Stable emulsion of cationic waterborne polyurethanes with cellulose nanocrystals for enhanced nanocomposite performance

The direct blending of a negatively charged cellulose nanocrystal (CNC) suspension in water with an anionic emulsified polymer emulsion is an environmentally friendly method of preparing CNC nanocomposites. However, the use of cationic polymer emulsions in the preparation of CNC nanocomposites is typically limited by emulsion precipitation. In this study, a stable emulsion of CNCs in an aqueous medium was achieved by using a cationic waterborne polyurethane (cWPU) aqueous suspension with electrostatic interactions between the cWPU and CNCs. In contrast to CNC nanocomposites prepared from anionic waterborne polyurethane (aWPU) emulsions, which have nearly identical particles sizes ranging from 16.5 nm (neat aWPU) to less than 25.6 nm (the aWPU-CNC emulsions) in this study, the particle sizes of the cWPU emulsion could be increased from 41.6 nm (neat cWPU) to 1769.7 nm (cWPU-CNC-5.0). This increase was attributed to electrostatic attraction and agglomeration between the polymer emulsion and CNCs. In particular, the suspensions with CNC loadings lower than 1 wt% were stable; they did not precipitate over 6 weeks of storage. Because of the additional electrostatic interactions in cWPU-CNC composites, their Young's modulus was 2600% higher than that of neat cWPU. Moreover, the cWPU composites (with charge opposite to that of the CNCs) had relatively larger elongation at break than the aWPU composites and hence had superior mechanical performance.

Automated summary

In this study, a stable emulsion of cellulose nanocrystals (CNCs) was achieved by using a cationic waterborne polyurethane (cWPU) aqueous suspension. The particle sizes of the cWPU emulsion were increased compared to anionic waterborne polyurethane (aWPU) emulsions, and the resulting cWPU-CNC composites exhibited superior mechanical performance.