What is rewritable recording? It is defined by the academic journal of Soc. of Electrophotography of Japan (Vol.34, No.4 (1995), p.441) as "A rewritable marking technology is a technology to form a visible image, by giving energies, such as heat, light, magnetism, electrical field, and/or pressure, where the image is maintained without giving any energy, and when an energy is given again, the image is deleted to allow its repetition."
Rewritable recording is assumed to have stemmed from the dream to erase written images easily. For several decades, chromic materials have been examined to show color transition; materials include photochromic material and thermo-chromic material. Phenomena in which a color changes or an image disappears charm people, who tend to focus on them. Applications for repeated use rather than throwing away copier paper etc. have also been proposed from market side.
A major application of rewritable recording is to display part of the information stored on memory medium, such as magnetic or a card IC. Representative examples are data display cards like reward cards to display the points accumulated when buying goods with points added according to the purchase price, and commuter passes to display the expiry date. Further, the application is being expanded to industrial use by combining it with RF-ID or laser recording. Thermal rewritable recording materials using heat for rewrite energy are mainly used for these applications.
Characteristics of the leuco dye/long-chain developer type thermal rewritable recording material, a typical rewritable recording material, and their coloring and decoloring mechanisms are explained below.
Thermal rewritable recording materials (the combination of leuco dye with a developer having a long-chain alkyl group) feature arbitrary coloring, such as black, blue, and red, depending on the kind of leuco dye. The developer molecule, having a long-chain alkyl group, can extract developer from the leuco dye using the crystallization energy of the developer itself, and can control the coloring and decoloring modes. The coloring and decoloring modes are controlled by aggregation and separarion leuco dye and a long-chain type developer by heating, as shown in Fig. 1.
Fig. 1: Coloring and decoloring control by aggregation and separarion of leuco dye and long-chain type developer
The typical coloring and decoloring process of leuco dye/long-chain developer type thermal rewritable recording material and the mechanism of coloring/decoloring phenomenon are shown in Fig. 2. When a developer is heated from a decoloring mode (A) beyond the melting point, it melts and reacts with the leuco dye; color develops (B). When is quenched from here, the developer clumps together with regularity while maintaining a bond with the leuco dye and a coloring mode is fixed (C). When the temperature rises from coloring mode (C), this condensed state begins to collapse at a temperature lower than coloring temperature (D). When the temperature rises further, the developer associates (the phenomenon of two same kind molecules joining together and acting as one molecule), builds a crystal independently, and sputters out leuco dye to decolor (E). When cooled from this state, it returns to the original decolored state (A). The developer stays in the crystallized state at (A), which is the most stable state (C).
Fig. 2: Coloring and decoloring mechanism
Some issues must be solved to put the leuco dye/long-chain developer type thermal rewritable recording material into practical use. Ricoh has approached these technical issues as follows:
(1)Strike a balance between shortening decoloring time and coloring stability
To improve stability of the coloring mode, reactivity of the leuco dye and developer must be improved. Beyond that, leuco dye from the developer must be separated promptly to decolor quickly. To reconcile these, Ricoh achieved both qualities by creating a new material, compounding a long-chain type developer.
(2) Improve light-fastness
Although leuco dye is also used for the thermal paper commonly used for receipts and the like, being heated and printed, yellowing and browning of the backgrounds and images occurs. This happens because leuco dye is naturally weak to light. It is especially important for rewritable recording material to be used longer to prevent this discoloration. In addition, the formed images remain thin even if erasure is tried. This phenomenon is peculiar to rewritable recording materials, but the degradation is based on ultraviolet rays and oxygen.
Ricoh improved light-fastness to a practical level for card applications by placing a layer containing ultraviolet absorbers above the recording layer to block ultraviolet rays. Further, we improved light-fastness by dozens of magnitude over the past by compounding a new material and preparing a new oxygen interception layer to block ultraviolet rays until just before visible light for application expansion in an outdoor environment, such as for physical distribution. This technology is used for the rewritable laser system.
(3) Improve repetitive durability
To improve practicality by taking advantage of the "rewritable" feature, durability of the recording medium itself must be improved. Ricoh improved durability of the recording layer itself by forming cross-links in matrix resins holding leuco dye and the long-chain type developer, which comprise the main material of a recording layer (to make a 3D network by bridge bonding high molecules). Further, we improved repetitive durability by preparing a hard protection layer to protect the surface against heat and pressure when heated by a thermal print-head. As a result, we achieved repeatability of 300 times image formation/erasure, which is required for card applications. Moreover, we changed the recording process from the conventional thermal print-head to a laser, deleting physical loads, such as pressure on the media by a thermal print-head. Further, controlling laser scanning to uniform heating improved repetitive durability to 1,000 times. Such technical developments have made it possible for Ricoh to achieve a highly efficient rewritable recording system heretofore not possible.
Images are formed when the heat is spread quickly around and cooled rapidly, after a recording layer is heated momentarily with small energy using a thermal print-head or laser. Because decoloring is seldom influenced by cooling speed, heating must be done in a fixed temperature range where decoloring is possible although deletion of the image is possible using various heating systems. With rewritable cards (Fig. 3) and the complex RF-ID tag media (Fig. 4), decoloring of images is achieved with big heating areas such as heat roller or ceramic heater.
Fig. 3: Rewritable card and printer
Fig. 4: RF tag media and printer
Laser systems, as shown in Fig. 5, irradiate a laser of several millimeters width lineally to scan at right-angles to the beam for decoloring. In any case, enlarging the medium area heated at one time makes it possible to heat the medium uniformly to an delitable temperature to finely delete images without unnecessarily prolonging decoloring time.
Fig. 5: Decoloring with Ricoh rewritable laser system
Ricoh's rewritable recording technology was first put into practical use for card displays, such as point cards, commuter passes, etc., where the display needs to be rewritten. RECO-View RF tags were then commercialized, combined with RF-ID used for production control etc., further expanding the application to a rewritable laser system where the media for a physical distribution application affixed on returnable containers can be rewritten with no contact.