Nanoparticles (such as nano-titanium dioxide, nano-iron oxide, and organic nanocolored pigments) exhibit high coverage, high brightness, and excellent color fastness when incorporated into color coatings due to their small particle size (typically <1 μm) and large specific surface area; consequently, they find wide application in fields such as water-based coatings, inks, cosmetics, and high-end ceramics. However, their dispersion process faces two major core challenges:
- Agglomeration challenge: Nanoparticles exhibit high surface energy, making them prone to forming hard agglomerates via van der Waals forces or hydrogen bonding, thereby resulting in poor uniformity of the color paste;
- Inherent color instability: Agglomerated particles can disrupt the principles of light scattering and absorption, leading to reduced coverage, duller colors, and diminished gloss, thereby preventing the full utilization of the performance advantages of nanoparticles.
Zirconia beads, as high-efficiency grinding media, leverage their high hardness (Mohs hardness: 8.5–9), low wear rate, and excellent toughness to break down pigment agglomerates through a "shear-friction" mechanism; furthermore, they are less prone to introducing impurities that could contaminate the color paste, making them a critical factor in controlling the particle size distribution and color intensity of the color paste.
The grinding of nanomaterial pigments should adhere to the principle that "the particle size of the grinding medium must be compatible with the target pigment particle size": the smaller the zirconia bead size, the more concentrated the shear force applied to the pigment agglomerates, enabling the breakdown of finer agglomerated particles; however, excessively small beads may result in insufficient impact energy, thereby prolonging the grinding cycle. For example, when using 0.1 mm zirconia beads for a 2-hour grinding process, the median particle size (D50) of the color paste can reach 120 nm, with a narrow particle size distribution. In contrast, when using 0.5 mm and 1.0 mm beads under the same grinding conditions, the D50 values increase to 250 nm and 480 nm, respectively, accompanied by the formation of more large agglomerates. Therefore, for color pastes targeting a target particle size range of 100–300 nm, it is recommended to use 0.1–0.3 mm zirconia beads.
The filling ratio (the ratio of the volume of zirconia beads to the volume of the grinding chamber) directly affects both the collision frequency between beads and their shear strength:
- Excessively low filling rate (<60%): large particle spacing results in reduced impact–shear cycles, low grinding efficiency, and failure to adequately break down pigment agglomerates;
- Excessively high filling rate (>80%): Poor fluidity of the pearl body may lead to "agglomeration," resulting in localized overheating (temperature increase of 5–10 °C), which in turn compromises the dispersion stability of the pigment;
- Optimal range: 65%–75%; at this range, the bead collision frequency can reach 10⁴–10⁵ collisions/min, enabling efficient grinding energy transfer.
Unsurface-treated zirconia beads have hydroxyl groups on their surface that readily form hydrogen bonds with nanoparticles, leading to the adsorption of pigments onto the bead surface and resulting in "secondary agglomeration." In contrast, zirconia beads subjected to surface modification—such as those coated with SiO₂ or PDMS (polydimethylsiloxane)—can effectively reduce this adsorption phenomenon. Data show that zirconia beads coated with PDMS can reduce the pigment adsorption rate from 12.5% to below 3%, optimize the D50 particle size of the color paste from 280 nm to 180 nm, and significantly lower the agglomeration rate after static storage.
Zirconia beads exhibit an extremely low abrasion rate (≤0.005%), which is significantly lower than that of glass beads and alumina beads. This low abrasion rate implies fewer impurity ions introduced during the grinding process, thereby helping to maintain the chemical stability and particle size distribution of the color paste. For example, grinding debris generated by high-abrasion media may act as impurities, leading to an increase in the particle size of the color paste and a decline in its performance.
The color-producing property of nanoparticles stems from the "interaction between light and pigment particles": the narrower the particle size distribution and the more uniform the particles, the more regular the light scattering (source of coverage) and absorption (source of color) become, resulting in superior coloration performance.
Principle: When the pigment particle size is approximately 1/4 of the incident light wavelength (for visible light, the optimal particle size ranges from approximately 100 to 190 nm), light scattering reaches its maximum value, and the covering power achieves its peak level. By optimizing zirconia beads to stabilize the D50 of nano-titanium dioxide at 150 nm, its covering power (contrast ratio) can be increased from 75% to over 90%, reaching an industry-leading performance level.
Pigments with uniform particle size exhibit more concentrated absorption of light at specific wavelengths and produce more saturated colors. The color difference ΔE is a standard metric for quantifying color deviation. In most industrial applications, a ΔE value <1.5 is regarded as a high-quality standard, beyond the resolution capability of the human eye. By optimizing the grinding process, it is possible to achieve a ΔE value for the color paste below 1.5, reaching a level that is indistinguishable to the naked eye.
The finer and more uniformly distributed the pigment particles are, the smoother the surface of the film after coating will be, and the higher its glossiness will be. Through optimization, the glossiness of the film layer (at 60°) can be increased from 45 GU to above 80 GU.
Water-based architectural coatings: It is recommended to use SiO₂-coated zirconia beads with a particle size of 0.2–0.3 mm, a filling rate of 70%, a target D50 value of 150–250 nm, and a coverage ratio>90%.
High-end ink: Recommended for use of 0.1–0.2 mm PDMS-coated zirconia beads with a filling rate of 65%, where the target D50 particle size of the color paste is 100–180 nm and ΔE <1.0.
Cosmetics (foundation): It is recommended to use uncoated high-purity zirconia beads with a particle size of 0.05–0.1 mm; the D50 of the target color paste should be between 50–100 nm, ensuring that no impurities are leached out.
Ceramic glaze: It is recommended to use 0.3–0.5 mm alumina beads or composite beads with a filling rate of 75%, and a target D50 particle size for the color paste between 200–300 nm, to meet the high-temperature resistance requirements.
- Customized technology optimization: includes the development of customized "core-shell structured" zirconia beads, integration of an intelligent grinding system with online particle size monitoring, and exploration of auxiliary dispersion techniques such as ultrasonic treatment to reduce energy consumption.
- Existing challenges primarily include: the large-scale production and cost control of ultrafine zirconia beads with diameters ranging from 0.05 to 0.1 mm, as well as how to ensure excellent flowability and uniform dispersion of the beads within high-viscosity pigment dispersion systems.
Zirconia beads leverage a fourfold mechanism— "particle size matching – energy regulation – surface anti-adsorption – low impurity contamination" —to effectively optimize the particle size distribution of nano-color pigments, thereby significantly enhancing their hiding power, color saturation, color accuracy, and glossiness. In practical applications, precise selection of the appropriate model is required based on specific requirements; ongoing technological innovations in the future will further drive the improvement of nano-color pigment performance.