A carrier for a developer

Description

[0001] The present invention relates to a carrier for a developer used for electrophotography, electro­static printing, and the like, and more particularly, relates to a magnetic carrier for a developer, pro­viding properly adjusted developing and cleaning char­acteristics and capable of forming high-density, and clear images at either a low or high developing speed.

[0002] In electrophotography which utilizes a two-­component developer comprising a mixture of toner and magnetic carrier, the toner being composed of colored resin powder prepared from a resinous binder having a colorant dispersed therein, the two-component developer is supplied to the circumferential surface of a devel­oping sleeve having a magnet disposed therein, and an electrostatic latent image formed on an image bearing member that is disposed facing the developing sleeve is developed with the toner. The two-component developer supplied to the developing sleeve forms a magnetic brush thereon consisting of a number of bristles, each bristle being formed by magnetic carrier particles that are linked together in a chain form. The toner in the developer is electrified by friction with the magnetic carrier that forms the bristles of the magnetic brush and adheres thereto. When the bristles of the magnetic brush rub the electrostatic latent image formed on the image bearing member, the toner adhering to the magnet­ic carrier on the bristles is attracted to the electro­static latent image by Coulomb force, thereby forming a toner image on the image bearing member.

[0003] In the above described electrophotography, when the speed difference between the image bearing member and the magnetic brush formed on the developing sleeve is great enough to ensure a sufficient brushing contact between the image bearing member and the mag­netic brush, a clear and high-density image is formed.

[0004] While the magnetic carrier forming the magnetic brush is required to have a good frictional electrification effect on the toner, it is also important to set the electrical resistance of the magnetic carrier which significantly influences the formation of an image within a predetermined range. Usually, the electric resistance of the magnetic carrier is so set that when a DC voltage is applied to the magnetic carrier (sample carrier) filled in a vessel of a predetermined volume, the measured re­sistivity comes within the range of 1 x 10⁶ to 1 x 10¹³Ω·cm.

[0005] However, even when the magnetic carrier has an electric resistance within the above-mentioned range, the magnetic carrier provides varied developing characteristics with variations in the composition and sintering conditions, etc., of a magnetic material for the magnetic carrier. Also, in the case of the magnetic carrier in which the magnetic substance (carrier core) is coated with a resin, the developing characteristic greatly varies with the variation in the electric resistance of the magnetic substance itself even when the electric resistance of the magnetic carrier is held within the above range as a whole. Therefore, with many kinds of magnetic carriers which have an electric resistance in the above-mentioned range, a decrease in a copy density, image trailing edge missing, image blurring and image tailing, etc., often occur, as the copying speed varies, resulting in a decrease in the quality of images. Moreover, there has not been sufficient knowledge presented so far about the electric characteristics of a magnetic carrier that constantly provides a satisfactory developing characteristic regardless of variations in copying conditions such as the copying speed.

[0006] The carrier for a developer of the present invention, which overcomes the above-discussed and numerous other disadvantages and deficiencies of the prior art, forms, in combination with toner, a magnetic brush on the circumferential surface of a developing sleeve disposed in a manner to face an image bearing member, said magnetic brush functioning to develop with the toner an electrostatic latent image that is formed on the image bearing member by means of a brushing contact with the electrostatic image, wherein said carrier satisfies the following relation:
1.7 ≦ X/Y ≦ 3.3

[0007] wherein X is the resistivity (Ω ·cm) of the carrier in a compressed state to which a DC voltage of 1000 V is applied, and Y is the resistivity (Ω ·cm) of the carrier in a state in which the carrier particles are linked together in a chain form in a magnetic field of 1600 gauss to which a DC voltage of 1000 V is applied.

[0008] In a preferred embodiment, the carrier for a developer is composed of particles made of a magnetic material.

[0009] In a preferred embodiment, the carrier for a developer is composed of particles made of a magnetic material that are coated with a resin.

[0010] Thus, the invention described herein makes possible the objective of providing a carrier for a developer which constantly provides satisfactory developing characteristics regardless of variations in copying conditions, in particular, the copying speed, so that high-density and clear images can be formed without causing image blurring, image tailing and car­rier development, etc., thereby making the carrier applicable to various copying systems with different copying conditions.

[0011] Generally, a magnetic carrier for a developer forms, in combination with toner in the developer, a magnetic brush on a developing sleeve. As the develop­ing sleeve revolves, the magnetic brush rubs an electrostatic latent image formed on an image bearing member that is disposed facing the developing sleeve. The magnetic brush consists of a number of bristles formed on the circumferential surface of the developing sleeve, each bristle being formed by the carrier particles that are linked together in a chain form, and through the frictional contact of the magnetic brush with the electrostatic latent image, the toner adhering to the carrier particles forming the magnetic brush is attracted to the electrostatic image by Coulomb force, thereby developing an electrostatic latent image into a toner image on the image bearing member. At the start of the developing process when the magnetic brush begins to rub the electrostatic latent image, since the carrier that is disposed on the circumferential surface of the developing sleeve is positioned away from the image bearing member with a relatively wide clearance provided therebetween, the bristles of the magnetic brush on the developing sleeve rub the electrostatic latent image independently of one another, but thereafter, the clearance therebetween gradually narrows as the developing sleeve revolves, causing the bristles of the magnetic brush to be compressed within the narrowed clearance, when they rub the image bearing member. In this state, the toner is transferred from the carrier forming the bristles of the magnetic brush to the electrostatic latent image on the image bearing member, while the bristles with reduced amount of the toner adhering thereto scrape the excessive amount of toner applied on the electrostatic latent image as well as the toner adhering to non-image forming areas. As the developing sleeve revolves further from the above-­mentioned state, the clearance between the carrier that is disposed on the circumferential surface of the developing sleeve and the image bearing member widens to release the bristles of the magnetic brush from the compressed state. The bristles of the magnetic brush are now made to contact the electrostatic latent image on the image bearing member independently of one anther, to develop the electrostatic latent image, after which the bristles are moved away from the image bearing member.

[0012] As described above, the magnetic carrier particles are in a state in which they are linked together in a chain form without being compressed in the early and final stages of the developing process, and are in the compressed state in other stages.

[0013] Since the state of the magnetic carrier changes as mentioned above during in the developing process, if the electric resistance of the magnetic carrier is set not only on the basis of the resistivity in the compressed state thereof but also in considera­tion of the resistivity of the magnetic carrier when the carrier particles are linked together in a chain form, which is close to the state of the magnetic carrier in the first and final stages of the developing process, it is possible to produce a magnetic carrier which provides an excellent developing characteristic.

[0014] If a magnetic carrier is used in which the ratio of X to Y satisfies the following relation:
1.7 ≦ X/Y ≦ 3.3
wherein X is the resistivity (Ω ·cm) of the carrier in the compressed state and Y is the re­sistivity (Ω ·cm) of the carrier when the carrier particles are linked together in a chain form in a magnetic field, the amount of the toner transferred to the latent image and the amount of the toner scraped by the magnetic brush from excessively applied areas and non-image forming areas will be maintained in a good balance at any copying speed, either high or low, so that a uniform copy density is obtained, thereby ensuring the formation of clear images without the fog of images, brush marking and carrier tailing.

[0015] In the present invention, the resistivity of the magnetic carrier in the compressed state was measured as follows: A sample carrier of 13.6 g was filled in a cylinder having a main electrode of 1.0 cm² in size at its bottom and a guard electrode surrounding the main electrode; the sample carrier filled in the cylinder was compressed by applying a load of 200 g/cm² on the cross section of an upper electrode; a DC voltage of 1000 V was applied to the sample carrier; and then the measurement was carried out. On the other hand, the resistivity of the carrier in the state in which the carrier particles were linked together in a chain form in a magnetic field was measured as follows: A sample carrier of 200 mg was introduced into a 2.0 mm clearance formed between a pair of electrodes, (each having an area of 1.2 cm²); a magnetic field with a surface magnetic flux density of 1600 gauss was applied to cause the sample carrier to be aligned in a chain form; a DC voltage of 1000 V was applied to the sample carrier; and then the measurement was carried out.

[0016] When the ratio of X to Y in the carrier does not satisfy the above-mentioned relation 1.7 ≦ X/Y ≦ 3.3, the following defects are caused. The carrier having a ratio greater than 3.3 can be used satisfactorily in a low-speed copying system, but in a medium- or high-speed system, tends to cause an in­crease in toner consumption, resulting in image blur­ring and image tailing, thereby causing an unsatisfac­tory transfer of a toner image onto a copy sheet and insufficient cleaning of the image bearing member. On the other hand, a carrier having the ratio smaller than 1.7 can be used satisfactorily in a high-speed copying system, but in a low-speed copying system results in a failure to obtain the appropriate copy density.

[0017] Examples of magnetic materials which can be used for the carrier of the present invention include iron oxide, reduced iron, copper, ferrite, nickel, cobalt, etc. and alloys of these metals mixed with zinc, aluminum, etc., but it is preferable to use particles made of ferrite which have a relatively stable electric resistance against an environmental change and change with time and which are capable of forming soft bristles.

[0018] Examples of ferrite appropriate for use are zinc ferrite, nickel ferrite, copper ferrite, manganese ferrite, nickel-zinc ferrite, manganese-magnesium ferrite, copper-magnesium ferrite, copper-zinc ferrite, manganese-copper-zinc ferrite. Especially, copper-zinc ferrite is preferable. The carriers made of these magnetic materials which can be used have a particle size of 10 to 200 µm, preferably 30 to 150 µm, and a saturation magnetization of 35 to 70 emu/g, preferably 40 to 65 emu/g.

[0019] The particles made of these magnetic materi­als may be directly used as the carrier, or may be coated with a resin, with the particles as the core.

[0020] As a resin used to coat the carrier core, a resin obtained from homopolymerization of the following monomers can be used:
styrene analogs such as p-chlorostyrene, methylstyrene; halogenated vinyl monomer such as vinyl chloride, vinyl fluoride; vinyl esters such as vinyl acetate, vinyl propionate, vinyl benzoate; esters of α-­methylene aliphatic monocarboxylic acid such as vinyl acrylate, methyl acrylate, ethyl acrylate, n-butyl acrylate, isobutyl acrylate, dodecyl acrylate, n-octyl acrylate, 3-chloroethyl acrylate, phenyl acrylate, methyl α-chloroacrylate, butyl methacrylate; vinyl ethers such as acrylonitrile, methacrylonitrile, acryloamide, vinyl methyl ether, vinyl isobutyl ether, vinyl ethyl ether and vinyl ketones such as vinyl ethyl ketone, vinyl hexyl ketone, methyl isopropenyl ketone.

[0021] Examples of other kinds of resins which can be used are epoxy resins, rosin-modified phenol­formaldehyde resins, cellulose resins, polyether resins, polyvinylbutyral, polyester resins, styrene­butadiene resin, polyurethane resins, polyvinylformal, melamine resins, polycarbonate, and fluorocarbon resins such as tetrafluoro-ethylene. One or more kinds of the above-mentioned resins can be used to coat the carrier core.

[0022] It is preferable to select a magnetic materi­al for a carrier, a resin for coating the carrier core and coating amounts of the resin so that the resistivity of the carrier, whether coated or not coated with a resin, comes within the range of 1 x 10⁸ to 1 x 10¹⁰Ω ·cm, when the carrier is in the compressed state, with the application of 1000 V, and within the range of 1 x 10⁷ to 1 x 10⁹Ω ·cm, when the carrier particles are linked together in a chain form in a magnetic field, with the application of 1000 V. As a toner which is mixed with the carrier to form a developer, resin powder of a particle size of 5 to 30 µm is used which is prepared from a resinous binder having a colorant, a change-controlling agent, and a magnetic material, if desired, dispersed therein. The resinous binder is selected in consideration of the fixing property as well as the frictional electrifica­tion property thereof with respect to the carrier.

[0023] Examples of resinous binders which can be used are aromatic vinyl resins such as polystyrene; and generally used thermoplastic resins and thermosetting resins such as acrylic resins, polyvinylacetal resins, polyester resins, epoxy resins, phenol resins, petroleum resins and olefin resins.

[0024] As colorants, previously known ones generally used in this field can be used. Examples of colorants which can be used are carbon black, lampblack, chrome yellow, Hansa yellow, benzidine yellow, threne yellow G, quinoline yellow, permanent orange GTR, pyrazolone orange, Vulcan orange, Watchung Red, permanent red, Brilliant Carmine 3B, Brilliant Carmine 6B, du Pont oil red, pyrazolone red, Lithol Red, Rhodamine B Lake, Lake Red C, rose bengal, aniline blue, ultramarine blue, chalco oil blue, methylene blue chloride, phthalocyanine blue, phthalocyanine green, malachite green oxalate; and oil soluble dyes such as C.I. Solvent Yellow 60, C.I. Solvent Red 27, and C.I. Solvent Blue 35. A single colorant or a mixture of more than one kind can be used for the toner. Also, as charge-controlling agents, previously known ones generally used in this field can be used. Examples of charge-controlling agents which can be used are oil soluble dyes such as Nigrosine base, oil black, and Spiron black; and metal salts of naphthenic acids, fatty acid metallic soaps and resin acid soaps.

[0025] Other additives which can be added to the toner include fluidity improvers, and fixing assistants such as silica and aluminum oxide, and various kinds of waxes as offset inhibitors.

[0026] Also, additives can be added to and mixed in the developer composed of a mixture of the carrier and toner. Examples of these additives are abrasives, fluidity improvers, filming inhibitors, and others gen­erally used in this field. Examples of these additives are inorganic fine powder made of aluminum oxide, silica, etc., and fine powder made of polymers such as silicone resins, acrylic polymers, styrene-acrylic polymers and fluorocarbon resins.

[0027] The following example illustrates this invention.

Example

[0028] Toner powder with a mean particle size of 17 µm was prepared from the following mixture of components, using a conventional method:

Component	Parts by weight
Styrene-acrylic copolymer	100
(resinous binder)
Carbon black	8
(colorant)
Metal complex monoazodye	1.5
(charge-controlling agent)
Polypropylene of low molecular weight	1.5
(offset inhibitor)

[0029] To 100 parts by weight of the thus produced toner, 0.2 parts by weight of hydrophobic silica and 0.2 parts by weight of aluminum oxide were added to form a toner composition.

[0030] Then, with Fe₂O₃·CuO·ZnO as the main compo­nent, several kinds of ferrite particles as the carrier core were produced using various sintering conditions (e.g., with conditions varied between the sintering temperatures 1000°C to 2000°C). Acryl-modified silicone resin KR-9706® (Silicone-type resin; produced by Shin-Etsu Co., Ltd.), acrylic resin BR-83® (produced by Mitsubishi Rayon Co., Ltd.), or New Gamet PC Clear® (fluorocarbon resin; produced by Toa Paint, Co., Ltd.) was used to coat the carrier core at a proportion of 0.8 to 1.6%, resulting in ten kinds of resin-coated carriers.

[0031] Finally, as previously mentioned, a voltage of 1000 V was applied to each of the ten carriers to measure the resistivities in the compressed state and in the linked state in a magnetic field.

[0032] Table 1 shows the characteristics of the respective carriers in each state.

Table 1

Carrier	Resistivity in compressed state (X)	Resistivity in linked state (Y)	X/Y
	Ω·cm	Ω·cm
a	9.8×10⁸	6.4×10⁸	1.5
b	1.7×10⁹	9.6×10⁸	1.8
c	1.8×10⁹	7.2×10⁸	2.5
d	2.9×10⁹	9.0×10⁸	3.2
e	3.3×10⁹	6.6×10⁸	5.0
f	3.0×10⁹	8.5×10⁸	3.5
g	9.5×10⁸	5.2×10⁸	1.8
h	3.2×10⁸	9.7×10⁸	3.3
i	4.0×10⁹	1.8×10⁹	2.2
j	2.5×10⁹	1.6×10⁹	1.6

[0033] Then, each kind of carrier was mixed with the above described toner to produce a developer. Each developer was used in copying tests on two electro-­photographic copying machines to examine the character­istics of copied images provided by each carrier. The two electro-photographic copying machines used for the tests had different copying speeds: One was Model No. DC-111 (A4: 11 sheets/minute) and the other was Model No. DC-5585 (A4: 55 sheets/minute), both machines being manufactured by Mita. The results are shown in Tables 2 and 3.

Table 2

	X/Y		Initial copy density	Toner consumption	Transfer efficiency	Image blurring	Image tailing	Image trailing edge missing	Internal machine contamination
				(mg/A4)	(%)
Carrier a	1.5	DC-111	1.20	29	89	○	○	×	○
		DC-5585	1.45	41	88	○	○	○	○
Carrier b	1.8	DC-111	1.41	37	89	○	○	○	○
		DC-5585	1.45	42	87	○	○	○	○
Carrier c	2.5	DC-111	1.44	40	86	○	○	○	○
		DC-5585	1.43	42	88	○	○	○	○
Carrier d	3.2	DC-111	1.44	43	83	○	○	○	○
		DC-5585	1.45	46	82	○	○	○	Δ
Carrier e	5	DC-111	1.48	55	75	Δ	×	○	Δ
		DC-5585	1.52	63	69	×	×	○	×

Table 3

	X/Y		Initial copy density	Toner consumption	Transfer efficiency	Image blurring	Image tailing	Image trailing edge missing	Internal machine contamination
				(mg/A4)	(%)
Carrier f	3.5	DC-111	1.45	52	78	Δ	×	○	×
		DC-5585	1.46	58	70	×	×	○	×
Carrier g	1.8	DC-111	1.40	41	88	○	○	Δ	○
		DC-5585	1.42	43	88	○	○	○	○
Carrier h	3.3	DC-111	1.45	46	82	○	Δ	○	○
		DC-5585	1.46	57	72	Δ	Δ	○	Δ
Carrier i	2.2	DC-111	1.43	41	85	○	○	○	○
		DC-5585	1.44	42	84	○	○	○	○
Carrier j	1.6	DC-111	1.19	28	86	○	○	×	○
		DC-5585	1.40	39	87	○	○	Δ	○

[0034] In Tables 2 and 3, the toner consumption means the toner amount consumed for making one copy of an A4 size original document (image forming area of 8%), while the transfer efficiency is obtained by dividing the difference between the total toner consumption amount and the toner amount collected through a cleaning operation by the total toner consumption amount.

[0035] Image blurring, image tailing, image trailing edge missing were estimated visually. The evaluation was made as follows:

[0036] ○: Not observed, Δ: Scarcely observed, and X: Frequently or continuously observed.

[0037] The internal machine contamination was estimated by applying a double-sided adhesive tape to a specific position inside the machine, and by measuring the amount of the toner which adhered to the tape with the use of a reflection densitometer as well as visual estimation. The evaluation was made as follows:

[0038] ○:contamination hardly observed, Δ: Slight contamination observed, and X: Considerable contamina­tion observed.

[0039] The results of the above-mentioned tests show that clear and high-density images are formed without causing image blurring, image trailing edge missing or other defects on either the low or high speed copying machines, when a carrier is used in which the ratio of the resistivity in the compressed state to the resistivity in the linked state in a magnetic field is approximately within the range of 1.7 to 3.3. The results also show that in the case of the ratio being smaller than 1.7, the toner amount transferred to the latent image on the image bearing member decreases with the decrease of the copying speed, causing a drop in copy density, and image trailing edge missing, etc., and that in the case of the ratio being greater than 3.3, the toner amount transferred to the latent image goes out of balance with respect to the toner amount scraped therefrom. The torn is consumed in excess with an increase in a copying speed, thereby frequently causing image blurring and image tailing to hamper the formation of clear images as well as the frequent contamination of the internal area of the copying machine due to the scattering toner.

Claims

1. A carrier for a developer forming, in combination with a toner, a magnetic brush on the circumferential surface of a developing sleeve disposed in a manner to face an image bearing member, said magnetic brush functioning to develop with the toner an electrostatic latent image that is formed on the image bearing member by means of a brushing contact with the electrostatic image, wherein said carrier satisfies the following relation:
1.7 ≦ X/Y ≦ 3.3
wherein X is the resistivity (Ω ·cm) of the carrier in a compressed state to which a DC voltage of 1000 V is applied, and Y is the resistivity (Ω ·cm) of the carrier in a state in which the carrier particles are linked together in a chain form in a magnetic field of 1600 gauss to which a DC voltage of 1000 V is applied.

2. A carrier for a developer according to claim 1, which is composed of particles made of a magnetic material.

3. A carrier for a developer according to claim 1, which is composed of particles made of a magnetic material that are coated with a resin.

4. A carrier for a developer according to any preceding claim which has a resistivity of from 1 x 10⁸ to 1 x 10¹⁰Ω.cm when the carrier in the compressed state with the application of 1000 V and within the range of 1 x 10⁷ to 1 x 10⁹Ω.cm when the carrier particles are linked together in a chain form in a magnetic field, with the application of 1000 V.

5. A carrier for a developer as claimed in any preceding claim, which has a particle size of 10 to 200 µm and a saturation magnetization of 35 to 70 emu/g.

6. A carrier for a developer as claimed in any preceding claim, wherein the toner is a resin powder of a particle size of 5 to 30 µm prepared from a resinous binder having a colourant, a charge controlling agent and optionally a magnetic material dispersed therein.

7. A carrier for a developer according to claim 6, wherein the resinous binder is an aromatic vinyl resin, an acrylic resin, a polyvinyl acetyl resin, a polyester resin, an epoxy resin, a phenol resin, a petroleum resin or an olefin resin.