Rewritable recording/erasing method and apparatus

Abstract: In a rewritable record display using a reversible heat-sensitive record medium, there are provided a rewritably recording and erasing method and apparatus having a display contrast and maximum iterative number which are substantially similar to the limit at which the recording and erasing reactions can be carried out due to the physical or chemical change then the erasing voltage applied to the medium is ranged between 0.3 times and 0.9 times the recording voltage, the image formed on the medium can be completely erased improving the maximum iterative number. In addition, the use of dye other than the organic low-molecular substance showed the characteristics similar to those of the organic low-molecular dye.

Description: BACKGROUND OF THE INVENTION: The present invention relates to a method for erasing and /or recording using reversible thermal recording medium (rewritable recording medium), on which recording and erasing image are derived from the heat sensitive nature of the medium layer. Description of the Prior Art: Recently, reversible thermal recording mediums, on which recording and erasing image can be done iteratively using heating, have developed very widely. As conventional types of such mediums, for example, so called an organic low-molecular type and leuco dye type are known. In the low-molecular type, thermal recording and erasing are carried out using physical phase transition of its nature which has cloudy and transparent phase, and the phase transition happens iteratively by the given temperature, and either of the phase is kept stably below a specified temperature (see Japanese Patent Laid-Open No. Sho 55-154198). In a leuco dye type, thermal recording and erasing are carried out by using chemical reaction including some compounds in the medium, which includes leuco dyes, developers and reducers, and these developers and reducers usually have hydroxyl group and carboxyl group in a compound molecule to release hydrogen ion. The color of the dye appears when a temperature is given to the medium, and it disappears when the lower temperature than before is given (see Japanese Patent Laid-Open No. Hei 2-188294). More particularly, the organic low-molecular type medium is comprised of the matrix substance made of thermoplastic resins and organic low-molecular substances dispersed in the matrix. When the medium is kept over the specified temperature T0, its phase is changed according to the temperature given to the medium. Namely, the medium has two phase transition temperatures T1 and T2 (T1<T2) which are higher than T0. After heating the medium T2 or higher and maintained at the temperature, and then it is cooled to T0 or lower, the medium becomes cloudy to provide the maximum light intercept phase. After heating the cloudy layer between T1 or more and T2 or less, and then it is cooled to T0 or lower, the layer becomes transparent. These phase transition mainly derives from structural changes of the organic low-molecular substances including in it. On the other hand, the phase transition of the leuco dye type medium can be controlled simply by the given heat energy. That is, the lactone ring including the leuco dye in the media is opened when the higher transition temperature is given by the energy, and the ring closed when the lower transition temperature is given to return the leuco compound colorless. Such phenomena derives from the structures of the developer and reducer, and the reversible nature of the leuco dye which enables to open and close the ring repeatedly. As the examples of the developer and reducer, a salt formed by gallic acid and aliphatic amine or the like are known. In order to increase the iterative number of recording and erasing of the thermal recording medium, the former low-molecular type employed a transparent protection layer formed on the recording layer, and this has been described in Japanese Patent Laid-Open Nos. Sho 57-82086, Hei 2-117891, Hei 2-131984, Hei 2-81672, Hei 2-566 and so on. The latter leuco dye type employed also a protection layer made of thermoplastic resin or the like. In the reversible thermal recording medium of the prior art, heat from a condensing lens or optical beam are irradiated to, or Joule heat is conducted to the medium to record or erase image on the medium. However, the prior art neither disclose a reliable method for providing a desired optical density and for erasing the recorded image, nor any optimum conditions for comprising the apparatus. They are still not established now. For example, "Thermal Recording Techniques", Vol. 117, Section 6 published by Triceps Co. Ltd. discloses some recording conditions. But, our experiments under the conditions described in the book showed that a rewritable recording medium had so less maximum iterative number that we could not repeat the recording and erasing the physical and chemical limit of the medium for use of which number is from about 1,000 to about 10,000. In the experiment, low-molecular type medium is used and non-contact method is employed and the procedure is as follows. That is, the transparent medium is put in a high temperature chamber at 120°C and then cool it down at room temperature to transit cloudy phase, and put it in a low temperature chamber at 80°C and then cool it down to transit transparent phase. The maximum iterative number obtained from the experiment is at least 2,000, or more. However, when the medium is heated under the known conditions, its surfaces are roughened, a heating means absorbs tails, either of the resin or organic low-molecular substance are oxidized and loose their properties, or the surface the medium is abraded to make random reflection at the iterative number about 100 or more. When the iterative number is further increased, for example, to more than 500, the large friction between the record medium and the heating means makes recording itself on the medium impossible. In the other experiments using the leuco dye type medium, the medium has less the maximum iterative number than former type, and recording itself becomes impossible about 50 iterations. SUMMARY OF THE INVENTION It is an object of the present invention in order to overcome the aforementioned problems, and to provide the maximum iterative number which is substantially equivalent to a physical or chemical limit of the medium, which is derived from the components of the medium, for the recording and erasing the image on it. We believed that these problems could be overcome by (1) improving the record medium itself, and (2) optimizing the recoding and erasing conditions or heating means. The former can be accomplished by employing the method for forming a recording layer using capsules, as described in Japanese Patent Filing No. Hei 3-118310 (Japanese Patent Laid-Open No. Hei 4-345886) assigned to the applicant. Such a method can provide only the maximum iterative number ranged between about 100 and about 1,000. Consequently, the present invention was done by establishing the optimum conditions of the latter method. Namely, the recording conditions in the present invention are selected such that any recorded trace brought from heat and pressure will not be left and also the surface of the recording layer is kept smooth without being roughened. Furthermore, the erasing conditions are selected such that the erasing can be reliably performed and also the surface of the recording layer does not receive any configuration change on it. An apparatus for use in the present method does not use both erasing-heating means and control means having particular accuracies, and generally used devices in the thermal recording or thermography are available for the present invention. An erasing-heating means used in the apparatus can erase the recorded image certainly whilst increasing the maximum iterative number of the medium. As described in Claim 1, the present invention provides a method for repeated recording and erasing for a thermoreversible recording medium having an optical density comprising the steps of: (a) recording an image on the medium by applying to the medium an energy, using heating means, ranged between 0,8 times and 1,5 times the energy level at which saturation of the optical density of said medium occurs, wherein the energy level at which saturation occurs is defined as a level at which the optical density varies by less than 10 %, even if a further energy is applied to said medium; and (b) erasing the image on the medium by applying a voltage to the heating means ranged between 0,3 times and 0,9 times the voltage applied in the step of recording. As described in Claim 2, the method of claim 1 is characterized in that the heating time for erasing the image on the medium is at least twice that required for recording. As described in Claim 3, the method of claim 1 is characterized in that the speed of erasing the image on the mediumis greater than or equal to Tmm/s, and the width of an erasing/heating means is Tµm or more, and that a pressure appied to the erasing/heating means is set to be ranged between 8 x 10 3 Pa and 5 x 10 4 Pa. As described in Claim 4, the method of claim 1 is further charaterized in that the erasing voltage is controlled to be ranged between 0,68 times and 0,86 times the recording voltage if the heating time on erasing is equal to that on recording. As described in Claim 5, the method of claim 1 is further characterized in that the erasing voltage is controlled to be ranged between 0,4 times and 0,7 times the recording voltage when the heating time is twice that on recording. As described in Claim 6, the method of claim 1 is further characterized in that the erasing voltage is ranged between 0,5 times and 0,75 times the recording voltage. As described in Claim 7, the method of claim 1 is further characterized in that the recording energy is ranged between 1.05 times and 1,2 times the energy at the start of saturation. In the rewritable recording/erasing method of the present invention, the desired record of the image was provided through optimizing the energy applied for recording without any deformation on the surface of the recording layer, the certain erasing was provided through optimizing the voltage applied or heating time for erasing with keeping the surface of the recording layer smooth, and also it makes the maximum iterative number increased. In addition, Tmm/s of the erasing speed, Tµm or more of the width of the erasing-heating means, and from 8 x 10 3 Pa to 5 x 10 4 Pa of the pressure are employed in the apparatus, they make further improved apparatus which enables to erase recorded image without any trace and also incresases the maximum iterative number. Therefore, the problems of the prior art including degradation of image quality and so on can be avoided with a remarkable improvement in the maximum iterative number. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1(a) and (b) are schematic views of two different structures for rewritably displaying a record. Figs. 2(a) and (b) are schematic views of two different structures for rewritably displaying a record. Fig. 3(a) shows the recording characteristics curve - energy. Fig. 3(b) shows the optical density - maximum iterative number. Fig. 4(a) shows the contrast ratio-energy. Fig. 4(b) shows the maximum iterative number - energy. Fig. 5(a) shows the dispersion of optical density - energy. Fig. 5(b) shows the relationship between the energy and the number of disfigurements. Fig. 6 shows the erasing characteristics. Fig. 7 shows the relationship between the heating time and ΔOD. Fig. 8(a) shows the erasing characteristics. Fig. 8(b) shows ΔOD - the maximum iterative number. Figs. 9(a) and (b) show the relationship between the erasing speed and the width of the heating means. Fig. 10(a) shows the relationship between the pressure and ΔOD. Fig. 10(b) shows the relationship between the pressure and the maximum iterative number. Detailed Description of the Preferred Embodiments First Example Fig. 1(a) illustrates the schematic structure of the apparatus used in the embodiment. Referring to Fig. 1(a), there is a rewritable recording medium 10 (which is a reversible thermal recording medium comprising a protection layer, recording layer and support member and further including a colored layer, magnetic recording layer and so on, if required, although they are not illustrated). In this figure, recording-heating means 12 is such as a thermal head having a resolution of 8 dots/mm. Reference numeral 14 designates a platen roller; 16 recording control means (RCM) for electrically controlling the recording-heating means (RM) 12; 18 erasing-heating means (EM) such as a thermal head; 20 a platen roller; 22 erasing control means (ECM) for electrically controlling the erasing-heating means 18. The medium 10 may comprise a transparent or opaque support member (not shown) formed of paper, polyethylene terephthalate film, metallic sheet (light reflecting layer) or the like, and a recording layer containing an organic low-molecular substance as a main component and having its property reversibly variable depending on temperature. The organic low-molecular substance may be a mixture of behenic acid with dodecanodioic acid in a ratio of 7:3. Such a material was used to form a recording layer having 10µm thickness. The principal of the phase transition of the medium which is shown in the present invention causes from the transition of the crystalline structure deformation of the low-molecular organic substances; the medium is the cloudy phase when it is cooled at room temperature after given the high temperature, and it is returned to the transparent phase when it is cooled to room temperature after given the lower temperature. More particularly, the organic low-molecular substance is poly-crystallized with its crystalline orientation being disturbed when it is cooled after first being molten at the raised temperature. Such a disturbed crystalline orientation scatters the incident light to induce the cloudy phase in the medium. On the other hand, the organic low-molecular substance is partially molten when it is given the lower temperature. The organic low-molecular substance then forms the single crystals with maintaining orientation. Therefore the organic low-molecular substance is in the transparent phase wherein the incident light can pass through the crystalline layer or be reflected by the support member, without scattering. In operation, the platen roller 20 is driven by any suitable drive means like motors. The medium 10 is moved in a direction of arrow A under the friction between the rotating platen roller 20 and the erasing-heating means 18. As an erasing signal is inputted from the erasing control means 22 to the erasing-heating means 18, a current is applied to a heating member (not shown) at a given timing so that the organic low-molecular substance 10 is heated by the heating member to erase an image thereon. When a portion of the image corresponding to one line is erased, then the image on the medium is erased line by line iteratively with such operation. When such a cycle is repeated, the image on the medium 10 can be erased two-dimensionally. For recording the image, the recording control means 16 receives a recording signal corresponding to the image from a signal terminal (not shown), as in the erasing cycle. The recording signal is then fed to the recording-heating means 12 to record a desired image. The optimum recording conditions were selected from the following experiments. Fig. 3(a) shows the basic profile of optical density obtained when the recording-heating means 12 in the aforementioned arrangement was used to heat the medium 10. In Fig.3(a), a vertical axis represents optical density while a horizontal axis represents in energy to be applied to the recording-heating means 12. As shown in Fig. 3(a), the medium 10 is gradually clouded as the energy applied is increased. It is noted that when the applied energy exceeds 0.6 mj/dot, the optical density is saturated at about 0.3. The optical density is measured using a densitometer (e.g. RD914 manufactured by Macbeth Co. Ltd.) after the medium 10 is given various voltage applied or heating time with the recording-heating means 12 and then cooled at room temperature (25°C). The optical density of the medium 10 itself is represented by a descendent curve since its phase turns gradually from transparent into cloudy. Fig. 3(b) shows the optical density profile after recording and erasing were repeated. In Fig. 3(b), a vertical axis represents the optical density while a horizontal axis represents the maximum iterative number. In Fig. 3(b), a straight line B shows ideal profile of the optical density, and a curve C shows the actual optical density after repeating erasing. On the other hand, a curve D shows the actual optical density after repeating recording and a straight line E shows the ideal optical density. These results were obtained from 10,000 iterations in each of which the medium 10 is heated and recorded by the recording-heating means 12, the image on the record medium being then erased by the erasing-heating means 18. When the recording energy which is 0.9 mj/dot or lower is applied to the record medium, the optical density for recording is substantially along with the straight line E and the optical density for erasing is substantially along with the straight line B. However, the optical density curve went over the line E,when the given energy is 0.6 mj/dot or lower. On the other hand, when the energy applied was 0.9 mj/dot or higher, the deviation from each of the straight lines B and E became large following the iterative number was increased, shown as the curves C and D. Such a tendency became remarkable as the applied energy increased. Fig. 4(a) is a graph in which a vertical axis represents the contrast ratio between the transparent and cloudy parts after the recording and erasing cycle have been repeated 10,000 times while a horizontal axis represents the applied energy. The contrast ratio is a value obtained when the optical density after 10,000 erasing is divided by the optical density after 10,000 recordings. From Fig. 4 (a), it is understood that the contrast is deteriorated with the applied energy equal to or lower than 0.5 mj/dot since the heated part is less cloudy after recording and also that the contrast is similarly deteriorated with the applied energy equal to or higher than 0.9 mj/dot since the optical density after recording increases with increase of the maximum iterative number (failure on recording) and the optical density after erasing decreases to increase the non-erased area. Fig. 4(b) is a graph in which a vertical axis represents the maximum iterative number and a horizontal axis represents the applied energy. Fig. 4(b) shows how many recording and erasing cycles can be repeated with clear contrast of the optical density before the recording and after the erasing. In other words, Fig. 4(b) represents the maximum iterative number for an optical density which is invariable against the respective energies on recording. It is understood from Fig. 4(b) that if the applied energy is lower than 0.9 mj/dot, the maximum iterative number can be about 10,000. In brief, (1) if the energy applied to the medium is decreased below about 0.5 mj/dot (which is equal to 0.8 times the energy on the start of saturation), the maximum iterative number is improved, but the contrast in the image formed on the medium is deteriorated; and (2) if the energy applied to the medium is increased above about 0.9 mj/dot (which is equal to 1.5 times the energy on the start of saturation), both the maximum iterative number and contrast are deteriorated. As the above results, it is desirable that the optimum recording condition is ranged between 0.8 times and 1.5 times the energy on the start of saturation. This is because as the applied energy becomes higher than 0.9 mj/dot, the medium is overheated to roughen the surface thereof or to oxidize the recording layer so that the recording and erasing cycle cannot be repeated. For example, if the voltage is increased to increase the energy to be applied to the medium, the heating member may be so heated that it becomes damaged. This may also lead to damage in the both protection and recording layers in the medium. More particularly, the protection layer may be fused to the thermal head to degrade the image formed on the medium or increase the friction between the thermal head and the medium so that the recording cycle cannot be performed. Further, the cycle becomes impossible due to oxidization of the recording layer or separation of the organic low-molecular substance out of the recording layer. Similar problems may be deteriorated without any affection against the surface smoothness in the medium and other factors. As described, the contrast in the image formed on the medium as well as the iterative number of cycles that the medium can be used are maintained better if the energy applied to the medium is ranged between 0.8 times and 1.5 times the energy at the start of saturation in the optical density. Particularly, if the recording energy applied to the medium is ranged between 1.05 times and 1.2 times the energy at the start of saturation, the contrast is further improved and the medium can be repeatedly subjected to 10,000 cycles with reduction of all the damage of the medium, the abrasion of the surface and the random reflection. This is because the range of the applied energy between 0.8 times and 1.05 times the energy at the start of saturation causes the optical density to be unstable. For example, Fig. 5(a) shows results when a dispersion in the optical density was determined with respect to a standard deviation which had been obtained from 10 times the energy of 0.6 mj/dot in Fig. 3(a). From Fig. 5(a), it is understood that the dispersion in the optical density is unstably increased at a level of energy equal to or lower than 1.05 times the energy at the start of saturation (about 0.63 mj/dot) even if the energy applied to the medium is invariable. On the other hand, the surface of the medium is slightly damaged after the recording and erasing cycle has been repeated within the range of energy between 1.2 times and 1.5 times the energy at the start of saturation (but the maximum iterative number will not be affected). This will be described with reference to Fig. 5(b) in which a vertical axis represents the number of cracks on the medium 10 per 10 cm 2 and a horizontal axis represents the energy applied to the medium. In Fig. 5(b), the number of iterations is 5,000. It is understood from Fig. 10 that it is preferred that the energy applied to the medium is equal to or lower than 1.2 times the energy at the start of saturation. In actual applications, when one makes greater account of the number of iterations than the optical density, the energy applied to the medium is preferably ranged between 0.8 times and 1.05 time the energy at the start of saturation. For example, such a range of energy is suitable for test printing in the word processing or the other application in which any visual judgment is permissible. When the energy applied to the medium is ranged between 1.2 times and 1.5 times the energy at the start of saturation, it is suitable for case when one makes greater account of the contrast in the image formed on the medium than the optical density thereof, such as a commutation ticket, an image display and the others. When the energy applied to the medium is ranged between 1.05 times and 1.2 times, it is applicable to all the above cases. The rewritable record displaying apparatus is characterized by an erasing means in the field of heat recording which is similar to the heat-sensitive recording process such as FAX and the like and to the heat transfer process such as color printer and the like. It is a primary object of the present invention to set recording conditions in which the complete erasing cycle is reliably performed to permit the iteration of the recording and erasing cycle such that the recording cycle will be performed without affection to the erasing cycle or with improvement of the recording characteristics. The recording conditions selected in accordance with the present invention is basically different from those of the prior art heat recording device. With the heat-sensitive recording device, for example, the medium can be recorded by an energy two times that at the start of saturation in the optical density without any disadvantage. There is substantially no problem unless the recording means such as thermal head is destroyed. The present invention provides the optimum recording conditions that an image formed on a record medium can be completely erased with the maximum iterative number of the record medium being improved and that the contrast in the image formed on the medium can be improved. As described, the aforementioned results were obtained by using the organic low-molecular type medium in which the recording layer was formed of a mixture of behenic acid with dodecanodioic acid at the ratio of 7:3 and into a thickness equal to 10µm. We have found that the energy at the start of saturation in the optical density is variable depending on the thickness of the recording layer, the material forming the recording layer and so on. Experiments showed that when the energy at the start of saturation is variable and if the recording energy applied to the medium is substantially ranged between 0.8 times and 1.5 times the energy at the start of saturation, the image formed on the medium can be erased completely and repeatedly with an improved contrast in the image formed on the medium. In addition, the use of dye other than the organic low-molecular substance showed the characteristics similar to those of the organic low-molecular dye. The erasing conditions were selected by the following procedure. Fig. 6 is a graph showing the erasing characteristics obtained from experiments by the use of the device shown in Fig. 1(a), in which a horizontal axis represents voltages applied to the erasing-heating means 18 and a vertical axis represents the optical densities after erasing. Experiments were carried out under such a condition that an image was recorded on the reversible heat-sensitive record medium 10 by the recording-heating means 12 with the optical density being equal to 0.3. Thereafter, the voltage applied to the erasing-heating means 18 was varied to erase the image. The voltage applied to the recording-heating means 12 was 14 volts with the heating time being 2 ms. A curve F in Fig. 6 shows results from the heating time of the erasing-heating means 18 which is set to be 2 ms. From these results, it is understood that when the voltage applied to the erasing-heating means 18 is varied from about 9.5 V to about 12 V, the image formed on the medium can be substantially completely erased. In other words, it is preferred that the erasing voltage is controlled to be ranged between 0.68 times and 0.86 times the recording voltage if the heating time on erasing is equal to that on recording. On the other hand, a curve G was obtained when the heating time in the erasing-heating means 18 is set to be equal to one-half the aforementioned heating time (i.e. one ms). From the results, it is understood that it is preferred to set the erasing voltage at a level equal to about 0.9 times the recording voltage (i.e. 12.6 V). It is however to be noted that the optical density after erasing becomes slightly lower than that in the heating time equal to 2 ms. It is believed that this results from the fact that the heating time required in the erasing cycle does not reach a predetermined time. On the other hand, if the erasing voltage was higher than 0.9 times the recording voltage, no erasing could be made even when the heating time is reduced. It is believed that this results from the fact that the heating time became too short while at the same time the temperature at the erasing-heating means exceeded the range of erasing temperature. When the heating time at the erasing-heating means 18 is 4 ms, the characteristics are represented by a curve H. In other words, the erasing cycle can be carried out when the erasing voltage is ranged between 0.4 times and 0.7 times the recording voltage. As the heating time is increased, the voltage applied to the erasing-heating means 18 tends to decrease. When the erasing voltage is smaller than 0.3 times the recording voltage, it was impossible to erase the image formed on the medium on the basis of the characteristics in the heating means such as thermal head. This means that since the medium is insufficiently heated, the temperature at the medium itself cannot reach the range of erasing temperature. Even when several kinds of experiments were carried out with different resistances at the erasing-heating means 18, the image formed on the medium could be erased by the erasing voltage ranged between 0.3 times and 0.9 times and more preferably between 0.5 times and 0.75 times the recording voltage. The above values are effective when the recording-heating means is also used as the erasing-heating means or when the erasing-heating means has the same structure as that of the recording-heating means. Particularly, the former is superior in cost performance since only a single heating means is required. Under the optimum erasing conditions described, the first example enables both the recording and erasing to be carried out even if the voltage applied to the heating means is only varied. In this case, the recording and erasing cycles can be carried out at the same speed. As described, the aforementioned results were obtained by using the organic low-molecular type medium in which the recording layer was formed on the support member from a mixture of behenic acid with dodecanodioic acid at the ratio of 7:3 and into a thickness equal to 10µm. Experiments showed that even if various factors such as the recording layer's thickness, the material forming the recording layer and so on are varied and when the erasing voltage applied to the medium is ranged between 0.3 times and 0.9 times the recording voltage, the image formed on the medium can be completely erased improving the maximum iterative number. In addition, the use of dye other than the organic low-molecular substance showed the characteristics similar to those of the organic low-molecular dye. This example was provided by using the rewritable record displaying device of the structure shown in Fig. 1(a). As the recording conditions, the energy to be applied to the recording-heating means was set at a level ranged between 0.8 times and 1.5 times the energy at the start of recording saturation in the recording layer formed of the organic low-molecular type heat-sensitive material which contains a mixture of behenic acid with dodecanodionic acid with a ratio of 7:3. As the erasing conditions, the erasing-heating means having the same structure as that of the recording-heating means was used with the erasing voltage ranged between 0.3 times and 0.9 times the recording voltage. The recording and erasing cycles were repeatedly carried out by iterating the heating step and the cooling-at-room-temperature step for both the recording and erasing cycles. By selecting time required to apply the voltage to the medium depending on the erasing voltage as described in connection with Fig. 6, the measuring process described in connection with Figs. 3(a) and 4(a) was used to measure the optical densities after recording and erasing. The optical densities after recording and erasing respectively became 0.3 and 1.25 while the contrast therebetween was 4.2. After the recording and erasing cycle had been repeated 10,000 times, it was proven that substantially the same measurements could be ensured. Although the first example has been described as to the reversible heat-sensitive record medium including the support member, the reversible heat-sensitive record medium may have no support member if the recording layer is formed to have its thickness and mechanical stability sufficient to maintain the configuration of the medium. Second Example In the second example, the experiment was carried out using the same apparatus under the same recording conditions in the first experiment, and erasing conditions were different from those in the first experiment. The erasing conditions were selected from the following experiments. Fig. 7 shows the relationship between ΔOD and heating time. In Fig.7, a vertical axis represents ΔOD and a horizontal axis represents heating time. The term "ΔOD" used herein represents the transparency after erasing the image, and it is shown as a value that DD-0.3. DD is the optical density when the image on the medium 10, which was recorded at 0.3 of the optical density using the recording/heating means 12, was erased using said erasing/heating mean 5. Namely, as the value of ΔOD is large, the recorded image is well erased, and if the value is zero, it means that the erasing is impossible. Fig. 7 shows data obtained from optical density at the various heating time when an image was recorded on the medium using the recording-heating means 12 under the conditions of 14V for 2 ms, and thereafter erased by the erasing-heating means 18 on which 7V is applied. If the heating time on erasing is substantially two times longer than that on recording, the erasing was completely carried out. If the voltage applied to the erasing-heating means 18 is lower than 7V, the heating time will be prolonged, however, the erasing was completely done when the heating time is two times longer than that on recording as in Fig. 7. If the erasing voltage applied is higher than 7V, the heating time will be shortened, however, the erasing was more reliable when the heating time is two times longer than that on recording. If the erasing voltage applied was 10V or higher, the erasing was not completely done because 10V is over the range of the erasing temperature when the heating time on erasing was two times or longer that on recording. In short, when a thermal head is used for erasing the image recorded on the medium completely, the voltage should be reduced and the heating time is two times or longer than that of recording with lower temperature than the recording one. If the erasing-heating means is not the thermal head but a heat roller or planar heating member which can be maintained at a constant temperature (e.g. 80°C), the heating time for the erasing was two time or longer than that on recording. It is believed that if the organic low-molecular type is used, it takes much longer time when the polycrystalline phase (cloudy) transit to single crystalline phase (transparent) than reverse transition of the phase. In the cloudy phase, molecule in the recording layer of the medium 10 is partially molten, and if the heating time is enough longer the polycrystalline changes into the single crystalline (transparent) phase. Similar results was obtained from the other the erasing-heating means laser beam, flash light or the like. Even if a leuco dye type medium was used, the heating time for erasing was required two times or longer than that for recording. The experiment conducted here employed the same apparatus and recording conditions as the first example, and also by setting the heating time for erasing is two times or longer than that for recording. The initial characteristics and the maximum iterative number of the medium were similar to those of the first example. Third Example In the third example, the temperature range of the medium for erasing in the apparatus shown in Fig.1 was set to 15 C or higher by selecting the materials which comprising the medium. Fig. 8(a) shows the erasing characteristics of the medium. A hot stamp was used as the erasing/heating means, and the temperatures and optical density for recording (or erasing) with 100 kPa (one kg/cm 2 ) pressure for two second were shown in Fig.8. The characteristics using the hot stamp shows that if the heating temperature is between 75°C and 100°C, the image recorded on the medium 10 can be erased. Namely, the range of erasing temperature of the medium is 25°C. The range of erasing temperature depends on the composition of the material forming the recording layer in the medium 10, and can be varied into about 5°C - about 50°C. Fig. 8(b) is a graph in which a vertical axis represents the value of Δ and a horizontal axis represents the maximum iterative number. Parameters used were selected that they make the erasing temperature range of the medium 10 at 5°C, 10°C, 12.5°C, 15°C, 20°C, 25°C, 30°C and 40°C possible. For example, if the erasing temperature ranges is 5°C, it means that when the medium is heated between 77.5°C and 82.5°C, the image recorded thereon can be erased. The erasing temperature range is 10°C means that the image can be erased by heating the medium between 75°C and 85°C. Experiments were carried out using such a device as shown in Fig. 1 with thermal head as heating means and either of thermal head or heat roller as erasing means. The results showed that if the temperature range for erasing is 12.5°C or lower, the maximum iterative number is decreased. If the erasing temperature range is 15°C or higher, the image on the medium is completely erased. When the erasing temperature range is 25°C, 30°C or 40°C, the similar results at 15 °C or 20°C were obtained (not shown). The reason that the erasing temperature range below 15 °C gives less iterative number of the medium is considered as follows. Firstly, given temperature is not well controlled in 7.5°C of desired temperature using the erasing-heating means 18, because the shape of the resister in the means, the variability of the heating resistor itself, or the uneven pressure from the means. The thermal head, heat roller and their control means, which are shown in Fig. 1(a) and used in the experiments, have accuracies similar to those normally used in the field of therm recording, therm transfer recording and so on. If the given temperature is well controlled below 7.5°C, these means must be manufactured with different specifications from usual ones. Naturally, this leads to increase of the manufacturing cost. Secondly, when the erasing speed is fast, the medium has no time to cool down from the precedent to the next step, and then heat is accumulated in the medium and it over the erasing temperature range. In order to increase the maximum iterative number and to erase the image on the medium reliably with no extra cost for the equipment, it is necessary 15°C or higher for the erasing temperature range in the medium 10. The erasing means may be a thermal head, heat roller, planar heating member or light irradiating means. In addition, the difference of the material does not affect to control the erasing temperature range, if the medium has the range higher 15°C. In this example, if the erasing temperature range of the medium is 15°C or higher, both of heating means and heating control means, which are used widely for thermal printing and thermography as the heating means and heating control means with usual regulation mode to control temperature, are avairable for the apparatus. Particularly, the apparatus of the present invention does not need any heating means and heating control means with high accuracy. Fourth Example The fourth example relates to the erasing-heating means in the apparatus. Fig. 9(a) shows the relationship between the width of the erasing-heating means 18, that is, the length of the same means 18 which moves to the direction and the erasing speed of the image on the medium. In Fig. 9(b) illustrates the width of the erasing-heating means 18 and the direction that the medium is moved to. In Fig. 9(a), a symbol "○" shows a completely erasable area; a symbol "▵" shows an erasable area; and a symbol "x" shows an unerasable area. The recording conditions were 14V of applied voltage, 2 ms of applied time, and 0.3 of optical density. The erasing speed and the width of the erasing-heating means 18 are variable. As the results, when the erasing speed is 50 mm/sec, and the width of the erasing-heating means 18 should be larger than 50µm under the above conditions, the image can be completely erased. It is thus preferred that the width of the erasing-heating means is 150µm or more, when the erasing speed is 150 mm/s. This relationship comes from that lines or dots erased need assure time to heat them. The relationship between the width of the erasing-heating means and erasing speed should be satisfy that when the erasing-heating means is T mm/s and the erasing speed is Tµm or more. To be equal to or larger than Tµm. If such a relationship between the erasing speed and the width of the erasing-heating means is broken, the energy required on erasing becomes insufficient. Accordingly, the dots and image recorded on the medium is not erasable when the medium is not sufficiently heated. Therefore, the apparatus of the present invention enables to erase the image on the medium with the same speed as recording them. Fig. 10(a) shows the relationship between the pressure applied to the erasing-heating means 18 and the above ΔOD. This result is obtained from 500 iterations of recording and erasing the image. Higher the pressure, better the erasing characteristics, and 8 x 10 3 Pa of the pressure is enough to erase. Fig. 10(b) shows the relationship between the pressure and the maximum iterative number. From Fig. 10(b), higher the pressure, less the iterative number. When unnecessary pressure is applied to the heating means, the heating means receives damage with deforming or distraction in the worst case, the medium has roughened surface with physical trace of the heating means, and these are causes which make the iterative number less than lower pressure is applied. A larger drive means (motor or the like) is required to move the medium 10, and it leads to increase the manufacturing cost high. These problems derives from that the unnecessarily high energy is applied to the medium and heating means. When both of heating and applying pressure are necessary for erasing the image on the medium, the relationship between them should be defined very precisely. However, as the result of our experiment, the pressure should be set between 8 x 10 3 Pa and 5 x 10 4 Pa, with no affection of the heating time. When the erasing-heating means in the apparatus shown in Fig. 1(a) was used to record and erase the image 10,000 times while determining the width of the erasing-heating means dependent on the erasing speed and optimizing the pressure applied to the erasing-heating means between 8 x 10 3 Pa and 5 x 10 4 Pa, the image is erased completely. As the erasing-heating means 18 may be any one of thermal head, heat roller, planer heating member and the like. Although the apparatus abovementioned comprises thermal heads as recording-heating means 12 and erasing-heating means 18 shown in Fig. 1(a), the erasing-heating means 18 may comprise a planar heating member such as ceramic heater and a support member 24 formed of a plastic material coated with rubber or Teflon instead of a platen roller shown in Fig. 1(b). Further, the erasing-heating means 18 may be a heat roller 26 shown in Fig. 2(a). In this case, the width of the erasing-heating means is defined as that portion of the heat roller 26 contacting the medium 10. In place of the platen roller 20, the heat roller 26 may be located on each side of the medium 10 or under the medium 10. In addition, a single heating means 28 which is used for both recording and erasing shown in Fig. 2(b). Such a single heating means 28 may be electrically controlled by a recording and erasing control means 30. Such kinds of modifications of the apparatus are including in the scope of the invention. Although the examples described above are employed the organic low-molecular type dye to form the medium 10, the present invention is not limited to such a material, and known reversible mediums are avairable. For other examples, the leuco or other dyes may be useful in the present invention. The same effect was obtained from when an organic low-molecular type dye which has cloudy phase at high temperature and transparent phase at lower temperature, and vice versa. As will be apparent from the foregoing, the present invention can optimize the recording and erasing conditions in the rewritable record display. Thus, the present invention can provide a display contrast and maximum iterative number which are substantially similar to the limits in the recording and erasing reactions due to the physical or chemical change in the heat-sensitive material forming the recording layer. When the range of erasing temperature for the reversible heat-sensitive record medium in the rewritable record displaying apparatus is set to be equal to or higher than 15 C, the medium used in the present invention can be subjected to the reliable erasing reaction by the use of a simplified structure comprising the erasing-heating means and the control means. When the erasing speed is equal to T mm/s in the rewritable record displaying apparatus and if the width of the erasing-heating means is equal to or larger than Tµm and the pressure applied to the erasing-heating means is ranged between 8 x 10 3 Pa and 5 x 10 4 Pa, the rewritable record displaying apparatus can be superior in the maximum iterative number and perform the reliable and complete erasing function.

Claims: Method for repeated recording and erasing for a thermoreversible recording medium having an optical density, characterized by the steps of: (a) recording an image on the medium by applying to the medium an energy, using a heating means, ranged between 0,8 times and 1,5 times the energy level at which saturation of the optical density of said medium occurs wherein the energy level at which saturation occurs is defined as a level at which the optical density varies by less than 10 % even if a further energy is applied to said medium; and (b) erasing the image on the medium by applying a voltage to the heating means ranged betweeen 0,3 times and 0,9 times the voltage applied in the step of recording to the heating means. Method according to claim 1, characterized in that the heating time for erasing the image on the medium is longer than twice the heating time for recording the image on the medium. Method according to claim 1, characterized in that the speed of erasing the image on said medium is greater than or equal to Tmm/s, and the width of an erasing/heating means is Tµm or more, and that a pressure is applied to said erasing/heating means equal to a value between 8 x 10 3 Pa and 5 x 10 4 Pa. Method according to claim 1, characterized in that the erasing voltage is controlled to be ranged between 0,68 times and 0,86 times the recording voltage if the heating time on erasing is equal to that on recording. Method according to claim 1, characterized in that the erasing voltage is ranged between 0,4 times and 0,7 times the recording voltage when the heating time is twice that of the heating time in recording. Method according to claim 1, characterized in that the erasing voltage is ranged between 0,5 times and 0,75 times the recording voltage. Method according to claim 1, characterized in that the recording energy is ranged between 1,05 times and 1,2 times the energy at the start of saturation.

Inventors: NAKATSU YUJI, OHNISHI MASARU, WATANABE NIRO, YAMADA KEIKI, NAKATSU, YUJI, OHNISHI, MASARU, WATANABE, NIRO, YAMADA, KEIKI, Nakatsu, Yuji, c/o Toppan Printing Co., Ltd., Ohnishi, Masaru, c/o Mitsubishi Denki K.K., WATANABE, NIRO, C/O TOPPAN PRINTING CO., LTD., YAMADA, KEIKI, C/O MITSUBISHI DENKI K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1992
Main classification: B41M5
IPC codes: B41M5, B41J2