CA1178744A - Polymeric electrolytes - Google Patents

Abstract

ABSTRACT
There is provided a polymer useful as a water absorbing agent. The polymer preferably contains the reaction product of an olefinically-unsaturated carboxylic acid, an alkyl acrylate and minor amounts of a crosslinking agent all in specified proportions.

Description

POLYMERIC POLYELECTROLYTES

This invention relates to the preparation of polymeric materials, and particularly to solid, water-swellable, water-insoluble polymers which have utility in absorbent articles.
Various systems have been proposed to produce water-swellable, water-insolu~le polymeric materials.
Such materials have been prepared from carboxyl-containing polymers by polymerization of carboxylic acid monomers in the presence of a crosslinking agent. Such monomers have included acrylic acid, maleic acid, maleic anhydride and the like. Such linear carboxylate polymers include acrylic acid-acrylate copolymers, acrylic acid-acrylamide copolymers, acrylic acid-olefin copolymers.
U.S. Patent No. 3,980,663 discloses producing such poly-mers by reacting a water-soluble low molecular weight polyelectrolyte which cc.ntains nucleophilic carboxyl ions with a crosslinking agent containing two or more sites which are subject to nucleophilic attack by the carboxy-late groups. The process is exemplified by the reaction in an aqueous solution of a sodium salt of an acrylic acid-methyl acrylate copolymer (prepared by adding sodium hydroxide to a latex of the copolymer) crosslinked with glycerine diglycidyl ether. The result-ing product was cast into a film and cured for one hour in an oven. This process and Product~ however, have varicus disadvantages. The crosslinking reaction is slow, ~r r ~ q requiring special large driers to provide the necessary residence time for curing. In addition, crosslinking is achieved by forming diester bridges between linear polymer chains. Such links are subject to hydrolysis and therefore polymer degradation. A further disadvan-tage of this patent is the preparation of a film which can only be formed into a particulate material by grind-ing. This results in a product having a low surface/
weight ratio inherent in a film which results in slow water absorption and the tendency for the swollen surface of the film to block the further penetration by water during an absorption test. Thus, the core of the film contributes its swelling capacity only slowly if at all.
Other polymeric systems have also been proposed with the underlying consideration being the formation of a water-swellable, water-insoluble material that is capable of absorbing large amounts of water without dis-integrating or being solubilized. It has been noted byVerbrugge in an article entitled "Mechanism of Alkali Thickening of Acid-Containing Emulsion Polymers" reported in the Journal of Applied Polymer Science, Vol. 14, pages 897 to 928, 1970, that acid containing latexes during neutralization all undergo a common swelling process leading to complete solubilization fcr the more hydrophilic polymers. Verbrugge described specific neutralization reactions in a series of polymers of varying Tg arld hydrophilicity of the general formula methyl methacrylate/ethyl acrylate/methacrylic acid (.~MA/EA/MAA) utilizing 20 mole % of ~AA and varying ratios of MMA/EA frc,m 50/0 to 0/80. Observations were made of both viscosity changes and visual changes in a light microscope. Work is also disclosed at the higher hydrophilic range of monomer, e.g., utilizing high EA/MAA contents such as 80/20 and 70/30 EA/MAA with zero MMA. The light microscope shows gradual swelling of the particles as the carboxylic acid groups were progressively neutralized. At 80% neutralization, the particles are disclosed as being so highly swollen as to make the phase change from solid to liquid medium barely distinguishable. At 90% and 100% neutralization, the partic-les are gone and a true solution is formed. Polymer latex particles which are less hydrophilic, e.g., those observed from high MMA levels and lower EA and MMA, e.g. 60/20/20 MMA/EA/MAA, result in viscous gels upon neutralization and are seen in the light microscope as barely discernable swollen gel particles with no elear boundary between the swollen par-ticle and the solution. Electron microscope examination of the dried 100% neutralized latex particle shows numerous tentacles projecting from the central core which result in association or sticking together of particles which in turn result in high viscosity. Verbrugge then discloses the prep-aration of four EA/MAA copolymers which are highly cross-linked with ethylene glycol dimethacrylate at 15 and 30%
levels. MAA content is 20 and 30%. The latexes pre~ared at 15% erosslinking swelled upon neutralization and resulted in much higher viscosity than obtained for the uncrosslinked polymer. The 30% crosslinked material because of its extremely high crosslink density shows neither swelling or viseosity.
The present invention is an improvement over the prior art water-swellable, water-insoluble polymers in that it combines specifie short ehain monomers with minor amounts of a erosslinking agent during polymerization. The resulting polymer when neutralized forms an absorbent product which when swollen forms gelled microspheres having a dry-to-the-touch feel. This eoupled with the absorbents' unusual retention of absorbed fluids makes the absorbent of the invention of substantial importance for all articles of manufacture requiring an absorption capability and, for example, in such diverse artieles as surgical and dental sponges, catamenial tampons, diapers, meat trays, paper towels, and disposable litter mats for household pets.
In accordance with the present invention, a polymer has been unexpectedly discovered which comprises (a) from about 15% to about 50% by weight of an olefinically-unsaturated carboxylic acid containing an activated carbon-to-carbon ,~
,,~^ ;.~, ~'c olefinic double bond and at least one carboxyl group; (b) from about 49.07% to about 82% by weight of an alkyl acrylate wherein the alkyl group has from 1 to 6 carbon atoms; and (c) from about 0.03% to about 3.0% by weight of a crosslinking agent which are polyunsaturated polymerizable vinyl monomers containing two or more free radical polymerizable ethylenic groups, said crosslinking agent being introduced directly into the carbon-carbon backbone of the polymer. All indicated amounts are by weight and based upon the total weight of the components of the polymer.
Also provided is an alkaline salt and organic amine salt of the above-defined polymer which materials are useful as a water absorbing agent. Exemplary alkaline salts may be selec-ted from sodium, potassium, lithium and ammonium. Exemplary organic amine salts may be selected from trimethyl amine and triethanol amine.
In the production of the polymers of this invention, a monomeric mixture which contains an olefinically-unsaturated carboxylic acid and an alkyl acrylate is polymerized in the presence of minor amounts of a vinyl crosslinking agent.
Crosslinking is achieved by introducing the vinyl groups of the crosslinking agent directly into the carbon-carbon back-bone of the polymer. Where long term resistance to degrada-tion is required, hydrolytically stable crosslinking agents such as divinyl benzene or triallylcyanurate may be employed.
The olefinically-unsaturated carboxylic acids useful in the invention are those materials containing an activated carbon-to-carbon olefinic double bond and at least one car-boxyl group, that is an acid containing an olefinic double bond. Representative examples of suitable carboxylic acids include acrylic acid, methacrylic acid, ethacrylic acid, crotonic acid, muconic acid, aconitic acid and similar com-pounds as well as mixtures thereof. The preferred carboxylic acids are acrylic acid and methacrylic acid.
The alkyl acrylates useful in the invention are those alkyl acrylates wherein the alkyl group has from 1 to 6 carbon atoms and preferably 1 to 4 carbon atoms. Representative examples of suitable acrylates include methyl acrylate, ethyl acrylate, propyl acrylate, n-butyl acrylate and iso-butyl acrylate.

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The preferred acrylates are methyl acrylate, ethyl acrylate and n-butyl acrylate with methyl acrylate being most preferred. ~ligher chained acrylates have a tendency to become hydrophobic causing the final polymer salt to exhibit lower swelling and water absorption in water and saline solutions.
Other monomers, which do not fall within the description of the monomers described above may be em-ployed in minor amounts, that is up to about 8% by weight,provided they do not adversely affect the basic and novel characteristics of the polymers of this invention.
For example, acrylamide, 2-ethylhexyl acrylate, hydroxy-ethyl acrylate and hydroxyethyl methacrylate may be employed as partial replacement for methyl acrylate or ethyl acrylate, and itaconic acid, maleic acid or maleic anhydride employed as partial replacement for methacrylic acid or acrylic acid.
The crosslinking technique used in the invention to transform water-soluble polymers into insoluble, water-swellable salt polymers is well known as free radical addition polymerization. Crosslinking agents usable according to the invention are polyunsaturated polymerizable vinyl monomers containing two or more free radical polymerizable ethylenic groups. Substan-tially any monomer having more than one polymerizable ethylene group can be used which monomer must be able to enter into vinyl addition polymerization reactions with the foregoing mentioned acids and acrylates. Illustrative examples include:
(1) diacrylate esters and dimethacrylate esters of glycols such as ethylene glycol dimethacrylate, propylene glycol dimethacrylate, butylene glycol dimeth-acrylate and hexylene glycol dimeth-acrylate;
~2) diacrylate esters and dimethacrylate esters of ether or polyether glycols such as diethylene glycol diacrylate, diethylene glycol dimethacrylate, triethylene or tetra-ethylene glycol diacrylate and triethylene or tetraethylene glycol dimethacrylate;
(3) allyl esters of polymerizable unsaturated carboxylic acids such as allyl acrylate, methallyl acrylate, allyl methacrylate, allyl ethacrylate, ethallyl acrylate, methallyl methacrylate;
(4) di or trivinyl aromatic compounds such as divinyl benzene, and trivinyl benzene;

(5) di or triallyl esters of di and tribasic acids such as diallyl phthalate, triallyl cyanurate, diallyl maleate, diallyl succinate, triallyl phosphate, diallyl oxalate, diallyl malonate, diallyl citrate, diallyl fumarate, diallyl ether;
and (6) acrylate or methacrylate esters of polyols such as di or triacrylate or methacrylate esters of trimethylol ethane, trimethylol propane or pentaerythritol.
In order to achieve the desired polymer pro-perties, it is important that the monomers be polymerized together in certain specified proportions, although the exact proportions will vary dependiny on the polymer characteristics desired. The olefinically-unsaturated carboxylic acids of the invention are employed in amounts of from about 15% to about 50~ by weight and most pre-ferably from about 20% to about 40% by weight, based on the total weight of the monomers used. If the amount of carboxylic acid employed exceeds about 50% by weight, the resulting polymer salt becomes excessively hydro-philic while absorbing excessive amounts of water leadingto a (1) soluble polymer, (2) viscous solution or suspen-sion, and (3) loss of polymeric structural integrity.
If the amount of carboxylic acid is less than about 15%, the resulting polymer salt is insufficiently hydrophilic and exhibits poor water absorption.

The alkyl acrylates are employed in amounts of about 49.07% to about 82% by weight and most preferably from about 59.02% to about 78% by weight, based on the total weight of the monomers used. If the amount of the acrylate employed exceeds about 82% by weight, then the resulting polymer in salt form is insufficiently hydro-philic and exhibits poor water absorption. If the amount of acrylate is less than about 49.07~, the resulting poly-mer becomes excessively ~ydrophilic while absorbingexcessive amounts of water leading to a (1) soluble polymer, (2) viscous solution or suspension, and (3) loss of polymeric stxuctural integrity.
The amount of crosslinking agent er~:ployed is desirably limited to an amount from about 0.03% to about 3.0% by weight, preferably from about 0.08% to about

2.0%. This low amount of crosslinking agent has been found sufficient to render the polymer salt water-insoluble while retaining a high degree of water absorbency. Use of less than 0.03% results in a polymer which upon neutralization functions primarily as a thickening agent lacking discrete particle identity.
As the amount of crosslinking agent is increased above 0.03%, the more discrete and rigid the resulting polymer becomes rendering expansion of the salt particles less possible. Water absorbency drops to a commercially un-acceptable level when amounts greater than 3.0% cross-linking agent are used.
The preferred polymer is prepared from a mixture containing as essential ingredients from about 20% to about 40% by weight of an olefinically-unsaturated carboxylic acid selected from methacrylic acid and acrylic acid; from about 59.02% to about 78 by weight of an alkyl acrylate selected from methyl acrylate, ethyl acrylate and n-butyl acrylate, and from about 0.08% to about 2.0~ by weight of a cross-linking agent.
The polymeric constituents should be reacted as completely as possible during polymerization. The polymer may be made by conventional polymerization techniques such as by solution, suspension or emulsion polymerization on a batch, continuous or semi-continuous basis.
Suspension polymeriæation is preferred since this technique results in an acid polymer product as a high surface area granular precipitate having an average particle size between 50 and 400 microns, which product appears to be composed of accretions of smaller particles in the 10 to 50 micron range. A large proportion of these smaller particles appear as high surface area donuts of collapsed spherical shapes with 2 to 5 micron protuberances on their surfaces.
The polymerization reaction is carried out in the presence of a catalyst. The catalysts which form free radicals necessary for the reaction are conventional and are usually organic peroxides, inorganic persulfates and free radical generating azo compounds. The amount of catalyst used is normally from about 0.01 to about 2.0 parts by weight per 100 parts by weight of the total monomeric material to be reacted. Representative examples of organic peroxides include benzoyl peroxide, acetyl per-oxide, bis(p-bromobenzoyl)peroxide, di-t butyl peroxide, t-butyl hydroperoxide, dicumyl peroxide, cumene hydro-peroxide, bis(p-methoxybenzoyl)peroxide, 2,2'-azobisiso-butyronitrile and the like. Exemplary inorganic persul-fates include ammonium, sodium and potassium persulfates.
These may be used alone or in conjunction with sodium or potassium bisulfite. While polymerization is preferably carried out with a free radical catalyst, radiation induced polymerization can also be employed such as high energy X-rays or gamma rays.
Suitable conventionally employed surface active agents and/or colloids may also be used during the polymerization reactions.
Polymerization times, and temperatures may vary considerably depending on the monomer system and catalyst used. The polymerization reaction will generally be completed within at least 30 minutes to several hours at temperatures from around 0C to 100C and preferably within 1 to 4 hours at 65C to 90C for maxim~m efficiency.

In the preferred embodiment suspension poly-merization is conducted in the following marner. A
reactor is charged with deionized water and a suspension agent and deaerated with an inert gas. The reactor may be optionally heated to dissolve the suspension agent.
Previously determined amounts of olefinically-unsaturated carboxylic acid and alkyl acrylate are added either separately or in admixture. Addition may occur at room temperature or at the reaction temperature. The cross-linking agent and catalyst may be added simultaneously with the monomer mixture or separately.
The reactor contents are then agitated by conventional means and heated to commence polymerization at a temperature around the lowest boiling point of the monomers. When methyl acrylate is polymerized, this temperature is about 70C. The reaction is then allowed to continue to polymerize to completion whereupon the reactor contents are cooled. The polymer product may be alternately recovered from the slurry by conventional filtratlon means or directly converted in the slurry to its salt form. The final product slurry can be steam-treated at about 100C to remove any traces of unreacted monomers. Alternately, a highly reactive redox catalyst is added to essentially provide a 100% yield. The slurry can then be filtered to recover the polymer in its non-swelling acid form or neutralized by adding to the final slurry calculated amounts of an aqueous alkaline solution to convert the free acid groups to the salt form. Alter-natively, the filtered polymer cake may be redispersed inwater and neutralized as above or dried, pulverized and then neutralized with alkali.
Typically the solids content of the final reaction mixture is from about 15~ to about 50%. Lower solid contents can be used but are generally undesirable from an economic standpoint.
The resulting polymer products in dried or slurry form may then be stored or used directly in an absorbent article such as by treating a substrate. For example, the article which may be a substrate is treated with the acid polymer which is then neutralized with an alkaline solution to form a water-swellable polymer by conventional neutralization methods. It is to be under-stood for the purpose of this invention that the treatingstep implies a complete particle dispersion on tne sub-strate or discontinuous dispersion. The substrates to be treated may vary widely depending on the end use but include fibrous substrates such as wood pulp, cellulose batting, paper, woven or non-woven cloth and the like.
Any suitable organic or inorganic base may be used during neutralization. Representative compounds include sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonium hydroxide, ammonia, and sodium carbonate. Neutralization may also be achieved with organic amines such as ethanolamine, diethanolamine, tri-ethanolamine, methyl diethanolamine, butyl diethanolamine, diethyl amine, dimethyl amine, trimethyl amine, triethyl amine, tributyl amine, and so forth, and mixtures t~!ereof.
The most preferred base is sodium hydroxide. In short, any basic material may be used which does not adversely affect the polymer composition.
It is critical in the process of the invention, and particularly during neutralization reaction to employ the minimum quantity of water required to effect complete and uniform reaction and neutralization. Effective amounts of water usable in the invention have been found to range fro~ a weight ratio of about 4 to 10:1 and pre-ferably about 5 to 7:1 for tota] water to polymer ratio.
When a ratio of less than 4:1 is employed, there is a danger that all of the free water will be rapidly ab-sorbed during neutralization by only a portion of the acid polymer as it is neutralized leaving little if any water to conduct the remaining neutralizing agent to un-neutralized acid polymer particles. On the other hand,a ratio above 10:1 results in excessive energy expendi-tures during drying to remove the unnecessary large amount of absorbed water.
In order to effect complete neutralization, stoichiometry should be observed between the equivalents of base dissolved in the alkaline neutralizing solution and the carboxylic acid equivalents in the polymer slurry. Thus, at least one equivalent of base, such as sodium hydroxide, per equivalent of carboxylic acid con-taining monomer moiety should be employed. However, as much as a 20 equivalent percent excess of base may be employed to insure that all the carboxylic acid groups are reached by t}!e neutralizing solution. In fact, a slight excess is desirable to drive the reaction and achieve maximum swelling. The excess alkalinity is not harmful since it reacts upon drying by partially saponi-fying the acrylate ester segments, particularly the methyl acrylate, to yield additional sodium acrylate monomer units in the polymer chain backbone.
Drying of the polymer salt may be done by conventional means such as fluid bed drying, rotary kiln drying, tray drying, vacuum drying and oven drying, at sufficient temperatures to remove essentially all of the water associated with the polymer product without re-sulting in polymer decomposition. Products containing up to 5% residual absorbed water are acceptable materials.
Preferred drying temperatures may be between 25C and 150C and most preferably between 80C and 100C.
During drying, the polymer salt particles become agglomerated forming a friable mass of solid particles. ~his mass may be ground to form discrete particles for ease of handling and shipping. Depending on the particular end use desired, it has been found that particles between abou~ 50 microns and about~ millimeters result in a particle that rapidly absorbs water and enables water to pass a mass of previously swollen particles to reach as yet untouched material. It should be recognized that as particle size increases the particle surface area to volume decreases which results in slower water absorption and ultimately larger swelled particles. Likewise, small particle size material when used in bulk tend to absorb water at a slower rate than larger particles as a resultof dec~d int~rticle p3re size and acoon~ngly dccreased peraolation rate.

The polymer product of the invention has an indeterminate "weight average" molecular weisht because of its crosslinking and insolubility in solvents commonly used in the determination of molecular weights. The neutrali~ed polymer is capable of absorbing surrounding water many times its own weight. In doing so, each individual absorbent particle swells or enlarges several times its individual diameter without destruction of the particles' particulate integrity. Absorption of distilled water in amounts greater than 100 X weight have been noted. In 1% sodium chloride solutions up to 30 X
weight increases have occurred whereas in 15% to 25%
sodium chloride solutions up to 10 X weight increases have been noted. This amount of absorption is signifi-cant, especially when it is recognized that the polymer particle substantially immobilizes the same therein and the resulting particulate material retains its structure integrity.
The water-insoluble, water-swellable polymer of this invention is preferably used in an absorbent dressing or article of manufacture requiring water retention ability.
The polymer material when used in the foregoing applications is preferably in ~olid, granular form to provide the largest surface area for absorption and insure that maximum surface area is available for ab-sorbency. There should be at least about 1~ by weight, with possibly up to 90% by weight of the polymer salt present in the absorbent articles based on the total weight of the composite absorbent article.
Depending upon the specific kind of absorbent article, the polymer salt may be dispersed on a carrier sheet or flexible support comprising an absorbent layer of textile fibers, wood pulp fibers, cotton linters and the like as well as mixtures thereof. Such absorbent layers may be those which are commonly used in making absorbent dressings, and articles of manufacture dis-cussed above such as surgical and dental sponges, catamenial tampons, diapers, meat trays, household pet ~ Z'.

litter mats, water separation devices such as filter cores, and the like. These materials may contain multiple layers of carrier sheets or supports and outer covering sheets all well known in the art.
Such absorbent articles may be prepared when the polymeric material is incorporated into the absorbent dressing by conventional procedures according to the in-vention, thus insuring that the desired particulate poly-mer is maintained in the final structure. In the case ofa catamenial tampon, this can be accomplished by spreading the polymer particles as a layer over a layer of absorbent fibers and then winding the layer into the form of a roll so the polymer particles are entrapped therein. Steaming and drying the composite will increase the adhesion of the particles to the fibers. The resultant absorbent material will maintain its particular character as it absorbs water. Other well known methods for preparing such articles are contemplated to be within the scope of the invention.
The water-insoluble, water-swellable polymers of this invention may also be used as a soil modifier to improve water retention and increase air capacity. When blended with soils of varying sand-silt-clay compositions and particularly with high sand contents, significantly improved water retaining capacity is noted. The higher water retaining capacity is observed even at pressures from 1 to 15 bars.
Use of the polymer salts are clearly superior to untreated control soils, the effect being most signifi-cantly seen in the sandier more porous soils. Such soils characteristically lose excessive amounts of water through both evaporation and drainage and the polymers strongly counteract this effect. A concomitant effect of excessive drainage in sandy soils is the loss of nutrients through wash out. The inventive polymers likewise counteract this effect so that nutrients added to the soil are re-tained for longer periods of time.
In view of the outstanding water retention properties of these polymers, they are especially suited for use in preventing wilting of plants grown in sandy, free draining soils such as are found in the arid regions of the earth. They are beneficial in maintaining plant viability during periods of drought and are particularly useful in turfs constructed of sand or free draining sandy soils such as are used in golf greens, athletic arenas and walks. They are also useful in nurseries, green-houses, home gardens, in artificial soils and in potted plants. The expense and need for constant watering and surveilance is substantially reduced. Thus, they ease water management burdens by holding more water in a plant bed and reduce the amount of field irrigation or greenhouse watering thus saving labor. They will also improve marginal lands by increasing water holding capabilities of soils making them suitable for crop production.
Finally, they are useful in loosening soils which have been compacted by natural or manmade processes such as grassed or bare pathways, plow pans or fragipans. They are also useful in transplants where the water retention properties and the close contact between the root tendrils and the water swollen polymer particles reduces transplant shock and loss of plant stock. A further use of this polymer is in seed coats which provide a water rich, drought resistant environment for germination and initial stages of growth.
When used in soils, the preferred pol~mer material is the sodium salt of 65/35/.1 EA/MAA/EGDMA.
While generally not equal to the sodium salt in terms of water retention properties, the potassium salt offers an advantage in supplying a nutrient which is not easily washed out and is available to the plant over an extended ~rowing period. This is due to the fact that the potassium in the form of the counter ion to the polymer is only slowly released in comparison to the solubilization rate of typical soluble synthetic potassium containing fertili-zers. Potassium fertilization and water retention can thus be optimized by utilizing blends of both sodium and potassium polymer salts.

As used herein, the term soil refers to any medium in which plants can be grown and which provides a means for support, oxygen, water and nutrients. Exemplary materials include natural growth media and artificial or unnatural growth media such as glass beads, foamed organic materials such as foamed polystyrene or polyurethane, calcined clay particles, comminuted plastic and the like.
The following examples are given to further illustrate the invention but are not deemed to be limiting thereof. All parts and percentages given are based upon total weight unless otherwise indicated.
The following procedure is employed in the examples where applicable to determine water and saline solution absorption. Add one gram of the polymer salt to 100 grams of a 1% NaCl solution or 200 grams distilled water. Allow to stand without agitation for two hours.
Filter the material using a Whatman No. 4 filter paper until no water passes through (usually 5 to 15 minutes).
Weigh the swollen product, subtract the weight of the polymer, i.e. one gram, and repor the results as swell-ing value. The rate of absorption is determined by adding one gram of the polymer salt into 30 grams distilled water in a small jar. The contents of the jar are stirred gently by shaking. The time necessary for the polymer to turn completely solid, that is, the product does not flow freely when the jar is inverted, is observed and recorded.

EXAMPLE I
This Example illustrates a method for preparing the polymers of this invention by the preferred suspen-sion polymerization technique.
A polymerization reactor was charged with2,000 grams of deionized water and 3 grams of Cellosize QP-4400 (product o~ ~nion Carbide which is a hydroxyethyl cellulose powder having a 2% viscosity of 4,000 to 6,000 cps) as a suspending agent. The contents of the reactor were heated to 65C until the hydroxyethyl cellulose dissolves and then cooled to 35C.
To the reactor was then added with agitatio~, a mixture containing 325 grams methyl acrylate, 175 grams glacial methacrylic acid, 0.5 grams ethylene glycol dimethacrylate as crosslinking agent and 0.5 grams azobisisobutyronitrile as catalyst. The contents of the reactor were deaerated by purging with nitrogen which is passed therethrough at a moderate flow rate (e.g., 100 milliliters per minute). The temperature was raised to 70C and the mixture allowed to polymerize for three hours. In the last hour the reactor temperature was raised to 80C to complete the reaction. The total reaction time was three hours. The contents of the reactor was continuously agitated during the entire polymerization reaction.
Upon completion of the reaction, the reactor slurry was cooled to 25C and filtered by pa~sing the slurry through a vacuum filter. Tne filter cake weighed 1,000 grams. To analyze the product and determine yield, a one-fifth portion of the filter cake is dried for 16 hours in a vacuum oven at 80C and ground to an average 40 mesh size (U.S. Standard Sieve size) to prepare a granular acid polymer containiny approximately 34 to 36%
methacrylic acid. The percent conversion of monomers to polymer product was 97.0%.

EXAMPLE I (continued) The remaining portions of the filter cake were neutralized with a basic solution of sodium hydroxide by separately dispersing each remaining one-fifth portion of the cake in 400 grams of deionized water followed by rapid and complete addition of 195 grams of a 10% sodium hydroxide solution (e.g., a 20% excess over the theoreti-cal amount necessary to neutralize the polymer). The resulting masses were dried at 80C in a vacuum oven to remove contained water and then ground to a size to pass through a 20 mesh screen, U.S. Standard Sieve size.
The saline absorbency of the polymer salt was determined in a 1~ sodium chloride solution. The filter cake weighed 24 grams which corresponds to a water pickup of 23 times its own weight. In the water absorb-ency test, water pickup of about 100 times the weight of the polymer was achieved.

EXAMPLES 2 to 17 Examples 2 to 17 prepare polymers by the sus-pension polymerization technique of Example 1 using different weight ratios of methyl acrylate (MA), methacrylic acid (MAA), and ethylene glycol dimetha-crylate (EDMA) as crosslinking agent in the amounts as set forth in Table 1. The polymers of Examples 2 to 12 were neutralized according to the process of Example 1 except that a lithium hydroxide solution was used in lieu of the soc'ium hydroxide solution. The procedure of Example 1 was used for neutralizing the acid polymers of Examples 13 to 17.
In Table I, the absorbency capacity of the alkaline polymers is set forth in amounts times the dry weight of the polymer. Absorbency values are given for deionized water, a 15~ calcium chloride solution and a 15~ sodium chloride solution. For convenience of computa-tion, the recited amounts of crosslinking agent represent that amount of material added to the other two monomers used.
The results clearly indicate that the polymers of this invention have a high absorbency capability for water and saline solutions over a wide range of monomer and crosslinking agent concentrations. The data also shows that water pickup decreases as the crosslinking agent level increases. Products obtained having about 95%
water absorption content consisted of hard swollen particles retaining structural integrity which are dry to the touch. At higher water absorption levels the swollen particles are slightly softer but are discrete and nonagglomerated particles being semi-dry to the touch.

t~j' <
--lg--TABLE I
Aksorption TLmes Dry Weight 5Example r_ r~EDM~ ~ter 15% CaC12 15% NaCl

3 8020 1 18 3.2 4.8

4 8020 0.5 22 4.9 5.5 7525 1 18 4.8 6.
6 70 30 1 17 5.5 6.3 7 6040 1 16 4.8 NT
8 8515 1 13 1.8 NT
9 7525 0.5 28 5.8 6.9 7525 0.3 37 6.1 8.6 1175 25 0.2 44 5.3 8.4 12 752~ 0.1 60 8.1 10.9 13 7030 1 17 5.5 6.3 14 7030 0.5 31 6.2 7.8 7030 0.3 57 7.7 10.1 1670 30 0.1 86 9.1 13.0 17 7030 0.05 105 8.6 13.0 M~ = methyl acrylate M~A ' methacrylic acid E~MA = et.~ylene glyool dimethacrylate EXAMPLES 18 to 24 Examples 18 to 24 demonstrate the preparation of polymers according to the procedure of Example 1 using different weight ratios of various monomers.
The polymers of Examples 18, 20 and 21 were neutralized according to the process of Example 1 except that a lithium hydroxide solution was used in lieu of the sodium hydroxide solution. Examples 19 and 22 to 24 employed the neutralization process of Example 1 with sodium hydroxide. The amounts of polymer components used and absorbency test results are recited in Table II.
For convenience of computation, the recited amounts of crosslin~ing agent represent that amount of material added to the other monomers used.
The results indicate that acrylic acid yields approximately equivalent results as methacrylic acid in swelling in water. For swelling in the presence of high electrolyte concentrations, the ~ethacrylic acid is pre-ferred. While the results indicate that all of the polymers tested are sensitive to high CaC12 and NaCl concentrations, the polymers still maintain a high level of absorbency while retaining structural integrity.
~xample 24 also demonstrates the utility of a polymer having a portion of the carboxylic acid monomer re-placed with a minor amount of a different acid monomer.

5 ~

1 0~ ~ ~ N ~ ~ ~

.~ ~ 2-o o o ~

~1 ~ o o L-+~
3s ~ æ y ~ 5 æ ~ ~

EXAMP1ES 25 and 26 Examples 25 and 26 used polymers prepared by the suspension polymerization technique of Example 1 using different weight ratios of monomers and crosslink-ing agents. The acid polymers were neutralized with basic solutions prepared from lithium hydroxide, sodium hydroxide, potassium hydroxide and triethanolamine.
The monomers employed are m~l acrylate (MA), methacrylic acid (MAA), and ethylene dimethacrylate (EDMA1. The amounts of monomers employed and the results are set forth in Table III. The amount of crosslinking agent represents that amount added to the two other monomers used.
The results demonstrate the effect of varying the neutralization agent and accordingly the resultant polymer salt product. The results indicate that sodium and lithium polymers have essentially equal absorption in water although the lithium compound exhibits superior results in electrolytic solutions. The potassium polymer exhibits lower swelling efficiency although it has par-ticular use in agricultural applications because of its potential nutrient value. The triethanolamine salts are still lower in absorption capacity, yet would be quite useful in cosmetic applications.

TABLE I I I

AbsorptioIl Times ~ ight 10 Exan~le M~ M~A E~ Salt Water 3.5% CaCl~

80 20 2 Li 11 3 . 2 Na 12.4 2 K 9 1.8 TE~ 5 1. 3 26 80 20 1 Na 16 2 . 3 Li 18 3.2 ~0 EXAMPLES 27 to 40 This example demonstrates the effect of vary-ing the amount of polymer components to prepare water-swellable, water-insoluble polymers. The procedure of Example 1 was repeated to prepare the polymer-sodium salt. The amounts of monomers used and results are set forth in Table IV.
A single system consisting of methyl acrylate, methacrylic acid and ethylene dimethacrylate (MA/M~A/EDMA~
was studied over an MA range of 50 to 80, MAA from 20 to 50, and EDMA from .03 to .3. The results indicate that for each crosslinking level maximum swellinq in 1% NaCl was obtained with polymers around 35-40% MAA. The poly-mer particles which gave swollen particles of highest structural integrity were found within the ~A concen-tration range of about 20 to 40%. Beyond 50% MAA, and within the .3-.03 crosslinking range, structural integrity of the swollen particle was lost. Structural integrity was likewise degraded at less than .03 crosslinking ~o that the neutralized polymer in water gave undefined, unfilterable particles which were indistinguishable from the gelled viscous slurries or solutions of the prior art.

.

Absorption of 1% NaCl Example MA ~A EDMA Times Dry Weight 27 80 20 0.3 5 28 75 25 0.3 8.5 10 29 65 35 0.3 8.7 50 50 0.3 8.7 31 75 25 0.1 17 32 65 35 0.1 20.5 33 60 40 0.1 20 15 34 50 50 0.1 17.5 75 25 0.05 18 36 70 30 0.05 24 37 50 50 0.05 25 38 80 20 0.03 17 20 39 65 35 0.03 32.5 50 50 0.03 26.5 MA = methyl acrylate M~ = methacrylic acid El~ = ethylene glyo~1 dimethacrylate EXAMPLES 41 to 46 Examples 41 to 46 prepare polymers by the suspension polymerization technique of Example 1 using methyl acrylate and methacrylic acid with various amounts of different crosslinking agents. The results are set forth in Table V.
The results indicate that the polymer prepared with diethylene glycol dimethacrylate at 0.1% and 0.05 gives swelling ratios in 1% NaCl of 25 and 34X, respectively. Thus, the lower the crosslinking level gives the more easily hydratable and expandable polymer.
The useful lower limit is reached, however, at .03~ be-cause beyond that point the particles no longer expand to an equilibrium absorption level where the swollen particles maintain their structural integrity, but rather absorb so much water that structural integrity is lost. In the latter instance, the particles become very soft, fuse together and eventually give a viscous gel or even a viscous polymer solution. Extending beyond diethylene glycol dimethacrylate, higher glycols such as tetraethylene glycol, as its dimethacrylate derivative, has been used to increase the hydrophilicity of the crosslinking unit. Allyl methacrylate is an example of a crosslinking agent where one vinyl group is in the alkyl group of an acrylate ester and the second is in the carboxvlate group, i.e., the methacrylate group.
Divinyl benzene is an example of a hydrocarbon cross-linking agent containing no hydrolyzable ester groups and, accordingly, the resulting polymer is structurally completed with hydrolytically stable carbon-carbon links througho-~t itsbackbone and crosslink bridges.

-27,-TliBLE V

Absorptian of 1% NbCl EXample M~ M~A X-linker Times Dry W~ight 41 65 35 0.1 diethylene glycol 25 dimethacrylate 42 65 350.05 diethylene glyoDl 34 dimethyacrylate 43 65 350.1 trimethylol propane 23.5 trimethacrylate 44 65 350.1 allyl nethacrylate 21 350.2 tetraethylene glycol 18 dimLth=~ylate 46 65 35 0.1 divinylbenzene 23 M~ = ~ethyl acrylate M~A = methacrylic acid Thi~ example illustrates a method for preparing the poly~ers of this invention and neutralizing the polymer slurry in the presence of various surfactants.
A polymerization reactor was charged with 2,000 grams of deionized water and 3 grams of Cellosize QP-4400 (product of Union Carbide which is a hydroxyethyl cellulose powder having a 2~ viscosity of 4,000 to

6,000 cps) as a suspending agent. The contents of the reactor were heated to 65C until the hydroxyethyl cellulose dissolves and then cooled to 35C.
To the reactor was then added with agitation, a mixture containing 325 grams methyl acrylate, 175 grams glacial methacrylic acid, 0.5 grams ethylene glycol di-methylacrylate as crosslinking agent and 0.5 grams azobisisobutyronitrile as catalyst. The contents of the reactor were then deaerated by purging with nitrogen which is passed therethrough at a moderate flow rate ~e.g., 100 milliliters per minute). The temperature was raised to 70C and the mixture allowed to polymerize for three hours. In the last hour the reactor temperature was raised to 80C to complete the reaction.
The total reaction time was three hours. The contents of the reactor was continuously agitated during the entire polymerization reaction.
Upon completion of the reaction, the reactor slurry was cooled to 25C. The slurry was diluted with 500 grams water in order to allow better agitation and flow.
One-fifth aliquot portion (600 grams) of the slurry was transferred to a steel beakPr and agitated very rapidly (3000 RPM) with a dispersator. To this was added 5% Aerosol OT as a surfactant (a dioctyl sodium sulfosuccinate from American Cyanamid). To the suspension was then added 195 grams of 10% NaOH and the slurry solidified at once into a soft swollen, friable, granular mass. The weight ratio of water to polymer in the neutralization step was 6.76 to 1. The friable solid d `~ ` ~

sodium polymer salt was then dried at 80C in a vacuum oven to remove contained water and then ground to a size to pass through a 20 mesh screen, U.S. Standard Seive size. The resulting dried product absorbed 100 times its weight in water. In a 1~ NaCl solution, 21 times its weight was absorbed. The rates of absorption were 8 seconds and 21 seconds, respectively.

4~

This Example demonstrates the preparation of various absorbent articles with the polymers of this invention.
A. 0.5 Grams of finely ground dry polymer salt from Example 1 was added slowly to a 250-gram slurry of defiberized cellulose containing 5 grams cellulose.
After mixing for 2 minutes, the slurry was diluted in the headbox of a conventional Noble and Wood Laboratory Sheet Machine and formed into a 12" x 12"
handsheet dried and tested for water absorbency.
The article contained 10% polymer salt. The dry paper absorbed 12 times its weight of water.
B. A polymer suspension, prepared according to Example 47 was diluted to 15% solids content and neutralized with 20 mole ~ excess of 10~ NaOH solu-tion. The neutralized polymer swelled and absorbed all of the water to form a solid, friable paste.
This was then diluted with water to 0.5% solids.
100 Grams of the slurry was added to a 250-gram slurry of a beaten cellulose pulp containing 5 grams cellulose. This was also converted to a 12" x 12"
handsheet as discussed above and tested for water absorbency. The dry paper absorbed 6.6 times its weight of water.
C. Six sheets of 4" x 10" pieces of paper toweling weighing 3.5 grams were layered upon one another in a stack and dusted evenly between the layers with 3.5 grams of finely ground polymer salt from Example 1. The resulting multilayered pad was sprayed with water mist until the weight increased to 20 grams. The wet pad was then dried at 80C to fuse the layers together. The pad was tightly rolled into an oblong shape, immersed in a 1% NaCl solution for a few minutes, and then pressed under 1.5 lbs./sq. in. pressure for 5 minutes to determine the extent of saline absoxption. The pad absorbed 9 times its weight of 1~ NaCl solution. A similar ~.a ~1~7 ~ ~

control towel processed as discussed without the polymer absorbed 3 times its weight of the saline solution.

~.a ~

CO~IPARATIVE E~PLES A to G

This example compares the water and saline solution absorption rates of various commercially available water-swellable, water-insoluble polymer products with the dried polymer salt prepared according to Example 1. The polymers tested and their known chemical description and results are set forth in Table VI. The results indicate that the polymers of the invention are unexpectedly superior water-swellable, water-insoluble compositions.
In the example, comparat~ve material A is identified commercially as AQUALON and is a product of Hercules Inc. Comparati~ material B is identified commercially as VITERRA-l and is a product of Union Carbide Corporation. Com~arative material C is identified commercially as VITERRA-2~and is a product of Union Carbide Corporation. Comparative material D is identi-fied commercially as P~RMASORB-10 and is a product of National Starch & Chemical Corporation. Comparative material E is identified commercially as PERMASORB-30 and is a product of National Starch & Chemical Corporation.
Co~parative material F is identified commercially as H-SPAN~and is a product of General Mills Chemicals, Inc. The material is produced under license from the U.S. Department of Agriculture. Compa~ative material G
is identified commercially as DOWFLA~E and is a product of Dow Chemical Company and prepared according to U.S. Patents 3,980,663 and 3,993,616.

;, ,~
r~

TABLE VI

Absorption Times Dry Weight Water 1% NaCl Inventive Product 100 21 10 Comparative A 16 5 fibers of internally crosslinked sodium carboxymethylcellulose -Comparative B 13 crosslinked polyethylene oxide 15 Comparative C Gelled Gelled mass. mass.
acrylamide-Na Cannot Cannot acrylate copolymer filter. filter.
Comparative D Gelled Gelled mass. mass.
acrylic po~ymer with Cannot Cannot hydrophilic carboxylate filter. filter.
-Comparative E
acrylic polymer with Cannot 20.4 hydrophilic carboxylate groups filter.
Comparative F
hydrolyzed starch polyacrylamide Cannot Cannot _ graft filter. filter.
Comparative G Gelled Gelled crosslinked salt of mass. mass.
linear carboxylic acid Cannot Cannot polyelectrolyte filter. filter.

t -3~-The invention being thus described, it wi~l be obvious that the same may be varied in many ways.
Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications are intended to be included within the scope of the following claims.

Claims (11)

Claims:

1. A polymer which comprises:
(a) from about 15% to about 50% by weight of an olefinically-unsaturated carboxylic acid containing an activated carbon-to-carbon olefinic double bond and at least one carboxyl group;
(b) from about 49.07% to about 82% by weight of an alkyl acrylate wherein the alkyl group has from 1 to 6 carbon atoms; and (c) from about 0.03% to about 3.0% by weight of a crosslinking agent which are polyunsaturated polymerizable vinyl monomers containing two or more free radical polymer-izable ethylenic groups, said crosslinking agent being introduced directly into the carbon-carbon backbone of the polymer.

2. The polymer of Claim 1 comprising from about 20%
to about 40% by weight of the olefinically-unsaturated carboxylic acid selected from the group consisting of methacrylic acid and acrylic acid.

3. The polymer of Claim 2 wherein up to 8% by weight of the total amount of olefinically-unsaturated carboxylic acid present is replaced with a material selected from the group consisting of itaconic acid, maleic acid, and maleic anhydride.

4. The polymer of Claim 1 comprising from about 59.02% to about 78% by weight of the alkyl acrylate selected from the group consisting of methyl acrylate, ethyl acrylate and n-butyl acrylate.

5. The polymer of Claim 1 comprising about 0.08% to about 2% by weight crosslinking agent.

6. A polymer salt useful as a water absorbing agent, which comprises the organic or inorganic base salt of a polymer consisting essentially of the suspension polymerization reaction product of (a) from about 15% to about 50% by weight of an olefinically-unsaturated carboxylic acid containing an activated carbon-to-carbon olefinic double bond and at least one carboxyl group;

(b) from about 49.07% to about 82% by weight of an alkyl acrylate wherein the alkyl group has from 1 to 6 carbon atoms; and (c) from about 0.08% to about 2.0% by weight of a crosslinking agent which are polyunsaturated polymerizable vinyl monomers containing two or more free radical polymer-izable ethylenic groups, said crosslinking agent being introduced directly into the carbon-carbon backbone of the polymer.

7. The water-insoluble polymer of Claim 6 comprising from about 20% to about 40% by weight of the olefinically-unsaturated carboxylic acid selected from the group eon-sisting of methacrylic acid and acrylic acid.

8. The water-insoluble polymer of Claim 7 wherein up to 8%
by weight of the total amount of olefinically-unsaturated carboxylic acid present is replaced with a material selec-ted from the group consisting of itaconic acid, maleic acid and maleic anhydride.

9. The water-insoluble polymer of Claim 6 comprising from about 59.02% to about 78% by weight of the alkyl acrylate selected from the group consisting of methyl acrylate, ethyl acrylate and n-butyl acrylate.

10. The water-insoluble polymer of Claim 6 comprising about 0.08% to about 2% by weight crosslinking agent.

11. The water-insoluble polymer of Claim 6 wherein the salt portion of the polymer is selected from the group consisting of sodium, potassium, lithium and ammonium.