Exploring Ultrafine Grinding in Aqueous Dispersion Systems

Aqueous dispersion systems involve dispersing insoluble solid particles evenly in water to create stable suspensions. This technology finds applications in various industries, including coatings, paints, inks, and formulations. Achieving stability in aqueous dispersion systems is critical for product quality. Factors affecting system stability include the use of surfactants, water-soluble polymers, electrolyte conditions, and other environmental variables. One key factor influencing system stability is the particle size and size distribution of solid particles. To ensure long-term stable suspensions, a central production challenge is achieving efficient ultrafine grinding and milling of solid particles.

Sand mills are commonly used for wet ultrafine grinding and milling. They are favored for their grinding effectiveness, high efficiency, and low energy consumption, making them integral in the production of aqueous dispersion systems. Different types of sand mills vary significantly in their grinding characteristics. Optimizing the use of sand mills for efficient and high-quality ultrafine grinding is a critical concern in this field. In this study, we prepared water-based dispersion systems using carbon black as the dispersed phase. We examined the impact of different types of sand mills, grinding methods, and grinding quantities on grinding effectiveness and efficiency and proposed a multi-stage combination grinding process suitable for large-scale, batch ultrafine grinding.

Materials and MethodsMaterials:

• Cabot Carbon Black CSX865, Shanghai Chentan Trading Co., Ltd.

• Dispersant MIO1, Rohm and Haas Company

• Dispersant AEC, Daily Chemical Research Institute

• Dispersant 5029, Santnopo Company

• Propylene Glycol, Dow Chemical Company

• Diethylene Glycol, Yanshan Petrochemical

• Polyethylene Glycol 200, Fushun Jiachua

• Triethanolamine, Fushun Jiachua

Equipment:

• ZWS-5 Horizontal Sand Mill, Jiangyin Fine Chemical Co., Ltd.

• M5 Nanosize Sand Mill, Jiangyin Mike Machinery Co., Ltd.

• LS-5 Basket Sand Mill, Jiangyin Fine Chemical Co., Ltd.

• TopSizer Laser Particle Size Analyzer (range: 0.2 µm - 2000 µm), Zhuhai Omek Instrument Co., Ltd.

Grinding Conditions:

1. Grinding Media: Pure zirconia beads were used as grinding media. The specifications of zirconia beads were 1.8 mm to 2.0 mm for the basket sand mill, 0.6 mm to 0.8 mm for the horizontal sand mill, and 0.3 mm for the nanosize sand mill.

2. Grinding Chamber Filling Rate: The filling rate in the grinding chamber was 75%.

Grinding Methods:

1. Grinding: Grinding refers to the method where the material flows between the container and the sand mill, and the material should be continuously stirred during grinding.

2. Multiple Passes Grinding: Multiple passes grinding ensures that all material flows through the sand mill multiple times under the same conditions, thereby achieving uniform grinding.

Particle Size Distribution of Dispersion Systems:The particle size distribution of dispersion systems was measured using a laser particle size analyzer. The D90 value, representing the particle size in the dispersion system after grinding, was obtained, and the M-value was used to characterize the width of the particle size distribution curve.

Results and Analysis:

Particle Size Distribution Curves:The laser particle size analyzer measured the particle size distribution curves of the experimental samples. The cumulative distribution curve is represented by Curve 2.

Impact of Different Types of Sand Mills on Grinding Time and Particle Size Distribution Curves:The results showed that different types of sand mills have different grinding fineness limits. Under experimental conditions, the nanosize sand mill achieved the smallest grinding particle size, with a D90 value as low as 0.29 µm. The horizontal sand mill followed, while the basket sand mill had the largest D90 value, which was 10.82 µm. In terms of grinding efficiency at the initial stage (0 min - 30 min), the basket sand mill was more efficient than the horizontal sand mill and the nanosize sand mill.

Effect of Grinding Media on Grinding Efficiency:The characteristics of grinding media, such as the diameter, density, sphericity, and surface finish, have a significant impact on grinding efficiency. In sand mills, materials are crushed through the mechanical force generated when materials come into contact with the grinding media. This mechanical force is generated within a specific area after contact between the two. Therefore, smaller grinding media provides a more concentrated impact area, leading to finer particle size. Larger grinding media, such as the ones used in basket sand mills, are more efficient in the initial grinding stage. Using smaller grinding media, as in the nanosize sand mill, results in higher efficiency in later grinding stages.

Comparison of Different Grinding Methods:Using the nanosize sand mill, the same material was ground through both single-pass and multiple-pass grinding, and the particle size distribution curves and M-value curves were compared. The results indicated that multiple-pass grinding achieved slightly higher grinding efficiency compared to single-pass grinding, resulting in a narrower particle size distribution. However, it was observed that multiple-pass grinding was more prone to foam formation, which could impact grinding efficiency and effectiveness. Therefore, when employing multiple-pass grinding, it is advisable not to use too many passes.

Efficiency Curve of the Nanosize Sand Mill:The production of ultrafine dispersion systems (D90 < 0.5 µm) mainly depends on the nanosize sand mill. The efficiency curve of the nanosize sand mill was studied by grinding the same material under different feeding conditions and plotting grinding time vs. particle size distribution curves (D90-T curves).

Conclusion:

1. Different types of sand mills produce varying grinding effects. In terms of grinding fineness, basket sand mills produce coarser results, followed by horizontal sand mills, and nanosize sand mills produce the finest results. In the initial grinding stage, basket sand mills are more efficient than horizontal sand mills, and nanosize sand mills are less efficient.

2. Multiple-pass grinding exhibits slightly higher grinding efficiency compared to single-pass grinding, resulting in a narrower particle size distribution. However, the formation of foam may limit the number of passes used.

3. With an increase in the quantity of material to be ground, the grinding efficiency of nanosize sand mills decreases significantly.

4. A multi-stage combination grinding process, involving basket sand mills, horizontal sand mills, and nanosize sand mills, is suitable for large-scale, batch ultrafine grinding of materials, resulting in improved grinding efficiency.