Polymeric binders having saturated ring of single layer positively charged organic photoconductor

Abstract: Composites comprising polymeric binders and phthalocyanine pigments to form a single layer positive organic photoconductor are provided for use in electrophotography. The polymeric binders comprise an aliphatic polymer or copolymer having a saturated ring for each repeat unit either included in the polymer chain or pendant therefrom and about 4 to 35% of functional groups such as -OH, -SH, >N-, >NH, and -NH 2 per repeat unit. The saturated ring portion, being essentially non-polar, or at least less polar than an unsaturated ring, maintains the specific morphology of the phthalocyanine pigments commonly employed in positive charge organic photoconductors (OPCs) and results in a stable dispersion required for the stable performance of the OPC. Keeping the functional groups listed above to less than about 35% ensures that the photoresponse is not reduced to an unacceptable level. Heating of the composite is used to control the concentration of the functional groups. One or more separate thermal carrier generation control agents comprising compounds containing the functional group(s) may be used to provide part or all of the functional groups in the composite.

Description: The present invention relates generally to image transfer technology and, more specifically, to electrophotography, employing a positive charging, organic photoconductor material including polymeric binders. Electrophotographic laser printing employs a toner containing pigment components and thermoplastic components for transferring a latent image formed on selected areas of the surface of an insulating, photoconducting material to an image receiver, such as plain paper, coated paper, transparent substrate (conducting or insulative), or an intermediate transfer medium. There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, usually special hydrocarbon liquids. With liquid toners, it has been discovered that the basic printing color (yellow, magenta, cyan, and black) may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multi-colored image. Specific morphologies of phthalocyanine (Pc) pigment powder have been known to exhibit excellent photoconductivity. These phthalocyanine pigments have been used as a mixture in polymeric binder matrices in electrophotographic photoconductors, deposited on a conductive substrate. In these phthalocyanine/binder photoconductors, the photo-generation of charge and the charge transport occur in the particles of the phthalocyanine pigment, while the binder is inert. Therefore, the photoconductor may be made of a single layer of phthalocyanine/binder. These single-layer photoconductors are known to be very good positive (+) charging OPCs due to the hole (positive charge) transportability of the phthalocyanine pigment. In these single-layer photoconductors, then, there is no need to add charge transport molecules, nor to have a separate charge transport layer. The phthalocyanine pigment content may be in the range of about 10 to 30 wt%, high enough to perform both charge generation and charge transport functions, with the binder content comprising the balance, i.e., in the range of about 90 to 70 wt%. The single photoconductor layer is usually more than about 3 micrometers (µm) thick in order to achieve the required charge acceptance and resulting image contrast. It would be desirable to provide a phthalocyanine-type positive-charging OPC which exhibits stable electrical prop-erties, including charge acceptance, dark decay and photodischarge, in a high cycle, high severity electrophotographic process. Modern digital imaging systems wherein the writing head is an LED array or a laser diode have very high light intensities (about 2 to 3 mW/cm 2 ) over very short exposure time spans (less than 50 nanoseconds), resulting in severe conditions for the OPC compound compared to optical input copiers with light intensities between about 10 to 30 erg/cm 2 and exposure times between several hundred microseconds to milliseconds. These light sources operate in the range of about 700 to 1100 nm, which, due to the absorbance of the phthalocyanine compounds in the higher end of this range, is why these compounds are employed. Unfortunately, there is no product on the market today which provides the stable electrical properties described above. This is because the phthalocyanine-type positive-charging OPC exhibits instability when it is frequently exposed to the corona charger and the intense light source in the electrophotographic process. The instability is more pronounced at the strong absorption, high light intensity, short exposure time conditions required for the laser printing process. The instability is exhibited in the significant increase of the dark decay after a small number of repeat cycles of laser printing. Also, the instability is exhibited in the decrease in surface potential. These instabilities cause deleterious changes in image contrast, and raise the issue of the reliability of image quality. These instabilities in the phthalocyanine/binder photoconductor appear to be independent of the chemical structure or morphology of the pigment. Instead, they appear to be dependent on the nature of the contact between individual pigment particles. These are recent observations, and there is no published report or suggestion in the prior art of these observations or how to effectively address and solve the problem of photoconductor instability in the high cycle, high severity electrophotographic process. Phthalocyanine pigments having specific morphology associated with particle size in sub-micrometer range have been observed to show different effects, depending on the type of the binder, such as agglomeration or aggregation. These properties are associated with the unstable dispersion of the pigment in the binder due to the poor compatibility between the two components. The above-mentioned unstable dispersion can cause the problem of non-uniformity of the coating, resulting in defects of the xerographic image quality, such as high noise and poor resolution. The poor dispersion of these pigments in binder also causes the unstable performance of the device, such as reduced life at different operating environments (ambient and elevated temperatures). The specific morphology with sub-micrometer particle size can be found in the following types of phthalocyanine pigments: the metal-free crstalline forms (α-, β-, τ-, and x-H 2 -phthalocyanines), α-copper phthalocyanine, α-titanyl phthalocyanine, Y-titanyl phthalocyanine, amorphous titanyl phthalocyanine, α-tetrafluorotitanyl phthalocyanine, α-haloindium phthalocyanines (halo = Cl, Br, I, F), α-vanadyl phthalocyanine, α-zinc phthalocyanine, β-zinc phthalocyanine, x-magnesium phthalocyanine, and α-chloroaluminum phthalocyanine. When conventional binders for the phthalocyanine pigment, such as acrylic resins, phenoxy resins, vinyl polymers including polyvinylacetate and polyvinyl butyral, polystyrene, polyesters, polyamides, polyimides, polycarbonates, methylmethacrylates, polyurethanes, polyureas, melamine resins, polysulfones, polyarylates, diallylphthalate resins, polyethylenes, and halogenated polymers, including polyvinylchloride, polyfluorocarbon, etc., are used, acceptable charge acceptance and photodischarge are obtained. However, among these polymers which result in good performance for charge acceptance and photodischarge, none of them exhibit the desirable stability under the LED array or laser diode exposure conditions. Also, any binders, and accompanying solvents, which do not form a stable dispersion with the phthalocyanine pigment usually exhibit very slow charge acceptance, high residual voltage, or dark decay, and are therefore unacceptable. The conventional polymeric binders, such as polycarbonates, polyesters, phenoxy resin, phenolic resin, polystyrene, polyvinyl toluene, polyvinyl carbazol, polyimide, and the like, contain unsaturated rings. On the other hand, some functional groups in the binder, especially hydroxy groups (-OH) and thiols (-SH), as well as >NH, -NH 2 , >N-, seem to exhibit strong interactions (e.g., hydrogen bonding) with the lone pair nitrogen of the phthalocyanine molecules. These interactions are observed to restrict the photoresponse of the photoconductor devices under space charge limited condition, such as exposing to strong light intensity in a very short time of several tens of nanoseconds. Preferably, desirable electrophotographic performance may be defined as high charge acceptance of about 30 to 100 V/µm, low dark decay of less than about 5 V/s, and photodischarge of at least 70% of surface charge with the laser diode beam of 780 nm or 830 nm frequency, through the optical system including beam scanner and focus lenses, synchronized at 0.05 µs for each beam. Thus, there remains a need to provide binders for the positive single layer OPC using sub-micrometer morphology phthalocyanine pigment as a photoconductive element to satisfy (a) stable dispersion, (b) high photoresponse to laser exposure, and (c) stable performance over a wide range of operating temperature (ambient to 75°C). Organic photoconductor materials suitable for use in electrophotographie equipment are known in which the material comprises a phthalocyanine pigment in a polyvinyl acetal or polyvinyl butyral resin binder. Such photoconductor materials are described in EP-A-0538889, EP-A-0470729, EP-A-0443563, EP-A-0573084, EP-A-0632333, and in EP-A-0658814 (which forms part of the state or the art by virtue of Article 54(3) EPC). The present invention provides a single layer positive organic photoconductor as defined in the accompanying claims. The present invention also provides a method of making such a photoconductor as defined in the accompanying claims. The polymeric binders of the composites according to the present invention have a saturated ring pendant from an aliphatic polymer chain. The saturated ring portion, being non-polar, or at least less polar than an unsaturated ring, maintains the specific morphology of the phthalocyanine pigments commonly employed in positive charge OPCs and results in a stable dispersion required for the stable performance of the OPC. Keeping the functional groups -OH, -SH, -N<, -NH-, -NH 2 in the composite to less than 35% per repeat unit of the polymer ensures that the photoresponse is not reduced to an unacceptable level. On the other hand, there must be at least 4% of the above functional groups present, since at a level of less than 4%, the OPC exhibits poor thermal stability. The polymeric binders of the invention maintain the specific morphology of the previously-mentioned phthalocyanine pigments and result in a stable dispersion of the pigments required for the stable operation of the apparatus. Formulating composites comprising polymeric binders and the above-mentioned phthalocyanine pigments, in which the polymeric binders contain saturated rings which are less polar or are non-polar, can maintain the specific morphology of the phthalocyanine pigments and result in a stable dispersion required for the stable performance of the device. The content of the functional groups -OH, -SH, -N<, >NH, and -NH2 in the composite, which cause the reduced photoresponse, must be kept below 35% per repeat unit of the polymer. This type of specific binder containing saturated rings exhibits the general chemical structure described below: wherein B is selected from the group consisting of: The polymeric binders are preferably polyvinylcyclohexane and its copolymers, having the general structural formula: where R is alkyl, substituted alkyl, aryl substituted aryl, alkoxy, aryloxy, and amino or substituted amino and where n ranges from 5 to 20,000, x ranges from 0.001 to 0.5, and y ranges from 0.5-0.999, where the sum of x+y = 1.0 As indicated above, the amount of -OH, -SH, -NH 2 , >NH, and >N- ranges from 4 to 35% per repeat unit of the polymeric or copolymeric binder. There must be some amount of functional group present, in order to provide thermal stability to the phthalocyanine pigment. However, a value of greater than 35% results in poor photoconductive properties of the pigment. The amount of the functional group is controllable by baking the OPC at a temperature and for a time that depends on the thickness of the layer and the amount of functional group. In general, the temperature is within the range of 80°C to 300°C and the time of heating is within the range of about several seconds to several hours. The heating causes chemical reaction or cross-linking, depending on the presence of other substituents, thereby reducing the content of the functional group. The photoconductive phthalocyanine pigment has a particle size less than 1 µm and is substantially uniformly dispersed in the polymeric binder. The uniform dispersion is judged by the glossiness of the finished surface. Preferably, the phthalocyanine pigments employed in the practice of the invention are those previously mentioned above. A single layer positive OPC may be fabricated employing the polymeric binder of the invention by combining the pigment and the polymeric binder, and, optionally, one or more thermal carrier generation control agents, to form a composite. While as discussed above, the presence of the functional groups -OH, -SH, >N-, >NH, and NH 2 is required in the amount of 4 to 35% per repeat unit of polymer, these functional groups can be provided in whole or in part by the addition of specific chemicals, called thermal carrier generation control agents, which include such functional groups, so that the total of these functional groups, whether on binder or on thermal carrier generation control agent(s) or both, remains within the required range. These functional groups form weak bondings with the nitrogen atoms or with the chelate metal of the phthalocyanine molecule. The amount of pigment in the composite is in the range of 13 to 17 wt%, the balance the binder. The addition of thermal control agent(s), if used, does not alter the ratio in the composite. Comparative Example 1 : The crystalline (x) form of phthalocyanine (Pc), x-H 2 Pc, in a matrix of an unsaturated polymer binder, high molecular weight polycarbonate dispersion (Makrolon™, available from Mobile Chemical Co.), in which the amount of x-H 2 Pc was 16 wt% and the amount of polycarbonate was 84 wt%, exhibited a non-glossy surface (agglomeration of pigment) and significantly reduced charge acceptance after 7.5K cycles at the lab ambient. The initial charge acceptance was about 550 V, but after 7.5K cycles had a value of about 150 V, which meant that the OPC no longer accepted charge well. Comparative Example 2 : x-H 2 Pc (16% wt) in unsaturated ring binder comprising phenoxy resin (PKHH, available from Union Carbide) containing 18% -OH groups exhibited low laser response plus significant reduction of charge acceptance after 10K life test at the lab ambient. Specifically, the dark decay initially was 3 V/sec; after 10K cycles, the dark decay was 10 V/sec, which meant that the OPC did not hold a charge well. Also, the initial charge acceptance was 550 V, but dropped to 200 V after 10K cycles due to poor dispersion. Comparative Example 3 x-H 2 Pc (16% wt) in polyvinyl butyral (PVB) with 5% content of -OH exhibited excellent dispersion and relatively high laser response, with a slight change of charge acceptance after 10K life test at the lab ambient. Comparative Example 4 Example 1 was repeated except that increasing the dispersion time from 48 hr ball milling to 78 hr resulted in a more stable charge acceptance after 10K life test. Comparative Example 5 Example 1 was repeated except that a quick dry (<8min) at higher temperature (150° to 230°C) was done in order to lower the content of -OH from the partial cross-linking of the PVB in the surface to yield a reduced change of charge acceptance after 10K life test at 50°C, i.e, increased thermal stability and laser response. Comparative Example 6 x-H 2 Pc (16% wt) with a PVB binder containing 33% of -OH exhibited good dispersion, slower laser response, and very little change of charge acceptance after 10K life test at the lab ambient. Comparative Example 7 Example 4 was repeated except that the OPC was baked quickly (<8 min) at high temperature (150° to 225°C) to cause a partial cross-linking, which reduced -OH content from 33% to 15%. Higher laser response and very little change of charge acceptance after 10K life test at 50°C were observed. This result shows a balance of -OR can maintain good laser response and better thermal stability. The positive organic photoconductor comprising phthalocyanine pigment and binder of the invention is expected to find use in electrophotographic printing, particularly in color electrophotographic printing. Thus, there has been disclosed a novel binder for use with phthalocyanine pigments in electrophotographic printing.

Claims: A single layer positive organic photoconductor comprising a composite comprising a polymeric binder and at least one photoconductive phthalocyanine pigment having a particle size less than 1 µm and uniformly dispersed in said polymeric binder, said phthalocyanine pigment including nitrogen atoms in its structure and, optionally, a chelate metal, and said polymeric binder formed from a polymer or copolymer having an aliphatic main chain, said composite further comprising at least one functional group selected from the group consisting of -OH, -SH, -N<, -NH-, and -NH 2 which can form weak bondings with the nitrogen atoms or with the chelate metals of said phthalocyanine pigment, said at least one functional group being present in an amount ranging from 4% to 35% per repeat unit of said polymer or copolymer, and being provided by at least one of said binder or at least one separate compound comprising said functional group selected from -OH, -SH, -N<, -NH-, and -NH 2 , wherein said pigment is present in an amount in the range of 13 to 17 wt.%, the balance being the binder and the said at least one separate compound, it used, and wherein said polymeric binder has a general chemical structure given by wherein B is selected from the group consisting of: and wherein B may carry at least one functional group R selected from alkyl, cycloalkyl, allyl, or aryl with or without first substituent functional groups; wherein R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , R 9 , R 10 , R 11 , R 12 are independently hydrogen, halogen, alkyl, alkoxy, allyl, or aryl with or without second substituent functional groups selected from the group consisting of -COOR, -OH,-CN,-O-CO-R, -NH 2 and -NO 2 ; and wherein m ranges from 0.15 to 1.0, and n and p each independently range from 0 to 0.85, with m + n + p = 1.0. A single layer positive organic photoconductor comprising a composite comprising a polymeric binder and at least one photoconductive phthalocyanine pigment in an amount in the range of 13 to 17 wt.%, the balance being the binder, having a particle size less than I µm and uniformly dispersed in said polymeric binder, said phthalocyanine pigment including nitrogen atoms in its structure and, optionally, a chelate metal, and said polymeric binder formed from a polymer or copolymer having an aliphatic main chain, said composite further comprising at least one functional group selected from the group consisting of -OH, -SH, -N<, -NH-, and -NH 2 which can form weak bondings with the nitrogen atoms or with the chelate metals of said phthalocyanine pigment, said at least one functional group being present in an amount ranging from 4% to 35% per repeat unit of said polymer or copolymer, and being provided by at least one of said binder or at least one separate compound comprising said functional group selected from -OH, -SH, -N<, -NH-, and -NH 2 , wherein said polymeric binder is selected from the group consisting of polyvinylcyclohexane and its copolymers given by the formula: where R is alkyl, substituted alkyl, aryl, substituted aryl, alkoxy, aryloxy, and amino or substituted amino and where n ranges from 5 to 20,000, x ranges from 0.001 to 0.5, and y ranges from 0.5 to 0.999, and where the sum of x + y = 1.0. The single layer positive organic photoconductor of claim 1 or 2 wherein said phthalocyanine pigment is selected from the following group of the phthalocyanine pigments: x-H 2 -phthalocyanine, α-H 2 -phthalocyanine, τ-H 2 -phthalocyanine, β-H 2 -phthalocyanine, α-copper phthalocyanine, α-titanyl phthalocyanine, Y-titanyl phthalocyanine, amorphous titanyl phthalocyanine, α-tetrafluorotitanyl phthalocyanine, α-haloindium phthalocyanine, α-vanadyl phthalocyanine, α-zinc phthalocyanine, β-zinc phthalocyanine, x-magnesium phthalocyanine, and α-chloroaluminum phthalocyanine. A method for providing a single layer positive organic photoconductor according to claim 1, 2 or 3, comprising the steps of: providing a composite comprising a polymeric binder as defined in claim 1 and a phthalocyanine pigment as defined in claim 1, said composite further comprising at least one functional group selected from the group consisting of-OH, -SH, -N<, -NH- and NH 2 , which can form weak bondings with the nitrogen atoms or with the chelate metals of said phthalocyanine pigment, said at least one functional group being provided by at least one of said binder and at least one separate compound comprising a functional group selected from -OH, -SH, -N<, -NH- and NH 2 ; followed by heating said composite to a temperature ranging from 80°C to 300°C for a period of time ranging from several seconds to several hours, said heating leaving a concentration of from 4% to 35% per repeat unit of said polymer of said functional groups when completed.

Inventors: GANAPATHIAPPAN SIVAPACKIA, HA TAN, NGUYEN KHE C, GANAPATHIAPPAN, SIVAPACKIA, HA, TAN, NGUYEN, KHE C.
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1995
Main classification: G03G5
IPC codes: G03G5