Resorcinol substitutes, tannin

Progress in the use of condensed tannins in adhesive formulations might be expected to be more rapid than is the case for lignins because of the impetus provided by the commercialization of wattle tannin-based adhesives and because of the extraordinarily high reactivity of tannins in reactions with formaldehyde. This reactivity offers an opportunity to substitute tannin for resorcinol (currently priced at about 1.80/lb) instead of phenol (about 0.40/lb). Now that wattle tannins have been successfully introduced, their application can be expected to continue to expand. The situation remains difficult, on the other hand, for use of conifer bark tannins in adhesives. Herb Hergert is certainly... 

As tannins contain many phenolic -type subunits (Fig. 3), one may be tempted to think that they will exhibit a similar reactive potential to that of phenol, and that therefore procedures used in standard PF production can be transferred to those containing tannin. This, however, is not the case. The real situation is that tannin is far more reactive than unsubstituted phenol due to the resorcinol and catchecol rings present in the tannin. This increase in hydroxyl substitution on the two aromatic rings affords an increase in reactivity to formaldehyde by 10 to 50... 

Condensed Tannins as Substitutes for Resorcinol in Bonding Polyester and Nylon Cord to Rubber... 

Condensed tannins have considerable promise as substitutes for resorcinol used in resin formulations for bonding of nylon or, particularly, polyester cord to rubber. Although much more work needs to be done, preliminary results suggest that refinement of extract properties and adhesive formulations could lead to a large, high-value market for condensed tannin extracts. 

Phenol-formaldehyde resin is the most common adhesive for exterior applications due to its water resistance, low initial viscosity and its ability to bond various types of wood substrates [1], Because of its resemblance to phenolic moieties, studies on tannin have been oriented towards an alternative formulation to replace the current synthetic phenol-formaldehyde or phenol-resorcinol-formaldehyde adhesives [2-4], A few suitable alternative natural resources such as oil palm shell, pecan shell nut, lignin, starch, rice bran and tannin are also available for this purpose. Among these materials, tannins represent the best immediate substitute for phenol in wood adhesive production [1],... 

Comparatively little attention has been directed to other specialty polymer uses for condensed tannins over the past 10 years, possibly due to the strong focus on their application as wood adhesives (88, 186). This seems unfortunate because condensed tannins offer excellent opportunities for use as specialty polymers due to the high degree of nucleophilicity of the phloroglucinol or resorcinol A-rings, the excellent potential for complexation to the catechol or pyrogallol B-rings and, in the procyanidins or prodelphinidins, the comparative ease with which the in-terflavanoid bond can be cleaved to permit the formation of derivatives substituted at the C-4 carbon. 

Hamed G R, Chung K H, Hemingway R W (1989) Condensed tannins as substitutes for resorcinol in bonding polyester and nylon cord to rubber. In Hemingway R, Conner A H (eds) Adhesives from renewable resources. ACS Symp Ser Washington DC 242-253... 

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