US3589928A - Process for the production of a light sensitive material for electrophotography - Google Patents

Abstract

A LIGHT SENSITIVE MATERIAL SUITABLE FOR ELECTROPHOTOGRAPHY AND THE METHOD FOR PRODUCING THE MATERIAL. ADMIXED IN AQUEOUS SOLUTION ARE A CADMIUM ION WHICH IS SELECTED FROM THE GROUP CONSISTING OF CADMIUM HALIDE, CADMIUM SULPHATE AND CADMIUM NITRATE; A CARBONATE ION WHICH IS SELECTED FROM THE GROUP CONSISTING OF SODIUM CARBONATE, POTASSIUM CARBONATE AND AMMONIUM CARBONATE; AND A SULFIDE ION WHICH IS SELECTED FROM THE GROUP CONSISTING OF SODIUM SULFIDE, AMMONIUM SULFIDE AND HYDROGEN SULFIDE. FURTHER, A SELENIDE MAY DISPLACE THE SULFIDE ION. A PRECIPITATE IS THUS FORMED WHICH IS THEREAFTER CALCINED AT A TEMPERATURE LESS THAN ABOUT 400*C.

Description

June 29, 1971 IWAO SAWATO ETAL 3,589,928

. PROCESS FOR THE PRODUCTION OF A LIGHT SENSITIVE MATERIAL FOR ELECTROPHOTOGRAPHY Filed Aug. 4, 1966 VOLTAGE I 2 3 5 7 I0 I00 SEC.

HALF DELAY EXPOSURE TIME INVENTORS. IWAO SAWATO KATSUO MAKINO ATTORNEYS,

United States Patent Int. Cl. otisc 1/00 US. Cl. 11734 6 Claims ABSTRACT OF THE DISCLOSURE A light sensitive material suitable for electrophotography and the method for producing the material. Admixed in aqueous solution are a cadmium ion which is selected from the group consisting of cadmium halide, cadmium sulphate and cadmium nitrate; a carbonate ion which is selected from the group consisting of sodium carbonate, potassium carbonate and ammonium carbonate; and a sulfide ion which is selected from the group consisting of sodium sulfide, ammonium sulfide and hydrogen sulfide. Further, a selenide may displace the sulfide ion. A precipitate is thus formed which is thereafter calcined at a temperature less than about 400 C.

This invention relates to a process for the production of a photoconductive powder consisting of cadmium sulfide and cadmium carbonate and in particular, a light-sensitive material for electrophotography.

As a light-sensitive material for electrophotography, glassy selenium or zinc oxide has been used. In addition, cadmium sulfide, zinc sulfide, antimony trisulfide, titanium oxide, mercury sulfide or lead oxide may be used. One of them is applied to a suitable support to form a uniform layer as in the case of using glassy selenium or dispersed in an electrically insulating binder to form a thin layer when using zinc oxide. The electrophotographic light-sensitive body consisting of a uniform layer of glassy selenium has excellent characteristics except that skilled technique is required for the production thereof. On the other hand, the electrophotographic light-sensitive body wherein a photoconductive fine grain is dispersed in a binder may be produced in a relatively simple manner, but is not generally favorably compared with glassy selenium with respect to characteristics and usage.

The present invention is concerned with a process for the production of a photoconductive fine powder suitable for use as an electrophotographic light-sensitive material with a binder.

The characteristics generally required for electrophotographic light-sensitive materials are that the electrically insulating property be large enough so as to maintain electrostatic charges for a long time in a dark place and that the photoconductive property be displayed by radiation of a radiant ray such as visible light to discharge the electrostatic charges rapidly. Moreover, such characteristics and conditions are preferred that there is a high sensitivity to a radiant ray, such as visible light, that is, the electrostatic charges are rapidly discarged by a small quantity of radiation, the characteristics being independent of the state of the material before use. It is charged electrostatically positive or negative and exhibits excellent, similar sensitivities to both the polarities, while keeping the cost of the material low. With respect to the widely used zinc oxide, the foregoing requirements and conditions are not necessarily satisfied, as the sensitivity is low, the characteristics are influenced by a state of the material before use to a great extent and the light-sensitivity is restricted to the negative polarity. In order to overcome these disadvantages of the zinc oxide, a number of studies have been disclosed. For example, the sensitivity is increased by color sensitizing with organic dyes. However, this is accompanied by a lowering of the heat stability and a bad influence upon the characteristics by the history of the material, in particular, by pre-exposure. On this point too, a number of improvements have been proposed, but it has been impossible to obtain a material useful within a short period, such as less than 10 seconds.

One object of this invention is to provide a new photoconductive fine powder having a light-sensitivity to either the positive or negative static charges and being free of influence upon its characteristics by the history of the material.

Another object of this invention is to provide a process for producing the photoconductive fine powder in a simple and stable manner.

A still further object of this invention is to provide an insulating photoconductive layer, in particular, a lightsensitive material for electrophotography by dispersing the photoconductive fine powder in a binder, applying the mixture to a support to form a layer and drying.

A still further object of the invention is to provide a photoconductive powder consisting of the photoconductive fine powder and a binder.

It has been well known that cadmium sulfide may be used as a photoconductive material and dispersed in an electrically insulating binding agent, may also be used as a light-sensitive material for electrophotography. However, this material has a high sensitivity to negative polarity, but not to positive polarity. Furthermore, since the response speed of photoconductivity is low and pre-exposure effect remains for several seconds, it is unsuitable for a repeated use within a period of about 10 seconds.

We have found that powder particules consisting of cadmium carbonate and cadmium sulfide give a relatively high response velocity of photoelectric current, rather than pure cadmium sulfide fine powder and an electrophotographic light-sensitive body produced by dispersing the powder in an insulating resin and applying to an electrically conductive support has a sufiicient property to be charged in a dark place and high light sensitivity. Further, it has been found that this light-sensitive body is well charged positively and negatively and exhibits a similar degree of light sensitivity to both.

A feature of the invention consists in producing a photoconductive fine powder consisting of cadmium sulfide and cadmium carbonate by the wet precipitating reaction followed by calcination, wherein cadmium carbonate is coprecipitated with cadmium sulfide by addition of a carbonate ion.

This wet reaction precipitate may be prepared by mixing an aqueous solution containing carbonate ion and sulfide ion with an aqueous solution containing cadmium ion, an aqueous solution containing carbonate ion with an aqueous solution containing cadmiumion in the presence of hydrogen sulfide or an aqueous solution containing carbonate ion in which hydrogen sulfide is dissolved under pressure with an aqueous solution containing cadmium ion. A small amount of other ions may be added to improve the characteristics. The precipitate thus obtained contains no solid solution of sulfide and carbonate as is proved by the X-ray analysis. Although its structure has not yet been made clear, it is not a mere mixture of cadmium carbonate powder and cadmium sulfide powder.

As the cadmium salt, cadmium halide, cadmium sulfate or cadmium nitrate may be used. As the carbonate, sodium carbonate, potassium carbonate or ammonium carbonate may be used. As the sulfur-containing compound, sodium sulfide, ammonium sulfide or hydrogen sulfide may be used. Sodium selenide or hydrogen selenide may be added to an aqueous solution containing carbonate ion to displace a part of the sulfur with selenium so as to control the precipitation.

In the production of a photoconductive fine powder consisting mainly of cadmium sulfide and cadmium carbonate by the wet process, the coprecipitation thereof comprising mixing an aqueous solution containing cadmium ion with an aqueous solution containing carbonate ion and sulfide ion is carried out always, but another procedure may be carried out comprising preparing a fine white precipitate of cadmium carbonate firstly and then adding a solution containing sulfur so as to convert a part of the white precipitate of cadmium carbonate into cadmium sulfide. In the latter case also, a part or all of the sulfur may be displaced by selenium.

Our invention will now be illustrated by the following examples, but it will be understood it is not limited thereby.

EXAMPLE 1 142.5 g. of cadmium chloride (CdCl -2 /2H O) was dissolved in distilled water to prepare 1250 ml. of the aqueous solution (B sodium). 37.2 g. of sodium carbonate (Na CO -H O) and 15.6 g. of sodium sulfide (Na s) were dissolved in distilled water to prepare 500 ml. of the aqueous solution (A solution). The B solution was added dropwise to the A solution with stirring at room temperature, the dropping velocity being about mL/ min, to yield a yellow precipitate. After decanting and filtering, the precipitate was washed with ethanol and dried in vacuum to yield a dried fine powder. The powder was slightly yellow and had a grain size of 0.1 micron or below. The observation by an electron microscope showed that cadmium sulfide and cadmium carbonate appeared to be one substance intermingled uniformly into a fine grain.

This resulting fine powder was charged to a crucible of quartz, covered up and calcined in the air at 200 C. for 8 hours. The appearance of the calcined powder was not changed, as shown by an electron microscope. The X-ray diffraction showed that the calcined powder gave a diffraction figure suggesting that it was amorphous, the X-ray diffraction intensity of cadmium sulfide being low and the width being wide as much as that of the noncalcined powder. When cadimum sulfide powder is calcined at 200 C., for 8 hours, the crystallization to some extent is observed, but no crystallization occurs in our powder. This means that there is at least an interaction physical and crystallographical, not mechanical, with cadmium carbonate.

The calcined powder and non-calcined powder were dispersed respectively in a binder in a ball mill of porcelain to provide a light-sensitive paint as shown in Table 1.

TABLE 1 Powder Binder, g.

No. of light Charge, Thinner, sensitive paint Class g. Solid Charge ml.

A Non-calcined 60 21.6 53.2 70 B Calcined 60 21. 6 53. 2 70 Blending time in the ball mill: 42 hours.

Binder: A thermo-setting acryl paint, Magicron No. 200 Clear, manufactured by Kansai Paint Co. Ltd.

Thinner: A thinner belonging to the foregoing paint.

tential, the decay of the surface potential during darkening and the attenuation during exposure with respect to each of positive and negative polarities, after charging them by positive and negative corona discharge. The exposure was conducted by the use of a tungsten lamp having a color temperature of 2660 K. and the intensity of illumination on the surface of the sample was approximately 15 lux. The measurements showed that the characteristics in positive and negative charging were substantially the same. With the increase of the coating thickness, the initial surface potential increased and the light sensitivity decreased. That is, the decay velocity during exposure is lowered with an increase in coating thickness. The decay during darkening was hardly changed by the coating thickness. The characteristics were improved by baking, remarkably in the non-calcined powder, but not so in the calcined powder.

In FIG. 1 there is shown a relation of the initial surface potential to the half decay exposure time (time required for half-value of the surface potential to decay exposure) as to samples of electrophotographic lightsensitive body which are prepared by the use of the lightsensitive paint A or B under a baking condition of 250 C. at 30 minutes. In this figure, the more off the curve to the left the better the characteristics. It is evident from this that the calcined powder compares favorably with the non-calcined powder and the positive charging and negative charging give substantially similar charac teristics in each of the calcined and non-calcined powder.

EXAMPLE 2 The calcined powder of Example 1 was mixed with the binder used in Example 1 in a proportion of 70 g. of the former to 15 g. of the latter and dispersed adequately. The mixture was dried by hot air at 6080 C. with vigorous stirring. The dried mixture was in small cake-like forms which had tho be finely divided. The dried, small blocks, were subjected to heat treatment at C., for 60 minutes to harden the binder completely, and charged into a ball mill of porcelain with ethyl alcohol, finely pulverized and then the ethyl alcohol was evaporated to yield a fine dry powder. This powder was then spread uniformly on an electrically conductive support, charged uniformly by corona discharge and exposed to light. The discharging of the charges on the powder occurred selectively according to the intensity of illumination. When applying an air jet to the surface, an image according to the intensity of illumination of the exposure was obtained. The light sensitivity was somewhat unfavorable when compared with Example 1 but substantially the same to either the positive or negative polarities.

EXAMPLE 3 A solution was prepared by dissolving 172.1 g. of cadmium bromide (CdBr -4H O) in distilled water to 1000 ml. of solution and B solution was prepared by dissolving 37.2 g. of sodium carbonate (Na CO -H O) and 15.6 g. of sodium sulfide (Na S) in distilled water to make 500 ml. of solution. The A solution was added dropwise to the B solution with stirring to yield a yellow precipitate and the resulting precipitate was fired at 200 C., for 6 hours in the air to give a photoconductive powder. The subsequent blending with an organic binder and treatment were carried out similarly as in Example 1.

EXAMPLE 4 A solution was prepared by dissolving 128.3 g. of cadmium carbonate (CdCO -8/3H O) in distilled water to make 1000 ml. of solution and B solution was prepared by dissolving 34.3 .g. of ammonium carbonate and 13.6 of ammonium sulfide in distilled water to make 1000 ml. of solution. The subsequent procedures and treatments were similar to Example 3.

EXAMPLE 5 A solution was prepared by dissolving 142.5 g. of cadmium chloride (CdCl -2 /2H O) in distilled water to make 1000 ml. of solution and B solution was prepared by dissolving 37.2 g. of sodium carbonate (Na CO -H O), 9.8 g. of sodium sulfide and 11.0 g. of sodium selenide (Na Se) in distilled water to make 1000 ml. of solution. The subsequent procedures and treatments were similar to, Example 3.

EXAMPLE 6 A light-sensitive paint was prepared from the calcined powder of Example 1 according to the following mixing ratio and applied to an aluminum plate, followed by drying to produce a sample of electrophotographic lightsensitive body. The drying was carried out at 50 C. for 2 hours. No baking was done.

Calcined powder of Example 1-88 g.

Silicone resin KR-211 (made by Shin-Etsu Chemical Co.)-22.9 g.

Epoxy ester D-4 (made by Nippon Oil & Fat Co.)-

Toluene-50 ml.

The blending was carried out in a ball mill of porcelain for 42 hours and the binder was capable of being dried at normal temperature. The measurement of the electrophotographic characteristics under a similar condition to Example 1 gave substantially the same results as in Example 1.

EXAMPLE 7 100 g. of the calcined photoconductive fine powder obtained in Example 1 was mixed with g. of Beckacite as a solvent and the mixture was dried in vacuum to yield a porous cake. The resulting cake was finely divided to give a powder consisting of the binder and photoconductive fine powder, which was suitable for use as a photoconductive toner for electrophotography. That was fused and fixed by means of a solvent such as trichrene or by heating.

EXAMPLE 8 A solution was prepared by dissolving 212 g. of anhydrous sodium carbonate in distilled water to make 1.5 of solution. B solution was prepared by dissolving 513 g. of cadmium sulfate (CdSO -8/3H O) in distilled water to make 1.5 1., and C solution was prepared by dissolving 78 g. of anhydrous sodium sulfide in distilled water to make 0.3 l. of solution. The B solution was dropwise added to the A solution with stirring to yield a white precipitate of cadmium carbonate, to which the C solution was then added dropwise to convert a part of the cadmium carbonate into cadmium sulfide. The thus obtained yellow precipitate was washed with water, completely dried and fired in the air at 200 C. for 24 hours to give a photoconductive powder. The powder was mixed with a binder in the similar manner to Example 1 to obtain a sample of light-sensitive body for electrophotography.

It will be understood that the precipitate formed by mixing an aqueous solution containing cadmium ion with an aqueous solution containing sulfide ion and carbonate ion has excellent photconductive properties and is further improved by calcination. The addition of other ions or dyes during precipitating, after precipitating or before calcining be employed for the purpose of improving the characteristics as an applicable variation of our invention. The calcination or firing is not always carried out in the air, but may be in oxygen gas or inert gas in accordance with the desired characteristics. The calcination may be conducted to such an extent as to decompose the formed cadmium carbonate, but preferably at a temperature below 400 C.

Since the photoconductive fine powder of our invention turns thin yellow, in a mixed disperlsion system, to the photoconductive fine powder and binder, may be added other inorganic orv organic pigments for improving or coloring the reflected ray, and dyes for color sensitizing.

What is claimed is:

1. A process for producing photoconductive fine powder which comprises admixing in aqueous solution cadmium ions, carbonate ions and sulfide and/or selenide ions, said materials admixed in a molar ratio of about 22 to about mol percent cadmium ions, from about 15 to about 57 mol percent carbonate ions and about 6 to about 40 mol percent of sulfide and/or selenide ions whereby a precipitate is formed, and thereafter calcining the precipitate at a temperature of less than about 400 C.

2. A process for producing a photoconductive element wherein the calcined precipitate of claim 1 is dispersed into an electrically insulating binder to form a liquid dispersion, applying the dispersion to a suitable support and drying.

3. A process for producing a photoconductive fine powder as in claim 2 wherein said calcining is carried'out after said drying.

4. A process as in claim 1 where said cadmium ion is selected from the group consisting of cadmium halides, cadmium sulphate and cadmium nitrate, said carbonate ion is selected from the group consisting of sodium carbonate, potassium carbonate and ammonium carbonate, and said sulfide ion is selected from the group consisting of sodium sulfide, ammonium sulfide and hydrogen sulfide.

5. A process as in claim 1 where said calcining occurs at 200 C.

6. A product manufactured in accordance with the process of claim 1.

References Cited UNITED STATES PATENTS 2,496,587 2/1950 Marcot 23-134 NORMAN E. TORCHIN, Primary Examiner J. R. HIGHTOWER, Assistant Examiner U .3. Cl. X.R. 252-501; 96-15

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