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Paint Constituents

Paint Constituents

Most paints and coatings comprise of 3 basic ingredients :

•  Pigment (omitted in clear lacquers and varnishes)
•  Resin (sometimes called the binder)
•  Solvent (sometimes called the thinner)

 

Pigment

Insoluble coloured powders added to the formulation to give some or all of the following properties: colour, opacity ('cover' the underlying surface), anti-corrosive properties, 'sandability' and 'matt ness'. Coloured pigments are either; naturally occurring minerals (or manufactured re-creations of these) such as Yellow Iron Oxide, termed 'inorganic pigments' or synthetic substances termed 'organic pigments'. Some pigments used in paints designed for industry are based on Lead Chromate (yellow & red), which are considerably cheaper than their synthetic equivalents and usually have better opacity and light fastness than their alternatives but are toxic and hence have restrictions of use applied to them. Very strict legislation governs the use of paints containing lead and chromium based pigments; they must not be used in areas likely to be chewed by children eg toys & pencils and must not be sold to the general public. Decorative paints used by the general public must not contain lead pigments, although older properties may still have old lead based paint on window frames etc, applied many years ago, great care must be taken when dealing with these (e.g. sanding).

Combinations of pigments may have to be used to achieve the desired colour, opacity, anti-corrosive properties and gloss level at an economical rate. Some very low cost pigments impart little colour but add bulk (i.e. reduce cost), fill surface defects and improve film build, these pigments are called fillers or extenders and are typically used in primers, fillers and undercoats. Putty is a dispersion of one of these fillers (Calcium Carbonate) in linseed oil (the resin); no solvent is used, as the product is required in a heavy paste form.

Resin

The 'glue' which binds together the pigment particles into a cohesive film and adheres to the substrate being coated. The higher the resin content, the higher the gloss of the film but the lower will be the opacity (the film will contain proportionally less pigment).

Resins used in paints can be conveniently divided into two types according to the mechanisms by which they dry.

Non-convertible coatings dry by loss of solvent alone, they remain soluble in the original solvents for their life eg Nitro cellulose, bitumen or solvent borne acrylics (nail varnish is an example of a non convertible coating & is easily removed by a solvent).

Convertible coatings dry initially by evaporation of the solvent, but then a secondary, irreversible chemical reaction takes place. This can be a reaction with oxygen in the air (eg alkyd types such as decorative gloss paint) or by reaction with a separate chemical added at the time of application termed a 'hardener' or 'catalyst'  (such as a Two Pack Epoxy). Single pack moisture-curing polyurethanes are also in this category, although in this case the 'hardener' is moisture in the atmosphere. These convertible coatings, once fully dried or cured are not soluble in the original solvent and are therefore considerably more solvent resistant than non-convertible coatings.

Resin types

Various resin systems are employed for coatings, each has its own benefits and drawbacks.

Alkyds

A synthetic resin manufactured by reacting a natural oil with a polyester resin. The ratio of oil to resin is expressed as oil length, alkyds with a high proportion of oil are called 'long oil' alkyds e.g. Alkysil, whereas those with a low proportion are termed 'short oil' alkyds e.g. QD3. Alkyds cure by reaction with atmospheric oxygen i.e. they oxidize, because of this ongoing reaction they tend to become brittle over time. Most alkyds will gradually yellow over time.

Acrylics

A synthetic resin manufactured by polymerizing an acrylic monomer such as butyl methacrylate. Acrylic resins can be employed as simple solutions that dry by solvent evaporation although certain types may be reacted with other polymers to create a two pack or stoving system. Most acrylics are essentially non-yellowing and do not become brittle with age.

Cellulose

Cellulose paints are usually based on Cellulose Nitrate, manufactured by reacting wood pulp with nitric acid. Dry purely by solvent evaporation and typically seen in wood finishes and automotive re-finish paints. Cellulose Nitrate is only soluble in solvents that are highly flammable, making the end products potential use restricted.

Epoxy

Usually encountered as a two-component product. Epoxy resins exhibit very good chemical resistance, superb adhesion characteristics but degrade badly (chalk) on exposure to UV light (i.e. daylight), for this reason their use is normally restricted to primers or chemical resistant finishes for application where that feature is the most important criteria. They are perfectly satisfactory for interior use eg as floor coatings e.g. Epidac 2S.

Polyurethane

Encountered in three forms, a single pack product, usually as a polyurethane / alkyd blend, a two pack variety or a moisture curing version.

Polyurethane alkyds, e.g. Dathane, show slightly better weathering and abrasion resistance than a pure alkyd. The two pack varieties e.g. 85 line, exhibit good chemical resistance coupled with good exterior durability (although certain types, mainly used in wood finishes, do not have good exterior durability). A third type e.g. Dacpol V8, cures by reaction with atmospheric moisture to give a very tough, chemical resistant film although prone to yellowing.

Chlorinated Rubber

Dry purely by solvent evaporation, are very chemical (but not solvent) resistant and possess good water resistance, hence their widespread use in pool enamels.

Vinyl

A term used to describe a large family of resin types, the commonest being based on Poly Vinyl Acetate which is usually encountered in an emulsion form. Vinyl resins are also available in solid format that are used in etch primers and anti corrosive primers and finishes e.g. Vinadac.

The above is only a few examples of the commonest resins available to the paint chemist, many others exist and more are being introduced.

Emulsion Resins

Some liquid resins are supplied finely dispersed in water in the form of an emulsion. They are manufactured as a dispersion of a liquid resin into another liquid, usually water, this second liquid must be a poor solvent for the resin i.e. the resin must not dissolve. A common example of an emulsion is milk, which is an emulsion of fat in water. In an paint emulsion, the dispersed micro droplets of resin are kept apart by molecular forces and coalesce into a continuous film only after the coating is applied and the water evaporates. This process is aided by the addition of a small amount of 'coalescing solvent' which later evaporates from the film.

Proper film formation (the flowing together of the resin globules) can only occur if the resin is sufficiently liquid to allow this to happen. Cold conditions will retard the evaporation of the water, cause a thickening of the resin and prevent the resin from forming a continuous film. Poorly coalesced films exhibit a powdery nature, as the resin exists within the dried coating as individual spheres rather than as an integral whole. Because the film contains air where the resin globules have failed to unite, the film often exhibits a white caste.

Solvent

Solvents are used to reduce the viscosity of the paint to application levels or to dissolve the solid resins sometimes used in paint formulations. Solvents help the applied film flow out into a smooth coating. Quite often, a blend of solvents is used to gain the optimum properties. No single solvent is ideal for dissolving all resins or is suitable for all application methods. For example Acetone is a good solvent for most resins but evaporates far too quickly for use in brushing finishes and is highly flammable, whereas White Spirit is less flammable, suitable for brushing paints but is a poor solvent for most resin types. It is the solvent that is now referred to as 'VOC' (volatile organic compound) in the Environmental Protection Act, which limits the amount of solvent that can be used in a paint formulation to be used by industry (in a factory environment only at present). Some coatings use water as the solvent component (e.g. emulsion paints). Coatings with comparatively little or no solvent content are known as 'high solids' coatings.

Other ingredients

Paints rarely have only the three components discussed so far, often as many as 10 or 12 ingredients make up a typical formula. Other ingredients can include flow promoters, thickeners, driers, anti-skinning agents, de-foamers, electrostatic additives, wetting agents etc.

Paint Manufacture

Principally involves the breaking down of the pigment agglomerates (clumps) in a finely dispersed form by mechanical means. Various types of high-speed mixer, bead mill, roller mill or ball mill can be utilised to carry out the dispersion operation. The process stops when the required degree of dispersion has been achieved. Fine dispersion is important to maximise the covering power of the pigment and achieve the greatest value for money as well as to produce a glossy, bit free film. Poorly dispersed pigments will settle out more readily than finely dispersed ones. Once dispersion of the pigment is complete the remaining ingredients are added and such processes as colour matching are carried out.

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