Control of electrical tree at initiation stage in LDPE by mixed addition of Al2O3 nano-particle and azobenzoic compound

A new polymer nano-composite was prepared by the mixed addition of azobenzoic compound ('Az compound') and Al2O3 nano-particle ('Al2O3') into LDPE to improve the electrical treeing resistance under ac voltage application. Three kinds of azobenzoic compound were tested: azobenzen ('Ab'), nitro-azobenzen ('Ni_Ab') and aminoazobenzen ('Am_Ab'). No dispersant was used for the particles. The test materials of the mixed addition were processed in our laboratory by solution casting method. The experimental results showed that the most effective tree resistance was observed in the mixed addition compound where 'Ab' was used. Tree initiation voltage (TIV) was increased by the mixed addition. A 300% increase in ratio was obtained when the concentrations of 'Ab' and 'Al2O3' were 2.0 wt% and 3.0 wt%, respectively. The incubation time (the period of time from voltage application to tree initiation) was significantly elongated by the mixed addition. The incubation time for mixed addition under a constant voltage of 13 kV was about 100 times longer than that of LDPE without any additive. The mixed addition was also effective in the control of tree development in its initiation stage. PD due to tree initiation was reduced to an undetectable level under the constant applied voltage of 10 kV following a short time application of over voltage. The undetectable state continued for more than 2000 min under constant voltage application, whereas, in the case of LDPE without any additive, PD could not be restricted to the undetectable level. It was revealed that the mixed addition of 'Ab' and 'Al2O3' was significantly effective in the control of tree development in the initiation stage. On the other hand, the mixed addition has no significant effect on the control of tree development after its initiation stage.