Trapping skinning: An anomalous drying behavior of polymer solvent systems

Drying gas flow rate, temperature and solvent composition are three important operating conditions in any industrial dryer. The operating conditions are manipulated to produce defect-free coatings that meet residual solvent specifications. As the drying conditions become more intense, the initial drying rate becomes faster and one might expect the residual solvent to be lower in dried coating. The residual solvent, however, can be higher in coatings dried under more intense conditions. This behavior is called trapping skinning and is not well understood and remains undocumented in the literature. This thesis reports experimental measurements of trapping skinning and a mathematical model that predicts it.; Industrial dryers have evolved from festoon dryers with low airflow to state-of-the-art hybrid dryers with high airflow with a corresponding increase in the rate at which heat can be transferred to the coating (or solvent removed). Laboratory experiments, however, have not evolved to similar heat transfer coefficients because of fluctuations in weight measurements in conventional gravimetric experiments at high airflows. This thesis demonstrates a High Airflow Drying Experimental Set up (HADES) that simulates drying in spiral and single-side impingement dryers. Experiments from HADES show that poly (methyl methacrylate)/acetone coatings exhibit trapping skinning at higher gas flows and the anomalous behavior is due to a glass transition occurring during drying and stress-driven diffusion. A diffusion Deborah number, De, characterizes stress development in a coating and the magnitude of stress-driven diffusion to mass transfer. Glassy polymer coatings are likely to show trapping skinning if the De approaches O(1) from below during drying. The stress develops due to coating shrinkage during drying and decays with a relaxation time that depends on solvent concentration and temperature.; Fick's law of diffusion with concentration dependent diffusion coefficient cannot explain trapping skinning. So, a non-Fickian non-isothermal model is developed to capture this behavior. The non-Fickian model includes the solvent mass transport due to stress gradients. This thesis shows how stress relaxation inside the coating causes trapping skinning. This thesis also illustrates the implications of the non-Fickian model on dryer design.