PVC products in 100 ~ 150℃ decomposition, ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. The thermal oxygen aging of PVC is complicated. In reality, we can inhibit the decomposition of PVC by adding an appropriate amount of PVC heat stabilizer in the production process of PVC. We can divide the thermal degradation process of PVC into two steps.
1, dehydrochlorination: PVC polymer molecular chain to remove the active chlorine atoms to produce hydrogen chloride, while generating conjugated polyolefins;
2. Formation of longer chain polyolefin and aromatic ring: with the further degradation, the chlorine atom on the allyl group is extremely unstable and easy to be removed, resulting in longer chain conjugated polyolefin, namely the so-called "zipper" dehydrogenation. At the same time, there is a small amount of C-C bond fracture and cyclization, resulting in a small amount of aromatic compounds.
Decomposition of dehydrochlorinated hydrogen is the main cause of PVC aging. The degradation mechanism of PVC is complicated, and there is no unified conclusion. The main mechanisms proposed by researchers include free radical mechanism, ion mechanism and single molecule mechanism. In the process, the thermal decomposition of PVC does not change much for other properties, mainly affecting the color of the finished product, adding heat stabilizer can inhibit the initial coloring of the product. When the mass fraction of HCl removed reaches 0.1%, the color of PVC begins to change. Depending on the number of conjugated double bonds formed, PVC will appear in different colors (yellow, orange, red, brown, black).
If there is oxygen in the thermal decomposition process of PVC, there will be the formation of colloidal carbon, peroxide, carbonyl and ester compounds. However, in the long time of product use, the thermal degradation of PVC has a great impact on the performance of the material. Adding heat stabilizer can delay the degradation time of PVC or reduce the degree of PVC degradation. Adding heat stabilizer in the process of PVC processing can inhibit the degradation of PVC, so the main role of heat stabilizer is: by replacing unstable chlorine atoms, absorption of hydrogen chloride, and unsaturated parts of the addition reaction to inhibit the degradation of PVC molecules.
The ideal heat stabilizer should have a variety of functions:
1. Replace active and unstable substituents, such as chlorine atoms connected to tertiary carbon atoms or allyl chlorine, to generate stable structures;
2, absorb and neutralize the HCl released in the process of PVC processing, eliminate the autocatalytic degradation of HCl;
3, neutralization or passivation of metal ions and other harmful impurities that play a catalytic role in degradation;
4, through a variety of forms of chemical reactions can block the continued growth of unsaturated bonds, inhibit degradation coloring;
5, can have a protective shielding effect on ultraviolet light.