Fu Yu-Wei, Sun Wei-Feng, Wang Xuan
Key Laboratory of Engineering Dielectrics and Its Application, Ministry of Education, School of Electrical and Electronic Engineering, Harbin University of Science and Technology, Harbin 150080, China.
Polymers (Basel). 2020 Feb 12;12(2):420. doi: 10.3390/polym12020420.
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.
通过第一性原理计算，从理论上研究了聚乙烯的紫外（UV）辐照交联反应以及光引发剂和反应产物的电子性质。在实验中，采用紫外引发交联技术和不同的光子引发体系制备了交联聚乙烯（XLPE）材料。结合电子结构计算，测试了紫外引发的XLPE的红外光谱和交流介电击穿强度，以探究反应产物对击穿特性的影响。理论计算表明，与聚乙烯相容的月桂酸4 - 羟基二苯甲酮在紫外光子激发下能有效引发聚乙烯分子的交联反应，并会产生羰基自由基的反应副产物；作为大分子辅助交联剂，二油基 - 2,2',4,4'-四烯丙基异氰脲酸酯的单体或均聚物在光子引发反应过程中可与多个聚乙烯分子形成化学连接，充当交联节点。反应副产物的羰基、羟基或酯基能够捕获热电子，防止聚乙烯分子发生碰撞电离，从而提高击穿电场。大分子辅助交联剂、大分子光子引发剂及其反应副产物可将捕获的高能电子的能量转化为热量，起到电压稳定剂作用。红外光谱的分子表征表明，随着交联反应的进行，大分子光子引发剂中羰基和大分子辅助交联剂中烯丙基的特征吸收峰强度逐渐降低，这与理论计算结果一致。与光子引发交联过程中常用的小分子光子引发体系相比，利用大分子光子引发体系制备的XLPE具有更高的介电击穿场强，这与电子亲和能和电离势的计算结果相符。实验和第一性原理计算的一致结果阐明了紫外引发交联技术的基本机理，并为开发高压XLPE绝缘材料提高绝缘强度提供了一种有前景的方法。
