In order to improve the fluidity of the PVC melt and prevent the molten mixture from adhering to the processing equipment, the additive added is called a lubricant. In polymer processing, during the melting and transport process, due to the shear force, there will be friction between the polymer molecules and the contact surface between the polymer and the processing equipment. Appropriate friction can improve the mixing and melting effect of the PVC compound by generating frictional heat, which is beneficial to improve the plasticization quality of the melt. However, excessive friction can cause the flow resistance to increase. When the frictional heat is too high, PVC can be discolored or decomposed due to overheating. To reduce excessive friction, add lubricant. Lubricants are often classified as external lubricants and internal lubricants. It has been proved that the viscosity of the PVC melt containing foaming agent is more sensitive to the shear rate than the PVC melt without foaming agent. Under the same temperature and shear conditions, the melt viscosity of the PVC containing the blowing agent is smaller than that of the PVC melt without the blowing agent. The reason for the analysis may be that the gas molecules play a role in isolating and lubricating the PVC molecules. Therefore, the PVC low-foaming product lubrication and the PVC non-foaming product are different.
The purpose of using a lubricant is to make the PVC extrusion process easy. The difference in the effect of internal lubricant and external lubricant is: the external lubricant is used to reduce the friction caused by the contact of the melt with the metal part (such as: screw, barrel, cavity); internal lubricant by changing the intermolecular Friction changes the rheological behavior and viscosity of the melt and has the function of improving melt flow and homogeneity. The difference between the two types of lubrication is the compatibility between the polymer and the lubricant. Internal lubricants have greater compatibility, while external lubricants have less compatibility. However, the compatibility tends to improve as the temperature and pressure increase. At this time, the original external lubricant can become an internal lubricant, so there is no strict boundary between the two. In many cases, a lubricant tends to act both internally and externally, but with a certain lubricating effect.
One factor to be aware of during the low foaming extrusion of PVC is that the melt should flow smoothly through the cavity to form a smooth, non-breakable surface. Lubricants affect the surface finish of the cavity and the mixing and distribution of the foaming gas, thereby affecting the cells of the PVC foam. Several lubricants are commonly used in PVC formulations.
(1) Metal soaps: calcium stearate, lead stearate, barium stearate, zinc stearate, cadmium stearate, etc.;
(2) Saturated hydrocarbons: white oil (liquid paraffin), paraffin wax, polyethylene wax, high temperature lubricant OA9, etc.;
(3) Fatty acids and their lipids: higher fatty acids, higher fatty alcohols, polyhydric alcohols, stearic acid, stearyl alcohol, and the like.
OA9 is a new type of polymer oxidation polymer. It is a kind of functional processing aid with excellent performance. It is suitable for increasing the plasticization of PVC foamed products, improving the mold release in the later period, and enhancing the continuity of production. It is a special high-temperature lubricant for PVC products. It is suitable for PVC foam board, PVC transparent film, PVC transparent film, PVC high calcium products, PVC flooring and other products.
In the extrusion formula of PVC low-foaming products, these three kinds of lubricants often need to exist at the same time, Qihong Technical Department has done a lot of tests on this. Lubrication has a great influence on the quality of extruded products, the energy consumption in the extrusion process, and the length of the die cleaning cycle. Extruded products of the same quality have a difference in energy consumption of 30 to 100% and a clearance period of 1 to 5 times. It can be seen that the lubricant plays a very important role in the extrusion process of PVC low foaming products.
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