1 Photocatalyst found in Japan
The study found that the photocatalyst represented by titanium oxide has the ability to oxidize and decompose and has strong hydrophilic properties when exposed to sunlight or fluorescent lamps. In recent years, photocatalyst products using this property have been commercialized in a wide range of fields, and the market has grown rapidly. The photocatalyst was discovered in Japan (1967, Hondo Fujishima effect). Its product market is mainly formed and developed in Japan, it can be said that it is an original technology from Japan.
In order to promote the comprehensive development and popularization of photocatalyst technology, at the Kanagawa Institute of Science and Technology, President Fujishima Sho, known as the "father of photocatalysts", provided funds to operate the photocatalyst open laboratory and photocatalyst museum. At the same time, it also conducts basic research and applied research, and carries out joint research and development with enterprises. Among them, a joint research and development with Tokyo Yinghua Industry Co., Ltd., namely "development of PS plates for offset printing using photocatalyst".
2 Characteristics of photocatalyst
The definition of a photocatalyst is: a substance that does not change itself by absorbing light energy, but can initiate various chemical reactions. The most widely used photocatalyst is titanium oxide. The study found that titanium oxide has the ability to oxidize and decompose and has strong hydrophilic properties when exposed to light (sunlight, fluorescent light). Oxidative decomposition ability can be explained by comparison with photosynthesis of plants. Photosynthesis is that chlorophyll in leaves absorbs energy from sunlight, synthesizes water and carbon dioxide into organic matter (starch), and then discharges oxygen; and photocatalysts such as titanium oxide decompose organic matter into water and carbon dioxide by absorbing sunlight and other light energy. That is to say, photosynthesis and photocatalysts cause completely opposite reactions. Chlorophyll can also be said to be a type of photocatalyst.
The oxidative decomposition function of the photocatalyst can decompose and remove odor or VOC gas, and can also remove the discharged nitrogen oxides to achieve air purification, or remove organic contaminants in the pool to achieve water purification, and can also be used for viruses, Antibacterial and sterilization of bacteria and other microorganisms.
In addition, the photocatalyst also showed another performance, that is, its affinity with water under light conditions-super hydrophilic. That is to say, under the light condition of the photocatalyst, the water on the surface of the photocatalyst will expand into a very thin water film. To prevent the mirror from fogging or being stained with oil, coat a layer of titanium oxide on the mirror. Under light conditions, water will enter under the oil stain to make it float and be washed away with the water.
The use of this super-hydrophilic property has introduced many applications, such as waterproof fog coatings for car rearview mirrors, anti-dirty coatings for exterior walls and windows of buildings or residences, anti-dirty roofing materials (tents), and so on.
At present, application research and development are being carried out in a wide range of fields using photocatalyst's oxidative decomposition ability and super hydrophilic property. The Kanagawa Institute of Science and Technology and Tokyo Yinghua Industry Co., Ltd. are working together to use the performance of photocatalysts in application research and development in offset printing.
3 Application on PS plate of offset printing
In ordinary offset printing, an anodized aluminum plate (PS plate) coated with photoresist is used to expose light, and an image is formed by a hydrophilic portion and a water-repellent portion. In offset printing, the PS plate is installed on the plate cylinder, and the ink is transferred to the rubber cylinder for printing through the hydrophilic part of the plate and the water-repellent part.
The offset platemaking process is to develop and apply gum after exposure. In this process, developer and gum waste liquid will be generated and need to be processed. In addition, the PS plate after printing is discarded because it cannot be reused, resulting in a large amount of discarded aluminum plates.
Taking advantage of the oxidative decomposition ability and super hydrophilic property of titanium oxide photocatalyst, Kanagawa Institute and Tokyo Application Chemical Co., Ltd. conducted a research and development of "photocatalyst offset PS plate" which can reuse PS plate. First, the traditional PS plate production process and film were used to conduct a photocatalyst hydrophilic / water repellent image production test. The specific process is as follows: firstly, the titanium oxide photocatalyst coating is applied on the anodized aluminum plate to replace the original photoresist; then a self-organized monomolecular film SAM (made of a water-repellent organic substance) is formed on it Self Assembled Monolayer). Next, use a photo-printing film, which is exposed to ultraviolet light. The SAM component underneath part of the film allowing ultraviolet rays to be decomposed and removed under the action of the photocatalytic oxidative decomposition mechanism, exposing the titanium oxide coating. The titanium oxide coating shows extremely strong hydrophilic properties. In addition, the SAM of the printing plate part blocked by ultraviolet rays by the printing film remains as it is and becomes a water-repellent part. In this way, a hydrophilic / water-repellent image obtained by the photocatalyst is formed.
In this process, the conventional development and gumming processes after exposure are no longer necessary, so no corresponding waste liquid will be generated.
For the selected printing test pattern, under the same conditions as the lithographic printing test, obtained a positive result: the use of this printing plate is completely successful and feasible.
Next, the scope of the test was expanded, and a printing press for the web rubber press of the printing plant was conducted. In this test, a printing plate with a size of 570 mm × 309 mm was used, and a monochrome printing test with a resolution of 45 lines / inch was performed. In the 5000-page printing test conducted, good printing quality was obtained.
4 Application of Inkjet Technology-Non-flexible Photocatalyst Offset Printing Plate
At present, the computerization process in the printing field is advancing. The digitization of data enables the plate-making process to enter the so-called CTP stage in which printing plates are directly scanned by a computer to produce printing plates. This eliminates the traditional printing plate used, and greatly reduces the plate-making process, resulting in the realization of short delivery time and cost reduction.
The second step, with CTP as the goal, discussed the non-flexibility of photocatalyst offset printing plates. The process is that after coating the titanium oxide photocatalyst coating and the SAM self-organizing monomolecular film on the printing plate, the inkjet method is used to spray the water-based ink on the printing plate to form an image to replace the original film. Under ultraviolet irradiation, the SAM on the printing plate that is not sprayed with water-based ink is decomposed and removed by the action of the photocatalyst and the photocatalyst layer is exposed to become a hydrophilic part. On the part of the printing plate sprayed with water-based ink, the SAM is retained because the water-based ink blocks ultraviolet rays. Next, the water-based ink is removed by water washing, and a photocatalyst plate having hydrophilicity / water repellency is prepared. Using this printing plate (printing plate size is 570mm × 309mm), a color printing test was carried out on a proofing machine in a printing factory, and a good printing effect was obtained.
5 PS version recycling
The third step is to discuss the purpose of this technology-the recycling of PS version. The PS plate used in the aforementioned color printing test was completely irradiated with ultraviolet rays to completely remove the SAM layer on the surface. Then apply SAM again → inkjet to form water-based ink image → ultraviolet irradiation → water washing, make a regenerated PS plate, and then send it to the printing house for color printing test. The results of the test confirm that the printing quality of the regenerated printing plate is exactly the same as that of the new printing plate. This has achieved the construction of practical goals.
6 Facing practicality
Compared with the traditional PS plate, the photocatalyst PS plate developed this time has simplified the plate-making process, greatly reduced the amount of waste liquid, and at the same time can be recycled. Therefore, the photocatalyst PS plate is a cost-effective method for printing It is expected that the technology associated with a significant reduction in environmental load. In addition, this technology is no different from the CTP process that is now popularized. The use of inkjet printers does not require expensive laser exposure machines, so the equipment investment is small and the purchase is easy.
The current problems are the number of times the PS plate is regenerated and the service life caused by the abrasion of the PS plate during the printing process. These problems are now being resolved.
PS version of the base material-aluminum plate has poor printing resistance, which has become an unfavorable factor in recycling, so the titanium plate base PS version is currently being developed. The surface of the titanium metal plate is treated by heat treatment or anodization, and a photocatalytic layer of titanium oxide is formed on the surface. This titanium-based PS version has now been made. Using this titanium-based PS plate, the same printing test and regeneration test as above were carried out, and it was confirmed that it could be used for printing, which laid the technical foundation. It can be expected that the number of recycling of the PS version of the photocatalyst can also be greatly improved.
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