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(38a) Enhanced Hardness and Strength of Zero-Coalescent Waterborne Acrylic Coatings with Cellulose Nanocrystals

Dogan-Guner, E. M. - Presenter, Georgia Institute of Technology
Brownell, S., The Dow Chemical Company
Schueneman, G., US Forest Service
Shofner, M. L., Georgia Institute of Technology
Waterborne acrylic coatings with low or no coalescent are very desirable to manufacturers and users of commercial paints and protective coatings. Volatile coalescents evaporate during drying of the coatings, resulting in air pollution and health concerns. On the other hand, coating producers use non-volatile coalescent alternatives, but they result in coatings that are often tacky and lack adequate hardness by remaining in the coating long-term. Achieving adequate mechanical performance in the acrylic coatings without the utilization of volatile organic solvents or coalescents is essential. To address this need, we investigated cellulose nanocrystals (CNCs) as performance additives to improve early hardness and strength of zero-coalescent latex coatings. CNCs are stiff rod-shaped nanomaterials having high elastic modulus and tensile strength. CNCs were added into waterborne acrylic latexes at different loadings by a post-synthesis blending approach. We used two types of coalescent-free and ambient film forming latexes produced by using a monomer mixture of butyl acrylate, methyl methacrylate and methacrylic acid (MAA). The latexes differed in the amount of MAA, which imparts acid groups that make the latex more hydrophilic, increasing the potential hydrogen bonding between the CNCs and the carboxylated acrylic polymer matrix. The resulting CNC loaded (0-15 wt%) acrylic latex films were characterized to determine film formation, morphology of CNCs in the latex matrix, and mechanical performance. We observed that CNCs aggregated in the interstitial areas between latex particles as water dries. This morphology did not negatively affect the film formation and thermal properties of the coatings. All coatings had an MFFT (minimum film forming temperature) of 0°C and a glass transition temperature lower than the MFFT. Film transparencies of CNC loaded (1-15 wt%) composite films were in the range of 93.1 – 72.3%. Also, these films did not retain a measurable amount of water after drying. The addition of CNCs significantly enhanced the tensile strength, Young’s modulus and hardness. We obtained almost 230% improvement in Koenig hardness and 10x improvement in the nanoindentation hardness in 15 wt% CNC loaded composite films compared to neat acrylic coatings. The higher MAA composition in the latex formulations slightly increased the tensile strength and modulus of the latex films. This work showed the remarkable effect of using CNCs to improve the mechanical performance of coalescent-free acrylic coatings. We found these results to be competitive with the conventional methods using volatile or non-volatile coalescents.