Grind Gauges

Grind Gauges

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The grain size (also called grain fineness) of a liquid coating can be determined quickly and easily with the grind gauge.

Coatings, paints and pigment pastes (etc.) contain pigments and fillers which don´t dissolve in the medium, but are dispersed. That means the pigments and fillers are finely distributed in the liquid binder-medium (hydro or solvent-borne), but are not dissolved like in a solution (for example salt in water = solution).

The color strength of the pigments rises, the more finely they are distributed (= the smaller the grain size). The grain size can be reduced by grinding with a disperser  or bead mill. In the process the pigment-agglomerates are mechanically destroyed. Depending on the application area usually an upper limit of the grain size is defined and then the pigments are ground until they are smaller than the defined limit. Mostly this limit is set in the range between 100 and 5 μm.

Therefore, in the coating, pigment, printing ink, paper, ceramics, pharmaceutical and food industries a determination of grain size is always needed. Because of the time saving and accuracy the grind gauges are used wherever the grain size needs to be about 5 μm or more. For grain sizes less than 5 μm a microscope must be used. Only with a microscope the exact grain size (diameter) and a grain size distribution can be identified.

The grind gauge consists of a stainless steel block. On the upper side of it there are one or more wedge-shaped channels (for example from 0 – 100 μm). Approximately 0.5 g of the substance to be tested is placed at the lowest point of the channel and is then wiped with the scraper (also called steel blade) through the upward going channel. It is recommended to immediately evaluate the grain size at eye level and in suitable light conditions. As soon as the solvent in the channel evaporates false results may be interpreted.

Only the grains “standing out” of the channel are visible and can be determined by using the scale (μm and in Hegman) beside the channel. If, for example, the grain is visible in the channel from 0 to 20 μm and a smooth film is visible from 20 μm upwards, the grain size is less than 20 μm. It should be noted that normally no clear boundary, but a crossover, is observed.