The present invention relates generally to improved asbestos-free porous polymeric sheet materials which are suitably patterned into separators for use in electrolytic cells, such as chlor-alkali cells and other types of liquid - liquid processing apparatuses like fuel cells, osmotic cells, diffusional cells, and the like. More particularly, the invention is concerned with microporous polytetrafluoroethylene (PTFE) in sheet form patterned into individual panels when bonded together with an intermediate non-melt processable perfluoroalkylene polymer layer/coating, produces stronger more reliable seals. It has been observed that statistically there are fewer occurrences of joint separation and separator failure when PTFE panels are sealed according to the disclosed invention.
Diaphragm cells have long been used for the manufacture of chlorine and caustic soda. In such cells, anolyte and catholyte have been previously separated by a diaphragm of deposited asbestos fibers, usually on a wire mesh cathode structure. In recent years however, such diaphragms in some instances have been replaced by ion-permeable membranes or porous separators. Of the porous separators, those which are microporous and made, for example, from PTFE are gaining increased favor, primarily for reasons of environmental safety, lower electrical power consumption, and lower cell maintenance costs.
Because polymeric separators are in most instances produced in sheet form and are not deposited onto wire mesh cathode structures, as in the case of a deposited asbestos diaphragms various methods have been suggested for retrofitting separator sheets onto such cathode structures. For example, U.S. Pat. No. 4,076,571 teaches restoration of elasticity to heat sealed portions of an electrolytic diaphragm, including the formation of an envelope by heat sealing together sections of diaphragm material at edges thereof. In the installation of such a diaphragm, an envelope is formed of the diaphragm material and slipped over an electrode, after which the envelope is closed by clamping means. U.S. Pat. No. 4,283,264 is a further example of a porous PTFE material retrofitted onto a chlor-alkali cell wherein a plurality of open ended tubular panels of a height greater than the cell cathodes are equipped with polymer flange portions at one or both ends of the panels. The anode compartments of the cell are sealed off from the cathode compartments by bonding adjoining halves of two adjacent separator tubes or panels by heat sealing their flanges together. Alternatively, U.S. Pat. No. 4,283,264 suggests that multiple tubular panels be joined together by means of an added peripheral polymeric sealing member by bonding the flanges of the tubular panels thereto. The peripheral sealing members in turn are held tightly in place by gaskets to prevent leakage between anolyte and catholyte compartments of the cell. Regardless of embodiment, the fitting of an electrolytic cell with polymeric microporous separators invariably requires multiple PTFE panels precut to fit the particular cell geometry or cathode configuration where the edges of such panels are bonded together, typically by forming lap joints or where edges of separator panels are butted up to each other and sealed by the application of heat and pressure.
Most applications for microporous separators, and especially in the case of electrolytic cell applications allow virtually no tolerance for seals which after a period of time begin to leak. It has been applicant's experience that even a relatively small separation of a sealed joint holding together separator panels will adversely affect cell performance in terms of lower current efficiencies, higher power consumption, as well as deterioration of product purity and concentration. Consequently, a key element to the successful conversion of an asbestos diaphragm cell to a non-asbestos containing PTFE microporous separator equipped cell lies in making highly reliable seals for bonding separator panels together.
Heretofore, various methods have been proposed for sealing PTFE panels. For example, U.S. Pat. No. 4,156,639 discloses the formation of an "endless-belt diaphragm" fabricated from multiple strips of fluorinecontaining polymers. In a preferred embodiment, a porous PTFE separator is bonded to a "window frame" sheet comprised of melt-processable fluorine-containing polymer strips, such as fluorinated ethylene/propylene copolymer. A belt is formed by joining two such frames together by means of lap joints bonded together by hot pressing the joint or by the application of a cement such as a low molecular weight, low melting point PTFE. A method similar to that of U.S. Pat. No. 4,156,639 is also disclosed in U.S. Pat. No. 4,153,530 whereby porous PTFE separator panels are connected onto upper and lower slotted supports by means of melt-processable fluorine-containing polymers.
U.S. Pat. No. 4,263,121 provides for making fluid impermeable seals for microporous separators by sealing the edges of separator panels with fluorinecontaining polymers. The sealing polymer should be a fusible substance and though it may be of the same chemical composition as the separator, it is essential that the fusible material be chemically and/or physically more readily fused than the separator material. Embodiments include porous PTFE separator panels bonded together with PFA and FEP both of which have lower melting temperatures than PTFE and are melt-processable .
Another process for sealing microporous separators is disclosed by U.S. Pat. No. 4,165,248 whereby microporous diaphragms are welded together by means of fluorocarbon polymer positioned as an intermediate material in the joint. However, before sealing the fluorocarbon material is treated with a strong base to cause a swelling action. Thereafter, the edges of the diaphragms and intermediate material are bonded together by application of heat and pressure.
The previously described processes for sealing microporous PTFE sheet materials, particularly for chlor-alkali cell applications, have a number of shortcomings. Early methods were not convenient or were commercially impractical or lacked reliability in terms of leak free performance over extended time periods. For example, more conventional melt-processable, thermoplastic sealant materials which are either only partially fluorinated or non-fluorinated tend to have shorter life expectancies, particularly when operating in the highly corrosive environment of a chlor-alkali cell over prolonged time periods. Furthermore, perfluorinated hydrocarbon separators, like PTFE, when fabricated from several preformed pieces and welded together to form a diaphragm sleeve or envelope, in addition to being costly in terms of manhours required for assembly, also presents a statistically greater risk of separator failure due to joint separation and leakage.
In addition to the foregoing disadvantages associated with earlier methods of bonding cell separators, previous efforts failed to address the problem of repairing damaged, defective or renewing "worn" porous PTFE cell separators without necessarily refitting the entire cell. Perfluorinated polymers like PTFE also have hydrophobic properties and in order to employ such materials in electrolytic cells they first must be made hydrophillic. Thus, PTFE separator materials are in some instances treated with chemical agents. One example of chemical treatment of fluorocarbon separators is disclosed in U.S. Pat. No. 4,252,878 (C. A. Lazarz, et al) wherein the separator surfaces are coated with fluorinated surface active agents. Such agents effectively impart wettability properties to separator sheets, but such treatments do require new methods for bonding, since previous methods for joint welding fail to provide fail-safe seals to PTFE surfaces coated with films.
Accordingly, it has now been discovered that damaged or defective panels of perfluorinated polymeric separators may be renewed by welding replacement panels to the operative and usable sections of the separator by joining and sealing said panels to separator edges which have been previously sealed by application of heat and pressure. In addition, dependable, leak-free seals can now be made in those circumstances where polymeric cell separators have been coated during manufacturing. Seals are formed by placing non-melt processable perfluoroalkylene polymer sealant between separator panels and bonding the panels together by the application of heat and pressure. In practice, the intermediate polymer is usually the same chemical composition as the separator material and will have substantially the same melting and fusing properties as the polymer panels being sealed. That is to say, the sealant may be comprised of non-porous polymer strips cut from sheet material, as well as porous polymer strips made from microporous separator sheets.
Accordingly, it is a principal object of the present invention to provide a means for sealing panels of perfluorinated polymeric materials into microporous separators wherein their seals exhibit the strength and dependability needed for long term operations for electrolytic cell applications.
A further object of the immediate invention is solution to the problem of resealing edges of perfluorinated materials which were previously sealed under heat and pressure and subsequently broken.
A still further object of the present invention is the fabrication of perfluorocarbon microporous separators by bonding together individual panels treated and coated with films and chemical agents.
These and other objects, features, and advantages will become apparent to those skilled in the art after a reading of the following more detailed description.