Nanoporous superabsorbent particles with low non-solvent content

Abstract

Superabsorbent particles having less than 1000 ppm non-solvent, a median size of from about 50 microns to about 2,000 microns and comprising nanopores having an average cross-sectional dimension of from about 10 nanometers to about 500 nanometers are provided. The superabsorbent particles exhibit a vortex time of about 80 seconds or less.

Description

Background Technology

[0001] Superabsorbent materials are used in a wide variety of applications to help absorb fluids. These materials are typically capable of absorbing several times their own weight in fluid (e.g., water, salt water, etc.). However, one problem associated with many conventional superabsorbent materials is that the absorption rate can be relatively slow when they first come into contact with the fluid.

[0002] To improve absorption rates, phase inversion has been proposed for superabsorbent materials. Phase inversion involves swelling the superabsorbent material in a solvent, washing the swollen superabsorbent material in a non-solvent, and removing the non-solvent by drying. Although phase inversion has produced superabsorbent materials with very fast absorption rates, significant amounts of non-solvent (e.g., up to 13%) have been found to remain in the superabsorbent material. This ultimately reduces the material's absorbability under load (AUL) and results in an unpleasant alcohol odor being released from the material during use. Furthermore, some jurisdictions have imposed restrictions on the amount of non-solvent that may be present in certain types of articles.

[0003] Therefore, it would be beneficial to provide superabsorbent materials with reduced non-solvent content. A further benefit would be to provide a superabsorbent material that, in addition to improved absorbency under load, also has a reduced non-solvent content. Summary of the Invention

[0004] According to one aspect of this disclosure, superabsorbent particles are disclosed, comprising less than about 1000 ppm of nonsolvent, having a median size of about 50 micrometers to about 2000 micrometers, and comprising nanopores with an average cross-sectional size of about 10 nanometers to about 500 nanometers. The superabsorbent particles exhibit a vortex time of about 30 seconds or less.

[0005] Other features and aspects of the invention are discussed in more detail below. Attached Figure Description

[0006] The complete and feasible disclosure of the invention (including its best mode) for those skilled in the art is set forth in more detail in the remainder of the specification with reference to the accompanying drawings, in which:

[0007] Figure 1 An apparatus for measuring the under-load absorbance (“AUL”) of the porous superabsorbent particles of the present invention is described; and

[0008] Figure 2 It shows Figure 1 AUL components.

[0009] The repeated use of reference numerals in this specification and drawings is intended to represent the same or similar features or elements of the invention. Detailed Implementation

[0010] Reference will now be made in detail to various embodiments of the invention, one or more of which are illustrated below. Each example is provided by way of explanation rather than limitation. Indeed, it will be apparent to those skilled in the art that various modifications and variations may be made to the invention without departing from its scope or spirit. For example, a feature illustrated or described as part of one embodiment may be used in another embodiment to produce another embodiment. Therefore, the invention is intended to cover such modifications and variations that fall within the scope of the appended claims and their equivalents.

[0011] Generally, the present invention has found that by carefully controlling the formation of superabsorbent particles, a significantly reduced content of nonsolvents can be retained in the superabsorbent particles after phase inversion treatment. In particular, as will be discussed in more detail below, this disclosure has found that by initially drying the superabsorbent particles and then rehydrating them before further drying or final drying, the superabsorbent particles result in a very low nonsolvent content while maintaining high porosity and rapid absorption performance. For example, in one aspect, the superabsorbent particles comprise a nonsolvent amount of about 1000 ppm or less, such as about 900 ppm or less, such as about 800 ppm or less, such as about 700 ppm or less, such as about 600 ppm or less, such as about 500 ppm or less, such as about 400 ppm or less, such as about 300 ppm or less, such as about 200 ppm or less, such as about 100 ppm or less, such as about 50 ppm or less, such as about 30 ppm or less.

[0012] Furthermore, in one aspect, the superabsorbent particles may have a median size (e.g., diameter) of about 50 micrometers to about 2,000 micrometers, in some embodiments about 100 micrometers to about 1,000 micrometers, and in some embodiments about 200 micrometers to about 700 micrometers. As used herein, the term "median" size refers to the "D50" size distribution of the particles, meaning that at least 50% of the particles have the indicated size. The particles may also have a D90 size distribution within the aforementioned range (at least 90% of the particles have the indicated size). The diameter of the particles can be determined using known techniques such as ultracentrifugation, laser diffraction, etc. For example, the particle size distribution can be determined according to standard test methods such as ISO 13320:2009. The particles may also have any desired shape, such as flakes, nodules, spheres, tubular shapes, etc. The size of the particles can be controlled to optimize performance for a specific application. The specific surface area of ​​the particles may also be relatively large, such as about 0.2 m² / g (m³). 2 / g) or larger, approximately 0.6m in some embodiments 2 / g or larger, and in some implementations about 1m 2 / g to approximately 5m 2 / g, such as that determined according to the BET test method as described in ISO 9277:2010.

[0013] Regardless of their specific size or shape, superabsorbent particles are inherently porous and typically possess a porous network that may contain a combination of closed pores and open chambers. The total porosity of the particles can be relatively high. For example, the particles may exhibit approximately 2 m² / g (m³ / g). 2 / g) or larger, in some embodiments about 5 to about 150m 2 / g, and in some embodiments about 15 to about 40m 2 The total pore area is approximately 5% per gram. The porosity percentage can also be about 5% or greater, about 10% to about 60% in some embodiments, and about 15% to about 60% in some embodiments. Another parameter characteristic of porosity is packing density. In this regard, the packing density of the superabsorbent particles of the present invention can, for example, be less than about 0.7 g / cm³. 3 In some implementations, the concentration is approximately 0.1 to approximately 0.65 g / cm³. 3 And in some embodiments, it is about 0.2 to about 0.6 g / cm³. 3 For example, it can be determined by mercury porosimetry at a pressure of 0.58 psi.

[0014] To achieve the desired pore properties, the porous network typically comprises multiple nanopores having an average cross-sectional size (e.g., width or diameter) of about 10 to about 500 nanometers, in some embodiments about 15 to about 450 nanometers, and in some embodiments about 20 to about 400 nanometers. The term "cross-sectional size" generally refers to a reference size (e.g., width or diameter) of the pore, which is substantially orthogonal to its long axis (e.g., length). It should be understood that various types of pores can be present within the network. For example, micropores with an average cross-sectional size of about 0.5 to about 30 micrometers, in some embodiments about 1 to about 20 micrometers, and in some embodiments about 2 micrometers to about 15 micrometers can also be formed. However, nanopores can be present in relatively high quantities in the network. For example, nanopores can account for at least about 25% of the total pore volume of the particles, in some embodiments at least about 40% of the total pore volume, and in some embodiments from about 40% to 80% of the total pore volume. The average volume percentage occupied by nanopores within a given unit volume of the material can also be per cm³. 3The particles comprise approximately 15% to 80%, in some embodiments approximately 20% to 70%, and in some embodiments approximately 30% to 60% per cubic centimeter. Various subtypes of nanopores can also be employed. For example, in some embodiments, a first nanopore can be formed with an average cross-sectional size of approximately 80 to 500 nanometers, in some embodiments approximately 90 to 450 nanometers, and in some embodiments approximately 100 to 400 nanometers, while a second nanopore can be formed with an average cross-sectional size of approximately 1 to 80 nanometers, in some embodiments approximately 5 to 70 nanometers, and in some embodiments approximately 10 to 60 nanometers. The nanopores can have any regular or irregular shape, such as spherical, elongated, etc. Regardless of size, the average diameter of the pores within the porous network will typically be approximately 1 nanometer to 1,200 nanometers, in some embodiments approximately 10 nanometers to 1,000 nanometers, in some embodiments approximately 50 nanometers to 800 nanometers, and in some embodiments approximately 100 nanometers to 600 nanometers.

[0015] Partly due to the unique properties of the porous network and the formation method of the superabsorbent particles, the inventors have discovered that the resulting superabsorbent particles can exhibit an increased absorption rate during the specific time period during which they begin to contact the fluid (such as water, aqueous solutions of salts (e.g., sodium chloride), bodily fluids (e.g., urine, blood, etc.), and the like). This increased rate can be characterized in a variety of ways. For example, the particles can exhibit a short vortex time, where vortex time refers to the amount of time (in seconds) required for a given amount of superabsorbent particles to close a vortex formed by stirring a given amount of 0.9% by weight sodium chloride solution, according to the test described below. More specifically, the superabsorbent particles can exhibit vortex times of about 80 seconds or less, about 60 seconds or less in some embodiments, about 40 seconds or less in some embodiments, about 35 seconds or less in some embodiments, about 30 seconds or less in some embodiments, about 20 seconds or less in some embodiments, and about 0.1 seconds to about 15 seconds in some embodiments. Alternatively, after being placed in contact with an aqueous solution of sodium chloride (0.9 wt%) for 0.015 kiloseconds (“ks”), the absorption rate of the particles can be about 300 g / g / ks or higher, in some embodiments about 400 g / g / ks or higher, in some embodiments about 500 g / g / ks or higher, and in some embodiments about 600 to about 1,500 g / g / ks. High absorption rates can even be maintained for relatively long periods. For example, after being placed in contact with an aqueous solution of sodium chloride (0.9 wt%) for 0.06 ks or even up to 0.12 ks, the absorption rate of the particles can still be about 160 g / g / ks or higher, in some embodiments about 180 g / g / ks or higher, in some embodiments about 200 g / g / ks or higher, and in some embodiments about 250 to about 1,200 g / g / ks.

[0016] It is noteworthy that the increased absorption rate can be maintained without sacrificing the total absorbable capacity of the particles. For example, after 3.6 ks, the total absorbable capacity of the particles can be 10 g / g or higher, in some embodiments about 15 g / g or higher, and in some embodiments about 20 to about 100 g / g. Similarly, the particles can exhibit a centrifugation retention capacity (“CRC”) of about 20 g liquid / g superabsorbent particles (g / g) or higher, in some embodiments about 25 g / g or higher, and in some embodiments about 30 to about 60 g / g. Finally, the superabsorbent particles can also exhibit a free swelling gel bed permeability (“GBP”) of about 40 Darcy or less, in some embodiments about 25 Darcy or less, and in some embodiments about 0.1 to about 10 Darcy.

[0017] Superabsorbent particles are typically formed from a three-dimensional cross-linked polymer network comprising repeating units derived from one or more olefinically (e.g., monoolefinically) unsaturated monomeric compounds having at least one hydrophilic group, such as carboxyl, carboxylic anhydride, carboxylate, sulfonic acid, sulfonate, hydroxyl, ether, amide, amino, or quaternary ammonium salt groups. Specific examples of suitable olefinically unsaturated monomeric compounds for forming superabsorbent particles include, for example: carboxylic acids (e.g., (meth)acrylic acid (including acrylic acid and / or methacrylic acid), maleic acid, fumaric acid, crotonic acid, sorbic acid, itaconic acid, cinnamic acid, etc.); carboxylic anhydrides (e.g., maleic anhydride); salts of carboxylic acids (alkali metal salts, ammonium salts, amine salts, etc.) (e.g., sodium (meth)acrylate, trimethylamine (meth)acrylate, triethanolamine (meth)acrylate, sodium maleate, methylamine maleate, etc.); vinyl sulfonic acids (e.g., vinyl sulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, etc.); (methyl... Acrylic sulfonic acids (e.g., methyl methacrylate, 2-hydroxy-3-(meth)acryloyloxypropyl sulfonic acid, etc.); vinyl sulfonic acids or salts of (methyl)acrylic sulfonic acids; alcohols (e.g., (meth)allyl alcohol); ethers or esters of polyols (e.g., hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, triethylene (meth)acrylate, poly(ethylene oxide-oxypropylene) glycol mono(meth)allyl ether (wherein the hydroxyl group may be etherified or esterified), etc.); vinylformamide; (meth)acrylamide, N-alkyl (meth)acrylamide (e.g., N-methylacrylamide, N-hexylpropylene). Acrylamides, etc.); N,N-dialkyl(methyl)acrylamides (e.g., N,N-dimethylacrylamide, N,N-di-n-propylacrylamide, etc.); N-hydroxyalkyl(methyl)acrylamides (e.g., N-hydroxymethyl(methyl)acrylamide, N-hydroxyethyl-(methyl)acrylamide, etc.); N,N-dihydroxyalkyl(methyl)acrylamides (e.g., N,N-dihydroxyethyl(methyl)acrylamide); vinyl lactams (e.g., N-vinylpyrrolidone); amino-containing esters of carboxylic acids (e.g., dialkylaminoalkyl esters, dihydroxyalkylaminoalkyl esters, morpholinoalkyl esters, etc.) (e.g., (methyl)acrylamide, etc.) Dimethylaminoethyl fumarate, diethylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, dimethylaminoethyl fumarate, etc.; heterocyclic vinyl compounds (e.g., 2-vinylpyridine, 4-vinylpyridine, N-vinylpyridine, N-vinylimidazole, etc.); monomers containing quaternary ammonium salt groups (e.g., N,N,N-trimethyl-N-(meth)acryloyloxyethyl ammonium chloride, N,N,N-triethyl-N-(meth)acryloyloxyethyl ammonium chloride, 2-hydroxy-3-(meth)acryloyloxypropyltrimethylammonium chloride, etc.); and so on, as well as any combination of the foregoing compounds. In most embodiments, (meth)acrylate monomer compounds and their salts are used to form superabsorbent particles.

[0018] The monomeric compounds mentioned above are generally soluble in water. However, it should be understood that compounds that can be made water-soluble through hydrolysis can also be used. Suitable hydrolyzable monomers can include, for example, olefinically unsaturated compounds having at least one hydrolyzable group (such as an ester, amide, or nitrile group). Specific examples of such hydrolyzable monomers include methyl (meth)acrylate, ethyl (meth)acrylate, 2-ethylhexyl (meth)acrylate, vinyl acetate, allyl (meth)acetate, (meth)acrylonitrile, etc. Furthermore, it should be understood that additional monomers can be used to form the resulting particles as copolymers (such as random, grafted, or block copolymers). If desired, the one or more comonomers can be selected from the group of monomers listed above. For example, the one or more comonomers can be (meth)acrylate, salts of (meth)acrylate, maleic anhydride, etc. For example, in one specific embodiment, the copolymer can be formed from acrylic acid (or its salts) and maleic anhydride. In other embodiments, as described in more detail below, comonomers containing crosslinkable functionality, such as alkoxysilanes, can also be used. Regardless of the type or types of comonomers used, it is generally desirable that the one or more major olefinic unsaturated monomers constitute at least about 50 mol% of the monomers used to form the polymer, about 55 mol% to about 99 mol% in some embodiments, and about 60 mol% to about 98 mol% in some embodiments, while the one or more comonomers constitute no more than about 60 mol% of the monomers used to form the polymer, about 1 mol% to about 50 mol% in some embodiments, and about 2 mol% to about 40 mol% in some embodiments.

[0019] To form a water-absorbing network, it is generally desirable for the polymer to be crosslinked during and / or after polymerization. For example, in one embodiment, the one or more olefinically unsaturated monomer compounds can be polymerized in the presence of a crosslinking agent to provide a crosslinked polymer. Suitable crosslinking agents typically have two or more groups that are capable of reacting with olefinically unsaturated monomer compounds and are at least partially water-soluble or water-dispersible, or at least partially soluble in or dispersible in an aqueous monomer mixture. Examples of suitable crosslinking agents may include, for example, tetraallyloxyethane, N,N'-methylenebisacrylamide, N,N'-methylenebismethylacrylamide, triallylamine, trimethylolpropane triacrylate, glycerol propoxy triacrylate, divinylbenzene, N-hydroxymethylacrylamide, N-hydroxymethylacrylamide, glycidyl methacrylate, polyvinylpolyamine, ethylenediamine, ethylene glycol, glycerol, triallyl ethers of tetraallyloxyethane and pentaerythritol, aluminates, silica, aluminosilicates, etc., and combinations thereof. The amount of crosslinking agent can vary, but is typically present in an amount of about 0.005 to about 1.0 mol% based on the molar number of the one or more olefinic unsaturated monomer compounds.

[0020] In the above embodiments, crosslinking typically occurs during polymerization. However, in other embodiments, the polymer may contain latent functionality that can become crosslinked when needed. For example, the polymer may contain alkoxysilane functionality that forms silanol functional groups upon exposure to water, and these silanol functional groups condense to form a crosslinked polymer. A specific example of such functionality is a trialkoxysilane having the following general formula:

[0021]

[0022] R1, R2, and R3 are alkyl groups that independently have 1 to 6 carbon atoms.

[0023] To incorporate this functionality into the polymer structure, monomeric compounds containing the aforementioned functionality can be used, such as olefinically unsaturated monomers containing a trialkoxysilane functional group. Particularly suitable monomers are (meth)acrylic acid or its salts, such as methacryloxypropyltrimethoxysilane, methacryloxyethyltrimethoxysilane, methacryloxypropyltriethoxysilane, methacryloxypropyltripropoxysilane, acryloyloxypropylmethyldimethoxysilane, 3-acryloyloxypropyltrimethoxysilane, 3-methacryloyloxypropylmethyldiethoxysilane, 3-methacryloyloxypropylmethyldimethoxysilane, 3-methacryloyloxypropyltri(methoxyethoxy)silane, etc. In addition to copolymerizable monomers containing a trialkoxysilane functional group, monomers that are copolymerizable and subsequently react with compounds containing a trialkoxysilane functional group or with water to form a silanol group can also be used. Such monomers may contain, but are not limited to, amines or alcohols. The amine groups incorporated into the copolymer can then be reacted with, for example, but not limited to (3-chloropropyl)trimethoxysilane. The alcohol groups incorporated into the copolymer can then be reacted with, for example, but not limited to tetramethoxysilane.

[0024] The superabsorbent polymer particles of the present invention can be prepared by any known polymerization method. For example, the particles can be prepared by any suitable bulk polymerization technique, such as solution polymerization, reverse suspension polymerization, or emulsion polymerization, as described in U.S. Patent Nos. 4,076,663, 4,286,082, 4,340,706, 4,497,930, 4,507,438, 4,654,039, 4,666,975, 4,683,274, or 5,145,906. For example, in solution polymerization, the one or more monomers are polymerized in an aqueous solution. In reverse suspension polymerization, the one or more monomers are dispersed in an alicyclic or aliphatic hydrocarbon suspension in the presence of a dispersant (such as a surfactant or protective colloid). If desired, the polymerization reaction can be carried out in the presence of a free radical initiator, a redox initiator (reducing agent and oxidizing agent), a thermal initiator, a photoinitiator, etc. Examples of suitable reducing agents may include, for example, ascorbic acid, alkali metal sulfites, alkali metal bisulfites, ammonium sulfite, ammonium bisulfite, alkali metal bisulfite, ammonium bisulfite, iron-containing metal salts (e.g., ferrous sulfate), sugars, aldehydes, primary and secondary alcohols, etc. Examples of suitable oxidizing agents may include, for example, hydrogen peroxide, octanoyl peroxide, benzoyl peroxide, cumene peroxide, tert-butyl phthalate, tert-butyl perbenzoate, sodium percarbonate, sodium peracetate, alkali metal persulfates, ammonium persulfate, alkyl hydroperoxides, peresters, diacryloyl peroxide, silver salts, etc.

[0025] If desired, the resulting particles can also be reduced in size to achieve the desired dimensions described above. For example, impact reduction sizing, typically employing a grinder with rotating grinding elements, can be used to form the particles. Repeated impact and / or shear stresses can be generated between the rotating grinding elements and stationary or counter-rotating grinding elements. Impact reduction sizing can also utilize airflow to carry the material and impact it against the grinding disc (or other shear element). A particularly suitable impact reduction device is commercially available from Pallmann Industries (Clifton, NJ) under the name [missing information - likely a brand name]. The model is PLM. In this device, a highly active air vortex is generated within a cylindrical grinding chamber between the stationary and rotating grinding elements of the impact mill. Due to the large air volume, the particles can be impacted and reduced in size to the desired particle size. Other suitable impact size reduction methods can be granted. PallmannU.S. Patent Nos. 6,431,477 and 7,510,133 describe this. Another suitable microparticle formation method is cold extrusion to reduce size, which typically employs shear and compressive forces to form particles of the desired size. For example, material can be forced through a die at a temperature below the melting point of the matrix polymer. Solid-state shear pulverization is another suitable method that can be used. Such methods typically involve continuously extruding material under high shear and compressive conditions while cooling the extruder barrel and screw to prevent the polymer from melting. Examples of such solid-state pulverization techniques are, for example, those granted by U.S. Patent Nos. 6,431,477 and 7,510,133. Khait US Patent No. 5,814,673, granted Furgiuele US Patent No. 6,479,003, etc., granted Khait US Patent No. 6,494,390, granted to [names of individuals]. Khait U.S. Patent No. 6,818,173, and the grant of Torkelson As described in U.S. Publication No. 2006 / 0178465, et al., another suitable particle-forming technique is called cryogenic disc milling. Cryogenic disc milling typically uses a liquid (e.g., liquid nitrogen) to cool or freeze the material before and / or during milling. In one embodiment, a single-channel disc milling apparatus with a fixed disc and a rotating disc can be used. The material enters between the discs via a channel near the center of the discs and forms particles through the frictional force generated between the discs. A suitable cryogenic disc milling apparatus may be named... The cryogenic grinding system was obtained from ICO Polymers (Allentown, PA).

[0026] Although not strictly necessary, additional components may be incorporated with the superabsorbent polymer before, during, or after polymerization. For example, in one embodiment, inclusions with a high aspect ratio (e.g., fibers, tubes, sheets, wires, etc.) may be used to help create an internally interlocking reinforcing framework that stabilizes the swollen superabsorbent polymer and enhances its elasticity. The aspect ratio (average length divided by median width) can range, for example, from about 1 to about 50, from about 2 to about 20 in some embodiments, and from about 4 to about 15 in some embodiments. Such inclusions may have a median width (e.g., diameter) of about 1 to about 35 micrometers, from about 2 to about 20 micrometers in some embodiments, from about 3 to about 15 micrometers in some embodiments, and from about 7 to about 12 micrometers in some embodiments, and a volume-average length of about 1 to about 200 micrometers, from about 2 to about 150 micrometers in some embodiments, from about 5 to about 100 micrometers in some embodiments, and from about 10 to about 50 micrometers in some embodiments. Examples of such high aspect ratio inclusions can include high aspect ratio fibers (also known as “whiskers”) derived from carbides (e.g., silicon carbide), silicates (e.g., wollastonite), etc.

[0027] If desired, hydrophobic substances can also be combined with superabsorbent polymers, such as substances containing hydrocarbon groups, substances containing hydrocarbon groups with fluorine atoms, substances with polysiloxane structures, and so on. Examples of such substances and superabsorbent particles formed therefrom are, for example, granted Fujimura As described in U.S. Patent No. 8,742,023, the entire contents of which are incorporated herein by reference. Suitable hydrophobic materials may include, for example, polyolefin resins, polystyrene resins, waxes, long-chain fatty acid esters, long-chain fatty acids and their salts, long-chain aliphatic alcohols, long-chain aliphatic amides, and mixtures thereof. In one specific implementation, long-chain fatty acid esters may be used, which are esters of fatty acids having 8 to 30 carbon atoms and alcohols having 1 to 12 carbon atoms, such as methyl laurate, ethyl laurate, methyl stearate, ethyl stearate, methyl oleate, ethyl oleate, glyceryl monolaurate, glyceryl monostearate, glyceryl monooleate, pentaerythritol monolaurate, pentaerythritol monostearate, pentaerythritol monooleate, sorbitol monolaurate, sorbitol monostearate, sorbitol monooleate, sucrose monopalmitate, sucrose dipalmitate, sucrose tripalmitate, sucrose monostearate, sucrose distearate, sucrose tristearate, tallow, etc. In another embodiment, long-chain fatty acids or their salts containing 8 to 30 carbon atoms, such as lauric acid, palmitic acid, stearic acid, oleic acid, dimer acid, benzyl acid, etc., and their zinc, calcium, magnesium and / or aluminum salts, such as calcium palmitate, aluminum palmitate, calcium stearate, magnesium stearate, aluminum stearate, etc., can be used.

[0028] Regardless of the specific manner in which the particles are formed, a variety of different techniques can be employed in which a porous network is formed within the pre-formed particles. For example, in one specific embodiment, a technique called "phase inversion" can be used, in which a polymer dissolved or swollen in a continuous phase solvent system is transformed into a continuous phase solid macromolecular network formed by the polymer. This transformation can be induced by several methods, such as by removing the solvent via dry methods (e.g., evaporation or sublimation), adding a non-solvent, or adding it to a non-solvent via wet methods. For example, in dry methods, the temperature (or pressure) of the particles can be changed so that the solvent system (e.g., water) can be transformed into another state of matter, which can be removed by venting or purging with gas without excessive shrinkage. For example, freeze-drying involves cooling the solvent system below its freezing point and then allowing it to sublimate under reduced pressure to form pores. On the other hand, supercritical drying involves heating the solvent system under pressure above its supercritical point to form pores.

[0029] However, wet processes are particularly suitable because they do not rely on a significant amount of energy to achieve the desired conversion. In wet processes, the superabsorbent polymer and solvent system can be provided as a single-phase homogeneous composition. The polymer concentration is typically in the range of about 0.1% to about 20% by weight / volume of the composition, and in some embodiments about 0.5% to about 10% by weight / volume of the composition. The composition is then contacted with the non-solvent system using any known technique, such as immersion in a bath, countercurrent washing, spray washing, belt spraying, and filtration. The difference in chemical potential between the solvent and non-solvent systems causes solvent molecules to diffuse out of the superabsorbent polymer, while non-solvent molecules diffuse into the polymer. Ultimately, this causes the polymer composition to undergo a transformation from a single-phase homogeneous composition to an unstable two-phase mixture containing polymer-rich and polymer-poor fractions. Micellar droplets of the non-solvent system in the polymer-rich phase also act as nucleation sites and become coated with the polymer, and at a certain point, these droplets precipitate to form a continuous polymer network. The solvent composition within the polymer matrix also collapses and forms voids. The matrix can then be dried to remove the solvent and non-solvent systems and form stable porous particles.

[0030] The exact solvent and non-solvent systems used to complete the phase transition are not particularly critical, as long as they are chosen synergistically based on their miscibility. More specifically, the solvent and non-solvent systems can be chosen such that they have a specific difference in their Hildebrand solubility parameter δ, where δ is a predictive indicator of the miscibility of the two liquids; a higher value generally indicates a more hydrophilic liquid, and a lower value indicates a more hydrophobic liquid. Generally, a difference in the Hildebrand solubility parameter (e.g., δ) between the solvent and non-solvent systems is desirable. 溶剂 –δ 非溶剂 (Approximately 1 to 15 calories) 1 / 2 / cm 3 / 2 In some implementations, approximately 4 to 12 calories 1 / 2 / cm 3 / 2 And in some implementations, approximately 6 to 10 calories. 1 / 2 / cm 3 / 2 Within these ranges, the solvent / non-solvent will have sufficient miscibility to allow solvent extraction to occur, but not so great that phase inversion cannot be achieved. Suitable solvents used in solvent systems can include, for example, water, aqueous alcohols, brine, glycerol, and combinations thereof. Similarly, suitable non-solvents used in non-solvent systems can include acetone, n-propanol, ethanol, methanol, n-butanol, propylene glycol, ethylene glycol, and combinations thereof. In one aspect, non-solvents include methanol, ethanol, isopropanol, or combinations thereof.

[0031] Typically, the volume ratio of the solvent system to the non-solvent system ranges from about 50:1 to about 1:200 (volume / volume), in some embodiments from about 10:1 to about 1:180 (volume / volume), in some embodiments from about 1:1 to about 1:160 (volume / volume), in some embodiments from about 1:60 to about 1:150 (volume / volume), in some embodiments from about 1:1 to about 1:60 (volume / volume), and in some embodiments from about 1:1 to about 1:2 (volume / volume).

[0032] After contact with a non-solvent and completion of the phase inversion, any suitable technique can be used to dry and / or remove the liquid phase, such as by using any suitable equipment (e.g., forced-air ovens and vacuum ovens) to increase control over temperature, time, vacuum, and / or flow rate. For example, in one instance, the superabsorbent particles are subjected to high-temperature drying at a temperature of about 80°C or higher, such as about 90°C or higher, such as about 100°C or higher, such as about 110°C or higher, such as about 120°C or higher, such as about 130°C or higher, such as about 140°C or higher, such as about 150°C or higher, up to about 175°C, or any range or value therebetween, for a drying time sufficient to evaporate all ethanol in the sample except for about 30% by weight or less of ethanol, such as about 25% by weight or less of ethanol, such as about 20% by weight or less of ethanol, such as about 16% by weight of ethanol. However, in one respect, the superabsorbent particles can instead be dried until the temperature of the superabsorbent particles is about 75% or higher of the drying temperature, such as about 80% or higher, such as about 85% or higher, such as about 90% or higher, such as about 95% or higher, such as about 99% or higher.

[0033] Nevertheless, as described above, after the initial drying of the superabsorbent particles, the particles are rehydrated to a moisture content, such as water, of about 10% to about 40% by weight, such as about 15% to about 35% by weight, such as about 20% to about 30% by weight, or any range or value between therewith. In particular, the present invention has found that rehydrating the superabsorbent particles to a specific moisture content allows non-solvents to escape from the superabsorbent particles without sacrificing the porous structure or rapid absorption performance.

[0034] However, in one aspect, this disclosure also finds that the desired moisture content can be based on the molecular weight of the nonsolvent. For example, higher molecular weight nonsolvents can most effectively escape to the upper limit of the aforementioned range, while lower molecular weight nonsolvents can escape at lower moisture contents.

[0035] Regardless of the chosen rehydration percentage, rehydration can be achieved by spraying, impregnation, or exposure to a humid environment. The humid environment can be provided by any known method in the art capable of providing a controlled humid environment at ambient or high temperatures. Examples include humidity chambers, fluidized beds, or disc dryers. Furthermore, in one aspect, one or more methods can be chosen. For example, the superabsorbent particles can be first sprayed with water and then placed in a humid environment until the desired moisture content is achieved.

[0036] In one respect, if a humid environment is used, a humidified environment with a relative humidity of about 40% or greater, such as about 50% or greater, such as about 60% or greater, such as about 70% or greater, up to about 90%, or any range or value therein, can be used. Humidification can be carried out at room temperature, or heating can be used in addition to a relative humidity of about 40°C to about 100°C, such as about 50°C to about 90°C, such as about 60°C to about 80°C, or any range or value therein.

[0037] Regardless of the moisture content and method used, after rehydration, the superabsorbent particles can be dried again to a target moisture content of about 1% to about 12.5% ​​by weight, such as about 1.5% to about 11% by weight, such as about 2% to about 10% by weight, or any range or value therein.

[0038] In addition, surface crosslinking agents can be used to treat the superabsorbent particles. Surface crosslinking treatment can increase the gel strength of the superabsorbent particles and improve the balance between CRC and GBP.

[0039] As a surface crosslinking agent, any conventional surface crosslinking agent can be used (polyglycidyl groups, polyhydric alcohols, polyamines, polyaziridines, polyisocyanates, silane coupling agents, alkylene carbonates, polyvalent metals, etc.). Among these surface crosslinking agents, polyglycidyl groups, polyhydric alcohols, or polyamines are preferred considering economic efficiency and absorption characteristics. Surface crosslinking agents can be used alone or in mixtures of two or more of them.

[0040] When performing surface crosslinking treatment, the amount (by weight%) of the surface crosslinking agent is not particularly limited, as the amount can vary depending on the type of surface crosslinking agent, the crosslinking conditions, the target performance, etc. Considering the absorption characteristics, based on the weight of the superabsorbent particles, the amount is preferably 0.001 to 3% by weight, more preferably 0.005 to 2% by weight, and particularly preferably 0.01 to 1% by weight.

[0041] Surface crosslinking is performed by mixing superabsorbent particles with one or more surface crosslinking agents, followed by heating. Suitable methods are described in more detail in Japanese Patent Nos. 3648553, JP-A-2003-165883, JP-A-2005-75982, and JP-A-2005-95759, each of which is incorporated herein by reference to the extent that it does not conflict with this document. The mixing of the superabsorbent polymer with the surface crosslinking agent can be accomplished using any suitable equipment, including any conventional equipment (cylindrical mixer, screw mixer, screw extruder, turbulence mixer, Nauta mixer, kneader mixer, flow mixer, V-type mixer, shredder, ribbon mixer, air-jet mixer, disc mixer, conical mixer, rolling mixer). The surface crosslinking agent can be diluted with water and / or a solvent.

[0042] There are no particular limitations on the mixing temperature of the superabsorbent particles and the surface crosslinking agent. The preferred mixing temperature is 10 to 150°C, more preferably 20 to 100°C, and most preferably 25 to 80°C.

[0043] The superabsorbent particles can be surface-crosslinked after being mixed with a surface crosslinking agent and then heated. The surface crosslinking temperature is preferably 100 to 180°C, more preferably 110 to 175°C, and most preferably 120 to 170°C. The heating time for surface crosslinking can be appropriately controlled according to the temperature. From the viewpoint of absorption performance, the surface crosslinking time is preferably 5 to 60 minutes, more preferably 10 to 40 minutes.

[0044] Surface crosslinking of superabsorbent particles can be performed before and / or after the phase inversion process, or before or after the rehydration process. However, in one aspect, to avoid aggregation of superabsorbent particles during the phase inversion process, surface crosslinking can be performed before the phase inversion process. However, to further improve the balance between CRC and GBP, surface crosslinking can be performed after the phase inversion process. Therefore, in one aspect, depending on the focus of crosslinking, surface crosslinking can be performed before and after the phase inversion process, and / or before or after rehydration.

[0045] The invention can be better understood by referring to the following examples.

[0046] Test methods

[0047] Pore ​​properties

[0048] The pore properties of superabsorbent particles (e.g., average pore size, total pore area, bulk density, pore size distribution, and porosity percentage) can be determined using mercury porosimetry (also known as mercury intrusion porosimetry), as is well known in the art. For example, commercially available porosimeters, such as the AutoPore IV 9500 from Micrometrics, can be used. Such devices typically characterize porosity by applying various levels of pressure to a sample immersed in mercury. The pressure required to infuse mercury into the pores of the sample is inversely proportional to the pore size. Measurements can be performed at an initial pressure of 0.58 psi and a final pressure of approximately 60,000 psi. Average pore size, total pore area, and bulk density can be measured directly during the mercury intrusion test. The total pore size distribution can be obtained from a graph of differential intrusion and pore diameter (μm). Similarly, considering that approximately 50% of the volume is occupied by empty space due to particle packing, the porosity percentage can be calculated based on the reduction in bulk density (assuming the particle size, packing, and shape remain constant). More specifically, the porosity percentage can be determined according to the following formula:

[0049] 100 x 0.5 x [(Bulk density of control sample – Bulk density of test sample) / Bulk density of control sample]

[0050] Bulk density (g / cm³) 3 The value was determined by mercury porosimetry at a pressure of 0.58 psi.

[0051] Absorption capacity

[0052] The absorption capacity of superabsorbent particles can be measured using an absorbability under load (“AUL”) test, a well-known test for measuring the ability of superabsorbent particles to absorb 0.9 wt.% sodium chloride distilled aqueous solution (test solution) while the material is under load. For example, 0.16 g of superabsorbent particles can be confined to 5.07 cm⁻¹ under nominal pressures of 0.01 psi, 0.3 psi, or 0.9 psi. 2 The sample is subjected to an area-loaded absorbent (“AUL”) cylinder. The sample absorbs the test solution from a dish containing excess fluid. At predetermined time intervals, after any excess interstitial fluid has been removed from the cylinder by a vacuum apparatus, the sample is weighed. The absorption rate is then determined at each time interval using the weight versus time data.

[0053] For example, see Figure 1An embodiment of an apparatus 910 for determining absorbance capacity is shown. Apparatus 910 includes an AUL assembly 925 having a cylinder 920, a piston 930, and a counterweight 990. The counterweight 990 may be a 100-gram counterweight. A side-arm flask 960 may be used, which has a rubber stopper 945 and a tube 955 at the top of the flask to help trap any fluid removed from the sample before it enters the vacuum system. Rubber or plastic tubing 970 may be used for the side-arm flask 960 and the AUL chamber 940. Additional tubing 970 may also be used to connect a vacuum source (not shown) to the side arm 980 of the flask 960. See also Figure 2 The cylinder 920 can be used to contain the superabsorbent particles 950 and can be made of acrylic tubing with an inner diameter of one inch (2.54 cm), which is slightly machined to ensure concentricity. After machining, a mesh 414 (e.g., 400 mesh) can be attached to the bottom of the cylinder 920 using a suitable solvent, which makes the sieve adhere firmly to the cylinder. The piston 930 can be a 4.4 g piston made of a solid material (e.g., acrylic) with a diameter of one inch (2.5 cm) and can be machined to fit snugly without sticking to the cylinder 920. As mentioned above, the device 910 also includes an AUL chamber 940, which removes interstitial fluid collected during the swelling of the superabsorbent particles 950. This testing device is similar to the GATS (Gravity Analysis Absorbency Test System) available from M / K Systems and the system described by Lichstein on pages 129–142 of the INDA Technological Symposium Proceedings, March 1974. It also utilizes an opening plate 935 with an orifice limited to a 2.5 cm diameter area.

[0054] To perform the test, the following steps can be taken:

[0055] (1) Wipe the inside of AUL cylinder 920 with an antistatic cloth and weigh cylinder 920, counterweight 990 and piston 930;

[0056] (2) Record the weight as the container weight (in grams), accurate to the nearest milligram;

[0057] (3) Slowly pour 0.16±0.005 g of superabsorbent particles 950 into cylinder 920 so that the particles do not come into contact with the side of the cylinder or can adhere to the wall of the AUL cylinder.

[0058] (4) Weigh the cylinder 920, counterweight 990, piston 930 and superabsorbent particles 950, record the value on the balance as the dry weight (in grams), accurate to the nearest milligram.

[0059] (5) Gently tap the AUL cylinder 920 until the superabsorbent particles 950 are evenly distributed on the bottom of the cylinder;

[0060] (6) Gently place the piston 930 and the counterweight 990 into the cylinder 920;

[0061] (7) Place the test fluid (0.9% by weight aqueous sodium chloride solution) in a fluid bath with a large mesh screen at the bottom;

[0062] (8) Start the timer while placing the superabsorbent particles 950 and the cylindrical assembly 925 onto the sieve in the fluid bath. The liquid level in the bath should provide a positive pressure head of at least 1 cm above the base of the cylinder;

[0063] (9) Gently vortex the sample to release any trapped air and ensure that the superabsorbent particles are in contact with the fluid.

[0064] (10) Remove the cylinder 920 from the fluid bath at specified time intervals, immediately place the cylinder on a vacuum device (opening plate 935 on top of AUL chamber 940) and remove excess interstitial fluid for 10 seconds.

[0065] (11) Wipe the outside of the cylinder with a paper towel or tissue;

[0066] (12) Immediately weigh the AUL assembly (i.e., cylinder 920, piston 930, and counterweight 990) containing the superabsorbent particles and any absorbed test fluid, and record the weight and time interval, wherein the weight is wet weight (in grams), accurate to the nearest milligram; and

[0067] (13) Repeat the test for all required time intervals.

[0068] At least two (2) samples are typically tested at each predetermined time interval. These time intervals are typically 15, 30, 60, 120, 300, 600, 1800, and 3600 seconds (or 0.015, 0.030, 0.060, 0.120, 0.300, 0.600, 1.8, or 3.6 kiloseconds). The “absorption capacity” of the superabsorbent particles at the specified time interval is calculated as grams of liquid / grams of superabsorbent material using the following formula:

[0069] (Wet weight - Dry weight) / (Dry weight - Container weight)

[0070] Absorption rate

[0071] The “absorption rate” of superabsorbent particles can be determined at specified time intervals by dividing the above-mentioned absorption capacity (g / g) by the specific time interval of interest (kiloseconds, ks) (such as 0.015, 0.030, 0.060, 0.120, 0.300, 0.600, 1.8 or 3.6 kiloseconds).

[0072] Centrifugation Retention Capacity (CRC)

[0073] The centrifugal retention capacity (CRC) test measures the ability of superabsorbent particles to retain liquid after being saturated and centrifuged under controlled conditions. The resulting retention capacity is expressed as grams of liquid retained per gram of sample weight (g / g). The test sample is prepared from particles pre-sieved through a US Standard 30-mesh sieve and retained on a US Standard 50-mesh sieve. The particles can be pre-sieved manually or automatically and stored in a sealed, airtight container until testing. The retention capacity is measured by placing 0.2 ± 0.005 g of the pre-sieved sample into a water-permeable bag that will contain the sample, while allowing the test solution (0.9 wt% sodium chloride distilled aqueous solution) to be freely absorbed by the sample. Heat-sealable tea bag material (such as heat-sealable filter paper model name 1234T) may be suitable. The bag is formed by folding a 5 inch × 3 inch sample of the bag material in half and heat-sealing two of the open edges to form a 2.5 inch × 3 inch rectangular pouch. The heat seal can be approximately 0.25 inches inside the material edge. After placing the sample into the pouch, the remaining open edges of the pouch can also be heat-sealed. An empty pouch can serve as a control. Prepare three samples (e.g., a filled pouch and a sealed pouch) for testing. Test the filled pouch within three minutes of preparation, unless immediately placed in a sealed container; in the latter case, the filled pouch must be tested within thirty minutes of preparation.

[0074] Place these bags on two straps with 3-inch openings. Between coated glass fiber sieves (Taconic Plastics, Inc., Petersburg, NY), immerse the bags in a dish of test solution at 23°C, ensuring the sieves are held in place until the bags are completely wetted. After wetting, hold the samples in the solution for approximately 30 ± 1 minutes, then remove them from the solution and temporarily lay them on a non-absorbent, flat surface. For multiple tests, after 24 bags have saturated the dish, the dish should be emptied and refilled with fresh test solution.

[0075] The moistened bag is then placed in the basket of a suitable centrifuge capable of subjecting the sample to approximately 350 g-force. A suitable centrifuge is the Heraeus LaboFuge 400, which features a water collection basket, a digital tachometer, and a machined drainage basket suitable for holding and draining the bag sample. In the case of centrifuging multiple samples, these samples can be placed in opposing positions within the centrifuge to balance the basket during rotation. The bags (including the moistened empty bag) are centrifuged at approximately 1,600 rpm (e.g., to achieve a target gravity of approximately 350 g-force) for 3 minutes. The bags are removed and weighed, first the empty bag (control), and then the bag containing the sample. The amount of solution retained by the sample, taking into account the solution retained in the bag itself, is the centrifugation retention capacity (CRC) of the sample, expressed as grams of fluid per gram of sample. More specifically, the centrifugation retention capacity is determined as:

[0076] Weight of sample bag after centrifugation – Weight of empty bag after centrifugation – Weight of dried sample

[0077] Dry sample weight

[0078] Three samples were tested, and the results were averaged to determine the retention capacity (CRC) of the superabsorbent material. The samples were tested at 23°C and 50% relative humidity.

[0079] Vortex time

[0080] Vortex time is the amount of time (in seconds) required for a predetermined mass of superabsorbent particles to close a vortex formed by stirring 50 mL of 0.9 wt% sodium chloride solution at 600 rpm on a magnetic stirring plate. The time taken for the vortex to close is an indication of the free swelling absorption rate of the particles. The vortex time test can be performed at a temperature of 23°C and 50% relative humidity according to the following procedure:

[0081] (1) Measure 50 ml (±0.01 ml) of 0.9% sodium chloride solution into a 100 ml beaker.

[0082] (2) Cover the 7.9 mm x 32 mm area without the ring. Magnetic stirring rods (such as those with trade name S / Place the single-pack round stirring rod with a removable pivot ring (obtained from the brand) into the beaker.

[0083] (3) Place the magnetic stirring plate (such as the one described by the trade name) The model 721 (the kind obtained through commercial purchase) is programmed to 600 RPM.

[0084] (4) Place the beaker in the center of the magnetic stirring plate to activate the magnetic stirring rod. The bottom of the vortex should be close to the top of the stirring rod. The superabsorbent particles are pre-sieved through a US Standard #30 mesh sieve (0.595 mm opening) and retained on a US Standard #50 mesh sieve (0.297 mm opening).

[0085] (5) Weigh the required mass of the superabsorbent particles to be tested on weighing paper.

[0086] (6) While stirring the sodium chloride solution, quickly pour the absorbent polymer to be tested into the brine solution and start the stopwatch. Add the superabsorbent particles to be tested into the brine solution between the center of the vortex and the side of the beaker.

[0087] (7) Stop the stopwatch when the surface of the salt solution becomes flat and record the time. The time (recorded in seconds) is reported as vortex time.

[0088] Free swelling gel bed permeability (GBP) test

[0089] As used herein, the free-swelling gel bed permeability (GBP) test determines the permeability of a superabsorbent material's swelling bed under conditions commonly referred to as "free-swelling" conditions. The term "free-swelling" means allowing the superabsorbent material to swell without a swelling constraint load while absorbing the test solution, as will be described. This test is performed under conditions where... Qin As described in U.S. Patent Publication No. 2010 / 0261812, which is incorporated herein by reference. For example, a testing apparatus comprising a sample container and a piston may be employed, which may include a cylindrical LEXAN shaft having concentric cylindrical bores drilled downward along the longitudinal axis of the shaft. Both ends of the shaft may be machined to provide an upper end and a lower end. A counterweight may be placed on one end having a cylindrical bore drilled through at least a portion of its center. A circular piston head may be positioned on the other end and has a concentric inner ring consisting of seven holes (each hole having a diameter of approximately 0.95 cm) and a concentric outer ring consisting of fourteen holes (each hole having a diameter of approximately 0.95 cm). These holes are drilled from the top to the bottom of the piston head. The bottom of the piston head may also be covered with a biaxially stretched stainless steel sieve. The sample container may comprise a cylinder and a 100-mesh stainless steel cloth sieve, the cloth sieve being biaxially stretched to tension and attached to the lower end of the cylinder. During testing, the superabsorbent particles can be supported on a sieve inside a cylinder.

[0090] The cylinder can be drilled from a transparent LEXAN rod or equivalent material, or it can be cut from LEXAN tubing or equivalent material, and has an inner diameter of approximately 6 cm (e.g., approximately 28.27 cm). 2The cylinder has a cross-sectional area of ​​approximately 1.5 cm, a wall thickness of approximately 0.5 cm, and a height of approximately 5 cm. A drain hole can be formed in the side wall of the cylinder, approximately 4.0 cm above the sieve, to allow liquid to drain from the cylinder, thereby maintaining the fluid level in the sample container at approximately 4.0 cm above the sieve. The piston head can be machined from a LEXAN rod or equivalent material and has a height of approximately 16 mm and a diameter determined to allow it to fit within the cylinder with minimal wall clearance while still sliding freely. The shaft can be machined from a LEXAN rod or equivalent material and has an outer diameter of approximately 2.22 cm and an inner diameter of approximately 0.64 cm. The upper end of the shaft is approximately 2.54 cm long and has a diameter of approximately 1.58 cm, forming an annular shoulder to support an annular counterweight. The inner diameter of the annular counterweight is then approximately 1.59 cm, allowing it to slide onto the upper end of the shaft and rest on the annular shoulder formed thereon. The ring weight can be made of stainless steel or other suitable materials that are corrosion-resistant in the presence of a test solution, which is a 0.9 wt.% distilled sodium chloride aqueous solution. The combined weight of the piston and ring weight is approximately 596 grams, corresponding to a depth of approximately 28.27 cm. 2 Approximately 0.3 pounds per square inch, or approximately 20.7 dynes per cm, is applied to the sample area. 2 The pressure. When the test solution flows through the test apparatus during the test as described below, the sample container is typically placed on a 16-mesh rigid stainless steel support sieve. Alternatively, the sample container may be placed on a support ring whose diameter is determined to be substantially the same as that of the cylinder, such that the support ring does not restrict flow from the bottom of the container.

[0091] To perform gel bed permeability testing under “free swelling” conditions, a piston with a weighted counterweight is placed in an empty sample container, and the height from the bottom of the counterweight to the top of the cylinder is measured using calipers accurate to 0.01 mm or a suitable gauge. The height of each sample container can be measured as empty, and when multiple testing devices are used, it is possible to track which piston and counterweight are being used. When the sample subsequently swells after saturation, the same piston and counterweight can be used for measurement. The test sample is prepared from superabsorbent particles pre-sieved through a US Standard 30-mesh sieve and retained on a US Standard 50-mesh sieve. The particles can be pre-sieved manually or automatically. Approximately 0.9 g of sample is placed in a sample container, and then the container without the piston and counterweight is immersed in the test solution for approximately 60 minutes to allow the sample to saturate and swell without any constraint load. At the end of this phase, the piston and counterweight assembly is placed on the saturated sample in the sample container, and then the sample container, piston, counterweight, and sample are removed from the solution. The thickness of the saturated sample is determined by re-measuring the height from the bottom of the counterweight to the top of the cylinder using the same calipers or gauges as previously used, ensuring the zero point remains constant relative to the initial height measurement. The height measurements obtained from the empty sample container, piston, and counterweight are subtracted from the height measurements obtained after saturating the sample. The resulting value is the thickness of the swollen sample, or height "H".

[0092] Permeability measurements are initiated by delivering a flow of test solution into a sample container containing a saturated sample, a piston, and a counterweight. The flow rate of the test solution entering the container is adjusted to maintain the fluid level approximately 4.0 cm above the bottom of the sample container. The amount of solution passing through the sample relative to time is measured by gravimetric analysis. Once the fluid level has stabilized and remained at approximately 4.0 cm, data points are collected every second for at least twenty seconds. The flow rate Q through the swollen sample is determined in grams per second (g / s) by performing a linear least-squares fit of the fluid (in grams) through the sample against time (in seconds). Permeability is obtained using the following formula:

[0093] K = (1.01325 × 10 8 )*[Q*H*Mu] / [A*Rho*P]

[0094] in

[0095] K = Permeability (Darcy),

[0096] Q = flow rate (g / s),

[0097] H = Sample height (cm)

[0098] Mu = Liquid viscosity (poise) (The test solution used in the test is approximately 1 centipoise).

[0099] A = Cross-sectional area of ​​the liquid flow (cm²) 2 ),

[0100] Rho = Liquid density (g / cm³) 3 (The test solution used in the test is approximately 1 g / cm³) 3 ),and

[0101] P = hydrostatic pressure (dynes / cm) 2 (Typically about 3,923 dynes / cm) 2 It can be calculated from Rho*g*h, where Rho = liquid density (g / cm³). 3 g = gravitational acceleration, usually 981 cm / s² 2 And h = fluid height, for example, 4.0 cm.

[0102] Test at least three samples and average the results to determine the free swelling gel bed permeability of the samples. Samples were tested at 23°C and 50% relative humidity.

[0103] Moisture percentage

[0104] To measure the moisture percentage in the superabsorbent particles, a moisture analyzer from an A&D Model MX50 was used. A heating temperature of 140°C was used to determine the moisture percentage.

[0105] Quantitative analysis of non-solvents in SAM

[0106] 0.03 g of SAM was placed in a 40 mL vial, and then 17 g of water was added. The vial was then capped and placed on a wrist shaker for 30 minutes. The vial was then removed from the shaker and allowed to homogenize for 1 minute. 3 mL of the homogenized mixture was transferred to a syringe and filtered through a glass fiber / 0.45 μm nylon membrane into a 2 mL GC autosampler vial, collecting 0.5 mL to 1 mL of the filtrate. The non-solvent content of the filtrate was then analyzed by the FC-FID method.

[0107] Analytical method conditions for quantification of non-solvents

[0108] Instrument: Agilent 6890GC

[0109] Column: DB-ALC1, 30m x 0.53mm x 3.0μm thin film

[0110] Carrier gas: Nitrogen

[0111] Injector: 0.5 μL, at a 5:1 split, temperature 250℃

[0112] Detector: FID, at 300°C

[0113] Temperature program: 40℃ for 3 minutes, increase to 200℃ at a rate of 25℃ / minute, hold for 5 minutes.

[0114] Run time: 14.4 minutes

[0115] Retention times: Methanol - 1.8 min, Ethanol - 2.4 min, IPA - 2.9 min

[0116] Quantitative calibration

[0117] Preparation standards for methanol, ethanol and isopropanol: 7 standards, 1-200 ppm, R2≥0.99.

[0118] Example 1A

[0119] First, 5 kg of commercially available cross-linked polyacrylate superabsorbent particles are provided. These particles are supplied as granted... Fujimura The sample was formed as described in U.S. Patent No. 8,742,023, with an initial vortex time of 35 seconds and a CRC of approximately 27.5 g / g. The particles were swollen with 50 kg of water containing 20% ​​wt% ethanol (the ethanol used was denatured with isopropanol). Using a vacuum filtration system, the swollen SAM particles were washed with 25 kg of isopropanol-denatured ethanol to remove excess liquid. This step was repeated at least four times. Initial drying of the sample was carried out at 170°C for 6 hours.

[0120] After initial drying, the superabsorbent particles were placed in a humid environment with a relative humidity of 60% and a temperature of 69°C for 4 hours. After rehydration, the moisture percentage of the superabsorbent particles was 29%. The superabsorbent particles (SAM) were then dried again at 65°C for 30 minutes.

[0121] Example 1B

[0122] Except for placing the particles in a humid environment for 12 hours, the particles were formed as described in Example 1A. After rehydration, the moisture percentage of the superabsorbent particles was 28%.

[0123] Example 1C

[0124] Except for placing the particles in a humid environment for 48 hours, the particles were formed as described in Example 1A. After rehydration, the moisture percentage of the superabsorbent particles was 27%.

[0125] Example 1 - Control

[0126] Except that the particles were not placed in a humid environment or re-dried, the particles were formed as described in Example 1A. The results of Example 1 are shown in Table 1.

[0127] Table 1

[0128]

[0129] Example 2A - Comparative

[0130] First, 10 kg of commercially available cross-linked polyacrylate superabsorbent particles are provided. These particles are supplied as granted... Fujimura The sample was formed as described in U.S. Patent No. 8,742,023, with an initial vortex time of 35 seconds and a CRC of approximately 27.5 g / g. The particles were swollen with 90 kg of deionized water. The swollen SAM particles were washed with 100 kg of methanol using a vacuum filtration system to remove excess liquid. This step was repeated at least twice. An additional 13.6 kg of methanol was used to wash the particles, followed by vacuum filtration to remove excess liquid. Initial drying of the sample was performed at 80°C for one hour, followed by four hours under vacuum at 140°C. The particles were then rehydrated for 2.4 hours in a humid atmosphere at 20% relative humidity and 69°C. After rehydration, the moisture content of the particles was 6%. However, the particles were not dried again.

[0131] Example 2B – Comparative

[0132] Except for using a relative humidity of 40%, the particles were formed as described in Example 2A. After rehydration, the moisture content of the particles was 12%.

[0133] Example 2C – Comparison

[0134] Except for maintaining the particles in a humid atmosphere for 64 hours, the particles were formed as described in Example 2B. After rehydration, the moisture content of the particles was 11%.

[0135] Example 2D

[0136] Except for the presence of a humid atmosphere with a relative humidity of 60%, particles were formed as described in Example 2A. After rehydration, the moisture content of the particles was 27%.

[0137] Example 2E

[0138] Except for drying the particles again on a heating plate at 140°C, the particles are formed as described in Example 2D.

[0139] Example 2 - Control

[0140] Except that the particles were not placed in a humid environment or re-dried, the particles were formed as described in Example 2A. The results of Example 2 are shown in Table 2.

[0141] Table 2

[0142] Example Vortex [300-600 μm] (seconds) Methanol (ppm) in SAM 2A 18 63951 2B 17 13466 2C 17 997 2D 15 Not detected 2E 26 224 2-Control 16 66259

[0143] Example 3A - Comparative

[0144] First, 40 kg of commercially available cross-linked polyacrylate superabsorbent granules are provided. These granules are prepared as granted... Fujimura Formed as described in U.S. Patent No. 8,742,023, with an initial vortex time of 35 seconds and a CRC of approximately 27.5 g / g. The particles were swollen with 400 kg of water containing 20% ​​wt% ethanol. The swollen SAM particles were washed with 200 kg of ethanol using a vacuum filtration system to remove excess liquid. An additional 100 kg of ethanol was used for washing with the swelling solution using a vacuum filtration system to remove excess liquid. This washing with 100 kg of ethanol was repeated at least five times. The initial drying of the sample was carried out at 120°C for one hour, followed by drying at 150°C for one hour, and then drying at 170°C for one hour. Furthermore, the particles were rehydrated at 80°C at approximately 50% relative humidity for two hours. After rehydration, the moisture content of the particles was 7%. The particles were not dried again.

[0145] Example 3B - Comparative

[0146] Except for rehydrating the particles for four hours, the particles were formed as described in Example 3A. After rehydration, the moisture content of the particles was 10%.

[0147] Example 3C - Comparison

[0148] Except for rehydrating the particles for six hours, the particles were formed as described in Example 3A. After rehydration, the moisture content of the particles was 11%.

[0149] Example 3D - Comparative

[0150] Except for rehydrating the particles for six hours, the particles were formed as described in Example 3A. The particles were then dried again at 80°C for one hour. After this second drying, the moisture content of the particles was 6%.

[0151] Example 3E

[0152] First, 5 kg of commercially available cross-linked polyacrylate superabsorbent particles are provided. These particles are designed to be used in... Fujimura et al.The particles are formed as described in U.S. Patent No. 8,742,023, having a particle size distribution of 90-300 micrometers, and undergoing surface cross-linking during the formation process. The particles are mixed with 10 kg of denatured ethanol. Then 10 kg of water is added and mixed for 5 minutes. An additional 30 kg of water is added to further swell the particles. Using a vacuum filtration system, the swollen SAM particles are washed multiple times with denatured ethanol to remove the acquisition liquid. The particles are washed with ethanol until approximately 1% by weight of water is measured in the collected acquisition liquid. Initial drying of the sample is carried out at 120°C for one hour and at 170°C for 2.5 hours. The particles are rehydrated at 70-75°C for six hours at a relative humidity of approximately 50-62%. After rehydration, the moisture content of the particles is 17%. The particles are then dried again at 70°C for one hour.

[0153] Example 3F

[0154] Except for the initial drying of the particles at 170°C for 3.5 hours, the particles were formed as described in Example 3E. The particles were then rehydrated at 70°C for six hours at a relative humidity of approximately 76%. After rehydration, the moisture content of the particles was 30%. The particles were then dried again at 70°C for one hour.

[0155] Example 3 - Control

[0156] Except that the particles were not placed in a humid environment or re-dried, the particles were formed as described in Example 3A. The results of Example 3 are shown in Table 3.

[0157] Table 3

[0158] Example Vortex [original particle size] (seconds) Ethanol (ppm) in SAM 3A 22 73562 3B 18 65578 3C 17 67154 3D 16 63034 3E 7 117215 3F 9 Not detected 3-Control 17 112837

[0159] Example 4A

[0160] First, 50 kg of commercially available cross-linked polyacrylate superabsorbent particles are provided. These particles are in the form of... Fujimura et al. The particles are formed in the manner described in U.S. Patent No. 8,742,023, having a particle size distribution of 90-300 micrometers, and undergoing surface cross-linking during the formation process. The particles are mixed with 100 kg of denatured ethanol. Then 100 kg of water is added and mixed for 5 minutes. An additional 300 kg of water is added to further swell the particles. Using a vacuum filtration system, the swollen SAM particles are washed multiple times with denatured ethanol to remove the acquisition liquid. The particles are washed with ethanol until approximately 1% by weight of water is measured in the collected acquisition liquid.

[0161] The particles were initially dried in a fluidized bed at 160°C until the particle temperature reached 130°C. The particles were then discarded from the fluidized bed. The particles were then fluidized using a humidifying gas and reintroduced into the fluidized bed for 3.5 hours until a moisture percentage of 19% was achieved at a particle temperature of 76°C. The particles were not dried again.

[0162] Example 4B

[0163] Except that the particles were placed in a fluidized bed using a humidifying gas for 4.5 hours and the particles were formed at a particle temperature of 77°C to achieve a moisture percentage of 23%, the particles were formed as described in Example 4A. The particles were not dried again.

[0164] Example 4C

[0165] Except for placing the particles in a fluidized bed using humidifying gas for 5.5 hours and achieving a moisture percentage of 28% at a particle temperature of 77°C, the particles were formed as described in Example 4A. The particles were then dried again at 70-90°C for 7.5 hours, resulting in a final moisture content of 5%.

[0166] Table 4

[0167] Example Vortex [original particle size] (seconds) Ethanol (ppm) in SAM 4A 6 22265 4B 8 855 4C 8 Not detected

[0168] While the invention has been described in detail with reference to specific embodiments thereof, it will be appreciated that those skilled in the art, upon gaining an understanding of the foregoing, will readily conceive of alternative forms, variations, and equivalents of these embodiments. Therefore, the scope of the invention should be assessed as encompassing the appended claims and any of their equivalents.

Claims

1. A superabsorbent particle having a median size of 50 micrometers to 2,000 micrometers and comprising nanopores with an average cross-sectional size of 10 nanometers to 500 nanometers. The superabsorbent particles contain less than 1000 ppm of non-solvent; The superabsorbent particles exhibit a vortex time of 80 seconds or less; and The superabsorbent particles have a moisture content of 1% to 12.5% ​​by weight.

2. The superabsorbent particles of claim 1, wherein the particles contain less than 500 ppm of non-solvent.

3. The superabsorbent particles of claim 1 or 2, wherein the particles exhibit an absorption rate of 300 g / g / ks or higher after being placed in contact with an aqueous solution of 0.9% sodium chloride for 0.015 kiloseconds.

4. The superabsorbent particles of claim 1 or 2, wherein the superabsorbent particles exhibit an absorption rate of 500 g / g / ks or higher after being placed in contact with an aqueous solution of 0.9% sodium chloride for 0.015 kiloseconds.

5. The superabsorbent particles of claim 1 or 2, wherein the superabsorbent particles exhibit an absorption rate of 160 g / g / ks or higher after being placed in contact with an aqueous solution of 0.9% sodium chloride for 0.120 kiloseconds.

6. The superabsorbent particles of claim 1 or 2, wherein the superabsorbent particles exhibit a total absorption capacity of 10 g / g or higher after being placed in contact with an aqueous solution of 0.9% sodium chloride for 3.6 kiloseconds.

7. The superabsorbent particles of claim 1 or 2, wherein the particles exhibit a centrifugal retention capacity of 20 g / g or higher.

8. The superabsorbent particles of claim 1 or 2, wherein the particles further comprise micropores.

9. The superabsorbent particle of claim 1 or 2, wherein the nanopores occupy at least 25% of the volume of the pores in the particle.

10. The superabsorbent particles of claim 1 or 2, wherein the particles exhibit a total pore area of ​​2 m² / g or greater.

11. The superabsorbent particles of claim 1 or 2, wherein the particles exhibit a porosity percentage of 5% or greater.

12. The superabsorbent particles of claim 1 or 2, wherein the particles exhibit a bulk density of less than 0.7 g / cm³, as determined by mercury porosimetry at a pressure of 0.58 psi.

13. The superabsorbent particles of claim 1 or 2, wherein the particles have an average pore size of 1 nanometer to 1,200 nanometers.

14. The superabsorbent particles of claim 1 or 2, wherein the particles have a specific surface area of ​​0.2 m² / g or greater, as determined according to ISO 9277:2010.

15. A method for forming superabsorbent particles according to any one of the preceding claims, the method comprising: A composition comprising a superabsorbent polymer and a solvent system is formed; The composition is contacted with a non-solvent system to initiate pore formation through phase inversion; Dry the superabsorbent particles; The particles are rehydrated to a moisture content of 10% to 40% by weight based on the weight of the superabsorbent particles; and After rehydration, the superabsorbent particles are dried again to the target moisture content.

16. The method of claim 15, wherein the superabsorbent particles are rehydrated to a moisture content of 15% to 35% by weight.

17. The method of claim 15 or 16, wherein the superabsorbent particles are rehydrated by spraying, impregnation, placement in a humid atmosphere, or a combination thereof.

18. The method of claim 15 or 16, wherein the non-solvent comprises acetone, n-propanol, ethanol, methanol, n-butanol, propylene glycol, ethylene glycol, or combinations thereof.

19. A superabsorbent particle formed by the method according to any one of claims 15 to 18.

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