Synthesis of single-component urethane sealants

Article abstract of DOI:10.1023/A:1012725924899

Single-component urethane sealing formulations with aldimines as cross-linking agents were developed. The temperature dependence of the curing time of the sealants was examined. The influence of the -CH=N-/-NCO ratio and of fillers (industrial carbon and chalk) on their physicochemical properties was studied.

Full text of DOI:10.1023/A:1012725924899

Russian Journal of Applied Chemistry, Vol. 74, No. 5, 2001, pp. 860 863. Translated from Zhurnal Prikladnoi Khimii, Vol. 74, No. 5,
2001, pp. 833 837.
Original Russian Text Copyright
2001 by Elchueva, Aristova, Bortnikov, Tabachkov.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
Synthesis of Single-Component Urethane Sealants
A. D. Elchueva, N. V. Aristova, I. V. Bortnikov, and A. A. Tabachkov
Kazan State Technological University, Kazan, Tatarstan, Russia
Received October 5, 2000; in final form, February 2001
Abstract Single-component urethane sealing formulations with aldimines as cross-linking agents were
developed. The temperature dependence of the curing time of the sealants was examined. The influence of
the CH=N / NCO ratio and of fillers (industrial carbon and chalk) on their physicochemical properties was
studied.
Polyurethane elastomers are widely used in various
branches of industry. With progress of studies in the
field of development of single-component formula-
tions, their performance is improved and the applica-
tion field is expanded. The procedures used in indus-
try for curing of polyurethane sealants have significant
drawbacks. All sealants of this type are two-compo-
nent; they are not quite convenient in service and re-
quire performing certain manipulations directly before
use. Their curing is effected with diamines and gly-
cols. Curing of a single-component urethane sealant
based on an isocyanate-containing prepolymer results
in formation of defects (blisters, cavities) due to re-
lease of carbon dioxide on contact of isocyanate
groups with atmospheric moisture. The goal of this
work was to develop single-component urethane for-
mulations with aldimines as cross-linking agents.
Aldimines, when reacting with atmospheric moisture,
release active curing agents which react with isocya-
properties of composites based on various diisocya-
nates showed [1] that strong intermolecular interaction
in polymers results in increased modulus of elasticity
and tensile strength. The rigid segments in the chain
(bulky aromatic diisocyanates and aromatic amines)
also enhance the cohesion strength. Flexible groups
(e.g., aliphatic amines) contribute to the elastic prop-
erties of the elastomers.
Thus, polyurethanes can be considered as block
copolymers [2] with flexible polyether or polyester
segments and rigid segments formed by urethane or
urea fragments. Flexible segments increase the elastic-
ity and relative elongation at break, whereas rigid
segments with enhanced intermolecular interactions
increase the hardness, tensile strength, melting point,
and glass transition point.
The structure of the three-dimensional polyurethane
network is determined by the synthesis conditions, in
particular, by the preparation procedure (single-stage
or two-stage). When the polymer is prepared by a
two-stage procedure via a prepolymer, formation of a
more regular network can be expected. Elastomers
prepared via prepolymer [3] exhibit higher cohesion
strength but lower values of the adhesion strength and
modulus of elasticity and are less elastic.
nate groups without releasing CO , so that formation
2
of defects in the cured sealant can be avoided.
Urethane elastomers show considerable promise for
practice. Polyurethanes are prepared from compounds
containing highly reactive isocyanate groups. Their
transformations yield polymeric structures with di-
verse types of chemical bonds and allow preparation,
within the same class of polyurethanes, of materials of
widely varying properties.
In a two-stage procedure for production of poly-
urethanes the prepolymers are usually highly viscous,
and special equipment is required for their mixing and
feeding to reaction vessels without air access. The
properties of the final product can largely depend on
the accuracy of temperature control and on the storage
time and stability of the prepolymer.
An important factor determining the physicome-
chanical properties of polyurethanes, especially at
elevated temperatures, is the nature of cross-links in
the three-dimensional polymer network, depending on
the curing agent used.
The advantage of the single-stage preparation of
polyurethanes is the stability of the reaction mixture.
With aromatic diamines, an additional advantage is
Studies of the effect of diamines and water as
curing agents in single-component systems on the
1070-4272/01/7405-0860$25.00 2001 MAIK Nauka/Interperiodica

SYNTHESIS OF SINGLE-COMPONENT URETHANE SEALANTS
the possibility of dosing these relatively high-melting Initial substances for synthesis of aldimines
861
crystalline substances using standard dosing units.
The major problem, however, is to balance the reac-
tivities of the backbone polyol and curing agent to
attain the required degree of cross-linking. If both
components are diols, it is not difficult, but with
amine cross-linking agents it is necessary to add a
catalyst accelerating the reaction of the hydroxide
with the isocyanate. The advantages of the single-
stage procedure are also simpler process and wider
range of suitable raw materials.
Diamine
Aldehyde
mp, C
Ethylenediamine
Benzaldehyde
Nitrobenzaldehyde
Furfural
Benzaldehyde
Nitrobenzaldehyde
Furfural
Benzaldehyde
Nitrobenzaldehyde
Furfural
42 44
145
51 52
(l)
86 88
43
110 113
85 90
(l)
Hexamethylenediamine
Phenylenediamine
Analysis of the present state of production of
single-component polyurethane elastomers [4] reveals
the following trends: improvement of the curing sys-
tem and development of optimal formulations.
Diaminodiphenylmethane Benzaldehyde
Nitrobenzaldehyde
Diaminodiphenylmethane Furfural
121
172
(l)
The goal of this study was preparation of single-
component urethane formulations with azomethines
(Schiff bases) as cross-linking agents. These systems,
reacting with atmospheric moisture without release of
carbon dioxide, facilitate reaction of the polymer
backbone chain with isocyanate groups.
1
2
3
where R , R = H, Alk, Ar; R = Alk, Ar, OH, OR,
NHR, Hal.
Aldehydes and dialkyl ketones smoothly condense
with primary amines to give the corresponding Schiff
bases, whereas the reactivity of aryl ketones is rela-
tively low, and in this case longer reaction time and
Brønsted acid catalysis are required.
EXPERIMENTAL
Hydrolysis of azomethines to the initial amines and
carbonyl compounds is an example of the nucleophilic
attack at the C=N bond. This two-stage process in-
volves addition of water molecules to the C=N bond
to form a carbinolamine intermediate which subse-
quently decomposes into the amine and carbonyl
compound. The rates of azomethine hydrolysis and
formation are pH-dependent.
In our study we used toluene 2,4-diisocyanate
[TDI, TU (Technical Specifications) 6.03331 79),
Laprol-5003 polyether (TU 6-05-1513 87), P-803
industrial carbon [GOST (State Standard) 7885 77),
and chalk (GOST 842 52). The deformation and
strength properties were determined according to
GOST 21751 76, and the Shore hardness, according
to GOST 263 75.
Cycloaddition of Schiff bases to simple isocya-
nates VI is preceded by reversible formation of zwit-
ter-ionic intermediate VII [Eq. (2)] which in some
cases can be isolated. The further transformations are
determined by the stability of the zwitter-ionic inter-
mediate (i.e., mainly by the steric and electronic ef-
fects of substituents in the azomethine molecule) and
by the reaction conditions (kinetic or thermodynamic
control). On the whole, when the benzylidene moiety
of Schiff base V contains electron-donor para substit-
uents and the reaction is performed at low tempera-
tures (i.e., under conditions of kinetic control), direct
ring closure in the zwitter-ion to give [2+2]-cyclo-
adduct VIII is favored:
Aldimines were prepared by reaction of an alde-
hyde with a diimine in a 2 : 1 molar ratio (see table).
Schiff bases are prepared [5] by condensation of
aldehydes and ketones I with various amino com-
pounds II, resulting in elimination of water and for-
mation of the corresponding azomethines III. The
kinetic data suggest a two-stage mechanism [Eq. (1)]
of formation (and reverse hydrolysis) of azomethines,
involving formation of a tetrahedral intermediate,
carbinolamine IV, which in some cases can be iso-
lated (e.g., reaction of chloral with hydroxylamine) or
1
detected in solution by H NMR spectroscopy:
_
O
R1
R2
R1
H
+ R3 NH2
Ar2
H
C=O
C
N +
R3
Ar1
R
+
Ar1
C H NPh
H
I
II
.
H
NPh
C
N
N
(1)
+
C
_-
OH
O
Ar2N
C
O
R1
R2
R1
R2
Ar2N
O
H
R3
C=NR3 + H2O
III
,
C
IV
N
(2)
VIII
VII
Ph
VI
V
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 5 2001

862
ELCHUEVA et al.
in particular, occurs on a support:
T, C
R CH=N R" N=CH R + H2O
+ H2N R" NH2.
2R CHO
(3)
The released diamine reacts with the OCN groups
of the prepolymer to give a three-dimensionally cross-
linked polymer.
Curing of sealants depends on several factors, in
particular, on the temperature and structure of substit-
uent R in the aldehyde molecule (Fig. 1). With in-
creasing temperature, the time of sealant curing de-
creases. Curing is also accelerated when electron-
withdrawing substituents such as NO , Br, I, or OCH
, h
Fig. 1. Correlation between the temperature T and time
of sealant curing with aldimines based on (1) benzaldehyde
and (2) nitrobenzaldehyde and furfural.
2
3
are introduced into the para position of benzaldehyde.
These substituents decrease the electron density on
the electrophilic center ( C =N carbon atom) and
+
E, % Gt, MPa
H, arb. units
enhance its affinity for water, accelerating the hydrol-
ysis. The amino groups released in the course of
hydrolysis react with the isocyanate group even at
0 25 C; as a result, the sealant curing accelerates.
1b
Figure 2 shows the dependences of the physicome-
chanical properties of sealants on the ratio of the reac-
tive groups NCO/ CH=N in the oligomeric system.
As this ratio is increased, the strength and hardness
of the urethane elastomers increase. This is due to the
increased degree of cross-linking. The increase in the
NCO/ CH=N ratio to 0.5 has a weak effect on the
properties. However, at further increase the strength
of the sealants is enhanced, reaching a maximum at
the NCO/ CH=N ratio equal to 0.8 0.85. At the
NCO/ CH=N ratio higher than unity the physico-
mechanical parameters start to decrease.
2b
1c
2c
Fig. 2. Influence of the CH=N / NCO ratio on the phys-
icomechanical properties of sealants containing 60 wt parts
of industrial carbon, cured with (1a 1c) aromatic and
(2a 2c) aliphatic imines: (1a, 2a) relative elongation E,
The deformation and strength characteristics of
sealants are influenced by the structure of the amine
moiety in aldimines. For example, the selanats cured
with aromatic imines considerably surpass in strength
those cured with aliphatic analogs, owing to incor-
poration of aromatic ring into the polymer structure.
(1b, 2b) nominal tensile strength G , and (1c, 2c) Shore
t
hardness H; the same for Fig. 3.
It was suggested to use Schiff bases for preparing
composites from polyisocyanate and aldimine for
coatings stable in storage [6, 7]; also, Schiff bases can
be used as latent hardeners for low-temperature curing
of epoxy oligomers [8].
Unfilled sealants are used seldom because of their
poor physicomechanical parameters and high cost.
To improve the properties and reduce the cost, various
fillers are added. In this work we used active (industri-
al carbon) and neutral (chalk) fillers. Figure 3b shows
the dependences of the main physicomechanical pa-
rameters of the sealants on the content of industrial
carbon and chalk. These data show that, irrespective
of the imine type and component ratio, the parameter
values as functions of the filler content pass through
maxima. The nominal tensile strength and the relative
elongation vary in parallel with varying filler content.
However, studies by Arbizov and his disciples [9]
showed that Schiff bases react with isocyanates only
at 100 160 C. Therefore, preparation of cold-curable
sealants using the reactions of Schiff base formation is
practically impossible. We have studied the hydrolysis
of Schiff bases as sealant components.
It is known [10] that aldimines IX in the presence
of atmospheric moisture are hydrolyzed to the initial
compounds: aldehydes and diamines. This reaction,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 5 2001

SYNTHESIS OF SINGLE-COMPONENT URETHANE SEALANTS
863
(a)
E, %
Gt, MPa
H, arb. units
E, %
Gt, MPa
(b)
H, arb. units
1b
1b
2b
1c
1c
2c
2c
2b
C2, wt parts
C1, wt parts
Fig. 3. Influence of the content of (a) P-803 industrial carbon C and (b) chalk C on the properties of sealants cured with
1
2
(1a 1c) aromatic and (2a 2c) aliphatic imines.
The hardness of the composites increases monotoni-
cally. The physicomechanical properties of polyure-
thane sealants filled with industrial carbon are consid-
erably better than those of the sealants filled with
chalk. However, at high filling (more than 60 wt parts
of industrial carbon or more than 90 wt parts of chalk
per 100 wt parts of oligomer) the curing becomes
complicated because of hindered diffusion. Highly ac-
tive industrial carbon should be introduced in smaller
amounts than chalk; the optimal dosage is 40 wt parts.
This filler strongly affects the efficiency of macro-
molecule binding and formation of the physical net-
work comprising from 60 to 80% of the total content
of bonds in the polymer. Replacement of industrial
carbon by neutral chalk at the same filler content
(60 wt parts per 100 wt parts of prepolymer) decreases
the physicomechanical properties of sealants.
through maxima at the following filler content: indus-
trial carbon 60 wt parts and chalk 90 wt parts.
REFERENCES
1. Lipatov, Yu.S., Kercha, Yu.Yu., and Sergeeva, L.M.,
Struktura i svoistva poliuretanov (Structure and Prop-
erties of Polyurethanes), Kiev: Naukova Dumka, 1970.
2. Saunders, J.H., J. IRI, 1968, vol. 2, pp. 21 22.
3. Rausch, R. and Sayigh, A., Ing. Eng. Chem., 1965,
vol. 57, pp. 92 94.
4. Tematicheskii obzor TsNIITEneftekhim. Seriya: Pro-
myshlennost’ sinteticheskogo kauchuka. Proizvodstvo
uretanovykh elastomerov v SShA (Topical Review of
the Central Research Inst. of Technical and Economi-
cal Studies in Petrochemical Industry, Ser.: Chemical
Rubber Industry. Production of Urethane Elastomers
in the United States), 1979.
5. Comprehensive Organic Chemistry. The Synthesis and
Reactions of Organic Compounds, Barton, D. and Ol-
lis, W.D., Eds., vol. 2: Nitrogen Compounds, Oxford:
Pergamon, 1979.
CONCLUSIONS
(1) Single-component urethane sealants with ald-
imines as cross-linking agents were developed.
6. US Patent 5569706.
7. US Patent 5523376.
8. Blagonravova, A.A. and Nepomnyashchii, A.I., Lako-
vye epoksidnye smoly (Varnish Epoxy Resins), Mos-
cow: Khimiya, 1970.
9. Zobova, N.N., Rusanov, G.N., and Arbuzov, B.A.,
Izv. Akad. Nauk SSSR, Ser. Khim., 1972, no. 2,
pp. 2016 2020.
(2) The effect of temperature on the curing time
of sealants was studied. With increasing temperature
from 20 to 80 C, the curing time decreases from
12 15 to 2 4 h.
(3) The ratio of the CH=N and NCO groups
affects the physicomechanical properties of the seal-
ants, with its optimal value being 0.8 0.85.
(4) The properties of sealants were studied in rela-
tion to the filler content. The strength parameters pass
10. Sorokin, M.F., Onosova, L.A., and Tarasov, A.V.,
Zh. Prikl. Khim., 1986, vol. 2, no. 8, pp. 17 21.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 74 No. 5 2001