Water absorbent acrylic copolymers.

The present invention relates to a method of manufacturing polyacrylate copolymer resins having improved water absorbing properties and more particu­larly to an improved process of preparing cross-linked copolymers of acrylic acid, styrene, and polyvinyl monomers.

Water absorbing resins have found wide use in sanitary goods, hygenic goods, water retaining agents, dehydrating agents, sludge coagulants, thicken­ing agents, condensation preventing agents and release control agents for various chemicals. Water absorbing resins heretofore known include hydrolysis products of starch-acrylonitrile graft polymers, carboxymethyl­cellulose, cross-linked polyacrylate products and other resins such as polyvinyl alcohol, polyethylene oxide and polyacrylonitrile resins. Of these water absorbing resins, the hydrolysis products of starch and acrylo­nitrile graft polymers have comparatively high ability to absorb water but require a cumbersome process for production and have the drawbacks of low heat resist­ance and decaying or decomposing easily due to the presence of starch.

One of the processes for polymerizing acrylic acid and acrylates is aqueous solution polymerization. The polymer obtained by this process is soluble in water and, therefore, is cross-linked to modify the polymer into a useful water absorbing resin. However, even if the modification is effected by reacting a cross-linking agent concurrently with or after aqueous solution polymerization, the resulting reaction product is in the form of a highly viscous aqueous solution or a gel containing absorbed water which is difficult to handle. Thus, the aqueous solution or gel must be dehydrated (dried) to obtain a water absorbing resin in the desired solid or powder form. It is neverthe­less difficult to dry the reaction product efficiently by the usual rotary drum roller method or spray drying method because care must be taken to avoid excessive cross-linking which results from over-heating during drying and insufficient drying results in reduced cross-linking density. Extreme difficulties are there­fore encountered in preparing a product of a desired low water content and good water absorbing ability.

It is an object of the present invention to provide a process for preparing a water absorbing cross-linked acrylate resin copolymer of low water content by aqueous solution polymerization without any additional dehydrating or drying step.

Another object of the present invention is to provide a process for preparing a cross-linked polyacrylate resin copolymer by copolymerization of partially or fully neutralized acrylic acid; a styrene monomer; and a water-­miscible to water soluble polyvinyl monomer in a com­bined concentration of 30 to 80% by weight partially or fully neutralized acrylic acid and 1% to 25% styrene and/or methyl methacrylate, based on the weight of acrylic acid, in water and initiating polymerization.

According to the present invention there is provided a process for preparing a solid water absorbing resin, characterised in that it comprises mixing a monomer solution of (A) acrylic acid neutralized 70-100 mole percent; (B) styrene in an amount of 1% to 25% based on the weight of acrylic acid and (C) a water-miscible to water-soluble polyvinyl monomer in a combined concen­tration of (A), (B) and (C) of at least 30 wt. %; with water to form a mixed monomer solution and initiating polymerization of said monomers such that during poly­merization, the exothermic heat of reaction is substan­tially the only heat energy used to accomplish poly­merization, cross-linking and to drive off sufficient water to obtain a solid cross-linked resin having a water content of 15 percent by weight or less.

In accordance with an important embodiment of the present invention, a heated aqueous solution comprising (A) acrylic acid neutralized 70 to 100 mole percent, for example with ammonia, and/or caustic alkali and/or an amine; and (B) styrene in an amount of 1% to 25% based on the amount of acrylic acid and acrylate, computed as based on acrylic acid; and (C) a water-­miscible to water-soluble polyvinyl monomer; water and, when desired, an organic solvent having a boiling point of 40 to 150°C, and having a combined monomer concen­tration of (A) plus (B) plus (C) of 30 to 80 wt. % is subjected to copolymerization in the presence of a polymerization initiator without external heating while allowing water to evaporate off.

An embodiment of the present invention will now be more particularly described by way of example and with reference to the drawings wherein: <ul id="ul0001" list-style="none"><li>FIG. 1 is a graph obtained using a gravi­metric absorbancy system (GATS) showing the water absorbancy vs. time for an acrylic hompolymer;</li><li>FIG. 2 is a graph obtained using a gravi­metric absorbancy system (GATS) showing the water absorbancy vs. time for an acrylic-styrene copolymer having 2% styrene; and</li><li>FIGS. 3 and 4 are graphs obtained using a gravimetric absorbancy system (GATS) showing the water absorbancy vs. time for acrylic-styrene copolymers having 4% and 6% styrene, respectively.</li></ul>

A cross-linked polyacrylate resin copolymer is prepared by aqueous solution polymerization while dehydrating or drying the reaction product during poly­merization by utilizing the exothermic heat from the copolymerization and cross-linking reactions for drying.

It has been found that acrylic acid neutralized in the range of 70 to 100 mole percent will copolymerize with styrene and cross-link rapidly with a polyvinyl monomer cross-linking agent to drive away excess water leaving a solid water absorbing resin copolymer having a desired degree of polymerization as well as new and unexpected water absorbing capacity. One or more poly­merization catalysts or initiators can be added to the aqueous monomer mixture to aid in polymerization.

A hot aqueous solution is prepared first which comprises acrylic acid neutralized 70 to 100 mole percent; styrene; a water-miscible or water-soluble polyvinyl monomer; water and, when desired, an organic solvent having a boiling point of 40 to 150°C, and which contains the acrylate monomer; styrene; and the polyvinyl monomer in a combined concentration of 30 to 80 wt. %.

To achieve the full advantage of the present invention, the acrylate; styrene; and polyvinyl monomers are present in a combined concentration of less than 70 weight percent of the monomer solution. In accordance with another important embodiment of the present invention, the combined concentration of the acrylate, styrene, and polyvinyl monomers is less than 55 weight percent of the monomer solution. The concentration of the monomers is deliberately determined considering the state of the solution (i.e., as to whether or not the monomers can be completely dissolved in water), ease of the reaction of the monomers, escape of the monomers due to the scattering during the reaction, and the like.

The aqueous solution can be prepared easily usually by placing acrylic acid, a strong alkali such as potassium hydroxide and/or ammonium hydroxide or a basic amine for neutralizing the acid, styrene, and the polyvinyl monomer into water in such amounts that the resulting solution has the above-mentioned monomer concentration. To thoroughly mix the monomers, the mixture can be heated to an elevated temperature. Any strongly basic alkali metal compound can be used for neutralization of the acrylic acid, such as potassium hydroxide, sodium hydroxide, lithium hydroxide, cesium hydroxide, potas­sium carbonate or sodium carbonate. Although it is desirable to use the neutralizing agent usually in an amount sufficient to neutralize acrylic acid 100 mole %, there is no particular need to neutralize the acid 100% and the neutralizing agent, e.g., hydroxide, is used in such an amount as to achieve not less than about 70% neutralization. Accordingly, the aqueous solution may contain up to about 30 mole percent of free acrylic acid based on the amount of acrylic acid and acrylate. However, a large quantity of free acrylic acid, if present in the aqueous solution, is likely to partly splash out of the reaction vessel, resulting in a loss during the reaction, leading to a reduced degree of polymerization. Use of an excessive amount of the neutralizing agent will not raise any particular problem, but the excess does not participate in the polymerization reaction and is therefore use­less.

We have also found that when the aqueous solution further contains an organic solvent having a boiling point of 40 to 150° C, the temperature of the aqueous solution is controllable with great ease and the resulting cross-linked resin copolymer has remark­ably improved ability to absorb water at an initial rate.

When incorporating an organic solvent accord­ing to the invention, the aqueous monomer solution has a solidifying point which is about 10 to about 20° C lower than otherwise. This increases the allow­able range of temperature control at least about 3 times. The organic solvent used is vigorously evaporated along with water by the heat of polymeriza­tion of the monomers. Since the latent heat of the evaporation of the organic solvent is considerably smaller than that of water, the organic solvent func­tions as a blowing agent in the polymerization reaction systems, consequently rendering the resulting resin porous. The resin exhibits about 2 to about 5 times higher initial rate of water absorption than the one obtained without using the organic solvent while pos­sessing high water absorbing ability. Thus, the or­ganic solvent, when added to the aqueous monomer solu­tion, produces improved effects without in any way impairing the advantages resulting from the use of the monomer solution.

Examples of organic solvents to be used in the invention when desired and having a boiling point of 40 to 150° C are methanol, ethanol, propanol and like alcohol solvents, acetone, methyl ethyl ketone and like ketone solvents, cyclohexane, n-hexane, n-­heptane and like hydrocarbon solvents, benzene, toluene and like aromatic hydrocarbon solvents, and tetrahydro­furan and like furan solvents. These solvents may be used singly or in admixture. The solvent is used in an amount of 0.5 to 15 wt. %, preferably 1 to 10 wt. %, based on the combined amount of the acrylic acid, styrene, and polyvinyl monomers. With less than 0.5 wt. % of the solvent present, a sufficient blowing action will not take place, while the solidifying point of the monomer solution will not lower greatly. Conversely if more than 15 wt. % of the solvent is used, the resulting resin is likely to exhibit reduced water absorbing ability although achieving a high initial rate of water absorption. Moreover the monomers are likely to separate out, hence objectionable. Because the monomer solution is heated prior to polymerization and further because the organic solvent evaporates along with water, the boiling point of the solvent is more preferably in the range of 55 to 120°C.

Acrylic acid neutralized 70-100 mole percent is mixed with 1% to 25%, based on the weight of acrylic acid, styrene; and a water-miscible or water-soluble polyvinyl monomer in an aqueous solution at a temperature of about 20 to 100° C. The solution is subjected to a polymerization reaction and a cross-linking reaction by the addition of a polymerization initiator. The poly­merization reaction proceeds sufficiently within a very short period of time and if the monomer concentration is at least 30 percent by weight of the aqueous monomer mixture, the heat of the copolymerization and cross-­linking reactions will evaporate water rapidly from the reaction system to form a dry solid (less than 15 percent by weight water) water absorbing resin without the need for any subsequent drying step. The solid can be easily pulverized into a powder suitable for any desired use.

A hot, i.e., at least 25°C, aqueous solution is prepared first including acrylic acid neutralized 70 to 100 mole percent; styrene in an amount of 1% to 25% based on the weight of acrylic acid; optionally acrylamide; a water-miscible or water-soluble polyvinyl monomer; and water. The aqueous solution can be prepared easily by placing (A) acrylic acid, and an amine, and/or a caustic alkali and/or ammonia for neutralizing the acid; (B) styrene in an amount of 1% to 25% based on the weight of acrylic acid; (C) acrylamide (0-30 mole percent based on acrylic acid); and (D) a polyvinyl monomer into water to form a mixed monomer solution. To dissolve the monomers thoroughly, the mixture can be heated to an elevated temperature up to the boiling point of water, i.e., 100° C.

The polyvinyl monomer to be used in accord­ance with the present invention should be miscible with or soluble in water so that the monomer will be uniformly dissolved or dispersed in the aqueous solu­tion of the monomer mixture. Examples of such poly­vinyl monomers include trimethylol propane triacrylate, bisacrylamides such as N,Nʹ-methylenebisacrylamide and N,Nʹ-methylenebismethacrylamide; polyacrylic (or polymethacrylic) acid esters represented by the follow­ing formula (I); and diacrylamides represented by the following formula (II). Among these, especially pre­ferably are N,Nʹ-methylenebisacrylamide, N,Nʹ-methy­lenebismethacrylamide and like bisacrylamides.

Formula (I)

<chemistry id="chem0001" num="0001"><img file="EP0242478A2_D0001.tif" /></chemistry> wherein X is ethylene, propylene, trimethylene, hexa­methylene, 2-hydroxypropylene, (CH₂CH₂O)_nCH₂CH₂ -or <chemistry id="chem0002" num="0002"><img file="EP0242478A2_D0002.tif" /></chemistry> n and m are each an integer of from 5 to 40, and k is 1 or 2.

The compounds of formula (I) are pre­pared by reacting polyols, such as ethylene glycol, propylene glycol, trimethylolpropane, 1,6-hexanediol, glycerin, pentaerythritol, polyethylene glycol and polypropylene glycol, with acrylic acid or methacrylic acid.

Formula (II):

<chemistry id="chem0003" num="0003"><img file="EP0242478A2_D0003.tif" /></chemistry> wherein ℓ is 2 or 3.

The compounds of the formula (II) are ob­tained by reacting polyalkylenepolyamines, such as diethylenetriamine and triethylenetetramine, with acrylic acid.

The polyvinyl monomer is used in an amount of about 0.001 to 0.3 wt. % based on the amount of acrylic acid, styrene, methyl methacrylate and acryl­amide monomers in the aqueous monomer mixture.

The polyvinyl monomer cross-linking agent should be present in the aqueous solution in an amount of at least .2 wt. % based on the total weight of monomers to provide a resin sufficiently cross-linked to have a "dry feel" after significant water absorp­tion. If the polyvinyl monomer is included in the aqueous solution in an amount of .2 to .6 weight per­cent based on the weight of neutralized acrylic acid and polyvinyl monomers, the resulting polymer will have an exceedingly "dry feel" on absorption of water.

The aqueous mixed monomer solution is heated and thereafter subjected to copolymerization and cross-­linking reactions with the addition of a polymerization initiator. Although the temperature of the aqueous mixed monomer solution is not particularly limited since the mixed monomer solution is initiated into polymerization by the addition of the initiator, the temperature is usually about 50 to about 85° C, pre­ferably about 60 to about 75° C.

Various polymerization initiators are usable which are known for use in preparing polyacrylates. Examples of useful initiators are redox initiators comprising a reducing agent, such as a sulfite or bisulfite of an alkali metal, ammonium sulfite or ammonium bisulfite, and an initiator, such as a per­sulfate of an alkali metal or ammonium persulfate, in combination with the reducing agent; azo initiators including azobis-isobutyronitrile, 4-t-butylazo-4ʹ-­cyanovaleric acid (V-50), 4,4ʹ-azobis(4-cyanovaleric acid) and 2,2ʹ-azobis(2-amidinopropane)-hydrochloric acid salt; and the like. These initiators can be used singly or in a suitable combination. Of these, especially preferable are a redox initiator composed of ammonium persulfate and sodium hydrogensulfite, and azo initiators such as azobisisobutyronitrile and 2,2ʹ-azobis(2-amidinopropane)-hydrochloric acid. The initiators are advantageously used usually in the form of an aqueous solution but can be used as diluted with a suitable solvent. The initiator is used in a usual amount, i.e., in an amount, calculated as solids, of about 0.1 to about 10%, preferably about 0.5 to about 5%, of the combined weight of the monomers, namely acrylate (and free acrylic acid); styrene; acrylamide; and polyvinyl monomer cross-linking agent. Depending on the amount and kind of the initiator, the initiator is usable together with isopropyl alcohol, alkylmercaptan or other chain transfer agents to control the molecular weight of the polyacrylate copolymer to be obtained.

By the addition of the polymerization ini­tiator, the mixed monomer solution is subjected to copolymerization with evaporation of water without heating the system from outside. More advantageously, the reaction is carried out by admixing a predetermined amount of the initiator or an aqueous solution thereof with the mixed monomer solution and causing the result­ing mixture to flow down onto and spread over a travel­ing conveyor belt or reacted continuously. The ini­tiator can be added to the mixed monomer solution as it is poured onto the conveyor belt.

The copolymerization proceeds rapidly after admixing the initiator with the mixed monomer solution and is completed within a short period of time, usu­ally in about 30 seconds to about 10 minutes. The reaction is exothermic, so that the reaction system is rapidly heated to about 100 to about 130° C by the heat of polymerization. Consequently, particularly where the monomer concentration in the mixed solution is at least 50 percent by weight, the water evaporates from the system rapidly to give a relatively dry, solid copolymer of low water content without resorting to any external heating. The water content of the copolymer is usually up to about 15%, and generally about 8 to 12% by weight as recovered. Subsequently, the dry solid copolymer can be made into the desired powder easily by a usual method, for example by pul­verization, without a drying step.

Polystyrene and/or methyl-­cellulose can be added to the mixed monomer solution in an amount of 0.5 to about 10 percent based on the total weight of monomers in the mixed monomer solution to increase the porosity and water absorbing capacity of the copolymers. It has been found, quite surpris­ingly, the polystyrene and methylcellulose will sub­stantially increase the water absorbing capacity of the resin copolymer described herein. To achieve the full advantage of the present invention, the polysty­rene and methylcellulose should be added in an average grain size of less than or equal to 5 micrometers.

The powder thus obtained has outstanding water absorbing ability and is useful for sanitary goods, paper diapers, disposable diapers and like hygenic goods, agricultural or horticultural water retaining agents, industrial dehydrating agents, sludge coagulants, thickening agents, condensation preventing agents for building materials, release control agents for chemicals and various other applications.

It has been found that by copolymerizing partially neutralized or fully neutralized acrylic acid with styrene, the total absorbency of the resulting copolymer as well as the retention capacity of the copolymer is unexpectedly superior to the absorbent homopolymer made without styrene Furthermore, when the styrene is included in the mixed monomer solution in an amount of 5-25% based on the weight of acrylic acid and acrylate (computed based on the molecular weight of acrylic acid), the initial rate of the water absorption of the resulting copolymer is unexpectedly superior to the homopolymer manufactured without the styrene copolymerization. It appears that when the styrene is added to the mixed monomer solution in an amount of 1 to about 5% based on the weight of acrylic acid, only the total water absorption and total water retention is increased. However, when the styrene is added to the mixed monomer solution in an amount of about 5% to about 25% based on the weight of acrylic acid, not only is the total water absorption and the total water retention far superior to that of the homopolymer, but the initial rate of water absorp­tion is far superior to that of the acrylic homopolymer and to that of the acrylic acid acrylamide copolymer.

In order to show the new and unexpected results achieved by the co- or ter-polymerization of partially or fully neutralized acrylic acid with styrene, the water absorption and retention properties of an acrylic acid homopolymer was studied, as shown in FIG. 1. The acrylate homopolmer was prepared in the manner as described with respect to Example 1 using the following mixed monomer solution <tables id="tabl0001" num="0001"><img file="EP0242478A2_D0004.tif" /></tables>

The results shown in FIG. 1 were obtained using a gravometric absorbency testing system (GATS). The polymer made in accordance with Example 1 using the mixed monomer solution listed above is placed in contact with water under a retaining disc exerting 0.5 psi pressure on the polymer for 20 minutes to determine an initial rate of water absorption and, after 20 minutes, the pressure above the polymer is reduced to .02 psi with a lighter retaining disc for 10 minutes to determine a total water absorption. At a 30 minute time interval, the pressure above the polymer is again increased to 0.5 psi to determine a total amount of water retained by the polymer. Water absorption readings determined by increase in polymer rate were taken at the 1 and 3 minute time periods to determine an initial rate of water absorption. A reading was taken at the 20 minute time interval to determine the total water absorbed under the 0.5 psi retaining disc and water absorption readings taken at the 30 minute time interval to determine the total water absorbed by the polymer. Readings taken at a 35 minute time interval with the 0.5 psi retaining weight on the polymer were taken to determine the total amount of water retained by the polymer. The readings on the lower coordinate, or x axis of the graph of FIG. 1 represent the grams of water absorbed per 0.1 gram of polymer and are as follows: 1 minute, 1.45; 3 minutes 1.72; 20 minutes, 2.22; 30 minutes, 3.19, 35 minutes, 2.63.

This same procedure outlined above was used to determine initial water retention, total water retention and total absorbency of the copolymers or terpolymers of the present invention wherein the acry­late is copolymerized with styrene , as shown in FIGS. 2-4. The copolymer of FIG. 2 was prepared in the same manner as that of FIG. 1 except with the addition of styrene by copoly­merizing the following mixed monomer composition <tables id="tabl0002" num="0002"><img file="EP0242478A2_D0005.tif" /></tables>

As shown in FIG. 2, when the mixed monomer solution contains 2% styrene based on the weight of acrylic acid, the total water absorbency measured at the 30 minute time interval increased from a value of 3.19 (see FIG. 1 homopolymer data) to a value of 4.21 and the total amount of water retained increased from a value of 2.63 (see FIG. 1 homopolymer data) to a value of 3.11 representing an increase in total water absorbed of more than 30% and an increase in total water retention of almost 20%. These results, and similar results shown in FIGS. 3 and 4 (4% and 6% styrene, respectively), are most surprising in the art of water absorbant polymers.

The present invention will be described in greater detail with reference to the following examples.

Example 1

The following are combined, wherein per­cents are weight percents based on the total weight of the monomer solution formed, 59.99% acrylic acid first, then 12.12% potassium hydroxide and 12.12% ammonium carbonate and 15.15% ammonium hydroxide serv­ing as neutralizing agents and 1% styrene based on the weight of acrylic acid. Thereafter .03% of N,N-­methylenebisacrylamide as a polyvinyl monomer is added to prepare an aqueous solution of potassium acrylate and ammonium acrylate which is 23.84% water having a neutralization degree of about 90% and a combined monomer concentration of 60.58 wt. %.

The aqueous solution is maintained at 70° C, and with the solution is admixed 0.15% of 2,2¹-­azobis-(2-amidino-propane) hydrochloric acid in an aqueous solution. The final solution is as follows: <tables id="tabl0003" num="0003"><img file="EP0242478A2_D0006.tif" /></tables>

The mixture is poured onto a traveling end­less belt and spread thereover in the form of a layer about 10 mm in thickness. About 30 seconds thereafter, the mixture starts to polymerize, and the reaction is completed in about 1 minute. The maximum temperature of the mixture during the reaction is about 120° C.

The copolymer is allowed to complete curing for about 30 minutes at ambient temperature to give a dry solid strip of potassium/ammonium polyacrylate and polyacrylic-polystyrene product having a water content of 11% and a residual monomer concentration of 1200 ppm. The strip is made into a powder by a pulverizer.

Examples 2 to 5

Copolymers are prepared in the same manner as in Example 1 with the exception of varying at least one of the combined concentration of monomers, the amount of polyvinyl monomer (N,N-methylenebisacryl­amide), the kind and amount (degree of neutralization) of neutralizing agent, and the amounts based on the combined amount of the monomers, of azo polymerization initiator. The following compositions are copoly­merizable: <tables id="tabl0004" num="0004"><img file="EP0242478A2_D0007.tif" /></tables><tables id="tabl0005" num="0005"><img file="EP0242478A2_D0008.tif" /></tables>

The amount of polyvinyl monomer listed is expressed in % by weight based on the combined amount of potassium acrylate, free acrylic acid, styrene and the polyvinyl monomer, and the concentration of initia­tor is expressed in % by weight based on the combined amount by weight (calculated as solids) of the monomers and the initiator, the same as hereinbefore.

EXAMPLE 6

To 22.2 g of deionized water are added 72.1 g of acrylic acid first, then 49.5 g of potassium hydroxide having a purity of 85% and serving as a neutralizing agent, and thereafter 3.0 grams of styrene and 0.01 g of N,N-methylenebisacrylamide as a polyvinyl monomer to prepare an aqueous solution of potassium acrylate having a neutralization degree of 75%, sty­rene, and a combined monomer concentration of about 75 wt.%.

The aqueous solution is maintained at 70° C, and with the solution are admixed 2.9 g of 18% aqueous solution of ammonium persulfate (0.67 wt. % based on the combined weight of the potassium acrylate, free acrylic acid styrene and N,N-methylenebisacryl­amide, the same as hereinafter). The mixture is poured onto a traveling endless belt and spread thereover in the form of a layer about 10 mm in thickness. About 30 seconds thereafter, the mixture starts to poly­merize, and the reaction is completed in about 1 minute. The maximum temperature of the mixture during the reaction is about 120 ° C.

The reaction gives a dry solid strip of cross-linked potassium polyacrylate-styrene copolymer product. The strip is made into a powder by a pul­verizer.

Examples 7 to 10

Polymers are prepared in the same manner as in Example 6 with the exception of changing at least one of the amount of N,N-methylenebisacrylamide and the kind and amount of the polymerization initiator as listed in Table 1 below. <tables id="tabl0006" num="0006"><img file="EP0242478A2_D0009.tif" /></tables>

Examples 11 to 18

Polymers are prepared in the same manner as in Example 1 except that the compounds listed in Table 2 below are used as polyvinyl monomers in the listed amounts. <tables id="tabl0007" num="0007"><img file="EP0242478A2_D0010.tif" /></tables>

*Polyethylene glycol diacrylate used in Examples 17 and 18 is represented by the following formula: <chemistry id="chem0004" num="0004"><img file="EP0242478A2_D0011.tif" /></chemistry>

Example 19

48.1 gr. of acrylic acid and 11.9 gr. of acrylamide are dissolved in 159 gr. of distilled water and then 22.7 gr. of NaOH is added for partial neutrali­zation of acrylic acid in an amount of 85 mole percent. 3.0 grams of styrene then is added. 0.006 gr. of N,Nʹ-methylenebisacrylamide is then added as the polyvinyl monomer. In addition, 0.048 gr. of ammonium persulfate and 0.048 gr. of sodium hydrogensulfite are added as the polymerization initiators. In this case, the initial temperature of the mixed monomer solution is 40°C.

Example 20

90. 1 gr. of acrylic acid and 9.9 gr. of acryl­amide are dissolved in 118.8 gr. of distilled water and 52.6 gr. of KOH is added for 75 mole percent partial netutralization of acrylic acid. 2 grams of styrene is added to the solution. 0.018 gr. of N,Nʹ-methylenebis­acrylamide is added as the polyvinyl monomer, and 0.08 gr of ammonium persulfate and 0.08 gr. of sodium hydrogen­sulfite are added as the polymerization initiators. The initial temperature of the mixed monomer solution is 30°C.

Example 21

52.7 gr. of acrylic acid and 17.3 gr. of acrylamide are dissolved in 25 gr. of distilled water and they are partially neutralized 80 mole percent with the addition of 32.8 gr. of KOH. 2.5 grams of styrene and 0.007 gr. of N,Nʹ-methylenebisacrylamide is added. For the polymerization catalyst 0.7 gr. of 2,2ʹ-azobis­isobutyronitrile dissolved in 10 cc. of acetone is added. The solution is kept at 80°C. in a TEFLON coated, glass fiber reaction chamber until completion of polymerization and cross-linking reactions yielding a solid resin.

Example 22

17.3 gr. of acrylamide is dissolved in 52.7 gr. of acrylic acid and partial (70 mole percent) neutralization of acrylic acid is accomplished by the addition of 30 gr. of aqueous ammonia having a concen­tration of 29 weight percent. In this case, for the polyvinyl monomer, 0.007 gr. of N,Nʹ-methylenebis­acrylamide is added. 0.7 grams of styrene is added for copolymerization and, as the catalyst, 0.7 gr. of 2,2ʹ-azobis(2-amidinopropane)hydrochloride dissolved in 8 gr. of distilled water is added. The polymerization is started at 80°C. to yield a solid resin.

Example 23

21 kg. of acrylic acid and 7 kg. of acrylamide are dissolved in 9.5 kg. of distilled water and the acrylic acid is partially neutralized with 12 kg. of KOH. 0.036 kg. of methylenebisacrylamide as a polyvinyl monomer was added as well as one gram of styrene and one gram of methyl methacrylate to provide an aqueous mixed monomer solution. The mixed monomer solution is mixed with 0.28 kg. of 2,2ʹ-azobisisobuty­ronitrile, dissolved in 2 kg. of acetone as a poly­merization initiator. This mixture at a temperature of 60°C. is transferred on an endless belt (600-700 mm. in width, and 7 m. in length) at a thickness of about 1 cm. The polymerization is initiated promptly on the belt resulting in a solid resin.

Example 24

52.7 gr. of acrylic acid and 17.3 gr. of acrylamide are dissolved in 20 gr. of distilled water. 32.8 gr. of KOH is added for 80 mole percent partial neutralization of acrylic acid. Then 0.007 gr. of N,Nʹ-methylenebisacrylamide polyvinyl monomer is added with 2.6 gr of methylcellulose. The aqueous monomer mixture is homogenized with agitation. Next, as an initiator, 0.7 gr. of 2,2ʹ-azobisisobutyronitrile dissolved in 10 cc of acetone is added. This mixed solution is kept at 80°C. in a water bath surrounding the reaction chamber. Polymerization is initiated as the temperature of the mixture increases as a result of the surrounding water bath, resulting in a solid porous resin.

Example 25

120.8 gr. of acrylamide is dissolved in 49.2 gr. of acrylic acid and 28.0 gr. of aqueous ammonia (29% concentration) is added for 70 mole percent neutraliza­tion of the acrylic acid. Next, 0.01 gr. of N,Nʹ-­methylenebisacrylamide, 3.5 grams of styrene and 5 gr. of methylcellulose are added and stirred to homogenize. As an initiator, 0.7 gr. of 2,2ʹ-azobis(2-aminidinopropane)-­hydrochloride dissolved in 5 gr. of distilled water is added. The polymerization is initiated in a TEFLON coated glass reaction chamber kept at 80°C, yielding a solid porous resin having a water content of less than 15% by weight.

Example 26

Same as Example No. 24, but having no methyl­cellulose.

Example 27

Same as Example No. 25, but having no methyl­cellulose.

Example 28

52.7 gr. of acrylic acid and 17.3 gr. of acrylamide are dissolved in 20 gr. of distilled water. 32.8 gr. of potassium hydroxide is added to neutralize 80 mole percent of the acrylic acid. Next, 2.5 grams of styrene, 0.007 gr. of N,Nʹ-methylenebisacrylamide, and 5 gr. of a polystyrene emulsion are added and homogenized under stirring. The polystyrene emulsion has a concen­tration of 50 weight percent and a polystyrene grain size of about 0.5 micrometers. Then, as the initiator, 0.7 gr. of 2,2ʹ-azobisisobutyronitrile dissolved in 10 cc of acetone solution is added. The mixed monomer solution is kept at 80°C. in the reaction chamber immersed in a water bath. With the increasing temperature of the mixed monomer solution polymerization is initiated resulting in a solid, porous resin.

Example 29

20.8 gr. of acrylamide is dissolved in 49.2 gr. of acrylic acid. 28.0 gr. of aqueous ammonia (29% concentration) is added to neutralize 70 mole percent of the acrylic acid. Then 10 grams of styrene and 0.01 gr. of N,Nʹ-methylenebisacrylamide are added. 3 gr. of the polystyrene emulsion of Example 40, but having a 1 micro meter average grain size, is added and the mixed solution is homogenized with agitation. Next, as the initiator, 0.7 gr. of 2,2ʹ-azobis(2-amidinopropane)-hydrochloride dissolved in 5 gr. of distilled water is added. The solution is kept at 80°C. and polymerization is initiated with increased temperature resulting in a solid, porous resin.

Example 30

Same as Example 28, but having no polystyrene.

Example 31

Same as Example 29, but having no polystyrene.

In one embodiment a process for preparing a solid, water absorbing, cross-linked resin comprises combining a monomer mixture of (A) potassium acrylate; (B) styrene in an amount of 1% to 25% based on the amount of acrylate computed based on the weight of acrylic acid; and (C) a polyvinyl monomer, with water in an amount of at least 30 combined weight precent of (A) plus (B) plus (C) based on the total weight of (A) plus (B) plus (C) plus water to form a monomer mixture mixture wherein the monomers of the monomer mixture consist essentially of (A), (B) and ( C); adding a polymerization initiator to said monomer mixture capable of initiating, and in an amount sufficient to initiate polymerization of said monomer mixture; copolymerizing said monomer mixture while utilizing the exothermic heat of reaction as substan­tially the only non-ambient energy source to drive water away from said polyacrylate resin copolymer to form said cross-linked resin copolymer having a water content sufficiently low to be powdered without an intermediate drying step.