Natural Adhesives, Binders, and Matrices for Wood and Fiber Composites: Chemistry and Technology

Natural Adhesives, Binders, and Matrices for Wood and Fiber Composites: Chemistry and Technology

Antonio Pizzi (Lorraine University, Epinal, France & King Abdulaziz University, Jeddah, Saudi Arabia)
Copyright: © 2014 |Pages: 51
DOI: 10.4018/978-1-4666-4554-7.ch004
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Recent developments and trends in the field of bio-based adhesives are reviewed. The more recent developments in tannin adhesives without the use of aldehyde-yielding compounds under the conditions of processing, or even without the use of hardeners, are described. Lignin adhesives are discussed next. The combination of these two types to yield natural environmentally friendly matrices for non-woven fiber mats is also reviewed. Several new trends in the developments of protein adhesives and in carbohydrate adhesives are then addressed. Unsaturated oil adhesives based on epoxidized unsaturated vegetable oils are also described as well as an example of cashew nut shell oil modified by a new and inexpensive method to yield an adhesive by self-condensation of the material. The chapter addresses last the new process of solid wood friction welding without the use of adhesive, in which the wood interface itself is used as the binder.
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Tannin Adhesives

The word tannin has been used loosely to define two different classes of chemical compounds of mainly phenolic nature: hydrolysable tannins and condensed tannins. The former are mixtures of simple phenols such as pyrogallol and ellagic acid and of esters of a sugar, mainly glucose, with gallic and digallic acids (Pizzi, 1983). Their lack of macromolecular structure in their natural state, the low level of phenol substitution they allow, their low nucleophilicity, limited worldwide production, and higher price somewhat decrease their chemical and economical interest.

Condensed tannins, on the other hand, constituting more than 90% of the total world production of commercial tannins (200,000 tons per year), are both chemically and economically more interesting for the preparation of adhesives and resins. Condensed tannins and their flavonoid precursors are known for their wide distribution in nature and particularly for their substantial concentration in the wood and bark of various trees. These include various Acacia (wattle or mimosa bark extract), Schinopsis (quebracho wood extract), Tsuga (hemlock bark extract), and Rhus (sumach extract) species, from which commercial tannin extracts are manufactured, and various Pinus bark extract species.

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