Heat stabilizer is one of the indispensable main additives for PVC processing, and the number of copies used in PVC heat 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 relatively stable. Commonly used heat stabilizers in PVC processing include alkaline lead salt stabilizers, metal soap stabilizers, organotin stabilizers, rare earth stabilizers, epoxy compounds, etc. The degradation mechanism of PVC is complex, and the mechanism of action of different 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 salts and metal oxides will greatly accelerate the decomposition of PVC. The thermo-oxidative aging of PVC is complex, 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 formation of longer chain conjugated polyolefins, that is, the so-called "zipper" dehydrogenation, and at the same time, a small amount of C-C bond breakage and cyclization produce a small amount of aromatic compounds. Among them, decomposition and dehydrochlorination are the main causes of PVC aging. The degradation mechanism of PVC is complex, and there is no unified conclusion, and the main ones proposed by researchers 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 other properties, mainly affecting the color of the finished product, and the addition of heat stabilizer can inhibit the initial 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 number 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. However, the thermal degradation of PVC has a great impact on the performance of the material for a long time, and the addition of heat stabilizers can delay the degradation time of PVC or reduce the degree of PVC degradation.
The degradation of PVC can be inhibited by adding heat stabilizers in the process of PVC processing, and the main functions of heat stabilizers are: inhibiting the degradation of PVC molecules by replacing unstable chlorine atoms, absorbing hydrogen chloride, and having addition reactions with unsaturated parts. The ideal heat stabilizer should have a variety of functions: (1) replace a reactive and unstable substituent, such as a chlorine atom or allyl chloride attached to an tertiary carbon atom, to generate a stable structure; (2) Absorb and neutralize the HCl released during PVC processing, and eliminate the automatic catalytic degradation of HCl; (3) neutralize or passivate metal ions and other harmful impurities that play a catalytic role in the degradation; (4) Through various forms of chemical reactions, the continuous growth of unsaturated bonds can be blocked, and 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 categories: (1) simple lead salt stabilizers, most of which are salt-based salts containing PbO; (2) Thermal stabilizers with lubricating effect, mainly neutral and salt-based salts of fatty acids; (3) Compound lead salt stabilizers, and solid and liquid composite stabilizers containing a synergistic mixture of lead salts and other stabilizers and components.
Lead salt stabilizer has strong thermal stabilization, good dielectric properties, and low price, and a reasonable ratio of lubricant can make PVC resin processing temperature range wider, processing and post-processing product quality is stable, is the most commonly used stabilizer at present. Lead salt stabilizers are mainly used in hard products. Lead salt stabilizer 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 contact with food, nor can they be made into transparent products, and they are easily 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, cadmium, zinc, barium, etc.) and stearic acid, lauric acid, etc. There are many types of products, each with its own characteristics. In general, lubricating stearic acid is preferred to lauric acid, while PVC-compatible lauric acid is superior to stearic acid.
Metallic soaps can absorb HCl, and some varieties can also replace the Cl atoms at the active site with fatty acid groups through the catalytic action of metal 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 compound, but usually a combination of several metal soaps. The most common is calcium and zinc soap stabilizers. According to the Frye-horst mechanism, the 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 calcium soap, zinc soap reacts with chlorine chloride to form unstable metal chloride. At this time, the auxiliary stabilizer as an intermediate medium transfers the chlorine atoms to the calcium soap, so that the zinc soap is regenerated, and the generation of zinc chloride with the effect of promoting hydrogen dechlorination is delayed.
Calcium and zinc stabilizers can be used as non-toxic stabilizers in food packaging, medical devices and pharmaceutical packaging, but their stability is relatively low, and the transparency is poor when the calcium stabilizer is used in large amounts, and it is easy to spray frost. Calcium-zinc stabilizers generally use polyols and antioxidants to improve their performance, and transparent calcium-zinc composite stabilizers 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 recognized in Europe, and United States uses more methyl tin. There are three categories of commonly used organotin stabilizers: (1) aliphatic salts, mainly referring to dibutyltin dilaurate, di-n-octyltin dilaurate, etc.; (2) Maleates, mainly referring to dibutyltin maleate, bis(monobutyl maleate) dibutyltin, di-n-octyltin maleate, etc.; (3) thiolates are thiolated salts, of which bis(thiocarboxylic acid) esters are the most used.
Organotin heat stabilizer has good performance and is a good variety for PVC hard products and transparent products, especially octyltin has almost become an indispensable stabilizer for non-toxic packaging products, but its price is more expensive.
Organotin heat stabilizer (tin thioglycolate) has a good stabilizing effect on PVC. In particular, liquid organotin stabilizers can be better mixed with PVC resins than solid heat stabilizers. Organotin stabilizers (tin thioglycolate) can replace unstable Cl atoms on polymers, giving PVC resins long-term stability and initial color retention. The stability mechanism of tin thioglycolate was proposed: (1) S atom could replace unstable Cl atom, thus inhibiting the formation of conjugated polyolefins. (2) HCl, as a product of thermal degradation of PVC, can accelerate the formation of conjugated polyolefins. Whereas, tin thioglycolate can absorb the HCl produced.
3.4 Rare earth stabilizers
Rare earth heat stabilizers mainly include the abundant light 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 stabilizers is as follows: (1) the special electronic structure of rare earth lanthanides (2 electrons in the outermost shell, 8 electronic structures in the subouter shell, and many empty orbitals), the energy level difference of the empty orbits is very small, and the outer or subouter electrons are intensified under the action of external thermal oxygen or polar groups, which can coordinate with the unstable Cl on the PVC chain, and can form a coordination complex 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 controlling free chlorine elements, so as to prevent or delay the automatic oxidation chain reaction of hydrogen chloride and play a role in thermal stabilization. (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 enter 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 stabilizers, the activation energy of PVC de-HCl can be improved, thereby delaying the thermal degradation of PVC plastics. (3) The appropriate anionic group in rare earth compounds can play a role in replacing 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 soap 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 elements 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 be 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-stabilizers 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, octyl ester and other epoxy compounds are commonly used secondary heat stabilizers 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 products to be exposed to sunlight, usually not used for hard PVC products, its disadvantage 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 material, it is found that sunflower oil has a good synergistic effect with metal soap salts, which can enhance the thermal stability of PVC materials, and the reasons for the synergistic effect are analyzed: the HCl produced by degradation is absorbed by sunflower oil and metal soap salts, 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 improve the thermal stability of PVC.
3.5.2 Polyhydroxyl groups
Pentaerythritol, xylitol and other polyhydroxyl compounds have a certain thermal stabilizing effect on PVC, and are commonly used secondary heat stabilizers for PVC.
Through the dechlorination rate and thermal stability experiments, it was found that the thermal stability time of PVC/polyolol compounds without heavy metals and zinc heat stabilizers was extended to 200°C, and its stabilization effect was related to the type and number of hydroxyl groups of polyhydroxyl compounds, especially the polyhydroxyl 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 stabilizers for PVC, absorb the HCl produced, and delay the discoloration 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 heavy metal stabilizers are intensifying, so that the production and consumption of heat stabilizers are further developing in the direction of non-toxic, low-toxicity, compound and high efficiency, lead-free and cadmium-free have attracted the general attention of developed countries, and alternative products continue to appear and apply, the application of lead, cadmium (especially cadmium) stabilizers has gradually declined, and some non-toxic or low-toxicity heat stabilizers (such as organotin compounds, calcium and zinc soap salts, rare earth stabilizers, etc.) have emerged.
Although considerable achievements 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 deficiencies and gaps compared with the world's advanced level (such as fewer varieties and small production scale). The production and application of new heat stabilizers in China are far from meeting the development of the domestic PVC industry, and the heat stabilizers required for some relatively high-grade PVC products are mainly dependent on imports. The rapid development of China's PVC industry has provided a good market guarantee and broad development space for the development of the heat stabilizer industry, and also put forward higher requirements for the heat stabilizer industry. To strengthen the research and development of new heat stabilizers in China, we should pay attention to the following points: (1) strengthen the research and improvement of the original lead-free cadmium-free calcium and zinc stabilizers to improve the quality of the original products; (2) According to the source of raw materials and market distribution, gradually establish a relatively centralized large-scale auxiliary production plant group; (3) Cooperate with the development and production of other PVC additives, develop multi-compound products, further reduce resource waste and environmental pollution, and promote the sustainable development of the "green" additives industry.