CONDUCTIVE FLUOROPOLYMER COATED ELASTOMERIC ROLLERS AND STRUCTURES

Abstract: The present invention is an improved release coating for use on a variety of elastomeric structures, such as printer rollers and belts. The coating of the present invention is a thin expanded polytetrafluoroethylene (PTFE) skin (18) that has a porous surface (20) attached to a substratum (16) and an impermeable surface (22) exposed to the contact surface, in which the PTFE is filled with a particulate filter. This use of dual surfaces on a single thin coating provides both an excellent bond to substratum materials, such as silicone elastomer layer, as well as being highly wear resistant to chemical attack.

Description: TITLE OF THE INVENTION CONDUCTIVE FLUOROPOLYMER COATED ELASTOMERIC ROLLERS AND STRUCTURES CROSS-REFERENCE TO RELATED APPLICATIONS This application is based on Provisional Application Serial No 60/003724 BACKGROUND OF THE INVENTION 1 Field of the Invention The present invention relates to release coatings used in a variety of applications where elastomeric structures are employed such as elastomeπc rollers used in xerographic reproduction technologies (e g , copiers telefaxes laser printers, and other printing devices) and other paper handling applications 2 Description of Related Art Resilient rollers are presently used for a variety of purposes In the area of xerographic and offset printers (hereinafter referred to collectively as "printer devices"), such rollers play an integral role in the proper operation of these devices Generally, it has been desirable to have a roller that possesses a number of properties, including good conformance (i e , good grip around a three-dimensional object, such as a sheet of paper), good chemical resistance good durability (i e , resistance to abrasion and wear), low surface energy or good release properties, and good electrical and or thermal conductivity. Prior attempts to provide a balance of these properties have not been entirely successful The ability to effectively combine these properties is further complicated by the fact that many pπnting and paper handling industries are highly cost sensitive Traditional rollers have generally comprised a rubber-like material such as silicone, that is coated over a solid core, such as metal For most applications, this construction works well at least initially, but tends to display a number of problems over time First, silicone material generally has limited wear resistance, eventually resulting in worn and uneven surfaces Second silicone has only limited release properties, resulting in dirt and contamination problems that can ruin image quality Wear of the silicone material tends to only worsen tne release properties of this material Third, the silicone material often will become deformed on the surface due to shear stress leading to wrinkles. 1ιstortιon, or cracking of the resilient surface and imperfections that can rum printer performance Furthermore, silicone material will swell and degrade over time, especially when it is exposed to certain solvents oils and vapors In addition, the thermal and electrical conductivity of silicone is limited by the amount of thermally and electrically conductive filler that can be added Finally, the addition of high levels of fillers reduces the mechanical integrity of the silicone The filler will shed and abrade from the surface A number of solutions have been proposed to try to improve some of these problems For example, in United States Patent 3,345,942. to Meltz, fluoropolymer particles were used to fill a rubber roller coating This approach is believed to provide some improvement in release, but chemical resistance of the roller is still limited due to the exposed rubber Further, effective surface energy is shared between the rubber and the fluoropolymer, thus reducing release properties relative to the use of fluoropolymers alone Another approach has been to spray on a top coat of fluoropolymer particulate, as is disclosed in United States Patent 4,789,565 to Kon et al , and Japanese Laid-Open Patent Application JP 4-361026 to Suzuki et al In these instances, many thin layers of fluoropolymer are built up on the roller to prevent the coating from cracking While this approach may present a better release coated surface, it tends to be very complex, requiring tremendous effort to prevent the rubber intermediate layer from getting damaged due to the thermal cycling required to cure or sinter the PTFE particles to form a continuous PTFE layer Further, a number of other processing problems can result, including being prone to contamination-related defects and a significant risk of pinholes developing during processing Still another approach is to employ a sleeve of fluoropolymer material over the roller surface Examples of this approach are described in United States Patents 5,180,899 to Inasaki, 4,219,327 to Idstein, and 3.912,901 to Strella et al This approach does protect the intermediate resilient layer from chemical attack and thermal degradation, but it seriously diminishes the conformability of the rollers. Since fluoropolymers are relatively hard when compared to rubbers, the final effective durometer reading of such materials will increase significantly when coated in this manner This causes problems with conformance to products being processed, such as conformance to paper passing between a fuser roll and a sleeved pressure roller Such lack of conformance may result in improper fusing of toner and poor durability of the pπnt. In addition, delivery of release agents to the rollers of this construction can be uneven on the paper edges, causing poor uniformity and buildup Another problem with fluoropolymer sleeves is that they hav-- a great chance of de-bonding due to the shear stresses created by the compression and extension of the interface between fluoropolymer film and rubber Other deficiencies of fluoropolymer tubes are their lack of reliable concentricity and their inability to be produced consistently below 0.002" (0.05 mm) wall thickness. A good example of the problem of de-bonding is illustrated in United States Patent 5, 180,899 to Inasaki. This patent discloses a multiple layered roller structure having a sponge layer coated with a silicone rubber layer that is then coated with a fluoropolymer tube layer. The combination of the properties of these various layers is said to improve conformability over silicone material alone. The fluoropolymer (e.g., full density polytetrafluoroethylene (PTFE)) tube layer is reported to contribute better aging characteristics and is less likely to collect soil or become deformed on its surface. Despite these improvements, this structure is still limited in a number of respects. For example, bonding between the PTFE tube and the substrate may be difficult. A full density PTFE material generally will not readily adhere to other materials without etching or similar treatment. Even with such treatments, the bond between PTFE and other materials tends to be somewhat tenuous. This can result in shifting of the tube layer and wrinkles and other problems. Further, full density PTFE has generally poor tensile strength and creep characteristics, again leading to shifting and its resulting distorted surface problems. Finally, constructions requiring many layers of material, particularly those employing one or more tubes of materials, are generally difficult to assemble and may be more prone to separation or distortion during use. The use of an extra intermediate sponge layer is stated to reduce stresses between the layers during use. Unfortunately, this approach is believed to be less than a fully satisfactory solution, with conformance characteπstics still limited and the complexity of manufacture significantly increased. Fluoropolymer elastomer rubber coatings are yet another technology path used to attempt to solve the above mentioned problems. Examples of these are disclosed in United States Patents 5,061 ,965 to Ferguson et al. and 4,430,406 to Newkirk et al. Again the problem of complexity in manufacture and poor release of aggressive chemicals has prevented this technology from meeting all the needs required by printer manufacturers. United States Patent 5,061 ,965 attempts to address some of these concerns by applying still an additional top coat of silicone over the fluoroelastomer to prevent buildup of contamination Again this increases the complexity of this device and is believed to further limit conformability Still another approach has been to provide expanded polytetrafluoroethylene (ePTFE) reinforcement of the silicone material Examples of this approach are disclosed in United States Patents 5 123 151 to Uehara et al . 4 887 340 to Kato et al and 3 345 942 to Meltz and Japanese Laid-Open Patent Application 5-134574 assigned to Sumitomo Electric Industries, Ltd All of these references teach that ePTFE membranes can help in extending the durability of the rubber They are all deficient in that they attempt to solve the problems with silicone material by embedding ePTFE mateπal. leaving an exposed layer of silicone only on the surface of the roller For example, Japanese Laid-Open Patent Application No 5-134574 to Sumitomo discloses a fixing roller for use in a printer that comprises an exposed elastic layer reinforced with a substrate of a fluororesin material (e g , tetrafluoroethylene resin) The fluororesin material is provided with a roughened surface to improve bonding between the elastic layer and the underlying reinforcement layer This patent application reports that this construction prevents shifting of the resilient layer and reduces the risk of cracking or similar problems As a result, the patent claims longer life and better durability Despite the improvements of this approach, reinforced silicone is not fully satisfactory as a roller surface Since silicone polymer remains as the release surface, this structure continues to be limited by some of the same wear and release deficiencies of conventional silicone material Among these problems are toner build-up, poorer paper release, susceptibility to damage from stripper fingers and the like, and image problems Further, an exposed silicone material continues to be susceptible to swelling and damage from exposure to vaπous contaminates. Another proposed approach has been plasma/monomer treatment of PTFE, as is described in Japanese Laid-Open Patent Application 5-147163 to Ayano, et al This approach attempts to address the issue of poor bond of solid fluoropolymers to rubber Unfortunately, this process is very complex and requires the use of monomers, reactors which require vacuum, and special plasmas for the polymerization to occur Although the bonding issues are improved significantly, the complexity is greatly increased Further, the basic problem of conformance is not addressed here and this will still be a problem in printing or other applications All of the above cited references address only the coating of the roller surface to improve mechanical and release properties None of these references discuss and te cn how to improve the thermal and electrical properties of the elastomeric coatings There nave been a number of attempts to incorporate thermal and electrical conductivity into coatings for roller surfaces, all of which address some of the issues but have limitations One approach to address the thermal and electrical conductivity issues is to add fillers to the rubber or change the chemistry of the rubber mateπal For example it is known in the industry to add iron oxide or other fillers to alter the thermal and electrical properties of silicone rubbers As described in Japanese Laid-Open Patent Application (Kokai) No 6-240145, a semiconductive rubber material may be obtained through covulcanization of a blend of an epichlorohydππ polymer and a fluorosilicone polymer This chemistry will provide a material with electπcal resistivities of 10 8 to 10 12 ohm cm As described, these approaches do improve the thermal and electrical properties, but do not address the release or abrasion issues In addition, release agents will easily diffuse into these rubber material and cause them to swell and crack in fuser applications Another approach as described by Japanese Laid Open Patent Application (Kokai) No. 55-17944 involves coating the roller surface with a fluoropolymer dispersion which contains 5 to 30% of a suitable thermally conductive filler. This method addresses the thermal and release problems discussed, but has some drawbacks Because a dispersion is used to hold the filler, the filler concentration is limited to only 30% This limits the thermal conductivity that may be achieved. In addition, the sintered dispersion coating is very abrasion sensitive, and the thickness is difficult to control Finally, the dispersion coating is more susceptible to pin holes and cracking. Yet another approach as descπbed by Japanese Laid Open Patent Application (Kokai) No. 62-285839 involves coating the surface of the elastomer of a roller with a filled expanded PTFE (ePTFE) membrane which is filled with silicone rubber The expanded PTFE membrane in this Patent is filled with a suitable thermally conductive or electrically conductive filler material. This addresses many of the issues and problems associated with some of the other technologies described above but does however have some drawbacks. Because the ePTFE membrane is filled with a silicone rubber material, the surface will behave as any other silicone rubber material The surface energy and the release characteristics will be those of the silicone rubber The ePTFE membrane will help to prevent the silicone rubber material from swelling in the presence of silicone oil and other fluids but does not prevent the oils and fluids from penetrating This technology is a definite improvement but does not provide the surface release character of fluoropolymers which is needed in many of the printer applications It is the intent of this invention to address all of the above mentioned problems of good conformance good thermal and/or electrical properties or conductivity, excellent durability, excellent resistance to chemicals excellent resistance to the buildup of contamination and simplicity and cost effectiveness of manufacturing Accordingly it is a primary purpose of the present invention to provide an improved resilient surface that has both good release and good wear properties It is another purpose of the present invention to provide an improved release surface that is protected from contamination and degradation to provide long-term resilient properties It is still another purpose of the present invention to provide an improved release surface of PTFE that is firmly attached to substrate material, with vastly diminished risk of separation or surface distortion It is yet another purpose of the present invention to provide an improved release surface that has good thermal conductivity It is another purpose of the present invention to provide an improved release surface that is antistatic or electncally conductive These and other purposes of the present invention will become evident from review of the following specification SUMMARY OF THE INVENTION The present invention is an improved release coating for use on a variety of contact surfaces, and particularly for roller and belt surfaces, such as those found in various printer technologies The release coating of the present invention comprises a thin, expanded, particulate-filled polytetrafluoroethylene (ePTFE) skin coated over a substratum, such as a silicone elastomer coating To establish a good mechanical bond to the substratum, the skin has a porous adherable surface, with adhesive imbibed within the pores of the skin This produces a bond that avoids many of the previous problems of delamination, wrinkles and distortions that previous coated rollers have experienced However, in order to protect the substratum from chemical attack, the skin includes an opposing exposed surface that has been rendered non-porous As a result, the release coating of the present invention is highly resistant to wear and attack Pi 3ferably, the present invention achieves a non-porous exposed surface on the release coating through one of two methods. First, after attachment to a substratum, the exposed surface is subjected to elevated heat and pressure to densify the expanded PTFE into a continuous, impermeable surface. Alternatively, the exposed surface may be coated with a solution of fluoropolymer that completely fills the pores and renders the exposed surface impermeable. The release surface of the present invention is far more compliant and resilient than most previous fluoropolymer coatings. Moreover due to the porous nature of the adherable surface, without etching or other surface treatments, the expanded PTFE forms a very secure bond that is not prone to separation or distortion Finally, since only fluoropolymer material is present on the exposed surface, the material of the present invention is very durable and resistant to wear and attack. DESCRIPTION OF THE DRAWINGS The operation of the present invention should become apparent from the following description when considered in conjunction with the accompanying drawings, in which: Figure 1 is a cross-section view of one embodiment of a pressure roller incorporating a release coating of the present invention. Figure 2 is a cross-section view of a flattened segment of a pressure roller incorporating another embodiment of a release coating of the present invention prior to densification. Figure 3 is a cross-section view of the flattened segment of the pressure roller shown in Figure 2, with the release coating shown rendered non-porous through densification. Figure 4 is a cross-section view of a flattened segment of a pressure roller incorporating another embodiment of a release coating of the present invention prior to final treatment of the exposed surface of the roller. Figure 5 is a cross-section view of the flattened segment of a pressure roller shown in Figure 4, with the release coating shown rendered non-porous. Figure 6 is a cross-section view of a flattened segment of a pressure roller incorporating still another embodiment of a release coating of the present invention prior to densification. Figure 7 is a cross-section view of the flattened segment of a pressure roller shown in Figure 6. with the release coating shown rendered non-porous through densification. Figures 8 and 8a are a cross-section view of a flattened segment of a pressure roller incorporating yet another embodiment of a release coating of the present invention Figure 9 is a schematic representation of one embodiment of apparatus used to produce the release coating of the present invention Figure 10 is a schematic representation of another embodiment of apparatus used to produce the release coating of the present invention. Figure 1 1 is a schematic representation of one fuser and pressure roller configuration used in many xerographic printer devices, an example of one application for a pressure roller incorporating the release coating of the present invention. DETAILED DESCRIPTION OF THE INVENTION The present invention provides an elastomer material which is covered with a particulate filled fluoropolymer, which can be made in any number of different geometries for a variety of purposes, such as belts, rollers, sheets, etc. Although not limited to such application, the composite material of the present invention is particularly suitable for use in various printer technologies where resilient rollers, belts, and the like are employed. The material of the present invention has significantly improved properties, including strength, durability, adhesion between the layers, release, chemical inertness, electrical and thermal conductivity, and manufacturability, over the conventional technologies. Throughout the development of the material of the present invention, several different methods of producing a fluoropolymer covered elastomer were developed, all of which provide the properties claimed by the present invention. The general makeup of the fluoropolymer covered elastomer of the present invention incorporates bonding at least one layer of a porous, expanded polytetrafluoroethylene (ePTFE) material to a substratum layer (e.g., an elastomer) using an adhesive on one "adherable" side of the ePTFE and then rendering a second "exposed" side non-porous. The exposed side may be rendered non-porous through either filling the exposed pores of the material completely with a solution of another fluoropolymer. or using heat and pressure to fuse the exposed microporous fluoropolymer layer together and thereby elimiπating the microporous structure. Further, for some applications, it may be desirable to render the exposed side non-porous by performing first the solution treatment, followed by densification. The effect of this procedure is two fold. First, by providing an adhered side of porous material, a very good mechanical bond can be established between the expanded PTFE material and a substratum This bond is believed to be far superior to previous attempts to chemically bond PTFE to other materials or to chemically etch PTFE material in order to improve chemical bonding. While mechanical bonds are generally much stronger than chemical bonds, mechanical roughening of surfaces is still an extra processing step that may not always be successful at improving adherence. In the present invention, processing is simplified in that etching or other attempts to modify or roughen the surface of the PTFE need not occur. Second, the exDosed side of non-porous PTFE material provides an excellent release surface for various applications. The expanded PTFE material is very strong, has excellent wear properties, and is highly chemically resistant. Since the PTFE material is the only surface that is exposed during operation, it effectively shields underlying layers from chemical attack and wear. Moreover, the nature of the present invention allows it to be a very thin layer of material, (e.g., on the order of less than 0.002 - 0.004 inches (0.05- 0.10 mm)) making it highly compliant. In this manner, the release coating does not seriously diminish resilience and compliance of underlying substrata materials. The expanded polytetrafluoroethylene (ePTFE) of the present invention has a microporous structure composed of polymeric nodes interconnected with fibrils and having particles entrapped therein. The expanded PTFE material is preferably made in accordance with United States Patents 3.953,566 to Gore and 4.985,296 to Mortimer, Jr., both incorporated by reference. The filled ePTFE material may be made in the following manner. First, a dispersion of PTFE material, such as that available from E. I. duPont de Nemours and Company, Wilmington, DE, is mixed with a desired particulate filler in which pnmary particles may be agglomerated and thus vary in size. Mixing can occur through any suitable means, such as through blending in a baffled mixing vessel. Once a slurry is created, the mixture is then dried, such as in a convection oven. The dried material is then cooled in a freezer and screened through a 6.25mm x 6.25mm mesh. The screened material is then lubricated, which assists processing. A sufficient amount of mineral spirits is used to lubricate the material during the subsequent extrusion process The lubricated material is placed into a pelletizer and compressed and then extruded through a ram extruder to form a tape The tape is then c_ tndered to a desired thickness, dπed ^ er heated drying drums and mechanically expanded both longitudinally and transversely Stretching should occur at a temperature of about 180-240°C and at a ratio of 1 1 to 1 up to 100 to 1 or more The preferred amount of expansion for use in the present invention is believed to be about 10 to 60 1 The rate of expansion may be anywhere between 0 10% to 10 000% per second preferred for the present invention being about 50 to 500% per second The filled expanded PTFE membrane can be made in a number of thickness ranging from 0 00025 inches to 0 125 inches (6 μm to 3 mm) with the preferred thickness for the present invention being from 0 00025 to 0 003 inches (6 to 75 um) The expanded PTFE membrane can be made with porosities or void volume ranging from 20% to 98%, with the preferred porosity for the present invention being 50-95% The filler concentration by weight for the present invention depends on the specific properties desired, but generally the concentrations are between 5% and 95%, with the preferred for most applications being between 10 and 50% Useful fillers to impart conductivity include carbon black, graphite, zinc oxide, chromium oxide, and metal oxides in general, chromium or tin nitride, silver and silver compounds, silver coated carbon black, and the like Once the filled expanded PTFE membrane is obtained, it can be bonded to the elastomeric substrate material using a number of different adhesives The choice of adhesive may be application dependent For bonding to a silicone-type elastomer of the kind typically found on printer pressure rollers, suitable adhesives may compnse. without limitation, epoxy, cyanate ester organic, thermoplastic or thermoset, polyolefin. and silicone adhesives For adhesion to silicone rubber elastomers, the preferred adhesive is silicone and most preferably an addition-cure silicone adhesive In addition, to further increase thermal and electrical properties, a filled silicone adhesive may be used The adhesives generally come in flowable fluids, but may also come in the form of sheets or powders The adhesive may be coated onto the elastomer through a number of processes which include, but are not limited to, roll coating, spraying, dip coating, gravure coating, sheet wrapping, and the like The viscosity for a silicone adhesive of the present invention depends on the type of composite that is desired, but is typically from 100 to 1 ,000,000 centipoise and the preferred range is from 10,000 to 50,000 centipoise For use with most printer roller applications of the present invention the preferred silicone adhesive is an addition-cure silicone adhesive with fillers to increase thermal stability, and adhesion promoters An e.. ample of this type of material is a SLA 7401 silicone adhesive available from General Electric Silicones of Waterford. New York This type of silicone adhesive allows for the most control on curing the silicone It is important for the present invention to be able to cure the silicone adhesive at a specific time in the making of the composite of the present invention The voids of the filled expanded PTFE on the exposed surface of the present invention may be rendered non-porous through at least two distinct methods filling or densifying In both instances, the intent is to produce a continuous surface of fluoropolymer on the surface of the composite It is preferred that the porous voids on the exposed side of the expanded PTFE material are densified into a non-porous surface More specifically, the voids of the expanded PTFE are exposed to heat and pressure that fuses the PTFE of the expanded PTFE and eliminates the voids PTFE will melt flow at temperatures ranging from 320 to 550°C, with the preferred range of temperatures being 350 to 450°C The heat may be applied through a number of processes which include, but are not limited to, pressing a roller against a heated roll, passing sheet materials through a heated roll nip roller, hot air stream, ultrasonically heating, or the like The amount of pressure applied may vary depending upon application and the properties desired. For most applications, a pressure of approximately 5 to 500 psi (34 kPA to 3450 kPA) applied for 5 to 600 seconds against the heated surface will provide the required amount of densification As is explained in more detail below, the preferred method of densifying the exposed surface of the release coating for roller applications is to press a roller coated with the release coating of the present invention against a heated metal roller. For producing a sheet, the sheet material may be passed through a heated nip. In both cases, the ePTFE will compress and flow with pressures ranging from 35 kPa (5 psi) up to 13,800 kPa (2000 psi) or more, with the preferred being in the range of 207 to 3445 kPa (30 to 500 psi) The speed at which the composite is passed through the nip or the speed at which the roller will rotate against the hot roll is approximately between 0 3 m/min (1 fpm) to 30.3 m/min (100 fpm) with the preferred being between 0 67 and 15 m/min (2 to 50 fpm) Alternatively, the voids in the exposed surface of the expanded PTFE material may be filled with a dispersion or solution A number of different polymer solutions exist that will fill the voids in the surface of an expanded PTFE and provide a continuous non-porous surface These include PTFE fluoπnate d ethylene propylene (FEP) perfluoroalkoxy polymer (PFA) ethylene- tetrafluoroethylene (ETFE), amorphous fluoropolymers and other fluoropolymer dispersions and solutions available from a number of sources such as E I duPont de Nemours and Company Wilmington Delaware The preferred solutions for use in treating the exposed surface of the release coating of the present invention are those disclosed in PCT Application WO 93/105100 to E I duPont de Nemours and Company incorporated by reference This material comprises perfluoroperhydrophenanthrene with tetrafluoroethylene hexafluoropropyleπe copolymer Its beneficial properties include that it is a true fluoropolymer solution and it wets and penetrates into the voids of the ePTFE membrane Coating with any of the above identified solutions may be accomplished through any appropriate process, such as. but are not limited to dipping, painting/roll coating, spraying and the like Once applied the solution or dispersion may be dried or cured through a number of processes which include, but are not limited to, baking in an oven, passing a hot air stream over the material, passing the material through a heated nip and the like As has been previously noted, for certain applications it may be desirable to first perform a solution treatment followed by densification using heat and pressure to render the exposed surface of the ePTFE skin non-porous The structure of the present invention may be better understood with reference to the drawings Figure 1 shows a cross-section of a pressure roller 10 suitable for use in xerographic printers (e g , copiers laser printers fax machines and the like) In this instance, the roller 10 comprises a center shaft 12 (typically metal), a solid core 14 (again, typically metal) and an elastomer coating 16 around the core 14 The elastomer coating 16 is selected and sized to provide the necessary amount of compliance to the roller For example in the case of a silicone rubber elastomer coating 16 on an approximately 2 54 cm outer diameter roller for use in a laser printer, a typical silicone elastomer coating 16 may comprises about 0 63 to 05 cm in thickness Depending upon the particular application requirements, any suitable underlying layer of material, including either the elastomer coating 16 or the core 14 may serve as a substratum for the release coating of the present invention The release coating of the present invention comprises a filled expanded PTFE membrane skin 18 mounted over the substratum For most applications of the present invention this membrane 18 typically will have a thickness of about 0.0127 to 0.250 mm In the illustrated embodiment, the expanded PTFE membrane 18. comprising an adherable surface 20 and an exposed surface 22. is held to the elastomer coating 16 substratum by a layer of adhesive 24 The adhesive material adheres to the elastomer coating 16 and partially permeates the porous expanded PTFE skin 18 through its adherable surface 20 The exposed surface 22 has been rendereα non-porous as previously described to effectuate a continuous barrier to protect the substratum from wear or attack. One construction of the release coating of the present invention is shown in greater detail in Figures 2 and 3. Figure 2 illustrates release coating 26 prior to being rendered non-porous. The release coating includes filled porous expanded PTFE 18 on its exposed surface 22 and an adhesive-permeated porous PTFE 28 on its adhered surface 20 Figure 3 shows the same structure once the exposed surface 22 has been rendered non-porous through densification. The thickness of densified layer 22 is typically 0.006 mm to 0.1016 mm. As can be seen, the exposed surface has been compacted in thickness and has lost its porous structure. By contrast, the adherable surface 20 continues to have a structure with adhesive permeated into its interstices. Thus, the release coating of the present invention provides an excellent mechanical bond to the elastomer coating 16 or other substratum while presenting an impermeable protective layer on its exposed surface 22. Another embodiment of the present invention is shown in Figures 4 and 5. Figure 4 comprises a filled porous fluoropolymer skin 30, preferably a filled expanded PTFE, attached to a substratum 32 by an adhesive layer 34 The porous skin 30 is rendered non-porous by the addition of one or more coatings of a fluoropolymer dispersion or solution that will fill its voids and cure or dry to an impermeable layer within the skin 30. as described above. Preferably the coatings of fluoropolymer are applied by dipping the roll in the solution and then placing it in an oven at 150°C for 30 minutes. The roll is removed, dipped and dried repeatedly (preferably about 2 to 15 times) until the void space is completely filled. After the voids are completely filled, the roller is placed in the oven at 200°C for 15 minutes. Once applied in this manner, as is shown in Figure 5. the skin 30 presents a continuous impermeable barrier on its exposed surface 36. As was true with the previously described embodiment, the adhered surface 38 of this embodiment is permeated with adhesive to form a strong mechanical bond to the substratum 32. Figures 6 and 7 demonstrate yet another release coating of the present invention. Figure 6 shows a filled expanded PTFE skin material 40 with adhered surface 42 and exposed surface 44 The adhered surface 42 is bonded to a substratum 46 by an adhesive layer 48. Again, the adhesive layer 48 is permeated into the interstices on one side of the expanded PTFE skin 40 . Figure 7 shows this structure once the exposed side has been treated to render it non-porous. In this instance, a fluoropolymer coating 50 has been applied to the exposed surface and it has partially permeated into the porous structure of the filled expanded PTFE skin 40 This construction is accomplished by dipping the roll in the solution and then placing in the oven at 150°C for 30 minutes. The roll is removed, dipped and dried until the void space is completely filled (approximately 2 to 15 times) After the voids are completely filled, the roller is placed in the oven at 200°C for 15 minutes. When formed in this manner, the skin 40 is effectively impermeable to chemical infiltration while leaving a layer 52 of unfilled porous PTFE material within the interior of the skin. Yet another embodiment of a release coating of the present invention is shown in Figure 8. In this instance, the material includes a filled porous fluoropolymer skin 60 adhered to a substratum 56 by an adhesive layer 48. The porous skin 60 is completely impregnated with adhesive from the adhesive layer 48. The surface of the material therefore comprises fluoropolymer 62 from the fluoropolymer skin 60 and adhesive 64 imbibed therein. An adhesive layer 48 is applied to the substratum 56. The porous fluoropolymer 60 is then laid over the adhesive 48. The adhesive 48 penetrates and fills the voids of the porous fluoropolymer layer 60. The adhesive 48 penetrates to the surface of the porous fluoropolymer 60 but does not completely cover the surface. The adhesive is then cured, leaving a surface of both the adhesive 64 and fluoropolymer 62. If desired, as shown in Figure 8a, a layer 66 of a fluoropolymer, e.g., unfilled ePTFE, can be applied to the outer surface of 62, and subsequently densified. The preferred method of densification of the present invention comprises employing a hot roller that can be urged toward a roller to be processed, with the two rollers moving against one another, to heat and densify the release coating of the present invention. One example of such an apparatus 66 is shown in Figure 9. A hot # roller 68 is mounted into contact with a roller 70 of the present invention. The roller 70 is mounted on a rotating axis 72 or in some other non-yielding manner to hold it in steady contact with hot roller 68. Operating the hot roller 68 at a temperature of between 250° and 550°C and an applied pressure of 35 to 13.800 kPa, the rollers 68. 70 are rolled against one another until an even densified release coating is formed on the exposed surface of the roller 70 Another embodiment of apparatus 74 to densify the release coating of the present invention is shown in Figure 10. In this instance, two rollers are used. a hot roller 72 and a απven roller 76. A roller 78 incorporating the release coating of the present invention is placed between the two rollers 72. 76. By urging the two rollers 72. 76 toward one another, the roller 78 can be densified in accordance with the present invention. Figure 1 1 illustrates one application for a pressure roller 80 of the present invention In a conventional electrostatic printer/copier application, a sheet of paper 82 with toner 84 applied thereto is driven past a fuser roll 86 to adhere the image to the sheet 82. In order to maintain the fuser roll 86 in close contact with the sheet of paper 82, a pressure roiler 80 is mounted in contact with the fuser roll 86. The pressure roll 80 must be sufficiently conformable to allow the paper to pass between the two rolls 80, 86 while maintaining it against the fuser roll 86. The rollers of the present invention provides the necessary conformance for this application while avoiding the deficiencies previously encountered with existing pressure rolls. It should be appreciated that the release layer of the present invention has a wide number of applications beyond use in xerographic or electrostatic printers. Among such uses are: as a coating on conveyor belts, such as paper handling belts, used in various industπes: the printing industry, textile industry, electronics industry, contamination sensitive roller process application, including pharmaceutical and food; and applications that include use or handling of adhesives. pastes and other tacky substances including, but not limited to, paints, glues, etc. Without intending to limit the scope of the present invention, the following examples illustrate how the present invention may be made and used. Example 1 A 1.25" (3.175cm) diameter fuser roller was coated with a thermally conductive PTFE layer. A filled expanded PTFE membrane was wrapped and adhered to the user roller and subsequently subjected to heat and pressure which densified the membrane. The fuser roller used was coated initially with a thin layer of silicone rubber that was substantially filled with iron oxide. The rubber layer was approximately 0.010 " (0.254mm), thick with an apparent hardness on the metal roller of approximately 70 shore A. The membrane used was filled with boron nitride from Advanced Ceramics Cleveland OH at a level of 50% by weight The boron nitride which has a mean particle size of approximately 10 micrometers was mixed with water and PTFE dispersion from ICI America Bayonne N J at a 10% solids level The coagulated material was then dried and frozen The frozen material was screened through a 6 35mm x 6 35mm mesh and lubricated with an adequate amount of hydrocarbon solvent The lubricated material was then placed into a pelletizer and compressed and subsequently ram extruded to produce a tape of 0 08 inches (2 mm) in thickness The extruded tape was then rolled down through calendering rolls to a thickness of 0 010 ' (0 254 mm) The tape was then expanded in both the longitudinal and transverse directions 6 1 at a rate of 500%/sec in transverse and 100%/sec in longitudinal The final thickness was 0 0015" (0 0381 mm) The final material had a Gurley time of approximately 2 5 seconds and a density oτ 0 40 g/cc The filled membrane had an average thermal resistance of 0 09368 C/W This is a substantial increase over what is expected with unfilled ePTFE which has a density of approximately 0 5 g/cc which is equal approximately to 0 2421 C W These measurements were made under the conditions of ASTM Standard D5470-93, the standard test method for thermal transmission properties of thin thermally conductive solid electrical insulative materials This filled ePTFE material was then wrapped and bonded to the fuser roller A thin layer of silicone adhesive was first applied to the fuser roller The adhesive used was Silicone Rubber 1081-60292 from General Electric Waterford NY and the thickness was approximately 0 0003-0 0008" (7 62 -20 3 micrometers) The filled expanded membrane was then wrapped around the roller 3 5 times, and the adhesive penetrated into pore spaces of the membrane The roller was placed into the oven at approximately 180°C for 15 minutes to allow the adhesive to cure The roller was then pressed against a heated metal roller approximately 6 inches (15 cm) diameter, and held with two elastomeric coated rollers approximately 4 inches (10 cm) in diameter The heated roller was kept at a temperature of 375°C, and the fuser roller was pressed against it with a force of 160 pounds Initially, the wrapped fuser roll was brought into contact with the heated roll rotating at a speed of approximately 25fpm (8 5 meters/min) for 30 seconds The speed was then reduced to 7fpm (2 meters/min) and the force was increased to approximately 700 pounds After 3 minutes the speed was increased to 25fpm (8 5 meters/min) and the force was reduced to approximately 550 pounds, the roller was allowed to rotate under these conditions for 1 minute The resulting roller consisted of a full density void free boron nitride filled PTFE covering over the silicone rubber coated rubber T, e roller was placed into a Xerox 5328 copier and 1000 copies were made The fuser roller showed no sign of wear or deterioration and the copy image quality was good The toner was properly fused into the paper and there was no sign of offsetting Example 2 A 1 25 inch (3 175 cm) diameter fuser roller was coated with an electrically conductive PTFE layer More specifically a filled expanded PTFE membrane was wrapped and adhered to the fuser roller and subsequently subjected to heat and pressure which densified the membrane The fuser roller used was coated initially with a thin layer of silicone rubber that was substantially filled with iron oxide The rubber layer was approximately 0 010 inch (0 254 mm) thick with an apparent hardness on the metal roller of approximately 70 shore A The membrane used was filled with ketjchen black from Akzo Chemical, Chicago, IL at a level of 15% by weight The ketjchen black was mixed with water and PTFE dispersion from ICI America of Wilmington DE at a overall 29 7% solids level The coagulated material was then dried and frozen The frozen material was screened through a 6 35 mm x 6 35 mm mesh and lubricated with an adequate amount of hydrocarbon solvent The lubricated material was then placed into a pelletizer and compressed and subsequently ram extruded to produce a tape of 0 10 inch (2 54 mm) thick The extruded tape was then dried at 10fpm at 250°C over a drum drier and subsequently rolled down through calendering rolls to a thickness of 0 023 inch (0 584 mm) The tape was then expanded in the longitudinal direction to a ratio of 4 1 at rate of 10%/sec and then in transverse and to a ratio of 5 5 1 at a rate of 30%/sec in longitudinal The final thickness was 0 0048 inch (0 122 mm) The final material had a Gurley time of approximately 2 5 seconds and a density of 0 2 g/cc The filled membrane had a electπcal resistivity of 917 ohm x cm This is a substantial decrease over what is expected with unfilled ePTFE which is typically on theorder of 10 12 to 10 13 ohm x cm These measurements were made using the following procedure The membrane was placed between two 1 1/16 inch (17 5mm) diameter copper electrodes and a weight was place on top for the electrodes to provide a 16 pound per square inch (1 10 Kpa) pressure Using a Hewlett Packard 3478A multimeter the through resistance 02 PC17US96/12826 -18- was measured. Using the resistance, the thickness of the sample and the size of the electrodes, the volumetric resistivity was calculated. This filled ePTFE material was then wrapped and bended to the fuser roller. A thin layer of silicone adhesive was first applied to the fuser roller The adhesive used was Silicone Rubber 1081-60292 from General Electric. Waterford. NY and the thickness was approximately 0 0003-0 0008 inch (7 62 - 20.3 micrometers). The filled expanded membrane was then wrapped around the roller 2.5 times, and the adhesive penetrated into pore spaces of the membrane The roller was placed into the oven at approximately 180°C for 15 minutes to allow the adhesive to cure. The wrapped fuser roller was then pressed against a heated metal roller approximately 6" (15 cm) diameter, and held with two elastomeric coated rollers approximately 4" (10 cm) in diameter The heated roller was kept at a temperature of 375°C. and the fuser roller was pressed against it with a force of 160 pounds. Initially, the wrapped fuser roll was brought into contact with the heated roll at a speed of approximately 25fpm (8.5m/mιn) for 30 seconds. The speed was then reduced to 7fpm (2m/mιn) and the force was increased to approximately 700 pounds and the fuser roller was left rotating at these conditions for 2 minutes. After 2 minutes the speed was increased back to 25fpm (8.5m/mιn) and after 30 seconds the fuser roller was removed from the heated roller. The resulting roller consisted of a full density, void free ketjchen black filled PTFE covering over the silicone rubber coated roller. The electrical properties of the full density roller coating were measure by placing a electrodes on the coating surface 10" apart from one another along the surface on the axis of the coated fuser roller. The standard fuser roller had a resistance of greater than 200 mega ohms at a 500V input and the coated roller had a resistance of less than 100 Kohms at a 500 V input. This demonstrates that the coated roller would effectively eliminate static charge from the surface of the roller. The measurements were made using a Amprobe Megohmmeter AMB-4D from Amprobe Instruments, Lynbrook, NY. Example 3 A 1.125 inch (2.85 cm) diameter pressure roller was coated with an electrically conductive fluoropolymer layer. A filled expanded PTFE membrane was wrapped and adhered to the fuser roller and subsequently filled with a fluoropolymer from a solution of fluoropolymer. The pressure roller had a thin, 0.0625 inch (0.158 cm), layer of silicone rubber which was substantially filled with iron oxide and had an apparent hardness on the metal roller of approximately 70 shore A The membrane used was filled with ketjchen black from Akzo Chemical Chicago IL at a level of 25% by weight The carbon was mixed with water and PTFE dispersion from E I duPont de Numerous Inc Wilmington DE at an overall solids level of 7 1 % The coagulated material was then dried and frozen The frozen material was screened through a 6 35 mm x 6 35 mm mesh and lubricated with an adequate amount of hydrocarbon solvent The lubricated material was then placed into a pelletizer and compressed and subsequently ram extruded to produce a tape of 0 030 inch ( 762 mm) thick The extruded tape was then dried over a drum drier and subsequently rolled down through calendering rolls to a thickness of 0 022 inch (0 558 mm) The tape was then expanded in the longitudinal direction to a ratio of 4 1 at rate of 15%/sec and then in transverse and to a ratio of 10 1 at a rate of 120%/sec in longitudinal The final thickness was 0 0038 inch (0 096 mm) The final material had a Gurley time of approximately 2 3 seconds and a density of about 0 5 g/cc The filled membrane had a volumetric electrical resistivity of 134 ohms x cm This is a substantial decrease over what is expected with unfilled ePTFE which is typically on the order of 10 *12 to 10 *13 ohm x cm These measurements were made using the same procedure as described in Example 2 This filled ePTFE material was then wrapped and bonded to the fuser roller A thin layer of silicone adhesive was first applied to the fuser roller The adhesive used was Silicone Rubber 1081-60292 from General Electric Waterford NY and the thickness was approximately 0 0003-0 0008 inch (7 62 - 20 3 micrometers) The filled expanded membrane was then wrapped around the roller 1 5 times, and the adhesive penetrated into pore spaces of the membrane The roller was placed into the oven at approximately 180°C for 15 minutes to allow the adhesive to cure The roller was then filled with a TEFLON® fluoropolymer solution The roller was mounted 0 5" (1 25cm) above a heated hot plate at 85°C the roller was rotated at 60 rpm Using a foam brush, a thin coat of a 1% solution of a copolymer of TFE/HFP in perfluoroperhydrophenathrene was applied to the surface of the roller The heat from the hot plate slowly drove the solvent out This step was repeated until the pores of the membrane were completely filled with fluoropolymer The roller was then placed in an oven at 180°C for 10 minutes The oven temperature was then increased to 220°C and the roller was retained in the oven for approximately 30 minutes. The roller was then removed and allowed to cool. The electπcal properties of the full density roller coating were measured by placing electrodes along the axis of the coated surface of the fuser, 10 inches (25 4 cm) apart from one another. The standard fuser roller had a resistance of greater than 200 mega ohms at a 500V input and the coated roller had a resistance of less than 100 Kohms at a 500 V input. This demonstrates that the coated roller effectively eliminated static charge from the surface of the roller The measurements were made using a Amprobe Megohmmeter AMB-4D from Amprobe Instruments. Lynbrook. NY Example 4 A 1.265" (3 175 cm) diameter fuser roller was coated with an electrically conductive PTFE layer A filled expanded PTFE membrane was wrapped and adhered to the fuser roller and subsequently subjected to heat and pressure which densified the membrane. The fuser roller used was coated initially with a thin layer of silicone rubber that was substantially filled with iron oxide The rubber layer was approximately 0.010 inch (0.254 mm), thick with an apparent hardness on the metal roller of approximately 70 shore A. The membrane used was filled with ketjchen black from Akzo Chemical, Chicago, IL., at a level of 7.5% by weight. The carbon was mixed with water and PTFE dispersion from E. I. duPont de Nemours, Inc. of Wilmington. DE., at an overall 11 % solids level. The coagulated material was then dried and frozen. The frozen material was screened through a 6.35 mm x 6.35 mm mesh and lubricated with an adequate amount of hydrocarbon solvent. The lubricated mateπal was then placed into a pelletizer and compressed, and subsequently ram extruded to produce a tape of 0.030 inch (0.762 mm) thick. The extruded tape was then dried over a drum drier and subsequently rolled down through calendeπng rolls to a thickness of 0.012.5 inch (0.318 mm) The tape was then expanded in the longitudinal direction to a ratio of 1.5.1 at rate of 15%/sec and then in transverse and to a ratio of 8:1 at a rate of 100%/sec. The final thickness was 0.0035 inch (0.088 mm). The final material had a Gurley time of approximately 4.5 seconds and a density of about 0.5 g/cc. The filled membrane had a electrical resistivity of 183.000 ohm x cm. This is a substantial decrease over what is expected with unfilled ePTFE which is typically on the order of 10 12 to 10 13 ohm x cm. These measurements were made using the same procedure as described in Example 2. This filled ePTFE material was then wrapped and bonded to the fuser roller A thin layer of silicone adhesive was first applied to the fuser roller The adhesive used was Silicone Rubber 1081-60292 from General Electric Waterford. NY and the thickness was approximately 0 0003-0 0008 inch (7 62 - 20 3 micrometers) The filled expanded membrane was then wrapped around the roller 2 5 times, and the adhesive penetrated into pore spaces of the membrane The roller was placed into the oven at approximately 180°C for 15 minutes to allow the adhesive to cure The wrapped fuser roller was then pressed against a heated metal roller approximately 6 inches (15 cm) diameter, and held with two elastomeric coated rollers approximately 4 inches (10 cm) in diameter The heated roller was kept at a temperature of 375°C. and the fuser roller was pressed against it with a force of 160 pounds Initially, the wrapped fuser roll was brought into contact with the heated roll rotating at a speed of approximately 30 fpm (10m/mιn) for 30 seconds The speed was then reduced to 10 fpm(3m/mιn) and the force was increased to 700 pounds and the fuser roller was left rotating under these conditions for 2 minutes After 2 minutes the speed was increased back to 30 fpm(10 m/min) and after 30 seconds the fuser roller was removed from the heated roller The resulting roller consisted of a full density, void free ketjchen black filled PTFE covering over the silicone rubber coated roller The roller was then placed back into the oven at a temperature of 350 β C for 1 hour The electrical properties of the full density roller coating were measured by placing electrodes along the axis of the coated surface of the fuser, 10 inches apart from one another. The roller had a 6,000 ohm resistance over this 10 inch distance. This low resistance will adequately dissipate static The measurements were made using a Universal LCR Meter, model 878, from BK Precision of Chicago, IL. The fuser roller was placed in a 5028 Xerox photocopier and 5000 copies were made. All of the copies looked good, there was no visible offsetting or image quality problem In addition, the toner was properly fused into the paper The fuser roller was removed and examined after the 5000 copies were made, and the fuser roller had no sign of wear While particular embodiments of the present invention have been illustrated and described herein, the present invention should not be limited to such illustrations and descriptions It should be apparent that changes and modifications may be incorporated and embodied as part of the present invention within the scope of the following claims.

Claims: WE CLAIM 1 A composite release coating that comprises an expanded polytetrafluoroethylene (ePTFE) skin having a non-porous exposed surface and a porous adherable surface, said ePTFE being filled with particulate filler an adhesive layer the adhesive layer imbibed within the porous ePTFE skin on the adherable surface, and a substratum bonded to the ePTFE skin by the adhesive layer 2 The composite of claim 1 wherein the exposed surface of the skin is rendered non-porous by a fluoropolymer imbibed therein 3 The composite of claim 1 wherein the exposed surface of the skin is rendered non-porous by densification of the exposed surface 4 The composite of claim 1 wherein the substratum comprises a resilient material 5 The composite of claim 1 wherein the substratum comprises a non- resihent material 6 The composite of claim 1 wherein the substratum comprises a flexible material 7 The composite of claim 1 wherein the adhesive layer comprises an elastomeric material 8 The composite of claim 1 wherein the adhesive layer penetrates through the skin to render the skin non-porous 9 A method of producing a release coating that comprises providing a skin of porous expanded polytetrafluoroethylene (ePTFE) material filled with a particulate filler, the skin having a first side and a second side; coating the first side of the skin with an adhesive, the adhesive penetrating into the pores of the ePTFE mateπal; attaching the first side of the skin to a substratum, the adhesive forming a bond between the skin and the substratum and leaving the second side of the skin exposed; treating the second side of the skin to render it non-porous so as to present a smooth release surface 10 The method of claim 9 that further comprises treating the second side of the skin by coating it with a dissolved fluoropolymer material that penetrates its pores and renders the second side non-porous 1 1 The method of claim 9 that further comprises treating the second side of the skin so as to density the ePTFE material and render the second side non-porous 12 The method of claim 1 1 that further comprises densifying the second side of the skin by applying heat and pressure 13 The method of claim 9 that further comprises causing the adhesive layer to penetrate and fill the voids in the skin to its exposed surface rendering the second side of the skin non-porous 14 The method of claim 9 that further comprises treating the skin to render it non-porous by both densifying the second side and coating it with a fluoropolymer material 15 A roller assembly for mounting in a printer device having at least one contact surface said roller assembly comprising a composite release coating comprising an expanded PTFE (ePTFE) skin having a non-porous exposed surface and a porous adherable surface said ePTFE being filled with particulate filler and an adhesive layer the adhesive layer imbibed within the porous ePTFE skin on the adherable surface and a printer roller bonded to the ePTFE skin by the adhesive layer 16 The roller assembly of claim 15, wherein said printer roller comprises a fuser roller 17 The roller assembly of claim 15 wherein said printer roller comprises a pressure roller 18 The roller assembly of claim 15, wherein the exposed surface of the skin is rendered non-porous by a fluoropolymer imbibed therein 19 The roller assembly of claim 15 wherein the exposed surface of the skin is rendered non-porous by densification of the exposed surface 20 The roller assembly of claim 15, wherein the adhesive layer comprises an elastomeπc material 21 The roller assembly of claim 15, wherein the adhesive layer penetrates through the skin to render the skin non-porous 22 The roller assembly of claim 15, wherein said composite release coating is antistatic or electrically conductive 23 A method of using the roller assembly of claim 15 in a printer device comprising mounting said roller assembly within said printer device, in contact with at least one contact surface of said printer device, moving said roller assembly relative to said contact surface to provide at least one of pressure and heat to said contact surface, whereby said composite release coating permits said roller assembly to move without degrading the surface of said roller assembly or said contact surface The method of claim 23, wherein said roller assembly comprises a fuser roller which provides heat and pressure to said contact surface The method of claim 23, wherein said roller assembly comprises a pressure roller which provides pressure to said contact surface

Inventors: HOBSON ALEX RAY, LAU TIT-KEUNG, SASSA ROBERT L, SPECK STEVEN H, HOBSON, ALEX, RAY, LAU, TIT-KEUNG, SASSA, ROBERT, L., SPECK, STEVEN, H.
Applicants: GORE & ASS, W.L. GORE & ASSOCIATES, INC.
Application year: 1996
Main classification: B32B27
IPC codes: B32B5, B32B27, G03G15