Method and apparatus for determining the absorption capacity of a superabsorbent material

Abstract

The present invention relates to a method for measuring the absorption capacity of superabsorbents, comprising a measuring device for measuring the absorption capacity, a reservoir container for the aqueous solution to be absorbed, and a balance, the measuring device being flexibly connected to the reservoir container on the balance by a hose, the absorption capacity being calculated from the change in weight of the reservoir container, the hose having a length of at least 10 cm.

Description

[Technical Field]

[0001] The present invention relates to a method for measuring the absorption capacity of superabsorbents, comprising a measuring device for measuring the absorption capacity, a reservoir container for the aqueous solution to be absorbed, and a balance, the measuring device being flexibly connected to the reservoir container on the balance by a hose, the absorption capacity being calculated from the change in weight of the reservoir container, the hose having a length of at least 10 cm. [Background technology]

[0002] Superabsorbents are used to make diapers, tampons, sanitary napkins and other hygiene articles, but are also used as moisture-retaining agents in commercial horticulture. Superabsorbents are also called water-absorbing polymers.

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

[0004] DE 38 31 272 A1 discloses a method for measuring the absorption rate by conductivity measurements.

[0005] EP 2535027 A1 describes a method, called the "K(t) method", for determining the time dependence of permeability and absorption rate of superabsorbents.

[0006] WO 2021 / 001221 describes a method for measuring the absorption capacity of a superabsorbent under pressure. Summary of the Invention [Problem to be solved by the invention]

[0007] The object of the present invention was to provide an improved method for measuring the absorption capacity of superabsorbents, which method in particular simplifies and accelerates the measurement of the absorption capacity at different pressures in a simple manner, can be carried out in a substantially automated manner and allows measurements to be made with a low standard deviation. [Means for solving the problem]

[0008] This object was achieved by a method for measuring the absorption capacity of a superabsorbent, comprising a measuring device for measuring the absorption capacity, a reservoir container for the aqueous solution to be absorbed, and a balance, the measuring device being flexibly connected to the reservoir container on the balance by a hose, the absorption capacity being calculated from the change in weight of the reservoir container, the hose having a length of at least 10 cm.

[0009] The length of the hose is preferably 20 to 100 cm, more preferably 30 to 80 cm, and most preferably 40 to 60 cm.

[0010] The length of the hose preferably corresponds to at least 1.5 times, more preferably at least 2.0 times, and most preferably at least 2.5 times the distance between the measuring cell and the balance, which is the shortest distance the hose spans. Therefore, the hose must be longer than the distance between the measuring cell and the balance. The hose can be laid, for example, in the form of a loop.

[0011] The hose is preferably made from a material having a Shore A hardness to DIN 53505 of 10-90, more preferably 20-80, most preferably 30-70.

[0012] Suitable materials are, for example, silicone rubber, perfluoroalkoxy polymers, thermoplastic elastomers, styrene-butadiene rubber, polyvinyl chloride, polyurethane and natural rubber.

[0013] The hose preferably has a wall thickness of 0.5 to 3.0 mm, more preferably 1.0 to 2.5 mm, and most preferably 1.5 to 2.0 mm.

[0014] The inner diameter of the hose is preferably 2 to 20 mm, more preferably 4 to 15 mm, and most preferably 6 to 10 mm.The outer diameter of the hose is preferably 2.5 to 23 mm, more preferably 4.5 to 18 mm, and most preferably 6.5 to 13 mm.

[0015] The method of the present invention can be used to determine the absorption capacity with high precision, i.e. with a low standard deviation.

[0016] The pressure applied to the superabsorbent sample can be reduced stepwise during the measurement, in this way it is possible in each case to determine the absorption capacity at the current pressure and its dependence on the measurement period.

[0017] In the process of the present invention, the absorption capacity of a superabsorbent for a liquid is determined. Such methods are known. For this purpose, a sample of the superabsorbent is first filled under pressure into a measuring cell with a liquid-permeable base, and the superabsorbent comes into contact with the liquid through the base so that it constantly draws in the liquid while swelling against the applied pressure, but is not hydrostatically forced into the measuring cell. The superabsorbent absorbs a certain amount of liquid, which is determined gravimetrically and expressed as grams of liquid absorbed per gram of superabsorbent.

[0018] The method of the present invention is applicable to all superabsorbents, especially commercially available particulate superabsorbents based on crosslinked, partially neutralized acrylic acid. Commercially available particulate superabsorbents for use in hygiene products typically have a particle size distribution in which at least 95% by weight of all particles are within a particle size range, generally at least 50 μm, preferably at least 100 μm, generally at most 1000 μm, preferably at most 850 μm. The method of the present invention is suitable not only for the somewhat broad particle size distribution within this range that typically occurs as a result of grinding, but also for very narrow particle size distributions within this range that occur, for example, in suspension polymerization, spray polymerization, or dropwise polymerization. Agglomerates within this particle size range that are also composed of smaller primary particles can also be analyzed by the method of the present invention.

[0019] Superabsorbents generally have a density of at least 10 x 10 -7 cm 3 s / g permeability ("Saline Flow Conductivity", "SFC", test method described below). In addition, it generally has a Centrifuge Retention Capacity ("CRC", test method described below) of at least 10 g / g.

[0020] Generally, in the method of the present invention, a sample amount of at least 0.5 g, preferably at least 1 g, and at most 30 g, preferably at most 20 g, more preferably at most 5 g of superabsorbent is sufficient. Particularly preferably, a sample amount of 1 g or less, preferably 0.9 g, is used. The size and shape of the measurement cell can be adjusted. Typically, a cylindrical measurement cell with an inner diameter ranging from at least 5 cm to at most 7 cm is used so that the sample covers a circular area with a diameter of at least 5 cm to at most 7 cm. Although a cylindrical geometry of the measurement cell is preferred, any other desired geometric shape, such as an oval, square, triangle, hexagon, or star, can also be used.

[0021] The measurement time at a given pressure is typically the time required to achieve a stable value. In particular, a value is stable if values ​​measured over a longer measurement time do not cause further statistically significant changes in the accuracy of subsequent evaluations. Generally, the measurement time is at least 1 minute, preferably at least 15 minutes, more preferably at least 30 minutes, and at most 240 minutes, preferably at most 120 minutes, more preferably at most 90 minutes. In most cases, a stable value is obtained after a measurement time of 60 minutes. Measurement at a given pressure level can also be automatically and flexibly terminated after a stable value is reached at the given pressure level, and measurement is started at the next pressure level.

[0022] To obtain comparable results, it is particularly important to use a defined liquid, since the absorption capacity of the superabsorbent depends heavily on it. Normal saline (0.9% by weight sodium chloride solution in water) or simulated urine solutions of various salts are usually used to model the absorption capacity under realistic conditions of use of the superabsorbent.

[0023] By far the most widely used test method for determining the absorption capacity of superabsorbents for liquids under pressure is gravimetric determination of the absorption of superabsorbents under pressure ("Absorption Against Pressure," "AAP," also referred to as "Absorption Under Load," "AUL") according to standard method NWSP 242.0.R2 (15). This method, referred to as "NWSP," is one of the standard test methods in this area of ​​industry, as listed in "Nonwovens Standards Procedures," 2015 edition, published jointly by EDANA (European Disposables and Nonwovens Association, Avenue Herrmann Debroux 46, 1160 Brussels, Belgium, www.edana.org) and INDA (Association of the Nonwoven Fabrics Industry, 1100 Crescent Green, Suite 115, Cary, North Carolina 27518, USA, www.inda.org). This publication is available from both EDANA and INDA.

[0024] This method involves gravimetrically determining the mass of a 0.9 wt % aqueous saline solution absorbed by a tested superabsorbent under standard test conditions, including applied pressure, over a period of 60 minutes.

[0025] The present invention is described below with reference to this embodiment in accordance with standard method NWSP 242.0.R2(15). For details of measurements in accordance with NWSP 242.0.R2(15) not mentioned below, explicit reference is made to the method description. Deviations from NWSP 242.0.R2(15) and the following explanations are possible, for example, in the shape or structure of the measurement cell, the absorption liquid used, the measurement time, or the applied pressure. However, in specific cases, when results from different methods are compared, the comparability of the results with the standard method or other embodiments of the present invention should be verified. In general, any differences between different methods are correlated and can be corrected, if necessary, based on prior comparative measurements so that the results are comparable.

[0026] This deviates from the rigorous method NWSP242.0.R2(15) established in the industry by replacing the Petri dish containing the reservoir of the absorbed liquid with a device that keeps the liquid level constant within an established plane (the surface of the glass frit) over multiple measurements. This simplifies and accelerates the measurement of multiple consecutive samples, since in this way only the respective measuring cell with the sample needs to be replaced, while the liquid level adjustment must be performed just before the consecutive measurements of a series of prepared measuring cells. A simple and known device of this kind is a reservoir bottle with a base outlet that supplies liquid to the Petri dish or equivalent container of method NWSP242.0.R2(15) by a hose connection; the liquid level in the Petri dish or equivalent container can be adjusted by venting the reservoir bottle with a dip tube. Such an apparatus is shown, for example, in FIG. 1 of EP 1 611 949 A1, which also provides an example of a modification of the apparatus according to method NWSP 242.0.R2(15), achieving the same results as method NWSP 242.0.R2(15) with the same sample volume, pressure, and measurement time. The lower end of the vent tube in the reservoir bottle is always at the same height as the liquid level in the measuring device and can therefore be adjusted by moving the vent tube in the reservoir bottle. Using the same physical principle, the liquid level in EP 1 516 884 A2 is also kept constant in the measuring device by the reservoir bottle and dip tube, but in this case, it is above the sample to determine another characteristic of the sample.

[0027] A further advantage of such a reservoir bottle for refilling liquid is that it can be placed on a balance, allowing the amount of liquid absorbed by the sample to be determined directly by gravitational measurement. Here, the connection to the reservoir bottle on the balance is selected so that its weight does not significantly distort the measurement. A relatively thin hose made of silicone rubber, typically with an outer diameter of about 10 mm, provides a good fit.

[0028] According to the method description, the AAP of the superabsorbent is 0.7 psi (49 g / cm 2, 4826 Pa) and gives a value typically referred to as "AAP 0.7 psi" or "AUL 0.7 psi". However, by varying the weight used in the measuring cell, other pressures can be established. More commonly measured and reported are those above 0.3 psi (21 g / cm). 2 The AAP is determined similarly at a pressure of 2068 Pa (AAP 0.3 psi). Values ​​at other pressures, such as AAP 0.1 psi (7 g / cm 2 , 689Pa) or AAP 0.9psi (63g / cm 2 , 6205 Pa) is similarly determined by appropriate variation of the weight. Without weight, "AAP 0 psi" or AAP 0.0 psi (0 g / cm 2 , 0 Pa). In this industry, the designation of the pressure index in the unit "psi" is the international standard, so the repetition of additional figures for metric values ​​will be omitted below. The pressure index is also always based only on the pressure load on the superabsorbent as a result of its weight, and not on absolute or ambient pressure.

[0029] A preferred embodiment of the method of the present invention is, in principle, an AAP measurement according to NWSP 242.0.R2(15), but in order to determine at least one further AAP value at a lower pressure, the AAP value is first determined at a higher pressure, and then the weight used in the measuring cell is reduced. It is preferred to first determine an AAP of 0.7 psi, then reduce the weight and determine an AAP of 0.3 psi, and finally remove the weight and determine an AAP of 0 psi. Furthermore, not only is the total mass of saline absorbed at a given pressure after each measurement time (preferably 60 minutes) determined, but the absorbed mass is also recorded as a function of time.

[0030] Unlike NWSP242.0.R2(15), the measuring cell is not weighed to allow the amount of liquid absorbed as a function of time; instead, in the device according to NWSP242.0.R2(15), the Petri dish used therein, or a separate dish in which the measuring cell stands on the filter plate (method description, point 6.1), is connected by a connecting tube, preferably a hose, for example a silicone hose, to a reservoir bottle for absorbing 0.9 wt. % saline solution, which stands on a balance, as described above and as shown, for example, in Figure 1 of EP 1 611 949 A1.

[0031] In this way, it is possible to continuously gravimetrically detect the amount of liquid absorbed by the superabsorbent, which can be achieved in a simple way by using a digital balance that sends an output of the current weight of the bottle to a computer at regular time intervals, for example every second.

[0032] In a preferred embodiment of the present invention, a dip tube is used, the diameter of which, particularly at the lower end, leads to a stream of fine bubbles rather than a "pulse" of large bubbles entering the reservoir bottle. This avoids measurement fluctuations from the balance caused by relatively large bubbles. While such fluctuations caused by rapid pressure equalization in the bottle do not necessarily distort statistically evaluated measurements, they can unnecessarily complicate evaluation, especially automated evaluation. However, it is not usually necessary to use a continuous capillary as the dip tube. For example, the dip tube used is a glass tube drawn to its tip at the lower end. It is preferable to use glass tubes with diameters of 1 to 9 mm, more preferably 2 to 8 mm, even more preferably 3 to 7 mm, and especially preferably 4 to 6 mm. However, other possible embodiments exist, such as silicone rubber hoses perforated with stoppers or any form of tube with a constriction in general. The optimal shape can be adjusted through some routine testing to determine the acceptable degree of pulse for a particular measurement setup and gas supply. However, in order to allow sufficient gas to enter the reservoir bottle, it is always necessary that the rate of absorption of liquid by the superabsorbent be dependent on the superabsorbent alone and not on the gas supply.

[0033] In the process of carrying out the measurement, as with the static procedure according to NWSP 242.0.R2(15), it is necessary to ensure that the liquid level in the Petri dish or other dish is high enough so that the filter plate is completely saturated without the liquid resting on the filter plate or being forced into the measurement cell. Therefore, the lower end of the dip tube should be adjusted in height by moving it within the reservoir bottle or by changing the height of the reservoir bottle, as described in NWSP 242.0.R2(15), so that it is positioned at the height of the upper edge of the filter plate. Additionally or alternatively, it is of course also possible to adjust the height of the measurement device.

[0034] It is also necessary to ensure that the upper edge of the filter plate, and therefore the Petri dish or other dish, is level. Therefore, it is usually advantageous to be able to adjust not only the liquid level but also the Petri dish or other dish with respect to its height, and more preferably, its inclination, by changing the height of the reservoir bottle or the movable dip tube in the reservoir bottle. In the simplest case, for this purpose, the measuring cell with the Petri dish is placed in a Labjack. However, individual height-adjustable feet are preferable. Such instrument feet are well known and standard for many instruments. For example, a rotatable threaded bolt is placed in the internal thread of the frame, and the structure stands at its lower end. A frame with three feet of this type is the easiest to adjust. It can also be an integral structure with the dish, on which the filter plate and then the measuring cell are placed.

[0035] The height of the liquid level is then set by adjusting the height of the bottom end of the dip tube in the reservoir bottle (or by changing the height of the entire bottle) and by the feet of the base structure of the measuring cell.

[0036] In an advantageous embodiment of the present invention, the filter plate used in accordance with the description of NWSP 242.0.R2(15) (method description, point 6.2) is replaced with a perforated plate. The perforated plate can be made of any material that is neither soluble nor swellable in the liquid used in the determination of absorbency under pressure in the present invention, and that is corrosion-resistant to this liquid to the extent that it does not distort the measurement, and that does not require undesirable frequent replacement. In principle, for example, stone, ceramic, metal, wood, glass, quartz, and plastic can be used. The surfaces of the materials used may be coated, for example, hydrophilized, enameled, or antibacterial coated. Transparent or see-through materials are preferred to allow easy visualization of impurities or air bubbles in the device. Examples of these are transparent or see-through inorganic, water-insoluble substances such as polycarbonate, polymethacrylate, styrene-acrylonitrile copolymer, and glass or quartz.

[0037] The perforated plate has a thickness of at least 3 mm, preferably at least 5 mm, and at most 20 mm, preferably at most 15 mm. The thickness is, for example, 6 mm or 10 mm.

[0038] The perforated plate has holes that allow the unhindered passage of liquid into the measuring cell. The holes may have any desired shape. Round holes are preferred because they are easy to drill and clean. The holes generally have a diameter of at least 0.5 mm, preferably at least 1 mm, and at most 5 mm, preferably at most 3 mm. The diameter is, for example, 2 mm. If other hole cross-sectional shapes are selected, their size is generally selected so that their opening area corresponds to the opening area of ​​a hole with the specified diameter.

[0039] The distribution of the holes can be regular or irregular. A uniform distribution on the perforated plate is preferred. The number of holes as well as their shape and arrangement on the perforated plate must be distributed such that the same measurement results are obtained for the superabsorbent when using a perforated plate as when using a glass frit according to NWSP 242.0.R2(15). If these results (within the measurement accuracy defined in NWSP 242.0.R2(15)) are not achieved, the number of holes must be increased, their distribution over the area of ​​the plate must be homogenized, and / or the cross-section of the holes must be increased. If this reduces the support capacity of the perforated plate to the extent that the surface of the measurement cell is no longer flat, its thickness must be increased.

[0040] Even if measurements obtained using a perforated plate and a glass frit according to NWSP 242.0.R2(15) should not be identical for a superabsorbent, it is sufficient that there is a linear correlation between the measurements determined by the perforated plate and those determined according to NWSP 242.0.R2(15). The number, diameter, and location of the holes in the perforated plate must be such that the correlation of the measurements is linear, with a correlation coefficient (r) of at least 0.90, preferably at least 0.95, and most preferably between 0.98 and 1.00. 2), there is a known fixed relationship between the results obtained by the two methods so that they can be interconverted and evaluated. [Brief explanation of the drawings]

[0041] [Figure 1] A top view of the main body 1 is shown. [Figure 2] A cross-sectional view of the main body 1 is shown. DETAILED DESCRIPTION OF THE INVENTION

[0042] Good results have been achieved with the method of the invention using a perforated plate having the general shape of perforated plate 7 shown in Figure 1. It consists of Plexiglas 6 mm thick and 89.5 mm in diameter, provided with 52 round holes (drilled holes) arranged concentrically, each 2 mm in diameter.

[0043] 1 and 2 show an exemplary configuration of a base structure in a measurement setup used according to the invention with an integrated pan and perforated plate.

[0044] Figure 1 shows a top view of the main body 1, and Figure 2 shows a cross-sectional view. A central conical recess 2 is connected at its lowest point to a supply pipe 5, which enters the main body 1 through a lateral hole. An annular channel 3 around the central recess 2 is connected to an outlet pipe 4, which also enters the main body 1 through a lateral hole. Around the central conical recess, the main body has a collar 6 that serves as a mounting base for a removable perforated plate 7. The collar's depth is the same as the thickness of the perforated plate, so that its surface lies in the same plane as the surface of the main body 1. Three threaded bolts 8, each with a screwdriver slot at its upper end, are positioned in the main body's threaded holes. The main body rests on these threaded bolts, and its height can be adjusted and horizontally aligned by turning them with a screwdriver. Naturally, height adjustment could also be achieved by using threaded bolts with a grip or foot attached that can be turned manually, or by a motor. Height adjustments other than threaded bolts could also be used. The type of height adjustment is not essential to the invention, as long as the device is mechanically stable and positionable. Such height adjustment devices are well known, examples of which include hydraulic devices, air cushions, liquid cushions, gear / rack devices, cables, or electromagnetic devices.

[0045] Figure 2 shows the device in a cross-sectional view, where, by way of example and in contrast to Figure 1, the supply 5 and drain 4 as well as one of the threaded bolts 8 are shown in the plane of the cross section, which is not necessary for construction purposes and would in any case be disadvantageous for the manufacture and operation of the device for reasons of space.

[0046] In the context of the present invention, the measuring device comprises an integrated dish with a perforated plate and the actual measuring cell. Instead of an integrated dish with a perforated plate, it is also possible to use any other suitable dish with a separate perforated plate, as shown in Figure 2. It is also possible to use a glass frit instead of the perforated plate shown in Figure 1.

[0047] The measuring cell with the superabsorbent is placed on a perforated plate. The supply 5 is connected by a hose to a reservoir bottle on the balance, and the drain 4 is likewise connected to a collection vessel large enough to accommodate any unabsorbed liquid, ideally at least as large as the reservoir bottle into which the liquid is absorbed.

[0048] The dimensions of such a base structure should be adjusted to the desired measuring cell. NWSP 242.0.R2(15) specifies a minimum filter plate diameter of 80 mm for a measuring cell with a diameter of 60 mm. For example, a 90 mm diameter perforated plate 7 placed in a 160 mm diameter body 1 has given good results for this measuring cell diameter. The diameter of the drain 4 can preferably be larger than that of the supply 5. Alternatively, the drain 4 and supply 5 have the same diameter. For the example body 1 with a diameter of 160 mm, a supply with an outer diameter of 8 mm and an inner diameter of 5 mm (to attach a 10 mm outer diameter silicone hose for connection to a reservoir bottle) and a drain with an outer diameter of 8 mm and an inner diameter of 5 mm have proven useful.

[0049] A well-established empirical method for verifying the correct adjustment of the liquid level in embodiments of the present invention using perforated plates (particularly perforated Plexiglas plates) rather than the filter plates described in NWSP 242.0.R2(15) is to observe the droplets that form at the outlets of the holes on the top surface of the plate when raising or lowering the dip tube in the reservoir bottle, reservoir bottle, and / or the dish in which the plate is placed (e.g., Body 1 according to Figures 1 and 2). When a filter paper of the same size and shape as the plate, also used according to the instructions in NWSP 242.0.R2(15) (Method Description, point 6.12) (standard laboratory "black band" filter paper is an example of a suitable filter paper), is placed on the plate, and the liquid that passes through the holes in the plate is absorbed by it. The amount of absorbed liquid can be determined by weighing the filter paper. Height adjustment is correct if the filter paper weighs 1.5 to 3.0 g for a 90 mm diameter dish after 5 minutes of contact time. Below that, the dish is too high or the bottom of the reservoir bottle or dip tube is too low. Above that, the dish is too low or the bottom of the reservoir bottle or dip tube is too high. For larger or smaller plates, the gram value will need to be adjusted accordingly.

[0050] In preferred embodiments of the present invention, it is not necessary to place filter paper between the filter plate and the measuring cell in accordance with NWSP 242.0.R2(15). In particular, in embodiments with a perforated plate, e.g., a perforated plastic plate, it has not been found to be necessary.

[0051] The weight change after each measurement time (preferably 60 minutes) can be accomplished by removing the weight used for each higher pressure and immediately replacing it with a weight for each lower pressure. It is possible to divide the weight so that only the portion corresponding to the excess weight for the higher pressure is removed. In the example of point 6.5 of NWSP 242.0.R2(15) (for a 39 mm plunger), a 1340 g weight is specified for determining an AAP of 0.7 psi and a 574 g weight is specified for determining an AAP of 0.3 psi. Therefore, in the method of the present invention, it is possible to remove the 1340 g weight after one hour of measurement (with this plunger diameter) and replace it with a 574 g weight. Similarly, it is also possible to use the 574 g weight and an additional 766 g weight that is removed after the measurement of an AAP of 0.7 psi, leaving the 574 g weight for the subsequent measurement of an AAP of 0.3 psi in the battery. Other weights or weight combinations for measurements at other pressures or more than two pressures can be easily calculated as well. For the final unweighted measurement ("AAP 0 psi" or "AAP 0.0 psi"), all weights are removed.

[0052] The material of the weight is of little importance. Metal weights, especially stainless steel weights, are often used. It is not necessary for the entire weight or partial weights to be made of the same material. However, if partial weights are used, it may be advantageous for the lowest partial weight on the sample, i.e., the one used to measure the AAP at the selected lowest non-zero pressure, to be made from a denser material, such as metal, especially stainless steel, in order to save plastic and volume. Producing the lightest weight from plastic allows for easier processing, particularly molding of the lower side. It may also be advantageous to use a hydrophobic plastic to prevent adhesion of the swollen superabsorbent when this weight is removed. Suitable materials are, for example, polyhaloolefin polymers, such as partially or fully fluorinated polymers such as PTFE (e.g., Teflon®) or fluorinated partially alkoxylated polymers such as PFA (e.g., Teflon®-PFA).

[0053] The sample remains the same, which means that the sample is not exchanged when the pressure level changes; instead, measurements at individual pressure levels are performed consecutively on the same sample in the same measurement cell.

[0054] The pressure change in the method of the present invention can be efficiently automated. For example, the weight can be exchanged with a robotic arm, thus eliminating the additional weight used for the previous higher pressure level. It is also possible to exchange the weight with a mechanically, hydraulically, or electrically driven plunger that presses the measuring cell with the appropriate weight.

[0055] The pressure in the measuring cell constantly decreases between the individual pressure levels. Superabsorbents, by their nature, do not easily release absorbed liquid again; moreover, in the measuring cell used in the method of the present invention, the provided liquid is absorbed but is not pushed back there, or at least not completely pushed back into the reservoir bottle on the balance, so that even if the superabsorbent releases again to a corresponding extent when the pressure increases, this will not be accurately detected in the measurement.

[0056] In a preferred embodiment of the present invention, a filter paper or another thin nonwoven fabric with the same area and shape as the inside of the measuring cell is placed on the superabsorbent sample in the measuring cell, i.e., in the case of a cylindrical measuring cell, a circular filter paper with the inner diameter of the measuring cell is typically placed on top. For this purpose, a filter paper such as that specified in NWSP242.0.R2(15) (method description, point 6.12) is preferably used. An example of a suitable filter paper is standard laboratory "black band" filter paper. Experience has shown that the amount of liquid absorbed by the filter paper is very small compared to the amount absorbed by the superabsorbent, so it does not significantly distort the measurement value.

[0057] Furthermore, in a preferred embodiment of the present invention, a spacer is placed between the underside of the weight and the superabsorbent or the filter paper thereon, the effect of which is that the weight does not lie perfectly flat on the superabsorbent or filter paper. In principle, a suitable spacer is at least one object that establishes a gap between the filter paper and the weight at at least one point, but which does not itself absorb any liquid to avoid distortion, and preferably has such low weight that it does not distort the measurement beyond its customary error range. Otherwise, the pressure on the sample caused by the weight of the spacer must be included in the measurement calculation or compensated for by a lower weight, and the spacer must be removed for measurements with an AAP of 0.0 psi. In the simplest case, a suitable spacer is simply at least one thin wire, preferably bent, such as one or more paper clips, between the filter paper and the weight. However, preferably, the underside of the weight or plunger, where the weight or plunger comes into contact with the superabsorbent or filter paper, has a correspondingly non-planar configuration, so that ridges are present as spacers. This can be done, for example, by machining parallel or concentric grooves. Suitable grooves are at least 1 mm deep and at most 5 mm, e.g., 2 mm deep, and spaced at least 0.5 mm apart and at most 5 mm apart, e.g., 2 mm apart. The groove sides may be perpendicular to the surface, but they may also have flatter angles, as long as they are not so flat that the filter paper reattaches when the plunger is raised. For example, good results have been obtained with a plunger having parallel grooves 2 mm wide, 2 mm deep, and 2 mm apart on its underside. When partial weights are used, it is sufficient to configure the spacer so that it is located below the lowest partial weight, i.e., the one used to determine the AAP at the selected lowest non-zero pressure.

[0058] The latter two measures, i.e., the filter paper between the superabsorbent sample and the weight or plunger and the spacer below it, prevent the expanded superabsorbent from sticking to the underside of the weight or plunger when the latter is lifted. In determining "AAP 0.0 psi," i.e., the absorbent capacity of the superabsorbent without compressive stress at the end of the measurement, this sticking would easily result in a distortion of this value. If "AAP 0.0 psi" is not specified, these two measurements do not contribute to the measurement and are therefore unnecessary, but they do not distort, or at least do not significantly distort, the measurement under pressure.

[0059] It may also be advantageous for handling if the lower outer edge of the plunger or weight is cut off, i.e. tapered all around at an angle of, for example, 45°.

[0060] Experience has shown that in each case, after 60 minutes at one pressure, the cumulative liquid mass measured by the method of the present invention in each preferred embodiment over that described in NWSP 242.0.R2(15) (comprising a 6 mm thick, 89.5 mm diameter Plexiglas perforated plate with 52 concentric holes, each 2 mm in diameter) corresponds exactly to the value obtained when measured at the same pressure according to NWSP 242.0.R2(15), with a variance of no more than 0.5 g / g.

[0061] The present invention further provides a measurement device for the performance of measurements according to the method of the present invention. [Example]

[0062] Example 1 (Comparative Example) The measuring device according to Figures 1 and 2 was used to measure the absorption capacity of the superabsorbent at different pressures and to determine the standard deviation. A reservoir container containing the liquid to be absorbed was placed on a balance and connected to the measuring device by a silicone rubber hose (Shore A hardness 60). The hose had an inner diameter of 8 mm, a wall thickness of about 1.8 mm, and a length of about 5 cm. The distance between the measuring cell and the balance was about 4 cm.

[0063] The results are shown in Table 1.

[0064] Example 2 The procedure was the same as in Example 1. The hose was made of silicone rubber and was about 16 cm long. The distance between the measuring cell and the balance was about 14 cm.

[0065] Example 3 The procedure was the same as in Example 1. The hose was made of silicone rubber and had a length of about 50 cm. The distance between the measuring cell and the balance was about 15 cm.

[0066] [Table 1]

Claims

1. A method for measuring the absorption capacity of a superabsorbent, comprising a measuring cell for measuring the absorption capacity, a reservoir container for the aqueous solution to be absorbed, and a balance, said measuring cell being flexibly connected to said reservoir container on said balance by a hose, said hose having a length of at least 10 cm.

2. 2. The method of claim 1, wherein the hose has a length of 40 to 60 cm and / or the length of the hose corresponds to at least 2.5 times the distance between the measuring cell and the balance.

3. 3. The method according to claim 1 or 2, wherein the hose consists of a material with a Shore A hardness (DIN 53505) of 50 to 70 and / or the hose has a wall thickness of 1.5 to 2.5 mm.

4. 4. The method according to any one of claims 1 to 3, wherein the progress of absorption under pressure versus time is used to calculate the swelling constant k or the characteristic swelling time τ.

5. The method according to any one of claims 1 to 3, wherein the magnitude of the difference in absorbent capacity at two different pressures is determined.

6. The method of claim 5, wherein the magnitude of the difference in absorbent capacity at non-zero pressure and at no pressure is determined.

7. 7. The method according to claim 4, wherein the swelling constant k or the characteristic swelling time τ or the difference in absorption capacity is used to calculate said parameter by a previously determined correlation between the magnitude of the swelling constant k, the characteristic swelling time τ or the difference in absorption capacity and at least one further parameter of the superabsorbent.

8. The parameters are T20 value, permeability (SFC) or gel strength G ｅ The method of claim 7, wherein

9. The applied pressure is initially 49 g / cm ２ and gradually increases to 21 g / cm ２ and 0 g / cm ２ The method according to any one of claims 1 to 8, wherein the

10. The method according to any one of claims 1 to 9, wherein the period during which the particular pressure is applied to the sample is at least 30 minutes and at most 90 minutes.

11. The method according to any one of claims 1 to 10, wherein the amount of superabsorbent used is between 0.5 and 5 g, said superabsorbent covering a circular area with a diameter of between 5 and 7 cm.

12. The method according to any one of the preceding claims, wherein at least 95% by weight of the superabsorbent has a particle size in the range of 100 μm to 1000 μm.

13. The superabsorbent is at least 10×10 －７ cm ３ 13. The method of any one of claims 1 to 12, having a saline flow conductivity (SFC) of s / g.

14. The method according to any one of the preceding claims, wherein the superabsorbent has a centrifuge retention capacity (CRC) of at least 10 g / g.

15. Apparatus for carrying out the method according to any one of claims 1 to 14.

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