Integrating optical coherence tomography with gravimetric and video analysis (OCT-Gravimetry-Video method) for studying the drying process of polystyrene latex system

Abstract

Latex, an aqueous dispersion of sub-micron polymer particles, is widely used as polymer binder in waterborne coatings and adhesives. Drying of a latex is inhomogeneous, during which the spatial distribution of particles is non-uniform and changes with time, usually resulting in a compromise of the integrity of a dried film. To study drying inhomogeneity of latex, we developed a system integrating optical coherence tomography (OCT) with gravimetric and video analysis (OCT-Gravimetry-Video method) to non-destructively monitor the drying process of non-film-forming latexes consisting of hard polystyrene spheres over time. OCT structural and speckle images of the latex’s internal structure show the packing process of particles, the detachment of latex and the formation of apparent shear bands in cross-sectional views. Video recordings show the formation of cracks and the propagation of the drying boundary in the horizontal direction. The drying curve, measured by gravimetry, shows the drying rate and the water content of the latex at each drying stage. Furthermore, we find that the particle size affects packing and cracking phenomena remarkably. The OCT-Gravimetry-Video method serves as a general and robust approach to investigate the drying process of waterborne latex system. This method can be employed for fundamental studies of colloids and for evaluations of industrial latex products.

Nondestructive Characterization of Drying Processes of Colloidal Droplets and Latex Coats Using Optical Coherence Tomography

“…The dark center inside the L latex droplet inferred that the particles were active in Brownian motions. With these analyses, we confirmed our observation that particles' packing occurred from the droplet edges and propagated inward, similar to the drying latex coat in the Petri dish [30]. The packing of particles was also observed in S latex droplet, in both OCT structural images (Figure 3D-F) and speckle images ( Figure 3J and K).…”

“…S latex: particle size, ~53 nm; initial solid content, 33.43 wt%; surfactant solid content, 1.4 wt%). To monitor the drying process of the latex droplets, the integrated OCT-gravimetry-video platform [30] was employed. The spectral-domain OCT system in the integrated platform had a central wavelength of 1320 nm and a spectral bandwidth of 110 nm.…”

“…The outer layer had lower scattering intensity, and inner layer had higher scattering intensity. OCT speckle contrast analysis [30,54] was further carried out on the same data. In the outer layer, the higher speckle contrast indicated that motions of particles in these regions were restricted.…”

“…Previously, Lawman and Liang [61] and Saccon et al [62] have reported using OCT to monitor the drying process of varnish, in which average refractive index, surface roughness, and thickness have been characterized. Huang et al furthers the application of OCT on investigating the drying phenomena of latex coat, including the cross-sectional imaging and particles' dynamics analysis [30,54].…”

“…Combining OCT with gravimetric and video measurements can fully characterize the drying process of polystyrene latex coat [30]. Among these modalities, time-lapse OCT imaging can show the local microstructures, 1D vertical drying process, and drying inhomogeneity from the cross-sectional view of the drying latex.…”