What is the role of Fischer-Tropsch wax in PVC processing?

2022-01-08   Pageview:689

In the plastics processing industry, waxes are used as lubricants for polyvinyl chloride resins (PVC). During PVC processing, friction occurs between PVC and between the PVC melt and the metal surface of the processing machinery, and PVC lubricants are added mainly to reduce the viscous flow resistance of the melt.

PVC materials with heat-sensitive characteristics are processed and plasticized with 80% of the heat required from the frictional shear heat between the raw material and the screw, and it is these hot spots that make PVC susceptible to the risk of degradation. How to control the friction between the screw/sleeve and the PVC to delay plasticization and achieve good external lubrication? How to increase the friction between PVC particles to promote plasticization and thus improve internal lubrication? Which wax can be added to achieve excellent balance of internal and external lubrication?

Fischer-Tropsch wax has a 97% n-alkane structure, a melting point of 110°C and excellent thermal stability and light resistance. Fischer-Tropsch waxes have a superb external lubrication and can be operated in a wide window. Its narrow molecular weight distribution results in lower precipitation risk and higher product surface finish, even greatly satisfying the need for high speed and high throughput extrusion external lubrication such as tubes and sheets. Longer machine clean-up cycles during use result in better processing efficiency.

Since wax is a non-polar substance, it is difficult for small molecular compounds to penetrate uniformly into the layers of particles of PVC resin molecules, resulting in poor compatibility of wax lubricant with PVC.

According to the mechanism of PVC lubrication, wax as a lubricant mainly performs as external lubrication, and the external lubrication is very strong, and also has good lubrication in the middle and late stages of molding process, and is non-toxic.

After PVC is plasticized, wax forms a very thin film on the surface of PVC, which reduces the adhesion and friction between PVC and metal surface of processing machinery, thus can prevent the decomposition of resin caused by local overheating and improve the thermal stability and fluidity.

The wax lubricant should not be used in excessive amount, and the excessive use will appear “spray frost” phenomenon on the surface of PVC. The melting point of wax as lubricant is generally 60℃~95℃, and the molecular chain should be long, because the wax with long carbon chain can make the two friction surfaces farther away from each other, and the lubrication effect is better.

Applications: PVC, pellets, sheets, pipes, profiles and cable materials, etc.


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