Liquid purification or separation – Filter – Supported – shaped or superimposed formed mediums
Reexamination Certificate
1998-03-13
2001-01-30
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Supported, shaped or superimposed formed mediums
C210S500360, C210S500270, C264S041000, C427S244000, C427S245000
Reexamination Certificate
active
06179132
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a porous polyperfluorocarbon membrane having a water wettable surface. More particularly, this invention relates to a porous polyperfluorocarbon membrane having a water wettable surface formed of a perfluorocarbon polymer including hydrophilic functional groups.
2. Description of the Prior Art
Polyperfluorocarbon membranes are useful in a wide variety of environments due to the chemical inertness of the membrane. By the term “polyperfluorocarbon” as used herein is meant homopolymers of a perfluorocarbon as well as polymers formed from more than one monomer at least one of which is a perfluorocarbon including copolymers or terpolymers or a polymeric blend of such homopolymers and/or polymers or the like. Examples of polyperfluorocarbons include polytetrafluoroethylene (PTFE), fluorinated ethylene-propylene copolymer (FEP) and perfluoro-alkoxy polymer (PFA or MFA).
Porous membrane filters are utilized in a wide variety of environments to separate materials within a fluid stream. Membranes are formed from a solid polymeric matrix and have highly precisely controlled and measurable porosity, pore size and thickness. In use, the membrane filters generally are incorporated into a device such as a cartridge which, in turn, is adapted to be inserted within a fluid stream to effect removal of particles, microorganisms or a solute from liquids and gases.
To be useful, membrane filters must be resistant to the fluid being filtered so that it maintains its strength, porosity, chemical integrity and cleanliness. For example, in the manufacture of microelectronic circuits, membrane filters are used extensively to purify various process fluids to prevent contaminants from causing circuit failures. Fluid filtration or purification is usually carried out by passing the process fluid through the membrane filter under a differential pressure across the membrane which creates a zone of higher pressure on the upstream side of the membrane than on the downstream side. Thus, liquids being filtered in this fashion experience a pressure drop across the membrane filter. This pressure differential also results in the liquid on the upstream side having a higher level of dissolved gases than the liquid on the downstream side. This occurs because gases, such as air, have greater solubility in liquids at higher pressures than in fluids at lower pressures. As the liquid passes from the upstream side of the membrane filter to the downstream side, dissolved gases come out of solution in the membrane resulting in outgassing of the liquid. Outgassing of a liquid can also occur spontaneously without a pressure differential as long as the liquid contains dissolved gases and there is a driving force for the gases to come out of solution, such as nucleating sites on the surfaces of a membrane where gas pockets can form and grow. Outgassing liquids typically used in the manufacture of semiconductors and microelectronic devices usually include very high purity water, ozonated water, organic solvents such as alcohols, and others which are generally significantly chemically active, such as concentrated and aqueous acids or bases which can contain an oxidizer. These chemically active liquids require the use of a chemically inert filter to prevent membrane degradation. Membrane degradation leading to the chemical breakdown of the membrane composition usually results in extractable material which is released from the filter during use, thus compromising the purity, integrity and cleanliness of the fluid being filtered. Polyperfluoro-carbon-based membrane filters made from fluorine-containing polymers such as polytetrafluoroethylene, or PFA are commonly utilized in these applications. Fluorine-containing polymers are well known for their chemical inertness, or excellent resistance to chemical attack. One disadvantage of fluorine-containing polymers is that they are hydrophobic and therefore membranes made from such polymers are difficult to wet with aqueous fluids or other fluids which have surface tensions greater than the surface energy of the membrane. In order to wet the surface of a hydrophobic membrane with water or an aqueous fluid, it is current practice to first wet the surface with an organic solvent, followed by contact of the surface with a mixture of water and an organic solvent and then followed by contact with water or an aqueous fluid. Alternatively, hydrophobic membranes can be wet with H
2
O under pressure. This process is time consuming, expensive and often ruptures the membrane. Moreover, this process does not ensure that a substantial portion of the pores in the membrane are completely intruded with water.
Another problem often encountered during the filtration of outgassing liquids with a hydrophobic membrane filter is that the membrane provides nucleating sites for dissolved gases to come out of solution under the driving force of the pressure differential, during the filtration process. Gases which come out of solution at these nucleating sites on the hydrophobic membrane surfaces, including the interior pore surfaces and the exterior or geometric surfaces, form gas pockets which adhere to the membrane. As these gas pockets grow in size due to continued outgassing, they begin to displace liquid from the pores of the membrane ultimately reducing the effective filtration area of the membrane. This phenomenon is usually referred to as dewetting of the membrane filter since the fluid-wetted, or fluid-filled portions of the membrane are gradually converted into fluid-nonwetted, or gas-filled portions where filtration ceases and which results in a reduction of the overall filtration efficiency of the filter.
In contrast, self wetting hydrophilic membranes are spontaneously wet upon contact with an aqueous liquid so that a treatment process for wetting its surface is not required. That is, no prior treatment with an organic solvent or pressure intrusion, or mechanical energy such as by stirring is required in order for the membrane surface to be wet with water. It has been proposed in U.S. patent application Ser. No. 08/848,809, filed May 1, 1997, which is incorporated herein by reference, to provide a process for modifying a surface of a porous membrane such as a polyperfluorocarbon membrane with a bound perfluorocarbon copolymer composition to render the entire surface non-dewetting. A porous membrane substrate is intimately contacted with a perfluorocarbon copolymer composition in a solvent or diluent. Excess perfluorocarbon copolymer composition is removed from the surface with a solvent or diluent for the copolymer. The solvent or diluent does not remove the perfluorocarbon copolymer composition bound to the membrane surface. The membrane having the copolymer composition bound to its surface then is heat treated to improve the bond between the membrane substrate and the surface modifying perfluorocarbon copolymer composition. The perfluorocarbon copolymer composition is utilized in concentrations and amounts so that the membrane surface is completely modified while avoiding substantial blocking or plugging of the membrane pores. Complete surface modification can be determined by staining with Methylene Blue dye. In order to wet the surface modified membrane with water, it is first necessary to prewet the membrane with an organic solvent such as isopropanol (IPA). Thus, the surface modified membranes are not directly wet with water. In addition, these membranes must be maintained in contact with water or an aqueous solution in order to prevent the membrane from drying out. If the membranes are allowed to become even partially dried, the dried portion of the membrane must be wet via the complex process of contact with organic solvent, then with a mixture of an organic solvent and water and then with water or an aqueous solution.
When modifying a membrane surface it is essential that the surface modification be effected without substantially reducing the membrane porosity. Thus, sufficient surface modifying composition must b
Cook, Esq. Paul J.
Fortuna Ana
Hubbard, Esq. John Dana
King, Esq. Timothy J.
Millipore Corporation
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