EP0000565B1 - Aqueous solutions of copolymers, process for their preparation and their use as shell forming materials for the production of microcapsules by complex coacervation - Google Patents

Description

The invention relates to aqueous copolymer solutions, processes for their preparation and their use as shell formers for the production of microcapsules by complex coacervation. The aqueous polymer solutions according to the invention are those whose polymer content is 5 to 30% by weight and consists of a mixture of (A) a copolymer with an intrinsic viscosity [ _'I] of 0.05 to 1 (dl / g) to its statistically distributed structural units 65 to 90 mol% consist of residues of acrylamide and 10 to 35 mol% of residues of maleic acid or maleic anhydride and (B) a corresponding hydrolyzed copolymer with an intrinsic viscosity [q] of 0.05 to 1.5 (dl / g) whose statistically distributed structural units accordingly consist of residues of acrylamide, acrylic acid and maleic acid, the maleic acid residues being at least partially in salt form, and which is characterized in that the weight ratio of the copolymers (A) and (B) is 1: 2 to 20: 1 is.

The copolymers (A) can be obtained by radical copolymerization of 65-90 mol% acrylamide and 10-35 mol% maleic anhydride or maleic acid.

The copolymerization of acrylamide and maleic anhydride is advantageously carried out as a precipitation polymerization in organic solvents which are inert to anhydride groups. Particularly cheap solvents are acetone and acetic acid esters of alcohols with 1-3 carbon atoms because of their good solvent power for the monomers and not least because of their low volatility. The polymerization can be initiated with the usual free-radical initiators such as aliphatic azo compounds or organic per-compounds such as peroxides or peresters. The polymerization temperature is determined by the decomposition properties of the initiator and, when using azoisobutyronitrile in batches, is 50 ° -80 ° C. In this process, the copolymers are obtained as finely divided powders and can easily be filtered off and freed of residual monomers by washing.

The copolymerization of acrylamide and maleic acid to give copolymers (A) can be carried out both as precipitation polymerization in organic solvents and also as solution polymerization in water.

In the precipitation copolymerization of acrylamide and maleic acid, in addition to acetone and acetic acid esters of alcohols with 1-3 carbon atoms, lower aliphatic alcohols with 1--4 carbon atoms can also be used successfully. The polymerization and isolation of the polymer powders is carried out analogously to that of acrylamide-maleic anhydride copolymers.

The precipitation copolymers can be dissolved in solvent or, after drying, in water to 5-30% by weight solutions. The anhydride groups of the acrylamide-maleic anhydride copolymers pass into maleic acid groups. When using the solvent-moist polymers, the organic solvent is then separated off by decanting and / or distillation.

Another preferred procedure for the preparation of aqueous solutions of the copolymers (A) is the solution copolymerization of acrylamide and maleic acid in water. The maleic acid can either be prepared by dissolving the appropriate amount of the andydride in water before the polymerization or used as such. Suitable initiators are water-soluble per compounds such as persulfates or hydrogen peroxide, optionally in combination with a reducing agent and / or heavy metal salt. Azo compounds with sufficient water solubility are also suitable initiators. Azoisobutyronitrile is sufficiently soluble in water, especially at elevated temperatures, and is particularly preferred because it does not affect the salt content of the solutions.

Like the precipitation polymerization, the solution polymerization can be carried out batchwise with presentation of the entire batch or according to a semi-batch feed process with metering in of the monomers. The preferred method of solution polymerization is that the aqueous solution of the monomers, which contains the initiator in dissolved or finely suspended form, is added at about 80 ° -120 ° C. with the exclusion of oxygen to a stirred charge of oxygen-free water in about 1-10 hours. The azoisobutyronitrile can also be added dropwise as a solution in an organic solvent in synchronism with the monomer solution, it being possible for the solvent to be distilled off continuously. After the monomer addition has ended, the mixture is stirred for 15-120 minutes and the solution is then cooled to room temperature.

The solutions of the copolymers (A) obtained by solution polymerization still contain small amounts of residual monomers, but these generally have no adverse effect on the application properties of the solutions. In addition, copolymers (A) prepared in aqueous solution already contain small amounts of acrylic acid and maleic acid salt units formed by hydrolysis during the polymerization.

Copolymers (A) according to the invention are obtained by copolymerization of 65-90 mol% of acrylamide and 10-35 mol% of maleic acid or maleic anhydride. Copolymers (A) of 70-85 mol% of acrylamide and 15-30 mol% of maleic acid or maleic anhydride are preferably used. The copolymers (A) contain the structural units made of acrylamide and Maleic anhydride in statistical distribution and in proportions that correspond to the compositions of the monomer mixtures.

The free radical initiators are added in amounts of 0.1-10% by weight, preferably 0.3-3.0% by weight, based on monomers. In precipitation polymerizations, the polymer yields are generally 75-95%. The copolymers (A) have intrinsic viscosity [ _{1 /]} of 0.05-l ', 0 [dl / g], preferably 0.08 to 0.4 [dl / g] (determined in 0.9% aqueous sodium chloride solution) .

The solutions of the copolymers (B)) can be obtained by heating the aqueous solutions of the copolymers (A) at temperatures of 60 ° -150 ° C., preferably 80-130 ° C. for a long time, optionally under pressure in an autoclave. Hydrolysis under reflux conditions at normal pressure is particularly preferred. Partial hydrolysis of acrylamide units occurs during heating. The resulting ammonia neutralizes some of the maleic acid building blocks and thereby causes an increase in the pH of the solution. As a result of the hydrolysis, the acrylamide-maleic acid copolymer converts into a copolymer which consists of acrylamide-acrylic acid and maleic acid units and in which the maleic acid units are at least partially in salt form.

If aqueous copolymer solutions (A) which have been prepared from thoroughly washed precipitation polymers are hydrolyzed, the viscosity of the solution does not change appreciably in the course of the hydrolysis, so there is neither degradation of the polymer nor postpolymerization.

When copolymer solutions (A) prepared by solution polymerization in water are heated, the solution viscosity can increase to a greater or lesser extent by post-polymerization of the residual monomers. However, this increase does not lead to a significant change in the coacervation behavior of the polymers.

Solutions of copolymers (B) according to the invention are obtained by heating solutions of the copolymers (A) according to the invention to 60 ° -150 ° C., preferably 80 ° -130 ° C. and particularly preferably to about 100 ° C. (reflux under normal pressure) until the pH of the solutions has risen to 3.8 to 4.9, preferably 4.0 to 4.6. As a result of any subsequent polymerization of residual monomers, the intrinsic viscosity of the copolymers (B), determined in 0.9% aqueous sodium chloride solution, is 0.05 to 1.5 [dl / g], preferably 0.08 to 1.0 [dl / g ].

The solutions of the copolymers (A) and the solutions of the copolymers (B) also form coacervates on their own with gelatin, if necessary after adjusting the pH with ammonia or alkali metal hydroxide solution or with acetic acid. However, these coacervates do not form a shell or a closed shell and / or are very sticky.

If you mix the aqueous solutions of the copolymers (A) and (B) according to the weight ratio of the dissolved copolymers (A): (B) as 1: 2 to 20: 1, surprisingly polymer solutions are obtained which, together with gelatin, provide coacervates which form uniform, closed shells and are therefore particularly suitable for the production of microcapsules. The optimal mixing ratio is mainly influenced by the composition of the copolymer (A) and by the pH of the solution of the copolymer (B), which is a measure of the degree of hydrolysis, and must be determined experimentally for each pair of copolymers. Of course, solutions of copolymers (A) can also be mixed with solutions of copolymers (B) which have been obtained from copolymers (A ') with a different composition by hydrolysis (for example copolymer (A) with 70 mol% acrylamide and 30 mol% % Of maleic acid units, and copolymer solution (B) from copolymer (A ') with 80 mol% of acrylamide and 20 mol% of maleic acid units). In some cases, changing the pH of the polymer solution by adding ammonia or alkali hydroxide or acetic acid can have a favorable effect on the coacervation behavior.

The process of microencapsulation by complex coacervation is known in principle. It is based on dispersing the encapsulating substance in finely divided form in an aqueous solution suitable for coacervate formation, and then triggering the coacervation. This is done by diluting, changing the pH or cooling or by a combination of these measures, depending on the system used.

The aqueous polymer solutions of the invention form complex coacervates with gelatin by dilution and / or cooling. They can be mixed, for example, in 10-15% by weight aqueous solution with a 10-15% by weight gelatin solution and coacervated by dilution and cooling. It is astonishing that such highly concentrated solutions can be used. It is also surprising that the coacervates obtained, which are a separate soft gel phase, already contain _20-30 % by weight of solids. This high solids content makes it very easy to dry the coacervates into powders. Since the coacervates form extremely coherent shells or coatings, they are particularly suitable for the production of microcapsules. During coacervation, the separate gel phase forms as a coherent layer on small, liquid or solid separate particles dispersed in the solution and envelops these particles.

The combination of polymer solutions according to the invention and gelatin can be used to microencapsulate a large number of liquid or solid dispersible or water-insoluble or sparingly water-insoluble or water-resistant substances.

Examples include: organic solvents, paraffin oils, perfume oils, silicone defoamers, Phosphoric acid esters, liquid crystals and color pigments, as well as pharmaceuticals and pesticides.

example 1

• a) Polymerization: (copolymer (A)) 240 g of acrylamide, 60 g of maleic anhydride and 3 g of azoisobutyronitrile are dissolved in 2.7 l of ethyl acetate. The solution is freed from oxygen by repeatedly evacuating the apparatus and filling with nitrogen and stirred at 60 ° C. for 20 hours with the exclusion of oxygen. The precipitated polymer is filtered off, washed thoroughly with ethyl acetate and dried in vacuo at 60 ° C. This gives 273 g of a finely powdered polymer with a viscosity number (intrinsic viscosity) determined in 0.9% strength aqueous NaCl solution [q] = 0.14 [dl / g].
• b) hydrolysis: (copolymer (B)) 100 g of the dry polymer acc. 1 a) are dissolved in 900 ml of demineralized water, and the solution is heated to reflux up to pH 4.6 at normal pressure (approx. 12-18 hours). Immediately after dissolving and when pH 3.0 is reached; 3.5; 4.1 and 4.6, the solution viscosity is determined using a falling ball viscometer. All solutions show the same viscosity within the error limit.
• c) Preparation of the polymer solution and encapsulation 150 g of dry polymer according to 1 a) are dissolved in 1,350 ml of demineralized water. The solution shows a pH of ~ 2.2. 150 ml of this solution are mixed with 50 ml of the hydrolyzed solution according to 1 b). The mixture has a pH of 3.5. To 50 g of the 10% strength by weight solution of the copolymer mixture thus obtained, 50 g of a 10% strength by weight aqueous solution of an acid-washed pig skin gelatin having a pH of 5.6 are added at 50.degree. 40 g of heat transfer oil ("Marlotherm ^(R)" ; Chem. Werke Hüls) are added and distributed at 50 ° C. with a high-speed stirrer to form droplets with a diameter of about 25 μm. After adding 125 g of demineralized water, the dispersion is cooled to +10 to + _{5 ° C.} with stirring. The oil droplets cover themselves with a closed, uniformly thick coacervate.

Examples 2-9

Polymer solutions according to Example 1 a) and 1 b) are mixed in the ratios given in the table. The mixtures are puffed according to the method given in Example 1 c). The encapsulation behavior is said to be "good" if the oil droplets are surrounded by a closed, regularly shaped and uniformly thick shell. A "medium" is a closed, irregularly shaped and unevenly thick shell. A "bad" encapsulation behavior occurs if the casing is not closed and coherent, the capsules stick very strongly or no casing is formed at all.

Examples 10-39

330 g of acrylamide and 170 g of maleic acid are dissolved in 3,500 ml of demineralized water at room temperature. After adding 4 g of finely powdered azoisobutyronitrile, 90 minutes. stirred vigorously at 20-25 ° C and then filtered off from the undissolved initiator. The solution obtained in this way is added dropwise in 2 hours to an initial charge of 1 l of oxygen-free boiling water. After stirring for one hour under reflux, the polymer solution is cooled. It has a pH of 3.0 and a viscosity number (intrinsic viscosity) [η] of 0.22 [dl / g] in 0.9% aqueous saline solution.

2 kg of polymer solution are heated to boiling again. As soon as the pH value reaches 3.8; 4.0; 4.2; 4.4; 4.6 and 4.9 increased, 250 ml of solution are removed and mixed with unheated solution.

Examples 40--49

300 ml of demineralized water are placed in a 2 l multi-necked flask with stirrer, reflux condenser, dropping funnel and nitrogen line. The apparatus is evacuated 3 times to -30 mbar and filled with nitrogen. The water is then heated to boiling with a slow passage of nitrogen.

150 g of monomers in the weight or molar ratios given in the table are dissolved in 1,050 g of demineralized water. When maleic acid is used, it is dissolved at room temperature. If maleic anhydride is used, it is expedient first to dissolve it under hydrolysis in water at 40 ° -50 ° C. and to add the acrylamide after cooling to room temperature. Then 1.0 g of finely powdered azoisobutyronitrile is added, the mixture is stirred at room temperature for 1 hour and undissolved fractions are filtered off. The monomer solution thus obtained is added dropwise uniformly to the boiling charge in 3 hours. After stirring at 100 ° C. for one hour, the polymer solution is cooled. 1,000 g of solution are stored at room temperature, and the remaining solution is heated to boiling until it has the pH given in the table. At the beginning, the pH is checked every 2 hours from pH≈4.0 per hour.

The mixed solutions are tested according to the method described in Example 1 c). The encapsulation behavior is assessed according to Example 2.

Examples 50-53

200 ml of boiling oxygen-free water are placed in a 1 liter multi-necked flask with stirrer, reflux condenser, dropping funnel and nitrogen line. 50 g of acrylamide and 20 g of maleic acid are dissolved in 430 ml of demineralized water. After adding the amount of azoisobutyronitrile shown in the table, the monomer / initiator mixture as a solution (L) or well-stirred suspension (S) is added dropwise uniformly at 100 ° C. over 2.5 hours. After stirring for one hour, _~ 400 ml of solution are removed and stored at room temperature. The remaining solution is hydrolyzed in 12-18 hours at 100 ° C to the specified pH. The blends are tested according to the procedure given in Examples 1 and 2.

Claims (3)

1. An aqueous copolymer solution, the polymeric content of which amounts to 5 to 30% by weight and consists of a mixture of

A) a copolymer with an intrinsic viscosity [η] of 0.05 to 1 [dl/g], 65 to 90 mole percent of whose statistically distributed structural units consist of residues of acrylamide and 10 to 35 mole percent of residues of maleic acid or maleic acid anhydride, and

B) a corresponding hydrolysed copolymer having an intrinsic viscosity [_1]] of 0.05 to 1.5 [dl/g] whose statistically distributed structural units accordingly consist of residues of acrylamide, arylic acid and maleic acid, at least some of the maleic acid residues being present in salt form,

characterised in that the ratio by weight of copolymer A to copolymer B is from 1:2 to 20:1.

2. A process for producing the aqueous copolymer solution according to claim 1, characterised in that

a) for producing copolymer (A) ther monomers are dissolved in an organic solvent or in water, polymerised with radical initiators at 50 to 120°C and the polymer is optionally isolated and dissolved in water and

b) for producing the copolymer (B) a part of copolymer (A) is hydrolysed by heating its aqueous solution to 60 to 150°C until a pH value of from 3.8 to 4.9 is obtained and then

c) both copolymer solutions are combined in a ratio of copolymer (A) to copolymer (B) of from 1:2 to 20:1.

3. The use of the aqueous copolymer solution according to claim 1 as shell-forming component in the production of microcapsules by complex coacervation.