Polyphenols, bark-derived

More recently, there has been a renewed recognition of the potential of bark-derived polyphenols for adhesives as a result of improved understanding of the chemical structure of these materials (, 5), new types of formulations (5), and the fact that tannins are being commercially used in adhesives in South Africa (7), thus serving as a prototype for utilization in other parts of the world. In order to properly assess the current developments in this field, this overview will provide a historical perspective on adhesives based on tannins as well as a summary of the extraction techniques and chemical structures. Finally, areas where additional work could be fruitful will be suggested. 

The differences between castoreum from different species and sources still need to be investigated. As suggested by Tang et al [69] it is quite likely that these differences could be diet related because the phenols are most likely derived from the diet of these animals. Indeed, already in 1949 Lederer [73] recognized this possibility and pointed out that 2, 3"-dihydroxydibenz-2-pyrone and 4,4 -dihydroxydiphenic acid dilactone 12 (Fig. 3) are closely related to ellagic acid, which is abundant in the bark of trees. A comprehensive chemical analysis of the polyphenols present in the preferred diet of the beavers and a comparison of these compounds with the phenols present in castoreum from animals fed exclusively on this diet could shed some light on this unresolved problem. 

The rapid growth of the pulp and paper industry following World War II coupled with a renewed scientific interest in utilization of bark and wood residues led to investigative programs on bark and wood tannins. The leather industry was continuing to decline in importance, so other alternatives were needed. One of these was replacement of phenol in whole or in part in phenol-formaldehyde adhesive formulations. This work progressed to the point where commercial quantities of polyphenolic extractives were made and sold for adhesive application. Excessive capacity and low petrochemically derived phenol prices in the 1960 s led to the demise of this effort in the United States (2,5). 

Going back to Pycnogenol and how it helped Cartier s men fight scurvy, it took centuries to learn that the pine needles provided a small amount of vitamin C and the bark yielded vitamin C-supercharging bioflavonoids, members of the polyphenol family of plant-derived substances. In fact, it was only in 1984 that... 

Vegetable tans, however, are based on wood, bark, leaves and roots of mimosa, quebracho, chestnut and oak. - Tannin is the active ingredient, a polyphenol (sugar derivative of gallic acid), which binds to the collagen (- proteins) by hydrogen bonds. Due to a m.w. of 500-5000, the penetration into the fibrils is slower than with chromium tans and results in heavier and fuller leathers. 

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