DE19640329A1 - Zwitterionic hydrogels useful as water reservoirs, soil improvers etc - Google Patents

Abstract

Zwitterionic hydrogels are claimed, and their preparation by mixing together and reacting crosslinked cationic polymer gels (CPG) and crosslinked anionic polymer gels (APG).

Description

The invention relates to novel synthetic zwitterionic hydrophilic gels, the process on their manufacture and examples of their use.

Natural hydrogels, disperse systems made of a solid, colloid, have long been known distributed substance with water as a dispersant. Their particles adhere through secondary or Main valences to one another, starting from strongly polar atomic groups such as -OH, -NH, CONH- and -SH.

Acid anions or ammonium cations occur in the natural hydrogels as structure-building groups are rare, e.g. B. in the proteins at the chain ends; natural hydrogels are gelatin, collagen or the polysaccharides agar-agar, carragen or the so-called phycocolloids from algae ingredients, which are mannose, fucose, Contain glucose, fructose and galactose. The natural hydrogels have in the Food industry and in cosmetics their areas of application; technical Find use, use as a soil conditioner or moisturizer So far, the natural hydrogels have not been, since they are easily biodegradable and therefore theirs Lose hydrophilic gel properties quickly.

For technical use as absorbent or liquid storage agent or as Soil improvers are made from synthetic ionic polymers; one gets out unsaturated alkylammomium salts by radical polymerization with the addition of polyfunctionally unsaturated crosslinkers polymeric cationic gels (DD-PS 291.448), the as a moisturizing principle in cosmetics (DD-PS 206.878), as Soil improver with water-storing effect (DD-PS 201.455, DD-PS 209,644, A. Kullmann et al, Arch. Acker- u. Crop production and soil science Berlin 27 (1983) pp. 491-497) or in the technical field as temporary blocking agents in the Have the natural gas industry used (DD-PS 159.015, DD-PS 213.942).

Polymers are obtained from acrylonitrile or acrylamides by partial saponification their nitrile or amide group to carboxylate groups become highly hydrophilic and swellable. Obtained by direct synthesis, by copolymerization of acrylamides with acrylic acids these products with a specified anion content, which makes the Have the properties of the synthetic hydrogels predetermined. This anionic Polyacrylamide gels are also used as absorbents or moisturizers and serve like the cationic polypyrrolidinium gels (H. Benkenstein et al., Wiss. Z. Humboldt University Mat. Nat. R. 36 (1987) p. 330) as water-storing substances Soil improvement.

A well-known example is their use in the KALLIDENDRON process, in which they the role of water storage in the planting of young trees under unfavorable Take over dry conditions (G. Kallistratos, Naturwissenschaftliche Rundschau (1987) 300-302, G. Kallistrator and I.M.G. Kallistratos, Journal of Oriental and African Studies Vol. 3, (1991), 194-208).

The water absorption of the synthetic cationic and anionic gels is of a number the ionic groups in the gel on the one hand and the salt concentration of the surrounding Water, on the other hand, d. H. with distilled water there are higher degrees of swelling than achieved with tap water; you take one or more after the swelling Water changes to wash out synthesis-related accompanying salts, so you can Ionic hydrogels achieve swelling levels of 1: 400. Degree of swelling below Practical conditions - especially in the soil - are usually far below these in laboratory terms achievable degrees of swelling. To compare different gels, different ones Synthesis batches and charge densities in the polymer, however, also increase the degree of swelling distilled water because there is no other objective benchmark.  

Anionic polymer gels differ from cationic gels in another point: their degree of swelling is not only dependent on salt concentrations, but in particular to a large extent also from the pH value. Low pH values work through protonation of Reducing carboxylate groups to the number of charge carriers, so that pH Lowering the degree of swelling of anionic polymer gels declines sharply.

In addition to reversible water absorption, ionic polymer gels are also used to exchange Ion-enabled; in the case of the cationic polymer gels, the quaternary ammonium ions are in the gel matrix - the associated anions are like in any ion exchanger flexible and can be exchanged. In the anionic polymer gels are in contrast, the cations are mobile and interchangeable.

Cationic polymers that are still soluble due to the lack of crosslinking and none Form gel phase, react with also still soluble anionic polymers Saturation of their positive or negative charges to water-insoluble ones Polymer compounds (simplexes) that are neither soluble nor swellable in water (B. Philipp and G. Reinisch, Fundamentals of Macromolecular Chemistry, Akademie-Verlag Berlin 1976, page 342). This intermolecular reaction of anionic with cationic groups in polyelectrolytes to simplexes also takes place when cation and anion are part of a (sufficiently large) chain, so that when trying to use cationic monomers subjecting anionic monomers to copolymerization, always poorly soluble, less swellable polymers result, which act as hydrogels and ion exchangers are unusable.

It is the object and aim of the present invention to overcome this difficulty and To produce gel structures, which are both cationic and anionic groups contain and nevertheless hydrophilic properties, strong swelling with water and Show exchange capacity for cations and anions.

The problem is solved by first creating a cross-linked cationic or anionic Polyelectrolyte gel is produced and the cationic is then crosslinked anionic polyelectrolyte gel or the crosslinked anionic polymer then with a cross-linked cationic polymer is combined.

It has been found that anionic polymer gels in their hydrated gel form cationic hydrated gels enter into compounds such that in terms of Morphology in the molecular area of the interfaces of the disperse anionic and cationic gel particles a statistical charge exchange over larger structural areas occurs, probably with fluctuating ionic bridges and not as ordered Structures between two oppositely charged groups. Such "Superstructures" between disperse polyelectrolyte gel particles differ in their Properties of the respective homopolymer gels; a disperse arises "Polyampholyte gel particles" which are still hydrophilic, reversibly swellable and Ion exchange is capable, however, a different gel structure and due to different Binding relationships z. B. shows a different strength and elasticity of the gel particles. Typical characteristics for polyelectrolytes, such as swellability, strong influence of the pH value, the ionic strength and type of the low molecular counterion on molecular form and Transport processes are qualitatively preserved in the superstructures, they become quantitative however, opposed to the strong interactions at the interfaces of the charged surfaces of the disperse gel particles forming the superstructure.

The procedure according to the invention is such that macroscopic stoichiometric anionic and cationic charges correspond to each other, especially if it is a target of the Gel formation is to have an "exchange gel" available for cations and anions. However, stoichiometry of the charges is not absolutely necessary to form the connection,  since an almost stoichiometric charge balance only by reaction to the Interfaces of the disperse gel particles occur, but still inside the gel particles in front of homopolymeric structures with their freely selectable movable, d. H. interchangeable Counterions exist. Therefore, one can also form structures that are macroscopic either the anionic or, on the other hand, the cationic polymer gel predominates; This can then be of application technology advantage if, for. B. polyvalent anions or polyvalent cations are to be exchanged or partially fixed; in these cases can with polyvalent metal ions also a complex bond on the anionic Enter the polymer gel area. In addition to the possibility of cationic and anionic hydrogel to mix and bring to reaction, you can technically z. B. with Advantage so proceed that an unsaturated ammonium salt, for. B. Dimethyl diallylammonium chloride in the presence of conventional crosslinking agents such as methyltriallyl ammonium chloride, tetraallylammonium chloride, fumaric acid diallyl ester, methylenebis acrylamide (or mixtures of these crosslinkers) polymerized and shortly before reaching the In the gel state, the anionic polyelectrolyte gel is added and mixed intimately, in this case occurs the reaction at the interfaces of gel particles already present during the Gelling process of the crosslinking polymerization of the unsaturated ammonium salt is still progressing and is being completed. For practical purposes, it is also still possible to add powder-dried pure cation gels with powder-dried anion gels mix, the gel formation to the superstructured "ampholyte gels" then takes place automatically upon contact with water, which must take place in every application.

You can also use powder-dry anion gel, e.g. B. an acrylamide-acrylate copolymer, in one Granulator, mixer or in a fluidized bed dryer with water dispersed cationic gel - e.g. B. a cross-linked polymeric dimethyldiallylamonium chloride - by Apply spray and dry the product together, this can also be done in the presence further solid particles take place. In this way, solids can be combined with hydrogels can be mixed or coated, which makes them hydrophilic, water-storing and gives ion exchange properties.

Use zwitterionic hydrogels z. B. in cosmetics as a moisturizing base gel, the interchangeability - especially the anions - opens up possibilities for anion exchange for anions with active substance character - so that the hydrogels can also be used as an active substance depot. Zwitterionic hydrogels find their most important field of application as soil improvers. They are inexpensive to manufacture and with low mass use, 100 to 400 times the amount of water can be reversibly absorbed. Dry periods can be bridged and young plants can be supported during the growing phase. It is both the direct introduction into the soil and the combination with other soil materials, e.g. B. compost, bentonites, marl, lime, clay, NPK fertilizers and a combination of these soil materials, possible. Trace elements that are so important for plant growth (Zn, Cu, Mn, etc., and especially the trivalent transition metals are stored in the anionic groups of the hydrogel. On the other hand, there is also a desired exchange capacity for the fertilizer cations K⁺, NH₄⁺, Ca ⁺⁺ , Mg ^{+ +.} fertilizing anions such as phosphate, nitrate or Wirkstoffanionen such. as thiocyanate can be stored temporarily. the exchange capacity for anions and cations makes the zwitterionic hydrogels superior to previously known types of gel.

Giving can also be given to growing plants in dry areas of the Stage of unfavorable salinization of the soil by irrigation with river water  counteract. Water absorption and water discharge are associated with large volume changes the gel particles connected, so that this constantly repeating process in the soil results in soil loosening that is beneficial to the crop.

Dissolved fertilizers can be administered with the irrigation water and are available to the plant available longer and therefore more effectively due to the exchange processes. At the same time the pollution of the groundwater by washed down, not from the plants more accessible fertilizer components reduced. There is also the possibility To produce combinations with pesticides or growth agents. Hydrogels with selected accompanying substances can also be used for seed treatment and seed coating can be used, the water-binding property and e.g. B. pesticides can be used only on the seed, without the Spreading arable soil over large areas with unnecessarily higher doses.

The cationic gel part makes it resistant to degrading Microorganisms reached, so that a long, inexpensive with zwitterionic hydrogels The soil is effective for several years.

Execution and application examples example 1

6 kg of a by copolymerization of dimethyldiallyl ammonium chloride with 1.78 mol% Methyltriallylammonium chloride produced polypyrrolidinium gel with 50% water content were swollen with 6 kg of water and stirred in a steam heated open Stirred kettle heated to 90 ° C. With further stirring, 9 kg were then added N, N-methylene bisformamide crosslinked copolymer of 3 moles of acrylamide and 1 mole Potassium acrylate entered. The obtained, largely dry, crumbly gel structure becomes a zwitterionic by further heating (approx. 1 hour) with stirring Mixed gel dried and solidified.

The degree of swelling was first 1: 100, after a second treatment with fresh deionized water 1: 150.

The gel can serve as a water reservoir 1: 100 mixed with soil.

Example 2

The procedure was as described in Example 1 and the reaction was carried out during the drying phase Stir 100 ml of a 1:10 concentrated HOAGLAND trace element solution as well as 200 g of Thomas flour and 50 g of a long-term N-based fertilizer Urea / aldehyde too; the degree of swelling of these equipped with fertilizer Mischel sample was 1:90.

The gel can be used as a water depot when mixed with soil Provides trace elements and nitrogen fertilizer for healthy plant development.

Example 3

5 kg of one made of dimethyl-diallyl-ammonium chloride and methylenebisacrylamide as crosslinker gels obtained with 50% residual moisture were heated to 90 ° without addition of water and with  7.5 kg of a finely powdered, crosslinked arylamide / acrylic acid copolymer with stirring and heating offset. A zwitterionic was obtained more quickly than described in Example 1 Mixed gel of crumbly to powdery consistency. The coarser gel fractions were from so hard that they could be crushed in a disk mill.

The degree of swelling was 1: 100.

Part of the product was as described in Example 2, with plant nutrients and Mixed trace elements.

Example 4

161.7 g of 50% aqueous dimethyl diallyl ammonium chloride solution are mixed with 1.52 g (2.2 mol%) potassium carbonate, dissolved in 5 ml of water. In this solution 2.38 g (2 mol%) sodium peroxodisulfate and 0.462 g (0.6 g mol%) N, N-methylenebisacrylamide solved. The polymerization begins, the temperature of. Within 45 minutes 25 ° rises to 31.5 ° C.

After 12 hours, the polymerization is ended by gradually warming to 90 ° C. guided.

The cationic gel obtained has a degree of swelling of 1:78.

With stirring, 210 g of a dry acrylamide / acrylate gel are added immediately, which removes water from the cationic gel by reaction to the zwitterionic mixed gel, a crumbly, easily processable product being obtained, which is dried by further reheating.
Degree of swelling: 1: 105

Example 5

The procedure of Example 4 is followed, but the acrylamide / acrylate polymer is already added during the polymerization of the cationic polymer gel is brought to an end by heating.

A zwitterionic gel with largely the same properties but with one is obtained lower degree of swelling (1:68).

Example 6

A cationic polymer gel based on dimethyldiallylammonium chloride and a Pre-swell anionic acrylamide / acrylate gel separately with water (factor 50-100), mixes the gel samples in the ratio that the cation content and anion content in the Equivalent ratio and mixes and blends to the swollen form of the zwitterionic gels, which result from reaction of the surfaces of the gel particles with each other in a few minutes. By (repeated) water changes you can desalt and increase the degree of swelling to 1: 200 and above.

Accordingly, by mixing unstoichiometric cation and anion Gel contents zwitterionic hydrogels generated with excess anion or excess cation if the intended use with special ability for ion exchange is required.

The ion exchange can be demonstrated by the swollen and extensive salt-free gel in contact with a dilute calcium nitrate solution of known content brought. Analysis of the calcium content before and after the gel treatment of the solution shows that the calcium content of the solution decreases in favor of the calcium content of the gel.

Claims (8)

1. Zwitterionic hydrogels and processes for their preparation and use, characterized in that cross-linked cationic polymer gels are mixed with cross-linked anionic polymer gels and reacted with one another. 2. The method according to claim 1, characterized in that cross-linked cationic Mixes polymer gels with cross-linked anionic polymer gels in their hydrated form and react. 3. The method according to claim 2, characterized in that a networked, anionic, powdery polymer gel progressing to the reaction mixture polymerizing solution of a cationic monomer is added. 4. The method according to claim 1, characterized in that cross-linked cationic Mixes polymer gels with cross-linked anionic polymer gels in dry powder form and then reacted by adding water. 5. The method according to claim 1 to 3, characterized in that the crosslinked cationic polymer gels with the crosslinked anionic polymer gels - based on their charge-bearing groups - in stoichiometric or sub-stoichiometric ratio mixes and makes them react. 6. The method according to claim 1 to 4, characterized in that one of the Components or the mixture of solids, such as compost, marl, bentonite, lime, Clay, fertilizer or trace elements. 7. The method according to claim 1 to 6, characterized in that the mixing in Reactors, granulators or fluid bed dryers. 8. Use of zwitterionic hydrogels as water reservoirs, soil improvers, Base gel for cosmetic preparations, ion exchangers and as an active ingredient depot.