Quantifying the Open Time and Film Formation of Waterborne Coatings with Laser Speckle Imaging

In drying waterborne paints, the time span between the deposition of the liquid dispersion and the formation of a uniform solid film harbors a procession of complex phenomena each of which determines the final properties of the dry paint film. One of the major challenges in the design of sustainable, water-based paints is to match or surpass the drying performance of their solvent-based counterparts. In particular, the so-called “open time”─the time during which the paint remains wet and susceptible to alterations without affecting the aesthetics of the final film─is often much shorter in waterborne systems than in solvent-borne equivalents. This short time window hastens the painter, limits remodeling of defects, and makes it difficult to smoothly blend the edges of a previously and freshly deposited paint without introducing permanent brush marks. Optimizing and tailoring the open time is thus desired. This endeavor would greatly benefit from methods capable of determining this critical parameter in a fast and objective way. Yet, experimental access to the internal dynamics accompanying the drying process is challenging. Paints are often opaque, which precludes the use of traditional optical methods. Additionally, the drying phenomena usually populate a multitude of time and length scales. To address these obstacles, we deploy the optical technique laser speckle imaging (LSI), which allows probing nanoscale motions deep inside turbid paints. We apply this method to quantitatively and objectively determine the open time. We develop a set of scaling relations that accurately predict the experimentally measured open time as a function of key parameters governing the drying process. Additionally, we harness the wide temporal dynamic range of LSI to capture phenomena that occur during the later drying stages, including deformation and coalescence of the polymer particles.