Water-borne composite coatings using nanoparticles modified with dopamine derivatives

• We used dopamine (DA) and dopamine sulfonate (DS) as water-based coupling agents.
• DA-modified boehmite nanoparticles (BNPs) were well-dispersed in acidic solution.
• Well-dispersed BNPs were synthesized using DS in a wide range of pH.
• DS-BNPs showed high compatibility to water-based polymers with high solid contents.
• The modifiers in aqueous solution can satisfactorily replace conventional silanes.

Water-based organic/inorganic composites have gained attention for their use in organic-solvent-free processes, and for being safe and environmentally friendly. In order to improve the mechanical properties of the resin, we modified boehmite nanoparticles (BNPs) and then blended them into a water-based epoxy resin. The BNPs were synthesized via the sol–gel method at a relatively low temperature (80 °C) to achieve a smaller size (13 nm on average), good transparency (over 80%), and be stable enough to be dispersed in water for extended periods of time. The BNP surfaces were then modified with the aqueous modifier dopamine, used here as a human- and eco-friendly water-based coupling agent, in order to prepare water-dispersible nanoparticles, producing dopamine-modified BNPs (DA-BNPs). Dopamine/polydopamine could be effectively coated on the BNP surfaces, allowing them to be uniformly dispersed in water. In order to disperse nanoparticles in alkaline solutions, a charged coupling agent, dopamine sulfonate (DS), was synthesized and used to form DS-modified BNPs (DS-BNPs). The prime nanoparticles were agglomerated in the mixture of water-based epoxy resin, while DS-BNPs were dispersed well in epoxy resin. In pencil hardness tests, DS-BNP/epoxy film displayed a hardness of 3H-4H, much higher than the original polyethylene terephthalate substrate's 1H and prime epoxy coating's 2H. In addition, the transparency was found to be over 80%. Those water-based BNPs could be also dispersed very well in ethylene-vinyl acetate copolymer emulsion. Dopamine and its derivative showed their potential as water-based modifiers to replace some silane-based coupling agents used in fabrication of water-based composites.