US3494789A - Photoconductive insulating material - Google Patents

Description

Feb. 10, 1970 KATSUO mo ET AL 3,494,789

PHOTOCONDUCTIVE INSULATING MATERIAL Filed June 2, 1966 3 Sheets$heet 2 FIG. 2

VOLTS- 4 5 I000 5 g 700- a m g g 300 2 E T 0-52= 2CdS- 30000 70 D-60 CdS 50 CHARGE 7.0 KV

EXPOSURE 2566K TUNGSTEN LAMP SURFACE INTENSITY 0F 30 ILLUMINATION l LUX 1 l l 1 l l l l 03 0.50.70 '23 710 20305070|00$ec Y wsxvosuas n ma H LF-DECAY I200 FIG. 3

3 E 800 u.| 4f l Z5 KV }Pr 6.5KV u? g 6% q. E -z5 KV M3 |.P T "-asxv 0 TORS 0 5 I0 '15 20 2s NUMBERS OF'REPEATING KATSUO MAKING BY IWAO SAWATO I j r'h aw ATTORNEYS.

Feb. 10 l9 70 v Tsuo MIAKENO ETAL I 3,494,789

ruorocounucwivn- INSULATING MATERIAL Filed June 2. 1966 I :s Sheets-Sheet 3 FIG. 4

SPECIFIC INTENSITY OF DIFFRACTED X-RAY casvm SPECIFIC INTENSITY v 0F DIFFRACTED X-RAY INVENTORS.

KATSW MAKING 7 BY IWAC SAWATO momma United States Patent US. Cl. 117-201 2 Claims ABSTRACT OF THE DISCLOSURE The invention is directed to a photoconductive insulating material mainly used for electrophotography. Said material comprises an electrically insulating binder material having a bulk resistivity of at least ohm-cm. at room temperature, and dispersed therein finely divided particles of the reaction product of cadmium carbonate and sulfur.

This invention relates to a photoconductive insulating material mainly used for electrophotogrpahy.

As the photoconductive insulating material used for electrophotography, it has been known to employ some sort of photoconductive inorganic compounds.

Such substances, for example, as sulfur, selenium, and further compounds, such as oxides, sulfides, or selenides of zinc, cadmium, mercury, bismuth, lead, etc., and also titanium oxide are known. Those substances are coated stratiformly on a plate, e.g., a metal plate, paper and so on. If necessary, the photoconductive insulating layer could be formed by dispersing these substances in the electric insulating resin binder. Among these substances, selenium and zinc oxide are the best known and are practically used up to now.

The procedure to get an electrophotographic image on the electrophotographic sensitive body having a photoconductive insulating layer employing these substances first begins with the placing of an electrostatic charge on the surface of the photoconductive insulating layer to bestow photosensitivity by corona discharge for instance, in the dark. Next follows exposure by usual photorgaphic procedure. The latent image got by exposure is then developed with charged toning powder into the visible image. The toning powder image is fixed intact upon the layer of photoconductive insulating material or alternatively transferred upon other suitable supporting body and fixed. When the image is transferred, the residual toning powder on the layer of the photoconductive insulating material is swept away so that the said layer could be reused. The above-mentioned process is a most general one in the art of electrography and further, pluralities of modified process or many kinds of methods and means to enable those processes are being considered, but fundmaental properties of those photoconductive insulating layers could be regarded essentially as identical.

Selenium, now Widely known and employed for practical use, is employed as a reusable photosensitive material coated on a metal plate, particularly on an aluminum plate, in the form of vitreous selenium layer. The photosensitivity of it corresponds to ASA 2-10 which is considered to be the highest among those electrophotographic sensitive materials up to now. In addition, the vitreous selenium is exceedingly well fitted for practical reuse because it has a very hard, mirror surface. On the other hand, another practically utilized zinc oxide is used to form an electrographic sensitive body, called a photosensitive paper, by a process of dispersing it in an electric insulating resin binder and coating it on a sheet of paper 3,494,789 Patented Feb. 10, 1970 which is then dried. This photosensitive paper, having a photosensitivity in the range of about ASA 0.01-0.2, looks almost white so that an image could be formed and fixed directly upon the paper. The said dispersed layer of zinc oxide could be also coated on a metal body, but it has not been used as a reusable photosensitive material for the reasons that it is far inferior to a photosensitive material employing selenium in such properties as lower film hardness and lower photosensitivity. Moreover, once exposed to light, the properties of the layer are impaired to a state unfit for continuous and repeated use. Other than selenium and Zinc oxide, only the properties of titanium oxide in practical use have been reported.

We should designate one method, the direct image producing process, comprising fixing the toning powder directly on the layer of photoconductive insulating material. Another method may be designated the indirect image producing process. This comprises either fixing the transferred image on other transferring material which is formed on the layer of photoconductive insulating material or developing and fixing on the transferred material, after transferring of the latent electrostatic image, formed on the layer of the photoconductive insulating material, to other transferring material, i.e., the method to form an image on other material with an aid of the layer of photoconductive insulating material.

In the direct image producing process, though it has some advantages resulting from its own properties, the layer of photoconductive insulating material imposes limitations in regard to the whiteness which is possible in copying the documents and the pliability of the coated sheet, for example, a paper. The whiteness of photoconductive insulating layer does no cause any problem in getting a copy image consisting of black and white, but it becomes a fatal defect in copying color printed matter. It could be understood that the limitation of employing a supporting body of paper and the like prevents the material from fully exhibiting the special properties of a photoconductive insulating layer. On the contrary, in the indirect image producing process, there is almost no limitation in employing the color and kinds of the supporting body of the photoconductive insulating layer, and they may be freely chosen so that there becomes possible combinations by which each special characteristic could be exhibited fully. And further, in the latter process an additional advantage exists, i.e., the possibility of continuous and repeated use.

It has previously been known to employ vitreous selenium as an electrophotographic sensitive material for continuous and repeated use. The said material has excellent properties such as photosensitivity, film hardness and pre-exposure effect, as the photoconductive insulating layer. On the other hand, some defects, for example, the extreme difificulty in producing it, exist. Nowadays, it is formed by depositing on a plate by vacuum deposition as a film of uniform vitreous selenium, and the necessity of a large sized vacuum evaporating apparatus for manufacturing it has become the cause of lowering production efficiency. As an additional defect, controlling additive elements added for improving the properties, such as photosensitivity, becomes extremely difiicult by vacuum evaporating process. Moreover, the vitreous selenium, being a sort of solid phase of selenium in a super-cooled state, tends to crystallize by heat, the moisture in air or contamination with other substances, which bring undesirable results to its properites with accelerated impairment of qualities and shortened life when used in high temperature and high humidity. However, in the case of employing zinc oxide, some trials for promoting photosensitivity by adding certain dyes have been done, but those dyes are thermally weak, and the deterioration of properties by corona discharge and light radiation in continuous and repeated use is relatively rapid.

Therefore, the material still is inferior to selenium sensitive material at the point of short life for useful time.

To overcome the above-mentioned disadvantages, a layer of photoconductive insulating material has been developed and prepared by dispersing a photoconductive powder of cadmium sulfide whose mean particle size is about 0.2 in a solvent solution of a thermosetting resin having volume specific resistivity at least more than 10" ohm-cm, and coating it on a suitable plate, i.e., a metal plate, and promoting the hardening by baking while the heat treatment of the powder of cadmium sulfide and the resin system is done simultaneously. The photoconductive cadmium sulfide employed in this desirably does not contain any impurity and is a stoichiometrically perfect compound.

But so far as using such a kind of cadmium sulfide power, it has been found that using it for a short period continuously and repeatedly is unsatisfactory because of the disadvantage of a too late response speed of photocurrent.

An object of this invention is to overcome these disadvantages inherent in vitreous selenium, zinc oxide and cadmium sulfide, etc., and further lies in proposing an improved photoconductive insulating material which has a photosensitivity equal to or more than that of vitreous selenium, and which is thermally stable, having a sufficiently low pre-exposure effect, being endurable in repeated and continuous use, as well as proposing a simple method of producing the said material.

In the accompanying drawing:

FIG. 1 shows the decay of the electrostatic potential of the surface of the electrophotographic sensitive materials of an example according to this invention and the prior art known hitherto;

FIG. 2 shows the relation between the initial surface potential and the exposure time for half decay, of the electrophotographic materials prepared according to one example of the invention and made of pure cadmium sulfide;

FIG. 3 illustrates the comparison of the variation of the surface potential immediately after charging (LP) in the case of employing continuously and repeatedly with the period of 10 sec. for one cycle, and that of the surface potential after exposure (R.P.) in repeated and continuous use, each for the photosensitive materials made of pure cadmium sulfide and of the example according to the invention; and

FIG. 4 shows the diagrammatic record of the X-ray diffraction analysis for the photoconductive powder used in an example of the invention, compared with that of pure cadmium sulfide.

Usually, an electrophotographic photosensitive body, which is prepared by coating a photoconductive insulating material onto a supporting member, is charged in one polarity by a corona discharger and it is not used in another polarity. The charging polarity is decided by the potential acceptance and the light sensitivity of the photosensitive member. Vitreous selenium, for example, is charged with positive ions and zinc oxide and cadmium sulfide with negative ions. But, by adding tellurium to vitreous selenium, some new electrophotographic sensitive material is considered whose photosensitivity is almost equal when charged either positive and negative and whose charge ability is almost equal in positive and negative charge by inserting an inter-layer between the supporting body and photo-conductive insulating layer. In the case that either positive and negative polarity is usable, an advantage lies in being able to get the directly developed image (negative negative or positivepositive) and the reversably developed image (negative psitive or positivenegative) by only changing the polarity of the corona scharge. A further object of this invention exists in offering a photoconductive insulated material fitted for the use in either the positive and the negative charge.

Hereafter, a description is made mainly of what is fitted for use in electrophotography, but there will be wide usage beside it such as for example, in converting means of light or radiation signals into electrical signals as in a photocell, photoamplifier, image converter, phototape, etc., and also where powdered photoconductive insulating materials are dispersed in a suitable binder.

For instance, in electrophotography, to which this invention is applied, a description is made in the case of employing a Xerox Processor Model 914.

In the machine, a cylindrical electrographic sensitive body, which is coated with photoconductive insulating substance on its surface, is forced to rotate at constant speed in continuously and repeated use. It completes one cycle of the process by steps consisting of (1) charging, (2) exposing, (3) developing, (4) transferring, (5) cleaning off, (6) pre-exposing. The step of (6) pre-exposing, is for the sake of accomplishing the preparation for proceeding the operation into the next cycle by discharging all the residual surface charges. The photosensitive body rotates a round in about 11 seconds. That is, within the period of 11 seconds, the process is performed repeatedly. It takes 10.4 secs. from after the step of (2) exposing to the step of 1) charging of next cycle, during this time there is required the complete dissipation of the influence caused by the step (2) exposing. Further, the preexposure step (6) of the previous cycle should be performed before the charging step (1) and the allowed time for it is only 0.6 sec. Therefore, the electrophotographic sensitive material must at least be able to reach the state in 0.6 sec. where nothing of the influence of the exposing effect is left. The photosensitive material prepared by dispersing the powder of pure cadmium sulfide in a thermosetting resin and coating it on a metal plate which then is hardened by a heat treatment, could not be employed satisfactorily in Xerox Processor Model 914. However, electric charge ability in the dark and the photosensitivity is sufficiently high.

This is attributed to the response speed of photoelectric current. Particularly, the decaying time constant of photocurrent is approximately 2 sec., by half value time, so that though the dark resistance has remained in a lower state at the time of charging (1), after pre-exposure (6) of the pre vious cycle, and the chargeability is lowered, the surface charge of the place that corresponds to the shadow part of the exposure decreases rapidly during a period of 34 see. until the end of developing process, after charging process. Thus, electrostatic contrast is lowered respectively and consequently. A disadvantage is the lowering of the contrast of the copied image.

To explain this phenomena, FIG. 3 should be employed where the abscissa represents the number of continuous repetitions in the case of using in the course of the aforementioned process with the period of 10 sec., and the ordinate represents the degree of surface potential immediately after the charging (l) designated LR,

and that at the time point corresponding to the end of' the developing process, designated 'R.P. From the diagram, it should be understood that in the case of e.m-

ploying the photosensitive material #A-13 which is made of pure cadmium sulfide, the said LP. decreases rapidly after only a few times of use.

We inventors have found the fact that instead of powder of cadmium sulfide a material comprising cadmium sulfide incorporated with cadmium carbonate has a relatively rapid response time to photocurrent and an electrophotosensitive member produced by dispersing it in an insulating resin and coating on a metal supporting body has sufficient potential acceptance in the dark as well as high photosensitivity. Moreover, such photosensitive material could be charged either in positive or in negative polarity, as well as having sufi'icient photosensitivity. The powder consisting of cadmium sulfide and cadmium carbonate can be produced by firing cadmium carbonate with sulfur powder at the temperature range between 400- 600 C. As practical methods for this process, there is a pot firing or a calcination of cadmium carbonate in sulfur vapor. By employing a wet method, it would be obtained as a precipitant by dropping an aqueous solution of cadmium chloride into an aqueous solution of ammonium carbonate and ammonium sulfide.

EXAMPLE 1 6 found that the potential acceptance increases in proportion to the film thickness, but the photosensitivity decreases as the film thickness increases.

Consequently, the initial surface potential, when charged in the dark, increases proportionately to the film thickness, and the amount of exposure to light required to reduce the potential to half of the initial state increases. A relation between the exposure time for half decaying of the potential and the initial surface potential in cases of varying them by changing the film thickness is such A time Powder Consisting of Cadmium sulfide and 10 as shown in FIG. 2. For convenience of comparison, the millm carbonate Was Obtained y firing the mixture of data of photosensitive paint #D-60 which consists of 0.1-0-2 i r n diameter Cadmium Carbonate POWder pure cadmium sulfide, is also illustrated in FIG. 2. From Weight P and sulfur fine Powder Weight P FIG. 2, materials having satisfactory early surface potenat the temperature about 450 Q some results of Y tial and exposure time for half-decay could be deterdiiffaetieh analysis of the Photoeohduetive Powder, thus mined for the purpose of obtaining a desired electrostatic Obtained, is shown in Accordingly, it Could he contrast. The measurement conditions in FIG. 2 is as understood that there exists a considerable amount of f ll (1) h i was d b corona-discharge t cadmium carbonate as well as cadmium sulfide as a poly- :10 kv. and, (2) exposure by tungsten lamp of 2666 K., merpheus mixture of its hexagonal, cubic and amorphous, where the intensity of illumination of a sample surface and the constitutional ratio of cadmium sulfide and cadwas above 15 l Th f ti d h t iti ig- Inium Carb nate (C C 3) is approximately X-1.5. ment D52 was coated on the outer surface of an alumi- Then, 155 of said POWder is dispersed in 138-5 gof num pipe having a diameter about 200 mm. by a spray thermoplastic acrylic resin solution commercially named h d, hi h was th d i d f 30 i t 70 c, ft Magieroll deal (489% of Whieh is which drying, it was heat-treated for min. at 150 C. Thus, is avai a m Kansai Paint p y, With its thihhel' a cylindrical, reusable electrophotographic sensitive body as a solvent. The material is mixed in a ball mill suffiwas d d, whose t d fil thi k was b ut eiehtiy to Obtain a photosensitive P The Paint is 3.1 mg./cm. after the heat-treatment. FIG. 3 shows the coated on an aluminum Sh t Whi h is dried pr p r y results from measuring the variation of the surface potenfor 30 nat 7 After y g, a further heat treat- 30 tial of said material in a method as described formerly, m nt f 30 min. at is done and thus a P for example, in Xerox Processor Model 914, in the case Conductive insulating hlyer having high film hardness is of employing it continuously and repeatedly with a period generateii As a typical eXampie Of an eleetrophotegraphie of 10 sec. for one cycle. Charging is done by applying p p y inherent in electrophotographic sensitive mern- 6.5 kv. or 7.5 kv. to a single wire contron and exthus Obtained, the Variation of the surface posing to light was done at 250 lux sec., once. The initial potential is shown in FIG. 1. FIG. 1 also shows the desurface potential i di l aft r h h i Proscrease of surface Potential of an p 'aphi ess (1) (LR) decreases rapidly early in several repeated member employing vitr u Selenium, member #1001 uses in the case when photosensitive member #A-13 comprising CdS and a binder, and the member #BZC made of pure cadmium sulfide is used, but in the case comprising a dye-sensitized ZnO and a binder. Each 40 of employing #A25, according to an example of this ordinate of FIG. 1 is plotted logarithmically with a value i vention, such variation can be scarcely observed in reof the surface potential for each sensitive member. The peated use. The surface potential after exposure (R.P.) a s a is Plotted wtih a time by Sec unit, Passed after at the time point corresponding to the finished time of charging. developing is shown only for #A13, but for #A-25, it

Selenium is a most excellent material at the surface poi too small to be illustrated and it hardly varies during tential property in the dark and p otosensitivity but, it repeated use. The values at the first time and the thirtieth is said so only when selenium is charged with positive and time of repeated use are all as follows, as shown in when charged negative, its property is very undesirable, Table 1.

TABLE 1 Initial surface potential After exposure surface Voltage of (1.1 pot entiai (R.P.)

corona Photosensitive discharge, 1st 30th 1st 30th member kv. time, volts time,volts time,volts time,vo ts 93? 553 it; it 53 --i 9:? as $18 2 2 though not shown in FIG. 1. CdS and ZnO exhibit ex- FIG. 3 and Table 1 show that in a photosensitive memcellent properties in negative charge. On the other hand, ber of the example according to this invention, the response the photosensitive member #1445, an eXamPIe of h time of the photocurrent and, particularly, the decaying invention, eXhihits exerting Properties either in Poslth/e time constant of the photocurrent have been improved to and negative charg The measurements above cases be shorter than that of aphotosensitive member employing e peffermefl under the following condltlons: the CdS, and to a degree where no inconvenience exists in FPPhed potential. for th chargmg corona'dlsczharge practical use with such a repeating period as is encountered 1s 7.0 kv. elther in positive or negatnl e case, 2226i I){e in employing Xerox Processor Model 914. Posure was Performed by a tungsten amp at 0 There exists an optimum ratio for mixing the resin, as its color temperature.

The intensity of the illumination on the surface of a sample was about 15 lux. In the course of examining the chargeability and photosensitivity of the sensitive pigment D-52, an example of this invention, coating it on binder of the photosensitive layer and the photo-conductive powder, and the range of mixing ratio of practically usuable materials are alike to the photoconductive insulting material comprising other photoconductive powan aluminum plate in various film thicknesses, it was ders and resin binders.

7 EXAMPLE 2 The same photoeonductive powder comprising cadmium sulfide and cadmium carbonate,

as employed in Example 1 is used. As an insulating binder there is used a mixture of a silicone resin KR-211 (available from Shinetsu Chemical Industries Co. Ltd.), a room temperature driable resin binder, and an epoxyester (avail able from Nippon Oil and Fats Co. Ltd.). The photoconductive powder and the binder mixed at a ratio shown in Table 2, were mixed and kneaded in a porcelain ballmill to disperse the powder, and thus photosensitive paints were produced. The said photosensitive paints were coated on an aluminum sheet of 0.1 mm. thickness by a spraymethod, and were dried at room temperature. The amount of coated film after drying weighed 3.54.0 mg./cm.

TABLE 2.--PREPARATION CONDITIONS OF PHOTOSENSI- TIVE PAINTS can be dried to form a hard layer at room temperature, a poor thermostable substrate such as paper or plastics may be used to produce a photosensitive member. Further, this paint can be coated over such a plate that can hardly be heated, for example, iron or steel plate, to form a large electrophotographic photosensitive plate.

Further, it is similar to other photosensitive insulating material comprising a photoeonductive powder and a resin binder in which many other kinds of thermoplastic resins and thermoset-ting resins are widely usable. Further, a glass material could be employed.

In a series of photoeonductive powders comprising various content ratios of cadmium sulfide and cadmium carbonate, the most preferred have the formula CdSnCdCO the existence of a trace of selenium, as cadmium selenide, is allowed.

TABLE 3.PROPERTIES OF ELECTROPHOTOGRAPHIO SENSITIVE MATERIALS PRO- DUCED EMPLOYING THE PHOTOSENSITIVE PIGMENT SHOWN IN FIGURE 2.

Charged 7.6 kv. by Corona Discharge Pre-exposure: 2000 Lux, 2 min. (Fluorescent Lamp At 15 see. after pre- At 5 min. after pre- Exposure Rest exposure exposure time for Number of photo- I half decay, sensitive pigment I.P., v. D.D mm. I.P., v. D.D., mm. I.P., v. D.D., min. sec.

LP. =Initial surface potential.

D.D.=Dark half decay time.

Some measurement results of properties of the members, thus prepared, are shown in Table 3. Employing the said table, we should explain how the mix ratio of a resin, working as a binder of a photoconductive paint, and a photoeonductive powder affects the characteristics of the product when employed as an electrophotographic sensitive body. The electrophotographic sensitive body was charged by a corona discharge at 7.0 kv. The decay of the surface potential in the dark was measured for each electrophotographic sensitive member (1) left in the dark place for suflicient time, (2) illuminated for 2 min. at 2000 lux with a fluorescent lamp and charged 15 sec. after the illumination finished, and (3) illuminated for 2 mins. at 2000 lux with a fluorescent lamp, and charged 5 min. after the illumination finished. Thus, consequently, the pre-exposure effect on decay in the dark is examined. Further, the photosensitivity is shown as a value of time which is required for half decay of the surface potential when the said body is illuminated at 15 lux of light intensity of 2.660" K. tungsten lamp, after being charged at -7.0 kv. 'by a corona discharge.

When the content of photoeonductive. powder is decreased, the initial surface potential and decaying in the dark is improved, but the photosensitivity falls down. From the above relation, a content ratio of these compositions fit for an object could be determined. Still more, it would be understood from the table that a pre-exposure effect could hardly be recognized. Although in the above examples of this invention an aluminum plate is employed as a backing member, the said member should not be restricted to aluminum. Other material could be used as the substrates. Because of employing a binder material that What is claimed is:

1. A method for producing a photoeonductive insulating material comprising firing a mixture of cadmium carbonate and sulfur to produce a material represented by the formula CdSnCdCO where n is greater than 0 and has a value up to about 4,

forming a dispersion of CdSnCdCO in a resinous binder and solvent for the binder,

coating said dispersion on a substrate, and

drying and thermally curing said coating.

2. A photoconductive insulating material comprising an electrically insulating binder material having a bulk resistivity of at least 10 ohms-cm. at room temperature, and finely divided particles of a photoeonductive material dispersed in said binder material, said photoeonductive material being represented by the formula CdC-n(CdCO in which n represents O n54.

References Cited WILLIAM L. JARVIS, Primary Examiner US. Cl. X.R. 96-1; 117-230; 346-

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