The use of bark extracts in adhesives

The literature research deals with extractants of bark. The rise in costs of fossil and non-renewable raw materials used by the chemical industry has prompted an increase in the research concerning the pure chemicals and aliquot raw materials which can be derived from bark. This interest has been directed, in particular, to phenolic compounds which may be derived from bark. Initially, these compounds were tried experimentally as raw material for adhesives used in the plywood and particle board industry. Tannins or the phenolic components of bark are obtained by extraction from wood bark. The quality of both the yields and the derivatives varies according to the solvent used and the conditions of extraction. Tannins are aliquots of polymeric polyphenols which also include small amounts of monomeric polyphenols and carbohydrates. Tannins are used in glues as the phenols which they contain react with formaldehyde in the same way as most phenols, thought at a faster rate. A condensation product from the reaction hardens when heated in a manner typical of phenol resins. Owing, however, to the volume of polymeric polyphenols, the tanninformaldebyde resins have a significantly higher viscosity than their synthetic counterparts in the same concentrate. Additionally, the adhesive bond is weaker than that of the synthetic phenol resins. The utilisation potential of tannins has captured the attention of scientists around the world for two reasons in particular. Firstly, they are found in renewable natural resources, and secondly, they have less harmful effect on the environment than the products found in oil-based raw materials.