Water-soluble O-acetyl galactoglucomannan (GGM) is a softwood-derived polysaccharide, which can be extracted on an industrial scale from wood or mechanical pulping waters and now is available in kilogram scale for research and development of value-added products. To develop applications of GGM, information is needed on its stability in acidic conditions. The kinetics of acid hydrolysis of GGM was studied at temperatures up to 90 degrees C in the pH range of 1-3. Molar mass and molar mass distribution were determined using size exclusion chromatography with multiangle laser light scattering and refractive index detection. The molar mass of GGM decreased considerably with treatment time at temperatures above 70 degrees C and pH below 2. The molar mass distribution broadened with hydrolysis time. A first-order kinetic model was found to match the acid hydrolysis. The reaction rate constants at various pH values and temperatures were calculated on the basis of the first-order kinetic model. Furthermore, the activation energy, E, was obtained from the Arrhenius plot. The activation energy E was 150 kJ mol (-1) for acid hydrolysis of spruce GGM. The apparent rate constant during acid hydrolysis increased by a factor of 10 with a decrease in pH by 1 unit, regardless of temperature. In addition, gas chromatography and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry were applied to study the released GGM monomers and oligomers.
By aiming at tailoring the bonding strength of a thermosetting lignin-containing phenol-formaldehyde (LPF) wood adhesive, different fractions of an industrial hardwood alkaline lignin have been prepared through sequential solvent fractionation (i-PrOH, EtOH, and MeOH). Those fractions were comprehensively characterized by GPC, GC, Py/GC−MS, and NMR techniques. Lignin fractions with low molar mass and narrow dispersity, including the i-PrOH-soluble and EtOH-soluble ones, were of high purity and had more reactive sites for LPF adhesive synthesis and better accessibility due to lower degree of condensation than the high molar mass ones. Some recalcitrance of integrating high molar mass fractions covalently into the PF adhesive was observed, which was also true in the case of lignin phenolation. The tailored bonding strength of the LPF adhesive, tested by gluing wood pieces, provided strong evidence for molecular structure−performance correlation; the i-PrOH-PF had the lowest activation energy, the highest curing enthalpy, and the strongest bonding strength of 2.16 MPa. This study demonstrates a clear structure−property-application relationship of technical hardwood lignin in the LPF adhesive field, which might pave the way for a more effective bulk valorization.
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