View Article Online
Paper
Materials Advances
However, despite the notable decrease in WVTR of CNF after chemical crosslinking and the addition of nano-clay, it cannot be concluded that the internal pores have been minimized to their lowest possible extent. These remaining pores may con- tinue to facilitate the transmission of water vapor. In other words, while the modifications have demonstrably improved the barrier properties, there is potential for further optimiza- tion of the CNF structure to minimize water vapor transmis- sion. Therefore, additional polymers are needed to fill up the free volume and block the tunnels for water molecules to pass through. Polyvinyl alcohol (PVA) has long been recognized as a suitable candidate for food packaging due to its biodegrad- ability, high oxygen barrier, good film-forming properties, etc. Most researchers focused on incorporating CNMs as fillers inside PVA matrix to improve the barrier performance, 22–24 but very few explored the reverse approach. Chowdhury et al. 25 incorporated CNC with PVA and discovered that the WVTR of CNC/PVA composite was lower than either pure CNC or pure PVA due to the lower free volume. In this study, we propose an additive modified CNF coating using PAE as the crosslinker, Cloisite-Na + as the nano-clay and PVA as the extra polymer for a premier combination. Molded pulp (MP) is chosen as a low-cost substrate as lidded tray are already used in short-term storage, but improvement in barrier properties could extend preservation ability. Water vapor trans- mission rate and oxygen permeation tests have shown that an improvement was achieved for the formulated CNF compared to the unmodified counterparts, and the contact angle mea- surements confirmed the hypothesis regarding the effect of each component on improving barrier performance, and SEM morphology test proved the increase of smoothness of the formulated CNF. Oil and grease resistance and food sauce stain resistance revealed that the formulated samples were at least equal to or potentially greater than the uncrosslinked CNF/ CMC coated samples. Furthermore, the mechanical test showed that the Young’s modulus decreased for the formulated CNF samples, but UTS and strain at break increased, and the formulated CNFs still had some degree of strength even com- pletely soaked in water. While previous work on oil resistant coatings for paper has been established, to our knowledge, this is the first time that a waterproof, stain- and oil-resistant CNF coating with good water vapor barrier has been developed. While in this case it is for molded pulp, the concept of using polymer and clay additive, and crosslinker can be used on many other paper goods.
oxygen transmission rates is crucial for preserving food quality and extending shelf life. Studies have demonstrated the effec- tiveness of CNFs in enhancing oxygen barrier properties. 12 However, the oxygen barrier properties of CNFs drop sharply at relatively high humidity conditions due to the hydrophilicity of CNFs. Aulin et al. 13 studied the oxygen barrier properties of micro fibrillated cellulose (MFC) free-standing films and coat- ings at different relative humidity (RH) at room temperature and found that the oxygen permeability (OP) of MFC films significantly increased at a RH higher than 70%. It was also found that the oxygen transmission rate (OTR) increased shar- ply above a moisture content of 15%. The water vapor transmis- sion rate (WVTR) of CNFs is also high compared with most commercial polymers, and CNFs are considered a poor moisture barrier. 12 Various approaches have been utilized to enhance the moisture barrier properties of CNFs. For example, Spence et al. 14 studied the relationship of water adsorption, WVTR, and initial contact angle vs. lignin content and found that higher lignin content samples showed higher contact angles but worse water barrier performance at the same time. Although lignin is more hydrophobic than cellulose nanofibrils, high lignin content could also introduce more non-adsorbing large pores, therefore increasing WVTR values. Besides incorporating more hydrophobic materials, much research has focused on modification of cellulosic nanomaterials to reduce WVTR by crosslinking. Hasan et al. 15 reported cross- linked self-standing films of lignin-containing cellulose nano- fibrils (LCNFs) derived from a recycled old, corrugated cardboard (OCC) pulp. Crosslinking was achieved by soaking in Al 3+ or polyamidoamine epichlorohydrin (PAE) water bath. Results showed that both high humidity oxygen permeability and water vapor permeability was reduced for Al 3+ and PAE cured LCNF, while PAE-cured samples were more pronounced, which could be due to reduced porosity, increased hydrophobicity from the PAE to the film, and the ester linkage. Furthermore, the researchers also applied cold and hot pressing during the film preparation, which could attribute to the increase in moisture and oxygen barrier performance. It has been found that PAE- crosslinked CNF showed a transition from hydrophilic to more hydrophobic. 16,17 Apart from chemical crosslinking, addition of nano-sized clay could enhance the barrier properties against water vapor as impermeable filler particles create a more tortuous diffusion path for gas or water molecules. 18 Tayeb et al. 19 crosslinked CNFs using two crosslinking agents, PAE and Acrodur thermo- set acrylic resin (ACR), and added colloidal montmorillonite nano-clay (MMT). Results showed that the synergistic effect of crosslinker and MMT substantially reduced the WVTR. Simi- larly, Khairuddin et al. 20 reported a comparison study of applying different types of clay in starch and tested the water barrier performance. It was found that Cloisite-Na + , a sodium montmorillonite that can be well dispersed in water, showed the lowest WVTR among other types of nano-clay. Similarly, Farmahini-Farahani et al. 21 utilized the Cloisite-Na + nano-clay in microcrystalline cellulose and reported decrease in WVTR.
2. Experimental section 2.1. Materials
Mechanically fibrillated cellulose nanofibrils (CNFs) were pur- chased from the University of Maine, Orono, ME, USA ( B 15%, B 90% retained fines). Sodium carboxymethyl cellulose (CMC) (Lot #MKCK7917, average M w B 250 000, degree of substitution 0.9), chitosan (Batch #04609LD, medium molecular weight), acetic acid (Lot # MKCL7705, glacial ReagentPlus s , Z 99%),
2834 | Mater. Adv. , 2025, 6 , 2833–2844
© 2025 The Author(s). Published by the Royal Society of Chemistry
Made with FlippingBook flipbook maker