Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
2001-07-26
2003-12-16
Aftergut, Jeff H. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S294000, C156S296000, C210S321800
Reexamination Certificate
active
06663745
ABSTRACT:
BACKGROUND OF THE INVENTION
A hollow fiber membrane is a tubular filament comprising an inner diameter, an outer diameter, with a wall thickness, usually porous, between them. The inner diameter defines the hollow portion of the fiber and is used to carry fluid, either the feed fluid to be filtered, or the permeating fluid, if the fluid being filtered contacts the outer surface. The inner hollow portion is sometimes called the lumen.
Hollow fiber membranes are used for diverse applications, including gas separation, reverse osmosis, ultrafiltration and particle and bacteria removal with microporous membranes. In these applications, the membrane acts as a permeable barrier, allowing the passage of the carrier fluid and some dissolved or dispersed species, and retaining other selected species due to differences in specie size, permeation rates, or other physical or chemical attributes.
In practical applications, fiber is cut or otherwise made to a specific length and a number of fibers gathered into a bundle. A portion of one or both ends of the fiber bundle are encapsulated in a material which fills the interstitial volume between fibers and forms a tube sheet. This process is sometimes called potting the fibers and the material used to pot the fibers is called the potting material. The tube sheet acts as a seal in conjunction with a filtration device. If the encapsulation process closes and seals the fiber ends, one or both ends of the potted fiber bundle are cut across the diameter or otherwise opened. In some cases, the open fiber ends are closed and sealed before encapsulation to prevent the encapsulation material from entering the open ends. If only one end is to be opened to permit fluid flow, the other end is left closed or is sealed. The filtration device supports the potted fiber bundle and provides a volume for the fluid to be filtered and its concentrate, separate from the permeating fluid. In use, a fluid stream contacts one surface and separation occurs at the surface or in the depth of the fiber wall. If the fiber outer surface is contacted, the permeating fluid and species pass through the fiber wall and are collected in the lumen and directed to the opened end or ends of the fiber. If the fiber inner surface is contacted, the fluid stream to be filtered is fed into the open end or ends and the permeating fluid and species pass through the fiber wall and are collected from the outer surface.
A variety of materials are used to form the seal. Epoxy resins and urethanes are commonly used as components for the seal. Thermoplastic polymers are another important class. These are polymers that can be flowed and molded when heated and recover their original solid properties when cooled. As the conditions of the application for which the filtration device is being used become more severe, the materials that can be used to form the seal becomes limited. For example, the organic solvent-based solutions used for wafer coating in the microelectronics industry will dissolve or swell and weaken urethane or epoxy based seals. The high temperature stripping baths in the same industry consist of highly acidic and oxidative compounds, which will destroy seals made of common polymers. Seals made from perfluorinated thermoplastics have exceptional resistance to chemical and thermal degradation and would provide excellent sealing material.
Membranes made of perfluorinated thermoplastic polymers are very useful in filtration applications requiring high degree of chemical and thermal resistance. To fully benefit from the properties of the perfluorinated thermoplastic membranes, a filter element using such membranes must be made of materials having similarly resistant properties. For high temperature operation it is preferrable that the melting temperature of the potting material be as close as possible to the melting temperature of the hollow fiber membranes. This will maximize the operational temperature because the operational temperature will be limited to approximately the lower melting temperature of the filter components. Also, it is difficult to pot perfluorinated thermoplastic membranes with dissimilar potting materials and obtain good bonding between the potting material and the perfluorinated thermoplastic membranes. For filtration of ultrapure solutions, exceedingly low levels of extractable residual matter is required of the filtration device. Perfluorinated thermoplastics are commonly used in applications which require very low extractable matter and a filter made entirely of perfluorinated thermoplastic materials would have an advantage in such applications. For these reasons, it is desirable to have a method of potting perfluorinated thermoplastic membranes in a perfluorinated thermoplastic potting material.
Manufacturing a filter element from thermoplastic hollow fiber membranes using thermoplastic polymers as a sealing material is more difficult than with typical resinous materials such as reactive epoxies and urethanes. Epoxies and urethanes used for this application as chosen to have good flow properties so that they can easily flow around the fibers to be sealed. These materials comprise low molecular weight reactive components, having low viscosities, which react to form the final pot after being flowed or otherwise loaded into a portion of a vessel containing a fiber bundle. Thermoplastic polymers are polymers which can be flowed and molded when heated and recover their original solid properties when cooled. Thermoplastic polymers are high molecular weight materials and have high viscosity. They do not easily flow around the fibers of a fiber bundle, and are not prone to flow uniformly around or through a mass of fibers. Thermoplastic materials have to be heated to melt or soften them in order to flow. The hot thermoplastic material can have detrimental effects on the fibers being sealed. Perfluorinated thermoplastics are particularly difficult to use as potting materials because of their high melting temperature and high viscosity. Perfluorinated thermoplastic polymers have to be heated to above their melting point to be extruded or injected. Too long a contact of the perfluorinated thermoplastic potting material heated above its melting point with porous hollow fiber membranes of similar melting points will cause melting and collapse of the hollow fibers. If the perfluorinated thermoplastic potting material cools too quickly as it is being flowed into the fiber bundle, it will not completely fill the interstitial spaces between the fibers. It will instead tend to form occluded volumes from the cooled potting material not being able to easily flow. These will result in weaknesses and possible leaks. Practitioners have attempted to overcome these difficulties in a variety of complicated schemes.
U.S. Pat. Nos. 4,980,060 and 5,066,397 discloses methods of making a filter element comprising a plurality of porous hollow fiber membranes of a thermoplastic resin, fusion bonded at the periphery of the end portions to form a terminal block. In one embodiment, thermoplastic hollow fiber membranes containing fine particle inorganic filler are dipped in a mixture of gypsum and water to seal the end openings. The end portions are dipped in a solvent for the inorganic filler to wash away the filler only from the surface of the end portions. The extraction operation may be effected efficiently by carrying by dipping the end portions of the membranes in the solvent while subjecting the solvent to ultrasonic treatment. The sealed end portions are arranged in a bundle in a lengthwise direction and the end portions are heated to at least the softening temperature of the resin used to make the membranes. The peripheries of the end portions of mutually adjacent membranes should be kept in contact during the heating step, as by winding a non-adhesive tape around the end portions prior to the heat treatment. The tape is removed after the heat treatment. In another embodiment, a powdery thermoplastic resin is applied to the peripheries of the end portions of the membranes. The resin is
Cheng Kwok-Shun
Doh Cha P.
Aftergut Jeff H.
Cook Paul J.
King Timothy J.
Mykrolis Corporation
Mykrolis Corporation
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