Cellulose Nanocrystal (CNC)–Latex Nanocomposites: Effect of CNC Hydrophilicity and Charge on Rheological, Mechanical, and Adhesive Properties

Abstract

Cellulose nanocrystals (CNCs), a sustainable nanomaterial, are in situ incorporated into emulsion‐based pressure‐sensitive adhesives (PSAs). Commercially available CNCs with different surface hydrophilicity and surface charge (CNC101 and CNC103 from CelluForce) are used to explore their role in PSA property modification. Viscosity measurements and atomic force microscopy reveal differences in degree of association between the CNCs and the latex particles depending on the surface properties of the CNCs. The more hydrophilic and higher surface charge CNCs (CNC101) show less association with the latex particles. Dynamic strain sweep tests are used to analyze the strain‐softening of the nanocomposites based on CNC type and loading. The CNC101 nanocomposites soften at lower strains than their CNC103 counterparts. This behavior is confirmed via dynamic frequency tests and modeling of the nanocomposites’ storage moduli, which suggest the formation of CNC aggregates of, on average, 3.8 CNC101 and 1.3 CNC103 nanoparticles. Finally, PSA properties, i.e., tack, peel strength, and shear strength, simultaneously increase upon addition of both CNC types, although to different extents. The relationship between the PSA properties and CNC surface properties confirms that the less hydrophilic CNCs lead to improved CNC dispersion in the PSA films and therefore, enhance PSA properties.

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

“…Compared to the use of hydrophobic CNCs in MEP, hydrophilic and partially hydrophobic CNCs improved tack, peel strength, and shear strength (Figure 5). [ 39 ] Hydrophilic CNCs considerably increased the shear up to 0.75 wt% loading (Figure 5C), which was attributed to the formation of long CNC aggregates in the final film. Further increases in hydrophilic CNC loadings resulted in highly elastic polymers, which led to a reduction in all three adhesive properties and making these films inappropriate for use as PSAs (Figure 5).…”

“…The increase in viscosity was much lower than previously found with the use of water‐dispersible CNCs in CEP (up to 350 mPa s). [ 39 ] Figure 2 shows the evolution of monomer conversion, particle size, N p and N p / N d for the runs shown in Table 3.…”

“…Further increases in hydrophilic CNC loadings resulted in highly elastic polymers, which led to a reduction in all three adhesive properties and making these films inappropriate for use as PSAs (Figure 5). [ 39 ] The partially hydrophobic CNCs continuously improved tack and peel strength up to 1 and 1.25 wt%, respectively, while the improvement in shear strength was lower than that of hydrophilic CNCs up to 1 wt% (Figure 5). The partially hydrophobic CNCs were better dispersed in the final latex, and thus formed shorter aggregates.…”

“…As a result, a less elastic behavior was observed, which allowed for improvement in the tack and peel strength at higher CNC loadings. [ 39 ]…”

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

“…While unmodified CNCs are slightly amphiphilic [28] and can stabilize monomer/water interfaces [29,30], they have not been demonstrated on their own as the sole stabilizer in latex-based PSA synthesis. However, recent work has shown that CNCs surface modified to be sufficiently amphiphilic to be surface active resulted in even better PSA performance due to their improved affinity with the monomer/polymer phase [31]. The effect of this modification is inferred to be twofold, the higher affinity between the CNCs and the polymer (i) improved CNC dispersion throughout the latex, ensuring no aggregation upon drying and a homogeneous PSA film [31] and (ii) enabled the CNCs to act as entanglement points for the polymer chains, improving the cohesive strength of the PSA [32].…”