehicle undergo the following changes:
"Wetting" of the pigment surface by the vehicle.
"Grinding" of the pigment aggregates to small primary particles.
"Stabilisation" of these small particles to prevent re-aggregation.
A resin which is to perform well as a pigment dispersant must therefore be composed of molecules consisting of two parts:
An "Anchor" group which wets the pigment surface and becomes associated with it.
A "Stabilising Chain" which is soluble in the solvent used and therefore stretches out into it.

Pigment particle surfaces contain areas of electrical charge. The "Anchor Group" in a dispersing resin is chosen for its ability to be attracted to some of these changed areas. Int his way during dispersion, the original surface of the aggregate and the new surfaces exposed during grinding are covered by molecules of the dispersing resin. This process is known as "Wetting." Once the anchor group is attached to the pigment surface, the rest of the resin molecule stretches out into the solvent surrounding the pigment particle. A protective barrier is built up around each particle, which prevents the particles from re-aggregating.

This is known as steric stabilisation. Pigment dispersions in organic solvent (e.g. petrol or white spirit) are stabilised this way. In the case of water-borne paints (e.g. emulsion paints), the situation is more complicated and also involves "charge stabilisation." This arises from groups in the molecule ionising in the water to give a "Cbud" of balanced positive and negative charges around each particle. This again prevents the particles from re-aggregating.