1.Hydrotalcite type auxiliary heat stabilizers
Hydrotalcite layered dihydroxyl composite metal hydroxides (LDH) are inorganic crystal materials with special structures and properties. The common chemical composition of hydrotalcite includes magnesium-aluminum composite hydroxides, lamellar hydroxyl groups, carbonate ions and crystalline water. The crystal structure features are as follows: nanoscale layers are orderly arranged, atoms within the layers are connected by covalent bonds, and the layers are linked by weak chemical bonds (ionic bonds, hydrogen bonds) and have exchangeable anions. The main layer is alkaline. Its special chemical composition and crystal structure endow it with a series of unique properties and functions. Its thermal stability effect is better than that of barium soap, calcium soap and their mixtures. In addition, it has the advantages of transparency, insulation, weather resistance and good processability. It is not contaminated by sulfides, is non-toxic and can be combined with zinc soaps and organotin, etc.
Heat stabilizers play a synergistic role and are a highly promising type of non-toxic auxiliary heat stabilizers. The thermal stability effect of hydrotalcite during the processing of PVC is generally believed to be due to the fact that the hydroxyl groups on its surface absorb the HCl gas released by the thermal decomposition of PVC, thereby inhibiting the catalytic effect of HCl on the decomposition of PVC. In addition, some scholars have proposed the mechanism of the exchange of CO32- between HCl and the interlayer of hydrotalcite. When hydrotalcite is used as a heat stabilizer for PVC, the HCl generated by its thermal decomposition reacts with the CO32- between the layers of hydrotalcite, which can also effectively inhibit the decomposition of PVC.
2. Phosphite esters
Phosphite esters are the most widely used auxiliary stabilizers in Ca/Zn composite stabilizers and are indispensable components in composite stabilizers. The phosphites mainly used as auxiliary stabilizers include triphenyl phosphite, tridecyl phosphite, trinonylphenyl phosphite, and trioctyl phosphite, etc. For soft PVC, phosphite esters are generally used in combination with β -diketone, epoxy soybean oil, etc. Phosphite esters have plasticizing effects and are not suitable for rigid PVC. It has antioxidant capacity, can capture hydrogen chloride, and add polyolefins, which can greatly improve the stability performance of the PVC stabilization system. The addition amount in liquid stabilizers is generally 10% to 35%(by mass fraction), and the main varieties include phenyl diisooctyl phosphite, octyl phosphite, diphenyl decyl phosphite, diphenyl decyl phosphite, trononyl phosphite, etc. At present, hydrolyzed diisooctyl phosphite is mostly selected in China. It can effectively improve the coloring, thermal stability, transparency, anti-scaling and weather resistance of PVC products. Phosphite esters are the most widely used auxiliary stabilizers and have long been commonly applied in calcium-zinc non-toxic liquid compound stabilizer applications. The most effective ones are alkyl/aryl phosphite esters. For example, the Mark-1500 developed by Adeka-Argels of Japan has excellent initial coloring performance for stabilizers.
3. Epoxy compounds
Among epoxides, epoxy soybean oil has traditionally been used as an auxiliary stabilizer. Recent studies have shown that bisphenol A diglycidyl ether, bisphenol F diglycidyl ether, glycidyl ether of phenolic resin, glycidyl ether of tetraphenylethane, alicyclic epoxy resin, triglycidyl trimellate, diepoxypropyl terephthalate, etc. all have relatively high stability efficiency. Epoxides react with hydrogen chloride to form chloroethanol. Under the catalytic action of metal soaps such as calcium and zinc, they replace the unstable chlorine atoms in PVC to exert a stabilizing effect. In static stability tests, the role of epoxy compounds is to inhibit the yellowing of PVC. The effect is not good when used alone. When used in combination with phosphite esters, its stability effect can be significantly improved. Epoxy-based auxiliary heat stabilizers generally include epoxy-based soybean oil, epoxy-based flaxseed oil, epoxy-based butyl stearate, octyl ester and other epoxy-based compounds. When used in combination with the Ca/Zn system, they have a high synergistic effect and possess the advantages of light stability and non-toxicity. They are suitable for soft PVC products, especially those exposed to sunlight, and are usually not used for hard PVC products. Its drawback is that it is prone to exudation. The synergistic mechanism [6] can be regarded as the HCl produced by degradation being absorbed by epoxy groups and metal soap salts, reducing the HCl concentration and slowing down the HCl removal rate of PVC (HCl has a catalytic effect on the degradation of PVC), thereby enhancing the thermal stability of PVC. In addition, under the catalysis of Zn salts, epoxides can also effectively replace allyl chlorine atoms.
4. Polyols
The polyols mainly used as auxiliary stabilizers in the Ca/Zn composite system include pentaerythritol, dipentaerythritol, polyvinyl alcohol, tetramethylcyclohexanol, trimethylolpropane, carbitol, as well as sorbitol, mannitol, xylitol, maltitol, isomaltitol, lactitol and their dehydrated and semi-dehydrated products, etc. When this type of variety is used in combination with β -diketone, epoxides and hydrotalcite in soft PVC, it has an excellent synergistic effect. It should be noted that although polyols have good thermal stability, some varieties still have shortcomings due to their own dehydration and coloring during processing. New varieties such as inulin and tris (α -hydroxyethyl) isocyanurate can overcome the above-mentioned defects. In addition, polyols are prone to sublimation. During processing, the sublimated substances deposit on the equipment, hindering the processing. To overcome these shortcomings, many polyols partially esterified with fatty acids have now been developed, such as Tohtlixer-101 launched in Japan. It is a polyol modifier that can well overcome the shortcomings of general polyols. When used in combination with the Ca/Zn stabilizing system, it shows good light stability, processability and storage stability. Polyols can chelate metal ions, prevent the catalytic degradation of chlorides, and in the presence of metal soaps, they can displace allyl chlorine, thereby stabilizing PVC. In addition, the more hydroxyl groups in polyols can form colorless ligands with metal ions, thereby alleviating the catalytic acceleration effect of zinc stearate and preventing the formation of colored ligands formed by the combination of metal ions and PVC polyene structures until an auxiliary stabilizing effect is achieved. As the number of hydroxyl groups increases, the stabilizing effect of polyols also increases. The main polyols include pentaerythritol, dipentaerythritol, polyvinyl alcohol, tetramethylcyclohexanol, carbitol, etc., as well as sorbitol, mannitol, xylitol, maltitol, isomaltitol, ltol and their dehydrated and semi-dehydrated products. These varieties, when used in combination with β -diketone, epoxides and hydrotalcite in soft PVC, have an excellent synergistic effect. Regarding its mechanism of action [9], it is generally believed that pentaerythritol can form a complex with ZnSt2, and then the complex undergoes a substitution reaction according to the following formula to generate ZnCl2 and pentaerythritol complexes, thereby inhibiting the catalytic degradation of PVC by ZnCl2 and the "zinc burning" phenomenon, and prolonging the thermal stability time of PVC.
5. β -diketone
β -diketone is an indispensable auxiliary stabilizer in the Ca/Zn composite stabilizer system. It plays a significant role in enhancing thermal stability, light stability and inhibiting "zinc burning". The main varieties include stearoyl benzoylmethane, dibenzoylmethane, isopentyl benzoylmethane, octyl benzoylmethane, etc. The basic dosage is generally 8 to 12 parts of Ca/Zn composite stabilizer or 0.2 to 0.3 parts of PVC resin. The prominent role of β -diketone is to improve the coloring performance of products, and it generally has no antagonistic effect with other components. Among this type of auxiliary stabilizers, stearoylbenzoylmethane is the top choice. It is a variety approved by the US FDA(Food and Drug Administration) for use in food packaging materials. Secondly, there is benzoylmethane, which is a classic variety. Currently, it is also produced domestically and some is exported. In addition to the above two solid varieties, there are also two main varieties of liquid β -diketones. One is isopentylbenzoylmethane developed by Rodia Company, and the other is liquid β -diketone T-247 developed by Shanxi Chemical Research Institute. In recent years, research on β -diketones has been very active. For instance, Ciba Company has developed 1.3-pyrimidine diketones and polyketone compounds (DATHP), and Akcros Company has developed pyrrolidin-2.4-diketones, which have better thermal stability and color control effects than the traditionally used β -diketones [5]. β -diketone is the most effective type of compound for improving initial coloring. The main varieties include stearoyl benzoylmethane, dibenzoylmethane, isopentyl benzoylmethane, octyl benzoylmethane, etc. The basic dosage is generally 8% to 12% of Ca/Zn stabilizer, or 0.2% to 0.3% of PVC resin. The prominent role of β -diketone is to improve the coloring performance of products, and it generally has no adverse side effects with other components. The mechanism of action [7-8] can be regarded as that the methylene group sandwich-between two carbonyl groups has relatively high activity and is prone to losing protons. Therefore, it can displace allyl chlorine through a carbon alkylation reaction to form a firm carbon-carbon structure, thereby halting the growth of the conjugated chain caused by the removal of HCl and achieving a stabilizing effect. However, due to the slow reaction rate, the stabilizing effect is not high. When β -dione is added to the Ca/Zn system, on the one hand, β -dione will complex with zinc salts in the system to form zinc β -dione, and then zinc β -dione rapidly displace allyl chlorine atoms through carbon-alkoxylation (or oxygen-alkylation) reactions. On the other hand, ZnCl2 can also catalyze the aforementioned carbon-alkylation reaction, enabling it to proceed rapidly.
6. Amino crotonate and α -phenylindole
When used alone, the thermal stability of amino croton ester is average and it is rarely used as the main stabilizer. Amino croton ester is mainly used in combination with Ca/Zn composite stabilizers and epoxides, which can greatly improve the thermal stability effect of Ca/Zn composite stabilizers.
α -phenylindole is not a very good stabilizer when used alone, especially due to its poor initial colorability, and it can only be used for alkali-stabilized emulsion PVC. When α -phenylindole is used in combination with Ca/Zn system compounds in suspended PVC, the performance of this system can be significantly improved.
