Causes and solutions of PVC instability

- Aug 22, 2017 -

Causes and solutions of PVC instability


PVC is polymerized by radical polymerization of vinyl chloride monomer. In the reaction, the molecular chain in the growth process, will happen chain transfer reaction to generate tertiary carbon atoms, chlorine and hydrogen atoms connected with tertiary carbon atoms, because the distribution of electron cloud density and bond energy is low, it is easy to become active atoms, and the adjacent H and Cl from a HCl.

If the PVC resin is purely linear, and the secondary carbon atoms combine with chlorine atoms, then its stability is better.

But in fact, even high purity PVC resins begin to decompose HCl at temperatures above 100 degrees Celsius, indicating that there are still unstable factors in their molecular structures.

The unstable structure of PVC is believed to be involved in the radical polymerization of vinyl chloride, and the molecules may contain the following structures

In the structure, one end contains secondary and tertiary chlorine, and one end has beta unstable groups, and oxygen is present in the structure. The reason may be that some of them are produced by the oxidation of the initiator and the oxygen structure may be caused by the polymerization of trace oxygen or by the oxidation of the polymer.

Terms of settlement:

Because the thermal stability of PVC is not good, it is necessary to add the corresponding stabilizer to repair the defects of PVC chain and absorb the HCL produced by PVC dechlorination at the same time. At the same time, because of the high viscosity and large shearing characteristics of PVC, the corresponding lubricant, such as polyethylene wax, must be added into the stabilizer.

Polyethylene wax, PE wax, low molecular weight polyethylene, ethylene by direct polymerization, by different synthesis process, catalytic system for synthesis of products in molecular weight, molecular weight distribution and molecular chain structure are different, the performance of the corresponding products will also have significant differences. PE wax is usually a white powder with an average molecular weight of 1500-5000 and a melting point of 100-120 degrees. It has excellent external lubrication effect in PVC processing, and can effectively improve the flowability, yield, dispersion, surface brightness and release properties of PVC processing. Because of its high molecular weight, high melting point and high temperature stability, it shows a strong external lubrication effect under high temperature and high shear conditions.

Polyethylene wax products can effectively improve the fluidity of PVC dynamic thermal stability and processing process, increase the efficiency of extrusion of PVC products, improve product surface gloss, and effectively reduce the precipitation process of PVC products.

PE wax on the domestic market is mostly divided into several kinds:

Ethylene homopolymerization PE wax synthesis, PE wax produced by this method has good lubricity, high gloss, narrow molecular weight distribution, very stable quality, representing products such as the United States Honey Well Company's A-C series;

Low molecular weight polyethylene pyrolysis products, commonly known as the wax cracking, cracking process is simple and easy to operate, but the molecular weight distribution of the product with the production process there are some fluctuations, the quality is relatively good, there will still be a small part of the low melting point component;

A by-product of ethylene polymerization process, commonly known as cards wax, through the refining process, for purification of PE wax, the product of low viscosity, high melting point, and excellent lubrication performance, low price, but there is a certain fluctuation with the quality of raw materials and process, due to the characteristics of the refining process of low melting point groups of products there will be more points.



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