This paper reviews stress development mechanisms and stress measurement techniques for polymer coatings. Most polymeric coatings shrink during and after solidification due to chemical reaction, solvent evaporation, phase separation, or some combination thereof. Coating adhesion, however, prevents shrinkage from occurring freely; this frustration of in-plane shrinkage leads to a tensile stress in the plane of the coating. At the same time, stress accumulates, it may be relaxed by processes such as molecular motion. The measured stress at any time is the result of the competition between stress buildup from frustrated shrinkage and stress relief from relaxation. Accumulation of stress is a problem because it can lead to defects such as cracks. An understanding of stress development in various types of polymeric coating systems will lead to strategies for material selection, process optimization, and defect elimination. In this paper, background on stress development is provided, followed by an overview of stress measurement methods for polymer coatings. The remainder of the paper focuses on stress development during the drying and curing of polymer coatings, drawing many examples from previous stress measurement studies.
Bibliographical noteFunding Information:
The authors acknowledge support from the Center for Interfacial Engineering, an NSF sponsored Engineering Research Center at the University of Minnesota through its Coating Process Fundamentals Program, the industrial sponsors of the Coating Process Fundamentals Program, and DuPont through a young professor grant. JAP and DMV also acknowledge the Graduate School at the University of Minnesota for Doctoral Dissertation Fellowships. The authors also thank Dr. S-Y. Tam, Dr. H. Lei, Dr. M. Wen, V. Rajamani, Professor H. K. Stolarski, Professor W. W. Gerberich, Professor R. F. Cook and especially Professor L. E. Scriven for insight into the development of stresses in polymer coatings.