When you open your tin of paint, it may be some emulsion to make your home walls that new shade of blue you saw in a magazine. It could be some metal paint in a workshop to protect a chassis, they are all made up of 5 special ingredients. Each ingredient has a very special purpose and without it your paint wouldn’t work as you hoped. Here’s what each ingredient in paint is and what it does.

The Big 5 In Paint

The main ingredients of paint which could cover residential paints or industrial paint can be grouped as follows:

A. Binder

B. Pigment

C. Extender

D. Solvent

E. Additives

The ingredients used and the relative amounts in the paint are of paramount importance for the final properties of the paint. To formulate a modern paint which satisfies technical, health, safety, environmental and economical requirements is very complicated. Small variations in the relative amount of the paint ingredients may lead to great variations in the final properties of the paint.

Lets look at each main ingredient one by one as a deep dive into paint construction.

A - BINDER

Binders decide the properties of the paint and so are absolutely critical:

• Bind the pigment

• Give the coating adhesion to the substrate

• Promote chemical and atmospheric resistance

• Impart flexibility to the film

• Increase the abrasion resistance

The binder is the non-volatile portion of the paint. Resin is the binder that holds the pigment particles together and provides adhesion of the paint to the surface. Most paints or coatings are named by the generic type of resin (i.e. vinyl, epoxy, acrylic etc.). The resin, or binder, is responsible for most of a coating’s physical and chemical properties, chemical resistance, weather resistance, adhesion properties and also influence the hardness and abrasion resistance.

Different binders will have different curing mechanisms. In general the curing mechanism will determine how good protection a coating will offer. Oxidatively curing & physically drying coatings are usually single pack and offer poorer protection than their chemically curing, multi component counterparts.

If you haven’t used enough binder or any binder in your paint mix, you may find as a result that your metal paint won’t adhere to metal railings or that once dried it almost instantly cracks or wrinkles.

Often referred to as resins, binders can be classified as thermoplastic or thermoset. Thermoplastic resins can be repeatedly softened by heating and hardened by cooling. They can also be dissolved by the original solvent used in the coating; they are resoluble (i.e. vinyl, chlorinated rubber, acrylic etc.) They are usually one-pack products.

Thermosetting resins undergo a chemical reaction by the action of heat, catalysts, UV-light etc. They do not melt by heating or re-dissolve in solvent. Epoxy, polyurethane and silicates are such resins. They are usually two-pack products, but can also be one-pack coatings, cured by oxygen in the atmosphere, like alkyds.

The choice of the binder is determined by the purpose for which the paint is intended. For example epoxy for good chemical resistance and polyurethane for weather and gloss resistance. The binders can be modified to give certain properties. For example pure epoxy has very good chemical resistance but must be applied on a blast cleaned surface. Paints based on coal tar (now withdrawn in many countries) have very good water resistance and penetrating properties (surface tolerant), but are not very resistant to chemicals such as solvents. A combination of the binders give the paint fairly good chemical resistance as well as good resistance to water and good penetrating properties.

B – Pigment

The function of the pigments is to:

• Impart opacity

• Impart decorative colour

• Give strength to the paint film

• Increase chemical and atmospheric exposure resistance

• Contribute to regulation of viscosity

There four types of pigments:

• Colour pigments

• Extenders

• Barrier pigments

• Active / Reactive pigments

Colour Pigments

Colour pigments, provide the pleasing colour and decorative characteristics of a coating.

They can;

• Be natural or synthetic pigments

• Be organic or inorganic pigment

• Give colour to the film

• Hide the substrate / previous coats (opacity)

• Almost all colours consist of a blend of pigments

• A colour can be changed or adjusted by altering the pigment blend

Titanium dioxide is a commonly used pigment for white and light tints, and has high hiding power. In general the hiding power of pigments will vary by pigment colour and chemistry.

C - Extender Pigments

Extender pigments (also called fillers or reinforcing pigments) have their own function in the paint. An extender is a powder with different particle size and shapes (needles, fibres, balls or plates), which is practically insoluble in liquids. The colour of the dry powder is usually white or slightly coloured, but dispersed in the vehicle they become more or less transparent.

Barrier Pigments

Some pigments have a very special function. Paints pigmented with aluminium flakes have a very low water and water vapour permeability, which gives the paint higher water resistance. These flakes will lie parallel to the substrate and have the same effect as tiles or fish scales. These flakes are very impermeable and water, oxygen etc. must therefore work its way through long and intricate paths before it reaches the metal paint surface.

Active / Reactive Pigments

This category of pigments takes an active role in ensuring that the paint performs as expected:

Anticorrosive

1. Zinc-Phosphate

2. Zinc-Dust

3. Red Lead

4. Lead Chromate

5. Zinc Chromate

Antifouling

1. Cuprous Oxide

Adding zinc to the coating has a different effect. Zinc coatings provide protection by two methods. Firstly, they can serve as a barrier. Alternatively, they can provide galvanic protection of steel surfaces by allowing the zinc to act as a sacrificial anode. This action tends to protect the base material from corrosion while the pigment itself is corroding.

Some paints contain biocides to prevent marine fouling. These compounds are in most cases based on copper.

D - SOLVENTS

The function of the solvent in a coating is extremely important but as legislation progresses coming under more and more restriction to limit Isocyanate damage to both environment and painter health. Solvents:

• To transport the pigment binder combination to the substrate

• To control viscosity

• To control the flow of binder

• To wet the substrate

Solvents, diluents and thinners are liquids used to dissolve the binder and to adjust the viscosity of the paint. They also influence the flow properties, drying speed, spraying or brushing characteristics and gloss. Certain binders, such as drying oils, themselves have a low viscosity and therefore do not need to be dissolved. Most binders, however, are either solid materials or highly viscous liquids, and must therefore be dissolved in organic solvents. A few types of binders, such as certain silicates are dissolved in water, while emulsions and water based epoxies can be diluted with water.

The solvent is a liquid, single or blended, which is volatile under normal conditions and is capable of dissolving the binder completely. The only reason for having a solvent in the paint is to make the paint applicable. As soon as the paint is applied on to a structure the solvent evaporates from the paint layer.

A diluent is a volatile liquid, single or blended, which will not dissolve the binder. It may be used in conjunction with the solvent without causing any deleterious effects. Normally, a diluent is mixed with the binder solution (vehicle) to obtain certain effects, e.g. regulation of the evaporation pattern, adjustment of the paint viscosity or cost reduction of the volatile part of the paint.

Some painters do not realise the importance of solvents or thinners in the formation of a paint film. Too much thinning of a paint may cause sagging and formation of pinholes during, or shortly after, application. Using the wrong solvent can create porosity, discolouration, poor gloss, floating of pigment, fish eyeing, poor coating strength and lack of adhesion.

E - ADDITIVES

Finally the addition to binder, pigments and solvents, most coating formulations contain additives. They are usually added in small quantities, with the level of use seldom exceeding 1 - 2% of the entire formulation. They are a must for having homogeneous industrial paints and contribute to ease in manufacture, improve package stability and application, and also to improve appearance and quality of resistance.

For example, an Alkyd contains dryers to promote curing. Hard, brittle resins, such as vinyl, contain plasticizers to produce a more flexible film. Other additives may be incorporated into a coating formulation to control pigment settling, give a certain gloss or improve sag resistance.

Other catalysts, which have an influence on the curing rate, are used in other paints and coatings. For example, hydrogen peroxide in polyester – yes the same product use to make your hair bleach blonde also ensures your paint dries at a nice even rate.

Hope this article has answered any questions you have about paint composition and look out for the next articles on total paint shop support.