In order to optimize the extrusion process temperature of the extruder, the first step is to understand and master the benchmark for setting the process temperature. Mass production practice has proved that the following three conditions can be used as benchmarks:
1. Thermal stability of PVC extruder resin
PVC resin is a thermosensitive polymer. The pure PVC resin begins to degrade at 100 ℃, and the degradation accelerates at 150 ℃. On the other hand, PVC began to change from glassy state to viscoelastic state at 160 ℃. Therefore, pure PVC resin cannot be processed directly, and the thermal stability of the resin must be improved by adding heat stabilizer. In general, the stabilizer test of PVC resin is carried out at 180 ℃/30min and 200 ℃/20min. Therefore, the plasticizing temperature and time of PVC resin should not exceed this range.
2. Plasticization degree
Plasticization degree, also known as gel degree, in PVC extruder plastics, plasticization degree is a sign of product crystallization degree and fusion degree of PVC primary particles. A large number of research and test data show that the impact resistance of unmodified PVC-U is the strongest when the plasticization degree is 60% - 65%, that is, the primary particles in the product are not fully plasticized, and only when most of the fusion occurs, the breaking strength is the highest when the plasticization degree is 60%, and the elongation at break is the highest when the plasticization degree is 65%. When the melt temperature is below 150 ℃, the plasticization degree is zero; When the melt temperature is below 190 ℃, the primary particles in the product are clearly visible, and the plasticization degree is below 45%; When the melt temperature is about 200 ℃, most of the primary particle boundaries in the product disappear, only a few primary particles are visible, and the plasticization degree is 70%; When the melt temperature is above 200 ℃, the primary particles of the product are fully plasticized, and the degree of plasticization can reach more than 80%.
3. Processing temperature of blends with CPE
All PVC extruder products are toughened and modified by adding CPE, while the temperature band of CPE impact modifier is relatively narrow. A large number of tests have proved that the products formed by CPE modified PVC at 190 ℃ and 200 ℃ have very different microscopic morphology. At 190 ℃, the modifier particles formed a network structure covering PVC primary particles, which can obtain good impact resistance and toughening effect; At 200 ℃, the primary particles of PVC completely melted, the network structure disappeared and changed into spheres, which were dispersed in the PVC resin matrix, resulting in a significant decline in impact resistance. From the above discussion, it can be seen that the processing conditions of PVC modified by CPE blending are relatively harsh.