Multi-functional peripherals (MFPs) and digital printing systems form images by the electrophotographic process, which is based on the principle of static electricity, and photoconductors play an important role in this process.
Photoconductors generally have a three-layer structure, with an undercoat layer, a charge generation layer, and a charge transport layer on top of an aluminum cylinder, and have the property that they generate a charge when light is shone on them.
The electrophotographic printing process first applies a negative charge to the surface of the photoconductor to generate an electrostatic charge. Light is then irradiated on the areas to which toner should adhere (colored areas). This is known as “exposure.” Positive and negative charges are generated in the charge generation layer of the photoconductor, with the negative charge migrating via the undercoat layer to the aluminum substrate and the positive charge via the charge transport layer to the surface of the photoconductor. When this positive charge transfers to the surface, it neutralizes the negative electrostatic charge that had previously been generated. This means that some areas of the photoconductor surface are negatively charged while others are not, creating an invisible “electrostatic latent image” composed of static electricity.
Toner adheres to the areas on the surface of the photoconductor that are not negatively charged. This toner is then transferred to paper and heated in a process known as “fusion fixation” to complete printing.
Image quality during printing is determined in part by the stability of the sensitivity of the charge generation material (CGM) used in the charge generation layer of the photoconductor. Konica Minolta has been a global leader in photoconductor development for many years, and in 1989 the company broke new ground in CGM development within the industry with its development of Y-form titanylphthalocyanine crystals. Digital printing systems have become widely used in the commercial printing sector in recent years, and high image quality comparable with that of offset printing is now required. Variations in humidity affect the sensitivity properties of conventional photoconductors, but such tiny alterations in image quality are no longer acceptable, and stable, high-quality imaging has now become essential.
Conventional CGMs include adsorbed water as part of their crystalline structure, and this water increases their quantum efficiency. The amount of adsorbed water changes according to the humidity, causing minute variations in sensitivity. To improve this humidity dependence, we designed a CGM with a molecular structure containing organic molecules in place of water molecules, and used this to develop a new CGM that is not subject to humidity-induced variations in sensitivity.
We used this CGM as the charge generation layer to produce a photoconductor with higher sensitivity compared with conventional products and stable sensitivity properties irrespective of the environment.
Most materials that combine new functions and high performance have not only never been commercialized, in most cases they have never even been made before. Being able to develop compounds with particular properties in-house is an advantage when using such novel materials in products.
Since launching Japan's first color photographic film in 1940, Konica Minolta has amassed great expertise in organic synthesis via our research and development of light-sensitive film materials and additives. Organic synthesis requires experience in basic chemistry in terms of the properties and handling of a wide range of reagents, knowledge of how to handle hazardous materials, and knowledge and expertise concerning synthesis reactions. Not only all these, but isolation and purification techniques such as chromatography for isolating only the target novel compounds are also required, as are structural analysis technologies such as NMR, IR, UV, and X-ray crystallography for determining that the final compound is actually the desired novel compound.
This novel compound must also be synthesized in large quantities at the factory level in order for it to be incorporated into products, and research in manufacturing technology is also essential in order to scale up the synthesis reaction from the laboratory level while maintaining a good yield.
CGM must be coated evenly on the photoconductor drum if high image quality is to be achieved. Even a high-performance CGM will produce patchy images if its coating is patchy.
This is achieved by dispersing the CGM homogeneously within the coating liquid at the submicron level, and using manufacturing technology to coat the surface of the drum with an even thickness. Konica Minolta's photoconductor drums are produced in-house by our manufacturing subsidiary, where these processes are controlled by sophisticated manufacturing technology.