Stress, fracture, and microstructure development of drying aqueous particulate coatings

Particulate coatings are ubiquitous, acting as sunscreens, architectural paints, and anticorrosive coatings. The performance of these coatings most often requires that the coating is defect free, and is dependent on the coating microstructure that forms during solidification. Regulations and societal concern over the volatile organic compounds (VOCs) released during drying continue to drive innovation toward aqueous formulations. However, aqueous particulate coatings develop stress during drying, making them prone to stress-induced defects such as cracking, curling, and delamination. Additionally, aqueous formulations often rely on latex particles to coalesce and form a continuous film at room temperature. In this work, two characterization methods were developed to support the research and development of aqueous particulate coating formulations. The first method measures the stress development of uniformly drying suspensions that are prone to fracture. The second characterization method uses freeze-drying as a sample preparation method for visualizing film forming latex particle microstructure with Scanning Electron Microscopy (SEM).The stress development of drying aqueous metal oxide particle suspensions was quantified through cantilever beam deflection. Cantilever walls were used to encourage uniform drying for each coating. However, the selection of particle size and density was critical to ensuring drying uniformity, even with the cantilever walls. Coatings prepared from suspensions of zinc oxide particles (D50 ∼ 0.4 µm) were small enough for the coating to crack at a practical thickness, yet dense enough to limit particle accumulation at the evaporative free surface, mitigating lateral drying. The stress development profiles of uniformly dried coatings showed a unique two-stage stress decline coincident with fracture, which means cantilever beam deflection is well suited for assessing processing and formulation strategies that mitigate fracture. Using the freeze-drying preparation method developed in this work, the microstructural evolution of drying latex particle suspensions was visualized with Scanning Electron Microscopy (SEM). Latex suspensions dried either above or below their minimum film formation temperature (MFFT) were flash-frozen in a liquid cryogen to halt microstructural development. The frozen samples were freeze-dried to remove the frozen water, and then stored and sputter coated at temperatures below the MFFT. Evidence of all three film formation stages (consolidation, compaction, and coalescence) was visible in the SEM images of the freeze-dried latex coatings. Additionally, the freeze-drying preparation method produced results that aligned with those from cryogenic SEM, using a fraction of the resources cryoSEM requires. Thus, freeze-drying was shown to be an effective preparation method for characterizing the transient microstructure of film forming latex coatings without cryoSEM.

Author dc:creator

• Moorhead, Annie

• aqueous
• coatings
• cracking
• drying
• film formation
• latex

Language dc:language.iso

en

Handle dc:identifier.uri

https://hdl.handle.net/11299/278047

OAI identifier oai:identifier

oai:conservancy.umn.edu:11299/278047