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(i) Binder

It is the most important component of paint. The Binder gives the paint most of itproperties such as adhesion to the substrate, resistance to weathering, water,chemicals, temperature etc. Binders can be divided into groups (generic types)depending on the drying or curing process which takes place after the paint hasbeen applied to the substrate. The first groups are known as the paint absorbsoxygen from the air and- dries. Example: alkyd paints.

'The next group are called physically drying. When the paint is applied to thesubstrate, the solvent simply evaporates. Examples: chlorinated rubber and acrylic.The third and final group contains chemically curing paints. These paints are usuallytwo-component, e.g. epoxy.

Solvents Evaporating

Polymer o = Curing Agent (Hardener)

Polymer and Hardener Molecules Reacted to Form a NewChemical Substance

Figure 6.3: Curing Mechanisms of Binders

Oxidising Physically dryingChemically curing

Alkyd Chlorinated rubber Epoxy

Vinyl Polyurethane

Acrylic Polyester

Asphalt Silicate

Tar

AN

I

Glossy

PVC 15-25

Semi-Glossy

PVC 30-40

Flat

PVC 35-50

I Binder

Colourpigments

AExtenders

Figure 6.4: Types of Paints

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Paint Technology(ii) PigmentsIn a paint, colour pigments give the paint opaqueness, the desired shade etc.Colour pigments, both organic and inorganic, are available in many shades.

Titanium dioxide is a strong colouring, white pigment with good opaqueness and isused in white and pale colours. Rust-inhibiting pigments include zinc (cathodicprotection) and zinc phosphate (inhibiting protection). Such pigments are usedonly in primers (first coat).

(iii) Extenders or FillersSuch as dolomite, talcum, etc. have little or no opaqueness and are transparent in thebinder. They have different forms e.g. balls, needles, fibres, etc. and are used to give asealed film, the right gloss, etc. In antifouling paints, for example, copper oxide isused as a pigment to prevent fouling of ships' hulls.

(iv) Solvents

These are added to the paint to adjust the viscosity so the paint can be applied bybrush, roller or spray gun. Different binders require special solvents or solventmixtures to be able to dry or cure in such a way that the paint properties are notdamaged.

(v) AdditivesThese are a small, but important, parts of the paint. Such substances includeantisettling agents (to prevent fouling), thickeners (to prevent sagging/running),antifoaming agents (to prevent air entrapment), etc.

Paint Systems

What does a paint system consist of ?

Coating systems. may consists of any number of coats and combinations ofmaterials. In most cases, coating systems will consist of 2 to 4 coats. We usuallysplit a coating system into three main parts:

Primers (may include shop primer)

Intermediate coats (midcoats)

Topcoats

Each part of the paint systems has a definite function, although some paints canplay a dual role in a paint system.

A coating system however, need not to be composed of the three different parts;even a single coat can provide a coating system, depending upon the requirements ofthe coating. Inorganic zinc coatings, for example, provide an excellent one-coatsystem for the storage of refined oil products and many solvents.

(i) PrimersThe primer is the first coat of a paint system. The primer is the base on which therest of the coating system is applied. As a base, it must have strong adhesion to thesubstrate surface and must forma good basis for the next layer of the coating system.

Primers normally provide corrosion protection during the application period andthe whole service life of the paint system. The primer is considered to be one ofthe most important components of the coating system.

Most primers for steel contain anti-corrosive pigments such as red lead, zinc powder(zinc dust), zinc chromate, zinc phosphate, calcium phosphate, etc. Due to their

toxicity, red lead and zinc chromate are used less frequently nowadays and certainlynot in prefabrication (shop-priming) as they will release noxious or toxic fumesduring welding and flame-cutting.

The primer must also provide a proper and compatible base for the intermediatecoat. It must provide a surface that can be thoroughly wetted by the subsequent 9

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coat. The generally flat, non-glossy surface must also provide some physical adhesion

to the next coat. Primers, then have dual requirements; adhesion to the substrate and

provision of a surface which will allow proper adhesion of the following coats.

(ii) Intermediate CoatsIn some paint systems intermediate coats are applied between primers and topcoat(s).

Intermediate coats, often also called undercoats or body coats, must adhere well to

the primer; they are essential for obtaining a sufficient "build" to the paint system.

Furthermore, they should have high mechanical strength, good covering power

and their colour should preferably be nearly the same as that of the top coat, butsufficiently different to allow the contrast to be obvious when applying the final

coat. The colour should be lighter than the topcoat to allow good hiding by the

topcoat. Often a white, or nearly

white, intermediate coat is beneficial for the

hiding power of the topcoat.

The formulation of the intermediate coats is important. Coatings that build up high

thickness of the paint per coat are often termed "High Build coatings". The body

coat (or intermediate coat) must also provide strong adhesion to the primer, as

well as a good base for the topcoats. The intermediate coat must have a surface

ideal for good adhesion of the topcoat. Without the ability of this material to

properly adhere to the primer and to provide proper adherence to the topcoats, theproblem of inter-coat adhesion would cause early coating breakdown.

(iii) Top CoatsThe top coat is the last coat(s) of a paint system, designed to protect the coats

beneath from the environment, to contribute to the overall corrosion protection

offered by the system and to give the required colour and gloss.

The finishing paint or top coat has many

functions. Beside being

decorative, it

should be weather-resistant. The topcoat is the first li ne of defence against

different environments such as: Wind and weather, highly

polluted atmospheres,

water and aggressive chemicals. It is the initial barrier of the coating system. This

means that the finishing paint also must have several properties: Good colour- and

gloss retention, non chalking in bright sunlight and good adhesion under humid

conditions.

Moreover, the finishing paint may also be required to have good mechanical properties,

i.e. resistance against scratching, physical impact and abrasion and resistance to growth.

Glossy topcoats are formulated with a low pigment to vehicle ratio. In order to

satisfactorily apply a topcoat over a reactive base coat containing Zinc, the topcoats

must be highly alkali-resistant. Such topcoats would include vinyls, chlorinated

rubbers, epoxies and coal tar epoxies.

6.4 GENERIC PAINT TYPES

It is not possible to develop a "universal" paint which fulfils all possible functions. Paints

are therefore developed for different jobs. As a result, each paint has a different set of

properties. Paints are often grouped according to the chemical composition of the binder,

which we refer to as `generic paint types'. To ensure that you choose the right paint or

paint system with the right properties, it is importa nt to know the strengths and

weaknesses of the various generic types.

(i) Alkyd Paints

Alkyd paints are made from alcohol and acid with the addition of fatty acid or oil.

The addition of fatty acid and/or oil can be varied to give alkyds with differentproperties.

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