As the environmental protection consciousness gradually strengthened, the traditional PVC heat stabilizer such as powdered lead salt stabilizer, barium, cadmium class stabilizer by certain limit, the current has accordingly developed a series of low toxicity, non-toxic heat stabilizer, mainly including dust-free composite lead salts, composite calcium, zinc, organotin, rare-earth, organic antimony, hydrotalcite, etc. The development status, characteristics, stability mechanism and development direction of several typical low toxicity, non-toxic, high efficiency and compound heat stabilizers are reviewed.

PVC with its excellent flame retardant, insulation, wear resistance and other performance won the broad market, widely used in building materials, light industry, agriculture, packing, electric power, public utilities and other departments, especially in the construction of plastic, agricultural areas such as plastics, plastic packaging materials, daily occupies an important position. In 2005, the production of PVC resin in China was 649.20 million t, second only to the United States.

As is known to all, PVC resin and its products because of their structural defects, there are easy to aging and thermal degradation, its processing temperature (160 C) above (120-130 - C) than the decomposition temperature is higher, so to PVC products, must be adding heat stabilizer in PVC processing and forming process, to slow or stop the thermal degradation of PVC resin. Lead salt is the first used thermal stabilizer, which has a strong stabilizing effect and low price, but it can cause certain environmental pollution in the process of production and use. Since 2000, European countries such as Norway, Finland, Sweden, Denmark and the United Kingdom have taken action to ban lead salt stabilizers. Since 2003, China has also been concerned about lead salt, and Beijing and Shanghai have adopted a decision to ban lead in water pipes. In 2004, the ministry of construction made it clear that the supply of PVC - U tubes used in the whole country must be non-lead salt stabilizer. Our country in PVC plastic products the beginning of total ban lead has been opened, PVC plastic heat stabilizer is moving toward low toxic, non-toxic, pollution-free, compound, efficient direction development. In this paper, the development of the low toxic heat stabilizer of polyvinyl chloride is reviewed.

1 dust - free composite lead salt heat stabilizer.

Dust-free composite lead salt heat stabilizers is in heating and mixing conditions will have a synergistic effect of all kinds of lead salt stabilizer, auxiliary heat stabilizer and lubricant inside and outside after fully decentralized hybrid composite made of granular or flake of lead salt stabilizer [1]. This kind of heat stabilizer (grain materials, sheet materials and packing materials in advance) is to keep the good thermal stability characteristics of lead salt, and overcomes the drawback of toxicity of lead salt dust big, very favorable for occupational health and environmental protection. In 1985, wenzhou tiansheng group plastic assistant general factory first introduced the compound lead salt of PVC cable to the market, and later developed the type 301 and 101 composite lead salt heat stabilizer. Zheng De [2] in 1994, such as application of uniform design experiment optimization method, with the aid of computer aided design, obtain a better performance of modified low lead dust-free multifunctional composite heat stabilizer TS - D, after the performance test and structure characterization, TS - D is multi-functional modified compound stabilizer and lubricant, good processability, thermal stability of PVC is better than the traditional lead salt stabilizer, and can achieve good low dust environment. At present our country to use more dust-free composite lead salt heat stabilizers have German bear brand SMS318 type, henkel company (Henke1) STA - B Ⅱ DX2840 type.

2 composite calcium and zinc heat stabilizer.

Calcium soap thermal stabilizer belongs to long - effect heat stabilizer, which has poor stability and strong coloring, but it is non - toxic and has excellent lubricity. The stability of the zinc soap stabilizer for PVC is poorer, belongs to a short-acting heat stabilizer, and prone to "burn" zinc (mainly produce ZNCL, as the strong lewis acid, has a catalytic role of HC1) off phenomenon, but has good initial coloration, strong weatherability, etc. The composite calcium and zinc thermal stabilizer is the synergistic effect of the two, making it the most active field in compound stabilizer in recent years. The world famous thermal stabilizer manufacturers all have this product launch. The AkcrostabCZ series, OM's PlastiStab series, Witco's Mark series, etc. Since the beginning of the 20th century, China has conducted research on this field and has achieved fruitful results. Sun qi equality [3] found that the Ca/Zn composite heat stabilizer not only increases the whiteness of PVC products, but also improves the thermal stability of the products. Jia-you xu [4] by spectrum study found pentaerythritol and zinc stearate and zinc chloride to form complex, suppresses the zinc chloride catalytic degradation of polyvinyl chloride (PVC), thus inhibiting the "burn" zinc, pentaerythritol plays auxiliary stabilizer in calcium zinc stabilizer: wenzhou tiansheng plastic additives co., LTD. Is a CZ 601 type calcium zinc compound stabilizer, through processing and performance test, thermal stability and health indicators, etc. That are comparable to imported products, make the research and development of the our country in this field to a new stage.

Organic tin thermal stabilizer.

The organic tin stabilizer can be expressed in XnSnY(4-n) and (n= 1-3). , x group can be alkyl, such as methyl, butyl, octyl, also can be ester base, such as methyl acrylate and butyl acrylate Y groups can be a fatty acid root, root can also be mercaptan (thiol esters), etc. The properties of different alkyl groups are also different. The thermal stability sequence is methyl > butyl > octyl, and the thermal stability sequence of the alkyl phase is thiol tin > maleic acid monoester tin > lauric acid tin.

The mechanism of organic tin stabilizer is to replace the active chlorine atoms in PVC molecules into groups that are not easily decomposed. Reaction process, the chlorine atoms on the PVC molecular chain coordination with organic tin compound, when the presence of HC1, ligand complex emergence, PVC chain molecules of the chlorine atom is organic tin Y group replacement, so as to inhibit the decomposition reaction. Has the advantage of organic tin stabilizer has excellent thermal stability, excellent transparency, good compatibility, good liquidity, scaling and non-toxic, but deficiency is less lubricity, and the manufacturing cost is expensive.

Since organic tin thermal stabilizer is one of the best and most promising thermal stabilizers for PVC, the research on it is very active.

3.1 improve and improve the performance of original varieties.

People found that introducing epoxy group in the study of organic tin compounds, can significantly improve the stability, even the trialkyl tin tin and four alkyl derivatives have outstanding thermal stability, and when used with other stabilizer to better [6].

3.2 development of new organic tin thermal stabilizer.

A new type of organic tin thermal stabilizer is developed, such as the introduction of benzene ring in stabilizer molecules.

The molecular weight of organic tin thermal stabilizer was improved 3-3.

To improve the molecular weight of organic tin thermal stabilizer and to form polymerized organic tin thermal stabilizer, it can avoid volatilization of small molecular heat stabilizer and improve stability performance [8]. Rong-bao wei, etc. [9] use of double (B a alkyl carbonyl alkyl) stannous chloride with two acid, phenol, two and two mercaptan interfacial polycondensation reaction, synthesis of a series of organic tin polyester, polyether, and polyethylene sulfide stabilizer. The experimental results show that the stable effect order is organic tin polysulfide > organic tin polyester > organic tin polyether.

3-4 develop new preparation technology.

 Ai-dong tang/l0 aqueous phase method is used to synthesis double formyl yiding oxygen ethyl (B) tin (thioglycolic acid, diisooctyl), measured the thermal decomposition temperature of 232 ~ C, through the theoretical calculation, get the product decomposition reaction apparent activation energy of 307.53 kJ. Mol, ratio of ester base tin chloride as raw materials of decomposition reaction apparent activation energy (1 l 1.24 kJ. Mol. 196.29 kJ. Mol. 1, to illustrate the thermal stability of product performance is better than the ester base tin chloride: Hoch [11], such as bulk polymerization method is presented in this paper, in the patent in benzoyl oxide (BPO), maleic acid was synthesized in the presence of tin a styrene, acrylic acid butanol (DBTM - St - BA) terpolymer, under the condition of the same tin content copolymerization product effect on the thermal stability of PVC and the influence of processing performance is better than that of DBTM, can be used as a multi-functional PVC heat stabilizer.

3.5 develop odorless organic tin products.

Environmental protection plasticizer is plastic additives in the emphasis and direction of the development in the new century, organic tin stabilizer development is becoming more and more people attach importance to environmental protection, if the improvement in technology of synthesis, avoid to generate toxic trimethyl tin compounds, or eliminate the odor of tin products,

Although the production and development of heat stabilizer in China have achieved considerable achievements, there are still many deficiencies and large gaps compared with the advanced level of the world. First, there are few varieties and unreasonable structure; Second, the production scale is small, the product quality is poor.

4 rare earth thermal stabilizer

             The thermal stability mechanism of rare earth is determined by their electronic structure. According to the theory of quantum mechanics, the rare earth ions have many energy levels 4 and 5 d f empty (electronic orbit), they are as a coordination center ion can accept 6 ~ 12 ligand lone electron pair, at the same time they have larger ionic radius, thus may form 6 to 12 keys can complexation of keys. These features make rare earth heat stabilizer in addition to outside and 3-4 HC1 molecules form ionic bonds, possibly several adsorption HC1 molecules to bond can range from complex, effectively decreased as the concentration of the thermal degradation of catalyst HC1, effectively reduces the speed of HCI catalytic degradation of magic and rare earth ions can also with PVC chain unstable chlorine atoms on the complexation, suppresses free PVC drop HC1 reaction.

                 Rare earth heat stabilizer has the advantage of good thermal stability, especially good long-term stability, high transparent, non-toxic, odourless, dispersion, precipitation resistance, plasticizing effect is good, and have light stability; The main drawback is that it has early coloring. In 1971, the Japanese takada unhappy people [l3] first study of the field, and reports the stearic acid lanthanum and cerium rare earth organic weak acid salt, the thermal stability of PVC and points out that they have low toxicity, good lubricity, products high transparency, good light stability etc. France's Louis et al. [14,15] followed closely, exploring the rare earth thermal stabilizer and opening up a new field of PVC thermal stabilizer. However, due to the scarcity of rare earth resources in these countries, their research and application in this field are severely limited. In the early 1980s, China began to study in this field. Baotou plastic institute first developed the solid rare earth thermal stabilizer. Although our country started relatively late in this field, due to abundant rare earth resources in China, it has achieved remarkable results in this field and has studied several systems.

4.1 rare earth stearate.

         On thermal stability of rare earth stearate is similar to calcium stearate, has the characteristics of heat stabilizer for a long time, in addition of rare earth stearate is a kind of wide lubricity, processing AIDS, and concurrently stabilizer effect of non-toxic transparent type PVC heat stabilizer for a long time. Such as Yang of red [l6] to study the single rare earth stearate and rare earth stearate synthesis method, found that rare earth stearate to alkali treatment, not only improve the content of rare earth products, also can extend the application field of rare earth.

4.2 rare earth maleate.

         Maleic acid monoester of rare earth and rare earth stearate, try at the beginning of the thermal aging or mass produce coloring, but have the function of the type of thermal stabilizer for a long time characteristics, and has strong PVC color control capability. Wu maoying et al. [17] studied the properties of single esters of maleate, and found that the thermal stability and transparency of monolate monesters were better than those of stearic acid, while the effect of pressure analysis and spray frost was lower.

4.3 rare earth epoxide.

            Rare earth than epoxy fatty acid rare earth stearate has a better long-term thermal stability, epoxy fatty acid molecules containing epoxy group, rare earth and auxiliary heat stabilizer with epoxy compounds are similar, with auxiliary heat stable effect. Wu Maoying [18] such as studied by the new technology of epoxy soybean oil synthetic epoxy fatty acid rare earth, the epoxy soybean oil in aqueous ethanol saponification reaction with NaOH, then the generated epoxy sodium soap solution with rare earth chloride solution in double decomposition reaction, can be synthesized epoxy group retained intact epoxy fatty acid with high purity rare earth.

4.4 carboxylic ester rare earth.

        Carboxylic acid ester rare earth with polar ester base and longer alkyl, has better compatibility with PVC, thus conducive to the performance of thermal stability. Yue-jian liu [19] to study and compare the carboxylic acid ester, such as rare earth and organic tin on the light aging stability of PVC, found that carboxylic ester rare earth out of HC1 resistance is superior to the organic tin, antioxidant capacity than organic tin, but the compound stabilizers have synergistic effect.

4.5 rare earth of salicylic acid.

          Salicylic acid rare earth is usually referred to as salicylic acid rare earth alkali metals or alkaline earth metal salts. Guang-ye liu etc. [20] with rare earth nitrate and sodium salicylate reaction made salicylic acid, rare earth salt on the thermal stability test showed that salicylic acid rare earth effect on the stability of PVC than commonly used metal soap stabilizer. Lead and cadmium stearate stearate.

4.6 composite rare earth thermal stabilizer.

            Due to the shortcoming of early coloring of rare earth heat stabilizer can be generally, generally cannot be used alone, but with other heat stabilizers and the auxiliary heat stabilizer has good synergy, mixture composite heat stabilizer can be complex. Such as Wu Maoying [21] developed on the basis of the rare earth stearate in rare earth stearate. Zinc stearate a mercaptan dotl composite heat stabilizer RHS 2, is a kind of effective non-toxic transparent PVC heat stabilizer, used to replace soft transparent PVC products in the production of Ba/Cd class toxic heat stabilizer, can improve the product quality, reduce product cost, and expand the application range of products.

         Power is rare earth resources in our country (80% of the world total capacity), is also the production of the first big country, again because of type ore enrichment easier, smelting cost is low, these factors are the development of rare earth heat stabilizer provides a unique advantage. It is of great economic and social significance to comply with the world's increasingly strict environmental requirements, and low lead and low toxic rare earth thermal stabilizers.

5 organic antimony thermal stabilizer.

         The organic antimony thermal stabilizer generally refers to the trivalent mercaptan antimony. The product was industrialized in the late 1970s, and 60% of the PVC water pipes in the United States in 1988 used organic antimony as the thermal stabilizer. In 1986, the Beijing institute of AIDS developed the trivalent sulphide antimony, and in the 1990s, some units, such as zhongnan university of technology, put into production the organic antimony thermal stabilizer. Organic antimony heat stabilizers, resistance to early coloring excellent stability, thermal stability at low dosage is better than that of organotin heat stabilizer, but has the versatility and improve, improve physical performance and machining performance advantage of the products [22].

       Organic antimony heat stabilizers in PVC stability mechanism in addition to being able to fight off HC1, can also molecules by esterification reaction of carboxyl and thiol group and PVC unstable chlorine atoms in molecules, or addition to the instability of the PVC molecules of carbon atoms, or addition to the thermal decomposition of PVC generated on the instability of the molecular structure. In recent years, liu has been waiting.