Enabling zero added-coalescent waterborne acrylic coatings with cellulose nanocrystals

Abstract

Waterborne acrylic coatings contain volatile and non-volatile coalescents. Volatile organic compound (VOC) coalescents enable film formation of hard acrylic polymers at ambient temperature, but their evaporation results in air pollution and health concerns. Non-volatile alternatives are available, but they remain in the coating long-term and as a result films are often tacky and lack adequate hardness. Reinforcing mineral fillers are available, but are associated with sustainability challenges inherent to mineral mining. An alternative approach demonstrated here utilizes a renewably-sourced, plant-based hardening filler dispersed in the water phase, with soft polymer latexes that don’t require VOCs for film formation. We reinforced coalescent-free and ambient film-forming butyl acrylate/methyl methacrylate/methacrylic acid latexes with cellulose nanocrystals (CNCs) by a post-synthesis blending approach. CNCs possess high elastic modulus and tensile strength, making them promising candidates to improve the mechanical performance of soft ambient film-forming acrylic latexes. CNCs form aggregates within the interstices between latex particles as water dries, however film coalescence is not compromised. Films with 15 wt% CNC showed almost 230 % improvement in Koenig hardness and 10x improvement in the nanoindentation hardness compared to neat films, achieving properties similar to hard VOC-containing acrylic binders in a zero-added VOC formulation.