UV-Initiated Crosslinking Reaction Mechanism and Electrical Breakdown Performance of Crosslinked Polyethylene.

Yu-Wei Fu,Xuan Wang,Wei-Feng Sun

doi:10.3390/polym12020420

Abstract

The ultraviolet (UV) irradiation crosslinking reactions of polyethylene and the electronic properties of photo-initiators and reaction products are theoretically investigated by the first-principles calculations. The crosslinked polyethylene (XLPE) materials are prepared in experiments that employ the UV-initiated crosslinking technique with different photon-initiation systems. Infrared spectrum and the alternating current dielectric breakdown strength of UV-initiated XLPE are tested to explore the effect of reaction products on the breakdown characteristics in combination with the electron structure calculations. The theoretical calculations indicate that the 4-hydroxybenzophenone laurate, which is compatible with polyethylene, can effectively initiate crosslinking reactions of polyethylene molecules under UV photon excitation and will produce reaction by-products from carbonyl radicals; as a macromolecular auxiliary crosslinker, the monomer or homopolymer of dioleyl-2,2′,4,4′-tetraallyl isocyanurate can form chemical connections with multiple polyethylene molecules acting as a crosslinking node in a photon-initiated reaction process. The carbonyl, hydroxyl, or ester groups of reaction by-products are capable of capturing hot electrons to prevent polyethylene molecules from impact ionization, and thus will increase the breakdown electric field. The macromolecular auxiliary crosslinker and the macromolecular photon initiator as well as its reaction by-product can convert the energy of their captured high-energy electrons into heat, which can act as a voltage stabilizer. The molecule characterization of infrared spectra demonstrates that the characteristic absorption peaks of the carbonyl in the macromolecular photon initiator and the allyl in the macromolecular auxiliary crosslinking agent are gradually decreasing in intensity as the crosslinking reaction proceeds, which is consistent with the conclusion from theoretical calculations. Compared with the small molecular photon-initiation system generally used in the photon-initiated crosslinking process, the higher dielectric breakdown field of XLPE being prepared by utilizing a macromolecular photon-initiation system is in good agreement with the calculation results of electronic affinity and ionization potential. The consistent results of the experiments and first-principles calculations elucidate the fundamental mechanism of the UV-initiation crosslinking technique and suggest a prospective routine to improve the insulation strength for developing high-voltage XLPE insulating materials.

Highlights

At present, submarine power cables are primarily fabricated by using crosslinked polyethylene (XLPE) materials to realize electrical insulation in high voltage grade [1,2]
In comparison to the ground state S0 and excited state S1, the photon initiators in excited state T1 provide double radicals on ketone carbonyl (C=O) as indicated by the significantly higher positive and lower negative Mulliken charges on C7 and O8 atoms respectively, while the charges populated on other carbon atoms are almost unchanged
In order to elucidate the underlying physical and chemical mechanisms of UV-initiated polyethylene crosslinking reactions, the excited states of the photon initiator, the transition states, the energies of radical producing and connecting reactions, and the electronic structure of crosslinking by-products are calculated by the all-electron numerical orbit first-principles method for both micromolecular and macromolecular photon-initiation systems

Summary

Introduction

Submarine power cables are primarily fabricated by using crosslinked polyethylene (XLPE) materials to realize electrical insulation in high voltage grade [1,2]. The continuous cable production employing the chemical crosslinking technique with peroxide can not persist for sufficiently long time, which limits its manufacture to obtain a single cable with enough length for avoiding redundant connections of relaying cables. The ultraviolet (UV) crosslinking technique is a non-heat-sensitive process of fabricating XLPE with the advantages of a high production rate, long-time continuous production, small fundamental investment, low raw material cost, and low energy consumption, which is promised to realize the industrial production of high voltage grade and extra-long XLPE insulated cables [3,4,5,6]

Methods

Results

Conclusion