Developing composition for electrophotography

Abstract: This invention relates to a developing composition used in electrophotography. More specifically, this invention concerns composition of a developer for developing an electrostatic pattern formed on a photosensitive material such as a photoconductive layer by the electrophotographical method such as electrographic recording, electrographic printing or the like. The present invention provides a developing composition comprising a toner having dielectric dissipation factor with in a range of 1.5 × 10⁻³ to 3.0 × 10⁻³ and a silicon-coated carrier having electric current with in a range of 1.5 to 15 µA, and also found a developing composition comprising a toner having dielectric dissipation factor with in a range of 3.0 × 10⁻³ to 5.0 × 10⁻³ and a silicon-coated carrier having electric current with in a range of 0.1 to 1.5 µA.

Description: BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a developing composition used in electrophotography. More specifically, this invention concerns the composition of a developer for developing an electrostatic pattern formed on a photosensitive material such as a photoconductive layer by the electrophotographical method such as electrographic recording, electrographic printing or the like. Prior Art It has previously been proposed to provide a developing composition comprising a toner and a carrier for developing an electrostatic pattern formed on a photosensitive material such as a photoconductive layer by the electrographic printing method or the like. In general, composition of the toner is selected from styrene-acrylate copolymer, polyester and the like, and also composition of the carrier is selected from iron powder, ferrite and the like. The constituents of these compositions in the toner and the carrier are usually determined according to the electric properties (i.e., triboelectrification) of the developing composition against the developing condition of the electrographic system. The developing condition is not only influenced by the electric properties of the developing agent but is also influenced by the field intensity generated between the photosensitive material and the developing composition. However, the suitable relationship between the electric properties of the developing composition and the field intensity have not been cleared by the persons who are skilled in the art. Accordingly, it is difficult to determine the above factors in appropriate manner and thus there are some problems in the conventional developing process, i.e., scattering of the toner, carrier on the copy, increase of back ground, lacking distinction of the picture on the photosensitive material, and the like. SUMMARY OF THE INVENTION It is an object of the present invention to provide a developing composition used in an electrophotographic developing process having predetermined field intensity and improved processing characteristics. As the result of our researches to remove the above disadvantages, we have found an electrophotographic developer composition comprising a toner having a dielectric dissipation factor tan δ from 1.5 X 10⁻³ to 3.0 X 10⁻³ at a frequency of 1 kHz and a silicon-coated carrier having an electric current value from 1.5 to 15 µA or a toner having a dielectric dissipation factor tan δ from 3.0 X 10⁻³ to 5.0 X 10⁻³ at a frequency of 1 kHz and a silicon-coated carrier having an electric current value from 0.1 to 1.5 µA. Furthermore, we have found that these developers provide the optimum developing condition when the developing system has a predetermined electrical field intensity characteristics within a quadrangular region bounded by a straight line running from a point A to a point B, a straight line running from the point B to a point C, a straight line running from the point C to a point D, and a straight line running from the point D to the point A, such that when the electrical field intensity E of a photosensitive material such as a phoconductive layer in volts per meter is given by the relationship: E = V/d ( where V is the surface potential of the photoconductive layer in volts and d is the distance between a developing roller and the photoconductive layer in meters), the points A, B, C, D are defined by : (a) the surface potential is 600 V at the point A when an electrical field intensity of 0.6 x 10⁶ V/m is applied at the point A; (b) the surface potential is 800 V at the point B when an electrical field intensity of 0.8 x 10⁶ V/m is applied at the point B; (c) the surface potential is 800 V at the point C when an electrical field density of 1.6 x 10⁶ V/m is applied at the point C; and (d) the surface potential is 600 V at the point D when an electrical field intensity of 1.2 x 10⁶ V/m is applied at the point D. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows the surface potential as a function of the field intensity and the distance between a developing roller and a photoconductive layer. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The surface potential is determined by a voltmeter prove which is provided in a developing unit of the developing system. A result obtained by the measurement is applied to the following formula which calculates a value of the field intensity: E = V /d wherein V is the surface potential of the photoconductive layer with in a range of 600 to 800 volts; and d is the distance between a developing roller and the photoconductive layer with in a range of 1 x 10⁻³m to 5 x 10⁻⁴ m. It is preferable that the resulting value of the field intensity is in the dashed region of Figure 1. Because when E and V are out of the dashed region of Figure 1, several disadvantages such as scattering of the toner and/or, increasing of back ground on photoconductive layer, lacking distinction of the picture, or the like, can be produced. In this invention, the dielectric dissipation factor of the toner is measured by an electrode for solid material(AS-20438, SE-70. Ando Electronic Co., LTD.) and a capacitance-measuring assembly (1620 Capacitance-Measuring Assembly, Tokyo Electron Co., LTD.). After measuring, dielectric dissipation factor (tan δ) is calculated by following formula: tanδ = G / 2πfC wherein C is a value of the capacitance; f is a value of the frequency; and G is a value of the conductance. A sample provided for the above measurement is prepared by forming the toner (2.5 g) into a pellet with an oil pressure pump (under 200 kg/m² for 30 minutes). In a preferred embodiment of the present invention, the dielectric dissipation factor is in the range of 1.5 X 10⁻³ to 3.0 X 10⁻³ at a frequency of 1 kHz when a value of electric current of the carrier is in the range of 1.5 to 15 µA . In another preferred embodiment of the present invention, the dielectric dissipation factor is in the range of 3.0 X 10⁻³ to 5.0 X 10⁻³ at a frequency of 1 kHz when a value of electric current of the carrier is in the range of 0.1 to 1.5 µA. The value of the dielectric dissipation factor is varied by changing the composition of the toner or by modifying a mixing process thereof. For example, the value of the dielectric dissipation factor rises in proportion to the rise in the content of carbon-black(e.g., Kechen Black EC. Lion Akzo Co., LTD.) which is a coloring agent of the toner. Also, the value of the dielectric dissipation factor rises in proportion to decrease in the viscosity of the toner by mixing thereof at a high temperature. In contrast, the value of the dielectric dissipation factor is decreased when the content of methylmetacrylate (hereinafter, referred as MMA) in the monomer composition, or when the resistance of the binding resin rises. In the case of the composition in which the monomer of the binding resin is styrene/butyl acrylate/MMA, the resistance value of the polymer can be varied in proportion to the rise or decrease in the content of MMA. The value of the dielectric dissipation factor can also be varied according to the resistivity of the binding resin. For example, the value of the dielectric dissipation factor can be decreased according to rising the resistivity of the binding resin depended by increasing the weight average molecular weight (hereinafter, referred as Mw) of the composition of the monomer without increasing the number average molecular weight (hereinafter, referred as Mn) of the composition. Furthermore, the value of the dielectric dissipation factor can be varied by adding inorganic particles into the toner or by placing the particles on a surface of the toner. This inorganic powder is preferably selected from magnesium oxide, zirconium oxide, aluminum oxide, tungsten carbide, tungsten trioxide, silicone nitride, titanium oxide, carbon, magnetite and the like. The conventional resin, such as styrene resin, polyacrylate ester resin, styrene-acrylate ester copolymer resin, polyvinyl chloride resin, polyvinyl acetate resin, phenolic resin, epoxy resin, polyester resin, and the like, can be used as binding resin for the present developing composition. Also, the conventional coloring agent, such as carbon black, mono-azo dye of red, dis-azo dye of yellow, a quinacridon magenta dye, anthraquinone dye, and the like, can be used as a coloring agent for the present developing composition. Furthermore, a charge-controlling agent, such as nigrosine dye, alkoxy amine, quaternary ammonium salt, alkylamide, metal complex salt of mono-azo dye, electron acceptable organic complex, or polyolefine chloride, can be used in the developing composition of the present invention and the agent can also be selected according to the predetermined electrification. Another additional agents are follows: resin powder such as polyethylene, polyacryle, or the like, powder such as titanium dioxide, conductive titan, zinc, or the like, lubricant agent such as metal salt of high quality fatty acid, hydrophobic silica, colloidal silica, and the like. For preparing the toner, the above described resin, colouring agent, charge-controlling agent and another additional agents are mixed with each other according to predetermined formula. Then, the mixture is subjected to melting and grinding processes to obtain the toner. The developing composition according to the present invention comprises a resin coated carrier, which is prepared by coating the surface of the core material with a silicone resin, solidifying at room temperature by the conventional method. The core material is preferably iron powder, ferrite, or the like, and also the silicone resin is preferably selected from the group of the compounds having following structures or the like. A current value of the silicone coated carrier can be restricted by a type of core material composition and the method thereof, or content of silicone resin coated on the surface of the core material. The invention will be more clearly understood with reference to the following examples. Unless otherwise stated, quantities are expressed as parts by weight. Example 1: In the first step, following compounds were mixed by a super mixer to obtain a mixture. styrene-acrylate co-polymer resin 100 parts (Mn = 3500, Mw = 136,500, Mw / Mn = 39) carbon black 5 parts (#40, Mitsubishi Chemical Industries Co., LTD.) quaternary ammonium salt 2 parts (Bontron P-51, Orient Chemical Industries Co., LTD.) polypropylene 3 parts (Viscol 550P, Sanyo Chemical Industries Co., LTD.) The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 2.6 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz of the frequency. In the second step, a developing composition of the present invention was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 6.8 µA of current value by ball mill for 1 hour. Example 2: In the first step, following compounds were mixed by a super mixer to obtain a mixture. styrene-acrylate co-polymer resin 100 parts (Mn = 5200, Mw = 260,000, Mw / Mn = 50) carbon black 5 parts (#40, Mitsubishi Chemical Industries Co., LTD.) quaternary ammonium salt 2 parts (Bontron P-51, Orient Chemical Industries Co., LTD.) polypropylene 3 parts (Viscol 550P, Sanyo Chemical Industries Co., LTD.) The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 1.6 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz of the frequency. In the second step, a developing composition of the present invention was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 8.2 µA of current value by ball mill for 1 hour. Example 3: In the first step, following compounds were mixed by super mixer to obtain a mixture. styrene-acrylate co-polymer resin 100 parts (Mn = 4000, Mw = 124,000, Mw / Mn = 31) carbon black 5 parts (#40, Mitsubishi Chemical Industries Co., LTD.) quaternary ammonium salt 2 parts (Bontron P-51, Orient Chemical Industries Co., LTD.) polypropylene 3 parts (Viscol 550P, Sanyo Chemical Industries Co., LTD.) The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 4.4 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz of the frequency. In the second step, a developing composition of the present invention was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 1.5 µA of current value by ball mill for 1 hour. Example 4: In the first step, following compounds were mixed by a super mixer to obtain a mixture. styrene-acrylate co-polymer resin 100 parts (Mn = 4500, Mw = 140,000, Mw / Mn = 31.1) carbon black 5 parts (#40, Mitsubishi Chemical Industries Co., LTD.) quaternary ammonium salt 2 parts (Bontron P-51, Orient Chemical Industries Co., LTD.) polypropylene 3 parts (Viscol 550P, Sanyo Chemical Industries Co., LTD.) The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 3.8 X 10⁻³ of dielectric dissipation factor was obtained. In the second step, a developing composition of the present invention was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 0.25 µA of current value by ball mill for 1 hour. Control 1: In the first step, following compounds were mixed by a super mixer to obtain a mixture. The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 1.3 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz of the frequency. In the second step, a developing composition was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 13.8 µA of current value by ball mill for 1 hour. Control 2: In the first step, following compounds were mixed by a super mixer to obtain a mixture. The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 7.2 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz of the frequency. In the second step, a developing composition was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 11.2 µA of current value by ball mill for 1 hour. Control 3: In the first step, following compounds were mixed by super mixer to obtain a mixture. styrene-acrylate co-polymer resin 100 parts (Mn = 4000, Mw = 116,000, Mw / Mn = 29) carbon black 5 parts (#40, Mitsubishi Chemical Industries Co., LTD.) quaternary ammonium salt 2 parts (Bontron P-51, Orient Chemical Industries Co., LTD.) polypropylene 3 parts (Viscol 550P, Sanyo Chemical Industries Co., LTD.) The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 5.2 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz. In the second step, a developing composition was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 10.6 µA of current value by ball mill for 1 hour. Control 4: In the first step, following compounds were mixed by a super mixer to obtain a mixture. styrene-acrylate co-polymer resin 100 parts (Mn = 4500, Mw = 207,000, Mw / Mn = 46) carbon black 5 parts (#40, Mitsubissi Chemical Industries Co., LTD.) quaternary ammonium salt 2 parts (Bontron P-51, Orient Chemical Industries Co., LTD.) polypropylene 3 parts (Viscol 550P, Sanyo Chemical Industries Co., LTD.) The mixture was ground to powder after subjecting to melting and mixing processes. As a result, the toner having 12 µm of an average particle size and 1.8 X 10⁻³ of dielectric dissipation factor was obtained at 1 kHz of the frequency. In the second step, a developing composition was obtained by mixing 4 parts of the above resulting toner with 100 parts of silicone-coated ferrite carrier having 1.2 µA of current value by ball mill for 1 hour. All of the developers described above were tested by printing 50,000 sheets of papers by the printer (presetting to 750 volts of the surface potential and 1.5 X 10⁶ V/m of the field intensity) consecutively . The results of each test was shown in table I to VIII. In these tables, tone grade was measured by Machbeth illuminometer, the back ground was determined by color and color difference meter, extent of toner dispersion was determined visually and checked by a standard ( each symbol means as follows: o ○ means negative; ○ means a small quantity of dispersed toner; Δ means a medium quantity of dispersed toner; and × means a large quantity of dispersed toner), and tone grade was determined by copying Kodak Gray Scale pattern (Eastman Kodak Co., LTD.). In table I to VIII, the tone grade is expressed by the number as the grade becomes higher one in proportion to increase the number. As shown in table I to VIII, the developing composition of the present invention provides fine prints up to 50,000 printing in contrast with the control one.

Claims: An electrophotographic developer composition comprising a toner having a dielectric dissipation factor tan δ 1.5 x 10⁻³ to 3.0 x 10⁻³ at a frequency of 1 kHz and a silicon-coated carrier having an electrical current value from 1.5 to 15 µA or a toner having a dielectric dissipation factor tan δ from 3.0 x 10⁻³ to 5.0 x 10⁻³ at a frequency of 1 kHz and a silicon-coated carrier having an electrical current value from 0.1 to 1.5 µA. An electrophotographic developer composition according to claim 1, the developer being used in a developing system having electrical field intensity characteristics within a quadrangular region bounded by a straight line running from a point A to a point B, a straight line running from said point B to a point C, a straight line running from said point C to a point D, and a straight line running from said point D to said point A, such that when the electrical field intensity E of a photoconductive layer in volts per meter is given by the relationship: E = V/d, where V is the surface potential of said photoconductive layer in volts and d is the distance between a developing roller and said photoconductive layer in meters, said points A, B, C, D are defined by : (a) the surface potential is 600 V at said point A when an electrical field intensity of 0.6 x 10⁶ V/m is applied at said point A; (b) the surface potential is 800 V at said point B when an electrical field intensity of 0.8 x 10⁶ V/m is applied at said point B; (c) the surface potential is 800 V at said point C when an electrical field intensity of 1.6 x 10⁶ V/m is applied at said point C; and (d) the surface potential is 600 V at said point D when an electrical field intensity of 1.2 x 10⁶ V/m is applied at said point D.

Inventors: SHIBATA KOHSEI C O TOMOEGAWA P, SHIBATA, KOHSEI, C/O TOMOEGAWA PAPER CO.
Applicants: TOMOEGAWA PAPER CO LTD, TOMOEGAWA PAPER CO. LTD.
Application year: 1990
Main classification: G03G9
IPC codes: G03G9