Simplified method for predicting residual solvent content in polymer coatings

Polymer coatings have a wide variety of uses in a number of industries. The polymeric films are coated on a substrate in a wet state and are dried to meet residual solvent content (RSC) specifications. Drying models predict the residual solvent content in the films. Previous models are elaborate and involve coupled mass and heat transfer equations. These equations are non-linear partial differential equations and are solved using numerical methods such as the finite element and finite differences methods. These models predict evolution of solvent concentration and temperature through the entire drying operation. At long drying times, the rate of solvent removal falls to an extremely low value and residual solvent plateaus at a nearly constant value. This apparent shut-off in drying is due to strong diffusional resistance to solvent transport through a solvent-depleted layer near the coating surface. The drying period is called the diffusional plateau, where drying rate becomes negligible and RSC plateaus at a non-zero value. The focus of this thesis is to predict RSC in the system at long times with a simple model. This thesis develops a simple, quasi-steady state, one-dimensional model to predict RSC in the drying polymer-solvent coatings for a given drying temperature and dry polymer film thickness. The use of only these key operating parameters makes this model is much easier to use than previous drying models and is amenable to calculation by standard spreadsheet software. Ease of these calculations provides a wide scope of use in industry where RSC specifications are to be met. The quasi-steady state model results agree well (9-15%) with results obtained from a finite element method prediction and with experiments (10-20%) done using a thermo gravimetric analyzer for the system of poly (vinyl acetate)-toluene. A detailed analysis on how the three main physical parameters of temperature, drying rate and dry polymer film thickness affect the residual solvent for this system has been done. Other polymer-solvent systems were also examined and the quasi-steady state method was found to give good comparison with results from finite element method predictions for these systems.