JP7776115B2 - Toner for developing electrostatic latent images - Google Patents

Description

The present invention relates to a toner for developing electrostatic latent images, and more specifically to a toner for developing electrostatic latent images (hereinafter sometimes simply referred to as "toner") used in image forming devices that utilize electrophotography, such as facsimiles, printers, and copiers.

For example, in electrophotographic image forming devices such as facsimiles, printers, and copiers, a toner image transferred to a transfer material such as paper is generally heated, melted, and fixed to the transfer material. In response to growing awareness of energy conservation in recent years, there is a demand for toner that can be fixed at even lower temperatures than conventional toners.

Various methods have been proposed for fixing toner at low temperatures, including adjusting the molecular weight and molecular weight distribution of the binder resin that makes up the toner (Patent Document 1) and using a combination of an amorphous resin and a crystalline resin as the binder resin (Patent Document 2).

Japanese Patent Application Laid-Open No. 2002-82484 JP 2013-160886 A

While the proposed method achieves a certain degree of low-temperature fixability for toner, it cannot be said to be fully compatible with the increasing speed of image forming devices in recent years, and further improvements in the low-temperature fixability of toner are desired.

The present invention was made in consideration of these conventional problems, and its purpose is to provide a toner that can further improve low-temperature fixability.

The toner according to the present invention, which achieves the above-mentioned object, is a toner for developing electrostatic latent images, which comprises toner base particles and silica particles externally added to the toner base particles, wherein the toner base particles contain at least a binder resin and a colorant, the silica particles are hydrophobized with an alkylsilane coupling agent, and the actual treated amount of the alkylsilane coupling agent, calculated from the following formula, is 0.01% by mass or more and less than 30% by mass:
Actual treatment amount with alkylsilane coupling agent (mass%) = loss on ignition - loss on drying Loss on ignition (mass%): mass loss rate when silica particles are kept at a temperature of 500°C for 2 hours Loss on drying (mass%): mass loss rate when silica particles are kept at a temperature of 105°C for 2 hours

In the toner having the above configuration, it is preferable that the silica particles be fumed silica having an average primary particle diameter of 5 nm to 70 nm or colloidal silica having an average primary particle diameter of 50 nm to 200 nm.

In the toner having the above configuration, it is preferable that the alkylsilane coupling agent has an unsubstituted, linear alkyl group having 3 or more carbon atoms.

Furthermore, in the toner having the above-described configuration, it is preferable that the amount of the silica particles added externally is in the range of 0.5% by mass or more and 10% by mass or less relative to the toner base particles.

Furthermore, for a toner having the above-described configuration, it is preferable that the volume average particle size of the toner base particles is in the range of 4 μm or more and 15 μm or less.

The present invention also provides a developer for developing electrostatic latent images, characterized by comprising any of the toners described above and a carrier.

The toner of the present invention enables further improvement in low-temperature fixability. Furthermore, the developer of the present invention can also accommodate faster image forming devices.

After extensive research into improving the low-temperature fixability of toner, the inventors discovered that in addition to the binder resin, the external additives on the toner base particles also affect the low-temperature fixability of toner. More specifically, they discovered that hydrophobically treated silica particles used as external additives increase the viscosity of molten toner, thereby reducing the fixability of the toner image to the transfer material. Further research led the inventors to discover that the increase in viscosity of molten toner can be suppressed by adjusting the type and amount of hydrophobizing agent used on the silica particles, which led to the present invention.

In other words, the toner according to the present invention is a toner comprising toner base particles and silica particles externally added to the toner base particles, and one of its major features is that the silica particles have been hydrophobized with an alkylsilane coupling agent.

To date, silica particles have been widely used as an external toner additive to improve the fluidity and charging properties of toner. Furthermore, because silica particles themselves are hydrophilic and susceptible to temperature and humidity, hydrophobically treated silica particles have been used favorably. HMDS (hexamethyldisilazane) and PDMS (polydimethylsiloxane) have primarily been used as hydrophobizing agents.

However, according to the results of the inventors' research, it was found that although toner to which silica particles treated with HMDS or PDMS are externally added improves toner fluidity and charging properties, the viscosity of the toner increases when the toner melts, impairing the toner's fixability to the transfer material. In addition, it was found that using an alkylsilane coupling agent as a hydrophobic treatment agent for silica particles can suppress the increase in viscosity of the molten toner, and that it is best to keep the amount of alkylsilane coupling agent used within a specified range. The alkylsilane coupling agent used in the present invention is described below.

(Alkylsilane coupling agent)
The alkylsilane coupling agent used in the present invention is preferably a trialkoxyalkylsilane coupling agent represented by the following formula:
C _n H _2n+1 -Si(OC _m H _2m+1 ) ₃
(wherein n is an integer of 3 or more, and m is an integer of 1 to 3)
If n is less than 3, the hydrophobic effect may be reduced. The preferred maximum value for n is 18. The larger n is, the more the thickening effect of the molten toner is suppressed. A more preferred range for n is an integer from 8 to 12. Furthermore, if m is greater than 3, the reactivity of the hydrophobic treatment may decrease. A more preferred range for m is an integer from 1 to 2. Furthermore, it is preferable that the alkyl group does not have a substituent.

The effective treatment amount of alkylsilane coupling agent relative to silica particles, calculated from the above formula, is in the range of 0.01% by mass or more and less than 30% by mass. If the effective treatment amount of alkylsilane coupling agent is less than 0.01% by mass, hydrophobicity cannot be imparted to the silica particles. On the other hand, if the effective treatment amount of alkylsilane coupling agent is 30% by mass or more, the thickening effect of the molten toner becomes greater. The preferred range of the effective treatment amount of alkylsilane coupling agent is in the range of 1% by mass or more and 10% by mass or less.

The ignition loss of silica particles is the mass loss rate when the silica particles are held at 500°C for two hours, and refers to the mass proportion of organic matter, volatile components, and moisture present in the silica particles. On the other hand, the drying loss is the mass loss rate when the silica particles are held at 105°C for two hours, and refers to the mass proportion of volatile components and moisture present in the silica particles. Therefore, subtracting the drying loss from the ignition loss calculates the actual amount of organic matter present in the silica particles, i.e., the amount of alkylsilane coupling agent present.

There are no particular limitations on the method for hydrophobizing silica particles with an alkylsilane coupling agent, and conventional methods can be used. Examples include directly mixing silica particles and an alkylsilane coupling agent using a mixer such as a Henschel mixer, spraying the alkylsilane coupling agent onto silica particles, or adding silica particles to a solution in which an alkylsilane coupling agent is dissolved or dispersed in a suitable solvent, followed by removing the solvent.

(Silica particles)
The silica particles used in the present invention can be any particles produced by a conventionally known method, but from the viewpoint of the dispersibility of the silica particles, those produced by a dry method or a high-temperature hydrolysis method are preferred. Specifically, fumed silica or colloidal silica is preferably used. When fumed silica is used as the silica particles, the average primary particle diameter is preferably 5 nm or more and 70 nm or less. When colloidal silica is used as the silica particles, the average primary particle diameter is preferably 50 nm or more and 200 nm or less.

The average primary particle size of silica particles is the average value of the diameters or maximum diameters of several (100) particles randomly selected from photographs of silica particles taken with a scanning electron microscope (SEM).

The amount of silica particles externally added to the surface of the toner base particles is preferably in the range of 0.5% to 10% by weight relative to the toner base particles. If the amount of silica particles externally added is less than 0.5% by weight, the fluidity and chargeability of the toner may not be improved. On the other hand, if the amount of silica particles externally added exceeds 10% by weight, the viscosity of the toner when melted may increase, resulting in a decrease in low-temperature fixability.

(Toner base particles)
The toner base particles used in the present invention contain at least a binder resin and a colorant. There are no particular limitations on the binder resin, and examples thereof include styrene-acrylic resin and polyester resin. Of course, these resins may be used in combination with other resins as needed.

Examples of monomers that form the base of styrene-acrylic resins include styrene derivatives such as styrene, α-methylstyrene, p-methylstyrene, p-t-butylstyrene, p-chlorostyrene, and hydroxystyrene; methacrylic acid, methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, butyl (meth)acrylate, glycidyl (meth)acrylate, methoxyethyl (meth)acrylate, propoxyethyl (meth)acrylate, methoxydiethylene glycol (meth)acrylate, and ethoxyethyl (meth)acrylate. Examples of (meth)acrylic acid esters include diethylene glycol (meth)acrylate, benzyl (meth)acrylate, cyclohexyl (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, (meth)acrylonitrile, (meth)acrylamide, N-methylol (meth)acrylamide, ethylene glycol di(meth)acrylate, 1,3-butylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, and trimethylolethane tri(meth)acrylate.

Polyester resins are primarily obtained by condensation polymerization of polycarboxylic acids and polyhydric alcohols. Examples of polycarboxylic acids include aromatic polycarboxylic acids such as phthalic acid, isophthalic acid, terephthalic acid, succinic acid, 1,2,4-benzenetricarboxylic acid, 2,5,7-naphthalenetricarboxylic acid, 1,2,4-naphthalenetricarboxylic acid, and pyromellitic acid; aliphatic dicarboxylic acids such as maleic acid, fumaric acid, succinic acid, adipic acid, sebacic acid, malonic acid, azelaic acid, mesaconic acid, citraconic acid, and glutaconic acid; alicyclic dicarboxylic acids such as cyclohexanedicarboxylic acid and methylmedic acid; and anhydrides and lower alkyl esters of these carboxylic acids. One or more of these may be used.

The content of trivalent or higher valent components here depends on the degree of crosslinking, and the amount added can be adjusted to achieve the desired degree of crosslinking. Generally, the content of trivalent or higher valent components is preferably 15 mol% or less.

On the other hand, examples of polyhydric alcohols used in polyester resins include alkylene glycols such as ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol, 1,4-butenediol, neopentyl glycol, 1,5-pentane glycol, and 1,6-hexane glycol; alkylene ether glycols such as diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol, and polytetramethylene glycol; alicyclic polyhydric alcohols such as 1,4-cyclohexanedimethanol and hydrogenated bisphenol A; bisphenols such as bisphenol A, bisphenol F, and bisphenol S, and alkylene oxides of bisphenols. These can be used alone or in combination of two or more.

Monocarboxylic acids and monoalcohols may be used as needed to adjust the molecular weight and control the reaction. Examples of monocarboxylic acids include benzoic acid, parahydroxybenzoic acid, toluenecarboxylic acid, salicylic acid, acetic acid, propionic acid, and stearic acid. Examples of monoalcohols include benzyl alcohol, toluene-4-methanol, and cyclohexanemethanol.

The binder resin used in the present invention preferably has a glass transition temperature in the range of 45°C to 90°C. If the glass transition temperature is below 45°C, there is a risk of solidification inside the toner cartridge or developing device. On the other hand, if the glass transition temperature exceeds 90°C, the toner may not be sufficiently fixed to the transfer material, such as paper.

Colorants used in the present invention include, for example, black pigments such as carbon blacks, including acetylene black, orchid black, and aniline black; yellow pigments such as yellow lead, zinc yellow, cadmium yellow, yellow iron oxide, mineral fast yellow, nickel titanium yellow, navel yellow, naphthol yellow S, Hansa yellow G, Hansa yellow 10G, benzidine yellow G, benzidine yellow GR, quinoline yellow lake, permanent yellow NCG, and tartrazine lake; orange pigments such as red yellow lead, molybdenum orange, permanent orange GTR, pyrazolone orange, Balkan orange, induthrene brilliant orange RK, benzidine orange G, and induthrene brilliant orange GK; red pigments such as red iron oxide, cadmium red, red lead, mercury cadmium sulfide, permanent red 4R, and lithol red. Examples of suitable pigments include pyrazolone red, watching red calcium salt, lake red D, brilliant carmine 6B, eosin lake, rhodamine lake B, alizarin lake, and brilliant carmine 3B; purple pigments include manganese violet, fast violet B, and methyl violet lake; blue pigments include iron blue, cobalt blue, alkali blue lake, Victoria blue lake, phthalocyanine blue, metal-free phthalocyanine blue, partially chlorinated phthalocyanine blue, fast sky blue, and indanthrene blue BC; green pigments include chrome green, chromium oxide, pigment green B, malachite green lake, and Fanal yellow green G; white pigments include zinc oxide, titanium oxide, antimony white, and zinc sulfide; and white pigments include baryte powder, barium carbonate, clay, silica, white carbon, talc, and alumina white. These colorants may be used alone or in combination.

The total content of colorants is preferably in the range of 0.1 to 20 parts by weight, and particularly 1 to 15 parts by weight, per 100 parts by weight of binder resin.

(Preparation of Toner Base Particles)
The toner base particles of the present invention can be produced by a method known per se, such as a pulverization classification method, a melt granulation method, a spray granulation method, or a suspension/emulsion polymerization method, but the pulverization classification method is preferably used in terms of production equipment, productivity, etc. Such a pulverization classification method is described below.

First, toner components such as binder resin, colorant, and optionally charge control agent, release agent, and magnetic powder are premixed using a Henschel mixer or V-type mixer, and then melt-kneaded using a melt-kneading device such as a twin-screw extruder. After cooling, the molten mixture is coarsely pulverized and finely pulverized, and then classified as necessary to obtain toner base particles with a predetermined particle size distribution. The volume average particle size of the toner base particles is preferably in the range of 4 μm to 15 μm. The volume average particle size of the toner base particles is measured using a Coulter Multisizer II manufactured by Coulter.

Here, any known charge control agent can be used. For example, positively chargeable charge control agents include nigrosine dyes, fatty acid-modified nigrosine dyes, carboxyl-containing fatty acid-modified nigrosine dyes, quaternary ammonium salts, amine compounds, and organometallic compounds. Negatively chargeable charge control agents include metal complexes of oxycarboxylic acids, metal complexes of azo compounds, metal complex dyes, and salicylic acid derivatives. The amount of charge control agent added is preferably in the range of 0.1 to 10 parts by weight per 100 parts by weight of binder resin.

Various waxes and low-molecular-weight olefin resins can be used as release agents. Examples of waxes that can be used include polyhydric alcohol esters of fatty acids, higher alcohol esters of fatty acids, alkylene bis-fatty acid amide compounds, and natural waxes. Examples of low-molecular-weight olefin resins that can be used include polypropylene, polyethylene, and propylene-ethylene copolymers with number-average molecular weights in the range of 1,000 to 10,000, particularly 2,000 to 6,000, with polypropylene being particularly preferred. The amount of release agent added is preferably in the range of 0.1 to 10 parts by weight per 100 parts by weight of binder resin.

Examples of magnetic powders include black iron tetroxide (Fe ₃ O ₄ ), red iron oxide (γ-Fe ₂ O ₃ ), zinc iron oxide (ZnFe ₃ O ₄ ), yttrium iron oxide (Y ₃ Fe ₅ O ₁₂ ), cadmium iron oxide (CdFe ₂ O ₄ ), gadolinium iron oxide (Gd ₃ Fe ₅ O ₁₂ ), copper iron oxide (CuFe ₂ O ₄ ), lead iron oxide (PbFe ₁₂ O ₁₉ ), nickel iron oxide (NiFe ₂ O ₄ ), neodymium iron oxide (NdFeO ₃ ), barium iron oxide (BaFe ₁₂ O ₁₉ ), magnesium iron oxide (MgFe ₂ O ₄ ), manganese iron oxide (MnFe ₂ O ₄ ), and lanthanum iron oxide (LaFeO _{3 )} . ), iron powder (Fe), cobalt powder (Co), nickel powder (Ni), etc. The amount of magnetic powder added is preferably in the range of 0.1 to 5 parts by mass, more preferably 0.5 to 3.0 parts by mass, per 100 parts by mass of the binder resin.

The toner of the present invention is obtained by adhering the silica particles to the surface of the toner base particles prepared in this manner. The silica particles can be mixed into the toner base particles using conventional methods, such as a Henschel mixer, V-type mixer, Turbula mixer, or hybridizer, where the toner particles and silica fine particles are placed and stirred.

In addition, a surface treatment agent may be added to the toner base particles as needed to adjust the chargeability, fluidity, etc. of the toner. Examples of surface treatment agents include one or more of the following: inorganic fine powders such as alumina, zinc oxide, magnesium oxide, and calcium carbonate; organic fine powders such as polymethyl methacrylate; and fatty acid metal salts such as zinc stearate. The amount of surface treatment agent added is preferably in the range of 0.1 to 2.0 parts by weight per 100 parts by weight of the toner base particles.

(Electrostatic latent image developer)
The toner of the present invention can be used as a one-component developer or a two-component developer. When used as a two-component developer, the carrier used is not limited. Examples of the carrier include magnetic metals such as iron, nickel, and cobalt, and their alloys, or alloys containing rare earth elements; soft ferrites such as hematite, magnetite, manganese-zinc ferrite, nickel-zinc ferrite, manganese-magnesium ferrite, and lithium ferrite; iron oxides such as copper-zinc ferrite; and mixtures thereof. These magnetic particles can also be produced by sintering and atomizing magnetic materials, as well as magnetic particles whose surfaces are coated with a resin. A magnetic material-dispersed resin can also be used as the carrier. In this case, the magnetic material can be any of the magnetic materials described above, and the binder resin can be, for example, a vinyl resin, a polyester resin, an epoxy resin, a phenolic resin, a urea resin, a polyurethane resin, a polyimide resin, a cellulose resin, a polyether resin, or a mixture thereof.

The particle size of the carrier is generally preferably 30 μm to 200 μm, particularly 50 μm to 150 μm, as measured by electron microscopy. The apparent density of the carrier, when mainly made of a magnetic material, differs depending on the composition and surface structure of the magnetic material, but is generally preferably in the range of 2.4 g/cm ³ to 3.0 g/cm ³ .

The toner concentration in the two-component developer consisting of the toner and carrier is 1% to 10% by weight, and preferably 1% to 7% by weight. If the toner concentration is less than 1% by weight, the image density will be too low, while if the toner concentration exceeds 10% by weight, toner may scatter within the developing device, causing problems such as contamination inside the device or toner adhering to background areas on transfer paper, etc.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

(Preparation of Toner Base Particles I)
100 parts by mass of polyester resin (manufactured by Mitsubishi Chemical Corporation, product name: ER-508), 6 parts by mass of carbon black (manufactured by Cabot Corporation, product name: BP-L), 1.0 part by mass of zinc salicylate complex, and 2.0 parts by mass of polypropylene wax (manufactured by Sanyo Chemical Industries, Ltd., product name: 660P) were mixed using a Henschel mixer, and then melted and kneaded using a twin-screw extruder. The resulting mixture was rolled using a cooling press roller and coarsely pulverized using a feather mill. The mixture was then pulverized using an air jet type fine pulverizer and classified using an air flow classifier to obtain toner base particles I having a volume average particle size of 7.5 μm.
The glass transition temperature (Tg) and softening temperature (T1/2) of the toner base particles I were measured by the following methods, and found to be 63°C and 128°C, respectively.

(Preparation of Toner Base Particles II)
A crystalline polyester resin having a melting point of 70° C. and Mw=10,000 was prepared by condensation polymerization of 238 parts by mass of adipic acid, 265 parts by mass of 1,6-hexanediol, and 1 part by mass of hydroquinone.
Toner base particles II were prepared in the same manner as toner base particles I, except that 93 parts by mass of a polyester resin (manufactured by Mitsubishi Chemical Corporation, product name: ER-508) and 7 parts by mass of the crystalline polyester resin prepared above were used as the polyester resin. The glass transition temperature (Tg) and softening temperature (T1/2) of toner base particles II were measured in the same manner as toner base particles I, and the Tg was 50°C and the T1/2 temperature was 104°C.

(Glass transition temperature (Tg))
Using a differential scanning calorimeter (Shimadzu Corporation, DSC-60), 10 mg±0.5 mg of a sample was weighed into an aluminum pan, and the glass transition temperature Tg was measured from the intersection of the baseline of the chart and the tangent to the endothermic curve at a heating rate of 5°C/min.

(Softening temperature (T1/2))
Using a capillary rheometer (Shimadzu Corporation, Flow Tester CFT-500D), the temperature was increased at a rate of 6°C/min, the nozzle was 1.0 mmφ x 1 mm, the load was 10 kgf, and the sample amount was 1.5 g. The temperature was measured when the piston reached the midpoint in the process from the start to the end of the outflow.

(Preparation of Silica Particles A)
100 parts by mass of untreated fumed silica powder (BET specific surface area 130 m ² /g) was mixed with 100 parts by mass of n-propanol using a stirrer. 50 parts by mass of n-propanol and a small amount of diethylamine were added to this mixture and stirred for 1 hour. 100 parts by mass of n-propanol and 4.0 parts by mass of isobutyltriethoxysilane were added to this mixture and heated to reflux with stirring in an inert atmosphere for 5 hours, and then allowed to cool to room temperature. The solvent was removed from this mixture using a rotary evaporator under superheated reduced pressure, and then it was dried in a vacuum oven for 18 hours to produce silica particles A.

(Preparation of Silica Particles B)
Silica particles B were prepared in the same manner as silica particles A, except that isobutyltriethoxysilane was replaced with 5.0 parts by mass of octyltriethoxysilane.

(Preparation of Silica Particles C)
Silica particles C were prepared in the same manner as silica particles A, except that isobutyltriethoxysilane was replaced with 7.0 parts by mass of decyltriethoxysilane.

(Preparation of Silica Particles D)
Silica particles D were prepared in the same manner as silica particles A, except that the untreated fumed silica powder was replaced with one having a BET specific surface area of 50 m ² /g and isobutyltriethoxysilane was replaced with 2.5 parts by mass of decyltriethoxysilane.

(Preparation of Silica Particles E)
100 parts by mass of untreated fumed silica powder (BET specific surface area 200 ^m2 /g) was placed in a reaction vessel, and the powder was fluidized by stirring under a nitrogen atmosphere, and 3.0 parts by mass of octyltrimethoxysilane was sprayed onto it. With continued stirring, the temperature was raised from room temperature to 340°C and maintained at that temperature for 30 minutes. Silica particles E were obtained by cooling.

(Preparation of Silica Particles F)
A small amount of aqueous ammonia and 2.0 parts by weight of octyltriethoxysilane were added to 100 parts by weight of a commercially available hydrophilic colloidal silica dispersion (BET specific surface area: 30 ^m /g, solid content: 40%), and the mixture was stirred at room temperature with an overhead stirrer for 24 hours. The mixture was then dried in a forced circulation constant temperature dryer at 128°C to obtain silica particles F.

(Preparation of Silica Particles G)
As silica particles G, commercially available hydrophobic silica particles (fumed silica particles surface-treated with HMDS, manufactured by Clariant, trade name: H13TM, average primary particle diameter: 16 nm) were prepared.

(Silica particles H)
As silica particles H, commercially available hydrophobic silica particles (fumed silica particles surface-treated with silicone oil, manufactured by Teika Corporation, trade name: MSN-002, average primary particle diameter: 16 nm) were prepared.

(Silica particles I)
As silica particles I, commercially available hydrophobic silica particles (fumed silica particles surface-treated with silicone oil, manufactured by Aerosil Co., Ltd., trade name: RY-50, average primary particle diameter: 40 nm) were prepared.

(Silica particles J)
As silica particles J, commercially available hydrophobic silica particles (fumed silica particles surface-treated with silicone oil, manufactured by Cabot Corporation, trade name: TG-308F, average primary particle diameter: 12 nm) were prepared.

(loss on drying)
The loss on drying of the silica particles A to J prepared as above was measured by the following method. The measurement results are shown in Table 1.
- Place 3 g of sample in a container that has been dried, cooled, and weighed accurately in advance, and weigh it accurately.
Using a heat-drying moisture meter MX-50, dry at 105°C for 2 hours. Allow to cool in a desiccator for 30 minutes, then weigh the mass and calculate using the following formula.
Loss on drying (%) = Loss on drying (g) / Sample weight (g) × 100

(Ignition loss)
The ignition losses of the silica particles A to J prepared as described above were measured by the following method. The measurement results are shown in Table 1.
・Place 0.5 g of sample in a magnetic crucible that has been dried, cooled, and precisely weighed in advance, and then weigh it precisely.
Ignite for 2 hours in an electric furnace (small electric furnace Black mini-BS1) set to 500°C. Allow to cool in a desiccator for 1 hour, then weigh the mass and calculate it using the following formula.
Loss on ignition (%) = Loss on ignition (g) / Sample weight (g) × 100

(Viscoelasticity measurement)
The viscoelasticity of the silica particles A to J prepared as above was measured by the following method. The measurement results are shown in Table 1.
A sample was prepared by mixing 10 parts by mass of each of the silica particles A to J with 90 parts by mass of polyester resin (manufactured by Mitsubishi Chemical Corporation, product name: ER-508) using a Super Mixer Piccolo SMP-2 manufactured by Kawata Co., Ltd., and viscoelasticity measurements of 1.0 g of the sample were performed using a viscoelasticity measuring device (manufactured by TA Instruments, ARES-G2 rheometer) under the following conditions. From the measured data, the ratio of the storage modulus G' to the loss modulus G'' at 110°C (G''/G') was read as tan δ(110°C).
Dynamic viscoelasticity measurement conditions Parallel plate diameter: 25 mm
Measurement frequency: 1.0Hz
Measured distortion: 0.2%
Measurement temperature: heated from 70°C to 200°C at a rate of 3.0°C per minute.

Example 1
100 parts by mass of toner base particles I and 2.0 parts by mass of silica particles A were charged into a Super Mixer Piccolo SMP-2 manufactured by Kawata Corporation, mixed at 3000 revolutions per minute for 6 minutes, and then sieved through a sieve with 74 μm openings to prepare the toner of Example 1. The minimum fixing temperature, low-temperature fixing property, and storage stability of the prepared toner were measured and evaluated by the following methods. The results are also shown in Table 2.

Examples 2 to 6 and Comparative Examples 1 to 5
Toners of Examples 2 to 6 and Comparative Examples 1 to 5 were prepared in the same manner as in Example 1, except that the type and amount of silica particles were changed to those shown in Table 2. The minimum fixing temperature, low-temperature fixability, and storage stability of the prepared toners were measured and evaluated by the following methods in the same manner as in Example 1. The results are also shown in Table 2.

Comparative Example 5
A toner of Comparative Example 5 was prepared in the same manner as in Comparative Example 2, except that the toner base particles were changed to toner base particles II. The minimum fixing temperature, low-temperature fixing property, and storage stability of the prepared toner were measured and evaluated by the following methods in the same manner as in Example 1. The results are also shown in Table 2.

The amount of silica particles added in the examples and comparative examples was determined by geometric calculation from the particle sizes of the toner base particles and silica particles so that the coverage of the surface of the toner base particles with silica particles was approximately 100%.

(Lower limit fixing temperature)
The prepared toner was uniformly applied to a commercially available 75 g/ ^m² PPC paper sheet to a density of 0.40 mg/ ^cm² . This PPC paper sheet was fixed using an external fixing tester employing a commercially available heat roller-type toner fixing device, with the fixing speed (heat roller peripheral speed) set at 375 mm/sec and the heat roller temperature set in 10°C increments within the range of 150 to 230°C, to obtain a fixed image. The image density of the resulting fixed image was measured using an X-Rite eXact, followed by a peel test using the method described below. The image density was measured again after peeling. The fixation rate was calculated using the following formula, and the lowest temperature among the heat roller temperatures at which the fixation rate was 90% or higher was designated the minimum fixation temperature.
Fixation rate (%)=Fixed image density after peeling test/Fixed image density before peeling test×100

(Peel test method)
A mending tape is applied to the fixed image, and a 500 g weight is moved back and forth on the tape at 1 cm/sec 5 times, and then the mending tape is peeled off at 1 cm/sec.

(Low temperature fixability)
Based on the results of the minimum fixing temperature, the low-temperature fixing property was evaluated according to the following criteria.
◯: The minimum fixing temperature is 170°C or less. △: The minimum fixing temperature is more than 170°C and less than 180°C. ×: The minimum fixing temperature is more than 180°C.

(Storability)
20.0 g of toner was placed in a sealed container and allowed to stand at 50°C for 8 hours. After standing, the sealed container was tapped, and the entire amount of toner was transferred to a sieve and sieved using a powder tester under the following conditions. After sieving, the mass of toner remaining on the sieve was measured, and the storage stability was evaluated according to the following criteria: Good: Less than 0.2 g Fair: 0.2 g or more but less than 1.0 g X: 1.0 g or more Sieving condition device: Powder Tester PT-X (manufactured by Hosokawa Micron Co., Ltd.)
Sieve opening: 355 μm
Vibration width: 1mm
Vibration time: 10 seconds Operation method: A sieve is set on the vibration table of the powder tester, toner is placed on the sieve, and the sieve is vibrated at a vibration amplitude of 1 mm for 10 seconds, and the weight of the toner remaining on the sieve is measured.

As is clear from Table 2, the toners of Examples 1 to 6 all exhibited excellent low-temperature fixing performance, with a minimum fixing temperature of 170°C, and also had good storage stability.

In contrast, the toner of Comparative Example 1, in which silica particles hydrophobized with hexamethylsilazane were externally added, and the toners of Comparative Examples 2 to 4, in which silica particles hydrophobized with polydimethylsiloxane were externally added, had lower limit fixing temperatures of 180°C and 190°C, respectively, which were higher than the toners of Examples 1 to 6. Furthermore, the toner of Comparative Example 5, in which silica particles hydrophobized with polydimethylsiloxane were externally added to toner base particles II, had a lower limit fixing temperature of 160°C, which showed good low-temperature fixing performance, but poor storage stability.

The toner of the present invention enables further improvement in low-temperature fixability.

Claims (6)

A toner for developing an electrostatic latent image, comprising toner base particles and silica particles externally added to the toner base particles,
the toner base particles contain at least a binder resin and a colorant,
The silica particles are
C _n H _2n+1 —Si(OC _m H _2m+1 ) ₃ (wherein n is an integer of 3 or more, and m is an integer of 1 to 3)
The hydrophobic treatment is carried out using only an alkylsilane coupling agent represented by the formula :
1. A toner for developing electrostatic latent images, wherein the actual amount of the alkylsilane coupling agent to be treated is 0.01% by mass or more and less than 30% by mass, as calculated by the following formula:
Actual treatment amount with alkylsilane coupling agent (mass%) = loss on ignition - loss on drying Loss on ignition (mass%): mass loss rate when silica particles are kept at a temperature of 500°C for 2 hours Loss on drying (mass%): mass loss rate when silica particles are kept at a temperature of 105°C for 2 hours The toner for developing electrostatic latent images according to claim 1, wherein the silica particles are fumed silica having an average primary particle diameter of 5 nm to 70 nm or colloidal silica having an average primary particle diameter of 50 nm to 200 nm. The toner for developing electrostatic latent images according to claim 1 or 2, wherein the alkylsilane coupling agent has an unsubstituted, linear alkyl group having 3 or more carbon atoms. The toner for developing electrostatic latent images according to any one of claims 1 to 3, wherein the amount of the silica particles added externally is in the range of 0.5% by mass to 10% by mass relative to the toner base particles. The toner for developing electrostatic latent images according to any one of claims 1 to 4, wherein the volume average particle size of the toner base particles is in the range of 4 μm or more and 15 μm or less. A developer for developing electrostatic latent images, comprising the toner according to any one of claims 1 to 5 and a carrier.