Induced accelerated autocondensation of polyflavonoid tannins for phenolic polycondensates. III. CP‐MAS ¹³C‐NMR of different tannins and models

Tannins: Major Sources, Properties and Applications

“…The self-condensation reactions characteristic of polyfl avonoid tannins have only recently been used to prepare adhesive polycondensates hardening in the absence of aldehydes [85] . This self-condensation reaction is based on the opening under alkaline and acid conditions of the O1ᎏC2 bond of the fl avonoid repeating unit and the subsequent condensation of the reactive centre formed at C2 with the free C6 or C8 sites of a fl avonoid unit on another tannin chain [85][86][87][88][89] . Although this reaction may lead to considerable increases in viscosity, gelling does not generally occur.…”

“…Although this reaction may lead to considerable increases in viscosity, gelling does not generally occur. However, gelling does occur when the reaction takes place (i) in the presence of small amounts of dissolved silica (silicic acid or silicates) catalyst and some other catalysts [85][86][87][88][89][90] , and (ii) on a lignocellulosic surface [89] . In the case of the more reactive procyanidin-and prodelphinidin-type tannins, such as pine tannin, cellulose catalysis is more than enough to cause hardening and to produce boards of strength satisfying the relevant standards for interior-grade panels [89] .…”

Applications of Tannin Resin Adhesives in the Wood Industry

“…Gelling accelerates as silicate content increases and stabilizes after reaching a certain value. Although tannin resin adhesive that was manufactured through autocondensation increases the dry strength of panels, the strength of the resulting crosslinking is insufficient for exterior-graded panels [18]. Aldehyde curing agents should be added for the preparation of exterior-graded panels.…”

Tannin-Based Biofoams-a Review

“…Tannin-based carbon aerogel foams innovatively based on the ionic and radical autocondensation of tannins under alkaline conditions promoted by their reaction with silica and silicates [67][68][69][70][71] have also been prepared recently [72]. It is of interest to note that upscaling to pilot plant level of the preparation of these types of foams has also been reported [73].…”