Preparation method of biodegradable slow-release fertilizer and biodegradable slow-release fertilizer

Abstract

The application relates to the technical field of fertilizers, and particularly discloses a preparation method of a biodegradable slow-release fertilizer and the prepared biodegradable slow-release fertilizer. The preparation method of the biodegradable slow-release fertilizer provided by the application adopts an end-capped isocyanate aqueous solution to perform wetting treatment on the surface of fertilizer particles in a pretreatment stage, so that the hydrophobic polyester mixed powder can be easily coated on the surface of the fertilizer particles in a coating step, and the problem of poor compatibility between the polyester mixed powder and the fertilizer particles is effectively solved; after the coating is completed, the end-capped isocyanate aqueous solution is sprayed, and then the temperature is increased to perform a reaction; the end-capped isocyanate is unblocked under heating, the released isocyanate groups chemically react with hydroxyl groups and carboxyl groups at chain ends of PBAT, and finally a dense reticular coating structure is formed on the surface of the fertilizer particles, so that the mechanical strength, water resistance and slow-release stability of the coating layer are greatly enhanced.

Description

Technical Field

[0001] This invention relates to the field of fertilizer technology, and in particular to a method for preparing a biodegradable slow-release fertilizer and the resulting biodegradable slow-release fertilizer. Background Technology

[0002] Traditional chemical fertilizers release nutrients too quickly after application, making it difficult for crops to absorb them efficiently and promptly. This results in the loss of a large amount of nutrients through volatilization, leaching, or fixation. This not only leads to serious waste of resources and increased production costs, but also causes severe environmental pollution problems such as eutrophication of water bodies and soil compaction and acidification.

[0003] To overcome the aforementioned shortcomings, slow-release fertilizers and controlled-release fertilizers have emerged. Existing slow-release fertilizers mainly include coated slow-release fertilizers, chemically synthesized slow-release fertilizers, and matrix-compound slow-release fertilizers. Among these, coated slow-release fertilizers are currently the most technologically mature and widely used type, achieving their slow-release function by coating the fertilizer granules with a semi-permeable or insoluble membrane material. Based on the different coating materials, coated slow-release fertilizers can be divided into: (1) Non-biodegradable polymer-coated fertilizer: using synthetic polymers such as polyethylene, polypropylene, polystyrene, and epoxy resin as coating materials; (2) Inorganic material coated fertilizer: Sulfur, mineral powder, etc. are used as coating materials; (3) Bio-based or biodegradable polymer-coated fertilizers: Commonly used coating materials include starch, cellulose, lignin, polylactic acid (PLA), polybutylene adipate / terephthalate (PBAT), etc.

[0004] Polymer-coated slow-release fertilizers face the following pressing technical challenges: I. Currently widely used coating materials (such as polyethylene, polypropylene, epoxy resin, etc.) are difficult to biodegrade in soil. After fertilizers are absorbed by crops, these polymer coatings will remain in the soil in the form of microplastics for a long time and accumulate in the soil, damaging soil structure and microbial ecology, causing persistent "white pollution", which violates the development concept of green agriculture. Second, existing polymer coating processes typically require complex equipment, processes, high-temperature conditions, and the use of large amounts of organic solvents, resulting in high production costs and limiting their large-scale application in common field crops. For example, when using PBAT as a coating material, its high viscosity necessitates the use of special solvents for dissolution and spraying, making the process complex, requiring large amounts of solvent, and also environmentally unfriendly. Summary of the Invention

[0005] The main technical problem solved by this invention is to provide a method for preparing biodegradable slow-release fertilizer and the resulting biodegradable slow-release fertilizer.

[0006] To solve the above-mentioned technical problems, the technical solution adopted by the present invention is as follows.

[0007] In a first aspect, the present invention provides a method for preparing a biodegradable slow-release fertilizer, comprising the steps of: S1: Pretreatment of fertilizer granules: The fertilizer granules are put into a rotary drum coating machine preheated to 40℃-55℃, and the first end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. S2: Coating the wetted fertilizer granules: Maintain the temperature of the rotary drum coating machine at 40℃-55℃, add polyester mixed powder into the rotary drum through the powder feeding device, and coat the surface of the wetted fertilizer granules with the polyester mixed powder to complete the coating process; wherein, the polyester mixed powder includes PBAT prepolymer powder and inorganic powder filler. S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 40℃-55℃, and spray the coated fertilizer granules with a second end-capped isocyanate aqueous solution through a spraying device; after spraying, raise the temperature of the rotary drum coating machine to 80℃-150℃ and keep it at the temperature for 5-10 minutes for reaction; then cool it to ≤40℃, discharge and screen to obtain biodegradable slow-release fertilizer.

[0008] In one embodiment of the present invention, the polyester mixed powder is composed of PBAT prepolymer powder and inorganic powder filler, wherein the mass ratio of the PBAT prepolymer powder to the inorganic powder filler is (1-3):1. Specifically, the PBAT prepolymer powder and the inorganic powder filler can be mixed in a mass ratio (PBAT prepolymer powder: inorganic powder filler) of 1:1, 2:1, 3:1, or any other ratio within the range of (1-3):1.

[0009] As one embodiment of the present invention, the preparation process of the polyester mixed powder is as follows: PBAT prepolymer powder and inorganic powder filler are taken in a mass ratio, and then placed in a mixer and mixed at room temperature (about 25°C) for 2-5 minutes to obtain a uniformly mixed polyester mixed powder.

[0010] As one embodiment of the present invention, the PBAT prepolymer powder is obtained by crushing PBAT prepolymer and passing it through a 50-100 mesh sieve, and the intrinsic viscosity of the PBAT prepolymer is 0.2-0.8 dL / g.

[0011] In one embodiment of the present invention, the inorganic powder filler is calcium carbonate and / or bentonite.

[0012] In one embodiment of the present invention, the amount of the polyester mixed powder is 4%-8% of the fertilizer granules mass. Specifically, the amount of the polyester mixed powder can be 4%, 5%, 6%, 7%, 8% of the fertilizer granules mass, or any other value within the range of 4%-8%.

[0013] As one embodiment of the present invention, the fertilizer granules can be any one of urea, compound fertilizer, and potassium chloride.

[0014] As one embodiment of the present invention, the first capped isocyanate aqueous solution and the second capped isocyanate aqueous solution are both aqueous solutions containing at least one of water-soluble capped HDI (hexamethylene diisocyanate), capped IPDI (isophorone diisocyanate), capped TDI (toluene diisocyanate), and capped MDI (diphenylmethane diisocyanate).

[0015] In one embodiment of the present invention, the mass percentage concentration of the first and second capped isocyanate aqueous solutions is 15%-25%. Specifically, the mass percentage concentration of the first and second capped isocyanate aqueous solutions can be 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, or any other value within the range of 15%-25%.

[0016] In one embodiment of the present invention, the first capped isocyanate aqueous solution and the second capped isocyanate aqueous solution have the same or different compositions.

[0017] As a further embodiment of the present invention, a catalyst is also added to the first and second terminal isocyanate aqueous solutions, wherein the catalyst is selected from at least one of stannous octoate, dibutyltin dilaurate, and di(dodecylthio)dibutyltin. Adding a catalyst helps to lower the desealing temperature of the terminal isocyanate and increase the rate of subsequent crosslinking reactions.

[0018] In one embodiment of the present invention, the amount of catalyst added to the first end-capped isocyanate aqueous solution is 0.1%-0.5% of the mass of the end-capped isocyanate contained in the first end-capped isocyanate aqueous solution. Specifically, the amount of catalyst added can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5% of the mass of the end-capped isocyanate contained in the first end-capped isocyanate aqueous solution, or any other value within the range of 0.1%-0.5%.

[0019] In one embodiment of the present invention, the amount of catalyst added to the second end-capped isocyanate aqueous solution is 0.1%-0.5% of the mass of the end-capped isocyanate contained in the second end-capped isocyanate aqueous solution. Specifically, the amount of catalyst added can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5% of the mass of the end-capped isocyanate contained in the second end-capped isocyanate aqueous solution, or any other value within the range of 0.1%-0.5%.

[0020] In a preferred embodiment of the present invention, the first end-capped isocyanate aqueous solution and the second end-capped isocyanate aqueous solution have the same composition.

[0021] In one embodiment of the present invention, the mass ratio of the first capped isocyanate aqueous solution to the second capped isocyanate aqueous solution is (3-5):(5-7). Specifically, the mass ratio of the first capped isocyanate aqueous solution to the second capped isocyanate aqueous solution can be 3:7, 4:6, 5:5, or any other ratio within the range of (3-5):(5-7).

[0022] In one embodiment of the present invention, the sum of the mass of the terminal isocyanate contained in the first and second aqueous solutions of the terminal isocyanate is 4%-6% of the mass of the PBAT prepolymer powder contained in the polyester mixed powder. Specifically, the sum of the mass of the terminal isocyanate contained in the first and second aqueous solutions of the terminal isocyanate can be 4%, 5%, 6%, or any other value within the range of 4%-6% of the mass of the PBAT prepolymer powder contained in the polyester mixed powder.

[0023] In one embodiment of the present invention, the rotational speed of the rotary drum coating machine is 20-40 rpm. Specifically, the rotational speed of the rotary drum coating machine can be 20 rpm, 25 rpm, 30 rpm, 35 rpm, 40 rpm, or other values ​​within the range of 20-40 rpm.

[0024] Secondly, the present invention provides a biodegradable slow-release fertilizer prepared by the preparation method described in the present invention.

[0025] The present invention provides a method for preparing biodegradable slow-release fertilizer. In the preparation process, a mixture of capped isocyanate and polyester powder is used to treat fertilizer particles. Specifically, in the pretreatment stage, an aqueous solution of capped isocyanate is used to wet the surface of the fertilizer particles. This aqueous solution acts as a binder, ensuring that the hydrophobic polyester powder can easily coat the surface of the fertilizer particles during the coating step, effectively solving the problem of poor compatibility between the polyester powder and the fertilizer particles. After coating, an aqueous solution of capped isocyanate is sprayed on top, followed by heating for reaction. The capped isocyanate is decapsulated under heating, and the released isocyanate groups (-NCO) react chemically with the hydroxyl (-OH) and carboxyl (-COOH) groups at the ends of the PBAT chains. Simultaneously, the PBAT prepolymer molecular chains undergo cross-linking reactions at the powder interface, forming a robust polyurethane / polyester cross-linked network. Ultimately, a dense network coating structure is formed on the surface of the fertilizer particles, thus greatly enhancing the mechanical strength, water resistance, and slow-release stability of the coating layer.

[0026] The method for preparing biodegradable slow-release fertilizer provided by this invention adopts a segmented temperature control process. The pretreatment and coating steps are carried out at low temperatures, specifically 40℃-55℃, to ensure that the capped isocyanate does not decapsulate during the coating stage, thus guaranteeing the stability and uniformity of the coating operation. Subsequently, the temperature is raised to 80℃-150℃ to precisely trigger the cross-linking and curing reaction, forming a dense network coating structure on the surface of the fertilizer particles.

[0027] This invention utilizes capped isocyanates, specifically isocyanates that have undergone capping treatment. The isocyanate can be selected from HDI (hexamethylene diisocyanate), IPDI (isophorone diisocyanate), TDI (toluene diisocyanate), MDI (diphenylmethane diisocyanate), etc. The capping agent used in the capping treatment is a monofunctional active hydrogen compound. The capping agent reacts with the isocyanate to generate a stable adduct B-NCO (where B represents the capping agent group and -NCO is the protected isocyanate group), i.e., the capped isocyanate, thus "blocking" the activity of the isocyanate. The adduct is stable at lower temperatures (e.g., 40℃-55℃). Upon heating (e.g., 80℃-150℃), the chemical bonds of the end-capped isocyanate break, and the end-capping agent group detaches in the form of BH, simultaneously releasing highly reactive isocyanate groups -NCO. These released reactive isocyanate groups rapidly react with the hydroxyl (-OH) and carboxyl (-COOH) groups at the ends of the PBAT prepolymer chain, forming a three-dimensional cross-linked network structure, resulting in a dense coating structure with high mechanical strength and good water resistance. The end-capped isocyanate used in this invention has a certain degree of water solubility and can be dissolved and dispersed in water.

[0028] The biodegradable slow-release fertilizer prepared by this invention can be completely biodegraded in the soil without environmental residues. Furthermore, the production process is simple, the cost is low, and the slow-release performance is stable, offering the following advantages: (1) The coating layer can be completely biodegraded in the soil environment without secondary pollution; (2) It can provide a stable and controllable nutrient release curve, effectively matching the crop growth cycle; (3) The preparation process is simple, efficient and low-cost, and suitable for large-scale industrial production; (4) The coating layer has good mechanical strength, water resistance and processing adaptability; (5) Water is used as the medium during preparation, which avoids the large-scale use of organic solvents, reduces VOC emissions, and is green and environmentally friendly; (6) Inorganic fillers such as calcium carbonate and bentonite are added to the polyester mixed powder. They are widely available and inexpensive, which not only reduces the cost, but also the introduced inorganic filler particles can form tortuous microchannels in the coating layer, which is conducive to the precise control of the nutrient release curve and regulates the degradation and release rate of biodegradable slow-release fertilizer. Detailed Implementation

[0029] The technical solution of the present invention will be described in detail below through specific embodiments. These embodiments are intended to further illustrate the content of the present invention, but are not intended to limit the scope of protection of the claims of the present invention.

[0030] Unless otherwise specified, all reagents used in the following examples or comparative examples are commercially available products.

[0031] Unless otherwise specified, the experimental methods used in the following examples or comparative examples are conventional experimental methods.

[0032] Example The specific components of the fertilizer granules, capped isocyanate and polyester mixed powder used in Examples 1-3 are shown in Table 1.

[0033] Table 1 The preparation process of the polyester mixed powder is as follows: PBAT prepolymer powder and inorganic powder filler (calcium carbonate or bentonite) are taken in a mass ratio, and then placed in a mixer and mixed at room temperature (about 25°C) for 5 minutes to obtain a uniformly mixed polyester mixed powder.

[0034] PBAT prepolymer powder is obtained by pulverizing PBAT prepolymer and passing it through a 100-mesh sieve. The intrinsic viscosity of PBAT prepolymer is 0.6 dL / g (measured in phenol / tetrachloroethane solution).

[0035] All capped isocyanates were purchased from Lanxess.

[0036] Example 1 This embodiment provides a method for preparing a biodegradable slow-release fertilizer, including the following steps: S1: Pretreatment of fertilizer granules: Urea fertilizer granules are put into a rotary drum coating machine preheated to 50°C. The rotary drum coating machine rotates at 40 rpm. The end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. The terminated isocyanate aqueous solution is an aqueous solution of terminated HDI, wherein the mass percentage concentration of terminated HDI is 20%, and the terminated HDI aqueous solution contains stannous octoate catalyst, the amount of stannous octoate added is 0.3% of the mass of terminated HDI contained in the terminated HDI aqueous solution; S2: Coating the moistened fertilizer granules: Maintain the temperature of the rotary drum coating machine at 50℃, add polyester mixed powder into the rotary drum through the powder feeding device, and coat the surface of the moistened fertilizer granules with the polyester mixed powder to complete the coating process; the amount of polyester mixed powder used is 5% of the mass of the fertilizer granules; S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 50°C, and spray the coated fertilizer granules with a terminal isocyanate aqueous solution (the composition is the same as the terminal isocyanate aqueous solution in step S1) through a spraying device. After spraying, raise the temperature of the rotary drum coating machine to 90°C and keep it at the temperature for 10 minutes to react. After naturally cooling to 38°C, the material is discharged, screened to remove adhering clumps, and biodegradable slow-release fertilizer is obtained.

[0037] The mass ratio of the end-capped isocyanate aqueous solution used in step S1 to the end-capped isocyanate aqueous solution used in step S3 is 3:7. The sum of the mass of end-capped isocyanate contained in the end-capped isocyanate aqueous solution used in step S1 and the end-capped isocyanate aqueous solution used in step S3 is 5% of the mass of PBAT prepolymer powder contained in the polyester mixed powder.

[0038] Example 2 This embodiment provides a method for preparing a biodegradable slow-release fertilizer, including the following steps: S1: Pretreatment of fertilizer granules: Urea fertilizer granules are put into a rotary drum coating machine preheated to 45°C. The rotary drum coating machine rotates at 30 rpm. The end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. The isocyanate aqueous solution is an aqueous solution of capped HDI, wherein the mass percentage concentration of capped HDI is 20%, and the aqueous solution of capped HDI contains a catalyst, dibutyltin dilaurate, the amount of which is added is 0.3% of the mass of capped HDI contained in the aqueous solution of capped HDI; S2: Coating the moistened fertilizer granules: Maintain the temperature of the rotary drum coating machine at 45℃, add polyester mixed powder into the rotary drum through the powder feeding device, and coat the surface of the moistened fertilizer granules with the polyester mixed powder to complete the coating process; the amount of polyester mixed powder used is 5% of the mass of the fertilizer granules; S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 45°C, and spray the coated fertilizer granules with a terminal isocyanate aqueous solution (the composition is the same as the terminal isocyanate aqueous solution in step S1) through a spraying device. After spraying, raise the temperature of the rotary drum coating machine to 100°C and keep it at the temperature for 8 minutes to react. After naturally cooling to 39°C, the material is discharged, screened to remove adhering clumps, and biodegradable slow-release fertilizer is obtained.

[0039] The mass ratio of the end-capped isocyanate aqueous solution used in step S1 to the end-capped isocyanate aqueous solution used in step S3 is 5:5. The sum of the mass of end-capped isocyanate contained in the end-capped isocyanate aqueous solution used in step S1 and the end-capped isocyanate aqueous solution used in step S3 is 5% of the mass of PBAT prepolymer powder contained in the polyester mixed powder.

[0040] Example 3 This embodiment provides a method for preparing a biodegradable slow-release fertilizer, including the following steps: S1: Pretreatment of fertilizer granules: Potassium chloride fertilizer granules are put into a rotary drum coating machine preheated to 55°C. The rotary drum coating machine rotates at 25 rpm. The end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. The isocyanate aqueous solution is an aqueous solution of IPDI with a mass percentage concentration of 20%. The aqueous solution of IPDI contains a catalyst, dibutyltin dilaurate, and the amount of dibutyltin dilaurate added is 0.2% of the mass of IPDI contained in the aqueous solution of IPDI. S2: Coating the moistened fertilizer granules: Maintain the temperature of the rotary drum coating machine at 55℃, add polyester mixed powder into the rotary drum through the powder feeding device, and coat the surface of the moistened fertilizer granules with the polyester mixed powder to complete the coating process; the amount of polyester mixed powder used is 5% of the mass of the fertilizer granules; S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 55°C, and spray the coated fertilizer granules with a terminal isocyanate aqueous solution (the composition is the same as the terminal isocyanate aqueous solution in step S1) through a spraying device. After spraying, raise the temperature of the rotary drum coating machine to 130°C and keep it at the temperature for 5 minutes to react. After naturally cooling to 35°C, the material is discharged, screened to remove adhering clumps, and biodegradable slow-release fertilizer is obtained.

[0041] The mass ratio of the end-capped isocyanate aqueous solution used in step S1 to the end-capped isocyanate aqueous solution used in step S3 is 4:6. The sum of the mass of end-capped isocyanate contained in the end-capped isocyanate aqueous solution used in step S1 and the end-capped isocyanate aqueous solution used in step S3 is 5% of the mass of PBAT prepolymer powder contained in the polyester mixed powder.

[0042] Comparative Example The specific components of the fertilizer granules, capped isocyanate, and polyester mixed powder used in Comparative Examples 1-2 are shown in Table 2.

[0043] Table 2 The preparation process of the polyester mixed powder in Comparative Example 1 is as follows: PBAT prepolymer powder and calcium carbonate are taken in a mass ratio, then placed in a mixer and mixed at room temperature (about 25°C) for 5 minutes to obtain a uniformly mixed polyester mixed powder.

[0044] The polyester blend powder in Comparative Example 2 was made entirely of PBAT prepolymer powder, without the addition of any inorganic powder fillers.

[0045] The PBAT prepolymer powder and end-capped isocyanate used in the comparative example are the same as those in the above examples.

[0046] Comparative Example 1 This comparative example provides a method for preparing a slow-release fertilizer, including the following steps: S1: Pretreatment of fertilizer granules: Urea fertilizer granules are put into a rotary drum coating machine preheated to 50°C. The rotary drum coating machine rotates at 40 rpm. The end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. The terminated isocyanate aqueous solution is an aqueous solution of terminated HDI, wherein the mass percentage concentration of terminated HDI is 20%, and the terminated HDI aqueous solution contains stannous octoate catalyst, the amount of stannous octoate added is 0.3% of the mass of terminated HDI contained in the terminated HDI aqueous solution; S2: Coating the moistened fertilizer granules: Maintain the temperature of the rotary drum coating machine at 50℃, add polyester mixed powder into the rotary drum through the powder feeding device, and coat the surface of the moistened fertilizer granules with the polyester mixed powder to complete the coating process; the amount of polyester mixed powder used is 5% of the mass of the fertilizer granules; S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 50°C, and spray the coated fertilizer granules with a terminal isocyanate aqueous solution (the composition is the same as the terminal isocyanate aqueous solution in step S1) through a spraying device. After spraying, raise the temperature of the rotary drum coating machine to 90°C and keep it at the temperature for 10 minutes to react. After naturally cooling to 38°C, the material is discharged, screened to remove adhering clumps, and slow-release fertilizer is obtained.

[0047] The mass ratio of the end-capped isocyanate aqueous solution used in step S1 to the end-capped isocyanate aqueous solution used in step S3 is 3:7. The sum of the mass of end-capped isocyanate contained in the end-capped isocyanate aqueous solution used in step S1 and the end-capped isocyanate aqueous solution used in step S3 is 5% of the mass of PBAT prepolymer powder contained in the polyester mixed powder.

[0048] Comparative Example 2 This comparative example provides a method for preparing a slow-release fertilizer, including the following steps: S1: Pretreatment of fertilizer granules: Urea fertilizer granules are put into a rotary drum coating machine preheated to 50°C. The rotary drum coating machine rotates at 40 rpm. The end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. The terminated isocyanate aqueous solution is an aqueous solution of terminated HDI, wherein the mass percentage concentration of terminated HDI is 20%, and the terminated HDI aqueous solution contains stannous octoate catalyst, the amount of stannous octoate added is 0.3% of the mass of terminated HDI contained in the terminated HDI aqueous solution; S2: Coating the wetted fertilizer granules: Maintain the temperature of the rotary drum coating machine at 50℃, add PBAT prepolymer powder into the rotary drum through the powder feeding device, and coat the surface of the wetted fertilizer granules with the PBAT prepolymer powder to complete the coating process; the amount of PBAT prepolymer powder used is 5% of the mass of the fertilizer granules; S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 50°C, and spray the coated fertilizer granules with a terminal isocyanate aqueous solution (the composition is the same as the terminal isocyanate aqueous solution in step S1) through a spraying device. After spraying, raise the temperature of the rotary drum coating machine to 90°C and keep it at the temperature for 10 minutes to react. After naturally cooling to 38°C, the material is discharged, screened to remove adhering clumps, and slow-release fertilizer is obtained.

[0049] The mass ratio of the end-capped isocyanate aqueous solution used in step S1 to the end-capped isocyanate aqueous solution used in step S3 is 3:7, and the sum of the mass of end-capped isocyanate contained in the end-capped isocyanate aqueous solution used in step S1 and the end-capped isocyanate aqueous solution used in step S3 is 5% of the mass of PBAT prepolymer powder.

[0050] Performance testing 1. Coverage The coating conditions of the slow-release fertilizers prepared in the above embodiments and comparative examples were tested, and the details are shown in Table 3.

[0051] Table 3 As can be seen from the above, adding an appropriate amount of inorganic powder filler to polyester mixed powder can effectively prevent the adhesion between particles during the coating process and improve the coating effect.

[0052] 2. Thin film performance testing According to the mass of the end-capped isocyanate and polyester mixed powder used in the above examples and comparative examples, the end-capped isocyanate and polyester mixed powder were weighed respectively, and then mixed evenly to obtain the mixture of Example 1, Example 2, Example 3, Comparative Example 1, and Comparative Example 2 respectively. After spraying silicone oil onto a stainless steel tray, spread one of the above mixtures on top, then compact it with a flat plate and place it in an oven. After heating and cooling at the following experimental temperatures and times, a 25μm film sample was taken and subjected to tensile strength and elongation at break tests according to ISO 527 standard. The coating performance was compared by measuring tensile strength and elongation at break.

[0053] The experimental temperature and time for Example 1 were as follows: the product was processed in an oven at 90°C for 10 minutes. The experimental temperature and time for Example 2 were as follows: the product was processed in an oven at 100°C for 8 minutes. The experimental temperature and time for Example 3 were as follows: the product was processed in an oven at 130°C for 5 minutes. The experimental temperature and time for Comparative Example 1 were as follows: the product was treated in an oven at 90°C for 10 minutes. The experimental temperature and time for Comparative Example 2 were as follows: the sample was treated in an oven at 90°C for 10 minutes.

[0054] The tensile strength and elongation at break test results of each embodiment and comparative example are shown in Table 4.

[0055] Table 4 As can be seen from the above, when the amount of inorganic powder filler added to the polyester mixed powder is too high (Comparative Example 1), the resulting coating performance is poor and there is a risk of it being easily damaged.

[0056] 3. Decomposition rate test The slow-release fertilizers obtained from each example and comparative example were composted according to ASTM D5988-2003. The decomposition rate (organic component) of the coating was determined by testing the amount of carbon dioxide released. The decomposition rate results are shown in Table 5.

[0057] Table 5 As can be seen from Table 5, the biodegradability of the coating of the slow-release fertilizer in each embodiment meets the degradation requirement of ≥90%.

[0058] 3. Release rate test The water dissolution rate method is a commonly used laboratory method for evaluating the nutrient release characteristics of slow-release fertilizers. This method refers to the national standard GB / T 23348-2009. Slow-release fertilizer samples obtained from each example and comparative example were weighed and placed in purified water at a constant temperature (usually 25℃). The nutrient content in the water was measured periodically, and the cumulative release rate was calculated to obtain the initial release rate and nutrient release period parameters. The results are shown in Table 6.

[0059] Table 6 As can be seen from the table above, the initial dissolution rate (day 1) and differential dissolution rate (day 7) of the slow-release fertilizers in Examples 1-3 were significantly lower than those in Comparative Example 1, and the cumulative release rate after 28 days was all <45%, which meets the national standard. Comparative Example 1 had a high content of inorganic powder, resulting in poor coating performance and premature degradation and rapid release. The particles in Comparative Example 2 were not tested due to clumping.

[0060] The above description is merely an embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent transformations made based on the content of the present invention specification, or direct or indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A method for preparing a biodegradable slow-release fertilizer, characterized in that, Including the following steps: S1: Pretreatment of fertilizer granules: The fertilizer granules are put into a rotary drum coating machine preheated to 40℃-55℃, and the first end-capped isocyanate aqueous solution is sprayed onto the fertilizer granules through a spraying device to wet the surface of the fertilizer granules. S2: Coating the wetted fertilizer granules: Maintain the temperature of the rotary drum coating machine at 40℃-55℃, add polyester mixed powder into the rotary drum through the powder feeding device, and coat the surface of the wetted fertilizer granules with the polyester mixed powder to complete the coating process; wherein, the polyester mixed powder includes PBAT prepolymer powder and inorganic powder filler. S3: Crosslinking and curing treatment: Maintain the temperature of the rotary drum coating machine at 40℃-55℃, and spray the coated fertilizer granules with a second end-capped isocyanate aqueous solution through a spraying device; after spraying, raise the temperature of the rotary drum coating machine to 80℃-150℃ and keep it at the temperature for 5-10 minutes for reaction; then cool it to ≤40℃, discharge and screen to obtain biodegradable slow-release fertilizer.

2. The preparation method according to claim 1, characterized in that, The polyester mixed powder is composed of PBAT prepolymer powder and inorganic powder filler, and the mixing mass ratio of the PBAT prepolymer powder and the inorganic powder filler is (1-3):

1.

3. The preparation method according to claim 1, characterized in that, The PBAT prepolymer powder is obtained by crushing PBAT prepolymer and passing it through a 50-100 mesh sieve. The intrinsic viscosity of the PBAT prepolymer is 0.2-0.8 dL / g.

4. The preparation method according to claim 1, characterized in that, The inorganic powder filler is calcium carbonate and / or bentonite.

5. The preparation method according to claim 1, characterized in that, The amount of the polyester mixed powder is 4%-8% of the fertilizer granule mass.

6. The preparation method according to claim 1, characterized in that, The first and second end-capped isocyanate aqueous solutions are aqueous solutions including at least one of water-soluble end-capped HDI, IPDI, TDI, and MDI, and the mass percentage concentration of the first and second end-capped isocyanate aqueous solutions is 15%-25%. The first end-capped isocyanate aqueous solution and the second end-capped isocyanate aqueous solution may have the same or different compositions.

7. The preparation method according to claim 6, characterized in that, The first and second end-capped isocyanate aqueous solutions also contain a catalyst selected from at least one of stannous octoate, dibutyltin dilaurate, and di(dodecyl sulfide)dibutyltin; and / or, The amount of catalyst added in the first end-capped isocyanate aqueous solution is 0.1%-0.5% of the mass of the end-capped isocyanate contained in the first end-capped isocyanate aqueous solution; the amount of catalyst added in the second end-capped isocyanate aqueous solution is 0.1%-0.5% of the mass of the end-capped isocyanate contained in the second end-capped isocyanate aqueous solution.

8. The preparation method according to claim 1, characterized in that, The mass ratio of the first capped isocyanate aqueous solution to the second capped isocyanate aqueous solution is (3-5):(5-7).

9. The preparation method according to claim 1, characterized in that, The sum of the mass of the end-capped isocyanate contained in the first end-capped isocyanate aqueous solution and the mass of the end-capped isocyanate contained in the second end-capped isocyanate aqueous solution is 4%-6% of the mass of the PBAT prepolymer powder contained in the polyester mixed powder.

10. A biodegradable slow-release fertilizer prepared by any one of claims 1-9.

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