In the last few articles, we have covered dispersion. In this article, I discuss the robustness of a coatings system in reference to pigment dispersion. Upcoming articles will cover resins and binders.
When formulating a coating there are many things to consider, and pigment selection is one of them. Selecting pigments is important as they greatly affect the following:
- Color stability
- Gloss retention and exterior durability
- Hiding power and tint strength
- Ease of manufacture
- Physical, chemical, and other properties
While all these properties are well known, one pigment area not normally covered is ease of dispersion. You can have the same pigment made by different manufacturers, made by the same manufacturer at different locations, or different grades of the same pigment from the same manufacturer, and have significantly different dispersion experiences. The same is true for dispersants. Choosing raw materials only from lab experiences without considering how scale up and production friendly they are can lead to production delays, longer grind times, and even failure of the product.
With raw material substitutions due to global logistics issues, substituting one pigment or dispersant for another requires work. There is no such thing as a drop-in replacement, no matter what raw material suppliers tell you. Some properties will always change. A robust formulation will resist changes better.
If you plot property versus usage level, you will get different curves. In Figures 1, 2, and 3 you see three products plotted, with dispersant level versus Hegman grind, contrast ratio, and tint strength respectively. Table 1 gives the physical properties of the dispersants.
In all cases, dispersant usage level was based on lowering the solids to 40% (lowest sample level) with water so the dispersant actives on pigment solids remained the same. It is assumed economics are similar for all three dispersants.
While all three products have the similar maximum properties, the shape of the curve will have a huge influence on the efficiency of production and final properties of the coating. You can formulate on a plateau or a pinhead. On a plateau, variations in addition levels have less effect than if you formulate on a pinhead.
Product C has the highest results in most cases, but will be difficult for your plant. If you look at Hegman grind and your process allows for 0.59-0.61% addition, then product A, B, or C all work. However, for all practical applications, that is not possible in production, especially due to lot-to-lot variability in the dispersant and pigment manufacture.
Another issue you will see is product A is a waxy solid. This requires heating up the material for it to flow, making use in manufacturing more difficult. You must remember to put it in an oven the day before you use it, or heat trace lines.
Product B would be the choice at around 0.7% usage level since it has the highest resistance to level of addition variations and is easy for the plant to handle. The key is to look at the whole picture, and anticipate issues in production to make the most robust system possible.
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