Research On The Effects Of Molecular Structure And Crosslinking Degree On The Water-resistant Tree Aging Properties Of Polyethylene

In damp environment,water molecules will permeate into the insulation under the action of electric field,which contribute to the formation of water tree.The existence of water tree will degradate the mechanical and dielectric properties of insulation,and the water tree with long-term development will transfer into water tree and cause permanent damage.Therefore,research and improvement on water tree aging of polyethylene is very important to the safe operation of power cable.In this paper,the water knife electrode method is adopted to perform accelerated aging experiment on samples.And the experiment is divided into two main parts:(1)the effect of molecular structure on resistance of water tree of polyethylene.The samples are LDPE with long side chain on main chain,LLDPE with a large number of short side chain and HDPE with few short side chain on main chain;(2)the influence of crosslinking degree on the resistance of water tree.XLPE samples with different crosslinking degree were prepared by adding different content of crosslinking agent into polyethylene based resin.Peroxide crosslinking method was adopted on LDPE,while ultraviolet irradiation crosslinking method on LLDPE.DSC measurement was perform to obtain fusion-crystallization curve of samples.PLM and SEM were used to observe the spherulite and wafer respectively.Dynamic thermal mechanical curves and stress-strain curves were drawn by the use of DMA and electronic tensile machine.The influence of molecular structure and crosslinking degree on anti-water tree aging characteristics was explained on the basis of crystalline morphology and wafer chain entanglement,combining growth mechanism of the water trees.The study found that:Compared with the LDPE and HDPE,LLDPE has better capacity of resisting water tree aging.Because LLDPE has dense short molecular chain structure which promotes entanglement between wafers.Water molecules in the LLDPE amorphous phase requires more energy and time to destroy these tangling molecular chains,and the tangling molecular chains to some extent will restrain the wafer from slipping under extrusion of water molecules,which is equivalent to weaken the damaging effects of electrically induced stress and inhibit the growth of water trees.From the observation of melting-crystallization curve and PLM ? SEM,it can be acknowledged that with the increase of sample crosslinking degree,the three-dimensional network structure was enhanced and the growth of the spherulites and wafer was inhibited.Thus,the crystallinity decreased.In theory,the increase of amorphous area will accelerate the growth of water tree,but the forming 3D-mesh structure restrict the capacity of water molecules to press material to form water trees.The experimental results shows that the 3D-mesh structure plays a leading role on improving the capacity to inhibit water tree of materials.Dynamic thermal mechanical curve and the stress-strain curve shows that the higher the degree of crosslinking,? relaxation strength weaker,more obvious the stress hardening phenomenon.the cross-linking bond can effectively strengthen the entanglement of molecular chains in amorphous region.Under the action of electrically induced stress,slipping and damage between wafers become more difficult,and the ability to inhibit the growth of water tree was enhanced.