Dispersion process

#This invention relates in general to dispersion processes and, more specifically, to a process for forming dispersions of particles in a solution of a film forming binder.

#A photoconductive layer for use in electrophotography may be a single layer or it may be a composite layer. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes a photosensitive member having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the photoconductive layer is sandwiched between the contiguous charge transport layer and the supporting conductive layer. In another embodiment, the charge transport layer is sandwiched between the supporting electrode and a photoconductive layer.

#Photosensitive members (photoreceptors) having at least two electrically operative layers as described above provide excellent images when charged with a uniform electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles. A key component of modern photoreceptors is the charge generation layer (CGL) which absorbs imaging light and produces the conducting charge that is used to discharge the charge on the photoreceptor surface and hence form an electrostatic image. The material component of the charge generation layer that performs this photogeneration of charge is a polycrystalline pigment. For reasons of electrical performance and cycling stability it is desirable that the charge generation layer be as thin as possible, yet absorb more than 90 percent of the light to which it is exposed. Thus, it is desirable to coat charge generation layer to a thickness of about 0.1 micrometer to about 0.2 micrometer (100 nanometers to 200 nanometers) taking into account the binder polymer. To coat such thin layers requires that the pigment particles be smaller than about 0.1 micrometer. (100 nanometers) Furthermore, the presence of larger pigment agglomerates causes a non-uniformity of photoelectrical response and is one of the causes of undesirable charge deficient spots (CDS). Hence, pigment dispersions require milling of the pigment in the presence of a binder polymer and solvent. Techniques such as shaking or rolling the dispersion with steel or other hard material ball (shot) are used to accomplish this milling.

#A 50/50 weight percent ratio of 0.8 gram of x-form metal free phthalocyanine pigment agglomerates and 0.8 gram of poly(4,4′-diphenyl-1,1′-cyclohexane carbonate (Z-type polycarbonate) were mixed together in a vessel with 40 milliliters of methylene chloride solvent. A magnetic stir bar was used to stir the mixture. The temperature of the resulting mixture was ramped up from ambient room temperature on an adjustable hotplate which was also initially at ambient room temperature. The temperature control setting of the hot plate was then set to “Low” to raise the temperature of the mixture to ˜30° C. Five minutes later, the temperature control setting of the hot plate was set to “1” to raise the temperature of the mixture to about to ˜40° C. About 5-10 minutes later, the temperature control setting of the hot plate was set to “2” to raise the temperature of the mixture to about to ˜50° C. Ramping of the temperature was important because placement of a cool vessel and contents on a preheated hot-plate can cause a glue-like sludge to form on the bottom that is extremely difficult to disperse. The mixture was then boiled with some frothing for 20 minutes. The vessel was uncapped during boiling. As solvent evaporated during the boiling stage, a thin stream of solvent was slowly (so as not to rapidly cool the mixture) added to maintain the solid/solvent ratio and to wash down larger particles that were deposited on the vessel walls by the boiling and frothing. After 20 minutes of boiling, the heat setting was lowered to “1”, the vessel opening was lightly capped with a glass cap, and the mixture was allowed to simmer for ˜30 minutes. Solvent was added periodically in about 10-15 minute intervals to maintain solvent/solid ratios. The same process has been repeated with an identical mixture using a condenser attached to the flask so that no solvent is lost to evaporation and the same times were required for the various process steps. The mixture was stirred with the magnetic stir bar during the ramping, boiling and simmering heating periods. Upon completion of the 30 minutes of simmering, the vessel was removed from the hotplate. The resulting dispersion was judged by the naked eye. The liquid looked like paint and showed no “clouds” on swirling. When ˜1 milliliter of the dispersion was squirted onto a glass plate, the mixture was uniform, smooth, and no particles (agglomerates) were visible. This dispersion contained 0.8 gram of the phthalocyanine particles and 0.8 gram poly(4,4′-diphenyl-1,1′-cyclohexane carbonate in 35 milliliters methylene chloride and was stable for at least 4 weeks, after which it was all used up in coatings.

#A 50/50 weight percent ratio of 0.5 gram of perylene dimer pigment (described in U.S. Pat. No. 5,645,965 and pending U.S. patent application Ser. No. 09/316,587, entitled “Perylenes”, filed in the names of Hsiao et al., on May 21, 1999, the entire disclosures of the patent and patent application being incorporated herein by reference) and 0.5 gram of poly(4,4′-diphenyl-1,1′-cyclohexane carbonate (Z-type polycarbonate) were mixed together in a vessel with 40-60 ml of methylene chloride solvent. The temperature of the resulting mixture was ramped up from ambient room temperature on an adjustable hot plate to ˜40° C. over 15 minutes and then boiled for 1 hour. Ramping of temperature was important because placement of a cool vessel and contents on a preheated hot-plate can cause a glue-like sludge to form on the bottom that is extremely difficult to disperse. The mixture was then simmered for 2 hours. During the ramping, boiling and simmering heating periods, the mixture was stirred with a magnetic stir bar. As solvent evaporated during the boiling stage, a thin stream of solvent (so as not to rapidly cool the solution) was added to maintain the solid/solvent ratio and to wash down larger particles that are deposited on the vessel walls by agitation of the mixture due to boiling. During the simmering stage, solvent was added periodically (10-15 minutes) to maintain the solid/solvent ratio. The container was uncapped during boiling, but lightly capped during simmering. The dispersion was stable for at least 8 weeks.

#Small strips of conductive substrate were dip coated twice with the x-form metal free phthalocyanine dispersion prepared in Example I to form a dried charge generating layer having a thickness of ˜0.5 micrometer. Transmission electron microscopy (TEM) examination of one coated sample coated with the charge generating layer alone revealed pigment sizes below the resolution (<0.01 μm) of the TEM.