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convenience of characterization. The specimens were dried inside an oven at 90 1 C for 2 hours, followed by drying at the corresponding cure temperature for 30 minutes for crosslink- ing. For each crosslinking strategy, a specimen was first diluted 100 times using DI water and stirred at 1000 rpm for 24 hours. Then, 100 m L of each supernatant was diluted with 17 mL of DI water. Finally, the suspensions were characterized by a nephelometric turbidimeter (Vernier s , Beaverton, OR). Before testing, the turbidimeter was calibrated using one cuvette containing 100 NTU standard and another cuvette containing only DI water (0 NTU). The diluted supernatant samples were added into the turbidity bottles and inserted inside the holder of the turbidimeter for testing. Turbidity values were recorded at 1 per second for 3 minutes. 2.4. Preparation of formulated CNF dry sheets and coated MP trays The process of making formulated CNF dry sheets and coated molded pulp (MP) tray was similar to previous literature with some modifications. 26,28 The B 20 wt% wet-formulated CNF- based pastes were first rolled into sheets 1 mm thick using a roller pin and a stainless steel sheet. Two 3D-printed rings were securely fixed at both ends of the rolling pin, with their inner diameters matching the outer diameter of the rolling pin. Each ring exhibited a thickness of 1 mm throughout its cross- section. The CNF-based wet sheets were then trimmed to desired shapes. For dry sheet making, the wet formulated CNF sheets were sandwiched between a series of different sheets. The struc- tural composition, proceeding from the inner core to the outer shell, was arranged as follows: wet CNF sheets are positioned at the center, followed by stainless steel meshes (‘‘Dutch Weave’’), then layers of copy papers are applied, succeeded by rubber sheets, and finally enclosed by stainless steel plates, as shown in Fig. S1 (ESI † ). Finally, the stacked layers were hot-pressed at 120 1 C. During the hot-pressing process, the pressure gradually increased from 20 kPa to 60 kPa in 30 minutes to maintain the shape of CNF sheets and avoid over pressing. Before coating formulated CNF onto the MP trays, a few droplets of an aqueous primer solution, consisting of 2.5 wt% PAE, 2 wt% chitosan, 0.8 wt% acetic acid, were first smeared on the MP trays to increase the adhesion between the coating and substrate. Then the wet formulated CNF sheets were hand-coated onto the surface of trays as described in previous literature. 26 Folds, wrinkles, and air pockets were flattened by hand, and a sharp blade was used to trim off the leftover sheet, leaving the edge of the trays uncoated. Then the wet samples were placed inside the 3D printed molds with air holes for drying. The molds were wrapped with rubber hands and placed in the oven for at 90 1 C for 5.5 hours, followed by 120 1 C for 30 mins. 2.5. Water vapor transmission rate (WVTR) measurement Water vapor transmission rate (WVTR) measures water barrier properties of samples by quantifying the mass of water vapor that passes through a unit area of the film in a given time. WVTR evaluations were performed using Gardco Inc.’s small permeation cups, following ASTM D1653-13 standards. The tests were
toluene (product no, 244511, Lot #S HBH2498V, anhydrous, 99.8%), castor oil (product no, 259853, Lot # MKCP1892), polyvinyl alcohol (PVA) (Lot # MKCC9501, M w 89 000–98 000, 99+% hydrolyzed), pectin from citrus peel (product no., P9135- 100G, Lot #000311421), alginic acid sodium salt (180947-250G, Lot #1003666881), starch (product no, S9755-500G, soluble, ACS reagent), 1,2,3,4-butanetetracarboxylic acid (BTCA) (Lot # MKCC8769), citric acid (CA) (Lot # MKCG2579, ACS reagent, Z 99.5%), and sodium hypophosphite monohydrate (SHP) ( Z 99%) were purchased from Sigma-Aldrich. Ultrapure water was produced using a Barnstead Ultrapure water system. n -Heptane (product no., 32441, Lot # P11H726, spectrophoto- metric grade, 99+%) was purchased from Alfa Aesar. Polyamide epichlorohydrin (PAE, Polycup 9250, solid content = 25%) was kindly provided by Solenis LLC (Wilmington, DE). Cloisite-Na + was kindly provided by BYK USA, Inc. (St Louis, MO). Mixing cups and lids of 200 mL volume (Max 200 Cup – translucent, 501 220t) were purchased from FlackTek Inc., Landrum, SC, USA. Small Perm Cups (10 cm 2 , MO-1652) were purchased from Gardco, Columbia, MD, USA. Stainless steel meshes (‘‘Dutch weave,’’ 316SS) with a mesh size of 165 800, and two temperature resistant silicone rubber sheets with a hardness of 60A were purchased from McMaster Carr Supply Company (Elmhurst, IL). 2.2. Preparation of CNF-based paste The method of CNF-based paste preparation was the same as stated in previous literature. 26 Large CNF chunks were first dried to around 20% solid concentration using a Memmert HCP 50 Humidity Chamber at 28.5 1 C and 95% RH to achieve a homogeneous concentration. Then the CNF chunks were bro- ken into small pellets using a blender. Later, 48 g of the 20 wt% CNF pellets were added into a 200 mL Flacktek mixing cup. CMC powders were added with the dry weight ratios of CMC to CNF of 1 : 10. Other components (crosslinker, nano-clay, extra polymers) were also added at specific weight ratios. The mix- ture was mixed using the SpeedMixer system (Flacktek Inc.) at 2000 rpm for 8 min and allowed to cool at room temperature for at least 10 min. For the second stage of mixing, the mixture was added into a high shear torque mixer (Plasti-Corder PL 2100 Electronic Torque Rheometer, C. W. Brabender, South Hackensack, NJ) equipped with Banbury-type mixing blades at a mixing speed of 120 rpm and temperature of 55 1 C. A plateau in torque vs. time curve indicated a relatively homogeneous mixture had been achieved. The mixing generally requires 40 min to complete. The samples were collected and stored inside a refrigerator at 4 1 C for future tests. Shorthand name of each formulation are given by weight ratios of contents. For example, a 10 : 1 dry weight ratio of CNF : CMC mixture is represented by 10CNF/1CMC. 2.3. Turbidity test Procedures for turbidity were adopted from El Awad Azrak et al. 27 with some modifications. Small specimens were taken from the CNF-based pastes containing different crosslinkers. The weight of each specimen was lower than 1 g for the
Mater. Adv. , 2025, 6 , 2833–2844 | 2835
© 2025 The Author(s). Published by the Royal Society of Chemistry
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