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Comparison of the properties of urea-formaldehyde resins by the use of
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Article in Journal of Adhesion Science and Technology · October 2018
DOI: 10.1080/01694243.2018.1492780
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ISSN: 0169-4243 (Print) 1568-5616 (Online) Journal homepage: http://www.tandfonline.com/loi/tast20
Comparison of the properties of ureaformaldehyde resins by the use of formalin or
urea formaldehyde condensates
Ali Dorieh, Nosratollah Mahmoodi, Manouchehr Mamaghani, Antonio Pizzi
& Massoud Mohammadi Zeydi
To cite this article: Ali Dorieh, Nosratollah Mahmoodi, Manouchehr Mamaghani, Antonio Pizzi &
Massoud Mohammadi Zeydi (2018): Comparison of the properties of urea-formaldehyde resins
by the use of formalin or urea formaldehyde condensates, Journal of Adhesion Science and
Technology
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JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
https://doi.org/10.1080/01694243.2018.1492780
Comparison of the properties of urea-formaldehyde
resins by the use of formalin or urea formaldehyde
condensates
Ali Dorieha, Nosratollah Mahmoodia, Manouchehr Mamaghania,
Antonio Pizzib and Massoud Mohammadi Zeydia
a
Department of Organic Chemistry, University Campus 2, University of Guilan, Rasht, Iran;
Lermab-Enstib, University of Lorraine, Epinal, France
b
ABSTRACT
ARTICLE HISTORY
In industry, formalin and urea formaldehyde condensates (UFC)
are used for the preparation of urea-formaldehyde adhesives. The
aim of this study was to evaluate the characteristics of a UF resin
prepared with formalin (42% concentration) and two kinds of UFC
with various urea content (F/U 5 and 4.3 malar ratio). The resins were prepared with final molar ratio F/U ¼ 1.1. The UF resin
with formalin showed better mechanical properties than those
with UFC. The 13C NMR and FT-IR results indicated that methylene, methylene ether and urons groups in a UF resin prepared
with formalin were present in higher proportion than observed in
industrial UF resins. The DSC (differential scanning calorimetry)
results showed that the curing temperature of a UF resin prepared with formalin is lower than that of a UF resin prepared
with UFC. The mechanical properties of MDFs prepared with the
three types of adhesives showed that MDFs bonded with a UF
prepared with formalin yields a better mechanical properties than
MDFs prepared with a UF resin based on UFC. Moreover, resin
synthesized with UFC (F/U ¼ 5) result in higher IBS (internal bond
strength) MOR (modulus of rupture), MOE (modulus of elasticity),
TS (thickness swelling) and formaldehyde emission (FE) than resin
synthesized with UFC (F/U ¼ 4.3). The MDF bonded with the UF2
resin presented poor results, with the highest TS and the lowest
IBS, MOE, MOR and FE.
Received 6 January 2018
Accepted 21 June 2018
KEYWORDS
Formalin; UFC; ureaformaldehyde resin; mole
ratio; 13C NMR; IR; DSC
1. Introduction
Formaldehyde is a key material to prepare various kinds of resins such as phenol formaldehyde (PF), urea formaldehyde (UF), polymeric methylene diphenyldiisocyanate
(pMDI), and melamine–urea–formaldehyde (MUF) resins [1,2]. UF is one of the resins that have multifold application in the fiberboard and particleboard industry [3].
UF resins, beside their low prices, have many advantages such as good solubility in
water, ease of usage under different curing conditions, rapid curing reaction in the
CONTACT Nosratollah Mahmoodi
mahmoodi@guilan.ac.ir
Campus 2, University of Guilan, Rasht, Iran
ß 2018 Informa UK Limited, trading as Taylor & Francis Group
Department of Organic Chemistry, University
2
A. DORIEH ET AL.
hot press, excellent thermal properties, and resistance to abrasion [4]. The UF resins
have, however, also several disadvantages such as low resistance to water and cause
formaldehyde emission from the boards. These disadvantages limit somewhat their
use as wood panel adhesives for interior applications [5,6]. Formaldehyde emission
(FE) must be controlled in boards and UF adhesives by altering the synthesis condition of the resin and by adding various modifiers. One of the optimum procedures to
control and reduce formaldehyde release is to change the molar ratio of formaldehyde
to urea (F/U) during resin synthesis [7,8]. Several approaches for the production of
UF adhesives can be found in the literature. The most common method of UF resin
synthesis is the alkaline-acidic-alkaline process, which includes three steps. First the
hydroxymethylation reaction is conducted under mild alkaline conditions, followed
by the polycondensation reaction under mild acidic conditions and finally the neutralization and addition of the last urea [9,10]. However, under alkaline conditions
monomethylol, dimethylol, and tri-methylol ureas are formed. During the condensation under acidic conditions methylene bridges instead of methylene ether bridges are
preferentially formed. Different types of UF structural groups are present in the resin.
Thus, methylene linkages, ether linkages, hydroxymethylol, amide groups, uron and
their derivatives formed during the reaction of urea and formaldehyde and a number
of possible side reactions can have a bearing on the resin rate of curing, its structure,
the panel strength properties and consequently also on formaldehyde emission
[11–14]. 13C NMR analysis is a good technique to investigate the changes of the components that are prepared at various temperatures and F/U molar ratios. For example,
at alkaline pH, with F/U ¼ 2/1 at 80 C and 90 C more than 30% of the formaldehyde used produces methylene ether bridges (–NR–CH2–O–CH2–NR–). While for F/
U ¼ 1/1 at 80 C, the formation of linear methylene bridges (–NR–CH2–NR–) just
begins to compete with that of ether bridges. At the F/U ¼ 1/1 at 90 C methylene
bridges predominated [14]. The F/U molar ratio directly affects the panel properties
and its formaldehyde emission. Therefore, for adhesives of low F/U molar ratio, the
formaldehyde emission rate is lower [15]. Another way to decrease formaldehyde
emission in boards is to use formaldehyde catchers. PVA, lignin, acrylic emulsion,
phenol and melamine and others which also help to adjust the performance of a UF
resin [16]. Reducing the free formaldehyde content of the UF resin and formaldehyde
emission in bonded products have been studied [17,18]. The resin manufacturing
parameters which need to be manipulated to produce urea-formaldehyde (UF) resins
capable of producing low formaldehyde emission particleboards of class E1 have been
presented [19]. The UF resin prepared by formalin with higher concentration, showed
better mechanical properties than that of a normal resin [20]. It has been reported
that by increasing catalyst and resin solids contents and decreasing the pH, the gel
time of the UF resin was notably reduced [21].
The influence of the reaction pH on the reactivity, molecular structures, and reaction kinetics of the UF resins determined by FT-IR and solid-state 13C NMR spectroscopies has been reported [10,12,22]. UF adhesives having higher F/U molar ratio,
have been shown to present considerably lower values of onset and peak temperatures
compared to adhesives with lower the F/U molar ratio [23]. Medium density fiberboard (MDF) manufacture has been increasing in new factories, mainly cabinetry,
JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
3
flooring, and home furniture, have come into operation, mainly in the developed
regions of the world. Therefore in recent years global MDF production increased to
more than 60 million m3 per year [24]. MDF is extensively used particularly in the
furniture manufacturing industry as a result of some of its advantages, e.g. good
physical and mechanical properties. Nevertheless, MDF also presents some disadvantages, such as higher formaldehyde emission content, biological decomposition, low
dimensional stability, etc. These drawbacks can be reduced by using some chemical
additives [25,26].
The present work studies the effect of three kinds of UF resins which were prepared by either using non-alcohol stabilized formalin or a urea-formaldehyde concentrate (UFC) containing two different urea percentages (19% and 22%). 13C NMR and
DSC analysis were carried out and the final mechanical properties and
formaldehyde emission from medium density fiberboard was measured. The effect of
the F/U ¼ 1.1 molar ratio on resin viscosity, pH, and free formaldehyde content was
also examined.
2. Experimental
2.1. Materials and methods
Two kinds of UFC containing 47% formaldehyde with urea percentage of 19 and
22% and formaldehyde aqueous solution (industrial grade, 42%) were purchased from
Chassbsaz Co., ltd, Iran. The aqueous solutions of formic acid (10 wt%) and NaOH
(25 wt%) were used to adjust the pH level during the UF resin synthesis. The 20 wt%
solution of ammonium sulphate (NH4)2SO4 was used as hardener. The fiber used in
this study was obtained from the Arian Sina Co., ltd, Iran.
2.2. Preparation of UF resin
The UF resins with two mole ratio, specifically F/U ¼ 1.1, 1.3 were prepared as follow:
1500 parts of formaldehyde 42 wt% (UF1) and 1200 parts of UFC with 47% formaldehyde with 19 wt% urea (UF2) and 22 wt% urea (UF3) were charged to a 3-L glass
reactor equipped with a heating jacket, condenser, mechanical stirring and thermometer. The pH of the solution was adjusted to 8–8.5 with 25% NaOH solution, and the
temperature was raised to 40 C. The first amount of urea (U1) was then added to
give an F/U1 molar ratio of 2.1. The mixture was then heated to 88 ± 1 C. The
hydroxymethylation was ended by heating during 40 min at 88 ± 1 C. Then the pH
was adjusted to 4.8–5.2 with formic acid and the acid promoted reaction was
accomplished at 90 ± 1 C to obtain the desired viscosity. The viscosity of resulted
mixture was measured by NO.3 Ford cup (at 65 C). Afterwards, the reaction
mixture was cooled during 20 min, until a temperature of 50 C was attained.
At this point, a second amount of urea is added to provide a final F/U
mole ratio. The reaction is terminated by cooling the mixture to a temperature
of 25 C.
4
A. DORIEH ET AL.
2.3. Resin properties determination
Viscosity, pH, gel time and solid content were determined at the end of each synthesis. The final viscosity of the resins was measured by a NO.4 Ford cup (at 20 C).
The resins pH was measured using a combined glass electrode. The non-volatile
solids contents was determined by evaporation of volatiles in 1.5 g of resin for
3 h at 105 C. The resin reactivity was determined by measuring the gel
time of a resin sample at 100 C after addition of the (NH4)2SO4 (20 wt%) as
cure catalyst.
2.4. Determination of free formaldehyde
Free formaldehyde of synthesized UF resins was determined by modified Na2SO3
method. The 25 mL solution of Na2SO3 (1 mol) was mixed with 10 mL 1N HCl was
added to solution of 2–3 g sample of UF resin in 100 mL distilled H2O were placed in
250 mL Erlenmeyer flask. The mixed solution including about five drops of 0.1% thymolphthalein indicator solution was neutralized with 1N NaOH. The percent of free
formaldehyde was determined by the equivalent of the amount of the consumed
NaOH titration.
3. Characterization
3.1. Infrared (IR) spectroscopy
The liquid UF resin was covered on the surface of KBr pills. Then the dried pills
were sent to IR Shimadzu 470 spectrometer.
3.2.
13
CNMR spectroscopy
The liquid state 13C nuclear magnetic resonance (13C NMR) spectra of selected sample were obtained with a Bruker Avance 400 MHz and 100 MHz spectrometer,
respectively. The 400 ll liquid sample was directly mixed with 50 ll deuterium DMSO
for 13CNMR determination. About 400 scans were accumulated for each spectrum.
13
C NMR chemical shifts were measured with respect to tetramethylsilane (TMS) as
the internal standard.
3.3. DSC measurement
In the current study, all DSC measurements were carried out with a DSC (TA
METTLER TGA/STTA 851, Switzerland) equipped with a thermal analysis (TA) system and software. The sample resins were tested by placing about 3 mg of each sample into a hermetic pan. Heating rates of 10 C/min and a scanned temperature range
of 30 to 200 C were select and a pressure of 4MPa with nitrogen gas was used during the DSC scanning.
JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
5
3.4. Medium density fiber board evaluation
To evaluate the mechanical properties of the panels, the modulus of elasticity (MOE),
and modulus of rupture (MOR) (4 samples from each board) were determined
according to EN 310: 1999. The internal bond strength (IBS) (6 samples from each
board) was measured according to EN 319: 1999. Thickness swelling (TS) after
24 hours soaking in cold water (3 samples from each board) were measured according
to EN 317: 1999. Free formaldehyde was measured in accordance with EN 120: 1999.
Mechanical properties of MDF were measured in Zwickroal Z010 instrument Arian
Sina co, ltd, Iran. All the obtained results were statistically analyzed.
4. Results and discussion
The amount of urea that was added to the different preparation stages of the UF
resin is given in Table 1. The UF resins were prepared either using formalin 42% or
two kinds of UFC containing 47% by weight of formaldehyde (22%wt urea or 19%wt
urea respectively).
4.1. Characteristics of UF resins prepared with formalin and UFC
Table 2 shows the properties of UF1, UF2 and UF3 resins. With higher molar ratio
F/U, the final viscosity was higher. With increasing F/U molar ratio, free formaldehyde content in all the three resins increased, as well as their non-volatile solids content decreased. Table 2 shows that the resins synthesized with UFC at F/U ¼ 1.3 need
a lower amount of water than those with molar ratio F/U¼ 1.1.
Gel times and curing time of the UF resins increased with the F/U molar ratio
decreasing, as shown in Table 2. In particular, the gel time lengthened when the F/U
mole ratio decreased from 1.3 to 1.1. Conversely, these results confirm that the curing
reactivity of a UF resin decreases by decreasing the F/U mole ratio. This is possibly
explained by a decrease in the availability of formaldehyde at lower F/U mole ratio.
The gel time indicates that the curing reaction of the UF1 adhesive is faster than for
the UF3 and UF2 adhesives.
4.2. FT-IR Analysis
The comparison of the FT-IR absorption band assignments of UF1, UF2 and UF3
spectra in the range of 650–4000 cm1 are shown in Figure 1 and Table 3. The spectra for the three resins are nearly similar. The peak at 3300–3400 cm1, is attributed
Table 1. The addition amount of urea at different stages.
Sample
UF1
UF2
UF3
UF1
UF2
UF3
Material
Formalin
UFC (22%
UFC (19%
Formalin
UFC (22%
UFC (19%
Urea)
Urea)
Urea)
Urea)
Final mole ratio F/U
U1 (g)
U2 (g)
Water (g)
1.1
1.1
1.1
1.3
1.3
1.3
600
273
309
600
273
309
545
491
488
369
334
331
–
218
175
–
166
123
6
A. DORIEH ET AL.
Table 2. Properties of UF resins used in this work.
Properties
Viscosity
Sample Mole ratio F/U Solids content (%) 25 C (cps)
1.1
57.4
110
UF1
1.1
62.3
230
UF2
1.1
62.6
210
UF3
1.3
55.9
36
UF1
1.3
62.8
95
UF2
1.3
63
86
UF3
pH Gel time (s) Density (gr/cm3) Free formaldehyde (%)
8.25
53
1.215
0.188
8.32
77
1.255
0.105
8.26
60
1.257
0.163
7.89
44
1.206
0.302
8.15
60
1.258
0.194
8.41
49
1.261
0.276
Figure 1. FT-IR spectra of UF1, UF2 and UF3 resins.
Table 3. Absorption band assignment of FT-IR spectra of UF1, UF2 and UF3.
Observed band(cm1)
Range absorption band (cm1 )
3340–3355
2960–2963
1646–1664
1545–1560
1440–1465
1380–1400
1330–1380
1300–1320
1250–1260
1130–1150
1030–1050
1000–1020
650–900
Chemical structure assignment
UF1
UF2
UF3
NH stretching of primary aliphatic amines
–O–CH3, aliphatic ethers
C ¼ O stretching of primary amide- NH2 (urea)
C–N stretching of secondary amines
C–H bending in NCH2N, CH2O,OCH3
C–H mode in CH2 and CH3
CON stretching of CH2ON
¼C–N or ¼ CH–N of tertiary cyclic amides
C–N and N-H stretching of tertiary amides
C–O stretching of aliphatic ether
C–N or NCN stretching of methylene linkages (NCH2N)
COO stretching of methylol group
N–H bending of primary aliphatic amines
3355
2962
1661
1546
1456
1385
1346
–
1254
1135
–
1015
779
3350
2962
1663
1550
1458
1386
1355
–
1254
1136
–
1013
902, 779
3353
2963
1656
1548
1457
1386
1356
–
1255
1135
–
1014
778
to the ¼ NH groups. The infrared spectra and band assignments to the chemical
structures involved give useful information on the structure of synthetic resins. The
C ¼ O stretching of primary amides was the most dominant absorption band and
occurs at 1650 cm1. The band from 1030 to 1050 cm1 can be assigned to the C–N
stretching from the C–N or C–N2, amide groups in the resins [27,28].
4.3.
13
C NMR analysis
The chemical structures of synthetic resins base on their 13C NMR analysis of UF1
(Figure 2), UF2 (Figure 3) and UF3 (Figure 4) with the equivalent molar ratio F/U ¼ 1.1
were obtained and summarized as listed in Table 4 [29–31]. The relationship between
JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
Figure 2.
13
Figure 3.
13
7
CNMRspectrum of sample UF1.
CNMRspectrum of sample UF2.
different functional groups and the final properties of the resins are shown in Figures 2,
3 and 4. Based on the integration of the peak areas in the 13C NMR spectra the molar
distribution of formaldehyde between different functional groups containing methylene
carbons in the 40–100 ppm range was assigned. The regions of carbonyl carbons
(150–170 ppm) depends on the different substitutions in urea compounds. In conclusion,
8
A. DORIEH ET AL.
Figure 4.
13
CNMRspectrum of sample UF3.
the ratios of all carbons and quantities of different species were calculated. The signals at
39 ppm belong to DMSO-d6 and the small signal at 50 ppm (not list in Table 1) is
assigned to MeOH. The signals at 64–65 and 71–72 ppm are assigned to the hydroxymethylation (–NH–CH2OH) produced under alkaline conditions. These belong to the
MMU and DMU groups, respectively. Furthermore, the peaks of the linear methylene
and methylene ether linkages (–CH2OCH2–) appears at 68, 75 and 78 ppm respectively.
The peak at 78 ppm refers to the carbon of the CH2 groups of uron derivatives. The
peak at 68–70 belongs to the methylene carbons in the triazine ring [32]. The increase
in methylene ether signals at 68–70 ppm more possibly shows the formation of methylene ethers of diurea in the UF2 resin. The various carbons of the methylene linkages
peaks appears at 47, 54, and 60 ppm. The signals at 154–158 ppm belong to uron- type
methylene ether groups [33,34]. The formation of urons, presenting greater stability,
occurs at pH 3-6 and especially at pHs lower than 3. The lower uron stability and lower
uron proportion occur at pH 4.5–6.0, but particularly pH 5.0–5.5 [33]. Hence, the cyclic
intermolecular ether groups (uron) also contributes to the content in ether linkages.
Their chemical shift, other than those at 156–158 ppm, may overlap with the ether bonds
at 75–77 and 78–80 ppm. The total content of methylol groups, methylene and ether
bridges shown in Table 4, confirm that most of the formaldehyde was consumed in UF2
JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
9
Table 4. Structures and chemical shift.
Structures
Methylenes, R
–NH–CH2–NH– (I)
–NH–CH2–N¼ (II)
¼N–CH2–N¼ (III)
Dimethylene ethers, R
–NH–CH2OCH2-NH– (I)
–NH–CH2 O CH2-N¼ (II)
¼N–CH2OCH2N¼ (III)
Hydroxymethyls, R
–NH–CH2OH (I)
–N(–CH2)–CH2OH (II)
N(CH2 OH)2
HO–CH2–OH
HOCH2–O–CH2–OCH2OH
HOCH2–O–CH2–OCH2OH
H(CH2O)nOCH2OCH3
Methoxymethylenes, R
–NH–CH2–O–CH3
Carbonyl region
NH2–CO–NH–
–NH–CO–NH–
–NH–CO–N¼
Uron
Chemical shifts/ppm
UF1
UF2
UF3
46-49
53 55
60 61
Total
5.51
17.75
–
23.30
8.27
8.88
–
17.16
9.17
9.18
–
18.35
68 70
75 77
78 80
Total
12.01
0.62
0.38
13.01
23.10
7.64
5.69
36.43
17.54
3.80
2.54
23.88
64 66
71 72
58.23
–
44.63
0.52
54.15
–
Total
82 84
86 88
90 91
94 95
Total
72 74
58.23
0.55
0.48
0.67
0.31
2.01
3.45
45.15
0.86
–
–
–
0.86
0.40
54.15
0.92
–
–
–
0.92
2.70
161 162
159 161
21.09
77.26
32.11
65.02
28.54
70.26
154–158
1.65
2.87
1.2
and UF3, which explains the lower content of free formaldehyde of UF2 and UF3 with
molar ratio F/U ¼ 1.1 than in UF1 [35]. Table 4 indicates that the content of methylene
bridges on the amount of formaldehyde of UF2, 17.16%, was lower than that of UF3,
18.35% and UF1, 23.30%, respectively. However, there were more ether bridges in UF2
than in UF3 and UF1, specifically, 36.43 vs 23.88 and 13.01%.
As methylene ether bonds are considered to be not as stable as methylene bridges,
and can rearrange readily to methylene bridges by splitting off formaldehyde [4], it is
suggested to try to avoid such ether bridges in UF resins. The UF1 resin produced
higher content of hydroxymethyl groups (mostly –NH–CH2OH) and methylene
groups but the least dimethylene ether groups compared to other resins (UF2 and
UF3), as well as yielding the most di- or tri-substituted urea and lower content of
monosubstituted urea (NH2–CO–NH–). The most significant is the content of dimethylol groups of UF1, 0.52%, which is not observed in UF2 and UF3. The mechanism of this is primarily the methylolation (hydroxymethylation) of urea and secondly
the polycondensation reactions occurring during UF resin synthesis [36]. The first
compound formed, monosubstituted urea, further condenses in situ with urea to
form immediately methylene diurea. This is attributed to the 160-161 ppm signals in
the carbonyl and 47-49 ppm in the methylene ranges respectively. Further methylenelinked linear oligomers are also present, also characterized by the carbonyl signals
appearing at 160–161 ppm and by a change in the ratio of the signals at 47–49 ppm.
The UF2 resin gave the higher proportion of monosubstituted urea (NH2–CO–NH–)
and methylene bridges but the lower amount of methylol groups (especially –NH–CH2OH).
10
A. DORIEH ET AL.
Figure 5. DSC curves of UF1, UF2 and UF3 adhesives.
Table 5. Onset, peak temperatures for the curing reaction of
UF1, UF2 and UF3 adhesives by DSC measurements at 10 C/min
heating rate.
Adhesive
UF1
UF2
UF3
Onset
Peak temperature
End set
Exothermic DH (J/g)
84
87
84
89
97
95
93
103
102
51.06
95.20
95.25
In summary, the higher free formaldehyde content, as well as the higher content
of hydroxymethyl groups are present in UF1 compared to UF3 and UF2, confirming
and explaining the shortest gel time in UF1.
4.4. DSC analysis
Figure 5 depicts typical DSC curves for UF1, UF2 and UF3 resins at a heating rate of
10 C/min. (NH4)2SO4 was added at 0.2 weight% level based on resin solids.
Generally, each DSC spectra starts with an exothermic peak and is followed by an
endothermic peak. Table 5 summarizes the results of the exothermic peaks for the
UF1, UF2 and UF3 resins from the DSC thermograms. The exothermic peak appearing before 100 C belongs to the polycondensation reaction between primary amide
linkages of unreacted urea and the hydroxymethyl groups (–CH2OH). The crosslinking reaction, in all three cases takes place at a similar temperature range (80–85 C)
and the reaction heats evolved are similar. The endothermic heats of reaction vary
widely within the temperature range of 100–150 C as they are certainly related to the
evaporation of condensed water, free formaldehyde, and decomposition of the methylene-ether linkages [37]. The UF1 resin resulted in the lowest peak temperatures and
the smallest heat of the exothermic reaction. The smaller DH value for the UF1 resin
confirmed the shorter gel time of the UF1 resin compared with those of the other
resins. The lower value of the UF1 exothermic peak temperature is probably related
to the higher proportion of methylene linkage and hydroxymethyl groups confirmed
by 13CNMR analysis.
JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
11
Table 6. MDF panels results.
Properties particleboards
IBS (internal bond strength)
MOE (modulus of elasticity)
MOR (modulus of rupture)
TS (thickness swelling)
FF (free formaldehyde)
Unit
UF1
UF2
UF3
MPa
MPa
MPa
%
mg/100 g oven dry board
0.516
2326
23
13
12.8
0.461
2134
22
16
9.4
0.502
2185
23
14
10.7
The onset temperature is an indication of UF resin cure or reactivity, the peak
temperature is an important parameter for comparing the reactivity of UF resins
[38]. Both the onset and peak temperatures suggest that the UF1 adhesive presents
the higher reactivity which could result in an adhesive with a shorter curing time.
Conversely, the peak temperature for the other UF resins (with UFC) was increased.
4.5. Production of medium density fiberboard
The industrial fibers used were fully dry and did not require further drying. The resin
in liquid form was mixed with the hardener and H2O for the preparation of the glue
mix. The UF adhesives cure under acid conditions, hence, wood acidity can affect the
rate at which UF adhesives harden [39]. 10% adhesives solids w/w on oven dry
weight of wood particles was used for each type of adhesive to produce panels of
250mm 350 mm 10 mm thick (10 in. 14 in. 0.4 in.) by hot pressing the glued
wood furnish at 180 C and at 4 MPa pressure, for a pressing time of 5 min. The
MDF panels were produced at an average specific density above 700 kg/m3. The
MDFs accordingly obtained were maintained at room temperature (20 C and 65%
relative humidity) for 2 weeks before testing.
Table 6 shows that the MDF bonded with the UF1 resin yielded the highest
strength panel of all for IB (Figure 6), MOE (Figure 7) and MOR (Figure 8) and the
lower TS values (Table 6 fourth row). The most significant effect of the MDF produced by UF2 resin is the lower amount FF emission (Figure 9). However, there no
significant difference in MOR and MOE for the MDFs made from the UF1, UF2, or
UF3 resins.
The MDFs bonded with UF2 yielded the highest thickness swelling of all because
of its weak bonding. Conversely, the MDF bonded with UF1 resulted in the lowest
thickness swelling. These were confirmed by the NMR and DSC spectra. It should be
noted that the MDFs bonded with the UF3 resin (UFC with 19% urea) yielded better
mechanical properties than the MDF bonded with the UF2 resin (UFC with 22%
urea). Furthermore, the MDF manufactured with the UF2 resin gave a poor performance with the highest thickness swelling and lowest internal bond (IB), modulus of
elasticity (MOE), and modulus of rupture (MOR), due to the lowest proportion of
hydroxymethyl groups and methylene bonds. This could indicate that using UFC in a
resin at the higher F/U ¼ 5 molar ratio yields a better performance than the UF resin
prepared by using a UFC at the lower F/U ¼ 4.3 molar ratio. This probably relates to
the proportions of mono-, di- and tri-methylol ureas in the UFC used.
In general, the MDF panels bonded with the UF1 resin includes the highest
amount of branched-type methylene bridges (i.e. 17.75. Table 4) to give a high
strength of the cured resin, good resistance to water, as well as the highest proportion
12
A. DORIEH ET AL.
0.52
IB(MPa)
0.5
0.48
0.46
0.44
0.42
UF1
UF2
Adhesive type
UF3
Figure 6. IBS values using different adhesives.
MOE (MPa)
2400
2300
2200
2100
2000
UF1
UF2
Adhesive type
UF3
Figure 7. MOE values using different adhesives.
MOR (MPa)
23
22.5
22
21.5
UF1
UF2
Adhesive type
UF3
Free formaldehyde
(mgr/100gr)
Figure 8. MOR values using different adhesives.
15
10
5
0
UF1
UF2
UF3
Adhesive type
Figure 9. Free formaldehyde values using different adhesives.
of hydroxymethyl groups (–CH2OH) for higher resin reactivity and thus better
strength. UFC generally yielded lower strength MDFs than formalin. Since UFC has
been one of the usual raw materials used for adhesive for panel products, the poorer
JOURNAL OF ADHESION SCIENCE AND TECHNOLOGY
13
bond strength obtained with it demonstrates the possibility of optimizing a better UF
adhesive by using formalin.
5. Conclusions
The overall conclusions deriving from the synthesis of UF resins prepared in this
study under various synthesis procedures are that: NMR and IR spectra of UF resins
show differences in the chemical structure of the synthesized UF resin;
1.
2.
3.
4.
5.
MDFs bonded with the UF1 adhesive resulted in the best overall panel performance with the exception of the free formaldehyde (FF) content. The UF1 adhesive
with raw formalin yielded the highest IB strength, modulus of elasticity (MOE),
and modulus of rupture (MOR) and the most water resistant boards with the
least thickness swelling (TS) but the highest free formaldehyde (FF) in the resin.
With the lowest free formaldehyde (FF) the resin prepared with UFC at F/U¼
4.3 resulted in the poorer performance with the highest thickness swelling (TS)
and lowest internal bond strength (IBS). This poor performance could be related
to the molecular mobility of the cured UF resin network.
The UF1 adhesive showed significantly lower values of onset and peak temperatures compared to UF3 and UF2. Both the onset and peak temperatures suggest
that the UF1 adhesive has higher reactivity and can be regarded as a faster curing
adhesive.
The 13C-NMR spectra indicated that the UF1 resin presents the highest proportion of methylene linkages and hydroxymethyl groups, thus resulting in the best
overall board properties.
The UF3 adhesive yielded better mechanical performance (IB, MOE, MOR and
TS) than UF2, which probably relates to the higher F/U molar ratio in UFC.
Acknowledgements
We are thankful to University of Guilan and the Arian Saeed Industrial Group (ASIG) for
their partial support.
Disclosure statement
No potential conflict of interest was reported by the authors.
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