Effect of humidity during the coating of Stِber silica sols

Various silica sols (varying in surface chemistry and solvent) were synthesized by the Stِber process and then subsequently coated on substrates at various humidities. For ethanol-based sols, films prepared by spin or dip coating at low humidities had a higher refractive index, lower thickness, and greater microcracking than those prepared at high humidities. The change in film properties followed an abrupt, instead of a gradual, change with humidity. This change in film microstructure can be explained by the ability/inability of capillary condensed liquid in the micropores of the colloid to evaporate prior to achieving full film strength and hence allowing the micropores to collapse. The magnitude of the shrinkage and the relative humidity at which the pores collapsed were found to depend on the colloid surface chemistry and the coating method. In contrast to the ethanol-based sols, humidity during spin coating had a negligible effect on film properties for sec-butanol and decane-based sols. This is likely due to the lower vapor pressure and/or lower water solubility of these solvents such that the pores in the latter stages of drying the films did not contain much water. Understanding this behavior has been important for improving the performance and process repeatability of using these films as anti-reflective coatings in high-peak-power laser systems.