PVC (polyvinyl chloride) is one of the five general plastics in the world, which has excellent properties such as low price, non-flammable products, corrosion resistance, and good transparency. The so-called PVC stabilizer usually refers to the heat stabilizer of PVC and its copolymers. Broadly speaking, all additives that can improve the thermal stability of polymers are called heat stabilizers. Due to the poor thermal stability of PVC, severe degradation will occur at its processing temperature (160-220℃), so heat stabilizers must be added to restrain its degradation in the processing process.
- Lead salt heat stabilizers: Lead salt stabilizers mainly work through the chemical reaction between lead salt and hydrogen chloride. Lead salt stabilizers have a low cost but excellent properties. At present, lead salt stabilizers can be divided into three categories: (1) organic lead salt heat stabilizers, such as lead stearate and lead malate; (2) inorganic lead salt heat stabilizers, such as tribasic lead sulfate and disalt lead phosphite; (3) compound lead salt heat stabilizers, such as lead stearate and calcium and zinc stabilizers in different proportions.
Figure 1. The structural formula of lead stearate.
- Metal soap heat stabilizer: Metal soap stabilizers refer to higher fatty acids formed by the complex decomposition of carboxylic acids or phenol with metal salts such as Cd, Ba, Ca, Zn, and Mg. According to the mechanism and function of stabilizers, metal soap stabilizers are divided into two categories, which are main metal soap stabilizers and auxiliary metal soap stabilizers with good thermal stability and good processability.
- Organotin heat stabilizers: Having many properties, such as capturing HCl produced by thermal degradation of PVC, replacing active chlorine atoms in PVC molecules, and double bond addition and antioxidation, organotin stabilizers have become the best PVC heat stabilizers. The general formula of organotin heat stabilizers is RnSnY4-n, that R in the formula represents methyl, n-butyl, n-octyl, ester group, etc., and Y is lauric acid, maleic acid, stearic acid, mercaptan, and so on. At present, the commonly used organotin heat stabilizers can be divided into three categories that are aliphatic salts, maleates, and mercaptan salts.
Figure 2. The structural formula of dibutyltin oxide heat stabilizer.
- Organic antimony heat stabilizer: Organic antimony heat stabilizer is a kind of low toxic and efficient stabilizer, which is mainly used in hard PVC, and has excellent initial coloring and long-term thermal stability, lubrication function, and low melt viscosity during processing. It can be used as a synergistic stabilizer for other heat stabilizers, and the effect is good. Organic antimony heat stabilizers mainly include antimony carboxylate, antimony mercaptocarboxylate, antimony mercaptan, and antimony mercaptoester carboxylate. There are many researches on sulfur-containing organic antimony heat stabilizers.
- Rare earth heat stabilizer: Lanthanum, cerium, and neodymium are the rare earth elements mainly used as heat stabilizers. The organic weak acid rare earth salts and inorganic rare earth salts of these elements can be used as stabilizers. The main organic weak acid type rare earth salts are fatty acid type rare earth salt, stearic acid type rare earth salt, citric acid type rare earth salt, salicylic acid type rare earth salt, lauric acid type rare earth salt, glutaric acid type rare earth salt, and so on. The most important feature of rare earth heat stabilizers is versatility. The excellent properties such as non-toxicity, anti-radiation, high efficiency, and environmental protection make them widely used in soft PVC products, hard PVC products, transparent PVC products, and opaque PVC processing products.
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- Troitskii, V.A. Dozorov.(1975) “The simplest mathematical model of the process of the thermal dehydrochlorination of polyvinyl chloride.” European Polymer Journal. 11:277-281.