Method for determining the absorption capacity of superabsorbers, and device

Abstract

The invention relates to a method for measuring the absorption capacity of superabsorbers, comprising a measuring device for measuring the absorption capacity, a storage container for the aqueous solution to be absorbed and weighing scales, wherein the measuring device is flexibly connected to the storage vessel on the weighing scales by means of a tube, and the absorption capacity is calculated from the change in weight of the storage vessel, characterised in that the tube has a length of at least 10 cm.

Description

[0001] METHOD FOR DETERMINING THE ABSORPTION CAPACITY OF SUPERABSORBERS, AND DEVICE

[0002] Description

[0003] The present invention relates to a method for measuring the absorption capacity of superabsorbents, comprising a measuring apparatus for measuring the absorption capacity, a storage vessel for the aqueous solution to be absorbed and a balance, wherein the measuring apparatus is flexibly connected to the storage vessel located on the balance by means of a hose and the absorption capacity is calculated from the change in the weight of the storage vessel, characterized in that the hose has a length of at least 10 cm.

[0004] Superabsorbents are used in the manufacture of diapers, tampons, sanitary pads, and other hygiene products, as well as as water-retaining agents in agricultural horticulture. Superabsorbents are also known as water-absorbing polymers.

[0005] The production of superabsorbents is described in the monograph “Modern Superabsorbent Polymer Technology”, FL Buchholz and AT Graham, Wiley-VCH, 1998, pages 71 to 103.

[0006] DE 38 31 272 A1 discloses a method for measuring the absorption rate by conductivity measurement.

[0007] EP 2 535 027 A1 describes a method called the “K(t) method” for determining the time dependence of the permeability and absorption kinetics of superabsorbents.

[0008] WO 2021 / 001221 describes a method for measuring the absorption capacity of superabsorbents under pressure.

[0009] The object of the present invention was to provide an improved method for measuring the absorption capacity of superabsorbents. This method should, in particular, simplify and accelerate the measurement of absorption capacity at different pressures, be as automated as possible, and provide measured values ​​with a low standard deviation.

[0010] The object was achieved by a method for measuring the absorption capacity of superabsorbents, comprising a measuring apparatus for measuring the absorption capacity, a storage vessel for the aqueous solution to be absorbed and a balance, wherein the measuring apparatus is flexibly connected to the storage vessel located on the balance by means of a hose and the absorption capacity is calculated from the change in the weight of the storage vessel, characterized in that the hose has a length of at least 10 cm.

[0011] The length of the hose is preferably 20 to 100 cm, particularly preferably 30 to 80 cm, most preferably 40 to 60 cm.

[0012] The length of the hose is preferably 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 scale, with the distance being the shortest distance to be bridged by the hose. The hose should therefore be longer than the distance between the measuring cell and the scale. The hose can be arranged, for example, in the form of a loop.

[0013] The hose is made of a material with a Shore A hardness according to DIN 53505 of preferably 10 to 90, particularly preferably 20 to 80, most particularly preferably 30 to 70.

[0014] Suitable materials include silicone rubber, perfluoroalkoxy polymers, thermoplastic elastomers, styrene-butadiene rubber, polyvinyl chloride, polyurethane and natural rubber.

[0015] The tube has a wall thickness of preferably 0.5 to 3.0 mm, particularly preferably 1.0 to 2.5 mm, most preferably 1.5 to 2.0 mm.

[0016] The inner diameter of the tube is preferably 2 to 20 mm, particularly preferably 4 to 15 mm, most preferably 8 to 10 mm. The outer diameter of the tube is preferably 2.5 to 23 mm, particularly preferably 4.5 to 18 mm, most preferably 6.5 to 13 mm.

[0017] With the method according to the invention, the absorption capacity can be determined with high accuracy, ie with a low standard deviation.

[0018] The pressure applied to a sample of the superabsorbent can be gradually reduced during the measurement. In this way, the absorption capacity at the prevailing pressure and its dependence on the measurement duration can be determined. The method of the present invention determines the absorption capacity of a superabsorbent for liquid. Such methods are known. For this purpose, a sample of a superabsorbent is placed under pressure in a measuring cell with a liquid-permeable bottom and brought into contact with liquid through the bottom in such a way that the superabsorbent constantly absorbs the liquid, swelling against the applied pressure, but it is not hydrostatically forced into the measuring cell. The superabsorbent absorbs a certain amount of liquid, which is determined gravimetrically and expressed in grams of absorbed liquid per gram of superabsorbent.

[0019] The process according to the invention is applicable to all superabsorbents, in particular to 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 in the particle size range from generally at least 50 μm, preferably at least 100 μm, to generally at most 1000 μm, preferably at most 850 μm. The process according to the invention is suitable not only for the somewhat broader particle size distributions within this range typically resulting from grinding, but also for very narrow particle size distributions within this range, such as those resulting, for example, from suspension, spray, or droplet polymerization.Agglomerates in this grain size range, which are composed of smaller primary particles, can also be analyzed using the method according to the invention.

[0020] The superabsorbent generally has a permeability (“Saline Flow Conductivity”, “SFC”, measurement method described below) of at least 10 x 10' 7 cm 3 s / g. Furthermore, it generally has a centrifuge retention capacity (CRC, measurement method described below) of at least 10 g / g.

[0021] In general, sample quantities of at least 0.5 g, preferably at least 1 g, and at most 30 g, preferably at most 20 g, and particularly preferably at most 5 g of superabsorbent are sufficient in the process according to the invention. Particular preference is given to using sample quantities of up to 1 g, preferably 0.9 g. The size and shape of the measuring cell can be adapted. Typically, a cylindrical measuring cell with an inner diameter in the range of at least 5 cm to at most 7 cm is used, so that the sample occupies a circular area with a diameter of at least 5 cm to at most 7 cm. A cylindrical geometry of the measuring cell is preferred, but any other geometry—for example, oval, square, triangular, hexagonal, or star-shaped—can be used. The measurement duration at a given pressure is typically until a stable value is reached.The value is considered stable when the values ​​measured over a longer measurement period no longer change the accuracy of the subsequent evaluation in a statistically significant manner. Generally, the measurement time is at least one minute, preferably at least 15 minutes, particularly preferably at least 30 minutes, and at most 240 minutes, preferably at most 120 minutes, particularly preferably at most 90 minutes. In most cases, a stable value is reached after a measurement time of 60 minutes. The measurement at a given pressure level can also be automatically and flexibly ended after a stable value has been reached at a given pressure level, and the measurement can be started at the next pressure level.

[0022] To obtain comparable results, it is particularly important to use a defined liquid, as the absorption capacity of superabsorbents depends heavily on it. To simulate the absorption capacity under realistic conditions of superabsorbent use, physiological saline (0.9 wt% aqueous sodium chloride solution) or a solution of various salts mimicking urine is usually used.

[0023] By far the most widespread and common method for measuring the liquid absorption capacity of a superabsorbent under pressure is the gravimetric determination of absorption against pressure (“AAP”, also known as “Absorption under Load”, “AUL”) according to standard method NWSP 242.0. R2 (15). This method is one of the standard test methods designated “NWSP” and commonly used in this field of technology, which are described in “Nonwovens Standards Procedures”, Edition 2015, jointly published 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] In this method, the mass of 0.9 wt% aqueous sodium chloride solution absorbed by the tested superabsorbent over a period of 60 minutes is determined gravimetrically under standardised measuring conditions, including the applied pressure.

[0025] In the following, the invention is described with reference to this embodiment according to the standard method NWSP 242.0.R2 (15). For details of the measurement according to NWSP 242.0.R2 (15) not mentioned below, express reference is made to the method description. Deviations from NWSP 242.0.R2 (15) and the following description are possible, for example in the shape or design of the measuring cell, the absorption liquid used, the measurement time or the applied pressure. However, the comparability of the results with the results of the standard method or other embodiments of the invention must be checked in individual cases if results from different methods are to be compared. As a rule, any deviations between different methods are correlated and can be corrected if necessary using previous comparative measurements so that the results are comparable.

[0026] An established deviation from the exact specification NWSP 242.0.R2 (15) in this field of technology is to replace the Petri dish containing the liquid to be absorbed with a device that maintains the liquid level constant at the set level (surface of the glass frit) even over multiple measurements. This simplifies and accelerates the measurements of several samples in succession, since only the respective measuring cell with the sample needs to be exchanged, but the liquid level only needs to be adjusted once before consecutive measurements of a series of prepared measuring cells. A simple and well-known device of this kind is a storage bottle with a bottom drain, which replaces the Petri dish of specification NWSP 242.0.R2 (15) or a comparable vessel is supplied with liquid via a hose connection, and in which the level of the liquid in the Petri dish or comparable vessel can be adjusted by aerating the storage bottle using a dip tube. Such a device is shown, for example, in Fig. 1 of EP 1 611 949 A1, which also represents an example of a modification of the apparatus according to standard NWSP 242.0. R2 (15), with which the same results as according to standard NWSP 242.0. R2 (15) can be achieved with the same sample quantities, pressures, and measurement times. The lower end of the aeration tube in the storage bottle is always at the same level as the liquid level in the measuring device, which can thus be adjusted by moving the aeration tube in the storage bottle.Using the same physical principle, in EP 1 516 884 A2 the liquid level in a measuring device is kept constant by means of a storage bottle and dip tube, but in this case the liquid level above the sample is used to determine another property of the sample.

[0027] A further advantage of such a storage bottle for liquid replenishment is that it can be placed on a scale, allowing the amount of liquid absorbed by the sample to be determined directly gravimetrically. The connection to the storage bottle on the scale is chosen so that its own weight does not significantly distort the measured value. A relatively thin silicone rubber tube, typically in the order of 10 mm outside diameter, is well suited. The determination of the AAP of a superabsorbent according to NWSP 242.0. R2 (15) is carried out according to the method description under a pressure of 0.7 psi (49 g / cm2 , 4826 Pa) and results in the value commonly referred to as "AAP 0.7 psi" or "AUL 0.7 psi." However, it is also possible to set a different pressure by varying the weight used in the measuring cell. The AAP 0.3 psi is frequently measured and specified, which is analogous to a pressure of 0.3 psi (21 g / cm 2 , 2068 Pa). Values ​​at other pressures, such as the AAP 0.1 psi (7 g / cm 2 , 689 Pa) or the AAP 0.9 psi (63 g / cm 2 , 6205 Pa), are determined analogously by varying the weight accordingly. Without weight, an “AAP 0 psi” or AAP 0.0 psi (0 g / cm 2, 0 Pa). In this field of technology, terms indicating pressure in the unit "psi" are internationally common, so the following will omit the additional metric values. The pressure specification always refers only to the pressure exerted by the weight on the superabsorbent, not to absolute pressure or ambient pressure.

[0028] A preferred embodiment of the method of the present invention is, in principle, an AAP measurement according to NWSP 242.0.R2 (15), in which, however, an AAP value is first determined at a higher pressure, then the weight used in the measuring cell is reduced to determine at least one further AAP value at a lower pressure. Preferably, the AAP 0.7 psi is first determined, then the weight is reduced and the AAP 0.3 psi is determined, and finally the weight is removed and the AAP 0 psi is determined. Furthermore, not only is the total mass of saline solution absorbed after each measurement time - preferably 60 minutes - at the given pressure determined, but the absorbed mass is also recorded as a function of time.

[0029] In order to enable the amount of liquid absorbed to be determined as a function of time, unlike in NWSP 242.0.R2 (15), the measuring cell is not weighed, but in the apparatus according to NWSP 242.0.R2 (15), the Petri dish or other dish used there, in which the measuring cell stands on a filter plate (method description section 6.1), is connected by means of a communicating tube, preferably a hose, for example a silicone hose, to a storage bottle for the 0.9% aqueous sodium chloride solution to be absorbed, which storage bottle stands on a balance, as described above and for example also shown in Fig. 1 of EP 1 611 949 A1.

[0030] In this way, the amount of liquid absorbed by the superabsorbent can be continuously measured gravimetrically. This can be easily achieved by using a digital scale that outputs the current weight of the bottle to a computer at regular intervals, for example, every second.

[0031] In a preferred embodiment of the invention, a dip tube is used whose diameter, particularly that at the lower end, does not result in "bursts" of large gas bubbles entering the storage bottle, but rather in a stream of fine gas bubbles. This avoids the fluctuations in the balance's measured value caused by larger gas bubbles. Such fluctuations caused by intermittent pressure equalization in the bottle do not necessarily distort statistically evaluated measured values, but can unnecessarily complicate evaluation, particularly automatic evaluation. However, it is usually not necessary to use a continuous capillary as the dip tube. For example, a glass tube with a point at the lower end is used as the dip tube. A glass tube with a diameter of 1 to 9 mm is preferred, more preferably 2 to 8 mm, particularly preferably 3 to 7 mm, and especially preferably 4 to 6 mm.However, other designs are also possible; for example, a silicone rubber tube with a perforated stopper, or generally any type of tube with a constriction, can be used. The optimal geometry can be adapted to the specific measurement setup and the acceptable level of shock during the gas supply with a few routine tests. In any case, sufficient gas must be able to enter the reservoir bottle at any time so that the rate at which the liquid is absorbed by the superabsorbent depends solely on the superabsorbent and not on the gas supply.

[0032] When performing the measurement, as with the static procedure according to NWSP 242.0.R2 (15), it is important to ensure that the liquid level in the Petri dish or other dish is high enough to completely wet the filter plate without any liquid standing on the filter plate or being hydrostatically forced into the measuring cell. Therefore, the lower end of the immersion tube must be adjusted by moving it in the reservoir bottle or changing the height of the reservoir bottle so that it is level with the top edge of the filter plate, as described in NWSP 242.0.R2 (15). In addition, or instead, the height of the measuring device can of course also be adjusted.

[0033] It is also important to ensure that the top edge of the filter plate is level, so that the Petri dish or other dish is also level. It is therefore usually advantageous if, in addition to adjusting the liquid level using a movable dip tube in the storage bottle or by changing the height of the storage bottle, the height of the Petri dish or other dish can also be adjusted, and preferably also its inclination. In the simplest case, the measuring cell with the Petri dish is placed on a laboratory lifting platform. However, individual height-adjustable feet are better. Such device feet are well known and are commonly used for numerous devices. For example, rotatable threaded rods are arranged in an internal thread of the frame, with the structure resting on their lower ends. A frame with three such feet is the easiest to adjust.This can also be a single construction with the bowl in which the filter plate and then the measuring cell are arranged.

[0034] The height of the liquid level is then adjusted by adjusting the height of the lower end of the dip tube in the storage bottle (or changing the height of the entire bottle) and the feet of the substructure of the measuring cell.

[0035] In an advantageous embodiment of the invention, the filter plate used according to the description of NWSP 242.0.R2 (15) (method description section 6.2) is replaced by a perforated plate. The perforated plate can be made of any material that is neither soluble nor swellable in the liquid used for the inventive determination of absorbency under pressure and is also sufficiently corrosion-resistant to this liquid that neither measurements are distorted nor the perforated plate needs to be replaced undesirably frequently. Examples of materials that can generally be used include stone, ceramic, metal, wood, glass, quartz, and plastic. The surfaces of the materials used can be coated, for example, hydrophilic, enameled, or antimicrobially coated. Transparent or translucent materials are preferred to ensure easy detection of impurities or air bubbles in the apparatus.Examples include polycarbonate, polymethacrylate, styrene-acrylonitrile copolymers, and transparent or translucent inorganic water-insoluble materials such as glass or quartz.

[0036] This 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.

[0037] The perforated plate has holes that allow unhindered passage of liquid to the measuring cell. The holes can have any shape. Round holes are preferred, as they are easy to produce by drilling and are also easy to 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 chosen so that their opening areas correspond to the opening area of ​​the holes at the specified diameter. The distribution of the holes can be regular or irregular. A uniform distribution across the perforated plate is preferred.The number of holes, their shape, and their arrangement must be distributed across the perforated plate in such a way that the same measurement results on a superabsorbent are obtained using the perforated plate as when using the glass frit according to NWSP 242.0.R2 (15). If these results are not achieved within the measurement accuracy defined in NWSP 242.0.R2 (15), the number of holes must be increased, their distribution across the plate surface must be made more uniform, and / or their cross-section enlarged. If this reduces the load-bearing capacity of the perforated plate for the measuring cell to such an extent that its surface is no longer flat, its thickness must be increased.

[0038] Even if the measured values ​​obtained with a perforated plate and those obtained with glass frit according to NWSP 242.0. R2 (15) for a superabsorbent are not identical, it is sufficient to linearly correlate the measured values ​​determined with the perforated plate with the measured values ​​determined according to NWSP 242.0. R2 (15). If the number, diameter and placement of the holes in the perforated plate are selected such that the correlation of the measurement results is linear and a correlation coefficient (r 2 ) of at least 0.90, preferably at least 0.95, most preferably from 0.98 to 1.00, there is a known fixed relationship between the results obtained with both methods, so that they can be converted and evaluated.

[0039] Good experience in the process according to the invention has been gained with a perforated plate having the general shape of the perforated plate 7 shown in Fig. 1, which consists of Plexiglas with a thickness of 6 mm and is provided with 52 round holes (bores) of 2 mm diameter each arranged in concentric circles and has a diameter of 89.5 mm.

[0040] Figures 1 and 2 show an exemplary construction of a substructure in a measuring setup to be used according to the invention with an integrated shell and perforated plate.

[0041] Fig. 1 shows a plan view and Fig. 2 shows a cross-section of a body 1 with a central conical recess 2 which is connected at its lowest point to an inlet pipe 5 which is inserted into the body 1 through a lateral hole. An annular channel 3 around the central recess 2 is connected to an outlet pipe 4 which is also inserted into the body 1 through a lateral hole. Around the central conical recess, the body has a collar 6 which serves as a bearing for a removable perforated plate 7. The depth of the collar is the same as the thickness of the perforated plate so that its surface lies in the plane of the surface of the body 1. Three threaded rods 8 with a slot for a screwdriver at the upper end are arranged in threaded holes in the body so that the body stands on these threaded rods and can be adjusted in height and aligned horizontally by turning them using a screwdriver.It is of course also possible to use threaded rods with attached handles or feet that can be rotated manually, or to perform the height adjustment motorically. Height adjustment devices other than threaded rods can also be used. The type of height adjustment is not essential to the invention, as long as the mechanical stability and alignment of the device are ensured. Such height adjustment devices are known; examples include hydraulic devices, air cushions, fluid cushions, gear / rack devices, cable pulls, or electromagnetic devices.

[0042] Fig. 2 shows the device in cross-section. For clarity, unlike in Fig. 1, the inlet 5 and outlet 4, as well as one of the threaded rods 8, are shown in the plane of the cross-section. However, this is not a design requirement and, due to space constraints, is rather disadvantageous for the manufacture and operation of the device.

[0043] The measuring apparatus according to this invention comprises the integrated dish with the perforated plate and the actual measuring cell. It is also possible to use any other suitable dish with a separate perforated plate instead of the integrated dish with perforated plate shown in Fig. 2. Instead of the perforated plate shown in Fig. 1, a glass frit can also be used.

[0044] The measuring cell containing the superabsorbent is placed on the perforated plate. The inlet 5 is connected to the storage bottle on the balance via a hose, and the outlet 4 is also connected to a collecting vessel of sufficient size to collect unabsorbed liquid, ideally at least the size of the storage bottle for the liquid to be absorbed.

[0045] The dimensions of such a substructure must be adapted to the desired measuring cells. 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, good experience has been gained for this measuring cell diameter with a perforated plate 7 with a diameter of 90 mm, which is arranged in a body 1 with a diameter of 160 mm. Preferably, the diameter of the outlet 4 can be larger than that of the inlet 5, or the diameters of the outlet 4 and the inlet 5 can be the same. For the example body 1 with a diameter of 160 mm, an inlet with an outer diameter of 8 mm and an inner diameter of 5 mm (suitable for a silicone hose with an outer diameter of 10 mm for connecting to the storage bottle) and an outlet with an outer diameter of 8 mm and an inner diameter of 5 mm have proven successful.An established, experience-based method for verifying the correct setting of the liquid level in the embodiment of the invention with a perforated plate (in particular, a perforated Plexiglas plate) instead of the filter plate specified in NWSP 242.0.R2 (15) consists in observing the droplets forming on the upper side at the exit of the holes when the immersion tube is raised or lowered in the storage bottle, the storage bottle, and / or the dish (such as the body 1 according to Figs. 1 and 2) with the plate inside it. If a paper filter, as used in the description of NWSP 242.0.R2 (15) (method description, section 6.12) (standard laboratory "black band" filter paper is an example of a suitable filter paper) of the size and shape of the plate is placed on the plate, the liquid that has passed through the holes in the plate will be absorbed by it.The amount of liquid absorbed can be determined by weighing the filter paper. If, with a plate diameter of 90 mm and the paper filter remaining in place for 5 minutes, it is between 1.5 and 3.0 g, the height setting is correct. If it is below this value, the dish is too high, or the storage bottle or the lower end of the dip tube is too low; if it is above this value, the dish is too low, or the storage bottle or the lower end of the dip tube is too high. For larger or smaller plates, the gram value must be adjusted accordingly.

[0046] In a preferred embodiment of the invention, the filter paper arranged between the filter plate and the measuring cell according to NWSP 242.0. R2 (15) is omitted. It has proven unnecessary, particularly in the embodiment with a perforated plate, for example, a perforated plastic plate.

[0047] The weight can be adjusted after the respective measurement period—preferably 60 minutes—by removing the weight used for the higher pressure and immediately replacing it with the weight for the lower pressure. It is possible to divide the weight so that only the portion corresponding to the additional weight for the higher pressure needs to be removed. In the example in Section 6.5 of NWSP 242.0.R2 (15) (for a plunger diameter of 59 mm), a weight of 1340 g is specified for determining the AAP 0.7 psi and a weight of 574 g for determining the AAP 0.3 psi.In the method according to the invention, the 1340 g weight can therefore be removed (with this plunger diameter) after one hour of measurement and replaced with one of 574 g. However, it is also possible to use a 574 g weight and an additional 766 g weight, which is removed after measuring the AAP 0.7 psi, with the 574 g weight remaining in the cell for the subsequent measurement of the AAP 0.3 psi. Other weights or weight combinations for measurements at other or more than two pressures can be easily calculated in a similar way. For the final measurement without weight ("AAP 0 psi" or "AAP 0.0 psi"), all weights are removed. The material of the weights is largely irrelevant. Metal weights are often used, especially stainless steel weights. It is not necessary for all weights or partial weights to be made of the same material.When using partial weights, it can actually be advantageous to make the lowest partial weight on the sample, i.e. the one used to determine the AAP at the lowest selected pressure other than 0, out of plastic, and the other weights out of a denser material such as metal, particularly stainless steel, to save volume. Making the bottom weight out of plastic allows for easier processing, particularly shaping the underside. It can also be advantageous to use a hydrophobic plastic to prevent swollen superabsorbent from sticking when this weight is removed. Suitable materials include polyhaloolefin polymers, such as partially or fully fluorinated polymers like PTFE (e.g. Teflon®) or fluorinated partially alkoxylated polymers such as PFA (e.g. Teflon®^PFA).

[0048] The sample remains the same, so it is not replaced when the pressure level is changed, but the measurements at the individual pressure levels are carried out one after the other with the same sample in the same measuring cell.

[0049] The pressure change in the method according to the invention is easily automatable. For example, the weight can be replaced by a robotic arm, or the additional weight used for the previous, higher pressure stage can be removed. It is also possible to replace the weight with a mechanically, hydraulically, or electrically driven piston that presses against the measuring cell with a corresponding weight force.

[0050] The pressure in the measuring cell is always reduced between the individual pressure levels. Superabsorbents, by their very nature, do not readily release absorbed liquids. Furthermore, although the measuring cell used in the method according to the invention absorbs available liquid, even if the superabsorbent were to release it in a corresponding amount upon pressure increase, it would not be pushed back, or at least not completely, into the reservoir bottle on the scale, and thus would not be accurately recorded in the measurement.

[0051] In a preferred embodiment of the invention, a filter paper or other thin fleece of the same surface area and shape as the interior of the measuring cell is placed on the superabsorbent sample in the measuring cell. In the case of a typically cylindrical measuring cell, this means a circular filter paper with the inner diameter of the measuring cell. For this purpose, a filter paper such as that described in NWSP 242.0.R2 (15) (Method Description Section 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 so small compared to that absorbed by the superabsorbent that it does not significantly distort the measurement.

[0052] Furthermore, in a preferred embodiment of the invention, a spacer is arranged between the underside of the weight and the superabsorbent or the filter paper lying on top of it, which ensures that the weight does not lie flat on the superabsorbent or the filter paper over its entire surface. In principle, at least one object that creates a gap between the filter paper and the weight at at least one point is sufficient as a spacer, but which does not absorb any liquid itself to avoid falsifications and whose weight is preferably so low that the measurement is not falsified beyond its usual error range; otherwise, the pressure on the sample generated by the weight of the spacer must be taken into account in the measurement or compensated for with a lower weight, and the spacer must be removed to measure an AAP of 0.0 psi.In the simplest case, at least a piece of thin wire, preferably bent, such as one or more paper clips, is sufficient as a spacer between the filter paper and the weight. However, the underside of the weight or stamp, with which it comes into contact with the superabsorbent or filter paper, is preferably not designed to be flat, so that raised areas act as spacers. This can be achieved, for example, by milling parallel or concentric grooves. Suitable grooves are those with a depth of at least 1 mm and a maximum of 5 mm, for example 2 mm, and spaced from each other by at least 0.5 mm and a maximum of 5 mm, for example 2 mm. The flanks of the grooves can be perpendicular to the surface, but can also have shallower angles, as long as they are not so shallow that the filter paper sticks again when the stamp is lifted.For example, good results have been achieved with stamps whose undersides had parallel grooves 2 mm wide, 2 mm deep, and 2 mm apart. If partial weights are used, it is sufficient to design the underside of the lowest partial weight, i.e., the one used to determine the AAP at the lowest selected pressure other than 0, with a spacer.

[0053] The last two measures mentioned, namely the filter paper between the superabsorbent sample and the weight or plunger, and the spacer on the underside, prevent swollen superabsorbent from sticking to the underside of the weight or plunger when the weight or plunger is lifted. When determining the "AAP 0.0 psi," i.e., the absorption capacity of the superabsorbent without pressure at the end of the measurement, this sticking could easily lead to falsified values. If no "AAP 0.0 psi" is determined, these two measures contribute nothing to the measurement and are therefore unnecessary; however, they do not, or at least not significantly, distort the measured values ​​under pressure.

[0054] It may also be advantageous for handling purposes to break the outer edges of the underside of the stamp or weight, i.e. to bevel them all around, for example at an angle of 45°.

[0055] The cumulative liquid masses measured with the method according to the invention in the embodiments preferred compared to the description of NWSP 242.0.R2 (15) (including the perforated plate made of Plexiglas with a thickness of 6 mm, a diameter of 89.5 mm with 52 holes of 2 mm diameter each arranged in concentric circles) after 60 minutes at a pressure correspond, according to experience, to the values ​​obtained when measuring exactly according to NWSP 242.0.R2 (15) at the same pressure with a deviation of at most 0.5 g / g.

[0056] A further object of the present invention is a measuring device for carrying out measurements according to the method according to the invention.

[0057] Examples

[0058] Example 1 (comparison example)

[0059] Using a measuring apparatus as shown in Fig. 1 and Fig. 2, the absorption capacity of a superabsorbent was measured at various pressures, and the standard deviation was also determined. The storage vessel containing the liquid to be absorbed was placed on a scale and connected to the measuring apparatus with a silicone rubber tube (Shore hardness A 60). The tube had an inner diameter of 8 mm, a wall thickness of approximately 1.8 mm, and a length of approximately 5 cm. The distance between the measuring cell and the scale was approximately 4 cm.

[0060] The results are summarized in Table 1.

[0061] Example 2

[0062] The procedure was as in Example 1. The hose was made of silicone rubber and had a length of approximately 16 cm. The distance between the measuring cell and the scale was approximately 14 cm. Example 3

[0063] The procedure was as in Example 1. The hose was made of silicone rubber and had a length of approximately 50 cm. The distance between the measuring cell and the scale was approximately 15 cm.

[0064] Tab. 1: Absorption capacities with standard deviation (in brackets) *) Comparison example

Claims

Patent claims 1. A method for measuring the absorption capacity of superabsorbents, comprising a measuring cell for measuring the absorption capacity, a storage vessel for the aqueous solution to be absorbed and a balance, wherein the measuring cell is flexibly connected to the storage vessel located on the balance by means of a hose, characterized in that the hose has a length of at least 10 cm.

2. Method according to claim 1, characterized in that 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 scale.

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

4. Method according to one of claims 1 to 3, wherein the swelling constant k or the characteristic swelling time T is calculated from the time course of the absorption under pressure.

5. Method according to one of claims 1 to 3, wherein the amount of the difference in absorption capacity is determined at two different pressures.

6. The method according to claim 5, wherein the amount of the difference in absorption capacity is determined at a pressure other than zero and without pressure.

7. Method according to one of claims 4 to 6, characterized in that at least one further characteristic of the superabsorbent is calculated from the swelling constant k or the characteristic swelling time T or the difference in absorption capacity by means of a previously measured correlation between the swelling constant k, the characteristic swelling time T or the amount of the difference in absorption capacity and the characteristic.

8. Method according to claim 7, characterized in that the parameter is the T20 value, the permeability (SFC) or the gel strength G e is. Process according to claims 1 to 8, characterized in that the applied pressure is initially 49 g / cm 2 and gradually increases to 21 g / cm 2 and 0 g / cm 2is reduced. Method according to claims 1 to 9, characterized in that the time for which the respective pressure is applied to the sample is at least 30 minutes and at most 90 minutes. Method according to one of claims 1 to 10, characterized in that the amount of superabsorbent used is from 0.5 to 5 g and the superabsorbent occupies a circular area with a diameter of 5 to 7 cm. Method according to one of claims 1 to 11, characterized in that at least 95 wt. % of the superabsorbent has a grain size in the range of 100 pm to 1000 pm. Method according to one of claims 1 to 12, characterized in that the superabsorbent has a liquid conductivity (SFC) of at least 10 x 10 -7 cm 3s / g. Process according to one of claims 1 to 13, characterized in that the superabsorbent has a centrifuge retention capacity (CRC) of at least 10 g / g. Apparatus for carrying out a process according to one of claims 1 to 14.

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