Method of producing a phthalocyanine photoconductive layer

BACKGROUND OF THE INVENTION

This invention relates to electrophotography and more particularly to the preparation of a binder plate usable in xerography.

In the art of xerography as originally disclosed by Carlson in U.S. Pat. 2,297,691, an electrostatic latent image is 25 formed on a photoconductive insulating layer and is developed thereon by finely divided electroscopic developing materials. The developed image may then be fixed in place or transferred to a copy sheet where it is permanently fixed. Generally the photoconductive insulating layer is 30 first charged to sensitize it and is then exposed to a light image or other pattern of activated electromagnetic radiation to dissipate the charge in radiation struck areas. Thus the charge pattern formed conforms to the electromagnetic radiation pattern which impinges upon the plate. This 35 charge pattern may then as above discussed be developed or made visible by a chargewise deposition on the plate of an electroscopic or electrostatically attractable, finely divided colored material which is referred to in the art as “toner.”

As disclosed in the above noted Carlson patent, suitable inorganic and organic materials may be used to form the photoconductive insulating layer on which the latent electrostatic image is formed. Other photoconductive materials have been disclosed in the prior art as being useful in 45 similar electrophotographic processes such as in U.S. Pats. 2,357,809; 2,891,001; and 3,079,342. Some of these materials are vitreous selenium, polymers such as polyvinylcarbazole, and resin suspensions of inorganic photoconductive pigments such as, for example, zinc oxide and cad- 50 mium sulfide. While most of these materials have evidenced some commercial utility, there are certain inherent disadvantages to the commercial use of each of the suggested compositions.

The discovery of the photoconductive insulating prop- 55 erties of highly purified vitreous selenium has resulted in this material becoming the standard in commercial xerography. Vitreous selenium, however, is sensitive only to wavelengths shorter than about 5,800 A.U. In addition, xerographic plates made with selenium are expensive to 60 manufacture since this material must be applied to the supporting substrate by vacuum evaporation under carefully controlled conditions. Also, vitreous selenium layers are only metastable and may be re-crystallized into inoperative crystalline forms at temperatures only slightly in 65 excess of those prevailing in conventional xerographic copying machines.

Other known xerographic plates made with certain aromatic organic photoconductors have relatively low sensitivity to light and have most of this sensitivity in the ultra- 70 violet range, which is not fully satisfactory for use in conventional electrophotographic copying devices. Even the

979 alpha-phthalocyanine is being finely and uniformly dispersed, it is also being consistently crystallized to the more photosensitive beta-form under these conditions. The pigment dispersion is diluted to about 35% weight percent solids with toluene and then applied by Mayer Rod to a conductive substrate of about 5 mil aluminum foil. The resultant pigment-binder layer is bluish-green in color. The substrate is coated to a dry thickness of about 0.3 mil. The photodischarge characteristics of the pigment-binder coating is determined by corona charging the layer to about 500 volts positive (measured by a Keithley Model 610 BR electrometer with DC probe) followed by exposure to a tungsten lamp (quartz iodine at 2850° color temperature). Under these conditions, an exposure of about 4 foot-candle seconds is sufficient to reduce the potential to about 60 volts. Using conventional xerographic equipment such as the Xerox Model D copier, the phthalocyanine pigment-binder coating is similarly charged, exposed to a standard positive test target with tungsten fight, and then developed with dry toner by conventional cascade means. The toned image is then transferred to ordinary bond paper. A high quality image is obtained.

EXAMPLE Π

About 500 grams of the formulation defined in Example I is added to a ball milling jar, Va full of % diameter flint pebbles and roller milled for about 1 hour at about 140 r.p.m. The dispersion is coated as described in Example I. The resultant pigment-binder layer is deep blue in color. When tested for photosensitivity as described in Example I, the coating accepts only about 120 volts and requires about 6 foot-candle seconds of exposure to reach 60 volts residual. No image is obtained using the Model D equipment.

EXAMPLE ΙΠ

About 50 grams of the formulation described in Example I is placed in a glass jar and milled for about 1 hour on a Gardner paint shaker using Vs steel burnishing balls 4θ as a milling aid. The deep blue dispersion is then coated and tested as described in Example I. The coating accepts only about 140 volts and discharges to about 60 volts with about 2 foot-candle seconds of exposure. No image is obtained using the Model D equipment. Examples IV and V illustrate cases where sand milling a phthalo binder slurry gives an acceptable photosensitive dispersion where pebble milling does not. It is evident that for the same pthalocyanine used, pebble milling is difficult if not incapable of conversion.

EXAMPLE IV

A premix of 196 grams Chlorowax, 196 grams Arotap EP8911—7—7, 66 grams SR-82, 22 grams of alpha-pthalocyanine (obtained from American Cyanamid) and 150 grams of toluene is added to the L-3-J sand mill and milled as described in Example I for about 2 hours. The milled mixture is coated and tested as described in Example I, the bluish-green layer is found to accept 500 volts and discharge to 60 volts with 4.8 foot-candle seconds of exposure. An electrostatic image is produced, developed and transferred to paper as described in Example I.

EXAMPLE V

The following materials are combined in a gallon ball mill jar, ½ full of Vz diameter flint pebbles and are roller milled for about 20 hours at about 140 r.p.m.:

144 g. of alpha-form metal free-phthalocyanine (obtained from American Cyanamid);

960 g. of Arotap EP8911-7-7;

328 g. of Silicone Resin SR-82;

144 g. of Syloid #244, a silica pigment (obtained from W. R. Grace and Co.);

960 g. of Chlorowax 70-LP; and

3000 g. of toluene.

The dispersion is coated as described in Example I. The coating is deep blue in color which is characteristic of

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This process is much more rapid than pebble milling and/or ball milling. In fact, dispersions are prepared in hours rather than days. Moreover, through the use of sandmilling, as opposed to ball milling and/or pebblemilling, a less sensitive alpha phthalocyanine starting 5 material is recrystallized to the much more sensitive beta form in a surprisingly short period of time. In some instances sandmilling has been found to be the only method that results in this conversion of alpha to beta phthalocyanine. Commercially acceptable dispersions may have a 10 pigment content of up to about 15-20% by weight. Whatever pigment concentration is employed, however, for a given pigment concentration, dispersions prepared by short term ball milling and/or pebble milling do not yield electrophotographically acceptable coatings. 15

The above described process works particularly well when one wishes to prepare a beta metal free phthalocyanine-binder dispersion starting with the alpha form to be converted and amounts of beta, X-form or mixtures thereof. In addition to the alpha form which is to be con- 20 verted, any suitable phthalocyanine may be employed. Typical phthalocyanines include metal phthalocyanines and metal-free phthalocyanines such as alpha, beta and X-form phthalocyanine.

Any suitable resin may be employed in the process of 25 the present invention. Typical resins include petroleum hydrocarbons, styrene-acrylonitriles, epoxys, polycarbonates, polysulfones, styrene-butadiene copolymers, polyesters, phenolics, alkyds, silicone-alkyds, coumarone-indenes, phenoxys, polyvinylcafbazoles and polyurethanes. ³⁰A preferred composition for use in the process of the present invention comprises a combination of a phthalocyanine pigment with an alkyd-acrylate resin blend, a silicone resin, and a chlorinated hydrocarbon, more fully described in U.S. Pat. No. 3,640,710. 35

The sandmilling may be carried out for any suitable time. A preferred time period ranges from about 0.2 hour to about 2.0 hours. Optimum results are achieved when said sandmilling is carried out for about 0.75 hour employing about 50% by volume sand and maintaining temperatures of about 120-180° F.

After the materials are combined to form a photoconductive layer, said layer may be positioned upon any suitable support substrate. Typical support substrates include paper, aluminum, brass and plastics. 45

The pigment-binder-solvent slurry may be applied to substrates by any of the well-known painting or coating methods, including spray, flow coating, knife-coating, electro-coating, Mayer bar drawdown, dip coating, reverse roll coating, etc. Spraying in an electric field may 50 be preferred for smoothest finish and dip coating for convenience in the laboratory.

DESCRIPTION OF PREFERRED EMBODIMENTS

To further define the specifics of the present invention 55 the following examples are intended to illustrate and not limit the particulars of the present invention. Parts and percentages are by weight unless otherwise indicated.

EXAMPLE I _ _ .,60

The following materials are premixed and then placed in a Lr-3-J laboratory sand mill (Chicago Boiler Company) previously half-filled with 20-30 mesh Ottawa sand:

grams of alpha-form metal free phthalocyanine (obtained from Holland-Suco Color Co.);

192 g. of Arotap EP8911-7-7, an acrylic resin (obtained from Ashland Chemicals);

192 g. of Chlorowax 70-LP a chlorinated unpolymerized resinous paraffin (obtained from Diamond Shamrock Co.);70

g. of silicone resin SR-82, (obtained from General Electric Co.); and

250 g. of toluene.

The slurry is milled for 1 hour at 2400 r.p.m. The temperature is maintained between 120°-180° F. While the 75

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5.

the alpha form of phthalocyanine. The material is found to accept only 240 volts and requires about 3.6 foot-candle seconds of exposure to discharge to about 60 volts residual. A suitable electrostatic image is not obtained on the Model D equipment. It is evident that only a portion of the pigment is converted to the beta-form, thus limiting the photosensitivity of the coating.

Although the present examples were specific in terms of conditions and materials used, any of the above listed typical materials may be substituted when suitable in the above examples with similar results. While sand is the preferred milling media other small bead-type media may be employed in the process of the present invention such as glass beads, Coors Ceramedia and Minimedia (the latter two are small particle ceramic beads). In addition other materials may be incorporated in the system of the present invention which will enhance, synergize or otherwise desirably affect the properties of the systems for their present use. For example, a silica pigment may be incorporated in the sand milling process to serve as an antiblocking agent.