Heat-treated Wood Samples Research Articles

Effect of Heat Treatment on Hygroscopicity of Chinese Fir (Cunninghamia lanceolata [Lamb.] Hook.) Wood

Chinese fir (Cunninghamia lanceolata [Lamb.] Hook.) is a widely planted species of plantation forest in China, and heat treatment can improve its dimensional stability defects and improve its performance. The wood samples were heat-treated at various temperatures (160, 180, 200, and 220 °C) for 2 h. To clarify the effect of heat treatment on wood hygroscopicity, the equilibrium moisture content (EMC) was measured, the moisture adsorption and desorption rates were determined, the hygroscopic hysteresis was examined, and the Guggenheim, Anderson, and de Boer (GAB) model was fitted to the experimental data. The moisture absorption isotherms of all samples belonged to the Type II adsorption isotherm, but the shape of the desorption isotherm was more linear for heat-treated wood samples, especially when the heat treatment temperature was higher. According to the results analyzed with ANOVA, there were significant differences in equilibrium moisture content between the control samples and the heat-treated samples under the conditions of 30%, 60%, and 95% relative humidity (RH, p < 0.05), and the results of multiple comparisons were similar. The decrease in hygroscopicity was more pronounced in wood treated at higher temperatures. The EMC of the 160–220 °C heat-treated samples of the control samples was 14.00%, 22.37%, 28.95%, and 39.63% lower than that of the control sample at 95% RH. Under low RH conditions (30%), water is taken up mainly via monolayer sorption, and multilayer sorption gradually predominates over monolayer sorption with the increase in RH. The dynamic vapor sorption (DVS) analysis indicated that the heat-treated wood revealed an increase in isotherm hysteresis, which was due to the change in cell wall chemical components and microstructure caused by heat treatment. In addition, the effective specific surface area of wood samples decreased significantly after heat treatment, and the change trend was similar to that of equilibrium moisture content.

A Primary Study on Mechanical Properties of Heat-Treated Wood via in-situ Synthesis of Calcium Carbonate

This study aims to improve the value of fast-growing wood and extend the heat-treated wood utilization using inorganic calcium carbonate (CaCO3) crystals via an in-situ synthesis method. CaCl2 and Na2CO3 solutions with a concentration ratio of 1:1 were successively introduced into the thermally modified poplar wood obtained by steam heat treatment (HT) at 200°C for 1.5 and 3 h, resulting in the in-situ synthesis of CaCO3 crystals inside the heat-treated wood. The filling effect was best at the concentration of 1.2 mol/L. CaCO3 was uniformly distributed in the cell cavities of the heat-treated wood, and some of the crystals were embedded in the fissures of the wood cell walls. The morphology of CaCO3 crystals was mainly spherical and rhombic polyhedral. Three main types of CaCO3 crystals were calcite, vaterite, and aragonite. The HT of poplar wood at 200°C resulted in degrading the chemical components of the wood cell wall. This degradation led to reduced wood mechanical properties, including the surface hardness (HD), modulus of rupture (MOR), and modulus of elasticity (MOE). After CaCO3 was in-situ synthesized in the heat-treated wood, the HD increased by 18.36% and 16.35%, and MOR increased by 14.64% and 8.89%, respectively. Because of the CaCO3 synthesization, the char residue of the 200°C heat-treated wood samples increased by 9.31% and the maximum weight loss rate decreased by 19.80%, indicating that the filling with CaCO3 cannot only improve the mechanical properties of the heat-treated wood but also effectively enhance its thermal stability.

Modelling some physical and mechanical properties of heat-treated Scotch pine using artificial neural network

In this study, some physical and mechanical properties of yellow pine wood (Pinus sylvestris), which is used extensively in furniture industry, were tested after heat treatment. The findings obtained were modelled by artificial neural network (ANN) and interval values related to temperature and time variations were tried to be estimated. This study, which makes it easier to reach intermediate values, aims to save the relevant researchers from trial load all of the heating parameters during the furniture design/production stages. In the study scotch pine samples were heat-treated at 150, 160, 170, 180, 190 and 200 °C for 2, 4 and 6 hours, under normal atmosphere conditions. Color changes, weight losses and compression strength parallel to grain values of heat-treated samples were determined. After experimental study, modelling procedure was performed by ANN using two different learning algorithm- Levenberg-Marquardt (LM) and Scaled Conjugate Gradient (SCG) algorithm- 15 different hidden neurons. The best model was obtained from 2-7-6 structure using LM learning algorithm. Mean absolute percentage error (MAPE) of the best model was found below 8.0% for estimated color parameters. The weight loss and compression strength parallel to grain were 5.79% and 1.50%, respectively. It was concluded that ANN can be used successfully to predict all studied parameters of heat-treated wood samples.

Weathering performance of thermally modified wood coated with polyacrylate containing olive leaf extract as a bio-based additive

Heat-treated (HT) wood is now a popular building material, which has been welcome by consumers due to its dark brown color, dimensional stability and bioresistance. This wood has many advantages. However, its color is susceptible to changes due to weathering. Therefore, for any outdoor application, protection of the wood with proper coatings is obligatory. In the current research work, olive leaf extract has been added to polyacrylate coating to study any resistance against natural or accelerated weathering. The polyacrylate polymer blended with additives of nanoparticles of TiO2 or ZnO as well as 2-(2-hydroxyphenel)-benzotriazole (BTZ) type UV-absorber in combination with olive leaf extract or hindered amine light stabilizer (HALS) was investigated. For this purpose, HT wood samples were coated with a polyacrylate coating containing the mentioned additives. Afterwards, they were exposed to natural weathering for one year. The best samples, such as polyacrylate coated thermally treated wood with TiO2-olive leaf extract 20%, BTZ-olive leaf extract 10% and 20% were selected after one year and exposed to accelerated weathering. The results showed olive leaf extract in combination with TiO2, BTZ had less discoloration compared to other coatings during natural and accelerated weathering. Moreover, these compositions exhibited the best resistance to mold growth and development of cracks.

Shear and adhesion strength of open and closed system heat-treated wood samples

This paper investigated the effects of heat treatment in open (atmospheric pressure) and a closed (under pressure) system on the shear and adhesion strength of Scots pine (Pinus sylvestris) wood. In addition, pull-off adherence testing was carried out of the coatings with water-based, polyurethane-based and oil/wax-based varnishes. Shear strength decreased significantly after heat treatment in Scots pine (31 % to 39 %) in open system, while it decreased between 2 % and 38% in the closed system without glue. The shear strength of the wood samples glued with glue was higher than the samples without glue at laboratory scale. The lower shear strength of modified wood could be attributed to other factors, such as the reduced chemical bonding or mechanical interlocking of adhesives, and the reduced strength of brittle modified wood substrate. With increasing heat treatment temperature adherence decreased. Maximum pull-off adhesion (4,80 MPa) was observed in the control samples coated with PUR

Modeling bending strength of oil-heat-treated wood by near-infrared spectroscopy

Refractory wood species like American beech (Fagus grandifolia Ehrh.) and pin oak (Quercus palustris) are difficult to be treated by impregnation with wood preservative formulations or chemical modification agents. They need to be subjected to a preservative process that uses a heat treatment in a deep fryer with vegetable oil at 220 °C for 120 min. Such heat treatment influences several parameters related to the chemical and mechanical wood properties, such as the bending strength of earlywood (Ew) and latewood (Lw) growth rings. In this study, models to estimate bending strengths of Ew and Lw growth rings from near-infrared spectroscopy (NIRS) data have been proposed for oil-heat-treated wood samples of American beech and pin oak. The models were developed using the partition to latent structures (PLS) regression technique. For both species, the bending strength was significantly lower for the heat-treated wood compared to the untreated wood. In the case of beech, the mean bending strength was 112.87 ± 8.55 MPa for the untreated wood samples and 80.78 ± 4.79 MPa for the heat-treated wood samples. In the case of pin oak, the mean bending strength was 99.57 ± 5.34 MPa for the untreated wood samples and 83.69 ± 5.71 MPa for the heat-treated wood samples. All the PLS models were significant at p value < 0.0001. For the Ew tissues, the R2 with the validation data set was 0.78 for beech and 0.95 for pin oak. The corresponding root mean square errors of prediction (RMSEp) were 44.57 MPa for beech and 34.10 MPa for pin oak. The corresponding ratios of prediction to deviation (RPD) were 2.2 and 4.8 for beech and pin oak, respectively. For the Lw tissues, R2 was 0.86 and 0.92 for beech and pin oak, respectively. The corresponding RMSEp were 50.01 MPa for beech and 47.87 MPa for pin oak. RPD values were 5.8 and 3.8 for beech and pin oak, respectively. For both the Ew and Lw tissues and species, B coefficients of the response surface models (ranging from − 108.8 to 195.8) were statistically significant indicating that the models can be used to detect which wavelengths of the spectra have the highest impact on the effect of heat treatment on the bending strength.

Anti-fungal and anti-termite activity of extractives compounds from thermally modified ash woods

Thermal modification of wood is a promising alternative to chemical and biocidal modification processes, increasing the biological durability and dimensional stability of wood. However, the wood-decay resistance properties of heat-treated wood are still not well known. The main objective of this study was to determine the biological resistance of heat-treated ash wood, and assess the antifungal and anti-termite activity of extractive compounds from heat-treated ash woods, depending on the intensity of the modification process (2 hours at 170, 200, 215, 228 (°C) - steam pressure). Untreated and heat-treated wood samples were extracted with water or acetone. The extracts were then used to determine inhibition effectiveness against white-rot (Trametes versicolor) and brown-rot (Rhodonia placenta) fungi. Whatman papers impregnated with extractives were used to evaluate the inhibition of termite feeding. Lastly, the extractives were analyzed by Gas Chromatography - Mass Spectrometry (GC-MS) and compared for their level of anti-termite and antifungal activity. The results showed that the degree of antifungal activity of these extracts depended on the solvent used during the extraction process and varied depending on heat treatment intensity. The extracts were more effective against brown-rot than white-rot fungi. However, the anti-termite activity of heat-treated ash wood extracts was not really significant. A GC-MS analysis showed that the main share of the extractives in untreated wood was removed. In addition, new chemical elements were generated by the thermal degradation of wood polymers (lignin and hemicelluloses), including aliphatic acids, monosaccharides and other products resulting from their dehydration reaction. The most abundant element was syringaldehyde, from lignin derived compounds, which might explain the antifungal activities of thermally treated ash wood extracts.

Combined effects of ZnO particle deposition and heat treatment on dimensional stability and mechanical properties of poplar wood

This study proposed a one-step wood modification method by combining the deposition of ZnO particles on wood surface and heat treatment. The effects of ZnO particles and heat treatment on mechanical properties and dimensional stability of poplar wood were examined. Samples were sorted into 4 groups, i.e., control, heat-treated, impregnation/heat-treated, and hydrothermal-treated samples. The mechanical properties and dimensional stability of impregnation/heat-treated and hydrothermal-treated wood samples were measured in comparison with those of the control and heat-treated wood samples. Compared with the control ones, the reduction of the flexural strength of the heat-treated, impregnation/heat-treated and hydrothermal-treated samples were about 11.57%, 8.53% and 15.90%, respectively. The modulus of elasticity of the heat-treated and hydrothermal-treated samples decreased by 13.78% and 23.78%, respectively, while the impregnation/heat-treated samples increased by about 8.57% due to the ZnO particles growth. The dimensional stabilities of the heat-treated, impregnated/heat-treated and hydrothermal-treated samples were improved in comparison with that of the control sample. All samples were characterized by the scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X - ray diffraction (XRD).

COLOR CHANGE — MASS LOSS CORRELATION FOR HEAT-TREATED WOOD

Heat treatment is renowned as the most environmentally friendly process of dimensional stabilization that can be applied to wood, in order to make it suitable for outdoor uses. It also darkens wood color and improves wood durability. The intensity of heat treatment can be appreciated by means of two parameters: the color change occured in wood due to the high temperature, and the mass loss, which is a measure of the degree of thermal degradation. In order to find a mathematical correlation between these two parameters, an experimental study was conducted with four European wood species, which were heat-treated at 180°C and 200ºC, for 1-3 hours, under atmosheric pressure.The paper presents the results concerning the color changes and mass losses recorded for the heat-treated wood samples compared to untreated wood. For all four species, the dependency between the color change and the mass loss was found to be best described by a logarithmic regression equation with R2 of 0.93 to 0.99 for the soft species (spruce, pine and lime), and R2 of 0.77 for beech. The results of this study envisage to simplify the assessment procedure of the heat treatment efficiency, by only measuring the color – a feature that is both convenient and cost-effective.

Properties of solid wood and laminated wood lumber manufactured by cold pressing and heat treatment

Physical, mechanical, and morphological properties of solid wood lumbers which were cold pressed in a press and then heat treated in a kiln. Two different kinds of domestic thinning small-diameter softwood (Ginko biloba L.) and hardwood (Tilia amurensis Rupr.) were used in this study. First 50mm thick lumbers were cold pressed until 35mm (30% of control lumber) using a stopper for 5min. Then the cold pressed lumbers were heat treated in an electric kiln at 180°C for 6, 12, 24, or 48h. To increase the utilizability of woods, the LVLs were produced from 4mm thick veneers prepared from the heat treated lumbers using a veneer saw. Each LVL sample consisted of 5 layers which were subsequently 48h-, 24h-, 12h-, and 6h-treated veneers and untreated veneer (from top layer to bottom layer). The shrinkage rates of softwood and hardwood were considerably decreased with increasing temperature. The mechanical properties of heat treated samples were better than those of unpressed control samples. The bending strength and modulus of elasticity of the LVLs manufactured from cold pressed and then heat treated lumbers were slightly lower than those of untreated woods. The colour values obtained from the heat treated wood samples showed a clear effect of the temperature on the colour changes.

Effect of artificial weathering on the properties of heat treated wood

This study aims to investigate the change in chemical composition, surface characteristic and mechanical properties of heat treated four wood species (ash, iroko, Scots pine and spruce) during artificial weathering from 400 h to 1600 h in relation to their color changes, surface roughness, compression strength, modulus of elasticity, modulus of rupture and surface composition. Original color of wood species was significantly changed by heat treatment and artificial weathering. Artificial weathering decreased color change of heat treated wood samples except for iroko. Color change significantly increased with longer weathering exposure for heat treated wood samples however similar trend was not observed for control samples exposed to weathering factors. In general, heat treatment alone did not have a considerable effect on surface roughness of wood. Heat treatment seemed to protect wood surface to become rougher after weathering for softwoods. Compression strength and MOR of samples decreased while MOE increased during heat treatment. Compression strength, MOR and MOE of samples decreased considerably with longer weathering exposure both for heat treated and control samples. Softwood species seemed to be more affected by heat and weathering than hardwood species did with respect of loss on the strength properties. Severe delignification and hemicellulose degradation occurred in heat treated and control samples for all wood species during weathering from 400 h to 1600 h evident from rapid decrease at 1504/1508 cm−1 and 1730 cm−1, respectively. Heat treatment did not fully protect hemicellulose degradation and delignification occurred by weathering with longer exposure periods.

A spectrocolorimetric and chemical study on color modification of heat-treated wood during artificial weathering

Effect of artificial weathering on the wood surface color modifications of three North American species (jack pine, aspen, and birch) heat-treated under different temperatures was studied by spectrocolorimetric colormeter (datacolor, CHECK TM). Data was analyzed using the reflectance spectra (400–700nm) as well as the CIE-L*a*b* system and ΔE. Kubelka–Munk (K–M) spectra of samples were recorded as a function of artificial weathering time to obtain the absorption maxima of the chromophore woods formed during artificial weathering. The results were compared with those of the respective untreated (Kiln-dried) species. Analysis of chemical components shows that the lignin percent of jack pine, aspen, and birch increased after heat treatment (28.66–35.9%, 20.27–26.41%, and 19.04–22.71% respectively) which might be due to smaller influence of heat treatment on lignin content than hemicelluloses. This improves the resistance of heat-treated wood to photo-degradation. This is also supported by the smaller change observed in K–M spectra and total color parameters in CIE-L*a*b* system of heat-treated wood samples compared to those of untreated wood when weathered for72h. However, the lignin percent of heat-treated woods reduce to maximum 2.5% after artificial weathering of 1512h. This suggests that the weathering degrades most lignin matrix; consequently, both the colors of heat-treated woods and untreated woods are lighter and very similar after a long period of artificial weathering.

Comparison of VOC emissions between air-dried and heat-treated Norway spruce ( Picea abies), Scots pine ( Pinus sylvesteris) and European aspen ( Populus tremula) wood

Heat-treated wood is an increasingly popular decoration material. Heat-treatment improves dimensional stability of the wood and also prevents rot fungus growth. Although production of heat-treated wood has been rapidly increasing, there is only little information about the VOC emissions of heat-treated wood and its possible influences on indoor air quality. In the present study, VOC emissions from three untreated (air-dried) and heat-treated wood species were compared during a four weeks test period. It appeared that different wood species had clearly different VOC emission profiles. Heat-treatment was found to decrease VOC emissions significantly and change their composition. Especially, emissions of terpenes decreased from softwood samples and aldehydes from European aspen samples. Emissions of total aldehydes and organic acids were at the same level or slightly higher from heat treated than air-dried softwood samples. In agreement with another recent study, the emissions of furfural were found to increase and those of hexanal to decrease from all the wood species investigated. In contrast to air-dried wood samples, emissions of VOCs were almost in steady state from heat treated wood samples even in the beginning of the test.

The effects of heat treatment on technological properties in Red-bud maple ( Acer trautvetteri Medw.) wood

Heat treatment is often used to improve the dimensional stability of wood. In this study, the effects of heat treatment on technological properties of Red-bud maple ( Acer trautvetteri Medw.) wood were examined. Samples obtained from Düzce Forest Enterprises, Turkey, were subjected to heat treatment at varying temperatures (120 °C, 150 °C and 180 °C) and for varying durations (2 h, 6 h and 10 h). The technological properties of heat-treated wood samples and control samples were tested. Compression strength parallel to grain, bending strength, modulus of elasticity in bending, janka-hardness, impact bending strength, and tension strength perpendicular to grain were determined. The results showed that technological strength values decreased with increasing treatment temperature and treatment times. Red-bud maple wood could be utilized by using proper heat treatment techniques with minimal losses in strength values in areas where working, and stability such as in window frames, are important factors.

Color stability of heat-treated wood during artificial weathering

The color change of wood after exposure to UV light may produce aesthetical damages. The stability of the color to light exposure is an important issue. This study describes experiments of testing the color stability of heat-treated wood samples. Heat treatment was done at 240°C during 2 hours, under nitrogen. Heat-treated samples of ash, beech, maritime pine and poplar heartwood were exposed to UV-light (QUV equipped with fluorescent lamps UVA-340) during 835 hours. Color measurements during accelerated weathering were made at intervals throughout the test period. The results are presented in ΔE and L* a* b* coordinates according to the CIELab system. The experiments show that the color stability for heat-treated wood is better during the 835 hours of exposure when compared to untreated wood. The properties of heat treated wood (lignin modifications, and monomers of phenolic compounds) are probably involved in the retified wood resistance against UV light under experimental conditions.

Comparing the VOC emissions between air-dried and heat-treated Scots pine wood

The emissions of volatile organic compounds (VOCs) from air-dried Scots pine wood and from heat-treated Scots pine wood were compared with GC-MS analysis. Air-dried wood blocks released about 8 times more total VOCs than heat-treated (24 h at 230°C) ones. Terpenes were clearly the main compound group in the air-dried wood samples, whereas aldehydes and carboxylic acids and their esters dominated in the heat-treated wood samples. Only 14 compounds out of 41 identified individual compounds were found in both wood samples indicating considerable changes in VOC emission profile during heat-treatment process. Of individual compounds α-pinene, 3-carene and hexanal were the most abundant ones in the air-dried wood. By contrast, in the heat-treated wood 2-furancarboxaldehyde, acetic acid and 2-propanone were the major compounds of VOC emission. Current emission results reveal that significant chemical changes have occurred, and volatile monoterpenes and other low-molecular-weight compounds have evaporated from the wood during the heat-treatment process when compared to air-dried wood. Major chemical changes detected in VOC emissions are explained by the thermal degradation and oxidation of main constituents in wood. The results suggest that if heat-treated wood is used in interior carpentry, emissions of monoterpenes are reduced compared to air-dried wood, but some irritating compounds might be released into indoor air.