| PATENT ASSIGNEE'S COUNTRY | USA |
| UPDATE | 08.99 |
| PATENT NUMBER | This data is not available for free |
| PATENT GRANT DATE | 17.08.99 |
| PATENT TITLE |
Porous polytetrafluoro-ethylene and preparation |
| PATENT ABSTRACT |
This invention relates to porous polyfluoroethylene (PTFE), shaped articles prepared therefrom, and to methods of preparing said articles. |
| PATENT INVENTORS | This data is not available for free |
| PATENT ASSIGNEE | This data is not available for free |
| PATENT FILE DATE | 23.07.98 |
| PATENT REFERENCES CITED | This data is not available for free |
| PATENT PARENT CASE TEXT | This data is not available for free |
| PATENT CLAIMS |
What is claimed is: 1. A laminated structure comprising (i) a porous shrink-resistant polytetrafluoro-ethylene (PTFE) shaped article having a mean pore size of less than about 10 .mu.m, a porosity of at least 40%, a DSC melting endotherm in the range of 315 to 333.degree. C. with an associated heat of fusion of at least 35 J/g, and having no DSC melting endotherm at temperatures above 370.degree. C., and (ii) a second, different shaped article bonded to (i). 2. The laminated structure according to claim 1 wherein a bonding agent bonds article (i) to second article (ii), and said bonding agent is present in the pore network of portions of article (i) that are adjacent to said second shaped article (ii). 3. A filtration medium comprising a porous, shrink-resistant polytetrafluoro-ethylene (PTFE) shaped article having a mean pore size of less than about 10 .mu.m, a porosity of at least 40%, a DSC melting endotherm in the range of 315 to 333.degree. C. with an associated heat of fusion of at least 35 J/g, and having no DSC melting endotherm at temperatures above 370.degree. C. 4. A gas/liquid separatory medium comprising a porous, shrink-resistant polvtetrafluoro-ethylene (PTFE) shaped article having a mean pore size of less than about 10 .mu.m, a porosity of at least 40%, a DSC melting endotherm in the range of 315 to 333.degree. C., with an associated heat of fusion of at least 35 J/g, and having no DSC melting endotherm at temperatures above 370.degree. C. 5. The separatory medium according to claim 4 wherein the gas is air or water vapor and the liquid is water. 6. An article of protective clothing comprising the separatory medium according to claim 5. 7. The article according to claim 6 wherein the protective clothing is selected from the group consisting of space-suits, cleanroom suits, waterproof rainwear, gloves, footwear, socks, undergarments and medical garments. 8. An article of outdoor equipment comprising the separatory medium according to claim 5. 9. The article according to claim 8 wherein the outdoor equipment is a tent or sleeping bag. -------------------------------------------------------------------------------- |
| PATENT DESCRIPTION |
FIELD OF THE INVENTION This invention relates to porous polytetrafluoroethylene (PTFE) compositions, shaped articles made therefrom, and their preparation. TECHNICAL BACKGROUND U.S. Pat. Nos. 4,360,488 and 4,385,026 disclose formation of "non-draining" gels by heating PTFE with a highly fluorinated high-boiling material at a temperature close to the crystalline melting point of the polymer (330-350.degree. C.). A solution or swollen mass containing from about 1 to about 50 weight % polymer is formed on heating, from which is recovered, on cooling, a sponge-like gel, said gel being without defined shape and retaining no "memory" of the crystallinity of the original PTFE. The gel, after removal of the fluorinated material by extraction in refluxing solvent such as Freon.RTM.-113 (bp 45.8.degree. C.), is described as porous, and could be formed into porous shapes, e.g., into porous sheet by pressing between platens. The process appears to employ granular PTFE only and, because crystalline memory is lost during processing, the initial gels are shapeless globs which require post fabrication into shaped articles. The porous products have increased crystallinity and a partially fibrillar structure. Use as filter membranes or diaphragms for electrochemical cells is disclosed. Microporous PTFE films and sheeting are known. U.S. Pat. No. 3,664,915 discloses uniaxially stretched film having at least 40% voids and a highly fibrillar structure. U.S. Pat. Nos. 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. Pore size in the above films is at least 0.2 .mu.m. Unsintered, paste-extruded PTFE film is stretched at rates of over 2000%/sec and as high as 40,000%/sec to achieve porosity, followed by sintering nder constraint at 327 to 370.degree. C. Such stretching rates are far higher than those employed in conventional film preparation. U.S. Pat. No. 4,110,392 discloses microporous PTFE films having pore sizes as low as 0.01 .mu.m, achieved by stretching unsintered PTFE as above, followed by sintering without constraint, then stretching a second time at high speed. Porosities of these films are 10 to 50%. Japanese Application 3-221541 discloses microporous PTFE film with pore size of 0.1 to 0.2 .mu.m. The films are prepared by a modification of the above-described art procedures and have different film morphology and improved air permeability. However, ultrahigh stretching rates are again required. The art does not provide the means of improving porous PTFE articles by stretching at conventional rates. U.S. Pat. No. 5,110,527 discloses porous PTFE films exhibiting pore sizes of at least 15 .mu.m and voids of 80% and greater fabricated by a method in which a blend of PTFE resins of high and low molecular weight is paste-extruded into a sheet, stretched at temperatures below the melting point, and finally sintered while held in the stretched state by exposure to temperatures of at least 327.degree. C. SUMMARY OF THE INVENTION The present invention provides for porous products made from high molecular weight PTFE, and the processes for the production thereof. The present invention provides a novel process (I) for introducing porosity into PTFE, said process comprising (a) contacting PTFE with a fluid which penetrates and swells, but does not significantly dissolve the polymer or eliminate viscoelastic memory therefrom, at a temperature in the range of about 250-400.degree. C.; (b) cooling and separating the penetrated, swollen polymer from unabsorbed fluid, said polymer containing up to about 80% by weight of absorbed fluid, preferably up to 50% by weight of absorbed fluid; and (c) removing the absorbed fluid, to form a porous product having a single DSC melting endotherm, said endotherm being in the range of about 315.degree. C. to 333.degree. C. with an associated heat of fusion of at least 35 J/g. In step (a), temperatures within the range of about 250.degree. C. to 400.degree. C., should be sufficiently high for the selected fluid to extensively penetrate and swell the PTFE under process conditions, but low enough to avoid significant dissolution of the polymer or loss of viscoelastic memory therefrom. Preferably, the temperature is at or near the melting point of the PTFE under process conditions. Usually, this temperature is in the range of about 290.degree. C. to 360.degree. C.--toward the lower end for sintered or recrystallized PTFE, and toward the higher end for virgin or unsintered PTFE. Some of the porous products have a fibrillar structure. For certain embodiments, preferred fluids are liquids at 25.degree. C. and 1 bar. Halogenated organic liquids containing fluorine and/or chlorine and, optionally, also intra-chain ether oxygen, are most preferred. The invention also provides a variation of the above process (Process II) wherein a porous, shrink-resistant PTFE shaped article is prepared by subjecting the porous product from step (b) or step (c) to low-rate uniaxial or biaxial stretching, or wherein step (c) and stretching are performed simultaneously. Suitable fluids for use in Process II include halogenated organic fluids which may also contain intra-chain ether oxygen, and non-halogenated aromatic hydrocarbons, optionally also containing one or more substituents that are inert under process conditions. Halogen is preferably fluorine and/or chlorine. The rate, extent, and temperature of stretching in process II of the invention are much lower, more easily controlled, less energy-intensive, and less demanding on machinery and on the PTFE itself than stretching methods of the art. Thus, the present process is an improvement over the prior art. Moreover, the final, oriented shaped articles of this invention resist shrinkage in many wetting fluids, including those selected and exemplified in Example 67, hereinbelow. Shrink resistance is believed to result from lower orientation imposed by the relatively mild conditions of stretching in the present process compared to the prior art, and also to the unusually high degree of crystallinity of said oriented articles. The porous, shrink-resistant PTFE shaped article prepared by process II has a mean pore size of less than about 10 .mu.m, a porosity of at least about 40%, and a DSC melting endotherm in the range of about 315 to 333.degree. C. with an associated heat of fusion of at least 35 J/g, and has no DSC melting endotherm at temperatures above 370.degree. C. Preferably, the mean pore size of the shaped article prepared by process II, is less than 2 .mu.m, more preferably less than 1 .mu.m. Preferred porosity is at least about 50%, more preferably at least 60%. The heat of fusion associated with the 315.degree. C. to 333.degree. C. melting endotherm is preferably at least 40 J/g and accounts for at least 80% of the total heat of fusion of the article. Also included in this invention are: (1) non-draining compositions consisting essentially of PTFE and, absorbed therein, up to about 80% by weight of a fluid penetrant; (2) Unstretched, porous PTFE containing up to about 81% voids, preferably about 50% voids; (3) shaped articles of (1) and (2); (4) extruded articles such as wire, cable, fiber or tubing coated with the compositions (1) or (2), and a process for their preparation; (5) composites comprising PTFE and up to about 50% by weight of one or more polymers, and a process for preparing said composites. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a scanning electron micrograph showing the interior of a sample comprising a mass of interconnecting fibers as produced by Example 1. FIG. 2 is a scanning electron micrograph showing the product of Example 23. FIG. 3 is a scanning electron micrograph showing the product of Example 26. FIG. 4 is a graph of the maximum pore size of the film plotted against the heat of fusion. FIG. 5 is a graph of the heat of fusion of the film plotted against the reactor pressure. FIG. 6 (a-e) show differences in shrinkage between the files of the present invention and those of the art. FIG. 6a shows the film of Comparative Example 1 after exposure to isopropyl alcohol for 88 hours at room temperature. FIG. 6b shows a film of Example 65 after exposure to isopropyl alcohol for 88 hours at room temperature. FIG. 6c shows a film of Example 33 after exposure to isopropyl alcohol for 88 hours at room temperature. FIG. 6d shows a film of Example 55 after exposure to isopropyl alcohol for 88 hours at room temperature. FIG. 6e shows a cardboard disk of 2" diameter which represents the template from which samples 6a-d were prepared prior to exposure to the alcohol. DETAILS OF THE INVENTION In the process (I) of the present invention for introducing porosity into PTFE, 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. The PTFE is contacted with an excess of a fluid which penetrates and swells but does not dissolve or eliminate viscoelastic memory from the PTFE under process conditions, and heated therein at a temperature in the range of about 250-400.degree. C. By "excess fluid" is meant substantially more fluid than is required to penetrate and swell the polymer fully under process conditions; i.e., substantially more than about 80% of the volume occupied by the unswollen polymer. When the fluid is a relatively volatile liquid, vapor or gas, the process may be operated under pressure. Operating pressure for the invention process is from about atmospheric pressure to about 300 MPa or higher. The starting PTFE may be immersed in unheated fluid and then heated to the operating temperature, or immersed in fluid previously heated to the operating temperature. It is preferable, but not essential, to completely envelope the PTFE in fluid. The polymer should preferably remain completely enveloped by the fluid throughout the fluid contacting procedure. Precautions should be taken to insure that fluids causing adverse chemical reactions under process conditions are avoided. In the invention process (II) for preparing stretched, porous, shrink-resistant PTFE shaped articles, conventionally formed shaped articles, such as film, fiber or billet, of PTFE, are contacted with an excess of an appropriate organic fluid in which the PTFE is insoluble or of limited solubility under process conditions, and heated therein at a temperature at or near the melting point of the PTFE under said process conditions. The starting article may be immersed in unheated fluid and then heated to the operating temperature, or immersed in fluid previously heated to the operating temperature. The polymer should preferably remain completely enveloped by the fluid throughout the fluid contacting procedure. As indicated above, the appropriate temperature in the invention process should be sufficiently high to permit high fluid uptake but must not exceed that required to maintain the shape of the starting PTFE article under process conditions. Suitable process temperatures are at or near the melting point of the PTFE under process conditions. Factors affecting the appropriate temperature include the swelling power of the selected fluid, the crystalline morphology of the PTFE, the duration of exposure in step (a), and the surface/volume ratio of the PTFE article. Temperatures in the range of 290-360.degree. C. are usually suitable. Preferred fluids for use in processes I and II include halogenated organic liquids containing fluorine and/or chlorine and, optionally, also ether oxygen. Non-limiting examples of such liquids include chlorobenzene, fluorobenzene, 1,2-dichlorobenzene, benzotrifluoride, perfluorodimethylcyclobutane, perchloroethylene, Freon.RTM.-113 (1,1,2-trichloro-1,2,2-trifluorethane), Freon.RTM.-114 (1,2-dichloro-1,1,2,2-tetrafluoroethane, 1,1-difluoro-1,2,2,2-tetrachloroethane, 1,2-difluoro-1,1,2,2-tetrachloroethane, perfluoro-n-butyltetrahydrofuran, Krytox.RTM. perfluorinated oils (perfluoropropene oxide oligomers), pentafluoroethane, chloroform, methylene chloride, 1,1,2-trichloroethylene, carbon tetrachloride, and mixtures thereof. Higher boiling perfluorinated alkanes and perfluorinated cycloalkanes, such as perfluorotetradecahydrophenanthrene, which are strong solvents for PTFE, may be suitable when mixed with one or more fluids which reduce the solvent power of said alkanes or cycloalkanes or, alternatively, the PTFE can be contacted with said alkanes or cycloalkanes for a sufficiently short duration and/or at a sufficiently low temperature that the PTFE is not significantly dissolved and suffers no significant loss of shape. The perfluorinated alkanes and cycloalkanes can be made by fluorination of the corresponding hydrocarbon compound, see, for example, British Patent 1,281,822. By "dimer" herein is meant a byproduct from the fluorination of phenanthrene using a combination of CoF3 and fluorine, as described in British Patent 1,281,822. When phenanthrene is thus fluorinated to perfluorotetradecahydrophenanthrene, a higher boiling fraction is obtained upon fractional distillation of the crude liquid product. This fraction has a boiling point in the range of 280.degree. C. to about 400.degree. C. at atmospheric pressure, typically about 316-340.degree. C. It has a small amount of olefin and a very small amount of hydrogen in it which can be further reduced by post-fluorination. It is believed that most of this mixture consists of the general structure ##STR1## 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 ##STR2## 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. Other fluids suitable for processes I and II include non-halogenated aromatic organic liquids such as, for example, toluene, benzene, anisole, and mixtures thereof with halogenated fluids. Other non-halogenated fluids suitable for process I include cyclohexane, diethyl ether, ethyl acetate, C.sub.1-4 alcohols, tetraethoxysilane, water, and mixtures thereof with halogenated fluids. Inorganic liquids such as the tetrachlorides of titanium, silicon and tin, and the trichlorides of iron and bismuth are also suitable fluids for process I, as are gases such as ammonia, nitrogen, and sulfur dioxide. Fluid swelling power can be increased by operating the process at elevated pressure. In the present processes, heating and pressure are maintained for a period of time dependent on the temperature, pressure, solvent power and volatility of the penetrating fluid being used, and on the dimensions of the sample. The required time may extend from less than 1 minute to several hours, generally decreasing as the swelling power of the penetrating fluid increases. As previously noted, the contact time between PTFE and fluid penetrant under processing conditions should be sufficiently short to avoid significant dissolution of the polymer, or loss of viscoelastic memory therein. "Viscoelastic memory" is herein defined as the ability of the PTFE starting material to retain or regain its original shape, although its original dimensions may increase under the conditions of the invention process. One skilled in the art will understand that a "shaped article" is one that essentially has its own shape without external confinement or support. It is important in the practice of the invention that viscoelastic memory and hence the shape of the PTFE starting material be retained throughout the process. For example, if the polymer is introduced in the form of film or rod, film or rod are recovered at the end of the procedure without significant change in shape, although the dimensions of said shape may increase. It should be understood that PTFE is not removed, e.g., by extraction, during the invention process; as demonstrated in the Examples, no loss in weight of polymer is detected. The polymer content of the initial (unextracted) products of the present process is greater than about 20% by weight. These products are not free-draining. Higher boiling fluids retained by these products may not be removed easily, e.g., by pressing or evaporation, and may require hot extraction with a solvent such as Freon.RTM.-113. Removal of high boiling liquids is particularly difficult and, for this reason, use of thinner sections of PTFE and more volatile fluids is preferred. In process II, if the solvent-treated, unstretched, porous intermediate article is not in a form suitable for stretching, it may be converted to such a form by conventional procedures, such as, for example, skiving. Forms suitable for stretching include, but are not limited to, fibers, hollow fibers, film and sheet. By "porous" products is meant products having a significant void content which are permeable to gases. The terms "void content," "porosity," and "% voids" are herein used synonymously. The method of calculation of % voids used herein is to determine the difference in density between the starting resin and that of the porous product made therefrom in the practice of this invention, and divide said difference by said starting resin density. The result multiplied by 100 is the % voids. Density of the starting resin is determined preferably by a direct volumetric technique such as helium pycnometry, but any generally accepted technique may be employed. Density of the porous product of this invention was determined by measuring the weight of a specimen of known dimensions. The unstretched porous products of processes I and II have void contents (porosity) of up to at least about 81%. Both open and closed voids are thought to be present. Preferred void content of the unstretched porous articles of process II is about 15 to 35%. Stretching of the latter articles in process II increases porosity up to 50% and higher. The porous products of process I of the invention may be fibrillar (FIGS. 1-3), have a relatively high surface area, and are semi-crystalline with a heat of fusion of at least 35 J/g. The products are particularly useful as insulation, as gas permeable articles including membranes and diaphragms, and as catalyst supports and filters. As demonstrated in the Examples, porosity of the products is shown by the reduced density of the PTFE products measured on dry, essentially liquid-free samples, by increased surface area, and/or by measurement of gas transmission through membranes of the product. A major advantage of the present process is the ability to introduce porosity into shaped articles of PTFE without loss of shape. Thus, film, fiber, wire and cable coatings, tubing and the like can be rendered porous without the necessity to refabricate the article. As retention time under processing conditions can be quite short, the process is suitable for continuous operation. For example, PTFE film or fiber, or PTFE coatings on wire or cable, can be rendered porous in a continuous operation wherein the article is drawn through a bath containing an appropriate fluid penetrant (Example 31) heated to an operating temperature of at least about 250.degree. C. The latter method is particular suitable in process II for preparing porous film or fiber for subsequent uniaxial or biaxial stretching to form the final invention products. Alternatively, a billet of PTFE fabricated by compaction of virgin PTFE powder may be subjected to fluid penetration in a sealed pressure vessel. After optional fluid removal, the billet is skived into film and stretched to form the final product. As indicated, porosity of the intermediate invention products is substantially further improved by subjecting said products to uniaxial or biaxial stretching. The porous PTFE products of process I of this invention can be used to prepare composites of PTFE with one or more additional polymers, wherein said additional polymers occupy pores or voids in a matrix structure formed by the porous PTFE. The composites may be prepared by infusing one or more liquid and/or gaseous monomers, under pressure if necessary, together with appropriate polymerization initiators, into the porous PTFE previously prepared according to the invention process. Suitable monomers include, but are not limited to, ethylene, halogenated ethylenes, methacrylate and acrylate esters such as methyl, ethyl and butyl methacrylates and methyl, ethyl and butyl acrylates, styrenes, urethanes, polymerizable epoxides, difunctional monomers such as dimethacrylates and diacrylates and diglycidyl methacrylate to induce crosslinking, and the like. The selection of appropriate polymerization initiators will depend on the monomers chosen and will be apparent to those skilled in the art of polymerization. For example, a,a'-azobisisobutyronitrile (VAZ0-64.RTM.) is a well known free radical polymerization catalyst suitable for the polymerization of monomers such as methacrylates, acrylates, di(meth)acrylates, styrenes and the like. The monomers may be polymerized in situ by the application of heat and/or irradiation such as UV or electrons. The PTFE composites prepared as described exhibit low friction similar to PTFE itself but have generally higher modulus and creep resistance. The composites are useful in load bearing applications including gaskets and seals. A separate utility for the non-draining PTFE/absorbate composition of the invention is as an improved lubricating surface, e.g., for brake cable. When the PTFE/absorbate composite is flexed, a small amount of the absorbate exudes to the surface at the flex point; it is promptly reabsorbed when the stress is removed. Any type of fully polymerized, high molecular weight, crystalline or partly crystalline PTFE is operable in the invention processes. 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 polytetrafluoroethylene homopolymer and copolymers of polytetrafluoroethylene, which may contain minor amounts of repeat units of other monomers, providing said copolymers are of high molecular weight, crystalline, and non melt-fabricable, and their viscoelastic memories are not significantly diminished by the penetrating fluids under process conditions. The PTFE polymer suitable for the practice of this invention is characterized by a melting point in the range of 315 to 333.degree. C., preferably about 327.degree. C., and a heat of fusion of less than about 35 J/g, preferably 25-30 J/g, which melting point and heat of fusion is determined by Differential Scanning Calorimetry (DSC; ASTM D3418-82, D4591-87) from a specimen which has, prior to said determination of melting point and heat of fusion, been heated in the DSC at least 20.degree. C. above its melting point and recrystallized from the melt at a cooling rate of 1.degree. C./min from 20.degree. C. above its melting point to about 250.degree. C. or lower. The PTFE resin preferred for the practice of this invention, as hereinabove defined, is known in the art to be of a molecular weight of about 10-30 million. "Melting point" refers to the temperature at the peak of the DSC melting endotherm. By "crystalline PTFE" is meant PTFE having a heat of fusion of about 65 J/g in the virgin, as-polymerized state. DSC is also employed for characterizing the porous products of this invention in their as-fabricated state. For this purpose, the as-fabricated sample is subject to a single heating at 10.degree. C./min from ca. 100.degree. C. to ca. 380.degree. C., the number of endotherms being determined and the melting points and heats of fusion associated with each endotherm determined. (ASTM D3418-82, Paragraphs 10.1.1 and 10.1.2). The actual heat of fusion and crystallinity of a given starting PTFE will depend on its fabrication history. The starting PTFE employed in the invention process, again depending on its fabrication history, has at least one crystalline melting point in the temperature range of about 315.degree. to about 350.degree. C. However, the unstretched porous products of process I of the invention have one crystalline melting point, said melting point lying in the range of about 315 to 333.degree. C. It has been discovered that pore size in the porous products of process II of the invention can be controlled by controlling the fluid used, fluid pressure and rate of cooling of the liquid-treated porous PTFE precursor articles. Such cooling-step control also affects the degree of crystallinity in said articles. Fluid-treated porous PTFE articles (porous precursors) of process II that are suitable for conversion by stretching into the final porous products of the invention are characterized by a melting point of ca. 325-330.degree. C. and an associated heat of fusion in the range of 35-65 J/g, which heat of fusion represents at least 80%, and usually 100%, of the total heat of fusion of the specimen. It is known in the art that conventional fabrication of the PTFE resin suitable for the practice of this invention results in fabricated articles exhibiting a melting point of ca. 325-330.degree. C. and an associated heat of fusion in the range of 25-30 J/g. It is further known in the art that degradation of PTFE during processing can result in a higher heat of fusion associated with lower molecular weight polymers produced by that degradation. However, in that case, the higher heat of fusion is retained upon a second heating following recrystallization at 10.degree. C./min or more slowly after the first heating in the DSC, as described hereinabove. In the case of the porous articles of this invention, the higher than expected heat of fusion is observed only upon the first heating in the DSC. Following melt-recrystallization and reheating, the heat of fusion is found to be ca. 30 J/g, as is expected for the undegraded resin suitable for the practice of this invention. Thus, the unexpectedly high heat of fusion of the porous articles of this invention is inherent in those products and not a result of molecular weight degradation during processing. It has been found in the practice of the invention process that porous precursors of lower crystallinity, as indicated by a heat of fusion toward the lower end of the above range, are more difficult to stretch. Small defects appear to be more detrimental to the integrity of a less crystalline precursor during stretching, particularly when stretching more than fourfold on an areal basis. However, precursor film of lower crystallinity made from fine powder PTFE of higher molecular weight proved more likely to stretch without breaking than that made from granular PTFE. For this reason, the finest pore size films were made from the fine powder PTFE. In one embodiment of process II wherein unstretched, porous precursors are prepared and then converted into the final stretched, porous products of the invention, virgin PTFE resin in a mold is first cold compacted into a billet or cylindrical preform by placing the mold between the platens of a hydraulic press and applying pressure. The powder compaction step may be carried out below or above room temperature with appropriate adjustments made in pressure and dwell time (time held at pressure) to ensure that the resulting billet has sufficient integrity for further handling. At room temperature (about 20 to 35.degree. C.), pressures of about 2000 to 5000 psi (14 to 35 MPa) were suitable, with dwell times of about 1 to 120 minutes. Suitable billets can be formed from either granular or fine powder PTFE resins; the former normally require lower pressure and shorter dwell time. Billets so formed should have a specific gravity of at least about 1.7, preferably at least 1.9. The billet so formed may be conventionally sintered before further handling, but such sintering is unnecessary in the practice of this invention. The billet is placed in a pressure vessel and immersed in an appropriate fluid and heated to a temperature close to the melting point of the polymer in the presence of the fluid under process conditions, and held for a time sufficient for the fluid to uniformly penetrate the billet, but not change the shape of the billet, as discussed above. Temperatures in the range of about 320 to 360.degree. C. are suitable for many fluids. Following the hold period at the selected processing temperature, the billet is subjected to slow, controlled cooling to about 50 to 100.degree. C. below the processing temperature. The hold period and cooling rate will depend upon the solvent power of the fluid, process temperature and pressure, and billet thickness. Following treatment in the pressure vessel, the billet so processed is freed from unabsorbed fluid. Absorbed fluid may optionally be removed at any convenient point in the process, by any convenient means such as, for example, extraction, evaporation or suction. The billet may be skived into continuous film or sheet of any desired thickness, by conventional methods. The skived film or sheet is then stretched either biaxially or uniaxially about 300-1000% on an areal basis at rates of about 1% to 1,000%/second, preferably about 10% to 1,000%/second, most preferably about 100% to 500%/second, at temperatures in the range of about 20 to 150.degree. C., preferably 80.degree. to 120.degree. C. Stretching at higher rates within the specified range results in somewhat finer pores. Stretching may be accomplished by any convenient means including batchwise using, e.g., a pantograph, or continuously using a machine direction stretcher and a tenter frame in tandem, or using a continuous biaxial orientation machine. The porous, shrink-resistant PTFE shaped articles of this invention exhibit a mean pore size of less than about 10 .mu.m, a porosity of about 40% or higher, a melting transition with a peak in the range of 315 to 333.degree. C. with an associated heat of fusion of at least 35 J/g and no endotherm above 370.degree. C. The shaped articles of the invention exhibit a morphology comprising interconnecting nodes and fibrils, said nodes being smaller or equal in size to said fibrils. The porous, shrink-resistant PTFE shaped articles have outstanding chemical inertness and resist undesirable physical changes over a wide temperature range. The porous articles can be provided in many shapes such as, for example, film, sheet, filaments, tubing, rings and rod, and are useful in a wide variety of applications, including, but not limited to: filtration media for separating solids from fluids (gases and liquids); semi-permeable membranes for separating gases or gases and liquids; thermal and electrical insulation; protective clothing (for example, space suits, cleanroom suits, waterproof rainwear, gloves, footwear, socks, undergarments, medical drapings and garments,); sports equipment (for example, tents, sleeping bags); medical materials (for example, vascular, ligament or tendon prostheses, suture needle holders, damming materials, dental floss); seam and sealing tape; gaskets and other load-bearing articles. In many of the applications where the articles of this invention find utility, advantage is taken of the pore structure which allows the selective transmission of fluids through the article, permitting many types of filtration or separations. An example is transmission of air and water vapor, but not liquid water, through articles of the invention. It is often advantageous to bond or laminate the invention articles to other materials for, e.g., greater support, comfort or durability; bonding agents are able to significantly penetrate the pore network and, after curing, become locked therein. Certain perfluorinated alkanes and cycloalkanes such as perfluoro(tetradecahydrophenanthrene) and "dimer" described hereinabove, having high solvent power for PTFE, can effectively bond or laminate the invention articles without significantly penetrating the pore network. Any adhesive capable of bonding directly to PTFE will be suitable for forming laminates of the invention articles. Methods of using the present porous, shrink-resistant PTFE shaped articles, including laminated materials, electrical and thermal insulation, protective materials, bearing materials, film, tubes, filaments, rods and the like are essentially those already described by Gore and Associates and in related art for porous PTFE articles prepared by different processes. Such methods, including laminations and bonding, are described in U.S. Pat. Nos. 5,095,779, 5,128,209, 5,086,914, 4,187,390, 4,194,041, 4,978,813 and 4,208,745 herein incorporated by reference; and in numerous other U.S. Patents assigned to W. L. Gore & Associates, Inc. or related entities. The invention is further illustrated with reference to the following examples: Examples 1-31 relate to the introduction of porosity into PTFE articles by treatment with fluid penetrants at elevated temperatures. Example 32 relates to the use of a porous product to prepare a polymer composite. Temperatures are expressed in degrees Celsius and percentages are by weight unless otherwise indicated. |
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