Dispersants have a number of functions in a pigment dispersion; one of these is preventing particle agglomeration, which can adversely affect the desired properties of a coating or ink. During the premix stage of milling, the dispersant needs to dissolve in the milling medium and then adsorb onto the pigment particles. By preventing re-agglomeration during milling, the dispersant can reduce milling time, improve tinctorials and improve other properties of the coating or ink. Designing effective pigment dispersants to ensure they can successfully do their job can be a challenge—especially with new demands on the formulator that reflect a changing world.
The more long-standing challenges in the art of formulating to disperse a pigment in water include accommodating different pigment surfaces, incorporating co-solvents and other electrolytes, and inter- and intramolecular associations. These challenges are fundamental to pigment dispersions and must be overcome when the dispersing agent is designed, else pigment performance can be lost.
The more recent challenges that now need to be met are biocide restrictions, a drive toward fewer stock keeping units (SKUs), stability of smaller particle size dispersions, better water resistance, and VOC level reductions. These newer challenges aren’t inherent to the dispersion itself, but are more a result of evolving regulations and the move to be more environmentally friendly or more operationally efficient. Here’s a closer look at these challenges:
- Biocide restrictions—anything sold in water will tend to contain some form of preservative, especially a container which gets opened and closed (like cosmetics), in order to stop bacterial growth. In the coatings industry, the biocides MIT and BIT are widely used, but are now being increasingly restricted by legislation in Europe. If a biocide exceeds a certain amount within a coating, the container must carry a warning label, which manufacturers want to avoid. For dispersant makers, this is a challenge because if the dispersant is sold as a solution in water, a biocide has to be used to stop bacterial growth. The more biocide a dispersant contains, the less flexibility a paint manufacturer has in formulating the final product. At Lubrizol, we have developed a dispersant that is 100% active with no water content, which removes the need to add biocide. Without water added, transporting materials also becomes cheaper and greener as we’re transporting less volume of more-concentrated product.
- Fewer SKUs—formulators are asking for fewer dispersants that will work with more of their products to reduce the SKUs on their shelves. Lubrizol has worked to change the structure of dispersants so they work across a wider range of pigment surfaces.
- Stability of smaller particle size dispersants—typically, particles are dispersed into the micron size. But for high performance applications, like inkjet printing and automotive coatings, they need to be even smaller. Keeping these smaller dispersant particles stable is a major challenge because they tend to agglomerate very quickly.
- Better water resistance—making a pigment dispersion in water, the coating is applied and then the water dries off (hydrophilic). After it dries, the opposite is desired (hydrophobic) to provide water resistance, stain resistance and higher durability.
- VOC reduction—most countries have legislation around volatile organic compound (VOC) content in coatings. As VOC continue to be restricted, the toolbox of additives which can be used gets smaller. As an example, some companies include a humectant in their formulations to prevent coatings from drying too quickly. The humectant sits on the surface and slows down the drying rate without the paint/coating cracking too quickly. Mono propylene glycol (MPG) is one humectant that is commonly used because it is inexpensive and effective - but contributes to VOC and sVOC’s (semi-volatile organic compounds). To address the new limits, Lubrizol has developed new humectant technology that is not based on mono propylene glycol and that contributes to lower VOCs and sVOC’s while remaining effective in use.
By careful modification of dispersant and humectant structures, Lubrizol has been able to deliver new additives solutions that are better tuned to meet the requirements of coating and ink formulators challenged by the needs of the today's modern industries.