13 августа, 2015 
Malyar The word tannin has been used loosely to define two different classes of chemical compounds of mainly phenolic nature: hydrolyzable tannins and condensed tannins. The former, including chestnut, myrabalans (Terminalia and Phyllantus tree species), and divi-divi (Caesalpina coriaria) extracts, are mixtures of simple phenols such as pyrogallol and ellagic acid and of esters of a sugar, mainly glucose, with gallic and digallic acids [2]. They can and have been used successfully as partial substitutes (up to 50%) for phenol in the manufacture of phenol-formaldehyde resins [8,9]. Their chemical behavior towards formaldehyde is analogous to that of simple phenols of low reactivity and their moderate use as phenol substitutes in the above-mentioned resins does not present difficulties. Their lack of macromolecular structure in their natural state, the low level of phenol substitution they allow, their low nucleophilicity, limited worldwide production, and their higher price somewhat decrease their chemical and economical interest.
Condensed tannins, on the other hand, constituting more than 90% of the total world production of commercial tannins (200,000 tons per year), are both chemically and economically more interesting for the preparation of adhesives and resins. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees. These include various Acacia (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), and Rhus (sumach extract) species, from which commercial tannin extracts are manufactured, and various Pinus bark extract species. Where the bark and wood of trees were found to be particularly rich sources of condensed tannins, commercial development ensued through large-scale afforestation and/or industrial extraction, mainly for use in leather tanning. The production of tannins for leather manufacture reached its peak immediately after World War II and has since progressively declined. This decline of their traditional market, coupled with the increased price and decreased availability of synthetic phenolic materials due to the advent of the energy crisis of the early 1970s, stimulated fundamental and applied research on the use of such tannins as a source of condensed phenolics.
The structure of the flavonoid constituting the main monomer of condensed tannins may be represented as follows:
this flavonoid unit is repeated 2 to 11 times in mimosa tannin, with an average degree of polymerization of 4 to 5, and up to 30 times for pine tannins, with an average degree of polymerization of 6 to 7 for their soluble extract fraction [10-12].
The nucleophilic centers on the A ring of a flavonoid unit tend to be more reactive than those found on the B ring. This is due to the vicinal hydroxyl substituents, which cause general activation in the B ring without any localized effects such as those found in the A ring.
Formaldehyde reacts with tannins to produce polymerization through methylene bridge linkages at reactive positions on the flavonoid molecules, mainly the A rings. The reactive positions of the A rings are one of positions 6 or 8 (according to the type of tannin) of all the flavonoid units and both positions 6 and 8 of the upper terminal flavonoid units. The A rings of mimosa and quebracho tannins show reactivity toward formaldehyde comparable to that of resorcinol [13-15]. Assuming the reactivity of phenol to be 1 and that of resorcinol to be 10, the A rings have a reactivity of 8 to 9. However, because of their size and shape, the tannin molecules become immobile at a low level of condensation with formaldehyde, so that the available reactive sites are too far apart for further methylene bridge formation. The result may be incomplete polymerization and therefore weakness. Bridging agents with longer molecules should be capable of bridging the distances that are too long for methylene bridges. Alternatively, other techniques can be used to solve this problem.
In condensed tannins from mimosa bark the main polyphenolic pattern is represented by flavonoid analogs based on resorcinol A rings and pyrogallol B rings. These constitute about 70% of the tannins. The secondary but parallel pattern is based on resorcinol A rings and catechol B rings [2,14]. These tannins represent about 25% of the total of the mimosa bark tannin fraction. The remaining part of the condensed tannin extract is the “nontannins’’ [14]. They may be subdivided into carbohydrates, hydrocolloid gums, and small amino and imino acid fractions [2,14]. The hydrocolloid gums vary in concentration from 3 to 6% and contribute significantly to the viscosity of the extract despite their low concentration [2,14]. Similar flavonoid A — and B-ring patterns also exist in quebracho wood extract (Schinopsis balansae and Schinopsis lorentzii) [13-15], but no phloroglucinol A-ring pattern, or probably a much lower quantity of it, exists in quebracho extract [15-17]. Similar patterns to wattle (mimosa) and quebracho are followed by hemlock and Douglas fir bark extracts. Completely different patterns and relationships do instead exist in the case of pine tannins [18-20] which present instead only two main patterns: one represented by flavonoid analogs based on phloroglucinol A rings and catechol B rings [18,20]. The other pattern, present in much lower proportion, is represented by phloroglucinol A rings and phenol B rings [18,20]. The A rings of pine tannins then possess only the phloroglucinol type of structure, much more reactive toward formaldehyde than a resorcinol-type structure, with important consequences in the use of these tannins for adhesives.
In condensed polyflavonoid tannin molecules the A rings of the constituent flavo — noid units retain only one highly reactive nucleophilic center, the remainder accommodating the interflavonoid bonds. Resorcinolic A rings (wattle) show reactivity toward formaldehyde comparable to, though slightly lower than that of resorcinol [21]. Phloroglucinolic A rings (pine) behave instead as phloroglucinol [22]. Pyrogallol or catechol B rings are by comparison unreactive and may be activated by anion formation only at relatively high pH [16]. Hence the B rings do not participate in the reaction except at high pH values (pH 10), where the reactivity toward formaldehyde of the A rings is so high that the tannin-formaldehyde adhesives prepared have unacceptably short pot lives [21]. In general tannin adhesive practice, only the A rings are used to cross-link the network. With regard to the pH dependence of the reaction with formaldehyde, it is generally accepted that the reaction rate of wattle tannins with formaldehyde is slowest in the pH range 4.0 to 4.5 [23]; for pine tannins, the range is between 3.3 and 3.9.
Formaldehyde is generally the aldehyde used in the preparation, setting, and curing of tannin adhesives. It is normally added to the tannin extract solution at the required pH, preferably in its polymeric form of paraformaldehyde, which is capable of fairly rapid depolymerization under alkaline conditions, and as urea-formalin concentrates. Hexamethylenetetramine (hexamine) may also be added to resins due to its potential formaldehyde releasing action under heat. Hexamine is, however, unstable in acid media [24] but becomes more stable with increased pH values. Hence under alkaline conditions the liberation of formaldehyde might not be as rapid and as efficient as wanted. Also, it has been fairly widely reported, with a few notable exceptions [25], that bonds formed with hexamine as hardener are not as boil resistant [26] as those formed by paraformaldehyde. The reaction of formaldehyde with tannins may be controlled by the addition of alcohols to the system. Under these circumstances some of the formaldehyde is stabilized by the formation of hemiacetals [e. g., CH2(OH)(OCH3)] if methanol is used [2,22]. When the adhesive is cured at an elevated temperature, the alcohol is driven off at a fairly constant rate and formaldehyde is progressively released from the hemiacetal. This ensures that less formaldehyde is volatilized when the reactants reach curing temperature and that the pot life of the adhesive is extended. Other aldehydes have also been substituted for formaldehyde [2,21,23,25].
In the reaction of polyflavonoid tannins with formaldehyde two competitive reactions are present:
1. The reaction of the aldehyde with tannin and with low-molecular-weight tannin — aldehyde condensates, which are responsible for the aldehyde consumption.
2. The liberation of aldehyde, available again for reaction. This reaction is probably due to the passage of unstable — CH2-O-CH2- ether bridges initially formed to — CH2- linked compounds.
In the case of some tannins, namely quebracho tannin, a third reaction of importance is present,
3. The simultaneous hydrolysis of some interflavonoid bonds, hence a depolymerization reaction, partly counteracting and hence slowing down hardening [26-28]. Notwithstanding that the two major industrial polyflavonoid tannins which exist, namely mimosa and quebracho tannins, are very similar and both composed of mixed prorobinetinidins and profisetinidins one could not explain this anomalous behavior of quebracho tannin. It has now been possible to determine by both nuclear magnetic resonance (NMR) [26] and particularly by laser desorption mass spectrometry (MALDI-TOF) for mimosa and quebracho tannins and some of their modified derivatives [28] that: (i) mimosa tannin is predominantly composed of prorobinetinidins while quebracho is predominantly composed of profisetinidins, (ii) mimosa tannin is heavily branched due to the presence of considerable proportions of ‘‘angular’’ units in its structure while quebracho tannin is almost completely linear [28]. This latter structural difference contributes to the considerable differences in viscosity of water solutions of the two tannins and which (iii) induces the interflavonoid link of quebracho to be more easily hydrolyzable, due to the linear structure of this tannin, confirming NMR findings [26,28] that this tannin is subject to polymerization/depolymerization equilibria. This also showed that the decrease of viscosity due to acid/base treatments to yield tannin adhesive intermediates does also depend in quebracho on a certain level of hydrolysis of the tannin itself and not only of the carbohydrates present in the extract (see Section IV). This tannin hydrolysis does not appear to occur in mimosa tannin in which the interflavonoid link is completely stable to hydrolysis.
It is interesting to note that while — CH2-O-CH2- ether bridged compounds have been isolated for the phenol-formaldehyde [24] reaction, their existence for fast-reacting phenols such as resorcinol and phloroglucinol has been postulated, but they have not been isolated, as these two phenols have always been considered too reactive with formaldehyde. They are detected by a surge in the concentration of formaldehyde observed in kinetic curves due to methylene ether bridge decomposition [19].
When heated in the presence of strong mineral acids, condensed tannins are subject to two competitive reactions. One is degradative leading to lower-molecular-weight products, and the second is condensative as a result of hydrolysis of heterocyclic rings (p-hydroxybenzyl ether links) [16]. The p-hydroxybenzylcarbonium ions created condense randomly with nucleophilic centers on other tannin units to form ‘‘phlobaphenes’’ or ‘‘tanner’s red’’ [16,29-31]. Other modes of condensation (e. g. free radical coupling of B-ring catechol units) cannot be excluded in the presence of atmospheric oxygen. In predominantly aqueous conditions, phlobaphene formation or formation of insoluble condensates predominates. These reactions, characteristic of tannins and not of synthetic phenolic resins, must be taken into account when formulating tannin adhesives.
Sulfitation of tannin in one of the oldest and most useful reactions in flavonoid chemistry. Slightly sulfited water is sometimes used to increase tannin extraction from the bark containing it. In certain types of adhesives, the total effect of sulfitation, while affording the important advantages of higher concentration of tannin phenolics in adhesive applications due to enhanced solubility and decreased viscosity, and of higher moisture retention by the tannin resins, allowing slower adhesive film dry-out, hence longer assembly times [32], also represents a distinct disadvantage in that sulfonate groups promote sensitivity to moisture with adhesive deterioration and poor water resistance of the cured glue line even with adequate cross-linking [32-35].
In recent years the importance of the marked colloidal nature of tannin extract solutions has come to the fore [27,36-45]. It is the presence of both polymeric carbohydrates in the extract as well as of the higher molecular fraction of the polyphenolic tannins which determines the colloidal state of tannin extract solutions in water [26,36]. The realization of the existence of the tannin in this particular state affects many of the reactions that lead to the formation and curing of tannin adhesives, to the point that reactions not thought possible in solution become instead not only possible but the favored ones [26,36], while reactions mooted to be of determinant importance when found on models not in the colloidal state have in reality been shown to be inconsequential to tannin adhesives and their tannin applications [43,44].
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