JP7719293B2 - Recording paper - Google Patents

Description

The present invention relates to recording paper.

In recent years, inkjet printers have become capable of producing images that are comparable to those produced by multicolor offset printing or color electrophotography. In addition, inkjet printers have become widely used due to the fact that their running costs for color printing are lower than those of electrophotography printers. In particular, inkjet printers that use water-based ink, which pose fewer environmental and safety issues than oil-based ink, have become mainstream in recent years.

As a result, the use of inkjet recording paper has expanded to include posters and drafting. As a result, there is a greater demand for recording paper and recording ink with higher print quality and abrasion resistance than ever before.

Furthermore, in fields such as commercial printing, so-called on-demand printing methods have been introduced, in which variable information is digitized and printed at high speed, and inkjet printers employing on-demand printing methods have also appeared. Such on-demand printing methods are suitable for printing small runs because they allow information to be printed directly onto media such as paper without plate-making. Recently, with the expansion of applications due to significant advances in the speed and resolution of devices, there has been a strong demand for improved drying properties for recording paper.
For example, Patent Document 1 proposes a porous resin film that uses calcium carbonate powder treated with a surface treatment agent having an HLB value of 5 to 100, and that has a liquid absorption capacity of a specific value or more.

Japanese Patent Application Publication No. 2001-164017

However, according to the study of the present inventors, it has been found that even if the liquid absorption volume is relatively large, if the liquid absorption speed (ink absorption speed) is relatively slow, sufficient drying properties may not be obtained.
In addition, from the viewpoint of improving the drying properties of ink during printing, it is preferable that the liquid absorption rate of the print-receiving layer surface is high. However, it has also been found that if the liquid absorption rate is too high, the ink may become discolored or granular patterns may appear due to bubbles spraying from the openings on the surface of the print-receiving layer.

Therefore, the present invention aims to provide recording paper with excellent print quality and drying properties.

After extensive research, the inventors discovered that by incorporating a filler with a specific surface treatment into the print-receiving layer of recording paper, the liquid absorption rate of the layer surface can be set within a specific range and the amount of liquid absorption can be increased to a specific value or more, thereby producing recording paper with good print quality and drying properties, and thus completed the present invention.

That is, the present invention relates to the following 1 to 6.
1. A recording paper having a print-receiving layer and a liquid-absorbing layer,
the liquid-absorbing layer and the print-receiving layer are both porous layers containing a thermoplastic resin,
the print-receiving layer contains a hydrophilic surface-treated filler and a hydrophobic surface-treated filler;
the liquid absorption rate on the surface on the print-receiving layer side is 3.5 to 10 cc/ ^m2 ·0.5 s;
Recording paper with a liquid absorption capacity of 10 cc/ ^m2 ·180 s or more.
2. The recording paper according to 1 above, wherein the surface on the print-receiving layer side has a surface strength of 1.1 kg-cm or more.
3. The recording paper according to 1 or 2 above, wherein the liquid-absorbing layer is a layer containing a filler.
4. The recording paper according to any one of 1 to 3 above, wherein the content of the filler in the print-receiving layer is 45 to 65% by mass.
5. The recording paper according to any one of 1 to 4 above, wherein the ratio of the hydrophilic surface-treated filler to the hydrophobic surface-treated filler (hydrophilic surface-treated filler/hydrophobic surface-treated filler) is 95/5 to 55/45 by mass.
6. The recording paper according to any one of 1 to 5 above, wherein the print-receiving layer further contains a thermoplastic elastomer.

According to the present invention, the liquid absorption speed of the surface of the print-receiving layer side of the recording paper is within a specific range, and the amount of liquid absorption is a specific value or more, thereby providing recording paper with excellent print quality and drying properties.

FIG. 1 is a cross-sectional view showing an example of a recording sheet according to this embodiment.

The recording paper according to an embodiment of the present invention will be described in detail below, but the present invention is not limited thereto. In the following description, the term "(meth)acrylic" refers to both acrylic and methacrylic. Similarly, the term "(meth)acryloyl" refers to both acryloyl and methacryloyl, and the term "(meth)acrylate" refers to both acrylate and methacrylate.

(Recording sheet)
The recording paper according to this embodiment has a print-receiving layer and a liquid-absorbing layer, and both the liquid-absorbing layer and the print-receiving layer are porous layers containing a thermoplastic resin. The print-receiving layer contains a hydrophilic surface-treated filler and a hydrophobic surface-treated filler. The liquid absorption rate on the surface of the print-receiving layer side is 3.5 to 10 cc/ ^m2 ·0.5 s, and the liquid absorption amount is 10 cc/ ^m2 ·180 s or more.

Figure 1 is a cross-sectional view showing an example of a recording paper according to this embodiment. In Figure 1, the recording paper 1 has a print-receiving layer 12 and a liquid-absorbent layer 11. The liquid-absorbent layer 11 is preferably provided in contact with the print-receiving layer 12. As will be described later, the recording paper according to this embodiment preferably has a support layer 10 on the side opposite the print-receiving layer 12 from the liquid-absorbent layer 11. In other words, Figure 1 is a diagram illustrating a recording paper 1 configured such that the liquid-absorbent layer 11 and the print-receiving layer 12 are laminated in this order on the support layer 10.

The recording paper according to this embodiment has a liquid absorption rate on the surface facing the print-receiving layer of 3.5 to 10 cc/ ^m2 ·0.5 s. The liquid absorption rate is 3.5 cc/ ^m2 ·0.5 s or more, preferably 4 cc/ ^m2 ·0.5 s or more, and more preferably 5 cc/ ^m2 ·0.5 s or more. An absorption rate of this value or more can improve the ink drying property during printing and further suppress print bleeding. On the other hand, the liquid absorption rate is 10 cc/ ^m2 ·0.5 s or less, and preferably 8.5 cc/ ^m2 ·0.5 s or less. An absorption rate of this value or less can suppress ink color settling and the occurrence of granular patterns due to bubbles spraying from the openings in the print-receiving layer surface, thereby improving print quality.
The liquid absorption rate on the surface on the print-receiving layer side is the amount of liquid transferred, measured in accordance with the liquid absorbency test method by the Bristow method described in Japan Tappi No. 51:2000, and means the amount of liquid absorbed per unit area 500 milliseconds after the test solution is dropped.

The amount of liquid absorption from the surface on the print-receiving layer side of the recording paper according to this embodiment is 10 cc/ ^m2 ·180 s or more, preferably 20 cc/ ^m2 ·180 s or more, and more preferably 30 cc/ ^m2 ·180 s or more. Having a liquid absorption amount equal to or greater than the above value improves drying properties. On the other hand, from the viewpoint of improving productivity and suppressing dry-down, the amount of liquid absorption is preferably 50 cc/ ^m2 ·180 s or less, and more preferably 40 cc/ ^m2 ·180 s or less. The amount of liquid absorption from the surface on the print-receiving layer side may be 10 cc/ ^m2 ·180 s to 50 cc/ ^m2 ·180 s.
The liquid absorption amount of the recording paper refers to the value measured for the amount of liquid absorbed from the surface on the print-receiving layer side, and is the Cobb water absorbency value measured in accordance with the provisions of JIS P8140: 1998. However, instead of water, a 70% by mass aqueous ethanol solution was used as the test solvent, and the contact time was 180 seconds.

In the recording paper according to this embodiment, the liquid absorption rate on the surface facing the print-receiving layer is 3.5 to 10 cc/ ^m² ·0.5 s, and the liquid absorption amount is 10 cc/ ^m² ·180 s or more. That is, in this embodiment, it is preferable that both the liquid absorption rate and the liquid absorption amount are within the above-mentioned ranges. As described above, the inventors have discovered that even if the liquid absorption volume is relatively large, sufficient drying may not be achieved if the liquid absorption rate is relatively slow. Additionally, from the perspective of improving ink drying during printing, a high liquid absorption rate on the surface of the print-receiving layer is preferable, but it has also been discovered that an excessively fast liquid absorption rate can cause ink discoloration and granular patterns due to bubbles spraying from the openings on the surface of the print-receiving layer. In contrast, the present invention has discovered that recording paper with good print quality and drying properties can be obtained by ensuring that the liquid absorption rate on the surface facing the print-receiving layer is within a specific range and that the liquid absorption amount is at least a specific value.
The print-receiving layer contains a hydrophilic surface-treated filler and a hydrophobic surface-treated filler. By containing both the hydrophilic surface-treated filler and the hydrophobic surface-treated filler, it becomes easier to adjust the liquid absorption rate to an appropriate range, i.e., 3.5 to 10 cc/ ^m2 ·0.5 s.

The surface strength of the surface on the print-receiving layer side is preferably 1.1 kg-cm or more, more preferably 1.2 kg-cm or more, and even more preferably 1.3 kg-cm or more. A surface strength of at least this value can improve the abrasion resistance of the print. It can also improve the suitability of the recording paper for lay-flat binding. There is no particular upper limit for the surface strength, but since increasing the surface strength of the layer tends to reduce the surface opening ratio, from the viewpoint of improving drying properties, it is preferably 2.0 kg-cm or less, and more preferably 1.5 kg-cm or less. The surface strength of the surface on the print-receiving layer side may be 1.1 kg-cm to 2.0 kg-cm.
The surface strength of the surface on the print-receiving layer side can be measured using an internal bond tester.

Furthermore, it is preferable that the liquid-absorbent layer is also a layer containing a filler. It is more preferable that both the print-receiving layer and the liquid-absorbent layer are stretched layers containing a filler. For example, by co-extruding the print-receiving layer and the liquid-absorbent layer and then co-stretching them, or by extrusion-laminating the print-receiving layer onto the liquid-absorbent layer and then co-stretching them, it is possible to make both the print-receiving layer and the liquid-absorbent layer stretched layers, which is preferable because it simplifies the manufacturing process and reduces manufacturing costs.

In addition, in the case of general coated printing paper, a coating layer containing a pigment and a binder is provided on the surface of a substrate such as paper. To provide appropriate ink receptivity, calcium carbonate or kaolin is typically used as the pigment. However, such coating layers contain large amounts of pigment, making them brittle and prone to cracking when bent. Therefore, coated printing paper with such a coating layer is often difficult to use in lay-flat bookbinding. In contrast, when the print-receiving layer is a layer made porous by stretching a filler-containing resin composition, it is easy to make it sufficiently porous with a smaller amount of filler than the aforementioned coating layer, making it easier to achieve the desired liquid absorption rate and amount. Furthermore, the porous layer obtained in this manner is resistant to bending.
In particular, when the print-receiving layer is a stretched layer containing a filler and the surface strength of the surface on the print-receiving layer side is 1.1 kg-cm or more, sufficient lay-flat bookbinding performance tends to be obtained. When the print-receiving layer is formed by stretching, it is preferable to use the materials described below, as this makes it easier to achieve the various physical properties described above. In addition, preferred embodiments of the liquid-absorbent layer will be described later.

(Print-receiving layer)
The print-receiving layer is a porous layer containing a thermoplastic resin, a hydrophilic surface-treated filler, and a hydrophobic surface-treated filler. As described above, the print-receiving layer is preferably a stretched layer. That is, the print-receiving layer is preferably a stretched layer containing a thermoplastic resin, a hydrophilic surface-treated filler, and a hydrophobic surface-treated filler. When printing is performed on the surface of the print-receiving layer side of the recording paper according to this embodiment, colorants such as pigments and dyes typically remain on the surface of the print-receiving layer and develop color. Meanwhile, the ink solvent passes through the print-receiving layer and moves to the lower layer (liquid-absorbing layer).

<Thermoplastic resin>
Examples of thermoplastic resins used in the print-receiving layer include olefin polymers, polyamides, polyesters, polycarbonates, polystyrenes, poly(meth)acrylates, polyvinyl chloride, and mixed resins thereof. Among these, olefin polymers are preferred from the viewpoint of water resistance and solvent resistance.

As the olefin polymer, a propylene polymer, an ethylene polymer, etc. can be preferably used.
Examples of propylene-based polymers include propylene homopolymers such as isotactic homopolypropylene and syndiotactic homopolypropylene obtained by homopolymerizing propylene, and propylene copolymers obtained by copolymerizing propylene as the main component with α-olefins such as ethylene, 1-butene, 1-hexene, 1-heptene, 1-octene, and 4-methyl-1-pentene. The propylene copolymer may be a binary system or a multi-component system of ternary or higher. Furthermore, the propylene copolymer may be a random copolymer or a block copolymer.

Examples of ethylene polymers include high density polyethylene, medium density polyethylene, linear low density polyethylene, copolymers of ethylene as a main component with α-olefins such as propylene, butene, hexene, heptene, octene, and 4-methylpentene-1, maleic acid-modified ethylene-vinyl acetate copolymers, ethylene-vinyl acetate copolymers, and ethylene-
Examples include (meth)acrylic acid alkyl ester copolymers, ethylene-(meth)acrylic acid copolymers and metal salts thereof (metals include zinc, aluminum, lithium, sodium, potassium, etc.), ethylene-cyclic olefin copolymers, maleic acid (anhydride) modified polyethylene, maleic acid (anhydride) modified polypropylene, etc. Here, the term "maleic acid (anhydride)" refers to both maleic anhydride and maleic acid.

Among the above olefin polymers, propylene homopolymers, i.e., polypropylene, or high-density polyethylene are preferred from the viewpoints of improving moldability and reducing costs, and maleic anhydride-modified polyethylene or maleic anhydride-modified polypropylene are preferred from the viewpoints of achieving favorable pore-forming properties and filler binding properties.
Among the above thermoplastic resins, one type may be used alone, or two or more types may be used in combination.

<Thermoplastic elastomer>
From the viewpoint of increasing the surface strength, the print-receiving layer preferably contains a thermoplastic elastomer (hereinafter, sometimes simply referred to as "elastomer").
The elastomer in the print-receiving layer can be appropriately selected from thermoplastic elastomers such as polystyrene-based (TPS), olefin-based (TPO), polyvinyl chloride-based (TPVC), polyurethane-based (TPU), polyester-based (TPC), or polyamide-based (TPAE). The elastomer is preferably one that is highly compatible with the resin that is the main component of the print-receiving layer, and is preferably an elastomer of the same type as the thermoplastic resin contained in the print-receiving layer. For example, when an olefin-based polymer is used as the thermoplastic resin, it is preferable to use an olefin-based elastomer.
Olefin-based elastomers have a polyolefin such as polypropylene or polyethylene as a hard segment and a rubber component such as ethylene-propylene rubber (EPM, EPDM) as a soft segment. Olefin-based elastomers can be broadly classified into three types: a blend type of polyolefin and a rubber component, a dynamically crosslinked type (sometimes referred to as thermoplastic vulcanizates (TPV)), and a polymerization type (reactor-TPO (R-TPO)). The elastomer used in this embodiment may be any of these.

By including an elastomer in the print-receiving layer, the toughness of the print-receiving layer containing a thermoplastic resin, preferably an olefin polymer, can be improved, thereby improving the surface strength of the recording paper.

The elastomer content is preferably, for example, 6 to 14% by mass, and more preferably 8 to 12% by mass, relative to the total amount of the thermoplastic resin and elastomer contained in the print-receiving layer. Having an elastomer amount equal to or greater than the lower limit mentioned above tends to provide a sufficient modification effect. Furthermore, having an elastomer amount equal to or less than the upper limit mentioned above can prevent void formation, thereby preventing a decrease in dot gain, a decrease in ink anchoring ability or ink fixability on the print-receiving layer surface, etc.

<Filler>
The hydrophilic surface-treated filler and hydrophobic surface-treated filler used in the print-receiving layer include inorganic particles or organic particles that have been subjected to a surface treatment described below.
The inorganic particles and the organic particles can be used alone or in combination. When a resin composition containing a filler and a thermoplastic resin is stretched, a large number of fine pores with the particles as nuclei can be formed inside the stretched layer. This allows a porous layer to be obtained.

The filler content in the print-receiving layer is preferably 45% by mass or more, and more preferably 50% by mass or more, from the viewpoint of favorable pore formation. A filler content of 45% by mass or more in the print-receiving layer increases the porosity, facilitating the penetration of ink into the liquid-absorbing layer and increasing the amount of liquid absorption. On the other hand, the filler content in the print-receiving layer is preferably 65% by mass or less, and more preferably 60% by mass or less, from the viewpoint of suppressing the occurrence of surface defects and preventing excessive porosity and liquid absorption rate. The filler content in the print-receiving layer may be 45% to 65% by mass.

From the viewpoint of obtaining favorable pore formation, the average particle diameter of the filler contained in the print-receiving layer is preferably 0.1 μm or more, and more preferably 0.3 μm or more. From the same viewpoint, the average particle diameter of the filler is preferably 5 μm or less, and more preferably 3 μm or less. If the average particle diameter of the filler is 0.1 μm or more, a sufficient number and size of pores are formed in the layer, making the porous layer porous and facilitating the permeability of aqueous pigment inks. If the average particle diameter of the filler is 5 μm or less, the formation of coarse pores is suppressed, making it easier to improve the clarity of inkjet printed images. The average particle diameter of the filler contained in the print-receiving layer may be 0.1 μm to 5 μm. Here, the average particle diameter of the filler refers to the average primary particle diameter (D50). This is the volume-based median diameter measured by laser light diffraction/scattering.

<<Inorganic particles>>
The inorganic particles are not particularly limited, but examples thereof include heavy calcium carbonate, light calcium carbonate, calcined clay, talc, titanium oxide, barium sulfate, alumina, silica, zinc oxide, zeolite, mica, glass fiber, hollow glass beads, etc. Among these, heavy calcium carbonate, calcined clay, diatomaceous earth, etc. are preferred because they are inexpensive, easily form many pores by stretching the resin composition that forms the porous layer, and allow for easy adjustment of the porosity. In particular, heavy calcium carbonate or light calcium carbonate is preferred because its average particle size or particle size distribution can be easily adjusted to a range that facilitates pore formation. Among the above inorganic particles, one type can be used alone, or two or more types can be used in combination.

<<Organic particles>>
The organic particles are not particularly limited, but are preferably organic particles that are incompatible with the thermoplastic resin, have a melting point or glass transition temperature higher than that of the thermoplastic resin, and are finely dispersed under the melt-kneading conditions of the thermoplastic resin. For example, when an olefin-based resin is used as the thermoplastic resin contained in the print-receiving layer, it is preferable to use a resin selected from polyethylene terephthalate, polybutylene terephthalate, polycarbonate, nylon-6, nylon-6,6, homopolymers of cyclic olefins, copolymers of cyclic olefins and ethylene, etc., having a melting point of 120 to 300°C or a glass transition temperature of 120 to 280°C.

<<Hydrophilic surface treatment filler>>
By including a hydrophilic surface-treated filler whose surface has been hydrophilized as the filler contained in the print-receiving layer, the print-receiving layer has good affinity with aqueous ink, and the liquid absorption rate of the print-receiving layer surface is improved. The hydrophilic surface-treated filler can be obtained, for example, by applying a surface treatment agent to the filler surface using the method described below.

The surface treatment agent may include a water-soluble cationic copolymer and a water-soluble anionic surfactant.
The water-soluble cationic copolymer is preferably a copolymer of a monomer (d1) selected from diallylamine salts, alkyldiallylamine salts, dialkyldiallylammonium salts, and tetraalkylammonium salts substituted with (meth)acryloyloxy groups with a nonionic hydrophilic vinyl monomer (d2). The amine salt in (d1) is preferably a compound in which the amino group moiety is salted with hydrochloric acid, sulfuric acid, nitric acid, acetic acid, or the like. The anion that forms a salt with the ammonium structure is preferably selected from chloride ions, bromide ions, sulfate ions, nitrate ions, methyl sulfate ions, ethyl sulfate ions, and methanesulfonate ions. Furthermore, the alkyl group in (d1) is preferably an alkyl group having 1 to 4 carbon atoms.

Specific examples of (d1) include diallylamine salt, methyldiallylamine salt, ethyldiallylamine salt; salts such as dimethyldiallylammonium, (meth)acryloyloxyethyltrimethylammonium, (meth)acryloyloxyethyldimethylethylammonium, and acryloyloxyethyldimethylammonium; and quaternary ammonium salts obtained by alkylating N,N-dimethylaminoethyl (meth)acrylate with epoxy compounds such as epichlorohydrin, glycidol, and glycidyltrimethylammonium chloride. Among these, diallylamine salt, methyldiallylamine salt, and dimethyldiallylammonium salt are particularly preferred. These may be used alone or in combination.

Specific examples of (d2) include acrylamide, methacrylamide, N-vinylformamide, N-vinylacetamide, N-vinylpyrrolidin, 2-hydroxyethyl (meth)acrylate, 2-hydroxy(meth)acrylate, 3-hydroxypropyl (meth)acrylate, (meth)acrylic acid methyl ester, (meth)acrylic acid ethyl ester, and (meth)acrylic acid butyl ester, with acrylamide or methacrylamide being preferred. These may be used alone or in combination.

The copolymerization ratio of (d1) to (d2) is arbitrary, but preferred ranges are 10 to 99 mol% for (d1), more preferably 50 to 97 mol%, and even more preferably 65 to 95 mol%, and 90 to 1 mol% for (d2), more preferably 50 to 3 mol%, and even more preferably 35 to 5 mol%. The cationic copolymer can be produced, for example, by the method described in JP-A-5-263010.

Examples of the water-soluble anionic surfactant include one or more sulfonates selected from the group consisting of sulfonates of alkylene oxide adducts of monohydric alcohols, alkyl sulfonates, and alkyl benzene sulfonates. Examples of the cations that form salts with the sulfonic acid structure include sodium ions, potassium ions, lithium ions, and ammonium ions.
Among these, alkyl sulfonates are preferred, and specifically, those having an alkyl group having 8 to 20 carbon atoms, such as octanesulfonate, dodecanesulfonate, tetradecanesulfonate, hexadecanesulfonate, and octadecanesulfonate, are particularly preferred. These may be used alone or in combination.

There are no particular limitations on the method for hydrophilizing the filler surface. For example, a surface treatment agent can be added to a slurry containing inorganic or organic particles, followed by stirring and drying. When using calcium carbonate (inorganic particles) produced by wet milling as the filler, an aqueous medium (preferably water) is added to the calcium carbonate so that the calcium carbonate/aqueous medium (preferably water) mass ratio is, for example, in the range of 70/30 to 30/70, preferably 60/40 to 40/60. Then, 0.05 to 2 parts by mass, preferably 0.1 to 1 part by mass, of cationic copolymer is added per 100 parts by mass of calcium carbonate in terms of solids, and wet-milled by conventional methods. Alternatively, an aqueous medium in which the cationic copolymer is dissolved in the amount specified above may be prepared, mixed with calcium carbonate, and wet-milled by conventional methods.

By including a hydrophilic surface-treated filler as the filler in the print-receiving layer, a print-receiving layer with a relatively fast liquid absorption rate on its surface can be obtained. However, as mentioned above, if the liquid absorption rate is too fast, the ink tends to sink and granular patterns tend to appear due to bubbles spewing from openings in the surface of the print-receiving layer. The openings are voids that are formed on the surface of the print-receiving layer, which is a porous layer. By replacing part of the filler with a hydrophobic surface-treated filler, it becomes easier to appropriately adjust the liquid absorption rate.

<<Hydrophobic surface treated filler>>
The hydrophobized surface-treated filler contained in the print-receiving layer is preferably inorganic or organic particles that have been hydrophobized with paraffin or a fatty acid having 12 to 22 carbon atoms or a salt thereof, and more preferably inorganic or organic particles that have been hydrophobized with a fatty acid having 12 to 22 carbon atoms or a salt thereof.

Examples of the fatty acid having 12 to 22 carbon atoms for the hydrophobic surface treatment include lauric acid, myristic acid, palmitic acid, stearic acid, arachidic acid, behenic acid, oleic acid, linoleic acid, linolenic acid, and eleostearic acid.
The method of surface treatment is not particularly limited, and can be carried out, for example, by introducing an aqueous solution of a treatment agent into a slurry of inorganic or organic particles, similar to the preparation of the hydrophilic surface-treated filler described above. This makes it possible to obtain surface-treated inorganic or organic particles, i.e., inorganic or organic particles having, on their surfaces, a surface treatment layer containing paraffin or a fatty acid having 12 to 22 carbon atoms or a salt thereof.

The ratio of the hydrophilic surface-treated filler to the hydrophobic surface-treated filler in the print-receiving layer (hydrophilic surface-treated filler/hydrophobic surface-treated filler) is preferably 95/5 to 55/45 by mass, and more preferably 85/15 to 65/35. This range of ratio of the hydrophilic surface-treated filler to the hydrophobic surface-treated filler is preferred because it makes it easier to adjust the liquid absorption rate to an appropriate range, i.e., 3.5 to 10 cc/ ^m2 ·0.5 s.

The total amount of the hydrophilic surface-treated filler and the hydrophobic surface-treated filler in the print-receiving layer is preferably 90% by mass or more, more preferably 95% by mass or more, based on the total amount of fillers contained in the print-receiving layer. This total amount may be 100% by mass. When the total amount of the hydrophilic surface-treated filler and the hydrophobic surface-treated filler is within the above range, it becomes easier to obtain a print-receiving layer with a precisely adjusted liquid absorption rate.
As the filler contained in the print-receiving layer, an untreated filler that has not been subjected to a surface treatment may be used in combination, as long as the effect of the present invention is not impaired.

<Other ingredients>
The print-receiving layer may optionally contain known additives, such as antioxidants, light stabilizers, ultraviolet absorbers, dispersants for fillers, crystal nucleating agents, antiblocking agents, plasticizers, slip agents such as fatty acid amides, dyes, pigments, release agents, and flame retardants.
From the viewpoint of improving outdoor durability, the substrate layer preferably contains an antioxidant, a light stabilizer, and the like.
Examples of the antioxidant include sterically hindered phenol-based antioxidants, phosphorus-based antioxidants, and amine-based antioxidants.
Examples of the light stabilizer include sterically hindered amine light stabilizers, benzotriazole light stabilizers, and benzophenone light stabilizers.
The content of the antioxidant and the light stabilizer is preferably 0.001 to 1% by mass relative to the base layer.

The print-receiving layer is porous, with a porosity of 40-60%, preferably 45-55%. A porosity in this range is preferable because it makes it easier to achieve both high liquid absorption speed and high surface strength. The porosity can be calculated as the area ratio of pores on a cross section by observing the cross section of the target layer using a scanning electron microscope, importing the observed image into an image analyzer, and performing image analysis of the observed area.

The basis weight of the print-receiving layer is preferably 1 to 15 g/ ^m² , more preferably 3 to 13 g/ ^m² , and even more preferably 4 to 10 g/ ^m² . Having a basis weight within this range is preferred because it makes it easier to adjust the liquid absorption rate to the desired level. The thickness of the print-receiving layer, although it depends on the porosity of the layer, is preferably 1.0 to 18.0 μm, more preferably 4.0 to 16.0 μm, and even more preferably 5.0 to 12.0 μm. Having a thickness within this range is preferred because it makes it easier to adjust the liquid absorption rate to the desired level.

(Liquid absorption layer)
The liquid-absorbing layer is a layer that absorbs ink that has passed through the print-receiving layer, and is preferably provided in contact with the print-receiving layer. The liquid-absorbing layer is preferably a porous layer containing a thermoplastic resin and a layer containing a filler. The liquid-absorbing layer is more preferably a stretched layer containing a filler. That is, the liquid-absorbing layer is more preferably a stretched layer containing a thermoplastic resin and a filler.

<Thermoplastic resin>
The thermoplastic resins used in the liquid-absorbing layer include the same thermoplastic resins as those listed in the section on the print-receiving layer, and the preferred ones among them are also the same as those mentioned above.

<Filler>
The filler used in the liquid-absorbing layer may be the same as the filler listed in the section on the print-receiving layer. However, the filler used in the liquid-absorbing layer may or may not have been subjected to at least one of the above-mentioned hydrophilic surface treatment and hydrophobic surface treatment. From the viewpoint of obtaining the desired liquid absorption capacity, it is preferable that the filler has not been subjected to a surface treatment.

The filler content in the liquid-absorbent layer is preferably 45 to 65% by mass, and more preferably 50 to 60% by mass. A filler content at or above the lower limit mentioned above tends to ensure a sufficient amount of pore space in the liquid-absorbent layer. On the other hand, from the perspective of achieving favorable extensibility of the layer and suppressing surface defects, it is preferable that the filler content be at or below the upper limit mentioned above.

<Other ingredients>
The liquid-absorbing layer may contain other optional components such as the various additives described above, as in the print-receiving layer.

The thickness and porosity of the liquid-absorbent layer may be adjusted so that the amount of liquid absorbed from the print-receiving layer is 10 cc/m2 or ^more . A layer with a high porosity may be provided thinly, or a layer with a lower porosity may be provided thickly. For example, when the thickness of the liquid-absorbent layer is about 40 to 50 μm, the porosity is preferably 40% or more, more preferably 45% or more. On the other hand, from the viewpoint of improving productivity, the porosity is preferably 50% or less. The porosity of the liquid-absorbent layer may be 40% to 50%.

(Support layer)
In the recording paper according to this embodiment, a support layer may be laminated on the liquid-absorbent layer on the side opposite the print-receiving layer. Having such a support layer in the recording paper allows the recording paper to have an appropriate thickness and stiffness suitable for printing. That is, by adjusting the thickness of the support layer, the thickness of the recording paper can be adjusted, stiffness suitable for printing can be imparted, and opacity and paper feed/discharge properties can be adjusted. From the viewpoint of obtaining sufficient stiffness, the thickness of the support layer is preferably 15 μm or more, more preferably 20 μm or more, and even more preferably 30 μm or more. Furthermore, from the viewpoint of improving handleability during printing, the thickness of the support layer is preferably 400 μm or less, more preferably 300 μm or less, and even more preferably 200 μm or less. The thickness of the support layer may be 15 μm to 400 μm.
The support layer may be a single layer or a laminate of two or more layers.

There are no particular restrictions on the material that makes up the support layer, but for example, the support layer is preferably a thermoplastic resin layer with excellent water resistance. The same resins as those listed in the print-receiving layer section can be used as the thermoplastic resin. The support layer, like the print-receiving layer and the liquid-absorbing layer, may contain a filler, and may be a porous layer. The support layer may also contain other optional components, such as the additives mentioned above.

(Other layers)
The recording paper according to this embodiment may have layers other than those described above, provided that the effects of the present invention are not impaired. For example, the recording paper may have an adhesive layer between the liquid-absorbent layer and the support layer, or a pressure-sensitive adhesive layer on the surface of the support layer opposite the liquid-absorbent layer. Furthermore, any layer may be present between the print-receiving layer and the liquid-absorbent layer, as long as it does not prevent ink from penetrating from the print-receiving layer to the liquid-absorbent layer.

(Manufacturing method)
The method for producing the recording paper of the present invention is not particularly limited, but examples include the following methods. For example, after forming a thermoplastic resin film that constitutes the support layer, a laminate resin film that constitutes the print-receiving layer and the liquid-absorbing layer may be laminated on it. In this case, the print-receiving layer and the liquid-absorbing layer may be co-extruded using a multi-layer die system using a feed block and a multi-manifold, and then co-stretched to form a laminate resin film in which both layers are porous. Alternatively, multiple dies may be used to extrude and laminate one layer onto the surface of the other layer, and the resulting laminate may be stretched to form a laminate resin film in which both layers are porous.

Alternatively, the support layer, liquid-absorbent layer, and print-receiving layer may all be co-extruded and then co-stretched; the support layer and liquid-absorbent layer may be co-extruded, and then the print-receiving layer may be extrusion-laminated onto the surface of the liquid-absorbent layer and then co-stretched; or the liquid-absorbent layer and print-receiving layer may be extrusion-laminated onto the support layer and then co-stretched, thereby simultaneously making the liquid-absorbent layer and print-receiving layer porous and laminating them to the support. From the standpoint of process simplicity and reduced manufacturing costs, co-extrusion and/or extrusion lamination followed by co-stretching is preferred for production. Any known stretching method can be used.

(printing)
The recording paper according to this embodiment can be printed on the surface of the print-receiving layer side. The printing method used for the print-receiving layer is not particularly limited, and in addition to various known plate-based printing methods such as gravure printing, offset printing, flexographic printing, seal printing, and screen printing, digital printing using various printers such as inkjet printers, electrophotographic printers, and liquid toner printers, and thermal melt transfer printing can also be used.

Depending on the printing method, various inks can be used, such as UV-curable ink, oil-based ink, oxidative polymerization curable ink, melt thermal transfer recording ink, water-based ink, powder toner, or liquid toner (electroink).

In particular, the recording paper according to this embodiment is suitable for use in inkjet printing, particularly inkjet printing using aqueous ink. The recording paper according to this embodiment has a liquid absorption rate within a specific range and an amount of liquid absorption equal to or greater than a specific value, resulting in excellent print quality and drying properties.

The present invention will be explained in more detail below using examples, but the present invention is not limited to the following examples.

[Raw materials]
The raw materials used in the examples and comparative examples are as shown in Table 1. In the table, MFR means melt flow rate. The average particle size (D50) of the surface-untreated calcium carbonate shown in Table 1 is the average primary particle size (D50), which is the volume-based median size measured by a laser light diffraction/scattering method using a Microtrac (manufactured by Nikkiso Co., Ltd.).

[Production Example 1: Production of water-soluble cationic copolymer]
A reactor equipped with a reflux condenser, thermometer, dropping funnel, stirrer, and gas inlet tube was charged with 500 parts by weight of diallylamine hydrochloride (60% aqueous solution), 13 parts by weight of acrylamide (40% aqueous solution), and 40 parts by weight of water. The temperature inside the system was raised to 80°C while nitrogen gas was introduced. With stirring, 30 parts by weight of ammonium persulfate (25% aqueous solution) was added dropwise over 4 hours using the dropping funnel. After the completion of the addition, the reaction was continued for 1 hour, yielding a viscous pale yellow liquid. 50 parts by weight of this was poured into 500 parts by weight of acetone, resulting in the formation of a white precipitate. The precipitate was filtered, thoroughly washed twice with 100 parts by weight of acetone, and then vacuum dried to obtain a white solid polymer (water-soluble cationic copolymer). The weight-average molecular weight of the resulting polymer was determined by GPC to be 55,000.

[Production Example 2: Production of hydrophilic surface-treated calcium carbonate]
40% by mass of heavy calcium carbonate (average particle size 8 μm, manufactured by Nippon Cement Co., Ltd., dry-ground product) and 60% by mass of water were thoroughly mixed and stirred to form a slurry, to which 0.06 parts by mass of the water-soluble cationic copolymer produced in Production Example 1 was added per 100 parts by mass of heavy calcium carbonate, and the mixture was wet-ground using a table-type attritor-type media stirring mill (glass beads with a diameter of 1.5 mm, a filling rate of 170%, and a peripheral speed of 10 m/sec).
Next, 50 parts by mass of a mixture (aqueous solution with a concentration of 2% by mass) of sodium alkylsulfonate having 14 carbon atoms as the main component and sodium alkylsulfonate having 16 carbon atoms as the main component was added and stirred. Thereafter, the mixture was classified through a 350-mesh screen, and the slurry that passed through the 350-mesh screen was dried in a fluidized bed dryer (MSD-200, manufactured by Nara Machinery Works, Ltd.). The average primary particle size of the obtained calcium carbonate was measured using a Microtrac (manufactured by Nikkiso Co., Ltd.) and found to be 1.5 μm.

[Production Example 3: Production of hydrophobic surface-treated calcium carbonate]
500 parts by mass of synthetic calcium carbonate (precipitated calcium carbonate) having a BET specific surface area of 16 m ² /g was added with water, and the mixture was stirred at 40°C to prepare a calcium carbonate slurry having a solids content of 10% by mass. Next, a 10% by mass aqueous solution of sodium laurate was prepared at 90°C, and the prepared solution was mixed with the calcium carbonate slurry and stirred to hydrophobize the calcium carbonate surface. This hydrophobized calcium carbonate slurry was dried until the solids content reached 60%. Thereafter, the mixture was dehydrated using a dryer to obtain calcium carbonate having been subjected to a hydrophobized surface treatment. The average primary particle size of the obtained calcium carbonate was measured by ultrasonic dispersion using an ultrasonic disperser Model US-300T (manufactured by Nippon Seiki Co., Ltd.) using ethanol as a solvent under conditions of 300 μA for 60 seconds, and was found to be 0.23 μm.

[Example 1]
(Production of print-receiving layer resin composition)
40 parts by mass of polypropylene (trade name: Novatec PP MA3Q, manufactured by Japan Polypropylene Corporation), 0.7 parts by mass of maleic anhydride-modified polypropylene (trade name: Modic P908, manufactured by Mitsubishi Chemical Corporation), 5.0 parts by mass of olefin-based elastomer (trade name: Tafmer PN20300, manufactured by Mitsui Chemicals, Inc.), 13 parts by mass of the hydrophobic surface-treated calcium carbonate obtained in Production Example 3, 37 parts by mass of the hydrophilic surface-treated calcium carbonate obtained in Production Example 2, and 3.6 parts by mass of surface-untreated calcium carbonate (trade name: Softon #1800, manufactured by Bihoku Funka Co., Ltd.) were blended and stirred and mixed in a mixer, and then subjected to an extrusion process to obtain Resin Composition A.

(Production of liquid-absorbing layer resin composition)
38 parts by mass of polypropylene (trade name: Novatec PP MA3Q, manufactured by Japan Polypropylene Corporation), 3.5 parts by mass of high-density polyethylene (trade name: Novatec LL US070G, manufactured by Japan Polyethylene Corporation), and 58 parts by mass of surface-untreated calcium carbonate (trade name: Softon #1800, manufactured by Bihoku Funka Co., Ltd.) were blended and stirred in a mixer, and then extruded to obtain resin composition B.

(Production of Support Layer Resin Composition)
60 parts by mass of polypropylene (trade name: Novatec PP MA3Q, manufactured by Japan Polypropylene Corporation), 10 parts by mass of high-density polyethylene (trade name: Novatec LL US070G, manufactured by Japan Polyethylene Corporation), and 30 parts by mass of surface-untreated calcium carbonate (trade name: Softon #1800, manufactured by Bihoku Funka Co., Ltd.) were blended and stirred in a mixer, and then subjected to an extrusion process to obtain a resin composition C for forming a molding support layer.

(Film molding)
The resin composition C was melt-kneaded in an extruder set at 250°C, extruded through a die into a sheet, and cooled to 70°C in a cooling device to obtain a single-layer unstretched film. This unstretched film was reheated to 145°C and then stretched 5 times in the longitudinal direction using the difference in peripheral speed between rolls to obtain a longitudinal uniaxially stretched film.
Next, the above resin compositions A and B were fed into an extruder, and each resin composition was extruded into a two-layer sheet, which was then laminated so that resin composition B was in contact with the above uniaxially stretched film, thereby obtaining a three-layer laminate.
The resulting laminate was reheated to 160°C in an oven and then stretched 9 times in the transverse direction using a tenter stretching machine, followed by heat treatment at 170°C to prepare a recording paper having a three-layer structure consisting of one biaxially stretched layer and two uniaxially stretched layers.
The basis weight of the resulting recording paper was 8 g/m ² for the print-receiving layer, 32 g/m ² for the liquid-absorbing layer, and 56 g/m ² for the support layer.
The resulting recording paper was measured for the liquid absorption rate, liquid absorption amount, and surface strength of the print-receiving layer surface by the following methods. The following evaluations were also carried out. The results are shown in Table 2.

(Liquid absorption speed)
The amount of liquid transferred (V) measured using a liquid dynamic absorbency tester (Bristow Tester Model KM500, manufactured by Kumagai Riki Kogyo Co., Ltd.) in accordance with the Bristow method (Japan Tappi No. 51:2000) was defined as the liquid absorption rate of the print-receiving layer. A test solution prepared by mixing 2% by mass of stamp ink (red) (manufactured by Shachihata Co., Ltd.) as a coloring dye in a mixed solvent of 30% by mass of distilled water and 70% by mass of ethanol was used, and the absorption amount per unit area 500 milliseconds after the test solution was dropped was determined.

(Liquid absorption amount)
The Cobb water absorbency was measured and used as the value of the amount of liquid absorbed in accordance with the provisions of JIS P8140: 1998. However, instead of water, a 70% by mass aqueous ethanol solution was used as the test solvent, and the contact time was 180 seconds.

(Surface strength)
An adhesive tape (manufactured by Nichiban Co., Ltd., product name "Cellotape (registered trademark)", product number: "CT-18") was applied to the surface of the print-receiving layer so as to prevent air from getting in, and the strength at the time of peeling the adhesive tape was measured using an internal bond tester (manufactured by Kumagai Riki Kogyo Co., Ltd.) in accordance with the method described in JAPAN TAPPI No. 18-2.

<Evaluation>
To evaluate the print quality, printing was carried out on the print-receiving layer side of the recording paper by the following method.
An N5 image of JIS X9201:2001 (High-definition color digital standard image (CMYK/SCID)) was printed by inkjet printing on the print-receiving layer surface of the recording paper obtained in each Example and Comparative Example. For printing, an aqueous pigment inkjet printer (model name: TM-C3500, manufactured by Seiko Epson Corporation) and the printer's standard cyan, magenta, yellow, and black aqueous pigment inks (model number: SJIC22) were used. The resulting printed recording paper was used to carry out the following various evaluations. The results are shown in Table 2.

(drying)
The state of the ink on the printed image immediately after printing was visually observed, and the ink drying property was evaluated by pressing a finger onto the printed image as follows. The evaluation criteria were as follows:
A (Excellent): No rubbing, no ink on fingers, completely dry surface.
B (Good): No rubbing and no ink on fingers.
C (Acceptable, lower limit of practical use): Scratches, ink stains on fingers, or the surface is damp.
D (unacceptable, not suitable for practical use): There are scratches, ink on fingers, and the surface is wet.

(image quality)
The printed image was visually observed and evaluated for two items, bleeding and roughness (granular pattern, bubbles), according to the following criteria.
(bleed)
The sharpness of the image contours was judged.
A (Excellent): The image outline is clear.
B (Good): The image outline is slightly blurred.
C (Acceptable, practical lower limit): The outline of the image is blurred.
D (unacceptable, not suitable for practical use): The image outline is unclear.

(Rough)
The density uniformity of the solid image area and the presence or absence of surface defects were evaluated according to the following criteria:
A (good): The density of the solid image portion of the image is constant.
B (Acceptable, practical lower limit): The density of a part of the solid image portion of the image is reduced, but is at a practically acceptable level.
C (unacceptable, not suitable for practical use): The density of a part of the solid image portion of the image is reduced, and is not at a level acceptable for practical use.
D (unacceptable, not suitable for practical use): A granular pattern (bubble formation) occurred in the solid image portion of the image, and the level was not acceptable for practical use.

(ink density)
The density of the solid image area was measured using a spectral color densitometer 530JP (manufactured by X-Rite Co., Ltd.) and the evaluation criteria were as follows:
A: The density of the solid image portion exceeds 1.7.
B: The density of the solid image portion is more than 1.5 and 1.7 or less.
C: The density of the solid image portion is more than 1.3 and 1.5 or less.
D: The density of the solid image portion is more than 1.0 and 1.3 or less.

(Ink fixability)
The ink fixation was evaluated using a color fastness rub tester FR-20 (manufactured by Suga Test Instruments Co., Ltd.) The evaluation conditions were 100 rubs and a load of 400 g, and the image was visually observed after the test.
The evaluation criteria are as follows:
A: The image is not peeled off.
B: Part of the image peeled off, and the density of the image area decreased.
C: The image is partially peeled off, and the surface of the support layer is visible.
D: The image was entirely peeled off, and the surface of the support layer was visible.

(Bookbinding suitability)
After printing, the recording paper was folded 180 degrees with the solid image area facing inward, rubbed with a finger 10 times, and then opened, and the surface of the folded area was visually inspected for any damage. The evaluation criteria were as follows:
A: The entire surface of the folded portion is not torn.
B: Part of the surface of the folded portion is torn, and the support layer is exposed.
C: Half of the surface of the folded portion is torn, and the support layer is exposed.
D: The entire surface of the folded portion is torn, and the support layer is exposed.

[Examples 2 to 11, Comparative Examples 1 to 3]
Recording papers were prepared in the same manner as in Example 1, except that the raw materials for the print-receiving layer and the liquid-absorbing layer were changed as shown in Table 2. The liquid absorption rate, liquid absorption amount, and surface strength were measured and various evaluations were performed in the same manner as in Example 1. The results are shown in Table 2.

As shown in Table 2, the recording paper of the examples achieved both excellent print quality and ink drying by having an absorption rate within a specific range and an absorption amount above a specific value. On the other hand, Comparative Example 1, which had a low absorption rate, resulted in significant image bleeding, while Comparative Example 3, which had an absorption rate that was too high, resulted in rough images, and neither resulted in sufficient image quality. Furthermore, Comparative Example 2, which had a low absorption amount, resulted in insufficient print drying.

Although the present invention has been described in detail and with reference to specific embodiments, it will be apparent to those skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the present invention. This application is based on Japanese patent applications filed on March 29, 2022 (Patent Application No. 2022-054469) and August 26, 2022 (Patent Application No. 2022-135388), the contents of which are incorporated herein by reference.

1 Recording paper 10 Support layer 11 Liquid-absorbing layer 12 Print-receiving layer

Claims (6)

A recording paper having a print-receiving layer and a liquid-absorbing layer,
the liquid-absorbing layer and the print-receiving layer are both porous layers containing a thermoplastic resin,
the print-receiving layer contains a hydrophilic surface-treated filler and a hydrophobic surface-treated filler;
the liquid absorption rate on the surface on the print-receiving layer side is 3.5 to 10 cc/ ^m2 ·0.5 s;
Recording paper with a liquid absorption capacity of 10 cc/ ^m2 ·180 s or more. Recording paper according to claim 1, wherein the surface strength of the surface on the print-receiving layer side is 1.1 kg-cm or more. The recording paper described in claim 1, wherein the liquid-absorbing layer is a layer containing a filler. The recording paper described in claim 1, wherein the filler content in the print-receiving layer is 45 to 65% by mass. The recording paper described in claim 1, wherein the ratio of the hydrophilic surface-treated filler to the hydrophobic surface-treated filler (hydrophilic surface-treated filler/hydrophobic surface-treated filler) is 95/5 to 55/45 by mass. The recording paper described in claim 1, wherein the print-receiving layer further contains a thermoplastic elastomer.