# Utilization of Partially Liquefied Bark for Production of Particleboards
HTL produces bio-oil as main products while MACL mainly produces bio-polyol or phenolated compounds depending on the solvents that are used [9]. Liquefied wood (LW) from MACL with polyhydric alcohols and phenols has been used in different adhesive systems, such as polyurethane, UF, MUF, phenol-formaldehyde, and epoxy systems [14; 15; 16; 17; 18; 19; 20; 21]. LW has high reactivity with other adhesives precursors and reactive sites due to a large amount of phenolic and alcoholic hydroxyl groups in their compositions [16]. [PERSON] et al. [16] produced particleboard from melamine-formaldehyde (MF) or MUF adhesives with added LW, where spruce was liquefied in glycerol-diethylene glycol mixture as a solvent and \(p\)-toluenesulfonic acid as a catalyst for 3 h at 180 \({}^{\circ}\)C. The results showed that the addition of 50% LW to the MF and MUF adhesives did not influence the mechanical properties of particleboard but significantly reduced the formaldehyde emissions. [PERSON] et al. [22] bonded particleboard with MF that was partially substituted by LW. Adhesive mixtures containing 20% of LW had the largest improvements in the mechanical properties of the particleboard compared to the reference boards. Substitution of MF by LW of up to 30% resulted in a comparable mechanical strength to board bonded only with MF, while significantly reducing formaldehyde emissions. LW worked as a plasticizer and increased the mobility of the MF resin molecules. However, the thermal stability of MF substituted by LW was reduced because LW prolonged the curing time of the final adhesive, decreased the cross-linking degree, and accelerated the thermal degradation. [PERSON] et al. [23] produced particleboard with a mixture of UF and LW (ratio 4:1) and investigated the influence of different liquefying solvents, e.g., a mixture of solvents from the polyhydroxy alcohol group, including glycerine, ethylene glycol, propylene glycol, diethylene glycol, and dipropylene glycol, on the chemical structure, physical and mechanical properties of the particleboard. For all liquefying solvents, boards exhibited comparable properties to the ones produced without LW. Bark, as an industrial residual material, with heterogeneous structure and diverse chemical composition, can be used for the production of a variety of and bio-composites and bio-compounds such as tannin and polyphenols, bio-oil, antioxidants, and bio-based adhesives [24; 25; 26; 27; 28; 29; 30; 31]. Particleboard based on larch bark has been reported by Tudor with a much lower formaldehyde emission content than the wood-based panels, which means bark work as formaldehyde scavenger in the particleboard [31]. Limited publications can be found related to the utilization of liquefied bark in wood adhesives and wood-based composites, while, all of them followed a complete liquefaction process. [PERSON] [24] liquefied bark in polyhydric alcohols and \(p\)-toluenesulfonic acid at 120 \({}^{\circ}\)C for 2 h, and then prepared three-layer particleboard by using an adhesive mixture of 80% MUF and 20% liquefied bark (LB) with the addition of 1 M NaOH or 25% ammonium hydroxide for neutralizing the adhesive mixtures. The boards made with MUF-LB adhesives had 10-20% lower modulus of elasticity, 22-29% lower bending strength, and 25% lower tensile strength in comparison with the ones made with commercial MUF adhesives. It was also determined that replacing of MUF with LB led to a slight reduction in formaldehyde content. [PERSON] and [PERSON] [32] prepared particleboard bonded by LB-based resol adhesives, which was prepared from LB of Taiwan Acacia (_Acacia confusa_) and China fir (_Cunningham lancelata_) with two types of catalysts, i.e., sulfuric acid and hydrochloric acid. The results showed that the resol type adhesives prepared from bark, liquefied with sulfuric acid, had a higher viscosity compared to those made from liquefied bark with hydrochloric acid. Adhesives from liquefied China fir had a higher viscosity than those from liquefied Taiwan acacia with the same acid catalyst.