1. Field of the Invention
This invention relates to a magnetic toner for rendering latent images visible in image-forming processes such as electrophotography, electrostatic recording, magnetic recording and toner jetting, and also relates to an image-forming method making use of the magnetic toner.
2. Related Background Art
A number of methods are conventionally known as electrophotography. In general, copies or prints are obtained by forming an electrostatic latent image on an electrostatic latent image bearing member (hereinafter also xe2x80x9cphotosensitive memberxe2x80x9d) by utilizing a photoconductive material and by various means, subsequently developing the electrostatic latent image by the use of a toner to form a toner image as a visible image, transferring the toner image to a transfer medium such as paper as occasion calls, and then fixing the toner image to a recording medium by the action of heat, pressure, or heat-and-pressure.
Apparatus for such image formation include copying machines and printers. In recent years, as printers, LED printers or LBP printers are prevailing in the recent market. As a trend of techniques, there is a tendency toward higher resolution. More specifically, those which hitherto have a resolution of 240 dpi or 300 dpi are being replaced by those having a resolution of 600 dpi, 800 dpi or 1,200 dpi. Accordingly, with such a trend, developing systems are now required to achieve a high minuteness. Copying machines have also made progress to have high functions, and hence they trend toward digital systems. In this trend, chiefly employed is a method in which electrostatic latent images are formed by using a laser. Hence, the copying machines also have come to have a high resolution. Also, with an improvement in image quality, it is much sought to achieve a higher speed and a longer service life.
In developing systems used in such printers and copying machines, toner images formed on the photosensitive member in the step of development are transferred to a recording medium in the step of transfer via, or not via, an intermediate member. Any transfer residual toner and fogging toner at non-image areas, left on the photosensitive member are removed in the step of cleaning, and is stored in a waste toner container. In this cleaning step, blade cleaning, fur brush cleaning, roller cleaning and so forth are conventionally used. When viewed from the standpoint of apparatus, the whole apparatus must be made larger in order to provide such a cleaning means. This has been a bottleneck in attempts to make apparatus compact. In addition, from the viewpoint of ecology, a system that may produce no waste toner is long-awaited in the sense of effective utilization of toner. Thus, it is sought to provide a toner having a high transfer efficiency and less causing fog.
From the viewpoint of making apparatus compact, one-component developing systems are preferable because they require no carrier particles such as ferrite particles or iron powder which are required in two-component developing systems. Also, since in the two-component developing systems the concentration of toner in two-component developers must be kept constant, a device for detecting toner concentration so as to supply the toner in the desired quantity is required, resulting in a large size for the developing assemblies. In the one-component developing system, on the other hand, such a device is not required, and hence the developing assemblies can also be made small and light-weight as being preferable. Magnetic toners used in such image-forming processes are commonly chiefly composed of a binder resin and a magnetic material and besides contain additives such as a charge control agent and a release agent which are used to bring out properties necessary as toners. As a colorant of the magnetic toner, the magnetic material is used as it is as the colorant, or a non-magnetic inorganic compound, organic pigment or dye is used together with the magnetic material. As the release agent, used are waxes sparingly compatible with the binder resin, as exemplified by low-molecular weight polyethylene and low-molecular weight polypropylene.
However, developing systems making use of an insulating magnetic toner have a problem concerning the insulating magnetic toner used. The problem is that, in insulating magnetic toner particles, a finely powdery magnetic material is mixed and dispersed in a considerable quantity, and it affects fluidity and triboelectric chargeability of the magnetic toner because magnetic fine particles constituting the magnetic material stand partly uncovered to the surfaces of toner particles, consequently causing variation or deterioration of various performances required for the magnetic toner, in relation to developing performance and running performance of the magnetic toner. This is presumed to be due to the fact that magnetic fine particles having a relatively lower electrical resistance than the resin constituting the magnetic toner particles are present at the surfaces of the magnetic fine particles. Also, the chargeability of the magnetic toner also has a great influence on development and transfer, and is closely concerned with image quality. Accordingly, it is sought to provide a magnetic toner which can stably provide a high charge quantity.
To cope with this problem, proposals concerning magnetic iron oxides to be contained in magnetic toners are hitherto made, but there is room for further improvement.
For example, Japanese Patent Application Laid-Open No. 62-279352 discloses a magnetic toner containing a magnetic iron oxide incorporated with silicon element. In such a magnetic iron oxide, the silicon element is intentionally brought into existence inside the magnetic iron oxide, but there is room for further improvement in the fluidity of the magnetic toner containing the magnetic iron oxide. Japanese Patent Publication No. 3-9045 discloses adding a silicate to control the shape of magnetic iron oxide to be spherical. In the magnetic iron oxide thereby obtained, the silicon element is rich distributed inside the magnetic iron oxide fine particles because of the use of the silicate for the controlling of particle shape of magnetic fine particles and the silicon element is less present at the surfaces of the magnetic iron oxide fine particles, thus, because of a high smoothness of the magnetic iron oxide fine particles, the fluidity of the magnetic toner can be improved to a certain extent. However, it is preferable to more improve the close adhesion between the binder resin constituting magnetic toner particles and the magnetic iron oxide. Japanese Patent Application Laid-Open No. 61-34070 discloses a process for producing triiron tetraoxide by adding a hydroxosilicate solution to triiron tetraoxide in the course of oxidation reaction. The triiron tetraoxide fine particle obtained by this process has silicon element in the vicinity of its surface, but the silicon element is present in layer in the vicinity of the surface of the triiron tetraoxide fine particles. Hence, there is a problem that the surface is weak to mechanical shock such as friction.
Meanwhile, toners are produced by melt-mixing a binder resin, colorant and so forth and uniformly dispersing them, followed by pulverization by means of a fine griding mill and then classification by means of a classifier to obtain toners having the desired particle diameters (pulverization process). To make toners have fine particle diameters, there is a limit to the range of material selection. For example, colorant-dispersed resin compositions must be brittle enough to be pulverizable by means of economically available production apparatus. Since the colorant-dispersed resin compositions are made brittle because of such a requirement, particles having particle diameters in a broad range tend to be formed when such compositions are actually pulverized at a high speed, so that, in particular, fine particles (particles having been pulverized in excess) having a relatively large proportion are formed in a large quantity and also the magnetic fine particles tend to come off from the resin during pulverization. Moreover, such highly brittle materials tend to be further pulverized or powdered when used as developing toners in copying machines or printers.
As a countermeasure therefor, Japanese Patent Application Laid-Open No. 2-256064 disclose, in the production of pulverization toners, a magnetic toner production process in which magnetic fine particles standing free are removed by classification after pulverization. In pulverization processes, however, the magnetic iron oxide fine particles essentially come to stand uncovered to the surfaces of magnetic toner particles, and hence a problem tends to arise in fluidity of magnetic toner particles and in charging stability in severe environment, resulting in a low transfer performance. Thus, there is further room for improvement.
In the pulverization process, it is also difficult to uniformly disperse solid fine particles such as a magnetic powder and a colorant in the resin. Depending on the degree of such dispersion, this can be one of the causes of an increase in fog and a decrease in image density.
In the pulverization process, making toner particles finer in order to achieve high minuteness and high image quality accompanies a lowering of uniform chargeability and fluidity of toners.
In order to overcome such problems of toners which are ascribable to the pulverization process and further to satisfy requirements stated above, proposed are processes for producing toner particles by suspension polymerization.
Toner particles produced by suspension polymerization (hereinafter xe2x80x9csynthetic toner particlesxe2x80x9d or xe2x80x9csynthetic tonerxe2x80x9d) can readily be produced in fine particles. In addition, toner particles obtained have a spherical shape and hence have a superior fluidity. This is advantageous for achieving a high image quality.
However, incorporation of magnetic fine particles into such synthetic toner particles tends to make the toner particles have a low fluidity and a low charging performance, tending to cause a lowering of developing performance. This is because the magnetic fine particles are commonly hydrophilic and hence tend to be present at toner particle surfaces in the suspension polymerization, which makes use of an aqueous medium. This is also because, in the step of granulation carried out when magnetic synthetic toner particles are produced, hydrophilic magnetic fine particles may partly move to the aqueous medium to become present as free magnetic fine particles having come off from the magnetic toner particles. In order to solve this problem, it is important to modify the surface properties the magnetic fine particles have.
To improve dispersibility and enclosure property of the magnetic fine particles in the synthetic toner particles, proposals are made in a large number in regard to surface modification of the magnetic fine particles. For example, Japanese Patent Applications Laid-Open No. 59-200254, No. 59-200256, No. 59-200257 and No. 59-224102 disclose techniques for treating magnetic fine particles with silane coupling agents of various types, and Japanese Patent Application Laid-Open No. 63-250660 discloses a technique for treating silicon-element-containing magnetic fine particles with a silane coupling agent.
Such treatment brings about a certain improvement in the dispersibility in the magnetic toner particles. However, the magnetic fine particle surfaces must be made uniformly hydrophobic, and the uncovering of the magnetic fine particles to the magnetic toner particle surfaces must be more controlled.
Meanwhile, with regard to the quantity of magnetic fine particles at the magnetic toner particle surfaces, a toner having a special structure in which any magnetic fine particles are not present in toner particle surface layers is proposed as disclosed in Japanese Patent Application Laid-Open No. 7-209904. This toner is advantageous in that it ensures superior enclosure of magnetic fine particles and can be free from any uncovering of the magnetic fine particles to the magnetic toner particle surfaces. However, such a toner must be produced by a complicate process, and can be produced with difficulty in an industrial manufacture scale. Also, its repeated use over a long period of time in an environment of low humidity may cause a lowering of image quality which is due to charge-up of the magnetic toner. Thus, more improvement has been necessary for the stability of charging of the magnetic toner.
Further, a technique for making the particle diameter of the magnetic toner particles smaller to achieve higher image quality is disclosed in Japanese Patent Application Laid-Open No. 1-112253. However, with the magnetic toner particles having such a smaller diameter, it is more difficult to obtain the uniform dispersion of the magnetic powder and the enclosure thereof, tending to give rise to the above-mentioned various problems.
For the purpose of improving the fluidity and charging performance of toners, methods are also proposed in which inorganic fine powder is added as an external additive, and are put into wide use. For example, Japanese Patent Applications Laid-Open No. 5-66608, No. 4-9860 and so forth disclose external addition of an inorganic fine powder having been subjected to hydrophobic treatment or an inorganic fine powder having been subjected to hydrophobic treatment and thereafter further to treatment with a silicone oil. Japanese Patent Applications Laid-Open No. 61-249059, No. 4-264453 and No. 5-346682 disclose use of a hydrophobic-treated inorganic fine powder and a silicone-oil-treated inorganic fine powder in combination. Such methods are known in the art.
Methods in which conductive fine particles are externally added as the external additive are also proposed in a large number. For example, carbon black as conductive fine particles is known to be used as an external additive for the purpose of providing conductivity to toners or controlling any excess charging of toners to make their triboelectric distribution uniform. Also, Japanese Patent Applications Laid-Open No. 57-151952, No. 59-168458 and No. 60-69660 disclose external addition of conductive fine particles such as tin oxide, zinc oxide and titanium oxide, respectively, to high-resistance magnetic toner particles. Japanese Patent Applications Laid-Open No. 61-275864, No. 62-258472, No. 61-141452 and No. 2-120865 disclose addition of graphite, magnetite, polypyrrole conductive particles or polyaniline conductive particles to toners.
These proposals, however, have room for further improvement to solve the above problems, when toner particles having small particle diameters are used in order to achieve a higher resolution.
In recent years, as copying machines and printers are being made compact, it has also become an important subject to achieve space saving, cost reduction and low power consumption. With regard to fixing assemblies, too, they have become required to be compact, structurally simple, and small power consumption. With this trend, toners are made to have a low viscosity at the time of melting to enlarge the area for their adhesion to fixing base materials or toner particles are incorporated with a release agent so that the toners can exhibit a sufficient fixing performance at a low amount of heat and a low pressure. Accordingly, binder resins used are required to have a low glass transition point (Tg) and a low molecular weight. However, for toners composed chiefly of soft components, it is difficult to achieve both fixing performance and high-temperature anti-offset properties simultaneously. Such toners also have a problem that they tend to cause a lowering of developing performance during long-term service or to stick or cling to the photosensitive member.
Meanwhile, with regard to the improvement of fixing performance, various proposals have been made from old times. For example, Japanese Patent Publication No. 51-23354 discloses a pulverization toner improved in high-temperature anti-offset properties and low-temperature fixing performance, obtained by polymerizing a monomer such as styrene in the presence of a cross-linking agent and a molecular-weight modifier to obtain an appropriately cross-linked resin, and kneading this resin and a colorant such as carbon black, followed by pulverization. Japanese Patent No. 2681791 discloses a pulverization toner obtained by melt-kneading a styrene type binder resin containing a THF-insoluble matter in an mount of 10 to 60% by weight based on the weight of the resin, together with a charge control agent and a wax, followed by pulverization. These publications teach that molecular chains of the THF-insoluble matter (cross-linked component) of the binder resin are cut by melt kneading to form a high-molecular weight component, to thereby obtain a toner improved in both high-temperature anti-offset properties and low-temperature fixing performance. However, as a result of such thermal and mechanical cutting of the insoluble matter of the binder resin, soluble components formed by the cutting of molecular chains may have a fairly broad molecular weight distribution. Hence, medium-molecular-weight components which may damage low-temperature fixing performance also tend to be formed in a large quantity.
Such pulverization toner particles further have a problem that they have so low a circularity as to have a low transfer efficiency. Also, since the magnetic fine particles stand uncovered to toner particle surfaces in a large number, the toners tends to have a low fluidity and a low uniform chargeability.
On the other hand, in the case of the synthetic toners, different from the pulverization toners, toner particles can directly be produced without the step of melt kneading, and hence molecular chains of the insoluble matter (cross-linked component) formed at the time of polymerization are by no means cut. Thus, they are advantageous in that toner particles having very high anti-offset properties can be obtained, but on the other hand the insoluble matter tends to damage the low-temperature fixing performance. Accordingly, the low-temperature fixing performance and the high-temperature anti-offset properties must be balanced by controlling the insoluble matter. Also, when the magnetic fine particles are made insufficiently hydrophobic, the magnetic fine particles tend to stand uncovered to the toner particle surfaces in a large number, tending to make fixing performance poor and cause a deterioration of fixing assemblies.
Japanese Patent Application Laid-Open No. 11-38678 discloses non-magnetic synthetic toner particles having 0 to 20% of components having a molecular weight of 1,000,000 or more and 0 to 60% of THF-insoluble matter, the total of the both being 1 to 60%. This publication, however, discloses a technique concerning non-magnetic toner particles, and there is room for improvement in respect of magnetic synthetic toner particles containing magnetic fine particles. In addition, Japanese Patent No. 2749234 discloses a process for producing magnetic toner particles in which a wax component in toner particles is present in a fibrous form. As disclosed in this publication, a polymerizable cross-linking agent is added in a monomer composition containing magnetic particles, which is then polymerized in the presence of an azo type polymerization initiator to obtain magnetic synthetic toner particles. Also, Japanese Patent No. 2749122 discloses a method of surface-treating magnetic particles with a polymer having a specific reactive group. As disclosed in this publication, a polymerizable cross-linking agent is added in a monomer composition containing magnetic particles, which is then polymerized in the presence of an azo type polymerization initiator to obtain magnetic synthetic toner particles. However, as presumed from the amount of the cross-linking agent, the type and amount of the polymerization initiator and the polymerization temperature which are described in these publications, the medium molecular weight component produced because of any excess formation of THF-insoluble matter or its cross-linking in a very weak state may be in a large proportion. Hence, in the case of the magnetic toner containing the magnetic fine particles in a large quantity, there is a problem on fixing performance. Also, in the magnetic synthetic toner particles obtained by the processes disclosed in these publications, the hydrophobic treatment of the magnetic fine particles used is insufficient, and there are problems also on fluidity and charging performance. Moreover, the achievement of both developing performance and fixing performance is also insufficient.
With regard to the image-forming method, as methods by which the electrostatic latent image is formed into a visible image, developing systems such as cascade development, magnetic brush development and pressure development are known in the art. Another method is also known in which, using a magnetic toner and using a rotary sleeve internally provided with a magnet, the magnetic toner is caused to fly across a photosensitive member and a developing sleeve by the aid of an electric field. For example, Japanese Patent Application Laid-Open No. 54-43027 discloses a method in which a magnetic toner is thinly coated on a magnetic toner-carrying member and this is triboelectrically charged, which is then served to develop an electrostatic latent image under application of a magnetic field. According to this method, the thin coating of a magnetic toner on a magnetic toner-carrying member enables the magnetic toner to be sufficiently triboelectrically charged. Moreover, the electrostatic latent image is developed while the magnetic toner is supported by the action of magnetic force. Hence, the magnetic toner can be kept from spreading to non-image areas, so that any fog can be kept from occurring and highly minute images can be obtained. Also, with regard to transfer efficiency, use of a toner having a uniform charge quantity distribution brings about a high transfer efficiency, but it is sought to make further improvement.
Spherical toner particles are esteemed to have a high transfer efficiency. Concerning such particles, Japanese Patent Application Laid-Open No. 61-279864 discloses a proposal on a toner whose shape factors SF-1 and SF-2 are specified. Japanese Patent Application Laid-Open No. 63-235953 discloses a proposal on a magnetic toner made spherical by the action of mechanical impact force. However, toners are sought to be more improved in transfer efficiency.
Such spherical toner particles have on the one hand an advantage that they have a higher transfer efficiency than toner particles produced by pulverization, but on the other hand have a nature that they can be removed by cleaning with difficulty because of their sphericity. Moreover, since toner particles trend toward smaller particle diameters as state previously, toner particles may escape at the time of cleaning, and it has become more difficult to remove transfer residual toner completely by cleaning. However, an improvement of cleaning assemblies can keep the toner particles from escaping to a level that may cause no great problem. In image-forming methods having a conventional corona charging system, images having no problem in practical use can be formed.
However, in recent years, from the viewpoint of environmental protection, in place of the primary charging and transfer process which have utilized corona discharging conventionally used, it is becoming prevailing to employ primary charging (contact charging) and transfer process (contact transfer) each of which make use of a member brought into contact with the photosensitive member surface, having great advantages of low ozone and low power consumption. For example, Japanese Patent Applications Laid-Open No. 63-149669 and No. 2-123385 disclose processes concerning the contact charging process and contact transfer process. In these processes, a conductive flexible charging roller is brought into contact with a photosensitive member and the photosensitive member is uniformly charged applying a voltage to the conductive roller, followed by exposure and development to form a toner image. Thereafter, another conductive roller to which a voltage is kept applied is pressed against the photosensitive member, during which a transfer medium is passed between them, and the toner image held on the photosensitive member is transferred to the transfer medium, followed by the step of fixing to obtain a fixed copy image.
However, in such a contact charging process and a contact transfer process, too, there is room for further improvement. Stated specifically, in the case of the contact charging, the charging member is kept in pressure contact with the surface of the photosensitive member by pressing the former against the latter. Hence, the presence of any transfer residual toner tends to lower the contact between the contact charging member and the photosensitive member to tend to lower charging performance. In reverse development, the toner tends to spread to non-image areas to tend to cause fog. Also, any accumulation of toner on the charging performance tends to make it difficult to charge the photosensitive member uniformly, tending to cause a decrease in image density or cause coarse images. In addition, since the charging member is kept in pressure contact, melt-adhesion of toner tends to occur. These tendencies appear more remarkably as the transfer residual toner is in a large quantity.
Then, in the case of the contact transfer, the transfer member is brought into contact with the photosensitive member through the transfer medium at the time of transfer, and hence the toner image is pressed when the toner image formed on the photosensitive member is transferred to the transfer medium, tending to cause a problem of partial faulty transfer, which is called xe2x80x9cblank areas caused by poor transferxe2x80x9d. Moreover, as a trend of techniques in recent years, there is a demand for developing systems of higher resolution and higher minuteness. To meet such a demand, toners are directed to have a smaller particle diameter. However, as toners are made to have a smaller particle diameter, the attraction force (e.g., mirror force or van der Waals force) of toner particles on the photosensitive member may increase to tend to result in an increase in the transfer residual toner, tending to cause faulty transfer.
Thus, in the image-forming methods making use of the contact charging process and contact transfer process which are very preferable taking account of environment, it is sought to bring forth a magnetic toner, and an image-forming method, which promise a high transfer performance and a superior charging stability and may hardly cause melt-adhesion of toner.
Meanwhile, with regard to the toner having a high transfer efficiency as stated above, also proposed is a technique called a development-cleaning (also called cleaning-at-development) system or cleanerless system in which development and cleaning are carried out in the same step.
Disclosure of conventional techniques concerning the development-cleaning or cleanerless system is, as seen in Japanese Patent Application Laid-Open No. 5-2287, focused on positive memory or negative memory appearing on images because of an influence of the transfer residual toner. However, in these days where electrophotography is utilized on and on, it has become necessary to transfer toner images to various recording mediums. In this sense, it is sought to make further adaptation to various recording mediums.
The prior art having disclosed the cleanerless system is seen in Japanese Patent Applications Laid-Open No. 59-133573, No. 62-203182, No. 63-133179, No. 64-20587, No. 2-302772, No. 5-2289, No. 5-53482 and No. 5-61383. These, however, neither mention any desirable image-forming methods nor refer to how the toner be constituted.
As developing systems in which the development-cleaning system or cleanerless system is preferably applied, in conventional development-cleaning systems basically having no cleaning assembly, it has been considered essential for the system to be so made up that the photosensitive member surface is rubbed with the toner and toner-carrying member. Accordingly, studies have largely made on contact developing systems in which the toner or toner-carrying member comes into contact with an image-bearing member. This is because, in order to collect the transfer residual toner in a developing means, it is considered advantageous for the system to be so made up that the toner or toner-carrying member comes into contact with and rub the image-bearing member. However, in the development-cleaning system or cleanerless system making use of a contact development system, its long-term service tends to cause deterioration of toner, deterioration of toner-carrying member surface and deterioration or wear of photosensitive member surface, but any satisfactory solution has not been made for running performance. Accordingly, it is sought to provide a development-cleaning system according to a non-contact developing system.
Here, consider an instance in which the contact developing system is applied to a image-forming method employing the development-cleaning system or cleanerless system. In the image-forming method employing the development-cleaning system or cleanerless system, any cleaning member is provided and hence the transfer residual toner remaining on the photosensitive member surface comes into contact with the contact charging member as it is, to come to adhere to or mix in the contact charging member. Also, in the case of a charging system predominantly governed by a discharge charging mechanism, the transfer residual toner tends to adhere to the charging member because of a deterioration due to discharge energy. When insulating toners commonly used adhere to or mix in the contact charging member, the charging performance tends to lower.
In the case of the charging system predominantly governed by a discharge charging mechanism, the charging performance of the member to be charged tends to lower abruptly around the time when a toner layer having adhered to the contact charging member surface comes to have a resistance which may obstruct the discharge voltage. On the other hand, in the case of a charging system predominantly governed by a direct injection charging mechanism, the charging performance of the member to be charged may lower where the transfer residual toner having adhered or mixed has lowered the probability of contact between the contact charging member surface and the member to be charged.
This lowering of uniform charging performance of the member to be charged appears as a lowering of contrast and uniformity of electrostatic latent images after imagewise exposure to tend to cause a decrease in image density or make fog occur seriously.
In the image-forming method employing the development-cleaning system or cleanerless system, the point is that the charge polarity and charge quantity of the transfer residual toner on the photosensitive member is controlled so that the transfer residual toner can stably be collected in the step of development and the collected toner may not make the developing performance poor. Accordingly, the charge polarity and charge quantity of the transfer residual toner on the photosensitive member is controlled by means of the charging performance.
This will be described specifically taking the case of a commonly available laser beam printer. In the case of reverse development making use of a charging member for applying a voltage with negative polarity, a negatively chargeable photosensitive member and a negatively chargeable toner, in the transfer step the toner image is transferred to the recording medium by means of a positively chargeable transfer member. The charge polarity of the transfer residual toner varies from positive to negative because of its relation to the type of the recording medium (differences in thickness, resistance, dielectric constant and so forth) and the areas of images. However, the charging member having a negative polarity, used to charge the negatively chargeable photosensitive member, can uniformly adjust the charge polarity to the negative side even if even the polarity of the transfer residual toner has been shifted to the positive side in the transfer step. Hence, when the reversal development is employed as the developing system, the transfer residual toner, which stands negatively charged, remains at light-area potential areas to be developed by toner. At dark-area potential areass not to be developed by toner, the toner is attracted toward the toner carrying member in relation to the development electric field and is collected without remaining on the photosensitive member having a dark-area potential. That is, the development-cleaning system can be established by controlling the charge polarity of transfer residual toner simultaneously with the charging of the photosensitive member by means of the charging member.
However, where the transfer residual toner has adhered to or mixed in the contact charging member beyond the contact charging member""s capacity to control toner""s charge polarity, it becomes difficult to uniformly adjust the charge polarity of the transfer residual toner. Also, even where the transfer residual toner has been collected on the toner-carrying member by mechanical force such as rubbing, the transfer residual toner may adversely affect the charging performance of toner on the toner-carrying member, resulting in a lowering of developing performance, unless its charge has not uniformly been adjusted.
More specifically, in the image-forming method employing the development-cleaning system or cleanerless system, the charge control performance at the time the transfer residual toner passes the charging member and the manner in which the transfer residual toner adheres to or mixes in the charging member are closely concerned with the running performance and image quality characteristics.
In order to prevent uneven charging to effect stable and uniform charging, the contact charging member may be coated with a powder on its surface coming into contact with the surface of the member to be charged. Such constitution is disclosed in Japanese Patent Publication No. 7-99442.
The contact charging member (charging roller) is follow-up rotated as the member to be charged (photosensitive member) is rotated (without no velocity differential drive), and hence may remarkably less cause ozone products compared with corona charging assemblies such as Scorotron. However, the principle of charging is still chiefly the discharge charging mechanism like the case of the roller charging mentioned previously. In particular, a voltage formed by superimposing AC voltage on DC voltage is applied in order to attain more stable charging uniformity, and hence the ozone products caused by discharging may more greatly occur. Accordingly, when the apparatus is used over a long period of time, difficulties such as smeared images due to ozone products tend to come out. Moreover, when applied in cleanerless image-forming apparatus, any inclusion of the transfer residual toner makes it difficult for the powder coated, to stand adhered uniformly to the charging member, so that the effect of carrying out uniform charging may lower.
Japanese Patent Application Laid-Open No. 5-150539 also discloses that, in an image-forming method making use of contact charging, at least image-developing particles and conductive fine particles having an average particle diameter smaller than that of the image-developing particles are contained in a toner in order to prevent any charging obstruction which may be caused when toner particles or silica particles having not completely be removed by a cleaning means such as a cleaning blade come to adhere to and accumulate on the surface of the charging means during repetition of image formation for a long time. However, the contact charging used here, or proximity charging, applies the discharge charging mechanism, which is not the direct injection charging mechanism, and has the above problem ascribable to the discharge charging. Moreover, when applied in the cleanerless image-forming apparatus, nothing is taking into consideration about any of the influence on charging performance that is exercised when the conductive fine particles and transfer residual toner pass the charging step in a larger quantity than the apparatus having a cleaning mechanism, the influence on the collection of these large-quantity conductive fine particles and transfer residual toner in the developing step, and the influence on toner""s developing performance that is exercised by the conductive fine particles and transfer residual toner thus collected. Furthermore, when the direct injection charging mechanism is applied in the contact charging, the conductive fine particles can not be fed to the contact charging member in necessary quantity to tend to cause faulty charging due to the influence of the transfer residual toner.
In the proximity charging, it is also difficult to uniformly charge the photosensitive member because of the large-quantity conductive fine particles and transfer residual toner, and the effect of leveling patterns of the transfer residual toner can not be obtained, to cause pattern ghost because the transfer residual toner may shut out pattern-imagewise exposure light. In-machine contamination due to toner may further occur when a power source is instantaneously put off or paper jam occurs during image formation.
In the image-forming method employing the development-cleaning system, development-cleaning performance can be improved by improving charge control performance required when the transfer residual toner passes the charging member. As a proposal therefor, Japanese Patent Application Laid-Open No. 11-15206 discloses an image-forming method making use of a toner having toner particles containing specific carbon black and a specific azo type iron compound and having inorganic fine powder. It is also proposed, in the image-forming method employing the development-cleaning system, to improve development-cleaning performance by reducing the quantity of transfer residual toner, using a toner having a superior transfer efficiency the shape factors of which have been specified. However, the contact charging used here also applies the discharge charging mechanism, which is not the direct injection charging mechanism, and has the above problem ascribable to the discharge charging. Moreover, these proposals may be effective for keeping the charging performance of the contact charging member from lowering because of the transfer residual toner, but can not be expected to be effective for positively improving the charging performance.
In addition, among commercially available electrophotographic printers, image-forming apparatus are also available which are designed for carrying out the development-cleaning system, in which a roller member coming into contact with the photosensitive member is provided between the transfer step and the charging step so that the performance of collecting the transfer residual toner at development can be assisted or controlled. Such image-forming apparatus have good development-cleaning performance and the waste toner can greatly be reduced, but involve a high cost and may damage the advantage inherent in the development-cleaning system also in view of compact construction.
As countermeasures for these, Japanese Patent Application Laid-Open No. 10-307456 discloses an image-forming apparatus in which a toner containing conductive charge-accelerating particles having particle diameter which is xc2xd or smaller than average particle diameter of toner particles or toner is applied in an image-forming method employing the development-cleaning system making use of the direct injection charging mechanism. According to this proposal, an image-forming apparatus for carrying out the development-cleaning system can be obtained, which can greatly reduce the quantity of waste toner and is advantageous for making the apparatus compact at a low cost, and good images are obtainable without causing any faulty charging and any shut-out or dispersion of imagewise exposure light.
Japanese Patent Application Laid-Open No. 10-307421 also discloses an image-forming apparatus in which a toner containing conductive particles having particle diameter which is {fraction (1/50)} to xc2xd of average particle diameter of the toner is applied in an image-forming method employing the development-cleaning system making use of the direct injection charging mechanism and the conductive particles are made to have a transfer accelerating effect. Japanese Patent Application Laid-Open No. 10-307455 still also discloses that, a conductive fine powder is controlled to have particle diameter not larger than the size of one pixel of constituent pixels, and the conductive fine powder is controlled to have particle diameter of from 10 nm to 50 xcexcm in order to attain better charging uniformity.
Japanese Patent Application Laid-Open No. 10-307457 discloses that, taking account of human visual sensation, conductive fine particles are controlled to have particle diameter of about 5 xcexcm or smaller, and preferably from 20 nm to 5 xcexcm, in order to make any influence of faulty transfer on images visually recognizable with difficulty.
Japanese Patent Application Laid-Open No. 10-307458 also discloses an image-forming method which employs the development-cleaning system making use of the direct injection charging mechanism and in which a conductive fine powder is controlled to have particle diameter not larger than the average particle diameter of a toner to thereby prevent the conductive fine powder from obstructing the behavior of the toner at the time of development or prevent development bias from leaking through the conductive fine powder, and the conductive fine is controlled to have particle diameter larger than 0.1 xcexcm to thereby eliminate a difficulty that the conductive fine powder may become buried in the image-bearing member to shut out imagewise exposure light, thus superior image recording can be materialized.
Japanese Patent Application Laid-Open No. 10-307456 discloses an image-forming apparatus which carries out the development-cleaning system and in which a conductive fine powder is externally added to toner particles so that the conductive fine powder contained in the toner particles may adhere to an image-bearing member in the step of development, at least at a contact zone between a flexible contact charging member and the image-bearing member, and may remain and be carried on the image-bearing member also after the step of transfer so as to stand between them, to thereby obtain good images without causing neither faulty charging nor shut-off of imagewise exposure light.
In all these proposals, however, there is room for further improvement in stable performances required when the apparatus are repeatedly used over a long period of time and in performances required when toner particles having a small particle diameter are used in order to achieve a higher resolution.
An object of the present invention is to provide a magnetic toner having solved the problems the prior art has had, and an image-forming method making use of the magnetic toner.
Another object of the present invention is to provide a magnetic toner having a good fixing performance, having superior environmental stability and charging stability and can form images in a high density and a high minuteness even over long-term service, and an image-forming method making use of such a magnetic toner.
Still another object of the present invention is to provide an image-forming method which can well carry out the development-cleaning system.
A further object of the present invention is to provide an image-forming method which can stably achieve a good charging performance and enables image formation by the cleanerless system.
To achieve the above objects the present invention provides a magnetic toner comprising magnetic toner particles containing at least a binder resin, a magnetic material containing a magnetic iron oxide, and a release agent.
the magnetic toner having;
a weight-average particle diameter of from 3 xcexcm to 10 xcexcm;
a magnetization intensity (saturation magnetization) of from 10 Am2/kg to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds);
an average circularity of 0.970 or more;
a ratio of weight-average particle diameter to number-average particle diameter, of 1.40 or less;
iron and an iron compound which stand liberated from the magnetic toner particles at a liberation percentage of from 0.05% to 3.00%; and
a resin component having a tetrahydrofuran (THF)-insoluble matter in an amount of from 3% by weight to 60% by weight.
The present invention also provides an image-forming method comprising;
a charging step of charging an image-bearing member electrostatically by applying a voltage to a charging member kept in contact with the image-bearing member, forming a contact zone between them;
an electrostatic latent image forming step of forming an electrostatic latent image on the charged surface of the image-bearing member;
a developing step of forming a toner image by developing the electrostatic latent image by causing a magnetic toner to move to the electrostatic latent image at a developing zone where an alternating electric field is kept formed; the developing zone being formed between the image-bearing member for holding thereon the electrostatic latent image and a toner-carrying member for carrying the magnetic toner on its surface which are face to face disposed leaving a preset space between them, and a layer of the magnetic toner being formed on the surface of the toner-carrying member in a thickness smaller than that space; and
a transfer step of transferring the toner image to a transfer material via, or not via, an intermediate transfer member;
the steps being repeated to form images;
wherein the magnetic toner comprises magnetic toner particles containing at least a binder resin, a magnetic material containing a magnetic iron oxide, and a release agent.
the magnetic toner having;
a weight-average particle diameter of from 3 xcexcm to 10 xcexcm;
a magnetization intensity (saturation magnetization) of from 10 Am2/kg to 50 Am2/kg (emu/g) under application of a magnetic field of 79.6 kA/m (1,000 oersteds);
an average circularity of 0.970 or more;
a ratio of weight-average particle diameter to number-average particle diameter, of 1.40 or less;
iron and an iron compound which stand liberated from the magnetic toner particles at a liberation percentage of from 0.05% to 3.00%; and
a resin component having a tetrahydrofuran (THF)-insoluble matter in an amount of from 3% by weight to 60% by weight.