Paper transparentizing agent

Abstract

To provide a solvent-based paper transparentizing agent that does not require UV curing and that has excellent impregnation properties, transparency, and drying properties in a resin composition that is impregnated into opaque paper to allow the underlying printed matter or pattern to be seen through.SOLUTION: A paper transparentizing agent for making paper transparent according to the present invention contains an acrylic oligomer and an aliphatic ester solvent, wherein the weight average molecular weight of the oligomer is 1,000 to 15,000, the ratio of the aliphatic ester solvent to the total solvent is 30 wt.% or more, and the solid content is 25 to 65 wt.%.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a paper clarifying agent that is impregnated into paper to make it transparent. [Background technology]

[0002] Conventionally, one technique for impregnating paper with a resin to make it translucent is to impregnate the paper with a UV-curable ink to make the paper transparent. For example, the applicant previously invented a resin composition for transparent paper with a viscosity of 500 to 3000 mPa·s, which contains a bifunctional EO-modified bisphenol A diacrylate, a multifunctional acrylic monomer, and a photopolymerization initiator (Patent Document 1).

[0003] This composition had good penetration into paper and a very fast curing rate, making it an excellent invention as a photocurable resin composition for transparent paper. However, because such photocurable resin compositions require ultraviolet irradiation, while they have the advantage of high productivity, they also have the disadvantage of requiring capital investment in ultraviolet irradiation equipment. Therefore, there has been a strong demand for solvent-based varnishes that do not require ultraviolet irradiation and can make paper transparent in a relatively short time.

[0004] Patent documents related to such non-reactive clarifying agents include, for example, a paper clarifying agent consisting of a mixture of an alicyclic saturated hydrocarbon resin and at least one hydrocarbon solvent selected from a hydrocarbon solvent and an aliphatic ester solvent (Patent Document 2). However, this paper clarifying agent has poor productivity due to insufficient drying properties, and the film tends to be brittle. Therefore, there has been a demand for a solvent-based clarifying resin that is excellent in impregnation, transparency, and drying properties. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 6793485 [Patent Document 2] Patent No. 5722537 Summary of the Invention [Problem to be solved by the invention]

[0006] An object of the present invention is to provide a solvent-based paper transparentizing agent that does not require ultraviolet curing and that is excellent in impregnation properties, transparency, and drying properties in a resin composition that is impregnated into opaque paper to process it so that an underlying printed matter or pattern can be seen through. [Means for solving the problem]

[0007] In order to achieve the above object, the invention of claim 1 provides a paper clarifying agent for making paper transparent, comprising an acrylic oligomer (A) and an aliphatic ester solvent (B), wherein the weight-average molecular weight of (A) is 1,000 to 15,000, the proportion of (B) in the total solvent is 30% by weight or more, and the solid content is 25 to 65% by weight.

[0008] The invention of claim 2 provides the paper clarifying agent according to claim 1, characterized in that the viscosity at 20°C is 3 to 500 mPa·s.

[0009] The invention of claim 3 provides transparent paper coated or impregnated with the paper transparentizing agent of claim 1 or 2.

[0010] The invention of claim 4 is characterized in that the paper clarifying agent of claim 1 or 2 is applied to paper in an amount of 10 to 40 g / m 2 The present invention provides a method for producing transparent paper, characterized in that the paper is dried after coating or impregnation. [Effects of the Invention]

[0011] The resin composition of the present invention has excellent impregnation properties as well as excellent transparency and drying properties, and is therefore useful as a solvent-based paper clarifying agent that can be applied to opaque paper such as envelopes to make part or all of the paper translucent so that the underlying printed matter or pattern can be seen through. DETAILED DESCRIPTION OF THE INVENTION

[0012] The composition of the present invention comprises an acrylic oligomer (A) and an aliphatic ester solvent (B). In this specification, the term "(meth)acrylate" encompasses both acrylate and methacrylate.

[0013] The acrylic oligomer (A) used in the present invention is a polymer of an acrylic monomer, which is a main component of the clarifying resin. It may or may not have a reactive functional group, but it is preferably non-functional from the viewpoint of the storage stability of the composition and the stability over time after application.

[0014] The weight-average molecular weight (hereinafter referred to as Mw) of (A) is 1,000 to 15,000, preferably 3,000 to 12,000, and more preferably 4,000 to 10,000. If it is less than 1,000, the coating tends to be brittle, while if it exceeds 15,000, penetration into paper tends to be insufficient, resulting in reduced transparency. Mw was measured and calculated by gel permeation chromatography using a column with a styrene-divinylbenzene-based packing material and a tetrahydrofuran eluent, relative to standard polystyrene.

[0015] The glass transition point (hereinafter referred to as Tg) of (A) is preferably 30 to 90° C., more preferably 35 to 80° C., and particularly preferably 40 to 75° C. Tg is a factor that affects the properties of the film, and by setting it to 30° C. or higher, blocking after curing can be sufficiently suppressed, while by setting it to 90° C. or lower, sufficient film flexibility can be ensured.

[0016] The monomer constituting (A) is not particularly limited. Examples of such monomers include linear alkyl monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, n-propyl (meth)acrylate, and n-butyl (meth)acrylate; branched alkyl monomers such as ethylhexyl (meth)acrylate, isodecyl (meth)acrylate, isomistyryl (meth)acrylate, and isostearyl (meth)acrylate; alicyclic monomers such as cyclohexyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentanyl (meth)acrylate, and isobornyl (meth)acrylate; hydroxyl group-containing monomers such as hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, and hydroxybutyl (meth)acrylate; and amide group-containing monomers such as N,N-dimethyl (meth)acrylamide, N,N-diethyl (meth)acrylamide, N,N-dipropyl (meth)acrylamide, and acryloylmorpholine. These monomers may be used alone or in combination of two or more. Among these, it is preferable to use a methacrylate-based monomer having a relatively high Tg and constituting 50 mol % or more of the polymer. Furthermore, the methacrylate-based monomer preferably contains methyl methacrylate (hereinafter referred to as MMA) because of its high Tg and ease of handling and availability.

[0017] The amount of (A) based on the total solid content is preferably 15% by weight or more, more preferably 30% by weight or more, particularly preferably 50% by weight or more, and particularly preferably 80% by weight or more. By adjusting the amount to 15% by weight or more, it is possible to ensure sufficient coating stability and film flexibility.

[0018] The aliphatic ester solvent (B) used in the present invention is an organic solvent obtained by the condensation reaction of a carboxylic acid and an alcohol. It has a high affinity with (A) and moderate volatility, which allows (A) to penetrate the paper easily and ensures sufficient time for penetration, resulting in stable transparency.

[0019] The SP value of (B), which is close to that of (A), is preferably 7 to 11, more preferably 8 to 10, and particularly preferably 8.2 to 9.5. The boiling point of (B), which is related to volatility, is preferably 60 to 150°C, more preferably 70 to 140°C, and particularly preferably 75 to 130°C. Examples include ethyl acetate (SP value 9.1, boiling point 77°C), butyl acetate (SP value 8.5, boiling point 126°C), and propyl acetate (SP value 8.4, boiling point 102°C), which can be used alone or in combination of two or more. Among these, ethyl acetate and butyl acetate are preferred in terms of their compatibility with (A), cost, safety, and availability.

[0020] The solvent other than (B) is not particularly limited, and examples thereof include alcohol-based solvents such as ethanol, n-propyl alcohol, isopropyl alcohol, n-butyl alcohol, isobutyl alcohol, and diacetone alcohol; ketone-based solvents such as acetone, methyl ethyl ketone (hereinafter referred to as MEK), methyl isobutyl ketone, and cyclohexanone; ether-based solvents such as propylene glycol monomethyl ether (hereinafter referred to as PGM), diethyl ether, and diisopropyl ether; and hydrocarbon-based solvents such as cyclohexane and methylcyclohexane, which may be used alone or in combination of two or more.

[0021] The blending ratio of (B) to the total solvent is 30% by weight or more, preferably 50% by weight or more, more preferably 80% by weight or more, and particularly preferably 100% by weight. If it is less than 30% by weight, when mixed with a highly volatile solvent, the penetration time into the paper may not be sufficient, resulting in uneven transparency. Furthermore, when mixed with a less volatile solvent, it may take a long time to dry, which may reduce productivity, or it may become more susceptible to staining, resulting in reduced yield.

[0022] The clarifying agent of the present invention may contain, as necessary, various additives such as a solid content adjuster, a leveling agent, an ultraviolet absorber, an antifoaming agent, a wetting agent, an antiblocking agent, an antioxidant, and a flame retardant, as well as organic fine particles and inorganic fine particles, within a range that does not impair the performance.

[0023] The solid content adjuster refers to a resin that increases the solid content ratio without increasing the viscosity, and examples thereof include alicyclic hydrocarbon resins. Specific examples include petroleum resins, terpene resins, rosin ester resins, and hydrogenated versions of these. These can be used alone or in combination of two or more. Among these, rosin ester resins are preferred because of their good compatibility with (A).

[0024] The softening point of the solids content adjuster is preferably 65 to 150°C, more preferably 75 to 140°C, and particularly preferably 85 to 130°C. By setting the softening point to 65°C or higher, tackiness is less likely to remain in the coating after drying, ensuring sufficient blocking resistance, while by setting the softening point to 150°C or lower, sufficient toughness against bending can be ensured. The softening point can be measured by the ring and ball method in accordance with ASTM D36.

[0025] The amount of the solid content adjuster is preferably 85% by weight or less, more preferably 80% by weight or less, based on the total solid content. Within this range, sufficient toughness against bending can be ensured. Examples of solid content adjusters include Pine Crystal KE-100 and KE-311 (product names: rosin ester resins, manufactured by Arakawa Chemical Industries, Ltd.).

[0026] The solids content of the clarifying agent of the present invention is 25 to 65 wt%, preferably 28 to 62 wt%, more preferably 30 to 60 wt%, and particularly preferably 35 to 55 wt%. If the solids content is less than 25 wt%, the fine voids between the cellulose fibers that make up the paper cannot be sufficiently filled, which tends to make it difficult to form a resin film. If the solids content is more than 65 wt%, the fluidity decreases, and similarly the voids cannot be sufficiently filled, resulting in low total light transmittance and uneven coating.

[0027] The viscosity of the clarifying agent of the present invention at 20° C. is preferably 3 to 500 mPa·s, more preferably 5 to 400 mPa·s, and particularly preferably 10 to 300 mPa·s. A viscosity of 3 mPa·s or more ensures a sufficient amount of coating on the base paper and sufficient control of bleeding, while a viscosity of 500 mPa·s or less ensures sufficient penetration into the substrate, resulting in stable transparency.

[0028] The basis weight of the base paper to which the clarifying agent of the present invention is applied is 20 to 100 g / m 2 is preferable, and 30 to 80 g / m 2 More preferably, 40 to 70 g / m 2 is particularly preferred. 2 By setting the density to 100g / m or more, bleeding can be sufficiently suppressed. 2 The following conditions can ensure sufficient penetration of the clarifying agent. The basis weight of the base paper is measured in accordance with JIS P 8124:2011.

[0029] The density of the base paper to which the clarifying agent of the present invention is applied is 0.50 to 0.95 g / m 3 is preferable, and 0.60 to 0.90 g / m 3 More preferably, 0.65 to 0.85 g / m 3 is particularly preferred. 3 By setting the density to 0.95 g / m or more, bleeding can be sufficiently suppressed. 3 The density of the base paper is measured from the basis weight and paper thickness in accordance with JIS P 8118:2014.

[0030] The method for applying the clarifying agent of the present invention to the base paper may be a known coating method such as gravure printing, flexographic printing, screen printing, roll coating, bar coating, or blade coating, but gravure printing is preferred because it allows for easy multiple printings in a single conveyance. Furthermore, the application to the base paper is preferably carried out from both the front and back sides because this improves permeability and ensures transparency more stably.

[0031] The coating amount of the clarifying agent of the present invention is 50 g / m2 2 In the case of 5 to 50 g / m 2 is preferable, and 10 to 40 g / m 2 More preferably, 15 to 30 g / m 2 is particularly preferred. 2 By increasing the weight to 50g / m or more, transparency is possible. 2 By setting the weight below this, sufficient drying properties can be achieved and the cost of the production process can be reduced. The recommended weight is 0.1 to 1.2 times the basis weight of the base paper.

[0032] After application of the clarifying agent of the present invention, the viscosity may be reduced by increasing the temperature in a drying oven, thereby promoting resin penetration into the substrate. Specific drying conditions include a drying time of approximately 60 seconds in a hot air drying oven set at a temperature of 80°C to 100°C. Under these conditions, no special equipment such as a far-infrared irradiator is required, and the temperature of the paper surface is kept below 100°C, making for safe working conditions, and a transparent appearance can be consistently obtained.

[0033] The present invention will be described in more detail below with reference to examples and comparative examples, but these are intended to be specific examples and are not intended to limit the scope of the present invention. Unless otherwise specified, measurements were carried out at a room temperature of 25°C and a relative humidity of 65%. The blend amounts are expressed in parts by weight as solid content. [Example]

[0034] As the oligomer (A), oligomers 1 to 7 having the monomer structures shown in Table 1 were prepared.

[0035] Table 1 JPEG2025147276000001.jpg53135 BMA: butyl methacrylate, 2-HEMA: 2-hydroxyethyl methacrylate ST: styrene, BZMA: benzyl methacrylate, MAA: methacrylic acid SMA: stearyl methacrylate, MMA: methyl methacrylate

[0036] Examples 1 to 12 The paper transparentizing agents of Examples 1 to 13 were prepared by using oligomers 1 to 6 as (A), butyl acetate and ethyl acetate as (B), KE311 (trade name: Arakawa Chemical Industries, Ltd., rosin ester resin, softening point 90 to 100°C) as a solids content adjuster, and IPA (isopropyl alcohol) as a solvent, in the formulations shown in Tables 2 and 3, and stirring and degassing until uniformly dissolved.

[0037] Comparative Examples 1 to 7 In addition to the materials used in the examples, Oligomer 7, which has a large Mw and does not fall under (A), was used. The paper clarifying agents of Comparative Examples 1 to 7 were prepared by stirring and degassing the formulations shown in Table 4 until uniformly dissolved.

[0038] Table 2 JPEG2025147276000002.jpg82135

[0039] Table 3 JPEG2025147276000003.jpg92135

[0040] Table 4 JPEG2025147276000004.jpg82135

[0041] The evaluation method was as follows.

[0042] Measurement sample The base paper was pure white roll paper Hamayu (product name: Hokuetsu Corporation, 50 g / m²). 2 ) and Kishu Wrap, a bleached kraft paper (product name: Hokuetsu Corporation, 50 g / m2) 2 The resin composition was applied to the substrate using a #20 bar coater and left to dry in an 80°C environment for 3 minutes. The reverse side of the coated substrate was then coated and dried under the same conditions to prepare a measurement sample with two coats.

[0043] Viscosity: Measurements were taken using a Toki Sangyo cone-plate viscometer RC-550 with a cone angle of 3°R17.65 at 25±1°C, at a rotation speed of 100 rpm for viscosities of 200 mPa·s or less, 50 rpm for viscosities of 200 to 800 mPa·s, and 10 rpm for viscosities over 800 mPa·s. Viscosity was rated as ◎ for 10 to 300 mPa·s or less, 〇 for 3 mPa·s to less than 10 mPa·s and 〇 for 300 mPa·s to less than 500 mPa·s, and × for less than 3 mPa·s and 500 mPa·s or greater.

[0044] Total light transmittance: Measured in accordance with JIS K7361-1 using a Hazeguard manufactured by Toyo Seiki Seisakusho, with the following criteria: for one coating, 65% or more was rated as good, 60-65% as fair, and less than 60% as bad. For two coatings, over 70% was rated as good, 65-70% as fair, and less than 65% as bad.

[0045] Coating unevenness: The transparent coating was visually inspected, and a mark of ◯ was given for no unevenness, a mark of △ for slight unevenness, and a mark of × for unevenness.

[0046] Transparency assessment: For the eight evaluation items of total light transmittance and coating unevenness, a rating of ◎ was given when all the evaluations were rated as 〇, a rating of 〇 when there was one △, and an rating of × when there were two or more △ or at least one ×.

[0047] The evaluation results are shown in Tables 5 to 7. Table 5 JPEG2025147276000005.jpg128152

[0048] Table 6 JPEG2025147276000006.jpg137143

[0049] Table 7 JPEG2025147276000007.jpg132157

[0050] The paper clarifying agents of the examples were evaluated to be good in both viscosity and transparency.

[0051] On the other hand, Comparative Example 1, in which the solid content exceeded the upper limit, and Comparative Examples 2, 4, and 5, in which the blending amount of (B) was less than 30 wt%, were judged as × in transparency. Comparative Examples 3 and 6, in which the solid content was below the lower limit, both had low viscosity, Comparative Example 3 had too much bleeding and film formation was not successful, and Comparative Example 6 was judged as × in transparency. Comparative Example 7, in which the Mw of the oligomer exceeded the upper limit, was judged as × in transparency, and all were not suitable for the present invention.

Claims

1. A paper clarifying agent for making paper transparent, comprising: an acrylic oligomer (A) and an aliphatic ester solvent (B), wherein the weight-average molecular weight of (A) is 1,000 to 15,000; the proportion of (B) in the solvent is 30% by weight or more; and the solid content is 25 to 65% by weight.

2. 2. The paper clarifying agent according to claim 1, wherein the viscosity at 20° C. is 3 to 500 mPa·s.

3. 3. Transparent paper coated or impregnated with the paper clarifying agent of claim 1 or 2.

4. The paper clarifying agent of claim 1 or 2 is applied to paper in an amount of 10 to 40 g / m ２ A method for producing transparent paper, characterized by coating or impregnating the paper and then drying the paper.

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