ASYMMETRIC MEMBRANES OF POLYTETRAFLUOROETHYLENE

AND THEIR PREPARATION

FIELD OF THE INVENTION This invention relates to asymmetric membranes of polytetrafluoroethylene (PTFE) and their preparation.

TECHNICAL BACKGROUND There is much literature available concerning the production and uses of synthetic membranes. See, for example, Resting, R. E., Synthetic Polymeric Membranes Structural Perspective, 2d Ed., John Wiley and Sons, Inc., New York, 1985 and Porter, M. C, Ed. Handbook of Industrial Membrane Technology, Noyes Publications, 1990, Chapter 1, "Membranes and Their Preparation", (chapter by Heiner Strathmann) , 1990. In order to be of practical utility for uses such as ultrafiltration and separation, a membrane must have an asymmetric structure to provide both the desired separation and acceptable flux. Such asymmetric membranes possess one surface, the barrier layer, which is either non-porous (solution- diffusion membranes) or has extremely fine pores (ultrafiltration membranes) while the opposing surface and much of the interior is less dense and exhibits a higher degree of porosity. Various synthetic materials have been used to produce asymmetric membranes. While

PTFE has been used in single layer microporous films for filtration and for waterproof linings (in apparel) , it has proven difficult to use PTFE in creating asymmetric membranes due to the difficulty of finding suitable solvents to dissolve the PTFE or, in the absence of a suitable solvent, of melt processing a suitably thin selective layer of PTFE on a less selective porous support .

A method for producing asymmetric membranes from polymers that are soluble is to cast them from a polymer

solution onto a substrate by some method which causes the polymer to precipitate or remain on the substrate in the form of a film. See Handbook of Industrial Membrane Technology, Mark C. Porter, Ed., Noyes, 1990, Chapter 1; U. S. Patent 4,374,891 and U.S. 4,784,880 (Reexam No. 34,115), both of which are incorporated herein by reference . Membranes can be produced via methods that cause the polymeric material to deposit in an asymmetric structure. Such methods include: (a) thermogelation;

(b) evaporation of a volatile solvent from the solution; or

(c) addition of a non-solvent or a solvent- nonsolvent mixture. Commercially available PTFE resins are of ultra- high molecular weight. Until recently, it has been difficult or impossible to fabricate asymmetric PTFE membranes because of the poor solubility of PTFE resins in known solvents . Applicants have discovered, however, that dilute PTFE solutions can be formed at high temperatures in certain perfluorocycloalkanes, and that such solutions can be cast onto a substrate to produce asymmetric membranes by the methods referred to above . WO 8805687 discloses microporous asymmetric composite fluoropolymer membranes made by laminating 2 calendered fluoropolymer sheets and sintering the laminate .

JP 59068344 creates an asymmetric membrane by coating porous PTFE with an amphoteric fluorosurfactant . U.S. 4,889,626 discloses an asymmetric tubular membrane made using a PTFE alloy.

U.S. 4,248,924 discloses a porous asymmetric PTFE film wherein the asymmetry is produced by compression and a temperature gradient .

U.S. 4,863,604 discloses a composite asymmetric film made up of two or more sheets of microporous fluorocarbon polymer films .

U.S. 4,619,897 discloses a fluorine resin membrane which has been treated with a fluorinated surface active agent .

R. West et al., Proc.-Inst. Environ., Sci., Performance of New Dual Asymmetric PTFE Membrane 37th (1991) . The filter is characterized by having three layers, a thin inner layer of fine pores with layers above and below it having relatively larger pores.

F. Vego et al . , Journal of Applied Polymer Science, Vol 21, 3269-3290 (1977) disclose the preparation of asymmetric PTFE membranes using sintering of PTFE emulsions in the presence of salts. The porous supports formed were then deposited on a dense file of PTFE.

S. Munari et al., ^* Journal of Applied Polymer Science, Vol 20, 243-253 (1976) disclose hyperfiltration membranes prepared by radiochemical grafting of styrene onto PTFE.

P. Tancrede et al., Journal of Biochemical and Biophysical Methods, 7 (1983) 299-310, disclose the formation of asymmetrical planar lipid bilayer membranes from characterized monolayers . Microporous PTFE films and sheeting are known. U.S. Patent 3,664,915 discloses uniaxially stretched film having at least 40% voids and a highly fibrillar structure. U.S. Patents 3,953,566, 3,962,153 and 4,187,390 disclose porous PTFE films having at least 70% voids, said films consisting of nodes and fibrils wherein the nodes are at least 1000 times thicker than the fibrils.

SUMMARY OF THE INVENTION

The invention comprises asymmetric membranes of PTFE characterized by one surface being substantially

free of pores or having pores no greater than about 0.1 microns in diameter, and by a second surface having numerous pores greater than or equal to about 0.1 micron, preferably greater than 1 micron, in diameter.

The invention further discloses a process for making asymmetric membranes comprising:

(a) contacting a substrate with a solution of PTFE having molecular weight of at least 1 million, in a perfluorinated cycloalkane solvent having a critical temperature of at least 340°C, said solution being at a temperature about 300°C to about 360°C, thereby coating a film of said solution to be coated onto said substrate;

(b) removing the coated substrate from the hot solution;

(c) removing the solvent from said coated substrate and allowing the PTFE to cool and coalesce on the substrate so that the one surface is less porous and the opposite surface is more porous; and, optionally,

(d) separating the coalesced film from the substrate.

The PTFE solution is preferably heated to about 340°C. In step (c) the solvent can be removed by evaporation, or by addition of a second "solvent" (called hereinafter "non-solvent") that selectively dissolves the solvent and can be washed away, but will not dissolve the PTFE. The non-solvent may also be introduced as a solvent/non-solvent mixture. The non- solvent may also be added in step (a) .

DETAILS OF THE INVENTION In the present process, as-polymerized PTFE and melt-recrystallized PTFE are equally suitable starting materials. These may be in the form of particles or shaped articles such as film, sheet, fiber, rod or billet. To make the solution, the PTFE is contacted with an excess of solvent which dissolves the PTFE when the mixture is heated to at least about 300°C to about 360°C, preferably about 340°C, at ambient atmospheric pressure. The starting PTFE may be immersed in unheated solvent and then heated to the operating temperature, or immersed in the solvent which has been previously heated to the operating temperature .

Suitable solvents for PTFE herein are perfluorinated cycloalkanes, as described in co-pending Serial No. 07/936,449, which is incorporated herein by reference in its entirety. By perfluorinated cyclo¬ alkanes are meant saturated cyclic compounds, which may contain fused or unfused rings. In addition, the perfluorinated cycloalkane may be substituted by perfluoroalkyl and perfluoroalkylene groups. By perfluoroalkyl group is meant a saturated branched or linear carbon chain. By perfluoroalkylene group is meant an alkylene group which is branched or linear and is bound to two different carbon atoms in carbocyclic rings. -The total number of carbon atoms in all of the perfluoroalkyl and perfluoroalkylene groups in a molecule of the solvent must be less than the total number of carbon atoms in the carbocyclic rings of a solvent molecule. It is preferred if there are at least twice as many carbon atoms in the rings of the solvent molecule as there are atoms in the perfluoroalkyl and perfluoroalkylene groups.

In order to insure that the solvent will actually dissolve the polymer (PTFE) , the critical temperature of

the solvent should be 340°C or higher, preferably about 360°C or higher. Critical temperatures of many compounds can be found in standard references, and may be measured by methods known to those skilled in the art. The process of dissolving the polymer is carried out under autogenous pressure at the temperature required to dissolve the polymer or at atmospheric pressure, whichever is greater. By autogenous pressure is meant the sum of the vapor pressures of the constituents of the process at the process temperature. Stirring or other forms of agitation will increase the rate of dissolution of the polymer.

Compounds useful as solvents herein include, but are not limited to, perfluoro (tetradecahydrophen- anthrene) , perfluorodimer and perfluoro [ (cyclohexyl- methyl) decalin] . Preferred solvents are perfluoro- (tetradecahydrophenanthrene) , and perfluro[ (cyclohexyl- methyl)decalin] . Another preferred solvent is perfluorodimer. By "perfluorodimer" herein is meant a byproduct from the fluorination of phenanthrene using a combination of C0F ₃ and fluorine, as described in British Patent 1,281,822, which is incorporated herein by reference.

When phenanthrene is thus fluorinated to perfluoro- tetradecahydrophenanthrene, a higher boiling fraction is obtained upon fractional distillation of the crude liquid product. This fraction has a boiling range of 280°C to about 400°C at atmospheric pressure, typically about 320-340°C. It has a small amount of olefin and a very small amount of hydrogen in it, both of which can be further reduced by postfluorination. It is believed that most of this mixture consists of the general structure

wherein z is 0, 1 or 2. Also believed to be present in smaller quantities are compounds from ring fusion and/or ring opening of the above compounds or their precursors such as

from the compound where z is 0 (it is not possible to say with assurance that this particular isomer is in the mixture - it is merely illustrative of one possible structure consistent with the analytical data and the synthetic method) . Similar fused structures from the compounds where z _. is 1 or 2 are also believed to be present. Although traces of hydrogen are present, the location has not been determined. The term "perfluorodimer" is used throughout this Application to describe this material . The PTFE used in the present process is any type of fully polymerized, high molecular weight, crystalline or partly crystalline compound. The PTFE may be in any form; for example granular, fine powder, or fabricated into shaped articles. By "fine powder" is meant a

coagulated and dried PTFE product of emulsion or dispersion polymerization. By "granular" is meant a product of suspension polymerization which may optionally be milled. By "PTFE" is meant polytetra- fluoroethylene homopolymer and copolymers of polytetra- fluoroethylene, which may contain minor amounts of repeat units of other monomers .

Substrates may be coated by contacting, as by dipping, the substrate into a solution of the polymer, removing the excess solution, if any, by allowing it to flow off and then removing the solvent, as by heating (e.g., in a drying oven) or using a non-solvent.

The choice of solvent (s) and any additives used may influence the final membrane structure, as does the rate of cooling, evaporation or any non-solvent addition to the solution e.g., for removal of excess solvent. The various additives and conditions for cooling and evaporation would be known to those skilled in the membrane art. Thus, one will usually use a starting solution whose composition or temperature is near the point of phase separation (into either two liquid phases or a liquid and a solid phase) , with additives being chosen to adjust both the starting thermodynamic state and/or the rates of membrane formation. For most membrane polymers, having membranes produced by coating a dissolved material on a solid substrate, lower solution concentrations lead to structures with "fingered" pores which may be unacceptable for certain uses. Higher concentrations (e.g. 15-30%) usually lead to "spongy" structures. For PTFE, it is preferred if the solution concentration is about 2-3%, which gives acceptable membrane morphology. For membranes cast as films on liquids, as disclosed in U.S. 4,374,891, low concentrations can yield acceptable membrane layers.

Separation of the membrane from the substrate can be done by peeling. Care must be exercised to avoid fissures caused by the peeling process .

Substrates that may be used in the process include, but are not limited to, aluminum, glass or stainless steel plates.

The invention is further illustrated with reference to the following examples in which temperatures are expressed in degrees Celsius and percentages are by weight unless otherwise indicated.

EXAMPLE 1 60 g of Teflon® MP1500, available from E. I. du Pont de Nemours and Company, Wilmington, Delaware 19880, was dissolved in 2940 g of perfluorodimer. The addition of the Teflon® to the oligomer was carried out in a glass resin kettle. The perfluorodimer was first added to the kettle. The Teflon® was added incrementally over a period of 5 days while the kettle contents were maintained at temperatures ranging from 310 to 330°C. The material was maintained at temperature for an additional 3 days with occasional scraping of the kettle sides and stirrer. Agitation was maintained throughout the 8 days except while scraping. This resulted in a uniform appearing solution of 2% MP1500.

The solution was then cooled to ambient temperature and later reheated for transfer to a heated box made of high nickel alloy metal, the interior dimensions of said box being 7" x 1" x 12"h. The solution was heated to 320°C and held without agitation for 1.5 hours. After that period, a polished aluminum plate which had been pre-heated in an oven to 250°C was removed from the oven and quickly immersed in the 320°C solution, and held for several minutes (ca. 5) . Upon removal of the plate, it was found to be

heavily coated with a thick and uneven coating. The plate was allowed to cool in the air for several minutes, and then immersed in F113 for a couple of hours, after which it was removed and air-dried. An uneven coating was observed on the plate, a large portion of which was very bright white. A portion of the white coating was removed by scraping with a new single-edge razor blade. The coating peeled off quite readily with no sense of being pulled or stretched. The film so produced ranged in thickness from ca 100 μm to ca. 400 μm.

Scanning electron micrography revealed that the opposing surfaces of the film were quite different in morphology, one surface being smooth and almost featureless, containing a few relatively large holes as defects. The opposite surface consisted of extensive areas containing numerous holes ranging in size from a couple of microns to submicron size.

EXAMPLE 2 A second specimen from the same plate revealed a surface which resembled a foam structure, containing a large number of pores less than 0.1 μm in diameter.

EXAMPLE 3

In a second experiment, a similarly pre-heated polished stainless steel plate was immersed in the solution used in Example 1 at 335°C for five minutes. Upon removal, the coating appeared to be much more uniform than it had been on the aluminum plate. This plate, too, was air cooled, then immersed in Freon®-113 (B.P. 45.8°C) for a couple of hours.

The resultant plate also showed extensive areas of white coating, but not as extensive as in the case of the aluminum plate. The film was considerably more difficult to peel off. SEM revealed a somewhat porous surface containing large, gaping holes, submicron

fissures, and a great deal of fibrillation. There appeared to be some evidence of porosity but it was less pronounced and less well-defined than in the aluminum plate case.