Patent Application
20080064279
The conductive articles of the invention contain a densified core material generally made of a nonwoven or woven fabric. The conductive articles additionally contain an effective amount of at least one electromagnetically conductive material. The electromagnetically conductive material may include one or more electromagnetically conductive organic or inorganic particulate materials, including metals such as copper or nickel, or organic particulates such as carbon black. The fabric, which preferably is made in a flexible sheet-like form, may optionally include an adhesive on one or more of its surfaces. The adhesive may include an additional amount of one or more electromagnetically conductive materials. The article may include a seal coat opposite the surface or side on which an adhesive layer is placed. Alternatively, the article may include a seal coat applied to each side of the densified fabric. The article may also include a release layer or liner adjacent the adhesive.
The densified core materials of the invention can include any woven or nonwoven fabric or fabric-like material that includes a degree of interstitial separation or space within the fibers or threads making up the fabric-like material. Although webs or sheets of natural or synthetic woven fibers or threads are useful in the articles of the invention nonwoven materials will generally be preferred because of their relative cost and ease of manufacture.
Fibers having a diameter of about 100 microns (μm) or less, and particularly so-called “microfibers” having a diameter of no greater than about 50 μm, are useful in the manufacture of nonwoven web-based materials. These fibers and microfibers are typically used in the form of nonwoven webs that can be used in the manufacture of a wide variety of products, including face masks and respirators, air filters, vacuum bags, oil and chemical spill sorbents, thermal insulation, first aid dressings, medical wraps, surgical drapes, disposable diapers, wipe materials and the like. Nonwoven webs of fibers are particularly desirable because they provide a material with a high surface area and generally have a high degree of porosity.
The fibers can be made by a variety of melt processes, including by known spunbond and melt-blown processes. In a spunbond process, fibers are extruded from a polymer melt stream through multiple banks of spinnerets onto a rapidly moving, porous belt thereby generally forming an unbonded web. This unbonded web is then passed through a bonder (typically a thermal bonder) that bonds some of the fibers to adjacent fibers and provides integrity to the web. In a typical melt-blown process, fibers are extruded through fine orifices using high velocity air attenuation onto a rotating drum to form an autogeneously bonded web. In contrast to a typical spunbond process, a melt-blown process generally requires no further processing. Both of these processes are detailed in a variety of publications, including by Wente in “Superfine Thermoplastic Fibers,” Industrial Engineering Chemistry, vol. 48, pp. 1342 et seq. (1956).
Any material capable of forming a fiber by melt processing, including in the processes described immediately above may be employed in making a suitable nonwoven material. Useful, generally preferred exemplary polymeric materials include polyesters such as polyethylene terephthalate; polyalkylenes such as polyethylene or polypropylene; polyamides such as nylon 6; polystyrenes; and polyarylsulfones. Also useful are slightly elastomeric materials including olefinic elastomeric materials such as some ethylene/propylene or ethylene/propylene/diene elastomeric copolymers and other ethylenic copolymers such as ethylene vinyl acetates.
The woven or nonwoven core material is densified prior to its incorporation into the finished articles of the invention. Densification refers to any process by which the interstitial area or space in the woven or nonwoven material is reduced by the application of pressure, or by the application or removal of heat, or by both the application of pressure and the application or removal of heat, or by any other method of reducing interstices in the woven or nonwoven material. Densification may be accomplished, for example, by standard calendering processes whereby a web of the core material is passed through a pair or a series of rollers which are held under pressure. The roller may be either heated or cooled. The core material may also be pressed by the application of heated or cooled plates such as with the use of a Flatten Press.
Densification, once achieved, may be evidenced in any one or more of several ways, including by one or more of the following: a reduction in the thickness of the article, an increase in the density of the article, a reduction in air permeability, a reduction in porosity or a change in the surface resistivity of the core material. Importantly, no absolute threshold can be defined for the thickness, density, permeability, porosity or surface resistivity of the core materials before and after densification. Because the invention provides a relative increase in the performance of electromagnetically conductive articles, the core materials of the articles of the invention will generally exhibit a relative reduction in one or more of its cross-sectional thickness, air permeability, porosity or surface resistivity or an increase in its density after densification. This change provides for the ability, once constructed, for the articles to exhibit the same or even improved electromagnetic radiation shielding properties compared with articles constructed of non-densified materials.
By way of example, a typical thickness of the woven or nonwoven core material can range from about 1 to about 10 mil, more typically from about 3 to 8 mil. Generally, depending on the material chosen for the woven or nonwoven core, the core will be calendered, pressed or otherwise processed (i.e., densified) to reduce its thickness by about 10 to 80 percent, more preferably from about 25 to 60 percent. When so densified, the air permeability of the core material (and/or an article made of the material) will generally be reduced. Typically, the air permeability of the woven or nonwoven core material measured along a plane dissecting the material through its smallest cross-sectional dimension will be no greater than about 0.5 m3/min, preferably no greater than about 0.25 m3/min and more preferably no greater than about 0.2 m3/min.
The conductive articles of the invention also include one or more electromagnetically conductive organic or inorganic particulate materials disposed on or within the densified core woven or nonwoven material. Useful electromagnetically conductive particulates include: noble metals; non-noble metals; noble metal-plated noble or non-noble metals; non-noble metal-plated noble or non-noble metals; noble or non-noble metal plated non-metals; conductive non-metals; conductive polymers; and mixtures thereof. More particularly, the conductive particulates may include noble metals such as gold, silver, platinum; non-noble metals such as nickel, copper, tin, aluminum, and nickel; noble metal-plated noble or non-noble metals such as silver-plated copper, nickel, aluminum, tin, or gold; non-noble metal-plated noble and non-noble metals such as nickel-plated copper or silver; noble or non-noble metal plated non-metals such as silver or nickel-plated graphite, glass, ceramics, plastics, elastomers, or mica; conductive non-metals such as carbon black or carbon fiber; conductive polymers such as polyacetylene, polyaniline, polypyrrole, polythiophene, poly sulfurnitride, poly(p-phenylene), poly(phenylene sulfide) or poly(p-phenylenevinylene); and mixtures thereof. Generally preferred will be those noble and non-noble metals (and mixtures of such metals) that exhibit conductivity to electromagnetic radiation across a wide frequency spectrum. Because of their relative abundance, specific preferred metals include silver, nickel and copper and mixtures thereof.
The electromagnetically conductive material (or mixture of materials) may be applied to the woven or nonwoven core material by coating or plating (electro- or chemically) an effective amount of the conductive material onto the core material. The conductive material may be applied to the core material before or after densification. Any amount of conductive material may be employed that provides a desired amount of shielding property, and this amount will necessarily vary based on the chosen electromagnetically conductive material and on the application to which the article will be employed. Where the chosen electromagnetically conductive material is a metal, exemplary application of the metal to the core material can range from 5 to 100 g/m2, from 10 to 80 g/m2 or from 20 to 50 g/m2.
The articles of the invention can include an adhesive layer on at least a portion of one exterior surface of the woven or nonwoven core material or layer. Where the core material is in the form of a substantially flat web or sheet, an adhesive layer can be placed on at least a portion of one or both of the top and bottom surfaces. Any suitable adhesive may be employed for this purpose, and the type or composition of the adhesive will be chosen to be compatible with the substrate onto which the article will be adhered. Generally, when the articles are to be used for the protection of electronic components, a suitable electronics grade adhesive will be selected. Any among numerous known pressure sensitive adhesives (or “PSAs”) may be used, including natural or synthetic tackified rubber PSAs, repositionable PSAs or acrylic-based PSAs. Generally preferred will be acrylic-based adhesives and specifically those containing at least fifty percent by weight or more acrylate functionality. One suitable acrylic-based adhesive is disclosed in U.S. Pat. No. Re 24,906 which describes a 95.4/4.5 weight percent isooctyl acrylate/acrylic acid copolymer pressure sensitive adhesive. Also useful are photopolymerizable acrylic-based adhesives. The selected adhesive composition may be applied to one or more surfaces of the woven or nonwoven core material by any suitable known method, including by solvent or holt melt coating or processing techniques.
The adhesive composition may also be formulated to contain one or more electromagnetically conductive materials. When added to the adhesive, such materials can aid in further enhancing the shielding or protective properties of the article. The electromagnetically conductive material chosen for incorporation into the adhesive may be the same or may be different from that chosen to be used with the densified core material. Generally, when present, the conductive material will be added to the adhesive to constitute between 0 and 75 percent by weight of the adhesive composition, preferably from 10 to 50 weight percent. When the electromagnetically conductive article is made in the form of an adhesive tape, a release liner may also be applied to the outer surface of the adhesive. The adhesive composition may also include other functional components or additives such as one or more corrosion inhibitors or one or more corrosion resistance additives.
A seal or top coating may optionally be applied to the outer surface of the electromagnetically conductive article. This coating can be used to protect the woven or nonwoven core material and seal or help secure the conductive material within the article. Any material that may be used to seal the core material may be used as a top or seal coat. One such useful material is a vinyl polymer and specifically a clear or substantially clear vinyl acetate-vinyl alcohol-vinyl chloride copolymer. The seal or top coat may be coated onto the core substrate to any desired weight, but will generally be applied in an amount sufficient to fill or substantially fill the surface voids in the core material to provide a substantially smooth surface. As with the adhesive, the seal or top coat can also be formulated to include an additional amount of one or more electromagnetically conductive materials. When added to the top coat (as when added to the adhesive) such materials can aid in further enhancing the shielding or protective properties of the article. The electromagnetically conductive material chosen for incorporation into the top coat may be the same or may be different from that chosen to be used with the densified core material and/or the adhesive. Generally, when present, the conductive material will be added to the adhesive to constitute between 0 and 75 percent by weight of the coating composition, more preferably from 10 to 50 weight percent.
Any number of conventional or optional additives or adjuvants may be added to one or more of the layers or components of the electromagnetically conductive articles of the invention. Anti-oxidants, ultraviolet stabilizers, and/or corrosion inhibitors may, for example, be added to the adhesive or seal coat (or both) to provide protection for the electromagnetically conductive articles. Other functional or nonfunctional additives or adjuvants may similarly be added.
The articles of the invention can be used in any application where an electromagnetic shield is desired. The articles, for example, can be formed into tapes and used for shielding applications relating to electronic devices, circuits, RFID devices such as RFID tags, or other devices benefiting from electromagnetic shielding. The articles may also be used to contain, block or mask radiation emitted from the devices or components which they might be used to shield. When used in the application of shielding a device, the electromagnetically conductive article or densified core material thereof should be positioned in close proximity to the device, such as, for example, within 25 mm from the device, and preferably less than 5 mm from the device.
By employing a densified woven or nonwoven core material, the articles of the invention provide several potential advantages. By providing for a more efficient and concentrated use of one or more electromagnetically conductive materials within the densified interstitial area of the woven or nonwoven core substrate material, the articles provide for a greater shielding effectiveness per unit volume of the article. This provides an ability for the construction of thinner shielding articles that possess equivalent or improved shielding properties compared with articles that employ nondensified core substrate materials. The articles of the invention also generally provide improved surface resistivities and reduced physical and/or electrical permeabilities (i.e., reduced current leakage, improved electrical conduit properties and improved electrical sealing properties). The densified core materials can provide more consistent cross-sectional dimensions (e.g., thicknesses) and provide enhanced adhesion to substrates to which they may be attached. A reduction in porosity and/or permeability of the core materials also allows for more efficient use of adhesive and top coat materials. Encapsulation of the electromagnetically conductive materials within the densified core materials reduces corrosion and aids in the prevention of other deleterious effects of moisture and humidity. The densified materials are also less susceptible to physical abrasion and fraying, provide for the more effective addition of pigments and other additives and provide a greater degree of durability.
Five product samples were prepared for testing and evaluation as provided in Table 1 below:
The 6.0 mil uncalendered core material sample and the 4.0 mil calendered core material sample (Sample Nos. 2 and 5 respectively) were prepared by plating the core material with copper and nickel metals on a polyethylene terephthalate (PET) fabric. The 6.0 mil uncalendered product sample, 4.0 mil uncalendered product sample and 4.0 mil calendered product sample (Sample Nos. 1, 3 and 4 respectively) were prepared by first plating copper and nickel metals on PET fabric. For these samples (Sample Nos. 1, 3, and 4 respectively), an acrylic adhesive loaded with nickel particles was subsequently laminated to one side of the PET fabric and a seal coat consisting of a vinyl binder and silver was laminated to the other side of the PET fabric.
The graphs of
Each of the Samples were evaluated for shielding effectiveness according to ASTM D4935-99 using a Hewlett-Packard™ 8510 Network Analyzer and Transverse Electromagnetic (TEM) cell. The graph shown in
Surface resistivity measurements were performed on the Samples according to ASTM F43 using a Delcom™ 717 eddy current detection system and/or a four-point measurement system. The results are shown in Table 3 and in
Air permeability measurements were performed on the Samples using a Frazier™ 2000 Differential Pressure Air Permeability Tester. The results are shown below in Tables 2 and 3 and in
Each of the Samples were tested for taber abrasion using a Teledyne™ Model 503 abrasion tester was used with CS-5 felt wheels. Prior to testing, each Sample was weighed and measured for initial resistance. The Samples were weighed again after the completion of 1000 and 2000 cycles to determine weight loss and measured for resistance after the completion of 100, 200, 400, 1000, and 2000 cycles. The results are shown in
This application claims the benefit of U.S. Provisional Patent Application No. 60/825,216, filed Sep. 11, 2006, the disclosure of which is incorporated by reference herein in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 60825216 | Sep 2006 | US |
