Pickering emulsions of thyme oil in water using oxidized cellulose nanofibers: Towards bio-based active packaging.

The antioxidant and antimicrobial properties of thyme essential oil (TEO) are useful for active food packaging, but its poor aqueous solubility restricts its applications. This work involves anionic cellulose nanofibers (CNFs) as the sole stabilizing agent for TEO-in-water emulsions, with oil concentrations ranging from 10 mL/L to 300 mL/L. A double mechanism was proposed: the adsorption of CNFs at oil/water interfaces restricted coalescence to a limited extent, while thickening (rheological stabilization) was required to avoid the buoyance of large droplets (>10 µm). Thickening effects comprised both higher viscosity (over 0.1 Pa·s at 10 s-1) and yield stress (approximately 0.9 Pa). Dilute emulsions had good film-forming capabilities, whereas concentrated emulsions were suitable for paper coating. Regarding antimicrobial activity, CNF-stabilized TEO-in-water emulsions successfully inhibited the growth of both Gram-negative (E. coli, S. typhimurium) and Gram-positive bacteria (L. monocytogenes). As for the antioxidant properties, approximately 50 mg of paper or 3-5 mg of film per mL of food simulant D1 were required to attain 50 % inhibition in radical scavenging tests. Nonetheless, despite the stability and the active properties of these bio-based hydrocolloids, providing this antioxidant and antimicrobial activity was incompatible with maintaining the organoleptic properties of the foodstuff unaltered.

Enzymatic pretreatment for cellulose nanofiber production: Understanding morphological changes and predicting reducing sugar concentration.

Enzymatic pretreatment plays a crucial role in producing cellulose nanofibers (CNFs) before fibrillation. While previous studies have explored how treatment severity affects CNF characteristics, there remains a lack of suitable parameters to monitor real-time enzymatic processes and fully comprehend the link between enzymatic action, fibrillation, and CNF properties. This study focuses on evaluating the impact of enzyme charge (using a monocomponent endoglucanase) and treatment time on cellulose fiber morphology and reducing sugar generation. For the first time, a random forest (RF) model is developed to predict reducing sugar concentration based on easily measurable process conditions (e.g., stirrer power consumption) and fiber/suspension characteristics like fines content and apparent viscosity. Polarized light optical microscopy was found to be a suitable technique to evaluate the morphological changes that fibers experience during enzymatic pretreatment. The research also revealed that endoglucanases initially induce surface fibrillation, releasing fine fibers into the suspension, followed by fiber swelling and shortening. Furthermore, the effect of enzymatic pretreatment on resulting CNF characteristics was studied at two fibrillation intensities, indicating that a high enzyme charge and short treatment times (e.g., 90 min) are sufficient to produce CNFs with a nanofibrillation yield of 19-23 % and a cationic demand ranging from 220 to 275 µeq/g. This work introduces a well-modeled enzymatic pretreatment process, unlocking its potential and reducing uncertainties for future upscaling endeavors.

Luminescent Papers with Asymmetric Complexes of Eu(III) and Tb(III) in Polymeric Matrices and Suggested Combinations for Color Tuning.

Complexes of lanthanide ions, such as Eu(III) (red light emission) and Tb(III) (green light emission), with proper ligands can be highly luminescent and color-tunable, also attaining yellow and orange emission under UV radiation. The ligands employed in this work were poly(sodium acrylate), working as polymeric matrix, and 1,10-phenanthroline, taking advantage of its antenna effect. Possibilities of color display were further enhanced by incorporating a cationic polyfluorene with blue emission. This strategy allowed for obtaining cyan and magenta, besides the aforementioned colors. Uncoated cellulose paper was impregnated with the resulting luminescent inks, observing a strong hypsochromic shift in excitation wavelength upon drying. Hence, while a cheap UV-A lamp sufficed to reveal the polyfluorene's blue emission, shorter wavelengths were necessary to visualize the emission due to lanthanide ions as well. The capacity to reveal, with UV-C radiation, a full-color image that remains invisible under natural light is undoubtedly useful for anti-counterfeiting applications. Furthermore, both lanthanide ion complexes and polyfluorenes were shown to have their luminescence quenched by Cu(II) ions and nitroarenes, respectively.

Stabilization of Beeswax-In-Water Dispersions Using Anionic Cellulose Nanofibers and Their Application in Paper Coating.

Beeswax is a bio-sourced, renewable, and even edible material that stands as a convincing option to provide paper-based food packaging with moisture resistance. Nonetheless, the difficulty of dispersing it in water limits its applicability. This work uses oxidized, negatively charged cellulose nanofibers along with glycerol to stabilize beeswax-in-water emulsions above the melting point of the wax. The synergistic effects of nanocellulose and glycerol granted the stability of the dispersion even when it cooled down, but only if the concentration of nanofibers was high enough. This required concentration (0.6-0.9 wt%) depended on the degree of oxidation of the cellulose nanofibers. Rheological hindrance was essential to prevent the buoyancy of beeswax particles, while the presence of glycerol prevented excessive aggregation. The mixtures had yield stress and showed pseudoplastic behavior at a high enough shear rate, with their apparent viscosity being positively influenced by the surface charge density of the nanofibers. When applied to packaging paper, the nanocellulose-stabilized beeswax suspensions not only enhanced its barrier properties towards liquid water (reaching a contact angle of 96°) and water vapor (<100 g m-2 d-1), but also to grease (Kit rating: 5) and airflow (>1400 Gurley s). While falling short of polyethylene-coated paper, this overall improvement, attained using only one layer of a biobased coating suspension, should be understood as a step towards replacing synthetic waxes and plastic laminates.

Comparative Study on the Stiffness of Poly(lactic acid) Reinforced with Untreated and Bleached Hemp Fibers.

Composite materials containing natural reinforcement fibers, generally called biocomposites, have attracted the interest of both researchers and manufacturers, but the most environmentally advantageous combinations include a bio-based matrix, as well. With this in mind, a poly(lactic acid) (PLA) matrix was reinforced with natural fibers from hemp, both untreated strands (UHSs) and soda-bleached fibers (SBHFs). The preparation of the subsequent fully bio-sourced, discontinuously reinforced composites involved kinetic mixing, intensive single-screw extrusion, milling, and injection molding. Up to a fiber content of 30 wt%, the tensile modulus increased linearly with the volume fraction of the dispersed phase. Differences between SBHFs (up to 7.6 Gpa) and UHSs (up to 6.9 Gpa) were hardly significant (p = 0.1), but SBHF-reinforced composites displayed higher strain at failure. In any case, for the same fiber load (30 wt%), the Young's modulus of PLA/hemp biocomposites was greater than that of glass fiber (GF)-reinforced polypropylene (5.7 GPa), albeit lower than that of PLA/GF (9.8 GPa). Considering all the measurements, the contribution of each phase was analyzed by applying the Hirsch model and the Tsai-Pagano model. As a concluding remark, although the intrinsic tensile modulus of SBHFs was lower than that of GF, the efficiency of those natural fibers as reinforcement (according to the rule of mixtures) was found to be higher.

Sustainability of cellulose micro-/nanofibers: A comparative life cycle assessment of pathway technologies.

Cellulose micro- and nanofibers (CNFs) are commonly regarded as "greener" than petro-based materials. The high energy input that their production still demands, along with the use of chemicals or heat in some pretreatments, asks for a critical view. This paper attempts a life cycle assessment of CNFs produced from bleached hardwood kraft pulp via three different pre-treatments before mechanical homogenization. First, a fully mechanical route, based on a Valley beating pre-treatment. Second, an enzymatic route, based on endoglucanases and requiring certain temperature (~50 °C). Third, a TEMPO-mediated oxidation route, considering not only the impact of the chemical treatment itself but also the production of TEMPO from ammonia and acetone. The main output of the study is that both, mechanical and TEMPO-mediated oxidation routes, present lower impacts than the enzymatic pre-treatment. Although the mechanical route presents slightly milder contributions to climate change, acidification, eutrophication, and other indicators, saying that TEMPO-mediated oxidation is environmentally unfeasible should be put under question. After all, and despite being disregarded in most assessment publications up to date, it is the only well-known way to selectively oxidize primary hydroxyl groups and thus producing kinds of CNFs that are unthinkable by other ways.

Response of Polypropylene Composites Reinforced with Natural Fibers: Impact Strength and Water-Uptake Behaviors.

Composites from polypropylene (PP) reinforced with hemp strands (HS) are prepared in the current work with the aim of deepening on the influence of this reinforcement on the impact performance of these specific composites. Despite all the research conducted in this field, the effect of this natural reinforcement on the absorbed energy during crack formation and propagation is not fully tackled in previous research works. From the methodology and samples' geometry, the results concluded that the quality of the interface has a noticeable role in the impact resistance of these materials. The interface strength, fiber dispersion and fiber pullout are the main contributors to crack formation, whereas fiber pullout is the main one responsible for crack propagation. Maximum values of absorbed energy were found for PP composites comprising 20-30 wt% of HS and 8 wt% of the coupling agent for the un-notched samples, whereas maximum absorbed energy values corresponded to PP composites with 40 wt% of HS and 4 wt% of coupling agent for the notched samples. The water-absorption behavior in different humid environments is also examined. From the kinetic study, the water diffusion followed a Fickean behavior showing low-diffusion coefficients, increasing with fiber content. This systematic investigation represents a contribution to the analysis of the potential of reinforcing conventional polymers with natural materials, as a strategy towards more sustainable development.

Durable Nanocellulose-Stabilized Emulsions of Dithizone/Chloroform in Water for Hg2+ Detection: A Novel Approach for a Classical Problem.

The use of dithizone (DTZ) for colorimetric heavy-metal detection is approximately one century old. However, its pending stability issues and the need for simple indicators justify further research. Using cellulose nanofibers, we attained DTZ-containing emulsions with high stability. These emulsions had water (at least 95 wt %) and acetic acid (1-8 mL/L) conforming the continuous phase, while dispersed droplets of diameter <1 µm contained chloroform-solvated DTZ (3 wt %). The solvation cluster was computed by molecular dynamics simulations, suggesting that chloroform slightly reduces the dihedral angle between the two sides of the thiocarbazone chain. Nanocellulose concentrations over 0.2 wt % sufficed to obtain macroscopically homogeneous mixtures with no phase separation. Furthermore, the rate of degradation of DTZ in the nanocellulose-stabilized emulsion did not differ significantly from a DTZ/chloroform solution, outperforming DTZ/toluene and DTZ/acetonitrile. Not only is the emulsion readily and immediately responsive to mercury(II), but it also decreases interferences from other ions and from natural samples. Unexpectedly, neither lead(II) nor cadmium(II) triggered a visual response at trace concentrations. The limit of detection of these emulsions is 15 µM or 3 mg/L, exceeding WHO limits for mercury(II) in drinking water, but they could be effective at raising alarms.

Behavior of the Flexural Strength of Hemp/Polypropylene Composites: Evaluation of the Intrinsic Flexural Strength of Untreated Hemp Strands.

The growing demand for plant fiber-reinforced composites offers new opportunities to compete against glass fiber (GF)-reinforced composites, but their performance must be assessed, revised, and improved as much as possible. This work reports on the production and the flexural strength of composites from polypropylene (PP) and hemp strands (20-50 wt.%), using maleic anhydride-grafted PP (MAPP) as a compatibilizer. A computational assessment of the reaction between cellulose and MAPP suggested the formation of only one ester bond per maleic anhydride unit as the most stable product. We determined the most favorable MAPP dosage to be 0.06 g per gram of fiber. The maximum enhancement in flexural strength that was attained with this proportion of MAPP was 148%, corresponding to the maximum fiber load. The modified rule of mixtures and the assumption of similar coupling factors for tensile and flexural strength allowed us to estimate the intrinsic flexural strength of hemp strands as 953 ± 116 MPa. While falling short of the values for sized GF (2415 MPa), the reinforcement efficiency parameter of the natural fibers (0.209) was found to be higher than that of GF (0.045).

Tensile Strength of Poly(lactic acid)/Bleached Short Hemp Fiber Fully Green Composites as Replacement for Polypropylene/Glass Fiber.

The compatibility between poly(lactic acid) (PLA) and natural fibers to develop bio-sourced, recyclable, and biodegradable composites remains a commonplace issue. This work highlights that, at least in the case of hemp, pulping and bleaching towards delignified short fibers attained remarkable improvements over untreated hemp strands. This approach differs from usual proposals of chemically modifying hydroxyl groups. Soda-bleached hemp fibers (SBHFs) granted a relatively large bonding surface area and a satisfactory quality of the interphase, even in the absence of any dispersant or compatibilizer. To attain satisfactory dispersion, the matrix and the fibers were subjected to kinetic mixing and to a moderately intensified extrusion process. Then, dog-bone specimens were prepared by injection molding. Up to a fiber content of 30 wt.%, the tensile strength increased linearly with the volume fraction of the dispersed phase. It reached a maximum value of 77.8 MPa, signifying a relative enhancement of about 52%. In comparison, the tensile strength for PLA/hemp strands was 55.7 MPa. Thence, based on the modified rule of mixtures and the Kelly & Tyson modified equation, we analyzed this performance at the level of the constituent materials. The interfacial shear strength (over 28 MPa) and other micromechanical parameters were computed. Overall, this biocomposite was found to outperform a polypropylene/sized glass fiber composite (without coupling agent) in terms of tensile strength, while fulfilling the principles of green chemistry.