Liquid Developer For Use In Electrophotography

Abstract

A liquid developer for use in electrophotography and being of the type in which a toner is dispersed in a carrier liquid, said carrier liquid consisting of an organic solvent having a high electric resistance of at least 10.sup.10 .OMEGA.cm. and a low dielectric constant of a maximum of 3, said toner being selected from the group of toners of the following types (a),(b),(c) and (d): A. Toners of the type in which pigment particles are coated, by the kneading technique, with a homopolymer of a monomer selected from the group consisting of polymeric monomers having an epoxy radical (-0-) and polymeric monomers having a carbinol radical, B. toners of the type that said homopolymer is graftcopolymerized to said pigment particles, C. toners of the type in which the pigment particles are coated, by the kneading technique, with a copolymer in which is contained, by copolymerization, a monomer selected from the aforesaid group of polymeric monomers, and D. toners of the type in which said copolymer is graftcopolymerized to the pigment particles.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is concerned with a liquid developer for use in electrophotography and of the type in which a toner is dispersed in a carrier liquid and the toner has a positive charge, said carrier liquid consisting of an organic solvent having a relatively high electric resistance and a low dielectric constant, said toner comprising pigment particles and a polymer selected from the group consisting of homopolymers obtained from the polymerization of monomers having specific radicals and the group consisting of copolymers obtained from the copolymerization of those monomers having specific radicals and other types of monomers, said polymer being either graft copolymerized to or coated on said pigment particles.

2. Description of the Prior Art

Liquid developers for use in electrophotography are, in general, prepared by kneading a mixture of pigment particles, a polarity regulating agent, a dispersing agent and a fixing agent, and dispersing the kneaded mixture in a solvent consisting of a paraffinic or an isoparaffinic hydrocarbon. More specifically, it has been an indispensible requirement of the liquid developers of the prior art that the pigment particles which constitute the basic material of the toner are coated with the aforesaid various agents, i.e., a polarity regulating agent which is adapted to impress the toner with a distinct positive or negative electric charge to cause the deposition, by electrophoresis, of the toner onto the area of the electrostatic latent image which is formed on the photoconductive layer of the electrophotographic copying material, a fixing agent which is adapted to insure the adhesion of the toner on the surface of said photoconductive layer, and a dispersing agent which is adapted to have the toner stably dispersed in the carrier liquid. It has been the practice to use, as the polarity regulating agents and the fixing agents, those materials such as naturally occurring rosin, asphalt, synthetic resins and lauryl methacrylate-dimethylaminoethyl methacrylate copolymers, and also to use, jointly with the aforesaid materials, surface active agents, such as metal soap, in order to impart a distinct polarity to the toner and to improve the dispersibility of the toner in the carrier liquid. It is also known to affect the regulation of the polarity of the toners by the combined use of two or more kinds of pigments. In spite of the past efforts which have been paid to improve the ability of the toners as desired, however, the following drawback of the toners has not been successfully solved to date.

Specifically, the aforesaid drawback of the toners was represented by the fact that, when the liquid developers of the prior art were stored for an extended period of time or when the same liquid developers were used repeatedly, it occurred that the polarity (meaning the positive or negative electric charge) of the toners became indistinct so as to result in a deterioration of the developing ability of the toners. The evidence of the deterioration of the developing ability of the toners was detected by observing the lowered concentration of the toner in the liquid developer due to the repeated use thereof and caused by the elevated concentration of the additional agents such as the surface active agent in the liquid developer, and in some cases, the deterioration was detected by observing the sedimentation of the particles of the toner in the bottom of the layer of the liquid. When the liquid developer is converted to either one of the aforesaid conditions, not only will the copied image which is formed on the surface of the photoconductive layer of the electrophotographic copying material lose clarity, but also there will occur a dark solid background on the copying material and an insufficient fixation of the toner, and the copy will not meet the practical requirements.

As has been described above, it has been extremely difficult for the liquid developers of the prior art to retain, for an extended period of time, the initial satisfactory developing ability which was possessed by them at the time they were prepared.

SUMMARY OF THE INVENTION

It is, therefore, the object of the present invention to provide a liquid developer for use in electrophotography which has overcome the aforesaid drawbacks of the liquid developers of the prior art and which, above all, is extremely stable and does not cause any deterioration of the developing ability during its repeated use.

We have discovered the following facts that, when a toner which is prepared either (1) by conducting the graft copolymerization of a homopolymer of a polymeric monomer having an epoxy radical or a carbinol radical or the graft copolymerization of a copolymer which contains the aforesaid polymeric monomer to pigment particles, or (2) by coating the pigment particles with said polymeric monomer or with said copolymer by kneading the mixture of said pigment particles and said polymeric monomer or said copolymer, is dispersed in a carrier liquid consisting of a hydrocarbon, the resulting toner will bear a distinct positive electric charge, that the resulting toner exhibits a satisfactory dispersibility and a satisfactory developing ability without the combined use of other additives, and further that these desirable properties of the toner do not deteriorate after the storage of the liquid developer containing said toner for an extended period of time and during the repeated use of such a liquid developer. It has been found also by us that the liquid developer containing the aforesaid graft copolymerized toner and the liquid developer containing the aforesaid coated toner both possess a substantially equal developing ability.

The present invention has been worked out based on the aforesaid knowledge.

The carrier liquid which is used with the toner in the present invention can be the carrier liquids which have been employed in general in electrophotography. It is to be noted, however, that, since the polarity regulating agents (resins) and the other additives which were used in the liquid developers of the prior art were selected, with preference, from among those having a low solubility in the carrier liquids, it is desirable, in the present invention, to use those polymeric monomers having an epoxy radical or a carbinol radical, in the form of a copolymer containing such a substance as lauryl methacrylate as the counterpart constituent of the copolymer. Those toners which are prepared either by graft copolymerizing the aforesaid copolymers to pigment particles or by coating the pigment particles with such copolymers by kneading the mixture of the pigment particles and the copolymers, exhibit a much better dispersibility in the aforesaid carrier liquids, as compared with the dispersibility which is exhibited by the toners prepared either by graft-copolymerizing homopolymers to pigment particles or by coating the pigment particles with homopolymers by kneading the mixture of these two constituent materials.

As the polymeric monomers having an epoxy radical which are applicable to the present invention, those monomers such as glycidyl methacrylate, 2,3-epoxypropyl acrylate, allyl-2,3-epoxypropyl maleate, and allyl-2,3-epoxybutyrate are suitable. Also, as the polymeric monomers having a carbinol radical which are applicable to the present invention, those monomers such as hydroxyethyl methacrylate, allylalcohol, 2-ethyl-2-hydroxylethyl-aminoethyl acrylate, diethyl-vinylethynyl carbinol, .beta.-hydroxylbutyl acrylate, methyl-phenyl-vinylethynyl carbinol, methyl-ethyl-vinylethynyl carbinol, n-dipropyl-vinylethynyl carbinol and n-propyl-vinylethynyl carbinol are suitable. Furthermore, as the monomers which are copolymerized with one of the aforesaid polymeric monomers, those monomers including alkyl methacrylate such as butyl methacrylate and lauryl methacrylate, styrene, styrene derivatives such as aminostyrene, hydroxystyrene, chlorostyrene and methylstyrene; acrylamide, acrylamide derivatives and acrylic acid derivatives such as dibutylacrylamide, dimethyl-acrylamide and .alpha.-methyl-acrylamide; diene derivatives such as 2-n-butylbutadiene, 1,3-cyclohexadiene and 2-n-amylbutadiene-1,3, dibasic acid such as anhydrous maleic acid and crotonic acid; and acrylonitrile derivatives and methacrylonitrile derivatives such as acrylonitrile and .alpha.-ethylacrylonitrile, are suitable.

In case a copolymer of the aforesaid type is used with the pigment particles, the object of the present invention will be attained by the employment of a two or more component copolymer in which is contained a polymeric monomer having the aforesaid specific radical in amount at least 15 percent by mole, as will be understood clearly from the embodiments of the present invention which will be described later.

As has been described, the present invention contemplates the provision of a liquid developer for use in electrophotography which is of an extremely simple structure such that a toner which is comprised of a combination of only pigment particles and a polymer is dispersed in a carrier liquid. It is to be noted, however, that, in spite of such a simple structure of the liquid developer, the toner bears a very distinct polarity and also that it has a satisfactory dispersibility in the carrier liquid and that, accordingly, when the liquid developer of the present invention is used in the developing process, the copied images obtained are satisfactory in both the clarity and the fixation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

EXAMPLE 1

To 0.2 mole of glycidyl methacrylate, 0.2 mole of butyl methacrylate, 0.41 mole of styrene and 0.3 mole of lauryl methacrylate was added 0.005 mole of azobisisobutyronitrile, and there further was added 1 mole of toluene. The resulting mixture was allowed to react in a nitrogen atmosphere at 90.degree. C. for 9 hours, and thus, a highly viscous copolymer was prepared. To 7 parts by weight of said copolymer were added 1 part by weight of carbon-black (produced under the trade name of Asahi XC-550, by Asahi Carbon, Ltd. of Japan) and 32 parts by weight of Naphtha No. 6 (trade name of a petroleum hydrocarbon, i.e., an aliphatic hydrocarbon solvent containing 25 weight percent aromatics, flash point 40.degree. C., specific gravity 0.789, produced by ESSO Standard Oil Company of U.S.A.). The resulting mixture was kneaded in a ball mill for 8 hours, and as a result, a highly concentrated toner was prepared. Four grams of the concentrated toner thus obtained was introduced in 1 liter of Isopar H (trade name of an isoparaffinic hydrocarbon, flash point 123.degree. F., Kauri-Butanol value 27, specific gravity 0.7571, produced by ESSO Standard Oil Company of U.S.A.). Thus, a liquid developer for use in electrophotography of the type in which the toner consisted of the pigment particles coated with the aforesaid copolymer and having a polarity regulated to the positive charge was dispersed in the carrier liquid was obtained.

Using a commercially available electrophotographic copying material, an electrostatic latent image was given, this copying material according to a known technique, and then the resulting copying material was developed by immersing the latter in the liquid developer of the present invention. The result was that a very clear positive copied image was formed. Also, the liquid developer of the present invention was left to stand for more than 6 months and furthermore, the liquid developer thus obtained was subjected to repeated use for such an extended period of time. However, neither of these liquid developers of the present invention showed any appreciable deterioration of their developing abilities.

EXAMPLE 2

To 7 parts by weight of a monomer-constituting liquid consisting of:

styrene 0.2 mole lauryl methacrylate 0.2 mole hydroxyethyl methacrylate 0.3 mole azobisisobutyronitrile 0.005 mole toluene 1.0 mole

was added 1 part by weight of carbon-black (produced under the trade name of Statexdeness by Columbian Carbon Company of U.S.A.). In a manner similar to that described in example 1, the resulting mixture was allowed to react, and thus, a graft-copolymeric toner wherein the copolymer is united with the pigment particles was prepared. To 1 part by weight of this toner was added 4 parts by weight of Naphtha No. 6, and the resulting mixture was kneaded in a ball mill for 8 hours to disperse the components thoroughly relative to each other. As a result, a highly concentrated toner was obtained. Four grams of this concentrated toner was dispersed in 1 liter of Naphtha No. 6, and thus a liquid developer for use in electrophotography wherein the polarity of the toner was regulated to the positive charge was obtained. Using this liquid developer, an electrophotographic copying material which had been given an electrostatic latent image by a known technique was developed. The result was that a very clear positive copied image was formed. This liquid developer was left to stand or subjected to repeated use for a period of more than 6 months, but it showed no appreciable deterioration of its developing ability in each of these tests.

EXAMPLE 3

A copolymer was prepared, in a manner similar to that described in example 1, by the use of the copolymer-constituting liquid having the following composition:

lauryl methacrylate 1.0 mole hydroxyethyl methacrylate 0.6 mole azobisisobutyronitrile 0.005 mole toluene 1.0 mole

By blending this copolymer with carbon-black (produced under the trade name of Mitsubishi No. 44, by Mitsubishi Chemical Industries, Ltd. of Japan) and also with Naphtha No. 6, the resulting mixture was kneaded thoroughly in a ball mill. Thus, a highly concentrated toner was prepared. Four grams of this concentrated toner was dispersed in 1 liter of Naphtha No. 6, and as a result, a liquid developer for use in electrophotography wherein the polarity of the toner was regulated to the positive charge was obtained. The liquid developer thus obtained was applied to the development of the electrophotographic copying material which was similar to that described in example 1. The result was that a very clear positive copied image was formed. The liquid developer thus obtained was left to stand or subjected to repeated use for a period of more than 4 months, but no appreciable deterioration of the developing ability was noted in each of these tests.

EXAMPLE 4

A copolymer was prepared, in a manner similar to that described in example 1, by the use of a copolymer-constituting liquid having the following composition:

lauryl methacrylate 0.6 mole allyl alcohol 0.4 mole toluene 1.0 mole azobisisobutyronitrile 0.005 mole

7 parts by weight of this copolymer, 1 part by weight of Spirit Black (a product of Orient Chemical Industries, Ltd. of Japan) and 32 parts by weight of Isopar H were kneaded together in a ball mill for 8 hours, and thus, a highly concentrated toner was prepared. Four grams of this concentrated toner was dispersed in 1 liter of Isopar H, and as a result, a liquid developer for use in electrophotography was obtained. The liquid developer thus obtained was applied to the development of an electrophotographic copying material similar to that described in example 1. The result was that a very clear positive copied image was formed. This liquid developer was left to stand or subjected to repeated use for a period of more than 4 months, but no deterioration of the developing ability was noted in each of these tests.

Claims

What is claimed is:

1. A liquid developer for use in electrophotography consisting essentially of a liquid hydrocarbon having a high electric resistance of at least 10.sup.10 .OMEGA.cm and a low dielectric constant of a maximum of 3 and a toner dispersed in said liquid hydrocarbon, said toner consisting essentially of particles of a pigment selected from the group consisting of Carbon Black (C.I. No. 77,266) and Spirit Black (C.I. No. 50,440) and a coating-layer coated on said particles, said coating-layer consisting essentially of a polymer selected from the group consisting of:

a. homopolymers of a monomer selected from the group consisting of glycidyl methacrylate, 2,3-epoxypropyl acrylate, allyl-2,3-epoxypropyl maleate, allyl-2,3-epoxybutyrate, hydroxyethyl methacrylate, allylalcohol, 2-ethyl-2-hydroxylethylaminoethyl acrylate, diethyl-vinylethynyl carbinol, .beta.-hydroxylbutyl acrylate, methyl-phenyl-vinylethynyl carbinol, methyl-ethyl-vinylethynyl carbinol, n-dipropyl-vinylethynyl carbinol and n-propyl-vinylethynyl carbinol; and

b. copolymers of at least 15 mol percent of a monomer selected from said group of monomers named in (a) and the balance consisting of at least one monomer selected from the group consisting of butyl methacrylate, lauryl methacrylate, styrene, aminostyrene, hydroxystyrene, chlorostyrene, methylstyrene, acrylamide, dibutylacryl-amide, dimethylacrylamide, .alpha.-methyl-acrylamide, 2-n-butylbutadiene, 1,3-cyclohexadiene, 2-n-amylbutadiene-1,3, anhydrous maleic acid, crotonic acid, acrylonitrile and .alpha.-ethyl acrylonitrile.