Plastic and nonmetallic article shaping or treating: processes – Pore forming in situ – By treating occluded solids
Reexamination Certificate
2001-09-20
2004-03-09
Kuhns, Allan R. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Pore forming in situ
By treating occluded solids
C521S061000
Reexamination Certificate
active
06702965
ABSTRACT:
The invention relates to a porous composite product, in particular in the form of a film, in particular with a high specific surface, and to a process for the preparation of such a product.
It also relates to the precursor composite product of use in carrying out the said process.
It also relates to the application of the porous composite product in the form of a film as electrode for the entire range of electrochemical products and to the application in general of the porous composite product, with a high specific surface, in the field of selective membranes, of packaging or of catalysis.
Low density porous films are already known, in particular by Patent Application EP-A-283,187, which are obtained by spinning, at the melting temperature, a mixture of a first thermoplastic polymer and of a second thermoplastic polymer and then removing the second polymer by means of an appropriate solvent. Such a porous film can be used for various applications, in particular in the field of filtration or of separation.
Patent Application EP-A-430,439 discloses a process for improving the production of such films, in which a mixture of a first thermoplastic polymer and of a second immiscible thermoplastic polymer is extruded through a die and then removing the immiscible polymer by means of a solvent, the process being characterized in that a perforated barrier plate is interposed upstream of the die, so as to obtain a porous structure comprising a longitudinal region of low porosity and another longitudinal region of higher porosity.
Furthermore, polarizable electrodes are known which can be used in electrical capacitors of the double-layer type which are capable of being charged with, or of discharging, a large amount of electric charge.
The polarizable electrodes which can be used in supercapacitors are based on an ideally polarizable material which is light and which has a large exchange surface area, such as active charcoal, which is a carbonaceous material with a high specific surface, in particular greater than 1000 m
2
/g.
For an electrode to exhibit the maximum efficiency, it must have a maximum proportion of active mass and an optimum accessibility to this mass. The latter property requires that it should have an open porous structure. This is the case, for example, with electrodes made of activated fabric: an active charcoal fabric is manufactured from a fabric based on viscose or on polyacrylonitrile, which fabric is carbonized and then activated.
However, such electrodes are expensive and exhibit a high and uneven thickness (generally greater than 300 &mgr;m). Furthermore, although such a production method makes it possible, at least in theory, to employ a spooling technology, it turns out in practice that such an operation is difficult to carry out.
Electrodes with a very high proportion of active mass (generally greater than 98%) can also be obtained by sintering. Active charcoal and various additives, in particular conducting black, are mixed mechanically with a liquid until a suspension is obtained. The solution obtained is poured over a filtering partition, which is placed under partial vacuum. After a certain time, all the components are deposited homogeneously on the filtering partition, whereas the liquid has passed through this partition. The partial vacuum creates a degree of cohesion between the components, equivalent to compacting under pressure. The electrode is the dry material recovered on the partition.
However, as above, this technology exhibits numerous disadvantages. In particular, it lends itself with difficulty to the use of a spooling technology and the thickness, homogeneity and evenness of the electrodes are difficult to control. Furthermore, the processes are limited in the choice of the polymers. In particular, polyolefins cannot be used.
The carbonaceous filler can also be mixed mechanically with a binding polymer in a small proportion, for example 3% of Teflon, until a very viscous paste is obtained, and then rolling in order to give a sheet which is cut up using a hollow punch in order to produce an electrode.
This process results in the same disadvantages as the preceding production methods.
A process for manufacture by coating is also mentioned, in which process the active filler and one or more additives, such as a binding polymer, are mixed with a solvent until a paste of controlled viscosity is obtained. The latter is coated onto a support sheet which can act subsequently as current collector. The sheet passes into an oven in order for the solvent to be evaporated.
The deposit can be relatively thin (down to a few microns) and homogeneous and the proportion of active mass is high.
It is nevertheless a process which is difficult to implement because of the possible use of solvents which can be toxic.
The electrodes in the form of films, in particular of polyolefin films, which make it possible to employ a spooling technology are also known.
These polarizable electrodes are based on a carbonaceous material, for example an active charcoal with a high specific surface, in particular at 1000 m
2
/g, and on a binder, such as polyolefins, in particular polyethylene or polypropylene, or other polymers, such as polyesters, polycarbonates or polyimides.
Polarizable electrodes using a polyethylene or polypropylene binder and an active charcoal powder have been provided, for example (JP-A-22062/92).
However, polarizable electrodes based on a binder such as polyethylene or polypropylene exhibit a very low porosity.
Such phenomena also occur with the other binders cited above.
Document BE-A-693,135 discloses porous sheets of polytetrafluoroethylene entirely in the form of fibrils comprising conducting filler materials, such as graphite or a metal, up to 98% of the weight of the sheet.
This type of structure is obtained by mixing an aqueous dispersion of polytetrafluoroethylene particles with an extractable polymer, followed by milling. This milling constitutes a critical stage which results in the shearing of the polytetrafluoroethylene particles and the conversion of the particles into a network of elongated fibers. Extrusion is then carried out and then the extractable polymer is removed. The final structure exhibits pores of greater than 0.1 &mgr;m. These sheets can be used as electrode in fuel cells.
The abstract of Japanese document JP-A-57100142 discloses the production of a porous membrane which consists in extruding a mixture, by volume, of 15-60% of a polyolefin resin; 3-40% of a polyether; 20 to 80%, by volume, of an extractable, finely divided powder; and 0.5 to 10% of an insoluble powder and in then extracting the polyether and the extractable powder. The Applicant has confirmed that it is impossible by the process disclosed in this document to obtain sheets comprising a higher proportion of filler without seriously affecting the mechanical properties.
It would therefore be desirable to produce porous electrodes formed-of a binder and of fillers, in particular with a high specific surface, which can be produced in a large amount which make it possible to employ a spooling technology.
The aim of the present invention is specifically to provide a solution to this technical problem.
One object of the present invention is to provide novel porous composite products having in particular a high specific surface.
Another object of the present invention is to provide composite films, in particular with a high specific surface, with a high content of fillers, which make it possible to employ a spooling technology.
Another object of the present invention is to make it possible to use a broad choice of polymers.
Another object of the present invention is to provide porous composite products or films which are inexpensive to manufacture.
Another object of the present invention is to make it possible to obtain products of varied shape, because of the extrusion technique used, which can be employed, such as pipes, rods, films or any other extruded object.
Another object of the present invention is to provide carbonaceous electrodes in the form of thin, homogene
Capitaine François
Le Goff Philippe
Penneau Jean-François
Blakely & Sokoloff, Taylor & Zafman
Bollore
Kuhns Allan R.
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