Method of pneumatically conveying superabsorbent particles

Abstract

The invention relates to a method of pneumatically conveying superabsorbent particles, mixing the superabsorbent particles with an aqueous wax dispersion before the pneumatic conveying, the wax having a glass transition temperature of at least 65°C, and using from 0.020 to 0.20 wt.-% of wax relative to the untreated superabsorbent particles.

Description

[0001] The present invention relates to a method for pneumatically conveying superabsorbent particles, wherein the superabsorbent particles are mixed with an aqueous wax dispersion prior to pneumatic conveying, the wax having a glass transition temperature of at least 65°C, and 0.020% to 0.20% by weight of wax is used based on the untreated superabsorbent particles.

[0002] Superabsorbent polymers are used in the manufacture of diapers, tampons, sanitary napkins, and other hygiene products, as well as in commercial gardening as water-retaining agents. Superabsorbent polymers are also known as absorbent polymers.

[0003] The preparation of superabsorbents is described in the monograph "Modern Superabsorbent Polymer Technology", FLBuchholz and A.T. Graham, Wiley-VCH, 1998, pp. 71-103.

[0004] The use of wax-coated superabsorbents is documented, for example, in EP 0 755 964 A2 and WO 2008 / 077779 A1.

[0005] One object of the present invention is to provide an improved superabsorbent, particularly a superabsorbent with improved conveying performance for pneumatic conveying.

[0006] This objective is achieved by a method for pneumatically conveying superabsorbent particles, wherein the superabsorbent particles are mixed with an aqueous wax dispersion prior to pneumatic conveying, the wax having a glass transition temperature of at least 65°C, and 0.020% to 0.20% by weight of wax is used based on the untreated superabsorbent particles.

[0007] In principle, there are three different types of pneumatic conveying:

[0008] In dilute phase transport, the laws governing free-flowing individual particles apply approximately within the range of high gas velocities. This is the conventional type of pneumatic transport. No product deposition occurs under any circumstances. The transported material is distributed substantially uniformly within the transport pipe.

[0009] If the gas velocity decreases, the conveying enters the range of fluidized dilute phase conveying, where the conveyed material flows particularly in the lower half of the conveying pipe. Dilute phase conveying exists in the upper half of the conveying pipe.

[0010] - At low gas velocities, transport is carried out particularly gently in the form of dense-phase transport (piston transport, pulse transport) with high pressure drop.

[0011] In principle, pressure transport can be carried out at a slower transport rate than suction transport because the pressure reserve is greater under high pressure than under low pressure, and the density of the transport gas driving the product forward increases with increasing pressure.

[0012] Because the transported gas is compressible, there is no constant pressure in the transport pipeline; instead, the pressure is higher at the beginning than at the end. However, this also alters the gas volume, resulting in a lower gas velocity dominating at the beginning under higher pressure, and a higher gas velocity dominating at the end under lower pressure.

[0013] In the fluidized dilute phase transport range, excessively low transport rates are problematic because stable transport is impossible in the unstable region between dense phase transport and fluidized dilute phase transport. Instead, the resulting mechanical stress can lead to severe damage to the transport system, to the point that the transport pipes tear from their mounts.

[0014] In pneumatic conveying, the optimal initial gas velocity depends on the diameter of the conveying pipe. This correlation is described by the Froude number:

[0015]

[0016] Fr Froude number

[0017] v gas velocity

[0018] D. Inner diameter of the delivery pipe

[0019] g acceleration due to gravity

[0020] In pneumatic conveying, the initial gas velocity preferably corresponds to a Froude number of 2 to 40, more preferably 5 to 30, and most preferably 10 to 20. In pneumatic conveying, the load of the conveyed material is preferably 1 to 30 kg / kg, more preferably 5 to 25 kg / kg, and most preferably 10 to 20 kg / kg, wherein the load of the conveyed material is the quotient of the mass flow rate of the conveyed material and the mass flow rate of the gas.

[0021] The glass transition temperature of the wax is preferably at least 70°C, more preferably at least 75°C, and most preferably at least 80°C.

[0022] According to G. Kanig (Kolloid-Zeitschrift & Zeitschrift für Polymere, Vol. 190, p. 1, Equation 1), the glass transition temperature Tg is the limit that the glass transition temperature tends to with increasing molecular weight. Tg is determined by DSC method according to DIN ISO 11357-2:2014-07 (differential scanning calorimetry, 20 K / min, midpoint measurement). The Tg values ​​of homopolymers of most monomers are known and are listed, for example, in Ullmann's Encyclopedia of Industrial Chemistry, VCH Weinheim, 1992, 5th edition, Vol. A21, p. 169 and below; other sources for the glass transition temperature of homopolymers are, for example, J. Brandrup, E. H. Inmmergut, Polymer Handbook, 15th edition, J. Wiley, New York 1996; 2nd edition, J. Wiley, New York 1975; and 3rd edition, J. Wiley, New York 1989. To achieve the desired glass transition temperature Tg of the wax by selecting appropriate types and amounts of monomers, the Fox formula (TGFox, Bull. Amer. Phys. Soc. (Ser. II) 1956, Vol. 1, p. 123 and below) is useful, according to which a good approximation of the glass transition temperature of the copolymer is:

[0023] 1 / Tg = x1 / Tg1 + x2 / Tg2 + ... + x n / Tg n

[0024] Where x1, x2, ... x n The mass fraction of the monomer, Tg1, Tg2, ..., Tg n The glass transition temperature, in Kelvin, is the temperature of the homopolymer formed from one of monomers 1, 2, ..., n in each case.

[0025] The amount of wax used is preferably from 0.025% to 0.15% by weight, more preferably from 0.030% to 0.1% by weight, and most preferably from 0.035% to 0.08% by weight, in each case based on untreated superabsorbent particles.

[0026] In a preferred embodiment of the invention, the wax is a copolymer of 70 to 95 mol% of at least one olefinically unsaturated hydrocarbon and 5 to 30 mol% of at least one olefinically unsaturated carboxylic acid. Ethylene-acrylic acid copolymers and ethylene-methacrylic acid copolymers are particularly preferred. Examples of such copolymers are... EAS 5 (ASF SE; Ludwigshafen; Germany), 960 (DuPont Company; Wilmington; United States of America) and 5980I (The Dow Chemical Company; Midland; United States of America).

[0027] The wax is preferably used in the form of an aqueous dispersion, with a solid content preferably less than 50% by weight, more preferably less than 40% by weight, and very preferably less than 30% by weight. A wax dispersion consisting only of water, wax, and an alkali metal hydroxide as a dispersing aid is very preferred.

[0028] Such wax dispersions are typically prepared by dispersing a wax melt in water and stabilizing it with a suitable dispersing agent.

[0029] Examples of such wax dispersions are WE 1、 WE 4 CE 18 (purchased from BASF SE; Ludwigshafen; Germany respectively), 001 (Honeywell International; Morris Plains; United States of America), Emulsion 34935 (Michelman, Inc.; Cincinnati; United States of America).

[0030] In a highly preferred embodiment of the invention, an aqueous wax dispersion composed of an ethylene-acrylic acid copolymer and an ethylene-methacrylic acid copolymer is used, which is subsequently stabilized using only sodium hydroxide or potassium hydroxide as a dispersing aid.

[0031] The pH of the wax dispersion is preferably at least 7, very preferably at least 8, and most preferably at least 9.

[0032] The median particle size of the superabsorbent particles is preferably 150 to 850 μm, more preferably 200 to 600 μm, and most preferably 250 to 500 μm.

[0033] In a preferred embodiment of the invention, the average sphericity (ASPHT) of the superabsorbent particles is preferably greater than 0.72, more preferably greater than 0.76, and most preferably greater than 0.80.

[0034] During pneumatic conveying, the temperature of the superabsorbent particles is preferably at least 20°C, more preferably at least 30°C, and most preferably at least 40°C, and is preferably at least 20°C, more preferably at least 25°C, and most preferably at least 30°C lower than the glass transition temperature of the wax.

[0035] This invention is based on the discovery that the conveying performance of superabsorbent particles can be improved by treating them with an aqueous wax dispersion. Wax-coated superabsorbent particles generate a smaller pressure differential during pneumatic conveying, increasing the capacity of the conveying pipeline. To avoid adversely affecting the absorption performance of the superabsorbent particles, the amount of wax must not be too high, and the wax must not form a continuous film on the particle surface. The latter is ensured by a sufficiently high glass transition temperature.

[0036] The preparation of the superabsorbent is described in detail below:

[0037] Superabsorbents can be prepared by polymerizing a monomer solution or suspension containing the following substances:

[0038] a) At least one olefinic unsaturated monomer with an acidic group that can be at least partially neutralized.

[0039] b) at least one crosslinking agent, and

[0040] c) At least one initiator

[0041] Furthermore, the superabsorbent is typically insoluble in water.

[0042] Monomer a) is preferably water-soluble, that is, its solubility in water at 23°C is generally at least 1 g / 100 g water, preferably at least 5 g / 100 g water, more preferably at least 25 g / 100 g water and most preferably at least 35 g / 100 g water.

[0043] Suitable monomers a) are, for example, olefinically 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.

[0044] The proportion of acrylic acid and / or its salts in the total amount of monomer a) is preferably at least 50 mol%, more preferably at least 90 mol%, and most preferably at least 95 mol%.

[0045] Monomer a) typically contains polymerization inhibitors, preferably hydroquinone monomethyl ether (MEHQ) as a storage stabilizer.

[0046] The monomer solution preferably contains up to 250 ppm by weight, more preferably up to 130 ppm by weight, more preferably up to 70 ppm by weight, and more preferably at least 10 ppm by weight, more preferably at least 30 ppm by weight, and particularly about 50 ppm by weight of hydroquinone monomethyl ether (MEHQ), in each case based on the amount of unneutralized monomer a). For example, the monomer solution can be prepared by using an olefinically unsaturated monomer with an acidic group containing an appropriate amount of hydroquinone monomethyl ether (MEHQ).

[0047] Suitable crosslinking agent b) is a compound having at least two suitable crosslinking groups. Such groups are, for example, olefinic unsaturated groups that can be free radically polymerized into polymer chains, and functional groups that can form covalent bonds with the acid groups of monomer a). Furthermore, polyvalent metal salts that can form coordination bonds with at least two acid groups of monomer a) are also suitable as crosslinking agent b). Other suitable crosslinking agents b) are "nano-clay" as described in US 2017 / 361305, water glass as described in WO2000 / 31157 A1, and aluminates as described in WO 99 / 55767 A1.

[0048] Crosslinking agent b) is preferably a compound having at least two polymerizable groups that can be polymerized into a polymer network by free radicals. Suitable crosslinking agents b) are, for example, ethylene glycol dimethacrylate, diethylene glycol diacrylate, polyethylene glycol diacrylate, allyl methacrylate, trimethylolpropane triacrylate, triallylamine, tetraallyl ammonium chloride, tetraallyloxyethane, as described in EP 0 530 438 A1; diacrylates and triacrylates, as described in EP 0 547 847 A1, EP 0 559 476 A1, EP 0 632 068 A1, WO 93 / 21237 A1, WO 03 / 104299 A1, WO 03 / 104300 A1, WO 03 / 104301 A1 and DE 103 31 450 A1; mixed acrylates containing other olefinic unsaturated groups in addition to acrylate groups, such as DE 103 31 456 A1 and DE As described in 103 55 401 A1; or a mixture of crosslinking agents, such as those described in, for example, DE 195 43 368 A1, DE 196 46 484 A1, WO 90 / 15830 A1 and WO02 / 032962 A2.

[0049] Preferred crosslinking agent b) is pentaerythritol triallyl ether, tetraallyloxyethane, methylene dimethylacrylamide, 15-ethoxylated trimethylolpropane triacrylate, polyethylene glycol diacrylate, trimethylolpropane triacrylate, and triallylamine.

[0050] The amount of crosslinking agent b) is preferably from 0.25% to 1.5% by weight, more preferably from 0.3% to 1.2% by weight, and most preferably from 0.4% to 0.8% by weight, in each case based on the total amount of monomer a) used. With increasing crosslinking agent content, the centrifugal retention capacity (CRC) decreases, and at 21.0 g / cm³... 2 Absorption under pressure reaches its maximum value.

[0051] The initiator used (c) can be any compound that generates free radicals under polymerization conditions, such as a thermal initiator, a redox initiator, or a photoinitiator. Suitable redox initiators are sodium persulfate / ascorbic acid, hydrogen peroxide / ascorbic acid, sodium persulfate / sodium bisulfite, and hydrogen peroxide / sodium bisulfite. A mixture of thermal and redox initiators, such as sodium persulfate / hydrogen peroxide / ascorbic acid, is preferred. The reducing component used is preferably a sodium salt of 2-hydroxy-2-sulfoacetic acid, or a mixture of a sodium salt of 2-hydroxy-2-sulfoacetic acid, a disodium salt of 2-hydroxy-2-sulfoacetic acid, and sodium bisulfite. Such a mixture can be used as… FF6 and FF7 (Brüggemann Chemicals; Heilbronn; Germany) can be obtained. Alternatively, pure 2-hydroxy-2-sulfoacetic acid or its salts can be used as the reducing agent, especially when ascorbic acid is also used.

[0052] Before or during polymerization, chelating agents and 2-hydroxycarboxylic acids, such as those described in WO 2017 / 170604 A1, may be added to the monomer solution.

[0053] Typically, aqueous solutions of the monomer are used. The water content of the monomer solution is preferably 40% to 75% by weight, more preferably 45% to 70% by weight, and most preferably 50% to 65% by weight. Monomer suspensions, i.e., monomer solutions of monomers a) such as sodium acrylate, with a solubility exceeding that of the monomer, can also be used. As the water content increases, the energy consumption in subsequent drying increases, and as the water content decreases, the heat of polymerization can only be insufficiently removed.

[0054] The acid groups in the resulting polymer gel are typically partially neutralized. Neutralization occurs at the monomer stage. This is usually accomplished by incorporating a neutralizing agent in aqueous solution or preferably in solid form. The degree of neutralization is preferably 25 to 85 mol%, more preferably 30 to 80 mol%, and most preferably 40 to 75 mol%, for which conventional neutralizing agents can be used, preferably alkali metal hydroxides, alkali metal oxides, alkali metal carbonates, or alkali metal bicarbonates and mixtures thereof. Ammonium salts can also be used instead of alkali metal salts. Particularly preferred alkali metals are sodium and potassium, but very particularly preferred are sodium hydroxide, sodium carbonate, or sodium bicarbonate and mixtures thereof.

[0055] The following is a description of solution polymerization:

[0056] Suitable reactors for solution polymerization include, for example, kneading reactors or belt reactors. In a kneader, the polymer gel formed in the polymerization of monomer aqueous solutions or suspensions is continuously pulverized by, for example, a counter-rotating stirring shaft, as described in WO 2001 / 038402 A1. Polymerization on a belt is described, for example, in DE 38 25 366 A1 and US 6,241,928. Polymerization in a belt reactor forms a polymer gel that must be pulverized in, for example, an extruder or a kneader.

[0057] To improve drying performance, the pulverized polymer gel obtained by kneading can be extruded separately.

[0058] The polymer gel is then dried using an air-circulating bag dryer until the residual moisture content is preferably 0.5 to 10% by weight, more preferably 1 to 6% by weight, and most preferably 1.5 to 4% by weight, determined by EDANA recommended test method No. WSP 230.2-05 "Mass Loss Upon Heating". If the residual moisture content is too high, the dried polymer gel has an excessively low glass transition temperature (Tg), making further processing difficult. If the residual moisture content is too low, the dried polymer gel is too brittle, and in the subsequent grinding step, an unwanted large amount of polymer particles with excessively small particle sizes ("fine powder") is obtained. Prior to drying, the solid content of the polymer gel is preferably 25% to 90% by weight, more preferably 35% to 70% by weight, and most preferably 40% to 60% by weight. The dried polymer gel is then crushed and optionally coarsely ground.

[0059] Subsequently, the dried polymer gel is typically ground and graded. The equipment used for grinding is usually a single-stage or multi-stage roller mill (preferably a two-stage or three-stage roller mill), pin mill, hammer mill, or vibratory mill.

[0060] The average particle size of the polymer particles removed as the product fraction is preferably 150 to 850 μm, more preferably 250 to 600 μm, and particularly 300 to 500 μm. The average particle size of the product fraction can be determined by EDANA recommended test method No. WSP220.2-05 "Particle Size Distribution", in which the mass proportions of the sieve fractions are plotted cumulatively and the average particle size is determined in the graph. Here, the average particle size is the sieve aperture size value that reaches 50% by weight cumulatively.

[0061] The proportion of polymer particles with a particle size greater than 150 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0062] Polymer particles that are too small in size reduce gel bed permeability (GBP). Therefore, the proportion of excessively small polymer particles (“fine powder”) should be small.

[0063] Therefore, excessively small polymer particles are typically removed and recycled back into the process, preferably before, during, or shortly after polymerization (i.e., before the polymer gel dries). The excessively small polymer particles can be wetted with water and / or an aqueous surfactant before or during recycling.

[0064] The excessively small polymer particles can also be removed in subsequent process steps (e.g., after surface post-crosslinking or another coating step). In this case, the recycled excessively small polymer particles are subjected to surface post-crosslinking or coated in another manner, such as with pyrolytic silica.

[0065] If a kneading reactor is used for polymerization, it is preferable to add excessively small polymer particles during the last third of the polymerization process. However, excessively small polymer particles can also be incorporated into the polymer gel in a kneader or extruder connected downstream of the polymerization reactor.

[0066] If too small polymer particles are added to the monomer solution at a very early stage, such as actually at the time, the centrifugal retention capacity (CRC) of the resulting polymer particles will be reduced. However, this can be compensated for, for example, by adjusting the amount of crosslinking agent b).

[0067] The proportion of polymer particles with a particle size of up to 850 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0068] The proportion of polymer particles with a particle size of up to 600 μm is preferably at least 90% by weight, more preferably at least 95% by weight, and most preferably at least 98% by weight.

[0069] Excessively large polymer particles reduce the swelling ratio. Therefore, the proportion of excessively large polymer particles should also be small. Consequently, excessively large polymer particles are usually removed and recycled back into the grinding process.

[0070] The following is an explanation of droplet polymerization:

[0071] In droplet polymerization, a monomer solution is metered into a reactor through at least one orifice for forming droplets. Droplet polymerization is described, for example, in WO 2014 / 079694 A1 and WO 2015 / 110321 A1.

[0072] The orifices can be, for example, in a droplet plate. The number and size of the orifices are selected based on the required capacity and droplet size. The droplet diameter is typically 1.9 times the diameter of the orifice. It is important here that the liquid to be dropletized does not pass through the orifice too quickly, and the pressure drop across the orifice is not too large. Otherwise, the liquid will not be dropletized, but will instead break up (eject) due to high kinetic energy. The Reynolds number based on the throughput and orifice diameter is preferably less than 2000, more preferably less than 1600, particularly preferably less than 1400, and most preferably less than 1200.

[0073] The droplet plate preferably has at least 5, more preferably at least 25, and most preferably at least 50 holes, and more preferably up to 750, more preferably up to 500, and most preferably up to 250 holes. The diameter of the holes is selected according to the desired droplet size.

[0074] The diameter of the pores is preferably 50 to 500 μm, more preferably 100 to 300 μm, and most preferably 150 to 250 μm. The spacing between the pores is preferably 10 to 50 mm, more preferably 12 to 40 mm, and most preferably 15 to 30 mm. Too small a spacing leads to the formation of agglomerates.

[0075] The temperature at which the monomer solution passes through the pore is preferably 5 to 80°C, more preferably 10 to 70°C, and most preferably 30 to 60°C.

[0076] The carrier gas flows through the reactor. The carrier gas can flow through the reactor in a parallel or counter-current manner with the free-falling droplets of the monomer solution, preferably in a parallel manner, i.e., from bottom to top. After one pass, the carrier gas is preferably at least partially recycled back into the reactor as recirculating gas, preferably to at least 50% and more preferably to at least 75%. Typically, a portion of the carrier gas is removed after each pass, preferably up to 10%, more preferably up to 3%, and most preferably up to 1%.

[0077] The oxygen content of the carrier gas is preferably 0.5 to 15% by volume, more preferably 1 to 10% by volume, and most preferably 2 to 7% by volume.

[0078] In addition to oxygen, the carrier gas preferably also contains nitrogen. The nitrogen content of the carrier gas is preferably at least 80% by volume, more preferably at least 90% by volume, and most preferably at least 95% by volume. Gas mixtures may also be used. The carrier gas may also be filled with water vapor and / or acrylic acid vapor.

[0079] The gas velocity is preferably set in such a way that the flow in the reactor is guided, for example, there is no convection opposite to the general flow direction, and is typically 0.1 to 2.5 m / s, preferably 0.3 to 1.5 m / s, more preferably 0.5 to 1.2 m / s, particularly preferably 0.6 to 1.0 m / s and most preferably 0.7 to 0.9 m / s.

[0080] The carrier gas flowing through the reactor is preheated to the reaction temperature upstream of the reactor.

[0081] Advantageously, the gas inlet temperature is adjusted so that the gas outlet temperature (i.e., the temperature at which the carrier gas leaves the reactor) is typically 90 to 150°C, preferably 100 to 140°C, more preferably 105 to 135°C, particularly preferably 110 to 130°C, and most preferably 115 to 125°C.

[0082] The reaction can be carried out under high pressure or under reduced pressure; preferably under reduced pressure up to 100 mbar relative to ambient pressure.

[0083] The reaction exhaust gas, i.e., the gas leaving the reactor, can be cooled, for example, in a heat exchanger. This causes water and unconverted monomer a) to condense. Subsequently, the reaction exhaust gas can be at least partially reheated and recycled back into the reactor as recirculating gas. A portion of the reaction exhaust gas can be discharged and replaced with fresh carrier gas, in which case water and unconverted monomer a) present in the reaction exhaust gas can be removed and recycled.

[0084] An integrated heating system is particularly preferred, which means that some of the waste heat from exhaust gas cooling is used to heat the circulating gas.

[0085] The reactor can be trace-heated. The heat tracing is adjusted so that the wall temperature is at least 5°C higher than the internal temperature of the reactor, and condensation on the reactor wall is reliably prevented.

[0086] The following is an explanation of reverse suspension polymerization:

[0087] Reverse suspension polymerization involves suspending a monomer solution in a hydrophobic solvent during polymerization. Reverse suspension polymerization is described, for example, in WO 2008 / 068208 A1 and WO 2015 / 062883 A2.

[0088] The hydrophobic solvents that can be used are all solvents known to those skilled in the art for suspension polymerization. Aliphatic hydrocarbons, such as n-hexane, n-heptane, n-octane, n-nonane, n-decane, cyclohexane, or mixtures thereof, are preferred. The hydrophobic solvent has a solubility in water of less than 5 g / 100 g, preferably less than 1 g / 100 g, and more preferably less than 0.5 g / 100 g at 23 °C.

[0089] The hydrophobic solvent boils in a range of preferably 50 to 150°C, more preferably 60 to 120°C, and most preferably 70 to 90°C.

[0090] The ratio of hydrophobic solvent to monomer solution is 0.5 to 3, preferably 0.7 to 2.5, and very preferably 0.8 to 2.2.

[0091] If no aggregation occurs, the average diameter of the monomer solution droplets in the suspension is preferably at least 100 μm, more preferably 100 to 1000 μm, more preferably 150 to 850 μm, and most preferably 300 to 600 μm. The droplet diameter is determined by light scattering and expressed as the volume average diameter.

[0092] The diameter of the monomer solution droplets can be adjusted by introducing stirrer energy and by using appropriate dispersing agents.

[0093] To disperse monomer aqueous solutions in hydrophobic solvents or to disperse the resulting superabsorbent particles, dispersing aids are preferably added. These dispersing aids may be anionic surfactants, cationic surfactants, nonionic surfactants, or amphoteric surfactants, or natural, semi-synthetic, or synthetic polymers.

[0094] Anionic surfactants include, for example, sodium polyoxyethylene dodecyl ether sulfate and sodium dodecyl ether sulfate. Cationic surfactants include, for example, trimethyloctadecyl ammonium chloride. Amphoteric surfactants include, for example, carboxymethyl dimethylacetamide. Nonionic surfactants include, for example, sucrose fatty acid esters, such as sucrose monostearate and sucrose dilaurate, dehydrated sorbitan esters such as dehydrated sorbitan monostearate, and polyoxyalkylene compounds based on dehydrated sorbitan esters, such as polyoxyethylene dehydrated sorbitan monostearate.

[0095] Dispersants are typically dissolved or dispersed in hydrophobic solvents. Based on the monomer solution, the amount of dispersant is 0.01% to 10% by weight, preferably 0.2% to 5% by weight, and more preferably 0.5% to 2% by weight. The diameter of the monomer solution droplets can be adjusted by the type and amount of dispersant.

[0096] Advantageously, polymerization is carried out in series with several stirred reactors. By conducting post-reactions in other stirred reactors, monomer conversion can be improved and backmixing can be reduced. In this case, it is further advantageous when the first stirred reactor is not too large. As the size of the stirred reactor increases, the size distribution of the dispersed monomer solution droplets inevitably widens. Therefore, a relatively small first reactor makes it possible to prepare superabsorbent particles with a particularly narrow particle size distribution.

[0097] The reaction is preferably carried out under reduced pressure, for example, at 800 mbar. The pressure can be used to set the boiling point of the reaction mixture to the desired reaction temperature.

[0098] If polymerization is carried out under sufficient reflux, inertization can be omitted. In this case, dissolved oxygen is removed from the polymerization reactor along with the evaporated solvent.

[0099] Superabsorbent particles can be azeotropically dehydrated in a polymer dispersion and separated from the polymer dispersion. The separated superabsorbent particles can be dried to remove any adhering residual hydrophobic solvent.

[0100] The following is an explanation of post-crosslinking of the surface:

[0101] To further improve performance, the polymer particles can be thermally surface-crosslinked. Suitable surface-crosslinking agents are compounds containing groups capable of forming covalent bonds with at least two carboxylic acid ester groups of the polymer particles. Suitable compounds are, for example, polyfunctional amines, polyfunctional amide amines, polyfunctional epoxides, as described in EP 0 083 022 A2, EP 0 543 303A1 and EP 0 937 736 A2; difunctional or polyfunctional alcohols, as described in DE 33 14 019 A1, DE 35 23 617A1 and EP 0 450 922 A2; β-hydroxyalkylamides, as described in DE 102 04 938 A1 and US 6,239,230; or oxazolines, as described in EP 0 999 238 A2.

[0102] Other suitable surface post-crosslinking agents described are cyclic carbonates in DE 40 20 780 C1, 2-oxazolidinones and their derivatives, such as 2-hydroxyethyl-2-oxazolidinone, in DE 198 07 502 A1; bis-2-oxazolidinones and poly-2-oxazolidinones in DE 198 07992 C1; 2-oxotetrahydro-1,3-oxazines and their derivatives in DE 198 54 573 A1; N-acyl-2-oxazolidinone in DE 198 54 574 A1; cyclic urea in DE 102 04 937 A1; bicyclic amide acetal in DE 103 34 584 A1; oxetane and cyclic urea in EP 1 199 327 A2; and oxetane and WO in EP 2 204388 A1. Morpholin-2,3-dione and its derivatives in 03 / 031482 A1.

[0103] Preferred surface post-crosslinking agents are ethylene carbonate, ethylene glycol diglycidyl ether, the reaction product of polyamide and epichlorohydrin, and a mixture of propylene glycol and 1,4-butanediol.

[0104] Very particularly preferred surface post-crosslinking agents are 2-hydroxyethyl-2-oxazolidinone, 2-oxazolidinone and propane-1,3-diol.

[0105] In addition, surface post-crosslinking agents containing additional polymerizable olefinic unsaturated groups, as described in DE 3713 601 A1, may be used.

[0106] The amount of the surface post-crosslinking agent is preferably from 0.001% to 3% by weight, more preferably from 0.02% to 1% by weight, and most preferably from 0.05% to 0.2% by weight, in each case based on polymer particles.

[0107] Post-crosslinking of the surface is typically carried out by spraying a solution of a post-crosslinking agent onto dry polymer particles. After spraying, the polymer particles coated with the post-crosslinking agent are subjected to post-crosslinking and drying, and the post-crosslinking reaction can occur before and during drying.

[0108] The spraying of the surface crosslinking agent solution is preferably carried out in a mixer with a moving mixing tool, such as a screw mixer, disc mixer, or paddle mixer. Horizontal mixers, such as paddle mixers, are particularly preferred, and vertical mixers are very particularly preferred. Horizontal and vertical mixers are distinguished by the position of the stirring shaft; a horizontal mixer has a horizontally mounted stirring shaft, while a vertical mixer has a vertically mounted stirring shaft. Suitable mixers are, for example, horizontal... Plowshare mixer (Gebr.) Maschinenbau GmbH; Paderborn; Germany), Vrieco-Nauta continuous mixers (Hosokawa Micron BV; Doetinchem; the Netherlands), Processall Mixmill mixers (Processall Incorporated; Cincinnati; USA) and Schugi (Hosokawa Micron BV; Doetinchem; the Netherlands). However, a surface crosslinking agent solution can also be sprayed in a fluidized bed.

[0109] Post-crosslinking agents are typically used in aqueous solutions. The penetration depth of the post-crosslinking agent into polymer particles can be adjusted by the content of the non-aqueous solvent and the total amount of solvent.

[0110] When water is used as the solvent alone, it is advantageous to add a surfactant. This improves wetting properties and reduces the tendency to form clumps. However, it is preferable to use solvent mixtures, such as isopropanol / water, propane-1,3-diol / water, propylene glycol / water, 2-methylpropane-1,3-diol / water, ethylene glycol / water, diethylene glycol / water, triethylene glycol / water, tetraethylene glycol / water, or polyethylene glycol / water, wherein the mass mixing ratio is preferably from 20:80 to 40:60.

[0111] Post-crosslinking of the surface is preferably carried out in a contact dryer, more preferably a paddle dryer, and most preferably a disc dryer. A suitable dryer is, for example, a Hosokawa dryer. Horizontal paddle dryer (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Disc dryer (Hosokawa Micron GmbH; Leingarten; Germany) Dryers (Metso Minerals Industries Inc.; Danville; USA) and Nara paddle dryers (NARA Machinery Europe; Frechen; Germany). Fluidized bed drying can also be used.

[0112] Post-crosslinking of the surface can be carried out within the reactor itself by heating the jacket or by blowing in warm air. Downstream dryers, such as disc dryers, rotary tube furnaces, or heated screw dryers, are also suitable. Mixing and hot post-crosslinking of the surface are particularly advantageous in fluidized bed dryers.

[0113] The preferred reaction temperature range is 100 to 250°C, more preferably 120 to 220°C, even more preferably 130 to 210°C, and most preferably 150 to 200°C. At this temperature, the preferred residence time is at least 10 minutes, more preferably at least 20 minutes, and most preferably at least 30 minutes, and typically at most 60 minutes.

[0114] In a preferred embodiment of the invention, the polymer particles are cooled after surface post-crosslinking. Cooling is preferably performed in a contact cooler, more preferably a paddle cooler, and most preferably a disc cooler. A suitable cooler is, for example, a Hosokawa cooler. Horizontal paddle cooler (Hosokawa Micron GmbH; Leingarten; Germany), Hosokawa Disc cooler (Hosokawa Micron GmbH; Leingarten; Germany) Coolers (Metso Minerals Industries Inc.; Danville; USA) and Nara paddle coolers (NARA Machinery Europe; Frechen; Germany). Fluidized bed coolers can also be used.

[0115] In the cooler, the polymer particles are cooled to a preferred temperature of 40 to 90°C, a more preferred temperature of 45 to 80°C, and a most preferred temperature of 50 to 70°C.

[0116] To further improve performance, polymer particles can be coated or re-wetted.

[0117] Rewetting is preferably performed at 40 to 120°C, more preferably 50 to 110°C, and most preferably 60 to 100°C. At too low a temperature, polymer particles are prone to agglomeration, while at higher temperatures, water has evaporated significantly. The amount of water used for rewetting is preferably 1% to 15% by weight, more preferably 2% to 10% by weight, and most preferably 3% to 8% by weight. Rewetting improves the mechanical stability of polymer particles and reduces their tendency to become electrostatically charged.

[0118] Suitable coatings for improving swelling ratio and gel bed permeability (GBP) include, 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 include, for example, polyols. Suitable coatings for counteracting the unwanted agglomeration tendency of polymer particles include, for example, pyrolytic silica. 200, and surfactants, such as 20. Suitable coatings for binding dust, reducing the tendency to agglomerate and improving mechanical stability are polymer dispersions as described in EP 0 703 265 B1 and waxes as described in US 5 840 321.

[0119] method:

[0120] The standard test method described below, collectively referred to as "WSP," is documented in the following literature: "Standard Test Methods for the Nonwovens Industry," 2005 edition, jointly published by Worldwide Strategic Partners EDANA (Avenue Eugène Plasky, 157, 1030 Brussels, Belgium, www.edana.org) and INDA (1100 Crescent Green, Suite 115, Cary, North Carolina 27518, USA, www.inda.org). This publication is available from both EDANA and INDA.

[0121] Unless otherwise specified, measurements should be performed at an ambient temperature of 23±2℃ and a relative humidity of 50±10%. The superabsorbent particles should be thoroughly mixed before measurement.

[0122] Residual monomers

[0123] The residual monomer content was determined by the test method recommended by EDANA, No. WSP 210.2(05) "Residual Monomers".

[0124] Moisture content

[0125] Moisture content was determined using EDANA recommended test method No. WSP 230.2(05) "Mass Loss Upon Heating".

[0126] Centrifugation retention capacity

[0127] The centrifugation retention capacity (CRC) was determined using the test method recommended by EDANA, No. WSP 241.2(05) "Fluid Retention Capacity in Saline, After Centrifugation".

[0128] At 21.0 g / cm 2 Absorption under pressure (absorption under load)

[0129] The concentration was determined at 21.0 g / cm³ using EDNA's recommended test method No. WSP 242.2(05) "Absorption Under Pressure, Gravimetric Determination". 2 Absorption under pressure (AUL).

[0130] At 49.2 g / cm 2 Absorption under pressure (absorption under high load)

[0131] Similar to the EDNA recommended test method No. WSP 242.2(05) "Absorption Under Pressure, Gravimetric Determination", the concentration was determined at 49.2 g / cm³. 2 Absorption under pressure (AUHL) differs in that the pressure is set at 49.2 g / cm³. 2 (0.7 psi), instead of a pressure of 21.0 g / cm. 2 (0.3psi).

[0132] Extractables

[0133] The content of extractables in the absorbent polymer particles was determined by EDANA recommended test method No. WSP 270.2(05) "Extractable".

[0134] Bulk density

[0135] Bulk density (ASG) was determined using EDANA recommended test method No. WSP 260.2(05) "Density, Gravimetric Determination".

[0136] saline conductivity

[0137] As described in EP 0 640 330 A1, the saline conductivity (SFC) of the swollen gel layer at a pressure of 0.3 psi (2070 Pa) is determined as the gel layer permeability of the swollen gel layer of the absorbent polymer particles. An improvement on the apparatus described on page 19 and in Figure 8 of the cited patent application is made so that the glass frit (40) is no longer used, and the piston (39) is made of the same polymer material as the cylinder (37), and now includes 21 equally sized holes evenly distributed across the entire contact surface. The measurement procedure and evaluation remain unchanged compared to EP 0 640 330 A1. Flow rate is automatically detected.

[0138] The saline conductivity (SFC) is calculated as follows:

[0139] SFC[cm 3 s / g]=(Fg(t=0)x L0) / (dx A x WP)

[0140] Where Fg(t=0) is the flow rate of the NaCl solution, in g / s, obtained by linear regression analysis of the flow rate measurement Fg(t) data and extrapolation to t=0; L0 is the thickness of the gel layer, in cm; and d is the density of the NaCl solution, in g / cm³. 3 Calculate; A is the area of ​​the gel layer, in cm² 2 Calculated; and WP is the hydrostatic pressure above the gel layer, expressed in dyn / cm. 2 count.

[0141] Eddy current test

[0142] 50.0 ml ± 1.0 ml of a 0.9 wt% sodium chloride aqueous solution was introduced into a 100 ml beaker containing a 30 mm x 6 mm magnetic stir bar. The sodium chloride solution was stirred at 600 rpm using a magnetic stirrer. Then, 2000 g ± 0.010 g of hygroscopic polymer particles were added as quickly as possible, and the time taken for the stirrer eddies to disappear due to the absorption of sodium chloride solution by the hygroscopic polymer particles was measured. At this time, all contents of the beaker should still be rotating in a uniform gel mass, but the surface of the gelled sodium chloride solution should no longer show any individual turbulence. The time taken was recorded as the eddy current.

[0143] Mean Sphericity (ASPHT)

[0144] use Mean sphericity (ASPHT) was determined using a 3001L particle analyzer (Microtrac Europe GmbH; Germany).

[0145] The sample to be analyzed is introduced into the funnel. The computer-controlled measurement system activates the metering device and ensures a continuous, concentration-adjustable flow of particles. Particles fall individually through the measurement axis, producing high-contrast shadow images between the light source and the high-resolution camera. The light source is camera-driven, and due to the very short exposure time, accurate image information can be generated in real time for multiple evaluations of each individual particle.

[0146] In the 3D method, each particle is analyzed repeatedly, thus producing absolute results for length, width, thickness, area, and perimeter. Size and shape are calculated using the number of pixels covered by the particles.

[0147] Average particle size

[0148] The average particle size was determined using EDANA recommended test method No. WSP 220.2(05) "Particle Size Distribution". The average particle size was determined by plotting the mass percentage of the screening fractions in a cumulative manner. The average particle size here is the sieve aperture size value at which 50% by weight is achieved cumulatively.

[0149] Dust measurement

[0150] Dust measurements were performed using a dust meter (Heubach GmbH, Langesheim, Germany). Figure 1 The dust meter (1) is shown with an optical fiber. 2070 aerosol sensor (2) and A schematic setup of a 3000H diffused light aerosol spectrometer system (3) (Pelas GmbH, Karlsruhe, Germany). The supply of dry air (relative humidity <10%) as the transport gas is adjusted to 0.1 mbar and 1.00 mbar via control valve (4). 3A flow rate of (STP) / hour was introduced into the dust meter via a short suction tube (5). For measurement, 25 ± 1 g of superabsorbent was introduced into a modified container (6). The container (6) consisted of a roller (6a) of 60 mm in length and 140 mm in diameter, a protective net (6b), a baffle (6c), three lifting flights (6d) of 60 mm in length and 22 mm in height, a cover (6e), and a clamping device (6f). During measurement, the container (6) was moved at a speed of 45 rpm.

[0151] With the help of The 3000H scattered light aerosol spectrometer system counts the number of dust particles within 10 minutes and normalizes them to 1g of superabsorbent per minute. The dust particles are classified into three levels:

[0152] a) Level <1μm

[0153] b) Grade 1–10μm

[0154] c) Grade >10μm Example

[0155] Example 1

[0156] The superabsorbent was prepared similarly to Example 1 of WO 2016 / 134905 A1. The monomer solution used additionally contained 1.07% by weight of the disodium salt of 1-hydroxyethylidene-1,1-bisphosphonic acid.

[0157] The gas inlet temperature of the reaction zone (5) is 167°C, the gas outlet temperature of the reaction zone (5) is 107°C, the gas inlet temperature of the internal fluidized bed (27) is 100°C, the product temperature in the internal fluidized bed (27) is 78°C, the gas outlet temperature of the condenser column (12) is 57°C, and the gas outlet temperature of the gas drying unit (37) is 47°C.

[0158] The prepared superabsorbent (base polymer) had a bulk density (ASG) of 0.73 g / ml and a centrifugal retention capacity (CRC) of 49.4 g / g, with a concentration of 21.0 g / cm³. 2 The absorption under pressure (AUL) was 10.5 g / g, the residual monomer content was 5200 ppm, the extractable content was 4.5% by weight, and the moisture content was 8.0% by weight.

[0159] The superabsorbent has the following particle size distribution:

[0160]

[0161] The median particle size (d50) of the superabsorbent is 377 μm and the average sphericity (ASPHT) is 0.81.

[0162] Subsequently, the base polymer was surface-crosslinked similarly to Examples 11 to 15 of WO 2015 / 110321 A1. 2.0 wt% ethylene carbonate, 5.0 wt% water, and 0.3 wt% aluminum sulfate were used, based on the base polymer in each case. The product temperature was 160°C, and the weir height was 75%.

[0163] In the cooler, after surface post-crosslinking, first add 2.35 wt% of an aqueous solution of 0.2 wt% dehydrated sorbitol monolaurate, then add 2.35 wt% of... Diluted aqueous polymer dispersion of CE 18 (BASF SE; Ludwigshafen; Germany). CE 18 is an aqueous wax dispersion of 21 wt% ethylene-acrylic acid copolymer (composed of 20 wt% acrylic acid and 80 wt% ethylene), stabilized with potassium hydroxide. The glass transition temperature of the wax is 80°C. Calculations were performed on the diluted aqueous polymer dispersion to allow for the addition of 500 ppm of wax in solid form, based on the amount of superabsorbent particles on the polymer.

[0164] The temperature of the superabsorbent particles was 75°C when they were added.

[0165] The prepared surface-crosslinked superabsorbent had a bulk density (ASG) of 0.794 g / ml and a centrifugal retention capacity (CRC) of 40.1 g / g at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 32.9 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 23.3 g / g, and the saline conductivity (SFC) is 5 x 10⁻⁶. -7 cm 3 The concentration of the extractables was s / g, the eddy current was 68s, the moisture content was 3.2% by weight, the residual monomer content was 399ppm, and the extractable content was 3.2% by weight.

[0166] The post-crosslinked superabsorbent has the following particle size distribution:

[0167]

[0168] The median particle size (d50) of the superabsorbent is 379 μm and the average sphericity (ASPHT) is 0.80.

[0169] Subsequently, the superabsorbent particles are pneumatically conveyed.

[0170] The conveying pipe used is a smooth aluminum pipe with a length of 164 mm and an inner diameter of 100 mm. The conveying pipe consists of two horizontal sections and two vertical sections connected by elbows. The total vertical height is increased to 13 m. The conveying pipe has an internal bypass of an inward-flow type (Zeppelin Systems GmbH; Friedrichshafen; Germany). The product is conveyed into the conveying pipe via a CFH250 star feeder (Zeppelin Systems GmbH; Friedrichshafen; Germany).

[0171] The conveying capacity is 7.5 tons of superabsorbent particles per hour, the star feeder speed is 13.5 rpm, and the gas delivery rate is 560 m³ / h. 3 (STP) / hour, with a gas velocity of 11 m / s at the start of the conveying pipeline and 11.1 m / s at the end. The pressure within the conveying pipeline ranges from +660 to 0 mbar, based on an ambient pressure gauge. During stable conveying, pressure fluctuations are ±50 mbar, with an average pressure of 560 mbar. The load on the conveyed material is 11 kg / kg, and the Froude number at the start of conveying is 11.

[0172] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 39.5 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 31.1 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 23.1 g / g, and the saline conductivity (SFC) is 4 x 10⁻⁶. -7 cm 3 s / g.

[0173] The change in initial pressure over time in the delivery pipeline is shown in the figure. Figure 2 middle.

[0174] Table 1 compares the number of dust particles in Example 1 before and after pneumatic conveying.

[0175] Example 2 (Comparative Example)

[0176] The procedure is as described in Example 1. The addition level of CE 18 has been reduced to 125 ppm, based on the superabsorbent particle count.

[0177] The prepared surface-crosslinked superabsorbent had a bulk density (ASG) of 0.761 g / ml and a centrifugal retention capacity (CRC) of 39.3 g / g at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 32.7 g / g, and at 49.2 g / cm³...2 The absorption under pressure (AUHL) is 23.1 g / g, and the saline conductivity (SFC) is 5 x 10⁻⁶. -7 cm 3 The concentration of the sample was s / g, the eddy current was 65s, the moisture content was 3.1% by weight, the residual monomer content was 428ppm, and the extractable content was 3.0% by weight.

[0178] The post-crosslinked superabsorbent has the following particle size distribution:

[0179]

[0180]

[0181] The median particle size (d50) of the superabsorbent is 380 μm and the average sphericity (ASPHT) is 0.80.

[0182] The conveying capacity is 7.6 tons of superabsorbent particles per hour, the star feeder speed is 13.5 rpm, and the gas delivery rate is 560 m³ / h. 3 (STP) / hour, with a gas velocity of 7.7 m / s at the start of the conveying pipeline and 16.4 m / s at the end. The pressure within the conveying pipeline ranges from +1600 to 0 mbar, based on an ambient pressure gauge. The load on the conveyed material is 12 kg / kg, and the Froude number at the start of conveying is 7.7.

[0183] The gas delivery rate is 560m 3 The pneumatic conveying system, operating at (STP) / hour, cannot function uniformly. During the unstable conveying process, pressure fluctuations are ±450 mbar, with an average pressure of 1120 mbar.

[0184] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 39.3 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 30.7 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 21.6 g / g, and the saline conductivity (SFC) is 3 x 10⁻⁶. -7 cm 3 s / g.

[0185] The change in initial pressure over time in the delivery pipeline is shown in the figure. Figure 3 middle.

[0186] Table 1 compares the number of dust particles in Example 2 before and after pneumatic conveying.

[0187] Example 3 (Comparative Example)

[0188] The procedure is as described in Example 1. No additional steps were taken. CE 18.

[0189] The prepared surface-crosslinked superabsorbent had a bulk density (ASG) of 0.78 g / ml and a centrifugal retention capacity (CRC) of 39.6 g / g at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 33.2 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 24.8 g / g, and the saline conductivity (SFC) is 5 x 10⁻⁶. -7 cm 3 The concentration of the sample was s / g, the eddy current was 66s, the moisture content was 3.5% by weight, the residual monomer content was 386ppm, and the extractable content was 3.0% by weight.

[0190] The post-crosslinked superabsorbent has the following particle size distribution:

[0191]

[0192] The median particle size (d50) of the superabsorbent is 383 μm and the average sphericity (ASPHT) is 0.79.

[0193] The conveying capacity is 7.2 tons of superabsorbent particles / hour, the star feeder speed is 13.5 rpm, and the gas delivery rate is 560 m³ / h. 3 (STP) / hour, with a gas velocity of 7.5 m / s at the start of the conveying pipeline and 16.4 m / s at the end. The pressure within the conveying pipeline ranges from 2400 to 0 mbar, based on an ambient pressure gauge. Pressure fluctuations during conveying are ±1000 mbar. The conveyed material load is 11.3 kg / kg, and the Froude number at the start of conveying is 7.5. Pressure peaks during conveying are clearly visible as loud popping sounds within the pipeline.

[0194] The gas delivery rate is 560m 3 The pneumatic conveying system, operating at (STP) / hour, cannot function uniformly. During this highly unstable conveying process, pressure fluctuations reach ±900 mbar, with an average pressure of 1160 mbar.

[0195] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 38.6 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 32.0 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 20.6 g / g, and the saline conductivity (SFC) is 2 x 10⁻⁶. -7 cm 3 s / g.

[0196] The change in initial pressure over time in the delivery pipeline is shown in the figure. Figure 4 middle.

[0197] Table 1 compares the number of dust particles in Example 3 before and after pneumatic conveying.

[0198] Example 4

[0199] Subsequently, the base polymer of Example 1 was subjected to surface post-crosslinking similar to Examples 11 to 15 of WO 2015 / 110321 A1. 2.0 wt% ethylene carbonate, 5.0 wt% water, and 0.05 wt% aluminum sulfate were used, based on the base polymer in each case. The product temperature was 159°C, and the weir height was 75%.

[0200] In the cooler, after surface post-crosslinking, first add 4.35 wt% of an aqueous solution of 0.23 wt% aluminum lactate, then add 1.66 wt% of... A diluted aqueous polymer dispersion of CE 18 (BASF SE; Ludwigshafen; Germany) and sorbitol monolaurate. Calculations were performed on the diluted aqueous polymer dispersion to allow for the addition of 500 ppm of wax in solid form and 25 ppm of sorbitol monolaurate, based on superabsorbent particles.

[0201] The prepared surface-crosslinked superabsorbent had a bulk density (ASG) of 0.75 g / ml and a centrifugal retention capacity (CRC) of 38.1 g / g at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 34.4 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 25.9 g / g, and the saline conductivity (SFC) is 5 x 10⁻⁶. -7 cm 3 The concentration of the sample was s / g, the eddy current was 69s, the moisture content was 4.5% by weight, the residual monomer content was 263ppm, and the extractable content was 2.6% by weight.

[0202] The post-crosslinked superabsorbent has the following particle size distribution:

[0203]

[0204] The median particle size (d50) of the superabsorbent is 396 μm and the average sphericity (ASPHT) is 0.80.

[0205] The superabsorbent particles obtained therefrom were pneumatically transported under different conditions (Examples 4a to 4d).

[0206] Example 4a

[0207] The initial conveying operation achieved a conveying capacity of 7.2 tons of superabsorbent particles per hour, with a star feeder speed of 13.5 rpm and a gas delivery rate of 550 m³ / h. 3 (STP) / hour, with a gas velocity of 10.7 m / s at the start of the conveying pipeline and 17.3 m / s at the end. The pressure within the conveying pipeline ranges from 740 to 0 mbar, based on an ambient pressure gauge. The load on the conveyed material is 10.9 kg / kg, and the Froude number at the start of conveying is 11. During stable conveying, pressure fluctuations are ±50 mbar, and the average pressure during conveying is 580 mbar.

[0208] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 38.7 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 34.2 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 26.2 g / g, and the saline conductivity (SFC) is 5 x 10⁻⁶. -7 cm 3 s / g.

[0209] Table 1 compares the number of dust particles in Example 4a before and after pneumatic conveying.

[0210] Example 4b

[0211] Subsequently, in the second conveying operation, the conveying capacity was 11.2 tons of superabsorbent particles / hour, the star feeder speed was 20 rpm, and the gas delivery rate was 660 m³ / h. 3 (STP) / hour, with a gas velocity of 11.0 m / s at the start of the conveying pipeline and 20.4 m / s at the end. The pressure within the conveying pipeline ranges from 910 to 0 mbar, based on an ambient pressure gauge. Pressure fluctuations during conveying are ±50 mbar. The conveyed material load is 14.2 kg / kg, and the Froude number at the start of conveying is 11. During stable conveying, pressure fluctuations are ±50 mbar, and the average pressure during conveying is 825 mbar.

[0212] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 38.6 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 33.6 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 25.3 g / g, and the saline conductivity (SFC) is 4 x 10⁻⁶. -7 cm 3 s / g.

[0213] Example 4c

[0214] The third conveying operation then carried out at a rate of 16.4 tons of superabsorbent particles per hour, with the star feeder speed at 30 rpm and the gas delivery rate at 560 m³ / h. 3 (STP) / hour, with a gas velocity of 10.6 m / s at the start of the conveying pipeline and 23.1 m / s at the end. The pressure within the conveying pipeline ranges from 1230 to 0 mbar, based on an ambient pressure gauge. Pressure fluctuations during conveying are ±50 mbar. The load on the conveyed material is 18.3 kg / kg, and the Froude number at the start of conveying is 10. During stable conveying, pressure fluctuations are ±50 mbar, and the average pressure during conveying is 1160 mbar.

[0215] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 38.6 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 33.0 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 24.2 g / g, and the saline conductivity (SFC) is 4 x 10⁻⁶. -7 cm 3 s / g.

[0216] Example 4d

[0217] Subsequently, in the fourth conveying operation, the conveying capacity was 20.8 tons of superabsorbent particles per hour, the star feeder speed was 40 rpm, and the gas delivery rate was 770 m³ / h. 3 (STP) / hour, with a gas velocity of 9.1 m / s at the start of the conveying pipeline and 23.2 m / s at the end. The pressure within the conveying pipeline ranges from 1800 to 0 mbar, based on an ambient pressure gauge. Pressure fluctuations during conveying are ±50 mbar. The conveyed material load is 23.1 kg / kg, and the Froude number at the start of conveying is 9. During stable conveying, pressure fluctuations are ±200 mbar, and the average pressure during conveying is 1500 mbar.

[0218] After delivery, the centrifugal retention capacity (CRC) of the superabsorbent was 38.6 g / g, at 21.0 g / cm³. 2 The absorbance under pressure (AUL) was 32.8 g / g, and at 49.2 g / cm³... 2 The absorption under pressure (AUHL) is 23.9 g / g, and the saline conductivity (SFC) is 3 x 10⁻⁶. -7 cm 3 s / g.

[0219] The changes in initial pressure over time in the delivery pipelines in Examples 4a to 4d are shown in the figures. Figure 5 middle.

[0220] Table 1: Standardized quantity of particles P discharged from the Heubach dust meter at 5 l / min dry air (standardized to per minute and per g of superabsorbent, classified and summarized).

[0221]

[0222] *) Comparative Examples

Claims

1. A method for pneumatically conveying surface-crosslinked superabsorbent particles, wherein the surface-crosslinked superabsorbent particles are mixed with an aqueous wax dispersion prior to pneumatic conveying, the wax having a glass transition temperature of at least 65°C, and 0.025% to 0.20% by weight of wax is used based on untreated surface-crosslinked superabsorbent particles, the wax not forming a continuous film on the particle surface.

2. The method according to claim 1, wherein the glass transition temperature of the wax is at least 80°C.

3. The method according to claim 1 or 2, wherein 0.035% to 0.08% by weight of wax is used in the superabsorbent particle meter based on surface post-crosslinking.

4. The method according to claim 1 or 2, wherein the wax is a copolymer of 70 to 95 mol% of at least one olefinically unsaturated hydrocarbon and 5 to 30 mol% of at least one olefinically unsaturated carboxylic acid.

5. The method according to claim 1 or 2, wherein the average particle size of the surface-crosslinked superabsorbent particles is 250 to 500 µm.

6. The method according to claim 1 or 2, wherein the average sphericity of the surface-crosslinked superabsorbent particles is greater than 0.

72.

7. The method according to claim 1 or 2, wherein during pneumatic transport, the temperature of the surface-crosslinked superabsorbent particles is at least 40°C and at least 20°C lower than the glass transition temperature of the wax.

8. The method according to claim 1 or 2, wherein in pneumatic conveying, the initial gas velocity corresponds to a Froude number from 2 to 40.

9. The method according to claim 1 or 2, wherein in pneumatic conveying, the load of the conveyed material is 1 to 30 kg / kg, and the load of the conveyed material is the quotient of the mass flow rate of the conveyed material and the mass flow rate of the gas.

10. A composition comprising surface-post-crosslinked superabsorbent particles and wax particles, wherein the wax particles are located on the surface of the surface-post-crosslinked superabsorbent particles, the wax particles are dispersible in water and have a glass transition temperature of at least 65°C, the wax particles are present in a proportion of 0.025% to 0.2% by weight based on the surface-post-crosslinked superabsorbent particles, the wax does not form a continuous film on the particle surface, and the composition has an absorption of at least 10 g / g at a pressure of 4.83 kPa.

11. The composition of claim 10, wherein the glass transition temperature of the wax is at least 80°C.

12. The composition of claim 10 or 11, wherein, Based on the superabsorbent particle meter with post-crosslinked surface, 0.035% to 0.08% by weight of wax was used.

13. The composition according to claim 10 or 11, wherein the wax is a copolymer of 5 to 30 mol% of at least one olefinically unsaturated carboxylic acid and 70 to 95 mol% of at least one olefinically unsaturated hydrocarbon.

14. The composition according to claim 10 or 11, wherein the average particle size of the surface-crosslinked superabsorbent particles is 250 to 500 µm.

15. The composition according to claim 10 or 11, wherein the average sphericity of the surface-crosslinked superabsorbent particles is greater than 0.72.

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