Alcohol (methanol or ethanol) and water showed synergistic effects on biomass direct liquefaction, and the 50 wt % co-solvent of either methanol-water or ethanol-water was found to be the most effective solvent for the liquefaction of eastern white pine sawdust. The 50 wt % aqueous alcohol at 300°C for 15 min produced a bio-oil yield at approximately 65 wt % and a biomass conversion of >95%. At a temperature higher than 300°C, conversion of bio-oil to char was significant by repolymerization. The Fourier transform infrared spectroscopy (FTIR) and gas chromatography-mass spectrometry (GC-MS) analyses of the obtained bio-oils confirmed the presence of primarily phenolic compounds and their derivatives (such as benzenes), followed by aldehyde, long-chain (and cyclic) ketone and alcohol, ester, organic acid, and ether compounds. Gel permeation chromatography (GPC) results suggested that hotcompressed ethanol as the liquefaction solvent favored lignin degradation into monomeric phenols. The X-ray diffraction (XRD) patterns of sawdust before and after the liquefaction displayed that the cellulosic structure of the feedstock was completely converted into amorphous carbon at around 300°C and into crystalline carbon at about 350°C.

Phenolic bio-oil produced by the direct liquefaction of Eastern white pine (Pinus Strobus L.) sawdust in a hot-compressed ethanol-water (1:1 w/w) medium at 300 C was used to partially substitute for phenol in the synthesis of bio-oil-phenol-formaldehyde (BPF) resol resins. Bio-based resol resins with high levels of phenol substitution (up to 75 wt%) could be used as plywood adhesives because of the low molecular weights found for the phenolic bio-oil (weight-average molecular weight ¼ 1072 g/mol, numberaverage molecular weight ¼ 342 g/mol). The properties of the BPF resol resins were analyzed by differential scanning calorimetry, Fourier transform infrared spectroscopy, gel permeation chromatography, and thermogravimetric analysis. All of the experimental BPF resins possessed broad molecular weight distributions but had similar chemical/ thermal properties compared to a conventional phenolformaldehyde (PF) resol resin reference (or 0 wt % BPF resin). The BPFs exhibited the typical properties of a thermosetting PF resin, for example, an exothermic curing temperature of 140-150 C and an acceptable residual carbon yield of 48-72 wt % nonvolatile content at 700 C. The experimental BPFs were applied as adhesives in the assembly of plywood, and then, the dry/wet tensile strengths were evaluated. The tensile strengths of the dry plywood samples bonded with the BPF resins up to a high ratio value of 75 wt % bio-oil exceeded or were comparable to that of the conventional pure PF resin adhesive. All of the BPF-resin-bonded plywood samples gave wet tensile strengths comparable to those of the conventional PF adhesive.