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Wood: Simplified Method Shows Complex Structure


Cross-section of a mechanically isolated wood fibre by electron microscopy.

Mechanically-isolated wood fibres show several different properties in comparison to chemically-isolated fibres. This is one of the most recent results of a project funded by the Austrian Science Fund FWF at the University of Natural Resources and Applied Life Sciences, Vienna. The project yields significant findings on the structural changes in wood fibres after exposure to moisture and tension. The current results are important for both the structural analysis of wood as well as for the investigation of innovative applications for this classic material.

Wood is one of the most common and versatile natural organic materials. It harmoniously combines high strength with high deformation capability. How these seemingly contradictory properties may be explained, is a topic of today´s wood research. For the analysis of wood’s numerous properties, individual wood fibres had been isolated by means of a chemical procedure until now - although researchers have suspected for a long time that this chemical procedure leads to changes in wood so that scientific results might be doubtful.

In response to this problem, scientists at the University of Natural Resources and Applied Life Sciences, Vienna developed an alternative isolation method for wood fibres. Fibres are isolated from wood in a mechanical procedure using fine tweezers. "We have thus succeeded in isolating wood fibres whose cell walls are not changed or destroyed by chemical substances", says Prof. Stefanie Stanzl-Tschegg at the Institute of Physics and Material Sciences when explaining the advantages of the method. "If we now compare mechanically isolated wood fibres with those that have been traditionally isolated with chemicals then we are able to better understand the weaknesses of individual methods. In this way, we obtain much new information about the structure and properties of wood." Additionally, the scientists were able to show that the mechanical isolation method is also capable of isolating single fibres of other natural materials such as hemp or flax in a much better way than previously possible.

Wet & Dry

An important property that Prof. Stanzl-Tschegg and her colleagues were able to elucidate through their latest findings was the drying behaviour of wood. Previous projects on this topic with chemically-isolated wood fibres showed that fibres twist in an anti-clockwise direction as a result of the drying procedure. Responsible for this phenomenon are spiral-shaped structures in the cell walls of wood fibres. These are formed by so-called cellulose fibrils, which are embedded parallel to each other, strengthening the material. However, tests carried out by the team of Prof. Stanzl-Tschegg indicated that wood fibres twisted much less during drying when mechanically isolated. By means of special microscopic methods the researchers analysed the matrix consisting of the complex molecules lignin and hemi-cellulose. Contrary to the chemically isolated wood fibres, this matrix remains intact in mechanically-isolated wood fibres where it encompasses individual cellulose fibrils. Hence the matrix resembles a corset that lends stability to wood fibres in wet condition by counteracting distortions during the drying process.

Tension & Pressure

This result falls in line with a remarkable list of fundamental findings on the natural material of wood by Prof. Stanzl-Tschegg and her team. Similarly, they had succeeded in detecting yet another functional feature of wood fibres: a molecular mechanism within the wood fibres works like a Velcro connection. When cellulose fibrils deform as a result of tension or pressure, their bonds disconnect from the matrix of lignin and hemi-cellulose and consequently allow for deformation of the wood. As soon as the stress is released, however, the bonds lock-in at the new position and continue to maintain the original stiffness of the material - a property that had so far been associated with metallic materials rather than with wood.

It is the discovery of such previously unknown properties of the traditionally proved natural material of wood that permits its specific and secure use in new applications. In that manner, this material research project funded by the Austrian Science Fund FWF also contributes to securing the future of a significant branch of industry in Austria, a country rich in forests.

Till C. Jelitto | alfa
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