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PVC material ratio is processed with gravimetric blender


source:[Gravimetric blender]   release time:2021-05-03 10:59:31




Polyvinyl chloride plastics have different forms and great differences, as well as various processing methods, such as pressing, extrusion, injection, coating and so on. The particle size, fish eyes, bulk density, purity, foreign impurities, and porosity of polyvinyl chloride resin have an impact on the processability; for paste resin, the viscosity and gelling properties of the paste should be considered.
Polyvinyl chloride is an amorphous polymer with low shrinkage. The powder should be preheated before processing to remove moisture, enhance the plasticizing effect, and prevent bubbles. Moreover, PVC is easy to decompose, especially when it comes in contact with steel and copper at high temperatures (decomposition temperature 200 degrees). The molding temperature range is small, and the material temperature must be strictly controlled. When using a screw injection machine and a straight-through nozzle, the aperture should be large to prevent dead ends from stagnating. The mold pouring system should be thick, the gate section should be large, the mold should be cooled, the mold temperature is 30-60°C, and the material temperature is 160-190°C.
Below the glass transition temperature (Tg, 80°C), polyvinyl chloride is glassy; at Tg→viscous flow temperature (Tf, about 160°C), it is a highly elastic rubber with plasticity; at Tf→thermal decomposition temperature (Td) For viscous flow, the higher the temperature, the easier the flow. When the temperature exceeds Td, PVC decomposes a large amount of hydrogen chloride (HCl), and the material loses chemical stability and physical properties. Therefore, Td is the upper limit temperature for processing and molding. Due to the great force between the polyethylene molecules, the Tf is very high, even close to the decomposition temperature, so it is necessary to add a plasticizer to reduce the Tf. On the other hand, it is also necessary to add stabilizers to increase the Td of PVC in order to be processed and formed.
The glass transition temperature (Tg) is only related to the molecular chain segment structure, and has little to do with the molecular weight, while the viscous flow temperature (Tf) is the temperature at which macromolecules start to move, and is related to the molecular weight. The larger the molecular weight, the higher the Tf. Therefore, for some processing molding (such as injection molding), it is necessary to appropriately reduce the molecular weight of the resin. According to the different molecular weight, domestic suspension PVC resin is divided into 1-7 grades, the larger the serial number, the smaller the molecular weight. XJ-4 (XS-4) to XJ-7 (XS-7) resins are often used in the manufacture of hard pipes, hard boards, etc. Other resins with lower models and larger molecular weights need to be added in large quantities due to higher Tf. The agent reduces Tf, so it is often used in the manufacture of soft products. Polyvinyl chloride with an average degree of polymerization below 1000 is called low-polymerization degree polyvinyl chloride, which has good processing properties, and less plasticizers can be added during processing, so that the product will not be accelerated due to the migration of plasticizers Ageing. Polyvinyl chloride products with low degree of polymerization have good transparency and are widely used in building materials, food and pharmaceutical packaging materials and to replace organic glass products.
Polyvinyl chloride melt is a non-Newtonian pseudo-fluid. The higher the shear rate, the smaller the apparent viscosity, and the change is quite sensitive. When the temperature is increased, the viscosity will not decrease much. Even if the plastic is below the decomposition temperature, due to the high temperature for a long time, it will cause heat and oxidative degradation, which will affect its performance. Therefore, to improve the fluidity of PVC melt, the main consideration should be to increase the shear rate (increase the pressure). In fact, increasing the external force will help the movement of the macromolecules, so that the Tf will be reduced, and the macromolecules can flow at a lower temperature.
With so many PCVs, the mixing ratio of PVC with a Gravimetric blenderis more accurate.[Gravimetric blender]