A method for preparing a papermaking filler using a mixed solution of organic fibers and inorganic compounds, and its use.

Abstract

This invention relates to a method for producing a papermaking filler using a mixed solution of organic fibers and inorganic compounds, and to the use thereof. Paper containing the papermaking filler produced by the method according to the present invention has a significantly improved bulk compared to paper using existing pulverized calcium carbonate fillers, and therefore exhibits superior tear length and superior smoothness. These characteristics allow for the production of high-quality paper even when using a larger amount of filler than that used with pulverized calcium carbonate, thereby improving economic efficiency, reducing the amount of organic fibers used, and achieving energy savings during drying.

Description

[Technical Field]

[0001] The present invention relates to a method for preparing a papermaking filler using a mixed solution of organic fibers and inorganic compounds, and to the use thereof. [Background technology]

[0002] The raw materials used to manufacture printing paper include wood pulp, fillers, and other additives. Of these, wood pulp is the main raw material, followed by fillers, which have a high content. Fillers are used to improve paper quality, such as opacity, brightness, and printability. However, because fillers are less expensive than wood pulp, cost savings can be expected by replacing pulp with fillers.

[0003] In recent years, the paper industry has faced a critical challenge in developing technologies for the efficient use of fillers, which are not only cheaper than pulp but also have advantages over pulp in terms of drying load. This is due to the rising prices of both pulp raw materials and oil. However, the use of these fillers is limited because they have drawbacks, such as hindering the formation of hydrogen bonds between fibers and reducing the tensile strength and stiffness of the paper. To overcome these problems, various measures have been explored. Many technologies have been developed and applied to increase the filler content while preventing a decrease in paper strength, including pulp and filler mixtures (fiber-filler composites), prefrocculation, lumen loading, and hybrid calcium carbonate.

[0004] For example, preflocculation technology refers to the technique of mixing calcium carbonate and an ionic polymer to form aggregates of calcium carbonate particles, and then creating stable preflocs of uniform size by forming strong vortices. These preflocs are used as raw materials for papermaking, and when preflocs are added to the raw materials to manufacture paper, it is possible to produce paper with higher tensile strength than paper using ordinary calcium carbonate. However, its use is limited because it does not increase bulk.

[0005] As prior art for manufacturing papermaking fillers, Korean Patent Application Publication No. 2009-0040682 discloses a method for manufacturing paper by applying preflocculation of a filler using amphoteric polyacrylamide, Korean Patent Application Publication No. 2005-0023824 discloses a method for manufacturing paper using cationic starch, and Korean Patent Application Publication No. 2015-1510313 discloses a method for manufacturing hybrid calcium carbonate by preflocculating calcium carbonate and calcium compounds with an ionic polymer, and then injecting carbon dioxide to agglomerate the calcium carbonate and the newly generated calcium carbonate. However, a method for preparing papermaking fillers using a mixed solution of organic fibers and inorganic compounds of the present invention, as well as its use, has not yet been disclosed. [Overview of the project] [Problems that the invention aims to solve]

[0006] The present invention has been devised to meet the above-mentioned needs, and provides a method for preparing a papermaking filler using a mixed solution of organic fibers and inorganic compounds, as well as its use, and the present invention is completed by confirming that paper produced using a filler prepared by the filler preparation method of the present invention has superior bulk and tear length compared to paper produced by conventional preparation methods. [Means for solving the problem]

[0007] To achieve the above objective, the present invention provides a method for preparing a papermaking filler, comprising the steps of: (1) preparing a mixed aqueous solution containing an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100, and having a solid content of 1 to 60% by weight; (2) adding an ionic polymer to the mixed aqueous solution and stirring to form a prefloc; and (3) preparing a papermaking filler by adding 10 to 1000 parts by weight of a calcium compound to 100 parts by weight of the inorganic compound contained in the mixed aqueous solution in which the prefloc has been formed, and then injecting carbon dioxide at 10 to 80°C until the pH is maintained at 7.0 ± 1.0.

[0008] According to embodiments of the present invention, step (1) may include (1-1) preparing a mixed solution having a solid content of 1 to 60% by weight by mixing organic fibers and inorganic compounds in a weight ratio of 1:5 to 100 and then adding water; and (1-2) preparing a mixed aqueous solution having a solid content of 1 to 60% by weight by grinding the mixed solution, which contains an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100.

[0009] In addition, the inorganic compound may be one or more selected from calcium carbonate (CaCO3), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), magnesium carbonate (MgCO3), gypsum, kaolin, calcined clay, talc, perlite, diatomaceous earth, zinc carbonate, lithium carbonate, magnesium hydroxide, and trihydrated alumina.

[0010] In addition, the calcium carbonate may be one or more selected from ground calcium carbonate (GCC) and precipitated calcium carbonate (PCC).

[0011] In addition, the fibrous material may contain one or more selected from cellulose and chitin.

[0012] In addition, the ionic polymer may be one or more selected from anionic polymers and cationic polymers.

[0013] In addition, the anionic polymer may be one or more selected from polysaccharides, proteins, polyvinyl alcohol, polyvinyl acetate, cellulose derivatives, epoxy acrylates, polyesters, polyurethanes, polyester acrylates, polyether acrylates, polyolefin dispersions, polyamides, vinyl copolymers, and polyacrylates.

[0014] In addition, the cationic polymer may be one or more selected from polyamidoamine-epihalohydrin polymers, polyalkyldiallylamine-epihalohydrin polymers, polyethyleneimines, polyacrylamides, polyamines, polyvinylamines, and cationic starches.

[0015] In addition, the amount of ionic polymer added in step (2) may be 0.01 to 10 parts by weight of one or more types of ionic polymers per 100 parts by weight of the mixed aqueous solution.

[0016] In addition, in step (3), the calcium compound may be one or more selected from calcium oxide, calcium hydroxide, calcium sulfate, and calcium phosphate.

[0017] In addition, the present invention provides a papermaking filler having a shape in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fibrous fibrils, wherein the papermaking filler contains 1 to 20 volume percent of fibrous fibrils and 80 to 99 volume percent of metal carbonates (this filler is hereinafter referred to as hybrid flexible calcium carbonate (HFCC)).

[0018] In addition, the present invention provides a method for manufacturing paper, comprising: (1) mixing 1 to 60% by weight of the above-mentioned papermaking filler (HFCC) with 40 to 99% by weight of beaten natural pulp or recycled pulp; and (2) introducing the mixture from step (1) into a paper machine to manufacture paper.

[0019] In addition, the present invention provides a method for producing ultra-high filler-containing paper in which inorganic material accounts for 50% by weight or more of the total weight, the method comprising: (1) mixing 50 to 99% by weight of the above-mentioned papermaking filler (HFCC) with 1 to 40% by weight of beaten natural pulp or recycled pulp, and adding 1 to 30% of an ionic polymer to improve strength; and (2) introducing the mixture from step (1) into a paper machine to produce paper.

[0020] In addition, the present invention provides a method for producing a composite material, comprising: (1) mixing 50 to 99% by weight of the above-mentioned papermaking filler (HFCC) with 1 to 50% by weight of beaten natural pulp or recycled pulp; (2) introducing the mixture from step (1) into a paper machine to produce a sheet in the form of paper; and (3) after step (2), impregnating, spraying, or curtain coating 100 parts by weight of the produced sheet with 50 to 5000 parts by weight of a synthetic polymer or biodegradable polymer to produce a composite material.

[0021] In addition, the present invention provides a method for producing stone paper, comprising: (1) mixing 50 to 99% by weight of the above-mentioned papermaking filler with 1 to 50% by weight of beaten natural pulp or recycled pulp; (2) introducing the mixture from step (1) into a paper machine to produce a sheet in the form of paper; and (3) after step (2), producing stone paper by coating or impregnating 100 parts by weight of the produced sheet with 1 to 50 parts by weight of a synthetic polymer or biodegradable polymer.

[0022] In addition, the present invention provides a paper formed by including a filler, wherein the filler has a structure in which a plurality of fiber fibrils having an average width of 5 nm to 10 μm are present in a central region and a metal carbonate is disposed on the fiber fibrils, and the filler contains 1 to 20% by volume of fiber fibrils and 80 to 99% by volume of metal carbonate.

[0023] In this case, the paper may contain 1 to 60% by weight of the filler (HFCC).

[0024] In addition, the present invention provides a super-high filler-containing paper formed by including a filler, wherein the filler has a structure in which a plurality of fiber fibrils having an average width of 5 nm to 10 μm are present in a central region and a metal carbonate is disposed on the fiber fibrils, and the filler contains 1 to 20% by volume of fiber fibrils and 80 to 99% by volume of metal carbonate.

[0025] In this case, the super-high filler-containing paper may contain the filler (HFCC) in an amount of 50% by weight or more based on the total weight of the super-high filler-containing paper.

[0026] In addition, the present invention provides a composite material formed by including a filler, wherein the filler has a structure in which a plurality of fiber fibrils having an average width of 5 nm to 10 μm are present in a central region and a metal carbonate is disposed on the fiber fibrils, and the filler contains 1 to 20% by volume of fiber fibrils and 80 to 99% by volume of metal carbonate.

[0027] In addition, the present invention provides a stone paper formed by including a filler, wherein the filler has a structure in which a plurality of fiber fibrils having an average width of 5 nm to 10 μm are present in a central region and a metal carbonate is disposed on the fiber fibrils, and the filler contains 1 to 20% by volume of fiber fibrils and 80 to 99% by volume of metal carbonate.

Advantages of the Invention

[0028] This invention relates to a method for preparing a papermaking filler using a mixed solution of organic fibers and inorganic compounds, and to the use thereof. Paper containing the papermaking filler prepared by the preparation method according to the present invention has significantly improved bulk, superior tear length, and excellent smoothness compared to paper using conventional crushed calcium carbonate fillers. By utilizing these characteristics, it becomes possible to produce paper of superior quality even when using a larger amount of filler than that used for crushed calcium carbonate, thus offering superior economic feasibility and the benefits of reducing the amount of organic fibers used and drying energy. [Brief explanation of the drawing]

[0029] [Figure 1] This is a process flowchart illustrating the process of manufacturing paper using a papermaking filler prepared according to the present invention. [Figure 2] This is a comparison of the bulk (A), tear length (B), and internal bonding strength (C) of the paper prepared in Examples 4-9 and Comparative Examples 1-4 of the present invention. [Figure 3] Figure 3A shows the initial state of the carbon dioxide reaction, Figure 3B shows the intermediate state of the carbon dioxide reaction, and Figure 3C shows the final state of the carbon dioxide reaction. [Modes for carrying out the invention]

[0030] The present invention relates to a method for preparing papermaking filler (HFCC), comprising the steps of: (1) preparing a mixed aqueous solution containing an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100, and having a solid content of 1 to 60% by weight; (2) adding an ionic polymer to the mixed aqueous solution and stirring to form a prefloc; and (3) preparing a papermaking filler by adding 10 to 1000 parts by weight of a calcium compound to 100 parts by weight of the inorganic compound contained in the mixed aqueous solution in which the prefloc has been formed, and then injecting carbon dioxide at 10 to 80°C until the pH is maintained at 7.0 ± 1.0.

[0031] In this case, step (1) may also be carried out by a method comprising: (1) preparing a separate mixed aqueous solution containing an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100; (2) preparing by mixing an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100; or (3) preparing a mixed aqueous solution having the same size and weight ratio by mixing and grinding organic fibers and an inorganic compound.

[0032] According to embodiments of the present invention, step (1) may include (1-1) preparing a mixed solution by mixing organic fibers and an inorganic compound and then adding water, and (1-2) preparing a mixed aqueous solution containing an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100, and having a solid content of 1 to 60% by weight, by grinding the mixed solution.

[0033] In this case, in step (1-1), the organic fibers and inorganic compounds may be mixed in a weight ratio of 1:5 to 100, and the mixed solution may have a solid content of 1 to 60% by weight.

[0034] In step (1-2), the step of grinding the mixed solution of organic fibers and inorganic compounds not only reduces the size of the inorganic compounds to an appropriate level, but also allows the organic fibers to be fibrillated with low energy by friction with the inorganic compounds in a strongly alkaline state due to hydration of the inorganic compounds. Therefore, compared to methods of preparing and using microfibrils or nanofibrils separately, the energy consumption in the preparation of organic fiber fibrils can be significantly reduced. Grinding is preferably carried out using a grinder or a ball mill, but is not limited to these.

[0035] In steps (2) and (3), a prefloc is prepared using the mixed inorganic compound and organic fiber fibrils, and the reaction is carried out in water by adding calcium oxide and injecting carbon dioxide, thereby depositing the metal carbonate onto the surface of the prefloc, and thus a robust, elongated metal carbonate can be prepared.

[0036] In the present invention, preflocculation means transforming inorganic compound particles and organic fiber fibrils into aggregates by treating them with a coagulant and a flocculating agent.

[0037] The stirring in step (2) is preferably carried out at 500 to 5,000 rpm. If stirring is carried out at less than 500 rpm, the size of the aggregate particles formed becomes very large, which leads to a problem where the physical properties of the paper deteriorate when these particles are used to manufacture paper. If stirring is carried out at more than 5,000 rpm, the particle size becomes too small, which is ineffective.

[0038] The particle size of the pre-floc formed in step (2) can be 1 to 100 μm.

[0039] If the particle size of the prefloc formed in step (2) is less than 1 μm, there is a problem that the size of the prefloc is so small that it does not have the effect of improving the physical properties of the paper manufactured using it. If the particle size exceeds 100 μm, there is a problem that it is difficult to uniformly distribute the filler when manufacturing paper using it.

[0040] The inorganic compound may contain one or more selected from calcium carbonate (CaCO3), calcium oxide (CaO), calcium hydroxide (Ca(OH)2), magnesium carbonate (MgCO3), gypsum, kaolin, calcined clay, talc, perlite, diatomaceous earth, zinc carbonate, lithium carbonate, magnesium hydroxide, and trihydrated alumina, preferably a metal carbonate containing one or more selected from calcium carbonate, magnesium carbonate, aluminum carbonate, lithium carbonate, and zinc carbonate, and more preferably calcium carbonate, but is not limited thereto.

[0041] Calcium carbonate is preferably one or more selected from pulverized calcium carbonate (GCC) and precipitated calcium carbonate (PCC), but is not limited thereto. The fibrous material preferably includes one or more selected from cellulose and chitin, but is not limited thereto.

[0042] The ionic polymer is preferably one or more ionic polymers selected from anionic polymers and cationic polymers, but is not limited to these.

[0043] The anionic polymer is preferably one or more selected from polysaccharides, proteins, polyvinyl alcohol, polyvinyl acetate, cellulose derivatives, epoxy acrylates, polyesters, polyurethanes, polyester acrylates, polyether acrylates, polyolefin dispersions, polyamides, vinyl copolymers, and polyacrylates, but is not limited thereto. The cationic polymer is preferably one or more selected from polyamidoamine-epihalohydrin polymers, polyalkyldiallylamine-epihalohydrin polymers, polyethyleneimines, polyacrylamides, polyamines, polyvinylamines, and cationic starches, but is not limited thereto.

[0044] The amount of ionic polymer added in step (2) is preferably 0.01 to 10 parts by weight of ionic polymer, more preferably 0.1 to 0.3 parts by weight of ionic polymer, per 100 parts by weight of the mixed aqueous solution, but is not limited to these amounts.

[0045] In step (3), the calcium compound is preferably one or more calcium compounds selected from calcium oxide, calcium hydroxide, calcium sulfate, and calcium phosphate, and more preferably calcium oxide or calcium hydroxide, but is not limited to these.

[0046] When calcium hydroxide is used as a calcium compound, calcium hydroxide prepared by reacting sodium hydroxide with calcium chloride may be used. That is, since calcium hydroxide can be formed by reacting sodium hydroxide with calcium chloride, calcium carbonate can be formed by reacting it with carbon dioxide without directly adding calcium hydroxide.

[0047] In step (3), calcium contained in the calcium compound component is dissolved in the form of a salt such as calcium hydroxide in the dilution solution in which the prefloc is formed, and calcium carbonate can be synthesized by a carbonation reaction by injecting carbon dioxide therein. In this case, it is preferable to react the carbon dioxide until the pH of the dilution solution generally reaches 7.0, and the reaction must be continued until the injected carbon dioxide stops reacting. In addition, in step (3), the temperature at which the dilution solution is reacted by injecting carbon dioxide is preferably 10 to 80°C.

[0048] In addition, the present invention provides a papermaking filler having a shape in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fibrous fibrils, wherein the filler contains 1 to 20 volume percent of fibrous fibrils and 80 to 99 volume percent of metal carbonates.

[0049] Because the filler has a structure in which metal carbonates are arranged on fibrous fibrils, when forming paper containing the filler, it may be more advantageous in that the bulk of the paper is greatly improved, it has excellent tear length, and it has excellent smoothness. In addition, because the filler satisfies the content range of fibrous fibrils and metal carbonates, when forming paper containing the filler, it may be more advantageous in that the bulk of the paper is greatly improved, it has excellent tear length, and it has excellent smoothness.

[0050] In addition, the present invention provides a method for manufacturing paper, comprising: (1) mixing 1 to 60% by weight of the above-mentioned papermaking filler (HFCC) with 40 to 99% by weight of beaten natural pulp or recycled pulp; and (2) introducing the mixture from step (1) into a paper machine to manufacture paper.

[0051] In addition, the present invention provides a method for producing ultra-high filler-containing paper in which inorganic material accounts for 50% by weight or more of the total weight, the method comprising: (1) mixing 50 to 99% by weight of the above-mentioned papermaking filler with 1 to 50% by weight of beaten natural pulp or recycled pulp, and adding 1 to 30% of an ionic polymer to improve strength; and (2) producing ultra-high filler-containing paper by introducing the mixture from step (1) into a paper machine.

[0052] In addition, the present invention provides a method for producing a composite material, comprising: (1) mixing 50 to 99% by weight of the above-mentioned papermaking filler with 1 to 50% by weight of beaten natural pulp or recycled pulp; (2) introducing the mixture from step (1) into a paper machine to produce a sheet in the form of paper; and (3) after step (2), impregnating, spraying, and / or curtain coating 100 parts by weight of the produced sheet with 50 to 5000 parts by weight of a synthetic polymer or biodegradable polymer to produce a composite material.

[0053] Composite materials are materials that exhibit new properties different from the original materials by combining two or more materials with different characteristics. The difference from synthetic materials is that the constituent components remain separated and unchanged. Composite materials can replace metal fittings or metal sheets in aircraft or automobiles, and their uses are endless, including mobile phone cases, tennis rackets, golf clubs, industrial pipes or tanks, with continuous development of new applications. In this invention, synthetic polymers or biodegradable polymers are added to metal carbonates, resulting in improved strength and dimensional stability, making them suitable for a wide range of applications.

[0054] In addition, the present invention provides a method for producing stone paper, comprising: (1) mixing 50 to 99% by weight of the above-mentioned papermaking filler with 1 to 50% by weight of beaten natural pulp or recycled pulp; (2) introducing the mixture from step (1) into a paper machine to produce a sheet in the form of paper; and (3) after step (2), producing stone paper by coating or impregnating 100 parts by weight of the produced sheet with 1 to 50 parts by weight of a synthetic polymer or biodegradable polymer.

[0055] Although it uses the name "paper," stone paper can replace paper or plastic. Stone paper is strong and waterproof, yet it decomposes quickly after use like biodegradable plastic. It can be processed into various shapes and folded, and its printability is superior to that of ordinary paper.

[0056] Currently manufactured stone paper has a density of 1.0 or higher, which is a disadvantage as it is heavy when used to produce books or printed materials. However, the stone paper produced using metal carbonates according to the present invention has a density of 1.0 or lower, making it comparable to ordinary paper. Furthermore, it possesses high hydrophobicity, high printability, and biodegradability, making it an inexpensive material that can replace paper made from non-biodegradable plastics or wood.

[0057] In addition, the present invention provides a paper formed with a filler, wherein the filler has a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fibrous fibrils, and the filler comprises 1 to 20 volume percent of fibrous fibrils and 80 to 99 volume percent of metal carbonates.

[0058] In this case, the paper may contain 1 to 60% by weight of filler (HFCC).

[0059] In addition, the present invention provides an ultra-high filler-containing paper formed with a filler, wherein the filler has a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fibrous fibrils, and the filler contains 1 to 20 volume percent of fibrous fibrils and 80 to 99 volume percent of metal carbonates.

[0060] In this case, ultra-high filler paper contains filler (HFCC) in an amount of 50% by weight or more, based on the total weight of the ultra-high filler paper.

[0061] In addition, the present invention provides a composite material characterized in that the composite material is formed by including a filler, wherein the filler has a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region and metal carbonates are arranged on the fibrous fibrils, and the filler contains 1 to 20 volume percent of fibrous fibrils and 80 to 99 volume percent of metal carbonates.

[0062] In addition, the present invention provides a stone paper characterized in that the stone paper is formed by including a filler, the filler having a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region and metal carbonates are arranged on the fibrous fibrils, and the filler comprises 1 to 20 volume percent of fibrous fibrils and 80 to 99 volume percent of metal carbonates.

[0063] Paper and ultra-high filler-containing paper may be more advantageous in that they have significantly improved bulk, excellent tear length, and superior smoothness due to the use of fillers (HFCC).

[0064] In addition, since the filler meets the weight range of fibrous fibers and metal carbonates, it may be more advantageous in terms of the production of bio-composites and stone paper.

[0065] Modes for carrying out the invention The present invention will be described in more detail below using examples. It will be clear to those skilled in the art that these examples are provided solely to illustrate the present invention in more detail, and that the scope of the present invention is not limited thereto.

[0066] Example 1. Preparation of papermaking filler 1 Step 1-1: A mixed solution was prepared by adding 2 g of organic fiber and 40 g of calcium oxide (CaO. Taekyung Industrial., approximately 10 μm in diameter) to 200 ml of water.

[0067] Organic fibers were used after beating bleached hardwood chemical pulp to the standard filtration water level of 200 ml CSF. The average width of the pulp was confirmed to be approximately 15.5 μm.

[0068] Step 1-2: The mixed solution prepared in Step 1-1 was placed in a ball mill and processed for 1 hour. After the ball milling process was complete, the size of the calcium oxide and the width of the organic fiber fibrils were measured using an electron microscope. As a result, it was confirmed that the average diameter of the calcium oxide was 3.1 μm and the average width of the organic fibers was approximately 678 nm.

[0069] Step 2: To 100 parts by weight of the total solids in the mixed aqueous solution of calcium oxide and organic fiber fibrils after ball milling, 0.1 parts by weight of a cationic polymer, polyacrylamide (PAM, Ciba Chemical Korea), was added to impart weak cationic properties. Then, while a vortex was formed, 0.1 parts by weight of an anionic polymer, micropolymer (Eka Chemical Korea), was added to 100 parts by weight of the total solids so that the overall zeta potential, i.e., the charge, remained neutral. Here, the exact amount of ionic polymer added was based on the zeta potential. The diluted solution was then stirred at a rotation speed of 2,000 rpm for 10 minutes to form a pre-floc.

[0070] Step 3: After Step 2, 5 g of calcium oxide was added to the mixed aqueous solution, and then carbon dioxide was continuously injected into the diluted solution while stirring at 2000 rpm at 30°C until the pH reached 7.0, thereby preparing the papermaking filler. The reaction endpoint was also set at the point where the carbon dioxide injection rate and the carbon dioxide discharge rate were equal.

[0071] In this case, a total of 45 g of calcium oxide reacted to form 80 g of precipitated calcium carbonate, which in turn formed a filler attached to 2 g of cellulose fibril (cellulose fibril:precipitated calcium carbonate = 1:40).

[0072] Example 2. Preparation of papermaking filler 2 Step 1-1: A mixed solution was prepared by adding 2 g of organic fiber and 40 g of pulverized calcium carbonate (GCC, Omyakorea Co., Ltd., diameter 5 μm or larger) to 200 ml of water.

[0073] Organic fibers were used after beating bleached hardwood chemical pulp to the standard filtration water level of 200 ml CSF. The average width of the pulp was confirmed to be approximately 15.5 μm.

[0074] Step 1-2: The mixed solution prepared in Step 1-1 was placed in a ball mill and processed for 1 hour. After the ball milling process was complete, the size of the pulverized calcium carbonate and the width of the organic fiber fibrils were measured using an electron microscope. As a result, it was confirmed that the average diameter of the calcium carbonate was 3.2 μm and the average width of the organic fibers was approximately 825 nm.

[0075] Step 2: To 100 parts by weight of the total solids in the mixed aqueous solution of pulverized calcium carbonate and organic fiber fibrils after ball milling, 0.1 parts by weight of a cationic polymer, polyacrylamide (PAM, Ciba Chemical Korea), was added to impart weak cationic properties. Then, while a vortex was formed, 0.1 parts by weight of an anionic polymer, micropolymer (Eka Chemical Korea), was added to 100 parts by weight of the total solids, so that the overall zeta potential, i.e., the charge, remained neutral. Here, the exact amount of ionic polymer added was based on the zeta potential.

[0076] Subsequently, the diluted solution was stirred at a rotational speed of 2,000 rpm for 10 minutes to form pre-flocs of pulverized calcium carbonate-cellulose fibril.

[0077] Step 3: To the pre-floc formed in Step 2, 22.5 g of calcium oxide was added under a vortex of 2,000 rpm at 30°C. Then, carbon dioxide was continuously injected into the diluted solution until the pH reached 7.0 to prepare the papermaking filler. Here, the reaction endpoint could also be set as the point where the carbon dioxide injection rate and the discharge rate were equal. In this case, 40 g of pulverized calcium carbonate and 40 g of newly generated precipitated calcium carbonate formed a filler (HFCC) attached to the surface of cellulose fibrils.

[0078] Example 3. Preparation of papermaking filler 3 Step 1-1: A diluted mixture was prepared by adding 5.6 g of calcium oxide used in Step 1-1 of Example 1, 30 g of pulverized calcium carbonate used in Step 1-1 of Example 2, and 2 g of organic fiber to 200 ml of water.

[0079] Steps 1-2: Grinding was performed using a ball mill, as in Steps 1-2 of Example 2. After grinding, the average width of the cellulose fibrils was confirmed to be 782 nm.

[0080] Step 2: Prefloc was formed using the same method as in Step 2 of Example 2.

[0081] Step 3: In the same manner as in Step 3 of Example 2, 16.9 g of calcium oxide was added to the total amount of calcium oxide, excluding the 5.6 g added in Step 1-1, to bring the total amount of calcium oxide to 22.5 g. Carbon dioxide was then injected into the pulverized calcium carbonate at 30°C, and this was attached to the cellulose fibrils together with the newly generated precipitated calcium carbonate to prepare the filler. The reason for adding calcium oxide to the divided portion in this manner is to maintain an alkaline state during the ball milling process in order to assist in the fibrillation of the organic fibers.

[0082] Example 4. Preparation of paper containing papermaking filler 1 Step 1: As pulp for papermaking, bleached softwood pulp and bleached hardwood pulp were mixed in a ratio of 2:8, and beaten to achieve a water-free filtration rate of 500 ml CSF (Canadian Standard Water-Free Filtration). Then, the papermaking filler and pulp prepared in Example 1 were mixed in a weight ratio of 25:75.

[0083] Step 2: Using the papermaking filler and pulp mixed in Step 1, produce 60 g / m² of paper according to the paper testing method (ISO 5269 / 1). 2 Paper with the following basis weight was manufactured (Figure 1).

[0084] Example 5. Preparation of paper containing papermaking filler 2 Paper was produced under the same conditions as in Example 4, except that in Step 1 of Example 4, the papermaking filler and pulp prepared in Example 1 were mixed in a weight ratio of 35:65.

[0085] Example 6. Preparation of paper containing papermaking filler 3 Paper was produced under the same conditions as in Example 4, except that the papermaking filler prepared in Example 2 was used in Step 1 of Example 4.

[0086] Example 7. Preparation of paper containing papermaking filler 4 Paper was produced in the same manner as in Example 4, except that in Step 1 of Example 4, the papermaking filler and pulp prepared in Example 2 were mixed in a weight ratio of 35:65.

[0087] Example 8. Preparation of paper containing papermaking filler 5 Paper was manufactured in the same manner as in Example 4, except that the papermaking filler used in Step 1 of Example 4 was the same as the papermaking filler prepared in Example 3.

[0088] Example 9. Preparation of paper containing papermaking filler 6 Paper was produced in the same manner as in Example 4, except that in Step 1 of Example 4, the papermaking filler and pulp prepared in Example 3 were mixed in a weight ratio of 35:65.

[0089] Comparative Example 1. Step 1: As pulp for papermaking, bleached softwood pulp and bleached hardwood pulp were mixed in a 2:8 ratio and beaten to achieve a filtration rate of 500 ml CSF (Canadian Standard Filtration).

[0090] Step 2: Mix the crushed calcium carbonate (GCC, Omyakorea Co., Ltd. 2-3 μm) as filler with the pulp beaten in Step 1 in a weight ratio of 25:75, and measure 60 g / m² according to the paper testing method (ISO 5269 / 1). 2 Paper with a basis weight was manufactured.

[0091] Comparative Example 2. Paper was produced under the same conditions as in Comparative Example 1, except that the filler and pulp were mixed in a weight ratio of 35:65 in Step 2 of Comparative Example 1.

[0092] Comparative Example 3. Paper was manufactured under the same conditions as in Comparative Example 1, except that precipitated calcium carbonate (PCC, manufactured by Artone Paper, approximately 1.5 μm) was used instead of pulverized calcium carbonate in step 2 of Comparative Example 1.

[0093] Comparative Example 4. Paper was produced under the same conditions as in Comparative Example 1, except that in Step 2 of Comparative Example 1, precipitated calcium carbonate (PCC, manufactured by Artone Paper, approximately 1.5 μm) was used instead of pulverized calcium carbonate, and the filler and pulp were mixed in a weight ratio of 35:65.

[0094] The GCC fillers used in Comparative Examples 1 and 2 are fillers primarily used in general fine paper factories, while the PCC fillers used in Comparative Examples 3 and 4 are high-grade fillers used to increase bulk and brightness compared to GCC.

[0095] Experiment Example 1: Analysis of Paper Characteristics To analyze the characteristics of paper manufactured containing the papermaking filler according to the present invention, the following experiment was conducted.

[0096] (1) Analysis of paper volume and tear length To evaluate the effectiveness of the paper prepared in the examples and comparative examples of the present invention, the bulk and tear length of the prepared paper were analyzed. The basis weight of the paper prepared in Examples 4-9 and Comparative Examples 1-4 was 60 g / m². 2 In addition, bulk and tear length were analyzed. Bulk is the density of the paper (unit: g / cm³). 3 ) is measured, and then the reciprocal (in cm units). 3 The value was obtained by taking ( / g). The tear length (in km) is a value obtained by dividing the tensile strength (in kN / m) by the basis weight of the paper. This value eliminates the effect of basis weight deviations when comparing tensile strengths, and can be said to be a more reliable value when comparing strengths.

[0097] As a result, as shown in Table 1 below, the bulk of the paper prepared with GCC (ground calcium carbonate) in Comparative Example 1 and Comparative Example 2 was 1.71 and 1.68, respectively, which is much lower than the bulk of the paper prepared in Examples 4-9 (1.86-1.92). This resulted in a very large difference in thickness and bending stiffness, which are important physical properties of actual paper. In particular, since bending stiffness is proportional to the cube of the paper thickness, a very large difference occurs in bending stiffness (Table 1).

[0098] Increasing the filler content reduces bending stiffness, which is a key reason why the filler content cannot be increased. However, the filler according to the present invention was confirmed to exhibit very high bending stiffness (the bending stiffness of Examples 4-9 was 550-680, while the bending stiffness of Comparative Examples 1 and 2 was 150-250).

[0099] In Comparative Examples 3 and 4, the bulk of the paper prepared using PCC (precipitated calcium carbonate) was 1.84 and 1.83, respectively, which is higher than that of pulverized calcium carbonate and shows a bulk value similar to that of Examples 4 to 9.

[0100] However, it can be confirmed that precipitated calcium carbonate (PCC) has a significantly lower tensile strength compared to Examples 4-9 (Table 1 and Figure 2).

[0101] In addition, as shown in Table 1 below, the tear lengths of the paper produced using pulverized calcium carbonate (GCC) in Comparative Examples 1 and 2 were 2.18 when 25% filler was added and 1.84 when 35% was added. The tear lengths of the paper produced in Examples 4 to 9 were 3.13 to 3.53 when 25% filler was added and 3.07 to 3.14 when 35% filler was added. Therefore, it was confirmed that the tear lengths of Examples 4 to 9 of the present invention were significantly higher than those of Comparative Examples 1 to 2, under the same filler content.

[0102] For paper produced using precipitated calcium carbonate (PCC), the tear length was 1.88 when 25% filler was added and 1.65 when 35% was added. This was lower than in Examples 4-9, and even lower in Comparative Examples 1 and 2, which used pulverized calcium carbonate (GCC), at the same filler content.

[0103] Therefore, it was confirmed that paper produced using the papermaking filler according to the present invention has high bulk and tear length. Figure 2 compares the bulk, tear length, and internal bond strength when 25% and 35% filler are added. It was confirmed that the bulk, tear length, and internal bond strength of Examples 4 to 9 were all higher than when pulverized calcium carbonate or precipitated calcium carbonate was used as the filler.

[0104] (2) Analysis of paper brightness and smoothness To evaluate the effectiveness of the papers prepared in the examples and comparative examples of the present invention, the brightness and smoothness of the prepared papers were analyzed. Specifically, brightness analysis was performed using the ISO 2470 method, which is a method for measuring the brightness of paper, and smoothness was measured using the Bekk smoothness method TAPPI T479 cm-99.

[0105] As a result, as disclosed in Table 1 below, Comparative Examples 1 and 2, using powdered calcium carbonate, showed an average brightness of 86.3%. Comparative Examples 3 and 4, using precipitated calcium carbonate, showed an average brightness of 88.8%. In other words, precipitated calcium carbonate showed significantly higher brightness than pulverized calcium carbonate. The average brightness of Examples 4 and 5 was 88.5%, which was higher than that of pulverized calcium carbonate and at a similar level to precipitated calcium carbonate. However, the average brightness of Examples 6 and 7 was 87.3%, which was lower than that when using precipitated calcium carbonate alone. The average brightness of Examples 8 and 9 was 88.8%, which was at a similar level to the brightness of precipitated calcium carbonate, and this result was judged to be mainly due to the effect of precipitated calcium carbonate being synthesized and adhering to the filler surface of Examples 8 and 9. In the case of Examples 6 and 7, it was judged to be due to the presence of a significant amount of pulverized calcium carbonate.

[0106] It has been found that paper containing HFCC, a papermaking filler according to the present invention, has superior bulk, tear length, and bending stiffness compared to paper made with crushed calcium carbonate or precipitated calcium carbonate, while maintaining brightness and smoothness. [Table 1]

[0107] (3) Evaluation of the shape and composition of the filler By observing the papermaking fillers prepared in Examples 1-3 with an electron microscope during the intermediate filler formation process, we were able to confirm the process by which calcium carbonate adheres to and forms in the central region of multiple fiber fibrils, as shown in Figure 3. As can be seen in Figure 3, it was found that multiple fiber fibrils are involved when calcium carbonate is formed on the fiber fibrils, and when sufficient calcium carbonate adheres, the appearance of the fiber fibrils is no longer visible. As a result, it was confirmed that the papermaking fillers prepared in Examples 1-3 have multiple fiber fibrils present in the central region and have a shape in which calcium carbonate is attached to the fiber fibrils.

[0108] Furthermore, the content of fibrous fibrils and metal carbonates was measured using the standard method TAPPI T211 om-02 (ash in wood, pulp, and cardboard: combustion at 525°C). Specifically, water was removed from the filler samples using the prepared calcium carbonate metal salt at 105°C, and the total weight (A) was measured. After being treated again at 525°C for 1 hour, it was cooled and its weight (B) was measured. At this point, the calcium carbonate remains, but all the fibrous fibrils are removed as water and carbon dioxide. The value (AB) is the weight of fibrous fibrils, and B is the weight of calcium carbonate.

[0109] In addition, the content of fibrous fibrils and metal carbonates in the fillers prepared in Examples 1-3 was measured, and the results are shown in Table 2 below. It is known that the specific gravity of calcite-type calcium carbonate is 2.7, and the specific gravity of cellulose is 1.5. Therefore, if the weights of calcium carbonate and fibrous fibrils are known, their volumes can be calculated. [Table 2]

[0110] As can be seen from Table 2 above, Examples 1 to 3 of the present invention satisfy the range of fiber fibril and metal carbonate content according to the present invention, and therefore, it can simultaneously exhibit all of the effects of improved bulk, excellent break length and smoothness as shown in Table 1 above.

[0111] <Preparation of ultra-high filler content paper> Step 1: To produce ultra-high filler paper, bleached softwood pulp and bleached hardwood pulp were mixed in a 5:5 ratio and beaten to achieve a water-free filtration rate of 300 ml CSF (Canadian Standard Water-Free Filtration). Then, the papermaking filler and pulp prepared in Example 1 were mixed in a weight ratio of 60:40, and this was referred to as "Ultra-High Filler Paper 1" stock. In addition, papermaking filler and pulp were mixed in a weight ratio of 70:30, and this was referred to as "Ultra-High Filler Paper 2" stock.

[0112] Step 2: To each of the “ultra-high filler-containing paper 1” stock and the “ultra-high filler-containing paper 2” stock mixed in Step 1, 5 wt% of an epoxy resin based on the solids content was added, and paper having a basis weight of 60 g / m 2 was produced according to the paper testing method (ISO 5269 / 1). The physical properties of the produced ultra-high filler-containing paper are shown in Table 3 in comparison with the physical properties of Comparative Example 2. The content of the filler contained in the paper was calculated by comparing the residual solids after reacting the paper at 525° C. for 1 hour with the initial paper weight, and is shown in Table 3 as ash content. The bulk, breaking length, and smoothness of the ultra-high filler-containing paper to which 50% or more of the filler was added were significantly superior to those of Comparative Example 2. Therefore, it was confirmed that the filler prepared in Example 1 can be used for the preparation and use of ultra-high filler-containing paper.

Table 3

[0113] <Preparation Example 2: Preparation of Composite Material> Step 1: In order to prepare a sheet for producing a bio-composite material, beating was performed on bleached softwood pulp so that the freeness became 300 ml CSF (Canadian Standard Freeness), and then it was mixed with the papermaking filler prepared in Example 1 at a weight ratio of 10:90 (beaten bleached softwood pulp: filler of Example 1). To this stock, 8 wt% of an epoxy resin based on the solids content was added, and a paper test piece having a basis weight of 80 g / m 2 was prepared according to the paper testing method (ISO 5269 / 1).

[0114] Step 2: After melting PLA at 220 to 240° C., curtain coating was performed on the paper prepared in Step 1 at 350 g / m 2 to prepare a “bio-composite material”. For comparison, 430 g / m 2A sheet was prepared using only PLA, and this was referred to as "bio-composite PLA." The nozzle temperature was maintained at 240°C. In the case of paper test pieces, coating was performed when the temperature rose to 100°C, and a vacuum plate was applied to the opposite side of the coated surface, maintaining a vacuum of 0.6 atm to adjust the effective internal penetration of the PLA.

[0115] Step 3: After the surface of the paper test piece, which had been thoroughly impregnated with PLA, was smoothed using a roller, the bio-composite material was completed.

[0116] The weight and density of the prepared bio-composite were measured at room temperature to calculate the PLA content, and the calcium carbonate content was determined by measuring the weight after treatment at 525°C for 3 hours. In addition, the tensile strength was measured and is shown in Table 4. [Table 4]

[0117] Table 4 shows that the bio-composite PLA was so brittle that its tensile strength could not be measured. On the other hand, the bio-composite recorded an elongation of 2.3%, which is the strain at the fracture point under tensile force. The bio-composite PLA recorded an elongation of 0.0, indicating no tensile stress at all. The tensile strength of the bio-composite was recorded as 34.7 MPa, which was not significantly different from that of a typical PLA composite.

[0118] <Preparation Example 3: Preparation of Stone Paper> Step 1: To prepare a sheet for making stone paper, bleached softwood pulp was beaten to a water-filtration rate of 300 ml CSF (Canadian Standard Water-Filtration). This was then mixed with the papermaking filler prepared in Example 1 in a weight ratio of 10:90 (beaten bleached softwood pulp: filler from Example 1). To this stock, 8% by weight of epoxy resin relative to the solid content was added, and the paper was made to 80 g / m² according to the paper testing method (ISO 5269 / 1). 2 Paper test pieces with the following basis weight were prepared.

[0119] Step 2: 0.935 g / cm³ 3 After melting HDPE with a density of 220-240°C, 20 g / m² of the resulting mixture is applied to the paper prepared in Step 1. 2 Curtain coating was performed using the specified amount. At this time, with the temperature of the paper test piece rising to 100°C, a 0.6 atm vacuum plate was applied to the opposite side of the coated surface to adjust for effective internal penetration of the HDPE.

[0120] Step 3: After the surface of the paper test piece, which had been thoroughly impregnated with HDPE, was smoothed using a roller, the "stone paper" was completed.

[0121] The weight and density of the prepared stone paper were measured at room temperature to calculate the HDPE content, and the calcium carbonate content was determined by measuring the weight after treatment at 525°C for 1 hour. In addition, the tensile strength was measured and is shown in Table 5. [Table 5]

[0122] Although embodiments of the present invention have been described above, the technical concept of the present invention is not limited to the embodiments presented herein. Those skilled in the art who understand the technical concept of the present invention can easily propose other embodiments within the scope of the same technical concept by adding, changing, deleting, or adding components, and these will also fall within the scope of the technical concept of the present invention.

Claims

1. A method for preparing papermaking fillers, (1) A step of preparing a mixed aqueous solution containing an inorganic compound having an average size of 0.1 to 10 μm and fiber fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100, and having a solid content of 1 to 60% by weight, (2) Adding an ionic polymer to the mixed aqueous solution and stirring to form a prefloc, (3) A method comprising the step of preparing the papermaking filler by adding 10 to 1,000 parts by weight of a calcium compound to 100 parts by weight of the inorganic compound contained in the mixed aqueous solution in which the prefloc is formed, and then injecting carbon dioxide at 10 to 80°C until the pH is maintained at 7.0 ± 1.

0.

2. The above step (1) is, (1-1) Mixing organic fibers and inorganic compounds in a weight ratio of 1:5 to 100, and then adding water to prepare a mixed solution having a solid content of 1 to 60% by weight, (1-2) The method according to claim 1, comprising preparing the mixed aqueous solution by pulverizing the mixed solution, which contains the inorganic compound having an average size of 0.1 to 10 μm and the fibrous fibrils having an average width of 5 nm to 10 μm in a weight ratio of 1:5 to 100 and has a solid content of 1 to 60% by weight.

3. The inorganic compound is calcium carbonate (CaCO3). ３ ), calcium oxide (CaO), calcium hydroxide (Ca(OH) ２ ), magnesium carbonate (MgCO3) ３ The method according to claim 1, wherein one or more are selected from gypsum, kaolin, calcined clay, talc, perlite, diatomaceous earth, zinc carbonate, lithium carbonate, magnesium hydroxide, and alumina trihydrate.

4. The method according to claim 3, wherein the calcium carbonate is one or more selected from pulverized calcium carbonate (GCC) and precipitated calcium carbonate (PCC).

5. The method according to claim 1, wherein the fibrous material comprises one or more selected from cellulose and chitin.

6. The method according to claim 1, wherein the ionic polymer is one or more selected from anionic polymers and cationic polymers.

7. The method according to claim 6, wherein the anionic polymer is one or more selected from polysaccharides, proteins, polyvinyl alcohol, polyvinyl acetate, cellulose derivatives, epoxy acrylates, polyesters, polyurethanes, polyester acrylates, polyether acrylates, polyolefin dispersions, polyamides, vinyl copolymers, and polyacrylates.

8. The method according to claim 6, wherein the cationic polymer is one or more selected from polyamidoamine-epihalohydrin polymer, polyalkyldiallylamine-epihalohydrin polymer, polyethyleneimine, polyacrylamide, polyamine, polyvinylamine, and cationic starch.

9. The method according to claim 1, wherein in step (2), the amount of the ionic polymer added is 0.01 to 10 parts by weight of one or more ionic polymers per 100 parts by weight of the mixed aqueous solution.

10. The method according to claim 1, wherein in step (3), the calcium compound is one or more selected from calcium oxide, calcium hydroxide, calcium sulfate, and calcium phosphate.

11. A papermaking filler having a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in a central region, and a metal carbonate is disposed on the fibrous fibrils, comprising 1 to 20 volume percent of the fibrous fibrils and 80 to 99 volume percent of the metal carbonate.

12. A method for manufacturing paper, (1) A step of mixing 1 to 60% by weight of the papermaking filler according to claim 11 with 40 to 99% by weight of beaten natural pulp or recycled pulp, A method comprising the steps of (2) introducing the mixture from step (1) into a paper machine to produce paper.

13. A method for producing ultra-high filler-containing paper in which inorganic materials constitute 50% by weight or more of the total weight, wherein the method is (1) A step of mixing 50 to 99% by weight of the papermaking filler according to claim 11 with 1 to 50% by weight of beaten natural pulp or recycled pulp, and adding 1 to 30% of an ionic polymer to improve strength, A method comprising (2) introducing the mixture from step (1) into a paper machine to produce the ultra-high filler-containing paper.

14. A method for manufacturing composite materials, (1) A step of mixing 50 to 99% by weight of the papermaking filler according to claim 11 with 1 to 40% by weight of beaten natural pulp or recycled pulp, (2) A step of introducing the mixture from step (1) into a paper machine to produce a sheet in the form of paper, (3) A method comprising the step of producing the composite material by coating or impregnating 100 parts by weight of the sheet produced in step (2) with 50 to 5000 parts by weight of a synthetic polymer or biodegradable polymer.

15. A method for manufacturing stone paper, (1) A step of mixing 50 to 99% by weight of the papermaking filler according to claim 11 with 1 to 50% by weight of beaten natural pulp or recycled pulp, (2) A step of introducing the mixture from step (1) into a paper machine to produce a sheet in the form of paper, (3) A method comprising the step of producing stone paper by coating or impregnating 100 parts by weight of the sheet produced in step (2) with 1 to 50 parts by weight of a synthetic polymer or biodegradable polymer.

16. A paper formed with a filler, wherein the filler has a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fibrous fibrils. A paper in which the filler comprises 1 to 20 volume percent of the fiber fibrils and 80 to 99 volume percent of the metal carbonate.

17. An ultra-high filler-containing paper formed with a filler, wherein the filler has a structure in which a plurality of fiber fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fiber fibrils. The filler comprises 1 to 20 volume percent of the fiber fibrils and 80 to 99 volume percent of the metal carbonate. Ultra-high filler content paper, wherein the inorganic material accounts for 50% or more of the total weight of the paper.

18. A composite material formed with a filler, wherein the filler has a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and a metal carbonate is arranged on the fibrous fibrils. A composite material wherein the filler comprises 1 to 20 volume percent of the fiber fibrils and 80 to 99 volume percent of the metal carbonate.

19. Stone paper formed with a filler, wherein the filler has a structure in which a plurality of fibrous fibrils having an average width of 5 nm to 10 μm are present in the central region, and metal carbonates are arranged on the fibrous fibrils. Stone paper wherein the filler comprises 1 to 20 volume percent of the fiber fibrils and 80 to 99 volume percent of the metal carbonate.

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