This document describes the synthesis and properties of a new urea-formaldehyde (UF) resin modified with hydroxymethyl furfural (HMF). HMF was introduced to replace some of the formaldehyde in order to improve thermal stability, reduce formaldehyde emissions, and utilize a bio-based monomer. UF and urea-HMF-formaldehyde (UHF) resins were synthesized using an alkaline-acid method and characterized. Particleboards made with UHF resin showed improved mechanical properties, lower water absorption and thickness swelling, and reduced formaldehyde emissions compared to UF resin particleboards. The UHF resin demonstrated potential as an alternative adhesive for wood panels with benefits over traditional UF resin.

1. ORIGINAL
Hydroxymethyl furfural-modified urea–formaldehyde resin:
synthesis and properties
N. Esmaeili1 • M. J. Zohuriaan-Mehr1,2 • S. Mohajeri1 • K. Kabiri1,2 • H. Bouhendi1
Received: 30 June 2015 / Published online: 24 June 2016
Ó Springer-Verlag Berlin Heidelberg 2016
Abstract Considering the importance of urea–formalde-
hyde (UF) resins in the wood industry, this work reports on
a new bio-based modification of UF resins. The use of
5-hydroxymethyl furfural (HMF) is motivated by the cur-
rent concerns about the effects of formaldehyde on human
health. UF and urea–HMF–formaldehyde (UHF) resins
were synthesized by an alkaline-acid method and charac-
terized by FTIR, thermogravimetric analysis, and differ-
ential scanning calorimetry. The UHF, as a newly modified
polymeric resin, was thermally characterized, and it was
found that its thermo-stability and char yield was
improved. In order to investigate the performance of the
UHF, the preparation of particleboards with the UHF as
adhesive, as well as its film formation ability have been
studied. The UHF films formed on wood panels were
uniform without any crack. Film formation ability of the
UHF resin was improved due to the presence of more
hydroxyl groups as well as furan rings of the HMF moieties
resulting in more activated groups to be bonded by wood.
Furthermore, formaldehyde release of the particleboards
bonded by UHF was significantly lower than that of which
bonded by the UF resin. Lab particleboards using the UHF
resins showed higher modulus of rupture, modulus of
elasticity, and internal bond compared to boards with UF
resins, as well as lower water absorption and thickness
swelling. Based on these results UHF resin can be con-
sidered as a possible candidate as adhesive for wood and
wood based panels.
1 Introduction
In the past decades, urea–formaldehyde (UF) resins have
been used in wood industry for production of wood panels,
such as medium density fiberboard (MDF) or others. Other
adhesives such as phenol–formaldehyde resins (Fink 2005)
have also been used in wood industry. However, because of
their very low costs, non-flammability, high curing rate,
and the light color, UF resins are still used in vast amounts
and as the most important adhesive by far in the production
of wood based panels (Zorba et al. 2008; Abdullah and
Park 2010; Basta et al. 2011; Patel et al. 2013). UF resins
show some disadvantages, for example, low water-sus-
ceptibility (Christjanson et al. 2002, 2006), low heat-sta-
bility (Dim 2011), non-uniform film formation (Singh et al.
2013) and, most importantly, emission of carcinogenic free
formaldehyde (Birkeland et al. 2010). For solving the latter
problem, reducing the formaldehyde (F)/urea (U) molar
ratio in the synthesis is one the most favorable approaches
(Tohmura et al. 2000; Ferra 2010). Some drawbacks in the
manufacturing of the UF resin with lower F/U molar ratio
occur (Han et al. 2008); for example, the mechanical
strength of the resins was deteriorated, due to lack of suf-
ficient cross-linkages formed by formaldehyde. However,
some approaches for improvement have been reported,
such as increasing the polycondensation time in order to
create more crosslinks (Park and Kim 2008; Park et al.
2006). A usual approach for reducing free formaldehyde
emission is addition of some formaldehyde scavengers to
the particleboards (Park et al. 2008).
& M. J. Zohuriaan-Mehr
mjzohuriaan@yahoo.com;
bcst.ippi@gmail.com
1
Adhesive and Resin Department, Polymer Processing
Faculty, Iran Polymer and Petrochemical Institute,
PO Box 14965-115, Tehran, Iran
2
Biomass Conversion Science and Technology (BCST)
Division, Iran Polymer and Petrochemical Institute,
PO Box 14965-115, Tehran, Iran
123
Eur. J. Wood Prod. (2017) 75:71–80
DOI 10.1007/s00107-016-1072-8

2. Another way for minimizing the content of free
formaldehyde is its replacement by other aldehydes par-
tially or completely (Zhang et al. 2014, Li et al. 2009).
Though abundance of lab results is available in technical
and patent literature, the breakthrough in industrial appli-
cation is still missing.
For many applications, the weakness of aminomethylene
linkages against water and hydrolytic degradation of resin
structure limits the use of UF resins (Paiva et al. 2012). The
main reason for the low water resistance of UF resins is the
reversibility of the linkage in aqueous media. By introducing
some relatively hydrophobic monomers (e.g., melamine) in
the UF resin structure for decreasing the possibility of water
attack to the resin chains, the stability of the resin against
water is improved (No and Kim 2007). The better chemical
resistance of the C–N linkages in MUF resins is due to the
quasi-aromatic behavior of the p electrons as well as the pH
value in the hardened resin, which is still in the acidic range
but higher compared to hardened UF resins.
In this work, hydroxymethyl furfural (HMF) was used as
co-monomer for synthesizing urea–HMF–formaldehyde
(UHF) resins by alkaline-acid method. According to Gan-
dini (2010) furan polycondensates are now of great interest
of both, fundamental and industrial research. To the best of
the authors‘ knowledge, HMF-based modification of UF
resin has not been reported so far. HMF was introduced to
the structure for its good thermo- and hydrolytic stability.
Furthermore, replacing the carcinogenic formaldehyde by
HMF with higher safety and less volatility was considered.
Another important issue is that formaldehyde is originated
petro-chemically, but HMF is a bio-based monomer pre-
pared from biomass resources such as fructose (Van Putten
et al. 2013). The structure of the UHF resin and curing
process were studied by FTIR spectroscopy. The thermal
behavior of the resins was analyzed by thermogravimetric
analysis (TGA) and differential scanning calorimetry
(DSC). Other experiments were performed for further
investigations, such as gelation time and water durability.
2 Experimental
2.1 Materials
Fructose, urea, paraformaldehyde, glacial acetic acid,
sodium hydroxide, hydrochloric acid, ammonium chloride,
and methyl-isobutyl ketone (MIBK), all of analytical
grades, were purchased from Merck and used as received.
2.2 HMF synthesis from fructose
The monomer 5-HMF was synthesized by conventional
heating method (Van Putten et al. 2013; Esmaeili et al.
2016). Briefly, fructose, distilled water, HCl and MIBK
were added to a flask and magnetically stirred for 2.5 h at
80 °C. After all, HMF was extracted by MIBK and then
recrystallized in MIBK to yield purified product.
2.3 Synthesis of UF resin
The UF resin was synthesized by alkaline-acid method
(Table 1) (Ferra et al. 2012). Paraformaldehyde and dis-
tilled water were added into a 250 mL two-necked glass
flask equipped with a magnetic stirrer, thermometer and
reflux condenser. In the first step, NaOH solution (0.1 M)
was added dropwise into the flask to adjust pH *10,
heated to 80 °C and kept for 0.5 h to dissolve
paraformaldehyde completely. Then, 7.7 g urea was
added to the flask and kept for 1 h. As the second step,
glacial acetic acid solution (25 %) was added for adjust-
ing pH *4, second urea (5.5 g) was then added and
polymerization was carried out during 0.5–4 h. In the
third step, the mixture was neutralized with NaOH solu-
tion (0.1 M) and final urea (4.8 g) was added and mixed
until dissolving the total urea, then cooled to ambient
temperature. UF1–UF5 distinguish by the duration of the
acidic condensation step, hence in the degree of conden-
sation. For further analysis and characterization sample
UF2 (Table 1) was chosen.
2.4 Synthesis of UHF resin
The UHF resin synthesis procedure was the same method
as for the UF resin, but in the first step, HMF and
paraformaldehyde were added to the flask simultaneously.
In the second step, the polymerization time was 2 and 4 h,
respectively (Table 1). Sample UHF3 with the highest
replacement rate of paraformaldehyde by HMF was chosen
for further analysis and characterization. In this resin,
approx. 29 % of the paraformaldehyde was replaced by
HMF based on the amount of moles.
2.5 Curing of resins
An aqueous ammonium chloride solution (15 %) was used
as curing agent. Thus, 0.50 g of the solution was added to
10.0 g resin, gently mixed and put in an oven at 105 °C for
3 h (Bono et al. 2003).
2.6 Characterization
The viscosity of the resins was analyzed by a Brookfield
viscometer (Visco Star R, Selecta Co., Spain) by LCP
spindle at 23 °C, in the range of 1–50 rpm. The spectral
and thermal characterization of the resins was performed
by FTIR spectroscopy, TGA, and DSC, respectively.
72 Eur. J. Wood Prod. (2017) 75:71–80
123

3. FTIR spectroscopy (Bruker, IFS48, Germany) was used
for characterization of functional groups of the uncured and
cured resins. The samples were dried (1.0 g resin, 105 °C,
3 h, without hardener) and prepared as KBr pellets with
1 wt% of the dried material.
Thermal analysis was carried out with TGA (Mettler
Toledo, TGA 1500, England) and DSC (Netzsch, 200-F3
Maia, Germany). For the TGA, cured samples were placed
in alumina crucibles. Samples were heated from ambient
temperature to 600 °C under N2 flow rate of 50 mL min-1
and heating rate of 10 °C min-1
. For the DSC, powdered
but still uncured samples were prepared by drying at
105 °C for 3 h, and then mixed with 0.01 g ammonium
chloride and put into the high pressure steel crucibles for
analysis. After all, samples were heated from -20 to
220 °C, with a heating rate of 10 °C min-1
.
2.7 Solid content
Solid content of the resin was determined by heating at
105 °C for 3 h according to the procedure described by
Bono et al. (2003).
2.8 Film formation properties
For the investigation of the film formation ability, the
resins were mixed with the hardener solution and then
applied onto the surface of particleboards
(10.0 9 10.0 9 0.5 cm3
); the amount of resin mix was
selected in order to form a film with 500 lm thickness; the
coated particleboard with the film was then heated at
105 °C for 3 h. The structure of the final film on the sub-
strate was visualized for probable cracks on the surface.
2.9 Determination of formaldehyde release
from particleboards
The content of formaldehyde release from the lab parti-
cleboards was determined by the flask method (based on
EN 717-3:1996).
2.10 Particle board preparation and testing
One-layer laboratory particleboards having 13 mm thick-
ness were prepared by adding 10 % total resin solids on dry
wood particles. The boards were pressed at a maximum
pressure of 1.5 MPa at a press temperature of 180 °C for
12 min. This remarkably long press time was selected in
order to guarantee full curing of the experimental resin
UHF3. All tests were carried out in accordance with the
relevant test methods as described in the various EN
standards on a universal testing machine (STM-20, Santam,
Iran). The tests performed on the specimens were: internal
bond strength perpendicular to the plane of the board (IB;
EN 319), modulus of rupture (MOR, EN 310), modulus of
elasticity (MOE, EN 310), water absorption (WA, EN 317),
and thickness swelling (TS, EN 317). The WA and TS
samples were fully immersed in distilled water at 25 °C for
Table 1 Reaction conditions for preparation of the UF and UHF resins
Resin
code
1st stepa
(F ? HMF)/U molar
ratio
2nd stepb
, time
(h)
(F ? HMF)/U molar
ratio
3rd stepc
, viscosity
(cP)
(F ? HMF)/U molar
ratio
UF1 10 g para-F 2.59 0.5 1.51 325 1.11
UF2 10 g para-F 2.59 1 1.51 449 1.11
UF3 10 g para-F 2.59 2 1.51 478 1.11
UF4 10 g para-F 2.59 3 1.51 523 1.11
UF5 10 g para-F 2.59 4 1.51 826 1.11
UHF1
d
9 g para-F, 3.8 g
HMF
2.57 4 1.50 456 1.10
UHF2
e
8 g para-F, 8 g
HMF
2.57 4 1.50 457 1.10
UHF3
f
7 g para-F, 12.2 g
HMF
2.57 2 1.50 459 1.10
a
Para-F indicates paraformaldehyde; 20 g water, 7.7 g urea, 1 h, 80 °C, pH *10
b
5.5 g urea, 80 °C, pH *4
c
4.8 g urea, 0.5 h, 25 °C, F/Utotal = 1.11, pH *10, viscosity measured by a Brookfield viscometer at 1 rpm, 23 °C, LCP spindle
d
9 % replacement of formaldehyde by HMF (mole by mole)
e
19 % replacement of formaldehyde by HMF (mole by mole)
f
29 % replacement of formaldehyde by HMF (mole by mole)
Eur. J. Wood Prod. (2017) 75:71–80 73
123

4. 2 and 24 h, respectively. For accelerated aging tests (water
resistance test), specimens were boiled in water for 2 h,
dried at 105 °C for 16 h, and then tested for IB.
3 Results and discussion
3.1 Resin synthesis
Urea–formaldehyde and HMF-modified urea–formalde-
hyde (UHF) resins were synthesized through known alka-
line-acid procedure (Ferra et al. 2012) (Fig. 1).
In order to achieve a viscosity range of 400–460 mPa s,
the duration of the acidic polycondensation was varied
(Table 1). For the UF resin, necessary duration of the
condensation was 1 h, but it was 2 h for the UHF resin in
order to produce a resin with similar viscosity. Therefore,
as the intended viscosity range was 400–460 mPa s, the
UF2 sample with viscosity of 449 mPa s and UHF3 sample
with viscosity of 459 mPa s were chosen for further
analyses.
The difference of the polycondensation time implies that
the reactivity of the HMF is much lower than that of
formaldehyde due to (1) higher steric hindrance near the
carbonyl group in HMF compared to the formaldehyde,
and (2) decreased electrophilicity of the carbonyl group
due to its resonance with the aromatic furan ring (Gancarz
1995). Regarding the steric hindrance, nucleophilic attack
of the nitrogen atom of urea on formaldehyde is easy, but
such attack on the carbonyl group of HMF is not favored
by the neighboring bulky furan ring. The carbonyl group in
the HMF is also involved in the resonance by the furan ring
with the result, that the nucleophilic addition to it will be
disfavored. Further, the furan group of HMF has an
inductive electron donor property, so that the elec-
trophilicity of its carbonyl group is decreased; this will
impede the nucleophilic attack of the NH2 group of urea.
Therefore, longer condensation time was needed to achieve
the UHF resin with the targeted viscosity.
The main properties of four selected resins (UF2 sample
as well as three UHF samples) are summarized in Table 2.
As the reactivity of HMF aldehyde group is lower than
that of the formaldehyde (Gancarz 1995), the majority of
formaldehyde is consumed in the condensation reactions
before HMF will contribute to the reaction. The existence
of a furan ring near the aldehyde functional group in HMF
makes the nucleophilic attack to be difficult. So, a con-
siderable proportion of the total nucleophilic addition had
taken place with formaldehyde instead of HMF during the
first period of reaction. Furthermore, it should be consid-
ered that the methylol group of HMF could contribute to
the condensation reactions during resin synthesis. In the
first stage at basic pH (Fig. 1c), urea reacts with aldehyde
group but not with methylol group of HMF; so there is no
competition between aldehyde and methylol group of HMF
for reacting with urea. In the second stage of the reaction at
acidic pH, the methylol group can condense with urea or
other methylol groups (see Fig. 1d). Though the HMF
molecule owns two active functional groups, the aldehyde
group is considered as the reactive group during
formaldehyde replacement calculations.
3.2 Resin properties
The various solid contents of the resins according to their
recipe (64 % for UF2 vs. 60–61 % for the UHF resins)
were not corrected prior to further analysis.
The gelation time was 105 s for the UF resin and
240–270 s for the UHF resins (Table 2). The gelation time
increased significantly by replacing even only a smaller
part (9 %) of the formaldehyde by HMF in the structure of
the resin. The curing process is, as it is the continuation of
the condensation process in the reactor, based on the
reaction of the aldehyde groups with the amino groups. As
already noticed during the acidic condensation step, the
reactivity of HMF is lower than that of formaldehyde due
to the lower reactivity of its aldehyde group in comparison
with that of formaldehyde (Gancarz 1995); hence, the
curing reaction of the UHF resin (as the continuation of the
acidic condensation step in the resin preparation) also was
slower than UF resin. Surprisingly already a small
replacement rate in UHF1 by 9 % HMF (means replace-
ment of 9 % of the moles of formaldehyde by HMF)
increased the time of curing already to more than the
double value. Higher HMF contents, however, did not
show further significant increase in the gelation time
(samples UHF2 and UHF3).
Figure 1e proposes a structure of the cured UHF resin. A
block-like copolymeric structure is more probable rather
than a random-like one. It might be similar to the structure
reported for a urea–CH2O–furfural resin system (Zhang
et al. 2014).
3.3 FTIR spectroscopy
FTIR spectroscopy showed characteristic bands (Kandel-
bauer et al. 2007) proving the synthesis of resins (Fig. 2).
During the first step of the addition reaction occurring
between formaldehyde and urea, dimethylolurea (1,3-
bishydroxymethyl urea) is produced. The specific bands
proving the structure of both UF and UHF resins are as
follows: 3,370 cm-1
(alcohol O–H stretching, broad;
hydrogen bonded with water and methylol groups),
3,030 cm-1
(C–H aromatic stretching), 1,655 cm-1
(C=O
stretching of primary amide), 1,644 cm-1
(secondary
amide, appearance as a shoulder of the C=O primary amide
74 Eur. J. Wood Prod. (2017) 75:71–80
123

5. (a) Methylolation step in the synthesis of UF resin
H2N NH2
O
+
H H
O
H2N N
H
O
OH
pH 10
(b) Condensation step in the synthesis of UF resin.
H2N N
H
O
OH NH2
N
H
O
HO
+
H2N N
H
O
O N
H
NH2
O
pH 4
H2N N
H
OH
O
+
H2N NH2
O
pH 4
H2N N
H
N
H
O
NH2
O
(c) Methylolation step between urea and HMF in the synthesis of UHF resin.
H2N NH2
O
+
O
OHC CH2OH
O
CH2OH
N
H
O
H2N
OH
pH 10
(d) Condensation step of HMF methylols in the synthesis of UHF resin
O
CH2OH
N
H
O
H2N
OH
2
O
N
H
O
H2N
OH
O
N
H
O
NH2
OH
O
pH 4
(e) Proposed structure of the UHF resin after the curing step
Fig. 1 Main reactions in the
synthesis of UF and UHF resins
(a–d) and structure of the cured
UHF resin (e)
Eur. J. Wood Prod. (2017) 75:71–80 75
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6. band), 1,547 cm-1
(N–H bending amide), 1,513 cm-1
(N–
C–N of a methylene bridge), 1,509 cm-1
(C=C of furan
ring), 1,380 cm-1
(C–N bending vibration of amide),
1,292 cm-1
(CH2 methylol groups of urea moieties), and
1,251 cm-1
(C–N bending of amide).
In the UHF samples, three bands appeared at 1,010,
1,505, and 3,020 cm-1
and can be attributed to C–O–C
ether, aromatic C=C bond, and C=C–H of furan ring,
respectively. The absence of C–H stretching vibration of
the aldehyde group (at 2,830 cm-1
) proved successful
incorporation of HMF into the UHF resin structure (Taylor
et al. 2013). C–O–C ether linkages were observed in the
structure UF resins as well (ether bridges from the reaction
of 2 methylol groups), but these linkages appear at
1,100–1,104 cm-1
(Ahamad and Alshehri 2014).
FTIR spectroscopy was also used for the investigation of
the cured resins. As shown in Fig. 2, the curing results in
decreased intensity of the O–H alcohol stretching at
3,370 cm-1
. This spectral observation verifies the reduc-
tion of the O–H groups. During the curing process, the –
CH2–OH groups react further to form either –CH2–O–
CH2– ether bridges or –CH2– methylene bridges (Kan-
delbauer et al. 2007).
In the curing process of the UHF resin, intensity of the
C=C aromatic furan ring in the 1,505 cm-1
was decreased.
This fact can be attributed to possible Diels–Alder reac-
tions of the furan rings during the curing (Bobrowski and
Grabowska 2012).
3.4 Thermal characterization
DSC was used for monitoring the curing behavior of the
resins. Curing of the UF resin started at lower temperature
in comparison with the UHF resin, due to the higher
reactivity of formaldehyde rather than that of HMF and
therefore the higher reactivity of the resin as such. Steric
hindrance of the aldehyde group by the furan ring in the
HMF molecule causes delayed curing of the UHF resin
compared to the UF resin (Fig. 3). In addition, no glass
transition temperature (Tg) is observed, probably due to
Table 2 Characteristics of the synthesized UF and UHF resins
Resin codea
Initial HMF (g) Gelation time (s) Solid content (%) pH WSb
Film formation Fc
(mg kg-1
)
UF2 (0 %) 0 105 64 10.1 0.559 Weak 14
UHF1 (9 %) 3.8 240 61 10.0 0.315 – –
UHF2 (19 %) 8.0 260 61 10.2 0.276 – –
UHF3 (29 %) 12.2 270 60 10.0 0.245 Good 8
a
HMF: mol % based on original number of moles of formaldehyde in the UF recipe; numbers are given in parenthesis
b
Water solubility (resin:water, g g-1
)
c
Formaldehyde emission from lab particleboard
Fig. 2 FTIR spectra of typical UHF (upper curves) and UF resins
(lower curves) before and after curing
Fig. 3 DSC thermograms of resins UF2 and UHF3
76 Eur. J. Wood Prod. (2017) 75:71–80
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7. hiding by the broad curing peak. It can be confirmed by the
endothermic shift of the base line of both resins (Ebewele
1995). As the curing peak of the UHF sample appears at
much higher temperature (130 °C for UHF vs. 80 °C for
UF), the possible Tg of the UHF sample seems to be higher
than that of the UF sample. It can be attributed to the
existence of the aromatic groups in the UHF resin structure
(Gao et al. 2008; Sperling 2005).
TGA and differential TGA (DTG) diagrams for the
cured resins are shown in Fig. 4. The UF resin thermogram
showed a three-step weight-loss but a four-step pattern for
the UHF resin. For both resins, the weight-loss observed in
the range of 50–110 °C is attributed to adsorbed moisture.
In the temperature range of 100–180 °C formaldehyde is
released from dimethylene ether bridges (–CH2–O–CH2–)
to form more stable methylene (–CH2–) bridges (Liu et al.
2008).
For the UF resin, the major thermo-degradation steps
occurred above 180 °C. In this stage, chain splitting hap-
pened and the C–C bonds were homolitically disconnected
to form free radicals causing additional thermo-degradation
(Ahamad and Alshehri 2014).
At even higher temperatures (400 °C), the methylene
bridges started to be quickly decomposed, causing the
major weight loss of the UF resin. The char yield of the UF
resin was only 11 % (Roumeli et al. 2012; Taylor et al.
2013).
The UHF resin was decomposed in four stages. After
releasing the absorbed moisture at around 70–110 °C, the
second step, attributed to the formaldehyde emission, also
happened at 100–180 °C. These two steps were similar to
those mentioned for the UF resin.
The UHF resin structure is more complicated due to the
presence of the furan rings; one part of the structure is
formed by the reaction of urea with formaldehyde and
another part from the reaction of urea with HMF. Also, as it
has been mentioned before, due to the lower reactivity of
the carbonyl group in HMF in comparison to formalde-
hyde, the UHF resin is assumed to bear a block-like
copolymeric structure (Fig. 1e). There are some chains
formed only by urea and formaldehyde; after all
formaldehyde has reacted, HMF molecules will be attacked
by urea to form the HMF-urea structure. The latter part
induces heat stability to the UHF resin. Whereas the
decomposition of UF moieties takes place at 180–320 °C
(the 3rd decomposition stage) (Jiang et al. 2010), there is
another step of the UHF resin decomposition, appearing at
380 °C; this temperature stage is not observed for the UF
resin and leads to a high char yield (34 %). This is attrib-
uted to formation of some stable carbonic cyclic species
originated from furan rings which can tolerate more heat
than linear structures do. These cyclic structures increase
the char yield of the UHF resin during combustion (Tu-
molva et al. 2009).
3.5 Film formation
In order to study the film formation property, the ability of
the resins to generate uniform coatings on wood surfaces
was investigated. UF resins cannot form a non-cracked and
uniform coating due to high brittleness. Singh et al. (2013)
have also mentioned the brittleness of UF resin and their
problems for making a smooth and uniform surface to
investigate the morphology of UF resin by SEM and TEM.
In the current work, the UF resin did not generate such a
film on a wood plate too. On the contrary, the UHF resin
could obtain a uniformed film without cracks (Fig. 5). The
capability of the UHF resins in the forming of films on
wood panel may be partially attributed to the existence of
more hydroxyl groups (as methylols) in its structure, so that
hydroxyl groups can intra-molecularly interact (e.g., via
hydrogen bonding) with lignocellulose, i.e., wood surface.
Fig. 4 TG/DTG thermograms of resins UF2 and UHF3
Fig. 5 Appearance of the UF2 and UHF3 resins applied onto wood
panels
Eur. J. Wood Prod. (2017) 75:71–80 77
123

8. 3.6 Formaldehyde emission
The formaldehyde release content of particleboards is usu-
ally determined by the chamber test method (EN 717-1), gas
analysis (EN 717-2), or flask method (EN 717-3) (Ferra et al.
2012; Sperling2005). In this work, the flask method was used
to measure the formaldehyde release from the lab particle-
boards; the results were 14 and 8 mg kg-1
for UF and UHF
bonded particleboards, respectively. This means that the
incorporation of HMF reduced formaldehyde emission. It
can be attributed to replacing remarkable parts of
formaldehyde by HMF, which simply reduces the content of
formaldehyde in the system.
The usual method for decreasing the content of free
formaldehyde in UF resin is the reduction of the formalde-
hyde/urea molar ratio. Beside the direct reduction of F/U this
aim can be achieved by replacing the formaldehyde by HMF.
So it could be expected that UHF resin should have a lower
amount of extractable formaldehyde than that of the UF
resin, though more parameters than only the content of free
formaldehyde in the liquid resin determine the
extractable formaldehyde from cured resins, like curing
conditions and achieved degree of curing.
3.7 Mechanical properties of particleboards
The results are shown in Fig. 6 and demonstrate the
influences of the resin type on the properties of experi-
mental particleboards. As can be seen, particleboards
bonded with UHF resin exhibit higher mechanical prop-
erties than those made with UF resins. The IB for parti-
cleboards bonded by UHF3 was 0.55 MPa and higher than
that of UF2 with 0.39 MPa. Water resistance analysis was
performed and the IB strength decreased to 0.38 and
0.1 MPa for particleboards bonded by UHF3 and UF2,
respectively. After the same test, the average MOR and
MOE for UHF3 resin decreased from 18.7 to 18.0 MPa and
from 3,120 to 2,980 MPa, respectively. Moreover, by
performing this resistance test on particleboards bonded
with UF2, the average MOR and MOE values decreased
from 14.2 to 11.3 MPa and from 2,650 to 2,010 MPa,
respectively. Due to more hydrophobicity of the UHF resin
in comparison with the UF resin, the aging process was
retarded for the UHF and a minor decrease was observed
during the test.
Based on European Standards (EN), MOR, MOE, and
IB values of 16, 2,300, and 0.40 MPa, respectively, are the
minimum requirements of structural particleboards for use
under dry conditions (P4). According to the results, parti-
cleboards in this work produced by UHF3 showed prop-
erties that are higher than the EN requirements. However,
the long press time used for the production of the lab
particleboards has to be considered for such comparison.
3.8 Physical properties
Incorporating a hydrophobic monomer like HMF to the
resin structure decreased hydrophilicity of the system
resulting in higher insolubility and resistance to water.
Fig. 6 Average values of IB, MOR, and MOE of particleboards
bonded with UF and UHF resins, tested dry and after boiling and re-
drying
78 Eur. J. Wood Prod. (2017) 75:71–80
123

9. WA and TS for the lab particleboards are presented in
Fig. 7. WA for UHF3 and UF2 were 19 and 56 % for 2 h
immersion, and 26 and 75 % for 24 h immersion, respec-
tively. TS for UHF3 and UF2 were 2 and 8 % for 2 h
immersion, and 3 and 12 % for 24 h immersion, respec-
tively. In addition, according to EN 312 (board type P4,
[10–13 mm), the maximum TS for 24 h requirement is
16 %. An important problem of UF resins is their insta-
bility against water. It is mainly originated from hydrolytic
cleavage of methylene and especially ether bridges.
4 Conclusion
An alkaline-acid method was used for the synthesis of UF
and UHF resins. For the UHF resins, HMF was partly used
as alternative aldehyde replacing up to 29 % of the
formaldehyde (calculated as moles) in the recipe. Some
important targets for improving the features such as water
durability, film formation capability, and subsequent
formaldehyde emission from lab particleboards have been
investigated.
Improved hydrolytic stability of the HMF containing
resin (UHF) was achieved, as well as superior heat stability
of the resin with char yield of 34 % compared to the value
of 11 % for the UF resin. Film formation of the UHF resin
on wood panels was uniform without any crack, which
could not be achieved with the UF resins. The gelation time
of the UHF resin was significantly longer due to incorpo-
ration of HMF into the resin structure. Improved mechan-
ical strength was observed in particleboards bonded with
UHF resin instead of UF resin. The formaldehyde release
from the particleboards bonded with UHF was lower than
that bonded with UF. MOR, MOE, IB, WA, and TS of the
lab particleboards were also improved with UHF resin
compared to UF resins based on the adjusted sufficiently
long press times in order to achieve full curing of the UHF
resin.
Despite its lower reactivity, but owing to the above
mentioned improvements, the HMF-modified UF resin can
be seen as a potential alternative to be used as adhesive in
the wood based panels industry.
Acknowledgments The authors are very much obliged to one of the
reviewers due to his/her highly informative and deep comments
causing evolutionary improvement of this article.
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