A C-13 NMR-STUDY OF POLYFLAVONOID TANNIN ADHESIVE INTERMEDIATES .1. NONCOLLOIDAL PERFORMANCE DETERMINING REARRANGEMENTS

Chemical and heat treatments introduce noticeable modifications on the structure of four different polyflavonoid tannin extracts during their modification for use as tannin-formaldehyde polycondensates. Such modifications are performance determining and account for the profound differences in behaviour of the four tannin extracts when used as tannin-formaldehyde adhesives. Such modifications are directly followed by C-13 NMR on the tannin extracts. Although some of the modifications observed by C-13 NMR appear to correlate well with previous findings by model compounds studies, others could only be observed by C-13 NMR of the modified tannin extracts themselves. Structural rearrangements to phlobatannins, depolymerization, recyclation, rearrangement of the type of interflavonoid monomer links, and autocondensation have been followed by C-13 NMR and found to occur to different extents in the four tannins accounting for many of their differences in adhesive performance and characteristics. A novel reaction of autocondensation was observed for pecan nut pith tannins, which is different in applied characteristics from other polyflavonoid tannin adhesives. (C) 1994 John Wiley & Sons, Inc.