JP4638324B2 - Inkjet recording medium - Google Patents

Description

  The present invention relates to an ink jet recording medium, and particularly to an ink jet recording medium having high gloss, sufficient ink absorption, suitable for pigment ink, and good surface strength of a coating layer, and close to the texture of photographic paper for silver salt photography. Is.

  In recent years, due to remarkable progress of inkjet printers and plotters, full-color and high-definition images can be easily obtained.

  The ink jet recording system records images, characters, and the like by causing micro droplets of ink to fly and adhere to a recording material such as paper according to various operating principles. Inkjet printers and plotters are rapidly spreading in various applications in recent years as hard copy creation apparatuses for image information such as letters and various graphics created by a computer. In particular, color images formed by the multi-color ink jet method can be recorded inferior to multi-color printing by the plate making method and printing by the color photographic method, and in applications where the number of copies is small. Is widely applied because it is cheaper than printing and photographic technologies. In particular, with the recent rapid digitization and the spread of digital cameras, the ink jet recording method has been used as an output method for digital data, that is, as an alternative method for silver salt photography.

  In particular, with the advancement of ink jet printer technology, price reduction, and the spread of digital cameras, it has become possible to easily output photos at home. In order to reproduce gradation smoothly or to make ink dots that form an image inconspicuous, current printers use inks that are lighter than conventional ones to reduce the size of ink droplets or reduce the size of ink droplets. Accordingly, by changing the size of the recording medium, a recorded material having an image quality equal to or higher than that obtained by a general silver salt photography method is being obtained. As a result, the amount of ink used for image formation has increased compared to the prior art, and along with this, ink jet recording media are required to have a property of absorbing and drying ink sufficiently and quickly. In addition, the same texture and storability as photographic paper used in silver salt photography, that is, gloss and scratch resistance, have been demanded.

  Further, in the case of a use for outputting a photographic image, the use is for indoor / outdoor exhibition or personal record storage, so that weather resistance and image storage stability of images are required more than ever. In response to such demands, inks and papers have been improved, and much better storage stability than before can be obtained. However, in particular, the light resistance has not yet reached the level of silver salt photography, and the current situation is that the requirements are not satisfied.

  In order to satisfy such demands, pigment type inks have recently been used. It is known that the pigment ink has little light deterioration and does not re-dissolve with water, so that the weather resistance and the image storage stability are improved as compared with the dye type ink. However, the color material pigment used in the pigment ink is insoluble in the solvent unlike the dye. Therefore, it is necessary to stably disperse the color material pigment, and the ratio of the color material pigment in the pigment ink cannot be easily increased. In addition, the coloring efficiency is not high like a dye type ink, and it is difficult to obtain a clear color. Therefore, the amount of ink used for image formation tends to be larger than that of dye ink. In particular, boundary bleeding (bleeding) is likely to occur between heavy color portions with a large amount of ink.

  As a method for improving the coloring efficiency of the pigment ink, that is, the color developability, a method of reducing the dispersed particle diameter of the colorant pigment is conceivable (for example, see Patent Document 1). The study of atomizing the average dispersed particle size of the pigment ink has been made for a long time (for example, see Non-Patent Documents 1, 2, and 3), and is a very effective means for improving the color developability. However, reducing the average dispersed particle size reduces the light resistance that is characteristic of pigment inks. Further, the surface energy of the particles increases due to the atomization of the colorant pigment particles, and the dispersion stability is impaired. If the colorant pigment particles themselves are atomized and an excessive dispersant is added to ensure dispersion stability, the pigment ink becomes more penetrating into the ink jet recording paper, and the colorant pigment ink jet recording paper. Since the abundance ratio in the vicinity of the surface decreases, the color developability may decrease.

  In particular, along with recent progress in inkjet technology, inkjet recording paper with gloss has been frequently used in response to the fact that inkjet recording has been used as an alternative to silver halide photography. Conventionally, cast coated paper as proposed in Patent Documents 2 and 3 has been used as an inkjet recording medium having gloss. However, although the recording medium manufactured by these technologies has a certain level of gloss, it cannot be said that it has the same texture as photographic paper used for silver salt photography, and an inkjet printer that can realize the current photographic image quality. When used in the ink, the ink absorbability and the sharpness of the image were not sufficient.

  Further, as disclosed in Patent Document 4, in order to realize high gloss, an ink jet having a three-layer structure in which an intermediate layer composed of cationic fine particles is provided on a lower layer and a gloss layer mainly composed of colloidal silica is provided thereon. Recording paper has been proposed. Although it is possible to obtain high gloss by this method, it is difficult to avoid cracks on the surface of the coating layer, and when printed with pigment ink, the color developability is lowered. Further, as in Patent Document 5, by using colloidal silica in which the elongation percentage of the binder of the ink receiving layer is within a specific range and spherical colloidal silica having a primary particle diameter of 10 to 100 nm is aggregated in a tuft shape as a pigment. Ink jet recording paper in which cracks on the outermost surface of the coating layer are controlled has been proposed. However, even by this method, cracks cannot be completely eliminated, and the surface strength of the coating layer is lowered.

  In order to achieve the same texture, high gloss, ink absorbency and color development as photographic paper for silver salt photography, a glossy layer containing alumina hydrate and polyvinyl alcohol is provided as in Patent Document 6, for example. Ink jet recording paper has been proposed in which this is treated with a treatment liquid containing boric acid or borate, and then pressed and dried on a mirror drum heated in a wet state. By this method, cracks on the outermost surface of the coating layer can be reduced and high gloss can be obtained, but the thickness of the coating layer is increased to absorb the ink in a single layer, and a crosslinking agent is used. Therefore, embrittlement of the coating layer becomes remarkable, and cracking of the coating layer may easily occur when stress such as bending is applied. In addition, the surface of the coating layer formed of alumina hydrate tends to diffusely reflect light due to rubbing, and some scratches are very conspicuous and the texture is impaired.

Further, as in Patent Document 7, a porous material containing boehmite and polyvinyl alcohol on the treated surface of a substrate surface-treated with one or more selected from the group consisting of boric acid or borate and a paper surface treating agent An ink jet recording material having an ink absorbing layer or a coating comprising a borax aqueous solution, fine silica particles and polyvinyl alcohol on a first coating layer comprising a pigment, polyvinyl alcohol, and a cationic compound, as in the example of Patent Document 8. Inkjet recording paper has been proposed in which a working solution is applied wet-on-wet, dried, and further provided with a third coating layer made of colloidal silica and polyvinyl alcohol. However, when the aqueous solution of the crosslinking agent of the hydrophilic binder is applied to the coating layer as described above, the coating unevenness caused by the coating layer state and the coating method to be applied causes uneven crosslinking, and a uniform surface without cracks. It was difficult to form, and it was very difficult to provide a paper satisfying the same texture as silver salt photographic printing paper, scratch resistance of the coating layer surface, and suitability for pigment ink.
JP 2003-55951 A (page 4) JP-A-63-264391 Japanese Patent Laid-Open No. 2-27487 JP 2004-195781 A JP 2005-1334 A JP 2002-293004 A JP 11-291621 A JP 2004-167959 A (page 12) Color materials lecture material "Current status of ink-jet color materials and recent research trends", Color Material Association, October 2000, pages 23-34 IS &T's NIP13 、 D. E. Burner, 1997, 667-669. Coloring material, 1998, Vol 7, [7], pp. 458-464.

  An object of the present invention is to provide an ink jet recording medium which has been difficult to realize in the past, has particularly high gloss, sufficient ink absorbability, suitability for pigment ink, and good surface strength of a coating layer.

As a result of studying this problem, after coating at least two coating layers of an upper layer and a lower layer on the inkjet recording medium of the present invention, that is, a gas-permeable support, the coating layer was applied with a wetting liquid. In an ink jet recording medium manufactured by a casting method in which the substrate is rewetted and pressure dried on a mirror drum heated in a wet state, the lower layer contains a water-absorbing inorganic pigment, latex, boric acid or a salt thereof , and colloidal silica . A coating composition is formed, and the upper layer is formed from a coating composition containing a submicron pigment and polyvinyl alcohol, and the wetting liquid contains at least colloidal silica having an average primary particle size of 40 nm or less. Texture of photographic paper for silver salt photography with high gloss, sufficient ink absorbency, good pigment ink, and good surface strength of the coating layer It has become possible to provide a near ink jet recording medium.

  In particular, since the sub-micron pigment of the upper layer coating composition is alumina hydrate, it has become possible to obtain particularly high gloss.

  Further, by using colloidal silica whose ionicity of colloidal silica contained in the wetting liquid is anionic, it has become possible to obtain an ink jet recording medium having high gloss and excellent surface strength of the coating layer.

  In addition, by using wet synthetic silica as the water-absorbing inorganic pigment of the lower coating composition, it is possible to obtain an ink jet recording medium having excellent ink absorbability.

  According to the present invention, there can be obtained an ink jet recording medium having a particularly high gloss, sufficient ink absorptivity and suitability for pigment ink, which has not been obtained conventionally, and having a good surface strength of the coating layer.

  Hereinafter, the ink jet recording material of the present invention will be described in detail.

  In order to reproduce the texture of photographic paper for silver salt photography, the present inventor has intensively studied to obtain an ink jet recording medium having high gloss, sufficient ink absorbability, suitability for pigment ink, and high coating layer surface strength. As a result, after coating at least two coating layers, an upper layer and a lower layer, on a gas-permeable support, the coating layer was rewetted with a wetting liquid and pressure-dried to a mirror drum heated in a wet state. In the inkjet recording medium produced by the caulking cast method, the lower layer is formed of a coating composition containing a water-absorbing inorganic pigment, latex, boric acid or a salt thereof, so that the polyvinyl alcohol crosslinking agent contained in the upper layer is formed. Boric acid and its salts can be uniformly distributed in the coating layer, and it is possible to form a uniform, non-cracking upper layer, and it does not contain a water-soluble synthetic polymer such as polyvinyl alcohol. Further, the stability of the lower layer coating composition is very high, and there is no occurrence of coating defects. Furthermore, by forming a lower layer having high surface smoothness and thermoplasticity, it is possible to form an extremely smooth and highly glossy upper layer. In addition, the upper layer is formed from a coating composition containing a submicron pigment and polyvinyl alcohol, thereby forming a coating layer having no color cracking and excellent ink absorbability on the surface of the coating layer. Is possible. Further, the wetting liquid for re-wetting the coating layer contains at least colloidal silica having an average primary particle diameter of 40 nm or less, thereby improving the coating layer strength on the outermost surface of the coating layer, particularly scratch resistance, and high. It has been found that gloss and ink absorbability can be achieved.

  In the present invention, paper is usually used as the air-permeable support, but a support other than paper, such as a nonwoven fabric, can be used as necessary.

  In the present invention, the lower layer refers to a coating layer formed on the side closer to the support of the recording medium, and in the present invention, the upper layer refers to a coating layer coated and formed on the lower layer. These coating compositions are usually used in the form of an aqueous liquid in which each material is dissolved or dispersed in water. In the present invention, the lower layer and the upper layer are usually adjacent to each other, but one or two or more intermediate layers may be provided between the lower layer and the upper layer, as long as the interaction between the lower layer and the upper layer is not strongly inhibited. One or two or more coating layers may be provided between the lower layer and the support and on the upper surface side.

  In the present invention, the cast treatment refers to pressing a coated layer in a wet state onto a heated mirror surface, transferring the mirror surface shape to the surface of the coating layer, and removing moisture from the coated layer to dry the recording medium. Say the process to give. The apparatus used for the casting process (casting apparatus) is usually an apparatus having a structure in which the coated paper is continuously crimped to the surface of the cylinder whose surface is chrome-plated using an elastic roll. An apparatus having another structure having the same action may be used. The so-called rewetting method, in which the coating method used in producing the ink jet recording medium of the present invention is a so-called rewetting method in which the coating composition is once dried after being applied and then re-applied with moisture, and then pressure-bonded to the mirror surface of the casting apparatus. Examples of the method such as a so-called direct method in which the coating composition or the coating composition of the upper and lower layers is applied to the mirror surface of the casting apparatus without being dried can be exemplified, but are not limited thereto. is not.

  In the present invention, the water-absorbing inorganic pigment means that water is used in place of linseed oil, and when a test according to the test method for oil absorption defined in JIS K 5101 is performed, 100 ml or more of water per 100 g is reached by the end point. Examples of inorganic pigments used include heavy calcium carbonate, light calcium carbonate, wet synthetic silica, diatomaceous earth, calcium silicate, talc, magnesium hydroxide, halloysite, activated clay, acid clay, hydrotalc. Site, alumina hydrate, aluminum hydroxide, bentonite clay, zeolite, kaolin, calcined kaolin, gypsum, titanium oxide, barium sulfate and the like. One of these pigments may be used alone, or two or more thereof may be used in combination.

  Among such water-absorbing inorganic pigments, wet synthetic silica is particularly preferably used because an inkjet recording medium having good ink absorbability can be obtained even with a small coating amount. In the present invention, wet synthetic silica refers to porous silicon dioxide obtained by mixing a silicate such as sodium silicate and an acid such as sulfuric acid, followed by steps such as washing, aging and pulverization. As the production method, a precipitation method and a gel method are known, and any of them can be preferably used for the lower layer coating composition of the present invention.

  The oil absorption amount of the wet synthetic silica used in the lower layer coating composition of the present invention is preferably higher from the viewpoint of ink absorbency. On the other hand, if the oil absorption amount is too high, the strength of the coating layer may decrease. Absent. Specifically, the oil absorption of the wet synthetic silica used for the lower layer coating composition of the present invention is preferably 1 to 5 ml / g, more preferably 2 to 4 ml / g. Moreover, since the particle size of the wet synthetic silica used for the lower layer coating composition of the present invention is too small, the viscosity of the coating composition becomes high and the coating operation may be difficult. Since the gloss of the recording medium after the casting process may be difficult to express, it is not preferable. Specifically, the particle size of the wet synthetic silica used in the lower layer coating composition of the present invention measured by the Marvin method is preferably 2 to 20 μm, more preferably 3 to 15 μm.

The amount of boric acid or its salt added to the lower coating composition of the present invention depends on the pH of the coating composition, but if it is too small, cracks will occur in the upper layer after drying and the gloss will decrease. If the amount is too large, the strength of the coating layer may be reduced. Therefore, the water-absorbing inorganic pigment is converted to H ₃ BO _{3 based} on the boron atom, and the content is 0.2 to 10% by mass. It is preferable that the content be 0.4 to 5% by mass.

In the present invention, it is preferable that the lower layer contains colloidal silica because an ink jet recording medium having excellent ink absorbability can be obtained. In the present invention, colloidal silica is a method in which an acid such as sulfuric acid is allowed to act on an aqueous silicate solution such as sodium silicate, or a metal ion is removed by a cation exchange resin and then a process such as aging is performed. It is an aqueous dispersion of a silicate polymer to be produced, and is usually composed of spherical silicate particles having a specific surface area of 15 to 700 m ² / g by BET method.

As the colloidal silica used in the lower layer of the present invention, when the specific surface area is too small, the gloss after casting may be difficult to express, and when the specific surface area is too large, the ink absorption Since the effect of improving the properties may not be sufficient, those having a specific surface area by the BET method of 50 to 500 m ² / g are preferably used, and those having a specific surface area of 100 to 350 m ² / g are more preferably used. In addition, if the amount of colloidal silica added in the present invention is too small, the effect of improving ink absorbency may not be sufficient, and if it is too large, gloss after casting may be difficult to express. It is preferable to set it as 10-100 mass% with respect to the addition amount of, and it is more preferable to set it as 15-60 mass%.

  In the present invention, latex refers to a liquid material in which a substantially water-insoluble thermoplastic polymer compound is dispersed in water, and once dried, the latex is substantially insoluble in hot water. Is distinguished from the aqueous solution. Examples include vinyl acetate polymer, ethylene-vinyl acetate copolymer, (meth) acrylic acid ester polymer, styrene- (meth) acrylic acid ester copolymer, styrene-butadiene copolymer, acrylonitrile-butadiene. Copolymers, isoprene copolymers, chloroprene copolymers, urethane-based polymers, copolymers in which two or more of the monomers constituting these polymers are combined in a random, graft or block manner, etc. And an aqueous dispersion of the synthetic polymer compound, natural rubber latex, and the like. One of these latexes may be used alone, or two or more thereof may be used in combination.

  In the present invention, the amount of latex added to the lower layer coating composition is too small, the resulting recording medium may not have sufficient mechanical strength, if too large, the ink absorbency may be insufficient, Since the gloss after the cast treatment may not be sufficiently obtained, the mass ratio of latex / (water-absorbing inorganic pigment + colloidal silica) is preferably 10/100 to 60/100, and 15/100 to 40 / More preferably, it is 100.

  A water-soluble polymer compound such as a vinylpyrrolidone polymer, a (meth) acrylic acid polymer, or a (meth) acrylamide polymer is added to the lower layer coating composition of the present invention to the extent that the effects of the present invention are not impaired. You can also. However, it is preferable that the water-soluble polymer compound having high crystallinity such as polyvinyl alcohol is not substantially contained since the gloss of the ink jet recording medium obtained after the casting treatment is remarkably lowered.

  In the present invention, the submicron pigment means that when the dispersion is dispersed on a substrate and observed with a scanning electron microscope, 80% or more of the area occupied by the particles in the observation field is composed of particles having a long side of 1 μm or less. It refers to the occupied inorganic pigment. The type of submicron pigment used in the upper layer coating composition of the present invention is not particularly limited, but examples include gel method silica, precipitation method silica, colloidal silica, gas phase method silica, gas phase method alumina, and pseudoboehmite. it can. In particular, it is preferable to use an inorganic pigment in which particles having a long side of 400 nm or less occupy 80% or more of the area occupied by the particles in the observation field because particularly high surface gloss can be obtained. Further, when an inorganic pigment occupying 80% or more of the area occupied by the particles in the observation field and particles having a long side of 100 nm or more is used, the coating layer is torn even if the upper coating composition is dried at high speed after coating. It is preferable because defects such as

As the submicron pigment used in the upper layer coating composition of the present invention, a pigment having a specific surface area of a certain level or more is preferable because a high printing density can be obtained. On the other hand, if the specific surface area is too large, the ink absorptivity may be lowered, which is not preferable. Specifically, it is preferable to use one having a specific surface area of 60 to 600 m ² / g by the BET method, and more preferably 150 to 400 m ² / g. In addition, the pigment having such a specific surface area is usually a pigment formed by forming a higher order structure in which primary particles having a diameter of several tens of nm or less are combined and voids are formed therein.

In the present invention, it is preferable to use alumina hydrate as the submicron pigment of the upper layer coating composition because an inkjet recording medium having particularly high gloss can be obtained. The alumina hydrate used in the present invention can be represented by the general formula Al ₂ O ₃ .nH ₂ O. Alumina hydrates are classified into dipsite, vialite, norstrandite, boehmite, boehmite gel (pseudoboehmite), diaspore, amorphous amorphous, etc., depending on the composition and crystal form. In particular, in the above formula, when the value of n is 1, it represents an alumina hydrate having a boehmite structure, and when n is more than 1 and less than 3, it represents an alumina hydrate having a pseudo boehmite structure, where n is 3 or more represents an amorphous alumina hydrate. In particular, the preferred alumina hydrate for the present invention is an alumina hydrate having a pseudo boehmite structure in which at least n is more than 1 and less than 3.

  In order to stabilize the dispersion of alumina hydrate, various acids are usually added to the dispersion. Examples of such acids include nitric acid, hydrochloric acid, hydrobromic acid, acetic acid, formic acid, ferric chloride, aluminum chloride and the like, but the present invention is not limited thereto.

  The alumina hydrate used in the present invention can be produced by a known method such as hydrolysis of aluminum alkoxide such as aluminum isopropoxide, neutralization of aluminum salt with alkali, hydrolysis of aluminate. The physical properties of alumina hydrate such as particle size, pore size, pore volume, and specific surface area should be controlled by conditions such as precipitation temperature, aging temperature, aging time, solution pH, solution concentration, and coexisting compounds. Can do.

  As a method for obtaining alumina hydrate from alkoxide, JP-A-57-88074, JP-A-62-56321, JP-A-4-275717, JP-A-6-64918, JP-A-7-10535, JP-A-7-267633, US Pat. No. 2,656,321 and the like disclose a method for hydrolyzing aluminum alkoxide. These aluminum alkoxides include isopropoxide, 2-butoxide and the like.

  The alumina hydrate used in the present invention is preferably an alumina hydrate having an average primary particle size of 3 nm to 40 nm. A particularly preferable average primary particle diameter is 5 nm to 30 nm.

  As the polyvinyl alcohol used in the upper layer coating composition of the present invention, polyvinyl alcohol having various saponification degrees from 70 mol% to 100 mol% can be used, but when the rewetting method is used as the method of the casting process used in the present invention. It is preferable to use one having a saponification degree of 95 mol% or less because gloss is easily developed. Also, modified polyvinyl alcohol in which various functional groups such as silyl group, carboxyl group, amino group, and acetoacetyl group are introduced, and other monomers such as ethylene are introduced randomly, grafted, or in blocks are also used. Can do. If the amount of polyvinyl alcohol added in the present invention is too small, cracking may occur during drying, or the surface strength of the formed inkjet recording medium may be insufficient. On the other hand, the amount of polyvinyl alcohol added is large. If the amount is too large, the ink absorbability may decrease, which is not preferable. Specifically, the addition amount of polyvinyl alcohol is preferably 2 to 40% by mass, more preferably 5 to 25% by mass of the submicron pigment.

  Since the polyvinyl alcohol of the upper coating composition of the present invention has a higher viscosity of the aqueous solution, cracking is less likely to occur when the coated coating composition is dried even if the addition amount is relatively small. The amount added can be reduced, and as a result, the ink absorbability can be improved, which is preferable. However, if the viscosity of the aqueous solution is too high, the viscosity of the coating composition obtained by the present invention becomes too high, which may make the coating operation difficult. Specifically, the viscosity of an aqueous solution having a solid content concentration of 4% by mass measured at 25 ° C. using an Ubbelohde viscometer based on JIS Z8803 is preferably 15 to 400 mPa · sec, and 30 to 200 mPa · sec. More preferably. One of these polyvinyl alcohols may be used alone, or two or more thereof having different saponification degree, viscosity, modification and the like may be used in combination.

  In the present invention, the wetting liquid means that the coating layer coated on the support is rewet, and the coating layer is pressure-dried to a heated mirror drum in a wet state so that the surface of the mirror drum is coated. It has the function of a so-called rewetting liquid that can be efficiently transferred to the coating layer, and a further surface modification effect due to the components of the wetting liquid. In the present invention, the wetting liquid contains at least colloidal silica having an average primary particle diameter of 40 nm or less. By incorporating this in the wetting liquid, it becomes possible to efficiently distribute the colloidal silica in the vicinity of the upper layer surface, and as a result, it is possible to obtain high gloss and ink absorbency, and surface strength of the coating layer. It was.

  The colloidal silica contained in the wetting liquid of the present invention has an average primary particle size of 40 nm or less, preferably 5 to 35 nm, and more preferably 15 to 30 nm. The particle size is necessary to obtain high gloss, ink absorbency, and coating layer strength on the outermost surface of the coating layer. When the average primary particle size exceeds 40 nm, the gloss of the coating layer decreases. In addition, scratches on the surface of the coating layer at the time of rubbing are also noticeable. In addition, the average primary particle diameter of the colloidal silica is measured using a dynamic scattering method.

  The ionicity of the colloidal silica used in the wetting liquid of the present invention is not particularly limited, and is usually treated with a general anionic colloidal silica, a cationic resin such as a quaternary ammonium salt, an aluminum compound, or the like. Cationic colloidal silica can be used. In particular, anionic colloidal silica reacts strongly with an ionic cationic cationic upper layer coating composition without a water-soluble binder such as polyvinyl alcohol, and has a surface coating strength that does not impair ink absorbability. It is preferable because it increases greatly.

  In the present invention, it is preferable that the wetting liquid contains a surfactant together with colloidal silica. By adding a surfactant to the wetting liquid, adhesion of the wetting liquid to the coating layer becomes uniform, and the uniformity of the finally formed surface is significantly improved. The surfactant used here is not particularly limited as long as there is no problem with miscibility with colloidal silica, and is usually a general surfactant, for example, a dialkylsulfosuccinate such as sodium bis (2-ethylhexyl) sulfosuccinate. Anionic surfactants such as ammonium salts of polyoxyethylene alkyl ether sulfates, sodium dodecyl benzene sulfonate and sodium laurate; cationic surfactants such as fatty acid amine salts, quaternary ammonium salts, sulfonium salts and phosphonium; Non-ionic such as polyoxyethylene alkyl ether, polyoxyethylene alkyl ester, polyoxyethylene sorbitan fatty acid ester, polyoxyethylene alkyl phenyl ether, polyoxyethylene alkylamine, acetylene glycol Such as sexual surfactants can be used. In particular, acetylene glycol-based surfactants are preferable because the wetting liquid can be uniformly attached to the surface of the coating layer without foaming and can impart an appropriate swelling effect.

  The surfactant adjusts the wettability between the upper layer surface and the wetting liquid, has an effect of uniformly wetting with the wetting liquid, and an effect of promoting re-swelling of the coating layer to be rewet. The concentration of the surfactant in the wetting liquid is not particularly limited. However, if the amount is too small, the surface of the coating layer cannot be uniformly wetted. If the amount is too large, the penetration of the wetting liquid into the coating layer becomes remarkable. In order to cause a decrease in coating layer strength and poor gloss, the amount is preferably in the range of 0.005% by mass to 0.05% by mass.

In the present invention, it is desirable that the wetting liquid is provided in an amount necessary and sufficient for re-swelling the coating layer. The amount of the wetting liquid depends on the cast treatment equipment, the time from wetting to the cast treatment, the coating layer for re-swelling, etc. However, it is preferably about 40 to 120 g / m ² in terms of the amount of adhesion immediately after wetting. Here, when the adhesion amount of the wetting liquid is less than 40 g / m ² , the coating layer is not sufficiently re-swelled, and the gloss may be deteriorated or the gloss may be partially lost. . Moreover, when the adhesion amount exceeds 120 g / m ² and is excessive, irregularities appear on the surface of the coating layer after the casting treatment, and the surface quality may be deteriorated.

The dry adhesion amount of the colloidal silica adhered on the coating layer by the wetting liquid in the present invention is not particularly limited as long as it does not deteriorate various properties as an ink jet recording medium. the characteristics, preferably 0.3 to 5 g / m ^2, more preferably 1~ 3 g / m ^2. Further, the concentration of colloidal silica in the wetting liquid in the present invention is not particularly limited, but the above-mentioned colloidal silica dry adhesion amount necessary for maintaining good characteristics as an inkjet recording medium and the coating layer From the amount of wet liquid adhering required for re-swelling, 0.5 mass%-5 mass% are preferable, and 1-3 mass% is still more preferable. Here, when the concentration of colloidal silica is too low, that is, when the dry adhesion amount is small, particularly the effect of improving the surface strength may be low, and when the concentration of colloidal silica is high, that is, when the dry adhesion amount is large, The color developability may decrease, and the glossiness of the surface may become glaring.

  A water-soluble binder, a thermoplastic resin and an emulsion, an ink fixing agent, and the like can be added to the wetting liquid in the present invention as long as the characteristics of the final inkjet recording medium are not deteriorated.

  For each coating layer of the present invention, various additives such as a surfactant, an antifoaming agent, a thickening agent, a color adjusting agent, a fluorescent brightening agent, an antioxidant, and an ultraviolet absorber are added as necessary. It can also be added.

  In the present invention, there is no particular limitation on the method of applying the coating composition of each coating layer or the method of adhering the wetting liquid to the coating layer. Air knife coater, blade coater, rod blade coater, bar coater, reverse Various coating methods such as a roll coater, a comma coater, a gate roll coater, a film transfer coater, a lip coater, a die coater, and a curtain coater can be used. Among these, the curtain coater has a high uniformity of the obtained coating layer, and furthermore, when the upper layer is applied, the coating composition hardly penetrates into the lower layer and gives a very good surface quality. It is preferably used for coating a work layer, particularly an upper layer coating composition.

The amount of the coating composition of the present invention to be coated on the support is not particularly limited, but in order to achieve both ink absorbency and economy, each layer is usually 5 to 20 g / m ² as a solid content. It is preferable to apply 10 to 30 g / m ² in the total of both layers. However, particularly when it is required to absorb a large amount of ink, it may be preferable to apply a maximum of about 50 g / m ^{2 in} total for both layers.

  It is also possible to perform cast treatment after increasing the smoothness with a calender device such as a soft calender or super calender after or after applying and drying the lower layer coating composition or the upper layer coating composition. is there.

  In addition, a release agent such as an oily substance or an aqueous dispersion of an oily substance is added to the coating composition or the wetting liquid of the present invention for the purpose of improving the peelability from the mirror surface in the casting apparatus, or a casting treatment. It is preferable that the coating is performed prior to the coating because stable production is possible over a long period of time. Examples of substances that can be used as a release agent in the present invention include higher alkylamines such as dimethyloctylamine and dimethyloctadecylamine or salts thereof, higher alkyl quaternary ammoniums such as trimethyloctylammonium chloride and trimethyloctadecylammonium chloride. Examples thereof include salts, higher carboxylic acids such as oleic acid, stearic acid and caprylic acid or salts thereof, higher alcohols such as octyl alcohol and octadecyl alcohol, liquid paraffin, polyethylene wax and silicone oil.

  Examples of the present invention will be described below. In the examples, unless otherwise specified, parts are parts by mass, and% is mass%.

[ Reference Example 1]
[Preparation of lower layer coating composition 1]
6 parts of sodium tetraborate decahydrate (0.97 parts in terms of H ₃ BO ₃ ) was dissolved in 350 parts of water, and 100 parts of wet synthetic silica (Fine Seal X-37B manufactured by Tokuyama Corporation) was added thereto. It was fully dispersed using a disperser having a post-saw blade. Subsequently, 83.3 parts (solid content 40 parts) of styrene-butadiene (SBR) latex (JSR Co., Ltd. 0623N) having a solid content of 48% was added and mixed to obtain a lower coating composition 1. In addition, about the wet synthetic silica used here, water was used instead of linseed oil, and when a test according to the oil absorption test defined in JIS K 5101 was performed, 300 ml of water per 100 g was obtained by the end point. Used.

[Preparation of Alumina Hydrate Sol]
1 part of acetic acid is mixed with 299 parts of water, 100 parts of alumina hydrate having a pseudo boehmite structure (Dispersal HP14 manufactured by Sasol, average primary particle diameter: 14 nm) is added, and the mixture is stirred as it is for 2 hours to peptize, An alumina hydrate sol having a solid concentration of 25% was obtained. When this dispersion was diluted 100 times with water, dispersed on a glass substrate and observed with a scanning electron microscope, 80% or more of the area occupied by particles in the observation field was occupied by particles having a long side of 100 to 400 nm. It was.

[Preparation of upper layer coating composition 1]
To 100 parts (solid content 25 parts) of the alumina hydrate sol obtained above, a 10% aqueous solution of polyvinyl alcohol (PVA235 manufactured by Kuraray Co., Ltd.) having a saponification degree of 88 mol% and a 4% aqueous solution having a viscosity of 95 mPa · s at 25 ° C. 70 parts (7 parts of solid content) were added, and the upper-layer coating composition 1 was prepared.

[Preparation of Wetting Solution 1]
Colloidal silica (Snowtex 20; average primary particle diameter 20 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) 5 parts (solid part 1 part) is added to 94.975 parts of water, and then solids 100 % Wetting Liquid 1 was obtained by adding and mixing 0.025 part of a surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.).

[Preparation of inkjet recording medium]
The lower layer coating composition 1 is coated on a base paper having a basis weight of 157 g / m ² (Diafoam manufactured by Mitsubishi Paper Industries Co., Ltd.) using an air knife coater so that the coating amount after drying is 10 g / m ^2. Then, it was dried using a hot air dryer. Next, after the obtained coated paper was treated using a soft calender, the upper layer coating composition 1 was coated using an air knife coater so that the mass after drying was 10 g / m ^2, and was heated and dried. It was dried using a machine. Furthermore, the obtained coated paper was processed using a soft calendar. The coated surface of this coated paper was wetted by contacting with the wetting liquid 1 for 5 seconds, and then pressure-bonded to a mirror surface chrome plating cylinder of a casting apparatus heated to a temperature of 95 ° C. at a linear pressure of 20 kN / m and a speed of 25 m / min. The ink jet recording medium of Reference Example 1 was prepared by peeling from the cylinder after drying. At this time, the adhesion amount of the wetting liquid before drying was 62 g / m ² , and the dry adhesion amount of colloidal silica was 0.6 g / m ² .

[ Reference Example 2]
An inkjet recording medium of Reference Example 2 was obtained in the same manner as Reference Example 1, except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 2. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of Wetting Solution 2]
10 parts (2 parts solids) of colloidal silica (Snowtex 20; average primary particle size 20 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) having a solid content of 20% are added to 89.975 parts of water, and then solids of 100 % Surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.) was added and mixed to obtain a wetting liquid 2.

[ Reference Example 3]
An inkjet recording medium of Reference Example 3 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 3. The dry adhesion amount of colloidal silica at this time was 2.8 g / m ² .
[Preparation of Wetting Solution 3]
Colloidal silica with 20% solid content (Snowtex 20; average primary particle size 20 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) 25 parts (solid content 5 parts) is added to 74.975 parts of water, and then solid content 100 % Surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.) was added and mixed to obtain a wetting liquid 3.

[ Reference Example 4]
An inkjet recording medium of Reference Example 4 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 4. The dry adhesion amount of colloidal silica at this time was 0.2 g / m ² .
[Preparation of Wetting Solution 4]
To 98.475 parts of water, 1.5 parts of solid colloidal silica (Snowtex 20; average primary particle size 20 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) 1.5 parts (solid part 0.3 parts) is added. A wetting liquid 4 was obtained by adding and mixing 0.025 part of a 100% solid surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.).

[Example 1 ]
An ink jet recording medium of Example 1 was obtained in the same manner as Reference Example 1 except that the lower layer coating composition 1 of Reference Example 1 was changed to the following lower layer coating composition 2. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of lower layer coating composition 2]
6 parts of sodium tetraborate decahydrate (0.97 parts in terms of H ₃ BO ₃ ) are dissolved in 350 parts of water, and colloidal silica having a BET specific surface area of 230 m ² / g and a solid content of 40% (Nissan Chemical Industries) 25 parts (Snow part 10 parts) of Snowtex 40) were added. Further, after adding 100 parts of wet synthetic silica (Fine Seal X-37B manufactured by Tokuyama Corporation), the mixture was sufficiently dispersed using a disperser having a saw blade. Subsequently, 83.3 parts (solid content 40 parts) of SBR latex (JSR company 0623N) with a solid content of 48% were added and mixed to obtain a lower coating composition 2.

[ Reference Example 5 ]
An inkjet recording medium of Reference Example 5 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 5. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of Wetting Solution 5]
To 89.975 parts of water, 10 parts of colloidal silica (Snowtex XS; average primary particle size 5 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) 10 parts (2 parts solids) is added, and then solids 100 % Surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.) was added and mixed to obtain a wetting liquid 5.

[ Reference Example 6 ]
An inkjet recording medium of Reference Example 6 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 6. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of Wetting Solution 6]
To 95.975 parts of water, 4 parts of colloidal silica (Snowtex 50; average primary particle size 30 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) 50% solids (2 parts solids) is added, followed by 100 parts solids. % Wetting Liquid 6 was obtained by adding and mixing 0.025 part of a surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.).

[ Reference Example 7 ]
An inkjet recording medium of Reference Example 7 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 7. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of Wetting Solution 7]
Colloidal silica having a solid content of 40% (Snowtex XL; average primary particle size 40 nm, anionic, manufactured by Nissan Chemical Industries, Ltd.) 5 parts (solid content 2 parts) is added to 94.975 parts of water, and then solid content 100 % Surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.) was added and mixed to obtain a wetting liquid 7.

[ Reference Example 8 ]
An inkjet recording medium of Reference Example 8 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 8. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of Wetting Solution 8]
To 89.975 parts of water, 10 parts of colloidal silica (Snowtex AK; average primary particle size 20 nm, cationic, manufactured by Nissan Chemical Industries, Ltd.) 10 parts (2 parts of solids) is added, followed by 100 parts of solids. % Surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.) was added and mixed to obtain a wetting liquid 8.

[ Reference Example 9 ]
Change the upper coating composition 1 of Example 1 below upper coating composition 2 using the following fumed silica dispersion, except that the coating amount of the upper layer and 8 g / m ² and Reference Example 1 Similarly, an ink jet recording medium of Reference Example 9 was obtained. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .
[Preparation of vapor phase silica dispersion]
While stirring 392 parts of water, 8 parts of a 50% aqueous solution of a dimethyldiallylammonium chloride polymer having a viscosity of 400 mPa · s (solid content 4 parts), gas phase method silica (specific surface area 90 m ² / g by BET method) 100 Part was added and predispersed using a blade type disperser. The obtained preliminary dispersion was treated with a colloid mill to obtain a vapor phase silica dispersion having a solid concentration of 20.8%. When this dispersion was diluted 100 times with water, dispersed on a glass substrate and observed with a scanning electron microscope, 80% or more of the area occupied by the particles in the observation field was occupied by particles having a long side of 100 to 400 nm. It was.

[Preparation of upper layer coating composition 2]
To 100 parts (solid content 20.8 parts) of the vapor phase silica dispersion obtained above, a saponification degree of 88 mol%, a 4% aqueous solution of polyvinyl alcohol having a viscosity of 95 mPa · s at 25 ° C. (PVA235 manufactured by Kuraray Co., Ltd.) 100 parts of a 10% aqueous solution (solid content 10 parts) was added to prepare an upper layer coating composition.

[ Reference Example 10 ]
In Reference Example 1, the upper layer coating composition 1 was applied on the lower layer, dried at room temperature for 15 seconds, then contacted with the wetting liquid 1 for 5 seconds to be wetted, and heated to a temperature of 95 ° C. for mirror chrome plating. The ink jet recording medium of Reference Example 10 was produced by pressure-bonding to a cylinder at a linear pressure of 20 kN / m and a speed of 20 m / min, and peeling from the cylinder after drying. At this time, the adhesion amount of the wetting liquid before drying was 57 g / m ² , and the dry adhesion amount of colloidal silica was 0.9 g / m ² .

[ Reference Example 11 ]
In Reference Example 1, immediately after applying the upper layer coating composition 1 on the lower layer, after being pressure-bonded and dried on a mirror surface chrome plating cylinder of a casting apparatus heated to a temperature of 95 ° C. at a linear pressure of 20 kN / m and a speed of 20 m / min. Then, it is wetted by contacting with the wetting liquid 1 for 5 seconds, and is again pressure-bonded to the mirror-plated chrome plating cylinder of the casting apparatus heated to a temperature of 95 ° C. at a linear pressure of 20 kN / m and a speed of 25 m / min. Thus, an ink jet recording medium of Reference Example 11 was produced. At this time, the adhesion amount of the wetting liquid before drying was 70 g / m ² , and the dry adhesion amount of colloidal silica was 1.3 g / m ² .

[Comparative Example 1]
An inkjet recording medium of Comparative Example 1 was obtained in the same manner as in Reference Example 1 except that the wet treatment was performed with a wetting liquid not containing colloidal silica in Reference Example 1.

[Comparative Example 2]
An inkjet recording medium of Comparative Example 2 was obtained in the same manner as Reference Example 1 except that the wetting liquid 1 of Reference Example 1 was changed to the following wetting liquid 9.
[Preparation of Wetting Solution 9]
Colloidal silica having a solid content of 40% (Snowtex YL; average primary particle size 75 nm, cationic, manufactured by Nissan Chemical Industries, Ltd.) 5 parts (solid content 2 parts) is added to 94.975 parts of water, and then solid content 100 % Surfactant (Olfin E1010; manufactured by Nissin Chemical Industry Co., Ltd.) was added and mixed to obtain a wetting liquid 9. The dry adhesion amount of colloidal silica at this time was 1.1 g / m ² .

[Comparative Example 3]
On the inkjet recording medium of Comparative Example 1, 60 g / m ^{2 of} wet liquid 1 was applied in a coating amount before drying, and dried with a hot air drier to obtain an inkjet recording medium of Comparative Example 3.

[Comparative Example 4]
An inkjet recording medium of Comparative Example 4 was obtained in the same manner as Reference Example 1 except that the lower layer coating composition 1 of Reference Example 1 was changed to the lower layer coating composition 3 shown below.
[Preparation of lower layer coating composition 3]
After adding 100 parts of wet synthetic silica (Fine Seal X-37B manufactured by Tokuyama Co., Ltd.) to 350 parts of water, it was sufficiently dispersed using a disperser having a saw blade. Next, 83.3 parts (solid content 40 parts) of a styrene-butadiene (SBR) latex having a solid content of 48% (JSR Corporation 0623N) was added and mixed to obtain a lower coating composition 3.

[Comparative Example 5]
[Preparation of lower layer coating composition 4]
2 parts of sodium hydroxide was dissolved in 350 parts of water, 100 parts of wet synthetic silica (Fukuseal X-37B manufactured by Tokuyama Co., Ltd.) was added thereto, and then sufficiently dispersed using a disperser having a saw blade. Next, 200 parts of a 12% aqueous solution (24 parts solids) of a silyl-modified polyvinyl alcohol (R polymer 1130 manufactured by Kuraray Co., Ltd.) having a saponification degree of 98.5 mol% and a 4% aqueous solution with a viscosity of 25 mPa · s was added and mixed to form a lower layer coating. Composition 4 was obtained.

[Preparation of Intermediate Layer Coating Composition 1]
4 parts of sodium tetraborate decahydrate was added to 96 parts of water, and dissolved by stirring to prepare an intermediate layer coating composition 1.

[Preparation of inkjet recording medium]
^On the base paper (Diafoam made by Mitsubishi Paper Industries) having a basis weight of 157 g / m ² , the lower layer coating composition 4 is applied using an air knife coater so that the coating amount after drying becomes 10 g / m ^2. Then, it was dried using a hot air dryer. Next, the obtained coated paper was treated using a soft calender, and then the intermediate layer coating composition 1 was coated so that the coating amount after drying was 0.2 g / m ^2, and a hot air dryer And dried. Furthermore, the upper layer coating composition 1 was applied using an air knife coater so that the mass after drying was 10 g / m ^2, and dried using a hot air dryer. Furthermore, the obtained coated paper was processed using a soft calendar. The coated surface of this coated paper was wetted by contact with water for 5 seconds, and then pressure-bonded to a mirror chrome plating cylinder heated to 90 ° C. at a linear pressure of 20 kN / m at a speed of 25 m / min. The ink jet recording medium of Comparative Example 5 was produced by peeling from the cylinder.

[Gloss evaluation]
For the evaluation of gloss, the 60-degree specular gloss was measured according to JIS-Z8741. Here, a recording medium having a glossiness of 60% or more has high gloss.

[Evaluation of image clarity]
The evaluation of image clarity was performed using a image clarity measuring device manufactured by Suga Test Instruments Co., Ltd. at a measurement angle of 45 ° and an optical comb width of 2.0 mm. In the measurement by this method, in general, a recording medium giving 60 or more image clarity has a high gloss feeling, and a recording medium giving 65 or more image clarity has a very high gloss feeling.

[Evaluation of ink absorbency]
A predetermined evaluation image was printed by an inkjet printer (PIXUS 850i) manufactured by Canon Inc., and the ink absorbability of each recording medium was classified into the following four stages based on the legibility of image details. Here, D is a practically problematic level.
A: No unevenness or blurring in the print portion, very good B: Some blurring is observed in the boundary portion of different colors in the print portion, but good C: Bleeding in the boundary portion of different colors becomes remarkable, and the solid portion Absorption unevenness is slightly confirmed in D: In general, bleeding can be confirmed at the boundary of the color, and uneven absorption in the solid portion is also remarkable.

[Evaluation of suitability of pigment ink]
A black gradation pattern (100% -0% gradation) is printed by an ink-jet printer (PX-G900; using pigment ink) manufactured by Seiko Epson Co., Ltd. The standard was classified into the following four stages. Here, D is a level having a problem in practical use.
A: There is no color unevenness or poor color developability in the print portion, and it is very good. B: Although some color unevenness is seen in the print portion, there is no color developability failure, and C: Color unevenness is scattered in the print portion. D: Defect in color development is observed D: Remarkable color unevenness is visible in the print area, and color development is also poor due to ink drop.

[Evaluation of strength]
Place a commercially available high-quality paper on the glossy surface of the recording medium, and place a weight on which three rubber plates with a diameter of 5 mm are attached to the bottom of a cubic weight with a side of 5 cm and a mass of 1 kg. The paper was pulled and the degree of scratches on the glossy surface was visually judged. Here, D is a level causing a practical problem.
A; No scratches can be confirmed on the surface B; Some scratches can be visually recognized on the surface C; Thin scratches can be visually recognized on the surface D;

Table 1 shows the gloss, image clarity, ink absorbability, pigment ink suitability, and strength of the recording media obtained in Example 1, each reference example , and each comparative example.

From Table 1, the lower layer is formed from a coating composition containing a water-absorbing inorganic pigment, latex, boric acid or a salt thereof, and the upper layer is formed from a coating composition containing a submicron pigment and polyvinyl alcohol. Example 1 and Reference Examples 1 to 11 in which the working layer was treated with a wetting liquid containing at least a colloidal silica having an average primary particle size of 40 nm or less and cast-treated were high gloss, good ink absorbability, and suitability for pigment ink. It can be seen that the scratch resistance of the outermost surface of the coating layer is good. In Example 1 in which colloidal silica is contained in the lower layer coating composition, the ink absorbability is greatly enhanced.

  Comparative Example 1 in which the treatment with the wetting liquid is not performed and Comparative Example 2 in which the average primary particle diameter of the colloidal silica used in the wetting liquid is large are low in the strength of the coating layer surface or inferior in glossiness, and scratches are easily noticeable. . In Comparative Example 3 in which the upper layer was applied and the wetting liquid was applied after the casting process was performed, a uniform and flat coating layer could not be formed and the gloss was inferior, and polyvinyl alcohol was used for the lower layer coating composition. In Comparative Example 5 in which borax was applied as the intermediate layer, it was difficult to obtain high gloss because the coating layer was hard and the thermoplasticity was low. In Comparative Example 4 that does not use borax, the crack cannot be controlled, and the pigment ink penetrates into the coating layer, so that a uniform and vivid image cannot be obtained.

Claims (3)

A casting method in which at least two coating layers of an upper layer and a lower layer are coated on an air-permeable support, and then the coating layer is rewetted with a wetting liquid, and is pressed and dried on a mirror drum heated in a wet state. In the ink jet recording medium manufactured by the method, the lower layer is formed from a coating composition containing a water-absorbing inorganic pigment, latex, boric acid or a salt thereof, and the upper layer is a coating containing a submicron pigment and polyvinyl alcohol. An ink jet recording medium formed from a composition, wherein the wetting liquid contains at least colloidal silica having an average primary particle diameter of 40 nm or less, and the lower layer coating composition contains colloidal silica .   2. The ink jet recording medium according to claim 1, wherein the submicron pigment of the upper layer coating composition is an alumina hydrate.   3. The ink jet recording medium according to claim 1, wherein the ionicity of the colloidal silica contained in the wetting liquid is anionic.