Surface levelling of thermosetting powder coatings: theory and experiment

The deposition of protective coatings from thermosetting polymer powders is an ecological, economic and energy-efficient technology. A frequent problem encountered with powder coatings is a rough surface (with undulations on a length scale much greater than the powder particle size) that detracts from the visual appearance. The levelling (i.e. flattening) of the surface of a polymer melt is driven by the minimization of the surface energy but opposed by the (possibly time-dependent) viscosity of the melt. We address the problem by developing a model of surface levelling to consider flow in two directions, building upon a one-dimensional model already in the literature. We have performed simulations to predict the final coating profiles starting with a Gaussian profile and using experimentally determined values of polymer viscosity. To compare to the simulations, we have measured experimentally the dimensions of surface undulations on coatings formed from thermosetting acrylic powder layers having purposely created features of known dimensions. There is good agreement between simulation and experiment. Both find that the levelling proceeds to a greater extent with increasing coating thickness and with decreasing lateral dimension of the surface undulation. Our results open up the possibility of predicting final surface topography given the rheological properties of a polymer.