Reduction of Millbases

When the vehicles and solvents required to make a finished product are added to a ground millbase, it is best to add the reducing liquids slowly with appropriate agitation. Rapid reduction may cause the stiff millbase to break into small or large chunks that cannot be broken by the agitator because of the relatively thin liquid surrounding the chunks.

Sometimes, even with slow reduction and good agitation, numerous coarse and fairly tough particles are created when a millbase is reduced. This phenomenon is frequently called “shock.” The reducing liquid, instead of blending into and diluting the millbase, seems to suck the vehicle, or possibly just the solvent, from the millbase—leaving the pigment relatively dry. This effect can be observed by smearing a layer of millbase on a panel and covering it with a layer of reducing liquid. In a few minutes, if the tendency to “shock” is present, the millbase will become stiff and dry; often cracks, like those seen in dried mud, will form. Vulnerability to “shock” appears to be independent of pigment type or grade.

The exact mechanism of “shock” has not been clearly established. It generally occurs only when the resin content is considerably higher in reducing liquid than it is in the millbase vehicle. Addition of straight solvent to a millbase will seldom, if ever, produce “shock.” The specific solvents used in the grinding and reducing vehicles may also have some effect. “Shock” is most likely to be a problem when the vehicle in the millbase is very low in resin solids. Such millbases are likely to be used in pebble milling to obtain fast grinding. This grinding technique is sometimes called “slush grinding.”

“Shock” can usually be avoided by employing a millbase vehicle no lower in resin solids than the reduction vehicle. If avoiding “shock” is not feasible, some sort of milling after reduction can usually be used to disperse the agglomerates formed in reduction. This is usually not difficult when pebble milling because it can be done by adding the reducing liquids to the mill and continuing milling for a half hour or so before emptying. Because in many operations it is very inconvenient to mill after reduction, it is best to avoid “shock” if possible.

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