POSITIVELY-CHARGEABLE TONER FOR DEVELOPING ELECTROSTATIC IMAGES AND METHOD FOR PRODUCING THE SAME

Abstract

Provided is a positively-chargeable toner for developing electrostatic images excellent in printing durability, charge stability, and conveyance amount stability. The toner comprises colored resin particles comprising a binder resin and a colorant, and an external additive, wherein the external additive comprises spherical silica particles with a number average particle diameter of 70 to 200 nm. wherein the external additive has a particle size distribution in which a percentage of a number of particles having a particle diameter of 50 nm or more is 75% or more, which is calculated based on a SEM image.

Claims

1. A positively-chargeable toner for developing electrostatic images, comprising colored resin particles comprising a binder resin and a colorant, and an external additive, wherein the external additive comprises spherical silica particles; wherein a number average particle diameter of the spherical silica particles is from 70 nm to 200 nm; and wherein the external additive has a particle size distribution in which a percentage of a number of particles having a particle diameter of 50 nm or more is 75% or more, which is calculated based on a SEM image of the external additive.

2. The positively-chargeable toner for developing electrostatic images according to claim 1, wherein a content of the external additive is from 0.5 part by mass to 5.0 parts by mass with respect to 100 parts by mass of the colored resin particles.

3. The positively-chargeable toner for developing electrostatic images according to claim 1, wherein the positively-chargeable toner comprises inorganic oxide particles having a number average particle diameter of from 5 nm to 25 nm, as the external additive.

4. The positively-chargeable toner for developing electrostatic images according to claim 3, wherein a content of the inorganic oxide particles is from 0.01 part by mass to 0.4 part by mass with respect to 100 parts by mass of the colored resin particles.

5. A method for producing a positively-chargeable toner for developing electrostatic images, the method comprising the steps of: producing colored resin particles comprising a binder resin and a colorant, and external addition in which an external additive is added on surfaces of the colored resin particles by mixing and stirring the colored resin particles with the external additive, wherein, in the external addition step, an external additive which comprises spherical silica particles having a number average particle diameter of from 70 nm to 200 nm and has a particle size distribution in which a percentage of a number of particles having a particle diameter of 50 nm or more is 75% or more, which is calculated based on a SEM image of the external additive, is used.

6. The production method according to claim 5, wherein the external additive comprises inorganic oxide particles having a number average particle diameter of from 5 nm to 25 nm, and wherein the inorganic oxide particles are added on the colored resin particles, and the spherical silica particles are further added on the colored resin particles on which the inorganic oxide particles are added.

Description

EXAMPLES

[0137] Hereinafter, the present disclosure will be described further in detail with reference to examples and comparative examples. However, the present disclosure is not limited to these examples. Herein, parts and % are based on mass unless otherwise noted.

[0138] Test methods performed in the examples and the comparative examples are as follows.

[0139] 1. Preparation of Spherical Silica Particles

[Production Example 1]

[0140] In a 3 L glass reactor equipped with a stirrer, a dropping funnel and a thermometer, 623.7 g of methanol, 41.4 g of water, and 49.8 g of 28% aqueous ammonia were added and mixed, and the temperature of the mixed solution was adjusted to be 35 C.

[0141] While stirring the mixed solution in which the temperature was adjusted, dropwise addition of 1250 g of a mixture of tetramethoxysilane and tetrabutoxysilane and dropwise addition of 418.1 g of 5.4% aqueous ammonia were started at the same time. Here, 1250 g of the mixture of tetramethoxysilane and tetrabutoxysilane was added dropwise over 8.5 hours, and the 5.4% aqueous ammonia was added dropwise over 5 hours, respectively.

[0142] Even after finishing the dropping, the hydrolysis was performed by further continuing the stir of the mixed solution for 0.5 hour to obtain a suspension of spherical silica particles.

[0143] Subsequently, an ester adapter and a condenser tube were mounted on the 3 L glass reactor, and the obtained suspension of spherical silica particles was heated up to a temperature of 60 C. to 70 C. to distill off (distill and remove) methanol. Then, water was added thereto, and the suspension was heated up to a temperature of 70 C. to 90 C. to completely distill off (distill and remove) methanol, thereby obtaining an aqueous suspension of spherical silica particles.

[0144] While stirring the obtained aqueous suspension of spherical silica particles, dropwise addition of methyltrimethoxysilane was started at room temperature and 11.6 g of methyltrimethoxysilane was added dropwise over 0.5 hours. Even after finishing the dropping, hydrophobizing treatment was performed by further continuing the stir of the aqueous suspension for 12 hours.

[0145] To the hydrophobized aqueous suspension, 1440 g of methyl isobutyl ketone was added, thereafter, the aqueous suspension was heated up to a temperature of 80 C. to 110 C. to distill off (distill and remove) an azeotropic mixture over 10 hours, and then the aqueous suspension was cooled to room temperature.

[0146] After 1000 g of methanol was added to the cooled aqueous suspension and the mixture was stirred for 10 minutes, the mixture was processed with a centrifuge at 3000 G for 10 minutes to separate a supernatant liquid. After distilling off the solvents, methyl isobutyl ketone and methanol, from the residual liquid, the obtained matter was dried to obtain spherical silica particles.

[0147] To 100 g of the dried spherical silica particles, 10 g of hexamethyldisilazane and 10 g of a compound represented by the following formula 1 as a cyclic silazane were added as hydrophobizing agents at room temperature, and then the mixture was heated up to 110 C. and reacted for hours, whereby the spherical silica particles were hydrophobized.

[0148] Subsequently, the mixture was heated up to 80 C. under reduced pressure (6650 Pa) to completely distill off (distill and remove) the solvent to prepare spherical silica particles (silica a, number average particle diameter: 90 nm, sphericity: 1.12).

##STR00001##

[Production Example 2 to Production Example 6]

[0149] Spherical silica particles (silica b to silica f) of Production Example 2 to Production Example 6 were prepared similarly as in Production Example 1 except that, in Production Example 1, the dropping amount and dropping time of the mixture of tetramethoxysilane and tetrabutoxysilane were changed as shown in Table 1 below, and subjected to the tests.

[0150] Preparation conditions of the silica in Production Examples 1 to 6 and physical properties of the each silica thus produced are shown in Table 1 below. The dropping amount and dropping time in Table 1 below mean a dropping amount and dropping time of the mixture of tetramethoxysilane and tetrabutoxysilane.

TABLE-US-00001 TABLE 1 Production Production Production Production Production Production Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Silica a Silica b Silica c Silica d Silica e Silica f Preparation Dropping 1250 1250 1250 1700 1500 1300 condition amount (g) Dropping 8.5 7.0 8.0 4.0 5.0 6.0 time (h) Property Number 90 80 85 30 50 70 average particle diameter (nm) Sphericity 1.12 1.11 1.12 1.13 1.15 1.13

[0151] 2. Production of Positively-Chargeable Toner for Developing Electrostatic Images

Example 1

[0152] 78 parts of styrene and 22 parts of n-butyl acrylate as polymerizable monomers, and 5 parts of carbon black (manufactured by Mitsubishi Chemical Corporation, product name: #25BS) as a black colorant were dispersed using an in-line type emulsifying and dispersing machine (manufactured by Pacific Machinery & Engineering Co., Ltd., product name: MILDER) to obtain a polymerizable monomer mixture.

[0153] To the polymerizable monomer mixture, 1.0 parts of a charge control resin (quaternary ammonium group-containing styrene acrylic resin) as a charge control agent, 5.0 parts of a fatty acid ester wax (behenyl behenate) as a release agent, 0.3 part of a polymethacrylic acid ester macromonomer (manufactured by TOAGOSEI CO., LTD., product name: AA6) as a macromonomer, 0.6 part of divinylbenzene as a crosslinkable polymerizable monomer, and 1.6 parts of t-dodecyl mercaptan as a molecular weight modifier were added, then mixed and dissolved to prepare a polymerizable monomer composition.

[0154] On the other hand, an aqueous solution containing 7.2 parts of sodium hydroxide (alkali metal hydroxide) dissolved in 50 parts of ion-exchanged water was gradually added to an aqueous solution containing 12.2 parts of magnesium chloride (water-soluble polyvalent metal salt) dissolved in 250 parts of ion-exchanged water at room temperature while stirring to prepare a magnesium hydroxide colloid (hardly water-soluble metal hydroxide colloid) dispersion.

[0155] The polymerizable monomer composition was charged into the magnesium hydroxide colloid dispersion at room temperature and stirred. After charging 4.4 parts of t-butylperoxydiethyl acetate as a polymerization initiator thereinto, the mixture was subjected to a high shear stirring at 15,000 rpm for 10 minutes and dispersed, using an in-line type emulsifying and dispersing machine (manufactured by Pacific Machinery & Engineering Co., Ltd., product name: MILDER), to form droplets of the polymerizable monomer composition.

[0156] The suspension in which the droplets of the polymerizable monomer composition were dispersed (polymerizable monomer composition dispersion) was charged into a reactor furnished with a stirring blade and the temperature thereof was raised to 90 C. to start a polymerization reaction. When the polymerization conversion reached almost 100%, 1 parts of methyl methacrylate as a polymerizable monomer for shell and 0.3 part of 2,2-azobis(2-methyl-N-(2-hydroxyethyl)-propionamide) (manufactured by Wako Pure Chemical Industries, Ltd., product name: VA-086, water-soluble) dissolved in 10 parts of ion-exchanged water were added. After continuing a reaction at 90 C. for 4 hours, the reactor was cooled by water to stop the reaction to obtain an aqueous dispersion of colored resin particles having a core-shell type structure.

[0157] The aqueous dispersion of colored resin particles was subjected to acid washing, in which sulfuric acid was added dropwise to be pH of 6.5 or less while stirring at room temperature. Subsequently, filtration separation was performed, and 500 parts of ion-exchanged water was added to the obtained solid content to make a slurry again, then a water washing treatment (washing, filtration and dehydration) was repeatedly performed several times. Next, filtration separation was performed, and the obtained solid content was placed in a container of a dryer and dried at 45 C. for 48 hours to obtain dried colored resin particles. The obtained colored resin particles had a volume average particle diameter (Dv) of 7.5 m, a number average particle diameter (Dn) of 6.6 m, a particle size distribution (Dv/Dn) of 1.14, and an average degree of circularity of 0.987.

[0158] To 100 parts of the above-obtained colored resin particles, 0.1 part of positively-chargeable silica particles having a number average particle diameter of 7 nm (manufactured by Cabot, product name: TG820F), the surfaces of which had been hydrophobized by hexamethyldisilazane and a cyclic silazane as hydrophobizing agents was added, and an external addition treatment was performed by mixing and stirring at a circumferential speed of 40 m/s for 10 minutes, using a high speed stirrer (manufactured by NIPPON COKE & ENGINEERING. CO., LTD., product name: FM MIXER). Thereafter, 3.6 parts of silica a (number average particle diameter: 90 nm, sphericity: 1.12) as spherical silica particles was added to the obtained mixture, and an external addition treatment was performed by mixing and stirring at a circumferential speed of 40 m/s for 10 minutes, using a high speed stirrer to obtain a toner of Example 1. The test results are shown in Table 1.

[Example 2 to Example 9 and Comparative Example 1 to Comparative Example 4]

[0159] Toners of Example 2 to Example 9 and Comparative example 1 to Comparative example 4 were prepared similarly as in Example 1 except that, in the external addition treatment of Example 1, the type and content of the spherical silica particles and the content of the inorganic oxide particles were changed as shown in Table 2 below, and subjected to the tests.

[0160] 3. Evaluation of Spherical Silica Particles

(1) Measurement of Number Average Particle Diameter

[0161] The number average particle diameter of the silica a to silica f was measured by the following method.

[0162] First, scanning electron micrographs of each spherical silica particle were taken, and diameters of the equivalent circles corresponding to projected areas of the particles in the micrographs were calculated under conditions of an area ratio of particles to a frame area of up to 2% and a total number of processed particles of 100, by an image processing analyzer (manufactured by NIRECO CORPORATION, product name: LUZEX IID). The arithmetic mean value thereof was taken as the number average particle diameter of the spherical silica particles.

[0163] (2) Measurement of Sphericity

[0164] The sphericity of the silica a to silica f was measured by the following method.

[0165] First, scanning electron micrographs of each spherical silica particle were taken. Using the same image processing analyzer and under the same analysis conditions as in the measurement of the number average particle diameter, the area (Sc) of a circle with a diameter being the same as an absolute maximum length of a particle and the real projected area (Sr) of the particle in the micrographs were analyzed. The sphericity (Sc/Sr) was calculated from these analytical values, and the arithmetic mean value thereof was taken as the sphericity of the spherical silica particles.

[0166] 4. Calculation of Particle Size Distribution of External Additive

[0167] The particle size distribution of the external additive used for each of the toners of Example 1 to Example 9 and Comparative example 1 to Comparative example 4 was calculated by the following method.

[0168] First, a surface of the toner particles after external addition was observed using a scanning electron microscope (SEM) (manufactured by JEOL Ltd., model number: JSM-7610F), and 20 SEM images were taken at a magnification of 45,000 times. The particle size distribution of the external additive was obtained by performing image processing and image analysis on the SEM images and calculating the size (particle diameter) of the external additive present on the toner particle surface and the number frequency of each size. From this particle size distribution, a percentage of a number of particles having a particle diameter of 50 nm or more was calculated.

[0169] 5. Evaluation of Positively-Chargeable Toner for Developing Electrostatic Images

[0170] Evaluation of toner was carried out for each of the toners of Example 1 to Example 9 and Comparative example 1 to Comparative example 4. Details are as follows.

[0171] (1) Printing Durability Test

[0172] In this test, using a commercially-available, non-magnetic one-component development printer (printing speed: 40 sheets of A4 size/min), the toner was charged into a toner cartridge of a development device, and then printing sheets were set.

[0173] After being left in a normal temperature and normal humidity (N/N) environment (temperature: 23 C., humidity: 50%) for 24 hours, in the same environment, 20,000 sheets were continuously printed at a printing density of 5%.

[0174] Black solid pattern printing (100% printing density) was performed at every 500th sheet, and the printing density of the black solid pattern printed image was measured using a reflection densitometer (manufactured by Macbeth, product name: RD918). Further, thereafter, white solid pattern (0% printing density) was printed, and the printer was stopped in the middle of the white solid pattern-printing, then the toner in a non-image area on a photoconductor after development was attached to an adhesive tape (manufactured by Sumitomo 3M Limited, product name: SCOTCH MENDING TAPE 810-3-18). Then, the tape was removed therefrom and attached to a printing sheet.

[0175] Next, a whiteness degree (B) of the printing sheet, on which the adhesive tape had been attached, was measured by means of a whiteness meter (manufactured by Nippon Denshoku Industries Co., Ltd., product name: ND-1). Similarly, only an unused adhesive tape was attached on the printing sheet, and a whiteness degree (A) thereof was measured. Then, a difference (B-A) between these whiteness degrees was taken as a fog value (%). The smaller value indicates that fog is less, and image quality is better.

[0176] The number of continuously printed sheets that could maintain an image quality at a printing density of 1.3% or more and a fog value of 3% or less was examined.

[0177] Incidentally, in Table 2 below, >20000 indicates that the image quality at a printing density of 1.3% or more and a fog value of 3% or less could be maintained even at the time of 20,000 sheets.

[0178] (2) Charge Stability and Conveyance Amount Stability Tests

[0179] The same printer as in the printing durability test was used, then the toner was filled and the printing sheets were set therein.

[0180] After being left in a normal temperature and normal humidity (N/N) environment (temperature: 23 C., humidity: 50%) for 24 hours, in the same environment, 20,000 sheets were continuously printed at a printing density of 5%.

[0181] At every 500th sheet, the charge amount and the toner suction amount were measured for a toner mounted on a developing roll by a suction-type actual machine charge amount measurement device.

[0182] A value (Q2/Q1) obtained by dividing the toner charge amount (Q2) at the time of printing of 20,000 sheets by the toner charge amount (Q1) at the start of continuous printing was used as an index of charge stability. As this indicator (Q2/Q1) is close to 1, it means that a fluctuation of the toner charge amount is smaller, which means that the toner is excellent in charge stability.

[0183] The toner suction amount was regarded as the conveyance amount of the toner. A value (M2/M1) obtained by dividing the conveyance amount of toner (M2) at the time of printing of 20,000 sheets by the conveyance amount of toner (M1) at the start of measurement was used as an index of the conveyance amount stability. As this indicator (M2/M1) is close to 1, it means that a fluctuation of the toner conveyance amount is smaller, which means that the toner is excellent in conveyance amount stability.

[0184] The measurement and evaluation results of the toners of Example 1 to Example 9 and Comparative example 1 to Comparative example 4 are shown in Table 2 below together with the information of the external additive.

TABLE-US-00002 TABLE 2 Example 1 Example 2 Example 3 Example 4 Example 5 Example 6 Example 7 Type of Spherical Silica a Silica b Silica c Silica c Silica c Silica c Silica a silica particles Number average 90 80 85 85 85 85 90 particle diameter (nm) Sphericity 1.12 1.11 1.12 1.12 1.12 1.12 1.12 Content (parts) 3.6 2.4 0.70 2.0 2.7 1.2 3.6 Content of Inorganic 0.1 0.1 0.1 0.1 0.1 0.1 0 oxide particles (parts) Whole content of 3.7 2.5 0.8 2.1 2.8 1.3 3.6 External additive (parts) Percentage of a number 89 85 80 87 88 84 91 of particles having a particle diameter of 50 nm or more in External additive (%) Printing durability >20000 >20000 >20000 >20000 >20000 >20000 20000 (sheets) Charge stability (Q2/Q1) 0.73 0.99 0.86 0.72 0.74 0.85 0.72 Conveyance amount 0.95 1.07 1.58 1.45 1.03 1.35 1.04 stability (M2/M1) Comparative Comparative Comparative Comparative Example 8 Example 9 Example 1 Example 2 Example 3 Example 4 Type of Spherical Silica b Silica c Silica d Silica e Silica f Silica f silica particles Number average 80 85 30 50 70 70 particle diameter (nm) Sphericity 1.11 1.12 1.13 1.15 1.13 1.13 Content (parts) 2.4 2.7 1.1 1.9 0.70 1.0 Content of Inorganic 0 0 0.1 0.1 0.5 0.3 oxide particles (parts) Whole content of 2.4 2.7 1.2 2.0 1.2 1.3 External additive (parts) Percentage of a number 88 92 1 48 52 68 of particles having a particle diameter of 50 nm or more in External additive (%) Printing durability 20000 20000 7000 10000 12000 14000 (sheets) Charge stability (Q2/Q1) 0.96 0.73 0.34 0.48 0.51 0.64 Conveyance amount 1.11 1.07 1.37 2.04 1.61 1.50 stability (M2/M1)

[0185] 6. Consideration

[0186] Hereinafter, the evaluation results of the toner will be studied with reference to Table 2.

[0187] From Table 2, the toner of Comparative example 1 is a toner containing silica d having a number average particle diameter of nm. In the toner of Comparative example 1, the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is 1%.

[0188] From Table 2, the number of sheets evaluated for printing durability of Comparative example 1 is as low as 7,000 sheets, and the charge stability (Q2/Q1) is as low as 0.34. In particular, the number of sheets evaluated for printing durability is the smallest among the toners evaluated this time. In addition, the value of charge stability (Q2/Q1) is the lowest among the toners evaluated this time. Accordingly, it can be seen that the toner of Comparative example 1 which contains spherical silica particles having a number average particle diameter of less than 70 nm, and in which the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is much less than 75% is inferior in printing durability and charge stability.

[0189] From Table 2, the toner of Comparative example 2 is a toner containing silica e having a number average particle diameter of 50 nm. In the toner of Comparative example 2, the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is 48%.

[0190] From Table 2, the number of sheets evaluated for printing durability of Comparative example 2 is as low as 10,000 sheets, the charge stability (Q2/Q1) is as low as 0.48, and the conveyance amount stability (M2/M1) is as high as 2.04. In particular, the value of conveyance amount stability (M2/M1) is the highest among the toners evaluated this time. Accordingly, it can be seen that the toner of Comparative example 2 which contains spherical silica particles having a number average particle diameter of less than 70 nm, and in which the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is less than 75% is inferior in printing durability, charge stability, and conveyance amount stability.

[0191] From Table 2, the toner of Comparative example 3 is a toner containing silica f having a number average particle diameter of 70 nm. In the toner of Comparative example 3, the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is 52%.

[0192] From Table 2, the number of sheets evaluated for printing durability of Comparative example 3 is as low as 12,000 sheets, the charge stability (Q2/Q1) is as low as 0.51, and the conveyance amount stability (M2/M1) is as high as 1.61. Accordingly, it can be seen that the toner of Comparative example 3 in which the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is less than 75% is inferior in printing durability, charge stability, and conveyance amount stability.

[0193] From Table 2, the toner of Comparative example 4 is a toner containing silica f having a number average particle diameter of 70 nm. In the toner of Comparative example 4, the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is 68%.

[0194] From Table 2, the number of sheets evaluated for printing durability of Comparative example 4 is as low as 14,000 sheets, and the charge stability (Q2/Q1) is as low as 0.64. Therefore, it can be seen that the toner of Comparative example 4 in which the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is less than 75% is inferior in printing durability and charge stability.

[0195] On the other hand, from Table 2, the toners of Example 1 to Example 9 are toners containing any one of silica a to silica c having a number average particle diameter of from 80 nm to 90 nm. In the toners of Example 1 to Example 9, the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is from 80% to 92%.

[0196] From Table 2, the number of sheets evaluated for printing durability of Example 1 to Example 9 is 20,000 sheets or more, the charge stability (Q2/Q1) falls within the range of 0.72 to 0.99, and the conveyance amount stability (M2/M1) falls within the range of 0.95 to 1.58.

[0197] Accordingly, it can be seen that the toners of Example 1 to Example 9 in which the number average particle diameter of spherical silica particles is from 70 nm to 200 nm, and the percentage of the number of particles having a particle diameter of 50 nm or more in the external additive is 75% or more are toners excellent in printing durability, charge stability, and conveyance amount stability.