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(11) | EP 0 468 811 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Carrier for developing electrostatic image |
| (57) A carrier suitable for developing an electrostatic image which comprises core particles
coated with a layer comprising fluorocarbon particles and porous resin particles; wherein the porous resin particles are composed of a plurality of elementary resin particles fused together at the surfaces thereof, the elementary resin particles having: (a) a particle size of not more than 0.5 µm; (b) a BET specific surface area of from 5 to 150 m²/g; and (c) a volume average particle size of from 1.5 to 5.0 µm. |
[0001] The present invention relates to carrier for developing electrostatic image employed in electrophotography, electrostatic recording or electrostatic printing, more specifically, to carrier for developing electrostatic image obtained by coating the surfaces of carrier core particles with resin particles by a dry method.
BACKGROUND OF THE INVENTION
[0002] Two-component developer used in electrophotography consists of toner and carrier. Carrier serves to give toner an appropriate amount of electrostatic charge of suitable polarity.
[0003] As the carrier, resin-coated carrier obtained by providing on each surface of carrier core particles a resin layer has been employed widely due to its various merits. The following are already proposed resin layers:
(1) A fluororesin coating layer that contains 0.5 to 65 wt% of a fluorocarbon (see Japanese Patent Examined Publication No. 48782/1982); and
(2) A resin coating layer containing fluorocarbon particles as conductive powder (see Japanese Patent Open to Public Inspection No. 48050/1985)
[Subject matter to be dissolved by the present invention]
[0004] The above resin layers, due to the presence of a fluorocarbon, enable carrier to have a lower surface energy, and eventually make it less subject to toner spent. But, the provision of the above resin layers by a wet process involves such a problem that the fluorocarbon particles are unstably present in a coating composition in the form of large secondary particles which are formed by the agglomeration of fluorocarbon elementary particles that cannot be dispersed as they are due to their strong cohesiveness. The poor dispersion stability of the agglomerated fluorocarbon particles makes the handling of the coating composition more difficult, and, in addition, inhibits the uniform dispersion of the fluorocarbon particles in the resin layer, and prevents the fluorocarbon particles from being in close contact with the resin particles.
[0005] Carrier having such resin coating layer is poor in durability. When it is employed in repeated copying, its characteristics are likely to change considerably with the lapse of time, since the fluorocarbon particles contained in the resin layers tend to be removed therefrom.
[0006] Meanwhile, the frictional charging of resin-coated carrier with toner is greatly affected by the characteristics of the carrier particle's outermost surface. Carrier having the above-mentioned resin layer on its respective core particle, since fluorocarbon particles are dispersed ununiformly in its outermost surface, cannot make toner uniformly charged by mutual friction due to a difference in chargeability between the fluorocarbon and the coating resin, causing such image troubles as fogging and a lowering in solid image density.
[0007] Various dry methods were proposed for the provision of a resin layer on the surface of a carrier core particle. For instance, Japanese Patent Publication Open to Public Inspection No. 87168/1990 discloses a method of coating the surfaces of magnetic particles with resin particles by a dry process which comprises adding to magnetic particles with a weight average particle size of 10 to 200 µm resin particles of which the weight average particle size is less than 1/200 of that of the magnetic particles to form a homogeneous mixture, and giving impact repeatedly to this mixture in a mixing apparatus of which the temperature has been set at 50 to 110°C to allow the surfaces of the magnetic particles to be coated with the resin particles.
[0008] The inventors found that the application of a dry process in the provision of a resin-fluorocarbon coating layer is not always successful. Since resin particles are extremely small in size, they tend to fly during the production process, and cannot be mixed sufficiently with core particles. Further, when coating is performed in a mixer provided with a rotator, where air purge is usually done to protect the sealed portion of a bearing, resin coating efficiency, i.e., the weight ratio of resin particles that are formed into a layer to those as raw material, is low due to serious fly loss of resin particles.
[0009] Due to such low resin coating efficiency, considerable amounts of resin particles or agglomerated resin particles are allowed to remain on the surface of a carrier particle in a free state without forming a film (these particles and agglomerated particles will often be referred to as "white powder"). The white powder may stick to the surface of a carrier particle electrostatically to hinder the frictional charging with toner, making toner charged only weakly. This phenomenon causes fogging at the early stage of image forming.
[0010] When a large amount of white powder is present on the surface of a carrier particle, it tends to be transferred selectively to a photoreceptor during development, affecting adversely developing and cleaning conditions. That is, since white powder has a charging polarity opposite to that of toner, it selectively sticks to the non-image forming portion of a photoreceptor, and is sent to a cleaning part without being transferred. This leads to the overloading of the cleaning part, and then to insufficient cleaning. If cleaning cannot be performed sufficiently, the surface of a photoreceptor is subjected to filming. As a result, the light sensitivity of the photoreceptor is lowered, causing an image to be fogged.
SUMMARY OF THE INVENTION
[0011] The object of the invention is to provide carrier for developing electrostatic image comprising core particles each coated with a fluorocarbon-containing resin layer and having the following advantage:
(1) the resin layer can be formed efficiently;
(2) the fluorocarbon particles are dispersed uniformly in the resin layer, allowing the carrier to keep its good frictional chargeability throughout repeated developing; and
(3) the fluorocarbon particles are hardly removed from the surface of the carrier, allowing the carrier to retain its good low surface energy characteristics and low resistance characteristics.
[0012] By the use of porous resin particles with a specific BET specific surface area and a specific volume average particle size together with fluorocarbon particles, resin coating efficiency can be increased drastically, the uniform dispersion of fluorocarbon particles in a resin layer can be attained to a sufficient level, and the thickness of a resin layer can be controlled freely.
[0013] The carrier of the invention is obtained by coating the surface of each carrier core particle with porous resin particles (secondary resin particles) which satisfy the following requirements and fluorocarbon particles:
(1) being composed of elementary resin particles with a volume average particle size of not more than 0.5 µm that are fused on their surfaces;
(2) having a BET specific surface area of 5 to 150 m²/g; and
(3) having a volume average particle size of 1.5 to 5.0 µm.
[0014] When such specific porous resin particles and fluorocarbon particles are employed in combination in forming a resin coating layer on the surface of a core particle by a dry process, it is possible to increase resin coating efficiency drastically, to control the thickness of a resin coating layer freely, to attain uniform dispersion of fluorocarbon particles in a resin coating layer, to prevent effectively the removal of fluorocarbon particles from a resin coating layer, and to impart the carrier with an appropriate level of conductivity.
[0015] Small resin particles are improved in spreadability, but have a drawback that they can form only a single layer when applied on the surface of a core particle by a dry process. If attempts are made to provide multiple layers from small-sized resin particles by a dry process, there may be a serious lowering in coating efficiency due to the fly of the resin particles. To solve this problem, in the invention, use is made of porous resin particles which each comprise small-sized elementary resin particles fused on their surfaces, and have a specific BET specific surface area and a specific volume average particle size. These porous resin particles exhibit good compatibility with fluorocarbon particles, allowing a homogeneous mixture to be formed. Such uniform mixture, when applied on the surface of a carrier core particle, permits the formation of a resin coating layer in which fluorocarbon particles are quite uniformly dispersed.
[0016] The porous resin particles of the invention enjoy the following advantage:
- They have good spreadability, which is inherited from the small-sized elementary particles constituting them;
- Due to their relatively large sizes, they hardly fly during dry processing, and hence, can be mixed sufficiently with fluorocarbon particles and core particles; and
- They can be kept in close contact with fluorocarbon particles.
[0017] By the use of such resin particles, it is possible to obtain a strong, uniform, and thickness-controllable resin coating layer at a higher efficiency.
[0018] The carrier of the invention, which is obtained by coating core particles with these resin particles and fluorocarbon particles, contains an extremely small amount of white powder, and therefore, is capable of giving toner an appropriate amount of electrostatic charge.
[0019] Since the fluorocarbon particles are hardly removed from its surface, the carrier of the invention can maintain its low surface energy characteristics and low resistance characteristics for a long time, and eventually has good durability.
[0020] The carrier of the invention, in which the fluorocarbon particles are uniformly dispersed in its resin coating layer to form "composites" with the porous resin particles, can give toner an appropriate amount of charging of suitable polarity. By using the carrier of the invention, it is possible to produce repeatedly a high quality copy image free from fogging and other troubles. In addition, the use of this carrier effectively prevents toner particles from flying within a copy machine. Further, since the fluorocarbon particles contained lower the energy of its surface, the carrier of the invention can be free from toner spent.
[0021] Meanwhile, carrier containing fluorocarbon particles is employed as the negative-charged carrier that makes toner charged positively, since fluorocarbon particles have strong negative chargeability. However, in the invention, the chargeability of the carrier depends on the charging characteristics of the porous resin particles, and hence, the carrier can be used also as the positive-charged carrier that gives toner negative charge. The charging characteristics of the porous resin particles can be fully manifested without being affected by those of the fluorocarbon particles, and at the same time, the fluorocarbon particles can exhibit their toner spent prevention effect sufficiently.
DETAILED DESCRIPTION OF THE INVENTION
BRIEF EXPLANATION OF THE DRAWINGS
[0022]
Fig. 1 shows one form of a high-speed stirring type mixer suitable for use in the invention; and
Fig. 2 shows the plane view of the horizontal rotator of the mixer shown in Fig. 1.
[0023] The carrier of the invention comprises carrier core particles each coated with a resin coating layer. The resin coating layer consists of porous resin particles and fluorocarbon particles.
[0024] The porous resin particles must satisfy the following requirements: (1) being composed of elementary resin particles with a volume average particle size of not more than 0.5 µm that are fused on their surfaces; (2) having a BET specific surface area of 5 to 150 m²/g;, preferably 10 to 120 m²/g;, more preferably 20 to 100 m²/g; and (3) having a volume average particle size of 1.5 to 5.0 µm.
[0025] The BET specific surface area is measured with, for instance, a micromeritics flow sorb (Type II2300; manufactured by Shimazu Corp).
[0026] The volume average particle size is measured by means of, for instance, a laser diffraction type size distribution measuring machine (HEROS; sold by Japan Electronics Corp). In the measurement, the dispersion of the porous resin particles is performed over a period of two minutes by means of a ultrasonic homogenizer with an output power of 150 W after the resin particles, a surfactant and water as a disperse medium are put in a beaker of 50 cc capacity.
[0027] The BET specific surface area of the porous resin particles are satisfactory when it is in the range of 5 to 150 m²/g;. Since impact power to be applied to the porous resin particles during dry coating depends on the sizes of the core particles, it is preferred that the porous resin particles have a larger BET specific surface area when the sizes of the core particles are small. If the BET specific surface area is large enough, the porous resin particles can exhibit good spreadability to the surface of the carrier core particle with minimum impact power. Meanwhile, simple, non-porous resin particles with particle sizes of about 2 µm have a BET specific surface area of smaller than 5 m²/g;.
[0028] If the porous resin particles have a BET specific surface area smaller than 5 m²/g;, they have poor spreadability to the surface of the core particle, making it difficult to obtain a coating layer of uniform thickness. In addition, the porous resin particles tend to agglomerate to form white powder, which may stick to the respective particle surface of the carrier electrostatically, hindering successful development. Further, since a considerable amount of the porous resin particles are present in a free state without forming a layer on the surface of the core particle, there may be a substantial lowering in resin coating efficiency.
[0029] In the case of a BET specific surface area exceeding 150 m²/g;, it is difficult to handle the porous resin particles because of their extremely small particle sizes, and as a result, fly loss of the resin particles tends to occur, causing resin coating efficiency to be lowered. Such lowering in resin coating efficiency is observed most frequently when coating is performed by a dry method where a rotary mixer with air purge function is employed.
[0030] When the volume average particle size is smaller than 1.5 µm, though spreadability is improved due to an increased BET specific area, handling of the resin particles is difficult because of their small particle sizes, and as a result, fly loss of the resin particles tends to occur, resulting in a lowered resin coating efficiency.
[0031] If the porous resin particles have a volume average particle size exceeding 5.0 µm, their spreadability to the surface of the core particle is lowered due to the excessive agglomeration of the elementary resin particles. In this case, since the porous resin particles have a smaller BET specific surface area, and their film-forming property is so poor as will cause themselves to agglomerate to form white powder. The presence of such white powder, as mentioned before, hinders successful development.
[0032] The elementary resin particles that constitute the porous resin particle of the invention are small particles with a volume average particle size of not more than 0.5 µm. By the fusion of these small-sized elementary particles on their surfaces, it is possible to obtain without fail the porous resin particles with the above-defined BET specific surface area and volume average particle size. When the volume average particle size of the elementary particles exceeds 0.5 µm, the spreadability of the porous resin particles is poor since they have an extremely small BET specific surface area.
[0033] Resins for the elementary resin particles are not limitative. In the present invention, since the application of the porous resin particles is performed by a dry process, resins hardly soluble in solvents are also usable. Therefore, there is a wide choice in the kind of usable resin. Examples of usable resins include styrene resins, acryl resins, styrene-acryl resins, vinyl resins, ethylene resins, rosin-modified resins, polyamide resins, polyester resins, silicone resins, fluororesins and mixtures thereof. Of them, styrene-acryl resins and acryl resins are preferable. A styrene-acryl resin is prepared by the copolymerization of a styrene monomer and an acryl monomer.
[0034] The specific examples of styrene monomers are styrene, o-methylstyrene, m-methylstyrene, p-methylstyrene, α-methylstyrene, p-ethylstyrene, 2,4-dimethylstyrene, p-butylstyrene, p-t-butylstyrene, p-hexylstyrene, p-octylstyrene, p-nonylstyrene, p-decylstyrene, p-dodecylstyrene, p-methoxystyrene, p-phenylstyrene, p-chlorostyrene, 3,4-dichlorostyrene, and mixtures thereof.
[0035] The specific examples of acryl monomers are acrylic acid and its esters such as acrylic acid, methyl acrylate, ethyl acrylate, butyl acrylate, isobutyl acrylate, propyl acrylate, octyl acrylate, dodecyl acrylate, lauryl acrylate, 2-ethylhexyl acrylate, stearyl acrylate, 2-chloroethyl acrylate, phenyl acrylate and methyl α-chloroacrylate; methacrylic acid and its esters such as methacrylic acid, methyl methacrylate, ethyl methacrylate, propyl methacrylate, butyl methacrylate, isobutyl methacrylate, octyl methacrylate, dodecyl methacrylate, lauryl methacrylate, 2-ethylhexyl methacrylate, stearyl methacrylate, phenyl methacrylate, dimethylaminoethyl methacrylate and diethylaminoethyl methacrylate; and mixtures thereof.
[0036] In preparing a styrene-acryl resin, the weight ratio of a styrene monomer to an acryl monomer is preferably 9:1 to 1:9. The styrene component makes the resin coating layer harder, and the acryl component makes it sturdier. By adjusting the weight ratio of the styrene component and the acryl component adequately, it is possible to control considerably the charging amount of toner in the frictional charging of the carrier and the toner.
[0037] The fluorocarbon particles used in the invention are the particles of carbon monofluoride, polydicarbon monofluoride or polytetracarbon monofluoride, which can be obtained by heating at a higher temperature a carbon source such as carbon black, crystalline graphite and petroleum coke in the presence of fluorine gas, and generally described simply as CFx, wherein x represents the content of fluorine (normally, not more than 1.2, preferably not more than 0.5). These fluorocarbon particles can impart the carrier of the invention with a suitable level of conductivity. As a result, the specific resistance of the carrier can be in an appropriate range, allowing the carrier to have an improved developability.
[0038] It is preferred that the fluorocarbon particles are contained in the resin coating layer in a proportion of 5 to 60 wt%. When the fluorocarbon content exceeds 60 wt%, the fluorocarbon particles are likely to be removed from the surface of the carrier, since they cannot be dispersed sufficiently in the resin coating layer. If the fluorocarbon content is smaller than 5 wt%, the carrier has poor durability.
[0039] To protect the resin coating layer from abrasion, and to make the carrier of the invention more durable, it is preferred that the combined amount of the porous resin particles and the fluorocarbon particles accounts for 0.3 to 5 wt% of the amount of the core particles.
[0040] As the carrier core particles, magnetic particles can be employed preferably. To attain a sufficient frictional charging of the carrier with toner, and to prevent the carrier from sticking to a photoreceptor, the magnetic particles preferably have a weight average particle size of 10 to 200 µm, as measured by Microtrack Type 7981-OX (manufactured by Leeds & North Rup).
[0041] Substances usable as the magnetic particles include those which are strongly magnetized by a magnetic field in its direction, such as ferromagnetic metals (e.g. iron, cobalt, nickel) and alloys or compounds containing such ferromagnetic metals (e.g. ferrite, magnetite).
[0042] "Ferrite" is a general term for iron-containing magnetic oxides, and represented by MO·Fe₂O₃, wherein M represents a divalent metal such as nickel, copper, zinc, manganese, magnesium and lithium.
[0044] The core particles, porous resin particles and fluorocarbon particles according to the invention are mixed uniformly by means of a normal stirrer. To this mixture, impact is repeatedly given over a period of 10 to 60 minutes, preferably 15 to 30 minutes, by means of a high-speed stirring mixer of which the temperature has been set at 50 to 110°C. By this dry process, the porous resin particles and the fluorocarbon particles are allowed to stick to and spread over the surface of each core particle, thus forming a resin coating layer thereon.
[0045] The intensity of impact to be applied to the mixture of the porous resin particles, the core particles and the fluorocarbon particles is not limitative, as long as it is not too much to crush the core particles. The film-forming properties of the porous resin particles and the fluorocarbon particles are improved by increasing impact power within such a range as will not cause the core particles to be crushed.
[0046] Fig. 1 shows a high-speed stirring mixer suitable for use in a dry process. The upper lid 2 of a mixing chamber 1 is provided with a raw material input port 4, a filter 5 and an inspection port 6. The raw material input port is equipped with an input valve 3.
[0047] Raw material particles are put in the chamber from the raw material input port through the input valve 3, and stirred by rotary blades 8a, 8b and 8c of a rotator 8 that rotates horizontally by a motor 7. By this, impact is given to the raw material particles. As shown in Fig. 2, this rotator consists of a central part 8d, and three rotary blades (8a, 8b and 8c) provided symmetrically with respect to the central part 8d. Each rotary blade extends obliquely from the bottom 1a of the mixing chamber. By these blades, the raw material particles are stirred upwardly. The particles then collide with the upper or lower inner wall of the chamber, and then fall down within the rotating range of the rotator blades 8a, 8b and 8c. A rotator 9 that rotates vertically is provided above the rotator 8. This rotator 9 has two rotator blades that rotate vertically to collide with the particles that have been rebounded from the inner wall of the chamber. The rotator 9 serves to accelerate the stirring of the raw material particles and to prevent them from agglomeration.
[0048] Thus, the raw material particles repeatedly collide with the horizontal rotator 8, the vertical rotator 9, and the inner wall of the chamber 1, or collide with each other. By this, the raw material particles are given mechanical impact, and the porous resin particles and the fluorocarbon particles are caused to stick to the surface of each core particle to form a resin coating layer thereon. The so-obtained carrier particles open a discharging valve 10, and are withdrawn from a product discharging port 11.
[0049] A jacket 12 serves, for example, to heat the raw material particles when they are stirred, and to cool them after stirring. It covers 3/4 of the height of the mixing chamber's outer wall (i.e. covers to a position at which the vertical rotator is provided). The mixer temperature is measured by a thermometer 13.
[0051] In respect of the improved reproducibility of a solid image, characters or lines, it is preferred that the carrier have a resistivity of 10⁷ to 10¹⁴ Ω·cm, more preferably 10⁸ to 10¹² Ω·cm.
[0052] The carrier of the invention is mixed with toner to form two-component developer. In this case, the kind of toner is not limitative.
EXAMPLES
[0053] The present invention will be described in more detail according to the following examples, which should not be construed as limiting the scope of the invention.
[0054] In the following examples, "parts" means "parts by weight", BA means butyl acrylate, BMA means butyl methacrylate, and St means styrene.
<Porous resin particles A>
[0055] Porous resin particles each being composed of MMA/BA (weight ratio: 75/25) copolymer particles (elementary particles) with a volume average particles size of 0.1 µm as measured upon the completion of polymerization, which are fused to each other on their surfaces; having a BET specific surface area of 39 m²/g; and having a volume average particle size of 3.0 µm.
<Porous resin particles B>
[0056] Porous resin particles each being composed of MMA/BMA (weight ratio: 70/30) copolymer particles (elementary particles) with a volume average particles size of 0.20 µm as measured upon the completion of polymerization, which are fused to each other on their surfaces; having a BET specific surface area of 150 m²/g; and having a volume average particle size of 1.6 µm.
<Porous resin particles C>
[0057] Porous resin particles each being composed of MMA/BA (weight ratio: 75/25) copolymer particles (elementary particles) with a volume average particles size of 0.20 µm as measured upon the completion of polymerization, which are fused to each other on their surfaces; having a BET specific surface area of 5 m²/g; and having a volume average particle size of 4.9 µm.
<Porous resin particles D>
[0058] Porous resin particles each being composed of MMA/St (weight ratio: 60/40) copolymer particles (elementary particles) with a volume average particles size of 0.08 µm as measured upon the completion of polymerization, which are fused to each other on their surfaces; having a BET specific surface area of 75 m²/g; and having a volume average particle size of 2.9 µm.
<Non-porous resin particles a>
[0059] Non-porous resin particles each being composed of MMA/BMA (weight ratio: 80/20) copolymer particles (elementary particles) with a volume average particles size of 0.10 µm; and having a BET specific surface area of 65 m²/g.
<Porous resin particles b>
[0060] Porous resin particles each being composed of MMA/BA (weight ratio: 75/25) copolymer particles (elementary particles) with a volume average particles size of 0.06 µm as measured upon the completion of polymerization, which are fused to each other on their surfaces; having a BET specific surface area of 4.5 m²/g; and having a volume average particle size of 3.9 µm.
<Porous resin particles c>
[0061] Porous resin particles each being composed of MMA/St (weight ratio: 70/30) copolymer particles (elementary particles) with a volume average particles size of 0.04 µm as measured upon the completion of polymerization, which are fused to each other on their surfaces; having a BET specific surface area of 10 m²/g; and having a volume average particle size of 5.1 µm.
[Example 1]
[0063] The above ingredients were stirred for 15 minutes in a high-speed stirring mixer. Hot water was then circulated in the mixer to make its temperature be 80°C. Impact was given to the mixture by main stirring blades for 20 minutes, thus coating the surfaces of the core particles with the porous resin particles and the fluorocarbon particles by a dry process. As a result, a resin-coated carrier sample was obtained.
[0064] In the following examples and comparative examples, the substantially the same procedure as mentioned above was repeated to produce resin-coated carrier samples except that the ingredients were varied to those shown below:
[Example 2]
[Example 3]
[Example 4]
[Example 5]
[Example 6]
[Example 7]
[Comparative Example 1]
[Comparative Example 2]
[Comparative Example 3]
[Comparative Example 4]
[Evaluation]
[0075] Each of the carrier samples obtained above was examined for resin coating ratio, resin coating efficiency, white powder transmittance and carrier resistance. The results of the examination are shown in Table 1. The examination was performed by the following methods:
<Resin Coating Ratio>
[0076] The resin coating ratio is defined by the following formula:
1. Resin coating ratio (wt%)
2. Weight of resin applied
3. Weight of carrier
[0077] The measurement of the weights of the resin particles applied and the carrier obtained was performed as follows:
1. The tare weight of a glass-made sample tube of 30 cc- capacity was measured accurately by means of a chemical balance. This weight was designated as A.
2. About 3 g of a resin-coated carrier sample was put in a tared sample tube of 30 cc-capacity, and weighed accurately by means of a chemical balance. This weight was designated as B.
3. About 20 cc of methyl ethyl ketone was put in the above sample tube. The tube was covered, and stirred for 10 minutes by a wave rotor (Model WR-60; manufactured by Thermonics Corp), thereby allowing the resin to be molten.
4. The procedure 3 was repeated five times to remove the resin completely. The tube was then put in an oven heated to 60°C for drying, then cooled to room temperature. The weight after the removal of the resin was measured. This weight was designated as C.
[0078] From A, B and C, the weight of the resin particles applied and the weight of the carrier obtained were calculated by the following equations:
Weight of resin applied=B-C
Weight of carrier=B-A
(2) Resin coating efficiency
[0079] Resin coating efficiency is defined by the following formula:
1. Resin coating efficiency (%)
2. Resin coating ratio
3. Theoretical amount of resin applied
[0080] If there is no loss in the amount of resin particles applied, resin coating efficiency becomes 100%. The resin coating ratio in the above formula is the value obtained by the method (1), and includes the amount of white powder (explained later).
(3) Transmittance of white powder
[0081] The measurement of white powder transmittance is aimed at examining the amount of resin particles or agglomerates thereof that fail to form a film and electrostatically stick to and remain on the surface of a carrier particle in a free state. The higher the white powder transmittance, the larger the amount of white powder. No practical difficulty arises with a white powder transmittance of not less than 90%.
[0082] The white powder transmittance was measured by a process comprising introducing 20 g of each carrier sample and 15 ml of methanol into a 20 ml-sample tube, stirring by a wave rotor at 46 rpm, and putting the supernatant into a cell for an electrimetric colorimeter (wavelength: 522 nm) to examine the transmittance of white powder.
<Carrier Resistance>
[0083] A voltage of 100 V was applied to a 0.5-thick carrier layer (electrode area: 1 cm², load: 1 kg). The current after 30 seconds from the beginning of the voltage application was measured, and the carrier resistivity was calculated from the value obtained.
[0084] As is evident from Table 1, in Examples 1 to 7, good results were obtained for resin coating ratio, resin coating efficiency and white powder resistance, and the carrier samples obtained in these examples each had a sufficiently low surface resistance.
[0085] In Comparative Example 1 where the non-porous resin particles were employed, resin coating efficiency and white powder transmittance were lower than in Examples 1 to 7.
[0086] In Comparative Example 2 where the BET specific surface area of the porous resin particles was smaller than 5 m²/g, resin coating efficiency and white powder transmittance were lower than in Examples 1 to 7.
[0087] In Comparative Example 3 where the volume average particle size of the porous resin particles exceeded 5 µm, resin coating efficiency and white powder transmittance were lower than in Examples 1 to 7.
[0088] In Comparative Example 4 where the resin coating layer contained no fluorocarbon particles, though relatively good results were obtained for resin coating efficiency and white powder transmittance, the obtained carrier sample had an excessively high surface resistance.
<Imaging Test>
[0089] Each of the above-obtained carrier samples was mixed with toner for a copying machine (Type: U-Bix, manufactured by Konica Corp) at a mixing ratio shown in Table 2, thereby to obtain two-component developer samples.
[0090] Using each of the developer samples, imaging was performed by the above-mentioned copying machine. Evaluation was made for fogging, solid image density and carrier durability, and the results are shown in Table 2.
[0091] Of the development conditions, Dsd (the distance between a development drum and a development sleeve) and Hcut (the distance between a thickness-controlling blade and a development sleeve) were determined appropriately according to the particle size of each carrier sample.
<Fogging>
[0092] The density of the white background of a copy was measured by means of a Sakura densitometer (produced by Konica Corp), and indicated as a value relative to the density (reflection density: 0.0) of the white background of an original. Evaluation was made according to the following criterion:
Relative density smaller than 0.01= A
Relative density smaller than 0.02= B
Relative density not less than 0.02= C
<Solid Image Density>
[0093] The density of the solid portion of a copy was measured by means of a Sakura densitometer (manufactured by Konica Corp) and indicated as a value relative to the density of the solid portion of an original (reflection density: 1.2). Evaluation was made according to the following criterion:
Relative density larger than 1.2= A
Relative density of 1.0 to 1.2= B
Relative density smaller than 1.0= C
<Durability>
[0094] The density of the solid portion of a copy was measured by means of a Sakura densitometer (manufactured by Konica Corp) and indicated as a value relative to the density of the solid portion of an original (reflection density: 1.2). Durability was expressed in terms of the number of copies that were taken over which this relative solid image density fell below 1.0.
1. A carrier suitable for developing an electrostatic image which comprises core particles
coated with a layer comprising fluorocarbon particles and porous resin particles;
wherein the porous resin particles are composed of a plurality of elementary resin particles fused together at the surfaces thereof, the elementary resin particles having:
wherein the porous resin particles are composed of a plurality of elementary resin particles fused together at the surfaces thereof, the elementary resin particles having:
(a) a particle size of not more than 0.5 µm;
(b) a BET specific surface area of from 5 to 150 m²/g; and
(c) a volume average particle size of from 1.5 to 5.0 µm.
2. A carrier according to claim 1 wherein the BET specific surface area is from 10 to
120 m²/g.
3. A carrier according to claim 2 wherein the BET specific surface area is from 20 to
100 m²/g.
4. A carrier according to any one of the preceding claims wherein the elementary resin
is a styrene resin, acryl resin, styrene/acryl resin, vinyl resin, ethylene resin,
rosin-modified resin, polyamide resin, polyester resin, silicone resin or fluororesin.
5. A carrier according to any one of the preceding claims wherein the core particles
are magnetic particles having a size of from 10 to 200 µm.
6. A carrier according to any one of the preceding claims wherein the fluorocarbon particles
are present in the layer in an amount of from 5 to 60 wt%.
7. A carrier according to any one of the preceding claims wherein the porous resin particles
and the fluorocarbon particles are together present in an amount of from 0.3 to 5
wt% relative to the weight of the core particles.
8. A process for producing a carrier as defined in any one of the preceding claims which
comprises mixing together the core particles, fluorocarbon particles and porous resin
particles in the dry state.
9. A two-component developer which comprises a toner and a carrier as defined in any
one of claims 1 to 7.