JPS5920693B2 - Conductive polyolefin composition - Google Patents

Abstract

A mixture containing a homogeneous mixture of a propylene-ethylene copolymer containing from 20 to 35 mol% of ethylene and at least 30 parts by weight of finely divided conductive carbon per 100 parts by weight of copolymer can be prepared by mixing at a high shear rate, for example in a Banbury mixer, at a propylene-ethylene copolymer temperature of at least 100 DEG C. A mixture of this type is suitable for the production of a two-pole plate for an electrochemical cell.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to producing high quality electrically conductive materials from polyolefins.

Many attempts have been made to produce conductive or semiconductive materials from polymeric plastics mixed with conductive solids such as carbon black, graphite or finely divided metals.

The use of thermoplastic and thermosetting resins of all kinds has been proposed, including melamine, phenolic aldehyde and more general polyolefins such as polyethylene and its graft copolymers and polytetrafluoroethylene. Generally, relatively low resistance compositions can be prepared by dry mixing a finely divided thermoplastic polymer and an electrically conductive filler and molding the mixture under heat and pressure.

Such products are usually porous and non-homogeneous in structure and are therefore unsuitable for some sophisticated applications requiring thin, opaque, highly homogeneous composition electrical conductors. An example of such an application is a bipolar plate for a fuel cell or battery. A much higher degree of homogeneity than can be obtained by dry molding methods is obtained using known mixing means such as Pampari mixers or roll mills.

FR 1305140 describes the preparation of a number of formulations in carbon black or graphite crystalline polypropylene in a Pampari mixer, with or without amorphous copolymer plasticizers. The resistance of all of these formulations is on the order of a few tag ohms and centimeters, making them suitable for use as thermistors and semiconductors. It should be noted that the mixing time in one Banbury mixer is on the order of 30 minutes, which inevitably causes some thermal decomposition of the polypropylene, resulting in a deterioration of its physical and chemical properties. When mixing carbon black into rubber or polymers such as polypropylene in a Banbury or similar mixer, there is an upper limit to the amount that can be incorporated to obtain a homogeneous product.

Beyond this limit, inhomogeneous particles or carbon are present in the mixture, which can be ascertained by various different methods. The more carbon a given polymer can incorporate, the better its so-called binder efficiency. Three test methods used to measure binder efficiency are briefly described below. In both cases, the carbon-loaded polymer is ground into particles approximately 2 um in diameter. Plate Out Test The white polypropylene compound is melted at 100° C. on a mill roll for 5 minutes.

Once a good rolling pencil is obtained, add 3t of carbon-laced particles and immediately peel off the sheet. If the carbon black is properly integrated, the individual particles will be incorporated into the white polypropylene without contaminating it. If this is not the case, the white sheet will be locally gray contaminated by free carbon. Extraction Test The carbon-loaded particles described above are extracted with isopropanol in a Soxle apparatus.

If there is free carbon black powder, it will appear as a black turbidity. Black test The formulation to be tested is extruded through a flat die into sheets 0.25 ml thick and corrugated between rolls.

Then visually inspect for cracks or tears. If present, it indicates the presence of free carbon and at the same time indicates that the sheet does not have a uniform resistance over its entire surface. When carbon black is used as a rubber reinforcing agent, especially in tires, a typical concentration is 45
phr. That is, 45 parts by weight of black per 100 parts by weight of rubber.
Parts by weight. So-called "masterbatches", which are intended to be diluted with additional rubber, contain 70 to 75 phr.
Manufactured to contain black. The resistance of such a masterbatch would be on the order of several hundred ohm-cm. As previously indicated, low resistance formulations can be produced by processing dry blended formulations of finely divided carbon black and polymer under heat and pressure. They are heterogeneous, have relatively low mechanical properties, and are porous. Since such formulations are always heterogeneous, the concept of binder efficiency as a measure of maximum carbon content discussed for homogeneous formulations is no longer applicable. Now, according to the invention, by appropriate selection of the starting materials and preferably by using a specific compounding technique, products with exceptionally low resistance, homogeneous electrical properties and good mechanical properties can be obtained. It has now been discovered that it is possible to produce electrically conductive carbon-polyolefin blends.

Such materials are particularly suitable for making bipolar plates for fuel cells and batteries. This is because, in addition to the properties mentioned above, this material is chemically inert and does not have a poisonous effect on any type of catalyst of the fuel cell that would coat the plates. According to the present invention, the electrically conductive non-porous polyolefin composition comprises propylene-propylene having at least 20 mole % ethylene.
It consists of a homogeneous blend of ethylene copolymer and at least 30 parts by weight of finely divided conductive carbon per 100 parts by weight of copolymer.

The volume resistivity perpendicular to the plate plane is the most important for the end use mentioned above, and the value given here is A.
Measured according to STM-D-257-61.

A desirable value for the product is approximately 1 ohm-cm. Experience has shown that when using crystalline polypropylene homopolymers, the binder efficiency is too low to give a homogeneous product close to the desired performance.

In sharp contrast, if the polyolefin used is a highly crystalline thermoplastic propylene-ethylene copolymer, the binder efficiency is much higher, yet the conductivity can be greatly improved even at comparable carbon loadings. found.

The copolymer should have a minimum ethylene content of 20 mole percent, and for practical purposes the ethylene content should not exceed about 35 mole percent. This is because above this level thermoplasticity tends to be lost and the product becomes an elastomer. Moreover, if this copolymer is intended for fuel cells, it is preferably highly purified, e.g. by extraction with a solvent such as chloroform or isoprobanol, to avoid the possibility of acting as a catalyst poison in the fuel cell. Certain catalyst residues and stabilizers have been removed. Obviously, this is not necessary if the end use is such that their presence is benign. The desired copolymer type has a melt flow rate (MF
R) is at least 5, preferably from 5 to 10, although copolymers with a lower MFR, for example 1.5, can still be used. A preferred form of fine conductive carbon is carbon black.

This is because it is inexpensive, easily available, and provides excellent results. Finely divided conductive carbon, such as carbon black, should preferably be present in an amount of 90 to 100 parts by weight per 100 parts by weight of copolymer. Part or all of the carbon black can be replaced with graphite. Acetylene black is preferred due to its good electrical properties, but obviously furnace blacks and channel blacks can also be used.

A suitable commercially available black is Vulcan (Vu
(e.g. Vulcan 3, Vulcan XXX and Vulcan XC-72). Typically, fine conductive carbon has an active surface area of 300 to 500 I/y.

As already mentioned, in the art, when using a Banbury mixer to mix carbon black into polypropylene, mixing times on the order of half an hour are employed, which inevitably leads to decomposition of the polymer. aggravation and thus a deterioration of its physical and chemical properties.

This degradation can be substantially reduced if the filler is mixed into the polymer under severe conditions including high shear, high temperature and relatively short mixing times. In accordance with the present invention, an electrically conductive, non-porous polyolefin composition is prepared by combining a propylene-ethylene copolymer with finely conductive carbon under conditions of high shear at an elevated temperature of at least 100°C. by mixing at least 30 parts by weight to 1 part carbon and continuing mixing until a homogeneous blend is obtained. Mixing equipment such as a Banbury mixer or roll mill can be used as used in the rubber industry, with Banbury being preferred.

The conventional use of such equipment is to gradually heat the charge through the energy expended in shearing, so that the polymer becomes sufficiently plastic or fluid. The result is that it takes a relatively long time before a homogeneous dispersion of the filler is ensured, during which time the decomposition described above may occur. However, in a preferred embodiment of the invention, the mixing is carried out at a pre-established relatively high temperature and a relatively short period of time. For this reason, the Banbury mixer and the charged copolymer are preheated to a temperature of 100°C or higher, preferably 150°C or higher, preferably 200°C. The mixing step then takes 10 minutes or less, typically 3 to 5 minutes.
minutes. It will be appreciated that this is contrary to the usual usage in Banbury, where the tougher the mixture is and therefore the colder it is possible, the theoretically maximum mixing will take place. However, the copolymer formulations prepared in this way are hardly affected by decomposition, are very homogeneous, and have extremely low resistances of at most a few tens of ohm-cm, which values vary depending on the conditions and 0.5-10 depending on the amount of integrated carbon black
It can range from ohms to centimeters.

The compound product having a resistance value of less than 10 ohm-cm, preferably less than 1 ohm-cm, is then formed into a thin plate having a thickness of about 250 microns, for example by calendering or extrusion. Can be molded.

Such plates can be given a corrugated or similar cross-section by suitable molding techniques. Comparative data then demonstrates the unexpected benefits of the present invention.

Blends of Vulcan XC-72 car pump hooks in various proportions were prepared using polypropylene homopolymer and propylene-ethylene copolymer (PP-PE copolymer) containing 27 mole percent ethylene.

In powder form, both resins have an MFR of 6 and crystallinity of approximately 9.
3% (before extraction), and was previously extracted with isopropanol to remove catalyst residues and the like. Capacity 3.51 11
The formulation was prepared in a Banbury mixer of capacity 3.51 which had been preheated for half an hour with steam at a power of kg/Cd to raise the temperature to about 180°C.

The polymer and carbon black were charged simultaneously to the mixer in the proportions shown in the table, giving a total charge of 3 kg. The rotor speed is 78.5 RPM and the piston ram pressure is 4
．． It was 8 kg/Cd. Mixing times ranged from about 7 minutes for low carbon formulations to about 10 minutes for high carbon formulations. The resistance of the resulting formulation is as per ASTM-D-25
7-61, and binder efficiency was determined by plate-out test and crack test.

The results are shown in the table below. As shown in the table, the binder efficiency of the copolymers is up to 1% per 100 parts of copolymer.
A comparison of a black content of 10 parts with a maximum of only 75 parts based on polypropylene shows that it is far superior to that of the homopolymer.

It will also be seen that the lowest resistance obtained when using polypropylene is around 130 ohm-cm, which is many times too high for the intended fuel cell use. In sharp contrast, over the same range of ratios, the copolymer formulations all effectively fall within the targeted 1-10 ohm-cm range, providing not only higher black receptivity but also greater It will be seen that it also shows an effective internal distribution. 100 parts of the copolymer and 110 parts of Black's product were ground into granules with a particle size of approximately 2 mm.

This was fed to a 30 mm extruder with L/D of 25:1 equipped with a 14 (V7!) flat die with a rib opening of 250 microns. Twelve samples of the extruded sheet were taken at 5 minute intervals. 250±1 at 10CTrL angle
Regarding the test material 12 having a thickness of microns, the electrical resistance was measured at nine points along the diagonal line using a four-electrode direct current indicator, and the isotropy of the resistance was examined. All meter readings were within ±10% of the average value of 0.27 ohm-cm. It will be appreciated that this raw material formulation can be formed into electrically conductive articles of any desired shape or nature using conventional techniques such as extrusion or molding.

Claims (1)

Claims: 1 At least 20 mole % ethylene, preferably 20
Electrically conductive, characterized in that it is a homogeneous mixture of a propylene-ethylene copolymer containing ~35 mole % ethylene and at least 30 parts by weight of finely conductive carbon, preferably carbon black, per 100 parts by weight of this copolymer. and non-porous polyolefin compositions. 2 at least 20 mol% ethylene, preferably 20
An electrically conductive, non-porous, homogeneous mixture of a propylene-ethylene copolymer containing ~35 mole percent ethylene and at least 30 parts by weight of finely conductive carbon, preferably carbon black, per 100 parts by weight of the copolymer. In the process for producing polyolefin compositions, under high shear conditions, preferably in a Banbury mixer or rubber mill, and at a temperature of at least 100°C,
A method characterized in that propylene-ethylene copolymer and finely divided electrically conductive carbon are mixed and the mixing is continued until a homogeneous blend is obtained.