Sulphonic fluorinated ionomers

Details Sulphonic fluorinated ionomers Sulphonic fluorinated ionomers

2001-08-07

2004-03-16

Lipman, Bernard (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S247000, C526S250000, C526S252000

Reexamination Certificate

active

06706834

ABSTRACT:

The present invention relates to sulphonic fluorinated ionomers crosslinked by radical route suitable for the preparation of membranes for electrochemical applications, in particular for fuel cells, and as ionic exchange resins acting as catalysts.
Specifically, the invention relates to sulphonic fluorinated ionomers crosslinked by radical route characterized by a high hydration degree, both at room temperature and at high temperature (up to 180° C.), without substantially compromising the physical integrity of the obtained membranes, wherefore they are usable also at high temperatures in the range of 120° C.-180° C.
It is known in the prior art the use of the class of polymers called by the term “ionomers” in electrochemical applications, such for example in fuel cells, chlorosoda cells, lithium batteries and in reactors in which the ionomer acts as a solid catalyst. These applications implies the contact of the ionomer with a liquid, in particular water, having affinity with the ionic functional groups of the ionomer itself.
Generally, the larger the amount of ionic groups present in the chain, the better the efficiency of the ionomer application, both in terms of capability of ionic exchange in electrochemical applications, and in terms of catalyst activity in catalysis applications. From this point of view, an important parameter is the equivalent weight of this ionomer. The lower the equivalent weight, the higher the percentage of sulphonic groups present in the chain. Therefore, ionomers having a low equivalent weight are desirable since they give a higher application efficiency.
In electrochemical applications, for example in fuel cells, there is a direct correlation between the polymer conductivity and the water retention from the ionomer itself. The ionic conductivity of the polymer, besides being increased by the greater presence of ionic groups in the polymer, results increased, within an upper limit, also by the larger amount of water that the polymer is capable to retain (swelling degree). However, the excessive affinity of the ionomer with water has as a consequence the drawback of an excessive polymer swelling, which assumes a gelatinous state consequently losing its physical integrity. The ionomer becomes therefore 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 shape and the physical integrity of the final membrane, the ionomer must however show a physical consistency sufficient to prevent the release thereof from the support and it must be insoluble in the liquid medium with which it comes into contact during the use.
Besides, the ionomeric membrane must be activated before the use, wherefore the chemical transformation of the precursor groups —SO
2
F into the corresponding sulphonic 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 impossible to recover the ionomer and separate it from the other products of the transformation reaction.
In the prior art, to obtain a limited ionomer hydration and sufficient physical integrity, polymers having a high equivalent weight, of the order of 1,000-1,200, are used i.e. having a low concentration of sulphonic groups, which represent the hydrophilic part of the polymer. Therefore, 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 having a lower ionic conductivity during the application. An example of said ionomers is represented by the commercial product NAFION®, used in fuel cells and having an equivalent weight of the order of 1,000-1,100. The membranes obtained from said ionomers have good mechanical properties. However, if these membranes are used at temperatures higher than 100° C., the interstitial water, which is the carrier of the protons H
+
in fuel cells, tends to reduce itself, wherefore the membrane tends to dehydrate and the membrane conductivity is drastically reduced. As a consequence, the membranes obtained by NAFION® are not efficiently 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 low, lower than 22,000. So low hydration values are indeed necessary for maintaining the physical integrity of the ionomer having equivalent weights lower than 725, provided that the equivalent weight is not lower than 500 (col. 6, 8-16). Therefore, according to the description of this patent, it is impossible to obtain sulphonic ionomers of equivalent weight lower than 500 having the property of the insolubility in water. Besides, no mention is made to the behavior of the membranes at high temperatures, of the order of 120° C.-160° C.
The need was felt to have available sulphonic fluorinated ionomers such that the obtained membranes have a high hydration percentage, both at room temperature and at high temperature (up to about 180° C.) without substantially compromising the membrane physical integrity, wherefore the membranes are usable also at high temperatures, of the order of 120° C.-180° C., in electrochemical applications. Said membranes can be used also as ionic exchange resins.
An object of the present invention are therefore crosslinked sulphonic fluorinated ionomers obtainable by radical crosslinking of:
A) crosslinkable sulphonic fluorinated ionomers, having equivalent weight 380-1300 g/eq, preferably 380-800 g/eq, and comprising:
from 48% to 85% by moles of monomeric units deriving from tetrafluoroethylene (TFE);
from 15% to 47% by moles of fluorinated monomeric units containing sulphonyl groups —SO
2
F;
from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula:
 wherein: m=2-10, preferably 4-8;
R
1
, R
2
, R
5
, R
6
, equal to or different from each other, are H or C
1
-C
5
alkyl groups;
B) a fluorinated compound as crosslinking radical initiator;
C) a fluorinated bis-olefin of the above structure (I) as crosslinking agent;
the radical crosslinking being carried out at a temperature in the range 250° C.-310° C., preferably 260° C.-300° C.
Among the fluorinated monomers containing sulphonyl groups —SO
2
F we can mention:
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 (vinylsulphonylfluoride);
F
2
C═CF—Ar—SO
2
F wherein Ar is an aryl ring.
Preferably the crosslinkable fluorinated sulphonic ionomers A) comprise:
from 54% to 71% by moles of monomeric units deriving from TFE;
from 45% to 28% by moles of monomeric units deriving from vinylsulphonylfluoride F
2
C═CF—O—CF
2
—CF
2
—SO
2
F;
an amount higher than 0.4% by moles up to 3% by moles, more preferably from 1% to 2.5% by moles of monomeric units deriving from the bis-olefin of formula (I).
As regards the radical initiators B) used in the radical crosslinking of the present invention, they are selected from:
(d)-branched perfluoroalkanes of formula: C
a
F
2a+2
wherein a=5-15, preferably 7-11;
(e)-halogenated compounds of formula: ClO
2
S(CF
2
)
n
SO
2
Cl wherein n=4-10;
(f)-peroxidic perfluoropolyether compounds having oxidizing power in the range 0.8-6, preferably 1-3.5, of structure
T—O—(R
f
)—(O)
c
—T′
wherein:
T,T′═—CF
3
,—CF
2
CF
3
, —CF
2
CF
2
CF
3
c is an integer such as to give the above oxidizing power;
R
f
perfluoropolyether chain having a number average molecular weight in the range 1,000 and 30,000, preferably 4,000-20,000, comprising one or

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