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Many people make black baking paint and color paste, and even though they choose blue phase carbon black, it tends to be reddish and grayish when ground, and the blue phase cannot be produced. Core sentence: The blue phase of carbon black is not achieved by simply grinding for a long time, but by fully wetting, dispersing aggregates, stabilizing particles without flocculation, and releasing the inherent optical color of carbon black.

1、 Basic principles

Blue phase is not the paint film turning blue, it is a black background with a cool blue color. Optical essence: The size of carbon black aggregates determines the wavelength of scattered light. The primary carbon black particles themselves have a blue/red phase background color; But dry carbon black is a large amount of agglomerated blocks. The goal of grinding is to break down micrometer sized aggregates into native aggregates and eliminate the scattering caused by large particles. Once there is still a large amount of aggregation, the coating will scatter randomly, and the blue phase will be lost, gray or reddish. On the colorimeter, the blue phase is reflected as a negative value of b, and the larger the negative value, the stronger the blue phase.

2、 Step 1: Selection of Charcoal Black Brand Number

Choosing the wrong raw materials, no matter how hard you grind them, you won’t be able to produce a pure blue phase. Prioritize the use of blue phase pigment carbon black, with moderate primary particle size and surface oxidation modified furnace/groove carbon black.

Misconception: The smaller the particle size, the bluer it is. Ultra fine high specific surface area carbon black itself has a reddish background color; Carbon black with medium particle size and controllable structure is easier to grind out a clean blue phase. The dispersion difficulty of high structure carbon black is higher, while low structure carbon black is easier to release color.

3、 Formula system

1. The selection of dispersant must be a high molecular weight dispersant specifically for carbon black. One end of the dispersant adsorbs onto the surface of carbon black, while the other end is compatible with the resin, forming steric hindrance. Carbon black has a large specific surface area, and the amount of dispersant added should match the specific surface area of carbon black, which cannot be estimated based on ordinary pigments. Insufficient dispersant: The opened carbon black immediately flocculates again, and the blue phase disappears; Excessive dispersant affects the gloss and water resistance of the paint film. Priority should be given to polyacrylic dispersants with anchoring groups in water-based systems; Polyurethane and polyester polymer dispersants are selected for solvent based applications.

2. Control the viscosity of the grinding base material and ensure that the viscosity of the sand slurry is neither too high nor too low. The viscosity is too high, which hinders the movement of grinding beads and prevents shear transmission; The viscosity is too low, the collision force of the beads is insufficient, and the agglomeration is poor. The grinding slurry should ensure sufficient pre wetting: first mix the resin, solvent/water, and dispersant evenly, then slowly add carbon black, stir at low speed to wet, and replace the air in the carbon black aggregation pores. Insufficient pre wetting results in ineffective work during subsequent sanding.

3. pH controlled water-based system, carbon black itself tends to be acidic; A strong alkaline environment can easily disrupt the adsorption of dispersants, causing particle coarsening and color deviation. The pH is generally controlled between 8.0 and 9.0.

4、 Practical operation of grinding process

1. Equipment: Sand grinder is preferred. High speed dispersion alone is not sufficient and can only break up large pieces. It cannot open the strong agglomeration inside the carbon black, and the release of blue phase is limited. Grinding medium: Use 0.3-0.8mm zirconia beads. Small beads have high shear density and are suitable for carbon black depolymerization; The filling amount of bead material is 70-85%.

2. Process sequence: Pre mixing: resin+dispersant+solvent/water, low-speed stirring, slowly adding carbon black; ② High speed pre dispersion for 30 minutes, fully wetted; ③ Sand grinding cycle grinding, do not grind vigorously for a long time at once, control the material temperature. Excessive temperature can disrupt the adsorption of dispersants, cause carbon black to flocculate, and result in loss of blue phase; ④ Sampling tracking: fineness, b-value of color paste, blackness. Reaching the fineness standard does not mean that the blue phase is in place, it is necessary to check the b-value on the color palette.

Key point: The endpoint of grinding is not only based on the fineness, but also on the stability of the color b value. Continuing excessive grinding will not only fail to enhance the blue phase, but also cause heating and secondary particle flocculation.

1. Filtration: After grinding, filter with a fine filter bag to remove residual trace amounts of large agglomerated particles and avoid scattering of stray light causing dust.

5、 Common pitfalls

1: Only extending the grinding time, insufficient dispersant. After grinding the sample, it looks black, but when placed back coarse, the blue phase runs away.

2: Carbon black is fed in large quantities at once, resulting in poor wetting and internal air entrapment, making it difficult to open due to agglomeration.

3: The thickness of the sample dry film is inconsistent, the light source is different, and the color can be misjudged by the naked eye. When comparing the blue phase, it is necessary to ensure that the dry film thickness is consistent and the substrate is the same. When comparing under a standard light source, it is best to use a colorimeter to quantify the b-value.

4: The grinding temperature is too high, the dispersant desorbs, the particles re agglomerate, and the blue phase changes to gray and reddish.

Summary

To grind pure carbon black blue phase: first choose intrinsic blue phase carbon black; Match sufficient specialized polymer dispersants and prepare for pre wetting; Using small zirconia bead sand grinding to control the viscosity and temperature of the grinding slurry; The endpoint of grinding is judged based on the stability of the color b value to avoid secondary particle flocculation. Grinding eliminates micron level agglomeration and reduces stray light scattering, in order to release the clean and cool blue phase of carbon black itself.