Modified pulp and method for producing modified pulp

Abstract

To provide a modified pulp that exhibits superior transparency.SOLUTION: The modified pulp of the present invention is a modified pulp in which at least some of hydroxyl groups of cellulose are substituted with sulfate ester groups, characterized in that a haze value of a dispersion prepared by dispersing the modified pulp in water so that the solid content concentration becomes 0.6 mass% is 90% or less.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to modified pulp and a method for producing modified pulp. [Background technology]

[0002] As a fine cellulose fiber into which an anionic functional group has been introduced, one having excellent transparency and a haze value of 15% or less when dispersed in water at a concentration of 0.2% by mass has been proposed (Patent Document 1). The fine cellulose fiber described in Patent Document 1 is a nanofiber that has been finely divided using a high-pressure homogenizer or the like. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-25468 Summary of the Invention [Problem to be solved by the invention]

[0004] In contrast, there has been no pulp fiber (for example, fiber length of 100 μm or more, fiber width of 1 μm or more) before being refined that has excellent transparency.

[0005] Therefore, an object of the present invention is to provide a modified pulp having excellent transparency and a method for producing the same. [Means for solving the problem]

[0006] In order to achieve the above object, the modified pulp of the present invention is A modified pulp in which at least a portion of the hydroxyl groups of cellulose are substituted with sulfate ester groups, The haze value of a dispersion obtained by dispersing the polymer in water so that the solid content concentration is 0.6% by mass is 90% or less.

[0007] The manufacturing method of the present invention comprises: an alkali treatment step of immersing the pulp in an alkaline alkali metal solution; a contacting step of contacting the pulp after the alkali treatment step with a reaction solution containing a sulfate ester group-donating compound and at least one of urea and a urea derivative; a reaction step of introducing sulfate ester groups into at least a portion of the hydroxyl groups of the cellulose constituting the pulp after the contact step, This is a method for producing modified pulp, in which the haze value of a dispersion obtained by dispersing the pulp in water to a solids concentration of 0.6% by mass is 90% or less. [Effects of the Invention]

[0008] According to the present invention, it is possible to provide a modified pulp having excellent transparency and a haze value of 90% or less, and a method for producing the same. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a flow chart showing an example of a method for producing modified pulp of the present invention. [Figure 2] FIG. 2 is a photograph showing the results of optical microscope observation of the aqueous dispersion of modified pulp. DETAILED DESCRIPTION OF THE INVENTION

[0010] The modified pulp of the present invention is one in which at least a portion of the hydroxyl groups of cellulose has been substituted with sulfate ester groups.

[0011] The modified pulp is a fibrous material composed of a plurality of cellulose fibers, in which at least a portion of the hydroxyl groups (-OH groups) of the cellulose (a chain polymer in which D-glucose is linked via β(1→4) glycosidic bonds) constituting the cellulose fibers contained therein have been substituted with sulfate ester groups represented by formula (1).

[0012] (-OSO3 - ) r Z r+ (1) In formula (1), r is an independent natural number between 1 and 7, Z r+ When r=1, is at least one selected from the group consisting of a hydrogen ion, an alkali metal ion, a monovalent transition metal ion, an ammonium ion, an aliphatic ammonium ion, an aromatic ammonium ion, and a cationic polymer; when r=2 or more, is at least one selected from the group consisting of an alkaline earth metal ion, a polyvalent metal ion, and a compound containing two or more cationic functional groups (e.g., diamine) in the molecule.

[0013] <Physical properties of modified pulp> The physical properties of the modified pulp, excluding the haze value described below, are not particularly limited, but are, for example, as follows.

[0014] <Amount of sulfate ester groups introduced> The amount of sulfate ester groups introduced per 1 g (solid mass) of the modified pulp is preferably adjusted to, for example, 0.6 mmol / g or more, 0.8 mmol / g or more, 1 mmol / g or more, or 1.2 mmol / g or more.

[0015] Although there is no particular upper limit, from the viewpoint of suppressing fiber breakdown and cost increases resulting from decreased crystallinity, for example, the amount of sulfate ester groups introduced per 1 g (solid mass) of the modified pulp is 9.9 mmol / g or less, or 5 mmol / g or less.

[0016] In particular, from the viewpoint of transparency in a dispersion liquid in which the modified pulp is dispersed in a dispersion medium, it is preferable that the amount of sulfate ester groups introduced per 1 g (solid content mass) of the modified pulp is adjusted to, for example, 0.8 mmol / g to 5 mmol / g, 1 mmol / g to 5 mmol / g, or 1.2 mmol / g to 5 mmol / g.

[0017] The dispersion medium constituting the modified pulp dispersion is not particularly limited, but is preferably one that can exhibit the aforementioned transparency. Examples of the dispersion medium include protic polar dispersion media such as water, ethanol, methanol, acetic acid, formic acid, 2-propanol, nitromethane, and aqueous ammonia; aprotic polar dispersion media such as acetone, ethyl acetate, tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, dimethyl sulfoxide (DMSO), dimethyl sulfide (DMS), and dimethylacetamide (DMA); and nonpolar dispersion media such as dimethyl ether, benzene, toluene, hexane, chloroform, and 1,4-dioxane. These may be used alone or in combination. From the standpoint of ease of handling, protic polar dispersion media such as water, ethanol, methanol, acetic acid, formic acid, and aqueous ammonia may be used. While the water may be tap water, ion-exchanged water or pure water is preferred, as will be the case hereinafter.

[0018] <Method for measuring the amount of sulfate ester groups introduced> The amount of sulfate ester groups introduced into the modified pulp can be evaluated by the amount of sulfur introduced due to the sulfate ester groups, or by directly measuring the sulfate ester groups. For example, the amount of sulfate ester groups introduced into the modified pulp can be measured using a CHNS / O elemental analyzer. Alternatively, the amount of sulfate ester groups introduced into the modified pulp can be calculated by measuring electrical conductivity.

[0019] <Crystal structure> The modified pulp may have, for example, a cellulose type I crystalline structure, or a cellulose type II crystalline structure, or may contain both a cellulose type I crystalline structure and a cellulose type II crystalline structure.

[0020] <How to confirm the crystal structure> The crystalline structure of the modified pulp can be confirmed, for example, using an X-ray diffraction device.

[0021] <Average fiber length> The average fiber length of the modified pulp is not particularly limited, and is, for example, 0.2 mm to 2 mm, 0.2 mm to 1.8 mm, 0.2 mm to 1.5 mm, or 0.2 mm to 1 mm.

[0022] <Short fiber rate (%)> The modified pulp may also include modified pulp with short fiber length as follows. Examples of modified pulp with short fiber length (hereinafter referred to as "short fiber") include modified pulp with a fiber length distribution of 0.04 mm or more and 0.2 mm or less. The content (%) of short fibers in the modified pulp (i.e., short fiber ratio (%)) is, for example, 10% or more, 15% or more.

[0023] From the viewpoint of transparency and handling, the modified pulp has a short fiber content (%) (i.e., short fiber ratio (%)) of, for example, 10% to 70%, 10% to 60%, 10% to 50%, 10% to 45%, or 15% to 45% in terms of fiber length distribution.

[0024] <Average fiber width> The average fiber width of the modified pulp is not particularly limited, and is, for example, 5 μm to 100 μm, 10 μm to 50 μm, 20 μm to 40 μm, or 20 μm to 30 μm.

[0025] <Method for measuring average fiber length, average fiber width, and fiber distribution> The average fiber length and average fiber width of the modified pulp can be measured, for example, using a fiber tester or fiber length distribution measuring instrument manufactured by Lorentzen & Wettley in accordance with ISO 16065-2: 2007. The fiber length distribution and fiber width distribution of the modified pulp can be measured, for example, using a fiber length distribution measuring instrument in accordance with ISO 16065-2: 2007.

[0026] <Kink> In order to improve transparency, the modified pulp preferably has little bending. Specifically, the modified pulp preferably has a low kink (1 / m) value. For example, the modified pulp has a kink (1 / m) of 1 to 300, 1 to 100, or 1 to 50.

[0027] <Kink measurement method> The kink of the modified pulp can be measured, for example, using a fiber length distribution measuring instrument in accordance with ISO 16065-2:2007.

[0028] From the viewpoint of reliability, it is desirable to measure the above-mentioned measured values ​​of the modified pulp in accordance with ISO 16065-2:2007, and measure at least 5,000 fibers.

[0029] <Viscosity> The viscosity of a dispersion (solid content concentration 1% by mass) obtained by dispersing the modified pulp in water is, for example, 1000 mPa·s or more, 5000 mPa·s or more, or 10000 mPa·s or more. In particular, if the average fiber length of the modified pulp is 1 mm or less, the viscosity of the dispersion tends to increase.

[0030] <Viscosity measurement method> The viscosity (mPa·s) of the modified pulp can be measured, for example, using a B-type viscometer at a measurement temperature of 20°C, with measurements being performed at rotation speeds of 6 rpm and 60 rpm, and the thixotropy index (TI) value can also be calculated from each viscosity value. TI value = (viscosity at 6 rpm) / (viscosity at 60 rpm)

[0031] The TI value can be adjusted as appropriate, and when a high TI value is required, the lower limit of the TI value is, for example, 3 or more, 4 or more, or 5 or more. The upper limit of the TI value is, for example, 10 or less, 8 or less, 6 or less, or 5 or less. On the other hand, when a low TI value is preferred, the lower limit is, for example, 1 or more, and the upper limit is, for example, 3 or less, or 2.5 or less.

[0032] <Haze value> The modified pulp is dispersed in water to a solid content of 0.6% by mass, and the resulting dispersion has a haze value of 90% or less. The haze value may be, for example, 86% or less, 75% or less, 70% or less, 60% or less, 40% or less, 30% or less, or 20% or less.

[0033] <Method for measuring haze value> The haze value can be determined, for example, by measuring a dispersion obtained by dispersing the modified pulp in water to a solid content concentration of 0.6% by mass using a spectrophotometer in accordance with JIS K 7105.

[0034] <Total light transmittance> The total light transmittance of the dispersion is, for example, 92% or more, 94% or more, or 95% or more within the above-mentioned haze value range. The total light transmittance can be determined, for example, by measuring a dispersion in which the modified pulp is dispersed in water to a solids concentration of 0.6% by mass using a spectrophotometer in accordance with JIS K 7105.

[0035] Next, an example of the method for producing the modified pulp will be described with reference to Fig. 1. However, the present invention is not limited to this example. Fig. 1 is a flow chart showing an example of the method for producing the modified pulp.

[0036] <Alkali treatment process> First, the pulp is immersed in an alkaline alkali metal solution (step S1).

[0037] In the present invention, pulp refers to a fibrous material comprising a plurality of cellulose fibers. These cellulose fibers are a collection of a plurality of fine fibers (e.g., microfibrils). These fine fibers are a collection of a plurality of cellulose molecules (hereinafter, sometimes simply referred to as "cellulose"), which are chain-like polymers in which D-glucose is bonded via β(1→4) glucoside bonds. It is also preferable to wash the pulp beforehand. For example, filtration and dehydration using water on a 200-mesh or 235-mesh sieve can remove excessively fine fibers and debris, which is desirable because it improves handleability during production. In other words, pulp is a collection of cellulose fibers of a size that can become residue on a 200-mesh or 235-mesh sieve.

[0038] Examples of the pulp include wood-based pulp (hereinafter simply referred to as "wood pulp"), dissolving pulp, cotton-based pulp such as cotton linter, non-wood-based pulp such as wheat straw, bagasse, paper mulberry, mitsumata, hemp, kenaf, and fruit, and recycled paper pulp prepared from recycled newspapers, magazines, cardboard, etc. One type of pulp may be used alone, or two or more types of pulp may be used in combination. From the viewpoint of ease of availability, wood pulp is easily used as the pulp.

[0039] There are various types of wood pulp, and there are no particular limitations on their use. Examples include paper pulp such as softwood kraft pulp (NBKP), hardwood kraft pulp (LBKP), and thermomechanical pulp (TMP).

[0040] The alkaline alkali metal solution may be, for example, a solution containing one or more of lithium hydroxide, sodium hydroxide, and potassium hydroxide as a solute.

[0041] The solvent for the alkali metal solution is not particularly limited, and examples thereof include protic polar solvents such as water, ethanol, methanol, acetic acid, formic acid, 2-propanol, nitromethane, and aqueous ammonia; aprotic polar solvents such as acetone, ethyl acetate, tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, dimethyl sulfoxide (DMSO), dimethyl sulfide (DMS), and dimethylacetamide (DMA); and nonpolar solvents such as diethyl ether, benzene, toluene, hexane, chloroform, and 1,4-dioxane. One type of solvent may be used alone, or two or more types may be used in combination. Water is particularly preferred because it is easy to handle and inexpensive.

[0042] The amount of alkali metal in the total amount of the alkali metal solution is not particularly limited, and is, for example, 2% by mass to 30% by mass.

[0043] The ratio of the mass of the alkali metal solution to the mass of the pulp to be immersed therein is not particularly limited, but may be, for example, a ratio of about 5 g of the pulp to 100 g of the alkali metal solution.

[0044] The time for which the pulp is immersed in the alkali metal solution is not particularly limited, and may be an extremely short time such as about 20 seconds.

[0045] <Cleaning process after alkaline treatment process, etc.> The alkali treatment step may be followed by a washing step of washing the pulp. This washing step is not particularly limited as long as it can wash the alkali metal solution from the pulp. For example, a method of washing the pulp on a sieve (e.g., 45 μm (300 mesh) mesh size) with pure water may be used. The washed pulp may be dehydrated using a dehydrator (e.g., Model RC-001 manufactured by Versos Co., Ltd.). The dehydration may be performed by squeezing, air drying, or the like, in addition to using a dehydrator. The solids concentration of the pulp after dehydration is not particularly limited and may be, for example, about 25% by mass. The dehydrated pulp may be stored at room temperature or refrigerated in a refrigerator (e.g., at 4°C).

[0046] <Contact process> Next, the pulp after the alkali treatment step is brought into contact with a reaction solution containing a sulfate ester group-donating compound and at least one of urea and a urea derivative (hereinafter referred to as "urea, etc." Examples of the urea derivative include thiourea, biuret, phenylurea, benzylurea, dimethylurea, diethylurea, tetramethylurea, benzoleinurea, and hydantoin, and one type may be used alone or two or more types may be used in combination) (step S2).

[0047] The sulfate ester group-donating compound is not particularly limited as long as it is a compound capable of donating sulfate ester groups to the pulp, and examples thereof include sulfamic acid, sulfamic acid salts, and sulfuryl compounds having a sulfonyl group with two oxygen atoms covalently bonded to sulfur. One of these compounds may be used alone, or two or more may be used in combination. The sulfate ester group-donating compound is preferably sulfamic acid because it has lower acidity than sulfuric acid and the like, has a high efficiency of introducing sulfate ester groups, is inexpensive, and is highly safe. Hereinafter, an example will be described in which sulfamic acid is used as the sulfate ester group-donating compound and urea is used as the urea.

[0048] This step is not particularly limited as long as it is a method that can cause the contact. For example, the pulp may be immersed in a reaction solution in which sulfamic acid and urea are dissolved in a solvent, thereby impregnating the pulp with the reaction solution, or the reaction solution may be applied to the pulp. Alternatively, sulfamic acid and urea may be separately applied to, impregnated into, or sprayed onto the pulp. Among these methods, the method of immersing the pulp in the reaction solution to impregnate the pulp with the reaction solution makes it easier to uniformly contact the pulp with sulfamic acid and urea.

[0049] The solvent for dissolving sulfamic acid and urea is not particularly limited, and examples thereof include protic polar solvents such as water, ethanol, methanol, acetic acid, formic acid, 2-propanol, nitromethane, and aqueous ammonia; aprotic polar solvents such as acetone, ethyl acetate, tetrahydrofuran (THF), dimethylformamide (DMF), acetonitrile, dimethyl sulfoxide (DMSO), dimethyl sulfide (DMS), and dimethylacetamide (DMA); and nonpolar solvents such as diethyl ether, benzene, toluene, hexane, chloroform, and 1,4-dioxane. One of the solvents may be used alone, or two or more may be used in combination. Water is particularly preferred because it easily dissolves sulfamic acid and urea.

[0050] The pulp in this state that has been brought into contact with sulfamic acid and urea in this step is sometimes referred to as "reaction liquid-impregnated pulp."

[0051] <Contact amount of reaction solution> The reaction solution is preferably contacted with the pulp so that the sulfamic acid and urea in the reaction solution are present in a predetermined ratio relative to the pulp. Specifically, the reaction solution is contacted so that the amounts of sulfamic acid and urea in the reaction solution relative to the pulp in the reaction solution-impregnated pulp when the pulp is subjected to the reaction step are appropriate. More specifically, the amount of sulfamic acid contacted relative to the pulp (solid mass, which is the dry mass) in the reaction solution-impregnated pulp immediately before the heating reaction in the reaction step is adjusted to be equal to or greater than the amount of urea contacted.

[0052] For example, the reaction liquid is prepared so that the mixing ratio of sulfamic acid and urea is such that the value (sulfamic acid / urea) obtained by dividing the parts by mass of sulfamic acid per 100 parts by mass of the solid content of the pulp in the reaction liquid-impregnated pulp immediately before being subjected to the heating reaction by the parts by mass of urea per 100 parts by mass of the solid content of the pulp in the reaction liquid-impregnated pulp immediately before being subjected to the heating reaction is 0.8 or more, 0.85 or more, or 1 or more.

[0053] Furthermore, for example, the contact amount of sulfamic acid is adjusted to be 70 parts by mass or more, 100 parts by mass or more, or 200 parts by mass or more per 100 parts by mass of the solid content of the pulp in the reaction liquid-impregnated pulp immediately before being subjected to the heating reaction.

[0054] Furthermore, for example, the contact amount of urea, i.e., the contact amount of urea relative to the mass of solids of the pulp in the reaction solution-impregnated pulp immediately before being subjected to the heating reaction, is adjusted to 20 parts by mass or more, 30 parts by mass or more, or 50 parts by mass or more per 100 parts by mass of the solids of the pulp while maintaining the above-mentioned relationship with sulfamic acid. The upper limit of the contact amount of urea is not particularly limited, and is, for example, 350 parts by mass or less, 300 parts by mass or less, or 250 parts by mass or less per 100 parts by mass of the solids of the pulp.

[0055] The contact amounts of sulfamic acid and urea per 100 parts by mass of the solid content of the pulp can be calculated appropriately depending on, for example, the state of the reaction liquid-impregnated pulp to be subjected to the reaction step.

[0056] <State of pulp impregnated with reaction liquid> The state of the reaction liquid-impregnated pulp to be subjected to the above-mentioned next reaction step may be, for example, the reaction liquid-impregnated pulp as it is, i.e., a state in which the pulp is in contact with the reaction liquid without actively removing water, or a state in which the water has been actively removed from the pulp in contact with the reaction liquid.

[0057] The former reaction liquid-impregnated pulp (in a state where water is not actively removed) includes pulp in a state where the pulp is in contact with the reaction liquid (including, for example, a slurry state), and pulp prepared by removing the pulp from a state where the pulp is in contact with the reaction liquid and leaving it to stand.

[0058] On the other hand, the latter reaction liquid impregnated pulp (in a state where water has been actively removed) refers to pulp that has had water intentionally removed from the state where the pulp has been in contact with the reaction liquid. Examples include pulp that has been prepared by removing the pulp from the state where the pulp has been in contact with the reaction liquid and then naturally drying it by air drying or the like, pulp that has been prepared by filtering and dehydrating the pulp that has been in contact with the reaction liquid, pulp that has been filtered and dehydrated and further air-dried, pulp that has been filtered and dehydrated and further dried using a circulating air dryer, pulp that has been filtered and dehydrated and further dried using a heated dryer, pulp that has been contacted with the reaction liquid and then dried using a circulating air dryer or a heated dryer, and the like.

[0059] Thus, the reaction solution-impregnated pulp to be subjected to the reaction step may be one that has not been subjected to the aforementioned active water removal, or one that has had a certain amount of water removed by active water removal. Furthermore, when removing water by drying, there is no particular problem even if the moisture content after drying is about 1%. In particular, the latter method can reduce the moisture content in the reaction solution-impregnated pulp to be subjected to the reaction step, thereby shortening the reaction time for the heating reaction in the reaction step. This has the advantage of improving the productivity of the modified pulp. Furthermore, the dehydration method has the advantage of enabling the reaction solution-impregnated pulp to be prepared more efficiently than when treating a large amount of reaction solution.

[0060] When an active drying method is used, the reaction solution-impregnated pulp may be dried until its moisture content reaches about 1%, or the moisture may be removed by drying it until it reaches an absolutely dry state with a moisture content significantly lower than 1%.

[0061] In this specification, the term "wet state" refers to a reaction solution-impregnated pulp that has a moisture content of 1% or more and is not in an absolute dry state. For example, the term "wet state" may refer to pulp that is still impregnated with the reaction solution, pulp that has been dehydrated to a certain extent, or pulp that has been dried to a certain extent.

[0062] In addition, in this specification, bone-dry means a state in which the moisture content is reduced to less than 1%, for example, by reducing the pressure in a desiccator containing a desiccant such as calcium chloride or diphosphorus pentoxide, or by subjecting the material to a long-term heat drying treatment.

[0063] Therefore, when the latter method (reaction method in a state where active moisture removal is performed) is used, the moisture content of the reaction solution-impregnated pulp may be brought to a non-bone-dry state or to a bone-dry state, but it is preferable to use the method in which the moisture content is brought to a non-bone-dry state.

[0064] The moisture content of the reaction liquid-impregnated pulp in this specification is calculated using the following formula. Moisture content of reaction liquid-impregnated pulp (%) = 100 - (mass of solid content of reaction liquid-impregnated pulp (g) / mass of reaction liquid-impregnated pulp at the time of moisture content measurement (g)) × 100 = {(mass of reaction liquid-impregnated pulp at the time of moisture content measurement (g) - mass of solid content of reaction liquid-impregnated pulp (g)) / mass of reaction liquid-impregnated pulp at the time of moisture content measurement (g)} × 100

[0065] The solids mass (g) of the reaction solution-impregnated pulp in the above formula refers to the dry mass of the reaction solution-impregnated pulp. Specifically, this refers to the dry mass adjusted to a constant weight by drying the sample using a dryer or other device. For example, by placing the reaction solution-impregnated pulp in a dryer and drying it under specified drying conditions (e.g., 105°C for 2 hours) and measuring the mass, the mass of the reaction solution-impregnated pulp after moisture removal (i.e., the mass of the reaction solution-impregnated pulp that remains after the drying conditions are met, including the pulp and reagents in the reaction solution) can be calculated. Furthermore, "constant mass" refers to a state in which moisture in the atmosphere and the raw materials in the treatment facility no longer appears to move in or out. Specifically, after drying for a set period of time (e.g., 2 hours), the change in mass between two consecutive measurements is within 1% of the mass at the start of drying (however, the second mass measurement should be at least half the drying time required for the first measurement).

[0066] The state of the pulp when it is brought into contact with the reaction liquid is not particularly limited, and may be, for example, a dry state or a wet state (i.e., a moist state).

[0067] <Pre-drying process in the contact process> In the above example, the method for preparing reaction solution-impregnated pulp in this step was described, in which the reaction solution-impregnated pulp is prepared by actively removing moisture. However, when using a method for removing moisture while heating (pre-drying step) in this method (for example, when directly heating and drying the pulp in a state where the reaction solution has been in contact with the pulp, or when heating and drying a dehydrated pulp), it is desirable to adjust the heating temperature to a predetermined temperature or lower. The drying temperature in this pre-drying step is not particularly limited, but is preferably adjusted to a temperature that can remove moisture contained in the reaction solution-impregnated pulp and ambient moisture and does not allow the reaction to proceed. For example, the drying temperature in the pre-drying step can be adjusted so that the ambient temperature of the reaction solution-impregnated pulp is 100°C or lower. On the other hand, from the viewpoint of workability, it is preferable to adjust the temperature to 50°C or higher. Therefore, the drying temperature in the pre-drying step in this step is preferably 50°C to 100°C, or 70°C to 100°C.

[0068] <Moisture adjustment process in contact process> This process may include a moisture adjustment step in which the moisture content of the pulp to be contacted with the reaction solution is adjusted to fall within a predetermined range. This moisture adjustment step involves drying or humidifying the pulp so that it has a predetermined moisture content. By including this moisture adjustment step, the moisture content in the pulp when contacted with the reaction solution or the like can be made somewhat uniform, which may improve product stability in continuous operation. Furthermore, drying the pulp to a certain extent to reduce the moisture content (for example, to a moisture content of 1% to 10%) has the advantage of improving storage stability.

[0069] <Reaction process> Next, sulfate ester groups are introduced into at least a portion of the hydroxyl groups of the cellulose constituting the pulp after the contacting step (step S3). This step involves reacting cellulose fibers contained in the reaction solution-impregnated pulp provided from the contacting step with sulfamic acid and urea to substitute at least a portion of the hydroxyl groups in the cellulose fibers with sulfate ester groups, thereby introducing sulfate ester groups into the cellulose fibers contained in the pulp. That is, this step is a step of carrying out a reaction to substitute at least a portion of the hydroxyl groups in the cellulose fibers contained in the reaction solution-impregnated pulp with sulfate ester groups.

[0070] This step is not particularly limited as long as it is a method capable of substituting at least a portion of the hydroxyl groups of the cellulose fibers in the reaction solution-impregnated pulp with sulfate ester groups, and for example, a method in which the reaction is accelerated by heating the reaction solution-impregnated pulp can be used. Hereinafter, the reaction will be described taking as an example a case in which the reaction is carried out by this heating method.

[0071] <Reaction temperature in the reaction process> The reaction temperature in this step is not particularly limited, but is preferably a temperature at which sulfate ester groups can be introduced into the cellulose fibers that make up the pulp while suppressing thermal decomposition and hydrolysis of the fibers. For example, the ambient temperature of the reaction solution-impregnated pulp subjected to this step is adjusted to 100°C to 200°C, 120°C to 200°C, 120°C to 180°C, or 120°C to 160°C. If the ambient temperature during heating is 200°C or less, thermal decomposition and discoloration of the fibers can be suppressed.

[0072] The heater or the like used in this step is not particularly limited, and for example, one that can directly or indirectly heat the reaction solution-impregnated pulp after the contact step while satisfying the above-mentioned requirements can be used, and known dryers, reduced pressure dryers, microwave heating devices, autoclaves, infrared heating devices, hot pressing methods using heat presses (e.g., AH-2003C manufactured by AS ONE Corporation), etc. can be used. In particular, from the viewpoint of operability, it is preferable to use a circulating air dryer because gas may be generated in this step.

[0073] <Reaction time in the reaction process> When the heating method is used in this step, the heating time (i.e., reaction time) is not particularly limited, but is, for example, 1 minute or more, 5 minutes or more, 10 minutes or more, or 15 minutes or more when the reaction temperature is adjusted to be within the above range, and from the viewpoints of operability and cost, it is 5 to 300 minutes, or 5 to 120 minutes.

[0074] By carrying out the above steps, the modified pulp can be produced.

[0075] <Cleaning process after reaction process> The reaction process may be followed by a washing process for washing the modified pulp. The modified pulp has an acidic surface due to the influence of sulfamic acid (a sulfate ester group-donating compound). Furthermore, unreacted reaction liquid is still present. Therefore, by providing a washing process for completely terminating the reaction and removing excess reaction liquid to neutralize the pulp, handling can be improved.

[0076] This washing step is not particularly limited, and may be performed, for example, so long as the modified pulp is substantially neutral. For example, a method of washing the modified pulp with pure water or the like until it becomes neutral may be used. Alternatively, neutralization washing using an alkaline solution or the like may be performed. When performing such neutralization washing, examples of the alkaline compound contained in the alkaline solution include inorganic alkaline compounds and organic alkaline compounds. Examples of inorganic alkaline compounds include hydroxides, carbonates, and phosphates of alkali metals. Examples of organic alkaline compounds include ammonia, aliphatic amines, aromatic amines, aliphatic ammonium, aromatic ammonium, heterocyclic compounds, and hydroxides of heterocyclic compounds.

[0077] The method for separating the modified pulp in the washing step is not particularly limited, and may be any method that allows the modified pulp to be separated from the wash water by filtration. For example, the modified pulp after the reaction can be washed using a stainless steel sieve with a mesh size of 243 μm (70 mesh) to 20 μm (635 mesh), 132 μm (120 mesh) to 45 μm (300 mesh), or 75 μm (200 mesh) to 45 μm (300 mesh).

[0078] The modified pulp can be widely used in various fields, for example, for producing transparent sheets and films. By using the modified pulp, it is possible to produce sheets and films that are excellent in transparency despite being made primarily of pulp. The transparent sheets and films can be widely used, for example, for labels on PET beverage bottles and transparent windows in windowed envelopes, which have previously been made of resin. [Example]

[0079] (Example 1-1) <Alkali treatment process> 20 g (5 g solids) of softwood bleached kraft pulp (NBKP) (hereinafter simply referred to as "pulp") was immersed in 100 g of a 7% by mass aqueous solution of sodium hydroxide for 1 hour. The softwood bleached kraft pulp (NBKP) used was manufactured by Marusumi Paper Co., Ltd. (25% solids, the remaining 75% was water), and the sodium hydroxide used was manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0080] <Cleaning process after alkaline treatment process, etc.> After immersion in the sodium hydroxide solution, the pulp was washed with pure water on a 300-mesh sieve. The washed pulp was placed in a homemade net using nylon mesh (manufactured by AS ONE Corporation, model number: PA-46μ, mesh size 46μm, 300 mesh), dehydrated for 2 minutes using a dehydrator (manufactured by Versos Corporation, model number: RC-001) until the solid content reached 25% by mass, and then stored in a refrigerator (4°C).

[0081] <Contact process> The pulp (8 g, containing 2 g of pulp after the alkali treatment step) was dehydrated to a solids concentration of 25% by mass and immersed in the reaction solution for 10 minutes to impregnate the pulp after the alkali treatment step. The reaction solution was prepared by adding 2.43 g (0.025 mol) of sulfamic acid and 1.20 g (0.020 mol) of urea to 12 g of pure water and stirring at room temperature (23°C). The sulfamic acid and urea were manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0082] The pulp impregnated with the reaction solution was spread thinly and uniformly on an acrylic plate and dried at 85°C for 3 hours using a thermostatic dryer (Yamato Scientific Co., Ltd., DKN602).

[0083] <Reaction process> The pulp impregnated with the reaction solution was subjected to a heating reaction at a reaction temperature of 140° C. for a reaction time (heating time) of 30 minutes.

[0084] <Cleaning process after reaction process, etc.> The pulp after the reaction step was neutralized on a 635 mesh sieve and washed with pure water to obtain modified pulp. An aqueous solution of sodium bicarbonate (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was used as the neutralizing agent. The obtained modified pulp was prepared into a 0.6% by mass aqueous dispersion using pure water and then stored in a refrigerator (4°C).

[0085] (Examples 1-2 to 1-22) A modified pulp was obtained in the same manner as in Example 1-1, except that the concentration of the aqueous sodium hydroxide solution used in the alkaline treatment process (amount of sodium hydroxide) and the amounts of sulfamic acid and urea in the reaction solution were changed as shown in Table 1.

[0086] (Comparative Examples 1-1 to 1-6) Modified pulp was obtained in the same manner as in Examples 1-1 to 1-6, except that the alkali treatment step was not carried out.

[0087] Table 1 shows the concentrations of the aqueous sodium hydroxide solutions used in the alkali treatment step in Examples 1-1 to 1-22, the amounts of sulfamic acid and urea in the reaction solutions used in producing the modified pulps in Examples 1-1 to 1-22 and Comparative Examples 1-1 to 1-6, and the haze values ​​and total light transmittances of the aqueous dispersions (solid content concentration: 0.6% by mass) of the modified pulps in Examples 1-1 to 1-22 and Comparative Examples 1-1 to 1-6. The haze values ​​and total light transmittances shown in Table 1 were measured using a spectrophotometer (manufactured by Nippon Denshoku Industries Co., Ltd., model number: SH 7000), and are the same in Tables 2 and 3 below.

[0088] [Table 1]

[0089] As shown in Table 1, modified pulps having excellent transparency and haze values ​​of 90% or less were obtained in Examples 1-1 to 1-22. On the other hand, the modified pulps of Comparative Examples 1-1 to 1-6, which were produced without carrying out the alkali treatment step, had haze values ​​exceeding 90% and were poor in transparency.

[0090] FIG. 2 shows the results of optical microscope observation of the aqueous dispersions of modified pulp of Examples 1-2, 1-10, 1-19, and Comparative Example 1-4. As shown in FIG. 2, in the aqueous dispersions of modified pulp of Examples 1-2, 1-10, and 1-19, the cellulose fibers in the modified pulp absorb water as a solvent and expand, compared to the aqueous dispersion of modified pulp of Comparative Example 1-4. It is presumed that the haze value is lower and the transparency is excellent, despite the absence of pulp defibration treatment. However, this mechanism is merely presumed, and the present invention is not limited thereto. The micrograph shown in FIG. 2 was taken using an optical microscope (Nikon Corporation, Model: ECLIPSE LV100ND) observed by polarization. The observation sample was prepared by dropping the aqueous dispersion of modified pulp onto a commonly used slide glass and covering it with a cover glass without drying or staining.

[0091] Example 2-1 A modified pulp was obtained in the same manner as in Example 1-18, except that in the alkali treatment step, an aqueous solution of lithium hydroxide having the same concentration was used instead of the aqueous solution of sodium hydroxide. The lithium hydroxide used was manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0092] (Example 2-2) A modified pulp was obtained in the same manner as in Example 1-18, except that in the alkali treatment step, an aqueous potassium hydroxide solution of the same concentration was used instead of the aqueous sodium hydroxide solution. The potassium hydroxide used was manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.

[0093] Table 2 shows the type of alkali metal used in the alkali treatment process in Examples 1-18, 2-1, and 2-2, the amount of sulfamic acid and urea in the reaction solution used in producing the modified pulp, and the haze value and total light transmittance of the aqueous dispersion of the modified pulp (solid content concentration 0.6% by mass).

[0094] [Table 2]

[0095] As shown in Table 2, it was confirmed that modified pulp with low haze value and excellent transparency could be obtained even when the type of alkali metal used in the alkali treatment step was changed.

[0096] (Examples 3-1 to 3-6) A modified pulp was obtained in the same manner as in Example 1-18, except that the immersion time of the pulp in the aqueous sodium hydroxide solution in the alkali treatment step was changed.

[0097] Table 3 shows the immersion time of the pulp in the aqueous sodium hydroxide solution in the alkali treatment process of Examples 1-18 and 3-1 to 3-6, the amount of sulfamic acid and urea in the reaction solution used in producing the modified pulp, and the haze value of the aqueous dispersion of the modified pulp (solid content concentration 0.6% by mass).

[0098] [Table 3]

[0099] As shown in Table 3, it was confirmed that modified pulp with low haze and excellent transparency could be obtained even when the pulp was immersed in the alkali metal solution for an extremely short time of 20 seconds.

[0100] Although the present invention has been described above with reference to the embodiments and examples, the present invention is not limited to the above-described embodiments and examples. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the scope of the present invention.

Claims

1. A modified pulp in which at least a portion of the hydroxyl groups of cellulose are substituted with sulfate ester groups, A modified pulp characterized in that the haze value of a dispersion obtained by dispersing the modified pulp in water to a solids concentration of 0.6% by mass is 90% or less.

2. an alkali treatment step of immersing the pulp in an alkaline alkali metal solution; a contacting step of contacting the pulp after the alkali treatment step with a reaction solution containing a sulfate ester group-donating compound and at least one of urea and a urea derivative; a reaction step of introducing sulfate ester groups into at least a portion of the hydroxyl groups of the cellulose constituting the pulp after the contact step, A method for producing modified pulp, wherein the haze value of a dispersion obtained by dispersing the pulp in water to a solids concentration of 0.6% by mass is 90% or less.

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