Investigation of wood adhesives from kraft lignin and polyethylenimine

Abstract

A new formaldehyde-free wood-adhesive system consisting of kraft lignin and polyethylenimine (PEI) was investigated for making two-ply plywood. The lignin-PEI adhesive was prepared by mixing a kraft lignin solution and a PEI solution at room temperature. The mixing time of the lignin and PEI had little effect on the shear strength when it was ≥40 min. The shear strength of the resulting wood composites increased slightly as a function of increased hot-press time from 2 to 9 min. The optimum hot-press temperature was 140°C. The lignin-PEI adhesive with a lignin/PEI weight ratio of 2:1 resulted in the highest shear strength and the highest water resistance. The wood composites bonded with the lignin-PEI adhesives retained very high shear strengths after undergoing a water-soaking-and-drying test, as well as a boiling water test. Storage of the lignin-PEI adhesive for three days did not have detrimental effect on plywood shear strength. The molecular weight (M w) of PEIs in the range of 70 000 to 150 000 had little effect on the shear strength of the resulting wood composites. The cure chemistry of this lignin-PEI adhesive was proposed to be similar to that of the quinone-tanning process.