First, the degradation of PVC
From the viewpoint of high thermal stability of 1,3,5-trichlorohexane, PVC having a similar basic structure should also be very stable. But this is not the case, PVC is far less stable than a simple chloride. When PVC is heated to 100 ° C, it is accompanied by the de-HCL reaction. When processed at 100-200 ° C, PVC undergoes violent degradation. In addition to de-HCL, macromolecular cross-linking occurs, resulting in blackening of the product and deterioration of physical properties. The degradation of PVC is accompanied by a double bond generated by the removal of HCL, which activates a chlorine atom on the carbon atom, that is, an allyl chloride atom, to cause the HCL molecule to escape. Oxygen is present to accelerate the degradation of PVC.
Second, the thermal degradation mechanism of PVC and the factors affecting the thermal degradation of PVC
The degradation theories generally recognized in the PVC industry are mainly free radical mechanisms, ionic mechanisms and single molecule mechanisms. Among them, the free radical mechanism is the most popular. The factors affecting the thermal degradation of PVC are generally considered to be eight, that is, the influence of structure, the influence of oxygen, the influence of HCL, the influence of metal chloride, the influence of solvent, the influence of plasticizer, the influence of critical size with some unpredictable factor.
Third, the inhibition of thermal degradation
In order to effectively prevent the degradation of PVC, it is necessary to control the degradation history of the degrading force acting on the polymer system during a certain period of time. The purpose of the stabilizer is to achieve comprehensive control of PVC color, rheological properties, mechanical properties, electrical properties, chemical resistance, thermal properties, optical properties, etc. through the control of degradation forces and degradation history. In actual production, the realization of PVC stability is mainly based on two approaches. First, strict control in the production process makes the obtained product itself as stable as possible. This method is generally called preventive stabilization technology. Another method is to complete the production of the resin, and to combine one or more substances before or during the processing to inhibit the degradation that has already begun. This method is generally called a stabilization technique for terminating degradability. The idea is also the role of heat stabilizers.
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