Drying and cracking of soft latex coatings

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.