Effect Of Delocalized Aromatic Molecules With Electron-donating And Electron-Withdrawing Groups On Insulating Properties Of Polyethylene Blends

With the rapid development of economic and social in the world today,higher requirements are imposed on the power network.Long-distance and large-capacity power transmission has also become a trend.Therefore,high-voltage cables with higher electrical resistance are required.The insulation layer of a high-voltage cable has an important effect on the dielectric strength.At present,the most widely used insulation materials are mainly cross-linked polyethylene(XLPE),which has good insulation properties,strong thermal stability and mechanical stability.However,the thermosetting characteristic makes it unrecyclable,prompting people to increase the research on thermoplastic materials such as low density polyethylene(LDPE)and high density polyethylene(HDPE)to replace thermosetting XLPE.For improving the common problems such as electrical treeing and space charge in the process of apply ing insulating materials,introducing nanoparticles,polymer blending,adding voltage stabilizers and chemical grafting are currently effective methods.Among them,the addition of voltage stabilizers is the most common.The most used voltage stabilizer usually contains conjugated structure aromatic ring and polar groups.Adding a voltage stabilizer to a polymer has the disadvantages of limited solubility and easy migration.Hence,researchers have tried to graft the voltage stabilizer onto the polyethylene molecular chains for improvement.In this paper,graphene oxide was first prepared by a modified Hummers method.In addition,nitrogen-doped graphene,boron-doped graphene and boron-nitrogen co-doped graphene were prepared by a vapor deposition method.Next,0.2 wt%of doped graphene was added to the polyethylene blending material(90%LDPE+10%HDPE,which is hereinafter referred to as LH)to prepare composites by a solution blending method.Finally,electrical properties were tested on the composites.For the electrical treeing test,the addition of the three kinds of doped graphene increases the tree initiation voltage(TIV)of the composites to different degrees.Among them,boron-nitrogen co-doped graphene has the greatest increase of 17%.Meanwhile,boron-doped graphene and nitrogen-doped graphene increases the TIV by 7.8%and 10.7%,respectively.In the space charge test,all the three kinds of doped graphene have a certain effect on inhibiting the accumulation of space charge and improving the space charge distribution in the composites.In the DC conductivity test,the addition of three kinds of doped graphene has greatly increased the conductivity of the composites.However,in comparison,boron-nitrogen co-doped graphene has the least increase of the conductivity.Seven kinds of aromatic molecules containing different electron-donating groups and electron-withdrawing groups were screened as voltage stabilizers.1 wt%of the above molecules were added to LH by the diffusion loading method to obtain blending materials.The same electrical performance tests were performed.It is found that 3-aminobenzoic acid performed best in the electrical treeing experiment,which can increase the TIV by about 50%compared with the reference sample LH.The five other molecules simultaneously connected to the electron-donating and electron-withdrawing groups increase the TIV by 8%-43%,respectively.4-cyanophenylboronic acid,which contains only electron-withdrawing groups,reduces the TIV by 5%.Further Gaussian simulations were performed to calculate the energy gaps Eg of the seven molecules.Moreover,a linear fit was made between the TIV and Eg,in which a good negative correlation is established.That is,the smaller the Eg is,the more advantageous it is to suppress tree initiation.In the space charge experiment,3-aminobenzoic acid and 3-aminophenylboronic-acid perform better,and both show significant space charge suppression effect.In the DC conductivity experiment,3-aminobenzoic acid still performs best,and it shows the effect of reducing the material conductivity under the test conditions of 303 K,313 K and 323 K.The best-performing 3-aminobenzoic acid was grafted onto the polyethylene molecular chains by the solution grafting method.FTIR characterization displays the characteristic peaks of C-N(alkyl carbon)and N-H(secondary amino)in the grafted samples,which proves that the grafting operation was successful.According to DSC and TGA characterization,it is observed that the effect of the grafting operation on the thermal properties of the grafting material is very small under this condition and can be even ignored.For the electrical performance test,the grafting sample has a 29% increase of TIV compared with LH,which has an obvious inhibitory effect on the electrical treeing.However,it is not as good as diffusion loading samples,presumably due to the very limited grafting rate.In terms of space charge inhibition,the grafted sample shows little effect.It is supposed that the limited grafting rate causes the number of traps formed in the polymer to be very limited,which cannot reach the threshold of space charge accumulation inhibition.While for electrical conductivity,at higher temperatures of 323 K and 333 K,the grafted samples still shows a significant decrease in electrical conductivity.The reason may be that the polar molecule,3-aminobenzoic acid,has a certain conductivity and the lower grafting rate is in favor of the DC conductivity performance.Based on the above results,it is considered that the voltage stabilizers have similarities in the three test mechanism.From the analysis of molecular structure characteristic,it is believed that the co-existence of matched electron-donating and electron-withdrawing groups in the aromatic molecules is favorable for the insulation properties.With regard to the suppression of electrical treeing,the molecules with this structure act like a miniature electric field,and the electron-withdrawing group and the electron-donating group behave like two poles,which can more thoroughly buffer the energy of the captured high-energy electrons.In terms of the inhibition of space charge accumulation and the decrease of DC conductivity,the aromatic ring in the molecule of the structure can serve as a carrier binding region,and the electron-withdrawing group and the electron-donating group can serve as electron and hole traps,respectively.Therefore,better capture of the carriers can be achieved.