- What is the fineness of grind?
- What are the consequences if the fineness of grind is poor?
- Why is it important to measure the fineness of grind?
- What factors affect the fineness of grind?
- What are the applications of fineness of grind?
- Which instrument evaluates the fineness of grind?
- How to interpret the results of the fineness of grind?
- What are the test methods to measure the fineness of grind?
What is the fineness of grind?
Fineness refers to the degree of smoothness or uniformity of the finish of the system.
The fineness of grind is the degree of uniformity and extent of fineness to which particles/pigments are ground in paints and inks. Most of the pigments and extenders used in paints are processed to very fine particle sizes. This particle size ranges from 2 to 50 Microns. However, this fine size also needs to be:
- uniformly processed to achieve a uniform dispersion and
- ensure the quality of dispersion in inks and paints.
High shear dispersion or milling process converts the powder/pigment lumps into aggregates. This is finally converted into fine particle suspension.
Know more about the dispersion process »
What are the consequences if the fineness of grind is poor?
It is very critical to control the fineness of grind for many reasons. The parameters of paints & inks which are inversely proportional to the fineness of grind are:
Particle Size vs Strength Development
Other critical failures include
inadequate adhesion, chalking, flaking, and floating. These defects occur due to inefficient dispersion or grinding.
Why is it important to measure the fineness of grind?
The fineness of grind is not only limited to finish and aesthetics. But it can also facilitate a lot of other optimization and standardization processes. Some of the reasons to check the fineness of grind are as follows:
- Ensure optimum grinding of paints, inks, etc., for physical and physiochemical properties
- Grinding/high shear dispersion is a costly process. Hence, evaluation and optimization of the fineness of grind can save a lot of costs and other resources.
- Inbound/outbound product and quality rationalization
- Optimization and selection of efficient processes and grinding media
In a nutshell, the particle size and optimum grinding provide many advantages if controlled and optimized. Hence, the fineness of grind plays a critical role in the same.
What factors affect the fineness of grind?
There can be many factors affecting the fineness of grind for inks and paints. A few of the most influential factors are as follows:
Choice of wetting and dispersing agent
The choice of wetting and dispersing agent (W & D) is the most important factor for the system. A wetting and dispersing agent should be compatible and in adequate dosage. As the particles and aggregates break down the dispersant demand of the system increases.
- Wetting agents provide quick wetting of the aggregates or particles so that they can be easily mixed and processed.
- The dispersant selection keeps the aggregates and ground particles apart. It also facilitates long-term grind stability.
Processing equipment
Different types of equipment play a critical role to determine the fineness of grind. Some of them are:
They facilitate a greater degree of dispersion and efficient grinding of pigments and extenders. The selection of right type of cowl or mixing blades ensures greater and quicker grinding efficiencies.
Similarly, the choice of milling mechanisms, beads, and other grinding media also ensures effective and efficient fineness of grind.
Time of processing
Generally, the fineness of grind is directly proportional to the processing time. That is, the longer the time of processing the finer the grinding.
Nature of pigments & extenders
The nature of pigment and extenders to be ground greatly affects the fineness of grind. Several factors of pigments and extenders that play a critical role in the fineness of grind are:
- surface charge,
- specific gravity,
- nature of the surface treatment, and
- morphology
Viscosity of the grinding phase
Optimum viscosity leads to a good vortex (doughnut) formation of the mill base. This results in faster particle size reduction. The high or low viscosity of the mill base results in inferior shear distribution. This affects the pigment grinding and particle size reduction.
Watch how to adjust the viscosity of the mill base in practice
What are the applications of fineness of grind?
Many industries use fineness of grind as a critical quality control parameter. Some areas of applications include:
- Paints, coatings, and inks
- Pigment dispersions and pastes
- Cosmetics and food industry
- Other liquid systems like slurries, greases, sauces, etc.
Which instrument evaluates the fineness of grind?
The fineness of grind testing is done with the help of a Hegman gauge which is also known as a grindometer or grind gauge. The Hegman gauge is a hard-stainless steel block. It has either one, two, or more grooves with graded slopes. It is a simplistic yet relatively accurate prediction of particle size distribution and uniform grinding.
Hegman Gauge with Rating Scales
Credit: Wikipedia
The scales are in mils, microns, NS (Hegman grading), or PCU (North). It is an easy-to-handle instrument. It requires moderate skills to perform testing and gives good results with low error.
| Hegman Units |
Mils |
Microns |
| 0 |
4 |
101.6 |
| 1 |
3.5 |
88.9 |
| 2 |
3 |
76.2 |
| 3 |
2.5 |
63.5 |
| 4 |
2 |
50.8 |
| 5 |
1.5 |
38.1 |
| 6 |
1 |
25.4 |
| 7 |
0.5 |
12.7 |
| 8 |
0 |
0 |
The Rating System of Hegman Gauge
The relation between microns and NU is as follows:
| Microns |
North Units |
| 0 |
100 |
| 10 |
90 |
| 20 |
80 |
| 30 |
70 |
| 40 |
60 |
| 50 |
50 |
| 60 |
40 |
| 70 |
30 |
| 80 |
20 |
| 90 |
10 |
| 100 |
0 |
Procedure to measure FOG using Hegman Guage
- The sample required for testing the fineness of the grind is as low as 2- 3 ml.
- The sample is to be poured on the deep grooves (upside) of the gauge.
- With the stainless-steel scraper provided the sample (paint or ink) is to be drawn down.
- This is to be done towards the shallow end of the gauge with a decent amount of pressure.
- The scraper is to be held in an oblique position that is slightly inclined towards the applicator (chemist).
Sizes available for measuring the fineness of grind
Hegman gauges come in different models which can be single scale, dual scale, or even three scales in one gauge. There are many vendors of Hegman gauges. Some of the reputed suppliers are BYK, TQC Sheen, GARCO, Elcometer, Gardner, etc.
Hegman gauges are available in the following ranges:
- 0 to 100 micrometers
- 0 to 50 micrometers
- 0 to 25 micrometers
- 0 to 15 micrometers
- 0 to 10 micrometers
How to interpret the results of the fineness of grind?
The interpretation of the results is simple. But it may need observational expertise to give good analytical inferences. By looking at the initial drawdown of the paint, one can see the point where the smooth, fine finish of the gauge is distorted by coarse particles. The visible line marks also suggest the coarse particle presence in the gauge.
The unground particles leave a track behind the paint or ink drawdown. This allows the chemist to determine the fineness of grind or the gauge reading. The following image demonstrates the Hegman gauge evaluation.
Interpretation of the Hegman Gauge Ratings
The given system shows the initial marks at 4 Hegman or 50 microns
What are the test methods to measure the fineness of grind?
The test methods for quantifying the degree of grinding or fineness of grind include:
- ASTM D1210-05: Standard test method for the fineness of dispersion of pigment — Vehicle systems by Hegman-type gauge.
Testing the Fineness of Grind (FOG)
Credit: Spectrochem
- ASTM D1316: Standard test method for the fineness of grind of printing inks — By the NPIRI grindometer.
- ISO 1524: Paints, varnishes, and printing inks — Determination of fineness of grind.
These tests measure the FOG of paints, inks, and other liquid systems. The above test methods exclude products containing pigments in flake form (e.g., glass flakes, micaceous iron oxides, etc.).
References
- https://en.wikipedia.org/wiki/Hegman_gauge
- https://industrialphysics.com/applications/fineness-grind-testing/
- https://www.mdpi.com/1996-1944/15/3/705