Introduction
In industries such as coatings, inks and masterbatches, the coloring power of color paste is one of the core indicators to evaluate its application performance. Coloring power is directly related to the color performance of the final product, production cost, and batch-to-batch stability. The preparation process of color paste, especially the dispersion and grinding links, plays a decisive role in the formation and exertion of coloring force. As a commonly used pigment dispersion and grinding equipment in the laboratory, the operating parameters and processes of the flat mill have a systematic impact on the performance of the prepared color paste. This paper will discuss in depth the influence mechanism of key process factors on the coloring force in the process of preparing color pulp using a flat mill, and provide a theoretical basis and practical reference for optimizing the color paste preparation process in combination with relevant technical standards.
Principle
The flat mill mainly exerts shear force and pressure on the pigment aggregates through the relative movement between a fixed grinding disc and a rotating grinding disc, causing it to depolymerize in the binder (color spreader) and disperse stably to form a uniform color paste. Its core purpose is to obtain an optimal dispersion state of pigment particles.
Coloring power is usually defined in technical standards as the ratio of the color depth presented by a certain amount of color paste to be mixed with the reference white pigment paste and the color depth presented by the standard color paste under the same conditions. It reflects the ability of pigments to contribute to the color of the system per unit of mass or volume, and its mathematical expression can be simplified as:
Coloring power (%) = (white paste mass of standard color paste / white paste mass of color paste to be tested) × 100%
The higher the value, the higher the pigment efficiency in the dispersion system.
When using a flat mill to prepare the color paste, several process parameters are correlated with each other, which together determine the dispersion quality and coloring power of the final color paste.
Grinding medium and time
The material, particle size and filling rate of abrasive media (such as glass beads and zirconium beads) directly affect the energy transfer efficiency. Smaller media particle sizes provide more contact points, facilitating fine particle grinding but may result in longer grinding times or media wear introducing impurities. The grinding time needs to match the medium, and the lack of time will lead to incomplete dispersion of pigment aggregates and incomplete coloring power. If the time is too long, it may cause damage to the pigment crystal structure or high energy consumption, and there is a risk of performance degradation.
Grinding speed and pressure
The speed of the grinding disc determines the magnitude of the shear force. Appropriate speed increase can enhance shear and promote dispersion. However, excessive speeds generate too much heat, which can lead to changes in the properties of the connector or pigment. The pressure applied by the flat grinder is also critical, enough pressure to ensure effective contact between the grinding medium and the material, but excessive pressure may exacerbate wear and temperature rise of the equipment.
The ratio and properties of pigments and connecting materials
The formulation of the color paste, i.e. pigment concentration (PVC), is the basic factor. Too high pigment concentration will make the viscosity of the system too large, reduce the grinding efficiency, and disperse unevenly. Too low a concentration is not economical. The wettability of the binder and the selection of dispersant are crucial, which can reduce the interfacial energy of the pigment surface and prevent re-flocculation, which is the prerequisite for obtaining high coloring power and stable color paste.
Experimental methods
To systematically evaluate the impact of these parameters, experiments can be designed according to relevant national or industry standards such as dispersion test methods in the field of coatings and pigments. The control variable method is usually used to prepare a series of color paste samples by fixing other conditions and changing the grinding time, speed, media type and other parameters in turn. Subsequently, each sample is mixed with the standard white paste in a fixed ratio, coated on the standard card, dried, and its color value (such as K/S value, correlated with color depth) is measured using a colorimeter or spectrophotometer, and the relative color power is calculated compared with the standard sample. Data recording and analysis can be done in the following table format:
| Experimental variables (e.g., grinding time/min) | Measured relative coloring force/% |
| 10 | 92 |
| 20 | 98 |
| 30 | 100 |
| 40 | 99 |
Analysis of results
Through the analysis of experimental data, it can usually be found that the coloring force rises rapidly with the increase of grinding time and then tends to be stable or even slightly decreased, and the inflection point corresponds to the better grinding time. There is a similar optimal range for grinding speed and pressure. The goal of optimizing the process is to find a balance between this series of parameters and ensure that the coloring force is fully utilized, while taking into account production efficiency and energy consumption. In addition, the scientific selection of pretreatment (such as pre-dispersion) and dispersants can often significantly improve the grinding efficiency, so that the color paste can achieve a higher coloring force platform in a shorter grinding time.
Conclusion
As a key equipment for color paste preparation, the operation process of the flat mill has a significant impact on the coloring force. The increase in coloring power is essentially the performance of the pigment in the binder to achieve the best microscopic dispersion. By systematically optimizing the grinding medium, time, speed, pressure, and formulation system, the coloring power of the paste can be effectively improved, thereby improving the color quality and economic benefits of the final product. In actual production, it is recommended to conduct systematic process experiments in combination with specific pigment and connector systems to determine the most suitable operating parameters of the flat mill.