Pushing the Limits with Cellulose Nanocrystal Loadings in Latex‐Based Pressure‐Sensitive Adhesive Nanocomposites

Abstract

Cellulose nanocrystals (CNCs) are a naturally sourced, nontoxic, nanoparticle known to improve tack, peel strength, and shear resistance simultaneously when incorporated into nanocomposite latexes produced for pressure-sensitive adhesive (PSA) applications. In this study, methods for incorporating CNCs into a butyl acrylate/styrene/acrylic acid (91.5/4.5/4.0 wt%) seeded semibatch emulsion polymerization for the production of PSAs are presented. Past work has revealed a limit of 1.0-2.0 wt% (based on monomer) CNC loadings due to latex instability. In this work, CNC loadings of up to 4.0 wt% in a stable latex with 40 wt% solids are achieved by utilizing all of the available water in the formulation to maximize CNC redispersion. The CNC addition method is studied (i.e., in the seed, in the feed, or partitioned in both) and adhesive performance results indicate that there are no significant benefits to where the CNCs are dispersed in situ during polymerization. However, the quality of the CNC-water dispersion is pivotal to ensuring latex homogeneity and therefore, adhesive film quality. Though CNCs improved adhesive strength at low concentrations, their enhancing effects are modest when used with a commercially competitive base-case PSA formulation.

How latex film formation and adhesion at the nanoscale correlate to performance of pressure sensitive adhesives with cellulose nanocrystals

“…They attributed this increased composite rigidity to the presence of cellulose-cellulose hydrogen bonds (in dried materials), and to the crystallinity and high aspect ratio of the CNCs [14]. Similar results have been reported in numerous different latex formulations [4,[6][7][8]75]. The smooth retraction curves of the probe when CNCs are present (figure 2a) is a clear indication that the rigidity of the film is improved upon the addition of CNCs.…”

In‐situ addition of carboxylated cellulose nanocrystals in seeded semi‐batch emulsion polymerization

“…[68][69][70][71] Although particle diameters in that range are not inherently problematic, latex particle size does impact film formation and ultimately, performance properties. In order to eliminate the effects of particle size on film formation, the initial and final latex particle sizes should be controlled and maintained from latex to latex, [38,72] which proved difficult to do with Triton X-405 in the current study. Lastly, the final conversion was below 70% (Figure 4A) and the solids content only reached 32% (Figure 4C).…”

Incorporating Hydrophobic Cellulose Nanocrystals inside Latex Particles via Mini‐Emulsion Polymerization

“…[59] The use of hydrophilic CNCs with water-based acrylic PSAs has proven to simultaneously improve three key PSA properties, i.e., tack, peel strength, and shear strength. [33][34][35][36][37] When tested against a stainless steel substrate, incorporation of CNCs improved the work of adhesion and therefore the tack. [33] By increasing and strengthening the entanglement points for polymer chains, the CNCs increased PSA elasticity, which improved shear strength.…”

“…[29][30][31] The use of hydrophilic CNCs in emulsion-based polymer systems has been studied. [32][33][34][35][36][37] Most recently, Limousin et al blended anionic CNC dispersions with latexes of different size and surface charge and attributed the improvements in mechanical properties of the dried films to the type of CNClatex interactions and structures formed thereafter. [38] Pakdel et al studied the effects of hydrophilic and partially hydrophobic CNCs on latex viscosity, flocculation of latex particles, and mechanical and PSA properties of poly(butyl acrylate/vinyl acetate/acrylic acid) (poly(BA/VAc/AA)) emulsion PSAs.…”