Preparation of sulphonic fluorinated polymer solutions

Details Preparation of sulphonic fluorinated polymer solutions Preparation of sulphonic fluorinated polymer solutions

1999-03-31

2001-03-06

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, C526S248000, C526S249000, C526S253000

Reexamination Certificate

active

06197903

ABSTRACT:

The present invention relates to a process for preparing fluorinated polymer solutions at low temperature having sulphonic functional groups and to liquid compositions obtainable from said process.
Specifically, the invention relates to solutions obtainable with a process in which a temperature between the room temperature and about 150° C., preferably between 50° C. and 100° C., is used. The liquid compositions obtainable from said process are stable solutions and/or dispersions wherein the particle sizes, determined by dynamic laser light scattering (DLLS), are in the range 30-100 nm, preferably 40-70 nm, the distribution being monomodal The invention liquid compositions contain an amount of fluorinated polymers having sulphonic functional groups (ionomers) ranging from about 0.5% and about 30% by weight. The equivalent weight of said fluorinated polymer is in the range of about 400-1300, preferably between 650-800 nm, still more preferably between 730-795 nm.
The preparation of solutions and/or dispersions of sulphonic (per)fluorinated ionomers having different equivalent weight (EW) is known in the art.
As equivalent weight, the polymer weight in acid form necessary to neutralize a NaOH equivalent is meant; the equivalent weight is determined by polymer titration in the acid form with a standard NaOH solution.
GB 1,286,859 describes ionomer dispersions having equivalent weight not higher than 1060 using an aqueous solution containing at least 5% by weight of an organic solvent. As organic solvents primary, secondary and tertiary C
1
-C
4
alcohols; amides such as dimethylacetamide, dimethylformamide; acetone, etc., are mentioned. The used organic solvent feature is to have a boiling point lower than 130° C. such that they are easily removable after polymer casting and a solubility in water of at least 5% by weight. The temperatures used in the examples for the dissolution process are comprised between the room temperature and 100° C.
Tests carried out by the Applicant have shown that by the process described in said patent, the dispersed particle sizes are in the range of about 150-300 nm. As it is well known, as the particle size increases, the polymer concentration being equal, the specific surface decreases. Therefore sizes of this order limit the sulphonic group availability on the particle surface and therefore reduce the ionomer efficiency in all those applications where an high presence of such groups is required, as in the case of impregnation treatments on porous supports, for example of PTFE or ceramic materials.
Dissolution processes with hydroalcoholic mixtures at temperatures higher than at least 180° C. and up to 350° C. for ionomers having equivalent weights higher than about 1000, are also known.
See for example U.S. Pat. No. 4,433,082 which describes a dissolution process in a liquid medium of ionic exchange (per)fluorinated polymers having —SO
3
M functional groups, wherein M is H, Na, K having an equivalent weight in the range 1025-1500. Water, optionally containing alcohols, is used. A mixture constituted by 20-90% by weight of water and by 10-80% by weight of an alcohol (C
1
-C
4
) and/or other solvents miscible in water in a dissolution temperature range from 180 to 300° C., preferably from 210° C. to 250° C., is for example mentioned. Such dissolution is carried out for at least 0.5 h in a closed vessel obtaining two liquid phases having a different density, wherefore it is necessary to separate the phase having a lower density from the one containing the polymer.
The disadvantage of this process is to operate at high temperatures leading to corrosion of the used containers (autoclaves). For this reason the used materials are special steels, e.g. Hastelloy. Also in this case it is however necessary a surface treatment since the corrosion takes place all the same even though in longer times.
In the publication by R. B. Moore, III, C. R. Martin, Macromolecules 22, 3594-3599 of 1989 the dissolution conditions for sulphonic perfluorinated ionomers having an equivalent weight in the range of about 600-1300 are reported. Also in this case the polymer is dissolved in hydroalcoholic mixtures and it is pointed out that the temperature necessary to dissolve the sulphonic perfluorinated ionomers changes with the used ionomer equivalent weight. It is meaningful to note that also for the ionomers having equivalent weight lower than 1000 the dissolution method requires the use of high operating temperatures, always higher than 240° C., and of dissolution times higher than one hour.
Patent Application WO 98/16581 relates to a dissolution process at high temperature (150-350° C.) to obtain solutions and/or dispersions comprising sulphonic perfluorinated ionomers. The dispersion medium is substantially water, free of alcohols. An example is reported wherein the dissolution of a sulphonic copolymer TFE/perfluoro(3-oxa-4-pentenesulphonylfluoride) in —SO
3
H form having an equivalent weight lower than 800 at a temperature of 230° C. for 5 hours under stirring, is shown. Besides, a dissolution method in a non aqueous medium constituted by alcohols and/or organic solvents miscible in water is described. Also in this case it is necessary to operate at temperatures in the range 150° C.-350° C. According to this patent at least 25% of the polymer particles have a size in the range of about 2-30 nm, determined by low angle X rays measurements.
Experiments carried out by the Applicant (see the Examples) have shown that the distribution is bimodal, wherein at least 50% of the particle distribution is about 120 nm. Therefore also in this case the same disadvantages described for GB 1,286,859 are present.
In the mentioned patents, the high temperature required to obtain the sulphonic (per)fluorinated ionomer dissolution in a dispersion medium involves the use of autoclaves constituted by special corrosion-resistant materials, as described in patent WO 98/16581, thus determining an increase of the plant costs. When not valuable steels are used, corrosion problems and consequent pollution of the final solution from the metals constituting the autoclave take place, making it not very suitable to be used in all the applications where an high purity, for instance in electrolytic and fuel cells, is required.
The need was felt to have available solutions and/or dispersions of ionomers obtainable by a process which does not require high temperatures, preferably without the use of equipments under pressure (autoclaves) and contemporaneously allows to obtain the dispersed polymer particle sizes in the range 30-100 nm, preferably 40-70 nm. As said, the particle size reduction involves an increase of the respective specific surface, the dispersed polymer concentration being equal, with consequent result improvement in impregnation, casting, etc. applications.
The Applicant has surprisingly and unexpectedly found that it is possible to solubilize sulphonic (per)fluorinated ionomers in suitable solvent mixtures at low temperature contemporaneously obtaining particles having sizes in the above indicated range, overcoming the above mentioned disadvantages of the prior art.
An object of the present invention is a process to prepare a solution and/or dispersion containing sulphonic (per)fluorinated ionomers having —SO
3
M functional groups, wherein M is selected from H, Li, Na, K, NR
4
(R equal to or different from each other are selected from H, CH
3
, C
2
H
5
), preferably H, characterized in that it dissolves the ionomers above defined at a temperature comprised between the room temperature and about 150° C., preferably between 50° C. and 100° C., in a monophasic ternary mixture essentially constituted by water in a percentage in the range 0.1%-50% by weight, by a C
1
-C
4
alcohol in a percentage in the range 50%-99% by weight and by a fluoro(poly)oxyalkylene having one hydrogen atom in at least a fluorinated end group, preferably in both terminals, in a percentage in the range 0.1%-40% by weight; wherein the dispersed polymer particle sizes, determined by dynamic laser light scattering (DLS

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