Synthesis and properties of polymeric materials prepared by polymerization of epoxy oligomers with N,N′-carbonyldiimidazole transformation products

Article abstract of DOI:10.1134/S1070427212030238

The reaction of N,N′-carbonyldiimidazole with ethanol and epoxy oligomers of diphenylolpropane diglycidyl ether and 4,4′- bis(glycidylamino)-3,3′-dichlorodiphenylmethane was studied by gas chromatography-mass spectrometry and differential scanning calorimetry. From these oligomers and N,N′-carbonyldiimidazole transformation products, epoxy polymers were synthesized, and their physicomechanical and adhesion properties were determined. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S1070427212030238

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2012, Vol. 85, No. 3, pp. 456−459. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © M.S. Fedoseev, V.A. Glushkov, A.A. Gorbunov, L.F. Derzhavinskaya, T.E. Oshchepkova, 2012, published in Zhurnal Prikladnoi
Khimii, 2012, Vol. 85, No. 3, pp. 485−489.
MACROMOLECULAR COMPOUNDS
AND POLYMERIC MATERIALS
Synthesis and Properties of Polymeric Materials Prepared
by Polymerization of Epoxy Oligomers
with N,N'-Carbonyldiimidazole Transformation Products
M. S. Fedoseev, V. A. Glushkov, A. A. Gorbunov, L. F. Derzhavinskaya,
and T. E. Oshchepkova
Institute of Technical Chemistry, Ural Branch, Russian Academy of Sciences, Perm, Russia
e-mail: msfedoseev@mail.ru
Received October 27, 2011
Abstract—The reaction of N,N'-carbonyldiimidazole with ethanol and epoxy oligomers of diphenylolpropane
diglycidyl ether and 4,4'-bis(glycidylamino)-3,3'-dichlorodiphenylmethane was studied by gas chromatography–
mass spectrometry and differential scanning calorimetry. From these oligomers and N,N'-carbonyldiimidazole
transformation products, epoxy polymers were synthesized, and their physicomechanical and adhesion properties
were determined.
DOI: 10.1134/S1070427212030238
Imidazoles are promising ionic curing agents for ep-
oxy oligomers, imparting to the resulting polymers high
adhesion, resistance to heat and water, and good electrical
and physicomechanical properties [1, 2].
place with boron trifluoride complexes. Therefore, it is
appropriate to develop latent curing agents of purely imid-
azole type for single-pack epoxy formulations containing
no foreign substances in the reaction system.
The polymerization rate of the oligomers is determined
by the rate of adduct formation with imidazoles, i.e., by
the nucleophilic power of the imidazoles. Latent curing
agents came into use, such as adducts of imidazoles
with isocyanates, lactones, anhydrides, or tetrabutylam-
monium bromide and products of reactions of imidazole
with acrylic acid and 1-(2-aminoethyl)piperazine [1, 3–5].
With these latent imidazole curing agents and also with
complexes of BF3 with Lewis bases, single-pack epoxy
formulations were developed, which allowed consider-
able prolongation of the spreadable life and control over
temperature and time conditions of curing [6]. However,
not all the latent curing agents ensure the required service
reliability of polymeric materials. For example, epoxy
polymers prepared in the presence of boron trifluoride
amine complexes have low heat resistance, which restricts
their wide use [7].
Here we report on polymerization of epoxy oligomers
of various structures under the action of N,N'-carbon-
yldiimidazole transformation products and on physico-
chemical and adhesion characteristics of the polymeric
compounds obtained.
EXPERIMENTAL
The following chemicals were used: N,N'-carbonyldi-
imidazole (CDI;AlfaAesar, the United Kingdom); ED-20
epoxy–4,4'-isopropylidenediphenol resin [GOST (State
Standard) 10587–84; weight fraction of epoxy groups
21.4% and of hydroxy groups 1.5%; dynamic viscos-
ity at 25°С 14 Pa s); ethanol (analytically pure grade);
4,4'-bis(glycidylamino)-3,3'-dichlorodiphenylmethane
oligomer (ESOD oligomer) synthesized by the reaction
of epichlorohydrin with 4,4'-diamino-3,3'-dichlorodi-
phenylmethane, followed by dehydrochlorination with
NaOH (weight fraction of epoxy groups 19.70%, dynamic
viscosity at 50°С 4.35 Pa s; weight fraction of NH groups
In thermal dissociation of latent imidazole curing
agents, the released products can negatively affect the
properties of the polymeric materials obtained, as it takes
456

SYNTHESIS AND PROPERTIES OF POLYMERIC MATERIALS
457
7.1%) [8]. Polymerization of epoxy oligomers with CDI
O
N
N
transformation products was studied by DSC with a DSC
822e calorimeter (Mettler–Toledo, Switzerland) in the
dynamic mode at a heating rate of 0.08 deg s–1 in the
temperature interval 25–300°С. From the thermograms,
we determined the reaction onset temperature Тon, peak
temperature Тp, and thermal effect of the reaction Q. The
effective activation energy Е was calculated by Kiss-
inger’s method [9]:
N
N
N
+ C2H5OH
OC2H5
O
N
N
NH
+
The kinetic data on polymerization of the epoxy
oligomers with N,N'-carbonyldiimidazole, obtained by
DSC, are well consistent with the data of gas chromatog-
raphy–mass spectrometry (GC–MS). The DSC patterns
of the kinetics of ED-20 curing with pure imidazole and
with imidazole released in the reaction of N,N'-carbonyl-
diimidazole with ethanol coincide in shape (Fig. 2). Two
peaks in the DSC curve characterize the known mecha-
nism of formation of imidazole adducts with epoxy resin
and of subsequent polymerization of the resin with the
adducts [2]. The limiting step of the process is the adduct
formation. The kinetic parameters of the polymerization
of the epoxy oligomers are given in Table 1. The polym-
erization onset temperature for the epoxy oligomers is
94–112°С, and the maximal thermal effect is observed at
105–135°С. In polymerization of ED-20 oligomer with
imidazole and CDI, it was 453 and 466 J g–1, respectively.
ESOD oligomer is polymerized by the above imidazoles
at higher temperatures but with a smaller thermal effect:
301 and 294 J g–1, respectively. The difference is associ-
ated with differences in the molecular structure of the
oligomers. In this case, the lower thermal effect of the
polymerization is a positive factor for reducing thermal
stresses and enhancing the crack resistance in fabrication
of massive articles from polymeric composite materials.
d[ln(β/Tp2)]/d(1/Tp) = –ER,
where β is the heating rate (deg min–1); Tp, peak tempera-
ture; and R, universal gas constant.
The glass transition point was also determined by
DSC.
The deformation and strength characteristics and adhe-
sion of the polymers and adhesive joints were determined
at 25 ± 2 and 150 ± 2°С with an Instron 3565 tensile-
testing machine (the United Kingdom).As adhesion sub-
strates we used carbon-reinforced plastic (ENFB binder,
LuP-0.1 tape, perpendicular reinforcement pattern) and
D16AT grade aluminum. Prior to gluing, the specimens
were cleaned with emery paper, degreased with acetone,
and dried for 20 min. The compounds applied as a thin
layer were kept at 60°С for 2 h and glued in molds, fol-
lowing the curing schedule for adhesive binders.
The rheological properties and spreadable life of
epoxy compounds after mixing were evaluated from
changes in the viscosity, monitored with a Rheotest-2
rotary viscometer (Germany) with a cone–plate working
unit at a constant shear rate of 180 s–1 and a temperature
of 25 ± 0.5°С.
Chemical transformations of N,N'-carbonyldiimid-
azole in the reaction with ethanol were studied with an
Agilent Technologies 6890N/5975B gas chromatograph–
mass spectrometer; HP-5ms column, 30 m × 0.25 mm,
0.025 μm, carrier gas He, electron impact ionization
(70 eV).
The chromatogram of an ethanol solution of N,N'-
carbonyldiimidazole at room temperature contains only
peaks of its transformation products, imidazole and N-
ethoxycarbonylimidazole, which were identified using
NIST08 library (Fig. 1). The imidazole amount found
corresponds to the CDI concentration in the solution.
Both products are readily soluble in ethanol:
τ, min
Fig. 1. Chromatogram of a solution of N,N'-carbonyldiimidazole
in ethanol. (I) Intensity and (τ) retention time. (1) Imidazole and
(2) N-ethoxycarbonylimidazole.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 3 2012

458
FEDOSEEV et al.
Table 1. Kinetic parameters of anionic polymerization of
epoxy oligomers with imidazoles
80–120–150°С within 4 h. The results obtained in study-
ing the polymerization kinetics and rheological properties
of the epoxy compounds allow these compounds to be
classed with single-pack polymer formulations with latent
curing systems.
Ton
Tp
Reaction system
Q, J g–1 E, kJ mol–1
°С
The polymeric materials prepared by polymerization
of widely used ED-20 epoxy resin (90 wt %) with N,N'-
carbonyldiimidazole transformation products (10 wt %)
exhibit fairly high physicomechanical characteristics:
cohesion tensile strength at 25°С 70 MPa, relative elonga-
tion 10%. The glass transition point is 115°С.
ED-20–imidazole
ED-20-CDI
94
97
105
110
120
453
466
301
89
102
95
ESOD oligomer–
imidazole
110
In practice, it is frequently necessary to glue polymeric
composite materials in units and parts of various struc-
tures operating at high temperatures.As a rule, high-soft-
ening-point adhesives are used for this purpose. Gluing
of aluminum and its alloys, glass textolites, ceramics, and
heat-insulation materials with high-softening-point epoxy
adhesives K-153, K-300, and K-400 has been reported.
The shear strength of the adhesive joint is 9–10 MPa at
20°С and 0.2–1.5 MPa at 200°С.
ESOD oligomer–
CDI
112
135
294
105
The data we obtained allow certain conclusions on
specific features of the polymerization of the epoxy
oligomers with N,N'-carbonyldiimidazole. In the first
step, the adduct formation of the released imidazole and
N-ethoxycarbonylimidazole with ED-20 resin occurs at
a lower rate compared to the adduct formation of pure
imidazole, taken separately, with the epoxy resin. This
feature positively affects the process of preparing adhe-
sive epoxy compounds; in particular, the spreadable life of
the reaction mixtures increases by a factor of 3 compared
to amine curing agents such as isophoronediamine and
by a factor of 1.5 compared to the most active imidazole
curing agent, 2-ethyl-4-methylimidazole (Fig. 3). The
second step of the polymerization and formation of
the structure and properties of the polymers occurs at
Tests of the epoxy compounds under consideration in
adhesive joints showed (Table 2) that the shear strength in
gluing aluminum and carbon-reinforced plastic with the
new adhesive based on commercial ED-20 epoxy resin
and N,N'-carbonyldiimidazole transformation products is
20–25 MPa at 20°С and 4.5–12 MPa at 150°С. High shear
strength of the adhesive joints was also attained with the
ESOD–N,N’-carbonyldiimidazole system. Enhancement
of the adhesion strength is ensured by the imidazole cur-
ing agent, which is incorporated in the polymeric matrix
η, Pa s
T, °C
τ, min
Fig. 3. Dynamic viscosity η of ED-20 epoxy–4,4'-isopropyl-
idenediphenol resin at 25°C with various curing agents as
a function of time τ. Curing agent: (1) isophoronediamine, (2)
2-ethyl-4-methylimidazole, and (3) N,N'-carbonyldiimidazole
transformation products.
Fig. 2. DSC curves of curing of ED-20 epoxy–4,4'-isopropyl-
idenediphenol resin with (1) imidazole and (2) N,N'-carbonyl-
diimidazole transformation products. (T) Temperature.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 85 No. 3 2012

SYNTHESIS AND PROPERTIES OF POLYMERIC MATERIALS
Table 2. Shear strength of adhesive joints
459
Adhesive joint strength, MPa
ESOD–CDI
Testing temperature, °C
ED-20–CDI
VK-9
Carbon-reinforced plastic–adhesive–carbon-reinforced plastic
25
21
20
–
–
Mixed break
Break within adhesive layer
150
6
14
Mixed break
Break within adhesive layer
Aluminum–adhesive–aluminum
25
26
12
14
Break within adhesive layer
Peeling of adhesive
150
16
12
4.5
Break within adhesive layer
Peeling of adhesive
in the course of polymerization and forms a polymer
with strong intermolecular interactions. The strength of
adhesive joints with VK-8 epoxy adhesive is given for
comparison.
reinforced plastic exhibit enhanced adhesion strength.
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Federation for Basic Research (project no. 11-03-96006-r
ural_a).
To sum up the data on the adhesion properties of com-
pounds based on epoxy oligomers and N,N'-carbonyldi-
imidazole transformation products, we should note their
advantages over commercial adhesives. Adhesive joints
based on ED-20 epoxy resin and CDI are characterized by
higher shear strength at both room and elevated tempera-
tures. The ESOD–CDI epoxy adhesive compound, which
can be classed with single-pack compounds with latent
polymerization system, has high softening point on cur-
ing and preserves high strength at elevated temperature.
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