Polyvinyl chloride (PVC) is one∏¥↓ of the five major general∑↔←→-purpose plastics, and its annua÷¶¶l sales volume in the world ranks second only tφδ₩o PE among thermoplastics. PVC has excellent corrλ≠βosion resistance and high mε ×echanical properties, and low price$♦, abundant resources, mature manufac↓™&₩turing process, so its produ↕>cts are widely used in various fields of induγ¶strial and agricultural production. However, PV≥↑C will be degraded due to thermal•€β and mechanical shear in p₩•✔>rocessing, and there are shor¶≠β tcomings such as poor thermal stability a∑β↕nd easy decomposition, so heat stabil↔ >izers must be used during γ★>processing to prevent or delay decompositiΩ≥$₩on. Commonly used heat stabilizers include lead sα₩₽alts, metal soaps, organotins, etc. Altho♣™ ±ugh lead salt stabilizers have excellent t§↕hermal stability, they are highly toxic and hav"₩↓e great harm to human health. Metal sΩ£→§oaps are usually used togethe•≈r, which are easy to precipitate during ✔ processing, and have low stability <efficiency. Organotin stabilπizers can maintain a high lev←Ωβδel of transparency in transparent <λ™↔products and are used in smaller quantities, bu↓€t they are expensive. So these stabilizers h≤δave been phased out or restricted. At presen→σt, the development and development of low/no→✘£♥n-toxic, high-efficiency, low-co ←"εst environment-friendly stabilizers has beco₹ε₽me an urgent issue in the field of PV> €™C processing.
At present, the new thermal stabilize→>≤≈rs of PVC at home and abroad m>♣♦ainly include calcium/zinc composite heat s↔π✔σtabilizers and rare earth heat stabilizers.←∑> Calcium/zinc stabilizers are the most active ♥£R&D field of composite st ←±↑abilizers in recent years, and✔¶ such heat stabilizers have been develope↕©✘±d abroad that can be used for soft and har₩<≤d products. At present, most of the calcium/z•₩inc stabilizers produced in China are inσ™× liquid form [1], which are only used for soft pr'¶¥oducts, and the research and devel×≥opment of hard products has just s↕λ↑tarted [2,3]. Rare earth compos<∑ite stabilizer is a new type of thermal sta€₹bilization system after the thermal stabiliφ£λ<zation system of PVC processin•₹'₽g such as lead, metal soap, orγ≥ganotin, etc., which was first developed by ♠China in the 80s of the 20tφ&£h century. Rare earth composite stabilizer has t←★®he characteristics of non-toxic, high efficiency ∏εand good comprehensive perfo♥∏rmance, and is suitable for β•all kinds of PVC products. Ch£±©ina is rich in rare earth resourσ φces, accounting for 8O of th↕÷&e world's total reserves
Therefore, making full use ©of resource advantages and vigoro₹→ usly developing and applying rare earth heat•& Ω stabilizers has broad prospects.↑☆ In this paper, the synthesis, thermal stabili©'ty, processing properties and mechanical proper<™ties of PVC rare earth heat $→¶stabilizers are reviewed, ≤★✘hoping to promote the research, develoσβpment and application of PVC rare earth h> eat stabilizers in universities, research σ institutes and enterprises.
1. Thermal stabilization mechanism of rare β®earth heat stabilizers
1.1 Thermal degradation mechanism ≈♥of PVC
The degradation mechanisms of§∏α♣ PVC mainly include free radical mec∑®§×hanism, ionic mechanism and unimolecular mech↔≠anism. Among them, the free radical mechanism is≠≥✘ the most common, which has become the theα₩β≥oretical basis for the study of stabili$±↕₹zers. The degradation of PVC is mainly related ≈≠≥to its molecular structure, ultraviolet light,"¶£" mechanical force, oxygen, ozone§↕, hydrogen chloride and active metal←< ions will accelerate the decompositi©€on of PVC. The thermal degradation of PVC often ✔ undergoes several chemical rλβδ±eactions at the same time, am♦÷ong which the decomposition and reδ≠βmoval of hydrogen chloride is theש∏φ main cause of PVC decomposiβ₩ tion. Under the initiation of the formed free rad₹•→icals, PVC undergoes de-HC1 degΩ★₹radation according to the chain mechani≤Ω♦sm, so that a certain number of conjugΩ ↑ated double bond structures are formed, whi≥π∑±ch changes the color of the pol<>$ymer until complex reaction∞×s such as cross-linking and oxidation πΩ™δoccur, resulting in material breakage and destr¥β±uction.
1.2 Thermal stabilization mechanism of r↑←≠are earth heat stabilizers
It is generally believed that rare e'∏arth heat stabilizers have excellent ther<♦mal stability, which is caused by the special"≈&♣ structure of rare earth elements. R↕↔&are earth metal ions have moΩ$ 're orbitals (6S4F5D6P) that c♥'↕an be used as lone pairs of central←∑®¥ ions to accept ligands, and at the s•↕ame time, rare earth metal i"β ons have a large ionic radius, whichπ™↓♣ can form ionic ligands with inorganic o✘←εr organic ligands through electrostatic attract≈↔>ion, so as to form stable complexes under the a±£ction of external light, heat or other compoundφ'♦s. According to the "soft and h€♦®ard acid-base principle" proposed by Pes☆φβa-son [4], hard alkali chloride ions a♠>nd strong acid rare earth metal ions ↕ are easy to form stable complexes. Th∏γ↕erefore, rare earth metal Ω☆ions (RE) has a strong coordinat€♥™γion complexation ability with× the chlorine atom (C11) on the PVπ ♣↔C chain, and [C1CHC1-]- and Cl- are no longe✘♥r involved in the catalytic ↔δ€reaction of HC1 decomposition, so theyβσ$ play a certain stabilizing role in PVC. The staΩbilization mechanism can bγ₩±e expressed in a chemical model as:
2. Synthesis of rare earth heat stabilizers
Rare earth heat stabilizers are generally div★α₹→ided into rare earth organi✔•✔φc compounds and inorganic compoun↓ λ d stabilizers. At present, rare earth organ↑♣×ic stabilizers are mainly used in PVC ✘♣§<processing, while rare earth inorganic stab• ♦ilizers are rarely studied [5].
Taking rare earth stearate as an example, the §λ↓common preparation method of rare earth heat staγ₹<bilizer usually adopts the traditional metathesis✔≥ method [6], and the reaction is carried©₹ out in two steps, such as equatio₩₹ε≠ns (1) and (2). The metathesis method rea✔™γαcts in an aqueous medium, due to the lar φge gelatinization energy of sodium'☆ soap, small solubility, and viscous solution≤α§☆, it is easy to combine with stearic acid to fo≈§rm acidic soap, and it is also Ω→easy to be salted out. To obtain highly÷Ω↓ pure rare earth salts of ste>$&↔arate, the reaction must be carried '>¶×out slowly under dilution con∑↓ditions. In production, there are Ω←>shortcomings such as large reaction vesse♦≠↑ls, low production efficiency, and high eneα₽✘rgy and water consumption. Therefore©↔ α, some researchers continue to improv ∞ e the metathesis method, and there®☆♥ are successively saponificati &"©on method, improved metathe€≤>↕sis method and one-step method.
Cai Weilong et al. [8] introduced the pr§βeparation process of improved meta♥¶♦thesis method and one-step method, and comp₹≈±ared the two processes and₹α products. The improved metathesis m≤α↓ethod is to add a pre-made rare ea<♦✔•rth nitrate solution to the stearate •→' ethanol solution, and then drop into t∞φ♦he sodium hydroxide soluti≈↓↔←on to react to prepare the rare earth ste<™€arate. The main advantages of tΩ☆his method are the acid-base neutralizat¥↔×♦ion reaction of rare earth hydro≥™€xide and stearic acid, the reaction speed is δ♦¶←fast, the product purity is high, and a lar♥¶→ge amount of solvent is not required, whic₹€h effectively overcomes the shortcomings of th™∑e traditional metathesis m↕×ethod. The reaction principle is as follow✘₹s in equations (5) and (6)★ ™∞.
The one-step method uses hydrogen peroxide§• as a catalyst, absolute ethanol as a sol₹♠÷vent, and stearic acid and lant∞"↓hanum oxide to obtain a stearate rar¶♥®&e earth product through a on'♠e-step reaction. The advantagesγ"♠≈ of this method are that the ∑↑∞☆reaction is completed in one step, the process i≤☆δ♥s simple and easy to operate, and th€€¥e "three wastes" and en>¶ergy consumption are small. The prinα ciple of the reaction is as sho★₽wn in equation (7).
Cai Weilong et al. also investigated the thermγ≤βal stabilization effect of rare earth✘φ stearate stabilizer prepared b←'y two methods on PVC, and the expe→≥☆rimental results showed that the thermal stabil₽★★ization effect of rare earth stear∑✔±¥ate prepared by one-step method and rare ea≤γrth stearate prepared by improve×α✘→d method was comparable to that of PVC, which₽€ was slightly better than that of traditionalφ• metathesis products, and the eff∏↓♦ect was significantly better than that ♣→ of zinc stearate and calcium stear§↑∏≥ate of metal soaps, and was ≤✔close to the thermal stabilization effect of ©☆↓organotin.
3. Performance of rare ear∏'th heat stabilizers
3.1 Thermal stability
Wu Maoying et al. [9] synthesized Rεδ'H
1. Rare earth stearate, which has the role of bot"☆h heat stabilizer and processing aid. The the'÷rmal stability of rare earth s✘ ↔←tearate is not as good as that of thiol octyltin φ∑↔17MOK, but similar to calc÷£ ₹ium stearate, which has the characteris♣εtics of a long-term heat staλ>®≥bilizer, but in terms of transparency, ∑∏∑rare earth stearate is close to 17MOK and sign™£₽ificantly better than calcium stearate. I&α ©n addition, rare earth stea≤♠σrate is used as a processing aid, and ¶®the plasticizing flow efficiency is more t☆©han twice that of ACR-201 (acr"±↑ylic acid copolymer).
Yang Zhanhong et al. [10] cφ∏'arried out alkaline treatmen§≥t of rare earth stearate acc₩→↕♥ording to the concept that increasing the m←☆etal content of metal soap₽÷§ε can increase thermal stability, an•φd synthesized two products: basic rare £₽✘earth monostearate (or rar∞♠✔e earth monostearate) and basic rare earth ™↑<distearate (or rare earth distearate), and the r$₽ φare earth content in the prβ $↓oducts was increased, reaching 31.01% and 19σ↕©φ.53%。 Zeng Dongming et al. [11] a↑₩♥βlso prepared rare earths of stearate, citr¶'ate, laurate and malate according to this p≤Ωrinciple, and studied the thermal stability ''of each rare earth product, and found th&≈↑↓at the thermal stability of ra¥↔Ωre earth malate was the best↑↕, and its long-term thermal stability was comp₹≥β←arable to that of organotin. The study also↔♥©≤ found that the compound of rare earth mala✔≥≠te and zinc stearate has a good synergistic≠Ω effect, and the sulfite-rare-earth-γλzinc composite system also ha↓$♣s a good synergistic effect.
Wu Maoying et al. [l2] found that monoester ma©>leate rare earth is similar to stearate r≠✘®are earth, and also has the funct≥§≠ion characteristics of long-§££term heat stabilizer, but monoester malσ≥eate rare earth has a strong abπ☆÷ility to inhibit PVC coloring, and £&the transparency of monoester maleate r✔×ε≠are earth is better than that of stearate rar§∏γe earth, which is similar to thiol octyltin 17MOK÷←$< Very close.
In particular, it should be pointed out that the♣₽< composite heat stabilizer composed o↔✔±&f monoester maleate rare earth has a w♣≤♥ ide range of applications and high cost α↓∞♦performance, which is not only suita'∑ble for soft products, but also can be used for→± the processing of semi-rigid PVC prod±₽→≤ucts.
Liu Jianping[l3]
The study also shows that monoester maleate r♣™'Ωare earth has a good therm≈₩αal stability effect on PVC, and thΩ₹e thermal stability improves and gradualΩ♠☆ly stabilizes with the increase of the ↔σ∏♥amount added. In addition, in ®£↕terms of impact performance and tensile propπφ∏>erties of PVC, the use of monoest ∞★er maleate rare earth is slightly h★'₹φigher than that of stearate©↔★> rare earth.
Wu Maoying et al. [14] developed ∞®a high-purity epoxy fatty acid rare earth§α with a well-preserved epoxy group. Similar to s¥<πtearate rare earth, epoxy fatty acid ra£αre earth has the thermal stability€Ω☆≈ characteristics of long-term heat stabi≠lizer, but its stable spec↔≠εimen is lighter in the later stage of heating, ₹∏>←indicating that epoxy fatty acid♣± rare earth has better lon ★g-term thermal stability. In a↓γ± ddition, the long-term ther<♦mal stability of epoxy fatty acid rare earths i©♥Ω s complementary to the outstanding ini₽♦™$tial thermal stability of tε♣hiol organotins, and there ∞ 'is a synergistic effect between th↓♦φ>e two.
Liu Guangye et al. [15] studied the stabili<→₩zing effect of rare salicylate throו∑γugh thermogravimetric loss, and the res$"¥ults showed that the stabilizing effect ↓♥of rare salicylate exceeded that of lead stearatλ☆≥♥e and cadmium stearate, which were c< ↑♥ommonly used. Han Huaifen et al.[16] It isφ believed that salicylic acid mixed £'→₹rare earth salts have a good thermal stabilizing ≤φ₩effect on PVC, which is significantly be ₹ tter than stearate. When the stabili☆∞ ↕zer is used together, the stearic acid system ×÷§has a good synergistic effect, but∏σ the salicylic acid rare earth"§ system does not, and the salicylic acid rare ear•£th and organotin have a positive synerg₽↕∏istic effect in a certain ratio range, which εφ±lays the foundation for the rare earth "δstabilizer to partially replace the expensive↑¶ ↑ organotin stabilizer.
Liu Yuejian et al. systema₹∏tically studied the thermal ☆ ₩stability of carboxylate rare eart♣•φhs [17-19]. Studies on static and dynami₹≤c thermal stability have shown that carbo★&xylate rare earths have excellent thermal stabil₹ ity, which is comparable to that o $™f organic thiol tin on PVC. They compared carbo↔₹×xylate rare earth and organotin pairs using ×≤≥artificial aging the photoaging and stabiliza®≠→tion effect of PVC showed th>Ωat the ability of carboxylat★©•×e rare earth to remove HC1 was better t•∏β•han that of organotin, but 'π≈÷the oxidation resistance was nα"§ot as good as that of organotin, but the co ×✘mposite stabilizer of the two had a synergistic ≤¶ effect. Carboxylate rare earths als™∑o have the effect of promoting the gela&•tion of PVC.
From the above, compared with the traditional me¥δπtal lead salts or metal so™↓Ω↕ap heat stabilizers, the ther←•mal stability performance of different rare&↕π← earth heat stabilizers is equival&←ent to or has been surpassed, and some va ☆ ¶rieties even reach the effectλ §> of organotin heat stabilizers. WhatΩ ×'s more, rare earth heat stabilizer÷♥s have excellent synergistic effects wΩ ♦ith other stabilizers, and can be use↔∞d efficiently through compounding.
3.2 Processing performance, m©×echanical properties and other↕≥₩s
Rare earth stabilizers have a plas↕≥ticizing effect on PVC because the∑¶ε≥ plasticization process of PVC compoun↕₽ds is actually PVC powder particles (~1O0μm<¶) is broken into primary particles (~1 &mu™↔δ;m) and finer particles (~O. 1 μm) the ≠εstrong interaction between the rare earth ato•♥m Reδ+ jitter and the chlorine atom C1&delⶀ•ta;- in the PVC molecule can enhance the t↔∑≈ransfer of force (especially the shear forceλ✔÷↕), thereby promoting the gelation of PVC₹≤. It is not difficult to understand that the m™α♥ore immediate effect of this int"ε☆∏eraction is to effectively improve the ₹♥compatibility of rare earth compounds €≥♦"with PVC. Therefore, the transparency of ★★₽<rare earth compounds~PVC sy©πstem is better [20]. Experimental st™←udies by Wu Maoying [12], Liu Yuejia ₽n et al. [21], Hu Shengfei et al. [22], andλ∑ Zhang Yonghua et al. [23] all p≠≈rove the above views.
Due to the special structure of ♥Ωrare earth elements, rare earth heat stabilπ₩'izers can improve the mechanical properties of♥∑φε PVC products. Hu Shengfei e£πt al. [22] and Zhang Yonghua et al. [★π₹23] compared rare earth com→α☆≥posite stabilizers with composite lead saφεlts, and found that the impact streng₽©th, flexural strength or tensile strength®∑✔ of materials using rare earth stabilizerγ₽s were significantly higher th ™→←an those of materials using lead salts,•÷ and the dimensional stability was g®ε£ood. On the other hand, the increase of inorgani€×♦c fillers does not affect φthe performance of its produc∏¥♠δts. Liu Jianping et al. [13×] also compared the differ₽<≤ences in mechanical properties between moδ♥noester maleate rare earth and stearate r✔δ↑are earth stabilizer. In comparison, tδγhe impact performance and tensile p₩₹Ωroperties of monoester maleate rare e♥λ αarth stabilizers are slightly higheσ€r.
Rare earth heat stabilizers can also be made int♠αo rare earth multifunctional compo≥Ω←site stabilizers with other additives, whφα₩÷ich have the effects of coupling,✔☆↑ compatibilizing, and toughening ∑γ$PVC systems [20], improving material fluidity >∏and improving the aging resistance of P∞₽₽→VC [18]. Hu Luguang et al.>>•Ω [24] applied rare earth composite mulφσtifunctional stabilizer to thφ e processing of micro-foamin♣₩αg plates, and used Brabender plasticizer to meaβ ₹sure the plasticization performance, a≈nd the experimental data sh₽₽λowed that with the increase of the a∞φδmount of rare earth stabilizer, the plastici$γ☆•zation temperature, torque and homogenizatioλ≈n section temperature of t¶α≥ he system decreased, which indic↓≥✔ated that the use of rare earth st•★abilizer improved the fluidity of tπ>π→he material, had a strong internal and external l↑☆ubrication effect, and the p♦rocessing rheological performan'αε"ce was better than that of theαδ composite lead series. Hu Luguang et φ✘al. further used scanning electro±∞™σn microscopy to study the structural p♥•♦roperties of microfoamed sheets, and found that↑≥ the gradual increase in the amount of rare £earth stabilizers brought about the π↑≠™enhancement of toughening and coupling can &>lead to the recovery of mechanical proλ©σperties, and the tensile and impact strengt∞☆¶h are also slightly improved. The authors poδ✘β✔int out that the rare eartβ☆$h composite multifunctional stabil÷÷≈'izer is suitable for the rapid extrusion of l&©arger products, and is PVC-U the p♠↓¥roduction of micro-foamed shπλ✘eets results in better processin←↕g flow properties.
4. Outlook
Rare earth heat stabilizer has the advan•×tages of non-toxic, high eff•λ$↑iciency and high cost performanceα≈★", and the multi-functional rare earσ th composite stabilizer made by usi®γ♠→ng synergistic effect can be widely βΩused in the processing of soft and hard prod¶✔×ucts such as PVC profiles, pipes, plates, artifi★£ ≠cial leather, transparent products, etc≠®←., suitable for extrusion, injectio' ₹n molding, calendering, blow mo≥σ♥±lding and other processing technologie®"s, and has low quantity, high efficie✘≈σ>ncy, good processing performance, excellent light≤♣ and heat stability and weather resistanφ&φce in product processing, and has the effects<↕ of coupling, solubilization and tougheni₩ ≈ng, which fully meets the developm♠≠₩ent requirements of environmentally friendly pl₹≠$₽astic additives.
PVC has a wide range of applications, includ₹☆↕ing soft products such as artificial☆↕σ leather and plastic shoes, a↕×nd hard products such as profile₽π÷s, pipes, and plates. In 2005, China's÷₽ total PVC output reached 6®"β.492 million tons, and the apparen↕₽↔t consumption of PVC was 7.918 ↕million tons, with an average annual value ♥∏∑↓of 10 the rate of growth around the right. Ta$β±→king building pipes as an example, by 2010, ther≤♣e were 80 building drainage pipes in the countr€☆€y plastic pipes will be used. Th£★∞e research and development of PVC β≤heat stabilizer and the development o∏™f PVC industry are closely related, so mak≠®±πe full use of China's rich rare earth€₩ resources to vigorously develop, produce₽ and apply rare earth heat staδ→★bilizer, which has huge economic and social ±₽→↔benefits.