Wrinkling of a two-layer coating, comprised of a glassy polyvinyl alcohol top layer and a rubbery polyethyl acrylate bottom layer, was studied. The coatings were prepared on polyethylene terephthalate substrates by depositing and drying the layers in sequence. Fine, short-wavelength (180-200 μm) wrinkles formed when the two-layer coating was exposed to humid atmosphere after the second drying treatment and coarse, long-wavelength (450-500 μm) wrinkles formed when the two-layer coating was heated to a temperature above the glass transition temperature of the top layer. Both types of wrinkles must have arisen from compressive stress in the top layer. The compressive stress developed by either moisture absorption (fine wrinkles) or differential thermal expansion on heating (coarse wrinkles). When compressive stress is high enough, the top layer deforms out of plane without detaching from the bottom layer to produce wrinkles. The balance of forces in systems of three adhering layers of different thicknesses and moduli in static mechanical equilibrium was analyzed, and it was found that the layer with the highest in-plane axial stiffness, the product of elastic modulus by thickness, typically dictates the dimension of the entire composite. When the mismatch of thermal expansivities of the layers generates the compressive stress that produces wrinkling, the difference between the thermal expansivity of the top layer and the axial stiffness-weighted average thermal expansivity of the three layers governs the compressive stress in the top layer. Because polymer modulus falls abruptly at the glass transition temperature Tg, the critical compressive stress required for wrinkling falls abruptly at Tg. Therefore, the two-layer coating atop a stiff substrate is more susceptible to wrinkling when the top layer is above its Tg. When absorption of moisture in the top layer generates the compressive stress that generates wrinkling, that stress goes through a minimum because modulus falls and compressive strain rises with rising moisture content. However, the critical compressive stress for wrinkling falls because modulus falls with rising moisture content. Therefore, the two-layer coating atop a stiff substrate is more susceptible to wrinkling the greater the moisture content in the top layer. The relative humidity (RH) of the sample atmosphere was raised to raise the moisture content in the top layer. The wrinkle wavelength fell with rising RH because the modulus, and therefore the bending stiffness, of the top layer fell with rising RH. The measured wavelengths were qualitatively predicted at low humidities and quantitatively predicted at high humidities.
Bibliographical noteFunding Information:
This research was supported by the Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME, www.iprime.umn.edu) and companies that participate in the Coating Process Fundamentals Program of that partnership. Dr. Peter E. Price, Jr. and Craig Miller of 3M Inc. brought the phenomenon of wrinkling in two-layer coatings that solidify by drying rather than curing to our attention and made valuable suggestions about selecting a model system. We thank Hui Luo for help with SEM images of the coating cross sections and Wieslaw Suszynski for invaluable advice.