Preparation And Insulation Properties Of XLPE Grafted With P-Oleoyloxy Acetophenone

Crosslinked polyethylene（XLPE）is widely used as the main insulation material of power cables due to its excellent insulation performance,mechanical and thermal properties.With the continuous improvement of the voltage level of transmission lines,higher requirements are put forward for the insulation performance of XLPE.In this paper,p-Oleoyloxy Acetophenone（APO）,a graftable voltage stabilizer,was prepared by modifying p-hydroxyacetophenone with oleoyl chloride,then melt blended with low density polyethylene,and finally APO grafted cross-linked polyethylene（XLPE-g-APO）was obtained hot pressing and cross-linking process.The effect of the amount of voltage stabilizer on the physical and chemical properties,as well as AC and DC insulation properties of XLPE-g-APO was studied,and the mechanism of APO improving insulation properties was analyzed by theoretical calculation.The structure of APO and the physicochemical properties of XLPE-g-APO were analyzed by Fourier transform infrared spectroscopy（FTIR）,nuclear magnetic resonance（¹H NMR）,gel content determination,determination of rheological properties and differential scanning calorimetry（DSC）.The results show that APO is successfully prepared by acylation reaction of oleoyl chloride and AP,and can be grafted onto XLPE molecular chain by free radical addition reaction during cross-linking process.When the amount of APO is 0.8 wt%,the maximum effective grafting rate is 71.3%.The addition of APO slightly reduces the cross-linking reaction rate and increases the cross-linking time.With the increase of APO content,the crosslinking degree and mechanical properties of XLPE-g-APO increased first and then decreases,while the crystallization temperature and melting point increases,and the crystallinity reaches the maximum when the APO content was 0.8wt%.The effect of grafting APO on the insulation performance of XLPE was studied combining with theoretical calculation and in comparision with XLPE and acetophenone（AP）blends（XLPE-AP）.APO has a low LUMO value and a narrow energy gap,which is easy to be excited to capture high-energy electrons,and dissipates high-energy electron energy through ketone-enol isomerization.At the same time,shallow traps are introduced into XLPE after grafting with APO,and XLPE with 0.8 wt%APO（XLPE-g-0.8APO）shows the best comprehensive insulation performance.In terms of DC insulation performance,the appropriate amount of APO can reduce the conductivity of XLPE under low field.When the electric field intensity is higher than 25 k V/mm,XLPE-g-APO exhibits field-induced conductivity characteristics,and the conductivity of XLPE-g-APO is slightly higher than that of pure XLPE,but always lower than that of XLPE-AP.When the content of APO is 0.4 wt%,it has a certain inhibitory effect on the injection and accumulation of space charge.XLPE-g-0.8APO has the highest DC breakdown field strength in a wide temperature range,the value is 452.6 k V/mm,393.8 k V/mm and 277.7k V/mm at 30 ^oC,50 ^oC and 70 ^oC,respectively,which is 19.0%,24.2%and 14.0%higher than that of pure XLPE.In terms of AC insulation performance,the introduction of APO can inhibit the initiation and growth of electrical trees.XLPE-g-0.8APO has the highest electrical tree initiation voltage of 7.23 k V,which is 30.9%higher than that of pure XLPE.The AC breakdown field strength of XLPE-g-0.8APO in a wide temperature range is also the highest,which is about 10%higher than that of pure XLPE.Meanwhile,the dielectric constant of XLPE-g-0.8APO remains between 2.44～2.46,and the dielectric loss is less than 5×10^-3.In addition,the grafted voltage stabilizer APO shows obvious resistance to migration compared with AP,and the decrease of AC and DC breakdown field strength of XLPE-g-APO after aging was significantly lower than that of XLPE-AP and XLPE.