Doctor of Philosophy
El-Aasser, Mohamed S.
Drying inhomogeneity of latex is the phenomenon where the spatial distribution of latex particles is non-uniform and evolving with time during the drying process. In order to study this phenomenon, this dissertation research employed the optical coherence tomography (OCT), a 3D microscopic imaging technique, to visualize the dynamics and the structure of latex in real time. The gravimetric and video analysis were also integrated to OCT (called “OCT-Gravimetry-Video” method) to simultaneously monitor the drying process of latex. For the polystyrene model latex (with Tg = 106 °C), multiple types of drying inhomogeneity were observed at room temperature. Vertical packing and horizontal drying front were the packing processes of particles from the top of latex and the edge of latex, respectively. Shear bands and cracks were the dislocations of latex structure after particles became packed, due to the lack of particles’ deformation to relax the internal stress. For the film-forming low-Tg model latexes (acrylate copolymer latexes with Tg < 0 °C) at room temperature, the particles on the top of drying latex are readily deformed and coalesced early into a polymer layer (called “skin layer”). The low-permeable skin layer impedes drying and retains water in the wet domain beneath skin. The film drying time can extend to weeks which is unacceptable in paint applications. In the architectural or roof coatings with millimeter-thick films, ~15 wt% water can be trapped beneath skin, leading to poor adhesion to substrate and bubbles inside films. The real-time OCT images show four drying stages: I) packing of particles, II) consolidation of particles, III) formation and maturation of incipient skin, IV) thickening of coalesced latex polymer layer (coalesced layer, which includes skin). The gravimetry shows a dramatic decrease of drying rate after Stage III, and the film appears white and opaque in the video. The entire film drying time 2 can be dominated by Stage IV. The coalesced layer thickness (ℎ푐표푎푙) is found to increase with the square root of time, and the “coalesced layer thickening model” is developed. It is found that, the post-added (added after polymerization) surfactant into the lowTg latex can delay the maturation of skin layer (Stage III), accelerate the increment of ℎ푐표푎푙 with time, and shorten the film drying time. A mechanism is proposed that, besides other chemical species in aqueous phase, the surfactant molecules adsorbed on particles’ surface (which increase repulsive forces between particles) and the surfactant molecules desorbed from particles’ surface (which are trapped within the interstices between particles) can delay the coalescence between particles. The longer the delay time for particles’ coalescence (∆푡푑푒푙), the more water evaporates before the packed particles on top coalesce into a matured skin, thus the water content in the wet domain beneath skin (푘푤 ∗ ) is reduced. According to the mathematical model, lower 푘푤 ∗ leads to faster increase of ℎ푐표푎푙 with time, and less time for drying the entire latex film. Based on the measured drying curves, ∆푡푑푒푙 is found to increase linearly with the percentage of latex particles’ surface coverage by surfactant (푆퐶푠푢푟). At the same 푆퐶푠푢푟, the anionic sodium lauryl sulfate surfactant gives a longer ∆푡푑푒푙 than the nonionic Triton X-100 surfactant does. For both the model latexes and the commercial latex studied in this dissertation research, with the appropriate type and amount of surfactant post-added into latex, not only is the film drying time significantly shortened, but also the water-resistance of the dried latex film is not compromised. Drying inhomogeneity is complex, which is not only determined by the physical properties of latex, but also influenced by the water-soluble molecules and their molecular interactions. The OCT-Gravimetry-Video method can be a useful tool to study the drying process and film formation of latex systems with more details.
Huang, Hao, "Drying Inhomogeneity during Film Formation of Waterborne Latex Systems A Mechanistic Study on Skin Layer Formation" (2018). Theses and Dissertations. 4238.