SYNOPSISAcid-and alkali-catalyzed polyflavonoid tannin-based rigid foams were prepared. These foams have comparable physical and mechanical properties to the synthetic phenolic rigid foam used as a comparative standard. The fluid polymer phase was based on a mimosa tannin-formaldehyde resin with a minor addition of a fortifier resin. Expansion of the fluid phase was brought about by a physical blowing agent, whereas dimensional stabilization was achieved through cross-linking at the desired density. In the case of the acid-catalyzed foam, a heat-generating agent in the form of furfuryl alcohol was employed. The polymer composition of tannin-formaldehyde / urea-formaldehyde systems as a function of p H was predicted from the respective gel times and rate constants, i.e., above pH 7, the copolymer proportion will tend to 100% and, that a t pH 3.4, the polymer blend proportion will tend to a maximum.
SYNOPSISPolyflavonoid tannins have been found to autocondense and harden when in presence of small amounts of SiO, at high pH. Small amounts of boric acid and AlC13 were found to have the same effect. The mechanism of tannin autocondensation to hardening was found to depend on the Lewis acid behaviour of the additives used. Such mechanism involves Lewis acid acceptance of electrons from the ether oxygen of the flavonoid unit pyran ring with subsequent facilitation of base-induced heterocycle opening. The reactive C2 site created by heterocycle opening proceeds to autocondense with the reactive A-ring of a flavonoid unit on another chain denying to the flavonoid the possibility of intramolecular rearrangements to catechinic acid and phlobatannin which occur in model compounds. Si has been shown to pass through a coordination state of 5, by CP-MAS "Si-NMR. The portion of Si, which has not been able to complete the reaction due to premature hardening, remains attached to the flavonoids, in this coordination state, in the hardened network. The Si portion, which has completed the reaction and caused the hardening, reverts instead to SiOz and is detached from the flavonoid. The Si02, H3B03, and A1C13 mechanism shows that their presence also reverses the relative facility of cleavage between interflavonoid bond and heterocycle opening in mainly procyanidin and profisetinidin tannins. 0 1994
SYNOPSISPolyflavonoid tannin autocondensation was found to be facilitated by the reaction occurring on cellulose and lignocellulosic substrates. Although the mechanism of polyflavonoid autocondensation induced by cellulose differs from that induced by the action of Lewis acids, the subsequent reaction of autocondensation appears to be similar. The determining step of tannin rearrangement pathways under alkaline conditions in cellulose-induced highertemperature autocondensation is the favoring of the heterocycle pyran ring opening over the normal interflavonoid bond cleavage and catechinic acid rearrangement. This is caused by relevant bond weakening and easier cleavage induced by strong attractive forces between flavonoid and cellulosic substrate. Applied bonding of cellulosic substrates by polyflavonoid tannin autocondensation reactions appears to be feasible and to occur as predicted from theoretical predictions. 0 1995 John Wiley & Sons, Inc.
I NTRODUCTIO NThe autocondensation of polyflavonoid tannins induced by Lewis acids as described in the preceding article' opens up the possibility for new applications of polymeric tannins as adhesives. Foremost could be the use of tannins as wood adhesives without the use of any formaldehyde, or any other aldehydes, leading to a n environmentally friendly, no formaldehyde emission, natural binder.Recently, the acceleration induced by a cellulose substrate on the polycondensation and hardening of synthetic phenol-formaldehyde resins was reported.' By the nature of this catalytic activation, a similar effect by a cellulose substrate could occur also in the type of autocondensation described in the preceding articles.'s2 This aspect needs to be investigated before application of the tannin autocondensation reaction to wood adhesives. This article deals, then, with the catalytic activation, autocondensation, and hardening of polyflavonoid tannins
EXPERIMENTAL Differential Scanning Calorimetry ( DSC)T h e samples were tested by placing in crucibles a small amount of pine tannin 40% solution a t pH 12, pine tannin 40% solution + 4% SiO, (Areosyl) a t p H 12, and pine tannin 40% solution a t p H 12 on filter paper for the mass proportion of tannin to paper of l : l. The sample crucibles were then heated a t different heating rates, namely, 5, 10, 15, 20, 25,
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