How to correctly check fineness of grind by Hegman gage

Fineness of grind is a test that is carried out all over the world in the production of paints by grinding in a dissolver or various types of mills. This test allows for a quick assessment of the fineness of pigments and fillers produced as suspensions (e.g. slurries, pigment concentrates, etc.) in the dispersing and grinding process, where the effectiveness of destroying aggregates and agglomerates is particularly important for the preparation of repeatable batches of various paints.

Grindometer during the fineness of grind test

The test is performed using the ASTM D1210 standard and is referred to as the fineness of grind. The instrument to perform the test is a hardened stainless steel block grindometer designated by ASTM D1210 as a tapered gage (machined tapered path) or stepped gage (each of 1 ich2 of path surface). Grindometers can be one path or two parallel pathes and are scaled from 0 µm (Hegman 8) to 100 µm (Hegman 0) or in narrow ranges, e.g. 0 to 50 µm, 0 to 15 µm, etc. Along with the grindometer, there is a scraper for scraping paint or slurry during the test. It is a two-edge profiled plate made of hardened stainless steel, the leading edge of which is rounded in a strictly defined radius.

Conversion chart

The scale of the grindometer is selected depending on the desired measuring range, its accuracy and the experience of the laboratory technician. Grindometers with very low ranges, e.g. 0-15 µm, require the most demanding of skill in evaluation. The selection of the scale should be made on the basis of grinding the fillers to be read, hence usually one universal wide-range grindometer 0-100 µm (or 0-50 µm) and supplementary narrow ranges are used. The conversion of Hegman units is given in the table above and as can be seen the scale increase takes place every 0.5 mils.

Various grindometers and scraper

When using grindometers, remember not to use metal spatulas or other metal tools to apply paint to the path, or to clean them, because the grindometer is a precision device that could be damaged by scratching with metal tools. Grindometers should be kept perfectly clean and cleaned with non-aggressive solvents immediately after the fineness of grind readings. When working with latex paints, it is advisable to clean with water and a soft cloth, as well as remove dried paint fragments in the grooves with a cloth soaked in ethyl alcohol.

Fineness of grind test after completing the slurry grinding

Grindometers are commonly used at various stages of the manufacturing process to determine if the dispersing and grind process is complete, and are also used to determine cleanlines (texture), typically using single wide lane grindometers for this evaluation. The degree of dispersion (fineness of grind) test is done by reading the spot in the track where clusters of pigment or filler particles start to appear. However, in order for this reading to be possible and performed correctly, there are three very important rules that are often overlooked when performing this daily test.

Rule #1 is for sample dilution. While slurries can have a low viscosity, latex paints usually have this viscosity at a much higher level. In assessing the fineness of grind, it is about determining the point of occurrence of clusters of primary particles or agglomerates (if the grinding is still not at the correct level) and to do this, the particle must be free to move, and more specifically to sink to the bottom of the path. Then the clusters are easily visible and do not cause scratches in the wet paint path. This is stated in Note 3 from the ASTM D1210 Test Method.

NOTE 3—For this test method to function properly, the pigment particles in the specimens to be tested should be free to settle to the bottom of the gage channel after the drawdown. Therefore, before testing, high-viscosity intermediate specimens that have little ability to flow should be reduced with a compatible liquid. Reduction should be in approximately the same proportion as the intermediate will be reduced in practice“. (Citation from ASTM D1210)

The photo below shows the appearance of a wet paint path in the grindometer without diluting the sample prior to testing. There are visible scratches in the wet paint film and no clusters of pigment particles and fillers are visible. In addition, a sample that is not diluted may dry out too quickly, making it difficult to read.

Sample before dilution

You can see the same sample below, but diluted with water. Clusters of pigments and fillers that are easy to identify have appeared in the grindometer path. Additional reduction of viscosity prevented the sample from drying out too quickly and does not hinder the evaluation by reducing the time needed to make it.

Sample after dilution with water to facilitate reading of the fineness of grind

Rule #2 is about how you make your judgment. In accordance with ASTM D1210, the assessment should be made by observing the wet paint film in the grooves first from deepest to shallow, then second measurement on a new sample from the shallowest to the deepest. This principle makes it possible to check the visual judgment for the correctness of the readings, which can be properly corrected based on the two viewing directions of the grindometer grooves. It is very important not to try to grasp the entire length of the path, and to make observations following the path according to this principle, and the grindometer should be perpendicular to our direction of observation.

Rule #3 is illumination. Assessment should be made according to the rules given, provided that there is adequate light on the grindometer and the paint paths. The ASTM D1210 standard indicates the illumination 250 mm above the grindometer and the illumination must be tube-shaped and parallel to the grindometer. The observation is made at an angle of 75 to 80°. This method significantly improves the repeatability and correctness of reading. The ASTM D1210 standard provides for the use of a light box, but is not mandatory. A fluorescent lamp of sufficient length is sufficient to illuminate the entire path of the grindometer. See how this reading looks like in our video – link below.

In addition to the above rules, you should remember about other activities that affect the precision of the test, such as the appropriate inclination of the scraper when distributing paint in the grooves of the grindometer. All the details described in ASTM D1210 allow to increase the precision of readings, hence familiarization with the test method in ASTM D1210 is mandatory and cannot be limited to reading the grindometer manual supplied with the instrument. Although this test is very simple, its importance is very great. On the basis of the result of the fineness of grind test, grinding or finishing is carried out and the production batch is sent for bottling. Therefore, careful execution is very important. With a high frequency of testing in production and to improve the efficiency of testing the degree of grinding, you can additionally use automatic grindometers that allow you to distribute paint at a constant speed, and in more advanced devices also assess the clusters using a digital camera and software analyzing the test result.

Published by Artur Palasz

Scientist, paint formulator and testing expert.

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