A multiple cross-linking strategy to develop an environment-friendly and water resistance wheat gluten protein wood adhesive.

Wheat gluten (WG) shows great promise to synthesize environment-friendly wood adhesives. However, their weak bonding strength and poor water resistance have limited its application in the commercial wood-based panel industry. In this study, a novel WG-based adhesive was developed by constructing a multiple cross-linking network generated by covalent and non-covalent bonds. The potential mechanism was revealed by FT-IR analysis. Furthermore, their surface morphology, thermal stability, viscosity, and residual rate of adhesives with different compositions were systematically characterized and compared. The results showed that the hydrogen bonding, reactions between amine groups and tannin, and ring opening reaction of epoxy, synergistically contributed to generate a highly crosslinked network. The wet/boil water strength of the plywood prepared from WG/tannin/ethylene imine polymer (PEI)-glycerol triglycidyl ether (GTE) adhesive with the addition of 15 % GTE could reach 1.21 MPa and 1.20 MPa, respectively, and a mildew resistance ability was observed. This study provides a facile strategy to fabricate high-performance plant protein-based adhesives with desirable water resistance for practical application.

Highly Conductive, Anti-Freezing Hemicellulose-Based Hydrogels Prepared via Deep Eutectic Solvents and Their Applications.

Hydrogels containing renewable resources, such as hemicellulose, have received a lot of attention owing to their softness and electrical conductivity which could be applied in soft devices and wearable equipment. However, traditional hemicellulose-based hydrogels generally exhibit poor electrical conductivity and suffer from freezing at lower temperatures owing to the presence of a lot of water. In this study, we dissolved hemicellulose by employing deep eutectic solvents (DESs), which were prepared by mixing choline chloride and imidazole. In addition, hemicellulose-based DES hydrogels were fabricated via photo-initiated reactions of acrylamide and hemicellulose with N, N'-Methylenebisacrylamide as a crosslinking agent. The produced hydrogels demonstrated high electrical conductivity and anti-freezing properties. The conductivity of the hydrogels was 2.13 S/m at room temperature and 1.97 S/m at -29 °C. The hydrogel's freezing point was measured by differential scanning calorimetry (DSC) to be -47.78 °C. Furthermore, the hemicellulose-based DES hydrogels can function as a dependable and sensitive strain sensor for monitoring a variety of human activities.

Strain-induced orientation facilitates the fabrication of highly stretchable and tough xylan-based hydrogel for strain sensors.

Stretchable and tough polysaccharide-based functional hydrogels have gained popularity for various applications. However, it still remains a great challenge to simultaneously own satisfactory stretchability and toughness, particularly when incorporating renewable xylan to offer sustainability. Herein, we describe a novel stretchable and tough xylan-based conductive hydrogel utilizing the natural feature of rosin derivative. The effect of different compositions on the mechanical properties and the physicochemical properties of corresponding xylan-based hydrogels were systematically investigated. Owing to the multiple non-covalent interactions among different components to dissipate energies and the strain-induced orientation of rosin derivative during the stretching, the highest tensile strength, strain, and toughness of xylan-based hydrogels could reach 0.34 MPa, 2098.4 %, and 3.79 ± 0.95 MJ/m3, respectively. Furthermore, by incorporating MXene as the conductive fillers, the strength and toughness of hydrogels were further enhanced to 0.51 MPa and 5.95 ± 1.19 MJ/m3. Finally, the synthesized xylan-based hydrogels were able to serve as a reliable and sensitive strain sensor to monitor the movements of human beings. This study provides new insights to develop stretchable and tough conductive xylan-based hydrogel, especially utilizing the natural feature of bio-based resources.

Skin-mimicking strategy to fabricate strong and highly conductive anti-freezing cellulose-based hydrogels as strain sensors.

Conductive hydrogels have attracted increasing attention for applications in wearable and flexible strain sensors. However, owing to their relatively weak strength, poor elasticity, and lack of anti-freezing ability, their applications have been limited. Herein, we present a skin-mimicking strategy to fabricate cellulose-enhanced, strong, elastic, highly conductive, and anti-freezing hydrogels. Self-assembly of cellulose to fabricate a cellulose skeleton is essential for realizing a skin-mimicking design. Furthermore, two methods, in situ polymerization and solvent replacement, were compared and investigated to incorporate conductive and anti-freezing components into hydrogels. Consequently, when the same ratio of glycerol and lithium chloride was used, the anti-freezing hydrogels prepared by in situ polymerization showed relatively higher strength (1.0 MPa), while the solvent-replaced hydrogels exhibited higher elastic recovery properties (94.6 %) and conductivity (4.5 S/m). In addition, their potential as strain sensors for monitoring human behavior was analyzed. Both hydrogels produced reliable signals and exhibited high sensitivity. This study provides a new horizon for the fabrication of strain sensors that can be applied in various environments.

A skin-inspired biomimetic strategy to fabricate cellulose enhanced antibacterial hydrogels as strain sensors.

With the development of wearable devices, the fabrication of strong, tough, antibacterial, and conductive hydrogels for sensor applications is necessary but remains challenging. Here, a skin-inspired biomimetic strategy integrated with in-situ reduction has been proposed. The self-assembly of cellulose to generate a cellulose skeleton was essential to realize the biomimetic structural design. Furthermore, in-situ generation of silver nanoparticles on the skeleton was easily achieved by a heating process. This process not only offered the excellent antibacterial property to hydrogels, but also improved the mechanical properties of hydrogels due to the elimination of negative effect of silver nanoparticles aggregation. The highest tensile strength and toughness could reach 2.0 MPa and 11.95 MJ/m3, respectively. Moreover, a high detection range (up to 1300%) and sensitivity (gauge factor = 4.4) was observed as the strain sensors. This study provides a new horizon to fabricate strong, tough and functional hydrogels for various applications in the future.

Ethyl cellulose based self-healing adhesives synthesized via RAFT and aromatic schiff-base chemistry.

In this work, reversible addition-fragmentation chain transfer (RAFT) polymerization and Schiff base chemistry was combined to fabricate self-healing adhesives. An esterification reaction was first performed to prepare ethyl cellulose based macroinitiators. Then, a "grafting from" RAFT of vanillin methacrylate and lauryl methacrylate was used to obtain graft copolymers. DSC result showed that the glass transition temperature was manipulated via changing the ratio of vanillin and fatty acids moieties. NMR spectrum analysis demonstrated the presence of aldehyde groups, which were available for the dynamic crosslinking to generate a network as self-healing adhesives. The adhesive test showed that the shear strength could reach 0.81 MPa with a self-healing efficiency of 98.7 %. The bottlebrush structures of copolymers and reversibility of Schiff base chemistry might collaboratively contribute to the high self-healing efficiency. This study provides a facile way to fabricate high-performance self-healing adhesives from ethyl cellulose and renewable resources.

Unexpected role of amphiphilic lignosulfonate to improve the storage stability of urea formaldehyde resin and its application as adhesives.

As the second-largest natural polymer, the utilization of lignin for practical applications has attracted increasing attention. In this study, lignosulfonate was employed to enhance the storage stability of urea formaldehyde (UF) resins. Cryo-scanning electron microscopy was firstly used to observe the influence of lignosulfonate addition on the colloidal morphology of UF resin. Moreover, adding lignosulfonate at different stages during the UF resins synthesis was also investigated to reveal its effect on storage stability. The potential interaction between lignosulfonate and UF resins was then analyzed via FT-IR, 13C CPMAS NMR, and zeta potential. It has been observed that lignosulfonate could increase the electrostatic repulsion of UF resins to avoid aging. No chemical reaction between UF resins and lignosulfonate was observed. After the elucidation of potential interaction, the effect of lignosulfonate on the curing process, thermal stability and adhesive performance of UF resins was systematically evaluated. Finally, as adhesives to fabricate eucalyptus plywood, the shear strength and formaldehyde release of UF resins with 20% addition of lignosulfonate could reach 0.88 MPa and 0.12 mg/L, respectively. Due to the excellent performance, low cost and wide availability of lignosulfonate, it might be industrially used as a stabilizer in the UF resins production.

Structures, Properties and Potential Applications of Corncob Residue Modified by Carboxymethylation.

In this study, corncob residue (CR) valorization was simply and efficiently realized via carboxymethylation, and its enhanced performance as fillers in urea-formaldehyde (UF) resin was investigated. The structures of corncob residue and carboxymethylated derivative were analyzed by nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR), and Raman techniques, respectively. The thermal stability, morphology, viscosity control, and adhesive strength were then investigated to evaluate its performance as fillers in UF resin composite. Similar to commercial flour, carboxymethylated CR could effectively disperse in UF resin. It also exhibited a better initial viscosity control between 30 and 50 °C. The adhesive test analysis showed that the shear strength of resin with carboxymethylated CR addition could reach 1.04 MPa, which was comparable to flour (0.99 MPa) and significantly higher than raw CR (0.45 MPa). Moreover, a low formaldehyde emission was observed.

Synthesis of Lignin-Based Polyacid Catalyst and Its Utilization to Improve Water Resistance of Urea-formaldehyde Resins.

In this study, a lignin-based polyacid catalyst was synthesized via two steps to enhance water resistance of urea-formaldehyde (UF) resins. The first steps involved a hydroxymethylation reaction to increase the hydroxyl content in lignin. Then, hydroxymethylated lignins were reacted with maleic anhydride to form maleated lignin-based polyacids. The acid groups were expected to function as acid catalysts to catalyze the curing process of UF resin. In order to elucidate the structural variation, 3-methoxy-4-hydroxyphenylpropane as a typical guaiacol lignin structural unit was used as a model compound to observe the hydroxymethylation and the reaction with maleic anhydride analyzed by 1H and 13C NMR. After the structural analysis of synthesized lignin-based polyacid by FTIR and 13C NMR, it was used to produce UF resin as an adhesive in plywood and medium density fiberboard (MDF) production, respectively. The results showed that when the addition of lignin-based polyacid was 5% in plywood, it could effectively improve the water resistance of UF resins as compared to commercial additive NH4Cl. It also exhibited a lower formaldehyde emission. Like plywood, lignin-based catalysts used in medium density fiberboard production could not only maintain the mechanical properties, but also inhibit the water adsorption of fiberboards.

Demethylation of Alkali Lignin with Halogen Acids and Its Application to Phenolic Resins.

Lignin, a byproduct from the chemical processing of lignocellulosic biomass, is a polyphenolic compound that has potential as a partial phenol substitute in phenolic adhesive formulations. In this study, HBr and HI were used as reagents to demethylate an alkali lignin (AL) to increase its hydroxyl content and thereby enhance its reactivity for the preparation of phenolic resins. Analyses by FT-IR, 1H-NMR and 2D-NMR(HSQC) demonstrated both a decrease in methoxyl groups and an increase in hydroxyl groups for each demethylated lignin (DL). In addition, the molar amounts of phenolic hydroxyls, determined by 1H-NMR, increased to 0.67 mmol/g for the HI-DL, and 0.64 mmol/g for the HBr-DL, from 0.52 mmol/g for the AL. These results showed that HI, a stronger nucleophilic reagent than HBr, provided a higher degree of AL demethylation. Lignin-containing resins, prepared by copolymerization, met the bonding strength standard for exterior plywood with DL used to replace as much as 50 wt.% of phenol. The increased hydroxyl contents resulting from the lignin demethylations also imparted faster cure times for the lignin-containing resins and lower formaldehyde emissions. Altogether, the stronger nucleophilicity of HI, compared to HBr, impacted the degree of lignin demethylation, and carried through to measurable differences the thermal properties and performance of the lignin-containing PF resins.