PROCESS FOR PRODUCING LONG-TERM COLOR STABLE SUPERABSORBENT POLYMER PARTICLES

Abstract

The invention relates to a process for producing long-term color stable superabsorbent polymer particles, comprising polymerization of a monomer solution, drying the resulting polymer gel, optionally grinding and classifying the resulting dried polymer gel and thermally surface post-crosslinking and cooling the resulting polymer particles, wherein a thermal surface post-cross-linker and hydrogen peroxide are added to the polymer particles prior to the thermal surface post-crosslinking.

Claims

1. A process for producing long-term color stable superabsorbent polymer particles, comprising polymerizing a monomer solution, comprising a) partly neutralized acrylic acid, b) optionally at least one crosslinker, and c) at least one initiator, drying a resulting polymer gel, optionally grinding and classifying the resulting dried polymer gel, and thermally post-crosslinking and cooling the resulting polymer particles, wherein a thermal post-crosslinker and 0.014 to 0.095% by weight of hydrogen peroxide, based on the polymer particles, are added to the polymer particles prior to the thermal post-crosslinking.

2. The process according to claim 1, wherein from 0.005 to 5% by weight of the thermal post-crosslinker, based on the polymer particles, is added the polymer particles prior to the post-crosslinking.

3. The process according to claim 1, wherein from 0.02 to 0.05% by weight of the peroxide, based on the polymer particles, is added the polymer particles prior to the post-crosslinking.

4. The process according to claim 1, wherein a temperature of the post-crosslinking is from 130 to 190° C.

5. The process according to claim 1, wherein a residence time of the post-crosslinking is from 20 to 60 minutes.

6. The process according to claim 1, wherein the monomer solution comprises a polymerization inhibitor.

7. The process according to claim 1, wherein the monomer solution comprises from 0.004 to 0.010% by weight of a polymerization inhibitor, based on acrylic acid.

8. The process according to claim 6, wherein hydroquinone monomethyl ether is used as polymerization inhibitor.

9. The process according to claim 1, wherein at least one chelating agent is added to the polymer particles during or after the cooling.

10. The process according to claim 1, wherein from 0.005 to 1% by weight of at least one chelating agent, based on the polymer particles, is added to the polymer particles during or after the cooling.

11. The process according to claim 9, wherein 1-hydroxyethane-1,1-diphosphonic acid and/or salts thereof are used as chelating agent.

12. The process according to claim 1, wherein the post-crosslinking is substantially without any irradiation with active energy rays.

13. Superabsorbent polymer particles obtained according to the process of claim 1, wherein the superabsorbent polymer particles have a centrifuge retention capacity of at least 40 g/g and an absorption under high load of at least 20 g/g.

14. The superabsorbent polymer particles according to claim 13, wherein the superabsorbent polymer particles have a yellowness index after 14 days of aging at 70° C. and a relative humidity of 80% of not more than 54.

15. The superabsorbent polymer particles according to claim 13, wherein the superabsorbent polymer particles have a yellowness index after 14 days of aging at 70° C. and a relative humidity of 80% of not more than 50.

Description

EXAMPLES

Example 1 (Base Polymer)

[0119] The example was done analogously to Example 1 of WO 2016/134905 A1.

[0120] The process was performed in a concurrent spray drying plant with an integrated fluidized bed (27) as shown in FIG. 1 of WO 2016/134905 A1. The reaction zone (5) had a height of 22 m and a diameter of 3.4 m. The internal fluidized bed (IFB) had a diameter of 3 m and a weir height of 0.25 m.

[0121] The drying gas was fed via a gas distributor (3) at the top of the spray dryer. The drying gas was partly recycled (drying gas loop) via a cyclone as dust separation unit (9) and a condenser column (12). The drying gas was nitrogen that comprises from 1% to 4% by volume of residual oxygen. Prior to the start of polymerization the drying gas loop was filled with nitrogen until the residual oxygen was below 4% by volume. The gas velocity of the drying gas in the reaction zone (5) was 0.79 m/s. The pressure inside the spray dryer was 4 mbar below ambient pressure.

[0122] The temperature of the gas leaving the reaction zone (5) was measured at three points around the circumference at the end of the cylindrical part of the spray dryer as shown in FIG. 3 of WO 2016/134905 A1. Three single measurements (43) were used to calculate the average temperature (spray dryer outlet temperature). The drying gas loop was heated up and the dosage of monomer solution is started up. From this time the spray dryer outlet temperature was controlled to 114° C. by adjusting the gas inlet temperature via the heat exchanger (20). The gas inlet temperature was 167° C.

[0123] The product accumulated in the internal fluidized bed (27) until the weir height was reached. Conditioned internal fluidized bed gas having a temperature of 105° was fed to the internal fluidized bed (27) via line (25). The gas velocity of the internal fluidized bed gas in the internal fluidized bed (27) was 0.65 m/s. The residence time of the product was 150 min. The temperature of the superabsorbent polymer particles in the internal fluidized bed (27) was 71° C.

[0124] The spray dryer offgas was filtered in cyclone as dust separation unit (9) and sent to a condenser column (12) for quenching/cooling. Excess water was pumped out of the condenser column (12) by controlling the (constant) filling level inside the condenser column (12). The water inside the condenser column (12) was cooled by a heat exchanger (13) and pumped counter-current to the gas. The water inside the condenser column (12) was set to an alkaline pH by dosing sodium hydroxide solution to wash out acrylic acid vapors.

[0125] The gas leaving the condenser column (12) was split to the drying gas inlet pipe (1) and the conditioned internal fluidized bed gas (25). The gas temperatures were controlled via heat exchangers (20) and (22). The hot drying gas was fed to the concurrent spray dryer via gas distributor (3). The gas distributor (3) consists of a set of plates providing a pressure drop of 2 to 4 mbar depending on the drying gas amount.

[0126] The product was discharged from the internal fluidized bed (27) via rotary valve (28) into sieve (29). The sieve (29) was used for sieving off overs/lumps having a particle diameter of more than 800 μm.

[0127] The monomer solution was prepared by mixing first acrylic acid with 3-tuply ethoxylated glycerol triacrylate (internal crosslinker), secondly with 37.3% by weight sodium acrylate solution and thirdly with aqueous of disodium 1-hydroxyethane-1,1-diphosphonic acid (HDPA). The temperature of the resulting monomer solution was controlled to 10° C. by using a heat exchanger and pumping in a loop. A filter unit having a mesh size of 250 μm was used in the loop after the pump. The initiators were metered into the monomer solution upstream of the dropletizer by means of static mixers (31) and (32) via lines (33) and (34) as shown in FIG. 1 of WO 2016/134905 A1. sodium peroxodisulfate solution having a temperature of 20° C. was added via line (33) and [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride solution together was added via line (34). Each initiator was pumped in a loop and dosed via control valves to each dropletizer unit. A second filter unit having a mesh size of 140 μm was used after the static mixer (32). For dosing the monomer solution into the top of the spray dryer three dropletizer units were used as shown in FIG. 4 of WO 2016/134905 A1.

[0128] A dropletizer unit consisted of an outer pipe (47) having an opening for the dropletizer cassette (49) as shown in FIG. 5 of WO 2016/134905 A1. The dropletizer cassette (49) was connected with an inner pipe (48). The inner pipe (48) having a PTFE block (50) at the end as sealing can be pushed in and out of the outer pipe (47) during operation of the process for maintenance purposes.

[0129] The dropletizer cassette (49) had 256 bores having a diameter of 170 μm and a bore spacing of 15 mm. The dropletizer cassette (49) consisted of a flow channel (56) having essential no stagnant volume for homogeneous distribution of the premixed monomer and initiator solutions and one droplet plate (53). The droplet plate (53) had an angled configuration with an angle of 3°. The droplet plate (53) was made of stainless steel and had a length of 630 mm, a width of 128 mm and a thickness of 1 mm.

[0130] The feed to the spray dryer consisted of 10.45% by weight of acrylic acid, 33.40% by weight of sodium acrylate, 0.018% by weight of 3-tuply ethoxylated glycerol triacrylate, 0.108% by weight of disodium 1-hydroxyethane-1,1-diphosphonic acid (HDPA), 0.072% by weight of [2,2′-azobis[2-(2-imidazolin-2-yl)propane]dihydrochloride, 0.072% by weight of sodiumperoxodisulfate solution (15% by weight in water) and water. The degree of neutralization was 71%. The feed per bore was 1.4 kg/h.

[0131] The resulting superabsorbent polymer particles were analyzed. The conditions and results are summarized in tables 1 to 3.

Example 2 (not Inventive)

[0132] 1200 g of the water-absorbent polymer particles prepared in Example 1 (base polymer) were put into a laboratory ploughshare mixer (model MR5, manufactured by Gebrüder Lödige Maschinenbau GmbH, Paderborn, Germany). A surface-postcrosslinker solution was prepared by mixing 30 g ethylene carbonate, 1.2 g aluminum sulfate and 60 g of deionized water, as described in Table 4, into a beaker. At a mixer speed of 200 rpm, the aqueous solution was sprayed onto the polymer particles within one minute by means of a spray nozzle. The mixing was continued for additional 5 minutes. The product was removed and transferred into another ploughshare mixer (model MR5, manufactured by Gebrüder Lödige Maschinenbau GmbH; Paderborn; Germany) which was heated to 150° C. before. After mixing for further 40 minutes at 150° C., the product was removed from the mixer and sifted through a 850 μm sieve.

[0133] A remoisturizing solution was prepared by mixing 3 g aluminum lactate, 25 mg Sorbitanmonodo-decanoate (Span® 20) and 80 g of deionized water, as described in Table 8, into a beaker. 1000 g of this product was transferred into another ploughshare mixer (model MR5, manufactured by Gebrüder Lödige Maschinenbau GmbH; Paderborn; Germany) which was heated to 80° C. before. After reaching the final temperature of 80° C. the heating of the ploughshare mixer was switch off and at a mixer speed of 200 rpm, the aqueous remoisturizing solution was sprayed onto the polymer particles within 2 minutes by means of a spray nozzle. The mixing was continued for additional 13 minutes. The product was removed and sifted through a 850 μm sieve.

[0134] The samples were analyzed. The formulation, conditions and results are summarized in Table 4 and 5.

Example 3 (Inventive)

[0135] The example was performed analogous to Example 2, except, that additionally 0.18 g hydrogene peroxide was added into the surface-postcrosslinker solution.

Example 4 (Inventive)

[0136] The example was performed analogous to Example 2, except, that additionally 0.36 g hydrogene peroxide was added into the surface-postcrosslinker solution.

Example 5 (Inventive)

[0137] The example was performed analogous to Example 2, except, that additionally 0.72 g hydrogene peroxide was added into the surface-postcrosslinker solution.

Example 6 (Inventive)

[0138] The example was performed analogous to Example 2, except, that additionally 1.08 g hydrogene peroxide was added into the surface-postcrosslinker solution.

Example 7 (not Inventive)

[0139] The example was performed analogous to Example 2, except, that additionally 1.80 g hydrogene peroxide was added into the surface-postcrosslinker solution.

Example 8 (not Inventive)

[0140] The example was performed analogous to Example 2, except, that additionally 3.60 g hydrogene peroxide was added into the surface-postcrosslinker solution.

Example 9 (not Inventive)

[0141] The example was performed analogous to Example 2, to Example 2, except, that additionally 0.18 g sodium persulfate was added into the surface-postcrosslinker solution.

Example 10 (not Inventive)

[0142] The example was performed analogous to Example 2, to Example 2, except, that additionally 0.36 g sodium persulfate peroxide was added into the surface-postcrosslinker solution.

Example 11 (not Inventive)

[0143] The example was performed analogous to Example 2, to Example 2, except, that additionally 0.72 g sodium persulfate peroxide was added into the surface-postcrosslinker solution.

Example 12 (not Inventive)

[0144] The example was performed analogous to Example 2, to Example 2, except, that additionally 0.15 g hydrogene peroxide was added into the remoisturizer solution.

Example 13 (not Inventive)

[0145] The example was performed analogous to Example 2, to Example 2, except, that additionally 0.30 g hydrogene peroxide was added into the remoisturizer solution.

Example 14 (not Inventive)

[0146] The example was performed analogous to Example 2, to Example 2, except, that additionally 0.60 g hydrogene peroxide was added into the remoisturizer solution.

TABLE-US-00001 TABLE 1 Process conditions of the polymerization (base polymer) T T T T T T gas inlet gas outlet gas IFB IFB CC GDU Unit ° C. ° C. ° C. ° C. ° C. ° C. Example 1 167 114 105 71 56 47 T gas inlet: temperature of the gas prior to the gas distributor (3) T gas outlet: temperature of the gas leaving the reaction zone (5) T gas IFB temperature of the gas entering the internal fluidized bed (27) via line (25) T IFB: temperature of the superabsorbent polymer particles in the fluidized bed (27) T CC: temperature of the gas leaving the condenser column (12) T GDU: temperature of the gas leaving the gas drying unit (37)

TABLE-US-00002 TABLE 2 Properties of the superabsorbent polymer particles (base polymer) Bulk Ext. Density Flowrate CRC AUL RAA 16 h Moisture Unit kg/m.sup.3 g/s g/g g/g wt. % wt. % wt. % Example 1 676 10.9 43.9 26.6 0.588 4.4 8.8 *) comparative example

TABLE-US-00003 TABLE 3 Particle size distribution of the superabsorbent polymer particles (base polymer) 150 150-200 200-250 250-300 300-400 400-500 500-600 600-710 710-850 >850 Unit μm μm μm μm μm μm μm μm μm μm Roundness Example 1 0.0 0.3 2.5 6.6 29.4 31.2 18.4 8.2 2.8 0.6 0.82

TABLE-US-00004 TABLE 4 Process conditions of the surface post-crosslinking (SXL) SXL Cooler/Remoisturizing Temp. EC Water Al-Sulfate H.sub.2O.sub.2 NaPS Water Al-Lactate Span ®20 H.sub.2O.sub.2 Example ° C. % bop % bop % bop ppm bop ppm bop % bop % bop ppm bop ppm bop  2*) 150 2 5 0.1 0 8 0.3 25  3 150 2 5 0.1 150 8 0.3 25  4 150 2 5 0.1 300 8 0.3 25  5 150 2 5 0.1 600 8 0.3 25  6 150 2 5 0.1 900 8 0.3 25  7*) 150 2 5 0.1 1500 8 0.3 25  8*) 150 2 5 0.1 3000 8 0.3 25  9*) 150 2 5 0.1 150 8 0.3 25 10*) 150 2 5 0.1 300 8 0.3 25 11*) 150 2 5 0.1 600 8 0.3 25 12*) 150 2 5 0.1 8 0.3 25 150 13*) 150 2 5 0.1 8 0.3 25 300 14*) 150 2 5 0.1 8 0.3 25 600 EC: ethylene carbonate Al-Lactate: aluminum trilactate Al-Sulfate: aluminum sulfate *)comparative example Span ®20: Sorbitanmonododecanoate H2O2: hydrogene peroxide bop: based on polymer

TABLE-US-00005 TABLE 5 Properties of the superabsorbent polymer particles (after surface post-crosslinking) and color stability of the superabsorbent polymer particles, storaged at 70° C. and 80% relative humidity in a climatic test cabinet for 0, 7 and 14 days Performance Color (70° C. and 80% humidity) CRC AUHL Caking L a b YI Example g/g g/g % 0 d 7 d 14 d 0 d 7 d 14 d 0 d 7 d 14 d 0 d 7 d 14 d  2*) 37.0 24.5 4.4 91.2 73.3 63.0 −1.5 3.6 6.4 9.7 15.4 18.0 17.7 40.8 58.2  3 37.7 24.7 3.3 91.8 75.4 65.0 −1.5 3.1 5.8 6.9 14.5 18.0 12.3 37.2 55.8  4 40.4 24.5 4.6 93.6 77.1 67.6 −1.5 2.7 5.2 6.0 13.3 16.8 10.3 33.3 50.0  5 42.8 23.5 4.7 94.2 79.1 71.6 −1.4 1.8 3.9 6.0 13.7 18.0 10.3 32.7 48.7  6 47.4 18.5 5.1 92.8 76.5 66.1 −1.8 3.0 5.8 6.0 13.1 17.0 10.2 33.3 52.2  7*) 61.9 7.7 4.8 94.5 74.6 61.9 −2.1 3.1 6.2 7.2 14.2 17.4 12.0 37.0 57.2  8*) 36.5 4.9 5.7 92.3 51.8 31.8 −2.7 5.9 7.4 10.3 15.1 10.9 17.8 60.3 77.7  9*) 38.6 23.3 4.9 90.1 75.3 66.3 −1.8 2.8 5.5 12.1 15.3 17.9 22.5 38.8 54.2 10*) 39.8 22.2 4.9 90.4 75.3 66.1 −1.8 2.8 5.5 11.0 15.2 18.1 20.5 38.6 54.9 11*) 38.7 19.8 4.4 89.3 73.5 63.7 −1.7 3.3 6.2 13.3 15.6 18.1 25.3 41.0 57.8 12*) 37.6 25.3 5.1 92.0 73.1 63.0 −1.4 3.6 6.3 9.6 14.7 17.2 17.6 39.6 55.9 13*) 37.4 25.8 5.2 91.0 76.0 69.7 2.0 1.8 3.3 2.6 16.7 20.7 6.7 41.0 56.5 14*) 36.8 25.4 3.1 92.9 71.1 59.1 −1.4 4.2 6.6 6.0 14.6 17.1 10.6 41.0 59.6 *)comparative example

[0147] FIG. 1 shows the Yellowness index (YI D1925) after storage at 70° C. and 80% relative humidity in a climatic test cabinet for 14 d in dependency of the hydrogene peroxide contents in surface-postcrosslinker solutions (Examples 2 to 8)

[0148] FIG. 2 shows the absorbency under high load (AUHL) in dependency of the hydrogene peroxide contents in surface-postcrosslinker solutions (Example 2 to 8)