Whether a paint reaches a workable viscosity at a high solid content, or ends up too thick to apply well, often comes down to how well the pigment was dispersed — and research on titanium dioxide dispersions shows that getting this right is a genuine formulation trade-off, not simply a matter of adding more dispersant.
What is it?
Dispersants are chemicals used to break up pigment agglomerates and keep individual particles separated within a paint or coating’s liquid vehicle. The industry’s standard way of verifying that this has actually worked is ASTM D1210, a test method that measures the “degree of dispersion” (commonly called fineness of grind) of a pigment-vehicle system using a Hegman-type gauge — judging whether agglomerates have been broken up enough that they won’t interfere with the smoothness of the finished coating film.
Why should I use it?
- A well-chosen dispersant lets a formulation reach a high solid content while keeping viscosity low enough to process and apply — see “How it’s used.”
- Getting the dispersant’s molecular weight right is a genuine trade-off between stability and viscosity, not a “more is better” decision — see Limitations.
How it’s used
A 2025 peer-reviewed study on aqueous titanium dioxide dispersions used poly(acrylic acid) sodium salt (PAAS) as the dispersant at a fixed dosage of 0.15% by weight, testing molecular weights ranging from 800 to 100,000 g/mol. The best-performing variant, at a molecular weight of around 6,000 g/mol, achieved a 50% solid content dispersion with a viscosity of just 43 mPa·s (at a shear rate of 100 s⁻¹), a hydrodynamic particle size of 343.8 nm, and a zeta potential of -55.6 mV — the combination that let the dispersion carry a lot of pigment without becoming too thick to handle.
Limitations — when it isn’t the right choice
- Choosing a dispersant’s molecular weight is a balancing act, not a simple “higher is better” decision. The same 2025 study found that lower molecular weight PAAS variants (800-5,100 g/mol) produced lower viscosity dispersions, but with weaker long-term stability. Higher molecular weight variants (8,000-100,000 g/mol) produced better stability, but at the cost of excessive viscosity caused by polymer chain entanglement. Only the mid-range molecular weight (around 6,000 g/mol) delivered both a workable viscosity and good stability. This means dispersant selection has to account for the specific solid content and processing conditions of a formulation — there’s no single “best” molecular weight independent of context. This finding is specific to a PAAS dispersant used with titanium dioxide in an aqueous system; other pigment chemistries or dispersant types (e.g., polyurethane- or polyacrylate-based) may behave differently and would need separate verification.
