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Color change and emission of volatile organic compounds from Scots pine exposed to heat and vacuum-heat treatment
Journal of Building Engineering 26 (2019) 100918
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Color change and emission of volatile organic compounds from Scots pine exposed to heat and vacuum-heat treatment
T
Hüseyin Sivrikayaa,*, Daniela Tesařováb, Eva Jeřábkováb, Ahmet Cana a b
Forest Industry Engineering, Faculty of Forestry, Bartin University, 74100, Bartin, Turkey Department of Furniture, Design and Habitat, Faculty of Forestry and Wood Technology, Mendel University in Brno, Zemědělská 1, Brno, 613 00, Czech Republic
A R T I C LE I N FO
A B S T R A C T
Keywords: Scots pine Heat treatment VOC Mass loss Color change
Heat treatment improves the dimensional stability and durability of wood. Thermal modification of wood has been extensively studied in the last two decades as it is an environmentally friendly method. Thermally treated wood has been industrialized under different brand names such as ThermoWood, Plato wood and Retified wood. These products differ from each other according to their process technology. During the heat treatment process, wood is chemically changed due to the higher temperature. This is followed by increased color change and the wood becomes darker. In addition, various volatile organic compounds (VOCs) in different quantities are emitted during or after heat treatment. In this study, evaluation of VOC emissions of Scots pine sapwood and heartwood was carried out after heat and vacuum-heat treatment. Higher mass loss occurred in sapwood samples with heat treatment as compared to vacuum-heat treatment. Lightness decreased and color change increased after the heat treatment, but these results were inhibited with vacuum-heat treatment. Vacuum-heat treatment increased the sum of emitted compounds (TVOC) when heartwood samples were used. Pentanal, hexanal and α-Pinene were emitted from air-dried samples in higher quantities than other compounds, with α-Pinene being the most frequently emitted compound from the air-dried or treated wood samples.
1. Introduction Thermal modification is potentially and commercially the most advanced method for improving the dimensional stability and decay resistance of wood. In this process, modification of wood can be performed at higher temperatures (180 °C–260 °C) in air, in a vacuum or under an inert atmosphere. It is an environmentally friendly process and does not present any hazard at the end of the service life [1]. A new technology in the modification of wood called Termovuoto has been introduced recently. In this process, the oxygen inside the reactor is reduced by a partial vacuum and heating is provided by forced convection [2]. The thermo-vacuum is classified as a dry process in an open system, where the vacuum pump continuously removes all volatile compounds from the reactor [3]. However, thermal treatment reduces the mechanical properties of wood [4,5]. In addition, after thermal treatment of wood at higher temperatures and longer times, the color is changed. Generally, lightness is reduced, which increases color change and makes the wood darker. Color change is evaluated based on the CIElab color system. The performance of the CIE L*a*b* color measurement system has been investigated for use in industrial quality control of thermally modified timber. A strong
*
correlation was found between color data and heat-treatment intensities, and it was proposed that color measurements be used for quality control of thermally modified timber since the color measurements are rapid, precise, and highly reproducible [6]. Color is also an important property of wood for the final consumer in terms of a visual decorative point for the selection of a specific wood. In addition, the darkening of wood by heat treatment would provide an attractive way to use a substitute material for tropical woods [7]. Gonzales-Pena and Hale [8] reported that color change (ΔE*) was highly influenced by the lightness behavior (ΔL) and also suggested that ΔE* in heat-treated wood originated from the chemical changes in the wood polymers, especially more in lignin than in polysaccharides. During thermal treatment, higher temperatures change not only the physical properties but also the chemical structure of wood. Chemical changes in heat-treated wood have attracted the interest of earlier researchers [9–12]. Candelier et al. [13] investigated the chemical composition of heattreated wood under vacuum, nitrogen and steam. Extractives were found to be lower in the samples heat-treated under vacuum, while lignin, hemicellulose and alpha-cellulose contents were higher under steam and nitrogen; the results were also confirmed by chromatography
Corresponding author. E-mail address: [email protected] (H. Sivrikaya).
https://doi.org/10.1016/j.jobe.2019.100918 Received 25 April 2019; Received in revised form 5 August 2019; Accepted 6 August 2019 Available online 07 August 2019 2352-7102/ © 2019 Elsevier Ltd. All rights reserved.
Journal of Building Engineering 26 (2019) 100918
H. Sivrikaya, et al.
placed in the oven at 103 °C until they reached oven-dried weight. In the first method, heat treatment was applied to the samples only at target temperatures of 180 and 200 °C for 2 h during the process. For this process, a laboratory oven (Memmert) was used in the presence of air. In the second method, the vacuum-heat treatment process was performed using a Jeio Tech vacuum-oven. The same temperatures and time were applied in the absence of oxygen by means of a vacuum pump (KNF Laboport) at the level of 0.08 MPa. After heat treatment, the treated samples were wrapped with aluminum foil and then kept in a sealed bag. In addition, air-dried wood samples were used as a reference. In both methods, the wood samples were not exposed to heating outside the target temperatures. The only difference between the two methods employed was the vacuum process applied during the heat treatment in the second method.
analyses. Boonstra and Tjeerdsma [14] studied a two-stage heat treatment of wood: in an aqueous environment at super atmospheric pressure, and as a second stage, under dry and mild atmospheric conditions below 200 °C. A series of chemical analyses including those of wood chemical components, acetyl and free hydroxyl group contents and CHNO-elemental analysis as well as UV-spectroscopy revealed significant differences in the chemistry of the two-stage compared to the one-stage heat treatment and contributed to the clarification of the reaction mechanism and its effect on wood properties. In the heat treatment of wood between 180 and 225 °C, cellulose, hemicellulose, and lignin, particularly hemicellulose, are significantly converted into volatiles and other pyrolysis products [15]. Degradation of hemicellulose results in the production of the main volatile compounds such as water, formic acid, acetic acid and furfural [16]. Increased emission of furfural from ash, beech, maple and spruce via thermal treatment has been attributed to the degradation of hemicellulose [17]. In the hydrothermal treatment of wood under moist conditions, acetic acid is formed due to the cleavage of the acetyl groups from the hemicellulose in wood at high temperatures. Spectroscopy via FTIR indicates that esterification reactions take place under dry conditions at elevated temperatures during the curing step and that they contribute to the decrease of hygroscopicity [18]. On the other hand, the emission of volatile organic compounds (VOCs) is of interest to science and industry due to the impact of these compounds on the indoor environment. Volatile organic compounds can be emitted from wood and wood-based panels, with the rate of their emission depending on certain conditions like drying and storage [19]. Heat treatment increases the emission of furfural and decreases the emission of hexanal from wood [20]. Chemical compounds produced by thermal treatment have been investigated by gas chromatography–mass spectrometry (GC-MS) and (C13) nuclear magnetic resonance (C NMR). Polynuclear aromatic hydrocarbon derivatives of phenantrene and other classes of polyaromatics compounds were found [21]. In oak wood, the degradation products were found to be 5-methylfurfural and 5-hydroxymethylfurfural [22], contrary to the furfural which resulted from the degradation of pentoses in hemicellulose. Higher amounts of furfural were found in wood subjected to thermal modification under saturated steam at the temperatures of 160 and 170 °C than in wood treated under superheated steam conditions at 170, 185 and 212 °C [23]. In addition, a higher acid content was reported in wood treated under saturated steam conditions than in wood treated under superheated steam conditions. The strong smell in birch treated under saturated steam conditions was associated with degradation reactions of the wood components and the wood species [24]. According to Hofmann et al. [25], the release of VOCs from wood can be a health concern. Therefore, further research is needed to optimize wood modification methods to improve wood properties and reduce the VOC emissions due to the complex reactions that occur during thermal modification. In the case of pine wood, the emission of VOCs can change depending on the sapwood or heartwood within the cross-section. Based on the literature mentioned above, the objective of this study was to compare heat-treated and vacuum and heat-treated Scots pine sapwood and heartwood in terms of post treatment mass loss, color change and emission of VOCs.
2.1. Mass loss The mass loss (ML) was determined based on the difference in mass before and after heat treatment, according to Equation (1):
ML (%) = 100(m 0 − m1)/ m 0
(1)
where m0 is the initial oven-dry weight of the sample before heat treatment and m1 is the weight of the same sample after heat treatment. 2.2. Color change Color change of the heat-treated samples was measured by a Konica Minolta CM-700d spectrophotometer according to the CIELab system using three replicates. Based on the L*, a*, b* color coordinate system, L* represents the black-and-white axis; for black, L * = 0 and for white, L* = 100; a* represents red-green color based on the positive and negative axes and b * represents yellow-blue color (positive value to yellow, negative value to blue). The total color change (ΔE*) was calculated according to Equation (2).
ΔE * = [(Δa*)2 + (Δb*)2 + (ΔL*)2] 1 2
(2)
ΔL*=L*f − L*i Δa*=a*f − a*i Δb*=b*f − b*i where i indicates the initial value before treatment and f describes the final value after treatment. 2.3. VOC test The Field and Laboratory Emission Cell (FLEC) was designed for measuring VOC emissions of material surfaces. The FLEC is made of stainless steel, with the inner surface produced on a lathe and then hand polished. The circular FLEC with a diameter of 150 mm provided a maximum test material surface area of 177 cm2 and a volume of 35 mL, with emission-free silicon rubber foam used for sealing the cell. The FLEC was placed on the circular rim of a special stainless steel pot. A sealant was applied to the circular aluminum plate. The FLEC was supplied with clean, humidified air (50 ± 3% RH) from an air supply control unit. All experiments were carried out in the laboratory at an ambient temperature of 20–25 °C. The VOCs emitted by the test samples were collected in two Thermal Desorption Tubes (786090–100, 4-mm inner diameter) and filled in with 100 mg of Tenax® TA (Scientific Instrument Services) using the Gilian –LFS air sampling pump. In the present study, ČSN EN ISO 16000–1 [26], ČSN EN ISO 16000–2 [27], ČSN EN ISO 16000–6 [28], ČSN EN ISO 16000–10 [29] and ČSN EN ISO 16000–11 [30] test standards were used for VOC analyses, respectively.
2. Material and methods Scots pine (Pinus sylvestris) was selected as the wood species as it is the main material used in the wood industry and for scientific research. Freshly sawn Scots pine timber free of defects was obtained from a sawmill in Bartın Province. The test samples were comprised of sapwood and heartwood sections of the timber cut to the dimensions of 4 × 4 × 1 cm. Before heat treatment, the sapwood and heartwood samples were
2.3.1. Materials for VOC analyses The internal standards used were Ethylacetate, Toluene, Hexanal, n2
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Butyl acetate, Ethylbenzene, m, p-Xylene, Styrene, o-Xylene, α-Pinene, 3-Ethylenetoluene, D-10o-Xylene, 1,2,4-Trimethylbenzene (Supelco, Sigma Aldrich), 3-σCarene and Butoxyethanol (Fluka). The samples were unwrapped from the aluminum foil immediately before testing. Sampling was performed using the FLEC together with the volume of the stainless steel pot. The sampling of the VOC emissions emitted by the samples of Scot pine was carried out via a splitter by pumping air through two pumps with an air flow of 12 l h−1 through two short-path thermal desorption tubes using TENAX TA sorbent; the organic components were adsorbed on the sorbent. The time alotted for one sampling was 180 min. The VOC emissions were collected through the Gilian–LFS 113 air sampler.
was found to be 3% at 200 °C under heat treatment, while it was 1.68% under vacuum-heat treatment. In the case of heartwood, vacuum-heat treatment resulted in the same mass loss as heat treatment only at 180 °C, while it was higher at a temperature of 200 °C. In addition, heartwood samples showed higher mass loss than sapwood samples under vacuum-heat treatment. The higher mass loss in heartwood samples might have been due to the releasing of more extractives during the heating process in contrast to the heat treatment at 200 °C. It can be said that the difference in mass loss was more apparent in sapwood than in heartwood. Because, sapwood under vacuum-heat treatment resulted in less mass loss than only heat treatment as shown in Table 1. Mass loss was found in different amounts by previous studies depending on the treatment process and parameters and wood species. Zaman et al. [32] found the mass loss for Scots pine at 205 °C to be between 5.7% for 4 h and 7.0% for 8 h. Esteves et al. [33] reported that mass loss for pine reached 7.3% with increasing treatment time and temperature after heat treatment at 190–210 °C for 2–12 h by steaming in the absence of air. Mass loss results for Norway spruce were between 1.5% at 180 °C for 4 h and 12.5% at 225 °C for 6 h [14]. Kutnar et al. [34] found lower mass losses of 1.46% at 180 °C and 4.3% at 210 °C with Norway spruce thermally modified in a vacuum. This result is very close to our results with Scots pine. Mass losses obtained in the present study are in agreement with the ThermoWood patent [35], which specifies a mass loss of 3% to improve dimensional stability and of at least 5% to improve durability. Mass loss was lower in our experiments than in some studies. This phenomenon can be explained by the fact that the wood samples in our study were exposed only to heating at 180 and 200 °C for specified times, which meant they were not subjected to temperatures under 180 or 200 °C while the oven was heating up.
2.3.2. GC-MS system and analysis of samples The samples were then subjected to gas chromatography (Agilent GC 6890) in conjunction with mass spectrometry and thermal desorption via the ChemStation program. The contents of the sorption sampling tube were thermally desorbed from the tube into the capillary column of the gas chromatograph and mass spectrometer detector. Subsequently, qualitative and quantitative data were evaluated using the ChemStation software. The peak areas in the total ion current (TIC) chromatogram were used to evaluate the sum of all emitted VOC compounds (TVOC) that were eluted from the chromatographic column between n-hexane and n-hexadecane. 2.3.3. SER (specific emission rate) Calculation of area-specific emission rates and expression results for a given test condition depended on the area-specific emission rate of the test specimen and the air flow rate through the emission test cell. For individual VOCs, the compounds found both in materials and in the background had to be subtracted compound by compound. For the TVOC, the measured background was subtracted. The relation between the area-specific emission rate (ϱx) and the area-specific air flow rate of the emission test cell (qA) is expressed as shown in Equation (3):
ρx = qA (L/ n) = qA/ q
3.2. Color change The ΔL* resulted in minus values after heat and vacuum-heat treatment, not only in sapwood but also in heartwood (Fig. 1). This meant that the wood had become darker after heat treatment. This can be explained by the decrease in lightness due to the higher temperature during the heating process. The blackish hue increased with the increase in temperature from 180 to 200 °C. In addition, the darkness was more intense in heartwood samples than in sapwood in all cases; this can be attributed to the physical and chemical properties of heartwood. When treatment processes were compared, lightness change was considerably lower in the samples subjected to vacuum-heat treatment than in those only exposed to heat treatment. For instance, the lightness value (L*) decreased from 79.5 to 69.5 when sapwoods were exposed to heat treatment at 180 °C for 2 h. However, when the same treatment was carried out under vacuum, the L* decreased from 79.1 to 75.2. At 200 °C, L* was decreased from 79.6 to 55.7 by heat treatment, whereas it was reduced from 80.6 to 72.3 by vacuum-heat treatment. Esteves et al. [7] found the decreases in L* in transverse sections of pine as 9.4 and 28.4% for 2 h at the temperatures of 170 and 200 °C, respectively. Similar results for the L* value, which was decreased from 80 to 40 under the thermos-vacuum process, were obtained by Allegretti et al. [36], who worked with Norway spruce and fir in the range of 160–220 °C. A remarkable decrease in L* was obtained by Srinivas and Pandey [37] at higher temperatures of 210, 225 and 240 °C under a vacuum of 400 mm Hg. The rubber wood L* decreased from 77.8 to 44.8, 29.9 and 25.6, respectively, after 8-hr heat treatment, while a* and b* increased initially but later decreased with longer times at all temperatures in the process. The Δa* values increased towards the red color more in the heartwood than in the sapwood samples in all cases (Fig. 2). However, the increase was about two times higher in sapwood samples when the temperature was increased from 180 to 200 °C. The redness was increased by the increase in temperature for both sapwood and
(3)
where n is the air change rate (changes per hour) and L is the product loading factor in m2/m3. Equation (3) shows that the area-specific air flow rate (q) equals the n/L. For a given product tested under given emission test cell conditions, the concentration of VOCx depends on the area-specific air flow rate. The measured concentration of a VOC in the outlet air from the emission test cell (ϱx) is converted to an area-specific emission rate (qA), and ϱx is the mean concentration of VOCx, calculated from duplicate air samples. The results are related to the duration of the emission measurement after placing the test specimen in the emission test cell and may be reported quantitatively as the area-specific emission rate of individual VOC s and/or the TVOC according to the objective of the test. The TVOC should be regarded only as a factor specific to the product studied and should be used only for comparison of the products with similar target VOC profiles. 3. Results and discussion 3.1. Mass loss Mass loss is one of the most important features in heat treatment and is used as a reference to determine the quality of a product. It depends on wood species, heating medium, temperature and treatment time [31]. In the present study, mass loss results of the treated samples are given in Table 1. Table 1 indicates that mass loss increased with the increasing temperature in both sapwood and heartwood samples. In the sapwood samples, mass loss was higher under heat treatment compared to vacuum-heat treatment at the same temperatures. For example, mass loss 3
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Table 1 Mass loss of the heat-treated and vacuum-heat treated samples. Wood sp.
Section
Treatment
Time (h)
Mass loss (%)
Scots Scots Scots Scots Scots Scots Scots Scots
Sapwood (Sw) Sapwood Sapwood Sapwood Heartwood (Hw) Heartwood Heartwood Heartwood
HT 180 °C (Heat treatment) HT 200 °C V-HT 180 °C (Vacuum-heat treatment) V-HT 200 °C HT 180 °C HT 200 °C V-HT 180 °C V-HT 200 °C
2 2 2 2 2 2 2 2
1.39 3.00 1.03 1.68 1.86 2.43 1.86 2.39
pine pine pine pine pine pine pine pine
(0.11) (0.07) (0.06) (0.13) (0.10) (0.14) (0.08) (0.13)
The data in parentheses are standard deviations. Sw: Sapwood, Hw: Heartwood, HT: heat treatment, V-HT: vacuum-heat treatment.
temperature and the treatment process. The highest change (27.05) was obtained in the heartwood samples subjected to heat treatment at 200 °C and in the sapwood heat-treated at 200 °C (24.84). At a temperature of 180 °C, the lowest color change value was obtained by vacuum heat treatment with sapwood samples. Increased temperature resulted in greater color change in all experiments. In the experiments, vacuum-heat treatment always resulted in lower changes in color than heat treatment. Bekhta and Niemz [42] pointed out that one of the main effects of heat treatment was the darkening of wood that increased when treatment temperature exceeded approximately 200 °C. Lignin was suggested as being more responsible than polysaccharides for color change, due to the darkening of lignin in heattreated wood. According to FTIR findings in one study [7], this was associated with the generation of chromophoric groups, particularly with the quinone. This has also been confirmed in several other works [42–44]. According to Kačíková et al. [45], results for Norway spruce showed an increase in color change (ΔE*) and decrease in lightness (ΔL*), with ΔE* values of 16.94, 42.43 and 55.36 at temperatures of 187, 221 and 271 °C, respectively, in addition to the ΔL* values of 12.69, −41.53 and −52.29 at the same temperatures. Yang et al. [46] applied thermovacuum treatment on alder birch hardwood at temperatures of 160–200 °C under a relative vacuum of −0.08 MPa. Lower ΔL* values and higher ΔE* values resulted from higher heat-treatment temperatures (180 °C or above) and longer treatment time. The L* decreased from 76.81 to 52.27. The highest ΔE* value was 25.21 after heat treatment at 200 °C for 4 h. In our study, vacuum-heat treatment at 200 °C gave the lower ΔE* values of 10.69 with sapwood and 15.30 with heartwood. This difference may have been due to the wood species or treatment time. Decrease in lightness and increase in the color change of wood under heat treatment were reported by Bourgois et al. [47], who attributed this to the decrease in hemicellulose, especially pentosans, due to the higher temperatures of 240–310 °C.
Fig. 1. Change in L*, a* and b* of samples under heat and vacuum-heat treatment.
Fig. 2. Total color change (ΔE*) on heat and vacuum-heat treated samples. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.3. VOC emissions
heartwood. However, the rate of the redness was lower in the samples heat-treated under vacuum. Over all experiments, both heat and vacuum-heat treatment increased the yellowish color (Δb*). However, this increase was higher with heartwood than sapwood. The highest Δb* values were obtained with heartwood samples subjected to vacuum-heat treatment at 180 °C (9.46) and 200 °C (9.85). Ahajji et al. [38] tested beech and spruce at the temperatures of 210, 235 and 250 °C and found that after heat treatment the b* decreased for beech, whereas the a* increased for spruce. It was found that L* was decreased whereas a* and b* was increased by heat and vacuum-heat treatment. The tendency in these parameters (L*, a* and b*) is in agreement with the previous authors [39–41]. The total color change (ΔE*) of the samples was affected by the
The SER values of the heat and vacuum-heat treated samples are given in Table 2. The total volatile organic compounds (TVOC) were considerably higher in air-dried heartwood (413.16 mg.m-2. h−1) than in air-dried sapwood (32.89 mg.m-2. h−1), as shown in Table 2. Especially, among the terpenes, α-Pinene was the dominating compound (714.39 mg.m-2. h−1) emitted from heartwood. It was mentioned that in radiata pine, αPinene, camphene, β-Pinen and limonene were the monoterpenes expected to be released during drying [48]. The TVOC increased from 180 to 200 °C as shown in Table 2. This increase was due to the initial extractive (pinene) rather than to extractives created due to heat treatment degradation. Moreover, the increased amount of TVOC between the temperatures of 180 and 200 °C 4
Journal of Building Engineering 26 (2019) 100918
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Table 2 SER and flow rates of VOC emissions of Scots pine sapwood and heartwood exposed to heat and vacuum-heat treatment (mg.m−2.h−1). Sample
Butanal Crotonaldehyde Benzene 1-Methoxy-2-Propanol Pentanal Trichlorethylen Toluene Hexanal Tetrachlorethylen n-Butyl acetate Furfural Ethylbenzene m.p-Xylene Styren Cyclohexanon o-Xylen Heptanal Butoxy-Ethanol α-Pinene Camphen Benzaldehyde 3-Ethyl-Toluene 4-Ethyl-Toluene 1.3,5-Trimethyl-Benzene β-Pinene 2-Ethyl Toluene Myrcen 1.2.4-Trimethyl-Benzene Octanal α-Phellandren 3-δ-Carene 1.2.3-Trimethyl-Benzene Limonen γ-Terpinen Decanal TVOCMS
Sw
0.27 – – – 16.88 – 0.15 51.32 0.03 – – 0.04 0.04 – – – – 0.4 76.52 1.73 – – – – 2.82 0.06 – 0.03 0.58 – 0.03 – 1.22 0.02 – 32.89
Hw
– – – – 1.86 – – 6.01 0.02 – – 0.04 – – – 0.38 – 714.39 8.61 0.09 – – – 7.95 0.09 – 0.02 – – – 0.17 5.26 – 0.51 413.16
Sapwood
Heartwood
180 °C
200 °C
V-180 °C
V-200 °C
180 °C
200 °C
V-180 °C
V-200 °C
2.45
0.42
0.08 – 0.85 – 0.13 2.68 – 0.08 – 0.02 0.05 0.05
0.025 – 0.92 – 0.27 4.53 0.07 0.16 0.5 0.05 0.18 0.07
0.01 1.1 0.20 54.36 1.79 – 0.055 0.05 0.02 3.02 0.06 0.11 0.01 0.41 0.02 0.14 0.04 1.02 0.03 0.1 32.76
0.06 0.17 0.10 111.37 3.40 – 0.055 0.04 0.03 4.73 0.08 0.17 0.04 0.18 0.05 0.33 0.06 2.41 0.06 0.12 62.53
– 0.05 – 4.17 – 0.02 4.20 0.01 0.02 – 0.01 – – – 0.01 0.12 – 0.09 55.91 1.39 – 0.02 0.02 0.02 2.33 0.04 0.07 – – 0.02 0.15 0.02 0.87 0.02 – 33.28
– 0.03 – 2.08 – 0.13 3.08 0.02 0.1 – 0.06 0.07 0.02 – 0.03 0.13 – 0.09 95.58 2.41 – 0.03 0.03 0.02 2.46 0.05 0.04 0.04 – 0.025 0.05 0.01 2.18 0.025 – 56.36
– 0.06 0.37 1.19 – 0.16 3.93 0.07 0.04 0.25 0.02 0.07 0.01 – 0.02 0.04 – 0.08 97.59 3.62 0.32 0.12 0.07 0.02 5.05 0.08 0.13 0.03 0.2 0.05 0.575 0.06 4.25 0.06 0.15 72.74
– 0.06 – 0.62 – 0.09 1.90 0.04 0.06 0.45 0.01 0.09 0.05 – 0.04 0.04 – 0.14 136.90 5.00 0.2 0.12 0.05 0.02 3.59 0.075 0.07 0.03 – 0.125 0.07 0.07 5.62 0.17 – 91.07
– – 0.05 1.20 – 0.13 3.68 – 0.07 – – – 0.02 – – 0.70 – 0.08 203.49 4.55 0.12 0.02 – 0.04 3.87 0.02 0.18 – – 0.06 – 0.04 3.65 0.10 0.96 125.65
– 0.06 – 4.23 – 0.12 14.23 0.04 0.11 0.36 0.02 0.06 0.06 – 0.01 0.37 – 0.08 173.25 6.09 0.96 0.18 0.09 0.05 5.94 0.09 0.17 0.01 0.11 0.11 0.01 0.10 8.84 0.12 0.46 121.10
The most abundant VOCs emitted from the air-dried wood samples were pentanal and hexanal from the aldehyde group and α-Pinene from the terpenes. The emissions of pentanal and hexanal were higher in sapwood than in heartwood. Of all the compounds, the most outstanding result was that of α-Pinene which was emitted in the highest amounts from air-dried heartwood (714.39). After heat treatment, the emission of α-Pinene was still higher compared to other compounds. Both heat and vacuum-heat treatments increased the emission of αPinene from sapwood samples at 200 °C compared to 180 °C. The emission of α-Pinen from heat-treated heartwood samples was higher than from heat-treated sapwood samples. In accordance with our study, Manninen et al. [51] reported that hexanal and α-Pinene were abundant in air-dried Scots pine, whereas they found the emission of these compounds very low (about 1%) in heat-treated wood. In addition, they found 3-carene in high quantities in air-dried wood, but not in heattreated wood. In our study, the 3-carene emitted from both air-dried and heat-treated woods was extremely low. The difference in air-dried woods regarding 3-carene may be attributed to the characteristics of Scots pine growing in different geographical locations. Pentanal was observed in much higher amounts in sapwood (16.88) than heartwood (1.86) under air-dried conditions, as well as in very low amounts in sapwood after heat treatment. However, it was not present in heartwood after heat treatment. This result was compatible with that of Manninen et al. [51], who also found the pentanal compound in airdried Scots pine, but not in heat-treated Scots pine. It is possible that the pentanal emission may be consumed during the heat-treatment process. With regard to hexanal from the aldehyde group, there was a difference in the sapwood samples between the emissions from heat treatment and from vacuum-heat treatment. Vacuum-heat treatment
depended on the increasing evaporation energy of the VOCs, and the higher release of emissions with higher evaporation energy. Our results were compatible with the results obtained by Czajka and Fabisiak [49], who examined the emission of VOCs from cross-sections of Scots pine sapwood and heartwood over 3.7 and 14 days, respectively. Within the entire test period, the amounts of VOCs were several times higher in heartwood than in sapwood and the terpenes were the dominant compounds. However, the concentrations of aliphatic and aromatic compounds and alcohols were found to be higher in the sapwood compared to the heartwood. Dix et al. [50] also reported that in pine species the heartwood emitted higher amounts of VOCs than sapwood. In the present study, sapwoods, both heat-treated and vacuum-heat treated, produced lower amounts of TVOC after heating at 180 °C compared to heating at 200 °C. In contrast to the sapwood samples, when heartwood was used, the TVOC amounts were higher in the samples exposed to vacuum-heat treatment at 180 °C and 200 °C; αPinene was also a major contributor in the emissions from heartwood samples. Hyttinen et al. [20] observed VOC emissions from Scots pine for four weeks. During this period, TVOC amounts decreased from 2000 μg m−2h−1 to 1000 μg m−2h−1 for air-dried samples and from 240 μg m−2h−1 to 40 μg m−2h−1 for heat-treated samples. Table 2 indicates that butanal was detected in small amounts in the sapwood under air-dried conditions (0.27), while it was not detected in the air-dried heartwood of the Scots pine samples. Butanal was also emitted in higher amounts from sapwood heat-treated at 180 °C (2.45), and in very low amounts from sapwood heat-treated at 200 °C, but was not detected in vacuum-heat treated sapwood or in heartwood under any treatments. 5
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compounds [54]. Monoterpene compounds such as α–Pinene, β–Pinene and 3–Carene originating from softwoods can be considered as the most important volatile organic compounds [55].
remarkably reduced the hexanal compound during the process, and thus emission of this compound was very low after vacuum-heat treatment. Risholm-Sundman et al. [52] analyzed the VOCs from Scots pine after 30 min at 40, 60 and 80 °C and found that the amount of hexanal was somewhat greater at the highest temperature and that it was generated by oxidation of unsaturated fatty acids. They indicated that there was a very high emission of terpenes (3700 μg/m2) from pine, and mainly α-Pinene, β-Pinene and 3-carene were emitted from softwood. They drew attention to the allergic reactions to these components in spite of their pleasant odor. Terpenes were also the main compounds in the VOC emission of air-dried Scots pine obtained from two different locations, and made up about 71% of the total VOCs, but these quickly decreased in heat-treated wood [53]. Alpha Pinene, delta3-Carene and hexanal were the main compounds measured from airdried Scots pine by Hyttinen et al. [20], with acetic acid, furfural and hexanal as the other main compounds in heat-treated Scots pine. The hexanal amount emitted from the heat-treated sample was lower than from the air-dried ones in their study, which reported results similar to ours. Detection of 1-Methoxy-2-Propanol from the VOCs was surprising, especially from sapwood after vacuum-heat treatment at 180 °C and 200 °C and from heartwood after vacuum-heat treatment at 200 °C. Wang et al. [53] investigated released VOCs in larch wood using HPLC and GC-MS during the vacuum-heat treatment, unlike to our research, which was carried out after the treatment process. They found more varieties of VOCs in the larch wood at higher temperatures, with the main compounds including α-Pinene, β-Pinene, limonene, furfural and cedrene. It can be inferred from Table 2 that at a temperature of 200 °C, heartwood emitted a high amount of toluene under vacuum-heat treatment. Camphen was found in high quantity in air-dried heartwood (8.61) rather than sapwood (1.73). In sapwood and heartwood, both heat and vacuum-heat treatments resulted in more emission of camphen at the temperature of 200 °C than at 180 °C. β-Pinene was found to be higher in air-dried heartwood than sapwood; however, the emission of β-Pinene was lower in all heat-treated samples than in air-dried heartwood. Similarly, limonen was seen in high quantity in air-dried heartwood, and emission of this compound was increased at a temperature at 200 °C under both treatment methods, and vacuum-heat treatment at 200 °C yielded the highest limonen. Furfural was emitted in very low quantities after treatment in our study. Large amount of furfural may be lost either during the heat treatment or during the time between heat treatment and the VOC experiments, because the VOC was measured in our samples after the heat-treatment process. The other volatile organic compounds such as crotonaldehyde, benzene, trichlorethylen, furfural, o-xylen, myrcen, 3-δ-carene and γ-terpinen were emitted in negligible quantities (below 1 mg m−2 h−1) (Table 2). For example, crotonaldehyde was not observed before heat treatment, whereas it was detected in very low quantity under vacuum-heat treatment in sapwood at 180 °C (0.05 mg m−2 h−1) and in heartwood at 200 °C (0.06 mg m−2 h−1). Benzaldehyde was only seen in heartwood after heat treatment, particularly at 200 °C (0.96 mg m−2 h−1) under the vacuum process. In an earlier study, similar to ours, both compounds were found in low concentrations. Crotonaldehyde was measured as 0.70 (mg. m−3) at 160 °C, but as 2.07 at 240 °C, and benzaldehyde as 0.42 and 1.07 (mg. m−3) at 160 and 240 °C, respectively, during the vacuum-heat treatment in larch [53]. The aldehydes pentanal and hexanal and the ketones furfural and acetone as well as 2-pentyl furan were exhibited in low emissions from exotic species such as merbau, wenge, zebrawood and teak, but this finding was associated with long storage of the material and the earlier drying process [54]. The amount of other volatile organic compounds released from wood greatly depends on the wood species. Hardwoods like beech and oak release a high amount of acetic and formic acid and lesser amounts of terpene compounds, whereas softwoods release much smaller amounts of organic acids, but much greater amounts of terpene
4. Conclusions In this study, heat and vacuum-heat treated wood were compared with regard to VOC emission and color change. Generally, lower weight losses were obtained in sapwood and heartwood samples of Scots pine after treatment. However, vacuum-heat treatment reduced the mass loss more than heat treatment in sapwood, but slightly increased it in heartwood. It can be concluded that the process in the present study focusing on the only target degree has been found successful for reducing mass loss, plays important role in strength properties, in modified samples without any defect. However, we cannot predict how it will produce results for large size timber in the industrial scale treatment. Vacuum-heat treatment hindered the color change during heat treatment. The change in color was higher on heat-treated samples than on vacuum-heat treated samples. In air-dried samples, the amount of VOC emission was found to be dependent on the individual compound as well as sapwood or heartwood. In the air-dried state, heartwood samples were much richer in α-Pinene content than sapwood. Increasing temperature usually increased the TVOC emission. Sapwood samples of Scots pine emitted fewer VOCs than those of Scots pine heartwood, especially from the point of TVOC emission. Vacuum-heat treatment increased the TVOC emission, particularly in the heartwood samples. It might be concluded that as heartwood is rich in extractive content, it may not be affected by heating under vacuum, in which the degradation is hindered by the absence of oxygen. Thermally treated sapwood may be recommended as a construction material for interiors, provided that the wood species and extractive content are taken into consideration. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jobe.2019.100918. References [1] C.A.S. Hill, Wood Modification-Chemical, Thermal and Other Processes. Wiley Series in Renewable Resources, John Wiley & Sons, Ltd, 2006. [2] S. Ferrari, I. Cuccui, O. Allegretti, Thermo-vacuum modification of some European softwood and hardwood species treated at different conditions, BioResources 8 (1) (2013) 1100–1109. [3] A. Sandak, J. Sandak, O. Allegretti, Quality control of vacuum thermally modified wood with near infrared spectroscopy, Vacuum 114 (2015) 44–48. [4] S. Poncsak, D. Kocaefe, M. Bouazara, A. Pichette, Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera), Wood Sci. Technol. 40 (2006) 647–663. [5] M.J. Boonstra, J. Van Acker, B.F. Tjeerdsma, E.V. Kegel, Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents, Ann. For. Sci. 64 (2007) 679–690. [6] C. Brischke, C.R. Welzbacher, K. Brandt, A.O. Rapp, Quality control of thermally modified timber: interrelationship between heat treatment intensities and CIE L*a*b* color data on homogenized wood samples, Holzforschung 61 (2007) 19–22. [7] B. Esteves, A.V. Marques, I. Domingos, H. Pereira, Heat-induced colour changes of pine (Pinus pinaster) and eucalypt (Eucalyptus globulus) wood, Wood Sci. Technol. 42 (2008) 369–384. [8] M.M. Gonzalez-Peña, M.D. Hale, Colour in thermally modified wood of beech, Norway spruce and Scots pine. Part 1: colour evolution and colour changes, Holzforschung 63 (2009) 385–393. [9] W. Sandermann, H. Augustin, Chemische Untersuchungen über die thermische Zersetzung von Holz, Holz Roh Werkstoff 21 (1963) 256–265. [10] F. Kollmann, D. Fengel, Änderungen der chemischen Zusammensetzung von Holz durch thermische Behandlung, Holz Roh Werkstoff 23 (1965) 461–468. [11] P. Topf, Versuche zur Frage der Selbstentzündung, des Gewichtsverlustes, des Brennwertes und der Elementaranalysen, Holz Roh Werkstoff 29 (1971) 295–300. [12] B.F. Tjeerdsma, M. Boonstra, H. Militz, Thermal modification of non-durable wood species II. IRG/WP vol. 98–40124, (1998), p. 10. [13] K. Candelier, S. Dumarçay, A. Pétrissans, L. Desharnais, P. Gérardin, M. Pétrissans, Comparison of chemical composition and decay durability of heat treated wood cured under different inert atmospheres: nitrogen or vacuum, Polym. Degrad. Stab.
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(1997) 678,324. [36] O. Allegretti, M. Brunetti, I. Cuccui, S. Ferrari, M. Nocetti, N. Terziev, Thermovacuum modification of spruce (Picea abies Karst.) and fir (Abies alba Mill.) wood, BioResources 7 (2012) 3656–3669. [37] K. Srinivas, K.K. Pandey, Effect of heat treatment on color changes, dimensional stability, and mechanical properties of wood, J. Wood Chem. Technol. 32 (2012) 304–316. [38] A. Ahajji, P.N. Diouf, F. Aloui, I. Elbakali, D. Perrin, A. Merlin, B. George, Influence of heat treatment on antioxidant properties and colour stability of beech and spruce wood and their extractives, Wood Sci. Technol. 43 (2009) 69. [39] V. Möttönen, T. Karki, Color changes of birch wood during high-temperature drying, Dry. Technol. 26 (2008) 1125–1128. [40] G. Gunduz, D. Aydemir, S. Korkut, The effect of heat treatment on some mechanical properties and color changes of uludag fir wood, Dry. Technol. 28 (2010) 249–255. [41] V. Kamperidou, I. Barboutis, V. Vasileiou, Response of colour and hygroscopic properties of Scots pine wood to thermal treatment, J. For. Res. 24 (2013) 571–575. [42] P. Bekhta, P. Niemz, Effect of high temperature on the change in color, dimensional stability and mechanical properties of spruce wood, Holzforschung 57 (2003) 539–546. [43] B. Tjeerdsma, M. Boonstra, A. Pizzi, P. Tekely, H. Militz, Characterisation of thermally modified wood: molecular reasons for wood performance improvement, Holz Roh-Werkst 56 (1998) 149–153. [44] K. Mitsui, H. Takada, M. Sugiyama, R. Hasegawa, Changes in the properties of lightirradiated wood with heat treatment: Part 1 Effect of treatment conditions on the change in colour, Holzforschung 55 (2001) 601–605. [45] D. Kačíková, F. Kačík, I. Čabalová, J. Ďurkovič, Effects of thermal treatment on chemical, mechanical and colour traits in Norway spruce wood, Bioresour. Technol. 144 (2013) 669–674. [46] Y. Yang, T.Y. Zhan, J.X. Lu, J.H. Jiang, Influences of thermo-vacuum treatment on colors and chemical compositions of alder birch wood, BioResources 10 (2015) 7936–7945. [47] P.J. Bourgois, G. Janin, R. Guyonnet, La mesure de couleur. Une methode d’etude et d’optimisation des transformations chimiques du bois thermolyse, Holzforschung 45 (1991) 377–382. [48] A.G. McDonald, P.H. Dare, J.S. Gifford, D. Steward, S. Riley, Assessment of air emissions from industrial kiln drying of Pinus radiata wood, Holz als Roh-und werkstoff 60 (2002) 181–190. [49] M. Czajka, E. Fabisiak, Emission of volatile organic compounds from cross section of pine wood (Pinus sylvestris L.). Annals of Warsaw University of Life SciencesSGGW, Forestry and Wood Technology vol. 77, (2012), pp. 150–154. [50] Dix B, Roffael E, Schneider T. reportAbgabe von flüchtigen Verbindungen (Volatile Organic Compounds, VOC) von Strands, hergestellt aus Kernund Splintholz der Kiefer. WKI-Kurzbericht 6/2004, WKI short report 6/2004. . [51] A.M. Manninen, P. Pasanen, J.K. Holopainen, Comparing the VOC emissions between air-dried and heat-treated Scots pine wood, Atmos. Environ. 36 (2002) 1763–1768. [52] M. Risholm-Sundman, M. Lundgren, E. Vestin, P. Herder, Emissions of acetic acid and other volatile organic compounds from different species of solid wood, Holz als Roh-und Werkstoff 56 (1998) 125–129. [53] Z. Wang, B.L. Sun, J.L. Liu, Investigation of volatile products released during vacuum heat treatment of larch wood, Wood Res. 62 (2017) 773–782. [54] A. Stachowiak-Wencek, W. Pradzynski, Emission of volatile organic compounds from wood of exotic species. Annals of Warsaw University of Life Sciences-SGGW. Forestry and Wood Technology vol. 86, (2014). [55] A.G. McDonald, J.S. Gifford, D. Steward, P.H. Dare, S. Riley, I. Simpson, Air emission from timber drying: high temperature drying and redrying of CCA-treated timber, Holz Roh- Werkst 62 (2004) 291–302.
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Color change and emission of volatile organic compounds from Scots pine exposed to heat and vacuum-heat treatment
T
Hüseyin Sivrikayaa,*, Daniela Tesařováb, Eva Jeřábkováb, Ahmet Cana a b
Forest Industry Engineering, Faculty of Forestry, Bartin University, 74100, Bartin, Turkey Department of Furniture, Design and Habitat, Faculty of Forestry and Wood Technology, Mendel University in Brno, Zemědělská 1, Brno, 613 00, Czech Republic
A R T I C LE I N FO
A B S T R A C T
Keywords: Scots pine Heat treatment VOC Mass loss Color change
Heat treatment improves the dimensional stability and durability of wood. Thermal modification of wood has been extensively studied in the last two decades as it is an environmentally friendly method. Thermally treated wood has been industrialized under different brand names such as ThermoWood, Plato wood and Retified wood. These products differ from each other according to their process technology. During the heat treatment process, wood is chemically changed due to the higher temperature. This is followed by increased color change and the wood becomes darker. In addition, various volatile organic compounds (VOCs) in different quantities are emitted during or after heat treatment. In this study, evaluation of VOC emissions of Scots pine sapwood and heartwood was carried out after heat and vacuum-heat treatment. Higher mass loss occurred in sapwood samples with heat treatment as compared to vacuum-heat treatment. Lightness decreased and color change increased after the heat treatment, but these results were inhibited with vacuum-heat treatment. Vacuum-heat treatment increased the sum of emitted compounds (TVOC) when heartwood samples were used. Pentanal, hexanal and α-Pinene were emitted from air-dried samples in higher quantities than other compounds, with α-Pinene being the most frequently emitted compound from the air-dried or treated wood samples.
1. Introduction Thermal modification is potentially and commercially the most advanced method for improving the dimensional stability and decay resistance of wood. In this process, modification of wood can be performed at higher temperatures (180 °C–260 °C) in air, in a vacuum or under an inert atmosphere. It is an environmentally friendly process and does not present any hazard at the end of the service life [1]. A new technology in the modification of wood called Termovuoto has been introduced recently. In this process, the oxygen inside the reactor is reduced by a partial vacuum and heating is provided by forced convection [2]. The thermo-vacuum is classified as a dry process in an open system, where the vacuum pump continuously removes all volatile compounds from the reactor [3]. However, thermal treatment reduces the mechanical properties of wood [4,5]. In addition, after thermal treatment of wood at higher temperatures and longer times, the color is changed. Generally, lightness is reduced, which increases color change and makes the wood darker. Color change is evaluated based on the CIElab color system. The performance of the CIE L*a*b* color measurement system has been investigated for use in industrial quality control of thermally modified timber. A strong
*
correlation was found between color data and heat-treatment intensities, and it was proposed that color measurements be used for quality control of thermally modified timber since the color measurements are rapid, precise, and highly reproducible [6]. Color is also an important property of wood for the final consumer in terms of a visual decorative point for the selection of a specific wood. In addition, the darkening of wood by heat treatment would provide an attractive way to use a substitute material for tropical woods [7]. Gonzales-Pena and Hale [8] reported that color change (ΔE*) was highly influenced by the lightness behavior (ΔL) and also suggested that ΔE* in heat-treated wood originated from the chemical changes in the wood polymers, especially more in lignin than in polysaccharides. During thermal treatment, higher temperatures change not only the physical properties but also the chemical structure of wood. Chemical changes in heat-treated wood have attracted the interest of earlier researchers [9–12]. Candelier et al. [13] investigated the chemical composition of heattreated wood under vacuum, nitrogen and steam. Extractives were found to be lower in the samples heat-treated under vacuum, while lignin, hemicellulose and alpha-cellulose contents were higher under steam and nitrogen; the results were also confirmed by chromatography
Corresponding author. E-mail address: [email protected] (H. Sivrikaya).
https://doi.org/10.1016/j.jobe.2019.100918 Received 25 April 2019; Received in revised form 5 August 2019; Accepted 6 August 2019 Available online 07 August 2019 2352-7102/ © 2019 Elsevier Ltd. All rights reserved.
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placed in the oven at 103 °C until they reached oven-dried weight. In the first method, heat treatment was applied to the samples only at target temperatures of 180 and 200 °C for 2 h during the process. For this process, a laboratory oven (Memmert) was used in the presence of air. In the second method, the vacuum-heat treatment process was performed using a Jeio Tech vacuum-oven. The same temperatures and time were applied in the absence of oxygen by means of a vacuum pump (KNF Laboport) at the level of 0.08 MPa. After heat treatment, the treated samples were wrapped with aluminum foil and then kept in a sealed bag. In addition, air-dried wood samples were used as a reference. In both methods, the wood samples were not exposed to heating outside the target temperatures. The only difference between the two methods employed was the vacuum process applied during the heat treatment in the second method.
analyses. Boonstra and Tjeerdsma [14] studied a two-stage heat treatment of wood: in an aqueous environment at super atmospheric pressure, and as a second stage, under dry and mild atmospheric conditions below 200 °C. A series of chemical analyses including those of wood chemical components, acetyl and free hydroxyl group contents and CHNO-elemental analysis as well as UV-spectroscopy revealed significant differences in the chemistry of the two-stage compared to the one-stage heat treatment and contributed to the clarification of the reaction mechanism and its effect on wood properties. In the heat treatment of wood between 180 and 225 °C, cellulose, hemicellulose, and lignin, particularly hemicellulose, are significantly converted into volatiles and other pyrolysis products [15]. Degradation of hemicellulose results in the production of the main volatile compounds such as water, formic acid, acetic acid and furfural [16]. Increased emission of furfural from ash, beech, maple and spruce via thermal treatment has been attributed to the degradation of hemicellulose [17]. In the hydrothermal treatment of wood under moist conditions, acetic acid is formed due to the cleavage of the acetyl groups from the hemicellulose in wood at high temperatures. Spectroscopy via FTIR indicates that esterification reactions take place under dry conditions at elevated temperatures during the curing step and that they contribute to the decrease of hygroscopicity [18]. On the other hand, the emission of volatile organic compounds (VOCs) is of interest to science and industry due to the impact of these compounds on the indoor environment. Volatile organic compounds can be emitted from wood and wood-based panels, with the rate of their emission depending on certain conditions like drying and storage [19]. Heat treatment increases the emission of furfural and decreases the emission of hexanal from wood [20]. Chemical compounds produced by thermal treatment have been investigated by gas chromatography–mass spectrometry (GC-MS) and (C13) nuclear magnetic resonance (C NMR). Polynuclear aromatic hydrocarbon derivatives of phenantrene and other classes of polyaromatics compounds were found [21]. In oak wood, the degradation products were found to be 5-methylfurfural and 5-hydroxymethylfurfural [22], contrary to the furfural which resulted from the degradation of pentoses in hemicellulose. Higher amounts of furfural were found in wood subjected to thermal modification under saturated steam at the temperatures of 160 and 170 °C than in wood treated under superheated steam conditions at 170, 185 and 212 °C [23]. In addition, a higher acid content was reported in wood treated under saturated steam conditions than in wood treated under superheated steam conditions. The strong smell in birch treated under saturated steam conditions was associated with degradation reactions of the wood components and the wood species [24]. According to Hofmann et al. [25], the release of VOCs from wood can be a health concern. Therefore, further research is needed to optimize wood modification methods to improve wood properties and reduce the VOC emissions due to the complex reactions that occur during thermal modification. In the case of pine wood, the emission of VOCs can change depending on the sapwood or heartwood within the cross-section. Based on the literature mentioned above, the objective of this study was to compare heat-treated and vacuum and heat-treated Scots pine sapwood and heartwood in terms of post treatment mass loss, color change and emission of VOCs.
2.1. Mass loss The mass loss (ML) was determined based on the difference in mass before and after heat treatment, according to Equation (1):
ML (%) = 100(m 0 − m1)/ m 0
(1)
where m0 is the initial oven-dry weight of the sample before heat treatment and m1 is the weight of the same sample after heat treatment. 2.2. Color change Color change of the heat-treated samples was measured by a Konica Minolta CM-700d spectrophotometer according to the CIELab system using three replicates. Based on the L*, a*, b* color coordinate system, L* represents the black-and-white axis; for black, L * = 0 and for white, L* = 100; a* represents red-green color based on the positive and negative axes and b * represents yellow-blue color (positive value to yellow, negative value to blue). The total color change (ΔE*) was calculated according to Equation (2).
ΔE * = [(Δa*)2 + (Δb*)2 + (ΔL*)2] 1 2
(2)
ΔL*=L*f − L*i Δa*=a*f − a*i Δb*=b*f − b*i where i indicates the initial value before treatment and f describes the final value after treatment. 2.3. VOC test The Field and Laboratory Emission Cell (FLEC) was designed for measuring VOC emissions of material surfaces. The FLEC is made of stainless steel, with the inner surface produced on a lathe and then hand polished. The circular FLEC with a diameter of 150 mm provided a maximum test material surface area of 177 cm2 and a volume of 35 mL, with emission-free silicon rubber foam used for sealing the cell. The FLEC was placed on the circular rim of a special stainless steel pot. A sealant was applied to the circular aluminum plate. The FLEC was supplied with clean, humidified air (50 ± 3% RH) from an air supply control unit. All experiments were carried out in the laboratory at an ambient temperature of 20–25 °C. The VOCs emitted by the test samples were collected in two Thermal Desorption Tubes (786090–100, 4-mm inner diameter) and filled in with 100 mg of Tenax® TA (Scientific Instrument Services) using the Gilian –LFS air sampling pump. In the present study, ČSN EN ISO 16000–1 [26], ČSN EN ISO 16000–2 [27], ČSN EN ISO 16000–6 [28], ČSN EN ISO 16000–10 [29] and ČSN EN ISO 16000–11 [30] test standards were used for VOC analyses, respectively.
2. Material and methods Scots pine (Pinus sylvestris) was selected as the wood species as it is the main material used in the wood industry and for scientific research. Freshly sawn Scots pine timber free of defects was obtained from a sawmill in Bartın Province. The test samples were comprised of sapwood and heartwood sections of the timber cut to the dimensions of 4 × 4 × 1 cm. Before heat treatment, the sapwood and heartwood samples were
2.3.1. Materials for VOC analyses The internal standards used were Ethylacetate, Toluene, Hexanal, n2
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Butyl acetate, Ethylbenzene, m, p-Xylene, Styrene, o-Xylene, α-Pinene, 3-Ethylenetoluene, D-10o-Xylene, 1,2,4-Trimethylbenzene (Supelco, Sigma Aldrich), 3-σCarene and Butoxyethanol (Fluka). The samples were unwrapped from the aluminum foil immediately before testing. Sampling was performed using the FLEC together with the volume of the stainless steel pot. The sampling of the VOC emissions emitted by the samples of Scot pine was carried out via a splitter by pumping air through two pumps with an air flow of 12 l h−1 through two short-path thermal desorption tubes using TENAX TA sorbent; the organic components were adsorbed on the sorbent. The time alotted for one sampling was 180 min. The VOC emissions were collected through the Gilian–LFS 113 air sampler.
was found to be 3% at 200 °C under heat treatment, while it was 1.68% under vacuum-heat treatment. In the case of heartwood, vacuum-heat treatment resulted in the same mass loss as heat treatment only at 180 °C, while it was higher at a temperature of 200 °C. In addition, heartwood samples showed higher mass loss than sapwood samples under vacuum-heat treatment. The higher mass loss in heartwood samples might have been due to the releasing of more extractives during the heating process in contrast to the heat treatment at 200 °C. It can be said that the difference in mass loss was more apparent in sapwood than in heartwood. Because, sapwood under vacuum-heat treatment resulted in less mass loss than only heat treatment as shown in Table 1. Mass loss was found in different amounts by previous studies depending on the treatment process and parameters and wood species. Zaman et al. [32] found the mass loss for Scots pine at 205 °C to be between 5.7% for 4 h and 7.0% for 8 h. Esteves et al. [33] reported that mass loss for pine reached 7.3% with increasing treatment time and temperature after heat treatment at 190–210 °C for 2–12 h by steaming in the absence of air. Mass loss results for Norway spruce were between 1.5% at 180 °C for 4 h and 12.5% at 225 °C for 6 h [14]. Kutnar et al. [34] found lower mass losses of 1.46% at 180 °C and 4.3% at 210 °C with Norway spruce thermally modified in a vacuum. This result is very close to our results with Scots pine. Mass losses obtained in the present study are in agreement with the ThermoWood patent [35], which specifies a mass loss of 3% to improve dimensional stability and of at least 5% to improve durability. Mass loss was lower in our experiments than in some studies. This phenomenon can be explained by the fact that the wood samples in our study were exposed only to heating at 180 and 200 °C for specified times, which meant they were not subjected to temperatures under 180 or 200 °C while the oven was heating up.
2.3.2. GC-MS system and analysis of samples The samples were then subjected to gas chromatography (Agilent GC 6890) in conjunction with mass spectrometry and thermal desorption via the ChemStation program. The contents of the sorption sampling tube were thermally desorbed from the tube into the capillary column of the gas chromatograph and mass spectrometer detector. Subsequently, qualitative and quantitative data were evaluated using the ChemStation software. The peak areas in the total ion current (TIC) chromatogram were used to evaluate the sum of all emitted VOC compounds (TVOC) that were eluted from the chromatographic column between n-hexane and n-hexadecane. 2.3.3. SER (specific emission rate) Calculation of area-specific emission rates and expression results for a given test condition depended on the area-specific emission rate of the test specimen and the air flow rate through the emission test cell. For individual VOCs, the compounds found both in materials and in the background had to be subtracted compound by compound. For the TVOC, the measured background was subtracted. The relation between the area-specific emission rate (ϱx) and the area-specific air flow rate of the emission test cell (qA) is expressed as shown in Equation (3):
ρx = qA (L/ n) = qA/ q
3.2. Color change The ΔL* resulted in minus values after heat and vacuum-heat treatment, not only in sapwood but also in heartwood (Fig. 1). This meant that the wood had become darker after heat treatment. This can be explained by the decrease in lightness due to the higher temperature during the heating process. The blackish hue increased with the increase in temperature from 180 to 200 °C. In addition, the darkness was more intense in heartwood samples than in sapwood in all cases; this can be attributed to the physical and chemical properties of heartwood. When treatment processes were compared, lightness change was considerably lower in the samples subjected to vacuum-heat treatment than in those only exposed to heat treatment. For instance, the lightness value (L*) decreased from 79.5 to 69.5 when sapwoods were exposed to heat treatment at 180 °C for 2 h. However, when the same treatment was carried out under vacuum, the L* decreased from 79.1 to 75.2. At 200 °C, L* was decreased from 79.6 to 55.7 by heat treatment, whereas it was reduced from 80.6 to 72.3 by vacuum-heat treatment. Esteves et al. [7] found the decreases in L* in transverse sections of pine as 9.4 and 28.4% for 2 h at the temperatures of 170 and 200 °C, respectively. Similar results for the L* value, which was decreased from 80 to 40 under the thermos-vacuum process, were obtained by Allegretti et al. [36], who worked with Norway spruce and fir in the range of 160–220 °C. A remarkable decrease in L* was obtained by Srinivas and Pandey [37] at higher temperatures of 210, 225 and 240 °C under a vacuum of 400 mm Hg. The rubber wood L* decreased from 77.8 to 44.8, 29.9 and 25.6, respectively, after 8-hr heat treatment, while a* and b* increased initially but later decreased with longer times at all temperatures in the process. The Δa* values increased towards the red color more in the heartwood than in the sapwood samples in all cases (Fig. 2). However, the increase was about two times higher in sapwood samples when the temperature was increased from 180 to 200 °C. The redness was increased by the increase in temperature for both sapwood and
(3)
where n is the air change rate (changes per hour) and L is the product loading factor in m2/m3. Equation (3) shows that the area-specific air flow rate (q) equals the n/L. For a given product tested under given emission test cell conditions, the concentration of VOCx depends on the area-specific air flow rate. The measured concentration of a VOC in the outlet air from the emission test cell (ϱx) is converted to an area-specific emission rate (qA), and ϱx is the mean concentration of VOCx, calculated from duplicate air samples. The results are related to the duration of the emission measurement after placing the test specimen in the emission test cell and may be reported quantitatively as the area-specific emission rate of individual VOC s and/or the TVOC according to the objective of the test. The TVOC should be regarded only as a factor specific to the product studied and should be used only for comparison of the products with similar target VOC profiles. 3. Results and discussion 3.1. Mass loss Mass loss is one of the most important features in heat treatment and is used as a reference to determine the quality of a product. It depends on wood species, heating medium, temperature and treatment time [31]. In the present study, mass loss results of the treated samples are given in Table 1. Table 1 indicates that mass loss increased with the increasing temperature in both sapwood and heartwood samples. In the sapwood samples, mass loss was higher under heat treatment compared to vacuum-heat treatment at the same temperatures. For example, mass loss 3
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Table 1 Mass loss of the heat-treated and vacuum-heat treated samples. Wood sp.
Section
Treatment
Time (h)
Mass loss (%)
Scots Scots Scots Scots Scots Scots Scots Scots
Sapwood (Sw) Sapwood Sapwood Sapwood Heartwood (Hw) Heartwood Heartwood Heartwood
HT 180 °C (Heat treatment) HT 200 °C V-HT 180 °C (Vacuum-heat treatment) V-HT 200 °C HT 180 °C HT 200 °C V-HT 180 °C V-HT 200 °C
2 2 2 2 2 2 2 2
1.39 3.00 1.03 1.68 1.86 2.43 1.86 2.39
pine pine pine pine pine pine pine pine
(0.11) (0.07) (0.06) (0.13) (0.10) (0.14) (0.08) (0.13)
The data in parentheses are standard deviations. Sw: Sapwood, Hw: Heartwood, HT: heat treatment, V-HT: vacuum-heat treatment.
temperature and the treatment process. The highest change (27.05) was obtained in the heartwood samples subjected to heat treatment at 200 °C and in the sapwood heat-treated at 200 °C (24.84). At a temperature of 180 °C, the lowest color change value was obtained by vacuum heat treatment with sapwood samples. Increased temperature resulted in greater color change in all experiments. In the experiments, vacuum-heat treatment always resulted in lower changes in color than heat treatment. Bekhta and Niemz [42] pointed out that one of the main effects of heat treatment was the darkening of wood that increased when treatment temperature exceeded approximately 200 °C. Lignin was suggested as being more responsible than polysaccharides for color change, due to the darkening of lignin in heattreated wood. According to FTIR findings in one study [7], this was associated with the generation of chromophoric groups, particularly with the quinone. This has also been confirmed in several other works [42–44]. According to Kačíková et al. [45], results for Norway spruce showed an increase in color change (ΔE*) and decrease in lightness (ΔL*), with ΔE* values of 16.94, 42.43 and 55.36 at temperatures of 187, 221 and 271 °C, respectively, in addition to the ΔL* values of 12.69, −41.53 and −52.29 at the same temperatures. Yang et al. [46] applied thermovacuum treatment on alder birch hardwood at temperatures of 160–200 °C under a relative vacuum of −0.08 MPa. Lower ΔL* values and higher ΔE* values resulted from higher heat-treatment temperatures (180 °C or above) and longer treatment time. The L* decreased from 76.81 to 52.27. The highest ΔE* value was 25.21 after heat treatment at 200 °C for 4 h. In our study, vacuum-heat treatment at 200 °C gave the lower ΔE* values of 10.69 with sapwood and 15.30 with heartwood. This difference may have been due to the wood species or treatment time. Decrease in lightness and increase in the color change of wood under heat treatment were reported by Bourgois et al. [47], who attributed this to the decrease in hemicellulose, especially pentosans, due to the higher temperatures of 240–310 °C.
Fig. 1. Change in L*, a* and b* of samples under heat and vacuum-heat treatment.
Fig. 2. Total color change (ΔE*) on heat and vacuum-heat treated samples. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
3.3. VOC emissions
heartwood. However, the rate of the redness was lower in the samples heat-treated under vacuum. Over all experiments, both heat and vacuum-heat treatment increased the yellowish color (Δb*). However, this increase was higher with heartwood than sapwood. The highest Δb* values were obtained with heartwood samples subjected to vacuum-heat treatment at 180 °C (9.46) and 200 °C (9.85). Ahajji et al. [38] tested beech and spruce at the temperatures of 210, 235 and 250 °C and found that after heat treatment the b* decreased for beech, whereas the a* increased for spruce. It was found that L* was decreased whereas a* and b* was increased by heat and vacuum-heat treatment. The tendency in these parameters (L*, a* and b*) is in agreement with the previous authors [39–41]. The total color change (ΔE*) of the samples was affected by the
The SER values of the heat and vacuum-heat treated samples are given in Table 2. The total volatile organic compounds (TVOC) were considerably higher in air-dried heartwood (413.16 mg.m-2. h−1) than in air-dried sapwood (32.89 mg.m-2. h−1), as shown in Table 2. Especially, among the terpenes, α-Pinene was the dominating compound (714.39 mg.m-2. h−1) emitted from heartwood. It was mentioned that in radiata pine, αPinene, camphene, β-Pinen and limonene were the monoterpenes expected to be released during drying [48]. The TVOC increased from 180 to 200 °C as shown in Table 2. This increase was due to the initial extractive (pinene) rather than to extractives created due to heat treatment degradation. Moreover, the increased amount of TVOC between the temperatures of 180 and 200 °C 4
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Table 2 SER and flow rates of VOC emissions of Scots pine sapwood and heartwood exposed to heat and vacuum-heat treatment (mg.m−2.h−1). Sample
Butanal Crotonaldehyde Benzene 1-Methoxy-2-Propanol Pentanal Trichlorethylen Toluene Hexanal Tetrachlorethylen n-Butyl acetate Furfural Ethylbenzene m.p-Xylene Styren Cyclohexanon o-Xylen Heptanal Butoxy-Ethanol α-Pinene Camphen Benzaldehyde 3-Ethyl-Toluene 4-Ethyl-Toluene 1.3,5-Trimethyl-Benzene β-Pinene 2-Ethyl Toluene Myrcen 1.2.4-Trimethyl-Benzene Octanal α-Phellandren 3-δ-Carene 1.2.3-Trimethyl-Benzene Limonen γ-Terpinen Decanal TVOCMS
Sw
0.27 – – – 16.88 – 0.15 51.32 0.03 – – 0.04 0.04 – – – – 0.4 76.52 1.73 – – – – 2.82 0.06 – 0.03 0.58 – 0.03 – 1.22 0.02 – 32.89
Hw
– – – – 1.86 – – 6.01 0.02 – – 0.04 – – – 0.38 – 714.39 8.61 0.09 – – – 7.95 0.09 – 0.02 – – – 0.17 5.26 – 0.51 413.16
Sapwood
Heartwood
180 °C
200 °C
V-180 °C
V-200 °C
180 °C
200 °C
V-180 °C
V-200 °C
2.45
0.42
0.08 – 0.85 – 0.13 2.68 – 0.08 – 0.02 0.05 0.05
0.025 – 0.92 – 0.27 4.53 0.07 0.16 0.5 0.05 0.18 0.07
0.01 1.1 0.20 54.36 1.79 – 0.055 0.05 0.02 3.02 0.06 0.11 0.01 0.41 0.02 0.14 0.04 1.02 0.03 0.1 32.76
0.06 0.17 0.10 111.37 3.40 – 0.055 0.04 0.03 4.73 0.08 0.17 0.04 0.18 0.05 0.33 0.06 2.41 0.06 0.12 62.53
– 0.05 – 4.17 – 0.02 4.20 0.01 0.02 – 0.01 – – – 0.01 0.12 – 0.09 55.91 1.39 – 0.02 0.02 0.02 2.33 0.04 0.07 – – 0.02 0.15 0.02 0.87 0.02 – 33.28
– 0.03 – 2.08 – 0.13 3.08 0.02 0.1 – 0.06 0.07 0.02 – 0.03 0.13 – 0.09 95.58 2.41 – 0.03 0.03 0.02 2.46 0.05 0.04 0.04 – 0.025 0.05 0.01 2.18 0.025 – 56.36
– 0.06 0.37 1.19 – 0.16 3.93 0.07 0.04 0.25 0.02 0.07 0.01 – 0.02 0.04 – 0.08 97.59 3.62 0.32 0.12 0.07 0.02 5.05 0.08 0.13 0.03 0.2 0.05 0.575 0.06 4.25 0.06 0.15 72.74
– 0.06 – 0.62 – 0.09 1.90 0.04 0.06 0.45 0.01 0.09 0.05 – 0.04 0.04 – 0.14 136.90 5.00 0.2 0.12 0.05 0.02 3.59 0.075 0.07 0.03 – 0.125 0.07 0.07 5.62 0.17 – 91.07
– – 0.05 1.20 – 0.13 3.68 – 0.07 – – – 0.02 – – 0.70 – 0.08 203.49 4.55 0.12 0.02 – 0.04 3.87 0.02 0.18 – – 0.06 – 0.04 3.65 0.10 0.96 125.65
– 0.06 – 4.23 – 0.12 14.23 0.04 0.11 0.36 0.02 0.06 0.06 – 0.01 0.37 – 0.08 173.25 6.09 0.96 0.18 0.09 0.05 5.94 0.09 0.17 0.01 0.11 0.11 0.01 0.10 8.84 0.12 0.46 121.10
The most abundant VOCs emitted from the air-dried wood samples were pentanal and hexanal from the aldehyde group and α-Pinene from the terpenes. The emissions of pentanal and hexanal were higher in sapwood than in heartwood. Of all the compounds, the most outstanding result was that of α-Pinene which was emitted in the highest amounts from air-dried heartwood (714.39). After heat treatment, the emission of α-Pinene was still higher compared to other compounds. Both heat and vacuum-heat treatments increased the emission of αPinene from sapwood samples at 200 °C compared to 180 °C. The emission of α-Pinen from heat-treated heartwood samples was higher than from heat-treated sapwood samples. In accordance with our study, Manninen et al. [51] reported that hexanal and α-Pinene were abundant in air-dried Scots pine, whereas they found the emission of these compounds very low (about 1%) in heat-treated wood. In addition, they found 3-carene in high quantities in air-dried wood, but not in heattreated wood. In our study, the 3-carene emitted from both air-dried and heat-treated woods was extremely low. The difference in air-dried woods regarding 3-carene may be attributed to the characteristics of Scots pine growing in different geographical locations. Pentanal was observed in much higher amounts in sapwood (16.88) than heartwood (1.86) under air-dried conditions, as well as in very low amounts in sapwood after heat treatment. However, it was not present in heartwood after heat treatment. This result was compatible with that of Manninen et al. [51], who also found the pentanal compound in airdried Scots pine, but not in heat-treated Scots pine. It is possible that the pentanal emission may be consumed during the heat-treatment process. With regard to hexanal from the aldehyde group, there was a difference in the sapwood samples between the emissions from heat treatment and from vacuum-heat treatment. Vacuum-heat treatment
depended on the increasing evaporation energy of the VOCs, and the higher release of emissions with higher evaporation energy. Our results were compatible with the results obtained by Czajka and Fabisiak [49], who examined the emission of VOCs from cross-sections of Scots pine sapwood and heartwood over 3.7 and 14 days, respectively. Within the entire test period, the amounts of VOCs were several times higher in heartwood than in sapwood and the terpenes were the dominant compounds. However, the concentrations of aliphatic and aromatic compounds and alcohols were found to be higher in the sapwood compared to the heartwood. Dix et al. [50] also reported that in pine species the heartwood emitted higher amounts of VOCs than sapwood. In the present study, sapwoods, both heat-treated and vacuum-heat treated, produced lower amounts of TVOC after heating at 180 °C compared to heating at 200 °C. In contrast to the sapwood samples, when heartwood was used, the TVOC amounts were higher in the samples exposed to vacuum-heat treatment at 180 °C and 200 °C; αPinene was also a major contributor in the emissions from heartwood samples. Hyttinen et al. [20] observed VOC emissions from Scots pine for four weeks. During this period, TVOC amounts decreased from 2000 μg m−2h−1 to 1000 μg m−2h−1 for air-dried samples and from 240 μg m−2h−1 to 40 μg m−2h−1 for heat-treated samples. Table 2 indicates that butanal was detected in small amounts in the sapwood under air-dried conditions (0.27), while it was not detected in the air-dried heartwood of the Scots pine samples. Butanal was also emitted in higher amounts from sapwood heat-treated at 180 °C (2.45), and in very low amounts from sapwood heat-treated at 200 °C, but was not detected in vacuum-heat treated sapwood or in heartwood under any treatments. 5
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compounds [54]. Monoterpene compounds such as α–Pinene, β–Pinene and 3–Carene originating from softwoods can be considered as the most important volatile organic compounds [55].
remarkably reduced the hexanal compound during the process, and thus emission of this compound was very low after vacuum-heat treatment. Risholm-Sundman et al. [52] analyzed the VOCs from Scots pine after 30 min at 40, 60 and 80 °C and found that the amount of hexanal was somewhat greater at the highest temperature and that it was generated by oxidation of unsaturated fatty acids. They indicated that there was a very high emission of terpenes (3700 μg/m2) from pine, and mainly α-Pinene, β-Pinene and 3-carene were emitted from softwood. They drew attention to the allergic reactions to these components in spite of their pleasant odor. Terpenes were also the main compounds in the VOC emission of air-dried Scots pine obtained from two different locations, and made up about 71% of the total VOCs, but these quickly decreased in heat-treated wood [53]. Alpha Pinene, delta3-Carene and hexanal were the main compounds measured from airdried Scots pine by Hyttinen et al. [20], with acetic acid, furfural and hexanal as the other main compounds in heat-treated Scots pine. The hexanal amount emitted from the heat-treated sample was lower than from the air-dried ones in their study, which reported results similar to ours. Detection of 1-Methoxy-2-Propanol from the VOCs was surprising, especially from sapwood after vacuum-heat treatment at 180 °C and 200 °C and from heartwood after vacuum-heat treatment at 200 °C. Wang et al. [53] investigated released VOCs in larch wood using HPLC and GC-MS during the vacuum-heat treatment, unlike to our research, which was carried out after the treatment process. They found more varieties of VOCs in the larch wood at higher temperatures, with the main compounds including α-Pinene, β-Pinene, limonene, furfural and cedrene. It can be inferred from Table 2 that at a temperature of 200 °C, heartwood emitted a high amount of toluene under vacuum-heat treatment. Camphen was found in high quantity in air-dried heartwood (8.61) rather than sapwood (1.73). In sapwood and heartwood, both heat and vacuum-heat treatments resulted in more emission of camphen at the temperature of 200 °C than at 180 °C. β-Pinene was found to be higher in air-dried heartwood than sapwood; however, the emission of β-Pinene was lower in all heat-treated samples than in air-dried heartwood. Similarly, limonen was seen in high quantity in air-dried heartwood, and emission of this compound was increased at a temperature at 200 °C under both treatment methods, and vacuum-heat treatment at 200 °C yielded the highest limonen. Furfural was emitted in very low quantities after treatment in our study. Large amount of furfural may be lost either during the heat treatment or during the time between heat treatment and the VOC experiments, because the VOC was measured in our samples after the heat-treatment process. The other volatile organic compounds such as crotonaldehyde, benzene, trichlorethylen, furfural, o-xylen, myrcen, 3-δ-carene and γ-terpinen were emitted in negligible quantities (below 1 mg m−2 h−1) (Table 2). For example, crotonaldehyde was not observed before heat treatment, whereas it was detected in very low quantity under vacuum-heat treatment in sapwood at 180 °C (0.05 mg m−2 h−1) and in heartwood at 200 °C (0.06 mg m−2 h−1). Benzaldehyde was only seen in heartwood after heat treatment, particularly at 200 °C (0.96 mg m−2 h−1) under the vacuum process. In an earlier study, similar to ours, both compounds were found in low concentrations. Crotonaldehyde was measured as 0.70 (mg. m−3) at 160 °C, but as 2.07 at 240 °C, and benzaldehyde as 0.42 and 1.07 (mg. m−3) at 160 and 240 °C, respectively, during the vacuum-heat treatment in larch [53]. The aldehydes pentanal and hexanal and the ketones furfural and acetone as well as 2-pentyl furan were exhibited in low emissions from exotic species such as merbau, wenge, zebrawood and teak, but this finding was associated with long storage of the material and the earlier drying process [54]. The amount of other volatile organic compounds released from wood greatly depends on the wood species. Hardwoods like beech and oak release a high amount of acetic and formic acid and lesser amounts of terpene compounds, whereas softwoods release much smaller amounts of organic acids, but much greater amounts of terpene
4. Conclusions In this study, heat and vacuum-heat treated wood were compared with regard to VOC emission and color change. Generally, lower weight losses were obtained in sapwood and heartwood samples of Scots pine after treatment. However, vacuum-heat treatment reduced the mass loss more than heat treatment in sapwood, but slightly increased it in heartwood. It can be concluded that the process in the present study focusing on the only target degree has been found successful for reducing mass loss, plays important role in strength properties, in modified samples without any defect. However, we cannot predict how it will produce results for large size timber in the industrial scale treatment. Vacuum-heat treatment hindered the color change during heat treatment. The change in color was higher on heat-treated samples than on vacuum-heat treated samples. In air-dried samples, the amount of VOC emission was found to be dependent on the individual compound as well as sapwood or heartwood. In the air-dried state, heartwood samples were much richer in α-Pinene content than sapwood. Increasing temperature usually increased the TVOC emission. Sapwood samples of Scots pine emitted fewer VOCs than those of Scots pine heartwood, especially from the point of TVOC emission. Vacuum-heat treatment increased the TVOC emission, particularly in the heartwood samples. It might be concluded that as heartwood is rich in extractive content, it may not be affected by heating under vacuum, in which the degradation is hindered by the absence of oxygen. Thermally treated sapwood may be recommended as a construction material for interiors, provided that the wood species and extractive content are taken into consideration. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jobe.2019.100918. References [1] C.A.S. Hill, Wood Modification-Chemical, Thermal and Other Processes. Wiley Series in Renewable Resources, John Wiley & Sons, Ltd, 2006. [2] S. Ferrari, I. Cuccui, O. Allegretti, Thermo-vacuum modification of some European softwood and hardwood species treated at different conditions, BioResources 8 (1) (2013) 1100–1109. [3] A. Sandak, J. Sandak, O. Allegretti, Quality control of vacuum thermally modified wood with near infrared spectroscopy, Vacuum 114 (2015) 44–48. [4] S. Poncsak, D. Kocaefe, M. Bouazara, A. Pichette, Effect of high temperature treatment on the mechanical properties of birch (Betula papyrifera), Wood Sci. Technol. 40 (2006) 647–663. [5] M.J. Boonstra, J. Van Acker, B.F. Tjeerdsma, E.V. Kegel, Strength properties of thermally modified softwoods and its relation to polymeric structural wood constituents, Ann. For. Sci. 64 (2007) 679–690. [6] C. Brischke, C.R. Welzbacher, K. Brandt, A.O. Rapp, Quality control of thermally modified timber: interrelationship between heat treatment intensities and CIE L*a*b* color data on homogenized wood samples, Holzforschung 61 (2007) 19–22. [7] B. Esteves, A.V. Marques, I. Domingos, H. Pereira, Heat-induced colour changes of pine (Pinus pinaster) and eucalypt (Eucalyptus globulus) wood, Wood Sci. Technol. 42 (2008) 369–384. [8] M.M. Gonzalez-Peña, M.D. Hale, Colour in thermally modified wood of beech, Norway spruce and Scots pine. Part 1: colour evolution and colour changes, Holzforschung 63 (2009) 385–393. [9] W. Sandermann, H. Augustin, Chemische Untersuchungen über die thermische Zersetzung von Holz, Holz Roh Werkstoff 21 (1963) 256–265. [10] F. Kollmann, D. Fengel, Änderungen der chemischen Zusammensetzung von Holz durch thermische Behandlung, Holz Roh Werkstoff 23 (1965) 461–468. [11] P. Topf, Versuche zur Frage der Selbstentzündung, des Gewichtsverlustes, des Brennwertes und der Elementaranalysen, Holz Roh Werkstoff 29 (1971) 295–300. [12] B.F. Tjeerdsma, M. Boonstra, H. Militz, Thermal modification of non-durable wood species II. IRG/WP vol. 98–40124, (1998), p. 10. [13] K. Candelier, S. Dumarçay, A. Pétrissans, L. Desharnais, P. Gérardin, M. Pétrissans, Comparison of chemical composition and decay durability of heat treated wood cured under different inert atmospheres: nitrogen or vacuum, Polym. Degrad. Stab.
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