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and after the testing, indicating that it passed the kit #12 level, which was the same level of performance as the CNF/CMC coated MP tray in the previous research. 26 This indicates that the formulated CNF coating also provided effective barrier properties against oil and grease penetration. However, due to the limitation of the testing standard, the highest kit number is 12. The SEM images and water contact angle test have shown that formulated CNF coatings are less porous and more hydro- phobic than the unmodified CNF/CMC coating, therefore, it is reasonable to speculate that the formulated CNF coatings might have even better oil and grease resistance than the CNF/CMC ones. Regardless, a kit of 12 typically needs totally impermeable materials such as plastic films or PFAS coatings for paper. Thus, all coatings show PFAS-type performance using a mostly natural formulation of non-toxic, biodegradable food- safe components. 3.4. Sauce stain test To evaluate the real-life performance of the formulated CNF coated MP tray as food containers, a sauce stain test was conducted. Fig. 3 and 4 showed the results of the test. It can be easily seen that both sauces left a stain on the uncoated MP trays, but not on CNF/CMC coated MP trays or the formulated CNF coated trays. However, turbidity testing has proven that the CNF/CMC coating could be dissolved in water, and the CNF/ CMC coated trays showed higher Cobb value than the formu- lated CNF coated ones as well. Therefore, it can be postulated that, over an extended period, residual marks are likely to occur on the CNF/CMC coated trays.
3.5. Mechanical performance As mechanical performance can be a criterion for rigid trays, uniaxial tension tests were conducted both CNF-based dry sheets and coated samples, and results were compared with the uncrosslinked counterparts. The thickness of the coatings was about 0.2 mm, and the thickness of the uncoated MP trays was about 1.5 mm. Tables 5 and 6 showed the mechanical test results CNF-based dry sheets and coated MP trays. Detailed stress vs. strain curves were shown from Fig. S5–S8 (ESI † ). For the It was observed that the Young’s decreased around 50% after PAE-crosslinking, but the UTS was similar to uncross- linked 10CNF/1CMC dry sheet, and strain-at-break increased. To explain this, we propose a hypothesis. After PAE crosslink- ing, the positions of fibers within the CNF structure are presumed to be more fixed than in the pre-crosslinked state. For uncrosslinked 10CNF/1CMC, all fibers are expected to show some degree of orientation in the direction of applied tension when being stretched. In this case, the Voigt model (axial loading model) fits this condition better. However, after cross- linking, the degree of fiber orientation may be diminished due to constrained movement, potentially resulting in a reduction in the number of fibers bearing the tensile load within the elastic deformation region. Consequently, this may manifest as a lower Young’s modulus, which might be explained by a combination of the Voigt model (axial loading model) and Reuss model (transverse loading). Also, the PAE itself is also a polymer. Therefore, unlike the common crosslinking by small molecules, PAE crosslinking may more closely resemble the connection of CNF chains with weak ‘‘springs’’, so that
Fig. 3 Ketchup stain test. From top to bottom were uncoated MP trays, CNF/CMC coated MP trays, and formulated CNF coated trays. From left to right there were trays before testing, during testing, and after testing.
Mater. Adv. , 2025, 6 , 2833–2844 | 2841
© 2025 The Author(s). Published by the Royal Society of Chemistry
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