Wrinkling of polymer coatings

PMSE 114

Soumendra K. Basu, soumendra.basu@gm.com1, Alon V. McCormick, mccormic@cems.umn.edu2, Lorraine F. Francis, lfrancis@umn.edu2, and L. E. Scriven, pjensen@cems.umn.edu2. (1) India Science Lab, General Motors R&D, Units 1-8, 3rd Floor, Creator, ITPL, Whitefield Road, 560066, Bangalore, India, (2) Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue S.E, Minneapolis, MN 55455
Wrinkles are wavy features on the surface of a coating. Wrinkles form either by design as in the case of wrinkle finishes, or by accident. Understanding the mechanism of wrinkle formation and the parameters that control wrinkle characteristics, such as wavelength, amplitude and regularity, is necessary for the design of wrinkles for different coating applications as well as for their prevention. Examination of several wrinkling coating systems, including epoxy powder coatings, UV cured acrylates and acrylic coatings, revealed the general steps in wrinkle formation. First, a depth-wise gradient in cure develops, leading to a crosslinked ‘skin' at the free surface with a viscous liquid beneath. Second, the skin swells due to absorption of monomer or solvent from beneath; the swelling is constrained and can only occur freely in the thickness direction and the skin develops an in-plane compressive stress. Third, the compressive stress reaches a critical level needed for buckling and the skin deforms out-of-plane to create the wrinkles. Lastly, the curing and sometimes drying continues until the entire coating is solid and the wrinkles are locked in place. In this talk, experimental evidence and theoretical models will be used to elaborate on these steps to wrinkle formation and the factors that control the wrinkle characteristics.