View Article Online
Paper
Materials Advances
was6.48 10 15 cm 3 [STP] cmcm 2 s 1 Pa 1 ., which was almost the same as pure CNF OP value reported by Chowdhury et al. , 31 which was 6.49 10 15 cm 3 [STP] cm cm 2 s 1 Pa 1 , meaning that the addition of CMC did not change the oxygen barrier properties of CNFs. Some degree of reduction in OP was observed, which could be attributed to a reduction in the free volume. CNFs have long been known as good oxygen barrier materials due to their hydrophilicity, which impedes the adsorp- tion and desorption of oxygen molecules during the permeation process. 3.3. Cobb test and oil and grease resistance The Cobb test was conducted to evaluate the ability of water penetration resistance by measuring the amount of water absorbed over a specific amount of time. The uncoated MP tray, CNF/CMC coated MP trays, and the formulated CNF coated MP tray were tested. Results showed that the water absorbency of the uncoated MP tray was 154 11 g m 2 , the value for CNF/CMC coated MP trays was 137 9gm 2 , and the formulated CNF coated MP tray exhibited a significantly lower value of 56.3 4.4 gm 2 . This substantial reduction in water absorbency demonstrates the effectiveness of the formulated CNF coating in enhancing the water resistance of the MP tray. One aspect that also must be stated is that this test is over relatively short exposure times. The CNF/CMC material is known to completely redisperse with time due to its low inter-fiber cohesiveness resulting from its high charge state. Thus, over a longer time, the CNF/CMC will completely come off, while we expect the formulated material will not. The oil and grease resistance test aims to evaluate the material resistance and barrier performance against oils and grease. This test is primarily applicable to packaging materials, food containers, and other products that meet oils and greases. The oil and grease resistance of the formulated CNF coated tray was evaluated using the TAPPI T559 kit test method. The results were shown in Fig. 2. After a kit #12 level oil test, it was observed that the formulated CNF coated MP tray sample did not exhibit any notable changes via visual inspection before
Therefore, the formulation of 100CNF/10CMC/1.5PAE/ 3Cloisite-Na + /10PVA, comprising PAE as crosslinker, Cloisite- Na + as the nano-clay to create tortuous paths for water vapor diffusion, and PVA as extra polymer to further reduce free volume, was established. Henceforth, unless otherwise speci- fied, this formulation will be referred to as ‘‘formulated CNF’’. Interestingly, water contact angle analysis showed that while crosslinking led to an increase in contact angle indicating that PAE made the material less hydrophilic, addition of clay, polymer and clay/polymer steadily made the contact angle decrease indicating an increase in hydrophilicity. While nor- mally this may mean higher WVTR, in this case the increased tortuosity and lower free volume counteracts it. The results are shown in Fig. S4 and Table S1 (ESI † ). The significantly lower dry cup WVTR values compared to wet cup values are to be expected due to lack of humidity plasticization. During wet cup testing, the coating faces an environment of 100% RH on one side and 50% RH on the external side, resulting in considerable moisture adsorption by the coating itself. Although the 3D network structure created by PAE crosslinking prevents the coating from dissolving, swelling still occurs, leading to increased free volume. Conversely, in dry cup testing, the coating faces 0% RH, and the absence of water molecules’ plasticizing effect results in lower porosity. This difference in moisture exposure and its consequent effects on the coating’s structure account for the observed disparity in WVTR values between wet and dry cup tests. Similarly to the wet WVTR testing, the formulated CNF-coated tray samples showed statistical lower WVTR values compared to other samples. As the formulation has been decided during the WVTR tests, the oxygen permeability (OP) tests were conducted on formulated CNF (100CNF/10CMC/1.5PAE/3Cloisite-Na + /10PVA) and CNF/CMC only. Due to the limitations of sample thickness for OP tests, the test specimens were dry sheets of pure coatings instead of coated MP trays. The test was con- ducted at room temperature and 0% RH. The results showed that the OP of formulated CNF was 2.31 10 15 cm 3 [STP] cmcm 2 s 1 Pa 1 , and OP value for the unformulated CNF/CMC
Fig. 2 Oil and grease resistance testing for uncoated MP tray, CNF/CMC coated MP tray, and formulated CNF coated MP tray samples. (a) Uncoated tray for test kit level 1 before testing. (b) Uncoated tray for test kit level 1 after testing. (c) CNF/CMC coated tray for test kit level 12 before testing. (d) CNF/CMC coated tray for test kit level 12 after testing. (e) Formulated CNF coated MP tray for test kit level 12 before testing. (f) Formulated CNF coated MP tray for test kit level 12 after testing. (a)–(d) Are reprinted with permission from Zhang, J.; Youngblood, J. P. Cellulose Nanofibril (CNF)-Coated PFAS-Free, Grease-Resistant All-Bio-Based Molded Pulp Containers for Food Packaging. ACS Appl. Polym. Mater. , 2023, 5 (7), 5696–5706. Copyright 2024 American Chemical Society.
2840 | Mater. Adv. , 2025, 6 , 2833–2844
© 2025 The Author(s). Published by the Royal Society of Chemistry
Made with FlippingBook flipbook maker