A C-13 NMR-STUDY OF POLYFLAVONOID TANNIN ADHESIVE INTERMEDIATES .2. COLLOIDAL STATE REACTIONS

Water solutions of 40-50% polyflavonoid tannin extracts appear to be in a colloidal state. This is caused by both the presence of noticeable amounts of hydrocolloid gums as well as the presence of higher molecular mass tannins. C-13 NMR analysis appears to confirm that during chemical treatment, reactions occur in these colloidal solutions that would not be likely to occur in noncolloidal solutions as used in model compound experiments. These reactions centre on the role of the organic anhydride used during treatment, and lead to intramicellar direct alpha-set attack or to acetylation of the tannin followed by a rapid rearrangement again to an alpha-set attack, which is similar to both a Kolbe reaction and a Fries rearrangement. This eventually leads to accelerated cross-linking of the tannin-formaldehyde adhesive prepared from the chemically modified tannin extract. It appears that such reactions and rearrangements are performance determining in the case of the two lower-reactivity tannins examined, mimosa and quebracho; have some noticeably positive effects on the higher reactivity pine tannin, and appear instead to have a deleterious effect on the other higher reactivity tannin, namely pecan nut tannin. The reasons for such behaviour are briefly discussed. (C) 1994 John Wiley & Sons, Inc.