The dispersion of carbon black in black masterbatch, a general-purpose plastic raw material, directly affects the blackness, gloss, and mechanical properties of the final product. Precise control of process parameters is the core means to achieve uniform carbon black dispersion. In the production process of black masterbatch, carbon black dispersion involves key steps such as premixing, melt extrusion, and cooling granulation. The process parameters of each step need to be optimized collaboratively to overcome challenges such as carbon black's tendency to agglomerate and its poor compatibility with resin.
Premixing is the fundamental stage of carbon black dispersion. Its core objective is to achieve a preliminary uniform distribution of carbon black particles within the resin carrier through physical action. This stage requires a high-speed mixer to break down the agglomerated structure of the carbon black through high shear force. Temperature control is particularly critical during mixing; the resin must be kept in an unmelted state to prevent premature agglomeration of carbon black due to excessive temperature. Simultaneously, the method of adding dispersant directly affects the premixing effect. Typically, the dispersant needs to be premixed with a small amount of resin to form a "masterbatch" before adding the carbon black to reduce the risk of agglomeration. Furthermore, the matching of mixing time and rotation speed must be adjusted according to the characteristics of the raw materials to ensure that the carbon black particles are fully coated by the resin, laying the foundation for subsequent melt dispersion.
Melted extrusion is the core step in carbon black dispersion. Twin-screw extruders achieve deep dispersion of carbon black in the resin through the synergistic effect of high temperature and strong shear force. Temperature control must balance the dual requirements of resin melting and carbon black dispersion: too low a temperature will result in incomplete resin melting, preventing carbon black dispersion; too high a temperature may cause resin degradation or carbon black coarsening. Therefore, the extruder needs to adopt a segmented temperature control design. The temperature in the feeding section must be below the resin melting point to ensure the stability of solid conveying; the temperature in the melting section must be above the melting point to promote resin melting; and the temperatures in the compression and metering sections need to be further increased to enhance the dispersing effect of shear force on the carbon black. Screw rotation speed is another key parameter. Increasing the rotation speed can enhance shear force and promote the deagglomeration of carbon black agglomerates, but excessively high rotation speed may lead to a shortened material residence time and insufficient dispersion. Therefore, the optimal rotation speed range needs to be determined through experiments to achieve a balance between shear force and residence time.
The design of the screw structure has a decisive impact on the dispersion effect of carbon black. The meshing zone of a twin-screw extruder promotes carbon black dispersion through strong shear force, while the combination of screw elements directly affects the material flow state. For example, an alternating design of shear blocks and conveying blocks can enhance local shear force while ensuring material conveying efficiency, thus improving the uniformity of carbon black dispersion. Furthermore, the length-to-diameter ratio of the screw must be optimized according to the characteristics of the raw material; an excessively small ratio may lead to insufficient dispersion, while an excessively large ratio may cause carbon black coarsening or increased energy consumption.
The cooling and granulation process is crucial to the stability of carbon black dispersion. The molten material after extrusion needs to be rapidly shaped by water cooling to prevent the carbon black from re-agglomerating due to temperature reduction. The temperature of the cooling water tank must be matched with the extrusion speed to ensure uniform cooling of the material and avoid particle deformation or carbon black migration due to excessively high local temperatures. The synchronization between the pelletizer speed and the extrusion speed also needs to be strictly controlled, as particle size uniformity directly affects the dispersion effect in subsequent processing. If the particles are too large, it may lead to uneven carbon black dispersion during injection molding or extrusion; if the particles are too small, it may cause dust problems and increase processing difficulty. The selection and dosage of dispersants are key chemical methods for controlling the dispersion of carbon black. Dispersants adsorb onto the carbon black surface, forming steric hindrance or electrostatic repulsion, preventing the carbon black particles from re-aggregating. Commonly used dispersants include EVA wax, EBS, and stearates, and their selection must be determined based on the resin type and carbon black characteristics. For example, EVA wax is often used as a dispersant in PE carrier black masterbatch because of its good compatibility with PE and its ability to effectively reduce the interfacial tension between carbon black and resin. The dosage of dispersant needs to be optimized through experimentation; excessive amounts may lead to precipitation on the product surface or affect mechanical properties, while insufficient amounts will not achieve adequate dispersion.
Real-time monitoring and adjustment of process parameters are important means to ensure the dispersion stability of carbon black. Online monitoring equipment can be used to monitor parameters such as temperature, pressure, and torque during the extrusion process in real time. Combined with microscopic observation or colorimeter detection, process conditions can be adjusted promptly. For example, if an abnormally high extrusion torque is detected, it may indicate poor carbon black dispersion leading to increased melt viscosity. In this case, the temperature or rotation speed should be appropriately increased to improve the dispersion effect. Furthermore, establishing a database of process parameters and product performance can provide data support for subsequent production and enable continuous process optimization.
The dispersion control of carbon black in black masterbatch, a general-purpose plastic raw material, requires synergistic optimization from multiple dimensions, including equipment, processes, and chemical additives. By precisely controlling process parameters in premixing, melt extrusion, and cooling granulation, combined with the rational selection and dosage adjustment of dispersants, uniform dispersion of carbon black in the resin can be achieved, thereby improving the performance stability of black masterbatch and the quality consistency of downstream products.
