Do you really know about stabilizers?
PVC stabilizer mechanism of action and use
Heat stabilizer is one of the main additives indispensable to PVC processing, PVC heat stabilizer used in small portions, but its role is huge. The use of heat stabilizers in PVC processing can ensure that PVC is not easy to degrade, more stable. PVC processing commonly used heat stabilizers are alkali lead salts stabilizers, metal soap stabilizers, organic tin stabilizers, rare earth stabilizers, epoxy compounds, etc. PVC degradation mechanism is complex, the mechanism of action of different stabilizers are different, the stabilization effect achieved also differs.
1. The mechanism of thermal degradation of PVC
The decomposition of PVC is obvious at 100-150°C. UV light, mechanical force, oxygen, ozone, hydrogen chloride and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. thermal oxygen aging of PVC is more complex, and some literature reports the thermal degradation process of PVC into two steps. (i) dechlorination of hydrogen chloride: the removal of active chlorine atoms from PVC polymer molecular chains generates hydrogen chloride and conjugated polyolefins; (ii) formation of longer chains of polyolefins and aromatic rings: as degradation proceeds further, the chlorine atoms on allyl are extremely unstable and easy to be removed, generating longer chains of conjugated polyolefins, which is called "zipper "The degradation is accompanied by a small amount of C-C bond breaking and cyclization, resulting in a small amount of aromatic compounds. Among them, the fractional dehydrogenation of hydrogen chloride is the main cause of PVC aging. The degradation mechanism of PVC is complicated and there is no unified definite theory, and the main ones proposed by researchers are [4] free radical mechanism, ionic mechanism and single molecule mechanism.
2. Thermal stabilization mechanism of PVC
During processing, the thermal decomposition of PVC does not change much for other properties, but mainly affects the color of the finished product, and the addition of heat stabilizers can inhibit the initial colorability of the product. When the mass fraction of HCl removed reaches 0.1%, the color of PVC starts to change. Depending on the number of conjugated double bonds formed, PVC will show different kinds of colors (yellow, orange, red, brown, black). If oxygen is present during the thermal decomposition of PVC, colloidal carbon, peroxides, carbonyl and ester compounds will be formed. However, the thermal degradation of PVC has a significant impact on the performance of the material over a long period of time, and the addition of heat stabilizers can delay the degradation of PVC or reduce the degree of PVC degradation.
The addition of heat stabilizers in the process of PVC can inhibit the degradation of PVC, then the main role played by heat stabilizers are: to inhibit the degradation of PVC molecules by replacing the unstable chlorine atoms, absorbing hydrogen chloride, and unsaturated parts of the addition reaction. The ideal heat stabilizer should have a variety of functions: (1) replacement of active, unstable substituents, such as chlorine atoms attached to the tertiary carbon atom or allyl chloride, to generate a stable structure; (2) absorption and neutralization of HCl released during the processing of PVC, eliminating the autocatalytic degradation of HCl; (3) neutralization or passivation of metal ions and other harmful impurities that play a catalytic role in degradation; (4) Through various forms of chemical reactions, the continued growth of unsaturated bonds can be blocked to inhibit the degradation of coloring; (5) preferably have a protective shielding effect on ultraviolet light.
3. PVC stabilizers, mechanism of action and applications
3.1 Lead salt stabilizers
Lead salt stabilizers [7] can be divided into three categories: (1) simple lead salt stabilizers, mostly salt-based salts containing PbO; (2) thermal stabilizers with lubrication, mainly neutral and salt-based salts of fatty acids; (3) compound lead salt stabilizers, and solid and liquid compound stabilizers containing synergistic mixtures of lead salts and other stabilizers and components.
Lead salt stabilizer has strong thermal stabilization, good dielectric properties, and low price, and reasonable ratio with lubricants can make PVC resin processing temperature range, processing and post-processing of product quality and stability, is currently the most commonly used stabilizer. Lead salt stabilizers are mainly used in hard products. Lead salt stabilizers have the characteristics of good heat stabilizer, excellent electrical properties, and low price. But lead salts are toxic, can not be used in contact with food products, but also can not make transparent products, and easy to be contaminated with sulfide to generate black lead sulfide.
3.2 Metal soap stabilizers
Stearic acid soap heat stabilizer is generally alkaline earth metals (calcium, cadmium, zinc, barium, etc.) and stearic acid, lauric acid, etc. saponification system. There are more types of products, each with its own characteristics. Generally speaking lubricity stearic acid is better than lauric acid, and compatibility with PVC lauric acid is better than stearic acid.
Metal soaps due to the ability to absorb HCl, some varieties can also be catalyzed by its metal ions to fatty acid roots to replace the active site of the Cl atom, so you can play a different degree of heat stabilization of PVC. PVC industry is rarely a single metal soap compounds, but usually several metal soap complexes. Commonly, calcium and zinc soaps are used as stabilizers. According to the Frye-horst mechanism, calcium / zinc complex stabilizer stabilization mechanism can be considered: first zinc soap and PVC chain allyl chloride reaction, and then calcium soap, zinc soap and chlorine chloride reaction to generate unstable metal chlorides. At this point, as an intermediate medium for the auxiliary stabilizer and then the chlorine atoms transferred to the calcium soap, so that the zinc soap regeneration, delayed with the promotion of de-hydrogen chloride effect of the generation of zinc chloride.
Calcium and zinc stabilizers can be used as non-toxic stabilizers, used in food packaging and medical devices, pharmaceutical packaging, but its stability is relatively low, the amount of calcium stabilizers when the transparency is poor, easy to spray frost. Calcium and zinc stabilizers are generally used polyol and antioxidants to improve its performance, there are already transparent calcium and zinc composite stabilizers for rigid pipes in China.
3.3 Organic tin stabilizer
The alkyl tin in organotin is usually methyl, n-butyl, n-octyl and other three. Most of the Japanese production is butyl tin class, Europe octyl tin class is more common, which is the standard non-toxic stabilizers recognized in Europe, the United States is more methyl tin with some. Commonly used organotin stabilizers are three categories: (1) aliphatic acid salts, mainly refers to dibutyltin dilaurate, dioctyltin dilaurate, etc.; (2) maleic acid salts, mainly refers to dibutyltin maleate, bis(monobutyl maleate) dibutyltin, dioctyltin maleate, etc.; (3) thiol salts, of which bis(thiocarboxylic acid) ester is the most amount. Organotin heat stabilizer performance is better, is used for PVC hard products and transparent products of better varieties, especially octyl tin almost become non-toxic packaging products indispensable stabilizer, but its price is more expensive. Organotin heat stabilizer (tin thioglycolate) has a very good stabilization effect on PVC. Especially liquid organotin stabilizer, compared with solid heat stabilizer, liquid organotin stabilizer can be better mixed with PVC resin. The organotin stabilizer (stannous thioglycolate) can replace the unstable Cl atoms on the polymer, giving long-term stability and initial color retention to the PVC resin. The stabilization mechanism of tin thioglycolate is also proposed: (1) S atoms can replace the unstable Cl atoms, thus inhibiting the generation of conjugated polyolefins. (2) HCl, as a product of thermal degradation of PVC, can in turn accelerate the generation of conjugated polyolefins. And tin thioglycolate can absorb the generated HCl.
3.4 Rare-earth stabilizers
Rare-earth heat stabilizers mainly include organic weak acid salts and inorganic salts of lanthanum, cerium and neodymium, which are abundant in resources. The types of organic weak acid salts are stearic acid rare earth, fatty acid rare earth, salicylic acid rare earth, citric acid rare earth, lauric acid rare earth, octanoic acid rare earth, etc. Rare earth stabilizer mechanism of action of preliminary research for: (1) rare earth lanthanide elements of the special electronic structure (the outermost two electrons, the next outer layer of eight electron structure, there are many empty orbitals) determined by the empty orbital energy level difference is very small, in the external thermal oxygen or in the role of polar groups, the outer or second outer layer of electrons are radicalized, can be unstable with the PVC chain Cl coordination, and can be processed with PVC At the same time, there is a strong attraction between rare earth elements and chlorine elements, which can play a role in controlling free chlorine elements, thus can prevent or delay the automatic oxidation chain reaction of hydrogen chloride and play a role in thermal stabilization. (2) Rare earth multifunctional stabilizer can physically adsorb oxygen and ionic impurities contained in PVC processing and PVC itself into the lattice cavities of rare earth multifunctional stabilizer, avoiding their impact vibration on the parent C-Cl bond. Therefore, through the action of rare earth multifunctional stabilizers, the activation energy of PVC deHCl can be improved, thus delaying the thermal degradation of PVC plastics. (3) Suitable anionic groups in rare earth compounds can play the role of displacing the allyl chloride atoms on PVC macromolecules, eliminating this degradation weakness and also achieving the purpose of stabilization. Rare earth stabilizers are more researched in China. Overall, rare earth heat stabilizers are better than metal soap stabilizers, have better long-term thermal stability, and a wide range of synergistic effects between other types of stabilizers, have good tolerance, not polluted by sulfur, storage stability, non-toxic environmental advantages. In addition, rare earth elements have a unique coupling effect with CaCO3, while promoting the plasticizing effect of PVC, thus increasing the amount of CaCO3, reducing the use of processing aids ACR, and effectively reducing costs. The stabilizing effect of rare earths on PVC is characterized by its unique synergistic effect. Rare earths, together with certain metals, ligands and co-stabilizers, can 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 in polyvinyl chloride by heat stabilizers, they are used with the above stabilizers have a high synergistic effect, with the advantages of light stability and non-toxic, suitable for soft, especially soft FVC products to be exposed to sunlight, usually not used in hard PVC products, the disadvantage is easy to bleed .
A study pointed out that the epoxy sunflower oil added to PVC containing different metal soap salts (Ba/Cd and Ca/Zn), through the determination of the thermal stability of the material, found that sunflower oil and metal soap salts have a good synergy to enhance the thermal stability of PVC materials, analyzed the reasons for the synergy: degradation of HCl generated by the sunflower oil and metal soap salts absorbed by the HCl concentration decreases while reducing the rate of deHCl of PVC (HCl has a catalytic effect on the degradation of PVC), which improves the thermal stability of PVC.
3.5.2 Polyhydroxy groups
Pentaerythritol, xylitol and other polyhydroxy compounds have a certain thermal stabilization effect on PVC, and are commonly used as side heat stabilizers for PVC. Through the rate of de-hydrogen chloride and thermal stability experiments, it was found that PVC without heavy metals and zinc heat stabilizers / polyhydroxy compounds thermal stability time extended to 200 ℃, the stabilization effect is related to the type of polyhydroxy compounds and the number of hydroxyl groups, especially the polyhydroxy compounds containing terminal hydroxyl groups to promote the long-term thermal stability of PVC and absorb the HCl generated during degradation.
3.5.3 Others
Phosphite, β-diketone, dihydropyrimidine, etc. can be used as auxiliary heat stabilizers for PVC to absorb the generated HCl and delay the discoloration of PVC.
4. Current status and development trend of PVC heat stabilizers
After entering the 21st century, due to the increasingly stringent global requirements for environmental protection, restrictions on heavy metal stabilizers regulations are increasingly aggravated, so that the production and consumption of heat stabilizers to further development of non-toxic, low toxicity, composite and efficient direction, lead-free, cadmium-free has attracted the general attention of developed countries, alternative products continue to appear and application, lead, cadmium (especially cadmium) stabilizer application has been gradually declining trend, the emergence of Some non-toxic or low-toxic heat stabilizers (such as organotin compounds, calcium and zinc soap salts, rare earth stabilizers, etc.).
Although in recent years, China's composite, non-toxic and low-toxic heat stabilizer production and development has made considerable achievements, but compared with the world's advanced level there are many shortcomings and more gaps (such as fewer varieties, production scale is small, etc.). China's new heat stabilizer production and application is far from meeting the development of the domestic PVC industry, some of the more high-grade PVC products require heat stabilizers are still mainly dependent on imports. The rapid development of China's PVC industry provides a good market guarantee and a broad development space for the development of the heat stabilizer industry, and also puts forward higher requirements for the heat stabilizer industry. Strengthen the research and development of new heat stabilizers in China, should pay attention to the following points: (a) strengthen the original lead-free cadmium-free calcium and zinc stabilizers research and improvement, improve the quality of the original product; (b) according to the source of raw materials and market distribution, and gradually establish a relatively concentrated group of large-scale production of additives; (c) with the development and production of other PVC additives, the development of multiple composite products to further reduce resource waste and environmental pollution, and drive the sustainable development of the "green" additives industry.
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