Abstract
To achieve high direct current (DC) dielectric performance of crosslinked polyethylene (XLPE) applied for insulated cable, the auxiliary crosslinking agent of trimethylolpropane trimethacrylate (TMPTMA) is employed in photon-initiated crosslinking process to the present polar-molecular group which will introduce deep traps for charge carriers. The space-charge accumulation and electrical conductance of XLPE are observably suppressed due to the deep traps deriving from the TMPTMA crosslinkers that are chemically connecting (grafted onto) polyethylene molecules. Thermally stimulated depolarization current tests and first-principles calculations consistently demonstrate a trapping mechanism of impeding charge injection and carrier transport in XLPE with TMPTMA crosslinkers. The characteristic cyclic anhydrides with coupled carbonyl groups are used as auxiliary crosslinkers to promote crosslinking efficiency and provide polar groups to polyethylene molecules which can be effectively fulfilled in industrial cable production. The results of infrared spectroscopy show that the auxiliary crosslinkers have been successfully grated to polyethylene molecules through the UV-initiation process. The space-charge characteristics achieve a significant improvement consistent with the theoretical estimation that deeper electronic traps can be introduced by auxiliary crosslinker and will consequently suppress space-charge accumulation through a trapping mechanism. Meanwhile, the conductivity of XLPE observably increases after using TMPTMA auxiliary crosslinkers at various temperatures of cable operation. The first-principles calculations also demonstrate that substantial electronic bound states have been introduced at the band edge of polyethylene molecules crosslinked by TMPTMA, leading to reduction in electrical conductivity. On the advantage of ameliorating DC dielectric performance by way of UV-initiated crosslinking process, the present research suggests a substantial strategy in XLPE cable industrial productions.
Highlights
At present, the effective methods of modifying polymer insulation materials have been presented as by polymer dielectric nanocomposites, ultra-clean process, blending, and chemical modification [1,2,3]
For photon-initiated crosslinking reactions, the prepared hot-pressed blend is first treated in a plate vulcanizer at 160 ◦ C with the pressure being increased by 5 MPa per 5 min from 0 to 15 MPa so as to make the material melt, and the melt material is irradiated by a light source array of UV LED units (NVSU233A-U365, Riya Electronics Chemistry Co., Ltd., Shanghai, China) for 2 s on an irradiation platform at normal pressure and room temperature in an air atmosphere
It is explicit that the XLPE crosslinking degrees being initiated by UV irradiation with trimethylolpropane trimethacrylate (TMPTMA) have reached an adequate 25% in thermal elongation to be mechanically qualified for cable productions, implying the competent efficiency of UV-initiation crosslinking technique with TMPTMA as the auxiliary crosslinking agent
Summary
The effective methods of modifying polymer insulation materials have been presented as by polymer dielectric nanocomposites (filling nanoparticles), ultra-clean process, blending, and chemical modification [1,2,3]. High concentration of inorganic nanofillers to obtain nonlinear composites generally used in insulation systems will inevitably deteriorate mechanical properties and break down strength. These drawbacks and difficulties in developing cable accessories by nanodielectrics technology make it almost impossible to be fulfilled in practical industrial productions. Recent reports indicated that excellent dielectric properties of modified polymers by chemically grafting are attributed to the trapping mechanism of space-charge suppression and break down strength improvement [7]
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