Action Mechanism And Application Of PVC Stabilizer
Heat stabilizer is one of the indispensable main auxiliaries in PVC processing.PVC heat stabilizer is used in a small number of parts, but the effect is huge. The use of heat stabilizers in PVC processing can ensure that PVC is not easy to degrade and is relatively stable. The commonly used heat stabilizers in PVC processing are alkaline lead salt stabilizers and metal soap stabilizers. The degradation mechanism of PVC is complex, the action mechanism of different stabilizers is different, and the stabilization effect is also different.
Thermal degradation mechanism of PVC.
The decomposition of PVC is obvious at 100-150 ℃. Ultraviolet light, mechanical force, oxygen, ozone, hydrogen chloride, and some active metal salts and metal oxides will greatly accelerate the decomposition of PVC. The thermal-oxidative aging of PVC is more complex, and some literature has reported that the thermal degradation process of PVC can be divided into two steps.
(1) Dehydrochlorination: the active chlorine atoms are removed from the PVC polymer molecular chain to produce hydrogen chloride and conjugated polyolefins are formed at the same time.
(2) the 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, forming longer-chain conjugated polyolefins, that is, the so-called "zipper" dehydrogenation. At the same time, there is a small amount of Cmurc bond breaking and cyclization, resulting in a small number of aromatic compounds. Among them, the decomposition of hydrogen chloride is the main reason for the aging of PVC. The degradation mechanism of PVC is complicated and there is no unified conclusion. The main mechanisms proposed by the researchers are  free radical mechanism, ion mechanism, and single-molecule mechanism.
Thermal stabilization mechanism of PVC.
In the processing process, the thermal decomposition of PVC does not change much for other properties, mainly affecting the color of the finished product, the addition of heat stabilizer can inhibit the initial coloring of the product. The color of HCl begins to change when the mass fraction of removed PVC reaches 0.1%. Depending on the number of conjugated double bonds formed, PVC appears in different colors (yellow, orange, red, brown, black). If oxygen exists in the thermal decomposition of PVC, colloidal carbon, peroxides, carbonyl, and ester compounds will be formed. However, in the long-term use of the product, the thermal degradation of PVC has a great impact on the properties of the material. The addition of a heat stabilizer can delay the time of PVC degradation or reduce the degree of PVC degradation.
The addition of heat stabilizer in the process of PVC processing can inhibit the degradation of PVC, so the main role of heat stabilizer is to inhibit the degradation of PVC by replacing unstable chlorine atoms, absorbing hydrogen chloride, and reacting with unsaturated sites.
The ideal heat stabilizer should have a variety of functions:
(1) to replace active and unstable substituents, such as chlorine atoms connected to tertiary carbon atoms or allyl chloride, to form a stable structure;
(2) to absorb and neutralize HCl, released during PVC processing to eliminate the automatic catalytic degradation of HCl;
(3) to neutralize or passivate metal ions and other harmful impurities that catalyze the degradation.
(4) the continued growth of unsaturated bonds can be blocked by various forms of chemical reactions, and the degradation and coloring can be inhibited.
(5) it is best to protect and shield ultraviolet light.
1. PVC stabilizer, action mechanism, and application.
1.1 lead salt stabilizer.
Lead salt stabilizers can be divided into three categories:
(1) pure lead salt stabilizers, mostly basic salts containing PbO;
(2) heat stabilizers with lubrication, mainly neutral and basic salts of fatty acids;
(3) compound lead salt stabilizers, and solid and liquid composite stabilizers containing synergistic mixtures of lead salts and other stabilizers and components.
The lead salt stabilizer has strong thermal stability, good dielectric properties, and a low price. A reasonable proportion of lead salt stabilizers with lubricant can widen the processing temperature range of PVC resin and stabilize the quality of processed and post-processed products, so it is the most commonly used stabilizer at present. Lead salt stabilizers are mainly used in hard products.
Lead salt stabilizers have the characteristics of good thermal stabilizers, excellent electrical properties, and low price. But lead salt is toxic, can not be used in contact with food products, can not be made transparent products, and is easily contaminated by sulfides to form black lead sulfide.
1.2 Metal soap stabilizers.
Stearic acid soap heat stabilizers are generally prepared by saponification of alkali earth metals (calcium, cadmium, zinc, barium, etc.) with stearic acid and lauric acid. There are many kinds of products, each of which has its own characteristics. Generally speaking, lubricating stearic acid is better than lauric acid, and lauric acid is better than stearic acid incompatibility with PVC.
Metal soap can absorb some kinds of HCl, and replace the Cl atom in the active part with fatty acid radical through the catalysis of metal ions, so it can play a thermal stabilizing effect on PVC in different degrees.
It is rare in the PVC industry to have a single metal soap compound, but it is usually a compound of several metal soaps. Common is calcium-zinc soap stabilizer. According to the Frye-horst mechanism, the stabilization mechanism of Calcium/Zinc composite stabilizer can be considered as follows: first, zinc soap reacts with allyl chloride on PVC chain, and then calcium soap and zinc soap react with chlorine chloride to form unstable metal chlorides. At this time, as an intermediate auxiliary stabilizer, chlorine atoms are transferred to calcium soap to regenerate zinc soap and delay the formation of zinc chloride which can promote dehydrochlorination.
Calcium and Zinc stabilizers can be used as non-toxic stabilizers in food packaging, medical equipment, and drug packaging, but their stability is relatively low, poor transparency, and easy to spray frost when the dosage of calcium stabilizers is large. Calcium-Zinc stabilizers generally use polyols and antioxidants to improve their properties, and transparent calcium-zinc composite stabilizers for hard pipes have appeared in China.
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