In the group of wood composite materials from fragmented wood, “valueless” biomass and residues are converted to valuable material. The problem of application of other necessary synthetic chemical components, particularly adhesives, which are essential non-wooden components in wood composite production (mainly oil derivatives), has been the subject of many scientific researches that gave positive results. Further to the above, many researches are focused on wood liquefaction and liquefied wood (LW) application as potential adhesives, related to gluing solid particles with liquid wood. Based on previous studies, maximum attention was given to LW applications in modification of phenol-formaldehyde (PF) resins, polyurethanes, saturated and unsaturated polyesters, isocyanates and epoxy resins, and their further application in novel material types. It is evident that because of their “inferior” properties, researches on urea-formaldehyde (UF) resins failed, as well as researches on preparations of UF base adhesives modified with LW. Therefore, the subject of this study was to research the influence of specific experimental parameters on liquefied wood compatibility with UF resins, its influence on polymer structure and adhesion-cohesion properties of modified UF adhesives, and particleboard physical mechanical properties and formaldehyde emission. The results showed that in all cases of UF resin replacement with LW, there was a significant reduction of formaldehyde emission in particleboards, which is one of the aims of this study. Furthermore, it was shown that LW does not show any polymer or adhesion properties, and in this regard laboratory synthesis was conducted of designed liquefied wood-formaldehyde (LWF) resin and LW was synthesized with formalin. With the LWF resin modification of particleboards, results showed increased mechanical properties and free formaldehyde emission as a direct influence of added formalin. To reduce the increased free formaldehyde emission, LWF resin was synthesized analogously to the production of PF resin novolak type, and prepared based on the percentage of lignin content (because of polyphenol properties) in investigated wood species (black poplar), which was applied in further researches.
