With growing interests in renewable energy, high voltage dc transmission has become a hot topic worldwide. Charge accumulation under high voltage dc is a major issue as its presence distorts the electric field, leading to premature failure. In the present project, we aim to chemically treat polymeric insulation and change charge transport characteristics of the material via fluorination process. Various samples will be electrically characterised and tested, so an optimal processing condition can be achieved to meet practical requirements as dc insulating material. Modelling and simulation of electric field distribution with new developed insulating material have been planned to help design an insulation spacer in high voltage dc GIS systems.