Oleophobic coated membranes

Details Oleophobic coated membranes Oleophobic coated membranes

2001-05-01

2003-02-18

Spitzer, Robert H. (Department: 1724)

Gas separation

Specific media material

With coating, impregnant, or bonding agent

C055SDIG005, C096S012000, C427S255600, C427S385500, C428S098000

Reexamination Certificate

active

06521012

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to oleophobic filtration media including polymeric membranes and other substrates that are coated with polymerized substituted or unsubstituted para-xylenes. A method of coating such substrates with polymerized substituted or unsubstituted para-xylenes is also provided. The coated substrates possess both hydrophobic (water repellent) and oleophobic (oil repellent) properties.
BACKGROUND OF THE INVENTION
Oleophobic filtration media, including membranes and substrates, are preferred for filtration of gases, in venting filters, and as gas vents. These oleophobic filtration media allow gases and vapors to pass through the filter while liquid water is repelled. Oleophobic filtration media are preferred for filters in healthcare and related industries, for example, as vent filters for intravenous (IV) fluids and other medical devices. In the health care industry, such filters must be sterilized before use.
Polytetrafluoroethylene (PTFE) has been the most common material in filters for gas venting. PTFE is chemically and biologically inert, has high stability, and is hydrophobic. PTFE filters therefore allow gases to be selectively vented while being impervious to liquid water. PTFE membranes can be sterilized for health-related applications with steam or by chemical sterilization without losing integrity.
Treating PTFE membranes with steam can cause pore blockage due to condensation of oil from the machinery that generates the steam. The resulting loss of air permeability reduces the membrane's ability to serve as an air vent. Although chemical sterilization minimizes exposure of the membrane to oil, chemical sterilization uses toxic chemicals and can generate byproducts which cause waste disposal problems. Ionizing radiation has also been preferred for sterilization of materials in medical and biological devices. PTFE is unstable toward ionizing radiation. Irradiated PTFE membranes have greatly reduced mechanical strength and are generally not preferred for applications where they are subjected to even moderate pressures.
Perhaps the two biggest drawbacks to PTFE as a filter for venting gases are the high cost and the low air permeability of PTFE membranes. PTFE membranes are formed by extruding and stretching PTFE. Both the PTFE raw material and the processing to form the PTFE membranes are expensive. Further, the extruding and stretching processes preferred for forming PTFE membranes create membranes which have relatively low air permeability.
The oleophobicity of PTFE can be improved by impregnating or coextruding the PTFE with siloxanes (for example, U.S. Pat. No. 4,764,560), fluorinated urethane (U.S. Pat. No. 5,286,279), or perfluoro-2,2-dimethyl-1,3-dioxole (U.S. Pat. No. 5,116,650). Although the oil resistance of the PTFE is improved, the treated PTFE membranes are expensive, and air permeability remains fairly low. As a result, efforts have been made to identify alternative substrates which are less expensive and have higher air permeability than PTFE and which can be modified by coating to be hydrophobic and oleophobic.
Coating filtration substrates allows one to retain the desirable bulk properties of the substrate while only altering the surface and interfacial properties of the substrate. Coating substrates to increase their hydrophobic and oleophobic properties has not been practical, because the coatings can reduce permeability. Furthermore, many of the coating materials are expensive.
Scarmoutzos (U.S. Pat. No. 5,217,802) modified the surface of substrates made of nylon, polyvinylidene difluoride (PVDF), and cellulose by treating the substrate with a fluorinated acrylate monomer. The substrate was sandwiched between two sheets of polyethylene, and the monomer was polymerized by exposing to ultraviolet light. The resulting composite filters had hydrophobic and oleophobic surfaces. The air permeability of the filters decreases with time.
Moya (U.S. Pat. No. 5,554,414) formed composite filters from polyethersulfone and PVDF membranes with a method similar to that of Scarmoutzos. The resulting filters did not wet with water or hexane. The disadvantage of the Moya filters is that air permeability of the treated filters was lower than the untreated substrates, and the fluorinated monomer is expensive.
Sugiyama et al. (U.S. Pat. No. 5,462,586) treated nylon fabric and PTFE membranes with solutions containing two different preformed fluoropolymers. The treated filters were resistant to water and oils. The durability of filters coated with preformed polymers is often less than that for filters where the coating is formed by polymerizing a monomer on the surface of the substrate.
Kenigsberg et al. (U.S. Pat. No. 5,156,780) treated a variety of membranes and fabrics with solutions of fluoroacrylate monomers and formed coatings on the substrate by polymerizing the monomer. The coating conferred oil and water repellency onto the substrate. However, the airflow through the treated membrane was reduced, compared to the untreated membrane.
Hydrophobic media suitable for garments have been made by extruding mixtures of polypropylene or PTFE and the fluorochemical oxazolidinone as disclosed in U.S. Pat. No. 5,260,360. The media made by extruding tend to have relatively low air permeability.
In copending U.S. application Ser. No. 09/323,709 filed Jun. 1, 1999, oleophobic and hydrophobic filters are prepared by forming a polydimethylsiloxane coating on a polymeric substrate by polymerizing vinyl terminated siloxane with a crosslinker such as hydrosilicon in the presence of a catalyst.
SUMMARY OF THE INVENTION
In a first embodiment of the present invention, a coated filter is provided including a polymeric substrate and a coating, the coating including a polymeric para-xylene.
In one aspect of the first embodiment, the coating renders the coated filter permanently oleophobic.
In another aspect of the first embodiment, the polymeric substrate includes polysulfone. The polysulfone may include polyalkylsulfone, polyethersulfone, or polyarylsulfone. Alternatively, the polymeric substrate may include a polymer such as polyvinylidene fluoride, polyolefin, polytetrafluoroethylene, poly(tetrafluoroethylene-co-ethylene), acrylic copolymer, polyamide, nylon, polyester, polyurethane, polycarbonate, polystyrene, polyethylene-polyvinyl chloride, polyacrylonitrile, cellulose, or mixtures thereof. The polyolefin may include polyethylene, polypropylene, or mixtures thereof.
In a further aspect of the first embodiment, the polymeric substrate includes a porous membrane. The porous membrane may include an isotropic membrane.
In yet another aspect of the first embodiment, the porous membrane may include an asymmetric membrane. The asymmetric membrane may have a supporting structure, a first porous face having a first average pore diameter and a second porous face having a second average pore diameter, wherein an asymmetry between the first average pore diameter and the second average pore diameter is at least about 2:1, 5:1, 10:1, 20:1, or 200:1
In a further aspect of the first embodiment, the supporting structure includes an isotropic region adjacent the second porous face, the isotropic region having a substantially constant pore size, the supporting structure further including an asymmetric region adjacent the isotropic region. The isotropic region may extend through from about 5% to about 80%, or from about 15% to about 50%, of the supporting structure. The second average pore diameter may be between about 0.01 &mgr;m and about 50.0 &mgr;m, between about 0.05 &mgr;m and about 20.0 &mgr;m, or less than about 0.01 &mgr;m.
In another aspect of the first embodiment, the polymeric substrate includes a material such as nonwoven material, woven material, or melt blown material.
In a further aspect of the first embodiment, the poly-para-xylene includes a polymonochloro-para-xylene or an unsubstituted poly-para-xylene.
In another aspect of the first embodiment, the coated filter further includes a support, wherein the coated filter is bonded to the sup

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