Processing Materials Of Pvc

PVC plastics have different shapes and processing methods, including pressing, extrusion, injection and coating. The particle size, fish eye, bulk density, purity, foreign impurities and porosity of PVC resin all affect the processability; For paste resin, the viscosity and gelling property of paste should be considered.
PVC is an amorphous polymer with small shrinkage. The powder should be preheated before processing to remove moisture, enhance the plasticizing effect and prevent bubbles. Moreover, PVC is very easy to decompose, especially in contact with steel and copper at high temperatures (decomposition temperature 200 ℃). The forming temperature range is small, and the material temperature must be strictly controlled. When using screw injection machine and straight nozzle, the hole diameter should be large to prevent material stagnation in dead angle. The mold pouring system should be large, the gate section should be large, and the mold should be cooled, with mold temperature of 30-60 ℃ and material temperature of 160-190 ℃.
Under the glass transition temperature (Tg, 80 ℃), PVC is in glass state; At Tg → viscous flow temperature (Tf, about 160 ℃), it is highly elastic rubber and plastic; At Tf → thermal decomposition temperature (Td), the flow is viscous. The higher the temperature is, the easier the flow is. When the temperature exceeds Td, PVC decomposes into a large amount of hydrogen chloride (HCl), and the material loses its chemical stability and physical properties. Therefore, Td is the upper limit temperature for processing and molding. Due to the large intermolecular force of polyethylene, Tf is very high, even close to the decomposition temperature, so plasticizers need to be added to reduce Tf. On the other hand, it is also necessary to add stabilizer, so as to improve the Td of PVC for processing and molding.
The glass transition temperature (Tg) is only related to the chain segment structure of the molecular chain, and has little relationship with the molecular weight. The viscous flow temperature (Tf) is the temperature at which the macromolecule starts 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 resins are divided into 1-7 grades. The larger the serial number, the smaller the molecular weight. XJ-4 (XS-4) to XJ-7 (XS-7. PVC with an average degree of polymerization below 1000 is called low degree of polymerization PVC, which has good processability. Plasticizer can be added less during processing, so that the products will not be accelerated due to the migration of plasticizer. Low polymerization degree PVC products have good transparency, and are widely used in building materials, food and drug packaging materials, as well as to replace plexiglass products.
Polyvinyl chloride melt belongs to non Newtonian pseudofluid. The higher the shear speed is, the smaller the apparent viscosity is, 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, it will also be heated and degraded due to oxidation, which will affect its performance. Therefore, increasing the shear rate (increasing the pressure) should be considered to improve the fluidity of PVC melt. In fact, increasing the external force is conducive to the movement of macromolecules, so that Tf is reduced and macromolecules can flow at a lower temperature.