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The setting of PVC formula system and the selection of various additives are important factors to ensure the quality of PVC profiles. In addition to the stable system and the modified system, the polyethylene wax of the lubricating system has a great influence on the processing properties of profiles, the appearance of products and various physical properties. Due to its small amount, its important role is often ignored. Many defects in PVC profile processing and products are closely related to the selection and proportion of lubricantpolyethylene wax. Therefore, the lubricant polyethylene wax plays a very important role in PVC profile formulation.
1. Classification and action mechanism of lubricants
According to the action mechanism of lubricant polyethylene wax, lubricant polyethylene wax can be divided into internal lubricant and external lubricant. Internal lubricant refers to the lubricant that can reduce the friction between polymer molecules after polymer melting; External lubricant refers to the lubricant that can reduce the mutual friction between particle polymers and between polymer melt and metal surface before and after polymer melting.
2. Properties of plastics affected by lubricants
The effect of lubricant polyethylene wax on the rheology of PVC is significant. Lubricants have a very significant impact on the dissolution. The fully divergent external lubricant covers the resin particles, reducing the heat generated by friction, delaying the coalescence and melting of particles by physical action, and hindering the mutual diffusion of short segments of polymer macromolecular chains. On the contrary, the fully dispersed internal lubricant acts as a pseudo plasticizer and can promote the melting process. General lubricants have the properties of internal and external lubricants, and the magnitude of these two functions varies with varieties. The basic principle for selecting lubricants is to try to obtain a controlled melting rate.
(2) Thermal stability
In general, the lubricant polyethylene wax can reduce the material temperature during processing, so it will have a good effect on thermal stability. More specifically, some lubricants, such as metal soaps, themselves have the function of stability; Some lubricants, such as glycerol monoester, act as a chelating agent; Other lubricants can interact with stabilizers.
(3) Mechanical properties
Lubricant polyethylene wax can affect mechanical properties such as tensile strength, elongation and impact strength. The influence of lubricant on the mechanical properties of profiles is mainly reflected in the fillet strength and low-temperature impact resistance of profiles. The influence of lubricant on the low-temperature impact of products is mainly reflected by the influence on the plasticizing effect of products. The better the plasticizing degree is, the higher the low-temperature impact strength is.
On the premise of fully ensuring the plasticization of materials, the strength of weld corner tends to decrease with the increase of lubricant amount. A large number of experiments show that the fillet strength is poor when the external lubricant is excessive, and it has a good low-temperature drop hammer impact strength. On the contrary, when the external lubricant is insufficient, the fillet strength is better and the low-temperature drop hammer impact strength is poor.
(4) Electrical performance
Generally speaking, the stronger the polarity or ionic property of the lubricant, the worse the performance, and the higher the temperature, the greater the difference. In addition to the volume resistivity and the associated conductor insulation resistance, the surface resistivity is also affected by lubricants. Lubricants with higher polarity can reduce the surface resistivity and have a certain anti-static effect.
(5) Surface characteristics
Lubricant polyethylene wax can migrate to the surface due to its low compatibility with PVC, thus affecting the surface characteristics of PVC plastics. On the positive side, some lubricants can make soft PVC have sliding and anti adhesion properties. However, when excessive use is made, excessive seepage will cause difficulties in lamination, adhesion, embossing and printing. In addition, lubricants can also affect the compatibility and seepage of auxiliary plasticizers. If the lubrication system of PVC profile is selected reasonably, the plasticization degree of the material will be fully guaranteed and the surface quality of the profile will be better. Excessive lubricant will affect the welding quality and strength of profiles.
Lubricants also have certain effects on the appearance of finished products, such as color, luster and transparency. Different lubrication systems may make the same pigment have different color effects. For PVC profiles, under the condition of ensuring plasticization, an appropriate amount of external lubrication will improve the surface finish. Excess lubricant will cause many defects of products, such as stripes and burn marks on materials, excessive precipitation from the molding die, and scratches on the profile surface.
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