Preparation of Softwood Kraft Lignin-Based Cold-Setting Wood Adhesives

Abstract

The preparation of softwood kraft lignin (NKL)-based thermosetting (LPF) and cold-setting (LPRF) wood adhesives was investigated. The LPF and LPRF adhesives were prepared by the following procedure. A mixture of NKL : phenol (8 : 2 ∼ 7 : 3) was hydroxymethylated under alkaline conditions at 80°C for 2 hours for LPF resins. The LPF resins solution obtained from NKL : phenol (8 : 2) was acidified with aqueous HCI and water insolubles (WI) were collected in a yield of 89% by centrifugation. Then, the WI reacted with small amounts of resorcinol at pH 11 and 80°C for the preparation of LPRF resins. Bonding of wood veneers using LPRF adhesives was performed by cold-setting at room temperature for 48 hours. A satisfactory adhesive was mainly composed of 60 parts of KKL, 15 parts of phenol and 25 parts of resorcinol. Powdered LPF, LPRF and resorcinol-formaldehyde (RF) adhesives were analyzed by differential scanning calorimetry (DSC). Major exothermic temperature peaks of LPF, LPRF and RF resins were observed at 145.5, 116.1 and 79.0°C, respectively. The LPRF adhesives provided high-quality bonds, both wet and dry equivalent to RF adhesives. However, high exothermic temperatures and activation energies of LPRF resins compared with those of RF resins indicate that improvements may be necessary for more fast-curing.