With the gradual strengthening ↔±of environmental awareness, tradi≤₽tional PVC heat stabilizers such as powdered le↔•ad salt stabilizers, barium, ↔©§®cadmium stabilizers have been liδδφmited to a certain extent, and a series oΩ≤f low-toxicity and non-toxic ★ αheat stabilizers have been developed γ accordingly, mainly includinσ£₹→g dust-free composite lead sal©♣ts, complex calcium, zinc, organotin, rε∞εare earths, organic antimony, hydrotalcite, etc. ε✘×≈This paper mainly reviews ÷δ₹the development status, characteristics, sta★♣bilization mechanism and development directi≤♠♣¥on of several typical low-toxici&™∞πty, non-toxic, high-efficiency and comp>osite heat stabilizers.
PVC has won a broad market wi☆↓th its excellent flame retardant, ™₹£ insulation, wear resistance and other pΩ ™™roperties, and is widely used ♣φ∏in building materials, light indust✔δδ★ry, agriculture, packaging, electric power€•β, public utilities and oth✔→<©er departments, especially in the fields ασ♥☆of building plastics, agricultura£l plastics, plastic packaging materials, d✔↕'aily plastics and other fields. In 200β←"5, China's PVC resin output was 6.492 milli✘≥↕on tons, second only to Uni✘♣ted States and second in the w≈©orld.
As we all know, PVC resin and iε×ts products due to their own structural defeγ≠≥cts, there are shortcoming×≈s of easy thermal degradation and aging, •§<its processing temperature (more than 160"C)®φ is higher than the decomposi★δtion temperature (120-130~C), so to ®>turn PVC into products, it isεγΩ necessary to add heat stabilizers φ≈¶in the PVC processing and m≤↔→olding process to delay or prevent the the×♦rmal degradation of PVC resin.™★σ Lead salt is the earliest used hea→∞♦§t stabilizer, its stabilizing effect is st≈Ω↔rong, and the price is low, but it will caλ™±✘use certain environmental poll>>≠ution in the process of production and us©•e. Since 2000, European countries such a✔↕s Norway, Finland, Sweden, Denm☆×₽™ark and United Kingdom have successively ta¥÷αφken action to ban lead salt stabilizers. ♠★Since 2003, China has also begun to ∑pay attention to the problem of l✔ε★ead salt, and Beijing and Shangha★i have passed the decision to ban lead in water s ↑upply pipes. In 2004, the annoπφ€uncement of the Ministry of Construction cl¶¥early pointed out that the P¥↑♦VC-U pipes used in the water supply •βpipes used nationwide must be non-lead ₹∞&salt stabilizers. China's©♥× prelude to a comprehensive ban on lead in PVC p₹× lastic products has been opened, and §'™PVC plastic heat stabilizers are develop™♣ing in the direction of low toxicity, n±on-toxicity, non-pollution, compos☆γ πite and high efficiency. This article willλ★ mainly review the current development statusλ↑€α of PVC low-toxicity heat stabilize ✘±≈rs.
1. Dust-free composite lead ↔×salt heat stabilizer
Dust-free composite lead salt heat stabilizer is βγmade into granular or flake lead salt §∑composite stabilizer by fully dispersing and↕↔ mixing various lead salt stabiliz€♦☆ers with synergistic effect and intσ©≥ernal and external lubricants under heating anσγ→↑d mixing conditions[1]. This ←£★kind of heat stabilizer (gr≈✘'•anules, flakes and pre-packaged↑™ materials) not only maintains the characteriφγstics of good thermal stability ₹>✘•of lead salt, but also overcomes the sho±★rtcomings of high toxicity of lead s∏≈₽¥alt dust, which is very beneficial to oεccupational health and environmenta©Ω €l protection. In 1985, Wenzhou ₽∞<Tiansheng Group Plastic Additives Genera✘£÷<l Factory took the lead in putting the γ÷λcomposite lead salt special→₹↑© for PVC cable materials on theγ≈×∑ market, and later developed 301 ∏≤πand 101 composite lead salt heat stabil¶ε✔✘izers. Zheng De et al. [2] in 1994 applied the↕§♣€ uniform design test optimization method and with<π↑δ the help of computer-aided Ωαdesign, obtained a modified m↑σulti-functional low-lead dust-free com€∑posite thermal stabilizer TS-D with better perfo≈δ♠πrmance, which was a multi-functio ©nal modified compound with both stabilizer a♠♣ nd lubricant after performance testi↔₽δ↕ng and structural characterization, with g♠Ωood processability, and the thermal stabilit™"£αy effect on PVC was better than that of ' the traditional salt-lead stabilizer, and a ★ good environment for low-dust operatio∑&÷n could be achieved. At present, t ♦•"he dust-free composite lead salt heat stabiliz∏✘ ₽er used more in China includes ©<₹✘Germany Bear SMS318 and Germa"♠φγny Henkel (Henke1) company STA≥ε•-BII.DX2840.
2. Compound calcium-zinc heaσ∏©t stabilizer
Calcium soap heat stabilizer is a long-term hea♦≤♥♣t stabilizer, with poor stabili↓←₽σty, strong colorability, but noβ±n-toxic and excellent lubricity.₽☆÷ Zinc soap stabilizer has poor stability to P≥'∞VC, belongs to short-acting heat s®♣₩ tabilizer, and is prone to "zinc burning&quo÷☆§t; (mainly the production of z♦←ncl, which is a strong Lewis acid, with the <™♣effect of catalytic de-HC1), but has the advantag¥↕es of excellent initial color•¶®ability and strong weather r××₽esistance. The thermal stabilizer of comΩα pound calcium and zinc is to mε↓πake use of the synergistic ±$effect of the two, making it the most acti≤₽ve field of compound stabili ×±zer in recent years. The world's leading >σ€₹heat stabilizer manufacturers have lauδ↔£ nched this product. Such as Akcros AkcrosTab CZσ☆™ series, OM PlastiStab series, Witco₩©© Mark series, etc. China has been advancing ₩π★>in this field since the 90s of th£♣±★e 20th century
The results have been fruitfu×l. Sun Qiping [3] found through $↑"experimental research that Ca≠↕≠♦/Zn composite heat stabilizer can n✘<β✘ot only increase the whiteness o☆∑←↓f PVC products, but also improve the♦ ≠☆ thermal stability of products. Xu Jiayo™γ¶u [4] et al. found through spectroscopic stud×αβ←ies that pentaerythritol and zinc∞¥ stearate and zinc chloride form a c'₩ omplex, inhibiting the catalytic d<≥β↔egradation of zinc chloride to polyvinyl chloridπ"★€e, thereby inhibiting "zinc burninβΩg", pentaerythritol plays the role o→ ♠f auxiliary stabilizer in calcium an $ d zinc stabilizers: Wenzhou Tianshe©₹ng Plastic Additives Co., Ltd. devel'γoped CZ-601 calcium zinc composi® ×←te stabilizer, through the processing performa↑↓πnce, thermal stability and health indicatorsφ≠ and other aspects of the test, are equivalen׶ ★t to imported similar products, s ÷Ωo that China's research in this field ha↑δ s reached a new level.
3. Organotin heat stabilizer
Organotin stabilizers can generally be expr•<₹essed by XnSnY(4-n) and (n=☆±>≠1—3). Among them, the X g←∏roup can be an alkyl group, such as m☆≠£∏ethyl, butyl, octyl, or an ester ♦group, such as methyl acrylate, butyl acrylate, a &nd the Y group can be a fat♠€ty acid group, or a thiol £ε₽εgroup (thiol ester group), etc₹€. The properties of different>←•φ alkyl groups are also differe$€nt, the thermal stability order is m₽±ethyl> butyl > octyl, an₩♣∏☆d when the alkyl group is the same,≥☆ the thermal stability order is α¶ thiol tin> monoester tin male ♦ate > tin laurate.
The mechanism of action of organ★£φotin stabilizer is to repl→✔λ€ace the active chlorine atom in the PVC moleculeα£ with a group that is not easy to deco♣¶×mpose and detach. During the €→≠reaction, the chlorine atom on the PVC molec< ular chain coordinates wit∏•÷h the organotin compound, 'and when HC1 is present, the coordi★"nation complex is detached, and the ®∏chlorine atom on the PVC chain is replaced bβγy the Y group in the organ∑™&∏otin molecule, thus inhibiting the decompos ∑←ition reaction. The advantages of organot★¥₩ in stabilizers are excellent therm€₹al stability, excellent transparency, g≥'♠λood compatibility, good fluidity, ₹♠non-fouling and non-toxicity, but the₹★ disadvantages are poor lubricity and ex<÷÷pensive manufacturing.
Because organotin heat stabilizer is one of₹↔ the best and most promising heat stabiliα¥×★zers for PVC at present, the resear÷π<ch on it is very active, anπ₽d there are mainly the following research hotspot ↓s.
3.1 Improve and improve the performancλ✘€e of the original variety
In studies, it has been found that the introdu₽¶ ↕ction of epoxy groups into organ₩↓otin compounds can significantly improve stabi>©☆★lization, and even trialkylt♥↔in and tetraalkyltin deriva®≥↔ tives have outstanding thermal≈☆ stability, and are more effective wh ≥en combined with other stabilizers [6].
3.2 Development of new organotin heat sta£ε↓&bilizers
Development of new organotin h←©eat stabilizers, such as the introducti¥ ←on of benzene rings into stabilizeπ♣r molecules.
3-3 Increase the molecular weight of or®<★ganotin heat stabilizers
Increasing the molecular weight ofΩ∞ ✔ organotin heat stabilizers to form ☆ <>polymeric organotin heat stabilize'®←rs can avoid the volatilization of smaα✔ll molecule heat stabilizers and improve staβγbility [8]. Wei Rongbao et al. [9] synthesi✔zed a series of organotin polyester, poly©←φether and polysulfide stabilizers by≥€γ✘ interfacial polycondensation reacti¶<δ∏on with diacid, diphenol, diol and ≠₹≤dithiol using bis(B-alkyl carbonyl) ti<≈>αn dichloride. Experiments have proved that th™₽≈eir stabilizing effect is ₩♠εin the following order: org€∑λanotin polysulfide> organotin•← polyester > organotin pol¶€αλyether.
3-4 Development of new prepa∏€₩ration processes
Tang Aidong et al. [l0] used the aqueous ph←≠♠↑ase method to synthesize bis(B-butoxyformσπ¥ylethyl)tin di(ethylhexyl thioglyctolaπ te), and measured its thermal decompositionδ$× temperature of 232~C, and after theoret¥πβαical calculation, the apparent activat←•ion energy of the product decomposition reaΩ×ction was 307.53kJ.mol, which €∞♥≈was higher than the apparent ♥ ↓activation energy of the decomposition rea₹σσ&ction of ester-based tin chloride as a raw materΩ÷πial (1l1.24kJ.mol. 196.29 kJ.mol high. 1. Il® lustrate that the thermal stabilit↑→y of the obtained product i♣λ∞γs better than that of ester tin chlo÷'β&ride: Hoch et al. [11] proposed in the patent"φ≠ to synthesize a terpolymer of dibutyltin malei¥β÷Ωc acid in the presence of ₹&©×benzoyl oxide (BPO) in the presence of oxid♠βized benzoyl (BPO), and the copolyπγmer product has a better eff©₩ ≠ect on the thermal stability and p×♥rocessing performance of PVC thδan DBTM under the same tin content, ★←which can be used as a multifunctional PVC heat sπ•β€tabilizer.
3.5 Develop odorless organotin products
Environmental protection plasticizer is the ÷±focus and direction of the devel£"opment of plastic additives in the new cen÷♦≈✔tury, and the development of organotinλ☆• human stabilizers also pays more and more attent≤±×≈ion to environmental protection.
Although China has made considera£'★γble achievements in the production a§≈πnd development of heat stabilizers, there are s₩₹till many deficiencies and large gaps compa±→red with the world's advanced levεel. First, there are few varieties and the struc§ε ture is unreasonable; Second, th<☆×€e production scale is small¥✔∑ and the product quality is poo≈₹r.
4. Rare earth heat stabili∑¥zer
The mechanism of thermal sta✘$bility of rare earths is determined by the★₽ir electronic structure. According to the♥ ® theory of quantum mechanics, rare ea÷π¥♦rth ions have many 4f and 5d empty ✔£±electron energy levels (electron orbitaΩ♣₽®ls), and they can accept lone ÷δ&>electron pairs of 6~12 ligand♦≠☆s as coordination center ions, σ₩©and they have a large ionic radius, ≥ so it is possible to form compl₽↔§ex bonds with 6-12 bond energies unequ®¶"÷al. These characteristics make ε₹↓the rare earth heat stabilizer not onlφ$¶y form ionic bonds with 3-4 HC1 mo× 'lecules, but also may adsorb•λ₽ several HC1 molecules to form complexes with ✔♦≈>unequal bond energy, which ≤≥σeffectively reduces the concentratio♥™≤n of HC1 as a thermal degradation ca∞♣ talyst, thereby effectivelyφ♣×" reducing the speed of HCI-catalyzed☆₹ degradation reaction, and rare earth ion₹λλs can also be complexed with unstable chlorine÷←$ atoms on the PVC chain, in≥'♦hibiting the degradation and de-∑ ± HC1 reaction of PVC.
The advantages of rare earth heat stabili™↔©→zer are good thermal stability, especially ex<→cellent long-term stability, high tr✔←¥γansparency, non-toxic, odorless, good d©£ispersion, precipitation resistance, good plasti×€cization effect, and light stability; The ma ®Ω"in disadvantage is that it has initia•&l colorability. In 1971, Yukito Takada [l3]↓ ≤φ of Japan was the first to conduct research i<&n this field, and reported ™↓that rare earth organic weak salts such →§©as lanthanum stearate and cerium have a t¥λ★hermally stable effect on PV↕ ♣C, and pointed out that they →Ωhave significant advantages such as low toxicit'£↑y, good lubricity, high productβ§× transparency and good light staφ≥≤bility. Louis et al. [14,1 &'5] of France followed suit, and th↓✔e in-depth research on rare earth₽©♦ heat stabilizers opened upε↔ε a new field of PVC heat stabili↓✘≥zers. However, due to the lack of rare earth res✘≥×ources in these countries, their• ≥☆ research and application in this≤↑↕ field are greatly limited. Chi φna began to get involved in research '∑≠πin this field in the early 80s, and Bao¶∑tou Plastics Research Instit₩✘δute first developed solid rare ear™≥☆λth heat stabilizers. Although China st÷™δ§arted late in this field, due to China& ₩"#39;s abundant rare earth r$esources, it has achieved ↕™☆remarkable results in this field, and several sy¥ ♦stems have been studied.
4.1 Rare earth stearate
In terms of thermal stability, rare earth stea®$rate is similar to calcium stearate and∞→<↕ has the characteristics of a long-term heat ↕γ♣stabilizer, in addition, rare≠• earth stearate is a non-toxic transparen$ ±t long-term PVC heat stabilizer witΩ¶←h lubricity, processing aids ₹↑↔and wide stabilizers. For example, Yang Zhanhong δ≤et al. [L6] studied the synthesiα&♣Ωs method of rare earth monostearate a•₩nd rare earth distearate, and ≈☆found that the alkaline trea∞♦↕tment of rare earth stearate not only improves ©₹&the rare earth content in the product, but "also expands the applicatio✔♥★n field of rare earth.
4.2 Maleate monoester rare earth
Monoester maleate rare earth is ★ε↔similar to stearic acid rare earth, and the §↑αtest piece will produce coloring®♦ at the early stage of heat aging, but i♥ €↕t has the characteristics of lon↔ •g-term heat stabilizer and h$φ≥as a strong ability to inhδibit PVC coloring. Wu Maoying et al. [17] s♦λtudied the characteristics of monoes←∏☆ter maleate rare earths and found tha★♣♥t the thermal stability and transparβ∏ency of monoester maleate rΩ★are earths were better than those of rare∞€© stearate rares, while the effect↕"s of compression and frosting were ≈✔ lower.
4.3 Epoxy fatty acids rare e≠•arths
Epoxy fatty acid rare earth has bett₩≥er long-term thermal stability thα ✔an stearate rare earth, and ep ✔oxy fatty acid rare earth molecuπλle contains epoxy group, which is similar to the®βε combined use of epoxy compound auxilia♣Ω♣€ry heat stabilizer, and has auxili₹€₩✔ary thermal stabilization e∏₹₽αffect. Wu Maoying et al. [18] studied a ≥♦new process for the synthesis of epoxy fatty ac←₽↔id rare earths from epoxy soybean oγ"γil, in which epoxy soybean oil wβ¥εγas saponified with NaOH in ethanol monohydrate ≠♠✔solution, and then the resulting sodi×£ um oxide soap solution was metathesσ≈¥is reacted with rare earth chloride sσ"olution, and the high-purity epoxy f☆↕€atty acid rare earth with €§εintact epoxy group could be >↓♣↕synthesized.
4.4 carboxylate rare earths
Carboxylate rare earths have &§>polar ester groups and long alkyl grΩ☆∑₹oups, 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 stπ≠↓×abilizing effects of carboxylate rare eart± ✔h and organotin on PVC, and fou•↔εnd that the resistance of carboxylate rare €♣γ€earth to HC1 removal is better than that>↕↔Ω of organotin, and the oxidation≠λ'→ resistance is not as goodδ★ as that of organotin, but th ↔e composite stabilizer of the two has a synergi®∑stic effect.
4.5 Salicylic acid rare earth
Salicylic acid rare earths general↕☆₽ly refer to salicylic acid rare earth alk®×ali metals or alkaline earth metal salts. Liu Guaφ€ββngye et al. [20] used rare eβ¶arth nitrate to react with sodium salicyφΩlate salt to prepare rare '±×∏salicylate rare, and the thermal stab☆ ♠ ility test showed that the stabilizing effect↓♦≠π of rare salicylate rare earth on PVC exc§≥₹eeded that of commonly used metal₽< soap stabilizers. Lead stearate an∏<d cadmium stearate.
4.6 Composite rare earth heat stabilizer
Due to the shortcomings of rare earth heat stabi∞'σ≥lizers, they generally cannot be used alone, but ✔←they have a good synergistic effect☆ ≤ with other heat stabilizers or auxiliary hπ₹₹αeat stabilizers, and can be compoundeδ<d into composite heat stabilizers. For σ€example, Wu Maoying et al. [21≈♥☆] developed a rare stearate based on rare earth s₩♥®tearate. Zinc stearate-thiol octyltin↔λ composite heat stabilizer RHS-2 is a high-effic≈λ✔iency and non-toxic transparent PV ♣$™C heat stabilizer, which is used to rπσeplace Ba/Cd toxic heat stabilizers in the pr✘≥₩oduction of soft transparent PVC products, wh♠ich can not only improve product qualiσty, reduce product costs, but al↓λ∑σso expand the application range of produδ ≠cts.
China is a big country in rare earth resoΩ÷urces (accounting for 80% of the world'∏•§;s total reserves), and it is also the l§₹®→argest country in terms of output, and €>↓due to the easy enrichment and low ±≠↔γsmelting cost due to the ore type, these factors provide unique favorable conditions for th™₩¶Ωe development of rare earth Ω↓↕↑heat stabilizers in China. In✔↓ response to the increasingly ♦♣✔stringent environmental protection r←♣γ£equirements around the world, low-lead, low-t↔£★☆oxicity rare earth heat stabilizers have very i&♦♣δmportant economic and social signific≠₩♥ance.
5. Organic antimony heat stab<∞ilizer
Organic antimony heat stabilizer σ£₹πgenerally refers to trivalent γ<antimony mercaptan, the product was industriali$γ ₹zed at the end of the 70s of the 20th century, δ United States 60% of the PVC water pip₹>εes in 1988 used organic antimony as a heat sta∏δεbilizer. In 1986, Beijing Additives§∏♥ Research Institute develope→•εd trivalent antimony thiol, and in the 90→∏₽↓s, Central South Universitλ↕₹y of Technology and other units su♥σ€ccessively put into production organ←♣ic antimony heat stabilizers. Organoantimony♥≤ heat stabilizers have excellent initia¶ l coloring resistance, and are b&<etter than organotin heat stabilizers at low dosaε♦πges, and have the advantages of versatilφ"ity and improving and machinability of produc₩<ts [22].
In addition to resisting the removal of HC1,↓< the stabilization mechanism of'δ£" organantimony heat stabilizers in PVC can alsφ§≈o make the carboxyl or thiol group€≤£s in the molecule interact ↑β≠with the unstable chlorine aδ✘✔≈toms in the PVC molecule, or add to the u₩∑nstable carbon atoms in the PVC molecule,♦π or add to the unstable mo&→δlecular structure generated by the thermal d♦★ecomposition of PVC. In recent years, Liu e↕₹t al. [23] synthesized antimony tris(ethyl m&∞≥ercaptorate), antimony penta€Ω(ethylhexyl thioglycolate), and antim ××ony ethyl mercaptoate carboxylate, resp$₹≥≤ectively, and investigated their thermal stabiβ✘lization effects on PVC, and obtained satisfacto®σ× ry results. Liu Jianping [24] synthesized a su ↕lfur-containing organantimony