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Home / News / Industry News / High-Concentration Black Paste Always Re-Coarsening? The Dispersion System May Not Be Right
Industry News
In solventborne coatings, inks, and automotive coatings, carbon black and organic pigments typically need thorough grinding to develop their full colouring capability. But the pigment is not finished after being "broken down" by mechanical equipment. The small particles formed during grinding have a large surface area, and if the system lacks sufficient stabilisation, these particles may still approach each other and re-agglomerate. Therefore, a situation frequently occurs in production: the colour paste looks good right after preparation, but after standing for a period, viscosity rises, re-coarsening appears, hue changes, and even flooding and colour mottle occur.
High Carbon Black Loading Causes Rapid Viscosity Rise
Carbon black has a large specific surface area. As loading increases, inter-particle interaction strengthens. If the dispersion system cannot fully stabilise the pigment, the colour paste readily shows obvious thickening.
Pigment May Still Re-Agglomerate After Grinding
Mechanical grinding only improves the initial agglomerate state. If pigment particle surfaces do not form a sufficiently stable protection layer, re-agglomeration may still occur during storage.
Blackness and Colour Development Difficult to Fully Achieve
When pigment is in an agglomerated state, the colouring surface area cannot be fully released, affecting blackness and potentially causing colour shade differences between colour paste batches.
Re-Coarsening and Flooding During Storage
After the colour paste stands for a long time, if dispersion stability is insufficient, pigment particles may re-agglomerate or migrate — ultimately manifesting as re-coarsening, settling, and flooding.
Why High-Concentration Colour Paste Is More Demanding on the Dispersant
When pigment proportion is high, the number of solid particles in the system increases significantly, and the chance of inter-particle interaction also increases accordingly. Especially with carbon black — which inherently has high specific surface area — if the solution is simply to extend grinding time, not only does production efficiency suffer, it may not resolve the re-agglomeration problem during storage.
The true approach is to allow the dispersant to more stably adsorb onto the pigment surface: one end forms stronger anchorage with the pigment, the other part reduces the chance of particles approaching each other again through steric hindrance and similar effects. This helps not only with full wetting during grinding, but also maintains the fine particle size state after grinding. For high-concentration carbon black colour paste, the dispersant's "stabilisation capability" is often more important than simple wetting speed.
DH-6329: Polymeric Hyperdispersant for Solventborne Systems
DH-6329 is a polymeric hyperdispersant with multiple anchor groups in its molecular structure that can interact with pigment surfaces, applicable for solventborne coatings, inks, and colour paste systems. For carbon black and organic pigment dispersion, it can help pigments achieve a more stable wetting and dispersion state. Especially for black paste systems, good flow can be maintained even at high carbon black loading, improving the problem of high-concentration colour paste readily thickening.
Beyond initial dispersion, DH-6329 also helps reduce the tendency for pigments to re-agglomerate during storage, so it has some improvement effect on systems prone to re-coarsening, viscosity change, and flooding. In automotive coatings and outdoor weathering coatings with high colour development, gloss, and storage stability requirements, it can also be used as a dispersion solution for carbon black and organic pigments. It also has good compatibility with various oil-based resins and can adapt to resin systems of different polarities. Reference dosage: inorganic pigments generally 5%–20% of pigment weight; organic pigments approximately 20%–50%; carbon black approximately 50%–100%.
The truly difficult problem to solve in high-concentration carbon black colour paste is not "can the pigment be ground apart" — it is whether a stable state can be maintained long-term after grinding. When carbon black loading increases, inter-pigment interaction increases significantly, and simply extending grinding time often cannot resolve the subsequent thickening, re-coarsening, and flooding problems. Selecting a polymeric hyperdispersant with multiple anchor groups can simultaneously improve wetting, viscosity reduction, and storage stability.
Frequently Asked Questions
Why is the recommended dosage for carbon black so much higher (50–100% of pigment) than for inorganic pigments?
The dosage is expressed as a percentage of pigment weight — and carbon black has an extremely high specific surface area (typically 50–1,500 m²/g depending on grade) compared to most inorganic pigments (often 5–30 m²/g). Much more dispersant per unit mass is needed to achieve adequate surface coverage and steric stabilisation of carbon black because there is so much more surface area to cover. The 50–100% figure reflects this high surface area requirement, not excessive addition relative to the ink or coating system overall.
Can DH-6329 be used in automotive topcoat systems as well as colour paste concentrates?
Yes — DH-6329 is specifically noted for use in automotive coatings and outdoor weathering coatings where high colour development, gloss, and storage stability are requirements. It can be used both in the colour paste concentrate and in the let-down system, though the specific addition point and dosage should be optimised for the specific automotive topcoat formulation to confirm the best performance outcome for the target properties.
