Purification And Mixing Of Insulating Materials

- Jun 28, 2019-

Generally, the principle of purification and mixing of insulating materials for making heat shrinkable pipes is unknown to those who use heat shrinkable sleeves. The following section of Kehong heat shrinkable pipe network describes the purification and mixing of insulating materials for you.

XLPE insulating material is a mixture of polyethylene resin, cross-linking agent and antioxidant. It requires that all components be clean, free of conductive and harmful impurities, and that the mixture be uniform and well dispersed. Uneven mixing and poorly dispersed mixtures will accelerate local aging and increase water absorption. If the antioxidant is not uniformly dispersed, it will also lead to a decline in electrical performance.

There are two kinds of impurities in insulating materials: external and self-generated. Foreign impurities contain a small amount of mixed metal powder, fine fibers and other conductive impurities. In-situ impurities include scorched PE resin (dark yellow, called "spark") and precipitated antioxidant "bloom". Conductive impurities can lead to a sharp increase in local electric field strength. Considering the electrical performance only, it is necessary to limit the size of impurities (and voids) to the range that will not lead to partial discharge under the working voltage. Assuming that the impurity is an ellipse suspended in the insulation, when the average electric field strength of the cable is 93 kV/mm and the ratio of the long axis to the short axis of the ellipse is 10:1, the maximum electric field strength of the ellipse tip will rise to 193 kV/mm, which is 50 times of the average electric field strength. The harmfulness of conductive impurities is not only related to particle size, but also to the shape and orientation of impurities in the electric field. However, the shape and orientation of very fine impurities have little effect on the electric field strength. Because its influence area and energy of high field strength are very small, it has no obvious influence on insulation and dielectric strength.

Since the discovery of the water tree phenomenon, the above design judgments about impurities and voids have exposed some shortcomings, especially for insulation over 35 K V. In order to prevent and reduce the water tree phenomenon, it is better to use dry cross-linking and clean (or ultra-clean) PE materials for cables over 35 kV.

In order to prevent the mixture from dispersing unevenly and mixing with foreign impurities, a special closed material treatment system must be adopted in the production plant. The mixing, mixing and extrusion must be limited to the current three mixing methods, and the second extrusion mixing granulation method is the most common one.