Hydrogel-forming polymer compositions for use in absorbent structures

Abstract

The present invention relates to improved hydrogel-forming polymer compositions which can be used as absorbents in absorbent structures and absorbent articles such as diapers, sanitary napkins and the like. Such hydrogel-forming polymer compositions are substantially water-insoluble, slightly cross-linked, partially neutralized polymers which are prepared from unsaturated polymerizable, acid group-containing monomers and cross-linking agents. These hydrogel-forming polymer materials, upon imbibing fluids, form hydrogels. Such polymer materials have relatively high gel volume and relatively high gel strength as measured by shear modulus of the hydrogen which forms therefrom. Such polymer materials also contain relatively low levels of extractable polymer material which can be extracted therefrom by contact with synthetic urine. Preferred hydrogel-forming polymers having these characteristics can be prepared by polymerizing the acid group-containing monomers in their free acid form at relatively low monomer concentrations, preferably using relatively low polymerization temperatures. Absorbent structures and absorbent articles containing these dried hydrogel-forming polymer materials are also disclosed.

Description

2000or "especially preferred" typesacid base required to titrate 20 ml of synthetic urine to pH 10.

N_a =The normality (in meq/ml) of the acid base nominally 0.10 meq/ml)

2. The total amount of polymerized acid base moieties (e.g. acrylic acid) plus polymerized neutralized acid moieties (e.g., sodium acrylate) (in mm) in the supernatant aliquot (M_t) is given by:

M_t =(V_b -V_bb)×N_b millimoles

where:

V_b =The volume (in ml) of base required to titrate the aliquot from pH 10 down to pH 2.7.

V_bb =The volume (in ml) of base acid required to titrate 20 ml of synthetic urine from pH 10 down to pH 2.7.

N_b =The normality (in meq/ml) of the base acid (nominally 0.10 meq/ml).

3. The amount of polymerized neutralized acid moieties (e.g., sodium acrylate) (in mm) in the original supernatant aliquot (M_b) is given by:

M_b =M_t -M_a

4. The total amounts of polymerized acid moieties (W_a) and polymerized neutralized acid moieties (W_b) (e.g., acrylic acid plus sodium acrylate) extracted (in mg) are given by:

W_a =M_a ×E_a ×D and W_b =M_b ×E_b ×D

where:

E_a =The equivalent weight of acid moiety in polyacid moiety (e.g., acrylic acid in polyacrylic acid=72 meq/mg). mg/meq.

E_b =The equivalent weight of neutralized acid moiety in neutralized polyacid moiety (e.g., sodium acrylate in sodium polyacrylate=94 meq/mg). mg/meq.

D=The dilution factor (75 ml/20 ml/=3.75).

5. The percent extractable polymer in the hydrogel-forming polymer sample (e) is given by:

e=[(W_a +W_b)×100]/W percent

where: W=The sample weight in mg.

2. Sulfonic Acid-Containing Hydrogel-Forming Polymers

Extractable polymer content of sulfonic acid-based hydrogel-forming polymers is determined by a gravimetric procedure wherein hydrogel samples are swollen overnight in distilled water, and the polymer content in the filtrate is gravimetrically determined. By comparing extractable content determinations for carboxylic acid-based hydrogel-forming polymers, using both the potentiometric method hereinbefore described and the gravimetric method, it has been determined that the extractables readings given by the gravimetric method using distilled water overnight provides acceptable correlation with extractables determined by the 16-hour synthetic urine procedure used in the potentiometric method.

The particular procedure of the gravimetric extractables determination are is set forth as follows:

Into a 500 ml Erlenmeyer flask is weighed accurately (to ±0.1 mg) about 0.25 grams of dry hydrogel-forming polymer (W_p). 250 ml of distilled water is added, and the mixture is stirred slowly for 1 hour. After this hour has passed, stirring is stopped, and the swollen gel is allowed to settle overnight. In the morning enough of the supernatant is filtered using a 3 ml disposable syringe and 0.22 micron filter to obtain at least 40 ml of filtrate. Exactly 40 ml of filtrate is placed into a clean 100 ml round-bottomed flask, and the solution is concentrated on a rotary evaporator (water aspirator vacuum, bath temperature 55° C). The remaining 2-3 ml of solution is transferred quantitatively to a tared weighing vial with the aid of additional distilled water. The solution in the weighing vial is reduced to dryness in an oven at 120°C The vial is cooled, reweighed, and the weight of residue (W_r) is determined using the tare weight of the vial. The percent extractable polymer (e) is calculated from the weight of dry polymer (W_p) and weight of residue (W_r) by the following equation. ##EQU4## As indicated, the extractables value obtained from this calculation is believed to approximately correspond to a 16-hour equilibrium extractables content value in synthetic urine.

EXAMPLE I

Four hundred and fifty parts of doubly distilled water were placed in a reaction vessel equipped with a mechanical stirrer, argon inlet, thermometer, and pressure-equalizing addition funnel containing 0.136 parts of N,N-methylenebisacrylamide dissolved in 92 parts of acrylic acid. The water and contents of the funnel were separately purged with argon through submerged gas dispersion tubes for 1 hour at 25°C The water was then cooled to 10°C while being vigorously stirred; 0.05 parts of ascorbic acid dissolved in 2 parts of water was added, immediately followed by 0.10 parts of a 30% hydrogen peroxide solution dissolved in 2 parts of water.

The contents of the addition funnel were then added to the solution. Within 5 minutes, the mixture formed a clear gel which could no longer be stirred. While maintaining external cooling at 6°C, the temperature of the gel rose to 30°C after 30 minutes and then began to fall. The gel was then heated to a temperature of 40°C for 3 hours. A portion of this gel (63.1 parts) was withdrawn from the flask and placed in a breaker containing 4.2 parts of sodium hydroxide dissolved in 80 parts of water.

The gel was chopped thoroughly until it had imbibed all of the surrounding fluid and was kept at 40°C for 16 hours. The transparent, rubbery particles were then added to 500 parts of methanol at 40°C and further chopped, converting them to opaque, sticky particles. The supernatant fluid was then removed, and 500 parts of fresh methanol were added and chopping was continued. A final decantation/addition/chopping produced hard particles which were stirred for 2 hours at 40°C and then isolated by filtration. These particles were dried under high vacuum at 60°C for 3 hours and pulverized to obtain 16.4 parts of a white powder.

Such powder had a gel volume of 59 g/g and an extractables content of 3.7%, The hydrogel formed from the powder had a shear modulus of 4.71×10³ dynes/cm². Degree of neutralization was 70%.

Example II

Another portion of the neutralized gel particles prepared as in Example I (before treatment with methanol) was dried directly at 80°C under high vacuum for 16 hours, pulverized, and redried for one hour. The resulting white powder has a gel volume of 45 g/g and an extractables content of 3.9% The hydrogel formed from this powder had a shear modulus of 9.68×10³ dynes/cm². Such a sample also has a degree of neutralization of 70%.

Example III

Four hundred and fifty parts of doubly distilled water, 92 parts of acrylic acid, and 1.87 parts of N,N'-methylenebisacrylamide were placed in a reaction vessel equipped with a mechanical stirrer, thermometer, and argon inlet. The solution was stirred at 25°C and purged with argon through a submerged dispersion tube for 1 hour. The solution was then vigorously stirred and cooled to 11°C; 0.025 parts of ascorbic acid dissolved in 2 parts of water, immediately followed by 0.050 parts of a 30% hydrogen peroxide, solution dissolved in 2 parts of water, were added. Within 3 minutes the mixture gelled and stirring was no longer possible. While maintaining external cooling at 2°C-5°C, the reaction temperature rose to 16°C after 30 minutes and then began to fall.

After heating the mixture to 40°C for three hours, the brittle gel was broken up and ground in a mortar and pestle to give small rubbery particles. A solution of 35.4 parts of sodium hydroxide in 1200 parts of water was added to the particles. After thorough mixing, all excess fluid had been imbibed.

The resulting gel particles were kept at 40°C for 16 hr. One portion of the gel particles was treated repeatedly with methanol and dried as described in Example I. After pulverization and an additional hour of drying, a white powder was obtained having a gel volume of 27 g/g and an extractables content of 0.4%. The powder forms a hydrogel having a shear modulus of 3.92×10⁴ dynes/cm². Degree of neutralization was 70%.

Another portion of the gel particles was directly dried under high vacuum at 80°C for 16 hours before grinding and for an additional one hour after drying to give a white powder having a gel volume of 26 g/g and an ectractables content of 0.9%. This powder forms a hydrogel having a shear modulus of 4.58×10⁴ dynes/cm². Degree of neutralization was also 70%.

EXAMPLE IV

A polymerization was carried out using the amounts of materials and methods of Example I, except that 0.748 parts of N,N'-methylenebisacrylamide were used. The final white powder has a gel volume of 37 g/g and an extractables content of 0.7%. The hydrogel formed from the powder has a shear modulus of 1.95×10⁴ dynes/cm². Degree of neutralization was 70%.

EXAMPLE V

A polymerization was carried out using the amounts of materials and methods of Example I, except that 0.374 parts of N,N'-methylenebisacrylamide were used. The final white powder obtained has a gel volume of 48 g/g and an extractables content of 2.1%. The hydrogel formed from the powder had a shear modulus of 1.11×10⁴ dynes/cm². Degree of neutralization was 70%.

EXAMPLE VI

A polymerization was carried out using the amounts of materials and methods of Example V, except that 0.025 parts of ascorbic acid dissolved in 2 parts of water and 0.050 parts of a 30% hydrogen peroxide solution dissolved in 2 parts of water were used as the initiator system. The final white powder that was obtained has a gel volume of 46 g/g and an extractables content of 1.1%. The hydrogel formed from this powder had a shear modulus of 9.94×10³ dynes/cm². Degree of neutralization was 70%.

EXAMPLE VII

Four hundred and forty parts of doubly distilled water, 92 parts of acrylic acid, and 0.374 parts of N,N'-methylenebisacrylamide were placed in a reaction vessel equipped with a mechanical stirrer, thermometer, argon inlet, and pressure-equalizing addition funnel. The contents of the flask were stirred gently while being purged with argon through a submerged dispersion tube for 1 hour at 25°C Half-way through this purge, the addition funnel was charged with 0.050 parts of a 30% hydrogen peroxide solution dissolved in 9 parts of water; this solution was also similarly purged with argon. After the purge period, the stirring rate was increased slightly, and the solution temperature was equilibrated with that of an exterior cooling bath at 12°C

In a slow, dropwise manner, a portion of the contents of the addition funnel was added until a 0.25°C reaction temperature rise coupled with a noticeable viscosity increase appeared. This occurred after about 2 parts of the hydrogen peroxide solution had been added in 13 minutes. Shortly thereafter, stirring was no longer possible. After 90 minutes, the gel reaction temperature reached a maximum of 22°C and began to fall.

At this time the gel was heated to 40°C for three hours. A portion of this gel (69.9 parts) was withdrawn and added to a solution of 4.93 parts of sodium hydroxide in 80 parts of water. The gel was chopped thoroughly until it had imbibed all of the surrounding fluid. It was then left at 40°C for 16 hours.

Treatment of the resulting rubbery particles with methanol, drying, and pulverizing by the method of Example I gave 20.1 parts of a white powder. A portion of this powder was pulverized further and dried for an additional 1 hour to give a material having a gel volume of 47 g/g and an extractables content of 0.9%. The hydrogel formed from this material has a hear modulus of 1.11×10⁴ dynes/cm². Degree of neutralization was 70%.

When a portion of the neutralized rubbery particles was directly dried under high vacuum at 80°C for 16 hours (no methanol treatment) followed by the same pulverization and redrying, a white powder was obtained having a gel volume of 45 g/g and an extractables content of 2.2%. This powder forms a hydrogel having a shear modulus of 1.24×10⁴ dynes/cm². Degree of neutralization was also 70%.

This example illustrates the hydrogel-forming materials of especially, low extractables content can be prepared using the minimum initiator concentration technique.

EXAMPLE VIII

A polymerization was carried out using the amounts of materials and methods of Example I except that 0.725 parts of trimethylol propane triacrylate were used in place of N,N'-methylenebisacrylamide. The final white powder that was obtained has a gel volume of 41 g/g and an extractables content of 2.1%. The hydrogel formed from this powder has a shear modulus of 1.35×10⁴ dynes/cm². Degree of neutralization was 70%.

EXAMPLE IX

Four hundred fifty parts of doubly distilled water, 92 parts of acrylic acid, and 0.748 parts of N,N'-methylenebisacrylamide were placed in a reaction vessel equipped with a mechanical stirrer, thermometer, and argon inlet. The solution was stirred at 25°C and purged with argon by submerged dispersion tube for 1 hour. The temperature of the solution was raised to 64°C, the stirring speed was increased, and 0.051 parts of potassium persulfate dissolved in 3 parts of water were added.

Within 45 seconds, the solution had gelled, and stirring was no longer possible. While maintaining the external temperature at 65°C, the reaction temperature rose to 84°C after 19 minutes and then began to fall. After maintaining the external temperature at 40°C for three hours, 123.4 parts of this gel were chopped in the presence of 8.12 parts of sodium hydroxide dissolved in 160 parts of water. When all excess fluid had been imbibed, the resulting rubbery particles were left at 40°C for 16 hours. These particles were dried and ground without methanol in the manner of Examples VII.

A white powder having a gel volume of 41 g/g and an extractables content of 3.9% was obtained. The hydrogel formed from this powder had a shear modulus of 1.43×10⁴ dynes/cm². The degree of neutralization was about 70%.

EXAMPLE X

Three hundred sixty three parts of doubly distilled water, 187 parts of acrylic acid, and 0.080 part of N,N-methylenebiacrylamide were dissolved and purged as in Example IX. To the quickly stirred solution cooled to 10°C were added 0.05 parts of ascorbic acid dissolved in 3 parts of water quickly followed by 0.100 parts of hydrogen peroxide solution dissolved in 3 ml of water.

After 45 seconds, the reaction mixture had gelled and could no longer be stirred. Within 17 minutes, the reaction temperature had increased to 65°C while maintaining external cooling below 10°C When the temperature had fallen to 40°C, it was maintained there for 3 hours. A portion of the resulting extremely tough, rubbery gel (60.1 parts) was chopped in the presence of 7.9 parts of sodium hydroxide dissolved in 160 parts of water. It was left at 40°C for 16 hours.

After drying and grinding as in Example IX, a white powder having a gel volume of 38 g/g and extractables content of 9.0% was obtained. The hydrogel formed from this powder had a shear modulus of 1.24×10⁴ dynes/cm². Degree of neutralization was about 70%.

EXAMPLE XI

Five hundred forty parts of doubly distilled water, 60 parts of acrylic acid, and 1.22 parts of N,N'-methylenebisacrylamide were dissolved and purged as in Example X. To the quickly stirred solution cooled to 14°C were added 0.02 parts of ascorbic acid dissolved in 3 parts water followed by 0.04 parts of hydrogen peroxide solution dissolved in 3 parts water. After 7 minutes, the viscosity of the solution prevented further stirring. After 114 minutes, the reaction temperature reached a high point of 17°C and began to fall. The gel was then heated at 40°C for three hours. A portion (113.2 parts) of the resulting brittle gel was broken up into small particles in a mortar and pestle and treated with 4.39 parts of sodium hydroxide dissolved in 80 parts of water.

After standing at 40°C for 16 hours, the particles were dried and ground as in Example X to give a white powder having a gel volume of 43 g/g and an extractables content of 0.6%. The hydrogel formed from this powder had a shear modulus of 1.31×10⁴ dynes/cm². Degeee of neutralization was about 70%.

EXAMPLE XII

Four hundred and fifty parts of doubly distilled water, 92 parts of acrylic acid, and 0.040 parts of N,N'-methylenebisacrylamide were placed in a reaction vessel equipped with a mechanical stirrer, thermometer, and argon inlet. The solution was stirred at 25°C and purged with argon by submerged dispersion tube for 1 hour. The stirring speed was then increased, and the solution temperature was lowered by external cooling to 11.8°C 0.025 parts of ascorbic acid dissolved in 2 parts of water were added. A solution of 0.052 parts of 30% hydrogen peroxide in 9 parts of water were placed in an addition funnel and added drop by drop to the stirred reaction mixture. When six drops had been added, a viscosity increase was noted, and the addition was terminated Within 3 minutes, stirring was stopped, and after 41 minutes the reaction temperature reached 23°C and then began to fall. At this time the reaction mixture was heated at 40°C for 3 hours.

A portion of this gel (118 parts) was thoroughly chopped in the presence of 7.74 parts of sodium hydroxide dissolved in 160 parts in of water until all excess fluid had been imbibed. The resulting rubbery particles were kept at 40°C for 16 hours and then treated with methanol and dried in the manner of Example I. A white powder was obtained which has a gel volume of 74 g/g and an extractables content of 13%. The hydrogel formed from this powder had a shear modulus of 3.27×10³ dynes/cm². Degree of neutralization was about 70%.

EXAMPLE XIII

This example illustrates preparation of a polyacrylate hydrogel-forming material using an inverse suspension polymerization technique.

A four-neck, 1 liter round-bottom resin kettle equipped with a stirrer, a reflux condenser, a dropping funnel, and an inert gas dispersion tube was charged with 430 ml of cyclohexane and 2.57 g of ethyl cellulose (Aldrich Chemical Company, Inc., ethoxyl content 48%). Argon gel was blown into the flask to purge dissolved oxygen, and the temperature elevated to 65°C In a separate flask, 28.0 g of acrylic acid was dissolved in 111.8 g of distilled water. The monomer concentration in the aqueous phase was 20% by weight. Then, 0.0128 g of potassium persulfate and 0.1798 g of N,N'-methylenebisacrylamide were dissolved in the aqueous solution, and argon was introduced into the solution to remove oxygen present therein.

The contents of the latter flask were added dropwise to the contents of the above mentioned four-neck flask over a period of one-half hour. After completion of the dropwise addition, reaction was carried out at 65°C for four hours, and then the reaction mixture was cooled to ambient temperature. Separately, in a flask, 23.2 g of 50.2% (w/w)sodium hydroxide solution is dissolved in 8.8 g distilled water and added dropwise to the contents of the four-neck flask. Following complete addition, the contents of the flask are allowed to stir for 20 minutes before the swollen polymer was isolated by filtration, allowed to stand overnight and then dried under reduced pressure at 80°C to obtain spherical particles of a polymer product with gel volume, v, of 34 g/g, a hydrogel shear modulus of 21,300 dynes/cm² and extractables, e, of 3.5%. Degree of neutralization was 75%. The value for (0.23v-3.0) is 4.82 which is greater than the 3.5% value for e.

EXAMPLE XIV (Comparative)

This example also illustrates preparation of a polyacrylate-type hydrogel-forming material using an inverse suspension polymerization technique. In this example, however, a monomer concentration above that used in the process of the present invention is employed. Furthermore, the monomer used was 75% neutralized (sodium acrylate) instead of being predominantly in the free acid form as required by the process of the present invention.

In this example the same polymerization vessel as used in Example XIII was charged with 470 ml of cyclohexane and 2.80 g ethyl cellulose. Inert gas was introduced to expel dissolved oxygen therefrom, and the temperature raised to 65°C In a separate flask, 57.0 g acrylic acid was neutralized with 47.3 g of 50.2% (w/w) sodium hydroxide solution dissolved in 54.9 g distilled water while externally cooling the charge. The monomer concentration in the aqueous phase was 44% by weight. Then, 0.0260 g potassium persulfate and 0.0073 g N,N'-methylenebisacrylamide were dissolved in the aqueous solution, and argon was blown into the solution to remove dissolved oxygen.

The contents of the latter flask were added dropwise over a period of one-half hour to the aforementioned four-neck flask, after which reaction was carried out at 65°C for four hours before temperature was returned to ambient.

The polymer is isolated in a swollen state by filtration and dried under reduced pressure at 80°C to yield spherical particles of polymer having a gel volume, v, of 35 g/g, a hydrogel shear modulus of 11,800 dynes/cm², and extractables, e, of 10.3%. (Some characteristics of the polymer were determined as an average of several runs.) The degree of neutralization was 75%. The value for (0.23v-3.0) is 5.05 which is not greater than the 10.3% value for e. Thus, this example illustrates the importance of utilizing acid form monomer at relatively low concentration if "preferred" hydrogel-forming material of especially low extractables is to be realized.

EXAMPLE XV

This example illustrates a reverse suspension polymerization procedure wherein water is removed from the polymer by azeotropic distillation prior to neutralization of the polymer.

A four-neck, 1 liter round bottom resin kettle equipped with a stirrer, a reflux condenser, a dropping funnel, and an inert gas-dispersion tube was charged with 430 ml cyclohexane and 2.57 g ethyl cellulose (Aldrich Chemical Company, Inc., ethoxyl content 48%). Argon gas was blown into the flask to purge dissolved oxygen, and the temperature elevated to 65°C In a separate flask, 28.0 g acrylic acid was dissolved in 111.9 g distilled water. The monomer concentration in the aqueous monomer solution was 20% (water content: 80%). Then, 0.0064 g of potassium persulfate and 0.0599 g N,N'-methylenebisacrylamide were dissolved in the aqueous solution, and argon was introduced to remove oxygen therein. The resulting monomer solution was fed dropwise to the four-necked flask in an argon atmosphere in the course of 0.75 hours to effect polymerization and was subsequently held at 65°C for four hours to complete the polymerization.

Thereafter, the water content of the polymer suspended in cyclohexane was adjusted to 55% by azeotropic distillation. The polymer beads were isolated by filtration and resuspended in 450 ml of fresh cyclohexane at ambient temperature. Separately, in a flask, 23.2 g of 50.2% (w/w) sodium hydroxide solution is dissolved in 87.4 g distilled water and the resulting solution is added dropwise over a period of 0.5 hours to the polymer suspended in cyclohexane. Following complete addition, neutralization is continued for 1.5 hours before the swollen polymer is isolated by filtration and dried under reduced pressure at 80°C to obtain spherical beads of polymer product with a gel volume of 51 g/g, a hydrogen shear modulus of 7,570 dynes/cm², and extractables of 4.8%. The degree of neutralization of the material was 75%. Some characteristics of the polymer were determined as an average of several runs.

EXAMPLE XVI

This example illustrates the preparation of a sulfonic acid-containing polymer which forms a hydrogel having the characteristics of those of the present invention by virtue of the use of the free acid monomers in relatively low concentrations.

A 4-necked, 1-liter round-bottomed resin kettle equipped with a stirrer, a reflux condenser, a jacketed dropping funnel, and an inert gas dispersion tube was charged with 510 ml of cyclohexane and 3.06 g ethyl cellulose (Aldrich Chemical Co., Inc., ethoxyl content 48%). Argon gas was introduced to expel dissolved oxygen, and the temperature was elevated to 65°C

In a cooled flask, 25.2 g of acrylic acid was dissolved in 132.8 g of distilled water, to which was added 0.0128 g of potassium persulfate and 0.1797 g N,N'-methylenebisacrylamide. Then 8,0538 g of 2-acrylamido-2-methylpropane sulfonic acid (Lubrizol special process reaction grade II) was added to the aqueous solution, and argon was blown into the solution to remove oxygen present therein. The monomer concentration in the aqueous phase was 20% by weight. The contents of this flask were added from the cooled addition funnel dropwise over a period of about one-half hour to the contents of the aforementioned 4-necked kettle.

After completion of the dropwise addition, reaction was carried out at 65°C for 4 hours, and then the reaction mixture was cooled to ambient temperature. Separately, in a flask, 23.2 g of 50.2% (w/w) NaOH solution was dissolved in 127.8 g of distilled water and added dropwise over the course of an hour to the contents of the 4-necked kettle. Following complete addition, the contents of the kettle were allowed to stir for 4 hours before the swollen polymer was isolated by filtration and dried under reduced pressure at 80°C to obtain spherical particles of 75% neutralized polymer. This polymer has a gel volume of 38 g/g, and extractables of 11% and a hydrogel shear modulus of 13,100 dynes/cm² . Some characteristics of the polymer were determined as an average of several runs.

EXAMPLE XVII (Comparative)

This example illustrates preparation of a sulfonicacid-containing polymer which forms a hydrogel having a higher extractables content than the hydrogels formed from polymers of this invention. Such high extractable materials result from the polymerization of sodium salt monomers in relatively high concentration.

The same polymerization vessel as in Example XVI was charged with 475 ml hexanes and 3.75 g SPAN 60 surfactant (sorbitan monostearate). Inert gas was introduced to expel dissolved oxygen therefrom, and the temperature was raised to 40°C

In a separate flask, 51.3 g of acrylic acid was neutralized with 41.1 g of 50% (w/w) NaOH solution dissolved in 47.9 g distilled water while externally cooling the charge. Following neutralization, 35.1 g of 51.6% (w/w) sodium 2-acrylamido-2-methylpropane sulfonate solution was added along with 0.0074 g N,N'-methylenebisacrylamide and 0.00260 g potassium persulfate. The monomer concentration in the aqueous phase was 46% by weight. Argon gas was blown into the aqueous phase to remove any dissolved oxygen. The contents of the latter flask were added dropwise over a period of one-half hour to the four-necked kettle, after which reaction was carried out at 65° for 3 hours before the temperature was allowed to return to ambient. The swollen polymer was isolated by filtration and dried at 80°C under reduced pressure to yield spherical particles of a 75% neutralized polymer product. This polymer has a gel volume of 35 g/g, and extractables of 49% and a hydrogel shear modulus of 9,460 dynes/cm². Some characteristics of the polymer were determined as an average of several runs.

EXAMPLE XVIII

A disposable diaper is prepared comprising a polypropylene topsheet, two tissue plys, an absorbent core, a liquid impervious polyethylene backing sheet containing elastic leg bands along each side of the completed diaper and two tape fasteners. The absorbent core is an hourglass-shaped mixture of wood pulp fibers (airfelt) and particles of a water-insoluble, slightly cross-linked, partially neutralized, substantially dry hydrogel-forming polymer of this invention. The diaper is hand assembled using double-sided tape to fasten the individual components together. The diaper core is described in greater detail in the following Table I:

TABLE I
______________________________________
Feature            Value
______________________________________
Core shape         Hourglass
Hydrogel-forming polymer
Polyacrylate of Example 1
Airfelt concentration
85% by weight
Hydrogel-forming polymer
15% by weight
concentration
Total surface area 93.6 in.2 (604 cm2)
Crotch area        46 in.2 (297 cm2)
Basis weight of crotch
1.64 g/in.2 (0.254 g/cm2)
Core weight        33.0 grams
______________________________________

Such a diaper article is especially effective with respect to total fluid capacity and low incidence of diaper failure.

EXAMPLE XIX

Disposable diapers containing partially (i.e., about 75%) neutralized, hydrogel-forming polymer absorbents of varying characteristics are prepared in a manner substantially similar to that described in Example XVIII. These diapers are tested by panels of fifty mothers over a ten-day period in comparison with a control diaper which is a commercially marketed disposable diaper product containing no absorbent hydrogel-forming polymer. Each panelist receives sixty diapers, thirty of the test diaper and thirty of the control.

Each of the two diaper types is tested for five days. Mothers are asked to keep diaries concerning the percentage of each diaper type which leaks and to provide an indication of their overall preference of diaper type. In tabulating results concerning the percentage of panelists having a preference for the test diapers, it should be noted that the percentage of mothers expressing no preference for either the test or control diaper was divided equally between the test and control diapers.

A description of the characteristics of the hydrogel-forming polymer used in the test diapers as well as test results are set forth in Table II.

TABLE II
__________________________________________________________________________
Hydrogel-                     % of     Favorable
Forming
Gel     1-Hour  16-Hour Test
% of Preference
Polymer
Volume  Extractables
Extractables
Diaper
Control
For Test
No.   (grams/grams)
(% by weight)
(% by weight)
Leaked
Leaked
Diaper (%)
__________________________________________________________________________
1.    27.3    2.4      6.3    11  18   76
2.    25.5    3.0      5.4     7  13   66
3.    26.6    4.4      8.2     9  18   67
4.    21.7    4.4     10.6    NA  NA   74
5.    27.9    5.6     11.8    10  15   79
6.    32.3    7.2     16.1     9  15   76
7.    27.3    4.5      5.5    10  16   69
8.    37.4    14.9    16.5    NA  NA   51
9.    27.9    16.3    31.7    18  15   40
__________________________________________________________________________
NA = Not Available

It can be seen from the Table II data that diapers containing hydrogel-forming polymers having the characteristics as set forth in the present invention generally had fewer instances of diaper leakage and were generally highly preferred by mothers in comparison with the control diaper. Diapers with Polymer Numbers 8 and 9, on the other hand, are those wherein the polymers are those having significantly higher levels of extractables than the polymers of the present invention. Diapers with these two polymer types have a significantly higher incidence of leakage and are furthermore not significantly preferred by mothers over the control diaper.

Claims

1. A substantially water-insoluble, slightly cross-linked, partially neutralized, hydrogel-forming polymer composition consisting essentially of

(a) from about 50 mole percent to 99.999 mole percent of polymerized unsaturated, polymerizable, acid group-containing monomers; and

(b) from about 0.001 mole percent to 5 mole percent of a cross-linking agent; wherein said composition has a degree of neutralization of at least about 25% and is substantially free of graft polymerizable polymer moieties; and further wherein said polymer composition, upon neutralization to a degree of neutralization of at least 50%, has or would have a gel volume of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer exhibits a shear modulus of at least about 2000 3270 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium extractable polymer content, at equilibrium in synthetic urine, of no more than about 17% by weight of hydrogel-forming polymer.

2. A hydrogel-forming polymer composition according to claim 1 wherein

(a) said composition has a degree of neutralization of at least about 50%;

(b) said composition has a gel volume of from about 25 to 80 grams of synthetic urine per gram of hydrogel-forming polymer;

(c) the hydrogel formed from said composition has a gel strength such that the hydrogel exhibits a shear modulus of from about 2500 4710 to 92,000 dynes/cm²,

(d) said composition has an initial extractable polymer content, after one hour in synthetic urine, of no more than about 5% by weight of hydrogel-forming polymer; and

(e) said composition has an equilibrium extractable polymer content, at equilibrium in synthetic urine, of no more than about 10% by weight of hydrogel-forming polymer.

3. A hydrogel-forming polymer composition according to claim 2 wherein

(a) said acid-group containing monomers are selected from acrylic acid, methacrylic acid, and 2-acrylamido-2-methylpropane sulfonic acid, and combinations thereof; and

(b) said cross-linking agent is selected from

(i) di- and polyvinyl compounds;

(ii) di- and polyesters of unsaturated mono- and polycarboxylic acids with polyols;

(iii) bisacrylamides;

(iv) carbamyl esters obtained by reacting polyisocyanates with hydroxyl group-containing monomers;

(v) di- and polyallyl ethers of the polyols;

(vi) di- and polyallyl esters of polycarboxylic acids;

(vii) esters of unsaturated mono- and polycarboxylic acids with mono-allyl esters of polyols; and

(viii) di- and triallyl amines.

4. A substantially water-insoluble, slightly cross-linked, partially neutralized, hydrogel-forming polymer composition consisting essentially of:

(a) from about 75% to 99.99% mole percent of polymerized acrylic acid monomers; and

(b) from about 0.01% to 3% mole percent of a cross-linking agent selected from N,N'-methylenebisacrylamide, trimethylol propane triacrylate and triallyl amine;

wherein said composition has at least 50% of its acrylic acid groups neutralized with sodium cations and is substantially free of graft polymerized polymer moieties, and further wherein said polymer composition has a gel volume, v, of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer exhibits a shear modulus, s, of at least about 2000 3270 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium extractable polymer content, e, at equilibrium in synthetic urine, of no more than about 17% by weight of hydrogel-forming polymer.

5. A hydrogel-forming polymer composition according to claim 4 wherein the relationship between gel volume, v, and equilibrium extractable polymer content, e, is defined by the equation: e≦0.23v-3∅

6. A hydrogel-forming polymer composition according to claim 4 wherein the relationship between gel volume, v, and equilibrium extractable polymer content, e, is defined by the equation: e≦0.23v-3.0; e≦0.073v-0.37 and wherein the relationship between gel volume, v, and shear modulus, s, is defined by the equation:

log s≧-2.494 log v+8.090.

7. A substantially water-insoluble, slightly cross-linked, partially neutralized, hydrogel-forming polymer composition consisting essentially of

(a) from about 75 mole percent to 99.99 mole percent of polymerized unsaturated, polymerizable, acid group-containing monomers; and

(b) from about 0.01 mole percent to 3 mole percent of a cross-linking agent;

wherein said composition has a degree of neutralization of at least about 25% and is substantially free of graft polymerizable polymer moieties; and further wherein said polymer composition, upon neutralization to a degree of neutralization of at least 50%, has or would have a gel volume, v, of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer exhibits a shear modulus, s, of at least about 2000 3270 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium extractable polymer content, e, at equilibrium in synthetic urine, of no more than about 17% by weight of hydrogel-forming polymer, and further wherein the relationship between gel volume, v, and equilibrium extractable polymer content, e, is defined by the equation: e≦0.23v-3∅

8. A hydrogel-forming polymer composition according to claim 7 wherein

(a) said composition has a degree of neutralization of at least about 50%;

(b) said composition has a gel volume of from about 25 to 80 grams of synthetic urine per gram of hydrogel-forming polymer;

(c) the hydrogel formed from said composition has a gel strength such that the hydrogel exhibits a shear modulus of from about 2500 4710 to 92,000 dynes/cm²

(d) said composition has an initial extractable polymer content, after one hour in synthetic urine, of no more than about 5% by weigth of hydrogel-forming polymer; and

(e) said composition has an equilibrium extractable polymer content, at equilibrium in synthetic urine, of no more than about 10% by weigth of hydrogel-forming polymer.

9. A hydrogel-forming polymer composition according to claim 8 wherein

(a) said acid group-containing monomers are selected from acrylic acid, methacrylic acid, 2-acrylamido-2-methylpropane sulfonic acid, and combinations thereof; and

(b) said cross-linking agent is selected from N,N'-methylenebisacrylamide, trimethylol propane triacrylate, and triallyl amine.

10. A substantially water-insoluble, slightly cross-linked, partially neutralized, hydrogel-forming polymer composition consisting essentially of

(a) from about 75 mole percent to 99.99 mole percent of polymerized unsaturated, polymerizable, acid group-containing monomers; and

(b) from about 0.01 mole percent to 3 mole percent of a cross-linking agent;

wherein said composition has a degree of neutralization of at least about 25% and is substantially free of graft polymerizable polymer moieties; and further wherein said polymer composition, upon neutralization to a degree of neutralization of at least 50%, has or would have a gel volume, v, of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer exhibits a shear modulus, s, of at least about 2000 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium extractable polymer content, e, at equilibrium in synthetic urine, of no more than about 17% by weight of hydrogel-forming polymer, and further wherein the relationship between gel volume, v, and equilibrium extractable polymer content, e, is defined by the equation: e≦0.23v-3.0;

and further wherein the relationship between gel volume, v, and gel strength as measured by shear modulus, s, is defined by the equation:

log s≧-2.494 log v+8.090.

11. A hydrogel-forming polymer composition according to claim 10 wherein

(a) said composition has a degree of neutralization of at least about 50%;

(b) said composition has a gel volume of at least about 25 to 80 grams of synthetic urine per gram of hydrogel-forming polymer;

(c) the hydrogel formed from said composition has a gel strength such that the hydrogel exhibits a shear modulus of from about 2500 to 92,000 dynes/cm² ;

(d) said composition has an initial extractable polymer content, after one hour in synthetic urine, of no more than about 5% by weight of hydrogel-forming polymer; and

(e) said composition has an equilibrium extractable polymer content, at equilibrium in synthetic urine, of no more than about 10% by weight of hydrogel-forming polymer.

12. A hydrogel-forming polymer composition according to claim 11 wherein

(a) said acid group-containing monomers are selected from acrylic acid, methacrylic acid, 2-acrylamido-2-methylpropane sulfonic acid, and combinations thereof; and

(b) said cross-linking agent is selected from N,N'-methylenebisacrylamide,

trimethylol propane triacrylate and triallyl amine. 13. A process for preparing a substantially water-insoluble, slightly cross-linked, partially neutralized hydrogel or hydrogel-forming polymer material suitable for use in absorbent products, said process comprising

(a) preparing a reaction mixture consisting essentially of from about 5% to 35% by weight of unsaturated, polymerizable, acid group-containing monomers in the free acid form, from about 0.001 mole percent to 5 mole percent, based on moles of polymerizable monomers, of a cross-linking agent and from 0 mole percent to about 5 mole percent, based on the moles of polymerizable monomers, of a free radical initiator in an aqueous medium which is substantially free of graft polymerizable polymer moieties;

(b) subjecting said aqueous reaction mixture to polymerization conditions which are sufficient to produce therein a substantially water-insoluble, slightly cross-linked polymer material which, upon subsequent neutralization to a degree of neutralization of at least 50% and upon subsequent drying, has or would have, a gel volume, v, of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer material exhibits a shear modulus, s, of at least about 2000 3270 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, an equilibrium extractable polymer content, e, at equilibrium in synthetic urine, or no more than about 17% by weight of hydrogel-forming polymer; and a relationship between gel volume, v, and equilibrium extractable polymer content, e, defined by the equation: e≦0.23v-3.0; and

(c) neutralizing at least a portion of the acid groups of the polymer material formed in the aqueous reaction mixture with salt-forming cations to form a partially neutralized polymer material having a degree of

neutralization of at least about 25%. 14. A process according to claim 13 wherein

(a) the acid groups of the polymer material formed in the aqueous reaction mixture are neutralized with salt-forming cations to form a polymer material having a degree of neutralization of at least about 50%;

(b) said neutralized polymer material has a gel volume of from about 25 to 80 grams of synthetic urine per gram of hydrogel-forming polymer;

(c) said neutralized polymer material forms a hydrogel having a gel strength such that said hydrogel exhibits a shear modulus of about 2500 4710 to 92,000 dynes/cm² ; and

(d) said neutralized polymer material has an initial extractable polymer content, after one hour in synthetic urine, of no more than about 5% by weight of hydrogel-forming polymer and an equilibrium extractable polymer content, at equilibrium in synthetic urine, of no more than about 10% by

weight of hydrogel-forming polymer. 15. A process according to claim 14 wherein

(a) said acid group-containing monomers are selected from acrylic acid, methacrylic acid, 2-acrylamido-2-methyl propane sulfonic acid, and combinations thereof; and

(b) said cross-linking agent is selected from

(i) di- and polyvinyl compounds;

(ii) di- and polyesters of unsaturated mono- and polycarboxylic acids with polyols;

(iii) bisacrylamides;

(iv) carbamyl esters obtained by reacting polyisocyanates with hydroxyl-group containing monomers;

(v) di- and polyallyl ethers of polyols;

(vi) di- and polyallyl esters of polycarboxylic acids;

(vii) esters of unsaturated mono- and polycarboxylic acids with mono-allyl esters of polyols; and

(viii) di- and triallyl amines. 16. A process according to claim 15 wherein the aqueous reaction mixture comprises

(a) from about 8% to 24% by weight of the acid group-containing monomers;

(b) from about 0.01 mole percent to 3 mole percent of the cross-linking agent; and

(c) from about 0.001 mole percent to 0.5 mole percent of the free radical initiator; and

wherein the polymerization conditions to which said reaction mixture is subjected include a polymerization temperature of from about 5°C

to 40°C 17. A proces according to claim 16 wherein the free radical initiator comprises a peroxygen compound or comprises a redox initiator system formed by combining a peroxygen compound with a reducing

agent. 18. A process according to claim 17 wherein the peroxygen compound initiator is selected from sodium, potassium, and ammonium persulfates, caprylyl peroxide, benzoyl peroxide, hydrogen peroxide, cumene hydroperoxides, t-butyl diperphthalate, t-butyl perbenzoate, sodium peracetate and sodium percarbonate, and wherein the redox initiator system comprises the combination of any of said peroxygen compound initiators with a reducing agent selected from sodium bisulfite, L-ascorbic acid and

ferrous salts. 19. A process according to claim 18 wherein the initiator, or a component thereof, is incrementally added to the aqueous reaction mixture only in such amounts as are sufficient to promote complete polymerization of the acid group-containing monomers and cross-linking

agents. 20. A process according to claim 13 which comprises the additional step of drying the polymer material produced in said aqueous reaction

mixture. 21. A process according to claim 20 wherein said drying step is accomplished either by subjecting said polymer material to a temperature of from about 40°C to 150°C for a period of time sufficient to form a semi-solid mass of hydrogel-forming polymer material, by treating said polymer material with a dewatering solvent or by removing

water from said polymer material via azeotropic distillation. 22. A process according to claim 13 wherein the aqueous reaction mixture is suspended in the form of droplets in a water-immiscible organic solvent, and wherein said droplets are subjected to polymerization conditions using

inverse suspension or inverse emulsion polymerization procedures. 23. A process for preparing a substantially water-insoluble, slightly cross-linked, partially neutralized hydrogel-forming polymer material suitable for use in absorbent products, said process comprising

(a) preparing a reaction mixture consisting essentially of from about 8% to 24% by weight of polymerizable acrylic acid monomers in the free acid form, from about 0.01 mole percent to 3 mole percent, based on total moles of polymerizable monomers, of a cross-linking agent selected from N,N'-methylenebisacrylamide, trimethylol propane triacrylate and triallyl amine, and from about 0.001 mole percent to 0.5 mole percent, based on the total moles of acrylic acid plus cross-linking agent monomers, of a free radical initiator in an aqueous medium which is substantially free of graft polymerizable polymer moieties;

(b) subjecting said aqueous reaction mixture to polymerization conditions, including a temperature of from about 5°C to 40°C, which are sufficient to produce therein a substantially water-insoluble, slightly cross-linked polyacrylic acid-based polymer material having, upon subsequent neutralization and drying, a gel volume, v, of from about 25 to 80 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that said hydrogel formed from said polymer material exhibits a shear modulus, s, of from about 2500 4710 to 92,000 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 5% by weight of hydrogel-forming polymer, an equilibrium extractable polymer content, at equilibrium in synthetic urine, of no more than about 10% by weight of hydrogel-forming polymer, and a relationship between gel volume, v, and equilibrium extractable polymer content, e, defined by the equation; e≦0.23v-3.0, and a relationship between gel volume, v, and shear modulus, s, defined by the equation: log s≧-2.494 log v+8.090; and

(c) neutralizing at least a portion of the carboxyl groups of said polyacrylic acid-based polymer material formed in the aqueous reaction mixture with sodium cations to form a partially neutralized polymer

material having a degree of neutralization of at least about 50%. 24. An absorbent structure suitable for use in disposable absorbent articles, said absorbent structure comprising:

(a) from about 50% to 98% by weight of said structure of hydrophilic fiber material; and

(b) from about 2% to 50% by weight of said structure of discrete particles of substantially water-insoluble, slightly cross-linked, partially neutralized, substantially dry, hydrogel-forming polymer material; wherein said polymer material has a degree of neutralization of at least about 25% and is substantially free of graft polymerizable polymer moieties; and further wherein said polymer material, upon neutralization to a degree of neutralization of at least 50%, has or would have a gel volume, v, of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer material exhibits a shear modulus, s, of at least about 2000 3270 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium extractable polymer content, e, at equilibrium in synthetic urine, of no more than about 17%

by weight of hydrogel-forming material. 25. An absorbent structure according to claim 24 wherein said hydrogel-forming polymer material

(a) has a degree of neutralization of at least about 50%;

(b) has a gel volume of from about 25 to 80 grams of synthetic urine per gram of hydrogel-forming polymer;

(c) forms a hydrogel material which exhibits a shear modulus of from about 2500 4710 to 92,000 dynes/cm²,

(d) has an initial extractable polymer content, after one hour in synthetic urine, of no more than about 5% by weight of hydrogel-forming polymer; and

(e) has an extractable polymer content, at equilibrium in synthetic urine,

of no more than about 10% by weight of hydrogel-forming polymer. 26. An absorbent structure according to claim 24 wherein the hydrophilic fiber material comprises from about 65% to 90% by weight of said structure and wherein said polymer particles comprise from about 10% to 35% by weight of

said structure. 27. An absorbent structure according to claim 24 which has

a density of from about 0.06 to 0.3 grams/cm³. 28. An absorbent structure according to claim 24 wherein said polymer material, prior to neutralization, comprises polymerized monomers selected from acrylic acid, methacrylic acid, 2-acrylamido-2-methyl propane sulfonic acid and combinations thereof, cross-linked with a cross-linking agent selected from (a) di- and polyesters of unsaturated mono- and polycarboxylic acids

with polyols, (b) bisacrylamides and (c) di- and triallyl amines. 29. An absorbent structure according to claim 28 wherein said absorbent structure has a density of from about 0.09 to 0.18 g/cm³, a basis weight of from about 0.02 to 0.12 g/cm² and wherein said absorbent structure comprises a mixture of wood pulp fibers and polymer particles having a

particle size of from about 50 microns to 1 mm. 30. An absorbent structure according to claim 24 wherein, for the hydrogel-forming polymer particles, the relationship between gel volume, v, and equilibrium extractable

polymer content, e, is defined by the equation: e≦0.23v-3∅ 31. An absorbent structure according to claim 24 wherein, for the hydrogel-forming polymer particles, the relationship between gel volume, v, and equilibrium extractable polymer content, e, is defined by the equation: e≦0.23v-3.0; and wherein the relationship between gel volume, v, and shear modulus, s, of the hydrogel formed from said particles is defined by the equation:

log s≧-2.494 log v+8.090to e≦0.073v-0.37 .

2. An absorbent article comprising:

(a) a liquid impervious backing sheet;

(b) a liquid pervious, relatively hydrophobic topsheet; and

(c) an absorbent core comprising an absorbent structure according to claim

24 positioned between said backing sheet and said topsheet. 33. A diaper article comprising:

(a) a liquid impervious backing sheet;

(b) a relatively hydrophobic, liquid pervious topsheet; and

(c) an absorbent core positioned between said backing sheet and said topsheet, said absorbent core comprising an absorbent structure which consists essentially of

(i) from about 65% to 90% by weight of said structure of hydrophilic fiber material; and

(ii) from about 10% to 35% by weight of said structure of discrete particles of substantially water-insoluble, slightly cross-linked, partially neutralized, substantially dry hydrogel-forming polymer material comprising polymerized acrylic acid monomers, cross-linked with a cross-linking agent selected from N,N'-methylenebisacrylamide, trimethylol propane triacrylate and triallyl amine; wherein said polymer material has at least 50% of its acrylic acid groups neutralized with sodium cations and is substantially free of graft polymerized polymer moieties, and further wherein said hydrogel-forming polymer material has a gel volume, v, of at least about 20 grams of synthetic urine per gram of hydrogel-forming polymer, a gel strength such that the hydrogel formed from said polymer material exhibits a shear modulus, s, of at least about 2000 3270 dynes/cm², an initial extractable polymer content, after one hour in synthetic urine, of no more than about 7.5% by weight of hydrogel-forming polymer, and an equilibrium extractable polymer content, e, after 16 hours in synthetic urine, of no more than

about 17% by weight of hydrogel-forming polymer. 34. A diaper article according to claim 33 wherein the absorbent core comprises an hourglass-shaped absorbent structure formed from an air-laid mixture of

hydrophilic fiber material and hydrogel-forming polymer particles. 35. A diaper article according to claim 33 wherein the polymer-containing absorbent structure comprises an insert positioned underneath an upper layer of the diaper core, which upper layer is hourglass-shaped and consists essentially of wood pulp fiber material and from 0% to about 8%

by weight of particles of hydrogel-forming polymer material. 36. A diaper article according to claim 33 wherein, for said polymer particles, the relationship between gel volume, v, and equilibrium extractable polymer

content, e, is defined by the equation: e≦0.23v-3∅ 37. A diaper article according to claim 33 wherein, for said polymer particles, the relationship between gel volume, v, and equilibrium extractable polymer content, e, is defined by the equation: e≦0.23v-3.0 and wherein the relationship between gel volume, v, and shear modulus, s, of the hydrogel formed from said particles is defined by the equation:

log s≦-2.494 log v+8.090 e≦0.073v-0.37 .