Patent Application
20100025879
This invention relates to highly filled composites and to methods for their preparation.
Fuel cells typically are constructed using end plates and separator plates made from highly filled composites containing thermoplastic resin and conductive fillers. References describing such composites include U.S. Pat. Nos. 5,798,188, 6,083,641, 6,180,275, 6,251,978 and 6,261,495; U.S. Patent Application No. 2002/0039675 A1; European Patent Application No. EP 1 059 348 A1; Japanese Patent Application Nos. 8-1663, 2000-200142, 2000-348739 and 2001-122677; Taiwan Patent Application No. 434930 and PCT Patent Application Nos. WO 97/50138, WO 97/50139, WO 00/30202, WO 00/30203, WO 00/44005 and WO 01/89013.
Many researchers have sought molding compositions that could be used for compression or injection molding of fuel cell separator plates and other conductive components. For example, in some of the above-mentioned references pellets of a highly filled composite are formed by combining thermoplastic resin and conductive filler in an extruder, converting the output from the extruder into pellets using a pelletizer, and feeding the thus-formed pellets to a suitable molding apparatus. The pellets typically have fairly regular shapes, e.g., cylinders. Less highly filled pellets containing thermoplastic resin and conductive filler are also commercially available, e.g., VECTRA™ A230 carbon-fiber reinforced liquid crystal polymer, commercially available from the Ticona Division of Celanese AG. Japanese Patent Application No. 8-1663 reports preparation of flake-form pellets using an extruder operated without a die and breaker plate. U.S. Patent Application No. 2002/0039675 A1 reports preparation of pellets that may be mixed with finer particles and preferably are separated therefrom.
We have found that a particularly useful molding composition can be formed by combining thermoplastic resin and filler (e.g., conductive filler) in a multiple screw extruder operated without an exit manifold (a so-called “8-0” adapter), die, breaker plate or pelletizer. The resulting extrudate is “autogranulating” or will “autogranulate”, that is, the extrudate will exit the extruder barrel as irregularly shaped granules without requiring pelletization, chopping, pulverization, crushing or other comminution techniques for forming pellets or other shaped particles. An autogranulating extrudate does not have to be pelletized, and in preferred embodiments is sufficiently highly loaded that it can not readily be pelletized. The extrudate does not have to be classified by separation and removal of finer particles, and in preferred embodiments is not so classified. The extrudate can be used in its as-extruded autogranulated form as a thermoplastic composite for molding shaped articles. Thus the present invention provides, in one aspect, a process for forming thermoplastic composite granules comprising extruding through a multiple screw extruder:
a) thermoplastic resin; and
b) sufficient filler so that an autogranulating extrudate exits the extruder barrel.
In another aspect, the invention provides an autogranulating thermoplastic composite comprising a blend of irregularly shaped granules containing thermoplastic resin and filler.
Preferred embodiments of the thermoplastic composite granules can be used as a molding compound for forming highly filled articles (e.g., fuel cell separator plates and end plates) by compression molding, injection molding or compression-injection molding.
In the practice of the present invention, “irregularly shaped” granules are granules the majority of which do not have the regular cylindrical shapes characteristically found in a pelletized extruded thermoplastic.
A thermoplastic resin can be added to extruder 50 at input end main feed port 52, and filler can be added to extruder 50 at one or more locations along the length of barrel 22 such as feed ports 54 and 56. Provided that sufficient filler is added to the extruder (e.g., an amount of about 40 wt. % or more based on the total weight of the extrudate), an autogranulating extrudate can form granules 58 as it exits extruder 50, and can be collected in hopper 60 placed below exit face 28. The autogranulating process efficiently forms highly filled granules in a range of sizes, with a minimum of equipment and processing cost. Pelletizer 46 of
Traditional pelletized molding compositions typically have very regular shapes and uniform sizes, for example cylindrical shapes or pillows that are approximately the same size from pellet to pellet. The autogranulated extrudate of the invention typically will be a blend of irregularly-shaped granules having a range of shapes and sizes, and will lack the uniform appearance of traditional pelletized molding compositions. Despite such non-uniform appearance, the autogranulated extrudate can provide an excellent molding composition, e.g., for compression molding highly filled conductive components having complex shapes such as fuel cell separators and endplates.
Suitable extruders are available from a variety of suppliers. If desired, extruders having more than two screws can be employed, e.g., three or four screw extruders. As will be appreciated by those skilled in the art, the screw configuration and extruder operating conditions may benefit from optimization or adjustment depending on the materials and equipment employed and the desired end use for the autogranulated extrudate. Representative extruders and extruder screws are shown in U.S. Pat. Nos. 4,875,847, 4,900,156, 4,911,558, 5,267,788, 5,499,870, 5,593,227, 5,597,235, 5,628,560 and 5,873,654.
A variety of thermoplastic resins can be employed in the invention. Suitable resins include polyphenylene sulfides, polyphenylene oxides, liquid crystal polymers, polyamides, polycarbonates, polyesters, polyvinylidene fluorides and polyolefins such as polyethylene or polypropylene. Other suitable resins are listed in the above-mentioned references or described in publications such as “High Performance Plastics from Ticona Improve Fuel Cell Systems” (Ticona division of Celanese AG). Representative commercially available polyphenylene sulfides include those available from the Ticona division of Celanese AG under the trademark FORTRON and those available from Chevron Phillips Chemical Company LP under the trademark RYTON. Representative commercially available polyphenylene oxides include those available from GE Plastics under the trademark NORYL. Representative liquid crystal polymers include those available from the Ticona division of Celanese AG under the trademark VECTRA, those available from Amoco Performance Products, Inc. under the trademark XYDAR and those available from E. I. duPont de Nemours and Company under the trademark ZENITE. Liquid crystal polymers are particularly preferred. The resin can be employed in a neat (viz., unfilled) form (e.g., VECTRA A950 liquid crystal polymer) or in a form that already includes one or more fillers (e.g., VECTRA A230 30% carbon fiber reinforced liquid crystal polymer and VECTRA A625 25% graphite filled liquid crystal polymer). Recycled autogranulated extrudate (and if desired, recycled and reground molded products made from such extrudate) can be added in suitable amounts to the thermoplastic resin.
A variety of fillers can be employed in the invention, in a variety of forms including particles, flakes, fibers and combinations thereof. Conductive fillers are especially preferred, including carbon (e.g., graphite, carbon black, carbon nanofibers and carbon nanotubes), metals (e.g., titanium, gold and niobium), metal carbides (e.g., titanium carbide), metal nitrides (e.g., titanium nitride and chromium nitride) and metal-coated particles, flakes or fibers (e.g., nickel-coated graphite fibers). Graphite is a particularly preferred conductive filler. Suitable nonconductive fillers include silica, calcium carbonate, magnesium carbonate, aluminum hydroxide, magnesium hydroxide, alumina, zinc oxide, clay, talc, glass powder, glass microbubbles, barium sulfate, plastic beads (e.g., polyester or polystyrene beads), olefin-based fibers (e.g., polyethylene fibers and polypropylene fibers), aramid fibers (e.g., NOMEX™ or KEVLAR™ fibers), rock wool, glass flakes and mica. The filler can have a variety of sizes (e.g., particle diameters, fiber lengths, or fiber length/diameter ratios) and a variety of surface areas. For example, when graphite particles are employed in the invention they preferably have a particle diameter of about 0.1 to about 200 micrometers, more preferably about 0.1 to about 25 micrometers, and a surface area of about 1 to about 100 m2/g, more preferably about 1 to about 10 m2/g as measured using the BET method. Carbon black particles preferably have a particle size less than about one micrometer and a surface area greater than about 500 m2/g. Carbon nanofibers and carbon nanotubes preferably have diameters ranging from a few nanometers to several hundred nanometers, and aspect ratios ranging from about 50 to about 1,500.
The autogranulated extrudate can contain very high filler loading levels. Loading levels of at least 40 wt. % filler are preferred, and loading levels of 50 to 95 wt. %, 60 to 95 wt. %, 70 to 95 wt. % or 80 to 95 wt. % filler are more preferred. The filler level should not be so low that autogranulation of the extrudate does not occur, and should not be so high so that the extrudate can not be compression molded using conventional molding equipment and a temperature of 300° C. or less into a self-supporting unitary article. At higher loading levels the extrudate is not readily pelletizable, that is, its rheological behavior is such that the extrudate can not be extruded through a strand die and chopped into pellets using conventional filled thermoplastic resin pelletizing equipment. The autogranulated extrudate typically contains a blend of granules whose average particle diameter may range from about 40 to about 4000 micrometers. The blend can have a unimodal or polymodal (e.g., bimodal) particle size distribution. It generally will not be necessary to screen or otherwise classify the extrudate, and it can be molded as is without removal of fine particles from the blend. The ability to use the extrudate without screening is especially desirable for compression molding. If desired, autogranulated extrudates containing differing weight fractions of filler can be combined with one another, e.g., by dry mixing.
The thermoplastic composite granules may contain other adjuvants such as dyes, pigments, indicators, light stabilizers and fire or flame retardants. The types and amounts of such adjuvants will be familiar to those skilled in the art.
The thermoplastic composite granules typically will be molded or otherwise subjected to further processing after they exit the extruder. The granules are especially suited for compression or injection molding. Suitable molding equipment and conditions will be familiar to those skilled and the art. The resulting molded or otherwise processed articles have a wide variety of uses, including fuel cell separator plates and end plates, battery electrodes, medical device electrodes, electromagnetic radiation absorbing materials, thermally or electrically conductive shields, trays and heat sinks. As will be appreciated by those skilled in the art, the final processed article can have a solid, hollow, foamed or other suitable configuration, contingent upon attainment of the desired level of surface or volume resistivity. For electrically conductive articles, volume resistivity values of about 0.1 ohm-cm or less, more preferably about 0.01 ohm-cm or less, are preferred, as evaluated using the four-point probe method described in Blythe, A. R., “Electrical Resistivity Measurements of Polymer Materials”, Polymer Testing 4, 195-200 (1984).
The invention is further illustrated in the following illustrative examples, in which all parts and percentages are by weight unless otherwise indicated.
Powdered polyphenylene sulfide resin (FORTRON™ 203B6, commercially available from the Ticona Division of Celanese AG) was twin-screw compounded with 70 wt. % No. 8920 graphite flakes (commercially available from Superior Graphite Co.) in a Model ZE40A twin screw extruder (commercially available from the Berstorff division of Krauss-Maffei Corp.), operated without an 8-0 adapter, pelletizing die or breaker plate. Upon exiting the extruder barrel, the extrudate spontaneously formed irregularly-shaped granules in a range of granule sizes. The individual granules were primarily flattened chunks having rounded and flattened portions, some surface striations and a shiny grey appearance. Despite the irregular size and appearance of the granules they were not subjected to pelletization, and were instead evaluated as a molding composition in their as-extruded form. The granules were compression molded using a heated laboratory press (commercially available from Carver, Inc.). The press was first brought to 300° C. at 34.5 Mpa. After reaching 300° C. the pressure was increased to 137.9 MPa and held at this pressure for 3 minutes to form the granules into a 102×102×3.2 mm flat rectangular plate. The resulting molded part had a uniform, low gloss matte appearance with fairly well-formed corners.
The mixture of resin and graphite flakes employed in Example 1 was dry-blended rather than extruded. The resulting blend could not be molded into well-formed separation plates using the Carver laboratory press. Several additions of the blend interspersed with molding cycles were required to obtain dense molded parts. However, the parts delaminated when the mold was opened.
The mixture of resin and graphite flakes employed in Example 1 was extruded through a reciprocating single screw extruder of an injection molding machine (150 Ton molding machine commercially available from Engel Machinery Inc.) equipped with a manifold and a 1.5 mm diameter die. The extrudate was formed during the injection cycle usually used during purging operations or when making an air shot. The extruded strands were manually chopped into pellets having a length of about 4 mm. The resulting pellets could not be molded into well-formed separation plates using the Carver laboratory press or using a larger heated compression press (commercially available from Hull Corp.) operated at 20 Mpa and a temperature of 300° C. The molded parts had poorly-formed corners whose “cottage cheese” appearance appeared to be due to projecting fragments of partly-fused pellets.
Pellet form liquid crystalline polymer resin (VECTRA™ A950, commercially available from Ticona Division of Celanese AG) was added to the inlet end of the twin screw extruder employed in Example 1. No. 2937 G graphite flakes (commercially available from Superior Graphite Co.) were added to the extruder at the main feed port to provide a 70 wt. % graphite loading level in the extrudate. Upon exiting the extruder barrel, the extrudate spontaneously formed irregularly-shaped granules in a range of granule sizes. The individual granules were primarily flattened chunks having rounded and flattened portions, some surface striations and a shiny grey appearance. The granules had an average diameter of about 586 micrometers as determined using W. S. Tyler Sieve Trays of 4 to 400 mesh size. As further illustrated in
Despite the irregular size and appearance of the granules they were not subjected to pelletization, and were instead evaluated as a molding composition in their as-extruded form. The granules were poured into a 100×100×2.5 mm flat rectangular plate mold. The granules were compression molded using the Hull Press employed in Comparison Example 2 at a temperature of 300° C. and a pressure of 20 MPa. The resulting molded part had a uniform, low gloss matte appearance with well-formed sharp-edged corners. Using the above-mentioned four point probe test, the average volume resistivity of the molded part was determined to be about 0.274 ohm-cm
Using the method and materials of Example 1, thermoplastic composite granules containing 80 wt. % or 90 wt. % filler were prepared and compression molded to form fuel cell separator plates. The plates exhibited four point probe test average volume resistivity values of 0.0996 or 0.02094 ohm-cm, respectively. These values represent very low resistivity.
Using the method of Example 1, thermoplastic composite granules were prepared using a XYDAR™ liquid crystal polymer (commercially available from Amoco Performance Products, Inc.) and the graphite flakes employed in Example 1. The density of the liquid crystal polymer resin was 1.38 g/cm3 and the density of the graphite flake filler was 2.25 g/cm3. Set out below in Table I are the Example No., weight percent filler, weight percent resin, calculated extrudate density, calculated volume percent filler, calculated volume percent resin, and extrudate appearance and moldability.
As shown in Table I, autogranulating extrudates could be formed at very high filler loading levels and molded into useful articles.
Various modifications and alterations of this invention will be apparent to those skilled in the art without departing from the scope and spirit of this invention. This invention should not be restricted to that which has been set forth herein only for illustrative purposes.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10299144 | Nov 2002 | US |
| Child | 11768621 | US |
