The function of polyethylene wax lubricant
1. Increase the melt index
The melt index of polymer melt is an important parameter to characterize its processing fluidity. Generally, as the temperature increases, the melt index increases and the melt flow rate increases. Due to the limitation of material thermal stability and other factors, the processing temperature cannot be increased indefinitely. Moreover, reducing the melt viscosity by increasing the processing temperature will also increase energy consumption. At this time, adding an appropriate amount of internal lubricant can change the rheological behavior of the resin melt, effectively increase the melt index, and achieve the purpose of reducing the melt viscosity and improving the melt fluidity.
2. Reduce endogenous heat
Endogenous heat is the heat generated by internal friction during polymer flow. In the high-speed processing and molding process, high shear converts a large amount of mechanical energy into heat energy. The greater the melt viscosity, the stronger the shear force, and the more endogenous heat is generated.
The adverse consequences of endogenous heat can lead to local overheating of the polymer melt and reduce its thermal stability. Adding internal lubricant can effectively reduce the melt viscosity and effectively reduce the endogenous heat. The possible adverse consequence is that the low melt viscosity may make processing such as injection molding difficult. In this case, we should choose a lubricity that can impart high enough lubricity to the internal structural units of the resin. At the same time, it is a lubricant with minimal solvation effect on the polymer. In this way, the processing stability can be improved by reducing the frictional heat generation at the internal interface of the resin or dissipating the generated endogenous heat as soon as possible, and also avoids problems such as the reduction of the melt viscosity and the reduction of the thermal deformation temperature of the product.
3. Demoulding effect
In the later stage of polymer processing, the product is formed. At this time, it needs to be easier to fall off the abrasive tool to improve processing efficiency and product quality. At this time, a lubricant with mold release effect can be added. The lubricants for mold release are generally external lubricants, mostly polar compounds. They have limited compatibility with resins and can migrate from the melt to the surface. The polar groups have a certain affinity with the metal surface. In this way, a relatively stable and isolated molecular layer is formed between the melt and the metal surface, thus inhibiting the adhesion between the polymer melt and the surface of the processing machine.
4. Delayed plasticization
Delayed plasticization is another manifestation of external lubricity in plastic processing. In PVC processing, PVC resin is melted under the action of shear deformation and mixed with various additives uniformly, and the requirements for resin melting are not consistent under specific circumstances. For example, in the initial stage of the molding process, it is often not desirable to melt the resin particles prematurely, and sometimes the resin is not required to be completely melted in order to obtain the best mechanical properties of the final product, which is especially obvious in the processing of high-impact polyvinyl chloride. By mixing a lubricant with low compatibility in the resin, it can migrate to the surface of the resin particles or the melt at the processing temperature, so as to form a lubricating layer on the surface of the resin particles or the melt and the metal surface of the processing machine to produce good sliding. , weaken the shear deformation effect, and achieve the purpose of delaying the plasticization of the resin.
5. Prevent melt fracture
High viscosity at high shear rates leads to a superficial phenomenon called melt fracture, as the precipitated species undergo regular stick/slip transitions within the flow channel.
6. Improve the self-lubrication of the product
Many plastic products require good surface lubrication, such as gears. Lubricants with external lubrication can often form a self-lubricating layer by migrating or exuding to the surface of the product, resulting in permanent lubrication.
7. Smooth and anti-blocking effect
For plastic products with large specific surface area, surface adhesion often brings many difficulties to their production or application. Many lubricants with external lubrication function migrate to the surface of the product, and can form a uniformly distributed thin coating after cooling. , Because the two interfaces are inserted into the isolation layer, it not only gives the surface sliding property of the product, but also does not cause the surface roughness. This functional additive is called a slip agent.
8. External lubricant
The most widely used external lubricants on foamed products are generally paraffin wax and PE wax. Paraffin wax is easy to precipitate, so PE wax is generally used.
Insufficient external sliding, the temperature of the 4th and 5th zones of the extruder is not easy to control, and it is easy to overheat. Large bubbles, clusters of bubbles, yellowing, and rough surface of the board appear on the board surface; frictional shear heat increases, causing material decomposition, and board surface yellow, paste;
Excessive slippage and poor plasticization lead to scaling in the cavity of the mold and precipitation on the surface of the product, which is prone to leucorrhea, uneven wall thickness and some symptoms that move back and forth on the surface irregularly;
9. Internal lubricant
Commonly used internal lubricants are stearic acid, 60, monoglyceride, 316 and so on.
Insufficient internal lubrication, poor material dispersibility, uneven plasticization, difficult to control the thickness of the product, thick foam plate in the middle and thin on both sides, there may also be leucorrhea, adhesion to the mold cavity and local overheating and other phenomena;
Too much internal lubrication, the foamed products become brittle, the heat resistance is reduced, and under the action of a certain temperature and melting pressure, it will be transformed into external lubrication, resulting in unbalanced lubrication;
Both internal and external lubrication are insufficient, the melt viscosity is large, the plasticizing torque is large, the melt sticking to the wall is serious, the surface of the material has yellow decomposition lines, the surface smoothness is poor, and the mechanical properties of the product are reduced;
Both internal and external lubrication are excessive, the plasticizing torque is small, and the melt plasticization is obviously insufficient. Although the product has good smoothness, the pressure point adhesion is poor, which seriously affects the mechanical properties of the product;
Less internal lubrication, more external lubrication, significantly prolonged plasticizing time, reduced plasticizing torque, difficult molding and brittle products;
The internal lubrication is more, the external lubrication is less, the plasticizing time is obviously shortened, there is a serious sticking phenomenon, the thermal stability time is shortened, and the surface of the product has a yellow line of decomposition.
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