Dissolving Pulp Sheet

Abstract

To provide a dissolving pulp sheet having excellent transparency of a solution, which is obtained by dissolving a dissolving pulp sheet or a cellulose derivative obtained by using the dissolving pulp sheet as a raw material in a solvent.SOLUTION: There is provided a dissolving pulp sheet made from dissolving pulp using a broad-leaved tree as a raw material, in which the cellulose purity obtained by the following formula is 97.0% or more, the burst strength is over 350 kPa and 900 kPa or less, and the density is 0.3 g / cm3 or more and 0.9 g / cm3 or less. Formula: Cellulose purity(%)=glucose content / (glucose content+xylose content)×100 (In the formula, the glucose content and the xylose content are the amount of glucose and the amount of xylose in the dissolving pulp sheet.)SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to a dissolving pulp sheet. [Background technology]

[0002] To produce dissolving pulp from lignocellulosic materials, it is necessary to selectively remove hemicellulose and lignin from the lignocellulosic materials and increase the cellulose purity. The alpha-cellulose content is generally used as an index of cellulose purity, and it is said that the higher the value, the higher the quality of the dissolving pulp. Two methods have long been known for producing dissolving pulp: acid sulfite cooking and prehydrolysis-kraft cooking. In the acid sulfite cooking method, most of the hemicellulose and lignin in the lignocellulosic materials are removed at once in the cooking process. On the other hand, in the prehydrolysis-kraft cooking method, most of the hemicellulose is removed by acid hydrolysis in the prehydrolysis process, and a small amount of hemicellulose and most of the lignin are removed in the subsequent kraft cooking. In the prehydrolysis process, the acetyl groups in the hemicellulose are released to produce acetic acid by simply adding water to the lignocellulosic materials and heating them, automatically creating an acidic state and allowing acid hydrolysis to proceed, so the process is generally carried out without adding any acid from outside. Comparing the acid sulfite cooking method and the prehydrolysis-kraft cooking method, if the focus is only on producing dissolving pulp, the acid sulfite cooking method is more efficient because it can remove hemicellulose and lignin in one process. However, if the focus is also on separating and effectively utilizing the by-products hemicellulose and lignin rather than discarding them, the prehydrolysis-kraft cooking method is more advantageous. In recent years, the separation of cellulose, hemicellulose, and lignin from lignocellulosic materials, which are biomass raw materials, and the production of valuable substances from each of them has been called biorefinery, and has been attracting attention, and the importance of the prehydrolysis-kraft cooking method has been reaffirmed.

[0003] As such a pulp sheet of dissolving pulp, Patent Document 1 discloses a dissolving pulp sheet having a specific ratio between the maximum and minimum thicknesses of the pulp sheet for the purpose of improving alkali solubility. Furthermore, Patent Document 2 discloses a production method for the purpose of obtaining a high-quality, low-moisture dissolving pulp sheet, in which dissolving pulp is dehydrated by passing it through a wire part, a press part, and a dryer part in that order, and then finished into a flat or rolled shape and continuously finished into product packaging. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-143355 A [Patent Document 2] JP 2015-206138 A Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, the use of dissolving pulp sheets has expanded to include rayon, cellophane, excipients for pharmaceutical manufacturing, food additives, liquid crystal films, etc., and for these applications, cellulose derivatives and fibers obtained from dissolving pulp sheets are required to have higher transparency than ever before. In addition, when manufacturing precision fibers and films, insoluble materials and unreacted materials that do not dissolve in solvents have caused problems in the manufacturing process and product quality. Therefore, an object of the present invention is to provide a dissolving pulp sheet which is excellent in transparency of a solution obtained by dissolving a dissolving pulp sheet or a cellulose derivative obtained using the dissolving pulp sheet as a raw material in a solvent. [Means for solving the problem]

[0006] As a result of intensive research, the present inventors have found that a dissolving pulp sheet made from dissolving pulp made from hardwood, having a cellulose purity of a specific value or more, and having burst strength and density of specific values ​​or less, can solve the above-mentioned problems. That is, the present invention provides the following: <1> ~ <7> Regarding. <1> A dissolving pulp sheet made of dissolving pulp made from hardwood, which has a cellulose purity of 97.0% or more, calculated by the following formula, a burst strength of more than 350 kPa and not more than 900 kPa, and a density of 0.3 g / cm 3 More than 0.9g / cm 3 Below is a dissolving pulp sheet. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp sheet.) <2> The transmittance of the reaction solution after the acetylation reaction of the dissolving pulp sheet (concentration of 3.7% by mass in terms of raw pulp) at 670 nm is 80.0% or more; <1> The dissolving pulp sheet according to claim 1. <3> The sheet moisture content of the dissolving pulp sheet is 6% by mass or more and 8% by mass or less. <1> or <2> The dissolving pulp sheet according to claim 1. <4> The ash content of the dissolving pulp sheet is 0.1% or less. <1> ~ <3> 13. A dissolving pulp sheet according to any one of claims 1 to 12. <5> The length-weighted average fiber length of the dissolving pulp is 0.50 mm or more and 0.80 mm or less. <1> ~ <4> 13. A dissolving pulp sheet according to any one of claims 1 to 12. <6> The dissolving pulp sheet has a burst strength of 750 kPa or less. <1> ~ <5> 13. A dissolving pulp sheet according to any one of claims 1 to 12. <7> The cellulose purity of the dissolving pulp calculated by the following formula is 97.0% or more. <1> ~ <6> 13. A dissolving pulp sheet according to any one of claims 1 to 12. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp.) Effect of the Invention

[0007] According to the present invention, it is possible to provide a dissolving pulp sheet having excellent transparency when a solution obtained by dissolving a dissolving pulp sheet or a cellulose derivative obtained using the dissolving pulp sheet as a raw material in a solvent is obtained. Therefore, when the dissolving pulp sheet of the present invention is used for various purposes, it is possible to improve the quality of the product at the destination of the purpose and further improve the productivity. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The dissolving pulp sheet of the present invention is a dissolving pulp sheet made of dissolving pulp made from hardwood, and has a cellulose purity calculated by the following formula of 97.0% or more, a burst strength of more than 350 kPa and not more than 900 kPa, and a density of 0.3 g / cm 3 More than 0.9g / cm 3 The following is the result. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp sheet.) In the following description, the expression "A to B" indicating a numerical range means a numerical range including the endpoints A and B.

[0009] Generally, the α-cellulose content is used as an index for adjusting the solubility of dissolving pulp, and attempts have been made to improve the solubility of a dissolving pulp sheet by increasing the α-cellulose content to a specific amount or more. However, in the study of the present invention, it has been found that the α-cellulose content alone is not sufficient as an index of the transparency. In the present invention, by setting the cellulose purity, burst strength, and density represented by the above formula to specific values, a dissolving pulp sheet made from hardwood can be obtained, which has excellent transparency when the solution obtained by dissolving the dissolving pulp sheet or a cellulose derivative obtained by using the dissolving pulp sheet as a raw material in a solvent is obtained.

[0010] The dissolving pulp sheet of the present invention has a cellulose purity of a specific value or more, thereby improving the transparency of a solution obtained by using the dissolving pulp sheet of the present invention as a raw material. Furthermore, the dissolving pulp sheet of the present invention has a bursting strength and density of a specific value or less, thereby improving the defibration property of the pulp sheet. Therefore, it is considered that the transparency of a solution obtained by using the dissolving pulp sheet of the present invention as a raw material can be excellent. Furthermore, by making the burst strength and density of the dissolving pulp sheet of the present invention equal to or greater than specific values, the strength and bone dry basis weight of the dissolving pulp sheet can be made equal to or greater than a certain level, thereby improving productivity and suitability for transportation.

[0011] [Dissolving pulp] The dissolving pulp sheet of the present invention is made of dissolving pulp made from hardwood. Considering production efficiency, it is preferable to use hardwoods with high volumetric density as the raw material for dissolving pulp. Hardwoods have high volumetric density and are preferable, and among hardwoods, some Eucalyptus plants and Acacia plants, which have high volumetric density, are preferably used, and Eucalyptus plants are more preferable. Examples of such hardwoods include Eucalyptus globulus, Eucalyptus grandis, Eucalyptus eurograndis, Eucalyptus camaldrensis, Eucalyptus pelita, Eucalyptus brassiana, and Acacia melanthii. Expressed in volumetric density, these are 450 to 700 kg / m 3 Preferably, it is 500 to 650 kg / m 3 The volume weight is 450kg / m 3 If the volumetric density is 700kg / m or more, it is advantageous in terms of pulp production efficiency. 3 If the hardwood has a molecular weight of 100 or less, the hardwood can be advantageously used alone or in combination with other hardwoods, and the combination is not limited thereto.

[0012] The dissolving pulp sheet of the present invention has a cellulose purity of 97.0% or more as calculated by the above formula, and the dissolving pulp constituting the sheet also has the same cellulose purity. That is, the dissolving pulp constituting the dissolving pulp sheet of the present invention preferably has a cellulose purity calculated by the following formula of 97.0% or more. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp.)

[0013] The cellulose purity of the dissolving pulp is preferably 97.0% or more, more preferably 97.1% or more, even more preferably 97.2% or more, still more preferably 97.5% or more, and still more preferably 98.0% or more. The higher the cellulose purity, the better, so there is no upper limit, and it is 100% or less. The cellulose purity is calculated by the above formula from the glucose and xylose amounts in the dissolving pulp measured by the NREL method (NREL: The National Renewable Energy Laboratory, USA), and specifically, can be calculated by the method described in the Examples. The unit of cellulose purity, "%", is "% by mass".

[0014] As described above, the dissolving pulp constituting the dissolving pulp sheet of the present invention preferably has a cellulose purity of 97.0% or more as calculated by the above formula, and further preferably has the following properties. The ash content of the dissolving pulp constituting the dissolving pulp sheet of the present invention is preferably 0.1% or less, more preferably 0.08% or less, even more preferably 0.07% or less, and even more preferably 0.06% or less. When the ash content is in the above range, transparency is improved, which is preferable. The ash content can be measured by the method described in the Examples.

[0015] The length-weighted average fiber length of the dissolving pulp constituting the dissolving pulp sheet of the present invention is preferably 0.50 to 0.80 mm, more preferably 0.55 to 0.75 mm, even more preferably 0.60 to 0.75 mm, and even more preferably 0.67 to 0.70 mm. The length-weighted average fiber length in the above range is preferable because it improves transparency. The length-weighted average fiber length of the dissolving pulp is a length-weighted average fiber length conforming to ISO 16065-2 (2007), and specifically, can be measured by the method described in the Examples.

[0016] The dissolving pulp constituting the dissolving pulp sheet of the present invention has a cellulose purity of a specific value or more, which improves the transparency of the sheet when dissolved, and it is preferable that the α-cellulose content of the dissolving pulp is also within the following range. The α-cellulose content of the dissolving pulp constituting the dissolving pulp sheet of the present invention is preferably 93.0% or more, more preferably 94.0% or more, and even more preferably 95.0% or more. There is no upper limit, but it is 100% or less. The α-cellulose content can be measured by the method described in the Examples. The moisture content of the dissolving pulp constituting the dissolving pulp sheet of the present invention is preferably 20% by mass or less, more preferably 15% by mass or less, in order to reduce transportation loss during transportation for external sales. From the viewpoint of improving reactivity and transparency, the moisture content is preferably 3 to 10% by mass, more preferably 5 to 8% by mass, even more preferably 6 to 8% by mass, still more preferably 7 to 8% by mass, and even more preferably 7 to 7.5% by mass. If the moisture content is within the above range, transparency is improved, which is preferable.

[0017] <Method of manufacturing dissolving pulp> The dissolving pulp constituting the dissolving pulp sheet of the present invention may be produced by any production method as long as the cellulose purity of the resulting dissolving pulp sheet falls within the above-mentioned range, but is preferably produced by a method including a prehydrolysis step and an alkaline cooking step. A method for producing dissolving pulp by a method including prehydrolysis and alkaline cooking will be described below, but the method is not limited to this. The desired cellulose purity can be adjusted by appropriately adjusting various conditions in the production of dissolving pulp. Specifically, this is achieved by increasing the intensity of prehydrolysis, as described below, and increasing the value of the P factor.

[0018] In the present invention, first, the hardwood lumber (hardwood material) is heated in the presence of water to perform prehydrolysis. Here, the amount of water (liquid ratio) relative to the hardwood material is preferably 1.0 to 10, more preferably 1.5 to 8.0, even more preferably 2.0 to 7.0, and even more preferably 2.7 to 5.0. When the liquid ratio is 1.0 or more, the hydrolysis proceeds sufficiently and the reaction becomes uniform, which is preferable. When the liquid ratio is 10 or less, the amount of heat required to heat to the desired temperature is reduced, which is economical, and therefore preferable. The method of adding water is not particularly limited, and water may be added from the outside, water originally contained in the hardwood material may be used, or water contained in the steam may be used when steam is used during heating. In addition, chemicals that directly or indirectly assist the hydrolysis of polysaccharides, such as alkali, acid, and chelating agent, may be added together with water. The strength of prehydrolysis is preferably 200 to 1000 as a P factor, more preferably 330 to 600, even more preferably 380 to 500, and even more preferably 430 to 500. The temperature is preferably 140 to 200°C, more preferably 150 to 170°C, and the treatment time is determined according to the treatment temperature, and is preferably 50 to 300 minutes, more preferably 100 to 200 minutes, even more preferably 100 to 170 minutes, even more preferably 130 to 170 minutes, and even more preferably 130 to 150 minutes. The P factor is the product of the temperature and time during prehydrolysis, and is represented by the following formula 1.

[0019]

number

[0020] The apparatus used in the prehydrolysis step is not particularly limited as long as it can hold the hardwood in a pressurized, hydrated state for a desired period of time, but is preferably a general-purpose continuous digester, batch cooker, etc. In the prehydrolysis step of the present invention, after the reaction is completed, the wastewater is dehydrated or diluted and washed, dehydrated, and sent to the next alkaline digestion step. The wastewater after prehydrolysis is sent to a flash tank and separated into a gas layer and a liquid layer, and the furfurals contained in the gas layer in large amounts can be extracted and used, and the decomposition products of hemicellulose contained in the liquid layer in large amounts can be extracted and used.

[0021] The apparatus used in the alkaline cooking step is not particularly limited, but preferably a general-purpose continuous digester, batch cooker, etc. can be used. As the alkaline cooking method, known cooking methods such as kraft cooking, polysulfide cooking, soda cooking, and alkaline sulfite cooking can be used, but in consideration of pulp quality, energy efficiency, etc., the kraft cooking method is preferably used. When kraft cooking hardwood materials, the sulfidity of the kraft cooking liquor is preferably 5 to 75%, more preferably 20 to 35%. The effective alkali addition rate is preferably 5 to 30% by mass, more preferably 10 to 25% by mass per bone-dry wood mass. The cooking temperature is preferably 140 to 170°C, and a cooking method in which cooking white liquor is added in portions may be used, and the cooking method is not particularly limited. However, the amount of water relative to the hardwood material (liquor ratio) is preferably 1.0 to 10, more preferably 1.5 to 8.0, even more preferably 2.0 to 7.0, and even more preferably 2.7 to 5.0.

[0022] In cooking, one or more of known cyclic keto compounds, such as benzoquinone, naphthoquinone, anthraquinone, anthrone, phenanthroquinone, and the above quinone compounds with alkyl, amino, or other nucleus substitutions, or hydroquinone compounds such as anthrahydroquinone, which are reduced forms of the above quinone compounds, and 9,10-diketohydroanthracene compounds, which are stable compounds obtained as intermediates in the anthraquinone synthesis method by the Diels-Alder method, may be added to the cooking liquor used as cooking aids. The addition rate is preferably 0.001 to 1.0% by mass per bone dry mass of wood chips.

[0023] The kappa number after cooking is not particularly limited, but in consideration of the pulp quality and subsequent bleachability, the kappa number is preferably 6 to 18.

[0024] The unbleached pulp obtained by the kraft cooking is washed, roughly screened and cleaned, and then bleached by a known bleaching method. Preferably, the pulp is first delignified by an oxygen delignification method. The medium- or high-consistency method can be used as the oxygen delignification method, but the medium-consistency method, which is performed at a pulp concentration of 8 to 15% by mass, is preferred because it does not require a special dehydration device and has good operability. Caustic soda or oxidized kraft white liquor can be used as the alkali used in the oxygen delignification method. As the oxygen gas, oxygen from a cryogenic separation method, oxygen from a PSA (Pressure Swing Adsorption), oxygen from a VSA (Vacuum Swing Adsorption), etc. can be used. The oxygen gas and the alkali are added to a medium-consistency pulp slurry in a medium-consistency mixer and mixed thoroughly, and then sent to a reaction tower that can hold a mixture of pulp, oxygen and alkali under pressure for a certain period of time, where the mixture is delignified. The partial pressure of oxygen gas is preferably 0.1 to 2 MPa, and more preferably 0.2 to 1 MPa, the oxygen gas addition rate is preferably 0.5 to 3 mass% per bone dry pulp mass, and the alkali addition rate is preferably 0.5 to 4 mass% per bone dry pulp mass. The reaction temperature is preferably 80 to 120°C, the reaction time is preferably 15 to 100 minutes, and the pulp concentration is preferably 8 to 15 mass%. Other conditions may be known. In the present invention, in the oxygen delignification step, it is preferable to carry out the above oxygen delignification several times in succession to advance delignification as much as possible. The pulp that has been subjected to oxygen delignification is sent to the washing stage to obtain oxygen-bleached pulp.

[0025] It is preferable to provide a washing stage for each bleaching stage, not limited to the washing stage after oxygen delignification. Washing machines used in the washing stages include pressure diffusers, diffusion washers, pressurized drum washers, horizontal long-net washers, press washers, etc., and are not particularly limited. Each washing stage can be served by one washing machine, or multiple washing machines can be used. In the present invention, it is preferable that the washing water in each washing stage has a low metal content, especially iron, and the metal content may be removed by ion exchange or the like. Washing aids such as alkali, acid, chelating agent, and surfactant can be added to the washing water in each washing stage. In addition, countercurrent washing can be performed in which washing wastewater is reused as washing water for the previous washing stage.

[0026] The unbleached pulp is preferably sent to a multi-stage bleaching step after passing through an oxygen delignification step. In the multi-stage bleaching step, known ECF bleaching steps such as chlorine dioxide (D), alkali (E), oxygen (O), hydrogen peroxide (P) and ozone (Z) can be used in combination, and the above-mentioned washing step can be provided after each bleaching step. In addition, during the multi-stage bleaching step, a high-temperature acid treatment step (A), an acid washing step, a high-temperature chlorine dioxide bleaching step, a chelating agent treatment step using ethylenediaminetetraacetic acid (EDTA) or diethylenetriaminepentaacetic acid (DTPA) or the like can also be introduced. In the multi-stage bleaching step, it is preferable to bleach the pulp so that the brightness of the pulp is 89.0 to 92.0% ISO. The brightness is more preferably 91.0 to 92.0% ISO, and even more preferably 91.5 to 92.0% ISO. A brightness of 89.0% ISO or more is preferable because the brightness can be increased by the subsequent treatment with peroxide under acidic conditions, improving the quality of the dissolving pulp. A brightness of 92.0% ISO or less is preferable because it is economical since the amount of bleaching chemicals used in the multi-stage bleaching process is not too large, and the quality of the dissolving pulp is improved without introducing unnecessary functional groups into the dissolving pulp.

[0027] The dissolving pulp obtained as described above is preferably used as a raw material for the dissolving pulp sheet of the present invention. In producing the dissolving pulp sheet, the dissolving pulp obtained here is preferably supplied to the production of the dissolving pulp sheet as a pulp slurry. The solid content of the pulp slurry is not particularly limited, but is preferably 0.5 to 5% by mass. If necessary, internal papermaking aids such as retention aids, drainage aids, surfactants, paper strength agents, bulking agents, softeners, internal sizing agents, dyes, fluorescent brighteners, pH adjusters, defoamers, pitch control agents, and slime control agents may be added, and these internal aids constitute a part of the dissolving pulp that constitutes the dissolving pulp sheet of the present invention.

[0028] [Dissolving pulp sheet] The dissolving pulp sheet of the present invention is a dissolving pulp sheet made of dissolving pulp made from hardwood, and has a cellulose purity calculated by the following formula of 97.0% or more, a burst strength of more than 350 kPa and not more than 900 kPa, and a density of 0.3 g / cm 3 More than 0.9g / cm 3 The following is the result. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp sheet.) The dissolving pulp sheet in the present invention includes a sheet continuously cut into a flat shape and a roll-shaped (wound) sheet.

[0029] The dissolving pulp sheet of the present invention is made of dissolving pulp made from hardwood, and preferred dissolving pulp is as described above in the section [Dissolving Pulp].

[0030] The cellulose purity of the dissolving pulp sheet of the present invention is preferably 97.0% or more, more preferably 97.1% or more, even more preferably 97.2% or more, still more preferably 97.5% or more, and still more preferably 98.0% or more. Since the higher the cellulose purity, the better, there is no upper limit, and it is 100% or less. The cellulose purity is calculated by the above formula from the glucose and xylose amounts in the dissolving pulp sheet measured by the NREL method, and specifically, can be calculated by the method described in the Examples. The unit of cellulose purity, "%", is "% by mass".

[0031] The upper limit of the bursting strength of the dissolving pulp sheet of the present invention is 900 kPa or less, preferably 850 kPa or less, more preferably 800 kPa or less, and even more preferably 750 kPa or less. When the upper limit of the bursting strength of the dissolving pulp sheet is within the above range, the defibration property of the dissolving pulp sheet can be improved, so that the reactivity and solubility of the dissolving pulp sheet can be increased, and the transparency of the solution obtained using the dissolving pulp sheet of the present invention as a raw material can be improved. The lower limit of the bursting strength is more than 350 kPa, preferably 360 kPa or more, more preferably 370 kPa or more, and even more preferably 380 kPa or more. When the lower limit of the bursting strength is in the above range, the density or bone dry basis weight of the dissolving pulp sheet can be set within a desired range, and the productivity and transport suitability of the dissolving pulp sheet can be improved. The bursting strength of the dissolving pulp sheet is measured by the method described in the Examples. Furthermore, since the cellulose purity and burst strength of the dissolving pulp sheet of the present invention are within the above ranges, the solution of the dissolving pulp sheet made of dissolving pulp and the solution of the cellulose derivative obtained using the dissolving pulp sheet as a raw material have excellent transparency.

[0032] The upper limit of the density of the dissolving pulp sheet of the present invention is 0.9 g / cm 3 or less, preferably 0.8 g / cm 3 Less than or equal to 0.7 g / cm3 More preferably, 0.65 g / cm 3 When the upper limit of the density is within the above range, the defibration property of the dissolving pulp sheet can be improved, so that the reactivity and solubility of the dissolving pulp sheet can be increased, and the transparency of the solution obtained using the dissolving pulp sheet of the present invention as a raw material can be improved. The lower limit of the density is 0.3 g / cm 3 or more, preferably 0.4 g / cm 3 More preferably, it is 0.45 g / cm 3 More preferably, it is 0.5 g / cm 3 That is all. By setting the lower limit of the density within the above range, the burst strength of the resulting dissolving pulp sheet can be set within the above range, and the strength of the paper can be easily improved, so that the paper can be prevented from breaking during production. In addition, the transportation efficiency of the product dissolving pulp sheet can be improved. The density of the dissolving pulp sheet is measured in accordance with JIS P 8118:2014.

[0033] As described above, the dissolving pulp sheet of the present invention has a cellulose purity calculated by the above formula of 97.0% or more, a burst strength of more than 350 kPa and not more than 900 kPa, and a density of 0.3 g / cm 3 More than 0.9g / cm 3 The following characteristics are preferable, but the following characteristics are also preferable: The sheet moisture content of the dissolving pulp sheet of the present invention is preferably 20% by mass or less, more preferably 15% by mass or less, in order to reduce transportation loss during transportation for external sales. From the viewpoint of improving reactivity and transparency, it is preferably 3 to 10% by mass, more preferably 5 to 8% by mass, even more preferably 6 to 8% by mass, still more preferably 7 to 8% by mass, and even more preferably 7 to 7.5% by mass. When the sheet moisture content is within the above range, transparency is improved, which is preferable. The ash content of the dissolving pulp sheet of the present invention is preferably 0.1% or less, more preferably 0.08% or less, even more preferably 0.07% or less, and even more preferably 0.06% or less. The ash content in the above range is preferable because it improves transparency. The moisture content and ash content of the sheet can be measured by the method described in the Examples.

[0034] The upper limit of the bone dry basis weight of the dissolving pulp sheet of the present invention is preferably 1200 g / m from the viewpoints of improving defibration property and improving the transparency of a solution obtained using the dissolving pulp sheet of the present invention as a raw material. 2 Less than 1100g / m 2 More preferably, 1000 g / m or less 2 More preferably, 900 g / m or less 2 The lower limit of the bone dry basis weight is preferably 400 g / m from the viewpoint of providing a dissolving pulp sheet containing a sufficient amount of dissolving pulp and improving productivity. 2 More preferably, 500 g / m 2 More preferably, 600 g / m 2 More preferably, 700 g / m 2 That's all. The bone dry basis weight of the dissolving pulp sheet can be measured by the method described in the Examples.

[0035] The upper limit of the specific burst strength of the dissolving pulp sheet of the present invention is preferably 2.0 kPa / (g / m) from the viewpoint of improving defibration property and improving the transparency of a solution obtained using the dissolving pulp sheet of the present invention as a raw material. 2 ) or less, more preferably 1.5 kPa / (g / m 2 ) or less, more preferably 1.2 kPa / (g / m 2 ) or less, and even more preferably 1.0 kPa / (g / m 2 ) or less. The lower limit of the burst strength is preferably 0.3 kPa / (g / m) from the viewpoint of setting the density or bone dry basis weight of the dissolving pulp sheet within a desired range and improving the productivity of the dissolving pulp sheet. 2) or more, more preferably 0.4kPa / (g / m 2 ) or more, more preferably 0.45 kPa / (g / m 2 The burst strength ratio of the dissolving pulp sheet is obtained by dividing the burst strength by the basis weight, and is calculated by the following formula: Specific bursting strength (kPa / (g / m 2 )) = Bursting strength (kPa) / Basis weight (g / m 2 )

[0036] The upper limit of the thickness of the dissolving pulp sheet of the present invention is preferably 2200 μm or less, more preferably 1800 μm or less, even more preferably 1600 μm or less, and even more preferably 1500 μm or less, from the viewpoint of setting the burst strength, density, and bone dry basis weight of the dissolving pulp sheet within the above ranges and from the viewpoint of improving the strength of the dissolving pulp sheet and preventing paper tearing during production. The lower limit of the thickness is preferably 600 μm or more, more preferably 950 μm or more, and even more preferably 1150 μm or more, from the viewpoint of providing a dissolving pulp sheet containing a sufficient amount of dissolving pulp and improving productivity. The thickness of the dissolving pulp sheet can be measured by the method described in the Examples.

[0037] The dissolving pulp sheet of the present invention has a cellulose purity of a specific value or more, which improves the transparency of the sheet when dissolved. It is preferable that the α-cellulose content of the dissolving pulp sheet is also within the following range. The α-cellulose content of the dissolving pulp sheet of the present invention is preferably 93.0% or more, more preferably 94.0% or more, and even more preferably 95.0% or more. There is no upper limit, but it is 100% or less. The α-cellulose content can be measured by the method described in the Examples.

[0038] The dissolving pulp sheet of the present invention or a cellulose derivative obtained by using the dissolving pulp sheet as a raw material has excellent transparency when dissolved in a solvent. In particular, the transparency after an acetylation reaction is excellent, as shown below. Because of these properties, the dissolving pulp and dissolving pulp sheet of the present invention can improve the quality of products at the intended use and further improve productivity when used for various purposes. Specifically, the transmittance at 670 nm of the reaction solution after the acetylation reaction (concentration of 3.7% by mass calculated based on the raw pulp) is preferably 80.0% or more, more preferably 81.6% or more, even more preferably 83.1% or more, still more preferably 84.6% or more, and even more preferably 86.2% or more. Its high transmittance and excellent transparency result in fewer unreacted fibers in the solution after acetylation, and therefore no problems occur during production, such as nozzle clogging during spinning.

[0039] <Method for manufacturing dissolving pulp sheet> The dissolving pulp sheet of the present invention is preferably obtained using the above-mentioned dissolving pulp as a raw material. The dissolving pulp sheet of the present invention is preferably obtained using the dissolving pulp described in the above [Dissolving Pulp] section as a raw material, and among them, it is more preferable that the suitable dissolving pulp sheet is obtained using the suitable dissolving pulp described in the above [Dissolving Pulp] section as a raw material.

[0040] The dissolving pulp sheet of the present invention is preferably formed by making paper from a slurry of dissolving pulp (bleached pulp) obtained after the multi-stage bleaching process explained in the above <Method of manufacturing dissolving pulp>, by a known method. A preferred method for producing the dissolving pulp sheet of the present invention is to make a slurry of dissolving pulp having a cellulose purity of 97.0% or more, calculated by the following formula, and form a dissolving pulp sheet so as to have a burst strength of 900 kPa or less. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp.) That is, the dissolving pulp sheet of the present invention is preferably formed by papermaking a slurry of dissolving pulp as described above in the section [Dissolving Pulp], more preferably a slurry of dissolving pulp (bleached pulp) obtained after the multi-stage bleaching process as described above in <Production method of dissolving pulp>. Among them, the more preferred dissolving pulp is the more preferred dissolving pulp as described above in the section [Dissolving Pulp]. Specifically, it is preferable to obtain the dissolving pulp by passing it through a wire part, a press part and a dryer part in this order to dehydrate it.

[0041] The dissolving pulp sheet of the present invention may contain a bulking agent or softening agent that inhibits the bonding between pulp fibers, provided that the effect of the present invention is not impaired. By using such a bulking agent or softening agent, the density and thickness of the dissolving pulp sheet can be adjusted, making it easier to set the burst strength within a desired range and to improve the transparency of a solution obtained using the dissolving pulp sheet of the present invention as a raw material. Specific examples of the bulking agent and softening agent include fatty acid derivatives, fatty acid polyamide amines, polyhydric alcohol surfactants, oil-based nonionic surfactants, etc. The amount of the bulking agent and softening agent added is preferably 0.05% by mass or more and 2.0% by mass or less based on the dry mass of the pulp.

[0042] The type of the wire part is not particularly limited, and examples thereof include a fourdrinier type, a short net type, a cylinder type, and a twin wire type. The type of the press part is not particularly limited, and for example, a twin bar press, a tri-nip press, a tri-vent press, an ENP, a shoe press, a tandem shoe press, a baby press, a twin wire press, a heavy duty press, etc. can be used. One or a combination of two or more of these can be used. By increasing the pressure of the press part, the burst strength can be adjusted to the desired range.

[0043] The dissolving pulp sheet of the present invention can be adjusted to have the desired burst strength and density by adjusting the pressing pressure of the press part, and the transparency of the solution obtained by dissolving the cellulose derivative obtained using the dissolving pulp sheet of the present invention as a raw material in a solvent can be improved, and in particular the transparency after the acetylation reaction can be improved. By lowering the pressing pressure of the pulp sheet, the density of the dissolving pulp sheet decreases, and the inter-fiber and intra-fiber bonds of the dissolving pulp sheet can be reduced, thereby reducing the bursting strength. As a result, the defibration property of the dissolving pulp sheet can be improved, and the transparency of the resulting solution can be improved. In addition, by increasing the pressing pressure of the press part, the density and bursting strength of the dissolving pulp sheet can be kept within a certain range, thereby preventing paper breakage. In addition, the transportation efficiency of the product dissolving pulp sheet can be easily improved. On the other hand, if the pressing pressure in the press part is too low, the density and bursting strength become too small, so that the paper is easily cut, and the transportation efficiency of the product dissolving pulp sheet decreases.If the pressing pressure is too high, the density of the dissolving pulp sheet becomes too high, and the inter-fiber bonds and intra-fiber bonds of the dissolving pulp sheet increase, so that the bursting strength becomes too high, and the transparency of the obtained solution becomes poor. From the above viewpoints, the pressing pressure of the press part is preferably 35 kgf / cm as the surface pressure. 2 More preferably, 40kgf / cm 2 More preferably, 45kgf / cm 2 More preferably, 50 kgf / cm 2 and preferably 200 kgf / cm 2 Less than or equal to 190kgf / cm 2 Less than 180kgf / cm, more preferably 180kgf / cm 2 Less than 170kgf / cm 2 The following is the result.

[0044] The moisture content of the pulp sheet after the press part is not particularly limited, but by making it 70 mass % or less, the load on the dryer part can be reduced, enabling efficient production of the pulp sheet.

[0045] The type of the dryer part is not particularly limited, and for example, a cylinder dryer, an IR dryer, a hot air dryer, etc. can be used. One or a combination of two or more of these can be used. In order to achieve the above-mentioned suitable moisture content, it is preferable to condition the moisture at 100° C. or less. EXAMPLES

[0046] The features of the present invention will be described in more detail below with reference to examples and comparative examples. The materials, amounts used, ratios, processing contents, processing procedures, etc. shown in the following examples can be appropriately changed without departing from the spirit of the present invention. Therefore, the scope of the present invention should not be interpreted as being limited by the specific examples shown below.

[0047] [Analysis and evaluation methods] [Cellulose purity] According to the NREL / TP-510-42618 method, the dissolving pulp sheet was acid-hydrolyzed with sulfuric acid, and then quantitative analysis of monosaccharides was performed using ion chromatography. The cellulose purity was calculated from the obtained data using the following formula. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 In the above formula, the amounts of glucose and xylose were calculated by the following formulas, respectively. Amount of glucose (g) = glucose measurement result (μM) × 162 (molecular weight of glucose unit) / rate of overdecomposition of sugar by acid (%) Amount of xylose (g) = xylose measurement result (μM) × 132 (molecular weight of xylose unit) / sugar overdecomposition rate by acid (%)

[0048] [Bursting strength] Measured in accordance with JIS P 8131:2009.

[0049] [density] Measured in accordance with JIS P 8118:2014.

[0050] [α-Cellulose content] Measured according to Tappi T203.

[0051] [Moisture percentage] The dissolving pulp sheet was conditioned for 24 hours in a room at 23°C and 50% humidity, then dried at 105°C for 24 hours, and the moisture content was calculated from the difference in the mass of the pulp sheet before and after drying. The mass of the dissolving pulp sheet was measured in accordance with JIS P 8127:2010.

[0052] [ash] Measured in accordance with JIS P 8251:2003.

[0053] [Length-weighted average fiber length] Measured in accordance with ISO 16065-2 (2007).

[0054] [Absolute dry basis weight] Measured in accordance with JIS P 8124:2011.

[0055] Thickness Measured in accordance with JIS P 8118:2014.

[0056] [Transparency (acetylation reactivity)] The surface of the dissolving pulp sheet was scraped with a paper knife to make it fluff-like, and 1.0 g was precisely weighed out in terms of absolute dry weight. The fluffed pulp was placed in a 100 mL Erlenmeyer flask, to which a mixed solution of 18.75 g acetic acid / 7.50 g acetic anhydride / 0.09 g sulfuric acid was added on an ice bath and allowed to stand for 5 minutes. After 5 minutes, the resulting solution was stirred at 40°C with a hot plate stirrer and an acetylation reaction was carried out for 300 minutes to obtain a sample solution. 2 mL of the sample solution was taken out, and the transmittance at 670 nm of the sample solution and the blank solvent consisting of only the solvent (acetic acid / acetic anhydride) was measured using a spectrophotometer (HITACHI, U-2001 spectrophotometer), and the transmittance of the sample was calculated according to the following formula, and the transparency (acetylation reactivity) was evaluated according to the following criteria. Transmittance (%) = (Transmittance of sample solution at 670 nm) / (Transmittance of blank solvent at 670 nm) × 100 (Evaluation Criteria) 9: Transmittance is 86.2% or more 8: Transmittance is 84.6% or more and less than 86.2% 7: Transmittance is 83.1% or more and less than 84.6% 6: Transmittance is 81.6% or more and less than 83.1% 5: Transmittance is 80.0% or more and less than 81.6% 4: Transmittance is 78.5% or more and less than 80.0% 3: Transmittance is 77.0% or more and less than 78.5% 2: Transmittance is 75.4% or more and less than 77.0% 1: Transmittance is less than 75.4% The higher the transmittance, the better the acetylation reactivity and the more excellent the transparency.

[0057] [Manufacturing of dissolving pulp sheets] [Example 1] 240g of eucalyptus wood chips (bone dry mass) were placed in an autoclave, and ion-exchanged water was added so that the liquid ratio was 3, after which prehydrolysis treatment was carried out in the autoclave. The temperature was 155°C, the time was 120 minutes, and the P factor was 400. The chips after prehydrolysis treatment were placed in the autoclave again, and a specified amount of white liquor (active alkali (AA) concentration 18%, sulfidity 28%) was added at a liquid ratio of 3, and kraft cooking was carried out for 300 minutes at a cooking temperature of 155°C to obtain unbleached kraft pulp (hereinafter referred to as "UKP"). 70 g of the UKP was collected by bone dry weight, and 1.5% sodium hydroxide was added per bone dry pulp weight, and then the mixture was diluted with ion-exchanged water to adjust the pulp concentration to 10% by weight. This pulp suspension was placed in an autoclave and exposed to oxygen at 100°C for 90 minutes under an oxygen partial pressure of 0.5 MPa. After the oxygen exposure was completed, the pulp was thoroughly washed with ion-exchanged water to obtain oxygen-bleached pulp (hereinafter referred to as "OKP"). 60 g of the above-mentioned OKP was collected by bone dry mass, and the pulp concentration was adjusted to 10% by mass with 0.5% by mass of chlorine dioxide and ion-exchanged water per bone dry pulp mass, and immersed in a thermostatic water bath at 80°C for 40 minutes to perform D0 stage treatment. The obtained pulp was thoroughly washed with ion-exchanged water, and then the pulp concentration was adjusted to 10% by mass with 1.0% by mass of sodium hydroxide, 0.1% by mass of hydrogen peroxide, and ion-exchanged water per bone dry pulp mass, and then immersed in a thermostatic water bath at 80°C for 60 minutes to perform alkali extraction stage (EP stage) treatment. The obtained pulp was thoroughly washed with ion-exchanged water, and then the pulp concentration was adjusted to 10% by mass with 0.3% by mass of chlorine dioxide and ion-exchanged water per bone dry pulp mass, and then immersed in a thermostatic water bath at 80°C for 60 minutes to perform D1 stage treatment. The obtained pulp was thoroughly washed with ion-exchanged water to obtain bleached pulp (hereinafter referred to as "BKP"). However, in stages D0 and D1, an appropriate amount of sodium hydroxide was added before the reaction so that the pH after the reaction was about 3 and about 4, respectively. The BKP was diluted with ion-exchanged water, and then printed on a square hand-made papermaking machine to a bone dry basis weight of approximately 800 g / m 2 After hand-making, a hydraulic square press was used to prepare a hand-made sheet with a surface pressure of 110 kgf / cm. 2After dehydration under the above conditions, the sheet was air-dried, and then the humidity was adjusted under conditions of a room temperature of 23° C. and a humidity of 50%, to obtain a dissolving pulp sheet 1.

[0058] [Example 2] A dissolving pulp sheet 2 was obtained in the same manner as in Example 1, except that the prehydrolysis treatment time was changed to 146 minutes (P factor 475) and the AA concentration in the kraft cooking was changed to 17%.

[0059] [Example 3] A dissolving pulp sheet 3 was obtained in the same manner as in Example 1, except that the liquor ratio in the cooking was changed to 2.5.

[0060] [Example 4] Dewatering by hydraulic square press machine with surface pressure of 160kgf / cm 2 A dissolving pulp sheet 4 was obtained in the same manner as in Example 1, except that the above-mentioned step was changed to the above-mentioned step.

[0061] [Example 5] Dewatering by hydraulic square press machine with surface pressure of 90kgf / cm 2 A dissolving pulp sheet 5 was obtained in the same manner as in Example 1, except for changing the above.

[0062] [Example 6] Dewatering by hydraulic square press machine with surface pressure of 65kgf / cm 2 A dissolving pulp sheet 6 was obtained in the same manner as in Example 1, except for changing the above.

[0063] [Example 7] A dissolving pulp sheet 7 was obtained in the same manner as in Example 1, except that the BKP was diluted with tap water during hand-making.

[0064] [Example 8] A dissolving pulp sheet 8 was obtained in the same manner as in Example 1, except that the pulp sheet after dewatering with the hydraulic square press was air-dried and then dried in a dryer at 105° C. for 15 minutes.

[0065] [Example 9] A dissolving pulp sheet 9 was obtained in the same manner as in Example 1, except that the wood species was changed to beech and the prehydrolysis treatment time was changed to 180 minutes (P factor 580).

[0066] [Example 10] When hand-made, the bone dry basis weight is approximately 650 g / m 2 A dissolving pulp sheet 10 was obtained in the same manner as in Example 1, except that a hand-made sheet of the above was prepared.

[0067] [Example 11] When hand-made, the bone dry basis weight is approximately 1000g / m 2 A dissolving pulp sheet 11 was obtained in the same manner as in Example 1, except that a hand-made sheet of the above was prepared.

[0068] [Example 12] A dissolving pulp sheet 12 was obtained in the same manner as in Example 1, except that when the BKP was diluted with ion-exchanged water, PT8107 (a fatty acid derivative, manufactured by Seiko PMC Corporation) was added as a bulking agent in an amount of 0.5 mass% per bone-dry pulp mass.

[0069] [Comparative Example 1] Dewatering by hydraulic square press machine with surface pressure of 230kgf / cm 2 A dissolving pulp sheet C1 was obtained in the same manner as in Example 1, except that the above-mentioned step was changed to the above-mentioned step.

[0070] [Comparative Example 2] A dissolving pulp sheet C2 was obtained in the same manner as in Example 1, except that the prehydrolysis treatment time was changed to 95 minutes (P factor 320) and the AA concentration in the kraft cooking was changed to 19%.

[0071] [Comparative Example 3] When hand-made, the bone dry basis weight is approximately 1400g / m 2 A dissolving pulp sheet C3 was obtained in the same manner as in Example 1, except that a hand-made sheet of the above was prepared.

[0072] Table 1 shows the analysis and evaluation results of the dissolving pulp sheets obtained in the above Examples and Comparative Examples.

[0073] [Table 1]

[0074] As shown in Table 1, the dissolving pulp sheets of the examples have excellent transparency of the reaction solution after the acetylation reaction, and are therefore excellent in the acetylation reaction. Thus, the dissolving pulp sheet of the present invention, which has excellent solution transparency, can improve the quality of the products at the intended use and further improve productivity when used for various purposes.

Claims

1. A dissolving pulp sheet made of dissolving pulp made from hardwood, The cellulose purity calculated by the following formula is 97.0% or more, The burst strength is more than 350 kPa and not more than 900 kPa, Density is 0.3 g / cm ３ 0.9g / cm or more ３ Below is a dissolving pulp sheet. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp sheet.)

2. The dissolving pulp sheet according to claim 1, wherein a reaction solution (having a concentration of 3.7% by mass in terms of raw pulp) after an acetylation reaction of the dissolving pulp sheet has a transmittance of 80.0% or more at 670 nm.

3. The dissolving pulp sheet according to claim 1 or 2, wherein the sheet moisture content of the dissolving pulp sheet is 6% by mass or more and 8% by mass or less.

4. The dissolving pulp sheet according to any one of claims 1 to 3, wherein the ash content of the dissolving pulp sheet is 0.1% or less.

5. The dissolving pulp sheet according to any one of claims 1 to 4, wherein the length-weighted average fiber length of the dissolving pulp is 0.50 mm or more and 0.80 mm or less.

6. The dissolving pulp sheet according to any one of claims 1 to 5, wherein the dissolving pulp sheet has a burst strength of 750 kPa or less.

7. The dissolving pulp sheet according to any one of claims 1 to 6, wherein the cellulose purity of the dissolving pulp calculated by the following formula is 97.0% or more. Cellulose purity (%) = glucose amount / (glucose amount + xylose amount) × 100 (In the formula, the glucose amount and the xylose amount are the glucose amount and the xylose amount in the dissolving pulp.)

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