Drying of solvent-borne coatings with pre-loaded drying gas

To investigate the effect of a methanol pre-loaded gas phase on the drying of toluene-poly(vinyl acetate) (PVAc) solutions, the solvent concentration profiles in thin coatings were measured by means of Inverse-Micro-Raman-Spectroscopy (IMRS). Without pre-loading, the toluene-PVAc solution shows a diffusion controlled skinning effect, which leads to a strongly extended drying time. In contact with a methanol pre-loaded gas phase, methanol diffuses into the coating accelerating the evaporation of toluene according to the level of pre-loading. After a certain toluene content is obtained, the residual methanol can be removed easily due to its higher vapor pressure and its better diffusion characteristics in PVAc compared to toluene (no skin formation). The presented measurements demonstrate the possibility to reduce the overall drying time of skin-forming coatings by pre-loading the gas phase with a suitable additional solvent, which is more volatile and has better diffusion characteristics in the polymer than the trapped solvent.