Water-resistant humidity sensors based on sulfonated polyimides

Sulfonated polyimides (SPIs) derived from 1,4,5,8-naphthalenetetracarboxiylic dianhydride (NTDA) and different sulfonated diamines were tested as humidity-sensing membranes. Surface-resistance-type humidity sensors were prepared by dip-coating a SPI solution onto comb-shaped electrodes on ceramic substrates, and their sensor properties were investigated. The SPI in triethylammonium form derived from NTDA and 4,4′-bis(4-aminophenoxy)-biphenyl-3,3′-disulfonic acid was the most suitable for the humidity sensor from the view points of easy fabrication, higher conductivity and better adhesion property between membrane and substrate. The pretreatment of the substrates with an amine-type silane-coupling reagent significantly improved the water stability of the sensors. They displayed excellent durability against water, heat, high temperature and high humidity, heat-shock and dewy atmosphere. The relationship of the resistance (R) versus relative humidity (RH) was stably reproduced after the severe durability tests, which the conventional polymer sensors could not stand for. The SPI sensors showed a fast 90% response time of about 1 min for the humidification from 30%RH to 90%RH but a fairly slow one of about 4 min for the dehumidification from 90%RH to 30%RH at 25 °C. They showed fairly large hysterisis of 5–6%RH in the R versus RH relationship between humidification and dehumidification runs. The 90% response time was fast at 75 °C, but the hysterisis improved only slightly. They have high potential of the applications for bathroom fans and laundry driers, where the high durability against dewy atmosphere or high temperature and high humidity is essentially required rather than the quick response.