Urea-Formaldehyde Adhesive Resins: Anthony H. Conner
Urea-Formaldehyde Adhesive Resins: Anthony H. Conner
Urea-Formaldehyde Adhesive Resins: Anthony H. Conner
*
ADHESIVE RESINS
Anthony H. Conner
Forest Products Laboratory
USDA Forest Service
dustry for a variety of purposes.1 The resin is used in the conditions, especially in combination with heat. These
production of an adhesive for bonding particleboard (61% conditions lead to a reversal of the bond-forming reactions
of the urea-formaldehyde used by the industry), medium- and the release of formaldehyde. For this reason, urea-
density fiberboard (27%), hardwood plywood (5%), and a formaldehyde resins are usually used for the manufacture of
laminating adhesive for bonding (7%), for example, furni- products intended for interior use only. However, even when
ture case goods, overlays to panels, and interior flush doors. used for interior purposes, the slow release of formaldehyde
Urea-formaldehyde resins are the most prominent ex- (a suspected carcinogen) from products bonded with urea-
amples of the class of thermosetting resins usually referred formaldehyde adhesives is a major concern that has come
to as amino resins.2,3 Urea-formaldehyde resins comprise under close scrutiny by state and Federal regulatory agen-
about 80% of the amino resins produced worldwide. cies.
Melamine-formaldehyde resins constitute the remainder of
this class of resins, except for minor amounts of resins that
are produced from other aldehydes or amino compounds CHEMISTRY OF UREA-FORMALDEHYDE
(especially aniline), or both. RESIN FORMATION
Amino resins are often used to modify the properties of
other materials.2,3 These resins are added during the pro- Urea-formaldehyde resins are formed by the reaction of
cessing of such products as textile fabrics to impart perma- urea and formaldehyde. The overall reaction of urea with
nent press characteristics; automobile tires to improve the formaldehyde is quite complex and, although initially stud-
bonding of rubber to tire cord; paper to improve the tear ied early in this century, is not completely understood at the
strength, especially of wet paper; and alkyds and acrylics to present time.4
improve their cure. Amino resins are also used for molding The synthesis of a urea-formaldehyde resin takes place in
products, such as electrical devices, jar caps, buttons, and two stages. In the first stage, urea is hydroxymethylolated
dinnerware, and in the production of countertops. by the addition of formaldehyde to the amino groups
The use of urea-formaldehyde resins as a major adhesive (Figure 1). This reaction is in reality a series of reactions
by the forest products industry is due to a number of that lead to the formation of mono-, di-, and trimethy-
advantages, including low cost, ease of use under a wide lolureas. Tetramethylolurea is apparently not produced, at
variety of curing conditions, low cure temperatures, water least not in a detectable quantity.
solubility, resistance to microorganisms and to abrasion, The addition of formaldehyde to urea takes place over the
hardness, excellent thermal properties, and lack of color, entire pH range (Figure 2). The reaction rate is dependent
especially of the cured resin. on the pH. The rate for the addition of formaldehyde to
The major disadvantage associated with urea- successively form one, two, and three methylol groups has
formaldehyde adhesives as compared with other thermoset- been estimated to be in the ratio of 9:3:1, respectively.4 The
ting wood adhesives, such as phenol-formaldehyde and exact ratio, of course, is dependent on the reaction condi-
polymeric diisocyanates, is the lack of resistance to moist tions employed in the addition reaction.
FIGURE 1. Formation of mono-, di-, and trimethylolurea by the addition of formaldehyde to urea. Tetramethylolurea has not been
observed experimentally.
8498 UREA-FORMALDEHYDE ADHESIVE RESINS
FIGURE 3. Condensation reactions of methylolureas to form (a) methylene bridges between amido nitrogens, (b) methylene ether
linkages, and (c) and (d) methylene linkages. Reactions of these types produce higher molecular weight oligomers and polymers.
UREA-FORMALDEHYDE ADHESIVE RESINS 8499
adhesive resins is carried out in two major steps. The first This issue originated in the mid-1970s as the increasing use
step consists of the formation of methylolureas by the of formaldehyde-emitting panel products in the more tightly
reaction of urea and formaldehyde under basic conditions constructed homes led to numerous complaints. This has
with a pH of ~8-9. This step is carried out under basic resulted in an increasing scrutiny of formaldehyde emission
conditions to allow the methylolation reactions to proceed levels from building products used within homes by state
in the absence of reactions involving the condensation of the and national regulatory agencies, including the Housing and
methylolureas. Urban Development (HUD) and the Environmental Protec-
In the second step, the reaction mixture is brought to the tion Agency (EPA), and the adoption by industry of volun-
acid side, with a pH of about 5, and the condensation tary standards for the emission of formaldehyde from wood
reactions are carried out until a desired viscosity is reached. products bonded with urea-formaldehyde adhesive resin,
Then, the reaction mixture is cooled and neutralized. Water most notably particleboard. Similar concerns in Europe led
is removed by vacuum distillation to give a resin with a to the well-known E-l standard limiting formaldehyde
desired solids content (typically about 60-65%). Urea is emissions from particleboard used in construction. In re-
often added in two, or sometimes more, steps. The initial sponse to consumer concerns and regulatory actions, the
addition of urea is made during the methylolation step, in particleboard, medium-density fiberboard, and hardwood
which the formaldehyde-to-urea (F/U) ratio is typically plywood industries have made major strides in reducing
large (~ 1.6-2). Usually, the second addition of urea is made formaldehyde emission levels from products bonded with
during the condensation step. The second and any subse- urea-formaldehyde adhesive resins (Figure 4).
quent additions of urea lower the final F/U ratio to the The evolution of formaldehyde from urea-formaldehyde
desired level. materials is incontrovertible. Over the past 40 years, inves-
These procedures for the synthesis of urea-formaldehyde tigators have examined extensively the structure of compo-
adhesive resins offer a wide range of conditions which make nents of urea-formaldehyde resin systems and the physical
possible the synthesis of resins with important properties chemistry of their formation and degradation in aqueous so-
such as tack, gel time, and spreadability for the uncured lutions. Classical kinetic, chromatographic, and NMR tech-
resin. Formaldehyde emissions and the durability of the niques have been applied.8-28 We can conclude from these
cured resin can be controlled and specifically tailored for the studies that the reactions leading to the formation of the
final end use of the resin. urea-formaldehyde products formed during urea-formalde-
An acidic-cure catalyst is added to the urea-formaldehyde
resin before it is used as an adhesive. Ammonium chloride
and ammonium sulfate are the most widely used catalysts
for resins used by the forest products industry. A variety of
other acids can be used as a catalyst, including formic acid,
boric acid, phosphoric acid, oxalic acid, and acid salts of
hexamethylenetetramine.
Resin cure is normally conducted at a temperature of
~120 °C and a pH < 5. The reactions that occur during the
final cure of the resin are thought to be similar to those that
occur during the acid condensation of the methylolureas.
The traditional viewpoint is that these reactions lead to the
formation of a crosslinked polymeric network for the
hardened, cured resin. However, there is evidence that a
colloidal phase also occurs during resin cure.5-7 This
evidence illustrates the lack of a full understanding of the
physical and chemical processes leading to the cure of
urea-formaldehyde resin systems and the need for contin-
ued research.
hyde resin synthesis and cure are reversible. In the forward Recent research has suggested possible new methods to
direction, water is eliminated; therefore, the reverse reac- lower formaldehyde emission levels. This research involves
tions can be viewed as hydrolysis, which leads to the release two strategies: the modification of the chemistry of urea-
of formaldehyde.29 Because most, if not all, of these reac- formaldehyde resins and the replacement of the formalde-
tions are catalyzed by acid, the use of an acid catalyst to hyde component in urea-formaldehyde resins with a less
hasten bond cure unfortunately also increases the rate of hy- volatile aldehyde or its chemical equivalent.
drolysis and formaldehyde liberation. Research involving the first strategy sought to chemically
The reduction in formaldehyde emission levels from modify urea-formaldehyde resins with polyamines (Figure
products bonded with urea-formaldehyde adhesive resins 5), primarily to improve durability and stability by decreas-
has been achieved by employing one or more of several ing the internal stresses developed during resin cure and
technological methods.30 In general, these methods include: increase the ability of the cured resin to withstand cyclic
l changing the formulation of the urea-formaldehyde stresses.32-37
adhesive resin (e.g., lowering the F/U ratio); Urea-formaldehyde resins were modified by:
l adding formaldehyde-scavenging materials directly to l incorporating the polyamines directly during resin
the urea-formaldehyde adhesive resin; synthesis;
l separately adding formaldehyde-scavenging materials l incorporating the polyamines in the form of urea-
to the wood finish, capped derivatives during resin synthesis;
l treating panels after their manufacture either with a l using the polyamine hydrochloride salts as cure cata-
formaldehyde scavenger or by the application of coat- lysts in place of ammonium chloride; and
ings or laminates; and l using a combination of the last two methods.
l changing to an entirely different adhesive resin system.
The direct addition of the free amines during resin cure
The most widely used approach for reducing formalde- produced uncurable resins. In general, modification with
hyde emission levels has emphasized decreasing the mole urea-capped amines or curing with amine hydrochlorides
ratio of F/U. Ratios of about 1.6 that were common 10 to 15 provided cure rates comparable with that of unmodified
years ago have now been reduced to values as low as 1.0 urea-formaldehyde resins cured with ammonium chloride.
and, in some cases, lower. Unfortunately, lowering the F/U These modifications also reduced the tendency of the resin
ratio produces resins with less tolerance for processing to crack and fracture and substantially improved the resis-
variations and panels that often have poorer physical and tance of bonded joints to the stress imposed by cyclic
structural properties31 As a consequence, some panel wet-dry exposures. Resins cured with the amine hydrochlo-
manufacturers use adhesive resins with a higher F/U ratio rides had less formaldehyde liberation than those cured with
and employ other methods to achieve the necessary reduc- ammonium chloride. These results indicate that the incor-
tion in formaldehyde emission levels. poration of flexible polyamines offers promise for improv-
FIGURE 5. Structures of di- and triamines used to modify urea-formaldehyde resins to increase the stability and durability of the cured
adhesive resin.
UREA-FORMALDEHYDE ADHESIVE RESINS 8501
CONCLUSION
Urea-formaldehyde resin is a major commercial adhe-
sive, especially within the forest products industry. It offers
a number of advantages when compared with other adhesive
systems. However, despite the fact that great strides have
been made to offset its major disadvantage by lowering the
formaldehyde emission levels of products bonded with
urea-formaldehyde adhesive resin, the industry still faces
the possibility of more restrictive regulations on formalde-
hyde in dwellings. Moreover, tighter restrictions on form-
aldehyde levels in the workplace are also likely. New
research efforts are needed to address this concern if
urea-formaldehyde adhesive resins are to maintain their
prominent position as a versatile adhesive system.
REFERENCES
POLYMERIC
MATERIALS
ENCYCLOPEDIA
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University of Massachusetts, Lowell
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