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freezing/thawing and ultrasonic exfoliation. 46 Nevertheless, even in water suspensions of exfoliated MMT, a stable mass percen- tage typically does not exceed 1 wt%. Tayeb et al. 19 prepared a 1 wt% MMT water suspension by sonication. The MMT suspen- sion and PAE were separately added to 1 wt% CNF, and the film was made by solvent casting. Unfortunately, higher contents are difficult as MMT can easily aggregate in high-concentration suspensions, even after being exfoliated leading to poor moisture barrier performance. Initial testing of MMT and Cloisite-Na + was performed where Cloisite-Na + showed better overall dispersion and stability at both high and low concentration. Thus, Cloisite- Na + was chosen to be used. Previous researchers have also studied the proper range of nano-clay concentration in CNF matrix. Shanmugam 47 prepared the CNF/Cloisite-Na + compo- sites with different weight ratios and tested the water vapor permeability of the films. It was found that the WVTR value increased as the nano-clay concentration increased up to 5 wt%, where aggregation occurred. The onset concentration started at 5 wt%, meaning that the aggregation might have happened when the nano-clay concentration exceeded 5 wt%. Similarly, Farmahini-Farahani et al. 21 also tested the water vapor perme- ability (WVP) of regenerated cellulose/Cloisite-Na + nanocompo- site films. It was found that the WVP started to plateau at 3 wt% of Cloisite-Na + to cellulose, showed a slight decrease at 5 wt%, and increased again at 10 wt%. In these two studies, the weight percentage was defined as the percentage of nano-clay based on cellulose. Therefore, in order to investigate the optimal Cloisite- Na + concentration for current study, the water barrier perfor- mance tests for PAE-crosslinked CNF with different Cloisite-Na + concentration based on previous work (3 wt% and 5 wt% based on the CNF) were tested. Table 2 showed the WVTR values for the coated MP trays with different formulations. A T -test was con- ducted, and the results showed that WVTR values of the 3 wt% Cloisite-Na + sample were statistically lower than the 5 wt% Cloisite-Na + sample at a = 0.05. This difference may be due to the PAE-crosslinked CNF system. The crosslinked 3D network structure made it harder for the nano-clay to be well dispersed in the CNF matrix compared to the un-crosslinked CNF system. This led to a shift in the threshold concentration for nano-clay aggregation. Consequently, under the current conditions, the 3 wt% formulation exhibits superior water vapor barrier perfor- mance to the 5 wt% formulation. In addition to chemical crosslinking and the incorporation of nano-clays, the barrier performance of CNF can also be enhanced by introducing other polymers into the system. Researchers have found that the water barrier performance improved when incorporating CNF to hydrophilic polymers polyvinyl alcohol (PVA). 22–24,48 Although both components are
Table 3 Wet-cup WVTR values for PAE crosslinked, nano-clay incorpo- rated CNF-based coated MP trays with different polymers
WVTR [gm 2 day 1 ] 466.9 14.7 527.6 20.1
Samples (weight ratio)
100CNF/10CMC/1.5PAE/3Cloisite-Na + /10PVA 100CNF/10CMC/1.5PAE/3Cloisite-Na + /10starch 100CNF/10CMC/1.5PAE/3Cloisite-Na + /10pectin
503.1 14.9 100CNF/10CMC/1.5PAE/3Cloisite-Na + /10alginic acid 525.2 12.4 100CNF/10CMC/1.5PAE/3Cloisite-Na + 528.8 19.7
hydrophilic materials, the composite showed better water bar- rier performance after mixing than any single component alone. Similarly, we hypothesize that when CNF serves as the matrix when it is at high content, it also contains numerous internal pores. Extra polymers, even as a minor component, will contribute to a lower WVTR. Previously, we have shown that the addition of polymer can enhance oxygen barrier by a similar method in CNC. 29 As the maximum benefit was observed at 10% polymer, such was used here during screening. The results of WVTR values of crosslinked, nano-clay incorporated CNF- based coated trays with different polymers are shown in Table 3. It was found that the PVA showed statistically signifi- cant lower values compared to the other three samples, while the other three groups showed almost the same results as the one without extra polymers. This phenomenon could be due to multiple reasons. First, compared to other polysaccharides, PVA is much harder to dissolve in water at room temperature. Therefore, at room temperature and high humidity conditions, the degree of swelling of PVA might be lower compared to other polymers, therefore creating much less new free volume due to plasticization. Also, PVA is known to have good film-forming abilities which may simply improve the structure of the film ( i.e. less pinholes, fisheyes, etc. ). 24,49 Consequently, 10 wt% PVA was incorporated into the PAE- crosslinked CNF formulation containing 3 wt% Cloisite-Na + . The wet cup WVTR of MP trays coated with this formulation was subsequently measured. To demonstrate the effects of each component, a series of control groups were also tested. The results of wet-cup and dry-cup WVTR values are shown in Table 4. It was evident that the addition of the PVA external polymer further reduced the WVTR by around 65 g m 2 day 1 . The WVTR values for the PAE crosslinked CNF with only nano- clay or PAE showed statistical difference to the value of PAE crosslinked CNF with both nano-clay and PAE, meaning that both nano-clay and PAE contributed to reduced WVTR.
Table 4 Wet-cup and dry cup WVTR values for CNF-based coated MP trays with/without PAE crosslinking, nano-clay, or PVA
WVTR (wet) [gm 2 day 1 ]
WVTR (dry) [gm 2 day 1 ]
Table 2 Wet WVTR values of CNF-based coated MP trays with different weight ratio of Cloisite-Na +
Samples (weight ratio)
100CNF/10CMC
784.6 21.9 36.1 1.6 588.7 30.8 9.7 1.7 528.8 19.7 5.3 0.8 519.3 25.0 4.9 0.6 466.9 14.7 3.9 0.2
WVTR [gm 2 day 1 ]
Samples (weight ratio)
100CNF/10CMC/1.5PAE
100CNF/10CMC/1.5PAE/3Cloisite-Na + 100CNF/10CMC/1.5PAE/10PVA 100CNF/10CMC/1.5PAE/3Cloisite- Na + /10PVA
100CNF/10CMC/1.5PAE
588.7 30.8 528.8 19.7 557.4 23.1
100CNF/10CMC/1.5PAE/3Cloisite-Na + 100CNF/10CMC/1.5PAE/5Cloisite-Na +
Mater. Adv. , 2025, 6 , 2833–2844 | 2839
© 2025 The Author(s). Published by the Royal Society of Chemistry
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