When PVC is processed at high temperature, HCl is easily released and unstable polyene structure is formed. At the same time, HCl has autocatalytic effect, which will further degrade PVC. In addition, if there is oxygen or iron, aluminum, zinc, tin, copper and cadmium plasma, it will catalyze the degradation of PVC and accelerate its aging. Therefore, plastic will appear various bad phenomena, such as discoloration, deformation, cracking, mechanical strength decline, electrical insulation performance decline, brittle and so on. In order to solve these problems, stabilizers must be added to the formula, especially heat stabilizers. Stabilizers for PVC include heat stabilizers, antioxidants, UV absorbers and chelators. In the formulation design, different kinds and quantities of stabilizers are selected according to the product use requirements and processing technology requirements.

（1） The heat stabilizer must be able to capture the HCl released by PVC resin with autocatalytic effect or react with the unstable polyene structure produced by PVC resin to prevent or reduce the decomposition of PVC resin. Generally, the characteristics, functions and product requirements of the heat stabilizer selected in the formula shall be considered. For example:

Lead salt stabilizer is mainly used in hard products. Lead salt stabilizers are characterized by good heat stabilizers, excellent electrical properties and low price. But it is toxic and easy to pollute products, so it can only produce opaque products. In recent years, a large number of composite stabilizers have been developed, and single component stabilizers are in danger of being replaced. The composite stabilizer has the advantages of strong specificity, small pollution and simple ingredients for processing enterprises. However, there is no unified standard, so the composite stabilizers of different countries are quite different.

Barium cadmium stabilizers are a kind of heat stabilizers with good performance. It is widely used in PVC agricultural film. It is usually used with barium cadmium zinc, organic phosphite and antioxidant.

Calcium and zinc stabilizers can be used as non-toxic stabilizers in food packaging, medical equipment and drug packaging, but their stability is relatively low. When the amount of calcium stabilizers is large, the transparency is poor, and it is easy to spray frost. Generally, polyols and antioxidants are used to improve the performance of calcium zinc stabilizers. Recently, there have been calcium zinc composite stabilizers for hard pipes in China.

Organotin heat stabilizers have good performance and are good varieties for PVC hard products and transparent products. Especially, octyltin is almost an indispensable stabilizer for non-toxic packaging products, but its price is more expensive.

Epoxy stabilizers are usually used as auxiliary stabilizers. When this kind of stabilizers are used together with barium cadmium calcium zinc stabilizers, it can improve the stability of light and heat, but it is easy to exude. The auxiliary stabilizers are polyols and organic phosphite.

In recent years, there are also rare earth stabilizers and hydrotalcite stabilizers. The main characteristics of rare earth stabilizers are excellent processing performance, while hydrotalcite is non-toxic stabilizers.

（2） During the processing and using of antioxidant PVC products, due to the action of heat and ultraviolet, the oxidation degradation of antioxidant PVC products is related to the generation of free radicals. The main antioxidant is chain breaking terminator or or free radical scavenger. Its main function is to combine with free radicals to form stable compounds and to stop the chain reaction. The main antioxidant for PVC is generally bisphenol A. There are also auxiliary antioxidants or hydrogen peroxide decomposing agents. PVC auxiliary antioxidants are triphenyl phosphite and phenyldiisooctyl phosphite. The main antioxidant and auxiliary antioxidant can play a synergistic role. PVC edge veneer

（3） The PVC products used in outdoor use of UV absorbers, because of its sensitive wavelength range of UV irradiation, the molecules of PVC become excited state, or their chemical bonds are destroyed, resulting in free radical chain reaction, promoting the degradation and aging of PVC. In order to improve the ability of anti ultraviolet, ultraviolet absorbers are often added. The common UV absorbers for PVC are triazine-5, UV-9, UV-326, TBS, bad and obs. Triazine-5 has the best effect, but the film is yellow because of its yellow color, which can be improved by adding a small amount of phthalocyanine blue. UV-9 is commonly used in PVC agricultural film, the general dosage is 0.2 ~ 0.5 phr. TBS, bad and OBS, which belong to salicylic acid, have mild action. When they are used together with antioxidants, they will have good aging resistance. For non transparent products, the weatherability is generally improved by adding rutile titanium dioxide with shading. At this time, if the ultraviolet absorbent is added, a large amount is needed, which is not very cost-effective.

（4） In the stable system of PVC plastics, the phosphite which is often added is not only an auxiliary antioxidant, but also a chelating agent. It can form metal complexes with harmful metal ions which can promote PVC to remove HCl. The commonly used phosphites are triphenyl phosphite, phenyldiisooctyl phosphite and diphenyloctyl phosphite. In PVC agricultural film, the general dosage is 0.5 ~ 1 phr. When it is used alone, it is easy to dye at the initial stage, and its thermal stability is not good. It is generally used in combination with metal soap. PVC edge veneer