A method of making a hemicaustic superabsorbent polymer from a hemicaustic byproduct, and a hemicaustic superabsorbent polymer prepared by the method

Abstract

A method for making a hemicaustic superabsorbent polymer from a hemicaustic byproduct, such as a kraft (sulfate), prehydrolized kraft or sulfite pulping process hemicaustic byproduct. The hemicaustic byproduct can be extracted from a pulp slurry as a hemicaustic suspension by treating the cellulosic pulp slurry with a cold caustic solution, then isolating the hemicaustic suspension and grafting a polar water-soluble vinyl monomer on hemicelluloses present in the hemicaustic suspension to produce a hemicaustic based superabsorbent polymer.

Description

A METHOD OF MAKING A HEM ICAUSTIC SUPERABSORBENT POLYMER FROM A HEMICAUSTIC BYPRODUCT, AND A HEMICAUSTICSU PERABSORBENT POLYMER PREPARED BY THE METHODTECHNICAL FIELD

[0001] The present disclosure relates to a method of making a hemicaustic superabsorbent polymer ("Hem-SAP") from a hemicaustic byproduct, such as a byproduct of the pulp manufacturing industry. The present disclosure is further directed to a Hem-SAP prepared by the method. The Hem-SAP of the present disclosure can be free of acrylate cross-linking functionalities. Also, the Hem-SAP of the present disclosure can have absorbent properties comparable to commercial superabsorbent polymers used in absorbent articles, such as absorbent articles used for body waste management, even though the Hem- SAP is made from a hemicaustic byproduct.BACKGROUND

[0002] The wood pulp industry usually involves treating pulp with an aqueous caustic solution at various concentrations, thereby rendering the pulp suitable for various applications. For instance, cellulosic fibers may be treated with a caustic solution at high concertation of >16% by weight. During the treatment (referred to as "mercerization"), the morphology of the fiber structure changes from ribbon-like to a round shape. The change in morphology adds stiffness to the fiber and makes it suitable for use in filter applications.

[0003] Another treatment of pulp with a caustic solution involves pulp pre-activation for reaction with other reagents (e.g., CS2) to make the pulp, for instance, suitable for use in viscose production. Yet another treatment, known as cold caustic treatment, assists in removing fragmented byproducts that are deposited on fiber surfaces. The treatment is typically used when preparing pulp with high purity.

[0004] Pulp treatment with a caustic solution is usually carried out by mixing a suspension of pulp and a sodium hydroxide solution at a low consistency (i.e., the concentration of fibers in a pulp suspension) and allowing the mixture to soak until hemicelluloses and other byproducts in the pulp are dissolved and diffuse out of the pulp structure into the solution. Pulp treatment with a caustic solution is usually conducted at a temperature range from about 0.0 °C to 40 °C. The treated pulp is then washed to remove the caustic solution and the dissolved hemicelluloses. The effluent stream produced by this process is referred to herein as a "hemicaustic." The caustic extraction process is described in, forexample, U.S. Patents 6,896,810, 7,812,153, and 7,919,667, all of which are hereby incorporated by reference.

[0005] Hemicaustic can be further processed by passing it through filtration to increase its hemicellulose concentration. The presence of hemicelluloses in hemicaustic extracts makes the hemicaustic a possible waste source of hemicelluloses.

[0006] Hemicellulose is generally understood to be a polysaccharide with relatively short-chain polymers of predominately mannose and xylose monomers, which are present in wood and other plant materials. It differs from cellulose in many ways, such as cellulose being partially crystalline and hemicelluloses being amorphous. Structurally, cellulose is a long-chain polymer of anhydroglucose repeating units with a number average degree of polymerization (DP) that can range from 300 to 5000. Cellulose is a homopolysacharide polymer while hemicelluloses are heteropolyhydroxylated polymers. The sugar moiety of hemicellulose chains is similar to cellulose in the sense that each bears free hydroxyl groups that are available for functionalization. They are capable of undergoing reactions that are typical for primary and secondary alcohols. Hemicelluloses can be derivatized to produce materials that can be used as thickeners for foods, coatings, paints, explosive slurries, oil well fluids, cosmetics and other personal care products, and many other products. Hemicelluloses can also be derivatized and crosslinked to produce superabsorbent polymer.

[0007] A superabsorbent polymer ("SAP") is a polar hydrogel that can absorb and retain large amounts of water or aqueous solutions, which can range from 10 to 1000 g water / g SAP. Superabsorbent polymers can be natural-based or oil-based polymers that are cross-linked chemically, the crosslinking making the SAP polymer water-insoluble but water-swellable. Several approaches for making SAPs are known. The main approach for making a SAP is by solution polymerization of acrylic monomers, such as acrylic acid or its salts, in the presence of a crosslinking agent and an initiator. Natural product-based superabsorbent polymers are usually prepared by graft copolymerization in an aqueous solution or by radiation-induced copolymerization. A disadvantage of natural-based superabsorbent polymers made from cellulose or starch is the fact that they tend to form a tacky material with low gel strength.

[0008] SAPs prepared by solution polymerization usually have poor absorption against pressure and a low rate of liquid intake, which makes them, for instance, insufficient for application in absorbent articles intended for body waste management. Since semi-swollen gel can cause what is known as gel blocking, particles tend to swell rapidly on the top layer of the gel bed, which inhibits the flow of fluid to the inner parts of the gel bed and can lead to a liquid leak. This phenomenon (i.e., gel blocking) is usually solved by surface cross-linking. Surface cross-linking is usually performed on a dried, milled, andsized SAP as a final stage in the process of making SAP. A cross-linking solution is applied to the particles, which are then cured through heating at high temperatures (e.g., 150-200 °C) for about 30-60 min. Relatively high temperatures and long reaction times are required for the surface cross-linking reactions.

[0009] Despite the above-discussed advances, it remains a challenge to convert hemicaustic byproduct to a high-value product, such as a superabsorbent polymer, in a single-step process. Yet another challenge is obtaining the SAPs that have desired properties, such as high gel strength, high swelling capacity, high swelling rate, and superior absorbency under load without surface crosslinking.SUM MARY AN D OBJ ECT

[0010] The present disclosure provides a method of making a Hem-SAP from a hemicaustic byproduct, which is a wood pulp industry byproduct. The present disclosure also provides a Hem-SAP prepared by the disclosed method.

[0011] An object of the present disclosure is to provide a method of making a Hem-SAP with high gel strength, high swelling capacity, high swelling rate, and superior absorbency under load. A further object of the disclosure is to provide a Hem-SAP that can be used in absorbent articles, personal disposable hygiene products, agriculture biomedical applications, food, cosmetics, surgical pads, controlled release, spill control, packaging material, chemical sensors, filtration applications, wound dressing, and other applications. Yet another object of the present disclosure is to prepare the Hem- SAP in a single-step process that does not require surface crosslinking. Still yet another object of the present disclosure is to prepare the Hem-SAP with ester linkages, the Hem-SAP being free of acrylate or amide linkages.

[0012] An embodiment of the present disclosure includes a method of making a hemicaustic superabsorbent polymer from a hemicaustic byproduct, the method comprising mixing a free radical initiator with a homogenous reaction mixture comprising a hemicellulose, a water-soluble vinyl monomer, and a crosslinking agent, thereby producing the hemicaustic superabsorbent polymer

[0013] In an embodiment of the preceding method, the method further comprises treating a hemicaustic byproduct suspension comprising the hemicellulose with an oxidizing agent to obtain a treated hemicaustic byproduct suspension, and adding the water-soluble vinyl monomer and the crosslinking agent to the treated hemicaustic byproduct suspension to obtain the homogenous reaction mixture.

[0014] In an embodiment of the preceding method, the hemicaustic superabsorbent polymer is substantially free of monomers.

[0015] In an embodiment of the preceding method, the method further comprises drying the hemicaustic superabsorbent polymer such that the hemicaustic superabsorbent polymer has a water content of between 1% and 20% by weight.

[0016] In an embodiment of the preceding method, the method further comprises grinding the hemicaustic superabsorbent polymer into a powder.

[0017] In an embodiment of the preceding method, the homogenous reaction mixture has a consistency range from 20% to 70%.

[0018] In an embodiment of the preceding method, the hemicaustic byproduct suspension comprises 2% by weight to 12% by weight of the hemicellulose.

[0019] In an embodiment of the preceding method, the oxidizing agent is selected from the group consisting of hydrogen peroxide, tert-butyl peroxide, peracetic acid, perbenzioc acid, di-tert-butyl peroxide, and combinations thereof.

[0020] In an embodiment of the preceding method, the water-soluble vinyl monomer is a polar vinyl monomer.

[0021] In an embodiment of the preceding method, the water-soluble vinyl monomer is hydrophilically water soluble with a molecular formula H2C=CHX and able to undergo a free radical polymerization.

[0022] In an embodiment of the preceding method, X in the molecular formula H2C=CHX is selected from the group consisting of COOH, CONH2, CN, CH2OH, SO3H, CONHC(CH3OH)3, CONHCH(CH3)CH2SO3H, (COCH2CH2O-)n, n being I to 200, COOCH2CH2COOH, CH2SO3H, and combinations thereof.

[0023] In an embodiment of the preceding method, the water-soluble vinyl monomer is selected from the group consisting of an acrylic monomer, an acryl-amide, a methacrylamide, acrylonitrile, a vinyl- substituted heterocyclic compound, and combinations thereof.

[0024] In an embodiment of the preceding method, the water-soluble vinyl monomer is selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, a-acryloxypropionic acid and alkali metal salts and ammonium salts thereof, itaconic acid, acrylamide, methacrylamide, 2-acrylamido-2- methyl-l-propane sulfonic acid poly(ethylene glycol) methyl ether acrylate, dipentaerythritol penta- / hexa-acrylate, 4-acryloylmorpholine, 2-carboxyethyl acrylate, 2-methyl-2-propene-l-sulfonic acid and its salts and combinations thereof.

[0025] In an embodiment of the preceding method, the crosslinking agent is an alkylene glycol polyglycidy ether crosslinking agent.

[0026] In an embodiment of the preceding method, the crosslinking agent is an alkylene glycol or polyalkylene glycol derivatized with glycidyl ether groups.

[0027] In an embodiment of the preceding method, the alkylene glycol polyglycidy ether crosslinking agent is soluble in an aqueous solution.

[0028] In an embodiment of the preceding method, the crosslinking agent is selected from the group consisting of ethylene glycol diglycidyl ether, glycerol triglycidyl ether, glycerol diglycidyl ether, glycerol propoxylate triglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol glycidyl ether, 1,4- cyclohexanoldimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol glycidyl ether, and combinations thereof.

[0029] In an embodiment of the preceding method, the free radical initiator is selected from the group consisting of potassium persulfate, ammonium persulfate, sodium persulfate, alkali-metal persulfates, hydrogen peroxide, ammonium cerium nitrate, 2,2'-azobis-(2-amidinopropane) hydrochloride, and combinations thereof.

[0030] An embodiment of the present disclosure includes a hemicaustic superabsorbent polymer obtained by the method described above.

[0031] Other objects, features, and advantages of the present invention will be apparent to those of ordinary skill in the art in view of the following detailed description of the invention and accompanying drawings.BRI EF DESCRI PTION OF TH E DRAWI NGS

[0032] The drawings as provided herein set forth some exemplary embodiments of the method and Hem-SAP of the present application, the detailed description of which follows. The drawings are merely exemplary and are not intended to limit the invention.

[0033] FIG. 1 is an exemplary chemical scheme showing structures of a Hem-SAP prepared from a hemicaustic byproduct, a vinyl monomer, and a glycerol diglycidyl ether cross-linking agent.

[0034] FIG. 2 is a scanning electron microscope image of a conventional superabsorbent polymer at lOOx magnification.

[0035] FIG. 3 is a scanning electron microscope image of a Hem-SAP prepared in accordance with Example 1 of the present disclosure at 250x magnification.

[0036] FIG. 4 is a scanning electron microscope image of a Hem-SAP prepared in accordance with Example 1 of the present disclosure at 1200x magnification.

[0037] FIG. 5 is a chart showing the effect of using various hydrophilic monomers on the absorbency of prepared Hem-SAPs and commercial superabsorbent polymers.

[0038] FIG. 6 is a chart showing the vortex time of prepared Hem-SAPs and a commercial superabsorbent polymer.DETAI LED DESCRI PTION

[0039] The present disclosure generally provides a method of making a Hem-SAP from a hemicaustic byproduct of the pulp industry and a Hem-SAP prepared by the disclosed method. The pulp industry production stream produces several byproducts, one of which is a hemicaustic byproduct that can contain hemicelluloses, low molecular weight celluloses, and metal hydroxides. The hemicaustic byproduct is typically in the form of a suspension. Such a hemicaustic byproduct can be derived from any wood source (e.g., softwoods and hardwoods) and / or non-wood source (e.g., bamboo, bagasse, and annual crops such as straw, stalks, husk, hulls, and bran), each of which can be acceptably used in the method of the present disclosure. Also, the hemicaustic byproduct can be generated from, for example, kraft (sulfate), prehydrolized kraft, and / or sulfite processes.

[0040] As used herein, the term "hemicaustic superabsorbent polymer" or "Hem-SAP" refers to a superabsorbent polymer prepared from a hemicaustic byproduct. Alternatively, the term "hemicaustic superabsorbent polymer" or "Hem-SAP" can refer to a hemicaustic-based superabsorbent polymer. The term "hemicaustic superabsorbent polymer" or "Hem-SAP" does not suggest or imply that the "hemicaustic superabsorbent polymer" or "Hem-SAP" has a particular causticity.

[0041] As used herein, the term "substantially free of monomers" refers to the Hem-SAP containing monomers in an amount of not more than 0.00% wt. %.

[0042] Method of Making a Hemicaustic Superabsorbent Polymer (Hem-SAP) from a Hemicaustic Byproduct

[0043] In an embodiment of the present disclosure, a Hem-SAP can be prepared from a hemicaustic byproduct by a method that includes mixing a free radical initiator with a homogenous reaction mixture comprising a hemicellulose, a water-soluble vinyl monomer, and a crosslinking agent, thereby producing the Hem-SAP. The method can be a single-step process or a multi-step process. As discussed above, conventional techniques produce superabsorbent polymers by a process that includes a surface crosslinking step. No such surface crosslinking step is required in the method of the present disclosure. Therefore, the Hem-SAP of the present disclosure can be prepared without such surface crosslinking. The single-step process of the present disclosure can improve the overall efficiency of the process andallow for recovery of high-value products from industrial byproducts in an efficient manner. In general, the method of the present disclosure makes it possible to produce a Hem-SAP from a hemicaustic byproduct, such as a kraft (sulfate), prehydrolized kraft, or sulfite pulping process hemicaustic byproduct. The hemicaustic byproduct can be extracted from a pulp slurry as a hemicaustic suspension by treating the cellulosic pulp slurry with a cold caustic solution, then isolating the hemicaustic suspension and grafting a polar water-soluble vinyl monomer on hemicelluloses present in the hemicaustic suspension to produce a Hem-SAP.

[0044] As described above, the method can include additional steps before and / or after mixing a free radical initiator with the homogenous reaction mixture. In certain embodiments, the method disclosed herein can include treating the hemicaustic byproduct with an oxidizing agent. The oxidizing agent can be provided in an aqueous solution comprising the oxidizing agent. Even if a hemicaustic byproduct is purified by passing the hemicaustic byproduct through filtration before producing a Hem-SAP, the Hem- SAP obtained with the purified product can have a yellow color, which is generally undesirable, especially in absorbent articles used for body waste management. Such yellowing can be removed by treating the hemicaustic byproduct with an oxidizing agent in, for example, an aqueous solution.Examples of oxidizing agents suitable for this purpose include hydrogen peroxide, chlorine dioxide, peracetic acid, perbenzoic acid, chlorine, chlorine dioxide, ozone, sodium hypochlorite, and any combination thereof. Hydrogen peroxide and ozone are examples of oxidizing agents that are especially beneficial because they do not leave unwanted residual byproducts in the Hem-SAP. The oxidizing agent can be added to the hemicaustic byproduct in an amount of about 0.01% to about 1.0% by weight relative to the amount of hemicellulose in the hemicaustic byproduct. Alternatively, the oxidizing agent can be added to the hemicaustic byproduct in an amount of about 0.05% to about 1.0% by weight relative to the amount of hemicellulose in the hemicaustic byproduct.

[0045] The method can include a step of converting the hemicaustic byproduct suspension into a homogenous reaction mixture by treating the hemicaustic byproduct suspension with a caustic solution, as discussed in detail above. The caustic solution can, for example, contain an alkali metal salt, such as sodium hydroxide or potassium hydroxide, or other basic compounds such as ammonium hydroxide. Converting the hemicaustic byproduct suspension into a homogenous reaction mixture typically is conducted at the range of about 15° C. to 40° C.

[0046] The homogenous reaction mixture can contain, for example, a hemicellulose, a water-soluble vinyl monomer, and a crosslinking agent. The hemicellulose can be obtained from the hemicaustic byproduct. In general, the method converts the hemicellulose present in a hemicaustic byproductand / or homogenous reaction mixture to a Hem-SAP by grafting a water-soluble vinyl monomer onto the backbone of the hemicellulose. The grafting of the water-soluble vinyl monomer can be induced by adding a free radical initiator to the homogenous reaction mixture, which is discussed in detail below. The free radical initiator can be added in the form of a solution. The free radical initiator can be added under an inert atmosphere.

[0047] The method can also include a step of drying the Hem-SAP. The drying of the Hem-SAP can be performed until the water content is between 0.1% and 20% by weight. In certain embodiments, the water content of the dried Hem-SAP can be between 1% and 5% by weight.

[0048] The method can include a step of grinding the Hem-SAP. The grinding of the Hem-SAP can be performed to provide a powder of the Hem-SAP. In certain embodiments, the powder of the Hem-SAP can pass through a 20-mesh screen but be retained by a 30-mesh screen, which could be beneficial for use in, for example, diaper applications. In other applications, the particle size can be less than that required to pass through a 20-mesh screen but be retained by a 30-mesh screen, such as for agricultural applications.

[0049] The hemicellulose in the hemicaustic byproduct can include a variety of hemicelluloses. Typical chemical structures of some of the hemicelluloses contained in the hemicaustic byproducts are shown below.

[0050] Hardwood xylan

[0051]

[0052] Hardwood glucoinaiuiansoftwood xylaii

[0053] Softwood ^ucomaimari

[0054]

[0055] Hemicelluloses are linear heteropolymers generally composed of cyclic 5-carbon and 6-carbon sugars (polysaccharides) including xylose, arabinose, galactose, glucose, mannose, and 4-O-methyl-D- glucuronic acid residues. Hemicelluloses present in the hemicaustic byproducts of the present disclosure are mainly galactoglucomannan, arabinoglucuronoxylan, arabinogalactan, glucuronoxylan, and glucomannan. The significant differences between softwood and hardwood hemicelluloses are that 75-95% of the hardwood hemicelluloses are xylan, more specifically an O-acetyl-4-O-methylglucurono-R- D-xylan. The principal hemicelluloses of softwoods are glucomannans. Xylan contains an acidic group (i.e., glucuronic acid) on its repeat unit and has a molecular structure similar to cellulose after disregarding the side chains. Hemicelluloses generally have an average degree of polymerization (DP) ranging from 50 and 200.

[0056] The hemicaustic byproduct of the present disclosure can contain an aqueous suspension of hemicelluloses (e.g., 2% to 20 wt.%), caustic (e.g., 2% to 10 wt.%), lignin (e.g., < 1 wt.%), and other organic and inorganic materials (e.g., < 1 wt.%). The hemicaustic byproduct can be purified by filtration. After filtration, the hemicaustic byproduct suspension contains from about 4 wt% to about 15 wt%hemicelluloses and from about 4 wt% to about 16 wt% caustic compounds (e.g., caustic soda, potassium hydroxide, lithium hydroxide and calcium hydroxide). A concentrated hemicaustic suspension can contain from about 3 wt% to about 10 wt% of caustic compounds in certain embodiments. The hemicelluloses present in the hemicaustic byproduct of the present disclosure include the sugar monomers glucose, xylose, mannose, and arabinose, with the xylose monomer being the principal sugar monomer.

[0057] As shown in the chemical structures above and the scheme provided in FIG. 1, the repeat unit of the hemicelluloses has two hydroxyl groups in a vicinal position, which allows the formation of a free radical on the sugar repeat unit by treatment with a free radical initiator. In the presence of free radical polymerizable vinyl monomers, a polymer grafting on the hemicellulose chains can occur.

[0058] The vinyl monomer of the disclosed method can be capable of undergoing the desired graft polymerization reaction and be capable of providing the Hem-SAP with desirable properties. For instance, the vinyl monomer can be water soluble and have a polar group at the vinylic position as shown in the following molecular formula H2C=CHX, and able to undergo a free radical polymerization. Examples if X in the molecular formula H2C=CHX include but not limited to are COOH, CONH2, CN, SO3H, CONHCH2(CH3OH), CONHC(CH3OH)3, CONHCH(CH3)CH2SO3H, COCH2CH2O-)n n= 1 to 200, COOCH2CH2COOH, CH2SO3H. More specifically, the vinyl monomer may comprise an acrylic monomer, an acryl-amide, a methacrylamide, acrylonitrile, a vinyl-substituted heterocyclic compound, or mixtures thereof. Examples of acceptable vinyl monomers include acrylic acid, methacrylic acid, ethacrylic acid, a-acryloxypropionic acid and alkali metal salts and ammonium salts thereof; itaconic acid, acrylamide, methacrylamide, and 2-acrylamido-2-methyl-l-propane sulfonic acid poly(ethylene glycol) methyl ether acrylate, dipentaerythritol penta- / hexa-acrylate, 4-acryloylmorpholine, 2-carboxyethyl acrylate, 2- methyl-2-propene-l-sulfonic acid and its salts. The vinyl monomers can be acrylic acid, methacrylic acid, their respective salt forms such as alkali metal or ammonium salts, acrylamide, and mixtures thereof.

[0059] The grafting may be carried out using vinyl monomers that are not neutralized or that have been neutralized or partially neutralized before mixing with hemicaustic byproduct. In certain embodiments, the vinyl monomers used in the grafting process are not neutralized before mixing with the hemicaustic byproduct. While not being bound by any individual theory, the inventors discovered that adding metal hydroxide solution to the hemicaustic byproduct in an amount sufficient to bring the pH value of the reaction mixture to 4.0 to 6.0 after the addition of vinyl monomer results in producing a clear reaction mixture.

[0060] Metal hydroxides can be used to neutralize the acid groups of the vinyl monomer and are typically those that will sufficiently neutralize the acid groups without having a detrimental effect on the polymerization process. Examples of such metal hydroxides include alkali metal hydroxides, such as sodium hydroxide (NaOH), lithium hydroxide (LiOH), potassium hydroxide (KOH), rubidium hydroxide (RbOH), benzyltrimethylammonium hydroxide (BTMOH), and any combination thereof. In certain embodiments, sodium or potassium hydroxides are employed to neutralize the monomer. In determining the desired degree of neutralization, care typically is taken to ensure that the pH of the resulting grafted fiber does not exceed the defined range. Otherwise, the absorbent properties of the produced Hem-SAP can be affected.

[0061] The amount of monomer present in the reaction mixture can be any amount suitable to add to the Hem-SAP properties, as well as other desirable properties described herein. For instance, the amount of monomer can be within the range of from about 20% to about 600% by weight, based on the total weight of the hemicellulose present in the hemicaustic byproduct. In certain embodiments, the amount of monomer can be within the range of from about 40% to about 500%, and more preferably from about 100% to 400% by weight.

[0062] In order to increase the Hem-SAP absorbency and gel strength and to minimize the formation of a homopolymer, a cross-linking agent can be used in addition to the vinyl monomer. Suitable crosslinking agents include glycols and polyols with multi glycidyl groups that are capable of reacting simultaneously with hydroxyl and carboxyl groups present on hemicellulose repeat unit and X group of the vinyl monomer. In certain embodiments, the crosslinking agent is selected from alkylene glycols, polyalkylene glycols and aromatic amines derivatized with glycidyl ether groups and soluble in the aqueous solution of the vinyl monomers. Suitable cross-linking agents include, for example, ethylene glycol diglycidyl ether, glycerol triglycidyl ether, glycerol diglycidyl ether (GDGE), glycerol propoxylate triglycidyl ether, polyethylen glycol diglycidyl ether, propylene glycol glycidyl ether, 1,4- cyclohexanoldimethanol diglycidyl ether, diglycidyl 1,2-cyclohexanedicrboxylate, N,N-diglycidyl aniline, N,N-diglcidyl-4-glycidyloxyaniline, and diglycidyl 1,2,3,4-tetrahydrophthalate. A mixture of these crosslinking agents also may be used.

[0063] The crosslinking agent can be dissolved in the vinyl monomer and added as a solution to the hemicaustic byproduct. The crosslinking agent can be present in an amount effective to covalently crosslink the water-soluble vinyl monomer and hemicelluloses. The amount of crosslinking agent can be determined based on the desired degree of absorption capacity and absorption under load (AUL). In certain embodiments, the cross-linking agent can be in an amount ranging from about 0.01 to about 5wt. %, based on the total weight of the vinyl monomer(s) and the hemicelluloses present in the hemicaustic suspension. In other embodiments, the amount of crosslinking agent ranges from about 0.02 to about 2 wt %, based on the total weight of the monomer and fiber. Usually, the presence of a high amount of crosslinking agent produces a Hem-SAP that exhibits a reduced absorption capacity and absorbency under load. On the other hand, if the crosslinking agent is used in an amount less than about 0.01 wt %, the produced Hem-SAP may exhibit a lower absorption capacity, low absorbency under load, lower gel strength and become sticky. The amount of crosslinking agent should, therefore, be controlled to provide a Hem-SAP with appropriate performance.

[0064] The free radical initiator can be used in the disclosed method such that the hemicellulose can be grafted with the vinyl monomer in the presence of a free radical initiator. The free radical initiator can be water-soluble persulfates such as potassium persulfate, ammonium persulfate, sodium persulfate, and other alkali-metal persulfates. Other initiators suitable include, for example, hydrogen peroxide, ammonium cerium nitrate, and water-soluble azo-compounds such as 2,2'-azobis-(2-amidinopropane) hydrochloride. Some of these initiators, such as hydrogen peroxide, can be combined with reducing substances such as iron, sulfites, or amines to form known redox-type initiators necessary to initiate the grafting reaction. The initiators can be in the form of a mixture of two or more. In certain embodiments, persulfates can be used. The total amount of initiators used may range anywhere from about 0.005 to about 1.0 weight percent of the total weight of the reaction mixture, including from about 0.01 to about 0.5 weight percent, based on the total weight of the reaction mixture. The initiator can be added as a solution in water.

[0065] Adding, for example, a sodium alkali solution to the homogenous reaction mixture followed by the addition of the vinyl monomer and crosslinking agent can produce an exothermic neutralization which can cause the temperature of the reaction mixture to increase by at least 10 °C. The rise in the temperature is enough to cause fragmentation in the initiator and start the grafting process, which results in a severe exothermic reaction within less than 5 min for the adding the initiator. As a result, external heat may not be necessary. External heat could be applied in case the exothermic reaction does not start immediately. In certain embodiments, the reaction is continued after the addition of the initiator for not less than two hours.

[0066] The grafting process can be carried out in the water that is present originally in the hemicaustic byproduct. Water could be added to adjust the reaction concomitancy. In certain embodiments, no water is added, and the grafting process is carried out at high consistency ranging from 20% to 80%preferably from 30% to 70%. Running the reaction at consistency below 20% can result in the formation of a homopolymer at a high percentage that affects the absorbent properties of the Hem-SAP.

[0067] The Hem-SAP prepared in accordance with various embodiments of the present disclosure can possess superabsorbent characteristics that make the Hem-SAP desirable for applications in absorbent articles intended for body waste management, such as, for instance, but not limited to, baby diapers, training pants, feminine hygiene products, and adult incontinence products. The Hem-SAP of the present disclosure can have a centrifuge retention capacity of greater than about 15.0 grams of synthetic saline (0.9% by weight NaCI) per gram of hem-SAP (hereinafter "g / g"), and even more than about 18 g / g. The centrifuge retention capacity measures the ability of the Hem-SAP to retain fluid against a centrifugal force. In addition, the Hem-SAP can have an absorbent capacity of greater than about 30 g / g and absorbency under load of greater than 20 g / g. The absorbent capacity measures the ability of the Hem-SAP to absorb fluid while confined in a cell under a 0.01 psi load. The absorbency under load measures the ability of the Hem-SAP to absorb fluid while under a load of 0.3 psi. The absorbent capacity, absorbency under load, and centrifuge retention capacity are determined by the Hanging Cell Test described below.

[0068] Without being bound to an individual theory, the inventors believe that, during the polymerization reaction of hemicaustic byproduct and vinyl monomer, radicals are developed by the initiator on both the hemicellulose repeating units and the vinyl monomer. The free radicals cause a chain growth polymerization to occur on the hemicellulose repeating units. The heat evolved from the free radical polymerization reaction initiates a SN2 reaction between the hydroxyl group of hemicelluloses, the carboxylate anion of the vinyl radical and the hemicellulose and epoxy groups of the crosslinking agent which result in cross-linked polymer via an ester linkage as shown in the scheme provided in FIG. 1. Once the SN2 reaction begins, it liberates heat to the reaction mixture.

[0069] SEM micrographs of conventional SAP and Hem-SAP crosslinked with glycerol diglycidyl ether (GDGE) are shown in FIGS. 2-4. The differences between the polymer surfaces are noticeable. Conventional superabsorbent polymer made from polyacrylic acid crosslinked with N,N'- ethylenebisacrylamide (EBAM) (FIG. 2) has a solid smooth surface. While Hem-SAP (FIG. 3) shows a porous polymeric network, the porosity can be seen clearly in the 1.2kx image (FIG. 4). The image shows grain nanoparticles that are attached to each other. The porosity could be attributed to the cross-linking agent diglycidyl ethers of ethylene glycol (EGDGE) and the branching structure of the hemicelluloses polymeric chain as shown in the chemical structures shown above and the scheme provided in FIG. 1.

[0070] By varying the amounts of the hemicaustic suspension, vinyl monomer, cross-linking agents, reaction mixture pH value, and the polymerization reaction conditions Hem-SAP with a wide range of absorbent capacity and absorbency under load can be produced, and the desired properties can be achieved.

[0071] The preparation of Hem-SAP can be performed in various types of mixing equipment, such as in containers provided with agitators or in rotating chambers.

[0072] The Hem-SAP prepared by the disclosed method can have high gel strength, high swelling capacity, high swelling rate, and superior absorbency under load. For instance, the Hem-SAP can have an absorbency under load of 0.3 psi as high as 28.0 g of saline / g. The gel strength can be related to the absorbency under load such that, when the absorbency under load is higher, the gel strength will also be higher. The swelling rate can be considered relative to the vortex time, which is discussed in the examples. In general, when the vortex time is lower, the swelling rate will be higher.

[0073] The Hem-SAP prepared by the disclosed method can have ester linkages and ether linkages, which are generally shown in FIG. 1. In certain embodiments, the Hem-SAP prepared by the disclosed method is free of amide linkages. The ester linkages are related to the reaction between the carboxylate group on acrylate polymer or hemicellulose and the epoxy group of the crosslinking agent. The ether linkages are related to the reaction between the hemicellulose hydroxyl groups and the epoxy groups of the crosslinking agent. The nature of the crosslinking agent selected for this work could contribute the polymer high absorbency, since it hydrophilic and water soluble. While the crosslinking agent used for commercial SAP is hydrophobic and water insoluble.EXAMPLES

[0074] The hemicaustic suspensions used in the following examples were obtained by extracting hardwood pulp with 10% sodium hydroxide by weight at 40°C for 10 minutes and purifying the extract to produce a suspension containing about 4% sodium hydroxide and 6.0% hemicelluloses, in which xylan was 90% to 95% of the hemicelluloses content by weight.

[0075] General Procedure for the Preparation of Hem-SAP

[0076] The Hem-SAPs of the Examples described below were prepared according to the following procedure. A grafting reaction was carried out in a PYREX® reaction kettle (1.0 L). The reaction kettle was equipped with a thermocouple inserted through a rubber septum, a TEFLON® bladed agitator on a glass shaft attached to a mechanical mixer, a gas inlet polyethylene tubing inserted through an adapter fitted with a rubber septum, and a gas outlet adapter connected to a gas trap partially filled with oil. Ahemicaustic suspension in the amount of 100 g (containing about 12% hemicellulose) was added to the reaction kettle. The hemicaustic suspension in the kettle was treated with hydrogen peroxide (2.0 g, 30 wt%). Acrylic acid (100.0 g) was added to the hemicaustic suspension. The reaction kettle containing the reaction mixture was placed in an ice water bath and neutralized to a pH of 5.0 to 5.5 using NaOH (50% solution, 45.0 g).

[0077] The required amount of crosslinking agent, which can be about 0.01 to about 5 wt. %, based on the total weight of the vinyl monomer(s) and the hemicelluloses present in the hemicaustic suspension, was then added via a syringe. The reaction kettle was heated until a clear solution was obtained (65 °C). The mixture was stirred until a clear solution was formed (5 min). An aqueous solution of potassium persulfate (0.05 g) and sodium thiosulfate (0.05 g) in 5.0 mL water was added at once (can be added in solid form). Gelation accompanied by an exothermic reaction started within 1.0 min from the addition of the aqueous solution containing the initiator. The reaction temperature reached about 104 °C, and a mass of porous gel formed in less than 5 min from the addition of the aqueous solution containing the initiator. After the exothermic reaction subsided (10 min), the reaction mixture was left under these conditions for another 1 hr. The produced solid mass was dried for about 2 hours at about 85 °C until the mass dried. Then, the dried solid mass was ground in a WILEY® mill such that the ground product passed through a 20-mesh screen but was retained on a 30-mesh screen.

[0078] Hanging Cell Test

[0079] A hanging cell test method was used to determine the absorbent capacity, absorbency under load, and centrifuge retention capacity. The hanging cell test was carried out in a plastic cylinder with a one inch inside diameter having a 100-mesh metal screen adhering to the cylinder bottom "cell," which contained a plastic spacer disk having a 0.995 inch diameter and a weight of about 4.4 g. In the hanging cell test, the weight of the cell containing the spacer disk was determined to the nearest 0.0001 g, and then the spacer was removed from the cylinder and about 0.05 g (dry weight basis) of Hem-SAP was added to the cylinder. The spacer disk then was inserted back into the cylinder on the Hem-SAP, and the cylinder group was weighed to the nearest 0.0001 g.

[0080] A 100 g (0.3 psi) weight was placed on the top of the spacer disk. The cell and its contents were hung in a Petri dish containing enough saline solution (0.9 wt.%) to touch the bottom of the cell. The cell was allowed to stand in the Petri dish for 30 minutes, then it was removed and hung in another empty Petri dish and allowed to drip for 30 seconds. The 100.0 g weight then was removed, and the weight of the cell and contents was determined. The weight of the saline solution absorbed per gram Hem-SAP was determined and expressed as the absorbency under load (g / g). The cell containing the Hem-SAPwas hanged again under zero load in the tray containing the saline solution. The cell was left in contact with the saline solution for 30 min. The weight of the saline solution absorbed per gram of fiber then was determined and expressed as absorbent capacity (g / g).

[0081] The cell then was centrifuged for 3 min at 1400 rpm (Centrifuge Model HN, International Equipment Co., Needham NTS, USA), and weighed. The results obtained were used to calculate the weight of saline solution retained per gram of fiber and expressed as the centrifuge retention capacity (g / g)-

[0082] Examples 1 to 4

[0083] The general procedure described above was repeated several times using 1:1 ratio by weight of hemicaustic suspension and acrylic acid with various amounts of the crosslinking agent glycerol diglycidyl ether (GDGE). The pH value of each reaction was maintained at 4.2. The Commercial SAP is AQUA KEEP® HP600 manufactured by Sumitomo Seika Chemicals Co., Ltd. The results are summarized in Table 1 below.

[0084] [Table 1: the effect of using various amounts of crosslinking agent on absorbency of Hem-SAP]Commercial SAP reported in this work was extracted from commercial diaper.

[0085] Examples 5 to 7

[0086] The general procedure described above was repeated several times using 1:1 ratio by weight of hemicaustic suspension and acrylic acid at various pH values and a constant amount of the crosslinkingagent glycerol diglycidyl ether (GDGE) (0.1 g). The Commercial SAP is AQUA KEEP® HP600 manufactured by Sumitomo Seika Chemicals Co., Ltd. The results are summarized in Table 2 below.

[0087] [Table 2: the effect of pH value of the reaction mixture on the absorbency of Hem-SAP]

[0088] Examples 8 to 10

[0089] The general procedure described above was repeated several times using various amounts of hemicaustic suspension to determine the amount that produces Hem-SAP with the highest absorbency. In each of Examples 8-10, 0.05 g of ethylene glycidyl ether (GDGE) was used. The Commercial SAP is AQUA KEEP® HP600 manufactured by Sumitomo Seika Chemicals Co., Ltd. The results are summarized in Table 3 below.

[0090] [Table 3: the effect of using various amounts of hemicaustic suspension on the absorbency of Hem-SAP]

[0091] Examples 11 to 13

[0092] The general procedure described above was repeated several times using a fixed amount of hemicaustic suspension (50.0 g) and a combination of the vinylic monomers acrylic acid and acrylamide (50.0 g in various ratios). In each of Examples 11-13, 0.05 g of ethylene glycidyl ether (GDGE) was used and pH was maintained at 5.5. The Commercial SAP is AQUA KEEP® HP600 manufactured by Sumitomo Seika Chemicals Co., Ltd. The results are summarized in Table 4 below.

[0093] [Table 4: the effect of using a combination of vinyl monomers with various proportions on the absorbency of Hem-SAP]

[0094] Examples 14 to 16

[0095] The general procedure described above was repeated several times using a fixed amount of hemicaustic suspension (50.0 g) and a fixed amount and ratio of a combination of the vinylic monomersacrylic acid and acrylamide (50.0 g in a 4:1 ratio by weight). In each of Examples 14-16, ethylene glycidyl ether (GDGE) was used as a cross-linking agent. The pH value was maintained at 5.5. The Commercial SAP is AQUA KEEP® HP600 manufactured by Sumitomo Seika Chemicals Co., Ltd. The results are summarized in Table 5 below.

[0096] [Table 5: the effect of using various amounts of crosslinking agent on the absorbency of Hem- SAP produced using a combination of two vinyl monomers]

[0097] Effect of using various hydrophilic vinyl monomers on Absorbency of Hem SAP

[0098] The general procedure described above was repeated using hemicaustic suspension (50.0 g) and a fixed amount and ratio of a combination of the vinylic monomers acrylic acid and various monomers (50.0 g in a 4:1 ratio by weight). In each of the examples, ethylene glycidyl ether (GDGE, 0.05 wt%) was used as a cross-linking agent. The pH value was maintained at 5.5. The "Commercial SAP (Pampers)" refers to SAP removed from a PAMPERS SWADDLERS® Size 4 diaper. The "Commercial SAP (Huggies)" refers to SAP removed from a HUGGIES® SNUG & DRY™ Size 4 diaper.

[0099] The results are summarized in FIG. 5.

[0100] Vortex Time

[0101] A vortex time test was carried out by adding a known weight of Hem-SAP (2.0 g) to 50 mL of saline solution in a 400 mL beaker containing a magnetic stirrer and placed on a plate rotating at 600 revolutions per minute. The Hem-SAP was added at once to an area between the beaker side and the center of the vortex. The time required for the vortex to become flat was recorded in seconds and reported as the Vortex Time. The Vortex Time represents the free swell absorbing rate of the superabsorbent material. The shorter the time the higher the absorption rate The Commercial SAP isAQUA KEEP® HP600 manufactured by Sumitomo Seika Chemicals Co., Ltd. The results are summarized in FIG. 6.

[0102] In the foregoing description, the associated products and methods of the present application have been described with reference to specific embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. Throughout this specification, unless the context requires otherwise, the word "comprise” and its variations, such as "comprises" and "comprising," will be understood to imply the inclusion of a stated item, element or step or group of items, elements or steps, but not the exclusion of any other item, element or step or group of items, elements or steps. Furthermore, the indefinite article "a" or "an" is meant to indicate one or more of the item, element, or step modified by the article.

Claims

Claims1. A method of making a hemicaustic superabsorbent polymer from a hemicaustic byproduct, the method comprising: mixing a free radical initiator with a homogenous reaction mixture comprising a hemicellulose, a water-soluble vinyl monomer, and a crosslinking agent, thereby producing the hemicaustic superabsorbent polymer.

2. The method of claim 1, further comprising treating a hemicaustic byproduct suspension comprising the hemicellulose with an oxidizing agent to obtain a treated hemicaustic byproduct suspension, and adding the water-soluble vinyl monomer and the crosslinking agent to the treated hemicaustic byproduct suspension to obtain the homogenous reaction mixture.

3. The method of claim 1, wherein the hemicaustic superabsorbent polymer is substantially free of monomers.

4. The method of claim 1, further comprising drying the hemicaustic superabsorbent polymer such that the hemicaustic superabsorbent polymer has a water content of between 1% and 20% by weight.

5. The method of claim 4, further comprising grinding the hemicaustic superabsorbent polymer into a powder.

6. The method of claim 1, wherein the homogenous reaction mixture has a consistency range from 20% to 70%.

7. The method of claim 2, wherein the hemicaustic byproduct suspension comprises 2% by weight to 12% by weight of the hemicellulose.

8. The method of claim 2, wherein the oxidizing agent is selected from the group consisting of hydrogen peroxide, tert-butyl peroxide, peracetic acid, perbenzioc acid, di-tert-butyl peroxide, and combinations thereof.

9. The method of claim 1, wherein the water-soluble vinyl monomer is a polar vinyl monomer.

10. The method of claim 1, wherein the water-soluble vinyl monomer is hydrophilical ly water soluble with a molecular formula H2C=CHX and able to undergo a free radical polymerization.

11. The method of claim 10, wherein X in the molecular formula H2C=CHX is selected from the group consisting of COOH, CONH2, CN, CH2OH, SO3H, CONHC(CH3OH)3, CONHCH(CH3)CH2SO3H, (COCH2CH2O-)n, n being 1 to 200, COOCH2CH2COOH, CH2SO3H, and combinations thereof.

12. The method of claim 1, wherein the water-soluble vinyl monomer is selected from the group consisting of an acrylic monomer, an acryl-amide, a methacrylamide, acrylonitrile, a vinyl- substituted heterocyclic compound, and combinations thereof.

13. The method of claim 1, wherein the water-soluble vinyl monomer is selected from the group consisting of acrylic acid, methacrylic acid, ethacrylic acid, a-acryloxypropionic acid and alkali metal salts and ammonium salts thereof, itaconic acid, acrylamide, methacrylamide, 2- acrylamido-2-methyl-l-propane sulfonic acid poly(ethylene glycol) methyl ether acrylate, dipentaerythritol penta- / hexa-acrylate, 4-acryloylmorpholine, 2-carboxyethyl acrylate, 2- methyl-2-propene-l-sulfonic acid and its salts and combinations thereof.

14. The method of claim 1, wherein the crosslinking agent is an alkylene glycol polyglycidy ether crosslinking agent.

15. The method of claim 1, wherein the crosslinking agent is an alkylene glycol or polyalkylene glycol derivatized with glycidyl ether groups.

16. The method of claim 14, wherein the alkylene glycol polyglycidy ether crosslinking agent is soluble in an aqueous solution.

17. The method of claim 1, wherein the crosslinking agent is selected from the group consisting of ethylene glycol diglycidyl ether, glycerol triglycidyl ether, glycerol diglycidyl ether, glycerolpropoxylate triglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol glycidyl ether, 1,4-cyclohexanoldimethanol diglycidyl ether, polyethylene glycol diglycidyl ether, propylene glycol glycidyl ether, and combinations thereof.

18. The method of claim 1, wherein the free radical initiator is selected from the group consisting of potassium persulfate, ammonium persulfate, sodium persulfate, alkali-metal persulfates, hydrogen peroxide, ammonium cerium nitrate, 2,2'-azobis-(2-amidinopropane) hydrochloride, and combinations thereof.

19. A hemicaustic superabsorbent polymer obtained by the method of claim 1.

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