A sequential design approach for in situ incorporation of cellulose nanocrystals in emulsion-based pressure sensitive adhesives

Abstract

Cellulose nanocrystals (CNCs) are naturally-sourced nanoparticles that can be used to modify polymers and provide exceptional mechanical properties to nanocomposite materials. In this study, CNCs were incorporated into water-based pressure sensitive adhesives (PSAs) via in situ semi-batch emulsion polymerization to improve PSA properties. A sequential design approach was used to improve CNC/ poly(n-butyl acrylate/vinyl acetate) nanocomposite PSAs. In the first part of the design, the effects of acrylic acid (AA) and anionic surfactant sodium dodecyl sulfate (SDS) were examined in the presence of 0.5 wt% CNCs (based on polymer mass). While the SDS was crucial to maintaining latex stability, its excessive addition led to a decrease in PSA performance, namely tack and peel strength. The AA comonomer was pivotal in improving the shear strength of the PSAs, especially in the presence of CNCs. In all cases, the addition of CNCs led to improved PSA shear strength. In the second part of the design, the addition of 1-dodecanethiol (NDM) as a chain transfer agent (CTA) with CNC levels up to 1 wt% led to an improvement in tack and peel strength but at the cost of diminishing the shear strength. Generally, the addition of CNCs improved PSA performance. Thus, to maximize the impact of CNCs on PSA properties, a careful balance of SDS (for latex stability), AA (to improve shear strength) and NDM (to improve tack and peel strength) are needed.

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

“…In previous work, the shear strength of PSA films prepared with different monomer systems likewise continually increased with higher CNC loadings. [10][11][12]20,21] In our case, the manifestation of this effect only occurred when the CNC concentration was high. This was because the base case polymer was exceptionally soft, due to the 0.25 phm chain transfer agent (i.e., NDM) in the formulation, which reduced the polymer molecular weight.…”

“…[29] Thus, sharper decreases in tack are observed with the addition of CNCs due to their impact on an adhesive's viscoelastic properties. [10][11][12]20,21] Specifically, CNCs increase the elasticity of the polymer matrix (or decrease its fluidity) due to their rod-like morphology because the formation of a network reduces polymer chain mobility. This suggests that the adhesive's ability to elongate and resist an applied force was reduced relative to the base case formulation (run A1), which explains the decrease in tack beyond 0.5 phm CNCs.…”

“…The gel content (Figure 2a), representing the insoluble cross-linked portion of the polymer, is extremely low for all composite latexes relative to previous, similar work. [10][11][12]20,21] Though it increased, the gel content ranged between 3% and 8% for CNC loadings between 0 and 2.0 phm and at its maximum, the gel content only reached 11-15% at CNC loadings of 3.0-4.0 phm (Figure 2c, runs C3, C3′, and C4). Low gel content can be attributed to the addition of chain transfer agent, which led to shorter overall polymer chain lengths and greatly reduced potential polymer cross-linking.…”

“…[17] CNCs also have accessible and abundant surface groups that provide opportunities for functionalization, meaning that the nanocrystals can be tuned and compatibilized for dispersion in both hydrophilic and hydrophobic systems. [18,19] For example, latexes produced via emulsion polymerization for adhesive applications were enhanced with CNCs using both physical (blending) [12,20] and chemical (in situ) methods [12,21] and showed simultaneous improvements in…”

Sustainable polymer reaction engineering: Are we there yet?

“…Nanoparticles constitute a group of additives used to enhance a polymer's mechanical, and other, properties as composite materials. Despite their superb property modification capabilities, [ 51–53 ] nanoparticles can pose toxicity concerns. [ 54 ] In addition to the energy intensive production of nanoparticles, [ 55 ] their release to the surrounding environment during production and possibly over the lifetime of the use of the composite material is a major drawback for their use.…”