Wednesday, May 21, 2008

Phenolic Adhesive from Vegetable Tannins

Research in connection with the development of adhesives from vegetable tannins has been reported from various parts of the world, and it is possible that these phenol containing natural substances will one day figure prominently in the synthesis of phenolic wood glues, especially hot curing ones, or be used in association with them.

A variety of tannin extracts, containing different types of highly reactive phenols, is available commercially. They are derived mainly from trees, especially from barks and woods, but many represent a cheap source of mixed phenols containing nuclei of recorcinol and phlorogluecinol types. Most extracts are now available as spray dried, water soluble powder and the difficulty soluble extracts dissolve readily at a slightly alkaline pH.

The chemistry of the vegetable tannins has recently been surveyed in detail, both of surveyor whom have provided exhaustive bibliographies. Tannin extracts are described as of two kinds, hydrolysable tannins and condense tannins. The former are built up form a variety of phenolic acids (related structurally to gallic acid) which are joined by ester linkages to a central sugar residue. They are of little interest in the formation of phenolic adhesives. The latter type contain mixtures of "polyphenol" (i.e. polynuclear polyhydric phenol) that react rapidly with formaldehyde. The structure of the actual tannin molecules are uncertain but they are clearly related to various C15 compound.

The important of leather tannin extracts, has covered the synthesis of adhesives made wholly from quebracho or mimosa extract and formaldehyde, as well as the partial replacement of ordinary phenol in P.F. glues. Utilizing a tannin extract as the only phenolic compound could be marketed. Certain unpublished work has indicated two main reason for this. Firstly the pot-life is short; associated with this, the ratio of pot-life to curing time (at temperatures used in plywood manufacture) is relatively low. The probable explanation is that the resin contains molecules of a wide range of size and structure, a small percentage of highly reactive molecules being sufficient to cause gelation and termination of the pot-life, whereas satisfactory curing requires also the cross linking of less reactive molecules. Secondly, the resins lack good spreading and wetting properties, especially when fillers are added. The adhesive have excellent resistance to water, including boiling water.

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