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Comparison of VOC emissions between air-dried and heat-treated Norway spruce (Picea abies), Scots pine (Pinus sylvesteris) and European aspen (Populus tremula) wood
Atmospheric Environment 44 (2010) 5028e5033
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Comparison of VOC emissions between air-dried and heat-treated Norway spruce (Picea abies), Scots pine (Pinus sylvesteris) and European aspen (Populus tremula) wood Marko Hyttinen*, Marika Masalin-Weijo, Pentti Kalliokoski, Pertti Pasanen Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, Kuopio 70211, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2010 Received in revised form 11 June 2010 Accepted 12 July 2010
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 airdried 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. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: VOC Emissions Heat-treatment Wood Air-dried
1. Introduction Heat-treated wood is a popular decoration material and it has been used in floor, wall, and ceiling materials and in furniture. The total production of heat treated timber in industrial processes was ca. 250,000 m3 in 2007 and its annual grow is estimated to be 30% (www.westwoodcorporation.com). Heat-treatment darkens the wood and helps to sustain better UV radiation and rot fungus (Temiz et al., 2006; Boonstra et al., 2007). Water uptake of wood is also reduced by the heat-treatment process. In addition, heat insulation properties and dimensional stability increase after the treatment, but bending strength of the wood can decrease (Kamdem et al., 2000, 2002). Compress treatment combined to heating process increases the coating hardness and elasticity of the wood. Besides the wood species, the process parameters, temperature and pressure determine the properties of the product (Shi et al., 2007). Heat-treatment is an environmentally friendly way to process the wood because no toxic chemicals are used. However,
* Corresponding author. E-mail address: marko.hyttinen@uef.fi (M. Hyttinen). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.07.018
heat treatment process changes the composition of the wood permanently and also affects organic emissions from the material. In Nordic countries, common heat-treatment process is called Thermowood-process. Raw wood material used in process can be fresh or oven dry soft or hardwood (usually Norway spruce, Scots pine, European aspen or Silver birch). Thermowood-process can be divided in three stages: temperature rise, heat-treatment, and cooling and humidification. In the first stage, temperature of the chamber is risen quickly to 100 C by heated steam, and after that steadily to the 130 C when wood is heated steam-dry. Then the actual heat-treatment process starts and oven temperature is kept at 185e215 C for 3 h. Finally, the cooling period starts by water steam and temperature decreases to 80e90 C. Water steam prevents wood both to catch fire, and to split up. Heat treatment affects all the wood components, i.e., cellulose, hemicelluloses, lignin, and extractives. Emissions and degradation products of wood differ according to wood species. Especially, differences can be detected between the hard- and softwood, which have different cell types (Sjöström, 1993). Hemicelluloses usually crack down thermally easier than cellulose or lignin because of its heterogeneous structure and lack of crystallinity. Degradation of hemicelluloses occurs intensively at 200e260 C. Aliphatic carboxylic acids (mainly formic and acetic
M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033
acids) are the major volatile degradation products formed during the heat treatment (Kotilainen et al., 1999; Kotilainen, 2000; Sundqvist et al., 2006). On the other hand, large amounts of acetic acid were detected already from native wood samples (Peters et al., 2008). Smaller emissions of acetic acid were observed from spruce than from hardwood due to greater number of acetyl groups in hardwood hemicelluloses. In addition, increased emissions of furfural from ash, beech, maple, and spruce were detected due to thermal treatment, which was interpreted to be caused by degradation of hemicelluloses. On the other hand, the emissions of the prevailing aldehydes in natural wood, pentanal and hexanal were observed to decrease during thermal treatment (Peters et al., 2008). Extractives can be divided into saturated and unsaturated fatty acids, glycerides, resin acids, steroids, steryl esters, terpenoids, and waxes. Some of them can be degradated and cause emissions of volatile organic compounds (VOC) or semi volatile organic compounds (SVOC). Among the unsaturated fatty acids oleic, (cis-9octadecenoic acid) and linoleic (cis,cis-9,12-octadecadienoic acid) acid are the dominating compounds (Sjöström, 1993), which may degrade further at high temperatures. Terpenoids are emitted readily from the wood even without rise of temperature. They are among the dominating VOCs emitted especially from conifers. Softwood contains terpenes from monoto tetraterpenes, except sesterterpenes which are rare (Fengel and Wegener, 1989). Volatile wood oil consists of mainly monoterpenes and 80% of the emitted VOCs from fresh softwood are monoterpenes (Roffael and references therein, 2006). Monoterpenes can be divided to acyclic, monocyclic, and dicyclic compounds. The most important monoterpenes are alpha- and beta-pinene and limonene which all can be found in softwood. In addition, deltacarene, camphene, myrcene, and b-fellandrene are common monoterpenes (Fengel and Wegener, 1989). Wood oil content differs between wood species and even between single trees. Environmental factors and heredity can affect the composition (Fengel and Wegener, 1989; Nerg et al., 1994). In addition, content of wood oils is different in sapwood and heartwood; e.g., more VOCs are emitted from heartwood of pine than from sapwood. VOC emissions from the wood surface decrease quickly during the storage. VOC emissions from pine wood decrease ca. 50% after 14 days storage (Dix et al., 2004a,b; Roffael, 2006). Hardwood contains less terpenoids and more large organic compounds than softwood (Fengel and Wegener, 1989). The terpene content of softwood decreases during the heating process. In the case of Scots pine, terpene content decreased in heat treatment process from ca. 70% to 10% (Manninen et al., 2002). Degradation of monoterpenes appears already at 120 C occurring via dehydrogenations, epoxidations, double bond cleavages, allylic oxidations, and rearrangements (McGraw et al., 1999). At the same time when the emissions of terpenes from Scots pine decreased, emissions of aldehydes, carboxylic acids, and ketones increased (Manninen et al., 2002). Similar effect was observed for spruce (Peters et al., 2008). In the present study, VOC emissions of three air-dried and heattreated wood species were compared in emission chamber tests. The species included Norway spruce (Picea abies), Scots pine
5029
(Pinus sylvesteris), and European aspen (Populus tremula) wood, which are commonly used indoor decorations, furnitures and materials. The test protocol was similar to that required by the ISO 16000-9 standard. Thus, the present study provides information about the specific VOC emission rates of these three wood species. In addition, the early development of emissions from fresh wood samples was followed during the first four weeks. In the previous studies, several totally different testing approaches have been used which makes the comparison of their results difficult. In addition, merely the relative proportions of different VOCs were reported in the only earlier emission chamber experiment for air-dried and heat-treated Scots pine wood (Manninen et al., 2002). 2. Materials and methods Two identical metal chambers (volume 120L) were used in the tests. Compressed air was first cleaned from VOCs by activated carbon and air flows were adjusted by mass flow meters (air exchange rate 0.5 h1). Temperature of air was 23 C and the relative humidity (RH) of air was adjusted to ca. 50% by mixing dry air and air moistened by bubbling it through deionized water. Temperature and RH data were collected continuously. The test conditions fulfilled the Emission Classification test protocol for Finnish building materials (Saarela et al., 2004) and they are also similar to the those in the ISO 16000-9 standard. The chamber conditions are presented in Table 1. 2.1. Material samples Wood samples were obtained directly from the wood heattreatment plants. Untreated (air-dried) and heat-treated aspen and spruce samples were obtained from the same tree. Air-dried wood samples were kept in dry wood storage. Samples were taken from the normal manufacturing process, wrapped in aluminium foil and delivered to the test laboratory. The wood was cut into pieces (sizes presented in Table 1) and put into the test chamber. As emission rate of VOCs also depends on age, the samples were put into the chamber as soon as possible after the delivery from the plant. In the original test protocol, testing is done only once when the material is 28 days (2) days old. In the present study, air samples were collected continuously onto the Tenax GR until their required testing day was obtained. However, the emission test with European aspen lasted in constant conditions only for 21 days because of power break and consequent malfunction of mass flow meters. During the power break (days 21e27), the European aspen wood samples were kept inside the chamber without ventilation. The test was continued after the break normally until the 36th day. The sampling scheme is presented in Table 1. 2.2. Air samples VOCs were collected onto Tenax GR adsorbent (sampling rate 200 mL min1, time 40e60 min) simultaneously from both
Table 1 Heat-treatment conditions, material loading, thickness of sample, conditions in test chambers, and sampling days. Wood species
Heat-treatment temperature
Material loading, Sample area, m2
Thickness of sample (cm)
Conditions in test Chambers (temp, C and RH, %)
Sampling days
Norway Spruce Norway Spruce (air-dried) Scots Pine Scots Pine (air-dried) European Aspen European Aspen (air-dried)
190 C, 2e3 h e 212 C, 2e3 h e 190 C, 2e3 h e
0.0462 0.0460 0.0464 0.0431 0.0441 0.0462
0.25 0.25 0.15 0.15 0.15 0.15
22.1 1.1 C, 22.7 1.0 C, 20.8 0.7 C, 20.9 0.4 C, 20.9 0.3 C, 21.2 0.5 C,
0.25, 1, 2, 3, 8, 10, 15, 20, 28
51.2 4.9% 49.9 3.3% 52.7 4.5% 47.8 13.7% 52.9 1.7% 52.6 3.1%
0.25, 1, 2, 3, 6, 10, 15, 20, 28 0.25, 1, 2, 3, 6, 10, 15, 20, 28, 29, 30, 34, 36
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M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033
chambers (sampling days presented in Table 1). Air samples were analyzed with a gas chromatograph (HP 6890) equipped with a mass selective detector (MSD 5973) after thermal desorption at 250 C for 10 min (Perkin Elmer ATD 400). The column was HP-5MS (column length 50 m, film thickness 0.5 mm), and the identification of the compounds was accomplished by retention times, standard compounds, and GC-MS data library. The concentrations were determined as the sum of the areas of compounds compared to the area responses of known amounts of toluene. Under these conditions limit of quantification of interest compounds is ca. 1 mg m3. Toluene has a linear response in TCT-GC-MS in the mass range of 0 to at least 867 ng. The TVOC value is defined to be the integrated detector response value in toluene equivalents of compounds eluting between and including C6 to C16 as given in ISO 160006:2004. The use of toluene standards underestimates the true concentrations of the polar compounds (Hyttinen et al., 2007). However, it is still commonly used method in indoor VOC analyses where numerous organic compounds are measured in single analysis. The specific emission rate, SER was calculated from the measured chamber concentrations (equation below).
h i SER ¼ C n Vc A1 ¼ C n L1 mg m2 h1 ; where C ¼ mean value of the measured chamber concentrations of a single compound [mg m3], n ¼ air exchange rate [h1], Vc ¼ chamber volume [m3], A ¼ emitting surface area of test specimen [m2], L ¼ loading factor [m2 m3]. 3. Results 3.1. Norway spruce The TVOC emission decreased from 550 mg m2h1 to 300 mg m2h1 from air-dried Norway spruce and from 220 mg m2h1 to 80 mg m2h1 from heat-treated Norway spruce during 28 days. The decrease of TVOC emission with time for both air-dried and heat-treated Norway spruce samples is presented in Fig. 1. Alpha-pinene (from 120 mg m2h1 to 40 mg m2h1), limonene (from 140 mg m2h1 to 40 mg m2h1) and beta-pinene (from 66 to 20 mg m2h1) were the main compounds emitted from airdried wood samples. The emissions of all VOCs, except that of 1methyl-4-(1-methylethenyl)-benzene, decreased during the test period. Acetic acid (17e53 mg m2h1), furfural (2-furancarboxaldehyde) (14e23 mg m2h1), and alpha-pinene (3e18 mg m2h1) were the main emitted compounds from the heat-treated Norway spruce. The SERs of nonanal and decanal increased slightly during
Specific Emission Rate (mg m -2 h-1)
0.6
the test period. Emission rates of individual compounds for the airdried and for heat-treated Norway spruce tests are presented in Tables 2 and 3 where emissions of single VOCs are followed for the period of one month starting from the second test day. 3.2. Scots pine During the four weeks measurement period, TVOC emission decreased from 2000 mg m2h1 to 1000 mg m2h1 from the airdried wood sample and from 240 mg m2h1 to 40 mg m2h1 from the heat-treated sample. The development of the emissions is presented in Fig. 2. Alpha-pinene (410e890 mg m2h1), delta-3-carene (340e680 mg 2 1 m h ), and hexanal (22e70 mg m2h1) were the main compounds emitted from the air-dried wood sample. Acetic acid was the main compound emitted from heat treated pine and its SER varied from 4 to Furfural (11e26 mg m2h1) and hexanal 86 mg m2h1. (10e17 mg m2h1) were the other main compounds emitted from heat-treated Scots pine. However, the emission of hexanal was lower from heat-treated sample compared to the air-dried one. The main compounds emitted are presented in Tables 4 and 5. 3.3. European aspen TVOC emission decreased from 950 to 190 mg m2h1 from airdried aspen and from 140 to 40 mg m2h1 from heat-treated aspen (Fig. 3). High daily fluctuations were detected in the SER of acetic acid from the air-dried sample. Hexanal (110e330 mg m2h1), pentanal (15e46 mg m2h1), and acetic acid (6e54 mg m2h1) were the main compounds emitted from air-dried aspen. For heattreated aspen, the most significant compounds emitted were acetic acid (13e170 mg m2h1), furfural (7e37 mg m2h1), and benzoic acid (0e8 mg m2h1). Emissions of furfural, propanoic acid, 2ethylhexylether, and hexanoic acid remained almost constant or even increased slightly during the test period. Daily fluctuations were again detected in emissions of acetic acid. The SERs of individual compounds for the air-dried and for heat-treated European aspen tests are presented in Tables 6 and 7. The break of six days in air flows caused a large change in the development of emission. In the test day 28, the SER of TVOC was more than doubled (550 mg m2h1) from air-dried aspen when compared with the previous measurement (the SER was 230 mg m2h1 in the test day 20). Emissions from heat-treated samples also increased
Table 2 Specific emission rates of VOCs (mg m2h1) from air-dried Norway spruce wood sample during the 4 weeks test period. Compound
Sampling days (2 to 28) 2
0.5 0.4 y = -0.065ln(x) + 0.4283 R² = 0.8048
0.3 0.2 y = -0.013ln(x) + 0.1069 R² = 0.1561
0.1 0.0 0
5
10
15
20
25
30
Day Heat-treated
Untreated
Log. ( Heat-treated )
Log. (Untreated)
Fig. 1. Specific TVOC emission rate (mg m2h1) from untreated (air-dry) and heattreated Norway Spruce wood samples.
1-Methyl-4-(1-methylethenyl)benzene 1-Methyl-4-(1-methylethyl)benzene Acetic acid Alpha-pinene Beta-Fellandrene Beta-Myrsene Beta-pinene Camphene Delta-3-carene d-Limonene Furfural (2-Furanecarboxaldehyde) Hexanal Total
3
8
10
15
20
28
7
5
3
6
4
3
5
12
8
5
9
6
5
6
8 120 14 12 66 7 19 140 1
20 102 11 10 50 7 17 120 1
15 62 9 7 38 3 11 85 1
17 75 9 8 34 7 13 105 e
17 52 6 6 28 4 9 69 0
10 40 5 4 21 3 7 55 e
12 54 5 4 38 4 6 53 e
16
14
11
9
8
6
8
422
366
251
291
211
160
196
M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033 Table 3 Specific emission rates of VOCs (mg m2h1) from heat treated Norway spruce wood sample during the 4 weeks test period. Compound
Sampling days (2 to 28) 2
1-Methyl-4-(1-methylethyl)benzene Acetic acid Alpha-pinene bis(2-Ethylhexyl)ether Camphene Decanal d-Limonene Furfural Hexanal Nonanal Total
3
8
Table 4 Specific emission rates of VOCs (mg m2h1) from air-dried Scots pine sample during the 4 weeks test period. Compound
10
15
20
28
2
1
1
1
1
0
0
53 18 2 2 2 6 18 2 2
30 11 1 1 1 4 14 1 1
41 5 2 0 3 2 19 1 2
17 5 2 0 6 2 16 1 4
46 4 2 0 2 1 22 1 2
36 3 2 0 4 3 20 1 3
28 3 3 0 5 1 23 1 4
107
67
75
54
81
71
68
3 7 31 29 1 890 20 31 20 680 28 1 37 32
4 33 22 5 710 14 23 17 560 25 1 22 27
6 22 33 19 7 610 14 24 15 490 21 5 61 23
10 7 34 23 e 640 15 26 18 540 26 1 29 35
15 13 26 16 10 440 12 20 12 380 18 1 40 26
20
28
23 12 28 19 17 17 10 3 450 410 12 11 22 17 15 9 390 340 20 13 1 1 70 34 28 22
1808 1463 1343 1395 1015 1086 909
4. Discussion TVOC emissions were significantly lower from heat-treated than from normal air-dried wood samples. Terpenes were the main compounds emitted from softwoods (Scots pine and Norway spruce) and heat-treatment decreased especially their emissions. Terpenes partly evaporate and partly degrade during the heattreatment process (McGraw et al., 1999). 4-Methyl-1-(1-methylethenyl)-benzene (p-cymenene) and 1-methyl-2-(1-methylethyl)benzene (o-cymene) were detected in higher extent in air-dried than in heat-treated softwood samples. These compounds are degradation products of camphene, delta-carene, and limonene (McGraw et al., 1999). It is possible that those compounds were at least partly formed during analysis in the thermodesorption stage of the Tenax tubes at 250 C. In the case of native Norway spruce, its SER of limonene were relatively high (50e140 mg m2h1). The SERs of its degradation products (p-cymene and p-cymenene) varied from 5 to 12 mg m2h1. Native Scots pine contained high concentrations of delta-carene (340e680 mg m2h1), and the SERs of its presumed degradation products, 1-methyl-2-isopropyl benzene (o-cymene) and methyl(1-methylethenyl)-benzene (ocymenene) were 19e34 mg m2h1 and 22e29 mg m2h1, respectively. In both cases, the maximum artefact formation during thermodesorption of the samples is about 10% of the total terpene emission from air-dried wood samples.
Table 5 Specific emission rates of VOCs (mg m2h1) from heat treated Scots pine during the 4 weeks test period.
2.5 Specific Emission Rate (mg m-2 h-1)
Sampling days (2 to 28) 2
1- Pentanol 1-Methyl-2-iso-propylbenzene 2-beta-Pinene Acetic acid Alpha-pinene Beta-Fellandrene Beta-Myrsene Camphene Delta-3-carene d-Limonene Furfural Hexanal Methyl(1-methylethenyl)-benzene Total
substantially being even higher after the break than during the first test day (230 mg m2h1). Especially, the emission of acetic acid increased considerably after the break (the SER was 50 mg m2h1 in the test day 20 and 170 mg m2h1 in the test day 28). Emissions of organic acids and aldehydes were clearly different from European aspen than from softwoods (Scots pine and Norway spruce) (Table 8). The emissions of oxygen containing organic compounds, which dominated in the emissions from aspen, were considerably lower from the heat-treated samples. There was, however, such a change that while emissions of aldehydes dominated from the air-dried sample, acetic acid was the most abundant compound from the heat-treated wood. Emissions of aldehydes decreased generally substantially due to heat treatment from aspen samples. Acetic acid and hexanal were the main oxidized compounds from both air-dried softwood species. Their emissions also dominated from the heat-treated samples. In addition, furfural, nonanal and decanal were the main emitted compounds from the heat-treated softwood samples. However, their emissions were relatively low when comparing them to the emissions of terpenes. The total SERs of terpenes for the Norway spruce and Scots pine samples are presented in Table 9, which indicates that the emissions of terpenes remained very low and almost in steady state in heat-treated wood samples whereas their emissions were high and dominated in air-dried wood samples during the whole test period. However, terpene emissions were deduced to half from their original level after one month. No emissions of terpenes were detected from European aspen.
5031
Compound
2.0 y = -0.208ln(x) + 1.8398 R² = 0.8345
1.5 1.0 y = -0.01ln(x) + 0.1274 R² = 0.0503
0.5 0.0 0
5
10
15
20
25
30
Day Heat-treated
Untreated
Log. ( Heat-treated )
Log. (Untreated)
Fig. 2. Specific TVOC emission rates from untreated and heat-treated Scots Pine wood samples.
Sampling days (2 to 28) 2
3
6
1-Metyhyl-2-iso-propylibenzene Acetic acid Alpha-pinene Benzaldehyde Benzoic acid bis(2-Ethylhexyl)ether Delta-3-carene d-Limonene Furfural Hexanal Nonanal Pentanal Tetrahydrofurane
4 4 4 3 e 3 9 6 16 13 2 3 e
3 28 2 4 1 3 5 5 11 11 3 e 7
3 13 1 3 1 2 5 5 18 12 2 2 e
10 4 86 3 12 7 3 7 6 26 17 4 3 15
15 2 36 1 5 3 3 4 4 18 10 2 2 2
20 2 49 3 16 20 3 6 4 20 13 4 3 18
28 2 27 2 6 12 3 3 3 18 11 3 2 2
Total
65
83
68
192
92
161
94
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M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033 Table 7 Specific emission rates of VOCs (mg m2h1) from heat treated European Aspen wood sample during the 5 weeks test period. Compound
Sampling days (2 to 36) 2
Fig. 3. TVOC emissions from air-dried and heat-treated European Aspen wood samples.
Terpenes and cymenes were alike found in the heat-treated wood samples, but their concentrations were low, e.g. for Norway spruce the mean terpene (camphene, delta-carene, and limonene) SER was 5 4 mg m2h1 and presumed degradation product emission due to thermosdesorption max. 1 1 mg m2h1. For the Scots pine, the corresponding values were 13 8 mg m2h1 and 3 2 mg m2h1. As expected, terpene emissions were negligible from European aspen. Emissions of aldehydes (furfural and hexanal) and carboxylic acids (acetic acid) were the most dominating compounds in heattreated softwood samples. In agreement with Peters et al. (2008), emissions of furfural increased and those of hexanal decreased in heat-treated wood samples when compared to the air-dried ones. This phenomenon was observed in all the wood species. This could also be expected, because hexanal belongs to the prevailing aldehydes in natural wood whereas furfural is a major degradation product of hemicelluloses. The results are also consistent with another recent study for heattreated Scots pine (Manninen et al., 2002) where acetic acid, furfural, and acetone were the main products emitted. Acetone was also detected in present study but because the used method was not optimized for the analysis of acetone (low breakthrough volume at room temperature for Tenax GR) it was not presented in the results. Aldehydes (pentanal and hexanal) and carboxylic acids (acetic acid) dominated in air-dried VOC emissions of European aspen. Pentanal and hexanal can be formed when unsaturated fatty acids are oxidized (Risholm-Sundman et al., 1998). Both compounds have reported to emit from hardwoods (Risholm-Sundman et al., 1998; Table 6 Specific emission rates of VOCs (mg m2h1) from air dried European Aspen wood sample during the 5 weeks test period. Compound
Sampling days (2 to 36) 2
3
6
11
15
20
28
29
30
34
36
1-Hexanol 10 7 5 6 4 4 8 3 4 8 3 1-Pentanol 25 17 12 13 10 11 18 8 10 18 8 1-Penten-3-ol 17 11 9 10 7 8 10 5 6 10 5 1-Penten-3-one 7 5 e e e e e e e e e 2-Ethyl-furane 6 5 4 5 4 4 5 3 4 5 3 2-Pentenal 5 3 3 3 2 2 4 2 2 4 2 Acetic acid 54 6 24 8 34 18 51 17 15 51 17 Benzoic acid 1 e e 2 1 1 1 1 0 1 1 Furfural 0 0 e e e e e e e e e Hexanal 330 220 180 190 140 150 180 110 120 180 110 Hexanoic acid 9 2 3 e 11 5 9 3 2 9 3 Pentanal 46 34 22 23 19 20 29 15 17 29 15 Propanoic acid 3 e 4 5 5 4 5 3 3 5 3 Total
513 311 267 265 239 227 550 137 320 170 183
3
6
11
15
20
28
29
34
36
1 1 88 2 1 2 7 1 e 1 1 e 3 2
1 1 75 e 1 1 8 1 3 1 0 e 4 2
0 1 43 e 2 1 10 1 3 1 1 e 2 1
e 1 13 3 e 5 11 1 e 3 1 e e 1
e 1 63 e 7 2 11 1 2 1 e 3 1
e 1 52 e 1 3 11 1 3 2 1 e 2 1
e 2 170 e 2 2 37 1 e 2 1 1 7 2
e 2 92 e 1 3 11 1 5 3 1 e 3 4
e 2 37 6 8 2 9 1 1 2 1 1 4 e
2 2 71 e 1 2 9 1 5 2 1 e 3 1
3 1 100 5 7 3 7 5 e 2 1 e 3 5
109
99
65
38
94
78
228
90
125
73
99
1-Butanol 2-Ethylhexylether Acetic acid Benzaldehyde Benzoic acid Decanal Furfural Hexanal Hexanoic acid Nonanal Octanal Pentanal Propanoic acid Toluene Total
30
Otwell et al., 2000). As opposite to Norway spruce and Scots pine emissions, heat-treatment decreased the emissions of oxidized organic compounds from European aspen. In addition, alcohols (pentanol, hexanol) were also commonly found in air-dried aspen wood samples. Acetic acid was the main emission product in all the heattreated wood emissions (especially from aspen). The formation of acetic acid during heat treatment of birch wood has been reported earlier (Sundqvist et al., 2006). The emissions of acetic acid fluctuated considerably more for the heat-treated wood samples than air-dried ones. There is no clear explanation for that because case conditions were stable and variation in temperature and RH were less than 5%. One explanation can be diffusion of acetic acid from inside the wood material to the surface if it was not evenly distributed in the bulk samples. In addition, the HP-5 column used is not the ideal one for the determination of such very polar compounds as acetic acid. Furthermore, hemicelluloses of hardwood contain more acetylic groups than softwood, which can thermally split to produce acetic acid (Fengel and Wegener, 1989). Furfural was also commonly found and it is one of the degradation products of cellulose and hemicelluloses (Fengel and Wegener, 1989; Peters et al., 2008). The highest VOC emissions from European aspen were measured in the test day 28 after the six days break (no air flow between the days 21 to 27) in air flows. Even though the RH of air remained quite stable (40e60%), this resulted in high VOC emissions for several days after the air flow was turned on (test days 28 and 30). Only after that emissions decreased to the similar level as they were before the break in air flows. VOC emission profile changed dramatically during the heattreatment process. Although VOC emissions were lower from the heat-treated than air-treated wood samples, oxidized organic Table 8 Specific emission rates of organic acids and aldehydes (mg m2h1) from air-dried and heat-treated European Aspen, Norway Spruce and Scots Pine. Wood sample
Sampling days (2 to 28) 2
3
6
10
15
20
28
Aspen (air-dried) Aspen (heat-treated) Spruce (air-dried) Spruce (heat-treated) Pine (air-dried) Pine (heat-treated)
448 105 25 77 38 40
266 95 36 49 54 58
236 63 27 66 48 51
231 36 26 44 142 155
213 91 26 73 72 76
200 76 16 63 109 125
479a 224a 20 61 73 79
a
Samples taken after power break.
M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033 Table 9 Specific emission rates of terpenes (mg m2h1) from air-dried and heat-treated Norway Spruce and Scots Pine. Wood sample
Spruce (air-dried) Spruce (heat-treated) Pine (air-dried) Pine (heat-treated)
Sampling days (2 to 28) 2
3
6
10
15
20
28
340 26 1698 19
283 16 1371 12
195 7 1192 12
222 7 1289 16
156 6 898 9
121 3 926 13
150 4 818 9
compounds were also formed during the treatment. These have more unpleasant odor and are typically more irritating than terpenes which dominate in the emissions of native softwoods. However, air-dried wood samples also emitted oxidized organic compounds, and terpenes might be oxidized to aldehydes and acids during the usage of the wood product (especially when there is ozone present in air). In present study, emissions of wood species were tested a one month. Actually, VOC emissions from wood (as any other materials) in constant conditions keep decreasing at least one year. However, TVOC emissions from heat-treated wood products were relatively low already in the first test days. 5. Conclusions Heat-treatment decreases and changes the composition of VOC emissions. The present study, which was conducted by using standardized emission testing, confirmed largely earlier findings on the emissions of heat-treated wood obtained by applying miscellaneous test methodology. TVOC emissions are significantly lower after the heat-treatment than before that. Especially emissions of terpenes from softwood decrease substantially during the heat-treatment. Heat-treatment increases the emission of furfural and decreases the emission of hexanal from all the tested wood species. Even though emissions of aldehydes and carboxylic acids, which have low odour threshold values, dominate emissions of heat-treated wood, their emission rates remained around the same level in softwood samples and in case of European aspen lower than the corresponding emissions from the air-dried wood samples. Therefore, in addition to the beneficial other properties the heat-treatment, the treated wood can be regarded a safe construction material for indoor air quality. References Boonstra, M.J., van Acker, J., Kegel, E., Stevens, M., 2007. Optimisation of a two-stage heat treatment process: durability aspects. Wood Science and Technology 41, 31e57.
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Dix, B., Roffael, E., Schneider, T., 2004a. Abgabe von flüchtigen Verbindungen (Volatile Organic Compounds, VOC) von Strands, hergestellt aus Kern- und Splintholz der Kiefer. WKI Kurzbericht short report 6/2004. Dix, B., Roffael, E., Schneider, T., 2004b. Einfluss der Lagerung auf die Bildung und Agabe von flüchtigen organischen Verbindungen, WKI-Kurzbericht short report 7/2004. Fengel, D., Wegener, G., 1989. Wood e Chemistry, Ultrastructure, Reactions. Berliini, Saksa: Walter de Gruyter, 344 pp. Hyttinen, M., Pasanen, P., Björkroth, M., Kalliokoski, P., 2007. Odors and volatile organic compounds released from the ventilation filters. Atmospheric Environment 41, 4029e4039. Kamdem, D.P., Pizzi, A., Triboulot, M.C., 2000. Heat-treated timber: potentially toxic byproducts presence and extent of wood cell wall degradation. Holz als Rohund Werkstoff 58, 253e257. Kamdem, D.P., Pizzi, A., Jermannaud, A., 2002. Durability of heat-treated wood. Holz als Roh- und Werkstoff 60, 1e6. Kotilainen, R., Alen, R., Arpiainen, V., 1999. Changes in the chemical composition of Norway spruce (Picea abies) at 160e260 C under nitrogen and air atmospheres. Paperi ja Puu 81, 384e388. Kotilainen, R., 2000. Chemical changes in wood during heating at 150e260 C. Academic Dissertation, Department of Chemistry, University of Jyväskylä, Research Report No. 80, ISBN: 951-39-0848-8, 57 pp. Manninen, A.-M., Pasanen, P., Holopainen, J.K., 2002. Comparing the VOC emissions between air-dried and heat-treated Scots pine wood. Atmospheric Environment 36, 1763e1768. McGraw, G.W., Hemingway, R.W., Ingram Jr., L.L., Canady, C.S., McGraw, W.B., 1999. Thermal degradation of terpenes: camphene, D3-carene, limonene, and a-terpinene. Environmental Science and Technology 33, 4029e4033. Nerg, A., Kainulainen, P., Vuorinen, M., Hanso, M., Holopainen, J.K., Kurkela, T., 1994. Seasonal and geographical variation of terpenes, resin acids and total phenolics in nursery grown seedlings of scots pine (Pinus Sylvestris L.). New Phytologist 128, 703e713. Otwell, L.P., Hittmeier, M.E., Hooda, U., Yan, H., Su, W., Banerjee, S., 2000. HAPs release from wood drying. Environmental Science and Technology 34, 2280e2283. Peters, J., Fischer, K., Fischer, S., 2008. Characterization of emissions from thermally modified wood and their reduction by chemical treatment. Bioresources 3, 491e502. Risholm-Sundman, M., Lundgren, M., Vestin, E., Herder, P., 1998. Emissions of acetic acid and other volatile organic compounds from different species of solid wood. Holz als Roh- und Werkstoff 56, 125e129. Roffael, E., 2006. Volatile organic compounds and formaldehyde in nature, wood and wood based panels. Holz als Roh- und Werkstoff 64, 144e149. Saarela, K., Tirkkonen, T., Björkroth, M., Kukkonen, E., Seppänen, O., Tuomainen, M., 2004. Emission Classification of Building Materials: Protocol for Chemical and Sensory Testing of Building Materials. The Building Information Foundation RTS, Finland, 45 pp. Shi, J., Kocaefe, D., Zhang, J., 2007. Mechanical behaviour of Quebec wood species heat-treated using ThermoWood process. Holz als Roh- und Werkstoff 65, 255e259. Sjöström, E., 1993. Wood chemistry. Fundamentals and Applications, second ed. Academic Press, Inc., San Diego, pp. 1e293. Sundqvist, B., Karlsson, O., Westermark, U., 2006. Determination of formic-acid and acetic acid concentrations formed during hydrothermal treatment of birch wood and its relation to colour, strength and hardness. Wood Science and Technology 40, 549e561. Temiz, A., Terziev, N., Jacobsen, B., Eikenes, M., 2006. Weathering, water absorption, and durability of silicon, acetylated, and heat-treated wood. Journal of Applied Polymer Science 102, 4506e4513.
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Comparison of VOC emissions between air-dried and heat-treated Norway spruce (Picea abies), Scots pine (Pinus sylvesteris) and European aspen (Populus tremula) wood Marko Hyttinen*, Marika Masalin-Weijo, Pentti Kalliokoski, Pertti Pasanen Department of Environmental Science, University of Eastern Finland, P.O. Box 1627, Kuopio 70211, Finland
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 February 2010 Received in revised form 11 June 2010 Accepted 12 July 2010
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 airdried 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. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: VOC Emissions Heat-treatment Wood Air-dried
1. Introduction Heat-treated wood is a popular decoration material and it has been used in floor, wall, and ceiling materials and in furniture. The total production of heat treated timber in industrial processes was ca. 250,000 m3 in 2007 and its annual grow is estimated to be 30% (www.westwoodcorporation.com). Heat-treatment darkens the wood and helps to sustain better UV radiation and rot fungus (Temiz et al., 2006; Boonstra et al., 2007). Water uptake of wood is also reduced by the heat-treatment process. In addition, heat insulation properties and dimensional stability increase after the treatment, but bending strength of the wood can decrease (Kamdem et al., 2000, 2002). Compress treatment combined to heating process increases the coating hardness and elasticity of the wood. Besides the wood species, the process parameters, temperature and pressure determine the properties of the product (Shi et al., 2007). Heat-treatment is an environmentally friendly way to process the wood because no toxic chemicals are used. However,
* Corresponding author. E-mail address: marko.hyttinen@uef.fi (M. Hyttinen). 1352-2310/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2010.07.018
heat treatment process changes the composition of the wood permanently and also affects organic emissions from the material. In Nordic countries, common heat-treatment process is called Thermowood-process. Raw wood material used in process can be fresh or oven dry soft or hardwood (usually Norway spruce, Scots pine, European aspen or Silver birch). Thermowood-process can be divided in three stages: temperature rise, heat-treatment, and cooling and humidification. In the first stage, temperature of the chamber is risen quickly to 100 C by heated steam, and after that steadily to the 130 C when wood is heated steam-dry. Then the actual heat-treatment process starts and oven temperature is kept at 185e215 C for 3 h. Finally, the cooling period starts by water steam and temperature decreases to 80e90 C. Water steam prevents wood both to catch fire, and to split up. Heat treatment affects all the wood components, i.e., cellulose, hemicelluloses, lignin, and extractives. Emissions and degradation products of wood differ according to wood species. Especially, differences can be detected between the hard- and softwood, which have different cell types (Sjöström, 1993). Hemicelluloses usually crack down thermally easier than cellulose or lignin because of its heterogeneous structure and lack of crystallinity. Degradation of hemicelluloses occurs intensively at 200e260 C. Aliphatic carboxylic acids (mainly formic and acetic
M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033
acids) are the major volatile degradation products formed during the heat treatment (Kotilainen et al., 1999; Kotilainen, 2000; Sundqvist et al., 2006). On the other hand, large amounts of acetic acid were detected already from native wood samples (Peters et al., 2008). Smaller emissions of acetic acid were observed from spruce than from hardwood due to greater number of acetyl groups in hardwood hemicelluloses. In addition, increased emissions of furfural from ash, beech, maple, and spruce were detected due to thermal treatment, which was interpreted to be caused by degradation of hemicelluloses. On the other hand, the emissions of the prevailing aldehydes in natural wood, pentanal and hexanal were observed to decrease during thermal treatment (Peters et al., 2008). Extractives can be divided into saturated and unsaturated fatty acids, glycerides, resin acids, steroids, steryl esters, terpenoids, and waxes. Some of them can be degradated and cause emissions of volatile organic compounds (VOC) or semi volatile organic compounds (SVOC). Among the unsaturated fatty acids oleic, (cis-9octadecenoic acid) and linoleic (cis,cis-9,12-octadecadienoic acid) acid are the dominating compounds (Sjöström, 1993), which may degrade further at high temperatures. Terpenoids are emitted readily from the wood even without rise of temperature. They are among the dominating VOCs emitted especially from conifers. Softwood contains terpenes from monoto tetraterpenes, except sesterterpenes which are rare (Fengel and Wegener, 1989). Volatile wood oil consists of mainly monoterpenes and 80% of the emitted VOCs from fresh softwood are monoterpenes (Roffael and references therein, 2006). Monoterpenes can be divided to acyclic, monocyclic, and dicyclic compounds. The most important monoterpenes are alpha- and beta-pinene and limonene which all can be found in softwood. In addition, deltacarene, camphene, myrcene, and b-fellandrene are common monoterpenes (Fengel and Wegener, 1989). Wood oil content differs between wood species and even between single trees. Environmental factors and heredity can affect the composition (Fengel and Wegener, 1989; Nerg et al., 1994). In addition, content of wood oils is different in sapwood and heartwood; e.g., more VOCs are emitted from heartwood of pine than from sapwood. VOC emissions from the wood surface decrease quickly during the storage. VOC emissions from pine wood decrease ca. 50% after 14 days storage (Dix et al., 2004a,b; Roffael, 2006). Hardwood contains less terpenoids and more large organic compounds than softwood (Fengel and Wegener, 1989). The terpene content of softwood decreases during the heating process. In the case of Scots pine, terpene content decreased in heat treatment process from ca. 70% to 10% (Manninen et al., 2002). Degradation of monoterpenes appears already at 120 C occurring via dehydrogenations, epoxidations, double bond cleavages, allylic oxidations, and rearrangements (McGraw et al., 1999). At the same time when the emissions of terpenes from Scots pine decreased, emissions of aldehydes, carboxylic acids, and ketones increased (Manninen et al., 2002). Similar effect was observed for spruce (Peters et al., 2008). In the present study, VOC emissions of three air-dried and heattreated wood species were compared in emission chamber tests. The species included Norway spruce (Picea abies), Scots pine
5029
(Pinus sylvesteris), and European aspen (Populus tremula) wood, which are commonly used indoor decorations, furnitures and materials. The test protocol was similar to that required by the ISO 16000-9 standard. Thus, the present study provides information about the specific VOC emission rates of these three wood species. In addition, the early development of emissions from fresh wood samples was followed during the first four weeks. In the previous studies, several totally different testing approaches have been used which makes the comparison of their results difficult. In addition, merely the relative proportions of different VOCs were reported in the only earlier emission chamber experiment for air-dried and heat-treated Scots pine wood (Manninen et al., 2002). 2. Materials and methods Two identical metal chambers (volume 120L) were used in the tests. Compressed air was first cleaned from VOCs by activated carbon and air flows were adjusted by mass flow meters (air exchange rate 0.5 h1). Temperature of air was 23 C and the relative humidity (RH) of air was adjusted to ca. 50% by mixing dry air and air moistened by bubbling it through deionized water. Temperature and RH data were collected continuously. The test conditions fulfilled the Emission Classification test protocol for Finnish building materials (Saarela et al., 2004) and they are also similar to the those in the ISO 16000-9 standard. The chamber conditions are presented in Table 1. 2.1. Material samples Wood samples were obtained directly from the wood heattreatment plants. Untreated (air-dried) and heat-treated aspen and spruce samples were obtained from the same tree. Air-dried wood samples were kept in dry wood storage. Samples were taken from the normal manufacturing process, wrapped in aluminium foil and delivered to the test laboratory. The wood was cut into pieces (sizes presented in Table 1) and put into the test chamber. As emission rate of VOCs also depends on age, the samples were put into the chamber as soon as possible after the delivery from the plant. In the original test protocol, testing is done only once when the material is 28 days (2) days old. In the present study, air samples were collected continuously onto the Tenax GR until their required testing day was obtained. However, the emission test with European aspen lasted in constant conditions only for 21 days because of power break and consequent malfunction of mass flow meters. During the power break (days 21e27), the European aspen wood samples were kept inside the chamber without ventilation. The test was continued after the break normally until the 36th day. The sampling scheme is presented in Table 1. 2.2. Air samples VOCs were collected onto Tenax GR adsorbent (sampling rate 200 mL min1, time 40e60 min) simultaneously from both
Table 1 Heat-treatment conditions, material loading, thickness of sample, conditions in test chambers, and sampling days. Wood species
Heat-treatment temperature
Material loading, Sample area, m2
Thickness of sample (cm)
Conditions in test Chambers (temp, C and RH, %)
Sampling days
Norway Spruce Norway Spruce (air-dried) Scots Pine Scots Pine (air-dried) European Aspen European Aspen (air-dried)
190 C, 2e3 h e 212 C, 2e3 h e 190 C, 2e3 h e
0.0462 0.0460 0.0464 0.0431 0.0441 0.0462
0.25 0.25 0.15 0.15 0.15 0.15
22.1 1.1 C, 22.7 1.0 C, 20.8 0.7 C, 20.9 0.4 C, 20.9 0.3 C, 21.2 0.5 C,
0.25, 1, 2, 3, 8, 10, 15, 20, 28
51.2 4.9% 49.9 3.3% 52.7 4.5% 47.8 13.7% 52.9 1.7% 52.6 3.1%
0.25, 1, 2, 3, 6, 10, 15, 20, 28 0.25, 1, 2, 3, 6, 10, 15, 20, 28, 29, 30, 34, 36
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M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033
chambers (sampling days presented in Table 1). Air samples were analyzed with a gas chromatograph (HP 6890) equipped with a mass selective detector (MSD 5973) after thermal desorption at 250 C for 10 min (Perkin Elmer ATD 400). The column was HP-5MS (column length 50 m, film thickness 0.5 mm), and the identification of the compounds was accomplished by retention times, standard compounds, and GC-MS data library. The concentrations were determined as the sum of the areas of compounds compared to the area responses of known amounts of toluene. Under these conditions limit of quantification of interest compounds is ca. 1 mg m3. Toluene has a linear response in TCT-GC-MS in the mass range of 0 to at least 867 ng. The TVOC value is defined to be the integrated detector response value in toluene equivalents of compounds eluting between and including C6 to C16 as given in ISO 160006:2004. The use of toluene standards underestimates the true concentrations of the polar compounds (Hyttinen et al., 2007). However, it is still commonly used method in indoor VOC analyses where numerous organic compounds are measured in single analysis. The specific emission rate, SER was calculated from the measured chamber concentrations (equation below).
h i SER ¼ C n Vc A1 ¼ C n L1 mg m2 h1 ; where C ¼ mean value of the measured chamber concentrations of a single compound [mg m3], n ¼ air exchange rate [h1], Vc ¼ chamber volume [m3], A ¼ emitting surface area of test specimen [m2], L ¼ loading factor [m2 m3]. 3. Results 3.1. Norway spruce The TVOC emission decreased from 550 mg m2h1 to 300 mg m2h1 from air-dried Norway spruce and from 220 mg m2h1 to 80 mg m2h1 from heat-treated Norway spruce during 28 days. The decrease of TVOC emission with time for both air-dried and heat-treated Norway spruce samples is presented in Fig. 1. Alpha-pinene (from 120 mg m2h1 to 40 mg m2h1), limonene (from 140 mg m2h1 to 40 mg m2h1) and beta-pinene (from 66 to 20 mg m2h1) were the main compounds emitted from airdried wood samples. The emissions of all VOCs, except that of 1methyl-4-(1-methylethenyl)-benzene, decreased during the test period. Acetic acid (17e53 mg m2h1), furfural (2-furancarboxaldehyde) (14e23 mg m2h1), and alpha-pinene (3e18 mg m2h1) were the main emitted compounds from the heat-treated Norway spruce. The SERs of nonanal and decanal increased slightly during
Specific Emission Rate (mg m -2 h-1)
0.6
the test period. Emission rates of individual compounds for the airdried and for heat-treated Norway spruce tests are presented in Tables 2 and 3 where emissions of single VOCs are followed for the period of one month starting from the second test day. 3.2. Scots pine During the four weeks measurement period, TVOC emission decreased from 2000 mg m2h1 to 1000 mg m2h1 from the airdried wood sample and from 240 mg m2h1 to 40 mg m2h1 from the heat-treated sample. The development of the emissions is presented in Fig. 2. Alpha-pinene (410e890 mg m2h1), delta-3-carene (340e680 mg 2 1 m h ), and hexanal (22e70 mg m2h1) were the main compounds emitted from the air-dried wood sample. Acetic acid was the main compound emitted from heat treated pine and its SER varied from 4 to Furfural (11e26 mg m2h1) and hexanal 86 mg m2h1. (10e17 mg m2h1) were the other main compounds emitted from heat-treated Scots pine. However, the emission of hexanal was lower from heat-treated sample compared to the air-dried one. The main compounds emitted are presented in Tables 4 and 5. 3.3. European aspen TVOC emission decreased from 950 to 190 mg m2h1 from airdried aspen and from 140 to 40 mg m2h1 from heat-treated aspen (Fig. 3). High daily fluctuations were detected in the SER of acetic acid from the air-dried sample. Hexanal (110e330 mg m2h1), pentanal (15e46 mg m2h1), and acetic acid (6e54 mg m2h1) were the main compounds emitted from air-dried aspen. For heattreated aspen, the most significant compounds emitted were acetic acid (13e170 mg m2h1), furfural (7e37 mg m2h1), and benzoic acid (0e8 mg m2h1). Emissions of furfural, propanoic acid, 2ethylhexylether, and hexanoic acid remained almost constant or even increased slightly during the test period. Daily fluctuations were again detected in emissions of acetic acid. The SERs of individual compounds for the air-dried and for heat-treated European aspen tests are presented in Tables 6 and 7. The break of six days in air flows caused a large change in the development of emission. In the test day 28, the SER of TVOC was more than doubled (550 mg m2h1) from air-dried aspen when compared with the previous measurement (the SER was 230 mg m2h1 in the test day 20). Emissions from heat-treated samples also increased
Table 2 Specific emission rates of VOCs (mg m2h1) from air-dried Norway spruce wood sample during the 4 weeks test period. Compound
Sampling days (2 to 28) 2
0.5 0.4 y = -0.065ln(x) + 0.4283 R² = 0.8048
0.3 0.2 y = -0.013ln(x) + 0.1069 R² = 0.1561
0.1 0.0 0
5
10
15
20
25
30
Day Heat-treated
Untreated
Log. ( Heat-treated )
Log. (Untreated)
Fig. 1. Specific TVOC emission rate (mg m2h1) from untreated (air-dry) and heattreated Norway Spruce wood samples.
1-Methyl-4-(1-methylethenyl)benzene 1-Methyl-4-(1-methylethyl)benzene Acetic acid Alpha-pinene Beta-Fellandrene Beta-Myrsene Beta-pinene Camphene Delta-3-carene d-Limonene Furfural (2-Furanecarboxaldehyde) Hexanal Total
3
8
10
15
20
28
7
5
3
6
4
3
5
12
8
5
9
6
5
6
8 120 14 12 66 7 19 140 1
20 102 11 10 50 7 17 120 1
15 62 9 7 38 3 11 85 1
17 75 9 8 34 7 13 105 e
17 52 6 6 28 4 9 69 0
10 40 5 4 21 3 7 55 e
12 54 5 4 38 4 6 53 e
16
14
11
9
8
6
8
422
366
251
291
211
160
196
M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033 Table 3 Specific emission rates of VOCs (mg m2h1) from heat treated Norway spruce wood sample during the 4 weeks test period. Compound
Sampling days (2 to 28) 2
1-Methyl-4-(1-methylethyl)benzene Acetic acid Alpha-pinene bis(2-Ethylhexyl)ether Camphene Decanal d-Limonene Furfural Hexanal Nonanal Total
3
8
Table 4 Specific emission rates of VOCs (mg m2h1) from air-dried Scots pine sample during the 4 weeks test period. Compound
10
15
20
28
2
1
1
1
1
0
0
53 18 2 2 2 6 18 2 2
30 11 1 1 1 4 14 1 1
41 5 2 0 3 2 19 1 2
17 5 2 0 6 2 16 1 4
46 4 2 0 2 1 22 1 2
36 3 2 0 4 3 20 1 3
28 3 3 0 5 1 23 1 4
107
67
75
54
81
71
68
3 7 31 29 1 890 20 31 20 680 28 1 37 32
4 33 22 5 710 14 23 17 560 25 1 22 27
6 22 33 19 7 610 14 24 15 490 21 5 61 23
10 7 34 23 e 640 15 26 18 540 26 1 29 35
15 13 26 16 10 440 12 20 12 380 18 1 40 26
20
28
23 12 28 19 17 17 10 3 450 410 12 11 22 17 15 9 390 340 20 13 1 1 70 34 28 22
1808 1463 1343 1395 1015 1086 909
4. Discussion TVOC emissions were significantly lower from heat-treated than from normal air-dried wood samples. Terpenes were the main compounds emitted from softwoods (Scots pine and Norway spruce) and heat-treatment decreased especially their emissions. Terpenes partly evaporate and partly degrade during the heattreatment process (McGraw et al., 1999). 4-Methyl-1-(1-methylethenyl)-benzene (p-cymenene) and 1-methyl-2-(1-methylethyl)benzene (o-cymene) were detected in higher extent in air-dried than in heat-treated softwood samples. These compounds are degradation products of camphene, delta-carene, and limonene (McGraw et al., 1999). It is possible that those compounds were at least partly formed during analysis in the thermodesorption stage of the Tenax tubes at 250 C. In the case of native Norway spruce, its SER of limonene were relatively high (50e140 mg m2h1). The SERs of its degradation products (p-cymene and p-cymenene) varied from 5 to 12 mg m2h1. Native Scots pine contained high concentrations of delta-carene (340e680 mg m2h1), and the SERs of its presumed degradation products, 1-methyl-2-isopropyl benzene (o-cymene) and methyl(1-methylethenyl)-benzene (ocymenene) were 19e34 mg m2h1 and 22e29 mg m2h1, respectively. In both cases, the maximum artefact formation during thermodesorption of the samples is about 10% of the total terpene emission from air-dried wood samples.
Table 5 Specific emission rates of VOCs (mg m2h1) from heat treated Scots pine during the 4 weeks test period.
2.5 Specific Emission Rate (mg m-2 h-1)
Sampling days (2 to 28) 2
1- Pentanol 1-Methyl-2-iso-propylbenzene 2-beta-Pinene Acetic acid Alpha-pinene Beta-Fellandrene Beta-Myrsene Camphene Delta-3-carene d-Limonene Furfural Hexanal Methyl(1-methylethenyl)-benzene Total
substantially being even higher after the break than during the first test day (230 mg m2h1). Especially, the emission of acetic acid increased considerably after the break (the SER was 50 mg m2h1 in the test day 20 and 170 mg m2h1 in the test day 28). Emissions of organic acids and aldehydes were clearly different from European aspen than from softwoods (Scots pine and Norway spruce) (Table 8). The emissions of oxygen containing organic compounds, which dominated in the emissions from aspen, were considerably lower from the heat-treated samples. There was, however, such a change that while emissions of aldehydes dominated from the air-dried sample, acetic acid was the most abundant compound from the heat-treated wood. Emissions of aldehydes decreased generally substantially due to heat treatment from aspen samples. Acetic acid and hexanal were the main oxidized compounds from both air-dried softwood species. Their emissions also dominated from the heat-treated samples. In addition, furfural, nonanal and decanal were the main emitted compounds from the heat-treated softwood samples. However, their emissions were relatively low when comparing them to the emissions of terpenes. The total SERs of terpenes for the Norway spruce and Scots pine samples are presented in Table 9, which indicates that the emissions of terpenes remained very low and almost in steady state in heat-treated wood samples whereas their emissions were high and dominated in air-dried wood samples during the whole test period. However, terpene emissions were deduced to half from their original level after one month. No emissions of terpenes were detected from European aspen.
5031
Compound
2.0 y = -0.208ln(x) + 1.8398 R² = 0.8345
1.5 1.0 y = -0.01ln(x) + 0.1274 R² = 0.0503
0.5 0.0 0
5
10
15
20
25
30
Day Heat-treated
Untreated
Log. ( Heat-treated )
Log. (Untreated)
Fig. 2. Specific TVOC emission rates from untreated and heat-treated Scots Pine wood samples.
Sampling days (2 to 28) 2
3
6
1-Metyhyl-2-iso-propylibenzene Acetic acid Alpha-pinene Benzaldehyde Benzoic acid bis(2-Ethylhexyl)ether Delta-3-carene d-Limonene Furfural Hexanal Nonanal Pentanal Tetrahydrofurane
4 4 4 3 e 3 9 6 16 13 2 3 e
3 28 2 4 1 3 5 5 11 11 3 e 7
3 13 1 3 1 2 5 5 18 12 2 2 e
10 4 86 3 12 7 3 7 6 26 17 4 3 15
15 2 36 1 5 3 3 4 4 18 10 2 2 2
20 2 49 3 16 20 3 6 4 20 13 4 3 18
28 2 27 2 6 12 3 3 3 18 11 3 2 2
Total
65
83
68
192
92
161
94
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M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033 Table 7 Specific emission rates of VOCs (mg m2h1) from heat treated European Aspen wood sample during the 5 weeks test period. Compound
Sampling days (2 to 36) 2
Fig. 3. TVOC emissions from air-dried and heat-treated European Aspen wood samples.
Terpenes and cymenes were alike found in the heat-treated wood samples, but their concentrations were low, e.g. for Norway spruce the mean terpene (camphene, delta-carene, and limonene) SER was 5 4 mg m2h1 and presumed degradation product emission due to thermosdesorption max. 1 1 mg m2h1. For the Scots pine, the corresponding values were 13 8 mg m2h1 and 3 2 mg m2h1. As expected, terpene emissions were negligible from European aspen. Emissions of aldehydes (furfural and hexanal) and carboxylic acids (acetic acid) were the most dominating compounds in heattreated softwood samples. In agreement with Peters et al. (2008), emissions of furfural increased and those of hexanal decreased in heat-treated wood samples when compared to the air-dried ones. This phenomenon was observed in all the wood species. This could also be expected, because hexanal belongs to the prevailing aldehydes in natural wood whereas furfural is a major degradation product of hemicelluloses. The results are also consistent with another recent study for heattreated Scots pine (Manninen et al., 2002) where acetic acid, furfural, and acetone were the main products emitted. Acetone was also detected in present study but because the used method was not optimized for the analysis of acetone (low breakthrough volume at room temperature for Tenax GR) it was not presented in the results. Aldehydes (pentanal and hexanal) and carboxylic acids (acetic acid) dominated in air-dried VOC emissions of European aspen. Pentanal and hexanal can be formed when unsaturated fatty acids are oxidized (Risholm-Sundman et al., 1998). Both compounds have reported to emit from hardwoods (Risholm-Sundman et al., 1998; Table 6 Specific emission rates of VOCs (mg m2h1) from air dried European Aspen wood sample during the 5 weeks test period. Compound
Sampling days (2 to 36) 2
3
6
11
15
20
28
29
30
34
36
1-Hexanol 10 7 5 6 4 4 8 3 4 8 3 1-Pentanol 25 17 12 13 10 11 18 8 10 18 8 1-Penten-3-ol 17 11 9 10 7 8 10 5 6 10 5 1-Penten-3-one 7 5 e e e e e e e e e 2-Ethyl-furane 6 5 4 5 4 4 5 3 4 5 3 2-Pentenal 5 3 3 3 2 2 4 2 2 4 2 Acetic acid 54 6 24 8 34 18 51 17 15 51 17 Benzoic acid 1 e e 2 1 1 1 1 0 1 1 Furfural 0 0 e e e e e e e e e Hexanal 330 220 180 190 140 150 180 110 120 180 110 Hexanoic acid 9 2 3 e 11 5 9 3 2 9 3 Pentanal 46 34 22 23 19 20 29 15 17 29 15 Propanoic acid 3 e 4 5 5 4 5 3 3 5 3 Total
513 311 267 265 239 227 550 137 320 170 183
3
6
11
15
20
28
29
34
36
1 1 88 2 1 2 7 1 e 1 1 e 3 2
1 1 75 e 1 1 8 1 3 1 0 e 4 2
0 1 43 e 2 1 10 1 3 1 1 e 2 1
e 1 13 3 e 5 11 1 e 3 1 e e 1
e 1 63 e 7 2 11 1 2 1 e 3 1
e 1 52 e 1 3 11 1 3 2 1 e 2 1
e 2 170 e 2 2 37 1 e 2 1 1 7 2
e 2 92 e 1 3 11 1 5 3 1 e 3 4
e 2 37 6 8 2 9 1 1 2 1 1 4 e
2 2 71 e 1 2 9 1 5 2 1 e 3 1
3 1 100 5 7 3 7 5 e 2 1 e 3 5
109
99
65
38
94
78
228
90
125
73
99
1-Butanol 2-Ethylhexylether Acetic acid Benzaldehyde Benzoic acid Decanal Furfural Hexanal Hexanoic acid Nonanal Octanal Pentanal Propanoic acid Toluene Total
30
Otwell et al., 2000). As opposite to Norway spruce and Scots pine emissions, heat-treatment decreased the emissions of oxidized organic compounds from European aspen. In addition, alcohols (pentanol, hexanol) were also commonly found in air-dried aspen wood samples. Acetic acid was the main emission product in all the heattreated wood emissions (especially from aspen). The formation of acetic acid during heat treatment of birch wood has been reported earlier (Sundqvist et al., 2006). The emissions of acetic acid fluctuated considerably more for the heat-treated wood samples than air-dried ones. There is no clear explanation for that because case conditions were stable and variation in temperature and RH were less than 5%. One explanation can be diffusion of acetic acid from inside the wood material to the surface if it was not evenly distributed in the bulk samples. In addition, the HP-5 column used is not the ideal one for the determination of such very polar compounds as acetic acid. Furthermore, hemicelluloses of hardwood contain more acetylic groups than softwood, which can thermally split to produce acetic acid (Fengel and Wegener, 1989). Furfural was also commonly found and it is one of the degradation products of cellulose and hemicelluloses (Fengel and Wegener, 1989; Peters et al., 2008). The highest VOC emissions from European aspen were measured in the test day 28 after the six days break (no air flow between the days 21 to 27) in air flows. Even though the RH of air remained quite stable (40e60%), this resulted in high VOC emissions for several days after the air flow was turned on (test days 28 and 30). Only after that emissions decreased to the similar level as they were before the break in air flows. VOC emission profile changed dramatically during the heattreatment process. Although VOC emissions were lower from the heat-treated than air-treated wood samples, oxidized organic Table 8 Specific emission rates of organic acids and aldehydes (mg m2h1) from air-dried and heat-treated European Aspen, Norway Spruce and Scots Pine. Wood sample
Sampling days (2 to 28) 2
3
6
10
15
20
28
Aspen (air-dried) Aspen (heat-treated) Spruce (air-dried) Spruce (heat-treated) Pine (air-dried) Pine (heat-treated)
448 105 25 77 38 40
266 95 36 49 54 58
236 63 27 66 48 51
231 36 26 44 142 155
213 91 26 73 72 76
200 76 16 63 109 125
479a 224a 20 61 73 79
a
Samples taken after power break.
M. Hyttinen et al. / Atmospheric Environment 44 (2010) 5028e5033 Table 9 Specific emission rates of terpenes (mg m2h1) from air-dried and heat-treated Norway Spruce and Scots Pine. Wood sample
Spruce (air-dried) Spruce (heat-treated) Pine (air-dried) Pine (heat-treated)
Sampling days (2 to 28) 2
3
6
10
15
20
28
340 26 1698 19
283 16 1371 12
195 7 1192 12
222 7 1289 16
156 6 898 9
121 3 926 13
150 4 818 9
compounds were also formed during the treatment. These have more unpleasant odor and are typically more irritating than terpenes which dominate in the emissions of native softwoods. However, air-dried wood samples also emitted oxidized organic compounds, and terpenes might be oxidized to aldehydes and acids during the usage of the wood product (especially when there is ozone present in air). In present study, emissions of wood species were tested a one month. Actually, VOC emissions from wood (as any other materials) in constant conditions keep decreasing at least one year. However, TVOC emissions from heat-treated wood products were relatively low already in the first test days. 5. Conclusions Heat-treatment decreases and changes the composition of VOC emissions. The present study, which was conducted by using standardized emission testing, confirmed largely earlier findings on the emissions of heat-treated wood obtained by applying miscellaneous test methodology. TVOC emissions are significantly lower after the heat-treatment than before that. Especially emissions of terpenes from softwood decrease substantially during the heat-treatment. Heat-treatment increases the emission of furfural and decreases the emission of hexanal from all the tested wood species. Even though emissions of aldehydes and carboxylic acids, which have low odour threshold values, dominate emissions of heat-treated wood, their emission rates remained around the same level in softwood samples and in case of European aspen lower than the corresponding emissions from the air-dried wood samples. Therefore, in addition to the beneficial other properties the heat-treatment, the treated wood can be regarded a safe construction material for indoor air quality. References Boonstra, M.J., van Acker, J., Kegel, E., Stevens, M., 2007. Optimisation of a two-stage heat treatment process: durability aspects. Wood Science and Technology 41, 31e57.
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