The minimum film formation temperature (MFFT) is the minimum drying temperature needed for a latex coating to coalesce into an optically clear, dense crack-free film. To better understand the interplay of forces near this critical temperature, cryogenic scanning electron microscopy (cryoSEM) was used to track the latex particle deformation and water migration in coatings dried at temperatures just above and below the MFFT. Although the latex particles completely coalesced at both temperatures by the end of the drying process, it was discovered that particle deformation during the early drying stages was drastically different. Below the MFFT, cracks initiated just as menisci began to recede into the packing of consolidated particles, whereas above the MFFT, partial particle deformation occurred before menisci entered the coating and cracks were not observed. The spacing between cracks measured in coatings dried at varying temperatures decreased with decreasing drying temperature near the MFFT, whereas it was independent of temperature below a critical temperature. Finally, the addition of small amounts of silica aggregates was found to lessen the cracking of latex coatings near the MFFT without adversely affecting their optical clarity.
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Acknowledgments This research was supported by the University of Minnesota Industrial Partnership for Research in Interfacial and Materials Engineering (IPRIME) and Evonik Industries. C. C. R. gratefully acknowledges a Doctoral Dissertation Fellowship sponsored by the University of Minnesota graduate school. CryoSEM was performed with the help of Chris Frethem at the University of Minnesota Characterization Facility, which receives partial support from NSF through the MRSEC program. CryoSEM images were also obtained at the Technion—Israel Institute of Technology under the knowledgeable direction of Prof. Yeshayahu Talmon. Financial support from the University of Minnesota graduate school made this travel possible. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.
- Cryogenic scanning electron microscopy
- Latex film formation
- Minimum film formation temperature