STRUCTURE AND PROPERTIES
OF ORGANOSILICATE COMPOSITES
FOR HIGH-TEMPERATURE RESISTANT COATINGS
S. V. Chuppina, V. A. Zhabrev
Grebenshchikov Institute of Silicate Chemistry
Russian Academy of Sciences,
nab. Makarova 2, St. Petersburg, 199034 Russia
fax: (812) 328-15-97, zhabrev@isc.nw.ru, tchoup@rambler.ru
Introduction.
The design of functional coatings is a key stage in the
development of new materials with desired properties. A coating is a
layer or a film of a material that differs in the chemical composition and
structure from a substrate and fulfills a function not typical of the
substrate material.
Naturally, functional properties of coatings are governed by the set
of factors. Each researcher working in the field of coatings operates in
the three-dimensional space “coating property–coating composition–
preparation procedure.” The service reliability of a “substrate– coating”
composite is provided by a combination of physicomechanical and
physicochemical properties of constituents.
In this report the chemical processes occurring in organosilicate
composite at temperatures ranging from 20 to 1100°C are considered.
The main physical and chemical regularities of correlations
“organosilicate composite composition – technology for production and
application – structure – properties” are also mentioned.
Structure and properties. Heat-resistant organosilicate coatings
(OSCts) have been widely used over more than 50 years in various fields
of engineering in our country and abroad as film materials that possess
unique functional properties (Table 1) [1, 2], the majority of which are
manifestations of characteristics of a bulk material. In particular, the
anticorrosive properties can be considered as a function of adhesive–
cohesive interactions, permeability of the coating with respect to
different reactants, and the glass transition temperature of the coating.
The heat resistance depends on the bond energy of components,
structural regularity, molecular mobility, and intermolecular interaction
in the bulk of the material.
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Table 1. Properties of cured organosilicate coatings
Heat resistance, °C
short-term
long-term
Impact strength according to GOST (National
Standard) 4765, N cm (kgf cm)
Flexural Strength according to GOST (National
Standard ) 6806, mm
300–700
1200
250–500 (25–
50)
3–15
Hardness according to GOST (National Standard)
5283, arb. units.
Adhesion to steel, aluminum according to GOST
(National Standard ) 15140, (cross-hatch test), points
0.3–0.7
1–2
Adhesion to steel according to the 90º peel test, MPa
2–7
Volume electrical resistivity, Ώ cm:
at 20 °С
at 500 °С
Dielectric strength at 20 °C, kV/mm
Dielectric loss tangent
Dielectric permittivity
Temperature coefficient of linear expansion in an
interval of 20–300 °C, K-1
Heat conductivity, W/ (m K)
Specific heat capacity, kJ /(kg K)
Radiation resistance:
in beta and gamma radiations fields
at doses, Gr (rad)
in neutron fields (air–vapor medium) at integral
dose, thermal neutrons/cm2:
> 20 000 h, 280 °C
> 10 000 h, 150 °C
Diffuse reflectance of light from coatings:
white color and light tones
red, green, blue, brown, orange colors
black color
42
1012–1015
up to 109
10–50
0.008–0.1
3.0–6.0
(1–2)×10-5
0.3–0.6
0.6–1.5
106–108
(108–1010)
1×1014
1×1021
85–92
60–80
50–55
However, there are a number of properties predominantly
associated with the low-energy character of the surface of OSCts (such
as low dirt- holding capacity, decontamination ability, hydrofobicity,
etc.).
At present, it has been universally accepted that the surface energy
of the coating reflects not only the chemical nature of the film former
but also a change in its composition and properties in the course of
formation and operation [3, 4].
Organosilicate composites used in the technology of OSCts are
suspensions of finely dispersed layered silicates and inorganic or organic
pigments in solutions of organosilicon oligomers with different special
modifiers (vitreous additives, finely dispersed metal powders, etc.).
At present, film formers are represented by commercial
organosilicon varnishes prepared by hydrolytic copolycondensation of
mixtures of bi- and thrifunctional organochlorosilanes. Synthesized by
this method oligomers have rather complex structure with random
distribution of linear, branched, cyclolinear, and ladder fragments.
Polyorganosiloxanes (POS) contain methyl and phenyl radicals at silicon
atoms, and are characterized by a broad distribution of molecular
masses, have silanol terminal groups. Both unmodified organosilicon
polymers and those modified during so-called reactionary mixture of
components by organic polymers, for example, by polyesters, glyptal
and epoxy resins, are used extensively.
Numerous properties of POS are considered to be features
associated with the presence of the siloxane bond in the backbone, and
with the chemical structure.
OSCts are characterized by the following important feature. At
temperatures below the destruction temperature of the organosilicon film
former, the coating operates as a polymer material with a wide range of
operating temperatures (as a rule, from –60 to +300°C). At temperatures
above the destruction temperature, the OSCt operates as a hightemperature inorganic coating that is formed through the interaction of
products of thermooxidative destruction of the polymer with mineral
components and structural transformations in the material [2].
The structural features of macrochains of these POS are also
responsible for a number of disadvantages inherent in the film formers
under consideration. First and foremost, these are low
physicomechanical properties, unsatisfactory resistance in liquid
corrosive media and solvents, the necessity of high-temperature curing
and the presence of toxic aromatic solvents (toluene, xylenes).
43
These disadvantages can be eliminated with the use of following
techniques: filling and reinforcement of polymers; blending with
different organic pitches containing polar groups; using various curious
agents; introduction of functional groups into the organic fragments of
the molecule, etc.
Inorganic pigments and fillers are introduced into the composition
for heat-resistant protective coatings. For OS-materials the choice of
pigments is governed by the field of application and the corresponding
operating properties.
By present time a great positive experience of using organic
pigments and dyes in composition of weatherproof OSCts has been
accumulated. In formulations of chemically proof, heat-resistant, electro
insulating OSCts inorganic pigments such as oxides and salts of
transition metals have been predominantly used.
The most widespread fillers of OSCts are the following
mechanically dispersed natural fillers: chrysotile asbestos, muscovite
mica, talc, barite, and synthetic filler Aerosil.
Processes of formation of OSCts. In the case of low-temperature
curing, different curing agents are introduced into the composition of
organosilicate suspensions [2, 4]. The curing parameters affect the
technological properties (the viscosity, durability, flow properties,
drying time); hardness, adhesion, impact strength, flexural strength, and
other physicomechanical properties; sol–gel fraction content; protective
properties; heat resistance; and electrically insulating properties. As a
rule, the set of these properties determines the service life and service
reliability of OSCts. The energy characteristics of the surface of OSCts
and the related decontamination ability, a low dirt-holding capacity,
hydrophobicity, and anti-icing effect [5], i.e., the surface properties of
OSCts, should be sensitive to a change in the chemical composition of
the surface layer.
The type of film former and a way of curing define gel fraction
content of OSCts, the internal stresses, the glass transition temperature,
energy characteristics of a surface, the permeability and, finally, a
complex of operating properties.
The curing of OSCts in the course of heat treatment, as a rule, is
associated with the condensation of silanol groups of POS and the
removal of H2O thus formed from the coating. An increase in the
temperature accelerates these processes and makes it possible to achieve
a more complete curing. The kinetics of curing and aging of
organosilicate composites is affected by oxidants of the environment (for
44
example, oxygen) and the oxidizing and reducing agents introduced into
the composition of organosilicate composites.
Let us consider the main high-temperature interactions in heatresistant organosilicate composites and coatings based on
“polydimethylphenylsiloxane (PDMPS)–muscovite–chrysotile asbestos–
aluminoborosilicate glass–ZrO2–V2O5–BaO2” system at temperatures
ranging from 20 to 1100°C [6].
On the processes occurring in composite in the course of
heating. The differential thermal analysis (DTA) curve for composite
differs radically from those characteristics of OSC (Fig. 1). In order to
understand the processes occurring in composite in the course of
heating, we analyze the DTA, IR spectroscopic, and X-ray diffraction
data for the composite components (PDMPS, chrysotile asbestos,
muscovite mica), their binary systems with the silicate–polymer ratio
equal to 1 : 4, and some model systems (Table 2, Fig. 2 – 6) [6].
Fig. 1. The DTA-data for organosilicate composite of “PDMPS–muscovite–
chrysotile asbestos–aluminoborosilicate glass–ZrO2–V2O5–BaO2” composition.
45
PDMPS. During the heating in air, the oxidation of PDMPS
modified by the organic polyester manifests itself in a weak exothermic
effect with a maximum at a temperature of 110°C (corresponding to the
removal of the residual solvent, i.e., toluene) and a strong asymmetric
exothermic peak with a maximum at a temperature of 630°C and a
pronounced shoulder at 410°C (associated with the exothermic reactions
of release of volatile products of decomposition of the PDMPS) (Table
2, Fig. 2). The sample weight decreases smoothly in the temperature
range 20–400°C. The behavior of the sample weight changes sharply in
the range 390–410°C, so that the maximum weight losses are observed
in the temperature range 400–500°C. The aforementioned character of
the release of the decomposition products is associated with the presence
of the organic plasticizer in the film former and different types of bonds
in the PDMPS molecule (the energy of these bonds increases in the
order Si–Calk < Si–Carom < C–H < Si–O), as well as with the specific
feature of the behavior of linear fragments of polyorganosiloxanes in the
course of heating: the destruction (especially at the first low-temperature
stage) occurs with the participation of terminal hydroxyl groups and the
formation of volatile cyclic siloxanes.
Fig. 2. The DTA-data for PDMPS.
46
Table 2. Thermal analysis data for the coating components
Sample and total weight
losses ∆m, wt %
Organosilicate Composite
∆m =19.8 (20–1000°C)
PDMPS
∆m=46.1 (20–1060°C)
Muscovite mica
∆m=4.7 (20–1200°C)
Chrysotile asbestos
∆m=16.2 (20–1340°C)
PDMPS + mica
∆m=13.9 (100–1440°C)
PDMPS + asbestos
∆m=24.2
(100–880°C)
Temperature ranges of destruction (°C)
and ∆m (wt %)
120–270 (exo), ∆m = 1.6
270–380 (without thermal effect), ∆m=2.3
380–485 (shoulder) – 535 (shoulder) –
660 (exo), ∆m = 13.7
660–880 (endo), ∆m = 1.9
880–900, 900–930, 930–970 (endo),
∆m=0.4
20–110–240 (exo), ∆m = 1.75
240–410 (shoulder) (exo), ∆m = 12.45
410–720 (exo), ∆m = 30.2
695–800 (shoulder)–820 (shoulder)–1010
(endo), ∆m = 2.95
1010–1075 (shoulder) – 1180 (endo),
∆m=0.1
575–780 (endo), ∆m = 8.0
780–850 (exo), ∆m = 1.0
20–140–430 (exo), ∆m = 1.4
430–500 (shoulder) (exo), ∆m = 1.6
500–620–690–750 (exo), ∆m = 8.1
1000–1100–1170 (endo), ∆m = 0.2
1170–1250 (shoulder) – 1330 – 1360
(shoulder) – 1400 (exo), ∆m = 0.35
20–135–275 (exo), ∆m = 0.6
275–510 (shoulder) (exo), ∆m = 5.7
510–660 (exo), ∆m = 13.5
660–725 (exo), ∆m = 2.8
760–795 (exo), ∆m = 0.55
The thermooxidative destruction of the PDMPS usually leads to
the release of silicon-free volatile products, such as methane, benzene,
formic acid, formaldehyde, carbon dioxide, carbon monoxide, and water,
as well as trimer and tetramer methylcyclosiloxanes, i.e.,
hexamethylcyclotrisiloxane D3 and octamethylcyclotetrasiloxane D4; in
47
this case, the main contribution to the weight losses is made by the
compounds D3 and D4. Therefore, the thermal effects in the temperature
ranges 240–410 and 410–720°C can be attributed to the burnout of the
polyester, release of trimers D3 and tetramers D4, oxidation of the side
(methyl, phenyl) groups of the PDMPS, and formation of the oxidation
products. The important feature of the thermooxidative destruction of
the PDMPS is that the hydroxyl groups appear in place of organic
radicals removed as a result of destruction and in place of broken
siloxane bonds, for example, upon formation of cyclic products. A
number of these hydroxyl groups possessing acidic properties enter into
the anhydrocondensation reactions with the formation of new siloxane
bonds and more complex structures (branchings, three-dimensional
networks); i.e., the so-called “siloxane structuring” takes place. Other
silanol groups in linear segments of the macrochain favor the breaking
of siloxane bonds. To put it differently, the thermooxidative destruction
and structuring reactions proceed in the PDMPS in the course of heating.
The change in the PDMPS composition during the heating is in
agreement with the IR spectroscopic data.
At higher temperatures (720–1060°C), the weight losses (Table 2)
are small and vary very slowly. The nonvolatile decomposition product
is represented by silica, which, according to the X-ray diffraction data,
remains amorphous to a temperature of 900°C. In the temperature range
900–1200°C, the structure of amorphous silica undergoes a
transformation accompanied by the formation of the lattice of the hightemperature cristobalite phase (at 1150–1200°C), which upon cooling
transforms into the low-temperature modification.
By summarizing the above results, it should be noted that a high
strength of the siloxane bond and the siloxane structuring (in the
polymer sample, this process occurs especially intensively in the outer
layers, which, as a result, do not undergo destruction and retard the
access of oxygen inside the sample) are responsible for the high heat
resistance of the PDMPS. The following inference is important for the
further analysis: the thermooxidative destruction of the PDMPS in the
range 500–650°C is attended by the formation of the reactive amorphous
silica enriched in hydroxyl groups, which can contribute to the acid–base
interaction and to the formation of the silicon–oxygen matrix of the
coating.
48
The “PDMPS + 10 wt % ZrO2” Model System. In the temperature
range 20–1000°C, the ZrO2 oxide is inert, as is evidenced by the absence
of effects according to the DTA, TG, and DTG data. In this temperature
range, the ZrO2 oxide acts as inert filler with respect to the PDMPS,
because the curves in the derivatogram for the PDMPS–ZrO2 system and
the weight loss curves (with allowance made for the introduced filler)
exhibit the same behavior as for the PDMPS sample.
The “PDMPS –Silicate” Model Systems. As follows from the
thermal analysis data, the filling of the PDMPS with silicates results in a
complication of the destruction process. In the DTA, TG, and DTG
curves, this manifests itself in the form of additional shoulders, steps,
and kinks. Furthermore, the temperature of the onset of destruction
according to estimates from the temperature corresponding to ∆m = 4%
increases, the total weight losses decrease considerably (to a larger
extent as compared to the theoretical decrease in the weight losses), and
the temperature ranges of destruction are shifted toward higher
temperatures. A comparison of the thermal analysis data for the
asbestos-containing and muscovite-containing samples (Fig. 5 and 6)
indicates that the PDMPS samples filled with the muscovite have a
higher heat resistance.
The components of the system affect each other: the presence of
PDMPS has an effect on the thermal destruction of the silicates and vice
versa.
The DTA curve of the PDMPS–muscovite system does not
contain an endothermic effect with a maximum of 845°C characteristic
of the muscovite and exhibits a weak endothermic effect at 1100°C
associated with the breaking of the muscovite crystal lattice. In this case,
numerous exothermic reactions proceed in the temperature ranges 430–
750 and 1170–1400°C. According to the IR spectroscopic data, the
organic environment of PDMPS in the PDMPS–muscovite system is
completely removed to a temperature of 600°C. This is in agreement
with the results of the chemical analysis: the carbon content in the
sample is less than 1 wt %. The removal of hydroxyl groups in the
system under investigation begins at 600°C and is virtually completed
upon heating to a temperature of 650°C, at which this process in the
muscovite only starts. Recall that PDMPS or, more exactly, the product
of its thermooxidative destruction contains hydroxyl groups at
temperatures in the range 500–650°C and that the heating to
temperatures above 650°C leads to the disappearance of the absorption
49
bands associated with the vibrations of SiO–H bonds. Therefore,
compared to pure PDMPS, the process of the removal of hydroxyl
groups in the presence of the muscovite begins at higher temperatures, is
completed at lower temperatures, and proceeds in a narrower
temperature range (from 600 to 650°C). It should be noted that,
compared to the destruction of muscovite, the dehydroxylation in the
PDMPS–muscovite system is completed at a temperature lower by
approximately 150°C.
At a temperature of 900°C, the X-ray diffraction pattern of the
PDMPS–muscovite system virtually coincides with that of the
muscovite. At 1000°C, there appear new phases, such as γ-Al2O3,
mullite 3Al2O3 · 2SiO2, and spinel MgO · Al2O3. The content of these
phases increases with an increase in the temperature. The exception is
provided by the γ-Al2O3 phase, which transforms into the α-Al2O3
modification at temperatures above 1100°C.
The DTA curve of the PDMPS–chrysotile asbestos system does
not involve the endothermic effect at 670°C typical of the chrysotile
asbestos. However, the DTA curve of this system contains no less than
four exothermic effects, including the sharp exothermic effect with a
maximum at 780°C. This effect has a shape characteristic of the
chrysotile asbestos and manifests itself for the given systems at a lower
temperature as compared to the asbestos. According to the IR
spectroscopic data, the organic environment of the organosilicon film
former in the PDMPS–asbestos system is completely removed to a
temperature of 500°C. The removal of hydroxyl groups in the system
under investigation begins at 500°C and is virtually completed to 600°C.
This process in the asbestos is observed in the range 550–600°C.
Therefore, the process of the removal of hydroxyl groups in the presence
of the asbestos begins and is completed at lower temperatures than that
in the unfilled PDMPS sample. Compared to the destruction of asbestos,
the dehydroxylation in the PDMPS–asbestos system is completed at a
temperature lower by approximately 180°C.
As follows from the X-ray diffraction data, the first change in the
phase composition of the PDMPS–asbestos system occurs at a
temperature of 700°C: there appear reflections, which suggest the
formation of two new crystalline phases, namely, forsterite and enstatite.
However, not all reflections characteristic of these compounds clearly
manifest themselves. This can indicate that, in the presence of the
PDMPS or, more exactly, the products of its destruction, the above
phases are formed at a lower rate. Upon annealing of the sample at
50
temperatures of 1150–1200°C, there appear reflections, which
demonstrate the formation of the Mg(Fe,Cr)2O4 spinel.
Fig. 3. The DTA-data for muscovite
Fig. 4. The DTA-data for chrysotile asbestos
51
Fig.5. The DTA-data for PDMPS+mica
Fig.6. The DTA-data for PDMPS+asbestos.
52
Therefore, the removal of hydroxyl groups from the silicate structures in
the composite materials containing the chrysotile asbestos or muscovite
is observed at temperatures lower than that in pure minerals. The
hydroxyl groups of the amorphous product of polymer destruction
accelerate the dehydroxylation of silicates, which contain a large number
of defects after long-term mechanochemical treatment of components in
the ball mills used for preparing the organosilicate composites.
However, the given interaction leads to the formation of the polymer
composites with a higher heat resistance: the filling of the PDMPS with
silicates results in a substantial decrease in losses of the weight, which
exceeds substantially the theoretical value. This indicates that the silicate
fillers play an active role and that heat-resistant metastable structures are
formed in the PDMPS filled with the silicates upon heating.
The “PDMPS –Chrysotile Asbestos–Aluminoborosilicate Glass”
Model Composite. The aluminoborosilicate glass retards the
transformation of the asbestos into forsterite and silica by preventing the
interaction of the magnesium and silicon oxides and the formation of
magnesium silicate crystalline phases, thus leading to a decrease in the
depth of destruction due to the decrease in the fraction of cyclosiloxanes
in the volatile products of polymer decomposition [6]. This circumstance
explains the high heat resistance of composites containing vitreous
additives.
The “Polydimethylphenylsiloxane–Aluminoborosilicate Glass–
BaO2–V2O5” Model Composite. A comparison of the X-ray diffraction
patterns of composite A (containing BaO2) and composite B (without
BaO2) heat treated at 20, 300, and 500°C for 3 h demonstrates that heat
treatment is attended by a change in the intensities of the diffraction
lines attributed to the vanadium oxide V2O5 and barium peroxide for
composite A and the diffraction line assigned to the V2O5 oxide for
composite B. A broad smeared reflection in the range of 2 θ angles equal
to 8.0°–18.0° corresponds to the amorphous aluminoborosilicate glass.
The heat treatment results in a change in the intensity of the diffraction
reflections of the aluminoborosilicate glass for both samples. The ratio
between the reflection intensities for the samples heat treated at
temperatures of 20, 300, and 500°C is equal to 1.57 : 1.32 : 1.00 for
composite A and 1.38 : 1.21 : 1.00 for composite B. This suggests that
the chemical interactions in composite A containing the vanadium oxide
53
and barium peroxide are more pronounced and associated with the
incorporation of the barium peroxide into the aluminoborosilicate glass
matrix.
However, a comparison with the results obtained earlier
demonstrates that these changes are less pronounced than those in
composites that do not contain the aluminoborosilicate glass additive. In
the X-ray diffraction patterns, the lines corresponding to the barium
peroxide and vanadium oxide V2O5 disappear and the lines associated
with the zirconia remain unchanged. The initial aluminoborosilicate
glass is X-ray amorphous, and its X-ray diffraction pattern exhibits a
broad smeared peak in the range 2 θ = 10°–40°. The thermal aging at
temperatures up to 800°C leads to the formation of vanadium-containing
glass-ceramic phases. Therefore, one more type of chemical interactions
manifests itself during thermal aging: the silicon–oxygen network
formed by the asbestos, mica, and glass is broken, and vanadium oxide
is incorporated into this network. A comparison of the X-ray diffraction
patterns of the sample heat treated at 180°C for 3 h and the sample heat
treated at 980°C for 3 h shows that the ratio between the intensities of
the reflections attributed to the aluminoborosilicate glass for the initial
sample and the sample heat treated at the high temperature is equal to
1.14 : 1.00. This means that the content of the aluminoborosilicate glass
in the system decreases. Moreover, high-temperature treatment leads to a
decrease in the intensities of the diffraction lines associated with V2O5,
BaO2, chrysotile asbestos, and muscovite and, conversely, to an increase
in the relative intensity of the diffraction lines corresponding to the ZrO2
oxide. In the X-ray diffraction pattern of the composite heat treated at
980°C, there appear reflections attributed to the barium metavanadate
BaV2O6.
Conclusion
Thus, the main types of chemical interactions during the formation
of coatings and their influence on the coating structure were considered.
It was established that the introduction of the low-alkali
aluminoborosilicate glass and V2O5 and BaO2 oxides into the PDMPS–
muscovite–chrysotile asbestos–ZrO2 system leads to an increase in the
54
heat resistance and an improvement of the physiochemical properties of
the composites.
1.
2.
3.
4.
5.
6.
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