JP2005288700A - Inkjet recording medium and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an inkjet recording medium which has proper inkjet recording quality, particularly, proper ink absorbency, and which hardly causes the fall of powder. <P>SOLUTION: In this inkjet recording medium, at least one under layer containing a pigment and a binding agent is provided on at least one side of a substrate; at least one ink receiving layer containing a pigment and a binding agent is provided on the surface of the under layer; and the pigment of the under layer includes silica wherein an amorphous silica material manufactured by a precipitation method is treated so that a mean particle diameter can be set to be in the range of 0.75-3.5 μm by using a wet grinding method without going through a drying step. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an inkjet recording medium having an ink receiving layer and a method for producing the same.

  In recent years, ink-jet recording methods have become widespread rapidly because full color and high speed are easy and print noise is low. In this recording method, ink droplets are ejected from a nozzle toward a recording medium, and a large amount of solvent is contained in the ink in order to form stable droplets. Therefore, in order to obtain a good image quality, it is necessary to quickly absorb ink droplets.

  For this purpose, the ink-jet recording medium is provided with an ink receiving layer (recording layer) for absorbing ink (see, for example, Patent Documents 1 to 3). The ink receiving layer contains a pigment for absorbing ink instantaneously and in large quantities, a binder for binding the pigment on the support, and appropriately contains an ink fixing agent and various auxiliary agents. As the pigment, silica is generally used from the viewpoint of ink absorbency and color developability.

  By the way, silica is classified into dry silica obtained by a dry production method and wet silica obtained by a wet production method, and wet silica is further classified into a precipitation method or a gel method. Since these silicas can control a porous structure, a specific surface area, a surface state, etc. according to manufacturing conditions, the characteristics of an ink receiving layer can also be adjusted with silica.

  On the other hand, recently, high-definition digital video (camera), scanners and personal computers have become popular, and image data has also been refined. However, due to the high performance of inkjet printers, these image data have the same quality as silver halide photographs. Output (hard copy) is available. Along with this, inkjet recording media are required to have glossiness equivalent to silver salt photographs and high image quality characteristics.

  A so-called cast coating method has been disclosed as a technique for obtaining a highly glossy ink jet recording sheet (see, for example, Patent Document 4). In the cast coating method, a wet ink receiving layer is pressed against a heated mirror-finished surface (mirror drum) via a press roll and dried. At this time, a good glossy surface can be obtained by copying the glossy surface of the drum surface onto the ink receiving layer.

Japanese Patent Laid-Open No. 2001-10211 JP 2001-277712 A JP-A-11-240242 Japanese Patent Laid-Open No. 2-27487

  As described above, it is important that the silica used for the pigment of the ink receiving layer has excellent ink absorbability. However, in the case of the technique described in Patent Document 2, the silica used in the ink receiving layer is pulverized to a particle size of 1 μm or less. However, since the particle size is small, the ink receiving layer becomes too dense and the ink absorptivity. Decreases. In particular, when the ink receiving layer is formed by the above-described cast coating method, the ink absorbability is significantly reduced. In the case of the technique described in Patent Document 3, an aluminosilicate having a particle diameter of 5 nm to 1 μm is used in the ink receiving layer. In this case, the ink absorbability is lowered because the particle diameter of the alumina silicate is small.

  On the other hand, when the silica used for the pigment is produced by a dry process, thixotropy becomes strong because the primary particles of silica exist alone, and when mixed with the coating liquid, the viscosity of the coating liquid becomes high and is suitable for coating. There is an inferior problem. Also, in the wet method silica, the BET specific surface area is larger in the gel method than in the precipitation method. Even if the particle size is adjusted by wet pulverization of silica having a large BET specific surface area, adjustment becomes difficult because the slurry viscosity increases as the pulverization progresses and the particle size of the silica decreases. For this reason, the gel silica is pulverized in a powder form after the drying process and the particle size is adjusted (so-called dry pulverization). However, such dry powder may cause a decrease in handling properties such as powder scattering and dirt during transportation. Also, there are problems that require labor and labor when dispersed in the coating liquid. Therefore, there is a demand for the development of wet silica that is not dried during transportation or dispersion.

  Furthermore, if the amount of silica in the coating layer (ink receiving layer) is increased in order to improve ink absorption and improve image quality, the coating layer will fall off and paper dust will be generated when cutting and handling inkjet recording paper. There is a problem of doing. When this paper dust adheres to the surface of the recording paper, an image defect occurs. In particular, when the cast coating method is used, the above-mentioned problem of powder falling becomes remarkable. In addition, if the amount of silica is reduced and the proportion of the binder is increased in order to prevent the powder falling off, the ink absorbability is lowered.

  Accordingly, a first object of the present invention is to provide an ink jet recording medium having improved productivity by using silica produced by a precipitation method and a method for producing the same. A second object of the present invention is to provide an ink jet recording medium having good ink jet recording quality, particularly ink absorbability, and less dust generation.

As a result of intensive studies on the above problems, the inventors of the present invention usually have a BET specific surface area of 200 m ² / g or more, and the silica produced using a gas phase method or a gel method is pulverized by pulverizing such silica. It has been found that when the diameter is reduced, the viscosity becomes extremely high and the productivity is lowered. Also, when the silica particle size is reduced, the coating layer becomes too dense and the ink absorbency deteriorates.On the other hand, when the silica particle size is increased and the oil absorption is increased, the strength of the coating layer containing silica is increased. In order to decrease, it has been found that there is an optimum range for the particle size of silica, and the present invention has been achieved.

Accordingly, the present inventors have found that the occurrence of powder falling can be solved by providing an under layer containing specific silica in the lower layer of the ink receiving layer. That is, usually, when cutting an inkjet recording paper, a crack is generated from a thin (soft) portion of the coating layer caused by coating unevenness of the ink receiving layer. For this reason, it is considered that the cut edge is not a vertical surface but irregularities, and the coating layer is peeled off from the convex portions to generate paper dust.
On the other hand, when the specific silica described above is used, the strength of the under layer is increased. The reason for this is considered to be that the amount of the effective binder that contributes to the strength improvement is increased by that amount because the amount of oil absorption of the silica is not too high and the binder in the under layer is not adsorbed so much. Since the under layer having a high strength carries the ink receiving layer on the support, it is considered that dropping of the pigment in the ink receiving layer is reduced.

  That is, in the ink jet recording medium of the present invention, one or more under layers containing a pigment and a binder are provided on at least one surface of a support, and an ink receiving layer containing the pigment and the binder is provided on the surface of the under layer. As described above, the pigment of the under layer was prepared by treating the amorphous silica raw material produced by the precipitation method so as to have an average particle diameter of 0.75 to 3.5 μm by the wet grinding method without passing through the drying step. It contains silica.

In addition, the ink jet recording medium of the present invention is provided with at least one under layer containing a pigment and a binder on at least one side of a support, and an ink receiving layer containing the pigment and the binder on the surface of the under layer. In the ink jet recording medium provided as described above, the pigment of the under layer contains silica having an oil absorption of 200 ml / 100 g or less and a BET specific surface area of 200 m ² / g or less and an average particle diameter of 0.75 to 3.5 μm. It is characterized by that.

  The ink receiving layer is preferably provided by a cast coating method.

  The method for producing an inkjet recording medium of the present invention is such that an amorphous silica raw material produced by a precipitation method is converted to silica having an average particle size of 0.75 to 3.5 μm by a wet grinding method without passing through a drying step. Then, without passing through a drying step, the silica is mixed with a binder to form a coating liquid, and the coating liquid is applied to the surface of the support to form an under layer. An ink receiving layer is formed on the surface of the substrate.

  According to the present invention, it is possible to obtain an ink jet recording medium that has good ink jet recording quality, particularly ink absorbability, and that is less likely to fall off during cutting. In particular, a powder fall-off suppressing effect is large in an ink jet recording medium using a cast coating method. Moreover, the productivity of an inkjet recording medium can be improved by using wet silica.

  Hereinafter, embodiments of the present invention will be described. In the ink jet recording medium of the present invention, an under layer and an ink receiving layer are provided in this order on at least one surface of a support.

<Support>
As the support, known materials can be appropriately selected and used, but it is particularly preferable to use paper (coated paper, uncoated paper, etc.). Raw paper pulp includes chemical pulp (coniferous bleached or unbleached kraft pulp, hardwood bleached or unbleached kraft pulp, etc.), mechanical pulp (ground pulp, thermomechanical pulp, chemithermomechanical pulp, etc.), deinked pulp Etc. may be used singly or in an arbitrary ratio. The pH of the paper may be any of acidic, neutral and alkaline. Moreover, the opacity of paper can be improved by containing a filler in paper. The filler can be appropriately selected from known fillers such as hydrated silicic acid, white carbon, talc, kaolin, clay, calcium carbonate, titanium oxide, and synthetic resin filler. Then, the above filler, sizing agent, other additives, etc. are added to the paper, and the paper is machined and dried, or after the paper making, the starch or polymer aqueous solution is size-pressed and dried. A support can be obtained by applying a calendar.

<Under layer>
The under layer contains a pigment and a binder. In the present invention, ink absorbability can be imparted by incorporating the following pigment in the under layer. In other words, in the present invention, ink absorption is ensured by adding a pigment to the ink receiving layer described later, but ink absorption is further improved by including the pigment in the under layer.
[Pigment]
A pigment containing silica having an average particle diameter of 0.75 to 3.5 μm is used. The silica is preferably prepared by adjusting the average particle diameter of an amorphous silica raw material produced by a precipitation method to the above range by a wet pulverization method without passing through a drying step. If it does in this way, since there is no drying process in the manufacturing process of a silica, when performing on-site manufacture in a factory etc., it can be transferred with a slurry state and is excellent in handling nature. Furthermore, since silica is not a powder, there are few problems of labor and cost during dispersion in the coating liquid. The sedimentation method is simpler in production equipment than the dry production method and gel method described above, and is suitable for wet grinding for the purpose of particle size control.

  The reason why silica is pulverized by a wet method is as follows. In other words, in the case of the sedimentation method, silica is produced in the state of a slurry dispersed in water. However, after the slurry is dehydrated to remove by-products and impurities, the silica is hydrated (hydrated) and pulverized. Done. However, when this pulverization is performed by dry pulverization, it is necessary to completely remove moisture by heating or decompression treatment, and the drying energy increases. Further, in the case of dry pulverization, since pulverization is not performed in a state of being dispersed in a solvent as in wet pulverization, the pulverized material tends to aggregate and is not suitable for the production of fine particles.

  The average particle diameter of the silica is 0.75 to 3.5 μm. Usually, when the particle size is reduced by pulverizing silica having a higher-order structure by agglomeration, a part of the internal pores of the silica is broken and the oil absorption amount is reduced. When the thickness is less than 75 μm, the ink absorbability of the under layer becomes insufficient. On the other hand, when the average particle diameter exceeds 3.5 μm, the strength of the under layer is lowered. This is because when the average particle size of silica after pulverization is reduced, the amount of oil absorption is reduced and the binder in the coating liquid adsorbed on the silica surface is reduced, which contributes to improving the strength. It is conceivable that the amount of binder (binder efficiency) increases and the under layer becomes dense, resulting in a decrease in ink absorbability. Conversely, when the average particle size of silica increases, the amount of oil absorption increases, and the binder in the coating solution is adsorbed on the silica surface, and the amount of effective binder that contributes to improving the strength (binder efficiency). As a result, the strength of the under layer may be reduced.

For the reasons described above, the average particle diameter of silica is 0.75 to 3.5 μm, and the oil absorption of silica is preferably 200 ml / 100 g or less, more preferably 100 to 180 ml / 100 g. Moreover, it became clear by examination of the present inventors that when the oil absorption amount exceeds 200 ml / 100 g, the paper powder tends to increase, so the above range is defined.
Normally, cutting of the ink jet recording medium is performed with a guillotine cutter or the like. The guillotine cutter has, for example, a cutting blade whose one side is flat and the other side is thin toward the tip, and the cutting blade is lowered from above the lithographic paper aligned on the metal flat plate and cut. It has become.

Further, the BET specific surface area of the silica preferably 200 meters ² / g or less, more preferably when the 30~180m ² / g, handling property of the coating liquid (productivity) can be improved. This is considered to be because when the BET specific surface area of silica exceeds 200 m ² / g, the viscosity of the silica dispersion or the underlayer coating liquid using the silica becomes high and the handling property is lowered. And in order to prevent such an inconvenience, if the silica dispersion concentration is reduced to about 10% or less, the concentration of the under layer coating solution using the silica dispersion is also lowered, and the productivity during coating is lowered. Further, when the concentration of the under layer coating liquid is low, the volume of the coating liquid increases, so that problems such as enlargement of the storage tank and the load on the liquid supply pump occur.

  As described above, a higher concentration of the under layer coating solution is preferable from the viewpoint of productivity during coating (reduction in drying load), while a lower viscosity of the silica dispersion or the under layer coating solution is preferable. It is preferable from the viewpoint of handling the coating liquid and finishing the coated surface, but it is difficult to achieve both. In the present invention, the concentration of the coating liquid and the like can be set to appropriate values by defining the oil absorption amount and / or the BET specific surface area of silica within the above ranges.

  It is preferable that the compounding quantity of a silica is 60-80 mass% with respect to the mass of an under layer, More preferably, 65-75 mass% is contained. When the amount of silica exceeds 80% by mass, the ink absorbability is improved, but the strength of the under layer is reduced, paper dust is generated due to the falling off of the layer, and handling is deteriorated. When the amount of the pigment is less than 60% by mass, the ink absorbability is deteriorated. When other pigments described below are used in combination, the total amount of pigment is used.

[Other pigments]
As the pigment for the under layer, in addition to the above silica, pigments such as kaolin, clay, calcium carbonate, talc, alumina, aluminum hydroxide, magnesium carbonate, satin white, titanium dioxide, montmorillonite, and plastic pigment are used alone or in two kinds You may use together. In this case, the above-mentioned pigment is preferably 0 to 80% by mass, more preferably 25 to 75% by mass, based on the entire pigment of the under layer. If the pigment other than silica exceeds 80%, the absorbency tends to deteriorate.

[Binder]
Examples of binders include casein, soy protein and synthetic protein, starches such as oxidized starch and esterified starch, polyvinyl alcohol, carboxymethylcellulose, hydroxycellulose, styrene / butadiene latex, acrylic emulsion, vinyl acetate emulsion, polyurethane. These may be used alone or in combination of two or more. The blending amount of the binder is preferably 3 to 50 parts by mass, and more preferably 5 to 30 parts by mass with respect to 100 parts by mass of the entire pigment of the under layer. When the blending amount of the binder is less than 3 parts by mass, the binding force between the pigments is reduced, the strength of the under layer is reduced, and the powder is likely to fall off. On the other hand, when the blending amount exceeds 50 parts by mass, the blending ratio of the pigment decreases and the ink absorbability tends to decrease.

  As already described, since the silica is used in the present invention, the adsorption with the binder is small, and the strength of the under layer is improved without increasing the blending amount of the binder. For example, normally, in order to improve the layer strength, it is necessary to add about 70 parts by mass of a binder. However, in this case, not only the ink absorbability is lowered, but also the layer becomes brittle and the powder falls off. The problem which becomes remarkable arises.

[Other ingredients]
In the under layer, a pigment dispersant, a fluidity improver, a water retention agent, a dye fixing agent, a thickener, an antifoaming agent, an antifoaming agent, a release agent, a foaming agent, a penetrating agent, a coloring dye, if necessary Various auxiliary agents such as coloring pigments, fluorescent brighteners, ultraviolet absorbers, antioxidants, preservatives, antibacterial agents, water-proofing agents, wetting agents, cationic polymer electrolytes and the like can also be added.

[Coating]
The above components are mixed to form a coating solution, which is coated on the support surface and dried to form an under layer. As described above, by using silica whose particle size is adjusted without going through a drying step, this liquid (slurry) is diluted to an appropriate concentration as it is, and a binder and other components are blended to form a coating liquid. Therefore, there is no problem of scattering or dirt when handling silica powder, and the process of redispersing the powder can be omitted, and the coating liquid preparation can be completed in a short time, and the energy required for water and stirring Industrial merit becomes large. The coating method will be described later.

The coating amount of the under layer is preferably 4 to ²⁰ g / m ² . When the coating amount is less than 4 g / m ² , the ink absorbability is not sufficient and causes bleeding, and when it exceeds 20 g / m ² , the coating amount is excessive and the productivity is deteriorated.

<Ink receiving layer>
An ink receiving layer including a pigment and a binder is provided on the surface of the under layer.
[Pigment]
The pigment is an essential component for improving the ink absorbability in the ink receiving layer. Examples of pigments include synthetic silica, alumina and alumina hydrate (alumina sol, colloidal alumina, pseudoboehmite, etc.), aluminum silicate, magnesium silicate, magnesium carbonate, light calcium carbonate, heavy calcium carbonate, kaolin, talc, calcium sulfate, and dioxide. Inorganic pigments such as titanium, zinc oxide, zinc carbonate, calcium silicate, and aluminum hydroxide, and organic pigments such as plastic pigments and urea resins can be used. In particular, alumina hydrate is preferable from the viewpoint of ink absorbability and gloss, and alumina is more preferable. Alumina and alumina hydrate are preferably used in combination with synthetic silica.
Different pigments are used for the under layer and the ink receiving layer.

Further, when no pigment is blended in the ink receiving layer, paper dust is not generated at the time of cutting, but the ink absorbability is lowered, so that an ink jet recording medium having high image quality cannot be obtained.

[Binder]
As a binder, the well-known binder used for the conventional coated paper can be used. In particular, when polyvinyl alcohol is used, the transparency of the ink receiving layer is improved, and a high gloss comparable to that of a silver salt photograph is obtained. Further, the print density is improved and a vivid recorded image can be obtained. The saponification degree and polymerization degree of polyvinyl alcohol can be appropriately selected. In addition to polyvinyl alcohol, other binders, such as starches such as oxidized starch and esterified starch, cellulose derivatives such as carboxymethyl cellulose and hydroxyethyl cellulose, polyvinyl pyrrolidone, casein, and gelatin, as long as the effects of the present invention are not impaired. , Soy protein, styrene-acrylic resin and derivatives thereof, styrene-butadiene latex, acrylic emulsion, vinyl acetate emulsion, vinyl chloride emulsion, urethane emulsion, urea emulsion, alkyd emulsion, and derivatives thereof may be used in combination.

Although the compounding quantity of a binder is not specifically limited as long as required layer strength is obtained, it is preferable to set it as 5-30 mass parts with respect to 100 mass parts of pigments, and it is more preferable to set it as 5-20 mass parts. When the blending amount is less than 5 parts by mass, the layer strength tends to decrease, and when it exceeds 30 parts by mass, the blending ratio of the pigment decreases and the ink absorbency tends to decrease.
In addition, when polyvinyl alcohol and another binder are used together as a binder, it is preferable that the ratio of polyvinyl alcohol with respect to the whole binder Z is 30 mass% or more, especially 50 mass% or more. When the proportion of polyvinyl alcohol is less than 30% by mass, the glossiness of white paper tends to be difficult to be obtained.

[Other ingredients]
In addition to the above components, the ink-receiving layer may contain a pigment dispersant, a water retention agent, a thickening agent, an antifoaming agent, an antiseptic, a colorant, a water resistance agent, a wetting agent, a fluorescent dye, an ultraviolet absorber, A cationic polymer electrolyte or the like can be appropriately added.

[Coating]
The coating amount of the ink receiving layer can be arbitrarily adjusted within the range that covers the surface of the support and sufficient ink absorbency can be obtained, but from the viewpoint of achieving both print density and ink absorbency, , and particularly preferably it is 5 to 30 g / ^{m 2} in terms of solid content is preferably, when considering the productivity 10 to 25 g / ^{m 2.} When the coating amount exceeds 30 g / m ² , the releasability from a mirror drum, which will be described later, decreases, and problems such as the coating layer adhering to the mirror drum occur. In the case where the ink receiving layer contains a pigment and polyvinyl alcohol, conventionally, when the coating amount exceeds 10 g / m ² , paper dust is generated at the time of cutting. However, according to the present invention, generation of paper dust can be suppressed, The coating can exceed 10 g / m ² .

<Coating method of under layer and ink receiving layer>

  Various methods such as blade coater, air knife coater, roll coater, brush coater, kiss coater, squeeze coater, curtain coater, die coater, bar coater, gravure coater, comma coater, and gate can be provided on the support. It can be appropriately selected from methods using known coating machines such as a roll coater, a short dwell coater, a flexographic gravure coater, a spray coater, and a size press. These various devices may be used on-machine or off-machine. It is also possible to perform surface treatment with a calendar device such as a machine calendar, a super calendar, or a soft calendar before and after coating each layer.

<Difference between under layer and ink receiving layer>
The under layer and the ink receiving layer can be easily distinguished by observing the cross section of the ink jet recording medium with an electron microscope or the like. In particular, when the ink receiving layer is provided by the cast coating method, a lot of voids are seen in the under layer, whereas the cast layer (ink receiving layer) has few voids and looks like a uniform layer with no distinction between pigment and binder. .

<Cast coat>
In the present invention, preferably, the ink receiving layer is formed by a cast coating method, whereby a high gloss comparable to a silver salt photograph can be obtained. In the cast coating method, a wet ink receiving layer is pressed against a heated mirror-finished surface (mirror drum) via a press roll and dried. At this time, a good glossy surface can be obtained by copying the glossy surface of the drum surface onto the ink receiving layer. In particular, a so-called “coagulation cast coating method” in which a liquid for coagulating an ink receiving layer (hereinafter referred to as “coagulating liquid”) is applied to a wet ink receiving layer and then pressed onto a mirror finished surface is a so-called “coagulation cast coating method”. Copying is easy, and minute unevenness on the surface of the ink receiving layer can be reduced, so that it is easy to obtain a high gloss equivalent to a silver salt photograph.
On the other hand, when the surface of the ink-receiving layer is made glossy by using the cast coating method, powder falling off at the time of cutting, roughening of the cut surface and fluffing are more noticeable than in the case of a normal mat surface. This can effectively prevent powder falling off.

  Examples of the coagulant (treatment solution) used in the coagulation casting method include formic acid, acetic acid, citric acid, tartaric acid, lactic acid, hydrochloric acid, sulfuric acid, carbonic acid, calcium, zinc, magnesium, sodium, potassium, barium, lead, cadmium, Examples thereof include salts with ammonium and the like, borax and various borates. In the present invention, at least one selected from these can be used.

  In particular, in the present invention, it is preferable to apply a coagulation cast coating method using polyvinyl alcohol as a binder in the ink receiving layer and using a treatment liquid for coagulating polyvinyl alcohol as the coagulation liquid. Examples of the treatment liquid include a liquid containing both boric acid and borate.

  Here, when borate is used alone, the coagulation action between borate and polyvinyl alcohol in the ink receiving layer is too strong, and the glossy surface of the mirror drum surface cannot be sufficiently copied to the ink receiving layer, which is good. It is difficult to obtain a glossy surface. On the other hand, when boric acid is used alone, the coagulation action of polyvinyl alcohol and boric acid in the ink receiving layer becomes insufficient, and the soft-coagulated ink receiving layer may adhere to the roll to which the treatment liquid is applied. It is difficult for the solidified state to become good. Even if the concentration of boric acid in the treatment liquid is increased to enhance the coagulation effect, the solubility of boric acid is low, so there is a limit to the increase in concentration, and it is difficult to coagulate to a desired hardness.

  For the above reasons, in the present invention, it is preferable to use a treatment liquid in which borate and boric acid are mixed. By doing so, it becomes easy to adjust the solidified state of polyvinyl alcohol, and it becomes easy to obtain an ink jet recording medium having a good gloss.

  The compounding ratio of borate and boric acid in the treatment liquid is a mass ratio after conversion to anhydride, and is preferably between borate / boric acid = 1/4 to 2/1. When the above blending ratio is less than 1/4, the proportion of boric acid is excessively large and the polyvinyl alcohol in the ink receiving layer is insufficiently solidified, and the soft-coagulated ink receiving layer adheres to the roll to which the treatment liquid is applied. In some cases, an ink receiving layer in a good wet state cannot be obtained. On the other hand, when the blending ratio exceeds 2/1, the polyvinyl alcohol in the ink receiving layer is too hard and solidified, and it is difficult to copy the glossy surface of the mirror drum surface, and it may be difficult to obtain a good glossy surface.

  Examples of the borate used in the present invention include borax, orthoborate, diborate, metaborate, pentaborate, and octaborate. In addition, although borate is not specifically limited to these, From the viewpoint of cost, availability, etc., it is preferable to use borax. The concentration of borate and boric acid in the treatment liquid can be adjusted as necessary, but the total concentration of borate and boric acid in the treatment liquid is in the range of 1 to 8% by mass in terms of anhydride. It is preferable. If the concentration of borate and boric acid, especially the concentration of borate is too high, the solidification of polyvinyl alcohol becomes too strong, and the glossiness of blank paper tends to decrease. On the other hand, when the concentration is high, boric acid is likely to be precipitated in the treatment liquid, so that the stability of the treatment liquid is deteriorated.

Treatment liquids include pigment dispersants, water retention agents, thickeners, antifoaming agents, preservatives, colorants, water resistance agents, wetting agents, fluorescent dyes, UV absorbers, cationic polymer electrolytes, etc. Can be added as appropriate.
The method for applying the treatment liquid to the ink receiving layer is not particularly limited, and may be appropriately selected from known methods (for example, roll method, spray method, curtain method, etc.).
Further, a release agent may be added to the ink receiving layer coating liquid and the treatment liquid in order to facilitate the peeling of the ink receiving layer from the mirror drum. Here, the melting point of the release agent is preferably 90 to 150 ° C, and particularly preferably 95 to 120 ° C. In the above range, since the melting point of the release agent is substantially equal to the metal surface temperature of the mirror finish, the release ability of the release agent is maximized.

  The present invention will be described more specifically with reference to the following examples. However, the present invention is not limited to these examples. “Part” and “%” represent “part by mass” and “% by mass”, respectively, unless otherwise specified.

(Manufacture of support)
A pulp composed of 100 parts of hardwood bleached kraft pulp (L-BKP) with a beating degree of 285 ml, 4 parts of titanium oxide, 0.4 parts of cationic starch (Kate 304L: product name manufactured by NSC Japan), 1.0 parts of aluminum sulfate Part, 0.1 part of a synthetic sizing agent, and 0.02 part of a yield improver were added to prepare a pulp slurry.

(Formation of under layer)
Starch is coated on both sides of the support by a gate roll device so that the dry coating amount per side is 1.5 g / m ^2, and the following coating solution A is applied on one side by a blade method. Was applied so that the dry coating amount was 7 g / m ² to form an under layer (basis weight of this coated paper: 180 g / m ² ).

  Coating liquid A: Prepared by the following first to fifth steps.

1. First Step In a reaction vessel (200 L), commercially available sodium silicate No. 3 (SiO ₂ : 20.0%, Na ₂ O: 9.5%) is diluted with water, and 6.7% by weight diluted sodium silicate in terms of SiO ₂ A 200 L solution was prepared. After heating this sodium silicate solution to 85 ° C., an amount of aluminum sulfate corresponding to 20% of the neutralization equivalent (a concentration of 8% by weight as Al ₂ O ₂ , hereinafter referred to as “band”) is added dropwise at 200 g / min. At a speed, it was added with sufficient vigorous stirring so that no coarse gel was generated, and then an amount of sulfuric acid (concentration 98% by weight) corresponding to 20% of the neutralization equivalent was also added. After completion of the addition, the partially neutralized liquid obtained was subjected to aging treatment with stirring, and at the same time circulated with a vertical sand grinder (capacity 7.57 L, 1% diameter glass beads filling rate 70%) targeting a particle size of 7 μm. Grinded. This aging and pulverization treatment was performed for 3 hours to obtain a slurry.

2. Second Step The temperature of the slurry is raised to 90 ° C., sulfuric acid having the same concentration as that used in the first step is added up to 80% of the neutralization equivalent under the same conditions as the first step, and aged for 32 minutes with stirring. did.

3. Third Step To the slurry after aging, sulfuric acid having the same concentration as that used in the first step was added at the addition rate of 76 g / min under the same conditions as in the first step, and the pH of the slurry was adjusted to 6.

4). Fourth Step The slurry after pH adjustment was filtered, washed with water, repulped into pure water, and silica slurry was recovered. The obtained silica slurry is diluted to a concentration indicating a liquid state, and wet pulverization is performed using a horizontal sand grinder (filling rate of glass beads having a bead diameter of 0.6 to 0.8 mm (manufactured by Toyo Ballotini) 80%). The final average particle size was 2.3 μm.

In addition, the average particle diameter, BET specific surface area, and oil absorption amount of silica contained in the silica slurry after completion of the fourth step were measured.
Average particle size: The above slurry was dropped and mixed in pure water to which a dispersant (sodium hexametaphosphate 0.2% by weight) was added to obtain a uniform dispersion. About this dispersion liquid, the average particle diameter was measured using the laser method particle size measuring machine (usage apparatus: Mastersizer S type made from Malvern).
BET specific surface area: The slurry was dispersed in ethanol as needed to adjust the solid content to 10%, dried at 105 ° C., and then used with a BET specific surface area measuring device (Gemini 2360, manufactured by Micromeritics). The BET specific surface area was determined from the nitrogen adsorption amount.
Oil absorption: The slurry was dispersed in ethanol as necessary to adjust the solid content to 10%, dried at 105 ° C., and then measured according to JIS K5101.

5). Fifth Step This silica slurry was dispersed at 2000 rpm for 20 minutes. Furthermore, 50 parts of polyvinyl alcohol (PVA117, manufactured by Kuraray Co., Ltd.) and 5 parts of a sizing agent (Polymaron 360: product name of Arakawa Chemical Industries, Ltd.) are added to 100 parts of silica solid content, and the B-type viscosity is about 500 cps. Thus, the concentration was adjusted to obtain a coating liquid A.

(Formation of ink receiving layer)
On the under layer, the following coating liquid B is applied with a roll coater so that the dry coating amount is 20 g / m ² to form an ink receiving layer. This layer is solidified using C, and then this layer is pressed against the surface of a mirror drum heated to 105 ° C. via a press roll for 20 seconds to copy the mirror surface, and an ink jet recording medium having a basis weight of 200 g / m ² Got.
Coating liquid B: Silica (Silojet 703C: Grace Japan Co., Ltd. product name, average particle size 330 nm) 40 parts, high-purity alumina (AKP-G015: Sumitomo Chemical Co., Ltd. product name, average particle size 100 nm) 30 parts), 30 parts of high-purity alumina (UA5605: product name of Showa Denko KK, average particle size 2.8 μm), 13 parts of polyvinyl alcohol (PVA224: product name of Kuraray Co., Ltd.) as a binder, and antifoaming agent 0.2 part was mix | blended and the coating liquid with a solid content concentration of 28% was prepared.
Coagulating liquid C: 4 parts of borax (anhydrous equivalent), 4 parts of boric acid, and 0.25 part of release agent (FL-48C: product name of Toho Chemical Industry Co., Ltd.) were blended, and the solid content concentration was 4.7%. A coagulation solution was prepared.

  The coating amounts of the under layer and the ink receiving layer were determined as follows. A sheet before providing a coating layer (under layer or ink receiving layer) (a support for measuring the under layer, a support having an under layer for measuring the ink receiving layer) is 23 ° C. and 50% RH. The mass after exposure for 4 hours or more to the environment of was previously measured. Next, a coating layer was provided on the surface of the sheet, the mass after being exposed to an environment of 23 ° C. and 50% RH for 4 hours or more was measured, and the coating amount of the coating layer was determined from the mass difference between the two.

  An ink jet recording medium was obtained in the same manner as in Example 1 except that in the coating liquid A, the blending amount of polyvinyl alcohol was 70 parts.

  An ink jet recording medium was obtained in exactly the same manner as in Example 1 except that in the coating liquid A, the blending amount of polyvinyl alcohol was 30 parts.

  In the coating liquid A, an ink jet recording medium was obtained in exactly the same manner as in Example 1, except that the wet pulverization in the fourth step was further sufficiently performed to make the final average particle size 1.3 μm.

<Comparative Example 1>
Instead of the silica slurry in the coating liquid A, after adding water to commercially available silica (Fine Seal X-37B, manufactured by Tokuyama Corporation, wet pulverization, average particle size 3.9 μm), the above fifth step is applied to obtain a solid. An ink jet recording medium was obtained in the same manner as in Example 1 except that the coating liquid A2 having a content of 21% was obtained and the under layer was coated with the coating liquid A2.

<Comparative example 2>
Instead of the silica slurry in the coating liquid A, water was added to commercially available silica (Mizukasil P-50, manufactured by Mizusawa Chemical Co., Ltd., silica prepared by the gel method, dry pulverization, average particle size 10 μm), An inkjet recording medium was obtained in exactly the same manner as in Example 1, except that the coating liquid A3 having a solid content of 21% was obtained by applying 5 steps and the under layer was coated with the coating liquid A3.

<Comparative Example 3>
Instead of the silica slurry in the coating liquid A, after adding water to commercially available silica (Fine Seal X-37B, manufactured by Tokuyama Corporation, wet pulverization, average particle size 3.9 μm), the above fifth step is applied to obtain a solid. An ink jet recording medium was obtained in the same manner as in Example 1 except that the coating liquid A4 having a content of 21% was obtained and the under layer was coated with the coating liquid A4. However, the blending amount of polyvinyl alcohol in the coating liquid A4 was 70 parts.

<Comparative example 4>
Instead of the silica slurry in the coating liquid A, water was added to commercially available silica (Mizukasil P-50, manufactured by Mizusawa Chemical Co., Ltd., silica prepared by the gel method, dry pulverization, average particle size 10 μm), An ink jet recording medium was obtained in the same manner as in Example 1, except that the coating liquid A5 having a solid content of 21% was obtained by applying 5 steps and the under layer was coated with the coating liquid A5. However, the blending amount of polyvinyl alcohol in the coating liquid A5 was 70 parts.

<Evaluation>
The following evaluations were performed on the ink jet recording media of the examples and the comparative examples.
(1) Surface strength A transparent adhesive tape with a width of 18 mm and a length of 10 mm (Nichiban brand name cello tape) is applied to the surface of the ink receiving layer in an environment of 23 ° C. and 50% RH, and a 2 kg rubber roller is reciprocated 10 times from above. And bonded. The sample was peeled 180 degrees at a speed of 300 mm / min using a Tensilon universal testing machine (RTC-1210A, manufactured by Orientec Corp.), and the load required for peeling was measured as the surface strength.

(2) Ink absorbability Using an inkjet printer (PM-950C: Seiko Epson Corporation product name (using dye ink)), a solid image of red (mixed color of magenta and yellow) and green (mixed color of cyan and yellow) An adjacent pattern was printed, and bleeding (bleed) at the boundary portion was visually evaluated according to the following criteria. The bleeding (bleed) at the boundary between red and green is black, and a stricter evaluation is possible.
◎: No bleeding at the boundary part ○: Almost no bleeding at the boundary part △: Slight bleeding at the boundary part ×: Remarkable bleeding at the boundary part

(3) Paper dust amount at the time of cutting 400 sheets of A4 size ink-jet recording media are stacked in the same direction, and the moisture in the paper specified in JISP8127 is adjusted to 6.5 ± 0.1% in a constant temperature and humidity chamber in advance. The total amount of paper dust when the sheet was cut four times in the width direction was measured (mg) from the surface opposite to the surface on the ink receiving layer side with a dampening machine (manufactured by Polar, Model No. 115EMC). The blade type of the cutting blade was high speed steel, the blade angle was 21 °, the blade thickness was 12.7 mm, and the clamp pressure was 1.3 t. Of course, the smaller the amount of paper dust, the better. The evaluation was made according to the following criteria according to the amount of paper dust.
○: Less than 10 mg of paper dust △: 10-20 mg of paper dust
×: Paper powder amount over 20mg

(4) Productivity Productivity was evaluated based on the coating speed of the under layer when the ink jet recording media of the examples and comparative examples were produced. Here, each inkjet recording medium is manufactured using a predetermined coating apparatus and an on-machine drying apparatus under the above-described experimental conditions. However, since the concentration of the underlayer coating liquid is different, a sufficient coating amount is obtained. It is necessary to change the coating speed in order to ensure proper drying. Accordingly, a speed at which problems such as insufficient drying occur does not occur depending on each ink jet recording medium.
○: Underlayer coating speed of 140 m / min or more ×: Underlayer coating speed of less than 140 m / min

  The obtained results are shown in Table 1.

As is apparent from Table 1, the ink jet recording media of the respective examples have high surface strength, and therefore, the amount of paper dust at the time of cutting is small, and the ink absorbability and productivity are also excellent.
On the other hand, in the case of Comparative Example 1 in which silica is dry-pulverized and the average particle diameter of silica exceeds 3.5 μm and the specific surface area exceeds 200 m ² / g, the surface strength is reduced and the amount of paper dust at the time of cutting. In addition, the concentration of the coating solution decreased and the productivity decreased. In addition, in the case of Comparative Example 2 in which silica was produced by a gel method and dry-pulverized and the average particle diameter of silica exceeded 3.5 μm and the specific surface area exceeded 200 m ² / g, paper dust at the time of cutting As the amount increased, productivity decreased. In the case of Comparative Example 2, since the oil absorption amount of silica was smaller than that of Comparative Example 1, the surface strength did not decrease so much, and the amount of paper dust decreased.
Also, in Comparative Examples 3 and 4 where the amount of PVA is large compared to Comparative Examples 1 and 2, the surface strength increased and the amount of paper dust was slightly improved, but the amount of paper dust was still large. Sex was still low. This is considered to be due to the fact that productivity increases as PVA increases, but paper powder increases when PVA is too much, so PVA cannot be blended to a level that improves productivity.

Claims (4)

An ink jet recording medium comprising at least one under layer containing a pigment and a binder on at least one side of a support, and one or more ink-receiving layers containing the pigment and the binder on the surface of the under layer, The pigment of the under layer includes silica obtained by treating an amorphous silica raw material produced by a precipitation method so as to have an average particle diameter of 0.75 to 3.5 μm by a wet grinding method without passing through a drying step. An ink jet recording medium. An ink jet recording medium comprising at least one under layer containing a pigment and a binder on at least one side of a support, and one or more ink-receiving layers containing the pigment and the binder on the surface of the under layer, The ink jet recording medium, wherein the pigment of the under layer contains silica having an oil absorption of 200 ml / 100 g or less, a BET specific surface area of 200 m ² / g or less, and an average particle diameter of 0.75 to 3.5 μm. The inkjet recording medium according to claim 1, wherein the ink receiving layer is provided by a cast coating method. The amorphous silica raw material produced by the sedimentation method is treated by a wet pulverization method so as to become silica having an average particle size of 0.75 to 3.5 μm without passing through the drying step, and then without passing through the drying step. In addition, the silica is mixed with a binder to form a coating liquid, the coating liquid is coated on the surface of the support to form an under layer, and an ink receiving layer is formed on the surface of the under layer. A method for producing an ink jet recording medium.