ISSN Online: 2377-424X
International Heat Transfer Conference 12
Drying of polymer varnishes: Solvent diffusion in glassy polymer films
Abstract
Understanding the drying kinetics of polymer solutions is a major issue in numerous industrial processes. Indeed,
most of lacquer, paint and varnish coatings are obtained by performing solvent evaporation from an initial dilute
solution. Drying kinetics generally shows two distinct regimes: the first step is rapid and mainly governed by the
exchanges between the solution and the environment. Then the solvent evaporation flux decreases strongly and
kinetics becomes governed by the physico-chemical properties of the system. Solvent transport in the second
step (polymer concentrated domain) involves several complex phenomena. First, the solvent/polymer diffusion
coefficient decreases by several orders of magnitude when solvent concentration decreases. Moreover, since the
glass transition temperature of a polymer solution depends on the solvent concentration, the film enters the
glassy state during the drying. The solvent diffusion through the polymer matrix is then coupled with the
relaxation of the viscoelastic stresses on the macromolecular chains and exhibits a non-fickian behaviour.
This study deals with the influence of the glass transition on solvent diffusion. A PMMA/PnBMA statistical
copolymer film was chosen to perform a series of differential drying and swelling experiments for various
solvent concentrations. A gravimetric technique was used, based on a quartz crystal microbalance placed in a
controlled solvent vapour pressure chamber. The coupling between diffusion and viscoelastic behaviour is
described with the "solubility" model that takes relaxation into account through a time variable boundary
condition at the film/vapour interface. In the concentrated domain, the solvent/polymer diffusion coefficient was
shown to decrease by about three orders of magnitude when solvent content decreases. Contrary to some authors,
no slowing down in the evolution of the diffusion coefficient was observed when crossing the glass transition.