Novel wood adhesives from condensed tannins and polyethylenimine

The wood composites industry is a major manufacturing sector in the United States. Increasing concerns about emissive formaldehyde to human health and our heavy dependence on petroleum and natural gas promote the need for a formaldehyde-free wood adhesive from renewable resources. Marine mussel adhesive protein is an excellent example of a formaldehyde-free adhesive from renewable resources. To cope with turbulent tides and waves, mussels stick to rocks or other substances in seawater by secreting an adhesive protein, commonly called marine adhesive protein. The two key functional moieties in the marine adhesive protein are 3,4-dihydroxyphenylalanine (a catechol moiety) and lysine (an amino moiety). Since marine adhesive protein is not readily available, condensed tannins, which contain large amounts of catechol groups, were considered for use in this adhesive system. This study revealed that a mixture of procyanidin-type condensed tannin and polyethylenimine (PEI) was an excellent surrogate for the marine adhesive protein. A tannin-PEI mixture performed successfully as a formaldehyde-free wood adhesive. Two-ply wood composites bonded with tannin-PEI adhesives had very high shear strengths and were very water resistant. The following variables of the tannin-PEI adhesives were investigated in detail for their effects on the shear strength and water-resistance of wood composites bonded with the tannin-PEI adhesives: the mixing time of tannin and PEI, the total solids content of the tannin PEI adhesives, the weight ratio between tannin and PEI, the cure time and temperature of the tannin-PEI adhesives, the storage time of the tannin-PEI adhesives, and the molecular weight of PEI. (C) 2003 Elsevier Ltd. All rights reserved.