EP0138885B1

EP0138885B1 - Anodized electrostatic imaging surface

Info

Publication number: EP0138885B1
Application number: EP19840901160
Authority: EP
Inventors: Richard A. Fotland; Leo A. Beaudet
Original assignee: Dennison Manufacturing Co
Current assignee: Dennison Manufacturing Co
Priority date: 1983-02-22
Filing date: 1984-02-21
Publication date: 1988-09-07
Also published as: DE3473943D1; EP0138885A4; EP0138885A1; CA1251984A; WO1984003366A1

Abstract

Dielectric sealing of porous anodized aluminum, in which moisture in the pores of the oxide coating formed by hard coat anodizing is removed, and the porous anodized surface then impregnated with a dielectric wax. Suitable wax sealants include carnauba and montan waxes. The anodized member (35) is preliminarily heated in order to drive off moisture and other substances from the pores; this heating process may be continued for the purposes of impregnating the pores with the wax sealant (35). Any excess material remaining on the member's surface is removed. After removing material from the member's surface, the member may be polished to a better than .5 micrometer finish. Dielectric imaging members (35, 36) fabricated using these techniques are suitably incorporated in high speed electrographic printers and copiers. One such printing system forms a latent electrostatic image on a dielectric cylinder (36) using an ion emitting print device (30); tones the image; transfers the toned image to an image receptor (50) using high pressure exerted between the dielectric cylinder (36) and a compliant transfer cylinder (48); scrapes off residual toner on the dielectric cylinder; and neutralizes any residual electrostatic image.

Description

The present invention relates to the sealing of anodized aluminum and aluminum alloy structures to achieve superior dielectric properties. More particularly, the invention relates to the production of hard, abrasion resistant dielectric members and to electrostatic imaging processes and apparatus utilizing such members.
Electrostatic printers have been proposed which make use of a member commonly in the form of a cylinder and consisting of an electrically conductive core coated with a dielectric material capable of receiving a pattern of electrostatic charge from a discharge device. This device is so controlled that a selected pattern of charge can be applied to the surface of the cylinder as it passes the device. Subsequently, this pattern is toned using, for example, particulate toner supplied by a suitable feed system, and then the toned image on the cylinder is transferred at a nip with a pressure roller to a receptor medium such as a sheet of paper as the paper passes through the nip. This transfer may or may not include toner fusing depending upon the nip pressure and also, for best results, on whether or not the cylinder and roller are skewed relative to one another. Subsequently, any remaining toner is scraped off mechanically and any electrostatic charge on the cylinder is dissipated as the cylinder passes a discharge device prior to receiving another selected pattern of charge. Apparatus of this type is disclosed in commonly assigned U.S. Patent No. 4,267,556.
In such a printer, the cylinder must satisfy a number of design criteria. Firstly, the surface should receive the desired pattern of charge accurately and without variations in electrostatic intensity within the pattern. The surface should maintain the pattern without significant dissipation before reaching the nip, and also the pattern must be dissipated by the discharge device leaving as nearly as possible no charge pattern on the cylinder. All of these criteria should be met ideally in a range of temperature and humidity variations which may be controlled within limits. Other desirable criteria relate to the mechanical requirements of the cylinder surface. The forces applied at the nip demand that the dielectric surface withstand a large distributed load which will, of course, result in some strain on the cylinder. Further, because the paper feeding into and out of the nip represents an impact loading and unloading, there are suddenly-applied local forces which the dielectric layer must resist. Also, when the cylinder and pressure roller are skewed, the paper is made to follow the pressure roller rather than the cylinder to cause sheer in the toner. The resultant relative movement between the dielectric layer and the paper could result in abrasion of the dielectric layer because the toner acts as an abrasive between the paper and the surface of the layer. The layer must withstand a mechanical scraper normally used to strip excesss toner off the cylinder after the majority of the toner has been transferred to the paper. Other potential problems relates to nonuse of the machine while a load is maintained at the nip, and also to ambient temperature and moisture variations, which should have no significant lasting effect on the cylinder.
U.S. Patent No. 4,195,927 discloses electrophotographic apparatus identical in construction to the '556 printing apparatus, except for the means for forming the latent electrostatic image on the dielectric cylinder. In the '927 apparatus, the latent electrostatic image is formed on a photoreceptor by conventional electrophotographic techniques, and transferred by TESI to the dielectric cylinder. The criteria for the '927 dielectric cylinder match those discussed above.
Hardcoat anodization of aluminum and aluminum alloys is an electrolytic proces which is used to produce thick oxide coatings with substantial hardness. Such coatings are to be distinguished from natural films of oxide which are normally present on aluminum surfaces, and from thin, electrolytically formed barrier coatings. The anodization of aluminum to form thick dielectric coatings takes place in an electrolytic bath containing an acid, such as sulfuric or oxalic acid, in which aluminum oxide is slightly soluble. The production techniques, properties, and applications of these aluminum oxide coatings are described in detail in The Surface Treatment and Finishing of Aluminum and Its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd., Paddington, England (chapter IX page 563). Such coatings are extremely hard and mechanically superior to uncoated aluminum. However, the coatings contain pores in the form of fine tubes with a porosity on the order of 10¹⁰ to 10¹² pores per square inch (1.6-10⁹ to 1.6-10" per square centimetre). Typical porosities range from 10 to 30 percent by volume. These pores extend through the coating to a very thin barrier layer of aluminum oxide, typically 300 to 800 Angstroms (30 to 80 nm).
Various treatment techniques have been applied to the processing'of anodized aluminum members to create a dielectric surface. Illustrative patents include U.S. Patent Nos. 3,615,405; 3,715,211; 3,907,560; 3,937,571; and 3,940,270. The foregoing references do not teach suitable techniques for the fabrication of an aluminum member of a type which is suitable for electrostatic imaging processes using pressure fusing and transfer, as discussed above.
U.S. Patent No. 3,782,997 discloses a method for treating anodized beryllium members to produce corrosion resistant dielectric surfaces. After anodizing, the beryllium members are cleaned, baked at 250°F (121°C) in a normal atmosphere, then at 200°F (94°C) in a vacuum to remove residual moisture. The article is cooled at 160°F (71°C) to seal the pores with an epoxy resin or similar material, using high pressure to facilitate impregnation. Excess material is removed by bleeding the member or rinsing it with a solvent. Finally, the member may be maintained at 212°F (100°C) for several hours to cure the impregnant material. This reference does not teach the production of a dielectric member having the surface properties required for good toner transfer under pressure.
Commonly assigned U.S. Patent Application Serial No. 072,524, which is a continuation-in- part of application Serial No. 822,865, now abandoned, discloses a method for forming a dielectric surface layer involving the preliminary dehydration of an anodized aluminum member followed by impregnation of surface apertures of the dehydrated member with an organic dielectric material. The preliminary dehydration may be accomplished by heating the anodized member in a vacuum or in air, or alternatively by storing it in a desicant container. This application discloses a class of impregnant materials broadly described as organic resins. The method disclosed therein has been found effective to fabricate a dielectric surface with improved resistivity, dielectric properties, and toner releases properties. It has been observed, however, that the dielectric properties are deleteriously affected by elevated humidities. Because these materials are usually applied at room temperature, special measures must be taken to control the environment during impregnation to minimize the risk of dehydration. Furthermore, it can be difficult to remedy the problem of an initially uneven application of the impregnant material.
Commonly assigned U.S. Patent No. 4,413,049 discloses an improvement to the above method wherein the impregnant materials are metallic salts of fatty acids. These are typically applied to seal the anodized aluminum member while the latter is maintained at an elevated temperature above the melting point of the impregnant material. These materials provide the advantages of ease of fabrication and improved dielectric properties at high humidities, but may suffer undesirably high dielectric absorption under certain conditions (such as prolonged storage in high humidities). In other words, under unfavourable operating conditions there will be a tendency toward retention of subsurface charge in the impregnated anodic layer. During neutralization of the dielectric surface this charge will migrate to the surface providing an undesirable residual potential.
US-A-3615405 discloses those features listed in the pre-characterising portions of the independent claims 1, 9, 15 and 22.
It is a primary object of this invention to provide desired dielectric properties in the treatment of members of porous anodized aluminum and aluminum-based alloys. A related object is to improve the dielectric strength and increase the resistivity of such members. Another related object is the achievement of thick dielectric surface layers with a high voltage acceptance and low charge decay rates.
It is a further object of the invention to provide a treated aluminum surface that will yield essentially total pressure transfer of a toned electrostatic image to plain paper and other substrates.
Yet another object of the invention is the achievement of a surface which maintains the above properties at elevated humidities.
Still another object of the invention is that the fabrication technique be easily implementable. As a related object, the technique should allow simple remedial steps to meet the above criteria where the initial fabrication is unsuccessful.
Further objects of the invention are hardness and abrasion resistance which would allow pressure transfer and fusing of electrostatic toner, while providing an extended operating life.
It is also desirable that such surfaces permit neutralization of most or all of any residual electrostatic image, i.e. have reduced dielectric absorption.
Thus, according to a first aspect of the invention a method of treating a member to form a dielectric surface layer is characterised by the features set out in Claim 1.
According to a second aspect, a printer is characterised by the features of Claim 9.
According to a third aspect, an electrostatic imaging apparatus is characterised by the features of Claim 15.
According to a fourth aspect, a combination of a cylinder and a roller for use in transferring a toned image from the cylinder to a receptor sheet is characterised by the features of Claim 22.
In a preferred embodiment of the invention a dielectric imaging cylinder produced according to the above method is incorporated in electrographic printing apparatus in which latent electrostatic images are formed on the surface of the cylinder in response to electronic input.
The process for manufacturing the dielectric member may include the anodizing of an aluminum or aluminum alloy member, dehydration of the anodic oxide surface layer, followed by impregnation of surface pores with the dielectric wax. The anodizing parameters are advantageously controlled to provide an oxide surface layer of a thickness in the range 12-100 micrometres, more preferably 20-35 micrometres.
In the preferred embodiment, the surface is then polished to a finish better than 0.5 micrometre rms, most preferably better than 0.25 micrometre rms.
Desirable properties of the impregnating wax include high resistivity and ther favourable dielectric properties; suitable impregnation characteristics; hydrophobicity; high melting point; low shrinkage during cooling from elevated temperatures typically on the order of 150°C; and resistance to degradation at such temperatures. The impregnant material is selected from carnauba wax and montan wax, and compounded wax formulations of these materials. Especially preferred impregnants include carnauba yellow No. 1 and refined Nos. 2 and 3 waxes. These waxes may be modified with resins or other additives for enhanced dielectric properties. Various paraffins and other petroleum-derived waxes, beeswax, and candelilla wax have not been found to provide comparable performance.
In the preferred embodiment of the invention, the preliminary dehydration is accomplished by heating the anodized member. The member is desirably heated to a temperature in the range from about 120 to 180°C, the preferred temperatures being around 150 to 170°C. The heated member may be maintained in a vacuum for enhanced dehydration. The processing at these elevated temperatures ensures sealing of the pores in an essentially moisture-free state, without causing oxidation or other degradation of the impregnant wax.
The impregnant material is applied to the anodized member while the latter is heated. Most preferably, the material is premelted and coated over the heated oxide surface. After the impregnant material thoroughly covers the heated surface, the member is maintained at the elevated temperature for a period and then allowed to cool to room temperature. The pores in the member's surface may be sealed by the impregnant in a substantially moisture-free condition, resulting in a thick, hard surface with a high charge acceptance, having a resistivity in excess of 10¹² ohm-centimeters and low dielectric absorption.
In another desirable embodiment, one may remedy undesirable characteristics (as, for example, an uneven or insufficient level of impregnant) resulting from a poor initial application of the impregnant material. These may be remedied subsequently to impregnation and preferably prior to polishing simply by reheating the aluminum member.
This process results in a member having a thick, hard, abrasion-resistant dielectric surface layer. Such a member is especially well suited to an electrostatic imaging process wherein a latent electrostatic image is formed _'on the dielectric surface layer, toned and transferred to a receptor medium using high pressure. The dielectric surface layer has a resistivity greater than 10¹² ohm-centimeters, and is characterized by high charge acceptance and dielectric strength. Such dielectric properties are maintained even at extremely high relative humidities. In the preferred embodiment, the member has a smooth, continuous surface providing good toner release over prolonged operation. The dielectric surface is characterized by low dielectric absorption, permitting substantially complete neutralization of electrostatic images. The dielectric surfaces of the invention are durable and abrasion resistant, and may be subjected to scraping for removal of residual toner during an extremely long service life.
In a further embodiment, a novel printer using a dielectric cylinder has an anodized layer impregnated with waxes and used in combination with an electrostatic device for creating a selected image on the cylinder, a toner mechanism for adding toner to this image, and a compliant roller for transferring the toned image to a receptor sheet.
In yet another embodiment, there is provided a dielectric cylinder in combination with a compliant roller, the axis of the cylinder and the axis of the roller being skewed in relation to one another for enhanced transfer of toned images from the cylinder to a receptor sheet. This mechanical arrangement has been observed to achieve markedly improved toner transfer efficiencies.
In the present specification and claims various well-known standard types of waxes are referred to. The reader is referred to any standard text such as the Condensed Chemical Dictionary, Van Nostrand, London 1977, 9th edition, Hawley, P. 924, or the Van Nostrand Encyclopedia of Chemistry, 4th edition, Considene, Section "Water Treatment", page 1011.
The above and additional aspects of the invention are illustrated in the detailed description which follows, taken in conjunction with the drawings in which:

Figure 1 is a partially sectioned schematic end view of electrostatic printing apparatus incorporating a cylinder fabricated in accordance with the invention;
Figure 2 is a diagrammatic representation of the skew existing between the cylinder and a pressure roller in the apparatus of Figure 1;
Figure 3 is a schematic plan view of electrostatic testing apparatus for dielectric members;
Figures 4-10 are time plots of surface potential for dielectric coupons tested in the apparatus of Figure 3, for various wax impregnants;
Figure 4 plots carnauba yellow No. 1, after polishing;
Figure 5 plots carnauba yellow No. 2, after polishing;
Figure 6 plots crude montan wax, after polishing;
Figure 7 plots carnauba yellow No. 1, after polishing and prolonged moisture exposure;
Figure 8 plots carnauba yellow No. 2, after polishing and prolonged moisture exposure;
Figure 9 plots montan wax, after polishing and prolonged moisture exposure; and
Figure 10 plots beeswax, after polishing and prolonged moisture exposure.

The method of the present invention comprises a series of steps for fabricating and treating anodized aluminum members. This method results in members having dielectric surfaces particularly suited to electrostatic imaging. Such members are effective in an imaging process in which they receive an electrostatic latent image, carry the image with minimal charge decay to a toning station, and transfer the toned image to a further member, using high pressure. After transfer of the toner image from the imaging member, the member may be scraped in order to remove residual toner. Finally, the member is typically treated to neutralize any remaining electrostatic image on the dielectric surface in preparation for reimaging. Preferred electrostatic printing and copying apparatus of this description is generally disclosed respectively in commonly assigned U.S. Patent Nos. 4,267,556, and 4,195,927. A number of properties of particular concern in this utilization include charge acceptance, hardness, tensile strength, abrasion resistance, toner release characteristics, and electrostatic discharge characteristics.
Reference is made first to Fig. 1 which shows somewhat schematically an electrostatic printer 30 embodying the invention, generally in accordance with U.S. Patent No. 4,267,556. A cylinder 32 is mounted for rotation about an axis 34 and has an electrically conductive core 35 coated in a dielectric layer 36. Cylinder 32 is capable of receiving an electrostatic image from a cartridge 38 driven by an electronic control system 40 and connected by mechanical connectors 42. As the cylinder rotates in the direction shown, an electrostatic image is formed by the cartridge 38 on the outer surface of the dielectric layer 36 and comes into contact with toner supplied from a hopper 44 by a feeder mechanism 46. The resulting toned image is carried by the cylinder 32 towards a nip formed with a pressure roller 48 having a compliant outer layer 49 positioned in the path of a receptor such as paper 50 which enters between a pair of feed rollers 52, is driven by the cylinder 32 and roller 48, and leaves between a pair of output rollers 54. The pressure in the nip is sufficient to cause the toner to transfer to the receptor 50, and with sufficient pressure, the toner will be simultaneously fused to the receptor. Typically cylinder 32 and roller 48 are pressed together by between 20 and 125 kilograms per linear centimeter of nip. In order to enhance this action, while reducing the pressure needed, the axes of rotation of the cylinder 32 and roller 48 are desirably skewed relative to one another as will be further described with reference to Fig. 2. This skewing not only improved load distribution, but unexpectedly enhances transfer and fusing of the toner as will also be described.
After passing through the nip between cylinder 32 and roller 48, any toner remaining on the surface of the dielectric layer 36 is removed by a scraper blade assembly 56, and any residual electrostatic charge remaining on the surface is neutralized by a discharge head 58 positioned between the scraper assembly 56 and the cartridge 38.
Reference is next made to Fig. 2 which is included to illustrate diagrammatically the "skew" between the cylinder 32 and the pressure roller 48. The respective axes are arranged such that the axis 34 of the cylinder is offset angularly with respect to the axis 80 of the pressure roller 48, the offset being equal at both ends of the nip between the cylinder and roller.
A measure of skew is the angle between the projected axes 34, 80 when projected vertically into a horizontal plane as illustrated by the angle P. Applicants have found skews within the angular range 0.5°-1.5° to successfully achieve the objects of high toner transfer efficiency, and excellent image integrity and permanence. In general, rollers which are relatively long in relation to their diameter would be skewed in the lower end of this range. An illustrative value of skew to effect the objects of the invention is 1.1°, measured as the offset at the bearing retainers of nine inch (23 cm) long rollers 32, 48.
Figure 2 also shows a geometric representation of the surface of the contact of the cylinder and roller at the nip, showing the direction of paper feed before and after engagement. As a receptor sheet of paper 50 (Fig. 1) travels in direction A and enters the nip, it is subjected to divergent forces in direction D (perpendicular to the projected axis of the cylinder 48 and E (perpendicular to the projected axis of roller 32). Because of the relatively high smoothness and modulus of elasticity of the surface of the cylinder 32, the paper 50 will tend to adhere to the compliant roller 48, continuing its travel in direction A. This results in a surface speed differential or "slip" between the surfaces of the paper and the cylinder 32; this phenomenon does not depend on the initial infeed direction.
Due to the compression of the roller 48 at the nip, paper will contact both the roller surface and the cylinder 32 over a finite distance in direction D. With reference to the resultant triangle shown in Fig. 2, the surface of the paper will undergo a proportional side travel N with respect to the surface of the cylinder 32, the factor of proportionality being related to the surface speed differential.
The skewing of the cylinder and roller in the above-described manner results in a surprising improvement in the efficiency of toner transfer from the surface of dielectric layer 36 to the image receptor, and with sufficient pressure results in simultaneously fusing the toner to the receptor using the anodized imaging surfaces of the invention. See Example 3. Toner transfer efficiency may be expressed quantitatively as the percentage of toner transferred to the image receptor 50, which can be measured by collecting residual toner scrapings and comparing these by mass to the toner image prior to transfer.
The simultaneous transfer and fusing of toner can be replaced by a two-step procedure. The skew would preferably be used to transfer the toner to the receptor but using a reduced pressure. Subsequently, the receptor would be subjected to heat and/or pressure to fuse the transferred toner to the receptor. Such a procedure would also benefit from the use of the inventive cylinder to be described in detail. However, the combination of the cylinder in a skewed arrangement with a roller is preferred so that simultaneous transfer and fusing can be effected at the nip efficiently.
From the foregoing description of Fig. 1, it will be evident that the load in the nip is maintained regardless of whether or not receptor 50 is in position between the cylinder 32 and the roller 48. Consequently, when a new receptor meets the nip, there is a shock load as the edge of the receptor enters the nip and there is also a similar effect when the trailing edge of the receptor leaves the nip. As a result, the cylinder 32 and its dielectric layer must withstand these sudden loadings without unacceptable deterioration. The layer 36 must be mechanically strong both from the standpoint of impact loading and also from the standpoint of localized deformation caused by the enhanced load in the nip resulting from the thickness of the paper. Further, because the roller 48 has a compliant layer on its outer surface to maximize the area of contact with the cylinder 32, the receptor moves with the roller rather than with the cylinder so that when the cylinder and roller are skewed, there is a resulting minor scuffing action cused by the toner carried between the receptor and the surface of the dielectric layer 36. This is an inherent result if skewing is to be used to enhance transfer and fusing of the toner. Consequently, the surface of the layer 36 must also withstand this mechanical difficulty.
It is also evident from the description of Fig. 1 that the dielectric layer must receive a pattern of charge and retain this charge accurately until such time as it can be discharged by the head 58. Consequently, the layer 36 must both exhibit predetermined dielectric qualities with minimum deterioration of the image prior to transferring the toner as well as the facility to permit the residual electrostatic image to be removed or neutralized evenly by the head 58.
All of the foregoing criteria must be met if an improved electrostatic printer is to be provided. The member 32 may be composed of aluminum or, advantageously, an aluminum alloy. In choosing an alloy of suitable composition, principal criteria include hardness, tensile strength, and abrasion resistance. The 6000 and 7000 series of alloys (in the Aluminum Association scheme) are especially preferred to meet these criteria.
The member is preferably fabricated to provide an even distribution of intermetallics at or near the surface, thereby reducing the risk of formation of surface pits or subsurface voids in the oxide layer during anodizing. It is beneficial for this reason to form the member by extrusion. In the preferred embodiment of the invention, the core is a solid extruded cylinder with a dielectric layer, but alternatively, it may be formed as a sleeve with the layer which is fitted onto a conductive mandrel.
The core of the member is machined before the second step of hard coat anodizing and the machining should provide a surface smoothness of better than 0.5 micrometer rms. A preferred machining technique for this step is grinding in order to minimize surface discontinuities which may lead to cracks during subsequent processing.
In the second processing stage, the machined aluminum member is hardcoat anodized according to the teachings of Wernick and Pinner; see The Surface Treatment and Finishing of Aluminum and its Alloys by S. Wernick and R. Pinner, fourth edition, 1972, published by Robert Draper Ltd., Paddington, England. The anodization is carried out to a desired surface thickness, on the order of 30 micrometers. This results in a relatively porous surface layer of aluminum oxide characterized by the presence of a barrier layer isolating the porous oxide from the conductive aluminum substrate. Precautions should be taken and the parameters of anodization chosen to avoid gas ruptures in the anodic oxide layer which will result in surface pits and subsurface voids. It is also desirable to avoid branching of the pores, which will interfere with the crucial impregnation step as explained below. It is highly desirable, furthermore, to avoid contamination of the oxide layer, for example with oils and waxes. Following anodization, the member's surface is advantageously thoroughly rinsed in deionized water in order to remove all anodizing bath and other residual substances from the surface and the pores. The oxide surface may be further rinsed in isopropyl alcohol to effect partial removal of moisture from the pores, and may also be vapor rinsed for removal of grease and like contaminants. The rinsed surface is preferably wiped dry to reduce surface moisture.
After anodizing the member, and prior to impregnating of the pores with a sealing material, the method of the invention requires a thorough dehydration of the porous surface layer. For best results, the dehydration is accomplished immediately after anodization. If there is a long delay between these two steps, however, it is advisable to maintain the member in a moisture-free environment. This is done in pursuance of the general objective of avoiding a reaction with ambient moisture which leads to the formation of boehmite [AIO(OH)_2] at pore mouths, effectively partially sealing the porous oxide so that subsequent impregnation is incomplete and dielectric properties are degraded. Such partial sealing can occur at room temperature in normal ambient humidity in a period of several days.
Removal of absorbed water from the porous oxide layer of an anodized aluminum structure may be realized by using either heat, vacuum, or storage of the article in a desiccator. The dehydration step requires thorough removal of water from the pores. Although all three techniques are effective, best results are realized by heating, optionally while maintaining the member in a vacuum. A preliminary step of dehydrating the member in a vacuum oven is especially preferred where the member has been stored in a moist environment for a period after anodization. Heating of the member in air, as compared with vacuum heating, results in only a slightly lower level of charge acceptance. Any thermal treatment of the oxide layer prior to impregnation preferably is carried out at a temperature in the range from about 100°C to about 180°C, most preferably in the range 150°C-170°C. It is an advantageous characteristic of the impregnant waxes of the invention, discussed below, that they do not undergo markedly degradative physical and chemical changes at these temperatures. Preferably, preliminary heating is effected for a limited duration, to avoid a significant loss of tensile strength of the anodized member; such periods are characteristicably shorter for alloys of the 7000 series as compared with the 6000 series alloys. An illustrative period would be one hour or less for 7075-T6 alloy. Where precautions have been taken after anodizing to minimize the retention and accumulation of moisture, the dehydration step may be accomplished in conjunction with the impregnation step, as explained below.
After removal of absorbed water from the oxide coating it is sealed with an impregnant material. In the present invention, the impregnant material consists essentially of a wax or compounded wax formulation having the requisite resistivity and other dielectric properties; favorable impregnation characteristics; and hydrophobicity. It is desirable to employ a material having low shrinkage during the cooling from the elevated impregnation temperature, typically on the order of 150°C, to ambient temperature, and having low moisture absorbance during and after impregnation. It has been found that particularly advantageous materials include carnauba wax and montan wax.
Carnauba wax, as a natural material, comes in various grades which have been found suitable in the present invention. Carnauba yellow no. 1 and refined nos. 2 and 3 have all been found to give the requisite charge acceptance, impregnation characteristics, and other properties. Carnauba yellow no. 1 is most preferred for reasons of purity. In the alternative embodiment, Montan wax is employed as the impregnant material. Any of the above waxes may be compounded with resins or other additives for enhanced dielectric and structural properties provided that they permit adequate impregnation.
In order to avoid introduction of moisture into the dehydrated porous surface layer, the member should be maintained in a substantially moisture-free state during impregnation. This will occur as a natural consequence of the preferred method of applying the impregnant materials of the invention. In the preferred embodiment of the invention, the member is preheated to an elevated temperature above the melting point of the impregnant wax, and maintained at or near this temperature during the impregnation step in order to melt the material or to avoid solidifying premelted material. These materials have sufficiently low viscosity after melting to impregnate the pores of the oxide surface iayer. The period of heating the member from room temperature to the impregnating temperature may provide the preliminary dehydration which is required to avoid trapped moisture in the pores, often without a prior separate dehydrating step. (See Examples 1 and 2).
It has generally been found unnecessary to maintain the heated member in a vacuum during impregnation, either to avoid absorption of moisture or to assist the impregnation of the pores through capillarity. In the preferred embodiment, the impregnant material may be applied to the oxide surface under moist ambient conditions because the heating of the aluminum member will tend to drive off any absorbed moisture from the oxide surface. Optionally, a vacuum may be employed in order to provide an extra precaution against reintroduction of moisture and to expedite impregnation. This may be contrasted to prior fabrication processes which require special measures to protect against reintroduction of moisture during the impregnation stage.
In the preferred embodiment of the invention, the impregnant material is applied to the surface of the aluminum member after heating the member to a temperature above the melting point of the material. Advantageously, the impregnant wax is premelted and applied to the oxide surface in liquid form (as by brushing the material onto the member or immersing the member in melted material). In either case, the material should then be allowed to spread over the oxide surface layer. This may be done by permitting a flow of the melted material, or by manually spreading the material over the surface using a clean, dry implement. The member should be maintained at or near this elevated temperature for a period of time sufficient to allow the melted material to completely impregnate the pores of the oxide surface layer. This period will be shorter when using a vacuum to assist impregnation.
It has been determined that a complete impregnation of the pores is important in achieving desired charging and discharging characteristics of the dielectric surface. In the preferred embodiment, if the member is allowed to cool prior to complete filling of the pores with the impregnant material, the material will tend to solidify leaving undesirable air pockets in the pores. It is a particularly advantageous aspect of this method that this problem may be remedied simply by reheating the aluminum member and allowing a more complete filling of the pores. The impregnant wax compositions effectively adhere to the pore walls. The member may be reheated for a subsequent impregnation step at any time subsequent to the initial impregnation, but preferably prior to polishing, as the impregnant material of the invention is not cross-linked. As previously mentioned, it is desirable to avoid branching of the pores inasmuch as this will interfere with a complete sealing of the pores.
Subsequent to impregnation of the pores, the aluminum is allowed to cool. During this period the impregnant wax will tend to shrink only slightly. The member is then treated (as by wiping or scraping) to remove any excess material from the surface, leaving only the material in the pores. In order to provide a surface with good release properties for electrostatic toner, a preferred embodiment of the invention includes a final step of polishing the member's surface to a finish better than 0.5 micrometer rms, preferably better than 0.25 micrometer rms.
The invention provides a simple and reliable technique for fabricating dielectric surface layers which are capable of supporting several thousand volts. Advantageously, the oxide layer 13 has a thickness in the range 12 um―100 Ilm, more preferably 20 um―35 µm. It is desirable for the dielectric surface layer 13 to have sufficiently high resistivity to support a latent electrostatic image during the period between latent image formation and toning. Consequently, the resistivity of the layer 13 should be in excess of j0¹² ohm-cm. The surface of the layer 13 should be hard and relatively smooth, in order to provide for complete transfer of toner to the receptor sheet 9. The dielectric layer 13 additionally should have a high modulus of elasticity so that it is not distorted by high pressures in the transfer nip. In order to provide a high service life it is desirable that layer 13 have high tensile strength and abrasion resistance. A dielectric cylinder produced in the manner described above satisfies all these requirements. A further characteristic of some importance in this application is the provision of a continuous surface, with minimal surface pitting, cracks, and other discontinuities. Such discontinuities will entrap toner particles, and cause severe wear in the scraper blades and cylinder surface.
It is furthermore desirable to reduce "dielectric absorption", or the tendency of the dielectric Jayer 13 to hold a charge below its surface. Subsurface charge will migrate to the surface after neutralizing at station 58 (Fig. 1) - a highly undesirable phenomenon. Dielectric absorption is generally aggravated by inadequate preliminary dehydration; poor, incomplete impregnation; decomposition of the impregnant material; formation of boehmite in the pores during the period after anodizing; or introduction of moisture during impregnation. The various processing steps of the invention are advantageously implemented to reduce dielectric absorption.
There is a tendency, as well, for worsening of this characteristic if the finished- dielectric member is stored or operated in high relative humidities. The impregnant materials of the invention have been found to provide dramatic improvements in discharging characteristics at high relative humidities.
The advantages of these methods and products will be further apparent from the following nonlimiting examples:

Example 1

A hollow aluminum cylinder of extruded 7075-T6_b1 alloy was machined to an outer diameter of 10.2 cm and length fo 22.9 cm, with a 19 mm wall thickness. The cylinder was machined to a 0.8 micrometer finish, then polished to a 0.06 um (2.25 microinch) finish. The cylinder was hardcoat anodized by the Sanford "Plus" process to a thickness between 42 and 52 um, then rinsed successively in deionized water, isopropyl alcohol, and a freon rinse for grease removal.
The cylinder was then placed for 30 minutes in a vacuum oven at 102 kPa (30 inches mercury), 160°C. The cylinder was maintained at this temperature and pressure for half an hour prior to impregnation.
A beaker of Carnauba Yellow No. 1 wax was preheated to 100°C to melt the wax. The heated cylinder was removed from the oven, and coated within 10 seconds with the melted carnauba wax using a paint brush. The cylinder was then placed back in the vacuum oven for a few minutes at 160°C, 30 inches mercury (102 kPa). The cylinder was removed from the oven and allowed to cool.
After cooling, the member was polished with successively finer SiC abrasive papers and oil. Finally, the member was lapped to a 0.11 micrometer finish by application of a lapping compound and oil with a cloth lap.
The cylinder's charge acceptance was measured at 980 volts using a Monroe Electronics electrostatic voltmeter, manufactured by Monroe Electronics, Middleport, NY. The cylinder was charged to 280-290 volts and then discharged using corona charging apparatus of the type described in the commonly assigned U.S. Patent No. 4,379,969. The corona device was grounded to the aluminum core 34 of cylinder 32. The cylinder showed a residual surface charge of 4-5 volts, indicating outstandingly low dielectric absorption.

Example 2

A dielectric cylinder was fabricated in accordance with Example 1, with the modification that the pores of the aluminum oxide surface layer were impregnated with Carnauba Yellow No. 2 wax. The cylinder exhibited comparable charge acceptance and dielectric absorption using the testing method of Example 1.

Example 3

A dielectric cylinder fabricated in accordance with Example 1 was incorporated in an electrographic printer of the type described with reference to Figure 1. Referring to this figure, the pressure roller 48 consisted of a solid machined 5 cm diameter aluminum core 60 over which was press fit a 5 cm inner diameter, 6.4 cm outer diameter polysulfone sleeve 49. The dielectric roller 32 was gear driven from an AC motor to provide a surface speed of 30.5 cm per second. The pressure roller 48 was held against the dielectric cylinder with a nip pressure of 490 N per cm (50 kilograms per linear centimeter) of contact. Rollers 32 and 48 were mounted with an end- to-end skew of 1.1°.
A charging head or cartridge 38 of the type described in commonly assigned U.S. Patent No. 4,160,257 was used to generate latent electrostatic images. The charging head was maintained at a spacing of 0.2 mm from the surface of the dielectric cylinder 32.
Under these conditions it was found that a 300 volt latent electrostatic image was produced on the dielectric cylinder in the form of discrete dots. The image was toned using single component toner from the toning feeder hopper 44 which was essentially identical to that employed in the Develop KG Dr. Eisbein and Company (Stuttegart) No. 444 copier. The toner employed was Hunt 1186 of the Phillip A. Hunt Chemical Corporation. The receptor 50 was plain paper injected into the pressure nip at the appropriate time from a sheet feeder.
Engineering plastic scraper blades were employed in the scraper assembly 56 to remove excess toner from the surface of the dielectric cylinder 32. The residual latent electrostatic image was erased using a corona charging/ discharge device 58 in accordance with commonly assigned U.S. Patent No. 4,379,969. After neutralization, a residual electrostatic image on the order of 4-5 volts remained on dielectric surface 36, allowing reimaging by the cartridge 38 with negligible ghost imaging.
No image fusing was required other than that occurring during pressure transfer. The transfer efficiency (i.e. percentage of toner transferred from the cylinder 32 to plain paper 50) was 99.9 percent.
The dielectric cylinder provided a service life of over one million copies.

Examples 4-6

The following examples were performed to demonstrate the electrical qualities of dielectric members produced according to the above-dis.^: closed technique using different impregnants. A series of 5 cm x 5 cm x 1.6 mm coupons fabricated of 7075T6 aluminum alloy sheet stock were cut down to 2.5 cm x 2.5 cm after impregnation to polish and test. The samples were anodized using the Sanford Plus process, rinsed with tap water, then heated five minutes on a 70-80°C laboratory hot plate for dehydration. The impregnants were melted onto the samples and the coupons were left on the hot plate for an additional minute. Excess impregnant was wiped off the coupons before solidifying, and the coupons were polished using a Buehler Minimet polishing/grinder unit, (Buehler, Ltd., Lake Bluff, Illinois) with successive 300, 400, and 600 grit dry disks.
The charging and discharging characteristics of the finished samples were tested using apparatus 70 schematically illustrated in Figure 3. The coupon 72 to be tested was mounted, anodized face upward, on a turntable 74 where the coupon would move at a surface speed of 25 cm per second as the turntable rotated. The conductive aluminum substrate of coupon 72 was grounded to the turntable 74. Once each cycle the sample was passed under an electrostatic charging/discharging device 76 of the type disclosed in commonly assigned U.S. Patent No. 4,379,969. The device 76 was selectively set to a 225-250 volt bias for charging, to ground for discharging, or disconnected. The potential of coupon 72 was measured using a Monroe electrostatic voltmeter 80 (Monroe Electronics, Middleport, N.Y.) with a probe spaced 2.5 mm from the dielectric surface of coupon 72. The readings from voltmeter 78 were recorded on a Gould chart recorder 90 (Gould Inc., Instruments Div., Cleveland, Ohio). This recorder produced charts shown in Figures 4 to 10 using a time division of 0.5 mm/second on the vertical scale (on which the readings proceed from bottom to top) and 25 volts/major division on the horizontal scale. Therefore, each horizontal line making up the charts represents the voltage reading for a given cycle.
With reference to the chart recordings of Figures 4-10, the test apparatus was operated with the following charging/discharging sequences identified by lettering corresponding to those used in the Figs.:

A. Repeated discharge
B. Repeated charge
C. Repeated discharge
D. Repeated charge
E. One discharge
F. Charging device disconnected
G. Repeated charge
H. Charging device disconnected.

The period F, which indicates the voltage profile after a single neutralization cycle, gives a measure of dielectric absorption. It is an important index of successful dielectric fabrication to achieve low potential readings during this period. The readings during period H give a measure of the charge decay characteristics ("self-decay").

Example 4

The testing apparatus 70 discussed above with reference to Figure 3 was used to record voltage readings taken from a series of coupons 72 fabricated as described above. The coupons were tested immediately after polishing, in a 18% R.H., 23°C laboratory environment. The coupons were impregnated with Carnauba yellow no. 1, Carnauba yellow no. 2, and crude montan waxes and the chart recordings are reproduced in Figures 4, 5 and 6 respectively.
The samples all exhibited excellent charge acceptance and outstandingly low dielectric absorption.

Example 5

The tests of Example 4 were repeated with the following modification. The sample coupons were stored for 17 hours in a desiccator at 95% R.H., 23°C. The samples were tested immediately after removal from the desiccator. The resulting charts for Carnauba yellow no. 1, Carnauba yellow no. 2, and montan waxes are reproduced respectively in Figures 7, 8 and 9. Again, the samples all exhibited excellent charge acceptance and low dielectric absorption, the latter being somewhat higher than recorded for the samples of Example 4. The carnauba wax samples were found to give somewhat superior readings to those for crude montan wax.

Example 6

Tests of the above-described type were conducted for a variety of impregnant waxes, including beeswax, candelilla wax, 180/185 microcrystalline wax, 170/175 microcrystalline wax, superla wax, 125/130 paraffin, and 160/165 paraffin (the various numerals indicate a range of melting points). The beeswax and candelilla wax samples were tested after polishing and 66 hours storage in an 85% R.H., 23°C desiccator. The remaining samples were tested shortly after cooling and removal of excess wax.
Figure 10 shows a reading taken during the periods B and C: repeated charging and repeated discharge, for beeswax. The remaining charts (not shown) were similar in their voltage profiles. These readings indicated poor dielectric properties for beeswax and candelilla wax after exposure to high relative humidities, while the remaining impregnants gave unacceptable results even before polishing.
While various aspects of the invention have been set forth in the drawings and the specification, it is to be understood that the foregoing detailed description is for illustration only. Dielectric cylinders manufactured according to the techniques of the invention have been disclosed in combination with particular electrographic printing apparatus, but dielectric members manufactured in accordance with the invention may be utilized in a wide variety of electrostatic imaging systems not discussed herein.

Claims

1. A method of treating a member to form a dielectric surface layer, comprising the steps of: hardcoat anodizing the member comprised of a material selected from the group aluminium and aluminium alloys, to form an oxide surface layer having a plurality of pores; heating the aluminium member to an elevated temperature; impregnating the pores of the oxide surface layer with an impregnant material, and removing any impregnant material on the member's exterior surface; characterised in that the impregnating is carried out while the member is at a temperature above the melting point of the impregnating material, to form a dielectric surface layer with a resistivity in excess of 10" ohm-centimetres, and in that the impregnated material is selected from the group carnauba wax, montan wax, compounded car- bauba wax and compounded montan wax.

2. A method as claimed in claim 1 in which the dielectric surface layer is polished to a finish better than 0.5 micrometer rms as a final step.

3. A method as claimed in any one of the preceding claims in which the impregnant material is selected from the group carnauba yellow no. 1 wax, carnauba yellow no. 2 refined wax, carnauba yellow no. 3 refined wax, and crude montan wax.

4. A method as claimed in any one of the preceding claims in which the member is heated to a temperature in the range 120°C to 180°C.

5. A method as claimed in any one of the preceding claims in which the heating step is effected in a vacuum.

6. A method as claimed in any one of the preceding claims in which the member is comprised of an aluminium alloy selected from the 6000 and 7000 series alloys of the Aluminium Association.

7. A method as claimed in any one of the preceding claims in which the member is reheated subsequent to the removing step.

8. A method as claimed in any one of the preceding claims in which the member is a cylinder.

9. A printer (30) of the type used to form a selected image on a receptor sheet (50), the printer having a cylinder (32) including a dielectric surface layer (36), means (38, 40) for forming a latent electrostatic image corresponding to the selected image on the surface of the cylinder, means (44, 46) for toning the latent electrostatic image, a compliant roller (60) forming a nip with the cylinder, and means for moving paper through the nip to receive the toner in the form of the selected image; characterised in that the cylinder (32) has at least an outer portion selected from the materials aluminium and aluminium alloys, with a surface layer of hardcoat anodizing impregnated with a wax selected from the group carnauba wax, montan wax, compounded carnauba wax, and compounded montan wax, the layer having a resistivity in excess of 10" ohm-centimetres.

10. A printer as claimed in Claim 9 in which the surface of the cylinder (32) is polished to a finish better than 0.5 micrometer rms.

11. A printer as claimed in any one of Claims 9 to 10 in which the respective axes (34, 80) of the cylinder (32) and roller (60) are skewed with reference to an axial centre point of the cylinder by an angle in the range of 0.5°-1.5°.

12. A printer as claimed in any one of Claims 9 to 11 in which the hardcoat anodizing has a thickness in the range 6-100 micrometers.

13. A printer as claimed in any one of Claims 9 to 12 including toner removal means (56) for stripping any excess toner from the cylinder (32) after toner transfer at the nip.

14. A printer as claimed in any one of Claims 9 to 13 including discharge means (58) for neutralizing any residual charge on the cylinder (32) after toner transfer and prior to reimaging by the means for forming a latent electrostatic image.

15. Electrostatic imaging apparatus, of the type including a dielectric imaging member (32), light exposure means (38) for forming a latent electrostatic image on said dielectric imaging member, means (44, 46) for toning said latent electrostatic image to form a visible toner image, and a further member (60) for exerting high pressure between the dielectric imaging member and an image receptor sheet (50) to transfer the visible toner image to said sheet; characterised in that the dielectric imaging member (32) has at least an outer portion selected from the materials aluminium and aluminium alloys, with a surface layer (36) of hardcoat anodizing impregnated with a wax selected from the group carnauba wax, montan wax, compounded carnauba wax, and compounded montan wax, the layer having a resistivity in excess of 10" ohm-centimetres.

16. An apparatus as claimed in Claim 15 in which the surface of the dielectric imaging member (32) is polished to finish better than 0.5 micrometre rms.

17. An apparatus as claimed in Claim 16 in which the dielectric imaging member (32) comprises a rotatably mounted cylinder, and the further member (60) comprises a rotatably mounted roller in contact with said cylinder under high pressure.

18. An apparatus as claimed in Claim 17 in which the respective axes (34, 80) of the cylinder and roller are skewed with reference to an axial center point.

19. An apparatus as claimed in any one of Claims 16 to 18 in which the dielectric imaging member (32) has a hard, smooth, abrasion resistant surface, and the further member (60) has a compliant surface.

20. An apparatus as claimed in any one of Claims 16 to 19 including a device (58) for neutralizing any residual latent electrostatic image after toner transfer but prior to reimaging by the means (38) for forming a latent electrostatic image.

21. An apparatus as claimed in any one of Claims 16 to 20.in which the surface layer (36) of hardcoat anodizing has a thickness in the range 6-100 micrometers.

22. The combination of a cylinder (32) and a roller (60) for use in transferring a toned image from the cylinder to a receptor sheet (50) fed between the cylinder and roller under high pressure, of the type including a support structure, a cylinder (32) rotatably mounted in the support structure for movement about its axis (34), and a compliant roller (60) rotatably mounted in the support structure, said cylinder and compliant roller being mounted to create a pressure nip therebetween for transferring a toned image from the cylinder (32) to the receptor sheet (50) under pressure as the sheet passes through the nip; characterised in that the cylinder (32) includes at least an outer portion (36) selected from the materials aluminium and aluminium alloys, having a surface layer of hardcoat anodizing impregnated with a wax selected from the group carnauba wax, montan wax, compounded carnauba wax, and compounded montan wax, the surface layer having a resistivity in excess of 10¹² ohm-centimetres.

23. A combination as claimed in Claim 22 in which the axes (34, 80) of said cylinder (32) and said roller (60) are skewed with reference to an axial center point of the cylinder by an angle in the range 0.5^0-1.5⁰.

24. A combination as claimed in Claim 22 or Claim 23 in which the surface of the cylinder (32) is polished to a finish better than 0.5 micrometer rms.

25. A combination as claimed in Claim 22 or Claim 23 or Claim 24 in which the cylinder (32) and roller (60) are pressed together by between 196 to 1230 Newtons per centimeter of nip (20 and 125 kilograms force per centimeter of nip).