Lubricants are essential additives in PVC processing. Adding a proper amount of lubricant in PVC can reduce the mutual friction between the particles before the PVC melt and the macromolecules in the PVC melt; reduce the mutual friction between the PVC melt and the plastic mechanical contact surface. The right amount of lubricant can improve the meltability of PVC melt, improve production efficiency, and prevent frictional heat generation from causing PVC degradation and improve the appearance quality of the product.
The external lubricant is poorly compatible with PVC and easily migrates from the inside of the resin melt to the surface to form a lubricant interface layer, thereby reducing the mutual friction between the particles before the PVC melt and between the PVC melt and the plastic mechanical contact surface.
The inner lubricant contains polar groups and has good compatibility with PVC, which can reduce the interaction between PVC molecules, reduce the melt viscosity and improve the melt fluidity.
The internal and external lubricants are only relatively speaking and there is no strict classification criteria. In resins with different polarities, the effects of internal and external lubricants may change. For example, stearic acid alcohol, stearic acid amine, butyl stearate and stearic acid monoglyceride serve as internal lubrication for polar resins such as PVC and PA, but for non-polar resins such as PE. For PP), it shows external lubrication. On the contrary, polymer paraffin and the like have poor compatibility with polar resins, such as an external lubricant in polar PVC, and an internal lubricant in non-polar resins such as PE and PP. At different processing temperatures, the effects of internal and external lubricants will change. For example, stearic acid and stearyl alcohol are used in the initial stage of PVC calendering. Due to the low processing temperature, the compatibility with PVC is poor, mainly for external lubrication. After the temperature rises, the compatibility with PVC increases, and it turns into an internal lubricant.