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.