Heat stabilizer is one of the indispensable main additives for PVC processing, and the number of copies used in PVC h÷eat stabilizer is small, but its effect is huge. The use of heat stabilizers in PVC processing can→ ensure that PVC is not easy to degrade and is relat∞ively stable. Commonly used heat stabilizers in PVC processin₽g include alkaline lead salt stabilizers, metal soap stabilizers, org™anotin stabilizers, rare earth stabilizers, epoxy compounds, etc. The degradation mechanism Ωof PVC is complex, and the mechanism of action of differentλ stabilizers is also different, and the stabilization effect achieved is also different.
1. Thermal degradation mechanism of PVC
PVC decomposes significantly at 100~150 °C, and ultraviolet light, mechanical force&, oxygen, ozone, hydrogen chloride and some active metal sαalts and metal oxides will greatly accelerate the decomposit™ion of PVC. The thermo-oxidative aging of PVC is co≥mplex, and some literature reports divide the thermal degradation process of PVC into two steps. (1) Dehydrochlorination: Hydrogen chloride is $produced by removing active chlorine atoms from the molecular chain ♥of PVC polymers, and conjugated polyolefins are generated at the same time ; (2) Formation of longer chain polyolefins and aromatic← rings: With the further degradation, the chlorine atoms on the €allyl group are extremely unstable and easy to remove, resulting in the formati÷on of longer chain conjugated polyolefins, that is, the so-called "zipper&q₽uot; dehydrogenation, and at the same time, a small amount of C-C bond breakage and cyclizati"on produce a small amount of aromatic compounds. Among them, decomposition and dehyd∑rochlorination are the main causes of PVC aging. The degradation mech≤anism of PVC is complex, and there is no unified conclusion, and the main ones proposed by reγsearchers are [4] free radical mechanism, ion mechanism® and single molecule mechanism.
2. Thermal stability mechanism of PVC
In the process of processing, the thermal decomposition of PVC does not change much for δother properties, mainly affecting the color of the finished product, and< the addition of heat stabilizer can inhibit the iεnitial colorability of the product. When the mass fraction♥ of HCl removed reaches 0.1%, the color of the PVC begins to change. Depending on the num≠ber of conjugated double bonds formed, PVC will exhibit different colors (yellow, orange, red,↕ brown, black). If oxygen is present during the thermal decomposition of PVC, colloidal carbons, peroxides, carbonyl and ester compounds will be formed. How ever, the thermal degradation of PVC has a great impact on the performance♦ of the material for a long time, and the addition of heat stabi₹lizers can delay the degradation time of PVC or reduce the degree of PVC degradation.
The degradation of PVC can be inhibited by adding heat st±abilizers in the process of PVC processing, and the main functions of he¶at stabilizers are: inhibiting the degradation of PVC molecules by replacing unstable chlorine at₩oms, absorbing hydrogen chloride, and having addition reactions with unsaturated pa rts. The ideal heat stabilizer should have a variety of functi÷ons: (1) replace a reactive and unstable substituent, such as a chlorine atom or allyl chloride attached to an tertiary carbon atΩom, to generate a stable structure; (2) Absorb and neutralize the HCl released during PVC p$rocessing, and eliminate the automatic catalytic degradation of HCl; (3) neutraαlize or passivate metal ions and other harmful impurities that play a ca±talytic role in the degradation; (4) Through various ∏forms of chemical reactions, the continuous growth of unsaturated bonds can be blocked, an✔d degradation and coloring can be inhibited; (5) It is best to have a protective shielding effect on ultraviolet light.
3. PVC stabilizer, mechanism of action and use
3.1 Lead salt stabilizers
Lead salt stabilizers [7] can be divided into three categori✘es: (1) simple lead salt stabilizers, most of which are s♠alt-based salts containing PbO; (2) Thermal stabilizers with lubricating effect,> mainly neutral and salt-based salts of fatty acids; (3) Compound lead salt stφabilizers, and solid and liquid composite stabilizers containing a synergistic mixture of lεead salts and other stabilizers and components.
Lead salt stabilizer has strong thermal stabilization, good d₽ielectric properties, and low price, and a reasonable ratio of lubricant can make PVC resin processing temperature range wider, processing and post-processing product βquality is stable, is the most commonly used stabilizer at present. Lea•d salt stabilizers are mainly used in hard products. Lead salt stabili∏zer has the characteristics of good thermal stabilizer, excellent electrical performance and low€ price. However, lead salts are toxic and cannot be used in products that come into cont∑act with food, nor can they be made into transparent products, and they are easil∑y contaminated by sulfides to produce black lead sulfide.
3.2 Metal soap stabilizers
Stearic acid soap heat stabilizer is generally prepared by← saponification of alkaline earth metals (calcium, cadmiu m, zinc, barium, etc.) and stearic acid, lauric acid, etc. There ar§e many types of products, each with its own characteristics. In general , lubricating stearic acid is preferred to lauric acid, while PVC-compatible lauric acidδ is superior to stearic acid.
Metallic soaps can absorb HCl, and some varieties can also replace the Cl atoms at t™he active site with fatty acid groups through the catalytic action of meta£l ions, so they can play a different degree of thermal stabilization ₽effect on PVC. In the PVC industry, there is rarely a single metal soap compou₹nd, but usually a combination of several metal soaps. The most common is calc'ium and zinc soap stabilizers. According to the Frye-horst mechanism, tαhe stabilization mechanism of calcium/zinc composite stabilizer can be considered♣ as follows: firstly, zinc soap reacts with allyl chlorine on the PVC chain, and then calci•um soap, zinc soap reacts with chlorine chloride to ₽form unstable metal chloride. At this time, the auxiliary stabilizer as an intermediate medium tran≤sfers the chlorine atoms to the calcium soap, so that the↓ zinc soap is regenerated, and the generation of zinc ch♦loride with the effect of promoting hydrogen dechlori¶nation is delayed.
Calcium and zinc stabilizers can be used as non-toxic stabilizers in foo≠d packaging, medical devices and pharmaceutical packaging, but their stability is ₽relatively low, and the transparency is poor when the calcium stabilizer is used iβn large amounts, and it is easy to spray frost. Calcium-zinc stabilizers generally use polyols and antioxidants to improve th eir performance, and transparent calcium-zinc composite stabi§lizers for rigid pipes have appeared in China.
3.3 Organotin stabilizers
Alkyl tin in organotins is usually methyl, n-butyl, n-¶octyl, etc. Most of the production in Japan is butyl tin, European octyl→ tin is more common, which is the standard non-toxic stabilizer recog$nized in Europe, and United States uses more methyl tin. There are three categories of commonly used organotin stabilizers: (1) alδiphatic salts, mainly referring to dibutyltin dilaurate, di-nα-octyltin dilaurate, etc.; (2) Maleates, mainly ref&erring to dibutyltin maleate, bis(monobutyl maleate) dibutyltin, di-n-octyltin maleate, etc.; (3) thiolates are thiolated salts, of whi$ch bis(thiocarboxylic acid) esters are the most used.
Organotin heat stabilizer has good performance and is ±a good variety for PVC hard products and transpare>nt products, especially octyltin has almost become an indispensable stabilizer for non-toxic paβckaging products, but its price is more expensive.
Organotin heat stabilizer (tin thioglycolate) has a good stabilizing effect on PVC. In parti cular, liquid organotin stabilizers can be better mixed with PVC resins than solid heat stabδilizers. Organotin stabilizers (tin thioglycolate) can replace unstable Cl atoms on polymers®, giving PVC resins long-term stability and initialσ color retention. The stability mechanism of tin thioglycolate was proposed: (1) ∞S atom could replace unstable Cl atom, thus inhibiting the formation of conjugated polλyolefins. (2) HCl, as a product of thermal degradation of PVC, can accelerate the formation of conjugated polyolefins. Wher↓eas, tin thioglycolate can absorb the HCl produced.
3.4 Rare earth stabilizers
Rare earth heat stabilizers mainly include the abundant ligλht rare earth lanthanum, cerium, neodymium organic weak salts and inorganic salts. The types of& organic weak salts include rare earth stearate, rare earth fatty acid, rare earth salicylate, rare earth citrate, rare earth laurate, rare earth caprylic acid,÷ etc.
The preliminary study of the mechanism of action of rare earth stabilαizers is as follows: (1) the special electronic structure of rare earth lanthanidesα (2 electrons in the outermost shell, 8 electronic structures in the£ subouter shell, and many empty orbitals), the energy leve☆l difference of the empty orbits is very small, and the outer or subouterΩ electrons are intensified under the action of external thermal oxygen or polar groups, which ca♣n coordinate with the unstable Cl on the PVC chain, and can form a coordination comp↓lex with the hydrogen chloride decomposed in PVC processing, and there is a strong ∏attraction between rare earth elements and chlorine elementsIt can play a role in cβontrolling free chlorine elements, so as to prevent or delay the automatic oxidation chain¶ reaction of hydrogen chloride and play a role in thermal stabiliza→tion. (2) The rare earth multifunctional stabilizer can physically adsorb the oxygen in the PVC≤ processing and the ionic impurities contained in the PVC itself, and e®nter the lattice cavity of the rare earth multifunctional stabilizer, avoiding their impact and •vibration on the parent C-Cl bond. Therefore, through the action of rare earth multifunctional stab÷ilizers, the activation energy of PVC de-HCl can be improved, thereby del↑aying the thermal degradation of PVC plastics. (3) The appropriate an↔ionic group in rare earth compounds can play a role in replacΩing the allyl chloride atom on the PVC macromolecule©, eliminating this degradation weakness, and also achieving the purpose of stability. There★ are many domestic studies on rare earth stabilizers.
In general, the stabilization effect of rare earth heat stabilizers is better than that of metal s¶oap stabilizers, with good long-term thermal stability, and has a wide range of synergistic effects with other types of stabilizers, with good tolerance, not polluted by sulfur, stable storage, non-toxic and environmentally friendly. In addition, rare earth elemen↓ts have a unique coupling effect with CaCO3 and promote the plasticization effect ≠of PVC, so that the amount of CaCO3 can be increased, the use of processing aid ACR can beε reduced, and the cost can be effectively reduced. The stabilizing effect of rare earths §on PVC is characterized by its unique synergistic effect. Rare earths can be properly combined with certain metals, ligands and co-sta♦bilizers to greatly improve the stabilization effect.
3.5 Other stabilizers
3.5.1 Epoxy
Epoxy soybean oil, epoxy linseed oil, epoxy tall oil, epoxy butyl stearate, oc<tyl ester and other epoxy compounds are commonly used secondary heat sσtabilizers for polyvinyl chloride, they have a high synergistic effect with ∏the stabilization of the above agents, with the advantages of photostability and non-toxicity,★ suitable for soft matter, especially soft FVC produ→cts to be exposed to sunlight, usually not used for hard PVC products, its disadvaγntage is easy to seepage.
Some studies have pointed out that by adding epoxy sunflower oil to PVC containing different metal ↔soap salts (Ba/Cd and Ca/Zn), through the determination of the thermal stability of the materγial, it is found that sunflower oil has a good synergistic effect with metal soap salts, whiσch can enhance the thermal stability of PVC materials, and the reasons for the synergistic effect a✘re analyzed: the HCl produced by degradation is absorbed by sunflower oil and metal soap salt÷s, and the HCl concentration decreases and the rate ★of PVC deHCl removal is reduced (HCl has a catalytic effect on the degradation of PVC) to i∑mprove the thermal stability of PVC.
3.5.2 Polyhydroxyl groups
Pentaerythritol, xylitol and other polyhydroxyl compounds have a certain thermal stabilizing effecλt on PVC, and are commonly used secondary heat stabilizers for PVC.
Through the dechlorination rate and thermal stability experiments, it was found th©at the thermal stability time of PVC/polyolol compounds without heavy metals and zinc heat s≈tabilizers was extended to 200°C, and its stabilization effect was related to the >type and number of hydroxyl groups of polyhydroxyl compounds, especially the polyhydroxyδl compounds containing terminal hydroxyl groups promoted the long-term thermal stability of PVC and absorbed the HCl generated "during degradation.
3.5.3 Miscellaneous
Phosphite, β-dione, dihydropyrimidine, etc. can be used as auxiliary heat stabilize™rs for PVC, absorb the HCl produced, and delay the discolor§ation of PVC.
4. The current status and development trend of PVC heat stabilizer
After entering the 21st century, due to the increasingly ♣strict requirements for global environmental protection, the regulations restricting hea→vy metal stabilizers are intensifying, so that the production and consumption of heat s$tabilizers are further developing in the direction of non-toxic, low-to♠xicity, compound and high efficiency, lead-free and cadmium-free have attracted the gener↔al attention of developed countries, and alternative products continue to appear and apply, the application of lead, cadmium (especially cadmium) stabilizers has gradually d★eclined, and some non-toxic or low-toxicity heat stabilizers (such as organotin compounds,£ calcium and zinc soap salts, rare earth stabilizeσrs, etc.) have emerged.
Although considerable achievements have been made in the production and d↔evelopment of complex, non-toxic and low-toxicity heat stabilizers& in China in recent years, there are many deficienγcies and gaps compared with the world's advanced level (such αas fewer varieties and small production scale). The production and application of new heat stabiliz↑ers in China are far from meeting the development of the domestic PVC industry, and the hea t stabilizers required for some relatively high-grade PVC products are mainly depen♥dent on imports. The rapid development of China's PVC industry has provided a good market gu✘arantee and broad development space for the development of the heat stabilizer industr®y, and also put forward higher requirements for the heat stabili÷zer industry. To strengthen the research and development of new he≠at stabilizers in China, we should pay attention to the following poi÷nts: (1) strengthen the research and improvement of th↑e original lead-free cadmium-free calcium and zinc stabilizers to improve th¥e quality of the original products; (2) According to the source of raw materials and market distrεibution, gradually establish a relatively centralized larg€e-scale auxiliary production plant group; (3) Cooperate with the developmen't and production of other PVC additives, develop multi-compou≠nd products, further reduce resource waste and environmental pollution, and promoεte the sustainable development of the "green" additives industry.