Classifications

• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/42—Use of materials characterised by their function or physical properties
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/20—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons containing organic materials
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61L—METHODS OR APPARATUS FOR STERILISING MATERIALS OR OBJECTS IN GENERAL; DISINFECTION, STERILISATION OR DEODORISATION OF AIR; CHEMICAL ASPECTS OF BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES; MATERIALS FOR BANDAGES, DRESSINGS, ABSORBENT PADS OR SURGICAL ARTICLES
  • A61L15/00—Chemical aspects of, or use of materials for, bandages, dressings or absorbent pads
  • A61L15/16—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons
  • A61L15/42—Use of materials characterised by their function or physical properties
  • A61L15/60—Liquid-swellable gel-forming materials, e.g. super-absorbents
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F8/00—Chemical modification by after-treatment
  • C08F8/30—Introducing nitrogen atoms or nitrogen-containing groups
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J3/00—Processes of treating or compounding macromolecular substances
  • C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
  • C08J3/245—Differential crosslinking of one polymer with one crosslinking type, e.g. surface crosslinking
• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
  • C08J2300/00—Characterised by the use of unspecified polymers
  • C08J2300/14—Water soluble or water swellable polymers, e.g. aqueous gels

Definitions

• the present invention relates to surface- crosslinked superabsorbent polymer particles, and to methods of producing the surface-crosslinked super- absorbent particles.
• the present invention also relates to the use of the surface-crosslinked particles in articles, such as diapers, catamenial devices,- and wound dressings. More particularly, the present invention relates to surface treating superabsorbent polymer (SAP) particles, such as a neutralized, crosslinked, homopolymer or copolymer of acrylic acid, with an oxazolinium ion to substantially improve the water absorption and water re- tention properties of the SAP particles.
• SAP superabsorbent polymer
• Water-absorbing resins are available in a variety of chemical forms, including substituted and unsubstituted natural and synthetic polymers, such as hydrolysis products of starch acrylonitrile graft polymers, carboxymethylcellulose, crosslinked polyacrylates, sulfonated polystyrenes, hydrolyzed polyacrylamides, polyvinyl alcohols, polyethylene oxides, polyvinylpyrrolidones, and polyacryloni- triles .
• Such water-absorbing resins are termed
• SAPs "superabsorbent polymers," or SAPs, and typically are lightly crosslinked hydrophilic polymers. SAPs are generally discussed in Goldman et al . U.S. Patent Nos. 5,669,894 and 5,559,335, the disclosures ' of which are incorporated herein by reference. SAPs can differ in their chemical identity, but all SAPs are capable of absorbing and retaining amounts of aqueous fluids equivalent to many times their own weight, even under moderate pressure. For example, SAPs can absorb one hundred times their own weight, or more, of distilled water. The ability to absorb aqueous fluids under a confining pressure is an important requirements for an SAP used in a hygienic article, such as a diaper.
• SAP particles refers to superabsorbent polymer particles in the dry state, i.e., particles- containing from no water up to an amount of water less than the weight of the particles.
• particles re- fers to granules, fibers, flakes, spheres, powders, platelets, and other shapes and forms known to persons skilled in the art of superabsorbent polymers.
• SAP gel and “SAP hydrogel” refer to a superabsorbent polymer in the hydrated state, i.e., particles that have absorbed at least their weight in water, and typically several times their weight in water.
• surface-treated SAP particle and “surface-crosslinked SAP particle” refer to an SAP particle having its molecular chains present in the vicinity of the particle surface crosslinked by a compound applied to the surface of the particle.
• surface crosslinking means that the level of functional crosslinks in the SAP particle in the vicinity of the surface of the particle is generally higher than the level of functional crosslinks in the SAP particle in the interior of the particle.
• SAP particles can differ in ease and cost of manufacture, chemical identity, physical properties, rate of water absorption, and degree of water absorption and retention, thus making the ideal water-absorbent resin a difficult composition to design.
• the hydrolysis products of starch-acrylonitrile graft polymers have a comparatively high ability to absorb water, but require a cumbersome process for production and have the disadvantages of low heat resistance and decay or de- composition due to the presence of starch.
• other water-absorbent polymers are easily and cheaply manufactured and are not subject to decomposition, but do not absorb liquids as well as the starch-acrylonitrile graft polymers. Therefore, it would be extremely advantageous to provide a method of increasing the water absorption properties of a stable, easy to manufacture SAP particles to match the superior water absorption properties of a difficult to manufacture particle. Likewise, it would be advantageous to further increase the liquid absorption properties of already superior SAP particles.
• conventional SAP particles all have a serious defect in that their rates of liquid absorption are lower than fluff pulp and paper.
• the urine when urine is excreted on a disposable diaper containing conventional SAP particles, the urine can remain in contact with the skin for a relatively long time and make the wearer uncomfortable. This is attributed to the low rate at which the diaper can absorb urine .
• SAP particles having a particularly high absorbency typically exhibit a poor gel strength, such that the gel deforms under pressure (e.g., the load of a body), and prevents further liquid distribution and absorption.
• a balanced relation between absorptivity (gel volume) and gel strength is desired to provide proper liquid absorption, liquid transport, and dryness of the diaper and the skin when using SAP particles in a diaper.
• gel volume gel volume
• gel strength is desired to provide proper liquid absorption, liquid transport, and dryness of the diaper and the skin when using SAP particles in a diaper.
• SAP particles have to perform additional functions with respect to liquid absorption and transport which previously were performed by the fluff pulp and paper, and which could not be accomplished satisfactorily with conventional SAP particles .
• Investigators have researched various methods of improving the amount of liquid absorbed and retained by SAP particles, especially under load, and the rate at which the liquid is absorbed.
• One preferred method of improving the absorption and retention properties of SAP particles is to surface treat the SAP particles.
• the surface treatment of SAP particles is well known.
• U.S. Patent No. 4,043,952 discloses the use of polyvalent metal compounds as surface treating compounds.
• U.S. Patent No. 4,051,086 discloses the use of glyoxal as a surface treatment to improve the absorption rate of SAP particles.
• the surface treatment of SAP particles with crosslinking agents having two or more functional groups capable of reacting with pendant carboxylate or other groups contained on the polymer comprising the SAP particle is disclosed in various patents.
• the surface treatment improves absorbency and gel rigidity to improve liquid flowability and prevent SAP particle agglomeration, as well as improving gel strength.
• the SAP particles are either mixed with the surface-crosslinking agent, optionally using small amounts of water and/or an organic solvent, or an SAP hydrogel containing 10% to 40%, by weight, water is dispersed in a hydrophilic or hydrophobic solvent and mixed with the surface-crosslinking agent.
• Prior surface crosslinking agents include diglycidyl ethers, halo epoxy compounds, polyols, polyamines, polyisocyanates, polyfunctional aziri- dine compounds, and di- or tri-alkylhalides .
• the agent used for the surface treatment has at least two reactive functional groups, and the SAP particles are heated after the surface crosslinking agent is applied to the surface of the SAP particles.
• Surface-crosslinked SAP particles in general, exhibit higher liquid absorption and retention values than SAP particles having a compar- able level of internal crosslinks, but lacking surface crosslinking.
• Internal crosslinks arise from polymerization of the monomers comprising the SAP particles, and are present in the polymer backbone. It has been theorized that surface crosslink- ing increases the resistance of SAP particles to deformation, thus reducing the degree of contact between surfaces of neighboring SAP particles when the resulting hydrogel is deformed under an external pressure.
• the degree to which absorption and reten- tion values are enhanced by surface crosslinking is related to the relative amount and distribution of internal and surface crosslinks, and to the particular surface crosslinking agent and method of surface crosslinking .
• surface-cross- linked SAP particles have a higher level of cross- linking in the vicinity of the surface than in the interior.
• surface describes the outer-facing boundaries of the particle.
• exposed internal surface also are included in the • definition of surface.
• the present invention is directed to a more reactive class of crosslinking agents, that does not require the high temperatures or prolonged heating associated with prior surface crosslinking agents.
• the present invention also is directed to surface-treated SAP particles that overcome the disadvantages associated with the use of prior surface crosslinking agents and with prior surface cross- , linked SAP particles.
• the present invention is directed to surface-treated SAP particles and to a method of surface treating SAP particles with a sufficient amount of an oxazolinium ion to substantially improve the water-absorption and water retention properties of the SAP particles.
• the present invention is- directed to treating SAP particles with oxazolinium ions, which can be formed in si tu, by applying an oxazolinium ion precursor, such as a hydroxyalkylamide (HAA) , to the surface of the SAP particles, then heating the precursor- treated SAP particles at about 90°C to about 170°C for about 60 to about 180 minutes to form the oxazolinium ion, which then surface crosslinks the SAP particles.
• an oxazolinium ion precursor such as a hydroxyalkylamide (HAA)
• SAP particles can be treated with the oxazolinium ion, and the resulting treated particles can be heated to surface crosslink the SAP particles.
• the stable oxazolinium ion is sufficiently reactive to surface crosslink the SAP particles at a temperature of about 90°C to about 170°C.
• SAP particles possess improved water absorption and water retention properties as a result of surface treatment with an oxazolinium ion.
• Treatment with - lo an oxazolinium ion is especially effective when performed on polyacrylate salts, hydrolyzed poly- acrylamides, or other polymers having a plurality of pendent neutralized carboxyl groups. Therefore, the present invention is directed to surface-treated SAP particles having about 0.001 to about 10 parts by weight of an oxazolinium ion, or oxazolinium ion precursor, per 100 parts by weight of SAP particles, applied to the surfaces of the SAP particles to crosslink molecular chains existing at least in the vicinity of the surface of the SAP particles.
• One aspect of the present invention is to provide such surface-treated SAP particles, and to a method of manufacturing the surface-treated SAP particles comprising applying a sufficient amount of an HAA to surfaces of the SAP particles and heating the surface-treated SAP particles at a sufficient temperature and for a sufficient time for the HAA to form an oxazolinium ion, which reacts with pendent groups on a polymer comprising the SAP particle to form surface crosslinks on the SAP particle.
• Yet another aspect of the present invention is to provide a method of surface crosslinking SAP particles comprising applying a sufficient amount of an oxazolinium ion to surfaces of the SAP particles and heating the surface treated SAP particles at about 90 °C to about 170 °C for about 60 to about 180 minutes for the oxazolinium ion to surface crosslink the SAP particles.
• Another aspect of the present invention is to provide a method of surface treating SAP particles comprising heating a sufficient amount of an HAA in the presence of the SAP particles at a sufficient temperature (e.g., about 100°C to about 160°C and for a sufficient time (e.g., about 90 to about 150 minutes) for the HAA to cyclize and thereby form an oxazolinium ion, which then surface crosslinks the SAP particles.
• a sufficient temperature e.g., about 100°C to about 160°C and for a sufficient time (e.g., about 90 to about 150 minutes) for the HAA to cyclize and thereby form an oxazolinium ion, which then surface crosslinks the SAP particles.
• Yet another aspect of the present invention is to provide surface-treated SAP particles exhibiting a high retention capacity, high gel strength, and high absorbency under load. This aspect is achieved by exposing a particle-shaped SAP to about 0.001% to about 10% by weight of an oxazolinium ion, either formed in si tu by heating an HAA in the presence of SAP particles, or by applying an oxazolinium ion to the particles, and heating to about 90°C to about 170°C.
• Another aspect of the present invention is to provide an SAP particle having surface crosslinks provided by an oxazolinium ion, which is generated by heating a hydroxyalkylamide (HAA) having the structure :
• R 1 selected independently, are hydrogen, straight or branched chain C 1-5 alkyl , or straight or branched chain C 1 .
• R 2 selected independently, are radicals selected from the group consisting of hydrogen and straight or branched chain C 1 _ 5 alkyl , or the R 2 radicals can be joined to form, together with the carbon atoms, a cycloalkyl ring;
• p and p' independently, are integers 1 to 4;
• n is an integer having a value of 1 or 2
• n' is an integer having a value 0 to 2, or when n' is 0, a polymer or copolymer (i.e., n has a value greater than 1, preferably 2 to 10) formed from the hydroxyalkyl- amide when A is an unsaturated radical .
• Another aspect of the present invention is to provide an SAP particles having surface crosslinks provided by an oxazolinium having the structure :
• R 1 , R 2 , p, p 1 , and n are as defined above with respect to an HAA.
• Such oxazolinium ions are stable at room temperature, and have a counterion such as tosylate, mesylate, or fluoride.
• the oxazolinium ions are sufficiently reactive to form surface crosslinks on an SAP particle at about 90°C to about 170°C.
• Still another aspect of the present invention is to provide SAP particles surface cross- linked with a oxazolinium ion in an amount sufficient to substantially improve the water absorbency and water retention properties of the SAP particles, such as retention capacity, absorption rate, and gel strength, and to maintain a "dry feel" for the SAP particles after significant liquid absorption.
• SAP particles are surface treated with an oxazolin- ium ion to substantially increase the rate of liquid absorption, amount of liquid absorption, and overall retention of liquids by the SAP particles.
• Surface- treatment of the SAP particles at any time after polymerization and sufficient drying to form solid SAP particles improves liquid absorption properties.
• the surface treatment is performed most advantageously immediately after the SAP particles are synthesized, dried to an appropriate water content, and sized, such as by grinding .
• SAP particles with an oxazolinium ion substantially improves the water absorption properties of the SAP particles, which can be either acidic or basic in nature.
• SAP particles containing a plurality of pendant, neutralized carboxyl groups along the polymer chain are particularly useful.
• the high temperature required to form surface crosslinks also causes the SAP particles to degrade in color from white or off-white to tan or brown.
• the tan to brown color of the SAP particles is esthetically unacceptable to consumers, who equate the tan to brown color of the SAP particles to an inferior product.
• the combination of color degradation and increased residual monomer can lead to SAP particles that do not meet production speci- fications and, therefore, are refused by the purchaser and/or consumer.
• the present invention overcomes these disadvantages associated with prior surface crosslinking agents by utilizing an oxazolinium ion as the surface crosslinking agent for the SAP particles.
• the identity of the SAP particles utilized in the present invention is not limited.
• the SAP particles are prepared by methods well known in the art, for example, solution or emulsion polymeriza- tion.
• the SAP particles therefore, can comprise an acidic water-absorbing resin, a basic water-absorbing resin, a blend of an acidic and basic water- absorbing resin, or a multicomponent SAP particle as disclosed in WO 99/25393, the disclosure of which is incorporated herein by reference.
• the SAP particles are prepared, for example, by: (1) copolymerizing an acrylate salt and a crosslinking monomer in aqueous solution, and drying the resulting gel-like hydrous polymer by heating;
• acidic water-absorbing resins have carboxylate, sulfonate, sulfate, and/or phosphate groups incorporated along the polymer chain.
• Polymers containing these acid moieties are synthe- sized either from monomers previously substituted with one or more of these acidic functional groups or by incorporating the acidic functional group into the polymer after synthesis.
• car- boxyl groups any of a number of ethylenically unsaturated carboxylic acids can be homopolymerized or copolymerized.
• Carboxyl groups also can be incorporated into the polymer chain indirectly by hydrolyzing a homopolymer or copolymer of monomers such as acrylamide, acrylonitrile, meth- acrylamide, and alkyl acrylates or methacrylates.
• An acidic water-absorbing resin present in an SAP particle can be either a strong or a weak acidic water-absorbing resin.
• the acidic water- " absorbing resin can be a single resin, or a mixture of resins.
• the acidic resin can be a homopolymer or a copolymer.
• the acidic water-absorbing resin typically is a neutralized, lightly crosslinked acrylic-type resin, such as neutralized, lightly crosslinked polyacrylic acid.
• the lightly crosslinked acidic resin typically is prepared by polymerizing an acidic monomer containing an acyl moiety, e.g., acrylic acid, or a moiety capable of providing an acid group, i.e., acrylonitrile, in the presence of a free radical crosslinker, i.e., a polyfunctional organic compound.
• the acidic resin can contain other copolymerizable units, i.e., other monoethyl- enically unsaturated comonomers, well known in the art, as long as the polymer is substantially, i.e., at least 10%, and preferably at least 25%, acidic monomer units.
• the acidic resin contains at least 50%, and more preferably, at least 75%, and up to 100%, acidic monomer units.
• the acidic resin is neutralized at least 50 mole %, and preferably at least 70 mole %, with a base prior to surface crosslinking.
• Ethylenically unsaturated carboxylic acid and carboxylic acid anhydride monomers, and salts, useful in the acidic water-absorbing resin include acrylic acid, methacrylic acid, ethacrylic acid, ⁇ - chloroacrylic acid, ⁇ -cyanoacrylic acid, ⁇ -methyl- acrylic acid (crotonic acid) , -phenylacrylic acid, ?-acryloxypropionic acid, sorbic acid, ⁇ -chloro- sorbic acid, angelic acid, cinnamic acid, p-chloro- cinnamic acid, S-stearylacrylic acid, itaconic acid, citraconic acid, mesaconic acid, glutaconic acid, aconitic acid, maleic acid, fumaric acid, tricar- boxyethylene, 2 -methyl-2 -butene dicarboxylic acid, maleamic acid, N-phenyl maleamide, maleamide, maleic anhydride, fumaric anhydride, it
• Sulfate-containing acidic resins are prepared by reacting homopolymers or copolymers containing hydroxyl groups or residual ethylenic un- saturation with sulfuric acid or sulfur trioxide.
• treated polymers include sulfated polyvinylalcohol, sulfated hydroxyethyl acrylate, and sulfated hydroxypropyl methacrylate.
• Phosphate- containing acidic resins are prepared by homopolym- erizing or copolymerizing ethylenically unsaturated monomers containing a phosphoric acid moiety, such as methacryloxy ethyl phosphate .
• Copolymerizable monomers for introduction into the acidic resin, or into the basic resin include, but are not limited to, ethylene, propyl- ene, isobutylene, C to C 4 alkyl acrylates and methacrylates, vinyl acetate, methyl vinyl ether, and styrenic compounds having the formula:
• R represents hydrogen or a C 1-6 alkyl group, and wherein the phenyl ring optionally is substituted with one to four C 1-4 alkyl or hydroxy groups .
• Suitable C ⁇ to C 4 alkyl acrylates include, but are not limited to, methyl acrylate, ethyl acrylate, isopropyl acrylate, n-propyl acrylate, n- butyl acrylate, and the like, and mixtures thereof.
• Suitable C to C 4 alkyl methacrylates include, but are not limited to, methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, n-propyl- methylmethacrylate, n-butyl methacrylate, and the like, and mixtures thereof or with C 1-4 alkyl acrylates.
• Suitable styrenic compounds include, but are not limited to, styrene, ⁇ -methylstyrene, p- methylstyrene, t-butyl styrene, and the like, and mixtures thereof or with C 1-4 alkyl acrylates and/or methacrylates .
• an acidic resin is lightly crosslinked, i.e., has a crosslinking density of less than about 20%, preferably less than about 10%, and most preferably about 0.01% to about 7%.
• a crosslinking agent most preferably is used in an amount of less than about 7 wt%, and typically about 0.1 wt% to about 5 wt%, based on the total weight of monomers.
• crosslinking polyvinyl monomers include, but are not limited to, polyacrylic (or polymethacrylic) acid esters represented by the following formula (I) ; and bisacrylamides, represented by the following formula (II) -
• X is ethylene, propylene, trimethylene, cyclohexyl, hexamethylene, 2 -hydroxypropylene, - (CH 2 CH 2 0) p CH 2 CH 2 - , or
• p and r are each an integer 5 to 40, and k is 1 or 2;
• the compounds of formula (I) are prepared by reacting polyols, such as ethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexane- diol, glycerin, pentaerythritol, polyethylene glycol, or polypropylene glycol, with acrylic acid or methacrylic acid.
• polyols such as ethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexane- diol, glycerin, pentaerythritol, polyethylene glycol, or polypropylene glycol
• acrylic acid or methacrylic acid acrylic acid or methacrylic acid.
• crosslinking monomers include, but are not limited to, 1, 4-butanediol diacrylate, 1, 4-butanediol dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, ethoxylated bisphenol A diacrylate, ethoxylated bisphenol A dimethacrylate, ethylene glycol dimethacrylate, 1 , 6-hexanediol diacrylate,
• the acidic resin can be any resin that acts as an SAP in its neutralized form.
• acidic resins include, but are not limited to, polyacrylic acid, hydrolyzed starch-acrylonitrile graft copolymers, starch-acrylic acid graft copolymers, saponified vinyl acetate-acrylic ester copolymers, hydrolyzed acrylonitrile copolymers, hydrolyzed acrylamide copolymers, ethylene-maleic anhydride copolymers, isobutylene-maleic anhydride copolymers, poly (vinylsulfonic acid), poly (vinylphosphonic acid), poly (vinylphosphoric acid), poly (vinylsul- furic acid), sulfonated polystyrene, poly (aspartic acid), poly (lactic acid), and mixtures thereof.
• the preferred acidic resins are the polyacrylic acids.
• sequence and the number of reactions e.g., polymerization, hydrolysis, and neutralization
• a basic water-absorbing resin present in the SAP particles can be a strong or weak basic water-absorbing resins.
• the basic water-absorbing resin can be a single resin or a mixture of resins.
• the basic resin can be a homopolymer or a copolymer.
• the identity of the basic resin is not limited as long as the basic resin is capable of reacting with an oxazolinium ion.
• the strong basic resins typically are present in the hydroxide (OH) or bicarbonate (HC0 3 ) form.
• the basic water-absorbing resin can be a lightly crosslinked acrylic-type resin, such as a poly (dialkylaminoalkyl (meth) acrylamide) .
• the basic resin also can be a polymer such as a lightly cross- linked polyethylenimme, a poly (vinylamine) , a poly- (allylamine) , a poly (allylguanidine) , a poly(di- methyldiallylammonium hydroxide) , a quatemized polystyrene derivative, such as
• guanidine-modified polystyrene such as
• Me is methyl
• R 4 is hydrogen or methyl
• n is a number 1 to 8
• q is a number from 10 to about 100,000, or a poly (vinylguanidine) , i.e., poly(VG), a strong basic water-absorbing resin having the general structural formula (III)
• a water-soluble basic resin can be crosslinked in solution by suspending or dissolving an uncrosslinked basic resin in an aqueous or alcoholic medium, then adding a di- or polyfunctional compound capable of crosslinking the basic resin by reaction with the amino groups of the basic resin.
• Such crosslinking agents include, for example, multifunctional aldehydes (e.g., glutaraldehyde), multifunctional acrylates (e.g., butanediol diacrylate, TMPTA) , halohydrins (e.g., epichlorohydrin) , dihalides (e.g., dibromopropane) , disulfonate esters (e.g., ZS (0 2 ) O- (CH 2 ) n -0F (0) 2 Z, wherein n is 1 to 10, and Z is methyl or tosyl) , multifunctional epoxies (e.g., ethylene glycol diglycidyl ether), multifunctional esters (e.g., dimethyl adipate) , multifunctional acid halides (e.g., oxalyl chloride), multifunctional carboxylic acids (e.g., succinic acid), carboxylic acid anhydrides (e.g., succ
• Patent No. 5,085,787 incorporated herein by reference, and in EP 450 923.
• the crosslinking agent is water or alcohol soluble, and possesses sufficient reactivity with the basic resin such that cross- linking occurs in a controlled fashion, preferably at a temperature of about 25 °C to about 150 °C.
• Preferred crosslinking agents are ethylene glycol diglycidyl ether (EGDGE) , a water-soluble diglycidyl ether, and a dibromoalkane, an alcohol -soluble compound .
• the basic resin can be any resin that acts as an SAP in its charged form.
• Examples of basic resins include a poly (vinylamine) , a polyethylenimme, a poly (vinylguanidine) , a poly (allylamine) , a poly- (allylguanidine) , or a poly (dialkylaminoalkyl (meth) - acrylamide) prepared by polymerizing and lightly crosslinking a monomer having the structure
• R 7 and R a independently, are selected from the group consisting of hydrogen and methyl
• Y is a divalent straight chain or branched organic radical having 1 to 8 carbon atoms
• R 9 is hydrogen
• R 10 is hydrogen or an alkyl radical having 1 to 4 carbon atoms.
• Preferred basic resins include a poly (vinyl- amine), polyethylenimme, poly (vinylguanadine) , poly (methylaminoethyl acrylamide), and poly (methyl- aminopropyl methacrylamide) .
• the SAP particles can be in the form of spheres obtained by inverse phase suspension polymerization, flakes obtained by drum drying, or irregularly shaped particles obtained by pulverizing solid polymer. From the standpoint of the speed of absorption, the SAP particles preferably are small, and typically the particle size is about 20 to about 2000 um, preferably about 50 about 850 ⁇ .
• the SAP particles comprising an acidic resin, basic resin, a blend of acidic and basic resin, or multicomponent SAP particles, are surface treated by applying a surface crosslinking agent to the surface of the SAP particles, followed by heating the particles.
• Surface treatment results in surface crosslinking of the SAP particles. It has been found that surface treating SAP particles with an oxazolinium ion, either directly or through an oxazolinium ion precursor, enhances the ability of the SAP particles to absorb and retain aqueous media under a load.
• surface crosslinking is achieved by contacting SAP particles with a solution of an HAA or a stable oxazolinium ion that wets predominantly only the outer surfaces of the SAP particles.
• Surface crosslinking of the SAP particles then is performed, preferably by heating at least the wetted surfaces of the SAP particles. Heating the HAA forms oxazolinium ions in si tu, which in turn surface crosslink the SAP particles.
• the oxazolinium ion can be applied to the outer surfaces of the SAP particles, followed by heating the SAP particles to surface crosslink the exposed surface of the SAP particles.
• the surface crosslinking agent utilized in the present invention is an oxazolinium ion.
• Simple oxazolinium ions have been isolated and characterized as disclosed in Stanssens et al . , Proc . -Int . Conf . Org. Coat . Sci . Technol . , p. 435 (1992).
• Reactions of oxazolinium ions with carboxylate an- ions to form esters of 2-hydroxyalkylamides have been disclosed in Winstein et al . , J. Am. Chem . Soc , 72, p. 4669 (1950), and McLasland et al . , J. Am. Chem. Soc ⁇ . , 72, p.
• stable oxazolinium ion is defined herein as an oxazolinium ion that is stable at room temperature and has sufficient reactivity with acidic (e.g., carboxyl) and basic (e.g., amino) moieties at about 90 °C to about 170 °C to form covalent bonds. Stability is imparted to an oxazolinium ion by judicious selection of a counterion, for example, a counterion comprising a conjugate base of an acid having a pKa of about 1.5 to about 6, preferably a pKa of about 2 to about 5, and most preferably about 2.5 to about 4.5.
• a counterion comprising a conjugate base of an acid having a pKa of about 1.5 to about 6, preferably a pKa of about 2 to about 5, and most preferably about 2.5 to about 4.5.
• the counterion is a conjugate base of an acid having a pKa of about 2.5 to about 3.5.
• specific examples of counterions include, but are not limited to, benzo- ate, haloacetate, formate, fluoride, sulfonate (e.g., tosylate or mesylate) , and the like.
• the reactivity of a stable oxazolinium ion can be increased by exposing the stable oxazolinium ion to a sufficiently polar solvent (e.g., propylene glycol) , optionally containing a sufficient concentration of a less acidic counterion to solvate or exchange the ion pair.
• a sufficiently polar solvent e.g., propylene glycol
• the reactivity of a stable oxazolinium ion can be increased prior to heating in the presence of the SAP particles to facilitate the surface crosslinking of the exposed surface of the SAP particles.
• the oxazolinium ion surface crosslinking agent utilized in the present invention can be generated in si tu from a hydroxyalkylamine .
• Hydroxyalkylamides are disclosed in Swift et al .
• An HAA useful in the present invention has the following formula:
• Preferred HAAs are wherein R 1 is H or C 1-5 hydroxyalkyl , n and n' are each 1,-A- is -(CH 2 ) ra -, m is 0-8, preferably 2-8, each R 2 on the ⁇ -carbon is H, and one of the R 2 radicals on the beta carbon in each case is H and the other is H or a C 1-5 alkyl, and q and q', independently, are an integer 1 to 3; that is, -OH
• both R 2 groups are H or both R 2 groups are -CH 3 .
• HAA compounds include, but are not limited to, bis [N,N-di ( ⁇ - hydroxyethyl) ] adipamide, bis [N,N-di ( ⁇ -hydroxypropyl) ] succinamide, bis [N,N-di ( ⁇ -hydroxyethyl) ] - azelamide, bis [N-N-di ( ⁇ -hydroxypropyl) ] adipamide, and bis [N-methyl-N- ( ⁇ -hydroxyethyl) ] oxamide .
• a commercially available ?-HAA is PRIMIDTM XL-552 from EMS-CHEMIE, Dornat , Switzerland.
• PRIMIDTM XL-522 has the structure
• HAA PRIMIDTM QM-1260 from EMS-CHEMIE, having the structure :
• An oxazolinium ion can be formed in si tu by heating an HAA at a sufficient temperature and for a sufficient time.
• an oxazolinium ion is formed from PRIMIDTM XL-552 as follows:
• the cyclic oxazolinium ion then reacts with a carboxyl or amino group on a polymer chain of an SAP to form a covalent bond.
• a PRIMIDTM XL-552 molecule then can form a second oxazolinium ion from a second ⁇ -hydroxyethyl group on the molecule.
• This second oxazolinium ion reacts with carboxyl or amino group on a second polymer chain of the SAP, and, together with the first oxazolinium ion, forms surface crosslinks.
• Other HAA compounds provide corresponding oxazolinium ions.
• the first and second oxazolinium ions generated in si tu from an HAA molecule can be formed simultaneously.
• a stable oxazolinium ion can be applied directly to the surface of a SAP followed by heating to surface-crosslink the SAP.
• R 1 , R 2 , p, p 1 and n are as defined above with respect to an HAA.
• the solution can be applied as a fine spray onto the surface of freely tumbling SAP particles at a ratio of about 1:0.01 to about 1:0.5 parts by weight SAP particles to solution of oxazolinium ion or oxazolinium ion precursor.
• the HAA or the oxazolinium ion is distributed evenly on the surfaces of the SAP particles.
• mixing is performed in suitable mixers, e.g., fluidized bed mixers, paddle mixers, a rotating disc mixer, a ribbon mixer, a screw mixer, milling rolls, or twin-worm mixers.
• suitable mixers e.g., fluidized bed mixers, paddle mixers, a rotating disc mixer, a ribbon mixer, a screw mixer, milling rolls, or twin-worm mixers.
• the amount of HAA or oxazolinium ion used to surface treat the SAP particles varies depending upon the identity of SAP particles. Generally, the amount of HAA or oxazolinium ion, used to surface treat the SAP particles is about 0.001 to about 10 parts by weight per 100 parts by weight of the SAP particles.
• a preferred amount of HAA or oxazolinium ion used to surface crosslink the SAP particles is about 0.01 to about 5 parts by weight per 100 parts, by weight, SAP particles.
• the amount of oxazolinium ion used as a surface crosslinking agent is about 0.05 to about 1 part, by weight, per 100 weight parts of SAP particles.
• the surface-treated SAP particles are heated for about 60 to about 180 minutes, preferably about 60 to about 150 minutes, to effect surface crosslinking. To achieve the full advantage of the present invention, the SAP particles are heated for about 75 to about 180 minutes.
• Ordinary dryers or heating ovens can be used for heating the surface-treated SAP particles and the oxazolinium ion precursor or oxazolinium ion.
• Such heating apparatus includes, for example, an agitated trough dryer, a rotating dryer, a rotating disc dryer, a kneading dryer, a fluidized bed dryer, a pneumatic conveying dryer, and an infrared dryer.
• any other method of forming or reacting the oxazolinium ion with the polymer of the SAP .particle to achieve surface crosslinking of the SAP particles such as microwave energy, can be used.
• the mixer can be used to perform simultaneous mixing and heating of the HAA or oxazolinium ion, and the SAP particles, if the mixer is of a type that can be heated.
• the degree and gradient of surface crosslinking can vary within a given type of SAP particle.
• variations in surface :volume ratio within the SAP particles e.g., between small and large particles
• it is typical for the overall level of crosslinking to vary over the group of SAP particles e.g., is greater for smaller particles
• Surface-crosslinked SAP particles of the present invention have advantages over conventional absorbent particles.
• the surface-crosslinked SAP particles of the invention can be produced at a low cost by a simple method which involves mixing SAP particles with an HAA (or other oxazolinium ion precursor) or a stable oxazolinium ion, and heating.
• the resulting surface-crosslinked SAP particles are less susceptible to fish eye formation than conventional absorbent resins and, therefore, exhibit a high rate of liquid absorption
• the present surface- crosslinked SAP particles also are white to off- white in color.
• SAP particles used in the following examples are lightly crosslinked polyacrylic acid polymers, neutralized about 75% to about 80% with sodium hydroxide .
• the sodium polyacrylate was made by methods well known in the art.
• a solution containing 1% to 5%, by weight, of PRIMIDTM XL-552 and 0% to 37.5%, by weight, propylene glycol in water was applied to the surface of SAP particles, at the rate of about 4 to about 10 grams of solution per 100 grams of SAP particles.
• the surface-treated SAP particles then were heat treated at about 150°C to about 170°C for about 60 to about 120 minutes such that the PRIMIDTM XL-552 generated oxazolinium ions to surface crosslink the SAP particles.
• the best results were achieved using a 3.5% PRIMIDTM XL-552/25% propylene glycol solution applied at about 7 grams of solution per 100 grams of SAP particles, then heat treating for about 120 minutes at 160 °C.
• the performance results are summarized in Table 1.
• a solution containing 1% to 5%, by weight, PRIMIDTM XL-552 and 0% to 25%, by weight, 1,3-butane- diol in water was applied to the surface of SAP particles, at the rate of about 4 to about 10 grams of solution per 100 grams of SAP particles.
• the surface-treated SAP particles then were heat treated at about 150°C to about 170 °C for about 60 to about 120 minutes such that the PRIMIDTM XL-552 generated oxazolinium ions to surface crosslink the SAP parti- cles.
• a solution containing 1% to 5%, by weight, PRIMIDTM XL-552 and 0 to 25% 1 , 4-butanediol in water was applied to the surface of SAP particles, at the rate of about 4 to about 10 grams of solution per 100 grams of SAP particles.
• the surface-treated SAP particles then were heat treated at about 150°C to about 170°C for about 60 to about 120 minutes such that the PRIMIDTM XL-552 generated oxazolinium ions to surface crosslink the SAP particles.
• the best results were achieved using a 3.5% PRIMIDTM XL-552/- 25% 1, 4-butanediol solution applied at about 7 grams of solution per 100 grams of SAP particles, then heat treating for about 120 minutes at 160°C.
• Table 1 The performance results are summarized in Table 1.
• a solution containing 1% to 5%, by weight, PRIMIDTM XL-552 and 0% to 25%, by weight, ethanol in water was applied to the surface of SAP particles, at the rate of about 4 to about 10 grams of solution per 100 grams of SAP particles.
• the surface-treated SAP particles then were heat treated at 150 °C to about 170°C for about 60 to about 120 minutes such that the PRIMIDTM XL-552 generated oxazolinium ions to surface crosslink the SAP particles.
• the best results were achieved using a 3.5% PRIMIDTM XL-552/- 25% ethanol solution applied at about 7 grams of solution per 100 grams of polymer, then heat treating for about 120 minutes at 160°C.
• Table 1 The performance results are summarized in Table 1.
• toluene 40 tnL was admixed with bis (N-methyl-N- (2- hydroxyethyl) adipamide (2 g) and p-toluene sulfonic acid monohydrate (2.92 g) under a dry argon atmos- phere .
• the resulting mixture was heated to reflux, and azeotropic distillation and collection in a Dean-Stark trap removed the produced water.
• the total reaction time was 6.75 hours.
• the toluene then was removed by rotary evaporation, and the residue was dried further in a vacuum oven for 60 hours at 1 Torr.
• the bisoxazolinium salt was an oil, and was used without further purification. The formation of the bisoxazolinium ion was confirmed by infrared spectroscopy. The bisoxazolinium salt of Example 6 was used to surface crosslink a sodium polyacrylate SAP. The results are summarized in Table 3.
• An SAP (0.160 g +/-0.001 g) is carefully scattered onto a 140-micron, water-permeable mesh attached to the base of a hollow Plexiglas cylinder with an internal diameter of 25mm.
• the sample is covered with a 100 g cover plate and the cylinder assembly weighed. This gives an applied pressure of 20 g/cm 2 (0.28 psi).
• the sample can be covered with a 250 g cover plate to give an applied pressure of 51 g/cm 2 (0.7 psi) .
• the screened base of the cylinder is placed in a 100mm petri dish containing 25 milliliters of a test solution (usually 0.9% saline), and the polymer is allowed to absorb for 1 hour (or 3 hours) .
• the CRC test was performed as follows:
• Teabag material (CH DEXTER) , cut and sealed to produce a teabag 6.25 x 8.5 cm Weigh paper or plastic weighing boat Timer
• Filled teabags were handled carefully with tongs, contacting only the edge of the teabag and not the area filled with polymer.
• typical hydrogel- forming SAPs exhibit SFC values of 1 x 10 "7 cm 3 sec/g or less.
• SFC values 1 x 10 "7 cm 3 sec/g or less.
• the boundaries of the hydrogel come into contact , and interstitial voids in the high SAP concentration region become generall bounded by hydrogel.
• the permeability or saline flow conductivity properties in this region is generally indicative of the permeability or saline flow conductivity properties of a hydrogel zone formed from the SAP alone.
• Increasing the permeability of these swollen high concentration regions can provide superior fluid handling properties for the absorbent structure, thus decreasing incidents of leakage, especially at high fluid loadings.
• a method of determining the SFC value of SAP particles is set forth in Goldman et al . U.S. Patent No. 5,599,335, incorporated herein by reference.
• QM-1260 is PRIMIDTM QM- 1260 ;
• SAP particles were surface crosslinked with the stable oxazolinium salt of Example 6.
• the amount of oxazolinium ion used was 2000 ppm based on the weight of the SAP particle.
• Table 3 illustrates that an SAP surface crosslinked with a stable oxazolinium ion provides improved absorption properties (compare Samples 14 and 15 to control Samples 9-11) .
• Preferred SAP particles of the present invention are surface crosslinked with an oxazolinium ion and have a 0.7 AUL of at least 15, and more preferably at least 20.
• the preferred surface crosslinked SAP particles typically have a 0.7 AUL of about 15 to about 50.
• Preferred SAP particles of the present invention also have a CRC of less than 32.4, and more preferably less than 31.5.
• Preferred SAP particles typically have a CRC of less than 32.4 to about 25.
• Preferred SAP particles of the present invention have an SFC value of at least about 15 x 10 "7 cm 3 sec/g, and preferably at least about 20 x 10" 7 cm 3 sec/g.
• the SFC value of the present surface-crosslinked SAP particles is at least about 25 x 10 "7 cm 3 sec/g, and can range to greater than 60 x 10 "7 cm 3 sec/g.

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