(667e) Effect of Particle Size and Shape on Drying and Stress Development in Colloidal Coatings

Coatings prepared from colloidal particle suspensions have a wide range of applications including barrier coatings, antireflective coatings and printed electronic devices. For coatings prepared from colloidal suspensions, major changes in microstructure and properties occur during drying. As-deposited, the liquid suspension is stress-free, but during drying stress develops and may cause defects, such as cracking, curling or delamination. The study of coatings stress development and microstructure change during drying provides a basis for developing formulations and processing conditions to control stress and defects and provide for high performance.

Mineral pigments like calcium carbonate, clay and talc are extensively used in the paper industry. Latex like styrene butadiene, styrene acrylic and polyvinyl acetate are used to provide good cohesion of the porous structure formed by rigid pigments. Different pigments size, shape and blending ratio can significantly impact dispersion properties, as well as the appearance and physical properties of the coated products.

Past research has demonstrated that coatings made from suspensions of large particles develop lower stresses and are less likely to crack compared with those made from suspensions of fine particles. In this work, the effect of particle size is further explored by studying coatings with similar average particle size but different particle size distribution. In addition, particulate coatings with clays of different shape, relative size and content ratio to other pigments in the coating formulation are also investigated. This research addresses the effects of particle size and shape on coatings stress development, microstructure formation and mechanical properties.

In this research, we studied aqueous coating systems containing ground calcium carbonate particles, latex particles and clays. Coating drying behavior, including weight loss and stress development, was monitored under controlled conditions (temperature and humidity). Drying stress was determined by substrate deflection method. SEM and cryo-SEM techniques were used to relate the coating stress development and cracking behavior to microstructure changes. The cracking behavior was studied by measuring the critical cracking thickness and mechanical properties (i.e., hardness, modulus and fracture toughness) using nanoindenter. Critical cracking thickness was found to increase with clay content in certain systems. The objective of this study is to understand this phenomenon and the effects of particle sizes and shape on coating stress development and mechanical properties, which are responsible for coating performance.