Raw material bark

Stable boards can be made from a waste product of the wood industry without glue

Tree bark may be suitable as a similarly versatile material as wood. A research team at the Max Planck Institute of Colloids and Interfaces in Potsdam has pressed tree bark into boards that have similar mechanical properties to particleboard, but do not contain any adhesive. Such boards could be used in the furniture or packaging industry, for example.

In the timber industry, tree bark has so far been regarded as an inferior waste product. Although it contains various useful chemicals, is widely used for mulching in the garden, and is sometimes burned to generate energy, it is not used in nearly as many ways as wood. Now, however, there could be new uses for tree bark. A team led by Charlett Wenig and Michaela, who conduct research at the Max Planck Institute of Colloids and Interfaces, have pressed the bark of birch, oak, larch and pine trees into boards that can withstand bending loads similar to those of particleboard.

For this purpose, the researchers first peeled towel-sized pieces of bark from the trees, made them into a flat shape by clamping the plates in a wooden frame, and dried them. Then they placed two pieces of bark with the barky outer sides on top of each other in such a way that they lay crosswise on top of each other in the direction of growth. They then pressed the barks together at 90 degrees Celsius for 20 minutes at pressures of 20 and 97 bars, respectively. And although they did not use any glue, the pieces of bark bonded together to form a stable structure, with the sandwiches made of oak bark proving to be significantly more stable than those made of larch bark. "A big advantage of ’pure’ single-component products is that there is no need to separate the components after they have been used," says Charlett Wenig. The researchers are not yet able to explain why the bark plates bond stably under pressure and with heat.

The team analyzed the structure and properties of the bark plates using various methods. In addition to the bending tests, they also determined the roughness of the surface, took density measurements and examined the materials using microcomputer tomography. They found, for example, that the boards became similarly smooth to sanded wood surfaces when pressed. In addition, the researchers investigated how much the boards swelled with changes in humidity - an important characteristic of wood-based materials, as the moisture content also influences their properties. According to the results, the samples expanded by a few percent at high humidity. Whereby this mainly depended on how much moisture the bark contained before it was processed into boards. Finally, the team proved that bark can also be shaped by pressing pieces of bark into U-shaped molds. Now the team wants to further investigate how the properties of the bark panels can be further improved by varying the pressure and temperature during processing.

MPIKG/PH

Charlett Wenig, Friedrich Reppe, Nils Horbelt, Jaromir Spener, Ferréol Berendt, Tobias Cremer, Marion Frey, Ingo Burgert, Michaela Eder