With the gradual strengthening of environmental awareness, traditional PVC heat stabilizers such as powdered lead salt stabilizers, barium, cadmium stabilizers have been limited to a certain extent, and a series of low-toxicity and non-toxic heat stabilizers have been developed accordingly, mainly including dust-free composite lead salts, complex calcium, zinc, organotin, rare earths, organic antimony, hydrotalcite, etc. This paper mainly reviews the development status, characteristics, stabilization mechanism and development direction of several typical low-toxicity, non-toxic, high-efficiency and composite heat stabilizers.
PVC has won a broad market with its excellent flame retardant, insulation, wear resistance and other properties, and is widely used in building materials, light industry, agriculture, packaging, electric power, public utilities and other departments, especially in the fields of building plastics, agricultural plastics, plastic packaging materials, daily plastics and other fields. In 2005, China's PVC resin output was 6.492 million tons, second only to United States and second in the world.
As we all know, PVC resin and its products due to their own structural defects, there are shortcomings of easy thermal degradation and aging, its processing temperature (more than 160"C) is higher than the decomposition temperature (120-130~C), so to turn PVC into products, it is necessary to add heat stabilizers in the PVC processing and molding process to delay or prevent the thermal degradation of PVC resin. Lead salt is the earliest used heat stabilizer, its stabilizing effect is strong, and the price is low, but it will cause certain environmental pollution in the process of production and use. Since 2000, European countries such as Norway, Finland, Sweden, Denmark and United Kingdom have successively taken action to ban lead salt stabilizers. Since 2003, China has also begun to pay attention to the problem of lead salt, and Beijing and Shanghai have passed the decision to ban lead in water supply pipes. In 2004, the announcement of the Ministry of Construction clearly pointed out that the PVC-U pipes used in the water supply pipes used nationwide must be non-lead salt stabilizers. China's prelude to a comprehensive ban on lead in PVC plastic products has been opened, and PVC plastic heat stabilizers are developing in the direction of low toxicity, non-toxicity, non-pollution, composite and high efficiency. This article will mainly review the current development status of PVC low-toxicity heat stabilizers.
1. Dust-free composite lead salt heat stabilizer
Dust-free composite lead salt heat stabilizer is made into granular or flake lead salt composite stabilizer by fully dispersing and mixing various lead salt stabilizers with synergistic effect and internal and external lubricants under heating and mixing conditions[1]. This kind of heat stabilizer (granules, flakes and pre-packaged materials) not only maintains the characteristics of good thermal stability of lead salt, but also overcomes the shortcomings of high toxicity of lead salt dust, which is very beneficial to occupational health and environmental protection. In 1985, Wenzhou Tiansheng Group Plastic Additives General Factory took the lead in putting the composite lead salt special for PVC cable materials on the market, and later developed 301 and 101 composite lead salt heat stabilizers. Zheng De et al. [2] in 1994 applied the uniform design test optimization method and with the help of computer-aided design, obtained a modified multi-functional low-lead dust-free composite thermal stabilizer TS-D with better performance, which was a multi-functional modified compound with both stabilizer and lubricant after performance testing and structural characterization, with good processability, and the thermal stability effect on PVC was better than that of the traditional salt-lead stabilizer, and a good environment for low-dust operation could be achieved. At present, the dust-free composite lead salt heat stabilizer used more in China includes Germany Bear SMS318 and Germany Henkel (Henke1) company STA-BII.DX2840.
2. Compound calcium-zinc heat stabilizer
Calcium soap heat stabilizer is a long-term heat stabilizer, with poor stability, strong colorability, but non-toxic and excellent lubricity. Zinc soap stabilizer has poor stability to PVC, belongs to short-acting heat stabilizer, and is prone to "zinc burning" (mainly the production of zncl, which is a strong Lewis acid, with the effect of catalytic de-HC1), but has the advantages of excellent initial colorability and strong weather resistance. The thermal stabilizer of compound calcium and zinc is to make use of the synergistic effect of the two, making it the most active field of compound stabilizer in recent years. The world's leading heat stabilizer manufacturers have launched this product. Such as Akcros AkcrosTab CZ series, OM PlastiStab series, Witco Mark series, etc. China has been advancing in this field since the 90s of the 20th century
The results have been fruitful. Sun Qiping [3] found through experimental research that Ca/Zn composite heat stabilizer can not only increase the whiteness of PVC products, but also improve the thermal stability of products. Xu Jiayou [4] et al. found through spectroscopic studies that pentaerythritol and zinc stearate and zinc chloride form a complex, inhibiting the catalytic degradation of zinc chloride to polyvinyl chloride, thereby inhibiting "zinc burning", pentaerythritol plays the role of auxiliary stabilizer in calcium and zinc stabilizers: Wenzhou Tiansheng Plastic Additives Co., Ltd. developed CZ-601 calcium zinc composite stabilizer, through the processing performance, thermal stability and health indicators and other aspects of the test, are equivalent to imported similar products, so that China's research in this field has reached a new level.
3. Organotin heat stabilizer
Organotin stabilizers can generally be expressed by XnSnY(4-n) and (n=1—3). Among them, the X group can be an alkyl group, such as methyl, butyl, octyl, or an ester group, such as methyl acrylate, butyl acrylate, and the Y group can be a fatty acid group, or a thiol group (thiol ester group), etc. The properties of different alkyl groups are also different, the thermal stability order is methyl> butyl > octyl, and when the alkyl group is the same, the thermal stability order is thiol tin> monoester tin maleate > tin laurate.
The mechanism of action of organotin stabilizer is to replace the active chlorine atom in the PVC molecule with a group that is not easy to decompose and detach. During the reaction, the chlorine atom on the PVC molecular chain coordinates with the organotin compound, and when HC1 is present, the coordination complex is detached, and the chlorine atom on the PVC chain is replaced by the Y group in the organotin molecule, thus inhibiting the decomposition reaction. The advantages of organotin stabilizers are excellent thermal stability, excellent transparency, good compatibility, good fluidity, non-fouling and non-toxicity, but the disadvantages are poor lubricity and expensive manufacturing.
Because organotin heat stabilizer is one of the best and most promising heat stabilizers for PVC at present, the research on it is very active, and there are mainly the following research hotspots.
3.1 Improve and improve the performance of the original variety
In studies, it has been found that the introduction of epoxy groups into organotin compounds can significantly improve stabilization, and even trialkyltin and tetraalkyltin derivatives have outstanding thermal stability, and are more effective when combined with other stabilizers [6].
3.2 Development of new organotin heat stabilizers
Development of new organotin heat stabilizers, such as the introduction of benzene rings into stabilizer molecules.
3-3 Increase the molecular weight of organotin heat stabilizers
Increasing the molecular weight of organotin heat stabilizers to form polymeric organotin heat stabilizers can avoid the volatilization of small molecule heat stabilizers and improve stability [8]. Wei Rongbao et al. [9] synthesized a series of organotin polyester, polyether and polysulfide stabilizers by interfacial polycondensation reaction with diacid, diphenol, diol and dithiol using bis(B-alkyl carbonyl) tin dichloride. Experiments have proved that their stabilizing effect is in the following order: organotin polysulfide> organotin polyester > organotin polyether.
3-4 Development of new preparation processes
Tang Aidong et al. [l0] used the aqueous phase method to synthesize bis(B-butoxyformylethyl)tin di(ethylhexyl thioglyctolate), and measured its thermal decomposition temperature of 232~C, and after theoretical calculation, the apparent activation energy of the product decomposition reaction was 307.53kJ.mol, which was higher than the apparent activation energy of the decomposition reaction of ester-based tin chloride as a raw material (1l1.24kJ.mol. 196.29 kJ.mol high. 1. Illustrate that the thermal stability of the obtained product is better than that of ester tin chloride: Hoch et al. [11] proposed in the patent to synthesize a terpolymer of dibutyltin maleic acid in the presence of benzoyl oxide (BPO) in the presence of oxidized benzoyl (BPO), and the copolymer product has a better effect on the thermal stability and processing performance of PVC than DBTM under the same tin content, which can be used as a multifunctional PVC heat stabilizer.
3.5 Develop odorless organotin products
Environmental protection plasticizer is the focus and direction of the development of plastic additives in the new century, and the development of organotin human stabilizers also pays more and more attention to environmental protection.
Although China has made considerable achievements in the production and development of heat stabilizers, there are still many deficiencies and large gaps compared with the world's advanced level. First, there are few varieties and the structure is unreasonable; Second, the production scale is small and the product quality is poor.
4. Rare earth heat stabilizer
The mechanism of thermal stability of rare earths is determined by their electronic structure. According to the theory of quantum mechanics, rare earth ions have many 4f and 5d empty electron energy levels (electron orbitals), and they can accept lone electron pairs of 6~12 ligands as coordination center ions, and they have a large ionic radius, so it is possible to form complex bonds with 6-12 bond energies unequal. These characteristics make the rare earth heat stabilizer not only form ionic bonds with 3-4 HC1 molecules, but also may adsorb several HC1 molecules to form complexes with unequal bond energy, which effectively reduces the concentration of HC1 as a thermal degradation catalyst, thereby effectively reducing the speed of HCI-catalyzed degradation reaction, and rare earth ions can also be complexed with unstable chlorine atoms on the PVC chain, inhibiting the degradation and de-HC1 reaction of PVC.
The advantages of rare earth heat stabilizer are good thermal stability, especially excellent long-term stability, high transparency, non-toxic, odorless, good dispersion, precipitation resistance, good plasticization effect, and light stability; The main disadvantage is that it has initial colorability. In 1971, Yukito Takada [l3] of Japan was the first to conduct research in this field, and reported that rare earth organic weak salts such as lanthanum stearate and cerium have a thermally stable effect on PVC, and pointed out that they have significant advantages such as low toxicity, good lubricity, high product transparency and good light stability. Louis et al. [14,15] of France followed suit, and the in-depth research on rare earth heat stabilizers opened up a new field of PVC heat stabilizers. However, due to the lack of rare earth resources in these countries, their research and application in this field are greatly limited. China began to get involved in research in this field in the early 80s, and Baotou Plastics Research Institute first developed solid rare earth heat stabilizers. Although China started late in this field, due to China's abundant rare earth resources, it has achieved remarkable results in this field, and several systems have been studied.
4.1 Rare earth stearate
In terms of thermal stability, rare earth stearate is similar to calcium stearate and has the characteristics of a long-term heat stabilizer, in addition, rare earth stearate is a non-toxic transparent long-term PVC heat stabilizer with lubricity, processing aids and wide stabilizers. For example, Yang Zhanhong et al. [L6] studied the synthesis method of rare earth monostearate and rare earth distearate, and found that the alkaline treatment of rare earth stearate not only improves the rare earth content in the product, but also expands the application field of rare earth.
4.2 Maleate monoester rare earth
Monoester maleate rare earth is similar to stearic acid rare earth, and the test piece will produce coloring at the early stage of heat aging, but it has the characteristics of long-term heat stabilizer and has a strong ability to inhibit PVC coloring. Wu Maoying et al. [17] studied the characteristics of monoester maleate rare earths and found that the thermal stability and transparency of monoester maleate rare earths were better than those of rare stearate rares, while the effects of compression and frosting were lower.
4.3 Epoxy fatty acids rare earths
Epoxy fatty acid rare earth has better long-term thermal stability than stearate rare earth, and epoxy fatty acid rare earth molecule contains epoxy group, which is similar to the combined use of epoxy compound auxiliary heat stabilizer, and has auxiliary thermal stabilization effect. Wu Maoying et al. [18] studied a new process for the synthesis of epoxy fatty acid rare earths from epoxy soybean oil, in which epoxy soybean oil was saponified with NaOH in ethanol monohydrate solution, and then the resulting sodium oxide soap solution was metathesis reacted with rare earth chloride solution, and the high-purity epoxy fatty acid rare earth with intact epoxy group could be synthesized.
4.4 carboxylate rare earths
Carboxylate rare earths have polar ester groups and long alkyl groups, which have good compatibility with PVC, so they are conducive to the exertion of thermal stability. Liu Yuejian et al. [19] studied and compared the photoaging and stabilizing effects of carboxylate rare earth and organotin on PVC, and found that the resistance of carboxylate rare earth to HC1 removal is better than that of organotin, and the oxidation resistance is not as good as that of organotin, but the composite stabilizer of the two has a synergistic effect.
4.5 Salicylic acid rare earth
Salicylic acid rare earths generally refer to salicylic acid rare earth alkali metals or alkaline earth metal salts. Liu Guangye et al. [20] used rare earth nitrate to react with sodium salicylate salt to prepare rare salicylate rare, and the thermal stability test showed that the stabilizing effect of rare salicylate rare earth on PVC exceeded that of commonly used metal soap stabilizers. Lead stearate and cadmium stearate.
4.6 Composite rare earth heat stabilizer
Due to the shortcomings of rare earth heat stabilizers, they generally cannot be used alone, but they have a good synergistic effect with other heat stabilizers or auxiliary heat stabilizers, and can be compounded into composite heat stabilizers. For example, Wu Maoying et al. [21] developed a rare stearate based on rare earth stearate. Zinc stearate-thiol octyltin composite heat stabilizer RHS-2 is a high-efficiency and non-toxic transparent PVC heat stabilizer, which is used to replace Ba/Cd toxic heat stabilizers in the production of soft transparent PVC products, which can not only improve product quality, reduce product costs, but also expand the application range of products.
China is a big country in rare earth resources (accounting for 80% of the world's total reserves), and it is also the largest country in terms of output, and due to the easy enrichment and low smelting cost due to the ore type, these factors provide unique favorable conditions for the development of rare earth heat stabilizers in China. In response to the increasingly stringent environmental protection requirements around the world, low-lead, low-toxicity rare earth heat stabilizers have very important economic and social significance.
5. Organic antimony heat stabilizer
Organic antimony heat stabilizer generally refers to trivalent antimony mercaptan, the product was industrialized at the end of the 70s of the 20th century, United States 60% of the PVC water pipes in 1988 used organic antimony as a heat stabilizer. In 1986, Beijing Additives Research Institute developed trivalent antimony thiol, and in the 90s, Central South University of Technology and other units successively put into production organic antimony heat stabilizers. Organoantimony heat stabilizers have excellent initial coloring resistance, and are better than organotin heat stabilizers at low dosages, and have the advantages of versatility and improving and machinability of products [22].
In addition to resisting the removal of HC1, the stabilization mechanism of organantimony heat stabilizers in PVC can also make the carboxyl or thiol groups in the molecule interact with the unstable chlorine atoms in the PVC molecule, or add to the unstable carbon atoms in the PVC molecule, or add to the unstable molecular structure generated by the thermal decomposition of PVC. In recent years, Liu et al. [23] synthesized antimony tris(ethyl mercaptorate), antimony penta(ethylhexyl thioglycolate), and antimony ethyl mercaptoate carboxylate, respectively, and investigated their thermal stabilization effects on PVC, and obtained satisfactory results. Liu Jianping [24] synthesized a sulfur-containing organantimony