Big quantities of residues and side-streams are generated annually from forest-based and agricultural industries all around the world and present a relatively unexplored renewable resource. Due to the absence of a regularly updated and systematic database of supply, industrial residues and side-streams usually end up in landfill disposal, are used for energy generation, or remain at the production sites. These renewable side-streams are mainly lignocellulosic materials that can be used for fuels, chemicals, and other value-added materials. However, the difficulty in recovering useful components from industrial wastes from a techno-economic point of view is hindering the use of these materials. There are different methods for converting biomass into fuels, chemicals, and materials, including thermochemical, biochemical, and physical conversion. Negative environmental impacts from direct incineration of waste materials and increasing interest in reducing the dependency on fossil-based sources have increased the need for the valorization of the industrial side-streams for material and chemical applications. 

Among the different thermochemical conversion methods, liquefaction of lignocellulosic materials is an efficient way to convert solid biomass into liquids. Liquefaction including hydrothermal liquefaction (HTL) and moderate acid-catalyzed liquefaction (MACL), is often carried out in an aqueous environment by employing organic solvents with or without catalyst under pressure or ambient conditions. A liquefaction process is influenced by many factors such as material type, solvent, catalyst, time, and temperature. All the parameters of the liquefaction are related to each other, and they affect the yields and the properties of the final products. Studies on the utilization of industrial waste and side-streams as feedstock for liquefaction have increased in recent years, generating significant interest from both academia and industry.  

This PhD study included a literature review on liquefaction technologies that provide liquefied products for wood adhesives, followed by experimental work on MACL and its optimization for different industrial side-streams, such as wood sawdust, bark, and oat husks. Liquefaction of those materials led to different liquefaction yields (LY) due to their different chemical compositions. When the same liquefaction conditions were applied, liquefied wood sawdust had the highest LY while liquefied bark had the lowest. This was mainly attributed to wood sawdust having a higher cellulose and lower lignin content, when compared to bark and oat husks. After optimizing the liquefaction of wood sawdust, obtained products were applied in wood adhesive formulations successfully. Crude liquefied wood (CLW) and purified liquefied wood (PLW) polyols were obtained from the liquefaction of wood sawdust with the highest LY of 99.7% and used for the synthesis of polyurethane (PU) adhesives by reacting them with polymeric diphenylmethane diisocyanate (pMDI). The bonding strength and penetration to wood adherends of the PU adhesives were affected by the molar ratios between the isocyanate groups (NCO) in pMDI and the hydroxyl groups (OH) in the CLW and PLW. The highest bonding strength of PU adhesives was achieved at an NCO:OH molar ratio of 1.5:1. The thermal stability of the PU adhesives was improved by increasing the NCO:OH molar ratio. PU adhesives based on CLW and PLW with the same adhesive formulation did not show significant differences in their properties while CLW polyol contained more water and alcohols than PLW.  

A novel method called partial liquefaction of lignocellulosic biomass was also proposed. Partially liquefied bark (PLB) was prepared and used to replace wood particles for producing particleboards (PB) with or without the presence of a commercial synthetic adhesive, i.e. melamine-urea-formaldehyde (MUF). PLB was shown to provide single-layer PBs with good adhesion, mechanical strength, and water repellency. The overall mechanical properties of non-MUF single-layer PBs were inferior to those of MUF-bonded PBs. Increasing the PLB content up to 9.5% led to enhanced mechanical properties for MUF-bonded PBs. PLB prepared from bark with a particle size less than 2 mm ensured good mechanical behavior of single-layer PBs. Moreover, three-layer particleboards prepared from PLB and wood particles had comparable mechanical properties to the reference PBs made solely from wood particles, and PLB had less influence on the mechanical properties of the PBs when used in the surface layer than in the core layer. Formaldehyde emissions from the three-layer PBs were below the limits required by European Standard EN 13986:2004 and major volatile organic compounds (VOCs) were carboxylic acids. 

This research provided a comprehensive understanding of converting different lignocellulosic materials by a MACL process into valuable polymers and raw materials, which are suitable for the synthesis of wood adhesives and for the manufacturing of particleboards. Due to time constraints related to conducting the PhD, it was not possible to conduct a full characterization of the liquefied products from the selected materials. Such studies should be part of future research in order to supplement our knowledge of MACL mechanisms. 

This paper presents a novel method of partially liquefying bark sawmilling waste for use in making particleboards. Maritime pine (Pinus pinaster Ait.) bark of different particle sizes (fine, medium, coarse, and mixed) was partially liquefied in the presence of ethylene glycol as a solvent and sulphuric acid as a catalyst at 180 degrees C for 30 minutes. Single-layer particleboards were prepared by mixing partially liquefied bark (PLB) and wood chips at a ratio of 0.25 with no adhesives (group A) and at ratios of 0.25 or 0.1 with melamine-urea-formaldehyde (MUF) adhesives for additional bonding (groups B and C respectively). Mechanical and physical properties of the particleboards were tested according to European standards. The results showed that the boards in group A had lower densities, inferior mechanical properties and higher moisture content than those in groups B and C. Bark particle size had a significant effect on the mechanical properties of particleboards within each group. Additional MUF bonding and avoidance of coarse bark particles had a positive effect on mechanical properties. The thickness swelling (TS) and water absorption (WA) values of MUF-bonded boards were lower than those of boards without MUF, and greater addition of PLB produced particleboards with better water resistance. Bark particle size was not as critical for TS and WA as for mechanical properties. The overall results suggested using a bark particle size of < 2 mm for further studies.

Polyurethane (PU) adhesives were prepared with bio-polyols obtained via acid-catalyzedpolyhydric alcohol liquefaction of wood sawdust and polymeric diphenylmethane diisocyanate(pMDI). Two polyols, i.e., crude and purified liquefied wood (CLW and PLW), were obtained fromthe liquefaction process with a high yield of 99.7%. PU adhesives, namely CLWPU and PLWPU,were then prepared by reaction of CLW or PLW with pMDI at various isocyanate to hydroxyl group(NCO:OH) molar ratios of 0.5:1, 1:1, 1.5:1, and 2:1. The chemical structure and thermal behavior of thebio-polyols and the cured PU adhesives were analyzed by Fourier transform infrared spectroscopy(FTIR) and thermogravimetric analysis (TGA). Performance of the adhesives was evaluated by singlelap joint shear tests according to EN 302-1:2003, and by adhesive penetration. The highest shearstrength was found at the NCO:OH molar ratio of 1.5:1 as 4.82 ± 1.01 N/mm² and 4.80 ± 0.49 N/mm² for CLWPU and PLWPU, respectively. The chemical structure and thermal properties of the cured CLWPU and PLWPU adhesives were considerably influenced by the NCO:OH molar ratio. 

Bark as a sawmilling residue can be used for producing value-added chemicals and materials. This study investigated the use of partially liquefied bark (PLB) for producing particleboard with or without synthetic adhesives. Maritime pine (Pinus pinaster Ait.) bark was partially liquefied in the presence of ethylene glycol and sulfuric acid. Four types of particleboard panels were prepared with a PLB content of 4.7%, 9.1%, 20%, and 33.3%, respectively. Another five types of particleboard panels were manufactured by using similar amounts of PLB and 10 wt.% of melamine–urea–formaldehyde (MUF) adhesives. Characterization of bark and solid residues of PLB was performed by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), and automated vapor sorption (AVS). Mechanical and physical properties of the particleboard were tested according to the European standards EN 310 for determining modulus of elasticity and bending strength, EN 317 for determining thickness swelling after immersion in water, and EN 319 for determining internal bond strength. The results showed that the increase in PLB content improved the mechanical strength for the non-MUF boards, and the MUF-bonded boards with up to 20% of PLB met the requirements for interior uses in dry conditions according to EN 312. The non-MUF boards containing 33.3% of PLB and the MUF-bonded boards showed comparable thickness swelling and water absorption levels compared to the reference board.

Utilizing biomass waste for wood-based composites has been driven by harsh competition for raw materials and environmental concerns for more sustainable products. Bark, as a by-product of the sawmilling and pulping industries, is a lignocellulosic material that is rich in lignin and extractives, and holds potential for producing chemicals and value-added materials. There are many possibilities of using bark in wood-based panel manufacturing such as making adhesives (e.g. bark tannin extractives, liquefied bark) or using it as a furnish in small amounts. Instead of using the completed liquefied bark products in the adhesive mixture, we have been working on a novel method of making particleboards by using partially liquefied bark as a furnish material with binding abilities. Thus, partially liquefied bark was mixed with wood chips with an aim to investigate the effect of different bark sizes on the properties of particleboards.Maritime pine (Pinus pinaster Ait.) bark was partially liquefied in the presence of ethylene glycol as solvent and sulphuric acid (H₂SO₄) as catalyst in 180˚C for 30 minutes. Four different sizes of bark were used: mix, coarse (> 2 mm), middle (1-2 mm), and fines (< 1 mm). One-layered 8-mm particleboards were prepared by mixing dry wood chips with the partially liquefied bark categories (9.1% or 20% w/w). Melamine-urea-formaldehyde (MUF) resin was 10% of the total weight of the furnish materials (dry wood chips and partially liquefied bark); while boards were also made without adding the resin. Mechanical and physical properties of the particleboards were tested according to the European standards, and ANOVA analysis of the results showed no statistically significant differences between varying bark sizes. Particleboards made with 9.1% of partially liquefied bark and with 10% of MUF resin met all the standard requirements for mechanical strength and thickness swelling. Particleboards made with 20% of partially liquefied bark and without adding MUF resin were inferior to those with MUF resin.From the current results we can conclude that it is possible to make particleboards from partially liquefied bark with competitive properties, and this supports our original idea of not completing the liquefaction process. In that respect, our work can contribute to energy and material savings when using liquefied products in wood panel manufacturing. More research is needed to optimize the process as well as to evaluate the formaldehyde emission level from this type of panels. 

Nanocellulose is a competitive reinforcement material for usein biocomposite structures and fibrous products. In this study, adhesive mixtures of dicarboxylic acid cellulose nanofibrils (CNF) were dispersed into commercial polyvinyl acetate (PVAc) and starch adhesives, whichwere applied to Norway spruce (Picea abies) to assess their performance in wood joining. Single-lap joints were prepared and tested with PVAc mixtures with0 to 0.64 wt% CNFand starch glue mixtures containing 0 to 1.07 wt% CNF. CNF suspensionshaving three concentrations(0.64, 0.96,and 1.28%)were compared. The results showed that the optimum amount of CNF, 0.48% suspensions, added to PVAc increased the average lap joint strength (EN 205:2003) by 74.5% when compared to control specimens with pure PVAc. Correspondingly, 0.96% CNF suspensions also enhanced the strength of starch adhesive by 34.5%. Lower and higher CNF concentrations showed clearly inferior performance. (PDF) Effect of Cellulose Nanofibrils on the Bond Strength of Polyvinyl Acetate and Starch Adhesives for Wood. 

Liquefaction, a useful method of turning whole biomass into liquids, provides advantages for energy andpolymers and finds applications in many sectors. This paper reviews the different liquefaction technologies andrecent advances in the development of sustainable wood adhesives. Current liquefaction technologies includehydrothermal liquefaction (HTL) and moderate acid-catalyzed liquefaction (MACL). HTL produces bio-oils asprimary products, and solid residues and gases as by-products. MACL depends on the solvent types used, whichare grouped to polyhydric alcohols and phenols. Bio-polyols from alcohol liquefaction, phenolated biomass fromphenol liquefaction and phenolic compounds rich-HTL bio-oils have been used in the production of liquefiedbiomass-based adhesives, which have shown competitive properties but face challenges for industrial uses. Yet, abetter understanding of reaction pathways and optimization of the liquefaction processes is needed.