WO1990008806A1

WO1990008806A1 - Superabsorbing copolymers of vinyl alcohol

Info

Publication number: WO1990008806A1
Application number: PCT/US1990/000019
Authority: WO
Inventors: Donna L. Visioli
Original assignee: E.I. Du Pont De Nemours And Company
Priority date: 1989-01-27
Filing date: 1990-01-04
Publication date: 1990-08-09
Also published as: CN1044470A; AU5034090A; CA2008692A1

Abstract

Superabsorbing polymers suitable for a wide variety of applications, e.g., in blends with cellulosic fibers for use in absorbent hygienic and sanitary articles, are made by alcoholysis in the presence of a small amount of an alkali metal alkoxide or hydroxide of a precursor copolymer VAc/X/Y, wherein VAc stands for vinyl acetate; X is a dialkyl ester of one of specified ethylenically unsaturated dicarboxylic acids; and Y is a copolymerizable ethylenically unsaturated monocarboxylic acid.

Description

TITLE SUPERABSORBING COPOLYMERS OF VINYL ALCOHOL BACKGROUND OF THE INVENTION This invention relates to certain copolyiners of vinyl alcohol characterized by their ability to absorb a large amount of water, that is, at least ten times the polymer weight of water. Polymers having such high water absorption capability are generally referred to as superabsorbing polymers.

Superabsorbing polymers find application in the medical industry, the food industry, and agriculture and, further, are used in many consumer products. Superabsorbing polymers swell to a large degree in contact with water but do not fully dissolve. They are usually prepared by either one of two methods, namely, crosslinking water-soluble polymers to make them water-insoluble, while allowing them to retain their ability to swell in water; or modifying water-insoluble polymers by introducing hydrophilic groups, to cause swelling in contact with water. The first method is the one most commonly used to prepare superabsorbing polymers. The crosslink density is of critical importance since too little crosslinking produces a soft, loose gel, which still is quite water-soluble, while too much crosslinking reduces polymer swelling and thus its ability to absorb water. To function as an absorbent for aqueous fluids, a polymer should have some ionic character since charge repulsion is an important factor in promoting polymer swelling in contact with aqueous fluids. However, highly ionized superabsorbing polymers are very sensitive to the ionic strength and pH of the aqueous fluid. The earliest superabsorbing polymers were crosslinked water-soluble polymers such as polyacrylamide, copolymers of acrylamide with acrylic acid, and poly( thylene oxide) . Those materials, however, did not find wide industrial acceptance. Later superabsorbing polymers, of greater industrial interest, include saponified graft copolymers of starch with acrylonitrile, graft copolymers of cellulose and cellulose derivatives, graft copolymers of starch and acrylic acid, and partially neutralized polyacrylic acid. Certain more recent synthetic superbasorbent polymers, which have not been commercialized, include copolymers of maleic anhydride, e.cr.. with isobutylene, and polyvinyl alcohol.

Superabsorbing polymers are of great interest in personal hygiene products such as diapers, incontinent pads, and feminine care products. Normally, they are used in their usual commercial granular or powdered form, in mixtures with various cellulosic fibers. Those fibers, for example, wood pulp or modified cellulose fibers, when used alone, have certain shortcomings in that the fluid is held mainly in the interstices between individual ibers and can be readily squeezed out under pressure, and the mechanical strength of the fibers is significantly reduced when wet. Because of that, cellulosic fibers by themselves are not ideal absorbent materials for such applications. The addition of superabsorbing polymers to the pulp improves the retention of fluid under pressure and gives a blend with increased absorptive capacity.

The most commonly used prior art superabsorbing polymers, i.e. , the starch-acrylic acid graft copolymers and partially neutralized polyacrylic acid, suffer from various performance deficiencies. Since they are highly ionic, they are sensitive to changes in ionic strength and pH of the aqueous fluids which they are intended to absorb. Where, e.g.. the aqueous fluid is urine, its ionic strength and pH can vary depending on the diet and health of the user. In the case of feminine care products, these highly ionic materials readily oxidize blood, causing dark color, which users find objectionable. Superabsorbing polymers with low pH (pH of about 5-5.5), such as partially neutralized polyacrylic acid, may exacerbate skin irritation in the presence of moisture, thus causing diaper rash. Those polymers usually also contain traces of free acrylic acid monomer, which is generally recognized as having some toxicity, in addition to its even lower pH. Finally, but importantly, the prior art commercial superabsorbing polymers normally do not have sufficient tack to adhere to the cellulosic fibers with which they are blended. As a result, they tend to separate from the cellulosic fibers and migrate within the pad or sift out of the pad during handling. Naturally, it would be desirable to fix the superabsorbing polymer in place in the pad, so that the polymer would not migrate.

U. S. Patent No. 4,102,842 to Fuji oto et al. discloses certain water-insoluble hydrophilic gels, which are based on copolymers of a vinyl ester with an ethylenically unsaturated carboxylic acid or derivative thereof. The ethylenically unsaturated carboxylic acid may be a monocarboxylic acid, such as acrylic or methacrylic acid, or a dicarboxylic acid, such as aleic, fu aric, or itaconic acid. The hydrophilic gels are prepared by saponifying the copolymers to form ionomers, acidifying the resulting solution to precipitate the copolymer, redissolving the copolymer at an appropriate pH between 4 and 12, and drying the solution until the water content is less than 100% by weight. These gels are said to have high water absorption capability and retentivity. However, the process by which they are prepared, which requires several pH adjustments and then a long drying period in order to insolubilize the gel material, does not appear to be commercially attractive.

There is a need for a superabsorbing polymer that can be prepared by a simple and convenient process and can be used in the usual applications for such materials without having the shortcomings of certain prior art polymers of this type.

SUMMARY OF THE INVENTION

According to this invention, there is now provided a superabsorbing copolymer which is obtained by alcoholysis in a homogeneous solution in a C₁-C₄ alcohol, optionally containing water in an amount of up to 20 weight percent of the solvent system, in the presence of a base selected from the group consisting of alkali metal hydroxides and C₁-C₄ alkoxides, of a copolymer represented by the formula VAc/X/Y, wherein VAc stands for vinyl acetate; X stands for a diester of an ethylenically unsaturated dicarboxylic acid selected from the group consisiting of maleic, fumaric, itaconic, citraconic, and mesaconic acids, the alcohol moiety of the diester being derived from an aliphatic or cycloaliphatic alcohol soluble in water at room temperature to the extent of at least about 4 g per 100 g of water; and Y stands for a copolymerizable ethylenically unsaturated monocarboxylic acid soluble in water at room temperature to the extent of at least about 4 g per 100 g of water; the weight proportion of X in the copolymer being about 4-60%, and the weight proportion of Y in the copolymer being 0 to about 2%; the amount of water initially present in the alcoholysis mixture always being less than the weight of the starting VAc/X/Y copolymer to be alcoholyzed; and the amount of base being about 1-20 weight percent of the starting VAc/X/Y copolymer.

DETAILED DESCRIPTION OF THE INVENTION

While it is believed that the base principally acts as a catalyst, the amount of base itself is not, strictly speaking "catalytic", as this term is generally understood. The amount of base will depend to a large extent, i.a. , on the temperature at which alcoholysis is run and on the efficiency of agitation. Thus, under plant conditions, run for about three hours at about 65°C with efficient agitation, the amount of base may be as little as 1% based on the weight of the starting VAc/X/Y copolymer; while under laboratory conditions, a reaction run for a few minutes at room temperature in a blender may require as much as 15% of base. Likewise, alcoholysis run in the presence of water normally is less efficient and may require more base; e.g., 5% instead of 1%. Under most plant conditions, the amount of base will usually be about 1-10% based on the starting VAc/X/Y copolymer, preferably about 1-5%, and the amount of water, if present, will be no more than about 10% of the weight of the solvent system.

Typical representative alcohols providing the alcohol moiety for the dicarboxylic acid esters X include methyl, ethyl, propyl, isopropy1, butyl, isobutyl, tertiary butyl, 2-pentyl, 3-pentyl, 2-methyl-3-pentyl, 3-methyl-2-pentyl, cyclopentyl, and cyclohexyl alcohol. Many of the diesters of this group are commercially available; others can be made by methods well known in the art.

Typical representative comonomers Y. include acrylic acid, methacrylic acid, 3-butenoic acid, 4-pentenoic acid, and 5-hexenoic acid. The amount of comonomer Y preferably is 0.02-1.0% of the total copolymer weight.

The starting copolymers of this invention are well known from scientific and patent literature, and some are commercially available. They usually are made by a free-radical initiated polymerization reaction, either in a batch process or in a continuous process, for example, as described in Polwinyl Alcohol Properties and Applications. C. A. Finch, Editor, New York, John Wiley and Sons, 1973, p. 67.

The preferred process for making the starting copolymer is a continuous process run in a methanol solution, the amount of methanol being about 5-35%, preferably 10-25% of the feed. The initiator advantageously employed in the preferred process is azobisisobutyronitrile (AIBN) , which is introduced in an amount of about 50-400 ppm, based on the monomer and solvent feed. The polymerization temperature usually is about 70 to 90°C.

In a preferred embodiment of the process employed in the laboratory for the alcoholysis, the starting copolymer VAc/X/Y is dissolved in methanol and contacted with a small amount of sodium methoxide, while the solution is vigorously agitated either at room temperature or at an elevated temperature, e.g.. 40 to 60"C. The alcoholysis temperature may be as high as the boiling temperature of the alcohol in which it is conducted. The product precipitates at this stage. The mixture is neutralized, and the product is filtered, washed with methanol and acetone, filtered again, and dried, preferably in a vacuum oven overnight. In lieu of methanol, another alcohol, such as, e.g.. ethanol, 1-propanol, isopropyl alcohol, 1-butanol, isobutyl alcohol, or tert-butyl alcohol may be used.

The base preferably will be an alkali metal alkoxide such as. e.g.. sodium methoxide, potassium methoxide, sodium ethoxide, potassium tert-butoxide, etc. Representative alkali metal hydroxides, which also may be used, are sodium hydroxide, potassium hydroxide, and lithium hydroxide. Sodium methoxide is the most practical and efficient base and is preferred even in systems containing water. Sodium hydroxide has a greater tendency to form under the reaction conditions sodium acetate, generally referred to as "ash" in the industry and considered undesirable.

While the inventor does not wish to be bound by any particular scientific theory, it appears that alcoholysis of the starting VAc/X/Y copolymer produces vinyl alcohol (VOH) groups by the alcoholysis of the vinyl acetate groups and that either some of those vinyl alcohol groups or some still remaining vinyl acetate groups enter into a transesterification reaction with the alkyl ester groups to form chain-to-chain crosslinks.

These crosslinks then account for the formation of insoluble polymer, which precipitates from the methanol solution. Compared with the alcoholysis product of U.S. Patent 4,102,842, which becomes insoluble only after drying, the insoluble copolymer of the present invention is formed during the alcoholysis step itself. While it is not known what proportion of the VOH groups or of the carboxyl groups of the diacid take part in this hypothetical reaction, it is believed that under the alcoholysis conditions all or nearly all ester groups, whether vinyl acetate or alkyl ester, are cleaved. Those carboxyl groups of the X monomer not participating in the formation of crosslinks, according to the above theory, predominantly form lactone rings with the neighboring hydroxyl groups formed by the cleavage of vinyl acetate groups on the same starting copolymer molecule. It has been observed that the alcoholyzed, superabsorbing polymers of the present invention made with an amount of base close to the lower end of the above-recited range at an elevated temperature are substantially devoid of either free carboxyl groups or neutralized carboxyl groups. However, even those laboratory-made superabsorbing polymers of this invention which contain some neutralized carboxyl groups have the same overall properties, precipitate in the same way out of the alcoholysis medium and can be recovered by filtration, centrifugation, or any other known technique used for separating solids from liquids, and are equally effective superabsorbents.

It is to be noted that the crosslinked product of this invention is quite different from the usual prior art crosslinked polyvinyl alcohol (PVOH) . This prior art crosslinked PVOH is often made by copolymerizing VAc with divinyl comonomer such as, e.g. , divinylbenzene and hydrolyzing the VAc groups to hydroxyl groups. In this prior art crosslinked PVOH, the amount of crosslinking as well as the polymerization itself are difficult to control. Further, the crosslinked precursor itself is not soluble in water or in most solvents and, therefore, cannot be made into a shaped article, e.g. , film and then hydrolyzed to a water-absorbing film. By contrast, the starting copolymer VAc/X/Y used in the process of the present invention is soluble in methanol and in several other solvents. It, therefore can be cast from solution into films, which then can be alcoholyzed to a superabsorbing polymer film. Generally speaking, in addition to the already discussed various sanitary and personal care articles, the superabsorbing copolymers of the present invention are useful in agriculture, e.g.. in soil reforming, in transporting plants, in keeping seedlings moist, and in shipping hydro-mulch insecticide and herbicide formulations; in medicine, e.g.. in films to be used in burn treatment and in relieving bed sores, in medical sheets, nonwoven towels and wipes, and surgical dressings; in industrial uses such as, e.g.. fuel filters, dehydrating agents, cosmetics, wallpaper, leakage-proof agents, ice packs, swellable gaskets, sludge dewatering, cleaning agents, bags for absorbing aqueous spills, and fire fighting fluids. The copolymers of the present invention preferably are shaped by casting. High temperature fabrication methods, such as, e.g. extrusion, are not recommended because the material under those conditions tends to be dehydrated excessively, forming extensive crosslinks, with concomitant loss of its superabsorbent quality. In many applications, the copolymers of the present invention can be blended with other materials, including cellulosic fibers or pulp, other polymeric materials, for example poly(vinyl acetate) or various carboxylic polymers and ionomers, conventional fillers, pigments, dispersants, stabilizers, and the like.

This invention is now illustrated by examples of certain preferred embodiments thereof, where all parts, proportions, and percentages are by weight unless indicated otherwise. Preparation of Precursor VAc/X Y Copolymer

Most of the examples were carried out with superabsorbing polymers of the present invention, where X' was X, as defined herein. However, in three comparative examples, X' was outside the scope of the present invention; it was either an ester of a monocarboxylic acid, or a monoester of a dicarboxylic acid, or a dicarboxylic acid anhydride.

The precursor copolymer was prepared in a continous stirred-tank reactor. Feed solutions were prepared batchwise; a portion of those solutions was charged batchwise as an initial reactor charge, and the remainder was fed continuously into the reactor. For lower X' monomer concentrations, the comonomers other than VAc were found to be totally converted to copolymer, so that the content of the comonomer in the copolymer was dependent on both the concentration of the comonomers in the feed stream and the percent solids in the resulting copolymer solution. Monomer proportions in the copolymer were therefore calculated in Examples 1, 3-12, and 14-19, from the feed monomer ratios. In Examples 2 and 13, those proportions were determined analytically. The amount of dimethyl maleate groups was determined by converting single carbon-oxygen bonds to carbon-bromine bonds via reaction with hot hydrobromic acid in the presence of a measured amount of a solution of isopropyl alcohol in o-xylene as an internal standard. Methyl bromide formed in this reaction was determined by gas chromatography on a methyl silicone column. The gas chromatographic determination was calibrated against methyl alcohol. The amount of acrylic acid groups was determined by titration. Precursor solutions were dried overnight in a vacuum oven to remove the volatile residual methanol and vinyl acetate comonomer.

Alcoholysis was carried out by adding 300 g of a 17% solution of VAc/XVY copolymer in methanol to a 4.75% solution of sodium methoxide in 200 ml of methanol and agitating for 10 minutes at room temperature in a high speed explosion-proof blender. The reaction was quenched by adding 10 ml of glacial acetic acid, and the product was filtered, washed four times with 1000 ml portions of methanol and twice with 1000 ml portions of acetone, then vacuum-dried overnight at 75°C.

The water absorbing characteristics of the alcoholyzed product were evaluated using the Mark II Gravimetric Absorbency Testing System (GATS) from M/K Systems, Inc., Danvers, MA 01923. This instrument consists of a reservoir of liquid placed on an electronic balance, which monitors the change in weight of the reservoir as it receives fluid from or gives it to the test specimen. The test specimen is mounted on a piece of filter paper, placed on a fritted glass plate and set at a height of 1.4 cm above the reservoir. A load of 100 g (0.5 kPa) is placed on top of the sample. As the fluid (a 0.9% solution of sodium chloride in distilled water) flows from the reservoir to the sample, the reservoir becomes lighter; this change in weight as a function of time is translated into absorbed volume as a function of time. The rate of liquid uptake is defined as the slope of the initial rising part of the absorption curve. The total absorption capacity is defined as the maximum amount of liquid taken up. After the absorption capacity is reached, a heavier load (712 g, 3.5 kPa) is placed on top of the sample, and some of the liquid is forced out of the sample; the amount of liquid retained by the sample is the retention under pressure.

Samples were tested both in their neat form and in pads. The pads were prepared using a blend of copolymer with cellulose pulp containing 15% of copolymer. Cellulose pulp, sold under the trademark Starbrite^®, was obtained from Temple-Eastex, Inc., Diboll, Texas. The material was 100% kraft fluff pulp. The blend was pressed on a Carver press using a force of 6600 N to a density of 0.2 g/cm³. Rate of uptake was expressed as grams of liquid taken up per minute. Absorption capacity was expressed as grams of liquid per gram of polymer for the neat polymer samples and grams per pad for the pad samples. Retention under pressure was expressed either as grams of liquid per gram of neat polymer, or as grams of liquid per 1.36 g pad, or as percent retention (grams with heavy final load/grams with initial light load) .

The rate of liquid uptake, the absorption capacity, and the retention under pressure are important parameters, but their relative importance depends on the particular end use.

The experimental data as well as the evaluation results are given below in Tables I and II, where the following abbreviations are used: AIBN = azobisisobutyronitrile

MeOH = methyl alcohol

DMM = dimethyl maleate

DEM = diethyl maleate

DMF = dimethyl fumarate

MA = methyl aerylate

DBM = dibutyl maleate

MAH = maleic anhydride

MAME = maleic acid, monoethyl ester

AA = acrylic acid

MAA = methacrylic acid

A, B, and C are comparative examples, outside the scope of the present invention.

Controls were commercial products based on superabsorbing polymers, as follows:

Cl = Crosslinked sodium polyacrylate, Aridall^® 1080 from Chemdal Corp., Arlington Heights, Illinois.

C2 = Crosslinked sodium polyacrylate. Favor SAB^® 922G from Paul Stockhausen G.m.b.H. , Federal Republic of Germany.

C3 = Crosslinked, acrylic acid grafted starch, Sanwet^® M-1000 from Sanyo Chemical Co., Japan.

C4 = Crosslinked, acrylic acid grafted starch, Water-Lok^® J550 from Grain Processing Co., Muscatine, Iowa.

C5 = Crosslinked sodium polyacrylate, Drytech^® from Dow Chemical Company, Midland, Michigan.

C6 = Starbrite^® cellulose fluff pulp from Temple-Eastex, Inc., Diboll, Texas. TABLE I

PREPARATION OF VAC/X'/Y COPOLYMERS

Feed Composition* _r Precursor Comp.* ppm % % % Temp. Solids % %

Example AIBN MeOH X^» X' Y Y °C % X¹ Y

1 350 10.0 5.0 DMM 0.4 AA 90 44.6 11.2 0.9

2 80 10.0 4.5 DMM 0.01 AA 70 8.4 38.9 0.33

3 200 10.0 3.5 DMM 0.20 AA 90 41.9 8.3 0.48

4 100 30.0 2.5 DMM 0.01 AA 90 23.3 10.7 0.04

5 50 10.0 1.5 DMM 0.20 AA 75 14.0 10.7 1.43

6 300 30.0 1.5 DMM 0.50 AA 90 36.4 4.1 1.37

7 200 25.0 1.1 DMM 0.2 AA 75 18.8 5.9 1.06

8 350 10.0 5.0 DMM 0.4 AA 90 46.5 10.7 0.86 <£

9 350 10.0 6.0 DEM 0.4 AA 90 46.7 12.8 0.86

10 350 10.0 5.0 DMF 0.4 AA 90 41.1 12.2 0.97

11 350 10.0 7.9 DBM 0.4 AA 90 47.3 16.7 0.85

12 400 10.0 5.0 DMM 0.5 MAA 90 43.5 11.5 1.14

13 50 10.0 5.0 DMM 0.01 AA 70 3.6 56.7 0.65

A 250 9.0 9.0 MA 0 _— 78 30.0 26.7 0

B 800 12.0 4.3 MAME 0.01 AA 85 39.9 10.8 0.025

C 1000 12.0 2.7 MAH 0.01 AA 85 13.1 20.6 0.08 * VAc is calculated by difference from: % VAc = 100% - (X' + Y)

TABLE II TEST DATA

Neat Polymer Pads ( 1.36 g)

Rate Abs. Retention Rate Abs. Retention

Example g/sec q/q g/g %_ g/sec q %_

1 14.0 19.1 15.5 81 28.2 12.7 10.5 83

2 22.0 25.5 21.5 84 32.5 12.4 10.1 82

3 21.5 22.8 16.9 75 28.3 11.2 9.5 85

4 13.5 23.3 21.0 90 29.3 10.5 8.6 82

5 21.5 25.9 21.1 81 27.2 9.0 7.9 88

6 16.0 20.6 18.8 91 19.0 7.4 6.4 86

7 1.9 19.2 19.0 98 18.0 7.8 6.8 87

8 11.8 20.5 19.7 96

9 8.8 17.8 17.6 98 10 14.2 22.4 21.9 97 11 8.0 17.0 16.8 99 12 28.0 17.2 15.9 92 13 13.0 22.0 21.1 96

TABLE II (cont •d.)

TEST DATA

Neat Polvmer Pads (1.36 g)

Rate Absorp. Retention Rate Abs. Retention

Example g/sec g/g g/g % g/sec q %

A Disperses but does not swell** — — —

B Dissolves in water — — —

C Dissolves in water — — —

Cl 1.8 20.0 15.7 78 40.7 12.8 11.2 87

C2 0.2 17.4 15.8 91 3.72 13.8 12.3 89 σ

C3 12.2 60.5 54.3 90 17.5 14.0 12.5 89

C4 10.5 49.3 42.5 86 18.0 14.5 13.0 90

C5 13.0 46.0 38.5 84 30.1 16.0 13.8 86

C6 — — — — 20.2 5.9 4.9 82

** This polymer appears to be highly crosslinked

As can be seen from Table II, the superabsorbing polymers of the present invention of Examples 1-13 have very good water absorption and retention properties, which are at least comparable to those of certain commercial products (Examples C1-C6) . It is to be noted that when the comonomer X' is other than one of the comonmers X required by the present invention, the resulting copolymer is not a superabsorbing polymer; it either does not swell (X =methyl acrylate) or dissolves in water (X'=monoethyl ester of maleic acid or maleic anhydride) . Examples 14-16

A VAc/X/Y copolymer, which contained 8.4% of copolymerized dimethyl maleate and 0.5% of copolymerized acrylic acid, was prepared by copolymerization under the general conditions described in connection with Examples 1-13, wherein the feed contained, in addition to vinyl acetate, 10% of methanol, 3.5% of dimethyl maleate, 0.2% of acrylic acid, and 200 ppm of azobisisobutyronitrile. The polymerization temperature was 90°C, and the reaction product contained 41.5% solids.

The VAc/X/Y copolymer was alcoholyzed by two different methods. In Example 14, the alcoholysis was run in exactly the same manner as described above for Examples 1-13. In Examples 15 and 16, 50 g of VAc/X/Y copolymer was dissolved in 300 ml of methanol, and the solution was poured into 180 ml of methanol to which 20 ml of 10% sodium methoxide in methanol had been added. The solution was then refluxed for 3 hours. The reaction mixture containing solid product was allowed to cool to room temperature and was acidified with 10 ml of glacial acetic acid. The solid product was filtered, washed on the filter with methanol, and dried. The superabsorbing polymer product was tested for absorption rate, absorption capacity, and retention, as described above. The following results were obtained.

TABLE III

Example Absorp. rate Absorption Retention g/sec g/g %

14 18.5 19.8 99

15 19.3 26.0 89

16 32.5 26.0 95

Examples 17-19

A VAc/X/Y copolymer, wherein VAc stands for vinyl acetate; X stands for dimethyl maleate, and Y stands for acrylic acid was prepared by copolymerizing the above three monomers in the same general manner as described above. The reactor feed was 10% methanol, 84.1% VAc, 5% dimethyl maleate, 0.4% acrylic acid, and 350 ppm of azobisisobutyronitrile, and the polymerization temperature was 90°C. The copolymer composition was 87.9% vinyl acetate, 11.2% dimethyl maleate, and 0.9% acrylic acid.

This copolymer was subjected to alcoholysis under the following conditions. In each case, 50 g of the VAc/X/Y copolymer was dissolved in 300 ml of methanol, and the solution was diluted with 40 g of deionized water. This solution was combined with the indicated volume of a 10% w/v solution of either sodium methoxide or sodium hydroxide in methanol and with an additional amount of methanol, and the resulting liquid was well mixed in a Waring blender, then transferred to a resin kettle and refluxed for 3 hours. The mixture was allowed to cool; it was then quenched with 10 ml of glacial acetic acid, and the precipitated reaction product was filtered, washed, and dried as in the earlier examples. The product was evaluated in the same manner as before.

Table IV, below, gives the experimental data and results.

Table IV

Base Absorp. sol'n. Add'l. rate Absorp. Retent

Ex. Base ml MeOH, ml g/sec g/g %

17 NaOMe 48 175 12.8 17.9 100

18 NaOH 53 147 13.3 15.5 100

19 NaOH 35 170 15.8 13.0 100

The product of Example 19, while satisfactory from the standpoint of its water absorption and retention properties, was marginal with respect to its physical properties, in that it formed a rather loose, difficult to handle gel.

Generally speaking, the superabsorbing polymers of the present invention do not have the deficiencies of many prior art commercial products. Those prior art polymers usually did not flow freely under humid conditions, and therefore presented difficulties in manufacturing articles. They also tended to move around in articles such as, e.g. , diapers; this probably was their most serious deficiency. By contrast, the superabsorbing polymers of the present invention can be fixed in place by exposing the article to mist or steam or can be thermally bonded. The superabsorbing polymers of the present invention are not sensitive to the ionic strength or pH of the aqueous liquid to be absorbed, contain no residual monomer which may cause skin rashes, do not have a low pH, and do not discolor blood. In addition, it has been surprisingly found that the superabsorbing polymers of the present invention, when used in diapers and incontinent devices, tend to exhibit a reduced level of urine odor, as compared with many of the presently available commercial products. This is an important practical advantage.

Claims

I CLAIM:

1. A process for producing a superabsorbing polymer, said process comprising subjecting to alcoholysis in the presence of a base selected from the group consisting of alkali metal C1-C4 alkoxides and alkali metal hydroxides, in a homogenoeus solution in a C1-C4 alcohol containing an amount of water of 0 to about 20 weight percent of the solvent system, of a copolymer represented by the formula VAc/X/Y, wherein VAc stands for vinyl acetate; X stands for a diester of an ethylenically unsaturated dicarboxylic acid selected from the group consisiting of maleic, fumaric, itaconic, citraconic, and mesaσonic acids, the alcohol moiety of the diester being derived from an aliphatic or cycloaliphatic alcohol soluble in water at room temperature to the extent of at least about 4 g per 100 g of water; and Y stands for a copolymerizable ethylenically unsaturated onocarboxylic acid soluble in water at room temperature to the extent of at least about 4 g per 100 g of water; the weight proportion of X in the copolymer being about 4-60%, and the weight proportion of Y in the copolymer being 0 to about 2%; the initial amount of water, if present, always being less than the amount of starting VAc/X/Y copolymer; and the initial amount of base being about 1-20% based on the weight of the starting VAc/X/Y copolymer; and recovering the resulting superbasorbing polymer by separating it from the medium in which the alcoholysis was carried out.

2. A process of Claim l wherein the alcoholysis is carried out in methanol, the concentration of the starting VAc/X/Y copolymer in methanol being about 10-60 weight percent.

3. A process of Claim 2 wherein the base is sodium methoxide or potassium methoxide.

4. A process of Claim 3 wherein the amount of base is about 1-5 % based on the weight of the starting VAc/X/Y copolymer.

5. A process of Claim 1 wherein the alcohol contains water.

6. A process of Claim 5 wherein the amount of water is at most about 10 weight percent based on the solvent system.

7. A process of Claim 1 wherein the alcoholysis is conducted at a temperature of from about room temperature to the boiling temperature of the alcohol in which the alcoholysis is conducted.

8. A process of Claim 7 which is conducted at a temperature of about 40-65°C.

9. A process of Claim 1 wherein X in the starting copolymer VAc/X/Y is dimethyl maleate or diethyl maleate, and Y is acrylic acid or methacrylic acid.

10. A process of Claim 9 wherein X in the starting copolymer VAc/X/Y is dimethyl maleate, and Y is acrylic acid.

11. A process of Claim 10 wherein the amount of dimethyl maleate in the starting VAc/X/Y copolymer is about 4.1-56.7 weight percent, and the amount of acrylic acid in the starting VAc/X/Y copolymer is about 0.12-1.43 weight percent.

12. A superabsorbing polymer made by subjecting to alcoholysis in the presence of a base selected from the group consisting of alkali metal ^C4-C4 alkoxides and alkali metal hydroxides, in a homogeneous solution in a C_1.-C4 alcohol containing an amount of water of 0 to about 20 weight percent of the solvent system, a copolymer represented by the formula VAc/X/Y, wherein VAc stands for vinyl acetate; X stands for a diester of an ethylenically unsaturated dicarboxylic acid selected from the group consisting of maleic, fumaric, itaconic, citraconic, and mesaconic acids, the alcohol moiety of the diester being derived from an aliphatic or cycloaliphatic alcohol soluble in water at room temperature to the extent of at least about 4 g per 100 g of water; and Y stands for a copolymerizable ethylenically unsaturated monocarboxylic acid soluble in water at room temperature to the extent of at least about 4 g per 100 g of water; the weight proportion of X in the copolymer being about 4-60%, and the weight proportion of Y in the copolymer being 0 to about 2%; the initial amount of water, if present in the C1-C4 alcohol in which the alcoholysis is performed, being always less than the amount of the starting VAc/X/Y copolymer; and the amount of base being about 1-20 percent based on the weight of the starting VAc/X/Y copolymer; and recovering the resulting superbasorbing polymer by separating it from the medium in which the alcoholysis was carried out.

13. A superabsorbing polymer of Claim 12 obtained by alcoholysis of a copolymer VAc/X/Y wherein X is dimethyl maleate or diethyl maleate, and Y is acrylic acid or methacrylic acid.

14. A copolymer of Claim 12 wherein the amount of water in the alcoholysis solvent system is 0-10 weight percent, and the amount of base is about 1-10 percent based on the weight of the starting VAc/X/Y copolymer.

15. A superabsorbing polymer of Claim 14 whwrein the amount of base is about 1-5% based on the weight of the starting VAc/X/Y copolymer.

16. A superabsorbing polymer of Claim 12 wherein X in copolymer VAc/X/Y is dimethyl maleate, and Y is acrylic acid.

17. A superabsorbing polymer of Claim 16 wherein the amount of dimethyl maleate in the copolymer VAc/X/Y from which it is derived is about 4.1-56.7 weight percent, and the amount of acrylic acid in said copolymer VAc/X/Y is about 0.12-1.43 weight percent.

18. A blend of a superabsorbing polymer of Claim 12 with cellulosic fibers.

19. A blend of Claim 18 wherein cellulosic fibers are in the form of cellulose pulp.

20. An absorbent article for sanitary or hygienic use containing an aqueous liquid-absorbing medium wherein the aqueous liquid-absorbing medium comprises a blend of Claim 18.

21. A film, sheet, or nonwoven fabric for medical use made of a superabsorbing polymer of Claim 12.

22. A medium for use in agricultural applications capable of providing moisture to plants or seedlings, said medium comprising a superabsorbing polymer of Claim 12.

23. A dehydrating agent for industrial use comprising a superabsorbing polymer of Claim 12.

24. A bag containing an absorbent medium for absorbing aqueous spills, wherein the absorbent medium comprises a superabsorbing polymer of Claim 12,