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Influence of wood structure on wood properties of tropical species
Baar, Jan
The presented thesis is focused on aesthetical and acoustic properties of tropical wood. The discussed tropical species are utilized in Europe mainly for their unusual appearance and colour in joinery and furniture production. The irreplacable acoustic properties like low internal friction predestine specific species for production of musical instruments. The colour of six selected tropical species - jatoba (Hymenea courbaril L.), massaranduba (Manilkara bidentata A. Chev.), muiracatiara (Astronium graveolens Jacq.), angelim amargoso (Vatairea spp.), angelim pedra (Hymenolobium spp.) and angelim vermelho (Dinizia excelsa Ducke) - were analyzed and expressed in CIEL*a*b* colour space. Subsequently the samples were exposed to artificial sunlight and the light-induced discolouration rate and progress was evaluated. The colour change was rapid in zhe first hours of exposure and all species showed a similar trend in lightness (L*) change - initial darkening replaced by gradual fading. The change in chromatic parameters (a* and b*) change was specific for individual species and probably related to the chemical composition of the extractives. The overall discolouration of lighter coloured wood samples is more distinctive. The extractive content of tropical species is usually high and determines wood colour. The wood of African Padauk (Pterocarpus soyauxii Taub) and Jatoba (Hymenaea courbaril L.) was extracted in a mixture of methanol, water and acetone. The total extractives and total phenolic content was measured. Extractive content was related to colour parameters and above all to lightness in both species. The increase in extractive content leads to a lighter and more yellow colouration of Jatoba wood while African Padauk was darker with a higher amount of extractives. In Jatoba, a positive correlation was found between extractive content and the rate of discolouration. The extractives are responsible for intensive discolouration of tropical woods in contrast to light coloured woods from the temperate zone, where the yellowing is primarily caused by lignin photodegradation. The sound velocity was measured by ultrasound and resonance longitudinal method in heartwood of doussie (Afzelia bipindensis), merbau (Intsia bijuga), wengé (Millettia stuhlmannii Taub.), muiracatiara (Astronium graveolens Jacq.) and zebrano (Microberlinia brazzavillensis). The sound velocity was not related to the density in any case. Although anatomical structural characteristics as fiber length or ray ratio positively influenced it. With longer fibers and slender rays the sound velocity increased. Other acoustic properties like dynamic young modulus of elasticity, specific modulus of elasticity, internal friction and acoustic conversion efficiency (ACE) were determined and were related to measured anatomical features. The differences in composition of wood tissue of individual species were the main source of departure between the assessed relationships. The wood with longer fibers and lower frequency of rays reached lower values of internal friction. The stiffness of wood characterized by the dynamic modulus of elasticity was correlated with fiber and ray tissue. A higher volume of fibers influenced the stiffness positively, while ray volume had the opposite effect on stiffness. Positive effects of fiber length and ray ratio were found for dynamic modulus of elasticity and for acoustic conversion efficiency as well. Both values increased with longer fibers and higher ray ratio. The sound velocity is the basis of some nondestructive techniques for determination of wood mechanical properties. The comparison of density and three dynamic moduli of elasticity (longitudinal and flexural resonance method and ultrasound method) in prediction of mechanical properties was carried out. The density was evaluated as a poor predictor when the samples are not straight-grained and defect free. The interlocked grain often occurred in tropical species and its presence influences the mechanical properties of wood but not wood density. The dynamic moduli of elasticity showed strong correlation with the static modulus of elasticity. Among nondestructive techniques, the weakest correlation was found for the ultrasound method, which is probably partially explained by different measuring mechanism. The prediction of the modulus of rupture is less accurate when the dynamic modulus of elasticity is compared with the static modulus of elasticity; however, it was still good in comparison with the density model.
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