METHOD FOR DETERMINING CHARACTERISTICS OF SUPER-ABSORBENTS

Abstract

In a method of measuring parameters of superabsorbents, the absorption capacity of superabsorbents is determined under pressure, by reducing the pressure applied to a sample of the superabsorbent stepwise and determining the absorption capacity at each pressure. In addition, the rise in absorption capacity after a reduction in pressure is measured as a function of time and this is used to calculate the swelling constant k or the characteristic swelling time τ. Swelling constant or characteristic swelling time or the magnitude of the difference in absorption capacity at two different pressures are used to determine further parameters of the superabsorbent.

Claims

1. A method of measuring an absorption capacity of a superabsorbent under pressure, comprising reducing the pressure applied to a sample of the superabsorbent stepwise during the measurement and determining an absorption capacity at each pressure and a dependence thereof on the measurement duration.

2. The method according to claim 1, wherein a progression of absorption under pressure against time is used to calculate a swelling constant k or a characteristic swelling time τ.

3. The method according to claim 1, wherein a magnitude of the difference in the absorption capacity at two different pressures is determined.

4. The method according to claim 3, wherein the magnitude of the difference in the absorption capacity at a non-zero pressure and without pressure is determined.

5. The method according to claim 2, wherein the swelling constant k or the characteristic swelling time τ or the difference in the absorption capacity is used to calculate at least one further parameter of the superabsorbent by means of a correlation measured beforehand between swelling constant k, characteristic swelling time τ or the magnitude of the difference in the absorption capacity and the parameter.

6. The method according to claim 5, wherein the parameter is a T20 value or a permeability (SFC) or a gel strength G.sub.e.

7. The method according to claim 1, wherein the pressure applied is 49 g/cm.sup.2 at first and is reduced stepwise to 21 g/cm.sup.2 and 0 g/cm.sup.2.

8. The method according to claim 1, wherein the period over which the particular pressure is applied to the sample is at least 30 minutes and at most 90 minutes.

9. The method according to claim 1, wherein the amount of superabsorbent used is from 0.5 to 5 g.

10. The method according to claim 9, wherein the superabsorbent covers a circular area having a diameter of 5 to 7 cm.

11. The method according to claim 1, wherein at least 95% by weight of the superabsorbent has a grain size in the range from 100 μm to 1000 μm.

12. The method according to claim 1, wherein the superabsorbent has a saline flow conductivity (SFC) of at least 10×10.sup.−7 cm.sup.3s/g.

13. The method according to claim 1, wherein the superabsorbent has a centrifuge retention capacity (CRC) of at least 10 g/g.

14. The method according to claim 1 for quality control in the continuous production of superabsorbents.

15. The method according to claim 1 for characterization of a superabsorbent in the development of new superabsorbents.

16. An apparatus for performing the method according to claim 1.

Description

EXAMPLES

Example 1: Determination of SFC and T20 for a Superabsorbent Surface Postcrosslinked to Different Degrees

[0088] The parameters sought in example 1 are SFC and T20. These parameters are dependent on the permeability of the superabsorbent. Different permeability was generated here by different degrees of completeness of surface postcrosslinking of a superabsorbent.

[0089] A commercial polyacrylate superabsorbent base polymer having the following properties was used:

[0090] CRC: 34.2 g/g

[0091] AAP 0.3 psi: 15.6 g/g (measured to NWSP 242.0.R2 (15))

[0092] ABD: 0.59 g/cm.sup.3

[0093] FLR: 9.86 g/sec

[0094] Particle Size Distribution:

[0095] >850 μm: 0% by wt.

[0096] 710-850 μm: 0.7% by wt.

[0097] 600-710 μm: 6.0% by wt.

[0098] 500-600 μm: 18.5% by wt.

[0099] 400-500 μm: 29.2% by wt.

[0100] 300-400 μm: 22.6% by wt.

[0101] 200-300 μm: 15.1% by wt.

[0102] 150-200 μm: 6.7% by weight

[0103] <150 μm: 1.2% by wt.

[0104] This base polymer was surface postcrosslinked as follows:

[0105] 1196 g of the base polymer formed an initial charge in a paddle drier of capacity 5 l, model M5RMK from Gebr. Lodige Maschinenbau GmbH; Elsener Strasse 7-9, 33102 Paderborn, Germany, and was stirred at 200 revolutions per minute. By means of a two-phase nozzle, a solution of 0.07% by weight of N-hydroxyethyl-2-oxazolidone, 0.532% by weight of isopropanol, 2.128% by weight of water, 0.07% by weight of propane-1,3-diol, 0.70% by weight of propane-1,2-diol and 0.50% by weight of aluminum lactate, based in each case on the weight of the initial charge of base polymer, was sprayed onto the stirred polymer within 62 seconds.

[0106] The base polymer thus sprayed was transferred into a Pflugschar® M5R plowshare mixer of capacity 5 l, model VT MSR, with a heating jacket from Gebr. Lodige Maschinenbau GmbH; Elsener Strasse 7-9, 33102 Paderborn, Germany. The temperature of the product in the mixer was measured by introducing a thermocouple into the opening provided for the purpose in the mixer to such an extent that its tip was at a distance from the heated inner wall of the mixer and was within the product, but could not be impacted by the mixing tools. The polymer was stirred here at 50 revolutions per minute. Oil heated externally to 250° C. flowed through the heating jacket of the mixer. The product reached the desired reaction temperature of 175-184° C. after about 15 minutes and was surface postcrosslinked in the process. After defined reaction times—measured from the juncture of product transfer into the heatable mixer—samples were taken and freed of lumps by means of a 700 μm screen.

[0107] The samples were analyzed. The AAP values were ascertained in accordance with the invention, in principle analogously to NWSP 242.0.R2 (15) (starting weight 0.90 g, test cell with diameter 60 mm). However, after a measurement duration of 60 minutes to ascertain AAP 0.7 psi, the weight was reduced, such that, after the AAP 0.7 psi, the AAP 0.3 psi was determined—again over a measurement duration of 60 minutes. Thereafter, the weight was decreased, such that, after the AAP 0.3 psi, the AAP 0.0 psi was determined—again over a measurement duration of 60 minutes. In addition, a base construction as shown in FIG. 1 was used, with a perforated Plexiglas plate of thickness 6 mm and diameter 89.5 mm, and with 52 holes, each of diameter 2 mm, arranged in concentric circles. The filter paper to be arranged between perforated plate and test cell was omitted; instead, a black band filter paper was placed onto the sample in the test cell. The plunger used was provided on its underside with parallel grooves of width and depth 2 mm, spaced apart by 2 mm. The test cell was supplied with physiological saline solution via a reservoir bottle, vented by a capillary, through a 6 mm silicone hose, and the amount of liquid absorbed was determined as a function of the measurement time by means of a balance on which the reservoir bottle stood. The AAP values of the superabsorbent were determined as the mass of liquid absorbed at the end of the measurement time; the value reported in each case is the average of a double measurement.

[0108] The SFC values of the samples were determined as described above, and the T20 values as per the “K(t) Test Method (Dynamic Effective Permeability and Uptake Kinetics Measurement Test Method)” described in EP 2 535 027 A1 on pages 13 to 18.

[0109] The results are collated in table 1.

TABLE-US-00001 TABLE 1 Reaction AAP AAP AAP SFC duration 0.7 psi 0.3 psi 0.0 psi AAP 0.0 psi − [10.sup.−7 T20 [min] [g/g] [g/g] [g/g] AAP 0.7 psi cm.sup.3s/g] [s] 25 11.9 26.1 50.4 38.5 0 1185 30 20.7 28.2 50.0 29.3 0 258 35 24.9 30.1 49.8 24.9 5 182 40 24.8 30.0 47.3 22.5 29 168 45 25.3 30.3 46.9 21.6 47 161 50 25.3 29.9 46.0 20.7 66 179

[0110] Table 1 shows how the surface postcrosslinking reaction proceeds over the reaction time and the permeability of the superabsorbent rises as a result (elevated SFC and reduced T20). Table 1 also shows, in the range in which there is actually measurable permeability (measured as SFC), a linear correlation between SFC and the magnitude of the difference of AAP 0.0 psi and AAP 0.7 psi (=AAP 0.0 psi minus AAP 0.7 psi). In the case of T20, there is likewise a correlation with this magnitude of the difference in AAP, provided that the superabsorbent has not yet attained its final T20 value. By the method of the invention, it is accordingly possible using correlations thus ascertained for types of superabsorbent to replace the laborious determination of values such as SFC or T 20 (or else other comparable values) with the readily automatable inventive determination of AAP values.

Example 2: Determination of the SFC Parameter on Various Superabsorbents

[0111] For the different superabsorbents listed in table 2 (all available from BASF SE, Carl-Bosch-Strasse 38, 67056 Ludwigshafen, Germany), as described in example 1, swelling curves as shown schematically in FIG. 3 were ascertained, except that the reduction in weight was undertaken by taking away partial weights by means of a robot arm. In addition, CRC and SFC of all samples were additionally measured by the previously known method described above, but the SFC here with a sample weight of 0.9 g.

[0112] The values ascertained are collated in table 2. “aAAP” represents the AAP ascertained in accordance with the invention at the given pressure, and τ with specification of a pressure value represents the characteristic swelling time ascertained from the same measurement at the pressure specified.

[0113] The data were evaluated with the above-described program R, and the CRC determined in the previously known manner, the AAP 0.7 psi determined in accordance with the invention, and the swelling time τ0.7 psi determined in accordance with the invention from the swelling for the measurement of the AAP 0.7 psi were used to calculate an SFC correlated to the SFC measured.

[0114] The formula used for the calculation of this predicted SFC in table 2 is: SFC˜CRC*(aAAP 0.0 psi −aAAP 0.7 psi)*τ0.7 psi. The correlation ascertained is shown in FIG. 4. The correlation coefficient r.sup.2 is 0.99. (With a similarly good correlation coefficient (r.sup.2=0.97), it is possible to ascertain the correlation on replacement of the swelling time τ 0.7 psi used here by the swelling time τ 0.3 psi.) What is meant here and hereinafter by correlation coefficient r.sup.2 is always the “adjusted r.sup.2” of the program R.

[0115] Example 2 shows that parameters such as the SFC here, even in the case of different superabsorbents having significantly or even very significantly different properties, can be ascertained efficiently in accordance with the invention and in a routine and automated manner with a minimal degree of complexity compared to previously known SFC measurements.

Example 3: Determination of Gel Strength G.SUB.e

[0116] For the superabsorbents used in example 2, the properties necessary for calculation of gel strength G.sub.e and gel bed porosity ϕ.sub.0 by the previously known method according to Buchholz were measured manually by previously known methodology. Table 3 gives an overview of the values.

[0117] The values determined in accordance with the invention for these superabsorbents were used in accordance with the invention, by correlation, to ascertain values of gel strength G.sub.e and gel bed porosity ϕ.sub.0. These values are reproduced in table 4. The formula used in R was G.sub.e˜(aAAP 0.0 psi−aAAP 0.7 psi)*τ 0.3 psi. FIG. 5 shows the correlation. The correlation coefficient r.sup.2 is 0.97.

[0118] Example 3 shows that parameters such as gel strength that are obtainable only laboriously by previously known methodology can be determined in quite a simple manner in accordance with the invention.

TABLE-US-00002 TABLE 2 predicted aAAP aAAP aAAP aAAP 0.0 psi − τ τ SFC SFC CRC 0.0 psi 0.3 psi 0.7 psi aAAP 0.7 psi 0.3 psi 0.7 psi [10.sup.−7 [10.sup.−7 Superabsorbent [g/g] [g/g] [g/g] [g/g] [g/g] [sec] [sec] cm.sup.3s/g] cm.sup.3s/g] HySorb ® C 9630 33.6 49.7 28.9 21.3 28.4 1087 544 4 6 HySorb ® T 9630 35.0 52.0 30.3 23.0 29.0 1050 484 3 3 HySorb ® T 6600 33.9 48.8 29.2 22.9 25.9 1031 400 8 11 HySorb ® B 7085 30.1 44.2 28.8 23.2 21.1 1007 526 27 28 HySorb ® B 7160 S 31.9 45.4 28.3 22.3 23.1 1012 428 29 25 HySorb ® B6600 34.4 47.0 28.4 21.8 25.3 1138 434 8 10 ASAP ® 720 26.8 47.2 30.4 25.7 21.6 682 206 90 91 HySorb ® T 5400 27.4 49.0 29.0 23.0 26.0 842 168 48 48 HySorb ® N 6830 34.4 51.9 29.1 21.7 30.1 1030 520 3 3 HySorb ® N7400 35.9 53.1 29.9 21.5 31.6 1114 600 4 3 HySorb ® N7059 38.7 53.5 27.0 19.0 35.0 1174 591 2 1 SAVIVA ® C300 40.2 55.9 28.4 18.1 37.7 773 771 0 0 SAVIVA ® B3 38.1 53.2 32.9 25.1 28.1 1063 592 6 3 SAVIVA ® B400 41.7 57.8 35.0 26.5 31.2 1047 734 4 5

TABLE-US-00003 TABLE 3 AAP AAP AAP CRC 0.0 psi 0.3 psi 0.7 psi G.sub.e Superabsorbent [g/g] [g/g] [g/g] [g/g] Φ.sub.0 [psi] HySorb ® C 9630 33.6 47.7 29.6 20.0 0.30 0.27 HySorb ® T 9630 35.0 48.7 30.7 20.2 0.28 0.27 HySorb ® T 6600 33.9 49.8 31.9 24.6 0.32 0.38 HySorb ® B 7085 30.1 39.7 28.7 23.4 0.24 0.51 HySorb ® B 7160 S 31.9 41.7 29.0 23.0 0.24 0.41 HySorb ® B6600 34.4 46.7 31.0 22.8 0.26 0.34 ASAP ® 720 26.8 44.9 30.6 25.0 0.40 0.64 HySorb ® T 5400 27.4 45.9 28.7 21.5 0.40 0.43 HySorb ® N 6830 34.4 50.7 30.6 18.1 0.32 0.24 HySorb ® N7400 35.9 48.9 30.1 18.3 0.27 0.22 HySorb ® N7059 38.7 49.4 28.6 14.4 0.22 0.14 SAVIVA ® C300 40.2 56.2 31.0 15.3 0.28 0.15 SAVIVA ® B3 38.1 50.7 33.4 25.4 0.25 0.34 SAVIVA ® B400 41.7 54.2 34.6 24.9 0.23 0.27

TABLE-US-00004 TABLE 4 aAAP aAAP aAAP 0.0 psi − τ predicted CRC 0.0 psi 0.7 psi aAAP 0.7 psi 0.3 psi G.sub.e Superabsorbent [g/g] [g/g] [g/g] [g/g] [sec] Φ.sub.0 [psi] HySorb ® C 9630 33.6 49.7 21.3 28.4 1087 0.30 0.29 HySorb ® T 9630 35.0 52.0 23.0 29.0 1050 0.28 0.29 HySorb ® T 6600 33.9 48.8 22.9 25.9 1031 0.32 0.37 HySorb ® B 7085 30.1 44.2 23.2 21.1 1007 0.24 0.49 HySorb ® B 7160 S 31.9 45.4 22.3 23.1 1012 0.24 0.44 HySorb ® B6600 34.4 47.0 21.8 25.3 1138 0.26 0.34 ASAP ® 720 26.8 47.2 25.7 21.6 682 0.40 0.63 HySorb ® T 5400 27.4 49.0 23.0 26.0 842 0.40 0.44 HySorb ® N 6830 34.4 51.9 21.7 30.1 1030 0.32 0.27 HySorb ® N7400 35.9 53.1 21.5 31.6 1114 0.27 0.22 HySorb ® N7059 38.7 53.5 19.0 35.0 1174 0.22 0.13 SAVIVA ® C300 40.2 55.9 18.1 37.7 773 0.28 0.14 SAVIVA ® B3 38.1 53.2 25.1 28.1 1063 0.25 0.31 SAVIVA ® B400 41.7 57.8 26.5 31.2 1047 0.23 0.24