2. Description of the Related Art Heat-resistant polyimide belts or tubes have found a wide variety of uses as flexible printed substrates, wire insulating materials, and belts for electrical or electronic pension devices. One particularly important application is in electrophotographic copiers, for example as a heat roller to fix toner or a transfer belt to transfer color. Transfer belts in electrophotographic copiers sequentially transfer and layer one or more component color images to form the copied image. Transfer belts typically have a multi-layer structure. Japanese Patent Application Laid Open No. 59-77467, for example, describes a transfer layer of silicone rubber disposed on a heat-resistant film of polyimide or the like. United States Patent No. 5,802,442 discloses a transfer layer comprising a hard, durable transfer layer (e. g., polyester, polyphenylene ether, polycarbonate, and the like) disposed on an elastomeric layer (e. g., natural rubber, urethane rubber, silicone rubber, and the like). United States Patent No. 5,208,638 discloses a conductive fluoroelastomer transfer surface, a metal layer, and a polyimide base layer.

One drawback to many of the aforementioned belts is the presence of a seam along the width of the belt. This seam is formed as a result of the welding of the opposite ends of the belt together during the manufacturing process. The seam defines a weak point in the belt, which may ultimately be the source of future maintenance problems, and can often result in the making of extraneous marks on the

paper during normal operation. While the presence of a seam can be compensated for by appropriate design and operation of electrophotographic copiers, seamless belts are far more desirable.

The aforementioned United States Patent No. 5,802,442, as well as United States Patent No 5,536,352, describes the method of centrifugation for the manufacture of seamless transfer belts. Centrifugation, however, requires investment in suitable centrifugal devices. A method for the manufacture of seamless polyimide tubes on a mandrel is disclosed in United States Patent No. 5,759,655, which discloses dipping the mandrel in a polyimide precursor solution, removing the coated mandrel, sliding a ring down the coated mandrel, and then drying and half-curing the polyimide precursor solution prior to application of the next layer. United States Patent No. 4,747,992 generally discloses forming seamless belts by coating mandrels by spray coating, dip coating, wire wound rod coating, powder coating, electrostatic spraying, sonic spraying, and blade coating. One drawback to many of these methods is the phenomenon known as"solvent popping", wherein an initial skin forms on the surface of the film and prevents the smooth, defect-free evaporation of solvent from under the film. Despite this multiplicity of methods, there accordingly remains a need in the art for improved, economical methods for the manufacture of seamless polyimide belts, particularly for electrophotographic copiers, which do not require expensive equipment or multiple steps.

SUMMARY OF THE INVENTION The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a method for the manufacture of seamless, polyimide belt, comprising coating a polyimide precursor solution onto a preheated mandrel to form a gel film, imidizing the precursor solution to form a cured transfer belt, and removing the transfer belt from the mandrel. In one embodiment, the mandrel is preheated in order to gel the polyimide precursor solution, which prevents slumping during subsequent processing. In another embodiment, the mandrel is flexible, which improves ease of removal of the cured transfer belt. The process is well suited to

coating high-viscosity liquids (i. e., above 20,000 cps) into high wet film thicknesses (i. e., above about 250 microns).

The above-discussed and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of a coating machine useful in the manufacture of seamless polyimide belts.

FIG. 2 is a plan view of the coating machine of FIG. 1 for use in the present method.

FIG. 3 is a perspective view of an alternate embodiment of the coating machine of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A method for the manufacture of seamless polyimide belts, particularly for electrophotographic copiers, comprises coating a polyimide precursor solution onto a mandrel, imidizing the precursor solution to form a cured belt, and removing the belt from the mandrel. The process is well-suited to coating high-viscosity liquids (i. e., above about 20,000 cps) into high wet film thicknesses (i. e., above about 250 microns). As used herein,"belt"refers to a continuous loop of any practical width, including tubes.

Polyimide precursor solutions are well-known in the art, being prepared, for example, by reacting aromatic tetracarboxylic acids and aromatic diamines in an organic, polar solvent. Exemplary tetracarboxylic acids include but are not limited to 2,2-bis [4- (3, 4-dicarboxyphenoxy) phenyl] propane dianhydride ; 4,4'-bis (3,4- dicarboxyphenoxy) diphenyl ether dianhydride; 4,4'-bis (3,4- dicarboxyphenoxy) benzophenone dianhydride ; 4,4'-bis (3,4- dicarboxyphenoxy) diphenyl sulfone dianhydride; 2,2-bis [4- (2, 3- dicarboxyphenoxy) phenyl] propane dianhydride; 4,4'-bis (2,3-

dicarboxyphenoxy) diphenyl ether dianhydride; 4,4'-bis (2,3- dicarboxyphenoxy) benzophenone dianhydride; 4,4'-bis (2,3- dicarboxyphenoxy) diphenyl sulfone dianhydride; 4- (2, 3-dicarboxyphenoxy)-4'- (3, 4- dicarboxyphenoxy) diphenyl-2,2-propane dianhydride; 4- (2, 3-dicarboxyphenoxy)-4'- (3,4-dicarboxyphenoxy) diphenyl ether dianhydride; 4- (2, 3-dicarboxyphenoxy)-4'- (3,4-dicarboxyphenoxy) benzophenone dianhydride and 4- (2, 3-dicarboxyphenoxy)-4'- (3,4-dicarboxyphenoxy) diphenyl sulfone dianhydride, as well as various mixtures thereof.

Suitable organic diamines include but are not limited to ethylenediamine, propylenediamine, 1,12-dodecanediamine, 1,18-octadecanediamine, 3- methylheptamethylenediamine, 2,5-dimethylheptamethylenediamine, 2,2- dimethylpropylenediamine,-methyl-bis (3-aminopropyl) amine, 3- methoxyhexamethylenediamine, 1, 2-bis (3-aminopropoxy) ethane, 1,4- cyclohexanediamine, bis- (4-aminocyclohexyl) methane, m-phenylenediamine, p- phenylenediamine, 2,4-diaminotoluene, 2,6-diaminotoluene, m-xylylenediamine, p- xylylenediamine, 2-methyl-4,6-diethyl-1,3-phenylene-diamine, 5-methyl-4,6-diethyl- 1,3-phenylene-diamine, benzidine, 3,3'-dimethylbenzidine, 3,3'-dimethoxybenzidine, 1,5-diaminonaphthalene, bis (4-aminophenyl) methane, bis (2-chloro-4-amino-3,5- diethylphenyl) methane, bis (4-aminophenyl) propane, 2,4-bis (b-amino-t-butyl) toluene, bis (p-b-amino-t-butylphenyl) ether, bis (p-b-methyl-o-aminophenyl) benzene, bis (p-b-methyl-o-aminopentyl) benzene, 1, 3-diamino-4-isopropylbenzene, bis (4- aminophenyl) sulfide, bis (4-aminophenyl) sulfone, bis (4-aminophenyl) ether, bis (3- aminophenyl) ether, bis (3-methoxy-4-aminophenyl) ether, 4,4'-diamino benzophenone, and 1, 3-bis (3-aminopropyl) tetramethyldisiloxane. Mixtures of these compounds may also be present.

Suitable solvents include, but are not limited to, N-methylpyrollidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAC), phenol, o-cresol, m-cresol, and p-cresol. The polyimide precursor solution may optionally be blended with fillers to impart desirable properties. For example, blending the polyimide precursor solution with carbon black may alter the electrical conductivity of the solution.

Likewise, blending the solution with boron nitride may enhance the thermal

conductivity. Suitable polyimide precursor solutions are available from E. I. DuPont High Performance Films division in Circleville, Ohio, those being used to produce type JP, type HA, and other types of KAPTONS polyimide film.

The preferred viscosity of the precursor solution is in the range from about 20,000 to about 200,000 centipoise (cps); preferably from about 50,000 to about 150,000 cps, and most preferably from about 80,000 to about 120,000 cps. Prior to coating, the precursor solution is agitated to insure homogeneity, filtered, and then degassed. To prolong the shelf life of the precursor solution, it is preferable to store it at 0°C or less.

Referring now to FIGS. 1 and 2, the polyimide precursor solution is applied to a cylindrical rotating surface (e. g., a mandrel or flexible belt) using a coating machine 10, which is essentially a modified lathe. Coating machine 10 comprises a tool post 12 having disposed on one end thereof (in place of the cutting tool of a conventional lathe) a doctor blade, shown generally at 14, which is a substantially"U"shaped member having tapered legs 15 protruding therefrom to form blade portions. Tapered legs 15 define a reservoir 17, which is the area of doctor blade 14 between tapered legs 15. The polyimide precursor solution, as it is applied, is pumped into reservoir 17 through a delivery hose 16.

A cylindrical coating mandrel 18 is rotatably mounted on coating machine 10 in the place that a workpiece being cut would normally be mounted on a conventional lathe. Mandrel 18 comprises a cylindrical metal body having extremely precise diameter control and low circular run out relative to the axis of rotation of mandrel 18 and is positioned axially between a lathe headstock 20 and a lathe tailstock 22.

Mandrel 18 is also positioned such that a gap 23, which can be best seen in FIG. 2, is defined between the ends of tapered legs 15 and an outer surface of mandrel 18, thereby allowing doctor blade 14 to freely traverse the length of mandrel 18. The surface of mandrel 18 is polished so as to result in a smooth inner surface of the finished polyimide belt once the polyimide belt is removed from mandrel 18. The surface may also have a coating, such as a fluoropolymer or a silicone release agent, disposed thereon to facilitate the release of the polyimide belt from mandrel 18.

Referring now to FIG. 3, an alternative embodiment of a coating machine 10 is shown. Coating machine 10 comprises a drive pulley 24 disposed between lathe headstock 20 and lathe tailstock 22, an idler pulley 26 located a suitable distance away and oriented to accept a flexible, thin belt mandrel 30, a parallelism adjustment mechanism 28 in mechanical communication with drive pulley 24, and an overall tensioning mechanism 32 in mechanical communication with idler pulley 26.

Parallelism adjustment mechanism 28 is configured to"steer"or move idler pulley 26 laterally relative to drive pulley 24, thereby ensuring that"creep", or lateral movement of thin belt mandrel 30 on drive pulley 24, is minimized. Overall tensioning mechanism 32 comprises a threaded rod 31 having an adjustment knob 33 threadedly disposed thereon and is configured such that rotation of adjustment knob 33 varies the distance between drive pulley 24 and idler pulley 26, thereby varying the tension on thin belt mandrel 30. Thin belt mandrel 30 is preferably a seamless belt, and may be fabricated of metal a high temperature polymer capable of withstanding the drying and imidization steps of the subsequently coated precursor solution.

The operation of coating machine 10 described below applies to the embodiments of either FIGS. 1 and 2. For the sake of brevity, reference will be made to the embodiment of FIGS. 1 and 2 only. To coat mandrel 18, toolpost 12 traverses the width of mandrel 18 while being driven by a lead screw (not shown) and guided by machine ways (not shown). Stops 34,36 are located at the extreme ends of mandrel 18 to indicate the start and end positions of the path of doctor blade 14 as it traverses mandrel 18. Toolpost 12 is mounted on a lathe bed (not shown) on an infeed mechanism (not shown), which allows doctor blade 14 to traverse the length of mandrel 18 while maintaining gap 23 with extreme precision. During operation of coating machine 10, gap 23 is varied by adjusting the positioning of the infeed mechanism.

In addition to coating machine 10, the inventive method provides for storage for unused precursor solution and a delivery apparatus for delivery of the precursor solution through delivery hose 16 to reservoir 17 at a controlled rate. In a preferred embodiment, a holding tank (not shown), which provides for low-shear continuous

mixing and which is held under constant vacuum, is used for storage. Continuous mixing and storage under vacuum aids in maintaining a homogeneous mixture of polymer, solvent, and any fillers present in the solution. Storage under vacuum also removes any air bubbles that would otherwise result in voids being formed in the final belt. It is also preferred to provide inline filters to remove any gel particles or other foreign material present in the precursor solution.

To coat mandrel 18, the polyimide precursor solution is delivered from the holding tank, at a controlled rate, by a gear pump (not shown) or other suitable pumping system through delivery hose 16. The flow rate is monitored and controlled by mass flow controllers. The precursor solution is pumped through delivery hose 16 to reservoir 17 of doctor blade 14. Doctor blade 14 is positioned and configured to maintain gap 23 between tapered legs 15 and the surface of mandrel 18 as doctor blade 14 traverses the length of mandrel 18. As mandrel 18 rotates, doctor blade 14 traverses the length of mandrel 18 at a predetermined rate of speed as the precursor solution is pumped to fill reservoir 17 and gap 23. The coating progresses in a spiral fashion, with each spiral overlapping slightly to form a continuous, wet film. The thickness of the wet film is determined by gap 23 and the flow rate of polyimide precursor solution to reservoir 17.

High quality, smooth, economical coatings may be obtained by matching the precursor solution flow rate with gap 23 as doctor blade 14 traverses the length of mandrel 18. This is achieved by proper adjustment of the headstock rotation speed and the toolpost traverse speed. The proper adjustment of these settings is a function of at least two parameters, viz., the flow rate of the polyimide precursor solution to reservoir 17 and the rotational speed of mandrel 18. In general, it is desirable to coat as quickly as possible, while simultaneously providing the correct amount of spiral overlap without causing turbulence (and hence air entrapment). Determination of the proper settings to provide smooth, non-interrupted coatings is well within the skill of those in the art.

Once the coating is complete, the mandrel continues rotation while an effective quantity of heat is applied to the coated surface. The heat causes the evaporation of the solvent contained within the polyimide precursor solution and the

formation of a gel film. Heating continues until the coating is dry enough to prevent dripping or slumping of the coating. However, as mentioned above, this heating step can cause initial formation of an exterior film, which may result in solvent popping.

Accordingly, in a preferred embodiment, mandrel 18 is preheated to a temperature effective to result in gelation of the polyimide precursor solution upon coating, which prevents dripping and slumping during subsequent processing.

Effective temperatures are readily determined by those of ordinary skill in the art, depending on the boiling point of the solvent and the viscosity of the polyimide precursor solution. If, during coating, the temperature of mandrel 18 is below the effective temperature, gelation is slowed, whereas if the temperature of mandrel 18 exceeds the effective temperature, the solvent evaporates too quickly and causes defects in the film. Effective preheat temperatures are generally from about 30°C to about 100°C, more preferably about 60°C to about 90°C, and most preferably from about 70°C to about 80°C. In general, mandrel 18 is preheated to approximately 75°C prior to being loaded onto coating machine 10. Suitable initial drying times are determined empirically for each size belt and coating thickness. In general, sufficient drying is achieved once the film comprises approximately 50% solids.

After this initial drying, the mandrel may be removed from coating machine 10 and baked in an oven for further drying and imidization (cure). The oven temperature is typically ramped from the drying temperature of approximately 150°C to approximately 350°C over a period of approximately one hour. Mandrel 18 may be suspended vertically or horizontally during the imidization bake, and may or may not be rotated during the baking process. If the process is being applied to thin belt mandrel 30, a support form may be needed to enable thin belt mandrel 30 to hold its shape and to withstand the hoop stress resulting from the forces of the shrinking polyimide coating during the imidization bake, regardless of whether or not thin belt mandrel 30 is being rotated. At the end of the imidization bake, mandrel 18 or thin belt mandrel 30 is allowed to cool to room temperature.

Once cooled to room temperature, the fully imidized belt is removed from mandrel 18 by sliding the belt laterally off mandrel 18. A flow of compressed air directed at the belt edge is useful to start separation. A polyimide belt removal is

most easily accomplished by flexing mandrel 18 inward towards the center of rotation while peeling the polyimide belt off the surface of mandrel 18. A belt that is thin and flexible is most easily removed during the belt removal step. If mandrel 18 is solid and has a rigid coating, this peeling method cannot be used, so greater care must be taken so as to not damage the belt or mar the surface of mandrel 18 during belt removal.

Prior to installation and use, the belt edges may be trimmed to remove the non-uniform edges resulting from the beginning and end of the coating spiral.

Trimming may be achieved by rotating the belt while employing stationary razor blades, or by laser cutting, waterj et cutting, or other suitable technique, provided it can result in a straight, nick-free edge. Such belts are suitable for use in electrophotographic copiers, as insulators, and other applications.

The above-described method provides numerous advantages over the prior art methods. For example, the prior art method of dip-coating a mandrel into precursor solution precludes effective use of a preheated mandrel, and requires the use of external heat to gel the film in order to prevent dripping or slumping. However, the need to apply external heat to the coated mandrel often results in solvent popping.

Accordingly, only very thin coatings very thin coatings can be successfully formed without dripping, slumping, or solvent popping. The present process, in contrast, allows for the formation of thicker coatings (e. g., greater than about 50 microns, preferably greater than about 100 microns, and preferably greater than about 150 microns) without solvent popping. Seamless polyimide belts having final thicknesses of about 20 to about 300 microns, and preferably about 25 to about 150 microns, may be manufactured using the present method. In addition, the present process is easily automated in a high volume production mode. Use of thin, flexible mandrels are more economical than solid, rigid mandrels. Thin, flexible mandrels are also lighter, and therefore easier to handle, and are more easily fabricated in large diameters than solid mandrels of comparable size.

The invention is further illustrated by the following non-limiting Example.

EXAMPLE A coating machine substantially as shown in FIG. 1 was employed to manufacture seamless polyimide belts. The coating machine was fitted with a mandrel having a diameter of 150 mm and a width of 400 mm and preheated to 75°C.

The doctor blade gap was set at 0.500 mm, the headstock rotation speed was set at 200 rpm, the toolpost traverse speed was set at 100 mm per minute, and the volumetric flow rate of the polyimide precursor solution (100,000 cps) was 24 cubic centimeters per minute. A gel film which did drip or slump prior to cure was formed without additional heating.

After coating, the gel film was cured by heating for 30 minutes at 150°C, 30 minutes at 200°C, 30 minutes at 250°C, 30 minutes at 300°C, and 30 minutes at 350°C.

While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.

What is claimed is: