Drying of thin films of polymer solutions coated over impermeable substrates

The manufacturing of adhesive tapes, photographic films, magnetic media, and many other products involves the coating and drying of thin films of polymeric solutions or emulsions over an impermeable substrate. Wet thickness may vary from 1 micron to 500 microns and solid content from 3% to 50% or more. The drying process generally occurs in convective ovens, where air temperature, air flow rate, and solvent partial pressure may be adjusted. In many cases, it is the manufacturing step that controls the productivity; therefore, it is very important to optimize the oven conditions to minimize the oven residence time without causing defects on the coated film and keeping the residual solvent inside the final product within specification limits. A drying model and a simulation software based on it were developed in order to solve the transient mass and heat transfer equations that describe the drying of a polymer solution containing two different solvents. The resulting system of equations shows strong non- linearity due to the free surface and the dependence of diffusion coefficients on solvent concentration and film temperature. The integration in time was performed with a fully implicit method, and at each time step, Newton's method was used to solve the resulting non- linear system of algebraic equations. An example of using the drying simulator to optimize an industrial adhesive tape line is discussed. The gain in productivity achieved by the analysis was greater than 40%.