Abstract
European ash (Fraxinus excelsior L.) is one of the species commonly used for wood thermal modification that improves its performance. The presented research aimed to investigate a moisture-dependent strength anisotropy of thermally-modified European ash in compression. Wood samples were modified at 180 °C and 200 °C. Their mechanical parameters were determined in the principal anatomical directions under dry (moisture content of 3%) and wet (moisture content above fibre saturation point) conditions. Effect of heat treatment temperature and moisture content on the ash wood mechanical parameters concerning each anatomical direction were determined. The results show that thermal treatment kept the intrinsic anisotropy of wood mechanical properties. It decreased wood hygroscopicity, which resulted in improved strength and elasticity measured for wet wood when compared to untreated and treated samples. Higher treatment temperature (200 °C) increased wood elasticity in compression in all the anatomical directions despite wood moisture content during the measurements. Multivariate analysis revealed that the modification temperature significantly affected the modulus of elasticity perpendicular to the grain, while in the case of compression strength, the statistically significant effect was observed only parallel to the grain. The results obtained can be useful from an industrial perspective and can serve as part of a database for further modelling purposes.
Highlights
Favoured by the increased pressure on the replacement of biocides with more environmentally friendly preservatives, thermal modification has been developed to increase wood biological durability and dimensional stability, and reduce its hygroscopicity
Thermal treatment of ash resulted in a decrease in wood density from 591 ± 53 kg × m−3 to 579 ± 51 and 563 ± 45 kg × m−3 for wood modified at 180 and 200 ◦ C, respectively, with a concomitant mass loss of about 1.8% for 180 ◦ C and 6% for 200 ◦ C
Besides the apparent influence of moisture content and the anatomical direction on modulus of elasticity (MOE), the results indicate a significance (p < 0.05) of a modification temperature on this parameter only perpendicular to the grain (Table 2)
Summary
Favoured by the increased pressure on the replacement of biocides with more environmentally friendly preservatives, thermal modification has been developed to increase wood biological durability and dimensional stability, and reduce its hygroscopicity. It is commercially by far the most advanced wood modification technology in the market with the highest economic importance. Numerous production plants worldwide utilise heat treatment techniques carrying out modification processes in open and closed systems that differ mainly in the applied treatment temperature Thermal modification results in chemical changes in wood cell wall polymers. An organic-acid mediated process causes decomposition of hemicelluloses. The extent of chemical reactions is Materials 2020, 13, 1647; doi:10.3390/ma13071647 www.mdpi.com/journal/materials
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