In electrostatographic imaging and recording processes such as electrophotographic printing, an electrostatic latent image is formed on a primary image-forming member such as a photoconductive surface and is developed with a thermoplastic toner powder to form a toner image. The toner image is thereafter transferred to a receiver member, e.g., a sheet of paper or plastic, and the toner image is subsequently fused or fixed to the receiver member in a fusing station using heat and/or pressure. The fusing station includes a heated fuser member which can be a roller, belt, or any surface having a suitable shape for fixing thermoplastic toner powder to the receiver member. Fusing typically involves passing the toned receiver member between a pair of engaged rollers that produce an area of pressure contact known as a fusing nip. In order to form the fusing nip, at least one of the rollers typically includes a compliant or conformable layer. Heat is transferred from a heated roller fuser member to the toner in the fusing nip, causing the toner to partially melt and attach to the receiver member.
Normally included in a compliant heated fuser member roller is a resilient or elastically deformable base cushion layer (e.g., an elastomeric layer). The base cushion layer is usually covered by one or more concentric layers, including a protective outer layer. The base cushion layer is typically bonded to a core member included in the roller, with the roller having a smooth outer surface. Where the fuser member is in the form of a belt, e.g., a flexible endless belt that passes around the heated roller, it commonly has a smooth outer surface which may also be hardened. Similarly, a resilient base cushion layer can be incorporated into a deformable pressure roller used in conjunction with a relatively hard fuser roller.
Simplex fusing stations attach toner to only one side of the receiver member at a time. In this type of station, the engaged roller that contacts the unfused toner is commonly known as the fuser roller and is a heated roller. The roller that contacts the other side of the receiver member is known as the pressure roller and is usually unheated. Either or both rollers can have a compliant layer on or near the surface. It is common for one of these rollers to be driven rotatably by an external source while the other roller is rotated frictionally by the nip engagement.
In a duplex fusing station, which is less common, two toner images are simultaneously attached, one to each side of a receiver passing through a fusing nip. In such a duplex fusing station there is no real distinction between fuser roller and pressure roller, both rollers performing similar functions, i.e., providing heat and pressure.
It is known that a compliant fuser roller, when used in conjunction with a harder or relatively non-deformable pressure roller, e.g., in a Digimaster 9110 machine made by Heidelberg Digital L.L.C., Rochester, N.Y., provides easy release of a receiver member from the fuser roller, because the distorted shape of the compliant surface in the nip tends to bend the receiver member towards the relatively non-deformable unheated pressure roller and away from the much more deformable fuser roller. On the other hand, when a conformable or compliant pressure roller is used to form the fusing nip against a hard fuser roller, such as in a DocuTech 135 machine made by Xerox Corporation, Rochester, N.Y., a mechanical device such as a blade is typically necessary as an aid for releasing the receiver member from the fuser roller.
A conventional toner fuser roller includes a rigid cylindrical core member, typically metallic such as aluminum, coated with one or more synthetic layers usually formulated with polymeric materials made from elastomers. An elastically deformable or resilient base cushion layer, which may contain filler particles to improve mechanical strength and/or thermal conductivity, is typically formed on the surface of the core member, which core member may advantageously be coated with a primer to improve adhesion of the resilient layer. Roller cushion layers are commonly made of silicone rubbers or silicone polymers such as, for example, polydimethylsiloxane (PDMS) polymers disclosed, e.g., by the Chen, et al., patent (U.S. Pat. No. 6,224,978, assigned to Eastman Kodak Company, Rochester, N.Y.).
The most common type of fuser roller is internally heated, i.e., a source of heat is provided within the roller for fusing. Such a fuser roller generally has a hollow core member, inside of which is located a source of heat, usually a lamp. Surrounding the core member can be an elastomeric layer through which heat is conducted from the core member to the surface, and the elastomeric layer typically contains fillers for enhanced thermal conductivity [see for example the Fitzgerald patents (commonly assigned U.S. Pat. Nos. 5,292,606 and 5,336,539) and the Fitzgerald, et al., patent (commonly assigned U.S. Pat. No. 5,480,724)]. An internally heated fuser roller can be made using a condensation-polymerized silicone rubber material including solid filler particles, such as for example used in a NexPress 2100 digital color press (manufactured by NexPress Solutions LLC, Rochester, N.Y.).
Less common is an externally heated fuser roller, which fuser roller is typically heated by surface contact with one or more heating rollers. An externally heated fuser roller can be made using an addition-polymerized silicone rubber material including solid filler particles. Externally heated fuser rollers are for example disclosed by the O'leary patent (U.S. Pat. No. 5,450,183, assigned to Eastman Kodak Company, Rochester, N.Y.), the Derimiggio, et al., patent (commonly assigned U.S. Pat. No. 4,984,027), the Aslam, et al., patent (commonly assigned U.S. Pat. No. 6,567,641), and the Chen, et al., patent (commonly assigned U.S. Pat. No. 6,490,430). Inclusion of thermal-conductivity-enhancing fillers enhances heat transfer from one or more external heating rollers typically used for the external heating of the fuser roller. Moreover, the thermal-conductivity-enhancing fillers enable intermittent use of an auxiliary heating device located within the roller.
Some fuser rollers rely on film splitting of a low viscosity oil to enable release of the toner and (hence) receiver member from the fuser roller. The release oil is typically applied to the surface of the fuser from a donor roller coated with the oil provided from a supply sump. A donor roller is for example disclosed in the Chen, et al., patent (commonly assigned U.S. Pat. No. 6,190,771) which is hereby incorporated by reference.
Release oils (commonly referred to as fuser oils) are composed of, for example, polydimethylsiloxanes. When applied to the fuser roller surface to prevent the toner from adhering to the roller, fuser oils may, upon repeated use, interact with PDMS material included in the resilient layer(s) in the fuser roller, which in time can cause swelling, softening, and degradation of the roller. To prevent these deleterious effects caused by release oil, a thin barrier layer made of, for example, a cured fluoroelastomer and/or a silicone elastomer, is typically formed around the resilient cushion layer, as disclosed in the Davis, et al., patent (U.S. Pat. No. 6,225,409, assigned to Eastman Kodak Company, Rochester, N.Y.) and the Chen, et al., patents (U.S. Pat. No. 5,464,698, and 5,595,823, assigned to Heidelberg Digital L.L.C., Rochester, N.Y.). A fluoro-thermoplastic random copolymer outermost coating can also be used for this purpose, as disclosed in the Chen, et al., patents (commonly assigned U.S. Pat. Nos. 6,355,352 B1 and 6,361,829 B1).
To rival the photographic quality produced using silver halide technology, it is desirable that electrostatographic multicolor toner images have high gloss. To this end, it is desirable to provide a very smooth fusing member contacting the toner particles in the fusing station. A gloss control outer layer (which also serves as a barrier layer for fuser oil) can be provided as disclosed in the Chen, et al., patent application (commonly assigned U.S. patent application Ser. No. 09/608,290). A fluorocarbon thermoplastic random copolymer useful for making a gloss control coating on a fuser roller is disclosed in the Chen, et al., patent (commonly assigned U.S. Pat. No. 6,429,249).
In the fusing of the toner image to the receiver member, the area of contact of a conformable fuser roller with the toner-bearing surface of a receiver member sheet as it passes through the fusing nip is determined by the amount of pressure exerted by the pressure roller and by the characteristics of the resilient cushion layer. The extent of the contact area helps establish the length of time that any given portion of the toner image will be in contact with and heated by the fuser roller. It is generally advantageous to increase the contact time by increasing the contact area so as to result in a more efficient fusing process. However, unless the effective modulus for deforming a compliant roller in the nip is sufficiently low, high nip pressures are required to obtain a large nip area. Such high pressures can be disadvantageous and cause damage to a deformable roller, e.g., such as pressure set or other damage caused by edges of thick and/or hard receiver members as they enter or leave the nip. Hence a low modulus deformable roller is desirable.
It is known from the Chen, et al., patent (commonly assigned U.S. Pat. No. 5,716,714) that use of a relatively soft deformable fusing-station roller (e.g., a deformable pressure roller having a low effective modulus for deformation) can advantageously reduce the propensity of a fusing station nip to cause wrinkling of receiver members passing through the nip.
One way to try to create a low modulus fusing-station roller is to use a foamed material, e.g., a cured material having an open-cell or a closed-cell foam structure, with the material inclusive of suitable strength-enhancing and/or thermal-conductivity-enhancing fillers. Attempts to utilize such foamed materials, for example as base cushion layers, have not generally been successful, for a number of reasons. The thermal conductivity of closed-cell structures tends to be disadvantageously low, even when squeezed in a fusing nip. Although an open-cell structure can be squeezed relatively flat in a fusing nip, the resilience typically becomes compromised because opposite walls within the foam tend to stick together under the heat and pressure of the nip. Moreover, foamed polymeric materials generally have poor tear strength, and shear strength also tends to be low. As a result, fusing-station rollers incorporating foamed base cushion layers are quite susceptible to damage and tend to age rapidly.
In particular, attempts to make fusing-station rollers with fluoroelastomer foamed materials, which have desirable low surface energy and high thermal stability, have not been successful because of the tendency to incur a “pressure set” under the high loading typically present in fusing station nips. For example, foam rollers made with VITON® fluoroelastomers are susceptible to “pressure set”.
Suitable thermal conductivity of synthetic layers used in fusing-station rollers is attainable by the use of one or more suitable particulate fillers, the thermal conductivity being determined by the filler concentration. The thermal conductivity of most polymers is very low and the thermal conductivity generally increases as the concentration of thermally conductive filler particles is increased. However, if the filler concentration is too high, the mechanical properties of a polymer are usually compromised. For example, the stiffness of the synthetic layers may be increased by too much filler, e.g., so that there is insufficient compliance to create a wide enough nip for proper fusing. Moreover, too much filler will cause the synthetic layers to have a propensity to delaminate or crack or otherwise cause failure of the roller. Because the mechanical requirements of fusing-station rollers require that the filler concentrations generally be moderate, the abilities of the layers to transport heat are thereby limited. In fact, the total concentration of strength-enhancing and thermal-conductivity-enhancing in prior art internally heated compliant fuser rollers has reached a practical maximum. As a result, the number of copies that can be fused per minute is limited, and this in turn can be the limiting factor in determining the maximum throughput rate achievable in an electrostatographic printer.
An auxiliary internal source of heat may optionally be used with an externally heated fuser roller, e.g., as disclosed in the Aslam, et al., patent (commonly assigned U.S. Pat. No. 6,567,641) and in the Chen, et al., patent (commonly assigned U.S. Pat. No. 6,490,430). Such an internal source of heat is known to be useful when the fusing station is quiescent and/or during startup when relatively cold toned receiver members first arrive at the fusing station for fusing therein. In order for such an auxiliary internal source of heat to be effective (when intermittently needed) the fuser roller must have a sufficiently large thermal conductivity. However, this requirement conflicts with a need to keep heat at the surface of an externally heated fuser roller, i.e., so as not to unnecessarily conduct heat into the interior which would compromise the fusing efficiency of the roller. On the other hand, it is important to have a high enough thermal conductivity at the surface of the fuser roller to ensure efficient transfer of heat to the fuser roller from one or more heating rollers contacting the surface. Moreover, in order to have high efficiency, externally heated fuser rollers rely to a certain extent on thermal conduction of heat around the surface of the roller.
Ways to improve upon the above-described limitations associated with externally heated elastically deformable fuser rollers (including an optional auxiliary internal source of heat) are disclosed in the Chen, et al., patent applications (commonly assigned U.S. patent application Ser. Nos. 10/139,486 and 10/139,464). In the Chen, et al., U.S. patent application Ser. No. 10/139,486, an externally heated fuser roller having improved efficiency includes a core member, a base cushion layer around the core member, a relatively thin heat storage layer around the base cushion layer, and an outer gloss control layer around the heat storage layer, wherein the heat storage layer is loaded with more thermally conductive filler than is the base cushion layer and hence has a higher thermal conductivity. In the Chen, et al., U.S. patent application Ser. No. 10/139,464, a thin heat distribution layer is further included between the heat storage layer and the outer gloss control layer. While the fusing efficiencies relating to U.S. patent application Ser. Nos. 10/139,486 and 10/139,464 are much improved, the fuser rollers (respectively having three-layer and four-layer structures around the core member) are relatively expensive to manufacture, and may be susceptible to delamination with prolonged use.
It is known that instead of solid fillers, certain hollow fillers can be included in an addition-polymerized silicone rubber for the purpose of lowering rather than increasing the thermal conductivity of a deformable fuser roller, as disclosed in the Meguriya patent (U.S. Pat. No. 6,261,214, assigned to Shin-Etsu Chemical Company, Ltd., Tokyo, Japan). In particular, the Meguriya patent discloses incorporation into the silicone rubber of hollow filler particles (also known as microballoons) manufactured under the tradename EXPANCEL® available from Expancel, Sundsvall, Sweden, and Duluth, Ga.
Hollow microballoons are well known and are disclosed for example in the Morehouse, et al., patent (U.S. Pat. No. 3,615,972, assigned to Dow Chemical Company, Midland, Mich.). Microballoons are made from thermoplastic microspheres which encapsulate a liquid blowing agent, typically a hydrocarbon liquid. Such microspheres are made in unexpanded form. The walls of the unexpanded microspheres are generally impermeable to the liquid blowing agent, i.e., diffusion of molecules of the liquid blowing agent through the walls is typically negligible. An expanded form of a microsphere, i.e., a microballoon, is obtained by heating an unexpanded microsphere to a suitable temperature so as to vaporize the blowing agent, thereby causing the microsphere to grow to a much larger size. Too high of a heating temperature can result in some loss of internal vapor pressure and a shrinking of the microballoon. Methods for expanding microspheres are disclosed in numerous patents, such as, for example, the Gunderman, et al., patent (U.S. Pat. No. 3,914,360, assigned to Dow Chemical Company, Midland, Mich.), the Edgren, et al., patent (U.S. Pat. No. 4,513,106, assigned to KemaNord AB, Stockholm, Sweden) and the Morales, et al., patent (U.S. Pat. No. 6,235,801 B1). Expansion is generally irreversible after cooling, and the expanded microballoon form is stable under normal ambient conditions and can be sold as a dry powder or alternatively as a slurry in a liquid vehicle. Expanded microspheres or microballoons which are available commercially can be incorporated into various materials, such as for example to make improved paints or lightweight parts. Unexpanded microspheres are also available commercially for incorporation into various types of materials (e.g., expandable inks) or for manufacture of solid parts, e.g., by thermal curing in a mold so as to expand the microspheres. The shell material of certain microsphere particles can include finely divided inorganic particles, e.g., silica particles.
The use of microspheres in a compressible layer of a digital printing blanket carcass is disclosed in the Castelli, et al., patent (U.S. Pat. No. 5,754,931, assigned to Reeves Brothers, Incorporated, Spartanburg, S.C.). The microspheres are uniformly distributed in a matrix material which includes thermoplastic or thermosetting resins.
The Dauber, et al., patent (U.S. Pat. No. 5,916,671, assigned to W.L. Gore & Associates, Incorporated, Newark, Del.) discloses a resilient gasket made of a composite of polytetrafluoroethylene and resilient expandable microspheres.
There remains a need to provide for an electrostatographic machine an improved fusing station having high productivity which includes fusing-station members that are simple in construction, are very durable, and are made of material that can resist gouges or pressure damage from edges of receiver members moving through a high pressure fusing nip.
Specifically, there remains a need for a tough, long lasting fuser roller which can have only one layer coated on a core member, and which is thereby simple in structure by comparison with multi-layer prior art fuser rollers. This layer is required to be chemically unreactive, stable at high temperatures, and impervious to fuser oil. Moreover, there remains a need for an improved pressure roller having a similarly simple structure and which has similar properties.
A crosslinked fluoroelastomer is a desirable material for making fuser rollers and pressure rollers, because of low surface energy, chemical inertness, imperviousness to fuser oil, and high-temperature stability.