AT391321B - METHOD FOR PRODUCING LIQUID-ABSORBING ACRYLIC RESINS - Google Patents

Description

No. 391 321

The invention relates to a method for producing liquid-absorbing, crosslinked acrylic resins using thermally decomposable, N-containing blowing agents.

Liquid-absorbent acrylic resins are mainly used for absorbent diapers and hygiene articles, as drying and water retention agents and in agriculture as soil improvers, especially in dry areas. Their preparation is already described in EP-A-206 808, EP-A-207 714 or US 4,677,174 by polymerizing acrylic acid, which is 70 to 100% neutralized, together with a polyvinyl monomer as crosslinking agent, and with the addition of a polymerization initiator described. Drying of the acrylic resin obtained, which is generally very difficult to do, is not necessary in this process. However, this has the disadvantage of a relatively high residual water content and residual monomer content.

In addition to a high liquid absorption capacity, liquid-absorbing acrylic resins are also required to have a high swelling rate. On the other hand, however, if the swelling speed is too high, especially on the surface of the resin particles, the undesirable effect of " gel blocking " in which the resin particles that swell too much on the surface form a barrier for the further liquid to be absorbed, and so further absorption of liquid is only possible slowly or not at all. It would therefore be desirable to be able to produce liquid-absorbent resins with a defined and adjustable swelling rate.

This object was surprisingly achieved in that thermally decomposable, N-containing blowing agents are added in the production of the acrylic resin, which split off gases during the drying process and thereby result in a very loose and easily grindable acrylic resin.

The invention accordingly relates to a process for the preparation of liquid-absorbing, crosslinked acrylic polymers, which is characterized in that an aqueous solution of acrylic acid or methacrylic acid or a mixture of the two, optionally containing further comonomers, with NaOH, KOH or NH3 to a degree of neutralization neutralized from 60 to 100% and polymerized with the addition of a crosslinking agent and a polymerization initiator, the polymer obtained dries and comminutes, either before or after the polymerization, but after the 60 to 100% neutralization and before drying, an N-containing blowing agent , which decomposes or sublimates during the drying at 80 to 250 ° C with gas formation, is added in an amount which corresponds to an increase in the degree of neutralization to 102 to 140%.

In addition to acrylic acid or methacrylic acid, all unsaturated compounds known in acrylic resin chemistry, such as. As acrylonitrile, methacrylonitrile, acrylamide, methacrylamide, maleic acid, fumaric acid, itaconic acid, vinyl sulfonic acid, vinyl pyridinium salts or vinyl pyrrolidone can be used.

Known as crosslinkers in acrylic resin chemistry are polyunsaturated compounds, such as are described, for example, in EP-A-207 714, EP-A-233 014 or DE-OS 36 09 545, e.g. B. Ν, Ν'-methylenebisacrylamide, Ν, Ν’-methylenebismethacrylamide, divinylbenzene, allyl acrylate, dialkyl phthalate, glycol diacrylate or tris-acryloylperhydro-s-triazine (TAT), in question. Tri- and polyunsaturated compounds, such as. B. TAT, are preferably used. About 0.05 to 1 mol% of the crosslinking agent, based on the total amount of the monomers, is usually used.

Suitable as N-containing blowing agents are all compounds which decompose or sublime under the drying conditions, that is at temperatures from 80 to 250 ° C., with the formation of gas. These are primarily ammonium salts, for example ammonium carbonate, bicarbonate, carbamate, formate, acetate, propionate, butyrate, benzoate, salicylate, cyanide, rhodanide, sulfide, hydrogen sulfide, sulfonate, fluoride, ammonium salts of higher fatty acids and urea. The hydrogen of the ammonium groups can be substituted one or more times by alkyl radicals having 1 to 4 carbon atoms. A preferred blowing agent is ammonium carbonate. The ammonium carbaminate partially used in the examples also forms ammonium carbonate when heated in aqueous solution.

It has surprisingly been found that the blowing agent content of the polymerized, not yet dried acrylic resin gel is an essential factor for the swelling rate of the dried and ground acrylic resin. The higher the blowing agent content, the greater the swelling rate. Accordingly, it is possible to control the required swelling rate in a targeted manner by correspondingly high blowing agent addition during the production of the acrylic resin. The blowing agent is preferably added in an amount which corresponds to an increase in the degree of neutralization to 105 to 130%, ie an excess of 5 to 30 neutralization equivalents after the complete neutralization of the monomer solution or the polymer gel.

The neutralization with NaOH, KOH or NH3 in the first stage is preferably carried out up to a degree of neutralization of 75 to 95%.

The acrylic polymers according to the invention are prepared, for example, by mixing the aqueous monomer solutions and the crosslinking agent, neutralization with NaOH, KOH or NH3 up to one

Degree of neutralization from 60 to 100% and subsequent addition of the blowing agent at temperatures of about 15 to 60 ° C. The mixture is then flushed with nitrogen, for example, and the polymerization is started by adding a polymerization initiator at temperatures of about 5 to 35 ° C. As -2-

No. 391 321

Polymerization initiators are known from the aqueous acrylic resin chemistry, for example compounds described in EP-A-207 714 or DE-OS 36 09 545, such as. B. dibenzoyl peroxide, dilauryl peroxide, dicyclohexyl peroxidicarbonate, ammonium persulfate, ammonium peroxodisulfate with sodium disulfite as catalyst and 2,2-azobis (2-amidinopropane) hydrochloride (V-50 from Wako, Japan). The polymerization starts without further supply of heat and the temperature rises and is complete after about 30 to 300 minutes after a temperature maximum of about 60 to 110 ° C. has been reached. The resulting gel is then dried and ground at temperatures of approximately 80 to 250 ° C. under normal pressure or in vacuo. A preferred grain size range of approximately 0.06 to 1 mm is sieved from the flaky or platelet-shaped powder present after grinding

In another possible production method, the blowing agent is added to the gel-like mass formed after the polymerization after comminution with stirring, and the product is then dried and ground.

Due to the blowing agent that decomposes or sublimates during drying, the polymer gel forms a solid, large-bubble polymer foam during drying, which after grinding and sieving produces a platelet-shaped powder with a high and controllable swelling rate that does not stick or gel when swollen. Blocking shows. The acrylic resin according to the invention has an approximately neutral pH in contact with body liquids and accordingly has good skin tolerance.

Another advantage of using the blowing agent is that the gel block obtained after the polymerization only has to be broken down slightly into chunks of approximately 2 to 10 cm in size, since the product that swells up during drying repeatedly creates new interfaces for heat transfer, which the drying process compared to a non-expanding product greatly facilitated and shortened or made possible. At the same time, it is possible to obtain polymer resins with a low residual monomer content and a low water content.

example 1

950 ml of water were placed in a thermostattable reaction container and 300 g of acrylic acid, 127.2 g of NaOH and 115 mg of tris-acryloylperhydro-s-triazine (TAT) (0.071 mol%) were dissolved therein with stirring at room temperature. The degree of neutralization was 75%. 50 g of ammonium carbaminate were then added, which increased the degree of neutralization of the solution to 106%.

The mixture was then drained into an isolated reaction vessel, flushed with nitrogen for one hour and the polymerization at 20 ° C. by adding 234 mg (NH4) 2S20g, 117 mg Na2S2Ü5 and 169 mg azobis (2-amidinopropane) hydrochloride (V-50 von Wako, Japan), which were dissolved in 1 ml of water. A weak stream of nitrogen was blown over the reaction mixture during the polymerization. The polymerization was associated with an increase in temperature to 80 ° C. and was ended after 99 minutes.

Parts of the gel block obtained were dried in the following manner: a) 80 ° C., 24 h, normal pressure, convection drying cabinet b) 150 ° C., 24 h, normal pressure, convection drying cabinet c) 180 ° C., 4 h, normal pressure, convection drying cabinet d) 250 ° C, 2 h, normal pressure, convection oven e) 150 ° C, 5 h, 20 mbar f) 150 ° C, 6 h, 20 mbar and 80 ° C, 16 h, normal pressure

The dried fractions were ground on a hammer mill and the com fraction from 0.25 to 0.71 mm was sieved.

The acrylic resins obtained were tested for swelling volume, swelling mass, swelling rate, solids content, pH and residual monomer content. The values obtained are shown in Table 2.

Examples 2 to 15

Acrylic resins were prepared analogously to Example 1, but, as summarized in Table 1, the type and amount of the blowing agent, the degrees of neutralization before (neutral grade I) and after (neutral grade II) the blowing agent addition, and the amount of water used varied were. In Example 9, 600 mg of TAT (0.15 mol%) was also used instead of 115 mg of TAT and no V-50 was added. In Example 12, instead of 0.071 mol% TAT, 0.071 mol% methylene bisacrylamide (MBA) was used as the crosslinking agent. Table 1 also shows the pH of the solution before (pH I) and after (pH II) the addition of blowing agent, and the polymerization time (t ^).

The following blowing agents were used:

Ammonium carbaminate NH2COONH4 ACa

Ammonium carbonate (N ^^ COg AC -3-

No. 391 321

Urea H

Triethylammonium carbonate (NfEt-jJH ^ CO ^ TC

Ammonium benzoate AB

Ammonium acetate AA

The properties of the acrylic resins obtained are summarized in Table 2.

Example 16

An acrylic resin was prepared analogously to Example 1, but the blowing agent was not added to the 10-20 mm cubes in the form of 92 g of ammonium carbonate as a 30% strength aqueous solution until after the polymerization and comminution of the gel block obtained which diffused into the gel at room temperature for 16 hours. The acrylic resin was then dried at 150 ° C. and 20 mbar for 5 hours in a vacuum drying cabinet.

The production parameters are shown in Table 1, the properties of the acrylic resin obtained in Table 2.

Example 17

883 ml of water were placed in a thermostattable reaction container and 358 g of methacrylic acid, 127.2 g of NaOH and 600 mg of TAT (0.15 mol%) were dissolved therein with stirring. Then 75 g of ammonium carbonate were added, which increased the degree of neutralization of the solution from 75 (neutral grade I) to 115% (neutral grade Π). The solution was then polymerized analogously to Example 1 and the methacrylic acid resin obtained was dried in a vacuum drying cabinet at 150 ° C. and 20 mbar for 5 hours

The production parameters are shown in Table 1, the properties of the resin in Table 2

Comparative Example V18

An acrylic resin was produced analogously to Example 1, but with the difference that no blowing agent was added.

The production parameters are shown in Table 1, the properties of the acrylic resin obtained in Table 2.

The properties of the acyl resins given in Table 2 were measured by the following methods:

Official volume. Oil mass. pH 1 g of the acrylic resin was placed in a 100 ml measuring cylinder in which 100 ml synthetic urine (20 g urea, 8.2 g NaCl, 0.85 g CaCl2.2H20.1.14 g MgSC ^. 7H20.1000 g H20) were slowly sprinkled in. The swelling volume is the volume of the completely swollen resin in ml, the swelling mass is the weight of the swollen resin in g after the contents of the measuring cylinder have been emptied onto a sieve and the supernatant liquid has drained off and let drain for 25 minutes has been.

The pH was measured on the supernatant liquid after the resin had swelled completely.

Swelling speed (measurement according to AT 88/2109)

A one-sided adhesive lamination film was covered with a glass frame, which left a square area with a side length of 10 cm free, on which 8 cm 2 parallel, 5 cm long silver wires were connected, which were connected to a conductivity meter. 1 g of the acrylic resin was evenly sprinkled on the square surface and the acrylic resin was then wetted evenly from a pipette with 25 ml of a 0.9% NaCl solution. The swelling process was followed by means of conductivity measurement. After adding the NaCl solution, the conductivity rose to a maximum value that corresponded to the end point of the swelling process. The swelling rate was given as the time in s from the addition of the NaCl solution to reaching the maximum conductivity. A high swelling rate therefore corresponds to a short swelling time.

Solids content

The acrylic resin was ground and dried to constant weight in a drying cabinet at 120 ° C. to remove the residual water content.

Residual monomer content

The determination was carried out by extracting the acrylic acid using a methanol-water mixture, converting the acrylic acid using diazomethane to pyrazoline according to Analytical Chemistry Vol. 40 (8), pp. 1347-1349 and determining the pyrazoline by gas chromatography using a Carbowax-Megapore 20M column (Alkali ionization detector). -4- 5

No. 391 321

Table 1

Polymerization of the acrylic resins

Example neutr. Propellant neutr. Water pH I pH II lR grade I type amount grade II (ml) (min) (%) (g) (%) 1 75 ACa 50 106 950 5.0 8.3 99 10 2 75 ACa 62.5 113 950 5.0 8.8 67 3 75 ACa 75 121 950 5.0 8.9 57 4 75 AC 92 121 933 5.0 8.4 86 5 75 AC 106.1 128 925 5, 0 8.9 71 6 75 AC 120.2 135 922 5.0 9.0 54 15 7 75 H 48.4 113 950 5.0 5.4 272 8 75 H 57.7 121 950 5.0 5.4 272 9 75 TC 254.1 121 1357 5.0 10 10 75 AB 150.5 121 950 5.0 8.2 143 11 75 AA 74.2 121 950 5.0 5.2 253 20 12 75 ACa 75 121 950 5.0 8.8 253 13 80 ACa 75 126 955 5.3 9.0 47 14 85 ACa 75 131 965 5.4 9.1 52 15 90 ACa 75 136 970 5.5 9.2 52 16 75 AC 92 121 950 '5.0 - 52 25 17 * 75 AC 75 115 883 5.7 9.0 V18 75 - - - 950 5.0 - 253 * Polymerization of methacrylic acid 30

Table 2 35

Properties of acrylic resins

Example dry-swell-swell-swell solid residual mo pH type volume mass speed substance nomer 40 (ml / g) (g / g) (s) (%) (ppm) 1 a 83 65.0 162 90.0 b 93 65.0 198 99.4 130 c 94 68.0 163 98, 6 45 d 77 63.0 118 99.2 710 e 76 57.0 108 98.0 930 7.0 2 a 85 66.0 180 91.7 b 95 67.0 174 99.4 210 c 90 69.0 146 98.8 50 d 55 51.0 90 99.4 510 e 70 52.0 96 98.4 800 6.8 3 a 83 67.0 192 90.3 b 96 69.0 204 99.5 260 c 86 66.0 144 99.1 55 'd 55 48.0 84 99.4 260 e 58 49.0 66 99.2 840 7.0 4 e 67 55.0 65 97.7 5 e 65 43.0 60 97 , 5 185 6 e 68 53.0 126 98.5 225 60 7 e 84 61.3 181 84.2 8 e 77 57.3 128 96.4 -5

Claims (6)

No. 391 321 continued Table 2 Example of dry-swell-swell-swell-solid residual moss pH volume volume mass. substance nomer (ml / g) (g / g) (s) (%) (ppm) 9 f 44 37.0 113 10 f 46 36.0 87 95.0 11 f 72 52.0 114 91.0 12 f 72 70.3 60 13 f 59 44.2 60 14 f 62 48.6 54 15 f 80 61.0 177 93.2 16 e 73 54.7 150 97.7 17 * e 30 21.0 66 97.4 6.8 V18 e 60 52.4 42 92.4 935 5.8 * methacrylic acid resin PATENT CLAIMS 1. Process for the preparation of liquid-absorbing, crosslinked acrylic polymers, characterized in that an aqueous solution of acrylic acid or methacrylic acid or of a mixture of both, neutralized with NaOH, KOH or NH ^ to a degree of neutralization of 60 to 100% and polymerized with the addition of a crosslinking agent and a polymerization initiator, the polymer obtained dries and comminutes, either before or after the polymerization, but after the 60th up to 100% neutralization and before drying, an N-containing blowing agent, which during drying at 80 to 250 ° C under Ga formation decomposes or sublimates, is added in an amount which corresponds to an increase in the degree of neutralization to 102 to 140% 2. The method according to claim 1, characterized in that urea or an ammonium salt is used as the N-containing blowing agent. 3. The method according to claim 2, characterized in that ammonium carbonate is used as the N-containing blowing agent. 4. The method according to any one of claims 1 to 3, characterized in that the N-containing blowing agent is added to the monomer solution neutralized up to a degree of neutralization of 60 to 100% before the polymerization. 5. The method according to any one of claims 1 to 4, characterized in that is neutralized with NaOH, KOH or NH3 up to a degree of neutralization of 75 to 95%. 6. The method according to any one of claims 1 to 5, characterized in that the N-containing blowing agent is added in an amount which corresponds to an increase in the degree of neutralization to 105 to 130%. -6-