Improving wood and flame protection by nanoparticles
Externally, one cannot see any signs of wood treatment. "It is impregnated, but the typical wood surface remains intact," says Thomas Hübert of BAM Federal Institute for Materials Research and Testing, pointing to a 5 by 10 centimetre piece of pine wood. Impregnation hardly changes the colour. The secret is silicon or titanium nanoparticles, compounds formed in the wood which protect it permanently against fungal infection. Moreover, as a result of the collaboration of research groups from BAM and the Burkhardt Institute of the Georg August University of Göttingen, the scientists were able to show that the wood's fire resistance and strength is improved by the treatment. "The addition of as little as one percent by mass of titanium dioxide is sufficient for this," says Muhammad Shabir Mahr, who writes his thesis on this topic at BAM and the University of Göttingen.
The project was also aimed at seeing if conventional methods, which often rely on harmful substances, can be replaced with environmentally friendly alternatives. For the first step, kiln dried pine sapwood (Pinus sylvestris L.) was selected. Sapwood comes from the outer layer of the trunk of the tree, where water and nutrients are transported. Therefore, this wood is particularly vulnerable to vermin attack. The wood samples were impregnated with ethanol- or isopropanol-based silicon and titanium alcoholate solutions in a vacuum. The alcoholates can be hydrolysed and heterogeneous mixtures of substances, so-called suspensions, result which contain nanoparticles.
The so-called sol-gel process was used for this purpose. Scientists call a suspension "sol" if it contains very small solid particles (with a size of a few nanometres) in a liquid medium. The sol-gel process is a synthesis method in which initially a sol is formed by a hydrolysis and condensation reaction, and then a solid gel is formed by crosslinking the sol particles. "These gels cover the cells of the wood evenly in thin layers," says Shabir Mahr. "We find them mainly in the cell cavities, the lumen. But they are also detectable in the cell walls."
As the researchers report in an article for the journal "International Biotdeterioration & Biodegradation"* the embedding leads to the materials having a significantly measurable protection against wood-destroying brown rot fungi (Coniophora puteana and Poria placenta). The samples were first exposed to a ten-week laboratory experiment. "But the 16-week results also suggest that the protection is permanent," says Thomas Hübert.
The impregnated samples showed reduced moisture absorption by up to 50 percent (compared to non-treated wood), while the flexural strength increased by up to 40 percent. The material also exhibits improved fire resistance. "Ideally," says Shabir Mahr, " the flame-retardant effect was up to 80 percent." Carbon monoxide formation as well as smoke development could be significantly reduced.
The scientists see great potential in their method which ultimately uses non-toxic materials that do not release toxic substances, as well in the easy handling and the relatively low cost. In the next step, they will look to replace the solvents currently used such as ethanol or isopropanol with water.
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