Rewritable Printing Technology
Rewritable Printing Technology Based on Coloring and Discoloring
The volume of paper printed using laser printers and other printing devices is increasing along with the digitization of information in offices and factories. Much of this paper is for printing for temporary use. Ricoh developers thought that printing paper stock that could be reprinted repeatedly would make it possible to reduce the wasting of paper. This idea sparked the development of rewritable printing technology.
This technology is an expanded application of a thermal printing technology widely used for product labels, train tickets, and parcel delivery service dockets. Thermal printing papers are coated with colorless colorants called leuco dyes and a developer to activate the dyes. The application of heat melts and blends together these materials, and their molecules bond together in a chemical reaction (the right side of Figure 1). The leuco dyes that have bonded with the developer produce black, blue, and other colors. Rewritable printing is a technology that makes it possible to erase the colors by once again separating the colored leuco dyes and developer (the left side of Figure 1).
Coloring and discoloring occur due to differences in heat application conditions (Figure 2). The materials melt when heat is applied at high temperature T1 (for instance, 160°C), and subsequent quenching causes coloring. When the paper is once again heated to slightly lower temperature T2, discoloring occurs. The use of a thermal head to selectively heat only areas where color is desired makes it possible to produce text, and the use of a heating roller to heat the entire sheet makes it possible to completely erase the text. These phenomena are repeatable (Figure 3).
The rewritable printing technology was conceived as a result of chemically analyzing the printing mechanism and linking the analysis results to a new concept involving reversible coloring and discoloring rather than considering direct-thermal printing technology to be a surface phenomenon.
Coloring and Discoloring Mechanism
Previously, complete discoloring required the application of heat for several minutes. However, advances in the development of long-chain developers have made possible discoloration with heat application of the order of several milliseconds and reprinting at a speed comparable to that of laser printing (Figure 4).
The mechanism for coloring and discoloring at will was created by attaching a long-chain hydrocarbon (CH2) to the leuco dye coloring developer. In leuco dye coloring, the molecular aggregated state is important; the concept is that stability of the colored state changes according to the molecular aggregated state. Accordingly, Ricoh introduced a long-chain hydrocarbon group that actively controls the molecular aggregated state (Figure 5).
Figure 6 shows the schematic chemical mechanism of the coloring and discoloring states. Quenching from melted state B results in coloring state C, in which the leuco dye is introduced between layers of the long-chain developer. Although the coloring state is stable at ordinary temperatures, when the temperature increases it breaks down owing to the thermal motion of the molecules (D). When this happens, the long-chain developer creates the most stable crystalline state and expels the leuco dye, which breaks the bond between the leuco dye and the developer and results in discoloring (E–›A). That is to day, the leuco dye coloring and discoloring reactions can be physically controlled by means of changes in the thermal molecular aggregated state of the long-chain developer. This is an unprecedented phenomenon, and this advanced materials technology focused on the molecular aggregated state realized rewritability, a previously unavailable product function.
Expanding Applications as a New Information Medium
Rewritable printing technology was first practically applied in loyalty cards, patient registration cards, commuter passes, and other rewritable cards. Now, the convenience of these cards has greatly increased thanks to the ability to view the digital information recorded on the magnetic recording strips on the back of the cards.
The technology has subsequently been applied to rewritable sheets, making it possible to sharply reduce the use of printed paper at factories and other sites, and to rewritable hybrid media (RHM), new information media incorporated into distribution and production management systems in combination with RF tags. The technology has been commercialized as RECO-View RF tags, and its use is increasing.
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