PVC material processing

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PVC material processing

Polyvinyl chloride plastics vary in shape and vary widely, and they can be processed in a variety of ways, including pressing, extrusion, injection, and coating. The particle size, fisheye, bulk density, purity, foreign matter, and porosity of the polyvinyl chloride resin all have an effect on the processability; the paste resin should consider the viscosity and gelation properties of the paste.

Polyvinyl chloride is an amorphous polymer with a small shrinkage. Before the powder is processed, it should be preheated to remove water, enhance the plasticizing effect and prevent air bubbles. Moreover, PVC is easily decomposed, especially in contact with steel and copper at high temperatures (decomposition temperature is 200 degrees). The molding temperature range is small, and the material temperature must be strictly controlled. When using a screw type injection machine and a straight-through nozzle, the aperture should be large to prevent dead angles from stagnation. The mold casting system should be coarse, the gate section should be large, the mold should be cooled, the mold temperature is 30-60 degrees, and the material temperature is 160-190.

Below the glass transition temperature (Tg, 80 ° C), polyvinyl chloride is in a glassy state; at Tg → viscous flow temperature (Tf, about 160 ° C), it is highly elastic rubbery, plasticity; at Tf → thermal decomposition temperature (Td) In the viscous flow state, 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 its chemical stability and physical properties, so Td is the upper limit temperature for processing. Due to the large inter-molecular force of polyethylene, the Tf is very high, even close to the decomposition temperature, so it is necessary to add a plasticizer to lower the Tf. On the other hand, it is also necessary to add a stabilizer to increase the Td of the PVC in order to be processed.

The glass transition temperature (Tg) is only related to the molecular chain segment structure, and has little relationship with the molecular weight. The viscous flow temperature (Tf) is the temperature at which the macromolecule starts to move, which is related to the molecular weight. The larger the molecular weight, the higher the Tf. Therefore, for some processing (such as injection molding), it is necessary to appropriately reduce the molecular weight of the resin. According to the difference in molecular weight, the domestic suspension polyvinyl chloride 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) type resins are commonly used in the manufacture of hard tubes, hard plates, etc., other models with lower molecular weight and larger molecular weight, due to higher Tf, need to add a lot of plasticization The agent lowers the Tf and is therefore commonly used in the manufacture of soft products. Polyvinyl chloride with an average degree of polymerization below 1000 is called a low degree of polymerization of polyvinyl chloride, which has good processing properties and can be added with less plasticizer during processing, so that the product is not accelerated due to migration of plasticizer. Ageing. Low-polymerization polyvinyl chloride products have good transparency and are widely used in building materials, food and pharmaceutical packaging materials, and in place of plexiglass 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 raised, the viscosity is not reduced much. Even if the plastic is below the decomposition temperature, it will heat up and oxidize and degrade due to the high temperature for a long time, which will affect its performance. Therefore, improving the fluidity of the polyvinyl chloride melt should mainly consider increasing the shear rate (increasing the pressure). In fact, increasing the external force contributes to the movement of the macromolecule, which reduces the Tf, and the macromolecule can flow at a lower temperature.