Super absorbent cellulosic material with tunable properties and its process of production

Abstract

It is provided a cellulose super absorbent material and method of producing same comprising the steps of providing a cellulose material from wood pulp fibres; carboxymethylation of the cellulose material producing a water insoluble carboxymethyl cellulose mixture; dissolving the carboxymethyl cellulose mixture in a cold alkali system producing a cellulose dope mixture; preparing a gel absorbent optionally by physical assembly in acidic media or chemical crosslinking from the cellulose dope mixture; and neutralizing and isolating the gel absorbent obtaining an isolated cellulose super absorbent material. The resulting cellulose super absorbent material has a high absorption of saline liquid and high centrifuge retention capacity (CRC)..

Description

SUPER ABSORBENT CELLULOSIC MATERIAL WITH TUNABLEPROPERTIES AND ITS PROCESS OF PRODUCTIONCROSS-REFERENCE TO RELATED APPLICATION

[0001] The present application is claiming priority from U.S. Provisional Application No. 63 / 612,519 filed December 20, 2023, the content of which is hereby incorporated by reference in its entirety.TECHNICAL FIELD

[0002] It is provided a cellulose super absorbent material and method of producing same.BACKGROUND

[0003] Super absorbent polymers (SAP) are typically cross-linked, insoluble materials that absorb large quantities of liquid. Super absorbent polymers are used mainly in the personal hygiene product sector. It is also important that the materials retain the absorbed liquid under external pressure to prevent leakage. Cross-linked polyacrylate or polyacrylamide polymers are used for that purpose in absorbent cores of diapers. Access to higher performance super absorbent materials enables design of lighter, less bulky hygiene products which can have increased comfort for the wearer as well as increased sustainability due to savings across the logistical chain (transportation costs and reduction raw material use).

[0004] As hygiene products such as a diaper or a sanitary napkin become thinner, superabsorbent polymers are required to have higher absorption performance.

[0005] U.S. 3,284,441 describes the manufacture of water-soluble sodium carboxymethylcellulose. First ground cellulose is soaked in a mixture of isopropyl alcohol with concentrated sodium hydroxide and stirred at 15 °C for 2 hours. Then monochloroacetic acid (MCA) is added and the mixture heated to 70 °C for 1.5 hours after which the slurry is cooled down and the excess sodium hydroxide is neutralized with acetic acid and purified with water-alcohol mixtures. With this process water- soluble carboxymethyl cellulose product with a DS between 0.28 and 0.72.

[0006] U.S. 9,353,191 describes a method to produce a polymer hydrogel from an aqueous solution of a soluble polysaccharide, such as sodium carboxymethyl cellulose, with polycarboxylic acid cross-linker such as citric acid. The mixture is isolated from thesolution by drying for example. The product is then heated to induce condensation reactions between the polysaccharide and the polycarboxylic acid. The inventors report that such a product may absorbed up to 100 g / g of a 11% simulated gastric fluid after 60 minutes. The highest absorption of saline liquid reported is 57 g / g after 30 minutes of test time. No CRC values are reported.

[0007] Alam et al. (2019, “Sustainable Production of Cellulose-Based Hydrogels with Superb Absorbing Potential in Physiological Saline”, ACS Omega, 29: 9419-9426) describes the preparation of carboxymethylated softwood kraft cellulose pulp by first soaking the dry-lap sheets in a concentrated solution of MCA (100 g in 130 g. water) and dispersed into a slurry. Then excess concentrated sodium hydroxide is added to neutralize the MCA start the etherification reaction. The mixture is left to react for 12 hours and purified using alcohol-water mixtures. To prepare the super-absorbent material, this CMC is dispersed in 6 wt. % sodium hydroxide at room temperature and cross-linked using epichlorohydrin at mass ratios of 0.5-1.5 relative to CMC. The result is a gel that absorbs as much as 118 g / g of saline liquid after 40 hours of test time. The CRC of this material is unreported.

[0008] U.S. 2022 / 0071819 application refers to the manufacture of paper-thin absorbent core. To achieve so, a high performance super-absorbent polymer is required. One way to increase the performance of super-absorbent polymers is by an addition surface post-cross-linking step. This strengthens the surface of the polymer particle and in part, increases the liquid retention under external force. The invention also produces the super absorbent poly by spray drying a solution containing neutralized acrylic acid with a cross linker which leads to spherical particles. The highest CRC report in the invention is 57.9 g / g of 0.9% NaCI.

[0009] Accordingly, various attempts have been made to improve the absorbency under load and the liquid permeability in order to prevent leakage of fluid during product use. It is thus still desired to be provided with new methods of producing super absorbent materials.SUMMARY

[0010] It is provided a cellulose super absorbent material having a high absorption of saline liquid and high centrifuge retention capacity (CRC).

[0011] It is further provided a process for producing cellulose super absorbent material comprising the steps of providing a cellulose material; carboxymethylating the cellulosic material producing a water insoluble carboxymethyl cellulose mixture; dissolving the carboxymethyl cellulose mixture in a cold alkali system producing a cellulose dope mixture; preparing a gel absorbent by physical assembly or chemical crosslinking from the cellulose dope mixture; and e) neutralizing and isolating the gel absorbent obtaining an isolated cellulose super absorbent material.

[0012] In an embodiment, the cellulose material is wood pulp fiber, or produced from woody biomass.

[0013] In a further embodiment, the cellulose material is carboxymethylated in a water-alcohol mixture.

[0014] In another embodiment, the cold alkali system comprises sodium hydroxide, zinc oxide or urea.

[0015] In an embodiment, the cold alkali system comprises 8% sodium hydroxide and is at 8 °C.

[0016] In a further embodiment, the cellulose dope mixture is a viscous solution at 6 wt.% cellulose content.

[0017] In another embodiment, the gel absorbent is prepared by physical selfassembly in acidic conditions and further cut, neutralized, and coagulated.

[0018] In a further embodiment, the gel absorbent is prepared by adding a crosslinker to the cellulose dope mixture producing the gel absorbent.

[0019] In an embodiment, an epoxide chemical cross-linker is used.

[0020] In another embodiment, the cross-linker is epichlorohydrin or a diglycidyl ether.

[0021] In a supplemental embodiment, process described herein further comprises washing with a water-alcohol mixture the isolated cellulose super absorbent material.

[0022] In an embodiment, the absorption in deionized water and saline solution is of at most 1000 g / g and / or at most 120 g / g respectively.

[0023] In another embodiment, the CRC is up to 90 g / g.

[0024] Preferably, the absorption in deionized water is in the range of 100-1000 g / g, preferably in the range of 300-1000 g / g.

[0025] In a further embodiment, the absorption in saline solution is in the range of 10 to 120 g / g, in the range of 40 to 120 g / g, or preferably is in the range of 90 to 120 g / g-

[0026] In a further embodiment, it is provided a cellulose super absorbent material obtained by the process described herein.

[0027] In an embodiment, the cellulose super absorbent material is produced in the form of a hydrogel, beads, a powder, a film or fibres.

[0028] It is also provided an absorbent product comprising the cellulose super absorbent material as described herein.

[0029] In an embodiment, the absorbent product is a diaper, a sanitary napkin or a wipe.BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Reference will now be made to the accompanying drawings.

[0031] Fig. 1 illustrates a flowchart of the process to produce the cellulose superabsorbent as described herein in accordance to an embodiment.

[0032] Fig. 2 illustrates pictures of the cellulose superabsorbent hydrogel particles as provided herewith before (Fig. 2A) and after swelling (Fig. 2B).

[0033] Fig. 3 shows the viscoelastic properties of a cross-linked hydrogel before and after soaking in water.DETAILED DESCRIPTION

[0034] In accordance with the present disclosure, there is provided cellulose super absorbent material.

[0035] It is provided a scalable process to produce a carboxymethylated (CMC) material with unique solubility and superabsorbent properties. Furthermore, thematerial can be assembled into various, tunable product forms which have unique superabsorbent performance properties (high liquid absorption and liquid retention capacities in water and saline solution) not previously reported in the literature or prior art. In addition, the colorless, odorless and non-toxic hydrogel is bio-based and is suitable for use in absorbent hygiene applications, wound dressings, wipes and medical applications.

[0036] Solubilized CMC material can be assembled into various product forms with tunable performance. The material can be assembled in acidic regeneration media such as and not limited to formic acid, sulfuric acid, hydrochloric acid, sodium bisulfite solution, or any other acidic media known to the anyone skilled in the art. Preferably a sodium bisulfite aqueous solution is used. Such physically assembled material can be produced in the form of hydrogel beads, powders, fibers, and films. Optionally a chemical crosslinker such as epichlorohydrin, diglycidyl ether of various spacer lengths or other crosslinking chemistries known to someone skilled in the art could be used to produce a chemically cross-linked hydrogel material with unique absorbency and tunable mechanical properties. Commercial carboxymethyl cellulose materials of equivalent degree of substitution do not form stable superabsorbent hydrogels.

[0037] The superabsorbent performance of aforementioned materials is tunable with free swell capacity (FSC) values in distilled water in the range of 100-1000 g / g, 300-1000 g / g more preferably, FSC in saline values in the range of 30 to 120 g / g or 70 to 120 g / g more preferably CRC in saline values in the range 10 to 90 g / g, 60 to 90 g / g more preferably.

[0038] It is thus provided the preparation of a cellulose super absorbent material to be used in personal hygiene products, prepared from carboxymethylated wood pulp fibres. Briefly, as depicted in Fig. 1 , cellulose pulp C is directly carboxymethylated 10 in water-alcohol mixtures, omitting the alkali cellulose mercerization pre-treatment step. This produces water insoluble carboxymethyl cellulose. The carboxymethylated pulp fibres are dissolved 11 in a cold alkali system (e.g., -8 °C, 8% sodium hydroxide) which yields a viscous solution at 6 wt.% cellulose content (dope). The cold alkali system is needed for the complete dissolution of the cellulose since it is not water-soluble. The material can be physically assembled to yield a gel 12 using an acidic media such as (formic acid, sulfuric acid, hydrochloric acid, sodium bisulfite solution, etc... ) or other acidic media. Preferably an aqueous sodium bisulfite solution is used. Optionally, a suitable cross-linker CL is added to yield a gel 12 which is then cut, neutralized, andcoagulated 14 to allow mechanical separation 16 of the material from the bulk media. As encompassed herein, a suitable cross-linker can be e.g., an epoxide, such as epichlorohydrin, diglycidyl ether of various spacer lengths or other crosslinking chemistries known to someone skilled in the art. The isolated material is purified 18 by washing with water-alcohol mixtures and dried. The purified gel PG shows very high absorption of saline liquid (115 g / g) as well as an excellent centrifuge retention capacity (CRC) of 86 g / g.

[0039] The proposed solution produces a carboxymethylated cellulose pulp without a swelling / mercerization pre-treatment in concentrated sodium hydroxide. Since the product isn’t water soluble, the cold caustic system was used and then a cross-linker CL was added to the mixture. Omitting the alkali cellulose step led to stronger gels after the cross-linking reaction with high saline absorption and retention values.

[0040] Contrary to known processes, the produced carboxymethylated cellulose pulp is unlike carboxymethyl cellulose commercially available and the method of hydrogel formation is not aided by physical assembly using nanostructured cellulose materials.

[0041] Furthermore, the method of carboxymethylated material production provided herewith has been tuned to produce a non water-soluble material that can dissolve in a cold caustic aqueous system without additives and without the need for depolymerization steps. The degree of substitution range has been tuned to produce a material with a degree of substitution in the range of 0.4-0.7. The product produced from the aforementioned process is formed into various product forms including hydrogels, beads, powders, films and fibers. In an embodiment, the absorbency in deionized water and saline (0.9% NaCI) solution is -1000 g / g and 100 g / g, respectively. The retention in saline (0.9% NaCI) solution after centrifugation is up to 90 g / g-

[0042] Accordingly, it is provided the preparation of a cellulose super absorbent material to be used in personal hygiene products, prepared from carboxymethylated wood pulp fibres.

[0043] Absorbent performance range of many samples listed in Table 1 below:Table 1. Performance of powder (physical assembly in acid) and chemically crosslinked super absorbent hydrogel

[0044] The specific carboxymethylation procedure provided herewith omits the typical mercerization step leading to a water insoluble grade of carboxymethylcellulose even at high DS (~0.7). This carboxymethylcellulose can however be solubilized in the well-known cold alkali solvent system without the use of other additives (zinc oxide, urea for example). Suitable cross-linkers such as multifunctional epoxides react with the hydroxyl groups of carboxymethylcellulose to form a gel with high saline absorption and retention properties. Suitable cross-linkers are e.g., epoxides which can be used as is or readily prepared from the reaction product of their respective multifunctional alcohol and epichlorohydrin. The cross-linking can be done in bulk sodium hydroxide medium, or alternatively the CMC / NaOH / crosslinker mixture may be dispersed in a non-miscible liquid such as heptane to produce a suspension of uniform gel spheres.

[0045] The cross-linking of insoluble carboxymethylcellulose in alkaline media leads to noteworthy behavior which suggests the material is precipitating / recrystallizing when removed from reaction medium. Fig 3 shows the elastic / loss moduli (G7G”) of a cross-linked hydrogel at a low CMC to cross-linker ratio. After the cross-linking procedure, the reaction mixture has predominantly liquid-like behavior since G”>G’ due to the low cross-linker ratio. When the material is soaked in water to remove the sodium hydroxide gelation eventually occurs, and the material now has a predominantly solid-like behavior. (G’>G”). It is hypothesized that removing the sodium hydroxide from the gel induces precipitation of the otherwise insoluble carboxymethylcellulose which enhances the mechanical properties of the hydrogel.

[0046] As comparison, the procedure was applied to a commercial water-soluble grade of carboxymethylcellulose and did not form a gel upon cross-linking. The liquid absorption and retention performance were also poor.EXAMPLE I FSC CRC protocol

[0047] Absorbency was tested by a 24 h free swell capacity test according to a modified WSP 240.2. (R3) standard test method. Since the pore size of bags used to enclose samples for testing is an important factor in preventing loss of cellulose material during testing, for this reason Rosin bags (25 gm pore) were used instead of the standard tea bag. The solution used for the 24-hour immersion was saline (0.9% NaCI) or deionized water. After immersion in the desired solution for 24 hours, samples are removed, allowed to drip for 10 minutes. Absorption capacity is defined as the grams of water absorbed per gram of cellulose material.

[0048] Swollen samples from the free-swell capacity measurements were tested for Centrifugal Retention Capacity (CRC) according to WSP 241.2. (R3) standard test method Briefly, samples were centrifuged at 250xg. for 3 minutes. Centrifuged samples were weighed, then oven-dried, and weighed again. CRC (g / g) is defined as grams of liquid per gram of cellulosic material after subjecting the sample to a force which removes unbound water and is calculated as follows:EXAMPLE IIPreparation of water insoluble carboxymethylcellulose precursor

[0049] A water insoluble carboxymethylcellulose with a degree of etherification varying from 0.3 to 0.70 as measured by ASTM D1439-15, is prepared from northern bleached softwood Kraft pulp. To do so, a dried pulp sheet is ground into a powder using a knife mill. The powder is then mixed in an 80 / 20 wt.% alcohol / water slurry and pre-heated to 80 °C. Upon reaching temperature sodium hydroxide and sodium monochloroacetate solutions are added to the mixture and stirred for 90 minutes. Varying the sodium hydroxide / sodium monochloroacetate content will yield material with different degrees of substitution. After the allotted time, the carboxymethylated pulp is separated from the reaction medium by filtration, obtaining a solid cake. This solid cake is then purified by redispersion in 70% vol ethanol and repeated filtration twice. Finally, the solid cake is dispersed in anhydrous ethanol and filtered again before air-drying. This material serves as the basis for producing superabsorbent powder andsuperabsorbent hydrogel. The absorption properties of this material at a DS if 0.65 are recorded in table 2.Table 2. Absorption properties for the precursor carboxymethylcellulose at DS 0.65EXAMPLE III Cross-linking procedure to produce superabsorbent hydrogel

[0050] The carboxymethylated cellulose pulp produced in Example II is water insoluble even at high substitution degrees. However, strong alkali solvent systems can be used to help dissolution. To do so, a carboxymethylated pulp possessing a degree of substitution of 0.65 is dissolved in a subzero (-8 °C) solution of 8 wt.% sodium hydroxide for a final carboxymethylcellulose content of 6 wt.%. This yields a viscous solution of carboxymethylcellulose otherwise known as “dope”. This mixture is then heated at 50 °C and an epoxy cross-linker is added to the mixture and mixed. The mixture gels and is left to react for 60 minutes. A stiff gel is obtained which is then diced and then placed in DI water prior to blending using a handheld blender a portion of ethanol is added, and the excess sodium hydroxide is then neutralized using concentrated hydrochloric acid until a neutral pH is obtained. Ethanol is then added to the mixture until the hydrogel particles start to coagulate. The material is then recovered by filtration followed by oven-drying at 105 °C. The absorption properties (FSC and CRC) of this material are found in Table 3.Table 3. Absorption properties of superabsorbent hydrogel from Example III.EXAMPLE IV Preparation of super absorbent powder

[0051] Carboxymethylcellulose prepared from procedure described in Example II can be solubilized in a cold alkali solvent system followed by regeneration in a sulfuric acid solution. To do so, first carboxymethylcellulose pulp with a DS of 0.40 is dissolved in a subzero (-8 °C) 8 wt. % sodium hydroxide solution, for a final cellulose content of 6 wt.%. This solution is then slowly added to a solution of 10 wt.% sulfuric acid while stirring. Upon contact with the acid, the carboxymethylcellulose solidifies. The solid material is then separated from the acid by centrifugation and washed 3 times by dispersing and repeated centrifugation in a 0.9% NaCI solution to prevent swelling and dissolution of the material. The centrifuged solids are then dispersed in a mixture in 100% ethanol and neutralized with sodium hydroxide before being centrifuged again. Finally, the material is dispersed in 100% ethanol again before being filtered and airdried. The absorption properties of this material after 24 hours are found in Table 4.Table 4. Absorption properties of the superabsorbent powder after 24 hours.EXAMPLE VPreparation of superabsorbent hydrogel from commercial water-soluble grade of carboxymethylcellulose

[0052] For comparison, the cross-linking procedure described in Example II is repeated using a commercial grade of carboxymethylcellulose with a DS of 0.70 as feedstock. After the reaction time, no gelation is observed. The mixture is poured in DI water and neutralized with hydrochloric acid before coagulation with ethanol. The material is separated by filtration and left to air-dry. The absorption properties are found in Table 5. Almost no absorption is found in DI water, due to solubilization of the material which causes it to escape the test bag. This suggests little, or no cross-linking occurred. This would be consistent with the lack of gelation upon addition of the crosslinking agent. Absorption of 0.9% NaCI is good since solubilization of carboxymethylcellulose is poor in saline solutions. Most of the saline retained is lostupon centrifugation which suggest poor mechanical strength of the swollen material cause by insufficient cross-linking.Table 5. Absorption properties of a cross-linked commercial carboxymethylcellulose.

[0053] While the present disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations and including such departures from the present disclosure as come within known or customary practice within the art and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A process for producing cellulose super absorbent material comprising the steps of: a) providing a cellulose material; b) carboxymethylating of said cellulose material producing a water insoluble carboxymethyl cellulose mixture; c) dissolving the carboxymethyl cellulose mixture in a cold alkali system producing a cellulose dope mixture; d) preparing a gel absorbent by physical assembly or chemical crosslinking from the cellulose dope mixture; and e) neutralizing and isolating the gel absorbent obtaining an isolated cellulose super absorbent material.

2. The process of claim 1 , wherein the cellulose material is wood pulp fibres, or produced from woody biomass.

3. The process of claim 1 or 2, wherein the cellulose material is carboxymethylated in a water-alcohol mixture.

4. The process of any one of claims 1-3, wherein the cold alkali system comprises sodium hydroxide, zinc oxide or urea.

5. The process of claim 4, wherein the cold alkali system comprises 8% sodium hydroxide and is at 8 °C.

6. The process of any one of claims 1-5, wherein the cellulose dope mixture is a viscous solution at 6 wt.% cellulose content.

7. The process of any one of claims 1-6, wherein the gel absorbent is prepared by physical self-assembly in acidic conditions and further cut, neutralized, and coagulated.

8. The process of any one of claims 1-6, wherein the gel absorbent is prepared by adding a cross-linker to the cellulose dope mixture producing the gel absorbent.

9. The process of claim 8, wherein an epoxide chemical cross-linker is used.

10. The process of claim 9, wherein the cross-linker is epichlorohydrin or a diglycidyl ether.

11. The process of any one of claims 1-10, further comprising washing with a water- alcohol mixture the isolated cellulose super absorbent material.

12. A cellulose super absorbent material having a high absorption of saline liquid and high centrifuge retention capacity (CRC).

13. The cellulose super absorbent material of claim 12, wherein the absorption in deionized water and saline solution is of at most 1000 g / g and at most120 g / g respectively.

14. The cellulose super absorbent material of claim 12 or 13, wherein the CRC is up to 90 g / g.

15. The cellulose super absorbent material of claim 13, wherein the absorption in deionized water is in the range of 100-1000 g / g.

16. The cellulose super absorbent material of claim 13, wherein the absorption in deionized water is in the range of 300-1000 g / g.

17. The cellulose super absorbent material of claim 13, wherein the absorption in saline solution is in the range of 10 to 120 g / g.

18. The cellulose super absorbent material of claim 13, wherein the absorption in saline solution is in the range of 40 to 120 g / g.

19. The cellulose super absorbent material of claim 13, wherein the absorption in saline solution is in the range of 90 to 120 g / g.

20. A cellulose super absorbent material obtained by the process of any one of claims21. The cellulose super absorbent material of any one of claims 12-20, produced in the form of a hydrogel, beads, a powder, a film or fibers.

22. An absorbent product comprising the cellulose super absorbent material of any one of claims 12-21.

23. The absorbent product of claim 22, wherein said product is a diaper, a sanitary napkin or a wipe.

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