Lubricants are essential additives in PVC processing. For lubricants, the effects commonly mentioned in the industry can be attributed to two points:
1. Can reduce the friction between the particles before the melting of PVC and the macromolecules in the PVC melt.
2. Reduce the mutual friction between the contact surface of PVC melt and plastic machinery. This is actually the most typical summary of the role of lubricant 2 (internal and external lubrication).
Discuss and analyze the internal and external lubrication of PVC lubricant from a technical level:
As for the internal lubrication of lubricant, as far as PVC is concerned, it can be regarded as the same kind of material as plasticizer, which plays a role of plasticization or softening. The difference is that the lubricant has a lower polarity and a longer carbon chain. Therefore, compared with plasticizers, the compatibility of lubricants and polyvinyl chloride is significantly reduced. It is precisely because of the low compatibility of lubricants and polyvinyl chloride ( And there is a certain compatibility), so only a small amount of lubricant molecules can penetrate between the molecular chains of the polymer like plasticizers, weakening the mutual attraction between the molecular chains, and when the polymer is deformed, the molecular chains It is easier to slide and rotate with each other, while not excessively lowering the glass transition temperature of the polymer.
External lubrication refers to the interface lubrication mechanism. The lubricant adheres to the surface of the molten resin or to the surface of the processing machine or mold to form a lubricant molecular layer. The lubricating interface is formed due to the presence of the lubricant molecular layer. As a result, the friction between the resin and the processing machine is reduced. The viscosity of the lubricating interface film and its lubrication efficiency depend on the melting point of the lubricant and the processing temperature. In general, lubricants with long molecular carbon chains can make the two friction surfaces farther away, so they have better lubrication.
The role of PVC lubricants in PVC processing is no less than that of stabilizers. Its use generally needs to follow strict principles, such as internal and external slip balance, moderate amount, etc.
If the importance of PVC lubricant is not enough, the principle should be followed when using it. What if PVC lubricant is not used well?
A. Unbalanced internal and external lubrication.
Too much external slippage, although the extrusion speed is fast, but the material is easy to produce, poor plasticization; too much internal slippage, large extrusion volume, the material plasticization is not good. If the initial lubrication is not good, the extrusion torque will be too large. Insufficient lubrication in the later period will lead to insufficient lubrication in the screw homogenization section, compression section, and die section, and the material shears violently, which will affect the surface quality and internal performance of the extruded product.
B. The amount of lubricant is excessive.
The amount of lubricant is not as large as possible. Due to the incompatibility of lubricants and PVC resins, the excessive addition of lubricants will adversely affect the PVC blending system. The internal and external balance of lubrication is the balance within a certain limit, because the lubricant and the polyvinyl chloride blend system produce adverse effects. In actual production, many processing problems and some problems related to thermal stability, and even many defects of products, the root cause may be caused by excessive lubricant consumption.