High-capacity superabsorbent materials and their preparation methods

Abstract

This document describes superabsorbent materials (SAMs) and methods for preparing SAMs. The superabsorbent materials are copolymerized with selected redox-active inorganic salts during a staged polymerization process. The copolymer-based superabsorbent materials exhibit significantly improved absorption properties. The compositions and methods described herein can be used in a variety of absorption products.

Description

Technical Field

[0001] This disclosure relates to superabsorbent materials (SAMs) and methods for preparing SAMs. SAMs are copolymerized with selected redox-active inorganic salts during a staged polymerization process. The copolymer-based SAMs exhibit significantly improved absorption properties. The compositions and methods according to this disclosure can be used in a variety of absorption products. Background Technology

[0002] Superabsorbent polymers (SAPs) are three-dimensional networks capable of absorbing and retaining hundreds of times their own dry weight in water (or other aqueous media) and physiological fluids such as urine and blood, typically depending on the ion concentration of the aqueous solution. SAPs have applications in numerous fields, including pharmaceuticals, personal care products, biomaterials, biosorbents, and agriculture. The first commercially available SAP was produced in 1970 through the alkaline hydrolysis of starch-g-polyacrylonitrile. While these polymers can absorb up to 500 g / g of water, they are mechanically weak in their swollen state. SAPs were developed industrially in Japan and the United States for hygiene applications in the early 1980s. SAPs have been found to have the potential to replace fluff, making their use in hygiene products such as baby diapers and sanitary napkins cost-effective.

[0003] Desired characteristics of SAP include high absorbance capacity, high and tunable swelling rate, high absorbability under load, good swollen gel strength, high gel fraction after crosslinking, excellent durability and stability during swelling and storage, non-toxicity, and low cost. While current hydrogel systems offer good performance in several of these different aspects, various formulations suffer from several drawbacks, including low absorbability under pressure, gel blockage (whereby the initial layer of SAP forms a relatively impermeable barrier to subsequent water diffusion and deeper absorption into the material), and high sensitivity to electrolyte solutions. Various strategies have been employed to address these challenges, including the formation of complex and nanocomposite hydrogels, interpenetrating polymer network (IPN) hydrogels, and various surface treatments; however, improvements are still needed to enable the use of minimal materials across the widest possible range of applications.

[0004] The properties, concentrations, and molar ratios of monomers and crosslinking agents are considered the most important factors affecting the absorbance capacity of SAPs. Acrylic acid, acrylamide, and methacrylic acid are the most widely used monomers in the commercial preparation of SAPs. However, the presence of residual acrylamide in the gel poses a challenge to the practical use of such hydrogels in human health and personal care products. Conversely, the water absorption and swelling properties of ionic SAPs (e.g., those based on acrylic acid or methacrylic acid) are significantly reduced in saline liquids, including physiological fluids such as urine and blood. This salt sensitivity is due to the presence of counterions such as sodium ions (Na+) in physiological fluids. +It can effectively shield the charge of the polymer backbone, causing the charged groups that bind to the counterions to the polymer to condense, and thus reducing the counterion entropy and direct chain-chain repulsion that can be used to drive the swelling response.

[0005] Conventional superabsorbent polymers (SAMs) are primarily made from two types of SAPs: synthetic polymers and natural polymers. Generally, synthetic superabsorbent polymers are charged polyelectrolytes, such as polyacrylic acid (PAA), polyvinyl sulfonic acid, salts of polyvinyl phosphate, and partially hydrolyzed maleic anhydride copolymers. Natural polymers include both neutral and charged polymers, such as carboxymethyl cellulose, sodium alginate, chitosan salts, and modified starches. All of these known SAMs have substantial limitations. For example, the most widely used commercial PAA-based SAMs have limited absorbability under load (AUL) and are also highly sensitive to salts.

[0006] Salt sensitivity of SAM leads to a reduction in gel swelling capacity. To compensate for this capacity reduction, personal care garments require more SAM material to ensure sufficient absorbency and prevent leakage. Therefore, the personal care industry urgently needs to improve or overcome the salt sensitivity of conventional SAM materials.

[0007] However, the salt sensitivity of SAM remains a challenging problem to be solved. While numerous attempts have been made in this field, all proposed solutions remain impractical and prohibitively expensive. It is well known in the art that the main component of SAM exists in its neutralized salt form, and the conventional idea is that all these neutralized salt units are considered to be in their fully ionized form, such as metal ions (e.g., Na+). + During swelling, all components can move freely. It is also known in the art that salt is detrimental to capacity, and that high-valent inorganic salts inhibit SAM capacity more than monovalent salts (such as sodium chloride). Therefore, past work on reducing the salt sensitivity of SAM materials has mainly focused on simply using more SAM material to compensate for capacity loss, removing / reducing salt from physiological fluids (such as urine), or attempting to design new structures with lower salt sensitivity.

[0008] PCT / US2020 / 053003 describes how copolymerization of SAM monomers with selected inorganic salts containing metal ions having an oxidation state of at least 2 can provide a significant increase in CRC performance without adversely affecting other performance properties, such as AUL. However, these copolymerizations are achieved using a non-staged one-pot synthesis procedure. While one-pot synthesis procedures are simple for fabrication, they have a significant drawback: they do not offer flexibility in controlling / designing the SAM polymer chain structure to mitigate salt sensitivity. For example, a uniform crosslink density throughout the gel network limits elastic expansion, thus reducing the potential capacity increase caused by chain charge-charge repulsion.

[0009] This article describes SAM and methods for its preparation. SAM is copolymerized with selected redox-active inorganic salts during a staged polymerization process. Due to the increased presence of permeation free ions, copolymer-based SAM exhibits significantly improved absorption properties. Due to the staged copolymerization procedure, copolymer-based SAM also possesses an interpenetrating network, thereby mitigating the elastic forces restricting chain expansion. Copolymer-based SAM also offers structural design flexibility, where each stage of polymerization can have its own composition, crosslink density, etc., which can further enhance the absorption capacity of SAM while providing unique SAM properties that may not be available with conventional homogeneous PAA-based SAM structures.

[0010] The compositions and methods disclosed herein can be used in a variety of absorbent products.

[0011] Purpose of the invention

[0012] The purpose of this disclosure is to address the salt sensitivity of AA-based SAMs by using a staged polymerization technique. Summary of the Invention

[0013] In one aspect, this document provides a superabsorbent polymer comprising a polymer backbone comprising (i) neutralized main-chain monomers, (ii) neutralized main-chain monomers complexed with a coordination complex comprising a metal ion having an oxidation state of at least 2, (iii) optional unneutralized main-chain monomers, and (iv) optional unneutralized main-chain monomers complexed with a coordination complex comprising a metal ion having an oxidation state of at least 2, and optional crosslinking agent; wherein the superabsorbent polymer comprises a non-uniform microstructure.

[0014] On the other hand, this document provides a method for preparing a superabsorbent polymer, the method comprising: (i) an initial polymerization stage, the initial polymerization stage comprising (ia) forming a mixture comprising a solvent, a monomer, a salt comprising a redox-active metal ion, an optional crosslinking agent and an initiator, and (ib) reacting the mixture at a first temperature; and (ii) a final polymerization stage, the final polymerization stage comprising (iia) optionally adding a crosslinking agent to the mixture; (iib) heating the mixture from the first temperature to a second temperature; and (iic) reacting the mixture at the second temperature.

[0015] In another aspect, this document provides a method for using a superabsorbent polymer comprising a polymer backbone comprising (i) neutralized main-chain monomers, (ii) neutralized main-chain monomers complexed with a coordination complex comprising a metal ion with an oxidation state of at least 2, (iii) optional unneutralized main-chain monomers, and (iv) optional unneutralized main-chain monomers complexed with a coordination complex comprising a metal ion with an oxidation state of at least 2, and optional crosslinking agent; wherein the superabsorbent polymer comprises a non-uniform microstructure. The method includes using the superabsorbent polymer in a consumer product. Attached Figure Description

[0016] Figure 1 A conventional process involving the non-stage SAM polymerization of stable salts such as FeCl3 is described.

[0017] Figure 2 According to an exemplary embodiment of this disclosure, the figure depicts FeCl2-induced staged SAM polymerization.

[0018] Figure 3A According to an exemplary embodiment of this disclosure, the figure depicts a two-dimensional view of the polymer microstructure resulting from staged SAM polymerization induced by FeCl2.

[0019] Figure 3B According to an exemplary embodiment of this disclosure, the figure depicts a three-dimensional view of the polymer microstructure resulting from FeCl2-induced staged SAM polymerization.

[0020] Figure 4 According to an exemplary embodiment of this disclosure, the figure depicts energy storage modulus data of polymers produced by staged polymerization and polymers not produced by staged polymerization.

[0021] Figure 5 According to an exemplary embodiment of this disclosure, the figure depicts energy storage modulus data of a polymer produced by staged polymerization.

[0022] Figure 6 According to an exemplary embodiment of this disclosure, the figure depicts a selected inorganic salt for increasing ionization.

[0023] Figure 7 According to an exemplary embodiment of this disclosure, the figure depicts a single iron(II) complex prior to polymerization.

[0024] Figure 8 According to an exemplary embodiment of this disclosure, the figure depicts the coordination modes of two iron(II) complexes prior to polymerization.

[0025] Figure 9According to an exemplary embodiment of this disclosure, the figure depicts a potential AANa-SIS unit formed by a single iron(II) complex in a polymeric network.

[0026] Figure 10 According to an exemplary embodiment of this disclosure, the figure depicts a potential AANa-SIS unit formed from a didentate iron(II) monomer complex in a polymeric network. Detailed Implementation

[0027] Superabsorbent polymers, produced by copolymerization of SAM monomers with selected inorganic salts having an oxidation state of at least 2, are described in PCT / US2020 / 053003, which is incorporated herein by reference. These copolymerizations were achieved using a non-stepwise one-pot synthesis procedure. Figure 1 An example of a non-stage SAM polymerization process involving stable salts such as FeCl3 is shown. In a conventional process, a prepolymer mixture is prepared at 0°C or room temperature, and a stable metal salt is added to the prepolymer mixture. These stable metal salts do not initiate polymerization. Instead, single-step polymerization occurs via single-step thermal radical polymerization, where the temperature is raised to 60°C–70°C and maintained at that temperature to complete the polymerization. This results in a nearly homogeneous polymer structure with a uniform crosslink density.

[0028] In contrast to non-staged one-pot synthesis procedures and the resulting polymers, this disclosure relates to the copolymerization of SAM with selected redox-active inorganic salts during staged polymerization. Figure 2 A staged polymerization process is described. This exemplary staged polymerization process has two stages. Typically, in such a two-stage process, a prepolymer mixture is prepared at a low temperature (such as 0°C and / or an ice bath) or at room temperature. A crosslinking agent may or may not be added to the prepolymer mixture. A compound having a redox-active metal (such as FeCl2) is then added to the mixture to initiate a first-stage redox-induced polymerization while providing ionization enhancement. This produces water-soluble oligomers. Without the addition of a crosslinking agent to the prepolymer mixture, there will be no crosslinking or reduced crosslinking. A crosslinking agent may or may not be added to the water-soluble oligomers. Next, a second-stage thermal free radical polymerization is initiated by raising the temperature to 60°C–70°C and maintaining it to complete the polymerization. This produces a polymer containing a shell-core microstructure, wherein the first-stage oligomers account for 1%–30% of the final polymer dry weight, and the second-stage polymer accounts for 60%–99% of the final polymer dry weight. After the second-stage polymerization, the polymer is water-insoluble.

[0029] The staged polymerization process according to this disclosure provides a first function for incorporating a small amount of selected inorganic salts (SIS) into a predominantly PAA-based superabsorbent material (SAM) to enhance its absorption capacity. Furthermore, in addition to the first capacity-enhancing function, the staged polymerization process according to this disclosure provides a second function for controlling the polymerization process, allowing for further improvement of the gel properties of the SAM polymer. In this second function, the SIS will be part of the polymerization initiation step and must be redox-active. In other words, when the SIS reacts with a suitable free radical polymerization initiator (e.g., KPS) to begin the polymerization process, they must be able to react from a lower oxidation state (e.g., M... n+ ) oxidized to a higher oxidation state (e.g., M) n+1 ). By initiating the polymerization process, oxidized metal ions (e.g., M) n+1 This will be incorporated into the polymer chain through complexation to provide a source of ionization enhancement. Non-redox active salts can still be added for ionization enhancement purposes, but they will not initiate polymerization.

[0030] Control of polymerization means that the secondary function of a redox-active SIS will allow the polymerization process to proceed in pre-designed stages such as Stage 1, Stage 2, Stage 3, etc. Stage 1 is the polymerization initiation step, and its polymerization parameters can be the same as or different from those of the other stages. These polymerization parameters include, but are not limited to, temperature, time, oxygen level, crosslinking agent, initiator, one or more monomers, the concentration (e.g., loading level) of each monomer, initiator, and crosslinking agent, and any specially designed reaction-related sequence (e.g., the order in which monomers, initiators, or crosslinking agents are added). For example, Stage 1 may or may not require a crosslinking agent, such that the polymer network formed in this step can be primarily a long-chain polymer without chemical bond-based crosslinking. In another example, both Stage 1 and Stage 2 may have crosslinking agents, but require two different temperatures and different polymerization times to complete their respective polymerization processes.

[0031] According to this disclosure, the polymer networks formed at different stages are not independent. Instead, the polymer networks formed at different stages are interconnected by chemical crosslinking, physical chain-chain entanglement, or chain charge-charge interactions. In some cases, such interconnection may result in a hierarchical network structure, while in others, the overall structure may resemble an interpenetrating network because certain portions of the network from different stages are effectively entangled together by chemical crosslinking, charge-charge interactions, and physical entanglement.

[0032] In a general embodiment, using an acrylic acid-based monomer as an example, a selected inorganic salt (SIS), including a redox-active salt, can be added to acrylic acid (AA) or a partially neutralized AA solution to form AA-SIS and AANa-SIS complexes prior to polymerization. After complex formation, a conventional polymerization procedure is performed to form the desired sample material. Alternatively, SIS can be added as a final agent to the polymerization procedure, which prevents premature polymerization initiation if they are added to the monomer first. Furthermore, SIS can be added in batches at desired ratios at each stage or at different stages. The preferred loading range of SIS can vary, but typically will not exceed 1% of the weight of the monomer involved.

[0033] The general structure of the SAM material disclosed herein can be described as a staged overall network, wherein each stage has its own sub-network:

[0034] [network] 第1阶段 [network] 第2阶段 ….[network] 第x阶段 [network] 第x1阶段 …

[0035] Typically, the number of stages can be any suitable number of stages known in the art. The number of stages can be one, two, three, four, five, or more than five. Preferably, the total number of stages should be controlled to five or less, and most preferably to three or less.

[0036] The network ratios of the different stages can be any desired ratio and can be controlled by changing the polymerization parameters as described above. In some embodiments, one stage will be dominant. In some embodiments, these stages are divided equally. In some embodiments, the transition from one stage to the next is considered a separate stage because the transition can be time-dependent, temperature-dependent, or similar. For example, the polymerization mixture requires time to transition from one temperature to another because raising the temperature can take time (e.g., a continuous or pre-prepared temperature bath).

[0037] Due to the complexity of precisely defining the exact network structure for each polymerization stage, this disclosure employs a simplified approach, defining the composition of the final polymer by using the total loading level of each monomer and the SIS complexation form of each monomer. For example, if the staged polymerization involves two monomers and a redox SIS salt, the composition of the final polymer can be described as [(monomer 1)]. x (Single 1-SIS) y (Single 2) z (Single 2-SIS) w], where x, y, z, and w are the molar or weight ratios of each component. In terms of weight ratio, the ratio of Y to W in the combination is typically limited to less than 1% of (x+y+z+w). In a specific example, for two monomers, acrylic acid and sodium acrylate, the composition of the staged copolymer can be described as [(AA)]. x (AA-SIS) y (AANa) z (AANa-SIS) w ], where x, y, z, w are the molar ratio or weight ratio of each monomer and their SIS complex forms.

[0038] In many embodiments, the superabsorbent polymer according to this disclosure comprises a polymer backbone comprising (i) neutralized backbone monomers, (ii) neutralized backbone monomers complexed with a coordination complex comprising a metal ion with an oxidation state of at least 2, (iii) optional unneutralized backbone monomers, and (iv) optional unneutralized backbone monomers complexed with a coordination complex comprising a metal ion with an oxidation state of at least 2, and optional crosslinking agent, wherein the superabsorbent polymer comprises a non-uniform microstructure.

[0039] In some embodiments, the superabsorbent polymer comprises multiple polymer moieties, each polymer moiety being generated in a different polymerization stage. In some embodiments, the superabsorbent polymer comprises a first polymer moieties and a second polymer moieties, wherein the first polymer moieties and the second polymer moieties are generated in different polymerization stages.

[0040] In some embodiments, the superabsorbent polymer comprises a first polymer portion present in an amount ranging from about 1 to about 40% by weight of the superabsorbent polymer and a second polymer portion present in an amount ranging from about 60 to about 99% by weight of the superabsorbent polymer. In some embodiments, the superabsorbent polymer comprises a first polymer portion present in an amount ranging from about 1 to about 30% by weight of the superabsorbent polymer and a second polymer portion present in an amount ranging from about 60 to about 99% by weight of the superabsorbent polymer.

[0041] Typically, superabsorbent polymers may contain any suitable non-uniform microstructures known in the art. Non-uniform microstructures result in non-uniform polymer structures. In some embodiments, the superabsorbent polymer contains non-uniform microstructures selected from the group consisting of: layered polymer structures having domains from different stages; core-shell polymer structures comprising a core from one stage and a shell from another stage; interpenetrating networks having one stage as a network and another stage as another network; and combinations thereof.

[0042] exist Figure 3A Two-dimensional view and Figure 3B An exemplary core-shell polymer microstructure is shown in the three-dimensional view.

[0043] Typically, superabsorbent polymers can have any suitable crosslinking density known in the art. In some embodiments, the superabsorbent polymer has a non-uniform crosslinking density. In some embodiments, the superabsorbent polymer comprises a first polymer portion having a first crosslinking density and a second polymer portion having a second crosslinking density, wherein the first and second crosslinking densities are different. In some embodiments, the superabsorbent polymer comprises a first polymer portion with no crosslinking or with minimal crosslinking and a second polymer portion with high crosslinking.

[0044] Typically, superabsorbent polymers possess unique absorption properties compared to conventional superabsorbent polymers. In some embodiments, superabsorbent polymers are configured to provide a storage modulus (G') reduction of at least 5.2% between 300 and 500 seconds after exposure to a 0.9% NaCl solution at room temperature. In contrast, polymers not produced through staged polymerization do not exhibit such a large reduction in storage modulus (G') under the same test conditions.

[0045] Typically, superabsorbent polymers are copolymerized from selected inorganic salts, neutralized monomers, unneutralized monomers, and optionally crosslinking agents. This copolymerization of superabsorbent polymers reduces salt sensitivity to significantly improve CRC performance without adversely affecting other performance properties, such as AUL. Higher AUL can also be achieved if the gel strength of the SAM according to this disclosure can be enhanced using well-known techniques, such as surface crosslinking of the SAM particles.

[0046] Metal ions with an oxidation state of at least 2 can have at least 2, at least 3, or at least 4 oxidation states. The use of stable, selected inorganic salts containing such metal ions is based on key findings in the analysis of interactions between metal salts and SAM polymer chains ion pairs. There are key differences when metal salts, especially those with metal ions in the salt having an oxidation state greater than 2, interact with SAM polymer chains from an external solution, and when the same salt is introduced into the polymer chains prior to polymerization by forming metal complexes with monomers. In the former case, the salt acts as an “external” ion pair approaching the polymer chain, primarily acting as a chain charge shielding agent (a monoclonal counterion such as Na+). + ) and high oxidation ions (such as Ca) 2+ And Al 3 + )) and chelating crosslinking agents (high oxidation ions (such as Ca) 2+ Al 3+The shielding and chelating effects of high-oxide-weight metal ions are deeper because they accumulate on the outer layer of SAM particles, thus significantly limiting and reducing the elastic portion of SAM swelling. However, in the latter case, the same high-oxide-weight metal ions, when they first form complexes with monomers such as AA or AANa and then polymerize, can actually help generate more free ions for the polymer chains, thereby increasing SAM absorbability, such as... Figure 6 As shown in the figure. This observation is consistent with the osmotic pressure theory, which states that osmotic pressure is proportional to the total particle count in the gel system.

[0047] These complexed salts are referred to as "internal" or "intrinsic" metal salts and metal ions.

[0048] However, even if SAM uptake can be increased, too much internal salt can still be detrimental to SAM uptake because they can also act as crosslinking agents through chelation. Therefore, uptake benefits and effective increases in SAM capacity can only be achieved when chelation between polymer chains is minimized during polymerization. For this purpose, the loading level of the metal salt used to form the complex must be managed to maximize ionization while minimizing crosslinking. Furthermore, selected salts of metals with an oxidation state greater than two are suitable as stable selected inorganic salts in this disclosure. This is because high oxidation state salts are more conducive to the formation of more free ions than salts of metal ions with low oxidation states. More specifically, metal ions with an oxidation state of two (i.e., M... 2+ Metal ions with an oxidation state of -1 (i.e., M) + It is more favorable in stable selected inorganic salts, and the oxidation state is a metal ion of three or greater (i.e., M). ≥3+ Metal ions with an oxidation state of two (i.e., M) 2+ It is more advantageous.

[0049] Since few metal ions are stable when the oxidation state is greater than three, the preferred salts are those with metal ions having an oxidation state of at least three. Some salts with M 3+ Salts that readily form chelate structures in aqueous solutions or polymer mixtures may still not be applicable to this disclosure.

[0050] Many inorganic salts containing metal ions with oxidation states greater than 2 (i.e., metal halides, metal sulfates, and metal nitrates) can be used to demonstrate the beneficial effects of this disclosure. However, most of these salts are not readily available and pose safety concerns in personal care applications. Therefore, the stable metal ions shown herein are common in nature and are associated with biologically relevant processes. Consequently, they are safe or considered safe in at least some personal care applications. In any case, this disclosure is not intended to be limiting, and any inorganic salt containing metal ions with oxidation states of at least two can be used as a stable selected inorganic salt in this disclosure.

[0051] The stable selected inorganic salts according to this disclosure include metal ions capable of forming coordination complexes with monomers in the prepolymer mixture; these structures will become part of the polymer chains or chain networks of the SAM after copolymerization. The stable selected inorganic salts are non-redox active salts that contribute to enhanced ionization but do not initiate polymerization.

[0052] The structure of coordination complexes is partly dependent on the coordination number of the metal ion. The coordination number of a metal largely depends on how many donor atoms can be accommodated around it, and it is, in turn, controlled by size. Regarding transition metals, later transition metals are smaller than earlier transition metals. Across the 3d transition series, nuclear charge and atomic number increase, but the shielding created by the filling of electron shells remains unchanged. This pulls the outer electrons closer together, thus the atoms and ions gradually become smaller. For zinc, the last and smallest of the 3d metals, four is the most common coordination number. Therefore, later transition metals generally form smaller complexes than earlier transition metals.

[0053] In some embodiments, a coordination complex comprising a metal ion having an oxidation state of at least 2 is coordinated with more than one neutralized or unneutralized main-chain monomer. It is conceivable that monomer complexes having more than one neutralized or unneutralized main-chain monomer or combinations thereof can initiate one or two or more polymer chains. Due to these geometrical constraints of the coordinating metal central ion, SAM materials with this structure will result in more favorable gel properties, such as improved gel stiffness and mechanical properties.

[0054] like Figures 7 to 10The depicted coordination modes between the monomer and the selected salt can have two main modes, such as coordination between the olefin double bond of the monomer and the metal center, and coordination between the carboxylic ester side of the monomer and the metal center. Coordination between the carboxylic ester and the metal center is more complex than coordination between the olefin double bond because it can exhibit different modes. It is conceivable that these different coordination modes can form prior to polymerization in the monomer complex stage, and some of these initially simple complexes can form more complex structures during polymerization or even later (such as during the drying process). It is also conceivable that these different interactions can occur within a single polymer chain or between different chains to form a network. For the purpose of enhancing ionization, networks formed by different chains here are not desirable and should be minimized, as they will act as crosslinking agents to eliminate ionization.

[0055] Because different coordination modes and the formation of different selected metal salts can make one or more modes superior to others, it should be understood here that those skilled in the art can fine-tune the polymerization and processing conditions for a given selected salt to maximize capacity enhancement. It is also conceivable that in some cases, CRC enhancement can be deeper than AUL enhancement, as under the polymerization conditions of this disclosure. In other cases, AUL enhancement can be deeper than CRC enhancement. In still other cases, both CRC and AUL enhancement can be achieved.

[0056] Transition metals are particularly advantageous in this disclosure because they have numerous oxidation states and coordination complexes. In some embodiments, the coordination complex comprises a metal ion selected from the group consisting of transition metals. Preferably, the coordination complex comprises a metal ion selected from the group consisting of transition metals in rows 1 and 2. Most preferably, the transition metal ion is selected from the group consisting of transition metals in row 1 and combinations thereof.

[0057] In some implementations, the coordination complex comprises a metal ion selected from the group consisting of: Ca 2+ Co 2+ Zn 2+ Mn 2+ Al 3+ Fe 3+ Co 3+ Mn 3+ And combinations thereof. In some embodiments, the coordination complex comprises a complex selected from the group consisting of: ML x 2+ ML x 3+ And their combinations, where L is an organic ligand and M is a metal ion selected from the group consisting of: Ca 2+ Co2+ Zn 2+ Mn 2+ Al 3+ Fe 3+ Co 3+ Mn 3+ Ru 3+以 And their combinations.

[0058] The coordination of organic ligands with metal centers can significantly alter the oxidation potential of the metal centers, thereby allowing for the modification of polymerization conditions to accommodate metal centers that are more sensitive to redox reactions. The use of organic ligands can be particularly helpful in stabilizing some highly unstable metal salts, such as Co. 3+ and Mn 3+ So that they can be like Fe 3+ It is also used as a stable salt. Suitable organic ligands (e.g., L) can include, but are not limited to, monodentate, bidentate, and multidentate ligands, such as those commonly used in inorganic / organic / organometallic chemistry. Specific examples include OR2, NR3, PR3, and OR2(CH2). n OR 2、 NR2(CH2) n NR2, PR2(CH2) n PR2, etc., where R is an alkyl group selected from the group consisting of: methyl, ethyl, butyl, and combinations thereof.

[0059] In some embodiments, a coordination complex comprising a metal ion having an oxidation state of at least 2 is coordinated with more than one neutralized or unneutralized main-chain monomer. Coordination with more than one neutralized or unneutralized main-chain monomer can occur along the same or different polymer chains.

[0060] For the purpose of enhancing ionization, stable, selected inorganic salts with non-redox activity can also be added, but they will not initiate polymerization. A particularly preferred stable, selected inorganic salt is FeCl3. FeCl3 is particularly effective and inexpensive. FeCl3 exists in water in various hydrated coordination complex forms. The hydrated forms can form monodentate, bidentate, and even higher-order polydentate complexes from the monomer, and these can be retained and become a source of more free ions in the polymerized SAM. Figures 7 to 10 As described and illustrated herein, the selection of inorganic salts can improve the salt sensitivity of SAMs through a number of mechanisms, primarily by increasing the free ions when they are directly incorporated into the polymer chains and chain networks of SAMs as intrinsic internal salts.

[0061] While not specifically targeted, SAMs with metal ions may possess other advantageous properties compared to SAMs without coordinating metal ions. For example, charged metal centers are known for trapping or killing bacteria, a property highly desirable for personal clothing such as diapers and trousers, as well as feminine pads. In another aspect, metal ions are known to absorb a variety of odor molecules, such as ammonia, amines, and sulfur compounds. These odor compounds are well known to be present in physiological fluids such as urine, blood, and sweat. In yet another aspect of this disclosure, metal ions in polymer chains and chains can act as initiating sites for degradation, allowing for more rapid decomposition in natural or engineered processes, which can be accelerated by means such as photoradiation (e.g., the commonly known OXO-biodegradation process). Therefore, SAMs with coordinating metal ions may be superior to other SAMs, at least because they exhibit better biodegradability.

[0062] Typically, neutralized main-chain monomers are present in an amount ranging from about 50 to about 99 mol% of the superabsorbent polymer. Neutralized main-chain monomers complexed with coordination complexes containing metal ions with at least two oxidation states are present in an amount ranging from about 0.001 to about 0.3 mol% of the superabsorbent polymer. Unneutralized main-chain monomers are present in an amount ranging from about 0 to about 40 mol% of the superabsorbent polymer. Unneutralized main-chain monomers complexed with coordination complexes containing metal ions with at least two oxidation states are present in an amount ranging from about 0.001 to about 0.3 mol% of the superabsorbent polymer. More neutralized main-chain monomers complexed with coordination complexes containing metal ions with at least two oxidation states can have a detrimental effect on the superabsorbent polymer. In some embodiments, the preferred total amount of neutralized and unneutralized monomers having a selected coordinating salt is in the range of about 0.01 mol% to about 0.3 mol% of the final polymer.

[0063] In some other embodiments, the preferred total amount of neutralized and unneutralized monomers having a selected coordination salt is in the range of about 0.01 mol% to about 0.15 mol% of the final polymer. In some other embodiments, the preferred total amount of neutralized and unneutralized monomers having a selected coordination salt is in the range of about 0.01 mol% to about 0.1 mol% of the final polymer. In some other embodiments, the preferred total amount of neutralized and unneutralized monomers having a selected coordination salt is in the range of about 0.01 mol% to about 0.05 mol% of the final polymer.

[0064] In some embodiments, the neutralized main-chain monomer is present in amounts greater than about 50 mol%, 55 mol%, 60 mol%, 65 mol%, 70 mol%, 75 mol%, 80 mol%, 85 mol%, 90 mol%, 95 mol%, 96 mol%, 97 mol%, or 98 mol% of the superabsorbent polymer.

[0065] In some embodiments, the unneutralized main-chain monomer is present in amounts greater than about 0 mol%, 5 mol%, 10 mol%, 15 mol%, 20 mol%, 25 mol%, 30 mol%, 35 mol%, 36 mol%, 37 mol%, 38 mol%, or 39 mol% of the superabsorbent polymer.

[0066] In some embodiments, the sum of the weight percentage of neutralized main-chain monomers complexed with coordination complexes containing metal ions with oxidation states of at least 2 and the weight percentage of unneutralized main-chain monomers complexed with coordination complexes containing metal ions with oxidation states of at least 2 is less than about 0.2%, less than about 0.075%, less than about 0.050%, less than about 0.025%, and less than 0.01% of the total weight percentage of the polymer main chain.

[0067] Depending on the end application, superabsorbent polymers can take many forms. In some embodiments, the superabsorbent polymer is in the form of particles, gels, fibers, beads, liquids, solids, pastes, or combinations thereof. In some specific embodiments, the superabsorbent polymer is a gel or particles, and preferably in the form of particles.

[0068] In some embodiments, the superabsorbent polymer is in the form of a diameter ranging from about 100 μm to about 1000 μm, about 100 μm to about 900 μm, about 100 μm to about 800 μm, about 100 μm to about 700 μm, about 100 μm to about 600 μm, about 100 μm to about 500 μm, about 100 μm to about 400 μm, about 100 μm to about 300 μm, or about 100 μm to about 200 μm. In some embodiments, the superabsorbent polymer is in the form of a diameter ranging from about 200 μm to about 900 μm, about 300 μm to about 800 μm, about 300 μm to 600 μm, about 400 μm to about 700 μm, or about 500 μm to about 600 μm.

[0069] In some specific embodiments, the superabsorbent polymer is in the form of a diameter ranging from about 300 μm to about 600 μm.

[0070] In several embodiments, the superabsorbent polymer exhibits substantially improved absorption and salt sensitivity properties in the uncrosslinked state. However, crosslinking agents can be used to further enhance the properties of the superabsorbent polymer. The superabsorbent polymer may contain one, two, or more crosslinking agents.

[0071] In some embodiments, the superabsorbent polymer comprises a crosslinking agent selected from the group consisting of methylene (bis)acrylamide (MBAA), polyethylene (diacrylate) (PEGDA), polyethylene (diacrylate) (EGDA), polyethylene (dimethacrylate) (EGDMA), polyethylene (dimethacrylate) (PEGDMA), and combinations thereof.

[0072] In some embodiments, the superabsorbent polymer comprises a slightly crosslinked polymer portion. In some embodiments, the superabsorbent polymer comprises a polymer portion with a crosslinker density ranging from about 0.1 mol% to about 2.0 mol%. In some embodiments, the superabsorbent polymer comprises a polymer portion with a crosslinker density ranging from about 0.2 mol% to about 2.0 mol%. In some embodiments, the superabsorbent polymer comprises a polymer portion with a crosslinker density ranging from about 0.5 mol% to about 2.0 mol%. In some embodiments, the superabsorbent polymer comprises a polymer portion with a crosslinker density ranging from about 1.0 mol% to about 2.0 mol%. In some embodiments, the superabsorbent polymer comprises a polymer portion with a crosslinker density ranging from about 1.5 mol% to about 2.0 mol%.

[0073] The neutralized and unneutralized main-chain monomers are each selected individually based on the final application of the superabsorbent polymer. Monomers known in the art for use in superabsorbent polymers are suitable as neutralized and unneutralized main-chain monomers. The neutralized and unneutralized main-chain monomers can be in the form of neutralized and unneutralized versions of the same monomer or in the form of neutralized and unneutralized versions of different monomers.

[0074] In some embodiments, the neutralized main-chain monomer and the unneutralized main-chain monomer are each individually selected from the group consisting of: acrylic acid, methacrylic acid, vinyl sulfonic acid, vinyl phosphoric acid, partially hydrolyzed maleic anhydride, and combinations thereof.

[0075] This document also discloses a method for preparing a superabsorbent polymer, the method comprising: (i) an initial polymerization stage, the initial polymerization stage comprising (ia) forming a mixture comprising a solvent, a monomer, a salt comprising a redox active metal ion, an optional crosslinking agent and an initiator, and (ib) reacting the mixture at a first temperature; and (ii) a final polymerization stage, the final polymerization stage comprising (iia) optionally adding a crosslinking agent to the mixture; (iib) heating the mixture from the first temperature to a second temperature; and (iic) reacting the mixture at the second temperature.

[0076] Salts containing redox-active metal ions are part of the polymerization initiation step. When reacting with a suitable radical polymerization initiator (e.g., KPS) to begin the polymerization process, the redox-active metal must be able to react from a lower oxidation state (e.g., M). n+ ) oxidized to a higher oxidation state (e.g., M) n+1 ). By initiating the polymerization process, oxidized metal ions (e.g., M) n+1 This will be incorporated into the polymer chain through complexation to provide a source of ionization enhancement. Non-redox active salts can still be added for ionization enhancement purposes, but they will not initiate polymerization.

[0077] In some embodiments, the salt containing redox-active metal ions comprises metal ions selected from the group consisting of: Co 2+ Mn 2+ Al 2+ Fe 2+ Co 2+ Mn 3+ Ru 2+ And combinations thereof. Typically, salts may contain any suitable salt anion known in the art. In some embodiments, the salt contains anion selected from the group consisting of: Cl... - SO4 2- NO3 - BF4 - , and their combinations.

[0078] A particularly preferred redox-active metal salt is FeCl2. FeCl2 is particularly effective and inexpensive. During polymerization, Fe... 2+ Oxidized into Fe 3+ .

[0079] In some implementations, redox-active metal ions are oxidized to metal oxide ions selected from the group consisting of: Co 3+ Al 3+ Fe 3+ Co 3+ Mn 3+ And their combinations.

[0080] In some embodiments, the mixture also contains a stable salt comprising a metal ion with an oxidation state of at least 2.

[0081] In some embodiments, the method further includes at least one intermediate polymerization stage between the initial polymerization stage and the final polymerization stage. The at least one intermediate polymerization stage may include any suitable conditions for inducing polymerization. In some embodiments, the at least one intermediate polymerization stage includes adding a crosslinking agent to the mixture. In some embodiments, the at least one intermediate polymerization stage includes a time-dependent transition or a temperature-dependent transition. As used herein, a time-dependent transition refers to a time period for inducing different polymerization stages, such as a time delay. As used herein, a temperature-dependent transition refers to the temperature at which different polymerization stages are induced.

[0082] Typically, the method may include any suitable number of aggregation stages known in the art. In some embodiments, the method includes one, two, three, four, five, or more than five aggregation stages. In some embodiments, the method includes multiple aggregation stages in the range of two to five.

[0083] Typically, the first temperature can be any temperature that promotes the reaction. In some embodiments, the first temperature is in the range of about -20°C to about 40°C. In some preferred embodiments, the first temperature is 0°C. In some preferred embodiments, the first temperature is in the range of about 20°C to about 25°C.

[0084] Typically, the second temperature can be any temperature that promotes the reaction. In some embodiments, the second temperature is in the range of about 20°C to about 100°C. In some embodiments, the second temperature is in the range of about 40°C to about 70°C. In some preferred embodiments, the second temperature is in the range of about 60°C to about 70°C.

[0085] The steps for initiating the reaction can be performed using standard materials and procedures known in the art, including adding an initiator, a promoter, or a combination thereof to the mixture. In some specific embodiments, the initiator is an oxidant.

[0086] The initiator can be an organic compound. In some embodiments, the initiator is selected from the group consisting of azo compounds, organic peroxide compounds, organic persulfate compounds, and combinations thereof.

[0087] The initiator can be an inorganic compound. In some embodiments, the initiator is selected from the group consisting of inorganic peroxide compounds. In some specific embodiments, the initiator is selected from the group consisting of potassium persulfate, ammonium persulfate, and combinations thereof.

[0088] Some reactions can proceed at a suitable rate in the absence of a accelerator. When a accelerator is used, it is an organic compound. In some embodiments, the accelerator is selected from the group consisting of: organic compounds, organic bases, tetramethylethylenediamine (TEMED), sodium metabisulfite (SPS), no accelerator, and combinations thereof.

[0089] An inert atmosphere improves the reaction by limiting air oxidation. An inert atmosphere can be static or dynamic. When the inert atmosphere is dynamic, an inert gas continuously flows over the reactants. Inert atmospheres include chemically inert gases. In some embodiments, the inert atmosphere is selected from the group consisting of: N2, CO2, rare gases, helium, neon, argon, krypton, xenon, and combinations thereof. In some specific embodiments, the method step of reacting the mixture includes reacting the mixture in an inert atmosphere of N2.

[0090] In some embodiments, the method further includes drying the reaction products. The reaction products may be dried in an oven or using other known drying methods. The drying temperature and time may vary depending on the relative moisture content and properties of the reaction products. In some embodiments, the method further includes drying the reaction products at a temperature ranging from about 50°C to about 100°C. In some specific embodiments, the method further includes drying the reaction products at a temperature ranging from about 70°C to about 90°C.

[0091] In some embodiments, the method further includes grinding the reaction products. The reaction products are ground to a desired particle size. Suitable methods are known in the art. In some embodiments, the method further includes grinding the reaction products using a grinding apparatus selected from the group consisting of mechanical stirrers, coffee grinders, crushers, pulverizers, grinders, mills, and combinations thereof.

[0092] In another aspect of this disclosure, the particles described herein can be further processed according to well-known procedures, such as particle surface crosslinking. Appropriately selected surface crosslinking agents can enhance the mechanical strength of the particles and help control the rate of ingestion, improve gel permeability, and enhance fluid distribution between and within a given particle.

[0093] Regarding the properties of the final superabsorbent polymer, the polymer's ability to absorb fluid under static load can be measured as absorbability under load (AUL). A typical AUL test format can be used. The ability of the pre-swelled polymer to retain water under force can be measured as centrifugal retention capacity (CRC). A typical CRC test format can be used.

[0094] Consumer products or superabsorbent materials may contain the superabsorbent polymer according to this disclosure. Superabsorbent polymers may also be used in consumer products or superabsorbent materials.

[0095] Suitable consumer products include, but are not limited to, personal hygiene products, wipes, napkins, bibs, disposable mattresses, wound dressings, food packaging, baby and adult diaper products, children's training pants, feminine pads and napkins, armbands, agricultural and pet products containing superabsorbent ingredients, disposable absorbent products, and combinations thereof.

[0096] Example

[0097] Without further detail, it is believed that those skilled in the art can make full use of the invention based on the foregoing description. Therefore, the following embodiments are to be interpreted as illustrative only and are not intended to limit this disclosure in any way.

[0098] Measurement technology.

[0099] Polymers according to this disclosure and embodiments are measured to determine their absorbability under load (AUL) and centrifugal retention capacity (CRC) properties.

[0100] Technique 1. AUL measurement of SAM particles.

[0101] AUL measures the ability of a polymer to absorb fluid under static load and can be considered a measurement of gel swelling in conjunction with gel strength. A typical AUL test format consists of a simple cylindrical apparatus with a macroporous sintered filter plate at one end of a sintered plastic cylinder. For the sample being tested, 160 mg (W1) of dried SAP sample is weighed and evenly placed on the surface of a polyester gauze placed on top of the sintered plastic. The desired load (e.g., a weight that produces 0.3–0.9 PSI pressure) is then placed on top of the dried SAP granules, with the load designed to allow it to move freely within the plastic cylinder. The apparatus is placed in a petri dish of 0.9% saline solution on top of a sieve (ensuring water is close to the bottom of the sintered plastic) for 1–72 hours or any desired measurement time. The swollen sample (W2) is weighed, and the AUL is calculated using Equation 1:

[0102]

[0103] Technique 2. CRC measurement of SAM particles.

[0104] In contrast to the AUL test, which measures the hydration of dry SAP powder under load, the CRC test measures the ability of pre-swelled SAP to retain water under force. For the test, approximately 100 to 200 mg (W1) of dried SAP sample is weighed into a pre-weighed tea bag. The tea bag is immersed in a 0.9 wt% saline solution for 1–2 hours or any desired test time to swell the SAP. The tea bag is then placed in a cylindrical centrifuge tube with a porous interior to allow drainage and centrifuged at 1600 rpm for 3 minutes. As a control, an empty tea bag is also placed in saline solution and centrifuged at the same speed. After centrifugation, the tea bag is weighed again (W2), and the difference between the dry and swollen samples is calculated. The CRC amount is calculated using Equation 2:

[0105]

[0106] SAM is copolymerized with selected stable inorganic salts.

[0107] The following examples illustrate a non-stage copolymerization method for preparing SAMs copolymerized with selected inorganic salts. These methods and the resulting polymers are known in the art, for example, in PCT / US2020 / 053003, which is incorporated herein by reference.

[0108] Selected stable inorganic salts (e.g., various inorganic salts, including metal ions with an oxidation state of at least 2) are copolymerized with sodium acrylate (e.g., a major component of the base monomers currently used in commercial SAMs). Samples synthesized under the same polymerization conditions without these salts are used as controls.

[0109] Comparative Example 1. Preparation of PAA-based SAM particles without using metal ions with an oxidation state of at least 2.

[0110] Add 15 g of acrylic acid (AA) monomer to a glass reaction flask (e.g., a 120 mL wide-mouth round jar, beaker, or round-bottom flask) equipped with a magnetic stir bar. The glass reaction flask should be fitted with a sealing cap or a glass or rubber stopper. Add approximately 45 mL of deionized (DI) water to the glass reaction flask containing the AA monomer, and then cool it to approximately 0°C using an ice-water bath. After cooling, add 5.84 g of NaOH flakes with a purity greater than 97% to the cold solution. Then stir the mixture with a magnetic stirrer until the NaOH flakes are completely dissolved. Dissolution under stirring typically takes about 10–20 minutes. Alternatively, neutralization can be achieved by adding a pre-prepared aqueous NaOH solution dropwise to the AA / water mixture, resulting in a final total water volume equal to the original volume.

[0111] The mixture is rigorously degassed with high-purity nitrogen or argon to remove residual air from the reaction mixture and displace air from the glass reaction flask. A degaussing time of at least 5–10 minutes is recommended. Approximately 5 minutes after entering the degaussing step, the initiator (approximately 45 mg of 99.99% potassium persulfate (KPS)) and crosslinking agent (approximately 68 mg of greater than 99% MBAA (N,N-methylenebis(acrylamide))) (both pre-dissolved in 4 mL of deionized water at room temperature) are added to the cooled, neutralized AA solution dissolved in deionized water. The degaussing process is then continued.

[0112] The glass reaction flask is then sealed and transferred to a preheated water bath equipped with a magnetic stirrer to begin the single-step thermal radical polymerization process. The polymerization temperature is controlled in the range of about 60°C to about 70°C, more specifically about 65°C. The flask is placed directly into the preheating bath, or into a temperature bath that starts at room temperature and gradually increases to about 65°C or any desired final polymerization temperature.

[0113] Monitor the gelation time (e.g., by determining when the magnetic stir bar stops stirring). Gelation of the control sample can only be observed when the temperature reaches 50°C-65°C for at least 5-20 minutes (e.g., the solution becomes viscous, and then the stir bar stops stirring). After gelation, retain the gel in the heating bath for another 4 hours to ensure polymerization is complete.

[0114] After polymerization, the gel is removed from the glass reaction flask. The large gel block is cut into smaller pieces with a knife or scissors for drying. A stirrer can be used to break the large gel block into smaller pieces. After cutting, the resulting gel blocks are dried in an oven at 85°C for at least 24–48 hours. Drying can also be accomplished by accelerating the drying process, using an air dryer, using the desired oven temperature, using a hot air stream, and combinations thereof. The drying process can also be carried out using a dynamic belt system combined with heating, vacuum, and air drying techniques.

[0115] The dried gel block is then placed in a coffee blender or any suitable tool to break it down to the desired particle size. Use a US standard sieve to collect particles with the desired size range (e.g., 300-600 micrometers).

[0116] Comparative Example 2. Preparation of PAA-based SAM particles using metal ions with an oxidation state of at least 2.

[0117] The reaction procedure was carried out according to Comparative Example 1, except that after the degassing step, the desired amount of the selected metal salt was added to the mixture, and then degassing continued for another 5-10 minutes.

[0118] Staged polymerization of SAM copolymerized with selected inorganic salts.

[0119] The following examples demonstrate the feasibility of staged polymerization for preparing SAM copolymerized with selected inorganic salts.

[0120] Example 1. Staged polymerization using ferric chloride (II).

[0121] Two-stage polymerization was carried out using two monomers, AA (30%) and AANA (70%), with MBAA as a crosslinking agent and KPS as a free radical polymerization initiator. The selected redox-active inorganic salt FeCl2 was used to initiate the first-stage copolymerization at an ice bath temperature of approximately 0°C to form the first-stage network. After the desired reaction time in the first stage, the temperature was increased to 65°C to complete the copolymerization via conventional free radical polymerization, thereby forming the second-stage network. Table 1 summarizes the capacity data from different FeCl2 loading levels, and it clearly shows that small amounts of FeCl2 can lead to a significant capacity increase, particularly for CRC. However, Table 1 also shows that excessive FeCl2 is undesirable as it may lead to a decrease in capacity. This is because excess redox FeCl2 can quench the polymerization by reacting directly with the initiator (e.g., consumption) to form a stable oxidized iron(III) salt. Complete quenching of the polymerization was confirmed by adding excess FeCl2, as no polymerization was observed when the loading level exceeded 30–40 mg.

[0122] Based on these data, the desired amount of FeCl2 is between 0.000066 and 0.1 wt% in the first-stage mixture, with an optimal CRC increase observed at approximately 0.083 wt% in the first-stage mixture. The preferred amount of FeCl2 is between 0.05 and 0.1 wt% in the first-stage mixture.

[0123] Table 1. Capacity data for staged polymerization using ferric chloride (II).

[0124]

[0125] Example 2. Changing the addition time of the crosslinking agent during the staged polymerization process.

[0126] Three-stage polymerization was carried out using two monomers, AA (30%) and AANa (70%), with KPS as a free radical polymerization initiator. MBAA was added at different times as a crosslinking agent to form stage 1, stage 2, and stage 3 networks. The selected redox-active inorganic salt FeCl2 was used to initiate stage 1 copolymerization at an ice bath temperature of approximately 0°C to form the stage 1 network, followed by stage 2 network formation by adding the crosslinking agent MBAA at the desired time. After the desired reaction time in stage 2, the temperature was increased to 65°C to complete the copolymerization via conventional free radical polymerization, thereby forming the stage 3 network. Table 2 clearly shows that changing the timing of crosslinking agent addition can potentially lead to higher capacity SAM materials.

[0127] Table 2. Effect of crosslinking agent addition time on SAM capacity during staged polymerization.

[0128]

[0129] Example 3. The amount and addition time of the crosslinking agent were varied during the staged polymerization process.

[0130] Three-stage polymerization was carried out using two monomers, AA (30%) and AANa (70%), with KPS as a free radical polymerization initiator. MBAA was added as a crosslinking agent at different times and in different ratios to form stage 1 and stage 2 networks. The selected redox-active inorganic salt FeCl2 was used to initiate stage 1 copolymerization at an ice bath temperature of approximately 0°C to form the stage 1 network, followed by the addition of crosslinking agent MBAA at desired times and loading levels to form the stage 2 network. After the desired reaction time in stage 2, the temperature was increased to 65°C to complete the copolymerization via conventional free radical polymerization, thereby forming the stage 3 network. Table 3 clearly shows that changing both the amount of crosslinking agent MBAA and the time of addition can potentially lead to materials with higher capacity.

[0131] Table 3. Effect of crosslinking agent addition time on SAM capacity during staged polymerization.

[0132]

[0133] Example 4. Control achieved through a phased aggregation process.

[0134] Staged polymerization can be used to control the structure of each stage and its total mass percentage in the final polymer by precisely controlling the polymerization time of each stage. FeCl2 and KPS were used to initiate the copolymerization of acrylic acid (approximately 30%) and sodium acrylate (approximately 70%) at an ice bath temperature of approximately 0°C to form the first-stage network, and the polymerization was then quenched by pouring the reaction mixture into methanol at the desired polymerization time. The precipitated polymer solids were then collected and dried to quantify the mass percentage of the first stage in the final polymer. The dried samples were found to be soluble in water, indicating that they predominantly constitute the properties of non-crosslinked long-chain polymers. The solubility properties are shown in Table 4.

[0135] Table 4. Time dependence of stage 1 polymerization without the addition of crosslinking agent MBAA during the staged polymerization process.

[0136]

[0137] Polymer property parameters, including molecular weight (weight fraction Mw, chain number fraction Mn, Z fraction Mz), polydispersity (Mw / Mn), hydrodynamic radius (Rh), intrinsic viscosity (IV), and MKα, were determined and are listed in Table 5. Table 5 clearly shows that longer reaction times result in longer polymer chains, and due to the low limitation of conventional crosslinking, higher capacity should be expected, especially for CRC. The data in Tables 4 and 5 support and corroborate each other.

[0138] Table 5. Polymer property parameters in stage 1.

[0139]

[0140] Example 5. Polymerization with three monomers with delayed addition of crosslinking agent MBAA.

[0141] Two-stage polymerization was carried out using three monomers: AA (30%), AANA (70%), and VSO3Na (sodium vinyl sulfonate), with MBAA as a crosslinking agent and KPS as a free radical polymerization initiator. The selected redox-active inorganic salt FeCl2 was used to initiate the first-stage copolymerization at an ice bath temperature of approximately 0°C without the use of crosslinking agent MBAA to form the first-stage network. After a desired reaction time of 10 minutes, the crosslinking agent was added to the polymerization mixture, and the temperature was then increased to 65°C to complete the copolymerization via conventional free radical polymerization, thereby forming the second-stage network. The results are shown in Table 6.

[0142] Table 6. Polymerization of three monomers with delayed addition of crosslinking agent MBAA (10 minutes after the start of polymerization).

[0143]

[0144] Comparative Example 3. Staged polymerization using non-redox SIS.

[0145] Non-redox SIS cannot induce a staged polymerization process. Non-redox active salts CaCl2, Al2(SO4)3, and FeCl3 were added to the polymerization mixture at an ice bath temperature of approximately 0°C, following the same procedure as for redox active salt FeCl2. Unlike FeCl2, no polymerization was observed regardless of how long the reaction mixture was held and stirred at this temperature. Polymerization only occurred when the temperature was raised to the temperature of conventional thermal radical polymerization, such as above 50°C–65°C, resulting in a final polymer with a homogeneous structure.

[0146] Comparative Example 4. Staged polymerization without using a selected inorganic salt (SIS).

[0147] A staged polymerization was performed using a control sample without added SIS. The polymerization mixture was prepared at an ice bath temperature of approximately 0°C using two monomers, AA (30%) and AANa (70%), with MBAA as a crosslinking agent and KPS as a free radical polymerization initiator. No polymerization was observed regardless of how long the reaction mixture was held and stirred at this temperature. Polymerization only occurred when the temperature was raised to the normal thermal free radical polymerization temperature, for example, above 50°C–65°C, resulting in a final polymer with a homogeneous structure.

[0148] Example 6. Synergistic effect of redox active SIS (FeCl2) and stable SIS (FeCl3).

[0149] The potential synergistic benefits of applying both redox-active and stable SIS in staged polymerization were demonstrated. First, stable SIS FeCl3 (with varying loading levels) was added to the prepolymer mixture, followed by redox-active SIS FeCl2 (with a fixed loading level) to initiate the polymerization process at an ice-bath temperature. After the first stage was formed at low temperature, the second and third stages could be achieved by gradually increasing the polymerization temperature to the conventional thermal radical polymerization temperature, for example, above 50°C–65°C. Table 7 clearly shows that the synergistic benefits can be achieved by mixing stable and redox-active SIS in the staged polymerization process.

[0150] Table 7. Synergistic effect of redox active SIS (FeCl2) and stable SIS (FeCl3).

[0151]

[0152]

[0153] Example 7. Comparison of mechanical properties.

[0154] Compared to uniformly crosslinked SAM, staged SAM exhibits unique swelling kinetics. The different crosslink densities in stages 1 and 2 demonstrate a shift in mechanical strength during swelling in brine solutions. As the liquid reaches lower crosslink density domains, particularly within the SAM particles, a decrease in mechanical strength is expected during swelling. This will be more pronounced through design for stages with less or no crosslinking agent.

[0155] Gel strength measuring instruments, such as ElastoSens TM Bio-instruments can be used to measure the mechanical properties of SAM particles during dynamic expansion.

[0156] The following procedure was used for sample measurement. 0.1g of SAM particles were placed in the ElastoSense container. TM The sample cup was then placed in the ElastoSense container. TM In the bio-sample measurement chamber, 5 mL of 0.9% NaCl solution was added to the sample cup, and the free swelling kinetics were recorded at 30-second intervals over 30 minutes to 1 hour.

[0157] Storage modulus data of polymers produced by staged polymerization and polymers not produced by staged polymerization. Figure 4 As shown in the figure, polymers produced by staged polymerization exhibit an initial decrease in storage modulus caused by the low crosslinking density region where the liquid reaches the particle surface, and a significant decrease in storage modulus between 300 and 500 seconds caused by the low crosslinking density region where the liquid reaches the interior of the particle. In contrast, polymers not produced by staged polymerization exhibit an initial increase in storage modulus, and no significant decrease in storage modulus between 300 and 500 seconds.

[0158] Specifically, the superabsorbent polymer produced by staged polymerization provides a 5.2% reduction in storage modulus (G') from approximately 883 Pa to approximately 837 Pa over a time period of approximately 360 to approximately 480 seconds. The superabsorbent polymer not produced by staged polymerization provides only a 1.5% reduction in storage modulus (G') from approximately 882 Pa to approximately 869 Pa over a time period of approximately 330 to approximately 390 seconds.

[0159] Additional energy storage modulus data of polymers produced through staged polymerization. Figure 5 As shown in the figure, the polymer produced by staged polymerization exhibits an initial decrease in storage modulus caused by the low crosslinking density region where the liquid reaches the particle surface, and a significant decrease in storage modulus between 300 and 500 seconds caused by the low crosslinking density region where the liquid reaches the interior of the particle.

[0160] Specifically, the superabsorbent polymer produced by staged polymerization provides a 10.8% reduction in storage modulus (G') from approximately 983 Pa to approximately 876.5 Pa over a period of approximately 330 to approximately 450 seconds.

[0161] This written description illustrates the present disclosure, including the best mode, by example, and also enables any person skilled in the art to practice the present disclosure, including the preparation and use of any composition or system and the performance of any included methods. The patentable scope of this disclosure is defined by the claims, and may include other instances that would occur to a person skilled in the art. Such other instances are intended to be within the scope of the claims if they have elements that are not different from the literal language of the claims, or if they have equivalent elements that are not substantially different from the literal language of the claims.

[0162] As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains,” “containing,” “characterized by,” or any other variations thereof are intended to cover non-exclusive inclusion, which is subject to any express indication of limitation. For example, a composition, mixture, process, or method that comprises a list of elements is not necessarily limited to those elements, but may include other elements not expressly listed or inherent to such compositions, mixtures, processes, or methods.

[0163] The transitional phrase "consisting of..." excludes any unspecified elements, steps, or ingredients. If in the claims, this would bring the claims close to including materials other than those listed, in addition to impurities typically associated with them. When the phrase "consisting of..." appears in a clause of the body of a claim, rather than immediately following the preamble, it only limits the elements set forth in that clause; other elements are not excluded from the claim as a whole.

[0164] The transitional phrase "consistently of..." is used to define a composition or method that includes materials, steps, features, components, or elements in addition to those explicitly disclosed, provided that these additional materials, steps, features, components, or elements do not substantially affect the essential and novel features of the claimed invention. The term "consistently of..." occupies an intermediate position between "comprising" and "consisting of...".

[0165] When the invention or a part thereof is defined using open-ended terms such as “comprising”, it should be readily understood (unless otherwise stated) that the specification should be interpreted as also using the terms “consistently made of” or “comprises of” to describe this invention.

[0166] Furthermore, unless explicitly stated otherwise, "or" refers to inclusive or rather than exclusive or. For example, conditions A or B are satisfied by any of the following: A is true (or exists) and B is false (or does not exist), A is false (or does not exist) and B is true (or exists), and both A and B are true (or exist).

[0167] Furthermore, the indefinite articles “a” and “an” preceding the elements or components of the invention are intended to be non-limiting in terms of the number of instances (i.e., occurrences) of the element or component. Thus, “a” or “an” should be understood to include one or at least one, and the singular form of the element or component also includes the plural, unless the number clearly indicates the singular.

[0168] As used in this article, the term “about” means plus or minus 10% of the value.

Claims

1. A method for preparing a superabsorbent polymer, the method comprising: The initial aggregation phase includes... Forming a mixture, the mixture comprising Solvent; monomer; Salts containing redox active metal ions with an oxidation state of at least 2; Optional crosslinking agent; as well as Initiator; and The mixture is reacted at a first temperature; and The final aggregation phase includes... Optionally, a crosslinking agent may be added to the mixture; The mixture is heated from the first temperature to the second temperature; as well as The mixture is reacted at the second temperature; The first temperature is in the range of -20°C to 40°C, and the second temperature is in the range of 40°C to 70°C; The superabsorbent polymer comprises: The polymer backbone comprises: Neutralized main chain monomers; Neutralized main-chain monomers that are complexed with coordination complexes containing metal ions with an oxidation state of at least 2; Optional unneutralized main-chain monomers; and Optional unneutralized main-chain monomers complexed with coordination complexes containing metal ions with an oxidation state of at least 2; and Optional crosslinking agent; The superabsorbent polymer described herein contains a non-uniform microstructure.

2. The method of claim 1, wherein the superabsorbent polymer is configured to provide a reduction of at least 5.2% in storage modulus (G') between 300 and 500 seconds after exposure to a 0.9% NaCl solution at room temperature.

3. The method of claim 1, wherein the superabsorbent polymer comprises a core-shell polymer microstructure.

4. The method of claim 1, wherein the superabsorbent polymer comprises a first polymer portion having a first crosslinking density and a second polymer portion having a second crosslinking density, wherein the first crosslinking density and the second crosslinking density are different.

5. The method of claim 1, wherein the superabsorbent polymer comprises a first polymer portion present in an amount ranging from 1 to 30% by weight of the superabsorbent polymer and a second polymer portion present in an amount ranging from 60 to 99% by weight of the superabsorbent polymer.

6. The method of claim 1, wherein the superabsorbent polymer is in the form selected from the group consisting of: gel, liquid, solid, paste or combination thereof.

7. The method of claim 6, wherein the solid is particles and / or fibers.

8. The method of claim 6, wherein the solid is a bead.

9. The method of claim 1, wherein the superabsorbent polymer is in the form of particles with a diameter in the range of 100 µm to 1000 µm.

10. The method of claim 1, further comprising at least one intermediate aggregation stage between the initial aggregation stage and the final aggregation stage.

11. The method of claim 10, wherein the at least one intermediate polymerization stage includes a time-dependent transition or a temperature-dependent transition.

12. The method of claim 11, wherein the at least one intermediate polymerization stage comprises adding a crosslinking agent to the mixture.

13. The method of claim 1, wherein the mixture further comprises a stable salt, the stable salt comprising a metal ion having an oxidation state of at least 2.

14. The method of claim 1, wherein the method comprises 2 to 5 polymerization stages.

15. The method of claim 1, wherein the method further comprises drying the reaction product.

16. The method of claim 1, wherein the method further comprises grinding the reaction product.

17. A method of using a superabsorbent polymer prepared by any one of claims 1 to 16, comprising using the superabsorbent polymer in a consumer product.

18. The method of claim 17, wherein the superabsorbent polymer is in the form of particles with a diameter in the range of 100 µm to 1000 µm.

19. The method of claim 17, wherein the consumer product is selected from the group consisting of: personal hygiene products, food packaging, armbands, agricultural and pet products containing superabsorbent ingredients, and combinations thereof.

20. The method of claim 17, wherein the consumer product is a wiping agent.

21. The method of claim 17, wherein the consumer product is a disposable absorbent product.

22. The method of claim 17, wherein the consumer product is a bib, a disposable mattress pad, a wound dressing, a napkin, baby and adult diaper products, training pants for children, feminine pads, and combinations thereof.

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