Incorporation of Polymer-Grafted Cellulose Nanocrystals into Latex-Based Pressure-Sensitive Adhesives

While the improvement of water-based adhesives with renewable additives is important as industry shifts toward more sustainable practices, a complete understanding of how the compatibility between additives and polymers affects adhesive performance is currently lacking. To elucidate these links, cellulose nanocrystals (CNCs) were first functionalized via surface-initiated atom-transfer radical polymerization with the hydrophobic polymers poly(butyl acrylate) (PBA) and poly(methyl methacrylate) (PMMA) to facilitate their incorporation into latex-based pressure-sensitive adhesives (PSAs). Next, PBA latexes were synthesized using seeded semibatch emulsion polymerization with unmodified or polymer-grafted CNCs added in situ at a loading of 0.5 or 1 phm (parts per hundred parts of monomer). Viscosity and electron microscopy suggested that the polymer-grafted CNCs were incorporated inside or on the latex particles. PSAs containing any CNC type had one or more improved properties (compared to the no-CNC "base case"); CNCs with a low degree of polymerization (DP) grafts exhibited improved tack (up to 2.5-fold higher) and peel strength (up to 6-fold higher) relative to PSAs with unmodified CNCs. The best performing PSA contained the low DP PMMA-grafted CNCs, which is attributed to the higher glass transition temperature and the higher wettability of the PMMA grafts compared to PBA, and the more uniform dispersion of polymer-grafted CNCs throughout the PSA film. In contrast, PSAs containing CNCs with high DP grafts resulted in reduced tack and peel strength (compared to low DP grafts) due to enhanced CNC aggregation. Unfortunately, all PSAs containing polymer-grafted CNCs exhibited inferior shear strength relative to PSAs with unmodified CNCs (and comparable shear strength to the no-CNC "base case"). Collectively, these results provide guidelines for future optimization of more sustainable latex-based PSAs.