Thermal stabilizer is one of the essential additives for PVC processing, and the number of the use of PVC thermal stabilizer is not much, but its effect is huge. The use of heat stabilizer in PVC processing can ensure that PVC is not easy to degrade and stable. The thermal stabilizers commonly used in PVC processing are alkaline lead salt stabilizers, metal soap stabilizers, organic tin stabilizers, rare earth stabilizers, epoxides and so on. The mechanism of PVC degradation is complex, and the mechanism of different stabilizers is different, and the stability effect is different.

1. Thermal degradation mechanism of PVC.

            PVC in 100 ~ 150 ℃ apparent decomposition, ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride and some active metal salt and metal oxide etc will greatly accelerate the decomposition of PVC. The thermal oxygen aging of PVC is more complicated. Some literatures report that the thermal degradation process of PVC is divided into two steps. (1) dehydrochlorination: the active chlorine atoms are removed from PVC polymer chains to produce hydrogen chloride, and the conjugated polyolefins are generated. (2) the formation of much longer chain olefins and aromatic ring: with the further of degradation, the chlorine atoms on the allyl unstable is easy to take off, generating a longer chain of conjugated olefins, referred to as the "chain" of the dehydrogenation, at the same time there is a small amount of C - C key fracture, cyclization, produce a small amount of aromatic compounds. Decomposition of hydrogen chloride is the main cause of PVC aging. The degradation mechanism of PVC is complicated and there is no unified conclusion. The researchers propose the mechanism of free radical, ion mechanism and single molecule mechanism.

2. Thermal stability mechanism of PVC.

                In the process of processing, the thermal decomposition of PVC has little change in other properties, mainly affecting the color of the finished product, and the addition of heat stabilizer can inhibit the initial coloring of the product. The color of PVC starts to change when the quality score of the removed HCl reaches 0.1%. According to the number of conjugated double bonds, PVC will present different colors (yellow, orange, red, brown and black). If there is oxygen in the thermal decomposition of PVC, there will be colloidal carbon, peroxide, carbonyl and ester-based compound. However, the thermal degradation of PVC has a great influence on the properties of the materials for a long time, and the addition of heat stabilizer can delay the degradation of PVC or reduce the degradation of PVC.

                 Adding heat stabilizer in PVC processing process can inhibit the degradation of PVC, so heat stabilizer plays the main role: by replacing unstable chlorine atoms, the absorption of hydrogen chloride, addition reaction with unsaturated parts such as way to suppress the degradation of PVC molecules. The ideal heat stabilizer should have a variety of functions :(1) replace the active and unstable substituents, such as the chlorine atom or allyl chloride attached to the tertiary carbon atom, to produce a stable structure; (2) to absorb and neutralize the HCl released in the process of PVC processing and eliminate the automatic catalytic degradation of HCl; (3) neutralization or passivation of metal ions and other harmful impurities that catalyze the degradation; (4) the continuous growth of unsaturated bonds can be blocked by various forms of chemical reactions, and the degradation of degradation is inhibited; (5) it is better to have protective shielding effect on ultraviolet light.

3. PVC stabilizer, mechanism and application.

3.1 lead salt stabilizer.

         Lead salt stabilizer [7] can be divided into three categories :(1) simple lead salt stabilizer, mostly salt base salt containing PbO; (2) the thermal stabilizer with lubricating effect is mainly the neutral and salt base salt of fatty acids; (3) compound lead salt stabilizers, and solid and liquid composite stabilizers containing lead salts and other stabilizers and components.

          The thermal stability of lead salt stabilizer is stronger, has good dielectric properties, and the price is low, and the lubricant reasonable ratio can make the PVC resin processing temperature range wide, processing and after processing product quality is stable, is by far the most commonly used stabilizers. Lead salt stabilizers are mainly used in hard products. Lead salt stabilizer is characterized by good thermal stabilizer, excellent electrical performance and low price. But lead salts are toxic, can not be used in food products, or transparent products, and can be easily contaminated by sulfide to produce black lead sulfide.

3.2 metal soap stabilizer.

          The hot stabilizer of stearic acid soap is alkaline earth metals (calcium, cadmium, zinc, barium, etc.) and saponification of stearic acid and lauric acid. There are many kinds of products, each with its own characteristics. In general, the lubricious stearic acid is better than the lauric acid, and the compatibility with the PVC is better than the stearic acid.

                   Metallic soap due to absorption of HCl, some varieties can also through its root of metal ions in the catalysis of fatty acids instead of the active site of Cl atom, so you can have different levels of heat stable effect of PVC. There are very few single metal soap compounds in the PVC industry, and usually several metal soap compounds. Calcium and zinc soap stabilizers are common. Based on the mechanism of Frye - horst and stability mechanism of calcium/zinc compound stabilizer can think: first of all, allyl chloride zinc soap and PVC chain reaction, and then zinc calcium soap, soap chlorine and chloride reaction of the unsteady metal chloride. At this time, as an auxiliary stabilizer of intermediate media, the chlorine atoms were transferred to the calcium soap, which caused the regeneration of zinc soap and delayed the generation of zinc chloride with the effect of promoting dehydrochlorination.

                Calcium and zinc stabilizers can be used as non-toxic stabilizers, which are used in food packaging and medical devices and medicine packaging, but their stability is relatively low, and the amount of calcium stabilizer is low and the transparency is poor. Calcium and zinc stabilizers generally use polyols and antioxidants to improve their performance, and there is a clear calcium and zinc compound stabilizer for hard tube.

3.3 organic tin stabilizer.

                 The alkyl tin in organic tin is usually methyl, n-butyl, n-octyl and so on. Most of the production in Japan is butyltin, which is more common in Europe. This is the standard non-toxic stabilizer approved by Europe, while the United States USES more methyl tin. There are three main types of organic tin stabilizers :(1) aliphatic acid salts, which mainly refer to dibutyltin of dilaurate and dioctyl cinnamic acid of dilaurate, etc. (2) maleic acid salts, mainly maleic acid dibutyl tin, dibutyl tin (maleate), dibutyltin maleate, etc. (3) mercaptan salts, of which two (sulphate carboxylic acid) esters are the most used.

              Organic tin heat stabilizer, with good performance are used in rigid PVC products and transparent products of good varieties, especially dotl almost become non-toxic packaging products indispensable stabilizer, but its price is more expensive.

                 Organic tin thermal stabilizer (thioglycol) has a good stabilizing effect on PVC. In particular, the liquid organic tin stabilizer, compared with the solid thermal stabilizer, liquid organic tin stabilizer can be better mixed with PVC resin. The organic tin stabilizer (thioglycoacetate) can replace the unstable Cl atoms on the polymer, making the PVC resin have long-term stability and initial color retention. The stable mechanism of thioglycolic acid was proposed :(1) S atom can replace the unstable Cl atom, thus inhibiting the generation of conjugated polyolefin. (2) HCl is the product of thermal degradation of PVC and can accelerate the generation of conjugated polyolefins. The HCl can be absorbed by thiothioacetate.

3.4 rare earth stabilizer.

              The rare earth thermal stabilizer mainly includes the rare earth lanthanum, cerium, neodymium and inorganic salt. The types of organic weak acid salts include rare earth, fatty acid rare earth, salicylic acid rare earth, citric acid rare earth, lauric acid rare earth, etc.

                A preliminary study on the mechanism of rare earth stabilizer is: (1) the special electronic structure of rare earth lanthanides (the outermost layer 2, outer eight electronic structure, there are many empty orbit), the empty orbital energy level difference is very small, under the effect of external heat oxygen or under the effect of polar group, the outer layer or outer electrons were intensified, with unstable Cl PVC chain coordination, and can be resolved and PVC processing of hydrogen chloride ligand complex formation, and rare earth elements with strong attraction between chlorine, can have the effect of control of free chlorine, which can prevent or delay the autooxidation chain reaction of hydrogen chloride, thermal stability. (2) rare earth multifunctional stabilizers for PVC processing PVC itself contains the oxygen and physical adsorption of ionic impurities, and into the lattice point of rare-earth multifunctional stabilizers, avoid them to maternal C - Cl key impact vibration. Therefore, the activation energy of PVC dehcl can be improved by the use of rare earth multifunction stabilizer, thus delaying the thermal degradation of PVC plastic. (3) the suitable anionic groups in rare earth compounds can be used to replace the allyl chloride atoms in PVC macromolecules to eliminate this degradation and achieve a stable goal. Rare earth stabilizer domestic research is more.

                    In general, the stability of rare earth heat stabilizer is better than metal soap stabilizer, has good long-term heat stability, and with other species have a wide range of synergies between stabilizer, has good tolerance, not polluted by sulfur, storage stability, the advantages of non-toxic environmental protection. In addition, rare earth elements have a unique coupling effect with CaCO3 and promote the plasticizing effect of PVC, thus increasing the amount of CaCO3 and reducing the use of processing auxiliary ACR, effectively reducing the cost. The stable effect of rare earth on polyvinyl chloride is characterized by its unique synergistic effect. The appropriate coordination of rare earth with certain metals, ligands and stabilizers can greatly improve the stabilizing effect.

3.5 other stabilizers.

3.5.1 track of epoxy

                  Epoxy soybean oil, flaxseed oil, epoxy epoxy can tall oil, epoxy butyl stearate, dioctyl epoxy compounds is polyvinyl chloride (PVC) commonly used heat stabilizer, they are used with the stabilizer have higher synergy, light stability and the advantages of non-toxic, suitable for soft, especially soft FVC products are exposed to the sun, usually is not used for rigid PVC products, its shortcoming is easy.

               Studies have pointed out that the epoxy sunflower oil added to contain different metallic soap salt (Ba/Cd and Ca/zinc) PVC, through the determination of the thermal stability of the material, found that sunflower oil and metal soap salt has the very good synergy, can enhance the thermal stability of PVC material, analyzes the causes of synergy: the degradation of HCl are absorbed by sunflower oil and metal soap salt, HCl concentration decreases at the same time reduce the PVC of HCl speed (HCl has catalytic effect to PVC degradation), improve the thermal stability of PVC.

3.5.2 hydroxyl groups.

Pentaerythritol, xylitol and other hydroxyl compounds have a certain thermal stabilization effect on PVC, which is used as the auxiliary thermal stabilizer of PVC.

               Through the dehydrochlorination rate and thermal stability experiment, found that do not contain heavy metals and zinc heat stabilizer of PVC/thermal stability of polyol extended to 200 ℃, its stable effect related to the types of polyol and hydroxyl number, especially containing the end hydroxyl polyol promote PVC thermal stability for a long time, produced by the absorption degradation HCl.

3.5.3 other

 It can be used as the auxiliary heat stabilizer of PVC, and it can absorb the HCl and delay the color change of PVC.

 4. Current status and development trend of PVC thermal stabilizer.

                          After entering the 21st century, due to the global demand for environmental protection is becoming more and more strict, the limit of heavy metals stabilizer of laws and regulations, make further heat stabilizer production and consumption to non-toxic, low toxicity, composite efficient direction, lead-free, no cadmium has been paid attention to by the developed countries, caused by substitute products appear constantly, and application of lead, cadmium cadmium) (especially the application of the stabilizer has have been gradually declining trend, there have been some non-toxic or low toxic heat stabilizer (such as organic tin compounds, calcium zinc soap and salt, the rare earth stabilizer, etc.).

                            Although in recent years, our country complex, non-toxic and low toxicity of heat stabilizer production and development has made considerable achievement, but compared with the world's advanced level there is a lot of deficiencies and more gap (such as varieties, small production scale, etc.). The production and application of new heat stabilizer in China is far from satisfying the development of domestic PVC industry, and some of the hot stabilizers needed for high-grade PVC products are mainly imported. The rapid development of PVC industry in China has provided a good market guarantee and broad development space for the development of the thermal stabilizer industry, and it also has higher requirements for the thermal stabilizer industry. To strengthen the research and development of new thermal stabilizers in China, we should pay more attention to the following points :(1) to strengthen the research and improvement of the original unleaded calcium and zinc stabilizer and improve the quality of the original products; (2) gradually establish a group of large-scale auxiliary production plants, based on the source of raw materials and market distribution; (3) cooperate with the development and production of other PVC auxiliary agents, develop diversified compound products, further reduce resource waste and environmental pollution, and promote the sustainable development of "green" auxiliary industry.