Patent Application
20070272606
In the present invention, a microporous coated substrate is produced wherein the coating contains two or more functionalities. Specifically, the multi-functional coating includes at least two additives and each additive provides at least one functionality. In one embodiment, the first additive may form a film around, encompass or otherwise bind the second additive to the microporous structure. The first additive provides some functionality to the composite, such as hydrophobicity, hydrophilicity, etc., and the second additive provides some further functionality to the composite. In another aspect of this invention, the multi-functional coating combines a first functional additive such as but not limited to surface energy modifying (e.g., lowering, etc.) polymeric binders with additional functional additives wherein the resulting multi-functional coating exists on the pore surfaces of the microporous substrate. As used herein, the term “functional additive” is intended to refer to any additional material which renders further functionality to the coated microporous substrate than what otherwise exists in the absence of the functional additive, such as by changing the chemical, physical or mechanical properties of the microporous substrate. One class of polymeric binders of this invention are fluoropolymer binders since they can be used to alter the surface energy or wetting characteristics of the otherwise inherently hydrophobic expanded PTFE substrate. Surprisingly, additional functional additives can be included in the polymeric binder mix so that the resultant coated microporous substrate exhibits both a surface energy change due to the fluoropolymer binder but also a second functional change due to the additional functional additive. While surface energy altering polymeric binders are of particular interest in this invention, other embodiments encompass incorporating a non-polymeric binder. In such embodiments where a non-polymeric binder is used, the multi-functionality results from the incorporation of more than one functional additive onto the surfaces of the microporous substrate, where at least one of the functional additives also acts as a binder. As used herein, the term “binder” refers to a material which adheres or otherwise attaches to at least a portion of the underlying microporous structure and assists in retention of the second functional component.
The multi-functional coatings of this invention can be applied to the microporous substrate by any means which produces the desired coating uniformity on the surfaces of the microstructure and preferably does not occlude the pores of the microporous substrate. One aspect of this invention dissolves the functional polymeric binder in an organic solvent into which the additional functional additives, such as particles are added. The organic solvent selected must be capable of wetting the microporous substrate surface. This multi-functional-additive, solvent mix is then coated onto the microporous substrate and the solvent vaporized. The functional polymeric binder and the functional additive particles contained therein are deposited onto substrate surface in order to create a desired effect. Examples of functions that can be provided in such a multi-functional coating include, but are not limited to, color change in the case of a pigment, or hydrophilicity change in the case of pH sensitive materials, and infrared reflectivity changes in the case of infrared absorbing materials. Carbon particles are of particular interest in applications where a change in an electromagnetic spectral response or electric or thermal conductivity of the substrate is desired.
A critical aspect of this invention is that the resultant microporous substrate must exhibit multiple functionalities such as that of the surface energy altering polymeric binder and a change in spectral, electromagnetic, or thermal response.
One important aspect of this invention is that the coating mix must be able to wet the substrate to which it is applied. In the case of microporous PTFE, the coating mix typically should have a surface tension around 30 dynes/cm or less.
In another embodiment of this invention, a multi-functional coating can be produced using an aqueous wetting system. In this embodiment, a polymeric binder and a second functional additive are stabilized in an aqueous wetting system which is then coated onto the desired microporous substrate. An aqueous wetting system containing a water insoluble alcohol (e.g., C5-C10 linear backbone) emulsified by a surfactant is one suitable system. The surfactant or surfactants can be selected to be compatible with the additives and any surfactants that may be used to stabilize the additives. The water is vaporized and the multi-functional coating is produced on the surfaces of the microporous substrate. Suitable functional additives include materials which have suitable stability to be delivered and which are either soluble in the wetting system (either the water or wetting agent) or dispersible in the wetting system. In one exemplary embodiment of the invention, if the substrate is a polymer layer that is not naturally oleophobic, it can be rendered oleophobic by incorporating within the aqueous delivery system a functional additive which is an oleophobic material. This unique feature of the invention provides significant advantages over conventional coated materials, in that the present invention can be tailored to readily facilitate coating at least a portion of the pore walls of the substrate.
In one embodiment of this invention, suitable microporous materials can include fluoropolymers, e.g. polytetrafluoroethylene or polyvinylidene fluorides, polyolefins, e.g. polyethylene or polypropylene; polyamides; polyesters; polysulfone, poly(ethersulfone) and combinations thereof, polycarbonate, polyurethanes. In instances where retention of air permeability or high breathability is desired, the present invention should be designed to preserve the open microporous structure, as filling the micropores may destroy or severely lessen the water-vapor transmitting property of the microporous substrate. Thus, the walls defining the voids in the microporous polymer preferably have only a very thin coating in such an embodiment. Moreover, to maintain flexibility of the substrate, the coating of the functional material should be sufficiently thin to not impact the flexibility of the substrate when coated.
Common oleophobic functional additive compositions suitable for this invention include oleophobic fluorocarbon compounds. For example, the fluorocarbon can be one that contains perfluoroalkyl groups CF3—(CF2)n—, where n is ≧0. The following compounds or classes of oleophobic materials, while not exhaustive, can be used:
The following polymeric particle solutions, while again not exhaustive, can also be used:
The concentration of the functional material provided by this invention can vary greatly depending on the desired outcome. When an oleophobic fluoropolymer is used as the functional additive material, such as but not limited to, polymers having —(CF2)n—CF3 pendant groups, functional materials of this type can impart very low surface energy values to the polymer and thus impart good oil and water resistance properties. Representative oleophobic polymers can be made from organic monomers having pendant perfluoroalkyl groups. These include fluoroalkyl acrylates and fluoroalkyl methacrylates having terminal perfluoroalkyl groups of the formula:
wherein n is a cardinal number of 1-21, m is a cardinal number of 1-10, and R is H or CH3; fluoroalkyl aryl urethanes, fluoroalkyl allyl urethanes, fluoroalkyl urethane acrylates; fluoroalkyl acrylamides; fluoroalkyl sulfonamide acrylates and the like. When a low surface energy coating is desired, concentrations from about 1% wt. up to about 30% wt. solids of oleophobic material based on total coating mix weight may be effective. When coating microporous substrates, the concentration of the oleophobic functional material preferably is between about 3% wt. up to about 12% wt. based on total coating mix weight.
Alternate embodiments of this invention include other functional additive materials. The present invention can be used to deliver particulate functional materials to surfaces, provided that the particulate can be dispersed in the wetting system. In some instances, it may be advantageous to disperse the particulates in a dispersing agent which can subsequently be dispersed in the wetting system. In applications involving particulates, such as carbon, concentrations ranging from about 0.1% wt. up to about 5% wt. based on total mix weight are often appropriate to impart the desired functional effect of this additional functional additive.
The optional functional material of the present invention may also be materials that are either soluble in the inventive aqueous wetting system or dispersible in the inventive aqueous wetting system. The list of soluble materials that can be used in conjunction with the present invention include but are not limited to polyacrylic acid, polyacrylamide, melamine, polyvinyl alcohol, salts, and dyes. The list of dispersible materials that can be used in conjunction with the present invention include but are not limited to fluoroacrylates, polystyrene, pigments, carbon black, and aluminum oxide. One requirement for these dispersible materials is that the particle size be sufficiently small so that then can physically enter the pores of the microporous substrate to which they are being applied. When the microporous substrate is inherently hydrophobic, such a coating can change the surface characteristic from hydrophobic to hydrophilic.
In additional embodiments of this invention, a wide range of functionalities can be incorporated into the low surface energy microporous substrate. Some examples include but are not limited to functional additives that can be provided to change water absorption, hydrophobicity, oleophobicity, light shielding, color change, flame resistance and fire retardancy, antimicrobial, antistatic, elasticity, infrared and near infrared absorption, UV absorption, catalysts, photo catalysts, biocompatibility, and controlled release of therapeutic agents. One primary aspect of this invention is the ability to provide two or more of these functionalities in the coating system.
In order to achieve some of the above embodiments, the functional additives that can be incorporated into these inventive constructs include, but are not limited to, carbon, metals, metal oxides (e.g., TiO2), fluoropolymers, acrylates, polyacrylic acid, heparin, permetherin, cerium oxide, benzophenone, nanoparticles, carbon nanotubes, quantum dots, cadmium selenium, lead selenium, dyes and pigments, antimony doped indium tin oxide.
Other useful permutations of this invention are also encompassed within the breadth of coated microporous substrates having a single coating with multiple functionalities provided to the substrate via the single coating. Additional advantages of this invention are that it can be used to provide both acidic and basic reactivities onto a single microporous substrate. In addition, because the addition of some particles may reduce the water entry pressure of a microporous substrate, this invention provides a way to maintain or even enhance the waterproofness of a microporous substrate while also providing the functionality of an additional functional additive or particulate. Additional functional additives of interest include but are not limited to metals, e.g. silver, metal carbonates, e.g. copper or zinc carbonates, metal oxides, e.g. cuprous oxide or molybdenum oxide, or organic materials such as triethylenediamine.
For the purposes of this application the following terms shall be recognized to have the meaning set forth below unless otherwise indicated:
“Air permeable” means that airflow is observed as determined by the Gurley test described below. It will be appreciated by one of skill in the art that an air permeable material will also be moisture vapor permeable.
“Air-impermeable” means that no airflow is observed for at least two minutes as determined by the Gurley test described below.
“Hydrophilic” material denotes a porous material whose pores become filled with liquid water when subjected to liquid water at a pressure of less than or equal to 1.0 psi.
“Microporous” term is used to denote a continuous layer of material comprised of interconnecting pores which create a passageway extending from one surface of the layer to the opposite surface of the layer.
“Oleophobic” means a material that has an oil rating of 1 or more, as measured by the Oil Repellency Test.
“Coating” refers to the presence of the material on at least a portion of a substrate.
“Conformal coating” refers to a coating which matches or follows the topography of the underlying substrate.
The resistance of samples to air flow was measured by a Gurley densometer (ASTM) D726-58) manufactured by W. & L. E. Gurley & Sons. The results are reported in terms of Gurley Number, which is the time in seconds for 100 cubic centimeters of air to pass through 6.54 cm2 of a test sample at a pressure drop of 1.215 kN/m2 of water. A material is air-impermeable if no air passage is observed over a 120 second interval.
In these tests, oil rating was measured using the AATCC Test Method 118-1983 when testing film composites. The oil rating of a film composite is the lower of the two ratings obtained when testing the two sides of the composite. The higher the number, the better the oil repellency. A value of greater than 1, preferably 2 or more, more preferably 4 or more, is preferred.
The test is modified as follows when testing laminates of the film composite with a textile. Three drops of the test oil are placed on the textile surface. A glass plate is placed directly on top of the oil drops. After 3 minutes, the reverse side of the laminate is inspected for a change in appearance indicating penetration or staining by the test oil. The oil rating of the laminate corresponds to the highest number oil that does not wet through the laminate or cause visible staining from the reverse side of oil exposure. The higher the number, the better the oil repellency. A value of greater than 1, preferably 2 or more, more preferably 4 or more, and most preferably, 6 or more, is preferred.
Spectral reflectance data is determined on the technical face of the sample (i.e., the camouflage printed side of the textile, laminate, or composite) and is obtained from 400 to 1100 nanometers (nm) at 20 nm intervals on a spectrophotometer (Data Color CS-5) (capable of measuring reflectance at wavelengths of 400-1100 nm or greater) relative to a barium sulfate standard. The spectral bandwidth is set at less than 26 nm at 860 nm. Reflectance measurements are made with the monochromatic mode of operation.
The samples were measured as a single layer, backed with six layers of the same fabric and shade. Measurements were taken on a minimum of two different areas and the data averaged. The measured areas were chosen to be at least 6-inches away from the selvage (edge). The specimen was viewed at an angle no greater than 10 degrees from the normal, with the specular component included.
Instrument: Photometric accuracy of the spectrophotometer shall be within 1 percent and wavelength accuracy within 2 nm. The standard aperture size used in the color measurement device shall be 1.0 to 1.25 inches in diameter for Woodland and Desert camouflage and 0.3725 inches in diameter for the Universal camouflage, MARPAT Woodland and MARPAT Desert. Any color having spectral reflectance values falling outside the limits at four or more of the wavelengths specified in MIL-DTL-31011A, MIL-DTL-31011B, or MIL-PRF-32142 shall be considered a test failure.
Results are reported in terms of average reflectance for a particular wavelength range, unless otherwise specifically noted.
Example 1 was run to demonstrate that a low surface energy microporous substrate could be produced that was oleophobic, air permeable, and near infrared suppressive. A microporous ePTFE membrane measuring 0.001 inch thick (0.2 μm nominal pore size, mass of 20 g/m2, obtained from W. L. Gore & Associates, Inc.) was coated with carbon black (Vulcan XC72, Cabot Corporation, Boston, Mass.) using a fluorocarbon polymer binder and wetting agents. The binder system was formulated by mixing 2.6 g of Witcolate ES2 (30% solution) (obtained from Witco Chemicals/Crompton Corporation, Middlebury, CT), 1.2 g of 1-Hexanol (Sigma-Aldrich Chemical Corporation, St. Louis, Mo.), and 3.0 g of fluoropolymer (AG8025, Asahi Glass, Japan) in 13.2 g of deionized water. 0.015 g of carbon black was added to the binder system. The mixture was sonicated for 1 minute. The membrane was hand coated with the mixture using a roller to a coating weight of approximately 3 g/m2. The coated membrane was cured at 190° C. for 2.5 minutes.
The resultant microporous expanded PTFE substrate had a Gurley number of about 29 seconds to about 49 seconds and a moisture vapor transmission rate of about 45,942 g/m2 (24 hours). The oil rating was 8. And this example gave an average reflection in the near infrared wavelength range of 720 nm to 1100 nm that was substantially reduced compared to the single functionality, control sample which was only oleophobic. Clearly, this example shows how a multi-functional coating system can be used to produce an air-permeable, microporous substrate having multiple functionalities.
Comparative A was produced similarly to Example 1 with the exception that no carbon was included in the fluorocarbon polymer binder and wetting agents. Average reflectance of the constructions was measured in the 720-1100 nm wavelength ranges. Results are reported as “Comparative A” in Table 1.
To demonstrate the present invention is capable of providing both a different aesthetic color and oleophobicity (Example 2), the following composition was prepared (1.5 g of Witcolate ES-2 (Witco Chemical Co.), 0.6 g of 1-hexanol, 6.7 g of de-ionized water, 1.5 g of AG8025 (Asahi Glass Co. Ltd.), and 0.2 g of blue dye (Techtilon Blue). These ingredients were added in the order of: water, surfactant, and alcohol. To this aqueous system, the remaining functional additives were added. This aqueous system was mixed by shaking for approximately one minute at ambient conditions. This aqueous mixture was applied to one side of a 20 g/m2 ePTFE membrane. The aqueous mixture wet the ePTFE substrate within a second. The coated membrane was cured in a lab oven at 190° C. for 2 min. An air permeable blue colored oleophobic membrane resulted. Air permeability was confirmed by the coated membrane having a Gurley measurement of about 28 seconds. The oil rating of the coated membrane was 8.
To demonstrate that a pH indicating coating and pH switchable hydrophilicity could be imparted to a microporous substrate. A mix of the following composition was prepared (1.3 g of Witcolate ES-2 (Witco Chemical Co.), 0.6 g of 1-hexanol, 6.3 g of de-ionized water, 2.0 g of polyacrylic acid (Aldrich Chemical, 19203-1), and 0.3 g of bromophenol blue. This mix immediately wet a 20 g/m2 ePTFE membrane. The coated membrane was cured in a lab oven at 190 C for 2 min. An air permeable yellow colored membrane resulted. When exposed to a water droplet, this membrane remained yellow and did not wet even under light finger pressure. When exposed to a NaOH solution (˜pH 10), the color of the membrane changed from yellow to blue and the membrane was penetrated by the dilute NaOH solution under light finger pressure. This membrane has hydrophilic properties towards basic solution and has a pH indicator to show the presence of a basic solution.
This example provided the additional functional element of light blocking. When the uncoated membrane was placed on a page of writing under normal indoor lighting conditions, the writing was visible. When this multi-functional coated membrane was placed on the same page of writing under the same lighting conditions, the writing was not visible.
