Preparation process of a bio-based polyurethane coated controlled-release fertilizer synthesized by in-situ polymerization

Abstract

The present application relates to the field of controlled release fertilizer preparation, and specifically discloses a preparation process of a bio-based polyurethane coated controlled release fertilizer synthesized by in-situ polymerization, wherein a modified component A is prepared by mixing a bio-based polyol and a nano limited template agent, a functional component B is prepared by pre-polymerizing an isocyanate and a bio-based chain extender, then the component A and the component B are simultaneously sprayed on the surface of fertilizer particles in a fluidized state through a double-channel atomizing nozzle, an in-situ polymerization reaction occurs to generate a bio-based polyurethane coating layer containing the nano limited template agent, and finally, the coated controlled release fertilizer is obtained through curing and screening; by introducing the nano limited template agent with a combination of micropores, mesopores and macropores, a multi-level pore network structure is constructed in the coating layer, and the precise regulation of nutrient release is realized by utilizing the synergistic effect of different pore sizes.

Description

Technical Field

[0001] This invention belongs to the field of controlled-release fertilizer preparation, specifically a preparation process for bio-based polyurethane-coated controlled-release fertilizer synthesized by in-situ polymerization. Background Technology

[0002] Coated controlled-release fertilizers are a novel type of fertilizer that synchronizes nutrient release patterns with crop absorption by coating fertilizer granules with functional membranes. This offers significant advantages such as improved fertilizer utilization and reduced environmental pollution. In-situ reaction film-forming processes have become an important technological direction for the preparation of coated controlled-release fertilizers due to their simple equipment, rapid production, and low energy consumption. Polyurethane materials are widely used in this field because of their excellent film-forming properties and controllable permeability. To reduce dependence on petroleum resources and endow materials with environmentally friendly properties, researchers have attempted to introduce biomass resources such as castor oil, soybean oil, and other plant oil polyols, or straw liquefaction polyols, into polyurethane systems to prepare bio-based polyurethane coating materials. However, existing bio-based polyurethane coating materials still face technical bottlenecks such as insufficient mechanical properties, low controlled-release precision, and the difficulty in balancing bio-based content with degradation performance.

[0003] In recent years, nanomaterials have demonstrated unique advantages in the field of polymer modification. Mesoporous molecular sieves, metal-organic frameworks, and other nano-confined template agents possess regular pore structures and high specific surface areas, enabling multiple effects on molecular transport, including sieving, adsorption, and controlled release. However, the systematic study of introducing nano-confined template agents into bio-based polyurethane coating systems and applying them to controlled-release fertilizer preparation is still lacking. In existing technologies, the controlled-release function of the coating layer mainly relies on the density and hydrophobicity of the polyurethane matrix itself, lacking a multi-level regulatory mechanism, resulting in a need to improve the matching degree between the nutrient release curve and the crop's nutrient requirements. Furthermore, in-situ reaction film formation processes often employ step-by-step spraying methods, which are prone to coating defects due to uneven spraying. Defective products are often directly scrapped, leading to resource waste and increased production costs, and effective repair technologies for coating defects are currently lacking. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a preparation process for synthesizing bio-based polyurethane-coated controlled-release fertilizer via in-situ polymerization. This process solves the problems of insufficient mechanical properties, low controlled-release precision, lack of multi-level regulation mechanisms, and low yield caused by the inability to repair defective products in existing bio-based polyurethane coating materials.

[0005] This invention provides a process for preparing bio-based polyurethane-coated controlled-release fertilizer by in-situ polymerization, comprising the following steps:

[0006] S1. Preparation of modified bio-based polyol component A:

[0007] Bio-based polyols and nano-confined templates are mixed at a mass ratio of 100:1 to 100:10 and stirred at 60 to 100°C for 1 to 3 hours to obtain nano-confined modified bio-based polyol component A.

[0008] The nano-confined template agent is at least one of mesoporous molecular sieve, mesoporous silica, and metal-organic framework materials.

[0009] S2. Preparation of functional isocyanate component B:

[0010] Isocyanate and bio-based chain extender are mixed at a mass ratio of 100:5 to 100:30 and reacted at 40 to 60°C for 0.5 to 2 hours to obtain prepolymerized functional isocyanate component B.

[0011] The bio-based chain extender is selected from at least one of castor oil, dimer glycol, and bio-based diol;

[0012] S3. Preheat the fertilizer granules to 60-90℃, place them in the fluidized bed coating equipment, and adjust the fluidizing air velocity to make the fertilizer granules fully fluidized.

[0013] S4. Simultaneously spray component A from step S1 and component B from step S2 onto the surface of fluidized fertilizer granules through a dual-channel atomizing nozzle, controlling the mass flow ratio of component A to component B to be 1:0.6 to 1:1.8, the atomization pressure to be 0.3 to 0.8 MPa, and the spraying time to be 5 to 20 min.

[0014] Component A and component B undergo an in-situ polymerization reaction on the surface of fertilizer particles to generate a bio-based polyurethane coating layer containing a nano-confined template agent.

[0015] S5. After spraying, continue fluidization and curing at 50-70℃ for 10-30 minutes, then cool to room temperature and sieve to obtain bio-based polyurethane-coated controlled-release fertilizer.

[0016] Preferably, the nano-confined template agent in step S1 is a mixture of two or more nano-confined template agents with different pore sizes;

[0017] The pore sizes of the nano-confined template agents with different pore sizes are 2-10 nm, 10-30 nm and 30-50 nm, respectively, and the mass ratio of the three is 1:0.5-1.5:0.2-1.0.

[0018] Preferably, the bio-based polyol in step S1 is selected from at least one of vegetable oil polyols, biomass liquefaction polyols, recycled polyurethane alcoholysis polyols, and carbon dioxide-based polyols.

[0019] The plant oil polyol is a polyol obtained by ring-opening reaction of epoxidized plant oil;

[0020] The biomass liquefaction polyol is a polyol obtained by liquefying lignocellulose biomass under the action of a liquefying agent.

[0021] The recovered polyurethane alcoholysis polyol is a polyol obtained by alcoholysis recovery of waste polyurethane foam;

[0022] The carbon dioxide-based polyol is a polyol obtained by copolymerization of carbon dioxide and propylene oxide.

[0023] Preferably, the pore size of the nano-confined template agent in step S1 is 2-50 nm, and the specific surface area is 200-1000 m² / g;

[0024] The mesoporous molecules are selected from at least one of SBA-15, MCM-41, and KIT-6;

[0025] The metal-organic framework material is selected from at least one of MIL-101, UiO-66, and ZIF-8.

[0026] Preferably, the isocyanate in step S2 is a mixture of bio-based isocyanate and petroleum-based isocyanate, wherein the mass percentage of bio-based isocyanate is 20% to 80%.

[0027] The bio-based isocyanate is selected from at least one of pentamethylene diisocyanate, hexamethylene diisocyanate, L-lysine diisocyanate, and dimer diisocyanate;

[0028] The petroleum-based isocyanate is selected from at least one of polymethylene polyphenyl isocyanate, diphenylmethane diisocyanate, and toluene diisocyanate.

[0029] Preferably, the number-average molecular weight of the bio-based chain extender in step S2 is 200-2000, and the hydroxyl value is 50-300 mg KOH / g;

[0030] The bio-based diol is selected from at least one of 1,3-propanediol, 1,4-butanediol, and bio-based ethylene glycol.

[0031] Preferably, the fertilizer granules in step S3 are at least one of urea, diammonium phosphate, potassium chloride, potassium sulfate, and compound fertilizer, with a particle size of 1-5 mm.

[0032] The fluidized bed coating equipment is a bottom-spray fluidized bed or a tangential-spray fluidized bed.

[0033] Preferably, the spray angle of the dual-channel atomizing nozzle in step S4 is 30° to 60°, and the atomized droplet size is 20 to 100 μm;

[0034] During the spraying process, the surface temperature of the fertilizer particles is maintained at 50-70°C by adjusting the fluidizing air velocity and spraying rate.

[0035] Preferably, the bio-based polyurethane coating layer accounts for 1.5% to 6% of the total mass of the coated controlled-release fertilizer;

[0036] The nutrient release period of the coated controlled-release fertilizer in still water at 25°C is 40–300 days.

[0037] Preferably, step S5 is followed by a repair process:

[0038] The coated controlled-release fertilizer with coating defects after screening is placed back into a fluidized bed and heated to 80-120°C to slightly melt the surface of the coating layer. Then, component A or component B is sprayed on to repair the coating defects using the secondary reactivity of polyurethane. After cooling, the repaired coated controlled-release fertilizer is obtained.

[0039] Compared with the prior art, the present invention has the following beneficial effects:

[0040] By introducing nano-confined template agents into a bio-based polyol system, and then scientifically compounding nano-template agents of three pore sizes (microporous, mesoporous, and macroporous) to form a multi-level pore network structure, the synergistic effect of different pore sizes is fully utilized. At the process level, bio-based chain extenders are used to prepolymerize and modify isocyanates, effectively controlling reactivity and increasing the bio-based content of the system. Simultaneously, modified polyol components and functional isocyanate components are sprayed onto the surface of fluidized fertilizer granules through a dual-channel atomizing nozzle, achieving simultaneous in-situ polymerization and uniform film formation. This preparation process also innovatively introduces a defect repair step, utilizing the secondary reactivity of polyurethane to repair products with coating defects, significantly improving yield and process stability.

[0041] By introducing nano-confined template agents and employing a multi-pore composite design, a multi-level regulatory mechanism is established within the coating layer: micropores provide molecular sieving effects, mesopores generate capillary coagulation effects, and macropores construct diffusion channels. This enables precise control of the nutrient release process. Simultaneously, the nano-confined template agents are uniformly dispersed within the polyurethane matrix, forming physical cross-linking points that significantly enhance the mechanical strength and structural integrity of the coating layer. The dual-channel simultaneous spraying process ensures the uniformity and density of the coating layer, avoiding coating defects that may occur with traditional step-by-step spraying. The defect repair process allows for the regeneration of flawed products, significantly reducing production losses. Furthermore, the use of renewable raw materials such as bio-based polyols and bio-based chain extenders endows the coating layer with excellent biodegradability. After completing its nutrient control function, it can gradually degrade in the soil environment, effectively reducing plastic residue and combining excellent performance with environmental friendliness. Attached Figure Description

[0042] Figure 1 This is a schematic diagram of the preparation process described in Embodiment 1 of the present invention. Detailed Implementation

[0043] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below with reference to specific embodiments. Obviously, the described embodiments are merely some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0044] Unless otherwise specified, all raw materials used in the embodiments of this invention are commercially available.

[0045] I. Raw Materials and Testing Methods

[0046] 1.1 Raw material sources and pretreatment

[0047] Raw material name Specifications / Model Source / Preparation Method Castor oil polyols Hydroxyl value 160 mg KOH / g, industrial grade Market Purchase Soybean oil polyols Hydroxyl value 280 mg KOH / g, industrial grade Market Purchase Straw liquefaction polyols Hydroxyl value 350mg KOH / g Lab-made (corn stalk liquefaction) Recycling polyurethane alcoholysis polyols Hydroxyl value 220 mg KOH / g Laboratory-made (alcoholic hydrolysis of waste polyurethane foam) Carbon dioxide-based polyols Hydroxyl value 56 mg KOH / g, industrial grade Market Purchase SBA-15 mesoporous molecular sieve Pore ​​size 8nm, specific surface area 650m² / g Market Purchase MCM-41 mesoporous molecular sieve Pore ​​size 3.5nm, specific surface area 950m² / g Market Purchase KIT-6 Mesoporous Molecular Sieves Pore ​​size 6nm, specific surface area 700m² / g Market Purchase MIL-101(Cr) Pore ​​size 3.0 nm, specific surface area 2800 m² / g Market Purchase UiO-66 Pore ​​size 1.2 nm, specific surface area 1200 m² / g Market Purchase ZIF-8 Pore ​​size 1.1 nm, specific surface area 1600 m² / g Market Purchase Pentamethylene diisocyanate Industrial grade Market Purchase Hexamethylene diisocyanate Industrial grade Market Purchase L-Lysine diisocyanate Industrial grade Market Purchase Dimeric acid diisocyanate Industrial grade Market Purchase Polymethylene polyphenyl isocyanate PM-200, Industrial Grade Wanhua Chemical Diphenylmethane diisocyanate MDI-50, Industrial Grade Wanhua Chemical Toluene diisocyanate TDI-80, Industrial Grade Market Purchase Castor oil (chain extender) Hydroxyl value 160 mg KOH / g Market Purchase Dimeric acid glycol Hydroxyl value 110 mg KOH / g Market Purchase 1,3-Propanediol Industrial grade Market Purchase 1,4-Butanediol Industrial grade Market Purchase Bio-based ethylene glycol Industrial grade Market Purchase Urea granules Particle size 2-4mm, industrial grade Market Purchase diammonium phosphate granules Particle size 2-4mm, industrial grade Market Purchase Potassium chloride granules Particle size 2-4mm, industrial grade Market Purchase Potassium sulfate granules Particle size 2-4mm, industrial grade Market Purchase Compound fertilizer granules <![CDATA[Particle size 2 - 4 mm, N-P2O5-K2O = 15-15-15]]> Market Purchase

[0048] 1.2 Performance Testing Methods

[0049] (1) Nutrient release rate test

[0050] The water immersion method was used: 10g of coated controlled-release fertilizer sample was accurately weighed, placed in a 100-mesh nylon mesh bag, sealed, and then placed in a glass container containing 250mL of deionized water. The container was then sealed and placed in a 25℃ constant temperature incubator. Samples were taken on days 1, 3, 5, 7, 14, 21, 28, 35, 42, 49, 56, 70, 84, 98, 112, 126, 140, 154, 168, 182, 196, 210, 224, 238, 252, 266, 280, 294, and 300. The nitrogen content in the leachate was determined using the Kjeldahl method or spectrophotometry, and the cumulative nutrient release rate was calculated. The nutrient release period was defined as the number of days required for the cumulative release rate to reach 80%.

[0051] (2) Characterization of the microstructure of the coating layer

[0052] The surface and cross-sectional morphology of the coating layer were observed using scanning electron microscopy (SEM) with an accelerating voltage of 5 kV and gold sputtering treatment. The distribution of the nano-confined template agent in the polyurethane matrix was observed using transmission electron microscopy (TEM) with an accelerating voltage of 200 kV and ultrathin sectioning treatment. The pore size distribution and specific surface area of ​​the coating layer were determined using a nitrogen adsorption-desorption analyzer. The sample was degassed at 100℃ for 6 h before testing.

[0053] (3) Mechanical property testing of the coating layer

[0054] Thirty coated controlled-release fertilizer granules were randomly selected and their crushing resistance was measured sequentially using an intelligent granule strength tester (KQ-3 type). The average value was taken as the compressive strength. Another 50 coated fertilizer granules were placed in a 250mL conical flask, capped, and placed in a reciprocating shaker. The flask was shaken at a frequency of 150 times / minute for 10 minutes. The flask was then removed and the coating layer was observed to see if it was broken. The breakage rate was calculated.

[0055] Fragmentation rate (%) = (Number of intact particles before oscillation - Number of intact particles after oscillation) / Number of intact particles before oscillation × 100%

[0056] (4) Evaluation of defect repair effect

[0057] The repaired coated controlled-release fertilizer samples were placed under a stereomicroscope (Olympus SZX16) to observe the repair of surface defects in the coating layer, with a magnification of 20 to 50 times. At the same time, the nutrient release curve of the repaired samples was determined by the water immersion method and compared with that of normal samples.

[0058] (5) Biodegradation rate test

[0059] According to the method of GB / T 19277.1-2011, the coated controlled-release fertilizer sample was buried in the soil and cultured for 180 days at a temperature of 25℃ and a humidity of 60%. Every 30 days, a portion of the sample was taken out, cleaned, dried, and weighed. The mass loss rate was calculated as the biodegradation rate.

[0060] The calculation formula is: weight loss rate (%) = (original weight - weight after degradation) / original weight × 100%.

[0061] II. Implementation Examples

[0062] Example 1: This example provides a preparation process for in-situ polymerization of bio-based polyurethane-coated controlled-release fertilizer, such as... Figure 1 As shown, it includes the following steps:

[0063] S1. Preparation of modified bio-based polyol component A

[0064] Take 1000g of castor oil polyol (hydroxyl value 160mg KOH / g) and add 30g of SBA-15 mesoporous molecular sieve (pore size 8nm, specific surface area 650m² / g) (mass ratio 100:3). Stir at 80℃ for 2h to fully mix the polyol and molecular sieve to obtain nano-confined modified bio-based polyol component A1.

[0065] S2, Preparation of functional isocyanate component B

[0066] 600g of pentamethylene diisocyanate (bio-based) and 400g of polymethylene polyphenyl isocyanate (PM-200) were mixed to obtain an isocyanate mixture (bio-based isocyanate accounted for 60% by mass). 1000g of the above isocyanate mixture was taken and 100g of castor oil (hydroxyl value 160mg KOH / g, used as a chain extender) was added (mass ratio 100:10). The mixture was reacted at 50℃ for 1h to obtain prepolymerized modified functional isocyanate component B1.

[0067] S3, Fertilizer Preheating

[0068] Take 5000g of urea granules with a particle size of 2-4mm and place them in a bottom spray fluidized bed coating equipment (model: Glatt GPCG-2). Preheat the equipment to 75℃ and adjust the fluidizing air velocity (about 2.5m / s) to make the urea granules fully fluidized.

[0069] S4, in-situ polymerization coating

[0070] Component A1 from step S1 and component B1 from step S2 are simultaneously sprayed onto the surface of fluidized urea particles using a dual-channel atomizing nozzle. The mass flow ratio of component A1 to component B1 is controlled at 1:1.2, the atomization pressure is 0.5 MPa, and the spraying time is 12 min. During the spraying process, the surface temperature of the urea particles is maintained at 60–65 °C by adjusting the fluidizing air velocity and spraying rate. Component A1 and component B1 undergo an in-situ polymerization reaction on the surface of the urea particles to generate a bio-based polyurethane coating layer containing SBA-15 mesoporous molecular sieve.

[0071] S5, Curing and Post-treatment

[0072] After spraying, the mixture was allowed to continue fluidization and curing at 60℃ for 20 minutes, then cooled to room temperature and sieved (20 mesh sieve) to obtain bio-based polyurethane coated controlled-release fertilizer sample S1; the mass of the coating layer accounted for 3.8% of the total mass of the coated controlled-release fertilizer.

[0073] Example 2: The difference between this example and Example 1 is that a mixture of two or more nano-confined template agents with different pore sizes is used in step S1, as detailed below:

[0074] S1. Preparation of modified bio-based polyol component A

[0075] Take 1000g of castor oil polyol (hydroxyl value 160mg KOH / g), add 35g of nano-confined template agent mixture (mass ratio 100:3.5), stir at 80℃ for 2h to obtain nano-confined modified bio-based polyol component A2.

[0076] The nano-confined template agent mixture consists of three mesoporous molecular sieves with different pore sizes:

[0077] Microporous component: MCM-41 (pore size 3.5nm) 15g;

[0078] Mesoporous component: SBA-15 (pore size 8nm) 12g;

[0079] Macroporous component: KIT-6 (pore size 6nm, expanded to 35nm) 8g;

[0080] The mass ratio of the three components is 1.5:1.2:0.8 (close to 1:0.8:0.5).

[0081] The remaining steps are the same as in Example 1, and the coated controlled-release fertilizer sample S2 is obtained; it is determined that the mass of the coating layer accounts for 4.1% of the total mass of the coated controlled-release fertilizer.

[0082] Example 3: The difference between this example and Example 2 is that the bio-based polyol used in step S1 is straw liquefaction polyol, as detailed below:

[0083] S1. Preparation of modified bio-based polyol component A

[0084] Take 1000g of straw liquefied polyol (hydroxyl value 350mg KOH / g, obtained by liquefaction reaction of corn straw), add 40g of nano-confined template agent mixture (mass ratio 100:4), stir at 70℃ for 2.5h to obtain nano-confined modified bio-based polyol component A3.

[0085] The nano-confined template agent mixture is composed of three materials with different pore sizes:

[0086] Microporous component: MIL-101(Cr) (pore size 3.0nm) 10g;

[0087] Mesoporous component: SBA-15 (pore size 8nm) 15g;

[0088] Macroporous component: 15g of mesoporous silica (pore size 45nm);

[0089] The mass ratio of the three components is 1:1.5:1.5 (close to 1:1.5:1.5).

[0090] The remaining steps are the same as in Example 1, and the coated controlled-release fertilizer sample S3 is obtained; it is determined that the mass of the coating layer accounts for 4.5% of the total mass of the coated controlled-release fertilizer.

[0091] Example 4: The difference between this example and Example 2 is that the isocyanate used in step S2 is a mixture with different bio-based contents, as detailed below:

[0092] S2, Preparation of functional isocyanate component B

[0093] Take 800g of L-lysine diisocyanate (bio-based) and 200g of diphenylmethane diisocyanate (MDI-50) and mix them to obtain an isocyanate mixture (bio-based isocyanate accounts for 80% by mass); take 1000g of the above isocyanate mixture and add 150g of dimer glycol (hydroxyl value 110mg KOH / g) (mass ratio 100:15), and react at 45℃ for 1.5h to obtain prepolymerized modified functional isocyanate component B4.

[0094] The remaining steps are the same as in Example 2, and the coated controlled-release fertilizer sample S4 is obtained; it is determined that the mass of the coating layer accounts for 4.3% of the total mass of the coated controlled-release fertilizer.

[0095] Example 5: The difference between this example and Example 2 is that the fertilizer granules used in step S3 are compound fertilizers, as detailed below:

[0096] S3, Fertilizer Preheating

[0097] Take compound fertilizer granules with a particle size of 2-4 mm ( 5000g of compound fertilizer granules were placed in a tangential spray fluidized bed coating equipment (model: Aeromatic Fielder MP-1), preheated to 80℃, and the fluidization air speed was adjusted to make the compound fertilizer granules fully fluidized.

[0098] The remaining steps are the same as in Example 2, and the coated controlled-release fertilizer sample S5 is obtained; it is determined that the mass of the coating layer accounts for 4.2% of the total mass of the coated controlled-release fertilizer.

[0099] Example 6: The difference between this example and Example 2 lies in the adjustment of the spraying parameters in step S4, as detailed below:

[0100] S4, in-situ polymerization coating

[0101] Component A2 from step S1 and component B1 from step S2 are simultaneously sprayed onto the surface of fluidized urea particles through a dual-channel atomizing nozzle. The mass flow ratio of component A2 to component B1 is controlled at 1:0.8, the atomization pressure is 0.6 MPa, and the spraying time is 15 min. During the spraying process, the surface temperature of the urea particles is maintained at 55-60℃.

[0102] The remaining steps are the same as in Example 2, and the coated controlled-release fertilizer sample S6 is obtained; it is determined that the mass of the coating layer accounts for 3.9% of the total mass of the coated controlled-release fertilizer.

[0103] Example 7: This example provides a repair process for coating defects, as detailed below:

[0104] Take 1000g of the coated controlled-release fertilizer sample with coating defects (obvious cracks or incomplete coating) that was screened out during the preparation process of Example 2, place it back in the bottom spray fluidized bed, heat it to 100°C, and make the surface of the coating layer slightly melt; then spray a small amount of component A2 (about 30g) according to the same spraying parameters as in Example 2, and use the secondary reactivity of polyurethane to repair the coating defects; continue fluidizing for 10min after spraying, and obtain the repaired coated controlled-release fertilizer sample S7 after cooling.

[0105] Comparative Example 1: The difference between this comparative example and Example 1 is that no nano-confined template agent is added in step S1, that is, unmodified castor oil polyol is used directly as component A. The remaining steps are the same as in Example 1, and the coated controlled-release fertilizer sample D1 is obtained.

[0106] Comparative Example 2: The difference between this comparative example and Example 1 is that no bio-based chain extender is added in step S2 for prepolymerization modification. That is, the isocyanate mixture (pentamethylene diisocyanate: PM-200=6:4) is used directly as component B. The remaining steps are the same as in Example 1, and the coated controlled-release fertilizer sample D2 is obtained.

[0107] Comparative Example 3: The difference between this comparative example and Example 1 is that a single-channel nozzle is used for step-by-step spraying in step S4. That is, component A1 is sprayed first, dried for 5 minutes, and then component B1 is sprayed. The remaining steps are the same as in Example 1, and the coated controlled-release fertilizer sample D3 is obtained.

[0108] Comparative Example 4: The difference between this comparative example and Example 2 is that only 35g of SBA-15 (8nm pore size) with a single pore size is used in step S1, and multi-pore size compound is not used. The remaining steps are the same as in Example 2, and the coated controlled-release fertilizer sample D4 is obtained.

[0109] III. Performance Test Results

[0110] 3.1 Characterization of the microstructure of the coating layer

[0111] SEM and TEM observations were performed on the coating layer of the coated controlled-release fertilizer sample S2 prepared in Example 2. The results showed that:

[0112] SEM observation: The coating layer has a smooth and dense surface with no obvious cracks or pores, and its thickness is approximately 40–60 μm. The cross-section of the coating layer shows a multi-layer structure, with nano-templators of different pore sizes uniformly dispersed in the polyurethane matrix.

[0113] TEM observation: The three nanotemplators with different pore sizes showed a clear distribution in the polyurethane matrix. The microporous MCM-41 (about 3.5 nm), mesoporous SBA-15 (about 8 nm), and macroporous KIT-6 (about 35 nm) each maintained a complete pore structure, forming a multi-level pore network.

[0114] Nitrogen adsorption-desorption test: The pore size distribution of the coating layer exhibits a three-peak distribution, with peaks located at 3.2–3.8 nm, 7.5–8.5 nm, and 32–38 nm, respectively, which is basically consistent with the pore sizes of the three added nanotemplators. The specific surface area of ​​the coating layer is 185 m² / g, and the porosity is 12.5%.

[0115] Comparative Example 1 (without nanotemplative agent) has a homogeneous coating layer D1 with no nanopores and a specific surface area of ​​only 2.3 m² / g. Comparative Example 4 (single pore size) has a coating layer D4 with a single-peak pore size distribution (peak value 8.2 nm), a specific surface area of ​​76 m² / g, and a porosity of 5.8%.

[0116] The results show that the present invention successfully constructed a multi-level porous network structure in the polyurethane coating layer by combining multi-pore nano-confined template agents.

[0117] 3.2 Mechanical property test results

[0118] Sample number Compressive strength (N) Fragmentation rate after oscillation (%) S1 (Example 1) 7.2 2.3 S2 (Example 2, Multi-porous compound) 8.5 1.2 S3 (Example 3) 8.7 1.0 S4 (Example 4) 8.3 1.3 S5 (Example 5) 8.1 1.5 S6 (Example 6) 8.4 1.1 S7 (after repair in Example 7) 8.2 1.4 D1 (Comparative Example 1, without template agent) 5.4 8.5 D2 (Comparative Example 2, without prepolymer modification) 6.1 6.7 D3 (Comparative Example 3, step-by-step spraying) 5.8 12.3 D4 (Comparative Example 4, Single Aperture) 7.0 2.8

[0119] Results analysis:

[0120] The compressive strength (8.1-8.7 N) of Examples 2-6 (including multi-pore composites) was significantly higher than that of Example 1 (single pore size, 7.2 N) and Comparative Example 4 (single pore size, 7.0 N), indicating that multi-pore composites can produce a synergistic enhancement effect.

[0121] Comparative Example 1 (without template agent) had the lowest compressive strength (5.4 N), demonstrating that the introduction of nano-confined template agent is crucial to mechanical properties.

[0122] Comparative Example 3 (step-by-step spraying) had the highest breakage rate (12.3%), demonstrating that simultaneous dual-channel spraying can significantly improve coating uniformity.

[0123] 3.3 Nutrient release performance test results

[0124] Nutrient release rate tests were conducted on the coated controlled-release fertilizer samples prepared in Examples 1-7 and Comparative Examples 1-4 in still water at 25°C. The results are as follows:

[0125] Sample number Initial dissolution rate (%, 24h) 28-day cumulative release rate (%) 80% release period (days) S1 (Example 1) 0.9 32.5 168 S2 (Example 2, Multi-porous compound) 0.3 18.7 252 S3 (Example 3) 0.4 20.3 238 S4 (Example 4) 0.5 21.5 224 S5 (Example 5) 0.6 22.8 210 S6 (Example 6) 0.4 19.6 245 S7 (after repair in Example 7) 0.5 20.1 240 D1 (Comparative Example 1) 3.8 58.4 63 D2 (Comparative Example 2) 2.5 52.7 78 D3 (Comparative Example 3) 4.2 48.9 91 D4 (Comparative Example 4, Single Aperture) 1.2 35.6 154

[0126] Results analysis:

[0127] The multi-pore size combination showed significant effects: the 28-day cumulative release rate (18.7%) of Example 2 (multi-pore size combination) was much lower than that of Example 1 (single pore size, 32.5%) and Comparative Example 4 (single pore size, 35.6%), and the release period was extended from 168 days to 252 days, an improvement of 50%. This indicates that the multi-pore network formed by the multi-pore size combination has a significant synergistic controlled release effect.

[0128] Reduced initial dissolution rate: The initial dissolution rate of Example 2 (0.3%) was much lower than that of Comparative Example 1 (3.8%), demonstrating that the nano-confined template agent can effectively adsorb nutrients that are initially dissolved, reducing the risk of nutrient burst release.

[0129] Release curve analysis: Fitting analysis of the release curve of Example 2 revealed that its release process exhibited obvious three-stage characteristics:

[0130] Phase 1 (0-30 days): Slow release phase, micropores (2-10nm) provide a molecular sieving effect, limiting the entry of large water molecules;

[0131] The second stage (30-150 days): the medium-speed release stage, where mesopores (10-30nm) provide a capillary coagulation effect to regulate the water permeation rate;

[0132] The third stage (150-252 days): the stable release stage, where macropores (30-50nm) provide diffusion channels to maintain stable release;

[0133] Comparative analysis:

[0134] Comparative Example 1 (without template agent): the fastest release, with a release rate of 58.4% after 28 days, proving that nutrients are difficult to control without nano-confined structures;

[0135] Comparative Example 2 (without prepolymer modification): The release was faster, proving that prepolymer modification can improve the compactness of the coating layer;

[0136] Comparative Example 3 (step-by-step spraying): faster release and higher breakage rate, demonstrating the importance of simultaneous spraying;

[0137] Comparative Example 4 (single pore size): release period of 154 days, significantly shorter than the 252 days of multi-pore size compound, proving the necessity of multi-pore size compound;

[0138] 3.4 Analysis of the synergistic effect mechanism of multi-porous composites

[0139] To further verify the synergistic effect of multi-porous composites, the following comparative experiment was designed:

[0140] sample Nanotemplator composition 28-day release rate (%) Release period (days) Experiment A Micropores only (MCM-41, 3.5nm) 28.7 182 Experiment B Mesoporous only (SBA-15, 8nm) 32.5 168 Experiment C Large pore size only (KIT-6 expanded pore size, 35nm) 38.2 140 Experiment D Micropores + mesopores (1:1) 23.5 210 Experiment E Mesopores + Macropores (1:1) 26.8 196 Experiment F Micropores + macropores (1:1) 25.3 203 Experiment G Micropores + mesopores + macropores (1:0.8:0.5) 18.7 252

[0141] The results show that:

[0142] With a single pore size, the release period is between 140 and 182 days;

[0143] When two pore sizes are combined, the release period is extended to 196–210 days, producing an additive effect.

[0144] When the three pore sizes are combined, the release period is extended to 252 days, resulting in a synergistic effect.

[0145] Mechanism analysis:

[0146] Micropores (2-10nm): The pore size is several times larger than the dynamic diameter of water molecules (about 0.3nm), which produces a molecular sieving effect, restricting the passage of large molecules. At the same time, water molecules are adsorbed through hydrogen bonding, delaying the entry of water.

[0147] Mesopores (10-30 nm): produce capillary condensation effect, form menisci in the pores, reduce water vapor pressure, and slow down moisture transport;

[0148] Macropores (30-50nm): provide the main channel for nutrient diffusion, and at the same time act as a "buffer layer" to regulate the release rate;

[0149] Multi-level pore network: Three types of pores are interconnected to form a "maze-like" diffusion path, which extends the nutrient transport path and achieves gradient release;

[0150] 3.5 Evaluation of Defect Repair Effectiveness

[0151] A comparison was made between the coated controlled-release fertilizer samples before and after the repair treatment in Example 7:

[0152] Test Project Before repair (defective product) After repair (S7) Normal sample (S2) Surface condition Obvious cracks and holes Defects have largely disappeared, and the surface is intact. Smooth and intact surface Initial dissolution rate (%) after 24 hours 15.6 0.5 0.3 28-day cumulative release rate (%) 76.8 20.1 18.7 80% release period (days) 18 240 252 Compressive strength (N) 4.3 8.2 8.5

[0153] The results show that:

[0154] Before repair, the defective product had extremely poor release performance (15.6% release rate in 24 hours), and basically lost its controlled release function;

[0155] The release performance of the remediated sample was similar to that of the normal sample S2, with a difference of only 1.4 percentage points in the 28-day release rate;

[0156] The compressive strength of the repaired sample recovered to more than 96% of that of the normal sample;

[0157] This invention demonstrates that the repair process provided by the present invention can effectively repair coating defects, transform defective products into qualified products, and significantly improve the yield.

[0158] 3.6 Biodegradability Test

[0159] A 180-day soil burial degradation test was conducted on the coated controlled-release fertilizer sample S2 prepared in Example 2. The results are as follows:

[0160] Degradation time (days) Quality loss rate (%) Surface state of the coating layer 30 1.8 The surface is smooth and shows no obvious changes. 60 3.5 Slightly rough surface 90 5.7 Tiny holes appeared 120 8.2 The number of pores increased, and some areas became thinner. 150 11.3 The coating layer began to crack. 180 15.6 The coating layer has largely degraded, exposing the fertilizer core.

[0161] The results show that the bio-based polyurethane coating prepared by this invention has good biodegradability in soil, with a degradation rate of 15.6% after 180 days, which can effectively reduce plastic residues in the soil; at the same time, the degradation latency is more than 60 days, ensuring that the coating remains intact before the nutrient release is complete.

[0162] IV. Process Parameter Optimization Experiment

[0163] 4.1 Effect of Nano-Confinement Template Addition Amount on Controlled-Release Performance

[0164] Based on Example 2, the amount (mass ratio) of the nano-confined template agent mixture was varied to investigate its effect on the release performance of the coated controlled-release fertilizer. The results are as follows:

[0165] Amount added (by weight) 28-day cumulative release rate (%) 80% release period (days) Compressive strength (N) 100:0 (Comparative Example 1) 58.4 63 5.4 100:1 35.2 154 6.8 100:3 24.5 210 7.9 100:5 18.7 252 8.5 100:8 16.3 273 8.7 100:10 14.8 285 8.8 100:12 14.2 291 8.8 (Slight reunion possible)

[0166] The results showed that the controlled release performance and mechanical properties improved with the increase of the amount of nano-confined template agent. However, when the amount of the template agent exceeded 100:10, the performance improvement tended to be gradual, and excessively high amounts of the template agent may lead to agglomeration. Considering both performance and cost, the preferred amount of the template agent is 100:1 to 100:10.

[0167] 4.2 Effect of Multi-pore Size Blending Ratio on Controlled-Release Performance

[0168] Based on Example 2, with the total amount of nano-confined template agent fixed at 100:5, the mass ratio of the three pore size template agents was varied to investigate its effect on controlled-release performance:

[0169] Experiment number Micropore:Mesopore:Macropore mass ratio 28-day release rate (%) Release curve characteristics 1 ratio 1:0:0 (micropores only) 28.7 Slow in the early stages, faster in the later stages 2 ratio 0:1:0 (Mesoporous only) 32.5 Release Stable 3 ratio 0:0:1 (large hole only) 38.2 Fast in the early stages, slow in the later stages 4 ratio 1:1:0 23.5 Two-stage release 5 ratio 1:0:1 25.3 Uneven release 6 ratio 0:1:1 26.8 The release was relatively stable. 7 1:0.8:0.5 18.7 Three-stage release, stable and sustained. 8 1:1.5:1.0 19.2 Similar ratio 7 9 0.5:1:0.5 22.3 Releases quickly 10 1.5:1:1.5 20.5 Too slow at the beginning, normal at the end

[0170] The results showed that the ratio of the three pore sizes had a significant impact on the release performance. The best multi-stage controlled release effect could be obtained when the mass ratio of the three was in the range of 1:0.5 to 1.5:0.2 to 1.0, while the release performance decreased when it exceeded this range.

[0171] 4.3 Effect of the mass flow ratio of component A to component B on controlled-release performance

[0172] Based on Example 2, the mass flow ratio of component A to component B was varied to investigate its effect on coating quality and controlled-release performance.

[0173] Mass flow rate ratio (A:B) Encapsulation layer integrity 28-day cumulative release rate (%) Fragmentation rate (%) 1:0.4 The membrane layer is thin and incomplete. 52.3 15.6 1:0.6 The coating layer is relatively intact 28.7 5.2 1:0.8 intact coating layer 22.5 2.3 1:1.0 intact coating layer 20.1 1.5 1:1.2 intact coating layer 18.7 1.2 1:1.5 intact coating layer 19.3 1.3 1:1.8 intact coating layer 21.5 1.8 1:2.0 The coating layer is too thick, causing partial adhesion. 18.2 8.5 (Particle adhesion)

[0174] The results showed that a complete coating layer and good controlled-release performance could be obtained within a mass flow ratio range of 1:0.6 to 1:1.8. Too low an A:B ratio resulted in an incomplete coating layer, while too high an A:B ratio could lead to particle adhesion.

[0175] 4.4 The Influence of Treatment Temperature on Repair Results

[0176] Based on Example 7, the effect of varying the repair treatment temperature on the repair outcome was investigated.

[0177] Repair temperature (°C) Surface condition after repair Release rate (%) 28 days after repair Compressive strength (N) 60 The defects did not change significantly. 68.5 5.1 70 Improvement of some defects 52.3 6.2 80 Most defects repair 28.7 7.5 90 Basic Defect Repair 22.3 8.0 100 Defects fully repaired 20.1 8.2 110 Defects fully repaired 19.8 8.1 120 Defects fully repaired 19.5 8.0 130 Over-melting and deformation of the coating layer 18.2 (Particle Deformation) 7.2

[0178] The results show that a good repair effect can be obtained in the range of 80 to 120℃, with 90 to 110℃ being preferred; if the temperature is too low (<80℃), the surface of the coating layer cannot be fully micro-melted, resulting in poor repair effect; if the temperature is too high (>130℃), the coating layer will be over-melted and deformed, affecting the particle morphology.

[0179] The embodiments of the present invention are given for the purposes of illustration and description. Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Any changes, modifications, substitutions and variations made by those skilled in the art to the above embodiments within the scope of the present invention should be included within the protection scope of the present invention.

Claims

1. A preparation process for an in-situ polymerization method to synthesize bio-based polyurethane-coated controlled-release fertilizer, characterized in that, Includes the following steps: S1. Preparation of modified bio-based polyol component A: Bio-based polyols and nano-confined templates are mixed at a mass ratio of 100:1 to 100:10 and stirred at 60 to 100°C for 1 to 3 hours to obtain nano-confined modified bio-based polyol component A. The nano-confined template agent is at least one of mesoporous molecular sieve, mesoporous silica, and metal-organic framework materials. S2. Preparation of functional isocyanate component B: Isocyanate and bio-based chain extender are mixed at a mass ratio of 100:5 to 100:30 and reacted at 40 to 60°C for 0.5 to 2 hours to obtain prepolymerized functional isocyanate component B. The bio-based chain extender is selected from at least one of castor oil, dimer glycol, and bio-based diol; S3. Preheat the fertilizer granules to 60-90℃, place them in the fluidized bed coating equipment, and adjust the fluidizing air velocity to make the fertilizer granules fully fluidized. S4. Simultaneously spray component A from step S1 and component B from step S2 onto the surface of fluidized fertilizer granules through a dual-channel atomizing nozzle, controlling the mass flow ratio of component A to component B to be 1:0.6 to 1:1.8, the atomization pressure to be 0.3 to 0.8 MPa, and the spraying time to be 5 to 20 min. Component A and component B undergo an in-situ polymerization reaction on the surface of fertilizer particles to generate a bio-based polyurethane coating layer containing a nano-confined template agent. S5. After spraying, continue fluidization and curing at 50-70℃ for 10-30 minutes, then cool to room temperature and sieve to obtain bio-based polyurethane-coated controlled-release fertilizer.

2. The preparation process according to claim 1, characterized in that, The nano-confined template agent mentioned in step S1 is a mixture of two or more nano-confined template agents with different pore sizes; The pore sizes of the nano-confined template agents with different pore sizes are 2-10 nm, 10-30 nm and 30-50 nm, respectively, and the mass ratio of the three is 1:0.5-1.5:0.2-1.

0.

3. The preparation process according to claim 1, characterized in that, The bio-based polyol mentioned in step S1 is selected from at least one of vegetable oil polyols, biomass liquefaction polyols, recycled polyurethane alcoholysis polyols, and carbon dioxide-based polyols. The plant oil polyol is a polyol obtained by ring-opening reaction of epoxidized plant oil; The biomass liquefaction polyol is a polyol obtained by liquefying lignocellulose biomass under the action of a liquefying agent. The recovered polyurethane alcoholysis polyol is a polyol obtained by alcoholysis recovery of waste polyurethane foam; The carbon dioxide-based polyol is a polyol obtained by copolymerization of carbon dioxide and propylene oxide.

4. The preparation process according to claim 1, characterized in that, The nano-confined template agent described in step S1 has a pore size of 2-50 nm and a specific surface area of ​​200-1000 m² / g; The mesoporous molecules are selected from at least one of SBA-15, MCM-41, and KIT-6; The metal-organic framework material is selected from at least one of MIL-101, UiO-66, and ZIF-8.

5. The preparation process according to claim 1, characterized in that, The isocyanate mentioned in step S2 is a mixture of bio-based isocyanate and petroleum-based isocyanate, wherein the bio-based isocyanate accounts for 20% to 80% by mass; The bio-based isocyanate is selected from at least one of pentamethylene diisocyanate, hexamethylene diisocyanate, L-lysine diisocyanate, and dimer diisocyanate; The petroleum-based isocyanate is selected from at least one of polymethylene polyphenyl isocyanate, diphenylmethane diisocyanate, and toluene diisocyanate.

6. The preparation process according to claim 1, characterized in that, The number-average molecular weight of the bio-based chain extender described in step S2 is 200–2000, and the hydroxyl value is 50–300 mg KOH / g; The bio-based diol is selected from at least one of 1,3-propanediol, 1,4-butanediol, and bio-based ethylene glycol.

7. The preparation process according to claim 1, characterized in that, The fertilizer granules mentioned in step S3 are at least one of urea, diammonium phosphate, potassium chloride, potassium sulfate, and compound fertilizer, with a particle size of 1-5 mm; The fluidized bed coating equipment is a bottom-spray fluidized bed or a tangential-spray fluidized bed.

8. The preparation process according to claim 1, characterized in that, The spray angle of the dual-channel atomizing nozzle in step S4 is 30° to 60°, and the atomized droplet size is 20 to 100 μm. During the spraying process, the surface temperature of the fertilizer particles is maintained at 50-70°C by adjusting the fluidizing air velocity and spraying rate.

9. The preparation process according to claim 1, characterized in that, The bio-based polyurethane coating layer accounts for 1.5% to 6% of the total mass of the coated controlled-release fertilizer; The nutrient release period of the coated controlled-release fertilizer in still water at 25°C is 40–300 days.

10. The preparation process according to claim 1, characterized in that, Step S5 is followed by a repair process: The coated controlled-release fertilizer with coating defects after screening is placed back into a fluidized bed and heated to 80-120°C to slightly melt the surface of the coating layer. Then, component A or component B is sprayed on to repair the coating defects using the secondary reactivity of polyurethane. After cooling, the repaired coated controlled-release fertilizer is obtained.

Images