Cross-linked polyethylene (XLPE) has been widely applied for high voltage cable insulation materials due to its excellent electrical and mechanical properties. The peroxide cross-linking reaction is the traditional process used for the synthesis of high voltage polyethylene cable insulation; however, this process has various disadvantages (e.g., slow production speed, high energy consumption, pre-cross-linking of the material on the surface of the extrusion die during long production periods).
The ultra-violet (UV) radiation cross-linking process may become a candidate for the fabrication of the high voltage cable material XLPE.With the aid of a photo-initiator, UV energy can easily penetrate the insulation wall and induce cross-linking when the insulating wall is transparent because polyethylene crystals melt after heating by extrusion.
The advantages of UV crossing compared to the traditional process include fast processing speed, small radiation zone, energy saving, and the production is not thermosensitive. Experimental investigations have shown that the rate of the UV radiation cross-linking reaction is not only influenced by power, radiation spectrum of the mercury lamp, and UV light emitting diode (LED) hybrid system, but also the type and content of photoinitiator and crosslinker.
With the use of the multi-functional crosslinker triallyl isocyanurate (TAIC), the crosslinking process of polyethylene via UV radiation can be as fast as milli-seconds, whereas the crosslinking rate is only on the minute timescale with the use of only photoinitiator.
However, the reaction mechanism of polyethylene cross-linking via UV radiation at the atomic and molecular levels are not very clear thus far, particularly with the use of a crosslinker. Therefore, to elucidate the chemical reactions occurring during the UV radiation cross-linking of polyethylene for the development of insulation wall materials for high voltage cables, the role played by the crosslinker should be clearly understood.
Under a high and divergent electric field, partial discharge and insulation failure are often initiated by electrical treeing.
The rated voltage of XLPE insulated power cables is limited to 500 kV even though XLPE is manufactured with super-clean technology. Research has shown that some organic polycyclic aromatic compounds or those with benzophenone-like structures, which serve as voltage stabilizers, can increase the resistance to electrical treeing effectively.
Using theoretical studies, our group first elucidated the mechanisms of aromatic carbonyl compounds as voltage stabilizers for increasing the electrical breakdown strength of XLPE in 2013.Acetophenone is an example of an aromatic carbonyl compound that can function as a voltage stabilizer; however, it easily migrates out of the polymeric matrix. Thus, aromatic carbonyl and benzil-type compounds with a larger alkoxy chain can effectively increase the compatibility with the polyethylene matrix and improve the electrical treeing inception level significantly.
This inspired us to investigate if polyethylene chains can be grafted with voltage stabilizer molecules to yield stationary products during the UV radiation cross-linking process for the fabrication of XLPE insulation materials possessing permanent insulation performances.