Patent Publication Number: US-2013251426-A1

Title: Fixing belt, fixing device, and image-forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-068173 filed Mar. 23, 2012. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to fixing belts, fixing devices, and image-forming apparatuses. 
     (ii) Related Art 
     Recently, fixing devices that heat a fixing belt by electromagnetic induction to perform fixing have been proposed for use with image-forming apparatuses. 
     SUMMARY 
     According to an aspect of the invention, there is provided a fixing belt including a heat-insulating layer formed of a glass fiber or a porous ceramic and a metal heat-generating layer disposed outside the heat-insulating layer. The metal heat-generating layer generates heat by electromagnetic induction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a schematic sectional view of a fixing belt according to an exemplary embodiment; 
         FIG. 2  is a schematic view of a fixing device according to an exemplary embodiment; and 
         FIG. 3  is a schematic view of an image-forming apparatus according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Exemplary embodiments of the present invention will now be described in detail. 
     Fixing Belt 
     A fixing belt according to an exemplary embodiment includes a heat-insulating layer formed of a glass fiber or a porous ceramic and a metal heat-generating layer disposed outside the heat-insulating layer. The metal heat-generating layer generates heat by electromagnetic induction. 
     The fixing belt is used, for example, for a fixing device capable of electromagnetic induction heating in electrophotographic image-forming apparatuses. To deliver high fixing performance, the fixing belt needs to efficiently transfer heat generated from the metal heat-generating, layer by electromagnetic induction to a material to be fixed, such as a toner, on the outer surface of the fixing belt. 
     The fixing belt according to this exemplary embodiment includes a heat-insulating layer formed of a glass fiber or porous ceramic, which contains pores or voids, inside the metal heat-generating layer. The use of such a material may reduce the loss of the heat generated from the metal heat-generating layer through the inner surface of the fixing belt, thus allowing the heat to be efficiently transferred to the outside of the fixing belt. This may reduce power consumption and shorten heating time (warm-up time). 
     The glass fiber or porous ceramic also has pores or voids in the surface thereof. These pores or voids may produce an anchor effect to provide good adhesion to the layers adjacent to the heat-insulating layer. The layers adjacent to the heat-insulating layer are, for example, the metal heat-generating layer disposed outside the heat insulating layer and an optional substrate layer disposed inside the heat-insulating layer. 
     A fixing belt capable of electromagnetic induction heating contacts and heats a material (e.g., a toner) transferred to a recording medium such as paper to fix the material. The fixing belt requires sufficient flexibility to release the recording medium having the material fixed thereto. At the same time, the fixing belt requires sufficient rigidity not to be twisted or fractured during rotation. 
     Because the heat-insulating layer of the fixing belt according to this exemplary embodiment is formed of a glass fiber or porous ceramic, it may have a good balance of flexibility and rigidity as the layer formed inside the metal heat-generating layer. This may allow the fixing belt according to this exemplary embodiment to have sufficient flexibility to release a recording medium and sufficient rigidity not to be twisted. 
     Thermal Conductivity 
     To more efficiently reduce the loss of the heat generated from the metal heat-generating layer through the inner surface of the fixing belt, the heat-insulating layer preferably has a thermal conductivity of 0.03 to 0.10 W/m-K or about 0.03 to about 0.10 W/m·K, more preferably 0.03 to 0.05 W/m·K or about 0.03 to about 0.05 W/m·K. 
     A thermal conductivity within the above upper limit may allow efficient reduction of the loss of heat through the inner surface of the fixing belt. A thermal conductivity within the above lower limit may provide the advantage of allowing the influence of temperature variations in the axial direction to be taken into account. 
     The thermal conductivity of the heat-insulating layer is measured as follows. The heat-insulating layer is cut into a 30 mm square film. The thermal conductivity of the film is measured using an ai-Phase Mobile thermal conductivity analyzer (from SII NanoTechnology Inc.). 
     The values disclosed herein are measured by the above procedure. 
     Elastic Modulus 
     To ensure that the fixing belt, has sufficient flexibility to release a recording medium and sufficient rigidity not to be twisted, the heat-insulating layer preferably has an elastic modulus of 1.0 to 10.0 GPa or about 1.0 to about 10.0 GPa, more preferably 1.0 to 7.0 GPa or about 1.0 to about 7.0 GPa. 
     An elastic modulus within the above upper limit may provide moderate flexibility so that the fixing belt smoothly releases a recording medium. An elastic modulus within the above lower limit may provide moderate rigidity so that the fixing belt is not twisted. 
     The elastic modulus of the heat-insulating layer is measured as follows. The heat-insulating layer is cut into a 4 mm by 20 mm film. The elastic modulus of the film is measured using a RHEOVIBRON dynamic viscoelastometer (from A&amp;D Company, Limited). 
     The values disclosed herein are measured by the above procedure. 
     Structure of Fixing Belt 
     The structure of the fixing belt according to this exemplary embodiment will, now be described with reference to the drawings. 
       FIG. 1  is a schematic sectional view of the fixing belt according to this exemplary embodiment. 
     As shown in  FIG. 1 , a belt  10  according to this exemplary embodiment includes, in order from inside to outside, a heat-insulating layer  10 A, a metal seed layer  10 B, a metal heat-generating layer  10 C, a metal protective layer  10 D, an elastic layer  10 E, and a release layer  10 F. 
     Although the structure of the fixing belt  10  according to this exemplary embodiment is illustrated in  FIG. 1 , it may have any other structure including at least the heat insulating layer  10 A and the metal heat-generating layer  10 C. For example, the metal seed layer  10 B, the metal protective layer  10 D, the elastic layer  10 E, and the release layer  10 F may be omitted from the structure illustrated in  FIG. 1 . The fixing belt  10  may further include a substrate layer inside the heat-insulating layer  10 A. 
     Heat-Insulating Layer 
     The heat-insulating layer  10 A may be any layer formed of a class fiber or porous ceramic. As used herein, the phrase “formed of a glass fiber or porous ceramic” does not necessarily mean that the heat-insulating layer  10 A is formed only of a glass fiber or porous ceramic; it may contain other materials in such amounts that the effect thereof is not impaired. 
     As noted above, the heat-insulating layer  10 A may have a thermal conductivity of 0.03 to 0.10 W/m·K or about 0.03 to about 0.10 W/m·K and an elastic modulus of 1.0 to 10.0 GPa or about 1.0 to about 10.0 GPa. 
     The glass fiber or ceramic contains pores or voids. The heat-insulating layer  10 A preferably has a porosity of 80% or more, more preferably 90% or more. 
     A porosity within the above lower limit may provide the advantage of implementing effective heat insulation. 
     The porosity of the heat-insulating layer  10 A may be derived from measurements of the density of the material (densitometer) and the basis weight and thickness of the heat-insulating layer  10 A (weight meter, dial gauge, or scale). The values disclosed herein are obtained from material manufacturers. 
     The heat-insulating layer  10 A preferably has a thickness of 20 to 180 μm, more preferably 20 to 80 μm. 
     A thickness within the above upper limit may contribute to low heat capacity, thus providing an energy-efficient fixing device. A thickness within the above lower limit may be effective for high paper release performance if the belt is bent during use. 
     The heat-insulating layer  10 A is formed of, for example, glass fiber paper (glass paper) or porous ceramic paper. 
     The heat-insulating layer  10 A may be formed of a commercial product. Examples of glass fiber paper include TGP (ultrathin glass paper; porosity: 85% or more; thickness: 20 μm) from Nippon Sheet Glass Co., Ltd. and AGM (ultrathin glass paper; porosity: 90% or more; thickness: 100 to 180 μm) from Nippon Sheet Glass Co., Ltd. 
     An example of porous ceramic paper is MARINETEX 02A (thickness: 180 μm) from Nichias Corporation. 
     Alternatively, a cylindrical glass fiber sheet or porous ceramic sheet may be used to form a seamless heat-insulating layer  10 A. 
     A cylindrical, sheet may be formed in a known manner, for example, by forming a fibrous sheet on a cylindrical mold, or by weaving fibers into a cylindrical shape. 
     Substrate Layer 
     The fixing belt  10  may further include a substrate layer inside the heat-insulating layer  10 A for improved sliding across the inner surface of the fixing belt  10 . 
     The substrate layer contains, for example, a resin as a major component. As used herein, the term “major component” means that the content thereof is 50% by mass or more, which applies hereinafter. 
     Examples of resins include polyimide, polyamideimide, fluorocarbon resins, aromatic polyamides, thermotropic liquid crystal polymers, polyester, polyethylene terephthalate, polyethersulfone, polyetherketone, and polysulfone, of which polyimide is preferred. 
     The resin used for the substrate layer may be for example, a foamed resin. The substrate layer may further contain a filler. 
     The substrate layer preferably has a thickness of 20 to 180 μm, more preferably 20 to 80 μm. 
     Metal Seed Layer 
     If the metal heat-generating layer  105 , described later, is formed by electroplating, the metal seed layer  10 B may be provided as a basis for forming the metal heat-generating layer  105  by electroplating because it is difficult to directly perform electroplating on the heat-insulating layer  10 A. 
     The metal seed layer  10 B is formed of an electroless layer. Examples of electroless layers include electroless nickel layers, electroless copper layers, electroless tin layers, electroless gold layers, and electroless nickel-tantalum layers, of which electroless nickel layers are preferred. 
     The metal seed layer  10 B has, for example, a thickness that does not impair the flexibility of the belt  10 , for example, 0.1 to 10 μm. 
     Metal Heat-Generating Layer 
     The metal heat-generating layer  10 C functions to generate heat by, for example, inducing eddy currents in a magnetic field. The metal heat-generating layer  10 C is formed of a metal capable of electromagnetic induction. 
     Examples of metals capable of electromagnetic induction include metals (e.g., nickel, iron, copper, gold, silver, aluminum, chromium, tin, and zinc) and alloys of two or more such metals (e.g., stainless steel). 
     In particular, suitable metals include copper, nickel, aluminum, iron, and chromium, and copper and copper-based alloys are preferred. 
     The metal heat-generating layer  10 C may be formed in a known manner. For example, electroless plating may be performed on the heat-insulating layer  10 A. Alternatively, as noted above, the metal seed layer  10 B may be provided on the heat-insulating layer  10 A before electroplating. 
     The appropriate thickness of the metal heat-generating layer  10 C varies depending on the material used. For example, if copper is used, the metal heat-generating layer  10 C preferably has a thickness of 3 to 50 μm, more preferably 5 to 20 μm. 
     Metal Protective Layer 
     The metal protective layer  10 D is disposed on the metal heat-generating layer  10 C to prevent cracking of the metal heat-generating layer  10 C after repeated deformation and to inhibit oxidative degradation after repeated heating for an extended period of time, thereby maintaining its heat generation performance. 
     The metal protective layer  10 D may be optionally provided. 
     The metal protective layer  10 D may be formed of, for example, an oxidation-resistant metal layer having high durability and oxidation resistance. For example, the metal protective layer  10 D may be formed of an electroplated layer for ease of processing as a thin film. In particular, the metal protective layer  10 D may be formed of an electroplated nickel layer, which has high strength. 
     The appropriate thickness of the metal protective layer  10 D varies depending on the material used. For example, it nickel is used, the metal protective layer  10 D may have a thickness of 2 to 20 μm. 
     Elastic Layer 
     The elastic layer  10 E conforms to irregularities of a toner image on a recording medium so that the surface of the fixing belt  10  comes into intimate contact with the toner image. 
     The elastic layer  10 E may be formed of a material that returns to its original shape after being deformed under a pressure of, for example, 100 Pa, Known elastic materials may be used, including heat-resistant rubbers such as silicone rubbers and fluorocarbon rubbers. Examples of such materials include SE6744 liquid, silicone rubber from Dow Corning Toray Co., Ltd. and Viton B-202 from DuPont Dow Elastomers LLC. 
     The elastic layer  101  preferably has a thickness of, for example, 0.1 to 3 mm, more preferably 0.15 to 1 mm. 
     Release Layer 
     If the fixing belt  10  as used as a heat-fixing belt to melt and fix an unfixed toner image to a recording medium, the release layer  10 F prevents the molten toner from adhering to the fixing belt  10 . The release layer  10 F may be optionally provided. 
     The release layer  10 F may contain, for example, a fluorinated compound as a major component. Examples of fluorinated compounds include fluorocarbon resins such as fluorocarbon rubbers, polytetrafluoroethylene (PTEE), perfluoroalkyl-vinyl ether copolymer (PFA), and ethylene tetrafluoride-propylene hexafluoride copolymer (FEP). 
     The release layer  10 F preferably has a thickness of, for example, 1 to 100 μm, more preferably 10 to 50 μm. 
     Thickness Measurement. 
     The thicknesses of the individual, layers are measured as follows. The thicknesses of the heat-insulating layer  10 A, the elastic layer  10 E, and the release layer  10 F are measured using an eddy-current thickness gauge (available from Fischer Instruments K.K.). The thicknesses of the metal seed layer  10 B, the metal heat-generating layer  10 C, and the metal protective layer  10 D are measured using an X-ray fluorescence thickness gauge (available from Fischer instruments K.K.), 
     Manufacture of Fixing Belt 
     An example of a method for manufacturing a fixing belt  10  will now be described. The method described herein forms a fixing belt including a heat-insulating layer; a metal heat-generating layer, a metal protective layer, an elastic layer, and a release layer outside the heat-insulating layer; and a substrate layer inside the heat-insulating layer. 
     The method begins with providing a heat-insulating layer such as glass fiber paper. The heat-insulating layer is wound around a core for manufacture of a fixing belt. The heat-insulating layer wound around the core is subjected to electroless plating to form a metal heat-generating layer (e.g., a 15 μm thick copper layer) and then to electroplating to form a metal protective layer (e.g., a 5 μm thick nickel layer). 
     An elastic material such as liquid silicone rubber is applied to the metal protective layer by dipping and is cured by baking to form an elastic layer. 
     An adhesive is applied to the elastic layer. The core coated with the adhesive is inserted into and covered with a release layer tube such as a PFA tube, with its hoe expanded. The tube is baked and is cut to remove unnecessary portions, thus forming a release layer. 
     A material for forming a substrate layer is applied to the inner surface of the heat-insulating layer and is baked to form a substrate layer (e.g., a polyimide layer) on the inner surface. Thus, a fixing belt is obtained. 
     Fixing Device 
       FIG. 2  is a schematic view of a fixing device according to an exemplary embodiment. 
     A fixing device  100  according to this exemplary embodiment is, for example, an electromagnetic induction fixing device including the fixing belt  10  according to the above exemplary embodiment. As shown in  FIG. 2 , the fixing device  100  includes a pressing roller (pressing member)  11  that presses a portion of the fixing belt  10 . For efficient fixing, the pressing roller  11  forms a contact region (nip) with the fixing belt  10 , which is curved along the circumferential surface of the pressing roller  11 . For sufficient releasability of a recording medium, the fixing belt  10  has bends at the ends of the contact region (nip). 
     The pressing roller  11  includes a substrate layer  11 A, an elastomeric layer  11 B disposed on the substrate layer  11 A, and a release layer  11 C disposed on the elastomeric layer  11 B. The elastomeric layer  11 B is formed of, for example, silicone rubber. The release layer  10 F is formed of, for example, a fluorinated compound. 
     The fixing device  100  further includes a counter member  13  disposed opposite the pressing roller  11  inside the fixing belt  10 . The counter member  13  is formed of, for example, a metal, heat-resistant resin, or heat-resistant rubber. The counter member  13  includes a support  13 A and a pad  13 B supported by the support  13 A. The pad  13 B contacts the inner surface of the fixing belt  10  to locally apply more pressure. 
     The fixing device  100  further includes an electromagnetic induction heating device  12  that incorporates an electromagnetic induction coil (exciting coil)  12   a  disposed opposite the pressing roller  11  (an example of a pressing member) with the fixing belt  10  therebetween. The electromagnetic induction heating device  12  supplies an alternating current to the electromagnetic induction coil  12   a  to generate a magnetic field. The exciting circuit varies the magnetic field to induce eddy currents in the metal heat-generating layer  10 C of the fixing belt  10 . These eddy currents are converted to heat (Joule heat) by the electrical resistance of the metal heat-generating layer  10 C, thus heating the surface of the fixing belt  10 . 
     The electromagnetic induction heating device  12  is not necessarily disposed at the position shown in  FIG. 2 . For example, the electromagnetic induction heating device  12  may be disposed upstream of the contact region of the fixing belt  10  in a rotational direction B, or may be disposed inside the fixing belt  10 . 
     In the fixing device  100  according to this exemplary embodiment, the fixing belt  10  is rotated in the direction indicated by the arrow B as driving force is transmitted to gears disposed at both ends of the fixing belt  10  by a drive unit (not shown). As the fixing belt  10  is rotated, the pressing roller  11  is rotated in the opposite direction, i.e., in the direction indicated by the arrow C. 
     A recording medium having an unfixed toner image  14  formed thereon is passed through the contact region (nip) between the fixing belt  10  and pressing roller  11  of the fixing device  100  in the direction indicated by the arrow A. The unfixed toner image  14  is melted and fixed to the recording medium  15  under pressure. 
     Image-Forming Apparatus 
       FIG. 3  is a schematic view of an image-forming apparatus according to an exemplary embodiment. 
     As shown in  FIG. 3 , an image-forming apparatus  200  according to this exemplary embodiment includes a photoreceptor (an example of an image carrier)  202 , a charging device  204 , a laser exposure device (an example of a latent-image forming device)  206 , a mirror  208 , a developing device  210 , an intermediate transfer member  212 , a transfer roller (an example of a transfer device)  214 , a cleaning device  216 , an erasing device  218 , the fixing device  100 , and a paper feed device. The paper feed device includes a paper feed unit  220 , a paper feed roller  222 , a registration roller  224 , and a recording medium guide  226 . 
     The image-forming operation of the image-forming apparatus  200  begins when the charging device  204 , which is disposed in proximity to the photoreceptor  202 , charges the surface of the photoreceptor  202  by non-contact charging. 
     The laser exposure device  208  emits a laser beam based on image information (signal) for each color. The mirror  208  directs the laser beam onto the surface of the photoreceptor  202  charged by the charging device  204  to form an electrostatic latent image. 
     The developing device  210  applies toners to the latent image formed on the surface of the photoreceptor  202  to form toner images. The developing device  210  includes developing units (not shown), each containing cyan, magenta, yellow, or black toner. As the developing device  210  is rotated in the direction indicated by the arrow, the developing device  210  applies the toners to the latent image formed on the surface of the photoreceptor  202  to form toner images. 
     The toner images formed on the surface of the photoreceptor  202  are transferred to the outer surface of the intermediate transfer member  212  at the contact between the photoreceptor  202  and the intermediate transfer member  212  by a bias voltage applied thereacross. The toner images are superimposed on top of each other such that they match the image information for the respective colors. 
     The intermediate transfer member  212  is rotated in the direction indicated by the arrow E, with the outer surface thereof in contact with the surface of the photoreceptor  202 . 
     In addition to the photoreceptor  202 , the transfer roller  214  is disposed around the intermediate transfer member  212 . 
     The intermediate transfer member  212  having the color toner image transferred thereto is rotated in the direction indicated by the arrow E. The toner image is transferred from the intermediate transfer member  212  to the surface of the recording medium  15  at the contact between the transfer roller  214  and the intermediate transfer member  212 . The recording medium  15  is fed to the contact in the direction indicated by the arrow A by the paper feed device. 
     The recording medium  15  is fed to the contact between the intermediate transfer member  212  and the transfer roller  214  as follows. The recording medium  15  contained in the paper feed unit  220  is lifted by a recording-medium lifting member (not shown) built into the paper feed unit  220  until the recording medium  15  contacts the paper feed roller  222 . When the recording medium  15  contacts the paper feed roller  222 , the paper feed roller  222  and the registration roller  224  are rotated to transport the recording medium  15  along the recording medium guide  226  in the direction indicated by the arrow A. 
     The toner image  14  transferred to the surface of the recording medium  15  is moved in the direction indicated by the arrow A. Upon reaching the contact region (nib) between the fixing belt  10  and the pressing roller  11 , the toner image  14  is fixed to the surface of the recording medium  15  as it is melted and pressed against the recording medium  15 . Thus, a fixed image is formed on the surface of the recording medium  15 . 
     After the toner image is transferred to the surface of the intermediate transfer member  212 , the cleaning device  216  cleans the surface of the photoreceptor  202 . 
     After the cleaning device  216  cleans the surface of the photoreceptor  202 , the erasing device  218  eliminates any charge therefrom. 
     EXAMPLES 
     The present invention is further illustrated by the following non-limiting examples. 
     Example 1 
     Fabrication of Fixing Belt 
     Glass fiber paper (TGP from Nippon Sheet. Glass Co., Ltd.; porosity: 85% or more; thickness: 20 μm), which is to form a heat-insulating layer, is wound around a core for manufacture of a fixing belt and is fixed at both ends with heat-resistant tapes. 
     The glass fiber paper wound around the core is subjected to electroless plating to form a metal heat-generating layer (15 μm thick cooper layer) and then to electroplating to form a metal protective layer (5 μm thick nickel layer). 
     A liquid silicone rubber (liquid injection molding (LIM) material from Shin-Esu Chemical Co., Ltd.) is applied to the metal protective layer by dipping and is cured by baking at 120° C. for 10 minutes to form an elastic layer having a thickness of 200 μm. 
     A silane coupling adhesive (from Dow Corning Toray Silicone Co., Ltd.) is applied to the elastic layer and is dried at 150° C. for 10 minutes. The core having the outermost surface thereof coated with the adhesive is inserted into and covered with a PIA tube (30 μm thick; from Kurabo Industries Ltd.), with its hole expanded. The PFA tube is baked at 200° C. for four hours and is cut at both ends to remove unnecessary portions, thus forming a release layer. 
     Thus, a fixing belt is obtained. 
     Example 2 
     A fixing belt including a seamless heat-insulating layer is fabricated by repeating the procedure of Example 1 except that the glass fiber paper (TOP from Nippon. Sheet Glass Co., Ltd.; porosity: 85% or more; thickness: 20 μm) is replaced by a cylindrical glass fiber sheet fabricated as follows. 
     The cylindrical glass fiber sheet is fabricated by forming a glass fiber sheet on a cylindrical mold and pressing the sheet into a cylindrical shape. The resulting belt has a thickness of 80 μm. 
     Example 3 
     A fixing belt is fabricated, by applying an N-methylpyrrolidone solution of a polyamic acid (U Imide from Unitika Ltd.; concentration: 20% by mass)) to the inner surface of the fixing belt fabricated in Example 1 and baking the coating at 360° C. for one hour to form a substrate layer (10 μm thick polyimide layer) on the inner surface. 
     Comparative Example 1 
     A fixing belt for comparison is fabricated by repeating the procedure of Example 1 except that, instead of forming the heat-insulating layer by winding the glass fiber paper around the core, a substrate layer is formed on the core. The metal heat-generating layer, the metal protective layer, the elastic layer, and the release layer are formed on the substrate layer by the procedure of Example 1. The substrate layer is formed as follows. 
     The substrate layer is formed by applying an N-methylpyrrolidone solution of a polyamic acid (U Imide from Unitika Ltd.; concentration: 20% by mass)) to the core and baking the coating at 360° C. for one hour to form a substrate layer (70 μm thick polyimide layer) on the core. 
     EVALUATIONS 
     Warm-Up Time 
     Each of the fixing belts fabricated in the Examples and Comparative Example is mounted as a fixing belt on a fixing device capable of electromagnetic induction heating in an electrophotographic image-forming apparatus (DocuCenter IV 2275 from Fuji Xerox Co, Ltd.) to measure the warm-up time thereof. The results are shown in Table 1. 
     The fixing belt of Example 1 has a 30% shorter warm-up time than the fixing belt of Comparative Example 1. 
     Power Consumption 
     With the above electrophotographic image-forming apparatus, 22 copies are printed to measure the power consumed during the print test. The results are shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                 Warm-up time 
                 Power consumption 
               
               
                   
                   
                 (s) 
                 (Wh) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Example 1 
                 4.2 
                 606 
               
               
                   
                 Example 2 
                 5.6 
                 960 
               
               
                   
                 Example 3 
                 5.0 
                 815 
               
               
                   
                 Comparative 
                 6.0 
                 980 
               
               
                   
                 Example 1 
                   
                   
               
               
                   
                   
               
            
           
         
       
     
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.