1. Field of the invention
The present invention relates to an image forming method carried out by developing an electrostatic latent image to form a toner image and transferring the toner image to a recording medium, followed by fixing. More particularly, it relates to an image forming method in which untransferred toner having remained on an electrostatic latent image bearing member after transfer is collected by a cleaning means and again used in the development of electrostatic latent images.
2. Related Background Art
A number of methods as disclosed in U.S. Pat. No. 2,297,691, Japanese Patent Publications No. 42-23910 and No. 43-24748 and so forth are conventionally known for electrophotography. In general, copies are obtained by forming an electrostatic latent image on a photosensitive member by utilizing a photoconductive material and by various means, subsequently developing the latent image by the use of a toner, and transferring the toner image to a transfer medium such as paper if necessary, followed by fixing by the action of heat, pressure, heat-and-pressure or solvent vapor.
Various methods or apparatus have been developed in relation to the above final step, i.e., the step of fixing the toner image to a sheet such as paper. A method most commonly available at present is the pressure heating system making use of a heating roller.
The pressure heating system making use of a heating roller is a method of carrying out fixing by causing an image-receiving sheet to pass over a heating roller whose surface is formed of a material having a releasability to toner while a toner image surface of the former is brought into contact with the surface of the latter under application of a pressure. Since in this method the surface of the heating roller comes into contact with the toner image of the image-receiving sheet under application of a pressure, a very good thermal efficiency can be achieved when the toner image is melt-adhered onto the image-receiving sheet, so that fixing can be carried out rapidly. This method is therefore very effective in high-speed electrophotographic copying machines. In this method, however, since the surface of the heating roller comes into contact with the toner image in a molten state under application of a pressure, part of the toner image may sometimes adhere and transfer to the surface of the fixing roller, which may re-transfer to the subsequent image-receiving sheet to cause an offset phenomenon, resulting in a contamination of the image-receiving sheet. Thus, it is one of important requirements in the heating roller fixing method to cause no toner to adhere to the surface of the heat-fixing roller.
For the purpose of causing no toner to adhere to the surface of a fixing roller, it has been attempted, for example, to form the surface of a roller by the use of a material having a good releasability to the toner, such as silicone rubber or fluorine resin, and further covering the roller surface with a thin film of a fluid having a good releasability, such as silicone oil in order to prevent offset to its surface and to prevent roller surface fatigue. Although this method is very effective for preventing the toner offset, it requires a device for feeding an offset preventing fluid, and hence has the problem that a fixing assembly becomes complicated.
Moreover, under the influence of such a device, the machine inside may be contaminated because of the evaporation of silicone oil by heat. Accordingly, from the thought that no device for feeding silicone oil should be used and instead the offset preventing fluid should be fed from the interior of the toner at the time of heating, a method is proposed in which a release agent such as low-molecular weight polyethylene or low-molecular weight polypropylene is added in the toner. When such an additive is used in a large quantity to make its addition very effective, it causes filming on the photosensitive member or contaminates the surface of a carrier or a toner carrying member such as a sleeve. As a result, deterioration of images is caused and there are problems in practical use. Accordingly, the release agent is added in toner in an amount small enough not to cause deterioration of images, and a releasing oil is fed a little or a cleaning device for removing the toner having offsetted is used in combination. The cleaning unit is a device comprising a cleaning means making use of a wind-up type member such as a web.
It is also known to incorporate a wax into toner as a release agent. For example, Japanese Patent Applications Laid-open No. 52-3304, No. 52-3305 and No. 57-52574 disclose such techniques.
Japanese Patent Applications Laid-open No. 3-50559, No. 2-79860, No. 1-109359, No. 62-14166, No. 61-273554, No. 61-94062, No. 61-138259, No. 60-252361, No. 60-252360 and No. 60-217366 disclose techniques by which waxes are incorporated into toners.
The waxes are used to improve anti-offset properties of toner in low-temperature fixing or high-temperature fixing or to improve fixing performance in low-temperature fixing.
In practice, however, good high-temperature anti-offset properties have been achieved but no satisfactory low-temperature fixing performance results, or good low-temperature anti-offset properties and low-temperature fixing have been achieved but unsatisfactory high-temperature anti-offset properties result. Thus, good low-temperature anti-offset properties and good high-temperature anti-offset properties have not been simultaneously achieved.
Now, as other methods, various attempts are made on techniques to improve of binder resins.
For example, in order to prevent the offset of toner, a method is known in which the glass transition point (Tg) or molecular weight of a binder resin in a toner is made higher to improve melt elasticity of the toner. If, however, such a method is used to improve anti-offset properties, the fixing performance may become unsatisfactory to cause the problem that fixing performance in low-temperature fixing (i.e., low-temperature fixing performance) becomes poor which is required when high-speed copying machines are used or energy saving is intended.
On the other hand, in order to improve the fixing performance of a toner, the viscosity of the toner at the time of melting must be decreased to increase the area in which the toner adheres to a fixing substrate. For this reason, it is required to make the Tg or molecular weight of the binder resin lower.
Since the low-temperature fixing performance and the anti-offset properties conflict each other in one aspect, it is very difficult to make an improvement in toners that can simultaneously satisfy these functions.
As proposals to solve these problems, for example, Japanese Patent Publication No. 51-23354 discloses a toner comprising a vinyl polymer appropriately cross-linked by adding a cross-linking agent and a molecular weight modifier, and Japanese Patent Publication No. 55-6895 discloses a toner comprising an xcex1,xcex2-unsaturated ethylene monomer as a component unit and whose molecular weight distribution has been broadened so that the ratio of weight average molecular weight to number average molecular weight comes to be 3.5 to 40. Another proposal is made for a toner comprising a vinyl polymer in which a blended resin having specific Tg, molecular weight, gel content and so forth is used.
It is true that these toners proposed can achieve a broader fixing temperature range between lowest fixing temperature (the lowest temperature at which fixing can be carried out) and offset temperature (the temperature at which the offset begins to occur) than a toner comprising a single-component resin having a narrow molecular weight distribution, but it is difficult to make the fixing temperature sufficiently low when a satisfactory offset preventing performance is imparted. On the contrary, there is a problem that the offset preventing performance becomes unsatisfactory when importance is attached to low-temperature fixing performance.
For example, a toner comprising a binder resin comprised of a low-molecular weight polymer and a high-molecular weight polymer is proposed in Japanese Patent Application Laid-open No. 56-158340. In practice, it is difficult for this binder resin to be incorporated with a cross-linking component, and hence it is necessary to make the molecular weight of the high-molecular weight polymer larger or to increase the proportion of the high-molecular weight polymer in order to improve the anti-offset properties. This aims at a remarkable decrease in grindability of resin compositions, and it is difficult to obtain satisfactory toners in practical use. As another proposal regarding a toner comprising a blend of a low-molecular weight polymer and a cross-linked polymer, Japanese Patent Application Laid-open No. 58-86558 discloses a toner having a resin component mainly comprised of a low-molecular weight polymer and an insoluble infusible high-molecular weight polymer. This method is considered capable of improving the anti-offset properties of toners and the grindability of resin compositions. However, the low-molecular weight polymer has a value of weight average molecular weight/number average molecular weight (Mw/Mn) of as small as 3.5 and the insoluble infusible high-molecular weight polymer is in an amount of as large as 40 to 90% by weight, and hence it is difficult to satisfy both the anti-offset properties of toners and the grindability of resin compositions at a high performance. In practice, it is very difficult to produce a toner that can satisfy both the fixing performance and the anti-offset properties unless a fixing machine having the device for feeding an offset preventing fluid is used. Moreover, in the course of heat kneading when the toner is produced, the melt viscosity greatly increases with an increase in the insoluble infusible high-molecular weight polymer, and hence the heat kneading must be carried out at a much higher temperature than usual, consequently bringing about a problem of a lowering of toner performance because of thermal decomposition of additives.
Japanese Patent Application Laid-open No. 60-166958 discloses a toner comprising a resin composition having a number average molecular weight of from 500 to 1,500, obtained by polymerization carried out in the presence of a low-molecular weight xcex1-methylstyrene polymer.
In particular, this publication discloses that the number average molecular weight (Mn) is preferably in the range of from 9,000 to 30,000. Making the Mn larger in order to improve anti-offset properties brings about problems in practical use, on the fixing performance and the grindability required when the toner is produced. Hence, it is difficult to satisfy both the anti-offset properties and the grindability of resin compositions at a high performance. Thus, the toner showing a poor grindability when the toner is produced is not preferable since it may cause a decrease in production efficiency of the toner produced, and also tends to cause inclusion of coarse toner because of properties of the toner, often resulting in occurrence of black spots around images.
Japanese Patent Applications Laid-open No. 56-16144 discloses a toner containing a binder resin component having at least one peak value in each of the regions of a molecular weight of 103 to 8xc3x97104 and a molecular weight of 105 to 2xc3x97106 This toner has superiority in the grindability of binder resin components, anti-offset properties of toner, fixing performance, prevention of filming or melt-adhesion to photosensitive members, and developing performance. It is sought to further improve the anti-offset properties and fixing performance in the toner. In particular, it is difficult for this resin to cope with the recent severe demand while further improving the fixing performance and also while maintaining or improving other various performances.
Thus, it is very difficult to achieve at a high performance both the performance concerning the fixing of toner (the low-temperature fixing performance and anti-offset properties) and the grindability in the production of toner. In particular, the grindability in the production of toner is a factor important to the recent trend where toners are made to have smaller particle diameters in answer to demands for making the quality level of copied images higher, making the resolution thereof higher and achieving higher fine-line reproducibility. The step of pulverization requires a very large energy, and hence the improvement in grindability is important also in view of energy saving. The phenomenon of melt-adhesion of toner to the inner walls of a pulverizing apparatus tends to occur in toners having a good fixing performance, sometimes resulting in a poor pulverization efficiency.
In the process of copying, there is a step in which the toner having remained on a photosensitive member after transfer is removed by cleaning. Nowadays, taking account of making apparatus more small-sized, light-weight and reliable, it is prevalent to carry out cleaning by means of a blade (i.e., blade cleaning). As photosensitive members are made to have a longer lifetime, drum-type photosensitive members are made to have a smaller diameter and systems are made more high-speed, requirements on toners becomes severer in respect of melt-adhesion resistance and filming resistance to photosensitive members. In particular, amorphous silicon photosensitive members having been recently put into practical use have a very high durability. OPC (organic photosensitive members) are also enjoying a longer lifetime. Hence, the performances required on toners have become higher.
To make apparatus small-sized, components must be well disposed in an narrow place. This is accompanied by a decrease in space through which cooling air flows and also a very near approach of a fixing assembly or a heat source of an exposure system to a toner hopper or a cleaner, so that toner is laid open to a high-temperature atmosphere. For this reason, none of toners can be now put into practical use unless they have much superior blocking resistance.
As a means for overcoming the problems discussed above, the present applicant has disclosed in Japanese Patent Application Laid-open No. 63-223662 a special resin to which a low-molecular weight resin is added during suspension polymerization. Even this method, however, can not achieve a satisfactory fixing performance when used in high-speed copying machines that can take copies on 50 or more A4-size sheets per minute. There has been found another problem that fixed images tend to be stained because of flow-out of toner from a cleaning member coming into contact with a fixing roller.
In low-speed or medium-speed copying machines, the quantity of offset matter on the fixing roller becomes reasonably large with an increase in the quantity of paper feed even though offset quantity per sheet is very small, which can be a cause of troubles of the fixing assembly. In order to remove this small quantity of offset matter, a fixing-step cleaning member such as a cleaning roller or web made of silicone rubber is fitted to the fixing roller in contact therewith. Conventional binder resins for toners are designed mainly with the intention of low-temperature fixing performance and anti-offset properties, and are not designed so that a high melt viscosity can be maintained even against a high temperature exceeding 200xc2x0 C. Hence, the toner substance having adhered to the fixing-step cleaning member comes to have a low melt viscosity as it stands there for a long time at a temperature set for the fixing roller. In addition, when the temperature of the fixing roller overshoots the temperature set for the fixing roller when a copying machine is switched on, the fixing roller may come to have a temperature higher than 200xc2x0 C., resulting in an extreme decrease in melt viscosity of the toner having adhered, which toner is again transferred to the fixing roller to cause contamination of recording mediums.
Japanese Patent Applications Laid-open No. 1-172843 and No. 1-172844 disclose a toner having peaks at a molecular weight of 3xc3x97103 to 5xc3x97103 and a molecular weight of 1.5xc3x97105 to 2.0xc3x97106, and having 40 to 60% of peak area in the region of a molecular weight of 1.5xc3x97105 to 2.0xc3x97106 or having 1 to 10% of gel content. However, it is hard to say that the toner has also completely well coped with the anti-offset properties and fixing performance, and the toner is sought to be further improved.
As discussed above, various performances such as developability, low-temperature fixing performance, anti-offset properties, blocking resistance, filming resistance and grindability (of resin compositions) required for toners often conflict with each other. In recent years, it is more sought to satisfy them altogether at high performances.
In the transfer step, the toner on a photosensitive member (an electrostatic latent image bearing member) is not transferred in its entirety, and about 10 to 20% by weight of the toner remains on the photosensitive member. The toner thus having remained on the photosensitive member (i.e., untransferred toner) is collected through a cleaning step and discharged out of the system as what is called a waste toner, which has not been reused. When such waste toner is discarded as waste (waste plastic material), there is a possibility of causing environmental pollution. Accordingly, nowadays, the waste toner is reused. That is, it is being widely studied to reuse the waste toner. If it becomes possible to reuse the waste toner, there can be advantages such that toners can be used effectively, machine space can be simplified and machines can be made compact.
Hitherto, however, when the waste toner is again used in the developing step, there have been various adverse effects such that reflection image density decreases, ground fog and reversal fog increase and toner scatter occurs.
As performances of the toner applied to such reusable systems, the toner is required not only to have the developability, low-temperature fixing performance, anti-offset properties, blocking resistance, filming resistance and grindability stated above, but also to have the properties such that it is tough to mechanical stress, has a good durability or running performance and shows a good transport performance when the waste toner is fed to the developing step.
To cope with these requirements, a variety of toners have been hitherto invented. For example, Japanese Patent Application Laid-open No. 63-220172 discloses a toner in which a non-linear polyester is used in a binder resin and a low-molecular weight polyolefin is incorporated therewith; Japanese Patent Application Laid-open No. 1-214874, a toner in which a specific polyester resin containing an aliphatic diol is used in a binder resin; and also Japanese Patent Application Laid-open No. 2-110572, a toner in which a metal-crosslinked styrene/acrylate copolymer is used in a binder resin and to which a polyolefin is added in a large quantity. These toners invented, however, all have a high possibility that some difficulties occur, e.g., the anti-offset properties become poor.
In recent years, there is an increasing demand for copying machines. With such demand, user""s demands for copying machines are varying. Under such circumstances, machine bodies are persistently required to be made compact particularly in the field of low-speed or medium-speed copying machines.
In recent years, not only high-speed copying machines but also such low-speed or medium-speed copying machines are sought to be made more highly durable and more highly reliable, and it is attempted to increase copy volume while maintaining always good image characteristics. Thus, with an increase in the copy volume, the quantity of the toner consumed increases, concurrently resulting in an increase in the quantity of the toner untransferred (i.e., waste toner). Hitherto, as previously mentioned, the untransferred toner is scraped off by a cleaning means such as a cleaning blade, delivered to a waste toner box and accumulated there and discharged out of the system. Thus, the waste toner has not been reused. The reason therefor is that the reuse of the waster toner has been accompanied by difficulties such that reflection image density decreases, ground fog and reversal fog increase, and toner scatter occurs. However, if it becomes possible to reuse the waste toner, not only toners can be used effectively, but also many advantages can be expected such that machines can be made compact since the waste toner box that has hitherto held a large volume in a machine body becomes unnecessary.
As discussed above, the performances required for toners often conflictory to each other. In recent years, also in the case when the waste toner is reused, it is more sought to satisfy them altogether at high performances.
An object of the present invention is to provide an image forming method that has solved the problems discussed above, that is, an image forming method employing a recycle system in which the untransferred toner is reused.
Another object of the present invention is to provide an image forming method that can obtain always sharp images without undergoing any mechanical damage even when copies are continuously taken while recycling the untransferred toner.
Still another object of the present invention is to provide an image forming method that can maintain an always high reflection image density and may cause no ground fog or toner scatter even when the untransferred toner is recycled.
A further object of the present invention is to provide an image forming method making use of a toner suited for a heat-roll fixing system in which no oil is applied.
A still further object of the present invention is to provide an image forming method making use of a toner that can achieve low-temperature fixing and has superior anti-offset properties.
A still further object of the present invention is to provide an image forming method making use of a toner that can achieve low-temperature fixing and may cause neither melt-adhesion nor filming to photosensitive members even in a high-speed system or during its use over a long period of time.
A still further object of the present invention is to provide an image forming method making use of a toner that can achieve low-temperature fixing, has a superior blocking resistance, and can also be used in a high-temperature environment in copying machines, in particular, small-sized machines.
A still further object of the present invention is to provide an image forming method making use of a toner that may cause less generation of coarse powder, because of a good grindability, and hence may cause less black spots around images and can form stable good developed images.
A still further object of the present invention is to provide an image forming method making use of a toner suited for a cleaning system employing a blade.
The present invention provides an image forming method comprising;
forming a toner image by developing through a developing means an electrostatic latent image formed on an electrostatic latent image bearing member;
said developing means holding a toner; said toner comprising a binder resin, a colorant and a release agent; said binder resin having at least one peak in the region of a molecular weight of from 2,000 to 50,000 and at least a peak or a shoulder in the region of a molecular weight of not less than 100,000, in molecular weight distribution as measured by gel permeation chromatography (GPC); and said release agent having a methylene chain;
transferring the toner image formed on the electrostatic latent image bearing member, to a recording medium;
cleaning the electrostatic latent image bearing member from which the toner image has been transferred to the transfer medium, to collect untransferred toner remaining on the electrostatic latent image bearing member;
feeding the toner collected, to said developing means so as to be again held in the developing means and used to form a toner image on the electrostatic latent image bearing member; and
fixing the toner image transferred to the recording medium, to the recording medium through a fixing means.