Classifications

• • 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
  • C08F2/00—Processes of polymerisation
  • C08F2/04—Polymerisation in solution
  • C08F2/10—Aqueous solvent
• • 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
  • C08F20/00—Homopolymers and copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride, ester, amide, imide or nitrile thereof
  • C08F20/02—Monocarboxylic acids having less than ten carbon atoms, Derivatives thereof
  • C08F20/04—Acids, Metal salts or ammonium salts thereof
  • C08F20/06—Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
• • 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

Definitions

• the present invention relates to a production process for a water-absorbent resin. More particularly, the present invention relates to a production process for a water-absorbent resin, comprising a polymerization step that includes the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time carrying out fine division of the resultant hydrogel, in which the various properties, polymerization rate, and drying rate are enhanced by carrying out the fine division of the gel uniformly.
• a polymerization step that includes the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time carrying out fine division of the resultant hydrogel, in which the various properties, polymerization rate, and drying rate are enhanced by carrying out the fine division of the gel uniformly.
• water-absorbent resins having excellent water absorbency are developed and widely used mainly for disposable uses, as absorbent articles (e.g. disposable diapers and sanitary napkins), and further water-retaining agents in agricultural and horticultural fields and water-holding materials in engineering works fields.
• the above water-absorbent resins are made water-swellable and water-insoluble by slightly crossliniking hydrophilic polymers.
• they have been obtained as powders by a process including the steps of: carrying out polymerization of water-soluble unsaturated monomers such as acrylic acid; and crosslinking the resultant polymers during or after the polymerization.
• many polymerization methods for obtaining powdery water-absorbent resins e.g. reversed-phase suspension polymerization, aqueous solution polymerization, and further precipitation polymerization which involves precipitating a polymer in a solvent, bulk polymerization which involves polymerization substantially in the absence of a solvent, and spray polymerization which involves polymerization in gas phase
• aqueous solution polymerization and reversed-phase suspension polymerization are mainly used in view of performance and easiness of controlling the polymerization.
• the reversed-phase suspension polymerization is a polymerization method which involves dispersing an aqueous monomer solution in a hydrophobic organic solvent in the form of particles having particle diameters of about 1 to about 0.1 mm, and has the advantage of obtaining gel particles having particle diameters of products at the same time as the polymerization (for example, as is mentioned in such as specifications of USP 4,093,776, USP 4,367,323, USP 4,446,261, and USP 4,683,274, and especially refer to the following patent document 1.).
• the control of the polymerization temperature is difficult in the reversed-phase suspension polymerization which is carried out by being dispersed in a large quantity of solvent. Especially, there is danger of explosion during the polymerization when the concentration is raised (for example, the concentration of the aqueous monomer solution is not less than 40 weight %). Therefore, there are problems such that the productivity in an increasing scale cannot be improved sufficiently.
• the aqueous solution polymerization is a method which involves polymerizing an aqueous monomer solution without using a dispersible solvent, and is excellent in costs, productivity, and safety of products because the polymerization can be carried out by using water only.
• aqueous solution polymerizations are further roughly divided into a method which involves static polymerization such as belt polymerization (for example, as is mentioned in such as specifications of USP 6,174,978 and USP 4,857,610, and especially refer to the following patent document 2.), and a method which involves polymerization while being stirred with such as kneaders (stirring polymerization).
• aqueous solution polymerization that is different from the reversed-phase suspension polymerization, obtained is a massive gel having a size of far larger than particle diameters of products. Therefore, the fine division of the gel is necessary for drying and production.
• polymerization vessels having shearing force e.g.
• the stirring polymerization has the advantage of easily removing polymerization heat and also having high productivity.
• the above stirring polymerization carried out by using such as kneaders is a production process for a water-absorbent resin, comprising a polymerization step including the steps of: supplying an aqueous solution of a water-soluble unsaturated monomer to a polymerization vessel having shearing force to carry out polymerization involving crosslinking; and at the same time finely dividing the resultant hydrogel (for example, as is exemplified in such as: specifications of USP 4,625,001, USP 4,985,514, and USP 5,124,416 filed by Nippon Shokubai Co., Ltd.; and a pamphlet of WO 01/38402 and specifications of USP 5,149,750, USP 4,769,427, and USP 4,873,299 filed by BASF, and especially refer to the following patent documents 3 and 4.).
• the gel is finely divided into pieces having particle diameters of about 1 mm during the polymerization, but there are many cases where the fine division of the gel carried out at the same time as the polymerization takes longer time than the polymerization time. Therefore, there are cases where the polymerization time is unnecessarily prolonged for carrying out the fine division sufficiently, or where the lowering of properties is caused because of the longtime fine division (shearing) of the gel.
• the hydrogel as discharged from the polymerization vessel in this way is finely divided into a particulate gel having a size of a few millimeter (favorably about 1 to about 3 mm) by shearing force during the polymerization, but the fine division of the gel during the ' polymerization is usually difficult to completely carry out.
• the resultant finely divided gel is contaminated with such a coarse gel as has a size of larger than a few centimeters in an amount of a few to about 10 weight %.
• the side-production of the coarse gel tends to increase if an attempt is made to raise the temperature when the polymerization is initiated, or to raise the concentration of the water-soluble unsaturated monomer, or to decrease the extractable content of the water-absorbent resin.
• a polymerization step including the steps of: carrying out polymerization involving crosslinking; and at the same time finely dividing the resultant gel
• a polymerization step including the steps of: carrying out polymerization involving crosslinking; and at the same time finely dividing the resultant gel
• the restriction occurs that the coarse gel is side-produced.
• the drying time of the finely divided gel depends upon its specific surface area.
• the contamination with the coarse gel in only a few percents has problems such that: not only the drying rate of the entire finely divided gel is greatly lowered and the longtime drying is necessary, but also the above unnecessarily drying causes the properties to lower, or the properties after the drying are changed or lowered in proportion to the gel particle diameter.
• the above coarse gel in an amount of few percents is an undried product even after the drying, and there is also a case where the procedure such as pulverization or classification after the drying cannot be carried out (e.g. the pulverization step or classification step after the drying is stopped.) because of the adhesion of the undried product (gel) with which the dried product is contaminated.
• a method which involves removing the coarse gel by classification after polymerization for example, as is mentioned in such as a gazette of JP-A-107800/1994, and especially refer to the following patent document 5.
• a method which involves classifying an undried product after drying for example, as is mentioned in such as a specification of USP 6,291,636, and especially refer to the following patent document 6.
• the above methods which involve removing the coarse gel or undried product after the polymerization not only the apparatuses are complicated and the classification efficiency is also low, but also are caused the problems of waste derived from the classified coarse gel and the lowering of yield accompanying the abandonment.
• an object of the present invention is to provide a method, which can solve the above hitherto problems at a stroke in the case where the aqueous solution polymerization (e.g. stirring polymerization and static polymerization) or reversed-phase suspension polymerization is carried out when a water-absorbent resin is produced.
• aqueous solution polymerization e.g. stirring polymerization and static polymerization
• reversed-phase suspension polymerization is carried out when a water-absorbent resin is produced.
• the present inventor diligently studied to solve the above-mentioned problems.
• a production process for a water-absorbent resin comprises a polymerization step that includes the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time carrying out fine division of the resultant hydrogel, with the production process being characterized in that the water-soluble unsaturated monomer contains furfural in an amount of 11 to 1,000 weight ppm (relative to the monomer).
• another production process for a water-absorbent resin comprises the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and carrying out fine division of the resultant hydrogel, with the production process being characterized in that:
• the water-soluble unsaturated monomer contains furfural in an amount of 11 to 1,000 weight ppm (relative to the monomer);
• the polymerization is initiated at a temperature of not lower than 30 °C and/or (B2) the water-soluble unsaturated monomer further contains a transition metal;
• the resultant finely divided hydrogel has a weight-average particle diameter of 0.3 to 4 mm, and the ratio of coarse gel particles having particle diameters of not smaller than 10 mm is not more than 5 weight % of the finely divided hydrogel.
• yet another production process for a water-absorbent resin comprises a polymerization step that includes the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time obtairiing a finely divided hydrogel, with the production process being characterized in that the water-soluble unsaturated monomer contains furfural in an amount of 11 to 1,000 weight ppm (relative to the monomer).
• the water-absorbent resin is said a water-swellable and water-insoluble resin as obtained by introducing a crosslinked structure into a polymer.
• water-swellable means ability of absorbing essentially at least 3 times, favorably 5 to 200 times, more favorably 20 to 100 times, as large a quantity of physiological saline as its own weight without load.
• water-insoluble means substantially water-insoluble such that the water-extractable content in the resin is essentially not more than 50 weight (mass) %, favorably not more than 25 weight %, more favorably not more than 15 weight %, still more favorably not more than 10 weight %.
• the water-absorbent resin includes an acrylic acid (and/or its salt) monomer (and further, favorably in a major proportion) as a water-soluble unsaturated monomer in view of properties in the present invention.
• the major proportion means that the total molar percentage of the acrylic acid and/or its salt as used is essentially not less than 30 mol %, favorably not less than 50 mol %, more favorably not less than 70 mol %, still more favorably not less than 90 mol %, particularly favorably substantially 100 %, relative to the entire monomer as used for the polymerization (except for crosslinking agents).
• the water-soluble unsaturated monomer means a monomer that is dissolved in water at room temperature in an amount of essentially not smaller than 1 weight %, favorably not smaller than 10 weight %, more favorably not smaller than 30 weight %.
• acrylic acid salt usable in the present invention favorably used in view of properties are acrylic acid monovalent salts (e.g. alkaline metal salts, ammonium salts, and amine salts), and more favorably used are acrylic acid salts of alkaline metals, and still more favorably used are acrylic acid salts as selected from the group consisting of a sodium salt, a lithium salt, and a potassium salt.
• the acid group of a polymer is favorably neutralized in a neutralization ratio of 20 to 99 mol %, more favorably 50 to 95 mol %, still more favorably 60 to 90 mol %.
• the monomer before the polymerization may be neutralized, or the polymer during or after the polymerization may be neutralized.
• both of the neutralization of the monomer and the neutralization of the polymer may be carried out together, but the neutralization of the acrylic acid is favorably carried out.
• Examples of basic substances usable for the neutralization include (hydrogen) carbonates, hydroxides of alkaline metals, ammonia, and organic amines.
• the treatment of strong bases namely, favorably hydroxides of alkaline metals such as sodium hydroxide, potassium hydroxide, and lithium hydroxide, particularly favorably sodium hydroxide
• the monomer favorably includes the acrylic acid and/or its salt in a major proportion. However, it may include other monomers together or may include them in a major proportion. Examples of other monomers as included together include water-soluble or hydrophobic unsaturated monomers as copolymerizable components (e.g.
• methacrylic acid maleic acid, maleic anhydride, fumaric acid, crotonic acid, itaconic acid, vinylsulfonic acid, 2-(mem)acrylamido-2-methylpropanesulfonic acid, (meth)acryloxylalkanesulfonic acid, and alkaline metal salts and ammonium salts thereof, N-vinyl-2-pyrrolidone, N-vinylacetamide, (meth)acrylamide, N-isopropyl(meth)acrylamide,
• crosslinking method usable in the present invention, but examples thereof include: (a) a post-crosslinking method which involves obtaining a hydrophilic polymer by cany out polymerization of acrylic acid and/or an acrylic acid salt (if necessary, together with the above water-soluble or hydrophobic unsaturated monomers as copolymerizable components), and thereafter adding a crosslinking agent during or after the polymerization; (b) a radical crosslinking method with radical polymerization initiators; and (c) an radiau ⁇ n-crosslinking method with such as electron beams.
• (d) a method which involves beforehand adding a predetermined amount of mtemal-crosslinking agent to acrylic acid and/or an acrylic acid salt, or the above water-soluble or hydrophobic unsaturated monomers as copolymerizable components to carry out polymerization, and subjecting to a crosslinking reaction at the same time as or after the polymerization.
• both of the crosslinking method (d) and the crosslinking methods (a) to (c) may be carried out together.
• an mtemal-crosslinking agent usable in the above method (d) is one or at least two intemal-crosslinking agents, such as N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glycol di(meth)acrylate, (polyoxyethylene)trimethylolpropane tri(meth)acrylate, trimethylolpropane di(meth)acrylate, polyethylene glycol di( ⁇ -acryloyloxypropionate), trimethylolpropane tri( ⁇ -acryloyloxypropionate), poly(meth)allyloxyalkane, (poly)ethylene glycol diglycidyl ether, ethylene glycol, polyethylene glycol, and glycerol.
• intemal-crosslinking agents such as N,N'-methylenebis(meth)acrylamide, (poly)ethylene glycol di(meth)acrylate, (poly)propylene glyco
• the amount of the mtemal-crosslinking agent as used is favorably in the range of 0.005 to 2 mol %, more favorably 0.01 to 1 mol %, still more favorably 0.05 to 0.2 mol %, relative to the aforementioned monomer component.
• the aforementioned amount of the intemal-crosslinl ⁇ ng agent as used is smaller than 0.005 mol % or larger than 2 mol %, there is a possibility that desirable absorption properties cannot be obtained.
• the monomer concentration in the above aqueous solution (hereinafter, refened to as an aqueous solution of a water-soluble unsaturated monomer) is not especially limited, but it is favorably in the range of 15 to 70 weight % in view of properties, more favorably in high-concentration conditions where coarse gels are easily caused particularly.
• the present invention is preferably applied, for example, in a monomer concentration of still more favorably not less than 20 weight %, particularly favorably not less than 30 weight %, most favorably not less than 40 weight %.
• solvents other than water may be used together, if necessary, and there is no especial limitation on the kinds of the solvents as used together.
• various properties of the water-absorbent resin may be improved by adding such as various foaming agents, hydrophilic polymers, surfactants, chelating agents, and water-absorbent resin fine powders, or fine powders of the water-absorbent resin may be recycled.
• various foaming agents e.g., hydrophilic polymers, surfactants, chelating agents, and water-absorbent resin fine powders, or fine powders of the water-absorbent resin may be recycled.
• foaming agents e.g.
• an aldehyde compound including furfural to the above water-soluble unsaturated monomer.
• the amount of this furfural as added is essentially in the range of 11 to 1,000 weight ppm, favorably 25 to 900 weight ppm, more favorably 30 to 600 weight ppm, still more favorably 50 to 400 weight ppm, particularly favorably 100 to 300 weight ppm, relative to the water-soluble unsaturated monomer.
• the weight (mass) of the water-soluble unsaturated monomer does not include that of such as a crosslinking agent that is added if necessary.
• the water-soluble unsaturated monomer in the present invention is also favorably prepared by a process including the steps of synthesizing acrylic acid that purposely contains the furfural, and using the resultant acrylic acid that contains the furfural in a predetem ⁇ ned amount.
• the protoanemonin content in the acrylic acid is thought more favorably not more than 10 weight ppm, still more favorably not more than 5 weight ppm, particularly favorably not more than 3 weight ppm, most favorably not more than 1 weight ppm.
• a specific polymerization process namely a production process for a water-absorbent resin, comprising a polymerization step that includes the steps of: supplying the aqueous solution of the water-soluble unsaturated monomer to a polymerization vessel having shear force; and carrying out fine division of the hydrogel as obtained at the same time as the polymerization involving crosslinking.
• agitation machines having at least two agitation shafts e.g. twin-arm kneaders
• More favorably used are machines having a rotation agitation shaft in such a manner that the aqueous solution of the water-soluble unsaturated monomer is continuously supplied to the polymerization vessel and the resultant hydrogel is continuously discharged (of the above, machines having at least two rotation agitation shafts).
• Examples thereof include triple-shaft kneaders (kneader-ruders) having two agitation paddles and one discharging screw, and twin-shaft-extuding kneaders or blenders.
• the most favorable machines are continuous kneaders having piston flowability, which are machines having two rotation agitation shafts in such a manner that the aqueous solution of the water-soluble unsaturated monomer is continuously supplied to the polymerization vessel and the resultant hydrogel is continuously discharged.
• the polymerization vessels usable in the present invention are exemplified or shown also in the above patent documents.
• the surface roughness of the inner faces of these polymerization vessels is favorably decreased by resin-coating with such as Teflon (registered trade mark) or by electrolysis grinding, and polymerization vessels having stainless-steel-made inner faces are particularly favorably used.
• the inner faces and agitation shafts of the polymerization vessel are favorably cooled or heated with a jacket.
• the volume of the polymerization vessel is fitly determined in the range of usually 0.001 to 10 m 3 , favorably 0.01 to 5 m 3 , and the aqueous monomer solution is favorably charged in an amount of 10 to 90 %, more favorably 20 to 70 %, relative to the volume.
• the rotation agitation shafts existing in these polymerization vessels are rotated for at least a predetermined time during the polymerization, and then the fine division of the hydrogel is carried out.
• the rotation speed may be constant or changeable, or the rotation may be stopped temporarily or intermittently.
• the static polymerization and the rotation polymerization may be carried out together in the polymerization vessel having shear force.
• these agitation shafts may be rotated in the same direction or in different directions (two directions), but the at least two agitation shafts are favorably in the two directions toward the inside.
• the rotation speeds of both may be identical or different.
• Paddle dryer (PADDLE DRYER, Nara Kikai Seisakusho Co., Ltd.);
• the fine division of the gel is easy during the polymerization, and the coarse gel (for example, having a size of not smaller than 1 cm) is hardly formed because the furfural is used in the present invention. Therefore, it is more favorable for polymerization as initiated at high temperature and polymerization as initiated in high concentration, in which a coarse gel has hitherto been easily formed.
• the polymerization as carried out is favorably initiated at a temperature of not lower than 20 °C, more favorably not lower than 30 °C, still more favorably not lower than 40 °C, particularly favorably not lower than 50 °C.
• the present invention is favorably applied in a concentration of the aqueous unsaturated monomer of not less than 20 weight %, more favorably not less than 30 weight %, particularly favorably not less than 40 weight %.
• the finely divided hydrogel as obtained in the present invention is discharged from the polymerization vessel and transferred to a subsequent step.
• the above hydrogel has a uniform particle diameter and very little coarse-gel amount.
• the weight-average particle diameter is in the range of 0.3 to 4 mm and the ratio of coarse gel particles having particle diameters of not smaller than 10 mm is not more than 5 weight %.
• the hydrogel is finely divided until the hydrogel favorably has a weight-average particle diameter of 0.3 to 4 mm, more favorably 0.5 to 3 mm, still more favorably 0.8 to 2 mm.
• the coarse gel as decreased relates to a state of having a size of not smaller than 5 cm (favorably not smaller than 1 cm), and the coarse-gel content in the finely divided gel as discharged is also thought favorably not more than 7 weight %, more favorably not more than 5 weight %, still more favorably not more than 3 weight %, particularly favorably not more than 1 weight %, of the entirety.
• reductants examples include: (bi)sulfurous acid (or its salts) such as sodium sulfite and sodium hydrogensulfite; L-ascorbic acid (or its salts); reducible metals (or their salts) such as ferrous salts; and amines.
• sulfurous acid or its salts
• L-ascorbic acid or its salts
• reducible metals or their salts
• ferrous salts such as ferrous salts
• amines examples include: (bi)sulfurous acid (or its salts) such as sodium sulfite and sodium hydrogensulfite; L-ascorbic acid (or its salts); reducible metals (or their salts) such as ferrous salts; and amines.
• reducible metals or their salts
• ferrous salts or amines.
• the amount of these polymerization initiators or reductants as used is in the range of usually 0.001 to 2 mol %, favorably 0.01 to 0.5
• the aforementioned aqueous monomer solution that will be polymerized favorably further contains a trace amount of a transition metal, and the polymerization is particularly favorably carried out in the presence of a trace amount of iron.
• the content of the transition metal as used is favorably in the range of 0 to 5 weight ppm (relative to the monomer in terms of its cation), more favorably 0.1 to 2 weight ppm, particularly favorably 0.2 to 1 weight ppm.
• the transition metal is in excess, the residual monomer and the water-extractable content tend to increase.
• the polymerization rate tends to decrease.
• the present invention in which the furfural is further used in an amount of 11 to 1,000 weight ppm, can also be applied to other specific polymerization method except the polymerization including carrying out fine division in the polymerization vessel having shear force.
• the present invention can be applied to the high-temperature polymerization or polymerization in the coexistence of a transition metal. Namely, the conditions described in the aforementioned (4) can also be applied to the following specific polymerizations.
• the present invention also provides a production process for a water-absorbent resin, which comprises the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and carrying out fine division of the resultant hydrogel, with the production process being characterized in that:
• the resultant finely divided hydrogel has a weight-average particle diameter of 0.3 to 4 mm, and the ratio of coarse gel particles having particle diameters of not smaller than 10 mm is not more than 5 weight % of the finely divided hydrogel.
• the polymerization is initiated at a temperature that is higher than an ordinary temperature and not lower than 30 °C and/or the polymerization initiator contains a transition metal in the prior polymerization, the stability of monomers is deteriorated before the polymerization.
• the furfural is further used in an amount of 11 to 1,000 weight ppm.
• the present invention can also be favorably applied to the reversed-phase suspension polymerization.
• dispersants selected from surfactants and protective colloids may be dissolved or dispersed in the aqueous solution of the water-soluble unsaturated monomer to contain them therein.
• this dispersant in the aqueous monomer solution causes the monomer or polymer to disperse in a particulate form in the hydrophobic organic solvent more uniformly, and then the particle diameter distribution of the water-absorbent resin as finally obtained becomes narrower.
• nonionic surfactants e.g. (polyoxyethylene)phosphates, such as polyoxyethylene octylphenyl ether phosphate and polyoxyethylene tridecyl ether phosphate (both produced by Dai-Ichi Industrial Pharmacy, trade name: PLYSURF), polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and sucrose fatty acid esters), and anionic surfactants (e.g.
• nonionic surfactants e.g. (polyoxyethylene)phosphates, such as polyoxyethylene octylphenyl ether phosphate and polyoxyethylene tridecyl ether phosphate (both produced by Dai-Ichi Industrial Pharmacy, trade name: PLYSURF)
• polyoxyethylene alkyl ethers polyoxyethylene alkyl phenyl ethers
• n-hexane n-heptane
• cyclohexane methylcyclohexane
• toluene or xylene.
• the ratio of the hydrophobic organic solvent and the aqueous monomer solution is favorably 3:2 to 4:1 approximately.
• the dispersant or hydrophobic organic solvent may be added during or after the polymerization.
• the reversed-phase suspension polymerization is favorable for improving the coloring of the water-absorbent resin.
• concentration for example, the concentration of the aqueous monomer solution is not less than 40 weight %) in the reversed-phase suspension polymerization which is carried out by being dispersed in a large quantity of solvent, the scale-increased industrial production (for example, in a reaction vessel having not smaller than 1 m 3 , particularly not smaller than 5 m 3 ) has hitherto been difficult because of danger of explosion of the polymerization, and the productivity has been low.
• the water-absorbent resin can also be obtained by such as washing with water or a hydrophilic organic solvent after the polymerization, or by adding such as decoloring agents or bleaching agents on the market.
• the above water-absorbent resin more favorably displays the above-mentioned or below-mentioned various properties.
• the crosslinked gel polymer as obtained in the polymerization step is finely divided with such as a meat chopper or a gel pulverizer that is exemplified in Japanese Patent Application No. 232734/2001 if it is necessary.
• the polymer is more favorably dried, and pulverized and classified if necessary.
• the present invention water-absorbent resin has high properties, and therefore its properties are further improved after the above step.
• the drying method adoptable are various methods, such as heat drying, hot-wind drying, drying under reduced pressure, infrared drying, microwave drying, drum-dryer drying, azeotropic dehydration with hydrophobic organic solvents, and high-humidity drying with steam having high temperature, and there is no especial limitation on the drying method.
• the azeotropic dehydration is favorably applied to the reversed-phase suspension polymerization.
• the surface-crosslinking of a water-absorbent resin is to make uniform crosslinked structure in a polymer, and to further make a portion having high crosslinking density on a surface layer of the water-absorbent resin.
• the water-absorbent resin as obtained in the present invention has little water-extractable content and high absorption capacity, and therefore there are advantages in that: the excellent surface-crosslinking effect is obtained, and further the high properties or performance can be displayed.
• the surface-crosslinking is to make uniform crosslinked structure in a resin, and besides, to make a portion having high crosslinking density on a surface layer, and it is carried out with the following surface-crosslinking agent.
• the surface-crosslinking agent may be permeated or coated onto the resin surface.
• the surface-crosslinking of the resin enhances absorption capacity and liquid permeability under a load.
• the water-absorbent resin favorably displays an absorption capacity of not less than 20 g/g for a physiological saline under a load (4.9 kPa), more favorably not less than 23 g/g, still more favorably not less than 25 g/g.
• the water-absorbent resin is thought to favorably display a liquid permeation amount (SFC) of not smaller than 10 x 10 "7 , more favorably not smaller than 20 x 10 "7 , still more favorably not smaller than 50 x 10 "7 .
• SFC liquid permeation amount
• crosslinking agent for carrying out the above surface-crosslinking there are used various agents, but generally used in view of properties are such as polyhydric alcohol compounds, epoxy compounds, polyamine compounds or their condensed products with haloepoxy compounds, oxazoline compounds, mono-, di-, or polyoxazolidinone compounds, multivalent metal salts, and alkylene carbonate compounds.
• the surface-crosslinking agent as used in the present invention is specifically exemplified in such as specifications of USP 6,228,930, USP 6,071,976, and USP 6,254,990.
• examples thereof include one or at least two of: polyhydric alcohol compounds, such as mono-, di-, tri-, tetra-, or polyethylene glycol, monopropylene glycol, 1,3-propanediol, dipropylene glycol,
• At least two of these agents may be used in combinations with each other.
• at least the polyhydric alcohol a polyhydric alcohol favorably having 2 to 10 carbon atoms, more favorably 3 to 8 carbon atoms
• the amount of the surface-croslinking agent as used is favorably in the range of 0.001 to 10 parts by weight, more favorably 0.01 to 5 parts by weight, relative to 100 parts by weight of the resin in terms of solid content.
• the temperature of a solution of the crossliriking agent is favorably adjusted to the range of 0 °C to a boiling point, more favorably 5 to 50 °C, still more favorably 10 to 30 °C, in view of blendability and stability.
• the temperature of the water-absorbent resin powder before blending is favorably in the range of 0 to 80 °C, more favorably 40 to 70 °C, in view of blendability.
• favorable is a method which involves: blending with the water and/or hydrophilic organic solvent if necessary; and thereafter spraywise or dropwise blending (more favorably spraying) the resultant aqueous solution with the water-absorbent resin.
• the size of the liquid drop as sprayed is favorably not larger than 300 ⁇ m, more favorably not larger than 200 ⁇ m.
• water-insoluble fine particle powders or surfactants may coexist in such a range as not to hinder the effects of the present invention when the blending is carried out.
• the favorable blending apparatus as used in the aforementioned blending can produce great blendability in order to ensure the uniform blending.
• various blenders are used, but favorable is a high-speed stirring-type blender, particularly a high-speed stirring-type continuous blender.
• Turbulizer trade name
• (Lodige mixer) (trade name) are usable.
• the water-absorbent resin is favorably heat-treated after the crosslinking agent is blended.
• the heating temperature is favorably in the range of 100 to 250 °C, more favorably 150 to 250 °C, and the heating time is favorably in the range of 1 minute to 2 hours.
• the favorable examples of the combination of the temperature and the time are 180 °C and 0.1 to 1.5 hours, and 200 °C and 0.1 to 1 hour.
• the heat treatment can be carried out with conventional dryers or heating furnaces.
• the dryers include groove-type blending dryers, rotary dryers, disc dryers, fluidized-bed dryers, air-stream-type dryers, and infrared dryers.
• the water-absorbent resin may be cooled after the heating.
• these surface-crosslinking methods are also mentioned in: various European patents, such as specifications of EP 0349240, EP 0605150, EP 0450923, EP 0812873, EP 0450924, and EP 0668080; various Japanese patents, such as gazettes of JP-A-242709/1995 and JP-A-224304/1995; various US patents, such as specifications of USP 5,409,771, USP 5,597,873, USP 5,385,983, USP 5,610,220, USP 5,633,316, USP 5,674,633, and USP 5,462,972; and various international published patents, such as pamphlets of WO 99/42494, WO 99/43720, and WO 99/42496, and these surface-crosslinking methods can also be applied to the present invention.
• the present invention process enables easy production of the water-absorbent resin, which has good absorption properties that are excellent in balance of the absorption capacity without load, absorption capacity under a load, and extractable content.
• the water-absorbent resin is widely used for water-retaining agents in agricultural and horticultural fields, water-holding materials in engineering works fields, hygroscopic agents, moisture-removing agents, and building materials, but the water-absorbent resin is particularly favorably used for sanitary materials such as disposable diapers and sanitary napkins.
• the present invention water-absorbent resin is excellent in the above three properties, and therefore the sanitary materials can be used generally in a high concentration (e.g.
• the colloidal titration of a water-soluble poly(acrylic acid salt) in the resultant filtrate as eluted from the water-absorbent resin was carried out with methyl glycol chitosan and potassium polyvinyl sulfate, and thereby the weight percentage of the water-extractable content in the water-absorbent resin (relative to the water-absorbent resin) was determined.
• the separately prepared filtrate after being stirred for 2 hours in the above (2) was UV-analyzed with liquid chromatography, and thereby the amount of the residual monomer in the water-absorbent resin (weight ppm/relative to water-absorbent resin) was also analyzed.
• the temperature of the monomer or polymerization gel during the polymerization was measured with a thermometer, and how long time passed since the addition of the initiator until the rise of temperature (minute) was defined as the induction time, and further how long time passed until the highest temperature of the polymerization system was defined as the peak time.
• the hydrogel was classified in a wet state to calculate its particle diameter distribution, and then the weight-average particle diameter (D50) was determined by plotting the particle diameter distribution on logarithmic probability paper.
• D50 weight-average particle diameter
• the color of the water-absorbent resin powder was measured as follows. That is to say, about 3 g of water-absorbent resin powder as obtained were all filled into a paste sample stand (30 mm ⁇ ), and then the surface color of the water-absorbent resin was measured in terms of the coloring degree (YI) with a spectral color difference meter, SZ- ⁇ 80 COLOR MEASURING SYSTEM (produced by Nippon Denshoku Kogyo Co., Ltd.), under the setting conditions (reflection measurement/attached powder-paste sample stand (30 mm ⁇ )/standard circular white plate NO2 for powder-paste as the standard/ 30 ⁇ floodlight pipe).
• YI coloring degree
• SZ- ⁇ 80 COLOR MEASURING SYSTEM produced by Nippon Denshoku Kogyo Co., Ltd.
• the above aqueous solution (1) of the water-soluble unsaturated monomer containing the furfural in an amount of 201.6 weight ppm was supplied to a reactor as prepared by lidding a jacketed stainless twin-arm kneader of 10 liters in capacity with two sigma-type blades as a polymerization vessel having shear force, and the atmosphere in the reactor was replaced with nitrogen for 20 minutes while the above aqueous solution was maintained at 25 °C.
• the above particulate crosslinked hydrogel polymer (1) was spread on a wire gauze with a mesh opening size of 300 ⁇ m to form a layer of about 50 mm in thickness. Subsequently, the hydrogel was hot-wind-dried by passing a hot wind having a temperature of 170 °C (dew point: 50 °C) at a speed of 1 m sec through the hydrogel in its vertical direction for 1 hour. Pulverized was a block-shaped dried material which was obtained in this way and comprised of a particulate dry polymer, and further the resultant pulverized product was classified with a JIS standard sieve having a mesh opening size of 850 ⁇ m, thus obtaining a water-absorbent resin powder (1).
• the above aqueous monomer solution (2) containing the furfural in an amount of 205.3 weight ppm was supplied to the same polymerization vessel as of Example 1, and the atmosphere in the reactor was replaced with nitrogen for 20 minutes while the above aqueous solution was maintained at 40 °C. Subsequently, while warm water of 40 °C was passed and the blades were rotated, 22.6 g of 20 weight % aqueous sodium persulfate solution and 12.5 g of 1.0 weight % aqueous L-ascorbic acid solution were added to the reactor. As a result, the polymerization was initiated after 10 seconds.
• Example 1 was changed, and thereby prepared was a comparative aqueous water-soluble monomer solution (1) with a concentration of 40 weight %, in which the furfural amount was adjusted to 0.3 weight ppm (relative to the monomer in terms of solid content).
• Example 1 The above comparative aqueous water-soluble monomer solution (1) containing the furfural in an amount of 0.3 weight ppm was supplied to the reactor as mentioned in Example 1. Since then, the atmosphere in the reactor was replaced with nitrogen in the same way as of Example 1, and the sodium persulfate and L-ascorbic acid were similarly added thereto. As a result, the polymerization was initiated after 10 seconds. The warm water was heated to 60 °C at the same time as the initiation of the polymerization. After 8.5 minutes, the reaction system reached its peak temperature, and thereafter the resultant crosslinked hydrogel polymer was sheared while the polymerization was carried out. The polymerization was completed after 20 minutes since the peak temperature was shown.
• the major proportion of the resultant comparative hydrogel polymer (1) was in the form of particles having particle diameters of about 1 mm in the same way as of Example 1, but the polymer was not finely divided sufficiently, and a coarse gel was partially observed therein.
• the hydrogel (coarse gel) particles having particle diameters of not smaller than 10 mm existed in an amount of 8.0 weight % of the entirety.
• Example 3 Prepared was an aqueous solution (3) of a water-soluble unsaturated monomer with a concentration of 40 weight %, including 447.6 g of acrylic acid, 4,736.5 g of 37 weight % aqueous sodium acrylate solution, 8.4657 g of polyethylene glycol diacrylate (average polyethylene glycol unit number: 8) as an mtemal-crosslinking agent, furfural in an amount of 205.3 weight ppm (relative to the monomer in terms of solid content), and 270.5 g of water.
• aqueous solution (3) of a water-soluble unsaturated monomer with a concentration of 40 weight % including 447.6 g of acrylic acid, 4,736.5 g of 37 weight % aqueous sodium acrylate solution, 8.4657 g of polyethylene glycol diacrylate (average polyethylene glycol unit number: 8) as an mtemal-crosslinking agent, furfural in an amount of 205.3 weight ppm (relative to the monomer in terms
• the above aqueous monomer solution (3) containing the furfural in an amount of 205.3 weight ppm was supplied to the same reactor as of Example 1, and the atmosphere in the reactor was replaced with nitrogen for 20 minutes while the above aqueous solution was maintained at 25 °C.
• the production process for a water-absorbent resin which comprises a polymerization step that includes the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time carrying out fine division of the resultant hydrogel, the water-soluble unsaturated monomer contains furfural in an amount of 11 to 1,000 weight ppm (relative to the monomer in terms of solid content), and thereby a coarse gel is not formed at all or hardly formed during the polymerization.
• Example 4 To 500 g of the water-absorbent resin powder (1) as obtained in Example 1, an aqueous surface-crosslinking agent solution including 1,4-butanediol, propylene glycol, and water in amounts of 0.32 weight %, 0.5 weight %, and 2.73 weight % (relative to the powder) respectively was added, and the resultant mixture was heat-stirred for 35 minutes in a mixer as heated in an oil bath at 212 °C, thus obtaining a surface-crosslinked water-absorbent resin powder (4). The results are listed in Table 2.
• a water-soluble unsaturated monomer with a concentration of 40 weight %, including 21.62 g of acrylic acid, 228.77 g of 37 weight % aqueous sodium acrylate solution, 0.292 g of polyethylene glycol diacrylate (average polyethylene glycol unit number: 8) as an mtemal-crosslinking agent, furfural in an amount of 203.5 weight ppm (relative to the monomer in terms of solid content), and 13.11 g of water.
• the above aqueous solution was added to a cylindrical polypropylene vessel of about 500 ml in capacity after the atmosphere in the vessel was replaced with nitrogen for 30 minutes while the above aqueous solution was maintained at 40 °C.
• the above polymerization vessel was lidded and heat-retained at 40 °C in an atmosphere of nitrogen, and then 1.44 g of 10 weight % aqueous sodium persulfate solution and 0.43 g of 0.5 weight % aqueous L-ascorbic acid solution as polymerization initiators were added thereto.
• the polymerization was initiated after 10 seconds. After 11 minutes, the peak temperature was shown, and thereafter the polymerization was completed after 10 minutes since the peak temperature was shown.
• the resultant crosslinked hydrogel polymer (5) was cut into small pieces having sizes of a few millimeters, and hot-wind-dried at 170 °C for 30 minutes, and thereafter the resultant dried product was pulverized with a vibration mill, and further the resultant pulverized product was classified with a JIS standard sieve having a mesh opening size of 850 ⁇ m, thus obtaining a water-absorbent resin powder (5).
• the results are listed in Table 3.
• Example 6 The replacement with nitrogen was carried out in the same way as of Example 5 except that the temperature of the water-soluble unsaturated monomer (5) was changed from 40 °C to 20 °C. Since then, the polymerization initiators were added to the water-soluble unsaturated monomer (5) of 20 °C in the same way as of Example 3. As a result, the polymerization was initiated after 30 seconds, and the temperature was raised by the polymerization heat. However, it took 84 minutes to reach the peak temperature, and the polymerization was completed after 10 minutes since the peak temperature was shown.
• the resultant comparative crosslinked hydrogel polymer (3) was cut and dried in the same way as of Example 5, and thereafter they were pulverized and classified similarly, thus obtaining a comparative • water-absorbent resin powder (3). The results are listed in Table 3. [Example 6] The replacement with nitrogen was carried out in the same way as of Example
• An aqueous solution (7) of a water-soluble unsaturated monomer with a monomer concentration of 35 weight % and a neutralization ratio of 75 % was obtained by using 18.0 g of acrylic acid, 190.6 g of 37 weight % aqueous sodium acrylate solution having a furfural content of 70 weight ppm (in terms of solid content), 0.2435 g of polyethylene glycol diacrylate (average polyethylene glycol unit number: 8) as an mtemal-crosslinking agent, and 43.1 g of deionized water.
• aqueous monomer solution (7) 0.048 g of sodium persulfate was dissolved, and the dissolved oxygen was removed by blowing nitrogen gas.
• a four-necked separable flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen-introducing tube, and a dropping funnel was charged with 389.5 g of cyclohexane, and 1.77 g of ethyl cellulose (produced by HERCULES INC., N-200) as a dispersant was added thereto and dissolved, and then the dissolved oxygen was removed by blowing nitrogen gas. Subsequently, the aqueous monomer solution (7) was added to the above separable flask, and the resultant mixture was dispersed with stirring at 225 rpm.
• aqueous solution (8) of a water-soluble unsaturated monomer with a monomer concentration of 40 weight % and a neutralization ratio of 75 % was obtained by using 18.0 g of acrylic acid having a furfural content of 250 ppm, 190.6 g of 37 weight % aqueous sodium acrylate solution having a furfural content of 190 weight ppm (in terms of solid content), 0.2435 g of polyethylene glycol diacrylate (average polyethylene glycol unit number: 8) as an mtemal-crosslinking agent, and 11.0 g of deionized water.
• aqueous monomer solution (8) 0.072 g of sodium persulfate was dissolved, and the dissolved oxygen was removed by blowing nitrogen gas.
• a four-necked separable flask equipped with a stirrer, a reflux condenser, a thermometer, a nitrogen-introducing tube, and a dropping funnel was charged with 389.5 g of cyclohexane, and 1.77 g of ethyl cellulose (produced by HERCULES INC., N-200) as a dispersant was added thereto and dissolved, and the dissolved oxygen was removed by blowing nitrogen gas.
• the use of the furfural also enables the polymerization such as the high-temperature polymerization or high-concentration polymerization mildly, and further a water-absorbent resin, which is not substantially colored and has a YI value of not more than 10, is obtained in the reversed-phase suspension polymerization.
• the present invention can provide a production process in which a coarse gel itself is made not to form, in a production process for a water-absorbent resin, which comprises a polymerization step that includes the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and at the same time carrying out fine division of the resultant hydrogel, or in a production process for a water-absorbent resin, which comprises the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acrylic acid (salt); and carrying out fine division of the resultant hydrogel.
• the present invention can provide a production process which can solve the problems such that the stability of the monomer is deteriorated when the high-temperature polymerization or polymerization in the presence of a transition metal is carried out, in a production process for a water-absorbent resin, comprising the steps of: carrying out polymerization, involving crosslinking, of an aqueous solution of a water-soluble unsaturated monomer including acryhc acid (salt); and canying out fine division of the resultant hydrogel.

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