JP2004317933A - Electrophotographic carrier - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an electrophotographic carrier which does not cause carrier scattering and gives stable printed image density, even when the average particle diameter of the carrier is made small. <P>SOLUTION: When volume resistivity of the electrophotographic carrier at an applied field intensity of 1,000 V/cm is represented by R<SB>1</SB>and volume resistivity of the carrier at an applied field intensity of 100 V/cm by R<SB>2</SB>, the carrier satisfies the expression: R<SB>1</SB>≥1/(108×R<SB>2</SB>). <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a carrier for electrophotography, and more particularly to a carrier used together with toner as a developer for an electrophotographic apparatus such as a copying machine or a printer.
[0002]
[Prior art]
For toners used in electrophotographic devices, colorants (mainly carbon black for black toners, pigments and dyes for color toners) and toner charge are adjusted in resins such as styrene acrylic and polyester. Charge controlling agent is mixed.
[0003]
As a method for producing a toner, the above resin, colorant and charge controlling agent are melt-kneaded, pulverized into fine powder by a pulverizer such as a jet mill, and a desired particle size (for example, a particle size of about 10 μm) is mainly determined by a classifier. Extremely small particle size (for example, 4 μm or less) and large particle size (for example, 20 μm or more) are excluded. Thereafter, a toner which can be used as a developer by externally adding a fine powder such as silica is obtained.
[0004]
In an electrophotographic apparatus, a photoconductor is charged, exposure is performed in accordance with image data, a charge distribution corresponding to an image pattern is formed on the photoconductor, and toner is supplied in accordance with the charge distribution in development to be visible. Appears on the photoreceptor as a toner image. Thereafter, the toner image is transferred to a recording material such as paper, and fixed on the recording material by heat fixing, thereby completing a print image.
[0005]
When transferring the toner on the photoconductor to the recording material, not all the toner on the photoconductor is transferred to the paper, and a part of the toner remains on the photoconductor without being transferred. In the case of a toner having a low ratio (so-called transfer efficiency) between the weight of the toner transferred to the recording material and the weight of the toner on the photosensitive member before transfer, the amount of toner remaining on the photosensitive member increases. If it is left as it is, it will overlap with the image to be formed next, and the quality of the image will be impaired. Therefore, the toner remaining on the photoconductor after the transfer is removed by the cleaning device. The toner removed from the photoconductor by this cleaning may be discarded or reused.
[0006]
In development, it is necessary to charge the toner in order to attach the toner in accordance with the charge distribution on the photoconductor. As a method of giving charge to the toner, a method of using a carrier, which is magnetic particles, and giving charge to the toner by frictional charging with the carrier is known. Since this method uses two components of a toner and a carrier, the developing method is generally called a two-component developing method, and a mixture of the toner and the carrier is called a two-component developer.
[0007]
In this two-component developing method, the carrier not only charges the toner, but also has a function of carrying the toner to the photoconductor. The carrier is provided with a magnet inside the developing device and is held on the surface of a developing roller on which an external cylinder rotates, and is conveyed to a portion facing the photoreceptor, where toner is attached according to the charge distribution on the photoreceptor surface. At this time, the development in which the carrier is brought into contact with the photoreceptor to attach the toner is called contact development.
[0008]
In this contact development, when the paint is applied, if the brush is fine, the paint can be applied evenly, but if the brush is rough, the applied toner can be applied in the same way The unevenness according to the size of the carrier occurs.
[0009]
It is necessary to reduce the carrier particle size in order to form fine brushes in order to uniformly and uniformly adhere the toner to the photoreceptor. As the carrier, a particle diameter of about 100 μm has been frequently used, but a carrier having a small particle diameter of 60 μm or less has been used in order to uniformly adhere the toner.
[0010]
[Problems to be solved by the invention]
As for the carrier particle size, the smaller the particle size, the less the unevenness, but there is also an adverse effect. One of the disadvantages is carrier jump. When the carrier jumps, the developer decreases from the developing device, so that the toner cannot be sufficiently charged, and the amount of the developer conveyed by the developing roller becomes uneven, which causes image unevenness. In addition, a transfer failure occurs in which the toner around the carrier is not transferred in the transfer of the carrier adhered to the photoconductor by the carrier jump.
[0011]
Carrier jump is a phenomenon in which when the electrical resistance of the carrier is low, charge of the opposite polarity to the charge of the toner is injected into the carrier and adheres to the photoconductor.However, even when the electrical resistance of the carrier is apparently high, it may occur. is there. When this carrier jump occurs, transfer failure occurs during transfer. Further, when the electric resistance of the carrier changes, there is a problem that the toner charge amount changes and the print image density changes.
[0012]
SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an electrophotographic carrier which does not cause carrier jump and can provide a stable print image density even when the average particle size of the carrier is reduced.
[0013]
[Means for Solving the Problems]
The above object is as follows. When the value of the volume resistivity at an applied electric field strength of 1000 V / cm is R ₁ and the value of the volume resistivity at an applied electric field strength of 100 V / cm is R ₂ , R ₁ ≧ 1/108 · R ₂ is satisfied. Achieved by obtaining a satisfactory electrophotographic carrier.
[0014]
Even when the electrical resistance of the carrier is apparently high, the carrier jump occurs because of the electric field intensity generated in the gap between the photoconductor and the developing roller due to the potential difference between the potential distribution due to the charge distribution on the photoconductor surface and the developing bias voltage applied to the developing roller. This is because there is a place where is locally strong, and when such a strong electric field acts on the carrier, the carrier resistance is extremely reduced and the charge is injected.
[0015]
Examining the relationship between the resistance of the carrier in which such carrier jump occurs and the electric field intensity applied to the carrier, it is found that the value when the volume resistivity of the carrier is approximately 1000 V / cm and the value when the electric field intensity is 100 V / cm are approximately. Was smaller than 1/108 of the value of.
[0016]
Further, even if the strength of the electric field applied to the carrier is the same, the amount of carrier jump may increase when the amount of the developer conveyed by the developing roller is increased. In such a case, when the developing bias voltage is applied to the developing roller under the condition that the photoconductor is charged but the toner is not developed without exposing, and the current flowing to the developing bias power supply is measured, carrier jump occurs. The current was larger than when the developer amount was not used. This is because the amount of the developer is increased to increase the density of the developer between the photoconductor and the developing roller, and the pressure applied to the developer is increased to reduce the resistance.
[0017]
Examining the relationship between the resistance of the carrier at which the carrier jump occurs and the load applied to the carrier, the value obtained when the volume resistivity is approximately 1 kg per square centimeter is 1 / the value obtained when 0.1 kg is applied per square centimeter. It was smaller than 10.
[0018]
With respect to the carrier whose resistance greatly decreases in response to the increase in the applied electric field strength and the load as described above, the charge is once injected after the resistance is lowered, and then, when the applied electric field strength and the load are reduced, the injected charge is injected. It is considered that the charge remains stored and carrier jump occurs.
[0019]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described with reference to the drawings. First, an image forming process of the electrophotographic apparatus will be described with reference to FIG. In FIG. 1, a photoreceptor 1 rotating clockwise has its surface uniformly charged by a charger 2, and an exposing unit 3 blinks light in accordance with image data. Becomes conductive and the charge on the surface disappears.
[0020]
The developing device 4 holds a two-component developer composed of a toner and a carrier, and is conveyed to a region facing the photoconductor 1 with the rotation of a developing roller 41 having a magnet therein. Although not shown, a bias voltage is applied to the developing roller 41, and the toner having the same charge polarity as the photoconductor is applied to a place where the electric charge on the photoconductor surface is lost due to the action of the electric field between the photoconductor and the developing roller. Adhere to.
[0021]
The toner image formed on the photoconductor 1 by the development is transferred to a recording material 7 such as a sheet by a transfer unit 5. The toner image transferred to the recording material 7 is melted by heating with a fixing device (not shown) and fixed on the recording material 7.
[0022]
Thereafter, the toner remaining on the photoreceptor 1 is removed by the cleaning device 6, and the image is formed in the same manner thereafter. The toner removed by the cleaning device 6 is collected by the toner hopper 42 and used again for development.
[0023]
When the toner is consumed by the development, the toner concentration in the developer of the developing device 4 decreases, the output value of the toner concentration sensor 44 changes, and the control unit 46 compares the output value with a reference value, and the toner concentration falls below a predetermined value. When the toner supply roller 43 is detected, the toner supply roller 43 is driven to supply toner from the toner hopper 42 into the developing device 4. When the output of the toner density sensor 44 reaches a value corresponding to the predetermined toner density, the control unit 46 stops the supply roller 43 so that the toner density in the developing device 4 does not become excessive.
[0024]
When the toner density sensor is of the magnetic bridge type, the toner density detection characteristic is such that the output decreases as the toner density increases, as indicated by the curve 13 in FIG. The control unit 46 captures the output signal (voltage value) of the toner density sensor, determines that the toner density has decreased if it becomes higher than the reference value, and replenishes the toner, and stops the supply if it becomes lower than the reference value. And stabilizes the toner concentration.
[0025]
The toner density sensor 44 is attached so that its detection surface faces the stirring screw 45 in the developing device 4, and the toner density of the developer conveyed on the surface of the toner density sensor 44 with the rotation of the stirring screw 45. Is detected. In order to stably control the toner concentration, it is necessary to adjust the fluidity of the developer so that the developer does not stay on the surface of the toner concentration sensor 44.
(Example 1)
The average particle diameter of the carrier of the developer in the developing device 4 is 40 μm, and the electric volume resistivity of the carrier is from the value of about 10 ¹⁵ Ω · cm at an applied electric field strength of 100 V / cm to the applied electric field strength of 1000 V / cm. Then, it is on the order of 10 ¹⁴ Ω · cm. Magnetite was used as the core material of the carrier. The resistivity of only the core material of the carrier was about 10 ⁵ Ω · cm, and the resistance of the carrier was adjusted by the amount of the coating material and the amount of the conductive material added to the coating material. The coating material is silicon resin, and the conductive material is carbon black.
[0026]
This carrier was formulated into a developer with a toner particle size of 8.5 μm and a toner concentration of 8% by weight. When this developer was used, transfer failure due to carrier skipping did not occur.
[0027]
Here, a method for measuring the electric volume resistivity of the carrier will be described. As shown in FIG. A ring-shaped concave disk 83 having an inner diameter of 70 mm and an outer diameter of 90 mm is arranged on a SUS disk 84 having a diameter of 40 mm with an insulating resin 86 having a width of 5 mm, and the carrier 85 is filled in the concave disk 83. From above, a voltage is applied to a metal cylinder 82 having a diameter of 50 mm serving as an electrode and a load, and a current flowing through a circular plate 84 having a diameter of 40 mm is measured while the ring-shaped concave disk 83 is grounded. A high resistance meter 81 that divides by the current to calculate a resistance value is used. The resistivity is obtained by multiplying the resistance value by the area of the disk 84 and dividing by the distance D between the electrodes 84 and 82.
[0028]
The resistivity of the carrier used in Example 1 is indicated by 91, 92, and 93 in FIG. 4, and the applied electric field strengths are 100 V / cm, 500 V / cm, and 1000 V / cm, respectively. When the applied electric field intensity was increased from 100 V / cm to 1000 V / cm, the resistivity was reduced to 1/6 at the maximum.
[0029]
Further, when the load was increased from 0.1 kg / cm ² to 1 kg / cm ² , the resistivity was reduced to 1/7 at the maximum.
[0030]
In this embodiment, magnetite is used as the core material of the carrier, but a ferrite carrier to which copper, zinc or magnesium is added can also be used. When a copper-zinc ferrite carrier was used, the saturation magnetization was reduced to 3/4 as compared with magnetite, but even when the resistivity was the same, transfer failure due to carrier jump did not occur.
(Example 2)
As compared with Example 1, carriers having resistivity characteristics when applied electric field strengths of 100 V / cm, 500 V / cm, and 1000 V / cm, which are indicated by 97, 98, and 99 in FIG. 4, were used. The applied electric field intensity of the carrier is between 10 ¹⁴ Ω · cm and 10 ¹⁵ Ω · cm at an applied electric field strength of 100 V / cm, and the resistivity at 1000 V / cm is about 10 ¹² Ω · cm to 10 ¹³ Ω · cm, and the resistivity at 1000 V / cm was reduced to 1/108 at the maximum with respect to the resistivity when the applied electric field strength was 100 V / cm.
[0031]
In addition, although slightly different depending on the applied electric field strength, the resistivity at 1 kg / cm ² was reduced to 1/10 at the maximum with respect to the resistivity at a load of 0.1 kg / cm ² .
[0032]
The core material of the carrier was magnetite, the coating material was an acryl-modified silicon resin, and the conductive material added to the coating material was carbon black. The dispersion in the coating material was better than in Example 1.
[0033]
Although the voltage dependence of the resistance is larger than that of Example 1, even when the carriers having the resistivity indicated by 97, 98, and 99 in FIG. 4 were used, the transfer failure due to the carrier skip did not occur. .
[0034]
Also in this case, even when the copper zinc ferrite carrier was used, when the resistivity was the same, no transfer failure due to carrier skipping occurred, but the carrier was sometimes slightly mixed in the image. This is because the ferrite carrier has a smaller saturation magnetization and thus has a weaker magnetic force than magnetite, so that carrier jumps are more likely to occur. However, the diameter of the carrier jumped is about 20 μm, which is about half of the average particle diameter of 40 μm, and it is considered that the carrier having a small particle diameter and a small magnetic force has selectively jumped.
[0035]
Carrier jump was found to have a volume resistivity of 10 ¹² Ω · cm, which is the boundary between whether carrier jump occurs or not.
(Comparative Example 1)
In contrast to Example 1, when carriers having resistivity characteristics indicated by 94, 95, and 96 in FIG. 4 were used, carrier skipping occurred, and a defect that a part of an image was not transferred occurred.
[0036]
The applied electric field strengths of 94, 95 and 96 in FIG. 4 are 100 V / cm, 500 V / cm and 1000 V / cm, respectively. The resistivity at an applied electric field strength of 1000 V / cm was at least as low as 1/1600 of the resistivity at 100 V / cm. Further, the resistivity of 1 kg / cm ^{2 was} 1 / 1,000,000 of the resistivity of 0.1 kg / cm ² when the load was 0.1 kg / cm ² .
[0037]
The core material of the carrier was magnetite, the coating material was a silicone resin, and the amount of carbon black added to the coating material was larger than in the examples. The amount of carbon black was adjusted to adjust the resistivity of 1 kg / cm ² to 1/100 or 1/1000 with respect to the resistance of 0.1 kg / cm ^2. When the amount of carbon black is smaller than / 10, the adjustment of 1/100 or 1/1000 cannot be performed stably, and the amount becomes 1/1000000, which is smaller than that.
[0038]
It is considered that when the resistance of the carrier core material is low and the load is large, the coating in which the amount of carbon black is increased in order to reduce the resistance causes dielectric breakdown and the resistance of the carrier core material appears. In such a carrier, carrier jump occurred and a defect that a part of an image was not transferred occurred.
[0039]
From the above Examples and Comparative Examples, the resistivity at 1000 V / cm is about 1/108 or more of the resistivity at an applied electric field strength of 100 V / cm, and the resistance at a load of 0.1 kg / cm ^2. It was found that when the resistivity at 1 kg / cm ² with respect to the ratio was about 1/10 or more, transfer failure due to carrier skipping did not occur. In addition, in terms of the resistivity of the carrier, when the resistivity was 10 ¹² Ω · cm or more, the transfer failure due to the carrier skip did not occur.
[0040]
In addition, it was found that a carrier having a small resistance decrease with respect to the applied electric field strength and the load was stable against environmental changes. FIG. 5 shows the change in resistivity at an applied electric field strength of 500 V / cm and a load of 0.1 kg / cm ² when the carrier was exposed to an environment of high humidity (20 ° C., 80% RH). In addition, the carrier 97 having a small decrease in resistance with respect to the load has a small decrease in resistance with respect to moisture absorption, and has a single digit or less. On the other hand, the resistance of the carrier 95 having a large decrease in resistance with respect to the applied electric field strength and the load was reduced by about three digits due to moisture absorption.
[0041]
When the resistance of the carrier changes, the two-component developer has a characteristic that the toner charge amount changes. When the resistance decreases, the absolute value of the charge amount of the toner decreases as compared with the case where the carrier resistance is high. If the carrier resistance is low, a discharge is likely to occur in a place where the electric field strength is locally increased on the carrier surface, and a large amount of charge cannot be accumulated, so that the charge of the toner having the same absolute value in the opposite polarity to the carrier also decreases (discharge). It is considered that the carrier charge returns to the toner.
[0042]
The decrease in the amount of charge of the toner is a result of a decrease in the amount of charge in the opposite polarity to the toner of the carrier, which causes carrier skipping, but this is a characteristic that seems to be an advantage at first glance that carrier skipping is reduced. is there. However, when the toner charge amount is caused by a change in the carrier resistance, there is actually no advantage and a problem occurs.
[0043]
For example, in the result of measuring the resistance under a load of 0.1 kg / cm ² in FIG. 5, the toner charge amount at 20 ° C. and 35% RH in the developer using the carrier 95 is about −20 μC / g. , 20 ° C. and 80% RH, when the resistance decreased by three orders of magnitude, the absolute value decreased to −12 μC / g. When printing was performed using this carrier, excessive development occurred at 20 ° C. and 80% RH, and fogging also occurred.
[0044]
On the other hand, when the carrier 97 shown in FIG. 5 is used, the toner charge amount at 20 ° C. and 35% RH is about −15 μC / g, but even at an environment of 20 ° C. and 80% RH, the toner charge amount is −14 μC / g. Yes, the absolute value only decreased by 1 μC / g, the print image density was stable, and no fog occurred. It should be noted that the print image density was stable even when carriers having resistance characteristics of 91, 92 and 93 shown in FIG. 4 were used. The reason why the print image density with respect to the environment is stable with the carrier having such a resistance characteristic is that the change in the resistivity with respect to the applied electric field strength and the load is small, so that the resistance is hardly reduced due to moisture in the environment, particularly in high humidity. It is thought that there is a characteristic.
[0045]
The decrease in resistance due to the applied electric field strength and load is due to the fact that substances that carry electrical conductivity are present in the carrier, but they start conducting under a strong electric field or high load. It is considered that, similarly to the above, there is an action to start conduction of those substances. It is considered that carriers having a small change in volume resistivity due to an applied electric field strength or load are stable to the environment because there are few substances which start electric conduction under such a strong electric field or high load.
[0046]
As described above, the value of the volume resistivity of the carrier used when the applied electric field strength is 1000 V / cm is 1/108 or more of the value when the applied electric field strength is 100 V / cm. By using a carrier whose value at 1 kg weight per square centimeter is 1/10 or more of the value at 0.1 kg weight per square centimeter, transfer failure due to carrier jump is prevented and the print image density is stabilized. it can.
[0047]
The volume resistivity of 10 ¹² Ω · cm or more is a value at which transfer failure due to carrier jump can be prevented.
[0048]
Note that the resistivity of the carrier can be adjusted by oxidizing the core material of the carrier or adding a type of an additional metal element such as Ba (barium) or Sr (strontium) in the case of a ferrite carrier. Magnetite to which (iron) is added can reduce the resistance. In addition, the resistance can be adjusted even in the coating treatment, and the resistance can be adjusted by the amount of the conductive agent added to the coating material, the mixing condition with the coating material, and the amount of the coating material. In that case, the resistance can be adjusted independently of the resistance of the core material, so the core material can be selected from the magnetic properties, and the resistance can be adjusted by coating. It is possible to select carriers having resistance and magnetic characteristics.
[0049]
In particular, when a magnetite carrier with a large saturation magnetization is used with a high resistance by coating, the carrier jump is suppressed by the magnetic force, and the action of preventing the carrier jump by preventing the charge injection by the resistance is combined. The effect of suppressing carrier skipping in the used electrophotographic apparatus is increased.
[0050]
【The invention's effect】
As described above, according to the present invention, it is possible to provide an electrophotographic carrier capable of obtaining a stable print image density without causing carrier skipping even when the average particle size of the carrier is reduced.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of an electrophotographic apparatus.
FIG. 2 is a characteristic diagram of a toner density sensor.
FIG. 3 is an explanatory diagram of a carrier resistance measuring device.
FIG. 4 is an explanatory diagram showing a measurement result of carrier resistance.
FIG. 5 is an explanatory diagram showing a moisture absorption characteristic of a carrier resistance.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Photoreceptor 2 Charger 3 Exposure device 4 Developing device 5 Transfer device 6 Cleaning device 7 Recording material

Claims (5)

When the value of the volume resistivity at an applied electric field strength of 1000 V / cm is R ₁ and the value of the volume resistivity at an applied electric field strength of 100 V / cm is R ₂ , R ₁ ≧ 1/108 · R ₂ is satisfied. Characteristic carrier for electrophotography. When the value of the volume resistivity when 1 kg is applied per square centimeter is R ₃ , and when the value of the volume resistivity when 0.1 kg is applied per square centimeter is R ₄ , R ₃ ≧ 1/10 · R ₄ is satisfied. An electrophotographic carrier, comprising: When the value of the volume resistivity at an applied electric field strength of 1000 V / cm is R ₁ , and the value of the volume resistivity at an applied electric field strength of 100 V / cm is R ₂ , R ₁ ≧ 1/108 · R ₂ is satisfied and 1 cm _2. Assuming that the value of the volume resistivity when 1 kg is applied per unit is R ₃ and the value of the volume resistivity when 0.1 kg is applied per square centimeter is R ₄ , R ₃ ≧ 1/10 · R ₄ is satisfied. An electrophotographic carrier, comprising: The electrophotographic carrier according to any one of claims 1 to 3, wherein the volume resistivity is 10 ¹² Ω · cm or more. The electrophotographic carrier according to any one of claims 1 to 4, wherein the core material constituting the electrophotographic carrier is magnetite.

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