Extraction of tannins from the bark of local conifers
Tannins can be used in adhesives to produce formaldehyde-free wood-based materials, which are used in furniture production. We have developed a profitable method to extract tannins from the bark of local conifers.
Project description (completed research project)
The aim of the project was to develop a suitable process for extracting tannins from local conifer barks. It had to be capable of being integrated into processes already in place in sawmills and wood-based industries. The researchers also formulated formaldehyde-free adhesives on the basis of the extracted tannins and analysed the physical and mechanical properties of the resultant wood-based materials.
Tannins can be used in adhesives that are needed to produce formaldehyde-free wood-based materials. Until now, commercially traded tannins have been extracted from the wood or bark of tropical or subtropical wood types, such as quebracho (schinopsis balansae) or mimosa (acacia mearnsii).
The barks of European conifers have so far not been used for commercial tannin extraction even though experiments in the laboratory have shown that, e.g., tannins from spruce bark (picea abies) are suitable for producing adhesive systems.
We wanted to develop a process that uses hot water for extracting tannins from local conifer barks. It had to be capable of being integrated into processes already in place in sawmills and wood-based industries. In this context, it was important to consider the influence of individual extraction parameters on the chemical structure and properties of the extracted tannins. The researchers also formulated formaldehyde-free adhesives on the basis of the extracted tannins and analysed the physical and mechanical properties of the resultant wood-based materials. Alongside the chemical-technical aspects, the researchers evaluated the economic viability of the new extraction process.
The extraction of tannins adds significant value to local barks and secures a better overall use of wood (cascaded use). Thanks to formaldehyde-free adhesive systems, sustainable wood-based materials are more marketable.
We were able to produce fibreboards and particleboards based on spruce bark adhesives at pilot scale. The next challenge is to produce the boards at industrial scale.
Adhesives based on spruce bark extracts displayed slightly lower strength than typical tannin adhesives based on mimosa bark. The main reasons for this difference are that the spruce extracts are diluted with cabonhydrates and that spruce bark tannins have fewer cross-links.
Extracts of fir, spruce, pine, larch and Douglas fir contain varying amounts of tannins, phenolic monomers, monosaccharides and pectins. Significant amounts of carbohydrates are present in all extracts. The tannins differ considerably from those extracted from tropical wood.
The yield of tannins is greater if the spruce bark has been seasoned out of doors. It can be improved if cold-water extraction is succeeded by hot-water extraction.
Extraction and characterization of bark tannin from domestic softwood species for bonding applications in wood-based materials