Lignins from enzymatic hydrolysis and alkaline extraction of steam refined poplar wood: Utilization in lignin-phenol-formaldehyde resins

• Characterization of enzymatic hydrolysis lignin (EHL) and alkaline extracted lignin (AEL) from three different steam refining conditions.
• Comparison of EHL and AEL with an organosolv (OL) and kraft lignin (KL) regarding chemical composition, molecular weight distribution, OH group quantification by 31P NMR spectroscopy and Mannich reactivity.
• EHL and AHL after mild steam refining showed high suitability as substitutes in LPF resins comparable to OL- and KL-PF references.
• High dry and wet internal bond of particleboards bonded by LPF resin with EHL and AEL.
• LPF with AEL fulfilled mechanical requirements of European Standard P7.

In this study, a steam refining process, optimized earlier with respect to carbohydrate yield, was used to extract lignins from non-debarked poplar wood from short growth plantations by enzymatic hydrolysis (EHL) and alkaline extraction (AEL) for the utilization in lignin-phenol-formaldehyde resins (LPF). Three different pretreatment conditions were chosen at 190 °C and 16 min, 210 °C and 15 min and 200 °C, 15 min, 2.5% SO2. Characterization of the lignins revealed that the carbohydrate content decreased about 47% (210 °C, 15 min) to 58% (200 °C, 15 min, 2.5% SO2) with intensification of the pretreatment conditions. Furthermore, the average molecular weight and polydispersity increased. The chemical and physico-chemical properties of lignin-phenol-formaldehyde resins synthesized with these lignins were evaluated and gluebond performance was tested by the Automated Bonding Evaluation System (ABES). LPF resins with AEL and EHL (190 °C, 16 min) were used to produce particleboards. LPF resins with Organosolv lignin (OL) and Kraft lignin as well as neat PF resins served as references. The mechanical properties of these panels were evaluated. The results showed high dry (EHL: 0.64 N mm−2, AEL: 0.73 N mm−2) and wet internal bond strength (EHL: 0.21 N mm−2, AEL: 0.25 N mm−2) of LPF bonded particleboards. LPF resin with AEL fulfills the requirements for the relevant standard specifications P7 according to DIN EN 312:2010-12.

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