Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to the preparation of electrically conducting
materials of high quality from polyolefins~
Many attempts have been made to make conductive or semi-conductive materials
from polymeric plastics loaded with conductive solids such as carbon black,
graphite, or finely divided metals, All manner of thermosetting and thermoplastic
resins have been proposed including melamine, phenol-aldehyde, and more commonly
polyolefins such as polyethylene and graft copolymers thereof, and
polytetrafluorethylene.
In general, compositions of relatively low resistance can be prepared by
dry mixing a finely divided thermoplastic polymer and conductive filler and
moulding the mixture under heat and pressure Such products are normally
porous and non-homogeneous in structure, and accordingly are not suitable for
certain sophisticated applications which require thin impermeable conductors of
highly uniform composition. An example of such an application is a bi-polar
plate for a fuel cell or battery.
A far higher degree of ho geneity than is obtained by the dry- ulding
process can be obtained by the use of known mixing devices such as a Banbury
mixer or roll mill, French Patent No. 1,305,140 describes the preparation in a
Banbury mixer of a number of blends of carbon black or graphite in a crystalline
polypropylene with or without an amorphous copolymer plasticiser. The
resistivities of these blends were all of the order of a number of megohms-cm
and they are described as suitable for use as thermistors and semi-conductors.
In addition, the mixing time in the Banbury is of the order of 30 minutes and
thifi inevitably results in some thermal degradation of the polypropylene with
consequent impairment of its physical and mechanical properties.
When carbon black is incorporated in a polymer such as rubber or
polypropylene in a Banbury or similar mixer, there is an upper limit to the amount
that can be incorporated to give a homogeneous product, When this limit is
exceeded, heterogeneous particles or carbon exist in the mixture and can be
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identified as such ln a number of different ways. The greater the amount
of carbon which a given polymer can take up, the better is what can be
called its binder efficiency. Three test methods used to measure
binder efficiency are briefly described below. In all cases the loaded
polymer was ground into granules of about 2 mm diameter.
Plate-out Test:
: '
A white polypropylene compound is melted for 5 minutes on a mill-
roll at 160C. When a good rolling pencil is obtained, 3 grams of the
carbon loaded granules are added and the sheet immediately stripped. If
the carbon black is properly incorporated, the individual granules are
taken up by the white polypropylene without the latter being stained. If
not the white sheet becomes locally stained grey by the free carbon.
Extraction Test:
The carbon loaded granules mentioned above are extracted with
isopropanol in a Soxhlet apparatus. Any free carbon black dust shows
itself as a black turbidity.
Cracks Test:
The examined compound is extruded through a flat die to a 0.25 mm
thick sheet and corrugated between rolls. It is then visually examined
for cracks or fi~sures. If present, these indicate the presence of free
carbon, and ~imultaneously show that the sheet cannot have uniform
resistance over its surface.
When carbon black is used as a reinforcing agent in rubber, and
particularly for tyres, a typical concentration is 45 phr, or 45 parts by
weight of black per 100 parts by weight of rubber. So-called "master
batches" intended for subsequent dilution with additional rubber may be
made up containing 70 to 75 phr of black. The resistance of such a master
batch would be of the order of a few hundred ohms-cm.
As already indicated, low resistance compounds may be made by the
treatment under heat and pressure of dry-mixed blends of finely divided
carbon black and polymer. These are heterogeneous, have relatively poor
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are porous. Since the blends are always heterogeneous, the concept of
binder efficiency as a measure of the maximum carbon content whlch can
be taken up to give a homogeneous blend is no longer applicable.
It has now been discovered, in accordance with the present invention,
that by proper aelection of the starting materials and preferably by
the use of a particular blending technique it is possible to prepare
csnductive carbon polyolefin blends of exceptionally low resistivity,
uniform electrical properties, and good mechanical properties. Such
materials are particularly suited for the manufacture of bi-polar plates
for fuel cells or batteries, since they are additionally chemically inert
and free from any poisoning effect on any fuel-cell type catalyst with
which the plate may be coated.
According to this invention an electrically conductive non-porous
polyolefin composition comprises a homogeneou6 mixture of a propylene-
ethylene copolymer having at least 20 mol % ethylene and at least 30 parts
by weight of finely divided conductive carbon per 100 parts by weight of
copolymer.
For such end-use the volume resistance perpendicular to the plate
face is the most significant, and the values given in this specification
were measured according to ASTM-D-257-61. A desirable value for the
product would be about 1 ohm-cm.
It was found experimentally that using a crystalline polypropylene
homopolymer, the binder efficiency was far too low to give a homogeneous
product which even approached the desired performance.
In sharp contrast, when the polyelfin used was a highly crystalline
thermoplastic propylene-ethylene copolymer the binder efficiency was
found to be much higher and the conductivity was greatly improved even at
equivalent loadings.
The copolymer has a minimum ethylene content of 20 mol % and for
practical purposes the ethylene content should not exceed about 35 mol %
since above this level the thermoplastic characteristics tend to be lost
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and the product becomes sn elastomer. In addition the copolymer, when
intended for fuel-cell use i~ preferably hlghly puriflet, e.g. by
extractlon wlth solvents such as chloroform or lsopropanol, to remove
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catalyst residue~ and stabilisers whlch could act as fuel-cell catalyst
poisons. Obviously this is not necessary when the end-u~e is one in which
their presence is harmless. A desirable class of copolymers is that
having a Melt Flow Rate (MFR) of at least 5 and preferably from 5 to 10,
although other copolymers with lower MFR, for example 1.5, are still usable.
The preferred form of finely divided conductive carbon is carbon
black since it is cheap, readily available and gives extremely good
results. The finely divided conductive carbon e.g. carbon black should
preferably be present in from 90 to 100 parts by weight per hundred parts -
by weight of copolymer. Part or all of the carbon black may be replaced
by graphite.
Acetylene blacks are preferred because of their good electrical
properties, but obviously furnace blacks and channel blacks can also be
used. Suitable commercially available blacks are available under the
Trade Mark of Vulcan (e.g. Vulcan 3, Vulcan XXX and Vulcan XC-72).
Usually the finely divided conductive carbon has an active surface
area of 300 to 500 m2/gm.
As stated earlier, the use of a Banbury mixer for incorporation of
carbon black into propylene as described in the art, involves mixing times
of the order of half an hour or 80 and this inevitably causes degradation
of the polymer and consequent deterioration of its physical and mechanical
properties. This degradation is substantially reduced if the filler is
incorporated into the polymer under severe conditions, involving a high
degree of shear, high temperature and relatively short mixing time.
In accordance with this invention the electrically conductive non-
porous polyolefin composition is prepared by a process comprising mixing
under conditions of high shear and at a temperature of at least 100C the
propylene-ethylene copolymer and the finely divided conductive carbon, the -
weight ratio being at least 30 parts of carbon per copolymer, the mixing
being continued until a homogeneous blend is obtainet.
Mixing equipment such as a Banbury mixer or Roll Mill, as used in the
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rubber industry, m~y be used, the Banbury being preferred. The normal use ~ ;
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involves the progre6sive heating of the charge by the energy expended in the
shearing action, and this may result in a comparatively long time before the
polymer is sufficiently plastic or fluid to ensure uniform dispersion of the
filler, and during that time the degradation referred to above has occurred.
However, in a preferred embodiment of the present invention the mixing
is carried out at a pre-established relatively high temperature and for a ~ -
relatively short time Thus the Banbury mixer and the copolymer charge may be
pre-heated to a temperature of above 100C, preferably above 150C, and
deslrably up to 200& . The mixing stage can then last for ten minutes or less ;~-
and typically from 3 to 5 minutes. It will be appreclated that this is
contrary to normal practice in a Banbury, where theoretically optimum mixing
is obtained, with as stiff and therefore as cold a mixture as possible.
However, it is found that the copolymer blends prepared in this way have
suffered very little degradation and are of exceptional uniformity in physical
structure and also have very low resistivity of a few tPns of ohms-cm at most
and which may range from about 0.5 to 10 ohms-cm according to circumstances and
the amount of carbon black incorporated.
The finished blend having a resistivity of below 10 and preferably bel~w
1 ohm-cm can then be formed, e.g. by calendering or extrusion into thin
plates having a thickness of about 250 microns. These plates can be corrugated
or given a similar profile by a suitable moulding techniqueO
The following comparative data will illustrate the unexpected benefits of
the invention.
Blends of Vulcan XC-72 carbon black in various proportions were made with
a polyprow lene homopolymer and a propylene-ethylene copolymer containing 27 mol
% ethylene (PP-PE copolymer). Both re3ins in powder form had a MFR of 6 and a
crystallinity of about 93% (before extraction) and had been previously extracted
with isopropanol to re ve catalyst residues etcO
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The blends were prepared in Banbury mixer of 3,5 litrs capacity which was
pre-heated for half an hour at the full s~eam pressure of 11 kg/rm2 whereby it3
temperature was raised to about 180& . Polymer and carbon black were
simultaneously charged to the mixer in the proportions indicated in the table
and in a total amount per charge of about 3 kg. The rotor speed was 78.5 RPM
and the Piston Ram pressure 4,8 kg/cm . Mlxing times were from about 7 minutes.
for the low carbon blends to 10 minutes for the high carbon blends.
The resistivities of the resultant blendg were determined by the method of
ASTM-D-257-61, and the binder efficiancy by plate-out test and cracks examination,
The results are shown in the following table.
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Binder Efficiency and Electrical Resistivity Measurements
. . . ,
Vulcan XC-72/PP homopolymer Vulcan XC-72/PP-PE copolymer
Binder Binder
Ratio Efficiency El. Resistivity Ra~io Efficiency El, Resistivity
_ ..... , ..
50/100 E 190 ohm-cm 50/100 E 11 ohm-cm
60/100 E 147 ohm-cm 60/100 E 3.5 ohm-cm
70/100 G 137 ohm-cm 70/100 E 2,5 ohm-cm
75/100 G 132 ohm-cm 80/100 E 1.4 ohm-cm
77/100 B _ 90/100 E 0.97 ohm-cm
80/100 B _ 100/100 E 0,48 ohm-cm
90/100 _ _ 115/100 B 0,27 ohm-cm
Rating: E = Excellent
G = Good
B = Bad
As shown by the table the binder efficiency of the copolymer is far better
than that of the homopolymer with a maximum content of 110 parts of black per
100 of copolymer compared with a maximum of only 75 parts for the polypropylene,
It can also be seen that using polypropylene the lowe3t resistivity obtainable
is around 130 ohms-cm which is many times too high for the intended fuel-cell
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use. In æharp contrast it can be seen that over the same range of propor-
tlons all of the copolymer blends are effectively within the target
range of 1 to 10 ohms-cm indicating not only a higher capacity for the
black but also a much more effective lnternal distribution.
The product wlth 110 parts of black to 100 of copolymer was ground to
glve granules of about 2 mm particle size. Thi5 was fed to a 30 mm
extruder with 25:1 L/D ratio equipped with a 14 cm flat die having a 250
micron lip opening. Twelve samples of the extruded sheet were taken at
five minute intervals and twelve teæt specimens 10 cm square and 250 + 1
micron thick were tested for electrical resistivity isotrophy by measuring
the resistivity at nine points along the diagonals using a four electrode
direct current indicator. All of the readings were within + 10% of the
average of 0.27 ohms-cm.
Obviously the raw blend can be formed to make conductive articles
of any desired shape or nature using conventional techniques such as
extrusion or moulding. For the bi-polar plates which are the preferred
embodiment of the lnvention the normal thickness will be from about 180
to 280 microns and these can be prepared by flat die extrusion as des-
cribed above, or by calendering.- In extrusion the melt viscosity is so
high that the width of the die has to be limited to about 50 cm but wider
sheets may be obtained by calendering. The finished plate is desirably
given a corrugated or similar profile for increased rigidity for example
by compression moulding.
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