In the wet process of plastic masterbatch, sand milling is a key step in controlling pigment particle size distribution. Its precision directly impacts the color uniformity, flowability, and processing stability of the final product. This step uses the synergistic effect of mechanical force and chemical additives to break down pigment agglomerates into micron-sized particles, ensuring a narrow particle size distribution range and laying the foundation for subsequent steps such as phase inversion and drying.
The core goal of sand milling is to refine pigment particles to less than 1 micron while avoiding reagglomeration and equipment wear caused by over-grinding. To achieve this goal, comprehensive control is required across three aspects: equipment selection, process parameters, and additive formulation. For example, sand mills typically use grinding media made of zirconium oxide or silicon carbide. Their hardness and density must be matched to the pigment's characteristics to ensure efficient grinding while minimizing impurities generated by media wear. The grinding media fill rate is typically controlled between 60% and 80%. A lower fill rate results in reduced grinding efficiency, while a higher fill rate can lead to excessive collisions between media, resulting in inefficient energy consumption.
When it comes to controlling process parameters, sand mill speed and grinding time are key variables. If the speed is too low, the grinding media will not receive enough kinetic energy to effectively break up pigment agglomerates. If the speed is too high, localized overheating may occur, altering the surface properties of the pigment particles and even leading to chemical degradation. In practice, the speed should be adjusted based on the pigment type (e.g., organic versus inorganic) and the target particle size. For example, organic pigments, due to their softer molecular structure, generally require a lower speed to avoid over-grinding. The grinding time should be dynamically adjusted by real-time sampling and monitoring the particle size distribution. Once the particle size reaches the target range, the grinding process should be stopped immediately to prevent over-grinding.
The selection and dosage of dispersant play a crucial role in particle size control. Dispersants adsorb on the surface of pigment particles, creating steric hindrance or charge repulsion, preventing reaggregation of broken fine particles. During the initial sand milling phase, the dispersant should be mixed with the pigment and water in the correct proportion to form a stable suspension. If the dispersant dosage is insufficient, the pigment particles are prone to secondary agglomeration due to excessive surface energy. If the dispersant dosage is excessive, the viscosity of the system may increase, affecting grinding efficiency. In actual production, the dispersant addition level is typically 5%-15% of the pigment mass, with the specific ratio determined through pilot experiments.
The stability of the cooling system is an often-overlooked yet crucial factor in the sand milling process. During the sand milling process, mechanical friction and particle breakage generate significant heat. Excessive temperatures can cause the pigment surface to melt or additives to decompose, leading to a wider particle size distribution. Therefore, the sand mill should be equipped with an efficient cooling jacket that maintains the temperature below 40°C via circulating cooling water. For heat-sensitive pigments, cryogenic grinding processes are also required to further minimize the impact of temperature on particle size.
Real-time monitoring of particle size distribution and feedback adjustments are key to ensuring consistent quality. Traditional methods involve sampling and then measuring with a laser particle size analyzer, but this is subject to lag. Modern production lines often utilize online particle size monitoring systems that analyze the particle size distribution of the grinding slurry in real time and automatically adjust sand mill parameters. For example, if the particle size distribution deviates from the target range, the system can automatically increase the speed or add dispersant, achieving closed-loop control.
The ultimate goal of the sanding process is to obtain a pigment slurry with uniform particle size and stable dispersion, providing high-quality raw materials for the subsequent phase inversion process. By optimizing the three-dimensional control of equipment, process, and additives, the wet process of plastic masterbatch ensures a narrow pigment particle size distribution and high repeatability, thereby improving the masterbatch's coloring efficiency and product appearance quality in processes such as injection molding and blow molding. This meticulous control is one of the core competitive advantages of the plastic masterbatch industry in achieving high-quality, low-cost production.
