The important role of lubricant in the formulation of PVC PROFILE
The setting of PVC formula system and the selection of auxiliaries are important factors to ensure the quality of PVC PROFILES The lubrication system has great influence on the processing performance of the profile, the appearance of the product and the physical properties. Because of its small amount, its important role is often neglected by people, PVC profile processing and products of many defects, and lubricant selection and mix proportion has a close relationship. Therefore, the lubricant in the PVC profile formulation has a very important role.
Classification of PVC LUBRICANTS
According to the action mechanism of lubricant, it can be divided into internal lubricant and external lubricant. The internal lubricant is the lubricant which can reduce the friction between the polymer molecules after the polymer melts; The term "external lubricant" refers to a lubricant that reduces friction between polymer particles and between polymer melts and metal surfaces before and after melting.
As for the mechanism of lubricant, there are many and complicated factors which affect the action of lubricant, so we can only give a brief explanation according to general experience.
1.1 INTERNAL LUBRICATION -- plasticizing or softening mechanism
In the case of PVC, lubricant and plasticizer can be regarded as the same kind of material, the difference is that the lubricant has lower polarity and longer carbon chain, so lubricant and PVC compatibility is significantly lower than plasticizer. Due to the poor (and somewhat compatible) compatibility of the lubricant with PVC, only a small number of lubricant molecules, like plasticizers, can penetrate between the molecular chains of the polymer, weakening the intermolecular attraction The polymer chains can slide and rotate easily during deformation without excessively lowering the glass transition temperature (Tg) of the polymer.
1.2 INTERFACE LUBRICATION MECHANISM
The lubricant is attached to the surface of the molten resin or to the surface of the processing machinery or the mould to form a lubricant molecular layer. The friction between the Resin and the machine is reduced as a result of the existence of the lubricant molecular layer. The viscosity of the lubricating interface film and its lubrication efficiency depend on the melting point and processing temperature of the lubricant. In general, the molecule of carbon chain length lubricant because of more two friction surface away, and thus have a greater effect of lubrication.
The effect of Lubricant on the properties of Plastics
2.1 rheological properties
The effect of Lubricant on the rheological properties of PVC is significant. The lubricant has a very significant effect on the melting. The fully dispersed outer lubricant covers the resin particles, reducing the heat generated by friction, and postponing the coalescence and melting of the particles by physical action And block the mutual diffusion of short segments of polymer macromolecular chains. In contrast, a sufficiently dispersed internal lubricant acts as a pseudo plasticizer that promotes melting. General Lubricants are internal and External Lubricant performance, the two role of the size varies with varieties. The basic principle in choosing a lubricant is to try to obtain a controlled melting rate.
Reasonable lubricating system is the most effective and common method to control plasticizing rate of Pvc-u, and it is also one of the main reasons that lubricant must be used in pvc-u processing. The external lubricant prolongs the plasticizing time by reducing the adhesion and friction to the metal surface. The compatibility of internal lubricant with PVC at high temperature weakens the cohesion between the polymer molecules, promotes the melting process, decreases the viscosity of the melt and increases the fluidity.
Any type of lubricant can cool the melt. The role of the lubricant is to reduce the effect of the added Shear Force by external sliding, whereas the role of the internal lubricant is to reduce the melt viscosity, which of course also reduces the heat generated by friction.
Melt breakdown, which is generally considered to be the result of turbulence in an elastic polymer melt, can also be controlled by lubricant equilibrium.
2.2 Thermal Stability
In general, the lubricant can reduce the material temperature during processing, so the thermal stability will have a good role. More specifically, some lubricants, such as metallic soaps, function as stabilizers in their own right; some lubricants, such as Monoglycerides, act as chelators; and others interact with stabilizers.
Lubricant on the appearance of finished products, such as color, gloss and transparency will also have a certain impact, different lubrication system may make the same pigment have different color effect. For PVC profiles, an appropriate amount of external lubrication under plasticized conditions will increase the Surface finish. Excessive lubricant will cause many defects in the product, such as markings and burn marks on the material, excessive precipitates in the molding The surface of the profile will be scratched.
2.4 physical properties of Dry Mixes
Guimon pointed out that lubricants can affect the strength of Pvc high-speed mixing of the degree of compaction and the ultimate apparent density and gelation. This is especially true in twin-screw extrusion. The amount of extrusion is proportional to the apparent density of the dry mixture, and the amount of extrusion needs less energy to melt the partially gelatinated particles.
2.5 Mechanical Properties
Lubricants Can Affect The tensile strength, elongation and impact strength and other mechanical properties. The influence of Lubricant on the Mechanical Properties of profiles is mainly reflected in the welding angle strength and low temperature impact resistance of profiles. The effect of lubricant on low temperature impact is mainly reflected by its effect on the plasticizing of products. The better the plasticization degree is, the higher the low temperature impact strength is. With the increase of the amount of lubricant, the Welding Angle Strength tends to decrease on the premise of fully guaranteeing the plasticization of materials. A large number of experiments show that the welding angle strength is poor when the external lubricant is excessive, and it has better impact strength of low temperature drop hammer. Conversely, when the external lubricant is insufficient, the welding angle strength is better and the impact strength of low temperature drop weight is worse.
2.6 thermal deformation temperature
Because lubricants generally have a low melting point, and some of them are liquid, when these substances are added, they will combine with RPVC and other components to affect the glass transition temperature (Tg) of PVC PLASTIC The temperature of hot deformation is lowered.
2.7 Electrical Properties
Generally, the more polar or ionic the lubricant, the worse the performance, and the higher the temperature, the greater the difference becomes. In addition to the volume resistivity and the associated conductor insulation resistance, the surface resistivity is also affected by the lubricant. The lubricant with larger polarity can reduce the surface resistivity and can produce a certain antistatic effect.
2.8 surface characteristics
The lubricant can migrate to the surface because of its low compatibility with PVC, thus it can affect the surface properties of PVC plastics. On the plus side, some lubricants can soft PVC with sliding and anti-adhesion properties. But when used in excess, will cause excessive exudation Lamination, adhesion, embossing and printing difficulties, in addition, lubricants can also affect the compatibility and exudation of auxiliary plasticizer. Reasonable selection of PVC profile lubrication system will fully ensure the degree of plasticization of materials, so that the profile surface quality is better. Excessive lubricant will affect the welding quality and strength of profiles.