Transparent paper

Abstract

We provide highly transparent paper. [Solution] The solution contains wood pulp with a filtration rate of 60°SR or higher as measured by the Schöpper-Riehller method specified in JIS P8121, and a water-soluble polymer whose B-type viscosity at 25°C (25°C, 60 rpm) as specified in JIS Z8803-1 is 100 mPa·s or higher, prepared to have a concentration of 0.5% by mass.

Description

[Technical Field]

[0001] This invention relates to a transparent paper that has low opacity, high transparency, and fewer defects caused by bundled fibers where pulp fibers are intertwined in clumps within the paper layer. [Background technology]

[0002] In paper manufacturing, it is common practice to improve the paper's texture by uniformly dispersing the pulp. Mechanical vibrations are often used to disperse the pulp. The transparency of paper is achieved by reducing the voids between pulp fibers, thereby suppressing the diffuse reflection of light within the paper. In glassine paper, a type of transparent paper, one method to improve its transparency is to use pulp fibers that have been beaten more intensely. However, this method can lead to problems such as difficulty in papermaking due to reduced dewatering capacity in the wire section, poor texture, and reduced tear strength. Patent Document 1 describes improving transparency by adding amphoteric starch to improve the texture, but the transparency is low, and further transparency is desired. Patent Document 2 describes improving transparency by including colloidal silica and an aqueous resin consisting of at least one of a water-soluble resin and a water-dispersible resin. Neither of the prior art documents describes uniformly dispersing fibers, and there was a need for transparent paper with higher transparency. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2014-196571 [Patent Document 2] Japanese Patent Publication No. 2016-199817 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] However, transparent papers that have been considered highly transparent until now are less transparent than petrochemical-derived films, and there is a demand for transparent papers with even higher transparency. Papers with low basis weight and thin thickness tend to show white, clumpy defects called bundled fibers if the pulp fibers in the papermaking raw materials are not sufficiently dispersed. This is especially true for glassine paper, where bundled fibers are more likely to form because the pulp fibers are strongly beaten, resulting in frequent contact between fibers, and because a large amount of long-fiber softwood kraft pulp is used to ensure paper strength. In addition, combined with the high transparency of glassine paper, the presence of bundled fibers in the paper makes the difference in transparency between the paper and the surrounding area noticeable, resulting in an uneven transparency. Therefore, for glassine paper, it is necessary to improve the dispersion of pulp fibers and reduce the amount of bundled fibers. Therefore, the present invention aims to provide a transparent paper that has high transparency, few binding fibers, and good form (uniform transparency). [Means for solving the problem]

[0005] The present invention provides the following [1] to [4]. [1] Basis weight 40g / m² 2 The following are transparent papers with an opacity of 30% or less: Wood pulp with a filtration efficiency of 60°SR or higher, as measured by the Schöpper-Leighler method specified in JIS P8121, A transparent paper containing a water-soluble polymer whose aqueous solution, prepared to a concentration of 0.5% by mass, has a B-type viscosity (25°C, 60 rpm) of 100 mPa·s or more, as specified in JIS Z8803-1. [2] The transparent paper according to [1], characterized in that the water-soluble polymer is anionic, nonionic, or amphoteric water-soluble polymer. [3] The transparent paper according to [1] or [2], characterized in that the water-soluble polymer is modified guar gum. [4] The transparent paper according to [1] or [2], characterized in that the amount of softwood kraft pulp in the wood pulp is 50% by weight or more of the total wood pulp. [Effects of the Invention]

[0006] According to the present invention, it is possible to provide transparent paper that has high transparency, small size and number of binding fibers, and good form (uniform transparency). Because the transparent paper of the present invention has high transparency and allows for visibility of contents, it can be widely used for various types of packaging, food packaging, pharmaceutical packaging, laminated paper, etc. [Modes for carrying out the invention]

[0007] The transparent paper of the present invention can be obtained by papermaking using a raw material that is a mixture of highly beaten pulp and a water-soluble polymer solution of a specific viscosity. In this invention, by adding a water-soluble polymer of a specific viscosity range to the raw material slurry, the water-soluble polymer adheres to the surface of the cellulose constituting the pulp, dispersing the cellulose fibers. This is thought to result in a uniform dispersion of the pulp, reducing fiber clumping and the size of clumped fibers, improving the texture, and enhancing transparency.

[0008] <Pulp> As the pulp constituting the glassine paper of the present invention, wood pulp such as softwood kraft pulp, hardwood kraft pulp, softwood sulfite pulp, hardwood sulfite pulp, dissolved pulp, and mercerized pulp, as well as non-wood pulp such as flax pulp, Manila hemp pulp, and kenaf pulp can be used. However, in order to improve the transparency of the transparent paper, it is preferable to use pulp with fewer voids between pulp fibers. Specifically, it is preferable to use pulp derived from softwood, and more preferably to use softwood kraft pulp. The content of softwood kraft pulp is preferably 50% by weight or more, more preferably 60% by weight or more, of the total wood pulp. Among softwood kraft pulp, it is preferable to use bleached kraft pulp (BKP).

[0009] The transparent paper of the present invention preferably uses pulp that has been beaten to 60°SR or higher in the Schöpper-Rieger method specified in JIS P8121. More preferably, it is 65°SR or higher. If the pulp is less than 60°SR, sufficient strength (such as tear strength) is achieved, but high transparency cannot be obtained. On the other hand, there is no particular upper limit, but it can generally be set to 95°SR. If the Schöpper-Rieger filtration rate is 95°SR or higher, high transparency can be obtained, but dewatering failure may occur in the wire section, potentially causing operational problems. The method for preparing the pulp's Shopper-Leighler filtration degree is not particularly limited, and a Clafflin, single-disc refiner, double-disc refiner, conical refiner, Niagara beater, etc., can be used.

[0010] <Water-soluble polymer> In the present invention, the water-soluble polymer added to the pulp can be a water-soluble polymer whose aqueous solution viscosity is 100 mPa·s or more when dissolved to a concentration of 0.5% by weight. When a water-soluble polymer has an aqueous solution viscosity of 100 mPa·s or higher, it can improve the dispersibility of pulp fibers in a papermaking raw material slurry when added. Although the principle is not clear, it is presumed that the water-soluble polymer with a certain degree of viscosity adheres to the surface of the pulp fibers, suppressing physical entanglement between the fibers. The water-soluble polymer of the present invention is not particularly limited as long as it has the viscosity described above and does not act as a binder between pulp fibers, but examples include anionic, nonionic, or amphoteric water-soluble polymers. Examples include various guar gums such as modified guar gum, carbomethylated guar gum, amphoteric guar gum, hydroxypropylated guar gum, and cationized guar gum, as well as water-soluble mucopolysaccharides such as hyaluronic acid, water-soluble polysaccharides such as pectin, xylan, glucomannan, tamarind seed gum, tragacanth gum, alginic acid, and carrageenan, various cellulose derivatives such as methylcellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, and hydroxypropylmethylcellulose, various starch derivatives such as modified starch, esterified starch, carboxymethylated starch, and hydroxyethylated starch, polyethylene oxide, polyacrylamide, polyacrylic acid, polyvinylpyrrolidone, polyvinyl alcohol, and polyamines. While not particularly limited, modified guar gum is preferred because it offers improved transparency, is easy to handle, and is highly safe for use in food packaging and pharmaceutical packaging applications.

[0011] In the present invention, the amount of water-soluble polymer added is preferably 0.05 to 1.0% by weight, and more preferably 0.1 to 0.8% by weight, based on 100% by weight of pulp. If the amount added is less than 0.05% by weight, the fibers will not be sufficiently dispersed, and sufficient transparency cannot be obtained. If it is 1.0% by weight or more, problems such as a decrease in the dewatering properties of the wire part will occur.

[0012] <Auxiliary agents, etc.> The transparent paper of the present invention may contain aluminum sulfate, polyamide epichlorohydrin-based wet strength enhancers, etc., from the viewpoint of improving transparency. Preferably, the amount of these additives (as a percentage of the pulp's oven-dry weight) is about 0.01 to 0.5% by weight for aluminum sulfate and about 0.1 to 2.0% by weight for polyamide epichlorohydrin-based wet strength enhancers.

[0013] <Manufacturing of transparent paper> As an example of the method for producing the transparent paper of the present invention, it can be obtained by making a raw material, which is a mixture of beaten pulp and the water-soluble polymer, into paper using a known paper machine. In the present invention, the paper machine is not limited, and a fourdrinier paper machine, cylinder paper machine, short-wire paper machine, combination paper machine and the like can be used. Among these, it is preferable to use a fourdrinier paper machine from the viewpoint that a large dewatering capacity can be obtained in the wire part. As an example of the method for producing the transparent paper of the present invention, it can be obtained by subjecting a sheet formed by papermaking with a paper machine to a hot-pressing treatment in a state containing moisture. The moisture content of the sheet to be subjected to the hot-pressing treatment is not limited, but the moisture content is preferably 20 to 35%, more preferably 25 to 30%. If it is 20% or less, it is difficult to obtain sufficient transparency, and if it exceeds 35%, operational problems such as instability of the moisture profile will occur. The sheet to be subjected to the hot-pressing treatment can be adjusted by drying to the above moisture content range, or can also be adjusted by applying moisture to a sheet that has been dried to below the above moisture content range. From the viewpoint of transparency, it is preferable not to let the moisture content fall below the above range. In addition, the temperature and pressure for the hot-pressing treatment are set to a temperature of 50°C to 120°C and a pressure (total linear pressure) of 20 MPa to 600 MPa. As apparatuses that can be used for the hot-pressing treatment, a super calender, soft calender, hot press roll, multi-stage super calender and the like can be used, and among them, it is preferable to use a multi-stage super calender from the viewpoint that a high pressure can be obtained.

[0014] <Basis Weight of Transparent Paper> The basis weight of the transparent paper of the present invention is preferably 40 g / m 2 or less. When it exceeds 40 g / m 2 , the thickness of the paper increases, the transparency decreases, and the opacity increases. As the lower limit, 15 g / m is preferable in consideration of the practically achievable papermaking range 2 or more.

[0015] <Opacity of Transparent Paper> The opacity of the present invention is preferably 30% or less. When the opacity exceeds 30%, the transparency decreases, which is not preferable. [Examples]

[0016] The present invention will be described below with reference to examples, but the present invention is not limited to the following examples.

[0017] (Example 1) A pulp slurry consisting of 100% by weight of coniferous kraft pulp (NBKP) was beaten using a double disc refiner with the Schöpper-Leighler method specified in JIS P8121 to achieve a filtration degree of 90°SR. To this pulp slurry, an aqueous solution of modified guar gum (manufactured by Sansho Co., Ltd.: Maploid 6700) (viscosity 150 mPa·s, 0.5% by mass) was added so that the modified guar gum solid content was 0.5% by weight relative to the weight of pulp in the slurry to prepare a papermaking slurry. Next, this papermaking slurry was used to manufacture base paper using a wirewheel papermaking machine. Next, the base paper was treated with a supercalender at a temperature of 80°C and a total linear pressure of 35 N·m, resulting in a basis weight of 35.0 g / m². 2 They manufactured transparent paper.

[0018] (Example 2) Transparent paper was produced in the same manner as in Example 1, except that the amount of modified guar gum added was 0.9% by weight relative to the pulp weight.

[0019] (Example 3) A pulp slurry consisting of 75% by weight of softwood kraft pulp (NBKP) and 25% by weight of hardwood kraft pulp (LBKP) was beaten using a double disc refiner with the Schöpper-Leighler method specified in JIS P8121 to achieve a filtration degree of 70°SR. Next, an aqueous solution of polyethylene oxide (PEO-PFZ Red, manufactured by Sumitomo Seika Co., Ltd.) (viscosity 130 mPa·s, concentration 0.08% by mass) was added to the pulp slurry as a water-soluble polymer, so that the solid content of polyethylene oxide in the slurry was 0.2% by weight relative to the weight of pulp, to prepare a papermaking slurry. Then, the papermaking slurry was used to manufacture base paper using a wirewheel papermaking machine. Next, the base paper was treated with a supercalender at a temperature of 80°C and a total linear pressure of 34.3 N·m to obtain a basis weight of 30.0 g / m 2 of transparent paper.

[0020] (Comparative Example 1) Transparent paper was produced in the same manner as in Example 1, except that no modified guar gum was blended.

[0021] (Comparative Example 2) Transparent paper was produced in the same manner as in Example 1, except that instead of modified guar gum, cationic starch, namely oxidized starch (Mermaid C50, manufactured by Shikishima Starch Co., Ltd., viscosity 22.5 mPa·s, 0.5 mass% viscosity) was used.

[0022] <Evaluation> The number of bundled fibers and opacity of the obtained transparent paper were measured by the following methods. The results are shown in Table 1.

[0023] <Measurement of the number of bundled fibers> The produced transparent paper was subjected to transmission inspection under an LED light source using a sheet surface defect inspection apparatus, the detected defect images were visually confirmed, and those recognized as bundled fibers of 1.0 mm in length × width 2 or larger were counted. Using the total number of counted bundled fibers and the formula below, the number of bundled fibers per 1,000 m 2 was determined. N (pieces / 1,000 m 2 ) = n (pieces) ÷ L (m) ÷ W (m) × 1,000 N: Number of generated bundled fibers per 1,000 m 2 n: Total number of bundled fibers in the inspection range L: Length of the inspected sheet W: Width of the inspected sheet

[0024] <Measurement of opacity> Opacity was measured by the method specified in JIS P 8152 using a spectroscopic whiteness meter and color difference meter (PF7000, manufactured by Nippon Denshoku Industries Co., Ltd.).

[0025] ​ [Table 1]

[0026] Examples 1 to 3, which are embodiments of the present invention, yielded transparent paper with fewer binding fibers, lower opacity, and better texture compared to the comparative example.

Claims

1. Basis weight 40 g / m² ２ The following are transparent papers with an opacity of 30% or less: Wood pulp with a filtration efficiency of 60°SR or higher, as measured by the Schöpper-Leighler method specified in JIS P8121, A transparent paper containing a water-soluble polymer whose aqueous solution, prepared to have a concentration of 0.5% by mass, has a B-type viscosity (25°C, 60 rpm) of 100 mPa·s or more, as defined in JIS Z8803-1.

2. The transparent paper according to claim 1, characterized in that the water-soluble polymer is anionic, nonionic, or amphoteric water-soluble polymer.

3. The transparent paper according to claim 1 or 2, characterized in that the water-soluble polymer is modified guar gum.

4. The transparent paper according to claim 1 or 2, characterized in that the amount of softwood kraft pulp in the wood pulp is 50% by weight or more of the total wood pulp.

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