Electrophotographic imaging members, e.g., photoreceptors, typically include a photoconductive layer formed on an electrically conductive substrate. The photoconductive layer is an insulator in the substantial absence of light so that electric charges are retained on its surface. Upon exposure to light, charge is generated by the photoactive or photosensitive pigment, and under applied field charge moves through the photoreceptor and the charge is dissipated.
In electrophotography, also known as xerography, electrophotographic imaging or electrostatographic imaging, the surface of an electrophotographic plate, drum, belt or the like (imaging member or photoreceptor) containing a photoconductive insulating layer on a conductive layer is first uniformly electrostatically charged. The imaging member is then exposed to a pattern of activating electromagnetic radiation, such as light. Charge generated by the photoactive pigment move under the force of the applied field. The movement of the charge through the photoreceptor selectively dissipates the charge on the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image. This electrostatic latent image may then be developed to form a visible image by depositing oppositely charged particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly (such as by a transfer or other member) to a print substrate, such as transparency or paper. The imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite single layer containing charge photogenerating and charge transporting compounds and other materials. In addition, the imaging member may be layered. These layers can be in any order, and sometimes can be combined in a single or mixed layer.
Typical multilayered photoreceptors have at least two layers, and may include a substrate, a conductive layer, an optional charge blocking layer, an optional adhesive layer, a photogenerating layer (sometimes referred to as, and used herein interchangeably, a “charge generation layer,” “charge generating layer,” or “charge generator layer”), a charge transport layer, an optional overcoating layer and, in some belt embodiments, an anticurl backing layer. In the multilayer configuration, the active layers of the photoreceptor are the charge generating layer (CGL) and the charge transport layer (CTL).
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, however, degradation of image quality was encountered during extended cycling. The complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits, on the imaging members. Thus, photoreceptor materials are required to exhibit, efficient charge generation and charge transport properties, and structural integrity and robustness so as to withstand mechanical abrasion during image development cycles.
Organic photosensitive pigments are widely used as photoactive components in charge generating layers. One such pigment used in the charge generating layer in electrophotographic devices is titanyl phthalocyanines or oxytitanium phthalocyanine (TiOPc). As explained, for example, in U.S. Pat. No. 5,164,493, which is hereby incorporated by reference in its entirety, polymorphism or the ability to form distinct solid state forms is well known in phthalocyanines and will affect its photoactive properties. There are several TiOPc crystal forms, or polymorphs, known to be useful in photoreceptor devices. For example, some crystal forms of TiOPc are described as Types I, II, III, X, Y and IV or V. TiOPc Type IV or Type V offer many attractive features as a photosensitive pigment, and they are especially of interest because of their high efficiency of charge generation. As such, TiOPc Type IV is incorporated into photoreceptor layers, such as disclosed in U.S. Pat. Nos. 5,164,493 and 5,384,222, which are hereby incorporated by reference in their entirety. Many processes for making the various crystal forms of TiOPc are disclosed in the literature. In general the processes involve the synthesis of the TiOPc as a first step. The desired polymorph, with the preferred photoactive properties, XRPD pattern and particle size is then obtained from the “as synthesized” TiOPc, usually Type I or II, in a second process step which involves the dissolution of the TiOPc in specific acidic solvent systems, followed by a precipitation in a non-solvent mixture or system. Further purification and solvent treatment of the material is usually required to obtain the desired photoactive properties. Such processes where the “as synthesized” TiOPc is dissolved in sulfuric acid, prior to precipitation and further purification, are described in U.S. Pat. Nos. 5,512,674 and 6,232,466 which are hereby incorporated by reference in their entirety. Other processes where the “as synthesized” TiOPc is dissolved in a mixture of haloacetic acid and alkylene halide, prior to precipitation and further purification, are described in U.S. Pat. Nos. 5,153,094, 5,153,313, 5,166,339, 5,189,156 and 5,182,382, which are hereby incorporated by reference in their entirety. These patents also describe the use of several non-solvent mixtures or systems to precipitate the dissolved TiOPc in order to obtain the desired crystal form, particle size and photoactive properties. The non-solvent system used is generally described as determining the type of crystal form obtained during the precipitation step, while another solvent treatment after the precipitation may also affect the polymorph, as well as the photoactive properties. Thus non-solvent systems used in the precipitation step described in the above mentioned patents include water, mixtures of water and an alcohol, an alcohol, a diol or a mixture of alcohol and alkylene halide. One of the preferred post precipitation treatments is the use of a halobenzene. Thus, U.S. Pat. No. 5,206,359, which is hereby incorporated by reference in its entirety, describes the use of a dissolution mixture comprised of a haloacetic acid and alkylene halide, a precipitation solvent mixture of an alcohol and water to obtain an intermediate crystal form TiOPc designated as Type X, followed by a treatment with a halobenzene to obtain the desired photoactive TiOPc crystal form identified as Type IV. U.S. Patent Publication No. 2006/0105254, which is hereby incorporated by reference in its entirety, describes the use of a dissolution mixture comprised of a haloacetic acid and alkylene halide, a precipitation solvent mixture of an alcohol and alkylene halide to precipitate an intermediate crystal form of TiOPc, assigned as type Y, followed by a treatment with monochlorobenzene to obtain the desired photoactive crystal form of TiOPc, identified as Type Y.
The control of the crystal form of phthalocyanines, such as titanyl phthalocyanine (TiOPc) is critical for obtaining the desired photoactive properties, such as high photosensitivity. Processes for obtaining phthalocyanines for electrophotographic application are generally complex, multistep processes and therefore the ability of obtaining the final product with all the right properties may not be very reproducible. Processes that involve obtaining an intermediate crystal form of the phthalocyanine are even more challenging: if the intermediate crystal form is not the right one, then the solvent treatment will generally not yield the desired crystal form of the phthalocyanine. Thus, there is a need for processes which improve the crystal form control of the phthalocyanines, such as TiOPc, which will result in a consistent and reproducible process for obtaining the desired crystal form and photoactive properties.
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