Highly swellable polymers

Abstract

The present invention relates to a method for producing polymers that are suitable for absorbing and storing aqueous liquids, and to polymers that can be obtained by this method. This invention further related to the use of such polymers. The method comprises the following steps: i. crosslinking free-radical polymerization of a monomer composition M comprising a) at least one monomer A having an ethylenic double bond and at least one neutralizable acid group or a group hydrolyzable to a neutralizable acid group, b) optionally one or more comonomers B which are different than the monomers A and have one ethylenic double bond, and c) 0.05 to 10% by weight, based on the total amount of monomers A and B, of at least one crosslinker C, in the presence of at least one polysaccharide-comprising substance S, in an aqueous liquid, where the weight ratio of the monomer composition M to the substance S is in the range from 9:1 to 1:9; and ii. at least partial neutralization of the acid groups and/or hydrolysis of the groups hydrolyzable to neutralizable acid groups in the polymer obtained in step i.;
wherein the polymerization and/or the neutralization is performed in the presence of urea.

Claims

1. A process for preparing polymers suitable for absorbing and storing aqueous liquids, comprising: i. crosslinking free-radical polymerization of a monomer composition M comprising a) at least one monomer A selected from monoethylenically unsaturated C.sub.3-C.sub.8-monocarboxylic acids, mixtures thereof and mixtures of at least one monoethylenically unsaturated C.sub.3-C.sub.8-monocarboxylic acid with one or more monoethylenically unsaturated C.sub.4-C.sub.8-dicarboxylic acids, b) optionally one or more comonomers B which are different than the monomers A and have one ethylenic double bond, and c) 0.05 to 10% by weight, based on the total amount of monomers A and B, of at least one crosslinker C, in the presence of at least one finely divided particulate cellulose material S comprising bound lignin, in an aqueous liquid, where the weight ratio of the monomer composition M to the substance S is in the range from 8:2 to 2:8; and ii. at least partial neutralization of the acid groups in the polymer obtained in step i.; wherein the polymerization and/or the neutralization is performed in the presence of urea and at least 90% by weight of the particles of the particulate cellulose material have dimensions in the range from 1 m to <500 m, determined by sieve analysis, wherein the amount of urea is from 5 to 60% by weight, based on the total amount of the monomer composition M and substance S, and wherein the polymerization is performed at a pH range from 1-4.

2. The process according to claim 1, wherein the polymer obtained in step i. is treated with urea before or during the neutralization in step ii.

3. The process according to claim 1, wherein the total amount of urea is 1 to 20% by weight, based on the total amount of monomer composition M, substance S and aqueous liquid.

4. The process according to claim 1, wherein the cellulose material is selected to an extent of at least 50% by weight, based on the total amount of substance S, from hemp dust, flax dust, sawdust, ground straw, ground olive stones, ground tree bark, reject material from pulp production, sugar beet peel, sugar cane waste, rice husks, cereal husks, and ground hemp fibers, ground flax fibers, ground Chinese silvergrass fibers, ground coconut fibers, ground kenaf fibers and ground wood fibers.

5. The process according to claim 1, wherein at least 90% by weight of the particles of the particulate cellulose material have dimensions in the range from 5 to <300 m, determined by sieve analysis.

6. The process according to claim 1, wherein the monomers A account for at least 50% by weight, based on the total amount of monomers in the monomer composition.

7. The process according to claim 1, wherein the monomers A comprise at least 50% by weight, based on the total amount of monomers A, of acrylic acid.

8. The process according to claim 1, wherein the crosslinkers C are selected from ethylenically unsaturated monomers having at least 2 ethylenically unsaturated double bonds.

9. The process according to claim 1, wherein the polymerization is performed under oxygen-comprising atmosphere.

10. The process according to claim 1, wherein a viscosity of the reaction mixture, determined to DIN EN 2555-2000 by means of a Brookfield viscometer at 23 C. at a shear rate of <10 sec.sup.1, in the range from 10 to 1000 mPa.Math.s is established at the start of the polymerization.

11. The process according to claim 1, wherein the neutralization is followed by separation of the polymer from the aqueous liquid and drying.

Description

I. PREPARATION EXAMPLES

Example 1

(1) 600 g of soluble lignin-free reject material (woodlike waste material obtained in pulp production for paper), 870 g of acrylic acid, 6 l of demineralized water, 6.45 g of methylenebisacrylamide, 1.5 g of ammonium persulfate were mixed well to give a slurry and introduced into a 10 l bucket. The bucket was placed into a water bath thermostated to 85 C. After about 2 hours, the material had reached a temperature of 78 C. The material was then solid. The polymerization mixture was left to stand for another 1 h while maintaining the bath temperature. The rubberlike, slightly tacky polymerization mixture was easily removable from the bucket in one piece.

(2) Total yield 6.62 kg.

(3) For final neutralization, the rubber cake was first comminuted, then admixed with 18 l of methanol and mixed well. This formed a thick slurry. Then 675 g of a 50% by weight aqueous KOH solution and 728 g of a 50% by weight aqueous urea solution were added together with 3 l of methanol and the whole lot was mixed vigorously for about 15 min. The product coagulated immediately on addition of the urea solution to become a chewing gum-like material, but this broke down very quickly to become a granular product. As soon as the pH was 6 to 7, which was the case after 15 min, the solid obtained was filtered off and dried under reduced pressure at 55 C.

(4) The light brown, free-flowing solid thus obtained exhibited a water absorption capacity at 22 C. of up to 550 g of tap water, dH=4, per g of solid.

Example 2

(5) 30 g of flax dust, 10 g of crude cellulose, 58 g of acrylic acid, 0.43 g of methylenebisacrylamide, 0.1 g of ammonium persulfate, 370 ml of demineralized water and 48 g of a 50% by weight aqueous urea solution and 0.4 g of a commercial wetting agent customary for textile fibers, for example a Triumph wetter (Contripon S from Zschimmer & Schwarz, 09218 Burgstadt) were mixed vigorously. This slurry mixture was introduced into a 1 l polyethylene bucket. The bucket was placed into a water bath thermostated to 85 C. After about 1 hour, the reaction mixture had already reached the maximum temperature of 71 C. The polymerization mixture was left to stand for another 1 h while maintaining the bath temperature. Subsequently, the rubberlike, slightly tacky polymerization mixture was removed from the bucket in one piece.

(6) Yield 509.1 g.

(7) Then the rubberlike cake was admixed with 1 l of methanol and comminuted with an Ultra-Turrax. This formed a thick brown slurry which, while stirring constantly and vigorously, was admixed with a solution of 250 ml of methanol and 45.1 g of a 50% by weight aqueous KOH solution. In the course of addition, a viscous lump formed immediately, but this very rapidly into a grainy material under vigorous stirring. After 10 min, the treatment was ended. The product obtained was filtered off and dried under the conditions specified for example 1. The water absorption capacity of the product for tap water, dH 4, was 400 g/g of product at 22 C.

Example 3

(8) 160 g of ground cellulose, ARBOCEL BC 1000 from Rettenmeier, 240 g of acrylic acid, 1600 g of demineralized water, 1.72 g of methylenebisacrylamide, 0.4 g of ammonium persulfate were mixed vigorously to give a homogeneous slurry and poured into a medium-sized bucket, and the latter was placed into a water bath at 87 C. After about 2 hours, the reaction mixture had reached the maximum temperature of 91 C. The mixture was left to react at bath temperature 87 C. for another 1 hour, and the material temperature of the contents of the bucket fell back down to 84 C. Subsequently, the rubberlike, slightly tacky product cake was removed from the bucket in one piece.

(9) Yield 1937.2 g.

(10) Then the rubberlike cake was admixed with 3.5 l of methanol and comminuted with an Ultra-Turrax unit. This formed a thick white slurry which, while stirring constantly and vigorously, was admixed with a solution of 1000 ml of methanol, 186.7 g of a 50% by weight aqueous KOH solution and 200 g of a 50% by weight aqueous solution of urea. In the course of addition, a viscous lump formed immediately, but this very rapidly broke down into a grainy material under vigorous stirring. After 10 min, the treatment was ended. The product obtained was filtered off and dried under the conditions specified for example 1. The water absorption capacity of the product for tap water, dH 4, was 400 g/g of product at 22 C.

II. STUDY OF BIODEGRADABILITY

(11) The following products were tested:

(12) polymer from example 1

(13) polymer from example 4

(14) commercial superabsorbent based on crosslinked polyacrylic acid (Luquasorb 1280 BASF SE)

(15) To study the biodegradability, the carbon mineralization of the polymers was studied in three soils in microcosms over a period of 271 days at 20 C. Straw served as the reference substance. After 271 days, for straw, it was possible to detect 62% of the carbon as CO.sub.2 in the middle of the soils, 33% in the case of the polymer from example 1, and 6% for the commercial superabsorbent. Since the rise in carbon mineralization for straw and the polymers, after initially intense mineralization, became linear after 90 days, it was possible to extrapolate the mineralization data. Extrapolated to two years, degradation rates of 92% for straw, 51% for the polymer from example 1 and 17% for the commercial superabsorbent were found. After correcting the mineralization for an annual average temperature of a soil of 10 C., mineralization rates of 20% for the polymer from example 1 and of nearly 7% for the commercial superabsorbent over two years were calculated.

III. STUDY OF SOIL-IMPROVING ACTION

(16) The following products were tested:

(17) polymer from example 2

(18) commercial polymer in the form of a potassium salt of a starch-grafted acrylic acid-acrylamide copolymer (Zeba Farm from Absorbent Technologies, Inc. Beaverton, Oreg. 97008)

(19) commercial superabsorbent based on crosslinked, partly neutralized polyacrylic acid (Luquasorb 1280 BASF SE)

(20) The study was conducted outdoors in a cultivation area in central Italy in a sandy soil. The respective products were incorporated into the soil in an application rate of 20 kg/ha. On Jul. 27, 2012, tomato seedlings (150 seedlings/40 m.sup.2) were planted into the soils thus treated. One trial plot was watered to an extent of 100%; a further trial plot was watered to an extent of 50%.

(21) During the trial period, i.e. up to harvesting on Oct. 1, 2012, the vigor (plant health) of the plant was assessed visually on the basis of plant height, intensity of green color of the plant, number of leaves and disorders affecting the plant, and rated on a scale from 0 to 10, 0 meaning particularly poor vigor and 10 particularly good vigor. In addition, the amount of tomatoes harvested was determined. The results are given in tables 1 and 2 below.

(22) TABLE-US-00001 TABLE 1 Plant vigor Vigor/100% watering Vigor/50% watering No addition 5 5 Luqasorb 1280 5.3 6.7 ZebaFarm 6.3 6.7 Polymer from ex. 2 7.0 8.3

(23) TABLE-US-00002 TABLE 2 Yield Yield/100% watering Yield/50% watering [kg/ha] [kg/ha] No addition 4035 1496 Luqasorb 1280 6695 10059 ZebaFarm 8356 7470 Polymer from ex. 2 9010 11297