Science Trends

01 Nov Science Trends

Better Coatings via Science-Based Formulation Using HSP

So you need to find a new solvent blend, or you need to incorporate a new polymer, or someone has offered a new pigment or nanoparticle that promises improved properties. How do you use the minimum effort to get the maximum balance of desired properties?

Clearly, it is important to understand the different solubility and compatibility issues between your components. You can try to do this via vague words like hydrophobic/hydrophilic or polar/non-polar, but these are too vague to be of much use. We need numbers to formulate scientifically, and in particular, we need a good measure of how “like” or “unlike” any two molecules might be. To get a measure of likeness, we must start with three numbers that describe each chemical, polymer, particle or additive used in our coating formulation.

Why three? Because two is too small and four is too complicated. Though, there is always an implicit fourth parameter, the molecular size, which is not discussed further in this article. This article describes:

• What those three numbers are;
• How you can determine them;
• How you can use them in three specific areas of coatings: coating a polymer, ensuring compatibility between the ingredients in the coating, and ensuring compatibility between coating and substrate so it sticks well.

What Are the Three Hansen Solubility Parameters?

The three parameters capture the essence of three familiar features of any molecule:

• Dispersive aspect (dD);
• Polar aspect (dP);
• Hydrogen-bonding aspect (dH).

Coatings formulators can readily appreciate the polar and hydrogen-bonding aspects. The dispersive part is less used, but is still intuitive. All molecules are held together via van der Waals or dispersive forces, and these forces differ for different molecules. Aromatics, for example, have a broad electron cloud, so they self-interact more strongly than the alkanes, which have a tight cloud. Many formulators, including myself, have gone wrong by not recognizing the importance of this apparently boring parameter.