Coating processes – With post-treatment of coating or coating material
Reexamination Certificate
1998-12-18
2003-03-25
Weisberger, Richard (Department: 1774)
Coating processes
With post-treatment of coating or coating material
C427S372200, C427S384000, C427S296000, C427S381000, C264S041000, C264S048000
Reexamination Certificate
active
06537614
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to filter materials. More particularly, the present invention relates to charge-modified filters.
Charge-modified filters are known in the art. They typically consist of microporous membranes or involve the use of materials which are blends of glass fibers and cellulose fibers or blends of cellulose fibers and siliceous particles. Charge modification generally was accomplished by coating the membrane or at least some of the fibers with a charge-modifying agent and a separate crosslinking agent in order to ensure the durability of the coating.
While microporous membranes generally are capable of effective filtration, flow rates through the membrane typically are lower than for fibrous filters. Moreover, microporous membranes also generally have higher back pressures during the filtration process than do fibrous filters. Accordingly, there is a need for fibrous filters having effective filtration capabilities for charged particles. Moreover, there also is a need for fibrous filters composed of glass fibers, without the need for cellulosic fibers or siliceous particles.
Apertured films, woven fabrics and nonwoven materials have been used as filter sheets for removing or separating particles from liquids. Such filter sheets generally rely on some form of mechanical straining or physical entrapment. Such filter sheets can pose limitations when the size of the particle to be removed is small relative to the average pore diameter of the filter sheet. For nonwoven materials, this is particularly true for particles of less than one micrometer in diameter.
Improved filters have been developed with modified surface charge characteristics to capture and adsorb particles by electrokinetic interaction between the filter surface and particles contained in an aqueous liquid. Such charge-modified filters typically consist of microporous membranes or involve the use of materials which are blends of glass fibers and cellulose fibers or blends of cellulose fibers and siliceous particles. Charge modification generally was accomplished by coating the membrane or at least some of the fibers with a charge-modifying agent and a separate crosslinking agent in order to ensure the durability of the coating.
While microporous membranes generally are capable of effective filtration, flow rates through the membrane typically are lower than for fibrous filters. Moreover, microporous membranes also generally have higher back pressures during the filtration process than do fibrous filters.
The use of fibers prepared from synthetic polymers is desirable because such fibers are inexpensive and they can be formed into nonwoven webs having porosities which are appropriate for the filtration of particles from a fluid stream. Many of such synthetic polymers, however, are hydrophobic, a characteristic which makes it difficult to durably coat fibers prepared from such polymers with a charge-modifying material. Accordingly, opportunities exist for improved charge-modified filter materials which are based on hydrophobic polymer fibers.
Stated differently, it would be desirable if certain inexpensive materials could be used to produce filters having modified surface charge characteristics for filtering very fine particles from aqueous liquids. For example, polyolefins are widely used in the manufacture of sheets of apertured films, woven fabrics, and nonwoven materials. Many types of polyolefin sheets tend to be hydrophobic and relatively inert. That is, the low surface free energy of polyolefins (e.g., polypropylene) and their relatively chemically inert natures render many unmodified polyolefins ill-suited for filter sheet applications in which a modified surface charge is desired for removing particles from an aqueous liquid. For example, many chemical charge modifiers (e.g., cationic resins, charged particles, etc.) adhere poorly, if at all, to conventional unmodified, hydrophobic polyolefin sheets.
In the past, chemical coatings and/or internal additives have been added to filter sheets made of inexpensive materials to impart desired properties. Many of these coatings and/or additives present problems related to cost, effectiveness, durability and/or the environment.
It has been proposed that biofunctional materials (e.g., proteins) can be deposited from solutions onto different substrates (i.e., sheets of materials) to modify the surface properties of the substrates and/or serve as a functionalized surface that can be chemically reactive. However, many of the economically desirable substrates (e.g., substrates formed of polymers such as polyolefins) have surfaces that are unsuitable for the rapid and inexpensive deposition of biofunctional materials, especially when durable, tightly bound coatings of satisfactory adherence are desired.
Even if inexpensive, durable, tenacious coatings could be adhered to an economically desirable substrate, coatings composed solely of biofunctional materials (e.g., proteins) may have limitations, especially if the coatings lacked the desired chemical characteristics such as, for example, modified surface charge characteristics.
Thus, there is still a need for a practical and inexpensive chemically charged modified filter for removing micron to sub-micron sized charged particles from an aqueous liquid. A need exists for such a filter formed from an unmodified, relatively inert, hydrophobic substrate such as, for example, an unmodified, relatively inert, polyolefin substrate. A need also exists for a method of removing charged, micron to sub-micron sized particles from aqueous liquids utilizing a practical and inexpensive chemically charge modified filter such as, for example, a chemically charge-modified filter formed from an unmodified, relatively inert, polyolefin substrate.
In addition to the needs described above, there is a need for a simple, practical and inexpensive chemically charge-modified filter for removing waterborne pathogens from aqueous liquid. This need also extends to a simple method for removing waterborne pathogens from aqueous liquids utilizing a practical and inexpensive chemically charge-modified filter.
One phenomenon observed with some filters having a modified surface charge characteristic is that the filters have different filtration efficiencies for different types of waterborne pathogens, such as, for example, different types of bacteria. That is, some filters having modified surface charges provide acceptable removal of some types of waterborne pathogens (e.g., some types of bacteria), but not others. The nature of this affinity appears to be difficult to predict. Since even relatively small differences in waterborne pathogen removal efficiency can be important, the discovery that a filter or filter system has an unpredictably strong affinity for a waterborne pathogen would be both unexpected and highly desirable, especially if the filter can be used to produce potable water. Meeting this need is important because removing waterborne pathogens from aqueous liquids in a practical and inexpensive manner remains a challenge in many parts of the world.
SUMMARY OF THE INVENTION
The present invention addresses some of the difficulties and problems discussed above by providing a hydrophobic polymer fiber having a cationically charged coating thereon, in which the coating includes a functionalized cationic polymer which has been crosslinked by heat. By way of example, the functionalized cationic polymer may be an epichlorohydrin-functionalized polyamine. As another example, the functionalized cationic polymer may be an epichlorohydrin-functionalized polyamido-amine.
The present invention also provides a fibrous filter which includes hydrophobic polymer fibers having a cationically charged coating thereon. The coating includes a functionalized cationic polymer which has been crosslinked by heat. As before, the functionalized cationic polymer is an epichlorohydrin-functionalized polyamine or an epichlorohydrin-functionalized polyamido-amine.
The present invention further provides a method of preparing a fi
Lyng Robert John
Varriale Monica Graciela
Wei Ning
Kimberly--Clark Worldwide, Inc.
Nelson Mullins Riley & Scarborough
Weisberger Richard
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