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THE FATE OF RESIDUAL SOLVENT IN DRYING COATINGS: CAN IT GET TRAPPED AND HOW?

2002 Dahlquist Award Winner

Richard Allan Cairncross
Drexel University, Philadelphia, PA

A common goal in industrial drying of polymer solution coatings is to reduce the residual solvent content (RSC) to a specified level. Industrial dryers consist of a series of zones operated at different air temperatures and airflow rates to meet the RSC specifications, and to produce defect-free coatings. A common observation is that, when drying at a constant temperature, the residual solvent content plateaus and the drying rate effectively drops to zero. Often the RSC can be reduced by further increasing the temperature. In homogeneous polymer solutions above the glass transition temperature of the polymer, the observed plateau in RSC is accurately predicted by Fickian diffusion with a concentration-dependent diffusion coefficient. We have developed a simple model which predicts the dependence of RSC on temperature, coating thickness, and the diffusion properties of the solution. In this case, solvent is retained by the diffusional resistance to mass transfer, and the diffusional resistance can be lowered by increasing temperature.

However, there are numerous claims that the RSC can also be reduced by using milder drying conditions, e.g. lowering airflow, lowering temperature or partially saturating the air with solvent vapor. Such behavior is anomalous and cannot be predicted by Fickian diffusion; we call this behavior anomalous skinning. We have measured anomalous skinning in PMMA/acetone coatings and have developed a non-Fickian model which predicts the anomalous behavior.