Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-09-17
2003-10-07
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C264S048000, C264S049000, C522S157000, C522S158000, C522S161000, C525S326100
Reexamination Certificate
active
06630518
ABSTRACT:
The present invention relates to a process according to the preamble of claim 1 for the preparation of a sulfonated polymer membrane.
The invention also relates to the polymer membrane according to the preamble of claim 12, its use according to claim 18 in electrochemical cells, and the electrochemical cell according to the preamble of claim 19.
Conductivity is one of the most essential properties in any application of a membrane such as the membrane according to the present invention. The membranes are also required to have mechanical strength, chemical stability, and good barrier properties against the permeation of non-desirable components from one side of the membrane to the other.
Ion-conductive membranes can be used in numerous applications. Some examples are their use as proton conductors in fuel cells or electrolytic cells. In a fuel cell, the energy released in the reaction is converted to electric current at a conversion rate of approximately 60-80%. Fuels suitable for such cells include hydrogen, natural gas and methanol. Fuel cells which have polymer membranes as electrolytes are regarded as one of the most interesting options for relatively small-scale energy production applications in which the power source is less than 150 kW. Such applications include vehicles and certain electric appliances.
At present, many polymer membranes are known which are suitable for use for the purposes mentioned above. In the state of the art (compiled in the work Davis, T. A., Genders, J. D. and Pletcher, D.,
A First Course in Ion Permeable Membranes
, pp. 35-57), two principal preparation processes are disclosed, of which, of course, several variations have been developed.
In the first prior art process, an unsubstituted alkene is copolymerized with a functionalized alkene which contains ionizable groups or, more probably, precursors of ionizable groups. It has been observed that perfluorinated membranes have the best properties in particular as regards stability and the chemical and physical properties. The first step in the preparation of a membrane such as this is monomer synthesis, whereupon the result obtained is a perfluorinated, substituted alkene having an ion-exchange group at the end of a side chain. Sulfonate or carboxylate groups are used commercially. The length of the side chain usually varies from 1 to 4 carbon atoms. This ionomer is thereafter copolymerized with polytetrafluoroethylene (PTFE). PTFE forms the backbone in almost all membranes prepared by the technique described above. The polymer is brought to film form before the conversion of the precursors to ion-exchange active groups.
According to another prior art process, the alkene is polymerized, whereafter ion groups are introduced into the polymer. Usually the membranes prepared in this manner are based on copolymers of styrene and divinyl benzene. There are numerous different alternative embodiments, for example, it is possible to irradiate a stable, inert polymer in order to enable this polymer to be grafted with some aromatic polymer. Ion-exchange groups, which become linked to the aromatic ring, are usually introduced into the structure by means of a strong sulfuric acid solution.
Furthermore, there are a number of membranes of special production, the best known of them being probably Gore Select. It is based on filling the pores of a material like Gore Tex, known as a weatherproof material and being based on PTFE material, with a ion-exchange active polymer, such as commercial Nafion®. The conductivity of Gore Select is, of course, not in the order of that of Nafion®.
The performance of membranes according to the first technique, one example being specifically Nafion®, is quite fair. The problem involved with these membranes is their difficult preparation process. For this reason the price of the product remains high and the amounts used remain low. It is also to be noted that the properties of the membrane are largely determined already during the monomer stage, and thus the modification of the properties in membrane form is nearly impossible. The processing of the membrane is also cumbersome. On the other hand, membranes prepared by the grafting process are seldom chemically resistant.
In prior art there is also disclosed a process in which reactive sites are provided in a polyvinyl fluoride film (PVF film) by electron or proton irradiation (Paronen, M., Sundholm, F., Rauhala, E., Lehtinen, T. and Hietala, S., Effects of Irradiation on Sulfonation of Poly(vinyl fluoride),
J. Mater. Chem.
, 1997, 7(12), 2401-2406). An irradiated film was treated with chlorosulfonic acid having a concentration of 2.5% by volume. It is noted in the publication that both the absorbed radiation dose and the mass of the irradiating particle affect the sulfonation. The conductivity of proton-irradiated membranes was at best 10-20 mS/cm when the absorbed doses were 400-1000 kGy. However, the distribution of sulfonic acid groups in the membrane is not discussed in the publication. Furthermore, it was observed in laboratory experiments that the membranes according to the publication, when treated, were not self-supporting or their conductivity was very low.
The object of the present invention is to eliminate the disadvantages associated with prior art and to provide a new process for the preparation of sulfonated polymer membranes.
According to the invention, a polymer film is irradiated with ions or gamma radiation in order to produce reactive sites. The irradiated membrane material is sulfonated in order to link sulfonic acid groups to it. The sulfonation is continued until the total concentration of sulfonic acid groups in the membrane is 0.4-3.0 meq/g and they are homogeneously distributed in the membrane in such a manner that their concentration in the middle of the membrane is at least 0.2 meq/g.
By the process according to the invention it is possible to prepare a membrane in which the sulfonic acid groups are linked directly to the repeating unit of the polymer chain and not to a side chain as in prior-art options, when the membrane material is non-aromatic.
More specifically, the process according to the invention is characterized by what is stated in the characterizing part of claim 1.
The membrane according to the invention, for its part, is characterized by what is stated in the characterizing part of claim 12.
The electrochemical cell according to the invention is characterized by what is stated in the characterizing part of claim 19.
Considerable advantages are achieved by means of the invention. By the process according to the invention, a membrane is obtained which is self-supporting and which can be used in various applications, such as fuel cells. The membrane can also be used as an ion-exchange active material in ion exchange, in the coating of material, in ion-selective purification, in applications exploiting filter or separator membranes, or in applications exploiting semi-permeable membranes. The homogeneous distribution of the sulfonic acid groups in such a manner that their concentration in the middle of the membrane is above 0.2 meq/g ensures that conductivity will not decrease too much. On the other hand, the homogeneous distribution of sulfonic acid groups in the membrane improves the mechanical properties of the membrane as compared with a situation in which the sulfonic acid groups are mainly located on the membrane surface.
By the process according to the invention it is possible to prepare, by a rapid and simple process, membranes the chemical and mechanical properties of which can be regulated by the selection of the starting membrane, by irradiation and by the sulfonation process. Owing to the simple process, the production costs are also much lower, and thus the selling price of the membrane is also significantly lower. This is of special significance considering the use of the electrochemical cell according to the invention, in particular a fuel cell, as a source of energy. Prior art membranes are so expensive that their use, for example, in cells intended for sources of energy for automo
Foelak Morton
Sughrue & Mion, PLLC
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