Bark is an important side-product of the wood processing industry. Due to the high moisture and ash content of the bark, combustion for energy production is not very profitable. Aside from restricted application in horticulture, relevant applications for bark by material utilisation are missing. Professor Frédéric Pichelin of Bern University of Applied Sciences explains why tannin extraction from domestic softwood barks holds huge potential for adhesives.
Condensed tannins are molecules that can be extracted from bark and used as a phenol substitute in many applications.
Until now, commercially traded tannins have been extracted from the wood or bark of tropical or subtropical wood types, e.g. quebracho (Schinopsis balansae) or mimosa (Acacia mearnsii). The extraction of the tannin is generally done in hot water. After spray drying, the extracts are sailed all over the world in powder form.
The barks of European conifers have so far not been used for commercial tannin extraction, even though experiments in the laboratory have shown that, for example, extracts from spruce bark (Picea abies) are suitable for producing adhesive systems.
The extraction process for gaining tannin from the bark of north European conifer species has been recently investigated. The gained tannins are to be used in low-emission adhesives for the production of wood-based materials.
First promising results in Switzerland
The global wood industry is the largest user of adhesives: about 80% of all wood and wood-based products involve some form of bonding, and 70% of the total volume of adhesives produced is consumed in the wood working industry. This big demand has motivated a Swiss team of researchers to investigate the potential of tannin and tannin extraction from different conifer barks as a substitute of synthetic adhesives in the production of formaldehyde-free wood-based products or other alternative applications.
The aim of this project is the development of an optimised tannin extraction process from domestic softwood barks for application as adhesive. The bark from Norway spruce (Picea abies), pine (Pinus sylvestris), European larch (Larix decidua), European silver fir (Abies alba), and Douglas fir (Pseudotsuga menziesii) has been included in the investigations.
The process should 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 and other factors (e.g. bark storage, pre-treatment) on the chemical structure and properties of the extracted tannins. Eventually, a link between the extracts’ characteristics and the properties of wood adhesives based on them should be identified.
The hot water extracts from the bark of softwood species proved to be composed only partially from condensed tannins. Relevant amounts of phenolic monomers, monosaccharides, and pectins were in fact also detected in most of the samples. The relative ratio of the different compounds in the extracts broadly varied among the species. Carbohydrates were detected in a considerable amount of all the samples. Peculiar structures of the condensed tannins, containing building units different from the commonly reported flavan-3-ols, were also identified.
Topochemical analysis of the bark tissues showed the considerable presence of non-extractable and non-leachable phenolic compounds.
The composition of the hot water extracts from spruce bark varied considerably during prolonged bark exposure to natural weathering, mainly due to the natural leaching of the most extractable compounds. A preliminary cold-water extraction is able to remove the compounds more responsible for variations observed during bark ageing. Following hot treatments could then result in extracts with a higher concentration of tannins, but always in combination with pectins.
Adhesive formulations based on spruce extracts showed slightly inferior properties than typical mimosa/hexamine adhesives. The tannin dilution effect of the co-extracted saccharides and a limited ability to develop effective crosslinks with hexamine due to molecular steric hindrances were related to such lower performances.
Further research is needed
Compared with commercial mimosa and quebracho extracts, the European softwood bark extracts showed a considerably lower extraction yield and higher carbohydrates fraction, hinting at possible limitations for their exploitation.
A two-step water extraction process was identified as a possible solution for gaining extracts with a higher tannin/carbohydrate ratio, but the loss of a considerable portion of them during the first extraction stage has to be carefully considered in the economic feasibility of the process.
The residual potential of the extracted bark as a still phenolic-rich substrate is worth being further valorised.
The performances of adhesives based on spruce extracts with a higher tannin/carbohydrate ratio supported the likelihood of fast-curing resins with a significantly reduced amount of added chemicals.
The presence of condensed tannins in the hot water extracts from the bark of several species makes them suitable for use in the formulation of glue systems, partially or fully substituting oil-based components. Nevertheless, despite the many valuable works about tannin extraction and use from tropical tree species (e.g. mimosa, quebracho), comparable data is missing about extraction yields and chemical composition of the extracts from the bark of domestic softwood species.
A major restriction of tannin extracts from domestic softwood bark is the high non-tannin portion (e.g. carbohydrates, phenolic monomers, inorganic salts) of the extracts. For bonding applications this is interfering with product performance by raising the hydrophilicity of the hardened resins and reducing the reactivity of the tannin-based adhesive system. The presence of polysaccharides in the extract is of particular concern, as they were associated to the measured high viscosity of their water solutions. Very little information has been published so far about the adaption of the extraction parameters with the aim of increasing the tannin/non-tannin ratio in the extract.
The industrial practice of tannin extraction is not a focus of further utilisation in bonding applications, but rather optimised for use in pharmaceutical, textile or food industry.
New business opportunities for the forestry sector and sawmill industry
The valorisation of bark offers new business opportunities for the sawmill industry, thus encouraging the use of local resources.
The project has demonstrated the possibility of using bark extract as an adhesive for wood-based panel production. Moreover, the specific properties of the identified molecules could open new applications in the field of food or for the pharmaceutical industry.
Research projects in the field of adhesive development for solid wood gluing are very important. At the moment, a lot of research is being carried out in the field of hardwood gluing for the production of glue-laminated timber or cross-laminated timber. PUR (polyurethane) adhesives are not performing for this application, and only synthetic PRF (platelet-rich fibrin) can be used. A natural tannin-based adhesive would be a nice alternative.
At the European level, many research projects in the field of natural composites have been launched. Softwood tannin extract could be used as a polymer for the production of bio-based composites, with a multitude of applications covering the automotive industry, packaging industry or building industry.
This product could basically replace synthetic phenolic or polyphenolic compounds. The innovation relies on the substitution of synthetic products through bio-based molecules with the same properties but recyclable.
Moreover, when used in the field of adhesives, tannin glues have the advantage of emitting no formaldehyde.
The research teams would like to thank the Swiss National Science Foundation for supporting this project.