TEMPO-oxidized cellulose nanofibers (TOCNs) as a green reinforcement for waterborne polyurethane coating (WPU) on wood

• Addition of TOCNs improved the mechanical properties of the WPU coating.
• Both hydrogen bonds and continuous TOCNs network in the coating contributed to the observed improvements.
• The results can be predicted by the Ouali model at the TOCNs content exceeding the percolation threshold.
• The pencil hardness of the coating was enhanced with the addition of TOCNs.

In this work, TEMPO-oxidized cellulose nanofibers (TOCNs) were investigated as a green additive to the waterborne polyurethane (WPU) based coating, for improving its mechanical properties. The structure, morphology, mechanical properties and performances of the WPU/TOCNs coating were determined. Results showed that TOCNs had good compatibility to the WPU coating, and significantly enhanced the mechanical properties of the coating. The Halpin-Tsai and Ouali models were used to fit for the Young’s modulus of the resulting coating, and good agreements were found between the Ouali model and experimental results when the TOCNs content exceeded the critical percolation threshold (0.7 vol% or 1.0 wt%). It was also found that the pencil hardness of the coating was improved with the addition of TOCNs. However, AFM and pull-off test revealed the negative effects of the TOCNs addition on the surface roughness and adhesion strength of the coating to the wood surface.