PVC products decompose significantly at 100～150℃. Ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride, and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. Thermal oxidative aging of PVC is more complicated. In reality, we add an appropriate amount of PVC heat stabilizer in the production process of PVC to inhibit the decomposition of PVC. We can divide the thermal degradation process of PVC into two steps.

1. Dehydrochlorination: The active chlorine atom is removed from the molecular chain of PVC polymer to produce hydrogen chloride, and at the same time, conjugated polyolefins are produced;

2. The formation of longer-chain polyolefins and aromatic rings: as the degradation progresses further, the chlorine atoms on the allyl groups are extremely unstable and easy to remove, resulting in longer-chain conjugated polyolefins, the so-called “zipper Formula “dehydrogenation, at the same time there is a small amount of cleavage and cyclization of the CC bond, and a small amount of aromatic compounds are produced.

Among them, decomposition and dehydrochlorination is the main cause of PVC aging. The degradation mechanism of PVC is relatively complicated, and there is no unified conclusion. The researchers put forward the free radical mechanism, ionic mechanism and single molecule mechanism. During the processing, the thermal decomposition of PVC does not change much of other properties, mainly affecting the color of the finished product. The addition of heat stabilizers can inhibit the initial coloring of the product. When the mass fraction of removed HCl reaches 0.1%, the color of PVC begins to change. According to the number of conjugated double bonds formed, PVC will show different colors (yellow, orange, red, brown, black).

If oxygen is present during the thermal decomposition of PVC, colloidal carbon, peroxide, carbonyl and ester-based compounds will be formed. However, during the long period of use of the product, the thermal degradation of PVC has a great impact on the performance of the material. The addition of thermal stabilizers can delay the time of PVC degradation or reduce the degree of PVC degradation. The addition of heat stabilizers in the process of PVC processing can inhibit the degradation of PVC. The main functions of heat stabilizers are: inhibition by replacing unstable chlorine atoms, absorbing hydrogen chloride, and reacting with unsaturated parts. Degradation of PVC molecules.

The ideal heat stabilizer should have multiple functions:

1. Replace active and unstable substituents, such as chlorine atoms or allyl chlorides attached to tertiary carbon atoms, to generate a stable structure;

2. Absorb and neutralize the HCl released during PVC processing to eliminate the autocatalytic degradation of HCl;

3. Neutralize or passivate metal ions and other harmful impurities that have a catalytic effect on degradation;

4. Through various forms of chemical reactions, it can block the continuous growth of unsaturated bonds and inhibit degradation and coloration;

5. It can protect and shield ultraviolet light.