JP2002533587A - Kraft wood fiber for carboxyalkyl cellulose - Google Patents

Abstract

  (57) [Summary] A method is disclosed for making kraft wood fibers having an alpha cellulose content greater than 97% and a viscosity greater than 40 centipoise. The method includes the steps of pre-hydrolyzing hardwood chips with water, kraft cooking, bleaching and caustic treatment. The resulting pulp can be converted to a carboxymethylcellulose superabsorbent having improved properties, especially high "absorbency under load".

Description

DETAILED DESCRIPTION OF THE INVENTION

TECHNICAL FIELD [0001] The present invention relates to a method for producing kraft wood pulp, for example, used as a superabsorbent.

BACKGROUND OF THE INVENTION The use of absorbent materials, commonly known as superabsorbents, in disposable absorbent personal care products is known. Such absorbent materials are commonly used in diapers, training pants, adult incontinence products, feminine hygiene products, and the like, reducing the overall bulk and increasing the absorption capacity of these products. Such absorbent materials are commonly present in absorbent products in fibrous matrices such as wood pulp fluff matrices. Wood pulp fluff matrix generally has an absorption capacity of about 6 grams of liquid per gram of fluff. Absorbent materials described above generally have an absorption capacity in water of at least about 10 times, preferably about 20 times, and often 100 times their weight. Obviously, the incorporation of such absorbent materials into personal care products increases the absorption capacity while reducing the overall bulk of such products.

[0003] A variety of materials have been described for use as absorbent materials in such personal care products. Such materials include agar, pectin, gums,
Natural base materials such as carboxyalkyl starch, carboxyalkylcellulose including carboxymethylcellulose and the like, as well as synthetic materials such as polyacrylates, polyacrylamides, hydrolyzed polyacrylonitriles and the like are included. Naturally based absorbent materials are known for use in personal care products, but such products are not widely used. The reason for their underutilization is due at least in part to their poor absorption properties compared to synthetic absorbent materials such as polyacrylates. In particular, many natural base materials tend to form a soft jelly-like mass when swollen with a liquid. When used in absorbent products, the presence of such soft jelly-like masses tends to impede the movement of liquids within the fibrous matrix into which the absorbent material has been incorporated. This phenomenon is known as gel blocking. Once gel blocking occurs, subsequent release of the liquid is not effectively absorbed by the product, causing the product to leak. further,
Many natural base materials exhibit poor absorption properties, especially when subjected to external pressure. In contrast, synthetic absorbent materials are often capable of absorbing large amounts of liquid while retaining their generally stiff, non-jelly-like properties. Thus, synthetic absorbent materials can be incorporated into absorbent products with minimal risk of gel blocking.

[0004] Carboxyalkyl polysaccharides and carboxyalkyl cellulose materials are well known in the art. Unfortunately, many known polysaccharides and cellulose do not have absorption properties comparable to many of the synthetic, highly absorbent materials.

SUMMARY OF THE INVENTION [0005] The present invention provides a method of making kraft wood pulp having an alpha cellulose content of greater than 97% and a viscosity (measured on a water soluble pulp by the 0.5% CED method) of greater than 40 centipoise. About. The method includes pre-hydrolyzing hardwood chips with water, kraft cooking the wood chips, bleaching the pulp, and caustic treating. The resulting pulp can be converted to a superabsorbent of carboxyalkyl polysaccharide, preferably carboxyalkylcellulose, most preferably carboxymethylcellulose, which has improved properties, especially about 20 or greater, More particularly, high Absorbency Under Load, which is 20 to about 25.
r Load).

DETAILED DESCRIPTION OF THE INVENTION In one embodiment, the present invention provides an alphacellulose content of greater than 97% and a viscosity of 30 centipoise or greater, more specifically 40 centipoise or greater. A kraft wood pulp having a viscosity (measured on a water-soluble pulp by the 0.5% CED method according to the Pulp and Paper Industry Technical Association (TAPPI) test method T230 om-89). In another embodiment, the wood pulp has a viscosity greater than about 42 centipoise. Preferably, the wood pulp comprises hardwood.

While the wood pulp of the present invention is based on hardwoods such as oak, eucalyptus, poplar, beech, and aspen, the methods of the present invention can include a wide variety of cellulosic fibers. Illustrative cellulosic fibers include, but are not limited to, wood products, such as wood and wood pulp fibers, from cotton, from straw and grass, such as rice and espart, and from stems and culms, such as bagasse. And non-wood paper fibers from stem stems having bast fibers such as jute, flax, kenaf, cannabis and ramie, and from leaf fibers such as Manila hemp and sisal. It is also possible to use a mixture of one or more cellulosic fibers. Preferably, the cellulosic fibers used are from a wood source. Preferred wood sources include softwoods such as pine, spruce and fir, and hardwoods such as oak, eucalyptus, poplar, beech and aspen.

As used herein, the term “fiber” or “fibrous” refers to a larger dimension of less than 10 mm, preferably less than 5 mm, often about 0.1 mm to 3 mm
And a length-to-diameter ratio (aspect ratio) of such a granular material.
Means greater than about 10. In contrast, the term "non-fibrous" or "non-fibrous" is meant to refer to a particulate material having a length to diameter ratio of such a particulate material of about 10 or less. .

[0009] It is generally desirable that the cellulose fibers used herein have wettability. As used herein, the term "wettable" is meant to refer to fibers or materials having a contact angle with water in air of less than 90 °. Cellulose fibers useful in the present invention have a contact angle with water in air of about 10 ° to about 5 °.
Suitably, between 0 ° and more preferably between about 20 ° and about 30 °. The wettable fiber is suitably a mixture of water and air in air at a temperature between about 0 ° C. and less than about 100 ° C., typically at an ambient temperature such as about 23 ° C. to 28 ° C. Fibers having a contact angle of less than 90 ° are referred to.

It is preferable that the cellulose fibers have wettability by nature. However, fibers that do not originally have wettability can also be used. It is possible to treat the surface of the fiber in a suitable manner to make it more or less wettable. When surface treated fibers are used, it is desirable that the surface treatment not be temporary. That is, it is desirable that the surface treatment not be washed off from the surface of the fiber upon the initial release or contact of the liquid. For the purpose of this application, surface treatment to non-wetting fibers is generally performed by measuring the majority of fibers in air with three consecutive contact angle measurements, drying between each measurement. It would be considered non-temporary if it showed that the contact angle with water was less than 90 °. That is, three individual contact angle measurements were made on the same fiber, and if all three contact angle measurements indicated that the contact angle of water in air was less than 90 °, the surface Processing would be considered non-temporary. If the surface treatment is temporary, the surface treatment will tend to be washed off the fiber during the first contact angle measurement, exposing the non-wetting surface of the fiber underlayer and the next contact angle measurement Will show a value greater than ゜. Useful wetting agents include polyalkylene glycols such as polyethylene glycol. The wetting agent is advantageously present in an amount of less than about 5 weight percent, preferably less than about 3 weight percent, more preferably less than about 2 weight percent of the total weight of the fiber, material or absorbent structure being treated. used.

In the present invention, the cellulosic fiber is desirably used in a form in which the cellulosic fiber has already been purified to pulp. In such a case, the cellulosic fibers would be substantially in the form of individual fibers, even if they were in an aggregated form such as a pulp sheet. Also, current methods are in contrast to known steam explosion methods, which generally treat cellulosic fibers, typically in the form of virgin wood chips and the like. Thus, the current method is a post-pulping cellulosic fiber modification method as compared to the known steam explosion method commonly used for high yield pulp production or waste recycling processes.

[0012] The starting material for the process of the invention will usually be wood chips having a suitable length of fiber for papermaking. Swarf can also be used, but sawdust is undesirable except for a small portion of the total feed because the fibers are partially cut. The chips must also be free of bark and foreign matter, as is well known. For the purposes of the present invention, it is desirable to avoid coarse chips. One problem with coarse chips is that cooking will be incomplete. It is best to use chopped or thin chips.

The process of the present invention involves the treatment of hardwood fibers or chips in four basic stages: pre-hydrolysis, kraft cooking, bleaching and caustic treatment. This process produces a kraft wood pulp having an alpha cellulose content of greater than 97% and a viscosity of 30 centipoise or greater (measured by the 0.5% CED method). Alpha cellulose is a major component of wood and paper pulp. This is the portion of cellulose that is insoluble in concentrated solutions of sodium hydroxide. Methods for measuring the alpha content of pulp include TAPPI method T203 and AST
Details are given in MD-588-42. Although conventional processing of hardwood is known to provide high alpha cellulose kraft pulp, the viscosity of these pulp is quite low, generally below about 20 centipoise.

[0014] The first step in the method of prehydrolysis present invention is a step of hydrolysis is used as a method of improving the amount of hemicellulose removed from lignocellulosic material while the degradation of cellulose to a minimum. The wood chips in the water are mixed in a M / K digester at 4 /
At a liquid / wood ratio of 1 it was heated to 170 ° C. in 60 minutes and brought to that temperature for 20 minutes. After the hydrolysis step was performed, the hydrolysis liquid was discharged from the digester.

The particular hydrolysis used in the method of the present invention depends on the type of wood and the degree of hemicellulose removal. This hydrolysis has a significant effect on the viscosity of the final high alpha cellulose. Due to the favorable degree of hydrolysis, H
The coefficients were used as control means. The definition of the H coefficient was defined in 1965 Intersc
Pul by Rydholm, published by ence Publishers
It can be found in typical pulp books such as ping Processes. Basically, this H factor is the variable used in kraft cooking, where the temperature and time variables are combined into a single variable that indicates the degree of cooking. For this hydrolysis step, the H factor is used to characterize the degree of hydrolysis. For the present invention, the temperature and time of the hydrolysis is such that the H factor is about 300 to about 1000, more specifically about 500 to about 800, and even more specifically about H, with a corresponding minimum chip yield of about 80%. It must be adjusted to be in the range of 600 to about 700. In the present invention, the preferred chip yield is 90%. Chip yield is defined as the ratio of the weight of the obtained chip (on an oven-dried basis) to the weight of the original chip (on an oven-dried basis).

In the kraft cooking stage, the hydrolyzed chips are placed in an M / K digester in a cooking liquor of sodium hydroxide and sodium sulfide at a liquid / wood ratio of 4/1 at 170 ° C. for 3 hours.
Heated for 5 to 60 minutes. The cooking liquor has an effective alkalinity of 15% and a sulphidity of 25.
%. The definition and calculation of the effective alkalinity and sulphidity are in
It can be found in representative pulp books, such as Pudding Processes by Rydholm, published by rscience Publishers. For the present invention, the effective alkalinity is in the range of 10-20%, and the sulfidity is in the range of 15-40%. The H factor is used to characterize the degree of cooking. The H-factor used to obtain the desired pulp depends on the available alkalinity and sulphidity. For the present invention, the effective alkalinity, sulfidity and H-factor must be adjusted to obtain an unbleached pulp with a minimum kappa number of 5 to achieve the desired end product. Kappa number is used to indicate the degree of lignin removal. Kappa number is based on TAPPI test method T23
Measured according to 6 cm-85.

Bleaching In the next stage, the unbleached pulp was subjected to a bleaching process to remove residual lignin in a series of steps using a selected combination of chemical reactants. Various combinations of chemical treatments have been proposed in the prior art. Furthermore, the individual chemical treatments have been rearranged in almost endless combinations and permutations. Therefore, in order to simplify the description of the various bleaching processes, the use of character codes has been used in combination to describe the individual chemical reactants used and the order of the process steps. Where appropriate, the character codes used thereafter are as follows: C = Chlorination Reaction with simple chlorine in acidic medium E = Alkali extraction Decomposition of reaction product using NaOH E (O) = Oxidative alkali Decomposition of reaction product using NaOH and oxygen D = Chlorine dioxide Acidic medium P = Peroxide Reaction with peroxide in alkaline medium O = Oxygen Reaction with elemental oxygen in alkaline medium Z = Ozone Reaction with ozone C / D With chlorine Mixture with chlorine dioxide H = hypochlorite Reaction with hypochlorite in alkaline solution

For the present invention, many combinations, such as DED, C / DED, are used to remove residual lignin and to reduce pulp brightness to at least 70%, and preferably to 85%. C (chlorinated) and H (hypochlorite) are not used because they break down the fibers and reduce the viscosity.

Caustic Extraction The next step is a caustic extraction to further remove residual hemicellulose in the bleached fibers. Caustic extraction conditions are treatment of the bleached fibers in a 6-12% sodium hydroxide solution at 15-65 ° C. for 10-100 minutes. A particular condition for obtaining an alpha cellulose content higher than 97% is the treatment of the bleached fibers in a 7.5% caustic soda solution at 25 ° C. for 60 minutes.

The treated wood pulp, prepared as described herein, is then
It can be converted to carboxyalkyl polysaccharides, preferably carboxyalkylcellulose, and most preferably carboxymethylcellulose (CMC) superabsorbent by methods well known in the art. A preferred method of conversion is assigned to the assignee of the present invention and is incorporated by reference herein in its entirety.
47,072 (Ning et al.). It is preferable that the obtained carboxyalkyl polysaccharide, carboxyalkyl cellulose, and carboxymethyl cellulose have relatively high molecular weights. It is generally most convenient to express the molecular weight of carboxymethylcellulose as the viscosity of a 2.0% by weight aqueous solution. Preferably, the carboxymethyl cellulose is from about 50 centipoise to about 80,000 centipoise, preferably from about 2,000 centipoise to about 8 centipoise.
0000 centipoise, most preferably about 20,000 centipoise to about 8
It has a viscosity of 2.0% by weight aqueous solution of 000 centipoise.

The carboxyalkyl cellulose has a pH in the range from about 5.0 to about 11.0, advantageously from about 6.0 to about 10.0, preferably from about 6.5 to about 9. It is suitable. It is generally desirable that the carboxyalkylcellulose be substantially neutral. The carboxyalkyl cellulose desirably has an absorbency under load (AUL) of at least about 17, preferably at least about 20, most advantageously at least about 24, and preferably at least about 27 grams per gram.

[0022] EXAMPLES The following examples illustrate the examples but not limited to the present invention. Unless otherwise noted, all parts, percentages, ratios, etc., are by weight.

[0023] For Example 1 Example 1, Terrace Bay aspen wood chips, 4: 1 water: mixed with water in a timber ratio. The mixture was heated to a temperature of about 170 ° C for 60 minutes and then held at 170 ° C for 20 minutes. When the pre-hydrolysis was completed, the liquid was discharged. The prehydrolyzed aspen was mixed with an alkaline solution (effective alkalinity 14.5%, sulphidity 25%) at a 4: 1 solution: chip ratio. The mixture was heated to a temperature of about 170 ° C. in 60 minutes and then held at 170 ° C. for 35 minutes. At the end of this kraft cooking stage, liquid was discharged. In the bleaching process, the kraft cooked and pre-hydrolyzed aspen fibers were processed in three stages. Wood fiber is diluted to 10% in an aqueous solution having 0.94% chlorine dioxide and 135%
Hold at ゜ F (57 ° C.) for 60 minutes. A hot caustic extraction step follows and the fibers are diluted to 10% in an aqueous solution with 1.5% sodium hydroxide and
Hold at 60 ° F. for 70 minutes. The chlorine dioxide step was repeated and the fibers were diluted to 10% in an aqueous solution having 0.6% chlorine dioxide and kept at 160 ° F (71 ° C) for 150 minutes. A caustic treatment (cold caustic extraction) is then performed, and the fibers are diluted to 10% in an aqueous solution with 7.5% sodium hydroxide,
It was kept at 7 ° F (25 ° C) for 60 minutes. The resulting kraft pulp has an alpha cellulose content of 97.8% and 42.
It had a viscosity of 9 centipoise.

Example 2 Example 2 was prepared as described in Example 1, except that mixed southern hardwood chips were used instead of aspen wood. The resulting kraft pulp has an alpha cellulose content of 98.7% and 4.
It had a viscosity of 5 centipoise.

COMPARATIVE EXAMPLE A Comparative Example A was made from kraft wood pulp ("ITr Rayonier"
anier "). This is probably pine pulp.

COMPARATIVE EXAMPLE B Comparative Example A was prepared from southern softwood kraft pulp (US Alliance Co.)
r54, available under the name of "CR54 Southern softwood kraft pulp").

The overall properties of Examples 1 and 2 and Comparative Examples A and B are shown in Table 1 below. Table 1 shows the percentage of alpha cellulose, the degree of polymerization in water (DP
w), and viscosity in centipoise (measured by TAPPI 0.5% CED method).

Table 1 Wood pulp properties

Absorbency Under Load (AUL) measures the ability of an absorbent material to absorb a liquid (a 0.9% by weight solution of sodium chloride in distilled water) when under load or restraint. Is the way. FIG. 1 describes an apparatus and method for measuring AUL. A perspective view of the device under test is shown. A test jack 1 having an adjustment knob 2 for raising and lowering a platform 3 is shown. A test stand 4 holds a spring 5 connected to a modified thickness gauge probe 6, which penetrates the housing 7 of the thickness gauge, and the housing 7 is fixed by the test stand. A plastic sample cup 8 containing a sample of the superabsorbent material to be tested has a liquid permeable bottom and is placed in a Petri dish 9 in which the saline solution to be absorbed is placed. Can be put in. A weight 10 is placed on a spacer plate (invisible) which is placed on a sample of the superabsorber (invisible).

The sample cup consists of a plastic cylinder having an inner diameter of 1 inch and an outer diameter of 1.25 inches. At the bottom of the sample cup, a 100 mesh metal mesh with 150 micron holes is formed by gluing to the end of the barrel, which heats the mesh to a temperature above the melting point of the plastic and heats the plastic barrel. This is done by pressing against a hot mesh, melting the plastic and bonding the mesh to the plastic cylinder.

[0031] A modified thickness gauge used to measure the swelling of a sample when absorbing saline is available from Mitutoyo Digimatic Indicator, IDC.
Series 543, Model 543-180, with a measurement range of 0-0.5 inches and an accuracy of 0.00005 inches (Mitutoyo Corporation)
The ion is 5-31-19 Shiba, Minato-ku, Tokyo 108, Japan). Mitutoyo C
When supplied by the corporation, the gage has a spring in the meter housing connected to the probe. Remove this spring,
A free fall probe having a downward force of about 27 grams is obtained. In addition, the cap covering the top of the probe attached to the top of the meter housing is also removed, and a hanging spring 5 (McMaster-
Carr Supply Co. (Available under the product number 964OK41 from Co., Ltd.), but this spring 5 opposes or reduces the downward force of the probe so that this force is about 1 gram ± 0.5 gram. A wire hook for connection to a suspension spring can be glued to the upper end of the probe. An extension needle (Mitutoyo Corporation) is also provided at the lower end of the probe.
ion, part number 131279) is provided to allow the probe to be inserted into the sample cup.

To perform the test, a 0.160 gram sample of the absorbent material, previously sieved to a particle size between 300 and 600 microns, is placed in a sample cup. The sample is then covered with a plastic spacer plate weighing 4.4 grams, which is slightly smaller than the inside diameter of the sample cup so that the sample is not disturbed during the test. Then a 100 gram weight is placed on the spacer plate,
This places a load of 0.3 pounds per square inch. The sample cup is
Place in the Petri dish on the platform of the test jack and raise until the tip of the probe makes contact. The meter scale is set to zero. Sufficient amount (50-10
0 ml) of saline is added to the Petri dish and the test is started. The distance of the weight pushed up by the sample expanded by absorbing the saline solution is measured by a probe. This distance multiplied by the area of the cross section inside the sample cup is a measure of the expanded volume of the sample based on absorption. If calculated using the density of saline and the weight of the sample,
The amount of saline solution absorbed is easily calculated. The weight of saline absorbed after 60 minutes is the AUL value, expressed in grams of saline absorbed per gram of absorber.
If desired, the corrected thickness gauge readings can be read continuously on a computer (Mitut).
oyo Digimatic Miniprocessor DP-2DX), a calculation is performed and an AUL reading is provided. As a mutual confirmation, the AUL can also be determined by measuring the difference in weight of the sample cup before and after the test, the difference in weight being the amount of solution absorbed by the sample.

The pulp of Examples 1 and 2 and Comparative Examples A and B was obtained from US Pat.
Carboxymethylcellulose (CMC) in the manner taught by
) Converted to superabsorbent. More specifically, 15 grams (0.0943 mole) of cellulose was first immersed in 400 ml of isopropanol in a reaction vessel equipped with a mechanical stirrer, an inert gas inlet and a temperature control probe. 8.31 g (0.
35 ml of an aqueous solution of NaOH (208 mol) were added (if the starting cellulose was in the form of wet pulp of about 30% strength, no water was needed and NaOH was added directly as pellets). The pulp slurry was stirred at room temperature for 30 minutes (1 hour if solid NaOH was used). Then, 8.9 g (0
. (0945 mol) chloroacetic acid (CAA) was added and the temperature was raised to 60 ° C. with stirring. The reaction was continued for 3 hours. After the slurry has been filtered, the product is mixed with a 70:30 (volume) mixture of methanol and water (400 m of solution for each wash).
Washed twice in l). During the first wash cycle, the pH of the slurry in the wash was adjusted to around 7.4 using acetic acid. Finally, the CMC fibers were washed one or more times with 100% methanol and dried at 50 ° C. This typically results in about 21 grams of dry CMC having a typical degree of conversion (DS) of 0.9.

Procedure for CMC-SAP Preparation : The CMC fibers were then dissolved in water to a 2% solution, dried at 50 ° C., and crushed into granules. Particle sizes ranging from 300 to 600 microns were collected for thermal cure and absorption testing. This part of the procedure is based on US Pat. No. 5,247,072, previously incorporated by reference. The resulting superabsorbent was tested for absorbency under load as described in the test procedure above.
The AUL results as a function of cure time are shown in Table 2 below.

Table 2 AUL results reported in grams / gram --- not tested

It will be understood that the foregoing description and examples have been set forth for purposes of illustration, and are not meant to be limiting on the scope of the invention, which is defined by the appended claims and all equivalents thereto. .

[Brief description of the drawings]

FIG. 1 is a diagram showing an apparatus for measuring a value of absorbency under load of an absorbent material.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) D21C 1/02 A41B 13/02 D 3/02 S 9/10 (81) Designated country EP (AT, BE, CH, CY, DE, DK, ES, FI, FR, GB, GR, IE, IT, LU, MC, NL, PT, SE), OA (BF, BJ, CF, CG, CI, CM, GA, GN, GW, ML, MR, NE, SN, TD, TG), AP (GH, GM, KE, LS, MW, SD, SL, SZ, TZ, UG, ZW), EA (AM, AZ, BY) , KG, KZ, MD, RU, TJ, TM), AE, AL, AM, AT, AU, AZ, BA, BB, BG, BR, BY, CA, CH, CN, CR, CU, CZ, D , DK, DM, EE, ES, FI, GB, GD, GE, GH, GM, HR, HU, ID, IL, IN, IS, JP, KE, KG, KP, KR, KZ, LC, LK, LR, LS, LT, LU, LV, MA, MD, MG, MK, MN, MW, MX, NO, NZ, PL, PT, RO, RU, SD, SE, SG, SI, SK, SL, TJ , TM, TR, TT, UA, UG, UZ, VN, YU, ZA, ZW (72) Inventor Sunton, Wisconsin, USA 54956 Nina Golf Bridge Drive 1715 Apartment 7 (72) Inventor, Ninjin, Georgia, United States 30022 Alpharetta Rod Coat 205 F Term (Reference) 3B029 BA12 BA17 BF03 4C098 AA09 CC02 CE06 DD06 DD27 4L055 AA03 AB02 AB04 AC06 AD01 AD05 BA08 BA20 BB13 BB15 BB22 EA01 EA02 EA04 EA20 EA24 EA40 GA50

Claims (8)

[Claims] 1. A method for producing an absorbable carboxyalkyl polysaccharide composition from fiber pulp having an alpha cellulose content of greater than 97% and a viscosity of greater than 30 centipoise, comprising: (a) hardwood using water. Pre-hydrolyzing the wood chips; (b) kraft cooking the pre-hydrolyzed wood chips to break the wood chips into fibers; and (c) bleaching the cooled fibers. (D) treating the bleached fiber with caustic to produce a treated fiber pulp; and (e) converting the treated fiber pulp to a carboxyalkyl polysaccharide having improved absorbency. Converting; and c. Converting. 2. The method of claim 1, wherein the hardwood chips are pre-hydrolyzed with an H factor of about 300 to about 1000, with a corresponding maximum chip yield of about 80%. 3. The method of claim 1, wherein the hardwood chips are pre-hydrolyzed with an H factor of about 500-800. 4. The method of claim 1, wherein the hardwood chips are pre-hydrolyzed with an H factor of about 600 to 700. 5. The pre-hardwood chips are cooked in a cooking liquor of sodium hydroxide and sodium sulfide to obtain an unbleached pulp having a Kappa number of 5 or greater. Item 1. The method according to Item 1. 6. The method of claim 1, wherein the caustic treatment of the bleached wood fiber is performed using a sodium hydroxide solution at about 15 to about 65 ° C. for about 10 to about 100 minutes. Method. 7. The method of claim 1, wherein said carboxyalkyl polysaccharide is carboxymethyl cellulose having an AUL of about 20 or greater. 8. The method of claim 7, wherein said AUL is between 20 and about 25.