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7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil
1/6
Adsorption and surface properties of silica withtransformer insulating oilsq
Amane Jadaa,*, Abdelouahed Ait Chaoua, Yves Bertrandb, Olivier Moreauc
aInstitute de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse, FrancebDivision Recherche et Developpements, Electricite de France, CIMA 8, Les Renardieres, BP 1. F-77818 Moret-sur-Loing Cedex, France
cDepartement Machines Electriques, Division Recherche et Developpements, Electricite de France, Service Materiel Electrique,
1 Avenue du General de Gaulle, 92 141 Clamart Cedex, France
Received 22 January 2001; revised 19 December 2001; accepted 7 January 2002; available online 12 February 2002
Abstract
The presence of polar species in transformer insulating oil may cause degradation and electric discharges in the power transformer. Such
oil polar fraction can originate either from the neat oil and/or from its oxidative degradation in the power transformer. The aim of this study is
to examine the relation between the insulating oil and the electrical properties of its polar fraction in order to predict eventual failure in the
power transformer.
We investigate at ambient temperature the adsorption of the polar fractions of various transformers insulating oils (a new and two used
oils) from the neat oils onto silica particles. The adsorbed amount was higher for the used oils as compared to the new one. Infrared spectra of
the polar fraction indicate the presence of hydroxyl, aromatic and carboxyl functional groups that are found in the asphaltenes compounds.
Microelectrophoresis study of the oil polar fraction covered silica particles gives negatively charged oil polar fraction. Such oil surface
charge depends on the pH and results from the ionisation of the oil acidic surface groups. Finally, we obtain a good correlation between the
amount of the oil polar fraction and the magnitude of the zeta potential at the water/oilsilica interface. q 2002 Elsevier Science Ltd. All
rights reserved.
Keywords: Asphaltenes; Adsorption; Silica; Zeta-potential
1. Introduction
Insulating oils are used in the electric power transformer
mainly to transfer heat. However, static electrication due
to the insulating liquid ow causes failure and electric
discharges in the power transformers [111]. Further, the
presence of small amount of sulphur in the oil can promote
dissolution of copper (cupric corrosion by sulphur), which
functions as degradation catalyst of the oil. In order to over-
come static electrication and oil degradation, we add the
benzotriazole (BTA) to the insulating oil in some manufac-tures researches [2]. This molecule adsorbs easily from the
oil onto the pressboard and metallic cooper. Hence, the
addition of the BTA to the oil inhibits charge separation
on the pressboard and reduces the oil degradation [2].
The static electrication, which is due to the liquid ow,
results from the charge generation at oil/pressboard
interface. In this process, it is generally assumed that the
pressboard and the oil acquire, respectively, a negative and a
positive charge, due to preferential adsorption onto the
pressboard of negative ions present in the oil (impurities,
additives). Static electrication occurs if the energy of oil
ow is sufcient to separate these ions from the oil/press-
board interface. Previous simulation studies [12], on static
electrication, have shown that charge generation at oil/
pressboard interface results not only from the transfer of
ionic species from the oil to the pressboard, but also from
the pressboard to the oil. In that work, the authors consider
two modelling approaches for simulation studies. Accordingto these models, charge generation at oil/pressboard inter-
face can be due either to adsorption onto the pressboard of
negative ions present in the oil, or to diffusion in the oil of
positive ions coming from the pressboard. Such positive
ionic species may be the protons H1 that result from the
dissociation of the alcohol radicals in the cellulose. The two
modelling approaches considered in the simulation studies
[12] were in accordance with the experimental observations.
Several authors [1322] have studied the process of the
electrical charge generation at solid/organic liquid interface.
From these works, it was concluded that the solid acquires
Fuel 81 (2002) 12271232
0016-2361/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0016-2361(02) 00019-4
www.fuelrst.com
* Corresponding author. Tel:133-3-896-08709; fax: 133-3-896-08799.
E-mail address: [email protected] (A. Jada).q Published rst on the web via Fuelrst.comhttp://www.fuelrst.com
7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil
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electrical charges either by preferentially adsorbing ions,
possibly still associated in structures, or by ion formation
resulting from surface groups dissociation, which are being
held in some lyophilic structure. Therefore, the occurrence
of the static electrication in power transformer suggests the
presence of an electrical double layer at the oil/pressboard
interface.
Mineral transformer oil is mainly a mixture of hydrocar-
bon compounds of three classes: alkanes, naphtenes and
aromatic hydrocarbon. Polar compounds found in the trans-
former oil are a minor part of the constituents, and usually
contain heteroatoms, such as, oxygen, nitrogen, or sulphur,
which may greatly inuence the chemical and electrical
properties of the oil. These heteroatoms are mainly
associated with the oil aromatic structures, such as pyridine,
pyrazine, pyrrole, carbazole, indole, or benzoic acid groups
and can result in their stabilisation in aqueous medium.
Further, nitrogen and oxygen atoms can be involved in
various acid and basic functional groups, such as carboxylic
acid, ether, ester, aldehyde, ketone, amine and amide[2325]. In addition, Waldo et al. [26] showed that sulphur
atom is essentially present in thiophene and organic sulphur
form.
The insulating oil polar fraction can result either from the
neat oil and/or from its oxidative degradation in the power
transformer. It is mainly made of charged particles in the oil,
which deteriorates transformer insulation by decreasing
electrical strength. It is useful to know the amount and the
charge of this polar fraction to predict future failure in
power transformer. The object of this work is to establish
a relationship between the chemical and the electrical prop-
erties of the transformer insulating oil polar fraction. Forthis purpose, we have extracted the polar fractions of
various transformers insulating oils from their neat oils by
using silica gel as solid support, and we have examined their
surface compositions and charges.
2. Experimental
2.1. Materials
Electricite de France supplied the three oils used in this
work. Two oils, respectively, UO1 and UO2, were used in
power transformers, while the third oil, NO, is new oil. Thesilica substrate used is a silica gel 60 for column chromato-
graphy purchased from Merck, having specic surface
area 480540 m2 g21. The solid particles size 40
60 mm, which are aggregates of primary particles having
size of about 12 nm.
2.2. Extraction and adsorption onto silica of the oil polar
fraction
The silica gel 60 was preliminary heated at 100 8C during
1 day, a weight amount (2 g) of the dried silica was placed in
a stoppered bottle and a known volume (50 cm3) of the neat
oil was added. The resulted dispersion was allowed to stand
few days at ambient temperature, until the adsorption equi-
librium was reached. Then, the oil fraction covered silica
substrate was separated from the dispersion and washed
several times with n-hexane to remove the residual non-
polar oil fraction adsorbed on silica. Finally, the oil polar
fraction covered silica solid substrate was placed in an oven,
until all the residual n-hexane was driven off into vapour.
2.3. Preparation of the oil polar fraction covered silica
particles aqueous dispersions
The silica dispersions were prepared by introducing a
given amount of the oil covered silica particles (0.1%) in
the 1023 M NaCl aqueous solutions. The resulted disper-
sions were ultrasonicated for few minutes and the pH values
of the systems were varied in the range 39 by adding to the
dispersions small amounts of sodium hydroxide (NaOH) or
hydrochloric acid (HCl) aqueous solutions. The nal silica
dispersions were then shaken for few days, until electricalequilibrium was reached. The nal pH values were
measured prior the zeta potential measurements.
2.4. Infrared measurements
Infrared spectra of the sample were recorded on
BRUKER (IFS 66, IFS 66/S, IFS 48) apparatus with a
golden gate single reection diamond attenuated total
reectance (ATR) accessory, purchased from GRASEBY
SPECAC. In this technique, the sample (the neat oil or the
oil polar fraction) is held in intimate contact on an ATR
crystal and internal reection occurs, when the infrared
radiation enters the ATR crystal (diamond). The crystaldesign enables total internal reection of the radiation that
creates an evanescent wave at the crystal surface. Such
evanescent wave extends into the sample, which is in
contact with the crystal. The spectra were an accumulation
of 100 scans and ranged from 4000 to 600 cm21.
We have recovered the oil polar fraction from the oil
polar fraction covered silica solid substrate by using ethanol
as eluent. The resulted ethanol solution was then placed in a
vacuum rotary evaporator to drive out the residual solvent
and to yield the pure oil polar fraction, which was investi-
gated by the infrared spectroscopy.
2.5. Zeta potential measurements
The electrokinetic potential or zeta potential of the silica
dispersions was measured at ambient temperature using the
microelectrophoresis method. The measurements were
made with a Zetaphoremeter II model Z3000, having cell
section of 0.5 0.2 cm2, apparent cell dimension of
1504 mm and a micrometer calibration with objective
20.X: 50 mm. This apparatus purchased from SEPHY,
converts the electrophoretic mobility Ue into the zeta poten-
tial z according to the Smoluchowski's equation [27,28]:
z h=1Ue; where h and 1 are, respectively, the viscosity
A. Jada et al. / Fuel 81 (2002) 1227 12321228
7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil
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and the permittivity of the aqueous medium. At least, we
achieve three experiments with each dispersion. The values
of the zeta potential were reproducible within ^2 mV.
3. Results and discussion
3.1. Infrared spectra of the neat oil and its polar fraction
Table 1 presents the amounts of the oil polar fractions
found in various insulating oils. As can be observed in the
table, the amounts of the polar fractions are higher in the
used oils as compared to the new one. The Fourier transform
infrared spectra (FTIR) of the neat oils NO, UO1 and UO2,
obtained in the region 4000600 cm21, are presented,
respectively, in the Fig. 1(a)(c). As can be seen in the
gures, the neat oils from different origins display similar
infrared spectra and are of high aliphatic index. These
spectra show three stretching and two bending absorption
peaks, respectively, in the regions 30002800 and 1450
1370 cm
21
, due to the alkyl CH bonds. In addition,Fig. 1(a)(c) indicates that the used oils UO1 and UO2 as
compared to the new one NO, are not degraded.
It is useful to know the structure of the insulating oil polar
fraction, which is the oil active specie that can cause electric
discharges and failure in the power transformer. Knowing
the structure of such oil polar fraction will allow us a better
understanding of its adsorption properties and the polarisa-
tion that occurs at the pressboard/insulating oil interface.
Fig. 2(a) and (b) shows infrared spectra of, respectively,
UO1 and UO2 polar fractions. Hence, we can observe differ-
ent features, when we compare the spectra to those of the
neat oils, in the region 18001600 and 34003100 cm21.
The spectra in Fig. 2(a) and (b) indicate also that the oilpolar fractions contain associated phenolic groups due to
hydrogen bonding, carboxylic groups, and aromatic groups.
These groups absorb, respectively, in the regions, 3400
3100, 18001700, and around 1600 cm21. Moreover,
Fig. 2(a) and (b) shows that the amount of the polar groups,
i.e. phenolic, carboxylic and aromatic groups, increase from
the UO1 to UO2 oil. On the other hand, the infrared spectra
of the asphaltenes as studied by others authors [2931],
present various absorption bands, which are found in the
spectra of oil polar fractions investigated in the present
work. These absorption bands are usually assigned to the
CH bonds of alkyl, the CH and the CyC bonds of
aromatic compound, the CyO bonds of carbonyl groups,
and the OH bonds of alcohol. Accordingly, the infrared
spectra of the UO1 and UO2 oil polar fractions presented in
Fig. 2(a) and (b) are consistent with the structure of the
asphaltenes. Thus, the asphaltene compounds contain
oxygen atoms, which are involved in various acidic func-
tional groups of linear or aromatic structures analogous to
carboxylic acid, and phenolic acid groups. Such groups, asdetermined by infrared spectroscopy, refer to the family of
compounds containing the carboxylic or phenolic, which are
bounded by alkyl chains to the condensed aromatic rings of
asphaltenes.
3.2. Electrical properties of the oil polar fraction
The electrical properties of the oil polar fractions were
determined by measuring the zeta potential of the oil polar
fraction covered silica particles. The analysis of the oil polar
fraction electrical properties provides a way of examining
charged particles in the oil, when the transformer is in equi-librium operation. Such charged particles can adsorb from
the oil onto the pressboard and hence cause failure in the
transformer.
Further, varying the pH of the aqueous dispersion allows
us to control the polarity of the oil polar fraction covered
silicawater interface and its inuence on the acidity of the
oil surface functional groups. Fig. 3 indicates the variation
of the zeta potential versus the pH, for various oil polar
fraction covered silica aqueous dispersions. The negative
values of the zeta potential of the aqueous dispersions
observed in the pH range 39 indicate negative oil polar
fraction surface charges. For the bare silica, the negative
surface charge observed is due to the ionisation of thesurface hydroxyl groups. The properties of such silica are
determined by the surface chemical activity, which in turn
depends on the concentration, the distribution, the type of
hydroxyl groups, the presence of siloxane bridges; and on
the porous structure of the silica. Several studies [32,33]
conrm the presence of silanol (SiOH) groups on silica
surface. In addition, in the capillary electrochromatography
on silica columns, used for separation of organic
compounds, the efciency depends on the silica particle
size [34].
The mechanism of the surface charge generation at the
A. Jada et al. / Fuel 81 (2002) 1227 1232 1229
Table 1
Amounts of the oil polar fractions adsorbed on the silica and maximum values of the zeta potential in water of the oil covered silica particles
Insulating oil Reference in
the text
Amount of the polar fraction
adsorbed on the silica (in
gram of the polar fraction per
100 cm3 of the neat oil) (%)
Maximum value of the oil
polar fraction covered
silica particles zeta
potential (mV)
New oil NO 0.052
35Used oil UO1 0.36 2 45
Used oil UO2 0.20 2 70
7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil
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A. Jada et al. / Fuel 81 (2002) 1227 12321230
Fig. 1. (a) Infrared spectra of the NO neat oil. (b) Infrared spectra of the UO1 neat oil. (c) Infra red spectra of the UO2 neat oil.
7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil
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bare SiO2water interface is mainly due to the adsorption of
hydroxyls (OH2) onto an amphoteric silica site. The
isoelectric point, i.e. the pH at which the zeta potential z
0; for this system can be obtained only by extrapolation,
since it occurs at about pH 2.5. Further, the acid side of
the z-pH curve is not accessible to electrokinetic
measurements.
On the other hand, the negative surface charge observed
in Fig. 3 for the oil polar fraction covered silicawater inter-
face, is attributed to the ionisation of the oil polar fraction
acidic groups. These groups, the carboxylic acid and pheno-
lic acid groups, have respectively, pKa, 4 and 4, pKa, 9
values [3537]. The important feature exhibited by these
used oil polar fractions, UO1, and UO2, when compared to
the bare silica, is the increase in the magnitude of the zeta
potential at pH values above 4. This behaviour is due to the
increase of the concentration of the oil surface groups, i.e.
the carboxylic acid and phenolic acid. However, the non-
used oil NO shows slight variations in the zeta potential,
within the experimental error range^2 mV. Fig. 3 indicatesalso an increase in the plateau levels zmax (see Table 1) from
the UO1 to the UO2, in a good agreement, with the increase
of their respective polar group amounts, as observed on their
FTIR spectra.
The change in the surface charge observed for the oil
polar fractions in the aqueous medium is related to the
nature and the amount of acidic groups present at their
surfaces. In addition, an arrangement of oil components
on the silica surface will occur in water, which will give
rise to the acidic groups development at the oilwater inter-
face. This surface arrangement is due to the differences in
the afnity of the various oil components for water and
silica.
It is likely that the polarity of charging at the oil polar
fraction/silica interface may resemble to the same charging
that occurs at the oil/pressboard interface in the power trans-
former. In fact, the surface of the paper or on the pressboard,
used in the transformer, contains polar groups, such as
hydroxyl (OH) that we nd also on the silica surface. In
these groups, the hydrogen and the oxygen atoms are,
respectively, positively and negatively polarised, due to
the large electronegativity of the oxygen atom. Such
positive polarised hydrogen has an afnity for the phenolic
groups that belongs to the oil polar fraction. Hence, in the
transformer, the pressboard may selectively adsorb the
family of compounds containing the phenolic groups,
which are bounded by alkyl chains to the condensed
aromatic rings of the oil polar fraction. Further, according
to Fowkes [38], the acidbase interactions between theacidic groups at the surface of a solid and the basic groups
of the molecules of a liquid are very specic. The energy of
such interaction is proportional to the enthalpy of the
complex or adducts formation of the acidbase pair and to
their surface concentration. Thus, in the pH range 39,
acidbase complexes between the acidic groups of the oil
polar fraction and the ions OH2 of the aqueous solution
form at the solidwater interface. Therefore, an increase
of the oil polar fraction surface charge with the increase
A. Jada et al. / Fuel 81 (2002) 1227 1232 1231
Fig. 2. (a) Infrared spectra of the insulating oil UO1 polar fraction. (b) Infrared spectra of the insulating oil UO2 polar fraction.
7/30/2019 Adsorption and Surface Tension of Silica in Transformer Oil
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of the pH should result from an increase of the concentration
of the acidbase surface complexes. Finally, the two
methods used in this work, microelectrophoresis and infra-
red spectroscopy, seem to be suited to investigate the
surface composition and functionalities of oil polar groups,
which may be present in the transformer insulating oil.Hence, we nd a good correlation between the amount of
the oil polar fraction, the peak area of various oil polar
groups, obtained by FTIR spectra, and the magnitude of
the zeta potential at the water/oilsilica interface. The use
of the two methods to investigate insulating oils is useful to
predict future failure in the power transformers.
4. Conclusions
At ambient temperature oil polar fractions adsorb from
various transformers, insulating oils (a new and two used
oils) onto the silica gel. The microelectrophoresis measure-ments in water of the various oil covered silica particles,
gives negatively charged oil polar fraction that resembles to
asphaltenes molecules. Further, the FTIR analysis of such
oil polar fraction indicates the presence of phenolic
carboxylic and aromatic polar groups. We nd a good corre-
lation between the amounts of the oil polar groups; their
functionalities and the magnitude of the zeta potential at
the water/oil covered silica interface.
In the power transformer, the oil polar fraction may
adsorb from the oil onto the pressboard leading to static
electrication. The use of microelectrophoresis and infrared
methods seems to be a good tool for analysis of the surface
charge and the structure of the particles included in the
insulating oil and hence to prevent electrical discharges in
power transformer.
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A. Jada et al. / Fuel 81 (2002) 1227 12321232
Fig. 3. Variation of zeta potential versus the pH for the aqueous dispersions
of the bare silica and the silica covered with insulating oil polar fractions.