Review on Nonconventional Fibrillation Methods of Producing Cellulose Nanofibrils and Their Applications.

The production of cellulose nanofibrils (CNFs) continues to receive considerable attention because of their desirable material characteristics for a variety of consumer applications. There are, however, challenges that remain in transitioning CNFs from research to widespread adoption in the industrial sectors, including production cost and material performance. This Review covers CNFs produced from nonconventional fibrillation methods as a potential alternative solution. Pretreating biomass by biological, chemical, mechanical, or physical means can render plant feedstocks more facile for processing and thus lower energy requirements to produce CNFs. CNFs from nonconventional fibrillation methods have been investigated for various applications, including films, composites, aerogels, and Pickering emulsifiers. Continued research is needed to develop protocols to standardize the characterization (e.g., degree of fibrillation) of the lignocellulosic fibrillation processes and resulting CNF products to make them more attractive to the industry for specific product applications.

Aqueous Polymer Modification of Cellulose Nanofibrils by Grafting-Through a Reactive Methacrylate Group.

Modifying the surface of cellulose nanofibrils (CNFs) produced by mechanical refinement with a variety of polymer functional groups in an entirely water-based system is challenging because only surface hydroxyl groups are accessible. To address this limitation, an entirely water-based, polymer modification scheme is developed. CNFs are functionalized with a reactive methacrylate functional group followed by subsequent grafting-through polymerization. This modification worked with a variety of water-soluble and water-insoluble (meth)acrylates and (meth)acrylamides, grafting up to 45 wt% polymer on to the CNFs. The reaction conditions introducing the methacrylate functional group are adjusted to vary the degree of functionality. Soxhlet extraction of modified samples demonstrates that the reactive methacrylate group is necessary to facilitate polymer grafting. The degree of functionalization of the polymers is studied via quantitative transmission IR spectroscopy and the morphology of the resulting cellulose nanofibrils is studied via a combination of optical, scanning electron, and atomic force microscopy. High levels of polymer modification do not significantly affect the micrometer-scale fibril morphology.

A Critical Evaluation and Modification of the Padé-Laplace Method for Deconvolution of Viscoelastic Spectra.

This paper shows that using the Padé-Laplace (PL) method for deconvolution of multi-exponential functions (stress relaxation of polymers) can produce ill-conditioned systems of equations. Analysis of different sets of generated data points from known multi-exponential functions indicates that by increasing the level of Padé approximants, the condition number of a matrix whose entries are coefficients of a Taylor series in the Laplace space grows rapidly. When higher levels of Padé approximants need to be computed to achieve stable modes for separation of exponentials, the problem of generating matrices with large condition numbers becomes more pronounced. The analysis in this paper discusses the origin of ill-posedness of the PL method and it was shown that ill-posedness may be regularized by reconstructing the system of equations and using singular value decomposition (SVD) for computation of the Padé table. Moreover, it is shown that after regularization, the PL method can deconvolute the exponential decays even when the input parameter of the method is out of its optimal range.

Tunable biocomposite films fabricated using cellulose nanocrystals and additives for food packaging.

Cellulose nanocrystals (CNCs) are considered a prospective packaging material to partially replace petroleum-based plastics attributed to their renewability, sustainability, biodegradability, and desirable attributes including transparency, oxygen, and oil barrier properties. However, neat CNC films are rigid and too brittle to handle or utilize for packaging applications. Hence different additives, including sorbitol, polyvinyl alcohol (PVA), chitin, and κ-carrageenan (CG) were selected to mix with CNCs for packaging film preparation. The influence of additive categories (plasticizer, nonionic polymer, weak cationic and anionic natural polysaccharide), and their concentrations on the performance of CNC suspensions as well as optical, barrier, mechanical, and thermal properties of CNC films were examined. The morphology and physical characterization including density, equilibrium moisture content, contact angle and water durability of the composite films were also determined. Sorbitol and PVA films had the best visible light transparency; mixing with chitin can effectively improve the water durability of CNC films, and CG changed the CNC film from hydrophilic to hydrophobic. Moreover, all CNC films exhibited sufficient oxygen barrier properties, high PVA content films attained the "very high" barrier grade. Thus, durable CNC films can be obtained by adding proper types and amounts of additives, which provides potential scenarios for practical application of CNC films in food packaging.

Characterization of CNC Nanoparticles Prepared via Ultrasonic-Assisted Spray Drying and Their Application in Composite Films.

The ultrasonic-assisted spray dryer, also known as a nano spray dryer and predominantly used on a lab scale in the pharmaceutical and food industries, enables the production of nanometer-sized particles. In this study, the nano spray dryer was applied to cellulosic materials, such as cellulose nanofibrils (CNFs) and cellulose nanocrystals (CNCs). CNC suspensions were successfully dried, while the CNF suspensions could not be dried, attributable to their longer fibril lengths. The nano spray drying process was performed under different drying conditions, including nebulizer hole sizes, solid concentrations, and gas flow rates. It was confirmed that the individual particle size of nano spray-dried CNCs (nano SDCNCs) decreased as the nebulizer hole sizes and solid contents of the suspensions decreased. The production rate of the nano spray dryer increased with higher solid contents and lower gas flow rates. The resulting nano SDCNCs were added to a polyvinyl alcohol (PVA) matrix as a reinforcing material to evaluate their reinforcement behavior in a plastic matrix using solvent casting. After incorporating the 20 wt.% nano SDCNCs into the PVA matrix, the tensile strength and tensile modulus elasticity of the neat PVA nanocomposite film increased by 22% and 32%, respectively, while preserving the transparency of the films.

Spray Drying Enzyme-Treated Cellulose Nanofibrils.

Enzyme-treated cellulose nanofibrils (CNFs) were produced via a lab-scale mass colloider using bleached kraft pulp (BKP) to evaluate their processability and power requirements during refining and spray-drying operations. To evaluate the energy efficiency in the CNF refining process, the net energy consumption, degree of polymerization (DP), and viscosity were determined. Less energy was consumed to attain a given fines level by using the endoglucanase enzymes. The DP and viscosity were also decreased using the enzymes. The morphological properties of the enzyme-pretreated spray-dried CNF powders (SDCNFs) were measured. Subsequently, the enzyme-pretreated SDCNFs were added to a PP matrix with MAPP as a coupling agent. The mixture was then compounded through a co-rotating twin-screw extruder to determine whether the enzyme treatment of the CNFs affects the mechanical properties of the composites. Compared to earlier studies on enhancing PMCs with SDCNF powders, this research investigates the use of enzyme-pretreated SDCNF powders. It was confirmed that the strength properties of PP increased by adding SDCNFs, and the strength properties were maintained after adding enzyme-pretreated SDCNFs.

Review on Hybrid Reinforced Polymer Matrix Composites with Nanocellulose, Nanomaterials, and Other Fibers.

The use of composite materials has seen many new innovations for a large variety of applications. The area of reinforcement in composites is also rapidly evolving with many new discoveries, including the use of hybrid fibers, sustainable materials, and nanocellulose. In this review, studies on hybrid fiber reinforcement, the use of nanocellulose, the use of nanocellulose in hybrid forms, the use of nanocellulose with other nanomaterials, the applications of these materials, and finally, the challenges and opportunities (including safety issues) of their use are thoroughly discussed. This review will point out new prospects for the composite materials world, enabling the use of nano- and micron-sized materials together and creating value-added products at the industrial scale. Furthermore, the use of hybrid structures consisting of two different nano-materials creates many novel solutions for applications in electronics and sensors.

Step aside, aluminum honeycomb.

A two-step process can turn hardwood into a strong and flexible moldable material.

Properties of Wood-Plastic Composites Manufactured from Two Different Wood Feedstocks: Wood Flour and Wood Pellets.

Driven by the motive of minimizing the transportation costs of raw materials to manufacture wood-plastic composites (WPCs), Part I and the current Part II of this paper series explore the utilization of an alternative wood feedstock, i.e., pellets. Part I of this study reported on the characteristics of wood flour and wood pellets manufactured from secondary processing mill residues. Part II reports on the physical and mechanical properties of polypropylene (PP)-based WPCs made using the two different wood feedstocks, i.e., wood flour and wood pellets. WPCs were made from 40-mesh wood flour and wood pellets from four different wood species (white cedar, white pine, spruce-fir and red maple) in the presence and absence of the coupling agent maleic anhydride polypropylene (MAPP). With MAPP, the weight percentage of wood filler was 20%, PP 78%, MAPP 2% and without MAPP, formulation by weight percentage of wood filler was 20% and PP 80%. Fluorescent images showed wood particles' distribution in the PP polymer matrix was similar for both wood flour and ground wood pellets. Dispersion of particles was higher with ground wood pellets in the PP matrix. On average, the density of composite products from wood pellets was higher, tensile strength, tensile modulus and impact strength were lower than the composites made from wood flour. Flexural properties of the control composites made with pellets were higher and with MAPP were lower than the composites made from wood flour. However, the overall mechanical property differences were low (0.5-10%) depending on the particular WPC formulations. Statistical analysis also showed there was no significant differences in the material property values of the composites made from wood flour and wood pellets. In some situations, WPC properties were better using wood pellets rather than using wood flour. We expect if the material properties of WPCs from wood flour versus wood pellets are similar and with a greater reduction in transportation costs for wood pellet feedstocks, this would be beneficial to WPC manufacturers and consumers.

Comparative Study of the Properties of Wood Flour and Wood Pellets Manufactured from Secondary Processing Mill Residues.

The generation of secondary processing mill residues from wood processing facilities is extensive in the United States. Wood flour can be manufactured utilizing these residues and an important application of wood flour is as a filler in the wood-plastic composites (WPCs). Scientific research on wood flour production from mill residues is limited. One of the greatest costs involved in the supply chain of WPCs manufacturing is the transportation cost. Wood flour, constrained by low bulk densities, is commonly transported by truck trailers without attaining allowable weight limits. Because of this, shipping costs often exceed the material costs, consequently increasing raw material costs for WPC manufacturers and the price of finished products. A bulk density study of wood flour (190-220 kg/m3) and wood pellets (700-750 kg/m3) shows that a tractor-trailer can carry more than three times the weight of pellets compared to flour. Thus, this study focuses on exploring the utilization of mill residues from four wood species in Maine to produce raw materials for manufacturing WPCs. Two types of raw materials for the manufacture of WPCs, i.e., wood flour and wood pellets, were produced and a study of their properties was performed. At the species level, red maple 40-mesh wood flour had the highest bulk density and lowest moisture content. Spruce-fir wood flour particles were the finest (dgw of 0.18 mm). For all species, the 18-40 wood flour mesh size possessed the highest aspect ratio. Similarly, on average, wood pellets manufactured from 40-mesh particles had a lower moisture content, higher bulk density, and better durability than the pellets from unsieved wood flour. Red maple pellets had the lowest moisture content (0.12%) and the highest bulk density (738 kg/m3). The results concluded that the processing of residues into wood flour and then into pellets reduced the moisture content by 76.8% and increased the bulk density by 747%. These material property parameters are an important attempt to provide information that can facilitate the more cost-efficient transport of wood residue feedstocks over longer distances.

Multi-layer oil-resistant food serving containers made using cellulose nanofiber coated wood flour composites.

Cost-effective, eco-friendly, and oil and grease-resistant food serving containers were made from wood flour with cellulose nanofibrils (CNF) or lignin-containing cellulose nanofibrils (LCNF) coating layers on the surface and in the bulk. The multi-layer wet-on-wet cellulose nanofiber composites were developed using a vacuum filtration process. All composites showed excellent oil/grease resistivity according to the "kit" test passing #12, the highest possible. The surface free energy and water contact angle showed that the composites with LCNF coating were more hydrophobic than the ones coated with CNF made from bleached pulp fiber. All composites had higher flexural and tensile properties compared with commercial food containers where the mechanical properties increased with increasing binder content and had acceptable thermal stability. Overall, the cellulose nanofiber composites possess excellent mechanical and barrier properties and can be considered as a wood-flour-based (pulp-free) and poly-fluoroalkyl substances (PFAs)-free alternative for oil-resistant commercial food serving containers.

Optimizing lignocellulosic nanofibril dimensions and morphology by mechanical refining for enhanced adhesion.

Using lignocellulosic nanofibrils as adhesive binders in structural composites is a growing field of interest attributable to their renewability, recyclability, and strength. A fundamental understanding of their adhesion mechanisms is crucial to tailor performance and optimize production costs. These mechanisms were elucidated by studying the morphology dependent adhesion in a model system composed of cellulose nanofibrils (CNFs) at different degrees of refinement and porous paper substrates. CNFs and lignin containing cellulose nanofibrils (LCNF) were characterized at different length scales using optical, atomic force, and scanning electron microscopy, revealing a complex distribution of sizes, spanning the macroscale to the nanoscale, which are modified unequally by refinement. Strong adhesion was correlated to a decrease in fiber size on the largest length scale and with an increase in relative fibril surface area. Flocculation hampered effective LCNF adhesion, but adding suspension stabilizers improved adhesion to levels comparable to CNF.

Recent Advances in Functional Materials through Cellulose Nanofiber Templating.

Advanced templating techniques have enabled delicate control of both nano- and microscale structures and have helped thrust functional materials into the forefront of society. Cellulose nanomaterials are derived from natural polymers and show promise as a templating source for advanced materials. Use of cellulose nanomaterials in templating combines nanoscale property control with sustainability, an attribute often lacking in other templating techniques. Use of cellulose nanofibers for templating has shown great promise in recent years, but previous reviews on cellulose nanomaterial templating techniques have not provided extensive analysis of cellulose nanofiber templating. Cellulose nanofibers display several unique properties, including mechanical strength, porosity, high water retention, high surface functionality, and an entangled fibrous network, all of which can dictate distinctive aspects in the final templated materials. Many applications exploit the unique aspects of templating with cellulose nanofibers that help control the final properties of the material, including, but not limited to, applications in catalysis, batteries, supercapacitors, electrodes, building materials, biomaterials, and membranes. A detailed analysis on the use of cellulose nanofibers templating is provided, addressing specifically how careful selection of templating mechanisms and methodologies, combined toward goal applications, can be used to directly benefit chosen applications in advanced functional materials.

Alignment of Cellulose Nanofibers: Harnessing Nanoscale Properties to Macroscale Benefits.

In nature, cellulose nanofibers form hierarchical structures across multiple length scales to achieve high-performance properties and different functionalities. Cellulose nanofibers, which are separated from plants or synthesized biologically, are being extensively investigated and processed into different materials owing to their good properties. The alignment of cellulose nanofibers is reported to significantly influence the performance of cellulose nanofiber-based materials. The alignment of cellulose nanofibers can bridge the nanoscale and macroscale, bringing enhanced nanoscale properties to high-performance macroscale materials. However, compared with extensive reviews on the alignment of cellulose nanocrystals, reviews focusing on cellulose nanofibers are seldom reported, possibly because of the challenge of aligning cellulose nanofibers. In this review, the alignment of cellulose nanofibers, including cellulose nanofibrils and bacterial cellulose, is extensively discussed from different aspects of the driving force, evaluation, strategies, properties, and applications. Future perspectives on challenges and opportunities in cellulose nanofiber alignment are also briefly highlighted.

Biopolymer blends of polyhydroxybutyrate and polylactic acid reinforced with cellulose nanofibrils.

Poly(lactic acid) (PLA) was used in an effort to enhance the mechanical properties of poly(hydroxybutyrate) (PHB) and the blends were reinforced with cellulose nanofibrils (CNF). The conventional and dynamic mechanical, morphological, thermal and rheological properties of the obtained composite blends were determined. The results showed that the mechanical properties of neat PHB noticeably increased attributable to the good interaction between the biopolymers and CNF from the scanning electron microscopy (SEM) characterization. Thermal stability of the neat PHB was improved by adding PLA, however differential scanning calorimetry results showed that PLA created enhanced thermal properties while adding CNFs did not provide any change in the composite thermal properties. Dynamic mechanical and rheological properties of the neat PHB generally improved with both PLA and CNFs, however, it decreased at high loadings of CNFs attributed to fiber aggregations and fiber pull-out in comparison to the low loading level of CNF.

Flexural Creep Behavior of High-Density Polyethylene Lumber and Wood Plastic Composite Lumber Made from Thermally Modified Wood.

The use of wood plastic composite lumber as a structural member material in marine applications is challenging due to the tendency of wood plastic composites (WPCs) to creep and absorb water. A novel patent-pending WPC formulation that combines a thermally modified wood flour (as a cellulosic material) and a high strength styrenic copolymer (high impact polystyrene and styrene maleic anhydride) have been developed with advantageous viscoelastic properties (low initial creep compliance and creep rate) compared with the conventional WPCs. In this study, the creep behavior of the WPC and high-density polyethylene (HDPE) lumber in flexure was characterized and compared. Three sample groupings of WPC and HDPE lumber were subjected to three levels of creep stress; 7.5, 15, and 30% of the ultimate flexural strength (Fb) for a duration of 180 days. Because of the relatively low initial creep compliance of the WPC specimens (five times less) compared with the initial creep compliance of HDPE specimens, the creep deformation of HDPE specimens was six times higher than the creep deformation of WPC specimens at the 30% creep stress level. A Power Law model predicted that the strain (3%) to failure in the HDPE lumber would occur in 1.5 years at 30% Fb flexural stress while the predicted strain (1%) failure for the WPC lumber would occur in 150 years. The findings of this study suggest using the WPC lumber in structural application to replace the HDPE lumber in flexure attributable to the low time-dependent deformation when the applied stress value is withing the linear region of the stress-strain relationship.

Material Extrusion Additive Manufacturing of Wood and Lignocellulosic Filled Composites.

Wood and lignocellulosic-based material components are explored in this review as functional additives and reinforcements in composites for extrusion-based additive manufacturing (AM) or 3D printing. The motivation for using these sustainable alternatives in 3D printing includes enhancing material properties of the resulting printed parts, while providing a green alternative to carbon or glass filled polymer matrices, all at reduced material costs. Previous review articles on this topic have focused only on introducing the use of natural fillers with material extrusion AM and discussion of their subsequent material properties. This review not only discusses the present state of materials extrusion AM using natural filler-based composites but will also fill in the knowledge gap regarding state-of-the-art applications of these materials. Emphasis will also be placed on addressing the challenges associated with 3D printing using these materials, including use with large-scale manufacturing, while providing insight to overcome these issues in the future.

Structural Performance of HDPE and WPC Lumber Components Used in Aquacultural Geodesic Spherical Cages.

Based on previous research, a novel wood-plastic composite (WPC) lumber has shown potential to replace high-density polyethylene (HDPE) lumber in the construction of aquacultural geodesic spherical cage structures. Six HDPE and six WPC assemblies, which are representative of typical full-size cage dimensions, were fabricated by bolting pairs of triangular panel components made with connected struts. Half of the panel assemblies had a plastic-coated steel wire mesh to simulate the actual restraint in field applications of the cages. The objective of the research was to characterize the structural performance of the panel assemblies under compressive loading. To determine the critical buckling load for the panel assemblies made from WPC and HDPE struts with and without wire mesh, Southwell's method was implemented. A two-dimensional (2D) linear finite element analysis model was developed to determine axial forces in the struts of the panel assembly for the applied load and boundary conditions. This model was used to determine strut compressive forces corresponding to the Southwell's method buckling load and the experimental failure load. It was found that the wire mesh increased the load capacity of both HDPE and WPC panel assemblies by a factor of two. The typical failure mode of the panels made from HDPE lumber struts, with and without wire mesh, was buckling of the struts, whereas the failure mode of the WPC panels, with and without wire mesh, was fracture at the notched section corresponding to the location of the bolts. The load capacity of the panel assemblies made from WPC lumber struts was three times and 2.5 times higher than the load capacity of the panel assemblies made from HDPE lumber struts with and without wire mesh, respectively.

Dewatering Behavior of a Wood-Cellulose Nanofibril Particulate System.

The novel use of aqueous suspensions of cellulose nanofibrils (CNF) as an adhesive/binder in lignocellulosic-based composite manufacture requires the removal of a considerable amount of water from the furnish during processing, necessitating thorough understanding of the dewatering behavior referred to as "contact dewatering". The dewatering behavior of a wood-CNF particulate system (wet furnish) was studied through pressure filtration tests, centrifugation, and characterization of hard-to-remove (HR) water, i.e. moisture content in the wet furnish at the transition between constant rate part and the falling rate part of evaporative change in mass from an isothermal thermogravimetric analysis (TGA). The effect of wood particle size thereby particle specific surface area on the dewatering performance of wet furnish was investigated. Permeability coefficients of wet furnish during pressure filtration experiments were also determined based on Darcy's law for volumetric flow through a porous medium. Results revealed that specific particle surface area has a significant effect on the dewatering of wet furnish where dewatering rate significantly increased at higher specific particle surface area levels. While the permeability of the systems decreased over time in almost all cases, the most significant portion of dewatering occurred at very early stages of dewatering (less than 200 seconds) leading to a considerable increase in instantaneous dewatering when CNF particles come in contact with wood particles.

Fully Bio-Based Hybrid Composites Made of Wood, Fungal Mycelium and Cellulose Nanofibrils.

Novel hybrid panel composites based on wood, fungal mycelium, and cellulose nanofibrils (CNF) were developed and investigated in the present study. In one set of experiments, mycelium was grown on softwood particles to produce mycelium-modified wood which was then hybridized with various levels of CNF as binder. The other set of experiments were conducted on unmodified wood particles mixed with CNF and pure mycelium tissue. It was found that the composites made of mycelium-modified wood and CNF resulted in enhanced physical and mechanical properties compared to the ones made by physically mixing wood, mycelium, and CNF. Scanning electron microscopy (SEM) images showed that mycelium modification covered wood particles with a network of fungal hyphae whereas CNF formed a uniform mycelial film over wood particles. Mycelium modification had a significant effect on reducing water absorption and thickness swelling of the hybrid composites and CNF increased the modulus of rupture and modulus of elasticity, optimally at 2.5% addition. We also present results and analysis pertaining to the development of unique lightweight composite systems with physical and mechanical properties optimized at 5% CNF addition with potential to be used in packaging and furniture applications.