Почему пигментные дисперсии повторно агломерируются после измельчения?

Pigment dispersion may appear stable immediately after grinding, but problems can develop during storage or subsequent formulation. Particle size may increase, viscosity may change, and sedimentation or color variation may occur. In many cases, the issue is not simply insufficient grinding, but inadequate stabilization of the newly dispersed particles. Selecting a suitable pigment dispersant is therefore important for maintaining dispersion stability after mechanical processing.

For coatings, printing inks, and other pigment-based formulations, understanding why particles associate again can help manufacturers identify formulation problems and evaluate additive performance more effectively. Polywill develops specialty additives for different formulation systems, including water-borne, solvent-borne, and UV applications.

What Happens to Pigments During Grinding?

Grinding breaks down pigment agglomerates and distributes the particles throughout the liquid medium. This process helps develop the required particle size and improves color development and other application properties.

However, grinding alone does not guarantee long-term stability. Newly separated particles may come together again if the formulation cannot maintain sufficient separation between them. As a result, a dispersion that initially appears uniform may gradually develop changes in viscosity, color strength, gloss, or storage behavior.

Почему пигментные дисперсии повторно агломерируются после измельчения?

1. Inadequate Surface Stabilization

Grinding exposes new pigment surfaces and increases the demand for effective stabilization. If the dispersant does not provide sufficient surface coverage, particles may associate again after grinding, even when the initial particle size is satisfactory.

2. Re-Agglomeration and Flocculation

After grinding, dispersed pigment particles may come together again due to attractive forces between their surfaces. This process is commonly described as flocculation and can lead to changes in viscosity, color strength, gloss, and storage stability.

The term re-agglomeration is also used to describe the renewed association of dispersed particles. However, the terminology may vary depending on the pigment system and the strength of particle interactions.

Some particle associations can be broken down by moderate shear, while others are more difficult to reverse. Therefore, maintaining effective pigment surface stabilization is essential for preventing undesirable particle association during storage and subsequent processing.

3. Pigment and Dispersant Compatibility

Different pigments have different surface characteristics, so a dispersant suitable for one pigment may not perform equally well with another. Organic pigments, inorganic pigments, titanium dioxide, and carbon black may require different dispersant chemistries.

Dispersant selection should consider pigment type, binder compatibility, and the intended formulation. Laboratory testing can help determine whether the selected additive provides consistent performance after formulation and storage.

4. Incorrect Dispersant Dosage

Too little dispersant may leave part of the pigment surface insufficiently stabilized. However, increasing the dosage does not always improve performance, as excessive amounts may affect viscosity, compatibility, or other formulation properties.

The appropriate dosage depends on the pigment characteristics, dispersant chemistry, and formulation requirements. It should be established through formulation trials rather than assumed to be the same for every pigment system.

5. Changes During Formulation and Letdown

A dispersion that performs well immediately after grinding may become unstable when incorporated into the final formulation. Changes in binder content, solvent composition, pH, or other additives can affect the interaction between pigment particles and the surrounding medium.

Therefore, dispersion performance should be evaluated not only in the grinding stage but also after letdown and the addition of other formulation components.

How Do Pigment Dispersants Stabilize Particles?

Pigment dispersants generally stabilize particles through electrostatic repulsion, steric stabilization, or a combination of both mechanisms.

Electrostatic stabilization is particularly important in many aqueous pigment dispersions, while steric stabilization plays a major role in solvent-borne systems and is also widely used in water-borne formulations.

For polymeric dispersants, pigment-affinic anchoring groups help the additive adsorb onto the pigment surface, while compatible polymer chains extend into the surrounding medium and provide steric stabilization.

The dominant stabilization mechanism depends on the pigment, dispersant chemistry, and liquid medium. Understanding these differences helps guide additive selection and subsequent stability testing.

How Can Storage Conditions Affect Dispersion Stability?

Temperature fluctuations, prolonged storage, changes in pH or solvent composition, and interactions between formulation components can affect dispersion stability. These changes may lead to flocculation, viscosity drift, sedimentation, or changes in color strength.

Because instability may develop gradually, a dispersion should not be judged only by its appearance immediately after production. Comparing samples before and after storage can help reveal changes that are not apparent during initial inspection.

How Can You Identify Pigment Dispersion Instability?

A practical stability evaluation should compare the dispersion before and after storage under defined conditions. No single test can identify every type of instability, so several indicators should be considered together.

1. Measure Viscosity Changes

Record the initial viscosity and compare it with the viscosity after storage. An increase may indicate particle association or the development of a more structured dispersion, while a decrease may reflect changes in the formulation or dispersion structure.

Viscosity results should be compared at consistent measurement temperatures and test conditions. A viscosity change alone does not prove re-agglomeration, but it can indicate that further investigation is needed.

2. Compare Particle Size Distribution

Measure the particle size distribution before and after storage using an appropriate method for the formulation. An increase in particle size or the appearance of a larger-particle fraction may indicate flocculation or renewed particle association.

Particle size data should be interpreted alongside other results because the measurement method, sample preparation, and dilution procedure can influence the findings.

3. Evaluate Color Strength and Development

Compare color strength and color development under consistent preparation and application conditions. Changes in color strength, shade, or uniformity may indicate that the dispersion no longer performs consistently.

For colored coatings and printing inks, this evaluation helps connect dispersion stability with practical application performance.

4. Perform a Rub-Out Test

A rub-out test can help identify color differences caused by uneven pigment distribution or flocculation. Apply the same formulation to a suitable substrate, then rub part of the wet film under consistent conditions and compare the rubbed and unrubbed areas.

A noticeable color difference may indicate a dispersion or compatibility problem. However, the result should be interpreted alongside other tests because rub-out behavior can also be affected by formulation and application conditions.

5. Check Sedimentation and Re-Dispersibility

Observe whether sediment forms during storage and whether the settled material can be redispersed using a defined mixing procedure. Soft sediment that redistributes readily behaves differently from hard sediment that is difficult to break up.

Record the storage duration, temperature, sediment appearance, and mixing conditions to make comparisons meaningful.

Suggested Stability Evaluation

These tests provide complementary evidence. For reliable evaluation, manufacturers should establish acceptance criteria appropriate to the pigment system and intended application rather than relying on one indicator alone.

How Can Re-Agglomeration Be Reduced?

Several practical steps can help improve the stability of pigment dispersions:

• Select a dispersant that is compatible with the pigment, binder, and liquid medium.
• Optimize the dispersant dosage through laboratory trials rather than assuming that a higher dosage will always produce better results.
• Control grinding conditions to achieve the required particle size without treating particle-size reduction as the only performance target.
• Evaluate the complete formulation after letdown to identify incompatibilities introduced by other components.
• Conduct storage tests and compare viscosity, particle size, color performance, sedimentation, and re-dispersibility before approving a formulation.

These measures help manufacturers distinguish between problems caused by grinding conditions, formulation changes, and inadequate dispersion stability.

Selecting a Suitable Pigment Dispersant

The selection of a pigment dispersing agent should begin with the pigment type, formulation medium, binder system, and required application performance. A suitable additive should support efficient dispersion while maintaining acceptable properties during subsequent processing and storage.

Different formulations may require different additive chemistries. For example, Polywill’s dispersant portfolio includes products for water-borne and solvent-borne systems:

• Coadd™ D-6239: A high-molecular-weight block copolymer recommended for water-borne coatings and printing inks.
• Coadd™ D-6075B: A high-molecular-weight multi-block copolymer wetting and dispersing agent recommended for high-performance water-borne industrial coatings.
• Coadd™ DO-6269: A polyacrylic block polymer with pigment-affinity groups, recommended for solvent-borne and solvent-free coating systems.
• Coadd™ DO-9950: Suitable for unsaturated polyester and epoxy resins, including certain solvent-free formulations containing inorganic pigments and fillers.

These products serve different formulation needs, so they should not be treated as interchangeable. The appropriate dispersant for pigment dispersion should be selected according to the specific pigment, resin, and processing requirements. Product documentation and laboratory compatibility testing should be consulted to confirm suitability for the intended application.

Conclusion

Pigment dispersions may re-agglomerate after grinding because particle association can develop during formulation, storage, or subsequent processing. Inadequate stabilization, pigment–dispersant incompatibility, incorrect dosage, and changes in formulation conditions can all contribute to instability.

Rather than relying only on initial appearance or grinding results, manufacturers should compare viscosity, particle size distribution, color strength, rub-out behavior, sedimentation, and re-dispersibility before and after storage. Combining these tests with appropriate additive selection can help identify the source of instability and improve the consistency of pigment-based formulations.

With experience in specialty additive development, Polywill provides dispersant solutions for different coating and ink systems. Matching additive chemistry to the formulation and validating performance through systematic testing are important steps toward achieving reliable dispersion stability.