In electrophotography, an image comprising a pattern of electrostatic potential, also referred to as an electrostatic latent image, is formed on a surface of an electrophotographic element comprising at least two layers: a photoconductive layer and an electrically conductive substrate. The electrostatic latent image can be formed by a variety of means, for example, by imagewise radiation-induced discharge of a uniform potential previously formed on the surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with an electrographic developer. If desired, the latent image can be transferred to another surface before development.
Among the many different kinds of photoconductive materials that have been employed in electrophotographic elements are phthalocyanine pigments such as titanyl phthalocyanine and titanyl tetrafluorophthalocyanines. Electrophotographic recording elements containing such pigments as charge-generation materials are useful in electrophotographic laser beam printers because of their capability for providing good photosensitivity in the near infrared region of the electromagnetic spectrum, that is, in the range of 700-900 nm.
The pigment titanyl phthalocyanine (PcTiO) as synthesized is not in the optimum crystal form for electrophotography. PcTiO can be converted to the optimum form by any of several techniques including acid pasting or some form of mechanical milling. For a discussion of the crystal forms of titanyl phthalocyanine used in electrophotography see P. M. Borsenberger and D. S. Weiss; Organic Photoreceptors for Imaging Systems (New York: Marcel Dekker, 1993), 356-361. Specific examples of crystal form interconversions include the following:
Acid pasting PcTiO with MeSO3H—CH2Cl2: R. S. Gaims, E. S. C. Simpson, J. A. Stewart, L. M. Traynor (Zeneca Ltd.); G. B. Patent 2322866 (Sep. 9, 1998).
Mechanically mixing PcTiO and F4PcTiO to form a cocrystalline mixture: M. F. Molaire, J. E. Kaeding (Eastman Kodak); U.S. Pat. No. 5,614,342 (Mar. 25, 1997).
Acid pasting FnPcTiO with H2SO4: M. F. Molaire, J. E. Kaeding, W. T. Gruenbaum (Eastman Kodak); U.S. Pat. No. 5,629,418 (May 13, 1997).
Mixing two or more pigments from PcH2, PcCu, PcTiO and PcVO in aqueous alcohol to effect a crystal structure change: T. Ohashi, M. Hayashi (Mitsubishi Chemical Corp.); E. P. 661353 (Jul. 5, 1995).
Acid pasting mixtures of RnPcTiO and R′nPcVO in H2SO4: A. Itami, K. Watanabe (Konica Corp.); U.S. Pat. No. 5,354,635 (Oct. 11, 1994).
Acid pasting PcTiO with CF3COOH—CH2Cl2: J. D. Mayo, J. M. Duff, T. L. Bluhm, C. K. Hsiao (Xerox Corp.); U.S. Pat. No. 5,182,382 (Jan. 26, 1993).
Salt milling PcTiO and F4PcTiO to form a cocrystalline mixture: K. C. Nguyen, T. R. Klose (Eastman Kodak); U.S. Pat. No. 5,112,711 (May 12, 1992).
Simultaneous vapor deposition of two phthalocyanines: S. Suzuki, J. Gouda, H. Toda, A Itsubo, T Sasaki, (Mitsubishi Petrochemical Co.); U.S. Pat. No. 4,981,767 (Jan. 1, 1991).
The disadvantage of acid pasting is the large quantity of concentrated sulfuric acid that must be used and the subsequent disposal of such sulfuric acid. In addition, PcTiO slowly decomposes in sulfuric acid. Organic acid pasting systems such as trifluoroacetic acid/dichloromethane have similar or worse disposal problems. Mechanical milling is more economical than acid pasting in terms of materials since no solvent is used and the milling media may be reusable. But, milling typically is a slower process than acid pasting and consumes considerable amounts of mechanical energy and milling vessel time. Therefore, the synthesis of a more easily milled form of titanyl phthalocyanine would be advantageous.