Oleophobic filter materials for filter venting applications

Details Oleophobic filter materials for filter venting applications Oleophobic filter materials for filter venting applications

1999-06-01

2002-03-12

Simmons, David (Department: 1724)

Gas separation

Specific media material

With coating, impregnant, or bonding agent

C055SDIG005, C428S391000, C428S447000, C442S080000

Reexamination Certificate

active

06355081

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to filters having both hydrophobic (water repellent) and oleophobic (oil repellent) properties. The properties are produced by forming a dimethylsiloxane coating on a substrate such as a hydrophobic or hydrophilic membrane or other filtration medium.
2. Background of the Invention
Hydrophobic filters are used in filtration of gases, in venting filters, and as gas vents. These hydrophobic filters allow gases and vapors to pass through the filter while liquid water is repelled by the filter.
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.
Hydrophobic membranes are used as filters in healthcare and related industries, for example, as vent filters for intravenous (IV) fluids and other medical devices. In the health industry, the membrane must be sterilized before use. PTFE membranes can be sterilized for these 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 used to generate 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 used for sterilization of materials used in medical and biological devices. PTFE is unstable toward ionizing radiation. Irradiated PTFE membranes have greatly reduced mechanical strength and cannot be used in 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 used to form PTFE membranes create a membrane which has 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 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 very 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. 5,462,5856) 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 air flow 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.
There is a need for an oleophobic and hydrophobic filter which is inexpensive and has high air permeability. Specifically, there is a need for a coating for filter medium substrates that makes the substrate oleophobic and hydrophobic, and for a more cost effective process of making oleophobic filters.
SUMMARY OF THE INVENTION
The present invention provides an oleophobic, hydrophobic, coated filter, including a substrate, the substrate having a pressure of water penetration, the coated filter further including a coating derived from a coating formulation, wherein the coating is permanently crosslinked to the substrate, and wherein the coating formulation includes a vinyl-terminated siloxane polymer, and wherein the coated filter has a pressure of water penetration at least 10 percent greater than the pressure of water penetration of the substrate without the coating. The substrate may include a porous polymeric membrane, a nonwoven material, or a woven material. The substrate may include a polymer such as polysulfone, polyethersulfone, polyarylsulfone, polyvinylidene fluoride, polypropylene, polyethylene, poly(tetrafluoroethylene), poly(tetrafluoroethylene-co-ethylene), nylon, or cellulosic esters. The siloxane polymer may include a vinyldimethyl-terminated siloxane. The coating formulation may further include a crosslinker, such as, for example, methylhydro,cyanopropylmethylsiloxane; methylhydro,phenylmethylsiloxane; methylhydro,methyl-octylsiloxane; methyltriacetoxy silane; or methyl silicone. The coating formulation may further include a crosslinker catalyst. The filter of the invention may also be bonded to a fabric.
In another aspect of the invention, there is provided a method of producing a hydrophobic, oleophobic filter, including the steps of providing a substrate having a first pressure of water penetration; contacting the substrate with a coating formulation including a vinyl-terminated siloxane polymer to produce a coated filter; crosslinking the coating formulation to the filter; and recovering an oleophobic, hydrophobic, permanently coated filter having a second pressure of water penetration, wherein the second pressure of water penetration is at least 10 percent greater than the first pressure of water penetration. In this method, the substrate may include a porous polymeric membrane, a nonwoven material, or a woven material. The substrate may include a polymer such as polysulfone, polyethersulfone, polyarylsulfone, polyvinylidene fluoride, polypropylene, polyethylene, poly(tetrafluoroethylene), poly(tetrafluoroethylene-co-ethylene), nylon, or cellulosic esters. The siloxane polymer may include a vinyldimethyl-terminated siloxane. The coating formulation may further include a crosslinker, such as, for example, methylhydro,cyanopropylmethylsiloxane; methylhydro,phenylmethylsiloxane; methylhydro,methyl-octylsiloxane; methyltriacetoxy silane; or methyl silicone. The coating formulation may further in

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