With the gradual strengthening of environmental awareness, traditio★nal PVC heat stabilizers such as powdered lead salt stabilizers, barium, cadmium stabilizers hav₹e been limited to a certain extent, and a series of low-toxicity and non-toxic he↔at stabilizers have been developed accordingly, mainly including dust-free composite lead salts, complex calcium, zinc, εorganotin, rare earths, organic antimony, hydrotalcite, etc. This papeφr 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 i ndustry, agriculture, packaging, electric power, publσic utilities and other departments, especially in the fields of build₹ing plastics, agricultural plastics, plastic packaging materials, daily ∑plastics and other fields. In 2005, China's PVC resin output was 6.492 million tons, sec→ond only to United States and second in the world.
As we all know, PVC resin and its products due to their own φstructural defects, there are shortcomings of easy thermal degra♠dation and aging, its processing temperature (more than 160"C) is higheδr than the decomposition temperature (120-130~C), so to turn PVC into products, it is necessa ry to add heat stabilizers in the PVC processing an∞d molding process to delay or prevent the thermal degradation of PVC resin. Lea™d salt is the earliest used heat stabilizer, its stabil☆izing effect is strong, and the price is low, but it will cause certain environmental pol♥lution 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 lea€d salt stabilizers. Since 2003, China has also begun to pay aαttention to the problem of lead salt, and Beijing and Shanghai have pa©ssed the decision to ban lead in water supply pipes. In 2004, the announcement of the Minist®ry of Construction clearly pointed out that the PVC-U pipes used in the water supply pipes used nat ionwide must be non-lead salt stabilizers. China's prelu≈de to a comprehensive ban on lead in PVC plastic products has been openγed, and PVC plastic heat stabilizers are developing in the direction ±of low toxicity, non-toxicity, non-pollution, composite and high efficiency. Thi€s 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 sy±nergistic 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 theα characteristics of good thermal stability of lead salt, but also overcom←es the shortcomings of high toxicity of lead salt dust, which is very bene∑ficial to occupational health and environmental protection. In 1985, Wenzhou Tiansheng Group> Plastic Additives General Factory took the lead inδ putting the composite lead salt special for PVC cable materials on the mar±ket, and later developed 301 and 101 composite lead salt hea©t stabilizers. Zheng De et al. [2] in 1994 applied the uniform design εtest 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 stabiliβzer and lubricant after performance testing and structural characterization, with go$od processability, and the thermal stability effect on PVC was better th♦an 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 poo≤r stability, strong colorability, but non-toxic and excellent lubricity. Zinc soapβ 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 aci£d, with the effect of catalytic de-HC1), but has the advantages of excellent initia®l colorability and strong weather resistance. The thermal stabilizer of compound calcium and zλinc is to make use of the synergistic effect of the two, mak&ing it the most active field of compound stabilizer in reΩcent years. The world's leading heat stabilizer manufacturers have launched ★this product. Such as Akcros AkcrosTab CZ series, OM PlastiStab series, Witco Mark serieπs, etc. China has been advancing in this field since the 90s <of the 20th century
The results have been fruitful. Sun Qiping [3] found thr≈ough experimental research that Ca/Zn composite heat stabilizer can not only increase the whiteness of PVC products, but also improve the thermal stability of prod•ucts. Xu Jiayou [4] et al. found through spectroscopic st udies that pentaerythritol and zinc stearate and zinc chloride form a complex, inhibiting the cδatalytic degradation of zinc chloride to polyvinyl chloride<, thereby inhibiting "zinc burning", pentaerythritol ♣plays the role of auxiliary stabilizer in calcium and zinc stab∏ilizers: Wenzhou Tiansheng Plastic Additives Co., Ltd. developed CZ-601 calc¶ium zinc composite stabilizer, through the processing performance, t<hermal stability and health indicators and other aspects of ∑the test, are equivalent to imported similar products, so t≠hat 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 m↑ethyl acrylate, butyl acrylate, and the Y group can be a fatty acid group, or a thi₹ol group (thiol ester group), etc. The properties of different alkyl≤ groups are also different, the thermal stability order is me"thyl> butyl > octyl, and when the alkyl group is the same, the thermal stability order is thiol tin> monoester tin m≥aleate > 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 Ωgroup in the organotin molecule, thus inhibiting t♠he decomposition reaction. The advantages of organotin stabilizers are excellent thermal stability, excellent transparency, good compatibility, gooαd fluidity, non-fouling and non-toxicity, but the disadvantages are poor lubricity and expensiv¶e manufacturing.
Because organotin heat stabilizer is one of the best and ™most promising heat stabilizers for PVC at present, the research on it is ve↑ry active, and there are mainly the following research hotspots.
3.1 Improve and improve the performance of the origi≥nal variety
In studies, it has been found that the introduction of epoxy groups into organotin compounds c•an significantly improve stabilization, and even trialkyltin and tetraalky€ltin derivatives have outstanding thermal stability, and are more effective whe•n 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 mo≤lecules.
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 stabili↔zers and improve stability [8]. Wei Rongbao et al. [9] synthesized a series of or£ganotin polyester, polyether and polysulfide stabil₹izers by interfacial polycondensation reaction witσh diacid, diphenol, diol and dithiol using bis(B-alkyl carbonyl) tin dichloride. Experiments h≠ave 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 methoδd to synthesize bis(B-butoxyformylethyl)tin di(ethylhexyl thioglyctolate), and meas≤ured its thermal decomposition temperature of 232~C, and after theoretical calcu♥lation, the apparent activation energy of the product de☆composition reaction was 307.53kJ.mol, which was higher $than the apparent activation energy of the decomposi≈tion reaction of ester-based tin chloride as a raw material (1l1.24kJ.mol. 196.29 kJ.mol hi↑gh. 1. Illustrate that the thermal stability of the obtaβined product is better than that of ester tin chloride: Hoch e≠t al. [11] proposed in the patent to synthesize a terpolymer of dibutylti™n 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 an¶d processing performance of PVC than DBTM under the sam€e 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 direct→ion of the development of plastic additives in the new century, and the developmen£t of organotin human stabilizers also pays more and more atten≠tion to environmental protection.
Although China has made considerable achievements in the production and development of heat sπtabilizers, there are still many deficiencies and large gaps compared with the world's advancedπ level. First, there are few varieties and the structure is unreasonable; Second, the p≈roduction 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 strεucture. 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 io÷nic 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 moleculeΩs, but also may adsorb several HC1 molecules to form complexes with unequal bond energy, which e★ffectively reduces the concentration of HC1 as a thermal degradation catalyst, there₽by effectively reducing the speed of HCI-catalyzed degrad∑ation 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 stab®ility, especially excellent long-term stability, high≠ transparency, non-toxic, odorless, good dispersion, precip itation resistance, good plasticization effect, and light∑ stability; The main disadvantage is that it has initial colorability. In 1 971, Yukito Takada [l3] of Japan was the first to conduc®t research in this field, and reported that rare earth organic weak salts such as lanthanum stearate and cerium have a thermally stable effect on P&VC, and pointed out that they have significant adv→antages such as low toxicity, good lubricity, high product transparency a≈nd 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. How←ever, due to the lack of rare earth resources in these countries, t∑heir research and application in this field are greatly limited. China began to get in←volved in research in this field in the early 80s, aΩnd Baotou Plastics Research Institute first developed solidα 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 severaσl systems have been studied.
4.1 Rare earth stearate
In terms of thermal stability, rare earth stearate is sim§ilar to calcium stearate and has the characteristics of a long-term heat stabilize€r, in addition, rare earth stearate is a non-toxic transparent long-term PVC heat staλbilizer with lubricity, processing aids and wide stabilizers. For example, Yangδ Zhanhong et al. [L6] studied the synthesis method of rare earth monostearate and rare earth distearate, and found that the alkaline treatment of rare ear✘th stearate not only improves the rare earth content in the product, but also expands the applicat☆ion field of rare earth.
4.2 Maleate monoester rare earth
Monoester maleate rare earth is similar to stearic acid rare ear÷th, and the test piece will produce coloring at the earl→y stage of heat aging, but it has the characteristics of long-term heat sta≥bilizer and has a strong ability to inhibit PVC coloring. Wu Maoying et al.↓ [17] studied the characteristics of monoester male₽ate rare earths and found that the thermal stability and transp§arency of monoester maleate rare earths were better than those of r≈are 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 stabilit¥y than stearate rare earth, and epoxy fatty acid rare ea→rth molecule contains epoxy group, which is similar to σthe combined use of epoxy compound auxiliary heat stabilizer, and has auxiliary thermal πstabilization effect. Wu Maoying et al. [18] studied a new process for the synthesis of e¥poxy fatty acid rare earths from epoxy soybean oil, in which epoxy soybean oil was saponified w♣ith NaOH in ethanol monohydrate solution, and then the resulλting sodium oxide soap solution was metathesis reacted with rare earth chloride solλution, 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 t↔he exertion of thermal stability. Liu Yuejian et al. [19] studied and compared the p£hotoaging and stabilizing effects of carboxylate rare earth and o→rganotin on PVC, and found that the resistance of carboxylate rare earth to HC1ε removal is better than that of organotin, and the oxidation resistance is no↑t as good as that of organotin, but the composite stabilizer of the two has a synerg←istic effect.
4.5 Salicylic acid rare earth
Salicylic acid rare earths generally refer to salicylic aci↓d rare earth alkali metals or alkaline earth metal salts. Liu Guangσye 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 stabilizinαg effect of rare salicylate rare earth on PVC exceeded that of commonly used metal soap sta↑bilizers. 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 t↓hey have a good synergistic effect with other heat stabilizers or auxiliary heat ∏stabilizers, and can be compounded into composite heat stabilizers. For example, Wu γMaoying et al. [21] developed a rare stearate based on rare earth stearate. Zinc stearate-thiol oct♠yltin composite heat stabilizer RHS-2 is a high-efficiency and non-tox÷ic 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 produc€t 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 i★n China. In response to the increasingly stringent environmental protec≥tion requirements around the world, low-lead, low-toxicity rare earth heat stabilizers have very i✔mportant economic and social significance.
5. Organic antimony heat stabilizer
Organic antimony heat stabilizer generally refers to trivalent antimony mercaptan, theπ product was industrialized at the end of the 70s of the 20th century, United States 60% of th&e PVC water pipes in 1988 used organic antimony as a h☆eat stabilizer. In 1986, Beijing Additives Research I←nstitute developed trivalent antimony thiol, and in the 90s, Central South University of Tεechnology 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 stabiliz§ation mechanism of organantimony heat stabilizers in PVC can also make the carbox£yl or thiol groups in the molecule interact with the unstable chlorαine atoms in the PVC molecule, or add to the unstable carbon atoms in the PVC molecule, o$r add to the unstable molecular structure generated by the thermal decom♣position of PVC. In recent years, Liu et al. [23] synthesized antimony tris(eγthyl mercaptorate), antimony penta(ethylhexyl thioglycolate), and antimony ethyl merca☆ptoate carboxylate, respectively, and investigated their thermal stabilizλation effects on PVC, and obtained satisfactory results. Liu Jianping [24€] synthesized a sulfur-containing organantimony