The basic operation of paint manufacture is the breakdown of pigment aggregates followed by wetting and dispersion of the individual particles. This is accomplished by applying a rubbing force or 'sheark’, sometimes accompanied by pressure, to the mixture of paint medium.

Shearing forces are exerted between the aggregates and the paint mill surface and is the main route in the manufacture of highly consistent pastes. With more fluid consistencies there is shear also between the aggregates; this is known as 'attrition' and is largely responsible for dispersion in many high speed mills.

It must be emphasised that paint milling is essentially a dispersion process and no breakdown of the ultimate particle occurs. Pigments are supplied to the paint manufacturer in a sufficiently fine state that the use of the term grinding in paint manufacture is incorrect.

Early artists mixed their paint by rubbing pigment and vehicle between glass and granite surfaces, using simple apparatus consisting, for example, of a mushroom-shaped muller which they rubbed the pigment and vehicle on a roughened plate of glass or granite until the desired fineness and smoothness of paste was achieved.

Mechanical milling devices were gradually introduced into paint making techniques, the first type being the simple system of two flat stones, one on top of the other, between which the pigment and vehicle were fed. The top stone rotating and the bottom one fixed. Squeezing and rubbing motions were provided by the weight of the top stone and its rotation over the bottom stone.

An improvement on flat stone mills was the cone mill in which a cone-shaped central stone rotated in a ring of stones, the paint mix being fed into the space between the stones, output was accelerated by this type of mill, roller mills were next introduced followed by ballmills, colloid mills, attfitor and high speed mills.

MAKING PAINT

A resin or mixture of resins, which forms the paint film, this is known as the binder and on its own gives a clear film. The type of resin used can be varied to give the paint whatever final properties are required pigments are used to give colour to the paint film and also to enable the coats of paint to hide what is underneath, solvents to dissolve the binder and give the paint the correct consistency (viscosity) for easy application. They must evaporate after application to allow the paint to dry.

A mixture of resin and solvent is called the 'vehicle' for the pigment. Most paints also contain small amounts of additives.

The manufacture of paint from its ingredients can be represented as follows:

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Pigments are supplied to the paint industry in the form of dry powders, although they are often made by mixing solutions of two or more ingredients together in water. Reaction between the ingredients produces the solid pigment particles which precipitate out of solution. At this stage the pigment particles are very small (primary particle size). The solid pigment is filtered and pressed to get rid of most of the water and the remainder of the water is evaporated.

Pigments are often composed of very complicated molecules, part of which carry small positive and negative electrical charges.

Although these charges balance each other overall, the surface of the pigment particle contains small areas of charge. During the drying process, when water is being removed from the pigment surface, a positively charged area of one particle is attracted to a negatively charged area in another particle and clumps of pigment particle are formed.

These clumps are known as aggregates and it is in this aggregated form that pigments are supplied. To produce good paints it is necessary that the pigment should be at least close to its primary particle size when mixed with the vehicle. Otherwise more of the expensive coloured pigment is used than is really necessary. The pigment will settle out and the dry film will not be smooth and glossy.

It is, therefore, necessary to break down the aggregates of pigment particles in the presence of the vehicle and to prevent them from re-aggregating. For this reason, it is not normally possible simply to stir the pigment into the vehicle, rather work has to be done on the mixture in order to break down the aggregates and ensure that the resin coats the small particle surface. This process known as pigment dispersion can be done in several ways.

For each method of dispersion, the correct mixture of pigment, resin and solvent must be used to produce a suitable viscosity for working the paste.

One of the first methods of dispersion was the use of roller mills. A cross section of a triple roll mill is shown. It consists of three rollers, rotating in the direction shown, each at a different speed. There is a very small gap between rollers 1 and 2 and between 2 and 3. A heavy paste (or premix) of pigment, resin and solvent is made by stirring the ingredients together. The paste is then fed into the mill and is carried around the roller as shown.

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Because of the difference in roller speed there is a shearing action between the first and second rollers and between the second and third rollers. This breaks up the pigment aggregates and rubs the vehicle into the pigment surface. This method puts a lot of work into the mixture, but is not much used in industry now because each machine needs constant attention and too much manpower is required to produce each litre of paint.

Ballmill consists of a hollow cylinder, about half full of small steel or steatite balls or pebbles. Pigment resin and solvent are fed into the cylinder which is then sealed and rotated in the direction shown. The balls are carried part way up the cylinder and then tumble back to the bottom. As they roll over each other, the pigment aggregates are sheared between them and are dispersed.

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Although this process must continue for 16-23 hours to get the correct degree of dispersion, it is still widely used in industry. Once loaded the mill requires little attention until the dispersed paste is filtered from the mill leaving the balls behind.

Attritor consists of a cylindrical vessel containing balls as used in a ballmill. A shaft with various side arms is rotated amongst the balls. Pigment resin and solvent are fed into the vessel and the movement of the balls against one another, caused by the stirrer produces the action necessary for pigment dispersion. This method is faster than the ballmill, about 4 hours and is being used increasingly.

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The sand grinder or bead mill consists of a cylinder mounted upright in which a metal shaft is rotated at high speed. Several flat discs are attached to the shaft and a quantity of sand or glass balls contained within the cylinder.

The premix, resin, pigment solvent paste is pumped in at the bottom and out at the top. The sand is held back at the top by a fine sieve. The high speed discs cause the sand to swirl vigorously and collide with pigment aggregates, breaking them up.

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The cylinder normally has a water jacket with cooling water to remove the heat generated by the grinding action. This method is fast and can be a continuous process and is gaining in popularity as a method of pigment dispersion in the paint industry.

High speed dispersion is one of the latest methods to be adopted by the paint industry. It is sometimes possible to disperse pigment in a vehicle using a high speed impeller disperser. This consists of a circular toothed disc attached to a vertically mounted shaft in a cylindrical tank. Pigment, resin and solvent are loaded into the tank and the dispersion is achieved by rotating the impeller at high Speed. No dispersion medium e.g. sand is required.

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This method, though very simple, is not capable of breaking down all the pigment aggregates. For this reason it is only used when the pigment is easy to disperse and when the paint in which the dispersion will be used is not required being glossy e.g. white emulsion paint.

Another type of mill is the heavy duty or pug mixer, in which roughly S-shaped blades revolve in opposite directions and at different speeds in adjacent troughs. A stiff paste is required and is roughly similar to kitchen dough mixer. These mixers are used for stoppers, fillers and other thick paints.

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The mixture of pigment, resin and solvent which has been through the pigment dispersion process is known as a millbase. In order to make the best use of the pigment dispersion equipment, mill bases contain a large proportion of pigment and a relatively small amount of vehicle. Most paints, on the other hand, usually contain a large amount of vehicle and a relatively small quantity of pigment.

To make paint, millbases of various pigments are stirred together in a mixing vessel to produce the required colour and then a quantity of vehicle is added to produce the required film properties. This vehicle may or may not be the same resin blend used in the dispersion process. Any necessary additives are also loaded at this stage. The final paint is then diluted or thinned with solvent to give the required viscosity.

A breakdown of possible process:

During manufacture the pigment and vehicle undergo the following changes:

1) Wetting of the pigment surface by the vehicle.

2) Grinding of the pigment aggregates to small primary particles.

3) 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:

     a) An 'anchor’ group which wets the pigments surface and becomes associated with it.

     b) A stabilising chain which is soluble in the solvents used and therefore stretches out onto it.

As mentioned previously, 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 charged areas. In this way, during dispersion, the original surface of the aggregate and the new surface exposed during grinding are coated 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.

Thus a protective barrier is built up around each particle, which prevents the particle from re-aggregating. This is known as steric stabilisation (e.g. petrol or white spirit) are stabilised in this way.

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In the case of waterborne paints (e.g. emulsions), the situation is more complicated and also involves charge stabilisation. This arises from groups in the molecule ionising in the water to give a cloud of balanced positive and negative charges around each particle. This again prevents the particle from re-aggregating. When the other ingredients are added to the millbase to produce paint, it is necessary that the protective barriers around the pigment particles are still maintained and the particles are kept apart. This is known as the deflocculated state.

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If a number of primary particles come together to form loose clusters, since each particle has been wetted up during the dispersing process the area between the particles are filled with free vehicle and not air, so they are unable to get very close to each other to form aggregates, but form a loose cluster. This is known as flocculation and can be recognized by a colour change if the paint is sheared, rubbed between the fingers, the shearing action of a spray gun or brush is usually sufficient to de-flocculate the pigment particles.

A dry pigment, in the form of a powder, generally consists of agglomerates. The strength of the forces which hold the pigment particles together is variable. In some cases agglomerates may be broken down) weak forces by shaking or stirring, in other cases strong forces to break down agglomerates and even here the process may not always be complete.

Pigments which are prepared by precipitation usually consist in the dry state, of agglomerates. The precipitate may contain residual soluble salts, owing to incomplete washing, or flocculating agents which have been added deliberately. During the drying of pigments, these substances may cement the primary particles together at points where they come into contact.

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Flocculate is the term used for loosely held collections of primary pigment particles or of agglomerates which may arise in dispersion in a state of rest. They may be broken down by stirring or shaking, but are reformed when the system is allowed to stand.

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It is clear that the boundaries between these types of particle are diverse. It may be difficult to know which type one is dealing with. It is easier to deal with by examination in liquid paint, agglomerate (2) consists of a cluster of primary particles of pigment which have not been separated during the milling operation; the interstices between the primary particles are filled with air, this leading to poor optical properties and fast settling.

In the flocculated state (1) which may be built up in a paint at rest, a number of primary particles have come together to form a loose cluster. Since each particle has been wetted during the dispersion process, the interstices are here filled with free vehicle and not with air, thus leading to degraded optical properties, softer settlement, resistance to sagging and runs, leveling impaired and easier brushing.

Deflocculated particles exist where only primary particles occur and where these particles are not in contact with each other, being separated by vehicle which leads to optimum optical properties, slow settling, excellent flow, tends to run and sag, difficult to brush.

The presence of flocculates will be seen from the summary of properties to have desirable properties for some paints, so that it is usual to attempt to obtain a certain degree of flocculation for brushing paints.

The exact composition of Stage I (Grinding) is found by experiment to give the minimum grinding time and the most stable complete dispersion. Stage II and III also require care, as hasty additions in an incorrect order can cause the pigment to re-aggregate or flocculate.

The amount of pigment in the formula is that required for the appropriate colour, hiding power, gloss, consistency and durability. As a rough guide, the amount may vary from one third of the binder weight to an equal weight for a glossy pastel shade.

Pigment/Binder relationship. The ratio of pigment to binder in paint will influence the properties of the dried film. The ratio can be expressed by weight: P/B ratio (pigment/binder ratio), or by volume: PVC (pigment volume concentration).

Stoppers and fillers have fairly high PVC to give optimum filling and flatting properties. Finishes have lower PVC, to give optimum gloss. Basecoats have a much higher PVC than single layer finishes, the same amount of colouring matter is compressed into a much thinner film.

PRODUCT SPECIFICATION

The identification of a product specification is carried out by market if surveying, identifying a hole in a product range. A future customer need or a current customer needs this information, gets distilled down into a list of specifications which may result in a product.

This specification is given to a formulating chemist who attempts to put together a product to meet that specification. The process is a laborious one which involves trial and error evaluations of the product through its development stage. Finally, it is likely to be fielding trialed prior to launch.

The product specification will include a list of properties which can be tested, in a later block will be listed some of the paint test methods used to evaluate both existing and development products. This will include application properties physical, mechanical and durability properties.

Development of existing products often involves feedback from the field in order to improve specific properties, but it is worth bearing in mind that paint properties are often a compromise. Improving one property reduces another (e.g. speed up drying reduces gloss}.