EP0631523A1 - Coacervation processes - Google Patents

Abstract

The invention relates to processes for forming particles which have a core of core material within a shell of polymeric material that has been deposited by coacervation. According to the invention there is provided a dispersion of particles of core material in an aqueous solution of a pH-sensitive polymeric material at a pH at which the polymeric material is soluble. The polymeric material is deposited as a shell around the core material by insolubilisation of the polymeric material caused by adjusting the pH of the solution to a pH at which the polymeric material is insoluble. The adjustment of the pH is conducted at a rate that is sufficiently slow and homogeneous throughout the solution so that the polymeric material deposits substantially only as a substantially uniform coacervate around the individual core particles.

Description

Coacervation Processes

This invention relates to processes for forming particles which have a core of core material within a shell of polymeric material that has been deposited by coacervation.

Coacervation processes are well known as a class and involve providing a dispersion of particles (liquid or solid) of core material in an aqueous medium containing coacervating polymer and causing the polymer to phase- separate and deposit around the particles. It is common to promote coacervation by adjusting the conditions in the aqueous medium so as to cause the polymer to deposit. For instance the concentration of a non-solvent may be increased to bring about deposition or two polymers in the medium may be caused to interact to form a polymeric complex as a coacervate.

Some polymeric materials are pH-sensitive, i.e., they are soluble'at one pH and insoluble at another pH. It is known to use pH-sensitive polymeric material as a coating or matrix around a pharmaceutically active ingredient to protect that active ingredient from ambient conditions at which the polymeric material is insoluble, and then to release the active ingredient by exposing the polymeric material to pH conditions at which it is soluble. Various techniques are known for depositing the pH-sensitive polymeric material around the active ingredient. Most of them involve forming relatively large particles but coacervation can be used to form small particles if the active ingredient can be supplied initially as a dispersion of small particles.

It is known to be desirable to encapsulate certain materials, such as the toxin derived from Bacillus thuringiensis (Bt) , within a pH-sensitive polymeric material. Processes for doing this are described in U.S. 4,948,586. These processes involve the use of organic solvents and achievement of encapsulation depends upon the choice of appropriate solvents, non-solvents, surfactants and temperature conditions.

Coacervation processes of this general type suffer from a number of disadvantages. The use of organic solvents in large quantities is undesirable because it necessitates the provision of suitable solvent handling apparatus and because the solvents themselves can exhibit some phytotoxicity. The changes in concentration or other conditions necessary to cause coacervation are liable to occur rather suddenly (for example at the point of addition of a non-solvent) and it is difficult to obtain uniform coacervation under these conditions.

It might be thought that it would be possible to achieve satisfactory coacervation of a pH-sensitive polymer by dispersing particles of core material in an aqueous solution of the polymeric material at a pH at which the polymeric material is soluble, and causing insolubilisation of the polymeric material and deposition of the insolubilised polymeric material as a shell around the core material by adding acid or alkali and thereby adjusting the pH of the solution to a pH at which the polymeric material is insoluble. Unfortunately, little or no deposition of insolubilised material occurs as a shell around individual core particles, as would occur in proper coacervation. Instead, a significant amount, and generally most or all, of the polymeric material deposits as agglomerates. A substantial proportion of the particles of core material remain substantially uncoated and/or large, but variable, numbers of the particles of core material are also trapped within each agglomerate.

It would be desirable to be able to encapsulate particles of core material within a shell of pH-sensitive polymeric material by a process that is simpler to operate and that will achieve more uniform coating of the individual particles of core material. A coacervation process for forming particles having core of core material within a shell of pH-sensitiv polymeric material comprises providing a dispersion of particles of core materia in an aqueous solution of the polymeric material at a pH a which the polymeric material is soluble, and depositing th polymeric material as a shell around the core material, characterised in that the polymeric material i deposited as a shell and is insolubilised by adjusting th pH of the solution to a pH at which the polymeric materia is insoluble, wherein the adjustment of the pH is conducted at rate that is sufficiently slow and homogeneous throughou the solution that the polymeric material deposit substantially only as a substantially uniform coacervat around individual particles.

The encapsulating polymer can change from a totall water soluble state to a totally water insoluble state ove a relatively small pH range. The polymer is usually one that has a very shar change from insolubility to solubility, for instance goin from a wholly soluble form to an insoluble form over a p change of less than 1 pH unit, often less than 0.5 pH unit and frequently is in the range 0.05 to 0.2 or 0.3 pH units For instance if the viscosity of the composition i measured, it may be found that this is at a substantiall constant high value down to a pH of, for instance, 6.8 an that it then drops very fast to a value at around 6.3 an that it then remains substantially constant with furthe decreases in pH. Alternatively, turbidity can be measure in which event it may be low at pH values down to about 6. (indicating full solution) and may then increase rapidly t a substantially constant high value at pH values below 6. (indicating an emulsion) . The precise pH band over whic this rapid change in solubility occurs will vary accordin to the particular composition of the polymer but is usuall located within the range 5.5-8.5, most usually 6 to 7.5. The rate of change of pH must be controlled i accordance with the invention so as to be slow an homogeneous as it passes through this critical range. Th rate of change approaching this range, and passing beyon it, is less important. In practice it is generall necessary for the desired slow rate to be controlled ove a range of at least 1 pH unit, and often at least 2 p units, because it is difficult to ensure control over th critical range unless this is done. Generally the chang in 2 pH units from above the insolubilisation point t below it should take at least 5 minutes, often at least 1 minutes, but it is usually unnecessary for it to tak longer than 60 minutes.

It is necessary that the change in pH through th critical range should occur slowly and homogeneousl throughout the solution. If it occurs slowly (fo instance as indicated by a pH meter in the bulk of th solution) but non-homogeneously then this is unsatisfactor since the slow but non-homogeneous change will necessaril mean that it is occurring fast in some parts of th solution and slow in others. The parts where it i occurring fast will give unsatisfactory results sinc significant amounts of the polymer will be deposited a particles that are free of core material and/or a agglomerates of variable size and content. Accordingly i is necessary that the pH change should not only be slow bu that it should be homogeneous in the sense that it is slo in all parts of the solution, and not just in some parts o the solution. For instance conventional pH adjustment by, fo example, addition of acid or alkali would change the p slowly in the bulk of the solution but would creat localised rapid pH changes, causing uncontrollabl precipitation of the polymer with little or n encapsulation of the core material, at the point o addition of the acid or alkali. If the pH adjustment is sufficiently slow and homogeneous throughout the solution, the encapsulating polymer will deposit predominantly around the individual particles of core material, resulting in efficient encapsulation. If the rate in any part of the solution or throughout the solution is too fast there will be an increasing tendency for polymer to be deposited in forms other than as a coacervate coating (with the result that core material remains uncoated) and/or as agglomerates of variable numbers of core particles.

One way of determining whether the process has been conducted satisfactorily is to subject the end product to centrifugation since this can be used to determine whether the great majority of particles are of the desired similar particle size and shape or whether the particles vary considerably in size, for instance between random agglomerates of polymer and small particles of core material.

Another good indication of the product can be obtained by scanning electron microscopy. By this technique it is easily possibly to differentiate between coated materials of approximately uniform size and random agglomerates and other particulate materials.

Preferably the majority of the polymeric material, for instance at least 75%, preferably at least 90% and most preferably at least 95% by weight is present as particles of substantially uniform size and that enclose core material, and the majority of core particles, generally at least 75%, preferably at least 90% and most preferably at least 95% by weight, are enclosed within a shell of polymeric material.

In some instances the process can be conducted in such a manner that coacervation occurs first and the coacervated particles may then aggregate to some extent and this can be satisfactory since the polymer is deposited as a substantially uniform coacervate around individual particles. This type of product is in contrast with a product in which the polymer deposits too fast and in an irregular manner leaving significant numbers of particles uncoated.

The preferred products of the invention have at least 90% by weight of the core material microencapsulated within the polymeric material and at least 90% by weight of the polymeric material present as coacervate coating around the core material.

In order to achieve the desired slow homogeneous change in pH it is preferred to dissolve a latent pH- adjusting material in the aqueous solution and cause this to react in the solution to liberate acid or base throughout the solution and thereby cause the adjustment in pH. Since the pH-adjusting material is dissolved in the solution, the pH change is homogeneous, and the desired slow rate can be achieved merely by controlling the rate of reaction, for instance by adjusting the temperature or the dilution of the solution.

When the polymer is soluble in acid and insoluble in alkali the pH-adjusting material should be one that will liberate alkali. Preferably the polymeric material is a material that is soluble in alkali and insoluble in acid in which event the latent pH-adjusting material should be one that will react in the solution to liberate acid in the solution.

The polymer itself may serve as the latent pH- adjusting material if it is included initially as a salt with ammonia or a volatile amine, since heating of the solution will drive off the ammonia and convert the polymer from its soluble, salt, form to an insoluble, acidic, form.

Alternatively, the solution may include an added heat decomposable salt of a volatile base (ammonia or a volatile amine) with an acid, so that when the solution is heated the salt decomposes, the base volatilises and the acid that is formed reduces the pH. It is necessary to ensure that the salt does not cause a significant change of pH before heat decomposition occurs, for instance as a result o being sufficiently acidic before decomposition.

The use of a salt of a weak acid with a volatile bas (for instance a salt of acetic acid or other organic wea acid with ammonia or a volatile amine) can therefore b convenient. However salts with stronger acids can b used, especially if the reaction temperature i sufficiently low that they decompose slowly.

It can also be useful to include a buffer in th solution so as to prevent unacceptable pH adjustment whe the salt is first added to the solution.

The need to heat the solution to decompose the sal can be undesirable, especially when the core materia comprises Bt toxin or other heat-sensitive material and s preferably the latent pH-adjusting material is one tha undergoes hydrolysis in the aqueous polymer solution t cause the desired liberation of acid (or alkali) . Th preferred materials are lactones that hydrolyse in th polymer solution to form a carboxylic acid. The preferre lactone is gluconolactone. Again, it can be useful t incorporate a buffer.

Provided sufficient care is taken, includin especially the application of sufficient agitation, othe ways of achieving the pH change may be possible. Fo instance a dispersion of pH-adjusting material may b distributed through the solution. For instance the bead may be of ion exchange material (generally a weak base o weak acid ion exchange material) or they may be bead comprising a polymeric matrix enclosing acid or alkali tha can permeate slowly through the matrix. For instance bead of polyacrylic acid scale inhibitor or the other weak aci in a polymeric matrix can be used to liberate acid. It i essential to apply vigorous stirring so as to prevent th concentration of acid or alkali at the surface of the bead increasing sufficiently to cause local precipitation.

Similarly, dialysis could be used to generate hydrogen o hydroxyl ions within the solution on a molecular scale, fo instance using one or more dialysis sheets, tubes or particles with acid or base on one side of the membrane and the polymer solution on the other. Again, however, very vigorous agitation will be needed to prevent the pH changing too fast locally, thus leading to unwanted polymer deposition.

Similarly, some other material may be added to the solution from outside, but again very vigorous agitation and^'careful addition will be required to prevent the pH changing too rapidly locally. Normal techniques of adding, for instance, acid dropwise to a solution that is being stirred at a conventional rate of stirring will not be satisfactory.

The pH-sensitive polymeric material is normally a copolymer of a blend of ionic and non-ionic ethylenically unsaturated monomers where both the blend and the polymer are insoluble at one pH and soluble at a different pH. When the ionic monomer is an amino or amido monomer, for instance a dialkylaminoalkyl (meth) -acrylate or acrylamide as acid addition or quaternary ammonium salt, the polymer can be insolublised by raising the pH. More usually, however, the enteric polymer is an anionic polymer, generally a copolymer of ethylenically unsaturated carboxylic acid monomer with water insoluble ethylenically unsaturated non-ionic monomer. Suitable carboxylic monomers include acrylic acid, ethacrylic acid, maleic acid or anhydride, crotonic acid and itaconic acid.

Suitable non-ionic monomer (for incorporation in anionic or cationic polymers) include aromatic ethylenically unsaturated monomers such as styrenes, vinyl halides, acrylonitrile, and alkyl esters of carboxylic monomers, for instance alkyl (meth) acrylates.

Preferred copolymers are formed from (meth) acrylic acid with styrene and/or alkyl (meth) acrylate. The amount of ionic monomer is generally in the range 5 to 40%, usually 15 to 30%, by weight, with the balance being insoluble non-ionic monomer. Copolymers of acrylic acid or other water soluble ionic monomer with a hydrocarbo monomer such as styrene are preferred. By appropriat selection of the comonomer blend and the polymerisatio conditions, it is possible to produce polymer having th desired pH sensitivity.

The polymer can be formed by polymerisation at a pH a which it is soluble or by polymerisation in an organi solvent or in an aqueous organic solvent mixture, fo instance at a pH at which it would be insoluble in th absence of the organic solvent. Preferably, however, i is formed by oil-in-water emulsion polymerisation at a p at which it is insoluble.

Instead of using copolymers of ethylenicall unsaturated monomers, it is also possible to use natura polymers or synthetic homopolymers, for instance sodiu alginate or polymethacrylic acid, provided they have th required solubility properties.

Although the polymer solution is usually free o organic solvent, in some instances it may include solven (for instance from the initial polymerisation) or solven may be included in a small amount (generally less than 50 and usually less than 10% by weight of the solution) i order to modify the polymerisation or coacervation process.

The molecular weight of the polymer must not be to low since otherwise it may fail to coacervate out o solution adquately and/or any coating that is formed ma have inadequate properties. Generally therefore th molecular weight is at least 50,000, preferably at leas 100,000 and usually at least 200,000. If the molecula weight is too high it may be difficult to cause the polyme to come out of solution sufficiently slowly and s generally the molecular weight is below 5 million, an preferably below 2 million and most preferably below million. The core material can be a variety of water insolubl liquid or solid materials. If liquid, the material shoul be broken down into a dispersion of droplets of th appropriate size by conventional mechanical homogenisation and/or the use of appropriate oil-in-water emulsifying agent. Preferably the core material (irrespective of whether it is liquid or solid) has a size of below 30μm and most preferably below lOμm and best results are achieved when the size is below 5μm, for instance 0.1 to 3μm. However in some instances (for instance when the core material is an ink) it can be desirable for the particle size to be much larger, e.g., 30 or 50 to lOOμm or up to 300μm or larger.

A particular advantage of the invention is that it is possible to control the coacervation so accurately that, even when the particle size is very small, substantially every particle in the final particulate composition is a monoparticle of the core material with a substantially complete coating of the pH-sensitive polymer. Whereas conventional.coacervation techniques tend to be unsuitable or difficult to operate if the particle size is to be, for instance, less than lOμm, in the invention it is easily possible to make coated monoparticles having a size of 0.1 to 3μm, typically up to around 1 or 2μm.

Another advantage of the invention is that this very accurate control of coacervation can be achieved without the need to include large amounts of organic solvents. A further advantage of the invention is that it is possible to synthesise the polymer from a monomer blend that is selected according to the physical properties that are required of the final polymer coating, provided always sufficient pH-sensitive monomer is included to render the polymer pH sensitive. Thus it is possible to optimise the polymer for its ultimate performance qualities. - This is in contrast to conventional coacervation techniques where the polymer normally had to be selected primarily from the point of view of its ability to form a coacervate when, for instance, the solvent concentration or temperature changed. The resultant polymers often gave rather poor wall properties, and so were often further reacted in an attemp at improving wall strength and other wall properties.

The core material may be a material that relies upo the pH-sensitive nature of the polymer coating to protec the material during storage or use, or it may be a materia where the pH-sensitive nature of the polymer is merel required for the formation of the coating. Thus the cor material can be, for instance, an ink, a fragrance, sythetic pesticide or a biologically produced material suc as enzyme (for instance a protease, especially of the typ used as a detergent enzyme) , a mycelium or insecticida toxin such as insecticidal virus, bacteria or fungi, suc as are described in U.S. 4,948,586.

The invention is of particular value for th encapsulation of crystals of the toxin of Bt, Bacillu thuringiensis. This material is preferably present in th form of crystals and/or spores and conveniently i introduced into the aqueous solution of polymer merely b combining a fermentation broth of the Bt toxin or othe microbiological product with the polymer solution.

Alternatively the broth may be combined with an emulsion o the polymer and the pH then adjusted to put the polyme into solution.

By the invention it is possible to provide a nove product which is a composition of a coating polymer and particulate biologically produced material wherein at leas 50% (of the weight of coating polymer and biologicall produced material) is present in the form of particle which have a size below 5μm (preferably below 3μm) an which have a core of the biologically produced material an a substantially continuous coacervate shell of the polymer, wherein the polymer is a pH-sensitive polymer.

Preferably at least 70%, and most preferably at leas 80 or 90%, by weight of the coating polymer an biologically produced material is present in the form o particles having a size of below 3μm. Most preferably th particles have a size of below 2μm, often around lμ . Th particle size is generally above O.lμm, frequently above 0.5μm. The particulate core material is preferably solid, e.g., Bt toxin crystals and/or spores. Preferably there is generally only one solid particle in each core. Thus by the invention it is possible to formulate Bt crystals into a form that is particularly suitable for efficient application and use.

The coated particles of the invention may be collected and dried as a powder, but usually they are produced and maintained as a suspension in the aqueous medium in which they are formed.

In the following examples, polymer A is a polymer prepared by oil-in-water emulsion polymerisation of 30% by weight acrylic acid with 70% by weight styrene, while polymer B is prepared in the same way from 15 weight percent methacrylic acid, 30 weight percent ethyl acrylate and 55 weight percent methyl methacrylate. EXAMPLE 1 Microencapsulation of Bt Bioinsecticide with Polymer A A fresh sample of Bacillus thuringiensis fermentation broth (100 parts) containing approximately equal amounts of Bt crystals and spores at pH 7 was vigorously stirred whilst a solution (25%) of polymer A in water at pH 8.8 was added. Stirring was continued whilst a mixture of 1,5- gluconolactone (5 parts) in water (10 parts) was added. The pH of this dispersion was monitored with time under constant mechanical agitation. After 30 minutes the pH had dropped from 8.2 to 6.4. At this time the smooth brown mixture became thixotropic and remained like that for a further 30 minutes when the pH had reached 6.0.

On examination by scanning electron microscopy (SEM) the starting broth was seen to contain roughly equal numbers of diamond shaped Bt crystals and rounded spores in the 1-2 micron size range. The reaction product by contrast contained very few diamond shapes and almost all particles had a rounded shape but still in the 1-2 micron region. Thus the alkali-soluble polymer A had been deposited as a film around the crystals when it was phase separated under acid conditions.

EXAMPLE 2

Preparation of Trifluralin Microcaosules An oil phase was prepared consisting of the insecticide Trifluralin 70 parts) , a solvent (Shellsol A; 25 parts) and an 0/W e ulsifier (Tensiofix B7416; 5 parts) . This oil phase (50 parts) was mixed vigorously with water (50 parts) to give a smooth orange O/W emulsion. This emulsion was next added to a clear solution of polymer A in water (6%) at pH 9. Glacial acetic acid was added dropwise to reduce the pH to 7. It was noted that during this stage local precipitation of Polymer A occurred and prolonged stirring at pH 7 was necessary to re-dissolve these polymer clumps.

When a smooth O/W emulsion was finally obtained (60 minutes) a freshly made up solution of 1,5-gluconolactone (5 parts) in water (95 parts) was added. The pH of this mixture dropped slowly but steadily from 7 to 6 over 120 minutes.

The final product consisted of finely dispersed yellowish orange particles in water. On standing for several days these particles tended to aggregate at the top of the solution with about 25% of the volume consisting of a clear colourless lower aqueous phase. With slight agitation the aggregates easily redispersed.

Evidence for microencapsulation is the fact that after standing for 1 month very few small orange Trifluralin crystals had been deposited on the sides of the glass container. Also under the visible microscope some capsules in the region 20-30 micron diameter could be seen with thin surrounding membranes. When pressure was applied to the cover slip the membrane could be seen to rupture and the orange internal phase leaked through the fracture into the surrounding colourless aqueous phase. EXAMPLE 3

Preparation of Capsules Containing Leuco-dve for Carbonless

Paper Manufacture

An aqueous phase (100 parts) at pH 7.5 containing 10% w/w Polymer B was prepared. A solution (2% w/w) of crystal violet lactone in a hydrocarbon solvent (15 parts) was added to the aqueous solution of polymer B to form an O/W emulsion. With continued mechanical agitation, a slurry of ammonium acetate (5 parts) in water (10 parts) was next added. The pH of this mixture at 20°C was 7.3. On warming and stirring the pH dropped steadily to pH 7 (50°C) and pH 6.2 (70°C) .

On cooling the mixture remained smooth and well dispersed. The essence of capsules was demonstrated by applying the mixture on to a paper sheet and drying. When this sheet was brought into contact with a second sheet of paper coated with acid clay and pressure applied a blue colour developed where the pressure had caused the capsules to rupture and bring the lactone dye into contact with acid clay.

Claims

CLAIM8

1. A coacervation process for forming particles having a core of core material within a shell of pH-sensitive polymeric material, the process comprising providing a dispersion of particles of core material in an aqueous solution of the polymeric material at a pH at which the polymeric material is soluble, and depositing the polymeric material as a shell around the core material, characterised in that the polymeric material is deposited as a shell and is insolubilised by adjusting the pH of the solution to a pH at which the polymeric material is insoluble, wherein the adjustment of the pH is conducted at a rate that is sufficiently slow and homogeneous throughout the solution that the polymeric material deposits substantially only as a substantially uniform coacervate around individual particles.

2. A process according to claim 1 wherein the polymeric material changes from a wholly soluble form to an insoluble form over^ *-H change of less than 1 pH unit.

3. A process according to claim 2 wherein the pH change is in the range 0.05 to 0.3 pH units.

4. A process according to any of claims 1 to 3 wherein at least 75% by weight of the polymeric material is deposited as a shell, and at. least 75% by weight of core particles are enclosed within a shell of polymeric material.

5. A process according to claim 4 wherein at least 90% by weight of the polymeric material is deposited as a shell around the core material, and at least 90% by weight of. core particles are enclosed within a shell of the polymeric material.

6. A process according to any preceding claim wherein the pH of the aqueous solution is adjusted by dissolving a latent pH-adjusting material in the aqueous solution and causing this material to react in the solution to liberate acid or base throughout the solution, thereby causing an adjustment in pH.

7. A process according to claim 6 wherein the latent pH- adjusting material comprises a salt of the polymeric material with ammonia or a volatile amine, whereby heating of the solution will drive off the ammonia and convert the polymeric material from a soluble form to an insoluble form.

8. A process according to claim 6 wherein the pH- adjusting material comprises a heat decomposable salt of a volatile base with an acid, whereby when the solution is heated the salt decomposes, the base volatilises and the acid that is formed reduces the pH.

9. A process according to claim 8 wherein the pH- adjusting material comprises a salt of a weak acid with a volatile base.

10. A process according to any of claims 6 to 9 further comprising a buffer in the aqueous solution.

11. A process according to any of claims 6 to 10 wherein the pH-adjusting material comprises a lactone that hydrolyses in the aqueous solution of polymeric material to form a carboxylic acid.

12. A process according to claim 11 wherein the lactone is gluconolactone.

13. A process according to any preceding claim wherein the pH-sensitive polymeric material is a copolymer of a blend of ionic and non-ionic ethylenically unsaturated monomers, wherein both the blend and the copolymer are insoluble at one pH and soluble at a different pH.

14. A process according to claim 13 wherein the amount of ionic monomer is. in the range 5 to 40%, the balance being non-ionic monomer.

15. A process according to claim 13 or claim 14 wherein the copolymer is formed from (meth) acrylic acid with styrene an /or alkyl (meth) acrylate.

16. A process according to any preceding claim wherein the molecular weight of the polymeric material is in the range

50,000 to 1,000,000.

17. A process according to any preceding claim wherein the core material has a size below 5μm.

18. A composition of a coating polymer and a particulate biologically produced material wherein at least 50% (of the weight of coating polymer and biologically produced material) is present in the form of particles which have a size below 5μm and which have a core of the biologically produced material and a substantially continuous coacervate shell of the polymer, wherein the polymer is a pH-sensitive polymer.