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introduction :
based on their band gabs between valence and conduction bands, materials can be
classified toinsulators, semiconductors and conductors [mayer and lau ,1990].this gap is large
for an insulator at room temperature and theavailable energy is not enough to transport
electrons from the valence band to the conduction band, where they would be able to
contribute to conduction [pascoe , 1974].
insulators, usually subjected to moisture, rain, fog, dew, roads and sea pollutants,
chemicals from industry, ultraviolet radiation, temperature extremes, overvoltage s and
mechanical loads due to wind and ice.
insulating materials should not be porous, free from unwanted impurities, possesses high
insulation resistance to prevent leakage current to the earth, have very high dielectric strength
to withstand the voltage stresses in high voltage system, mechanically strong enough to carry
tension and weight of conductors, their physical as well as electrical properties must be less
effected by changing temperature and must not be any entrance on their surfaces so that the
moisture or gases can enter in it [solymar and walsh , 1990]:
electrical insulators can be classified based on their materials to ceramic, porcelain,
polymer and composites insulators[bartnikas ,1987]. surfaces of porcelain insulators should be
glazed enough so that water should not be traced on it[hackam , 1999].
glass insulators sever from that moisture can easily condensed on glass surface and hence air
dust will be deposited on the wed glass surface which will provide path to the leakage current
of the system[mishra et.al, 2008].
although, polymeric insulators offer many advantages like light weight, ease of processing,
good vandalism resistance and easy transportation [tanaka , 2004], but they have two
disadvantages:
1. moisture may enter in the core if there is any unwanted gap between core and weather
sheds. this may cause electrical failure of the insulator.
2. over crimping in end fittings may result to cracks in the core which leads to mechanical
failure of polymer insulator.
composite insulators offer many advantageous like, light weight-lower construction and
transportation costs, vandalism resistance, high strength to weight ratio, better contamination
performance and improved transmission line aesthetics [james, 1993].
recently, a great deal of attention has been paid to the application of nano fillers, with particle
sizes between 1 and 100 nm, in the field of electrical insulating materials [tanaka , 2005]. the
key advantage of nano composites is their larger specific area as compared with micro
materials(about three orders larger than micro composites). also, the distance between
neighboring fillers are much smaller in nano composites than in micro composites. with this,
the interaction of polymers matrices with fillers is expected to be much more in
nanocomposites[ lau and piah, 2011].
nanoparticles as fillers in nanocomposites faces two problems. the first is their easily
agglomeration because of their high surface energy and the second is the incompatibility of the
hydropinghobic polymer with hydropinghilic nanoparticles, resulting in poor interfacial
interactions[v?zquez, 2009].
silicone rubber (sir) insulators are preferred in polluted environments, sea coast areas,
industrial areas and in desert areas. also, the reduced number of metallic parts in this kind of
insulator makes it suitable for saline areas, where the corrosion of the hardware is a significant
issue insulation is often damaged by the corrosion of the metallic parts.the main advantage of
sirinsulation is its low life cycle cost.
the main property of sir is its hydropinghobicity which enables the insulating surface to
prevents water filming,i.e., on a hydropinghobic surface, the water forms discrete dropinglets
[hackam,1999].
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silicone rubber (polysiloxanes or polydimethylsiloxanes) is generally non-reactive, stable, and
resistant to extreme environments and temperatures from -55 °c to +300 °c while still
maintaining its useful properties [roux and ange, 2007]
sir consists of backbone of silicon-oxygen linkages and two methyl groups on each silicon.
the silicon oxygen backbone provides a high degree of inertness to ozone, oxygen, heat (up to
315oc), uv light, moisture, and general weathering effects, while the methyl substituents
confer a high degree of flexibility.
short chain cyclic compounds, often referred to as low molecular weight lmw fluid, are
present in the base material that are not locked in by vulcanization and are free to diffuse to the
surface of the cured rubber and are responsible for the hydropinghobicity in silicones [v?zquez ,
2009].
there are two types of silicone rubber are widely used for outdoor high-voltage insulation.
they are the high-temperature vulcanizing(htv)and the room temperature vulcanizing(rtv)
[momen and farzaneh, 2010]. htv is cured at high-temperature and pressure, catalyzed by
peroxide-induced free radicals or by hydrosilylation. rtv is cured at lower temperature, i.e.
around room temperature, by condensation reaction as one component system [56]. rtv
contains cyclic lmw (5wt.%) polydimethsiloxane while htv has cyclic and linear lmw(3
wt.%) polydimethylsiloxanes. the different properties of htv and rtv may be attributed to
the linear lmw in htv, which diffuses better.
sir exhibits the ability to restore its hydropinghobicity even after a pollution layer has built up on
the surface,which can suppress the development of leakage currents, dry-band arcing and
flashover[v?zquez , 2009].
organic rubber has a carbon-to-carbon backbone which can leave it susceptible to ozone, uv,
heat and other ageing factors that silicone rubber can withstand well.polysiloxanes differ from
other polymers in that their backbones consist of si-o-si units unlike many other polymers that
contain carbon backbones. polysiloxane is very flexible due to large bond angles and bond
lengths when compared to those found in more basic polymers such as polyethylene. for
example, a c-c backbone unit has a bond length of 1.54 ? and a bond angle of 112?, whereas
the siloxane backbone unit si-o has a bond length of 1.63 ? and a bond angle of 130?. the
silicon-oxygen linkage in the silicone polymer chain is the same as that in sand, quartz, and
glass. this bond is responsible for the good high temperature stability of the silicones and their
resistance to weathering, corona discharge, and oxidation by ozone.
experimental part
materials:the used materials in this research are silicone rubber (sir) and silica nano
particles (snp).
the used type of silicone rubber(table 1) is room temperature vulcanized (rtv-630)
which consists of two-parts flow-able liquid silicon and curing agent (dbph type).the
usednano silica particlespossess 25-43 nm particle size with the following properties(table2).
samples preparation: in this work five samples of silicone rubber – nano silica composites
were prepared as shown in table (3).
procedure:the five nano composite samples were mixed, shaped (uniform sheet with 4 mm
thickness) and sampled. the mixing process consists of:
1-flowable part of the rtv-630 silicone rubber was mixed with the nano silica using ultrasonication
technique by using falc device for 10 minutes at 90oc.
2- curing part of the rtv-630 silicone rubber was mixed with the previous nano mixture for
two minutes.
tests:the breakdown strength test was carried out by oil tester (pgo s-3), while
acconductivity, loss tangent, dielectric constant tests were carried out by using (hioki 3532-
50 lcr hi tester which operates in the frequency range up to 5mhz.equations (1-3)used
synthesis and charaterization auda jabbar braihi
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to calculate dielectric constant (real part ? ),dielectric constant (imaginary part ?") and ac
conductivity (?ac).
? = cp / cair(1)
?" = ? . tan ? (2)
?ac = 2? f ?o ? tan ? (3)
where :cpand cair are capacitances of the pellet and air in faraday respectively , f is the
frequency in hertz, tan ? is the loss tangent and ?o is the constant permittivity of the free space
which is equal to 8.854* 10-12 f/m.
tensile properties such as tensile strength, deformation, elongation at break and elastic
modulus were measuredat room temperature using the universal tensile tester (model wdw
200 e) according to astm d-638-iv.the test starts by applying specified load (5kn) and the
cross head speed was 5 mm/min.
hardness testwas carried out on disks (10 mm diameter and 5 mm thickness) according to the
astm d 2240 using shore a hardness deviceat room temperature.the average of five
readings were adopted.
thermal conductivity test was carried out by using (thermal conductivity device) by placing
the sample between the upper and lower copper plate and ensure a good contact with each
other without too tightened or too loosened, setting the controller temperature as100?c and
record the temperature indication every 30 seconds.
contact angletest(circle fitting mode) was used to evaluate the wettability of the nano
composites using (sl200b optical dynamic / static contact angle meter ).
two types of aging tests were carried out. the first is uv aging test usinghardness samples
with radiation wavelength 375- 380 nm for 72 hour.the second isthermal aging test was
carried out on dumbbell samples for 72 hours at 250oc using electrical oven device.
morphology and chemical composition tests were carried out using scanning electron
microscope (inspect s50/fei company) which provided with edx technique.
ftir test was carried out using ir affinity-1 shimadzu , while dsc-60 – shimadzu device
was used to study the thermal transitions.
results and discussion
tem and sem images (shown in figures 1 and 2) proved that the prepared snps possess nano
scale dimensions of 25-43 nm.
figure (3) shows the effect of nano silica ratios on the values of the dielectric strength
breakdown of the pure silicone rubber as well as of composite results from the addition of nano
silica with four ratios (1,2,3,3.5 pphr). it s clear from this figure that the pure sample (0
pphrnano silica) cannot be used alone as an electrical insulator for high voltage because it s
breakdown strength less than 11 kv.
the 2 pphrnano silica ratio give the highest breakdown strength which is 13.9 kv/mm.this
behavior can be attributed to the maximum bonding at this ratio .this bonding occurs through
the chemical reaction between silanol groups on nano silica surface(figure 4) and function
groups on the surface of rtv silicone rubber as investigated by ftir test (figure 5). the
ftir test show that there is a chemical reaction between the rtv silicone rubber (matrix) and
the nano silica (filler) through si-c bonds, were the intensity of the si-c band increased from
2.96 (which belong to the intrinsic si-c bonds inrtv silicone rubber) to 19.2 .this is in good
agreement with [77].
figure (6) proved that the minimum ratio of the dissipated energy to the external applied
energy(loss tangent tan ?) occurs at 2 pphr nano silica ratio . this is because of the high
bonding at this ratio as shown earlier.
figure (7) showed that the maximum real part of the dielectric constant (? )was obtained at 2
pphr nano silica ratio. this means that 2 pphr ratio considered as the optimum ratio from the
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view point of the insulation application.this conclusion agree with dielectric breakdown
strength results.
figure (8) proved that at 2 pphr ratio, there is a minimum heat dissipation (dielectric loss
imaginary part ?") through the insulator.
figure (9)indicated that the maximum resistivity occurs at 2pphr ratio because the conductivity
is the reciprocal of the conductivity. this conclusion supports the using this ratio as an
electrical insulator for high voltages.
figure (10) shows the behaviors of both normal dumbbell samples and samples heated to
250oc for 72 hours. it s clear that the tensile strength increase with increasing nano silica ratios.
this is because the stiffness and the rigidity of the nano composite increased as the interactions
and bonding increased through the structure of the nano composite. also, it s clear from this
figure that all the tensile strength values of the aged dumbbell are lower than those of normal
dumbbell. this is due to the breakdown and cleavage of bonds under heat action which reduces
the stiffness and rigidity.
figure (11) showed the hardness values for the normal samples, samples aged by uv radiation
(wavelength 375- 380 nm) for 72 hour as well as for samples aged thermally for 72 hours at
250oc. it s clear that all hardness values increases with the increasing nano silica ratios due to
the increasing of the interactions .also, it s clear that thermal heating possess higher influence
on the hardness than the uv effect. this is due to the uv radiation induces crosslinking
through samples which enhance the hardness.
the silicon-oxygen linkage in the silicone polymer chain is the same as that in sand,quartz, and
glass. this bond is responsible for the good high temperature stability of thesilicones and their
resistance to weathering, corona discharge, and oxidation by ozone.organic rubber has a
carbon-to-carbon backbone which can leave it susceptible to ozone, uv, heat and other ageing
factors that silicone rubber can withstand well.
figure (12) shows the effect of the nano silica addition on the morphology of the rtv silicone
rubber. it s clear that the surface of the nano composite is smoother than the surface of the pure
sample. this is because the additional interactions due to silica addition. this conclusion was
supported by edx results (figure13)which show increasing si, o and c contents after nano
silica addition.
it s clear from figure(14) that the thermal conductivity proportional to the nano silica content
due to the thermal conductivity of silica (1.4 w/m.k) is more than the thermal conductivity of
rtv silicone rubber(0.3 w/m.k ) .
the wettability of the pure rtv silicone rubber and 2 pphr nano composite were evaluated by
contact angle measurement (circle fitting mode) as shown in figure(15). it clear that nano
silica addition decrease the hydropinghicity nature by reducing the contact angle values as shown
in table (4).hydropinghobicity is lost naturally due to the rain or artificially by washing the
insulating surface. fortunately, the hydropinghobicity is recovered after a few hours at normal
temperatures. this hydropinghobic recovery property can be attributed to two reasons:
1-transfer of low molecular weight (lmw) fluid from bulk to surface and the subsequent
adsorption process of lmw on the contaminant, either physical or chemical adsorption. the
microscopic diffusion of fluid serves to encapsulate contaminant particles and prevent moisture
absorption.
2-reorientation of the hydropinghobic methyl groups at the surface.
figure (16) shows the effect of nano silica addition on the melting behavior of rtv silicone
rubber .it s clear that the addition shifted the melting peak up word form 285.35oc to 305.85oc
.this is because the additional interactions increase the rigidity as well as due to that nano
silica itself possesses melting point higher than silicone rubber.
conclusions
1- optimum breakdown strength, loss tangent, dielectric constant (real part), dielectric loss
(imaginary part), resistivity and wettability obtained at 2 pphrnano silica ratio.
synthesis and charaterization auda jabbar braihi
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2-the agglomeration of nanoparticles prevent forming nano composite from more than 3,5
pphr nano silica ratio .
3- addition of the nano silica alters the morphology, melting behavior and the chemical
composition of the samples
4- nano silica addition causes chemical bonding through si-c bonds.
5- nano silica addition increases the mechanical properties (tensile strength and hardness).
6-thermal conductivity increases gradually with nano silica ratios.
table 1 properties of silicone rubber rtv-630
silicon rubber (rtv 630) model
white appearance
3%-5% mixing proportion of curing agent (%)
30-34 hardness (shore a)
1.08 density (g/cm3)
4 tensile strength (mpa)
? 250 elongation (%)
table 2 properties of the used nano silica particles
table 3 compositions of the prepared samples
silicone rubber(pphr) silica nano particles
curing part flow-able part (pphr)
3 97 0
3 96 1
3 95 2
3 94 3
3 93.5 3.5
value property
white powder appearance
99.5% purity
amorphous morphology
25-43 nm particle size
54. 86 o (wt%)
45.14 si (wt%)
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fig.1. tem image of snp
fig.2. sem images of snps
fig.3. dielectric strength of nano composites as a function of nano silica ratio
synthesis and charaterization auda jabbar braihi
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fig.4.structure of amorphous silica
fig.5. ftir spectrum of(a) pure rtv silicone rubber (b) nano composite with 2 pphr
silica nano particles
fig.6. tangent loss of nano composites as a function of nano silica ratio
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fig.7. dielectric constant (real part)of nano composites as a function of nano silica ratio
fig.8. dielectric constant (imaginary part) of nano composites as a function of nano silica ratio
fig.9. ac conductivity of nano composites as a function of nano silica ratio
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fig.10. tensile (for normal and aged states) of nano composites as a function of nano silica ratio
fig.11. hardness (for normal and aged states) of nano composites as a function of nano silica ratio
fig.12. sem images of (left) pure silicon rubber (right) nano composite with 2pphr silica
nano particles
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fig.13. edx graphs for (a) pure rtv silicone rubber (b) 2 pphr nano silica –sir composite
fig.14. thermal conductivity of nano composites as a function of nano silica ratio
a b
fig.15. contact angle of (a) pure rtv silicone rubber (b) 2pphr nano silica –sircomposite
table 4 contact angles for prepared samples
synthesis and charaterization auda jabbar braihi
of nano composite for ali yahya muneer
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fig.16. dsc curves of (a) pure rtv silicone rubber (b) 2pphr nano silica –sir composite
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