JP2023082985A - Recording medium and inkjet recording method - Google Patents

Abstract

To provide a recording medium which suppresses beading at the time of high speed printing using pigment ink, and enables formation of an image excellent in glossiness uniformity between a printed part and a non-printed part.SOLUTION: A recording medium has a base material, and an ink receiving layer on the base material, wherein the ink receiving layer contains an inorganic particle which is at least one selected from the group consisting of an alumina hydrate, alumina and vapor phase process silica and has an average primary particle diameter of 50 nm or less, a water-soluble resin (A) having an average polymerization degree of 1,000 or more, and a water-soluble resin (B) having an average polymerization degree of 450 or less, a zeta potential of the surface of the ink receiving layer is negative, and a pH of a paper surface of the ink receiving layer is 7.0 or more.SELECTED DRAWING: None

Description

The present invention relates to a recording medium and an inkjet recording method using the recording medium.

The inkjet recording method is widely used as a printing method for photographs in combination with glossy paper because it can obtain high-definition images and can easily be multicolored. Ink jet inks are broadly classified into dye inks in which a coloring material is dissolved in an aqueous solution and pigment inks in which a coloring material is dispersed in an aqueous solution. Compared to dye inks, pigment inks are superior in water resistance and discoloration resistance of printed matter. Therefore, when the printed matter is to be stored for a long period of time or used as a notice that is exposed to light, the printed matter printed with pigment ink is preferable. On the other hand, when pigment ink is used for printing on glossy paper for inkjet, there is a problem that grainy unevenness called beading occurs in the printed area, which tends to cause deterioration in image quality. Since beading is caused by the slow absorption of pigment ink, beading has been conventionally suppressed by reducing printer throughput. Therefore, in the printing method using the pigment ink, there is a problem that the printing speed is significantly inferior to that in the case of using the dye ink. Another problem with pigment inks is that when printed on glossy paper, unlike dye inks, colorants tend to exist on the surface of the recording medium. was there.

Patent Document 1 describes a recording medium having two ink-receiving layers mainly composed of fine alumina hydrate and capable of high-quality recording. Patent Document 2 describes a highly glossy recording medium having an ink-receiving layer formed using a dispersion liquid in which fumed silica is dispersed in the presence of an alkaline compound.

JP-A-6-199034 JP-A-2002-144701

According to studies by the present inventors, it has been found that beading is likely to occur in the recording medium described in Patent Document 1 when high-speed printing is performed using pigment ink. In addition, although the recording medium described in Patent Document 2 is excellent in the glossiness of the white paper portion, there is a problem that the glossiness difference between the pigment ink printed portion and the non-printed portion is large.

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a recording medium and an inkjet recording method capable of suppressing beading during high-speed printing using pigment ink and capable of forming an image with excellent gloss uniformity between printed and unprinted areas. to do.

The above objects are achieved by the present invention described below. That is, the present invention provides a recording medium having a substrate and an ink-receiving layer on the substrate, wherein the ink-receiving layer is selected from the group consisting of alumina hydrate, alumina, and fumed silica. Inorganic particles having an average primary particle diameter of 50 nm or less, a water-soluble resin (A) having an average degree of polymerization of 1,000 or more, and a water-soluble resin having an average degree of polymerization of 450 or less and a resin (B), wherein the surface of the ink receiving layer has a negative zeta potential, and the ink receiving layer has a paper surface pH of 7.0 or more.

The present invention also provides an inkjet recording method in which printing is performed on the above recording medium using an ink jet recording head and an ink containing a water-soluble resin having an anionic functional group.

According to the present invention, it is possible to provide a recording medium and an inkjet recording method capable of suppressing beading during high-speed printing using a pigment ink and forming an image with excellent gloss uniformity between printed and unprinted areas. can be done.

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below with reference to preferred embodiments.
Glossy paper used as photo paper for inkjet is produced by coating a base material with fine inorganic particles. By forming an ink-receiving layer having fine pores using these fine inorganic particles, both glossiness and ink absorbability are achieved. On the other hand, pigment ink differs from dye ink in that the coloring material is present in the ink in a dispersed state. Since the dispersed particle size is sufficiently larger than the pores of the glossy paper, the liquid component of the pigment ink printed on the glossy paper is absorbed by the ink-receiving layer. will remain. In general ink-jet printing, an image is formed by printing minute droplets in multiple batches. With pigment ink, when the ink is printed in multiple batches, the next droplet is applied on top of the previously printed pigment present on the surface of the ink-receiving layer, which greatly reduces the absorbability of the ink. Therefore, there is a problem that beading is likely to occur. On the other hand, if the size of the pores in the ink-receiving layer is made sufficiently large to form an ink-receiving layer that allows the pigment to enter, there is a problem that the glossiness is greatly impaired due to the large pores. As the inorganic particles forming the ink-receiving layer, cationic inorganic particles or cationized inorganic particles are generally used. gender is used. As a result, the ink-receiving layer as a whole has cationic properties. The inventors have found that beading can be improved by changing the charge of the ink-receiving layer-forming material to anionic one and making the surface of the ink-receiving layer negative in zeta potential. This is thought to be because the anionic water-soluble resin contained in the pigment ink agglomerates at the interface between the ink-receiving layer and the ink, preventing clogging of the pores and improving the ink absorbability. .

On the other hand, in the recording medium according to Patent Document 2, particles obtained by dispersing fumed silica as inorganic particles in the presence of an alkaline compound are used. According to the studies of the present inventors, when these particles are used, the degree of beading improvement is not sufficient, and there is a problem that the glossiness of the printed portion printed with the pigment ink is low. This is because although the vapor-phase silica particles as a whole exhibit anionic properties, alkaline compounds have cationic properties, and microscopically, anionic functional groups and cationic functional groups are mixed, and slightly It is presumed that this is due to aggregation due to existing cationic functional groups. On the other hand, as a method of anionizing inorganic particles, there is a method of adjusting the pH. In general, when an ink-receiving layer is formed using inorganic particles that have only been anionized by pH adjustment, the glossiness of the ink-receiving layer is reduced. smoothness and smoothness cannot be obtained. This is because the ink-receiving layer is formed by once dispersing inorganic particles in an aqueous solution, applying a coating liquid containing the dispersion, and drying the ink-receiving layer. This is because the dispersibility in the medium is unstable.

According to the recording medium of the present invention, the ink-receiving layer is mainly composed of inorganic particles having an average primary particle diameter of 50 nm or less, and the zeta potential of the surface of the ink-receiving layer is negative. beading during high-speed printing can be improved. Further, the ink-receiving layer contains at least a water-soluble resin (A) having an average degree of polymerization of 1,000 or more and a water-soluble resin (B) having an average degree of polymerization of 450 or less. It is possible to achieve both glossiness and color development of pigment ink.

<Recording medium>
The recording medium of the present invention has a substrate and at least one ink-receiving layer on the substrate. In the present invention, the recording medium is preferably an inkjet recording medium used in an inkjet recording method. Further, the opacity of the recording medium defined by JIS-P8149:2000 is preferably 97% or more.
Each component constituting the recording medium of the present invention will be described below.

[Base material]
Examples of the base material include those composed only of base paper and those having base paper and a resin layer, that is, base paper coated with resin. In the present invention, it is preferable to use a substrate having a base paper and a resin layer. In that case, the resin layer may be provided only on one side of the base paper, but it is preferably provided on both sides.

(base paper)
The base paper is mainly made of wood pulp, and is made by adding synthetic pulp such as polypropylene or synthetic fiber such as nylon or polyester, if necessary. Wood pulp includes bleached hardwood kraft pulp (LBKP), bleached hardwood sulfite pulp (LBSP), bleached softwood kraft pulp (NBKP), bleached softwood sulfite pulp (NBSP), hardwood dissolving pulp (LDP), softwood dissolving pulp ( NDP), hardwood unbleached kraft pulp (LUKP), softwood unbleached kraft pulp (NUKP), and the like. One or more of these can be used as necessary. Among wood pulps, it is preferable to use LBKP, LBSP, NBKP, NBSP, LDP, and NDP, which have a large amount of short fiber components. As the wood pulp, a chemical pulp (sulfate pulp or sulfite pulp) containing few impurities is preferable. Pulp that has been bleached to improve its whiteness is also preferred. A sizing agent, a white pigment, a paper strength enhancer, a fluorescent whitening agent, a moisture retaining agent, a dispersing agent, a softening agent, and the like may be appropriately added to the base paper.

In the present invention, the film thickness of the base paper is preferably 50 μm or more and 300 μm or less, more preferably 100 μm or more and 250 μm or less. Incidentally, in the present invention, the film thickness of the base paper is calculated by the following method. First, a section of the recording medium is cut out with a microtome, and the section is observed with a scanning electron microscope. Then, the film thickness of the base paper is measured at 100 or more arbitrary points, and the average value is taken as the film thickness of the base paper. The film thicknesses of other layers in the present invention are also calculated by the same method.
In the present invention, the paper density defined by JIS-P8118 of the base paper is preferably 0.6 g/cm 3 or more and 1.2 g/cm ³ or less, and 0.7 g/cm ³ or more and 1.2 g/cm ³ or more. It is more preferably ³ or less. Note that the paper density is calculated from the following formula.
Paper density (g/cm ³ ) = basis weight (g/m ² )/(film thickness (mm) x 1000)

(resin layer)
In the present invention, when the base paper is coated with a resin layer containing a resin, the resin layer may be provided so as to cover at least part of the surface of the base paper. However, the coverage of the resin layer is preferably 70% or more, more preferably 90% or more, and 100%, that is, the entire surface of the base paper is covered with the resin layer. is particularly preferred. In addition, the coverage of the resin layer is represented by the following formula.
Coverage (%) = (surface area of base paper coated with resin layer/total surface area of base paper) x 100

In the present invention, the film thickness (dry coating thickness) of the resin layer is preferably 20 μm or more and 60 μm or less, more preferably 30 μm or more and 50 μm or less. When the resin layers are provided on both sides of the base paper, it is preferable that the film thicknesses of the resin layers on both sides satisfy the above ranges.

A thermoplastic resin is preferable as the resin used for the resin layer. Examples of thermoplastic resins include acrylic resins, acrylic silicone resins, polyolefin resins, styrene-butadiene copolymers, and the like. Among these, it is preferable to use a polyolefin resin. In the present invention, polyolefin resin means a polymer using olefin as a monomer. Specific examples include homopolymers and copolymers of ethylene, propylene, isobutylene, and the like. One or two or more polyolefin resins may be used as necessary. Among these, it is preferable to use polyethylene. As polyethylene, it is preferable to use low density polyethylene (LDPE) or high density polyethylene (HDPE).

In the present invention, the resin layer may contain a white pigment, a fluorescent brightening agent, ultramarine blue, or the like, in order to adjust the opacity, whiteness, and hue. Above all, from the viewpoint of improving opacity, the resin layer preferably contains a white pigment. When resin layers are provided on both sides of the base paper, at least one of the resin layers preferably contains a white pigment. Examples of white pigments include rutile-type and anatase-type titanium oxide. In the present invention, the content of the white pigment in the resin layer is preferably 1 g/m ² or more and 20 g/m ² or less. Moreover, the content of the white pigment in the resin layer is preferably 25% by mass or less with respect to the content of the resin. If the content of the white pigment in the resin layer is 25% by mass or less, the dispersion stability of the white pigment can be maintained.

[Ink receiving layer]
In the present invention, the ink-receiving layer may be a single layer, or may be a multilayer of two or more layers. Also, the ink-receiving layer may be provided on only one side of the substrate, or may be provided on both sides. The film thickness (dry coating thickness) of the ink-receiving layer on one side of the substrate is preferably 15 μm or more and 60 μm or less, more preferably 30 μm or more and 45 μm or less. When the ink-receiving layer is composed of two or more layers, the total film thickness of the plurality of ink-receiving layers is preferably within the above range. The ink-receiving layer is a solidified product of the ink-receiving layer coating liquid, and is prepared by coating the ink-receiving layer coating liquid on a substrate and drying it, as described later.

(Zeta potential)
In the present invention, the surface of the ink-receiving layer has a negative zeta potential. The zeta potential of the surface of the ink receiving layer is preferably -5 mV or less. The zeta potential of the surface of the ink-receiving layer can be measured using a plate sample cell of a zeta potential meter (trade name: ELS-Z2, manufactured by Otsuka Electronics Co., Ltd.). This is a method of indirectly determining the zeta potential of a solid surface using the Mori-Okamoto equation by utilizing the electroosmotic flow generated between a quartz cell coated with polyacrylamide and the sample. Specifically, in the measurement of the zeta potential of the surface of the ink-receiving layer according to the present invention, polystyrene latex (particle diameter: 500 nm, manufactured by Otsuka Electronics Co., Ltd.) whose surface is coated with hydroxypropylcellulose is used as monitor particles for electrophoresis. , dispersed in a 10 mM NaCl solution. Then, the zeta potential of the surface of the ink-receiving layer is indirectly measured from the behavior of the monitor particles. When the ink-receiving layer is composed of two or more layers, the surface of the ink-receiving layer means the surface of the ink-receiving layer formed on the uppermost surface.

(Paper surface pH)
In the present invention, the paper surface pH of the surface of the ink receiving layer is 7.0 or higher. The surface pH of the ink-receiving layer is preferably 8.0 or higher. The paper surface pH of the surface of the ink-receiving layer in the present invention is determined according to JAPAN TAPPI Paper Pulp Test Method No. It can be measured by the method described in 49-1. Distilled water, ion-exchanged water, and an aqueous potassium chloride solution are generally used as the wetting liquid. Distilled water or ion-exchanged water is preferably used in order to avoid the influence of ions on pH. A general method for measuring the pH of paper surface is to measure the time using a stopwatch or the like after bringing an electrode into contact with the measurement surface moistened with a wetting liquid, and read the pH value after a certain period of time. The time can be set arbitrarily because the time required for the pH value to reach equilibrium differs depending on the type of paper. In the present invention, the pH value is read 30 seconds after wetting the measuring surface of the ink-receiving layer with ion-exchanged water.

(pore radius)
In the present invention, the pore radius of the surface of the ink receiving layer is preferably 50 nm or less, more preferably 5 nm or more and 25 nm or less, and even more preferably 5 nm or more and 20 nm or less. If the pore radius is 50 nm or less, the occurrence of beading can be suppressed by efficiently absorbing the liquid component contained in the ink, and the effects of the present invention can be obtained more efficiently. . This is because the smaller the pore radius, the greater the capillary force per unit area that absorbs the ink in the receiving layer. The pore radius can be measured by the BJH (Barrett-Joyner-Halenda) method of the nitrogen adsorption method, and the pore radius in the present invention refers to the median diameter of the pore distribution determined by the BJH method.
Materials that can be contained in the ink-receiving layer are described below.

(Inorganic particles)
In the present invention, the ink-receiving layer contains at least one kind of inorganic particles selected from the group consisting of alumina hydrate, alumina, and fumed silica. Among these, it is preferable to contain fumed silica as the inorganic particles. In addition, the average primary particle size of the inorganic particles is 50 nm or less. The average primary particle size of the inorganic particles is more preferably 40 nm or less, particularly preferably 20 nm or less. Also, the average primary particle size of the inorganic particles is preferably 5 nm or more. In the present invention, the average primary particle size of the inorganic particles is the number average particle size of the diameter of a circle having an area equal to the projected area of the primary particles of the inorganic particles when observed with an electron microscope. At this time, it is preferable to measure at least 100 points or more. Also, you may refer to the catalog value of the product.

In the present invention, the inorganic particles are preferably used in the ink-receiving layer coating liquid in a state of being dispersed with a dispersant. Dispersants will be described later. The average secondary particle size of the inorganic particles in the dispersed state is preferably 10 nm or more and 500 nm or less, more preferably 50 nm or more and 300 nm or less, and particularly preferably 100 nm or more and 250 nm or less. The average secondary particle size of inorganic particles in a dispersed state can be measured by a dynamic light scattering method.

In the present invention, it is desirable to disperse the inorganic particles in a pH range having a negative charge relative to the isoelectric point. The pH range is generally pH 9.0 or higher for alumina hydrate, pH 9.0 or higher for alumina, and pH 2.5 or higher for vapor-phase silica.

In the present invention, the ink-receiving layer may contain inorganic particles other than alumina hydrate, alumina, and fumed silica. However, 50% by mass or more of the total mass of the inorganic particles contained in the ink-receiving layer is preferably inorganic particles selected from alumina hydrate, alumina, and fumed silica. Other inorganic particles used in the present invention include, for example, silica other than vapor phase silica, colloidal silica, titanium dioxide, zeolite, kaolin, talc, hydrotalcite, zinc oxide, zinc hydroxide, aluminum silicate, Calcium silicate, magnesium silicate, zirconium oxide, zirconium hydroxide and the like. One or more of these inorganic particles can be used as necessary. Moreover, these inorganic particles are preferably used as particles dispersed with a dispersant.

In the present invention, the content (% by mass) of the inorganic particles in the ink-receiving layer is preferably 50% by mass or more and 98% by mass or less, more preferably 70% by mass or more and 96% by mass, relative to the total mass of the ink-receiving layer. % or less.
In the invention, the coating amount (g/m ² , dry coating amount) of the inorganic particles applied when forming the ink-receiving layer is preferably 8 g/m ² or more and 45 g/m ² or less. By setting the coating amount of the inorganic particles within the above range, it becomes easier to adjust the thickness of the ink-receiving layer within a preferable range.

[Alumina hydrate]
As the alumina hydrate used in the ink-receiving layer, one represented by the following general formula (X) can be suitably used.
General formula (X): Al ₂ O _3-n (OH) _2n ·mH ₂ O
(In general formula (X), n is 0, 1, 2, or 3, and m is 0 or more and 10 or less, preferably 0 or more and 5 or less, provided that m and n are not 0 at the same time.)
Note that m does not have to be an integer since mH ₂ O often represents a desorbable aqueous phase that does not participate in crystal lattice formation. In addition, m can become 0 when alumina hydrate is heated.

In the present invention, alumina hydrate can be produced by a known method. Specific examples include a method of hydrolyzing aluminum alkoxide, a method of hydrolyzing sodium aluminate, and a method of neutralizing an aqueous solution of sodium aluminate by adding an aqueous solution of aluminum sulfate or aluminum chloride. As the crystal structure of alumina hydrate, amorphous, gibbsite, and boehmite types are known depending on the heat treatment temperature. The crystal structure of alumina hydrate can be analyzed by X-ray diffraction. Among these, boehmite-type alumina hydrate (boehmite) or amorphous alumina hydrate is preferred in the present invention. Specific examples include alumina hydrates described in JP-A-7-232473, JP-A-8-132731, JP-A-9-66664 and JP-A-9-76628. Commercially available products include DISPERAL HP8, HP14, and HP18 (all trade names, manufactured by SASOL). One or two or more of these alumina hydrates may be used as necessary.

In the present invention, the specific surface area of alumina hydrate determined by the BET method is preferably 100 m ² /g or more and 200 m ² /g or less, more preferably 125 m ² /g or more and 175 m ² /g or less. preferable. Here, the BET method is a method in which molecules or ions of known sizes are adsorbed on the sample surface and the specific surface area of the sample is measured from the adsorption amount. In the present invention, gaseous nitrogen molecules (N ₂ : nitrogen gas) are used as molecules to be adsorbed on the sample.

〔alumina〕
As the alumina (aluminum oxide) used for the ink-receiving layer, vapor-phase alumina synthesized by a vapor-phase method is preferable. Specific examples of vapor-phase alumina include AEROXIDE Alu C, Alu 130, and Alu 65 (all trade names, manufactured by EVONIK).

In the present invention, the specific surface area of vapor phase alumina determined by the BET method is preferably 50 m ² /g or more, more preferably 80 m ² /g or more. Moreover, the specific surface area is preferably 150 m ² /g or less, more preferably 120 m ² /g or less. The average primary particle size of the vapor phase alumina is preferably 5 nm or more, more preferably 11 nm or more, and is preferably 30 nm or less, more preferably 15 nm or less.

[Vapor phase silica]
Silica is roughly classified into a wet method and a dry method (vapor phase method) according to its production method. As a wet method, a method is known in which hydrous silica is obtained by producing active silica by acid decomposition of silicate, polymerizing it moderately, and coagulating and sedimenting it. On the other hand, as a dry method (gas phase method), there is a method by high-temperature gas phase hydrolysis of silicon halide (flame hydrolysis method), and silica sand and coke are heat-reduced and vaporized by an arc in an electric furnace. is known to obtain anhydrous silica by air oxidation (arc method). The silica used in the present invention is silica (vapor phase silica) obtained by a dry method (vapor phase method). Vapor-phase method silica has a particularly large specific surface area, so that high ink absorptivity can be obtained. In addition, since fumed silica has a low refractive index, transparency can be imparted to the ink-receiving layer, and good color developability can be obtained. Specific examples of vapor-phase silica include AEROSIL (trade name, manufactured by Nippon Aerosil Co., Ltd.), Rheolosil QS type (trade name, manufactured by Tokuyama Corporation), and the like.

In the present invention, the specific surface area of the vapor phase silica measured by the BET method is preferably 50 m ² /g or more and 400 m ² /g or less, more preferably 200 m ² /g or more and 300 m ² /g or less.

(dispersant)
In the present invention, the ink-receiving layer contains at least a water-soluble resin (B) having an average degree of polymerization of 450 or less as a dispersant. The inorganic particles are preferably used in the coating liquid for the ink-receiving layer in a state of being dispersed by the dispersant. In the present invention, the dispersant means a material capable of stabilizing the dispersed state of inorganic particles in an aqueous solution. The water-soluble resin (B) as a dispersant is preferably a resin selected from polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and derivatives or copolymers thereof. Moreover, the water-soluble resin (B) preferably contains at least one anionic functional group selected from at least a carboxy group, a sulfo group, a phosphonic group, and the like, and is more preferably a sulfo group-modified polyvinyl alcohol. . One or more of these dispersants may be used as necessary. That is, as a dispersant, a water-soluble resin (B) having an average degree of polymerization of 450 or less and a dispersant other than the water-soluble resin (B) may be used in combination. As the dispersant other than the water-soluble resin (B), conventionally known ones can be used as long as the effects of the present invention are not impaired. In addition, among the resins exemplified as the water-soluble resin (B), those having an average degree of polymerization exceeding 450 can also be used. Further, an inorganic compound dispersing aid such as potassium hydroxide, sodium hydroxide, amine, etc. may be used in combination.

The content of the dispersant in the ink-receiving layer is preferably 0.5% by mass or more, and preferably 20% by mass or less, relative to the total mass of the inorganic particles contained in the ink-receiving layer. Further, the content is more preferably 1.0% by mass or more and 5.0% by mass or less with respect to the total mass of the inorganic particles contained in the ink receiving layer. The average degree of polymerization of the water-soluble resin (B) is preferably 250 or less. Also, the average molecular weight of the water-soluble resin (B) is preferably in the range of 3,000 to 50,000, more preferably in the range of 5,000 to 20,000. In addition, in the present invention, the viscosity average degree of polymerization obtained by the method of JIS-K6726 is used as the average degree of polymerization.

(binder)
In the present invention, the ink-receiving layer contains at least a water-soluble resin (A) having an average degree of polymerization of 1,000 or more as a binder. In the present invention, the binder means a material capable of binding inorganic particles and forming a film.
Examples of binders include starch derivatives such as oxidized starch, etherified starch and phosphate esterified starch; cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose; casein, gelatin, soybean protein, polyvinyl alcohol, and derivatives thereof; Conjugated polymer latex such as polyvinylpyrrolidone, maleic anhydride resin, styrene-butadiene copolymer, methyl methacrylate-butadiene copolymer; acrylic polymer latex such as acrylic acid ester and methacrylic acid ester polymer; ethylene-acetic acid Vinyl polymer latex such as vinyl copolymer; Functional group-modified polymer latex with functional group-containing monomer such as carboxyl group of the above polymer; Aqueous binder such as thermosetting synthetic resin such as melamine resin and urea resin ; Polymers and copolymers of acrylic acid esters and methacrylic acid esters such as polymethyl methacrylate; Synthetic resins such as polyurethane resins, unsaturated polyester resins, vinyl chloride-vinyl acetate copolymers, polyvinyl butyral, and alkyd resins . One or more of these binders may be used as necessary. That is, as the binder, a water-soluble resin (A) having an average degree of polymerization of 1,000 or more and a binder other than the water-soluble resin (A) may be used in combination.

Among the above binders, it is preferable to use polyvinyl alcohol or a polyvinyl alcohol derivative as the water-soluble resin (A). Examples of polyvinyl alcohol derivatives include anion-modified polyvinyl alcohol, silanol-modified polyvinyl alcohol, and polyvinyl acetal. Polyvinyl alcohol can be synthesized, for example, by saponifying polyvinyl acetate. The degree of saponification of polyvinyl alcohol is preferably 80 mol % or more and 100 mol % or less, more preferably 85 mol % or more and 98 mol % or less. The degree of saponification is the ratio of the number of moles of hydroxyl groups generated by the saponification reaction when polyvinyl acetate is saponified to obtain polyvinyl alcohol. shall be used. Moreover, the average degree of polymerization of the water-soluble resin (A) is preferably 2,000 or more, and more preferably 2,000 or more and 5,000 or less. In addition, in the present invention, the viscosity average degree of polymerization obtained by the method of JIS-K6726 is used as the average degree of polymerization.

In the present invention, from the viewpoint of ink absorption, the content of the binder in the ink-receiving layer is preferably 50% by mass or less, preferably 30% by mass, relative to the content of the inorganic particles in the ink-receiving layer. The following are more preferable. From the viewpoint of the binding property of the ink-receiving layer, the above ratio is preferably 5.0% by mass or more, more preferably 8.0% by mass or more.

When preparing the ink-receiving layer coating liquid, it is preferable to use a binder such as polyvinyl alcohol or a polyvinyl alcohol derivative as an aqueous solution. In that case, the solid content concentration of the polyvinyl alcohol (derivative) in the polyvinyl alcohol (derivative)-containing aqueous solution is preferably 3% by mass or more and 20% by mass or less.

(crosslinking agent)
In the invention, the ink-receiving layer may further contain a cross-linking agent. Examples of cross-linking agents include aldehyde-based compounds, melamine-based compounds, isocyanate-based compounds, zirconium-based compounds, amide-based compounds, aluminum-based compounds, boric acid, borate salts, and the like. One or more of these cross-linking agents can be used as necessary. In particular, when polyvinyl alcohol or a polyvinyl alcohol derivative is used as the binder, it is preferable to use boric acid or a borate among the above-mentioned cross-linking agents.
Boric acid includes orthoboric acid (H ₃ BO ₃ ), metaboric acid, diboric acid, and the like. The borate is preferably a water-soluble salt of boric acid. Examples thereof include alkali metal salts of boric acid such as sodium boric acid and potassium boric acid; alkaline earth metal salts of boric acid such as magnesium boric acid and calcium boric acid; and ammonium boric acid. Among these, it is preferable to use orthoboric acid from the viewpoint of the stability over time of the coating liquid and the suppression of cracks.

The amount of the cross-linking agent used can be appropriately adjusted according to the production conditions and the like. When a cross-linking agent is used in the present invention, the content of the cross-linking agent in the ink-receiving layer is preferably 1% by mass or more and 30% by mass or less, more preferably 5% by mass or more and 20% by mass, relative to the content of the binder in the ink-receiving layer. % by mass or less is more preferable. Furthermore, when the binder is polyvinyl alcohol and/or a derivative thereof, and the cross-linking agent is at least one selected from boric acid and borate salts, the ink receptivity to the content of polyvinyl alcohol in the ink-receiving layer increases. The total content of boric acid and/or borate in the layer is preferably 0.01% by mass or more and 1% by mass or less, and is preferably 0.05% by mass or more and 0.5% by mass or less. more preferred.

(Other additives)
In the present invention, the ink-receiving layer may contain additives other than those mentioned above. Specifically, pH adjusters, thickeners, fluidity improvers, antifoamers, foam inhibitors, surfactants, mold release agents, penetrants, coloring pigments, coloring dyes, fluorescent brighteners, and UV absorbers. agents, antioxidants, preservatives, antifungal agents, water-resistant agents, dye-fixing agents, curing agents, and weather-resistant materials.

[Undercoat layer]
In the present invention, an undercoat layer may be provided between the substrate and the ink-receiving layer for the purpose of improving the adhesion between the substrate and the ink-receiving layer. The undercoat layer preferably contains a water-soluble polyester resin, gelatin, polyvinyl alcohol, or the like. The film thickness of the undercoat layer is preferably 0.01 μm or more and 5 μm or less.

[Backcoat layer]
In the present invention, when the ink-receiving layer is provided only on one side of the base material, the side opposite to the side of the base material on which the ink-receiving layer is provided is characterized by handling properties, transport suitability, and continuous printing with a large number of sheets stacked. A back coat layer may be provided for the purpose of improving the transport scratch resistance. The backcoat layer preferably contains a white pigment, a binder, and the like. The back coat layer is preferably formed so that the dry coating amount is 1 g/m ² or more and 25 g/m ² or less.

<Method for manufacturing recording medium>
In the present invention, the method of manufacturing the recording medium is not particularly limited, but the recording medium according to the present invention can be manufactured, for example, according to the manufacturing method having the following steps. That is, the method for producing a recording medium according to the present invention comprises a step of preparing a substrate, a step of preparing an ink-receiving layer coating solution (coating solution preparation step), and preparing the ink-receiving layer coating solution. It is preferable to have a step of applying to a substrate to form a coating layer (coating layer forming step) and a step of drying the coating layer to form an ink receiving layer (ink receiving layer forming step). . A method for manufacturing the recording medium will be described below.

[Preparation of base material]
In the present invention, the base material is obtained, for example, by forming a resin layer on base paper. As a method for producing the base paper, a generally used papermaking method can be applied. Examples of the paper machine include a fourdrinier paper machine, a cylinder paper machine, a cylindrical machine, and a twin wire machine. In order to increase the surface smoothness of the base paper, the surface may be treated by applying heat and pressure during or after the papermaking process. Specific surface treatment methods include calendering such as machine calendering and super calendering.
The method of providing a resin layer on the base paper, that is, the method of coating the base paper with a resin includes a melt extrusion method, wet lamination, dry lamination, and the like. Among them, a melt extrusion method is preferred, in which one side or both sides of the base paper are coated with a molten resin by extrusion. For example, the conveyed base paper and the resin extruded from the extrusion die are brought into contact at the nip point between the nip roller and the cooling roller, and the resin layer is laminated on the base paper by crimping the nip (extrusion coating method) is widely adopted. When the resin layer is provided by the extrusion coating method, pretreatment may be performed so that the adhesion between the base paper and the resin layer becomes stronger. Examples of the pretreatment include acid etching treatment with a mixed solution of chromic acid sulfate, flame treatment with a gas flame, ultraviolet irradiation treatment, corona discharge treatment, glow discharge treatment, and anchor coating treatment such as alkyl titanate. Among these, corona discharge treatment is preferred. Moreover, when the resin layer contains a white pigment, the base paper may be coated with a resin composition in which a resin and a white pigment are mixed.

The method for producing a recording medium according to the present invention may have a step of winding the substrate prepared as described above into a roll on a winding core before forming the ink-receiving layer. A winding core having a diameter of 50 mm or more and 300 mm or less is preferably used. Moreover, the tension during winding is preferably 50 N/m or more and 800 N/m or less. The tension during winding may be constant from the start of winding to the end of winding. Moreover, in order to relieve the pressure concentration at the start of winding, the tension may be gradually decreased from the start of winding to the end of winding.

[Formation of ink-receiving layer]
In the method for producing a recording medium according to the present invention, examples of methods for forming an ink-receiving layer on a substrate include the following methods. First, an ink-receiving layer coating liquid containing materials for forming the ink-receiving layer is prepared. Then, in the coating layer forming step, the prepared coating liquid for the ink receiving layer is applied to the substrate to form the coating layer. As a method for applying the coating liquid, a curtain coater, a coater using an extrusion method, a coater using a slide hopper method, or the like can be used. Incidentally, the coating liquid may be heated during coating. Next, in the ink receiving layer forming step, the formed coating layer is dried. As a result, an ink-receiving layer can be formed on the substrate to obtain the intended recording medium. A drying method for the coating layer includes a method using a hot air dryer such as a straight tunnel dryer, an arch dryer, an air loop dryer, a sine curve air float dryer. Other examples include a method using a dryer using infrared rays, a heating dryer, microwaves, etc., and a low-temperature dehumidifying drying method in which drying is performed by applying low-temperature dehumidified dry air.

<Inkjet recording method>
The inkjet recording method of the present invention has a recording step of ejecting and applying ink to the recording medium of the present invention described above to record an image. Specifically, in the ink jet recording method of the present invention, a pigment ink is applied to the surface having the ink-receiving layer of the recording medium of the present invention having an ink-receiving layer on at least one side of a base material to form an image. has a recording step of recording In the recording step, an image is preferably recorded by ejecting ink using an inkjet recording head. Methods for ejecting ink include a method in which mechanical energy is applied to ink and a method in which thermal energy is applied to ink. In the present invention, it is preferable to adopt a method of applying thermal energy to the ink to eject the ink.

[ink]
The ink used in the inkjet recording method of the present invention preferably contains a water-soluble resin having an anionic functional group, and more preferably contains a pigment as a coloring material. Specifically, the ink contains a pigment dispersion in which a pigment is dispersed in a dispersion medium, a water-soluble resin having an anionic functional group, and an aqueous medium that is a mixed solvent of water and a water-soluble organic solvent. is more preferable.

(pigment)
As the pigment, all conventionally known pigments can be used. Preferable examples include azo pigments, quinacridone pigments, anthraquinone pigments, diketopyrrolopyrrole pigments, quinophthalone pigments, methine/azomethine pigments, phthalocyanine pigments, perylene pigments, perinone pigments and isoindolinone pigments.

(Water-soluble resin)
Examples of water-soluble resins include acrylic resins, polyurethane resins, polyester resins, derivatives and copolymers thereof. Moreover, a carboxy group, a sulfo group, a phosphonic group etc. are mentioned as an anionic functional group.

(aqueous medium)
As water, it is preferable to use deionized water such as ion-exchanged water or pure water. The water content (% by mass) in the ink is preferably 50.0% by mass or more and 95.0% by mass or less based on the total mass of the ink.
As the water-soluble organic solvent, any of those conventionally commonly used in inkjet inks can be used. Examples of water-soluble organic solvents include alkyl alcohols having 1 to 4 carbon atoms, amides, ketones, ketoalcohols, ethers, polyalkylene glycols, glycols, and alkylene groups having 2 to 6 carbon atoms. alkylene glycols, polyhydric alcohols, alkyl ether acetates, alkyl ethers of polyhydric alcohols, nitrogen-containing compounds, sulfur-containing compounds, and the like. One or more of these water-soluble organic solvents can be used as necessary. The content (% by mass) of the water-soluble organic solvent in the ink is preferably 3.0% by mass or more and 50.0% by mass or less based on the total mass of the ink.

(Other components in ink)
In addition to the above components, the ink may contain a moisturizing compound that is solid at room temperature, such as urea, a urea derivative, trimethylolpropane, and trimethylolethane, in order to maintain moisturizing properties. When such a compound is contained, the content (% by mass) of the compound in the ink is preferably 0.1% by mass or more and 20.0% by mass or less, and 3.0% by mass, based on the total mass of the ink. More preferably, it is 10.0% by mass or less.
In addition to the above compounds, the ink also contains various additives such as surfactants, pH adjusters, rust inhibitors, antiseptics, anti-mold agents, antioxidants, and anti-reduction agents. You may let

<Ink cartridge>
Further, an ink cartridge according to the present invention includes an ink containing portion for containing ink, and the ink according to the present invention described above is contained in the ink containing portion. As for the structure of the ink cartridge, there is a structure in which the ink containing portion is composed of an ink containing chamber for containing liquid ink. A negative pressure generating member accommodating chamber may be provided to accommodate the negative pressure generating member. Alternatively, the ink cartridge may have no ink containing chamber for containing liquid ink, and may have an ink containing portion configured to retain the entire amount of the contained amount by a negative pressure generating member. Further, the ink cartridge may be configured to have an ink containing portion and a recording head.

<Inkjet recording device>
An ink jet recording apparatus according to the present invention includes an ink containing section containing ink and a recording head for ejecting ink onto a recording medium according to the present invention, wherein the ink contained in the ink containing section is It is characterized by being the ink according to the present invention described above.

EXAMPLES The present invention will be described in more detail below using examples and comparative examples. The present invention is by no means limited by the following examples, as long as the gist thereof is not exceeded. In the description of the following examples, "parts" are based on mass unless otherwise specified.

[Example 1]
<Production of Recording Medium>
[Preparation of base material]
80 parts of LBKP with a Canadian standard freeness of 450 mLCSF, 20 parts of NBKP with a Canadian standard freeness of 480 mLCSF, 0.60 parts of cationic starch, 10 parts of ground calcium carbonate, 15 parts of light calcium carbonate, 0.10 parts of alkyl ketene dimer , and 0.030 parts of cationic polyacrylamide were mixed, and water was added so that the solid content concentration was 3.0 mass % to obtain a paper stock. Next, the stock was made into paper by a fourdrinier paper machine, wet-pressed in three stages, and then dried by a multi-tube dryer. Then, it was impregnated with an oxidized starch aqueous solution and dried by a size press so that the solid content after drying was 1.0 g/m ² . Furthermore, machine calendered base paper having a basis weight of 170 g/m ² , a Stockigt sizing degree of 100 seconds, an air permeability of 50 seconds, a Bekk smoothness of 30 seconds, a Gurley stiffness of 11.0 mN, and a film thickness of 200 μm. made. Next, a resin composition consisting of 70 parts of low-density polyethylene, 20 parts of high-density polyethylene, and 10 parts of titanium oxide was coated on one side of the base paper so that the dry coating amount was 25 g/m ² . (Dry coating thickness 30 μm, resin layer coverage 100%). In addition, let this coating surface be the surface of a base material. Furthermore, a resin layer is formed on both sides of the base paper by coating the other side of the base paper with low-density polyethylene (100 parts) (dry coating thickness: 30 μm, resin layer coverage: 100%). A base material was obtained.

[Preparation of Receptive Layer Coating Solution]
(Preparation of inorganic particle dispersion liquid 1)
100 parts of vapor phase silica (trade name: AEROSIL 200, manufactured by Nippon Aerosil Co., Ltd.) as inorganic particles, 1 part of potassium hydroxide, and sulfo group-modified polyvinyl alcohol (trade name: Gohsenex L) as water-soluble resin (B) -3266, an average degree of polymerization of 200, manufactured by Mitsubishi Chemical Corporation) was put into pure water so that the solid content concentration was 20% by mass. After that, the mixture was stirred for 30 minutes using ROBOMIX to obtain an inorganic particle dispersion liquid 1.

(Preparation of Receiving Layer Coating Solution 1)
Polyvinyl alcohol (trade name: PVA235, average degree of polymerization: 3500, manufactured by Kuraray Co., Ltd.) as a water-soluble resin (A) was added to the obtained inorganic particle dispersion liquid 1 so that the solid content concentration became an aqueous solution of 7% by mass. adjusted and added. At that time, the amount of polyvinyl alcohol added to 100 parts of the inorganic particles contained in the inorganic particle dispersion liquid 1 was adjusted to 25 parts. Then, pure water was added so that the solid content concentration was 14% by mass to obtain a receiving layer coating liquid 1.

[Formation of ink-receiving layer]
On the surface of the base material obtained above, the receiving layer coating solution 1 prepared above was applied so that the dry coating amount was 35 g/m ² to form a coating layer. Further, the coating layer was dried with hot air at 80° C. to obtain a recording medium 1. The film thickness of the ink receiving layer was 35 μm. The film thickness of the ink receiving layer was measured by cutting a part of the recording medium and observing the cross section with a microscope.

[Example 2]
In the preparation of Inorganic Particle Dispersion Liquid 1, except that the water-soluble resin (B) was changed to polyvinylpyrrolidone (trade name: Pitzcol K-17L, average polymerization degree 80, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), recording medium 1 and A recording medium 2 was obtained in the same manner.

[Example 3]
In the preparation of Inorganic Particle Dispersion Liquid 1, except that the water-soluble resin (B) was changed to polyvinylpyrrolidone (trade name: Pittscol K-30L, average degree of polymerization: 450, manufactured by Daiichi Kogyo Seiyaku Co., Ltd.), recording medium 1 and A recording medium 3 was obtained in the same manner.

[Example 4]
A recording medium 4 was obtained in the same manner as the recording medium 1, except that fumed silica (trade name: AEROSIL 50, manufactured by Nippon Aerosil Co., Ltd.) was used as the inorganic particles.

[Example 5]
A recording medium 5 was obtained in the same manner as the recording medium 1 except that fumed silica (trade name: AEROSIL 90, manufactured by Nippon Aerosil Co., Ltd.) was used as the inorganic particles.

[Example 6]
Recording medium 6 was obtained in the same manner as recording medium 1 except that boehmite (trade name: DISPERAL HP8, manufactured by SASOL) was used as the inorganic particles.

[Example 7]
A recording medium 7 was obtained in the same manner as the recording medium 1, except that fumed alumina (trade name: AEROXIDE Alu C, manufactured by EVONIK) was used as the inorganic particles.

[Comparative Example 1]
Recording medium 8 was prepared in the same manner as recording medium 1, except that 0.2 parts of acetic acid was added to 100 parts of inorganic particles to adjust the paper surface pH of the surface of the ink receiving layer during the preparation of receiving layer coating liquid 1. got

[Comparative Example 2]
A recording medium 9 was obtained in the same manner as the recording medium 1, except that the sulfo group-modified polyvinyl alcohol as the water-soluble resin (B) was not added when the inorganic particle dispersion liquid 1 was prepared.

[Comparative Example 3]
Except for using sulfo group-modified polyvinyl alcohol (trade name: ASP-05, average degree of polymerization: 500, manufactured by Japan Vinyl Acetate & Poval Co., Ltd.) as the water-soluble resin (B) in the preparation of inorganic particle dispersion liquid 1, recording A recording medium 10 was obtained in the same manner as the medium 1.

[Comparative Example 4]
A recording medium 11 was obtained in the same manner as the recording medium 1, except that the vapor phase silica was changed to boehmite (trade name: DISPERAL 60, manufactured by SASOL).

[Comparative Example 5]
(Preparation of inorganic particle dispersion liquid 2)
Gas phase silica (trade name: AEROSIL 200, manufactured by Nippon Aerosil Co., Ltd.) 100 parts, diallyldimethylammonium chloride polymer (trade name: PAS-H-1L, average degree of polymerization 50, manufactured by Nittobo Medical Co., Ltd.) 5 parts, was put into pure water so that the solid content concentration was 20% by mass. After that, the mixture was stirred for 30 minutes using ROBOMIX to obtain an inorganic particle dispersion liquid 2.

(Preparation of Receiving Layer Coating Solution 2)
Polyvinyl alcohol (trade name: PVA235, average degree of polymerization: 3500, manufactured by Kuraray Co., Ltd.) as the water-soluble resin (A) was added to the obtained inorganic particle dispersion liquid 2 so that the solid content concentration was 7% by mass. Boric acid was adjusted to an aqueous solution having a solid concentration of 5% by mass and added. At that time, the added amount of polyvinyl alcohol was set to 25 parts and the added amount of boric acid was set to 4 parts with respect to 100 parts of the inorganic particles contained in the inorganic particle dispersion liquid 2 . Then, pure water was added so that the solid content concentration was 14% by mass to obtain a receiving layer coating liquid 2.

(Formation of ink receiving layer)
A recording medium 12 was obtained in the same manner as the recording medium 1 except that the receiving layer coating liquid 2 was used.

<Measurement of zeta potential>
The zeta potential of the surface of the ink-receiving layer of the prepared recording media 1 to 12 was measured using a zeta potential meter (trade name: ELS-Z2, manufactured by Otsuka Electronics Co., Ltd.). Specifically, a sample is brought into close contact with the lower surface of a box-shaped quartz cell of a flat plate sample cell unit, and polyethylene latex (particle size: 500 nm, manufactured by Otsuka Electronics Co., Ltd.) is dispersed in a 10 mM NaCl solution inside the cell. was injected. Then, the zeta potential of the receiving layer surface was measured indirectly from the behavior of the particles.

<Measurement of paper surface pH>
JAPAN. TAPPI paper pulp test method no. 49-1, the paper surface pH of the surface of the ink-receiving layer was measured. Specifically, immediately after 40 mL of ion-exchanged water was dropped on the measurement surface, the flat glass electrode of a pH meter (trade name: LAQUA F-72, manufactured by Horiba Advanced Techno Co., Ltd.) was brought into contact with the dropping site, and after 30 seconds was read.

<Measurement of pore radius>
Each of the prepared recording media 1 to 12 was cut into strips of 10 cm x 5 mm, placed in a glass container, and measured using an automatic specific surface area/pore size distribution measuring device (trade name: Toyei Star II Plus, manufactured by Shimadzu Corporation). Adsorption and desorption isotherms of nitrogen gas were measured. Then, the median diameter (D50) was obtained from the measured adsorption-desorption isotherm of nitrogen gas using the BJH method. This median diameter (D50) was taken as the pore radius of the surface of the ink-receiving layer.

Table 1 shows constituent materials and physical properties of recording media 1 to 12 produced.

<Evaluation of Recording Medium>
The recording media 1 to 12 obtained above were evaluated for ink absorption in high-speed printing, image clarity of the printed area, and gloss as described below. As a recording apparatus, an apparatus obtained by modifying the print processing method of the inkjet recording apparatus Pro-10S (trade name, manufactured by Canon Inc.) was used. Pigment ink containing a cyan pigment (trade name: ink tank BCI-7C cyan, manufactured by Canon Inc.) and pigment ink containing a yellow pigment (trade name: ink tank BCI-7Y yellow, manufactured by Canon Inc.) are used as inks. was used. As for the printing method, the carriage speed was 12.5 inches/second, and the number of printing passes was variable. The number of passes here means that the print duty is divided by the number of passes and printed, and the 100% duty in this apparatus means that 30.4 ng of ink is applied to 600 dpi square. Table 2 shows the evaluation results.

(Evaluation of Ink Absorption)
Green solid images with recording duties of 140%, 120%, 100%, and 80% were printed on recording media using the inkjet recording apparatus while changing the total number of scans. The recording duty for each scan was divided by the total number of scans, and the ink containing the cyan pigment and the ink containing the yellow pigment were applied in half. For example, when the printing duty is 100%, the printing duty for ink containing cyan pigment is set to 50%, and the printing duty for ink containing yellow pigment is set to 50%. As the total number of scans decreases, the amount of ink applied per unit time increases, so the printing speed can be said to be faster. The ink absorbability was evaluated by visually confirming the presence or absence of the beading phenomenon in the resulting image. The beading phenomenon is a phenomenon in which ink droplets coalesce before being absorbed by a recording medium, and is known to be highly correlated with ink absorption. Even an image with a high recording duty can be judged to have high ink absorbability if the beading phenomenon does not occur.
5: The beading phenomenon did not occur even in the image of 140% duty with the total number of scans being 3 times.
4: A beading phenomenon was observed in the 140% duty image with a total of 3 scans, but no beading phenomenon occurred in the 120% duty image with a total of 3 scans.
3: A beading phenomenon was observed in the 120% duty image with a total of 3 scans, but no beading phenomenon occurred in the 100% duty image with a total of 3 scans.
2: A beading phenomenon was observed in the 100% duty image with a total of 3 scans, but no beading phenomenon occurred in the 80% duty image with a total of 3 scans.
1: The beading phenomenon was observed even in the image of 80% duty with the total number of scans being 3 times.

(Evaluation of image clarity in printed area)
Using the inkjet recording apparatus, a black image with a recording duty of 100% was printed on the recording medium under the condition that the total number of scans was three. The optical density of the printed portion was measured using a fluorescence spectrodensitometer (trade name: FD-7, manufactured by Konica Minolta), and the image clarity of the printed portion was evaluated according to the following evaluation criteria.
5: The optical density is 2.1 or more.
4: The optical density is 2.0 or more and less than 2.1.
3: The optical density is 1.9 or more and less than 2.0.
2: The optical density is 1.8 or more and less than 1.9.
1: The optical density is less than 1.8.

(Evaluation of glossiness)
The 20° glossiness of the white paper portion of the recording medium was measured using a glossmeter (trade name: VG2000, manufactured by Nippon Denshoku Industries Co., Ltd.), and the glossiness was evaluated according to the following evaluation criteria.
5: The 20° gloss is 10 or more.
4: The 20° gloss is 8 or more and less than 10.
3: The 20° gloss is 6 or more and less than 8.
2: The 20° gloss is 3 or more and less than 6.
1: 20° gloss less than 3;

Claims (12)

A recording medium having a substrate and an ink-receiving layer on the substrate,
The ink-receiving layer is
inorganic particles that are at least one selected from the group consisting of alumina hydrate, alumina, and fumed silica and have an average primary particle size of 50 nm or less;
a water-soluble resin (A) having an average degree of polymerization of 1,000 or more;
a water-soluble resin (B) having an average degree of polymerization of 450 or less;
including
A recording medium, wherein the surface of the ink-receiving layer has a negative zeta potential, and the paper surface pH of the surface of the ink-receiving layer is 7.0 or higher. 2. The recording medium according to claim 1, wherein said water-soluble resin (A) is polyvinyl alcohol or a derivative thereof. 3. The recording medium according to claim 1, wherein the inorganic particles have an average primary particle size of 20 nm or less. 4. The recording medium according to any one of claims 1 to 3, wherein the ink-receiving layer has a surface pore radius of 5 nm or more and 25 nm or less. The recording medium according to any one of claims 1 to 4, wherein the inorganic particles are fumed silica. The recording medium according to any one of claims 1 to 5, wherein the paper surface pH of the surface of the ink receiving layer is 8.0 or higher. The recording medium according to any one of claims 1 to 6, wherein the water-soluble resin (B) has an average degree of polymerization of 250 or less. The recording medium according to any one of claims 1 to 7, wherein the water-soluble resin (B) is selected from polyethylene glycol, polyvinylpyrrolidone, polyvinyl alcohol, and derivatives or copolymers thereof. The recording medium according to any one of claims 1 to 8, wherein the water-soluble resin (B) has an anionic functional group selected from the group consisting of a carboxy group, a sulfo group and a phosphonic group. The recording medium according to any one of claims 1 to 9, wherein the water-soluble resin (B) is sulfo group-modified polyvinyl alcohol. An inkjet recording method, wherein the recording medium according to any one of claims 1 to 10 is printed with an ink containing a water-soluble resin having an anionic functional group using an inkjet recording head. 12. The inkjet recording method according to claim 11, wherein the ink contains a pigment as a coloring material.