In the production of plastic masterbatches, the pigment pre-dispersion process is a key step in determining the final product's color uniformity, stability, and processing performance. Key control parameters require systematic and coordinated control across three key dimensions: equipment operation, process conditions, and raw material characteristics.
During the raw material pretreatment stage, the initial state of the pigment directly impacts dispersion. Some pigments that are prone to moisture absorption or agglomeration require drying to remove moisture to prevent pores and uneven dispersion caused by evaporation during mixing. Larger particle sizes require pre-dispersion using equipment such as a three-roll mill or sand mill to refine the pigment to micron-sized particles, reducing subsequent dispersion difficulties. Some specialty pigments also require surface treatment, such as coating with coupling agents or dispersants, to improve compatibility with the carrier resin and reduce agglomeration.
The selection and operating parameters of the mixing equipment are crucial for the initial mixing of the pigment and carrier resin. High-speed mixers utilize the shear and collision effects generated by the high-speed rotation of the impellers to achieve uniform contact between the pigment and the resin within a short period of time. The mixing process requires strict control of time and temperature. Too short a mixing time will result in incomplete mixing and uneven color distribution. Excessive mixing time may cause color decomposition or pre-softening of the carrier resin due to excessive material heating, impacting subsequent processing performance. Temperature control must balance dispersant activity and resin stability to avoid excessively high temperatures that could lead to dispersant inactivation or localized resin degradation.
Carrier resin handling is also crucial. Hygroscopic resins (such as PA, PET, and PC) must be pre-dried to remove moisture to prevent bubbles and degradation during processing. Non-hygroscopic resins must also be sieved to remove impurities and ensure purity before mixing with the color. The melting point, flowability, and other properties of the carrier resin must be compatible with the color. For example, high-melting-point resins require a high-temperature dispersion process, while low-flowability resins require optimized screw configuration to enhance shear.
The dispersant dosage and active temperature range are key parameters for controlling the color dispersion state. Dispersants wet the color surface and reduce surface tension, promoting uniform dispersion in the resin. The dosage must be precisely calculated based on the pigment type, particle size, and carrier resin properties. Excessive dosage may increase the mobility of the plastic masterbatches, while insufficient dosage may prevent full dispersion. Furthermore, the dispersant's active temperature range must be compatible with the mixing temperature. For example, some polyethylene wax dispersants work best at high temperatures; if the mixing temperature is too low, their wetting effect will be significantly reduced.
Controlling temperature and shear during the melt mixing stage is crucial for the final dispersion of the pigment. A twin-screw extruder uses the high-temperature shear force generated by the rotating screws to melt the premix and forcefully break up pigment agglomerates. During this stage, the temperature profile must be set according to the melting point of the carrier resin to ensure sufficient resin melting while preventing pigment decomposition. The screw speed must be adjusted based on the material's fluidity. High speeds enhance shear, but excessively high speeds may result in a short material residence time and inadequate dispersion.
Controlling parameters during the extrusion granulation stage directly impacts the morphology and properties of the plastic masterbatches. The extruder screw structure (single-screw or twin-screw) should be selected based on output and dispersion requirements. Twin-screw structures are more suitable for applications requiring high dispersion due to their greater shear force and more uniform mixing. The die temperature and pressure must be stably controlled to avoid surface roughness or internal defects in the plastic masterbatches caused by temperature fluctuations. The sharpness and rotational speed of the pelletizer blades affect pellet uniformity. Sharp blades ensure smooth pellet cuts and reduce dust generation.
From raw material pretreatment to extrusion pelletization, the pigment pre-dispersion process in plastic masterbatches production requires coordinated optimization of equipment, processes, and raw materials to achieve efficient dispersion. Strictly controlling parameters at each stage not only improves the color uniformity and stability of the plastic masterbatches, but also reduces production costs, improves efficiency, and provides reliable guarantees for the color quality of downstream plastic products.
