Search Results (20)

This work is devoted to the changes in polysaccharides in thermally treated wood after its accelerated aging with the aim of its optimal utilization after its original use has ended. Spruce wood samples were treated by the Thermowood process at temperatures of 160 °C, 180 °C, and 210 °C and subjected to accelerated aging in wet mode. The influence of treatment temperature and accelerated aging was monitored by wet chemistry, high-performance liquid chromatography (HPLC), X-ray diffraction (XRD), size exclusion chromatography (SEC), and Fourier-transform infrared spectroscopy (FTIR). During thermal treatment, hemicelluloses are mainly degraded. At the temperature of 210 °C, aromatic compounds formed as degradation products of lignin and hemicelluloses bind to cellulose fibers and increase cellulose yield. Preferential decomposition of the amorphous portion of cellulose leads to an increase in its crystallinity, while higher temperatures cause degradation of the crystal lattice. The degree of polymerization in both cellulose and hemicelluloses decreases due to the cleavage of glycosidic bonds. Accelerated aging does not significantly affect the changes in polysaccharides. The results obtained can be used in the processing of cellulose and hemicelluloses from thermally modified wood at the end of its life cycle in various industrial fields. Full article

Thermal modification improves the properties of wood, especially its stability and durability. We thermally treated spruce wood with the Thermowood process at three temperatures (160 °C, 180 °C, and 210 °C) and subjected it to accelerated aging in wet mode. We evaluated the chemical composition (wet chemistry, infrared spectroscopy), color, surface morphology, and wetting of the wood surface with water. Thermal treatment caused a significant decrease in hemicelluloses (up to 72.39% at a temperature of 210 °C), which initiated an increase in the content of more resistant wood components—cellulose and lignin. With accelerated aging, the hemicellulose content decreased by another 5%. The most significant differences between the infrared spectra of thermally modified wood before and after exposure to accelerated aging were in the absorption bands of lignin (1509 and 1596 cm⁻¹) and in the region of carbonyl groups between 1800 and 1630 cm⁻¹. Thermal treatment also caused a change in the color of the wood to dark brown; the overall color difference ΔE increased several times. The thermal-induced shortening of polysaccharide fibers and reduction in their width were even more manifested during accelerated aging. This work contains new knowledge about the properties critical for the reuse of thermally modified wood after accelerated aging, simulating the end of its life cycle. Full article

Thermal modification is an environmentally friendly process that does not utilize chemical agents to enhance the stability and durability of wood. The use of thermally modified wood results in a significantly extended lifespan compared with untreated wood, with minimal maintenance requirements, thereby reducing the carbon footprint. This study examines the impact of varying modification temperatures (160, 180, and 210 °C) on the lignin of spruce wood using the ThermoWood process and following the accelerated aging of thermally modified wood. Wet chemistry methods, including nitrobenzene oxidation (NBO), size exclusion chromatography (SEC), thermogravimetry (TG), differential thermogravimetry (DTG), and Fourier transform infrared spectroscopy (FTIR), were employed to investigate the alterations in lignin. At lower modification temperatures, the predominant reaction is the degradation of lignin, which results in a reduction in the molecular weight and an enhanced yield of NBO (vanillin and vanillic acid) products. At elevated temperatures, condensation and repolymerization reactions become the dominant processes, increasing these traits. The lignin content of aged wood is higher than that of thermally modified wood, which has a lower molecular weight and a lower decomposition temperature. The results demonstrate that lignin isolated from thermally modified wood at the end of its life cycle is a promising feedstock for carbon-based materials and the production of a variety of aromatic monomers, including phenols, aromatic aldehydes and acids, and benzene derivatives. Full article

Electron microscopy (EM) is a key tool for studying the microstructure of wood; however, observing uncoated samples poses a challenge due to surface charging. This study aims to identify the critical voltage that allows for the effective observation of uncoated wood samples without significant loading. As part of the experiment, samples of different wood species were tested, including Acacia (Robinia pseudoacacia L.), Oak (Quercus robur L.), Maple (Acer pseudoplatanus spp.), Ash (Fraxinus excelsior L.), Spruce (Picea abies (L.) Karst.), Thermowood (Thermal modifed Spruce), Garapa (Apuleia leiocarpa), Ipé (Handroanthus spp.), Merbau (Intsia bijuga), and Massaranduba (Manilkara spp.). Several methods were tested for surface preparation for SEM analysis, including the use of a circular saw, a hand milling machine, and a microtome. The results show that the optimal voltage for observing uncoated wood samples varied depending on the wood species. Regarding the selection of wood species and the results obtained, it was found that uncoated samples could be effectively observed. This finding suggests that practical observations can be accelerated and more cost-effective, as all wood species exhibited the required voltage range of 1 kV to 1.6 kV. Additionally, it was determined that using a secondary electron detector was optimal for such observations, as it provided a sufficiently strong signal even at relatively low voltages. Conversely, when using a backscattered electron detector, it was more beneficial to use coated samples to achieve a sufficient signal at higher voltages. This study brings new knowledge that will facilitate further research and applications of electron microscopy in the study of other wood species or wood-based materials. Full article

The circular economy forces societies to take actions aimed at giving post-consumer products a “second life”. As we know, wood is perfect for this. Moreover, reusing wood helps keep carbon in circulation, thus limiting its emissions into the atmosphere. It turns out that extensive research on determining the durability of wood is very useful and valuable for one more reason. Well, they can be used to create a model to determine the usefulness of wood, which has only apparently lost its utility value during many years of exposure to external factors. The research subject was samples of wood impregnated with protection agents and modified, originating from many years of field tests. The aim of the research was to correlate the results of wood durability determined after a period of exposure in open space with the results of determining the potential usefulness of such wood. On this basis, a model for determining the value of post-consumer wood was created. As a main result of post-consumer wood analysis, the high durabilities against C. puteana with mass loss below 3% were noticed for acetylated, furfurylated, and CCA-treated wood. Moreover, high color stabilities (ΔE < 10) were observed for thermowood and furfurylated wood. Full article

This work examines the effect of thermal modification temperature (180, 200, and 220 °C) in comparison with reference (untreated) samples on selected optical properties of six tropical wood species—Sp. cedar (Cedrala odorata), iroko (Chlorophora excelsa), merbau (Intsia spp.), meranti (Shorea spp.), padouk (Pterocarpus soyauxii), and teak (Tectona grandis). The main goal is to expand the existing knowledge in the field of wood thermal modification by understanding the related degradation mechanisms associated with the formation of chromophoric structures and, above all, to focus on the change in the content of extractive substances. For solid wood, the CIELAB color space parameters (L*, a*, b*, and ΔE*), yellowness (Y), ISO brightness, and UV-Vis diffuse reflectance spectra were obtained. Subsequently, these wood samples were extracted into three individual solvents (acetone, ethanol, and ethanol-toluene). The yields of the extracted compounds, their absorption spectra, and again L*, a*, b*, ΔE*, and Yi parameters were determined. With increasing temperatures, the samples lose brightness and darken, while their total color difference grows (except merbau). The highest yield of extractives (mainly phenolic compounds, glycosides, and dyes) from thermally modified samples was usually obtained using ethanol. New types of extractives (e.g., 2-furaldehyde, lactones, formic acid, some monomer derivatives of phenols, etc.) are already created around a temperature of 180 °C and may undergo condensation reactions at higher temperatures. For padouk, merbau, teak, and partially iroko modified at temperatures of 200 and 220 °C, there was a detected similarity in the intensities of their UV-Vis DR spectra at the wavelength regions corresponding to phenolic aldehydes, unsaturated ketones, quinones, stilbenes, and other conjugated carbonyl structures. Overall, a statistical assessment using PCA sorted the samples into five clusters. Cluster 3 consists of almost all samples modified at 200 and 220 °C, and in the other four, the reference and thermally modified samples at 180 °C were distributed. The yellowness of wood (Y) has a very high dependence (r = 0.972) on its brightness (L*) and the yellowness index of the extractives in acetone Yi(Ac), whose relationship was described by the equation Y = −0.0951 × Y(Ac) + 23.3485. Full article

Black locust is a fast-growing deciduous tree species with multiple industrial purposes due to its valuable traits. However, the heterogeneity of colour distribution between sapwood and heartwood limits its application. Thermal modification is an environment-friendly technology for improving various wood properties, especially dimensional stability, decay resistance, and colour homogeneity. In this work, black locust (Robinia pseudoacacia L.) wood samples were thermally modified at temperatures of 160, 180, and 210 °C. Extractives and main wood components were analysed by wet chemical methods, colour was measured by spectrometry, and structural changes by Fourier transform infrared spectroscopy. The obtained results show that the darkening of black locust wood, unlike other wood species of the temperate zone, is mainly caused by changes in extractives. Their content decreases during thermal treatment, but new chromophores are formed, especially in quinones. Degradation of hemicelluloses and the partial degradation of cellulose also contribute to colour changes. At higher temperatures, condensation reactions can occur in lignin, leading to the formation of some chromophores. Statistical analysis confirmed that temperature can be considered a very significant factor affecting the colour of the wood surface. Full article

This article presents a proposal of thermal modification of Norway spruce and sycamore maple for special wood products, mainly for musical instruments. Selected physical and acoustical characteristics (PACHs), including the density (ρ), dynamic modulus of elasticity along the wood grain (E_L), specific modulus (E_sp), speed of sound along the wood grain (c_L), resonant frequency (f_r) and acoustic constant (A), logarithmic decrement (ϑ), loss coefficient (η), acoustic conversion efficiency (ACE), sound quality factor (Q), and the timbre of sound, were evaluated. These two wood species were chosen regarding their use in the production or repair of musical instruments. For the thermal modification, a similar process to the ThermoWood process was chosen. Thermal modification was performed at the temperatures 135 °C, 160 °C and 185 °C. The resonant dynamic method was used to obtain the PACHs. Fast Fourier transform (FFT) was used to analyze the sound produced. The changes in the observed wood properties depended on the treatment temperature. Based on our results of all properties, the different temperature modified wood could find uses in the making of musical instruments or where the specific values of these wood characteristics are required. The mild thermal modification resulted in a decrease in mass, density, and increased speed of sound and dynamic modulus of elasticity at all temperatures of modification. The thermally modified wood showed higher sound radiation and lower loss coefficients than unmodified wood. The modification also influenced the timbre of sound of both wood species. Full article

The paper assessed the impact of the heat treatment of spruce wood, the (radial and tangential) side of the specimens exposed to fire, and the type of material (prism—higher density, floor—lower density) on the combustion process and the rate of fire spread. Five groups of specimens were used—untreated spruce wood specimens, two groups of heat-treated spruce wood specimens from the prism (higher density specimens), and two groups of heat-treated spruce wood specimens from the floor (lower density specimens). In one group, the flame was applied to the radial side, and in the other group to the tangential side of the specimens. The effect on the combustion process was assessed based on the parameters of mass loss and mass loss rate over time. The effect on the rate of fire spread across the specimens was assessed by the parameter fire spread rate. These parameters were determined using a simple test method where the specimens were exposed to a direct flame at an angle of 45°. To complement the results and to assess the processes involved, the temperatures at the specimen surfaces were also measured during the experiment. The main achieved results of the study are the findings on how the heat treatment, the density, and the side of the wood along which the fire spreads affect the burning process of the wood. The results indicated a significant effect of the density of the spruce thermowood on its combustion process. The higher density radial specimens exhibited a higher mass loss rate, and the overall average mass loss of the higher density samples was 27% of the original mass higher than that of the lower density samples. Additionally, the results suggested that the heat treatment of lower-density spruce wood (floor) does not significantly affect the mass loss and the mass loss rate. The difference in the overall average mass loss of the thermowood of floor and untreated wood samples was less than 2%, which is statistically insignificant. It was also found that for thermowood, fire will spread faster on the tangential side, where the fire spread rate is 29% higher compared with the radial side (for the floor samples). Based on the findings of other authors in a similar field, the results confirm that heat-treated spruce wood is more easily ignitable than untreated wood, which was proven by the spontaneous combustion of most of the thermowood samples during the experiment compared with the untreated wood samples. Full article

The thermal modification of wood has become the most-commonly commercialised wood modification process globally, with the ThermoWood^® process currently being the most dominant. As with all commercial processes, there is a need to have a robust quality control system, with several small–scale studies undertaken to date investigating quality control using a range of analytical methods, culminating in a multi-year assessment of colour as a means of quality control. This study, as an extension to this multi-year assessment, further explores the colour of Norway spruce and Scots pine commercially modified by the ThermoWood^® S and D processes, respectively, along with the mechanical properties and structural characterisation by Fourier transform infrared (FT–IR) spectroscopy and principal component analysis (PCA) to ascertain further correlations between colour and other measurable properties. Infrared spectroscopy indicated modifications in the amorphous carbohydrates and lignin, whereas the use of PCA allowed for the differentiation between untreated and modified wood. Colour measurements indicated reduced brightness, and shifting toward red and yellow colours after thermal modification, hardness values decreased, whereas MOE and MOR values were similar for modified wood compared to unmodified ones. However, by combining the colour measurements and PC scores, it was possible to differentiate between the two modification processes (Thermo–S and Thermo–D). By combining the mechanical properties and PC scores, it was possible to differentiate the untreated wood from the modified ones, whereas by combining the mechanical properties and colour parameters, it was possible to differentiate between the three groups of studied samples. This demonstrates there is a degree of correlation between the test methods, adding further confidence to the postulation of using colour to ensure quality control of ThermoWood^®. Full article

This paper deals with the effect of heat treatment on the selected physical properties of birch wood. Five stages of heat treatment were used, ranging from 160 °C to 200 °C, in 10 °C increments, having a peak treatment duration of 3 h for each level. Primarily, changes in thermal characteristics, namely conductivity, diffusivity, effusivity, volume heat capacity, changes in colour and gloss parameters, mass loss due to modification and different moisture content in wood under given equilibrium climatic conditions, were monitored. The ISOMET 2114 analyser was used to measure the thermal characteristics. The measurement principle of this analyser is based on the analysis of the thermal response of the analysed material to pulses of heat flow. Measurements of colour, gloss, density and moisture content were carried out according to harmonised EN standards. The aim was to experimentally verify the more or less generally known more positive perception of heat-treated wood, both by touch and sight, i.e., the warmer perception of darker brown shades of wood. In terms of thermal characteristics, the most interesting result is that they gradually decrease with increasing treatment temperature. For example, at the highest treatment temperature of 200 °C, there is a decrease in thermal conductivity by 20.2%, a decrease in volume heat capacity by 15.0%, and a decrease in effusivity by 17.7%. The decrease in thermal conductivity is nearly constant at all treatment levels, specifically at this treatment temperature, by 6.0%. The fact mentioned above is positive in terms of the tactile perception of such treated wood, which can have a positive effect, for example, in furniture with surface application of heat-treated veneers, which are perceived positively by the majority of the human population visually or as a cladding material in saunas. In this context, it has been found that the thermal modification at the above-mentioned treatment temperature of 200 °C results in a decrease in brightness by 44.0%, a decrease in total colour difference by 38.4%, and a decrease in gloss (at an angle of 60°) by 18.2%. The decrease in gloss is only one essential negative aspect that can be addressed by subsequent surface treatment. During the heat treatment, there is also a loss of mass in volume, e.g., at a treatment temperature of 200 °C and subsequent conditioning to an equilibrium moisture content in a conditioning chamber with an air temperature of 20 ± 2 °C and relative humidity of 65 % ± 5%, there was a decrease by 7.9%. In conclusion, the experiments clearly confirmed the hypothesis of a positive perception of heat-treated wood in terms of haptics and aesthetics. Full article

The paper deals with the issue of the colour tones of wood and furniture products. The main aim is to identify the supply of the colour tones of wood and furniture products and to map the interest in these wood colour tones at potential customers in Slovakia. By means of two independent surveys of supply and demand for colour tones of wood and furniture products, various shortcomings have been identified. It is necessary to increase the supply of wood and furniture products with the natural colour of the wood and at the same time in colour tones of grey, white and brown. The current demand for thermowood and modified alder wood is significantly higher than the supply of such products in the Slovak market. The results of the study represent an opportunity for Slovak woodworking and furniture enterprises to adjust their range of products according to the needs of potential customers, which will bring them higher turnover and help to overcome the current problems associated with the “COVID crisis”. Full article

Thermal modification is an environmentally friendly process in which technological properties of wood are modified using thermal energy without adding chemicals, the result of which is a value-added product. Wood samples of three tropical wood species (meranti, padauk, and merbau) were thermally treated according to the ThermoWood process at various temperatures (160, 180, 210 °C) and changes in isolated lignin were evaluated by nitrobenzene oxidation (NBO), Fourier-transform infrared spectroscopy (FTIR), and size exclusion chromatography (SEC). New data on the lignins of the investigated wood species were obtained, e.g., syringyl to guaiacyl ratio values (S/G) were 1.21, 1.70, and 3.09, and molecular weights were approx. 8600, 4300, and 8300 g·mol⁻¹ for meranti, padauk, and merbau, respectively. Higher temperatures cause a decrease of methoxyls and an increase in C=O groups. Simultaneous degradation and condensation reactions in lignin occur during thermal treatment, the latter prevailing at higher temperatures. Full article

Chemical composition influences the calorific power of wood, mainly due to the calorific power of structural compounds and extractives. Heat treatment changes the chemical composition of treated wood. This work studies the relationship between chemical composition and calorific power improvement by heat treatment. Samples were heat-treated by the ThermoWood process ^® for 1 h and 2 h. High heating value (HHV) and chemical composition; lignin, cellulose, hemicelluloses and extractives in dichloromethane, ethanol, and water were determined. The HHV of untreated wood ranged between 18.54–19.92 MJ/kg and increased with heat treatment for all the tested species. A positive linear correlation was found between HHV and Klason lignin (R² = 0.60). A negative trend was observed for holocellulose, cellulose, and hemicelluloses content against HHV, but with low determination coefficients for linear regression. The best adjust for polysaccharides was found for hemicelluloses content. A positive correlation could be found for dichloromethane extractives (R² = 0.04). The same was obtained in relation to ethanol extractives with R² = 0.20. For water and total extractives, no clear positive or negative trends could be achieved. The results showed that the HHV of wood increased with heat treatment and that this increase was mainly due to the increase in lignin content. Full article

Thermal modification is an environmentally friendly method to improve dimensional stability, durability, and aesthetic properties of wood. Changes in lignin as one of the main wood components markedly influence wood product properties and recycling possibilities of thermowood at the end of its life cycle. Teak and iroko wood samples were thermally treated at the temperatures of 160 °C, 180 °C and 210 °C following the Thermowood process. Dioxane lignin was isolated from treated and untreated wood and analysed by nitrobenzene oxidation (NBO), size exclusion chromatography (SEC) and Fourier transform infrared spectroscopy (FTIR). The yields of both acid-insoluble and dioxane lignins increased with an increasing treatment temperature. Dioxane lignins are GS-types containing more guaiacyl units compared to syringyl ones with S/G ratios of 0.91 and 0.84, respectively. In the process of thermal modification, several degradation and condensation reactions were observed. The cleavage of methoxyl groups and side chains, oxidation reactions, cleavage of the β-O-4 ether linkage and cross-linking radicals arising at higher temperatures were all confirmed. However, during the thermal treatment, teak lignin changed in a different way than iroko lignin, e.g., the molecular weight of iroko lignin decreased at all applied temperatures while it increased at 180 °C and 210 °C in teak lignin, and the change in S/G ratio and the cleavage of alkyl-aryl bonds are different in both wood species. Full article