US3672979A - Method of producing a phthalocyanine photoconductive layer - Google Patents

Abstract

A NOVEL PROCESS FOR THE PREPARATION OF A PHOTOCONDUCTIVE LAYER OF AN ELECTROPHOTOGRAPHIC PLATE WHICH COMPRISES COMBINING PHTAHLOCYANNINE PIGMENT PARTICLES AND A BINDER MATERIAL IN A LIQUID MEDIUM AND SANDMILLING AND THE COMBINATION IS DISCLOSED.

Description

United States Patent Ofice 3,672,979 Patented June 27, 1972 US. Cl. 117-401 Claims ABSTRACT OF THE DISCLOSURE A novel process for the preparation of a photoconductive layer of an electrophotographic plate which comprises combining phthalocyanine pigment particles and a binder material in a liquid medium and sandmilling and the combination is disclosed.

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 US. Pat. 2,297,691, an electrostatic latent image is 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 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 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 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 cadmium 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 properties 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 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 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 ultraviolet range, which is not fully satisfactory for use in conventional electrophotographic copying device s. Even the most sensitive organic photoconductive polymers leave much to be desired for commercial purposes. The choice of materials available for use in aromatic polymeric plates is, of course, limited because of the necessity of the selection of an already photoconductive material. In addition, all of the above noted xerographic plates lack abrasion resistance and stability of operation particularly at elevated temperatures.

Binder plates containing zinc oxide pigments, while comparatively inexpensive, are lower in sensitivity as compared with vitreous selenium plates and are not reusable. Also, as above noted, their visible sensitivity is quite limited. Furthermore, it is necessary to use such high percentages of photoconductive pigment in order to attain adequate sensitivity that it is difficult in zinc oxide plates to obtain smooth surfaces which lend themselves to efficient toner transfer and subsequent cleaning prior to reuse. An additional drawback in the use of commercial zinc oxide binder type plates is that they can be more readily sensitized by negative corona than by positive which results in poor print quality. This property makes them commercially undesirable since negative corona discharge generates much more ozone than positive corona discharge and is generally harder to control.

In copending application, Ser. No. 518,450, filed in the US. Patent Ofiice, Ian. 3, 1966, electrophotographic plates and processes employing phthalocyanine pigment dispersed in a binder material are disclosed. These plates may be reusable or non-reusable and have sensitivities which extend over the entire spectrum. In copending application Ser. No. 867,864, filed Oct. 16, 1969, abandoned for continuation-in-part application Ser. No. 889,718, filed Dec. 31, 1969, now Pat. No. 3,640,710, commonly assigned, preferred phthalocyanine binder formulations are disclosed and claimed.

SUMMARY OF THE INVENTION Generally, phthalocyanine pigment-binder plates are prepared by incorporating phthalocyanine pigments in a dissolved or melted binder through ball milling and/or pebble milling. These milling procedures usually must be carried on for days in order to produce acceptable dispersions. Further, when ball milling and/or pebble milling is employed, recrystallization of the alpha-form phthalocyanine starting material to the much more photosensitive beta polymorph is either extremely slow or does not take place at all.

It is, therefore, an object of this invention to provide a process for the preparation of phthalocyanine pigmentbinder dispersions that is devoid of the above disadvan tages.

Another object of this invention is to provide a much more rapid process for the preparation of phthalocyanine pigment-binder dispersions.

Still a further object of this invention is to provide a rapid process of preparing photosensitive beta-forn1 phthalocyanine pigment-binder dispersions using the less sensitive alpha polymorph as a starting material.

Yet another object of this invention is to provide a rapid and reliable process for the preparation of phthalocyanine pigment-binder dispersions wherein said phthalocyanine is present over a wide range of concentration.

A further object of this invention is to more economically produce an electrophotographic plate comprising phthalocyanine pigment dispersed in a binder material.

The above objects and others are accomplished in accordance with the present invention, generally speaking, by providing a process for the preparation of an electro photographic plate which comprises combining phthalocyanine pigment particles and a binder material in a liquid medium and sandmilling the combination.

This process is much more rapid than pebble milling and/or ball milling. In fact, dispersions are prepared hours rather than days. Moreover, through the use of sandmilling, as opposed to ball milling and/or pebblemilling, a less sensitive alpha phthalocyanine starting 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 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. 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- '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 the present invention. Typical resins include petroleum hydrocarbons, styrene-acrylonitriles, epoxys, polycarbonates, polysulfones, styrene-butadiene copolymers, polyesters, phenolics, alkyds, silicone-alkyds, coumarone-indenes, phenoxys, polyvinylcar'bazoles and polyurethanes. A 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 US. Pat. No. 3,640,710.

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 mploying 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 inelude paper, aluminum, brass and plastics.

The pigment-binder-solvent slurry may be applied to substrates by any of the welleknown 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 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 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.

EXAMPLEI The slurry is milled for 1 hour at 2400 r.p.m. The tem- I 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 suflicient 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 withtungsten light, 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 11 D equipment.

EXAMPLE III 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 steel burnishing balls as a milling aid. The deep blue dispersion is then coated and tested as described in Example I. The coating accepts only about 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 diflicult if not incapable of conversion.

EXAMPLE IV ple 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, full of V2" 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 EP-891 1-7-7;

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 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.

What is claimed is:

1. A process for the preparation of a photoconductive layer of an electrophotographic plate which comprises combining a phthalocyanine pigment and a binder in a liquid medium, said phthalocyanine being selected from at least one member of the group consisting of alpha, alpha and beta, and alpha and X-form phthalocyanine and sand milling the combination until at least a portion of said alpha phthalocyanine is converted to beta phthalocyanine.

2. The process as defined in claim 1 wherein said binder material comprises an alkyd-acrylate resin blend, a silicone resin, and a chlorinated hydrocarbon.

3. The process as defined in claim 1 wherein said sand milling is carried out for about 0.2 hour to about 2.0 hours.

4. The process as defined in claim 1 wherein said conversion is carried out at a temperature of about 120 l80 and about 50% by volume sand is used in said sand milling operation.

5. The process as defined in claim 1 wherein said layer is deposited on a conductive supporting substrate material.

6. A process for the preparation of a photoconductive layer of an electrophotographic plate which comprises combining phthalocyanine pigment particles said particles being selected from at least one member of the group consisting of alpha phthalocyanine, alpha and beta phthalocyanine, and alpha and X-form phthalocyanine, an alkydacrylate resin blend, a silicone resin, and a chlorinated hydrocarbon and sand milling the combination for about 0.75 hour in a sand mill filled by volume with sand at a temperature of about -180 F.

7. The process as defined in claim 6 wherein said layer is deposited on a conductive supporting substrate.

8. The process as defined in claim 7 wherein said substrate material comprises a conductive paper.

9. The process as defined in claim 7 wherein said substrate material comprises a conductive paper.

10. A process for the preparation of a photoconductive layer of an eletcrophotographic plate which comprises combining phthalocyanine pigment particles said particles being selected from at least one member of the group consisting of alpha, alpha and beta, and alpha and X-form phthalocyanine and milling the combination in a mill containing milling media said media being selected from the group consisting of glass beads and small particle ceramic beads until at least portion of said alpha phthalocyanine is converted to beta phthalocyanine.

References Cited UNITED STATES PATENTS 3,296,008 1/1967 Pugin 260--314.5 3,357,989 12/ 1967 Byrne et al 260314.5

OTHER REFERENCES Chemical Eng. News, Nov. 25, 1957, p. 61.

WILLIAM L. JARVIS, Primary Examiner U.S. Cl. X.R.