Electrostatic image-developing toner and developer

Abstract: An electrostatic image-developing toner comprising a binder resin, a quaternary ammonium salt and an acidic carbon black.

Description: The present invention relates to an electrostatic image-developing toner suitable for use in, for example, developing electrostatic latent images formed in electrophotography or electrostatic recording. Developer, for example used for electrocopiers, is once deposited on an image carrier such as a photoreceptor on which an electrostatic image has been formed, in a developing step. The deposited developer is then transferred from the photoreceptor to a transfer sheet in a transfer step and the transferred developer is finally fixed on copying paper in a fixing step. As the developer used for developing the electrostatic image formed on a latent image carrier, there are known two-component developers comprising a carrier and a toner, and one-component developers (magnetic toner) which require no carrier. For preparing a toner which is positively charged, a method is known in which a charge-imparting agent is added either internally or externally to a binder resin and a colorant. As the charge-imparting agent, there are known, for instance, Nigrosine dye, triaminophenylmethane compounds and quaternary ammonium salts. Among charge-imparting agents, quaternary ammonium salts, as compared with other agents such as Nigrosine dye, have advantages in that they can be used for color toner as they are colorless and in that they maintain good charging stability in continuous copying operations. On the other hand they have a drawback that the amount of charge imparted is relatively small. As the carrier used in combination with the toner in a two-component developer, there are known, for example, iron powder and ferrite powder. Ferrite powder is low in charge-imparting performance in comparison with iron powder. Thus, when using a toner containing a quaternary ammonium salt or a developer comprising the quaternary ammonium salt-containing toner and a carrier composed of a ferrite powder, there has been the problem that sufficient charge could not be obtained. Especially under high temperature and high humidity conditions or in a continuous copying operation, the charge is apt to decrease, causing a high background (BKG) level or making it impossible to obtain a stabilized copy density. Particularly just after the start of operation of a copying machine, as in the morning, copies are blotted black due to increase of the BKG level or brush marks are formed on the solid black of the copy. US-A-4,221,856 discloses a toner comprising a resin and a quaternary ammonium compound containing, as an anion, a sulfate, sulfonate, nitrate, borate, chlorate or halogen. US-A-4,980,258 discloses a toner comprising a binder resin, a quaternary ammonium compound and a neutral carbon black. JP-A-60/169,857 discloses a toner comprising a quaternary ammonium compound and a neutral carbon black. Encyclopedia of Chemical Technology, Kirk-Othmer, 3rd ed, vol 4, page 662 describes the types and applications of special carbon blacks. We have surprisingly found that by incorporating into a toner comprising a quaternary ammonium salt, an acidic carbon black (whose use as a component of a positively charged toner has previously been avoided since it is in itself an electron acceptive substance and has a nature to be charged negatively) the developer obtained has excellent charging characteristics and is capable of providing an excellent image quality and durability. The present invention provides an electrostatic image-developing toner comprising: (i) a binder resin; (ii) a quaternary ammonium salt of formula (I) or (II) : wherein R 1 , R 2 , R 3 and R 4 , which may be identical or different, are each a substituted or unsubstituted alkyl group or a substituted or unsubstituted aralkyl group; wherein R 5 , R 6 , R 7 and R 8 , which may be identical or different, are each a substituted or unsubstituted alkyl group or a substituted or unsubstituted aralkyl group; A is a benzene ring which may have a substituent(s) or a naphthalene ring which may have a substituent(s); and n is 2 or 3; and (iii) an acidic carbon black. The toner of the present invention may be used in an electrostatic image developer comprising (a) the toner, and (b) a resin-coated ferrite powder as a carrier. The developer comprising the toner has excellent static charging characteristics and has little probability of causing a change of image quality over time and being deteriorated by environmental factors. In formula (I), R 1 , R 2 , R 3 and R 4 , which may be identical or different, are preferably each a substituted or unsubstituted alkyl group having from 1 to 18 carbon atoms or a substituted or unsubstituted aralkyl group having from 7 to 15 carbon atoms. As substituent(s) of the alkyl group, a nitro group and a chlorine atom may be exemplified. As substituent(s) of the aralkyl group, a nitro group, a methyl group and a chlorine atom may be exemplified. It is especially preferred that R 1 is an alkyl group having from 1 to 8 carbon atoms, R 2 and R 3 , which may be identical or different, are each an alkyl group having from 1 to 18 carbon atoms and R 4 is an alkyl group having from 1 to 8 carbon atoms or a benzyl group; In formula (II), R 5 and R 7 are preferably each a substituted or unsubstituted alkyl group having from 1 to 8 carbon atoms or a substituted or unsubstituted aralkyl group having from 7 to 15 carbon atoms; and R 6 and R 8 are preferably each a substituted or unsubstituted alkyl group having from 1 to 30 carbon atoms or a substituted or unsubstituted aralkyl group having from 7 to 15 carbon atoms. As substituent(s) of the alkyl groups, a nitro group and a chlorine atom may be exemplified. As substituent(s) of the aralkyl group, a nitro group, a methyl group and a chlorine atom may be exemplified. As substituent(s) of the benzene ring and naphthalene ring in the group A, a hydroxyl group, an amino group and a (C 1 - C 4 ) alkyl group may be exemplified. Among them a hydroxyl group is preferred. Each of the rings may have two or more substituents. It is especially preferred that R 5 and R 7 are each a methyl group and the total number of carbon atoms possessed by R 6 and R 8 is 13 or greater preferably from 19 to 40, more preferably from 30 to 40. Examples of compounds of formula (I) are: Examples of compounds of formula (II) are: The binder resin used in the present invention can be selected from various known ones. For example, there can be used styrene resins (a homopolymer or copolymer containing styrene or styrene substituents) such as polystyrene, crotopolystyrene, poly-α-methylstyrene, styrenechlorostyrene copolymer, styrene-propylene copolymer, styrene-butadiene copolymer, styrene-vinyl chloride copolymer, styrene-vinyl acetate copolymer, styrene-maleic acid copolymer, styrene-acrylic ester copolymers (such as styrene-methyl acrylate copolymer, styrene-ethyl acrylate copolymer, styrene-butyl acrylate copolymer, styrene-octyl acrylate copolymer and styrene-phenyl acrylate copolymer), styrene-methacrylic ester copolymers (such as styrene-methyl methacrylate copolymer, styrene-ethyl methacrylate copolymer, styrene-butyl methacrylate copolymer and styrene-phenyl methacrylate copolymer), styrene-α-methyl chloroacrylate copolymer and styrene-acrylonitrile-acrylic ester copolymer; vinyl chloride resin; rosin-modified maleic acid resin; phenol resin; epoxy resin; polyester; low-molecular weight polyethylene; low-molecular weight polypropylene; ionomer resin; polyurethane; silicone resin; ketone resin; ethylene-ethyl acrylate copolymer; xylene resin; and polyvinyl butyral. The especially preferred resins in the present invention are styrene-acrylic ester copolymers, styrene-methacrylic ester copolymers, saturated or unsaturated polyesters and epoxy resins. These resins may be used either singly or in mixtures. The content of the quaternary ammonium salt in the toner is preferably from 0.1 to 10 parts by weight, more preferably from 0.2 to 6 parts by weight, based on 100 parts by weight of the binder resin. When the content of the quaternary ammonium salt is too small,it is difficult to obtain the expected effect of improving the charging characteristics, and when the content is too large, the quality of the toner produced tends to deteriorate. As the colorant an acidic carbon black obtainable by the furnace process may be exemplified. The pH of the acidic carbon black is preferably from 2 to 4. The carbon black content is preferably from 3 to 20 parts by weight, more preferably from 4 to 10 parts by weight, based on 100 parts by weight of the resin. Examples of the acidic furnace-process carbon blacks used in the present invention are MA-7, MA-8, MA-11, MA-100, #1000, #22008, #2350 and #24008 (which-are produced by Mitsubishi Kasei Corp.); MOGUL L, REGAL 400R and MONARCH 1000 (which are produced by Cabot Corp.); and 1035, 1040, 1255 and 3500 in RAVEN (produced by Columbia Corp.) . An acidic carbon black having a specific surface area, as measured by the BET method, of from 25 to 400 m 2 /g and a dibutyl phthalate (DBP) absorption of from 40 to 140 ml/100 g is preferably used. For good dispersion into the binder resin at the time of toner kneading, the acidic carbon black more preferably has a specific surface area of from 80 to 150 m 2 /g and a DBP absorption of from 50 to 120 ml/100 g. An acidic carbon black, at least a part of which has been treated with a metal salt of a carboxylic acid preferably having a melting point of from 70 to 250°C may also be added. The toner may comprise, in addition to a quaternary ammonium salt, other charge controlling agents such as a polyamine resin, Nigrosine dye or triaminotriphenylmethane compounds. The content of such other charge controlling agents is preferably not more than the content of the quaternary ammonium salt. The toner may further comprise other additives for improving the fixing property such as low-molecular weight olefin polymers, for example polyethylene and polypropylene. The content of the low-molecular weight olefin polymer is preferaby from 0.5 to 10 % by weight. The toner may further comprise other additives for the flowability of the toner such as fine silica powder, alumina and titania. The content of the silica powder is preferably from 0.05 to 5 % by weight. For the preparation of the toner, the component materials are mixed by a kneader or other suitable means and the resultant mixture is cooled, pulverized and classified. The average particle size of the obtained toner is preferably from 5 to 20 µm. The toner of the present invention may be used in a developer. The developer preferably comprises a mixture of a toner produced in the manner described above and a carrier comprising a ferrite powder having the particle surfaces coated with a resin. As the coating resin for the ferrite powder, there can be used, for example, fluorine resins, silicone-based resins, acrylic resins, styrene resins, epoxy resins, polyesters and polyamides. As the carrier for the developer it is preferred to use a ferrite powder coated with a silicone-based resin such as a silicone resin, a methyl silicone-containing resin, a phenyl silicone-containing resin or a mixture thereof. The coat of ferrite powder may be a monolayer structure or multi-layer structure. It is preferable that at least the uppermost layer of the coat is composed of the silicone resin, the methyl silicone-containing resin, the phenyl silicone-containing resin or the mixture thereof. The silicone resin contains giant molecules expanding in a network structure and has the silicon atoms bonded to each other through siloxane bonds. The silicon atom merely bonded to not more than 3 other silicon atoms through siloxane bonds on the surface of the giant molecule usually has hydroxyl groups. The resin in which these hydroxyl groups are at least partly replaced with methyl groups or methyl and phenyl groups, is here called a methyl silicone-containing resin or a phenyl silicone-containing resin. The particle size of the carrier is not specifically defined, but a preferable average particle size is from 10 to 200 µm. The mixing ratio of the carrier is preferably from 5 to 100 parts by weight based on 1 part by weight of the toner. The electrostatic image developer has an excellent charging performance such as an always moderate and stabilized charging property. It can minimize the BKG level or change the image density in a copying operation under high temperature and high humidity conditions, in a continuous copying operation or in an intermittent copying operation, which have been called in question in the past. Especially, it can prevent occurrence of increase of the BKG level in copying operation after being allowing to stand and can minimize the change of copied image quality. Thus, the developer is capable of forming good images irrespective of use conditions and its industrial benefit is immense. The present invention is further described in the following Examples. In the following Examples, all "parts" are "parts by weight" unless otherwise noted. Example 1 100 parts of a styrene-n-butyl acrylate copolymer (monomeric weight ratio of styrene/n-butyl acrylate = 82/18), 5 parts of an acidic carbon black (MA-7, produced by Mitsubishi Kasei Corp., pH = 3.0), 2 parts of a charge controlling agent composed of a quaternary ammonium salt [compound of the formula (11)] and 2 parts of a low-molecular weight polypropylene (Viscol 550P, produced by Sanyo Chemical Co., Ltd.) were blended, kneaded, pulverized and classified to obtain a black toner having an average particle size of 10 µm. To 100 parts of the thus-obtained black toner were externally added and mixed 0.2 parts of silica powder (R-972, produced by Nippon Aerosil Co., Ltd.) and 0.3 parts of magnetite powder (EPT-1000, produced by Toda Kogyo Corp.) by a Henschel mixer. 4 parts of the resultant mixture and 100 parts of a carrier composed of a ferrite powder having a methyl silicone-containing resin (average particle size about 100 µm) coated on the particle surfaces thereof were mixed and stirred to prepare a developer. This developer was subjected to a 100,000-sheet copying test under conditions of 40°C and 85-90% RH using a copying machine employing an organic photoconductor as a photoreceptor. The 100,000-sheet copying test was conducted by repeating 10 times the daily operating cycle of continuous copying of about 10,000 sheets and overnight (about 10-hour) suspension of operation. As a result, there was noted almost no change of image density and amount of charge during and after continuous copying of 10,000 sheets, and also there was no change of the BKG level in operation after overnight suspension (standing overnight) . Even after copying of 100,000 sheets, there was observed no increase of BKG level of the copy, and the uniformity and density of the solid black of the copy were also high. Example 2 A developer was prepared in the same procedure as Example 1 except that the carrier used was the one prepared by mixing 80 parts of a ferrite powder coated with methyl silicone-containing resin and 20 parts of a ferrite powder coated with phenyl silicone-containing resin. This developer was subjected to the 100,000-sheet copying test under high temperature and high humidity conditions. There was seen no increase of the BKG lrvel, and the uniformity and density of the solid black of the copy were high. The developer was also excellent in durability. Examples 3-6 Developers were prepared in the same way as Example 1 except for using the acidic carbon blacks shown in Table 1. The thus-obtained developers were subjected to the 100,000-sheet copying test under high temperature and high humidity conditions. There was no increase of the BKG level, and the uniformity and density of the the solid black ground of the copy were high. The developers were also excellent in durability. Comparative Examples 1 and 2 Developers were prepared in accordance with Example 1 except for using the carbon blacks shown in Table 1. These obtained developers were subjected to the 100,000-sheet copying test under high temperature and high humidity conditions. As a result, there were noted large changes of image density, large increase of the BKG level and large changes of the tribocharge in the course of continuous copying. Especially, the increase of the BKG level after overnight suspension rose sharply, and brush marks were formed on the black ground of the copy. Thus, these developers could not stand normal use. Remarks In the Comparative Examples, the difference of the level between the 10,000th copy and the 1st copy after overnight suspension was noted.

Claims: An electrostatic image-developing toner comprising: (i) a binder resin; (ii) a quaternary ammonium salt of formula (I) or (II): wherein R 1 , R 2 , R 3 and R 4 , which may be identical or different, are each a substituted or unsubstituted alkyl group or a substituted or unsubstituted aralkyl group; wherein R 5 , R 6 , R 7 and R 8 , which may be identical or different, are each a substituted or unsubstituted alkyl group or a substituted or unsubstituted aralkyl group; A is a benzene ring which may have a substituent(s) or a naphthalene ring which may have a substituent(s); and n is 2 or 3; and (iii) an acidic carbon black. A toner according to claim 1 which comprises 0.1 to 10 parts by weight of the quaternary ammonium salt and 3 to 20 parts by weight of the acidic carbon black, based on 100 parts by weight of the binder resin. A toner according to claim 1 or 2 wherein the acidic carbon black is obtainable by the furnace process. A toner according to any one of the preceding claims wherein the acidic carbon black has a pH of from 2 to 4. A toner according to any one of the preceding claims wherein the specific surface area of the acidic carbon black, as measured by the BET method, is from 25 to 400 m 2 /g, and the dibutyl phthalate absorption of the acidic carbon black is from 40 to 140 ml/100 g. A toner according to any one of the preceding claims wherein at least a part of the acidic carbon black has been treated with a metal salt of a carboxylic acid. A toner according to any one of the preceding claims wherein R 1 is an alkyl group having from 1 to 8 carbon atoms, R 2 and R 3 , which may be identical or different, are each an alkyl group having from 1 to 18 carbon atoms and R 4 is an alkyl group having from 1 to 8 carbon atoms or a benzyl group. A toner according to any one of claims 1 to 6 wherein R 5 and R 7 are each a methyl group and the total number of carbon atoms possessed by R 6 and R 8 is 13 or greater. A toner according to any one of the preceding claims wherein the binder resin is a styrene-acrylic ester copolymer, a styrene-methacrylic ester copolymer, a saturated or unsaturated polyester or an epoxy resin. A toner according to any one of the preceding claims which further comprises a low-molecular weight olefin polymer. A toner according to any one of the preceding claims which further comprises silica powder.

Inventors: HIRAMA KAZUHIRO, SHINTAKU TAKASHI, TSURUMORI MASAMI, HIRAMA, KAZUHIRO, SHINTAKU, TAKASHI, TSURUMORI, MASAMI
Applicants: MITSUBISHI CHEM CORP, MITSUBISHI CHEMICAL CORPORATION
Application year: 1992
Main classification: G03G9
IPC codes: G03G9