Tannin-based composites

Tannin compounds, extracted from the wood, bark, leaves, and galls of plants and used mainly in the leather industry, are now considered as another raw renewable material in the design of phenolic resins. Tannins are natural polyphenolic materials that can be classified by their chemical structure and properties into two 

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The loose term renewable resources adhesives has been used to identify polymerie eom-pounds of natural, vegetable origin that have been modified and/or adapted to the same use as some classes of purely synthetic adhesives [1]. At present two classes of these adhesives exist one already extensively commercialized in the southern hemisphere and the other on the slow way to commercialization. These two types of resins are tannin-based adhesives [2] and lignin adhesives [3 ]. Both types are aimed primarily at substituting synthetic phenolic resins. In some aspects, such as performance, they closely mimic, or are even superior to, synthetic phenolic adhesives, while in others they behave in a vastly different manner from their synthetic counterparts. In this chapter we focus primarily on tannin-based adhesives because they have already been in extensive industrial use in the southern hemisphere, in certain fields of application, for the past 20 years. These adhesives are of some interest not only for their excellent performance in some applications but also for their mostly environmentally friendly composition. Lignin adhesives are treated briefly here and in detail in Chap. 28. 

The structural proposal for humic acids by Stevenson (1982) had significant logic, based on the state of information at that time. The structure (Figure 1.3) has compositional aspects of phenols derived from lignin and from tannins, and paper chemistry exercises would allow reactions and interactions needed to give structures of the type predicted. The structure shown is highly aromatic, and the proposed aliphatic moieties are saccharide- and peptide-derived. Modern analytical procedures invariably show peptide and saccharide components to be associated with... 

The use of synthetic resins Umits somewhat the environmental attractiveness of such adhesives based on purely natural materials, while the use of tannins alone is limited at present by the relatively Umited supply of these materials [1]. Thus, the aim is to prepare an adhesive based on materials of natural origin, satisfying international standards for both performance and emission, which does not emit or even better does not contain any formaldehyde, the composition of which does not include any synthetic resins, and that is less costly and uses widely available materials. This will render wood panel adhesives based on natural materials much more acceptable both economically and environmentally. 

Carbon-based blacks have been treated as a composite group because the chemistry is far more complex than simply that of carbon (for example, they may contain a highly variable hydrocarbon phase). They are also conveniently hnked into hydrocarbons and other complex organic materials such as tannins. The carbon-based blacks are differentiated here from one another primarily by source material and production method, these pro-foimdly affecting composition and morphology. However, they... 

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