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Thermal Modification of Wood: Process and Properties

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A special issue of Forests (ISSN 1999-4907). This special issue belongs to the section "Wood Science and Forest Products".

Deadline for manuscript submissions: closed (5 January 2022) | Viewed by 22303

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Special Issue Editors

Dr. Carmen-Mihaela Popescu

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Petru Poni Institute of Macromolecular Chemistry of the Romanian Academy, 700487 Iasi, Romania
Interests: thermal and chemical modification of wood; degradation processes affecting historic wood; the effect of different degradative factors and the degradation/ageing mechanisms involved in wood degradation; formulations with superhydrophobic and antibacterial properties for wood; wood-based products and other organic substrate protection; cellulose nanocrystals, lignin nanoparticles and silica based nano-composites; pickering emulsion polymerization
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Prof. Dr. Holger Militz

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Department of Wood Biology and Wood Products, Georg-August-Universität, Büsgenweg 4, 37077 Göttingen, Germany
Interests: thermal and chemical modification of wood; processes and properties; degradation patterns by abiotic and abiotic influences; reactions on cell wall level; application of modified wood

Special Issue Information

Dear Colleagues,

Even though wood is widely used in many applications, due to its structure (cellulose, hemicelluloses, and lignin, and some low amounts of extractives and/or resins), its resistance to different environmental factors (biodegradation, weathering conditions) is sometimes limited.

Thermal treatment is largely used to improve the physical and biological characteristics of wood for some particular purposes, such as dimensional stability, reduced moisture uptake and increased durability, if it is to be exposed to chemicals or biological agents, or to frequently be used under natural environmental conditions. During the thermal treatment process, wooden structures undergo transformations induced by dehydration, decarboxylation, and oxidation reactions, coupled with heat and mass transfer. If a certain amount of relative humidity is present in the medium during the treatment, the reactions develop faster, and wood also undergoes hydrolysis reactions, and the crystallization of wood cellulose might be affected. All the reactions occurring in wood structure during the treatment, as well as its final properties, are directly related to treatment temperature, relative humidity during the treatment, presence or absence of oxygen (i.e., vacuum treatment), treatment time and by the nature of the wood species used. The resulting materials present lower hygroscopicity with a major influence on both dimensional stability and durability.

The aim of this Special Issue is to update the latest processes on thermal modification of wood and wood-based products with improved/modified treatment time, temperature, relative humidity or vacuum during the treatment, and to highlight the properties of the new materials. Therefore, we encourage review papers and research articles dealing with new processes for thermal modification of wood, improved or modified parameters, as well as physical, chemical, aesthetical, and sorption properties of the new materials and the use of thermally modified wood in wood-based composites.

Dr. Carmen-Mihaela Popescu
Prof. Dr. Holger Militz
Guest Editors

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Keywords

Wood thermal treatment
Thermal modification processes
Thermally modified wood properties
Thermally modified wood-based products

Published Papers (9 papers)

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Research

15 pages, 4945 KiB

Open AccessArticle

The Influence of High-Temperature and -Pressure Treatment on Physical Properties of Albizia falcataria Board

by Treza Chandra Julian, Hiroatsu Fukuda and Didit Novianto

Forests 2022, 13(2), 239; https://doi.org/10.3390/f13020239 - 04 Feb 2022

Cited by 5 | Viewed by 2144

Abstract

Albasia (Albizia falcataria), known as sengon wood, is a fast-growing tree species commonly found in Indonesian forests and community plantations. However, the low-density, hardness, and strength significantly restrict its commercial application. The purpose of this study was to determine the influence of densification on the physical properties of Albizia falcataria under high-temperature and -pressure. Different temperatures were applied to the Albizia falcataria board (100 °C, 120 °C, 140 °C, sandwich 140 °C). The densification process influences the density properties, color changes, thickness, compression ratio, equilibrium moisture content, and anatomical properties of the material. With this procedure, the density can be increased to 0.62 kg/L, a gain of approximately 112.78% over untreated wood. The density of wood increases, resulting in the decomposition of its chemical components, especially hemicellulose, which darkens the wood color and stabilizes equilibrium moisture control. As a result, the thermal compression modification treatment under high-temperature and -pressure is a highly effective method for enhancing the physical properties of fast-growing wood species, such as Albizia falcataria. Full article

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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13 pages, 15947 KiB

Open AccessArticle

Application of Temperature and Process Duration as a Method for Predicting the Mechanical Properties of Thermally Modified Timber

by Demiao Chu, Redžo Hasanagić, Atif Hodžić, Davor Kržišnik, Damir Hodžić, Mohsen Bahmani, Marko Petrič and Miha Humar

Forests 2022, 13(2), 217; https://doi.org/10.3390/f13020217 - 31 Jan 2022

Cited by 6 | Viewed by 1995

Abstract

This study aims to investigate the influence of thermal modification (TM) on the physical and mechanical properties of wood. For this purpose, the experimental part focused on selected influential parameters, namely temperature, residence time, and density, while the four-point bending strength is obtained as the output parameter. The obtained experimental data are stochastically modeled and compared with the model created by genetic programming (GP). The classical mathematical analysis obtained treatment parameters in relation to the maximum bending strength (T = 187 °C, t = 125 min

ρ

= 0.780 g/cm³) and compared with the results obtained by genetic algorithm (GA) (T = 208 °C, t = 122 min, and

ρ

= 0.728 g/cm³). It is possible to obtain models that describe experimental results well with stochastic modeling and evolutionary algorithms. Full article

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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14 pages, 1928 KiB

Open AccessArticle

Prediction of Mechanical Properties of Thermally Modified Wood Based on TSSA-BP Model

by Ning Li and Wei Wang

Forests 2022, 13(2), 160; https://doi.org/10.3390/f13020160 - 21 Jan 2022

Cited by 8 | Viewed by 2913

Abstract

In order to demonstrate whether the sparrow search algorithm can show good performance in optimization, this paper improves the prediction model by this algorithm and predicts the change data of wood mechanical properties under different conditions, which better reflects the connection between the process parameters of wood heat treatment and the change of wood mechanical properties. The article takes the five main mechanical property parameters of thermally modified wood: compressive strength along the grain, flexural strength, flexural elastic modulus, radial hardness, and tangential hardness, respectively, as the objects of study and improves the sparrow search algorithm by Tenting chaotic mapping and then optimizes the Back Propagation (BP) network model by this algorithm. The results show that the number of iterations of the optimized Tent-Sparrow search algorithm-Back Propagation network model (TSSA-BP) is only one-eighth that of the original BP network model, and the convergence speed is greatly improved, the root mean square error of the TSSA-BP model is at least one-half times that of the original BP model, and the optimized model fits the original data better in terms of predicted values; thus, this article provided a feasible prediction algorithm for the field related to the mechanical property changes of wood after heat treatment. Full article

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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13 pages, 4151 KiB

Open AccessFeature PaperArticle

Assessment of Structural Differences between Water-Extracted and Non-Extracted Hydro-Thermally Treated Spruce Wood by NIR Spectroscopy

by Carmen-Mihaela Popescu, Nanami Zeniya, Kaoru Endo, Takuma Genkawa, Miyuki Matsuo-Ueda and Eiichi Obataya

Forests 2021, 12(12), 1689; https://doi.org/10.3390/f12121689 - 02 Dec 2021

Cited by 3 | Viewed by 1487

Abstract

Sitka spruce wood samples were subjected to different conditions of hydro-thermal treatment by varying the relative humidity (RH) and period of exposure at a constant temperature of 120 °C. Near infrared (NIR) spectroscopy, principal component analysis (PCA) and two dimensional correlation spectroscopy (2D-COS) were employed to examine the structural changes which occur in the wood samples during the applied treatment conditions and to quantify the differences between non-extracted and water-extracted wood specimens after the treatment. Modifications were dependent on the amount of water molecules present the medium and also on treatment time. Higher variations were observed for samples treated at higher RH values and for longer periods. At the same time, it was also observed that during the hydro-thermal treatment a high amount of extractives remain in the wood structure, extractives which vary in quantity and composition. PCA and 2D-COS made it possible to discriminate modifications in the wood samples according to treatment time and relative humidity. Non-extracted and water-extracted samples were also examined to identify the sequential order of band modification. Full article

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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24 pages, 7115 KiB

Open AccessFeature PaperArticle

Correlation of Studies between Colour, Structure and Mechanical Properties of Commercially Produced ThermoWood^® Treated Norway Spruce and Scots Pine

by Petteri Torniainen, Carmen-Mihaela Popescu, Dennis Jones, Alexander Scharf and Dick Sandberg

Forests 2021, 12(9), 1165; https://doi.org/10.3390/f12091165 - 28 Aug 2021

Cited by 22 | Viewed by 2675

Abstract

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 article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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20 pages, 8260 KiB

Open AccessArticle

Haptic and Aesthetic Properties of Heat-Treated Modified Birch Wood

by Vlastimil Borůvka, Přemysl Šedivka, David Novák, Tomáš Holeček and Jiří Turek

Forests 2021, 12(8), 1081; https://doi.org/10.3390/f12081081 - 13 Aug 2021

Cited by 4 | Viewed by 2024

Abstract

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

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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12 pages, 2066 KiB

Open AccessArticle

Photodegradation of Unmodified and Thermally Modified Wood Due to Indoor Lighting

by Dace Cirule, Edgars Kuka, Matiss Kevers, Ingeborga Andersone and Bruno Andersons

Forests 2021, 12(8), 1060; https://doi.org/10.3390/f12081060 - 09 Aug 2021

Cited by 9 | Viewed by 2561

Abstract

Considering interior applications, sunlight, both direct through open window and through window glass, and artificial lighting are the main sources of radiation possessing sufficient energy to trigger photodegradation processes in wood. LED lamps, which emit mostly visible light, are becoming the dominant artificial light source in various interiors. In the present study, photodegradation of thermally modified (TM) and unmodified (UM) ash (Fraxinus excelsior), aspen (Populus tremula), and pine (Pinus sylvestris) due to exposure to UV radiation and LED lamps was evaluated and compared by analysing wood discolouration (CIELAB colour space), changes in reflectance and FTIR spectra, and formation of water-soluble components. The results show that, apart from UV radiation, LED lamps may cause considerable photodegradation of both TM and UM wood resulting in visually perceptible colour change, alteration in chemical structure and formation of water-soluble components. Improved photo-stability was observed for TM wood exposed to UV radiation, while even more changes in FTIR spectra were detected for TM than UM wood in the experiment with LED lamps. Comparing TM and UM wood, the changes due to photodegradation were quite similar for TM wood of all species while significant differences were observed in the case of UM wood. Full article

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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18 pages, 3643 KiB

Open AccessArticle

What Is the Supply and Demand for Coloured Wood Products? An Empirical Study in Slovakian Practice

by Mariana Sedliačiková, Mária Moresová, Patrik Aláč and Denisa Malá

Forests 2021, 12(5), 530; https://doi.org/10.3390/f12050530 - 25 Apr 2021

Cited by 14 | Viewed by 2134

Abstract

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

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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9 pages, 1232 KiB

Open AccessArticle

Effect of a Combination of Moderate-Temperature Heat Treatment and Subsequent Wax Impregnation on Wood Hygroscopicity, Dimensional Stability, and Mechanical Properties

by Lin Yang and Hong-Hai Liu

Forests 2020, 11(9), 920; https://doi.org/10.3390/f11090920 - 23 Aug 2020

Cited by 25 | Viewed by 2243

Abstract

Wood is an environmentally friendly material, but some natural properties limit its wide application. To study the effect of a combination of heat treatment (HT) and wax impregnation (WI) on wood hygroscopicity, dimensional stability, and mechanical properties, samples of Pterocarpus macrocarpus Kurz wood were subjected to HT at a moderate temperature of 120 °C and a high temperature of 180 °C, for a 4 h duration. Subsequently, half of the 120 °C HT samples were treated with WI at 90 °C. The results showed that 180 °C HT and WI decreased the capacity of adsorption and liquid water uptake and swelled the wood significantly, while WI had the biggest reduction. The effect of 120 °C HT was significant only on decreasing the capacity of adsorption and the swelling of liquid water uptake. The bending strength (MOR) of wood decreased only after 180 °C HT, and 120 °C/4h HT and WI had no significant influence on MOR. The bending stiffness (MOE) increased significantly after 180 °C HT and WI, while 120 °C/4h HT had no significant influence on MOE. Therefore, the combination of moderate-temperature HT can act synergistically in the improvement of certain aspects of wood properties such as capacity of water adsorption and liquid water uptake. WI effectively improved wood hygroscopicity, dimensional stability, and mechanical properties. Full article

(This article belongs to the Special Issue Thermal Modification of Wood: Process and Properties)

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