PVC is a heat-sensitive plastic, and its light stability is also very poor. Under the action of heat and light, the de-HCl reaction, that is, the degradation generally occurs. The reason why PVC degrades and its structure is closely related. The thermal degradation of PVC is a "chain" removal of hydrogen chloride to give a dilute propyl chloride product. In the presence of hydrogen chloride, the reaction can be accelerated. The structural defects of PVC are mainly caused by polymerization, which accelerates the rate of degradation, especially the initial color. Another known phenomenon of PVC degradation is chain scission and cross-linking, which is most likely a reaction that occurs in the last stage of aerobic conditions (auto-oxidation) or thermal degradation. The mechanical properties and rheological properties of PVC have changed significantly. An important factor affecting the rate of degradation is structural defects in PVC such as hydrogen chloride, any strong Lewis acid or base, and oxygen including auto-oxidation. The result of the degradation is that the strength of the plastic product is reduced, discoloration, black lines, and serious loss of use value of the product. Factors affecting PVC degradation include polymer structure, polymer quality, stability system, and molding temperature.

According to experience, the yellowing of PVC profiles is mainly due to the presence of paste at the die. The reason is that the die flow path is unreasonable or the local polishing in the flow channel is not good, and there is a stagnant zone. The yellow line of the PVC profile is mostly the yellow line on the PVC profile in the barrel, and the stagnant material is at the exit of the die; if the yellow line is not straight; in the presence of the paste, the main reason is the sieve plate (or transition) There is a dead angle between the sets) and the material flow is not smooth. , and mainly in the transition suite. If the yellow line appears when the formula and raw materials are unchanged, the cause should be found mainly from the mechanical structure, and the starting point of decomposition should be found and eliminated. If the cause is found on the mechanical structure, consideration should be given to problems with the formulation or process. There are several measures to avoid degradation:

1. Strictly control the technical indicators of raw materials, and use qualified raw materials;

2. Develop reasonable molding process conditions under which PVC materials are not easily degraded;

3. The molding equipment and mold should be well structured, and the dead angle or gap that may exist between the equipment and the material contact surface should be eliminated; the flow channel should be streamlined and suitable for length;

4. The heating device should be improved to improve the sensitivity of the temperature display device and the efficiency of the cooling system.

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