In coating aesthetics, jetness (blackness) and undertone are critical parameters that define premium finishes. A persistent misconception among formulation engineers is that "the finer you grind carbon black and the longer you mill it, the deeper the blackness and the stronger the blue undertone will be."

In reality, a pure blue undertone is not achieved through blind over-milling. Over-milling crushes the primary aggregates of carbon black, causing a massive surge in specific surface area. This destroys the polymer dispersant coverage and triggers secondary re-flocculation—turning the film yellowish, brownish, or dull gray.

True blue undertone is released by selecting the right carbon black grade, matching it with high-performance polymeric dispersants, and controlling shear and temperature during milling to stabilize the pigment inside a narrow particle size distribution (D50: 150–250 nm).

Below is a technical guide on mastering blue-undertone carbon black dispersion, complete with starting formulas using RD-9208 (Water-based) and RD-9618 (Solvent-based) dispersants.

1. Raw Material Selection: The Foundation of Blue Undertone

If you choose the wrong carbon black grade, no amount of milling optimization will yield a pure blue undertone.

1. Carbon Black Grade & Primary Particle Size

   Carbon blacks that release a blue undertone typically have moderate-to-larger primary particle sizes and deep surface oxidation treatment. Extremely fine high-jetness carbon blacks inherently possess a reddish undertone that is very difficult to turn blue.

2. Volatile Content (Surface Oxygen Groups)

   Opt for carbon blacks with higher volatile content (rich in surface carboxyl and hydroxyl groups). High surface functionality not only improves media wetting but also provides abundant anchoring sites for polymeric dispersants, preventing re-agglomeration after grinding.

Common Misconception: The blue undertone of high-end paints is a natural scattering effect of properly dispersed carbon black particles. Adding a small amount of Phthalo Blue is merely for fine-tuning. If the carbon black itself turns brown, tinting with blue pigments will not produce a deep, transparent jet-black finish.

2. Formulation & Polymer Dispersants: Preventing Re-Flocculation

Once carbon black is deagglomerated, its surface area expands exponentially. Without a robust polymer barrier, Van der Waals forces will quickly cause re-flocculation, driving up the $b^*$ value (shifting toward yellow/brown).

• Waterborne Systems (Resin-Free Concentrates / Industrial Coatings):We recommend RD-9208 Water-based Polymeric Dispersant. RD-9208 delivers outstanding viscosity reduction and stabilization, drastically lowering grinding viscosity for carbon black while offering broad compatibility with downstream emulsions.

• Solventborne Systems (Plastics, Wood, Industrial / Inks):We recommend RD-9618 Polymeric Block Dispersant. Featuring strong pigment-anchoring groups, RD-9618 anchors tightly onto carbon black surfaces, providing steric hindrance that prevents re-flocculation during heat storage and film drying.

• Crucial Addition Order:

  During pre-dispersion, always mix the solvent/water, resin, dispersant, and wetting agents thoroughly BEFORE slowly adding the carbon black powder. Never dump dry carbon black directly into a liquid mixture without dispersants, as trapped air will create hard agglomerates that bead mills cannot break.

3. Milling Process Controls: Shear Rate & Temperature

[Pre-Dispersion / High-Speed Dissolver]       ──► 8–10 m/s tip speed (De-aerate & wet out; reduces mill load by 30%)

[Bead Milling]                                                      ──► 0.3–0.6 mm Zirconia Beads | 10–14 m/s tip speed| 10–14 m/s tip speed

[Strict Temperature Control]                       ──► 40–50°C (STRICTLY KEEP < 55°C to prevent desorbing)

[Endpoint Determination]                                ──► Grind gauge ≤ 10 μm | Colorimetry b* minimum point| Colorimetry b* minimum point

1. Pre-Dispersion (De-aeration & Wetting)

   Set dissolver tip speed to 8-10 m/s. Start at low speed for wetting, then increase speed for 5–10 minutes to expel air trapped inside carbon black agglomerates. This step reduces bead mill load by roughly 30%.

2. Bead Milling Parameters

• Bead Selection: Use high-density 0.3-0.6mm Zirconia beads. The high-frequency micro-impacts efficiently break down agglomerates.

• Tip Speed: 10-14 m/s.

• Temperature Control (CRITICAL): Maintain mill base temperature at 40-50℃ and NEVER exceed 55℃. Temperatures above 55℃. cause polymeric dispersant anchoring groups to desorb from the carbon black surface, resulting in immediate re-flocculation and a loss of the blue undertone.

3. Beware of "Over-Milling"

   Excessive shear over long periods breaks carbon black's primary aggregates, exposing unpassivated surfaces that ruin dispersant coverage. This leads to bridging re-flocculation, causing the film to lose jetness, turn dull gray, and show color specks.

4. Determining the Endpoint

   Measure grind gauge fine-ness (≤ 10um) and verify dry-film color coordinates with a spectrophotometer (target the lowest b* value). Do not judge completion based solely on wet-film appearance or milling time.

4. High Blue-Undertone Starting Formulations

Below are starting formulations for waterborne and solventborne blue-undertone carbon black pastes using Ruike Chemical dispersants:

Formula 1: Waterborne Resin-Free Blue-Undertone Carbon Black Paste (RD-9208)

• Procedure: Mix water, RD-9208, wetting agent, and defoamer until homogeneous. Slowly add carbon black under high-speed stirring. Pre-disperse for 20 minutes, then transfer to a bead mill. Maintain milling temperature at 40-45℃ until b* reaches its minimum point.

Formula 2: Solventborne High Blue-Undertone Carbon Black Paste (RD-9618)

• Procedure: Fully dissolve RD-9618 in the solvents. Add carbon black and pre-disperse under high speed. Mill with 0.3-0.5mmZirconia beads, keeping the temperature below 50℃, until a fluid paste with maximum transparency and jetness is achieved.

5. Summary & Sample Requests

Achieving a clean blue undertone with carbon black requires a balanced approach: pigment grade selection, matching dispersants, temperature-controlled milling, and colorimetric evaluation. Avoiding over-milling and thermal desorption is the key to maintaining long-term stability and deep jetness.

To download Technical Data Sheets (TDS) for RD-9208 or RD-9618, or to request evaluation samples, visit our technical portal at www.rk-chem.com.

• Visit www.rk-chem.com to explore our full line of dispersants and coating additives.

• Contact our technical application specialists today to optimize your mill-base formulas and request free laboratory samples!