Classification and action mechanism of the hottest

2022-08-16
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Analyze the classification and action mechanism of PVC heat stabilizers

1 Classification of plastic heat stabilizers

heat stabilizers are a kind of additives that can prevent or reduce the degradation or cross-linking of polymers caused by heating in the process of processing and use, and prolong the service life of composites. The commonly used stabilizers can be classified into base, fatty acid soaps, organotin compounds, composite heat stabilizers and pure organic compounds according to the main components

1) base heat stabilizers: base stabilizers refer to inorganic and organic acid lead salts combined with "base". These stabilizers have excellent heat resistance, weather resistance and electrical insulation, low cost, poor transparency, and certain toxicity. The dosage is generally 0.5% - 5.0%. (article source: universal polyurethane)

2) fatty acid heat stabilizers: these heat stabilizers refer to compounds composed of fatty acid roots and metal ions, also known as metal soap heat stabilizers. Their properties are related to the types of acid roots and metal ions, and the general dosage is 0.1%~3.0%

3) organotin heat stabilizers: these heat stabilizers can form ligands with unstable chlorine atoms in PVC molecules, and the carboxylic ester group of organotin is replaced with unstable chlorine atoms in the ligands. This kind of heat stabilizer is characterized by high stability, good transparency and excellent heat resistance, but its disadvantage is that it is expensive

compared with other insulation materials

4) composite heat stabilizers: these heat stabilizers are liquid or solid complexes based on base or metal soaps and complexes based on organic tin, among which metal salts include calcium magnesium zinc, barium calcium zinc, barium zinc and barium cadmium; Commonly used organic acids such as organic fatty acids, naphthenic acids, oleic acids, benzoic acids and salicylic acids

5) organic compound heat stabilizer: in addition to a few main stabilizers that can be used alone (mainly nitrogen-containing organic compounds), this kind of heat stabilizer also includes polyols and phosphite with high boiling point. Phosphite is often used together with metal stabilizers, which can improve the weather resistance and transparency of composite materials and improve the surface color of products

2pvc heat stabilizer action mechanism

1) absorb and neutralize HCl and inhibit its autocatalytic effect. Such stabilizers include lead salts, organic acid metal soaps, organotin compounds, epoxy compounds, phenolic salts and metal mercaptans. They can react with HCl and inhibit the dehydrochlorination of PVC

2) substitution of unstable allyl chlorine atoms in PVC molecules inhibits depvc. For example, organotin stabilizers are coordinated with unstable chlorine atoms of PVC molecules, and organotin is replaced with unstable chlorine atoms in the ligand

3) addition reaction with polyene structure destroys the formation of large conjugated system and reduces coloring. The salt or ester of unsaturated acid contains double bonds, and diene addition reaction occurs with the conjugate double bonds of PVC molecules, so as to destroy its conjugate structure and inhibit discoloration

4) capture free radicals and prevent oxidation reaction. For example, the addition of phenolic heat stabilizer can block the removal of HCl, because the H atom free radical given by phenol can be coupled with the degraded PVC macromolecular free radical to form a substance that cannot react with O2, which has a heat stabilizing effect. The heat stabilizer can have one or several functions

the ideal PVC heat stabilizer should be a multifunctional substance or a mixture of some materials, which can achieve the following functions: first, replace active and unstable substituents; Second, absorb and neutralize HCl released during PVC processing to eliminate the autocatalytic degradation of HCl; Third, neutralize or passivate metal ions and other harmful impurities that play a catalytic role in degradation; Fourth, through various forms of chemical reactions, the continuous growth of unsaturated bonds can be blocked and degradation coloring can be inhibited; Fifth, it is best to protect and shield ultraviolet light. Usually, heat stabilizers are used together according to their special effects, and rarely used alone. Most varieties are powdery, and some are toxic, such as chemical substances requiring small fixture structure and large non-magnetic. In order to facilitate use, prevent dust poisoning, reduce toxic substances or replace them with non-toxic substances, many kinds of composite stabilizers have been developed at home and abroad in recent years, such as the world-famous German bear composite stabilizer series, and organotin or composite organotin stabilizers from the United States, Germany, Japan, the Netherlands and other countries, all of which occupy a considerable market in China. Therefore, it is an urgent need for the development of China's plastic industry to fully promote the application of new composite stabilizers with high efficiency, low cost, no dust pollution and no or low toxicity

principle analysis of heat stabilizer required for 3pvc processing

the ideal PVC structure is the head tail structure -ch2-chcl-ch2-chcl-, which is quite stable. However, up to now, the method of synthesizing PVC cannot make CIS butadiene undergo directional and regular polymerization under the action of Ziegel catalyst, as in the synthesis of CIS butadiene rubber

the polymerization of vinyl chloride is a random polymerization of free radicals. In addition to having a stable head tail structure, it also has a head head structure -ch2-chcl-chcl-ch-, a tail tail structure -chcl-ch2-ch2-chcl-; There is coupling disproportionation to produce vinyl junction ch2=ch-chcl-ch2- structure and allyl chloride-ch2-ch=ch-chcl-ch2, etc

the formation of allyl chloride, tertiary carbon chloride and double bonds in PVC synthesis are unstable factors in its molecular chain structure. The order of instability is: allyl chloride in PVC molecular chain> tertiary carbon chloride> Terminal allyl chloride> secondary chlorine. PVC is easy to degrade during processing because there are unstable factors in the structure of PVC molecular chain. If it is not stabilized, its decomposition temperature is about 130 ℃, but to process PVC resin into useful products, the molding temperature should be above 190 ℃. Therefore, it is necessary to add heat stabilizer to stabilize and improve its structure

4 evaluation of discoloration of various heat stabilizers

an ideal heat stabilizer should have the functions of absorbing HCl, eliminating active sites, adding to conjugated polyene chains, destroying carbocationic salts, preventing automatic oxidation, etc., without producing products that catalyze the degradation of PVC. Actual heat stabilizers show different thermal stability characteristics due to their different functions, which can be roughly divided into four types: initial type, long-term type equipped with high-speed angular displacement monitoring system, force detection sensor and amplifier, intermediate type and all-round type

1) cadmium and zinc soaps are typical early-stage heat stabilizers, which can quickly absorb HCl. It is predicted in the CD and Z markets that under the catalysis of N in 2018, carboxylate radicals will effectively replace the unstable chlorine atoms on the PVC chain, thus effectively inhibiting the initial degradation and coloring. However, due to their fast consumption, they will transform the products cdc12 and ZnC12, which are also high-efficiency catalysts for PVC dehydrochlorination, which will lead to the malignant degradation of PVC and the sudden blackening of materials, so the long-term thermal stability is poor

2) barium and calcium soaps are typical long-term heat stabilizers, which only have the function of absorbing HCl, so they can not effectively inhibit the coloring of PVC. However, because the conversion products bac12 and CaC12 do not have catalytic activity, they will not cause sudden blackening of PVC, and the long-term heat stability is good

3) fatty acid organotin is an intermediate type, which can not only absorb HCl, but also effectively replace the unstable chlorine atom on the PVC chain with carboxylate radical, and the conversion product has no catalytic activity

4) mercaptan organotin has omnipotent characteristics, can stabilize PVC by various mechanisms at the same time, and the conversion products do not have catalytic activity, so it has excellent initial and long-term thermal stability

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