Integrated water-retaining slow-release fertilizer suitable for semi-arid grassland and preparation method thereof

Abstract

The application discloses integrated water-retaining slow-release fertilizer suitable for semi-arid grassland and a preparation method thereof, and belongs to the technical field of slow-release fertilizer. The integrated water-retaining slow-release fertilizer comprises, from inside to outside, a nutrient storage core, a temperature-sensitive intermediate layer and a water-retaining slow-release outer layer. The core is composed of large-particle compound fertilizer, puffed diatomite and bamboo charcoal biochar; the intermediate layer is a temperature-sensitive coating material of starch grafted paraffin@SiO2 microcapsules; and the outer layer is soybean protein-bamboo charcoal hydrogel which swells and conducts water when encountering drip irrigation water, and can provide fertilizer after self-degradation. The preparation method comprises the following steps: bamboo charcoal limited oxygen pyrolysis, diatomite puffing, core disc granulation, paraffin@SiO2 microcapsule synthesis and layer-by-layer coating in a fluidized bed. The application realizes precise water-fertilizer coupling of drip irrigation triggered release and water-free dormant fertilizer locking through a double response mechanism of water swelling and temperature melting, and has the functions of water retention, slow release, degradability and soil improvement.

Description

Technical Field

[0001] This invention relates to the field of controlled-release fertilizer technology, and in particular to an integrated water-retaining controlled-release fertilizer suitable for semi-arid grasslands and its preparation method. Background Technology

[0002] Semi-arid grasslands are an important component of my country's grassland ecosystem and livestock production. These regions experience scarce and unevenly distributed rainfall, leading to intense soil evaporation. Water scarcity is a core limiting factor for improving grassland productivity. Furthermore, traditional fast-acting fertilizers applied to semi-arid grasslands are prone to rapid leaching and volatilization of nutrients through irrigation or rainfall, resulting in low fertilizer utilization rates. This increases production inputs and poses a risk of non-point source pollution.

[0003] Existing controlled-release fertilizers are mainly divided into two categories: temperature-dependent and water-dependent. Temperature-dependent resin-coated fertilizers release nutrients based on accumulated temperature. In semi-arid regions during hot and dry summers, nutrients are continuously released, but soil moisture is insufficient, preventing nutrients from migrating to the crop root zone for absorption and utilization. This easily leads to ineffective volatilization and soil fixation, resulting in a severe mismatch between nutrient release and water and fertilizer supply rhythms. Water-dependent controlled-release fertilizers, while releasing nutrients with water supply, exhibit large fluctuations in nutrient release under intermittent drip irrigation / rainfall irrigation modes. They tend to release rapidly in the initial stages and quickly stop after water cessation, making it difficult to match the stable nutrient requirements of grassland crops. Furthermore, existing water-retaining controlled-release fertilizers mostly use synthetic resin materials, which are difficult to biodegrade. Long-term application can easily lead to microplastic residues in the soil, making them unsuitable for ecologically sensitive semi-arid grassland ecosystems. On the other hand, biomass-based water-retaining materials generally suffer from poor film-forming properties, low mechanical strength, and difficulty in adapting to large-scale coating processes.

[0004] Therefore, developing a slow-release fertilizer that combines water retention and regulation, temperature-sensitive controlled release, full biodegradability, and compatibility with intermittent drip irrigation in semi-arid grasslands has significant application value. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing an integrated water-retaining slow-release fertilizer suitable for semi-arid grasslands and its preparation method.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: This invention first proposes an integrated water-retaining slow-release fertilizer suitable for semi-arid grasslands, which has a three-layer core-shell structure consisting of an inner core nutrient reservoir, a temperature-sensitive intermediate layer, and a water-retaining slow-release outer layer, from the inside out. The core material accounts for 85% to 90% of the total fertilizer mass. By mass percentage, the raw material composition is: 60% to 70% granular urea, 10% to 15% potassium dihydrogen phosphate, 5% to 10% potassium sulfate, 10% to 15% expanded diatomaceous earth, and 5% to 10% bamboo charcoal. The core uses large-particle urea, potassium dihydrogen phosphate, and potassium sulfate as the main nutrient sources, while expanded diatomaceous earth and bamboo charcoal jointly construct a porous adsorption network to achieve initial slow-release of nutrients. After being treated at high temperature, expanded diatomaceous earth has interconnected internal pores and a significantly increased specific surface area. It has both physical adsorption and capillary liquid retention functions, which can fix soluble nutrients inside the pores and slow down the rate of water contact and nutrient dissolution. Bamboo charcoal has abundant nanoscale pores and oxygen-containing functional groups on its surface. It can load nutrient molecules through electrostatic adsorption and pore retention, and at the same time, it can adsorb intermediate products in the dissolution process, thus further smoothing the release curve. The porous carrier also serves as a particle skeleton, enhancing the mechanical strength of the core particles, preventing particle breakage during subsequent coating processes, and ensuring the integrity of the multi-layer structure.

[0007] This layer only serves the functions of nutrient storage and initial slow release, and does not determine the core controlled release rhythm. Therefore, the total nutrient content and release potential of the core are comparable to those of conventional porous carrier slow-release fertilizers, and the pure fertilizer effect does not surpass existing technologies.

[0008] The temperature-sensitive intermediate layer accounts for 5% to 8% of the total fertilizer mass, with starch grafted modified material as the film-forming matrix and paraffin@SiO2 microcapsules dispersed as temperature-sensitive switching units; by mass, the raw material composition is: 100 parts starch, 30 to 50 parts paraffin@SiO2 microcapsules, and 10 to 20 parts caprolactone; the core material of the paraffin@SiO2 microcapsules is paraffin with a melting point of 45 to 55°C, and the wall material is silicon dioxide; The intermediate layer is the core controlled-release unit of this invention. It uses starch-grafted modified material as a continuous film-forming matrix and paraffin@SiO2 microcapsules as a dispersed temperature-sensitive switch, achieving the opening and closing regulation of nutrient channels through solid-liquid phase change. The phase change mechanism of paraffin@SiO2 microcapsules: Paraffin, the core material, is a typical solid-liquid phase change material. Below its melting point (45–55°C), it is solid, hydrophobic, and shrinks in volume, blocking the pores in the starch film layer and hindering water penetration and nutrient diffusion. Above its melting point, the paraffin melts and becomes liquid, expanding in volume and migrating into the film pores. The previously blocked diffusion channels are opened, allowing nutrients to be released outwards along with water. The SiO2 wall material prevents the paraffin from leaking out after melting, ensuring the stability of the switching function after multiple melt-solidification cycles. It also enhances the mechanical strength of the microcapsules, adapting them to the shear environment of fluidized bed encapsulation.

[0009] The role of starch graft matrix: After starch is modified by caprolactone ring-opening polymerization, its film-forming properties, water resistance and mechanical properties are significantly improved. It not only provides a dispersion carrier for microcapsules, but also constitutes the basic framework of the membrane layer. Its hydrophilicity and hydrophobicity are moderate, which can form a good interfacial bond with the porous inner core carrier and the outer hydrogel, thus avoiding the peeling of multiple membrane layers.

[0010] Unlike traditional monolithic temperature-sensitive membranes, this invention employs a distributed micro-switch structure, where the temperature-sensitive response occurs only locally within the microcapsule. The overall mechanical properties of the membrane are unaffected by phase transitions, thus solving the problems of high-temperature softening and easy breakage of traditional temperature-sensitive membranes. At the same time, it offers higher response sensitivity and better switching repeatability.

[0011] The water-retaining and slow-release outer layer accounts for 3% to 7% of the total fertilizer mass and is a soybean protein-bamboo charcoal composite hydrogel layer. By mass, the raw material composition is: 5 parts soybean protein isolate, 12 to 18 parts acrylic acid, 0.15 to 0.25 parts bamboo charcoal, 0.1 to 0.2 parts crosslinking agent, and 0.2 to 0.3 parts initiator.

[0012] The outer layer is a three-dimensional network hydrogel structure of soybean protein grafted with acrylic acid, with bamboo charcoal dispersed in the gel network as a reinforcing phase, realizing the triple functions of water storage, water conduction, and buffering. After being hydrolyzed by alkali, soybean protein exposes a large number of active amino and carboxyl groups, which then undergo graft copolymerization with acrylic acid to form a three-dimensional cross-linked network under the action of a cross-linking agent. The network is rich in hydrophilic groups (-COOH, -OH, -NH2), which can adsorb a large number of water molecules through hydrogen bonds. The water absorption ratio can reach tens of times its own mass. After being applied to the soil, it can store irrigation / rainfall water and release it slowly during dry periods.

[0013] The addition of bamboo charcoal can increase the cross-linking density and porosity of hydrogels, which not only enhances the mechanical strength and repeated swelling-shrinkage stability of the gel, but also provides continuous porous channels for water conduction, so that the water absorbed by the outer layer can be smoothly conducted to the middle layer, avoiding the sudden release of nutrients caused by the rapid entry and exit of water.

[0014] The hydrogel layer itself also has a certain nutrient retention effect. After the nutrients are released from the inner layer, they need to diffuse outward through the gel network, which can further smooth the release curve and achieve auxiliary slow release.

[0015] Preferably, the core particles have a particle size of 3-5 mm; the expanded diatomaceous earth is obtained by expanding 200 mesh at a high temperature of 600-900℃; and the bamboo charcoal is obtained by pyrolyzing bamboo at 700℃ for 2 hours with limited oxygen, with a particle size of 80-200 mesh.

[0016] Preferably, the paraffin@SiO2 microcapsules are prepared by the sol-gel method, and the wall material thickness is 50-200 nm; the crosslinking agent is N,N′-methylenebisacrylamide, and the initiator is potassium persulfate.

[0017] Preferably, in the water-retaining and slow-release outer layer, the amount of bamboo charcoal added is 4% of the amount of soybean protein isolate; the acrylic acid is partially neutralized by potassium hydroxide and then participates in graft copolymerization, with a neutralization degree of 70% to 80%.

[0018] The core inventive mechanism of this invention lies in the water-triggered and temperature-regulated dual-factor coupled release mode achieved through a three-layer structure, which is precisely adapted to the environmental characteristics of intermittent drip irrigation in semi-arid grasslands, large diurnal temperature range, and long hot and dry periods. The entire process can be divided into three stages: 1. Drought-induced dormancy stage: A dual-blocking nutrient lock-in mechanism: During periods of drought with no irrigation or rainfall, soil moisture content is low: The outer hydrogel is in a state of dry shrinkage, with closed network pores and almost no water conduction capacity, forming the first physical barrier. The intermediate layer of paraffin is solid due to ground temperature fluctuations or because it is below the melting point. It hydrophobically seals the pores of the membrane layer, forming a second temperature control barrier. With the synergistic effect of the two layers, external moisture has difficulty contacting the core nutrients, and the nutrients are hardly released and remain in a dormant state. This completely avoids the problem of ineffective nutrient loss caused by traditional temperature-type slow-release fertilizers that continuously release fertilizer at high temperatures but cannot be absorbed due to lack of water during the hot and dry summer weather in semi-arid regions.

[0019] 2. Drip irrigation activation phase: Synchronous release mechanism of water conduction temperature activation: After drip irrigation begins, the release process is initiated in stages to achieve simultaneous water and fertilizer application: Step 1: Outer layer water storage and conduction: The outer hydrogel quickly absorbs water and swells, storing a large amount of water and forming continuous water conduction channels, slowly transporting water to the middle layer, avoiding water from directly impacting the core and causing sudden release; Step 2: Temperature-sensitive switch activated: When the local temperature rises above the melting point of paraffin due to sunlight, the paraffin in the microcapsule melts, and the nutrient diffusion channels in the middle layer are opened; at this time, the middle layer has been moistened by the moisture conducted by the outer layer, and the moisture can enter the core to dissolve nutrients, and the nutrients are released outward steadily with the concentration gradient. The nutrient release rate at this stage is determined by both the water supply and the ambient temperature: drip irrigation provides the water base, and temperature determines the opening of the release channels. The two work together to match the crop's growth pattern of needing both water and fertilizer, achieving precise regulation of fertilizer release only when there is water and temperature.

[0020] 3. Water outage and subsequent decline phase: Automatic deceleration mechanism for temperature drop and shrinkage: After drip irrigation stops, soil moisture gradually evaporates, and nighttime temperatures drop simultaneously. When the temperature drops below the melting point of paraffin, the paraffin re-solidifies, sealing the pores of the film layer again, actively closing the release channels, and the nutrient release rate drops rapidly. As the water in the outer hydrogel gradually evaporates, the gel network slowly shrinks, the water conductivity decreases, and the release of nutrients is further slowed down. This mechanism enables a steady, pulsed release of nutrients during the drip irrigation cycle, avoiding the waste caused by continuous nutrient release after water cessation, while retaining a small amount of slow release to maintain the basic nutrient supply for crops, thus matching the application mode of intermittent drip irrigation.

[0021] This invention also proposes a method for preparing the aforementioned integrated water-retaining slow-release fertilizer suitable for semi-arid grasslands, comprising the following steps: S1 Core Particle Preparation: Mix large-particle urea, potassium dihydrogen phosphate, potassium sulfate, expanded diatomaceous earth and bamboo charcoal evenly according to the formula, and prepare core particles with a particle size of 3-5mm by disc granulation. Dry them until the moisture content is ≤2% and sieve them for later use. Preparation of S2 intermediate thermosensitive coating solution: Paraffin@SiO2 microcapsules were prepared by sol-gel method; starch was gelatinized with water and then mixed and dispersed with paraffin@SiO2 microcapsules, and caprolactone was added for grafting modification reaction to obtain a thermosensitive coating emulsion with a solid content of 20% to 25%. Preparation of S3 outer layer hydrogel coating solution: Soy protein isolate was alkali hydrolyzed and mixed with acrylic acid neutralization solution, bamboo charcoal was added and ultrasonically dispersed evenly, and then crosslinking agent and initiator were added to carry out prepolymerization reaction to obtain hydrogel coating suspension. S4 fluidized bed multilayer coating: The core particles are added to a fluidized bed coating machine for preheating. First, an intermediate temperature-sensitive coating emulsion is sprayed on and dried below 25°C to form an intermediate temperature-sensitive layer. Then, an outer hydrogel coating suspension is sprayed on and heat-treated to solidify to form a water-retaining and slow-release outer layer. S5 post-processing: Drying and sieving to obtain 3-6mm integrated water-retaining slow-release fertilizer granules, which are then sealed and moisture-proof packaged.

[0022] Preferably, the disc granulation parameters in step S1 are: disc inclination angle 45°~55°, rotation speed 30~50r / min, pelletizing rate ≥70%; drying temperature 60℃.

[0023] Preferably, the preparation of paraffin@SiO2 microcapsules in step S2 is as follows: paraffin is mixed with the emulsifier hexadecyltrimethylammonium bromide, heated to melt, and then homogenized and emulsified under high pressure of 50 MPa to obtain a paraffin emulsion; the pH is adjusted to 4-5, and tetraethyl orthosilicate and propyltriethoxysilane are added dropwise, and the mixture is hydrolyzed at 40-60°C for 4-6 hours, centrifuged, washed, and dried at 40°C to obtain the finished product; The preparation process of the temperature-sensitive coated emulsion specifically includes: Starch was mixed with water to prepare a starch slurry with an 18% solid content, and gelatinized at 85-90℃ for 30 min; cooled to 60℃, paraffin@SiO2 microcapsules were added, and the mixture was stirred and dispersed evenly at 600-800 r / min; stannous octoate (0.5%-1.0% by weight of starch) was added as a catalyst, and caprolactone was added. The mixture was then reacted at 60-80℃ for 4-8 h under nitrogen protection to obtain a thermosensitive coated emulsion with a total non-volatile solid content of 20%-25%. The non-volatile components include grafted modified starch and paraffin@SiO2 microcapsules.

[0024] Preferably, the conditions for the alkaline hydrolysis of soybean protein in step S3 are: stirring and reacting at 70°C under alkaline conditions for 1.5 hours; the conditions for the prepolymerization reaction are: reacting at 70°C for 20-30 minutes, and stopping when the viscosity of the system is suitable for atomized spraying.

[0025] Preferably, the intermediate layer coating parameters in step S4 are: inlet air temperature 40-50℃, material temperature 35-40℃, atomization pressure 0.3-0.4MPa, and spraying speed 0.8-1.2mL / min; the outer layer coating parameters are: inlet air temperature 50-60℃, atomization pressure 0.4-0.5MPa, and spraying speed 1.0-1.5mL / min; after coating, the material is heat-treated at 60℃ for 10-15min for curing. In fluidized bed processes, the material temperature is typically 10–20°C lower than the inlet air temperature. When the inlet air temperature is 50–60°C, the actual material temperature is approximately 35–45°C. During the outer coating process, the material temperature is near or below the lower limit of the paraffin melting point (45–55°C), and the coated middle layer of paraffin microcapsules may soften but not melt.

[0026] Preferably, in step S5, the moisture content of the finished product is ≤2%; the nutrient release rate of the finished product granules in the initial stage of static water after 24 hours is ≤15%, and the cumulative nutrient release rate after 28 days is 60% to 75%.

[0027] In-situ water retention mechanism to alleviate water stress: After the outer layer of hydrogel is applied to the soil, it can form a local micro-reservoir in the root zone: it absorbs and stores excess water during a single drip irrigation and slowly releases it into the soil during the dry period, prolonging the soil moisture period and reducing surface soil evaporation; at the same time, the bamboo charcoal and the diatomaceous earth and biochar in the core eventually enter the soil, which can improve soil porosity and water retention capacity. Long-term application can improve the soil water retention performance in semi-arid areas and alleviate the limitation of water stress on grassland growth.

[0028] Mechanism of biodegradation of all components: The entire system of this invention contains no recalcitrant synthetic resins, and all components can be gradually degraded in the soil environment. The outer layer of soybean protein hydrogel and the middle layer of starch matrix are both natural polymer materials that can be gradually decomposed into small molecule organic matter by enzymes secreted by microorganisms in the soil, and finally transformed into humus and carbon dioxide, with no plastic residue. In the paraffin@SiO2 microcapsules, paraffin can be slowly degraded, and SiO2 is a natural mineral component that is harmless to the soil; The core materials, diatomaceous earth and bamboo charcoal, are natural / biomass materials. Long-term storage can improve soil aggregate structure and increase soil organic matter content, which is suitable for the environmental protection requirements of semi-arid grassland ecologically fragile areas.

[0029] Compared with the prior art, the beneficial effects of the present invention are: 1. While existing micro-water-soluble gelled fertilizers possess a certain slow-release effect, their release is primarily driven by water alone, failing to differentiate between the start and end of drip irrigation. In semi-arid grasslands, this often results in ineffective nutrient loss due to release upon contact with water and continued release even after irrigation ceases. This invention innovatively constructs a dual-response mechanism of water swelling and temperature switching: the outer soybean protein hydrogel rapidly swells upon contact with drip irrigation water, forming a water conduction channel; the middle layer of paraffin@SiO2 microcapsules melts and dissolves when the soil temperature rises (above the melting point of paraffin) during drip irrigation, opening the nutrient release pathway. Both conditions are indispensable—heating without water or contact with water without sufficient temperature will not trigger efficient release, thus achieving precise water-fertilizer coupling for release upon drip irrigation start and dormancy upon drip irrigation stop, fundamentally solving the problem of fertilizer loss due to ineffective rainfall in semi-arid regions.

[0030] 2. Traditional coated slow-release fertilizers are mostly single-layer or double-layer structures, with a fixed nutrient release curve that is difficult to match the nutrient requirements of crops at different growth stages. This invention adopts a three-layer core-shell differentiated design: the core uses expanded diatomaceous earth and biochar as porous carriers to physically adsorb nutrients, providing a basic slow-release reservoir; the middle layer controls the timing and degree of the release switch through the temperature-sensitive melting behavior of paraffin microcapsules; the outer layer of soybean protein hydrogel regulates the water molecule permeation rate through cross-linking density, further delaying nutrient diffusion. The three layers each perform their respective functions and regulate step by step, allowing the nutrient release curve to be optimized in three segments: induction period, rapid release period, and slow release period, precisely adapting to the differentiated nutrient requirements of semi-arid grassland pastures during the greening-up, vigorous growth, and maturity stages.

[0031] 3. Existing water-retaining and slow-release fertilizers often involve simple physical mixing of water-retaining agents and fertilizers, resulting in a lack of chemical bonding between the water-retaining materials and the fertilizers. The water-retaining and slow-release functions are independent or even mutually restrictive. This invention introduces bamboo charcoal biochar into the three-dimensional cross-linked network of soybean protein through graft copolymerization, providing more cross-linking sites for the porous structure of the biochar and significantly increasing the density of the hydrogel network. Simultaneously, the abundant oxygen-containing functional groups on the surface of the biochar adsorb urea molecules through hydrogen bonds and electrostatic interactions, slowing down its exchange and release rate in water. The soybean protein hydrogel itself has a swelling ratio of over 222.3 g / g, which can increase the soil water-holding capacity to more than 1.93 times that of the control. The addition of bamboo charcoal further strengthens the network structure, enabling a positive synergy between water-retaining and slow-release properties within the same material system, rather than a trade-off. Furthermore, both soybean protein and starch are biodegradable materials, and the degraded small molecules can be absorbed and utilized by plants as a natural nitrogen source, achieving a four-fold unity of water retention, slow release, fertilization, and environmental friendliness. Attached Figure Description

[0032] Figure 1 This is a process flow diagram of the integrated water-retaining slow-release fertilizer proposed in this invention; Figure 2 This is a TEM image of a slice of the integrated water-retaining slow-release fertilizer of the present invention; Figure 3 This is a SEM image of the paraffin@SiO2 microcapsules of the present invention after a peeling test. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0034] Example 1: The integrated water-retaining slow-release fertilizer of this example has a core that accounts for 88% of the total mass, a middle temperature-sensitive layer that accounts for 7%, and a water-retaining slow-release outer layer that accounts for 5%.

[0035] Core (by weight): 65% large-particle urea, 12% potassium dihydrogen phosphate, 8% potassium sulfate, 10% expanded diatomaceous earth, and 5% bamboo charcoal; core particle size 3-4 mm.

[0036] Intermediate thermosensitive layer (parts by weight): 100 parts starch, 40 parts paraffin@SiO2 microcapsules, 15 parts caprolactone; paraffin melting point 50℃.

[0037] Water-retaining and slow-release outer layer (parts by weight): 5 parts soy protein isolate, 15 parts acrylic acid, 0.2 parts bamboo charcoal (4% of the soy protein content), 0.15 parts N,N′-methylenebisacrylamide, and 0.275 parts potassium persulfate.

[0038] The preparation steps are as follows: Core preparation: Weigh the raw materials according to the formula, mix them evenly in a high-speed mixer, add them to a disc granulator, with a disc inclination angle of 50° and a rotation speed of 40 r / min, spray water to granulate, and obtain 3-4 mm particles. Dry them at 60℃ until the moisture content is ≤2%, and sieve them for later use.

[0039] Preparation of paraffin@SiO2 microcapsules: 40 parts of paraffin and 3 parts of hexadecyltrimethylammonium bromide were added to deionized water, heated to 60℃ to melt, and homogenized and emulsified three times under high pressure of 50MPa to obtain a paraffin emulsion; the pH was adjusted to 4.5, and 15 parts of tetraethyl orthosilicate and 8 parts of propyltriethoxysilane were slowly added dropwise, and the mixture was stirred and hydrolyzed at 50℃ for 5h; the mixture was centrifuged, washed, and vacuum dried at 40℃ to obtain microcapsules.

[0040] Preparation of thermosensitive coating solution: 100 parts of starch were mixed to form a starch milk with a solid content of 18%, and gelatinized at 90℃ for 30 min; cooled to 60℃, paraffin@SiO2 microcapsules were added and dispersed evenly; 1.0% of stannous octoate by weight of starch was added as a catalyst, and 15 parts of caprolactone were added. The mixture was kept at 60℃ for 4 h to obtain a thermosensitive coating emulsion with a solid content of 25%.

[0041] Preparation of hydrogel coating solution: 5g soy protein isolate and 0.45g potassium hydroxide were added to 35ml deionized water and lysed at 70℃ for 1.5h; 13.4g potassium hydroxide was dissolved in 31ml water, cooled, and then neutralized with 15g acrylic acid to obtain potassium acrylate solution; the two were mixed and 0.2g bamboo charcoal was added, and the mixture was ultrasonically dispersed at 450W for 10min; crosslinking agent and initiator (dissolved in 14ml water) were added, and the mixture was prepolymerized at 70℃ for 20min to obtain a sprayable hydrogel suspension.

[0042] Fluidized bed coating: Core particles are added to a fluidized bed, and the inlet air temperature is 45℃ to preheat the material to 40℃; a temperature-sensitive coating emulsion is sprayed at an atomization pressure of 0.35MPa and a spraying rate of 1.0mL / min, with the coating covering 7% of the intermediate layer, and dried at 40℃ for 10min; a hydrogel suspension is then sprayed at an inlet air temperature of 55℃, an atomization pressure of 0.4MPa, and a spraying rate of 1.2mL / min, with the coating covering 5% of the outer layer; and the material is then heat-treated at 60℃ for 15min to cure.

[0043] Post-processing: Sieve particles of 3-6 mm to obtain the finished product.

[0044] Example 2: This example is basically the same as Example 1, except that: the core accounts for 90% of the total mass, the intermediate temperature-sensitive layer accounts for 6%, and the water-retaining slow-release outer layer accounts for 4%; the amount of paraffin@SiO2 microcapsules in the intermediate temperature-sensitive layer is 30 parts, and the melting point of paraffin is 45℃.

[0045] Example 3: This example is basically the same as Example 1, except that: the core accounts for 85% of the total mass, the intermediate temperature-sensitive layer accounts for 8%, and the water-retaining slow-release outer layer accounts for 7%; the amount of paraffin@SiO2 microcapsules in the intermediate temperature-sensitive layer is 50 parts, and the melting point of paraffin is 55℃.

[0046] Comparative Example 1: Commercially available conventional resin-coated slow-release compound fertilizer (N-P2O5-K2O=28-12-10, release period 60 days, temperature-dependent, existing mainstream technology product).

[0047] Comparative Example 2: Single-layer soybean protein-bamboo charcoal hydrogel slow-release fertilizer, with the core composition being the same as in Example 1, and the outer layer being a hydrogel layer with the same formula, with a total coating rate of 12% and no intermediate temperature-sensitive layer (existing biomass water-retaining slow-release fertilizer technology).

[0048] Performance testing: Nutrient release performance: According to GB / T-23348-2009 water immersion method, the initial release rate at 24h and the cumulative release rate at 28d were measured at 25℃; the cumulative release rate at 25℃ and 50℃ was measured at 7d respectively, and the temperature-sensitive response ratio (50℃ release rate / 25℃ release rate) was calculated.

[0049] Soil water retention performance: Add fertilizer at a ratio of 2% to sandy loam soil, saturate with water, place in a 25℃ incubator, and measure the soil moisture retention rate on the 15th day.

[0050] Crop growth effect: Ryegrass was used as the test crop. It was applied in pots with equal nitrogen content. The aboveground dry weight was measured after 60 days of growth to characterize the fertilizer-enhancing performance.

[0051] The test results are shown in the table below:

[0052] Data Analysis: for Figure 2 TEM images clearly show the complete transition region of the fertilizer particles from the core, middle layer, and outer layer. No large-scale structural collapse or interlayer miscibility was observed due to improper coating processes (such as excessively high fluidized bed temperatures). The microcapsule wall material showed no cracks, damage, or empty shells caused by paraffin leakage; no macroscopic cracks, pores, or dark gaps indicating discontinuity were observed. The two-phase contact boundaries were in close contact, and no mass loss was observed under electron beam penetration.

[0053] for Figure 3 Numerous ductile tear ridges, micropore aggregation, and traces of plastic rheological changes in paraffin material are visible on the fracture surface. This morphology is typical of cohesive fracture, meaning the fracture path extends through the paraffin phase itself, rather than along the interface between the two layers. In multilayer composites, peel strength is controlled by the strength of the weakest phase in the system. In this system, paraffin has the lowest modulus and lowest cohesive strength, yielding and undergoing plastic deformation before the interface under stress, until final fracture. In other words, even if the chemical bonding strength at the interface between the two phases is extremely high, the peel test still measures the cohesive strength of the paraffin phase, not the true bonding strength of the interface. Therefore, the interfacial bonding quality of this invention cannot be evaluated using interfacial peel test data.

[0054] In terms of nutrient release, the initial release rate of the example group at 24 hours was 9.8% to 12.5%, all of which were lower than the industry qualified threshold of 15%, significantly better than the 18.3% of the single-layer hydrogel control, and close to the 8.5% of traditional resin-coated fertilizers; the cumulative release rate at 28 days was 62.7% to 72.1%, which was at the same level as the 70.3% of existing high-quality slow-release fertilizers, and slightly lower than the single-layer hydrogel products. This indicates that the three-layer core-shell structure effectively suppressed the risk of nutrient burst release, the slow-release stability reached the level of existing technology, and the overall nutrient supply capacity did not exceed that of existing mainstream slow-release fertilizers.

[0055] The key differentiator is its temperature-sensitive response characteristic. In the example group, the 7-day release rate ratio at 50℃ and 25℃ reached 2.49–2.95, which is 1.6–1.9 times that of traditional resin-coated fertilizers, while the response ratio of the single-layer hydrogel product was only 1.13, with virtually no temperature regulation capability. This result verifies the on / off effect of paraffin@SiO2 microcapsules: at low temperatures, the solid paraffin blocks pores and hinders nutrient dissolution, while at high temperatures, it melts and opens release channels, enabling on-demand nutrient release based on diurnal temperature differences and drip irrigation rhythms in semi-arid regions.

[0056] In terms of water retention and crop effects, the soil moisture retention rate of the example group was 30.1% to 35.7% after 15 days, which is more than 65% higher than that of traditional slow-release fertilizers, and the water retention capacity is better than that of single-layer hydrogel products. In the crop pot experiment, the dry weight of the upper part of the rye grass was 3.28 to 3.42 g / pot, which is slightly lower than the 3.51 g of traditional resin-coated fertilizers and comparable to single-layer hydrogel fertilizers. This further confirms that the fertilizer enhancement performance of the product does not exceed the existing technology, and the core value is reflected in the enhanced water retention and improved adaptability to different scenarios.

[0057] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. An integrated water-retaining slow-release fertilizer suitable for semi-arid grasslands, characterized in that, The structure consists of a three-layer core-shell structure, from the inside out: a nutrient storage core, a temperature-sensitive intermediate layer, and a water-retaining and slow-release outer layer. The nutrient storage core accounts for 85% to 90% of the total fertilizer mass. By mass percentage, the raw material composition is: 60% to 70% granular urea, 10% to 15% potassium dihydrogen phosphate, 5% to 10% potassium sulfate, 10% to 15% expanded diatomaceous earth, and 5% to 10% bamboo charcoal. The temperature-sensitive intermediate layer accounts for 5% to 8% of the total fertilizer mass, with starch grafted modified material as the film-forming matrix and paraffin@SiO2 microcapsules dispersed as temperature-sensitive switching units; by mass, the raw material composition is: 100 parts starch, 30 to 50 parts paraffin@SiO2 microcapsules, and 10 to 20 parts caprolactone; the core material of the paraffin@SiO2 microcapsules is paraffin with a melting point of 45 to 55°C, and the wall material is silicon dioxide; The water-retaining and slow-release outer layer accounts for 3% to 7% of the total fertilizer mass and is a soybean protein-bamboo charcoal composite hydrogel layer. By mass, the raw material composition is: 5 parts soybean protein isolate, 12 to 18 parts acrylic acid, 0.15 to 0.25 parts bamboo charcoal, 0.1 to 0.2 parts crosslinking agent, and 0.2 to 0.3 parts initiator.

2. The integrated water-retaining slow-release fertilizer according to claim 1, characterized in that, The core particles have a particle size of 3-5 mm; the expanded diatomaceous earth is obtained by expanding 200 mesh at a high temperature of 600-900℃; the bamboo charcoal is obtained by pyrolyzing bamboo at 700℃ for 2 hours with limited oxygen, and has a particle size of 80-200 mesh.

3. The integrated water-retaining slow-release fertilizer according to claim 1, characterized in that, The paraffin@SiO2 microcapsules were prepared by the sol-gel method, with a wall thickness of 50–200 nm; the crosslinking agent was N,N′-methylenebisacrylamide, and the initiator was potassium persulfate.

4. The integrated water-retaining slow-release fertilizer according to claim 1, characterized in that, In the water-retaining and slow-release outer layer, the amount of bamboo charcoal added is 4% of the amount of soybean protein isolate; acrylic acid is partially neutralized by potassium hydroxide and then participates in graft copolymerization, with a neutralization degree of 70% to 80%.

5. A method for preparing an integrated water-retaining slow-release fertilizer suitable for semi-arid grasslands as described in any one of claims 1 to 4, characterized in that, Includes the following steps: S1 Core Particle Preparation: Mix large-particle urea, potassium dihydrogen phosphate, potassium sulfate, expanded diatomaceous earth and bamboo charcoal evenly according to the formula, and prepare core particles with a particle size of 3-5mm by disc granulation. Dry them until the moisture content is ≤2% and sieve them for later use. Preparation of S2 intermediate thermosensitive coating solution: Paraffin@SiO2 microcapsules were prepared by sol-gel method; starch was gelatinized with water and then mixed and dispersed with paraffin@SiO2 microcapsules, and caprolactone was added for grafting modification reaction to obtain a thermosensitive coating emulsion with a solid content of 20% to 25%. Preparation of S3 outer layer hydrogel coating solution: Soy protein isolate was alkali hydrolyzed and mixed with acrylic acid neutralization solution, bamboo charcoal was added and ultrasonically dispersed evenly, and then crosslinking agent and initiator were added to carry out prepolymerization reaction to obtain hydrogel coating suspension. S4 fluidized bed multilayer coating: The core particles are added to a fluidized bed coating machine for preheating. First, an intermediate temperature-sensitive coating emulsion is sprayed on and dried below 25°C to form an intermediate temperature-sensitive layer. Then, an outer hydrogel coating suspension is sprayed on and heat-treated to solidify to form a water-retaining and slow-release outer layer. S5 post-processing: Drying and sieving to obtain 3-6mm integrated water-retaining slow-release fertilizer granules, which are then sealed and moisture-proof packaged.

6. The preparation method according to claim 5, characterized in that, The parameters for disc granulation in step S1 are: disc inclination angle 45°~55°, rotation speed 30~50r / min, pelletizing rate ≥70%; drying temperature 60℃.

7. The preparation method according to claim 5, characterized in that, The preparation of paraffin@SiO2 microcapsules in step S2 is as follows: paraffin is mixed with the emulsifier hexadecyltrimethylammonium bromide, heated to melt, and then homogenized and emulsified under high pressure of 50 MPa to obtain a paraffin emulsion; the pH is adjusted to 4-5, tetraethyl orthosilicate and propyltriethoxysilane are added dropwise, and the mixture is hydrolyzed at 40-60℃ for 4-6 hours. After centrifugation and washing, the product is dried at 40℃ to obtain the final product. The preparation process of the temperature-sensitive coated emulsion specifically includes: Starch was mixed with water to prepare a starch slurry with an 18% solid content, and gelatinized at 85-90℃ for 30 min. After cooling to 60℃, paraffin@SiO2 microcapsules were added and stirred at 600-800 r / min to disperse evenly. Stannous octoate (0.5%-1.0% by weight of starch) was added as a catalyst, and caprolactone was added. The reaction was carried out at 60-80℃ for 4-8 h under nitrogen protection to obtain a thermosensitive coated emulsion with a total solid content of 20%-25% of non-volatile components.

8. The preparation method according to claim 5, characterized in that, In step S3, the conditions for alkaline hydrolysis of soybean protein are: stirring reaction at 70℃ for 1.5h under alkaline conditions; the conditions for prepolymerization reaction are: reaction at 70℃ for 20-30min, and the reaction is stopped when the viscosity of the system is suitable for atomized spraying.

9. The preparation method according to claim 5, characterized in that, In step S4, the parameters for the intermediate layer coating are: inlet air temperature 40-50℃, material temperature 35-40℃, atomization pressure 0.3-0.4MPa, and spraying speed 0.8-1.2mL / min; the parameters for the outer layer coating are: inlet air temperature 50-60℃, atomization pressure 0.4-0.5MPa, and spraying speed 1.0-1.5mL / min. After coating, the coating is cured by heat treatment at 60℃ for 10-15min.

10. The preparation method according to claim 5, characterized in that, In step S5, the moisture content of the finished product is ≤2%; the nutrient release rate of the finished product granules in the initial stage of static water after 24 hours is ≤15%, and the cumulative nutrient release rate after 28 days is 60%~75%.

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