How paint is made
Paint contains three main classes of material:
1) 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 enable the paint to have whatever final properties are required.
2) Pigments to give color to the paint film and also to enable the coat of paint to hide what is underneath.
3) Solvents to dissolve or suspend the binder and give the paint the correct consistency (viscosity) for easy application. They must also 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 quantities of various additives e.g. driers to speed up the drying process, substances to adjust pH of the paint, etc.
This section describes the way the paint chemist blends the “vehicle” and pigment together to make paint.
Why disperse pigments?
Pigments are supplied industry in the form of dry powders. These consist of “primary particles” which are sized to give the best results in paint films. For example, white pigments (Titanium dioxide TiO2) scatters light most efficiently (whiteness and opacity) when the particle size is 0.22 microns. Some pigments are much smaller, some larger.
The small primary particles of pigment clump together when the powder is dried and stored to form relatively large clumps (A ‘clump’ of Titanium oxide may contain 5,000,000 primary particles)
These clumps are known as aggregates and it is in this aggregated form that the pigments are supplied. To produce good paints, it is necessary that the pigment should be at its primary particle size when mixed with the vehicle. Otherwise most of the expensive colored pigment could not be used efficiently, and the dry film will not be smooth, glossy, opaque or have the correct color.
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 surfaces. This process is known as “pigment dispersion”, and the following processes must occur:
1) “Wetting” of the pigment surface by the vehicle.
2) De-aggregation of the pigment aggregates to small primary particles.
3) “Stabilization” of these small particles by the resin 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:
1) An “anchor group”, which wets the pigment surface and becomes associated with it.
2) A “stabilizing chain” which is soluble in the solvent used and therefore stretches out into 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 surfaces exposed during grinding are covered by molecules of the dispersing resin. This process is known as “wetting”.
The dispersion process
Many different types of dispersion machinery have been developed but they all use one or both of two mechanisms to break up aggregates.
The machinery falls into two groups:
1) Those that use “dispersion media”, usually ceramic balls 9 to 12mm in diameter or glass beads 1 to 3mm (like marbles) in diameter. Dispersion occurs where the balls touch each other by a combination of impact and shear depending on the conditions.
2) Those that do not use media - these disperse entirely by shear.
Converting pigment (millbases) into finished paints
Pigments are always dispersed at the highest concentration possible for technical and economic reasons. Finished paints contain only enough pigment to give the desired properties for the same reasons.
Millbases are therefore converted to paint by mixing with film forming resins, with solvents and any necessary additives. This must be done carefully, mixing the ingredients in the right order to get the best results.
Obviously the resins and solvents must be compatible but problems may arise if components with large differences in resin solids, viscosity or temperature are mixed - “pigment shock” - reaggregation of the pigment particles may occur. For these reasons pigment dispension are usually “second staged” when dispersion is complete - they are diluted with resin and solvent to give a recipe which is still more concentrated than the final paint but much easier to use than the dispersion recipe. Waterborne millbases and paints must have the pH adjusted at this stage.
To be converted to paint, dry pigment powder must be dispersed in resin solution. During this process, aggregates must be broken up into primary pigment particles, the surfaces of which must be coated with resin. This resin stabilizes the pigment dispersion and prevents the particles from reaggregating.
A lot of mechanical energy is needed to carry out this process. Various machines (or “mills) are used.
After pigment dispersion, the millbase is mixed with further resin, solvent and additives to give the final paint composition.ぐ颵ᇏ芻ꨀ봀SCROLL>