Controlled-release fertilizer and preparation method thereof

Abstract

The invention relates to the technical field of controlled release fertilizers, and discloses a controlled release fertilizer and a preparation method thereof. The controlled-release fertilizer comprises fertilizer particles; the fertilizer particles are sequentially coated with a first stearic acid layer, a starch layer, a second stearic acid layer and a polycaprolactone layer; the total mass of the first stearic acid layer and the second stearic acid layer is 7.5-13.5% of the mass of the fertilizer particles; and the mass ratio of the first stearic acid layer to the second stearic acid layer is 1: (0.9-1.2). According to the controlled-release fertilizer disclosed by the invention, the sustained release time of nutrients can be prolonged, the release of elements which are easy to dissolve out is slowed down, and the release of elements which are relatively difficult to dissolve out is accelerated.

Description

Technical Field

[0001] The invention relates to the technical field of controlled-release fertilizers, and in particular to a controlled-release fertilizer and a preparation method thereof. Background Art

[0002] Commercial fertilizers are important for meeting global food demand. A major problem with traditional fertilizers is that nutrients leach and lose too quickly and cannot be fully absorbed and utilized by plants. In addition, fertilizers used above the threshold level can pollute water bodies. Using controlled-release fertilizers (CRF) instead of traditional fertilizers can alleviate the above-mentioned leaching problems and environmental-related issues to a certain extent. Controlled-release fertilizers control the release of nutrients by coating the outside of fertilizer particles. Compared with traditional fertilizers, controlled-release fertilizers provide nutrients in a gradual and sustainable manner, which can prolong the continuous release time of nutrients in fertilizers, improve nutrient utilization, reduce fertilization rates, and reduce leaching problems, so they have good application prospects.

[0003] In fertilizers, elements that are easily soluble (such as nitrogen in some fertilizers) are released faster, while elements that are difficult to dissolve (such as phosphorus in some fertilizers) are released slower, which easily leads to excessive release of some elements, which cannot be fully utilized by plants, while insufficient release of some elements cannot meet the growth needs of plants. Although existing controlled-release fertilizers can extend the sustained release time of nutrients, it is difficult to achieve differentiated regulation of the release rate of elements with different solubility. Summary of the invention

[0004] In order to solve the above technical problem, that is, it is difficult for existing controlled-release fertilizers to achieve differentiated regulation of the release rates of different soluble elements, the present invention provides a controlled-release fertilizer and a preparation method thereof. The controlled-release fertilizer of the present invention can prolong the continuous release time of nutrients, slow down the release of easily soluble elements, and accelerate the release of difficultly soluble elements.

[0005] The specific technical scheme of the present invention is: In a first aspect, the present invention provides a controlled-release fertilizer, comprising fertilizer particles; the fertilizer particles are sequentially coated with a first stearic acid layer, a starch layer, a second stearic acid layer and a polycaprolactone layer; the total mass of the first stearic acid layer and the second stearic acid layer is 7.5-13.5% of the mass of the fertilizer particles; the mass ratio between the first stearic acid layer and the second stearic acid layer is 1:0.9-1.2.

[0006] In the controlled-release fertilizer of the present invention, the starch layer can absorb water by osmosis and can be decomposed into simple sugars in the soil, promoting microbial activity, thereby triggering the release of nutrients in the controlled-release fertilizer. The present invention can achieve the following effects by arranging a three-layer coating consisting of a first stearic acid layer / starch layer / second stearic acid layer outside the fertilizer particles: (1) The starch layer has a porous structure and swells microscopically after absorbing water, which further expands the pore structure within the layer. The stearic acid layer is more hydrophobic. By utilizing this difference between the two, the water permeation path can be regulated, making the water permeation path in the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer nonlinear, thereby prolonging the continuous release time of nutrients.

[0007] (2) Due to the difference in solubility and degradability between stearic acid and starch, microscopic pores are formed at the interface between the two. The "microchannels" formed by these pores can guide the directional diffusion of water and nutrients, thereby making the release of nutrients in the controlled-release fertilizer more uniform and avoiding the phenomenon of excessive accumulation or excessive release of local nutrients. This can extend the shelf life of the controlled-release fertilizer to a greater extent.

[0008] (3) In the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer, when any one of the two stearic acid layers is missing, even if the mass proportion of the other stearic acid layer is increased, it will affect the above-mentioned effects of (1) and (2), resulting in an accelerated nutrient release rate and a shortened effective period of the controlled-release fertilizer.

[0009] On this basis, the present invention can achieve differentiated regulation of the release rates of different solubilized elements by coating a polycaprolactone layer outside the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer. Specifically: the lower crystallinity and higher flexibility of polycaprolactone enable it to fill the pores on the surface of the second stearic acid layer, reduce the roughness of the surface of the controlled-release fertilizer and the number of large pores, thereby slowing down the rate at which water penetrates into the controlled-release fertilizer; at the same time, the present invention team found that under the system of the present invention, the introduction of the polycaprolactone layer can reduce the overall mechanical strength of the controlled-release fertilizer, making it easy for the controlled-release fertilizer to form fine gaps when subjected to external forces, and these gaps help to accelerate the release of elements. It may be due to the combined effect of the above two aspects that, after adding a polycaprolactone layer on the basis of the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer, the present invention can, on the whole, slow down the release of easily soluble elements (such as nitrogen in some fertilizers) and accelerate the release of difficultly soluble elements (such as phosphorus in some fertilizers). This can provide a more reasonable nutrient supply for plant growth and avoid the phenomenon of excessive release of some elements and insufficient release of some elements.

[0010] Preferably, the mass of the polycaprolactone layer is 1.0-5.6% of the mass of the fertilizer granules; the mass of the starch layer is 3.0-4.5% of the mass of the fertilizer granules.

[0011] When the mass of the polycaprolactone layer is controlled at 1.0-5.6% of the mass of the fertilizer particles, the layer can achieve differential regulation of the release rate of different soluble elements, effectively slowing down the release of easily soluble elements (such as nitrogen in some fertilizers) while releasing elements that are difficult to dissolve (such as phosphorus in some fertilizers). When the mass proportion of the polycaprolactone layer is too small, the easily soluble elements will be released too quickly and the difficult to dissolve elements will be released too slowly; when the mass proportion of the polycaprolactone layer is too large, the difficult to dissolve elements will also be released too slowly.

[0012] Preferably, the starch layer consists of potato starch.

[0013] Preferably, the polycaprolactone layer is composed of polycaprolactone having a weight average molecular weight of 40-50 kDa.

[0014] Preferably, the fertilizer particles are mushroom chaff.

[0015] In a second aspect, the present invention provides a method for preparing the controlled-release fertilizer, comprising the following steps: S1: soaking the fertilizer granules in stearic acid solution, taking them out and drying them; S2: soaking the product obtained in step S1 in a starch solution, taking it out, and drying it; S3: soaking the product obtained in step S2 in a stearic acid solution, taking it out, and drying it; S4: soaking the product obtained in step S3 in a polycaprolactone solution, taking it out, and drying it to obtain a controlled-release fertilizer.

[0016] Preferably, in step S1: the concentration of the stearic acid solution is 2-8% w / v.

[0017] Preferably, in step S2: the concentration of the starch solution is 1-3% w / v.

[0018] Preferably, in step S3: the concentration of the stearic acid solution is 2-8% w / v.

[0019] Preferably, in step S4: the concentration of the polycaprolactone solution is 2-3% w / v.

[0020] Preferably, in steps S1 to S4, the drying method is hot air drying.

[0021] Compared with the prior art, the present invention has the following advantages: (1) In the controlled-release fertilizer of the present invention, by providing a three-layer coating consisting of a first stearic acid layer / a starch layer / a second stearic acid layer on the outside of the fertilizer particles, the release rate of nutrients in the fertilizer particles can be better controlled, thereby extending the effective period of the controlled-release fertilizer.

[0022] (2) In the controlled-release fertilizer of the present invention, by providing a polycaprolactone layer outside the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer, the release of easily soluble elements (such as nitrogen in some fertilizers) can be slowed down, while the release of difficultly soluble elements (such as phosphorus in some fertilizers) can be accelerated, thereby providing a more reasonable nutrient supply for plant growth and avoiding the phenomenon of excessive release of some elements and insufficient release of some elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 The SEM images of the uncoated fertilizer particles and the controlled-release fertilizers obtained in Examples 1-2, Comparative Example 1 and Comparative Example 3 are shown.

[0024] Figure 2 It is the mechanical strength of the uncoated fertilizer granules and the controlled-release fertilizers obtained in Examples 1-2, Comparative Example 1 and Comparative Example 3. Data description: Mean±SD, n=3, *p<0.05. DETAILED DESCRIPTION

[0025] The present invention will be further described below in conjunction with the embodiments.

[0026] A controlled-release fertilizer, comprising fertilizer particles; the fertilizer particles are sequentially coated with a first stearic acid layer, a starch layer, a second stearic acid layer and a polycaprolactone layer; the total mass of the first stearic acid layer and the second stearic acid layer is 7.5-13.5% of the mass of the fertilizer particles; the mass ratio between the first stearic acid layer and the second stearic acid layer is 1:0.9-1.2.

[0027] In some specific embodiments, the mass of the polycaprolactone layer is 1.0-5.6% of the mass of the fertilizer granules; the mass of the starch layer is 3.0-4.5% of the mass of the fertilizer granules.

[0028] In some specific embodiments, the starch layer is composed of potato starch; the polycaprolactone layer is composed of polycaprolactone with a weight average molecular weight of 40-50 kDa; and the fertilizer particles are mushroom bran.

[0029] A method for preparing the above-mentioned controlled-release fertilizer comprises the following steps: S1: soaking the fertilizer granules in stearic acid solution, taking them out and drying them; S2: soaking the product obtained in step S1 in a starch solution, taking it out, and drying it; S3: soaking the product obtained in step S2 in a stearic acid solution, taking it out, and drying it; S4: soaking the product obtained in step S3 in a polycaprolactone solution, taking it out, and drying it to obtain a controlled-release fertilizer.

[0030] In some specific embodiments, in step S1: the concentration of the stearic acid solution is 2-8% w / v.

[0031] In some specific embodiments, in step S2: the concentration of the starch solution is 1-3% w / v.

[0032] In some specific embodiments, in step S3: the concentration of the stearic acid solution is 2-8% w / v.

[0033] In some specific embodiments, in step S4: the concentration of the polycaprolactone solution is 2-3% w / v.

[0034] In some specific embodiments, in steps S1 to S4: the drying method is hot air drying.

[0035] The present invention is described below by specific examples. It should be understood that these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention. Without departing from the spirit and scope of the inventive concept, changes and advantages that can be thought of by those skilled in the art are included in the present invention, and the attached claims and any equivalents thereof are the protection scope of the present invention.

[0036] Comparative Example 1: Preparation of controlled release fertilizer (CRF 1A) The controlled release fertilizer (CRF 1A) of this comparative example was prepared according to the following steps: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0037] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a 2% w / v stearic acid solution for later use.

[0038] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0039] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0040] (2.3) The secondary coated particles were immersed in the stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the secondary coated particles with a second stearic acid layer to obtain a controlled release fertilizer (recorded as "CRF 1A").

[0041] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 1A prepared in this comparative example, the masses of the first stearic acid layer, the starch layer, and the second stearic acid layer were 3.8%, 3.4%, and 4.0% of the mass of the uncoated fertilizer granules, respectively.

[0042] Example 1: Preparation of controlled release fertilizer (CRF 2A) The difference between this embodiment and comparative example 1 is that this embodiment is based on the CRF 1A prepared in comparative example 1, and a polycaprolactone layer is coated on the outermost layer. Specifically, the preparation steps of the controlled release fertilizer (CRF 2A) of this embodiment are as follows: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0043] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a 2% w / v stearic acid solution for later use.

[0044] (1.3) Add polycaprolactone with a weight average molecular weight of 45 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 2% w / v for later use.

[0045] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0046] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0047] (2.3) Immerse the secondary coated particles in the stearic acid solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the secondary coated particles with a second stearic acid layer to obtain tertiary coated particles.

[0048] (2.4) The thrice-coated granules were immersed in the polycaprolactone solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the thrice-coated granules with a polycaprolactone layer to obtain a controlled release fertilizer (recorded as "CRF 2A").

[0049] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 2A prepared in this embodiment, the masses of the first stearic acid layer, the starch layer, the second stearic acid layer, and the polycaprolactone layer were 3.8%, 3.4%, 4.0%, and 5.6% of the mass of the uncoated fertilizer granules, respectively.

[0050] Comparative Example 2: Preparation of controlled release fertilizer (CRF 3A) The difference between this comparative example and Example 1 is that: on the basis of Example 1, this comparative example increases the mass proportion of the polycaprolactone layer in the controlled-release fertilizer; the remaining steps are the same as Example 1. Specifically, the preparation steps of the controlled-release fertilizer (CRF 3A) of this comparative example are as follows: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0051] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a 2% w / v stearic acid solution for later use.

[0052] (1.3) Add polycaprolactone with a weight average molecular weight of 45 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 5% w / v for later use.

[0053] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0054] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0055] (2.3) Immerse the secondary coated particles in the stearic acid solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the secondary coated particles with a second stearic acid layer to obtain tertiary coated particles.

[0056] (2.4) The thrice-coated granules were immersed in the polycaprolactone solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the thrice-coated granules with a polycaprolactone layer to obtain a controlled release fertilizer (referred to as "CRF 3A").

[0057] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 3A prepared in this comparative example, the masses of the first stearic acid layer, the starch layer, the second stearic acid layer, and the polycaprolactone layer were 3.8%, 3.4%, 4.0%, and 7.2% of the mass of the uncoated fertilizer granules, respectively.

[0058] Comparative Example 3: Preparation of controlled release fertilizer (CRF 1B) The controlled release fertilizer (CRF 1B) of this comparative example was prepared according to the following steps: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0059] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a stearic acid solution with a concentration of 8% w / v, which is set aside.

[0060] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0061] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0062] (2.3) The secondary coated particles were immersed in the stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the secondary coated particles with a second stearic acid layer to obtain a controlled release fertilizer (recorded as "CRF 1B").

[0063] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 1B prepared in this comparative example, the masses of the first stearic acid layer, the starch layer, and the second stearic acid layer were 6.5%, 3.0%, and 6.9% of the mass of the uncoated fertilizer granules, respectively.

[0064] Example 2: Preparation of controlled release fertilizer (CRF 2B) The difference between this embodiment and comparative example 2 is that this embodiment is based on the CRF 1B prepared in comparative example 2, and a polycaprolactone layer is coated on the outermost layer. Specifically, the preparation steps of the controlled release fertilizer (CRF 2B) of this embodiment are as follows: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0065] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a stearic acid solution with a concentration of 8% w / v, which is set aside.

[0066] (1.3) Add polycaprolactone with a weight average molecular weight of 45 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 2% w / v for later use.

[0067] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0068] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0069] (2.3) Immerse the secondary coated particles in the stearic acid solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the secondary coated particles with a second stearic acid layer to obtain tertiary coated particles.

[0070] (2.4) The thrice-coated granules were immersed in the polycaprolactone solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the thrice-coated granules with a polycaprolactone layer to obtain a controlled release fertilizer (denoted as "CRF 2B").

[0071] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. After calculation, it was found that in the controlled-release fertilizer CRF 2B prepared in this embodiment, the masses of the first stearic acid layer, the starch layer, the second stearic acid layer, and the polycaprolactone layer were 6.5%, 3.0%, 6.9%, and 1.0% of the mass of the uncoated fertilizer granules, respectively.

[0072] Comparative Example 4: Preparation of controlled release fertilizer (CRF 3B) The difference between this comparative example and Example 2 is that: on the basis of Example 2, this comparative example reduces the mass proportion of the polycaprolactone layer in the controlled-release fertilizer; the remaining steps are the same as Example 2. Specifically, the preparation steps of the controlled-release fertilizer (CRF 3B) of this comparative example are as follows: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0073] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a stearic acid solution with a concentration of 8% w / v, which is set aside.

[0074] (1.3) Add polycaprolactone with a weight average molecular weight of 45 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 1% w / v for later use.

[0075] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0076] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0077] (2.3) Immerse the secondary coated particles in the stearic acid solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the secondary coated particles with a second stearic acid layer to obtain tertiary coated particles.

[0078] (2.4) The thrice-coated granules were immersed in the polycaprolactone solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the thrice-coated granules with a polycaprolactone layer to obtain a controlled release fertilizer (recorded as "CRF 3B").

[0079] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 3B prepared in this comparative example, the masses of the first stearic acid layer, the starch layer, the second stearic acid layer, and the polycaprolactone layer were 6.5%, 3.0%, 6.9%, and 0.4% of the mass of the uncoated fertilizer granules, respectively.

[0080] Example 3: Preparation of controlled release fertilizer (CRF 1C) The controlled release fertilizer (CRF 1C) of this example was prepared according to the following steps: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 3% w / v potato starch solution for later use.

[0081] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a stearic acid solution with a concentration of 8% w / v, which is set aside.

[0082] (1.3) Add polycaprolactone with a weight average molecular weight of 60 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 3% w / v for later use.

[0083] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a first stearic acid layer to obtain primary coated particles.

[0084] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0085] (2.3) Immerse the secondary coated particles in the stearic acid solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the secondary coated particles with a second stearic acid layer to obtain tertiary coated particles.

[0086] (2.4) The thrice-coated granules were immersed in the polycaprolactone solution, taken out within 30 s, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the thrice-coated granules with a polycaprolactone layer to obtain a controlled release fertilizer (denoted as "CRF 1C").

[0087] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. After calculation, it was found that in the controlled-release fertilizer CRF 1C prepared in this embodiment, the masses of the first stearic acid layer, the starch layer, the second stearic acid layer, and the polycaprolactone layer were 6.8%, 4.2%, 6.6%, and 2.9% of the mass of the uncoated fertilizer granules, respectively.

[0088] Test Example 1: Effect of polycaprolactone layer on the surface morphology of controlled-release fertilizer The uncoated fertilizer particles and the controlled-release fertilizers prepared according to the methods of Examples 1-2, Comparative Examples 1 and Comparative Examples 3 were characterized by scanning electron microscope SEM (FEI Inspect F50) for their structure and morphology. The specific method is as follows: the sample is dispersed on a carbon tape attached to the surface of a metal disk (stub) and coated with a platinum coating (Emitch K575X Peltier cooling). Then the instrument is calibrated and the surface morphology of the sample is observed. The results are shown in Figure 1 .

[0089] from Figure 1 It can be seen that the surface of the uncoated fertilizer particles is rough with clearly discernible pores; after coating with the first stearic acid layer, starch layer and second stearic acid layer (CRF 1A, CRF 1B), the surface smoothness is improved, but there are still many surface defects. On this basis, by coating with a polycaprolactone layer (CRF 2A, CRF 2B), the surface roughness can be further reduced and the number of large pores on the surface can be reduced.

[0090] Test Example 2: Effect of polycaprolactone layer on mechanical strength of controlled-release fertilizer The uncoated fertilizer particles and the controlled-release fertilizers prepared according to the methods of Examples 1-2, Comparative Examples 1 and Comparative Examples 3 were tested for mechanical strength. The test method was as follows: a mechanical strength tester (Instron 5969 test system) was used to apply compression force to a single particle to test the crushing strength. Three samples (n=3) were taken from each group for testing, and the results were averaged. The test results are shown in Figure 2 .

[0091] from Figure 2 It can be seen that the mechanical strengths of CRF 1A and CRF 1B are 0.36±0.03 MPa and 0.42±0.01 MPa, respectively. After CRF 2A and CRF 2B are respectively wrapped with polycaprolactone layers, the mechanical strengths are reduced (0.30±0.10 MPa and 0.24±0.01 MPa, respectively). The above results show that in the controlled-release fertilizer system of the present invention, the polycaprolactone layer can reduce the overall mechanical strength of the controlled-release fertilizer. The reason may be that polycaprolactone has a low degree of crystallinity and can interfere with the crystal structure of other more rigid phases in the material, thereby reducing the overall mechanical strength of the controlled-release fertilizer.

[0092] Test Example 3: Effect of polycaprolactone layer on the release rate of each element in controlled-release fertilizer Take uncoated fertilizer particles and controlled-release fertilizers prepared according to the methods of Examples 1 to 3 and Comparative Examples 1 to 4, and conduct nutrient release tests. The test method is as follows: Take 10 samples per group (the total mass of 10 samples is about 1.5 g), place them in a 50 mL sample tube containing 50 mL of water, set the temperature to 37°C, cover the sample tube with aluminum foil to prevent light interference, and stir the sample with a Pasteur pipette with nanograms of water. Take samples every day to detect the cumulative release concentration of N and P elements. After each sampling, make up with an equal amount of fresh water. The release rate test results of each element are shown in Tables 1 and 2.

[0093] Table 1 Nitrogen release rate test results

[0094] Table 2 Phosphorus release rate test results

[0095] From Table 1 and Table 2, we can see that: (1) Compared with uncoated fertilizer granules, CRF 1A, CRF 1B, CRF 2A, CRF 2B, CRF 3A, CRF3B and CRF 1C can effectively slow down the release of N and P elements by coating the outside of the fertilizer granules.

[0096] (2) Compared with CRF 1A, the rate of N release from CRF 2A slows down, while the rate of P release speeds up; compared with CRF1B, the rate of N release from CRF 2B slows down, while the rate of P release speeds up. This indicates that by coating the polycaprolactone layer outside the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer, the release of N element can be slowed down, while the release of P element can be accelerated. The possible reason for this is that polycaprolactone can reduce the roughness and the number of large pores on the surface of the controlled-release fertilizer, thereby slowing down the rate at which water penetrates into the controlled-release fertilizer; at the same time, under the system of the present invention, the introduction of the polycaprolactone layer can reduce the overall mechanical strength of the controlled-release fertilizer, making it easy for the controlled-release fertilizer to form fine gaps when subjected to external forces, and these gaps help to accelerate the release of elements. Under the combined effect of the above two aspects, after adding the polycaprolactone layer, it can show the overall effect of slowing down the release of elements that are easy to dissolve (such as nitrogen in this test example) while accelerating the release of elements that are difficult to dissolve (such as phosphorus in this test example).

[0097] (3) The results of CRF 1A, CRF 2A and CRF 3A show that when the mass proportion of the polycaprolactone layer is too large, the release of the difficult-to-dissolve element (P) will be too slow. The results of CRF 1B, CRF 2B and CRF 3B show that when the mass proportion of the polycaprolactone layer is too small, it is difficult to slow down the release of the easily-dissolved element (N) to a large extent and accelerate the release of the difficult-to-dissolve element (P).

[0098] Comparative Example 5: Preparation of controlled release fertilizer (CRF 4A) The difference between this comparative example and Example 1 is that the second stearic acid layer is not coated in this comparative example, and the mass proportion of the first stearic acid layer is increased; the remaining steps are the same as those in Example 1. Specifically, the preparation steps of the controlled release fertilizer (CRF4A) in this example are as follows: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0099] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a stearic acid solution with a concentration of 12% w / v, which is set aside.

[0100] (1.3) Add polycaprolactone with a weight average molecular weight of 45 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 2% w / v for later use.

[0101] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom chaff, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in stearic acid solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the fertilizer particles with a stearic acid layer to obtain primary coated particles.

[0102] (2.2) Immerse the primary coated particles in potato starch solution, take them out within 30 seconds, place them in a glass bowl, spread them gently, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a starch layer to obtain secondary coated particles.

[0103] (2.3) The secondary coated particles were immersed in the polycaprolactone solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the secondary coated particles with a polycaprolactone layer to obtain a controlled release fertilizer (recorded as "CRF 4A").

[0104] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 4A prepared in this comparative example, the masses of the stearic acid layer, the starch layer, and the polycaprolactone layer were 7.5%, 3.1%, and 5.3% of the mass of the uncoated fertilizer granules, respectively.

[0105] Comparative Example 6: Preparation of controlled release fertilizer (CRF 5A) The difference between this comparative example and Example 1 is that the first stearic acid layer is not coated in this comparative example, and the mass proportion of the second stearic acid layer is increased; the remaining steps are the same as those in Example 1. Specifically, the preparation steps of the controlled release fertilizer (CRF5A) in this example are as follows: (1) Preparation of dipping solution: (1.1) Add potato starch to water, heat in a microwave and stir until a transparent solution is formed to obtain a 1% w / v potato starch solution for later use.

[0106] (1.2) Add stearic acid to ethanol and dissolve it completely in an ultrasonic bath to obtain a stearic acid solution with a concentration of 12% w / v, which is set aside.

[0107] (1.3) Add polycaprolactone with a weight average molecular weight of 45 kDa to ethyl acetate and stir magnetically overnight to obtain a polycaprolactone solution with a concentration of 2% w / v for later use.

[0108] (2) Fertilizer granule coating: (2.1) Fertilizer particles with a particle size of 3-5 mm (mushroom husks, purchased from South Australian Peat Soil and Gardening Supplies Company) were immersed in potato starch solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby wrapping the fertilizer particles with a starch layer to obtain primary coated particles.

[0109] (2.2) Immerse the primary coated particles in the stearic acid solution, take them out within 30 seconds, place them in a glass bowl, spread them lightly, and dry them with a hot air gun to evaporate the solvent residue, thereby wrapping the primary coated particles with a stearic acid layer to obtain secondary coated particles.

[0110] (2.3) The secondary coated particles were immersed in the polycaprolactone solution, taken out within 30 seconds, placed in a glass bowl, gently spread, and dried with a hot air gun to evaporate the solvent residue, thereby coating the secondary coated particles with a polycaprolactone layer to obtain a controlled release fertilizer (denoted as "CRF 5A").

[0111] The mass of the uncoated fertilizer granules was measured, and the mass was measured again after each coating. It was calculated that in the controlled-release fertilizer CRF 5A prepared in this comparative example, the masses of the starch layer, the stearic acid layer, and the polycaprolactone layer were 3.2%, 8.0%, and 5.2% of the mass of the uncoated fertilizer granules, respectively.

[0112] Test Example 4: Effect of the first stearic acid layer / starch layer / second stearic acid layer on the release rate of controlled-release fertilizer Take uncoated fertilizer particles and controlled release fertilizers prepared according to the methods of Example 1, Comparative Example 1, Comparative Example 5 and Comparative Example 6, and conduct nutrient release tests. The test method is as follows: Take 3 samples from each group (the total mass of the 3 samples is about 150 mg), place them in a 50 mL sample tube containing 50 mL of water, set the temperature to 37°C, cover the sample tube with aluminum foil to prevent light interference, and use a Pasteur pipette to stir the sample with nanograms of water. Take samples every day (decant 2 mL from each sample tube) and add them to a quartz test tube with an optical path of 10 mm. Detect the absorbance at 300 nm and calculate the cumulative release concentration based on the standard curve. After each sampling, fill it with an equal amount of fresh water. The results of the nutrient release rate test are shown in Table 3.

[0113] Table 3 Nutrient release rate test results

[0114] From Table 3 we can see that: (1) Compared with uncoated fertilizer granules, CRF 2A, CRF 1A, CRF 4A and CRF 5A can effectively slow down the release of nutrients in fertilizers by coating the outside of fertilizer granules.

[0115] (2) Compared with CRF 2A, the nutrient release rates of CRF 4A and CRF 5A were faster. This indicates that the use of the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer can slow down the nutrient release rate of the controlled-release fertilizer to a greater extent. Among the three layers of coating, when any of the two stearic acid layers is missing, even if the mass proportion of the other stearic acid layer is increased, the nutrient release rate will be faster. The reason is that the starch layer has a porous structure and swells microscopically after absorbing water, which further expands the pore structure within the layer. The stearic acid layer is more hydrophobic. By using this difference between the two, the water permeation path can be regulated, making the water permeation path in the three-layer coating of the first stearic acid layer / starch layer / second stearic acid layer nonlinear; and due to the difference in solubility and degradability between stearic acid and starch, microscopic pores will be formed at the interface between the two. The "microchannels" formed by these pores can guide the directional diffusion of water and nutrients, thereby making the release of nutrients in the controlled-release fertilizer more uniform and avoiding the phenomenon of excessive accumulation or excessive release of local nutrients. Through the above two effects, the release of nutrients can be better slowed down.

[0116] (3) The nutrient release rates of CRF 2A and CRF 1A are similar, indicating that in the controlled-release fertilizer system of the present invention, the role of the polycaprolactone layer is not to reduce the nutrient release rate in the fertilizer, but to achieve differentiated regulation of the release rates of elements with different solubility, so that the release of easily soluble elements (such as nitrogen in some fertilizers) is slowed down, and the release of difficult-to-dissolve elements (such as phosphorus in some fertilizers) is accelerated (the experimental data on the controlled-release effect in this regard is shown in Test Example 3 above).

[0117] Unless otherwise defined, all technical terms and scientific terms used in the present invention have the same meanings as those commonly understood by ordinary technicians in the field to which the present disclosure belongs. Unless otherwise specified, the raw materials and equipment used in the present invention are conventional raw materials and equipment in the field and can be obtained from conventional commercial channels; unless otherwise specified, the methods used in the present invention are conventional methods in the field.

[0118] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any way. Any simple modification, change and equivalent transformation made to the above embodiment based on the technical essence of the present invention still falls within the protection scope of the technical solution of the present invention.

Claims

1. A controlled release fertilizer, characterized in that: The controlled-release fertilizer comprises fertilizer particles; the fertilizer particles are sequentially coated with a first stearic acid layer, a starch layer, a second stearic acid layer and a polycaprolactone layer; the total mass of the first stearic acid layer and the second stearic acid layer is 7.5-13.5% of the mass of the fertilizer particles; the mass ratio between the first stearic acid layer and the second stearic acid layer is 1:0.9-1.

2.

2. The controlled-release fertilizer according to claim 1, characterized in that The mass of the polycaprolactone layer is 1.0-5.6% of the mass of the fertilizer granules; the mass of the starch layer is 3.0-4.5% of the mass of the fertilizer granules.

3. The controlled-release fertilizer according to claim 1, characterized in that The starch layer consists of potato starch.

4. The controlled-release fertilizer according to claim 1 or 2, characterized in that: The polycaprolactone layer is composed of polycaprolactone with a weight average molecular weight of 40-50 kDa.

5. The controlled-release fertilizer according to claim 1, characterized in that The fertilizer particles are mushroom chaff.

6. A method for preparing the controlled-release fertilizer according to any one of claims 1 to 5, characterized in that: The following steps are involved: S1: soaking the fertilizer granules in stearic acid solution, taking them out and drying them; S2: soaking the product obtained in step S1 in a starch solution, taking it out, and drying it; S3: soaking the product obtained in step S2 in a stearic acid solution, taking it out, and drying it; S4: soaking the product obtained in step S3 in a polycaprolactone solution, taking it out, and drying it to obtain a controlled-release fertilizer.

7. The preparation method according to claim 6, characterized in that: In step S1: the concentration of the stearic acid solution is 2-8% w / v.

8. The preparation method according to claim 6, characterized in that: In step S2: the concentration of the starch solution is 1-3% w / v.

9. The preparation method according to claim 6, characterized in that: In step S3: the concentration of the stearic acid solution is 2-8% w / v.

10. The preparation method according to claim 6, characterized in that: In step S4: the concentration of the polycaprolactone solution is 2-3% w / v.

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