From Edis Audio Visual Wiki
Polyethylene or polythene (IUPAC name polyethene or poly(methylene)) is the most widely used plastic, with an annual production of approximately 80 million metric tons. Its primary use is within packaging (notably the plastic shopping bag).
Polyethylene is classified into several different categories based mostly on its density and branching. The mechanical properties of PE depend significantly on variables such as the extent and type of branching, the crystal structure and the molecular weight.
Polyethylene as a wire insulation material comes in many forms
Generic Polyethylene offers low cost, excellent electrical properties, high flammability, stiffer than vinyl.
Cross linked (XLPE) offers higher voltage capabilities
Irradiated Polyethylene offers moderate cost, excellent electrical, physical, and chemical properties, flame retardant, resistant to environmental stress
For low capacitance cables, particularly video cables a dielectric material as foamy and soft as possible; solid polyethylene is not good because its dielectric constant is relatively high and would keep capacitance up. Inclusion of air in the dielectric is ideal, foam low density polyethylene is the answer, because it combines the mechanical stability of PE with the best dielectric: air. This is the material for choice for these higher performance AV cables (other more expensive dieltrics such as TEF can also be used)