Rapid and unambiguous determination of the open time for waterborne paint films with Laser Speckle Imaging.

In drying coatings, the time span between the deposition of the wet film and the formation of an uniform solid harbors a procession of complex phenomena each of which carves the final properties of the dry paint film. Attaining a better control of the drying process requires the availability of a tool that allows monitoring the attending dynamics. The experimental access to the dynamics underlying the drying process is challenging as paint films are often opaque and the phenomena accompanying the drying process usually populate a multitude of time and length scales. To circumvent these obstacles we deploy Laser Speckle Imaging (LSI) as a technique that allow us to quantitatively probe the internal dynamics of drying waterborne paints and unambiguously determine a crucial handling parameter such as the open time. We develop a set of scaling relations which accurately predict the behavior of the experimentally determined open time as a function of a parameter set that governs the drying process. Additionally, we harness the big temporal dynamic range of LSI to monitor phenomena that inhabit the later stages of the drying process. Remarkably, we not only capture the film formation time, the time point in which phase inversion takes place, but also microscopically detect the deformation of polymeric particles that precedes it.