Possibilities of using trimethylpentaphenyltrisiloxane as coolant for removing low-potential heat

Article abstract of DOI:10.1134/S1070427214100218

The resistance of commercial trimethylpentaphenyltrisiloxane to heat and to thermal oxidation was studied. The limits of its use as coolant in systems for removing low-potential heat were determined. Instrumental methods were developed and tested for moni

Full text of DOI:10.1134/S1070427214100218

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2014, Vol. 87, No. 10, pp. 1529−1535. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © A.V. Lebedev, V.D. Sheludyakov, P.A. Storozhenko, V.S. Gurskii, Yu.V. Tsapko, I.M. Yasnev, A.G. Egorov, L.V. Osetrova, 2014, published
in Zhurnal Prikladnoi Khimii, 2014, Vol. 87, No. 10, pp. 1480−1486.
VARIOUS
TECHNOLOGIES
Possibilities of Using Trimethylpentaphenyltrisiloxane
as Coolant for Removing Low-Potential Heat
a
a
a
b
b
A. V. Lebedev , V. D. Sheludyakov , P. A. Storozhenko , V. S. Gurskii , Yu. V. Tsapko ,
b
c
c
I. M. Yasnev , A. G. Egorov , and L. V. Osetrova
a
State Research Institute of Chemistry and Technology of Organoelement Compounds,
sh. Entuziastov 38, 105118 Moscow, Russia
b
Aleksandrov Research Institute of Technology, a/ya 22, Sosnovyi Bor, Leningrad oblast, 188540 Russia
c
Lebedev Research Institute of Synthetic Rubber, ul. Gapsalskaya 1, St. Petersburg, 198035 Russia
e-mail: leanvik@yandex.ru, gurskyvs@yandex.ru
Received September 29, 2014
Abstract—The resistance of commercial trimethylpentaphenyltrisiloxane to heat and to thermal oxidation was
studied. The limits of its use as coolant in systems for removing low-potential heat were determined. Instrumen-
tal methods were developed and tested for monitoring the condition of siloxane in the course of its prolonged
operation as coolant.
DOI: 10.1134/S1070427214100218
In 1962, Crawley et al. [1] reported the development
of an ultra-high-vacuum working liquid for oil diffusion
pumps under DC-705 trade name. It was produced by
Dow Corning (the United States) and ensured the limiting
Therefore, it seemed topical to study the stability of
commercial compound I under critical conditions of its use
and to evaluate the efficiency of methods for monitoring
its stability.As critical conditions we chose temperatures
higher than 300°С and the presence of real industrial and
–
9
vacuum of up to 10 mmHg or even better when using
water-cooling traps. The base component of the product
was symmetrical trimethylpentaphenyltrisiloxane
MePh SiO(SiMePh)OSiMePh (I). Later this product
1
13
operation impurities in the suggested coolant. H, C,
2
9
and Si NMR spectroscopy and chromatography were
chosen as basic monitoring methods.
2
2
found very wide use in vacuum engineering owing to the
most favorable set of its characteristics. In particular, it
ensured extremely low residual pressure in the evacuated
system and exhibited high heat and radiation resistance.
In the former Soviet Union and then in the Russian
Federation, analogous substance is produced under
FM-1 liquid trade name. Recently China also started to
produce this substance under oil 275 trade name. High
heat and radiation resistance are priority characteristics
that should be exhibited by working liquids of modern
systems for removing low-potential heat in compact high-
power nuclear energy installation, which are now under
development, including working bodies of open space
cooling systems based on principles of carcass-free liquid
droplet radiators [2].
The chloride ions, which are capable to react at high
temperatures both with molecules of the tested siloxane
and with structural materials of the cooling system, were
detected in samples of FM-1 liquid by ion chromatography
on the level of 10–15 mg L . Such content may be due
to the use of water with impurities of chlorides and
hydrochloric acid proper in industrial synthesis of I.
We have found that chloride ions, which exert negative
corrosive effect, can be completely removed by twofold
extraction of a toluene solution of commercial siloxane I
with double-distilled water, followed by solvent removal
in a vacuum. Direct extraction is unfeasible because of
close densities of water and siloxane.
–
1
The content of dissolved gases in commercial
compound I was determined by gas chromatography. The
1
529

1530
LEBEDEV et al.
content of oxygen and nitrogen under normal conditions
–30.94, –31.03, –31.08, –31.10, and –31.16 ppm owing
to cis/trans isomerism. A weak triplet centered at δ =
–33.98 ppm was assigned to the methylphenylsiloxy units
of the linear pentamer MePh SiO[(SiMePh)O] SiMePh
–
1
was found to be 90 and 40–45 mL L atm, respectively.
Such a high solubility of oxygen in the siloxane liquid
could favor the occurrence of thermal oxidation processes
studied in this work.
2
3
2
(IV). It is manifested in the chromatograms as the peak
with the retention time of 12.2 min.
The water content of FM-1 samples was determined
by Fischer titration and was found to be 60–80 mg L .
–
1
The most unexpected feature of the 29Si NMR
spectra of FM-1 liquid is the presence of weak signals
of phenylsilyltrioxy moieties at –77 to –78 ppm. Their
assignment to a specific compound is extremely difficult,
and their presence is difficult to account for, because the
commercial product is isolated by vacuum distillation.
It should be noted that the 29Si NMR spectra are
largely informative only when studying neat commercial
trimethylpentaphenyltrisiloxane. For 5% solutions, the
measurement uncertainty can range from 20 to 50%.
The composition of FM-1 liquid and products of its
possible degradation was studied by GLC. However,
because of high boiling point of I, this method of analysis
requires setting high temperatures of the vaporizer and
detector, which can disturb the measurement results
because of the occurrence of pyrolytic processes.
Therefore, to monitor the coolant quality, we chose
the procedure based on high-performance liquid
chromatography with UV detection.
1
Along with GLC and HPLC, the initial samples
and their pyrolysis products were analyzed by NMR
spectroscopy. Combined use of the HPLC data, H NMR
The H NMR spectra allow more accurate quantitative
measurements. In these spectra, the phenyl groups give
a complex pattern. The signals of methyl groups are the
most informative. The pattern is considerably complicated
only in the case when the components exhibit structural
1
data for 5% solutions of the siloxanes in CDCl , and
3
2
9Si NMR data for the neat substances gives the best
isomerism, e.g., in the case of compounds A and A .
For the methyl protons in individual compounds I and
II, we determined the following chemical shifts: for I,
analytical results. In the chromatograms of solutions of
FM-1 liquid and its analog, DC-705, in acetonitrile, there
are well-defined peaks of the main substance I with the
retention time of 4.9 min and of the major impurity, linear
tetrasiloxane MePh SiO[(SiMePh)O] SiMePh (II), with
the retention time of 7.2 min in various ratios. The peak
area of any of the other impurity components does not
exceed 1.4% of the total peak area of all the components.
The 29Si NMR spectra of all the samples, recorded
with long signal accumulation, contained, along with
the signals of terminal Si atoms of compounds I and II
at δ = –11.14 ppm and signals of methylphenyl groups
of these compounds at –32.18 and –33.10, –33.12 ppm,
respectively, also other signals.Analysis of the published
data [3, 4] allowed the following assignment of these
signals: the signal at –10.2 ppm and the shoulder
3
4
0
.567 (2СН ) and 0.329 (СН ); for II, 0.591 and 0.589
3 3
(
2СН ), 0.326 (СН ), and 0.283 (СН ).
3 3 3
2
2
2
1
It should be noted that the Н NMR spectra bear
important information on impurities in individual
methylphenylsiloxanes. A conventional characteristic
of methylphenylsiloxane liquids is the index of the
Me Ph
phenyl substituent content, n = N /N , where N is
the number of the indicated groups in the molecule of
individual methylphenylsiloxane. This index can be
calculated from the experimental data by the formula
n = 5/3(Σ∫Me/Σ∫Ph), where Σ∫Me and Σ∫Ph are the
total integral intensities of methyl and phenyl proton
signals in the spectrum, respectively. The index n
calculated from the spectra is one of the most important
characteristics of methylphenylsiloxanes, determining
their physicochemical properties. When compared to
the theoretical value, it serves as a quantitative measure
of the impurity content, characterizing the prevalence of
methyl or phenyl substituents in the impurities.
(inflection) at –10.73 ppm are due to an impurity of the
linear disiloxane MePh SiOSiMePh (III), whose content
2
2
in different samples was 1.4 mol % at maximum. The
impurity signal at δ = –21.53 ppm (center of triplet) is
unambiguously assigned to the cyclic trimer present in
the initial samples, trimethyltriphenylcyclotrisiloxane
(
so-called product А ). Its content in the samples did not
To study the heat resistance of FM-1 liquid, the
samples of the commercial liquid and its individual
components, degassed in a vacuum and purged with
argon, were initially heated in a glass reactor or in a
metal autoclave at maximal temperatures, and then the
3
exceed1mol%.Theidentificationwasbasedonmeasuring
the spectra of FM-1 samples with additions of А and its
homolog, tetramethyltetraphenylcyclotetrasiloxane (so-
called product А ). The latter gives a set of Si signals at
3
4
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 10 2014

POSSIBILITIES OF USING TRIMETHYLPENTAPHENYLTRISILOXANE
1531
lower temperature boundary of the experiments was
gradually decreased to 300°С. The GLC and HPLC
analysis showed that noticeable decomposition of both
the individual compounds and their mixture, sample
of FM-1 liquid containing 90 wt % trisiloxane I, 9%
tetrasiloxane II, and 1% disiloxane III, occurred above
Table 1. Composition of liquid products of thermal
decomposition of FM-1 liquid. Pyrolysis time 5 h
a
b
Content in mixture, %
Т, °С
А3
Х1
III
А4
I
II
IV
Х2
4
4
4
5
25 0.6
50 3.1
75 5.6 4.9 4.51 8.2
00 1.9 2.5 0.76 1.8
–
–
0.8 1.83 88.3 7.7 0.44
–
–
5.31
0.27
9.8 4.90 72.5 6.2
–
–
–
4
00°С. After heating at this temperature in a titanium
13.0
0.74
–
–
autoclave for 5 h, only a small additional peak appeared
in the chromatogram. Its retention time was shorter
than that of the initial sample, and the peak area was
less than 2% of the total peak area. The pyrolyzates
consisted of a gas phase, a liquid phase, and a solid
residue unextractable with benzene. The qualitative
and quantitative composition of the liquid pyrolysis
products of individual compounds I–III is close to that
reported in [5]. The composition of the liquid pyrolysis
products of FM-1 liquid, which corresponds to the
content of individual components, is given in Table 1.
As can be seen, the thermal decomposition led to the
formation of cyclosiloxanes А and А , pentasiloxane
a
Products with more than 0.1% content are given.
Taking into account solid and gaseous products.
b
Table 2. Composition of gaseous products of thermal
decomposition of FM-1 liquid. Pyrolysis time 5 h
Gas phase composition, mol %
Т, °С
Н2
С Н
6
6
СН4
С Н
2
4
425
29.9
70.7
83.2
31.8
70.1
23.0
12.4
65.1
–
–
4
4
5
50
75
00
3.8
3.9
3.1
2.5
0.5
0.3
3
4
IV, a high-molecular-mass unidentified product Х , and
2
a low-molecular mass product Х presumably having
pyrolysis of FM-1 liquid and the known fact [5] that
disiloxane III exhibits the highest thermal stability among
the individual compounds studied, we can assume that
the limiting step of the thermal decomposition of FM-1
liquid is homolytic cleavage of the Si–O bond:
1
the 1,3-disila-2-oxaindan structure in accordance with
the mass spectrum. The possibility of formation of
such structures was assumed by Bochkarev et al. [6]
in a study of the fragmentation of molecular ions of
methylphenylsiloxanes.
As noted above, the pyrolysis yields the gas phase.
It consists (Table 2) of hydrogen, benzene, methane,
and ethylene, identified by gas chromatography–mass
spectrometry.
T
I–III
Ia–IIIa
The total amount of the gaseous products was as low as
0.3–0.7 wt % of the total weight of the pyrolysis products.
The gas formation is sharply enhanced at 500°С. At this
temperature, the amount of the gas phase in 5 h reached
The subsequent elimination of the phenyl radical
generates methylphenylsiloxy biradical participating in
numerous reactions:
9
–13 wt % relative to the initial sample.
Solid pyrolysis products start to prevail when
performing the pyrolysis at higher temperatures (475–
00°С). Elemental analysis shows that the weight fraction
5
of silicon relative to that of carbon increases by 20% for
all the tested compounds and FM-1 fluid, suggesting the
cleavage of Si–C bonds and formation of intermolecular
cross-links by the generated radicals.
n(V)
V + III
I,
IIa + C H
I.
Thus, FM-1 liquid exhibits high resistance to short
heating and decomposes insignificantly at temperatures
of up to 425°С. The decomposition rate sharply increases
with further heating. Taking into account data in Tables 1
and 2 on the compositions of the reaction mixtures from
6
5
Stabilization of the phenyl radical by proton detach-
ment yields benzene, and recombination of this radical
yields biphenyl.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 10 2014

1532
LEBEDEV et al.
W, % of initial
initial disiloxane and with its mass spectrum (m/z): 317
+
+
+
(
М – СН ) , 255 (М – С Н ) , 240 (М – СН – С Н ) ,
3 6 5 3 6 5
+
7
7 (С Н ) .
6 5
The resistance of FM-1 liquid to thermal oxidation was
studied starting from 300°С. The sample was heated in a
glass tube in air for 100, 300, and 500 h. In all the cases,
the I/II ratio somewhat increased, peaks of oxidation
products gradually appeared, and their areas increased
in proportion with the heat treatment time. Nevertheless,
we can state that the thermal oxidative degradation of
FM-1 liquid does not occur intensely at this temperature,
29
because the signals of trifunctional units in the Si NMR
spectra do not increase. It should be noted that the addition
of steel appreciably decreases the already low rate of
formation of oxidation products.
τ, h
Fig. 1. Main component (I) content W as a function of
thermolysis time τ.Atmosphere: (1) nitrogen, (2) nitrogen with
addition of oxygen, and (3) nitrogen with addition of water.
To determine the real operation characteristics of
FM-1 liquid in the course of prolonged operation,
we studied its thermal degradation at 380°С in the
experiments performed for 1500 h in stainless steel
autoclaves. FM-1 samples were placed in autoclaves and
purged with nitrogen to completely remove the dissolved
Apparently, theanalogouspathwaywiththeelimination
of the extremely reactive methyl radical is realized to a
considerably lesser extent. However, the contribution of
the methyl group to the process reaches a maximum in
the temperature interval 450–475°С. Apparently, in this
interval, the radical dehydrogenation processes, more
characteristic of the methyl group, which is less stable
under these conditions, become the most significant. At
–1
oxygen. In some autoclaves, water (5 mg mL FM-1)
–
1
or oxygen (1.1 mg mL FM-1) were introduced prior to
sealing. Liquid samples were taken at regular intervals
in the course of thermolysis, and the content of the main
component I and the concentrations of linear dimer III,
linear tetramer II, cyclic trimer A , and biphenyl were
5
00°С, the thermal decomposition becomes so vigorous
3
determined by HPLC. After sampling, oxygen or water
was introduced into the autoclaves again (in a nitrogen
atmosphere), and the thermolysis was continued. Figure 1
shows the dependences of the concentration of the main
component I on the thermolysis time.
that it results in rapid dephenylation with recombination
of the radicals to form high-molecular-mass derivatives.
In the case of compound III, which is the most
thermally stable among the compounds studied,
interesting intramolecular o-phenylene cyclization
occurs:
As follows from our data, in the examined thermolysis
time interval the concentration of the main component
gradually decreases. The largest concentration loss is
observed in the autoclave with water added: more than
6
0% in 1500 h. The effect of the water microimpurities
T
III
may be due to the fact that, despite low nucleophilicity,
water is capable of cleavage of Si–C bonds on the metal
surfaces at high temperatures. No significant effect of
microadditions of oxygen on the thermal degradation of
the main component was revealed in our experiments.
H
Figure 2 shows how the concentrations of compounds
III, II, and A vary in the course of thermolysis in a
3
nitrogen atmosphere.
As can be seen, the concentration of the major impurity
component, linear tetramer II, decreases with time,
The assumed structure of compound X is well
consistent with its shorter retention time compared to the
1
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 10 2014

POSSIBILITIES OF USING TRIMETHYLPENTAPHENYLTRISILOXANE
1533
c, μg mL–1
c, μg mL–1
τ, h
τ, h
Fig. 3. Biphenyl concentration c as a function of thermolysis
time τ of FM-1 liquid. Atmosphere: (1) nitrogen, (2) nitrogen
with addition of oxygen, and (3) nitrogen with addition of water.
Fig. 2. Concentration of impurity components c as a function
of thermolysis time τ. (1) А , (2) III, and (3) II.
3
whereas the concentrations of linear dimer III and cyclic
FM-1 viscosity is manifested to a significant extent only at
temperatures below 60°С. The results are shown in Fig. 4.
Hence, the formation of low-molecular-mass products
does not affect significantly one of the main parameters
of the coolant, viscosity at real operation temperatures.
trimer A increase with time.
3
Along with changes in the concentrations of the
above-indicated components, we observed an increase in
the peak areas of the degradation products with shorter
retention times. Figure 3 shows the dependence of the
EXPERIMENTAL
biphenyl (Т = 2.2 min) concentration on the thermolysis
r
time. It should be noted that we added double amount
The H and 29Si NMR spectra were recorded on a
1
–
1
of water (10 mg mL FM-1) into the autoclave after
sampling (400 and 880 h). It is also important that the
sensitivity of determining biphenyl is higher than the
sensitivity of determining the other identified components
by practically two orders of magnitude. In view of the
fact that biphenyl should be formed in the course of
degradation of all organosiloxanes containing phenyl
radicals, biphenyl can be considered as an indicator
compound for monitoring the decomposition of FM-1
liquid, and in certain cases the biphenyl determination can
be used for detecting the ingress of water into the coolant.
Bruker AM-500 spectrometer with СDCl as solvent.
3
The GLC analysis of the reaction mixtures was
performed with an LKhM-72 chromatograph equipped
The thermolysis results in appearance of weak
incompletely resolved peaks with long retention time
(12–35 min), absent in the chromatogram of the initial
liquid. The total relative area of these peaks reaches 25%
at the thermolysis time of 1500 h. The appearance of
these peaks is associated with possible cross-linking of
oligomeric molecules with the formation of polymeric
compounds. The polymerization can be a critical process
from the viewpoint of applications of FM-1 liquid,
because it leads to an increase in the viscosity. Indeed, our
experiments show that the viscosity gradually increases
with the thermolysis time. However, the increase in the
2
20 T
Fig. 4. Viscosity of FM-1 liquid η as a function of temperature
T. (1) Initial liquid and (2) liquid after thermolysis in a nitrogen
atmosphere for 880 h.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 87 No. 10 2014

1534
LEBEDEV et al.
with a thermal conductivity detector. The vaporizer
temperature was 550, and the detector temperature,
autoclave. Samples in an amount of 20 mL were
degassed for 1 h at a residual pressure of 100 Pa and
were purged for 30 min with argon, after which they
were thermostated for 5 h with collection of the gaseous
products in a trap cooled with liquid nitrogen. Prolonged
heating at 380°С was performed in autoclaves made
of 0Cr18Ni10Ti stainless steel. The autoclaves were
cylindrical vessels with an internal diameter of 28 mm
and a height of 60 mm. The autoclaves were charged
with 20 mL of the liquid being tested and a disk of the
same steel 20 mm in diameter and 12 mm thick as a
control sample intended for obtaining information on
the condition of the surface and presence of deposits.
The autoclaves were purged with nitrogen, hermetically
sealed using threaded lids with annealed copper gaskets,
heated for the above-indicated time, cooled to perform
chromatographic analysis and measure the viscosity,
and then again degassed and heated, repeating all the
operations up to the maximal heating time. In some
experiments, microadditions of water or oxygen were
introduced into the autoclaves in an inert atmosphere.
3
3
30°С. The carrier gas was helium with a flow rate of
–
1
0 mL min . The column temperature was programmed
–
1
with a heating rate of 16 deg min . The stainless
steel column was 1 m long and 5 mm in diameter.
The stationary phase was SE-30, 5% on Chromaton
N-AW. The gravimetric content of the substances was
determined using calibration coefficients calculated
from the chromatograms of the artificially prepared
mixtures of the individual substances. The relative
calibration coefficients were as follows: А , 0.66; А ,
3
4
0
.97; I, 1; II, 1.28; III, 0.81; and IV, 1.54. The relative
uncertainty of the measurements did not exceed 5%.
Reference samples of the compounds were used for
identifying the peaks.
Analysis of the gas and liquid phases of the pyrolysis
products by gas chromatography–mass spectrometry was
performed with an Agilent Technologies spectrometer
using a capillary column 250 μm in diameter and 30 m
long, coated with SE-30. A mass spectrometer was
used as detector. The carrier gas was helium, flow rate
Synthesis of siloxanes I–III. Mixture of siloxanes
I–III in 100/30/5 ratio was prepared by the procedure
suggested in [7], by the reaction of methyldiphenylsilane
with sodium hydroxide in toluene in the presence
of methanol, followed by azeotropic distillation of
water and treatment of the reaction mixture with
methylphenyldichlorosilane. Individual compounds I–
III containing no less than 99% main substance were
isolated by vacuum distillation (100 Pa) on a distillation
column 1.2 cm in diameter and 60 cm long, packed with
prismatic glass pieces.
–1
–1
1
mLmin , programmed heating at a rate of 16 deg min
to 300°С for the liquid phase and combined mode for
the gas phase: 3 min at 20°С, then heating at a rate
–
1
of 4 deg min . The sample volume was 0.5 μL. The
flow split ratio was 400 : 1. The chromatograms were
calculated by percent normalization with respect to peak
areas without calibration coefficients.
HPLC analysis of the samples was performed with a
Staier UF chromatograph using a Luna column, 5 μm,
1
1
50 × 4.6 mm, sample volume 20 μL, elution rate
mL min , UV detector (254 nm). Samples for analysis
–1
CONCLUSIONS
were prepared by dissolving the substances in acetonitrile
–
1
to obtain 5 mg mL solutions.
(
1) Commercial vacuum oil, FM-1 liquid, can be
The content of chloride ions in FM-1 liquid was
determined by ion chromatography of aqueous extracts
of FM-1 liquid using a Staier ion chromatograph
equipped with a conductometric detector. Commercial
FM-1 liquid produced by the State Research Institute
of Chemistry and Technology of Organoelement
Compounds was used.
successfully used as coolant for removing low-potential
heat. It can operate with high efficiency for a long time at
temperatures of up to 300°С. At higher temperatures, the
operation life of the liquid is limited. The condition of the
liquid can be monitored by HPLC, NMR spectroscopy,
and viscosity measurements.
(2) Biphenyl was suggested as indicator component
Thermal decomposition of FM-1 liquid and of
individual compounds I–III. A sample was kept for a
short time at a temperature in the interval 400–500 ±
for monitoring the thermal degradation of FM-1.
(3) To prolong the operation life as coolant, FM-1
liquid should be treated to remove microimpurities of
dissolved chloride ions and water.
1
0°С in a cylindrical glass reactor or a titanium
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POSSIBILITIES OF USING TRIMETHYLPENTAPHENYLTRISILOXANE
1535
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1
2
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