During the thermal degradation of PVC heat stabilizer, almost no monomer is produced, but a large amount of HCl is generated. The mechanism of thermal degradation of PVC is very complex, and the mechanism of action of PVC thermal stabilizer is also very complex. Based on the current research results, the effects of PVC heat stabilizer can be summarized into two types: preventive type (neutralizing HCl, replacing unstable Cl atoms and preventing automatic oxidation) and remedial type (reacting with unsaturated parts of PVC to destroy carbocation). Broadly speaking, all additives added for the purpose of improving the thermal stability of polymers can be called thermal stabilizers. Heat stabilizer in addition to a small number of applications in rubber and other resin processing, mainly used for the problem of thermal stability is very prominent PVC processing, therefore, usually said the heat stabilizer refers to PVC and PVC copolymer used by the heat stabilizer.

The thermal degradation process of PVC heat stabilizer is as follows:

1. Absorb and neutralize HCl, inhibit its automatic catalysis. These PVC stabilizers include lead salts, organic acid metal soaps, organotin compounds, epoxy compounds, amines, metal alkyls and phenols, and metal mercaptans, which can react with HCl and inhibit the reaction of PVC to remove HCl.

2. Replace unstable allyl chlorine atoms or tertiary carbon chlorine atoms in PVC molecules to inhibit depvc. For example, organotin stabilizer ligands with unstable chlorogen of PVC molecules. In the ligands, organotin displaces with unstable chlorine atoms.

3. Addition reaction occurs with polyene structure, which destroys the formation of large conjugated system and reduces coloration. The salt or ester of unsaturated acid contains double bond, and the conjugated double bond in PVC molecule occurs diene addition reaction, thus destroying its conjugated structure and inhibiting discoloration.

4. Capture free radicals and prevent oxidation reaction. For example, the addition of phenolic PVC heat stabilizer can retarding the removal of HCl, because the H atomic radical given by phenol can coupling with the degraded PVC macromolecular radical to form the material that cannot react with O2, and has the thermal stabilizing effect. This PVC stabilizer may have one or more functions.

Ideal PVC stabilizer, PVC heat stabilizer should be a multi-functional material, or a mixture of some materials, they can achieve the following functions:

1. Replace the active and unstable substituents, such as the Cl atom attached to tertiary Cl or allyl chloride, to form a stable structure;

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

3. Neutralization or passivation of metal ions and other harmful impurities that play a catalytic role in degradation;

4. Continuous growth of unsaturated bond can be blocked through various forms of chemical reactions, and degradation and coloring can be inhibited;

5. It is better to protect and shield ultraviolet light.