# Utilization of Partially Liquefied Bark for Production of Particleboards
## 3 Results and Discussion
### Performance of the Particleboard Containing PLB
Thickness swelling (TS) and water absorption (WA) of the particleboard after 2 and 24 h immersion in water are shown in Figure 8. The boards I and II exhibited remarkably high TS and WA after 2 h immersion in water, and they were decomposed after 24 h. This can be related to the very low internal bond strength between wood particles and PLB. Low interaction of PLB and wood particles may also be an additional reason, which caused poor interfaces. The increase of the PLB level in the non-MUF panels from 4.7% to 33.3% decreased the TS and WA suggesting that the PLB protected the wood particles against water. Similar trends were observed in TS of the boards made with MUF adhesives. This might be attributed to the increasing amount of compact PLB particles in the particleboard mat, which resulted in a reduction of liquid water penetration into the board. The lowest TS value obtained in the MUF-bonded boards containing the highest amount of PLB (33.3%), which was 3.15% after 2 h and 4.04% after 24 h immersion in water. According to EN 312:2010, the maximum thickness swelling within 24 h of particleboard for non-load bearing applications in humid conditions is 17%. Therefore, the boards produced of the highest amount of PLB (33.3%) without MUF adhesives (panel IV) as well as the panels manufactured with MUF adhesives and 9.1, 20, and 33.3% PLB (panels VII, VIII, and IX) fulfilled the standard requirement. The WA values of the particleboard manufactured with MUF adhesives and 4.7-33.3% PLB (VI, VII, VIII, and IX) were statistically lower than the reference (board V). The current result from TS and WA test indicated that PLB acted as an excellent water-resistant reagent in the particleboard. The overall results from the mechanical and water-related tests confirmed the hypothesis of this study that PLB particles provided an activated surface that can enhance its compatibility with wood particles. The SEM micrographs provided a visual explanation of the mechanical properties and TS changes due to the incorporation of different PLB levels in the particleboard. In detail, the SEM micrographs displayed that PLB caused a compact region with good interaction with wood particles (Figure 9). The compact PLB provided a less porous structure than wood particles and resulted in a reduction of water penetration in the particleboard. The PLB itself showed a homogeneous compact structure that can facilitate the transfer of stresses from the surface layers through the board [22]. An equal distribution of applied stresses at the PLB-wood particles interfaces could be achieved at a PLB level of 9.1% in the particleboard. Moreover, PLB has an advantage of modifying the surfaces of wood particles with the presence of unreacted solvent, acid and intermediates from partial liquefaction. Pressing of particleboard took place under the same temperature of 180 \({}^{\circ}\)C as partial liquefaction helps to transfer the liquid phases of PLB, containing remaining polyhydric alcohol and strong acids, to the wood surface for creating chemical bonding and self-adhesion ability. It can be seen from Figure 9b-d that cell walls of wood near the interface between PLB and wood particles are densified that can be caused by the chemical-active components of PLB.