Liquid purification or separation – Filter – Material
Reexamination Certificate
2000-05-24
2004-04-27
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500270, C210S500340, C264S041000
Reexamination Certificate
active
06726840
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to porous membranes of amorphous fluoropolymers for separation processes.
Specifically, the invention relates to membranes having improved porosity, in the range 5-500 nm, derived from amorphous (per)fluorinated polymeric films having a flat or cylindrical shape, and usable in separation processes such as ultrafiltration and nanofiltration, in particular of non-aqueous solutions of organic solvents containing a solute, for example a dissolved polymer. Another use is in the contactor field wherein an aqueous liquid phase containing a dissolved gas, for example O
2
or CO
2
, is contacted with the membrane, through which the gas permeation takes place allowing the control of the gas concentration in the liquid phase.
BACKGROUND OF THE INVENTION
The preparation of films and dense non-porous membranes of amorphous fluoropolymers obtained by dissolving an amorphous fluoropolymer in a solvent, filtering the solution and then using the spin coating method to coat glass substrata having a low thickness, is known in the prior art. This procedure is described in European Patent 416,528, wherein as fluoropolymer an amorphous perfluorinated copolymer formed of perfluoro-2,2-dimethyl-1,3-dioxole (PDD) in an amount between 30% and 99% by moles and of tetrafluoroethylene (TFE), is used. The obtained films have a substantially uniform thickness in the range 0.3-20 &mgr;m.
Dense membranes of this type have a chemical-structural porosity, pore size of about some Angstrom (about 10). See W. J. Davies and R. A. Pethrick, Eur. Polym. J. 30 (1994) 1289. These membranes can be used for the separation of a gas from a liquid. The productivity is rather good as regards the gas passing through the membrane. However, for the membranes to be used in ultrafiltration processes or for contactors, an even improved productivity is desirable.
U.S. Pat. No. 4,948,851 describes articles obtained by the same amorphous perfluorinated copolymers of EP '528, specifically self-supported films having a thickness of about 100-400 &mgr;m, prepared by molding of the melted amorphous polymer. With these copolymers it is possible to prepare self-supported films by casting having a thickness of the order of 20-25 &mgr;m. The obtained films are dense and do not show porosity as above said, therefore they have the drawbacks mentioned in EP '528.
U.S. Pat. No. 5,051,114 describes films obtained by deposition from casting having a thickness of the order of 25 &mgr;m. Such films can be used supported on porous substrata to form composite membranes useful for enriching and/or separating gaseous mixtures. In this patent, selectivity values for the couple O
2
/N
2
from 2.0 to 2.4 are reported, depending on the dioxole PDD content in the PDD/TFE copolymer. It is shown that by increasing the dioxole content, the oxygen permeability increases but the selectivity decreases. Further data relating to the selectivity to gases are given in Chapter 22 of “Modem Fluoropolymers” (1997) by P. R. Resnick and W. H. Buck, wherein in the case of membranes of copolymers PDD/TFE in molar ratio 87:13 obtained from casting, a value of 5.0 for the couple CO
2
/N
2
and 4.4 for the couple H
2
/N
2
is mentioned. The membranes described in this patent, even though they have a good gas-permeability, have no porosity and therefore they have the drawbacks mentioned in EP '528.
In European Patent Publication 969,025 in the name of the Applicant the preparation of non-porous manufactured articles of amorphous fluoropolymers formed of a copolymer of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxoles (TTD) with tetrafluorotilene, is described. The obtained membranes are dense and asymmetric and have selectivity towards gases, optionally used on supports suitable for obtaining composite membranes. In order to obtain said membranes under the form of flat dense films the solution is deposited on a smooth flat support with a stratifying knife and the solvent is eliminated by thermal treatment at high temperature, as mentioned in said patent. Tests carried out by the Applicant have shown that in order to obtain said selective membranes, thermal treatments at a temperature close to the boiling point of the used solvent and a subsequent treatment at a temperature near the amorphous polymer Tg, are used. In order to achieve the asymmetric membranes the phase inversion technique is used. According to this technique, after the solution deposition the support coated by the polymer is dipped into a coagulation bath maintained at a predefined temperature and formed of a non-solvent, preferably n-pentane. Such membranes show an improved selectivity in the gaseous separation, but since they have the layer effective to permeation with porosity of chemical-structural type (some Angstrom), they are not very suitable to be used as contactor membranes, for example for the control of the gas concentration in a liquid. Therefore, these membranes also show the drawbacks mentioned above.
Polytetrafluoroethylene (PTFE) porous films (of the order of thousands of Angstrom) are commercially known, obtained by a complex lubricated extrusion process at a high temperature, wherein the film porosity is induced by a complex stretching mechanical treatment of the extruded film. Films of this kind are known for example by GORETEX®. The membranes obtained from these products are used for ultrafiltration processes. The drawback of these membranes is that they are obtained with an expensive and complex process from the processing point of view.
The need was therefore felt to have available films of (per)fluorinated amorphous polymers characterized by an improved porosity and obtainable by a cheap industrial process, without using the complex thermomechanical treatments used for the porous films based on foam PTFE available on the market.
SUMMARY OF THE INVENTION
The present invention provides porous membranes of (per)fluorinated amorphous polymers having a porosity in the range of 5-500 nm, preferably 20-100 nm, measured by an atomic force electronic microscope. With respect to the membrane pore average size, about 80%-90% of the pores have a size ranging from minimum 5 nm to plus 5 nm of the value of the distribution peak. The porous membranes are suitable for use in separation processes such as ultrafiltration or nanofiltration as well as in the contactor field.
DETAILED DESCRIPTION OF THE INVENTION
The Applicant has surprisingly found that it is possible to obtain films and porous membranes of (per)fluorinated amorphous polymers suitable for ultrafiltration, nanofiltration processes and in the contactor field, not showing the prior art drawbacks.
An object of the invention is porous membranes of (per)fluorinated amorphous polymers having a porosity in the range 5-500 nm, preferably 20-100 nm, determined by an atomic force electronic microscope (FEM).
The membrane pore average size distribution of the (per)fluorinated amorphous polymers of the invention is very narrow, about 80%-90% of the pores have a size ranging from minus 5 nm to plus 5 nm of the value of the distribution maximum peak. Generally, the pore distributions are monodispersed for 80-90% close to the maximum peak value. For example, for a membrane having the distribution maximum peak close to about 60 nm, the pore sizes are for about 80-90% in the range 55-65 nm.
The pore number for mm
2
in correspondence of the distribution maximum peak ranges for example from about 20 to 150.
The porous membranes of (per)fluorinated amorphous polymers of the present invention are obtainable from copolymers of the following monomers:
CF
2
═CY
1
Y
2
(II)
wherein: Z is selected from F, R
f
, OR
f
, preferably OR
f
; R
f
is a perfluoroalkyl radical C
1
-C
5
; X
1
and X
2
are selected from F and CF
3
; Y
1
and Y
2
are selected from F, Cl, CF
3
, OR
f
, preferably F.
Also homopolymers of the monomers of structure (I) are usable.
Other amorphous (per)fluorinated polymers which can be used are those obtained from the cyclopolymerization of monomers having structure (
Arcella Vincenzo
Drioli Enrico
Gordano Amalia
Maccone Patrizia
Arent Fox Kintner Plotkin & Kahn
Ausimont S.p.A.
Fortuna Ana
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