Crosslinking of Epoxidized Natural Rubber with Borax for Self-Healing and Self-Repairing Properties: pH Dependence.

Epoxidized natural rubber (ENR) crosslinked using borax, which exhibits self-healing and self-repairing properties, is successfully developed. The crosslink formation of ENR by using borax under neutral and alkaline conditions is investigated. Fourier transform infrared spectroscopy (FTIR) shows that the borate-ester bond is formed in ENR prepared under both neutral and alkaline conditions, whereas boron nuclear magnetic resonance (11 B-NMR) results exhibit that the ENR prepared under alkaline conditions more actively forms crosslink networks with borax. Moreover, the crosslink density and gel content increase significantly with the presence of borax in alkaline conditions. The crosslink density and gel content of ENR with 10 phr borax are higher by 155% and 36%, respectively, than those of neat ENR. Furthermore, the formation of the crosslinking ENR by borax enhances self-healing and self-repairing properties. The healing efficiency significantly increases from 1.09% to 85.90%, when ENR is developed under alkaline conditions with 30 phr borax. These results represent the first successful demonstration of the efficient use of borax as a crosslinker in ENR, which exhibits its promising self-healing and self-repairing properties under atmospheric conditions without the need for external stimuli. The ENR prepared in this work holds great promise for various self-healing rubber applications.

Development of chitin nanofiber coatings for prolonging shelf life and inhibiting bacterial growth on fresh cucumbers.

The widespread usage of petroleum-based polymers as single-use packaging has had harmful effects on the environment. Herein, we developed sustainable chitin nanofiber (ChNF) coatings that prolong the shelf life of fresh cucumbers and delay the growth of pathogenic bacteria on their surfaces. ChNFs with varying degrees of acetylation were successfully prepared via deacetylation using NaOH with treatment times of 0-480 min and defibrillated using mechanical blending. With longer deacetylation reaction times, more acetamido groups (-NHCOCH3) in chitin molecules were converted to amino groups (-NH2), which imparted antibacterial properties to the ChNFs. The ChNF morphologies were affected by deacetylation reaction time. ChNFs deacetylated for 240 min had an average width of 9.0 nm and lengths of up to several µm, whereas rod-like structured ChNFs with a mean width of 7.3 nm and an average length of 222.3 nm were obtained with the reaction time of 480 min. Furthermore, we demonstrated a standalone ChNF coating to extend the shelf life of cucumbers. In comparison to the rod-like structured ChNFs, the 120 and 240-min deacetylated ChNFs exhibited a fibril-like structure, which considerably retarded the moisture loss of cucumbers and the growth rate of bacteria on their outer surfaces during storage. Cucumbers coated with these 120 and 240-min deacetylated ChNFs demonstrated a lower weight loss rate of ⁓ 3.9% day-1 compared to the uncoated cucumbers, which exhibited a weight loss rate of 4.6% day-1. This protective effect provided by these renewable ChNFs holds promising potential to reduce food waste and the use of petroleum-based packaging materials.

Development of water-resistant paper using chitosan and plant-based wax extracted from banana leaves.

On being exposed to water, cellulose paper swells and its mechanical properties become weak. In this study, natural wax with an average particle size of 12.3 µm extracted from banana leaves was mixed with chitosan to prepare coatings applied on paper surfaces. Chitosan efficiently dispersed banana leaf-extracted wax on paper surfaces. The mixed chitosan and wax coatings considerably influenced paper properties, including yellowness, whiteness, thickness, wettability, water absorption, oil sorption, and mechanical properties. The coating induced hydrophobicity in the paper, resulting in a significant increase in the water contact angle from 65.1 ± 7.7° (uncoated paper) to 123.2 ± 2.1°, and a decrease in water absorption by ⁓64 % to 52.6 ± 1.9 %. The coated paper demonstrated an oil sorption capacity of 212.2 ± 2.8 %, which was ⁓43 % greater than that of the uncoated paper (148.2 ± 5.5 %), and the tensile strength of the coated paper improved under wet conditions compared to the uncoated paper. Additionally, a separation of oil in water was observed for the chitosan/wax coated paper. Based on these promising results, the paper coated with chitosan and wax could be used for direct-contact packaging applications.

Property enhancement of epoxidized natural rubber nanocomposites with water hyacinth-extracted cellulose nanofibers.

Cellulose nanofibers (CNFs) have been widely used as reinforcement in various polymer matrices; however, limited studies of the use of CNFs in epoxidized natural rubber (ENR) have been reported. Here, we successfully prepared CNF-reinforced ENR nanocomposites with superior mechanical performance. CNFs were disintegrated from water hyacinth (Eichhornia crassipes) using high-pressure homogenization, and ENR nanocomposites with CNFs were fabricated by initial mixing and hot pressing. The crosslink densities of the nanocomposites with CNFs were higher than that of the neat ENR. Due to stronger interfacial interactions between the hydroxyl groups of the CNFs and the functional groups of the ENR, stress could be efficiently transferred from the ENR matrix to the stiff CNFs, resulting in a significant increase in the mechanical properties. Compared with those of the neat ENR, the tensile strength and Young's modulus of the ENR nanocomposites were improved by 80 and 39 %, respectively, with the incorporation of 2 parts per hundred rubber (phr) CNFs, whereas no loss in elongation at break was observed. The introduction of CNFs also improved the oil resistance of the nanocomposites. Therefore, CNFs could be the potential reinforcing agent in the ENR nanocomposites used in the various engineering applications of the rubber material.

Chitosan coating for the preparation of multilayer coated paper for food-contact packaging: Wettability, mechanical properties, and overall migration.

Cellulose-based paper is an alternative substitution for petroleum-based polymers for packaging applications, but its mechanical performance is poor when in contact with water. Herein, chitosan was applied on cellulose-based paper via a coating approach. The effects of chitosan coatings between none and five layers on the color properties, wettability, thermal properties, mechanical performance, and overall migration in food simulants of the paper were evaluated. After the application of chitosan, chitosan first filled cavities between cellulose fibers within a network, and the chitosan film was formed on the paper surface later. This resulted in a pronounced increase in wettability and mechanical properties associated with a loss of whiteness and an increase in yellowness of the coated paper. The chitosan-coated paper became hydrophobic with a water contact angle of 94.7 ± 2.8°, and a robust improvement of 156.4% for tensile strength and 114.8% for strain at break was observed for the paper coated with three layers of chitosan in wet conditions in comparison to the uncoated paper. A reduction in the migration of the low molecular residuals from the paper could be hindered by the chitosan coating. These enhanced features revealed that chitosan-coated paper could be used as a food-contact material.

Superabsorbent cellulose-based hydrogels cross-liked with borax.

Cellulose, the most abundant biopolymer on Earth, has been widely attracted owing to availability, intoxicity, and biodegradability. Environmentally friendly hydrogels were successfully prepared from water hyacinth-extracted cellulose using a dissolution approach with sodium hydroxide and urea, and sodium tetraborate decahydrate (borax) was used to generate cross-linking between hydroxyl groups of cellulose chains. The incorporation of borax could provide the superabsorbent feature into the cellulose hydrogels. The uncross-linked cellulose hydrogels had a swelling ratio of 325%, while the swelling ratio of the cross-linked hydrogels could achieve ~ 900%. With increasing borax concentrations, gel fraction of the cross-linked hydrogels increased considerably. Borax also formed char on cellulose surfaces and generated water with direct contact with flame, resulting in flame ignition and propagation delay. Moreover, the cross-linked cellulose-based hydrogels showed antibacterial activity for gram-positive bacteria (S. aureus). The superabsorbent cross-linked cellulose-based hydrogels prepared in this work could possibly be used for wound dressing, agricultural, and flame retardant coating applications.

Review of the recent developments in all-cellulose nanocomposites: Properties and applications.

Cellulose, the most abundant polysaccharide on Earth, has a number of desirable properties, including availability, biodegradability, low cost, and low toxicity and has been used in a variety of applications. Recently, all-cellulose composite materials have been made from a wide variety of cellulose sources, including wood and agricultural wastes, via impregnation or partial surface dissolution approaches utilizing a specific solvent. Due to the improved interfacial interactions between the cellulose matrix and cellulose reinforcement, all-cellulose composites exhibit superior mechanical properties when compared to biopolymers and petroleum-based polymers. The current article discusses the factors affecting the mechanical properties and interfacial bonding of all-cellulose composites. Additionally, the incorporation of inorganic nanoparticles is described to enhance the multi-functional properties of all-cellulose composites, such as their conductivity, permeability, and adsorption. Furthermore, this review summarizes the potential applications of all-cellulose composites in the following areas: composites, packaging, aerogels, hydrogels, fibers, tissue engineering, membranes, textiles, and coatings.

Surface and Interface Engineering for Nanocellulosic Advanced Materials.

How do trees support their upright massive bodies? The support comes from the incredibly strong and stiff, and highly crystalline nanoscale fibrils of extended cellulose chains, called cellulose nanofibers. Cellulose nanofibers and their crystalline parts-cellulose nanocrystals, collectively nanocelluloses, are therefore the recent hot materials to incorporate in man-made sustainable, environmentally sound, and mechanically strong materials. Nanocelluloses are generally obtained through a top-down process, during or after which the original surface chemistry and interface interactions can be dramatically changed. Therefore, surface and interface engineering are extremely important when nanocellulosic materials with a bottom-up process are fabricated. Herein, the main focus is on promising chemical modification and nonmodification approaches, aiming to prospect this hot topic from novel aspects, including nanocellulose-, chemistry-, and process-oriented surface and interface engineering for advanced nanocellulosic materials. The reinforcement of nanocelluloses in some functional materials, such as structural materials, films, filaments, aerogels, and foams, is discussed, relating to tailored surface and/or interface engineering. Although some of the nanocellulosic products have already reached the industrial arena, it is hoped that more and more nanocellulose-based products will become available in everyday life in the next few years.

Mechanical and antibacterial properties of the chitosan coated cellulose paper for packaging applications: Effects of molecular weight types and concentrations of chitosan.

Chitosan with low (25 kDa) and high molecular weight (2100 kDa) were used to enhance performances of paper made from steam-exploded bamboo fibers and nanofibrillated cellulose. Chitosan solutions with concentrations of 0-1.0 wt% were manually applied on paper surface using a facile coating approach with a wire bar. Effects of chitosan coatings on morphology, thermal stability, wettability, mechanical performances and antibacterial properties of the paper were investigated. The larger improvement in the mechanical properties and wettability of the chitosan coated paper was observed with increasing concentrations of chitosan due to the disappearance of empty pores between fibers within a cellulose network by the formed chitosan matrix. These improvements were significantly higher when high molecular weight chitosan was applied. Yet, the addition of chitosan slightly decreased the thermal stability of the coated paper, and the chitosan coating did not improve the antimicrobial properties of the paper. The antimicrobial activity of chitosan against Gram-positive (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli) was found to be diminished when a chitosan solution was entrapped within paper. Together with the overall migration of the paper in food simulants, the results suggested that the chitosan coated paper could be applied for non-food-direct-contact packaging materials.

Thermally Superstable Cellulosic-Nanorod-Reinforced Transparent Substrates Featuring Microscale Surface Patterns.

The recent rapid expansion of thin-film, bendable, and wearable consumer (opto)electronics demands flexible and transparent substrates other than glass. Plastics are the traditional choice, but they require amelioration because of their thermal instability. Here, we report the successful conversion of a soft and thermally vulnerable polymer into a highly thermally stable transparent nanocomposite material. This is achieved by the meticulous choice of a polymer with a glass-transition temperature below 0 °C that gives stable mechanics above room temperature, reinforcing the polymer with a load-bearing hierarchical network of the incredibly strong and stable natural material: cellulose nanorods. Owing to the Pickering emulsification process, the nanocomposites inherit the self-assembled structural hierarchy from the cellulose nanorod-encapsulated resin droplets. The ameliorated nanocomposites have highly desirable high-temperature endurance (∼150-180 °C) in terms of the thermomechanical, thermodimensional, and thermo-optical performance. Any photonic nano- or microstructures can be directly molded on the surface of the nanocomposites in high precision for better light management in photonic and opto-electronic applications. The highlight of this work is the demonstration of a highly thermally stable microlens array on the ameliorated transparent nanocomposite.

Effect of clay content on morphology and processability of electrospun keratin/poly(lactic acid) nanofiber.

This research work has concerned the development of volatile organic compounds (VOCs) removal filters from biomaterials, based on keratin extracted from chicken feather waste and poly(lactic acid) (PLA) (50/50%w/w) blend. Clay (Na-montmorillonite) was also added to the blend solution prior to carrying out an electro-spinning process. The aim of this study was to investigate the effect of clay content on viscosity, conductivity, and morphology of the electrospun fibers. Scanning electron micrographs showed that smooth and bead-free fibers were obtained when clay content used was below 2 pph. XRD patterns of the electrospun fibers indicated that the clay was intercalated and exfoliated within the polymers matrix. Percentage crystallinity of keratin in the blend increased after adding the clay, as evidenced from FTIR spectra and DSC thermograms. Transmission electron micrographs revealed a kind of core-shell structure with clay being predominately resided within the keratin rich shell and at the interfacial region. Filtration performance of the electrospun keratin/PLA fibers, described in terms of pressure drop and its capability of removing methylene blue, were also explored. Overall, our results demonstrated that it was possible to improve process-ability, morphology and filtration efficiency of the electrospun keratin fibers by adding a suitable amount of clay.

Effective Young's modulus of bacterial and microfibrillated cellulose fibrils in fibrous networks.

The deformation micromechanics of bacterial cellulose (BC) and microfibrillated cellulose (MFC) networks have been investigated using Raman spectroscopy. The Raman spectra of both BC and MFC networks exhibit a band initially located at ≈ 1095 cm(-1). We have used the intensity of this band as a function of rotation angle of the specimens to study the cellulose fibril orientation in BC and MFC networks. We have also used the change in this peak's wavenumber position with applied tensile deformation to probe the stress-transfer behavior of these cellulosic materials. The intensity of this Raman band did not change significantly with rotation angle, indicating an in-plane 2D network of fibrils with uniform random orientation; conversely, a highly oriented flax fiber exhibited a marked change in intensity with rotation angle. Experimental data and theoretical analysis shows that the Raman band shift rate arising from deformation of networks under tension is dependent on the angles between the axis of fibrils, the strain axis, the incident laser polarization direction, and the back scattered polarization configurations. From this analysis, the effective moduli of single fibrils of BC and MFC in the networks were estimated to be in the ranges of 79-88 and 29-36 GPa, respectively. It is shown also that for the model to fit the data it is necessary to use a negative Poisson's ratio for MFC networks and BC networks. Discussion of this in-plane "auxetic" behavior is given.

Micromechanics of TEMPO-oxidized fibrillated cellulose composites.

Composites of poly(lactic) acid (PLA) reinforced with TEMPO-oxidized fibrillated cellulose (TOFC) were prepared to 15, 20, 25, and 30% fiber weight fractions. To aid dispersion and to improve stress transfer, we acetylated the TOFC prior to the fabrication of TOFC-PLA composite films. Raman spectroscopy was employed to study the deformation micromechanics in these systems. Microtensile specimens were prepared from the films and deformed in tension with Raman spectra being collected simultaneously during deformation. A shift in a Raman peak initially located at ~1095 cm(-1), assigned to C-O-C stretching of the cellulose backbone, was observed upon deformation, indicating stress transfer from the matrix to the TOFC reinforcement. The highest band shift rate, with respect to strain, was observed in composites having a 30% weight fraction of TOFC. These composites also displayed a significantly higher strain to failure compared to pure acetylated TOFC film, and to the composites having lower weight fractions of TOFC. The stress-transfer processes that occur in microfibrillated cellulose composites are discussed with reference to the micromechanical data presented. It is shown that these TOFC-based composite materials are progressively dominated by the mechanics of the networks, and a shear-lag type stress transfer between fibers.