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Understanding of Formaldehyde Emissions from Solid Wood: An Overview

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Wood is known to contain and emit volatile organic compounds including formaldehyde. The emission of formaldehyde from wood increases during its processing to lumber and wood-based panels (i.e., particleboard and fiberboard). This increased emission can be attributed to the processing procedure of wood, which includes drying, pressing, and thermo-hydrolysis. Formaldehyde is emitted from wood under very high heat and is not expected to be a significant source of the emissions from composite wood products during normal service. Formaldehyde is also detectable even if wood has never been heated as well as under more or less ambient conditions. The presence of formaldehyde in the emissions from wood that does not contain adhesive resin has been explained by thermal degradation of polysaccharides in the wood. The emission levels of formaldehyde depend on factors such as wood species, moisture content, outside temperature, and time of storage. Additionally, the pyrolysis of milled wood lignin at 450 °C yields benzaldehyde, and the pyrolysis of spruce and pinewood at 450 °C generate formaldehyde, acetaldehyde, 2–propenal, butanal, and butanone, which can be attributed to the breakdown of the polysaccharide fraction of the wood.
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Salem and Böhm (2013). “Formaldehyde from wood,” BioResources 8(3), 4775-4790. 4775
Understanding of Formaldehyde Emissions from Solid
Wood: An Overview
Mohamed Z. M. Salem a,b* and Martin Böhm b
Wood is known to contain and emit volatile organic compounds including
formaldehyde. The emission of formaldehyde from wood increases
during its processing to lumber and wood-based panels (i.e.,
particleboard and fiberboard). This increased emission can be attributed
to the processing procedure of wood, which includes drying, pressing,
and thermo-hydrolysis. Formaldehyde is emitted from wood under very
high heat and is not expected to be a significant source of the emissions
from composite wood products during normal service. Formaldehyde is
also detectable even if wood has never been heated as well as under
more or less ambient conditions. The presence of formaldehyde in the
emissions from wood that does not contain adhesive resin has been
explained by thermal degradation of polysaccharides in the wood. The
emission levels of formaldehyde depend on factors such as wood
species, moisture content, outside temperature, and time of storage.
Additionally, the pyrolysis of milled wood lignin at 450 °C yields
benzaldehyde, and the pyrolysis of spruce and pinewood at 450 °C
generate formaldehyde, acetaldehyde, 2propenal, butanal, and
butanone, which can be attributed to the breakdown of the
polysaccharide fraction of the wood.
Keywords: Formaldehyde emission; Solid wood
Contact information: a: Forestry and Wood Technology Department, Faculty of Agriculture (ELShatby),
Alexandria University, Egypt; b: Department of Wood Products and Wood Constructions, Faculty of
Forestry and Wood Sciences, Czech University of Life Sciences, Prague, Czech Republic;
*Corresponding author: zidan_forest@yahoo.com
INTRODUCTION
The European Union, the USA, China, and Japan now have legislation regulating
the allowed levels of formaldehyde emission (FE) from wood and wood-based products,
and without doubt there will be increased focus and controls placed on products that are
known to release formaldehyde (Salthammer et al. 2010). The main sources of FE from
wood-based products such as medium density fiberboard (MDF), particleboard (PB), and
plywood are the resins used, such as urea-formaldehyde (UF), melamine-modified urea
formaldehyde (MUF), and phenol-formaldehyde (PF) (Salem et al. 2011a). Solid wood
grown in normal forest conditions releases low levels of formaldehyde, particularly
during the manufacturing process (Salem et al. 2012a). Furthermore, PF resins are
frequently used in the manufacture of cork products. For this reason, formaldehyde and
phenol are often measured together (Horn et al. 1998).
Formaldehyde has been classified as a known carcinogen by the State of
California, Proposition 65 (2008) and the International Agency for Research on Cancer
(IARC), a division of the World Health Organization (WHO) (IARC 2004). The National
Institute of Health’s National Toxicology Program (NTP) states that formaldehyde is
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Salem and Böhm (2013). “Formaldehyde from wood,” BioResources 8(3), 4775-4790. 4776
reasonably anticipated to be a human carcinogen (1998). The IARC has recently
established that formaldehyde is undetectable by smell at concentrations of less than 0.1
ppm. At concentrations between 0.1 ppm and 0.5 ppm, formaldehyde is detectable by
smell, with some sensitive individuals experiencing slight irritation to the eyes, nose, and
throat. At levels from 0.5 to 1.0 ppm, formaldehyde produces irritation of the eyes, nose,
and throat in most people, while at concentrations above 1.0 ppm, exposure to
formaldehyde produces extreme discomfort (IARC 2004). Formaldehyde can cause
contact dermatitis, associated with an allergic reaction to the chemical (Isaksson et al.
1999). Formaldehyde is a naturally occurring chemical in wood, as wood contains a
diminutive, but still detectable amount of free formaldehyde. Formaldehyde can be
formed from the main components of wood (cellulose, hemicelluloses, and lignin) as well
as from its extractives (Schäfer and Roffael 2000) to different extents depending on the
boundary conditions (pH value, temperature). On the other hand, the inorganic substances
in wood do not directly contribute to formaldehyde release.
The FE from solid wood increases at elevated temperatures and prolonged heating
times (Schäfer and Roffael 2000), even in the absence of wood resin (Jiang et al. 2002).
On the other hand, the degree of polymerization of cellulose seems to have no significant
influence on the emission of formaldehyde; also, raising the temperature to 100 and
150 °C, the formaldehyde liberation from starch is also very low even at high reaction
temperatures (Schäfer and Roffael 2000).
The emission of formaldehyde from wood is produced during hot-pressing of
composite panels, and it is generally accepted that FE from the wood itself is an
insignificant contributor to the total measurable level of FE in a composite wood product
(Birkeland et al. 2010). The present article review is focused on the emission of
formaldehyde from different wood species as reported in the literature. Additionally,
some attention has been given to the test methods used.
Test Methods for Formaldehyde Emission
Some common methods used for the determination of FE from solid wood include
the European small chamber method (EN 717-1 2004), gas analysis (EN 717-2 1994), the
perforator method (EN 120 1993), the flask method (EN 717-3 1996), a desiccator (JIS A
1460 2001), and the modified National Institute of Occupational Safety and Health
(NIOSH) test method 3500 (1994). The test conditions and properties of wood specimens
used to measure the FE with various test methods are presented in Table 1.
The perforator method measures the total extractable content of formaldehyde
present in the wood sample, while the other methods (EN 717-1, EN 717-2, and ASTM D
6007-02) measure the amount of formaldehyde emitted from the surface of the wood
specimens (Xiong and Zhang 2010; Salem et al. 2012a). The total formaldehyde
concentration measured by the perforator method cannot be all emitted at room
temperature (Xiong and Zhang 2010) and cannot be taken as a good index for the
pollution level of the tested wood materials. Furthermore, the products should be
evaluated by intra-laboratory and inter-laboratory comparisons to overcome the problems
with the emission levels of different products in different regions or countries, as
mentioned by the California Air Resources Board (CARB 2010).
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Table 1. The General Conditions used for some Standard Test Methods for the
Determination of Formaldehyde Emissiona
Test method
EN
7171
EN
7172
EN 120
EN 717-3
ASTM D
60072
Material
Volume
0.225 m3
4 L
Extractor
apparatus
500 mL flask
1 m3
Wall
material
Stainless
steel
Glass
Glass/perfor
ator
polyethylene
bottle with
bottle top
Aluminum
Test sample
Loading
ratio
1 m2/m3
0.4 ×
0.05 m
25 × 25 mm,
(110 g)
0.025 × 0.025
m, 20 g
0.43
m2/m3 (for
PB)
Edge
sealing
Yes
Yes
No
No
Yes
Sample
Conditioning
Temp.
(°C),
RH (%)
No
Varied
Not stated
Not stated
7 days at
(24 ±3°C),
(50±5%)
Test
conditions
Temp.
(°C)
23 ± 0.5
60 ± 0.5
Extraction
with 600 mL
toluene at
110 °C
40 °C
24 ± 3
RH (%)
45 ± 3
≤ 3 %
100%
50 ± 5
Air
exchange
(h1)
1.0 ±
0.05
(60 ± 3)
No
2
Air
velocity
0.10.3
m/s
1L/min
No
No
(25 m/s)
fan speed
Test
duration
24
weeks
4 h
3 h
3 hours
Until
steady-
state
Results
E1 ≤ 0.1
ppm or
0.124
mg/m3
E1 ≤
3.5
mg/m2.h
E1 ≤ 8
mg/100 g
o.d.
No official limit
values
published
CARB
Phase 1
and 2
(see
Table 3)
a: From Salem et al. (2012a) and Risholm-Sundman et al. (2007)
When increasing the temperature from 25.2 to 50.6 °C, the initial emittable
formaldehyde from dry building materials was increased significantly, by about 507%
(Xiong and Zhang 2010). This means that most of the formaldehyde in building materials
cannot be emitted at room temperature; the EN 120 uses temperatures around 110 °C, and
the EN 717-2 method uses temperatures of 60 °C (Salem et al. 2012a). Wiglusz et al.
(2002) reported that at 23 and 29 °C, the measurements did not show any emission of
formaldehyde; at a temperature of 50 °C, a high initial concentration of FE was found and
it decreased with time. The referenced chambers (EN 717-1 and ASTM D 6007-02) use
conditions common to an indoor environment (Salem et al. 2012b; Salem 2011b; Yu and
Crump 1999).
The C-history method for a closed chamber (Xiong et al. 2011; Yao et al. 2011),
multi-emission/flush regression (Xiong et al. 2009), and room temperature sorption/
emission (Wang and Zhang 2009) methods were developed to rapidly measure the initial
emittable formaldehyde concentration and to overcome the overestimation of formal-
dehyde content (FC) with the perforator method. The developed methods take less than
three days, in comparison to the reference methods, which require 7 to 28 days (Salem et
al. 2012a; Yu and Crump 1999). The new method was validated using the characteristic
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Salem and Böhm (2013). “Formaldehyde from wood,” BioResources 8(3), 4775-4790. 4778
parameters determined in a closed chamber experiment to predict the observed emissions
in a ventilated, full-scale chamber experiment (Xiong et al. 2011).
Regulations and Testing
Most European nations have passed laws that regulate formaldehyde, now known
as the E1-emission class. Standards such as EN 312 (2003) and EN 6225 (2003) all
require that the 0.1-mg/m3 h level be met. Testing for this mainly utilizes the EN 120 and
EN 7171 standard testing methods. In 2004, the EN 13986 (2005) established emission
classes E1 and E2 for use in construction (the E1 level is most common). These standards
basically require testing to be done on formaldehyde-containing wood products used in
construction (Table 2). In 2006, these same methods and the associated limits went into
effect for panel production. Because it is very difficult, if not impossible, to eliminate
formaldehyde from a building completely, the Japanese standard employs a tiered rating
system based on the amount of FE a building material gives off. This system is based on
one-star to four-star ratings, with four stars representing the lowest amount of FE (Table
2). The two Japanese desiccator methods JIS A1460 and JAS MAFF 233 both describe
determination of formaldehyde release from wood-based materials. Test pieces are placed
in a desiccator containing a vessel with water. The formaldehyde released from the test
pieces at 20 °C during 24 h is absorbed by the water and determined photometrically. As
in the flask method, the RH is very high ((RisholmSundman et al. 2007).
Table 2. Current Formaldehyde Emission Standards for WoodBased Panels in
Europe, Australia, the U.S.A., and Japan
Country
Standard
Test method
Board class
Limit value
Europe
EN 13986
EN 7171
E1PB,
MDF, OSB
≤ 0.1 ppm
EN 120
≤ 8 mg/100 g o.d. board
EN 7171
E1PLW
≤ 0.1 ppm
EN 7172
≤ 3.5 mg/(h.m2)
EN 7171
E2PB,
MDF, OSB
> 0.1 ppm
EN 120
> 8 30 mg/100 g o.d.
board
EN 7171
E2PLW
> 0.1 ppm
EN 7172
> 3.5 ≤ 8.0 mg/(h.m2)
Australia &
New Zealand
AS/NZS 18591
& 2
AS/NZS
4266.16
(Desiccator)
E0PB, MDF
≤ 0.5 mg/L
E1PB
≤ 1.5 mg/L
E1MDF
≤ 1.0 mg/L
E2PB, MDF
≤ 4.5 mg/L
USA
ANSI A 208.1 & 2
ASTM E1333
(large
chamber)
PB
≤ 0.18 or 0.09 ppm
MDF
≤ 0.21 or 0.11 ppm
Japan
JIS A 5908 &
5905
JIS A 1460
(Desiccator)
F**
≤ 1.5 mg/L
F***/“E0”
≤ 0.5 mg/L
F****/“SE0”
≤ 0.3 mg/L
PB: particleboard; MDF: medium density fiberboard; OSB: oriented strand board
F** class in Japan more or less equivalent to European E1-class
F*** and F**** are of much lower emission than the E1
F**** emission is close to the emission of solid untreated wood
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In contrast to building material standards in Europe and Japan, the proposed CARB
(2010) of Phase 1 and Phase 2 standards for hardwood plywood (HWPW), PB, and MDF,
with effective dates between 2009 and 2012, is productspecific. Based on the use of
published equations correlating the results of selected FE/FC tests (RisholmSundman et
al. 2007) and results from a study to compare the metrics used in the U.S. and Europe
(Groah et al. 1991), the relative stringency of the proposed standards has been estimated
and is shown in Table 3.
Table 2 lists the equivalent U.S. large chamber test value ASTM E 133396
(ASTM 2002) for the European E1, Japanese F***, and F**** standards applicable to
composite wood products subject to the proposed Airborne Toxic Control Measure
(ATCM 2009). Although the CARB regulation is only valid in California, many
composite wood product plants around the world have already been certified to satisfy
the CARB requirements, and the number of applications for certification is continuously
rising. In February 2009, the American National Standards Institute (ANSI) approved
revised national voluntary standards for ANSI A208.12009 for PB and ANSI A208.2
2009 for MDF for Interior Applications (Table 4).
FORMALDEHYDE EMISSION FROM SOLID WOODS
Wood as a Natural Material
Wood as a natural material contains formaldehyde (Meyer and Boehme 1997;
Que and Furuno 2007; Salem et al. 2011b), which can be released during thermal
treatment (Schäfer and Roffael 2000). Meyer and Boehme (1996) measured the FEs from
oak, Douglas fir, beech, spruce, and pine, and the emission of formaldehyde ranged
between 2 and 9 ppb. The results are presented in Table 5 as measured using a 1-m3
chamber, gas analysis, a perforator, and the flask method.
Table 3. Proposed Airborne Toxic Control Measure (ATCM) for Composite Wood
Products
Standard
Product(s)
Test Method
Numerical Value
≈ ASTM E 1333
(ppm)
CARBPhase1
HWPW
ASTM E 1333
0.08 ppm
0.08
,,,,
PB
,,,,
0.18 ppm
0.18
,,,,
MDF
,,,,
0.21 ppm
0.21
CARBPhase2
HWPW
,,,,
0.05 ppm
0.05
,,,,
PB
,,,,
0.09 ppm
0.09
,,,,
MDF
,,,,
0.11 ppm
0.11
E1
HWPW
EN 7171
0.12 mg/m3
0.14
,,,,
PB,MDF
,,,,
0.12 mg/m3
0.14
,,,,
All
EN 7172
3.5 mg/m2 h
N/A
,,,,
PB,MDF
EN 120
8 mg/100 g o.d.
board
0.10
F**
All
JIS A1460
1.5 mg/L
N/A
F***
All
,,,,
0.5 mg/L
0.07
F****
All
,,,,
0.3 mg/L
0.04
The Fstar standards apply to all wood products specified in the CARB standards. The
E1333” values were calculated using data in ASTM E 133396 (ASTM 2002), Battelle
(1996), RisholmSundman et al. (2007), and CARB (2007 and 2010).
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Table 4. The CARB New Standards Phase 1 and Phase 2 Formaldehyde
Emission for HWPW, PB, and MDFa
Effective
Date
Phase 1 (P1) and Phase 2 (P2) Emission Standards (ppm)
HWPWVC
HWPWCC
PB
MDF
Thin MDF
01.01.2009
P1: 0.08
P1: 0.18
P1: 0.21
P1: 0.21
01.07.2009
P1: 0.08
01.01.2010
P2: 0.05
01.01.2011
P2: 0.09
P2: 0.11
01.01.2012
P2: 0.13
01.07.2012
P2: 0.05
(a) Based on the primary test method [ASTM E 133396 (ASTM 2002)] in ppm. HWPWVC =
veneer core; HWPWCC= composite core
The formation of formaldehyde took place even when the wood was dried at a low
temperature (30 °C), and the low drying temperature was chosen because it has been
demonstrated (on wood particles) that drying under industrial conditions causes the
formation of formaldehyde (Marutzky and Roffael 1977). Furthermore, the emission
levels of formaldehyde depend on numerous factors such as wood species, moisture
content (MC), outside temperature, and storing time (Martínez and Belanche 2000;
Boehme 2000). It has been shown that an MC change from 0.0% to 4.0% results in a 6-
fold increase in FE and that the release is regulated by physical processes (Irle et al.
2008).
Table 5. Formaldehyde Parameters from Different Species of Solid Wooda
Wood
Moisture
content
Testing in the 1-m3
chamber
Gas
analysis
value
Perforator
value
Flask value
Testing
period
HCHO
concentration
3 hr.
24 hr.
(%)
(hr.)
(ppb)
(µg/m2 h)
(µg/100 g dry board)
Beech
53
360
2
114
359
2
22
7
336
3
34
155
8
12
Douglas-fir
117
384
4
397
517
4
55
9
240
5
82
207
6
75
Oak
63
360
9
431
597
17
80
8
360
4
51
188
6
44
Spruce
42
384
3
133
334
2
9
7
336
4
71
277
19
132
Pine
134
240
5
195
217
2
18
8
360
3
86
233
16
80
a: data adopted from Meyer and Boehme (1996).
Relationship between Wood Chemical Composition and Formaldehyde
Emissions
Figure 1 shows the formaldehyde release of unextracted and extracted spruce and
pine chips at different temperatures using the flask method. The results reveal that
extracted chips release significantly lower amounts of formaldehyde compared to
unextracted chips. Moreover, pine chips emit more formaldehyde than spruce chips
(Schäfer and Roffael 1999 and 2000). Additionally, the fatty acids release only minute
quantities of formaldehyde compared to resin acids, and abietic acid emits much higher
amounts of formaldehyde compared to saturated fatty acids. Pinewood has a higher
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extractive content and an especially higher amount of resin acids (Fengel and Wegener
1984), which are relevant to the release of formaldehyde. Furthermore, Schäfer (1996)
found that with increasing storage time, the spruce and pine particles emit less
formaldehyde than non-stored wood. Back et al. (1987) reported that the composition of
extractives changes during storage of wood: the content of extractives decreases and the
content of free sugars, lipophilic fats, fatty and resin acids, and steroles decreases
enormously.
Fig. 1. The released formaldehyde from unextracted and extracted pine and spruce particles
measured by the flask method (mg/1000 g O.D. wood) as affected by time and temperature. Data
has been replotted from Schäfer and Roffael (1999, 2000).
Additionally, it was reported that polysaccharides and lignin are a source of FE. A
pathway for the release of formaldehyde includes the transformation of polysaccharides
to hexoses, oxymethylfurfural, and its subsequent disproportionation to furfural and
formaldehyde (Schäfer and Roffael 2000). Fengel and Wegener (1984) reported that
softwood polyoses contain higher amounts of mannose and galactose than hardwood
polyoses, whereas hardwoods are rich in pentoses carrying higher amounts of acetyl
groups than softwoods.
At high temperatures, Schäfer and Roffael (2000) found that arabinose and xylose
release much more formaldehyde than starch and cellulose, as well as higher amounts
than glucose or galactose (Fig. 2). Additionally, the hardwood lignin content lies between
20 and 25%, while softwoods contain up to 32% lignin, and it is well known that
treatment of lignin with acid leads to liberation of formaldehyde (Freudenberg and
Harder 1927).
Effect of Wood Drying on Formaldehyde Emissions
Wood emits formaldehyde under very high heat but is not expected to be a
significant source of formaldehyde in composite wood products during normal service
(Salem et al. 2012; Böhm et al. 2012). On the other hand, oak wood in the green state
showed the highest FE, with 9 ppb, and beech wood had the lowest, with 2 ppb. The
values for Douglas fir, spruce, and pine were between 3 and 4 ppb. In the dry state, the
determined formaldehyde values were 1 to 2 units higher, except for oak. The value of 9
ppb determined in the green state for oak decreased to 4 ppb in the dry state (Meyer and
Boehme 1996).
0
50
100
150
200
250
40°C/3h 40°C/24h 100°C/3h 150°C/3h
13.5 6.7
103.3
2.8 15.3 13.9
232.9
Formaldehyde content
(mg/100 g O.D.)
Spruce Wood
Extracted Unextracted
0
50
100
150
200
250
300
40°C/3h 40°C/24h 100°C/3h 150°C/3h
1.4 5.3 18.8
166.5
4.1 24.7 44.1
288.1
Formaldehyde content
(mg/100 g O.D.)
Pine Wood
Extracted Unextracted
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0
2
4
6
8
10
12
14
Arabinose Xylose Glucose Galactose Starch Cellulose
Formaldehyde release
40°C/3h 100°C/3h 150°C/3h
Fig. 2. Formaldehyde release from arabinose, xylose, glucose, galactose, starch, and cellulose
measured by the flask method. Data has been replotted from Schäfer and Roffael (2000).
Boehme (2000) measured the formaldehyde release of different wood species in a
1-m3 chamber according to EN 717-1. The highest value was found for oak (9 ppb), and
the lowest was found for beech (2 ppb). The emission of formaldehyde from pine and
spruce lies in between. Figure 4 shows the formaldehyde release from undried wood in a
1-m3 chamber, as measured by Boehme (2000) at 30 °C. The emission of formaldehyde
from wood increases with thermal treatment during the drying and pressing processes
(Marutzky and Roffael 1977; McDonald et al. 2004).
Significantly, in softwoods (e.g., pine and spruce), extractives affect the formal-
dehyde release, and the removal of extractives decreases the formaldehyde emitted from
the wood by hydrothermal treatments; thermo-mechanical pulping (TMP) also enhances
the released formaldehyde in wood (Schäfer and Roffael 2000).
In the study of Young (2004), the air-dried wood of all the species tested produce
low emissions of formaldehyde, as seen in Fig. 3. Radiata pine has similar FE emission to
the other species tested.
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
Formaldehyde emission
(mg/L)
Air Dried 60°C 140°C
Fig. 3. Formaldehyde emission measured from air-dried wood species as affected by drying
periods. On this scale, 0.30 mg/mL JAS units is the Japanese low emission limit. Data has been
replotted from Young (2004).
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Emission levels from solid radiata wood increase after kiln drying but decrease to
low levels quite quickly after drying and stay low. FE from solid radiata pine did not
prevent the application of green labeling or sales in low emission markets like Japan. The
emission measured from radiata pine after treatment at 140 °C was higher than in
previous trials, and the result of 0.29 mg/mL is close to the F****/SE0 level of 0.30
mg/mL, as measured by JIS A 1460 (2001). This difference is due to the shorter period
from heat treatment to testing (16 compared to 20 days). FE from radiata pine was found
to be similar to the seven other wood types dried under identical conditions.
Recently, Böhm et al. (2012) found that the rate at which individual wood
species’ FE differed was associated with their steady state concentrations or emission
rates (Table 2). The values ranged between 0.0068 and 0.0036 ppm, as measured by EN
717-1, after a test period of 15 to 21 days, while they varied between 0.084 and 0.014
mg/m2 h when measured using EN 717-2. Beech wood showed the highest FE, at 0.0068
ppm and 0.084 mg/m2 h, as measured by EN 717-1 and EN 717-2, respectively, followed
by spruce wood (0.0055 ppm) and pine wood (0.0053 ppm). Birch wood had the lowest
amount (0.0036 ppm), as measured by EN 717-1, while poplar and oak woods (0.014
mg/m2 h) had the lowest values when measured using EN 717-2. Furthermore, when the
wood samples from the six species were air-dried (25 to 30 °C), formaldehyde was
formed with only relatively slight differences in the values between the wood species
(Table 6).
The values of FE could be affected by the anatomy of the respective wood species
(Salem et al. 2012a, 2013). For example, Böhm et al. (2012) found that plywood panels
produced from poplar veneer [low specific gravity (SG, 0.33)] with a simple anatomy
produce lower FE values. An increase in SG (beech and birch plywood) causes more
adhesive to be used to make the boards and consequently releases more formaldehyde.
Moreover, Aydin et al. (2006) reported that the FE from poplar and spruce plywood
decreased with increasing veneer moisture content. On the other hand, Nemli and Öztürk
(2006) found that increasing SG, shelling ratio, and pressure increased the FC of PB. For
instance, PB made from particles consisting of higher amounts of beech particles had
lower FC than that of panels from particles consisting of higher amounts of pine particles.
Table 6. Formaldehyde Emission Values Measured with EN 717-1 (ppm) and EN
717-2 (mg/m2 h) from Some Solid Woodsa
Wood
species
Formaldehyde emission values
ppm
mg/m2 h
Beech
0.0068
0.084
Poplar
0.0042
0.014
Birch
0.0036
0.049
Oak
0.0042
0.014
Pine
0.0053
0.016
Spruce
0.0055
0.069
a: data from Böhm et al. (2012)
At 23°C and 1013 hPa, the following relationship exists for formaldehyde measured by EN 717-
1: 1 ppm = 1.24 mg/m3 or 1 mg/m3 = 0.81 ppm.
Furthermore, the results in Fig. 4 reveal that with decreasing pine particle size, the
emanation of formaldehyde increases (Roffael et al. 2012); also, extended reaction time
(from 3 to 24 h) increased the difference in the formaldehyde release. Additionally, the
hot water extractive content of the particles increased in the same direction as the FE
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Salem and Böhm (2013). “Formaldehyde from wood,” BioResources 8(3), 4775-4790. 4784
from the particles. These differences can be related to the increase in the surface area of
the particles and its effects on decreasing the particle size. Previously, Schäfer (1996)
documented that ray cells with a high content of lipophilic extractives are enriched in the
fine fraction. Boruszewski et al. (2011) reported that FE from pine particles after cutting
was higher by 25% than that from the chips prior to cutting (Fig. 5). It was difficult to
compare the results with the requirements for PBs, as emission is expressed in mg/h m2.
However, it is possible to calculate the emission from the particles contained in a PB.
When the amount of absolutely dry particles contained in PB of a given density and
thickness is known, the obtained results may be recalculated to the surface of PB (EN
717-2 1994). Recalculated release of formaldehyde is shown in Fig. 5. Thus, it was found
that FE from pinewood, being an equivalent of PB, was 4.6% of the whole emission
permitted by EN 13986 (2005) standard for E1-class products (3.5 mg/h m2).
0
5
10
15
20
25
30
35
≥3-5 ≥1.25-3 ≥0.6-1.25 ≥0.315-0.6 ≥0.16-0.315
0.9 1.1 11.9 2.9
6.6 6.8 7.3
26.2
31.9
0.6 1.1 11.9 2.9
Value
Particle size (mm)
FC after 3h (mg/100g o.d.) FC after 24h (mg/100g o.d.)
Extractives content (%)
Fig. 4. Formaldehyde content from particles (pine wood) of different particle size, as measured
after 3 and 24 h by the flask method and the extractives content (hot water) of pine wood of
different particle size. Data has been replotted from Roffael et al. (2012).
Fig. 5. Formaldehyde emission from wood at the beginning of the processing chain with respect
to 125 g of absolutely dry material (*re-calculated to particleboard of density 650 kg/m3 and
thickness 16 mm) and from absolutely dry pine particles contained in particleboard of density 650
kg/m3 and thickness 16 mm. Data has been replotted from Boruszewski et al. (2011).
0
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
Chips Particles Particeboard
0.004 0.005
0.16
0.007*
Formaldehyde emission (mg/h m2)
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Salem and Böhm (2013). “Formaldehyde from wood,” BioResources 8(3), 4775-4790. 4785
Ҫolak et al. (2009) reported that values of FE of PB produced from eucalyptus
logs stored under water or indoor conditions and pressed at 150 °C were found to be 1.21
and 1.34 mg/100 g O.D. board, respectively, as measured by the EN 120 method. These
values are clearly lower than those of the panels produced from steamed log parts (1.88
mg/100 g O.D. board) and the log parts stored in outdoor conditions (1.92 mg/100 g O.D.
board). The FE values of the PBs pressed at 190 °C were found to be 0.72 mg/100 g O.D.
board for group I (indoor conditions for 2 months), 0.98 mg/100 g O.D. board for group
II (outdoor conditions for 4 months), 0.79 mg/100g O.D. board for group III (under water
for 3 months), and 0.82 mg/100 g O.D. board for group IV (steaming). There were
similar interactions among the FE values of the panels pressed at 190 °C and those of the
panels pressed at 150 °C. However, the differences among the emission values of the
panels pressed at 190 °C were lower. This may be due to the degradation and splitting of
the acetyl groups at this temperature.
MECHANISM OF ALDEHYDE AND KETONE EMISSION FROM WOOD
Mechanisms that may form aldehydes and ketones in extractives and wood
products include thermal, enzymatic, and microbial degradation. Research conducted on
the oxidative degradation of plant material has yielded some information about how
certain types of aldehydes and ketones are formed. However, these mechanisms do not
account for the variety of aldehydes and ketones observed in the wood product emissions,
and in some cases, the mechanisms occur under conditions that are distinctly different
from wood product manufacturing conditions.
The presence of formaldehyde in emissions from wood that does not contain
adhesive resin has been explained by thermal degradation of polysaccharides in the wood
(Schäfer and Roffael 1996), but this does not explain findings of the presence of FE from
wood that has never been heated (Meyer and Boehme 1997). In the work of Faix et al.
(1990 and 1991), the pyrolysis of milled wood lignin at 450 °C yielded benzaldehyde,
and pyrolysis of spruce and pinewood at 450 °C generated formaldehyde, acetaldehyde,
2propenal, butanal, and butanone, a result that is attributed to the breakdown of the
polysaccharide fraction of the wood. Conditions of pyrolysis are extreme and not
oxidative, and during the manufacture of wood products, only wood particles for PB are
likely to be exposed to such extreme conditions, and then only for a very brief time.
Enzymatic pathways for the oxidation of fatty acids to form hexanal and nonanal
have been described for nonwoody plants, but no such pathway has been described for
other aldehydes (HamiltonKemp and Andersen 1986). In short, although pathways exist
for some of the aldehydes and ketones that are observed in wood product emissions, there
are no mechanisms for other aldehydes (for example, pentanal, heptanal, and octanal).
With the exception of hexanal and nonanal, there is no explanation of how the aldehydes
and ketones could be formed at room temperature or under the relatively mild conditions
that are encountered in wood products manufacturing (Hatanaka et al. 1976).
Relationship between Formaldehyde Emission and Wood Pretreatments
Roffael et al. (2007) reported that the cold water extracts from pulps produced by
the chemothermomechanical technique (CTMP process) contain higher amounts of
formate and acetate ions compared to cold water extracts from pulps produced by the
TMP process. The FE from CTMP is lower than that from TMP due to the Cannizzaro
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Salem and Böhm (2013). “Formaldehyde from wood,” BioResources 8(3), 4775-4790. 4786
reaction catalyzed by alkali. Moreover, binderless fiberboards from CTMP are
significantly lower in the formaldehyde release compared to binderless boards from
TMP. The use of MUF resin increases the FE of the boards from TMP and CTMP. In
addition, Roffael (2008) found that FEs from binderless fiberboards using the flask
method after 24 h were 58.1 and 10.5 mg/100 g O.D. fibers with TMP and CTMP,
respectively.
The effects of waiting time before drying of alder (Alnus glutinosa) veneers on
various properties of plywood, including FE, were investigated by Ҫolakoğlu et al.
(2002). There were no significant differences among the FE values. It has been stated in
the literature that FE of plywood is related to the presence of acetyl groups in wood
(Ҫolakoğlu et al. 1998). Therefore, IR spectra were obtained to determine the effects of
waiting time before the drying process of veneers on acetyl groups. Then, the absorption
bands of carboxyl group (≈ 740/cm) were compared. Similar spectra were obtained for
each test group.
SUMMARY
1. Wood itself generates a significant amount of formaldehyde when exposed to
certain conditions common to the composite panel manufacturing process that is
caused by the thermal degradation of polysaccharides in the wood.
2. Relative to the formaldehyde release from wood, the chemical composition of
wood is much more important than its physical or anatomical structure.
3. Formaldehyde emission from solid wood has been shown to be impermanent, and
it rapidly decreases to levels below those set by the EN 717-1 and EN 717-2
standards.
4. The pyrolysis of wood generated formaldehyde, which is attributed to the
breakdown of the polysaccharide fraction of the wood during the hot pressing.
5. Softwood extractives affect formaldehyde release, and the removal of extractives
decreases the formaldehyde emitted from the wood by hydrothermal treatments;
thermo-mechanical pulping (TMP) also enhances the release of formaldehyde
from wood.
ACKNOWLEDGMENTS
Financial support for the preparation of this document was provided by grants
from the Internal Grant Agency, Faculty of Forestry and Wood Sciences, Czech
University of Life Sciences, Prague.
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Article submitted: April 29, 2013; Peer review completed: May 27, 2013; Revised
version received and accepted: June 4, 2013; Published: June 7, 2013.
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... All results were in the E1 and E0 emission classes, but the particleboards obtained with Recipe 3 had the lowest emissions, which meant that the addition of crosslinkers significantly reduced the formaldehyde emissions. In the case of Recipe 1, the value obtained for formaldehyde emission was low, 6.4 times lower than the standardized E1 limit and 2.5 times lower than the standardized E0 limit, which places these boards in the category of particleboards with ultra-low formaldehyde emission, close to that of natural wood [20]. By combining this value with the mechanical strengths (MOR and IB) of this type of particleboard, this board is recommended to be used in the field where no high-strength particleboards are required (i.e., paneling, decorative furniture, etc.). ...
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Adhesives represent an important part in the wood-based composite production, and taking into account their impact on the environment and human health, it is a challenge to find suitable natural adhesives. Starting from the current concerns of finding bio-adhesives, this paper aims to use magnesium lignosulfonate in three adhesive recipes for particleboard manufacturing. First, the adhesive recipes were established, using oxygenated water to oxidize magnesium lignosulfonate (Recipe 1) and adding 3% polymeric diphenylmethane diisocyanate (pMDI) crosslinker (Recipe 2) and a mixture of 2% polymeric diphenylmethane diisocyanate with 15% glucose (Recipe 3). The particleboard manufacturing technology included operations for sorting particles and adhesive recipes, pressing the mats, and testing the mechanical strengths and formaldehyde emissions. The standardized testing methodology for formaldehyde emissions used in the research was the method of gas analysis. Tests to determine the resistance to static bending and internal cohesion for all types of boards and recipes were also conducted. The average values of static bending strengths of 0.1 N/mm2, 0.38 N/mm2, and 0.41 N/mm2 were obtained for the particleboard manufacturing with the three adhesive recipes and were compared with the minimal value of 0.35 N/mm2 required by the European standard in the field. Measuring the formaldehyde emissions, it was found that the three manufacturing recipes fell into emission classes E1 and E0. Recipes 2 and 3 were associated with good mechanical performances of particleboards, situated in the required limits of the European standards. As a main conclusion of the paper, it can be stated that the particleboards made with magnesium-lignosulphonate-based adhesive, with or without crosslinkers, can provide low formaldehyde emissions and also good mechanical strengths when crosslinkers such as pMDI and glucose are added. In this way magnesium lignosulfonate is really proving to be a good bio-adhesive.
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In this report I discuss the chemical industry, its current unsustainability and international inequality, serious long-term impacts of toxic chemicals on human and environmental health, including persistent organic pollutants, pesticides and herbicides. Then I look at exposure to toxic chemicals at home, and occupational health impacts of industrial chemicals. I also discuss better responses to each of these issues, including assessing future sustainability of the chemical industry based on a proposed chemical waste management hierarchy, and the principles of the circular economy.
... In previous investigations, formaldehyde-based adhesives were effectively used on sorghum biomass particleboard [14,15] Formaldehyde-based adhesives such as urea-formaldehyde (UF) and phenol-formaldehyde (PF) were the commercial application for particleboard [16]. These adhesives are being used in the production process of particleboards because of their cheap cost [17]; however, they pose health and environmental hazards [17][18][19]. According to IARC [20], formaldehyde compounds cause cancer in humans. ...
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Natural adhesives made from non-fossil resources are expected to grow significantly in the future. This study analyzes the effects of maleic acid (MA) content and particle size class on the physical and mechanical characteristics of sorghum (Sorghum bicolor) biomass particleboard (SBMA-particleboard) as potential materials for table tennis blade. MA content was varied in the range of 5 to 20 wt%, while SBMA-particleboard were manufactured using various particles size class and target densities of 300 mm × 300 mm × 0.6 cm and 600 kg/m³, respectively. The result showed that the physical and mechanical properties of SBMA-particleboard improved with increasing MA content of 15 wt%. Furthermore, the MA content of 15 wt% effectively manufactured SBMA-particleboard, while the powder particle class provided higher dimensional stability and internal bond due to the more extensive contact area. Fourier transform infrared spectroscopy analysis showed the presence of ester linkages, indicating a reaction between the carboxyl groups of MA and the hydroxyl of the sorghum biomass to provide good physical and mechanical properties for SBMA-particleboard as potential materials for table tennis blade.
... In OMV, different inlet and outlet flow positions were considered To simulate the effect of different air distribution systems on formaldehyde concentration in the test room, Birch as a wooden material (emission rate: 49 μg m − 2 h − 1 [36]) was considered as the emission source of this VOC from the floor area. Among different wooden materials, including Beech, Birch, Spruce and etc., Birch had a lower formaldehyde emission rate [36,37]. Fig. 8 to Fig. 10 show the influence of air distribution layout on the air velocity and formaldehyde concentration distributions in the room and in the breathing zone. ...
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... Their widespread use is based on the fact that these adhesives exhibit versatile properties such as flexibility, low cost, high thermal stability, water and chemical resistance. However, formaldehyde emissions during their production and use associated with their fossil-based formulation have raised interest in environmentally sustainable and safe alternatives [22,30]. ...
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... Formaldehyde emissions released by furniture made using synthetic wood adhesives are toxic for human beings due to their carcinogenic nature (WHO 2004). Although the wood itself emits formaldehyde, its contribution is minor in comparison to the emissions due to the presence of synthetic adhesives (Salem and Böhm 2013). Formaldehyde emissions from wood furniture can be minimized by replacing the synthetic adhesives with biobased adhesive formulations during the manufacturing process. ...
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The wide use of hazardous formaldehyde (CH2O) in disinfections, adhesives and wood-based furniture leads to undesirable emissions to indoor environments. This is highly problematic as formaldehyde is a highly hazardous and toxic compound present in both liquid and gaseous form. The majority of gaseous and atmospheric formaldehyde derive from microbial and plant decomposition. However, plants also reversibly absorb formaldehyde released from for example indoor structural materials in such as furniture, thus offering beneficial phytoremediation properties. Here we provide the first comprehensive review of plant formaldehyde metabolism, physiology and remediation focusing on release and absorption including species-specific differences for maintaining indoor environmental air quality standards. Phytoremediation depends on rhizosphere, temperature, humidity and season and future indoor formaldehyde remediation therefore need to take these biological factors into account including the balance between emission and phytoremediation. This would pave the road for remediation of formaldehyde air pollution and improve planetary health through several of the the UN Sustainable Development Goals.
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In this study, the MDF sanding dust was examined as a precursor for synthesizing activated carbon (AC). For this purpose, MDF sanding dust was impregnated with KOH at two different weight ratios 1:0.50 and 1:1 (sanding dust/KOH). The process was done in one step at a temperature of 750 °C and a heating rate of 8 °C/min. The synthesized ACs at different content were added into urea-formaldehyde (UF) resin for medium density fiberboard (MDF) production. The laboratory MDF boards were produced using a hot press at a temperature of 190 °C and pressure of 40 MPa. The ACs content in UF resin was set at 0.25%, 0.50% and 1% based on the dry weight of the resin. The ACs were characterized by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The BET equation was used to determine the specific surface area of the AC samples. Water absorption, thickness swelling, internal bonding, flexural modulus and strength, and the formaldehyde emission from MDF boards were determined. The curing behavior of UF resins were evaluated using Differential Scanning Calorimetry (DSC). The results revealed that surface area and micropore volume of ACs increase with increasing KOH ratio; and the ACs exhibited three-dimensional porous network structures. The DSC results also showed that the ACs significantly increase the reaction enthalpy of UF resins. The addition of AC as an additive to UF resin significantly reduces formaldehyde release from MDF boards and improves their physical and mechanical properties.
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Meeting new, more strict limits for formaldehyde emission from wood based composites is hardly possible without essential changes in technology and processing. Since modifications of amino resins occurred to be not enough, other approaches must be considered. Thus, the present studies regard analysis of formaldehyde levels in raw materials for particleboard production at the beginning of processing chain: (1) chips from wood yard and (2) wet particles collected after knife ring flaker. The obtained results allowed to determine contribution of formaldehyde emission from wood material to overall emission from particleboard and to define procedures minimizing formaldehyde emission from the products in future.
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This study investigated the effects of adhesive type and press variables on the volatile organic compound (VOC) emissions arising from hot-pressing mixed-hardwood particleboard. Three adhesive types, urea-formaldehyde resin (UF), phenol-formaldehyde resin (PF), and polymeric methylene diisocyanate resin (pMDI), were evaluated in this study. A 25-1 fractional factorial design was used to evaluate the primary effect of five press variables (press temperature, press time, mat resin content, mat moisture content, and board density) and their interactions. A total of 27 chemical compounds were identified and quantified in the VOC emissions using four analytical techniques. Formaldehyde, methanol, acetic acid, and HMw VOCs with hexanal being the predominant chemical were the major compounds comprising the VOC emissions. The results revealed that formaldehyde and methanol emissions from UF particleboard, as well as the methanol emissions from PF particleboard were the most abundant components of the VOC emissions, and they contributed about 92 and 72 percent of the total identified VOCs, respectively. Lower levels of formaldehyde and acetic acid were released during the hot-pressing of PF particleboard. Acetic acid and HMw VOC emissions were the most abundant components of the VOC emissions arising from pMDI-bonded particleboard, pMDI significantly reduced the methanol emissions from the mixed-hardwood particleboard. The most significant press variables controlling VOC emissions were press time, mat resin content, press temperature, and interactions among these three variables. These press variables had different effects on the individually identified compounds based on the adhesive type used. In general, formaldehyde emissions arising from hardwood particleboard hot-pressing were significantly lower than those from softwood particleboard. However, formaldehyde emissions from UF-bonded hardwood particleboard were significantly higher than from the softwood UF-bonded particleboard.
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Formaldehyde parameters of five European wood species were determined using test methods applicable for wood-based materials. The five wood species studied showed differences in their formaldehyde emissions. Samples of green oak produced values of 9 parts per billion of formaldehyde (the highest value) and dry beech wood produced 2 parts per billion of formaldehyde (the lowest value). The emission values determined in the dry state were 1 to 2 units higher than those in the green state, with the exception of oak.
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Complaints about unpleasant odour from wall, ceiling and floor coverings made of composite cork, induced chamber tests to study the emissions of volatile organic compounds (VOC) from composite cork products for indoor use. Emissions of phenol and furfural were found to be high, particularly those from cork parquet. Emission factors after 1 week ranged from 150 to 650 μg m-2 h-1 and from 15 to 350 μg m-2 h-1 for phenol and furfural, respectively, and decreased only slowly over time, by a factor of approx. 10 for a 6-month period. The ranges of emission factors were found to be similar for some solvents such as cyclohexanone or toluene which are constituents of varnishes used to protect cork surfaces. The emission of furfural may result from chemical reactions in the cork during the production process or may be caused by additives such as binders.
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Concern about possible health effects from formalde hyde emitted from wood-based panel products manu factured with urea-formaldehyde and melamine-formal dehyde binding resins has led to the development and increasing use of low emission products. To control the emissions a range of tests has been developed which the wood-based panel industry can use to determine the potential of products to release formaldehyde. This pa per reviews laboratory test methods which are based on the extraction of formaldehyde and also the current requirements in European product standards. It further describes the use of chamber tests to measure formalde hyde emission under normal conditions of temperature and humidity. These tests are included in the labelling schemes of some countries and are the subject of a new European standard test method.
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The emission of formaldehyde from softwood particles, as measured by the flask method (EN 717-3), depends highly on the particle size. Therefore, no definite value for the formaldehyde release from wood can be given.
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The biosynthetic pathway of trans-2-hexenal, leaf aldehyde, in isolated chloroplasts of Thea sinensis leaves. was examined using a tracer experiment. A high and specific incorporation of radioactivity into cis-3-hexenal and trans-2-hexenal, was observed when linolenic acid-[U-14C] was incubated with the isolated chloroplasts. Thus, trans-2-hexenal was biosynthesized via cis-3-hexenal from linolenic acid in the chloroplasts.
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The emission of formaldehyde is an important factor in the evaluation of the environmental and health effects of wood-based board materials. This article gives a comparison between commonly used European test methods: chamber method [EN 717-1, 2004. Wood-based panels—determination of formaldehyde release—Part 1: formaldehyde emission by the chamber method. European Standard, October 2004], gas analysis method [EN 717-2, 1994. Wood-based panels—determination of formaldehyde release—Part 2: formaldehyde release by the gas analysis method, European Standard, November 1994], flask method [EN 717-3, 1996. Wood-based panels—determination of formaldehyde release—Part 3: formaldehyde release by the flask method, European Standard, March 1996], perforator method [EN 120, 1993. Wood based panels—determination of formaldehyde content—extraction method called perforator method, European Standard, September 1993], Japanese test methods: desiccator methods [JIS A 1460, 2001. Building boards. Determination of formaldehyde emission—desiccator method, Japanese Industrial Standard, March 2001 and JAS MAFF 233, 2001] and small chamber method [JIS A 1901, 2003. Determination of the emission of volatile organic compounds and aldehydes for building products—small chamber method, Japanese Industrial Standard, January 2003], for solid wood, particleboard, plywood and medium density fiberboard.