RU2777893C2 - Nano-porous super-absorbing particles - Google Patents

Abstract

FIELD: super-absorbing materials.

SUBSTANCE: super-absorbing material for absorbing fluids is disclosed, including super-absorbing particles characterized by a median size from 50 to 2000 micrometers, and containing a porous mesh, which includes a set of nano-pores characterized by an average cross-section size from 10 to 500 nanometers, where super-absorbing particles are characterized by vortex stop time of 80 seconds or less and free swell gel bed permeability (GBP) in the range from 5 Darcy to 100 Darcy, where particles are formed of cross-linked polymer, which contains repeated links obtained from one or more ethylene-unsaturated monomer compounds containing at least one hydrophilic radical. A method for the production of the specified super-absorbing particles is also disclosed.

EFFECT: group of inventions provides high absorption speed values at initial contact with fluid.

19 cl, 5 tbl, 27 ex, 8 dwg

Description

Superabsorbent materials are used in a wide variety of applications to aid in the absorption of liquids. These materials are typically capable of absorbing fluid (eg water, saline, etc.) in amounts several times their own weight. However, one problem with many conventional superabsorbent materials is that when they initially come into contact with a fluid, the rate of absorption can be relatively slow. Accordingly, there is currently a need for a superabsorbent material that exhibits relatively high absorption rates, especially upon initial contact with a fluid.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, superabsorbent particles are disclosed that have a median size of from about 50 to about 2000 micrometers and contain a pore network that includes a plurality of nanopores having an average cross-sectional size of from about 10 to about 500 nanometers. The superabsorbent particles are characterized by a vortex stop time of approximately 80 seconds or less and a gel layer free swelling permeability (GBP) of 5 Darcy or more.

In another aspect, a method for forming superabsorbent particles according to any one of the above claims includes forming a composition that contains a superabsorbent polymer and a solvent system; bringing the composition into contact with the system of precipitators to initiate the formation of a pore network through phase inversion; removal of the precipitant from the composition; and surface crosslinking of the superabsorbent particles.

Other features and aspects of the present invention are discussed in more detail below.

BRIEF DESCRIPTION OF GRAPHICS

A complete and sufficient disclosure of the present invention, including the best mode for its implementation, intended for a person skilled in the art, is set forth more specifically in the remainder of the description, where reference is made to the accompanying drawings, which show the following:

in fig. 1 depicts an apparatus that can be used to measure absorbency under load (“AUL”) of porous superabsorbent particles of the present invention;

in fig. 2 shows the AUL measurement assembly shown in FIG. one;

in fig. 3A-3F show SEM micrographs of the superabsorbent particles from Example 1, where FIG. 3A (456X), FIG. 3B (10000X, enlarged view) and FIG. 3C (55000X, enlarged view) shows particles before pore formation, and FIG. 3D (670X), Fig. 3E (10000X, enlarged view) and FIG. 3F (55000X, enlarged view) shows particles after pore formation;

in fig. 4 shows the pore size distribution of the control particles discussed in Example 1 before changing the solvent;

in fig. 5 shows the pore size distribution for the particles of Example 1 after changing the solvent to methanol;

in fig. 6 shows the pore size distribution for the particles of example 2 after changing the solvent to ethanol;

in fig. 7 shows the pore size distribution for the particles of example 3 after changing the solvent to isopropyl alcohol; and

in fig. 8 shows the pore size distribution of the particles from Example 4 after changing the solvent to acetone.

The recurring use of reference numbers in the present specification and drawings is intended to represent the same or similar features or elements of the present invention.

DETAILED DESCRIPTION

The following will be a detailed description with reference to various aspects of the present invention, one or more examples of which are given below. Each example is provided to illustrate the present invention and not to limit it. Indeed, it will be apparent to those skilled in the art that various modifications and changes can be made to the present invention without departing from the scope or spirit of the present invention. For example, features illustrated or described as part of one aspect may be used in another aspect to provide yet another aspect. Thus, the present invention is intended to cover such modifications and changes as fall within the scope of the appended claims and their equivalents.

In general, the present invention is directed to superabsorbent particles that typically have a median size (e.g., diameter) of from about 50 to about 2000 micrometers, in some aspects from about 100 to about 1000 micrometers, and in some aspects from about 200 to about 700 micrometers. micrometers. Used in this document, the term "median" size refers to the distribution of particles by size "D50", which means that at least 50% of the particles have the specified size. The particles can similarly have a D90 size distribution (at least 90% of the particles have the specified size) within the ranges indicated above. The particle diameter can be determined using known techniques such as ultracentrifugation, laser diffraction, etc. For example, the particle size distribution can be determined according to a standard test method such as ISO 13320:2009. The particles may also have any desired shape such as scaly, round, spherical, tubular, etc. Particle size can be adjusted to optimize performance for a particular application.

Regardless of their particular size or shape, superabsorbent particles are porous in nature and generally have a pore network that may contain a combination of closed and open pores. The overall porosity of the particles can be relatively high. For example, the particles may have a total pore area of about 2 square meters per gram (m ² /g) or more, in some aspects from about 5 to about 150 m ² /g, and in some aspects from about 15 to about 40 m ² /G. The percentage porosity can also be about 5% or more, in some aspects from about 10% to about 60%, and in some aspects from about 15% to about 60%. Another parameter characterizing porosity is bulk density. In this regard, the bulk density of the superabsorbent particles of the present invention, for example, may be less than about 0.7 grams per cubic centimeter (g/cm ³ ), in some aspects from about 0.1 to about 0.65 g/cm ³ and in some aspects from about 0.2 to about 0.6 g/cm ³ as determined at a pressure of 0.58 psi. inch for the introduction of mercury.

To achieve the desired pore properties, the pore network typically contains a plurality of nanopores having an average cross-sectional size (e.g., width or diameter) of from about 10 to about 500 nanometers, in some aspects from about 15 to about 450 nanometers, and in some aspects from about 20 to about 400 nanometers. The term "cross-sectional dimension" generally refers to the characteristic dimension (eg, width or diameter) of a pore that is substantially perpendicular to its major axis (eg, length). It should be understood that several types of pores may exist in a network. For example, micropores may also be formed that have an average cross-sectional size of from about 0.5 to about 30 micrometers, in some aspects from about 1 to about 20 micrometers, and in some aspects from about 2 micrometers to about 15 micrometers. However, nanopores may be present in a relatively high amount in the network. For example, nanopores can comprise at least about 25% by volume, in some aspects at least about 40% by volume, and in some aspects from about 40% by volume to 80% by volume of the total pore volume of the particles. The average volume percentage occupied by nanopores within a given unit volume of material can also be from about 15% to about 80% per cm ³ , in some aspects from about 20% to about 70%, and in some aspects from about 30% to about 60% per cubic centimeter of particles. Several subtypes of nanopores can also be used. In certain aspects, for example, first nanopores may be formed that have an average cross-sectional size of from about 80 to about 500 nanometers, in some aspects from about 90 to about 450 nanometers, and in some aspects from about 100 to about 400 nanometers, which may second nanopores are formed that have an average cross-sectional size of from about 1 to about 80 nanometers, in some aspects from about 5 to about 70 nanometers, and in some aspects from about 10 to about 60 nanometers. Nanopores can have any regular or irregular shape, such as spherical, elongated, etc. Regardless, the average pore diameter within a pore network will typically be from about 1 to about 1200 nanometers, in some aspects from about 10 nanometers to about 1000 nanometers, in some aspects from about 50 to about 800 nanometers, and in some aspects aspects from about 100 to about 600 nanometers.

The present inventors have found that, in part due to the particular nature of the pore network, the resulting superabsorbent particles may have an increased absorption rate for a particular period of time during which they come into contact with a liquid such as water, aqueous salt solutions (e.g. chloride sodium), body fluids (e.g. urine, blood, etc.), etc. This increased level can be characterized in several ways. For example, the particles may have a low swirl stop time, which refers to the amount of time in seconds it takes for a quantity of superabsorbent particles to stop a swirl formed by agitating a quantity of 0.9 weight percent sodium chloride solution as tested, described below. More specifically, the superabsorbent particles may have a swirl stop time of about 80 seconds or less, in some aspects about 60 seconds or less, in some aspects about 45 seconds or less, in some aspects about 35 seconds or less, in some aspects about 30 seconds. seconds or less, in some aspects about 20 seconds or less, and in some aspects from about 0.1 to about 10 seconds. Alternatively, after being contacted with an aqueous solution of sodium chloride (0.9 wt%) for 0.015 kiloseconds ("ks"), the impregnation rate of the particles can be about 300 g/g/ks or more, in some aspects about 400 g /g/ks or more, in some aspects about 500 g/g/ks or more, and in some aspects from about 600 to about 1500 g/g/ks. High absorption rates may even be maintained for a relatively long period of time. For example, after being contacted with an aqueous solution of sodium chloride (0.9 wt.%) for 0.06 ks or even no more than 0.12 ks, the impregnation rate of the particles can still be about 160 g/g/ks or more, in some aspects about 180 g/g/ks or more, in some aspects about 200 g/g/ks or more, and in some aspects from about 250 to about 1200 g/g/ks.

Notably, the increased absorption rate can be maintained without loss of the overall absorbency of the particles. For example, after 3.6 ks, the total absorbency of the particles may be about 10 g/g or more, in some aspects about 15 g/g or more, and in some aspects from about 20 to about 100 g/g. Similarly, the particles may have a centrifuge retention capacity ("CRC") of about 20 grams of liquid per gram of superabsorbent particles (g/g) or more, in some aspects about 25 g/g or more, and in some aspects from about 30 up to about 60 g/g. Finally, the superabsorbent particles may also have a gel layer free swelling ("GBP") permeability of about 5 Darcy or more, in some aspects about 10 Darcy or more, in some aspects about 20 Darcy or more, in some aspects from about 30 to 60 Darcy, in some aspects from about 30 to 100 Darcy, and in some aspects from about 60 to 100 Darcy. Superabsorbent particles are generally formed from a three-dimensional crosslinked polymer network that contains repeating units derived from one or more ethylenically unsaturated (e.g., monoethylenically unsaturated) monomeric compounds containing at least one hydrophilic radical such as a carboxyl group, a carboxylic acid anhydride group, carboxylic acid salts, sulfonic acid salts, sulfonic acid salts, hydroxyl, ether, amide group, amino group or quaternary ammonium salt group. Specific examples of suitable ethylenically unsaturated monomeric compounds for forming superabsorbent particles include, for example, carboxylic acids (e.g., (meth)acrylic acid (covers acrylic acid and/or methacrylic acid), maleic acid, fumaric acid, crotonic acid, sorbic acid, itaconic acid, cinnamic acid, etc.); carboxylic anhydrides (eg maleic anhydride); salts (alkali metal salts, ammonium salts, amine salts, etc.) of carboxylic acids (e.g. sodium (meth)acrylate, trimethylamine (meth)acrylate, triethanolamine (meth)acrylate, sodium maleate, methylamine maleate, etc.) ; vinyl sulfonic acids (for example, vinyl sulfonic acid, allyl sulfonic acid, vinyl toluene sulfonic acid, styrene sulfonic acid, etc.); (meth)acrylic sulfonic acids (eg sulfopropyl (meth)acrylate, 2-hydroxy-3-(meth)acryloxypropylsulfonic acid, etc.); salts of vinylsulfonic acids or (meth)acrylic sulfonic acids; alcohols (eg (meth)allylic alcohol); polyol ethers or esters (e.g., hydroxyethyl (meth)acrylate, hydroxypropyl (meth)acrylate, triethylene glycol (meth)acrylate, poly(oxyethyleneoxypropylene)glycol mono(meth)allyl ether (in which the hydroxyl groups may be esterified or esterified), and etc.); vinylformamides; (meth)acrylamides, N-alkyl(meth)acrylamides (e.g. N-methylacrylamide, N-hexylacrylamide, etc.), N,N-dialkyl(meth)acrylamides (e.g. N,N-dimethylacrylamide, N,N -di-n-propylacrylamide, etc.); N-hydroxyalkyl(meth)acrylamides (eg N-methylol(meth)acrylamide, N-hydroxyethyl(meth)acrylamide, etc.); N,N-dihydroxyalkyl(meth)acrylamides (eg N,N-dihydroxyethyl(meth)acrylamide); vinyl lactams (eg N-vinylpyrrolidone); esters containing an amino group (for example, dialkylaminoalkyl esters, dihydroxyalkylaminoalkyl esters, morpholino alkyl esters, etc.) of carboxylic acids (for example, dimethylaminoethyl (meth)acrylate, diethylaminoethyl (meth)acrylate, morpholinoethyl (meth)acrylate, dimethylaminoethyl fumarate, etc. .d.); heterocyclic vinyl compounds (eg 2-vinylpyridine, 4-vinylpyridine, N-vinylpyridine, N-vinylimidazole), etc.); monomers containing a quaternary ammonium salt group (e.g. N,N,N-trimethyl-N-(meth)acryloyloxyethylammonium chloride, N,N,N-triethyl-N-(meth)acryloyloxyethylammonium chloride, 2-hydroxy-3-(meth) acryloyloxypropyltrimethylammonium chloride, etc.), etc., as well as combinations of any of the above. In most aspects, monomeric compounds based on (meth)acrylic acid as well as its salts are used to form superabsorbent particles.

The monomeric compounds discussed above are generally soluble in water. However, it should be understood that compounds that can become water-soluble by hydrolysis can also be used. Suitable hydrolyzable monomers may include, for example, ethylenically unsaturated compounds containing at least one hydrolyzable radical such as ester, amide and nitrile groups. Specific examples of such hydrolyzable monomers include methyl (meth) acrylate, ethyl (meth) acrylate, 2-ethylhexyl (meth) acrylate, vinyl acetate, (meth) allyl acetate, (meth) acrylonitrile, etc. In addition, it should be understood that additional monomers may be used such that the resulting particles are formed as a copolymer, such as a random, graft or block copolymer. If necessary, the comonomer(s) may be selected from the group of monomers listed above. For example, the comonomer(s) may be (meth)acrylic acid, (meth)acrylic acid salt, maleic anhydride, etc. For example, in one particular aspect, the copolymer may be formed from acrylic acid (or a salt thereof) and maleic anhydride. In other aspects, described in more detail below, a comonomer that contains a crosslinkable functional group, such as an alkoxysilane group, can also be used. Regardless of the comonomer(s) used, it is generally desirable that the primary ethylenically unsaturated monomer(s) be at least about 50 mole %, in some aspects from about 55 mole % to about 99 mole %, and in some aspects from about 60 mol% to about 98 mol% of the monomers used to form the polymer, wherein the comonomer(s) is not more than about 60 mol%, in some aspects from about 1 mol% to about 50 mol%, and in some aspects, from about 2 mol.% to about 40 mol.% of the monomers used to form the polymer.

In order to form a network capable of absorbing water, it is generally desirable that the polymer be crosslinked during and/or after polymerization. For example, in one aspect, the ethylenically unsaturated monomeric compound(s) can be polymerized in the presence of a crosslinker to provide a crosslinked polymer. Suitable crosslinking agents typically contain two or more groups which are capable of reacting with the ethylenically unsaturated monomeric compound and are at least partially water-soluble or water-dispersible, or at least partially water-soluble or dispersible in an aqueous monomeric mixtures. Examples of suitable crosslinking agents may include, for example, tetraallyloxyethane, N,N'-methylenebisacrylamide, N,N'-methylenebismethacrylamide, triallylamine, trimethylol propane triacrylate, glycerol propoxy triacrylate, divinylbenzene, N-methylolacrylamide, N-methylol methacrylamide, glycidyl methacrylate, polyethylene polyamines, ethyl diamine, ethyl glycol, glycerin , tetraallyloxyethane and pentaerythritol triallyl ethers, aluminates, silicon dioxide, aluminosilicates, etc., as well as combinations thereof. The amount of crosslinker may vary, but is typically present in an amount of from about 0.005 to about 1.0 mole percent, based on moles of ethylenically unsaturated monomeric compound(s).

In the aspects described above, crosslinking generally occurs during polymerization. However, in other aspects, the polymer may contain a latent functional group that is capable of being cross-linked as needed. For example, the polymer may contain an alkoxysilane functionality which, when exposed to water, forms a silanol functionality which condenses to form a crosslinked polymer. One specific example of such a functional group is the trialkoxysilane group, which has the following general structure:

where R ₁ , R ₂ and R ₃ represent apkyl group, independently containing from 1 to 6 carbon atoms.

To introduce such a functional group into the polymer structure, a monomeric compound that contains a functional group, such as an ethylenically unsaturated monomer containing a trialkoxysilane functional group, can be used. Particularly suitable monomers are (meth)acrylic acids or salts thereof, such as methacryloxypropyltrimethoxysilane, methacryloxyethyltrimethoxysilane, methacryloxypropyltriethoxysilane, methacryloxypropyltripropoxysilane, acryloxypropylmethyldimethoxysilane, 3-acryloxypropyltrimethoxysilane, 3-methacryloxypropylmethyldiethoxysilane, 3-methacryloxypropylmethyldimethoxysilane, and propyl tris(methacryloxysilane) . In addition to copolymerizable monomers that contain a trialkoxysilane functional group, a copolymerizable monomer can also be used, which can then be reacted with a trialkoxysilane functional compound or moiety that reacts with water to form a silanol group. Such a monomer may contain, without limitation, an amino group or an alcohol group. The amino group included in the copolymer can then be reacted with, for example, but not limited to, (3-chloropropip)trimethoxysilane The alcohol group included in the copolymer can then be reacted with, for example, without limitation, tetramethoxysilane The superabsorbent polymer particles of the present invention can be prepared using any known polymerization method. For example, particles can be prepared using any suitable bulk polymerization technique such as solution polymerization, reverse suspension polymerization, or emulsion polymerization such as that described in US Pat. or 5,145,906. For example, in solution polymerization, the monomer(s) are polymerized in an aqueous solution. In reverse suspension polymerization, the monomer(s) are dispersed in a suspension medium based on an alicyclic or aliphatic hydrocarbon in the presence of a dispersing agent such as a surfactant or protective colloidal agent. If necessary, the polymerization reaction can be carried out in the presence of a free radical polymerization initiator, a redox polymerization initiator (reducing and oxidizing agents), a thermal polymerization 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 hydrosulfite, ammonium hydrosulfite, iron salts, such as ferrous sulfates, sugars, aldehydes, primary and secondary alcohols, and etc. Examples of suitable oxidizing agents may include, for example, hydrogen peroxide, caprylyl peroxide, benzoyl peroxide, cumene peroxide, tertiary butyl diperphthalate, tertiary butyl perbenzoate, sodium percarbonate, sodium peracetate, alkali metal persulfates, ammonium persulfates, alkyl hydroperoxides, peracid esters, diacrylic peroxides, silver salts, and etc.

If necessary, the obtained particles can also be reduced in size to achieve the desired size indicated above. For example, size reduction by impact, which typically uses a grinding device containing a rotating grinding element, can be applied with particle shaping. Repetitive impact and/or shear stress can be generated between the rotating grinding element and the stationary or counter-rotating grinding element. Downsizing by impact can also use airflow to move onto the chopping disc and impact the material with the chopping disc (or other shear element). One particularly suitable impact reduction device is commercially available from Pallmann Industries (Clifton, NJ) under the name TURBOFINER, type PLM. In this device, a highly active vortex air movement is created in the cylindrical grinding chamber between the stationary grinding element and the rotating grinding element of the impact mill. Due to the large volume of air, the particles can be impacted and the size can be reduced to the desired particle size. Other suitable impact reduction methods can be described in US Pat. Nos. 6,431,477 and 7,510,133, both issued to Pallmann. Another suitable method for forming microparticles is size reduction by cold extrusion, which typically uses shear and compressive forces to form particles of the desired size. For example, the material may be forced through the die at temperatures below the melting point of the matrix polymer. Solid phase shear pulverization is another suitable process that can be used. Such processes typically involve continuous extrusion of the material under high shear and compressive conditions while the extruder barrels and screw are cooled to prevent melting of the polymer. Examples of such solid state pulverizing techniques are described in, for example, US Pat. Nos. 5,814,673 to Khait; 6479003 issued to Furgiuele, et al.; 6494390 issued by Khait, et al.; 6818173 issued to Khait; and publication of US Patent Application No. 2006/0178465 to Torkelson, et al. Another suitable microparticle formation technique is the technique known as cryogenic disc milling. Cryogenic disc mill grinding typically uses a liquid (eg, liquid nitrogen) to cool or freeze the material prior to and/or during grinding. In one aspect, a single working disc grinder can be used that has a fixed disc and a rotating disc. Material enters the space between the disks through a channel near the center of the disk and is shaped into particles by means of frictional forces generated between the disks. One suitable device for grinding in a cryogenic disc mill is available under the name WEDCO cryogenic grinding system from ICO Polymers (Allentown, PA).

Although by no means necessary, it is also possible to combine additional components with the superabsorbent polymer before, during or after polymerization. For example, in one aspect, high aspect ratio inclusions (eg, fibers, tubes, plates, wires, etc.) can be used to help provide an internal interconnected reinforcing framework that stabilizes the swellable superabsorbent polymer and improves its resilience. For example, the aspect ratio (average length divided by median width) may range from about 1 to about 50, in some aspects from about 2 to about 20, and in some aspects from about 4 to about 15. Such inclusions may have a median width (e.g., diameter) from about 1 to about 35 micrometers, in some aspects from about 2 to about 20 micrometers, in some aspects from about 3 to about 15 micrometers, and in some aspects from about 7 to about 12 micrometers, as well as a volume average about 1 to about 200 micrometers long, in some aspects from about 2 to about 150 micrometers, in some aspects from about 5 to about 100 micrometers, and in some aspects from about 10 to about 50 micrometers. Examples of such high aspect ratio inclusions may include high aspect ratio fibers (also known as "whiskers") that are derived from carbides (eg silicon carbide), silicates (eg wollastonite), and the like.

Regardless of the particular method by which the particles are formed, a number of different techniques can be used to initiate the formation of the desired pore network. In certain aspects, control over the polymerization process itself can lead to the formation of pores in the resulting particles.

For example, the polymerization can be carried out in heterogeneous, two-phase or multi-phase systems with a monomer-rich continuous phase suspended in a solvent-rich discontinuous phase. As the monomer-rich phase begins to polymerize, pore formation can be induced by the solvent-rich phase. Of course, it is also possible to use techniques in which a pore network is formed in preformed particles. For example, in one particular aspect, a technique known as "phase inversion" can be used, in which a polymer dissolved or swollen in a system of solvents, which is a continuous phase, is converted into a solid macromolecular network, which is a continuous phase, formed by the polymer. This inversion can be induced by several methods, such as by removing the solvent by a dry method (eg evaporation or sublimation), adding a non-solvent, or additionally to the non-solvent by a wet method. For example, in dry processes, the temperature (or pressure) of the particles can be changed so that the solvent system (eg, water) can be converted to a different state of matter that can be removed without excessive shrinkage, either by vacuum or gas purge. Freeze drying, for example, involves cooling the solvent system below its freezing point and then allowing it to sublimate under reduced pressure such that pores are formed. On the other hand, supercritical drying involves heating the solvent system at a pressure above the supercritical point so that pores are formed.

However, wet methods are particularly suitable because they do not depend on significant amounts of energy to achieve the required inversion. In the wet process, the superabsorbent polymer and solvent system can be provided in the form of a single phase homogeneous composition. The polymer concentration typically ranges from about 0.1% to about 20% w/v, and in some aspects from about 0.5% to about 10% w/v of the composition. The composition is then brought into contact with the precipitant system using any known technique such as immersion bath, countercurrent washing, jet washing, ribbon rinsing and filtration. The chemical potential difference between the solvent and precipitant systems causes the solvent molecules to diffuse out of the superabsorbent polymer, while the precipitant molecules diffuse into the polymer. Ultimately, this causes the transition of the polymer composition from a single-phase homogeneous composition to an unstable two-phase mixture containing polymer-rich and polymer-depleted fractions. The micellar droplets of the precipitant system in the polymer-rich phase also serve as nucleation sites and become coated with the polymer, and at some point, such droplets precipitate to form a continuous polymer network. The composition of solvents inside the polymer matrix also breaks down and forms voids. The matrix can then be dried to remove the solvent and precipitant systems and form stable porous particles.

The exact solvent and precipitant systems used to achieve the phase inversion are not particularly critical, provided they are chosen consistently based on their miscibility. More specifically, solvent and precipitant systems can be chosen such that they have a particular difference in their Hildebrand solubility parameters, 8, which is a predictor of the miscibility of two liquids, with generally higher values suggesting a more hydrophilic liquid and lower values provide a more hydrophobic liquid. In general, it is desirable that the difference between the Hildebrand solubility parameters of the solvent system and the precipitant system (e.g., δ _solvent - ^δ _precipitants ) be from about 1 to about 15 ^cal . ^1/2 /cm ^3/2 and in some aspects from about 6 to about 10 calories ^1/2 /cm ^3/2 . In such ranges, the solvent/non-solvent will have sufficient miscibility to allow solvent extraction to proceed, but not too miscible such that a phase inversion does not occur. Suitable solvents for use in the solvent system may include, for example, water, aqueous alcohol, saline, glycerin, etc., as well as combinations thereof. Similarly, suitable non-solvents for use in the non-solvent system may include acetone, n-propyl alcohol, ethyl alcohol, methanol, n-butyl alcohol, propylene glycol, ethylene glycol, etc., as well as combinations thereof. Typically, the volume ratio of the solvent system to the precipitant system is in the range of from about 50:1 to about 1:200 (volume per volume), in some aspects from about 1:60 to about 1:150 (volume per volume), in some aspects about 1:1 to about 1:150 (v/v), in some aspects about 50:1 to about 1:60 (v/v), in some aspects about 10:1 to about 1:10 ( volume per volume), in some aspects from about 10:1 to about 1:2 (volume per volume), in some aspects from about 10:1 to about 1:1 (volume per volume), and in some aspects from about 1: 1 to approximately 1:2 (volume by volume). The amount of solvents used can be an important factor influencing the economics of the process.

After contact with the precipitant and completion of the phase inversion, drying/removal of the liquid phase is an important step in the preparation of materials. Typically, this involves any suitable drying technique, including one or more of temperature rise, time control, vacuum, and flow rates, using any suitable equipment, including forced air ovens and vacuum ovens.

Any precipitant that resides within the particle may be removed by any suitable method, including by increasing the moisture content of the particle at elevated temperatures. In one example, drying at high temperature at temperatures not exceeding 175°C may leave no more than 16 wt.% ethanol in the sample. The sample can then be placed in a humidification chamber at 69° C. at 50% relative humidity to reduce the ethanol content to less than 0.13%. Removal of residual solvent is beneficial to product safety.

In various aspects, the superabsorbent particles can be surface crosslinked with a surface crosslinker. The surface crosslinking treatment can provide high gel strength based on superabsorbent particles and improve the balance between CRC and GBP.

Any conventional surface crosslinkers (polyhydric glycidyls, polyhydric alcohols, polyhydric amines, polyhydric aziridines, polyhydric isocyanates, silane coupling agent, alkylene carbonate, polyvalent metals, etc.) can be used as surface crosslinkers. Among such surface crosslinkers, in view of economic efficiency and absorption characteristics, the surface crosslinker is preferably a polyhydric glycidyl, a polyhydric alcohol, or a polyhydric amine. Surface crosslinkers can be used alone or as a mixture of two or more types.

If surface crosslinking treatment is carried out, the amount (% by weight) of the surface crosslinker used is not particularly limited, since the amount may vary depending on the kind of surface crosslinker, crosslinking conditions, target efficiency, and the like. Considering absorption characteristics, 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 based on the weight of the superabsorbent particle.

The surface crosslinking treatment is carried out by mixing the superabsorbent particles with the surface crosslinking agent or agents, followed by heating. Suitable methods are described in more detail in Japanese Patent No. 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 they do not conflict with this document. The blending of the superabsorbent polymer with the surface crosslinker can be carried out using any suitable equipment, including any conventional equipment (drum-type mixer, screw-type mixer, screw-type extruder, turbulator, Nauta mixer, kneader, flow-type mixer, V-shaped mixer, shredder, ribbon mixer, airflow type mixer, disc type mixer, cone mixer, roller mixer). The surface crosslinker can be diluted with water and/or solvents.

The temperature at which the superabsorbent particles and the surface crosslinker are mixed is not particularly limited. The temperature for mixing the superabsorbent particles with the surface crosslinker is preferably 10 to 150°C, more preferably 20 to 100°C and most preferably 25 to 80°C.

Surface crosslinking of the superabsorbent particle can be carried out by heating after mixing with the surface crosslinker. 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 depending on the temperature. In the context of absorption efficiency, the surface crosslinking time is preferably 5 to 60 minutes, and more preferably 10 to 40 minutes.

The surface crosslinking of the superabsorbent particles can be carried out before and/or after the phase inversion process. In the context of avoiding aggregation of the superabsorbent particles during the phase inversion process, surface crosslinking is preferably carried out prior to the phase inversion process. Also, in the context of the balance between CRC and GBP, surface crosslinking is preferably carried out after the phase inversion process. Surface crosslinking is preferably carried out before and after the phase inversion process depending on the crosslinking priority.

EXAMPLES

The present invention may be better understood with reference to the following examples.

Test Methods

Pore properties

Pore properties (eg, mean pore diameter, total pore area, bulk density, pore size distribution, and percent porosity) of superabsorbent particles can be determined using mercury porosimetry (also known as mercury intrusion), well known in the art. For example, a commercially available porosimeter such as the AutoPore IV 9500 from Micrometrics can be used. Such devices generally characterize porosity by applying different levels of pressure to a sample immersed in mercury. The pressure required to inject mercury into the pores of the sample is inversely proportional to the pore size. Measurements can be made at a reference pressure of 0.58 psi. inch and at a final pressure of approximately 60,000 psi. inch. Mean pore diameter, total pore area and bulk density can be directly measured during the mercury injection test. The overall pore size distribution can be obtained from a differential plot of intrusion versus pore diameter (µm). Similarly, percent porosity can be calculated based on the reduction in bulk density (assuming constant particle size, packing, and shape), given that approximately 50% of the volume is occupied by void space due to particle packing. More specifically, percentage porosity can be determined according to the following equation:

100 × 0.5 × [(BVD of the control sample - BVD of the test sample)/BVD of the control sample],

where the bulk density (g/cm ³ ) is determined using the introduction of mercury at a pressure of 0.58 psi. inch.

absorbency

The absorbency of superabsorbent particles can be measured using the Absorbency Under Load (“AUL”) test, which is a well-known test for measuring the ability of superabsorbent particles to absorb a 0.9 wt% solution of sodium chloride in distilled water at room temperature (a solution used during testing), while the material is under load. For example, 0.16 grams of superabsorbent particles can be held within a 5.07 cm area of an absorbency under load ("AUL") cylinder at a nominal pressure of 0.01 psi. inch, 0.3 psi inch or 0.9 psi inch. Allow the sample to soak up the test solution from a container containing excess fluid. At predetermined time intervals, the sample is weighed after any excess of injected fluid in the cylinder has been removed by the vacuum apparatus. This weight versus time data is then used to determine absorption rate values at various time intervals.

As can be seen from FIG. 1, for example, shows one aspect of apparatus 910 that can be used to determine absorbency. Apparatus 910 includes an AUL measurement assembly 925 comprising a cylinder 920, a piston 930, and a weight 990. Weight 990 may be a 100 gram weight. A side arm flask 960 can be used that is fitted with a rubber stopper 945 and 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 can be used for sidearm bulb 960 and AUL measurement chamber 940. Additional tubing 970 may also be used to connect a vacuum source (not shown) to side arm 980 of bulb 960. As seen in FIG. 2, a cylinder 920 can be used to contain the superabsorbent particles 950 and can be made from a one inch ID acrylic tube lightly machined to provide concentricity. After machining, the mesh fabric 414 (eg, 400 mesh) can be attached to the bottom of the cylinder 920 using a suitable solvent that ensures the screen adheres tightly to the cylinder. Piston 930 may be a 4.4 g piston made from a 1 inch (2.5 cm) diameter hard material (e.g., acrylic) and may be machined to fit snugly without becoming stuck in cylinder 920. As above, the apparatus 910 also includes an AUF measurement chamber 940 which removes the embedded liquid absorbed during the swelling of the superabsorbent particles 950. This test apparatus is similar to the GATS (Gravimetric Absorption Test System) available from described by Lichstein on pages 129-142 in the INDA Technical Symposium Papers, March 1974. A holed disk 935 is also used, which has holes located in an area of 2.5 centimeters in diameter.

For testing, the following steps can be carried out:

(1) wipe the inside of the AUL measuring cylinder 920 with an antistatic cloth, and weigh the cylinder 920, weight 990, and piston 930;

(2) record the weight as CONTAINER WEIGHT in grams to the nearest value in milligrams;

(3) slowly pour a 0.16±0.005 gram sample of superabsorbent particles 950 into cylinder 920 such that the particles do not contact the sides of the cylinder or otherwise may adhere to the walls of the AUL cylinder;

(4) weigh cylinder 920, weight 990, piston 930, and superabsorbent particles 950 and record the value on the balance as DRY WEIGHT in grams to the nearest value in milligrams;

(5) gently press the AUL measuring cylinder 920 until the superabsorbent particles 950 are evenly distributed on the bottom of the cylinder;

(6) gently place piston 930 and weight 990 into cylinder 920;

(7) placing the test fluid (0.9 wt% aqueous sodium chloride solution) in a fluid bath with a large mesh screen at the bottom;

(8) simultaneously start a timer and place the superabsorbent particles 950 and cylinder assembly 925 on the screen in the fluid bath. The level in the bath should be at a height sufficient to provide at least 1 cm of positive free space above the base of the cylinder;

(9) gently agitate the sample to release any entrapped air and ensure that the superabsorbent particles are in contact with the fluid;

(10) remove the cylinder 920 from the fluid bath at a specified time interval and immediately place the cylinder in a vacuum apparatus (perforated disk 935 on top of the AUL measurement chamber 940) and remove excess injected fluid within 10 seconds;

(11) wipe the outside of the cylinder with a paper towel or cloth;

(12) immediately weigh the AUL measurement assembly (i.e., cylinder 920, piston 930, and weight 990) with the superabsorbent particles and any imbibed test fluid and record the weight as WET WEIGHT in grams to the nearest value in milligrams and interval time and

(13) is repeated for all required time intervals.

At least two (2) samples in total were tested at each predetermined time interval. 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 "absorbency" of the superabsorbent particles in the indicated time interval is calculated in grams of liquid per gram of superabsorbent material using the following formula:

Absorption rate

The "absorption rate" of superabsorbent particles can be determined in a designated time interval by dividing the absorbency (g/g) described above by the specific time interval (kilosecond, ks) of interest, such as 0.015, 0.030, 0.060, 0.120, 0.300, 0.600, 1.8 or 3.6 kiloseconds.

Centrifugation Retention Rate (CRC)

The Centrifuge Retention (CRC) test measures the ability of superabsorbent particles to retain liquid after being saturated and subjected to centrifugation under controlled conditions.

The retention capacity obtained is expressed as grams of retained fluid per gram of sample weight (g/g). The sample to be tested is prepared from particles previously sieved through a 30 mesh US standard sieve and held on a 50 mesh US standard sieve. Particles may be pre-sieved manually or automatically and stored in a sealed container until testing. The holding power is measured by placing 0.2 ± 0.005 grams of the pre-sieved sample in a permeable bag that will contain the sample, while allowing the sample to freely absorb the test solution (0.9 weight percent sodium chloride in distilled water). A heat sealable tea bag material such as model designation 1234T heat seal filter paper may be suitable. The pouch is formed by folding a 5 inch by 3 inch sample of pouch material in half and heat sealing the two open edges to form a 2.5 inch by 3 inch rectangular pouch. Bonded seams may be approximately 0.25 inches inside the edge of the material. After placing the sample inside the bag, the remaining open edge of the bag can also be thermally sealed. Empty pouches can also be made to serve as reference samples. Three samples were prepared for testing (eg filled and sealed sachets). Filled sachets are tested within three minutes of receipt, unless they are immediately placed in a sealed container, in which case the filled sachets must be tested within thirty minutes of receipt.

The sachets are placed between two ^TEFLON® coated 3 inch glass fiber screens (Taconic Plastics, Inc., Petersburg, NY) and immersed in a vessel of test solution at 23°C, making sure the screens are held until until the sachets are completely moistened. After wetting, the samples remain in the solution for approximately 30 ± 1 minutes, after which they are removed from the solution and temporarily placed on a non-absorbent flat surface. For several tests, the vial must be emptied and refilled with fresh test solution after saturation of 24 sachets in the vial.

The wet sachets are then placed in a basket of a suitable centrifuge capable of subjecting the samples to an overload of approximately 350 g. One suitable centrifuge is the Heraeus LaboFuge 400, which contains a water collection basket, a digital tachometer, and a machined drain basket configured to hold samples in bags and drain solution from them. If multiple samples are being centrifuged, the samples can be placed in opposite positions in the centrifuge to balance the basket as it rotates. The sachets (including wet empty sachets) are centrifuged at approximately 1600 rpm (eg to achieve an overload of approximately 350 g) for 3 minutes. The sachets are removed and weighed, with the empty sachets (controls) being weighed first, followed by the weighing of the sachets containing the samples. The amount of solution retained by the sample, taking into account the solution retained by the bag itself, is the centrifuge retention capacity (CRC) of the sample, expressed as grams of fluid per gram of sample. More specifically, centrifugation retention is defined as:

Weight of sample bag after centrifugation - Weight of empty bag after centrifugation - Weight of dry sample

Dry sample weight

Three samples are tested and the results are averaged to determine the retention capacity (CRC) of the superabsorbent material. Samples are tested at 23°C and 50% relative humidity.

Whirlwind stop time

The vortex stop time is the amount of time, in seconds, required for a predetermined mass of superabsorbent particles to stop the vortex created by agitating 50 milliliters of 0.9 weight percent sodium chloride solution at 600 rpm on a magnetic stirrer plate. The time it takes for the vortex to stop is indicative of the free-swell soak rate for the particles. The vortex stop time test can be performed at 23°C and 50% relative humidity according to the following procedure.

(1) Measure 50 milliliters (±0.01 milliliters) of 0.9 percent by weight sodium chloride solution into a 100 milliliter beaker.

(2) Place a 7.9 mm × 32 mm TEFLON® coated magnetic stirrer arm without rings (such as the armature commercially available under the brand name S/P® for one package of round stirrer arms with a removable support ring) in a beaker. .

(3) Set the program of the magnetic stirrer plate (such as the plate commercially available under the trade name DATAPLATE® Model No. 721) to 600 rpm.

(4) Place the beaker in the center of the magnetic stirrer plate so that the magnetic stirrer armature is activated. The bottom of the vortex should be near the top of the agitator armature. The superabsorbent particles are pre-screened through a 30 mesh US sieve (0.595 mm openings) and held on a 50 mesh US sieve (0.297 mm openings).

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

(6) While stirring the sodium chloride solution, quickly pour the absorbent polymer to be tested into the saline solution and start the stopwatch. The superabsorbent particles to be tested must be added to the saline solution between the center of the vortex and the wall of the beaker.

(7) Stop the stopwatch when the surface of the brine becomes flat and record the time. The time recorded in seconds is given as the stop time of the vortex.

Free-swell gel layer permeability test (GBP)

As used herein, the free swell gel layer (GBP) test measures the permeability of a swollen layer of superabsorbent material under conditions commonly referred to as "free swell" conditions. The term "free swelling" means that the superabsorbent material is allowed to swell without a swelling-limiting load during the absorption of the test solution, as will be described. This test is described in Qin US Patent Publication No. 2010/0261812, which is incorporated herein by reference. For example, a test apparatus may be used that includes a sample container and a piston that may include a LEXAN cylindrical shaft containing a concentric cylindrical hole drilled down the longitudinal axis of the shaft. Both ends of the shaft can be machined to provide top and bottom ends. The weight may be at one end which contains a cylindrical hole drilled through at least a portion of its center. A round piston head may be positioned at the other end and provided with a concentric inner ring of seven holes each having a diameter of approximately 0.95 cm and a concentric outer ring of fourteen holes each having a diameter of approximately 0.95 cm. from the top to the bottom of the piston head. The bottom of the piston head can also be covered with a biaxially stretched stainless steel mesh screen. The sample container may comprise a cylinder and a 100 mesh stainless steel screen that is biaxially stretched to tension and attached to the lower end of the cylinder. The superabsorbent particles may be maintained on the screen in the cylinder during the test.

The cylinder may be bored from a transparent LEXAN rod or equivalent material, or it may be cut from a LEXAN tubing or equivalent material, and it has an internal diameter of approximately 6 cm (for example, a cross-sectional area of approximately 28.27 cm ² ), a wall thickness of approximately 0 .5 cm and a height of about 5 cm. Drain holes may be formed in the side wall of the cylinder at a height of about 4.0 cm above the screen to allow liquid to drain from the cylinder, thereby maintaining the fluid level in the sample container at about 4.0 cm. 0 cm above the screen. The piston head can be made from a LEXAN rod or equivalent and is approximately 16 mm high and has a diameter such that it fits into the cylinder with minimal wall clearance, yet still moves freely. The shaft may be made 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 top end of the shaft is approximately 2.54 cm long and approximately 1.58 cm in diameter, forming an annular shoulder to support the ring weight. The annular weight, in turn, has an internal diameter of approximately 1.59 cm, so that it moves to the upper end of the shaft and remains on the annular shoulder formed thereon. The ring weight may be made of stainless steel or other suitable materials resistant to corrosion in the presence of the test solution, which is a 0.9% by weight solution of sodium chloride in distilled water. The combined weight of the piston and ring weight is about 596 grams, which corresponds to a pressure applied to the sample of about 0.3 psi or about 20.7 dynes/cm ² , for a sample area of about 28.27 cm ² . With the test solution flowing through the test apparatus during the test described below, the sample container rests as a whole on a rigid 16 mesh stainless steel support screen. Alternatively, the sample container may rest on a support ring substantially the same diameter as the cylinder, such that the support ring does not impede flow from the bottom of the container.

To perform the gel layer permeability test under “free swelling” conditions, place the plunger with weight on it in an empty sample container and measure the height from the bottom of the weight to the top of the cylinder using a caliper or suitable measuring device to an accuracy of 0.01 mm. The height of each sample container can be measured empty, and which plunger and weight is used can be tracked when multiple testers are used. The same piston and weight can be used to measure the subsequent swelling of the sample after saturation. The sample to be tested is prepared from superabsorbent particles previously sieved through a 30 mesh US standard sieve and retained on a 50 mesh US standard sieve. Particles can be pre-sifted manually or automatically. Approximately 0.9 grams of the sample is placed in the sample container, and the container, without the plunger and weight in it, is then immersed in the test solution for a period of approximately 60 minutes to ensure that the sample is saturated and that the sample swells without any limiting load. After this period, the piston and weight assembly is placed on the saturated sample in the sample container and then the sample container, piston, weight and sample are removed from the solution. The thickness of the saturated sample is determined by measuring again the height from the bottom of the weight to the top of the cylinder using the same caliper or feeler gauge as used previously, provided that the zero point has not been changed from the original height measurement. The height measurement obtained from the measurement of the empty sample container, plunger and weight is subtracted from the height measurement obtained after the sample has been saturated. The resulting value is the thickness or height "H" of the swollen sample.

The measurement of permeability is started by introducing the flow of the test solution into a sample container with a saturated sample, a plunger and a weight inside. The flow rate of the test solution in the container is adjusted to maintain the fluid height approximately 4.0 cm above the bottom of the sample container. The amount of solution passing through the sample as a function of time is measured gravimetrically. Data points are collected every second for at least twenty seconds once the fluid level has stabilized and is maintained at a height of approximately 4.0 cm. The flow rate, Q, through the swollen sample is determined in units of grams/second (g/s) by linear least-squares approximation of fluid flow through the sample (in grams) versus time (in seconds). The permeability is obtained using the following equation:

where

K = permeability (darcy),

Q = flow rate (g/s),

H = sample height (cm),

Mu = fluid viscosity (poise) (approximately 1 centipoise for the test solution used in this test),

A = cross-sectional area for fluid flow (cm),

Po = liquid density (g/cm ³ ) (about 1 g/cm ³ for the test solution used in this test) and

P = hydrostatic pressure (dynes/cm ² ) (typically about 3923 dynes/cm ² ) which can be calculated from Po*g*h where Po = fluid density (g/cm ³ ), g = gravitational acceleration, nominally 981 cm/sec. ² , and h = liquid height, for example 4.0 cm.

A minimum of three samples were tested and the results were averaged to determine the permeability of the free-swelling gel layer for the sample. Samples are tested at 23°C and 50% relative humidity.

Example 1

Initially, 15.00 grams of commercially available superabsorbent crosslinked polyacrylate particles were provided, which had an initial vortex stop time of 35 seconds, a CRC of 27.5 g/g, and a GBP of 48 darcy. The particles were swollen in excess of a good solvent (ie saline) for 60 minutes to achieve equilibrium swelling. The excess brine was then drained off and the embedded fluid was removed using a vacuum filtration technique. The vacuum filtration system included a Buchner funnel, moistened filter paper, a Buchner flask, a rubber stopper, and a vacuum tube. The swollen superabsorbent particles were then manually transferred into 1 kg of ACS high purity methanol with constant stirring in a 2 L Pyrex glass beaker. Stirring was carried out using a magnetic stirrer arm with dimensions L=5 cm, D=0.9 cm and a stirring speed of 800-1000 rpm. After 30 minutes, the solvent mixture was decanted and another 1 kg of fresh methanol was added to the superabsorbent particles. After 30 minutes, the solvent mixture was decanted again and the superabsorbent particles were transferred to Teflon Petri dishes and dried for 1 hour in a forced air oven at 85°C. The superabsorbent particles were then transferred to a vacuum oven with complete drying and removal of residual methanol. Drying was carried out at a temperature of 120-140°C and a pressure of 30 inches of mercury. within 4 hours. The dried superabsorbent particles were then adjusted using a set of sieves with a mesh size of 45-850 microns. Particles with a size of 300-600 microns in diameter were collected for additional evaluation.

Example 2

Particles were formed as described in Example 1, except that ACS 200 proof high purity ethanol was used during the solvent/non-solvent exchange step.

Example 3

Particles were formed as described in Example 1, except that isopropyl alcohol was used during the solvent/non-solvent exchange step.

Example 4

Particles were formed as described in Example 1, except that acetone was used during the solvent/non-solvent exchange step.

Also determined the various properties of the pores for examples 1-4 using the test described above. The pore size distribution for the samples is shown in FIG. 4-8 and the results are presented in the table below.

Example 5

The particles were formed as described in example 2, except that the particles were initially swollen in 5 wt.% sodium chloride solution.

Example 6

The particles were formed as described in example 2, except that the particles were initially swollen in 10 wt.% sodium chloride solution.

Example 7

The particles were formed as described in example 2, except that the particles were initially swollen in 15 wt.% sodium chloride solution.

Example 8

The particles were formed as described in example 2, except that the particles were initially swollen in a 20 wt.% solution of sodium chloride.

Example 9

Particles were formed as described in example 2, except that the particles were initially swollen in a 30 wt.% solution of high purity ethanol, corresponding to the ACS standard, 200 proof in deionized water.

Example 10

Particles were formed as described in example 2, except that the particles were initially swollen in a 40 wt.% solution of high purity ethanol, corresponding to the ACS standard, 200 proof in deionized water.

Example 11

Particles were formed as described in Example 2, except that the particles were initially swollen in a 50 wt.% solution of high purity ethanol, corresponding to the ACS standard, 200 proof in deionized water.

Example 12

Particles were formed as described in example 2, except that the particles were initially swollen in a 60 wt.% solution of high purity ethanol, corresponding to the ACS standard, 200 proof in deionized water.

Example 13

Particles were formed as described in example 2, except that the particles were initially swollen in 80 wt.% solution of high purity ethanol, corresponding to the ACS standard, 200 proof in deionized water.

Example 14

Particles were formed as described in Example 1, except that the solvent/non-solvent exchange time was reduced from 30 minutes to 15 minutes per step.

Example 15

Particles were formed as described in Example 1, except that the solvent/non-solvent exchange time was reduced from 30 minutes to 5 minutes per step.

Example 16

The particles were formed as described in example 1, except that the amount of methanol was reduced from 1 kg to 0.5 kg per stage.

Example 17

Particles were formed as described in Example 16, except that the solvent/poor solvent change time was reduced from 30 minutes to 15 minutes.

Example 18

Particles were formed as described in Example 16, except that the solvent/poor solvent change time was reduced from 30 minutes to 5 minutes.

Example 19

15.00 grams of the same superabsorbent particles provided in Example 1 were manually transferred into 1 kg of ACS high purity methanol with constant stirring in a 2 L Pyrex glass beaker. Stirring was carried out using a magnetic stirrer arm with dimensions L=5 cm, D=0.9 cm and a stirring speed of 800-1000 rpm. After 30 minutes, the solvent mixture was decanted and another 1 kg of fresh methanol was added to the superabsorbent particles. After 30 minutes, the solvent mixture was decanted again and the superabsorbent particles were transferred to Teflon Petri dishes and dried for 1 hour in a forced air oven at 85°C. The superabsorbent particles were then transferred to a vacuum oven with complete drying and removal of residual methanol. Drying was carried out at a temperature of 120-140°C and a pressure of 30 inches of mercury. within 4 hours. The dried superabsorbent particles were then adjusted using a set of sieves with a mesh size of 45-850 microns. Particles with a size of 300-600 microns in diameter were collected for additional evaluation.

Example 20

Particles were formed as described in Example 19, except that high purity ethanol was used to wash the superabsorbent particles.

Example 21

Particles were formed as described in Example 19, except that high purity isopropyl alcohol was used to wash the superabsorbent particles.

Example 22

Particles were formed as described in Example 19, except that high purity acetone was used to wash the superabsorbent particles.

Example 23

While stirring 100 grams of the obtained superabsorbent particles in Example 1 at high speed (using a "high-speed stirring turbulator" manufactured by Hosokawa Micron Corporation at a speed of 2000 rpm), a mixed liquid obtained by mixing 0.14 grams of ethylene glycol diglycidyl ether, 3 .43 grams of propylene glycol and 3.43 grams of water, by spraying and mixing evenly. The mixture was then allowed to stand at 150°C for 30 minutes to complete the surface crosslinking, thereby ensuring the formation of superabsorbent particles.

Example 24

Particles were formed as described in Example 23, except that superabsorbent particles were used to effect surface crosslinking, which were formed as described in Example 2.

Example 25

Particles were formed as described in Example 23, except that superabsorbent particles were used to effect surface crosslinking, which were formed as described in Example 16.

Example 26

Particles were formed as described in Example 23, except that the amount of ethylene glycol diglycidyl ether was reduced to 0.10 grams.

Example 27

Particles were formed as described in Example 23, except that the amount of ethylene glycol diglycidyl ether was reduced to 0.07 grams.

Samples from Examples 1-22 were tested for vortex stop time and CRC as discussed above. The results are outlined below.

Samples from Examples 1, 2, 16 and 23-27 were tested for vortex stop time, CRC and GBP as discussed above. The results are outlined below.

The superabsorbent particles of Example 1 were also tested for AUL (at 0.01 psi) before and after the solvent change procedure. The results obtained in relation to the properties are presented below.

In FIG. 3A-3F are also SEM micrographs showing the particles before and after the solvent change procedure. As indicated, the replacement of the solvent leads to the formation of particles containing a pore network, which includes many nanopores.

Although the present invention has been described in detail with respect to its specific aspects, it should be understood that those skilled in the art, upon reading and understanding the foregoing, will be fully aware of the alternatives, variations, and equivalents of these aspects. Accordingly, the scope of the present invention is to be defined as the scope of the appended claims and any equivalents thereof.

Claims (24)

1. A superabsorbent material for absorbing fluids comprising superabsorbent particles having a median size of 50 to 2000 micrometers and containing a pore network that includes a plurality of nanopores having an average cross-sectional size of 10 to 500 nanometers, wherein the superabsorbent particles characterized by a vortex stop time of 80 seconds or less and a gel bed free swelling permeability (GBP) ranging from 5 darci to 100 darci, where the particles are formed from a crosslinked polymer that contains repeating units derived from one or more ethylenically unsaturated monomeric compounds containing at least one hydrophilic radical. 2. The superabsorbent material of claim 1, wherein the particles have a GBP in the range of 10 Darcy to 100 Darcy. 3. The superabsorbent material of claim 1, wherein the particles have a GBP in the range of 20 Darcy to 100 Darcy. 4. The superabsorbent material of claim 1 wherein the particles have a GBP in the range of 60 Darcy to 100 Darcy. 5. The superabsorbent material of claim 1 wherein the particles have a GBP in the range of 90 Darcy to 100 Darcy. 6. The superabsorbent material of claim 1, wherein the particles have an absorption rate of 300 g/g/cc or more after contact with a 0.9 wt% aqueous sodium chloride solution for 0.015 kilosecond. 7. The superabsorbent material of claim 1, wherein the superabsorbent particles have an absorbency rate of 500 g/g/ks or more after contact with a 0.9 wt% aqueous sodium chloride solution for 0.015 kilosecond. 8. The superabsorbent material of claim 1, wherein the superabsorbent particles have an absorbency rate of 160 g/g/ks or more after being contacted with a 0.9 wt% aqueous sodium chloride solution for 0.120 kilosecond. 9. The superabsorbent material of claim 1, wherein the particles have a centrifugation retention of 20 g/g or more. 10. The superabsorbent material of claim 1, wherein the pore network further comprises micropores. 11. The superabsorbent material of claim 1 wherein at least 25% by volume of the pore network is formed by nanopores. 12. The superabsorbent material of claim 1 wherein the monomeric compounds are monoethylenically unsaturated compounds. 13. The superabsorbent material of claim 1, wherein the particles have a total pore area of 2 square meters per gram or more. 14. The superabsorbent material of claim 1, wherein the particles have a percentage porosity of 5% or more. 15. A method for forming superabsorbent particles according to any one of the preceding claims, wherein the method includes: formation of a composition that contains a superabsorbent polymer and a solvent system; bringing the composition into contact with the system of precipitators to initiate the formation of a pore network through phase inversion; removing the precipitant from the composition; and surface crosslinking of superabsorbent particles; where the difference between the Hildebrand solubility parameter of the solvent system and the Hildebrand solubility parameter of the precipitant system is from 1 to 15 calories ^1/2 /cm ^3/2 . 16. The method according to p. 15, where the composition is a single-phase homogeneous composition. 17. The method of claim 15, wherein upon contact with the precipitant system, a two-phase mixture is formed. 18. The method of claim 15 wherein the solvent system comprises water, aqueous alcohol, saline, glycerin, or a combination thereof. 19. The method of claim 15 wherein the precipitant system comprises acetone, n-propyl alcohol, ethyl alcohol, methanol, n-butyl alcohol, propylene glycol, ethylene glycol, or a combination thereof.

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