Carriers for developing electrostatic images

Abstract: The present invention provides a carrier comprising a core and a coating for developing electrostatic images, the core being formed from a specific polymer composition.

Description: Field of the Invention The present invention relates to a carrier comprising a core and a coating on the core for developing electrostatic images as claimed in Claim 1, the carrier constituting, along with a toner, an electrostatic image developer for use with an electronic photographic copying machine (hereinafter referred to simply as "carrier"). Background of the Invention Known carriers include those coated with a homopolymer comprising fluorinated acrylate or fluorinated methacrylate (Japanese Unexamined Patent Publication No.53-97,435). However, the polymer forms a coating low in durability, adhesion to the core material, strength, etc. Also known are carriers coated with a composition comprising a polymer having crosslinkable functional groups and a crosslinking agent (Japanese Unexamined Patent Publication No.60-59,369). However, the composition tends to insufficiently crosslink depending on the crosslinking conditions, forming a coating of low durability. Functional groups in the polymer such as organic acid residues, hydroxyl, epoxy, imino, etc. are hydrophilic and result in lower or unstable electrostatic charge capacity under humid conditions. Summary of the Invention It is the primary object of the present invention to provide a carrier comprising a core and a coating on the core, the coating being composed of a copolymer excellent in durability. It is another object of the invention to provide a carrier comprising a core and a coating on the core, the coating having a good adhesion to the core and high strength. It is a further object of the invention to provide a carrier comprising a core and a coating on the core, the coating having a great electrostatic charge capacity. Other objects and features of the invention will become apparent from the following description. We conducted extensive research to overcome the foregoing problems of the conventional techniques and found that specific copolymers exhibit outstanding properties when used for coating the carrier core. According to the invention, there is provided a carrier for developing electrostatic images, the carrier comprising a core and a coating on the core, the coating being formed from a copolymer or a composition containing the copolymer, the copolymer essentially comprising (a) about 45 to about 90 mole % of at least one monomer selected from the group consisting of chlorotrifluoroethylene, tetrafluoroethylene, trifluoroethylene and hexafluoropropylene and (g) about 10 to about 55 mole % of at least one of the monomers represented by the formula CH₂=CHOOCR wherein R is an alkyl, cycloalkyl or an aromatic group. According to the invention, there is also provided a carrier for developing electrostatic images, the carrier comprising a core and a coating on the core, the coating being formed from a composition containing a copolymer and a curing agent, the copolymer essentially comprising (a) about 40 to about 90 mole % of at least one monomer selected from the group consisting of chlorotrifluoroethylene, tetrafluoroethylene, trifluoroethylene and hexafluoropropylene; (g) about 9 to about 50 mole % of at least one of the monomers represented by the formula CH₂=CHOOCR wherein R is an alkyl, cycloalkyl or an aromatic group; and (h) about 1 to about 20 mole % of at least one monomer having a functional group and copolymerizable with the above monomer. A copolymer comprising at least one of (a) tetrafluoroethylene, trifluoroethylene, chlorotrifluoroethylene and hexafluoropropylene and at least one monomer represented by the formula (g) CH₂=CHOOCR wherein R is an alkyl or cycloalkyl group or an aromatic group, is used in the invention. Tetrafluoroethylene and chlorotrifluoroethylene are preferably used as the monomer (a). Of these, chlorotrifluoroethylene is more preferably used. Useful as an alkyl group represented by R₄ in the formula of the monomer (g) to be used in the invention are those straight or branched and substituted with halogen atoms or the like, such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, tert-butyl, hexyl, nonyl, decyl, undecyl, dodecyl, chloromethyl, etc. Examples of the cycloalkyl group are cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, etc. Usable as the aromatic group are phenyl which may be substituted with an alkyl group, halogen atom or a hydroxyl group or the like, naphthyl and the like, such as phenyl, methylphenyl, chlorophenyl, p-tert-butylphenyl, etc. Specific examples of the monomer (g) are as follows. CH₂=CHOOCCH₃, CH₂=CHOOCC₂H₅, CH₂=CHOOCC₃H₇, CH₂=CHOOCC₄H₉ CH₂=CHOOCCH(CH₃)₂, CH₂=CHOOCC(CH₃)₃, CH₂=CHOOC(CH₂)₉CH₃, CH₂=CHOOC(CH₂)₁₀CH₃, CH₂=CHOOCCH₂Cℓ, a compound (commertially available with trademarks such as "Veoba 10", product of Shell Chemical Co., Ltd.) represented by the formula etc. These monomers serving as the monomer (g) can be used singly or at least two of them are usable in mixture. The mixing ratio of the monomer (a) to the monomer (g) in the copolymer to be used in the invention is usually 45-90% : 55-10%, preferably 55-85% : 45-15%, more preferably 60-80% : 40-20%. If the amount of the monomer (a) is less than 45%, the fluorine content of the composition is reduced to afford the carrier an insufficient electrostatic charge capacity, resulting in a failure in producing the properties of the carrier to a full extent. In contrast, if the monomer (a) is used in an amount exceeding 90%, the solubility of the copolymer in the solvent is decreased, leading to the likelihood of encountering difficulty in coating the carrier core with the coating material. In order to improve the properties of the copolymer such as glass transition temperature (Tg), solubility in the solvent, electrostatic charge capacity and the like, the copolymer for use in the invention may further contain other monomers copolymerizable with the monomers (a) and (g) in an amount of up to 30% of the combined amount of the monomers (a) and (g) provided that such addition will not impair the properties of the copolymer. The kind of such additional monomer is not specifically limited. Usable as such monomer are, for example, styrenes such as styrene, α-methylstyrene, chloromethylstyrene and the like; alkyl acrylates or methacrylates unsubstituted or substituted in α-position such as methyl acrylate, ethyl acrylate, propyl acrylate, butyl acrylate, trifluoroethyl acrylate, pentafluoropropyl acrylate, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, trifluoroethyl methacrylate, pentafluoropropyl methacrylate, methyl α-fluoroacrylate, ethyl α-fluoroacrylate, propyl α-fluoroacrylate, butyl α-fluoroacrylate, trifluoroethyl α-fluoroacrylate, pentafluoropropyl α-fluoroacrylate, methyl α-chloroacrylate, ethyl α-chloroacrylate, propyl α-chloroacrylate, butyl α-chloroacrylate, trifluoroethyl α-chloroacrylate, pentafluoropropyl α-chloroacrylate and the like; vinyl ethers such as ethyl vinyl ether, 2-chloroethyl vinyl ether, propyl vinyl ether, butyl vinyl ether, 2,2,3,3-tetrafluoropropyl vinyl ether, cyclohexyl vinyl ether and the like; vinylketones such as methyl vinylketone, ethyl vinylketone, propyl vinylketone, butyl vinylketone, phenyl vinylketone and the like; olefins such as ethylene, propylene, isobutene, butadiene, isoprene and the like; and nitrogen-containing compounds such as N-methylpyrrolidone, N-methylcarbazole, 4-vinylpyridine, acrylonitrile, methacrylonitrile and the like; haloolefins such as vinyl chloride, vinylidene chloride and the like. The coating on the carrier core in the invention has a glass transition temperature (Tg) of 50°C or higher, preferably 60°C or higher. A glass transition temperature of below 50°C tends to render the coating soft and sticky in the step of production of the carrier or during the development by a developer with the carrier. The molecular weight of the copolymer for use in the invention is expressed in an intrinsic viscosity [η] of about 0.01 to about 2.0 as determined at 35°C using as a solvent 1,1,1-trichloroethane. The copolymer for use in the invention can be prepared by usual radical polymerization method such as bulk polymerization, suspension polymerization, emulsion polymerization or solution polymerization. In the case of the suspension polymerization and solution polymerization, there are used one or at least two of solvents, for example, chlorine-containing solvents such as 1,1,1-trichloroethane, 1,2-dichloromethane and the like; alcohols such as tert-butanol and the like; ester solvents such as ethyl acetate and the like; ketone solvents such as acetone, methyl ethyl ketone, methyl isobutyl ketone and the like; aromatic hydrocarbons such as toluene, xylene and the like; and fluorine-containing solvents such as 1,1,2-trichloro-1,2,2-trifluoroethane, 1,2,-dichloro-1,1,2,2-tetrafluoroethane and the like. In the case of the emulsion polymerization, there are employed one or at least two of emulsifying agents such as CF₃(CF₂)₆COONH₄, H(CF₂)₆COONH₄, sodium dodecylsulfate and the like. The carrier core may be coated with a composition comprising a resin and other additives in addition to the copolymer. Examples of useful resins are vinylidene fluoride, vinylidene fluoride-ethylene tetrafluoride copolymer and like fluorine-contained resins, silicone resin, acrylic resin and like resins, etc. Useful additives are silica flour, charge controlling agents, surfactants, lubricants, etc. The amount of these resins or additives used is preferably not more than 50% by weight of the copolymer. A wide range of organic solvents are useful unlike the case of conventional using fluorine-contained resins. Specific examples of organic solvents are ketone solvents such as acetone, methyl ethyl ketone, methyl propyl ketone, methyl isopropyl ketone, methyl isobutyl ketone and the like; acetate solvents such as ethyl acetate, cellosolve acetate, n-butyl acetate and the like; cyclic ethers such as tetrahydrofuran, dioxane and the like; aromatic hydrocarbons such as toluene, xylene and the like; halogenated hydrocarbons such as tetrachloroethylene, trichloroethylene, methylene chloride and the like; alcohols such as methyl alcohol, ethyl alcohol, butyl alcohol, tert-butyl alcohol, isopropyl alcohol and the like; fluorine-containing solvents such as 1,1,2-trifluorotrichloroethane, 1,2-difluorotetrachloroethane, hexafluorometaxylene, 1,1,2,3,4-hexafluorotetrachlorobutane and the like. These solvents are usable singly or at least two of them can be used in mixture. Preferred solvents have a boiling point of about 60 to about 140°C in view of the evaporation rate and the like. The kind of the monomer (h) to be used in the invention is not specifically limited insofar as the monomer (h) is copolymerizable with the monomers (a) and (g) and has a curable functional group. Examples of the monomer (h) are as follows. Hydroxyl group-containing monomers of the vinyl ether type such as CH₂=CHOCH₂CH₂CH₂CH₂OH CH₂=CHOCH₂CH(OH)CH₃ CH₂=CHOCH₂CH₂OH, those of the allyl ether type such as CH₂=CHCH₂O(CH₂)₄OH CH₂=CHCH₂O(CH₂)₂OH, those of the acrylate type such as CH₂=CHCOOCH₂CH₂OH, CH₂=C(CH₃)COOCH₂CH₂OH and the like; carboxyl-group containing monomers such as acrylic acid, methacrylic acid, itaconic acid, maleic anhydride, fumaric acid, maleic acid, α-fluoroacrylic acid, α-chloroacrylic acid and the like; epoxy group-containing monomers such as These monomers can be used singly or at least two of them are usable in mixture. The curing agent is not limited to a specific type and can be those usually used. Usable as such curing agent are tolylenediisocyanate, isophoronediisocynate and like isocyanates, blocked isocyanates, melamines, etc. These curing agents can be those commercially available. Specific examples of such curing agent which are commercially available are isocyanates with trademarks such as "Coronate EH" and "Coronate 2094" ( product of Nippon Polyurethane Co., Ltd.), "Desmodule Z4370 and N3390" (product of Sumitomo Byer Urethane Co., Ltd.), Sumidule N3200 (product of Sumitomo Byer Urethane Co., Ltd.) and the like, blocked isocyanates with trademarks such as "Coronate 2507, 2513 and 2515" (product of Nippon Polyurethane Co., Ltd.) and melamines with trademarks such as "Melane 28" (product of Hitachi Chemical Co., Ltd.), "Saimel 303" (product of Mitsui Toatsu Chemicals, Inc.) and the like. The mixing ratio of the monomers (a), (g) and (h) in the copolymer for use in the invention is usually monomer (a) / monomer (b) / monomer (h) = 40-90% : 50-9% : 20-1%, preferably 45-85% : 40-10% : 15-5%, more preferably 55-80% : 35-15% : 10-5%. If less than 40% of the monomer (a) is used, the content of fluorine atoms is diminished and thus the carrier is given an insufficient electrostatic charge capacity to result in a failure in producing the properties of the carrier to a satisfactory extent. On the other hand, if the amount of the monomer (a) exceeds 90%, the solubility of the copolymer in the solvent is reduced, entailing difficulty in coating the carrier core with the coating material. The use of the monomer (h) in an amount less than 1% renders the coating material less susceptible to curing, whereas the use of more than 20% of the monomer (h) reduces the quantity of electrostatic charge and imparts the charge to the carrier with impaired stability. Therefore the use of monomer (h) in an amount outside said range is undesirable. Preferably, the curing agent is used in such an amount that the number of functional group in the curing agent is about 1.0 to about 1.2 times the equivalence of the functional group in the copolymer. If the amount of the curing agent used is excessively small, the coating solution is less curable. In contrast, if the curing agent is used in an excessively large amount, an excessive amount thereof remains unreacted in the solution to deteriorate the electrostatic charge capacity of the carrier. Optionally the copolymer for use in the invention may further contain a copolymerizable monomer in an amount of up to about 30% by weight based on the combined amount of the monomers (a), (g) and (h) insofar as the addition will not impair the properties of the copolymer in order to improve the glass transition temperature (Tg) of the copolymer, the solvent solubility thereof and the electrostatic charge capacity of the carrier. Useful as such monomer are, for example, styrene and like additional monomers as exemplified above in the description. The coating on the carier core in the invention has a glass transition temperature (Tg) of 40°C or higher, preferably 50°C or more, after the curing of the coating. A glass transition temperature of less than 40°C tends to render the coating soft during the step of development, causing the toner to adhere to the surface of the carrier. The molecular weight of the copolymer to be used in the invention is expressed in an intrinsic viscosity of about 0.01 to about 2.0 as determined at 35°C using as a solvent chloroform or THF. The copolymer for use in the invention can be prepared as described above. Further, the composition used as a coating material for coating the carrier core in the invention may contain the same resin and/or additives as described above, such as fluorine-contained resin, silicone resin, acrylic resin and like resins and/or silica flour, charge controlling agents, surfactants, lubricants and like additives. The carrier core can be coated with the coating material by the same coating method as described above. The carriers of the invention are used in combination with a conventional toner for development of electrostatic images. Such toner is prepared by dispersing a coloring agent in a binder resin. Useful binder resins are homopolymers, copolymers or mixtures thereof, each polymer being composed of a monomer or monomers selected from the group consisting of styrenes such as styrene, p-chlorostyrene, α-methylstyrene and the like; α-methylene fatty acid monocarboxylic acid esters such as methyl acrylate, ethyl acrylate, n-propyl acrylate, butyl acrylate, lauryl acrylate, 2-ethylhexyl acrylate, methyl methacrylate, ethyl methacrylate, n-butyl methacrylate, lauryl methacrylate, 2-ethylhexyl methacrylate and the like; vinylnitriles such as acrylonitrile, methacrylonitrile and the like; vinylpyridines such as 2-vinylpyridine, 4-vinylpyridine and the like; vinyl ethers such as methyl vinyl ether, isobutyl vinyl ether and the like; vinylketones such as methyl vinylketone, ethyl vinylketone, methyl isopropenylketone and the like; unsaturated hydrocarbons and halides thereof such as ethylene, propylene, isoprene, butadiene and the like; and chloroprene and like halogen-type unsaturated hydrocarbons. Also usable as the binder resins are rosin-modified phenolformalin resin, oil-modified epoxy resin, polyester resin, polyurethane resin, polyimide resin, cellulose resin, polyether resin and like non-vinyl resins, mixtures of the non-vinyl resin and the above vinyl resin, etc. Examples of coloring agents for a toner are carbon black, Nigrosine, Aniline Blue, Calcoil Blue, Chrome Yellow, Ultramarine Blue, Methylene Blue, Rose Bengale, Phthalocyanine Blue, etc. The toner may contain wax, silica, zinc stearate and like additives, when so desired. The toner is mixed with the carrier usually in a ratio of about 0.3 to about 20 parts by weight of the former per 100 parts by weight of the latter, and the mixture is used as a developer for forming electrostatic images by magnetic brushing process, cascade process or the like. Effect of the Invention The coating layer of the carrier according to the present invention prepared from a copolymer or a composition containing the copolymer is excellent in strength, stably adheres to the core material and is therefore outstanding in durability. With this coating layer, furthermore, the first transition in the process of electrical charging of the carrier can be completed early to afford a great electrostatic charge capacity to the carrier. Examples Given below are examples and comparison examples to clarify the features of the present invention in more detail. In the examples there are used the following copolymers: Examples 1 to 3 A 15 g quantity of each of the copolymers A-1, A-2 and A-3 as mentioned hereinbefore was dissolved in a solvent of a mixture of ethyl acetate/n-butyl acetate (= 1/1) to prepare coating solutions. One kilogram of spherical iron particles (trademark "DSP 135C", product of Dowa Iron Powder Co., Ltd.) serving as the carrier core material was coated with each solution by the conventional method using a fluidized bed apparatus, giving three kinds of carriers having a coating layer of 2 µm thickness. Example 4 A 15 g quantity of the above-mentioned copolymer A-4 was dissloved in 500 ml of 3,3,4-hexafluorotetrachlorobutane to obtain a coating solution, and a carrier having a coating layer 2 µm in thickness was produced following the procedure employed in Example 1. Comparison Examples 1 and 2 Two kinds of comparative carriers having a 2 µm thick-coating layer were prepared in the same manner as in Example 1 with the exception of using a mixture of 2.8 g of an urethane resin (trademark "Coronate EH" product of Nippon Polyurethane Co., Ltd.) with 15 g of each of the copolymers represented by the following formula (1) (Comparison Example 1) and the formula (2) (Comparison Example 2), respectively. Test Example 1 Each carrier obtained in Examples 1 to 4 and Comparison Examples 1 and 2 was stirred by a ball mill for 100 hours and washed with a solvent of a 1 : 1 acetone/MEK mixture. Then the degree of peel resistance was evaluated by comparing the amounts of the coating dissolved out before and after the stirring. Table 1 shows the results. The evaluation of peel resistance was represented according to the following ratings: A....No peeling B....Peeling occurred over less than 5% of coating portion C....Peeling occurred over 5 to 10% of coating portion D....Peeling occurred over 10% or more of coating portion Table 1 shows that the carriers of the present invention had coatings of high strength with excellent adhesion. Test Example 2 A cluster of toner particles 10 µm in mean paticle size was produced by mixing together 100 parts by weight of a polystyrene-based resin (trademark "Piccolastic D135", product of Esso Standard Oil Co., Ltd.), 5 parts by weight of "Biales 155" (product of Columbia Ribbon and Manufacturing Co., Ltd.) and 5 parts by weight of "Oil Black BW" (product of Orient Chemical Ltd.). A 10 parts by weight quantity of toner thus obtained was admixed with 100 parts by weight of each of the copolymers prepared in Examples 1 to 4 and Comparison Examples 1 and 2, giving carriers. Thereafter using the carriers obtained, the quantity of electrostatic charge imparted to the toner was measured by the blow-off method with the results, together with the fluorine content of each copolymer, shown below in Table 2. In Table 2, each Roman numeral used has the following meaning. I.....Fluorine content of the copolymers (% by weight) II....The quantity of electrostatic charge Q/M (µc/g) imparted to the toners and measured by the blow-off method. Table 2 shows that each quantity of electrostatic charge imparted to the toners is large in the case of using the carriers of the present invention produced with the copolymer serving as a coating material and having a fluorine content of not less than 40% by weight. In contrast, the quantity of the electrostatic charge was noticeably small in the case of using the carriers of Comparison Examples 1 and 2 obtained with the copolymers serving as coating materials and having a fluorine content less than 40% by weight. Example 5 A 12 g quantity of the copolymer A-1 as indicated above and 3 g of an acrylic resin consisting of methyl methacrylate/ethyl mehacrylate/trifluoroethyl methacrylate (= 85/15/5, weight ratio) were dissolved in 500 ml of a solvent of a mixture of ethyl acetate/n-butyl acetate (= 1/1) to prepare a coating solution. Using this coating solution, a carrier was produced following the procedure in Example 1. The thus obtained carrier was evaluated for the degree of peel resistance with the result represented by the rating "A". Further the quantity of electrostatic charge imparted of the toner for the carrier was found to be + 24 µc/g. Examples 6 to 12 In a solvent of a mixture of acetone/methyl ethyl ketone/isopropanol (= 45/45/10, weight ratio) was dissolved each of the above-mentioned copolymers B-1 to B-4 as singly used or in the form of a mixture with the copolymers C-1 and C-2, respectively, giving coating solutions (concentration: 2%). A cluster of spherical iron particles (trademark "DSP 135C", product of Dowa Iron Powder Co., Ltd.) serving as a carrier core material was coated with each coating solution by the known fluidized spraying method, producing seven kinds of carriers having a 2 µm thick-coating layer. Table 3 shows the details of the copolymers used in Examples 6 to 12. Comparison Example 3 A carrier was obtained in the same manner as in Example 6 with the exception of using a coating solution prepared by dissolving a copolymer consisting of chlorotrifluoroethylene/alkyl vinyl ether/hydroxy-containing vinyl ether (trademark "Lumifron LF200", product of Asahi Glass Co., Ltd.) in xylene to a concentration of 2% and adding thereto isocyanate in a molar ratio of OH/NCO = 1/1.1. Test Example 3 Each carrier obtained in Examples 6 to 12 and Comparison Example 3 was stirred by a ball mill for 100 hours and washed with a solvent of a 1 : 1 acetone/MEK mixture. Then the degree of peel resistance was evaluated by comparing the amounts of the coating dissolved out before and after the stirring. Table 4 shows the results. The evaluation of peel resistance was represented according to the same ratings as in Test Example 1: Table 4 shows that the carriers of the present invention had coatings of high strength with excellent adhesion. Test Example 4 A cluster of toner particles 10 µm in mean paticle size was produced by mixing together 100 parts by weight of a polystyrene-type resin (trademark "Piccolastic D135", product of Esso Standard Oil Co., Ltd.), 5 parts by weight of "Biales 155" (product of Columbia Ribbon and Manufacturing Co., Ltd.) and 5 parts by weight of "Oil Black BW" (product of Orient Chemical Ltd.). A 10 parts by weight quantity of toner thus obtained was admixed with 100 parts by weight of each of the copolymers prepared in Examples 6 to 12 and Comparison Example 3, giving carriers. Thereafter using the carriers obtained, the quantity of electrostatic charge imparted to the toner was mesured by the blow-off method with the results shown below in Table 5. As clear from Table 5, the carriers according to the invention produced with specific copolymers can impart a large quantity of electrostatic charge to the toners. In comparison therewith, the quantity of the charge is remarkably low with regard to the carrier of Comparison Example 3. Example 13 A copolymer (intrinsic viscosity: 0.52) consisting of a 72% (percentage by weight, the same hereinafter) of chlorotrifluoroethylene (hereinafter referred to as "CTFE") and 28% of vinyl acetate (hereinafter referred to as "VAc") was dissloved in a solvent of a mixture of ethyl acetate/1,1,1-trichloroethane (= 1/1), giving a coating solution (concentration: 2.5%). A carrier core material (type: "DSPR-141", product of Dowa Iron Powder Co., Ltd.) was coated with the solution obtained above with use of a curtain flow coater (trademark "FL-MINI", manufactured by Freund Industry, Ltd.), giving a carrier having a coating layer 2 µm in thickness on dry basis. Apart from the above procedure, a cluster of toner particles having a mean particle size of 10 µm was prepared by mixing together 100 parts by weight of styrene/n-butyl methacrylate copolymer (molar ratio: 85 : 15, molecular weight: 80,000, Tg: 65°C), 2 parts by weight of a low-molecular-weight polypropylene (trademark "Viscol 660R", product of Sanyo Chemical Industry, Ltd.) and 5 parts by weight of carbon black (trademark "Regal 330R", product of Cabot Co., Ltd.), kneading and grinding the resulting mixture and classifying the particles. A developer was produced by admixing 100 parts by weight of the carrier and 3 parts by weight of the toner obtained above with use of a blender. Example 14 A developer was obtained by the same procedure as in Example 13 with the exception of using, as a starting material for production of a carrier, a copolymer (intrinsic viscosity: 0.66) consisting of 65% of CTFE and 35% of VAc. Example 15 A developer was produced in the same manner as in Example 13 with the exception of using, in the step of preparing a carrier, a copolymer (intrinsic viscosity: 0.42) consisting of 78% of CTFE and 22% of vinyl chloroacetate as dissloved in 1,1,1-trichloroethane. Example 16 A developer was produced in the same manner as in Example 13 with the exception of using, in the step of preparing a carrier, a copolymer (intrinsic viscosity: 0.28) consisting of 55% of CTFE, 35% of vinyl versate and 10% of cyclohexyl vinyl ether as dissloved in a solvent of a 1 : 1 methyl ethyl ketone/ethyl acetate mixture. Example 17 A developer was prepared by the same procedure as in Example 13 with the exception of using, in the step of preparing a carrier, a copolymer (intrinsic viscosity: 0.39) consisting of 75% of tetrafluoroethylene and 25% of vinyl benzoate as dissloved in a solvent of a mixture of 1,1,1-trichloroethane/ethyl acetate (= 1/1). Example 18 A developer was produced in the same manner as in Example 13 with the exception of using, in the step of preparing a carrier, a copolymer (intrinsic viscosity: 0.69) consisting of 55% of CTFE and 45% of vinyl pivalate as dissloved in ethyl acetate. Comparison Example 4 A developer was prepared by the same procedure as in Example 13 with the exception of using, in the step of preparing a carrier, a copolymer consisting of 80% of vinylidene fluoride and 20% of tetrafluoroethylene as dissloved in a solvent of a 1 : 1 methyl ethyl ketone/acetone mixture. Test Example 5 Using the developers obtained above in Examples 13 to 18 and Comparison Example 4, the quantity of electrostatic charge (Q/M, unit: µc/g) imparted to the toner was determined at a time immediately after the preparation of the developers and at a time after the standing thereof for 24 hours with use of a blow-off elctrostatic charge-quantity measuring apparatus (type: "TB-200", manufactured by Toshiba Chemical Co., Ltd.). Table 6 shows the results. Table 6 shows that the carriers of the present invention are electrically charged more stably than the one obtained in Comparison Example 4. Test Example 6 Each developer obtained above in Examples 13 to 18 and Comparison Example 4 was stirred by a ball mill for 300 hours and the surface of the carrier was observed with a scanning electron microscope. The coating layers of the carriers of Examples 13 to 18 exhibited no change, whereas the coating layer of the carrier produced in Comparison Example 4 was found to partially peel off. Example 19 A copolymer consisting of 55% of chlorotrifluoroethylene, 35% of vinyl versate and 10% of hydroxybutyl vinyl ether (hereinafter called "HBVE") was dissolved in a solvent of a mixture of methyl isobutyl ketone (hereinafter referred to as "MIBK")/butyl acetate (= 1/1) to a concentration of 5%. A 1 kg quantity of the resulting 5% solution was admixed with 11 g of an urethane resin (trademark "Coronate EH", product of Nippon Polyurethane Co., Ltd.), giving a coating solution. A carrier core material (type: DSPR 141, product of Dowa Iron Powder Co., Ltd.) was coated with the solution obtained above to a thickness of 2 µm on dry basis using a curtain flow coater (manufactured by Freund Industry, Ltd.), and the obtained product was heat-treated in a fluid state at a temperature of 150°C for 5 minutes. Then the product was sieved to remove the agglomerate, giving a carrier of the present invention having a mean particle size of 150 µm. Apart from the foregoing procedure, a cluster of toner particles having a mean particle size of 10 µm was prepared by mixing together 100 parts by weight of styrene/n-butyl methacrylate copolymer (molar ratio= 85/15, molecular weight: 80,000 and Tg: 65°C), 2 parts by weight of a low-molecular-weight polypropylene (trademark "Viscol 660R", product of Sanyo Chemical Industry, Ltd.) and 5 parts by weight of carbon black (trademark "Regal 330R", product of Cabot Co., Ltd.), kneading and grinding the mixture and classifying the particles. A developer was produced by admixing 100 parts by weight of the carrier and 3 parts by weight of the toner obtained above. Example 20 A developer was produced in the same manner as in Example 19 with the exception of using, as a starting material for preparing the carrier, a copolymer consisting of 60% of CTFE, 33% of vinyl acetate and 7% of HBVE. Example 21 A developer was produced by the same procedure as in Example 19 with the exception of using, in the step of preparing the carrier, a copolymer consisting of 52% of CTFE, 40% of vinyl acetate and 8% of ethylene glycol monoallyl ester as dissloved in a solvent of a mixture of toluene/MIBK/butyl acetate (= 2/1/1). Example 22 A developer was produced in the same manner as in Example 19 with the exception of using, in the step of preparing a carrier, a copolymer consisting of 50% of CTFE, 30% of vinyl pivalate, 10% of 2-hydroxypropyl vinyl ether and 10% of cyclohexyl vinyl ether as dissloved in ethyl acetate. Comparison Example 5 A developer was prepared in the same manner as in Example 19 with the exception of using, in the step of preparing a carrier, a copolymer consisting of 80% of vinylidene fluoride and 20% of tetrafluoroethylene as dissloved in a solvent of a 1 : 1 MEK/acetone mixture. Test Example 7 Using each developer obtained above in Examples 19 to 22 and Comparison Example 5 as placed into a 50 ml bottle, the quantity of electrostatic charge (Q/M, unit: µc/g) imparted to the toner was measured at a time after the stirring of the developer with a stirrer for 20 hours and at a time after the standing of the developer for 24 hours, using a blow-off electric charge-quantity-measuring apparatus (type: TB-200, manufactured by Toshiba Chemical Co., Ltd.). Table 7 shows the results. Table 7 shows that the carriers of the present invention are electrically charged more stably than the one obtained in Comparison Example 5. Test Example 8 Each developer obtained above in Examples 19 to 22 and Comparison Example 5 was stirred by a ball mill for 1 week and the surface of the carrier was observed with a scanning electron microscope. The coating layers of the carriers of Examples 19 to 22 exhibited no change, whereas the coating layer of the carrier produced in Comparison Example 5 was found to partially peel off. Example 23 A copolymer consisting of 43% of chlorotrifluoroethylene (hereinafter referred to as "CTFE") and 57% of vinyl chloride (hereinafter called "VCl") and having a molecular weight of 75,000 was dissolved in a solvent of a mixture of methyl ethyl ketone/1,2-dichloroethane (= 1/1), producing a coating solution having a solids content of 2%. A cluster of spherical steel particles having a particle size of 20 µm was coated with the solution by a known fluidized spraying method to prepare a carrier having a coating layer of 2 µm thickness on dry basis. Example 24 A copolymer consisting of a mixture of CTFE/vinylidene chloride (hereinafter referred to as "VdCl") (= 25/75, molar ratio) and having a molecular weight of 120,000 was dissloved in 1,1,1-trichloroethane, and a carrier having a coating layer 2 µm in thickness on dry basis was produced following the procedure in Example 23. Example 25 A carrier having a coating layer 2 µm in thickness on dry basis was prepared in the same manner as in Example 23 with the exception of using a coating solution obtained by dissolving a copolymer consisting of a mixture of CTFE/VCl/vinylidene fluoride (hereinafter called "VdF") (= 46.5/35/19.5, molar ratio) and having a molecular weight of 110,000 in methyl ethyl ketone. Example 26 A carrier having a coating layer 2 µm in thickness on dry basis was prepared in the same manner as in Example 23 with the exception of using a coating solution obtained by dissolving a copolymer consisting of a mixture of CTFE/VCl/vinyl acetate (= 52/42/6, molar ratio) and having a molecular weight of 80,000 in a solvent of a mixture of ethyl acetate/methyl ethyl ketone (= 2/8). Example 27 A carrier having a coating layer 2 µm in thickness on dry basis was produced by the same procedure as in Example 23 with the exception of using a coating solution obtained by dissolving a copolymer consisting of a mixture of tetrafluoroethylene (hereinafter referred to as "TFE")/VCl (= 42.8/57.2, molar ratio) and having a molecular weight of 60,000 in a solvent of a mixture of methyl ethyl ketone/acetone (= 1/1). Example 28 A carrier having a coating layer 2 µm in thickness on dry basis was prepared in the same manner as in Example 23 with the exception of using a coating solution obtained by dissolving a copolymer consisting of a mixture of TFE/VCl/styrene (= 33/49/18, molar ratio) and having a molecular weight of 75,000 in a solvent of a mixture of methyl ethyl ketone/trichloroethane (= 2/1). Comparison Example 6 A carrier having a coating layer 2 µm in thickness on dry basis was produced in the same manner as in Example 23 with the exception of using 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate copolymer (molecular weight: 100,000). Comparison Example 7 A carrier having a coating layer 2 µm in thickness on dry basis was prepared in the same manner as in Example 23 with the exception of using a copolymer (molecular weight: 100,000) consisting of a mixture of VdF/TFE (= 80/20, molar ratio) and a solvent of a mixture of acetone/methyl ethyl ketone (= 1/1). Test Example 9 Using the carriers obtained in Examples 23 to 28 and Comparison Examples 6 and 7, a test for measuring the quantity of electrostatic charge imparted to the toner was conducted as follows. A 100 parts by weight quantity of each carrier was admixed with 10 parts by weight of toner having a mean particle size of 10 µm and consisting of 100 parts by weight of styrene-based resin (trademark "Piccolastic D125", product of Shell Standard Oil Co., Ltd.), 10 parts by weight of carbon black (trademark "Regal 660R", product of Cabot Co., Ltd.) and 5 parts by weight of a low-molecular-weight polypropylene (trademark "Viscol 660P", product of Sanyo Chemical Industry, Ltd.), and the quantity of electrostatic charge imparted to the toner (Q/M, unit: µc/g) was determined by the blow-off method. Further, after the stirring of the mixture of the toner and the carrier by a ball mill for 1000 hours, the quantity of the charge imparted to the toner (Q/M, unit: µc/g) was determined once again by the blow-off method. Table 8 indicates the results. Table 8 shows that the carriers of the present invention are electrically charged more stably than those obtained in Comparison Examples 6 and 7. Example 29 A copolymer consisting of a mixture of CTFE/Pr (= 53/47, molar ratio) was dissolved in a solvent of a mixture of ethyl acetate/methyl ethyl ketone (= 1/1, weight ratio), giving a coating solution having a solids content of 2%. Thereafter a cluster of steel particles 200 µm in particle size serving as a carrier core material was coated with the solution by a known fluidized spraying method to produce a carrier having a resinous coating layer 2 µm in thickness. Example 30 A carrier having a resinous coating layer 2 µm in thickness was prepared in the same manner as in Example 29 with the exception of using a copolymer consisting of a mixture of CTFE/Pr/trifluoroethyl vinyl ether (= 51/35/14, molar ratio) and having a molecular weight of 80,000. Example 31 A copolymer consisting of a mixture of CTFE/Pr/hydroxybutyl vinyl ether (= 50/45/5, molar ratio) and having a molecular weight of 450,000 was dissolved in a solvent of a mixture of ethyl acetate/methyl ethyl ketone (= 1/1, weight ratio), giving a solution having a solids content of 2 wt%. To the solution thus obtained was added a hexanemethylenediisocyanate trimer (trademark "Coronate EH" product of Nippon Polyurethane Co., Ltd.) in an amount of 13% by weight based on the weight of the resin, producing a coating solution. Using the solution obtained, a carrier having a resinous coating layer 2 µm in thickness was prepared following the procedure in Example 29. To fully cure the resinous layer, the carrier of this example was evaluated for various properties after 7 days from the formation of the layer. Example 32 With 60 parts by weight of a fluorine-contained resin obtained in the same manner as in Example 29 was mixed 40 parts by weight of 2,2,3,3-tetrafluoromethyl methacrylate polymer (molecular weight: 100,000) to prepare a copolymer. Following the procedure in Example 29, the coploymer obtained was dissolved in a solvent and a carrier core material was coated with the thus obtained coating solution, whereby a carrier having a 2 µm-thick resinous coating layer was produced. Comparison Example 8 A comparative carrier having a 2 µm-thick coating layer was obtained in the same manner as in Example 29 with the exception of using 2,2,3,3,4,4,5,5-octafluoropentyl methacrylate polymer (molecular weight: 100,000). Comparison Example 9 A comparative carrier having a 2 µm-thick coating layer was obtained in the same manner as in Example 29 with the exception of using a copolymer (molecular weight: 100,000) consisting of a mixture of vinylidene fluoride/tetrafluoroethylene (= 80/20, molar ratio) and using a mixture of acetone/methyl ethyl ketone (= 1/1) as a solvent. Test Example 10 Using the carriers obtained in Examples 29 to 32 and Comparison Examples 8 and 9, a test for determining the quantity of electrostatic charge imparted to the toner was conducted as follows. A 100 parts by weight quantity of each carrier was admixed with 10 parts by weight of toner having a mean particle size of about 10 µm and consisting of 100 parts by weight of a styrene-based resin (trademark "Piccolastic D125", product of Esso Standard Oil Co., Ltd.), 10 parts by weight of carbon black (trademark "Regal 660R", product of Cabot Co., Ltd.) and a low-molecular-weight polypropylene (trademark "Viscol 660P", product of Sanyo Chemical Industry, Ltd.), and the quantity of electrostatic charge imparted to the toner (Q/M, unit: µc/g) was determined by the blow-off method. Further, after the stirring of the mixture of the toner and the carrier by a ball mill for 1000 hours, the quantity of the charge imparted to the toner (Q/M, unit: µc/g) was determined once again by the blow-off method. Table 9 shows the results. Table 9 shows that the carriers of the present invention are electrically charged with good stability. In contrast, the carriers of Comparison Examples 8 and 9 are electrically charged with markedly poor stability. Presumably, such poor stability is attributable to an unsatisfactory adhesion of the coating layer to the carrier core material.

Claims: A carrier for developing electrostatic images, the carrier consisting of a core and a coating on the core, the coating consisting of a fluorine containing copolymer or a composition containing the copolymer, the copolymer being prepared by copolymerisation of one of the following monomer compositions: (1) about 45 to about 90 mole % of at least one monomer selected from the group consisting of chlorotrifluoroethylene, tetrafluoroethylene, trifluoroethylene and hexafluoropropylene and about 10 to about 55 mole % of at least one of the monomers represented by the formula CH₂=CHOOCR wherein R is an alkyl, cycloalkyl or an aromatic group; (2) about 40 to about 90 mole % of at least one monomer selected from the group consisting of chlorotrifluoroethylene, tetrafluoroethylene, trifluoroethylene and hexafluoropropylene; about 9 to about 50 mole % of at least one of the monomers represented by the formula CH₂=CHOOCR wherein R is an alkyl, cycloalkyl or an aromatic group; and about 1 to about 20 mole % of at least one monomer having a functional group and copolymerizable with the above monomers.

Inventors: INUKAI HIROSHI, KITAHARA TAKAHIRO, KUBO MOTONOBU, INUKAI, HIROSHI, KITAHARA, TAKAHIRO, KUBO, MOTONOBU
Applicants: DAIKIN IND LTD, DAIKIN INDUSTRIES, LIMITED
Application year: 1989
Main classification: G03G9
IPC codes: G03G9