Method for preparing gelatin-based biodegradable microcapsules

Abstract

A method for encapsulating agrochemicals in a biodegradable capsule comprising complex coacervation of gelatin and a carboxylated polysaccharide.

Description

[Technical field]

[0001] The present invention relates to a method for preparing biodegradable microcapsules and to uses of the prepared microcapsules. [Background technology]

[0002] Microencapsulation is known in many technical fields, in the agrochemical field it can be beneficial, for example, to control the release rate of the active ingredient, to ensure the chemical stability of the active ingredient, and / or to protect workers from exposure to the active ingredient.

[0003] The process commonly employed for the preparation of microcapsules in the agrochemical field is the use of oil-soluble monomers selected from diisocyanates and polyisocyanates, which are reacted with water or water-soluble diamines and polyamines at the oil-water interface of an oil-water emulsion. This achieves the formation of a polyurea capsule wall. Such encapsulation techniques in the formulation of agrochemical active ingredients are well known to those skilled in the art (see, for example, PJ Mulqueen "Chemistry and Technology of Agrochemical Formulations", DA Knowles, editor, Kluwer Academic Publishers, 1998, pages 132-147).

[0004] Sustainability of pesticide formulations and development of products with low environmental impact have become important goals in the pesticide field. Therefore, biodegradability of microplastics has become an important topic, but polyurea-based microcapsules used in many pesticide formulations are not biodegradable. Therefore, it is necessary to provide a novel process for preparing biodegradable encapsulated pesticides. Summary of the Invention [Means for solving the problem]

[0005] Thus, a method for encapsulating pesticides in biodegradable capsules is provided which involves complex coacervation of gelatin and carboxylated polysaccharides.

[0006] This method results in capsules that exhibit biodegradable behavior while providing reduced grower exposure to any active ingredients as well as improved chemical / physical stability of the active ingredients. [Brief description of the drawings]

[0007] [Figure 1] Figure 1: (a) Cross-linked gelatin-NaCMC capsules (composition E) in dilute solution and (b) dried for 4 days. (c) Non-cross-linked capsules in dilute solution and (d) immediately after drying. Scale bar = 20 μm. [Diagram 2] FIG. 2 shows laser diffraction data recorded for both crosslinked (composition E, solid blue line) and non-crosslinked capsules (dashed red line). [Diagram 3-4] 3 and 4 are cryo-SEM images of non-crosslinked (left) and crosslinked (composition E, right) gelatin / NaCMC capsules containing a λ-cyhalothrin / Solvesso 200 ND mixture as the encapsulated core. [Diagram 5] Figure 5 shows (a) laser diffraction data recorded for gelatin / sodium alginate coacervate capsules prepared with a 2:1 ratio of gelatin:sodium alginate and 30 wt% oil phase, (b) optical micrograph of the same capsules in the diluted state, and (c) optical micrograph of the same capsules after drying for 2 hours. [Figure 6] Figure 6 shows (a) capsule size distribution obtained for a sample of gelatin / gum arabic coacervate capsules. The gelatin:gum arabic ratio was fixed at 1, the total polymer concentration was fixed at 2% and the oil phase contained 50% λ-cyhalothrin and 50% Solvesso 200 ND by weight. (b) Optical micrograph of capsules diluted to 0.1% by weight in water. (b) Optical micrograph of capsules shown in (b) after drying for 16 hours. [Figure 7] 7 shows optical micrographs obtained for gelatin / gum arabic capsules (composition I) before storage at elevated temperature: (a) non-crosslinked capsules in the wet state, (b) crosslinked capsules in the wet state, (c) non-crosslinked capsules in the dry state, and (d) crosslinked capsules in the dry state. Scale bars correspond to 100 μm in all cases. [Figure 8] FIG. 8 shows the release of λ-cyhalothrin from cross-linked gelatin / NaCMC by capsule size over a 24 hour period. [Figure 9] FIG. 9 shows the release of S-metolachlor from cross-linked gelatin / gum arabic and cross-linked gelatin / alginate capsules over a 90 minute period. [Figure 10] FIG. 10 is laser diffraction data recorded for gelatin / sodium CMC coacervate capsules (composition L) prepared in embodiment 2 using a 2:1 ratio of gelatin:sodium CMC and 40% by weight oil phase. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The term "biodegradable" is defined to mean a compound that passes the OECD Guidelines for the Testing of Chemicals, test no. 301 (OECD 301 test). In particular, a compound that is "biodegradable" is defined as a compound that has a mineralization of at least 30%, preferably greater than 40%, more preferably greater than 50%, and most preferably greater than 60%, measured as CO2 evolved or O2 consumed over a 28 day period, where mineralization is measured according to test methods OECD TG 301 B, C, D, F or OECD TG 310.

[0009] "Carboxylated polysaccharides" includes both naturally occurring polysaccharides that contain carboxylic acid groups and polysaccharides that have been chemically modified to contain carboxylic acid groups.

[0010] As used herein, the noun "pesticide" and the terms "pesticide active ingredient" are used interchangeably and include herbicides, insecticides, nematicides, molluscicides, fungicides, plant growth regulators and safeners; preferably, herbicides, insecticides and fungicides.

[0011] "Complex coacervation" itself is defined as the formation of a complex between two differently charged polyelectrolytes.

[0012] First embodiment In a first embodiment, the method advantageously comprises three successive steps. 1) forming an emulsion of an aqueous phase containing gelatin and an oil phase containing a pesticide; 2) adding a carboxylated polysaccharide; and 3) adding a cross-linking agent.

[0013] Step (1) The formation of the emulsion in step (1) may be carried out by high shear homogenization. Step (1) may be carried out at a temperature of 30 to 55°C.

[0014] Step (1) may be carried out at a pH of from 4.5 to 7.5, such as from 5 to 7, or even from 5.6 to 6.3.

[0015] Optionally, an antifoaming agent and / or an emulsifying agent can be added at this stage. The antifoaming agent may be present in an amount of 0.05 to 0.2% by weight. The emulsifying agent may be present in an amount of 0.01 to 0.2% by weight.

[0016] The gelatin may be type A or type B, preferably type B. The gelatin may be present in an amount of 1-6% by weight of the aqueous phase. It has been found that a higher concentration of gelatin in step (1) results in smaller emulsion droplet size and therefore smaller final coacervate capsules. Thus, gelatin is preferably present in the aqueous phase in an amount of 2-6%, such as 3-6%, 4-6% or even 4.5-5.8% by weight.

[0017] The oil phase preferably comprises a suitable hydrophobic solvent. By "suitable hydrophobic solvent" we mean one that has negligible solubility in water, i.e. less than 5 g / L, such as less than 4 g / L, less than 3 g / L, preferably less than 1 g / L. Examples include, but are not limited to, alkyl benzoates, seed oils, alkylated seed oils and aromatic fluids.

[0018] The concentration of the pesticide in the oil phase is preferably 1-100% by weight, such as 5-99%, 10-75%, 20-70%, 30-65%, 40-60%, preferably 45-55% by weight, etc. Advantageously, the concentration is greater than 45% by weight.

[0019] Preferably the pesticide is present in an amount of 0.01-65% by weight of the final formulation, such as 1-59%, 2-58%, 5-55%, 10-20%, 40-60% or 45-55% by weight.

[0020] Step (2) Step (2) preferably involves the addition of the carboxylated polysaccharide as an aqueous solution. Step (2) may be carried out at a temperature of from 30 to 55°C.

[0021] The carboxylated polysaccharide is preferably selected from one or more of gum arabic, sodium alginate and carboxymethylcellulose; and derivatives thereof.

[0022] Preferably, only one carboxylated polysaccharide is used, which reduces the material required to achieve the same or smaller capsule size and simplifies the process compared to processes using two or more carboxylated polysaccharides.

[0023] The ratio of gelatin to carboxylated polysaccharide is preferably 4:1 to 1:4, such as 3:1 to 1:3, most preferably 2:1 to 1:2, such as 1:1. It has been found that working with these ratios reduces aggregation.

[0024] Step (2) may also advantageously include a high shear homogenization step after addition of the carboxylated polysaccharide. Surprisingly, it has been found that an additional high shear homogenization step at this stage aids in reducing the droplet size.

[0025] Step (2) may be carried out under acidic conditions. Advantageously, the pH of the emulsion is between 3 and 6.5, and is reduced to between 3 and 5, or even to between 3.2 and 4.2, etc., after addition of the carboxylated polysaccharide. The change in pH is achieved with an acid, such as acetic acid, citric acid or hydrochloric acid, and helps induce complex coacervation.

[0026] Preferably, the temperature of the emulsion is below 15° C. and is gradually reduced, such as to below 12° C., such as between 5 and 11° C., to harden the capsules.

[0027] The carboxylated polysaccharide is preferably present in an amount of 0.25 to 3% by weight of the final formulation.

[0028] Step (3) The addition of a cross-linking agent increases the robustness and stability of the resulting capsule and therefore its tolerance to changes in pH, temperature, ionic strength (combinations of these) and the addition of co-formulants.

[0029] Crosslinking can occur by covalent bonds and / or "physical" crosslinks via secondary interactions such as hydrogen bonding.

[0030] The cross-linking agent is preferably selected from polyaldehydes (such as glutaraldehyde), polyacids (such as citric acid), carbodiimides (such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), polyphenolic compounds (such as tannic acid) and aldose sugars.

[0031] The crosslinker is present in an amount of 0.0001-2% by weight of the final formulation.

[0032] Dispersants may be added during step (3). Possible dispersants include lignosulfonates (e.g., Vanisperse CB, Ultrazine NA or Reax 80D), polymeric dispersants (e.g., Morwet D425), and / or surfactants.

[0033] After step (3), the composition may be allowed to return to ambient temperature or may be actively warmed to a temperature of 40-50° C. to promote crosslinking.

[0034] Second embodiment In a second embodiment: 1) forming an emulsion of an aqueous phase containing gelatin and a carboxylated polysaccharide and an oil phase containing a pesticide; and 2) Adding a crosslinking agent A method is provided that includes:

[0035] The second embodiment requires a reduced number of steps compared to the first embodiment, and is therefore more time efficient, and it has been found that the second embodiment typically results in smaller capsules.

[0036] Step (1) The formation of the emulsion in step (1) may be carried out by high shear homogenization. Step (1) may be carried out at a temperature of 30 to 55°C.

[0037] Step (1) may be carried out at a pH of from 4 to 7.5, such as from 5 to 7, or even from 5.6 to 6.3.

[0038] Optionally, an antifoaming agent and / or an emulsifying agent can be added at this stage. The antifoaming agent may be present in an amount of 0.05 to 0.2% by weight. The emulsifying agent may be present in an amount of 0.01 to 0.2% by weight.

[0039] The gelatin may be type A or type B, preferably type B. The gelatin may be present in an amount of 1-6% by weight of the aqueous phase. It has been found that a higher concentration of gelatin in step (1) results in smaller emulsion droplet size and therefore smaller final coacervate capsules. Thus, gelatin is preferably present in the aqueous phase in an amount of 2-6%, such as 3-6%, 4-6% or even 4.5-5.8% by weight.

[0040] The carboxylated polysaccharide is preferably selected from one or more of gum arabic, sodium alginate and carboxymethylcellulose; and derivatives thereof.

[0041] Preferably, only one carboxylated polysaccharide is used, which simplifies the process and reduces the amount of material needed to achieve the same or smaller capsule size.

[0042] The ratio of gelatine to carboxylated polysaccharide is preferably 4:1 to 1:4, such as 3:1 to 1:3, most preferably 2:1 to 1:2, such as 1:1. Advantageously, working with these ratios has been found to reduce aggregation.

[0043] The oil phase preferably comprises a suitable hydrophobic solvent. By "suitable hydrophobic solvent" we mean one that has negligible solubility in water, i.e. less than 5 g / L, such as less than 4 g / L, less than 3 g / L, preferably less than 1 g / L. Examples include, but are not limited to, alkyl benzoates, seed oils, alkylated seed oils and aromatic fluids.

[0044] The concentration of the pesticide in the oil phase is preferably 1-100% by weight, such as 5-99%, 10-75%, 20-70%, 30-65%, 40-60%, preferably 45-55% by weight, etc. Advantageously, the concentration is greater than 45% by weight.

[0045] Preferably the pesticide is present in an amount of 0.01-65% by weight of the final formulation, such as 1-59%, 2-58%, 5-55%, 10-20%, 40-60% or 45-55% by weight.

[0046] Step (2) The addition of a cross-linking agent increases the robustness and stability of the resulting capsule and therefore its tolerance to changes in pH, temperature, ionic strength (combinations of these) and the addition of co-formulants.

[0047] Crosslinking can occur by covalent bonds and / or "physical" crosslinks via secondary interactions such as hydrogen bonding.

[0048] Step (2) may be carried out under acidic conditions. Advantageously, the pH of the emulsion is between 3 and 6.5, and is reduced to between 3 and 5, or even to between 3.2 and 4.2, etc., prior to the addition of the crosslinker. The change in pH is achieved with an acid, such as acetic acid, citric acid or hydrochloric acid, and helps induce complex coacervation.

[0049] The cross-linking agent is preferably selected from polyaldehydes (such as glutaraldehyde), polyacids (such as citric acid), carbodiimides (such as 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide), polyphenolic compounds (such as tannic acid) and aldose sugars.

[0050] The crosslinker is present in an amount of 0.0001-2% by weight of the final formulation.

[0051] Dispersants may be added during step (2). Possible dispersants include lignosulfonates (e.g., Vanisperse CB, Ultrazine NA or Reax 80D), polymeric dispersants (e.g., Morwet D425), and / or surfactants.

[0052] After step (2), the composition may be allowed to return to ambient temperature or may be actively warmed to a temperature of 40-50° C. to promote crosslinking.

[0053] Microcapsules The chemical nature of the pesticide to be encapsulated is important when attempting encapsulation, especially with respect to achieving delayed release. Advantageously, the pesticide is hydrophobic. Without wishing to be bound by theory, it is believed that the hydrophilic, hydrated coacervate capsule wall thus provides a barrier to the hydrophobic pesticide, causing very slow diffusion. Advantageously, the pesticide has a solubility of 0.001-200 mg / L, such as 0.002-100 mg / L, 0.002-50 mg / L, preferably 0.002-20 mg / L, or even 0.002-1 mg / L. The pesticide may be λ-cyhalothrin, prosulfocarb and / or tefluthrin.

[0054] Preferably, the capsules prepared exhibit sustained release, which includes non-instantaneous release over a period of time, and thus encompasses sustained release, delayed release, and triggered release (e.g., by rupture of the capsule upon drying).

[0055] Advantageously, the process does not include additional emulsifiers, as the addition of additional emulsifiers (e.g., sodium dodecyl sulfate or poly(vinyl alcohol)) has been found to increase capsule cohesion, and the use of gelatin as the sole emulsifier avoids the need for the presence of additional emulsifiers.

[0056] Advantageously, the prepared capsules have a diameter (D) of less than 15 μm, such as less than 14 μm, less than 10 μm, less than 9 μm, less than 8 μm, or even less than 7 μm. 50 Preferably, the capsules have a diameter of 1 to 6 μm (microns), such as 2 to 5 μm (microns).

[0057] Thus, there is provided a composition comprising the microcapsules prepared by the methods described herein. There is also provided the use of such a composition in the treatment of weeds, pests, nematodes, mollusks and / or fungi.

[0058] The prepared composition may then be diluted, in which case the pesticide may be present in an amount of 0.01 to 45% by weight of the final formulation, such as 0.1 to 30%, 0.5 to 20%, 0.6 to 15%, or 1 to 10% by weight.

[0059] Use of the biodegradable microcapsules prepared by the methods described herein, as well as the use of the biodegradable microcapsules for the sustained release of lambda-cyhalothrin and / or tefluthrin, are also provided.

[0060] The invention is illustrated by the following non-limiting examples. EXAMPLES

[0061] composition A series of capsules were prepared according to the first embodiment of the present invention, the composition of which is shown in Table 1.

[0062] [Table 1]

[0063] A capsule according to the second embodiment of the present invention was prepared with the composition according to Table 2.

[0064] [Table 2]

[0065] analysis Composition E - Gelatin / sodium carboxymethylcellulose coacervate microcapsules Coacervate microcapsules prepared with gelatin and sodium carboxymethylcellulose are shown in Figure 1. These capsules were prepared with a 2:1 ratio of gelatin:sodium carboxymethylcellulose, a total polymer concentration of 2.25% by weight, and crosslinked with 0.25 g of glutaraldehyde.

[0066] Optical microscopy revealed that the capsules before and after cross-linking (Figures 1a and 1c, respectively) exhibited a spherical morphology, and both the non-cross-linked and cross-linked capsules maintained this morphology upon drying (Figures 1b and 1d, respectively).

[0067] Laser diffraction showed that the capsules were well dispersed without agglomeration, with a volume-mean diameter (D[4,3]) of 2.6 μm, Dv50=2.3 μm, and Dv95=5.3 μm. The non-crosslinked sample was characterized as having D[4,3]=2.6 μm, Dv50=2.0 μm, and Dv95=4.6 μm (Figure 2).

[0068] The structures of the non-crosslinked and crosslinked capsules were further characterized by cryo-SEM, where a thin but continuous coacervate complex wall was observed around each capsule (Figures 3 and 4).

[0069] The release properties of cross-linked gelatin / NaCMC capsules were characterized using a method based on the Collaborative International Pesticides Analytical Council (CIPAC) method 'MT 190-Determination of release properties of lambda-cyhalothrin cs formulations'. In this method, an aliquot of the formulation containing 75 mg of lambda-cyhalothrin was diluted to 6.0 g with water. An internal standard solution (a standard hexane solution with the ethanol removed) was added to the solution and placed on a roller where a 1 mL aliquot was withdrawn from the internal standard solution for sampling. One drop of trifluoroacetic acid was added to the vial, which was then capped for GC analysis.

[0070] The capsules were shown to slowly release lambda-cyhalothrin over a 24 hour period. However, it was also shown that changes in capsule size affect the level of sustained release, even for formulations of identical composition. As shown in Figure 8, smaller capsules released more lambda-cyhalothrin than larger capsules over a 24 hour period.

[0071] Composition D-Gelatin / Sodium Alginate Coacervate Microcapsules Representative examples of coacervate microcapsules prepared with gelatin and sodium alginate are shown in Figure 5. These capsules were prepared using a 2:1 ratio of gelatin:sodium alginate, a total polymer concentration of 1.5 wt%, and cross-linked with 0.3 g of glutaraldehyde. Laser diffraction showed that the resulting capsules had a D[4,3] of 6.6 μm (Figure 5a). Optical microscopy showed a well-defined spherical morphology for the dilute dispersion (Figure 5b). Moreover, these capsules retained their structure after drying for 2 h (Figure 5c).

[0072] Composition I - Gelatin / gum arabic coacervate microcapsules Representative examples of coacervate microcapsules prepared with gelatin and gum arabic are shown in Figures 6 and 7. These capsules were prepared with a 1:1 ratio of gelatin:gum arabic, a total polymer concentration of 1% by weight, and cross-linked with 0.2 g of glutaraldehyde. Laser diffraction showed that the resulting capsules had a D[4,3] of 34 μm (Figure 6).

[0073] Optical microscopy showed a distinct spherical morphology for the dilute dispersion (Figure 7b). Moreover, these capsules retained their structure after drying for 16 hours (Figure 7d). It can also be seen that the non-crosslinked capsules (Figures 7a and 7c) do not exhibit the same structural stability during the same process.

[0074] Compositions J and K - gelatin / gum arabic and gelatin / alginate capsules and S-MOC S-MOC was encapsulated with both gum arabic and alginate by the process previously described to form compositions J and K, respectively, without the additional high shear homogenization step in step 2. The capsules were shown to rapidly release S-metolachlor over a 90 hour period (Figure 9), in contrast to the hydrophobic pesticides listed above. The process was as previously described for composition E.

[0075] biodegradable Example B was tested for biodegradability in the OECD 301F test.

[0076] To perform such testing, the hydrophobic core material was first extracted from the capsules such that the remaining core material constituted no more than 10% by weight of the capsule, more preferably less than 5% of the capsule, and the resulting isolated wall material was then resuspended in water prior to OECD 301 testing.

[0077] Such capsules were found to achieve 68% mineralization within 28 days (data averaged from duplicate analyses).

[0078] The claimed process therefore results in the preparation of stable yet biodegradable microcapsules for pesticides.

[0079] The invention is defined by the claims.

Claims

1. 1. A method for encapsulating a pesticide in a biodegradable capsule, comprising complex coacervation of gelatin and a carboxylated polysaccharide: (1) forming an emulsion of an aqueous phase containing gelatin and an oil phase containing the pesticide; (2) adding the carboxylated polysaccharide; and (3) Adding a crosslinking agent wherein the process does not include an additional emulsifier.

2. 10. The method of claim 1, wherein step (2) comprises the addition of the carboxylated polysaccharide as an aqueous solution and / or is carried out under acidic conditions.

3. 3. The method of claim 1 or 2, wherein step (2) comprises a high shear homogenization step after said adding said carboxylated polysaccharide.

4. 3. The method of claim 1 or 2, wherein step (3) comprises adding a dispersant.

5. A method for encapsulating a pesticide in a biodegradable capsule, comprising: (1) forming an emulsion of an aqueous phase containing gelatin and a carboxylated polysaccharide and an oil phase containing a pesticide; and (2) Adding a crosslinking agent wherein the ratio of gelatin to carboxylated polysaccharide is 4:1 to 1:

4.

6. The method of claim 1 or 5, wherein the cross-linking agent is selected from polyaldehydes, polyacids, polyphenols, and aldose sugars.

7. 6. The method of claim 1 or 5, wherein the gelatin is present in an amount of 1 to 6% by weight of the final formulation.

8. 6. The method of claim 1 or 5, wherein the carboxylated polysaccharide is present in an amount of 0.25 to 3% by weight of the final formulation.

9. 6. The method of claim 1 or 5, wherein the cross-linking agent is present in an amount of 0.0001 to 2% by weight of the final formulation.

10. 6. The method of claim 1 or 5, wherein the pesticide is present in an amount of 0.01 to 60% by weight of the final formulation; and / or the pesticide has a solubility of 0.001 to 200 mg / L, preferably the pesticide is λ-cyhalothrin and / or tefluthrin.

11. 6. The method according to claim 1 or 5, wherein the carboxylated polysaccharide is selected from one or more of gum arabic, sodium alginate and carboxymethylcellulose, preferably only one carboxylated polysaccharide is used.

12. 6. The method of claim 1 or 5, wherein the capsules have a diameter of less than 15 microns, preferably less than 10 microns, more preferably less than 5 microns; and / or exhibit sustained release properties.

13. A composition comprising microcapsules prepared by the method of claim 1 or 5.

14. 14. Use of a composition according to claim 13 in the treatment of weeds, pests, nematodes, mollusks and / or fungi.

15. Use of biodegradable microcapsules prepared by the method of claim 1 or 5 for delayed release of said λ-cyhalothrin and / or tefluthrin.

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