View Article Online
Paper
Materials Advances
conducted in both dry and wet conditions. For dry tests, approximately 4 g of desiccant was placed inside the cup, while wet tests used about 5 g of water instead. The bottom flat portion of the formulated CNF coated MP trays were cut into circular samples that were slightly larger than the cup opening for full coverage. Then the testing samples were secured between two O-rings and tightly screwed onto the cup, ensuring complete coverage. The cups were weighed daily. Both dry and wet tests were carried out at room temperature (23 1 C) with 50% relative humidity. 2.6. Oxygen permeability measurement The oxygen permeability tests were conducted according to the procedures stated in the previous research. 29–31 Briefly, the dry CNF-based sheets were cut into a circle with a diameter of 25 mm and then inserted into a stainless-steel holder with an exposed area of 3 cm 2 . The system was first evacuated for around 10 min to remove residual air and moisture, and then the valve connected to the bottom of the holder was shut off to prevent outer environment air from entering the holder. The leak rate, which is defined as the inevitable pressure rise rate in the downstream volume due to the leakage of the system, was measured in the first 12 hours. After that, the valve connecting the upper part of the sample holder was opened to let the oxygen enter the holder. The upstream pressure was set at around 50 psi. The test takes around 3 to 5 days. Eqn (1) shows how the permeability is calculated. 32
2.9. Cobb test The Cobb test was performed to evaluate the sample’s resis- tance to water penetration and to measure the amount of water absorbed per unit area. The test followed the TAPPI T441 om-09 standard, with some modifications due to size limitations. Before testing, the circular samples were conditioned at room temperature and 50% relative humidity for 24 hours, then weighed using an analytical balance. The samples were secured inside the perm cups in the same manner as in the wet perm cup WVTR test described earlier. The perm cups were placed upside down to allow the water to contact the CNF-based coating, and a stopwatch was used to track the time. After 120 seconds, the perm cups were returned to their normal position, and the samples were removed and gently wiped with blotting paper to remove excess water. The samples were then reweighed, and the Cobb values were calculated. 2.10. Oil and grease resistance Oil and grease resistance tests were conducted according to the TAPPI T559 standard. Twelve vials containing a mixture of castor oil, toluene, and n -heptane were prepared in predeter- mined ratios, with kit 1 being the least aggressive and kit 12 the most aggressive. A single drop of the oil mixture was dropped to the surface of the CNF-based coatings, left for 15 seconds, and then quickly wiped off with a tissue. The test area was visually inspected to check for any darkening. Darkening indicated a failure, while no change in appearance indicated a pass. If the sample failed, testing was repeated with a lower kit number oil; if it passed, a higher kit number oil would be used for the next test. This process continued until the highest-numbered kit solution that the coating could pass was identified, which became the coating’s kit rating. 2.11. Sauce stain resistance testing of coated MP trays To evaluate the practical application of formulated CNF-coated MP trays, a sauce stain resistance test was conducted. The experiment involved food sauces to the surface of the formu- lated CNF-coated MP trays. Ketchup and yellow mustard were purchased from a local grocery store and used for testing. First, about 5 g of sauce was applied to the bottom part of the tray. After a 30-second interval, the sauces were wiped off using paper towels. Color comparisons of the tray surfaces before and after sauce application were performed via visual inspection. Additionally, uncoated MP trays and CNF/CMC-coated MP trays were subjected to identical testing procedures, serving as control groups for comparative analysis. 2.12. Mechanical test procedure A uniaxial tensile test was performed to characterize the mecha- nical properties of the CNF-based coating before and after chemical modifications using a TA.XTplusC Texture Analyzer (Stable Micro Systems, Godalming, United Kingdom). The CNF- based dry coating sheets and coated MP trays were first cut into dog-bone specimens using a steel rule die made by ACE Steel
d p d t ss
d p d t leak
V d l P 2 ART
(1)
P ¼
where P is the permeability of the film, V d is the downstream volume, l is the thickness of the film sample, P 2 is the upstream pressure, A is the exposed area of the film, R is the gas constant, and T is the absolute temperature, (d p /d t ) ss and (d p /d t ) leak are the steady-state pressure rise vs. time during the permeability and leak test, respectively. In our testing condition, V d =85cm 3 , A = 3 cm 2 . 2.7. Contact angle measurement The contact angle test was performed using a drop shape analyzer (DSA) (Kruss DSA30) by the sessile drop method. The DSA was equipped with a high-resolution CCD camera, and the angles were measured using the ADVANCE software. CNF-based dry sheets were first cut into small rectangular specimens and then placed on the sample holder. About 2 m L of DI water was dropped onto the surface of the specimens. Images of water droplets on the surface of the specimens were taken. Due to the hydrophilic nature of the CNF-based samples, only the initial contact angles were taken. In this study, the initial contact angles were defined as the contact angles measured within 1 second after the water droplet touching the surface of the specimens. 2.8. Surface characterization A PerkinElmer Spectrum 100 FTIR spectrometer (ATR-FTIR) was used to characterize different formulations of the CNF- based samples.
2836 | Mater. Adv. , 2025, 6 , 2833–2844
© 2025 The Author(s). Published by the Royal Society of Chemistry
Made with FlippingBook flipbook maker