Polyethylene (PE) has been widely used as a jacket for underground cables since it became commercially available in large quantities in about 1950. For use as a jacket, polyethylene may be compounded with carbon black or coloring material, and with stabilizers.
Carbon black gives the material the necessary sunlight protection for outdoor use. Polyethylene for jacketing is categorized under three different densities:
Low density 0.910 to 0.925 grams per cm3
Medium density 0.926 to 0.940 grams per cm3
High density 0.941 to 0.965 grams per cm3
Density generally affects the crystallinity, hardness, melting point, and general physical strength of the jacketing material. In addition to density, molecular weight distnibution is important since it influences the processing and properties of the polymer.
Polyethylene jackets are an excellent choice where moisture resistance is a prime design criteria since it has the best moisture resistance of any non-metallic jacket material. When polyethylene is used as a jacket material, it should be compounded with enough carbon black to prevent ultra-violet degradation.
Linear, low density, high molecular weight (LLDPE) is the most popular jacket material since it has better stresscrack resistance that the high density materials. High density provides the best mechanical properties, but may be very difficult to remove from the cable.
In evaluating fillers, both black and non-black, it has been found that although many of these materials improve the aging characteristics, carbon black is by far the best. It has also been found that the aging resistance increases with carbon black loading from 2 to 5 percent. Normally, a 2.5 to 3.0 percent loading is used.
Although PE has good moisture resistance and good aging properties in its temperature limits, it has poor flame resistance. This discourages using it as a jacket in many circumstances. Polyethylene jackets have good cold bend properties since they will pass a cold bend test at about -55 "C.
They are extremely diflicult to bend at low temperature because of their stiffness. Like PVC, PE is a thermoplastic material and will melt at elevated temperatures. This temperature will vary slightly with molecular weight and density, but melt occurs at about 105 "C.
High density polyethylene (HDPE) has been used extensively as the second (outer) layer for "ruggedized" thermoplastic in secondary and low voltage street light cables because of its toughness.While black polyethylene for jacketing is frequently an insulating material, with higher loadings of carbon-black it can be a semiconducting material. This material has been used for over 30 years in direct-buried applications to improve the grounding of the concentric neutral.