COMPOSITION
A composition provides a high-bulk cellulose fiber-containing sheet, in which the water-retaining ability of the cellulose fibers is sufficiently high, and the water-absorbing rate is large. The composition contains cellulose fibers having phosphoric acid groups or phosphoric acid group-derived substituents. In at least a part of the cellulose fibers, the phosphoric acid groups or the phosphoric acid group-derived substituents are crosslinked. The number of crosslinking points in the cellulose fibers, which is calculated according to the following Equation (1), is 0.20 mmol/g or more, and the water content is 50% by mass or less, with respect to the total mass of the composition: Number of crosslinking points=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 . . . Equation (1).
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The present invention relates to a composition. Specifically, the present invention relates to a cellulose fiber-containing composition comprising cellulose fibers having phosphoric acid groups.
BACKGROUND ARTConventionally, cellulose fibers have been broadly utilized in clothes, absorbent articles, paper products, and the like. As cellulose fibers, ultrafine cellulose fibers having a fiber diameter of 1 μm or less have been known, as well as cellulose fibers having a fiber diameter of 10 μm or more and 50 μm or less.
For example, when cellulose fibers are used in an absorbent article, the cellulose fibers that are in the form of a non-woven fabric or the like constitute various types of members of the absorbent article. In such a case, the non-woven fabric is required to have high water absorbency. Conventionally, in order to enhance the water absorbency of a non-woven fabric comprising cellulose fibers, a highly water-absorbent resin such as SAP has been deposited on the cellulose fibers. In addition, a technique of performing crosslinking modification on cellulose fibers to enhance the water absorbency of a non-woven fabric has also been studied.
Patent Document 1 discloses a fabric cloth containing crosslink-modified cellulose fibers. Herein, formaldehyde, a nitrogen-containing cyclic compound and the like are used as crosslinkers in the crosslinking modification. Moreover, Patent Document 2 discloses a method for producing a water-absorbent cellulose material having an excellent salt water-absorbing rate, which comprises immersing a water-swellable, crosslinked cellulose derivative having a carboxyl group in a strongly acidic aqueous solution, then adding alkali to the cellulose derivative in an organic solvent having compatibility with water so that the acid-type carboxyl group is converted to a salt type, and then drying the resultant. Also in this method, a technique of crosslinking cellulose fibers to enhance the water absorption of the cellulose material has been studied.
Patent Document 3 discloses a water-absorbent resin comprising cellulose and a polymer obtained by polymerizing a monomer aqueous solution having an acid group-containing unsaturated monomer as an essential component, wherein the resin has a crosslinked surface. Patent Document 3 describes phosphorylated crosslinked cellulose as cellulose, and also describes, as a crosslinker, an N-methylol compound such as dimethylol ethylene urea or dimethylol dihydroxy ethylene urea.
PRIOR ART DOCUMENTS Patent Documents Patent Document 1: JP-A-2000-129575 Patent Document 2: JP-A-H08-243388 (1996) Patent Document 3: JP-A-2011-213759 SUMMARY OF INVENTION Object to be Solved by the InventionIn general, when cellulose fibers are crosslinked, a cellulose fiber-containing sheet tends to have high bulk. However, in such high-bulk cellulose fiber-containing sheet, the water-retaining ability of cellulose fibers tends to be decreased. Thus, it has been desired to develop a cellulose fiber-containing sheet having sufficiently high water-retaining ability, although the sheet has high bulk. In addition, studies have also been conducted to enhance the water-absorbing rate of such a cellulose fiber-containing sheet.
Hence, in order to solve the problems of the prior art techniques, the present inventors have conducted studies directed towards providing a cellulose fiber-containing composition having sufficiently high water-retaining ability and being capable of exhibiting an excellent water-absorbing rate, even in the case of forming a high-bulk sheet from the cellulose fiber-containing composition.
Means for Solving the ObjectAs a result of intensive studies directed towards achieving the aforementioned object, the present inventors have found that the amount of the crosslinked structures of phosphorylated cellulose fibers in a composition comprising the phosphorylated cellulose fibers having crosslinked structures is set to be a predetermined amount or larger, so that sufficiently high water-retaining ability and an excellent water-absorbing rate can be exhibited, even in a case where a high-bulk sheet is formed from the aforementioned composition.
Specifically, the present invention has the following configurations.
[1] A composition comprising cellulose fibers having phosphoric acid groups or phosphoric acid group-derived substituents, wherein
in at least a part of the cellulose fibers, the phosphoric acid groups or the phosphoric acid group-derived substituents are crosslinked,
the number of crosslinking points in the cellulose fibers, which is calculated according to the following Equation (1), is 0.20 mmol/g or more, and
the water content is 50% by mass or less, with respect to the total mass of the composition:
Number of crosslinking points=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 Equation (1).
[2] The composition according to [1], which is a non-woven fabric.
[3] The composition according to [1] or [2], wherein when the composition is processed into a rectangular sample having a width of 5 mm and a length of 50 mm, then, an edge region ranging from the end of the rectangular sample in the longitudinal direction to 5 mm from the end is immersed in ion exchange water (electrical conductivity: 2 ρS/cm or less), and then, the time required for the ion exchange water to reach from the end of the longitudinal direction to a distance of 45 mm in the longitudinal direction is measured, a water-absorbing rate (mm/sec), which is calculated according to the following Equation (2), is 2.5 mm/sec or more and 100 mm/sec or less:
Water-absorbing rate (mm/sec)=40 (mm)/t (sec) Equation (2)
wherein t represents the time (see) required for the ion exchange water to reach from the end of the rectangular sample in the longitudinal direction to a distance of 45 mm in the longitudinal direction.
[4] The composition according to any one of [1] to [3], wherein the amount of the strongly acidic groups contained in the cellulose fibers is 1.60 mmol/g or more.
[5] The composition according to any one of [1] to [4], wherein the water retention capacity (%) of the cellulose fibers, which is calculated according to the following equation, is 150% or more:
Water retention capacity (%)=(weight of cellulose fibers after centrifugation treatment−absolute dry weight of cellulose fibers)/absolute dry weight of cellulose fibers×100,
wherein, in the above equation, the water retention capacity is measured in accordance with SCAN-C 62:00, and conditions for the centrifugation treatment are determined to be 20° C. and weight acceleration upon the centrifugation of 3950 g, and 15 minutes.
Advantageous Effects of InventionAccording to the present invention, a cellulose fiber-containing composition having sufficiently high water-retaining ability and being capable of exhibiting an excellent water-absorbing rate even in a case where a high-bulk sheet is formed from the composition can be obtained.
hereinafter, the present invention will be described in detail. The below-mentioned constituent features will be explained based on representative embodiments or specific examples in some cases. However, the present invention is not limited to such embodiments.
(Composition)The present invention relates to a composition comprising cellulose fibers having phosphoric acid groups or phosphoric acid group-derived substituents (hereinafter simply referred to as “phosphoric acid groups” at times). Herein, phosphoric acid groups or phosphoric acid group-derived substituents are crosslinked in at least a part of the cellulose fibers. The number of crosslinking points in the cellulose fibers, which is calculated according to the following Equation (1), is 0.20 mmol/g or more.
Number of crosslinking points=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 Equation (1)
In addition, the water content is 50% by mass or less, with respect to the total mass of the composition of the present invention. Besides, in the present description, the composition comprising cellulose fibers can be referred to as a “cellulose fiber-containing composition.”
Since the cellulose fiber-containing composition of the present invention has the above-described configuration, it can form a high-bulk cellulose fiber-containing sheet. However, in such a case as well, the water-retaining ability of the cellulose fibers can be kept at sufficiently high. Moreover, when the cellulose fiber-containing composition of the present invention is processed into a sheet, the sheet can exhibit an excellent water-absorbing rate. In the present invention, by appropriately regulating the crosslinked structures of phosphorylated cellulose fibers or the amount of the crosslinked structures, a cellulose fiber-containing composition capable of exhibiting novel physical properties has been successfully obtained.
The water content is 50% by mass or less, with respect to the total mass of the cellulose fiber-containing composition of the present invention. This means that the cellulose fiber-containing composition of the present invention is preferably, not in a slurry state but in a solid state. For example, the cellulose fiber-containing composition of the present invention is preferably in the state of gel, a sheet or a particulate, and more preferably in the state of a sheet. Among others, the cellulose fiber-containing composition of the present invention is preferably a non-woven fabric. Besides, in the present description, when the cellulose fiber-containing composition is in a sheet state, the cellulose fiber-containing composition can also be referred to as a “cellulose fiber-containing sheet.” The cellulose fiber-containing sheet is one embodiment of the cellulose fiber-containing composition.
The water content with respect to the total mass of the cellulose fiber-containing composition of the present invention may be 50% by mass or less, preferably 40% by mass or less, more preferably 30% by mass or less, further preferably 20% by mass or less, and particularly preferably 15% by mass or less. In addition, in the present invention, the water content in the cellulose fiber-containing composition may also be 0% by mass.
Herein, the water content can be calculated as follows. That is, the weight of a cellulose fiber-containing composition, which has been subjected to humidity conditioning up to an equilibrium state under conditions of 23° C. and a relative humidity of 50%, is measured, the cellulose fiber-containing composition is then dried at 105° C. overnight, and the weight of the resulting cellulose fiber-containing composition is then measured. Then, the water content can be calculated according to the following equation:
Water content (%)=(weight of cellulose fiber-containing composition before drying at 105° C.−weight of cellulose fiber-containing composition after drying at 105° C.)/weight of cellulose fiber-containing composition before drying at 105° C.×100.
When the cellulose fiber-containing composition of the present invention is in a sheet state, the density of the cellulose fiber-containing sheet is preferably 1.2 g/cm3 or less, more preferably 1.0 g/cm3 or less, and further preferably 0.8 g/cm3 or less. On the other hand, the density of the cellulose fiber-containing sheet is preferably 0.05 g/cm3 or more. Even in a case where the cellulose fiber-containing composition of the present invention is a non-woven fabric, the density of the non-woven fabric is preferably within the above-described range. When the cellulose fiber-containing composition of the present invention is a sheet, the density of the cellulose fiber-containing sheet is preferably within the above-described range, and a high-bulk sheet can be obtained by adjusting the density within the above-described range.
When the cellulose fiber-containing composition of the present invention is a sheet, the basis weight of the cellulose fiber-containing sheet is preferably 30 g/m2 or more, more preferably 50 g/m2 or more, and further preferably 100 g/m2 or more. On the other hand, the basis weight of the cellulose fiber-containing sheet is preferably 1000 g/m2 or less. By setting the basis weight of the cellulose fiber-containing sheet within the above-described range, water absorbency can be more effectively exhibited.
When the cellulose fiber-containing composition of the present invention is a sheet, the thickness of the cellulose fiber-containing sheet is preferably 5 μm or more, more preferably 10 μm or more, and further preferably 15 μm or more. On the other hand, the thickness of the cellulose fiber-containing sheet is preferably 50 mm or less, more preferably 40 mm or less, and further preferably 30 mm or less.
When the cellulose fiber-containing composition of the present invention is a sheet, the water-absorbing rate (mm/sec) calculated according to the following Equation (2) is preferably 2.5 mm/sec or more and 100 mm/sec or less. The water-absorbing rate (mm/sec) is more preferably 3.0 mm/sec or more, and further preferably 3.5 mm/sec or more.
Herein, the water-absorbing rate (mm/sec) calculated according to the following Equation (2) is a water-absorbing rate measured by the following procedures. First, a sheet-like cellulose fiber-containing composition is processed into a rectangular sample having a width of 5 mm and a length of 50 mm, and an edge region ranging from the end of this rectangular sample in the longitudinal direction to 5 mm from the end is then immersed in ion exchange water (electrical conductivity: 2 μS/cm or less). Thereafter, the time required for the ion exchange water to reach from the end of the longitudinal direction to a distance of 45 mm in the longitudinal direction is measured. After that, a water-absorbing rate (mm/sec) is calculated from the obtained time according to the following Equation (2):
Water-absorbing rate (mm/sec)=40 (mm)/t (sec) Equation (2).
In the above Equation (2), t represents the time (sec) required for the ion exchange water to reach from the end of the rectangular sample in the longitudinal direction to a distance of 45 mm in the longitudinal direction.
(Cellulose Fibers)The cellulose fiber-containing composition of the present invention comprises, as main components, cellulose fibers having phosphoric acid groups. Herein, the state in which cellulose fibers having phosphoric acid groups are comprised as main components in the cellulose fiber-containing composition means that the content of the cellulose fibers having phosphoric acid groups is 50% by mass or more, with respect to the total mass of the cellulose fiber-containing composition. The content of the cellulose fibers having phosphoric acid groups is preferably 60% by mass or more, more preferably 70% by mass or more, and further preferably 80% by mass or more, with respect to the total mass of the cellulose fiber-containing composition.
The cellulose raw material for obtaining cellulose fibers is not particularly limited, but pulp is preferably used from the viewpoint of availability and inexpensiveness. Examples of the pulp include wood pulp, non-wood pulp, and deinked pulp. Examples of the wood pulp include chemical pulps such as leaf bleached kraft pulp (LBKP), needle bleached kraft pulp (NBKP), sulfite pulp (SP), dissolving pulp (DP), soda pulp (AP), unbleached kraft pulp (UKP), and oxygen bleached kraft pulp (OKP). Further, included are, but not particularly limited to, semichemical pulps such as semi-chemical pulp (SCP) and chemi-ground wood pulp (CGP); and mechanical pulps such as ground pulp (GP) and thermomechanical pulp (TMP, BCTMP). Examples of the non-wood pulp include, but not particularly limited to, cotton pulps such as cotton linter and cotton lint; non-wood type pulps such as hemp, wheat straw, and bagasse; and cellulose isolated from ascidian, seaweed, etc., chitin, and chitosan. As a deinked pulp, there is deinked pulp using waste paper as a raw material, but it is not particularly limited thereto. The pulp of the present embodiment may be used singly, or in combination of two or more types. Among the above-listed pulp types, wood pulp and deinked pulp including cellulose are preferable from the viewpoint of easy availability.
In the present invention, the fiber width of a cellulose fiber having a phosphoric acid group is not particularly limited. For example, the fiber width of a cellulose fiber having a phosphoric acid group may be greater than 1000 nm, or may also be 1000 nm or less. Moreover, cellulose fibers having a fiber width of greater than 1000 nm may be present together with cellulose fibers having a fiber width of 1000 nm or less. When the fiber width of a cellulose fiber is 1000 nm or less, such a cellulose fiber may also be referred to as an “ultrafine cellulose fiber.”
Moreover, the cellulose fiber-containing composition of the present invention may also comprise cellulose fibers having no phosphoric acid groups, as well as cellulose fibers having phosphoric acid groups. In this case, the content of the cellulose fibers having no phosphoric acid groups is preferably 20% by mass or less, and more preferably 10% by mass or less, with respect to the total mass of the fiber raw material.
Herein, the fiber width of a cellulose fiber can be measured by electron microscopic observation according to the following method. First, an aqueous suspension of cellulose fibers having a concentration of 0.05% by mass or more and 0.1% by mass or less is prepared, and the suspension is casted onto a hydrophilized carbon film-coated grid as a sample for TEM observation. At this time, SEM images of the surface of the suspension casted onto glass may be observed. The sample is observed using electron microscope images taken at a magnification of 1000×, 5000×, 10000×, or 50000×, depending on the widths of the constituent fibers. However, the sample, the observation conditions, and the magnification are adjusted so as to satisfy the following conditions:
(1) A single straight line X is drawn in any given portion in an observation image, and 20 or more fibers intersect with the straight line X.
(2) A straight line Y, which intersects perpendicularly with the aforementioned straight line in the same image as described above, is drawn, and 20 or more fibers intersect with the straight line Y.
The widths of the fibers intersecting the straight line X and the straight line Y in the observation image meeting the above-described conditions are visually read. Three or more sets of images of surface portions, which are at least not overlapped, are thus observed, and the widths of the fibers intersecting the straight line X and the straight line Y are read in the each image. At least 120 fiber widths (20 fibers×2×3=120) are thus read.
The average fiber length of cellulose fibers is not particularly limited, but it is preferably 0.1 mm or more, and more preferably 0.6 mm or more. On the other hand, it is preferably 5 mm or less, and more preferably 2 mm or less. By setting the average fiber length of cellulose fibers within the above-described range, when the cellulose fiber-containing composition is processed into a sheet, the strength of the sheet can be enhanced. Herein, the average fiber length of cellulose fibers can be obtained, for example, by using Kajaani Fiber Size Analyzer (FS-200) manufactured by Kajaani Automation to measure the length weighted average fiber length. Otherwise, the average fiber length of cellulose fibers may also be measured by using a scanning electron microscope (SEM), a transmission electron microscope (TEM), etc., depending on the length of the fiber.
When the cellulose fibers are ultrafine cellulose fibers, the ultrafine cellulose fibers preferably have a type I crystal structure. In this regard, the fact that ultrafine cellulose fibers have a type I crystal structure may be identified by a diffraction profile obtained from a wide angle X-ray diffraction photograph using CuKα (λ=1.5418 Å) monochromatized with graphite. Specifically, it may be identified based on the fact that there are typical peaks at two positions near 2θ=14° or more and 17° or less, and near 2θ=22° or more and 23° or less.
The percentage of the type I crystal structure occupied in the ultrafine cellulose fibers is preferably 30% or more, more preferably 50% or more, and further preferably 70% or more. In this case, more excellent performance can be expected, in terms of heat resistance and the expression of low linear thermal expansion. The crystallinity can be obtained by measuring an X-ray diffraction profile and obtaining it according to a common method (Seagal et al., Textile Research Journal, Vol. 29, p. 786, 1959).
In the present description, cellulose fibers have phosphoric acid groups (phosphoric acid groups or phosphoric acid group-derived substituents). In the present invention, such cellulose fibers may also be referred to as “phosphorylated cellulose fibers.”
The phosphoric acid group comprised in the phosphorylated cellulose fibers is a divalent functional group corresponding to a phosphoric acid, from which a hydroxyl group is removed. Specifically, it is a group represented by —PO3H2. The phosphoric acid group-derived substituents include substituents, such as condensation-polymerized phosphoric acid groups, salts of phosphoric acid groups, and phosphoric acid ester groups, and they may preferably be ionic substituents.
In the present invention, the phosphoric acid group or the phosphoric acid group-derived substituent may be a substituent represented by the following Formula (1):
In the Formula (1), a, b, and n each represent a natural number (provided that a=b×m); at least one of α1, α2, . . . , αn and α′ is O−, and the rest are either R or OR. All of αn and α′ may also be O−. When n is 2 or greater and α′ is R or OR, at least one of αn is O− and the rest are R or OR. When n is 2 or greater and α′ is O−, all of an may be R or OR, or at least one of an may be O− and the rest may be R or OR. R each represents a hydrogen atom, a saturated straight chain hydrocarbon group, a saturated branched chain hydrocarbon group, a saturated cyclic hydrocarbon group, an unsaturated straight chain hydrocarbon group, an unsaturated branched chain hydrocarbon group, an unsaturated cyclic hydrocarbon group, an aromatic group, or a derivative group thereof.
Examples of the saturated straight chain hydrocarbon group include a methyl group, an ethyl group, an n-propyl group, and an n-butyl group, but are not particularly limited thereto. Examples of the saturated branched chain hydrocarbon group include an i-propyl group and a t-butyl group, but are not particularly limited thereto. Examples of the saturated cyclic hydrocarbon group include a cyclopentyl group and a cyclohexyl group, but are not particularly limited thereto. Examples of the unsaturated straight chain hydrocarbon group include a vinyl group and an allyl group, but are not particularly limited thereto. Examples of the unsaturated branched chain hydrocarbon group include an i-propenyl group and a 3-butenyl group, but are not particularly limited thereto. Examples of the unsaturated cyclic hydrocarbon group include a cyclopentenyl group and a cyclohexenyl group, but are not particularly limited thereto. Examples of the aromatic group include a phenyl group and a naphthyl group, but are not particularly limited thereto.
Moreover, examples of the derivative of the above-described R include functional groups such as a carboxyl group, a hydroxyl group or an amino group, in which at least one type selected from functional groups is added to or substituted with the main chain or side chain of the above-described various types of hydrocarbon groups, but are not particularly limited thereto. Furthermore, the number of carbon atoms constituting the main chain of the above-described R is not particularly limited, but it is preferably 20 or less, and more preferably 10 or less. If the number of carbon atoms constituting the main chain of the R exceeds 20, the molecules of phosphorus oxoacid groups containing R become too large, the groups can hardly permeate into a fiber raw material, so that the yield of ultrafine cellulose fibers is likely to be decreased.
βb+ is a mono- or more-valent cation consisting of an organic or inorganic matter. Examples of the mono- or more-valent cation consisting of an organic matter include an aliphatic ammonium and an aromatic ammonium, and examples of the mono- or more-valent cation consisting of an inorganic matter include alkali metal ions such as sodium, potassium or lithium ions, divalent metal cations such as calcium or magnesium ions, and hydrogen ions, but are not particularly limited thereto. These can be applied alone as a single type or in combination of two or more types. As such mono- or more-valent cations consisting of an organic or inorganic matter, sodium or potassium ions, which hardly cause the yellowing of a fiber raw material containing β upon heating and are industrially easily applicable, are preferable, but are not particularly limited thereto.
The content of the phosphoric acid groups comprised in the cellulose fibers is preferably 0.10 mmol/g or more, more preferably 0.20 mmol/g or more, even more preferably 0.50 mmol/g or more, further preferably 1.00 mmol/g or more, still further preferably 1.20 mmol/g or more, particularly preferably 1.30 mmol/g or more, and most preferably 1.60 mmol/g or more, per gram (mass) of the cellulose fibers. On the other hand, the content of the phosphoric acid groups is preferably 3.65 mmol/g or less, more preferably 3.5 mmol/g or less, and further preferably 3.0 mmol/g or less. Besides, in the present description, the content of the phosphoric acid groups comprised in the cellulose fibers is equal to the amount of strongly acidic groups of phosphoric acid groups in the cellulose fibers, as described later.
The content of the phosphoric acid groups in the cellulose fibers can be measured by a neutralization titration method. Upon the measurement by such a neutralization titration method, phosphoric acid groups are completely converted to acid type groups, and fibrillation is then performed by a mechanical treatment step (fibrillation step). Thereafter, while a sodium hydroxide aqueous solution is added to the obtained ultrafine cellulose fiber-containing slurry, changes in the pH are obtained, so that the amount of phosphoric acid groups introduced can be measured.
Conversion of the phosphoric acid groups to acid type groups is carried out by diluting the obtained phosphorylated cellulose fibers with ion exchange water, so that the concentration of cellulose fibers becomes 2% by mass, and then gradually adding a sufficient amount of 1 N hydrochloric acid aqueous solution to the resulting phosphorylated cellulose fibers, while stirring. In such conversion of the phosphoric acid groups to acid type groups, it is preferable to repeat the operation of dehydrating the above-described cellulose fiber-containing slurry to obtain a dehydrated sheet, then diluting the dehydrated sheet with ion exchange water again, and then adding a 1 N hydrochloric acid aqueous solution to the resultant, so that the phosphoric acid groups contained in the cellulose fibers can be completely converted to acid type groups. Then, after completion of the step of converting the phosphoric acid groups to acid type groups, it is preferable to repeat the operation of stirring the obtained cellulose fiber-containing slurry to uniformly disperse it, followed by filtration and dehydration to obtain a dehydrated sheet, so that redundant hydrochloric acid can be fully washed away.
In the mechanical treatment step (fibrillation step), ion exchange water is poured onto the obtained dehydrated sheet to obtain a cellulose fiber-containing slurry, in which the concentration of cellulose fibers is 0.3% by mass, and this slurry is then treated using a defibration treatment device (manufactured by M Technique Co., Ltd., CLEARMIX-2.2S) under conditions of 21500 rotations/min for 30 minutes. Thus, an ultrafine cellulose fiber-containing slurry is obtained.
In the titration using alkali, changes in the pH values indicated by the dispersion are measured while adding a 0.1 N sodium hydroxide aqueous solution to the ultrafine cellulose fiber-containing slurry. In this neutralization titration, in a curve obtained by plotting pH values measured with respect to the amount of alkali (sodium hydroxide aqueous solution) added, two points, in which the increment (the derivative of pH to the amount of alkali added dropwise) becomes maximum, are obtained (i.e., a point in which the increment becomes maximum, and a point in which the increment becomes second maximum). Among these, the amount of alkali required until the maximum point of the increment obtained first after addition of alkali (hereinafter referred to as a “first end point”) is equal to the amount of strongly acidic groups in the dispersion used in the titration, and the amount of alkali required until the maximum point of the increment obtained second after addition of alkali (hereinafter referred to as a “second end point”) is equal to the amount of weakly acidic groups in the dispersion used in the titration. The alkali amount (mmol) required until the first end point is divided by the solid content (g) in the ultrafine cellulose fiber-containing slurry to be titrated, to obtain a first dissociated alkali amount (mmol/g). This amount is defined to be the content of the phosphoric acid groups in the cellulose fibers.
It is considered that a crosslinked structure is formed by dehydration condensation of phosphoric acid groups introduced into cellulose fibers. Specifically, the crosslinked structure is a structure in which glucose units of cellulose individually bind to each one of two P atoms of pyrophosphoric acid via an O atom. Accordingly, when such a crosslinked structure is formed, weakly acidic groups are apparently lost, and thus, the amount of alkali required up to the second end point is reduced in comparison to the amount of alkali required up to the first end point. Herein, if phosphoric acid groups introduced into cellulose fibers are not condensed at all, the amount of strongly acidic groups introduced into cellulose fibers is equal to the amount of weakly acidic groups introduced into cellulose fibers. As such, the value obtained by dividing the amount of weakly acidic groups lost as a result of the formation of the crosslinked structure by 2 indicates the amount of crosslinked structures (the number of crosslinking points). That is, the amount of the crosslinked structures (the number of crosslinking points) is equal to the value obtained by dividing a difference between the amount of alkali required up to the first end point (first dissociated alkali amount) and the amount of alkali required up to the second end point (second dissociated alkali amount) by 2. The amount of the crosslinked structures (the number of crosslinking points) is represented by the following Equation (1):
Amount of crosslinked structures (number of crosslinking points)=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 Equation (1)
In the present invention, the amount of crosslinked structures (the number of crosslinking points) in cellulose fibers, which is calculated by the above Equation (1), may be 0.20 mmol/g or more, preferably 0.22 mmol/g or more, and more preferably 0.25 mmol/g or more. Besides, the upper limit value of the amount of the crosslinked structures (the number of crosslinking points) is a value obtained by dividing the amount of strongly acidic groups contained in cellulose fibers by 2. Thus, the upper limit value may be, for example, 1.82 mmol/g or less.
The water retention capacity of cellulose fibers is preferably 150% or more, more preferably 170% or more, and further preferably 200% or more. The upper limit value of the water retention capacity of cellulose fibers is not particularly limited, but it can be set, for example, at 1000%. Herein, the water retention capacity of cellulose fibers is a value measured in accordance with SCAN-C 62:00, and this value is calculated according to the equation below. Upon the measurement of the water retention capacity of cellulose fibers, the cellulose fibers are subjected to a centrifugation treatment for 15 minutes under conditions of 20° C. and 4400 rpm (weight acceleration upon centrifugation: 3950 g). The amount of cellulose fibers subjected to a centrifugation treatment is set at 0.5 g (absolute dry weight) for a single measurement (added basis weight: 1700±100 g/m2). As a centrifugal separator used herein, for example, H-3R manufactured by KOKUSAN Co. Ltd. can be used. It is to be noted that the larger the numerical value of water retention capacity, the higher the affinity of cellulose fibers with water that can be obtained.
Water retention capacity (%)=(weight of cellulose fibers after centrifugation treatment−absolute dry weight of cellulose fibers)/absolute dry weight of cellulose fibers×100
Cellulose fibers may have counterions. Such counterions may be either inorganic ions or organic ions. Examples of the inorganic ions include: monovalent metal ions including alkali metal ions as representative examples; divalent metal ions including alkaline earth metal ions as representative examples; and other non-metal cations including base metal ions such as ammonium ions, aluminum ions, tin ions or lead ions, and transition metal ions such as silver ions, copper ions, or iron ions. Examples of the organic ions include organic ammonium ions and organic phosphonium ions. When water retention capacity intends to be enhanced, monovalent cations are preferably used as counterions. From the viewpoint of versatility, ammonium ions and alkali metal ions are more preferably used as counterions, and sodium ions and ammonium ions are further preferably used as counterions. On the other hand, when functions such as deodorant and antibacterial functions intend to be imparted, functional cations such as copper ions, silver ions, or organic ammonium ions are preferably used as counterions.
(Optional Component)The cellulose fiber-containing composition of the present invention may comprise optional components other than cellulose fibers. Examples of such optional components may include antifoaming agents, lubricants, surfactants, ultraviolet absorbing agents, dyes, pigments, fillers, stabilizers, organic solvents miscible with water, such as alcohol, antiseptics, organic fine particles, inorganic fine particles, and resins (pellet-type and fibrous resins).
(Method for Producing Composition)The method for producing a composition (method for producing a cellulose fiber-containing composition) comprises a step of introducing phosphoric acid groups or phosphoric acid group-derived substituents into cellulose fibers, and then crosslinking the phosphoric acid groups or phosphoric acid group-derived substituents, so that the number of crosslinking points in the cellulose fibers, which is calculated according to the following Equation (1), can be 0.20 mmol/g or more. The water content of the thus obtained composition is 50% by mass or less.
Number of crosslinking points=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 Equation (1)
The step of introducing phosphoric acid groups or phosphoric acid group-derived substituents into cellulose fibers may be referred to as a “phosphoric acid group introduction step.” Besides, a step of crosslinking at least a part of phosphoric acid groups or phosphoric acid group-derived substituents is included in the phosphoric acid group introduction step. That is to say, the phosphoric acid group introduction step includes a step of phosphorylating cellulose fibers and a step of crosslinking at least a part of phosphoric acid groups or phosphoric acid group-derived substituents.
The phosphoric acid group introduction step may be performed by allowing at least one selected from a compound having phosphoric acid groups or phosphoric acid group-derived substituents and salts thereof (hereinafter, referred to as a “phosphorylating agent” or “Compound A”) to react with cellulose fibers. Such a phosphorylating agent may be mixed into the cellulose fibers in a dry or wet state, in the form of a powder or an aqueous solution. In another example, a powder or an aqueous solution of the phosphorylating agent may be added into a slurry of the cellulose fibers. That is to say, the phosphoric acid group introduction step includes, at least, a step of mixing cellulose fibers with a phosphorylating agent.
The phosphoric acid group introduction step may be performed by allowing a phosphorylating agent to react with cellulose fibers. This reaction may be performed in the presence of at least one selected from urea and derivatives thereof (hereinafter, referred to as “Compound B”).
One example of the method of allowing Compound A to act on the cellulose fibers in the presence of Compound B includes a method of mixing the cellulose fibers in a dry or wet state with a powder or an aqueous solution of Compound A and Compound B. Another example thereof includes a method of adding a powder or an aqueous solution of Compound A and Compound B to a cellulose fiber-containing slurry. Among them, a method of adding an aqueous solution of Compound A and Compound B to the cellulose fibers in a dry state, or a method of adding a powder or an aqueous solution of Compound A and Compound B to the cellulose fibers in a wet state is preferable because of the high homogeneity of the reaction. Compound A and Compound B may be added at the same time or may be added separately. Alternatively, Compound A and Compound B to be subjected to the reaction may be first added as an aqueous solution, which may be then compressed to squeeze out redundant chemicals. The form of the cellulose fibers is preferably a cotton-like or thin sheet form, but the form is not particularly limited thereto.
The phosphorylating agent (Compound A) is at least one selected from compounds having phosphoric acid groups and the salts thereof. Examples of the compound having phosphoric acid groups include, but are not particularly limited to, phosphoric acid, lithium salts of phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid. Examples of the lithium salts of phosphoric acid include lithium dihydrogen phosphate, dilithium hydrogen phosphate, trilithium phosphate, lithium pyrophosphate, and lithium polyphosphate. Examples of the sodium salts of phosphoric acid include sodium dihydrogen phosphate, disodium hydrogen phosphate, trisodium phosphate, sodium pyrophosphate, and sodium polyphosphate. Examples of the potassium salts of phosphoric acid include potassium dihydrogen phosphate, dipotassium hydrogen phosphate, tripotassium phosphate, potassium pyrophosphate, and potassium polyphosphate. Examples of the ammonium salts of phosphoric acid include ammonium dihydrogen phosphate, diammonium hydrogen phosphate, triammonium phosphate, ammonium pyrophosphate, and ammonium polyphosphate. Among these, phosphoric acid, sodium salts of phosphoric acid, potassium salts of phosphoric acid, and ammonium salts of phosphoric acid are preferably used.
Since the uniformity of the reaction is improved and the efficiency in introduction of a phosphoric acid group is further enhanced, the phosphorylating agent (Compound A) is preferably used as an aqueous solution. Although there is no particular restriction on the pH of an aqueous solution of the phosphorylating agent (Compound A), the pH is preferably pH 7 or less because the efficiency in introduction of phosphoric acid groups becomes high, and more preferably pH 3 or more and pH 7 or less from the viewpoint of suppression of hydrolysis of cellulose fibers. The pH of an aqueous solution of the Compound A may be adjusted, for example, by using, among compounds having phosphoric acid groups, a combination of an acidic one and an alkaline one, and changing the amount ratio thereof. The pH of an aqueous solution of the phosphorylating agent (Compound A) may also be adjusted by adding an inorganic alkali or an organic alkali to an acidic compound among compounds having phosphoric acid groups.
The amount of the phosphorylating agent (Compound A) added to cellulose fibers is not particularly limited. However, when the additive amount of the phosphorylating agent (Compound A) is converted to the amount of phosphorus atoms, the amount of the phosphorus atoms added to cellulose fibers (absolute dry mass) is preferably 0.5% by mass or more and 100% by mass or less, more preferably 1% by mass or more and 50% by mass or less, and most preferably 2% by mass or more and 30% by mass or less. If the amount of the phosphorus atoms added to cellulose fibers is within the above-described range, the yield of phosphorylated cellulose fibers can be further improved. By setting the amount of the phosphorus atoms added to cellulose fibers at 100% by mass or less, a balance can be kept between the effect of improving the yield and costs. On the other hand, by setting the amount of the phosphorus atoms added to cellulose fibers at the above-described lower limit value or more, the yield can be enhanced.
Examples of the Compound B used in the present embodiment include urea, biuret, 1-phenyl urea, l-benzyl urea, 1-methyl urea, and i-ethyl urea.
The Compound B is preferably used as an aqueous solution, as with the Compound A. Further, an aqueous solution in which both the Compound A and Compound B are dissolved is preferably used, because the uniformity of a reaction may be enhanced. The amount of the Compound B added to cellulose fibers (absolute dry mass) is preferably 1% by mass or more and 500% by mass or less, more preferably 10% by mass or more and 400% by mass or less, further preferably 100% by mass or more and 350% by mass or less, and particularly preferably 150% by mass or more and 300% by mass or less.
The reaction system may contain an amide or an amine, in addition to the Compound A and the Compound B. Examples of the amide include formamide, dimethylformamide, acetamide, and dimethylacetamide. Examples of the amine include methylamine, ethylamine, trimethylamine, triethylamine, monoethanolamine, diethanolamine, triethanolamine, pyridine, ethylenediamine, and hexamethylenediamine. Among them, particularly, triethylamine is known to work as a favorable reaction catalyst.
The phosphoric acid group introduction step preferably has a heating step (hereinafter also referred to as a “heat treatment step”). By establishing such a heat treatment step, phosphoric acid groups can be efficiently introduced into cellulose fibers, and further, at least a part of phosphoric acid groups or phosphoric acid group-derived substituents can be crosslinked. That is to say, the method for producing a composition of the present invention preferably comprises a step of mixing cellulose fibers with a phosphorylating agent, and a step of heating a mixture of the cellulose fibers and the phosphorylating agent.
With regard to the heat treatment temperature applied in the heat treatment step, it is preferable to select a temperature that allows an efficient introduction of phosphoric acid groups while suppressing the thermal decomposition or hydrolysis reaction of cellulose fibers. In addition, with regard to the heat treatment temperature, it is preferable to select a temperature at which phosphoric acid groups or phosphoric acid group-derived substituents are crosslinked so that the number of crosslinking points in cellulose fibers calculated according to the aforementioned Equation (1) can be 0.20 mmol/g or more. Specifically, the temperature is preferably 50° C. or higher and 300° C. or lower, more preferably 100° C. or higher and 250° C. or lower, and further preferably 130° C. or higher and 200° C. or lower. Moreover, a vacuum dryer, an infrared heating device, or a microwave heating device may be used for heating.
Upon the heat treatment, if the time for leaving the cellulose fibers to stand still gets longer while the cellulose fiber-containing slurry to which the Compound A is added contains water, as drying advances, water molecules and the Compound A dissolved therein move to the surface of the cellulose fibers. As such, there is a possibility of the occurrence of unevenness in the concentration of the Compound A in the cellulose fibers, and the introduction of phosphoric acid groups into the cellulose fiber surface may not progress uniformly. In order to suppress the occurrence of unevenness in the concentration of the Compound A in the cellulose fibers due to drying, the cellulose fibers in the shape of a very thin sheet may be used, or a method of heat-drying or vacuum-drying the cellulose fibers, while kneading or stirring with the Compound A using a kneader or the like, may be employed.
As a heating device used for heat treatment, a device capable of always discharging moisture retained by slurry or moisture generated by an addition reaction of phosphoric acid groups with hydroxy groups of the fiber to the outside of the device system is preferable, and for example, forced convection ovens or the like are preferable. By always discharging moisture in the device system, in addition to being able to suppress a hydrolysis reaction of phosphoric acid ester bonds, which is a reverse reaction of the phosphoric acid esterification, acid hydrolysis of sugar chains in the cellulose fibers may be suppressed as well.
The time required for the heat treatment (heating time) is, although affected by the heating temperature, preferably 1 second or more and 300 minutes or less, more preferably 5 seconds or more and 270 minutes or less, and further preferably 10 seconds or more and 15000 seconds or less, after the phosphorylating agent has been mixed with cellulose fibers and the obtained mixture has been then exposed to the heat source. For example, when the heating temperature is 100° C. or higher and 250° C. or lower, the heating time is preferably 10 seconds or more, more preferably 20 seconds or more, and further preferably 30 seconds or more. When the heat treatment temperature is 100° C. or higher and 250° C. or lower, the upper limit of the heating time is preferably set at 15000 seconds or less. In the present invention, by setting the heating treatment temperature and the heating time within an appropriate range, the amount of phosphoric acid groups introduced can be set within a preferred range.
The phosphoric acid group introduction step may be performed at least once, but may be repeated multiple times as well. This case is preferable, since more phosphoric acid groups are introduced.
<Disintegration and/or Washing Steps>
After completion of the phosphoric acid group introduction step, it is preferable to establish a disintegration step, and after completion of the disintegration step, it is preferable to further establish a washing step. The disintegration step is carried out in accordance with JIS P 8220. That is, the disintegration step is a step of converting the phosphorylated cellulose fibers obtained in the phosphoric acid group introduction step to a homogeneous pulp suspension. In this step, the phosphorylated cellulose fibers preferably have a size equivalent to common paper pulp (e.g., a width of 20 μm or more and 30 μm or less, and a length-average fiber length of 0.1 mm or more and 3.0 mm or less). When a suspension, in which the phosphorylated cellulose fibers are fully dispersed, can be obtained without performing such a disintegration step, the disintegration step may be omitted.
In the washing step, redundant chemicals such as a phosphorylating agent are washed away. In the washing step, it is preferable to repeat the operation of subjecting the phosphorylated cellulose fibers after completion of the disintegration treatment to filtration and dehydration, then pouring ion exchange water thereon, and then uniformly dispersing the solution by stirring.
<Alkali Treatment Step>After completion of the phosphoric acid group introduction step and the disintegration step, it is preferable to establish an alkali treatment step. By establishing such an alkali treatment step, the counterions of the phosphorylated cellulose fibers can be changed to various ions. For example, when sodium hydroxide is selected as alkali, the counterions of the phosphorylated cellulose fibers can be sodium ions. In the method for producing the cellulose fiber-containing composition of the present invention, the alkali treatment step may not be established, and in this case, one counterion of the phosphoric acid groups of the phosphorylated cellulose fibers is an ammonium ion, and the other counterion thereof is a hydrogen ion.
The method of alkali treatment is not particularly limited, but a method of immersing the phosphorylated cellulose fibers in an alkaline solution is applied, for example.
The alkali compound contained in the alkaline solution is not particularly limited, but it may be either an inorganic alkaline compound or an organic alkali compound. The solvent of the alkaline solution may be either water or an organic solvent. The solvent is preferably a polar solvent (water, or a polar organic solvent such as alcohol), and the solvent may also be an aqueous solvent. Among alkaline solutions, a sodium hydroxide aqueous solution, or a potassium hydroxide aqueous solution is particularly preferable, because of high versatility.
The temperature of the alkaline solution in the alkali treatment step is not particularly limited, but it is preferably 5° C. or higher and 80° C. or lower, and more preferably 10° C. or higher and 60° C. or lower.
The immersion time in the alkaline solution in the alkali treatment step is not particularly limited, but it is preferably 5 minutes or more and 30 minutes or less, and more preferably 10 minutes or more and 20 minutes or less.
The amount of the alkaline solution used in the alkali treatment is not particularly limited, but it is preferably 100% by mass or more and 100000% by mass or less, and more preferably 1000% by mass and 10000% by mass or less, with respect to the absolute dry mass of the phosphorylated cellulose fibers.
In order to reduce the amount of an alkaline solution used in the alkali treatment step, the phosphorylated cellulose fibers may be washed with water or an organic solvent before the alkali treatment step. After the alkali treatment, the alkali-treated phosphorylated cellulose fibers are preferably washed with water or an organic solvent in order to improve the handling property.
<Other Counterion Changing Treatments>Counterions can also be changed by allowing the phosphorylated cellulose fibers to come into contact with inorganic alkali salts or organic alkali salts, instead of performing the aforementioned alkali treatment step. For example, when sodium chloride is selected as an inorganic alkali salt, the counterion of the phosphorylated cellulose fibers can be set to be sodium. Alternatively, when alkyl ammonium chloride is selected as an organic alkali salt, the counterion of the phosphorylated cellulose fibers can be set to be alkyl ammonium.
<Defibration Treatment>When the cellulose fibers used in the present invention are ultrafine cellulose fibers having a fiber width of 1000 nm or less, a defibration treatment step may be established after completion of the alkali treatment step In the defibration treatment step, fibers are defibrated usually using a defibration treatment apparatus to yield a slurry comprising ultrafine cellulose fibers, and there is no particular restriction on a treatment apparatus, or a treatment method.
A high-speed defibrator, a grinder (stone mill-type crusher), a high-pressure homogenizer, an ultrahigh-pressure homogenizer, a high-pressure collision-type crusher, a ball mill, a bead mill, or the like can be used as the defibration treatment apparatus. Alternatively, for example, a wet milling apparatus such as a disc-type refiner, a conical refiner, a twin-screw kneader, an oscillation mill, a homomixer under high-speed rotation, an ultrasonic disperser, or a beater may also be used as the defibration treatment apparatus. The defibration treatment apparatus is not limited to the above. Examples of a preferred defibration treatment method include a high-speed defibrator, a high-pressure homogenizer, and an ultrahigh-pressure homogenizer, which are less affected by milling media, and are free from apprehension of contamination.
<Sheet Formation Step>Preferably, the method for producing the cellulose fiber-containing composition of the present invention further comprises a step of forming a sheet using the aforementioned phosphorylated cellulose fibers. In this case, the cellulose fiber-containing composition is preferably a sheet-like non-woven fabric. In the step of forming a sheet using phosphorylated cellulose fibers, the formation method can be selected, as appropriate, depending on the properties, shape, and the like of the sheet. In the present embodiment, methods such as, for example, a wet paper-making method or a dry paper-making method can be adopted.
The step of forming a sheet using cellulose fibers may be a step of forming a sheet according to a wet paper-making method. Hereafter, an example of the case of forming a sheet according to the wet paper-making method will be described.
In the wet paper-making step, first, ion exchange water is added to the phosphorylated cellulose fibers obtained in the aforementioned step to obtain a cellulose fiber-containing slurry.
Then, the cellulose fiber-containing slurry is subjected to the wet paper-making step. Examples of the paper machine used in the wet paper-making step include a Fourdrinier paper machine, a twin-wire paper machine, a cylinder paper machine, an inclined wire paper machine, a single net paper machine, and a Yankee paper machine. Moreover, paper-making may also be carried out using a handmade papermaking device.
The sheet obtained in the wet paper-making step is preferably subjected to a dehydration drying step. In the dehydration step, dehydration may be carried out by applying a pressure to the sheet. The pressure applied herein is preferably 1 MPa or more, more preferably 5 MPa or more, and further preferably 10 MPa or more. On the other hand, the pressure is preferably 100 MPa or less. Since the cellulose fiber-containing composition of the present invention is excellent in terms of resistance to compression, a high-bulk sheet is easily obtained, even in a case where the pressure is applied under the above-described conditions in the dehydration step.
In the case of using a dry paper-making method in the sheet formation step, the phosphorylated cellulose fibers are uniformly mixed in the air, and the air current containing the phosphorylated cellulose fibers is then ejected onto a mesh-like endless belt comprising a suction box in the lower side thereof, so as to form a sheet. Specifically, the dry paper-making method is a method comprising a step of mixing the phosphorylated cellulose fibers in the air and accumulating them. In the dry paper-making method, the above-described operations may be repeated multiple times, as necessary.
The method of drying the sheet in the dehydration drying step is not particularly limited, and hot air, steam, infrared ray, microwave, etc. may be utilized, as appropriate. In addition, a method of directly contacting a sheet with a metal plate or a metal roll used as a heat transfer medium, etc., may also be applied, as appropriate.
A pressurization operation may be further carried out on the sheet after completion of the dehydration drying step. The pressure applied herein is preferably 1 MPa or more, more preferably 5 MPa or more, and further preferably 10 MPa or more. On the other hand, the pressure is preferably 100 MPa or less. By this operation, the thickness and density of the sheet can be adjusted, as appropriate. Moreover, since the cellulose fiber-containing composition of the present invention is excellent in terms of resistance to compression, a high-bulk sheet is easily obtained, even in a case pressurization is carried out under the above-described conditions.
(Intended Use)The intended use of the cellulose fiber-containing composition of the present invention is not particularly limited. The cellulose fiber-containing composition is preferably a sheet, and more preferably a non-woven fabric. The cellulose fiber-containing composition is utilized, for example, in the state of a fluff pulp or a non-woven fabric, as a component of absorbent articles for absorbing sweat, urine, menstrual blood, hazardous chemicals, etc., or is also utilized for sanitary papers, filter materials, buffer materials, etc.
EXAMPLESThe characteristics of the present invention will be more specifically described in the following examples and comparative examples. The materials, used amounts, ratios, treatment contents, treatment procedures, etc. described in the following examples can be appropriately modified, unless they are deviated from the gist of the present invention. Accordingly, the scope of the present invention should not be restrictively interpreted by the following specific examples.
Example 1 <Phosphorylation Reaction Step>Pulp manufactured by Oji Paper Co., Ltd. (solid content: 96% by mass, basis weight: 213 g/m2, sheet-shaped), which was needle bleached kraft pulp, was used as a raw material. 100 Parts by mass (absolute dry mass) of the needle bleached kraft pulp were impregnated with a mixed aqueous solution of ammonium dihydrogen phosphate and urea, and were then compressed to result in 45 parts by mass of the ammonium dihydrogen phosphate, 120 parts by mass of the urea, and 150 parts by mass of ion exchange water, so as to obtain a chemical-impregnated pulp. The obtained chemical-impregnated pulp was subjected to drying and heating treatments in a hot-air dryer of 165° C. for 350 seconds, so that phosphoric acid groups and phosphoric acid-crosslinked structures were introduced into cellulose in the pulp, thereby obtaining phosphorylated cellulose fibers A.
<Disintegration and Washing Step>ion exchange water was poured onto the obtained phosphorylated cellulose fibers A, so that the concentration of cellulose fibers became 2% by mass, and the resultant was then subjected to a disintegration treatment for 20 minutes, using a desk-top disintegrator with a volume of 2 L. The obtained pulp slurry was subjected to filtration and dehydration to obtain a dehydrated sheet, and ion exchange water was poured onto the sheet again, followed by stirring for uniform dispersion. By repeating this operation, redundant chemicals were fully washed away, so as to obtain phosphorylated cellulose fibers B.
<Alkali Treatment Step>The obtained phosphorylated cellulose fibers B were diluted with ion exchange water, so that the concentration of cellulose fibers became 2% by mass, and thereafter, a 1 N sodium hydroxide aqueous solution was gradually added to the resulting phosphorylated cellulose fibers B, while stirring, so as to obtain a pulp slurry having a pH value of 12±0.2. Thereafter, this pulp slurry was dehydrated to obtain a dehydrated sheet, and ion exchange water was then poured onto the sheet again, followed by stirring for uniform dispersion. The resultant was subjected to filtration and dehydration to obtain a dehydrated sheet. By repeating this operation, redundant sodium hydroxide was fully washed away, so as to obtain phosphorylated cellulose fibers C comprising phosphorylated cellulose. Subsequently, according to the after-mentioned method, the water retention capacity of the phosphorylated cellulose fibers C was measured. Moreover, according to the after-mentioned method, the amount of phosphoric acid groups introduced into the phosphorylated cellulose fibers C and the content of crosslinked structures were measured.
<Sheet Formation and Pressing Step>Ion exchange water was poured onto the obtained cellulose fibers C, so that the concentration of cellulose fibers became 0.3% by mass, and thereafter, the resultant was subjected to dehydration and filtration to obtain a cellulose fiber-containing sheet having an area of 0.0043 m2 and a basis weight of 200 g/m2. This cellulose fiber-containing sheet was dried in a humidity conditioning chamber having a temperature of 23° C. and a relative humidity of 50%, until the weight became constant. Subsequently, the resulting sheet was pressed at a pressure of 11.57 MPa for 60 seconds to obtain a cellulose fiber-containing sheet A (cellulose fiber-containing composition). By measuring the thickness of the pressed sheet, the density of the cellulose fiber-containing sheet A was calculated. The density of the sheet was calculated in accordance with JIS P 8118: 1998. As a paper thickness gauge, a high-bridge paper thickness gauge (No. 735; manufactured by TAKAHASHI SEISAKUSHO Ltd.) was used. In addition, according to the after-mentioned method, the water content (moisture content) and water-absorbing rate of the cellulose fiber-containing sheet A were measured.
Example 2Phosphorylated cellulose fibers and a cellulose fiber-containing sheet were obtained in the same manner as that of Example 1, with the exception that the aforementioned <Alkali treatment step> was not carried out. The obtained phosphorylated cellulose fibers and cellulose fiber-containing sheet were subjected to the same measurements as those carried out in Example 1.
Example 3Phosphorylated cellulose fibers and a cellulose fiber-containing sheet were obtained in the same manner as that of Example 1, with the exceptions that the time required for the drying and heat treatment was set at 300 seconds in the aforementioned <Phosphorylation reaction step>, and further that the time required for the disintegration treatment using a disintegrator was set at 15 minutes in the aforementioned <Disintegration and washing step>. The obtained phosphorylated cellulose fibers and cellulose fiber-containing sheet were subjected to the same measurements as those carried out in Example 1.
Example 4Phosphorylated cellulose fibers and a cellulose fiber-containing sheet were obtained in the same manner as that of Example 3, with the exception that the aforementioned <Alkali treatment step> was not carried out. The obtained phosphorylated cellulose fibers and cellulose fiber-containing sheet were subjected to the same measurements as those carried out in Example 1.
Comparative Example 1 <Disintegration Step>Pulp manufactured by Oji Paper Co., Ltd. (solid content: 96% by mass, basis weight: 213 g/m2, sheet-shaped), which was needle bleached kraft pulp, was used as a raw material. Ion exchange water was poured thereon, so that the concentration of cellulose fibers became 2% by mass, and the resultant was then subjected to a disintegration treatment for 5 minutes, using a desk-top disintegrator with a volume of 2 L. The obtained pulp slurry was subjected to filtration and dehydration to obtain cellulose fibers A′. According to the after-mentioned method, the water retention capacity of the cellulose fibers A′ was measured.
<Sheet Formation and Pressing Step>Ion exchange water was poured onto the obtained cellulose fibers A′, so that the concentration of cellulose fibers became 0.3% by mass, and thereafter, the resultant was subjected to dehydration and filtration to obtain a cellulose fiber-containing sheet having an area of 0.0043 m2 and a basis weight of 200 g/m2. This cellulose fiber-containing sheet was dried in a humidity conditioning chamber having a temperature of 23° C. and a relative humidity of 50%, until the weight became constant. Subsequently, the resulting sheet was pressed at a pressure of 11.57 MPa for 60 seconds to obtain cellulose fiber-containing sheet A′. By measuring the thickness of the pressed sheet, the density of the cellulose fiber-containing sheet A′ was calculated. Moreover, according to the after-mentioned methods, the water content (moisture content) and water-absorbing rate of the cellulose fiber-containing sheet A′ were measured.
Comparative Example 2Phosphorylated cellulose fibers and a cellulose fiber-containing sheet were obtained in the same manner as that of Example 1, with the exceptions that the time required for the drying and heat treatment was set at 200 seconds in the aforementioned <Phosphorylation reaction step>, and further that the treatment using a disintegrator was not carried out in the aforementioned <Disintegration and washing step>. The obtained phosphorylated cellulose fibers and cellulose fiber-containing sheet were subjected to the same measurements as those carried out in Example 1. It is to be noted that, in Comparative Example 2, after addition of ion exchange water, the phosphorylated cellulose fibers were uniformly dispersed in water only by gently stirring the solution by hand, and thus, a disintegrator was not used.
Comparative Example 3Phosphorylated cellulose fibers and a cellulose fiber-containing sheet were obtained in the same manner as that of Comparative Example 2, with the exception that the aforementioned <Alkali treatment step> was not carried out. The obtained phosphorylated cellulose fibers and cellulose fiber-containing sheet were subjected to the same measurements as those carried out in Example 1. It is to be noted that, in Comparative Example 3, after addition of ion exchange water, the phosphorylated cellulose fibers were uniformly dispersed in water only by gently stirring the solution by hand, and thus, a disintegrator was not used.
(Analysis and Evaluation) <Measurement of Water Retention Capacity>The water retention capacity of cellulose fibers was measured in accordance with SCAN-C 62:00. Upon the measurement of the water retention capacity of cellulose fibers, the cellulose fibers were subjected to a centrifugation treatment for 15 minutes under conditions of 20° C. and 4400 rpm (weight acceleration upon centrifugation: 3950 g). The amount of cellulose fibers subjected to the centrifugation treatment was 0.5 g (added basis weight: 1700±100 g/m2) for a single measurement. As a centrifugal separator, H-3R manufactured by KOKUSAN Co. Ltd. was used. The water retention capacity was calculated according to the following equation:
Water retention capacity (%)=(weight of cellulose fibers after centrifugation treatment−absolute dry weight of cellulose fibers)/absolute dry weight of cellulose fibers×100.
It is to be noted that the larger the numerical value of water retention capacity, the higher the affinity of cellulose fibers with water that can be obtained.
<Measurement of Amount of Phosphoric Acid Groups Introduced>The amount of phosphoric acid groups introduced was measured by a neutralization titration method. Specifically, the phosphoric acid groups contained in cellulose fibers were completely converted to acid type groups, and fibrillation was then performed by a mechanical treatment step (fibrillation step). Thereafter, while a 0.1 N sodium hydroxide aqueous solution was added to the obtained ultrafine cellulose fiber-containing slurry, changes in the pH of the slurry (dispersion) were obtained, so that the amount of the phosphoric acid groups introduced was measured.
Conversion of the phosphoric acid groups to acid type groups was carried out by diluting the obtained phosphorylated cellulose fibers with ion exchange water, so that the concentration of cellulose fibers became 2% by mass, and then gradually adding a sufficient amount of 1 N hydrochloric acid aqueous solution to the resulting phosphorylated cellulose fibers, while stirring. Subsequently, this cellulose fiber-containing slurry was stirred for 15 minutes and was then dehydrated to obtain a dehydrated sheet. Thereafter, by repeating the operation of diluting the sheet with ion exchange water again, then adding a 1 N hydrochloric acid aqueous solution thereto, the phosphoric acid groups contained in the cellulose fibers were completely converted to acid type groups. Further, by repeating the operation of stirring this cellulose fiber-containing slurry for uniform dispersion, and then subjecting to filtration and dehydration to obtain a dehydrated sheet, redundant hydrochloric acid was fully washed away.
In the mechanical treatment step, ion exchange water was poured onto the obtained dehydrated sheet to obtain a cellulose fiber-containing slurry, in which the concentration of cellulose fibers was 0.3% by mass, and this slurry was then treated using a defibration treatment device (manufactured by M Technique Co., Ltd., CLEARMIX-2.2S) under conditions of 21500 rotations/min for 30 minutes. In the titration using alkali, changes in the pH values indicated by the dispersion were measured while adding a 0.1 N sodium hydroxide aqueous solution to the ultrafine cellulose fiber-containing slurry.
In this neutralization titration, in a curve obtained by plotting pH values measured with respect to the amount of alkali added, two points, in which the increment (the derivative of pH to the amount of alkali added dropwise) becomes maximum, are given (i.e., a point in which the increment becomes maximum, and a point in which the increment becomes second maximum). Among these, the amount of alkali required until the maximum point of the increment obtained first after addition of alkali (hereinafter referred to as a “first end point”) is equal to the amount of strongly acidic groups in the dispersion used in the titration, and the amount of alkali required until the maximum point of the increment obtained second after addition of alkali (hereinafter referred to as a “second end point”) is equal to the amount of weakly acidic groups in the dispersion used in the titration.
The alkali amount (mmol) required until the first end point was divided by the solid content (g) in the dispersion to be titrated, to obtain a first dissociated alkali amount (mmol/g). This amount was defined to be the amount of phosphoric acid groups introduced.
<Measurement of Number of Crosslinking Points>It is considered that a crosslinked structure is formed by dehydration condensation of phosphoric acid groups introduced into cellulose fibers. Specifically, the crosslinked structure is a structure in which glucose units of cellulose individually bind to each one of two P atoms of pyrophosphoric acid via an O atom. Accordingly, when such a crosslinked structure is formed, weakly acidic groups are apparently lost, and thus, the amount of alkali required up to the second end point is reduced in comparison to the amount of alkali required up to the first end point. Specifically, the number of crosslinking points is equal to a value obtained by dividing a different between the amount of alkali required up to the first end point (first dissociated alkali amount) and the amount of alkali required up to the second end point (second dissociated alkali amount) by 2.
<Measurement of Water Content>With regard to the water content, the weight of a cellulose fiber-containing sheet, which had been dried up to an equilibrium state in a humidity conditioning chamber at 23° C. and a relative humidity of 50%, was measured. Thereafter, the cellulose fiber-containing sheet was dried at 105° C. overnight, and the weight of the resulting cellulose fiber-containing sheet was then measured. After that, the water content was calculated according to the following equation:
Water content (%)=(weight of cellulose fiber-containing sheet before drying at 105° C.−weight of cellulose fiber-containing sheet after drying at 105° C.)/weight of cellulose fiber-containing sheet before drying at 105° C.×100.
With regard to the water-absorbing rate, the cellulose fiber-containing sheet was cut into a rectangular sample having a width of 5 mm and a length of 50 mm, and an edge region ranging from the end of this rectangular sample in the longitudinal direction to 5 mm from the end was then immersed in ion exchange water (electrical conductivity: 2 pS/cm or less). Thereafter, the time required for the ion exchange water to reach from the end of the longitudinal direction to a distance of 45 mm in the longitudinal direction was measured. After that, a water-absorbing rate (mm/sec) was calculated according to the following Equation (2):
Water-absorbing rate (mm/sec)=40 (mm)/t (sec) Equation (2).
In the above Equation (2), t represents the time (see) required for the ion exchange water to reach from the end of the rectangular sample in the longitudinal direction to a distance of 45 mm in the longitudinal direction.
The cellulose fibers obtained in the Examples had high water retention capacity, and the cellulose fiber-containing sheets exhibited an excellent water-absorbing rate. Although the cellulose fiber-containing sheets obtained in the Examples were high-bulk sheets (low-density sheets), these sheets achieved both excellent water-retaining ability and a high water-absorbing rate.
Claims
1. A composition comprising cellulose fibers having phosphoric acid groups or phosphoric acid group-derived substituents, wherein
- in at least a part of the cellulose fibers, the phosphoric acid groups or the phosphoric acid group-derived substituents are crosslinked,
- the number of crosslinking points in the cellulose fibers, which is calculated according to the following Equation (1), is 0.20 mmol/g or more, and
- the water content is 50% by mass or less, with respect to the total mass of the composition: Number of crosslinking points=(amount of strongly acidic groups contained in cellulose fibers−amount of weakly acidic groups contained in cellulose fibers)/2 Equation (1).
2. The composition according to claim 1, which is a non-woven fabric.
3. The composition according to claim 1, wherein when the composition is processed into a rectangular sample having a width of 5 mm and a length of 50 mm, then, an edge region ranging from the end of the rectangular sample in the longitudinal direction to 5 mm from the end is immersed in ion exchange water (electrical conductivity: 2 μS/cm or less), and then, the time required for the ion exchange water to reach from the end of the longitudinal direction to a distance of 45 mm in the longitudinal direction is measured, a water-absorbing rate (mm/sec), which is calculated according to the following Equation (2), is 2.5 mm/sec or more and 100 mm/sec or less:
- Water-absorbing rate (mm/sec)=40 (mm)/t (sec) Equation (2)
- wherein t represents the time (sec) required for the ion exchange water to reach from the end of the rectangular sample in the longitudinal direction to a distance of 45 mm in the longitudinal direction.
4. The composition according to claim 1, wherein the amount of the strongly acidic groups contained in the cellulose fibers is 1.60 mmol/g or more.
5. The composition according to claim 1, wherein the water retention capacity (%) of the cellulose fibers, which is calculated according to the following equation, is 150% or more:
- Water retention capacity (%)=(weight of cellulose fibers after centrifugation treatment−absolute dry weight of cellulose fibers)/absolute dry weight of cellulose fibers×100,
- wherein, in the above equation, the water retention capacity is measured in accordance with SCAN-C 62:00, and conditions for the centrifugation treatment are determined to be 20° C. and weight acceleration upon the centrifugation of 3950 g, and 15 minutes.
Type: Application
Filed: Sep 29, 2017
Publication Date: Sep 12, 2019
Applicant: OJI HOLDINGS CORPORATION (Tokyo)
Inventors: Yusuke TODOROKI (Chiba), Yuichi NOGUCHI (Tokyo), lkue HOMMA (Tokyo), Mengchen ZHAO (Chiba)
Application Number: 16/338,197
