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Materials Advances
Fig. 1 FTIR curved of the (a) BTCA crosslinked, (b) CA crosslinked, (c) PAE crosslinked, (d) CNF/CMC, and (e) pure CNF.
groups, which are more reactive than the carboxylic acid groups of BTCA and thus require less thermal energy to initiate the crosslinking reaction. Furthermore, BTCA-, CA-, and PAE-crosslinked CNF coated MP trays were compared visually, as shown in Fig. S3 (ESI † ). The varying crosslinking temperatures resulted in discernible differences in the coloration of the coating surfaces. The coat- ings crosslinked with BTCA and CA were found to exhibit a notably more pronounced brown hue compared to those crosslinked with PAE. From an aesthetic perspective for food packaging applications, the PAE-crosslinked coatings were determined to be superior to the other two alternatives. There- fore, PAE was selected as the crosslinker for CNF/CMC. 3.2. Formulation of CNF systems to improve barrier performance In the previous section, PAE was selected as the crosslinker for CNFs. However, although crosslinking prevents the dissolution of CNF under high humidity conditions, this does not indicate that relying solely on crosslinking to enhance water barrier performance is sufficient. Hence, a variety of formulations were screened by incorporation of nano-clay to increase tortuosity and water-soluble polymers to reduce free volume. As the main issue with CNF-based packaging is water transport, ‘‘wet cup’’ WVTR at high humidity (100% to 50% gradient) was used as a screening metric. Unfunctionalized montmorillonite (MMT) is one of the most prevalent forms of nano-clay. However, unfunctionalized MMT requires exfoliation to disperse into nanosheets, otherwise its ability of enhancing barrier performance would be greatly reduced due to agglomeration. Common methodologies include
to have different wavenumbers. Furthermore, a subtle inflec- tion point was observed at 1558 cm 1 for PAE crosslinked sample. This feature could potentially be attributed to the amide( II ) band absorbance. 19 However, its visibility was dimin- ished due to the intensity of adjacent absorption peaks. Never- theless, the emergence of the ester absorption peak provides evidence for the formation of crosslinking. To compare different crosslinkers and further prove that crosslinking has occurred, a turbidity test was conducted. Turbidity measurements can be employed as an effective method to assess the extent of crosslinking in CNF systems. As the crosslinking process progresses, a discernible decrease in turbidity is typically observed. This reduction can be attrib- uted to the formation of a three-dimensional network structure, which also proves the successful crosslinking. Table 1 shows the turbidity values for different crosslinking methods. The PAE-crosslinked samples showed the lowest turbidity values even at a relatively low weight ratio of 100 : 1.5 (CNF vs. cross- linker) compared to BTCA and CA. Also, when crosslinking with CNF, BTCA or CA generally requires higher temperatures 35 (140 1 C or higher) for the esterification reaction. PAE can crosslink at lower temperatures, often around 80–120 1 C. PAE forms covalent bonds with cellulose through its azetidinium
Table 1 Turbidity values for different crosslinking strategies. Data were collected at a sampling rate of one per second. The average and standard deviations are shown here
BTCA crosslinked
CA crosslinked
PAE crosslinked
Sample
CNF/CMC
Turbidity (NTU)
27.41 0.34 4.86 0.03 5.23 0.03 1.90 0.08
2838 | Mater. Adv. , 2025, 6 , 2833–2844
© 2025 The Author(s). Published by the Royal Society of Chemistry
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