Fluorinated ionomers

Details Fluorinated ionomers Fluorinated ionomers

2002-12-20

2004-07-27

Pezzuto, Helen L. (Department: 1713)

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

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S247000, C526S252000, C526S253000, C526S255000, C526S286000, C526S287000

Reexamination Certificate

active

06767977

ABSTRACT:

The present invention relates to sulphonic fluorinated ionomers and membranes obtained therefrom, said membranes usable also at high temperatures, of the order of 100° C. -180° C., in electrochemical applications, for example in fuel cells.
Specifically, the invention relates to membranes of sulphonic fluorinated ionomers having even very thin thicknesses up to a limit lower than 5 &mgr;m and having a high hydration degree and good mechanical properties under the use conditions.
More specifically, the sulphonic fluorinated ionomers used for the membrane preparation have equivalent weight, higher than 700, preferably between 720 and 1,700. Said membranes show, the equivalent weight being equal, an improved hydration degree compared with those of the prior art, combined with good mechanical properties.
The sulphonic fluorinated ionomers of the invention are partially crystalline and have an equivalent weight (EW) as above.
It is known in the prior art the use of the class of polymers called with the term ionomers in electrochemical applications, such for example in fuel cells, chloro-soda cells, lithium batteries, electrodialysis and in reactors in which the ionomer acts as a solid catalyst. Said applications implie the ionomer contact with an aqueous or polar liquid having affinities with the ionic functional groups of the ionomer.
In electrochemical applications, for example in fuel cells, there is a direct correlation between the ionomer conductivity and the water retention of the ionomer. The polymer ionic conductivity, besides being increased by a higher presence of ionic groups in the polymer, results significantly increased also by a larger amount of water that the polymer can retain (hydration degree).
The ionomer/membrane for the industrial application 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 with an alkaline aqueous solution and then with an acid solution (see later on).
In the prior art, to obtain membranes with sufficient physical integrity, polymers having an equivalent weight of about 1,100 are usually used. An example of such membranes is represented by the commercial product NAFION®, used in the fuel cells. Said membranes, to have a good physical integrity, are typically obtained with an ionomer having equivalent weight of about 1,100. Such membranes show a not high conductivity. Besides, if said membranes are used under dehydration conditions, or with unsaturated feeding fluids to the cell, in particular at cell temperatures higher than 100° C., they tend to dehydrate and the membrane conductivity is drastically reduced. Consequently the NAFION® membranes are not effectively usable, in particular at temperatures higher than 100° C. and under dehydration conditions of the feeding fluids to the cell.
The sulphonic ionomers described in the prior art do not allow to obtain membranes with an optimal combination of good physical integrity and high hydration. In particular in the car industry the need is felt to have available ionomeric membranes having a very high conductivity. This is obtainable when the membrane shows high hydration and good mechanical properties so as to be able to manufacture the membrane in extremely thin thicknesses, for example from 5 to 80 &mgr;m. Furthermore membranes having a very high conductivity allow to generate the same electric power with a smaller membrane surface. This is extremely desired in the car industry since it allows to reduce the size and thus the stack weight and cost. Besides, very thin membranes better resist critical dehydration conditions, since the water generated at the cathode side can more easily migrate to the anode side. Furthermore the dehydration is much higher as the cell working temperature is higher, the humidification degree of the feeding fluids thereto being equal. A high cell temperature, for example higher than 100° C., is desirable since it allows a more effective heat exchange.
Besides, the fuel cells of the prior art use very pure hydrogen not to have poisoning of the platinum-based electrodes. Indeed if reforming hydrogen is used, thus containing CO, there is a rapid poisoning of the platinum. According to the prior art, therefore, the hydrogen from reforming must be purified from CO before being used in the fuel cells. This phenomenon is remarkably reduced when the cell works at temperatures from 110 to 120° C., and it is practically absent at working temperature of about 150° C.
Therefore it is desirable that the membrane shows improved hydration properties, can be used also at high temperature, for example higher than 100° C., and shows improved mechanical properties so as not to lose its physical integrity even in extremely thin thicknesses.
Tests carried out by the Applicant have shown that with the ionomers reported in the prior art, the membranes do not show said property optimal combination.
Said membranes must be available for wide range applications, such for example in the automotive field, and therefore they must be obtainable by a process which allows its production on a large scale by continuous processes having a high efficiency, reliability and reproducibility.
The need was therefore felt to have available sulphonic fluorinated ionomers with EW higher than 700, and up to 1,700, having improved hydration properties combined with high mechanical properties able to give also thin membranes having a thickness up to a lower limit of 5 &mgr;m, usable both at room temperature and at high temperature (as above defined), without substantially compromising the physical integrity of the membrane.
The Applicant has surprisingly and unexpectedly found sulphonic fluorinated ionomers capable to solve the above technical problem.
An object of the present invention are semicrystalline sulphonic fluorinated ionomers having an equivalent weight higher than 700 g/eq, up to 1,700, preferably 720-1,500, 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 such amount to give the above equivalent weight, deriving from F
2
C═CF—O—(CF
2
)
q
—SO
2
F, q being an integer equal to 2 or 3;
and having the following properties for a TFE/(B) copolymer wherein q=2:
hydration, (see hereafter) expressed in % of H
2
O at 100° C. absorbed by the film prepared with the ionomer and after transformation from the —SO
2
F form into the —SO
3
H form, having the following values:
at
  750 EW
higher than
55%,
pref.
>70%
at
  850 EW
″
45%,
″
>55%
at
1,000 EW
″
35%,
″
>40%
at
1,100 EW
″
28%,
″
>22%
at
1,200 EW
″
23%,
″
>25%
The invention ionomers, for a TFE/(B) copolymer wherein q=2, show film extrudability, under the —SO
2
F form, with thickness of 20 &mgr;m having good mechanical properties.
The invention ionomers, for a TFE/(B) copolymer wherein q=2, show furthermore a melt flow index comprised between 0.01 and 100 g/10′, with a load of 5 kg measured at 280° C., and after transformation from the —SO
2
F form into the —SO
3
H form, as hereafter indicated.
The mechanical properties of the invention ionomers, for a TFE/(B) copolymer wherein q=2, are high, in particular there are the following mechanical properties of the film at 23° C. at:
  750 EW
tensile stress (MPa)
 >4,
pref.
 >5
  850 EW
″
>10,
″
>15
1,000 EW
″
>20,
″
>25
1,100 EW
″
>20,
″
>28
1,200 EW
″
>20,
″
>30
The fluorinated monomers of type (A) are selected from:
vinylidene fluoride (VDF);
C
2
-C
8
perfluoroolefins, preferably tetrafluoroethylene (TFE);
chlorotrifluoroethylene (CTFE), optionally in the presence of ethylene,
(per)fluoroalkylvinylethers (PAVE) CF
2
═CFOR
f
, wherein R
f
is a C
1
-C
6
(per)fluor

Affiliated with

Also associated with

Rating

Fluorinated ionomers does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Fluorinated ionomers, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Fluorinated ionomers will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-3208544