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modulus but higher UTS and strain at break, which could be ascribed to the improved ductility due to the introduction of PVA. Overall, we have shown a way to chemically modify the CNF/CMC coatings for better water barrier properties at high humidity conditions, which could be beneficial for food packag- ing applications. The combination of chemical crosslinking, nano-clay incorporation, and addition of extra polymer further enhanced the performance of CNF-based coatings on molded pulp trays. These modified coatings offer a promising sustainable alternative to conventional synthetic packaging materials. Author contributions JZ and JY both designed the experiments. JZ performed the experiment, collected, and analyzed the data. The first draft of the manuscript was written by JZ, and both authors commented on previous versions of the manuscript. Both authors read and approved the final manuscript.
the components. For the formulated CNF coated MP trays sample, similar trends were observed as in the dry sheet com- parison. However, the UTS was almost the same as the CNF/CMC coated samples. One possible reason could be the batch-to-batch difference in MP trays. To conclude, the unmodified CNF/CMC samples were stiff but brittle, while the formulated CNF samples were flexible but tougher. Wet strength is generally expressed as the ratio of the strength between the wet and dry state. Since PAE is a common wet strength enhancer, the wet strength test was also conducted to further characterize the mechanical properties of formulated CNF coatings at extreme conditions. The wet strength of the CNF/CMC could not be tested as the wet CNF/CMC sheets were so weak that it could be broken by their own weight, or easily broken when clamped. Detailed stress vs. strain curves are shown in Fig. S9 (ESI † ). The results showed that the totally wet formulated CNF coating had a Young’s modulus of 198.02 17.80 MPa, UTS of 7.48 1.08 MPa, and strain at break of 6.37% 0.32. Therefore, the formulated CNF coating had a wet strength of around 5.5%. According to the paper making industry, when the wet strength of the paper product exceeds 10–15%, the paper product is considered as ‘‘high wet strength’’. However, the dry UTS of most paper and paper product falls between 20–60 MPa, 51–53 which is much lower than the CNF/CMC dry sheets. 26,33 This leads to the anomalous result of having a formulated CNF/CMC of higher absolute wet strength than ‘‘high wet strength’’ paper but being considered ‘‘low wet strength’’ simply because it is so strong when dry. Thus, this standard may not be appropriate here. Additionally, having such a high absolute wet strength may (or may not) cause issues with repulpability or biodegradation/compostability, although such testing is not in scope here. 4. Conclusion In this work, in an effort to improve performance, a variety of crosslinkers, polymer additives and nano-clays were screened as additives to CNF. The ideal formulation was identified as PAE crosslinker, which made the surface more hydrophilic and reduced the WVTR. The addition of Cloisite-Na + as the nano-clay made the diffusion paths for water molecules more tortuous and further reduced WVTR. The addition of PVA lowered the WVTR further still by reducing the free volume (blocking the pores and void within the CNF matrix). Wet-cup and dry-cup WVTR values were reduced 21% and 89% respec- tively. The formulated CNF coatings also showed lower OP compared with the CNF/CMC coatings due to lower porosity. The formulated CNF/CMC was coated onto molded pulp trays. The Cobb value decreased from 137 9 of the CNC/CMC coated MP trays to 56.3 4.4 of the formulated CNF coated MP trays, and both formulated CNF coated MP trays and CNF/ CMC coated MP trays passed kit #12 level of oil and grease resistance and showed food sauce stain resistance. The for- mulated CNF coated trays and dry coatings had lower Young’s
Data availability
The data supporting this article have been included as part of the ESI. †
Conflicts of interest
The authors declare that they have no competing interests.
Acknowledgements
This research was funded in part by Purdue Research Founda- tion under Grant Number 60000034.
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