Лако-красочные материалы — производство

The autocondensation reactions characteristic of polyflavonoid tannins have only recently been used to prepare adhesive polycondensates hardening in the absence of aldehydes [70]. This autocondensation reaction is based on the opening under alkaline and acid condi­tions of the O1-C2 bond of the flavonoid repeating unit and the subsequent condensation of the reactive center formed at C2 with the free C6 or C8 sites of a flavonoid unit on another tannin chain [70-74]. Although this reaction may lead to considerable increases in viscosity, gelling does not generally occur. However, gelling occurs when the reaction occurs (i) in the presence of small amounts of dissolved silica (silicic acid or silicate) catalyst and some other catalysts [70-75], and (ii) on a lignocellulosic surface [74]. 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 [74]. In the case of the less reactive tannins, such as mimosa and quebracho, the presence of a dissolved silica or silicate catalyst of some type is essential to achieve panel strength as required by the relevant standards. Autocondensation reactions have been shown to contribute consider­ably to the dry strength of wood panels bonded with tannins, but to be relatively incon­sequential in contributing to the bonded panels’ exterior-grade properties which are rather determined by polycondensation reactions with aldehydes [74-76]. Combinations of tannin autocondensation and reactions with aldehydes, and combinations of radical with ionic reactions have been used both to decrease the proportion of aldehyde hardener used as well as to decrease considerably more the already low formaldehyde emission yielded by the use of tannin adhesives [74-76].