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his work examines the performance of three formaldehyde scavengers in wood-based panels. Sodium metabisulfite, ammonium bisulfite and urea were applied in different physical forms during particleboard production, and the resulting physico-mechanical properties (internal bond strength, thickness swelling, density and moisture content) and formaldehyde emission levels were compared. Formaldehyde content was measured using the perforator method, and formaldehyde emission was evaluated both by desiccator and gas analysis methods. The chemical reactions involved in each formaldehyde scavenging process are proposed and discussed. The tested scavengers showed distinct performances under the different emission testing conditions, which were interpreted in terms of the stability of the chemical compounds formed upon formaldehyde capture. Sodium metabisulfite proved to be an excellent scavenger for all formaldehyde methods allowing the production of particleboard panels with zero formaldehyde emission.
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ORIGINAL
Scavengers for achieving zero formaldehyde emission
of wood-based panels
Nuno A. Costa
Joa
˜
o Pereira
Joa
˜
o Ferra
Paulo Cruz
Jorge Martins
Ferna
˜
o D. Magalha
˜
es
Ade
´
lio Mendes
Luı
´
sa H. Carvalho
Received: 27 June 2012 / Published online: 16 July 2013
Ó Springer-Verlag Berlin Heidelberg 2013
Abstract This work examines the performance of three formaldehyde scavengers
in wood-based panels. Sodium metabisulfite, ammonium bisulfite and urea were
applied in different physical forms during particleboard production, and the
resulting physico-mechanical properties (internal bond strength, thickness swelling,
density and moisture content) and formaldehyde emission levels were compared.
Formaldehyde content was measured using the perforator method, and formalde-
hyde emission was evaluated both by desiccator and gas analysis methods. The
chemical reactions involved in each formaldehyde scavenging process are proposed
and discussed. The tested scavengers showed distinct performances under the dif-
ferent emission testing conditions, which were interpreted in terms of the stability of
the chemical compounds formed upon formaldehyde capture. Sodium metabisulfite
proved to be an excellent scavenger for all formaldehyde methods allowing the
production of particleboard panels with zero formaldehyde emission.
N. A. Costa J. Pereira J. Martins F. D. Magalha
˜
es A. Mendes L. H. Carvalho (&)
Laborato
´
rio de Engenharia de Processos, Ambiente e Energia (LEPAE), Faculdade de Engenharia,
Universidade do Porto, rua Dr. Roberto Frias, 4200-465 Porto, Portugal
e-mail: lhcarvalho@demad.estv.ipv.pt
N. A. Costa J. Pereira
Associac¸a
˜
o Rede de Compete
ˆ
ncias em Polı
´
meros (ARCP), Porto, Portugal
N. A. Costa J. Ferra P. Cruz
EuroResinas—Indu
´
strias Quı
´
micas, S. A., Sines, Portugal
J. Martins L. H. Carvalho
Departamento de Engenharia de Madeiras (DEMad), Instituto Polite
´
cnico de Viseu,
Campus Polite
´
cnico de Repeses, 3504-510 Viseu, Portugal
123
Wood Sci Technol (2013) 47:1261–1272
DOI 10.1007/s00226-013-0573-4
Introduction
Urea–formaldehyde (UF) resins are the most widely used adhesives in the
manufacture of wood-based panels. The production of particleboards (PB) and
medium density fiberboards (MDF) consumes a large fraction of UF resins produced
worldwide. The industrial success of these resins is associated with their high
reactivity, excellent adhesion to wood and low price (Dunky 1998). However, their
big disadvantage is formaldehyde emission due to hydrolysis of weak chemical
bonds during board production and lifetime use.
Formaldehyde was reclassified in 2004 by the International Agency for Research
on Cancer (IARC) as ‘carcinogenic to humans (Group 1)’’ (IARC 2006), compelling
companies to reduce formaldehyde emission to lower levels. Since 2009, California
Air Resources Board (CARB) imposed more restrictions on formaldehyde emission
limits, which had a great impact on the wood-based panels industry.
Reduction in F/U molar ratio has been a strategy adopted in the last decades to
decrease formaldehyde emission (Myers 1984). However, this reduction decreases
the reactivity of UF resins. Currently, reactivity of industrial UF adhesives is near
the minimum limit accepted for industrial panel production (Dongbin et al. 2006).
Substitution of UF resins by other formaldehyde-free adhesives does not
convince industrial producers due to their higher price or lower reactivity (Amazio
et al. 2011; Despres et al. 2010; Tang et al. 2011). In order to increase the degree of
cure and reduce free formaldehyde at the end of cure, new catalysts were studied,
but reactivity is still too low (Costa et al. 2012a; Gunnells and Griffin 1998).
The use of scavengers, such as natural or bio-based scavengers (Eom et al. 2006;
Kim 2009; Kim et al. 2006) or other compounds with good affinity to capture
formaldehyde (Boran et al. 2011; Costa et al. 2012b; Park et al. 2008), to reduce
formaldehyde emission from wood-based panels is commonly adopted. Costa et al.
(2012b) studied the use of sodium metabisulfite (Na
2
S
2
O
5
) as formaldehyde
scavenger in particleboards produced with UF and melamine–formaldehyde (MF)
resins with good results. The reaction of sodium metabisulfite with water forms
sodium bisulfite, also called sodium hydrogen sulfite (Eq. 1). The reaction between
formaldehyde and sodium bisulfite forms a bisulfite adduct (Eq. 2) (Barbera
´
et al.
2000; Walker 1944).
Na
2
S
2
O
5
þ H
2
O ! 2NaHSO
3
ð1Þ
NaHSO
3
þ HCHO ! NaSO
3
CH
2
OH adductðÞ ð2Þ
When neutralized with sodium hydroxide, sodium bisulfite forms sodium sulfite
(Na
2
SO
3
), as seen in Eq. 3. Sodium sulfite is used to quantify formaldehyde by
titration of the sodium hydroxide formed as by-product (Eq. 4) (Walker 1944).
NaHSO
3
þ NaOH ! H
2
O þNa
2
SO
3
ð3Þ
HCHO þNa
2
SO
3
þ H
2
O ! NaOH þNaSO
3
CH
2
OH ð4Þ
When strongly heated, sodium sulfite decomposes into sodium sulfate and
sodium sulfide (Eq. 5) (Gerrans et al. 2004). In the pulp and paper industry, sodium
sulfide combined with sodium hydroxide is used in kraft process, an alkaline
1262 Wood Sci Technol (2013) 47:1261–1272
123
chemical process used for treating wood chips in order to break the bonds between
lignin and cellulose and separate the fibers (Miller et al. 2009).
4Na
2
SO
3
! 3NaSO
4
þ Na
2
S ð5Þ
Ammonium and sulfite ions are in equilibrium in ammonium sulfite aqueous
solution (Eq. 6). In the presence of sodium ions (existent in the UF resin) and
formaldehyde, the sulfite ions react forming an adduct (Eq. 7). Ammonia in aqueous
solution reacts with formaldehyde forming hexamine (hexamethylenetetramine)
(Eqs. 8, 9) (Dreyfors et al. 1989; Walker 1944).
NH
4
HSO
3
$ NH
þ
4
þ HSO
3
ð6Þ
Na
þ
þ HSO
3
þ HCHO ! NaSO
3
CH
2
OH adductðÞ ð7Þ
NH
þ
4
þ HO
$ NH
3
þ H
2
O ð8Þ
4NH
3
þ 6HCHO ! CH
2
ðÞ
6
N
4
ð9Þ
There are several methods for the determination of formaldehyde emission on
wood-based panels. They can be divided into two main groups: emittable potential
and measurable emission of formaldehyde. The first type measures the formalde-
hyde content without establishing whether it will actually emit or in which time this
emission may occur. The second type determines the actually emitted formaldehyde
amount under the test conditions (Dunky et al. 2001).
Emittable potential is evaluated by the perforator method (EN 120), whereas
actually emitted formaldehyde can be evaluated by several methods: chamber
method (EN 717-1, ASTM E 1333 or ASTM D 6007), gas analysis (EN 717-2) and
desiccator method (JIS A 1460). Many authors have tried to relate formaldehyde
emissions measured by different methods (Park et al. 2011; Que and Furuno 2007;
Risholm-Sundman et al. 2007; Salem et al. 2012), but the relations are influenced by
other variables, such as resin type, type of wood-based panels, thickness or
manufacturing conditions. Salem et al. (2011a) and Kim and Kim (2005) studied
formaldehyde emission from different types of formaldehyde-based resins. Salem
et al. (2011b) showed that the manufacturing variables interfere significantly with
formaldehyde emissions, and correlations between methods to assess different
products were not possible.
The present work studies the performance of sodium and ammonium bisulfite and
urea as formaldehyde scavengers in particleboard production. Formaldehyde
emissions evaluated by perforator, desiccator and gas analysis methods are
compared and discussed.
Materials and methods
Materials
Last generation commercial UF resins (0.94 F/(NH
2
)
2
molar ratio) and urea were
provided by EuroResinas—Indu
´
strias Quı
´
micas, S.A. (Sines Portugal). Wood
Wood Sci Technol (2013) 47:1261–1272 1263
123
particles, paraffin and ammonium sulfate for the production of particleboard were
supplied by Sonae Indu
´
stria PCDM (Oliveira do Hospital, Portugal). Analytical
grade sodium metabisulfite and ammonium bisulfite solution (70 wt%) were used.
Particleboard production
The adhesive system is composed of UF resin, paraffin, catalyst (ammonium sulfate
30 wt% solution) and water for adjusting the mat moisture content. Wood particles
were blended with the adhesive in a laboratory glue blender. The solid resin load
was 7 wt% based on oven dry wood. Scavengers, when applied in liquid form, were
added to wood particles at the beginning of the blending operation prior to resin
blend. Scavengers applied in solid form were dispersed by hand on wood particles
after resin blending.
Three particle layer mats were hand formed in a square aluminum deformable
container with the dimensions of 220 9 220 9 80 mm
3
. Wood mass distribution
was 20 % in the upper face layer, 62 % for the core layer and 18 % in the bottom
face layer. Core and face layers differ in moisture content (11 % in face layer and
8 % in core layer) and size distribution of particles (smaller particles in face layer
and larger in core layers, as used in industrial production). Particleboards were
designed to obtain a target density between 650 and 700 kg/m
3
.
The pressing schedule of an industrial continuous process was adapted to a batch
cycle in a computer-controlled laboratory scale hot-press equipped with a linear
variable displacement transducer (LVDT), a pressure transducer and thermocouples.
For all series, six boards with a thickness of 16 mm were produced with a
pressing factor of 9.5 s/mm. Control series were produced using the same operating
conditions as for the other series.
Particleboard analysis
All boards were hermetically conditioned until tested. The boards were tested
according to European standards for density (D) (EN 323), internal bond (IB) (EN
319), moisture content (MC) (EN 322) and thickness swelling (TS) (EN 317). For
each series, one board was randomly selected for formaldehyde content (FC)
analysis according to EN 120 (perforator method) (the formaldehyde content was
adjusted to 6.5 % of moisture content according to EN 312). Three boards were
used for the desiccator method (JIS A 1460). In the second part of this work, one
panel of each series was submitted to the gas analysis method (EN 717-2) for
determination of formaldehyde emission.
Results and discussion
Evaluation of performance of different formaldehyde scavengers
Table 1 presents the properties of the particleboards produced with 5 wt%
scavenger (solid content based on solid resin) added in three different forms:
1264 Wood Sci Technol (2013) 47:1261–1272
123
(a) solid sodium metabisulfite (SMBS), (b) sodium bisulfite aqueous solution
(40 wt% sodium metabisulfite in water) (SB40) and (c) SB40 partially neutralized
with sodium hydroxide to pH = 5.8 (SB_NaOH).
As reported by Costa et al. (2012b), small amounts of sodium metabisulfite
reduce substantially the formaldehyde content of particleboards. Table 1 supports
these results and shows that formaldehyde emission is lower when solid sodium
metabisulfite is used. It should be noted that the precision of the perforator method
is not defined, but the exclusion criteria in EN 120 standard mention deviations
higher than 20 % between two replicates. The method precision can therefore be
assumed to be of that order, and smaller differences in formaldehyde content
between different test conditions should not be taken into account.
The release of small particles of sodium metabisulfite into air causes respiratory
tract irritation (Barbera
´
et al. 2000; Costa et al. 2012b), which can be avoided when
the scavenger is applied in liquid solution. However, dissolution of sodium
metabisulfite in water forms sodium bisulfite. Due to its proton donor ability, it shows
acidic characteristics, which can cause resin pre-cure in the blending operation
(Eq. 1). The sodium bisulfite solution, prepared with 40 wt% of sodium metabisul-
fite, has a pH of 3.7. Internal bond decrease and thickness swelling increase (Table 1)
can be related to pre-cure of the resin at this low pH, or to premature consumption of
formaldehyde due to higher mobility/dispersibility of the scavenger in liquid form.
The formaldehyde content obtained is similar using either metabisulfite in solid form
or in solution (Table 1). Differences between formaldehyde content of SMBS and
SB40 values are not significant. In case of formaldehyde emission of SB_NaOH and
control (without scavenger), the values are also similar and differences are not
significant. The lower effectiveness in reducing formaldehyde emission can be
related to migration of sodium bisulfite toward the core layer of the mat, dissolved in
the vapor phase formed during hot-pressing (Carvalho et al. 2010). Scavenger
depletion in the external layers reduces effectiveness of formaldehyde capture in
emission testing (desiccator method). Formaldehyde content measurements (perfo-
rator method) are not affected by scavenger distribution, since this test evaluates all
formaldehyde present in the sample.
The purpose of the trial performed with sodium bisulfite solution neutralized with
sodium hydroxide was to avoid resin pre-cure due to low pH, as discussed above.
Table 1 Properties of
particleboards produced with
sodium metabisulfite and
bisulfite
Control SMBS SB40 SB_NaOH
Internal bond (N/mm
2
) 0.58 0.57 0.41 0.45
Density (kg/m
3
) 690 696 690 679
Thickness swelling (%) 33.3 33.5 38.2 32.9
Moisture content (%) 7.3 7.3 7.8 7.3
Formaldehyde content
(mg/100 g oven dry
board)
2.2 1.8 1.7 2.6
Formaldehyde emission
(mg/L)
0.45 0.13 0.30 0.43
Wood Sci Technol (2013) 47:1261–1272 1265
123
However, as seen in Table 1, this led to similar emission to the control and slightly
higher formaldehyde content. Internal bond has decreased. Sodium bisulfite reacts
with sodium hydroxide forming sodium sulfite (Eq. 3). During cure, sodium sulfite
reacts with formaldehyde forming sodium hydroxide (Eq. 4) and increasing pH,
thus unfavoring the cure reaction. In addition, at high temperatures (190 °C),
decomposition of sodium sulfite into sodium sulfide (Eq. 5) can possibly cause
some degradation of wood components, which supports the reduction in physico-
mechanical properties of the boards (Dreyfors et al. 1989).
Table 2 shows the properties of particleboards produced with ammonium
bisulfite solutions (70 wt%, pH = 5.0) with two different incorporations: 5 %
(ABS_5) and 10 % (ABS_10) based on solid resin. As observed with sodium
bisulfite, the application of scavenger in liquid form reduces formaldehyde emission
but negatively affects the physico-mechanical properties of the particleboard.
Comparison of different formaldehyde emission methods
In the previous part of this work, it was shown that sodium metabisulfite presents
higher formaldehyde scavenging ability when applied in solid form. Ammonium
bisulfite also presented good performance, but negatively affects the internal bond
and thickness swelling. Another effective scavenger is urea, which has been widely
used in industry due to its good performance and low price (Costa et al. 2011;
Lehmann 1983; Park et al. 2008).
In this section, the scavenging performance of sodium metabisulfite in solid form,
ammonium bisulfite in liquid solution and urea is evaluated using three different
formaldehyde evaluation standard methods: perforator, desiccator and gas analysis.
Urea was applied in solution (30 wt%) in the face layer to avoid the ‘blistering
effect’ noted when a decorative paper is pressed over a urea prill. In the core layer,
urea was applied in solid form (prills) to avoid an increase in internal moisture
content that inhibits heat transfer (Carvalho et al. 2010). The amount of scavengers
added was 5, 10 and 15 wt% (solid scavenger based on solid resin). Physico-
mechanical properties (internal bond and thickness swelling) and formaldehyde
content (perforator method) and emission (desiccator and gas analysis method) were
evaluated for each series of particleboards, and the results are presented below. A
control series was produced with the same resin and operating conditions, but
without scavenger addition.
Table 2 Properties of
particleboards produced with
ammonium bisulfite
Control ABS_5 ABS_10
Internal bond (N/mm
2
) 0.54 0.33 0.12
Density (kg/m
3
) 669 684 643
Thickness swelling (%) 30.8 36.3 51.3
Moisture content (%) 7.9 7.9 8.6
Formaldehyde content
(mg/100 g oven dry board)
2.5 1.6 0.9
Formaldehyde emission (mg/L) 0.47 0.37 0.22
1266 Wood Sci Technol (2013) 47:1261–1272
123
All particleboards were produced with the same resin and using the same
pressing conditions (16 mm of thickness, 150 s of pressing at 190 °C). Figure 1
shows the internal bond and thickness swelling of the particleboards produced. As
previously concluded, particleboards produced with sodium metabisulfite do not
present a significant reduction in internal bond, neither suffer substantial penalty in
thickness swelling. Urea presents similar behavior. Ammonium bisulfite presents
higher penalty in internal bond and thickness swelling.
Figure 2 presents the formaldehyde content of particleboards produced with
different scavengers. Urea presents the lowest ability for scavenging formaldehyde.
Fig. 1 Comparison of internal bond and thickness swelling with different amounts of formaldehyde
scavengers. SMBS sodium metabisulfite, ABS ammonium bisulfite)
Fig. 2 Formaldehyde content by perforator method (EN 120) of particleboards produced with different
formaldehyde scavengers
Wood Sci Technol (2013) 47:1261–1272 1267
123
This result supports the idea that urea is no longer a preferred formaldehyde
scavenger to produce ultra-low emission wood-based panels. Sodium metabisulfite
and ammonium bisulfite present similar results using the perforator method. Boards
produced with 15 wt% sodium metabisulfite show a formaldehyde content near
values typical of solid wood (Meyer and Boehme 1997).
Figure 3 shows formaldehyde emission of particleboards analyzed by desiccator
method. Sodium metabisulfite presents zero formaldehyde emission when incorpo-
rated at 15 wt%, while ammonium bisulfite and urea present a significant reduction
in formaldehyde emission. Figure 4 shows formaldehyde emissions by the gas
analysis method. The test performed with ammonium bisulfite at 15 wt% is not
shown due to experimental errors during analysis.
The better performance of sodium metabisulfite in the desiccator method can be
related to higher hydrolysis resistance of the adduct formed in the reaction with
formaldehyde. The addition reaction between formaldehyde and urea used as
scavenger forms methylolureas, which tends to undergo hydrolysis in the presence
of moisture, releasing formaldehyde (Dunky 1998; Pizzi and Mittal 1994). This
explains the poor performance of urea addition in the desiccator method, since test
pieces are subjected to a high relative humidity. Ammonium bisulfite shows a
similar behavior, indicating that the compound formed by reaction with formalde-
hyde also has low moisture resistance.
In the gas analysis method, urea does not show scavenging ability. This may be
related to the low thermal stability of the oligomeric species formed by reaction of
urea scavenger with formaldehyde even at a relatively low temperature of 60 °C.
Ammonium bisulfite presents a similar trend. Compounds formed between
ammonium bisulfite and formaldehyde do not present the same stability as those
formed with sodium metabisulfite. The product formed in the presence of
ammonium ions is probably less stable than the sodium salt adduct described in
Fig. 3 Formaldehyde emission by desiccator method (JIS A 1460) of particleboards produced with
different formaldehyde scavengers
1268 Wood Sci Technol (2013) 47:1261–1272
123
Eq. 7. The higher temperature (60 °C) of the test can also reverse some of the
scavenging reactions.
Figure 5 shows the relation between formaldehyde emission obtained by the
desiccator method and formaldehyde content measured by the perforator method.
Sodium metabisulfite and urea exhibit a similar linear trend, unlike ammonium
bisulfite. Park et al. (2011) have compared both methods for particleboards within
the range between 2.9 and 16.2 mg per 100 g of oven dry board (o.d.b) and 0.3 and
3.0 mg/L, for perforator and desiccator values, respectively. No scavengers were
Fig. 4 Formaldehyde emission by gas analysis method (EN 717-2) of particleboards produced with
different formaldehyde scavengers
Fig. 5 Comparison of formaldehyde content by perforator method (EN 120) and formaldehyde emission
by desiccator method (JIS A 1460) of particleboards produced with different scavengers
Wood Sci Technol (2013) 47:1261–1272 1269
123
used in this case. The data of the present study show a similar slope to these authors,
but cover lower emission values, including zero emission (Fig. 5). Risholm-
Sundman et al. (2007) present a relation between both methods, also without
scavenger addition, for formaldehyde contents between 1.0 and 8.0 mg per 100 g
(o.d.b.) and emissions between 0.16 and 0.74 mg/L. In this case, the linear relation
obtained is different to the work here.
Conclusion
In this work, different formaldehyde scavengers were studied: powder sodium
metabisulfite, aqueous solutions of sodium, and ammonium bisulfite and urea, either
in aqueous solution or particulated. Formaldehyde emission of particleboards was
evaluated using three standard methods: perforator (EN 120), desiccator (JIS A
1460) and gas analysis (EN 717-2). Boards produced with sodium metabisulfite
exhibited formaldehyde content (perforator value) near solid wood levels and zero
formaldehyde emissions (desiccator method). The other tested scavengers yielded
much lower performances.
The scavenging performance of each scavenger is strongly dependent on the
formaldehyde analysis method. The test conditions, such as temperature, relative
humidity or air exchange, may interfere in different ways with the stability of the
chemical compounds formed in the scavenging reactions with formaldehyde. This
study confirms that comparisons between different formaldehyde emission standard
methods should be carefully analyzed.
Sodium metabisulfite showed the best formaldehyde scavenging performance for
all methods, even the gas analysis method, where the other additives did not exhibit
scavenging ability. Boards with higher content of sodium metabisulfite showed zero
emission without deteriorating physico-mechanical properties.
A relation between desiccator and perforator method values was given, but it was
not possible to establish a correlation between gas analysis and the other methods,
because this relation is strongly dependent on the type of scavenger used.
Acknowledgments This work is co-founded by FEDER (Fundo Europeu de Desenvolvimento
Regional)/QREN (E0_formaldehyde project with reference FCOMP010202FEDER005347) and national
funds through FCT (project PTDC/EQU–EQU/111571/2009) under the framework of COMPETE-
Programa Operacional Factor de Competitividade (POFC). The authors wish to thank Euroresinas and
Sonae Indu
´
stria PCDM for providing the equipment and raw materials needed for this work. Nuno Costa
wishes to thank FCT/MCTES for PhD grant SFRH/BDE/33655/2009.
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... The long-term problem of formaldehyde emission can be effectively improved through improving hot pressing technology, modification of adhesives, and developments in wood-based panel postprocessing technology (Basta et al. 2006;Mo et al. 2022). The use of nanotechnology materials, reduction of formaldehyde-urea molar ratio, and usage of formaldehyde scavengers can reduce free formaldehyde emissions (Costa et al. 2013;Gangi et al. 2013;Moubarik et al. 2013;Pizzi et al. 2020;Antov et al. 2021a,b,c;Bekhta et al. 2021a,b;Selakjani et al. 2021;Dorieh et al. 2022a;Dorieh et al. 2022b;Kristak et al. 2022;Kristak et al. 2022). Post-treatment techniques such as veneer and edging can also effectively reduce the formaldehyde emissions rate (Roffael 2011;Costa et al. 2013;Bekhta et al. 2018). ...
... The use of nanotechnology materials, reduction of formaldehyde-urea molar ratio, and usage of formaldehyde scavengers can reduce free formaldehyde emissions (Costa et al. 2013;Gangi et al. 2013;Moubarik et al. 2013;Pizzi et al. 2020;Antov et al. 2021a,b,c;Bekhta et al. 2021a,b;Selakjani et al. 2021;Dorieh et al. 2022a;Dorieh et al. 2022b;Kristak et al. 2022;Kristak et al. 2022). Post-treatment techniques such as veneer and edging can also effectively reduce the formaldehyde emissions rate (Roffael 2011;Costa et al. 2013;Bekhta et al. 2018). But most of the methods cannot be industrialized, and there is no way to completely solve the free formaldehyde Kristak et al. 2022). ...
Article
Wood-based panels, which contain wood raw materials along with urea-formaldehyde (UF) or phenol-formaldehyde (PF) resins, can increase the indoor air concentration of formaldehyde. Formaldehyde can stimulate the upper respiratory mucosa and cross-linking reaction with cell proteins and DNA, and this can result in degeneration and necrosis of respiratory cells and damaged cell proliferation. Formaldehyde can induce health hazards such as nasal cancer, leukemia, and destruction of the reproductive system. Acetaldehyde dehydrogenase 5 (ADH5) in the body cooperates with Fanconi anaemia complementation group D2 (FANCD2) to quickly metabolize formaldehyde into formate and maintain the balance of endogenous formaldehyde. However, when both ADH5 and FANCD2 proteins have defects or mutations, damaged DNA repair failure and cell proliferation induce a variety of health diseases. The damage has been found in the upper respiratory area, not on distal body tissues such as liver, kidney, and bone marrow. Meanwhile epidemiological survey has not shown a positive correlation between formaldehyde and health hazards. It is recommended that the use of wood formaldehyde-based products should be reduced, and pathogenesis genes and damage repair mechanism should be studied systematically and deeply to develop gene drugs to remove excess formaldehyde and activate the damage gene repair mechanism in the future.
... 38 The hydrolysis of sodium sulte to generate sodium bisulte, which reacts with formaldehyde to generate HOCH 2 SO 3 Na, were widely considered to be the initial reactions of the aliphatic dispersant. [39][40][41] To understand the substitution position of HOCH 2 SO 3 Na on the molecular chain of PSAF, the reaction (Fig. 3a) of HOCH 2 SO 3 Na with acetone was performed and the product was analyzed by NMR. As shown in Fig. 3b, peaks at 3.76 ppm could be ascribed to the chemical shi of the protons in -C-CH 2 -SO 3 Na, while the peak at 2.14 ppm could be assigned to the chemical shi of the protons in CH 3 -C]O and -C-CH 2 -C]O. ...
Article
Full-text available
Dispersants can have a substantial impact on the rheological characteristics of coal-water slurry (CWS). Due to their advantages in cost and synthesis, linear dispersants are currently most often employed in the commercial manufacturing of CWS. However, this kind of dispersant gives limited performance because of its weak adsorption and steric hindrance effect on the coal-water interface. This work describes a new linear dispersant (PSAF) with a significant steric hindrance effect that was created by incorporating phenolic groups into its molecular architecture, which gives higher maximum coal content (63.79 wt%) than that (63.11 wt%) from sulfonated acetone-formaldehyde (SAF). The synthesis mechanism was investigated using GPC, FT-IR and NMR. Various technologies were used to explore the rheological characteristics and dispersion mechanism for CWS prepared with PSAF. PSAF as well as SAF showed monolayer adsorption on the surface of coal and displayed a higher adsorption layer thickness (3.5 nm). PSAF dispersant presents stand-up adsorption rather than lie-down adsorption of SAF because of its strong p-p action, resulting in a stronger steric hindrance effect and improved rheological performance. This work can provide guidelines for the development of a high-performance dispersant as well as an understanding of the dispersal process for CWS.
... Introduction of propylamine can result in the pre-consumption of formaldehyde and decrease in a degree of crosslinking resulting from the fact that less formaldehyde is available for the reaction with the UF polymer which consequently could adversely affect strength properties of the resultant bonds. [50] The formaldehyde emission is crucial property for the practical application of UF adhesive-bonded plywood. The differences between variants containing PA in the amount ranging from 0.5% to 1.5% were statistically significant ( Figure 6). ...
Article
The aim of the study was to investigate the effect of propylamine addition to urea-formaldehyde resin on the properties of resultant plywood. The research consisted in investigating the properties of both liquid and cured resins, as well as the produced plywood. Studies have shown that the incorporation of propylamine adversely affected reactivity of the adhesive mixtures by increasing their pH. The evaluation of adhesive chemical structure with FTIR spectroscopy showed some slight changes indicating the reaction of formaldehyde with introduced modifier. The addition of propylamine contributed to the considerable reduction of free formaldehyde contained in the cured adhesive. Furthermore, the propylamine was also found to work effectively as a formaldehyde scavenger by significantly reducing the hazardous emission from plywood. However, based on the results of bonding quality, delamination, bending strength and modulus of elasticity it was found that application of propylamine resulted in a noticeable deterioration of plywood properties. Therefore, the amount of introduced modifier should not exceed 1% of solid UF resin.
... In recent decades, different approaches have been tested to reduce the emission of formaldehyde from formaldehyde-containing wood adhesives. Reductions can be achieved either by addition of formaldehyde scavengers that chemically bind free released formaldehyde (Costa et al. 2013) or through substitution of formaldehyde with other aldehydes to reduce the total amount of formaldehyde used in production. Examples include partial substitution with furfural (Zhang et al. 2014) and 5-hydroxymethylfurfural (HMF) (Esmaeili et al. 2017). ...
Article
Glycolaldehyde, produced from cracking of glucose, was tested as a substitute for formaldehyde in urea-based wood adhesives. Initially, different parameters (water content, aldehyde/urea-ratio, curing temperature, and time) were screened to identify the optimal curing conditions providing the highest bond strength. Afterwards, the system was reformulated as a 2-component system and compared to a urea-formaldehyde 2-component system, which showed a comparatively low strength of the resulting resin. Different hardeners were tested, and AlCl3 showed an 80% increase in bond strength for the resin compared to NH4Cl. Infrared and nuclear magnetic resonance analyses were performed to ensure formation of the desired aminal bond network, which showed that the hardener was essential for proper curing of the resin. Finally, a urea-glycolaldehyde-formaldehyde resin was tested that further indicated major differences between the reactivity of formaldehyde and glycolaldehyde.
... It has also been revealed that bark in particleboards acts as a formaldehyde scavenger, 44,45 due to the reaction of phenolic bark compounds with the formaldehyde. 46 Recently, Gößwald et al. 47 investigated the potential of low-density insulation boards made of spruce bark fibers, with different densities and fiber lengths, manufactured in a wet process. ...
Article
The climate change and the mineral resources reserves reduction highlight the necessity to find alternative ecological materials concerning the construction, energy and transport sector, among others. The development of bio-based thermal insulation products using agricultural and forest biomass has come recently to the forefront. The bark of the trees is a readily available, biological raw material of unique properties that outperforms as a bio-insulating material, alone or introduced in different polymeric matrices. In this study, bark powder of two pine species (scots pine and black pine) is chemically characterized and incorporated at 30% and 50% b.v. (of solids content) in cement matrix, aiming at the production of innovative insulation composites. The cement-bark composites were characterized, in terms of their resistance to compression, modulus of elasticity, thermal conductivity, thermal resistance and hygroscopic properties (swelling and absorption rate). The presence of bark in the cement matrix, concerning both bark species, decreased the compression strength of the composites, though a higher post-fracture resistance was observed. The bark share of 50% revealed lower strength compared to the share of 30%, as well as lower weight and density. The mild hydrothermal treatment was proved beneficial in the case of black pine bark specimens (30% share), which marked the highest compression strength after control specimens. The composites elasticity was increased by the bark introduction, especially of the hydrothermally treated bark. Additionally, the bark introduction improved the composites thermal resistance, with the bark share of 50% (especially the black pine one) to provide the highest thermal resistance. The hydrothermal treatment did not enhance further the composites thermal properties, but revealed a positive effect on their hygroscopic properties, namely lower water absorption and swelling values (especially the 30% bark content). The results demonstrated a high potential for the proposed cement-bark composites to be utilized in insulation, as well as several non-load bearing building applications.
... In addition, the low wetting angle of pMDI compared with water-based condensation resins can result in starved glue lines, which need to be adjusted (Hornus et al. 2020). Another viable options to reduce the free formaldehyde emission from wood-based panels include the addition of various organic, inorganic, and mineral compounds as formaldehyde scavengers in the adhesive systems, e.g., lignin, tannins, rice husk, hemp or wheat flour, bark, pulp and paper sludge, urea, phosphates, nanoparticles, charcoal, pozzolan, wollastonite, and zeolites (Eom et al. 2006;Kim 2009;Buyuksari et al. 2010;Darmawan et al. 2010;Migneault et al. 2011;Boran et al. 2012;Costa et al. 2013aCosta et al. , b, 2014de Cademartori et al. 2019;Antov et al. 2020a, b;Taghiyari et al. 2020a, b;Camlibel 2020;Barbu et al. 2020a, b;Kawalerczyk et al. 2020a, b;Réh et al. 2021); modification of hot pressing parameters, i.e., pressing temperature and pressing time (Puttasukkha et al. 2015;Bekhta et al. 2020); surface treatment or post-treatment of the finished composites (Roffael 2011;Hematabadi et al. 2012;Bekhta et al. 2018); and using environmentally sustainable, formaldehyde-free, biobased wood adhesives (Nordström et al. 2017;Hemmilä et al. 2017;Tisserat et al. 2019;Hosseinpourpia et al. 2019;Ghani et al. 2019;Sarika et al. 2020;Arias et al. 2021a;Saud et al. 2021). Addition of different metal and mineral nanomaterials (like nanosilver, nanosepiolite, and nanowollastonite) to formaldehyde-based resins was reported not only to improve physical and mechanical properties in wood-based composite panels (Taghiyari et al. 2011;Rangavar et al. 2013) but also to decrease hot-pressing time by means of improving thermal conductivity in panels and even to improve shear strength in polyvinyl acetate resin by graphene as well (Taghiyari Fig. 1 Natural raw materials for the production of eco-friendly bio-based wood adhesives (Arias et al. 2021b et al. 2011, 2020c, 2022). ...
Chapter
In recent years, bio-based wood adhesives have received a lot of attention as a sustainable and renewable alternative to the conventional synthetic adhesives used in the wood-based industry. Bio-based adhesives, on the other hand, such as protein, starch, lignin, and tannin, have inferior properties when compared with thermosetting synthetic resins. Reinforcement with nanomaterials with a high aspect ratio has the potential to improve the performance of bio-based wood adhesives. Therefore, this chapter discusses recent advances in the use of nanomaterials, such as nanocellulose, nanolignin, and nanoclay, in the synthesis of sustainable, bio-based wood adhesives for the production of wood-based composites with improved properties and a lower environmental footprint for advanced value-added applications. The majority of studies have found that nanomaterials have a positive reinforcing effect on adhesive performance. This chapter also discusses the challenges and future prospects of using these nanomaterials in bio-based wood adhesives.
Article
Urea‐formaldehyde (UF) resin with excellent intrinsic flame retardancy, high strength, and low cost has been widely used as adhesives, coatings as well as molding compounds, and it is a challenge to prepare UF resin with combined properties of high toughness/strength and low formaldehyde emissions. In this work, glutaraldehyde was introduced into the synthesis system of UF resin to partially replace formaldehyde, and urea‐glutaraldehyde‐formaldehyde (UGF) copolycondensation resin was prepared. It was found that glutaraldehyde participated in additional/condensation reactions of UF resin, and the crosslinking reaction of UGF resin was hindered with higher curing activation energy than that of neat UF resin. Due to the controllable curing kinetics and introduction of long methylene chains, UGF resin presented relatively low crosslinking density, and under external force, it underwent distinct yielding before fracture and many yield folds appeared on the fractured surface, showing high toughness and strength. Compared with neat UF resin, the tensile strength, elongation at break, impact strength, and critical stress intensity factor (KIC) of UGF resin increased by 26%, 42.30%, 14.6%, and 30%, respectively. Meanwhile, the free formaldehyde emission for UGF resin decreased by 47.5%, meeting the requirement of E0 grade. Such developed eco‐friendly UGF resin exhibited promising application potentials. Glutaraldehyde was introduced into the synthesis system of urea‐formaldehyde (UF) resin and urea‐glutaraldehyde‐formaldehyde (UGF) copolycondensation resin with combined properties of high toughness/strength and low formaldehyde emissions was prepared.
Article
The aim of this study is to obtain the allowable formaldehyde emission value for E1 class boards by reducing the formaldehyde content of the most preferred particleboards. For this purpose, a Formaldehyde Scavenger Solution (FS) was synthesized by using Mono Ethanol Amine (MEA), Ammonium Chloride (AC) and Distilled Water (DW) mixture. Urea formaldehyde resin was used as a virgin adhesive. Three-layer particleboards were produced and their mechanical and physical properties were determined in accordance with the relevant standards. Formaldehyde contents were specified per the EN 120 perforator method. As a result of this study, FS was successfully synthesized and has been successful in reducing formaldehyde content values in the manufactured particleboards. The lowest formaldehyde content was observed in the FS5 boards, which reduced the formaldehyde content by 51.57% compared to the control group. However, the usage of FS negatively affected the mechanical and physical properties. Considering all the mechanical and physical properties of the particleboard with low formaldehyde content, it is possible to say that the best results are achieved with the usage of FS at the lowest rates (2.5% and 5%). As a result, Formaldehyde Scavenger Solution might be utilized during particleboard manufacturing with the virgin Urea Formaldehyde resin.
Article
Full-text available
Formaldehyde scavenger microcapsules were introduced into particleboard to prepare an ecofriendly particleboard with a low pollution release in response to the problem of long-term unstable free formaldehyde release from particleboards. By analyzing key parameters of formaldehyde emission from particleboard, the effects of microcapsules on the diffusion, migration and inhibition of free formaldehyde in particleboard pore structures was discussed. The results showed that microencapsulated formaldehyde scavenger prepared by an emulsification cross-linking method with chitosan as the wall material and urea as the core material resulted in a good long-term controlled release effect on formaldehyde emission. Compared with that of the control panel, the formaldehyde emission of the particleboard with microcapsules decreased by 51.4 % and 25.8 % at 28 d and 180 d, respectively. The addition of formaldehyde scavenger microcapsules increased the particleboard macroscopic pore volume, which facilitated the conversion of adsorbed formaldehyde into free formaldehyde in the pore structure, thereby promoting its migration and diffusion in the particleboard pores. Moreover, the synergistic effect of the addition-condensation and nucleophilic cross-linking of the core and wall materials quickly captured the free formaldehyde in the panels and reduced the releasable concentration of formaldehyde in the material, thus achieving the long-term effective control of formaldehyde emission.
Article
Sustainable bio-based dialdehyde cellulose (DAC) was employed to transform crystalline urea–formaldehyde (UF) resins into amorphous ones for simultaneouly improving their adhesion strength and formaldehyde emission. Serial samples of the UF resins modified with DAC during the resin synthesis were extracted to understand the chemical reactions between the DAC and UF species. Fourier transform infrared, ¹H-nuclear magnetic resonance (NMR), and ¹³C-NMR spectroscopies confirmed the occurrence of reactions between the DAC and UF species. As the synthesis proceeded, the crystallinity of the modified UF resins decreased from 51.7% to 17.4%, transforming the crystalline domains into amorphous ones. Thermograms showed that the DAC in the modified UF resins was decomposed at temperatures over 200°C as degraded form, resulting in a lower cross-linking density than that of the neat UF resins. The adhesion strength of the modified UF resins was statistically similar to that of the neat UF resins, and the formaldehyde emission of the modified UF resins dramatically decreased to ∼64.6%. These results evidence the significant application potential of bio-based DAC in improving the sustainability and performance of UF resins.
Book
Full-text available
Wood-based panels is a general term for a variety of different board products, which have an impressive range of engineering properties. While some panel types are relatively new on the market, others have been developed and successfully introduced more than hundred years ago. However, even those panel types having a long history of continuous optimization are still a long way from being fully developed and they probably never will be. Technological developments on the one hand and new market and regulative requirements, combined with a steadily changing raw material situation, drive continuous improvements of woodbased panels and their manufacturing processes. Advances particularly in the fields of adhesive formulations, production technology, as well as online measuring and control techniques, have triggered a technology push. The adaptation of these technologies to the wood-based panels industry has been motivated by the requirement to improve product quality and reduce manufacturing costs at the same time, or, in other words, to secure the competitiveness of the wood-based panel producers. But there is also a continuous market pull that drives the panel manufacturers and research institutes towards product innovations. Examples of such developments are the increasing demand for light furniture, or the need to adapt panel properties so that new coating technologies can be applied. Considerable endeavours have been made to ensure that wood-based panels have no negative effects on human health. In particular, panel product formaldehyde emissions have been dramatically reduced over the last decades, and further reduction is still the subject of huge efforts applied by panel manufacturers, adhesive suppliers and researchers. In addition, a relatively new issue which is the detection and reduction of volatile organic compounds (VOCs) emitted from wood-based panels has come to the agenda. Again, product and process adaptations are needed to meet the new challenges. The third major driving force for the permanent further development of wood-based panels and the respective production processes is the continuously changing raw material situation. The raw material used in an individual production line usually depends on what is available in a relatively small catchment area, and therefore may vary considerably between different sites. However, there are not only regional variations of the raw material supply, but also changes over time caused by several factors. For example, forest management schemes have altered and will continue to do so. Moreover, the demand from other industries and of the energy sector for wood previously used mainly for panel manufacturing has dramatically increased in many regions. These changes force the panel producers to shift towards alternative sources, including recovered wood, and permanently push the panel industries to modify and optimize their processes in order to maintain a consistent quality level. Clearly, the high variability of the wood raw material constitutes a challenge not known by many other industries. The challenges listed here are considerable. On the other hand, a fibre- or particleboard is a hierarchically organized product, with quite elaborate structures on the different size scales from the molecular to the macroscopic level. Improving the engineering properties of panels requires several of these hierarchical levels to be considered simultaneously. This complexity of the material structure is what makes it so challenging to manipulate the panel properties. Or in other words, innovations are difficult without understanding the fundamental mechanisms at each of the hierarchical levels. The intention of this book is to give a general description of modern panel manufacture, but to also provide some state of the art information on a selected list of fundamental topics. Of course, it is not possible to include all important aspects of panel manufacture into such a book. Therefore, it is our intent to give examples and to stimulate the interested reader to continue his studies by means of the respective technical literature. This book may be used as a textbook for undergraduate and graduate students, as assistance for practitioners, and as reference work for scientists working in the field. It is an introduction into wood-based panel manufacture, but hopefully provides new insights into the fundamentals of the production technology even for specialists.
Article
Full-text available
This paper reports on different applications of sodium metabisulphite as a formaldehyde scavenger. The reduction in formaldehyde emission and the effect on physicomechanical properties of particleboards were studied. The scavenger was mixed with urea–formaldehyde resins with different formaldehyde to urea molar ratios and added separately in the production of particleboards. Several differences between the formaldehyde content and physicomechanical properties of boards were found. When applied to melamine–formaldehyde resins, the changes in formaldehyde emission were less significant, probably due to the incorporation of sodium bisulphite in polymeric matrix. The type of formaldehyde‐based resin and the form of scavenger addition are the major factors affecting formaldehyde scavenging efficiency.
Article
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.
Article
A critical review was made of the literature concerned with how the formaldehyde-to-urea mole ratio (F/U) affects formaldehyde emission from particleboard and plywood bonded with urea-formaldehyde (UF) adhesives, and how this ratio affects certain other adhesive and board properties. It is difficult to quantify the dependence of various properties on the mole ratio or determine lower limits of the mole ratio for a particular property because of the range in resin, board, and testing parameters used in the cited studies. However, the available data do at least suggest some limitations on F/U in conventional UF particleboard systems for the maintenance of acceptable properties.
Article
This study was to investigate the effect of addlng addltive as tannin, rlce husk and charcoal, for reducing the formaldehyde emission level, on the adhesion properties of urea-formaldehyde (UF) resin for particleboard. We controlled the hot-pressing time, temperature and pressure to determine the bonding strength and formaldehyde emission. Blends of various UF resinladditives (tannin, rice husk and charcoal) compositions were prepared. To determine and compare the effect of additives (tannin, rice husk and charcoal) content, 0, 5, 10 and 15%, by weight of UF resin, were used. NH4Cl as hardener added. To determine the level of formaldehyde emission, we used the desiccator, perforator and 20 L-small chamber method. The formaldehyde emission level decreased with increased additions of additive (except rice husk). Also, increased hot-pressing time decreased formaldehyde emission level. At a charcoal replacement ratio of only 1596, the formaldehyde emission level is under F& & & & grade (emit < 0.3 mgl 1 ). Curing of the high tannin additive content in this adhesive system indicated that the bonding strength increased. But, in the case of rice husk and charcoal, the bonding strength was much lower due to the inorganic substance. Furthermore, rice husk was poor in bonding strength as well as formaldehyde emission than tannin and charcoal.
Article
The purpose of this study was to investigate the physico-mechanical properties and characteristics on reduction of formaldehyde and total volatile organic compound (TVOC) emission from medium density fiberboard (MDF) as furniture materials with added volcanic pozzolan. Pozzolan was added as a scavenger to urea formaldehyde (UF) resin for MDF manufacture. The moisture content, density, thickness swelling, water absorption and physical properties of MDFs were examined. Three-point bending strength and internal bond strength were determined using a universal testing machine. Formaldehyde and TVOC were determined by desiccator and 20L small chamber methods as Korean standards method. With increasing pozzolan content the physical and mechanical properties of the MDFs were not significantly changed, but formaldehyde and TVOC emissions were decreased. Because pozzolan has a rough and irregular surface with porous form, it can be used as a scavenger for MDFs without any detrimental effect on the physical and mechanical properties.
Article
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.
Article
Five low molar ratio urea-formaldehyde (LUF) resins were synthesized in this study. The effects of molar ratio, free formaldehyde content, and catalysts on the curing characteristics of LUF resins were studied by measuring its free formaldehyde content, pH value change after catalysts added, curing rate, and pot life, observing its cured appearance, and analyzing its thermal behavior. The results indicate that: 1) The LUF resin with lower molar ratio than 1.0 can still cure; 2) Free formaldehyde content is not the main factor in affecting curing rate of LUF resin; 3) Compared with ammonium chloride as a traditional catalyst, persulfate salts markedly accelerate the curing rate of LUF resin, and result in the different appearance; 4) the addition of sodium chloride to catalysts can accelerate the curing rate of LUF resin, but the effect is moderate.