Can pure cellulose nanofibril films replace polyolefins as water vapor barriers in packaging?

Despite significant research into cellulose nanofibril (CNF) films as substitutes to synthetic plastic materials, commercial applications remain very limited. One major hindrance is the poor water vapor barrier properties of CNF films compared to polyolefins, a critical property for product protection, such as food safety and preservation. To date, it is unknown whether full moisture barrier properties can be achieved with materials made by the assembly of nanofibers and fibrils. A comprehensive understanding of the effect of film structure on water vapor transport properties is required. Here, over 200 films were produced with a wide range of grammages from 30 g/m2 to 580 g/m2 by casting and spray deposition. Their structures were quantified by µCT and SEM and related to their water vapor transmission rates (WVTRs). Porosity and pore connectivity decreased with increasing film grammage, which correlates with the exponential decrease in WVTR. However, the WVTR plateaued at 30 g/m2day, indicating that the known open space and adsorption diffusion mechanisms cannot be fully eliminated by producing high grammage films. Pure cellulose nanofibril films therefore cannot replace polyolefins in packaging applications, requiring modifications such as coating and nanofillers.

Modulating the chiral nematic structure of cellulose nanocrystal suspensions with electrolytes.

HYPOTHESIS: The iridescent optical properties of films made of cellulose nanocrystals (CNC) are controlled by the pitch and range of the chiral nematic structures. These are further tuned with the addition of electrolyte. EXPERIMENTS: Electrolyte type, valency and concentration were varied. The bulk CNC suspension properties were investigated by combining rheology, polarised optical photography and microscopy, while the spacing between crystals was determined using SAXS. FINDINGS: The addition of electrolyte to a CNC suspension containing chiral nematic structures first causes the nematic pitch to increase indicating the suspension has a weaker structure. Further increases in electrolyte concentration cause aggregation and complete breakdown of the chiral nematic structures. The univalent species cause larger changes to the chiral nematic structure with the onset and magnitude of structure breakdown occurring at lower ionic strengths compared with the divalent species. Cation size influences the chiral nematic structure with the order of influence being K+ > Na+ ≈ Ca2+ > Mg2+, which corresponds from the largest to smallest cation. This work demonstrates that both ion valency, concentration and species play a significant role in controlling the chiral nematic structures of CNC suspensions and will be a vital step in the development of CNC liquid crystals, optical materials and sensors.

Self-assembly of cellulose nanocrystals of different lengths.

Hypothesis The self-assembly (SA) of cellulose nanocrystals (CNC) in suspensions is important both from the fundamental and advanced technology development perspective. CNC of different lengths self-assemble differently in suspensions by balancing attractive and repulsive interactions which depends strongly on morphology, surface chemistry and concentrations. Experiments Two different commercial CNC samples (CNC-M and CNC-C) of different lengths were dispersed in Milli-Q water at different concentrations (0.5-10 wt%). CNC-M is provided as a gel at a solid concentration of 10.3 wt% which was diluted in Milli-Q water. CNC-C is sold as a powder which was dispersed in Milli-Q water with a mixer to achieve the desired concentrations. TEM was used to determine morphology of CNC. Polarised optical microscopy is performed to get microscale visualisation of the chiral nematic self-assembly. High flux synchrotron SAXS is applied to evaluate and compare the nanoscale self-assembly mechanisms of CNC of different lengths. Findings The SA of two different types of CNC rods of similar diameter but different lengths is investigated. SAXS analysis shows the short rods in suspension form an isotropic phase (randomly oriented) at lower concentration (0-4 wt%); as concentration is increased, the rods become systematically aligned in a nematic phase. The interrod distance d varies as c-0.33 at the lower concentration, which changes to c-0.5 and even c-1 at the higher concentrations. In contrast, the long rods in suspension remain in the isotropic phase throughout the measured concentration range from 0.5 to 10 wt%. The interrod distance also follows the isotropic power law slope of c-0.33. Suspensions made of the short CNC rods show long range order and large interrod distance compared to those formed by the long rods. POM agrees with the SAXS results. A specific equilibrium between attractive and repulsive forces is required to maintain SA and ordering of the rods. DLVO calculations reveal that the long rods maintain van der Waal attractive force dominating over the electrostatic repulsion, which hinders rods alignment in an ordered manner. However, for the short rods, the weaker attractive interactions are well compensated by the repulsive force which aligns rods in an ordered assembly. This fundamental understanding of the SA of rods in suspensions facilitates the engineering of novel CNC composites of unique optical properties which enables novel applications such as in sensors and bio-diagnostics.

Recent advancements, trends, fundamental challenges and opportunities in spray deposited cellulose nanofibril films for packaging applications.

Plastic packaging is causing a serious environmental concern owing to its difficulty in degrading and micro-particulates' emissions. Developing biodegradable films has gained research attention to overcome ecological and health issues associated with plastic based packaging. One alternative substitute for petroleum-based plastic is nanocellulose based films, having distinguishing characteristics such as biodegradability, renewability, and non-toxicity. Nanocellulose is classified into three major types, i.e., cellulose nanofibril, cellulose nanocrystals, and bacterial nanocellulose. However, the scope of this review is limited to cellulose nanofibril (CNF) because this is the only one of major types that could be turned into film at a competitive cost with petroleum derived polymers. This paper provides a concise insight on the current trends and production methods of CNF. Additionally, the methods for transforming CNF into films are also discussed in this review. However, the focus of this review is the CNF films produced via spray deposition, their properties and applications, and fundamental challenges associated with their commercialization. Spray deposition or spray coating is an ideal candidate as a large-scale production technique of CNF films due to its remarkable features such as rapidity, flexibility, and continuity. Spray deposited CNF films exhibit excellent mechanical properties and oxygen barrier performance, while, possessing limited moisture barrier performance. The possible pathways to improve the moisture barrier performance and optical properties of these films are also discussed in this review. The existing publications on spray deposited CNF films are also highlighted from the literature. Finally, the current status of industrial production of these films and opportunities for academics and industries are also presented, indicating that fibre production capacity needs to be enhanced.

Preparation and benchmarking of novel cellulose nanopaper.

Synthetic polymers and plastics which are currently used as barrier materials in packaging applications are neither renewable nor biodegradable. Nanopaper, which is obtained by breaking down cellulose fibers into nanoscale particles, have unique properties with the potential to replace synthetic packaging materials, but requires very high energy to mechanically process the fibers into nanopaper. This research investigates whether refining alone can be used to produce nanopaper with sufficient quality for packaging applications. Nanopaper was produced from Bleached Eucalyptus Kraft (BEK) refined with a PFI mill and from Northern Bleached Softwood Kraft (NBSK) refined in a pilot disc refiner. Both trials found a plateau for oxygen permeability and water vapour permeability that was reached after 1800 kWh/t and 12,000 kWh/t for refining in the pilot disc refiner and PFI mill, respectively. Refining beyond these optima produced either little or no reduction in permeability, while increasing the drainage time to form a sheet. However, elastic modulus, strain at break and sheet light transmittance did continue to increase. The plateau oxygen permeability of ~ 1.24 (cc µm)/(m2 day kPa) is 1-3 orders of magnitude lower than the oxygen permeability for PET and LDPE, respectively, while the plateau water vapour permeability ~ 3 × 10-11 g/m.s. Pa was 1-2 orders of magnitude higher than for PET and LDPE. The improved strength and barrier properties of nanopaper achieved at lab and pilot scale mechanical refining process promises a sustainable alternative to conventional packaging. Supplementary Information: The online version contains supplementary material available at 10.1007/s10570-022-04563-0.

Rapid Detection of Gram-Positive and -Negative Bacteria in Water Samples Using Mannan-Binding Lectin-Based Visual Biosensor.

Waterborne bacterial infection is a health threat worldwide, making accurate and timely bacteria detection crucial to prevent waterborne disease outbreaks. Inspired by the intrinsic capability of mannan-binding lectin (MBL) in recognizing the pathogen-associated molecular patterns (PAMPs), a visual biosensor is developed here for the on-site detection of both Gram-positive and -negative bacteria. The biosensor was synthesized by immobilization of the MBL protein onto the blue carboxyl-functionalized polystyrene microparticles (PSM), which is then used in a two-step assay to detect bacterial cells in water samples. The first step involved a 20 min incubation following the MBL-PSM and calcium chloride solution addition to the samples. The second step was to add ethanol to the resultant blue mixture and observe the color change with the naked eye after 15 min. The biosensor had a binary (all-or-none) response, which in the presence of bacterial cells kept its blue color, while in their absence the color changed from blue to colorless. Testing the water samples spiked with four Gram-negative bacteria including Acinetobacter baumannii, Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa and two Gram-positive bacteria of Enterococcus faecalis and Staphylococcus aureus showed that the biosensor could detect all tested bacteria with a concentration as low as 101.5 CFU/ml. The performance of biosensor using the water samples from a water treatment plant also confirmed its capability to detect the pathogens in real-life water samples without the need for instrumentation.

One-pot treatment of cellulose using iron oxide catalysts to produce nanocellulose and water-soluble oxidised cellulose.

This research explains the one-pot process of cellulose fibrillation to procure nanocellulose fibres and water-soluble oxidised cellulose using iron oxide catalysts with hydrogen peroxide. The process involved three stages. First, cellulose underwent depolymerisation to form nanocellulose fibres. Second, nanocellulose was oxidised to a water-soluble product, and the final stage was the complete decomposition. Factors such as the valency of ions in the crystal lattice of catalyst, its particle size, and reaction time influenced the treatment. By controlling these factors, either nanocellulose fibres or water-soluble oxidised cellulose with high yields of 83% and 38% produced, respectively. Nanocellulose had 73% crystallinity with DP (degree of polymerisation), 150, and the dimension of 30-110 nm thickness and 1-5 µm in length. The water-soluble product was oxidised with a carboxyl content of 2.9 mmol/g and DP, 25. This research gave an alternative method and eliminated the need for halogenated reagents, strong acids, and mechanical pretreatments.

Moulding of micropatterned nanocellulose films and their application in fluid handling.

HYPOTHESIS: Well-controlled micropatterned nanocellulose films are able to be fabricated via spray coating onto a micropatterned impermeable moulded surface. The micropattern size is able control the directionality of wicking fluid flow. EXPERIMENTS: Using photolithography and etching techniques, silicon moulds with channel widths of 5-500 µm and depths of 6, 12 and 18 µm were fabricated. Micropatterned nanocellulose sheets were formed by spray coating nanofibre suspensions onto the moulds. We also investigate the effect the dimensions of these micropatterned nanocellulose films have on wicking fluids. FINDINGS: Micropatterns were imparted on the surface of nanocellulose films which resulted in three well-defined regimes of conformation with the moulds: full, partial and no conformation. These regimes were driven by the aspect ratio (channel depth/width) of the moulds. Achieved channel widths and depths were compared to those possible with other micropattern fabrication techniques. The directionality of the wicking water droplets can be controlled with the micropatterned channel. Channels within the full conformation regime resulted in increased directionality of fluid flow compared with those not within this regime. This research demonstrates the industrially scalable process of spray coating has potential to serve as the foundation for a new generation of paper-based microfluidic devices.

An energy efficient production of high moisture barrier nanocellulose/carboxymethyl cellulose films via spray-deposition technique.

Nanocellulose (NC) films are considered as a prospective alternative to non-sustainable packaging materials, however, their higher embodied energy and limited moisture barrier properties are regarded as a huge constraint regarding their commercialization. This study aims to produce films with relatively low environmental impact and improved barrier performance. For this purpose, carboxymethyl cellulose (CMC) and NC were combined, and this resulted in multidimensional advantages. The mass production of films could be achieved in only 2 h (requiring at least 24 h under ambient conditions) when dried in an oven at 75 °C with enhanced mechanical properties and without compromising their dimensional stability. The moisture barrier properties of the NC/CMC films were improved up to 92 % compared with the NC films alone and the results achieved are comparable with packaging materials such as polyethylene terephthalate (PET), polycarbonates (PC) etc. Finally, the NC/CMC (1:1) films have low environmental impact compared with PET films.

Exfoliating B. mori silk into high aspect ratio nanofibrils facilitated by response surface methodology.

Silk fiber is formed by an assembly of fibrils. The fibrils can be isolated by a top-down mechanical process called microfibrillation and the fibrils are known as microfibrillated silk (MFS). The process involves chopping, milling, enzyme treatment and high-pressure homogenization. The milling is an important manufacturing step and to optimize the milling step, a response surface methodology was used in this work where the influence of fiber content in milled suspension, milling time and alkaline concentration were investigated. Output responses for the optimization were diameter distribution of fibrils, size and percentage of different diameter fractions, and the aspect ratio. The main and interaction effects of the milling parameters on these responses were statistically analysed. Milling time was the most significant factor for producing finer fibrils while the fiber content in milling had the maximum impact in reducing the number of large fibrils. Milling time had a positive correlation with the aspect ratio. The optimized milling resulted in MFS with an average diameter of 55.35 nm and 90% of MFS less than 100 nm based on high-magnification SEM image analysis. The aspect ratio of the MFS was 137. The MFS suspension was stable over the pH range 3-11.

Impact of structural changes in heteroleptic bismuth phosphinates on their antibacterial activity in Bi-nanocellulose composites.

To study and evaluate the effect of ligand choice and distribution in bismuth phosphinates on toxicity and antibacterial activity, a series of novel diphenyl mono-phosphinato bismuth complexes, [BiPh2(O(O[double bond, length as m-dash])P(H)Ph)] 1, [BiPh2(O(O[double bond, length as m-dash])PPh2)] 2, [BiPh2(O(O[double bond, length as m-dash])PMe2)] 3 and [BiPh2(O(O[double bond, length as m-dash])P(p-MeOPh)2)] 4, were synthesised, characterised and structurally authenticated by X-ray crystallography. Evaluation of their antibacterial activity towards Staphylococcus aureus (S. aureus), methicillin-resistant S. aureus (MRSA), vancomycin-resistant Enterococci (VRE), Escherichia coli (E. coli) and Pseudomonas aeruginosa (P. aeruginosa) showed all four mono-phosphinato bismuth complexes to be highly active. However, unlike their less soluble bis-phosphinato analogues, they displayed an increased level of toxicity towards mammalian cells (COS-7, human and murine fibroblasts), where it was shown the complexes disrupt cellular membranes leading to cytotoxicity. The mono-phosphinato bismuth complexes were used to produce antibacterial nanocellulose composites. Leaching studies showed that complex 1 had the highest levels of leaching, at 15% of the total available bismuth when the composite was soaked in water. The aqueous leachates of 1 were bacteriostatic towards MRSA and VRE at concentrations between 4.0 and 4.6 µM, while being bactericidal towards E. coli above 2.8 µM. At similar concentrations the complex showed toxicity towards human fibroblast cells, with cell viability reduced to 2% (1, 2.4 µM). The possibility to control leaching of the bismuth complexes from cellulose composites through structural changes is evidence for their potential application in antibacterial surfaces and materials.

Polyamide-amine-epichlorohydrin (PAE) induced TiO2 nanoparticles assembly in cellulose network.

HYPOTHESIS: TiO2-NPs-Cellulose composites functionality depends on the retention and dispersion of NPs in the composites. SAXS and SEM can be combined to reveal the effect PAE has on the NPs aggregation, retention and interaction mechanisms in the composites. EXPERIMENTS: TiO2-NPs-Cellulose sheets were made by first preparing PAE-cellulose suspensions of different PAE dosages (10 and 50 mg of PAE/g fibres). The TiO2 NPs suspension (at different NPs loading) was then added to the cellulose-PAE suspension. The final suspension was used to make flexible paper-like composites sheets. SEM and SAXS quantified NPs retention and aggregation state. FINDINGS: PAE dosage of 20 mg/g cellulose provides full surface coverage of cellulose fibres. A 10 mg of PAE/g cellulose covers half the cellulose surface area and no free PAE remains in the suspension. PAE dosage of 50 mg/g cellulose gives full cellulose surface coverage and provides a large amount of PAE (30 mg/g cellulose) free in the suspension. Surprisingly, at both PAE dosages, NP coagulates and the size of the aggregates increase with NPs loading. Aggregates of two particle sizes (10 and 35 nm) are formed and the number density of smaller particles is higher than larger particle. The NPs aggregates and their retention are similar at both PAE dosages, which is explained by different PAE-NPs bridging mechanisms.

Development of a Paper-Based Microfluidic System for a Continuous High-Flow-Rate Fluid Manipulation.

The current study describes the development of a disposable paper-based microfluidic system, which unlike its predecessors that are only capable of processing a small amount of fluid, can continuously process the fluid at a high flow rate of up to 1.5 mL/min. The fabrication procedure was clean-room-free and robust, involving the use of a CO2 laser to engrave the microchannels on a paper substrate, followed by alkenyl ketene dimer treatment to hydrophobize the paper and lamination. The microchannel down to a minimum depth of ∼80 µm with an average roughness of ∼8 µm was engraved on the substrate. As a proof of concept, the applicability of this system to enrich the microparticles based on the inertial focusing mechanism was tested. This new generation of paper-based microfluidic system can be potentially used for the diagnostic applications where the analyte is low in quantity and processing a large volume of fluid sample is required.

Cellulose nanofibers from recycled and virgin wood pulp: A comparative study of fiber development.

The global abundance of recycled pulp has introduced opportunities for cellulose nanofiber (CNF) production at lower energy due to the partially fibrillated nature of recycled pulp. This study investigated the potential of recycled pulp as a feedstock for CNF production, comparing recycled bleached de-inked pulp (DIP) predominantly from eucalyptus fibers with virgin bleached eucalyptus kraft (BEK) pulp. The specific energy consumption for CNF production with 10,000 PFI refiner revolutions and 1 homogenization pass was 7 % lower with recycled pulp. At this treatment level, fiber characterization experiments revealed that the CNF from recycled pulp had a median diameter of 19 nm and aspect ratio was 140, similar to that from virgin pulp. The tensile index of unrefined BEK sheets (30 Nm/g) almost doubled (55 Nm/g) when reinforced with only 20 wt% DIP CNF. This work demonstrates that recycled pulp is a viable alternative to virgin pulp feedstocks for CNF production.

Protein Paper from Exfoliated Eri Silk Nanofibers.

The exfoliation of silk fiber is an attractive method to produce silk micro- and nanofibers that retain the secondary structure of native silk. However, most fibrillation methods used to date require the use of toxic and/or expensive solvents and the use of high energy. This study describes a low cost, scalable method to produce microfibrillated silk nanofibers without the use of toxic chemicals by controlling the application of shear using commercially scalable milling and homogenization equipment. Manipulation of the degumming conditions (alkaline concentration and degumming temperature) and the shear in milling and/or homogenization enabled control over the degree of fibrillation. The microfibrillated silk was then characterized to determine structural change during processing and the stability of the resulting suspensions at different pH. Silk nanofibers obtained from milling degummed silk were characterized using atomic force microscopy. Nanofibers obtained both with and without high-pressure homogenization were then used to produce silk "protein paper" through casting. Silk degumming conditions played a critical role in determining the degree of microfibrillation and the properties of the cast silk papers. The silk papers produced from homogenized nanofibers showed excellent mechanical properties, high water absorption, and wicking properties. The silk papers were excellent for supporting the attachment and growth of human skin keratinocytes, demonstrating application possibilities in healthcare such as wound healing.

Characterizing highly fibrillated nanocellulose by modifying the gel point methodology.

The characterization of nanocellulose fibres (NC) length is a difficult and indirect measurement which relies on aspect ratio calculation and fibre diameter analysis. The aspect ratio can be directly calculated from the gel point, a parameter obtained from sedimentation experiments. The gel point has been used with macroscopic fibres and microfibrillated cellulose, that easily sediment by gravity. However, this methodology has not yield consistent results with highly charged nanofibres nor with fibres with sediment layer difficult to observe. In this study, the gel point methodology is modified: 1) dying the fibres with Crystal Violet to enable the visualization of the fibrils sedimentation line without affecting the fibre network; and 2) by optimizing the sedimentation time to ensure complete settling. The two types of fibrils characterized -low and high fibrillated NC (LF-NC, HF-NC)- behave differently due to the slower sedimentation of HF-NC. The time to reach a stable sedimented layer increases with the level of fibre fibrillation, the charge and the decrease of fibre dimension. Reproducible gel point can be measured after 2 days for LF-NC; however, 8 days are required for HF-NC. The modified methodology was validated by quantifying the influence of pH and salt concentration. As expected, low pHs and the addition of CaCl2 coagulate HF-NC into flocs which increase the ratio: final over initial fibres height (Hs/Ho); this decreases significantly the gel point, as a lower amount of HF-NC are required to interconnect all fibres. This modified method is a valuable tool for the accurate dimensional characterisation of highly charged and low diameter cellulose nanofibres.

Nanocellulose for gel electrophoresis.

HYPOTHESIS: Cellulose nanofibres produced by TEMPO-mediated oxidation can form gels. This study presents a proof-of-concept for gel electrophoresis with nanocellulose (NC). EXPERIMENTS: TEMPO-oxidised cellulose nanofibre dispersion is chemically cross-linked by inducing amide linkages to produce gel slabs for electrophoretic separation. Nanocellulose gel slabs 1 cm thick containing Tris/Borate/EDTA (TBE) buffer were casted. Different cross-linker types and ratios are investigated to assess the migration of conventional electrophoresis tracking dyes. FINDINGS: Tracking dyes (bromophenol blue and orange G) can diffuse within the gel at different rates and therefore separate. Changing the cross-linker length from EDA to HMDA (C2- to C6-chain) increases the overall network pore size resulting in a faster migration rate for both bromophenol blue and orange G. Increasing the cross-linker concentration stabilises the HMDA-NC gel (no extension) during the electrophoresis run without any effect on the dye migration rate. Increasing the voltage increases the migration rates for both orange G and bromophenol blue. Further development is required to cast the gels evenly and to prevent bubble formation during the cross-linking process. This will enable to effectively separate mixtures of proteins. Nanocellulose gels can become a novel substrate for sustainable biomedical separation and diagnostics by electrophoresis.

Carboxylated nanocellulose foams as superabsorbents.

HYPOTHESIS: Carboxylated nanocellulose fibres formed into foam structures can demonstrate superabsorption capacity. Their performance can be engineered by changing process variables. EXPERIMENTS: TEMPO-oxidised cellulose nanofibres of varying concentration and surface charge are produced from hardwood kraft pulp. Foams were prepared through a 2-step freezing and lyophilisation process. The absorption capacity of water and saline solution (0.9 wt%) were measured as a function of time and related to the foam structure. FINDINGS: The absorption capacity of nanocellulose foams can be manipulated from initial gel properties and processing conditions. Pore structure and distribution of nanocellulose foams are dictated by fibre content and charge density and freezing rate. The best performing foams are at 0.3-0.5 wt%, with a carboxylate concentration of 1.2 mmol/g and frozen at -86 °C before freeze-drying, which can absorb 120 g H2O/g fibre. Fibre surface charge influences the absorption capacity of the foams by dictating the amount of participating carboxylate groups. Absorption capacity in saline (60 g/g) is lower than in deionised water (120 g/g); but is only slightly lower than that of a commercial polyacrylic acid (PAA) SAPs (80 g/g). Nanocellulose foams are attractive renewable alternatives for superabsorbent applications, contributing to a reduction of plastic microspheres.

Pickering Emulsions Electrostatically Stabilized by Cellulose Nanocrystals.

Cellulose Nanocrystals (CNC) are explored to stabilize oil/water emulsions for their ability to adsorb at the oil/water interface. In this work, the role of electrostatic forces in the CNC ability to stabilize oil/water emulsions is explored using canola oil/water and hexadecane/water as model systems. Canola oil/water and Hexadecane/ water (20/80, v/v) emulsions were stabilized with the addition of CNCs using ultrasonication. Emulsion droplet sizes range from 1 to 4 µm as measured by optical microscopy. It is found that CNC can stabilize oil/water emulsions regardless of their charge density. However, reducing the surface charge density, by adding salts and varying pH, can reduce the amount of CNC's required to form a stable emulsion. Just by adding 3 mM Na+ or 1 mM or less Ca+2 to a CNC suspension, the amount of CNC reduced by 30% to stabilized 2 mL of Canola oil. On the other hand, adding salt increases the emulsion volume. The addition of 100 mM Na+ or the reduction of pH below 2 leads to the aggregation of CNC; emulsions formed under these conditions showed gel-like behavior. This work shows the potential of nanocellulose crystal in stabilizing food and industrial emulsions. This is of interest for applications where biodegradability, biocompatibility, and food grade requirements are needed.