PREPARATION OF SOFTWOOD KRAFT LIGNIN-BASED COLD-SETTING WOOD ADHESIVES

The preparation of softwood kraft lignin (NKL)-based thermosetting (LP F) and cold-setting (LPRF) wood adhesives was investigated. The LPF an d LPRF adhesives were prepared by the following procedure. A mixture o f NKL : phenol (8:2 similar to 7:3) was hydroxymethylated under alkali ne conditions at 80 degrees C for 2 hours for LPF resins. The LPF resi ns solution obtained from NKL:phenol (8:2) was acidified with aqueous HCl and water insolubles (WI) were collected in a yield of 89% by cent rifuga tion. Then, the WI reacted with small amounts of resorcinol at pH11 and 80 degrees C for the preparation of LPRF resins. Bonding of w ood 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. Po wdered LPF, LPRF and resorcinol-formaldehyde (RF) adhesives were analy zed by differential scanning calorimetry (DSC). Major exothermic tempe rature peaks of LPF, LPRF and RF resins were observed at 145.5, 116.1 and 79.0 degrees C, respectively. The LPRF adhesives provided high-qua lity bonds, both wet and dry equivalent to RF adhesives. However, high exothermic temperatures and activation energies of LPRF resins compar ed with those of RF resins indicate that improvements may be necessary for more fast-curing.