Heat stabilizer is one of tγ≈♠<he indispensable main additives for PVC ✔σprocessing, and the number ♦¶"of copies used in PVC heat stabilizer is small☆φ₩←, but its effect is huge. The usα®e of heat stabilizers in PVC processin±§"£g can ensure that PVC is not easy to degrade $∏and is relatively stable. Commonly used he✘'®€at stabilizers in PVC processing ¶™☆↑include alkaline lead salt stabilizers, met≠αal soap stabilizers, organotin¥↓δβ stabilizers, rare earth st₽¶abilizers, epoxy compounds, et÷↕c. The degradation mechanism of PVC is ™ΩΩcomplex, and the mechanism of action of dif←ferent stabilizers is also d ✘<ifferent, and the stabilization ¥ ✔effect achieved is also different.
1. Thermal degradation mechanism of PVC
PVC decomposes significantly at 100~150 °C, a≈↓φ•nd ultraviolet light, mechan↕Ω↓♣ical force, oxygen, ozone, hy£Ω$ drogen chloride and some active ¶←≠metal salts and metal oxides w ✘≥ill greatly accelerate the decomposi₩€tion of PVC. The thermo-ox₩×¥ idative aging of PVC is complex, and some liter•γ±αature reports divide the thermal degra✘≈≈dation process of PVC into two steps. (1) Dehy↓↔drochlorination: Hydrogen chloride$™ is produced by removing active chlorine∏₽ atoms from the molecular chain of $☆₩PVC polymers, and conjugated polyol≤™$₹efins are generated at the sam•ε↔e time; (2) Formation of longer chain po₩ ♣lyolefins and aromatic rings: With ↕γ'>the further degradation, the chl"$orine atoms on the allyl group are extre$< mely unstable and easy to remov≠®£♥e, resulting in the formation of longer chai© n conjugated polyolefins, that↑≥ is, the so-called "zipper" dehydroge×&βnation, and at the same time, a small amo$€unt of C-C bond breakage and cyclization p"&εroduce a small amount of aromatic compou¶≥nds. Among them, decomposition and d✔✔ehydrochlorination are the main causes of PVC agi₽₽↑ng. The degradation mechanism of PVC is compleσ₩≥©x, and there is no unified conclusion, and✔< the main ones proposed by researchersπ•ε are [4] free radical mechanism, ion mechaα$≠nism and single molecule mechanism.
2. Thermal stability mecha ☆¥→nism of PVC
In the process of processing, the the$ ∞rmal decomposition of PVC does not chφ©≤ange much for other properties, mainly affecting₹ the color of the finished pr✔πoduct, and the addition of heat stabilizer•÷♣ can inhibit the initial col§βφ♠orability of the product. Whenγ©ε the mass fraction of HCl removed r £ βeaches 0.1%, the color of the PVC₽•₩₽ begins to change. Depending✔Ω$™ on the number of conjugated double bonds& formed, PVC will exhibit different₩ colors (yellow, orange, red, brown, black).✘✘★ If oxygen is present during t→Ω≠φhe thermal decomposition of PVC, collo♦ $idal carbons, peroxides, ca↕≤>σrbonyl and ester compounds will be formed. Howeve✔≠r, the thermal degradation o÷₹♣f PVC has a great impact on ♣↔♣≤the performance of the material ±≈for a long time, and the a<σ∑ddition of heat stabilizers can delay the♥♠↑≥ degradation time of PVC or reduce the degre☆↔πe of PVC degradation.
The degradation of PVC can b§÷e inhibited by adding heat sta₽β<≥bilizers in the process of PVC pro≈≠'cessing, and the main functions of hea∞""☆t stabilizers are: inhibiting the de≠×gradation of PVC molecules by replacing u$÷>nstable chlorine atoms, absorbing hydrogen chlori≤Ωde, and having addition reac¥∑ tions with unsaturated parts.φ↓£ The ideal heat stabilizer should have a §>αvariety of functions: (1) replace a reactive ®δand unstable substituent, such as a chlorine ato↔♠↓m or allyl chloride attach×® ed to an tertiary carbon atom, to generate a stab <↑le structure; (2) Absorb and neutra• £lize the HCl released during PVC processing, an©λαd eliminate the automatic catalytic degradatio≈₹n of HCl; (3) neutralize or passivate me★✔₩tal ions and other harmful impurities t×≠↕hat play a catalytic role in the degradβ∏ation; (4) Through various forms of chemi±₹≈↔cal reactions, the continuo★δΩus growth of unsaturated bonds ca✔ πn be blocked, and degradation and coloring can be"≈ inhibited; (5) It is best to have a pro↔λtective shielding effect on ultraviol☆¶et light.
3. PVC stabilizer, mechanism of action and £≈use
3.1 Lead salt stabilizers
Lead salt stabilizers [7] can be div♦α∑ided into three categories: (1) simple lead salt ®∞ ♥stabilizers, most of which are salt-bas✘∑ed salts containing PbO; (2) Thermal stπ♥←abilizers with lubricating ef★ γfect, mainly neutral and salt-based salts of fa <tty acids; (3) Compound lead salt stabiliz✘δ¥₹ers, and solid and liquid composi£☆te stabilizers containing a synergistic ∞ mixture of lead salts and other£★ stabilizers and components.
Lead salt stabilizer has stron∞≥≥g thermal stabilization, good dielectric prεδ♣↕operties, and low price, and a reasonable rati©Ω©o of lubricant can make PVC resin processing ≈÷temperature range wider, processing and post-p≤←₩rocessing product quality is stable, is th₹♥∞§e most commonly used stabilizer at present. Le₹¥ad salt stabilizers are mainly ←∞€used in hard products. Lead salt stε$abilizer has the characteristics of g☆ ∞ood thermal stabilizer, exφβcellent electrical performance ¶&§₹and low price. However, lead∏α→ salts are toxic and cannot be u★γ∑sed in products that come into contact with foo™✔→←d, nor can they be made into t&γransparent products, and they are easily con♣÷®taminated by sulfides to produce black lead sulf ≠ ide.
3.2 Metal soap stabilizers
Stearic acid soap heat stabilizer is ge↕ ↕nerally prepared by saponi♦γ fication of alkaline earth me¥"∑tals (calcium, cadmium, zinc, barium, etc.) a££nd stearic acid, lauric acγφ€§id, etc. There are many types of products, e∞¥ach with its own characteristics. In general,←>' lubricating stearic acid is☆$β preferred to lauric acid, while •≤PVC-compatible lauric acid is superior →₽÷to stearic acid.
Metallic soaps can absorb HCl, and soπ↑me varieties can also replace the ☆πCl atoms at the active site with fatty acid gro☆≥Ωups through the catalytic action of metal ions, s♥×o they can play a different degre>↑e of thermal stabilization effect on PVC. ≤∑¶→In the PVC industry, there is ♠☆€rarely a single metal soap compound, but usua®♥lly a combination of several met§Ω₩al soaps. The most common is calcium and zinc so γap stabilizers. According to the ↓•Frye-horst mechanism, the stπ×λabilization mechanism of calc' ium/zinc composite stabilizer can be ♣β∏ considered as follows: firstly, zinc soap ✘€α♣reacts with allyl chlorine on↓α the PVC chain, and then calcium φφ© soap, zinc soap reacts with chlorine chloride tδπ♣&o form unstable metal chloride. At this timσ♦e, the auxiliary stabilizer as an i₩↔α≠ntermediate medium transfers the chlo<↑↑♠rine atoms to the calcium soap, so that the zinc♣™ soap is regenerated, and the≤₽© generation of zinc chloride with π$ the effect of promoting hydrogen dechlorination α♣∑is delayed.
Calcium and zinc stabilizers can be useφ♥d as non-toxic stabilizers≈π in food packaging, medical devices ♣∞φφand pharmaceutical packaging, but their stabilπ₽✘¶ity is relatively low, and the transpar✔₩ency is poor when the calcium st'€↕abilizer is used in large a ↓βmounts, and it is easy to spray fδβrost. Calcium-zinc stabilizer±¶✘s generally use polyols and antioxidants to i↓©λmprove their performance, and transparent calcε©σ₩ium-zinc composite stabilizers for rigid p©←≤γipes have appeared in China.
3.3 Organotin stabilizers
Alkyl tin in organotins is usually β✘®methyl, n-butyl, n-octyl, etc. Most of ' the production in Japan is butyl₩¶"> tin, European octyl tin is more common,÷α which is the standard non-toxic ₽>stabilizer recognized in Europe, and U§ nited States uses more methyl tin. ♣↑There are three categories of commonly→δ used organotin stabilizers: (1) aliphatic salts, δ mainly referring to dibutyltin dilaurate,≤↓→ di-n-octyltin dilaurate, e★™tc.; (2) Maleates, mainly '•$↑referring to dibutyltin mal∑'eate, bis(monobutyl maleate) dibutyltin, di-n-o™λδ↓ctyltin maleate, etc.; (3) thiolates are thiΩ×<olated salts, of which bis(thioc←≥★←arboxylic acid) esters are the most used.
Organotin heat stabilizer h®"as good performance and is a good variety for PV±©$™C hard products and transparent products, especi→↕ally octyltin has almost become an indispensa₹✔≥♥ble stabilizer for non-toxic pac>φkaging products, but its pri✔÷↓₽ce is more expensive.
Organotin heat stabilizer (t≈∏δin thioglycolate) has a goo♠ ↔™d stabilizing effect on PVC. In particular,λ↔ liquid organotin stabilizers can be better m≤γ ixed with PVC resins than solid heat stabilizers×♣. Organotin stabilizers (tin th♣×✘©ioglycolate) can replace unstable Cl atom' s on polymers, giving PVC resin$<s long-term stability and initial color ←&$retention. The stability mechanis→↔m of tin thioglycolate was $" δproposed: (1) S atom could÷® replace unstable Cl atom, thus inhibitin ↓g the formation of conjugated polyolefins. (2) ∏'∞HCl, as a product of thermal degradα→ation of PVC, can accelerate ×♥the formation of conjugated polyolefins. Whereas,¶€ tin thioglycolate can absorb the Hλ∑☆€Cl produced.
3.4 Rare earth stabilizers
Rare earth heat stabilizers ma→≤₩inly include the abundant light rare earth l±↓÷anthanum, cerium, neodymium organic wea↓αk salts and inorganic salts. The types o>←∏f organic weak salts include rare ÷÷↔earth stearate, rare earth fatty acid, rare eart ✔$γh salicylate, rare earth citrate↑↔&, rare earth laurate, rareε↔€ earth caprylic acid, etc.
The preliminary study of the mechanism of •γ•action of rare earth stabilizβ∑∏ ers is as follows: (1) the special electron♠∑∑≥ic structure of rare earth lanthanides (2 el★ ectrons in the outermost shell, 8 electro♦↕nic structures in the subouter shellφ☆&, and many empty orbitals), the energy level difδ✘λference of the empty orbitsα•✔∞ is very small, and the outer or s×ubouter electrons are intensified ' under the action of external ±©≤thermal oxygen or polar groupδ←✔αs, which can coordinate with the unst↑∑able Cl on the PVC chain, and ε≈can form a coordination complex with the♠> hydrogen chloride decomposed in PVC proc₽∞essing, and there is a strong attrac¥€↑λtion between rare earth elements and₹ γ chlorine elementsIt can pla¥σy a role in controlling free chlorine↔β≠ elements, so as to prevent o &r delay the automatic oxidation c≥≤≈"hain reaction of hydrogen chloride and play a •♠role in thermal stabilization. (2) Ω The rare earth multifunction✔☆$↕al stabilizer can physically a ÷dsorb the oxygen in the PVC processing an∞♠₹₽d the ionic impurities contained in the PVC<♠ itself, and enter the lattice cavity of the ra€₹re earth multifunctional stabilizer, avoiding ®¶✘®their impact and vibration on th✔€₽e parent C-Cl bond. Therefore↕→, through the action of rare earth mul<tifunctional stabilizers, the activa$€σεtion energy of PVC de-HCl can b®e improved, thereby delaying the thermalβ∞ degradation of PVC plastics. (3) The Ω"©appropriate anionic group in rare £∑↕earth compounds can play a role in repla×↓₽cing the allyl chloride atom on the →&★≈PVC macromolecule, eliminating this de₩↔gradation weakness, and also™ achieving the purpose of st∏γ δability. There are many domestic stud♠↔ies on rare earth stabilize→βrs.
In general, the stabilization effect of rareα✔∏ earth heat stabilizers is better than th↑>☆£at of metal soap stabilizers, with good long↕ ×-term thermal stability, and has a ↕βwide range of synergistic effects wi♠σ↔th other types of stabilizers, wi•★ε±th good tolerance, not polluted by sulfur, stabδ× ↕le storage, non-toxic and environment♣α$₹ally friendly. In addition, rare earth elem±σents have a unique coupling effect with CaCO3 ∏and promote the plasticization effect of PVC☆", so that the amount of CaCO3 can be increased, ♣₹εΩthe use of processing aid €λεACR can be reduced, and the cost can β"be effectively reduced. The stabilizing eff®ect of rare earths on PVC is ch₽✘>aracterized by its unique synergi∏Ω™Ωstic effect. Rare earths can be properly comb☆≤ined with certain metals, ligands and coφ-stabilizers to greatly improve the stabiliz↕≠♣ation effect.
3.5 Other stabilizers
3.5.1 Epoxy
Epoxy soybean oil, epoxy linseed<σ oil, epoxy tall oil, epoxy butyl stearate, ₽≥≈βoctyl ester and other epoxy compounds <≠are commonly used secondary heat stabiliz↔₹★πers for polyvinyl chloride, they have a∞•σ≠ high synergistic effect with the stabilizatio☆♠♥n of the above agents, with the advantage×∑s of photostability and non&←←≈-toxicity, suitable for soft matter, especially s÷$oft FVC products to be exposed to sunlight, usual★>∑ly not used for hard PVC products, its dis↕∏∑≈advantage is easy to seepage.
Some studies have pointed out thaε→∑γt by adding epoxy sunflower oil to PVC containi∏★ng different metal soap salts (Ba/Cd an↓εd Ca/Zn), through the determinati∏→on of the thermal stabilityσα♠→ of the material, it is found that$✔≤ sunflower oil has a good synergistic effect w∑♥&ith metal soap salts, which can enhance the £≥β₽thermal stability of PVC materials, and the rea♦×sons for the synergistic effect are a★≠₩nalyzed: the HCl produced ±♦•←by degradation is absorbed by sunflower oil and ★ metal soap salts, and the HCl conce∏βntration decreases and the rate of PVC deHCl rem™↓oval is reduced (HCl has a catalytic ∑ 'effect on the degradation of PVC) ♠≤♠to improve the thermal stabil&₩₩™ity of PVC.
3.5.2 Polyhydroxyl groups
Pentaerythritol, xylitol and other polyhydroxyl c↕Ω∏®ompounds have a certain thermal stabiliz₹$ing effect on PVC, and are commonly u×¥sed secondary heat stabilizers for PVC.
Through the dechlorination rate andσλ✔' thermal stability experiments, it was©♣ found that the thermal stability time of PVC/pol★<↑yolol compounds without heavy metals ✘Ω§πand zinc heat stabilizers was extended to 200♦®÷"°C, and its stabilizati>✔βon effect was related to the type and n♦÷umber of hydroxyl groups of polyh¶'ydroxyl compounds, especially the po∏✘←lyhydroxyl compounds containing terminal hydδφ∏roxyl groups promoted the long •→-term thermal stability of PVC ↕₽&÷and absorbed the HCl generated during de©gradation.
3.5.3 Miscellaneous
Phosphite, β-dione, dihydσγropyrimidine, etc. can be used as auxiliary π₹heat stabilizers for PVC, absorb tπ€ε×he HCl produced, and delay the discolorati¥φon of PVC.
4. The current status and developmentδ'♥ trend of PVC heat stabilizer
After entering the 21st century, due to€♥σ the increasingly strict requirements fδ÷π≠or global environmental protection, the regulatio±×¥ns restricting heavy metal stabilizers ar§φe intensifying, so that the production a§×"nd consumption of heat stabilizers are fuα×♦βrther developing in the direction of non-to↔&xic, low-toxicity, compound☆←γ and high efficiency, lead-f $✔Ωree and cadmium-free have attracted the general a&<©✔ttention of developed countries, and alternat♠₹ive products continue to appeaε∞↑r and apply, the application of lead, cadmium (es÷©pecially cadmium) stabilizers has graduallyγ±₩♣ declined, and some non-toxic or low-toxici₩γ★ty heat stabilizers (such as o∏ ↕¶rganotin compounds, calcium and zinc soap ✘φ☆salts, rare earth stabilize→ΩΩ¶rs, etc.) have emerged.
Although considerable achievγ ™ements have been made in the£← production and development±₩ of complex, non-toxic and ₹♠low-toxicity heat stabilizers in China ≠✔∏in recent years, there are many↔<π∞ deficiencies and gaps compa∞↑red with the world's advanced leve∏÷'l (such as fewer varieties and small ©¥≤∑production scale). The production and↔±≠★ application of new heat stabilizers in ↑σChina are far from meeting the develop♦ment of the domestic PVC industry, a∞"nd the heat stabilizers required for some relatiβ™ε≥vely high-grade PVC products are mainly dep≠≈₩↕endent on imports. The rapid d"↕€evelopment of China's PVC industry has provid₹' ed a good market guarantee and broad development ↔π♦space for the development of the•→ heat stabilizer industry, and a₩αlso put forward higher requirements for the h★§↕eat stabilizer industry. To strengthen the©λ♣ research and development of new heat st©♣÷abilizers in China, we should pay attention to♦δ the following points: (1) strengthen the res÷♥≥earch and improvement of the origina$✔l lead-free cadmium-free calc•™¥ium and zinc stabilizers to improve the qualit' ↕₩y of the original products; (2) According to∏β•& the source of raw materials a §nd market distribution, gradually e>↓stablish a relatively centralized large-scale a✘☆←uxiliary production plant group® ↑; (3) Cooperate with the development aΩγ×nd production of other PVC additives, develop mul↑≠∏ti-compound products, further reduce resource wasεδπ≤te and environmental pollution, ©γβ¥and promote the sustainable d÷₹©evelopment of the "green×★↔" additives industry.