Heat-treatment of water-absorbing polymeric particles in a fluidized bed at a fast heat-up rate

Abstract

The present invention relates to a method for heat-treating water-absorbing polymeric particles at a temperature equal to or above 150 C. in a fluidized bed dryer at a fast heat-up rate, the use of a fluidized bed dryer for heat-treating water-absorbing polymeric particles in continuous or batch mode as well as to the heat-treated polymeric particles obtained by the method of the present invention.

Claims

1. A method for heat-treating water-absorbing polymeric particles at a temperature T.sub.p1 equal to or above 150 C., wherein said water-absorbing polymeric particles comprise a residual amount of water being between 0.3 to 7 wt.-%, based on the composition, as determined by the EDANA standard test method WSP 203.3(10) when subjected to the heat treatment, the particles are heated in a fluidization chamber of a fluidized bed dryer from an initial particle temperature T.sub.0 equal to or below 50 C. to said temperature T.sub.p1 at a heat-up rate of at least 10 C. per minute to effect surface crosslinking of said water-absorbing polymeric particles.

2. The method according to claim 1, wherein the polymeric particles are heated from said initial particle temperature T.sub.0 to said temperature T.sub.p1 in less than 10 minutes and T.sub.p1 is in the range of from 170 to 245 C.

3. The method according to claim 1, wherein said polymeric particles are obtained by polymerising a monomer mixture comprising at least one type of ethylenically unsaturated monomer, at least one type of cross-linker and at least one type of initiator.

4. The method according to claim 3, wherein the monomer mixture additionally comprises at least one polyhydric alcohol as an additional cross-linker in an amount of at least 50 ppm, based on the total weight of ethylenically saturated monomer.

5. The method according to claim 4, wherein the polyhydric alcohol comprises glycerin.

6. The method according to claim 1, wherein after being heated to a temperature T.sub.p1, said polymeric particles are kept in the fluid bed dryer at a temperature T.sub.p2, wherein T.sub.p1 and T.sub.p2 both are in the range of from 170 to 245 C.

7. The method according to claim 1, wherein the water-absorbing polymeric particles are heated in the fluidization chamber for a residence time of from 5 to 60 minutes.

8. The method according to claim 1, wherein a solution comprising at least one organic or inorganic cross-linking agent is applied to the surface of said polymeric particles.

9. The method according to claim 8, wherein said solution comprises at least one compound selected from the group consisting of polyhydric alcohols, polyglycidyl compounds, cyclic carbonates, polyamines, alkoxysilyl compounds, polyaziridines, polyamidoamines, oxazolidones, bisoxazolines, water-soluble multivalent metal salts, metal oxides or mixtures thereof in an aqueous solvent.

10. The method according to claim 1, wherein said particles are contacted inside said fluidized bed dryer with at least one hot gas stream having a temperature T.sub.g inside said fluidization chamber of said fluidized bed dryer, said fluidization chamber, opening downwardly in at least one lower plenum chamber through at least one gas distribution bottom plate having openings formed there through for upward gas flow from said lower plenum chamber into said fluidization chamber and wherein the superficial gas velocity of said hot gas stream in the fluidized bed is in the range of from 0.1 to 0.57 m/s.

11. The method according to claim 10, wherein the pressure drop across the gas distribution bottom plate is in the range of from 100 to 900 Pa, and the total pressure drop across the bottom plate and the fluidized bed is in the range of from 2,500 to 5,000 Pa.

12. The method according to claim 2, wherein the polymeric particles are heated from said initial particle temperature T.sub.0 to said temperature T.sub.p1 in less than 10 minutes and T.sub.p1 is in the range of from 190 to 235 C.

13. The method according to claim 3, wherein said ethylenically unsaturated monomer is an acrylic acid present at least partly in form of a salt.

14. The method according to claim 1, wherein said polymeric particles comprise a residual amount of water being in the range of from 0.5 to 6 wt. %, based on the composition, as determined by the EDANA standard test method WSP 203.3(10).

15. The method according to claim 1, wherein before being heat-treated, said polymeric particles are wetted at their surface by an amount of water being in the range of from 1 to 5 wt.-% based on the whole composition.

16. The method according to claim 1, wherein before being heat-treated, said polymeric particles are wetted at their surface by an amount of water being in the range of from 1.5 to 3.5 wt.-% based on the whole composition.

17. The method according to claim 6, wherein after being heated to a temperature T.sub.p1, said polymeric particles are kept in the fluid bed dryer at a temperature T.sub.p2, wherein T.sub.p1 and T.sub.p2 both are in the range of from 190 to 235 C.

Description

LIST OF FIGURES

(1) FIG. 1 shows a fluidized bed dryer specifically adapted to the method of the present invention. Said fluidized bed dryer comprises a drying compartment formed of a fluidization chamber 1, opening downwardly in a plenum chamber 2 through a gas distribution bottom plate 3. A cooling chamber 4 is located adjacent to the drying compartment at said end of the drying compartment, which is opposed to the product inlet of the product charging system 5. The cooling chamber is equipped with a heat exchanger 12. From said cooling chamber the product is discharged by a product discharging system 6. To heat the gas stream entering the plenum 2 of the drying compartment, gas heaters 7a and 7b are used, from which a hot gas stream is fed into the drying compartment. After passing the fluidized bed in the fluidization chamber 1 the vent stream is directed into a filter system 8, where the elutriated polymeric particles are separated from the gas and are recovered. A part of the filtered vent stream 9 is recycled to the gas heaters 7a and 7b while the other part 10 is discharged to the atmosphere. The fluidized bed dryer furthermore may be equipped with means 11 for flushing the drying compartment with nitrogen or a noble gas. The fluidized bed dryer furthermore comprises means for introducing dry gas 14 into the cooling chamber 4.

(2) FIG. 2 illustrates a special device 15 for homogeneous feed distribution in the fluidization chamber 1.

(3) FIG. 3 illustrates the use of baffle plates 16 in the fluidization chamber 1 for obtaining a quasi plug flow of polymeric particles. A weir 17 serves to adjust the height of the fluidized bed in the fluidization chamber 1 and separates the fluidization chamber 1 from the cooling chamber 4.

(4) FIG. 4 illustrates the impact of particle amount on heat-up rate according to examples 4a-4c.

LIST OF REFERENCE SIGNS

(5) 1 fluidization chamber 2 plenum chamber 3 gas distribution bottom plate 4 cooling chamber 5 product charging system/product inlet 6 product discharging system/product outlet 7 gas heater 8 filter system 9 recycled filtered gas stream 10 discharged gas stream 11 means for flushing the drying compartment with nitrogen or noble gases 12 heat exchanger 13 Elutriated fines separated from the vent stream 14 dry gas 15 device for homogeneous feed distribution 16 baffle plates 17 weir

EXAMPLES

(6) 1. Analytical Methods

(7) Centrifuge Retention Capacity (CRC)

(8) The gravimetric determination of fluid retention capacity in a saline solution after centrifugation was carried out according to the EDANA standard test WSP 241.3(10).

(9) Adsorption Against Pressure/Adsorption under Pressure (AAP/AUP)

(10) The gravimetric determination of adsorption under pressure at a pressure of 0.3 or 0.7 psi/21 to 49 mbar using polymeric particles having a particle size in the range of from 150 to 850 m was determined according to the EDANA standard test WSP 242.3(10).

(11) Adsorption under Load at 0.9 psi (AUL.sub.0.9 psi)

(12) A nylon screen (5050 mm.sup.2; 100 mesh/149 m) was placed on top of a perforated metal plate, followed by a filter paper and finally by a hollow stainless steel cylinder of 26 mm inner diameter, 37 mm outer diameter and a height of 50 mm. 167 mg of water-absorbing polymer particles were placed in the cylinder and evenly distributed. A non-woven sheet having a diameter of 26 mm covered the polymer and was pressed down with a plastic piston of a diameter of 26 mm, which carried a weight. The total mass of the piston and the weight on top the piston was 328.2 gram to provide a load of 0.9 psi (62.1 mbar). The cylinder was immersed into a 0.9% saline solution such that the nylon screen and the solution surface had the same level to allow absorption of the liquid by the filter paper and the water-absorbing polymer particles without any hydrostatic pressure. The particles were soaked for one hour. The plate was removed from the water reservoir and excess liquid in the holes of the metal plate and the nylon screen was soaked up by paper tissue. Then the weight was removed from the swollen gel and the gel was weighed. The weight of saline solution absorbed under load by one gram of water absorbent polymer particles is reported as the absorption under load (AUL.sub.0.9psi).

(13) Extractables (Extr.)

(14) Determination of extractable polymer content by potentiometric titration was carried out using the EDANA Standard Test WSP 270.3(10).

(15) Residual Acrylic Acid (Res. AA)

(16) The amount of residual monomers in the superabsorbent materials, i.e. the amount of residual acrylic acid in the polyacrylate superabsorbent particles, was determined using the EDANA Standard Test WSP 210.3(10).

(17) Permeability under Load (PUL)

(18) The method used for determining the permeability under load is similar to the method for determining the AAP described above. For determining PUL, the above described AAP method was carried out, in which 0.9 g of the superabsorbent particles were placed in the AAP cell to obtain the AAP.sub.0.7 psi (0.9g) value. This method is then repeated with a weight of 5 g0.005 g from the same superabsorbent material to be tested in order to obtain the AAP.sub.0.7 psi (5g) value. The PUL-value is defined by the ratio of AAP.sub.0.7 psi (0.9g)/AAP.sub.0.7 psi (5g)100.

(19) Saline Flow Conductivity (SFC)

(20) The method described in U.S. Pat. No. 5,562,646 and U.S. Pat. No. 5,559,335 was used. For each test, an aliquot of 0.9 g of the superabsorbent polymeric particles having a particle size of from 150 to 850 m was used.

(21) Particle Size Distribution (PSD)

(22) The particle size distribution up to a size of 850 m of the super absorbent materials was determined using the EDANA Standard Test WSP 220.3(10).

(23) Moisture Content (Moisture)

(24) The residual moisture content of the superabsorbent materials, i.e. the evaluation of mass loss upon heating, was determined using the EDANA Standard Test WSP 230.3(10).

(25) Hunter Color (Color L, a, b)

(26) The hunter color was determined according to ASTM methods E 1164-94 and E 1347-97. In this method, the color (reflectance) of a material is measured using a Hunter Color Difference Meter. The sample color is reported in terms of three values; L is a correlate of lightness, a & b are termed color axes. Value a indicates redness or greenness if negative, and b indicates yellowness or blueness if negative.

(27) 2. Preparation of Superabsorbent Polymers

(28) Preparation of Polymer 1

(29) A monomer solution was prepared in batch-mode by mixing carefully 724.44 kg of sodium hydroxide, 3717.73 kg of process water (partially de-mineralized ground water) and 1401.3 kg of glacial acrylic acid (AA) (99.8%). To this solution were added 3.63 kg of a 40.2% active solution of the penta-sodium salt of diethylene triamine pentaacetic acid (commercially available under the trademark name Versenex 80 from the Dow Chemical Company), corresponding to an amount of 750 ppm, based on acrylic acid (b.o.AA), 10.32 kg of a 5% active solution of sodium chlorate (265 ppm b.o.AA), 11.68 kg PEG 600 (6000 ppm b.o.AA) and a mixture of 598 kg of glacial acrylic acid (99.8%) and 6.23 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (3200 ppm b.o.AA). During mixing, the temperature of the solution was controlled to be below 35 C.

(30) The monomer solution was then transferred into a horizontal single screw kneader reactor. During transport to the reactor, 1.3 kg of a 30% active solution of hydrogen peroxide (200 ppm b.o.AA) and 42.82 kg of a 10% active solution of sodium persulfate (2200 ppm b.o.AA) were mixed with the monomer solution. 368.1 kg of superabsorbent fines (18.4% b.o.AA) were added and homogeneously mixed into the monomer solution. The resulting mixture was de-oxygenated by purging it with nitrogen. The temperature was then adjusted to 30 C. Polymerization was initiated by adding 36.1 kg of a 1% active solution of sodium erythorbate (185 ppm b.o.AA) to the reactor under agitation. Once polymerization had started the temperature of jacket and shaft of the reactor was adjusted to 70 C. Once the reaction mixture reached a temperature close to 85 C., the pressure in the reactor was decreased such, that the peak temperature was maintained at 85 C. After having reached said peak temperature the polymer gel was cooled down to 70 C. by further reducing the pressure in the reactor. The vapor was condensed in a condenser above the reactor and redirected onto the gel in the reactor. 10 min after having reached the peak temperature, the granulated polymer gel was transferred into a slowly agitated hold tank for an average residence time of 100 min from where it was continuously withdrawn to be further sized in an extruder, spread onto the belt of a belt drier and dried in a hot-air stream at 170 C. for 20 min. The polymer layer obtained was crushed, ground in a roll mill (Bauermeister) and sieved (0.15 to 1.18 mm) to obtain polymer 1.

(31) Preparation of Polymer 2

(32) The procedure described for the preparation of polymer 1 was repeated, except that 5.84 kg of HE-TMPTA (3000 ppm b.o.AA) were used.

(33) Preparation of Polymer 3

(34) The procedure described for the preparation of polymer 1 was repeated, except that 4.28 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (1900 ppm b.o.AA) were used.

(35) The properties of polymers 1 to 3 are summarized in Table 1a.

(36) TABLE-US-00001 TABLE 1a Properties of polymers 1-3 prior to heat-treatment Particle size distribution CRC Moisture (mm, % on screen) Polymer (g/g) (%) 0.85 0.6 0.3 0.15 0.045 <0.045 1 38.1 1.5 0.1 15.8 66.8 3.8 3.6 0 2 38.9 1.9 0.1 16.7 62.3 15.9 4.8 0.1 3 38.1 1.5 0.1 15.8 66.8 3.8 3.6 0
Preparation of Polymer 4

(37) The procedure described for the preparation of polymer 3 was repeated, except that the monomer formulation contained no chlorate and only 1.06 kg of a 40.2% active solution of the penta-sodium salt of diethylene triamine pentaacetic acid (220 ppm b.o.AA). 2.28 kg of a 30% active solution of hydrogen peroxide (350 ppm b.o.AA), 27.25 kg of a 10% active solution of sodium persulfate (1400 ppm b.o.AA) and 42.82 kg of a 1% active solution of sodium erythorbate (220 ppm b.o.AA) were applied.

(38) Preparation of Polymer 5

(39) The procedure described for the preparation of polymer 4 was repeated, except that as a cross-linker 2.8 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (1443 ppm b.o.AA) was used and 5.42 kg of a 70% active solution of polyethylenglycol monoallyletheacrylacrylic acid ester (PEG-MAE-AE) with an average of 10 EO-units per molecule (1950 ppm b.o.AA) was added as an additional network cross-linker.

(40) Preparation of Polymer 6

(41) The procedure described for the preparation of polymer 4 was repeated, except that as a cross-linker 2.4 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (1235 ppm b.o.AA) was used and 5.64 kg of a 70% active solution of polyethylenglycoi monoallyletheacrylacrylic acid ester (PEG-MAE-AE) with an average 10 EO-units (2030 ppm b.o.AA) was added.

(42) Preparation of Polymer 7

(43) The procedure described for the preparation of polymer 4 was repeated, except that as a cross-linker 3.27 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (1685 ppm b.o.AA) was used and 6.47 kg of a 70% active solution of polyethylenglycol monoallyl ether-acrylacrylic acid ester (PEG-MAE-AE) with an average 10 EO-units (2330 ppm b.o.AA) was added.

(44) Preparation of Polymer 8

(45) The procedure described for the preparation of polymer 4 was repeated, except that as a crosslinker 4.7 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (2420 ppm b.o.AA) was used and 7.9 kg of a 70% active solution of polyethylenglycol monoallyl ether-acrylaciylic acid ester (PEG-MAE-AE) with an average 10 EO-units (2860 ppm b.o.AA) was added.

(46) The properties of polymers 4-8 prior to heat-treatment are summarized in Table 1b.

(47) TABLE-US-00002 TABLE 1b Properties of polymers 4-8 prior to heat-treatment Extr..sub.16 h Res. AA Moisture Hunter Color Polymer CRC (g/g) (%) (ppm) (%) L b 4 42.8 16.2 400 3.5 93.2 6.4 5 37.9 11.6 431 3.4 93.6 6.4 6 39.7 13.0 379 3.6 93.6 6.5 7 34.6 9.4 472 3.1 94.2 6.4 8 33.1 7.8 563 3.3 94.2 5.9
Preparation of Polymer 9

(48) The procedure described for the preparation of polymer 4 was repeated, except the monomer formulation comprised 2.8 kg of ethoxylated trimethylolpropane triacrylate (HE-TMPTA) with an average of 15 EO-units per molecule (1443 ppm b.o.AA) and 5.42 kg of a 70% active solution of polyethylenglycol monoallyl ether acrylacrylic acid ester (PEG-MAE-AE) with an average of 10 EO-units (1950 ppm b.o.AA). Furthermore 11% b.o.AA of superabsorbent fines were added and homogeneously mixed in the monomer solution.

(49) Preparation of Polymer 10

(50) The procedure described for the preparation of polymer 9 was repeated, except that the monomer formulation comprised 2.4 kg of ethoxylated trimethylolpropane triacrylate (HE-TMPTA) with an average of 15 EO-units per molecule (1235 ppm b.o.AA) and 5.64 kg of a 70% active solution of polyethylenglycol monoallyl ether acrylacrylic acid ester (PEG-MAE-AE) with an average 10 EO-units (2030 ppm b.o.AA).

(51) Preparation of Polymer 11

(52) The procedure described for the preparation of polymer 9 was repeated, except that the monomer formulation comprised 3.27 kg of ethoxylated trimethylolpropane triacrylate (HE-TMPTA) with an average of 15 EO-units per molecule (1685 ppm b.o.AA) and 6.47 kg of a 70% active solution of polyethylene glycol monoallyl ether acrylacrylic acid ester (PEG-MAE-AE) with an average of 10 EO-units per molecule (2330 ppm b.o.AA).

(53) Preparation of Polymer 12

(54) The procedure described for the preparation of polymer 9 was repeated, except that the monomer formulation comprised 4.7 kg of ethoxylated trimethylolpropane triacrylate (HE-TMPTA) with an average of 15 EO-units per molecule (2420 ppm b.o.AA) and 7.9 kg of a 70% active solution of polyethylene glycol monoallyl ether acrylacrylic acid ester (PEG-MAE-AE) having on average 10 EO-units (2860 ppm b.o.AA).

(55) Preparation of Polymer 13

(56) The procedure described for the preparation of polymer 12 was repeated, except that the monomer formulation comprised 9.88 kg of a 70% active solution of polyethyleneglycol monoallyl ether acrylacrylic acid ester (PEG-MAE-AE) with an average of 10 EO-units per molecule (3580 ppm b.o.AA) and that PEG 600 was omitted.

(57) The properties of polymers 4 and 9 to 13 are summarized in Table 1c.

(58) TABLE-US-00003 TABLE 1c Properties of polymers 4 and 9 to 13 Extr..sub.16 h Res. AA Moisture Hunter Color Polymer CRC (g/g) (%) (ppm) (%) L b 4 42.8 16.2 400 3.5 93.2 6.4 9 37.9 11.6 431 3.4 93.6 6.4 10 39.7 13.0 379 3.6 93.6 6.5 11 34.6 9.4 472 3.1 94.2 6.4 12 33.1 7.8 563 3.3 94.2 5.9 13 33.1 7.1 586 n.d..sup.1 91.9 5.8
Preparation of Polymer 14

(59) The procedure described for the preparation of polymer 3 was repeated, except that the concentration of ethoxylated trimethylolpropane triacrylate (HE-TMPTA) was changed to 2200 ppm b.o.AA and that 200 ppm b.o.AA of glycerin was added to the monomer solution.

(60) Preparation of Polymer 15

(61) The procedure described for the preparation of the polymer 3 was repeated, except that the concentration of the ethoxylated trimethylolpropane triacrylate (HE-TMPTA) was changed to 2700 ppm b.o.AA and that 600 ppm b.o.AA of glycerin was added to the monomer solution.

(62) The properties of polymers 14 and 15 are summarized in Table 1d.

(63) TABLE-US-00004 TABLE 1d Properties of polymers 14 and 15 Polymer CRC (g/g) Extr..sub.16 h (%) Res. AA (ppm) Moisture (%) 14 50.8 18.2 255 1.3 15 37.8 16.4 282 1.4
Preparation of Polymer 16

(64) A monomer solution was prepared by mixing carefully 2925.67 kg of a 25% active sodium hydroxide solution, 1495.13 kg of process water (partially de-mineralized ground water) and 1357.8 kg of glacial acrylic acid (AA) (99.9% active). To this solution were added 3.62 kg of a 40.2% active solution of Versenex 80, 1.71 kg of a 5% active solution of sodium chlorate (265 ppm b.o.AA), 22.61 kg of a 60% active solution of PEG 600 (7000 ppm b.o.AA) and a mixture of 581.9 kg of glacial acrylic acid (99.9% active) and 4.84 kg of ethoxylated trimethylolpropane triacrylate with an average of 15 EO-units per molecule (2500 ppm b.o.AA). In this monomer solution, 68.6% of the acrylic acid was neutralized. During mixing, the temperature of the solution was controlled to be below 35 C.

(65) The monomer solution was then transferred into a horizontal single screw kneader reactor. During transfer 1.3 kg of a 30% active solution of hydrogen peroxide (200 ppm b.o.AA) and 42.82 kg of a 10% active solution of sodium persulfate (2200 ppm b.o.AA) were mixed with the monomer solution. 319.72 kg of superabsorbent fines (16.5% b.o.AA) were added and homogeneously mixed into the monomer solution. The mixture was de-oxygenated by purging it with nitrogen. The temperature was adjusted to 30 C. and finally the polymerization was initiated by adding 36.1 kg of a 1% active solution of sodium erythorbate under agitation.

(66) After the start of polymerization the temperature of jacket and shaft of the reactor was adjusted to 70 C. Once the reaction mass reached a temperature close to 85 C., the pressure in the reactor was decreased to control the peak temperature at 85 C. After having reached said peak temperature, the polymer gel was cooled to 70 C. by further reducing the pressure in the reactor. The vapor was condensed in a condenser above the reactor and redirected onto the gel in the reactor. About 10 min after having reached the peak temperature the granulated polymer gel was transferred into a slowly agitated hold tank for an average residence time of about 100 min from where it was continuously withdrawn to be further sized in an extruder, spread onto the belt of a belt drier and dried in a hot air stream at about 170 C. for 20 min. The polymer layer obtained was crushed and ground in a roll mill (Bauermeister) and classified to obtain polymer 16 having a particle size between 100 and 800 m.

(67) Preparation of Polymer 17

(68) The procedure described lbr the preparation of polymer 16 was repeated, except that the degree of neutralization was adjusted to be 65%, the concentration of PEG 600 was reduced to 5000 ppm and the concentration of recycled fines to 11% (all b.o.AA).

(69) Preparation of Polymer 18

(70) The procedure described for the preparation of polymer 16 was repeated, except that the following concentrations were changed: PEG 600 to 8000 ppm, HE-TMPTA to 2700 ppm, Versenex 80 from to 500 ppm, hydrogen peroxide to 350 ppm and sodium persulfate to 1700 ppm. Furthermore, 200 ppm of glycerin (all concentrations b.o.AA), but no fines were added.

(71) Preparation of Polymer 19

(72) A monomer solution was continuously prepared consisting of 31.74 parts of acrylic acid (active content 99.9%), 43.85 parts of a 25% active aqueous solution of sodium hydroxide to neutralize the acrylic acid to a degree of 65%, and 15.63 parts of water, all based on 100 parts of final monomer solution. To this mixture were added 0.48 parts of a 5% active aqueous solution of Versenex 80 (750 ppm b.o.AA), 0.17 parts of a 5% active aqueous solution of sodium chlorate (265 ppm b.o.AA), 0.07 parts of HE-TMPTA (2200 ppm b.o.AA), 0.37 parts of a 60% active aqueous solution of PEG 600 (7000 ppm b.o.AA) and 0.00636 parts of glycerin (200 ppm b.o.AA). This monomer solution, having a temperature of about 28 C. and a total solids content of about 38%, was continuously transferred to a two screw reactor at a feed rate of 6500 kg/hour. Into said feed stream was continuously injected 0.26 parts of a 3% active aqueous hydrogen peroxide solution (250 ppm b.o.AA), 0.79 parts (2500 ppm active b.o.AA) of a 10% active aqueous sodium peroxide solution, 7% of superabsorbent fines and a stream of about 13.5 kg/h of nitrogen. To the feed zone of the reactor 0.84 parts of a 0.7% aqueous sodium erythorbate solution were added continuously (all parts based on 100 parts of final monomer solution). Furthermore 70 kg/h of steam were injected through a bottom valve in the reactor. Polymerization occurred in the reactor and the peak-temperature was controlled at 85 C. by reducing the pressure in the reactor to 850 mbar. Evaporated water was condensed in a condenser above the reactor and redirected onto the gel in zone 3 of the reactor. The free flowing, granulated gel was continuously discharged from the reactor into the hold tank where it resided for about one hour at a temperature of 83 C., was minced through a die plate having 6 mm wide slits which were radially arranged and dried on a belt drier in an air stream having a temperature of 170 C. for 20 min. After drying, the polymer was ground in a roll mill and sieved to obtain a particulate polymer having a particle size of between 150 and 800 mm.

(73) The properties of polymers 16-19 are summarized in Table 1e.

(74) TABLE-US-00005 TABLE 1e Properties of polymers 16 to 19 Polymer CRC (g/g) Res AA (ppm) Extr. (%) 16 33.9 174 13.8 17 37.2 194 17.7 18 49.6 527 21.9 19 43.5 281 21.7
Preparation of Polymers 20-22

(75) A monomer solution was prepared by mixing carefully 1263.67 parts of 99.9% active acrylic acid and 1993.66 parts of 24% active NaOH (resulting in a degree of neutralization of 68%) under cooling, so that the temperature of the mixture was permanently kept below 35 C. To this mixture were added 236.91 parts of water, 18.94 g parts of sodium sulphate, 2.9 parts of HE-TMPTA (2300 ppm b.o.AA), 12.62 parts of a 60% active solution of PEG 600 (6000 ppm b.o.AA) and glycerin (for polymer 20 250 mg/200 ppm b.o.AA, for polymer 21 500 ppm and for polymer 22 1000 ppm). Then 126.2 parts of SAP-fines (10% b.o.AA) and 0.25 parts of Versenex 80 (10 ppm b.o.AA) were added.

(76) The mixtures were kept at 22 C., while the initiator solutions were prepared and de-oxygenated. These initiator solutions were then fed to the monomer feed through T-fittings. The atmosphere of the head space of the polymerization reactor was kept inert by continuously purging it with a nitrogen stream of 200 L/h. The temperature of reactor jacket was set to 90 C. The monomer solution was continuously fed into this reactor at a temperature of 22 C. with a rate of 6.5 kg/h (2.1 kg/L of total reactor volume) which was de-oxygenated in the bubble column using a nitrogen stream of 20 L/h, a 35% active aqueous solution of hydrogen peroxide (350 ppm active b.o.AA), a 10% active aqueous solution of sodium persulfate (1700 ppm active b.o.AA), 20% of scrubber water (b.o.AA) containing 10% of sodium carbonate and 2% of NaOH and an 0.7% active aqueous solution of sodium erythorbate (200 ppm b.o.AA). The carbonate of the scrubber water completed deoxygenation and sodium ascorbate finally triggered immediate initiation of the polymerization reaction.

(77) At steady state conditions, the monomer feed was at least partially mixed with the polymer gel present in the reactor (on average about 1.7 kg) and polymer gel was continuously discharged through the opening and the discharge tube of the end plate into the gel receptacle. The discharged gel was kept under nitrogen for additional 60 min before being processed further. The granulated gel as discharged from the reactor still having a temperature above 60 C. was extruded through a kitchen type meat-mincer equipped with a die plate having openings of 8 mm. A portion of 800 g of the extruded gel was then placed in a basket made of a metal screen having a mesh size of 2 mm. Said basket was placed in a lab-size gel drier and was dried in a hot air stream of 5 m/s at a temperature of 180 C. for 20 min. The dry polymer obtained was ground in a roll mill and sieved in a Retsch sieve tower equipped with sieves having mesh sizes of 850 and 150 m. Properties of polymers 20 to 22 are summarized in Table 1f.

(78) TABLE-US-00006 TABLE 1f Properties of polymers 20-22 Glycerin CRC AAP.sub.0.3 psi Extr. Res AA Polymer (ppm) (g/g) (g/g) (%) (ppm) 20 200 39.5 7.7 19.8 400 21 500 38.1 7.6 19.8 642 22 1000 38.6 7.6 20.9 308
3. Heat-treatment in a Continuously Operated Fluidized Bed Dryer

Comparative Example 1

Heat-treatment of Polymer 2 in a Continuous FBD Using Standard Conditions

(79) Polymer 2 was produced during regular production for 10 month and heat-treated in a fluidized bed dryer with an essentially horizontal longitudinal axis equipped with a fine-hole plate (Conidur plate, Type 101, Hein & Lehmann, Germany) with a plate area of 8 m.sup.2 and holes of 0.35 mm, providing a pressure loss of 270 Pa with hot air having a temperature of 260 C., said plate being divided in two zones, two heat-exchanger for air heating (one for each zone) and a cooling chamber. The weir to the cooling chamber was set to provide a fluidized bed height of 40 cm. The fluidized bed dryer was operated with air as the fluidizing gas. The product temperature in the first zone was set to 220 C. and in the second zone to 230 C. To reach these temperatures, air was fed to the plenum of zone one with a feed rate of 5650 kg/h (feed stream), providing an superficial air velocity of 0.59 m/s and having a temperature of 260 C. and to the plenum of zone two with a feed rate of 3350 kg/h (feed stream) and a temperature of 235 C. Air of ambient temperature was introduced into the cooling chamber at a rate of 1450 kg/h and the same amount was discharged to the environment. Polymer 2 was fed into the fluidized bed dryer with a feed rate of 1754 kg/h. After an average residence time in the heating chambers of 40 min, the product was cooled down in the cooling chamber to a temperature of about 50 C.

(80) During a 10 month production period 24 cases of product decomposition occurred, all in zone one of the fluidized bed dryer which caused plant shut-downs to terminate decomposition, to clean the system and to separate material contaminated with brown and black particles of decomposed material. Due to these undesired shut-downs, a loss in production capacity of 5% and product loss of 2.9% based on total production incurred.

Example 1

Heat-treatment of Polymer 2 in a Continuous FBD According to the Present Invention

(81) Heat-treatment in the FBD described in comparative example 1 was continued with polymer 2 for further 7 month after the design of the fluidized bed dryer was optimized by exchanging the bottom plate to a more suitable type, e.g. the above mentioned CONIDUR fine-hole sheets types: 1 to 8, having a hole size of 0.3 mm and providing a pressure loss of 530 Pa with hot air having a temperature of 260 C. Other conditions were kept constant, except that air was fed to the plenum of zones one with a feed rate of 5260 kg/h, providing an superficial air velocity of 0.35 m/s and having a temperature of 260 C. and to the plenum of zone two with a feed rate of 2240 kg/h, a temperature of 235 C. and a heat-up rate of approximately 30 C./min. During continuous production campaign of 7 month, no decomposition was observed, so that neither production capacity nor product was lost.

Example 2

Heat-treatment of Polymer 1 in a Continuous FBD According to the Present Invention

(82) Polymer 1 was heat-treated in a fluidized bed dryer under the conditions described in example 1, except the polymer was fed to the fluidized bed dryer at a feed rate of 1050 kg/h and that the product temperature in the first zone was set to 205 C. with a heat-up rate of 25 C./min and in the second zone to 215 C. To reach said product temperatures, air was fed to the plenum of zone one with a feed rate of 5469 kg/h and a temperature of 246 C. and to the plenum of zone two with a feed rate of 3017 kg/h and a temperature of 235 C. After an average residence time in the heating chamber of 40 min, the product was cooled down in the cooling chamber to a temperature of about 50 C. A representative sample was taken at the outlet of the fluidized bed dryer.

Example 3

Heat-treatment of Polymer 2 in a Continuous FBD According to the Present Invention

(83) The procedure of example 1 was repeated except that polymer 2 was fed to the fluidized bed dryer at a feed rate of 1050 kg/h and that the product temperature in the first zone was adjusted to 230 C. with a heat-up rate of 35 C./min and in the second zone to 228 C. To obtain these temperatures the air to zone one was heated up to 287 C. and to zone two to 232 C. The product had also an average residence time in the heating chambers of 40 min. The product properties of the products obtained are summarized in Table 2 and 3.

(84) TABLE-US-00007 TABLE 2 Properties of polymers 1 and 2 after heat-treatment CRC AAP.sub.0.7 psi Extr. Res AA Example (g/g) (g/g) (%) (ppm) 2 31.8 22.0 7.0 397 3 33.3 24.4 8.9 255

(85) TABLE-US-00008 TABLE 3 Particle size distribution of polymers 1 and 2 after heat-treatment Example/ Particle size distribution (mm, % on screen) Polymer 0.85 0.6 0.3 0.15 0.045 <0.045 2/1 0 17.0 74.4 10.4 0.1 0 3/2 0 17.7 73.4 9.4 0.1 0

(86) The experiments surprisingly demonstrate that by using the method of the present invention, products having desirable absorption capacities, favorable ratios CRC to AAP and CRC to extractables, respectively, as well as low residual monomer contents can be obtained. In particular Example 3 further demonstrates the benefit of using a temperature for the hot gas stream well above 200 C., thus raising the heat-up rate. Higher CRC and AAP values were achieved although the cross-linker concentration was slightly reduced in comparison to Example 2.

(87) 4. Variation of Heat-up Rate

(88) Example 4 and 5 were performed in a batch-operated fluidized bed drier on a laboratory scale, type CTL (Allgaier-Werke KG, Uhingen, Germany), equipped with a conically shaped fluidization chamber with a Conidur fine-hole plate having a diameter of 20 cm on the bottom side, a ventilator, an air heater, a fresh air and an exhaust air filter which, if desired, can be de-dusted by pressured air blasts and a control box. The air stream is not circulated in this drier.

(89) The batch-operated fluidized bed dryer was pre-heated with a hot air stream of 5 m/s having a temperature indicated below (inlet air temperature). After the product zone of the heater had reached the desired temperature, the sample of polymeric particles to be heat-treated was filled in, thus being heated up and fluidized by said hot air stream. Once the product sample has reached the targeted temperature T.sub.p (e.g. 230 C.), fluidization and heat-treatment was continued for the desired heat-treatment time, e.g. for additional 40 min. During the heat-treatment time the inlet air temperature was adjusted such that the product was maintained at the desired heat-treatment temperature 2 C. After elapse of the heat-treatment time, the heat-treated product was discharged to a metal tray and cooled down to room temperature. The heat-treatment time given in the following examples is defined by the range of time from the point where the product has reached the targeted temperature until the point where fluidization is stopped and particles are discharged for cooling.

Examples 4a-4c

Varying the Heat-up Rate by the Amount of Polymer to be Treated

(90) Different amounts of polymer 2 (a: 100 g, b: 500 g and c: 1,000 g) were heat-treated for 40 min according to the heat-treatment procedure described under item 4.1. The temperatures in the product zone of the fluidized bed dryer were recorded and are shown in FIG. 4. The results of the Examples 4a-4c are gathered in Table 4.

(91) The results unambiguously show the positive impact of a fast heat-up rate on the absorption capacity. As illustrated in FIG. 4, the heat-up rate can efficiently be influenced by varying quantity of particles and bed height in the fluidization chamber, respectively.

(92) TABLE-US-00009 TABLE 4 Results obtained in examples 4a-c Polymer CRC AUL.sub.0.9 psi Res AA Extr Example Amount [g/g] (0.7 psi) [g/g] [ppm] [%] 4a 100 g 35.4 20.7 (23.7) 221 13.7 4b 500 g 33.0 20.2 (23.2) 214 13.0 4c 1000 g 30.3 20.3 (23.4) 268 9.5

Examples 5a-5c

Varying the Heat-up Rate by Inlet Temperature

(93) Samples of polymer 2 (500 g each) were heat-treated for 25 min according to the heat-treatment procedure described in example 4, except that the inlet-air temperature was varied as indicated in Table 5.

(94) TABLE-US-00010 TABLE 5 Results of the Examples 5a-c Time (min) CRC AUL.sub.0.7 psi to reach Example T.sub.g1 [g/g] [g/g] T.sub.p = 220 C. 5a 245 C. 29.7 24.1 10 5b 260 C. 32.9 24.3 5.2 5c 280 C. 35.2 24.6 3.4

(95) The results again confirm the impact of the heat-up rate on the absorption performance of the product. As demonstrated by these experiments, the inlet-air temperature is another tool to control this rate.

(96) 5. Surface-cross-linking of Dried Polymeric Particles

(97) 5.1 Surface Coating with an Aqueous Ethylene Carbonate (EC) Solution

(98) A portion of 1 kg of dried and ground SAP was filled into a plowshare mixer (Ldige) having a total volume of 6.1 L Under vigorous agitation 31.6 g of an 30% aqueous solution of ethylene carbonate (9500 ppm b. o. dry SAP) were sprayed onto the agitated product with the aid of a time-spraying nozzle at room temperature. No further additives or post-cross-linkers were added. When addition was completed, the rotation speed of the agitator then was reduced and the wetted product was held under moderate agitation for further 15 min.

(99) 5.2 Heat-treatment in a Small Laboratory Fluidized Bed Dryer

(100) The samples were heat-treated in a small laboratory fluidized bed dryer, in which a hot air stream was provided by a hot-air gun (Bosch, Gerlingen, Germany). The fluidization chamber was conically designed, having a lower diameter of 35 mm, an upper diameter of 60 mm and a 100 metal screen as a bottom plate. The chamber was covered by a lid containing an opening, which was covered by a 100 m metal screen. Two thermo couples were fixed for temperature measurement, one entering the hot air duct 1 cm below the bottom plate to measure the hot-air inlet temperature and the second entering the fluidization chamber 3 cm above the bottom plate to measure the temperature of the fluidized product. Both inlet temperature and product temperature could be controlled in the range of about 1 C.

(101) For heat-treatment, the fluidized bed dryer was pre-heated to the desired temperature (e.g. 230 C.) by providing a hot-air stream. Once the temperature in the fluidization chamber had reached said temperature, a product sample of 30 g (unless stated otherwise) was filled into said chamber, the sample being fluidized by the hot-air stream of the appropriate air inlet temperature. During a first period of time the particles were heated up with a rate of 45 C./min to the desired heat-treatment temperature (heat-up phase). Once the particles had reached the desired heat-treatment temperature T.sub.p, the actual heat-treatment time began and the inlet air temperature was adjusted accordingly to hold the desired particle temperature T.sub.p. The heat-treatment conditions at the desired temperature were maintained for 20 min unless indicated otherwise. At the end of the heat-treatment time the hot-air gun was switched off, the product was discharged and spread onto a plate for immediate cooling,

(102) 5.3 Heat-treatment of the Samples in an Erlenmeyer Flask Heated in an Oil Bath (Oil Bath Method)

(103) Heat-treatment was performed in a 300 mL Erlenmeyer flask containing a magnetic stirring bar (60 mm10 mm), the flask being heated in an oil bath, The whole heat-treatment system comprised two identical arrangements consisting each of an oil bath on a magnetic stirrer to heat and mix the polymer sample in the respective flask. Temperature control systems were used to set and control the oil bath and the product temperature as desired (2 C.). Usually, the temperature in the two oil baths is different. The first one is used for heating-up the polymer sample and the second one for holding the sample at the desired temperature T.sub.p during heat-treatment.

(104) A portion of 50 g of coated superabsorbent polymer was filled into the flask which was then placed into the pre-heated first oil bath. While gently being agitated, the polymer was heated up to the indicated temperature and then was immediately transferred from oil bath one into preheated oil bath two, where heat-treatment was performed at the indicated temperature T.sub.p for the indicated time. Thereafter, the product was removed from the flask and spread onto a plate for cooling,

Example 6

(105) Samples of polymer 3 were coated as described under item 5.1 and heat-treated at a particle temperature T.sub.p of 180 C. applying the method described under item 5.2. The sample amounts used and heat-treatment times applied are given in Table 6.

(106) TABLE-US-00011 TABLE 6 Conditions and results in examples 6a and 6b Polymer Heat-treating CRC AAP.sub.0.7 psi Extr..sub.16 h Example amount (g) time (min) (g/g) (g/g) (%) 6a 20 20 34.1 23.2 15.3 6b 50 40 33.4 23.3 17.5

(107) The results demonstrate that polymers containing chlorate can successfully be used to produce a SAP with excellent product properties by surface post-cross-linking Larger samples require longer heat-up and heat-treatment times, The ratio CRC/AAP.sub.0.7psi is, therefore, slightly inferior

Examples 7-11

(108) Samples (50 g each) of polymers 4-8 were coated as described under item 5.1 above and heat-treated applying the method described under item 5.2. Details are summarized in Table 7.

(109) CE 7-11

(110) Examples 7-11 were repeated except that heat-treatment was performed applying the method described under item 5.3. Conditions and results are summarized in Table 8. Herein, the abbreviations Ex and CE refer to examples and comparative examples, respectively. In addition, the mean parameters of the polymers obtained in examples 6 to 11 (referred to as Ex 6-11 in Tab. 7) are shown in comparison to those of the respective comparative examples (referred to as CE 6-11 in Tab. 7).

(111) TABLE-US-00012 TABLE 7 Conditions and results in examples and comparative examples 6-11 Example/ T.sub.g T.sub.p CRC AAP.sub.0.7 psi PUL Extr..sub.16 h SFC (Polymer) ( C.) ( C.) (g/g) (g/g) (%) (%) (cm.sup.3 .Math. s/g 10.sup.7) Ex 7/(4) 193 180 36.6 25.6 46.1 15.5 2 CE 7/(4) 197 180 39.1 21.5 27.2 13.3 0 Ex 8/(5) 193 180 33.1 26.3 69.2 11.6 11 CE 8/(5) 197 180 35.7 21.5 27.5 9.2 1 Ex 9/(6) 193 180 34.7 26.4 57.9 14.1 8 CE 9/(6) 197 180 36.5 23.0 31.0 10.6 2 Ex 10/(7) 193 180 31.3 26.8 72.9 9.1 12 CE 10/(7) 197 180 33.3 23.1 28.5 6.7 2 Ex 11/(8) 193 180 30.8 26.9 75.3 8.1 17 CE 11/(8) 197 180 31.1 26.6 42.6 5.9 4 Ex 6-11 33.3 26.4 64.3 11.7 9.9 CE 6-11 35.1 23.1 31.4 9.2 1.8

(112) These results clearly demonstrate the benefit of heat-treatment in a fluidized bed dryer. Significantly improved AAP.sub.0.7 psi, PUL and SEC values (even without the addition of multivalent metal salts) were obtained in the fluidized bed dryer.

Examples 12-19

(113) The coating procedure described for examples 6-11 was applied except that ethylene glycol (EG) or glycerin (Gly) was used as a surface cross-linker in concentrations as shown in Table 8. Polymer 5 was used in these examples.

(114) CE 12 and 16

(115) The procedure of examples 12 and 16, respectively, was repeated except that heat-treatment was performed applying the method described under item 5.3. Temperature conditions and results of the experiments are summarized in Table 8.

(116) TABLE-US-00013 TABLE 8 Conditions and results in examples 12-19 and comparative examples 12 and 16 Cross-linker T.sub.g1 T.sub.p CRC AAP.sub.0.7 psi PUL x/CE (ppm) ( C.) ( C.) (g/g) (g/g) (%) SFC (cm.sup.3 .Math. s/g 10.sup.7) Ex 12 6000 EG 197 180 33.5 23.5 68 9 CE 12 6000 EG 197 180 34.5 22.5 38 3 Ex 13 6000 EG 217 180 32.6 22.3 72 9 Ex 14 6000 EG 217 200 28.4 19.7 89 21 Ex 15 6000 EG 241 200 28.5 19.5 89 13 Ex 16 10000 Gly 197 180 32.9 20.7 67 7 CE 16 10000 Gly 197 180 33.1 21.8 35 0 Ex 17 10000 Gly 217 180 32.3 20.0 70 6 Ex 18 10000 Gly 217 200 28.9 19.4 84 11 Ex 19 10000 Gly 241 200 30.0 19.8 86 16

(117) These data show that by heat-treatment in a fluidized bed products of good quality can be obtained, also in the presence surface (post) cross-linkers. It can particularly be seen that using a fluidized bed dryer excellent PUL and SFC values can be obtained without compromising CRC; AAP or the ratio CRC/AAP.

Examples 20-22

(118) Samples of polymer 7 were coated and heat-treated as described above for examples 7-11, except that the coating solution further contained aluminum lactate in a concentration of 2,500 ppm, based on dry weight of the polymer. Temperatures and results of the experiments are summarized in Table 9.

Comparative Example 20

(119) The procedure of the example 20 was repeated except that heat-treatment was performed applying the method as described under item 5.3. Temperatures and results of the experiments are summarized in Table 9.

(120) TABLE-US-00014 TABLE 9 Conditions and results of examples 20-22 and comparative example 20 Aluminum T.sub.g1 T.sub.p CRC AAP.sub.0.7 psi PUL Ex/CE lactate (ppm) ( C.) ( C.) (g/g) (g/g) (%) SFC (cm.sup.3 .Math. s/g 10.sup.7) Ex 20 2500 197 180 29.1 26.7 79 19 CE 20 2500 197 180 29.8 26.3 65 7 Ex 21 2500 217 180 28.5 26.4 83 24 Ex 22 2500 217 190 27.2 24.4 91 51

(121) The results again demonstrate the advantage of using a fluidized bed dryer for heat-treatment. In particular, the PUL and SFC values are increased, indicating an improved permeability of the polymer. These data also demonstrate that a higher heat-up rate (given by the higher inlet temperature T.sub.g1) results in significantly improved product permeability.

(122) In general, the above experimental results also show that using a fluidized bed dryer for heat-treatment, surface-modifying additives such as aluminum salts or silica are not mandatory. This is an important point with respect to costs and process complexity. In addition, no additives are needed to reduce shear force during heat-treatment as it is the case for heat-treatment in a paddle dryer. A further important advantage of not using those additives is that they do not contaminate the streams to be recycled.

(123) 6. Surface-wetting of Previously Dried Polymeric Particles

Examples 23-34

(124) A portion of 1.0 kg of dried and ground polymeric particles was filled into a Plowshare mixer (Ldige) having a total volume of 6.1 L at room temperature. Under vigorous agitation 30 g of water were sprayed onto the agitated product with the aid of a spraying nozzle. No further additives and no post-crosslinkers were added to the product. Rotation speed of the agitator was then reduced and the moistened product was held under moderate agitation for further 15 min.

(125) The samples were then heat-treated in a small laboratory scale fluidized bed dryer as described under item 5.2, except that product samples of 20 g were used and each sample was heat-treated for 20 min. The conditions employed are given in Table 10. No polymer wetting was performed for the comparative examples 23 to 29, 33 and 34.

(126) TABLE-US-00015 TABLE 10 Conditions employed in examples and comparative examples 23-34 Polymer Water for T.sub.g1 T.sub.p Ex/CE applied Wetting (%) ( C.) ( C.) Ex 23 3 3 240 220 CE 23 3 0 240 220 Ex 24 3 3 252 230 CE 24 3 0 252 230 Ex 25 4 3 252 230 CE 25 4 0 252 230 Ex 26 9 3 252 230 CE 26 9 0 252 230 Ex 27 10 3 252 230 CE 27 10 0 252 230 Ex 28 11 3 252 230 CE 28 11 0 252 230 Ex 29 12 3 252 230 CE 29 12 0 252 230 Ex 30 9 3 277 230 CE 30 9 3 193 180 Ex 31 9 3 241 220 Ex 32 13 3 252 230 Ex 33 14 3 245 230 CE 33 14 0 245 230 Ex 34 15 3 245 230 CE 34 15 0 245 230

(127) The product samples as obtained from the various experiments were analyzed and the results are given in Tables 11a-11c.

(128) TABLE-US-00016 TABLE 11a Results of examples and comparative examples 23 and 24 CRC AAP.sub.0.7 psi Extr..sub.16 h SFC Ex/CE (g/g) (g/g) (%) (cm.sup.3 .Math. s/g 10.sup.7) Ex 23 36.0 26.4 12.3 7.0 CE 23 36.4 19.6 9.8 3.5 Ex 24 35.3 25.7 18.5 8.0 CE 24 37.1 20.7 15.4 2.5

(129) These results demonstrate the surprising positive effect of pre-wetting the polymeric particles prior to heat-treatment to improve AAP and SFC during heat-treatment.

(130) TABLE-US-00017 TABLE 11B Results of examples and comparative examples 25-32 CRC AAP.sub.0.7 psi PUL Extr..sub.16 h Hunter Color SFC Ex/CE (g/g) (g/g) (%) (%) L b (cm.sup.3 .Math. s/g 10.sup.7) Ex 25 35.9 21.9 39.3 6.7 90.1 11.1 2 CE 25 37.6 16.7 23.4 7.4 91.1 9.8 1 Ex 26 32.1 24.4 48.2 6.0 91.5 10.1 5 CE 26 30.3 21.6 26.7 5.7 92.4 10.3 3 Ex 27 32.7 23.3 47.7 5.7 91.3 10.1 5 CE 27 31.6 19.9 25.6 5.9 92.7 9.4 1 Ex 28 29.7 24.2 65.1 5.3 91.5 9.7 11 CE 28 27.6 22.4 38.9 5.3 92.9 9.3 1 Ex 29 28.8 24.2 58.0 4.9 91.0 9.4 7 CE 29 27.9 23.4 43.4 4.6 92.0 9.5 7 Ex 30 31.5 25.0 62.6 4.9 91.7 10 9 CE 30 40.2 8.3 28.5 11.3 93.1 7.6 0 Ex 31 33.4 24.7 42.1 5.6 92.2 9.0 3 Ex 32 31.1 23.7 32.3 4.9 89.7 9.6 5 Ex 23-32 32.6 20.6 31.6 2.8 CE 23-29 32.7 24.4 52.6 5.6

(131) The results show again the benefit of the present invention. Comparing the average of the comparative examples 23-29 with the average of the examples 23 to 32 it can be seen that by wetting the particles surface prior to heat-treatment in the fluidized bed dryer, the AAP after heat-treatment raises by almost 4 g/g, the PUL by 21 points and the SFC-value even doubles (in the absence of any permeability-promoting surface additives!), while the CRC stays substantially constant.

(132) Comparative examples 30 demonstrates that a heat-treatment temperature of 180 C. in many cases does not give a product of the desired performance, but that higher temperatures, often close to product decomposition temperature often are required.

(133) The beneficial effect of applying a high heat-up rate can be seen by comparing the results of example 26 and 30. The higher air inlet temperature applied in example 26 provides a higher heat-up rate leading to increased AAP, PUL and SFC values. In addition, the extractables level even dropped.

(134) TABLE-US-00018 TABLE 11c Results of examples and comparative examples 33-34 CRC AAP.sub.0.7 psi PUL Extr..sub.16 h SFC Ex/CE (g/g) (g/g) (%) (%) (cm.sup.3 .Math. s/g 10.sup.7) Ex 33 34.9 26.6 47 10.1 6 CE 33 33.7 22.4 50 8.0 5 Ex 34 29.1 26.0 88 10.8 27 CE 34 30.5 24.2 39 8.7 5

(135) Like the polymers used in examples 23 and 24 the polymers employed in examples 33 and 34 contain chlorate. In comparison to the results of examples 23 and 24, these examples demonstrate that the addition of even small amounts of glycerin positively influences the extractables concentration and the SFC value of the heat-treated polymeric particles.

(136) 7. Network Cross-linking Using Small Amounts of Glycerin

Example 35

(137) 100 g of polymer 18 were heat-treated at 230 C. for 15 min using the method described under item 4.

Comparative Example 35

(138) 500 g of polymer 16 were heat-treated at 230 C. for 40 min using the method described under item 4.

Example 36

(139) 500 g of polymer 18 were heat-treated at 230 C. for 15 min using the method described under item 4.

Comparative Example 36

(140) Comparative example 35 was repeated, except that a hot air stream having a gas velocity of 8 m/s was applied.

Example 37

(141) Example 35 was repeated, except that a hot air stream having a gas velocity of 8 m/s was applied.

Comparative Example 37

(142) 500 g of polymer 17 were heat-treated at 230 C. for 20 min using the method described under item 4.

Example 38

(143) Example 38 was repeated, except that 500 g of the polymer were applied.

Comparative example 38

(144) Comparative example 35 was repeated, except that a hot air stream having a gas velocity of 8 m/s was applied.

(145) The results of examples and comparative examples 35 to 38 are presented in Table 12.

(146) TABLE-US-00019 TABLE 12 Results of examples and comparative examples 35-38 CRC AUL.sub.0.9 psi Res AA Extr. Ex/CE [g/g] [g/g] [ppm] [%] Ex 35 38.9 23.8 459 17.7 CE 35 33.2 19.1 211 14.3 Ex 36 32.3 23.1 558 12.9 CE 36 33.1 20.2 214 13.0 Ex 37 37.2 24.1 562 11.8 CE 37 33.2 19.7 271 13.8 Ex 38 34.6 22.2 704 7.2 CE 38 36.0 20.4 267 12.7

Example 39

(147) Polymer 19 was continuously fed into a fluidized bed dryer at a rate of 2400 kg/h and heat treated according to the method described in example 1 at a temperature of 230 C. and a weir height of 100 mm, which leads to a residence time of about 10 to 15 min at 230 C. The polymeric particles obtained had the following characteristics: CRC 28.3 g/g, AUL.sub.0.9psi 23.3 g/g, Res AA 282 ppm, Extr. 6.5%.

(148) This example demonstrates that even in a continuously operated heat-treating process according to the present invention water-absorbing polymeric particles having a high AUL.sub.0.9psi can be obtained using small amounts of glycerin as a network cross-linker at heat-treatment temperatures above 200 C. and a fast heat-up rate. The heat-treated product furthermore comprises a very low amount of extractables and residual monomer.

Examples 40-46

(149) Amounts of 20 g of the chlorate-free, glycerin-containing polymers 20 to 22 were heat-treated as described under item 4 applying an inlet gas temperature T.sub.g1 of 255 C. and a particle temperature T.sub.p of 230 C. for the heat-treatment time t indicated in Table 13.

(150) TABLE-US-00020 TABLE 13 Results obtained in examples 40-46 Res. SFC Ex Gly t CRC AAP.sub.0.7 psi Extr. AAP.sub.0.7 psi/ AA PUL (cm.sup.3 .Math. s/g (Polymer) (ppm) (min) (g/g) (g/g) (%) CRC (%) (%) 10.sup.7) 40 (20) 200 15 26.0 20.3 7.9 1.28 599 41 6 41 (20) 200 20 25.8 20.2 7.3 1.28 538 51 9 42 (21) 500 2.5 26.1 20.3 8.3 1.29 766 41 10 43 (21) 500 5 25.1 20.7 7.3 1.21 805 51 15 44 (22) 1000 2.5 22.5 20.3 6.2 1.11 468 79 17 45 (22) 1000 5 21.2 19.8 5.9 1.07 505 85 41 46 (22) 1000 20 18.6 18.2 4.8 1.02 467 96 62 40-46 23.6 20.0 6.8 1.2 592.6 63.3 62

(151) Comparing these results to that of the chlorate-containing polymers 16 to 18 in examples 35 to 38 (Table 12), one can see that in examples 40 to 46 lower CRC values, but nevertheless good AAP.sub.0.7 psi values are obtained. In addition, good to excellent PUL and SFC values can be obtained as well as a low amount of extractables in the heat-treated glycerin-containing polymers without adding surface-modifying agent such as for example multivalent metal ions.

(152) Furthermore, only very short residence times are required using polymers containing glycerin as a network cross-linker.

Examples 47-52

(153) Examples 40 to 46 were repeated, except that the polymeric particles were wetted on their surface using an amount of 3 wt-% water immediately prior to being subjected to heat-treatment according to the method described under item 6.

(154) The results are presented in Table 14.

(155) TABLE-US-00021 TABLE 14 Results obtained in examples 47-52 Res. SFC Ex Gly t CRC AAP.sub.0.7 psi Extr. AAP.sub.0.7 psi/ AA PUL (cm.sup.3 .Math. s/g (Polymer) (ppm) (min) (g/g) (g/g) (%) CRC (%) (%) 10.sup.7) 47 (20) 200 15 26.6 21.5 7.9 1.24 591 58 10 48 (21) 500 2.5 27.1 20.9 8.7 1.30 755 52 10 49 (21) 500 10 23.7 21.1 6.6 1.12 801 81 28 50 (21) 500 20 20.4 20.1 5.6 1.01 707 94 110 51 (22) 1000 2.5 23.4 20.3 6.6 1.15 466 89 33 52 (22) 1000 5 21.4 19.8 5.5 1.08 516 96 69 47-52 23.8 20.6 6.8 1.2 639.3 78.3 43.3

(156) Wetting the surface of the polymeric particles prior to heat-treatment improves the absorption characteristics of the particles, as can be seen from the results presented in Table 14. In particular it significantly improves product permeability as indicated by the PUL and SFC values.

Example 53

(157) A sample of polymer 22 was surface coated using the method described under item 5.1. 50 g of the coated polymeric particles were heat-treated as described under item 4, using an inlet gas temperature T.sub.g1 of 190 C. and a particles temperature T.sub.p of 180 C. for 20 min.

Comparative Example 53

(158) Example 53 was repeated, except that a sample amount of 75 g of the surface-coated polymeric particles were heat-treated as described under item 5.3. The temperature in the first and in the second oil bath were both set to 180 C. and heat-treatment was carried out for 69 min.

(159) The results of example 53 and comparative example 53 are presented in Table 15.

(160) TABLE-US-00022 TABLE 15 Results obtained in example and comparative example 53 Res. SFC t CRC AAP.sub.0.7 psi Extr. AA PUL (cm.sup.3 .Math. s/g Ex/CE (min) (g/g) (g/g) (%) (%) (%) 10.sup.7) Ex 53 20 28.3 21.2 14.9 347 70 13 CE 53 69 31.2 23.3 14.9 334 43 5

(161) Even by applying a more than threefold heat-treatment time, the excellent product permeability obtained using the method of the present invention, as indicated by the PUL and SFC values, cannot be obtained by heat-treatment via surface contact.