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Basic information of calcium stearate
Calcium stearateis a white powder, insoluble in water, cold ethanol and ether, soluble in hot benzene, benzene, turpentine and other organic solvents, slightly soluble in hot ethanol and ether. When heated to 400, it slowly decomposes and is flammable. It decomposes into stearic acid and corresponding calcium salt in case of strong acid, with hygroscopicity.
Production method of calcium stearate
(1) Direct method
Add a certain amount of stearic acid and CaO into the reaction pot equipped with stirrer and thermometer, heat up to melting, slowly add catalyst H2O2 under continuous stirring, and vacuum, control the reaction temperature at 140~150, and react for 1.5~2.0h. After the reaction is completed, the product is obtained by discharging, cooling and crushing.
(2) Complex decomposition method
Dissolve stearic acid in 20 times the mass of hot water, add saturated caustic soda solution, and conduct saponification reaction at about 75 to generate dilute sodium stearate solution. Then add 1074 kg/m3 of calcium chloride solution to the sodium stearate solution, and carry out the double decomposition reaction at about 65, and the product calcium stearate is precipitated. The finished product is then filtered, washed and dried at about 90 ℃.
Application in PVC formula
Calcium stearate is used as a heat stabilizer for PVC and a lubricant and release agent for a variety of plastic processing. As a PVC lubricant, it is generally considered as an internal lubricant. Calcium stearate is a stearate of medium and strong alkali calcium with strong polarity. The chemical active point of PVC resin particles can form a relatively stable complex system with calcium stearate. However, the two long-chain alkyl groups of calcium stearate have little compatibility with polar resins such as PVC, and form a local micro-phase interface inside the PVC resin, that is, a lubricating film. The microscopic essence of its lubrication is that the force between the local lubrication interface formed by non-polar long-chain alkyl and the polar resin segment is less than the force between the resin segments, and because there are many small pores between the resin segments, when the resin segment is displaced, the alkyl that can be arbitrarily deformed is squeezed into other adjacent pores, thus reducing the resistance when the resin segment is displaced, and playing an internal lubrication role.
In practical application, calcium stearate, as a PVC lubricant, does not only play the role of internal lubrication, but its lubrication role is relatively complex. In other cases where the proportion of internal lubrication is relatively high, it will reflect the role of external lubrication. Moreover, it has synergistic effect with many lubricants.
Lubrication in PVC formula
Calcium stearate, originally used in PVC as an auxiliary stabilizer, is mainly used to absorb chloride ions produced by PVC degradation; At the same time, you should also find that calcium stearate is also widely used in polyolefin formulations. Probably not many people know the purpose of using calcium stearate in the original polyolefin formulation. In fact, the original use of calcium stearate in polyolefin formulations is to absorb some halogen components added during polyolefin polymerization, so there is a name called halogen absorbent.
So why is it called lubricant?
As we all know, there are many kinds of additives to be added in the formula design. Even if the polyolefin is not specially added with lubricant, the dispersant added is also a lubricant, which is equivalent to adding lubricant in a disguised form. In practice, it was found that when calcium stearate was present in the blend system, the lubricating properties would change significantly.
In the presence of other lubricants, calcium stearate will play a "rivet" role, strengthen the adhesion effect of other lubricants, and thus enhance the lubrication effect. That is to say, calcium stearate will play a synergistic effect. That is to say, it is of little significance to talk about the lubricating effect of calcium stearate, aside from other additives that play a role in lubrication.
Generally, in the field of styrene thermoplastic elastomer blending, calcium stearate is relatively less used, and zinc stearate is generally used. The main reason is that calcium stearate is relatively easy to precipitate in practical experience.
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