Polyurethane coated controlled release fertilizer and its preparation method

Abstract

This invention relates to the field of advanced petrochemical new materials technology, specifically to a polyurethane-coated controlled-release fertilizer and its preparation method, comprising the following components: a composite functional carrier system, dual-modified pigments, weather-resistant synergistic agents, and processing aids; the composite functional carrier system consists of the following components: a compatibility carrier resin, an interface modifier, a dynamic crosslinking agent, and the balance being a vinyl elastomer; the vinyl elastomer is an ethylene-octene copolymer; the compatibility carrier resin is a compound of homopolymer polypropylene and maleic anhydride-grafted polypropylene; this invention, through the optimization of component compounding, modification process, and processing parameters, completely solves the problems of poor dispersion, weak weather resistance, and insufficient compatibility of existing masterbatches, enabling PPR pipes to not only meet the basic requirements of sanitary water supply but also adapt to harsh outdoor environments, expanding its application range in municipal engineering, building exterior wall water supply, and other scenarios, and has significant practical value.

Description

Technical Field

[0001] This invention relates to the field of advanced petrochemical new materials technology, specifically to a polyurethane-coated controlled-release fertilizer and its preparation method. Background Technology

[0002] Controlled-release fertilizers enable stable nutrient release, reduce leaching and volatilization losses, and improve fertilizer utilization, making them an important technological pathway for fertilizer conservation, efficiency enhancement, and green production in modern agriculture. Polyurethane-coated fertilizers, due to their high coating strength, good formability, and adjustable controlled-release cycle, are widely used in field crops, cash crops, and horticulture.

[0003] Existing industrial polyurethane-coated controlled-release fertilizers mostly use ordinary polyester / polyether polyols and isocyanates for in-situ polymerization and coating. They generally suffer from problems such as insufficient adhesion between the coating and the particles, low mechanical strength, poor water resistance, insufficient controlled-release precision, and low production yield, making it difficult to meet the needs of large-scale production and long-term stable application.

[0004] For example, the prior art, with announcement number CN106800472B and invention titled "A Composite Modified Polyurethane Coating Liquid, Controlled-Release Fertilizer and its Preparation Method," employs a polyurethane coating system and introduces inorganic fillers to improve coating performance, thereby enhancing coating strength and slow-release effect to some extent. However, this technology still has significant drawbacks in actual production and application: First, the polyol used has not undergone end-group modification and precise hydroxyl value control, resulting in poor cross-linking uniformity with isocyanate. The bonding force between the inner adhesive layer and fertilizer particles is weak, making the coating prone to cracking and detachment during transportation and mechanical application, leading to excessive nutrient release and increased risk of seedling burn. Second, the inorganic particles are simply blended, resulting in poor compatibility with the polyurethane matrix and easy aggregation to form interface defects. This not only fails to stably construct controllable micropores but also reduces the coating's density and water resistance, making it prone to damage and swelling after long-term water immersion. Third, the coating is a single-layer homogeneous structure, making it difficult to simultaneously ensure coating adhesion, mechanical strength, and nutrient release control precision.

[0005] In summary, existing technologies cannot simultaneously solve a series of problems such as poor coating adhesion, low mechanical strength, insufficient water resistance, and general limited controlled release. There is an urgent need to develop a polyurethane-coated controlled-release fertilizer preparation technology that features reasonable raw material modification, scientific structural design, perfect process matching, and is green and environmentally friendly, in order to meet the needs of modern agriculture for efficient, safe, and sustainable fertilization. Summary of the Invention

[0006] The purpose of this invention is to provide a polyurethane-coated controlled-release fertilizer and its preparation method.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing a polyurethane-coated controlled-release fertilizer includes the following steps: (1) Preparation of modified polyester polyol: Copolymerized aliphatic polyester polyol is added to a reaction vessel and vacuum dehydrated for 50-70 min at a vacuum degree of -0.085~-0.095 MPa and a temperature of 90-100℃. Then, 0.2%-0.3% of ethylene glycol monomethyl ether by mass of polyester polyol is added and the reaction is kept at the temperature for 1.5-2 h. The hydroxyl value is adjusted to 300-320 mgKOH / g and the acid value is ≤0.8 mgKOH / g. The mixture is then cooled to room temperature and sealed for later use. (2) Preparation of composite modified attapulgite particles: obtained by three-step composite modification of attapulgite; (3) Core fertilizer pretreatment: Take the core fertilizer and put it into the coating drum, preheat and dry it at 85-95℃ for 35-45 minutes; (4) Inner layer high cross-linking adhesive layer coating: Take 35-40% of the total mass of modified polyester polyol and polypropylene glycol diol with hydroxyl value of 200-220 mgKOH / g and mix them at a mass ratio of 8:2-3. Add 0.3%-0.5% of the total mass of the mixed polyol catalyst and spray it evenly onto the surface of fertilizer granules with an atomization pressure of 0.35-0.45 MPa. Simultaneously spray composite isocyanate, control the cyanohydroxyl molar ratio of 1.12-1.18:1, the drum speed of 30-40 r / min, and spray for 15-20 min to obtain a dense inner layer adhesive layer; (5) Coating of the outer layer of low cross-linking microporous layer: Mix the remaining modified polyester polyol and modified attapulgite particles at a mass ratio of 100:2.5-3.5, and spray them evenly with atomization pressure of 0.25-0.35MPa. At the same time, spray the composite isocyanate, maintain the cyanohydroxyl molar ratio of 1.10-1.15:1, the drum speed of 28-32r / min, spray for 30-40min, and control the temperature at 85-95℃ throughout the process to obtain the outer layer of microporous controlled release layer; (6) Coupling and curing: After the coating is completed, the vacuum degree of -0.075~-0.085MPa is maintained in the drum and the temperature is kept at 85-95℃ for 40-50min. The cooling rate is controlled at 5-8℃ / min to reduce to room temperature. The broken particles are removed by screening with a 3.5-4.5mm standard sieve to obtain the finished controlled-release fertilizer.

[0008] As a further technical solution, in step (1), the co-polyaliphatic polyester polyol is synthesized by feeding adipic acid: propylene glycol: ethylene glycol = 1:0.6:0.4, with a number average molecular weight of 1200-1500 g / mol.

[0009] As a further technical solution, the method for preparing composite modified attapulgite particles in step (2) includes: Take 200-300 mesh natural attapulgite, first prepare a mixed acid solution of 1 mol / L dilute hydrochloric acid and 0.5 mol / L citric acid at a volume ratio of 3:1, activate and acid wash for 30 min at a solid-liquid ratio of 1:5, wash with deionized water until neutral, and dry at 105℃; then mix the activated attapulgite with adipic acid and propylene glycol at a mass ratio of 10:1:0.6, react in a sealed environment at 110℃ for 2 h, cool, add 1.2% of the particle mass of interfacial compatibilizer, stir at high speed of 800-1000 r / min for 15 min for dry coating, and dry at 105℃ to obtain composite modified attapulgite particles.

[0010] As a further technical solution, the preparation method of the interface compatibilizer is as follows: 100 parts by mass of soybean oil, 5-6 parts by mass of castor oil, 3-4 parts by mass of formic acid, and 0.3-0.5 parts by mass of 85% phosphoric acid are added to a reaction vessel, stirred and heated to 55℃, and 28-32 parts by mass of 30% hydrogen peroxide are added dropwise at a uniform rate for 1.5-2 hours, with the temperature controlled at 55-65℃ throughout the process. After the addition is complete, the reaction is maintained at this temperature for 3-4 hours. After the reaction, the mixture is allowed to stand and separate into layers, and the aqueous phase is discarded. The oil phase is washed with 5% sodium carbonate solution until the pH reaches 7.0-7.5, washed 2-3 times with deionized water until neutral, and then dehydrated under reduced pressure at -0.09MPa vacuum and 80-85℃ for 2-2.5 hours until the moisture content is ≤0.05%, thus obtaining the interface compatibilizer.

[0011] As a further technical solution, in step (3), the core fertilizer particles are large-particle urea or nitrogen-phosphorus-potassium-sulfur compound fertilizer with a particle size of 2.5-4.5 mm and a particle compressive strength of ≥50 N.

[0012] As a further technical solution, in step (4), the composite isocyanate is compounded by polymethylene polyphenyl isocyanate and diphenylmethane diisocyanate in a mass ratio of 3:2.

[0013] As a further technical solution, the catalyst in step (4) is dibutyltin dilaurate.

[0014] As a further technical solution, in step (4), the total mass of the inner dense bonding layer is 1.2%-1.5% of the mass of the core fertilizer particles, and the thickness is 6-10 μm.

[0015] As a further technical solution, in step (5), the total mass of the outer microporous controlled-release layer is 1.8%-2.4% of the mass of the core fertilizer particles, and the thickness is 9-14 μm.

[0016] The preparation method described above yields a polyurethane-coated controlled-release fertilizer.

[0017] Compared with the prior art, the beneficial effects of the present invention are: 1. This invention systematically addresses the shortcomings of existing technologies by combining modified polyester polyols, composite modified attapulgite particles, double-layer gradient coating, and vacuum coupling curing, thereby achieving the preparation of high-performance, high-stability, and high-yield controlled-release fertilizers.

[0018] 2. Modified polyester polyols are prepared by modifying aliphatic polyester polyols with ethylene glycol monomethyl ether. This allows for the control of hydroxyl and acid values, enhancing reactivity with isocyanates and improving crosslinking uniformity. Specific dehydration temperatures, vacuum levels, and reaction times result in more regular polyol molecular chain arrangement, forming a dense inner bonding layer after crosslinking. This significantly improves the adhesion between the coating and fertilizer granule surface, solving the problems of coating detachment and cracking. Composite modified attapulgite particles, activated by mixed acid, organically modified, and coated with an interface compatibilizer, exhibit enhanced surface activity and significantly improved compatibility with the polyurethane matrix. Because attapulgite is uniformly dispersed in the outer coating, it forms stable and controllable microporous channels, enabling slow and stable nutrient release, thus solving the problems of excessively rapid nutrient release and poor release control precision. The interface compatibilizer is prepared from natural raw materials such as soybean oil and castor oil. It has good biodegradability and is environmentally friendly. It can reduce the interfacial tension between inorganic particles and organic coatings, and eliminate agglomeration and interface defects. As the interface defects are effectively filled, the density and continuity of the coating are improved, thereby improving the mechanical strength and water resistance of the coating and solving the problem of easy damage after long-term water immersion.

[0019] 3. The dual-layer gradient coating process achieves structural differentiation: a highly cross-linked and densely bonded inner layer and a low-cross-linked microporous controlled-release outer layer. The inner layer uses a high proportion of modified polyester polyol, high atomization pressure, and high rotation speed to form a high-strength adhesion layer, ensuring the overall stability of the coating. The outer layer uses low atomization pressure, low rotation speed, and composite attapulgite particles to form a controlled-release microporous structure, balancing strength and controlled-release function. Due to the gradient matching of the inner and outer layer structures, the overall integrity and functional differentiation of the coating are synergistically enhanced, avoiding both an excessively thick inner layer affecting release and an excessively thin outer layer losing protection. The vacuum coupling curing process completes the curing after coating at a specific vacuum level and temperature, promoting the removal of residual small molecules and the complete cross-linking reaction. Combined with temperature control and cooling, the internal stress of the coating is released uniformly. Due to more complete curing and lower internal stress, the particle breakage rate is significantly reduced, and the yield after sieving is greatly improved, thus solving the problems of difficult molding and low yield in production.

[0020] 4. The present invention forms a complete synergistic system from components to the overall process: modified polyester polyol provides excellent crosslinking and adhesion properties, composite modified attapulgite provides controllable micropores and enhanced strength, interface compatibilizer ensures stable bonding of the multiphase system, double-layer coating achieves structural and functional differentiation, and coupling maturation ensures molding quality. This system not only makes the fertilizer nutrient release cycle adjustable, the release curve stable, and the utilization rate significantly improved, but also makes the particles have high compressive strength, good water resistance, and low breakage rate. At the same time, it uses a natural-based interface compatibilizer, which is green and environmentally friendly and meets soil safety requirements. Ultimately, it achieves high production efficiency, high finished product qualification rate, and stable application effect, and can be adapted to various crops and different fertilization scenarios, combining industrial practicality and agricultural application value. Attached Figure Description

[0021] Figure 1 The cumulative release rate over 60 days is shown in the example and comparative examples. Detailed Implementation

[0022] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0023] This invention provides a method for preparing polyurethane-coated controlled-release fertilizer, including the following steps: preparation of modified polyester polyol, preparation of composite modified attapulgite particles, pretreatment of core fertilizer, coating of inner high-crosslinked adhesive layer, coating of outer low-crosslinked microporous layer, and coupling curing molding.

[0024] This invention first involves the preparation of modified polyester polyol. Copolymerized aliphatic polyester polyol is added to a reaction vessel and vacuum-dehydrated for 50-70 minutes under vacuum conditions of -0.085 to -0.095 MPa and 90-100℃. Then, 0.2%-0.3% by weight of ethylene glycol monomethyl ether is added, and the reaction is maintained at this temperature for 1.5-2 hours. The hydroxyl value is adjusted to 300-320 mg KOH / g and the acid value is not greater than 0.8 mg KOH / g. The mixture is then cooled to room temperature and sealed for later use.

[0025] In this invention, the copolymerized aliphatic polyester polyol is synthesized by a molar ratio of adipic acid: propylene glycol: ethylene glycol = 1:0.6:0.4, with a number average molecular weight of 1200-1500 g / mol.

[0026] After the modified polyester polyol was prepared, composite modified attapulgite particles were prepared. The composite modified attapulgite particles were obtained by three-step composite modification of attapulgite.

[0027] In this invention, the method for preparing composite modified attapulgite particles is as follows: Take 200-300 mesh natural attapulgite, first prepare a mixed acid solution of 1 mol / L dilute hydrochloric acid and 0.5 mol / L citric acid at a volume ratio of 3:1, activate and acid wash for 30 min at a solid-liquid ratio of 1:5, wash with deionized water until neutral, and then dry at 105℃; then mix the activated attapulgite with adipic acid and propylene glycol at a mass ratio of 10:1:0.6, react in a sealed environment at 110℃ for 2 h, cool, add 1.2% of the particle mass of interfacial compatibilizer, stir at high speed of 800-1000 r / min for dry coating for 15 min, and dry at 105℃ to obtain composite modified attapulgite particles.

[0028] In this invention, the preparation method of the interface compatibilizer is as follows: 100 parts by mass of soybean oil, 5-6 parts by mass of castor oil, 3-4 parts by mass of formic acid, and 0.3-0.5 parts by mass of 85% phosphoric acid are added to a reaction vessel, stirred and heated to 55°C, and 28-32 parts by mass of 30% hydrogen peroxide are added dropwise at a uniform rate for 1.5-2 hours, with the temperature controlled at 55-65°C throughout. After the addition is complete, the reaction is maintained at this temperature for 3-4 hours. After the reaction, the mixture is allowed to stand and separate into layers, and the aqueous phase is discarded. The oil phase is washed with 5% sodium carbonate solution until the pH reaches 7.0-7.5, washed 2-3 times with deionized water until neutral, and then dehydrated under reduced pressure at a vacuum of -0.09 MPa and a temperature of 80-85°C for 2-2.5 hours until the water content is no more than 0.05%, thus obtaining the interface compatibilizer.

[0029] Next, the kernel fertilizer is pretreated by adding it to the coating drum and preheating and drying it at 85-95℃ for 35-45 minutes.

[0030] In this invention, the core fertilizer particles are large-particle urea or nitrogen-phosphorus-potassium-sulfur compound fertilizer with a particle size of 2.5-4.5 mm and a particle compressive strength of not less than 50 N.

[0031] After pretreatment, an inner high-crosslinking adhesive layer is coated. 35%-40% of the total mass of modified polyester polyol and polypropylene glycol diol with a hydroxyl value of 200-220 mgKOH / g are mixed at a mass ratio of 8:2-3. 0.3%-0.5% of the total mass of the mixed polyol is added as a catalyst. The mixture is then uniformly atomized and sprayed onto the surface of fertilizer granules at an atomization pressure of 0.35-0.45 MPa. Simultaneously, composite isocyanate is sprayed, controlling the cyanide-hydroxyl molar ratio at 1.12-1.18:1, the drum speed at 30-40 r / min, and the spraying time at 15-20 min to obtain a dense inner adhesive layer.

[0032] In this invention, the composite isocyanate is composed of polymethylene polyphenyl isocyanate and diphenylmethane diisocyanate in a mass ratio of 3:2; the catalyst is dibutyltin dilaurate; the total mass of the inner dense bonding layer is 1.2%-1.5% of the mass of the core fertilizer particles, and the thickness is 6-10 μm.

[0033] Subsequently, an outer low-crosslinking microporous layer is coated. The remaining modified polyester polyol and modified attapulgite particles are mixed evenly at a mass ratio of 100:2.5-3.5, and sprayed evenly with atomization pressure of 0.25-0.35MPa. Composite isocyanate is sprayed simultaneously, maintaining a cyanohydroxyl molar ratio of 1.10-1.15:1, a drum speed of 28-32r / min, and spraying for 30-40min. The temperature is controlled at 85-95℃ throughout the process to obtain the outer microporous controlled-release layer.

[0034] In this invention, the total mass of the outer microporous controlled-release layer is 1.8%-2.4% of the mass of the core fertilizer particles, and the thickness is 9-14 μm.

[0035] Finally, coupling and curing molding are carried out. After the coating is completed, the vacuum degree is maintained at -0.075~-0.085MPa and the temperature is maintained at 85-95℃ for 40-50 minutes in the drum. The cooling rate is controlled at 5-8℃ / min to reduce to room temperature. The broken particles are removed by screening with a 3.5-4.5mm standard sieve to obtain the finished controlled-release fertilizer.

[0036] The preparation method provided by this invention utilizes a double-layer coating structure constructed synergistically from modified polyester polyol and composite modified attapulgite particles. The high degree of cross-linking in the inner layer ensures the adhesion and density of the coating, while the low degree of cross-linking in the outer layer, combined with attapulgite, forms controllable micropores, enabling precise nutrient release. Simultaneously, the composite modified attapulgite enhances the mechanical strength and water resistance of the coating, the interface compatibilizer strengthens the compatibility of each component, and the coupled curing process optimizes the coating molding quality. This method solves the problems of low controlled release precision, easy coating damage, and uneven nutrient release in traditional polyurethane-coated fertilizers. The fertilizer nutrient release cycle is controllable, the utilization rate is high, and it is suitable for the growth stages of various crops.

[0037] Example 1: Preparation of modified polyester polyol: A copolymerized aliphatic polyester polyol with a molar ratio of adipic acid:propylene glycol:ethylene glycol = 1:0.6:0.4 and a number average molecular weight of 1200 g / mol was added to a reaction vessel and vacuum dehydrated for 50 min at a vacuum degree of -0.085 MPa and 90 °C. Then, 0.2% by weight of ethylene glycol monomethyl ether was added and the reaction was maintained at this temperature for 1.5 h. The hydroxyl value was adjusted to 300 mg KOH / g and the acid value was ≤0.8 mg KOH / g. The mixture was then cooled to room temperature and sealed for later use.

[0038] Preparation of interfacial compatibilizer: By mass, 100 parts soybean oil, 5 parts castor oil, 3 parts formic acid, and 0.3 parts 85% phosphoric acid were added to a reaction vessel, stirred and heated to 55℃, and 28 parts 30% hydrogen peroxide were added dropwise at a uniform rate over 1.5 hours, with the temperature maintained at 55℃ throughout. After the addition was complete, the reaction was kept at this temperature for 3 hours. After the reaction, the mixture was allowed to stand and separate into layers, and the aqueous phase was discarded. The oil phase was washed with 5% sodium carbonate solution until pH 7.0, washed twice with deionized water until neutral, and then dehydrated under reduced pressure at -0.09 MPa and 80℃ for 2 hours until the moisture content was ≤0.05%, thus obtaining the interfacial compatibilizer.

[0039] Preparation of composite modified attapulgite particles: Take 200 mesh natural attapulgite, first use a mixed acid solution prepared by 1 mol / L dilute hydrochloric acid and 0.5 mol / L citric acid at a volume ratio of 3:1, activate and acid wash for 30 min at a solid-liquid ratio of 1:5, wash with deionized water until neutral, and dry at 105℃; then mix the activated attapulgite with adipic acid and propylene glycol at a mass ratio of 10:1:0.6, react in a sealed environment at 110℃ for 2 h, cool, add 1.2% of the particle mass of interfacial compatibilizer, stir at high speed of 800 r / min for dry coating for 15 min, and dry at 105℃ to obtain composite modified attapulgite particles.

[0040] Core fertilizer pretreatment: Take large urea particles with a particle size of 2.5 mm and a particle compressive strength of ≥50 N, add them to the coated drum, and preheat and dry at 85℃ for 35 min.

[0041] Inner high cross-linking adhesive layer coating: 35% of the total mass of modified polyester polyol and polypropylene glycol diol with a hydroxyl value of 200 mg KOH / g are mixed at a mass ratio of 8:2. 0.3% of the total mass of the mixed polyol is added, and the mixture is uniformly atomized and sprayed onto the surface of fertilizer granules at an atomization pressure of 0.35 MPa. Simultaneously, a composite isocyanate of polymethylene polyphenyl isocyanate and diphenylmethane diisocyanate in a mass ratio of 3:2 is sprayed. The cyanohydroxyl molar ratio is controlled at 1.12:1, the drum speed is 30 r / min, and the spraying time is 15 min. A dense inner adhesive layer with a total mass of 1.2% of the core fertilizer granule mass and a thickness of 6 μm is obtained.

[0042] Outer low-crosslinking microporous layer coating: The remaining modified polyester polyol and modified attapulgite particles are mixed evenly at a mass ratio of 100:2.5, and sprayed evenly with atomization pressure of 0.25MPa. At the same time, the above-mentioned composite isocyanate is sprayed to maintain a cyanohydroxyl molar ratio of 1.10:1, a drum speed of 28r / min, and a spraying time of 30min. The temperature is controlled at 85℃ throughout the process to obtain an outer microporous controlled-release layer with a total mass of 1.8% of the core fertilizer particles and a thickness of 9μm.

[0043] Coupled curing and molding: After coating, the drum is kept at a vacuum of -0.075MPa and 85℃ for 40 minutes. The cooling rate is controlled at 5℃ / min to reduce the temperature to room temperature. The broken particles are removed by screening with a 3.5mm standard sieve to obtain the finished controlled-release fertilizer.

[0044] Example 2: Preparation of modified polyester polyol: A copolymerized aliphatic polyester polyol with a molar ratio of adipic acid: propylene glycol: ethylene glycol = 1:0.6:0.4 and a number average molecular weight of 1500 g / mol was added to a reaction vessel and vacuum dehydrated for 70 min at -0.095 MPa vacuum and 100 °C. Then, 0.3% (by weight) of ethylene glycol monomethyl ether was added, and the reaction was maintained at this temperature for 2 h. The hydroxyl value was adjusted to 320 mg KOH / g and the acid value was ≤0.8 mg KOH / g. The mixture was then cooled to room temperature and sealed for later use.

[0045] Preparation of interfacial compatibilizer: By mass, 100 parts soybean oil, 6 parts castor oil, 4 parts formic acid, and 0.5 parts 85% phosphoric acid were added to a reaction vessel, stirred and heated to 55℃, and 32 parts 30% hydrogen peroxide were added dropwise at a uniform rate over 2 hours, with the temperature controlled at 65℃ throughout. After the addition was completed, the reaction was maintained at this temperature for 4 hours. After the reaction, the mixture was allowed to stand and separate into layers. The aqueous phase was discarded, and the oil phase was washed with 5% sodium carbonate solution until pH 7.5, washed three times with deionized water until neutral, and then dehydrated under reduced pressure at -0.09 MPa and 85℃ for 2.5 hours until the moisture content was ≤0.05%, thus obtaining the interfacial compatibilizer.

[0046] Preparation of composite modified attapulgite particles: Take 300 mesh natural attapulgite, first use a mixed acid solution prepared by 1 mol / L dilute hydrochloric acid and 0.5 mol / L citric acid at a volume ratio of 3:1, activate and acid wash for 30 min at a solid-liquid ratio of 1:5, wash with deionized water until neutral, and dry at 105℃; then mix the activated attapulgite with adipic acid and propylene glycol at a mass ratio of 10:1:0.6, react in a sealed environment at 110℃ for 2 h, cool, add 1.2% of the particle mass of interfacial compatibilizer, stir at high speed of 1000 r / min for dry coating for 15 min, and dry at 105℃ to obtain composite modified attapulgite particles.

[0047] Core fertilizer pretreatment: Take nitrogen, phosphorus, potassium and sulfur compound fertilizer with a particle size of 4.5 mm and a particle compressive strength of ≥50 N, add it to the coated drum, and preheat and dry it at 95℃ for 45 min.

[0048] Inner layer high cross-linking adhesive layer coating: 40% of the total mass of modified polyester polyol and polypropylene glycol diol with a hydroxyl value of 220 mg KOH / g are mixed at a mass ratio of 8:3. 0.5% of the total mass of the mixed polyol is added to dibutyltin dilaurate. The mixture is then uniformly atomized and sprayed onto the surface of the fertilizer granules at an atomization pressure of 0.45 MPa. The above-mentioned composite isocyanate is sprayed simultaneously. The cyanohydroxyl molar ratio is controlled at 1.18:1, the drum speed is 40 r / min, and the spraying time is 20 min. A dense inner layer adhesive layer with a total mass of 1.5% of the core fertilizer granule mass and a thickness of 10 μm is obtained.

[0049] Outer low-crosslinking microporous layer coating: The remaining modified polyester polyol and modified attapulgite particles were mixed evenly at a mass ratio of 100:3.5, and sprayed evenly with atomization pressure of 0.35MPa. The above-mentioned composite isocyanate was sprayed simultaneously to maintain a cyanohydroxyl molar ratio of 1.15:1, a drum speed of 32r / min, and a spraying time of 40min. The temperature was controlled at 95℃ throughout the process to obtain an outer microporous controlled-release layer with a total mass of 2.4% of the core fertilizer particles and a thickness of 14μm.

[0050] Coupled curing and molding: After coating, the drum is kept at a vacuum of -0.085MPa and 95℃ for 50 minutes. The cooling rate is controlled at 8℃ / min to reduce the temperature to room temperature. The broken particles are removed by screening with a 4.5mm standard sieve to obtain the finished controlled-release fertilizer.

[0051] Example 3: Preparation of modified polyester polyol: A copolymerized aliphatic polyester polyol with a molar ratio of adipic acid:propylene glycol:ethylene glycol = 1:0.6:0.4 and a number average molecular weight of 1350 g / mol was added to a reaction vessel and vacuum dehydrated for 60 min at a vacuum degree of -0.09 MPa and 95 °C. Then, 0.25% by weight of ethylene glycol monomethyl ether was added and the reaction was maintained at this temperature for 1.75 h. The hydroxyl value was adjusted to 310 mg KOH / g and the acid value was ≤0.8 mg KOH / g. The mixture was then cooled to room temperature and sealed for later use.

[0052] Preparation of interfacial compatibilizer: By mass, 100 parts soybean oil, 5.5 parts castor oil, 3.5 parts formic acid, and 0.4 parts 85% phosphoric acid were added to a reaction vessel, stirred and heated to 55℃, and 30 parts 30% hydrogen peroxide were added dropwise at a uniform rate over a period of 1.75 h, with the temperature controlled at 60℃ throughout. After the addition was completed, the reaction was maintained at this temperature for 3.5 h. After the reaction, the mixture was allowed to stand and separate into layers, and the aqueous phase was discarded. The oil phase was washed with 5% sodium carbonate solution until the pH reached 7.25, washed twice with deionized water until neutral, and then dehydrated under reduced pressure at -0.09 MPa and 82℃ for 2.25 h until the moisture content was ≤0.05%, thus obtaining the interfacial compatibilizer.

[0053] Preparation of composite modified attapulgite particles: Take 250 mesh natural attapulgite, first use a mixed acid solution prepared by 1 mol / L dilute hydrochloric acid and 0.5 mol / L citric acid at a volume ratio of 3:1, activate and acid wash for 30 min at a solid-liquid ratio of 1:5, wash with deionized water until neutral, and dry at 105℃; then mix the activated attapulgite with adipic acid and propylene glycol at a mass ratio of 10:1:0.6, react in a sealed environment at 110℃ for 2 h, cool, add 1.2% of the particle mass of interfacial compatibilizer, stir at high speed of 900 r / min for dry coating for 15 min, and dry at 105℃ to obtain composite modified attapulgite particles.

[0054] Core fertilizer pretreatment: Take large urea particles with a particle size of 3.5 mm and a particle compressive strength of ≥50 N, add them to the coating drum, and preheat and dry at 90℃ for 40 min.

[0055] Inner high cross-linking adhesive layer coating: 37.5% of the total mass of modified polyester polyol and polypropylene glycol diol with a hydroxyl value of 210 mg KOH / g were mixed at a mass ratio of 8:2.5. 0.4% of the total mass of the mixed polyol was added to dibutyltin dilaurate. The mixture was atomized and sprayed uniformly onto the surface of fertilizer granules at an atomization pressure of 0.4 MPa. The above-mentioned composite isocyanate was sprayed simultaneously. The cyanohydroxyl molar ratio was controlled at 1.15:1, the drum speed was 35 r / min, and the spraying time was 17.5 min. A dense inner adhesive layer with a total mass of 1.35% of the core fertilizer granule mass and a thickness of 8 μm was obtained.

[0056] Outer low-crosslinking microporous layer coating: The remaining modified polyester polyol and modified attapulgite particles are mixed evenly at a mass ratio of 100:3, and sprayed evenly with atomization pressure of 0.3MPa. At the same time, the above-mentioned composite isocyanate is sprayed to maintain a cyanohydroxyl molar ratio of 1.12:1, a drum speed of 30r / min, and a spraying time of 35min. The temperature is controlled at 90℃ throughout the process to obtain an outer microporous controlled-release layer with a total mass of 2.1% of the core fertilizer particles and a thickness of 11.5μm.

[0057] Coupled curing and molding: After coating, the drum is kept at a vacuum of -0.08MPa and 90℃ for 45 minutes. The cooling rate is controlled at 6.5℃ / min to reduce the temperature to room temperature. The broken particles are removed by screening with a 4mm standard sieve to obtain the finished controlled-release fertilizer.

[0058] Comparative Example 1: The preparation method of Example 3 is adopted, except that: in the outer low cross-linking microporous layer coating step, no composite modified attapulgite particles are added, only the remaining modified polyester polyol is sprayed, and the other process parameters are exactly the same as the raw material ratio.

[0059] Comparative Example 2: The preparation method of Example 3 is adopted, except that the modified polyester polyol preparation step is not performed, and ordinary copolymer aliphatic polyester polyol is used directly, while the other process parameters and raw material ratios are exactly the same.

[0060] Comparative Example 3: The preparation method of Example 3 is adopted, except that no interfacial compatibilizer is added in the preparation step of composite modified attapulgite particles, and the other process parameters are exactly the same as the raw material ratio.

[0061] Comparative Example 4: The preparation method of Example 3 is adopted, except that: after the coating is completed, vacuum heat preservation and temperature control are not performed, and the material is directly cooled and sieved. The remaining process parameters and raw material ratios are exactly the same.

[0062] Experimental Section: Experiment 1: Nutrient cumulative release rate test: 1.1 Experimental Objective: The cumulative nutrient release rate of polyurethane-coated controlled-release fertilizers in Examples 1-3 and Comparative Examples 1-4 was tested under static water conditions at 25°C to verify the effects of composite modified attapulgite particles, modified polyester polyol, interface compatibilizer, and coupled maturation process on the controlled-release performance of fertilizers.

[0063] 1.2 Experimental Principle: Referring to the general method for testing the nutrient release rate of controlled-release fertilizers, the fertilizer to be tested was placed in deionized water at 25℃, and the nutrient concentration in the extract was measured periodically. The cumulative nutrient release rate at different times was calculated to reflect the precision and uniformity of fertilizer control.

[0064] 1.3 Experimental Instruments and Reagents: Experimental instruments: constant temperature water bath, ultraviolet-visible spectrophotometer, electronic balance, conical flask, funnel, volumetric flask; Experimental reagents: finished fertilizers of Examples 1-3 and Comparative Examples 1-4, deionized water.

[0065] 1.4 Test Methods: Weigh equal amounts of fertilizer samples from Examples 1-3 and Comparative Examples 1-4, place them in conical flasks containing 200 mL of deionized water, seal them, and place them in a 25°C constant temperature water bath. Samples were taken on days 7, 14, 28, and 60 to determine the total nutrient concentration in the extract and calculate the cumulative nutrient release rate. Each sample was tested in parallel three times, and the average value was taken as the final result.

[0066] 1.5 Experimental Data: Table 1

[0067] The fertilizers in Examples 1-3 exhibited slow initial release, with a release rate below 17% at 28 days and between 40% and 43% at 60 days, demonstrating excellent controlled-release performance. Comparative Example 1, lacking the addition of composite modified attapulgite particles, lacked a controllable microporous structure in the coating, resulting in rapid nutrient diffusion and a significant increase in release rates at each stage, leading to controllable release failure. Comparative Example 2, without modified polyester polyol, suffered from insufficient cross-linking and density in the coating, resulting in large pores, accelerated nutrient release, and decreased controlled-release effect. Comparative Example 3, without the addition of an interface compatibilizer, exhibited poor compatibility between attapulgite and the polyurethane coating, leading to defects and gaps in the coating, increased nutrient release rate, and reduced controlled-release precision. Comparative Example 4, without coupling maturation, resulted in insufficient coating formation and a loose structure, allowing nutrients to easily leach out, leading to a higher release rate than the examples and deteriorating controlled-release performance.

[0068] Experiment 2: Compressive strength test of coated particles: 2.1 Experimental Objective: The compressive strength of the coating of fertilizer granules in Examples 1-3 and Comparative Examples 1-4 was tested to verify the effect of each inventive feature on improving the mechanical properties of the coating.

[0069] 2.2 Experimental Principle: A particle strength tester is used to apply pressure to intact fertilizer particles and record the maximum pressure value when the particles break. This value is the compressive strength of the particles. The higher the value, the better the mechanical properties of the coating.

[0070] 2.3 Experimental Instruments and Reagents: Test instruments: particle strength tester, electronic balance; test reagents: finished fertilizers from Examples 1-3 and Comparative Examples 1-4.

[0071] 2.4 Test Methods: Thirty intact, undamaged fertilizer granules were selected from each sample. Test the compressive strength of each particle using a particle strength tester and record the data. The average value is calculated as the particle compressive strength of the sample.

[0072] 2.5 Experimental Data: Table 2

[0073] The particles in Examples 1-3 all exhibited compressive strengths exceeding 68 N, demonstrating excellent mechanical properties and meeting transportation and application requirements. Comparative Example 1, lacking composite-modified attapulgite particles, suffered from insufficient coating toughness and strength, resulting in a compressive strength of only 32.6 N, making it easily breakable. Comparative Example 2, using ordinary polyester polyol, exhibited an imperfect cross-linking structure in its coating, reducing its strength to 41.8 N and degrading its mechanical properties. Comparative Example 3, lacking an interfacial compatibilizer, suffered from uneven attapulgite dispersion and coating defects, resulting in a strength of 49.5 N, lower than the examples. Comparative Example 4, without coupling and curing, exhibited a weak coating bond, achieving a strength of 45.7 N, failing to meet the required strength for use.

[0074] Test 3: Water immersion integrity test of the coating: 3.1 Experimental Objective: The integrity of the fertilizer coatings in Examples 1-3 and Comparative Examples 1-4 was tested after long-term immersion in water at 25°C to evaluate the water resistance and stability of the coatings.

[0075] 3.2 Experimental Principle: Fertilizer granules were continuously soaked in deionized water at 25℃ for 28 days. The damage and detachment of the coating were observed, and the percentage of intact granules was counted. The higher the percentage, the better the water resistance of the coating.

[0076] 3.3 Experimental Instruments and Reagents: Experimental instruments: constant temperature water bath, petri dishes, tweezers; experimental reagents: finished fertilizers of Examples 1-3 and Comparative Examples 1-4, deionized water.

[0077] 3.4 Test methods: Weigh 100 fertilizer granules from each sample and place them in petri dishes containing deionized water, then soak them at a constant temperature of 25°C. After soaking for 28 consecutive days, the granules were removed and the number of granules with intact and undamaged coatings was counted. Calculate the percentage of intact particles, and take the average value of three parallel tests.

[0078] 3.5 Experimental Data: Table 3

[0079] After 28 days of immersion, the percentage of intact particles in Examples 1-3 was higher than 96%, indicating excellent water resistance and stability of the coating. Comparative Example 1, without composite-modified attapulgite particles, exhibited poor water resistance in its coating, easily swelling and breaking after long-term immersion, with only 42.5% intact. Comparative Example 2, without modified polyester polyol, had insufficient water resistance in its coating, resulting in a high breakage rate after immersion, with 58.3% intact. Comparative Example 3, without interfacial compatibilizer, had weak interfacial bonding, easily delaminating and detaching after water immersion, with 71.6% intact. Comparative Example 4, without coupling and curing, had a loose coating structure, poor water resistance, and significant breakage after immersion, with 65.9% intact.

[0080] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0081] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a polyurethane-coated controlled-release fertilizer, characterized in that, Includes the following steps: (1) Preparation of modified polyester polyol: Copolymerized aliphatic polyester polyol is added to a reaction vessel and vacuum dehydrated for 50-70 min at a vacuum degree of -0.085~-0.095 MPa and a temperature of 90-100℃. Then, 0.2%-0.3% of ethylene glycol monomethyl ether by mass of polyester polyol is added and the reaction is kept at the temperature for 1.5-2 h. The hydroxyl value is adjusted to 300-320 mgKOH / g and the acid value is ≤0.8 mgKOH / g. The mixture is then cooled to room temperature and sealed for later use. (2) Preparation of composite modified attapulgite particles: obtained by three-step composite modification of attapulgite; (3) Core fertilizer pretreatment: Take the core fertilizer and put it into the coating drum, preheat and dry it at 85-95℃ for 35-45 minutes; (4) Inner layer high cross-linking adhesive layer coating: Take 35-40% of the total mass of modified polyester polyol and polypropylene glycol diol with hydroxyl value of 200-220 mgKOH / g and mix them at a mass ratio of 8:2-3. Add 0.3%-0.5% of the total mass of the mixed polyol catalyst and spray it evenly onto the surface of fertilizer granules with an atomization pressure of 0.35-0.45 MPa. Simultaneously spray composite isocyanate, control the cyanohydroxyl molar ratio of 1.12-1.18:1, the drum speed of 30-40 r / min, and spray for 15-20 min to obtain a dense inner layer adhesive layer; (5) Coating of the outer layer of low cross-linking microporous layer: Mix the remaining modified polyester polyol and modified attapulgite particles at a mass ratio of 100:2.5-3.5, and spray them evenly with atomization pressure of 0.25-0.35MPa. At the same time, spray the composite isocyanate, maintain the cyanohydroxyl molar ratio of 1.10-1.15:1, the drum speed of 28-32r / min, spray for 30-40min, and control the temperature at 85-95℃ throughout the process to obtain the outer layer of microporous controlled release layer; (6) Coupling and curing: After the coating is completed, the vacuum degree of -0.075~-0.085MPa is maintained in the drum and the temperature is kept at 85-95℃ for 40-50min. The cooling rate is controlled at 5-8℃ / min to reduce to room temperature. The broken particles are removed by screening with a 3.5-4.5mm standard sieve to obtain the finished controlled-release fertilizer.

2. The preparation method according to claim 1, characterized in that, Step (1) The co-polyaliphatic polyester polyol is synthesized by feeding adipic acid: propylene glycol: ethylene glycol = 1:0.6:0.4, with a number average molecular weight of 1200-1500 g / mol.

3. The preparation method according to claim 1, characterized in that, The preparation method of composite modified attapulgite particles in step (2) includes: Take 200-300 mesh natural attapulgite, first prepare a mixed acid solution of 1 mol / L dilute hydrochloric acid and 0.5 mol / L citric acid at a volume ratio of 3:1, activate and acid wash for 30 min at a solid-liquid ratio of 1:5, wash with deionized water until neutral, and dry at 105℃; then mix the activated attapulgite with adipic acid and propylene glycol at a mass ratio of 10:1:0.6, react in a sealed environment at 110℃ for 2 h, cool, add 1.2% of the particle mass of interfacial compatibilizer, stir at high speed of 800-1000 r / min for 15 min for dry coating, and dry at 105℃ to obtain composite modified attapulgite particles.

4. The preparation method according to claim 3, characterized in that, The preparation method of the interface compatibilizer is as follows: 100 parts by mass of soybean oil, 5-6 parts by mass of castor oil, 3-4 parts by mass of formic acid, and 0.3-0.5 parts by mass of 85% phosphoric acid are added to a reaction vessel, stirred and heated to 55℃, and 28-32 parts by mass of 30% hydrogen peroxide are added dropwise at a uniform rate for 1.5-2 hours, with the temperature controlled at 55-65℃ throughout. After the addition is complete, the reaction is maintained at this temperature for 3-4 hours. After the reaction, the mixture is allowed to stand and separate into layers. The aqueous phase is discarded, and the oil phase is washed with 5% sodium carbonate solution until the pH reaches 7.0-7.

5. It is then washed 2-3 times with deionized water until neutral, and then dehydrated under reduced pressure at -0.09MPa vacuum and 80-85℃ for 2-2.5 hours until the moisture content is ≤0.05%, thus obtaining the interface compatibilizer.

5. The preparation method according to claim 1, characterized in that, In step (3), the core fertilizer particles are large-particle urea or nitrogen-phosphorus-potassium-sulfur compound fertilizer with a particle size of 2.5-4.5 mm and a particle compressive strength of ≥50 N.

6. The preparation method according to claim 1, characterized in that, In step (4), the composite isocyanate is composed of polymethylene polyphenyl isocyanate and diphenylmethane diisocyanate in a mass ratio of 3:

2.

7. The preparation method according to claim 1, characterized in that, The catalyst in step (4) is dibutyltin dilaurate.

8. The preparation method according to claim 1, characterized in that, In step (4), the total mass of the inner dense bonding layer is 1.2%-1.5% of the mass of the core fertilizer particles, and the thickness is 6-10 μm.

9. The preparation method according to claim 1, characterized in that, In step (5), the total mass of the outer microporous controlled-release layer is 1.8%-2.4% of the mass of the core fertilizer particles, and the thickness is 9-14 μm.

10. A polyurethane-coated controlled-release fertilizer is obtained by the preparation method according to any one of claims 1-9.

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