Pilot Study Investigating Effects of Changing Process Variables on Elastic and Energy-Absorbing Characteristics in Polyurethane/Agglomerated Cork Mix for Use in Micro-Transport Helmet.

This pilot investigation identifies the influence that changing the process variables of curing pressure, curing temperature, and mix ratio of a polyurethane/agglomerated cork matrix has on the mechanical properties of energy absorption, Young's modulus of elasticity, and spring stiffness in safety helmets intended for micro-transport riders. The results are compared to expanded polystyrene, a material commonly used in micro-transport helmets. Mechanical testing of the various samples found that, over the range tested, curing pressure had no effect on any of the mechanical properties, while increasing amounts of resin caused a stiffer structure, and increasing curing temperature led to increased energy absorption. Consistent with the elastic modulus findings, all polyurethane/agglomerated cork test samples demonstrated higher median levels of spring stiffness, ranging from 7.1% to 61.9% greater than those found for expanded polystyrene. The sample mixed at a 1.5:1 binder/cork ratio and cured at 40 °C displayed the closest spring stiffness to EPS. While the mechanical properties of the eco-friendly polyurethane/agglomerated cork matrix did not match those of expanded polystyrene, the difference in performance found in this study is promising. Further investigation into process variables could characterise this more ecologically based matrix with equivalent energy-absorbing and structural characteristics, making it equivalent to currently used expanded polystyrene and suitable for use in micro-transport helmets.

Additive Manufacturing of High-Temperature Thermoplastic Polysulfone with Tailored Microstructure via Precipitation Printing.

Current additive manufacturing processes for polymers, including material extrusion, vat photopolymerization, material jetting, and powder bed fusion, have limitations in manufacturing high-temperature thermoplastics including narrow material selection, compromised mechanical properties, and potential degradation of materials during processing. Polysulfone (PSU) is a high-temperature thermoplastic with outstanding chemical resistance, flame retardancy, and toughness. However, besides injection molding, additive manufacturing of PSU has only been achieved through extrusion or solvent-cast three-dimensional (3D) printing without obtaining high mechanical properties. In this work, precipitation printing is applied to fabricate high-temperature thermoplastics such as PSU for the first time, where tailoring of the microstructure and mechanical properties is enabled through control of solvents and printing parameters. The printed PSU can either be dense and strong with 2.47 GPa Young's modulus and 70.6 MPa tensile strength or porous and highly anisotropic. After drying at a maximum temperature of 190 °C, both the printed dense and porous PSU samples have a glass-transition temperature of about 200 °C, which allows them to be used in high-temperature environments. Thus, precipitation printing provides an alternative approach to manufacture high-temperature thermoplastics like PSU with scalability and tailorable properties.

An integrated wet-spinning system for continuous fabrication of high-strength nanocellulose long filaments.

The continuous production of high-strength nanocellulose long filaments (NCLFs) is critical in natural fiber-reinforced polymer composites. Despite the widespread availability of numerous filament production processes, the cost-effective and continuous fabrication of high-strength NCLFs on a large scale remains an ongoing challenge. Herein, we present an integrated wet-spinning system by incorporating a few previously researched filament production techniques to mass fabricate high-strength continuous NCLFs. The spinning speed is increased to improve NCLF productivity, and the bobbin winder speeds, collector bobbin winder location, and NCLF drying conditions are tuned. At the spinning speed of 510 cm/min, a production rate of 4.99 m/min is achieved, five times higher than the productivity of the former pilot system (0.92 m/min). Moreover, an AC electric field and mechanical stretching are introduced to highlight the versatility of the proposed integrated wet-spinning system, thereby enhancing the mechanical properties of NCLFs.

High-strength cellulose nanofiber/graphene oxide hybrid filament made by continuous processing and its humidity monitoring.

Human-made natural-fiber-based filaments are attractive for natural fiber-reinforced polymer (NFRP) composites. However, the composites' moisture distribution is critical, and humidity monitoring in the NFRP composites is essential to secure stability and keep their life span. In this research, high strength and humidity sensing filament was developed by blending cellulose nanofiber (CNF) and graphene oxide (GO), wet-spinning, coagulating, and drying, which can overcome the heterogeneous mechanical properties between embedded-type humidity sensors and NFRP composites. The stabilized synthesis process of the CNF-GO hybrid filament demonstrated the maximum Young's modulus of 23.9 GPa and the maximum tensile strength of 439.4 MPa. Furthermore, the achieved properties were successfully transferred to a continuous fabrication process with an additional stretching process. Furthermore, its humidity sensing behavior is shown by resistivity changes in various temperature and humidity levels. Therefore, this hybrid filament has excellent potential for in-situ humidity monitoring by embedding in smart wearable devices, natural fiber-reinforced polymer composites, and environmental sensing devices.

Environment-Friendly Zinc Oxide Nanorods-Grown Cellulose Nanofiber Nanocomposite and Its Electromechanical and UV Sensing Behaviors.

This paper reports a genuine environment-friendly hybrid nanocomposite made by growing zinc oxide (ZnO) nanorods on cellulose nanofiber (CNF) film. The nanocomposite preparation, characterizations, electromechanical property, and ultraviolet (UV) sensing performance are explained. CNF was extracted from the pulp by combining the 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) oxidation and the aqueous counter collision (ACC) methods. The CNF film was fabricated using doctor blade casting, and ZnO nanorods were grown on the CNF film by seeding and by a hydrothermal method. Morphologies, optical transparency, mechanical and electromechanical properties, and UV sensing properties were examined. The nanocomposite's optical transparency was more than 80%, and the piezoelectric charge constant d31 was 200 times larger than the CNF film. The UV sensing performance of the prepared ZnO-CNF nanocomposites was tested in terms of ZnO concentration, UV irradiance intensity, exposure side, and electrode materials. A large aspect ratio of ZnO nanorods and a work function gap between ZnO nanorods and the electrode material are essential for improving the UV sensing performance. However, these conditions should be compromised with transparency. The use of CNF for ZnO-cellulose hybrid nanocomposite is beneficial not only for electromechanical and UV sensing properties but also for high mechanical properties, renewability, biocompatibility, flexibility, non-toxicity, and transparency.

Cellulose Nanofiber-Based Nanocomposite Films Reinforced with Zinc Oxide Nanorods and Grapefruit Seed Extract.

Here, we report the fabrication and characterization of cellulose nanofiber (CNF)-based nanocomposite films reinforced with zinc oxide nanorods (ZnOs) and grapefruit seed extract (GSE). The CNF is isolated via a combination of chemical and physical methods, and the ZnO is prepared using a simple precipitation method. The ZnO and GSE are used as functional nanofillers to produce a CNF/ZnO/GSE film. Physical (morphology, chemical interactions, optical, mechanical, thermal stability, etc.) and functional (antimicrobial and antioxidant activities) film properties are tested. The incorporation of ZnO and GSE does not impact the crystalline structure, mechanical properties, or thermal stability of the CNF film. Nanocomposite films are highly transparent with improved ultraviolet blocking and vapor barrier properties. Moreover, the films exhibit effective antimicrobial and antioxidant actions. CNF/ZnO/GSE nanocomposite films with better quality and superior functional properties have many possibilities for active food packaging use.

Recent Research Progress on Lignin-Derived Resins for Natural Fiber Composite Applications.

By increasing the environmental concerns and depletion of petroleum resources, bio-based resins have gained interest. Recently, lignin, vanillin (4-hydroxy-3-methoxybenzaldehyde), and divanillin (6,6'-dihydroxy-5,5'-dimethoxybiphenyl-3,3'-dicarbaldehyde)-based resins have attracted attention due to the low cost, environmental benefits, good thermal stability, excellent mechanical properties, and suitability for high-performance natural fiber composite applications. This review highlights the recent use of lignin, vanillin, and divanillin-based resins with natural fiber composites and their synthesized processes. Finally, discussions are made on the curing kinetics, mechanical properties, flame retardancy, and bio-based resins' adhesion property.

Refractive Index Change of Cellulose Nanocrystal-Based Electroactive Polyurethane by an Electric Field.

A tunable optical lens can tune or reconfigure the lens material itself such that it can eliminate the moving part of the lens, which brings broad technological impacts. Many tunable optical lenses have been implemented using electroactive polymers that can change the shape of the lens. However, the refractive index (RI) change of electroactive polymers has not been well investigated. This paper investigated the RI change of CNC-based transparent and electroactive polyurethane (CPPU) in the presence of an actuating electric field. The prepared CPPU was electrically poled to enhance its electro-optical performance, and the poling conditions in terms of frequency and electric field were optimized. The poled CPPU was characterized using a Fourier transform infrared spectroscopy and a refractometer. To investigate the RI change in the presence of an actuating electric field, the poled CPPU was constrained between two electrodes with a fixed distance. The RI linearly increased as the actuating electric field increased. The RI change mechanism and the optimized poling conditions are illustrated. The tunable RI is a promising property for implementing a tunable optical lens.

Tannic-Acid-Cross-Linked and TiO2-Nanoparticle-Reinforced Chitosan-Based Nanocomposite Film.

A chitosan-based nanocomposite film with tannic acid (TA) as a cross-linker and titanium dioxide nanoparticles (TiO2) as a reinforcing agent was developed with a solution casting technique. TA and TiO2 are biocompatible with chitosan, and this paper studied the synergistic effect of the cross-linker and the reinforcing agent. The addition of TA enhanced the ultraviolet blocking and mechanical properties of the chitosan-based nanocomposite film. The reinforcement of TiO2 in chitosan/TA further improved the nanocomposite film's mechanical properties compared to the neat chitosan or chitosan/TA film. The thermal stability of the chitosan-based nanocomposite film was slightly enhanced, whereas the swelling ratio decreased. Interestingly, its water vapor barrier property was also significantly increased. The developed chitosan-based nanocomposite film showed potent antioxidant activity, and it is promising for active food packaging.

Chitosan Nanofiber and Cellulose Nanofiber Blended Composite Applicable for Active Food Packaging.

This paper reports that, by simply blending two heterogeneous polysaccharide nanofibers, namely chitosan nanofiber (ChNF) and cellulose nanofiber (CNF), a ChNF-CNF composite was prepared, which exhibited improved mechanical properties and antioxidant activity. ChNF was isolated using the aqueous counter collision (ACC) method, while CNF was isolated using the combination of TEMPO oxidation and the ACC method, which resulted in smaller size of CNF than that of ChNF. The prepared composite was characterized in terms of morphologies, FT-IR, UV visible, thermal stability, mechanical properties, hygroscopic behaviors, and antioxidant activity. The composite was flexible enough to be bent without cracking. Better UV-light protection was shown at higher content of ChNF in the composite. The high ChNF content showed the highest antioxidant activity in the composite. It is the first time that a simple combination of ChNF-CNF composites fabrication showed good mechanical properties and antioxidant activities. In this study, the reinforcement effect of the composite was addressed. The ChNF-CNF composite is promising for active food packaging application.

Simple centrifugal fractionation to reduce the size distribution of cellulose nanofibers.

Since cellulose nanofiber (CNF) has unique characteristics in terms of renewability, high specific elastic modulus and strength and transparency, it is attractive for a building block of future materials. CNF can be extracted from various natural resource by several means. However, the size of the extracted CNF is very broad and uniformity of the extracted CNF is very important for many applications. Thus, a fractionation process is necessary to obtain a uniformly sized CNF. In this paper, a simple centrifugal fractionation was carried out to reduce the size distribution of the extracted CNF suspension from hardwood pulp by the combination of TEMPO oxidation and aqueous counter collision methods. The original CNF suspension was diluted and centrifuged under low speed to remove cellulose microfibers then centrifuged under high speed to separate very small CNF. The centrifugation condition is 10 k rpm for 1 h followed by 45 k rpm for 4 h. The fractionated CNF was analyzed by an atomic force microscopy, and the length and width distribution histogram analysis was utilized. UV-visible analysis, FT-IR and XRD crystallinity analysis were carried out to analyze all fractionated CNFs and the original CNF. After centrifugal fractionation, the width and length distribution range were reduced by 62% and 70%, respectively. It is shown that the centrifugal fractionation is an easy and efficient method to fractionate a uniform CNF suspension.

Steered Pull Simulation to Determine Nanomechanical Properties of Cellulose Nanofiber.

Cellulose nanofiber (CNF) exhibits excellent mechanical properties, which has been extensively proven through experimental techniques. However, understanding the mechanisms and the inherent structural behavior of cellulose is important in its vastly growing research areas of applications. This study focuses on taking a look into what happens to the atomic molecular interactions of CNF, mainly hydrogen bond, in the presence of external force. This paper investigates the hydrogen bond disparity within CNF structure. To achieve this, molecular dynamics simulations of cellulose I ß nanofibers are carried out in equilibrated conditions in water using GROMACS software in conjunction with OPLS-AA force field. It is noted that the hydrogen bonds within the CNF are disrupted when a pulling force is applied. The simulated Young's modulus of CNF is found to be 161 GPa. A simulated shear within the cellulose chains presents a trend with more hydrogen bond disruptions at higher forces.

Effect of Process Orientation on the Mechanical Behavior and Piezoelectricity of Electroactive Paper.

This paper reports the effect of process orientation on the mechanical behavior and piezoelectricity of electroactive paper (EAPap) made from natural cotton pulp. EAPap is fabricated by a casting and wet drawing of cellulose film after dissolving cotton with LiCl and DMAc solvent. During the fabrication, permanent wrinkles, a possible factor for performance deterioration, were found in the films. Finite element method was introduced to identify the formation mechanism behind the wrinkles. The simulation results show that the wrinkles were caused by buckling and are inevitable under any conditions. The tensile and piezoelectric tests show that the orientation dependency of the stretched EAPap gives the anisotropic characteristics on both mechanical and piezoelectric properties. In this research, the anisotropic elastic moduli and Poisson's ratios are reported. The piezoelectric charge constant of EAPap in the linear elastic is calculated. The piezoelectric charge constants of EAPap are associated with the alignment angle in the order of 45° > 0° > 90° due to the strong shear effect. The higher stretching ratio gives the higher piezoelectricity due to the alignment of the molecular chains and the microstructure in EAPap. The highest piezoelectric charge constant is found to be 12 pC/N at a stretching ratio of 1.6 and aligning angle of 45°.

Preparation and characterization of synthetic melanin-like nanoparticles reinforced chitosan nanocomposite films.

This paper reports the preparation, characterization and properties of synthetic melanin-like nanoparticle (MNP) reinforced chitosan nanocomposite films. The MNP was prepared using dopamine hydrochloride and sodium hydroxide which followed by spontaneous oxidation. The prepared MNP was spherical in shape and in the size range of ∼100 nm. The MNP was used as a functional nanofiller to produce the chitosan/MNP nanocomposite films using simple solution mixing and casting method. The MNP are evenly dispersed and biocompatible with chitosan to form the nanocomposite films. The incorporation of MNP enhances the ultraviolet blocking, mechanical properties, swelling ratio, and hydrophobicity of the nanocomposite films. The reinforcement of MNP in chitosan does not deteriorate the thermal stability and water vapor barrier property of the nanocomposite films. Furthermore, the prepared nanocomposite films show strong antioxidant activity. The developed chitosan/MNP nanocomposite films can applied to active food packaging and biomedical packaging.

Alignment Effect on the Piezoelectric Properties of Ultrathin Cellulose Nanofiber Films.

This study aims at evaluating the piezoelectric property of an ultrathin cellulose nanofiber (CNF) film in a thickness direction. Cellulose is known to have piezoelectric properties. However, its measurement is not easy. By making an ultrathin CNF film and eliminating space charges, the pure piezoelectric property of CNF is intended to be measured. A 600 nm thick ultrathin CNF film was prepared using a microfabrication process. The effect of alignment methods on the piezoelectric property of the ultrathin CNF film in the thickness direction was investigated with the three alignment methods: corona poling, electrical in-plane alignment, and high magnetic field (HiMA) alignment. Piezoresponse force microscopy (PFM) was utilized to analyze the piezoelectric property. By applying AC voltages using PFM, the vertical displacement on the ultrathin CNF film surface was recorded and converted into the piezoelectric coefficient, d33. The aligned ultrathin CNF films show different piezoelectric coefficients. The corona-poled ultrathin CNF film shows the largest piezoelectric coefficient among the three alignment methods. Water contact angle measurement proves that the piezoelectric constant in the thickness direction is associated with hydroxyl groups in cellulose chains appearing on the surface of the ultrathin CNF films.

Swelling Behavior of Polyacrylamide-Cellulose Nanocrystal Hydrogels: Swelling Kinetics, Temperature, and pH Effects.

This paper reports swelling behavior of cellulose nanocrystal (CNC)-based polyacrylamide hydrogels prepared by a radical polymerization. The CNC acts as a nanofiller through the formation of complexation and intermolecular interaction. FTIR spectroscopy and XRD studies confirmed the formation of intermolecular bonds between the acrylamide and hydroxyl groups of CNC. The swelling ratio and water retention were studied in de-ionized (DI) water at room temperature, and the temperature effect on the swelling ratio was investigated. Further, the pH effect on the swelling ratio was studied with different temperature levels. Increasing the pH with temperature, the prepared hydrogel shows 6 times higher swelling ratio than the initial condition. The swelling kinetics of the developed hydrogels explains that the diffusion mechanism is Fickian diffusion mechanism. Since the developed hydrogels have good swelling behaviors with respect to pH and temperature, they can be used as smart materials in the field of controlled drug delivery applications.

Electroactive Hydrogels Made with Polyvinyl Alcohol/Cellulose Nanocrystals.

This paper reports a nontoxic, soft and electroactive hydrogel made with polyvinyl alcohol (PVA) and cellulose nanocrystal (CNC). The CNC incorporating PVA-CNC hydrogels were prepared using a freeze⁻thaw technique with different CNC concentrations. Fourier transform infrared spectroscopy, thermogravimetric analysis, X-ray diffraction and scanning electron microscopy results proved the good miscibility of CNCs with PVA. The optical transparency, water uptake capacity and mechanical properties of the prepared hydrogels were investigated in this study. The CNC incorporating PVA-CNC hydrogels showed improved displacement output in the presence of an electric field and the displacement increased with an increase in the CNC concentration. The possible actuation mechanism was an electrostatic effect and the displacement improvement of the hydrogel associated with its enhanced dielectric properties and softness. Since the prepared PVA-CNC hydrogel is nontoxic and electroactive, it can be used for biomimetic soft robots, actively reconfigurable lenses and active drug-release applications.

Cellulose nanofibers isolated by TEMPO-oxidation and aqueous counter collision methods.

In this research, cellulose nanofiber (CNF) was isolated by the combination of chemical 2,2,6,6-tetramethylpiperidine-1-oxylradical (TEMPO)-oxidation and physical aqueous counter collision (ACC) methods The combination of TEMPO-oxidation and ACC is an efficient method to isolate CNFs by reducing chemical usage in TEMPO-oxidation and saving energy in ACC along with controlling the size of CNFs. Two cellulose sources, hardwood bleached kraft pulp (HW) and softwood bleached kraft pulp (SW), were used for the CNF isolation with different TEMPO oxidation time and a defined number of ACC pass. The CNF properties were investigated and compared in term of morphology, crystallinity index, transparency and birefringence. The width of the isolated CNFs from HW is in the range of 15.1 nm-17.5 nm, and that of the SW CNFs is between 18.4 nm and 22 nm depending on the TEMPO oxidation time. This difference is due to the fact that SW is less oxidized than HW under the same chemical dosage, which results in larger width of SW-CNFs than HW-CNFs. The HW-CNF treated with TEMPO for over 2 h and isolated using ACC with 5 pass offers almost 90% transparency. Birefringence of CNFs exhibits that HW-CNFs show better birefringence phenomenon than SW-CNFs. The combination of TEMPO-oxidation and ACC methods is useful for isolating CNFs with its size control.

Cellulose long fibers fabricated from cellulose nanofibers and its strong and tough characteristics.

Cellulose nanofiber (CNF) with high crystallinity has great mechanical stiffness and strength. However, its length is too short to be used for fibers of environmentally friendly structural composites. This paper presents a fabrication process of cellulose long fiber from CNF suspension by spinning, stretching and drying. Isolation of CNF from the hardwood pulp is done by using (2, 2, 6, 6-tetramethylpiperidine-1-yl) oxidanyl (TEMPO) oxidation. The effect of spinning speed and stretching ratio on mechanical properties of the fabricated fibers are investigated. The modulus of the fabricated fibers increases with the spinning speed as well as the stretching ratio because of the orientation of CNFs. The fabricated long fiber exhibits the maximum tensile modulus of 23.9 GPa with the maximum tensile strength of 383.3 MPa. Moreover, the fabricated long fiber exhibits high strain at break, which indicates high toughness. The results indicate that strong and tough cellulose long fiber can be produced by using ionic crosslinking, controlling spinning speed, stretching and drying.

Performance improvement of miniaturized ZnO nanowire accelerometer fabricated by refresh hydrothermal synthesis.

Miniaturized accelerometers are necessary for evaluating the performance of small devices, such as haptics, robotics and simulators. In this study, we fabricated miniaturized accelerometers using well-aligned ZnO nanowires. The layer of ZnO nanowires is used for active piezoelectric layer of the accelerometer, and copper was chosen as a head mass. Seedless and refresh hydrothermal synthesis methods were conducted to grow ZnO nanowires on the copper substrate and the effect of ZnO nanowire length on the accelerometer performance was investigated. The refresh hydrothermal synthesis exhibits longer ZnO nanowires, 12 µm, than the seedless hydrothermal synthesis, 6 µm. Performance of the fabricated accelerometers was verified by comparing with a commercial accelerometer. The sensitivity of the fabricated accelerometer by the refresh hydrothermal synthesis is shown to be 37.7 pA g-1, which is about 30 times larger than the previous result.