CA1057948A

CA1057948A - Conductive polyolefin compositions

Info

Publication number: CA1057948A
Application number: CA228,347A
Authority: CA
Inventors: Angelo Murer; Ottavio Amadei
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1974-06-04
Filing date: 1975-06-03
Publication date: 1979-07-10
Also published as: FR2273843B1; BE829684A; SE411355B; IT1038259B; FR2273843A1; NL181875B; NL181875C; ZA753080B; DE2524640C2; CH618453A5; NL7506613A; DE2524640A1; JPS5920693B2; GB1495275A; BR7503341A; JPS5117937A; SE7506230L

Abstract

ABSTRACT
An electrically conductive non-porous polyolefin composition comprising a homogeneous mixture of a propylene-ethylene copolymer containing 20 mol % to 35 mol % ethylene and at least 30 parts by weight of finely divided conductive carbon e.g. carbon black, per 100?parts by weight of copolymer. This composition may be made by mixing under conditions of high shear e.g. in a Bambury mixer, and at a temperature of at least 100°C the propylene-ethylene copolymer and the finely divided conductive carbon, the mixing being continued until a homogeneousblend is obtained. A bi-polar plate for an electrochemical cell may be formed by obtaining a homogeneous blend of polyolefin and finely divided conductive carbon by the above-mentioned process and thereafter forming the composition into the shape of a bi-polar plate.

Description

l~S7~48 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 l~)S7~4~

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 1~)57~48 :-:
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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 ~

~1)57~48 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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- 4a --1~)57~4~i 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 ~ - 5 -1~t575~4~i~

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 4~

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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An electrically conductive non-porous polyolefin composition comprising a homogeneous mixture of a crystalline propylene-ethylene copolymer containing from 20 to 35 mol % ethylene and at least 30 parts by weight of finely divided conductive carbon per 100 parts by weight of copolymer, said composition having an electricresistivity below about 10 ohm-cm as determined by ASTM Test Method D257-61.

2. A composition according to 1 wherein the copolymer has a Melt Flow Rate (MFR) of 5 to 10.

3. A composition according to claim 1 wherein the finely divided conductive carbon is present in an amount of 90 to 100 parts by weight per 100 parts by weight of copolymer.

4. A composition according to any of claims 1, 2 and 3 wherein the finely divided conductive carbon is carbon black.

5. A composition according to any one of claims 1, 2 and 3 wherein the finely divided conductive carbon is acetylene black.

6. A composition according to any one of claims 1, 2 and 3 wherein the finely divided conductive carbon has an active surface area of 300 to 500 m2/gm.

7. A process for preparing an electrically conductive non-porous polyolefin composition having an electric resistivity below about 10 ohm-cm which comprises preheating to a temperature of above 100°C
a polymer charge comprising a crystalline propylene-ethylene copolymer containing from 20 to 35 mol % ethylene and at least 30 parts by weight per 100 parts by weight of copolymer of finely divided conductive carbon and mixing the polymer charge under conditions of high shear and at a temperature of at least 100°C, the mixing being continued until a homogeneous blend is obtained.

8. A process according to claim 7 wherein the mixing is carried out at a temperature in the range of 150°C to 200°C.

9. A process according to either of claims 7 or 8 wherein the mixing is carried out for less than 10 minutes.

10. A process according to claim 7 or 8 wherein the mixing is carried out in a Banbury mixer or rubber mill for less than 10 minutes.

11. A method of forming a bi-polar plate for an electrochemical cell which comprises preheating to a temperature of above 100°C a polymer charge comprising a crystalline propylene-ethylene copolymer having 20 to 35 mol %
ethylene and at least 30 parts by weight of finely divided conductive carbon per 100 parts by weight of copolymer and mixing the polymer charge under conditions of high shear and at a temperature of at least 200°C, continuing the mixing until a non-porous homogeneous composition is obtained and thereafter forming the composition into the shape of a bi-polar plate.

12. A method according to claim 11 wherein the polyolefin is first treated with a solvent to remove catalyst residues before it is mixed with the finely divided conductive carbon.

13. A method according to either of claims 11 or 12 wherein the pro-portion of finely divided conductive carbon is selected so that the resistivity across the plate is below 1 ohm-cm.