Biologische und technologische Eigenschaften von Buchenholz nach einer Modifizierung mit 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) (English shop)

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This study describes the investigation of the properties of chemically modified beech. Modification with three solution concentrations of 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) was carried out in an industrial process. The modification was based on a two-stage treatment, in which the water-based DMDHEU solution was introduced into the wood in the first step by means of vacuum-pressure impregnation and cured in the second step under hot steam conditions.

The results showed that modification of beech with DMDHEU is fundamentally possible. Solution uptake, WPG (weight percent gain due to the treatment) as well as the altered nitrogen content showed that DMDHEU penetrates into the wood and the cell wall and is fixed there. An increase in hardness determined by nanoindentation confirmed these results.

The equilibrium moisture contents between 20 and 90% relative humidity were affected by the treatment. On average they were clearly below those of the controls. However, no practically relevant difference between the treatments was measured. The treatment resulted in an anti-swelling efficiency (ASE) of approx. 30-40%. Water vapour uptake (water uptake per unit time) was reduced by approx. 50-65% as a result of the treatment.

Resistance to soft rot and basidiomycetes was increased by the treatment. Here it applied that a DMDHEU solution concentration of 2.3 M was necessary in order to achieve reliable protection against wood-destroying fungi in all investigations. Resistance to blue-stain fungi, by contrast, was not affected by DMDHEU treatment.

BRINELL hardness was increased by the treatment depending on the solution concentration, whereas JANKA hardness remained unchanged. Compressive strength was likewise increased, while shear and tensile strength were reduced. Bending strength was not significantly affected. The work absorbed by the modified wood under various force influences decreased considerably. This applied to tensile, bending and impact bending work. Following DMDHEU treatment, the modulus of elasticity behaved differently in the anatomical directions. In the axial direction the MOE was increased, in the radial direction reduced, and in the tangential direction it remained unchanged.

In all investigations, the formaldehyde emission of the treated wood met the equilibrium concentration of less than 0.1 ppm required by the German Chemicals Prohibition Ordinance (Chemikalienverbotsverordnung).

It was striking that, irrespective of the solution concentration, the beech tended after treatment towards increased crack formation parallel to the radial parenchyma tissue. The cracks did not become visible immediately after the modification process, but only after a longer period of storage. It was therefore investigated whether the altered cracking behaviour is attributable to 1) reduced strengths at constant stresses or 2) increased stresses at constant strengths, or whether a combination of the two factors led to this behaviour. Assumptions that increased stresses were caused by an altered swelling anisotropy were not confirmed. An examination of the stresses occurring at the surface by means of Electronic Speckle Pattern Interferometry (ESPI) during a tensile test likewise provided no indication of altered stress distributions or possible stress peaks. This result was surprising, since increased stress had been assumed on account of the differing MOE in the principal anatomical directions. An investigation of the cracking behaviour by means of the micro wedge splitting test and microscopic images of the cell structure suggest that the treatment led to a reduction in the strength of the S2 cell wall layer. The strength of the entire cell wall, by contrast, does not appear to be altered.

On the basis of the tests carried out in this work, it can be assumed that the change in cracking behaviour is influenced by the differing stiffness of the material depending on the anatomical direction as well as by the loss of plastic deformability. Further investigations are necessary in order to fully understand the cracking tendency of DMDHEU-treated beech.