WO1997033168A1

WO1997033168A1 - Method and device for measuring amount of absorbing gel material contained in absorbent pads

Info

Publication number: WO1997033168A1
Application number: PCT/US1996/016666
Authority: WO
Inventors: Shunji Ishigami; Kyoko Ida
Original assignee: The Procter & Gamble Company
Priority date: 1996-03-08
Filing date: 1996-10-17
Publication date: 1997-09-12
Also published as: JPH09243622A; JPH11506209A; JP3096067B2; AU7451796A

Abstract

A method for measuring an amount of an absorbing gel material held in an absorbent pad, wherein the absorbent gel material contains a functional group having an alkali metal ion, the method comprising the steps of: (a) bringing the absorbing gel material into contact with an acid solution containing a predetermined amount of hydrogen ions, the predetermined amount of hydrogen ions being more than the amount of the alkali metal ions contained in the absorbing gel material; (b) substantially substituting the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in the acid solution; (c) measuring the amount of hydrogen ions remaining after the substitution; and (d) calculating the amount of the absorbing gel material based on the measurement of the amount of the remained hydrogen ions. Preferably, the acid solution contains an excessive amount of hydrogen ions compared to the alkali metal ions contained in the absorbing gel material. More preferably, the method further comprises the step of applying a non-reactive gas to the absorbent pad through the acid solution so that the bubbles of the gas enter the absorbent pad and pass therethrough. In an alternative preferred embodiment, the method further comprises the step of applying an ultrasonic wave to the absorbent pad through the acid solution.

Description

METHOD AND DEVICE FOR MEASURING AMOUNT OF ABSORBING GEL MATERIAL CONTAINED IN ABSORBENT PADS

FIELD OF THE INVENTION The present invention relates to a method and device for measuring the amount of an absorbing gel material contained in an absorbent pad. The present invention has particular applicability to the measurements for absorbent articles such as diapers, adult incontinence pads, sanitary napkins, and the like.

BACKGROUND OF THE INVENTION

Absorbing gel material, which will be referred to as "AGM" hereinafter, can generally absorb water having weight of from several hundreds to about one thousand times as large as its own weight, and is used in various fields such as in sanitary articles including disposable diaper, etc. Figs. IA and IB show the structure of a disposable diaper using AGM of acrylic acid salt as the absorbing material, wherein Fig. IA is a plan view of the diaper and Fig. IB is a cross section of the diaper taken along the line A1-A2 of Fig. IA. Referring to these figures, an absorbent pad 103 consists of an air-felt 101 structured in layers of stacks of short fiber pulps and a number of AGM particles 102 having diameter of about 0.5 mm held in the air- felt 101. The absorbent pad 103 is enveloped by a tissue 104 and is held between a topsheet 105 and a backsheet 106. The topsheet 105 has a surface made of nonwoven fabric that contacts a skin and passes urine, and the backsheet 106 is made of a film for preventing urine from leaking outside. The tissue 104 is adhered to the inner surface of the backsheet 106 with a hot-melt adhesive 107, thus preventing the absorbent pad 103 from sliding aside. An eiastically contractible leg gather 108 is provided to the outer surface of the topsheet 105 along the longitudinal length of the diaper at each side.

When the above diaper is in use, the air-felt 101 absorbs urine owing to the capillary phenomenon immediately after the urine passes the topsheet 105, whereafter the AGM particles 102 absorb the urine retained among the short fiber pulps of the air-felt 101 , so that the wet air-felt 101 turns back dry and recovers the absorbing capacity for the next urination. Usually, an air-felt 101 can be used for a plurality of consecutive urinations effectively.

The absorbing capacity of a diaper as described above is determined by the amount of AGM in the absorbent pad 103. Therefore, in developing products or in controlling the quality of products, it is indispensable to measure the amount of AGM in the absorbent pad 103 per one diaper.

Before conducting the measurement, a sample for the measurement is conventionally prepared as follows. First, the adhering parts of the topsheet 105 and backsheet 106 are cut off with scissors or the like and the topsheet 105 is removed. Then the area of the backsheet 106 where the hot-melt adhesive 107 is applied is heated with a dryer or the like to melt the adhesive so that the absorbent pad 103 can be removed from the backsheet 106. After the absorbent pad 103 is taken out of the diaper, the absorbent pad 103 is cut into small pieces with scissors to facilitate the reaction between the AGM particles 102 in the absorbent pad 103 and the acid solution.

One of the methods for measuring the amount of AGM is disclosed in the Japanese Patent Laid-open Publication No. HI -260361, in which a reaction between AGM and an acid solution is utilized. In the process of the reaction between AGM particles and the acid solution, an alkali metal ion (e.g., Na⁺) of an alkali metal substituent (e.g., COONa) of the AGM is replaced by a hydrogen ion H⁺ in the acid solution, whereby a COOH substituent is produced and the alkali metal ion is released. Hence, the amount of AGM can be measured by measuring the amount of the alkali metal ions released. In the measurement, first, a series of solutions containing known concentrations of sodium ions are prepared. Next, the potential differences in the solutions are measured using a pH meter having a sodium- ion- detecting electrode. The potential difference values are plotted against the concentration values to obtain a calibration curve representing a relation between sodium ion concentration and potential difference. Then the acid solution that has reacted with the AGM is neutralized by aqueous ammonia, and the potential difference in the solution is measured by the same pH meter as that used above. The sodium ion concentration is determined based on the calibration curve, and the amount of AGM is calculated from the sodium ion concentration.

However, this method can not provide a high accuracy of measurement, in particular when the amount of AGM in the absorbent pad 103 is small. This is because the method needs to use a sodium-ion-detecting electrode to measure the sodium ions. The sodium-ion-detecting electrode unstable. More specifically, it changes its property considerably with respect to time and temperature, so that the reproducibility of measurements is poor. This fact often causes a low accuracy in the measurements. To improve the accuracy of measurement, it is therefore practically necessary to prepare new calibration curves very often and to maintain the electrode in a good condition for the measurement. However, these preparation and maintenance works increase the time required for the measurement.

Additionally, the following problems are also pointed out. (1) The absorbent pad 103 must be taken out of the diaper as being enveloped by the tissue 104. Therefore, it is necessary to heat and soften the hot-melt adhesive 107 with a dryer and strip the backsheet 106 off the absorbent pad 103 carefully. This work requires much time and skill so that the efficiency of measurement is considerably deteriorated. (2) To promote the reaction between AGM and acid solution, it is helpful to use a paddle mixer. In this case, since an adequate stirring efficiency cannot be obtained if the short fiber pulps twine around the paddle of the mixer, the absorbent pad 103 must be cut into small pieces before being put in a reaction bath. However, some of the AGM particles 102 held in the air-felt 101 are scattered and lost even when the cutting work is carried out with a great care, so that the amount of AGM cannot be measured accurately. (3) Since the AGM particle 102 is very hard before absorbing water, the cutting edges of the scissors are severely damaged when they are used to cut the absorbent pad 103. This incurs unnecessary cost for scissors. (4) Even if the absorbent pad 103 is cut into small pieces, it is impossible to completely prevent the pulps from twining around the paddle. Therefore, it is still necessary to wash the paddle to remove the pulps every time the stirring work of one sample is finished, which deteriorates the measurement efficiency. Furthermore, the work must be carried out manually, which is an obstacle to an automated measurement device.

SUMMARY OF THE INVENTION According to one aspect of the present invention, a method for measuring an amount of an absorbing gel material held in an absorbent pad, comprises the steps of: (a) bringing the absorbing gel material into contact with an acid solution containing a predetermined amount of hydrogen ions, the predetermined amount of hydrogen ions being more than the amount of the alkali metal ions contained in the absorbing gel material; (b) substantially substituting the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in the acid solution; (c) measuring the amount of hydrogen ions remaining after the substitution; and (d) calculating the amount of the absorbing gel material based on the measurement of the amount of the remained hydrogen ions. Preferably, the step (b) comprises the step of promoting the substitution of the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in said acid solution. More preferably, the promotion step comprises the step of applying a physical vibration to the absorbent pad through the acid solution. In a preferred embodiment, the physical vibration application step comprises the step of applying a physical vibration to the absorbent pad through the acid solution. In a more preferred embodiment, the physical vibration application step comprises the step of applying a non-reactive gas to the absorbent pad through the acid solution so that the bubbles of the gas enter the absorbent pad and pass therethrough. According to another aspect of the present invention, a device for measuring an amount of an absorbing gel material held in an absorbent pad, comprises: a means for bringing the absorbing gel material into contact with an acid solution containing a predetermined amount of hydrogen ions so that the alkali metal ions contained in the absorbing gel material are substantially substituted with hydrogen ions contained in the acid solution, the predetermined amount of hydrogen ions being more than the amount of the alkali metal ions contained in the absorbing gel material; a means for measuring the amount of hydrogen ions remaining after the substitution; and a means for calculating the amount of the absorbing gel material based on the measurement of the amount of the remained hydrogen ions.

BRIEF DESCRIPTION OF THE DRAWINGS Figs. IA is a plan view of a disposable diaper.

Figs. IB is a cross-sectional view of the disposable diaper taken along the line Al - A2 in Fig. IA. Figs. 2A, 2B, 2C and 2D are illustrations showing process steps of preparing a rolled sample in the embodiment;

Figs. 3A and 3B are illustrations for explaining the reacting process; Fig. 4 is a schematic view of an embodiment of an automatic device for measuring the amount of AGM in an absorbent pad; Fig. 5 is a side view of a sample rack used in the embodiment;

Fig. 6 is a perspective view of a waste container used in the embodiment; Figs. 7A, 7B, 8A and 8B are perspective views for explaining the operation of a multi-purpose nozzle used in the embodiment;

Fig. 9 is a graph showing the effect of the bubbling in the measurement; and Fig. 10 is a graph showing the effect of the ultrasonic wave in the measurement. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS According to one aspect of the present invention, a method for measuring an amount of an absorbing gel material held in an absorbent pad, comprises the steps of: (a) bringing the absorbing gel material into contact with an acid solution containing a predetermined amount of hydrogen ions, the predetermined amount of hydrogen ions being more than the amount of the alkali metal ions contained in the absorbing gel material; (b) substantially substituting the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in the acid solution; (c) measuring the amount of hydrogen ions remaining after the substitution; and (d) calculating the amount of the absorbing gel material based on the measurement of the amount of the remained hydrogen ions.

As used herein, the term "absorbing gel material" is referred to "AGM". The AGM which is often used in absorbent articles are commonly referred to as "water- insoluble and water-swellable", "hydrogel-forming", "hydrocolloids", or "superabsorbent" absorbent polymers. The AGM which can be measured in the present invention contains a functional group having an alkali metal ion such as a sodium ion (Na⁺) and a potassium ion (K⁺). In preferred embodiments, the AGM has a carboxyl group of a sodium salt (COONa).

The absorbent pad in the present invention can be any absorbent articles which contain the AGM such as diapers, adult incontinence pads, sanitary napkins, and the like. In a preferred embodiment, the absorbent pad is a disposable diaper which comprises an absorbent core including an air-felt material, AGM particles held therein, and a tissue for enveloping the air-felt material. The air-felt material is generally composed of short fiber pulps. In preferred embodiments, at least one portion of the absorbent pad is cut so that the AGM held therein can contact with the acid solution more easily. More preferably, at least one edge, more preferably two or four edges of the absorbent pad are cut to provide a better contact of the AGM and the acid solution.

For example, referring to Figs. 2A-2D, first, the adhering parts of the topsheet 105 and the backsheet 106 at the longitudinal sides of a diaper are cut off with scissors or other cutting devices, and leg gathers 108 are also removed (Fig. 2A). Next, longitudinal side edges of the tissue 104 enveloping the absorbent pad 103 are cut open with scissors (Fig. 2B). Then the absorbent pad 103 which is sandwiched by the tissue 104 is rolled together with the topsheet 105 and the backsheet 106 attached thereto so that the above cut open edges come at both end faces of the rolled cylinder (Fig. 2C). As a result, the absorbent pad 103 is exposed outside from the openings of tissue 104 at both ends of the rolled sample 109. The sample 109 is then inserted into an open-ended cylindrical sample tube 1 10 having inner diameter a little larger than the diameter of the rolled sample 109, whereby the sample 109 can be handled more easily and securely in storing or transferring (Fig. 2D).

The acid solution used in the present invention contains a predetermined amount of hydrogen ions. In preferred embodiments, the acid solution is selected the group consisting of a hydrochloric acid (HCL) solution, a sulfuric acid (H2SO4) solution, an acetic acid (CH3COOH) solution, a nitric acid (HNO3) solution, a phosphoric acid (H3PO4) solution, an oxalic acid ((COOH)2) solution and a formic acid (HCOOH) solution. The predetermined amount of hydrogen ions is more than the amount of the alkali metal ions contained in the AGM held in the absorbent pad. In preferred embodiments, the acid solution contains an excessive amount of hydrogen ions compared to the alkali metal ions contained in the AGM to promote the substitution reaction of the alkali metal ions with the hydrogen ions. By using the excessive amount of the hydrogen ions, sufficient amount of hydrogen ions remain after the acid solution has reacted with the AGM. Preferably, the acid solution has a concentration of hydrogen ions of from about 0.05 (mol/ml) to about 30 (mol/ml), more preferably from about 1.3 (mol/ml) to about 20 (mol/ml), and most preferably from about 2.7 (mol/ml) to about 13.7 (mol/ml). To bring the AGM into contact with the acid solution, any method for bringing materials contact with liquids can be used. In preferred embodiments, after the absorbent pad is put in a reaction bath, the bath is filled with the acid solution. In an alternative embodiment, after a reaction bath is filled with the acid solution, the absorbent pad is immersed in the acid solution. Preferably, the amount or volume of the acid solution is determined so that the whole absorbent pad is completely immersed.

Consequently, the alkali metal ions contained in the AGM are substituted with the hydrogen ions contained in the acid solution. Preferably more than 98%, more preferably more than 99.5% of the alkali metal ions contained in the AGM are substituted with the hydrogen ions.

In preferred embodiments, it is expected that the substitution reaction is completed in a short time since the time required for the substitution reaction relates to the total time required for the measurement. Thus, a step of promoting the substitution is additionally employed. Preferably, a physical vibration is applied to the absorbent pad through the acid solution to help the AGM be exposed to a fresh acid solution. Any physical vibration which helps the substitution reaction can be used. In a preferred embodiment, a non-reactive gas is applied to the absorbent pad through the acid solution so that the bubbles of the gas enter the absorbent pad and pass therethrough. Since the bubbles pass through the spaces formed between the pulps of the air-felt, the particles of AGM held in the air-felt are constantly exposed to a fresh acid solution and the reaction between the AGM and the acid solution can progress rapidly even without mechanically stirring the solution. Preferred non- reactive gases include any gases which are not reactive with the AGM nor the acid solution. Preferably, the non-reactive gas is a nitrogen gas or an air, most preferably noble gases. For example, referring to Figs. 3 A and 3B, a reaction bath 30 has such a diameter and depth that the whole sample 109 can be immersed completely. The reaction bath 30 has a number of holes 31 of an appropriate diameter at the bottom. An air supply tube 32 is connected to the side wall of the reaction bath 30 for supplying air through the holes 31 into the reaction bath 30. An absorbent pad or sample 109 is put in the empty reaction bath 30 while an air supplied through the holes 31. Then, maintaining the air flow at a preset rate, hydrochloric acid solution of a predetermined volume and concentration is applied into the reaction bath 30 so that the whole sample 109 is completely immersed. The bubbles of the air supplied from the holes 31 enter the absorbent pad 103 from the opening of the tissue 104 at the bottom end of the rolled sample 109, pass through the absorbent pad 103 upward and exit from the top opening of the tissue 104 (Fig. 3B). Here, the absorbent pad 103 is hardly deformed by the pressure of the bubbles since it is securely supported by the backsheet 106. Furthermore, since the backsheet 106 is impervious to liquid, the bubbles entering from the bottom opening of the tissue 104 pass almost straight upward through the absorbent pad 103. Therefore, any AGM particle 102 held in the absorbent pad 103 can contact with a fresh hydrochloric acid solution constantly during the bubbling, and the reaction can progress rapidly.

In preferred embodiments, the vibrator which generates an ultrasonic wave is immersed in the acid solution. The vibrator can be positioned anywhere in the acid solution in the reaction bath. In a preferred embodiment, the vibrator is positioned below the middle of the acid solution level in the reaction bath. Preferably, the ultrasonic wave has a frequency of from about 10 (kHz) to about 100 (kHz), more preferably from about 15 (kHz) to about 45 (kHz), and most preferably from about 20 (kHz) to about 30 (kHz). In preferred embodiments, the output power of the vibrator is determined by the time requirement for the measurement and the size of the absorbent pad. Preferably, the vibrator has an output power selected between about 50 (W) and about 1000 (W), more preferably between about 100 (W) and about 500 (W), and most preferably between about 150 (W) and about 300 (W). In a preferred embodiment, the ultrasonic wave is employed in addition to the use of the non- reactive gas which was already described above.

To measure the amount of hydrogen ions remaining after the substitution reaction, any methods for measuring residual hydrogen ions contained in liquids can be used. In preferred embodiments, a back-titration is employed by using a base solution. Preferably, the base solution is a hydroxide solution of an alkali metal. Preferred hydroxide solutions of alkali metals include a sodium hydroxide (NaOH) solution, a calcium di-hydroxide (Ca(OH)2) solution, a potassium hydroxide (KOH) solution, and a magnesium hydroxide (Mg(OH)2) solution. By using the back- titration, the amount of AGM can be measured with high and stable accuracy irrespective of whether the amount of the AGM is large or small.

More preferably, a sodium hydroxide solution which concentration has already been known is added little by little to the acid solution which contains residual hydrogen ions until the acid solution is neutralized. Preferably, a PH meter which has an ordinary electrode such as a silver chloride (AgCl) electrode is employed to detect the neutralization point. This means that the sodium-ion-detecting electrode which is unstable in general is not required for the measurement anymore. Since the ordinary electrode is stable (and also is commonly available in the technology field), it is sufficient to make the calibration curve for detecting the neutralization point once in a day. In addition, the reproducibility and reliability of measurements can be much improved.

Various materials can be used for the reaction bath. In preferred embodiments, the use of harder materials or less flexible materials for the reaction bath can more increase the effect of the application of the physical vibration. Preferred reaction bath materials include metals including a stainless steel, ceramics, and plastic materials including a polycarbonate material and a polypropylene material. More preferably, the reaction bath material is coated with a glass. Most preferably, a glass coated stainless steel is used. In a more preferred embodiment, a predetermined amount or volume of the acid solution after the substitution reaction is sampled in a titration cup i.e., a reaction bath. The sampling of the acid solution can be conducted by using a suction tube from the reaction bath 30, for example. Preferably, a metal meshed filter is used for removing pulp fibers of the air-felt which might be contained in the acid solution after the substitution reaction. Next, the amount of hydrogen ions in the sampled acid solution in the titration cup is measured through titration. More specifically, a sodium hydroxide (NaOH) solution having a predetermined concentration of hydroxyl cations is added little by little to the acid solution until the neutralization of the acid solution. The amount or volume of the sodium hydroxide solution required for the neutralization is measured. Based on the amount of the sodium hydroxide solution required for the neutralization, the amount of hydrogen ions contained in the sampled acid solution (residual hydrogen ions) is calculated. Thus, the amount of hydrogen ions consumed by the reaction with AGM is calculated by subtracting the amount of residual hydrogen ions from the predetermined amount of hydrogen ions contained in the same volume of the original acid solution before reacting with AGM. As a result, the weight of AGM can be calculated based on the amount of the consumed hydrogen ions.

For example, when the AGM held in a general baby diaper is measured, about 100 ml of hydrochloric acid solution with a concentration of 0.3 N is used for reaction, about 10-100 ml of the hydrochloric acid solution which has reacted with AGM is sampled in a titration cup, and a sodium hydroxide solution with a concentration of 0.1 N is used for titrating the hydrochloric acid in measuring the amount of the residual hydrogen ions.

The above described back-titration can be carried out by using a commercially available autotitrator, such as a METTLER AUTOTITRATOR manufactured by NIHON SIBER HEGNER K.K., and a personal computer. In a most preferred embodiment, a calibration curve is employed for simpler calculations. The calibration curve indicates the relationship between the total weight of AGM and the amount of the consumed hydrogen ions in the titration cup. The amount of the consumed hydrogen ions can be replaced with the amount or volume of the sodium hydroxide solution required for the neutralization. The calibration curve data is preliminarily prepared by measuring a set of known weights of AGM by the titration using the sodium hydroxide (NaOH) solution which is to be used in the measurements. By referring the calibration curve, the total weight of the AGM is obtained from the amount or volume of the sodium hydroxide solution required for the neutralization. In preferred embodiments, a number of absorbent pads or samples are handled sequentially by one device. In a preferred embodiment, the device comprises a sample provider for keeping a plurality of rolled samples of absorbent pads in stock and for providing the samples one by one; a solution provider for providing a predetermined quantity of an acid solution into the reaction bath; a solution sampler for sampling the acid solution from the reaction bath after the acid solution is contacted with the AGM; and a back-titrator for measuring through titration the amount of residual hydrogen ions in the sampled acid solution and for calculating the amount of the AGM based on the result of the titration.

Preferably, the device includes a reaction bath having a number of gas holes provided in the inner wall thereof and capable of containing one piece of the rolled sample. In an alternative embodiment, the device has a vibrator which applies an ultrasonic wave to the absorbent pad through the acid solution in the reaction bath.

In a preferred embodiment, the sample provider is a rack equipped with trays, wherein a plurality of roll samples are arrayed laterally on each tray and the tray is set on the rack with an appropriate inclination. When a sample at the lowest end of a tray is taken out, the next and subsequent samples roll down, so that the samples can be picked up at one and the same position.

Preferably, the solution sampler includes a weighing device, wherein the acid solution is charged into a titration cup set on a platform of the weighing device so that a predetermined quantity of the acid solution is sampled. In a preferred embodiment, the device has a cup dispenser for dispensing titration cups one by one, because it is recommended to use a new titration cup every time a new sample is measured. In more preferred embodiments, the device further includes a transferring mechanism installed close to the sample provider, the reaction bath, the back-titrator, the weighing device and the cup dispenser. The transferring mechanism does the following movements: taking one sample from the sample provider and setting the sample in the reaction bath; taking one titration cup from the cup dispenser and setting the cup on the platform of the weighing device; and taking a titration cup in which an acid solution is charged by the solution sampler from the platform of the weighing device and setting the cup in the back-titrator. The transfer mechanism can be realized by a well-known industrial robot having an arm with a hand at the end, where the arm can extend telescopically, move vertically and rotatably and the hand can grip and hold an object vertically and horizontally. The arm is mounted at about the center of a fixed table and transfers a sample or a titration cup among the sample provider, the reaction bath, the back- titrator, the weighing device and the cup dispenser disposed around the arm according to respective demands. The movements of the arm and the hand are controlled sequentially according to a program installed in a computer to synchronize with the operations of other devices. Thus the series of the measuring works from supplying a sample to calculating the result of measurement can be automated. In a preferred embodiment, the device further comprises a waste container for dumping the residual acid solution and the absorbent pad remaining in the reaction bath after the solution sampler has sampled a predetermined quantity of the acid solution. The waste container may include: a bucket having a mesh for sustaining wasted absorbent pads and passing the waste acid solution; a filter for filtering the acid solution that has passed through the mesh; a reservoir for holding the acid solution that has passed through the filter; and an injection tube for injecting a neutralizer for neutralizing the acid solution in the reservoir.

With the waste container, the solid waste and the liquid waste of the measurement, i.e. the absorbent pad after the AGM is extracted and the acid solution after reacted with the AGM, can be collected separately. Another advantage is that the liquid waste becomes almost harmless since the acid solution is neutralized in the waste container.

The reaction time of AGM in the reaction bath is normally longer than the titration time in the back-titrator. Therefore, it is preferable to employ a plurality of reaction baths and conduct the processes in parallel with their time schedules shifted in order to enhance the efficiency of automatic measurement. For example, while the acid solution sampled from a first reaction bath is titrated by the back-titrator, the first reaction bath is washed and, concurrently, the next reacting process with another sample is conducted in a second reaction bath. By increasing the number of reaction baths regarding the processing capacity of the back-titrator, a large number of samples can be processed with higher efficiency. Most preferably, most of the processes subsequent to the preparation of the sample rolls are automated. In other words, only a limited parts of the process, such as preparing the sample rolls and dumping waste samples after measured, requires manual operation, and they do not require any special skill. Therefore, the measurement cost is reduced and the measurement efficiency is greatly enhanced. An example of automatic measurement device is explained in the following with reference to Figs. 4-6. Referring to Fig. 4, a revolving stand 3 having a telescopic arm 2 is mounted at the center of a fixed table 1. Around the revolving stand 3 are disposed: three sample racks 4a, 4b and 4c for storing horizontally a number of samples 109 inserted in sample tubes 110; an electronic balance 5 having the platform at the top; three cylindrical reaction baths 30a, 30b and 30c; a titrator 6; a cup dispenser 7 storing a number of titration cups; and a hand exchanging section 8 where a plurality of hands of different sizes to be attached to the tip of the arm 2 are prepared. Three multi-purpose nozzles 50a, 50b and 50c that can move independently in the vertical direction are provided over the three reaction baths 30a, 30b and 30c, respectively. A solution sampling tube 1 1 with a solenoid valve (not shown) is connected to each of the multi-purpose nozzles 50a, 50b and 50c, and all the solution sampling tubes 11 coming from the multi-purpose nozzles 50a, 50b and 50c are merged and connected to the inlet of a solution sampling pump 12. A tube extending from the outlet of the solution sampling pump 12 is connected to a nozzle placed above the platform of the electronic balance 5.

To each of the multi-purpose nozzle 50a, 50b and 50c, a hydrochloric acid solution injecting tube 13 with a solenoid valve (not shown) is also connected. All the hydrochloric acid solution injecting tubes 13 coming from the multi-purpose nozzles 50a, 50b and 50c are merged and connected to the outlet of a hydrochloric acid solution injecting pump 14. A tube connected to the inlet of the hydrochloric acid solution injecting pump 14 leads to a hydrochloric acid solution measuring container 16 set on the platform of another electronic balance 15. By the operation of a hydrochloric acid solution measuring pump 17, hydrochloric acid solution stored in a hydrochloric acid solution storing tank 18 is supplied to the hydrochloric acid solution measuring container 16. In addition, a washing water supply tube with a solenoid valve (not shown) is also connected to each of the multi-purpose nozzles 50a, 50b and 50c.

Each of the reaction baths 30a, 30b and 30c can be turned upside down about a horizontal axis independently. A waste container 19 is provided under the reaction baths. To the waste container 19, a sodium hydroxide solution for neutralizing the hydrochloric acid solution is supplied from a neutralizer storing tank 21 by a neutralizer transferring pump 20. After neutralized in the waste container 19, the waste solution is discharged through a waste liquid discharging line 22 to the outside. Further, a cup dumping port 23 and a sample tube dumping port 24 are provided in predetermined places of the fixed table 1, and collecting buckets (not shown) are disposed under the both dumping ports 23, 24. Though not shown in Fig. 4, peripheral devices including a personal computer, a printer and so on are also provided, whereby the operation of the device is controlled, the amount of AGM is calculated based on the result of measurement obtained in the titrator 6, and various statistical calculations are conducted when necessary. Fig. 5 is a side view of a sample rack. The rack is equipped with three trays

41 fixed to frames 42 by an adequate inclination. Each tray 41 has a capacity of 12 pieces of samples 109. An end face 41a of each tray 41 is partially cut away so that a sample 109 lying there can be gripped by the hand 9, whose operations will be detailed later. When the sample 109 lying in the lowest position in the tray 41 is gripped and taken out with the hand 9, the next sample 109 rolls down and comes to the lowest position. In the present example, one sample rack has a capacity of 36 pieces of samples 109, hence the device shown in Fig. 4 has a total capacity of 108 pieces of samples 109. Of course the capacity can be designed according to a demand.

Fig. 6 is a perspective view of a waste container. The waste container includes three buckets 91 disposed in the positions corresponding to the three reaction baths 30a, 30b and 30c. The bottom of the bucket 91 is made of a net having relatively large mesh. Under the buckets 91 is provided a two-stage filter 92 for thoroughly catching pulp fibers of the air-felt 101. The solution (i.e., the hydrochloric acid solution which has reacted with AGM) passing through the two-stage filter 92 is collected in a reservoir 93 disposed at the bottom of the waste container. To the reservoir 93 are connected a neutralizer injecting tube 94 for injecting a sodium hydroxide solution, and the waste liquid discharging line 22 for discharging the waste liquid to the outside.

The automatic measurement device constituted as above operates as follows. First, sample tubes 110, each holding a sample 109 rolled manually or automatically by an appropriate machine in the sample preparation process, are charged in the sample rack 4a, 4b and 4c. The hand 9 for gripping the sample tube 110 is attached to the arm 2 in the hand exchanging section 8, and the arm 2 takes one sample tube 1 10 holding a sample 109 from the sample rack 4a (4b or 4c) with the hand 9. Then the arm 2, holding it horizontally, moves the sample tube 110 to an empty reaction bath, e.g., the reaction bath 30a. There, the arm 2 rotates the hand 9 by 90o to turn the sample tube 110 upright so that the sample 109 slips down from the sample tube 110 into the reaction bath 30a.

After the sample tube 1 10 is turned upright, the multi-purpose nozzle 50a is moved down into the sample tube 1 10. Therefore, even if the sample 109 in the sample tube 110 is held rather tightly so that the sample 109 cannot slip down when the sample tube 1 10 is turned upright, the sample 109 is pushed by the multi-purpose nozzle 50a and fall down into the reaction bath 30a. After that, the arm 2 transfers the empty sample tube 110 to the sample tube dumping port 24 and throws it from the sample tube dumping port 24 into the collecting bucket disposed thereunder. Sample tubes 1 10 collected in the collecting bucket may be used again in the next measurement.

Next, a predetermined concentration and volume of hydrochloric acid solution is supplied to the reaction bath 30a. In advance to that, a predetermined weight of hydrochloric acid solution is measured by the electronic balance 15 and stored in the hydrochloric acid solution measuring container 16. After the sample tube 110 is dumped as described above, the multi-purpose nozzle 50a is moved down closer to the reaction bath 30a in which the sample 109 is present. Then the hydrochloric acid solution injecting pump 14 is started, whereby the predetermined weight of hydrochloric acid solution stored in the hydrochloric acid solution measuring container 16 is flown through the hydrochloric acid solution injecting tube 13 and injected from the multi-purpose nozzle 50a into the reaction bath 30a. On starting the injection of hydrochloric acid solution into the reaction bath

30a, an air supply is also started from the holes 31 at the bottom of the reaction bath 30a. Here, of course, other gases such as nitrogen or oxygen may be used instead of air. When all the hydrochloric acid solution stored in the hydrochloric acid solution measuring container 16 is injected, the sample 109 in the reaction bath 30a is completely immersed in the hydrochloric acid solution. After that, the AGM particles 102 in the absorbent pad 103 react with the hydrochloric acid solution rapidly owing to the agitation by the bubbles. The period for bubbling is determined appropriately as explained above. Generally, almost all the AGM particles contained in a normal disposable diaper react with hydrochloric acid solution in a few minutes as will be described later.

During the bubbling, the arm 2 moves to the hand exchanging section 8 and exchanges the hand 9 with another one for gripping a titration cup. Then the arm 2 takes out a titration cup from the cup dispenser 7 and sets it in a predetermined position on the platform of the electronic balance 5. After the cup is taken out by the arm 2, the cup dispenser 7 holding a number of plastic cups piled up therein dispenses a new one to the preset take-out position.

After the bubbling is conducted for a predetermined period of time, the air supply from the holes 31 into the reaction bath 30a is stopped. Then the solution (i.e., the hydrochloric acid solution which has reacted with AGM) in the reaction bath 30a is pumped up by the solution sampling pump 12 via the multi-purpose nozzle 50a and injected into the titration cup set on the platform of the electronic balance 5. When the electronic balance 5 gives a reading of a predetermined weight, the solution sampling pump 12 is stopped.

When an ultrasonic wave is employed instead of the air bubbling, the reaction bath shown in Fig. 8C is used. After a sample to be measured (not shown) is put into the reaction bath 34, the acid supply tube 35 supplies the hydrochloric acid solution to the reaction bath 34 so that the whole sample is completely immersed in the supplied hydrochloric acid solution. The vibrator or prove 36 which generates the ultrasonic wave is immersed in the acid solution and an ultrasonic wave of about 20 (kHz) is applied to the sample through the hydrochloric acid solution in the reaction bath for about 20 seconds. The output power is about 200 (W) or 300 (W). The prove 36 and vibration oscillator (not shown) used is an Ultrasonic Homogenizer Model US-300T as obtainable from the Nihon Seiki Seisakusyo K.K., Tokyo, Japan. The acid solution after the reaction is pumped up through the sampling tube 38 which has the filter 37 at the suction end. and is injected into the titration cup.

After that, the arm 2 with the hand 9 grips the titration cup containing the solution, transfers it to the titrator 6, and put it on a platform of the titrator 6. On detecting the placement of the titration cup, the titrator 6 sucks the solution in the titration cup into a burette and starts titrating the solution to measure the amount of hydrogen ions contained therein.

Meanwhile, after the predetermined weight of the solution is sampled as described above, the multi-purpose nozzle 50a is raised and the reaction bath 30a is turned upside down about a horizontal axis, so that the solution and the sample 109 remaining in the reaction bath 30a are dumped into the waste container 19. Among those dumped, the solid sample 109 is caught by the bucket 91. The solution, on the other hand, is filtered by the two-stage filter 92 and collected in the reservoir 93. Then a sodium hydroxide solution is injected from the neutralizer injecting tube 94 to the waste solution to neutralize it. The neutralized liquid is discharged as a waste liquid through the waste liquid discharging line 22 to the outside. The pH of the waste liquid is monitored with a pH meter (not shown) provided in the waste liquid discharging line 22 and the flow rate of the sodium hydroxide solution is regulated so that the monitored pH of the waste liquid is kept at about 7.

The emptied reaction bath 30a is turned again about the horizontal axis back to its original position, whereafter the multi-purpose nozzle 50a is lowered down into the reaction bath 30a. Then water is jetted from the multi-purpose nozzle 50a, so that pulp fibers sticking to the surface of the tip of the multi-purpose nozzle 50a is washed off by the water jet and the multi-purpose nozzle 50a is cleaned. Furthermore, pulp fibers and residual hydrochloric acid solution sticking to the inner wall of the reaction bath 30a are also washed off by the water jet. When the washing is conducted for a preset time period, the multi-purpose nozzle 50a is raised again and the reaction bath 30a is turned upside down about the horizontal axis again, whereby the washing water held in the reaction bath 30a is discharged into the waste container 19.

When a titration by the titrator 6 is completed, the arm 2 grips the titration cup used with its hand, transfers it from the titrator 6 to the cup dumping port 23, and throws it into the collecting bucket disposed thereunder.

The results of the titration are transmitted from the titrator 6 to the personal computer, which calculates the weight of the AGM per one sample based on the quantity of consumed hydrogen ions obtained through the back-titration by the titrator 6. The measurement of one sample 109 is thus completed. In the above described processes, a titration by the titrator 6 can be completed in about 1 to 2 minutes, whereas the reaction in the reaction bath usually requires more than several minutes. Therefore, the present device is equipped with the three reaction baths 30a, 30b and 30c to enhance the efficiency of measurement. By this device, the bubbling processes are carried out in parallel with shifted time schedules, so that while a bubbling process in one reaction bath is completed and a solution sampled therefrom is titrated by the titrator 6, the bubbling processes with new samples are conducted in the other two reaction baths. The control of the whole device is conducted by the personal computer according to a predetermined program.

Next, the constitution and operation of the multi-purpose nozzle is described with reference to Figs. 7A, 7B, 8A and 8B. The multi-purpose nozzle 50 is composed of: a tip 51 shaped substantially spherical and having a number of holes; a filter 52 of a fine mesh attached on the inner surface of the tip 51 ; and a cylinder 53 to which the tip 51 is attached. The solution sampling tube 1 1, the hydrochloric acid solution injecting tube 13 and a water injecting tube 54 are connected to the cylinder 53. The multi-purpose nozzle 50 functions in various ways as follows: (1) In putting the sample 109 in the reaction bath 30, the multi-purpose nozzle 50 is used to push the sample 109 out of the sample tube 110 (Fig. 7A). That is, the sample 109 may be held by the sample tube 110 either loosely or tightly depending on the thickness of the air-felt 101. If it is held tightly, the sample 109 cannot slip down from the sample tube 110 when the sample tube 1 10 is turned upright. Even in such a case, the sample 109 surely drops in the reaction bath 30 as the multi-purpose nozzle 50 is moved into the sample tube 1 10. (2) The multi-purpose nozzle is used to supply hydrochloric acid solution in the reaction bath 30 (Fig. 7B). In this process, the hydrochloric acid solution coming from the hydrochloric acid solution injecting tube 13 into the cylinder 53 is showered from the holes of the tip 51 into the reaction bath 30. Therefore, while filling the reaction bath 30, the hydrochloric acid solution can permeate the absorbent pad 103 rapidly.

(3) The multi-purpose nozzle is used to pump up the solution (i.e., the hydrochloric acid solution which has reacted with AGM) from the reaction bath 30 (Fig. 8 A). In this process, the multi-purpose nozzle is moved down until the tip 51 is sufficiently immersed in the hydrochloric acid solution and the solution is pumped up through the solution sampling tube 11. Pulp fibers of the air-felt 101 borne in the solution are completely filtered at the tip 51 with the holes and the mesh filter 52 attached inside. (4) The multi-purpose nozzle is used to wash the reaction bath 30 after it is used (Fig. 8B). In this process, the multi-purpose nozzle is lowered into the reaction bath 30, and washing water (e.g., distilled water) is supplied through the water injecting tube 54 at high pressure. Then the washing water spouts from the holes of the tip 51 , whereby pulp fibers that have stuck to the holes when the solution is pumped is washed off. Furthermore, the washing water also washes off pulp fibers and residual solution sticking to the inner wall of the reaction bath 30. Of course the residual solution in the cylinder 53 is washed off thereby.

Fig. 9 is a graph showing an example of the effect of the bubbling method. The vertical axis shows the reaction rate (%) of the AGM and the acid solution and the horizontal axis shows the elapsed time (minutes) after the bubbling was started. In this example, 0.3N of hydrochloric acid (HCL) solution was used as the acid solution and 0.1N of sodium hydroxide (NaOH) solution was used as the base solution for titration. Each sample of the absorbent pads used contained 6.0 grams of the AGM. As a comparison purpose, an example of no application of bubbling also shown in the graph. The comparison measurement was conducted under the same measurement condition (without bubbling) as that of the bubbling method. As shown in the graph, the bubbling method achieved over 95% of reaction within 5 minutes after the bubbling was started. Thus, this example shows the effectiveness of the bubbling method for promoting the AGM and the acid solution in comparison with the no bubbling application.

Fig. 10 is a graph showing an example of the effect of the ultrasonic wave method. The vertical axis shows the reaction rate (%) of the AGM and the acid solution, and the horizontal axis shows the elapsed time (minutes) after the ultrasonic wave application was started. In this example, 0.3N of hydrochloric acid (HCL) solution was used as the acid solution and 0.1N of sodium hydroxide (NaOH) solution was used as the base solution for titration. Each sample of the absorbent pads used contained 9.0 grams of the AGM. The output power of the Ultrasonic Homogenizer (Model US-300T; Nihon Seiki Seisakusyo) was 150 (W) and the frequency was about 60 (kHz). As a comparison purpose, an example of no application of ultrasonic wave also shown in the graph. The comparison measurement was conducted under the same measurement condition (without ultrasonic wave) as that of the ultrasonic wave method. As shown in the graph, the ultrasonic wave method achieved over 98% of reaction within 4 minutes after the bubbling was started. Thus, this example shows the effectiveness of the ultrasonic wave method for promoting the AGM and the acid solution in comparison with the no ultrasonic wave application. It is obvious to the person skilled in the art that the preferred embodiments as described above are just illustrative and the present invention can be varied and modified within the spirit and scope thereof, and all the variations and modifications falling within the technical scope of the appended claims are intended to be included in the scope of the present invention.

Claims

WHAT IS CLAIMED IS:

1. A method of measuring an amount of an absorbing gel material held in an absorbent pad, wherein said absorbent gel material contains a functional group having an alkali metal ion, said method comprising the steps of:

(a) bringing the absorbing gel material into contact with an acid solution containing a predetermined amount of hydrogen ions, said predetermined amount of hydrogen ions being more than the amount of the alkali metal ions contained in said absorbing gel material;

(b) substantially substituting the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in said acid solution; (c) measuring the amount of hydrogen ions remaining after the substitution; and

(d) calculating the amount of the absorbing gel material based on the measurement of the amount of the remained hydrogen ions.

2. The method according to claim 1, wherein said step (c) comprises the steps of applying a base solution to the acid solution after the step (b) until the acid solution is neutralized, and measuring the amount of the remaining hydrogen ions based on the amount of the solution of base required for the neutralization.

3. The method according to claim 1, wherein the acid solution contains an excessive amount of hydrogen ions compared to the alkali metal ions contained in the absorbing gel material so as to provide a promotion of the substitution.

4. The method according to claim 3, wherein the acid solution has a concentration of hydrogen ions of from about 0.05 (mol/ml) to about 30 (mol/ml).

5. The method according to claim 1, wherein said step (a) comprises the step of immersing the absorbent pad in the acid solution stored in a reaction bath.

6. The method according to claim 5, wherein said step (b) comprises the step of promoting the substitution of the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in said acid solution.

7. The method according to claim 6, wherein said step of promoting the substitution comprises the step of applying a physical vibration to the absorbent pad through the acid solution.

8. The method according to claim 7, wherein said step of applying a physical vibration comprises the step of applying a non-reactive gas to the absorbent pad through the acid solution so that the bubbles of the gas enter the absorbent pad and pass therethrough.

9. The method according to claim 7, wherein said step of applying a physical vibration comprises the step of applying a ultrasonic wave to the absorbent pad through the acid solution.

10. The method according to claim 5 wherein the non-reactive gas is an air.

1 1. The method according to claim 9 wherein the ultrasonic wave has a frequency selected from the range between about 10 (kHz) and about 100 (kHz).

12. The method according to claim 1 wherein said acid solution is selected from the group consisting of a hydrochloric acid (HCL) solution, a sulfuric acid (H2SO4) solution, an acetic acid (CH3COOH) solution, a nitric acid (HNO3) solution, a phosphoric acid (H3PO4) solution, an oxalic acid ((COOH)2) solution and a formic acid (HCOOH) solution.

13. The method according to claim 2 wherein said base solution is selected from the group consisting of a sodium hydroxide (NaOH) solution, a calcium di- hydroxide (Ca(OH)2) solution, a potassium hydroxide (KOH) solution, and a magnesium hydroxide (Mg(OH)2) solution.

14. The method according to claim 1 wherein said alkali metal ion is the sodium ion (Na⁺).

15. The method according to claim 5, wherein said step of immersing the absorbent pad comprises the steps of rolling the absorbent pad before the immersion, and holding the rolled absorbent pad substantially upright in said reaction bath after the immersion.

16. A device of measuring an amount of an absorbing gel material held in an absorbent pad, wherein said absorbent gel material contains a functional group having an alkali metal ion, said device comprising: means for bringing the absorbing gel material into contact with an acid solution containing a predetermined amount of hydrogen ions so that the alkali metal ions contained in the absorbing gel material are substantially substituted with hydrogen ions contained in said acid solution, said predetermined amount of hydrogen ions being more than the amount of the alkali metal ions contained in said absorbing gel material; means for measuring the amount of hydrogen ions remaining after the substitution; and means for calculating the amount of the absorbing gel material based on the measurement of the amount of the remained hydrogen ions.

17. The device according to claim 16, further comprises a promotion means for promoting the substitution of the alkali metal ions contained in the absorbing gel material with hydrogen ions contained in said acid solution.

18. The device according to claim 17, wherein said promotion means comprises a means for applying a physical vibration to the absorbent pad through the acid solution.

19. The device according to claim 18, wherein said means for applying a physical vibration comprises a means for applying a non-reactive gas to the absorbent pad through the acid solution so that the bubbles of the gas enter the absorbent pad and pass therethrough.

20. The device according to claim 19, wherein said means for applying a physical vibration comprises a means for applying a ultrasonic wave to the absorbent pad through the acid solution.

21. The device according to claim 19 wherein the non-reactive gas is an air.

22. The device according to claim 20 wherein the ultrasonic wave has a frequency selected from the range between about 10 (kHz) and about 100 (kHz).

23. The device according to claim 17, wherein the acid solution contains an excessive amount of hydrogen ions compared to the alkali metal ions contained in the absorbing gel material so as to provide the promotion of the substitution.