FIELD OF INVENTION:
This invention relates to a silicone rubber polymer composites with single or multi
nanostructured fillers.
BACKGROUND OF THE INVENTION:
The polymer used in the non-ceramic out-door insulation is silicone rubber with
relatively low permittivity of the material and the one-piece shape of the devices result in
a voltage distribution that rapidly falls off from the ends, resulting in high electric fields
that appear around these ends. In these critical regions the intensity of electric field can
reach values above 20-22 kVrms/cm and corona discharge will occur. Even for electric
field below the corona threshold, it is considered that high local stress accelerates aging
reducing insulation life expectancy [1]. By reducing the intensity of the electric field at
the surface of the insulation the occurrence of these problems could be decreased. For
this an option is to use composite materials with high dielectric constant that can control
the electric field distribution [2, 3]. Numerous efforts and investigations have been made
so far to improve the dielectric properties of polymer composites for its use in various
applications including capacitors; however electrical insulator applications for high
voltage applications needs better understanding and development.
Recently Yang Rao et al. has disclosed in their patent US6864306 B2 (2005), on
preparation of high dielectric constant polymer composite with fillers. The prepared
composite is claimed to have a dielectric constant greater than 200 and its application is
for capacitors. The polymer is mainly thermoplastic and fillers are metallic powders and
ceramic materials. The volume percent of filler and polymer are 5-50% and 50-95%
respectively.
Meng et al., has also disclosed high dielectric constant thermoplastic composition
methods of manufacturing thereof and articles comprising the same, US 0054553 Al
(2009), A thermoplastic with 10-60% of dielectric filler was claimed. The claimed
dielectric constant is 25 at 900 MHz achieved by dispersing various non-metallic fillers
in the polymer. The application of the disclosure was making electrical articles such as
capacitors and circuits.
In another disclosure made by Niinobe et al., High-permittivity rubber compounds and
power cable members, US 6979707 B2 (2005); a high permittivity rubber for power cable
applications was disclosed. The rubber was prepared by mixing fillers such as barium
titanate in rubber. The relative permittivity of the compound is claimed to be around 20.
Kanakarajan et al., has filed a Patent application (US 0242823 Al (2009) relating to a
process for preparing polyimide based compositions useful in high frequency circuitry.
Polyimide materials having improved electrical and mechanical performance prepared by
mixing fillers was disclosed. The application targeted is for high frequency circuitry. The
fillers can be of various non-metallic materials including various oxides, titanate and their
mixtures.
Peiffer et al., teaches about Dielectric material with non-halogenated curing agent, US
0108309 Al (2011): dielectric material having non-halogenated curing agent was
disclosed. The main application is in electrical components including capacitors, resistor ,
inductor and their combinations. The above disclosures or inventions have claimed on
utilizing polymer composites with fillers either macro or nano based oxides, metallic
powders, however the disclosures or inventions have not addressed the high voltage
stress grading insulation applications and this invention addresses that.
Silicone rubber or poly(dimethyl siloxane) (PDMS) is a member of siloxane family and is
extensively used in the industry. The is because silicone rubbers possess unique physical,
chemical, and mechanical properties that are unmatched by any other polymeric
materials. However, unfilled silicone rubbers usually have low mechanical, electrical, and
thermal conductive properties. That is why silicone rubbers are often combined with
fillers. Enhancement of the dielectric property (k) of the silicon rubber composites for
outdoor voltage insulation applications can be achieved by mixing fillers such as BaTi03
(BT), SrTiO3 (ST) etc. By incorporating fillers into the dielectric polymer its relative
permittivity, can be increased thereby reducing the electrical stress and the possibility of
arcing on its surface [2-3].
The dielectric property of polymer composite is a function of many factors, including size
and shape of filler particles, dielectric constant of filler particles and polymer
respectively, morphology of filler, distribution of filler in the polymer matrix, volume
fraction of filler in the composite and the appearance of interfacial layer between filler
and polymer [3]. For a random ceramic filler-polymer composite, Lichtenecker's
logarithmic law of mixing is utilized to estimate relative permittivity of the polymer
composite, it is given by log ec= Vf log ef + vp log ep, where e, v are permittivity and
volume fraction of filler and rubber respectively. There are several other models to
theoretically determine the effective ec depending on their shape etc [2-3].
In-addition to above approach of mixing ceramic, ferroelectric or Para-electric fillers with
polymer another approach towards high dielectric constant materials is conductive
filler/polymer composites, which belong to conductor-insulator composite based on
percolation theory [2-6]. The percolative equation is given as
Where s is the effective dielectric constant, em is the dielectric constant of the matrix, fc
is the percolation threshold, and q is the critical exponent (~ 1 for 3-D composites). Ultra
high dielectric constant can be expected with conductive filler/polymer composites when
the concentration of the conductive filler is approaching the percolation threshold due to
the fact that the percolative approach requires much lower volume concentration of the
filler compared to traditional approach of high dielectric constant particle in a polymer
matrix. Therefore, this material option provides advantageous characteristics over the
conventional ceramic/polymer composites, especially ultra high dielectric constant with
balanced mechanical performance. A polymer matrix composite based on metal powers
such as Ni, Cu, Al etc has been fabricated with a high dielectric constant, and better
flexibility. Dang et al. has proposed a new composite based on LTNO (Li and Ti doped
NiO) and PVDF in which the dielectric constant can be optimized to 400 at room
temperature at 1 kHz with 30 vol.% LTNO. By doping with multi-walled carbon
nanotube (MWNT), it is reported a dielectric constant of about 900 at RT has been
achieved in PVDF monopolymer as the MWNT volume fraction increasing to 0.08 [4].
In-general the theoretical calculations based on several models indicate very high (greater
than 40%) filler loading content to achieve a dielectric constant greater than 100. At high
filler content the polymer losses its inherent properties and it is hard to mold such
composite into components required for insulation.
OBJECTS OF THE INVENTION:
An object of this invention is to propose a silicone rubber polymer composite with single
or multi nanostructured fillers;
Another object of this invention is to propose a method for preparing silicone rubber
polymer composites;
Further, object of this invention is to propose a silicone rubber polymer composite having
a dielectric constant more than 100;
Still another object of this invention is to propose a silicone rubber polymer composite
which can be applicable for stress grading insulation applications.
BRIEF DESCRIPTION OF THE INVENTION:
According to this invention there is provided a silicone rubber polymer composite
comprising filler material selected from a single non-metallic nanostructured filler
material,
A multi filler being combination of non-metallic nanostructured conductive filler
materials.
In accordance with this invention there is also provided a process of preparing silicone
rubber polymer composite comprising:
preparing filler by ball milling in surfactant or dispersant,
subjecting the filler to the step of calcination,
pulverizing the filler,
blending the filler with one part of polymer,
subjecting the mixed filler to the step of vacuum processing,
introducing the mixed filler in a mould and curing the said filler at a temperature between
room temperature to 200°C.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
Figure 1: is a flow chart illustrating the preparation method of polymer composite with
nano-filler.
Figure 2: SEM image of multi filler consisting of BaTiO3 and multiwall nanotubes.
DETAILED DESCRIPTION OF THE INVENTION:
Embodiments of the present invention provide for high dielectric silicone rubber polymer
composite with nano-fillers and their preparation methods. In this embodiment the
silicone rubber polymer composite can demonstrate relatively high dielectric constant at
sufficient filler loading with sufficient dispersion and adhesion of filler in the polymer
matrix.
As explained above the composite of silicone rubber and nano-fillers can overcome the
limitations of lower dielectric constant of pristine silicone rubber. In this regard the
composite can have a dielectric constant of greater than 100 and breakdown voltages can
range from 12-25 Kv/mm.
In-general, the fillers can include commercially available nanostructured CaCu3Ti4O12 or
BaTiO3 or SrTiO3 or polymer/CaCu3Ti4O12/BaTiO3/SrTiO3 such as polyaniline-
CaCu3Ti4Oi2/ BaTiO3/ SrTiO3 or nanostructured CaCu3Ti4O12/ BaTiO3/ SrTiO3 in
combination of conductive materials (figure 2). Nanostructured CaCu3Ti4O12/ BaTiO3/
SrTiO3 can have a particle sizes in the range of 10-500 nm and preferably below 100 nm.
The amount of nanostructured CaCu3Ti4O12/ BaTiO3/ SrTiO3 m the polymer matrix can
range from 10 volume % to 40 volume %.
The conductive material fillers can include materials such as 1-D nanostructure including
carbon nanotubes, multi, single, and thin-multi or double walled nanotubes, 2-D
nanostructure including grapheme and hexagonal boron nitride and transition metal
powders including and not limited to copper, nickel, aluminium. The tubular
nanostructure such as carbon nanotubes can have diameters of 10-50 nm and lengths of
10-100 micron. The transition metal powders materials can have particle sizes in the
range of 5-500 nm. The amount of conductive material fillers in the polymer matrix can
range from 0.01 volume % to 5 volume %.
The embodiment can be explained in terms of percolation theory and percolation
threshold for conductive material fillers. For small volume fraction of the conducting
material filler particles, the resistivity of composite is close to that of polymer matrix. As
the volume fraction of the conducting filler particles increase, the particles come into
contact with one another to form the conduction paths through the composite. As a result,
the resistivity drops by many orders of magnitude at a critical threshold. When a
saturation region of conducting filler particles is reached; there are a large number of
conduction paths, resulting in a low resistivity. This critical threshold where the
resistivity so rapidly drops is called percolation threshold. The value of percolation
threshold is no constant but depends of many factors and from one sample to another
fluctuates. The factors, which contribute to this, are: size, aspect ratio, structure,
allocation, roughness and kind of rubber. In other words the volume percent of
conductive filler cannot be increased beyond percolation threshold. Therefore the
embodiments of the polymer composite with conductive material filler can include
conductive material filler in the range of 0.01 volume % to 5 volume%.
The presence of high aspect ratio nanostructures such as nanotubes or layered structures
such as graphene below the per-location threshold will overcome the disadvantages of
adhesion with the polymer matrix when having single fillers.
The polymers utilized for making the polymer composite can include but are not limited
to commercially available two-part liquid silicone rubber, liquid nitirle rubber or liquid
neoprene rubber from Wacker Chemicals and Dupont etc. The volume percent of the
rubber in the composite can range from 60 volume % to 95 volume %.
Now having discussed various aspects of polymers, fillers and percolation theory,
embodiment on preparation of the polymer composites follows.
Process for preparing single or multi nanostructured fillers involves selection of single
fillers having high dielectric constant such as nanostructured CaCu3Ti4O12/ BaTiO3/
SrTiO3 or a composite nanostructured CaCu3Ti4O12/ BaTiO3/ SrTiO3 such as
polyaniline- CaCu3Ti4O12/ BaTiO3/ SrTiO3 or multi fillers such as nanotubes or graphene.
Followed by a mixing step in suitable dispersant/surfactant. It is preferable to ball-mill
the single or multi fillers in ball-milling unit to achieve functionalized fillers. After ball-
milling it is preferable to dry or calcinate or thermal treatment fillers followed by
pulverization.
Process for mixing the fillers in the polymer involves blending the fillers by mechanical
mixer or homogenizer either in part A or part B of the polymer. Thereafter mixing filler
in one part A or part B of the polymer. Thereafter mixing filler in one part of polymer,
another part of polymer is added to it and thoroughly mixed, followed be vacuum
processing. Process for preparing the polymer rubber composite involves pouring the
liquid polymer rubber composite involves pouring the liquid polymer with nano-filler in a
mould and curing.
Although various embodiments of this invention have been shown and described, it
should be understood that various modifications and substitutions, as well as
rearrangements and combinations of the preceding embodiments can be made by those
skilled in the art, without departing from novel spirit and scope of invention.
WE CLAIM:
1. A silicone rubber polymer composite comprising filler material selected from a single
non-metallic nanostructured filler material,
a multi filler being combination of non-metallic nanostructured conductive filler
materials.
2. The silicone rubber polymer composite as claimed in claim 1, wherein the
nanostructured non-metallic filler include commercially available nano-structured
CaCu3Ti4O12 (ACu3Ti4O12, perovskite-related structure, A is trivalent rare earth or Bi) /
BaTiO3 or polymer composite of CaCu3Ti4O12/BaTiO3/SrTiO3 including polyaniline-
CaCu3Ti4O12/BaTiO3/SrTiO3.
3. The silicone rubber polymer composite as claimed in claim 1, wherein the conductive
nanostructured materials include commercially available 1-D nanostructure including
nanotubes or 2-D nanostructure including graphene, hexagonal-boron nitride or metal
powders including Cu, Al, Fe, and Ni.
4. The silicone rubber polymer composite as claimed in claim 1, wherein the polymer
volume includes between 60-95%.
5. The silicone rubber polymer composite as claimed in claim 1, wherein the nano-
structured CaCu3Ti4O12/ BaTiO3/ SrTiO3 includes between 5-40 volume%.
6. The silicone rubber polymer composite as claimed in claim 1, wherein the nano-
structured polyanaline- CaCu3Ti4O12/BaTiO3/SrTiO3 includes between 5-40 volume%.
7. The silicone rubber polymer composite as claimed in claim 1, wherein the 1-D
nanostructure including nanotubes and 2-D nanostructures including graphene, hexagonal
boron nitride and metal powders including Cu, Al, Fe and Ni includes between 0.01 to 5
volume%.
8. The silicone rubber polymer composite as claimed in claim 1, wherein the volume of
conductive filler is less than the percolation threshold.
9. The silicone rubber polymer composite as claimed in claim 1, wherein the nano-
structured CaCu3Ti4O12/ BaTiO3/SrTiO3 having particle size between 10-500 nm and
preferably below 100 nm.
10. The silicone rubber polymer composite as claimed in claim 1, wherein the 1-D
nanostructure including nanotubes having diameters of 10-50 nm and lengths of 10-100
micron.
11. The silicone rubber polymer composite as claimed in claim 1, wherein the 2-D
nanostructures including graphene and hexagonal boron nitride having few layered (2-5)
and dimension in 5-30 nm.
12. A process of preparing silicone rubber polymer composite comprising:
preparing filler by ball milling in surfactant or dispersant,
subjecting the filler to the step of calcinations,
pulverizing the filler,
blending the filler with one part of polymer,
subjecting the mixed filler to the step of vacuum processing,
introducing the mixed filler in a mould and curing the said filler at a temperature between
room temperature to 200°C.

ABSTRACT

A silicone rubber polymer composite comprising filler material selected from a single
non-metallic nanostructured filler material,
a multi filler being combination of non-metallic nanostructured conductive filler
materials.