RESISTANCE HEATING ELEMENT WITH POLYIMIDE RESIN AND GRAPHITE PARTICLES

BACKGROUND

[0001 ] Some Image forming apparatuses include a fixing device that fixes the toner image on a sheet by heating and pressing the sheet on which the toner image has been transferred. Such fixing device may include an endless fixing belt, so that the sheet on which the toner image has been transferred is heated by the heat of the fixing belt. In order to generate heat in the fixing belt, a resistance heating element may be provided in the fixing belt, and power may be supplied to the resistance heating element to generate heat.

BRIEF DESCRIPTION OF DRAWINGS

[0002] Figure 1 is a schematic diagram of an example fixing device.

Figure 2 is a schematic diagram illustrating a layer configuration of an example fixing belt.

Figure 3 is a schematic diagram illustrating a layer configuration structure of another example fixing belt.

Figure 4 is an SEM image obtained by observing a cross section of the resistance heating element of Test Example 1 .

DETAILED DESCRIPTION

[0003] In the following description, with reference to the drawings, the same reference numbers are assigned to the same components or to similar components having the same function, and overlapping description is omitted. Hereinafter, examples of a resistance heating element will be described. The resistance heating element according to an example contains a polyimide resin and graphite particles.

[0004] The polyimide resin can be appropriately selected. Commercially available polyimide resins (for example, ON-20 (trade name), manufactured by New Japan Chemical Co., Ltd.) can be purchased and used. When the resistance heating element contains the polyimide resin, the strength of the resistance heating element may be improved. In addition, for example, in a case where the resistance heating element is formed in a sheet shape, graphite particles are easily segregated, and the resistance value of the resistance heating element may be decreased.

[0005] According to examples, the content of the polyimide resin may be 50% by volume or more, 60% by volume or more, 70% by volume or more, or 80% by volume or more, and may be 90% by volume or less, 80% by volume or less, or 70% by volume or less, based on the total volume of the resistance heating element.

[0006] According to examples, the graphite particles have an average particle diameter of 3 to 18 pm. The average particle diameter of the graphite particles may be 4 pm or more, 5 pm or more, or 6 pm or more, and may be 15 pm or less, 12 pm or less, 10 pm or less, 9 pm or less, 8 pm or less, 7 pm or less, or 6 pm or less.

[0007] According to examples, the graphite particles have an average aspect ratio of 1 .23 or more. According to examples, the average aspect ratio of the graphite particles may be 1 .3 or more, 1 .5 or more, 1 .8 or more, 2.0 or more, or 2.2 or more, and may be 3.0 or less, 2.8 or less, or 2.6 or less.

[0008] The average particle diameter and average aspect ratio of the graphite particles described herein are measured according to the following procedure. First, a graphite particle is observed at a magnification of 5,000 times using a scanning electron microscope (SEM), and the lengths of major axis and minor axis of the graphite particle are measured. The length of major axis of the graphite particle is defined as the maximum length of the graphite particle. The length of minor axis of the graphite particle is defined as the maximum length in a direction perpendicular to the major axis of the graphite particle. A graphite particle having a length of minor axis of 0.9 pm or less is excluded from the measurement target (because, for example, such measurement of 0.9 pm or less may correspond to the thickness of the graphite particle instead of the length in the minor axis). From the measured lengths of major axis and minor axis, the particle diameter and aspect ratio of the graphite particles are determined according to the following equations.

Particle diameter = (length of major axis + length of minor axis) / 2 Aspect Ratio = length of major axis / length of minor axis

The above particle diameter and aspect ratio are determined for 10 graphite particles, and the average values thereof are defined as the average particle diameter and average aspect ratio of the graphite particles, respectively.

[0009] According to examples, the content of graphite particles may be 10% by volume or more, 20% by volume or more, or 30% by volume or more, and may be 50% by volume or less, 40% by volume or less, 30% by volume or less, or 20% by volume or less, based on the total volume of the resistance heating element.

[0010] The resistance heating element may consist of the polyimide resin and the graphite particles, and may further contain other components. The other components may be selected from materials having a suitable heat resistance at a temperature at which the resistance heating element is used. Examples of other components indude resins other than polyimide resins, and conductive materials such as a carbon black, a carbon nanotube, and a metal powder.

[0011] The resistance heating element may be layered according to some examples. The layered resistance heating element is obtained by, for example, preparing a dispersion liquid containing the polyimide resin, the graphite particles, and a dispersion medium (for example, N- methylpyrrolidone (NMP)), applying the dispersion liquid to form a coating film, and then heating the coating film to remove the dispersion medium. The thickness of the layered resistance heating element may be, according to examples, 10 pm or more and 1000 pm or less.

[0012] When the resistance heating element is layered, the resistance heating element may have a single layer or may include a plurality of layers. In examples where the resistance heating element has a single layer, the polyimide resin and the graphite particles may be present substantially uniformly in the single layer. In examples where the resistance heating element includes a plurality of layers, the plurality of layers may include, for example, a polyimide layer in which the polyimide resin is unevenly distributed and a graphite layer in which the graphite particles are unevenly distributed. The polyimide layer may have a thickness of, for example, 1 .5 to 7 pm. The graphite layer may have a thickness of, for example, 1 to 5 pm. The plurality of layers may have, for example, a structure in which the polyimide layers and the graphite layers are alternately stacked. In this case, 3 to 10 of the polyimide layers and the graphite layers in a total may be present per 10 pm thickness of the resistance heating element.

[0013] The use of the resistance heating element is not particularly limited. The resistance heating element is used, for example, in an image forming apparatus. An example of the image forming apparatus includes a fixing device that fixes a toner image on a sheet by heating and pressing the sheet on which the toner image has been transferred. The image forming apparatus may further include known components other than the fixing device (for example, a photoreceptor, a charging device, an exposure device, a developing device, and a transfer device).

[0014] Fig. 1 is a schematic diagram illustrating an example fixing device 1 . The example fixing device 1 includes a fixing belt (fixing film) 2 and a pressure roller 3 located adjacent the fixing belt 2. The fixing belt 2 is an endless belt. The fixing belt 2 (endless belt) includes a resistance heating layer including the above-described resistance heating element. The pressure roller 3 has a shank 4 and an elastically deformable outer periphery 5 mounted around the shank 4. The fixing belt 2 and the pressure roller 3 are rotatable about respective axes. The pressure roller 3 presses the sheet P against the fixing belt 2. Accordingly, a nip region for fixing the toner image to the sheet P is formed between the fixing belt 2 and the pressure roller 3.

[0015] Fig. 2 is a diagram illustrating an example of a layer structure (or layer configuration) of the fixing belt 2. Namely, an example fixing belt 2A includes a resistance heating layer 6, a support layer 7 provided on an outer peripheral surface of the resistance heating layer 6, an elastic layer 8 provided on an outer peripheral surface of the support layer 7, and a surface layer 9 provided on an outer peripheral surface of the elastic layer 8.

[0016] Fig. 3 is a diagram illustrating another example layer structure (or layer configuration) of the fixing belt 2. Namely, an example fixing belt 2B includes a support layer 7, a resistance heating layer 6 provided on the outer peripheral surface of the support layer 7, an elastic layer 8 provided on the outer peripheral surface of the resistance heating layer 6, and a surface layer 9 provided on the outer peripheral surface of the elastic layer 8.

[0017] The resistance heating layer 6 includes the resistance heating element described above. That is, the resistance heating layer 6 contains the poiyimide resin and graphite particles according to the examples abovedescribed. Regarding the contents of the polyimide resins and the graphite particles in the resistance heating layer 6, the expression "based on the total volume of the resistance heating element" in the contents of the polyimide resins and the graphite particles of the resistance heating element described above, can be understood as "based on the total volume of the resistance heating layer 6". The thickness of the resistance heating layer 6 may be, according to examples, 10 pm or more or 50 pm or more and may be 1000 pm or less or 500 pm or less.

[0018] The support layer 7 may include a resin. Examples of the resin include polyimide resins, polyether ether ketone resins, polyamide-imide resins, and polyphenylene sulfide resins. According to examples, the support layer 7 may be, for example, 20 pm or more or 50 pm or more and may be 300 pm or less or 200 pm or less.

[0019] The elastic layer 8 may include a rubber. Examples of the rubber include fluorine rubbers and silicone rubbers. The thickness of the elastic layer 8 may be, for example, 100 pm or more and 300 pm or less. In other examples, a fixing belt may not include the above-described elastic layer.

[0020] The surface layer 9 can also be referred to as a release layer providing releasability. The surface layer 9 contains, for example, a fluororesin. Examples of the fluororesin include polytetrafluoroethylene (PTFE), tetrafiuoroethylene-perfluoroaikyl vinyl ether copolymer (PFA), and tetrafluoroethylene-hexafluoroethylene copolymer (FEP). In some examples, the surface layer 9 may contain a PFA tube. A known PFA tube can be used. The surface layer 9 may be, for example, 5 pm or more and 30 pm or less.

[0021 ] The fixing belt 2 further contains an electrode for supplying power to the resistance heating layer 6. The electrode is provided so as to be electrically connectable to the resistance heating layer 6, and is exposed so as to be electrically connectable to an external power source. The electrode may be made of, for example, Cu, Ni, Ag, Al, Au, Mg, or the like. The thickness of the electrode may be, for example, 1 pm or more and 50 pm or less.

[0022] The resistance heating element may be used in apparatuses other than the image forming apparatus described above. The resistance heating element may, in some examples, be used in an inkjet device, for example an industrial inkjet device. In some examples, the resistance heating element may be suitable as a heat source for drying inkjet ink ejected onto papers in the inkjet device.

Test Examples

[0023] Hereinafter, the resistance heating element will be described with reference to Test Examples, although it will be understood that the resistance heating element is not limited to such Test Examples.

[0024] A polyimide resin solution (ON-20 (trade name), manufactured by New Japan Chemical Co., Ltd.) was mixed with each of Graphite Particles No. 1 to 7 with a spatula according to the volumetric ratios shown in Table 1 . The Graphite Particles No. 1 to 7 have the following properties. _[0025]

_{Graphite Particles 1 :}

_{Average particle diameter: 2 μm , average aspect ratio: 1 .30, FT-2 (trade name) manufactured by Fuji Graphite Industry Co. , Ltd.}

_{Graphite Particles 2:}

_{Average particle diameter: 5 μm , average aspect ratio: 1 .31 , UP-5N (trade name), manufactured by Nippon Graphite Industry Co. , Ltd.}

_{Graphite Particles 3:}

_{Average particle diameter: 7 μm, average aspect ratio: 2.28, FS-5 (trade name) manufactured by Fuji Graphite Industry Co. , Ltd.}

_{Graphite Particles 4:}

_{Average particle diameter: 6 μm, average aspect ratio: 1 .23, CGB-6R (trade name) manufactured by Nippon Graphite Industry Co. , Ltd.}

_{Graphite Particles 5:}

_{Average particle diameter: 7 μm , average aspect ratio: 1 .21 , FT-7J (trade name) manufactured by Fuji Graphite Industry Co. , Ltd.}

_{Graphite Particles 6:}

_{Average particle diameter: 7 μm, average aspect ratio: 1 .56, PTG-7 (trade name) manufactured by Fuji Graphite Industry Co. , Ltd.}

_{Graphite Particles 7:}

_{Average particle diameter: 20 μm, average aspect ratio: 1 .37, MF-20N (trade name) manufactured by Fuji Graphite Industry Co. , Ltd.}

_{[0026] Subsequently, NMP was added to each of the m ixtures of polyimide resin solution and graphite particles so that the solid content was 15 to 20% by mass, and each m ixture was stirred with a homogenizer at 15,000rμm for 90 seconds to obtain a dispersion. The dispersion was applied onto a glass} substrate using a bar coater to form a coating film having a thickness of about 100 pm. The coating film was left to stand at room temperature (25 °C) for 30 minutes, and then heated in an oven at 200 °C for 270 minutes to obtain a resistance heating element having a thickness of about 10 pm.

[0027] Cross Section Observation of Resistance Heating Element

For the obtained resistance heating element of Test Example 1 , a cross section was exposed by cutting the resistance heating element perpendicularly to the glass substrate plane using a commercially available industrial scalpel, and the cross section was observed by a SEM. The obtained SEM image (1 ,000 magnifications) is shown in Fig. 4. It can be seen that the resistance heating element includes a plurality of layers.

[0028] Measurement of Resistance Value

For each of the resistance heating elements of the Test Examples and Comparative Test Examples, the resistance value was measured using a low resistivity meter (Loresta~GX MCP-T700 (trade name), manufactured by Nitto Seiko Analtec Corporation) equipped with an ASP probe. At that time, the thicknesses of the resistance heating elements for calculating the resistance values were measured using a thicknessmeter (Digimatic Indicator ID~A S112X (trade name), manufactured by Mitutoyo Corporation). The results are shown in Table 1. In Table 1 , "OL" indicates that the resistance value was too hiah to be measured. [0029] [Table 1 ]

[0030] As demonstrated above, in the resistance heating element described above, low resistance values are obtained by using graphite particles having the specific average partide diameter and average aspect ratio.

[0031] It is to be understood that not al! aspects, advantages and features described herein may necessarily be achieved by, or included in, any one particular example. Indeed, having described and illustrated various examples herein, it should be apparent that other examples may be modified in arrangement and detail is omitted.