1. Field of the Invention
This invention relates to toner for developing an electrostatic latent image, a process for manufacturing the same, a two-component electrostatic latent image developer using toner, and an image-forming method using the developer, which are used in an electrophotographic process and the like.
2. Description of the Related Art
Various electrophotographic processes, as such, are known. These processes are disclosed, for example, in U.S. Pat. No. 2,297,691, Japanese Patent Application Publication (JP-B) No. 42-23910 (U.S. Pat. No. 3,666,363), and the like. The electrophotographic process generally comprises basic steps consisting of an exposure step of forming an electrostatic latent image using various measures on a photosensitive layer utilizing a photoconductive material; a step of developing using a toner; a step of transferring the toner on a recording medium, e.g., paper; a step of fixing the toner image on the recording medium by means of heating, pressure, heat and pressure, solvent vapor, or the like; and a step of removing the toner remaining on the photosensitive layer.
There have lately been increasing demands for inexpensive and compact copy machines and printers using an electrophotographic method. It is important that a toner be fixed using lower power consumption and that, at the same time, a fixing process be simplified to design such copy machines and printers.
At present, a fixing process using a heat roller is adopted as the most common process for fusing toner to fix it on paper. It is not necessary to supply oil to the heat roller in systems for monochromatic copy machines or printers. However, in machines using a color toner, it is still necessary to provide a means of supplying oil to the heat roller to prevent an offset to the heat roller. This is obstructive to the design of a compact and inexpensive system because of the reasons that follow. Specifically, it is necessary to thermally fuse each of the toner layers in a full-color system which is required to produce a sharp multicolor image using color toners, especially cyan, yellow, and magenta toners. To this end, the fixing temperature of the heat roller must therefore be increased to a temperature high enough to fix the toner on paper.
Also, as a toner for developing an electrostatic latent image used in an electrophotographic field, resins which can fuse sharply and which allow the image surface to be smooth, specifically, resins having a low molecular weight and a narrow molecular weight distribution, are convenient. A very useful resin which has been recently adopted as the toner is polyesters which have a sufficient flexibility even if these are lowered in molecular weight. When such polyester resins for a color toner have low internal cohesive force, it is difficult to release toner from a roller during fusing. Accordingly, it is difficult to adopt polyester resins as a color toner in a fixing system without application of oil.
Approaches to solve these problems are, for example, disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 52-3304, 52-3305, 57-52574, 61-138259, 56-87051, and 63-188158. In these approaches, a toner to which a releasing agent is added is used so that the toner can be peeled off from a roller with ease, and these approaches have been widely investigated. The toner into which a releasing agent is incorporated is successful in the characteristic of release from the roller to some extent. However, not all characteristics required for a toner are necessarily satisfied. One of the reasons for this phenomenon, for example, is that the amount of releasing agents contained inside the toner and the amount existing on the surface of the toner cannot easily be controlled at the optimum condition. More concretely, in order to satisfy the characteristic of release from the roller, the amount of releasing agent added to the toner is preferably in a range of from 1% to 10% by weight. As the amount of releasing agent is increased, the releasing effect is higher. However, the powder fluidity and thermal cohesiveness of a toner prepared in a kneading-pulverizing process, which is a common method for preparing a toner, are impaired by adding the releasing agent only in an amount of from 1 to 3% by weight. This is because wax usually used as a releasing agent is deposited at a high concentration together with a pigment at the toner surface, that is, a pulverized surface. Further, when the toner is processed in apparatuses such as a pulverizer and a classifier used in production stages, the wax concentrated on the surface melts partially and spreads due to the collision between particles and frictional heat generated by friction in apparatuses. The spread wax finally covers the surface of the toner. According to Japanese Patent Application Laid-Open (JP-A) No. 02-87159, impact force was mechanically applied on a toner containing wax, which had been kneaded and pulverized, to make the toner spherical and then the amount of wax on the surface of the toner was measured by ESCA. As a result, the amount of wax was found to be on the order of 10% to 40% by weight. The ratio of wax remaining on the surface of kneaded-pulverized-type toner containing 1% to 10% by weight of wax is usually in a range of from 30% to 50% by weight. This wax on the surface causes a blocking phenomenon and, also, contaminates the surfaces of a photosensitive member and carrier, thereby causing the developing characteristics to be changed. Also, there are problems in that the adhesion to a photosensitive member and to a transfer intermediate increases, resulting in a reduction in transfer efficiency and that image smear occurs when a color image requiring multiple transfer is formed.
Processes for preparing novel toners have been proposed to overcome these drawbacks of the toner to which wax is added. One of these toners is a toner having a capsule-type structure which is disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 60-222868, 61-114247, and 59-162562. Using this capsule-type toner, the proportion of components with a low molecular weight such as wax and the like disposed on the surface of the toner particles can be reduced to 0% to 1% by weight. Problems related to the above-mentioned blocking and filming characteristics and also to the developing characteristics are comparatively difficult to solve. Such a toner provided with no wax on the surface thereof often fails in providing sufficient capability for being released from a roller. In order to avoid offset to a heat roller, it is necessary to instantly form a weak boundary layer composed of melted wax and the like between a toner fused at a fixing nip of a heat roller and the surface of the heat roller and thereby to prevent direct contact with a toner resin in an extremely short period of time. However, if wax is entirely inside the toner, a delay owing to the diffusion of wax to the surface of the toner occurs so that the weak boundary layer cannot be sufficiently formed. When a color image is formed using the capsule-type toner having a hard shell on the surface, a higher temperature is usually required to melt the toner image completely. It is further difficult to release toner from a roller, especially in processes for fixing toner at high-speed.
Another method for preparing toner is one of forming a so-called "pseudo-capsule structure". Specifically, in this pseudo-capsule structure, oil droplets of a monomer composition or a toner composition are formed in a dispersing medium with high polarity such as water. Polar components contained in the oil droplets are allowed to existing in a surface layer corresponding to the boundary layer between the water-phase and the oil-phase, and nonpolar components are not allowed to exist in the surface layer. A typical example of this toner is a toner prepared in the suspension-polymerization process which is disclosed in Japanese Patent Application Publication (JP-B) No. 36-10231. For a toner produced by a wet process, the amounts of wax existing on the surface and inside the toner can be controlled in a range in which the resistance to blocking and the oilless fixing capability are comparatively compatible by making use of such characteristics of the process. However, in toner produced by the suspension-polymerization process, the phase separation of nonpolar wax components from polymers including polar monomers as a major component is promoted as polymerization progresses, resulting in the formation of the above pseudo-capsule structure. However, phase separation during the polymerization process tends to vary depending on the composition of the monomers and polymerization conditions, and hence it is difficult to practice such structural control in actual industrial application in a stable manner. Japanese Patent Application Laid-Open (JP-A) No. 05-88409 describes that wax cannot exist in the surface layer to a depth of 0.15 times the particle diameter, exhibiting the same problem as the capsule-type toner. Also, compositions of polymerizable monomers of a resin to be toner are limited to monomers capable of being solution-polymerized such as styrene and its derivatives and .alpha.-methylene aliphatic monocarboxylates. Problems occur in which it is difficult to use polyesters which are required for producing a color toner as above-mentioned because of the toner resins.
The problem of the limitation in selection of a resin material for a toner is involved in the above-described kneading-pulverizing process. Specifically, a resin for a toner must be pulverized using an usual pulverizer and hence the resin is therefore required to be fragile. In this case, the problem arises that the resin is overly pulverized in a pulverizing step, whereby the size distribution is wider and contamination with bulky particles occur.
In view of this, a novel toner structure which meets the demands for wide selectivity as a toner resin, better compatibility of an oilless fixing capability with resistance to blocking, and excellent stability in development and transfer, and a novel process for manufacturing such a novel toner are required. Further, according to Japanese Patent Application Laid-Open (JP-A) No. 5-127422, a method is disclosed in which an oil-phase solution having a toner component dissolved therein is formed into particles in a water-phase containing a water soluble resin. Also, according to Japanese Patent Application Laid-Open (JP-A) No. 7-152202, a method is disclosed in which polyester is dissolved and then formed into particles in a water-phase containing an inorganic dispersant. Further, in Japanese Patent Application Laid-Open (JP-A) Nos. 7-168395 and 7-271099, tricalcium phosphate and hydroxyapatite, which have a particle diameter of 0.7 to 5 .mu.m, are proposed as an inorganic dispersant to be used. In these proposals, however, there are no descriptions of a releasing agent such as wax being added as a component for a toner, the amounts of wax on the surface and inside the particle are controlled in a specific range, and the compatibility of the resistance to blocking with the oilless fixing capability is maintained at a high level. When a toner was produced in practice by adding wax according to Japanese Patent Application Laid-Open (JP-A) Nos. 7-168395 and 7-271099, it was confirmed that polyester particles were prepared, but the size distribution was large, and the amount of toner micropowder of a size of 1 to 2 .mu.m or less, which was unsuitable for toner, increased and it was difficult to remove the powder by mechanical classification. Also, there are problems that colorants used for color toner include compounds which cause an increase in the amount of toner micropowders.
Regarding important functions required for copy machines or printers, there are demands in which a clear color image is outputted on media other than paper. Among these demands, those for forming a fixed image on a transparent film to make transparencies used for explanations using an overhead projector (OHP) are important in a field for color image production. However, color toners produced by a pulverizing process and by a polymerization process include those which are insufficient in color caused by the inadequate dispersion of wax. There are color toners which provide only an insufficient transparent image, especially, for an OHP image. Particularly, in the case of using a low-molecular-weight polyester as a binding resin and a low-molecular-weight wax as a releasing agent, a toner produced by a pulverizing method exhibits the above problems more strikingly since an adequate share is not applied in the steps of melting and kneading. At present, there is no adequate means for solving the above problem of insufficient compatibility of oilless fixing capability with high transfer capability relating to manufacturing of toner, specifically, relating to the homogeneous dispersion of wax as the releasing agent into the toner resin, especially into the polyester resin which is suitably used.