Formaldehyde-Free Wood Adhesives from Decayed Wood

Green Wood Adhesives from One-Pot Coacervation of Folic Acid and Branched Poly(ethylene imine)

Adhesives are extensively used in furniture manufacture, and most currently utilized furniture glues are formaldehyde-based chemicals, which emit formaldehyde throughout the entire life of the furniture. With increasing concerns about formaldehyde emission effects on human health, formaldehyde-free and environmentally friendly wood adhesives from bio-based resources are highly desired. In this study, we developed an eco-friendly, high-strength, and water-based wood adhesive from one-pot coacervation of the hierarchical self-assembly of folic acid (FA, a biomolecule, vitamin B9) with a commercially available biocompatible polymer-branched poly(ethylene imine) (b-PEI). The coacervation is caused by multiple hydrogen bonds between b-PEI and the stacks of FA quartets, which demonstrates a continuous robust 3D network, thus realizing adhesion and cohesion behaviors. This coacervate has the strongest adhesion toward wood compared with other substrates. The long-lasting shear bonding strength is up to 3.68 MPa, which is much higher than that of commercial super glue, but without releasing any toxic components. Since all the fabrication and application processes are under ambient conditions without any heating and high-pressure procedures, this work provides a facile yet powerful strategy to develop formaldehyde-free, eco-friendly, and high-performance bio-based waterborne adhesives for wood bonding.

Spruce Bark-Extracted Lignin and Tannin-Based Bioresin-Adhesives: Effect of Curing Temperatures on the Thermal Properties of the Resins

In this study, formaldehyde-free bioresin adhesives were synthesised from lignin and tannin, which were obtained from softwood bark. The extraction was done via organosolv treatment and hot water extraction, respectively. A non-volatile, non-toxic aldehyde, glyoxal, was used as a substitute for formaldehyde in order to modify the chemical structure of both the lignin and tannin. The glyoxal modification reaction was confirmed by ATR–FTIR spectroscopy. Three different resin formulations were prepared using modified lignin along with the modified tannin. The thermal properties of the modified lignin, tannin, and the bioresins were assessed by DSC and TGA. When the bioresins were cured at a high temperature (200 °C) by compression moulding, they exhibited higher thermal stability as well as an enhanced degree of cross-linking compared to the low temperature-cured bioresins. The thermal properties of the resins were strongly affected by the compositions of the resins as well as the curing temperatures.

A Review on the Lignin Biopolymer and Its Integration in the Elaboration of Sustainable Materials

Lignin is one of the wood and plant cell wall components that is available in large quantities in nature. Its polyphenolic chemical structure has been of interest for valorization and industrial application studies. Lignin can be obtained from wood by various delignification chemical processes, which give it a structure and specific properties that will depend on the plant species. Due to the versatility and chemical diversity of lignin, the chemical industry has focused on its use as a viable alternative of renewable raw material for the synthesis of new and sustainable biomaterials. However, its structure is complex and difficult to characterize, presenting some obstacles to be integrated into mixtures for the development of polymers, fibers, and other materials. The objective of this review is to present a background of the structure, biosynthesis, and the main mechanisms of lignin recovery from chemical processes (sulfite and kraft) and sulfur-free processes (organosolv) and describe the different forms of integration of this biopolymer in the synthesis of sustainable materials. Among these applications are phenolic adhesive resins, formaldehyde-free resins, epoxy resins, polyurethane foams, carbon fibers, hydrogels, and 3D printed composites.

Fungal Degradation of Wood: Emerging Data, New Insights and Changing Perceptions

Wood durability researchers have long described fungal decay of timber using the starkly simple terms of white, brown and soft rot, along with the less destructive mold and stain fungi. These terms have taken on an almost iconic meaning but are only based upon the outward appearance of the damaged timber. Long-term deterioration studies, as well as the emerging genetic tools, are showing the fallacy of simplifying the decay process into such broad groups. This paper briefly reviews the fundamentals of fungal decay, staining and mold processes, then uses these fundamentals as the basis for a discussion of fungal attack of wood in light of current knowledge about these processes. Biotechnological applications of decay fungi are reviewed, and an overview is presented on how fungi surmount the protective barriers that coatings provide on surfaces. Advances in biochemical analyses have, in some cases, radically altered our perceptions of how wood is degraded, and even the relationships between fungal species, while other new findings have reinforced traditional perspectives. Suggestions for future research needs in the coatings field relative to enhanced fungal and environmental protection are presented.

Multiple iron reduction by methoxylated phenolic lignin structures and the generation of reactive oxygen species by lignocellulose surfaces

Chelator-mediated Fenton chemistry is capable of reducing non-stochiometric amounts of iron via hydroquinone oxidation. These types of reactions have previously been demonstrated to be promoted by some lignocellulose degrading fungi in generating hydroxyl radicals to permit lignified plant cell wall deconstruction. Here we demonstrate that lignocellulose surfaces, when exposed by chemical treatment or fragmentation, can promote a similar multi-oxidative mechanism in the presence of iron. Iron reduction by lignin surfaces permits the generation of hydroxyl radicals in the cell wall to help explain fungal non-enzymatic cell wall deconstruction, and it also provides an explanation for certain phenomenon such as the anthropogenic generation of formaldehyde by wood. The mechanism also provides a basis for the generation of electrons by lignin that are required by certain fungal redox enzymes active in plant cell wall degrading systems. Overall, the data demonstrate that iron found naturally in lignocellulose materials will promote the oxidation of phenolic lignin compounds in the naturally low pH environments occurring within lignified plant cell walls, and that this activity is promoted by cell wall fragmentation.

Modification of the nanostructure of lignocellulose cell walls via a non-enzymatic lignocellulose deconstruction system in brown rot wood-decay fungi

Wood decayed by brown rot fungi and wood treated with the chelator-mediated Fenton (CMF) reaction, either alone or together with a cellulose enzyme cocktail, was analyzed by small angle neutron scattering (SANS), sum frequency generation (SFG) spectroscopy, Fourier transform infrared (FTIR) analysis, X-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Results showed that the CMF mechanism mimicked brown rot fungal attack for both holocellulose and lignin components of the wood. Crystalline cellulose and lignin were both depolymerized by the CMF reaction. Porosity of the softwood cell wall did not increase during CMF treatment, enzymes secreted by the fungi did not penetrate the decayed wood. The enzymes in the cellulose cocktail also did not appear to alter the effects of the CMF-treated wood relative to enhancing cell wall deconstruction. This suggests a rethinking of current brown rot decay models and supports a model where monomeric sugars and oligosaccharides diffuse from the softwood cell walls during non-enzymatic action. In this regard, the CMF mechanism should not be thought of as a "pretreatment" used to permit enzymatic penetration into softwood cell walls, but instead it enhances polysaccharide components diffusing to fungal enzymes located in wood cell lumen environments during decay. SANS and other data are consistent with a model for repolymerization and aggregation of at least some portion of the lignin within the cell wall, and this is supported by AFM and TEM data. The data suggest that new approaches for conversion of wood substrates to platform chemicals in biorefineries could be achieved using the CMF mechanism with >75% solubilization of lignocellulose, but that a more selective suite of enzymes and other downstream treatments may be required to work when using CMF deconstruction technology. Strategies to enhance polysaccharide release from lignocellulose substrates for enhanced enzymatic action and fermentation of the released fraction would also aid in the efficient recovery of the more uniform modified lignin fraction that the CMF reaction generates to enhance biorefinery profitability.

Preparation and characterization of lignin demethylated at atmospheric pressure and its application in fast curing biobased phenolic resins

Agricultural crop-based lignin was utilized to modify phenol-formaldehyde (PF) resin to prepare fast curing biobased phenolic resins by copolymerization.

Enzymatic monitoring of lignin and lignin derivatives biooxidation

Lignin oxidation was enzymatically monitored by measuring methanol released during the reaction. The methanol was oxidized to formaldehyde and hydrogen peroxide, and the latter used to oxidize ABTS to a product measured spectrophotometrically. The efficiency was comparable to the commonly used gas chromatography method. The assay was fast and inexpensive.

Nanoscale coatings on wood: polyelectrolyte adsorption and layer-by-layer assembled film formation

Surface chemistry of wood is based on the exposed surface that is the combination of the intact and cut cellular wall material. It is inherently complex and changes with processing history. Modification of wood surfaces through noncovalent attachment of amine containing water soluble polyelectrolytes provides a path to create functional surfaces in a controlled manner. Adsorption of polyethylenimine (PEI) and polydiallydimethylammonium chloride (PDDA) to wood was quantified as a function of solution conditions (pH and ionic strength). Polycation adsorption was maximized under basic pH without the addition of electrolyte. Added salt either had marginal influence or decreased adsorption of polycation, indicating interactions are strongly influenced by Coulombic forces. PEI adsorption could be modeled by both a Langmuir and Freundlich equations, although the wood surface is known to be heterogeneous. After adsorption of polycations, layer-by-layer assembled films were created on the wood surface. Layered films masked ultrastructural features of the cell wall, while leaving the microscale features of wood (cut lumen walls and openings) evident. These findings revealed for the first time that nanoscale films on wood can be deposited without changing the microscopic and macroscopic texture. Functionalized wood surfaces created by nanoscale films may have a future role in adhesives systems for wood composites, wood protection, and creating new functional features on wood.