Bi-chargeable electrophotographic materials including zinc oxide and a binder resin
United States Patent 3881925
An electrophotographic material chargeable both positively and negatively comprising a zinc oxide type electrophotographic layer containing as the binder for zinc oxide a copolymerized acrylic resin comprising 50-95% by weight acrylic ester and 1-5% by weight organic acid having vinyl group copolymerizable with the acrylic ester. The electrophotographic layer may contain further low-molecular weight polystyrene.
US Patent References:
/3578446.html
Sugarman - May 1971 - 3578446
/3672889.html
Baltazzi et al. - June 1972 - 3672889
/3681069.html
McNamee et al. - August 1972 - 3681069
/3578446.html
Sugarman - May 1971 - 3578446
/3672889.html
Baltazzi et al. - June 1972 - 3672889
/3681069.html
McNamee et al. - August 1972 - 3681069
Inventors:
Uchida, Koji (Kyoto, JA)
Suzuki, Shigeyoshi (Kyoto, JA)
Kuriu, Sadao (Kyoto, JA)
Suzuki, Shigeyoshi (Kyoto, JA)
Kuriu, Sadao (Kyoto, JA)
Application Number:
05/380027
Publication Date:
05/06/1975
Filing Date:
07/17/1973
View Patent Images:
Export Citation:
Assignee:
Mitsubishi Paper Mills, Ltd. (Tokyo, JA)
Primary Class:
Other Classes:
430/96
International Classes:
G03G5/05; G03G5/08
Field of Search:
96/1.8,1.7 252/501
Primary Examiner:
Torchin, Norman G.
Assistant Examiner:
Hightower, Judson R.
Attorney, Agent or Firm:
Sughrue, Rothwell, Mion, Zinn & Macpeak
Claims:
What is claimed is
1. A bi-chargeable electrophotographic photosensitive material containing photoconductive zinc oxide and a binder resin comprising a mixture of:
2. 50 to 90% by weight, based on the weight of said binder resin, of an acrylic copolymer comprising:
3. 10 to 50% by weight, based on the weight of said binder resin, of a polystyrene having a molecular weight of less than 5,000.
4. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 wherein said lower alkyl acrylate is methyl acrylate, ethyl acrylate, or butyl acrylate.
5. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 wherein said organic acid is acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride, or the monoester of each of said acids.
6. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 wherein said other copolymerizable vinyl monomer is methyl methacrylate, ethyl methacrylate, butyl methacrylate, a derivative of acrylic ester or methacrylic acid, styrene or a styrene derivative.
7. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 which consists essentially of the recited components.
1. A bi-chargeable electrophotographic photosensitive material containing photoconductive zinc oxide and a binder resin comprising a mixture of:
2. 50 to 90% by weight, based on the weight of said binder resin, of an acrylic copolymer comprising:
3. 10 to 50% by weight, based on the weight of said binder resin, of a polystyrene having a molecular weight of less than 5,000.
4. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 wherein said lower alkyl acrylate is methyl acrylate, ethyl acrylate, or butyl acrylate.
5. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 wherein said organic acid is acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride, or the monoester of each of said acids.
6. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 wherein said other copolymerizable vinyl monomer is methyl methacrylate, ethyl methacrylate, butyl methacrylate, a derivative of acrylic ester or methacrylic acid, styrene or a styrene derivative.
7. The bi-chargeable electrophotographic photosensitive material as claimed in claim 1 which consists essentially of the recited components.
Description:
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to electrophotography and more particularly, it relates to a bi-chargeable electrophotographic material which can be charged to any of positive charge or negative charge by an electrostatic charging step in electrophotographic process.
For facilitating the understanding of the present invention, the technical field of this invention will be explained briefly. As well known, the process of obtaining visible images by electrophotographic method is mainly composed of (1) electrostatic charging of a photosensitive or photoconductive layer, (2) image exposure (the formation of electrostatic latent images), and (3) development by charged colored particles or toners. In the 1st step, the photosensitive or photoconductive layer is charged for forming latent images as the patterns of electrostatic charge on the surface thereon in the subsequent image exposure step. That is, because an electrophotographic material has high insulative property in the dark, the electrophotographic layer can be charged by proper means. When the photosensitive or photoconductive layer of the electrophotographic material is, after uniformly charging the whole surface of the layer, imagewise exposed to light, the portions of the electrophotographic layer exposed to light become conductive to lose the electrostatic charge on the surfaces of those portions, while the portions of the layer which have not been exposed to light bear thereon the electrostatic charge, whereby a pattern of electrostatic charge is formed in conformity with the exposed image. This process is called "formation of electrostatic latent image." Such an electrostatic latent image can generally be developed by colored fine particles or toners having an electrostatic charge opposite to the electrostatic charge on the photosensitive or photoconductive layer, said fine particles or toners being used as a state of powder or a dispersion in highly insulative liquid. However, if a photosensitive or photoconductive layer is exposed imagewise to light in uncharged state, a conductive pattern is formed in conformity of the image on the photosensitive layer during the exposure or within a quite short period of time after exposure but such a pattern does not have a life as the aforesaid electrostatic pattern and further cannot be developed by charged colored fine particles. Thus, in electrophotographic method, the step of charging photosensitive or photoconductive layers is important and an indispensable step.
In general, an electrophotographic photosensitive layer is charged by conducting corona discharging by means of a high-voltage direct current electric source of 6000- 8000 volts to ionizing surrounding air and attaching the ionized air onto the surface of the photosensitive layer. The charging system of utilizing such a d.c. corona discharging can generate a positive charge or a negative charge by changing the poles of the d.c. electric source. However, when a conventional electrophotographic photosensitive layer is charged to definite one of a positive charge and a negative charge, duplication can be conducted well, but when the same electrophotographic photosensitive layer is charged to the opposite charge, duplication cannot be conducted well. Consequently, the charging device is set for giving the definite polarity of electrostatic charge fitted for the characteristics of the electrophotographic layer to be used.
For example, for a photosensitive or photoconductive plate composed of amorphous selenium known as Xerox (registered trade name), the charging device is set for providing a positive charge, while for a photosensitive or photoconductive layer composed of a zinc oxide-resin dispersion system as in Electrofax (resistered trade name), the charging device is set for providing a negative charge.
The reason why the aforesaid phenomena occur on the conventional electrophotographic layer has not yet been clarified. However, some people say that the charge on a photoconductor to be provided is selected according to whether the photoconducter is a P-type or a N-type. That is, a P-type photoconductor in which the current carrier is a positive hole is readily charged to a positive charge and further the positive charge vanishes smoothly when the photoconductor is exposed to light. On the other hand, a N-type photoconductor in which the current carrier is electron is fit for negative charging. According to the view, when a photosensitive layer prepared by using zinc oxide which is a N-type photoconductor is used, the electrophotographic process can be performed well in case of charging negatively the photosensitive layer.
However, if a novel electrophotographic photosensitive material which can be charged to any of a positive charge and negative charge and can perform well electrophotographic process is developed, various new processes that have never been attained will be developed. For example, if many originals have positives and negatives therein, duplications of positive to positive, positive to negative, negative to positive, and negative to negative can be desirably obtained by utilizing such novel electrophotographic materials using a same toner by only changing the poles of the charging device. On the other hand, in case of a conventional electrophotographic photosensitive layer which can be charged to only a definite pole of charge, such as a negative-chargeable photosensitive layer composed of a zinc oxide-resin dispersion system, a toner having a positive charge must be used for obtaining positive to positive duplications, while a toner having a negative charge must be used for obtaining negative to positive duplications. Similarly, a negatively charged toner must be used for obtaining positive to negative duplications, while a positively charged toner must be used for obtaining a negative to negative duplications. In other words, in the conventional case, the toner to be used must be changed or two specific equipments each containing a positive toner and a negative toner must be prepared. As compared with such conventional cases, the various kinds of duplications as stated above can be obtained in case of using the aforesaid electrophotographic layer which can be charged to any of a positive charge and a negative charge by simply changing the switch of a high d.c. voltage electric source for changing the poles.
The above-mentioned process will further be explained by referring to the accompanying drawings, in which
FIG. 1 is a view showing the step of making a duplication of a positive by exposing to the light image of a positive original a negatively charged electrophotographic photosensitive layer,
FIG. 2 is a view showing the step of making a duplication of a positive by exposing to the light image of a negative original a negatively charged electrophotographic photosensitive layer, and
FIG. 3 is a view showing the step of making a duplication of a positive by exposing to the light image of a negative original a positively charged electrophotographic photosensitive layer.
Now, FIG. 1 shows the step of obtaining duplications of positive from a positive original. The surface of an electrophotographic photosensitive layer 3 is first uniformly charged negatively by means of a corona discharging device 2 connected to a high voltage d.c. electric source 1 in the first step. The electrophotographic photosensitive layer 6 thus charged negatively is exposed to the light image of positive original 4 through, if necessary a lense 5 in the second step, whereby the portions of the layer exposed to light lose the electrostatic charge and the portions unexposed retain charge thereon, whereby an electrostatic latent image of a negative charge is formed on the layer. Then the electrophotographic paper 7 is developed by a positively charged toner in the third step, whereby the toner is attached to the latent image by electrostatic attractive force, whereby the developed positive image as shown in the final step of FIG. 1 is obtained. In addition, the mark - or + shows the electrostatic charge charged on photosensitive layer and the mark .crclbar. or ♁ shows the charge on toner.
FIG. 2 and FIG. 3 show the step of obtaining a positive duplicate from a negative original.
FIG. 2 shows an example of using a conventional electrophotographic photosensitive layer which can provide the positive duplicate by a single pole charge, e.g., a negative electrostatic charge in the illustrated case. Now, because the electrophotographic photosensitive layer fits, in the illustrated case, for negative charging, the electrophotographic paper 3 is charged negatively as in the case of FIG. 1 in the first step. Then, when the electrophotographic paper 6 is exposed to the light image of a negative original 4 through, if necessary, a lense 5 in the second step, the portions corresponding the negative image lose the electrostatic charge, while the non-image portions of the layer retain negative charge as shown in the figure according to the characteristics of the photoconductor, whereby an electrostatic latent image of the type where no electrostatic charge is present on the image portion is formed. Then, the latent image is developed by toner in the third step but since for attaching the toner to the portion bearing no electrostatic charge, such a toner that will be repulsed by the electrostatic charge at the non-imaged portions must be used, a negatively charged toner is used for the development. When the latent image is developed by a negatively charged toner, the toner attaches to the imaged portions as shown in the final step of FIG. 2 to provide the positive image.
On the other hand, FIG. 3 shows an example of using the novel bi-chargeable electrophotographic photosensitive layer. In this case, by changing the poles of the high voltage d.c. electric source 1, the layer 3 is positively charged as shown in the figure. Then, when the positively charged photosensitive layer 6' is exposed to the light image of a negative original 4' through, if necessary, a lense 5, the portions corresponding to the negative image or exposed to light lose the charge, while the non-image portions retain the charge thereon to form a latent image. In this case, however, the charge on the non-imaged portions is a positive charge. That is, in the case of FIG. 2, a negatively charged toner is used for developing the latent image to provide a positive image, while in the case of FIG. 3, a positively charged toner is used for developing the latent image. In other words, the same toner as used in the process of FIG. 1 is used in this case.
The above-mentioned embodiment is only one example of using the electrophotographic photosensitive layer which can be charged to any of a positive charge and a negative charge but from the example only, it will be understood that the usefulness of the bi-chargeable electrophotographic photosensitive layer be large.
An object of this invention is, therefore, to provide the aforesaid both-chargeable electrophotographic photosensitive materials which are expected to be used wide purposes.
As mentioned before, it has been considered almost definitely that in case of using a P-type or N-type photoconductor, it is principally difficult to render the photoconductor the bi-chargeable property. However, the inventors have investigated the possibility of obtaining the bi-chargeable property of an electrophotographic photosensitive layer of a zinc oxide-resin dispersion system using zinc oxide which is a N-type photoconductor, while changing the kinds of binders or vehicles and as the results thereof it has been discovered that in some case an electrophotographic process can be performed well even if the photosensitive layer is charged positively.
As well known, there are many kinds of binding resins for electrophotographic photosensitive layers using zinc oxide. Historically speaking, the binders for electrophotographic photosensitive layers were developed based on the resins for paints and thus the resins to be used as the binders for electrophotographic layers are in the range of the resins for paints. Typical examples of such resins are silicone resins, various polyester resins including alkyd resins, and various vinylic resins such as vinyl acetate, polyacrylate, and polystyrene.
Recently, however, various kinds of acrylic monomers become available easily and thus acrylic resins have frequently applied as the binders for zinc oxide. The binder resins for the bi-chargeable electrophotographic materials of this invention belong also to acrylic resins.
Acrylic resins include generally the resins prepared by polymerizing acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, the derivatives of the esters, styrene, styrene derivatives, and other monomer having polymerizable vinyl group.
The acrylic resins as the binder resins for zinc oxide of electrophotographic layers have hitherto been investigated about the composition of monomers and degrees of polymerization for obtaining the electrophotographic characteristics such as dispersibility of ZnO, adhesion to the substrates, mechanical strength or durability, charge acceptance, photosensitivity, moisture resistance and thermal resistance. For example, various kinds of acrylic resins for the purpose are disclosed in the specifications of Japanese Patent Publication Nos. 6395/69, 6392/69, 6393/69, 6394/69, 17,316/69, 11,636/71 and 18,116/71. In spite of such many investigations of acrylic resins, there are no disclosures of showing or suggesting the binders for the bi-chargeable purpose.
Only Japanese Patent Publication No. 951/70 which uses a vinyl chloride-vinyl acetate copolymer and Japanese Patent Publication No. 40,133/70 which uses cadmium compound series photoconductors teach the realization of both-chargeable electrophotographic photosensitive layers.
As the results of long investigations of acrylic resins and the relations between acrylic resins and bi-chargeable properties, the inventors have discovered the following matters.
That is, it has been discovered that the possibility of obtaining the bi-chargeable is determined by the amount of acrylic ester in acrylic resin compositions. As mentioned above, it has generally practiced to use acrylic ester and methacrylic ester as components for acrylic resins, that is, almost all of acrylic resins are mainly composed of such components. Also, the component ratios of the monomers are so selected that the mechanical strength, resistance, chargeability of electrostatic charge, and sensitivity required for the purposes can be obtained.
For the both-chargeable electrophotographic photosensitive layers of this invention, the necessary content of acrylic ester is 50-95% by weight, preferably 70-90% by weight. Among the remaining 5-50% components, 1-5% by weight is at least one organic acid monomer copolymerizable with the acrylic ester, such as acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride, and the monoesters of the above-mentioned dibasic acids. Other components than above may be any monomers copolymerizable with the acrylic ester, such as a methacrylic ester (e.g., methyl methacrylate, ethyl methacrylate, butyl methacrylate, etc.), a methacrylic ester derivative (e.g., hydrocyethyl methacrylate, glycidyl methacrylate, etc.), styrene, a styrene derivative (e.g., methyl styrene, chloromethylstyrene, etc.) and the like.
As the acrylic ester to be used, there are illustrated ethyl acrylate, propyl acrylate, and butyl acrylate. They may be used individually or as a combination thereof. The selection of the acrylic ester or esters is determined by the mechanical properties required.
As will be explained in the below-showing examples as comparison examples, when methacrylic ester is used in place of the acrylic ester, the bi-chargeable property will be lost.
As the methacrylic ester to be used as other component then the acrylic ester for the copolymerized acrylic resin, there are methyl methacrylate, ethyl methacrylate, propyl methacrylate, and butyl methacrylate each having different chain length of alkyl group as well as lauryl methacrylate having further longer chain length of alkyl group as in case of methacrylic esters. However, if the proportion of the methacrylic ester becomes higher than 50% by weight, the bi-chargeable property of the resin is, on the contrary, lowered.
The term "both-chargeable property" was used before frequently in this specification without giving any clear definition to the term and thus the term will be explained in detail a little for giving no misunderstanding. The term "both-chargeable property" used in this specification is somewhat different from the meaning capable of being simply charged to positive and negative electrostatic charges. Of course, it is the necessary factor of the term to be able to charge positive and negative electrostatic charges but the term also means that when the material having the property of being charged to positive and negative electrostatic charges is charged to the positive or the negative charge and developed by toner after imagewise exposure, it can give sharp images without forming fog. As easily understood by persons skilled in the art of this field, the electric potential characteristic curve of an electrophotographic photosensitive layer is cited as one of means of testing the characteristics of it. By the potential characteristic curve, the electric potential to be employed for charging the photosensitive layer and the decaying state of the electric potential of the layer when the layer is exposed to light can be obtained. However, the results obtained from the potential characteristic curve not always coincide with the real properties of the electrophotographic image obtained by applying an electrophotographic process. In case of testing the bi-chargeable property of an electrophotographic photosensitive or photoconductive material, the above difference in properties becomes remakable. For example, if an electrophotographic photosensitive layer is charged to -500 volts when charged negatively and to +500 volts when charged positively and also the decaying tendency by light exposure is completely the same in the both cases, that is, the potential characteristic curve completely symmetrical in the both areas of the zero potential base line is obtained, it frequently happens that the electrophotographic image obtained by processing the electrophotographic photosensitive layer in an electrophotographic process is sharp and has no fogs on blank areas in case of charging negatively the layer but becomes dim and has much fogs on blank areas. Thus, in such a case as above the electrophotographic photosensitive layer is not said to have the "both-chargeable property" in this invention even if the layer may be charged negatively or positively in same charged amount.
Now, it sometimes happens that if the content of methacrylic ester is larger in the above-indicated electrophotographic photosensitive layer, the charged amount of electrostatic charge on the layer is larger in both of negatively charged case and positively charged case than the electrophotographic photosensitive layer made of an acrylic ester resin by the results obtained from the electric potential characteristic curve thereof. However, in such case, an electrophotographic image have good qualities to some extent may be obtained by processing the layer practically in an electrophotographic process in case of charging the layer negatively but the image qualities become poor caused by dimness of image and the formation of much fogs in case of charging the layer positively.
However, the use of the methacrylic ester is not excluded in this invention. That is, as mentioned before, when 50-95% acrylic ester and 1-5% organic acid copolymerizable with the acrylic ester are used as the components for the acrylic copolymer to be used as the electrophotographic photosensitive or photoconductive layer in this invention, a methacrylic ester can be used as the rest component for the copolymer if any without deteriorating the both-chargeable property of the photosensitive layer. Furthermore, other components than the methacrylic ester, such as a derivative of the acrylic ester or the methacrylic ester, styrene, and a styrene derivative may be used as the other component for the acrylic copolymer. In particular, by using a monomer having high electric insulative property, such as styrene is used as the rest component for the acrylic copolymer of the electrophotographic photosensitive layer, the charge acceptance of negative or positive charge can be increased and the density of image can also be increased without substantially deteriorating the both-chargeable property of the layer.
Moreover, in case of obtaining electrophotographic images having succicient image density by means of a high speed developing machine in a short developing period of time, it is preferable to use as the binder of electrophotographic layer the acrylic copolymer resin with low molecular weight polystyrene having a molecular weight of up to about 5,000 in an amount of 10-50% by weight based on the whole weight of the resin for increasing further the amount of charge to be charged thereon. By using such an acrylic copolymer resin, the amount of charge acceptance can be increased without deteriorating the bi-chargeable property of the electrophotographic photosensitive layer, whereby the image density of the electrophotographic images formed thereon can be increased. The use of the low molecular weight polystyrene over 50% by weight of the whole resin is, however, undesirable since in this case the electrophotographic photosensitive layer is accompanied with the formation of fogs on the blank areas when the layer is positively charged, which results in causing indistinct images. The low molecular weight polystyrene to be used in this invention includes, besides styrene homopolymer, a copolymer of styrene and a small amount of an acrylic monomer such as acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, and derivatives of such monomers.
Now, the invention will be explained more practically by the following examples.
EXAMPLE 1
In a 500 ml. polymerization reaction vessel equipped with a stirring means were charged 60 g. of acrylic ester, 10 g. of methacrylic ester, 26 g. of methyl methacrylate, 2 g. of methacrylic acid, 2 g. of maleic anhydride, and 100 g. of xylene and after raising the temperature of the mixture to 80°C., 1 g. of azobisisobutyronitrile was added to the mixture and then the resultant mixture was heated with stirring for 8 hours to conduct the polymerization. A mixture of 80 g. of the resin solution thus prepared, 200 g. of electrophotographic zinc oxide (made by Sakai Kagaku K. K.), 200 g. of xylene, and 40 mg. of Bromophenol Blue as a dye sensitizer was mixed sufficiently by ultrasonic waves and the mixture was applied to the surface of an electrophotographic base paper, the surface of which had been subjected to conductive treatment, by means of a draw-down rod (a rod-form doctor having coiled fine copper wire thereon) in a coated amount as solid of 25 g./m. 2 and dried.
The sample thus prepared was seasoned overnight in the dark at a temperature of 20°C. and a humidity of 60% RH and then the bi-chargeable property of the sample was tested by the following manner.
a. Negatively chargeable test: After charging negatively the photosensitive layer of the sample by facing the layer to the negative pole side of a double corona discharging device connected to a high potential d.c. electric source, the photosensitive layer thus charged was exposed to light through a positive image original and then developed by means of positively charged liquid toner or a so-called positive toner, whereby a clear positive image having no fogs on blank area was obtained.
b. Positively chargeable test: After charging positively the photosensitive layer of the sample by facing the layer to the positive pole side of the same double corona discharging device, the photosensitive layer thus charged was exposed to light through a negative image original and then developed using the same positive toner as in the above case, whereby a sharp positive image having no fogs on blank area was also obtained.
EXAMPLE 2
The copolymers containing various quantities of ethyl acrylate as shown in Table 1 were prepared by the same way as in Example 1 and after preparing samples using the copolymers by the same manner as in Example 1, the samples were tested about the bi-chargeable property, the results of which are also shown in the same table.
Table 1 ____________________________________________________________ ______________ SAMPLE (1) (2) (3) (4) (5) (6) ____________________________________________________________ ______________ Monomer Ethyl acrylate 0 20 35 50 70 95 composition ( wt. % ) Buthyl methacry- 70 50 35 20 0 0 late Methyl methacry- 26 26 26 26 26 0 late Methacrylic acid 2 2 2 2 2 2.5 Maleic anhydride 2 2 2 2 2 2.5 Test result no fog no fog no fog no fog no fog no fog Negatively image quality image quality image quality image quality image quality image quality chargeable good good good good good good property density high density high density density density density low slightly almost same slightly lower than as sample (3) lower than samples (1) samples (3) and (2) and (4) Test result image image much fogs slightly no fog no fog unreadable difficult image fogs image image quality positively in readable indistinct image readable quality good chargeable good density low property ____________________________________________________________ ______________
The results shown in the above table showed the followings: That is, in the negative chargeable property test, the electrophotographic image formed could be read even if the proportion of ethyl acrylate increased but the image density tended to decrease gradually as the increase of the proportion of ethyl acrylate. On the other hand, in the positive chargeable property test, the electrophotographic image could be read when the proportion of ethyl acrylate was more than 50% by weight but when the proportion of ethyl acrylate was lower than 50% by weight, the image became dim and the formation of fogs on blank area increased to make unreadable the image.
EXAMPLE 3
Six kinds of copolymers were prepared by using methyl acrylate or butyl acrylate in place of the ethyl acrylate as the monomer component of the copolymers in Example 2 and the same test as in Example 2 was conducted about the samples prepared by using the copolymers as binder. The results showed that the samples showed almost the same tendency as the cases of using ethyl acrylate, that is, when the proportion of the acrylic ester component was higher than 50% by weight, the sample exhibited good positive chargeable property but when the proportion of the acrylic ester was lower than 50% by weight, the positive chargeable property of the samples was reduced.
EXAMPLE 4
A series of copolymers having the monomer compositions as shown in Table 2, that is, the copolymers containing various proportions of styrene were prepared and the same procedure as in Example 1 was followed by using the copolymers as the binder for electrophotographic photosensitive layers.
Table 2 ______________________________________ Monomer Sample Sample Sample Sample component 7 8 9 10 (wt.%) Ethyl acrylate 70 70 70 70 Methyl acrylate 21 16 11 6 Styrene 5 10 15 20 Methacrylic acid 2 2 2 2 Maleic anhydride 2 2 2 2 ______________________________________
The results showed that the image density was higher than that of the Sample 2 in Example 2 without deteriorating the both-chargeable property of the samples and the image density increased gradually as the proportion of styrene increased.
EXAMPLE 5
The resin having the same composition as the sample 10 of Example 4 was mixed with a low molecular weight polystyrene, Picolastic C-100 (registered trade mark, made by Esso Research and Engineering Co.) in the mixing ratio (by weight) as shown in Table 3 and samples prepared using the resultant mixture as the bonder for the electrophotographic photosensitive layers thereof, the results of which are also shown in the same table.
Table 3 ____________________________________________________________ ______________ Sample (11) (12) (13) (14) (15) (16) ____________________________________________________________ ______________ The resin of the sample (10) 10/0 8/2 6/4 4/6 2/8 0/10 Picolastic C-100 Test image sharp image sharp image sharp image sharp image image slightly Negatively density density density density slightly indistinct chargeable ordinary higher than higher than slightly indistinct density low property sample (11) sample (12) higher than density sample (12) high Test image sharp image sharp image sharp fog formed much fogs image Positively density density density image image indistinct chargeable ordinary higher than higher than slightly indistinct density low property sample (11) sample (12) indistinct ____________________________________________________________ ______________
The results showed that as the increase of the mixing ratio of the low molecular weight polystyrene, the image density increased but when the content of the low molecular weight polystyrene became higher than 50% by weight, fogs were formed on blak areas in the positively chargeable property test and the image became dim.
EXAMPLE 6
The same procedure as in Example 5 was followed using commercially available low molecular weight polystyrenes, Picolastic A-75, Picolastic D-100 and Picolastic D-125 (registered trade marks, made by Esso Research and Engineering Co.) or Himer ST-75, Himer ST-95, Himer ST-120, and Himer SU-120 (trade names, made by Sanyo Kasei K. K.) in place of Picolastic C-100. The results were almost same as those in Example 5.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope thereof.
The present invention relates to electrophotography and more particularly, it relates to a bi-chargeable electrophotographic material which can be charged to any of positive charge or negative charge by an electrostatic charging step in electrophotographic process.
For facilitating the understanding of the present invention, the technical field of this invention will be explained briefly. As well known, the process of obtaining visible images by electrophotographic method is mainly composed of (1) electrostatic charging of a photosensitive or photoconductive layer, (2) image exposure (the formation of electrostatic latent images), and (3) development by charged colored particles or toners. In the 1st step, the photosensitive or photoconductive layer is charged for forming latent images as the patterns of electrostatic charge on the surface thereon in the subsequent image exposure step. That is, because an electrophotographic material has high insulative property in the dark, the electrophotographic layer can be charged by proper means. When the photosensitive or photoconductive layer of the electrophotographic material is, after uniformly charging the whole surface of the layer, imagewise exposed to light, the portions of the electrophotographic layer exposed to light become conductive to lose the electrostatic charge on the surfaces of those portions, while the portions of the layer which have not been exposed to light bear thereon the electrostatic charge, whereby a pattern of electrostatic charge is formed in conformity with the exposed image. This process is called "formation of electrostatic latent image." Such an electrostatic latent image can generally be developed by colored fine particles or toners having an electrostatic charge opposite to the electrostatic charge on the photosensitive or photoconductive layer, said fine particles or toners being used as a state of powder or a dispersion in highly insulative liquid. However, if a photosensitive or photoconductive layer is exposed imagewise to light in uncharged state, a conductive pattern is formed in conformity of the image on the photosensitive layer during the exposure or within a quite short period of time after exposure but such a pattern does not have a life as the aforesaid electrostatic pattern and further cannot be developed by charged colored fine particles. Thus, in electrophotographic method, the step of charging photosensitive or photoconductive layers is important and an indispensable step.
In general, an electrophotographic photosensitive layer is charged by conducting corona discharging by means of a high-voltage direct current electric source of 6000- 8000 volts to ionizing surrounding air and attaching the ionized air onto the surface of the photosensitive layer. The charging system of utilizing such a d.c. corona discharging can generate a positive charge or a negative charge by changing the poles of the d.c. electric source. However, when a conventional electrophotographic photosensitive layer is charged to definite one of a positive charge and a negative charge, duplication can be conducted well, but when the same electrophotographic photosensitive layer is charged to the opposite charge, duplication cannot be conducted well. Consequently, the charging device is set for giving the definite polarity of electrostatic charge fitted for the characteristics of the electrophotographic layer to be used.
For example, for a photosensitive or photoconductive plate composed of amorphous selenium known as Xerox (registered trade name), the charging device is set for providing a positive charge, while for a photosensitive or photoconductive layer composed of a zinc oxide-resin dispersion system as in Electrofax (resistered trade name), the charging device is set for providing a negative charge.
The reason why the aforesaid phenomena occur on the conventional electrophotographic layer has not yet been clarified. However, some people say that the charge on a photoconductor to be provided is selected according to whether the photoconducter is a P-type or a N-type. That is, a P-type photoconductor in which the current carrier is a positive hole is readily charged to a positive charge and further the positive charge vanishes smoothly when the photoconductor is exposed to light. On the other hand, a N-type photoconductor in which the current carrier is electron is fit for negative charging. According to the view, when a photosensitive layer prepared by using zinc oxide which is a N-type photoconductor is used, the electrophotographic process can be performed well in case of charging negatively the photosensitive layer.
However, if a novel electrophotographic photosensitive material which can be charged to any of a positive charge and negative charge and can perform well electrophotographic process is developed, various new processes that have never been attained will be developed. For example, if many originals have positives and negatives therein, duplications of positive to positive, positive to negative, negative to positive, and negative to negative can be desirably obtained by utilizing such novel electrophotographic materials using a same toner by only changing the poles of the charging device. On the other hand, in case of a conventional electrophotographic photosensitive layer which can be charged to only a definite pole of charge, such as a negative-chargeable photosensitive layer composed of a zinc oxide-resin dispersion system, a toner having a positive charge must be used for obtaining positive to positive duplications, while a toner having a negative charge must be used for obtaining negative to positive duplications. Similarly, a negatively charged toner must be used for obtaining positive to negative duplications, while a positively charged toner must be used for obtaining a negative to negative duplications. In other words, in the conventional case, the toner to be used must be changed or two specific equipments each containing a positive toner and a negative toner must be prepared. As compared with such conventional cases, the various kinds of duplications as stated above can be obtained in case of using the aforesaid electrophotographic layer which can be charged to any of a positive charge and a negative charge by simply changing the switch of a high d.c. voltage electric source for changing the poles.
The above-mentioned process will further be explained by referring to the accompanying drawings, in which
FIG. 1 is a view showing the step of making a duplication of a positive by exposing to the light image of a positive original a negatively charged electrophotographic photosensitive layer,
FIG. 2 is a view showing the step of making a duplication of a positive by exposing to the light image of a negative original a negatively charged electrophotographic photosensitive layer, and
FIG. 3 is a view showing the step of making a duplication of a positive by exposing to the light image of a negative original a positively charged electrophotographic photosensitive layer.
Now, FIG. 1 shows the step of obtaining duplications of positive from a positive original. The surface of an electrophotographic photosensitive layer 3 is first uniformly charged negatively by means of a corona discharging device 2 connected to a high voltage d.c. electric source 1 in the first step. The electrophotographic photosensitive layer 6 thus charged negatively is exposed to the light image of positive original 4 through, if necessary a lense 5 in the second step, whereby the portions of the layer exposed to light lose the electrostatic charge and the portions unexposed retain charge thereon, whereby an electrostatic latent image of a negative charge is formed on the layer. Then the electrophotographic paper 7 is developed by a positively charged toner in the third step, whereby the toner is attached to the latent image by electrostatic attractive force, whereby the developed positive image as shown in the final step of FIG. 1 is obtained. In addition, the mark - or + shows the electrostatic charge charged on photosensitive layer and the mark .crclbar. or ♁ shows the charge on toner.
FIG. 2 and FIG. 3 show the step of obtaining a positive duplicate from a negative original.
FIG. 2 shows an example of using a conventional electrophotographic photosensitive layer which can provide the positive duplicate by a single pole charge, e.g., a negative electrostatic charge in the illustrated case. Now, because the electrophotographic photosensitive layer fits, in the illustrated case, for negative charging, the electrophotographic paper 3 is charged negatively as in the case of FIG. 1 in the first step. Then, when the electrophotographic paper 6 is exposed to the light image of a negative original 4 through, if necessary, a lense 5 in the second step, the portions corresponding the negative image lose the electrostatic charge, while the non-image portions of the layer retain negative charge as shown in the figure according to the characteristics of the photoconductor, whereby an electrostatic latent image of the type where no electrostatic charge is present on the image portion is formed. Then, the latent image is developed by toner in the third step but since for attaching the toner to the portion bearing no electrostatic charge, such a toner that will be repulsed by the electrostatic charge at the non-imaged portions must be used, a negatively charged toner is used for the development. When the latent image is developed by a negatively charged toner, the toner attaches to the imaged portions as shown in the final step of FIG. 2 to provide the positive image.
On the other hand, FIG. 3 shows an example of using the novel bi-chargeable electrophotographic photosensitive layer. In this case, by changing the poles of the high voltage d.c. electric source 1, the layer 3 is positively charged as shown in the figure. Then, when the positively charged photosensitive layer 6' is exposed to the light image of a negative original 4' through, if necessary, a lense 5, the portions corresponding to the negative image or exposed to light lose the charge, while the non-image portions retain the charge thereon to form a latent image. In this case, however, the charge on the non-imaged portions is a positive charge. That is, in the case of FIG. 2, a negatively charged toner is used for developing the latent image to provide a positive image, while in the case of FIG. 3, a positively charged toner is used for developing the latent image. In other words, the same toner as used in the process of FIG. 1 is used in this case.
The above-mentioned embodiment is only one example of using the electrophotographic photosensitive layer which can be charged to any of a positive charge and a negative charge but from the example only, it will be understood that the usefulness of the bi-chargeable electrophotographic photosensitive layer be large.
An object of this invention is, therefore, to provide the aforesaid both-chargeable electrophotographic photosensitive materials which are expected to be used wide purposes.
As mentioned before, it has been considered almost definitely that in case of using a P-type or N-type photoconductor, it is principally difficult to render the photoconductor the bi-chargeable property. However, the inventors have investigated the possibility of obtaining the bi-chargeable property of an electrophotographic photosensitive layer of a zinc oxide-resin dispersion system using zinc oxide which is a N-type photoconductor, while changing the kinds of binders or vehicles and as the results thereof it has been discovered that in some case an electrophotographic process can be performed well even if the photosensitive layer is charged positively.
As well known, there are many kinds of binding resins for electrophotographic photosensitive layers using zinc oxide. Historically speaking, the binders for electrophotographic photosensitive layers were developed based on the resins for paints and thus the resins to be used as the binders for electrophotographic layers are in the range of the resins for paints. Typical examples of such resins are silicone resins, various polyester resins including alkyd resins, and various vinylic resins such as vinyl acetate, polyacrylate, and polystyrene.
Recently, however, various kinds of acrylic monomers become available easily and thus acrylic resins have frequently applied as the binders for zinc oxide. The binder resins for the bi-chargeable electrophotographic materials of this invention belong also to acrylic resins.
Acrylic resins include generally the resins prepared by polymerizing acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, the derivatives of the esters, styrene, styrene derivatives, and other monomer having polymerizable vinyl group.
The acrylic resins as the binder resins for zinc oxide of electrophotographic layers have hitherto been investigated about the composition of monomers and degrees of polymerization for obtaining the electrophotographic characteristics such as dispersibility of ZnO, adhesion to the substrates, mechanical strength or durability, charge acceptance, photosensitivity, moisture resistance and thermal resistance. For example, various kinds of acrylic resins for the purpose are disclosed in the specifications of Japanese Patent Publication Nos. 6395/69, 6392/69, 6393/69, 6394/69, 17,316/69, 11,636/71 and 18,116/71. In spite of such many investigations of acrylic resins, there are no disclosures of showing or suggesting the binders for the bi-chargeable purpose.
Only Japanese Patent Publication No. 951/70 which uses a vinyl chloride-vinyl acetate copolymer and Japanese Patent Publication No. 40,133/70 which uses cadmium compound series photoconductors teach the realization of both-chargeable electrophotographic photosensitive layers.
As the results of long investigations of acrylic resins and the relations between acrylic resins and bi-chargeable properties, the inventors have discovered the following matters.
That is, it has been discovered that the possibility of obtaining the bi-chargeable is determined by the amount of acrylic ester in acrylic resin compositions. As mentioned above, it has generally practiced to use acrylic ester and methacrylic ester as components for acrylic resins, that is, almost all of acrylic resins are mainly composed of such components. Also, the component ratios of the monomers are so selected that the mechanical strength, resistance, chargeability of electrostatic charge, and sensitivity required for the purposes can be obtained.
For the both-chargeable electrophotographic photosensitive layers of this invention, the necessary content of acrylic ester is 50-95% by weight, preferably 70-90% by weight. Among the remaining 5-50% components, 1-5% by weight is at least one organic acid monomer copolymerizable with the acrylic ester, such as acrylic acid, methacrylic acid, fumaric acid, maleic acid, itaconic acid, maleic anhydride, itaconic anhydride, and the monoesters of the above-mentioned dibasic acids. Other components than above may be any monomers copolymerizable with the acrylic ester, such as a methacrylic ester (e.g., methyl methacrylate, ethyl methacrylate, butyl methacrylate, etc.), a methacrylic ester derivative (e.g., hydrocyethyl methacrylate, glycidyl methacrylate, etc.), styrene, a styrene derivative (e.g., methyl styrene, chloromethylstyrene, etc.) and the like.
As the acrylic ester to be used, there are illustrated ethyl acrylate, propyl acrylate, and butyl acrylate. They may be used individually or as a combination thereof. The selection of the acrylic ester or esters is determined by the mechanical properties required.
As will be explained in the below-showing examples as comparison examples, when methacrylic ester is used in place of the acrylic ester, the bi-chargeable property will be lost.
As the methacrylic ester to be used as other component then the acrylic ester for the copolymerized acrylic resin, there are methyl methacrylate, ethyl methacrylate, propyl methacrylate, and butyl methacrylate each having different chain length of alkyl group as well as lauryl methacrylate having further longer chain length of alkyl group as in case of methacrylic esters. However, if the proportion of the methacrylic ester becomes higher than 50% by weight, the bi-chargeable property of the resin is, on the contrary, lowered.
The term "both-chargeable property" was used before frequently in this specification without giving any clear definition to the term and thus the term will be explained in detail a little for giving no misunderstanding. The term "both-chargeable property" used in this specification is somewhat different from the meaning capable of being simply charged to positive and negative electrostatic charges. Of course, it is the necessary factor of the term to be able to charge positive and negative electrostatic charges but the term also means that when the material having the property of being charged to positive and negative electrostatic charges is charged to the positive or the negative charge and developed by toner after imagewise exposure, it can give sharp images without forming fog. As easily understood by persons skilled in the art of this field, the electric potential characteristic curve of an electrophotographic photosensitive layer is cited as one of means of testing the characteristics of it. By the potential characteristic curve, the electric potential to be employed for charging the photosensitive layer and the decaying state of the electric potential of the layer when the layer is exposed to light can be obtained. However, the results obtained from the potential characteristic curve not always coincide with the real properties of the electrophotographic image obtained by applying an electrophotographic process. In case of testing the bi-chargeable property of an electrophotographic photosensitive or photoconductive material, the above difference in properties becomes remakable. For example, if an electrophotographic photosensitive layer is charged to -500 volts when charged negatively and to +500 volts when charged positively and also the decaying tendency by light exposure is completely the same in the both cases, that is, the potential characteristic curve completely symmetrical in the both areas of the zero potential base line is obtained, it frequently happens that the electrophotographic image obtained by processing the electrophotographic photosensitive layer in an electrophotographic process is sharp and has no fogs on blank areas in case of charging negatively the layer but becomes dim and has much fogs on blank areas. Thus, in such a case as above the electrophotographic photosensitive layer is not said to have the "both-chargeable property" in this invention even if the layer may be charged negatively or positively in same charged amount.
Now, it sometimes happens that if the content of methacrylic ester is larger in the above-indicated electrophotographic photosensitive layer, the charged amount of electrostatic charge on the layer is larger in both of negatively charged case and positively charged case than the electrophotographic photosensitive layer made of an acrylic ester resin by the results obtained from the electric potential characteristic curve thereof. However, in such case, an electrophotographic image have good qualities to some extent may be obtained by processing the layer practically in an electrophotographic process in case of charging the layer negatively but the image qualities become poor caused by dimness of image and the formation of much fogs in case of charging the layer positively.
However, the use of the methacrylic ester is not excluded in this invention. That is, as mentioned before, when 50-95% acrylic ester and 1-5% organic acid copolymerizable with the acrylic ester are used as the components for the acrylic copolymer to be used as the electrophotographic photosensitive or photoconductive layer in this invention, a methacrylic ester can be used as the rest component for the copolymer if any without deteriorating the both-chargeable property of the photosensitive layer. Furthermore, other components than the methacrylic ester, such as a derivative of the acrylic ester or the methacrylic ester, styrene, and a styrene derivative may be used as the other component for the acrylic copolymer. In particular, by using a monomer having high electric insulative property, such as styrene is used as the rest component for the acrylic copolymer of the electrophotographic photosensitive layer, the charge acceptance of negative or positive charge can be increased and the density of image can also be increased without substantially deteriorating the both-chargeable property of the layer.
Moreover, in case of obtaining electrophotographic images having succicient image density by means of a high speed developing machine in a short developing period of time, it is preferable to use as the binder of electrophotographic layer the acrylic copolymer resin with low molecular weight polystyrene having a molecular weight of up to about 5,000 in an amount of 10-50% by weight based on the whole weight of the resin for increasing further the amount of charge to be charged thereon. By using such an acrylic copolymer resin, the amount of charge acceptance can be increased without deteriorating the bi-chargeable property of the electrophotographic photosensitive layer, whereby the image density of the electrophotographic images formed thereon can be increased. The use of the low molecular weight polystyrene over 50% by weight of the whole resin is, however, undesirable since in this case the electrophotographic photosensitive layer is accompanied with the formation of fogs on the blank areas when the layer is positively charged, which results in causing indistinct images. The low molecular weight polystyrene to be used in this invention includes, besides styrene homopolymer, a copolymer of styrene and a small amount of an acrylic monomer such as acrylic acid, methacrylic acid, acrylic ester, methacrylic ester, and derivatives of such monomers.
Now, the invention will be explained more practically by the following examples.
EXAMPLE 1
In a 500 ml. polymerization reaction vessel equipped with a stirring means were charged 60 g. of acrylic ester, 10 g. of methacrylic ester, 26 g. of methyl methacrylate, 2 g. of methacrylic acid, 2 g. of maleic anhydride, and 100 g. of xylene and after raising the temperature of the mixture to 80°C., 1 g. of azobisisobutyronitrile was added to the mixture and then the resultant mixture was heated with stirring for 8 hours to conduct the polymerization. A mixture of 80 g. of the resin solution thus prepared, 200 g. of electrophotographic zinc oxide (made by Sakai Kagaku K. K.), 200 g. of xylene, and 40 mg. of Bromophenol Blue as a dye sensitizer was mixed sufficiently by ultrasonic waves and the mixture was applied to the surface of an electrophotographic base paper, the surface of which had been subjected to conductive treatment, by means of a draw-down rod (a rod-form doctor having coiled fine copper wire thereon) in a coated amount as solid of 25 g./m. 2 and dried.
The sample thus prepared was seasoned overnight in the dark at a temperature of 20°C. and a humidity of 60% RH and then the bi-chargeable property of the sample was tested by the following manner.
a. Negatively chargeable test: After charging negatively the photosensitive layer of the sample by facing the layer to the negative pole side of a double corona discharging device connected to a high potential d.c. electric source, the photosensitive layer thus charged was exposed to light through a positive image original and then developed by means of positively charged liquid toner or a so-called positive toner, whereby a clear positive image having no fogs on blank area was obtained.
b. Positively chargeable test: After charging positively the photosensitive layer of the sample by facing the layer to the positive pole side of the same double corona discharging device, the photosensitive layer thus charged was exposed to light through a negative image original and then developed using the same positive toner as in the above case, whereby a sharp positive image having no fogs on blank area was also obtained.
EXAMPLE 2
The copolymers containing various quantities of ethyl acrylate as shown in Table 1 were prepared by the same way as in Example 1 and after preparing samples using the copolymers by the same manner as in Example 1, the samples were tested about the bi-chargeable property, the results of which are also shown in the same table.
Table 1 ____________________________________________________________ ______________ SAMPLE (1) (2) (3) (4) (5) (6) ____________________________________________________________ ______________ Monomer Ethyl acrylate 0 20 35 50 70 95 composition ( wt. % ) Buthyl methacry- 70 50 35 20 0 0 late Methyl methacry- 26 26 26 26 26 0 late Methacrylic acid 2 2 2 2 2 2.5 Maleic anhydride 2 2 2 2 2 2.5 Test result no fog no fog no fog no fog no fog no fog Negatively image quality image quality image quality image quality image quality image quality chargeable good good good good good good property density high density high density density density density low slightly almost same slightly lower than as sample (3) lower than samples (1) samples (3) and (2) and (4) Test result image image much fogs slightly no fog no fog unreadable difficult image fogs image image quality positively in readable indistinct image readable quality good chargeable good density low property ____________________________________________________________ ______________
The results shown in the above table showed the followings: That is, in the negative chargeable property test, the electrophotographic image formed could be read even if the proportion of ethyl acrylate increased but the image density tended to decrease gradually as the increase of the proportion of ethyl acrylate. On the other hand, in the positive chargeable property test, the electrophotographic image could be read when the proportion of ethyl acrylate was more than 50% by weight but when the proportion of ethyl acrylate was lower than 50% by weight, the image became dim and the formation of fogs on blank area increased to make unreadable the image.
EXAMPLE 3
Six kinds of copolymers were prepared by using methyl acrylate or butyl acrylate in place of the ethyl acrylate as the monomer component of the copolymers in Example 2 and the same test as in Example 2 was conducted about the samples prepared by using the copolymers as binder. The results showed that the samples showed almost the same tendency as the cases of using ethyl acrylate, that is, when the proportion of the acrylic ester component was higher than 50% by weight, the sample exhibited good positive chargeable property but when the proportion of the acrylic ester was lower than 50% by weight, the positive chargeable property of the samples was reduced.
EXAMPLE 4
A series of copolymers having the monomer compositions as shown in Table 2, that is, the copolymers containing various proportions of styrene were prepared and the same procedure as in Example 1 was followed by using the copolymers as the binder for electrophotographic photosensitive layers.
Table 2 ______________________________________ Monomer Sample Sample Sample Sample component 7 8 9 10 (wt.%) Ethyl acrylate 70 70 70 70 Methyl acrylate 21 16 11 6 Styrene 5 10 15 20 Methacrylic acid 2 2 2 2 Maleic anhydride 2 2 2 2 ______________________________________
The results showed that the image density was higher than that of the Sample 2 in Example 2 without deteriorating the both-chargeable property of the samples and the image density increased gradually as the proportion of styrene increased.
EXAMPLE 5
The resin having the same composition as the sample 10 of Example 4 was mixed with a low molecular weight polystyrene, Picolastic C-100 (registered trade mark, made by Esso Research and Engineering Co.) in the mixing ratio (by weight) as shown in Table 3 and samples prepared using the resultant mixture as the bonder for the electrophotographic photosensitive layers thereof, the results of which are also shown in the same table.
Table 3 ____________________________________________________________ ______________ Sample (11) (12) (13) (14) (15) (16) ____________________________________________________________ ______________ The resin of the sample (10) 10/0 8/2 6/4 4/6 2/8 0/10 Picolastic C-100 Test image sharp image sharp image sharp image sharp image image slightly Negatively density density density density slightly indistinct chargeable ordinary higher than higher than slightly indistinct density low property sample (11) sample (12) higher than density sample (12) high Test image sharp image sharp image sharp fog formed much fogs image Positively density density density image image indistinct chargeable ordinary higher than higher than slightly indistinct density low property sample (11) sample (12) indistinct ____________________________________________________________ ______________
The results showed that as the increase of the mixing ratio of the low molecular weight polystyrene, the image density increased but when the content of the low molecular weight polystyrene became higher than 50% by weight, fogs were formed on blak areas in the positively chargeable property test and the image became dim.
EXAMPLE 6
The same procedure as in Example 5 was followed using commercially available low molecular weight polystyrenes, Picolastic A-75, Picolastic D-100 and Picolastic D-125 (registered trade marks, made by Esso Research and Engineering Co.) or Himer ST-75, Himer ST-95, Himer ST-120, and Himer SU-120 (trade names, made by Sanyo Kasei K. K.) in place of Picolastic C-100. The results were almost same as those in Example 5.
While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modification can be made therein without departing from the spirit and scope thereof.