Principle of hot mixing technology

The process of transforming from solid, unidirectional, heterogeneous state to multiphase, homogeneous, and partial gel state under the action of shear force generated by the collision between the blade of the mixer and the material and the material and the inner wall of the mixer. . During compounding, compaction can be measured by apparent density increase. Usually the apparent density of PVC resin is 0.45-0.55g/ml (grams per milliliter). In high-speed mixing, due to heat and mutual impact, the high shear force generated by friction causes the temperature of the material to rise continuously. PVC particles and other components The parts continuously penetrate each other, and the surface of the PVC particles gradually absorbs or adsorbs stabilizers, melted lubricants, processing aids, etc., and the apparent density of the mixture can be increased to 0.63-0.70g/ml.

The mixing process is a process of diffusion, convection and shearing. Diffusion is beneficial to reduce the concentration difference between the various material components, so that the fine particles of the components migrate from the area with high concentration to the area with low concentration, so as to achieve uniform composition; convection and shear both use mechanical force for irregular flow , so that the material composition can achieve a fully uniform dispersion process. As the temperature continues to rise, the PVC particles continue to absorb stabilizers, lubricants, processing aids, etc., and are gradually covered by each component. When the temperature of the material rises to about 100 °C, it is easy to remove the resin and the auxiliary agent. moisture and other volatiles, avoiding the defect of air bubbles during extrusion. When the temperature rises to 110℃-120℃, it can form a uniform and stable dry mixed powder.

The mixing process is also the process of gelation of the PVC mixture. The unmixed PVC resin has different particle sizes and many small particles at room temperature, which is easy to cause uneven plasticization during the extrusion molding process. During the mixing process, the PVC material gradually melts, plasticizes, and forms a network structure during crystallization. From room temperature to below 50 °C, after the PVC agglomerated powder and larger particles are impacted by friction, although the apparent density does not change much, the large accumulated particles are sheared and broken and gradually become small particles, and in the seat After the mechanical energy of the heat is increased, the activity increases, and it gradually combines with other particles to increase the size of the particles. When the material temperature is below 90 °C, the heating rate is faster, and then the heating rate is slower. The three mixing forces of PVC before 90 °C are relatively intense. When the temperature reaches 115 °C, the small resin particles become large and uniform, the small particles gradually disappear, and the particles become transparent and translucent due to partial gelation. . The degree of its partial gelation is determined by the mixing discharge temperature under certain conditions. When the temperature reaches 120℃, the PVC particles tend to be stable, and the gelation degree is deeper. In the whole process, the particles are repeatedly combined and homogenized, the particles increase and play a densifying effect, so that the apparent density of the particles reaches the maximum value, and each component can give full play to the effective synergistic effect, which is beneficial to improve the fluidity of the dry mixture and promote extrusion. Uniform plasticization of molding, improve the output and welding of plastic products, impact resistance and other properties. For the dry-mixed powder with good uniformity and dry flow, it can be observed that the PVC resin has both particle refinement and uniform particle size morphological changes, and shows the characteristics of high density and partial gelation resistance.