Classifications

• • 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
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F13/00—Bandages or dressings; Absorbent pads
  • A61F13/15—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators
  • A61F13/53—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators characterised by the absorbing medium
• • 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
  • A61L15/22—Bandages, dressings or absorbent pads for physiological fluids such as urine or blood, e.g. sanitary towels, tampons containing macromolecular materials
  • A61L15/24—Macromolecular compounds obtained by reactions only involving carbon-to-carbon unsaturated bonds; Derivatives 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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J20/00—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof
  • B01J20/22—Solid sorbent compositions or filter aid compositions; Sorbents for chromatography; Processes for preparing, regenerating or reactivating thereof comprising organic material
  • B01J20/26—Synthetic macromolecular compounds
  • B01J20/261—Synthetic macromolecular compounds obtained by reactions only involving carbon to carbon unsaturated bonds
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B08—CLEANING
  • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
  • B08B1/00—Cleaning by methods involving the use of tools
  • B08B1/20—Cleaning of moving articles, e.g. of moving webs or of objects on a conveyor
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B08—CLEANING
  • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
  • B08B3/00—Cleaning by methods involving the use or presence of liquid or steam
  • B08B3/02—Cleaning by the force of jets or sprays
  • B08B3/022—Cleaning travelling work
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B08—CLEANING
  • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
  • B08B5/00—Cleaning by methods involving the use of air flow or gas flow
  • B08B5/02—Cleaning by the force of jets, e.g. blowing-out cavities
  • B08B5/023—Cleaning travelling work
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
  • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
  • B29C35/00—Heating, cooling or curing, e.g. crosslinking or vulcanising; Apparatus therefor
  • B29C35/02—Heating or curing, e.g. crosslinking or vulcanizing during moulding, e.g. in a mould
  • B29C35/0277—Apparatus with continuous transport of the material to be cured
• • 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
  • C08F220/00—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
  • C08F220/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
  • C08F220/04—Acids; Metal salts or ammonium salts thereof
  • C08F220/06—Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
• • 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
  • C08F222/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and at least one being terminated by a carboxyl radical and containing at least one other carboxyl radical in the molecule; Salts, anhydrides, esters, amides, imides, or nitriles thereof
  • C08F222/10—Esters
  • C08F222/1006—Esters of polyhydric alcohols or polyhydric phenols
  • C08F222/102—Esters of polyhydric alcohols or polyhydric phenols of dialcohols, e.g. ethylene glycol di(meth)acrylate or 1,4-butanediol dimethacrylate
• • 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
  • C08F6/00—Post-polymerisation treatments
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B17/00—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement
  • F26B17/02—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by belts carrying the materials; with movement performed by belts or elements attached to endless belts or chains propelling the materials over stationary surfaces
  • F26B17/023—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by belts carrying the materials; with movement performed by belts or elements attached to endless belts or chains propelling the materials over stationary surfaces the material being a slurry or paste, which adheres to a moving belt-like endless conveyor for drying thereon, from which it may be removed in dried state, e.g. by scrapers, brushes or vibration
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F26—DRYING
  • F26B—DRYING SOLID MATERIALS OR OBJECTS BY REMOVING LIQUID THEREFROM
  • F26B17/00—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement
  • F26B17/02—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by belts carrying the materials; with movement performed by belts or elements attached to endless belts or chains propelling the materials over stationary surfaces
  • F26B17/04—Machines or apparatus for drying materials in loose, plastic, or fluidised form, e.g. granules, staple fibres, with progressive movement with movement performed by belts carrying the materials; with movement performed by belts or elements attached to endless belts or chains propelling the materials over stationary surfaces the belts being all horizontal or slightly inclined
• • A—HUMAN NECESSITIES
  • A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
  • A61F—FILTERS IMPLANTABLE INTO BLOOD VESSELS; PROSTHESES; DEVICES PROVIDING PATENCY TO, OR PREVENTING COLLAPSING OF, TUBULAR STRUCTURES OF THE BODY, e.g. STENTS; ORTHOPAEDIC, NURSING OR CONTRACEPTIVE DEVICES; FOMENTATION; TREATMENT OR PROTECTION OF EYES OR EARS; BANDAGES, DRESSINGS OR ABSORBENT PADS; FIRST-AID KITS
  • A61F13/00—Bandages or dressings; Absorbent pads
  • A61F13/15—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators
  • A61F13/53—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators characterised by the absorbing medium
  • A61F2013/530481—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators characterised by the absorbing medium having superabsorbent materials, i.e. highly absorbent polymer gel materials
  • A61F2013/530583—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators characterised by the absorbing medium having superabsorbent materials, i.e. highly absorbent polymer gel materials characterized by the form
  • A61F2013/530591—Absorbent pads, e.g. sanitary towels, swabs or tampons for external or internal application to the body; Supporting or fastening means therefor; Tampon applicators characterised by the absorbing medium having superabsorbent materials, i.e. highly absorbent polymer gel materials characterized by the form in granules or particles
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J2220/00—Aspects relating to sorbent materials
  • B01J2220/50—Aspects relating to the use of sorbent or filter aid materials
  • B01J2220/68—Superabsorbents
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B08—CLEANING
  • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
  • B08B1/00—Cleaning by methods involving the use of tools
  • B08B1/10—Cleaning by methods involving the use of tools characterised by the type of cleaning tool
  • B08B1/16—Rigid blades, e.g. scrapers; Flexible blades, e.g. wipers
  • B08B1/165—Scrapers
• • 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
  • C08F6/00—Post-polymerisation treatments
  • C08F6/008—Treatment of solid polymer wetted by water or organic solvents, e.g. coagulum, filter cakes
• • 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
  • C08J2333/00—Characterised by the use of homopolymers or 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 of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
  • C08J2333/02—Homopolymers or copolymers of acids; Metal or ammonium salts thereof

Definitions

• the present invention relates to a method for producing superabsorbent particles by polymerizing a monomer solution or suspension, comprising drying the aqueous polymer gel obtained in a circulating air belt dryer, grinding, classification, and optionally thermal surface postcrosslinking, the aqueous polymer gel using an oscillating conveyor belt in the circulating air belt dryer is introduced, the underside of the returning conveyor belt is cleaned of adhering polymer gel by means of at least one scraper device and the underside of the returning conveyor belt is sprayed with water.
• Superabsorbents are used for the production of diapers, tampons, sanitary napkins and other hygiene articles, but also as water-retaining agents in agricultural horticulture.
• the superabsorbents are also referred to as water-absorbing polymers.
• the properties of the superabsorbers can be set, for example, via the amount of crosslinking agent used. As the amount of crosslinker increases, the centrifuge retention capacity (CRC) decreases and the absorption under a pressure of 21.0 g / cm 2 (AUL0.3psi) runs through a maximum.
• CRC centrifuge retention capacity
• superabsorbent particles are generally post-crosslinked. This increases the degree of crosslinking of the particle surface, whereby the absorption under a pressure of 49.2 g / cm 2 (AUL0.7psi) and the centrifuge retention capacity (CRC) can be at least partially decoupled.
• This surface post-crosslinking can be carried out in the aqueous gel phase.
• dried, ground and sieved polymer particles base polymer
• Crosslinkers suitable for this purpose are compounds which can form covalent bonds with at least two carboxylate groups of the polymer particles.
• WO 2008/087114 A1 describes the loading of the conveyor belts of circulating air belt dryers with aqueous polymer gel by means of oscillating conveyor belts.
• the object of the present invention was to provide an improved method for producing superabsorbers, in particular a simplified cleaning of the one used oscillating conveyor belt and a lower mechanical load on the conveyor belt itself.
• the object was achieved by a process for the production of superabsorbers by polymerizing a monomer solution or suspension, comprising a) at least one ethylenically unsaturated, acid-bearing monomer which is at least partially neutralized,
• e) optionally one or more water-soluble polymers including drying of the aqueous polymer gel obtained in a circulating air belt dryer, grinding, classification, and optionally thermal surface postcrosslinking, characterized in that the aqueous polymer gel is introduced into the circulating air belt dryer by means of an oscillating conveyor belt Underside of the returning conveyor belt is cleaned of adhering polymer gel by means of at least one scraper device and the underside of the returning conveyor belt is sprayed with water.
• the underside of the returning conveyor belt is the outside of the conveyor belt, onto which the polymer gel is dosed again after deflection.
• the top of the returning conveyor belt is the inside of the conveyor belt, which should not come into contact with polymer gel.
• the distance of the stripping device from the discharge end of the oscillating conveyor belt is preferably less than 20% of the length of the oscillating conveyor belt, particularly preferably less than 10% of the length of the oscillating conveyor belt, very particularly preferably less than 5% of the length of the oscillating conveyor belt, the Length of the oscillating conveyor belt is the distance of the swivel axis from the discharge end.
• the scraper is not restricted. Brushes arranged transversely to the running direction are suitable, for example. It is also possible to use a scraper.
• a scraper is a stripping device made of a non-flexible material that is arranged transversely to the conveying direction.
• a suitable non-flexible material is, for example, polytetrafluoroethylene.
• the scraper In order to avoid damage to the conveyor belt, the scraper should have as little or no direct contact with the conveyor belt.
• the scraper should be inclined in relation to the running direction of the returning conveyor belt. This promotes the peeling of the adhering polymer gel and prevents a jam between the conveyor belt and the scraper.
• the stripped polymer gel usually falls onto the conveyor belt of the circulating air belt dryer.
• the scraper is preferably inclined from 5 to 45 °, particularly preferably from 10 to 35 °, very particularly from 15 to 25 °, with respect to the vertical against the running direction of the conveyor belt.
• the distance between the scraper and the underside of the returning conveyor belt is preferably 0.1 to 5 mm, particularly preferably 0.2 to 2 mm, very particularly preferably 0.5 to 1.5 mm.
• the distance of the spray nozzles from the discharge end of the oscillating conveyor belt is preferably 1 to 50% of the length of the oscillating conveyor belt, particularly preferably 2 to 30% of the length of the oscillating conveyor belt, very particularly preferably 3 to 10% of the length of the oscillating conveyor belt, the Length of the oscillating conveyor belt is the distance of the swivel axis from the discharge end.
• the distance of the spray nozzles from the conveyor belt is preferably 5 to 50 cm, particularly preferably 10 to 30 cm, very particularly preferably 15 to 25 cm.
• the liquid is preferably sprayed on by means of at least one two-substance nozzle, particularly preferably by means of at least two two-substance nozzles.
• Two-substance nozzles enable atomization into fine droplets or a spray mist.
• a circular or elliptical full or hollow cone is formed as the form of atomization.
• Two fabric nozzles can be designed with external mixing or internal mixing. With the external mixing two-substance nozzles, liquid and atomizing gas leave the nozzle head via separate openings. They are only mixed in the spray jet after they have emerged from the spray nozzle. This enables the droplet size distribution and throughput to be controlled independently in a wide range.
• the spray cone of the spray nozzle can be adjusted via the air cap position.
• liquid and atomizing gas are mixed within the spray nozzle and the two-phase mixture leaves the nozzle head via the same hole (or via several holes connected in parallel).
• the quantity and pressure ratios are more strongly coupled with the internal mixing two-component nozzle than with the external mixing spray nozzle. Small changes in throughput therefore lead to changes in the drop size distribution.
• the adaptation to the desired throughput takes place via the selected cross section of the nozzle bore.
• Compressed air of 0.5 bar and more can be used as an atomizing gas.
• the droplet size can be set individually via the nozzle geometry, the nozzle type, the ratio of water mass flow to atomizer gas mass flow, as well as gas and water pressure.
• the amount of water used for spraying is preferably from 2 to 20 kg / h, particularly preferably from 6 to 16 kg / h, very particularly preferably from 8 to 12 kg / h.
• the weight ratio of atomizing gas to water is preferably from 2 to 20, particularly preferably from 6 to 16, very particularly preferably from 8 to 12. If too little water is used, the conveyor belt will only be insufficiently wetted. If too much water is used, excess water drips onto the perforated plates of the circulating air belt dryer and leads to blockages there.
• the use of two-component nozzles additionally improves the distribution of the water by preventing larger drops.
• the at least one spray nozzle is located behind the stripping device in the running direction of the conveyor belt. This prevents water from dripping from the scraper onto the conveyor belt of the circulating air belt dryer.
• the at least one spray nozzle is located in front of the conveyor belt of the circulating air belt dryer. This also prevents water from dripping onto the conveyor belt of the circulating air belt dryer.
• the conveyor belt has a length of preferably 2 to 10 m, particularly preferably from 2.5 to 8 m, very particularly preferably from 3 to 6 m, the length of the conveyor belt being the distance of the pivot axis from the discharge end.
• the conveyor belt has a width of preferably 0.5 to 1.5 m, particularly preferably 0.6 to 1.2 m, very particularly preferably 0.7 to 0.9 m.
• the conveyor belts customary for this purpose can be used.
• the surface of the conveyor belts, i.e. the side coming into contact with the polymer gel should be water-repellent and have a contact angle at 23 ° C. with respect to water of preferably at least 60 °, particularly preferably of at least 80 °, very particularly preferably of at least 100 °.
• the contact angle is a measure of the wetting behavior and is measured according to DIN 53900.
• the water content of the polymer gel on the conveyor belt is preferably from 20 to 80% by weight, particularly preferably from 30 to 70% by weight, very particularly preferably from 40 to 60% by weight.
• the temperature of the polymer gel on the conveyor belt is preferably from 60 to 105 ° C., particularly preferably from 70 to 100 ° C., very particularly preferably from 80 to 95 ° C.
• the present invention is based on the finding that the warm polymer gel adhering to the oscillating conveyor belt dries very easily.
• the dried polymer gel is difficult to remove from the conveyor belt.
• Polymer gel that has already dried is often the cause of further caking.
• the polymer gel can be kept moist by spraying with water and drying out can be avoided.
• the manufacture of the superabsorbers is explained in more detail below:
• the superabsorbers are produced by polymerizing a monomer solution or suspension and are usually water-insoluble.
• the monomers a) are preferably water-soluble, i.e. the solubility in water at 23 ° C. is typically at least 1 g / 100 g of water, preferably at least 5 g / 100 g of water, particularly preferably at least 25 g / 100 g of water, very particularly preferably at least 35 g / 100 g water.
• Suitable monomers a) are, for example, ethylenically unsaturated carboxylic acids, such as acrylic acid, methacrylic acid, and itaconic acid. Particularly preferred monomers are acrylic acid and methacrylic acid. Acrylic acid is very particularly preferred.
• Suitable monomers a) are, for example, ethylenically unsaturated sulfonic acids, such as styrene sulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
• styrene sulfonic acid such as styrene sulfonic acid and 2-acrylamido-2-methylpropanesulfonic acid (AMPS).
• AMPS 2-acrylamido-2-methylpropanesulfonic acid
• Impurities can have a significant impact on the polymerization. Therefore, the raw materials used should be as pure as possible. It is therefore often advantageous to specifically clean the monomers a). Suitable cleaning processes are described, for example, in WO 02/055469 A1, WO 03/078378 A1 and WO 2004/035514 A1.
• a suitable monomer a) is, for example, an acrylic acid purified according to WO 2004/035514 A1 with 99.8460% by weight acrylic acid, 0.0950% by weight acetic acid, 0.0332% by weight water, 0.0203% by weight % Propionic acid, 0.0001% by weight furfural, 0.0001% by weight maleic anhydride, 0.0003% by weight diacrylic acid and 0.0050% by weight hydroquinone monomethyl ether.
• the proportion of acrylic acid and / or salts thereof in the total amount of monomers a) is preferably at least 50 mol%, particularly preferably at least 90 mol%, very particularly preferably at least 95 mol%.
• the monomers a) usually contain polymerization inhibitors, preferably hydroquinone half ethers, as storage stabilizers.
• the monomer solution preferably contains up to 250 ppm by weight, preferably at most 130 ppm by weight, particularly preferably at most 70 ppm by weight, preferably at least 10 ppm by weight, particularly preferably at least 30 ppm by weight, in particular by 50% by weight .
• ppm hydroquinone half ether, each based on the unneutralized monomer a).
• an ethylenically unsaturated, acid-bearing monomer with a corresponding content of hydroquinone half ether can be used to prepare the monomer solution.
• Preferred hydroquinone half ethers are hydroquinone monomethyl ether (MEHQ) and / or alpha-tocopherol (vitamin E).
• Suitable crosslinkers b) are compounds with at least two groups suitable for crosslinking. Such groups are, for example, ethylenically unsaturated groups which can be radically polymerized into the polymer chain and functional groups which can form covalent bonds with the acid groups of the monomer a). Furthermore, polyvalent metal salts which can form coordinative bonds with at least two acid groups of the monomer a) are also suitable as crosslinkers b).
• Crosslinkers b) are preferably compounds having at least two polymerizable groups which can be polymerized into the polymer network by free radicals.
• Suitable crosslinkers b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallylammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1, di- and triacrylates
• Preferred crosslinkers b) are pentaerythritol triallyl ether, tetraallyloxyethane, methylene bismethacrylamide, 15-fold ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate and triallylamine.
• Very particularly preferred crosslinkers b) are the multiply ethoxylated and / or propoxylated glycerols esterified with acrylic acid or methacrylic acid to give di- or triacrylates, as described, for example, in WO 03/104301 A1.
• Di- and / or triacrylates of 3- to 10-fold ethoxylated glycerol are particularly advantageous.
• Di- or triacrylates of 1- to 5-fold ethoxylated and / or propoxylated glycerol are very particularly preferred.
• the most preferred are the triacrylates of 3 to 5-fold ethoxylated and / or propoxylated glycerol, in particular the triacrylate of 3-fold ethoxylated glycerol.
• the amount of crosslinker b) is preferably 0.05 to 1.5% by weight, particularly preferably 0.1 to 0.8% by weight, very particularly preferably 0.15 to 0.5% by weight, in each case calculated on the total amount of monomer a) used.
• the centrifuge retention capacity decreases and the absorption under a pressure of 21.0 g / cm 2 passes through a maximum.
• initiators c All compounds which generate free radicals under the polymerization conditions can be used as initiators c), for example thermal initiators, redox initiators, photoinitiators.
• Suitable redox initiators are sodium peroxodisulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium peroxodisulfate / sodium bisulfite and hydrogen peroxide / sodium bisulfite.
• thermal initiators and redox Initiators used such as sodium peroxodisulfate / hydrogen peroxide / ascorbic acid.
• the reducing component used is preferably the disodium salt of 2-hydroxy-2-sulfonatoacetic acid or a mixture of the sodium salt of 2-hydroxy-2-sulfinatoacetic acid, the disodium salt of 2-hydroxy-2-sulfonatoacetic acid and sodium bisulfite.
• Such mixtures are available as Brüggolite® FF6 and Brüggolite® FF7 (Brüggemann Chemicals; Heilbronn;
• aqueous monomer solution is usually used.
• the water content of the monomer solution is preferably from 40 to 75% by weight, particularly preferably from 45 to 70% by weight, very particularly preferably from 50 to 65% by weight. It is also possible to have monomer suspensions, i.e. To use monomer solutions with the solubility-exceeding monomer a), for example sodium acrylate. With increasing water content, the energy consumption during the subsequent drying increases and with decreasing water content, the heat of polymerization can only be dissipated insufficiently.
• the monomer solution can be pre-polymerized by inertization, i.e. Flowing through with an inert gas, preferably nitrogen or carbon dioxide, freed from dissolved oxygen.
• the oxygen content of the monomer solution is preferably reduced to less than 1 ppm by weight, particularly preferably to less than 0.5 ppm by weight, very particularly preferably to less than 0.1 ppm by weight, before the polymerization.
• Suitable reactors for the polymerization are, for example, kneading reactors or belt reactors.
• the polymer gel resulting from the polymerization of an aqueous monomer solution or suspension is continuously comminuted by, for example, counter-rotating stirring shafts, as described in WO 2001/038402 A1.
• the polymerization on the belt is described for example in DE 38 25 366 A1 and US 6,241,928.
• Polymerization in a belt reactor produces a polymer gel that has to be comminuted, for example in an extruder or kneader.
• the comminuted polymer gel obtained by means of a kneader can also be extruded.
• the acid groups of the polymer gels obtained are usually partially neutralized.
• the neutralization is preferably carried out at the monomer stage. This is usually done by mixing in the neutralizing agent as an aqueous solution or preferably also as a solid.
• the degree of neutralization is preferably from 25 to 85 mol%, particularly preferably from 30 to 80 mol%, very particularly preferably from 40 to 75 mol%, it being possible to use the customary neutralizing agents, preferably alkali metal hydroxides, alkali metal oxides, Alkali metal carbonates or alkali metal hydrogen carbonates and mixtures thereof.
• alkali metal salts ammonium salts can also be used.
• Sodium and potassium are particularly preferred as alkali metals, but sodium hydroxide, sodium carbonate or sodium hydrogen carbonate and mixtures thereof are very particularly preferred.
• Solid carbonates and hydrogen carbonates can also be used in encapsulated form, preferably in the monomer solution directly before the polymerization, during or after the polymerization into the polymer gel and before it dries.
• the encapsulation is carried out by coating the surface with an insoluble or only slowly soluble material (for example using film-forming polymers, inert inorganic materials or meltable organic materials), which delays the solution and reaction of the solid carbonate or hydrogen carbonate in such a way that Carbon dioxide is only released during drying and the resulting superabsorbent has a high internal porosity.
• an insoluble or only slowly soluble material for example using film-forming polymers, inert inorganic materials or meltable organic materials
• a surfactant can be added to the monomer solution before or during the polymerization and the monomer solution can then be foamed with an inert gas or water vapor or by vigorous stirring before or during the polymerization.
• the surfactant can be anionic, cationic, zwitterionic or non-ionic.
• a skin-friendly surfactant is preferably used.
• the polymer gel is then usually dried using a forced air belt dryer until the residual moisture content is preferably 0.5 to 10% by weight, particularly preferably 1 to 6% by weight, very particularly preferably 1.5 to 4% by weight, , with the residual moisture content being determined according to Test Method No. WSP 230.2-05 "Mass Loess Upon Heating" recommended by EDANA. If the residual moisture is too high, the dried polymer gel has a glass transition temperature T g which is too low and is difficult to process further. If the residual moisture is too low, the dried polymer gel is too brittle and in the subsequent comminution steps undesirably large amounts of polymer particles with too small a particle size (“fine”) are obtained.
• the solids content of the polymer gel before drying is preferably from 25 and 90% by weight, particularly preferably from 35 to 70% by weight, very particularly preferably from 40 to 60% by weight. The dried polymer gel is then broken up and optionally roughly crushed.
• the dried polymer gel is then usually ground and classified, it being possible to use single- or multi-stage roller mills, preferably two- or three-stage roller mills, pin mills, hammer mills or vibratory mills, for grinding.
• the average particle size of the polymer particles separated off as product fraction is preferably from 150 to 850 ⁇ m, particularly preferably from 250 to 600 ⁇ m, very particularly from 300 to 500 ⁇ m.
• the average particle size of the product fraction can be determined using test method No. WSP 220.2 (05) "Particle Size Distribution" recommended by EDANA, where the mass fractions of the sieve fractions are applied cumulatively and the average particle size is determined graphically.
• the average particle size is the value of the mesh size, which results for a cumulative 50% by weight.
• the proportion of polymer particles with a particle size of greater than 150 ⁇ m is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight.
• Polymer particles which are too small are therefore usually separated off and returned to the process, preferably before, during or immediately after the polymerization, i.e. before the polymer gel dries.
• the too small polymer particles can be moistened with water and / or aqueous surfactant before or during the recycling.
• the proportion of polymer particles with a particle size of at most 850 ⁇ m is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight.
• the proportion of polymer particles with a particle size of at most 600 ⁇ m is preferably at least 90% by weight, particularly preferably at least 95% by weight, very particularly preferably at least 98% by weight.
• Polymer particles with too large a particle size reduce the swelling rate. Therefore, the proportion of polymer particles that are too large should also be low. Polymer particles that are too large are therefore usually separated off and returned to the grinding.
• the polymer particles can be post-crosslinked thermally to further improve the properties.
• Suitable surface postcrosslinkers are compounds which contain groups which can form covalent bonds with at least two carboxylate groups of the polymer particles.
• Suitable compounds are, for example, polyfunctional amines, polyfunctional amidoamines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303 A1 and EP 0 937 736 A2, di- or polyfunctional alcohols, as in DE 33 14 019 A1, DE 35 23 617 A1 and EP 0 450 922 A2, or ⁇ -hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230.
• Preferred surface postcrosslinkers are ethylene carbonate, ethylene glycol diglycidyl ether, reaction products of polyamides with epichlorohydrin and mixtures of propylene glycol and 1,4-butanediol.
• Very particularly preferred surface postcrosslinkers are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and 1,3-propanediol.
• the amount of surface postcrosslinker is preferably 0.001 to 3% by weight, particularly preferably 0.02 to 1% by weight, very particularly preferably 0.05 to 0.2% by weight, in each case based on the polymer particles.
• the surface postcrosslinking is usually carried out in such a way that a solution of the surface postcrosslinker is sprayed onto the dried polymer particles. Following the spraying, the polymer particles coated with surface postcrosslinker are postcrosslinked and dried, and the surface postcrosslinking reaction can take place both before and during drying.
• a solution of the surface postcrosslinker is preferably sprayed on in mixers with moving mixing tools, such as screw mixers, disk mixers and paddle mixers.
• moving mixing tools such as screw mixers, disk mixers and paddle mixers.
• Horizontal mixers such as paddle mixers are particularly preferred, and vertical mixers are very particularly preferred.
• the distinction between horizontal mixers and vertical mixers is based on the mounting of the mixing shaft, ie horizontal mixers have a horizontally mounted mixing shaft and vertical mixers have a vertically mounted mixing shaft.
• Suitable mixers are, for example, Horizontal Ploughshare® Mixers (Gebr.
• solvent mixtures are preferably used, for example isopropanol / water, 1,3-propanediol / water and propylene glycol / water, the mixing mass ratio preferably being from 20:80 to 40:60.
• the surface postcrosslinking is preferably carried out in contact dryers, particularly preferably paddle dryers, very particularly preferably disc dryers.
• Suitable dryers are for example Hosokawa Bepex® Horizontal Paddle Dryer (Hosokawa Micron GmbH; Leingart; Germany), Hosokawa Bepex® Disc Dryer (Hosokawa Micron GmbH; Leingart; Germany), Holo-Flite® dryers (Metso Minerals Industries Inc .; Danville; USA ) and Nara Paddle Dryer (NARA Machinery Europe; Frechen; Germany). Fluidized bed dryers can also be used.
• the surface post-crosslinking can take place in the mixer itself, by heating the jacket or by blowing in warm air.
• a downstream dryer such as a tray dryer, a rotary kiln or a heated screw, is also suitable. It is particularly advantageous to mix in a fluidized bed dryer and postcrosslink the surface thermally.
• Preferred reaction temperatures are in the range 100 to 250 ° C, preferably 1 10 to 220 ° C, particularly preferably 120 to 210 ° C, very particularly preferably 130 to 200 ° C.
• the preferred residence time at this temperature is preferably at least 10 minutes, particularly preferably at least 20 minutes, very particularly preferably at least 30 minutes, and usually at most 60 minutes.
• the polymer particles are cooled after the surface postcrosslinking.
• the cooling is preferably carried out in contact coolers, particularly preferably blade coolers, very particularly preferably disc coolers.
• Suitable coolers include Hosokawa Bepex® Horizontal Paddle Cooler (Hosokawa Micron GmbH; Leingart; Germany), Hosokawa Bepex® Disc Cooler (Hosokawa Micron GmbH; Leingart; Germany), Holo-Flite® coolers (Metso Minerals Industries Inc. ; Danville; USA) and Nara Paddle Cooler (NARA Machinery Europe; Frechen; Germany). Fluidized bed coolers can also be used.
• the polymer particles are cooled in the cooler to preferably 40 to 90 ° C., particularly preferably 45 to 80 ° C., very particularly preferably 50 to 70 ° C.
• the surface post-crosslinked polymer particles can then be classified again, with polymer particles that are too small and / or too large being separated off and returned to the process.
• the surface post-crosslinked polymer particles can be coated or re-moistened to further improve the properties.
• the rewetting is preferably carried out at 40 to 120 ° C., particularly preferably at 50 to 110 ° C., very particularly preferably at 60 to 100 ° C. If the temperatures are too low, the polymer particles tend to clump together and at higher temperatures, water already clearly evaporates.
• the amount of water used for rewetting is preferably from 1 to 10% by weight, particularly preferably from 2 to 8% by weight, very particularly preferably from 3 to 5% by weight.
• the rewetting increases the mechanical stability of the polymer particles and reduces their tendency towards static charging.
• the rewetting in the cooler is advantageously carried out after the thermal surface postcrosslinking.
• Suitable coatings for improving the swelling rate and the gel bed permeability are, for example, inorganic inert substances, such as water-insoluble metal salts, organic polymers, cationic polymers and divalent or polyvalent metal cations.
• Suitable coatings for binding dust are, for example, polyols.
• Suitable coatings against the undesirable tendency of the polymer particles to cake are, for example, pyrogenic silica, such as Aerosil® 200, and surfactants, such as Span® 20.
• Suitable coatings for dust binding, to reduce the tendency to cake, and to increase mechanical stability are polymer dispersions, as in EP 0 703 265 B1 and waxes as described in US Pat. No. 5,840,321.
• coated and / or rewetted polymer particles can then be classified again, with polymer particles that are too small and / or too large being separated off and returned to the process.
• the present invention further relates to hygiene articles which contain superabsorbents produced by the process according to the invention.
• WSP Standard Test Methods for the Nonwovens Industry
• EDANA Herrmann-Debrouxlaan 46, 1 160 Oudergem , Belgium, www.edana.org
• INDA 1 100 Crescent Green, Suite 1 15, Cary, North Carolina 27518, USA, www.inda.org. This publication is available from both EDANA and INDA.
• the measurements should be carried out at an ambient temperature of 23 ⁇ 2 ° C and a relative humidity of 50 ⁇ 10%.
• the water-absorbing polymer particles are mixed well before the measurement.
• the centrifuge retention capacity (CRC) is determined according to test method no. WSP 241.2 (05) "Fluid Retention Capacity in Saline, After Centrifugation" recommended by EDANA.
• the content of extractable constituents of the water-absorbing polymer particles is determined in accordance with Test Method No. WSP 270.2 (05) "Extractable” recommended by EDANA.
• An acrylic acid / sodium acrylate solution was prepared by continuously mixing deionized water, 50% strength by weight sodium hydroxide solution and acrylic acid, so that the degree of neutralization corresponded to 71.3 mol%.
• the solids content of the monomer solution was 38.8% by weight.
• Polyethylene glycol 400 diacrylate (diacylate starting from a polyethylene glycol with an average molecular weight of 400 g / mol) was used as the polyethylenically unsaturated crosslinker. The amount used was 2 kg of crosslinking agent per ton of monomer solution.
• the throughput of the monomer solution was 20 t / h.
• the reaction solution had a temperature of 23.5 ° C. at the inlet.
• the aqueous polymer gel obtained was applied to the conveyor belt of a circulating air belt dryer by means of an oscillating conveyor belt.
• the circulating air belt dryer had a length of 48 m.
• the conveyor belt of the circulating air belt dryer had an effective width of 4.4 m.
• the oscillating conveyor belt was 5 m long.
• the conveyor belt had a width of 0.8 m and an effective width of 0.5 m.
• the angle of repose of the aqueous polymer gel on the conveyor belt was approximately 15 °.
• the cross section of the polymer gel bed on the conveyor belt was approximately 0.04 m 2 .
• the conveyor belt speed was 0.5 m / s.
• the oscillating conveyor belt was accelerated from 13 ° to an angular speed of 33 ° / s via a first swivel angle ⁇ i, decelerated from 20 ° to an angular speed of 17 ° / s via a second swivel angle ⁇ 2 and NEN third pivot angle ß 3 braked to the other end position.
• the total swivel angle was 50 °.
• a double stroke (from the first end position to the other end position and back again) took approx. 7 s.
• the rotating conveyor belt had a surface made of polytetrafluoroethylene (PTFE).
• the temperature of the aqueous polymer gel on the oscillating conveyor belt was 90 ° C.
• the stripping device was an elongated scraper attached transversely to the running direction of the returning conveyor belt.
• the scraper was inclined at 20 ° to the running direction of the returning conveyor belt.
• the distance of the stripping device from the discharge end was approx. 5 cm, i.e. the stripping device was located in the area of the deflection roller.
• the distance between the scraper device and the returning conveyor belt was 1 mm.
• the two-substance nozzles were arranged transversely to the running direction of the conveyor belt.
• the distance between the two fabric nozzles was approx. 20 cm each.
• the distance between the two-substance nozzles and the discharge end was approx. 20 cm.
• the distance between the two-substance nozzles and the returning conveyor belt was approx. 20 cm.
• a total of 10 kg / h of water and 100 kg / h of air were sprayed.
• the returning conveyor belt was easy to clean using the scraper. There were no major caking on the conveyor belt over a period of 6 months.
• the dried polymer gel was ground and sieved to a particle size fraction of 150 to 850 gm.
• the water-absorbing polymer particles obtained had a centrifuge retention capacity (CRC) of 34.9 g / g and an extractable content of 8.5% by weight.
• Example 2 (Comparative Example) The procedure was as in Example 1, but the spray nozzles on the oscillating conveyor belt were switched off.
• the returning conveyor belt was difficult to clean using the scraper. There were major caking on the conveyor belt, especially where polymer gel had already dried. After a few weeks of continuous production, the production had to be interrupted and the conveyor belt cleaned or the conveyor belt replaced due to damage.

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