PVC force facilitates the movement of macromolecules

Polyvinyl chloride plastics have different shapes and differences, and there are various processing methods, such as pressing, extrusion, injection, coating, etc. The particle size, fish eye, bulk density, purity, foreign impurities, and porosity of polyvinyl chloride resin have an impact on processability; paste resin should consider the viscosity and gelling properties of the paste.

Polyvinyl chloride is an amorphous high polymer with low shrinkage. The powder should be preheated before processing to remove moisture, enhance the plasticizing effect and prevent air bubbles. Moreover, PVC is easily decomposed, especially in contact with steel and copper at high temperature (decomposition temperature is 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 hole diameter should be large to prevent material stagnation in dead corners. The casting system of the mold should be large, the gate section should be large, the mold should be cooled, the mold temperature should be 30-60 °C, and the material temperature should be 160-190 °C.

Below the glass transition temperature (Tg, 80°C), polyvinyl chloride is in a glass state; at Tg→viscous flow temperature (Tf, about 160°C), it is highly elastic and rubbery, and has plasticity; at Tf→thermal decomposition temperature (Td) For 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 intermolecular force of polyethylene, the Tf is very high, even close to the decomposition temperature, so it is necessary to add plasticizers to reduce Tf. On the other hand, it is also necessary to add stabilizers to increase the Td of PVC before processing and molding.

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 begin 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 size of the molecular weight, domestic suspended 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) resins are often used in the manufacture of hard pipes, hard boards, etc. Other types of resins with lower molecular weight and higher molecular weight need to add a lot of plasticization 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 degree of polymerization polyvinyl chloride, which has good processing performance, and less plasticizer can be added during processing, so that the product will not be accelerated due to the migration of plasticizer. Ageing. Polyvinyl chloride products with low degree of polymerization have good transparency and are widely used in building materials, food and drug packaging materials and as substitutes for plexiglass products.

The polyvinyl chloride melt is a non-Newtonian pseudo-fluid. The larger the shear rate, the smaller the apparent viscosity, and the change is quite sensitive. When the temperature is raised, the viscosity does not decrease much. Even if the plastic is below the decomposition temperature, it will cause heat and oxidative degradation due to the high temperature for a long time, which will affect its performance. Therefore, to improve the fluidity of polyvinyl chloride melt, the main consideration should be to increase the shear rate (increase the pressure). In fact, increasing the external force helps the movement of macromolecules, which reduces Tf and enables macromolecules to flow at lower temperatures.