Dispersants provide a number of benefits for many different types of applications. That’s why we’re featuring a series of blog posts about them. This is the first post in the series, providing a high-level overview of dispersants—why they’re used and how they’re used. Future posts will focus on their use in specific markets.
Dispersants are highly specialized additives used to wet, disperse and stabilize solid particles in a variety of continuous phases, such as solvents, water and plastics. Dispersants are designed to reduce viscosity and increase the stability of a dispersion while enhancing the aesthetic properties of the final coating.
The solid particle being dispersed is typically a pigment, but it could also be a silica matting agent, a wax, a conductive particle (carbon, graphene, carbon nano tubes), inorganic fillers (calcium carbonate, talc, barytes), or even a precious metal (gold, silver and platinum)—any solid particle dispersed in a continuous phase (liquid or solid).
By reducing viscosity, dispersants make the dispersion more workable while also helping improve productivity economics by increasing pigment loading and rate of dispersion. This provides shipping advantages: with more pigment in the dispersion, the amount of water is reduced, making it easier to handle and transport. Formulation flexibility can also be improved with the ability to add dispersions to a wider range of base finishes.
A typical dispersant is a two-component structure, consisting of an anchoring group capable of being strongly adsorbed onto the particle surface, and a polymeric chain, which is attached to the anchor group and provides the stabilization. The anchor groups surround the particle and the chains sterically stabilize it to prevent the particles in the dispersion flocculating or turning into a gel.
There are many structures designed to achieve this stability because all particles have different surface natures and there are so many mediums—water, solvent, UV monomers, resins—that have even more variations of polarity. Different anchor groups are more receptive to different particle surfaces. It is the particular combination of anchoring group and polymeric chain which lead to the effectiveness of dispersants.
More detail on dispersant structure can be viewed in this Dispersants Technology & Benefits Video.
The Dispersion Process
The dispersion process involves three stages:
- Wetting of particles (pre-mixing)
- Separation of particles (achieved by mechanical means)
- Stabilization of particles (to overcome Van der Waals attractive forces)
The first and third stages can be improved by the use of effective dispersing/stabilizing agents, with the third stage being the most critical to optimize the performance of a system. That’s because the third stage controls the final quality and stability of the dispersion system.
Dispersants achieve stability via steric stabilization which is based on the adsorption of polymeric materials to particle surfaces to overcome Van der Waals attractive forces. The required properties for effective stabilization are:
- Strong adsorption of stabilizing polymer to the particle surface
- Optimum chain length (too long and it can fold back and compress the steric barrier; too short and it will not overcome Van der Waals forces)
- Good solubility in the medium being used during dispersion
- Compatibility with the resin after the solvent has evaporated
How Are Dispersants Added?
Most dispersants are pourable liquids, with some being waxy or granular solids. Ideally, the dispersant should be added before the mechanical process that breaks everything down—before applying mechanical energy. The best time to add the dispersant is during the mill base phase (where the main ingredients are resin, and solvent or water), ensuring the dispersant is dissolved before adding the pigment.
The best results come from the dispersant going onto the particle surface as the mechanical means expose the surface. The dispersant surrounds the available particle area and prevents the particles from coming back together, and resulting in a lower viscosity.
With that said, there are instances where dispersants can be post-added to improve the stabilization or color at the end of the process, but they won’t go through the original mill.
Improving the quality and stability of the dispersion also leads to improved coating quality. A reduction in the average pigment particle size increases color strength. A similar benefit is seen in gloss, transparency and brightness. However, if particle stability is not maintained in the dispersion, the final performance improvements can be reversed.
As mentioned previously, all dispersions have an anchor and a chain. Most Lubrizol hyperdispersants are either single-anchor, single-chain dispersants which have a lower molecular weight and are good for wetting, or a “comb” structure, or multiple-anchor, dispersants which provide better stabilization. Choosing the right dispersant for the application comes down to the level of experience and knowledge of dispersants that the manufacturer offers.
Of course, the dispersant must be able to do its job without introducing negative performance properties such as poor chemical resistance or surface softness, or impacting the adhesion and durability of the coating.
Lubrizol dispersants are used by formulators globally to meet a range of needs across many applications, including those in paints & coatings, printing, packaging, plastics & composites, and electronics applications.