Recording medium and recording method for using the same

There is provided a recording medium comprising a substrate and an ink-receiving layer containing alumina hydrate formed thereon, wherein the alumina hydrate is present unoriented in the ink-receiving layer and a diffraction intensity fluctuation .delta. in a diffraction pattern is not more than 5%, when irradiating an electron beam to a cross section of the ink-receiving layer.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to a recording medium to be suitably used for
 ink-jet recording. It also relates to an ink-jet recording method using
 such a recording medium.
 2. Related Background Art
 Known ink-jet recording systems normally comprise one or more nozzles for
 ejecting ink droplets onto a recording medium in order to produce and
 record pictures and/or characters on the medium. These systems are highly
 versatile in terms of colors and patterns to be used for recording and
 adapted to high speed recording without giving off particularly annoying
 noise and, unlike photography, requiring development and fixing steps.
 Therefore, they are finding increasingly diverse applications particularly
 in the field of information-related devices including printers, copying
 machines, word processors, facsimile machines and plotters. Additionally,
 in view of the recent development of marketing low cost digital cameras,
 digital video recorders and scanners and the widespread popularity of
 personal computers, ink-jet recording systems are expected to be popularly
 used as output devices for producing images stored in them. In fact,
 efforts have been made for the ink-jet recording system to meet the
 requirements of higher recording speed and enhanced high definition and
 full color recording capability in order to make it competitive with
 silver halide type color photography and multi color printing of a plate
 system. In the course of the recent technological development, however, it
 has been recognized that the recording medium is an important subject
 matter on which more stress has to be put.
 A number of different recording media have been proposed for ink-jet
 recording. For example, Japanese Patent Application Laid-Open No. 52-53012
 discloses a type of ink-jet recording paper prepared by applying a coating
 paint on low-sized paper. Japanese Patent Application Laid-Open No.
 53-49113 discloses another type of ink-jet recording paper prepared by
 impregnating paper incorporated with a powdery urea-formalin resin therein
 with a water-soluble polymeric substance. Japanese Patent Application
 Laid-Open No. 55-5830 discloses still another type of ink-jet recording
 paper prepared by forming an ink-absorbing coating layer on a surface of
 substrate. Japanese Patent Application Laid-Open No. 55-51583 describes
 the use of non-crystalline silica as pigment contained in the coating
 layer of ink-jet recording paper. Japanese Patent Application Laid-Open
 No. 55-146786 describes the use of a coating layer of a water-soluble
 polymeric substance.
 Recently, the use of alumina hydrate has been attracting attention for
 recording media, because it has advantages as compared with conventional
 recording media. Namely, alumina hydrate shows a remarkable fixing
 capability for a dye in an ink and an enhanced coloring potential due to
 its positive electric charge so that it can produce highly glossy images.
 Japanese Patent Application Laid-Open No. 7-232475 discloses a recording
 medium in which an alumina hydrate is used for enhancing ink-absorbency
 and for preventing bleeding. Also, U.S. Pat. Nos. 4,879,166 and 5,104,730
 and Japanese Patent Application Laid-Open Nos. 2-276670, 4-37576 and
 5-32037 respectively describe recording media comprising an alumina
 hydrate layer with a pseudo-boehmite structure.
 However, for a recording medium containing alumina hydrate to fully compete
 with silver halide type photography and multi color printing of a plate
 system in quickly producing a finely defined image, there are a lot of
 problems including the following that have to be solved.
 (1) In case of printing a finely defined color image in a short period of
 time, since a large volume of ink has to be applied to the surface of the
 recording medium, the applied ink cannot be fully taken up into the pores
 of the medium and can bleed and flow over the ink-receiving surface to
 degrade the image on the medium.
 (2) The recording medium is required to absorb ink rapidly for high speed
 printing, but beading may occur when the applied ink is not absorbed at a
 sufficiently high rate. The term "beading" as used herein refers to a
 phenomenon where some or all of the ink dots placed on the recording
 medium are mingled with adjacent ones to blur the image formed on the
 medium before the ink is sufficiently absorbed by the medium.
 (3) Japanese Patent Application Laid-Open No. 3-281384 describes an alumina
 hydrate, which is in columnar form and forms an aggrgation oriented in a
 certain direction, and a method for forming an ink-receiving layer using
 such alumina hydrate. Japanese Patent Application Laid-Open No. 2-276670
 describes a bundle of filaments of alumina sol. However, filament- or
 column-shaped particles of alumina hydrate can easily and densely
 agglomerate, presumably because they show a concentrated electric charge
 along the edges of the particles and hence it is difficult for them to
 permit an ink to infiltrate into the ink-receiving layer. As a result,
 such alumina hydrate is not adapted for ink to be used for producing high
 definition color images in a short period of time as beading can easily
 occur.
 SUMMARY OF THE INVENTION
 In view of the above identified problems, therefore, the first object of
 the present invention is to provide a recording medium for recording fine
 images that can be used with inks having different compositions and can
 absorb ink without producing bleeding and beading of ink. The second
 object of the invention is to provide an ink-jet recording method using
 such a recording medium.
 According to the present invention, there is provided a recording medium
 comprising a substrate and an ink-receiving layer containing alumina
 hydrate formed thereon, wherein the alumina hydrate is present unoriented
 in the ink-receiving layer and a diffraction intensity fluctuation .delta.
 in a diffraction pattern is not more than 5%, when irradiating an electron
 beam to a cross section of the ink-receiving layer.
 According to the present invention there is also provided an ink-jet
 recording method by ejecting and applying ink droplets onto a recording
 medium mentioned above.

DETAILED DESCRIPTION OF THE INVENTION
 A recording medium according to the present invention contains unoriented
 alumina hydrate as an essential ingredient. It comprises a substrate and
 an ink-receiving layer containing alumina hydrate formed on the substrate
 and a binding agent. The alumina hydrate is found unoriented in the
 ink-receiving layer. More specifically, as shown in the photograph of FIG.
 1 (taken through a transmission electron microscope with a magnifying
 power of 200,000), particles of alumina hydrate contained in a recording
 medium according to the present invention are not oriented in any
 particular direction (unoriented) and the alumina hydrate does not have
 any oriented crystal plane so that electron beams are not diffracted
 strongly by any particular crystal planes. Thus, as shown in the
 electron-diffraction pattern of FIG. 2, all the diffraction rings show a
 substantially identical intensity distribution pattern for all the crystal
 planes. On the other hand, bundles of filaments of alumina hydrate
 (boehmite) oriented in a certain direction in the photograph of FIG. 3
 (taken through a transmission electron microscope with a magnifying power
 of 200,000) shows a strong electron-diffraction that is produced by (020)
 plane and hence strong fluctuations in the diffraction rings of (020)
 plane as seen from the photograph of FIG. 4 (electron-diffraction
 pattern). For the purpose of the present invention, alumina hydrate meets
 the requirement defined by equation (1) below. That is to say, when the
 recording medium is cut from the surface of the medium to the bottom of
 the base and the exposed cross section of the ink-receiving layer is
 irradiated with electron beams, there is obtained a diffraction pattern of
 coaxially arranged rings. In this diffraction pattern, the diffraction
 intensity fluctuation .delta. represented by the equation (1) is not more
 than 5%:
EQU .delta.=[(Imax-Imin)/(Imax+Imin)].times.100 (1)
 where Imax represents the largest diffraction intensity of a ring in the
 diffraction pattern and Imin represents the smallest diffraction intensity
 of a ring in the diffraction pattern.
 The rate of ink absorption is particularly high to effectively prevent the
 occurrence of beading when the above requirement is met.
 For the purpose of the present invention, alumina hydrate is expressed by
 the general formula
EQU Al.sub.2 O.sub.3-n (OH).sub.2n.mH.sub.2 O (2)
 where n represents an integer of 0, 1, 2 or 3 and m represents a value
 between 0 and 10, preferably between 0 and 5, but both m and n should not
 be equal to 0 at the same time. In most cases, mH.sub.2 O in the formula
 (2) above represents water molecules that have nothing to do with the
 formation of crystal lattice and hence can easily be released from the
 compound so that m may or may not be an integer. Additionally, m can
 become equal to 0 when such a material is calcined. Alumina hydrate can be
 prepared by appropriate known means such as hydrolysis of aluminum
 alkoxide or sodium aluminate. Rocek et al. report that the porous
 structure of alumina hydrate is influenced by the deposition temperature,
 the pH value of the solution, the maturing time, the surfactant involved
 and other factors (Collect. Czech. Chem. Commun., Vol. 56, 1253-1262,
 1991). They also report that pseudo-boehmite may or may not take a
 cilia-like form in alumina hydrate (Rocek J. et al., Applied Catalysis,
 Vol. 74, 29-36, 1991). For the purpose of the present invention, alumina
 hydrate is spindle-shaped and shows an average aspect ratio between 1 and
 4. The average aspect ratio can be determined by dividing the major axis
 of each particle by the minor axis. The profile of each particle is
 observed through a transmission electron microscope by following a
 procedure as will be described hereinafter.
 A nitrogen adsorption/desorption technique can be used to simultaneously
 determine the BET specific surface area, the pore radius distribution and
 the pore volume of a given alumina hydrate and the pore radius
 distribution and the pore volume of the ink-receiving layer containing
 such alumina hydrate. For the purpose of the present invention, unoriented
 alumina hydrate preferably shows a BET specific surface area of 70 to 300
 m.sup.2 /g. If the BET specific surface area falls below the above defined
 lower limit, the pore radius distribution can be lopsided in favor of the
 large side so that the dye contained in the ink cannot be satisfactorily
 adsorbed nor fixed. If, on the other hand, it exceeds the upper limit, the
 alumina hydrate may not be dispersed satisfactorily in the ink-receiving
 layer to make it difficult to accurately control the pore radius
 distribution.
 For the purpose of the present invention, alumina hydrate is prepared
 through hydrolysis/deflocculation of aluminum alkoxide or of aluminum
 nitrate and sodium aluminate. As will be described hereinafter by
 referring to Examples, alumina hydrate in the form of spindle-shaped
 particles with an average aspect ratio between 1 and 4 can be obtained by
 means of a two-stage crystal growth process, although the present
 invention is not limited thereto by any means. Alternatively, for example,
 after forming alumina hydrogel slurry through hydrolysis of aluminum
 alkoxide or of aluminum nitrate and sodium aluminate, the obtained slurry
 may be spray-dried to produce powdery alumina hydrate, which is then
 dispersed into an acidic solution, to which sodium aluminate is added to
 prepare desired alumina hydrate through recrystallization and crystal
 growth. It should be noted that one tends to obtain unoriented and low
 anisotropic alumina hydrate particles, when raising the rate of crystal
 growth.
 The recording medium according to the present invention is prepared by
 applying a solution that contains unoriented alumina hydrate as described
 above as pigment and a binding agent (dispersive solution of alumina
 hydrate) to a substrate to form an ink-receiving layer. The physical
 properties of the ink-receiving layer are determined as a function of not
 only the unoriented alumina hydrate used, but also various parameters
 including the type of the binding agent used, the concentration, the
 viscosity and the dispersiveness of the coating solution, the applicator
 including the head, the rate of application and the drying conditions.
 Therefore, the conditions for manufacturing an ink-receiving layer for the
 purpose of the present invention have to be carefully adjusted for
 optimization.
 For the purpose of the present invention, the pores of the ink-receiving
 layer preferably show a maximum value found between 30 and 200 .ANG. for
 the pore radius distribution. If the maximum pore radius exceeds the above
 defined upper limit, the image formed on the recording medium can bleed
 due to poor adsorption and fixation of the ink applied to it. If, on the
 other hand, the maximum pore radius falls below the lower limit, the ink
 applied to it will be poorly absorbed by the recording medium to give rise
 to beading.
 Similarly, the pores of alumina hydrate in the ink-receiving layer
 preferably show a maximum value found between 30 and 200 .ANG. for the
 pore radius distribution. It should be noted that the maximum pore radius
 of the ink-receiving layer is a function of that of the alumina hydrate
 contained in it.
 The binding agent to be used with unoriented alumina hydrate in a recording
 medium according to the present invention can be selected from appropriate
 water soluble polymers including polyvinyl alcohol and modified products
 thereof, starch and modified products thereof, gelatin and modified
 products thereof, gum arabic, carboxymethyl cellulose, hydroxyethyl
 cellulose, hydroxypropylmethyl cellulose and other cellulose derivatives,
 SBR latex, NBR latex, latex of methylmethacrylate-butadiene copolymers and
 that of other conjugate diene copolymers, that of
 functional-group-modified polymers, latex of ethylene-vinyl acetate
 copolymers and that of other vinyl type copolymers, polyvinylpyrrolidone,
 maleic anhydride and its copolymers and acrylate copolymers. Any of these
 binding agents may be used solely or in combination. For the purpose of
 the present invention, the mixing ratio by weight of unoriented alumina
 hydrate to a binding agent is between 1:1 and 30:1, preferably between 5:1
 and 25:1. If the binding agent falls below the above defined range, the
 obtained ink-receiving layer will have inadequate mechanical strength and
 eventually give rise to cracks and exfoliation. If, on the other hand, it
 exceeds the above range, the pore volume will be reduced and therefor the
 ink absorbency of the ink-receiving layer may be lowered.
 For the purpose of the present invention, an alumina hydrate dispersant, a
 thickener, a pH modifier, a lubricant, a flowability modifier, a
 surfactant, a defoamer, a water-fastness imparting agent, a surface
 lubricant, a fluorescent brightening agent, a UV absorbing agent and/or an
 antioxidant may be added to the alumina hydrate and the binding agent, if
 necessary.
 For the purpose of the present invention, the substrate of the
 ink-receiving layer of a recording medium according to the present
 invention may be made of appropriately sized paper, unsized paper,
 resin-coated paper typically using polyethylene or paper of some other
 type or a sheet of some other material such as thermoplastic film or
 cloth, although it is not subjected to any particular limitations.
 To produce a recording medium that can compete with silver halide
 photography in terms of image quality, the substrate preferably has a
 basic weight of not less than 120 g/m.sup.2, more preferably between 150
 and 180 g/m.sup.2 and is made of a fibrous material such as wood pulp.
 For the purpose of the present invention, the ink-receiving layer may have
 a multilayer structure. For example, it may comprise a porous first
 ink-receiving layer containing barium sulfate and a second ink-receiving
 layer containing unoriented alumina hydrate laminated on a substrate in
 this order.
 When barium sulfate is used, it should be purified as much as possible in
 order to improve the whiteness and the light fastness of the recording
 medium. The barium sulfate of the lower first layer preferably has an
 average particle diameter between 0.4 .mu.m and 1.0 .mu.m, more preferably
 between 0.4 .mu.m and 0.8 .mu.m to improve the surface smoothness of the
 lower layer. If the average particle diameter falls below 0.4 .mu.m, the
 whiteness, the glossiness and the solvent absorbing ability of the
 recording medium will be degraded. If, on the other hand, the average
 particle diameter exceeds 1.0 .mu.m, the whiteness and the glossiness of
 the recording medium will also be degraded.
 Gelatin is preferably used as binder for binding barium sulfate in position
 because gelatin has a refractive index close to that of barium sulfate
 and, therefore, light will not significantly be reflected at any interface
 between them.
 For the purpose of the present invention, gelatin may be treated with acid
 or alkali. Preferably, 6 to 12 parts by weight of gelatin is added to 100
 parts by weight of barium sulfate when preparing a solution to be applied
 to the substrate for the purpose of the present invention. While chromium
 sulfate, chromium alum, formalin or triazine may typically be used for
 bridging gelatin, it is preferable to use chromium alum because it can be
 handled without difficulty. A bridging agent is added preferably at 0.2 to
 4 parts by weight to 100 parts by weight of gelatin.
 Barium sulfate is preferably applied to the substrate in a range of from 20
 to 40 g/m.sup.2 in terms of the solid content of the solution that
 contains barium sulfate in order to provide the recording medium with a
 sufficient ink-solvent absorbing ability and a required degree of
 smoothness. While the solution may be applied and dried with any method,
 it is preferable that a surface smoothing operation such as super calender
 is conducted as a finishing step, that the first ink-receiving layer has a
 whiteness of not less than 87% and that the Bekk smoothness of the surface
 is not less than 400 seconds.
 On the other hand, the Bekk smoothness of the surface is preferably not
 greater than 600 seconds, more preferably not greater than 500 seconds,
 because too smooth a surface absorbs ink poorly.
 As shown in FIG. 5, the substrate 1 of a recording medium according to the
 present invention may be provided with a release liner 4 on the rear side
 (the side opposite to the one carrying the ink-receiving layer 2) through
 a layer of an adhesive agent such as a pressure-sensitive adhesive agent
 layer 3 interposed therebetween in order to make the recording medium
 adherent. With this arrangement, the recording medium may be made to stick
 to an appropriate surface by peeling off the release liner 4.
 Further, in the present invention there may be provided a porous layer
 comprising thermoplastic resin particles as a surface layer on the
 ink-receiving layer, whereby an ink applied reaches an underlaying layer
 of the ink-receiving layer through the porous layer to form an image
 thereon, and then, when the porous surface layer is made nonporous, a
 print having a high optical density and excellent weather fastness can be
 obtained.
 The thermoplastic resin particles used in the present invention are
 preferably particles formed of a latex.
 For the purpose of the present invention, an ink-receiving layer may be
 formed on a substrate by applying a solution containing unoriented and
 dispersed alumina hydrate onto the surface of the substrate by means of an
 applicator and drying the applied solution. A blade coater, an air knife
 coater, a roll coater, a curtain coater, a bar coater, a gravure coater or
 a sprayer may be used as the applicator for the purpose of the present
 invention. The dispersive solution of unoriented alumina hydrate is
 applied to the surface of the substrate at a rate preferably between 0.5
 and 60 g/m.sup.2, more preferably between 5 and 45 g/m.sup.2, as dried
 coating. If necessary, the surface of the formed ink-receiving layer may
 be smoothed by means of a calender machine.
 An ink-jet recording method according to the present invention uses a
 recording medium as described above. Ink droplets are ejected onto a
 recording medium to produce and record images and/or characters on the
 medium. While either a bubble-jet system or a piezoelectric system may be
 used with an ink-jet recording method according to the present invention,
 a bubble-jet system may be preferable because it is more adapted to
 printing fine characters at high speed. Preferably, a water-based ink is
 used and may be colored by either a dye or a pigment.
 In the case that the recording medium of the present invention has a
 surface layer, the surface layer is made nonporous (transparent) by
 subjecting it to a heat treatment, after images are formed by applying an
 ink. When the porous layer is subjected to such a treatment, an image
 formed on the recording medium is improved in weather fastness such as
 water fastness and light fastness, and good gloss can be imparted to the
 image.
 Now, the present invention will be described in greater detail by way of
 examples, which do not limit the present invention by any means. The
 physical properties of the specimens were observed by the following
 methods.
 (1) BET Specific Surface Area, Pore Radius Distribution, and Pore Volume
 The specimens were heated and deaerated satisfactorily before being
 observed by means of a nitrogen adsorption/desorption method (using
 Omnisorp 360, trade name; available from COULTER Co.).
 (2) Observation of Alumina Hydrate (Aspect Ratio, and Particle Profile)
 The specimens were prepared either directly from powdery alumina hydrate or
 by dispersing it in deionized water to a concentration between 1 and 2%
 and then dipping out of the solution by means of a collodion-coated copper
 mesh to remove excess water. To observe the ink-receiving layer, the
 specimens were prepared by cutting each recording medium into very thin
 sections of 500 to 4,000 .ANG. by means of a microtome. The prepared
 specimens were then observed through a transmission electron microscope
 (H-800, trade name; available from Hitachi Co.). The average aspect ratio
 was determined by dividing the major axis of each particle by the minor
 axis.
 (3) Selected-Area Electron Diffraction Pattern and
 Measurement of the Diffraction Intensity Fluctuation
 The specimens were prepared by cutting each recording medium comprising a
 substrate and an ink-receiving layer into very thin sections of 700.+-.100
 .ANG. by means of a microtome. An area selected for diffraction was
 defined by 2,000 .ANG..phi. and the values obtained at 10 different cross
 sections were averaged. The electron diffraction of each cross section of
 the ink-receiving layer was observed by means of an electron
 diffractometer (H-800, trade name; available from Hitachi Co.) and the
 diffraction intensity of the diffraction pattern was transferred onto an
 imaging plate (available from Fuji Photo Film Co.) to observe the
 intensity distribution of the diffraction pattern for each lattice plane.
 The diffraction intensity fluctuation was determined by means of equation
 (1) above.
 (4) Printing Characteristics
 Ink-jet printing was conducted on the specimens using a color ink-jet
 printer with Y (yellow), M (magenta), C (cyan) and Bk (black) ink-jet
 heads, each having 128 nozzles arranged at a rate of 16 nozzles per mm,
 and inks having the compositions listed below. Then, they were observed
 for ink absorption, image density, bleeding and beading.
 &lt;1&gt; Ink Absorption
 The specimens were solid printed for both mono-color printing and
 multi-color printing with inks having the compositions listed below and
 each of the specimens was tested for surface ink absorption by touching
 the printed areas of the recording medium with a finger tip. The amount of
 ink per unit area at mono-color printing was defined to be 100%. A
 multi-color printing that did not smear the finger tip with ink when the
 amount of ink per unit area was 300% was ranked as "A", and a multi-color
 printing that smeared the finger tip with ink when the amount of ink per
 unit area was 300% but did not when the amount of ink per unit area was
 200% was ranked as "B".
 &lt;2&gt; Optical Density
 The solid prints obtained by using each Y, M, C and Bk inks with Ink
 Composition 1 below were observed for optical density by means of Macbeth
 Reflection Densitometer RD-918.
 &lt;3&gt; Bleeding and Beading
 The specimens were solid printed for both mono-color printing and
 multi-color printing with inks having Ink Composition 1 below and each of
 the specimens was observed for surface bleeding. As for beading, the
 specimens were solid printed for both mono-color printing and multi-color
 printing with two types of inks having the compositions listed below and
 each of the specimens was visually observed for beading. The amount of ink
 per unit area printed with a mono-color ink was defined to be 100%. A
 multi-color printing that did not show any bleeding and beading when the
 amount of ink per unit area was 300% was ranked as "A", and a multi-color
 printing that showed bleeding and/or beading when the amount of ink per
 unit area was 300% but did not when the amount of ink per unit area was
 200% was ranked as "B".
 The following compositions are expressed in terms of weight.

(Ink Composition 1)
 Dye (Y, M, C or Bk as shown below) 5 parts
 Ethylene glycol 10 parts
 Polyethylene glycol 10 parts
 Water 75 parts
 (Ink Composition 2)
 Dye (Y, M, C or Bk as shown below) 5 parts
 Ethylene glycol 15 parts
 Polyethylene glycol 10 parts
 Water 70 parts
 (Dye)
 Y: C. I. Direct Yellow 86
 M: C. I. Acid Red 35
 C: C. I. Direct Blue 199
 Bk: C. I. Hood Black 2
 EXAMPLES 1 to 3
 Aluminum octaoxide was synthetically prepared and hydrolyzed to produce an
 alumina slurry by a method described in U.S. Pat. No. 4,242,271 or U.S.
 Pat. No. 4,202,870. Water was added to the alumina slurry up to a solid
 content of alumina hydrate of 5%. Thereafter, the slurry was heated at
 80.degree. C. for 10 hours for a maturing reaction and the obtained
 colloidal sol was sprayed and dried to produce alumina hydrate. The
 obtained alumina hydrate was then mixed with and dispersed into deionized
 water, whose pH value was adjusted to 5 with nitric acid. Then, the
 mixture was heated to 95.degree. C. and sodium aluminate was added thereto
 until the pH rose to 10. Specimens were prepared for Examples 1 to 3 by
 maturing the mixture for 5 hours (Example 1), 10 hours (Example 2) and 15
 hours (Example 3), respectively. The colloidal sols were desalted and then
 deflocculated by adding acetic acid. When the alumina hydrate products
 obtained by drying the colloidal sols were observed by X-ray
 diffractometry, they were found to be pseudo-boehmite. When observed
 through a transmission electron microscope, all the alumina hydrate
 products were found in the form of spindle-shaped particles. The physical
 properties of the alumina hydrate products obtained by the above described
 measurements are listed in Table 1.
 Polyvinyl alcohol PVA117 (trade name; available from Kuraray Co.) was
 dissolved into deionized water to produce a 10% by weight solution. Each
 colloidal sol of the three alumina hydrate products was condensed to
 produce a 15% by weight solution. Then, the colloidal sol of alumina
 hydrate and the polyvinyl alcohol solution were mixed with each other such
 that the solid alumina hydrate and the solid polyvinyl alcohol showed a
 ratio by weight of 10:1 and the mixture was agitated to produce a
 dispersive solution. Subsequently, the dispersive solution was applied to
 a 100 .mu.m thick PET film (Lumirror, trade name; available from Toray
 Co.) by means of a die coater and dried to produce an ink-receiving layer.
 FIG. 1 is a photograph showing a cross section of the ink-receiving layer
 (taken through a transmission electron microscope with a magnifying power
 of 200,000). It will be seen that alumina hydrate is in the form of
 unoriented spindle-shaped particles. The cross section was then subjected
 to electron diffractometry to further look into it. FIG. 2 shows a
 photograph taken by electron diffractometry. Table 2 summarily shows the
 physical properties of the ink-receiving layer obtained by the above
 described methods.
 EXAMPLE 4
 A colloidal sol of alumina hydrate was synthetically prepared through
 hydrolysis of an aqueous solution of aluminum nitrate and that of sodium
 aluminate. The concentration and the amount of each of the materials was
 adjusted so as to be 5% of the concentration of solid alumina hydrate and
 the pH 9 of the product after adding sodium aluminate, respectively.
 Thereafter, the product was heated at 90.degree. C. for 10 hours for
 maturing. The obtained colloidal sol was desalted and then spray-dried to
 produce alumina hydrate. The obtained alumina hydrate was then mixed with
 and dispersed into deionized water, whose pH value was adjusted to 5 by
 means of nitric acid. Then, the mixture was heated to 95.degree. C. and
 sodium aluminate was added thereto to adjust the pH to 10. Colloidal sol
 was prepared by maturing the mixture for 15 hours. The obtained colloidal
 sol was desalted and then deflocculated by adding acetic acid. When the
 alumina hydrate product obtained by drying the colloidal sol was observed
 by X-ray diffractometry, it was found to be pseudo-boehmite. When observed
 through a transmission electron microscope, all the alumina hydrate
 products were found in the form of spindle-shaped particles. The physical
 properties of the alumina hydrate product obtained by the above described
 measurements are also listed in Table 1. An ink-receiving ,layer was
 formed, and its electron diffraction and physical properties were measured
 as in Examples 1 to 3. Table 2 summarily shows the obtained result.
 EXAMPLE 5
 A colloidal sol of alumina hydrate was synthetically prepared through
 hydrolysis of an aqueous solution of aluminum nitrate and that of sodium
 aluminate as in Example 4. Firstly, an aqueous solution of sodium
 aluminate was added to an aqueous solution of aluminum nitrate so as to be
 pH 5 to deposit crystals of alumina hydrate and then the mixture was left
 at 30.degree. C. for 2 hours while stirring the mixture constantly.
 Thereafter, sodium aluminate was added again to adjust the pH to 9 and the
 mixture was matured at 90.degree. C. for 10 hours. The concentration of
 solid alumina hydrate was so adjusted as to become equal to 5% after the
 synthesis.
 The obtained colloidal sol was then processed as in Example 4 to produce
 alumina hydrate. The physical properties of the alumina hydrate product
 were measured as in Example 1 and also listed in Table 1. A recording
 medium of the present invention was prepared and an electron
 diffractometry and physical properties of an ink-receiving layer were
 observed as in Example 1. Table 2 summarily shows the obtained result.
 EXAMPLE 6
 Alumina hydrate was prepared as in Example 5 except that the mixture was
 left for 4 hours after the deposition of crystals of alumina hydrate at pH
 5.
 The physical properties of the alumina hydrate product were measured as in
 Example 1 and also listed in Table 1. An ink-receiving layer was formed
 and observed by electron diffractometry and its physical properties were
 analyzed as in Example 1. Table 2 summarily shows the obtained result.
 EXAMPLE 7
 An ink-receiving layer was prepared as in Example 1 except that the
 substrate was replaced by a 75 .mu.m thick PET film and the dried
 ink-receiving layer had a thickness of about 30 .mu.m.
 A pressure-sensitive adhesive agent prepared for sticky labels by using
 acrylate type copolymer as a base polymer was applied to the release liner
 to a thickness of about 50 .mu.m by means of a blade coater. The release
 liner was then applied to the rear side of the PET film of the prepared
 recording medium to produce a recording sheet. The obtained recording
 medium could be made to stick to any appropriate surface by peeling off
 the release liner.
 COMATIVE EXAMPLE 1
 Alumina hydrate (sol) was synthetically prepared in the form of bundles of
 filaments (cilia-like form) through hydrolysis/deflocculation of aluminum
 isopropoxide. Then, an ink-receiving layer was prepared therefrom and a
 recording medium was produced by using the ink-receiving layer as in
 Example 1. A cross section of the ink-receiving layer was observed through
 a transmission electron microscope and also by electron diffractometry and
 the physical properties of the ink-receiving layer were measured. The
 obtained physical properties of the alumina hydrate and those of the
 ink-receiving layer are summarily shown in Tables 1 and 2. FIG. 6 shows
 the result of a measurement using a Bristow tester available from
 Toyo-Seiki Manufacturing and conducted on the specimens of the recording
 media of Example 1 and Comparative Example 1 for ink absorption. FIG. 6
 shows a relationship between a contact time (msec.sup.1/2) and a
 transferred amount of liquid (ml/m.sup.2). As seen from FIG. 6, ink is
 absorbed much quicker by a recording medium comprising an ink-receiving
 layer of unoriented alumina hydrate than by a recording medium comprising
 an ink-receiving layer of oriented alumina hydrate.
 TABLE 1
 Physical Comp.
 property/Sample Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1
 Average aspect 3 3 2 3 3 4 10
 ratio
 Max. pore radius 50 85 125 90 89 92 84
 BET specific
 surface area 231 158 75 150 153 156 187
 (m.sup.2 /g)
 Pore volume 0.65 0.78 0.84 0.81 0.79 0.80 0.83
 (cc/g)
 TABLE 2
 Physical Comp.
 property/Sample Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 1
 Max. pore radius 45 85 120 82 85 88 75
 (.ANG.)
 Pore volume 0.62 0.75 0.80 0.77 0.79 0.81 0.80
 (cc/g)
 Printing
 characteristics
 Absorption A A A A A A B
 Image density
 Y 1.70 1.68 1.63 1.70 1.67 1.68 1.61
 M 1.59 1.63 1.58 1.65 1.60 1.62 1.60
 C 1.72 1.70 1.71 1.72 1.72 1.70 1.65
 Bk 1.76 1.74 1.72 1.75 1.73 1.72 1.68
 Bleeding and A A A A A A B
 beading
 Ink Comp. 1 A A A A A A B
 Ink Comp. 2 A A A A A A B
 Fluctuation in
 diffraction (%) 0.6 0.8 1.2 0.9 3.4 5.0 23
 intensity
 EXAMPLE 8
 A solution to be applied was prepared by mixing 100 parts by weight of
 barium sulfate with an average particle diameter of 0.6 .mu.m produced by
 causing sodium sulfate to react with barium chloride, 10 parts by weight
 of gelatin, 3 parts by weight of polyethylene glycol and 0.4 part by
 weight of chromium alum. The solution was applied to a base paper to be
 coated with a basis weight of 150 g/m.sup.2, a Stockigt sizing degree of
 200 seconds and a Bekk smoothness of 340 seconds to a dried thickness of
 20 .mu.m by means of a die coater and then the paper was processed by a
 super calender to produce a recording medium with a surface smoothness of
 400 seconds.
 Aluminum octaoxide was synthetically prepared and hydrolyzed to produce an
 alumina slurry by a method described in U.S. Pat. No. 4,242,271 or U.S.
 Pat. No. 4,202,870. Water was added to the alumina slurry up to a solid
 concentration of alumina of 5%. Thereafter, the slurry was heated at
 80.degree. C. for 10 hours for a maturing reaction and the obtained
 colloidal sol was spray-dried to produce alumina hydrate. The obtained
 alumina hydrate was then mixed with and dispersed into deionized water,
 whose pH value was adjusted to 5 with nitric acid. Then, the mixture was
 heated to 95.degree. C. and sodium aluminate was added thereto until the
 pH rose to 10. The colloidal sols were desalted and then deflocculated by
 adding acetic acid. When the alumina hydrate products obtained by drying
 the colloidal sols were observed by X-ray diffractometry, they were found
 to be pseudo-boehmite. When observed through a transmission electron
 microscope, all the alumina hydrate products were found in the form of
 spindle-shaped particles.
 The solution was then applied to the above recording medium by means of a
 bar coater until the basis weight got to 20 g/m.sup.2 after the
 application and then dried at 100.degree. C. for 10 minutes in an oven.
 Thereafter, the alumina hydrate was baked at 150.degree. C. for 2 minutes
 to produce porous alumina hydrate for a recording medium according to the
 present invention.
 The finished recording medium was then used for printing and the printed
 image was tested for various physical properties. Table 3 summarily shows
 the obtained result.
 In Table 3, the smoothness was measured as follows. By means of a Bekk
 smoothness meter (available from Yoshimitsu-Seiki Co.) under the
 conditions of the range "1 cc" which is for high smoothness specimen, the
 readings multiplied by 10 were smoothness. The whiteness was measured by
 means of a Hunter Reflectometer (available from Toyo-Seiki Manufacturing
 Co.) to which a blue filter was attached. As for glossiness, the
 75.degree. glossiness was measured by means of a digital variable
 glossimeter (available from Suga Shikenki Co.) in accordance with JIS P
 8142.
 EXAMPLE 9
 Base paper and a barium sulfate solution the same as those of Example 8
 were used to form an ink-receiving layer to a dry thickness of 13 .mu.m
 and a recording medium with a surface smoothness of 320 seconds was
 prepared by means of a super calender.
 A coating solution containing pseudo-boehmite as used in Example 8 was
 applied onto the medium by means of a bar coater until the basis weight
 got to 20 g/m.sup.2 after the application and then dried at 100.degree. C.
 for 10 minutes in an oven. Thereafter, the alumina hydrate was baked at
 150.degree. C. for 2 minutes to produce a finished recording medium.
 The finished recording medium was then used for printing and the printed
 image was tested for various physical properties as in Example 8. Table 3
 summarily shows the obtained result.
 EXAMPLE 10
 A latex (an average particle size of 0.2 .mu.m) was applied to the ink
 receiving layer of the recording medium prepared as in Example 1 by a bar
 coater so as to have a dry thickness of about 5 .mu.m, and then dried in
 an oven at 60.degree. C. for 10 minutes. When printed on the thus obtained
 recording medium by means of an ink-jet printer, the ink passed through
 the resin layer formed of the latex, and thereby made images on the
 ink-receiving layer. Images veiled with a white resin layer formed of a
 latex were observed. When heated in an oven at 130.degree. C. for 10
 minutes, the resin layer formed of the latex as the surface layer was
 molten to form a transparent film, so that a high glossy image free from
 ozone fading (ozone resistant image) can be obtained.
 As described above, a recording medium according to the present invention
 contains alumina hydrate that is unoriented and shows a diffraction
 intensity fluctuation not exceeding 5% in the ink-receiving layer. Thus,
 ink is absorbed much quicker by a recording medium according to the
 present invention than by a recording medium comprising an ink-receiving
 layer of oriented alumina hydrate in the form of bundles of filaments
 (cilia-like form).
 TABLE 3
 Example 8 Example 9
 Bekk smoothness 400 320
 (second)
 Whiteness (%) 87.5 87.6
 75.degree. glossiness 61.0 51.6
 (%)
 Ink absorption A A
 Image density Y 1.65 1.63
 Image density M 1.66 1.60
 Image density C 1.69 1.66
 Image density Bk 1.72 1.66
 Bleeding and A A
 beading for
 Composition 1
 Bleeding and A A
 beading for
 Composition 2
 Fluctuation in 0.8 0.8
 diffraction
 intensity (%)