Supercritical foaming for constructing hydrophobic PI films with nanoporous structures: For low humidity hysteresis moisture sensing

Real-time and accurate detection of moisture content in insulating oil is crucial for the safe operation of oil-filled electrical equipment. This study presents a method for preparing hydrophobic porous PI films for moisture sensing using supercritical CO 2 foaming (SCF) technology. The resulting porous PI films have a uniform and dense nanometer-scale microporous structure, with a water contact angle of 111.7°, indicating strong hydrophobicity and enabling the mitigation of hysteresis-induced measurement inaccuracies. Compared to sensors made using traditional methods such as non-solvent induced phase separation (NIPS), the moisture sensors fabricated using this method exhibit a response time of 10 s, with humidity hysteresis reduced from 9.1 %RH to 2.3 %RH, achieving both fast response and low humidity hysteresis. This method also holds broad application prospects in areas such as permeation membranes and energy.