Microstructure development in drying latex coatings

The technical details of high-resolution cryo-scanning electron microscopy (cryo-SEM) in studying latex formation are presented. This technique was used to visualize the microstructure development during drying of monodisperse and bimodal latex coatings. The glass transition temperature of the monodisperse latexes ranged from 100 to −11 °C. The film formation process of these latexes, after they were coated onto silicon substrates, was followed. The micrographs showed particle ordering, formation of consolidation fronts, air invasion during drying, particles deformation, film coalescence and skinning phenomena, illuminating important physics that govern the drying process of latex coatings. The bimodal systems consisted of latex blends of large, hard and small, soft particles. Cryo-SEM revealed how porous, permeable structures were created by drying these latex blends. The porous structures were modulated by drying rate, the volume ratio and size ratio of the two kinds of particles. Binder migration phenomenon was observed at higher drying rate. In addition to microstructure investigation, permeability and strength in tension of the porous films were separately measured, illustrating the basic trade-off between integrity and strength on one hand, porosity and permeability on the other.