MXPA97002313A - Procedure for the preparation of hydrogels hydrophiles, porous, of elevated capacity of absorc - Google Patents

Abstract

The present application relates to a process for preparing hydrophilic, porous hydrogels with a high absorption capacity, characterized in that the water-swollen hydrophilic hydrogels with a high absorption capacity are freeze dried.

Description

PROCEDURE FOR THE PREPARATION OF HYDROGELS HYDROPHILES, POROUS, OF HIGH ABSORPTION CAPACITY The present invention relates to a process for the preparation of hydrophilic, porous hydrogels with a high absorption capacity, by freeze drying hydrophilic hydrogels with a high absorption capacity. The highly hydrophilic, hydrophilic hydrogels are especially polymers formed from hydrophilic (co) polymerized monomers, (co) polymers of grafting of one or more hydrophilic monomers, on an appropriate graft base, crosslinked cellulose or starch ethers, carboxymethylcellulose interlaced, partially entangled polyalkylene oxide or natural products swellable in aqueous fluids, such as, for example, guar derivatives. Said hydrogels are used as absorbing products of aqueous solutions for the preparation of diapers, tankers, sanitary napkins and other hygienic articles, but also with water containing means in the formation of agricultural parcels. A high absorption speed is also expected from said liquid absorbing hydrogels in addition to a high absorption capacity. However, with said high rate of absorption, the undesirable effect of "gel blocking" occurs, above all on the surface of the resin particles, whereby a barrier is formed on the surface of the resin particles which prevents they absorb the liquid and, in such a way, they can no longer absorb more liquid. The rate of absorption, however, can be increased by increasing the specific surface area of the absorbent resin, without the undesirable effect described above appearing as "gel blocking". The absorbent resin with porous structure thus exhibits advantages over absorbent resins which do not have a porous structure. Consequently, there has been no lack of research in order to provide hydrophilic porous hydrogels with a high absorption capacity. DE-A-38 31 261 describes the preparation of an interlaced, porous acrylic polymer, liquid absorber, by polymerizing a neutralized aqueous solution of acrylic or metaclic acid and of an entangled, active rnonomer. The porous structure of the acrylic polymer is obtained according to it, by the addition of an initiator containing N, which, during the drying of the acrylic polymer at 80-250 ° C, under gas formation, decomposes or sublimes US Pat. No. 5,354,290 discloses porous polymer structures consisting of a water-soluble water-absorbing polymeric water absorber.For the preparation of these porous polymeric structures, the appropriate water soluble monomers, together with an appropriate water-soluble interlayer, are polymerized in a oil-in-water emulsion When the polymers thus obtained are dried, the liquid oil phase is evaporated and the pores are produced in the polymeric material WO 94/22502 describes a superabsorbent polymer foam which can be prepared by the polymerization of the monomers in the presence of an initiator, which preferably has an evaporation temperature of less than 50 ° C. DE-A 36 37 057 is described and a process for the preparation of a porous polymer with water absorbing properties. The monomers suitable for the preparation of these porous polymers are polimerized in aqueous phase or in an oil / water / oil emulsion. By drying the polymer thus obtained, by evaporating the oil phase from the oil-in-water emulsion, a porous structure of the polymer is obtained. These processes have in common that they result in the porous structure of the water absorbing polyrnomer resin by the addition of an initiator, which is evaporated by drying the polymer resin. In these procedures, however, there is the drawback that the evaporation of the initiator is carried out according to a casual statistic, so that it is difficult to control the pore size and the pore distribution of the polymeric water absorbing resin. EP-A-0 105 634 ee discloses a cross-linked, porous, sulphonated polymeric material, which serve to absorb water and aqueous solutions of salt. This porous, interlaced polymeric material is prepared by the polymerization of water insoluble monomers, such as, for example, styrene, (meth.) Alkyl acrylate and divinylbenzene in high internal phase emulsion with water, as the internal phase, and followed by sulfonation of the obtained porous polymer. UO 93/04092, UO 93/04093, UO 93/04115 and UO 94/13704 describe absorbent foam materials, which are prepared by the polymerization of common "vitreous", such as styrene and styrene derivatives, with "rubbery" monomers "insoluble in water, such as 2-ethexyl acrylate, and with a water-insoluble interlayer, such as divinylbenzene, in a high internal phase emulsion, with water or an aqueous solution of CaCl 2 co or the internal phase. The ratio between the internal phase and the dispersed phase ranges between 20: 1 and 70: 1. The foam material thus obtained after drying is hydrophilized by means of washing with surfactant solutions. These processes have in common that the porous structure of the water-absorbing polymeric resin is obtained by the polymerization of water-insoluble monomers, high internal phase emulsion, water as the internal phase, and then by drying. These procedures, however, have a number of disadvantages. Due to the high ratios required between the internal phase and the dispersed phase, the use of space during the polymerization stage is very limited. For the preparation of an emulsion with high internal phase, it is necessary to use high amounts of surfactant, which remains in the porous, water absorbing polymer resin, and during the use of the product in hygienic articles, such as bandages and diapers, can be extracted again from the product. This is problematic, given the attractive force of the surfactant in general on the mucous membranes. In order that the porous, water-absorbing polymer resin can be formed according to the process described, from a content of water-insoluble, hydrophobic monomers, it must be subjected, after the polymerization step, to an additional step for the hydrophilicization of the polymeric resin. This is disadvantageous from the economic point of view. In the following, EP-A-421 264 describes a hydrophilic, absorbing polymer in the form of a highly porous polyhedral structure in the form of a foam. This polymeric absorbent, hydrophilic, are prepared by polymerization of appropriate hydrophilic monomers, in an aqueous medium containing a surfactant and the mononomers, and which is stabilized by means of a liquid hydrocarbon phase. The polymerization medium thus contains approximately 60 to 99% by weight, preferably hydrocarbon. This method, in fact, is economically disadvantageous, since, due to the high content of hydrocarbons in the polymerization medium, only a limited use of space during the polymerization can be obtained, and large quantities of the latter must be removed by drying. hydrocarbon, from the polymer. There remains, therefore, a need for a polymer for the preparation of abeorbent, hydrophilic, porous polymers, which lack the disadvantages described above, and which can be easily obtained; and that allows obtaining a defined scale of pore sizes and pore distribution in the polymer. It has surprisingly been found that this need is solved by freeze drying of hydrophilic, non-porous, absorbent polymers, which can be prepared by known methods. The present invention, moreover, relates to a process for preparing hydrogels of high absorption capacity, hydrophilic, porous, characterized in that the hydrogels with high absorption capacity, hydrophilic, swollen with water, are dried by freezing. Freeze drying is designed for the drying of an intensely cooled material under high vacuum, by cooling the solvent, which evaporates in the frozen state (drying by sublimation). During drying, the frozen material retains its original shape, so that after drying, a material with a very porous structure is obtained. The process according to the invention can be carried out in all freeze drying apparatuses.

These can be obtained commercially or are known to at least those skilled in the art. The freeze drying can be carried out intermittently or continuously. The hydrogels that are going to be dried by freezing are used in the absorbed state. This means that they exhibit a substantial water content. Thanks to this water content, the pore size and the pore distribution of the hydrogels to be prepared according to the invention can be controlled. The weight ratio of water: hydrogel is preferably between 2: 1 and 500: 1, especially between 4: 1 and 200: 1 and very especially between 5: 1 and 100: 1. The hydrogels prepared according to the process of the present invention exhibit a specific surface, according to BET (Brunnauer-Em ett-Teller), preferably from 2 to 15 m2 / g, especially from 3 to 10 rn2 / g. The specific pore volume of these hydrogels is preferably 1.3 x 10-2 to 2 x 10-1 cm3 / g, especially 5 x 10-2 to 1.5 x 10_ * cwß / g. The pore volume ratio and the average pore radius (dV / dR) is indicated as the distribution of pore radii; in general, as a distribution curve of Gaus, and is expressed as cm3 / g x angstrorns. The preferred pore radius distribution, in the hydrogels prepared according to the invention, is preferably from 1.5 to 5 cm3 gx angstrom, especially from 1.8 to 3 an3 / gx ang ost »Hydrophilic hydrogels, of high absorption capacity , which can be dried by freezing according to the invention, are especially polymers prepared from hydrophilic (co) polymerized monomers, graft (co) polymers, formed from one or more hydrophilic monomers, on a substrate. of appropriate graft; the crosslinked cellulose and starch ethers or in absorbable natural products in aqueous liquids, for example, guar derivatives. These hydrogels are known to those skilled in the art. Suitable hydrophilic hydrogels, which have a high absorption capacity, are suitable hydrophilic monomers, for example, polymeric acids, such as acrylic acid, methacrylic acid, vinyl sulphonic acid, vinylphosphonic acid, malic acid, especially its anhydride, fumaric acid, itaconic acid, 2-acrylarnido-2-methylpropanesulfonic acid, acrylamide or-2-methylpropanfoephonic acid, as well as their amides, their hydroxyalkyl esters and esters and amides containing amino groups or ammonium groups. In addition, the water-soluble N-vinylamines or the diallyl dimethiolonium chloride are also suitable.

Preferred hydrophilic species are compounds of general formula I: wherein: i means hydrogen, methyl or ethyl; R2 means the group -C00R *, the sulfonyl group, the phosphonyl group, the phosphonyl group esterified with alkanol of 1 to 4 carbon atoms, or a group of the formula: ? * $ N CH, H CHj 3 means hydrogen, methyl, ethyl or the carboxyl group; R * means hydrogen, amino or hydroxyalkyl of 1 to 4 hydrogen atoms; and R5 means the sulfonyl group, the phosphonyl group or the carboxyl group. Examples of the alkanols of 1 to 4 carbon atoms are: methanol, ethanol, n-propanol or n-butane! . The hydrophilic monomers acrylic acid and ethacrylic acid are especially preferred. Hydrophilic hydrogels, which can be obtained by polymerization of olefinically unsaturated compounds, are already known and described, for example, in US 4,057,521, US 4, 062,817, US 4,525,527, US 4,286,082, US. 4,340,706 and US 4,295,987. Hydrophilic hydrogels which can be obtained by graft copolyzing of unsaturated olefinic acids on different matrices are also already known, such as, for example, polysaccharides, polyalkylene oxide and its derivatives; and are described, for example, in the US ,011,892, US 4,076,663 or US 4,931,497. The appropriate grafting bases may be of natural or synthetic origin. Examples are starch, cellulose or cellulose derivatives, as well as other polysaccharides or oligosaccharides, polyalkylene oxide, especially polyethylene oxide and polypropylene oxide, as well as hydrophilic polyesters. Suitable polyalkylene oxides have, for example, the formula: wherein: R6 and R7, independently of each other, are hydrogen, alkyl, alkenyl or acryl; X is hydrogen or methyl and n means an integer from 1 to 10,000. R6 R7 preferably represents hydrogen, alkyl of 1 to 4 carbon atoms, alkenyl of 2 to 6 carbon atoms or phenyl. Preferred hydrogels are especially polyacrylates, polyethacrylates, such as the graft polymers described in US 4,931,497, US 5,011,892 and US 5,041,496. The content of those patent descriptions is incorporated herein as if it formed part of the present description. The hydrophilic hydrogels, which have a high absorption capacity, are preferably entangled, that is, they contain compounds with at least two double bonds, which are polymerized into a polymer network. Suitable interlators are, especially the rnetylenebis-acrylamide or -methacrylamide; the esters of unsaturated ono- or polycarboxylic acids, of polyols, such as diacrylate or riacrolylate, for example, butanediol diacrylate or methacrylate or ethylene glycol, as well as trinethylolpropane co-triacrylate and allyl compounds, such as (et) allyl acrylate, triallyl cyanurate, diallyl ester of rnaleic acid, polyallyl ester, tetraalyloxyethane, triallylamine, tetraalyletheiandia ina, allyl ester of phosphoric acid, as well as vinylphosphonic acid derivatives, such as, for example, those described in EP -A 343 427. The content of EP-A 343 427 is also incorporated in the present description as if it formed part of it. Especially preferred are the hydrophilic hydrogels, which have a high absorption capacity, are post-interlaced in a manner known per se in the aqueous gel phase, or are surface-entangled as ground and sieved polymer particles. Suitable crosslinkers are compounds which contain at least two groups which can form covalent bonds with the carboxyl groups of the hydrophilic polymer. Suitable compounds are, for example, the diglycidyl or polyglycidyl compounds, such as the glycidyl ester of phosphonic acid, the alkoxysilyl compounds, the polyaziridines, the polyanes or the polyamidoarnines, wherein said compounds can also be used in mixtures each other (see, for example, EP-A 83022, EP-1 543303 and EP-A 530438). Suitable crosslinking agents are described as crosslinking agents, especially in EP-A-349935. The content of the aforesaid patent descriptions is incorporated in this specification as if it formed part of it. Hydrophilic hydrogels of high absorption capacity can be prepared by polymerization processes known per se. Polymerization in aqueous solution is preferred, following the gel polymerization process mentioned above. In this manner, aqueous solutions of 15 to 50% by weight of one or more hydrophilic monomers and, optionally, an appropriate base are polyacrylated in the presence of a radical initiator, preferably without mechanical mixing, with the use of the Trommsdorff effect. Norrish (Bios Final Rep. 363.22; Makro ol. Chern. 1, 169 (1947)). The polymerization can be carried out on a temperature scale of 0 ° C to 150 ° C, preferably between 10 ° C and 100 ° C, under normal pressure or under high or reduced pressure. As usual, the polymerization can also proceed in an atmosphere of a protective gas, preferably under nitrogen. To facilitate the polymerization, electromagnetic energy beams or the usual chemical polymerization initiators can be used, for example, organic peroxides, such as benzoyl peroxide, tertbutyl hydroperoxide, ethylethyl ketone peroxide, curnyl hydroperoxide; azo compounds, such as azodiisobutyronitrile, as well as inorganic peroxy compounds, such as (NH4) 2 2? ß or K2S2O8 or H2H2, optionally in combination with reducing agents, such as sodium bisulfite, and iron (II) sulphate or reduction / oxidation (redox) sizers, which contain as an reducing component an aliphatic or aromatic sulphinic acid, such as benzenesulphonic acid and toluensulic acid, or derivatives of those acids, such as , for example, Mannich adducts formed from sulphinic acids, aldehydes and amino compounds, such as those described in DE-C 1 301 566. By subsequent heating for several hours of the polymeric gels, at With a temperature of 50 to 130 ° C, preferably 70 to 100 ° C, the quality properties of the polymers can be improved. The hydrogel to be freeze-dried, prepared according to the gel polymerization procedure, does not need to be isolated in the dried form; rather, the absorbed reaction product can be conducted directly to freeze drying. The degree of absorption of the polymer and with it the pore size of the product to be prepared-according to the invention, can be controlled by means of this method, by concentrating the monomer in the polymerization solution. Naturally, a dried hydrogel can also be absorbed and then dried by freezing, by adding the desired amount of water. The porous polymers prepared according to the invention can be comminuted mechanically in the dry state, can be mechanically ground according to the needs in the inlet zone and, if necessary, can be subjected to a subsequent surface treatment, such as, for example, the one described in DE-A-41 38 408. The advantages resulting from the porous structure are obtained in this way. For the freeze drying according to the following examples, a freeze drying installation obtained commercially of the type LYOVAC GT 2 of the company LEYBOLD-HERAEUS was used, in which a vacuum pump was integrated with an aspiration capacity, according to PNEUROP, of 7.6 m3 / hour and with an extreme partial pressure, without gas ballast, of only 2.5 x 10-* millibar. The drying procedure, in all cases, was identical. The gel that is going to be freeze-dried can reach different degrees of absorption, for which an absorption time is used with a duration of at least 48 hours; for which it is introduced in advance in a plastic bag, in a layer, with a layer thickness of between 1 and maximum 5 cm in one form, which is then cooled in the drying plate of the freeze dryer, in a rack common freezer and current in commerce, at temperatures between -10 and -20 ° C; in this way, it is transferred from the plastic bag on the drying plate of the freeze dryer, at an initial freezer plate temperature of -20 ° C, to a selected vacuum and with an ambient temperature between 20 and 25 ° C as ambient temperature, until obtaining a constant weight; it is maintained at a final vacuum temperature of about 0.15 millibars, and with an increase in the temperature of the drying plate to room temperature.

EXAMPLE 1 A polymeric gel that was adiabatically polymerized by polymerization in gel of 850 parts of acrylic acid and 42.5 parts of tetraalyloxyethane in 3240 parts of water, using 25 parts of a 4% solution of dihydrochloride of 2.2 is used for freeze drying '-azobis (2-amidinopropane), 11.5 parts of a 0.75% solution of hydrogen peroxide, as well as 14.5 parts of a 0.15% solution of ascorbic acid, starting with N2 gas at an initial temperature of 12 ° C. The acid gel is mechanically crushed by means of a meat grinder attached to a table, used in butcher shops, partially neutralized with 27% sodium hydroxide solution (4,000 parts of gel, 1297 parts of 27% NaOH) (corresponding to a degree of neutralization of the acrylic acid of 74 mol%) and for a homogenization improvement, it is passed a second time through the meat mill. The polymer gel thus obtained is freeze dried in the manner described above, without further dilution; a product in the form of a peanut shell is obtained, which is clearly looser and also with more open pores than an equal starting material, dried according to the usual technique and manner, in oven, with hot air or with hot rollers .

EXAMPLE 2 The same or gel as described in Example 1 is now diluted with demineralized water to a solids content of 3% by weight; it is absorbed and dried by freezing. A cotton-wool product with a residual weight clearly lower than that of the corresponding freeze-dried polymer of Example 1 is obtained. The specific surface, according to BET, in this product (example 2) is 5.05 rn2 / g. For comparison, a similar starting material, optionally 3%, is diluted, but is now dried in an oven, then milled and sieved at 100/800 μm; a product is obtained that exhibits a specific surface, according to BET, of 0.3 to 0.6 r / g.

EXAMPLE 3 Under conditions similar to those described in Example 1, but with the use of a lower concentration of interlayer, a polymeric gel was prepared which was then diluted, on the one hand, to a solids content of 10% in weight (a) and on the other, at a solids content of 1% by weight (b). Both degrees of absorption, after the corresponding absorption time, were dried by freezing. Both polymeric products obtained differ correspondingly to the degree of absorption, by the size of the pores, which is clearly recognized by a REM reception: the product (b) exhibits a degree of preabsorption of 99% which is clearly higher than that of the product (a), with a degree of preabsorption of 90%.

EXAMPLE 4 The SANUET (R) IM 7000 superabsorbent commercial product with a particle size distribution of 100-850 jrn is allowed to be absorbed in different amounts of demineralised water; it is dried by freezing, mechanically pulverized and the respective 100-850 pin granulometry fractions are subjected to the determination of the specific surface, according to BET, as well as the specific volume of the pores. The degree of absorption is converted as well as the measurement data to form Table 1.

TABLE 1 Exercise Degree of ab- Specific surface Specific volume of the fica according to BET of the pores (in Nurn SAP, (%). * - (in m2 / g) mS / g) 4a 80 2.61 12.6 4 b 85 2.72 19.8 4c 90 2.98 61.2 4d 95 4.31 97.4 4e 99 7.1.0 134.8 Co p. IM 7000 - 0.9 11.8 * Degree of absorption by weight of water in the water / SAP mixture before drying.

Claims (8)

NOVELTY OF THE INVENTION CLAIMS

1. - Process for the preparation of hydrophilic, porous hydrogels with a high absorption capacity, characterized in that the hydrophilic hydrogels, swollen with water, of high absorption capacity, are dried by freezing.

2. Process according to claim 1, further characterized in that in water-swollen hydrophilic hydrogels of high absorption capacity, the weight ratio of water: hydrogel is from 2: 1 to 500: 1.

3. Method according to claim 1 and / or 2, further characterized in that a hydrophilic, porous hydrogel with a high absorption capacity is prepared with a specific surface, according to BET, of 2 to 15 rn / g.

4. Method according to one or more of claims 1 to 3, further characterized in that a hydrophilic, porous hydrogel with a high absorption capacity is prepared with a specific pore volume of 1.3 × 10 ~ 2 to 2 × 10 -1 cm3 / g.

5. Method according to one or more of claims 1 to 4, further characterized in that a hydrophilic, porous hydrogel with a high absorption capacity is prepared with a pore radius distribution of 1.5 to 5 cm3 / g x angstrorn.

6. - Process according to one or more of claims 1 to 5, further characterized in that the hydrophilic hydrogels, of high absorption capacity, are polymers prepared from hydrophilic (co) polymerized monomers, graft (co) polymers of one or more hydrophilic monomers, on a graft basis, interlaced cellulose or starch ethers, or natural products swellable in aqueous liquids, such as, for example, guar derivatives.

7. - Method according to claim 6, further characterized in that the hydrophilic monomeric compounds are of the general formula I: \ / ((I) / \ wherein: R 1 signifies hydrogen, methyl or ethyl; R2 means the group -C00R *, the sulfonyl group, the phophonyl group, the phosphonyl group esterified with alkanol of 1 to 4 carbon atoms, or a group of the formula: CH, N CH, H CH, R3 means hydrogen, methyl, ethyl or the carboxyl group; R * means hydrogen, amino or hydroxyalkyl of 1 to 4 hydrogen atoms; and R5 means the sulfonyl group, the phosphonyl group or the carboxyl group.

8. Method according to claim 6 and / or 7, further characterized in that the hydrophilic monomers are acrylic acid or netacrylic acid.