In recent years, some problems of the old generation of polyolefins have been solved with the introduction of double-peak PE100 and PE80 materials. One of the basic problems of the previous generation polyolefins was the problem of sagging in the pipe production process. In the pipe production process, pipe cooling is done from the outer wall, which means that the newly formed pipe, after leaving the extruder die, is in contact with the cooling currents only from the outside surface, and its inner surface remains molten for some time. stayed The thicker the pipe wall, the longer this time will be. In this way, the molten materials on the inner surface of the tube will have the opportunity to flow down under the force of gravity and cause the unevenness of the wall thickness and the roughness of the inner surface of the tube. By changing the material from 80MRS to MRS100, the thickness of the pipe is reduced, and as a result, the inner surface cools down faster, and this problem is solved to some extent.
(Low Sagging or LS).
What polyolefins producers can do to fix the viscosity of the material is to change the viscosity of the polymer. As mentioned, melt viscosity of most polymers decreases with increasing shear stress. If we pay attention to the melt flow curve of common types of polyethylene, it can be seen that the viscosity increases as the shear rate decreases, but in the range of low shears, the curve becomes almost linear and with a further decrease in the shear rate, the viscosity will not increase significantly. Adding a percentage of polymer chain with a very high molecular weight to the material (bipolar distribution) causes the viscosity to continue to increase even at low shear rate, and at zero shear (the state that occurs in the slitting tube) the viscosity of polyethylene is higher and As a result, the amount of leaching is less.
By means of polyolefins with double distribution, it is possible to produce larger pipes with more wall thickness without changing the process conditions. The record obtained with these materials is a pipe with a diameter of 1600 mm and a wall thickness of 100 mm.
