русский
Español
Français
Home / News / Industry News / Waterborne Paint Re-Coarsens After Storage — How to Choose the Right Dispersant?
Industry News
Waterborne Coatings · Dispersant · Storage Re-Coarsening
Waterborne paint that shows a normal state right after dispersion, but then develops re-coarsening and increased particulate content after storage, is typically related to re-agglomeration of pigments and fillers. Beyond grinding fineness, the matching between the dispersant and the resin, pigment, and waterborne system is equally important. When selecting a dispersant, evaluation should be combined with pigment type, resin structure, and storage requirements.
Importantly: the evaluation of a dispersion system cannot be based only on the fineness right after grinding — the state after storage must also be included in the assessment.
6
Key diagnostic questions before changing the dispersant
1
Why Does Re-Coarsening Appear After Freshly Dispersed Paint Stands?
Re-coarsening does not necessarily mean the grinding stage was inadequate. After pigment particles are dispersed by equipment, they may look uniformly distributed — but without sufficient stabilisation, they may gradually re-approach and re-agglomerate during storage. Freshly produced state looking fine, then showing fine particles, settling, or re-coarsening after a period, is the typical presentation.
2
Why Dispersant Selection Affects Re-Coarsening
A dispersant's role is not only to help pigment "open up" — it also helps particles maintain a stable dispersed state in the waterborne system after grinding. If the dispersant's affinity for the pigment surface is insufficient, the grinding may look effective, but particles may still re-agglomerate during storage. This is why some dispersants show good fineness in small trials but the system state starts changing after several days or longer storage.
3
Waterborne Dispersant Selection Cannot Focus Only on "How Fast It Disperses"
In actual selection, many people primarily focus on grinding time and initial fineness. But for waterborne coatings, the storage stage equally deserves attention. A dispersant needs to simultaneously consider: pigment type, resin system, solid content, pH, and final storage requirements. Different pigments have different surface properties and different dispersant needs — one dispersant performing well in one pigment system does not predict equivalent performance in another.
4
Why Does Re-Coarsening Persist After Changing Dispersant?
If re-coarsening continues after changing the dispersant, other variables may be involved. Grinding equipment, grinding time, grinding media, pigment addition ratio, and system viscosity can all affect the final dispersion state. Additionally, the compatibility between the dispersant and the resin cannot be overlooked — if the dispersant can adsorb the pigment well but produces strong interaction with the resin after entering the complete formulation, the originally established dispersion state may change.
5
More Dispersant Does Not Mean Less Re-Coarsening
Dispersants have an appropriate use range. Insufficient addition may mean inadequate pigment surface coverage; too much may change system viscosity, flow state, and film surface performance. In waterborne systems, dispersant dosage changes may also affect the balance between other additives. Setting several gradient levels for comparison — observing initial fineness, particle size change after storage, viscosity, and film state simultaneously — is more reliable than simply increasing the addition level.
6
When Re-Coarsening Appears, How to Diagnose?
Investigate in this sequence: pigment type → dispersant type → dispersant dosage → grinding process → resin compatibility → storage conditions. First confirm at which stage re-coarsening occurs. If obvious coarse particles already exist right after grinding, prioritise checking the grinding process. If initial fineness is good but re-coarsening gradually appears after storage, focus more on the long-term stability of the dispersion system. By comparing different dispersants and addition ratios, it is easier to find an approach suited to the current system.
Summary
Waterborne paint re-coarsening is not only "the pigment was not ground fine enough." What truly affects the state after storage is the dispersant's ability to stabilise pigment surface dispersion, and the matching relationship between dispersant, pigment, and resin. When selecting a dispersant, the fineness and efficiency right after dispersion is complete cannot be the only measure — particle size, viscosity, and film state after storage also need to be included in the evaluation. For different pigments and resin systems, targeted dispersant screening and gradient testing is more likely to find the right formulation adjustment direction.
Frequently Asked Questions
Is particle size re-growth after storage always visible as obvious settling, or can it occur without obvious visual change?
Re-agglomeration can occur at a scale that is not yet visible as obvious settling but is detectable by particle size measurement. Soft agglomerates that form during storage may not immediately settle (they may remain suspended as larger, lower-density aggregates) but will affect film quality through increased film roughness and colour consistency issues. This is why relying only on visual settling assessment underestimates the problem — particle size measurement (laser diffraction or similar) at defined storage intervals gives a more complete picture of whether the dispersion state is truly stable.
Does storage temperature significantly affect re-coarsening rate in waterborne dispersions?
Yes. Elevated temperature increases the mobility of both pigment particles and dispersant molecules — accelerating the rate at which the dispersant can desorb and re-adsorb, and increasing the frequency of particle collisions. This makes re-agglomeration more likely at higher storage temperatures. Standard accelerated storage testing at 40–50°C is routinely used to predict longer-term ambient storage stability in a shorter test period. A formulation that shows adequate stability at ambient temperature but fails accelerated storage testing at 40°C may have marginal stability in real-world warm storage conditions.
