Humidity effects in the response of a porous pressure-sensitive paint

Fast-responding pressure-sensitive paints (PSP) based on luminescent oxygen sensor have porous structure in order to improve diffusive oxygen permeation into paint layer. This porous structure inevitably works as an absorbent of moisture in air, which has considerable influences on the response of the paint. The response of anodized alumina pressure-sensitive paint (AA-PSP), which is one of common fast-responding PSPs, was characterized in moist air. Tris(4,7-diphenyl-1,10-phenanthroline) ruthenium (II) dichloride was used as luminophore. Hydrophobic coating was applied to reduce the moisture adsorption. Emission characteristics of the PSP sample were measured using a calibration apparatus, in which pressure, temperature and humidity can be controlled independently. Moisture sorption isotherm of the PSP sample was measured by volumetric gas adsorption method. The experimental results show that the emission intensity of PSP increases in proportion to the relative humidity at a constant temperature and a pressure. This humidity sensitivity remains a constant value under various temperature conditions. The pressure sensitivity has a constant value irrespective of temperature and relative humidity. Moisture adsorption isotherm of the AA-PSP exhibits that the amount of water slowly increases in proportion to the relative humidity in the range from 20% to 80%. This implies that, within this range, water molecules are adsorbed and diffuse into the pore sidewalls to form thin water layer on the pore surface, which may play a major role in humidity sensitivity of the PSP.