An outer-coated composite hydrogel microsphere and a preparation method and application thereof

Abstract

The application discloses a kind of outer coating composite hydrogel microspheres and preparation method and application thereof, the outer coating composite hydrogel microspheres include hydrogel microsphere core and degradable shell, the hydrogel microsphere core is the mixture and / or reaction product of component including anionic biomaterial, cationic biomaterial, water, polyhydroxy compound I, polyvalent metal salt, the degradable shell includes degradable polymer, plasticizer, the component including polyhydroxy compound II.The outer coating composite hydrogel microspheres of the application have good synergies, and the effect of controlled release of fertilizer can be achieved by formula and process adjustment, the problem of easy burst release of existing anionic biomaterial fertilizer release coating material is solved, and the mechanical property is good, the problem that anionic biomaterial fertilizer release hydrogel microspheres poor mechanical property leads to easy breakage during storage, transportation and application and other processes and loses the problem of release effect.

Description

Technical Field

[0001] This invention belongs to the field of sustained release, and particularly relates to fertilizer sustained release. Specifically, it relates to an externally coated composite hydrogel microsphere, its preparation method, and its application. Background Technology

[0002] Fertilizers, as an important agricultural production material, play a vital role in increasing and maintaining crop yields. However, the use of conventional fertilizers has many environmental problems and drawbacks, such as waste and low efficiency.

[0003] To balance crop production and environmental protection, slow-release fertilizers are considered an effective way to overcome the shortcomings of conventional fertilizers and have therefore received great attention. This includes alginate-based slow-release fertilizers. For example, CN 107903111A discloses a method for encapsulating calcium alginate-based slow-release fertilizer, extending the release time of the internal fertilizer; CN 115093288A discloses a sodium alginate water-retaining slow-release fertilizer and its preparation method, extending the effective time of the fertilizer, improving soil aggregate structure and water retention, and increasing crop yield; CN115057741A discloses a method for preparing a non-swellable calcium alginate / chitosan microbead slow-release fertilizer, improving its resistance to swelling in humid and high-salt environments.

[0004] However, in existing technologies, on the one hand, the preparation of alginate-based slow-release fertilizers involves either dissolving the fertilizer in an aqueous solution for direct cross-linking or using the fertilizer as a core for heating and coating. This approach is wasteful and makes it difficult to quickly determine the encapsulation rate. Furthermore, the thermal processing conditions limit the types of fertilizers that can be coated. On the other hand, core-shell structured slow-release fertilizer preparation methods often use an oil-in-water phase followed by precipitation and solidification. The thickness of the resulting outer shell is difficult to control, and emulsifiers often play a crucial role in the bulk phase. The use of emulsifiers, cross-linking agents, and other organic reagents is detrimental to achieving cost reduction, green and harmless production, and environmental friendliness. These two aspects significantly limit the further control of fertilizer slow-release time for different crops and fertilization cycles. In addition, the lack of high mechanical strength in the alginate matrix makes it easily damaged, rendering it ineffective for slow release. This further restricts the application of alginate as a coating material for slow-release fertilizers. Summary of the Invention

[0005] To overcome the problems existing in the prior art, this invention provides an externally coated composite hydrogel microsphere, its preparation method, and its application. Taking sodium alginate as an anionic biomaterial and starch as a cationic biomaterial as an example, the high water absorption rate of the sodium alginate / starch composite hydrogel microspheres is utilized for post-adsorption of fertilizer solutions, achieving the goal of saving raw materials and flexibly adjusting fertilizer types and contents during processing. Simultaneously, this invention solves the technical problem of sodium alginate fertilizer slow-release hydrogel microspheres being easily damaged and losing their slow-release effect during storage, transportation, and application due to their poor mechanical properties. After complete fertilizer release, the outer shell and inner microspheres of the externally coated composite hydrogel microspheres described in this invention can naturally degrade to serve as a slow-release carbon source, or they can be composted for secondary utilization.

[0006] The first aspect of the present invention is to provide an externally coated composite hydrogel microsphere, comprising a composite hydrogel microsphere core and a biodegradable shell, wherein the hydrogel microsphere core is selected from a mixture of components including anionic biomaterials, cationic biomaterials, water, polyhydroxy compound I, and polyvalent metal salts and / or reaction products.

[0007] The anionic biomaterial is selected from at least one of sodium alginate, carboxymethyl cellulose, polyanionic cellulose, xanthan gum, and guar gum; and / or, the cationic biomaterial is selected from at least one of cationic modified starch and chitosan, preferably from at least one of cationic modified soybean starch, cationic modified corn starch, cationic modified sweet potato starch, and chitosan.

[0008] The polyhydroxy compound I is selected from at least one of glycerol, hydroxy silicone oil, ethylene glycol, glycerol, and sorbitol; and / or,

[0009] The polyvalent metal salt is selected from at least one of divalent, trivalent, and tetravalent metal salts, and preferably from at least one of calcium, aluminum, copper, and zinc salts, such as at least one of calcium chloride, aluminum chloride, zinc chloride, aluminum sulfate, and copper sulfate.

[0010] The water content in the core of the hydrogel microsphere is 0-10 wt%, preferably 0-2.5 wt%, and more preferably 0-10 wt%.

[0011] Optionally, the core of the hydrogel microspheres is further loaded with a target slow-release agent, preferably with fertilizer.

[0012] The biodegradable shell comprises components including a biodegradable polymer, a plasticizer, and a polyhydroxy compound II; preferably:

[0013] The biodegradable polymer is selected from at least one of polylactic acid, polycaprolactone, polyvinyl alcohol, and polyhydroxyalkanoate, and preferably the molecular weight of the biodegradable polymer is 10,000 to 300,000.

[0014] The plasticizer is selected from at least one of polyethylene glycol, glycerin, dibutyl phthalate, tributyl citrate, and epoxidized soybean oil.

[0015] The polyhydroxy compound II is selected from at least one of glycerol, hydroxy silicone oil, ethylene glycol, glycerol, and sorbitol.

[0016] In the biodegradable shell, the weight ratio of the biodegradable polymer to the plasticizer is (10-15):(3-8).

[0017] The weight ratio of the degradable polymer to the polyhydroxy compound II is (10-15):(1-3).

[0018] The second aspect of the present invention is to provide a method for preparing an externally coated composite hydrogel microsphere, preferably used to prepare the externally coated composite hydrogel microsphere described in the first aspect of the present invention. The preparation method includes: (1) preparing a hydrogel microsphere core using raw materials including the anionic biomaterial, the cationic biomaterial, water, polyhydroxy compound I, and the polyvalent metal salt; (2) mixing raw materials including the degradable polymer, the plasticizer, polyhydroxy compound II, and organic solvent to obtain a degradable shell forming liquid; and (3) spraying the degradable shell forming liquid onto the surface of the hydrogel microsphere core by atomization spraying to form the externally coated composite hydrogel microsphere.

[0019] In step (1), the weight ratio of the anionic biomaterial to the cationic biomaterial is (1-20):(1-5), preferably (1-10):(1-5), and / or, the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1-10):(1-10), preferably (1-5):(1-5), and / or, the weight ratio of the anionic biomaterial to the polyvalent metal salt is (1-10):(10-15).

[0020] Optionally, step (1') is performed after step (1) and before step (2): the core of the hydrogel microspheres obtained in step (1) is immersed in an aqueous solution of the sustained-release target substance; preferably, the concentration of the aqueous solution of the sustained-release target substance is 1 to 99 wt%.

[0021] In step (2), the weight ratio of the biodegradable polymer to the plasticizer is (10-15):(3-8).

[0022] The weight ratio of the degradable polymer to the polyhydroxy compound II is (10-15):(1-3).

[0023] The organic solvent in step (2) is selected from organic solvents with a boiling point of 20 to 80°C, preferably from at least one of dichloromethane, chloroform, acetone, and ethyl acetate; more preferably, the weight ratio of the biodegradable polymer to the organic solvent is (10 to 15):(100 to 150).

[0024] A third aspect of the present invention is to provide the application of the externally coated composite hydrogel microspheres described in one objective of the present invention or the externally coated composite hydrogel microspheres obtained by the preparation method described in another objective of the present invention in the sustained release of a target substance, particularly in the sustained release of fertilizers.

[0025] The composite hydrogel microspheres with external coating described in this invention exhibit excellent synergistic effects. Through formulation and process adjustment, they can achieve a controlled slow-release effect of fertilizer, fully synergizing with the crop growth cycle. This solves the problem of easy burst release of existing sodium alginate fertilizer slow-release coating materials. Furthermore, they have good mechanical properties, addressing the issue of easy breakage and loss of slow-release effect during storage, transportation, and application caused by the poor mechanical properties of sodium alginate fertilizer slow-release hydrogel microspheres. In addition, a secondary curing method is adopted in the process, utilizing the high water absorption of the composite hydrogel microspheres to post-adsorb the fertilizer solution, thereby saving raw materials and flexibly adjusting the type and content of fertilizers during processing.

[0026] The endpoints and any values ​​of the ranges disclosed in this invention are not limited to the precise ranges or values; these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. In the following, various technical solutions can, in principle, be combined with each other to obtain new technical solutions, which should also be considered as specifically disclosed herein.

[0027] Compared with the prior art, the present invention has the following beneficial effects:

[0028] (1) The outer-coated composite hydrogel microspheres of the present invention have good mechanical properties. When the sample is radially compressed between two rigid plates, its mechanical properties reach more than 100N (especially more than 120N).

[0029] (2) The outer-coated composite hydrogel microspheres of the present invention have good sustained-release controllability. When soaked in still water at 25°C for 24 hours, the initial nutrient release rate is less than 15% (preferably less than 5%), the cumulative nutrient release rate is less than 30% (preferably less than 15%) after 7 days, the cumulative nutrient release rate is less than 80% (preferably less than 25%) after 28 days, and the number of days with a cumulative nutrient release rate of 80% can be as long as 100 days or more (preferably more than 120 days and less than 180 days).

[0030] (3) The composite hydrogel microspheres with outer coating described in this invention have a good synergistic effect. Through formulation and process adjustment, the fertilizer can achieve a controlled slow release effect, which solves the problem of easy burst release of existing anionic biomaterial (e.g., sodium alginate) fertilizer slow release coating materials. In addition, the mechanical properties are good, which solves the problem that the slow release effect is easily damaged during storage, transportation and application due to the poor mechanical properties of anionic biomaterial (e.g., sodium alginate) fertilizer slow release hydrogel microspheres. Furthermore, the high water absorption of the composite hydrogel microspheres is used to post-adsorb the fertilizer solution in the process, so as to save raw materials and flexibly adjust the fertilizer type and content in the processing. Detailed Implementation

[0031] One objective of this invention is to provide an externally coated composite hydrogel microsphere comprising a hydrogel microsphere core and a biodegradable outer shell.

[0032] In a preferred embodiment, the core of the hydrogel microsphere is selected from a mixture and / or reaction products including anionic biomaterials, cationic biomaterials, water, polyhydroxy compound I, and polyvalent metal salts.

[0033] In a preferred embodiment, the anionic biomaterial is selected from at least one of sodium alginate, carboxymethyl cellulose, polyanionic cellulose, xanthan gum, and guar gum; and / or, the cationic biomaterial is selected from at least one of cationic modified starch (e.g., water-soluble starch) and chitosan, preferably from at least one of cationic modified soybean starch, cationic modified corn starch, cationic modified sweet potato starch, and chitosan.

[0034] In a preferred embodiment, the anionic biomaterial is selected from one or a combination of low-viscosity anionic biomaterials, medium-viscosity anionic biomaterials, and high-viscosity anionic biomaterials. The viscosity of the low-viscosity anionic biomaterial is 1–20 mPa·s, preferably 3–5 mPa·s; the viscosity of the medium-viscosity anionic biomaterial is 100–800 mPa·s, preferably 200–400 mPa·s; and the viscosity of the high-viscosity anionic biomaterial is 3000–15000 mPa·s, preferably 5000–10000 mPa·s.

[0035] The viscosity of the low-viscosity anionic biomaterial is 3, 3.5, 4, 4.5 or 5 mPa·s, the viscosity of the medium-viscosity anionic biomaterial is 200, 250, 300, 350 or 400 mPa·s, and the viscosity of the high-viscosity anionic biomaterial is 5000, 6000, 7000, 8000, 9000 or 10000 mPa·s.

[0036] In a further preferred embodiment, the anionic biomaterial is selected from one or a combination of two of medium-viscosity anionic biomaterials and / or high-viscosity anionic biomaterials with low-viscosity anionic biomaterials.

[0037] In a further preferred embodiment, the weight ratio of one or two of the medium-viscosity anionic biomaterials and / or high-viscosity anionic biomaterials to the low-viscosity anionic biomaterial is (1-20):1, preferably (1-10):1, for example 1:1, 2:1, 4:1, 6:1, 8:1, 10:1, 12:1, 14:1, 16:1, 18:1 or 20:1.

[0038] In a preferred embodiment, the polyhydroxy compound I is selected from at least one of glycerol, hydroxy silicone oil, ethylene glycol, glycerol, and sorbitol.

[0039] The polyhydroxy compound is beneficial for the plasticization or gelatinization of cationic biomaterials (e.g., cationic starch).

[0040] In a preferred embodiment, the polyvalent metal salt is selected from at least one of divalent, trivalent, and tetravalent metal salts, and preferably from at least one of calcium, aluminum, copper, and zinc salts, such as at least one of calcium chloride, aluminum chloride, zinc chloride, aluminum sulfate, and copper sulfate.

[0041] The initial curing occurs between cationic and anionic biomaterials, while the secondary curing involves further physical cross-linking between the cationic and anionic biomaterials under the action of polyvalent metal salts.

[0042] In a preferred embodiment, the core of the hydrogel microsphere is prepared using raw materials including the anionic biomaterial, the cationic biomaterial, water, polyhydroxy compound I, and the polyvalent metal salt.

[0043] In a further preferred embodiment, the weight ratio of the anionic biomaterial to the cationic biomaterial is (1-20):(1-5), preferably (1-10):(1-5), and / or the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1-10):(1-10), preferably (1-5):(1-5), and / or the weight ratio of the anionic biomaterial to the polyvalent metal salt is (1-10):(10-15).

[0044] For example, the weight ratio of the anionic biomaterial to the cationic biomaterial is (1, 2, 4, 6, 8 or 10):(1, 2, 3, 4 or 5), the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1, 2, 3, 4 or 5):(1, 2, 3, 4 or 5), and the weight ratio of the anionic biomaterial to the polyvalent metal salt is (1, 2, 4, 6, 8 or 10):(10, 11, 12, 13, 14 or 15).

[0045] In a preferred embodiment, the water content in the core of the hydrogel microsphere is 0-10 wt%, preferably 0-2.5 wt%, and more preferably free of water.

[0046] For example, the water content in the core of the hydrogel microspheres is 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 7 wt%, 8 wt%, 9 wt%, or 10 wt%.

[0047] In a preferred embodiment, the core of the hydrogel microspheres is optionally further loaded with a target slow-release agent, such as fertilizer.

[0048] In a further preferred embodiment, the fertilizer is a component capable of providing plant nutrients, including but not limited to at least one of nitrogen fertilizer, phosphorus fertilizer, potassium fertilizer, boron fertilizer, molybdenum fertilizer, manganese fertilizer, copper fertilizer, iron fertilizer, zinc fertilizer, and bio-fertilizer.

[0049] In a preferred embodiment, based on 100 wt% of the hydrogel microsphere core, the content of the loaded target sustained-release agent is 5 to 50 wt%, preferably 10 to 30 wt%, for example 5 wt%, 10 wt%, 15 wt%, 20 wt%, 25 wt%, or 30 wt%.

[0050] In a preferred embodiment, the biodegradable shell comprises components including a biodegradable polymer, a plasticizer, and a polyhydroxy compound II.

[0051] In a preferred embodiment, the biodegradable polymer is selected from at least one of polylactic acid, polycaprolactone, polyvinyl alcohol, and polyhydroxyalkanoate.

[0052] In a further preferred embodiment, the molecular weight of the degradable polymer is 10,000 to 300,000, for example, 10,000, 50,000, 100,000, 150,000, 200,000, 250,000 or 300,000.

[0053] Through extensive experimental research, the inventors discovered that when the molecular weight of the biodegradable polymer is below 10,000, the mechanical properties of the resulting coated composite hydrogel microspheres are poor. When the molecular weight of the biodegradable polymer exceeds 300,000, although the mechanical properties are better, the degradation cycle is too long, which is not conducive to fertilizer application in actual production processes.

[0054] In a preferred embodiment, the plasticizer is selected from at least one of polyethylene glycol, glycerin, dibutyl phthalate, tributyl citrate, and epoxidized soybean oil; and / or, the polyhydroxy compound II is selected from at least one of glycerin, hydroxy silicone oil, ethylene glycol, and sorbitol.

[0055] The polyethylene glycol is selected from at least one of polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, and polyethylene glycol 800.

[0056] In a preferred embodiment, in the biodegradable shell, the weight ratio of the biodegradable polymer to the plasticizer is (10-15):(3-8); and / or, the weight ratio of the biodegradable polymer to the polyhydroxy compound II is (10-15):(1-3).

[0057] For example, in the biodegradable shell, the weight ratio of the biodegradable polymer to the plasticizer is (10, 11, 12, 13, 14 or 15):(3, 4, 5, 6, 7 or 8); and / or, the weight ratio of the biodegradable polymer to the polyhydroxy compound II is (10, 11, 12, 13, 14 or 15):(1, 2 or 3).

[0058] Since the biodegradable shell is obtained by spraying, the content of each component in the biodegradable shell product is the same as the amount of raw materials used, except for organic solvents, because organic solvents will evaporate during the spraying process, after spraying, and during subsequent storage.

[0059] In a preferred embodiment, the thickness of the biodegradable shell is 10 to 20 μm, for example, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm or 20 μm.

[0060] After extensive experimental research, the inventors discovered that controlling the thickness of the biodegradable shell within the aforementioned range can effectively control the degree of slow release of the target slow-release agent, especially the initial slow release degree. Specifically, it can control the initial slow release degree to be lower and the number of days to reach 80% release to be longer.

[0061] The second objective of this invention is to provide a method for preparing an externally coated composite hydrogel microsphere, comprising: (1) preparing a hydrogel microsphere core by mixing raw materials including the anionic biomaterial, the cationic biomaterial, water, polyhydroxy compound I, and the polyvalent metal salt; (2) mixing raw materials including the biodegradable polymer, the plasticizer, polyhydroxy compound II, and organic solvent to obtain a biodegradable shell forming liquid; and (3) spraying the biodegradable shell forming liquid onto the surface of the hydrogel microsphere core by atomization spraying to form the externally coated composite hydrogel microsphere.

[0062] In a preferred embodiment, in step (1), the weight ratio of the anionic biomaterial to the cationic biomaterial is (1-10):(1-5), and / or the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1-5):(1-5), and / or the weight ratio of the anionic biomaterial to the polyvalent metal salt is (1-10):(10-15).

[0063] For example, in step (1), the weight ratio of the anionic biomaterial to the cationic biomaterial is (1, 2, 4, 6, 8 or 10):(1, 2, 3, 4 or 5), and / or, the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1, 2, 3, 4 or 5):(1, 2, 3, 4 or 5), and / or, the weight ratio of the anionic biomaterial to the polyvalent metal salt is (1, 2, 4, 6, 8 or 10):(10, 11, 12, 13, 14 or 15).

[0064] In a preferred embodiment, step (1) includes: (1.1) preparing an aqueous solution I of the anionic biomaterial, preparing an aqueous solution II of the cationic biomaterial and polyhydroxy compound I, and preparing an aqueous solution III of the polyvalent metal salt; (1.2) mixing the aqueous solution I and the aqueous solution II to obtain a mixed solution, stirring, allowing it to stand, and filtering to obtain preliminary microspheres; (1.3) (preferably under slow stirring) adding the preliminary microspheres to the aqueous solution III, collecting the microspheres after crosslinking and curing, and freeze-drying to obtain the core of the hydrogel microspheres.

[0065] In a further preferred embodiment, in step (1.1), the aqueous solution I of the anionic biomaterial is obtained as follows: the anionic biomaterial is mixed with water and stirred until it is completely dissolved; and / or, the aqueous solution II of the cationic biomaterial and polyhydroxy compound I is obtained as follows: the cationic biomaterial is mixed with water, the cationic biomaterial is gelatinized at 50-90°C (preferably 65-75°C), and the polyhydroxy compound is added and stirring is continued.

[0066] The aqueous solution I of the anionic biomaterial is obtained as follows: the anionic biomaterial is dissolved in water and mechanically stirred for 60–300 min at a stirring speed of 60–100 r / min until it is completely dissolved. The aqueous solution II of the cationic biomaterial and polyhydroxy compound I is obtained as follows: the cationic biomaterial is dissolved in water, and the cationic biomaterial is gelatinized in a boiling distilled water bath at 50–90°C (preferably 65–75°C). After adding polyhydroxy compound I, mechanical stirring is continued for 30–60 min at a stirring speed of 100–150 r / min.

[0067] Preferably, in preparing the aqueous solution I of the anionic biomaterial, the weight ratio of the anionic biomaterial to water is 1:(10-100), more preferably 1:(10-50). In preparing the aqueous solution II of the cationic biomaterial and polyhydroxy compound I, the weight ratio of the cationic biomaterial to water is 1:(50-200), more preferably 1:(50-100).

[0068] For example, in preparing the aqueous solution I of the anionic biomaterial, the weight ratio of the anionic biomaterial to water is 1:10, 1:20, 1:30, 1:40, 1:50, 1:60, 1:70, 1:80, 1:90, or 1:100. In preparing the aqueous solution II of the cationic biomaterial and polyhydroxy compound I, the weight ratio of the cationic biomaterial to water is 1:50, 1:60, 1:80, 1:100, 1:120, 1:140, 1:160, 1:180, or 1:200.

[0069] In a further preferred embodiment, the freeze-drying in step (1.3) is carried out by gradient cooling.

[0070] Optionally, the gradient freeze-drying specifically includes the following steps: First freezing: freezing at atmospheric pressure at a temperature of -40℃ to -30℃ for 1 to 2 hours; First freeze-drying: freezing and drying at atmospheric pressure at a temperature of -40℃ to -30℃ for 6 to 12 hours; Second freezing: freezing at atmospheric pressure at a temperature of -80℃ to -70℃ for 12 to 24 hours; Second freeze-drying: freezing and drying at atmospheric pressure at a temperature of -60℃ to -50℃ for 1 to 2 hours.

[0071] For example, the gradient freeze-drying specifically includes the following steps: First freezing: at atmospheric pressure, at temperatures of -40℃, -38℃, -36℃, -34℃, -32℃, or -30℃, freezing times of 1, 1.2, 1.4, 1.6, 1.8, or 2 hours; First freeze-drying: at atmospheric pressure, freezing and drying at temperatures of -40℃, -38℃, -36℃, -34℃, -32℃, or -30℃, for times of 6, 7, 8, 9, 10, ... 11 or 12 hours; Secondary freezing: at atmospheric pressure, at temperatures of -80°C, -78°C, -76°C, -74°C, -72°C, or -70°C, freezing time of 12, 14, 16, 18, 20, 22, or 24 hours; Secondary freeze-drying: at atmospheric pressure, at temperatures of -60°C, -58°C, -56°C, -54°C, -52°C, or -50°C, freezing and drying time of 1, 1.2, 1.4, 1.6, 1.8, or 2 hours. In a preferred embodiment, step (1') is optionally performed after step (1) and before step (2): the core of the hydrogel microspheres obtained in step (1) is immersed in an aqueous solution of the sustained-release target substance.

[0072] The microsphere core is immersed in an aqueous solution of the target substance to absorb and swell. After complete swelling and absorption, it is removed, the surface is rinsed with water, and then dried.

[0073] In a further preferred embodiment, the concentration of the sustained-release target aqueous solution is 1 to 99 wt%, for example, 1 wt%, 5 wt%, 10 wt%, 20 wt%, 30 wt%, 40 wt%, 50 wt%, 60 wt%, 70 wt%, 80 wt%, 90 wt%, or 99 wt%.

[0074] In a further preferred embodiment, the weight ratio of the sustained-release target aqueous solution to the core of the hydrogel microspheres in step (1) is (1-100):1, for example, 1:1, 5:1, 10:1, 20:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 90:1 or 100:1.

[0075] In a preferred embodiment, in step (2), the weight ratio of the degradable polymer to the plasticizer is (10-15):(3-8); and / or, the weight ratio of the degradable polymer to the polyhydroxy compound II is (10-15):(1-3).

[0076] In a preferred embodiment, the organic solvent in step (2) is selected from organic solvents with a boiling point of 20 to 80°C, preferably at least one of dichloromethane, chloroform, acetone, and ethyl acetate.

[0077] In a further preferred embodiment, the weight ratio of the degradable polymer to the organic solvent is (10-15):(100-150), for example (10, 11, 12, 13, 14 or 15):(100, 110, 120, 130, 140 or 150).

[0078] In a preferred embodiment, in step (3), the thickness of the sprayed biodegradable shell is controlled to be 10 to 20 μm, for example, 10 μm, 12 μm, 14 μm, 16 μm, 18 μm or 20 μm.

[0079] The preparation method described in this invention does not require a dry, high-temperature environment, nor does it require filtration, extrusion granulation, or other methods. This reduces fertilizer loss during the process, expands the range of available fertilizers, and makes it easier to determine the fertilizer encapsulation rate.

[0080] The preparation method described in this invention involves two curing processes. The first curing enhances the water absorption rate of the anionic and cationic biomaterials, while the second curing utilizes polyvalent metal salts for further physical cross-linking and reinforcement of the shape. The entire preparation process does not require high-temperature drying, filtration, extrusion granulation, or other methods, thus reducing fertilizer loss during the process and expanding the range of available fertilizers.

[0081] The preparation method may include:

[0082] Step A: Dissolve the anionic biomaterial in water and mechanically stir until it is completely dissolved.

[0083] Step B: Dissolve the cationic biomaterial in water, and change the method to make the solution homogeneous depending on the material (e.g., add polyhydroxy compound I).

[0084] Step C: Add the prepared cationic biomaterial solution from Step B to the anionic biomaterial solution from Step A, and stir. After stirring until homogeneous, pour into a beaker, let stand, filter, and obtain preliminary microspheres.

[0085] Step D: Prepare a multivalent metal solution using water. Under low-speed stirring, add the preliminary microspheres obtained in step C to the multivalent metal solution. After cross-linking and solidification, collect the microspheres and perform gradient cooling freeze-drying to obtain the microsphere core.

[0086] Step E: Dissolve the target sustained-release agent, such as urea, in water to prepare an aqueous solution of the target sustained-release agent. Immerse the microsphere cores obtained in Step D in the aqueous solution of the target sustained-release agent to allow them to absorb and swell. After complete swelling and absorption, remove the cores, rinse the surface with water, and dry them at room temperature.

[0087] Step F: Dissolve the biodegradable polymer, plasticizer, and polyhydroxy compound II in an organic solvent to prepare a biodegradable shell-forming solution.

[0088] Step G: The ultrasonic atomizing spraying equipment uniformly sprays the biodegradable shell-forming liquid from step F onto the surface of the microspheres prepared in step E to thicken the surface, thus obtaining the controlled-release fertilizer-coated composite hydrogel microspheres.

[0089] The third objective of this invention is to provide the application of the externally coated composite hydrogel microspheres described in the first objective of this invention or the externally coated composite hydrogel microspheres obtained by the preparation method described in the second objective of this invention in the sustained release of target substances, especially in the sustained release of fertilizers.

[0090] The present invention will now be described in detail with reference to specific embodiments. It should be noted that the following embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the content of the present invention are still within the scope of protection of the present invention.

[0091] It should also be noted that the various specific technical features described in the following embodiments can be combined in any suitable manner without contradiction. To avoid unnecessary repetition, the various possible combinations will not be described separately in this invention.

[0092] Furthermore, various embodiments of the present invention can be combined in any way, as long as they do not violate the spirit of the present invention. The resulting technical solutions are part of the original disclosure of this specification and also fall within the protection scope of the present invention.

[0093] Unless otherwise specified, the raw materials used in the examples and comparative examples are all disclosed in the prior art, such as those that can be directly purchased or prepared according to the preparation methods disclosed in the prior art.

[0094]

Example 1

[0095] Step A: Dissolve 7 parts by weight of sodium alginate of different viscosities (5 mPa·s: 5000 mPa·s: 7000 mPa·s = 4:2:1) in 100 parts of deionized water, and mechanically stir for 180 min at a stirring speed of 90 r / min until completely dissolved.

[0096] Step B: Dissolve 1 part by weight of cationic modified soybean starch in 100 parts of deionized water, gelatinize the starch in a boiling distilled water bath at 70°C, add 1 part by weight of glycerol and continue mechanical stirring for 30 minutes at a stirring speed of 100 r / min.

[0097] Step C: Add the prepared cationic modified soybean starch solution from Step B to the sodium alginate solution from Step A, and stir for 30 minutes at a stirring speed of 50 r / min. After stirring evenly, pour into a beaker, let stand for 15 minutes, filter, and obtain preliminary microspheres.

[0098] Step D: Add 15 parts by weight of calcium chloride to 100 parts by weight of deionized water to prepare a calcium chloride solution. Under low-speed stirring, add the preliminary microspheres obtained in Step C to the calcium chloride solution. After cross-linking and solidification, collect the microspheres and perform gradient cooling freeze-drying: at atmospheric pressure, at -40℃, freezing time for 2 hours; first freeze-drying: at atmospheric pressure, at -30℃, freezing and drying time for 6 hours; second freeze-drying: at atmospheric pressure, at -70℃, freezing time for 12 hours; second freeze-drying: at atmospheric pressure, at -50℃, freezing and drying time for 2 hours.

[0099] Step E: Dissolve urea in deionized water to prepare an aqueous solution with a concentration of 90 wt%. Soak the microspheres obtained in step D in the fertilizer solution to allow them to absorb and swell (the mass ratio of microspheres to fertilizer solution is 1:5). After complete swelling and absorption, remove the microspheres, rinse the surface with deionized water, and dry them at room temperature for 24 hours to obtain the microsphere core.

[0100] The microsphere core contains 2 wt% water and 18 wt% urea, based on a 100 wt% content.

[0101] Step F: Dissolve 15 parts by weight of polylactic acid (molar molecular weight 100,000), 5 parts by weight of polyethylene glycol 400, and 3 parts by weight of glycerol in 100 parts by weight of dichloromethane to prepare a biodegradable polymer solution.

[0102] Step G: The ultrasonic atomizing spraying equipment uniformly sprays the biodegradable polymer solution from step F onto the surface of the microsphere core prepared in step E, with a coating thickness of 10 μm, thus obtaining the controlled-release fertilizer-coated composite hydrogel microspheres.

[0103]

Example 2

[0104] Repeat steps A through F in Example 1;

[0105] Step G: The ultrasonic atomizing spraying equipment uniformly sprays the biodegradable polymer solution from step F onto the surface of the sodium alginate / starch composite hydrogel microspheres prepared in step E, with a coating thickness of 20 μm, thus obtaining the above-mentioned controlled-release fertilizer-coated composite hydrogel microspheres.

[0106]

Example 3

[0107] The process of Example 1 was repeated, except that 7 parts by weight of sodium alginate (viscosity 5 mPa·s) were used, and other conditions were the same as in Example 1.

[0108]

Example 4

[0109] The process of Example 1 was repeated, except that 7 parts by weight of sodium alginate (viscosity 5000 mPa·s) were used, and other conditions were the same as in Example 1.

[0110]

Example 5

[0111] The process of Example 1 was repeated, except that 7 parts by weight of sodium alginate (viscosity 7000 mPa·s) were used, and other conditions were the same as in Example 1. During the process, it was found that the system viscosity was too high, making dissolution and reaction difficult, and the preparation could not be completed smoothly according to the steps.

[0112]

Example 6

[0113] The process of Example 1 was repeated, except that 2 parts by weight of cationic modified soybean starch were dissolved in 100 parts of deionized water, and other conditions were the same as in Example 1.

[0114]

Example 7

[0115] The process of Example 1 was repeated, except that 5 parts by weight of cationic modified soybean starch were dissolved in 100 parts of deionized water, and other conditions were the same as in Example 1.

[0116]

Example 8

[0117] Repeat steps A through F in Example 1;

[0118] Step G: The ultrasonic atomizing spraying equipment uniformly sprays the biodegradable polymer solution from step F onto the surface of the sodium alginate / starch composite hydrogel microspheres prepared in step E, with a coating thickness of 5 μm, thus obtaining the above-mentioned controlled-release fertilizer-coated composite hydrogel microspheres.

[0119]

Example 9

[0120] Repeat steps A through F in Example 1;

[0121] Step G: The ultrasonic atomizing spraying equipment uniformly sprays the biodegradable polymer solution from step F onto the surface of the sodium alginate / starch composite hydrogel microspheres prepared in step E, with a coating thickness of 30 μm, thus obtaining the above-mentioned controlled-release fertilizer-coated composite hydrogel microspheres.

[0122] Comparative Example 1

[0123] The difference from Example 1 is that the molecular weight of polylactic acid is 10,000. All other conditions are the same.

[0124] Comparative Example 2

[0125] The difference from Example 1 is that the core does not contain starch or glycerol. During the implementation process, it was found that not all fertilizer solution could be absorbed. The encapsulation rate was determined based on the remaining amount and concentration of the fertilizer solution, and the actual encapsulation rate was used as the initial concentration for slow release. All other conditions remained the same.

[0126] Comparative Example 3

[0127] The difference from Example 1 is that it is not coated with a biodegradable polymer. All other conditions are the same.

[0128] Comparative Example 4

[0129] The difference from Example 1 is that the core does not contain starch or glycerol, and is not coated with a biodegradable polymer. During the implementation process, it was found that not all fertilizer solution could be absorbed; the encapsulation efficiency was calculated based on the remaining mass of the fertilizer solution. All other conditions remained the same.

[0130] Comparative Example 5

[0131] The difference from Example 1 is that a calcium chloride solution was prepared using 10 parts calcium chloride and 100 parts deionized water, and it was not coated with a biodegradable polymer. All other conditions were the same.

[0132]

Test Example 1

[0133] For the samples of the examples and comparative examples with a particle size between 3 and 5 mm, the mechanical properties of the samples were tested using a texture analyzer. Each sample was radially compressed between two rigid plates, and 20 samples of the same type were tested to obtain the average mechanical strength. The results are shown in Table 1 below.

[0134] Table 1

[0135]

[0136] [Test Example 2] Fertilizer Slow-Release Performance Test

[0137] The slow-release fertilizer products obtained from the examples and comparative examples were tested according to the national standard GB / T 23348 2009: the fertilizer nutrient release period was tested by water immersion under static water conditions at 25℃. If the cumulative nutrient release was still less than 80% after 28 days, the number of days when the cumulative nutrient release reached 80% was listed. For easy comparison, the urea granules used were tested together as samples. The results are shown in Table 2 below.

[0138] If the fertilizer solution is not completely absorbed, the encapsulation rate is determined based on the remaining amount of fertilizer solution, and the actual encapsulation rate is used as the initial slow-release concentration. Specifically, the urea content in the remaining fertilizer solution is determined using the Kjeldahl method to further determine the actual encapsulation rate. The urea content in the remaining fertilizer solution after absorption is expressed as a mass fraction (w) in percentages (%), calculated using the following formula:

[0139]

[0140] Where: v0 is the volume of hydrochloric acid standard solution consumed during blank treatment; v1 is the volume of sodium hydroxide standard titration solution consumed by the sample during determination; c is the concentration of the sodium hydroxide standard titration solution used; m is the sample volume; and x is the conversion factor between nitrogen content and urea.

[0141] The absorbed fertilizer solution, i.e., the actual encapsulation rate, is expressed as AE (in percentiles) and is calculated using the following formula:

[0142]

[0143] Where: M2 is the mass of the remaining fertilizer solution after absorption, w is the mass fraction of urea in the remaining fertilizer after absorption, M1 is the mass of the fertilizer solution before absorption, and W is the mass fraction of urea in the fertilizer solution before absorption.

[0144] Table 2:

[0145]

[0146]

[0147] Table 1 shows that the mechanical properties of the uncoated samples were poor, while the mechanical properties of the coated samples were mainly affected by the thickness and molecular weight of the polymer shell. Table 2 shows that the urea granules released nutrients quickly, and the coating significantly reduced this rate. Several samples achieved an initial nutrient release rate of less than 15% (preferably less than 5%) after 24 hours of soaking in still water at 25°C, and a cumulative nutrient release rate of less than 70% (preferably less than 25%) after 28 days. Examples 1, 2, 6, and 7 showed better sustained-release effects and mechanical properties, while Comparative Example 4, which used only a sodium alginate core, exhibited poor sustained-release effects and mechanical properties. Furthermore, the experiment revealed that lower molecular weight polymer shells resulted in faster nutrient release, especially in the later stages, compared to higher molecular weight shells. Shell thickness significantly affected the nutrient release rate; excessively thin or thick shells were not conducive to aligning with the growth cycles of common crops. Comparative Example 2 shows that the absence of starch significantly affected the fertilizer solution absorption efficiency and nutrient release efficiency during the preparation process. In summary, by changing the raw material ratio and the thickness of the polylactic acid spray coating, a controlled and slow-release effect can be achieved.

[0148] The present invention has been described in detail above with reference to specific embodiments and exemplary examples; however, these descriptions should not be construed as limiting the present invention. Those skilled in the art will understand that various equivalent substitutions, modifications, or improvements can be made to the technical solutions and embodiments of the present invention without departing from the spirit and scope of the invention, and all such modifications and improvements fall within the scope of the present invention. The scope of protection of the present invention is defined by the appended claims.

Claims

1. An externally coated composite hydrogel microsphere comprising a hydrogel microsphere core and a biodegradable shell, wherein the hydrogel microsphere core comprises a mixture and / or reaction products of components including anionic biomaterials, cationic biomaterials, water, polyhydroxy compound I, and polyvalent metal salts; and the biodegradable shell comprises components including a biodegradable polymer, a plasticizer, and polyhydroxy compound II.

2. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The anionic biomaterial is selected from at least one of sodium alginate, carboxymethyl cellulose, polyanionic cellulose, xanthan gum, and guar gum; and / or the cationic biomaterial is selected from at least one of cationic modified starch and chitosan.

3. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The cationic biomaterial is selected from at least one of cationic modified soybean starch, cationic modified corn starch, cationic modified sweet potato starch, and chitosan.

4. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The polyhydroxy compound I is selected from at least one of glycerol, hydroxy silicone oil, ethylene glycol, and sorbitol; and / or, The polyvalent metal salt is selected from at least one of divalent, trivalent, and tetravalent metal salts.

5. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The multivalent metal salt is selected from at least one of calcium salts, aluminum salts, copper salts, and zinc salts.

6. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The water content in the core of the hydrogel microspheres is 0~10wt%; and / or, The target sustained-release agent is further loaded into the core of the hydrogel microspheres.

7. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The water content in the core of the hydrogel microspheres is 0~2.5wt%; and / or, Fertilizer is further loaded into the core of the hydrogel microspheres.

8. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The biodegradable polymer is selected from at least one of polylactic acid, polycaprolactone, polyvinyl alcohol, and polyhydroxyalkanoates; and / or, The plasticizer is selected from at least one of polyethylene glycol, glycerin, dibutyl phthalate, tributyl citrate, and epoxidized soybean oil; and / or, The polyhydroxy compound II is selected from at least one of glycerol, hydroxy silicone oil, ethylene glycol, and sorbitol.

9. The externally coated composite hydrogel microspheres according to claim 1, characterized in that, The biodegradable polymer has a molecular weight of 10,000 to 300,000.

10. The externally coated composite hydrogel microspheres according to claim 8, characterized in that, In the biodegradable shell, the weight ratio of the biodegradable polymer to the plasticizer is (10~15):(3~8); and / or, The weight ratio of the degradable polymer to the polyhydroxy compound II is (10~15):(1~3).

11. A method for preparing externally coated composite hydrogel microspheres, used to prepare the externally coated composite hydrogel microspheres according to any one of claims 1 to 10, the preparation method comprising: (1) Prepare the core of hydrogel microspheres by mixing raw materials including the anionic biomaterial, the cationic biomaterial, water, polyhydroxy compound I, and the polyvalent metal salt; (2) Mix raw materials including the degradable polymer, the plasticizer, polyhydroxy compound II, and organic solvent to obtain a degradable shell forming liquid; (3) Spray the degradable shell forming liquid onto the surface of the core of the hydrogel microspheres by atomization spraying to form the outer-coated composite hydrogel microspheres.

12. The preparation method according to claim 11, characterized in that, In step (1), the weight ratio of the anionic biomaterial to the cationic biomaterial is (1~20):(1~5), and / or the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1~10):(1~10), and / or the weight ratio of the anionic biomaterial to the polyvalent metal salt is (1~10):(10~15).

13. The preparation method according to claim 11, characterized in that, In step (1), the weight ratio of the anionic biomaterial to the cationic biomaterial is (1~10):(1~5), and / or the weight ratio of the cationic biomaterial to the polyhydroxy compound I is (1~5):(1~5).

14. The preparation method according to claim 11, characterized in that, Step (1') is performed after step (1) and before step (2): the core of the hydrogel microspheres obtained in step (1) is immersed in the aqueous solution of the sustained-release target substance.

15. The preparation method according to claim 14, characterized in that, The concentration of the sustained-release target aqueous solution is 1~99wt%.

16. The preparation method according to any one of claims 11 to 15, characterized in that, In step (2), the weight ratio of the biodegradable polymer to the plasticizer is (10~15):(3~8); and / or, The weight ratio of the degradable polymer to the polyhydroxy compound II is (10~15):(1~3); and / or, The organic solvent in step (2) is selected from organic solvents with a boiling point of 20~80℃.

17. The preparation method according to claim 16, characterized in that, The organic solvent in step (2) is selected from at least one of dichloromethane, chloroform, acetone, and ethyl acetate.

18. The preparation method according to claim 16, characterized in that, The weight ratio of the biodegradable polymer to the organic solvent is (10~15):(100~150).

19. The application of the externally coated composite hydrogel microspheres according to any one of claims 1 to 10 or the externally coated composite hydrogel microspheres obtained by the preparation method according to any one of claims 11 to 18 in the sustained release of the target analyte.

20. The application according to claim 19, characterized in that, Application in slow-release fertilizers.

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