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IEEE Transactions on Dielectrics and Electrical Insulation Vol.
11, No. 5; October 2004 763
Polymer Nanocomposites as Dielectrics and
ElectricalInsulation-perspectives for Processing
Technologies,Material Characterization and Future Applications
T. TanakaGraduate School of Information, Production and System
LSI
Waseda University, 2-7 Hibikino, Wakamatsu-kuKitakyushu-shi,
808-0135, Japan
G. C. MontanariDepartment of Electrical Engineering
High Voltage and Material Engineering LaboratoryUniversity of
Bologna, 40136 Bologna, Italy
and R. Mulhaupt¨Institute of Macromolecular
ChemistryAlbert-Ludwig University of Freiburg
Stefan-Meier-Str. 31, D-7904 Freiburg i. Br., Germany
ABSTRACTPolymer nanocomposites are defined as polymers in which
small amounts ofnanometer size fillers are homogeneously dispersed
by only several weight percent-ages. Addition of just a few weight
percent of the nano-fillers has profound impacton the physical,
chemical, mechanical and electrical properties of polymers.
Suchchange is often favorable for engineering purpose. This
nanocomposite technologyhas emerged from the field of engineering
plastics, and potentially expanded itsapplication to structural
materials, coatings, and packaging to medicalrrrrrbiomedi-cal
products, and electronic and photonic devices. Recently these
‘hi-tech’ materi-als with excellent properties have begun to
attract research people in the field ofdielectrics and electrical
insulation. Since new properties are brought about fromthe
interactions of nanofillers with polymer matrices, mesoscopic
properties areexpected to come out, which would be interesting to
both scientists and engineers.Improved characteristics are expected
as dielectrics and electrical insulation. Sev-eral interesting
results to indicate foreseeable future have been revealed, some
ofwhich are described on materials and processing in the paper
together with basicconcepts and future direction.
Index Terms — Nanocomposite, polymer nanocomposite, nanofillers,
advancedmaterials, dielectrics, electrical insulation.
1 INTRODUCTIONHE development of nanocomposites represents aTvery
attractive route to upgrade and diversify proper-
ties of ‘‘old’’ polymers without changing polymer composi-tions
and processing. In contrast to conventional filledpolymers,
nanocomposites are composed of nanometer-
Ž .sized fillers ‘‘nanofillers’’ which are homogeneously
dis-tributed within the polymer matrix. Due to their very
highspecific surface areas, a few percent nanofillers can self-
Manuscript recei®ed on 25 February 2004, in final form 7 May
2004.
assemble to produce skeleton-like superstructures espe-cially
when anisotropic fillers with high lengthrdiameter
Ž .ratio aspect ratio are used. In comparison with the
con-ventional micrometer-sized fillers, the same volume frac-tion
of nanofillers contains billion-fold number ofnanoparticles. As a
result, most of the polymer ofnanocomposites is located at the
nanofiller rpolymer in-terface. The conversion of bulk polymer into
interfacialpolymer represents the key to diversified polymer
proper-ties. As a function of the nanofiller aspect ratio it is
possi-ble to reinforce the polymer matrix and to improve thebarrier
resistance against gas and liquid permeation. An
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important aspect of nanocomposite formation relates toprocessing
technology. While many prefabricatednanoparticles are difficult to
disperse and require specialsafety precaution for their handling,
modern nanocompos-ites are formed in situ via shear-induced
intercalation andexfoliation as illustrated by the effective
diffusion of poly-mer in between organophilic nanoparticles.
Let us compare filled polymers with filler-added poly-mers.
Nanofiller-added polymers or polymer nanocompos-ites might be
differentiated from micro-filler-filled poly-mers in three major
aspects that the nanocomposites con-tain small amounts, are in the
range of nanometers in sizeand have tremendously large specific
surface area. Allthese characteristics would reflect on their
material prop-erties. The first thing to attract interest arises
from thedifference in content. Conventional filled polymers
usu-ally contain a large amount of the fillers, e.g., more than50
wt%. Therefore those materials are really mixtures ofpolymers with
mineral fillers, resulting in big change ordifference in material
properties from polymers per se. Inthe case of nanofillers, the
amount less than 10 wt% isenough, so that some of intrinsic
polymeric properties, e.g.,density, must remain almost unchanged
even after theybecome nanocomposites. The second characteristics of
in-terest are expected from filler size difference. Size is
dif-ferent by three orders of magnitude in length between thetwo
kinds of materials, which would cause much more dif-ference, i.e.,
roughly by nine orders in their number den-sity. Therefore the
distance between neighboring fillers aremuch smaller in
nanocomposites than in conventionalfilled polymers. In many cases,
the inter-filler distancemight be in the range of nanometers, if
fillers are homo-geneously dispersed. The last, but not least,
difference isconcerned with the high specific surface area of
fillers.The specific surface is represented by the inverse size,
andthen is three orders larger for nonocomposites than thatfor
conventional filled polymers. Interaction of polymersmatrices with
fillers is expected to be much more in theformer than in the
latter. In contrast to many conven-tional fillers, some nanofillers
are composed of polyelec-trolyte nanoplatelets which disassemble
and get dispersedduring processing. This formation of nano-scaled
poly-electrolyte complexes can have a major impact on the
di-electric behavior.
These nanocomposite features offer new opportunitiesfor
designing a totally different world of dielectrics. In fact,the
size of fillers and the inter-filler distance are in therange of
nanometers, and the fillers would interact chemi-cally and
physically with polymer matrices, resulting in theemerging of
intermediate or mesoscopic properties thatbelong neither to atomic
nor macroscopic frame. Polymernanocomposites have attracted
scientists and engineers inthat these materials are potentially
endowed with unex-pectedly excellent properties. Recent scientific
discoveriesand technical breakthroughs in the materials indicate
their
upgrade from simple commodity plastics to ‘hi-tech’ mate-rials
with exceptional properties. The nanocomposites aremade in the form
of polymers containing a small amountof nanometer size fillers, or
nanofillers. Polymer matricesinteract with such nanofillers and are
chemically bondedtogether in many cases. These can be called
filler-addedpolymers rather than conventional filled polymers or
filledresins. Debye shielding length is in the range of nanome-ters
in metal, and so might be better watched in case thatthe effect of
electrode in contact with nanocomposites islarge.
There are several preceding areas of investigation
andapplication. In the field of engineering plastics,
materialselection has been made on the basis of property dia-
w x w 3xgrams such as Young’s modulus GPa vs. density Mgrmw x w
3xor the yield strength GPa vs. density Mgrm from me-
chanical properties’ point of view. Materials with lighterŽ
3.weight density: less than 1.5 Mgrm and more mechani-
Ž .cal strength E s 5�20 MPa, � s0.4 to 1.1 MPa areypreferred.
Much more expectation is naturally directed tothermal endurance or
thermal stability, stability againstaggressive chemicals,
impermeability against gas, waterand hydrocarbons, recyclability
through re-processing andless leakage of small molecules such as
stabilizers. In com-parison to traditional composites,
nanocomposites are
Ž . Ž .certainly advantageous in 1 homogeneous structure, 2Ž . Ž
.no fiber rupture, 3 optical transparency, and 4 im-
proved or unchanged processability.
In the field of food packaging industry, new materialswith low
permeability against oxygen, carbon dioxide, ni-trogen and water
vapor are expected. Often they shouldbe biodegradable. They should
be processed byinjectionrcompression molding, film blowing andror
cast-ing.
What can one expect from nanocomposite applicationsin
dielectrics and electrical insulation? Top priority wouldbe
dielectric breakdown strength. Generally polymerswould have
intrinsic breakdown strength as high as 10
Ž .MVrcm s1GVrm at room temperature, but in practicetheir
breakdown strength is much lower and is deter-mined by internal
defects. Therefore, nanofillers shouldhave the effect to increase
practical dielectric breakdownstrength, irrespective of defects
contained. In this regard,the resistance against partial discharge
and treeing isgreatly concerned. Mechanical strength and thermal
con-ductivity would be also involved if compact insulation de-sign
is required. Permittivity and dissipation factors as di-electric
properties should be as low as possible for electri-cal insulation,
while the former is required to be as highas possible for
capacitors. Flame retardancy is preferredfor cable insulation used
in the radiation field, whiletracking resistance is of interest for
outdoor insulators. Fordc application, formation of space charge in
insulationshall be contrasted. Recyclability is generally required
toprotect environments. All the performances described
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above, as well as other characteristics, might be improvedby
means of nonocomposite technology.
Processing methods suitable for nanocomposites as wellas their
applications have been developed by in situ reac-
Ž .tive blending, melt-mixing, thermo-kinetic sheared mix-ing,
extrusion, blowing and injection. Materials tried fornanocomposites
are polyolefin such as polyethylene,polypropylene and
polyethyleneterephthalate, epoxy, elas-tomers such as silicone
rubber and EPRrEPDM,polyamide, and polyimide for example.
Some of the favorable results have been obtained fordielectrics
such as polyethylene, silicone, epoxy,polyamide, and polyimide.
Fillers such as Al O , TiO ,2 3 2SiO and layered silicate have been
selected. Material2combinations are epoxy- TiO , PE- TiO , EPDM- Al
O ,2 2 2 3PE- SiO , polyimide- SiO , epoxy-layered silicate, and2
2polyamide-layered silicate. Outstanding results based onpioneering
works that several research groups have beentackling are described
in the paper.
On the basis of the above description, the followingitems are
reviewed in the paper: Concept of Nanocompos-ites, Materials of
Interest, Processing Methods, Experi-mental Work and Results and
Discussion on ElectricalProperties.
1.1 CONCEPT OF NANOCOMPOSITESIt seems difficult to define a term
of nanocomposites
clearly. The paper deals with what are called
polymernanocomposites. They are usually considered as
three-di-mensional composites of polymers with inorganicnanofillers
dispersed. In addition to those, two-dimen-sional nano-layered
structures are investigated as nano-metric dielectrics, too.
Nano-structured organic-inorganicmaterials and nano-structured
polymer-polymer sub-stances that are both chemically synthesized
from the be-
ginning are also an important field of investigation. Theformer
is out of scope in the paper and the latter is brieflyreviewed.
1.2 NANOMETRIC DIELECTRICSA term of ‘‘Nanometric Dielectrics’’
was used in 1994
w x1 as future research area of dielectrics. It was empha-sized
that our interest is shifted from ‘‘Debye relaxationalculture’’ to
‘‘a nanotechnical culture’’. The former is gov-erned by determining
statistically averaged properties,while the latter is largely
affected by a molecular order ofsmallness of systems and system
elements. Therefore, asshown in Table 1, it was suggested that
phenomena tooccur at nanometric scale should be explored to
clarifyfundamental properties and might open up a new field
ofapplications. This term might have been defined asnanometer size
dielectrics to investigate dielectric phe-nomena in nanometer
scale. A term of ‘‘NanoDielectrics’’
w xwas proposed in 2001 2 to explore nanometric dielectricsand
dielectrics associated with nanotechnology and toproduce
molecularly tailored materials. This conceptseems to be associated
with nanostructured ceramics andtailored nanocomposites.
Structures of nanometric dielectrics and their charac-teristics
are listed in Table 1 based on the description
w xmade in the reference 1 . This deals with bulks and lay-ered
structures, and especially focuses on the interfacialaspect of
nanometric phenomena, which are treated notonly by the classical
macroscoptic theories but also by thequantum mechanical theories.
This might represent thatthe phenomena we have to face in this
field are meso-scopic in between macroscopic and atomic scales. It
cov-ers not only electrical insulation but also electronics,
liv-
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ing systems, MEMS, batteries and the like. It is interest-ing to
note that, in connection to STM and AFM embed-
Ž .ded-polymers, a field effect display FED has been devel-Ž
.oped using carbon nanotubes CNT , which is considered
a next generation display device. Here STM and AFMstand for
scanning tunneling and atomic force micro-scopes, respectively.
1.3 COMPOSITE TO NANOCOMPOSITE[ ]MATERIALS 3
In general, composites consist of two phases, i.e., a ma-trix
and a disperse phase. Nanocomposites are named,when the disperse
phases are in the range of 1 to several
Ž .hundreds in nanometer nm . What is called a ‘‘single
nanoparticle’’ of 1 to 10 nm exhibits characteristics differentfrom
either atoms or low molecular bulk materials.Fullerens, carbon
nanotubes, dendrimers that have everattracted people are in the
range of 1 nm order, and thenmaterials can be called
nanocomposites, when they arecombined.
Composites were developed as structural materials,where carbon
fiber reinforced plastics developed in 1960’swere a typical
example. Emphasis was then placed on theimprovement of mechanical
strength and thermal en-durance. Therefore other functional
properties were setaside. However, the advent of nanotechnology has
changedsuch a situation and has opened a completely differentworld
of nanocomposites as functional materials, not sim-ply as
structural materials, but materials with optical,electrical,
electronic, magnetic, chemical and biologicalfunctions
Generally nanocomposites are, thus, a material systemof the form
that nanoparticles are dispersed in continuousmatrices. But
functional nanocomposites might includelayered structure devices
such as photovoltaic cells, junc-tion devices such as diodes and
transistors, and surfacemodified materials such as DNA chips the
surface of whichare fixed with biomolecules, if they are all in
nanometerscale. Layered devices consisting of organic and
inorganiccompounds and even mono-molecule transistors might besome
of the ultimate goals as nanostructured devices.
Table 2 shows many examples of nanocomposites andmaterial
combinations in such a wide range of applicationfields that include
structural and materials engineering,electronics and electrical
engineering, optics and opto-electronics, catalysts, filtration
membranes, bio-nano-technology and yarn. This table includes not
only polymernanocomposites but also inorganic-inorganic
nanocompos-ites for reference, and even somewhat different types
ofnanocomposites such as meso porous materials filled withpolymer
chains inside. Nanocomposites consist of a matrixpolymer and a
disperse filler for disperse type, and mate-rial 1 and material 2
for layered structure and copolymertypes.
1.4 DISPERSE PHASE AND[ ]NANO-PARTICLES 4
Nano-particles dispersed in nanocomposites are de-fined to lie
in either size between 1 to several 100 nm.Table 3 shows examples
of disperse phase ranging fromsub-nm to 1000 nm. Substances in size
of a few nanome-
Ž .ters 1 to 10 nm exhibit quite different characteristics
fromeither atoms and molecules smaller than 0.3 nm, or
bulkmaterials, as described above. Small-particle nanocompos-ites
are more important as functional materials thanstructural
materials. For example, melting point of wateris markedly lowered
due to the surface effect, if it is 10nm in size inside nano-porous
materials, and ice crystalsformed in carbon nanotubes smaller than
1 nm in innerdiameter exhibit behaviors different from bulk ice.
Fur-thermore, quantum effects emerge in the few-nanometerregion as
electrical and optical properties. Quantum dotsare one of the
examples. Polymer membrane exhibits re-verse osmosis, if its fine
holes are smaller in inner diame-ter than 1 nm, while it shows
ultra filtration, when theholes are in the range of 10 to several
tens nm. Fine holesin artificial dialysis membranes are close to 1
nanometerin inner diameter, and then prevent proteins from
passingthrough. Metal nanoparticles, fullerenes, carbon nan-otubes,
giant organic compounds, and mono or accumu-lated molecular layers
are some of the examples ofnanocomposites with particles having
very small nanome-ter size.
In the range of submicrometers, the size of dispersephase become
more apparent in structural materials thanin functional materials,
and then impact-resistant polymernanocomposites and high-toughness
ceramic nanocompos-ites are often optimized in filler size in this
range. Whenthe disperse phase is in the range of 10 to 100 nm,
struc-tural and optical properties are markedly affected. Rub-bers
and metals are reinforced by carbon black and byparticles,
respectively, where the smaller size would makebetter effect.
Optical glass prefers the transparency andthen limits the upper
size of fillers. Titanium dioxide isoptimized in its size for
cosmetics so that it may pass visi-ble light and scatter
ultraviolet light. Chemical tips underdevelopment, where a series
of chemical reactions such asmixing, reaction and analysis is held
on a tiny tip, wouldrequire technologies in size of this range.
Semiconductortips are finely processed in the range of 50 nm by
lithogra-phy.
Carbon black, which has been long used to reinforcerubbers, is
generally several tens of nm. On the other hand,commercial polymer
alloys include components hundred
Žnm in size. Among them, ABS acrylonitrile-butadiene-.styrene
terpolymer resin is endowed with optimum im-
pact resistance by controlling the size of disperse phase inthe
range of several hundred nm. Polymerrlayered claynanocomposites are
reinforced by introducing very thindisperse phase clay layers. For
example, in the case of
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nylonrmontmorillonite nanocomposite, montmorillonitelayers
become separated by 1 to 2 nm being exfoliatedinto nylon matrix.
Disperse phase of several tens of nm isobtained for aliphatic
polyamiderall aromatic polyamidealloy by the solution blend method,
resulting in significantmechanical reinforcement. However, this
method is veryexpensive, and then the melt compound method is
beingexplored for production on a commercial base. The meltcompound
method utilized for PETrall aromatic polyesteralloys gives
inadequate reinforcement, since dispersephase is limited to around
1 �m in size due to processingdifficulty. Methods for dispersion
should be improved inthis case, including development of
compatibilizers.
A functional material that has been used for a long timeis
silver salt for photo films. Size of silver bromide as
aphotosensitizer is usually larger than 1 �m, and is re-quired to
be several tens to hundreds in nm for more finepictures.
Nano-particles dispersed by the sol-gel methodfor functional
optical glass are several tens of nm in size.Blended polymer alloys
are often used as functional mate-rials. In this case, the size of
disperse phase is similar tothat for structural nanocomposites.
Block copolymers usedfor bulk materials and transparent materials
are providedwith micro phase separation structures of several
nanome-ters. In case of surface modification by graft, a
surfacelayer is thinner than that. Surface layers of enzymes
sur-
face-immobilized to biomaterials and physiologically ac-tive
substances, DNA probes, DNA chips, protein chips,and columns
immobilized to polymixins for septicemiatreatment are as thin as
below 1 nm. Nano thick films areoften formed by the vacuum
evaporation method and thespin coating method, and they are in the
range of severaltens to hundreds of nm. Monomolecular and
accumulatedmolecular layers typical for LB films are often thinner
than1 nm. Functional particles such as noble metal nano-par-ticles,
carbon nanotubes, fullerens and dendrimers are inthe order of 1
nm.
2 MATERIALS OF INTEREST[ ]2.1 POLYMER NANOCOMPOSITES 3, 4
Polymer nanocomposites are understood as polymers inwhich
nanofillers are homogeneously dispersed, but alsolayered structures
in nanometric scale shall be included inthis category. In disperse
type, nanofillers are usually or-ganically modified to be miscible
with companion poly-mers. Polymerrlayered silicate or clay
nanocomposites aremost popular now as polymer nanocomposites, in
whichsilicate or clay is micaceous in shape. They could be
datedback to 1990, when they were industrially manufacturedas
structural materials or engineering plastics for the firsttime.
This success initiated further significant research ac-
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tivities on a variety of nanocomposites for other purposes,too,
e.g, nanoparticles such as TiO , SiO and Al O can2 2 2 3be mixed
into polymers to form nanocomposites.
2.1.1 POLYMER/LAYERED SILICATE[ ]NANOCOMPOSITES 5
Various polymers are feasible as layered silicatenanocomposites,
which include vinyl polymers, condensa-
Ž .tion step polymers, polyolefins, specialty polymers,
andbiodegradable polymers, as shown in Table 4. Layered sil-icates
belong to the family of 2:1 phyllosilicates, as shown
w xin Figure 1 4 . Their crystal structure consists of
layersmade up of two tetrahedrally coordinated silicon atomsfused
to an edge-shared octahedral sheet of either alu-minum or magnesium
hydroxide. The layer thickness isaround 1 nm, and the lateral
dimensions of these layersmay vary from 3 nm to severalymicrometers
or larger,which depend on the particular layered silicate.
Stackingof the layers leads to a regular van der Waals gap
betweenthe layers called the interlayer or gallery. Isomorphic
sub-
Ž 3qstitution within the layers for example, Al replaced byMg2q
or Fe2q, or Mg2q replaced by Li1q generates neg-ative charges that
are counterbalanced by alkali and alka-
w xFigure 1. Structure of 2:1 phyllosilicates. Drawn based on 5
.
line earth cations situated inside the galleries. This type
oflayered silicate is characterized by a moderate surface
Ž .charge known as the cation exchange capacity CEC . Thischarge
in not locally constant, but varies from layer tolayer, and should
be considered as an average value over
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the whole crystal. Montmorillonite, hectorite and saponiteare
the most commonly used layered silicates.
There are two types of structure: tetrahedral-sub-stituted and
octahedral substituted. In case of tetrahe-drally substituted
layered silicates, the negative charge islocated on the surface of
silicate layers, and hence, thepolymer matrices can react more
readily with these thanwith octatedrally- substituted material. Two
distinct butinterrelated features characterizes layered silicates.
Thefirst is the ability of the silicate particles to disperse
intoindividual layers. The second is the ability to finely
tunetheir surface chemistry through ion exchange reactionswith
organic and inorganic cation. Pristine layered sili-cates usually
contain hydrated Naq or Kq ions. These are
Žonly miscible with hydrophilic polymers such as poly eth-. Ž .
Ž . Ž .ylene oxide PEO , or poly vinyl alcohol PVA . To make
these miscible with other polymers, their surface shouldbe
converted to an organophilic one, which can be real-ized by
ion-exchange reactions with cationic surfactantssuch as
alkylammonium and alkylphosphonium cations.
Three different types of polymerrlayered silicatenanocomposites
are thermodynamically achievable,namely intercalated, flocculated
and exfoliated nancom-
w xposites, as illustrated in nanometric scale in Figure 2 5 .In
intercalated nanocomposites, the insertion of a poly-mer matrix
into the layered silicate structure occurs in acrystallographically
regular fashion, regardless of the clayto polymer ratio.
Intercalated nanocomposites are nor-mally interlayer by a few
molecular layers of polymer.Properties of the composites typically
resemble those ofceramic materials. Flocculated nanocomposites are
con-ceptually the same as intercalated ones. However,
silicatelayers are sometimes flocculated due to
hydroxylatededge-edge interaction of the silicate layers. In
exfoliatednanocomposites, the individual clay layers are
separatedin a continuous polymer matrix by an average distance
thatdepends on clay loading. Usually, the clay content of
exfo-liated nanocomposites is much lower than that of interca-lated
nanocomposites.
2.1.2 POLYMER/METAL OXIDENANOCOMPOSITES
In a composite, the polymer in the vicinity of the filleris
strongly affected by the presence of the filler and thearea
surrounding the filler particle is called the interphase
w xor the interaction zone, as shown in Figure 3 6 .
Fornanocomposites, the volume fraction of the polymer ad-joining
the filler is large due to the large surface area. It iswell known
that the polymer chains interacting with thesurface of the filler
have altered properties such as crys-tallinity, crosslink density,
mobility or conformation.Therefore, the interface chemistry and
interfacial strengthis a much more critical parameter in
nanocomposites thanin traditional composites. Further, since these
interactionzones overlap at relatively low volume fractions, it has
been
Figure 2. Schematic illustration of three different types ofw
xpolymerrlayered silicate nanocomposites. Drawn based on 5 .
proposed that a small amount of nanofillers can have aprofound
affect on material properties. But unfortunately,such interaction
zones are not necessarily well character-ized. It was also
suggested that free volume in such inter-
w xaction zones would be affected by nano-fillers 7, 8 .
2.2 DIELECTRICS AND ELECTRICALINSULATING MATERIALS
Polymer matrices with dispersed inorganic nanoparti-cles are now
a target for investigation in the dielectric field.Performance
improvement as dielectrics and electrical in-sulation is expected.
Both thermoplastic resins and ther-moset resins are being
considered and their companion
Ž .nanoparticles are selected from clay layered silicates ,
sil-Ž . Ž . Ž .ica SiO , rutile TiO , and alumina Al O . Disperse2
2 2 3
phases, nanoparticles or nanolayers of interest vary in sizeas
indicated in Table 3. Layered silicates or clays are inthe range of
a few nm in thickness, and in the range of100 nm in other two
dimensions. Size of nanoparticles ofinorganic substances such as
SiO , TiO and Al O are2 2 2 3chosen to be 30 to 40 nm. Polymer
materials such as PE,PP, EVA, EPDM, PA, PI and epoxy are under
investiga-tion.
Figure 3. Illustrated morphology of polymers around metal
oxidesw x10 .
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w xFigure 4. Intercalation and exfoliation process for polymer
nanocomposites 19 .
3 PROCESSING METHODS UNDERDEVELOPMENT
Composite materials consist of at least two differentkinds of
material phases, and therefore inherently havethe interfaces with
which they both contact each other. Inorder to form a stable
composite system, it is requisite toincrease the compatibility
between them, and decrease theinterfacial tension between them to
the utmost. Especiallynanocomposite materials include nanoparticles
that aredispersed in nanometer scale separation even with
theirsmall addition as shown in Table 3, and are enormouslylarge in
their total surface areas. Thus it is technically re-quired to form
such a material system stably without ag-glomeration and phase
separation for production ofnanocomposites There are several
methods for nanocom-posites such as intercalation, sol-gel,
molecular compositeand direct dispersion, as shown in Table 5.
Interfacialstates between both of the two phases are modified
incertain ways so as to form their stable systems. In general,the
interfacial tension is required to be below 10y4 Nrmfor polymer
alloys of micrometer phase separation type,and below 5 x 10y4 Nrm
for polymer composites with mi-crometer-sized fillers. In case of
nanocomposites, thesevalues should be far less than those.
The layer exfoliation and intercalation method is one ofthe
present major methods for polymer nanocomposites.There are two
processes available at present to conduct it,i.e., the
polymerization process, and the melt compoundprocess. The former
was first available in public in 1987and has been favored since
then. The latter, however,seems to have been a main stream for this
type ofnanocomposites, since it costs less in equipment and
ren-ders the flexibility in application. The sol-gel method
nowattracts much more attention, because it might be com-paratively
easily modified to suit to industrial manufac-ture. Molecular
composites are to be manufactured by thesolution re-precipitation
method, and by the melt com-pound method. The latter is a new
method in this manu-facturing process that is expected to emerge.
It is neces-
Ž .sary to develop nanometric liquid crystal polyester
LCPfibrils to realize an industrial manufacture line for
themolecular composites.
3.1 INTERCALATION METHODThe intercalation method is the most
popular for poly-
mer nanocomposite formation. This is the method to in-tercalate
monomers or polymers between layers of inor-ganic layered
substances to cause to disperse them intopolymers during a process
of polymerization or melt com-pounding by exfoliating the layered
substances each byeach layer. Layered silicates are often used in
this method,as shown in Figure 4.
w xThere are three methods in this category 5 . The firstone,
i.e., intercalation of polymer or pre-polymer from so-lution is
based on a solvent system in which the polymeror pre-polymer is
soluble and silicate layers are swellable.The layered silicate is
first swollen in a solvent, such aswater, chloroform, or toluene.
When the polymer and lay-ered silicate solutions are mixed, the
polymer chains in-
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tercalate and displace the solvent within the interlayer ofthe
silicate. Upon solvent removal, the intercalated struc-ture
remains, resulting in polymerrlayered silicatenanocomposite. In the
second one, i.e., in-situ intercala-tive polymerization method, the
layered silicate is swollenwithin the liquid monomer or a monomer
solution so thatthe polymer formation may occur between the
interca-lated sheets. Polymerization is initiated either by heat
orby an organic initiator or catalyst fixed through cation
ex-change inside the interlayer before the swelling step. Thelast
one, i.e., melt intercalation method involves anneal-ing,
statically or under shear, a mixture of the polymerand organically
modified layered silicate above the soften-ing point of the
polymer. This method has great advan-tages over either in-situ
intercalative polymerization orpolymer solution intercalation.
Firstly, it is environmen-tally benign due to the absence of
organic solvents. Sec-ondly, it is compatible with current
industrial process, suchas extrusion and injection molding. The
melt intercalationmethod allows the use of polymers that were
previouslynot suitable for the other two methods.
3.2 SOL GEL METHODThe sol gel method is characterized by the
fact that in-
organic or composite organic-inorganic materials are madeat
relatively low temperatures, and in principle, consists
ofhydrolysis of the constituent molecular precursors andsubsequent
polycondensation to glass-like form. It allowsincorporation of
organic and inorganic additives during theprocess of formation of
the glassy network at room tem-perature. This method has been
traditionally utilized tofabricate glasses and ceramics. Recently,
at the same time,it has been used for polycrystals, porous
composites, andorganic-inorganic composites. Sol-gel reaction is
started
Ž .from metal alkoxide, M OR n. It should be melted inwater,
alcohol, acid, ammonia, and the like in order to behomogeneously
dispersed. Metal alkoxide is hydrolyzedthrough reaction with water
and turns out to be metalhydroxide and alcohol. There are many
kinds of metalsutilized such as Na, Ba, Cu, Al, Si, Ti, Zr, Ge, V,
W and
Ž .Y. Silicon alkoxides such as tetraethoxysilane TEOS andŽ
.tetramethoxysilane MTEOS are often used. In case of
TEOS, for example, an amorphous polymer with three di-mensional
network structures of silica is formed by poly-
w xmerization reaction followed by hydrolysis 3 .
Si OC H q H O™ OC H Si-OHqC H OHŽ . Ž .2 5 2 2 5 2 54
3�Si-OHqHO-Si�™�Si-O-Si �qH O2
�Si-OHq OC H Si-™�Si-O-Si �qC H OHŽ .2 5 2 53
The sol gel method was not considered suitable for
massproduction, since it used water as media in general. How-ever,
it is now expected to become a key technology in thenear future, as
new modified methods such as a continu-ous sol-gel have been
developed recently. Various compa-
nies have introduced highly functional organosols of silicacid,
produced either by solrgel condensation of te-traethoxsilane, or
acidification of sodium silicates, fol-lowed by functionalization
with various trisalkoxysilanes.Another very versatile solrgel route
led to the industrialpreparation of dispersable boehmite
nanofillers. In theSasol process aluminum or magnesium metal is
activatedby etching off the surface oxide layer. Reaction with
alco-hol produces alkoxides and hydrogen. Upon hydrolysis
thealuminumalcoxides from boehmite mineral that is ob-tained as
nanoparticle dispersion. The boehmite mineralscan be rendered
organophilic by reaction with carboxylicor benzenesulfonic acids.
The by-product alcohol is recy-cled in this process. In contrast to
natural organophilicboehmite, the solrgel reaction product is much
easier toredisperse and does not possess other metal ions as
impu-rities.
3.3 MOLECULAR COMPOSITEFORMATION METHOD: POSSIBILITY
FOR NANOCOMPOSITESMolecular composite is a material system
wherein dis-
crete reinforcement is achieved with molecular rods. It
wascharacterized originally by the fact that a rigid polymer
Ž .such as liquid crystal polyester LCP was dispersed in
aflexible polymer matrix in molecular or microfibril dimen-sion.
This past method was conducted by such a way thattwo kinds of
materials were melted in a co-solvent to beprecipitated afterwards.
It was not built up to mass pro-duction. A new method was developed
in 1990’s that engi-neering plastics were melt-compounded with
small
Ž .amounts of liquid crystal polyester LCP . This was foundto
produce composites with excellent properties, and have
w xattracted people’s attention since then 9 . Typical exam-ples
of this type are polyamide, poly phenylene ether al-loy,
polyethylenetelephthalate, and polycarbonate with all
Ž .aromatic polyester LCP . It should be noted that theyneed the
third substance that would function as compati-bilizers. The third
substance would work to promote theformation of fibrils and to
disperse it in the composites. Itis recognized that the molecular
composites have moreadvantage over pristine materials in mechanical
proper-ties. In this case, microfibrils to be used are 500 nm
intheir size. In order to put nanocomposites of this type
inreality, it is necessary to put disperse phase down to oneorder
smaller than that we have now
3.4 NANO-PARTICLE DIRECTDISPERSION METHOD
In this method, nano-particles are chemically modifiedon their
surfaces to increase compatibility with polymers,and are mixed with
a polymer and dispersed homoge-neously without agglomeration. There
are several exam-ples such as photo-hardening coating agents with
modi-fied silica nano-particles, nano-particle paste of gold
orsilver protected by comb-shaped block copolymers, and
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polyamide 6 nanocomposite with silica nano-particles
sur-face-treated by amino butyric acid.
4. EXPERIMENTAL WORK ANDRESULTS
4.1 SURFACE CHANGE OF POLYAMIDENANOCOMPOSITE CAUSED BY
PARTIAL
[ ]DISCHARGES 10Polyamide 6 used for experiments was
manufactured by
Unitica Co. in such a way that layered silicates, i.e.,
syn-thetic mica, were exfoliated to about 1 mm thick layers,and
they were uniformly dispersed in a polyamide 6 resin
Ž .by in-situ polymerization. Partial discharge PD degrada-tion
was investigated for polyamide 6 without nanoscale
Ž .fillers nanofillers and polyamide 6 nanocomposites with2, 4
and 5 wt% addition of organically, grafted in situ withpolyamide.
Such materials were subjected to partial dis-
Ž .charge under the IEC b electrode configuration for
eval-uation. Comparisons were made as to the surface rough-ness
observed by scanning electron microscopy and atomicforce
microscopy. It was found that the change in the sur-face roughness
was far smaller in specimens withnanofillers than those without
fillers, and that the 2wt%addition was sufficient for improvement,
as shown in Fig-ure 5. This result indicates that polyamide
nanocompositeis more resistive to PDs than polyamide 6
withoutnanofillers.
From SEM image observation, it seems that partial dis-charges
would attack the surface of nanocomposite, butdegrade the part of
polymer regions between nanofillersselectively, and even step aside
from the fillers to intrudeinto their backside. It is to be noted
that the nanofillersare more resistant against PDs, and larger in
permittivitythan the polymer per se.
A mechanism for PD degradation has been investi-gated. Figure 6
illustrates how PD possibly acts on thesurface of PArlayered
silicate nanocomposite for clarifi-cation of the mechanism. The
nanocomposite consists of agroup of nanometric spherulites formed
aroundnanofillers. Regions between such spherulites are filledwith
amorphous PA. PD resistance is stronger for layeredsilicate than
for polyamide. PA spherulite regions seem tohave stronger PD
resistance than PA amorphous regions.Permittivity of layered
silicate is about twice larger thanthat of polyamide itself. A
hypothetical model for expla-nation of PD resistance of this
material is considered as
Žfollows. PD will concentrate on the nanofillers about.twice on
the surface of a nanocomposite specimen due to
the difference in permittivity, but the nanofillers are
moreresistant against PD than the PA matrix. Spherulites mighthelp
to resist against PD, either. PD might be faint on theamorphous
regions that are less resistant against PD. Thisexplanation
supports the experimental fact thatPArlayered silicate
nanocomposite is more resistantagainst PD than pure PA. How PDs
degrade the
Figure 5. Relation between average surface roughness andw
xnanofiller content. V s 6 kV, t s 1 and 48 h 8 .
Figure 6. Schematic illustration of possible mechanisms for PD
re-Ž .sistance of polyamiderlayered silicate nanocomposite
hypothesis .
nanocomposite is illustrated in Figure 6, too. PDs concen-trate
on the layered silicates and shift toward polymer re-gions because
charge has been built up on the surface ofthe layered silicates.
PDs crawl selectively into the amor-phous regions that are less
resistant against PD thanspherulite regions.
Nevertheless, it is still uncertain how PD resistance willbe
affected by nanometric spherulites and ror ‘‘interac-tion zones’’
around nanofillers.
Results obtained are summarized as follows:Ž .i Polyamide
nanocomposites exhibited much stronger
PD resistance than pure polyamide.Ž .ii Surface erosion due to
PD was 5 times shallower
for polyamide nanocomposites than for pure polyamideunder a
certain condition.Ž .iii High PD resistant layered silicate is
considered to
play a significant role in PD resistance of PArlayered sili-cate
nanocomposite.Ž .iv Roles of spherulites and interaction zones
remain
still unsolved.
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Figure 7. Frequency dependence of dissipation factors for
poly-imide without fillers, and with 5 wt% 3�m microfillers and 5
wt% 40
w xnm nanofillers 9 .
4.2 ELECTRICAL CONDUCTION INPOLYIMIDE/SILICA
[ ]NANOCOMPOSITES 6Thermosetting polyimide with 2�10 wt%
nanofillers
were prepared by the solvent cast method. Silica was usedas
nanofillers. Their size was estimated to be 40 nm. Poly-imide films
were cured into free standing films of approxi-mately 25 to 50 �m
in thickness. Aluminum electrodes of1.27 cm in diameter were
deposited onto both surfaces ofthe films by sputtering prior to
electrical tests. The elec-trical properties of polyiminde-based
nanocompositeswere investigated by several ways such as isothermal
steadystate current, dielectric spectroscopy, and thermally
stim-ulated current measurements.
The effect of nanofillers on the dielectric properties
wasevaluated via interfacial polarization and electrical
con-duction mechanism investigation. As shown in Figure 7,
Ž .polyimide with micrometer size 3 �m fillers has a losspeak at
about 1 kHz, while pure polyimide has no peakaround the frequency.
This peak is attributed to theMaxwell-Wagner interfacial
polarization. This peak ismuch reduced for nanocomposites with 40
nm fillers. Thereduction of interfacial polarization was explained
in termsof the field mitigation with the reduction of filler
dimen-sion.
The electrical conduction of polyimide nanocompositeswas found
to be fairly consistent with the space charge
Ž .limited current SCLC mechanism. Polyimide with 2
wt%nanofillers showed some reduction in bulk electrical
con-ductivity compared to pure material and 10wt% micro-filler
polyimide at elevated temperatures. Thermally stim-
Ž .ulated current TSC peak shifted from 185�C to 200�C orhigher
temperatures. Results obtained are summarized asfollows:
Ž .i Peak dissipation factor of PI nanocomposite wassmaller at 1
kHz than that of PI microcomposite.
Ž .ii dc current of PI nanocomposite was smaller at
hightemperatures than that of pure PI and PI microcomposite.Ž .iii
A TSC peak shifted from 185�C for pure PI to 200
�C or higher temperatures for PI nanocomposite.The first
phenomenon was understood in terms of in-
terfacial polarization. The second and third phenomenawere
correlated to the introduction of deeper carrier traps.It was
emphasized that the nature of such traps and theinteraction between
filler and matrix would deserve fur-ther investigation.
4.3 THERMAL AND MECHANICALPROPERTIES OF POLYIMIDE
[ ]NANOCOMPOSITES 11Commercially available nanoparticles were
dispersed
evenly into standard thermosetting polyimide enamel af-ter
proper surface treatment or coating of nanoparticles.Nanoparticles
of 40 nm in thickness were added by 1 to10wt%. All the materials
were solvent cast and cured intofree standing films of
approximately 50 to 100 �m inthickness. Tensile strength, scratch
hardness and thermalconductivity were measured for these films. By
focusingon the relationship between nanoparticles and
polymermatrices inside nanocomposites, the following were
ob-tained:Ž .i Elongation and strength to failure improved if PI
was
converted into PI nanocomposite.Ž .ii Scratch hardness of PI
nanocomposite was larger
than that of PI microcomposite.Ž .iii Tensile modulus of PI
nanocomposite showed no
significant change between pure PI and PI nanocompos-ite.Ž .iv
Thermal conductivity was enhanced for PI
nanocomposite filled with coated nanoparticles as com-pared to
pure PI and PI microcomposite.
Surface modification or coating effect of the aboveŽ .statement
iv was found to increase thermal conductivity
as shown in Figure 8. It is interesting to cite that
surfacetreatment of nanofillers seemed to improve the
thermalconductivity of PI nanocomposite, which indicated
thatfiller-matrix interactions functioned for performance
im-provement. It was emphasized that polymer chains wouldinteract
with the surface of nanofillers to alter variousproperties such as
crystallinity, cross-link density, mobilityand conformation to
change mechanical and thermalproperties in the end.
4.4 ELECTRICAL PROPERTIES OFPOLYMER NANOCOMPOSITES BASED
UPON ORGANOPHILIC LAYERED[ ]SILICATES 12
Ž .Ethylenervinylacetate EVA and isotactic polypropy-Ž .lene
iso-PP nanocomposites were prepared from
organophlic layered silicates. Synthetic fluorohectorite
wasmodified by means of cation exchange of their interlayer
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Figure 8. Thermal conductivity vs. filler content
characteristics forŽpure PI, PI microcomposite, and PI
nanocomposites nanoparticles
. w xuncoated and coated 10 .
sodium cations for hydrophobic alkyl ammonium cationsin order to
expand the interlayer distance and to facilitateintercalation and
exfoliation during processing. Non-polarpolymers such as iso-PP
needed compatibilizers and highshearing stress during processing,
while polar polymerssuch as EVA did not. In case of iso-PP, the
addition ofmaleic anhydride grafted PP promoted exfoliation
andin-situ formation of nanocomposites
Fluorohectorite, modified by means of cation exchangeŽ .with
protonated octadecylamin ODA , was used as nan-
nofillers with different concentration, i.e., 2, 3 and 6
wt%.Specimens used were prepared in a two step process. In
Ž .the first step, the iso-PP PP HC 001 A-B1 from Borealiswas
melt-blended in conjunction with the PP-g-MA com-
Ž .patibilizer Licomont AR 504 from Clariant and
theŽorganohectorite Somasif ME 100 from CO-OP Chemical
.Japan , modified via ODA cation exchange, using a coro-Ž
.tation twin-screw extruder Collin; ZK 25T at a tempera-
ture of 220�C. Similar procedure was used to prepare EVAŽ
.ESCORENE ultra UL 00012 from Exxon Chemicalsnanocomposites. In the
second step, nanocomposite filmswere prepared using the same
extruder with a chill role
Ž .unit Collin; CR 72T . Obtained films were as thick asabout
400�m. Nanofillers in the films were checked byTEM.
Space charge, conduction current and dielectric
strengthmeasurements were performed for polymer nanocompos-ites
described above. The comparison among unfilled andnanofilled
materials, with different filler concentrations,showed that space
charge accumulation phenomena wereconsiderably affected by the
presence of nanofillers. Re-sults are summarized as follows:Ž .i
Space charge decreased at medium high electric field
by introduction of nanofillers for EVA and PP, while itincreased
at low fields.
Figure 9. Comparison of space charge values at poling field of
40Ž . w xkVrmm for EVA and PP base and nanofilled 6 wt% 11 .
Ž .ii Space charge inception threshold shifted to lowerŽ .values
e.g. from 14 to 5 kVrmm for PP for both EVA
and PP, if nanofillers were added. It decreased if
nanofillercontent increased.Ž .iii Depolarization charge was faster
in its decay rate
for nanocomposites than for base EVA and PP. EspeciallyPP with 6
wt% ODA showed a marked fast decay rate.Ž .iv dc conduction current
increased for nanocompos-
ites.Ž .v dc breakdown strength increased for nanocomposite
PP, while it did not change significantly for EVA
Space charge accumulated at a probable design stresssuch as
40kVrmm was found to be smaller for nanocom-posites than for pure
or base polymers, as shown in Figure9. This deviation was more
evident as nanofiller contentincreased up to a level of about 10%.
Further increase ofnanofiller concentration worsened electrical
properties. Itwas considered from space charge characteristics,
depo-larization and dc current measurements that ionic carrierswere
available and that carrier trap distribution was sig-nificantly
modified likely due to introduction of shallowtraps.
Some apparent contradiction should be pointed out asfor dc
current for future investigation. Namely dc currentdecreased, when
some nanofillers were added to PI, as
w xthe reference 9 indicated, while increased when
layeredsilicates were dispersed in EVA and PP, as in referencew x w
x w x11 . In both papers 9 and 11 , ionic carriers and carriertraps
were proposed for electrical conduction, but it wasnot revealed
whether the traps were associated with ionicconduction or
electronic conduction.
4.5 EPOXY WITH METAL-OXIDE FILLERS[ ]13
Ž . Ž .Titanium dioxide TiO micro 1.5 �m fillers and2Ž
.nanofillers 38 nm were dispersed in Bisphenol-A epoxy
Ž .Vantico CY1300 q HY956 . Such composites were me-chanically
stirred and molded into films of 500 to 750 �m
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in thickness. In case of nanofiller dispersion, the compos-ites
were subjected to large shear force in the mixing pro-cess to
obviate unwanted clustering or agglomeration of
Žnanofillers it was pointed out that nanoparticles wouldtend to
agglomerate to make dispersion in resins quite
.difficult . For most electrical characterization, the castfilms
with 1, 10 and 50 wt% micro- and nano-particleswere provided with
evaporated 100 nm aluminum elec-trodes. Results can be summarized
as follows.Ž . Ž .i Glass transition temperature Tg decreased or
re-
mained unchanged when an epoxy resin was filled bynanocomposite
particles, while Tg increased when it wasmicrocomposited.Ž .ii
Permittivity and loss tangent exhibited different
characteristics depending on voltage frequency and
tem-perature.Ž .iii Space charge behaved in a similar way as the
base
resin for nanocomposites, while substantial space chargewas
generated internally for microcomposites.Ž .iv Decay of charge in
nanocomposites was very rapid.
It was stated from Tg data that nanoparticles might be-have in a
similar way to filtered plasticizers rather than as‘‘foreign’’
substances creating a macroscopic interfaces.Dielectric spectra
were virtually indistinguishable betweenmicrocomposites with 10 wt%
particulates and the baseresin in the low frequency range, because
only the chargeat the electrode was involved. On the contrary, a
markeddifference was observed at low frequencies and high
tem-peratures between base resin and nanocomposites with 10wt%
nanofillers. The nanocomposites showed a flat tan�
Ž .response at low frequencies. In addition to iii , the decayof
charge was very rapid in nanocomposites compared tomicorcomposites.
It was emphasized from the above re-sults and some consideration
that nanometric fillers wouldmitigate the interfacial polarization
characteristic of con-ventional materials with a reduction in the
internal fieldaccumulation. Hypothetical ‘‘interaction zone’’ was
de-fined here as a short-range highly immobilized layers thatmight
develop near the surface of nanofillers to explainmajor phenomena
observed in nanocomposites. This willbe discussed later.
w xThe continuation of the same research, 14 , consideredŽ
.uniform field breakdown BDV specimens, prepared by
the use of a mold having multiple spherical protrusions soas to
form a plurality of recessed specimens in one opera-tion. Divergent
filed specimens used for electroclumines-
Ž .cence EL measurements were created by molding
aroundelectrolytically etched tungsten needles having a
well-characterized tip radius of about 4 �m. Laminar
moldedspecimens were subjected to thermally stimulated currentŽ
.TSC measurements. Experimental results are as follows:Ž .i BDV
remained almost same up to 10% nanoparticle
loading, while it decreased significantly for 10% micropar-ticle
loading.
Figure 10. Electroluminescence onset field as a function of
TiO2w xloading. a, 38 nm; b, 1.5 �m 13 .
Ž .ii EL onset field was higher for nanocomposites thanfor base
resins and microcomposites.
Ž .iii The �-TSC peak seemed to shift upward in tem-perature for
nanocomposites. Microcomposites exhibiteda significant �-TSC peak
of interfacial polarization above100 �C, while nanocomposites had
no peak at all.
As shown in Figure 10, EL onset field was a function offiller
loading with a peak at 10wt% loading for nanocom-posites. Based on
experimental results obtained above, in-terfacial space charge or
interfacial polarization was con-sidered crucial and was associated
with Maxwell-Wagnereffect. Then, it was emphasized that
nanoparticles mightmitigate effects of trapped entanglement on the
Maxell-Wagner effect.
4.6 STUDY OF THE PROPERTIES OF[ ]RTV NANOCOMPOSITE COATINGS
15
Specimens used for tracking tests were prepared andŽdivided in
three groups, that consisted of Group 1 virgin
.room temperature vulcanized silicone rubber, i.e., RTV ,Ž
.Group 2 RTV filled with 40 wt% ATH and Group 3
Ž .RTV with 5 wt% layered silicate nanocomposite . Sur-face SEM
observations confirmed that layered nanosili-cates were randomly
exfoliated and dispersed in RTV. Itwas considered that layered
nanosilicates were coveredwith organophilic cations when pristine
layered silicateswere processed by intercalation, so that the
surface en-ergy of normally hydrophilic silicates was lowered to
makelayered nanosilicates compatible with RTV coatings.Specimens of
120 mm � 50 mm � 6 mm in size weresubjected to a standard tracking
test. As shown in Table 6,the addition of nanoparticles to RTV
silicone rubber im-proved their tracking performances comparable to
that ofATH filled RTV. In addition to that, the maximum ero-sion
depth was much smaller for RTV nanocompositesthan for virgin RTV
and ATH filled RTV. Experimentsconfirmed a slight increase of tan �
, a slight decrease of� and a little change of � .r v
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Results are summarized as follows:
Ž .i Flame retardation of RTVrlayered silicatenanocomposite was
highly improved.
Ž .ii The needed amount of nano-layered silicate wasonly about
one tenth of that of the conventional ATHfiller.
Ž .iii Properties such as tan � , � and � remained, af-r vter
the addition of the nanofiller, satisfactory.
4.7 EFFECT OF SPACE CHARGE IN[ ]NANOCOMPOSITE OF LDPE/TIO
162
Ž .Low-density polyethylene LDPE r TiO nanocompos-2ites were
prepared by the solution blending method using
Žnanoparticles of TiO purity: 99.5 % or larger, average2diameter
30 nm, specific surface 150� m2rg, loose den-
3.sity: 0.05�0.06 grcm . Detailed procedure is described inw
xliterature 12 . Specimens for experiments were 30 �m in
thickness. The disperse performance of TiO in LDPE was2Ž
.observed with the scanning electron microscope SEM .
TiO was dispersed uniformly in LDPE up to 5wt%2through
controlling temperature and content of the solu-tion of LDPE; the
largest conglomeration diameter ofTiO in LDPE was less than 80 nm.
Space charge distri-2bution of such specimens with and without nano
TiO was2
Ž .measured by the pulsed electroacoustic PEA method.The
following results were obtained:
Ž .i Hetero-polar space charge near electrodes was muchless in
LDPEr TiO nanocomposites than that in pure2
Ž . ŽLDPE under lower direct current dc stress no more than.40
kVrmm .
Ž .ii Space charge inside the nanocomposites was muchmore
uniform than that in pure LDPE. Thus electricalstress concentration
was improved under dc stress in thenanocomposites.
Ž .iii Decay rate of the remnant of space charge in
LDPEspecimens containing TiO increased with increasing of2the TiO ,
when short-circuited after pre-stress at 502kVrmm for 1 h.
4.8 EFFECT OF � AND PHASE NANOAl O ON MECHANICAL PROPERTIES OF2
3
[ ]EPDM 17Nano Al O in � and phases was mechanically2 3
Žblended with sulphurized agent dicumyl peroxide or DCP:. Ž .2
wt% and 1010 type of antioxidant 0.2 wt% into ethy-
Ž .lene-propylene-diene methylene linkage rubber EPDM .Both
kinds of compounds were cross-linked and moldedinto specimens under
the same conditions. SEM observa-tions indicated that Al O in phase
was dispersed more2 3uniformly than Al O in � phase. The following
results2 3were obtained:
Ž .i Tensile strength and elongation of EPDM contain-ing nano Al
O in phase were better than those of2 3EPDM containing nano Al O in
� phase.2 3Ž .ii The smaller diameter of the Al O nanoparticles2
3
gave the larger improvement in mechanical properties.
Ž .iii Surface pretreatment of Al O nanoparticles2 3helped
improve the mechanical properties.
4.9 SYNTHESIS ANDCHARACTERIZATION OF
POLYIMIDE/SILICA NON-CLUSTERED[ ]COMPOSITES 18
Ž .A new class of PD partial discharge resistant filmsŽ .made of
‘‘nanocluster-trapped’’ polyimide PI rsilica
Ž .SiO nanocomposites that were synthesized by the
sol-gel2reaction was obtained by hydrolysis and poly-con-
Ž .densation of tetraethoxysilane TEOS ceramic precursorŽ .or
methyl-triethoxysilane MTEOS ceramic precursor in
Ž .the solution of polyamic acid PAA dissolved in N,N-Ž
.dimethyl-acetamide DMAc , followed by heating. Some
w xmore detailed description is given in literature 17 .
Thechemical surface and the surface morphology of the com-posite
films were characterized by using Atomic Force
Ž .Microscope AFM and Fourier Transform Infrared Spec-Ž
.troscope FTIR . Size of silica particles ranged from 127
to 506 nm, and MTEOS gave smaller sizes than TEOS.PD resistance
of the composite films was tested under highvoltage using rod-plate
electrode to derive PD lifetime. Asshown in Figure 11, PD lifetime
increases with the in-crease of silica content based on MTEOS. The
same be-havior was shown by TEOS, with life increasing
almostlinearly with silica content. PD lifetime was longer forMTEOS
than for TEOS. Therefore, life was consideredto be affected not
only by the type of precursor, but alsoby agglomeration extent of
inorganic particles. The resultsobtained are summarized as
follows:
Ž .i PD resistance was larger for PIrsilica nanocompos-ites than
for pure PI specimens.
Ž .ii Both agglomeration of particles and the type of pre-cursor
seemed to affect PD lifetime.
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Figure 11. PD lifetime vs. silica content for MTEOS PIrsilica
com-w xposite 17 .
4.10 PD-RESISTANCE OF POLYIMIDE[ ]NANOCOMPOSITES 19
w xSpecimens same as those used in the paper 18 werew xprepared.
Nanocluster-trapped PI rsilica composites 18
were renamed as polyimide-nano inorganic composites inw x19 .
Particle size was presumed to be in the range 127 to506 nm. PI
nanocomposite films were subjected to surfacepartial discharges
according to IEC-343 and ASTM-2275.PD breakdown time was measured
for 30�m films. PDbreakdown time was 3-12 times longer for
nanocompositePI than for pure PI. Silica loading was optimum at 8
wt%for PD resistance. Dielectric spectroscopy, such as C -pand tan
�- characteristics, was investigated both beforeand after PD aging,
and the surface morphology was char-
Figure 12. Temperature dependence of permittivty and tan � forw
xpure and nanocomposite epoxy resin 19 .
acterized by AFM. Dielectric spectroscopy data were ob-tained at
0, 30, and 45 minutes of aging time. Results re-garding capacitance
were uncertain. Permittivity and thepeak value of tan � decreased
steadily as aging time in-creased, while the frequency at tan� peak
shifted upwardfrom 1.16 to 1.27 MHz for pure PI and from 0.67 to
1.99MHz for nanocomposite PI.
The following results were obtained:
Ž .i PD resistance was stronger for nanocomposites thanfor pure
PI.
Ž .ii Silica loading of 8 wt% gave the best PD resistance.
Ž .iii Capacitance at tan� peak in the frequency depen-Ždence
decreased with aging time from 169 to 179 pF and
.from 194 to 274 pF .
Ž .iv The frequency corresponding to the peak value oftan �
shifted to higher values as aging time increased.
Ž . Žv Such frequency shift was larger from 0.67 to 1.99. Ž .MHz
for nanocomposite PI than from 1.16 to 1.27 MHz
for pure PI.
4.11 EPOXY-ORGANICALLY MODIFIEDLAYERED SILICATE
NANOCOMPOSITES
[ ]20Epoxy-silicate nanocomposites were prepared by dis-
persing synthetic layered silicates modified with
alkyl-am-Žmonium ions in an epoxy resin diglycidyl ether of
bisphe-
nol-A, DGEBA, Epikote 828, epoxide equivalent weights 184 to 194
or diglycidyl ether of bisphenol-F, DGEBF,Epikote 807, epoxide
equivalent weight s 160 to 175,
.Japan Epoxy Resin Co. . Two kinds of organically modi-Ž .fied
silicates STN and SEN provided by Co-op Chemical
Co. were used. In the dispersing process, the
organicallymodified layered silicates were mixed in epoxy resin
withshearing, and aggregations of the silicates were removedby
centrifugal separation after mixing epoxy resin and sili-cates.
Micrographs taken by transmission electron mi-
Ž .croscopy TEM indicated that the nanocomposites had amixed
morphology including both parallel silica layersŽ .0.1�0.5 �m, 5�15
layers and exfoliated silica layersŽ .nano-scale dispersion area.
Epoxy nanocomposite with3wt% DGEBArSEN showed similar mechanical
strengthto that with 6 wt% DGEBArSTN. As shown in Figure 12,the
permittivity and tan � were smaller for nanocompos-ite epoxy
especially at high temperatures than for pureepoxy. Marked
improvement was confirmed for tan � attemperatures above 120�C.
The following results were obtained:
Ž . Ž .i A glass transition temperature T of the nanocom-gŽ
.posite shifted to a higher temperature q20�C than pure
epoxy.
Ž .ii Mechanical strength improved by layered silicateŽ
.addition 3 to 6 wt% .
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Ž .iii Permittivity and tan � were smaller for nanocpm-posite
epoxy than for pure epoxy.Ž .iv Dispersion of modified silicate
prevented relative
Ž . Ž .permittivity � and dielectric loss tan � from
increas-ring at high temperatures above the glass transition
tem-perature.
4.12 CO-CONTINUOUSNANOSTRUCTURED POLYMER BY
[ ]REACTIVE BLENDING 21Polymer blends consist of a particulate
minority phase
dispersed in a matrix. There are several methods to blendtwo
homopolymers in continuous wt%: the ‘‘compatibi-lizer’’ method, and
the block copolymerrhomopolymerrhomopolymer blend method. A stable
co-continuouscopolymer was obtained on the basis of the graft
polymerblend method. Then, a nanostructured polyethyleneŽ . Ž .PE
-polyamide PA copolymer, especially with co-con-tinuous morphology,
was produced through a self-assem-bly process by reactive blending.
PE and PA were func-tionalized to drive reactive blending. The
functionalizedPE used for the backbone chain was a random
co-polymer
Ž .of ethylene, ethylacrylate and maleic anhydride MAH .PA6
synthesized by polycondensation, was terminated atone end by the
reactive functional group NH . It exhib-2ited excellent mechanical
properties over conventional PEand even classical PErPA blends, as
well as a nearly con-stant elastic modulus of 10 MPa between 100�C
and 200�C, high yield stress and high strain tensile behavior. It
didnot creep, when heated above the PE melting point.
Results obtained for mechanical properties of a newPErPA blend
are as follows:Ž .i It exhibited lower creep and greater heat
resistance.Ž .ii It remained stable at much higher temperatures
and
was provided with better mechanical properties than clas-sical
blends.
Ž .iii This technology would point the way towards thedesign of
stable co-continuous structure over a wide rangeof compositions and
polymer types.
4.13 NEW EPOXY RESINS WITHCONTROLLED HIGH ORDER
[ ]NANOSTRUCTURE 22, 23Epoxy resins with mono and twin mesogens
were for-
mulated. Mesogens were highly ordered in nanoscalethrough their
self-assembly process, possibly resulting inimprovement of thermal
conductivity by reducing as it isscattering. They would form
nanometric liquid crystal re-gions that were 5 to 30 nm for mono
mesogens, and in therange of 500 nm for twin mesogens. Thermal
conductivitywas obtained to be 0.33 WrmK for mono mosogens, and0.85
� 0.96 WrmK for twin mesogens. These values shouldbe compared to
0.17 WrmK for conventional epoxy. Asshown in Figure 13, epoxy
resins with nano-ordered struc-tures were obtained to have better
thermal conductivitythan conventional epoxy resins.
Results obtained are summarized as follows:
Ž .i Epoxy resins exhibited higher thermal conductivity,if
mesogens were highly ordered in nanometric scale.
Ž .ii Thermal conductivity was 5 times higher for epoxyresins
with twin mesogens than for conventional epoxyresins.
w xHigh thermal conductivity epoxy explained in 19 wasaimed at
low thermal expansion and low water absorp-tion, as well as high
Young’s modulus even at high tem-peratures, for use in
microelectronics printed circuitboards. Some other methods have
even been developed toincrease thermal conductivity of epoxy
resins. One is toform smectic liquid crystal structure in epoxy by
using largemesogens, and the other is to increase unidirectional
ther-
w xFigure 13. Thermal conductivity of various polymers for
commercial use 21, 22 .
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mal conductivity by driving liquid crystal regions
towardanisotropic orientation under high magnetic field, such as10
Tesla.
5 DISCUSSION5.1 DISCUSSION ON ELECTRICAL
PROPERTIES
5.1.1 EFFECT OF NANOMIZATION ONPERMITTIVITY AND TAN �
Ž . w x Ž .i Polyimidersilica nanocomposites 6 , Irwin GE :
Value of tan � tends to decrease in the order of purePI,
PIrsilica microcomposite and PIrsilica nanocompositeat the low
frequency region up to about 200 Hz between60 Hz and 1 MHz. A peak
appears at about 1 kHz in the
Ž .middle frequency region between 60 Hz and 1 MHz incase of
PIrsilica microcomposite. This peak is reducedfairly much in case
of PIrsilica nanocomposites. The peakis ascribed to the
Maxwell-Wagner interfacial polariza-tion. This reduction might be
caused by the mitigation ofthe field around fillers due to size
difference.
Ž . w xii EpoxyrTiO nanocomposites: Permittivity 13 , Nel-2Ž
.son RPI :
Above 50 or 60 Hz, the dielectric spectra are
virtuallyindistinguishable among base epoxy, epoxy microcompos-ite
and epoxy nanocomposite. This very low frequencyprocess might not
be due to particulates in bulk, but tocharges at the electrodes.
The microcomposite materialexhibits two peaks of tan � that are
observed at very lowfrequencies, such as 8 mHz and 100 mHz at two
differenthigh temperatures for the microcomposite,
respectively.Values of tan � at intermediate frequencies, around
100Hz, are smaller for nanocomposite than for microcompos-
w xite. This trend is similar to the data 9 obtained
forPIrsilica composite. This might be related to the functionof
nanofillers to immobilize polymer chains in what arecalled
‘‘interaction zones’’.
Ž .iii RTV silicone rubberr layered silicate nanocompos-w x Ž
.ites: 15 Lan Wuhan
Nanocomposites exhibit a slight increase of tan � and aslight
decrease of � at the commercial frequency. No dig-rital data are
available as for tan � , although some figuresare given.
Ž . w x Ž .iv PIrsilica nanocomposites: 19 , Lei Harbin
Nanocomposite PI gives lower permittivity than pure PIin the
frequency region between 100 kHz and 10 MHz.ŽNote: This appears to
be contradictory to the corre-
.sponding figures.
Ž . w xvi Epoxyrlayered silicate nanocomposites: 20 , ImaiŽ
.Toshiba
Permittivity and tan � are smaller for nanocompositeepoxy than
for base epoxy.
Interpretation of dielectric spectra seem to be compli-cated
when comparing base resin, microcomposites, andnanocomposites. It
is questionable whether or not thepermittivty and tan� are reduced
by nanomization at thecommercial frequency. Some data indicate a
certain re-duction, but some other data do not, resulting in
apparentcontradictions. This may depend on the way
nanofillersinteract with companion polymers. These results
requirefurther investigation, considering also methods of
dispers-ing fillers in polymer matrices. It is certainly crucial
toprevent nanofillers from agglomeration or to dispersefillers
homogeneously in polymer matrices for obtainingreproducible and
reliable data.
5.1.2
dc ELECTRICAL CONDUCTIVITY
Ž . w x Ž .i Polyimidersilica nanocomposites 6 , Irwin GE :dc
current at low field decreases at high temperatures
by nanomization.
Ž . w xii PP and EVArlayered silicate nanocomposites 12 ,Ž
.Montanari Bologna :
Ž .dc current at high field 30kVrmm increases at roomtemperature
by nanomization. Thus apparently oppositedata were obtained. But
this will depend on the modifica-tion of shallow trap depth levels
introduced by nanomiza-tion.
5.1.3 SPACE CHARGE, TSC AND ELBEHAVIORS
Ž .i Space Charge
� Space charge increases at low field and decreases atw w xhigh
field due to nanomizationy PP and EVArLS 12 ,
Ž .xMontanari BLN .
� Space charge inception field decreases due to nano-w w x Ž
.xmization PP and EVArLS 12 , Montanari BLN .
� Space charge is generated internally by nanomizationw w x Ž
.xEpoxyrTiO 13 , Nelson RPI .2
� wCharge decay time decreases due to nanomization PPw x Ž .xand
EVArLS 12 , Montanari BLN .
Charge decay time decreases due to nanomizationw w x Ž
.xEpoxyrTiO 13 , Nelson RPI .2Ž .ii TSC
� A TSC peak shifts toward higher temperatures due tow w x Ž
.xnanomization PIrSilica 6 , Irwin GE .
� A TSC peak shifts toward higher temperatures due tow w x Ž
.xnanomization EpoxyrTiO 14 , Nelson RPI .2
Ž .iii Electroluminescence
� EL onset field increases due to nanomizationw w x Ž
.xEpoxyrTiO 14 , Nelson RPI .2
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Modification of trap depth distribution due to the intro-duction
of nanofillers, particularly in reference to deeptraps, might
explain space charge, TSC and EL character-istics as described
above. Fast charge decay and otherspace charge characteristics seem
to be contradictory withthe deep trap theory and certainly need
some other con-sideration.
5.1.4 DIELECTRIC BREAKDOWN STRENGTH
� BDV increases for PP due to nanomization, whilew w xEVA
decreases a bit PP and EVArLS 12 , Montanari
Ž .xBLN .
� BDV remains same up to 10wt% loading, and tendsw w xto
decrease for more loading EpoxyrTiO2 14 , Nelson
Ž .xRPI .
As indicated above, dielectric breakdown strength mightnot be
greatly affected by nanomization under small load-ing of
nanofillers and proper dispersion conditions. Favor-able results
are obtained in some cases.
5.1.5 PD RESISTANCE
� wPD resistance improves due to nanomization PArLSw x Ž .x10 ,
Kozako Waseda .
� PD resistance improves due to nanomizationw w x Ž .xPIrSilica
18, 19 , Zhang Harbin .
It would be probable that nanomization improves PDresistance of
polymers, which certainly depends on hownanofillers are dispersed
in polymer matrices and arecompatible with them.
5.1.6 TRACKING RESISTANCE
� Flame retardancy improves due to nanomizationw w x Ž .xRTVrLS
15 , Lan Wuhan .
5.2 RELATED PROPERTIES5.2.1 GLASS TRANSITION TEMPERATURE
� Tg decreases or remains unchanged due to nanomiza-w w x Ž
.xtion EpoxyrTiO 13 , Nelson RPI .2
� w w xTg increases due to nanomization EpoxyrLS 20 , ImaiŽ
.xToshiba .
It is possible to increase Tg by nanomization. It shouldbe noted
that it would require homogeneous dispersion ofnanofillers for that
purpose.
5.2.2 THERMAL CONDUCTIVITY
� Thermal conductivity enhances due to nanomizationw w x Ž
.xPIrSilica 11 , Irwin GE .
� Thermal conductivity enhances by introduction ofwnannometric
liquid crystal polymers EpoxyrnanoLC poly-
w x Ž .xmer 22, 23 , Takezawa Hitachi .
Nanoparticles and nanomteric crystal parts have higherthermal
conductivity than base polymers, so that mixturesmight be more
thermally conductive than the base resins.It was suggested that
‘‘interaction zones’’ might have asgood heat transfer as
possible.
6 CONCLUSIONPolymer nanocomposites could be advantageous
over
traditional filled polymers in electrical and thermal
prop-erties as well as mechanical properties from the stand-point
of dielectrics and electrical insulation. This featurewill
technologically result in compact design of electricalequipments
with high reliability and thereby in significantcost reduction for
system integration and maintenance.Since this feature is originated
from mesoscopic charac-teristics of interaction zones between
polymer matricesand nanofillers, it will open a new academic arena
for di-electric and electrical insulation that will need
quantummechanics as well. Such interaction zones might be re-lated
to free volume and charge carrier trap distributionŽ .shallow and
deep traps , which should be further ex-plored. In order to obtain
excellent but low-cost polymernanocomposites, existing material
processing technologiesshould be more advanced so as to match
dielectrics andelectrical insulation. Results are summarized as
follows:
6.1 EFFECTS OF NANOMIZATIONŽ .1 dc conductivity increases and
decreases depending onmeasurement conditions. Introduction of deep
traps aresuggested.
Ž .2 Interfacial polarization can be reduced compared
tomicrocomposites.
Ž .3 There seems to be a certain reduction of permittivitydue to
nanomization. But change of permittivity as well astan� is
complicated, and not conclusive. Manufacturingprocesses should be
more investigated for homogenousdispersion of nanofillers.
Ž .4 Space charge, TSC and EL also give complex resultsin their
threshold field and quantity. Introduction of addi-tional levels of
shallow and deep traps, as well as increaseof trap density, might
be involved. These might be deeplyrelated to ‘‘interaction zones’’.
It is therefore necessary tocharacterize the interaction zones
between nanofillers andpolymer matrices chemically and
physically.
Ž .5 PD and tracking resistance improve. It is most proba-ble.
Role of nanofillers and interaction zones should bemore
clarified.
Ž .6 Thermal conductivity and glass transition temperaturecould
be increased by proper methods.
6.2 PROPERTIES OF POLYMERNANOCOMPOSITES
Ž .1 Electrical and thermal properties as well as mechani-cal
properties could be improved by nanomization of poly-
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mers. Polymer nanocomposites are advantageous overconventional
filled polymers, because a small amount ofnanofillers might not
modify the characteristics of basepolymers considerably.
Ž .2 Intercalation methods, sol-gel method, molecularcomposite
formation method and nanofiller direct disper-sion method are the
major processing technologies, andshould be more developed for
better and cheaper materi-als with excellent interaction zones.
Ž .3 Polymer matrices, nanofillers, and interaction zonesbetween
them are three major parts of nanocomposites.Their respective roles
should be investigated based onmaterial characteristics of
interest.
Ž .4 Especially interaction zones should be
characterizedchemically and physically.
Ž .5 Deep and shallow traps should be investigated andcorrelated
with physical and chemical characteristics ofinteraction zones.
Especially a theory for trap level anddensity modification should
be established.
Ž .6 Interaction zones are mesoscopic in nature. This willopen a
completely new aspect of dielectrics and insulationstudies, which
need consideration based on quantum andstatistical mechanics,
too.
Ž .7 Heat resistant thermoplastic nanocomposites are
en-vironmentally benign because of their recyclability. Thesecould
replace thermoset resins that cannot be recycled.
Ž .8 Biodegradable polymers such as polylactic acid can befilled
with nanofillers. PLA nanocomposites are expectedto be used for
eco-friendly electrical insulation.
7 APPLICATIONSŽ .1 Polymer nanocomposites have been investigated
forfuture use of electrical insulation for power apparatus,power
cables, outdoor insulators, and insulated wires forelectric power
technologies as well as printed circuitboards for electronics.Ž .2
Insulation could be more compact by using polymernanocomposites,
resulting in overall cost reduction in ap-paratus and
installation.Ž .3 Reliability could be improved by using
polymernanocomposites, resulting in lower maintenance cost.Ž .4
Polymer nanocomposites will give much innovation indielectric and
insulation technologies.
ACKNOWLEDGEMENTThis work was in part supported by
Grand-in-Aid
Ž .Fundamental Research B-14350171 for Scientific Re-search from
Japan Society for the Promotion of Science,Project Creative Energy
and Environments of WasedaUniversity Advanced Research Institute
for Science and
Ž .Technology, the Deutsche Forschungsgeminschaft DFGand the
‘‘Sonderforschungsbereich SFB428’’, to which theauthors are deeply
indebted.
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1070-9878rrrrr04rrrrr$20.00 � 2004 IEEE784
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