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International Journal of Adhesion & Adhesives 18 (1998) 95 —107
Urea—formaldehyde (UF) adhesive resins for wood
M. Dunky
KREMS CHEMIE AG, A 3500 Krems, Austria
Accepted 12 June 1997
Abstract
Urea—formaldehyde (UF) resins are the most important type of adhesive resins for the production of wood based panels. They
convince by their high reactivity and good performance in the production and by their low price, however they lack in water resistance
of the hardened resin owing to the reversibility of the aminomethylenelink and hence the susceptibility to hydrolysis. This need can be
overcome by introducing other components like melamine to the UF resin molecules. The former problem of subsequent formalde-
hyde emission can be considered as solved owing to the decrease of the content of formaldehyde in the resins during the last two
decades. Modern laboratory test methods enable a deep insight into the chemical structure and the gelling and hardening behaviour
of the resins. (1998 Published by Elsevier Science Ltd. All rights reserved.
Keywords: Urea—formaldehyde resins; molar ratio; degree of condensation; gelling and hardening behavior; water resistance and
hydrolysis; melanine
1. Introduction
Urea—formaldehyde (UF) resins are based on the mani-
fold reaction of two monomers, urea and formaldehyde.
By using different conditions of reaction and preparation
a more or less innumerable variety of condensed struc-
tures is possible. UF resins are the most important type
of the so-called aminoplastic resins. Currently, approx-
imately 6 billion tons are produced per annum world-
wide, based on a usual solids content of 66% by mass.
UF resins are thermosetting duromers and consist of
linear or branched oligomeric and polymeric molecules,
which also always contain some amount of monomer.
Non-reacted urea is often beneficial to achieve special
effects, e.g. better stability during storage. However, the
presence of free formaldehyde is ambivalent. On the one
hand, it is necessary to induce the hardening reaction. On
the other, it causes formaldehyde emission during the
press cycle as well as subsequent, displeasing, formalde-
hyde emission from the pressed boards, a fact that has led
to a total change in the formulation of UF resins during
the last 20 years. These days the problem of subsequent
formaldehyde emission has been solved, at least in Euro-
pe, where the most stringent formaldehyde emission re-
gulations in the world exist. Other countries have already
followed, will follow or should follow this trend.
After hardening UF resins form an insoluble, three-
dimensional network and cannot be melted or ther-
moformed again. In their stage of application UF resins
are still soluble or dispersed in water or in the form of
spray dried-powders, which, in most cases however, are
redissolved in water for application.
There are several papers and monographs concern-
ing UF resins in the literature [1—8], which contain
considerable additional information for the more in-
terested reader. This review deals rather with some
special aspects of UF resins, the industrial experience of
the author having biased the selection of the different
topics.
2. Chemistry of urea—formaldehyde adhesive resins
Despite the fact that UF resins consist of only two
main components, namely urea and formaldehyde, they
present a broad variety of possible reactions and struc-
tures. The basic characteristics of UF resins can be ex-
plained at the molecular level by:
ftheir high reactivity;
ftheir water solubility, which renders them ideal for use
in the woodworking industry; and
0143-7496/98/$19.00 (1998 Published by Elsevier Science Ltd. All rights reserved.
PII: S0143-7496(97)00054-4
fthe reversibility of the aminomethylene link, which
also explains the low resistance of UF resins against
the influence of water and moisture, especially at higher
temperatures. This is also one of the reasons for their
subsequent formaldehyde emission, when hardened
and in service.
2.1. Methylolation and condensation reaction
The reaction of urea and formaldehyde is basically
a two-step process: usually an alkaline methylolation
followed by an acid condensation. Methylolation refers
to the addition of up to three (four in theory) molecules of
the bifunctional formaldehyde to one molecule of urea to
give the so-called methylolureas. Each methylolation
step has its own rate constant, k*, with different k*values
for the forward and backward reactions. The reversibility
of this reaction is one of the most important features of
UF resins, and is responsible for both the low resistance
against hydrolysis caused by the attack of moisture or
water and the subsequent formaldehyde emission. This is
so because emittable formaldehyde results from the slight
hydrolysis of weakly bonded formaldehyde. The forma-
tion of methylol groups mostly depends on the F/U
molar ratio, with higher molar ratios increasing the ten-
dency to form highly methylolated species [9,10]. Prod-
ucts of side reactions are acetals, hemiacetals and etheri-
fied products, with residual methanol always present in
small amounts from the production of formaldehyde.
The methylol groups are more or less stable in slightly
alkaline conditions. Alkaline condensation might also
occur to a limited extent [11], but this is of no industrial
importance. Starting the reaction of urea and formalde-
hyde in the usual molar ratio but under acidic conditions
gives methylene-linked ureas which tend to be insoluble
in water with ca. five or six urea units [12—15]. Methy-
leneureas are used as long-term fertilizers, as neutral
fillers and as a white pigment, with some other ideas for
industrial and commercial use still being at the develop-
ment stage [16—18].
The UF polymer builds up in the acid condensation
step: the methylols, urea and free formaldehyde still pres-
ent in the system react to give linear and partly branched
molecules with medium and even higher molar masses.
The type of bond between the urea molecules depends on
the conditions used: low temperatures and only slightly
acidic pH favour the formation of methylene ether
bridges (—CH2—O—CH2—), while higher temperatures and
lower pHs lead to the more stable methylene (—CH2—)
bridges. Ether bridges can rearrange to methylene
bridges by splitting off formaldehyde. One ether bridge
needs two formaldehyde molecules and it is not as stable
as a methylene bridge. Hence it is recommended to avoid
such ether groups in UF resins under today’s common
conditions of low formaldehyde content by virtue of the
low final molar ratio of the resins.
The acid condensation step itself is still performed at
the same high molar ratio as given in the alkaline methyl-
olation step (F/U"1.8 to 2.5). Molar ratios lower than
ca. 1.8 lead to some precipitation during the acid conden-
sation step, causing inhomogenities in the solutions as
well as problems in the determination of the proper
endpoint of the reaction by water dilutability or by cloud
point. The low molar ratio of the final UF resin is
adjusted by the addition of the so-called second urea,
which also might be added in several steps [7]. Special
knowledge of this step is important for the production of
resins with good performance, especially at the low molar
ratios usually in use today in the production of particle-
boards and medium-density fibreboards (MDFs).
In the literature various other resin preparation pro-
cedures are also described, e.g. yielding of uron structures
[19,20] or triazinone rings in the resins [21,22]. The
latter are formed by the reaction of urea and an excess of
formaldehyde under basic conditions in the presence of
ammonia, a primary or a secondary amine, respectively.
These resins are used, among other applications, to en-
hance the wet strength of paper.
In the resin itself different chemical species are present:
ffree formaldehyde, which is in steady state with re-
maining methylol groups and the post-added urea;
fmonomeric methylol ureas, which have been formed
mainly by reaction of the post-added urea with the
high content of free formaldehyde at the still high
molar ratio of the acid condensation step;
foligomeric methylol ureas, which have not reacted
further in the acid condensation reaction or which
have been built up by the above-mentioned reaction
by post-added urea; and
fmolecules with higher molar masses, which are the
resin molecules in the closer sense of the word.
The condensation reaction and the increase in molar
mass can be monitored by gel permeation chromatogra-
phy (GPC) (Fig. 1) [23]. A longer condensation step
yields molecules with higher molar masses and the GPC
peaks move to lower elution volumes.
2.2. Influence of F/ºmolar ratio on the properties of ºF
adhesive resins and ºF-bonded wood-based panels
UF resins consist of the two monomers urea and
formaldehyde. Forced by the need to decrease the sub-
sequent formaldehyde emission, the F/U molar ratio of
the two monomers that is commonly used has changed
during the last two decades. However, it cannot be ex-
pected that under these circumstances no changes in the
properties and performance of the resins occur. Today’s
resins now have more or less the same performance
characteristics as many years ago, but with a distinctly
lower content of formaldehyde and hence a distinctly
lower subsequent formaldehyde emission—so low, in
96 M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107
Fig. 1. Monitoring of the acidic condensation process by GPC [23]. Steps of the condensation process: alkaline condensate (———), acidic step 2 min
(- - - - ), acidic step 4 min (—)—), acidic step 5 min (—))—). Conditions of GPC: coiled PTFE column 0.2 cm]200 cm, filled with Fractogel PVA 20 000;
mobile phase: dimethyl sulfoxide (DMSO); flow rate: 2 ml h~1.
fact, that the former problem of formaldehyde emission
can now be considered as solved.
The main differences between UF resins with high and
with low contents of formaldehyde are their reactivity as
a consequence of the different free formaldehyde content
and their degree of crosslinking in the cured network.
The degree of crosslinking is directly correlated to the
molar ratio of the two components. Taking into consi-
deration that an ideal linear UF chain has a molar ratio
of 1.0, assuming that there are no ether bridges, no
unreacted branch-site methylol groups and no other free
formaldehyde, then the small molar excess of formalde-
hyde above molar equality is what yields the final cross-
linking. In practice this calculation is not really exact,
because there are always ether bridges and some unreac-
ted methylol groups in the resin, even after hardening.
This is not only a question of the proper preparation
procedure but also a simple question of the mobility of
the individual molecules with already higher molar mass-
es during the hardening reaction and therefore often
a question of steric hindrance, which renders some reac-
tions impossible.
The higher the F/U molar ratio, the higher the content
of free formaldehyde in the resin. Assuming steady-state
conditions in the resins—which means, for instance, that
post-added urea has had enough time to react with the
resin—the content of free formaldehyde is very similar
even with different resin preparation procedures. On
a rough scale the content of formaldehyde in an unmodi-
fied UF resin is ca. 0.1% at F/U"1.1 and 1% at
F/U"1.8 [24]. This also decreases with time because
aging reactions in the resin consume parts of this free
formaldehyde. Additionally, determination of the free
formaldehyde requires exact conditions to avoid any
cleavage of weakly bonded formaldehyde [25]. Therefore
the content of free formaldehyde is only a coarse approxi-
mation of reality.
It has to be considered that it is neither the content of
free formaldehyde itself nor the molar ratio, that even-
tually should be taken as the decisive and only criterion
for the classification of a resin regarding the subsequent
formaldehyde emission from the boards produced. This
is because the composition of the adhesive mix as well as
the various process parameters during board production
determine the extent of formaldehyde emission. Depen-
ding on the type of board and process, sometimes it is
recommended to use a UF resin with an already low F/U
molar ratio (e.g. F/U"1.03) and, hence, a low content of
free formaldehyde. On the other hand, sometimes the use
of a resin with a higher molar ratio (e.g. F/U"1.10) and
the addition of a formaldehyde catcher will give better
results. Which of these possible ways will be the best in
practice can only be decided separately in each case by
trial and error.
2.3. Influence of molar mass distribution on the properties
of UF resin
The molar mass distribution is determined by (1) the
degree of condensation and (2) the addition of urea (and
M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107 97
sometimes also other components) after the condensa-
tion step; with this latter addition, again low molar
masses are present in the resin. This is the big difference
between the formaldehyde condensation resins [UF
resins, melamine—formaldehyde/melamine—urea—formal-
dehdye (MF/MUF) resins and also, to a smaller extent,
phenol—formaldehdye/phenol—urea—formaldehyde
(PF/PUF) resins] in comparison with polyaddition
resins and polymerized plastics. For this reason the mo-
lar mass distribution is much broader than for other
polymers: it starts at the low-molar-mass monomers (the
molecular weight of formaldehyde is 30, for urea it is 60)
and goes up to more or less polymeric structures. How-
ever, it is not clearly known what the highest molar
masses in a UF resin really are. Billiani et al. [26] and
Dunky and Lederer [27] have found molar masses of up
to 500 000 by light scattering. In particular, by the use of
low-angle laser light scattering (LALLS) coupled to
GPC, the shear conditions in the chromatographic co-
lumns [28] should guarantee that all physically bonded
associates are split off and that these high numbers (be-
tween 100 000 and 500 000) really describe the macro-
molecular structure of a UF resin in the right manner.
A second important argument for this statement is the
fact that, up to such a high number of the molar mass, the
on-line calibration curve gained in the GPC—LALLS run
is persistent and more or less linear. It does not contain
any sudden transition as would be the case for agglome-
ration; in the latter case the molar mass would increase
sharply but inconsistently.
The higher the molar mass (the higher the degree of
condensation), the lower the water dilutability of the resin
and the fewer the portions of the resin that remain soluble
in water. Diluting the resin with an excess of water causes
precipitation of parts of the resin. This resin portion con-
tains the higher-molar-mass molecules of the resin and its
proportion increases as the degree of condensation in-
creases [27]. At a given solids content, the viscosity in-
creases with a higher proportion of condensed structures.
2.4. Production of UF resins
The production of UF resins is performed in a discon-
tinuousaswellasinacontinuousway;thelatterisuse-
ful only for the production of large batches to avoid
too often the preparation of off-grade products. The reac-
tion is influenced by several parameters and requires pre-
cise control of purity, amount and sequence of addition of
the raw materials and of alkaline and acid catalysts. The
preparation conditions are adjusted and monitored with
respect to temperature, pH and concentration of the react-
ants. Usually, a three-stage process is used [8,29].
The first stage is always the methylolation step which
is usually performed under alkaline conditions and at
high molar ratio (F/U"1.8 to 2.5). This step is necessary
to build up the different methylolureas, the types and
proportions of the methylols formed being dependent on
the F/U molar ratio.
The polycondensation reaction takes place in the sub-
sequent acid condensation step to form polydisperse pat-
terns of oligomers and polymers of different molar mas-
ses. The determination of this molar mass distribution
can be done by means of GPC. The molar mass distribu-
tion (according to the degree of condensation) is one of
the most important characteristics of the resin and deter-
mines several of its properties, e.g. viscosity and flow
behaviour, wetting behaviour of a wood surface [30] and
penetration into the wood surface [31,32].
The last production step includes distillation of the
resin solution usually to 65—66% solids content, which is
performed by vacuum distillation in the reactor itself or
in a thin layer evaporator. Before or after this step an
addition of urea in one or more steps at different temper-
atures, including also maturing times, can take place in
order to decrease the final molar ratio. Different examp-
les for the production of UF resins are described in detail
in the literature [6,33].
2.5. Curing reaction
During the curing process a more or less three-dimen-
sional network is built up. This yields an insoluble resin
that is no longer thermoformable. The hardening reac-
tion is the continuation of the acid condensation step.
Whereas gelling in the reactor is to be avoided, the same
process needs to take place in the glueline. The acid
conditions can be adjusted by the addition of a so-called
latent hardener, or by the direct addition of acids (maleic
acid, formic acid, phosphoric acid and others) or acid
compounds which dissociate in water (e.g. aluminium
sulfate). Common latent hardeners are ammonium sul-
fate and ammonium chloride. The latter, however, has
not been used in the German and Austrian particleboard
and MDF industry for several years now, because of the
generation of hydrochloric acid the during combustion of
wood-based panels causing corrosion problems and the
suspected formation of dioxins. Ammonium sulfate re-
acts with the free formaldehyde in the resin to generate
sulfuric acid, which decreases the pH. This low pH and
the ensuing acid conditions enable resumption of the
condensation reaction and finally the gelling and harden-
ing of the resin. The rate of decrease of pH depends on
the amounts of available free formaldehyde and
hardener, and is greatly accelerated by heat [34,35].
3. Advantages and disadvantages of UF resins
3.1. UF resins have no water and weather resistance
The aminomethylene linkage is susceptible to hy-
drolysis and therefore is not stable at higher relative
98 M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107
humidities, especially at elevated temperatures [36].
Water also causes degradation of the UF resin, the effect
being more devastating the higher the water temperature.
This different behaviour of UF-bonded wood boards at
various temperatures is the basis for standard tests and
hence for the classification of bondlines, resins and
wooden products, respectively, into different bonding
classes. These include the lowest requirements (interior
use) for ‘run-of-the-mill’ UF-bonded boards up to more
or less water- and weather-resistant boards (V 100
boiling test, V 313 cycle test, WBP and others) according
to different national and international standards [37].
The incorporation of melamine (MUF, MF#UF)
and sometimes phenol (MUPF, PMUF) improves the
low resistance of UF bonds to the influence of humidity,
water and weather. However, this changes the character-
istics of the resins, especially concerning their reactivity.
Additionally, the costs for these modified and fortified
products are not comparable because of the much higher
price of melamine compared with urea. Therefore, the
content of melamine in these resins is always as high as
necessary but as low as possible, pure melamine—formal-
dehyde resins being in use only in mixtures with UF
resins. The advantage of higher hydrolysis resistance for
pure MF resins is counteracted by their low storage
stability in liquid form and their very high price.
The addition of melamine to a UF resin slows down
the pH drop after addition of the hardener [34]. The gel
time increases with the addition of melamine because of
the buffering capacity of melamine’s triazine ring [1].
This behaviour is basically the same if the melamine is
added to the UF resin just before gelation or if it is
incorporated chemically in any way during resin manu-
facture.
Melamine can also be added in the form of melamine
acetates [38,39], which decompose in the aqueous resin
mix only at higher temperatures and thus give some
savings of melamine for the same degree of water resis-
tance compared with totally reactor-made MUF resins.
Also, partially hydrolysed Nylon (polyamide) might be
a possible fortifier, having enough tertiary amide groups
to react with the UF methylols [40].
The weather durability of a glueline, which essentially
means its resistance to cyclic stresses arising from swell-
ing and shrinkage of the joint, as well as hydrolytic attack
on the chemical bonds, can be reduced by the incorpora-
tion of hydrophobic chains into the hardened network.
This has been done by Ebewele et al. [41—45], who
incorporated urea-capped di- and trifunctional amines
containing aliphatic chains into the resin structure and
used the hydrochloride derivates of some of these amines
as a curing agent. Wang and Pizzi [46] replaced formal-
dehyde by succinaldehyde, OHC—CH2—CH2—CHO,
a dialdehyde with a short hydrocarbon chain. Both ap-
proaches introduce some flexibility into the hardened
network, which should decrease internal stresses, as well
as impart some enhanced water repellancy of the cured
network owing to the presence of these hydrophobic
hydrocarbon chains.
Another approach to increase the resistance of UF
resins against hydrolysis is based on the fact that acid
hardening of the resin itself causes residues of acids or
acid compounds in the glueline. Myers [47] pointed out
that, the in case of an acid-hardening system, the decrease
in bond durability can be initiated by hydrolysis of the
wood cell-wall polymers adjacent to the glueline as well
as by an acid-catalysed resin degradation in the case of
UF-bonded products. A neutral glueline, therefore,
should show a distinctly improved hydrolysis resistance.
The amount of hardener (acids, latent hardeners) should
always be adjusted to the desired hardening conditions
(press temperature, press time and other parameters) and
never be like ‘the more the better’. In fact, just the oppo-
site occurs: an addition of too much hardener can cause
brittleness of the cured resin and a very high acid content
in the hardened glueline. However, neutralization must
not take place until the hardening reaction has finished,
otherwise it would delay or even prevent curing. These
divergent requirements are a problem which has not yet
been solved fully in practice.
Higuchi and Sakata [35] found that complete removal
of acidic substances by soaking plywood test specimens
in an aqueous sodium bicarbonate solution results in
a marked increase in the water resistance of UF gluelines.
Another attempt was made by this same research group
[48,49] through the use of glass powder as an acid
scavenger, which reacts only slowly with the remaining
acid of the glueline and therefore does not interfere with
the acidic hardening of the resin. Dutkiewicz [50] also
obtained some good results in neutralization of the in-
herent acidity of a hardened UF-bonded glueline by the
addition of polymers containing amino or amido groups.
Moreover, for PF-bonded joints, self-neutralization of
the glueline increases the durability of the joint, especially
preventing acid deterioration on solid wood [51].
3.2. Subsequent formaldehyde emission from UF-bonded
boards
The formaldehyde emission from panels in service is
caused, on the one hand, by residual formaldehyde pres-
ent in the UF-bonded boards trapped as gas in the
structure of the substrate as well as dissolved in the water
content of the boards (moisture). On the other hand,
hydrolysis of weakly bound formaldehyde from N-
methylol groups, acetals and hemiacetals, and, in more
severe cases, hydrolysis of methylene ether bridges (e.g. at
high relative humidity), also increase the content of
emittable formaldehyde. Contrary to phenolic and
polyphenolic resins, a permanent pool of emittable for-
maldehyde is generated from these structures. This ex-
plains the slow release of formaldehyde from UF-bonded
M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107 99
Fig. 2. Calculation of the decrease of equilibrium concentration, C%2, with time for C%2,0"1 (ppm), K"1(mh~1) [52]: (a) without hydrolysis;
(b) hydrolysis constant kH"4.0]10~8 (h~1); (c) hydrolysis constant kH"4.0]10~7 (h~1).
wood-based panels even over longer periods. However, it
depends also on the conditions—which might be de-
scribed overall as a hydrolysis constant—whether or not
this weakly bound formaldehyde pool leads to unpleas-
antly high emissions [52]. Fig. 2 shows the calculated
decrease of formaldehyde emission from a wood-based
panel on the basis of assumed values of the degree of
hydrolysis [52]. The higher the hydrolysis rate, the high-
er is the inherent reserve of formaldehyde which contrib-
utes to the subsequent formaldehyde emission.
The problem of the formaldehyde emission by UF-
bonded wood products was prevalent in former times.
Now it can be considered as solved, owing to the strin-
gent regulations introduced in many countries. The so-
called E1 emission class describes a formaldehyde emis-
sion that is sufficiently low to avoid irritation or inflam-
mation of the mucous membranes in the eyes, nose and
mouth. However, it is important that not only the boards
themselves, but also veneering and carpenter’s glue
resins, laquers and varnishes and other sources of formal-
dehyde are under control [53—55].
It has been the main challenge for UF chemists over
the last 20 years to reduce the content of formaldehyde in
UF resins, and this without any major changes in the
performance of the resins. In theory this is not possible,
because formaldehyde is one of the two main reactive
partners in the reaction with urea during the condensa-
tion reaction and during curing. Decreasing the F/U
molar ratio means lowering the degree of branching and
crosslinking in the hardened network, which unavoid-
ably leads to a lower cohesive bonding strength. UF
chemists did not manage to fully square the circle, but
they did revolutionize the chemistry of UF resins. For
example, for an unmodified particleboard UF resin, the
above-mentioned F/U molar ratio had been approxim-
ately 1.6 at the end of the 1970s. It is now in the range
1.02 to 1.08, but the board performance requirements as
given in the relevant quality standards [37] are still the
same.
Beside the degree of crosslinking of the cured resins,
which shows the difference between the types of resin
mentioned above, the rate of hardening also depends on
the availability of formaldehyde in the system. New types
of reaction and revolutionary ideas concerning both
basic UF chemistry and resin application [56] have
achieved an acceleration of the hardening reaction and
hence a steep increase in productivity on board produc-
tion lines. In the last decade and a half, the board produc-
tion lines have also changed. Most new lines now consist
of a continuous press with specific press times far below
the values that were common 20 years ago. Fortunately,
all of this progress clearly contradicts the concept held
10 years ago, that nothing was and is left to be developed
in the field of UF resins. The author is sure that even
now, after all this progress, we have not yet reached the
end of the line in new developments and that the chem-
istry of UF resins is still a good area for research and
development.
100 M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107
3.3. High reactivity and low press times, including full
hardening
UF resins can be distinguished from other formalde-
hyde resins, e.g. MF, MUF and PF, by their high reactiv-
ity and hence by the shorter press times achievable. With
the modern and long continuous press lines (up to 48 m,
soon even 54 m) in use today, specific press times as low
as 5 s per mm and even 4 s per mm are possible in the
production of medium-thickness particleboards. This re-
quires highly reactive UF resins, an adequate amount of
hardener, as high press temperatures as possible, and
a distinct gap between the moisture contents of glued
particles in the surface and core of the board. This moist-
ure gradient induces the so-called steam-shock effect,
even without the additional steam injection often used in
North American plants. The optimum moisture content
of the glued particles is 6—7% in the core and 11—13% in
the surface. The lower the moisture content in the core,
the higher the surface moisture content can be adjusted
in order not to exceed a certain moisture content level in
the total particleboard mat (which would cause problems
with steam elimination and even steam blisters in the
final panel). To achieve such a low moisture content of
the glued core particles it is necessary to limit the addi-
tion of water in the core. The lower the percentage of
adhesive used on the wood, the lower this amount of
water applied to the wood furnish and the lower the
moisture content of the glued core particles. For the
surface layers, on the other hand, additional water is
necessary and is added through the glue mix, to increase
the moisture content of the glued particles. However, this
additional water cannot be replaced by a higher moisture
content of the dried particles before the glueing blender,
because this water must be available at short notice to
induce a strong steam-shock effect. This is not the case
for the water present as the moisture content of internal
wood cell walls.
3.4. Clear glueline
All aminoplastic glue resins usually give clear and
unvisible gluelines after hardening, contrary to phenolic
or polyphenolic gluelines. However, it might happen that
the acid hardener causes some discoloration of the glue-
line, which might vary from light yellow to even dark red
or violet. This might occur especially when separate
application methods are used for the acids. In this case
the hardener is either spread before the adhesive or it is
applied onto the second, matching wood surface. De-
pending on the type of wood, it can be rather tedious to
find at least one acid or acid salt not causing such
discoloration. An excess of hardener salts, especially of
ammonium chloride, may also cause a slight change in
colour.
3.5. Cold tack
Cold tack means that the particle mat attains some
strength after the prepress, without any hardening reac-
tion. This strength is necessary, for instance, if the particle
mat is handed over from one caul to another. This is
particularly the case in multiopening particleboard
presses, in special form presses or in plywood mills, where
the glued veneer layers are prepressed to fit into the
openings of the presses. Also, some small level of cold
tack is necessary to avoid blow out of the fine surface
particles once the board mat reaches the inlet of a con-
tinuous press with a high belt speed. On the other hand,
cold tack can lead to agglomeration of fine particles and
fibres in the forming station.
Cold tack is generated by drying out of a glueline, up to
a certain maximum. Then it decreases again, when the
glueline is more or less dried out. Both the intensity of cold
tack and the optimum time span after the glue is spread
can be adjusted by the degree of condensation, as well as
by special reaction procedures [57—59]. Additionally, vari-
ous additives can increase the cold tack of the adhesive,
e.g. some thermoplastic polymers like poly(vinyl alcohol).
3.6. Aqueous system (absence of organic solvents)
UF adhesive resins are partly aqueous solutions and
partly dispersions in this aqueous solutions [27]. Usually
no organic solvent is used for UF resins in the wood-
based panels industry, neither for the adhesive nor for
impregnating resins. This avoids any necessity of safety
devices for the storage and use of inflammable materials.
The emission of organic carbon is, therefore, much lower
than for any solvent-borne system.
3.7. Synergic effects with other adhesives
Special advantages in the simultaneous use of two or
more resins and adhesives can be obtained by mixing
these different resins prior to their application. The addi-
tion of an MUF resin to a UF resin increases the moist-
ure and water resistance of the UF resins, whereby the
degree of resistance depends on the content of melamine
in this mix. However, as long as performance require-
ments can be fulfilled, a lower content of melamine in the
adhesive system will also be a cost advantage.
PMDI can be used as an accelerator and as a special
crosslinker for UF resins, with additions of less than 1%
in the first case and up to 2% in the latter, both numbers
being based on dry particles [60].
3.8. Non-flammability
Because of their high content of nitrogen, UF resins
are non-flammable and burn only with the support of
a flame.
M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107 101
3.9. Low price
Urea is rather inexpensive compared with melamine
and phenol. Formaldehyde also is relatively inexpensive,
and thus, together, they give a resin of low cost but
nevertheless high performance. However, such resins are
not as inexpensive as desired by the wood-based panels
industry. In some part, their low price is a question of
the current industrial overcapacity for such resins.
However, a multiple increase in the price of methanol,
as has already occurred once in this decade, leads to
much higher costs for formaldehyde and hence for UF
resins.
Addition of melamine, use of phenol, modification or
fortification of the UF resin—all of these factors increase
costs and thus the price compared with unmodified UF
resin. It needs the daily consideration of each mill man-
ager to use unmodified UF resins for most boards pro-
duced while using other, more expensive resins only for
special boards, e.g. for particleboards and MDFs with
very low thickness swelling such as those required for
flooring applications. On the other hand, a special,
higher cost resin might introduce savings to such an
extent, e.g. on the density of the board, that it would be
worth spending money on its use.
4. Characterization of urea–formaldehyde glue resins
The characterization of UF resins has undergone great
progress within the last two decades. It is now possible to
analyse the polydisperse structure of the resins and the
individual structural elements in the resins, even in
a partly quantitative way. The curing reaction can also be
monitored by means of adequate methods. Nevertheless,
there is great need for quick test methods for characteriz-
ing the resins, especially laboratory methods to predict
the bond strength achievable in practice.
4.1. Basic technological tests
Solids content, refractive index, density, viscosity, pH
and reactivity are all usually measured as part of the
laboratory quality systems of the resin producers and the
mills using these resins. These tests are generally easy to
perform, but nevertheless they yield important informa-
tion about the quality and performance of the resins.
Most of these test methods are standardized or at least
commonly acknowledged, but they can give wrong re-
sults if not performed conscientiously. The solids content
for instance, which is found by drying the sample for 2 h
at 120°C, can be influenced by the type of oven used
(fresh air or circulating air ventilation), the number of
dishes in the oven, possible openings of the oven and
other parameters which may appear unimportant to un-
skilled personnel. During the drying procedure not only
the water of the aqueous solution but also the dispersion
itself evaporates. There is also an advance in the conden-
sation process which leads to a nearly cured resin during
the test. This generates a considerable amount of conden-
sation water which, of course, also evaporates. Adding up
all components of the resins in their original form yields
a number ca.10—15% (in absolute figures) higher than
the solids content measured under the conditions de-
scribed above. Also, freeze drying of the resin increases
the solid content by approximately the same extent.
Other test methods for the solids content can give differ-
ent results: whether these are lower or higher depends on
how thorough the drying has been—the more severe the
test conditions, the lower the value of residual solids
content found.
The gel time is one of the most important resin para-
meters not only for their application, but also to under-
stand their curing reaction. The test method is performed
in boiling water (usually at 100°C) after the addition of
a certain amount of hardener (usually the resin mix is
100 g of the liquid resin plus 10 g of a 15% aqueous
solution of ammonium chloride). The result of this test
does not just include the time during which the resin gels.
Important further information may be obtained, e.g.
whether the resin gels sharply within 1 or 2 s, or whether
its gel point spans 10 s or more; the latter case fore-
casts a slow generation of cohesion bonding strength.
The behaviour of the resin in the test tube (e.g. foaming)
and also the consistency and strength of the gelled plug
can be evaluated. However, it is necessary to understand
that the gel test depends also on the individual doing it;
from time to time parallel tests with one and the same
resin but performed by different people should be done to
avoid false conclusions. In the author’s laboratory some
mechanical gel timers have been tested. On the whole,
however, the manual gel time test is still to be preferred.
4.2. Curing reaction and build-up of bonding strength
Condensation resins are systems that gel and harden,
and this reaction can be monitored by means of various
methods. The gel test, as described above, is a coarse
method to describe the gelling behaviour of a resin. More
sophisticated methods follow the thermal behaviour of
the resin during curing. The method of differential ther-
mal analysis (DTA) [61,62] measures the difference in
temperature between two cells, both heated up according
to a certain temperature program, whereby the one of the
two cells contains the sample under investigation. Differ-
ential scanning calorimetry (DSC) uses the same type of
instrument, but measures directly the heat flow necessary
to compensate for the temperature difference [63—66].
Usually one but sometimes two exothermic peaks can be
found in such a temperature scan. The temperatures at
the onset and top of an exothermic or endothermic peak,
the slope of the up curve and the width of the peak can all
102 M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107
Fig. 3. DSC plot of a modified UF resin, heating rate 10°C min~1. Resin mix: 100 g of the liquid UF resin#10 g of an aqueous solution of ammonium
chloride (15%) [59].
be evaluated to characterize the curing behaviour of the
UF sample (Fig. 3). The DSC run is generally per-
formed in sealed capsules or under a certain external
pressure to prevent the large endothermic peak due to
evaporating water, which would fully cover the curing
peak of interest.
Other methods directly monitor the generation of
bonding strength: e.g. dynamic mechanical analysis
(DMA), where the liquid resin is applied to a substrate
like a glass-fibre mat, or Automatic Bonding Evaluation
System (ABES), where real gluing of veneers takes place.
In the DMA method [59] the samples are heated up
following a special temperature program and a cyclic
deformation is applied. The response of the second bar,
which includes the behaviour of the resin during gelling
and hardening, is the basis for calculation of the storage
modulus, E@, the dissipative modulus, EA, and of the loss
factor, tan d"EA/E@(Fig. 4). More modern in situ ther-
momechanical analysis (TMA) tests have also been pro-
posed recently [67]; they give directly the average length
of polymer segments between crosslinking nodes of the
hardened adhesive network.
In the ABES test [68,69], joints containing the bonds
to be measured are prepared by pressing with heated
Fig. 4. DMA plot of an unmodified UF resin, heating rate 10°C min~1.
Resonance frequency is plotted as a function of time ("increasing
temperature). Resin mix: 100 g of the liquid UF resin#10 g of an
aqueous solution of ammonium chloride (15%) [59].
blocks for a certain time, cooled within few seconds and
then pulled in shear mode. This test is repeated for
different times and various temperatures of the heated
blocks. Fig. 5 shows a typical result of the ABES test with
M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107 103
Fig. 5. Comparison of two unmodified UF resins with the same solids content and the same F/U molar ratio, but with different reactivities, measured
by the ABES method [70]. UF resin A—newly developed unmodified UF resin (F/U"1.08) with high reactivity; UF resin B—standard unmodified
UF resin with the same molar ratio; the two resins differ in their mode of preparation.
the bond strength given as function of time [70]. The two
resins compared are two unmodified UF resins, in which
the first (resin A) was prepared according to a special
procedure to give a distinctly better reactivity than the
second (resin B), which was a standard UF resin.
4.3. Degree of condensation and molar mass distribution
As already mentioned, UF resins contain molecules
with different molar masses, from monomers to oligomers
and polymers. The determination of this molar mass dis-
tribution [molecular weight distribution (M¼D)] can be
performed by gel permeation chromatography [size ex-
clusion chromatography (SEC)]. This method divides the
molecules according to their hydrodynamic volume
which, to a high extent, is proportional to the molar
mass. The greatest problem with UF resins and GPC is
the poor solubility of the resins in most of the solvents
usually used for GPC. It is necessary to use dimethyl
formamide (DMF) or even DMSO [27] to get solubility
of higher molar masses. These solvents also have to be
used as eluents, shortening the lifetime of the chromato-
graphic columns. They additionally cause some other
problems, such as high pressures as a consequence of the
higher viscosity compared with other organic solvents,
and low refractive index increments. Moreover, calib-
ration of the columns can only be done with compounds
that often have poor similarity to UF resins, especially in
the oligomeric and polymeric region, because no UF
compounds with a definite monodisperse molar mass
and molecular structure are available. This fact induces
great uncertainty in the calculation of molar mass aver-
ages on the basis of chromatograms.
It is also possible to use the GPC together with light
scattering (GPC—LALLS) [71]. The eluent with the dis-
solved UF molecules passes a light scattering cell, the
weight-averaged molar mass is measured directly for
each chromatographic run, and no external calibration is
Fig. 6. GPC plots of three different UF resins. Column: Hibar LiCh-
rogel PS-20 (Merck) and Aquagel 100K (Chrompack); solvent and
mobile phase: DMF; detection: refractive index; temperature: 60°C
[59].
necessary [26]. However, only the high molar masses can
be determined by this method. Fig. 6 shows a chromato-
gram of several UF resins with different degrees of con-
densation and hence different molar mass distributions.
In the low-molecular-weight region (this is the part of
high elution volume) peaks of urea, monomethylol urea
and the dimethylolureas can be discerned. Fig. 7 uses
GPC coupled with LALLS to determine, directly on-line,
the weight-average molar mass in the elution volume.
Hence this method avoids the need for external calib-
ration with all problems the latter would generally entail.
4.4. Chemical composition of the resins and analysis of the
structural components
Determination of the chemical composition of UF
resins can be done by using several basic chemical
methods. The total amount of formaldehyde can be de-
termined after hydrolysis of the UF resin with concen-
trated sulfuric or phosphoric acid; thereby methylols,
methoxymethylols and ether and methylene bridges are
104 M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107
Fig. 7. GPC coupled with light scattering (LALLS) of an UF resin: e(»)"signal concentration; E(»)"normalized response of the LALLS detector;
log M8(»)"E(»)/e(»)"measured weight-average molar mass as a function of elution volume, »[26].
cracked under the formation of formaldehyde which can
be determined afterwards. The content of nitrogen is
accessible with the Kjeldahl method or by means of
elemental analysis. The latter method also can be used
for determining the amount of carbon in the resin; how-
ever, calculation of the amount of formaldehyde from
this carbon content involves consideration of a possible
small residue of methanol in the resin coming from the
residual content of methanol in formaldehyde. The
content of urea can be calculated for unmodified UF
resins from the content of nitrogen, if other possible
nitrogen sources are known. In one of the basic papers on
UF chemistry, Staudinger and Wagner [72] used elemen-
tal analysis for the analysis of various stuctures in UF
condensates.
Analysis of the structural components can be per-
formed by various spectroscopic methods such as infra-
red (IR) [64,73—78], nuclear magnetic resonance
(NMR)—i.e. 1H-NMR [79—82], 13C-NMR [33,64,
73,83—88] and 15N-NMR [89—91]—and Raman spectro-
scopy [92]. All of these methods allow some insight into
the structure and the nature of linkages in the resins to be
gained. In particular, they help UF chemists to obtain
correlations between (1) different preparation strategies
and the resulting structures, and (2) these structures and
the properties of wood-based panels produced with these
resins.
5. Trends and prospects in UF chemistry and in the
application of UF resins
The best way to characterize the trends and prospects
in UF chemistry is to relate a story that happened to the
writer some years ago. At that time, there was another
step down in the limit of the perforator value as a cri-
terion for formaldehyde emission, from 10 mg iodometric
value to 6.5 mg photometric value. Although, because of
the change in the analysis method, the real decrease
necessary was only ca.2—2.5 mg and not 3.5 mg, never-
theless this represented a reduction of 20—25% in the
former limit. We had a plant trial with a new resin having
a formaldehyde content lower than that of all other
previous resins, we added different types of scavenger, we
used accelerators to maintain a good reactivity of the
resin; in short, we used a really state-of-the-art resin to
fulfil these stringent requirements. We had not yet done
the work, when suddenly the managing director asked:
‘Can we run with this new system also faster than with
the actual one in use?’ Just this question is one of the
most important features concerning trends in the produc-
tion of bonded wood-based panels. Higher reactivity of
the resins and of the resin glue-mix (hence shorter press
times) and at the same time a decrease of the subsequent
formaldehyde emission (which itself increases with one
and the same resin with shorter press time), these are the
challenges in this industry we have to face.
Even if UF resins are the cheapest type of condensa-
tion resins, nevertheless they are an important cost factor
in the production of particleboards. Although the main
approach to a lower consumption of resin in production
must be better preparation of the particles (avoiding too
many fines [93]), the resin characteristics also can help
towards better effectiveness of the resin. This includes the
adjustment to an optimum viscosity based on the proper
molar mass distribution: a low degree of condensation
can lead to a too high penetration of the resin into the
wood surface, causing starved gluelines, whereas high-
molar-mass resins can cause insufficient wetting of the
substrate surface [30—32].
M. Dunky /International Journal of Adhesion & Adhesives 18 (1998) 95—107 105
A further trend is the need for improved analysis
methods. Determination of the molar mass distribution
is possible, but still rather troublesome. The sample prep-
aration, the duration of one chromatographic run, the
lifetime of the columns, the need to use rather difficult
and displeasing solvents and eluents like DMF and
DMSO, the long time necessary to get the system into
steady-state conditions and, last but not least, the old
story with the calibration of columns not only for
comparing chromatograms but also for evaluating molar
masses; this is a long list of problems to overcome.
An analysis system or a test to forecast reliable bond-
ing strengths would also be highly welcome. At the mo-
ment there is still a gap between all the analysis results,
which have been performed with the liquid resins, and
the bonding strengths of the wood-based panels after
gluing and hardening. There are some attempts that
promise success. Ferg et al. [73,94,95] have obtained
correlation equations evaluating the chemical structures
of various liquid UF resins with different F/U molar
ratios and different preparation methods, on the one
hand, and the achievable internal bond of the panel
(hence of the hardened resin) and the subsequent formal-
dehyde emission, on the other hand. Even if these equa-
tions cannot stand for all resins and all cases, they de-
scribe how such a universal equation might work. It will
be the task of chemists and technologists to evaluate in
detail all possible parameters and to judge their influence
on the performance of the resins and the wood-based
panels.
As stated above, formaldehyde emission is no longer
a problem nowadays as long as the stringent regulations
in force are fulfilled [96,97]. Nevertheless, new and even
more stringent regulations are under discussion and al-
ready partly in force [98]. The main task for the develop-
ment of new UF resins was and still is to decrease the
formaldehyde content in the resins and hence obtain an
even more marked decrease in the subsequent formalde-
hyde emission from wood panels.
6. Summary
In the future, UF adhesive resins will be the most
important adhesive systems for wood-based panels as
a consequence of the advantages of these resins, even
keeping in mind their limitations. The great progress
achieved during the last 20 years and in the very recent
past allows us to predict that, even in the future, the
various requirements on UF-bonded boards will be met.
Certainly co-condensation with other monomers and
with other resins will become more important, in order to
combine their individual properties and advantages with
the low price of the UF resins. This is possibly the most
important field of future developments and of new ap-
plications.
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[97] Austrian Formaldehyde Regulation 1990.
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