Pigment concentrates (pigment pastes) are monopigmented systems with as high as possible pigment content and as little as possible binder (grinding resin). They are used in two ways in the production of coatings.
For tinting: After the grinding and finalization, the pastes are used for the final fine adjustment of the color, i.e. for nuancing. In this case, only a small percentage of the pastes are used. The individual components of the pigment concentrates (especially the grinding resin) then have virtually no obvious influence on the quality of the coating that has been tinted.
For production: A coating can also be made up entirely of a blend of pastes and then let down with the desired binder (as a clear coating or white coating). Greater quantities of pigment pastes are required for this. As the pastes then have a significantly greater influence on the coating quality, the requirements on the quality of the pigment concentrates are also considerably greater.
While it has always been conventional practice to use tinting pastes, it is only in the recent past that the option of making coatings made entirely from paste blends has been more broadly received. Pastes are then used for production if the coating application is comparatively small, but a broad spectrum of colors and binder systems needs to be covered (typically, for example, with industrial coatings).
Manufacturing from pastes is quicker than manufacturing using a millbase and the coating manufacturer can respond more flexibly to customer requirements. Pigment concentrates can also then be used in automated coating production, as this is only possible if pumpable pastes are used instead of powdered pigments.
There are generally two possible variants. You can design a series of pigment pastes which are to be used only for one unique coating system. The actual coating binder is then also used as the grinding resin for the pastes. Such pastes can be optimally used in the coating system.
The other option is to develop a “universal” series of pastes which is compatible with most binders and can therefore be used to produce very different coating qualities. Of course, “universal pastes” will not necessarily be ideal for each type of coating binder. Compromises are inevitable in such cases.
The stability of the paste is one of the key requirements. No flocculation, no settling and no syneresis may occur during the, at times, very long storage periods. The paste should contain as much pigment as possible and contain as small as possible amounts of the other components. Nevertheless, the paste must be easy to handle, i.e. capable of flowing or being pumped, and it must not have a tendency to dry out.
It must be possible to easily incorporate the paste in all types of binder. On one hand, this is a question of compatibility; on the other hand, you have to exclude any shock phenomena (dispersion of the pigments).
Finally, the coating film properties, in particular the resistances, must not be negatively influenced by the paste content.
To comply with the specified requirements, it is absolutely necessary to use additives in the pastes. It is only by adding a suitable wetting and dispersing additive that the paste viscosity will reduce sufficiently for the pigment content to increase. Simultaneously, the pigment is deflocculated; a prerequisite for high-gloss and color-stable topcoats. Stabilizing the pigment dispersion by using additives also prevents shock phenomena when mixing the pastes and when finalizing.
In order to suppress floating as much as possible, alongside the deflocculation it should also be ensured that the pigment mobilities are adjusted and like pigment charges are provided.
The key component of a pigment concentrate is the pigment; the pigment content should be as high as possible. In addition, a grinding resin is required in most cases in order to obtain a storable paste and to ensure that the paste can be easily incorporated in the finalizing binder without any shock phenomena. The grinding paste is primarily chosen based on its compatibility. Its percentage of the paste should be a low as possible. If special dispersing additives are used, binder-free pigment concentrates of impeccable quality can also be formulated.
In addition to the solvent which is added via the grinding resin, it is usually necessary to add an additional solvent in order to produce a manageable paste. Choosing the correct solvent can also have an influence on the storage stability of the paste.
A suitable deflocculating wetting and dispersing additive is indispensable for a high-quality pigment concentrate. Since both inorganic and organic pigments need to be stabilized, polymeric additives are particularly ideal as paste additives. It must be ensured that the additive is compatible with the binder that is being used in the paste.