Research On Preparation And Properties Of Vinyl Carbazole Grafted Polyethylene

With outstanding electrical properties, good processability and low cost, polyethylene has been the first choice for the high voltage(HV) and extra high voltage(EHV) direct current(DC) power cable materials. However, the polyethylene, which was used in HVDC cables materials has the space charge accumulation problem. Recent studies showed that doping some inorganic nanoparticles and small organic conjugate molecules into the polyethylene could inhibit the amount of space charge. Because of the bad compatibility and agglomeration problems of the inorganic nanoparticles and the migration and volatility problems of small organic conjugate molecules, it is difficult for the composite materials to remain the properties stable for a long time. In order to suppress the space charge and avoid the agglomeration issues of the inorganic nanoparticles and the migration issues of the small organic conjugate molecules, the vinyl carbazole(VK), which can be used as space charge inhibitor because of the large conjugate structure and the good charge dispersion ability, was grafted to polyethylene, and was studied in this paper. Electrical properties and physical mechanical properties of the vinyl carbazole grafting polyethylene(VK-g-LDPE) were investigated also. This work could lay the foundation for the industrialization of polyethylene DC cable materials with high properties.The melting method and the hot pressing method were used to prepare grafting polyethylene in this paper. Based on the graft degree(GD), gel content and electrical properties research of the VK-g-LDPE materials, the melting method and the hot pressing method were compared, and the better grafting method were determined. Through the electrical properties, mechanical properties and space charge distribution comparison of the VK-g-LDPE and Nethyl carbazole/polyethylene composites, the more effective modification method was confirmed. The results showed that the melting method had less crosslinking side reaction and the products had higher GD. Moreover, the grafting was more uniform and the volume resistivity and the breakdown field strength of the materials were obviously better than that of the hot-pressing method. The chemical graft modification was significantly better than the physical blending modification in improving the volume resistivity, breakdown field strength of polyethylene in grafting polyethylene. And the VK-g-LDPE is obviously superior than N-ethyl carbazole/polyethylene composites in stability and suppressing the accumulation of space charge. The maximum volume resistivity of graft polyethylene is 2.1 times as that of polyethylene composites, the maximum breakdown field strength is 1.36 times as that of the composite materials. However, the two modification methods were similar in improving mechanical properties.In this paper, the synthesis of VK-g-LDPE was researched by melting method. Through the effects of grafting conditions on the GD, the optimum conditions were determined. Moreover, the influence of GD on the properties of graft polyethylene materials was investigated by testing the electrical properties, thermostability, mechanical properties and space charge distribution of grafted polyethylene materials. The results showed that the optimum conditions were as follows: the grafting temperature was 180 ℃; the rotor speed was 50 r/min; grafting time was 9 min; mass ratio of LDPE, DCP and VK was 150:1:50. The highest GD reached 15.24 %. With the increase of GD, the dielectric constant reduced first and then increased. The volume resistivity and breakdown field strength increased at first and then decreased when the GD increased. Moreover, when the GD was 10.12 %, the volume resistivity was 3.2 times of polyethylene. When the GD was 13.02 %, the breakdown voltage was 67.64 % higher than that of polyethylene. And at this time, the suppression effect of space charge in VKg-LDPE was the best, the maximum charge density was 0.31 C/m3. Compared with polyethylene, the grafting polyethylene has obviously improved in electrical properties and mechanical properties.