Fluorinated ionomers

Details Fluorinated ionomers Fluorinated ionomers

2001-06-22

2003-06-10

Bell, Bruce F. (Department: 1746)

Chemistry: electrical and wave energy

Apparatus

Electrolytic

C204S416000, C204S421000, C429S006000, C429S006000, C521S027000, C521S028000, C521S031000, C252S500000, C252S062200

Reexamination Certificate

active

06576100

ABSTRACT:

The present invention relates to sulphonic fluorinated ionomers suitable for the preparation of membranes working from room temperature to high temperatures, of the order of 120° C.-180° C., in electrolytic applications, for example in fuel cells.
Specifically, the invention relates to sulphonic fluorinated ionomers crosslinked without involving the —SO
2
F groups and capable to maintain a high degree of hydration, both at room and at high temperature (up to 120°-180° C.), without substantially compromising the physical integrity of the membrane.
More specifically, in the case of crosslinked sulphonic fluorinated ionomers and having a low equivalent weight, lower than about 750, the obtained membranes show a high capability of water absorption, both at room and at high temperature (up to 120°-180° C.), without substantially compromising the physical integrity of the membrane.
In the case of sulphonic fluorinated ionomers having an equivalent weight higher than about 750 and up to about 1,300, by the crosslinking of the invention it is possible to prepare membranes having an extremely thin thickness, for example in the range 10-80 &mgr;m, which maintain a good hydration also at high temperatures, of the order of 120° C.-180° C., still maintaining the physical integrity.
BACKGROUND OF THE INVENTION
It is known in the prior art the use of the class of polymers called by the term “ionomers” in electrochemical applications, such as for example in fuel cells, chloro-alkali cells, lithium batteries, electrodialysis and in reactors in which the ionomer acts as a solid catalyst. These applications imply the contact of the ionomer with an aqueous or polar liquid having affinity with the ionic functional groups of the ionomer.
Generally, the larger the amount of sulphonic groups (ionomers having a low equivalent weight), the better the efficiency of the ionomer in the application, both in terms of ion exchange capability in electrochemical applications, and in terms of the catalyst activity in catalysis applications. From this point of view, an important parameter is the equivalent weight of the ionomer. The lower the equivalent weight, the higher the percentage of ionic groups. Therefore, ionomers having a low equivalent weight are desirable since they give a higher efficiency in the application.
In electrochemical applications, for example in fuel cells, there is a direct correlation between the ionomer conductivity and the retention of water of the ionomer. The ionic conductivity of the polymer, besides being increased by the higher presence of ionic groups in the polymer, results increased, within certain limits, also by the larger amount of water that the polymer is capable to keep (swelling degree). However, the excessive affinity of the ionomer with water has the drawback of an excessive swelling of the ionomer, which takes a gelatinous state thus losing its physical integrity. The ionomer therefore becomes completely unusable in all the applications wherein it is required under a solid form.
Also in the applications wherein the ionomer is mixed with or deposited on a support material, suitable to guarantee the form and the physical integrity of the final membrane, the ionomer must however show a sufficient physical consistency to prevent the release from the support and it must be quite insoluble in water with which it comes into contact during the use. Besides, the ionomer/membrane must be activated before the use, wherefore the chemical transformation of the precursor groups —SO
2
F into the corresponding ionic groups —SO
3
H is necessary. The membrane activation is carried out first by contacting it with an alkaline aqueous solution and then with an acid solution. During this transformation phase, if the ionomer has a high swelling degree, it can partially or completely dissolve in the reaction medium. At this point, it is extremely difficult to recover the ionomer and separate it from the by-products of the transformation reaction.
In the prior art, to obtain a limited hydration of the ionomer and sufficient physical integrity, polymers having a high equivalent weight, of the order of 1,000-1,200, i.e. having a low concentration of sulphonic groups, are used. Ionomers having a high equivalent weight absorb a limited amount of water, which guarantees the polymer insolubility. On the other hand, having few ionic groups, they have the drawback to give membranes with a low ionic conductivity during the application. An example of said membranes is represented by the commercial product NAFION®, used in fuel cells. These membranes to have a good physical integrity must however have a high thickness, generally higher than 100 &mgr;m. Besides, if these membranes are used at temperatures higher than 100° C., the water contained in the membrane, due to the limited number of hydrophilic groups —SO
3
H and the high thickness, tends to diminish, wherefore the membrane tends to dehydrate and the membrane conductivity is drastically reduced. Consequently, the NAFION® membranes are not effectively usable at temperatures higher than 100° C.
U.S. Pat. No. 4,940,525 describes sulphonic ionomers having a low equivalent weight, lower than 725, used to obtain unsupported thick membranes for fuel cells, only if the hydration product of the polymer is lower than 22,000. According to this patent so low hydration values are indeed necessary to maintain the polymer physical integrity at equivalent weights lower than 725, provided that the equivalent weight is not lower than 500 (col. 6, 8-16). In this patent no mention is made either to the behaviour of these membranes at high temperatures, up to about 120° C.-180° C., or to the minimum usable thickness maintaining the physical integrity.
The need was therefore felt to have available sulphonic fluorinated ionomers able to give membranes usable both at room and at high temperature (up to 120°-180° C.), without substantially compromising the physical integrity of the ionomeric membrane for sulphonic fluorinated ionomers having a low equivalent weight, lower than 750; in the case of sulphonic fluorinated ionomers having an equivalent weight higher than about 750 and up to about 1,300, to have membranes having an extremely thin thickness, for example in the range 10-80 &mgr;m.
The Applicant has surprisingly and unxpectedly found sulphonic fluorinated ionomers able to solve the above mentioned technical problem.
SUMMARY OF THE INVENTION
An object of the present invention are crosslinked sulphonic fluorinated ionomers, where crosslinking does not involve the —SO
2
F groups, having an equivalent weight 380-1,300 g/eq, and comprising:
(A) monomeric units deriving from one or more fluorinated monomers containing at least one ethylene unsaturation;
(B) fluorinated monomeric units containing sulphonyl groups —SO
2
F in an amount such as to give the above equivalent weight.
The fluorinated monomers of type (A) are selected from:
vinylidene fluoride (VDF);
C
2
-C
8
perfluoroolefins, preferably tetrafluoroethylene (TFE);
C
2
-C
8
chloro- and/or bromo- and/or iodo-fluoroolefins, such as chlorotrifluoroethylene (CTFE) and bromotrifluoroethylene;
CF
2
═CFOR
f
(per)fluoroalkylvinylethers (PAVE), wherein R
f
is a C
1
-C
6
(per)fluoroalkyl, for example trifluoromethyl, bromodifluoromethyl, pentafluoropropyl;
CF
2
═CFOX perfluoro-oxyalkylvinylethers, wherein X is a C
1
-C
12
perfluoro-oxyalkyl having one or more ether groups, for example perfluoro-2-propoxy-propyl.
The fluorinated monomers of type (B) are selected from one or more of the following:
F
2
C═CF—O—CF
2
—CF
2
—SO
2
F;
F
2
C═CF—O—[CF
2
—CXF—O]
n
—CF
2
—CF
2
—SO
2
F
wherein X═Cl,F or CF
3
; n=1-10
F
2
C═CF—O—CF
2
—CF
2
—CF
2
—SO
2
F
F
2
C═CF—Ar—SO
2
F wherein Ar is an aryl ring.
Optionally the sulphonic fluorinated ionomers of the invention can contain from 0.01 to 5% by moles of monomeric units deriving from a bis-olefin of formula:
R
1
R
2
C═CH—(CF
2
)
m
—CH═CR
5
R
6
  (I)
wherein:
m=2-10, preferably 4-8;
R
1
, R
2
, R
5
, R
6

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