| High voltage direct current transmission?HVDC?has the advantages of small power loss,good stability,high reliability and low cost,has become one of the hot spots in today's research.Polypropylene?PP?has excellent dielectric,heat resistance and mechanical properties as a thermoplastic high voltage DC cable material of choice.However,as a material of HVDC transmission cable,PP accumulates in space charges,causes distortion of electric field easily and even causes partial breakdown and other problems,all those limited its applications in HVDC cable.Based on this,an advanced method was studied to modify PP to solve the problems.In this paper,PP molecule was grafted with a space charge inhibitor 4-allyloxy-2-hydroxybenzophenone?AHB?,PP was grafted with a space charge inhibitor which has conjugated structure,the PP grafted by AHB?PP-g-AHB?has less space charges and better dielectric,heat resistance and mechanical properties,lays a solid foundation for the development of thermoplastic HVDC cables of PP.In this paper,PP-g-AHB was prepared by melt grafting method,and the orthogonal design experiment and infrared spectroscopy were used to study the PP-g-AHB.The results showed that the optimum technological parameters of PP-g-AHB were as follows:mixing temperature 200?,rotor speed 50 r/min,mixing time 9min,material weight ratio PP:AHB:DCP=50:0.8:0.05,branching rate of 0.94%maximum;The existence of carbon-oxygen double bond stretching vibration peak at 1723.15 cm-1in infrared spectroscopy verifies that the AHB has been attached to PP successfully.The effects of grafting ratio of AHB on the electrical properties of PP-g-AHB were investigated by testing the properties such as the space charge,electrical tree,breakdown,volume resistivity,dielectric constant and dielectric loss etc.The results showed that the amount of space charge of product after AHB grafting obviously decreased,which was 74.45%lower than that of pure PP by the maximum space charge of grafted material of 2.21 C/m3.The grafting product also inhibited the electrical tree production.With the increase of the grafting rate,the initial priming voltage of the electrical tree increased first and then decreased,the probability of initiation of the electrical tree first decreased and then increased.When the grafting rate is 0.73%,the initial priming voltage and initiation probability for producing electrical tree of PP-g-AHB are 8.0 k V and 45%,respectively,which are increased by 23.1%and 30.8%respectively compared with 6.5 k V and 65%of pure PP.The grafted product the sensitivity of the breakdown field to decrease with increasing temperature.When the grafting ratio was 0.73%,the breakdown field strength of the material decreased by37.7%with the increase of temperature,however,the reduction of that of pure PP was up to 57.5%.The volume resistivity of AHB grafted product was enhanced significantly.When the graft ratio was 0.73%,the volume resistivity of the system was 1.69×1015?·m,which was 2.1 times higher than that of pure PP.With the increase of AHB grafting ratio,the dielectric constant firstly increases and then decreases.When the grafting ratio is 0.73%,the maximum dielectric constant is 2.19,which is 1.1 times higher than that of pure PP,the dielectric loss tangent does not change much.The mechanical properties and dynamic mechanical test analysis showed that the influence of grafting rate of AHB on the mechanical properties of PP-g-AHB.The results showed that the elastic modulus of PP-g-AHB gradually decreased with the increase of the grafting degree of AHB,tensile strength and elongation at break increased first and then decreased.When the grafting rate of AHB was 0.73%,the tensile strength and elongation at break reached the maximum of 29.64 MPa and 17.13%,respectively,which was 17.9%and 1.2times higher than that of pure PP.According to DMA test and analysis,with the increase of grafting ratio of AHB,the storage modulus first decreases and then increases,while the loss modulus and the loss tangent value have a tendency to rise but not obvious. |
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