A New Method for Hydrophobicity Evaluation Based on Tangential Transfer: Beyond Yes-or-No, Towards Discrimination

Room temperature vulcanized (RTV) silicone rubber coatings have been widely applied in power grids owing to their excellent hydrophobicity and hydrophobicity transfer. Among these, hydrophobicity transfer is a key property of outdoor insulation materials, enabling insulators to maintain high resistance to pollution flashover under long-term contamination. IEC 62039 recommends two methods for evaluating hydrophobicity transfer; however, their discriminative capability is limited and the acceptance criteria are relatively loose. As a result, some coatings perform well during initial installation but exhibit a rapid decline in hydrophobicity transfer during service. To address this limitation, a new quantitative evaluation method based on tangential transfer is proposed and validated experimentally on six RTV coatings. The results show that, when the inter-group coefficient of variation is used as the discriminative index, the method provides more than eightfold higher discrimination than the standard methods. It significantly amplifies the performance differences among coatings and enables effective ranking. Furthermore, in combination with the adsorption–cohesion theory, this study explores the correlation between the evaluation results and material properties. TG-DSC, swelling test, Soxhlet extraction, and GC–MS analysis are employed to determine the coating formulation, organic phase crosslink density, and the content and species of low molecular weight (LMW) siloxanes. The results reveal a strong linear correlation between the outcomes of the new method and the concentration of D4–D5. This finding is highly consistent with the transfer theory of LMW siloxanes, thereby confirming the scientific validity of the method. In summary, the proposed method provides a high-discrimination quantitative assessment of the intrinsic hydrophobicity transfer capability of RTV coatings and offers a scientific basis for material selection in engineering practice.