The shearing homogenizer is a crushing mechanical equipment used in the new micron technology. Its main working principle is to use the shearing action between the high-speed rotor and the stator to homogenize and crush the fluid materials.
The shearing homogenizer developed abroad adopts a stator-rotor structure homogenizer, driven by a high-speed motor (3 000 r/min-7 000 r/min), the material moves at a high speed in the gap between the rotor and the stator, forming Strong hydraulic shearing and turbulent flow, at the same time, under the combined force of centrifugation, extrusion and collision, it is fully dispersed and broken, and the emulsification effect that meets the requirements is achieved. Its core component, the stator-rotor structure, is shown in the figure. It is mainly composed of a stator and a concentric high-speed rotating rotor. The size of the gap between the stator and the rotor is the key factor to ensure the velocity field and shear force field in this space. The gap between the stator and the rotor can be very small, generally 0.2 mm to 1.0 mm.
Stator structures include: round hole type——used for circulating dispersion and emulsification of high-viscosity materials; mesh type——used for the preparation of low-viscosity fine emulsion and the rapid dispersion and refinement of tiny particles in the liquid; rectangular hole type Formula - used for mixing and dispersing medium and high viscosity materials.
Rotor structures include: two-blade paddle type—suitable for materials of various viscosities; comb type——can adjust the distance between comb bars according to the viscosity of various materials; turbine type——design according to different viscosity of materials Rotors for various viscosity requirements.
Suspension is a liquid dispersion system, and the role of homogeneous mixing is very important. Because, as long as the dispersed phase reaches a colloidal state, the internal and external phase stratification may occur. The stratification is mainly the result of the suspension of the dispersed matter under the action of gravity or centrifugal force. The stability of the suspended matter is not only related to the buoyancy, but also to the Brownian motion. Related, the smaller the particles of the dispersed phase in the dispersed system, the more stable the suspension; from a thermodynamic point of view, the general emulsified products are unbalanced mixtures, and oil and water will separate one day, so it is difficult to predict the product storage life.
However, suspension stability in cosmetics is an extremely important performance indicator. The suspension stability of the suspension is inversely proportional to the particle size of the dispersed phase. The particle size of the emulsion determines the suspension stability. The larger the particle size, the worse the stability, and the shorter the shelf life of the product. Therefore, there is a problem of incomplete emulsification in the cosmetics manufacturing process. Three months after the product is shipped, the product begins to separate from oil and water or spoil, which will cause serious product quality problems.
Therefore, cosmetics all need homogeneous emulsification, so that the particle size of the product is reduced, the distribution is uniform, and the system is stable, so as to prevent material stratification and improve product shelf life. Generally, at least 3 years of storage period are required, and the storage period of emulsified products is not only related to the formula , also has a very important relationship with the preparation process, such as "process variable factors" such as emulsification temperature, stirring method and cooling rate.
The principle of homogeneity is that the material is mechanically forced to pass through a very narrow gap at a high speed. Under the effect of hydrodynamics, the high tangential velocity generated by the high-speed rotation of the rotor is formed in the narrow gap between the rotor and the stator. The large velocity gradient, as well as the strong kinetic energy due to the high-frequency mechanical effect, make the material in the gap between the stator and the rotor subject to the combined effects of strong hydraulic shear, centrifugal extrusion, liquid layer friction, impact tearing and turbulence. , the incompatible solid phase, liquid phase and gas phase are dispersed uniformly and finely in an instant under the joint action of corresponding mature technology and appropriate amount of additives, and then the particles or droplets of the dispersed phase are broken up to reach For the purpose of homogeneous emulsification, a relatively stable high-quality emulsified product is finally obtained.
At the same time, from the point of view of the homogeneous mechanism, shear and pressure are the main factors that cause particle fracture, and the hydrodynamic effects that cause shear and pressure are turbulent flow effect and cavitation effect respectively. Generally, the fracture of coarse particles is mainly caused by shear action. . Among the materials entering the homogenizer, the particles of the dispersed phase are relatively coarse and unevenly dispersed. According to the Stokes theorem, the particle sedimentation velocity is proportional to the difference in gravity between the two phases, proportional to the square of the particle size, and inversely proportional to the viscosity of the material. Therefore, the material at this time is extremely unstable and the sedimentation is serious. In order to achieve the purpose of mixing and homogenization, shear The type homogenizer needs to fully stir and mix the materials, and at the same time break the coarse particles into smaller particles. The high-speed rotating rotor in the stator provides mechanical energy, and sufficient turbulent flow tension is generated in the rotor area.
