Polyethylene insulation is lapped on the core by the extrusion method. There are tree types of it: solid, pored and solid-pored. The stuff for it includes polyethylene, polyethylene-based and thermostabilizing additives, etc.
Polyethylene is a solid high-molecular material of alkene of ethylene polymerization C4H4. Depending on the production process there are high-pressure polyethylene (HPP) and low-pressure polyethylene (LPP). To get HPP the pressure have to be 140…250 MPa and the temperature 70…100ºC. Density of HPP is 0.918…0.930 g/sm3 and it is low-density polyethylene. Density of LPP is 0.949…0.967 g/sm3 and it is the high-density polyethylene. The way of polyethylene getting stipulates not only the density, but also the molecular structure – i.e. general properties.
Solid insulation consists of LPP based composition. It is also allowed to use HPP based compositions.
Foamed polyethylene insulation is a result of extruding polyethylene and some foaming concentrate on a core. Foaming concentrate is a high-pressure (low-density) polyethylene with evenly spread gas developing agents, gas developing agents decompounding activators and stabilizators in it.
PE foam’s density depends on the degree of porosity ƒ, which is equal to the ratio of air bubbles volume and solid dielectric in the foam material.
Advantages of foamed polyethylene insulation over solid insulation consist in lower specific inductive capacity and smaller cable diameter and mass as a result. The disadvantages: major moisture capacity, minor mechanical and electric strength.
Solid-foamed polyethylene insulation consists of two layers. Inner layer (80% of total thickness) provides equivalent specific inductive capacity insulation lowering. Outer layer is continuous and its function is to prevent moisture ingress for saving foam insulation’s characteristics in case the wet get into the cable or filler.