Effect of Molecular Structure Modification on Surface Discharge between Polypropylene and Silicone Rubber

Polypropylene (PP) as a thermoplastic polymer with high breakdown strength and thermal properties has been considered as a promising candidate for the new generation recyclable cable insulation. To study the potentiality of application in AC cable, the interface characteristics of PP/other polymers should be investigated, because it has been generally accepted that the weak point in a long cable route is cable accessory which connects each cable segment. The polymer surface often acts as a part of solid-gas or solid-solid composite insulation geometry. This paper reports on the effect of molecular structure modification on interface discharge characteristics between isotactic PP and silicone rubber under AC voltage. Direct fluorination was applied as the molecular structure modification method. The influence of fluorination time on discharge initiation and propagation stages has been estimated by measuring phase resolved partial discharge pattern and relationship between discharge rate and discharge magnitude. Attenuated total reflection Fourier transform infrared spectrum, water contact angle and surface roughness were also measured to gain a better understanding of the fluorination effect on surface chemical and physical structures of samples. Obtained results showed that with the increase of fluorination time, the average surface roughness of PP increased, whereas the water contact angle decreased. In addition, the discharge initiation time increased while the discharge propagation time decreased. It is suggested that at the initiation stage the C-F bond introduced by direct fluorination with higher bond energy and deeper trap depth should be responsible for the alleviation of polymer degradation. It could be concluded that although the direct fluorination has positive effect by delaying the appearance of discharge channel.