Recording apparatus

Abstract: A recording medium having a recording layer comprising a polymer liquid crystal is used for multi-value recording by using at least two different liquid crystal phases at least two levels of locally different liquid crystal proportion, or thicknesswise difference in an optical variable, such as transmittance a reflectance, generally through controlled heating and cooling. In effecting the recording, two heating sources may suitably be used, one for imagewise heating and the other for supplemental heating.

Description: FIELD OF THE INVENTION AND RELATED ART The present invention relates to a recording apparatus suitable for an erasable and rewritable recording medium, and also to a method of continuous multi-value recording in a recording system using the recording medium. Hitherto, there have been known recording and display media, which are susceptible of thermally reversible erasure and repetitive use, composed of a polymer or a mixture of a polymer and a low-molecular weight compound inclusive of, e.g., a polymer liquid crystal and phase separation polymer. Such a polymeric recording medium generally has a point of reversible transition, such as phase transition point, phase separation point, etc., at a high temperature, so that it involves a problem that a large application energy is required at the time of recording. Further, as the medium is instantaneously heated to a high temperature, there arises a problem of polymer degradation. On the other hand, there is a desire for a method of effectively controlling optical variable, such as optical transmittance or reflectance, of such a recording medium comprising a polymeric recording layer. Now, rewritable recording systems are generally explained in some detail. Known rewritable recording systems include: those utilizing inorganic materials, such as the optomagnetic system utilizing a magneto-optical characteristic of a magnetic material, the phase transition system utilizing a reversible phase transition between the amorphous and crystal phases of a chalcogenide film and a system utilizing a structural change in an amorphous phase temperature region. Further, known systems utilizing organic materials include the recording system of the photon mode with a photochromic material and the system utilizing a change in optical reflectance between the uniform alignment state and the non-alignment state of a polymer liquid crystal. In recent years, a data recording system capable of high-density recording is desired. One system capable of increasing a recording density may be a multi-value data recording system wherein a recording spot is provided with multiple states. As examples of the multi-value data recording system, there have been proposed systems wherein the reflectance of a cholesteric polymer liquid crystal is changed at multiple levels by changing the helical pitch or formation of pits in the non-aligned state (Japanese Laid-Open Patent Applications JP-A (Kokai) Sho.62-107448 and 62-12937). The above-mentioned rewritable recording systems respectively involve some problems or have left some room for improvement. For example, in the optomagnetic recording system wherein a recording state is read out by detecting the rotation of a polarization plane due to the Kerr effect or Farady effect accompanying spin inversion, the rotation angle is as small as 1 degree or less, so that it is difficult to realize multi-value recording with a broad dynamic range. In the phase transition recording system, the phase transition temperature range for selecting the amorphous state and the liquid crystal state is narrow so that it is difficult to effect multi-value recording by a temperature change. However, the above-proposed multi-value recording systems still involve problems, such that a sufficient dynamic range cannot be attained, the writing apparatus for multi-value recording is complicated, and a linear relationship is not attained at the time of multi-value recording between the irradiation energy or the intensity or anisotropy of the electric or magnetic field and the readout light quantity. SUMMARY OF THE INVENTION In view of the above problems, an object the present invention is to provide a recording system for recording on a recording medium having a polymer recording layer, whereby recording is effected at a low energy and the durability of the polymer is increased. According to the present invention, there is provided a recording apparatus comprising a recording medium comprising a polymer liquid crystal layer capable of assuming a transparent state or an opaque state; first heating means for heating the polymer liquid crystal layer to a temperature below a phase transition temperature for writing of the polymer liquid crystal; and a second heating means for imagewise heating the polymer liquid crystal layer to a temperature above the phase transition temperature for writing, thereby forming a written part therein, the heating by the first heating means being performed in advance of and, optionally, simultaneously with the heating by the second heating means. Preferred embodiments according to the invention are defined in the dependent claims. These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings wherein like parts are denoted by like reference numerals. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic perspective view of a display apparatus utilizing the present invention; Figure 2 is a schematic sectional view of a recording and readout section of the display apparatus; Figure 3 is a sectional view of a recording medium used in the display apparatus; Figure 4 is a schematic sectional view of the recording medium for illustrating a manner of writing; Figure 5 is a schematic perspective view partly broken of a display apparatus according to the present invention; Figure 6 is a sectional view of the display apparatus shown in Figure 5; Figures 7A through 7D are illustrations of other heating means; Figure 8 is a graph showing temperature changing characteristics of a polymer liquid crystal; Figure 9 is a perspective view illustrating another embodiment of the display apparatus according to the present invention; and Figure 10 is a partial sectional view of the apparatus shown in Figure 9. DESCRIPTION OF THE PREFERRED EMBODIMENTS The recording layer of a recording medium used in the present invention may suitably comprise a thermotropic polymer liquid crystal. Examples thereof may include a side chain-type polymer liquid crystal which comprise a main chain of a methacrylic acid polymer, a siloxane polymer, etc., and a mesogen or low-molecular weight liquid crystal unit in side chains thereof like pendants; and also a main chain-type comprising a mesogen unit in its main chain, such as those of the polyester-type or polyamide-type, as used in the field of high-strength and high-modulus, heat-resistant fiber or resin. These polymer liquid crystal may assume smectic phase, nematic phase, cholesteric phase or another phase or can also be a discotic liquid crystal. Another class of polymer liquid crystals suitably used in the present invention may include a polymer liquid crystal showing ferroelectricity by introducing an asymmetric carbon atom to assume SmC* (chiral smectic C) phase. Specific examples of the polymer liquid crystal used in the present invention are enumerated hereinbelow while other polymer liquid crystals can also be used in the present invention. (In the following formulas (1) - (13), p = 5 - 1000, 1 ≦ n 1 < 15) (In the following formulas (14) - (17), p = 5 - 1000, p1+p2 = 5 - 1000, q = 1 - 16, q1 = 1 - 16 and q2 = 1 - 16.) (In the following formulas (18) - (62), * denotes the location of an asymmetric carbon atom, and n2 = 5 - 1000.) The above polymer liquid crystals may be used singly or in combination of two or more species. In order to form a recording layer, a polymer liquid crystal may be dissolved in a solvent to form a solution for application. Examples of the solvent used for this purpose may include: dichloroethane, dimethylformamide (DMF), cyclohexane, tetrahydrofuran (THF), acetone, ethanol, and other polar or non-polar solvents, and mixtures of these. Needless to say, these solvents may be selected based on consideration of such factors as dissolving power of the polymer liquid crystal used, and the material of or wettability of the substrate or surface layer formed thereon to be coated thereby. A good recording layer may be formed by applying such a solution of a polymer liquid crystal in a solvent onto a substrate by bar coating, spin coating, dipping, etc., followed by drying. The thus formed polymer liquid crystal film can be heated to a temperature above the isotropic phase and then rapidly cooled to provide a better state. The recording layer may be formed in a thickness of generally on the order of 1 - 100 microns. In case where a thermal head is used for writing on the recording medium so as to control the liquid crystal proportion, the current supplied to the head is controlled along with the control of the voltage, duration, duty ratio, etc., of a pulse. In case where light illumination is used for writing, it is effective to add a light-absorbing layer or to incorporate a colorant or pigment in the recording layer. Examples of the colorant used for this purpose or in other modes of recording in the present invention may include the following: In the above-described manner, a continuous gradational recording may be effected by use of a thermal head or modulation of writing light to provide a controlled liquid crystal proportion, thereby to continuously change the optical density of a record portion. A specific example of the recording medium having a polymer recording layer which comprised a polymer liquid crystal represented by the following formula (I) (corresponding to formula (52) described hereinabove): The above polymer liquid crystal was dissolved in dichloroethane to form a 20 wt. % solution, and the solution was applied on a transparent polyester substrate which was washed in advance, followed by standing in the atmosphere of 95 °C for 10 minutes, whereby an about 10 micron-thick white scattering film was formed. The thus formed white sheet was scanned in contact with a thermal head in the pattern of letters and images, whereby a scanned pattern was fixed in a transparent state. The sheet was then placed on a black background having an optical density of 1.2, whereby a clear black display was formed against the white background. Then, the whole area of the above sheet having the above-mentioned pattern was heated to about 120 °C and then heated at about 90 °C for several seconds, whereby the original white scattering state was restored on the whole area and stably retained even if cooled to room temperature as it was, so that additional recording and display were possible. The above series of phenomena can be controlled based on the fact that the above-mentioned polymer liquid crystal can assume at least three states including a film state below the glass transition point where it retains a stable memory state, a liquid crystal state where it can be transformed into a substantially optical scattering state and an isotropic film state at a higher temperature where it assumes an isotropic molecular alignment. Figure 8 is a view showing a characteristic of the above-mentioned polymer liquid crystal on temperature increase. Referring to Figure 8, in case where the polymer liquid crystal is heated from room temperature without supplemental heating as indicated by curve 4 ○, heating up to T(max) is required in order to provide an isotropic phase necessary for writing. On the other hand, in case where supplemental heating has been effected in advance as indicated by curve 5 ○, it is possible to provide an isotropic phase necessary for writing by heating up to T'(max). In other words, by effecting supplemental heating, excessive heat application to the liquid crystal can be alleviated so that not only prevention deterioration and surface protection can be effected but also a heat energy from imagewise heating means can be decreased. Further, it is also effective for providing a clear image at a high contrast. Herein, "supplemental heating" refers to a heating which is effected in advance of imagewise heating to heat the recording layer to a temperature not causing a phase transition for writing in the recording layer (e.g., Tiso (isotropic phase transition temperature)) More specifically, the supplemental heating may be generally effected to heat the recording layer to a temperature (the neighborhood of the phase transition temperature) in the range of from a lower side temperature which is generally about a mid point between room temperature (25 °C) and the phase transition temperature for writing of the recording layer to an upper side temperature which is the lower one of a temperature of 2 - 5 °C below the phase transition temperature and a temperature which is lower than the phase transition temperature by 1/10 of the temperature difference between the phase transition temperature and room temperature. In case where the phase transition temperature is different whether it is on heating or cooling, the one on heating is used for the above purpose. Hereinbelow, the present invention will be explained more specifically based on examples. Example 1 Figure 1 is a schematic view of the display apparatus according to the present invention, Figure 2 is a sectional view of a writing section thereof, and Figure 3 is a schematic sectional view of a recording medium. In the display apparatus, a writing unit 72 provided with a thermal head 81 as shown in Figure 2 was moved by a driving unit (not shown) to form an image on a recording medium 71 as explained above. On the backside of the recording medium 71 is disposed a planar heat-generating member 73 for erasure of a written image. Referring to Figure 3, an example of such a recording medium 71 may be formed by coating a substrate 92 comprising a transparent resin such as polyethylene terephthalate, glass or ceramic with a polymer liquid crystal 93 and then with a protective layer 94 for protection from damage, dust or heat or damage by heat application means, which is a coating or laminated layer comprising a fluorine-containing resin, silicone resin, etc., or a polymer liquid crystal having a sufficiently high glass transition point Tg which is preferably higher than Tiso (isotropic phase transition temperature) of the polymer liquid crystal layer 93. For effecting the display, the polymer liquid crystal layer 93 is placed in a light-scattering state to provide a white display place, and an image formed on the recording medium 71 is formed in a transparent state or a state close thereto. In case where the substrate 92 and the planar heat-generating member 73 (e.g., when composed of ITO (indium-tin-oxide), tin oxide, etc.) are transparent, an improved image contrast may be obtained if a back plate 91 of a black or chromatic color is disposed on the back side. The back plate 91 can be disposed either separately as shown from or in contact with the recording medium 71. It is also possible to dispose a planar light source of a chromatic color instead of the back plate 91. A similar effect can be obtained by incorporating or applying a pigment or dye in or on the substrate 92. Further, instead of using the substrate 92, it is also possible to directly form the polymer liquid crystal layer on the back plate 91 or on the planar heat-generating member 73. Figure 4 is a schematic sectional view for illustrating a writing system. A thermal head 81 comprises a plurality of minute heat-generating elements 81a - 84g ... disposed in an array. In the illustrated state, an external OFF signal is applied to the minute heat-generating elements 81a and 81g, and no images are formed at the corresponding parts on the recording medium 71. Heat-generating elements 81b - 81e are supplied with external ON signals and generating heats. According to this embodiment, minute heat-generating elements may be supplied with external signals with varying voltages, pulse durations, waveforms, etc., to generate variable quantities of heat, whereby the polymer liquid crystal layer is caused to have locally different thickness regions with a temperature exceeding Tiso where transparent regions are formed. Thus, a locally different optical variable may be realized. In the illustrated state, the minute heat-generating elements generate varying heat quantities in the order of 81a = 81g < 81b < 81c < 81d < 81e < 81f, whereby the corresponding portions of the polymer liquid crystal layer assume multiple optical states ranging from a scattering state to a transparent state including intermediate states. In a specific example, the elements 81a and 81g were respectively supplied with a 1 msec-rectangular pulse of 0 V; 81f, 12V; 81b, 8 V; 81c, 9 V; 81d, 10 V; and 81e, 11 V. When a 10 micron-thick film of a polymer liquid crystal example of the formula (31) (n 2 =50) was subjected to the heat application, the corresponding portions of the layer provided transmittances of about 20 % (for 81b), about 40 % (81c), about 60 % (81d) and 80 % (81e), with a relative scale giving transmittances of 0 % for 81a and 81g and 100 % for 81f. Further, the thus recorded polymer liquid crystal layer was subjected to observation of a scattering state by placing a back plate on the side of the heat application and on the opposite side alternatively, whereby the opposite side showed an apparently larger surface scattering and it was confirmed that the transparent region was formed in the thicknesswise direction from the heat application side and with locally different thicknesses. According to the above-mentioned display system, it is possible to provide a particularly high-quality image with an intermediate density. Example 2 Figure 5 is a partly broken schematic perspective view of another display apparatus according to the present invention, and Figure 6 is a sectional view thereof. The display apparatus has, on one lateral side thereof, a halogen heater 160 comprising a halogen lamp 156 and a roller 157, and on the opposite side thereof, a drive roller 152. A recording medium 151 is wound about the two rollers to form a display section 154 on its front side, and on the backside of the display section 154 is disposed a back plate 155. in the neighborhood of the halogen heater 160, an erasure planar heater 158 for image erasure is disposed and connected with a temperature sensor 161. Further, near the drive roller 152 are disposed a planar heater 159 which is a first heating means and is served as a supplemental heating means, and a thermal head 153 which is a second heating means and is used for image writing. Incidentally, the drive motor 152 is driven by a motor (not shown) and other means may be also driven by electrical or electronic members (not shown). In a specific example, the recording medium 151 was prepared by coating a transparent polyethylene terephthalate substrate with a 20 wt. % solution in dichloroethane of a polymer liquid crystal of the following structural formula: by means of a wire bar, and dried in an oven at 90 °C for 15 min. to form a white scattering layer. Then the recording medium 151 was shaped into an endless form as indicated. For image writing, the drive roller 152 is rotated in the arrow direction, and the recording medium 151 is heated by the planar heater 159 as a supplemental heating means. At this time, the planar heater 159 is held at a temperature close to the transition temperature from the liquid crystal phase to the isotropic phase, e.g., at about 95 °C. The thus preliminarily heated recording medium 151 is then conveyed to pass by the thermal head 153. To the thermal head, an image signal, e.g., a facsimile signal sent from another facsimile transmitter, is supplied, whereby the heated portion of the recording medium in the form of a belt is transformed into a transparent image pattern. By continuing this operation, a transparent image is formed in an A4 page size and then stopped at the display section 154, whereby there is displayed on a white background a clear image of a chromatic or black color of the back late 155. According to our visual evaluation, it was found that, for example, a black paper having an optical density of 1.0 or above used as a back plate could provide an image of a sufficiently high contrast. Alternatively, if a color paper of another chromatic color, such as red, blue or green, may be used as a back plate 155, a clear image of each color can be formed on a white background. It was confirmed that the thus formed image was not changed after standing for 100 days. Image erasure is effected by using the halogen heater 160 and the erasure planar heater 158 and by rotating the drive roller 152 in the arrow direction after a desired image display. At this time, the halogen heater 160 is controlled at about 115 °C and the erasure planar heater 158 is controlled at about 95 °C based on the output from the temperature sensors 161. In this way, as for the appearance of the recording medium 151, the entirety of the medium 151 becomes almost transparent when it passes by the halogen heater 160 and is turned into a white scattering state when it passes by the erasure planar heater 158. Thus, by this operation, the above-mentioned display image is entirely erased and the white scattering state is restored. In a specific example, the erasure planar heater 158 had a width of about 40 mm and the entire face thereof was set to a temperature of at least 75 °C or higher. According to our experiments, the above-mentioned image writing and erasure could be stably repeated at least 200 cycles or more, and clear images were obtained at a moving speed of 40 mm/sec of the medium 151. This apparatus system can also be effectively operated by using an ordinary serial thermal head instead of the thermal head 153 and by effecting serial scan in the direction vertical to the movement of the medium 151 by using a drive system (not shown). Further, it is also possible to effect a gradational display by changing the magnitude or duration of a voltage pulse applied to respective dots of the thermal head 153. In this embodiment, a unitary planar heater 159 is used as a supplemental heating means, but this can be divided so as to locally effect the supplemental heating. It is also possible to use heating means such as a heat roller, a halogen lamp, and a thermal head. Figures 7A - 7D show embodiments using heating means other than a planar heater. Figure 7A shows an embodiment of using a halogen lamp 171 as a supplemental heating means. This embodiment is effective in respects of freeness from rubbing of the recording medium 151 and uniform heating because a non-contact heating means is used. Figure 7B shows an embodiment of using heat rollers 172, which rotate at a relative speed of zero with respect to the recording medium 151, so that uniform heating can be effected without rubbing as in the case of Figure 7A. Figure 7C shows an embodiment of disposing a halogen lamp 171 in a roller 173. In this case, supplemental heating of the recording medium 151 is effected not only in advance of but also simultaneously with image formation by using the thermal head 153, so that a good heat efficiency is attained. Further, it is possible to provide an image of a good contrast. Figure 7D shows an embodiment of using two thermal heads 153a and 153b, whereby selective supplemental heating can be effected corresponding to an image or locally. In the respective embodiments, the distance between the thermal head 153 and the supplemental heating means can be arbitrarily selected and adjusted. In this case, it is also possible to control the temperature of the supplemental heating means, e.g., by control of applied voltage or current, similarly as above. Further, in the above embodiment, a thermal head 153 is used as a writing means. It is however possible to use a laser beam if the recording medium includes a laser beam-absorbing layer. The above-mentioned polymer liquid crystal has a good heat resistance and film strength, so that it can be used for repetitive image formation while directly rubbing or scanning the polymer liquid crystal layer with a thermal head. In order to increase the strength, however, it is possible to laminate a protective layer of polyimide, aramide resin, etc., on the surface thereof as desired, by pressure-bonding or by the medium of an appropriate adhesive layer. In a specific example, lamination of a 3.5 micron-thick and 6 micron-thick aramide sheet by pressure-bonding both provided good results in image formation. Example 3 Figure 9 is a schematic perspective view of another display system according to the present invention, wherein a recording medium 195 constituting a display section 193 and a back plate 191 are fixed, and a recording unit 192 as a writing and reading means is moved. Figure 10 is a partial sectional view of the system. Referring to Figure 10, the recording medium 195 comprises a substrate 202 of a transparent resin such as polyethylene terephthalate, glass or ceramics, a polymer liquid crystal layer 203 and a protective layer 204 disposed on one side of the substrate 202, and a planar heat-generating member 201 of indium-tin-oxide, tin oxide, etc., as a supplemental heating means on the other side of the substrate 202. The back plate 191 may be a plate of a black or chromatic color. The recording unit 192 comprises a thermal head 205 for writing, a readout sensor 206 for reading an image written on the display section 193, and a light source 207 as an erasure means. The recording unit 192 is moved leftwardly and rightwardly by means of a drive motor (not shown). The system can further include a writing pen 194, such as a heat pen or a marker pen as used on an electronic blackboard. For effecting display by the above system, the polymer liquid crystal layer 203 is supplementary heated to a temperature below Tiso by energizing the planar heat-generating member 201 when the power is turned on or prior to driving of the recording unit 192. In this state, writing is effected by the thermal head 205 or by the writing pen 194. The recorded image on the display section 193 is read out by the readout sensor 206 and can be stored - in an external memory apparatus (not shown). Further, the erasure of the image is effected by heating from the light source 207. In this way, if the recording medium is provided with a supplemental heating means, it is possible to protect the recording medium and increase the heat efficiency at the time of image formation. Further, the planar heat-generating member 201 can also be used as an erasure means by controlling the heating temperature.

Claims: A recording apparatus comprising: a recording medium (151) comprising a polymer liquid crystal layer capable of assuming a transparent state or an opaque state; first heating means (159) for heating the polymer liquid crystal layer to a temperature below a phase transition temperature for writing of the polymer liquid crystal;and second heating means (153) for imagewise heating the polymer liquid crystal layer to a temperature above the phase transition temperature for writing, thereby forming a written part therein, the heating by the first heating means (159) being performed in advance of and, optionally, simultaneously with the heating by the second heating means (153). An apparatus according to claim 1, wherein said phase transition temperature for writing is a liquid crystal-isotropic phase transition temperature of the polymer liquid crystal. An apparatus according to claim 1 or 2, wherein said first heating means (159) heats the polymer liquid crystal layer to a temperature in a range from a temperature which is at the mid-point between room temperature and the phase transition temperature for writing, to a temperature which is the low of a temperature 2-5 °C below the phase transition temperature and a temperature which is lower than the phase transition temperature by 1/10 of the temperature difference between the phase transition temperature and room temperature. An apparatus according to one of the preceding claims, which further comprises third heating means (158) for erasing the written part. An apparatus according to one of the preceding claims, wherein the first heating means comprises a planar heater (159). An apparatus according to one of the preceding claims, wherein the first heating means comprises a heater roller. An apparatus according to one of the preceding claims, wherein the first heating means comprises a halogen lamp. An apparatus according to one of the preceding claims, wherein the first heating means comprises a thermal head. An apparatus according to one of the preceding claims, wherein the second heating means comprises a thermal head (153).

Inventors: EGUCHI TAKEO, ISAKA KAZUO, KANEKO SHUZO, KURABAYASHI YUTAKA, MOURI AKIHIRO, OHNISHI TOSHIKAZU, TOSHIDA YOMISHI, YOSHINAGA KAZUO, EGUCHI, TAKEO, ISAKA, KAZUO, KANEKO, SHUZO, KURABAYASHI, YUTAKA, MOURI, AKIHIRO, OHNISHI, TOSHIKAZU, TOSHIDA, YOMISHI, YOSHINAGA, KAZUO
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1988
Main classification: G11B7
IPC codes: G11B7, G02F1