Classification and action mechanism of PVC heat stabilizers-2

Analysis on the principle of heat stabilizer required for PVC processing

The ideal PVC structure is the head-to-tail structure -CH2-CHCl-CH2-CHCl-, which is quite stable. However, the method of synthesizing PVC so far cannot achieve directional and regular polymerization of butadiene under the action of Ziegel catalyst like the synthesis of butadiene rubber. The polymerization of vinyl chloride is a random polymerization of free radicals. In addition to a stable head-to-tail structure, it also has a head-to-head structure -CH2-CHCl-CHCl-CH- and a tail-to-tail structure -CHCl-CH2-CH2-CHCl. -; There is coupling and disproportionation to generate vinyl structure CH2=CH-CHCl-CH2- and allyl chloride-CH2-CH=CH-CHCl-CH2, etc.

The generation of allyl chloride, tertiary carbon chlorine and double bonds during the synthesis of PVC are unstable factors in its molecular chain structure. The order of instability is: allyl chloride within the PVC molecular chain > tertiary carbon chlorine > terminal allyl Basic chlorine>Secondary chlorine. PVC is easy to degrade during processing precisely because there are unstable factors in the structure of the PVC molecular chain. If it is not stabilized and modified, its decomposition temperature is about 130°C. However, to process PVC resin into useful products, the molding temperature must be Above 190℃. Therefore, heat stabilizers must be added to stabilize and improve its structure.

Analysis on the discoloration of various heat stabilizers when heated

An ideal heat stabilizer should simultaneously absorb HCL, eliminate active sites, add to conjugated polyene chains, destroy carbocation salts, prevent auto-oxidation, etc., without producing products that catalyze the degradation of PVC. Actual heat stabilizers show different heat stabilization properties due to their different functions, and can be roughly divided into four categories: initial type, long-term type, intermediate type and all-round type.

Cadmium and zinc soaps are typical early-stage heat stabilizers, which can quickly absorb HCL and effectively replace unstable chlorine atoms on the PVC chain with carboxylate groups under the catalysis of Cd and Zn, thus effectively inhibiting initial degradation and coloration. However, Because of its rapid consumption, the conversion products CdC12 and ZnC12 are also efficient catalysts for removing HCL from PVC. Therefore, they can cause malignant degradation of PVC and cause the material to suddenly turn black, so the long-term thermal stability is poor.

Barium and calcium soap are typical long-term heat stabilizers. They only have the function of absorbing HCL, so they cannot effectively inhibit the coloring of PVC. However, because the conversion products BaC12 and CaC12 do not have catalytic activity, they will not cause PVC to suddenly turn black, and their long-term thermal stability is poor. good.

Fatty acid organotin is an intermediate type, which can not only absorb HCL, but also effectively replace unstable chlorine atoms on the PVC chain with carboxylate groups, and the conversion product has no catalytic activity. organotin mercaptans have all-round characteristics and can stabilize PVC through various mechanisms at the same time. The conversion products do not have catalytic activity, so they have excellent initial and long-term thermal stabilization effects.

As the manufacturer of PVC stabilizer, NOVISTA's PROSTAB PVC Stabilizers contain the right amount of complex antioxidant system and other functional polymer additives, which can significantly increase finished product's performance and improve the surface luster.

If you have any requirements or demand, please feel free to contact us.

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