Sag in drying coatings: Prediction and real time measurement with particle tracking

Robert K. Lade, Jin Oh Song, Austin D. Musliner, Bryce A. Williams, Satish Kumar, Christopher W. Macosko, Lorraine F. Francis

Research output: Contribution to journalArticlepeer-review

8 Scopus citations


Abstract Sag is a coating phenomenon characterized by gravity-driven flow after deposition; excessive amounts of sag can lead to coating defects. In this work, a new method for evaluating and quantifying sag is investigated. The motion of micron-sized Lycopodium spores on an inclined coating surface is tracked during drying, and the resulting surface velocity data is used to determine sag length. This in situ particle tracking method is minimally invasive and permits real time measurements. Measured sag lengths and real time surface velocities in aqueous polyvinyl alcohol solution coatings compare well with a theoretical model. The model is also used to develop a predictive sag regime map, which anticipates the extent of sag given coating properties and process-specific parameters. This map also identifies viable processing windows and aids in intelligent coating design given specific process constraints. The predictions of the sag regime map are compared against experimental sag results from polyvinyl alcohol solution coatings as well as four commercial latex paints, revealing good agreement for coatings with Newtonian or 'Newtonian-like' rheologies.

Original languageEnglish (US)
Article number3635
Pages (from-to)49-58
Number of pages10
JournalProgress in Organic Coatings
StatePublished - Sep 1 2015

Bibliographical note

Funding Information:
The authors thank the industrial partners of the Coating Process Fundamentals Program (CPFP) of the Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME) for supporting this research. The authors would also like to extend their gratitude to Wieslaw Suszynski, for designing the inclined drying stage and for many helpful discussions. The authors would also like to thank Heng Zhang for SEM imaging. Parts of this work were carried out in the Minnesota Nano Center which receives partial support from NSF through the NNIN program.

Publisher Copyright:
© 2015 Elsevier B.V.


  • Drying
  • Lycopodium
  • Particle tracking
  • Regime map
  • Rheology
  • Sag


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