New thermostable heterocyclic binders and environmentally friendly technologies of production of composite materials

Article abstract of DOI:10.1134/S1070363211050355

Synthesis and processing of high-strength and high-heat-resistant polymer matrices (binders) have been studied. Stru?tural composite materials stable up to 400°? have been developed. Processing of the binders is carried out by ecologically safe technologies. Pleiades Publishing, Ltd., 2011.

Full text of DOI:10.1134/S1070363211050355

ISSN 1070-3632, Russian Journal of General Chemistry, 2011, Vol. 81, No. 5, pp. 1025–1031. © Pleiades Publishing, Ltd., 2011.
Original Russian Text © R.R. Mukhametov, K.R. Akhmadieva, L.V. Chursova, 2010, published in Rossiiskii Khimicheskii Zhurnal, 2010, Vol. 54, No. 1,
pp. 57–62.
New Thermostable Heterocyclic Binders
and Environmentally Friendly Technologies of Production
of Composite Materials
R. R. Mukhametov, K. R. Akhmadieva, and L. V. Chursova
All-Russian Scientific Research Institute of Aviation Materials Federal State Unitary Enterprise,
State Research Center of the Russian Federation (RIAM), ul. Radio 17, Moscow, 105005 Russia
e-mail: admin@viam.ru
Received December 1, 2009
Abstract—Synthesis and processing of high-strength and high-heat-resistant polymer matrices (binders) have
been studied. Struсtural composite materials stable up to 400ºС have been developed. Processing of the binders
is carried out by ecologically safe technologies.
DOI: 10.1134/S1070363211050355
INTRODUCTION
There are two key problems involved in the
development of such binders: to choose the structure of
the polymer matrix and to choose the way of
processing the latter for manufacturing CMs. The
polymer matrix should not soften up to 450°С, which
can be reached by introducing aromatic and hetero-
cyclic fragments into the polymer chain, i.e. by
forming double-chain step structures in the chain of
macromolecules to restrict mobility of the latter and
enhance intermolecular interaction, crystallinity, etc.
At present the replacement of traditional metal
materials by polymer and composite materials (CMs)
is one of the basic indicators of industrial and scientific
technical progress. The development of high tech-
nologies generates demands for principally new
polymer and structural materials capable of preserving
their high processing and performance characteristics
under the action of various destructive factors. The
expanding application range of polymer materials is
accompanied by a strengthening of requirements to the
quality of materials, their mechanical and strength
parameters, and power efficiency and environmental
friendliness of their processing technologies [1].
One of the ways to produce thermoreactive binders
capable of forming high-strength and high-heat-
resistance matrices for PCMs with working
temperatures above 170°С is polycyclotrimerization of
monomers with two and more functional groups with
multiple bonds with homo- and heteroatoms. The
reaction forms a cross-linked three-dimensional struc-
ture with identical fragments between cross-links and
stable six-membered aromatic carbo- and heterocycles
as nodes of the polymer network.
Polymer composite materials (PCMs) are finding
increasing application in the designs of basic com-
ponents of aerotechnics. The use of PCMs in new-
generation aircrafts and engines makes it possible to
radically decrease their weight, and, therefore, enhance
the fuel efficiency of aircrafts and increase the useful
load. To solve problems associated with the develop-
ment of power heat-loaded elements of gas turbine
engines (GTEs) and supersonic aircraft airframes,
materials with stable performance characteristics in
oxidative media up to 400°С are required. To this end,
unique polymer matrices combining high physico-
mechanical characteristics with maximum possible
heat resistance should be created.
Development of binders capable of forming not
only a high-strength and high-heat-resistance matrix,
but also ensuring realization of these properties in
PCMs, calls for a nontraditional approach in view of
the necessity to combine such mutually contradictive
characteristics of the polymer matrix as strength,
stiffness, heat resistance, plasticity and a high impact
fracture toughness.
1
025

1
026
MUKHAMETOV et al.
It was found that the reaction at 250°С for 5 h gives
Heterocyclic Binders for Thermostable PCMs
Composite materials with working temperatures of
compound I with a yield of no less than 80%. The
mass spectrum of compound I contains a molecular ion
peak at m/e 246. The IR spectrum of the product
up to 400°С are produced by the reactive molding
technology. The molding process involves the intra-
molecular isomerization polycyclization reaction
without evolution of low-molecular volatile products,
which occurs in the preliminarily synthesized low-
melting soluble oligomer (binder) on thermal treatment
directly on the filler. The cyclizations forms a
monolithic, high-strength, and high-heat-resistance
polymer matrix of a step macroheterocyclic structure.
Step polymers possess the highest, among organic
polymers, thermooxidative stability and deformative
thermostability [2]. Thus, the task of creation of a
polymer matrix stable at temperatures of up to 400°С
comes down to development of a method of synthesis
ща polyfunctional oligomers (binders) whose high-
temperature transformations form high strength and
high heat resistance macroheterocyclic structures of
the step type.
–
1
chows absorption bands at 3200 (NH) and 1627 cm
–
1
(
С=N), as well as a С≡N absorption band at 2233 cm .
The spectral data provide evidence for the suggested
structure of compound I. Further heating results in
isothermal cyclizations of product I to form isomer II
containing condensed isoindole and quinazoline rings.
It was shown that the polycondensation in melt of
stoichiometric amounts of tetrafunctional monomers:
3,3'-diamino-4,4'-dicyanodiphenylmethane and 4,4'-
(m-phenylenedioxy)diphthalonitrile forms an oligomer
(binder). The structure of its two possible isomers was
confirmed by elemental analysis, IR spectroscopy, and
thin-layer chromatography [4, 5]:
CH2
N
C
The collaborative research of the RIAM and
Institute of Organoelement Compounds of the Russian
Academy of Sciences (IOEC) showed that unique and
the most promising polymer matrices for PCMs with
working temperatures of up to 400°С are systems on
the basis of tetranils derived from aromatic tetra-
carboxylic acids and bis(о-cyanamines) [3]. In view of
the chemical structure of tetrafunctional monomers, it
was suggested the half-step macroheterocyclic struc-
ture formed by their reaction would contain condensed
isoindole and quinazoline rings conjugated with aro-
matic fragments.
CN
O
O
O
O
NH
C
NH
NH
C
N
C
CH2
NH NC
After grinding on a ball mill and acetone extraction
of admixtures and unreacted monomers, the resulting
oligomer is a fine cream-colored power readily soluble
in amide solvents and N-methylpyrrolidone. At 100–
To check this suggestion, a model reaction of
anthranitrile with phthalodinitrile in melt at a stoi-
chiometric reagent ratio was studied (Scheme 1).
1
30°С the oligomer forms a low-viscous mobile melt
whose formation occurs at 200°С within 15–25 min.
This binder can be applied on fillers from solutions,
melts, or by electrostatic sputtering to obtain prepregs
with almost indefinite pot lives. Intermediate products
and CMs on their basis were produced by a safe and
waste-free technology.
Scheme 1.
HN
C
NH NC
CN NC
+
C
N
New Thermostabilizers
CN NC
The service of organic polymeric matrices in an
oxidative medium at 400°С, i.e. under a very severe
exposure, is impossible without high-temperature
thermostabilizers (antioxidants). Research on the
thermo-oxidative stability of polymer matrices in the
presence of common antioxidants (sterically hindered
phenols, amines, phosphites, metal carbonyls, etc.)
I
HN
NH
C
N
C
C
N
II
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NEW THERMOSTABLE HETEROCYCLIC BINDERS
1027
showed that, irrespective of the chemical nature of the
antioxidant, stabilization is largely observed in the
temperature range 200–250°С. Raising the temperature
of isothermal testing to 350–400°С offsets completely
the contribution of antioxidants into the thermo-
oxidative destruction of binders.
groups during structuring of the oligomer. The spectra
provide evidence for the occurrence of intramolecular
isomerization polycyclization at the final stage of
formation of the polymer matrix. The binder curing
process was also studied by differential scanning
calori-metry (DSC) and IR spectroscopy to establish
the temperature and time parameters for the attainment
of a virtually 100% conversion of the functional groups.
The problem to stabilize organic polymers at 350–
4
00°С has raised demand for the development of
special compounds capable of building-in into the
polymer chain during binder synthesis. The colla-
borative research of the RIAM and IOEC resulted in
the development of high-temperature antioxidants:
thermostabilizers on the basis of о-carborane, speci-
fically 1,2-bis(cyanoethyl)-o-carborane:
The optimal curing regime was established by
studying the dependence of glass transition point on
temperature and of the physicochemical properties of
the matrix on thermal treatment conditions. The forma-
tion of the polymer matrix with high physico-
mechanical characteristics is almost complete after
thermal treatment at 200–250°С. Further heating at
N≡C−CH −CH −C
C−CH −CH −C≡N
2 2
2
2
3
00–400°С is needed to complete formation of the
polymer matrix and to reach the maximum possible for
this type of binders resistance to heat and thermal
oxidation.
B H
10
1
0
The compound builds-in into the main polymer
chain during binder synthesis and fulfills the role of
termostabilizer in the polymer matrix. The effect of
closo-12-carboranes is explained by the active reaction
of the icosahedron ВН groups with air oxygen and
boron-centered radicals with free radicals formed due
to thermo-oxidative destruction of the main polymer
chain. This prevents degenerative chain branching,
realizing in part the “non-chain” inhibition principle,
specifically, deactivation of compounds involved in
any reactions leading to polymer destruction. The
introduction into monomers at the stage of binder
synthesis of 10–20 wt % of 1,2-bis(cyanoethyl)-o-car-
borane allowed preparation of a thermostabilized
carborane-containing polymer matrix with a covalently
bound antioxidant. Thus stabilizer is easier to intro-
duce into polymer, an ideal molecular distribution is
reached, and antioxidant losses due to diffusion and
evaporation are excluded.
It should be noted that the polymer matrix
unmelting and insoluble) after thermal treatment is
(
formally not a cross-linked 3D polymer. The properties
characteristic of a cured thermoreactive binder sre
formed during formation of tightly packed super-rigid
half-step structure of regularly alternating aromatic and
heterocyclic fragments with a strong intermolecular
interaction. Such polymers exhibit quite high heat
resistance, nearly maximum possible for organic
polymers.
Effect of Curing conditions on the Properties
of Composite Materials
The fact that the developed binders are suitable for
manufacturing on their basis heat-loaded PCMs was
established by the results of thermomechanical
analysis (ТМА) of cured polymer matrices. Such tests
allow us to determine a real working temperature range
for structural PCMs. The ТМА was used to measure
the dependence of the glass transition point of the
binder cured at 200, 250, and 350°С (Fig. 1).
Structure of Stabilizers and Thermal Stability
of Composite Materials
The presence of free–С≡N groups imparts to the
oligomer the ability to undergo intramolecular poly-
cyclization on heating to form a half-step macrohetero-
cyclic structure containing condensed isoindole and
quinazoline rings. The binder curing process was
followed by IR spectroscopy by the decrease of the
relative intensity of the–С≡N absorption band over the
course of thermal treatment. On heating in the range
It was found that curing at 160–350°С leads to a
rigid macroheterocyclic half-step structure whose
transition from the rigid to highly elastic state is not
seen in the ТМА curves up to 550–600°С, when
thermal oxidation of the polymer commences. Under
ТМА conditions, no α-transitions, viz. the deformation
of the polymer matrix, associated with the segmental
mobility of macromolecules on polymer devitrifica-
tion, are not detected. Such behavior is characteristic
–1
1
70–400°С, the –С≡N bands at 2213 and 2233 cm
are decreasing, implying consumption of the –С≡N
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1
028
MUKHAMETOV et al.
1
00
Heated setting rollers
8
6
4
2
0
0
0
0
0
2
00°C
50°C
2
Drawing drum
350°C
Fusion
cage
1
00 200 300 400 500 600 700
Temperature, °C
Fig. 1. Thermomechanical curve of a polymer matrix cured
at various temperatures.
Feeding drum
of rigid-chain fully aromatic step polymers whose
mechanical loading induces no other deformations but
an elastic (Hookean) deformation associated with
changes in the average interatomic and intermolecular
distances, as well as valence angles in the
macromolecule. The lack in the developed polymer
matrix, up to 600°С, of a reversible highly elastic and
an irreversible viscoelastic deformation points to an
extremely high deformation heat resistance, addi-
tionally contributed by the thermostabilizer.
Alignment cage
Compressed air
Fig. 2. Scheme of the electrostatic powder spraying system.
The weight loss initiation temperature of thermo-
stabilized polymer matrices in an oxidative medium is
drum, alignment cage, fusion cage, heated setting
rollers, and a drawing drum (Fig. 2).
5
00–600°С. The optimal quantity of stabilizers, as
found by the DTG analysis of cured samples in air, is
0–20 wt %. If the fraction of antioxidants is higher
An environmentally friendly melt powder
techology for applying low-melting binders on fillers
to produce prepregs with an almost indefinite pot life
was developed. Since binders are cured by the
mechanism of multistage step isomerization poly-
cyclization without evolution of volatile products, a
simplified molding process can be used, which
involves molding at temperatures of up to 200°С in
tooling, followed by thermal treatment in the free state.
1
than 20 wt %, the degradation initiation temperature is
sometimes increases by 20°–30°, but the physico-
chemical properties of the polymer are much
deteriorated because of essential impact of the
thermostabilizing system on the binder curing process
and, as a consequence, violation of the optimal regime
of formation of the polymer matrix. The long-term
thermostability of cured polymer matrices was studied
under isothermal aging conditions in an oxidative
medium for 50 h at 400°С and for 10 h at 450°С,
which is quite a severe test for any organic polymers.
The binder samples comprising the carborane-
containing antioxidant showed a 20% weight loss at
Network Polymers on the Basis of Dicyanate Esters
for High-Strength Heat-Resistant Polymer
Composites
Epoxy oligomers are most frequently thermo-
reactive polymers used as binders for high-strength
composites with working temperatures of up to 170°С.
However, the chemical structure of network polymers
оn the basis of epoxy oligomers does not allow to raise
substantially the working temperature of polymer
composites, thereby preserving their high mechanical
properties. The chemical structures of epoxy oligomers
4
50°С, whereas those containing no antioxidant com-
pletely burned out under these conditions.
Prepregs on the basis of the thermostable hetero-
cyclic binder are produced by electrostatic powder
sputtering on a moving carbon tape [6]. The in-
stallation for prepreg production comprises a feeding
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 81 No. 5 2011

NEW THERMOSTABLE HETEROCYCLIC BINDERS
1029
Scheme 2.
N≡C−O
X
O−C≡N
Monomer
T
N≡C−O
O−C≡N
X
X
N
N
O
O
O
O
O
C
N
C
N
C
N
C
N
O
O
C
O
C
O
N
HC
N
CH
N
R
N
R
T
O
C
N
C
N
C
O
C
O
R
O
C
X
N
C
N
C
O
O
N
O−C≡N
Oligomer
Polymer
and their hardeners are quite diverse, but the thermal
characteristics of network polymers on their basis are
defined by the presence in the starting oligomers of
high concentrations of α-oxide rings, since the
aliphatic and heterochain fragments formed in the
polymers by α-oxide ring opening are fairly thermally
unstable.
thermostable aromatic S-triazine structures as polymer
network cross-links [7]. Such requirements are largely
met by the polycyclotrimerization of aryl dicyanates,
specifically dicyanate esters of bisphenols of various
chemical structure. The reaction involves no evolution
of volatile products and features a high selective
cyclotrimerization of N≡С–О groups and their almost
complete conversion (Scheme 2).
To enhance the heat resistance of network polymers,
systems whose intercross-links are formed by rigid
aromatic, heterocyclic, or organoelement radicals,
whereas thermally unstable groups are almost or
completely lacking.
The polycyclotrimerization of the diacyanate ester
of bisphenol А is performed in the presence of minor
amounts (about 1%) of ionic catalysts. The resulting
regular narrow-meshed network polycyanurates [poly
(
1,3,5-triazines) exhibit a complex of valuable
One of the most promising synthetic approaches to
a new generation of polymer matrices for PCMs with
working temperatures above 170°С is a poly-
cyclotrimerization reaction of monomers containing
two or more functional groups with multiple bonds
between homo- and heteroatoms, which leads
eventually to poly(1,3,5-triazine) structures with
properties: thermal and chemical stability, high di-
electric and adhesion parameters, resistance to ionizing
radiation, low gas permeability, and other properties
which radically distinguish them from other cross-
linked polymer systems. The cyclotrimerization re-
action can be stopped at the stage of formation of a
low-melting soluble resin-like oligomer whose
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1
030
MUKHAMETOV et al.
Polyimide matrix
Comparative characteristics of polymer matrices
Polycyanurate matrix
ВСТ-1210b
HexPly 954-3A
Epoxy
Binder IP-5a
Parameter
matrix
PMR-15
bismaleimides
Density, g cm–3
Tensile strength σ , MPa
1.23–1.30
60–120
1.32
48–56
3.9–4.5
1.4
1.25–1.35
40–80
1.23
80–90
3.17
3.2
1.19
56
1.24
85
+
+
Tensile elastic modulus Е , GPa
Relative elongation ε, %
Crack resistance G1С, J m–2
2.6–3.6
4.2–4.9
1.5–2.5
34–260
250–350
3.3
1.9
170
197
4.5
2.8–5.0
2.3
130–150
100–200
230–280
330–370
140
400
Glass transition temperature, °С
240
400–450
a
b
Heterocyclic binder developed at the RIAM, Technical Specifications 1-595-12-881-2005. Polycyanurate binder developed at the RIAM,
Technical Specifications 1-595-12-1042-2008.
solution or melt is a binder which can be processed by
different methods.
therewith, the maximum acceleration is already
observed at a 10% conversion of N≡С–О groups.
Starting from 2,2-bis(4-cyanatophenyl)propane, a
poly(1,3,5-triazine)-containing binder was synthesized
for further production of an oligocyanurate meeting
engineering requirements to binders suitable for
processing by the resin transfer molding (RTM)
technology. The developed synthesis gives the product
as a low-viscosity resin-like material readily soluble in
ketones, esters, alcohols, and aromatic hydrocarbons.
The cyclotrimerization of aryl dicyanate was
performed in melt at 140–150°С under vigorous
stirring. The IR spectra of samples taken from the
reaction mixture showed gradually decreasing N≡С–О
–
1
stretching vibration bands at 2236–2272 cm ;
therewith, strong bands characteristic of the S-triazine
–
1
ring appear at 1570 and 1370 cm . The stretching
absorption of the ether С–О–С stretching absorption
–
1
band appears at 1235–1160 cm . The resulting
evidence allowed us to assess the dependence of the
conversion of aryl dicyanate N≡С–О groups on the
time of isothermal oligocyclotrimerization (Fig. 3).
The binder is cured as a result of further
cyclotrimerization of N≡С groups and followed by IR
spectroscopy by the decreasing N≡С–О stretching
–
1
vibration bands at 2236–2272 cm and increasing
–
1
absorption bands at 1570 and 1370 cm , characteristic
of stretching vibrations of the 1,3,5-triazine ring. The
N≡С–О band disappears almost completely, when the
samples are heated at 200°С, and the yield of the gel
fraction is 98–100%. It should be noted that the binder
is cured without release of any low-molecular reaction
products, which makes it possible to obtain monolithic
matrix and PCMs on its basis and simplify the
technology of composite molding at 160–180°С in a
tooling, followed by thermal treatment at 200–220°С
in the free state. Post-curing of the material at 200°С
remains almost unaffected its deformation heat
resistance, but it is necessary to rearrange the supra-
molecular structure of the matrix, i.e. to realize
transition from the globular structure to fibril-like
formations favoring relaxation processes and relieving
internal strain [8].
The kinetic curve is evidently S-shaped, implying
an autocatalytic reaction which is strongly accelerated
with increasing concentration of aryl dicyanate
cyclotrimerization products in the reaction mixture;
5
4
3
2
1
0
0
0
0
0
0
1
2
3
4
5
6
7
The curing of the polycyanurate binder involves no
release of any low-molecular reaction products, which
allows this binder to be processed by the RTM
technology to obtain monolithic matrix and PCMs on
Time, h
Fig. 3. Dependence of oligomer (binder) N≡С–О group
conversion on heating time.
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NEW THERMOSTABLE HETEROCYCLIC BINDERS
1031
its basis. The PCMs produced by the RTM technology
from a polycyanurate network polymer are suitable for
manufacturing various components of aircraft
equipment with working temperatures of up to 200°С.
fact that binders are cured by the mechanism of
multistage step isomerization polycyclization without
release of volatile products, a simplified molding
process, including molding at temperatures of up to
2
00°С in a tooling and subsequent thermal treatment in
Optimization of the technologies of synthesis and
curing of thermostable binders opened the way to
unique polymer matrices combining the physico-
mechanical characteristics of the best epoxy matrices
with the deformation heat resistance and having no
analogs among organic polymer matrices (see table).
the free state.
A method of synthesis of a thermoreactive poly
(1,3,5-triazine) binder for high-strength and heat
resistant composite materials with working tempera-
tures of up to 200°С is developed.
CONCLUSIONS
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thermo-stable half-step fmacroheterocyclic structures
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