An experimental and theoretical investigation has been carried out into the evaporation of organic solvents (drying) from thin films of ink. By means of the infra-red technique, experimental drying curves have been recorded from stationary inked specimens during various conditions of forced convective drying. In the constant-rate period, the effect on the drying time of changes in air velocity, air temperature, coating thickness and percentage solvent in the ink, have been measured. Heat and mass transfer theory has been used to develop a criterion for the relative rate of drying of a range of solvents. Theoretical predictions of constant-rate drying time show good agreement with experimental measurements. In the falling-rate period, both the theoretical and experimental evidence appears to show that the rate of drying is limited by the rate at which solvent can diffuse through the polymer residue to the free surface. Analysis of experimental drying curves indicates that the diffusion coefficient is concentration dependent. To provide a theoretical solution of this process, numerical methods were used to solve the diffusion equation, with the diffusion coefficient exponentially dependent upon solvent concentration. The concept of the characteristic drying curve has been successfully used to correlate experimental drying rates in the falling-rate region. Non-dimensional results from the numerical solution of the diffusion equation have also been expressed in the form of a characteristic drying curve. Qualitative agreement between theory and experiment is good.
|Date of Award||1980|