Temperature-responsive starch-based gel coated slow-release fertilizer

Abstract

The invention relates to the technical field of fertilizer slow release, and discloses a temperature-responsive starch-based gel coated slow-release fertilizer, which comprises the following raw materials: amorphous starch: a starting material is common corn starch; the cross-linking agent is N, N '-methylene bisacrylamide (MBA); n-isopropylacrylamide (NIPAM) is used for endowing the gel with temperature responsiveness; acrylamide (AM): as a comonomer to improve gel performance; the urea is used as a core component of the slow-release fertilizer; the absolute ethyl alcohol is used for wetting the surface of urea and promoting uniform coating of the SNA powder. According to the invention, the gel can automatically adjust the internal microporous structure according to different soil temperatures, so as to accurately control the release rate of nutrients, effectively solve the problem that the traditional slow release fertilizer cannot dynamically adjust the nutrient release according to the environmental change, and meet the differentiated demands of plants for nutrients in different growth stages.

Description

Technical Field

[0001] The present invention relates to the technical field of fertilizer slow release, and specifically relates to a temperature-responsive starch-based gel-coated slow-release fertilizer. Background Art

[0002] Temperature-responsive hydrogels are a type of hydrogels that can undergo phase transitions during temperature changes within a certain temperature range. The reason for this change is mainly due to the presence of thermosensitive polymers in the hydrogels. These polymers generally have a lower critical solution temperature (LCST) or an upper critical solution temperature (UCST). Responsive polymers with LCST show hydrophilicity at lower temperatures and are prone to swelling behavior; when the temperature rises above the critical temperature, they show hydrophobicity and are prone to shrinkage behavior.

[0003] N-isopropylacrylamide (NIPAM) is a typical temperature-responsive polymer. The hydrogel prepared using NIPAM has typical temperature sensitivity. In the temperature-responsive starch-based gel-coated slow-release fertilizer, due to the thermosensitivity of the coating material, the nutrient release rate of the fertilizer can be adjusted with the change of soil temperature, so as to better meet the needs of plants at different growth stages; however, in the actual use process, due to the lack of research on N-isopropylacrylamide and insufficient understanding of its temperature-responsive performance and water retention and slow-release control performance in soils at different temperatures, it is impossible to use it as the coating material for temperature-responsive slow-release fertilizers, which severely restricts the development of temperature-responsive coated slow-release fertilizers. Based on this, the present invention designs a temperature-responsive starch-based gel-coated slow-release fertilizer to solve the above problems. Summary of the Invention

[0004] The purpose of the present invention is to provide a temperature-responsive starch-based gel-coated slow-release fertilizer, which solves the problem in the background art that the insufficient research on NIPAM restricts the development of coated slow-release fertilizers.

[0005] In order to solve the above technical problems, the present invention provides the following technical solutions:

[0006] A temperature-responsive starch-based gel-coated slow-release fertilizer, comprising the following raw materials:

[0007] Amorphous starch: The starting material is ordinary corn starch;

[0008] Crosslinking agent: N,N'-methylenebisacrylamide (MBA);

[0009] N-isopropylacrylamide (NIPAM): Used to endow the gel with temperature responsiveness;

[0010] Acrylamide (AM): As a comonomer to improve the gel performance;

[0011] Urea: As the core component of the slow-release fertilizer;

[0012] Absolute ethanol: Used to moisten the surface of urea and promote the uniform coating of SNA powder.

[0013] Preferably, the preparation method of the amorphous starch includes the following steps:

[0014] Step S101, Add 25 g to 35 g of ordinary corn starch into a three-necked round-bottom flask containing 100 mL to 150 mL of ethanol with a volume fraction of 45% to 55%;

[0015] Step S102, Place the flask in a water bath at 80°C to 90°C and mechanically stir at a speed of 140 rpm to 160 rpm for 1.5 hours to 2.5 hours;

[0016] Step S103, Perform suction filtration on the stirred mixed reaction solution, separate the solid matter, and repeatedly wash it at least three times with ethanol with a volume fraction of 45% to 55% to remove impurities and unreacted substances;

[0017] Step S104, Put the washed solid matter into a blast drying oven and dry it at 45°C to 55°C for 20 hours to 28 hours;

[0018] Step S105, Crush the dried solid matter and sieve it through a sieve with 80 meshes to 120 meshes to obtain the required amorphous starch powder.

[0019] Preferably, the preparation method of the temperature-responsive starch-based gel (SNA) includes the following steps:

[0020] Step S201, Weigh 4 g to 6 g of the amorphous starch prepared according to claim 1 into a three-necked round-bottom flask, and add 90 mL to 100 mL of distilled water, and stir to dissolve the starch evenly;

[0021] Step S202, Weigh N-isopropylacrylamide (NIPAM) and acrylamide (AM) in a specific molar ratio (such as NIPAM:AM = 1:1 to 3:1), and add a certain amount of N,N'-methylenebisacrylamide (MBA) as a cross-linking agent. The addition amount of MBA is 0.5% to 2% of the starch mass;

[0022] Step S203, Stir and pre-crosslink at a speed of 140 rpm to 160 rpm in a water bath at 50°C to 60°C for 25 minutes to 35 minutes;

[0023] Step S204: Weigh potassium persulfate (KPS) as the initiator, with its mass being 0.1% to 0.5% of the mass of starch. Dissolve it in 5 ml of water to form an initiator solution, and slowly add it dropwise to the flask.

[0024] Step S205: Heat up to the preset temperature (such as 60°C to 70°C) to cause the cross-linking reaction of the mixed solution until the reaction is complete.

[0025] Step S206: Take out the prepared gel, put it into a vacuum drying oven at 45°C to 55°C for drying for 44 hours to 52 hours, and grind part of the gel to the required particle size for standby.

[0026] Preferably, a preparation method of a temperature-responsive starch-based gel-coated slow-release fertilizer includes the following steps:

[0027] Step S301: Spray anhydrous ethanol in an amount of 5 ml to 15 ml on the surface of urea to make its surface uniformly wet.

[0028] Step S302: Take the SNA powder prepared according to claim 2 and ground to less than 100 mesh, and uniformly coat it on the surface of urea by the disk method, with the coating layer number being 2 to 4 layers.

[0029] Step S303: Dry at 45°C to 55°C for 44 hours to 52 hours to obtain a temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF).

[0030] Preferably, in step S302, the coating layer number of SNA powder is preferably 3 layers, and the particle size of urea particles is controlled between 1 mm and 3 mm.

[0031] Preferably, the temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF) can adjust the release rate of nutrients according to the change of environmental temperature, and is especially used in occasions for improving fertilizer utilization rate, reducing nutrient loss and environmental pollution.

[0032] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0033] 1. In the present invention, by copolymerizing NIPAM and acrylamide (AM) and introducing the cross-linking agent N,N'-methylenebisacrylamide (MBA), the present invention successfully prepares an SNA gel with excellent temperature responsiveness. This gel can automatically adjust the internal microporous structure according to different soil temperatures, and then precisely control the release rate of nutrients, effectively solving the problem that traditional slow-release fertilizers cannot dynamically adjust the nutrient release according to environmental changes, and meeting the different nutrient requirements of plants at different growth stages.

[0034] 2. In the present invention, the SNA-SRF coating layer can not only regulate nutrient release, but also significantly improve the water holding rate and water retention rate of the soil. Especially in arid and cold regions, its thermosensitive property helps to slow down water evaporation, provides a stable water environment for crops, helps to reduce the number of irrigation times, and improves water resource utilization efficiency.

[0035] 3. In the present invention, the temperature responsiveness of SNA-SRF enables it to adapt to climate changes in different seasons and regions. Especially in regions with large temperature differences, it shows excellent performance. This property makes the fertilizer have broad application prospects in various agricultural ecosystems and provides a more flexible and efficient nutrient management scheme for agricultural production. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 It is a diagram of the preparation method of the temperature-responsive starch-based gel-coated slow-release fertilizer of the present invention;

[0037] Figure 2 It is a diagram of the preparation method of amorphous starch and temperature-responsive starch-based gel of the present invention;

[0038] Figure 3 It is a formula table of temperature-responsive starch-based gel (SNA);

[0039] Figure 4 It is the FTIR spectra of amorphous starch (Aa), AM (Ab), SNA (Ac), SNA gels with different molar ratios of NIPAM to AM (B) and different molar ratios of St to monomers (C);

[0040] Figure 5 It is the XRD spectra of starch (Aa), amorphous starch (Ab), SNA gels with different molar ratios of NIPAM to AM (B) and different molar ratios of St to monomers (C);

[0041] Figure 6 It is the SEM photos of SNA gels with different molar ratios of NIPAM to AM;

[0042] Figure 7 It is the SEM photos of SNA gels with different molar ratios of St to monomers;

[0043] Figure 8 It is the trend chart of the water absorption and swelling ratio (A), equilibrium swelling ratio (B) and swelling equilibrium time (C) of SNA gels with the molar ratio of NIPAM to AM;

[0044] Figure 9 It is the trend chart of the water absorption and swelling ratio (A), equilibrium swelling ratio (B) and swelling equilibrium time (C) of SNA gels with different molar ratios of St to monomers;

[0045] Figure 10 Trend graphs of the water absorption and swelling ratio (A, D, G, J, M, P, S), equilibrium swelling ratio (B, E, H, K, N, Q, T), and swelling equilibrium time (C, F, I, L, O, R, U) of SNA gels with different NIPAM-to-AM molar ratios at different temperatures;

[0046] Figure 11 Trend graph of the temperature responsiveness of SNA gels with different NIPAM-to-AM molar ratios;

[0047] Figure 12 Trend graphs of the water absorption and swelling ratio (A, D, G, J, M), equilibrium swelling ratio (B, E, H, K, N), and swelling equilibrium time (C, F, I, L, O) of SNA gels with different St-to-monomer molar ratios at different temperatures.

[0048] Figure 13 Trend graph of the change in temperature responsiveness of SNA gels with different St-to-monomer molar ratios;

[0049] Figure 14 Water retention performance graph of SNA-SRF in soil;

[0050] Figure 15 Water conservation performance graph of SNA-SRF in soil;

[0051] Figure 16 Sustained and controlled release performance graph of SNA-SRF in water;

[0052] Figure 17 Sustained and controlled release performance graph of SNA-SRF in soil;

[0053] Figure 18 Data table of the slow release performance of SNA-SRF in soil. Detailed implementation manners

[0054] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0055] Embodiment 1;

[0056] In the embodiment of the present invention, a temperature-responsive starch-based gel-coated slow-release fertilizer includes the following raw materials:

[0057] Amorphous starch: The starting material is ordinary corn starch;

[0058] Crosslinking agent: N,N'-methylenebisacrylamide (MBA);

[0059] N-isopropylacrylamide (NIPAM): used to endow the gel with temperature responsiveness;

[0060] Acrylamide (AM): used as a comonomer to improve the gel properties;

[0061] Urea: used as the core component of the slow-release fertilizer;

[0062] Absolute ethanol: used to wet the surface of urea and promote the uniform coating of SNA powder.

[0063] Preparation method of amorphous starch, comprising the following steps:

[0064] Step S101, add 25 g to 35 g of ordinary corn starch into a three-necked round-bottom flask containing 100 mL to 150 mL of ethanol with a volume fraction of 45% to 55%;

[0065] Step S102, place the flask in a water bath at 80°C to 90°C and mechanically stir at a speed of 140 rpm to 160 rpm for 1.5 hours to 2.5 hours;

[0066] Step S103, perform suction filtration on the stirred mixed reaction solution, separate the solid matter, and repeatedly wash it at least three times with ethanol with a volume fraction of 45% to 55% to remove impurities and unreacted substances;

[0067] Step S104, put the washed solid matter into a blast drying oven and dry it at 45°C to 55°C for 20 hours to 28 hours;

[0068] Step S105, crush the dried solid matter and sieve it through a sieve with 80 meshes to 120 meshes to obtain the required amorphous starch powder.

[0069] Preparation method of temperature-responsive starch-based gel (SNA) comprises the following steps:

[0070] Step S201, weigh 4 g to 6 g of amorphous starch prepared according to claim 1 into a three-necked round-bottom flask, and add 90 mL to 100 mL of distilled water, and stir to dissolve the starch evenly;

[0071] Step S202, weigh N-isopropylacrylamide (NIPAM) and acrylamide (AM) in a specific molar ratio (such as NIPAM:AM = 1:1 to 3:1), and add a certain amount of N,N'-methylenebisacrylamide (MBA) as a crosslinking agent, and the addition amount of MBA is 0.5% to 2% of the starch mass;

[0072] Step S203, perform pre-crosslinking treatment by stirring in a water bath at 50°C to 60°C at a speed of 140 rpm to 160 rpm for 25 minutes to 35 minutes;

[0073] Step S204, weigh potassium persulfate (KPS) as an initiator, with its mass being 0.1% to 0.5% of the mass of starch, dissolve it in 5 mL of water to form an initiator solution, and slowly drop it into the flask;

[0074] Step S205, raise the temperature to a preset temperature (such as 60°C to 70°C) to cause the crosslinking reaction of the mixed solution until the reaction is complete;

[0075] Step S206, take out the prepared gel, place it in a vacuum drying oven at 45°C to 55°C and dry it for 44 hours to 52 hours, and grind part of the gel to the required particle size for standby.

[0076] A preparation method of a temperature-responsive starch-based gel-coated slow-release fertilizer, comprising the following steps:

[0077] Step S301, spray anhydrous ethanol in an amount of 5 mL to 15 mL on the surface of urea to make its surface evenly wet;

[0078] Step S302, take the SNA powder prepared according to claim 2 and ground to less than 100 mesh, and uniformly coat it on the surface of urea by the disk method, with the coating layer number being 2 to 4 layers;

[0079] Step S303, dry at 45°C to 55°C for 44 hours to 52 hours to obtain a temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF).

[0080] In step S302, the preferred coating layer number of the SNA powder is 3 layers, and the particle size of the urea particles is controlled between 1 mm and 3 mm.

[0081] The temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF) can adjust the release rate of nutrients according to environmental temperature changes, and is especially used in occasions for improving fertilizer utilization rate, reducing nutrient loss and environmental pollution.

[0082] The working principle of the embodiment of the present invention is: Using amorphous starch as the starting material, through heat treatment and mechanical stirring in an ethanol solution, the crystalline structure of starch particles is broken, and it is transformed into an amorphous form that is more easily processed and reacted. This process not only improves the solubility and reactivity of starch, but also lays a foundation for the subsequent copolymerization reaction with functional monomers.

[0083] Utilizing the temperature-sensitive property of N-isopropylacrylamide (NIPAM), along with acrylamide (AM) as a comonomer, under the action of the cross-linking agent N,N'-methylenebisacrylamide (MBA), a temperature-responsive starch-based gel (SNA) was formed through free radical polymerization. This gel can undergo volume phase transitions at different temperatures, thereby regulating the size and permeability of its internal microporous structure, and further affecting the nutrient release rate.

[0084] During the preparation of the slow-release fertilizer, urea, as the core nutrient component, was first evenly wetted with absolute ethanol to improve its surface properties, making it more easily and evenly coated with SNA powder. Subsequently, the SNA powder was coated on the surface of urea particles in multiple layers by the disk method, forming a temperature-responsive coating layer. This coating layer can not only effectively control the nutrient release rate but also automatically adjust the release rate according to the change of environmental temperature to achieve precise nutrient supply.

[0085] The prepared temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF) can slowly release nutrients at an appropriate soil temperature, reduce nutrient loss and environmental pollution, and improve the fertilizer utilization rate. At the same time, its temperature-responsive property enables the fertilizer to adapt to the climate conditions of different seasons and regions, providing a more scientific and efficient nutrient management scheme for agricultural production.

[0086] Example 2;

[0087] Results of structural characterization and performance testing

[0088] 1. FTIR characterization analysis of SNA

[0089] The FTIR analysis results of amorphous starch (Aa), AM (Ab), SNA (Ac), SNA gels with a molar ratio of NIPAM to AM of 2:1 (Ba), 3:2 (Bb), 1:1 (Bc), 2:3 (Bd), 1:3 (Be), and a molar ratio of St to monomers of 2:1 (Ca), 3:1 (Cb), 4:1 (Cc), 5:1 (Cd), 6:1 (Ce) are as Figure 4 shown. In curve Aa, the stretching vibration absorption peak of the hydroxyl group (-OH) on the starch molecular chain is at 3200 - 3400 cm -1 The stretching vibration absorption peak of the methylene group (-CH2-) in starch is at 2900 cm -1 The stretching vibration absorption peak of the carbonyl group (C=O) in starch is at 1650 cm -1 The stretching vibration absorption peak of the carbon-hydrogen bond (C-H) in starch is at 1400 cm -1 In curve Ab, the stretching vibration absorption peak of the imino group (-NH) in the AM amide group is at 3283 cm -1 The stretching vibration absorption peak of the methylene group (-CH2-) in starch is at 2850 cm-1 is the stretching vibration absorption peak of the carbon-hydrogen bond (C-H) in AM, 2370 cm -1 and 1586 cm -1 are the stretching vibration absorption peaks of the carbonyl group (C=O), 1457 cm -1 is the in-plane bending vibration of the mixed carbon-nitrogen bond (C-N) and nitrogen-hydrogen bond (N-H) in the amide group of AM, 1190 cm -1 is the stretching vibration absorption peak of the carbon-nitrogen bond (C-N). It can be observed in curve Ac that the stretching vibration absorption peak of the carbon-hydrogen bond (C-H) at 1400 cm -1 , the stretching vibration absorption peaks of the carbonyl group (C=O) at 1586 cm -1 and 1650 cm -1 and the stretching vibration absorption peak of the imino group (-NH) at 3283 cm -1 indicate that the cross-linking reaction occurred between amorphous starch and NIPAM and AM, and the SNA gel was successfully prepared.

[0090] With the increase in the proportion of NIPAM, the characteristic absorption peak of the carbonyl group (C=O) at 1586 cm -1 and the in-plane rocking vibration absorption peak at 1120 cm -1 in the starch-based gel gradually increase ( Figure 4 B), and with the increase in the proportion of starch, the stretching vibration absorption peak of the carbonyl group (C=O) at 1650 cm -1 in the SNA gel gradually weakens ( Figure 4 C).

[0091] 2. XRD Characterization Analysis of SNA

[0092] As Figure 5As shown, XRD was used to characterize the crystal structures of SNA gels with starch (Aa), amorphous starch (Ab), molar ratios of NIPAM to AM of 2:1 (Ba), 3:2 (Bb), 2:3 (Bc), 1:2 (Bd), and molar ratios of St to monomer of 2:1 (Ca), 3:1 (Cb), 4:1 (Cc), 5:1 (Cd), 6:1 (Ce). Diffraction peaks of starch appeared at 2θ of 18.2°, 22.6°, 34.2°, and 81.3° in curve Aa. The baseline of the spectrum was not stable, the diffraction peaks were relatively thin and had high intensity, and there were many sharp peaks between 10° and 25°, which might come from the impurity components in the starch. In curve Ab, the baseline of the spectrum was relatively stable, and the diffraction peaks at 18.2°, 22.6°, and 34.2° were relatively wide and had weak intensity, indicating that the crystallinity of amorphous starch was lower than that of starch. Gels prepared from amorphous starch would have a higher cross-linking degree. Compared with the XRD spectrum of amorphous starch, SNA gels only had relatively wide and blunt peaks between 2θ of 10° and 25°, the baseline of the spectrum was relatively stable, and the impurity components disappeared significantly.

[0093] With the increase in the proportion of NIPAM, the crystallinity of SNA gels first increased and then decreased ( Figure 5 B). When the molar ratio of NIPAM to AM was 1:1, the diffraction peak at 2θ of 81.3° disappeared. With the increase in the proportion of amorphous starch, the crystallinity of SNA gels first decreased and then increased ( Figure 5 C). When the molar ratio of St to monomer was 4:1, the crystallinity was the lowest, the hydrogen bonds in the starch were broken, and the area of the amorphous region increased, improving the swelling performance of the gel.

[0094] 3. SEM Characterization Analysis of SNA

[0095] Figure 6The SEM images of SNA gels at different scales with NIPAM to AM molar ratios of 3:1 (a, b, c), 2:1 (d, e, f), 3:2 (g, h, i), 1:1 (j, k, l), 2:3 (m, n, o), 1:2 (p, q, r) and 1:3 (s, t, u) are shown. When the molar ratio of NIPAM to AM is 3:1, the gel presents a dense three-dimensional network structure, the gel skeleton wall is thin and loose, the gel skeleton width is about 1 μm, and the pore diameter is between 5 and 10 μm. At this time, although the gel has good water absorption performance, it reaches swelling equilibrium quickly. After rapid water absorption and swelling, its structure is easily destroyed, and the sustained release performance is not excellent enough. When the molar ratio of NIPAM to AM is 3:1, the gel still presents a dense three-dimensional network structure, the gel skeleton width is about 2 μm, and the pore diameter is between 10 and 15 μm. When the molar ratio of NIPAM to AM is 3:2, the gel skeleton wall is still thin, but the gel skeleton width increases to 10μm, the pore diameter is between 15 and 30μm, and the water absorption and swelling properties of the gel will be enhanced. When the molar ratio of NIPAM to AM is 1:1, the gel skeleton wall begins to thicken, and the pore diameter remains between 20 and 30μm. When the molar ratio of NIPAM to AM is 2:3, the gel skeleton width increases to between 30 and 40μm, and the pore diameter increases to 70 to 90μm. At this time, the gel water absorption and swelling properties become superior, but the gel skeleton wall is thicker, which will hinder the release of fertilizers. When the molar ratio of NIPAM to AM is 1:2, the gel skeleton wall becomes denser and the pore diameter is reduced to between 15 and 20μm. When the molar ratio of NIPAM to AM is 1:3, the main monomer in the gel is AM, the gel skeleton wall thickness is about 10 μm, the pores are evenly distributed, the pore diameter is about 100 μm, the three-dimensional network structure of the gel becomes uniform and loose, and the water absorption and swelling properties are excellent.

[0096] As the NIPAM content gradually increases, the thickness of the skeleton wall of the SNA gel gradually decreases, the skeleton width gradually decreases, and the pore diameter gradually decreases. The addition of AM increases the width of the gel skeleton and enlarges the pore diameter, which enhances the water absorption and swelling performance of the gel to a certain extent. However, as the amount of AM added gradually increases, the gel skeleton wall will be too thick, and a large amount of nutrients will be adsorbed in the gel, which cannot be released in time. The gel can only be absorbed by the plant after it is completely degraded, which prolongs the nutrient release cycle and reduces the slow-release performance of the fertilizer.

[0097] Figure 7The SEM images of SNA gels with the molar ratio of St to monomer being 2:1 (a, f), 3:1 (b, g), 4:1 (c, h), 5:1 (d, i), and 6:1 (e, j) are shown at different scales. When the molar ratio of St to monomer is 2:1, the thickness of the gel's skeleton wall is relatively thick, the width of the gel skeleton is about 30 μm, and the pore diameter is between 40 and 50 μm. When the molar ratio of St to monomer is 3:1, the thickness of the gel's skeleton wall is still relatively thick, and the width of the gel skeleton is about 20 μm. When the molar ratio of St to monomer is 4:1, the thickness of the gel's skeleton wall significantly thins, the pore diameter is between 20 and 30 μm, and a relatively obvious three-dimensional network structure appears. When the molar ratio of St to monomer is 5:1, the skeleton wall of the gel begins to become thin and loose, the pore diameter is between 30 and 50 μm, and some unreacted starch granules are embedded in the gel skeleton. When the molar ratio of St to monomer is 6:1, its pore diameter suddenly increases to between 70 and 100 μm. Due to the overly large pores, during the fertilization process, a large amount of nutrients will be rapidly lost, and the slow-release effect cannot be achieved.

[0098] As the content of St gradually increases, the thickness of the skeleton wall of the SNA gel gradually thins, the width of the gel skeleton gradually decreases, the pore diameter first decreases and then increases, and the minimum value is obtained when the molar ratio of St to monomer is 4:1. When too much St is added, it can be clearly seen that many unreacted starch granules appear in the gel network, and the reaction is incomplete.

[0099] The working principle of the embodiments of the present invention is as follows: FTIR analysis reveals the changes in chemical functional groups in the SNA gel, thereby reflecting the successful progress of its cross-linking reaction. The cross-linking reaction of amorphous starch (Aa) with NIPAM (N-isopropylacrylamide) and AM (acrylamide) is verified in the FTIR spectrum, specifically manifested as the appearance and changes of characteristic absorption peaks such as carbonyl (C=O), carbon-hydrogen bond (C-H), and imino group (-NH); with the increase in the proportion of NIPAM, the enhancement of specific absorption peaks (such as the carbonyl at 1586 cm -1 ) indicates that the cross-linking degree may increase; while the increase in the proportion of starch leads to the weakening of some absorption peaks (such as the carbonyl at 1650 cm -1 ), which may be related to the change in the interaction between starch molecules. These changes together illustrate the adjustment of the chemical structure of the SNA gel, thereby affecting its performance.

[0100] XRD analysis provides information on the crystal structure of SNA gel. Compared with amorphous starch, SNA gel shows lower crystallinity in the XRD spectrum, indicating that the cross-linking reaction destroys the original crystal structure and forms a more disordered structure. With the increase of NIPAM ratio, the crystallinity of SNA gel increases first and then decreases, indicating that an appropriate amount of NIPAM helps to form a more stable cross-linked structure, but an excessive amount may destroy this balance. Similarly, the change in starch ratio also affects the crystallinity of the gel. The crystallinity is lowest when the molar ratio of St to monomer is 4:1, which may be related to the breaking of hydrogen bonds and the increase of amorphous regions, thereby enhancing the swelling properties of the gel.

[0101] The SEM images intuitively show the microscopic morphology of SNA gel, including key features such as its skeleton structure, wall thickness and pore size. With the change of the ratio of NIPAM to AM, the thickness, width and pore diameter of the gel skeleton wall all change significantly, which directly affects the water absorption and swelling properties and sustained release properties of the gel. For example, when the NIPAM content increases, the gel skeleton becomes thinner and the pore diameter decreases, but too much AM will cause the skeleton wall to be too thick, hindering the timely release of nutrients. As for the change of St content, a moderate St content helps to form a uniform three-dimensional network structure, but an excessive amount will lead to the appearance of unreacted starch granules, affecting the integrity and performance of the gel.

[0102] Embodiment 3;

[0103] 4. Effect of process conditions on the water absorption and swelling rate of SNA

[0104] The slow-release performance and water retention performance of hydrogel slow-release fertilizers depend on the water absorption and swelling characteristics of the polymer network, and the swelling capacity depends on the strength and elasticity of the hydrogel network. If the network is weak, it cannot bind water, resulting in low swelling capacity. On the other hand, if the hydrogel structure is too dense, the ability to bind water is also weak. In the study of this chapter, the swelling behavior was measured using a single-factor experiment, and the molar ratio of NIPAM to AM and the molar ratio of St to monomer were optimized. The equilibrium swelling of all prepared SNA hydrogels was studied in an aqueous environment at room temperature and pH = 7, and the water absorption swelling rate of the samples was measured every 3 hours.

[0105] (1) Effect of the molar ratio of NIPAM to AM on the water absorption and swelling rate of SNA

[0106] Figure 8 The water absorption swelling ratio (A), equilibrium swelling ratio (B) and swelling equilibrium time (C) of SNA gels with different NIPAM to AM molar ratios of 3:1, 2:1, 3:2, 1:1, 2:3, 1:2 and 1:3 are shown. Figure 8It can be seen that the water absorption and swelling ratios of all SNA gels with different NIPAM / AM molar ratios gradually increase with time and reach swelling equilibrium after a certain time. The SNA gel with a NIPAM / AM molar ratio of 3:1 reaches swelling equilibrium the fastest, with a swelling equilibrium time of 6 h, but its equilibrium swelling ratio is the lowest, only 4.16 g / g. This is because when the NIPAM content is too high, the gel adsorbs water molecules very quickly, but due to its fine and loose gel skeleton and overly dense three-dimensional network structure, it has insufficient deformation ability and less water absorption. The SNA gel with a NIPAM / AM molar ratio of 1:3 reaches swelling equilibrium at 102 h, and the equilibrium swelling ratio is 8.35 g / g. This is because the addition of AM increases the hydrophilicity of the gel, the width of the gel skeleton increases, and the pore diameter becomes larger, making it have better water absorption and swelling performance. The SNA gel with a NIPAM / AM molar ratio of 1:1 has the largest equilibrium swelling ratio (12.46 g / g) and the longest swelling equilibrium time (171 h). The results are consistent with the SEM characterization results. The appropriate gel skeleton thickness, width, and pore size enable it to have a good speed of adsorbing water molecules, and at the same time, its structure also has good deformation ability, making it have excellent water absorption and swelling performance.

[0107] The water absorption and swelling ratios of SNA gels gradually decrease according to the following NIPAM / AM molar ratios: NIPAM:AM = 1:1 > NIPAM:AM = 1:3 > NIPAM:AM = 1:2 > NIPAM:AM = 2:3 > NIPAM:AM = 3:2 > NIPAM:AM = 2:1 > NIPAM:AM = 3:1.

[0108] (2) Effect of the St / monomer molar ratio on the water absorption and swelling ratio of SNA

[0109] Figure 9 The water absorption and swelling ratios (A), equilibrium swelling ratios (B), and swelling equilibrium times (C) of SNA gels with different St / monomer molar ratios of 2:1, 3:1, 4:1, 5:1, and 6:1 are shown. From Figure 9A can be seen that the water absorption swelling rate of all SNA gels with different St to monomer molar ratios gradually increases with time and reaches swelling equilibrium after a certain period of time. The SNA gel with a St to monomer molar ratio of 6:1 reaches swelling equilibrium the fastest, with a swelling equilibrium time of 75h and a balanced swelling rate of 9.51g / g. This may be because when too much St content is added, the gel network pores are larger, which enables it to have a faster rate of adsorption of water molecules. The equilibrium swelling rate (9.40g / g) of the SNA gel with a St to monomer molar ratio of 5:1 is similar to that of the SNA gel with a St to monomer molar ratio of 6:1. The lower equilibrium swelling rate may be due to excessive St content, too little monomer content, and the gel cross-linking network is not tight enough. The SNA gel with a St to monomer molar ratio of 2:1 has the largest equilibrium swelling rate (13.08g / g) and the longest swelling equilibrium time (198h). This is due to the combined effect of hydrophilic monomers and starch gel, which makes it have excellent water absorption and swelling properties.

[0110] The water absorption swelling ratio of SNA gel gradually decreases according to the following molar ratio of St to monomer: St:(NIPAM+AM)=2:1>St:(NIPAM+AM)=4:1>St:(NIPAM+AM)=3:1>St:(NIPAM+AM)=6:1>St:(NIPAM+AM)=5:1.

[0111] 5. Study on the temperature response performance of SNA

[0112] Temperature responsiveness is the main indicator for measuring the performance of thermosensitive materials. In this patent, the temperature responsiveness of SNA gel is mainly tested by measuring the swelling equilibrium rate of the gel at different temperatures. The equilibrium swelling of all prepared SNA hydrogels is studied in water environments at 0°C, 10°C, 25°C, 35°C, and 45°C, and the water absorption swelling rate of the samples is measured every 3 hours.

[0113] (1) Effect of the molar ratio of NIPAM to AM on the temperature responsiveness of SNA

[0114] Figure 10Shows the water absorption swelling ratio, equilibrium swelling ratio and swelling equilibrium time of SNA gels with NIPAM to AM molar ratios of 3:1 (A, B, C), 2:1 (D, E, F), 3:2 (G, H, I), 1:1 (J, K, L), 2:3 (M, N, O), 1:2 (P, Q, R) and 1:3 (S, T, U) at different temperatures. The SNA gel with a NIPAM to AM molar ratio of 3:1 reaches the swelling equilibrium fastest at 25 °C, with a swelling equilibrium time of 6 h and an equilibrium swelling ratio of 4.16 g / g; it shows the lowest equilibrium swelling ratio (3.24 g / g) at 45 °C, but shows the optimal swelling equilibrium time (27 h) at this temperature; it shows the highest equilibrium swelling ratio (3.24 g / g) at 0 °C. The SNA gel with a NIPAM to AM molar ratio of 2:1 reaches the swelling equilibrium fastest at both 0 °C and 35 °C (45 h), with an equilibrium swelling ratio of 5.75 g / g at 0 °C and 5.45 g / g at 35 °C; it shows the lowest equilibrium swelling ratio (4.59 g / g) at 25 °C, but shows the optimal swelling equilibrium time (75 h) at this temperature; it shows the highest equilibrium swelling ratio (6.76 g / g) and the optimal swelling equilibrium time (75 h) at 10 °C. The SNA gel with a NIPAM to AM molar ratio of 3:2 reaches the swelling equilibrium fastest at 35 °C (27 h), and also shows a relatively good equilibrium swelling ratio (9.56 g / g); it shows the lowest equilibrium swelling ratio (4.65 g / g) at 25 °C; it shows the highest equilibrium swelling ratio (10.93 g / g) at 10 °C. The SNA gel with a NIPAM to AM molar ratio of 1:1 reaches the swelling equilibrium fastest at 35 °C (72 h), and also shows the lowest equilibrium swelling ratio (11.76 g / g); it shows the highest equilibrium swelling ratio (13.51 g / g) at 45 °C; it shows the optimal swelling equilibrium time (171 h) at 25 °C, and also shows a relatively good equilibrium swelling ratio (12.46 g / g). The SNA gel with a NIPAM to AM molar ratio of 2:3 reaches the swelling equilibrium fastest at both 10 °C and 25 °C, with a swelling equilibrium time of 78 h, and the highest equilibrium swelling ratio (9.20 g / g) at 10 °C is almost twice the lowest equilibrium swelling ratio (4.86 g / g) at 25 °C. The SNA gel with a NIPAM to AM molar ratio of 1:2 reaches the swelling equilibrium fastest at 10 °C (78 h); it shows the highest equilibrium swelling ratio (8.49 g / g) at 45 °C; it shows the lowest equilibrium swelling ratio (5.93 g / g) at 25 °C. The SNA gel with a NIPAM to AM molar ratio of 1:3 reaches the swelling equilibrium fastest at 35 °C (84 h); it shows the highest equilibrium swelling ratio (13.79 g / g) and the optimal swelling equilibrium time (198 h) at 45 °C; it shows the lowest equilibrium swelling ratio (8.36 g / g) at 25 °C.

[0115] Figure 11 shows the temperature responsiveness of SNA gels with NIPAM to AM molar ratios of 3:1, 2:1, 3:2, 1:1, 2:3, 1:2, and 1:3. As Figure 11 shown in A, the temperature responsiveness change trends of SNA gels with NIPAM to AM molar ratios of 3:1, 2:1, and 3:2 are similar. As the temperature increases, the image changes in an "M" shape. At all temperatures, as the NIPAM content decreases, its equilibrium swelling ratio gradually increases, indicating that the addition of AM has successfully improved the water absorption and swelling performance of NIPAM gels. As Figure 11 shown in B, the temperature responsiveness change trends of SNA gels with NIPAM to AM molar ratios of 1:2 and 1:3 are similar. As the temperature increases, the image changes in an "N" shape. The temperature responsiveness of SNA gels with NIPAM to AM molar ratios of 1:2 and 1:3 first decreases and then increases as the temperature increases. The temperature responsiveness of the SNA gel with a NIPAM to AM molar ratio of 1:1 changes in a "W" shape as the temperature increases, and it maintains a high equilibrium swelling ratio at each temperature. At 45 °C, with the addition of AM, the gels all have a high equilibrium swelling ratio, indicating that the addition of AM has successfully improved the shrinkage phenomenon of NIPAM gels above the critical phase transition temperature.

[0116] (2) Influence of the St to monomer molar ratio on the temperature responsiveness of SNA

[0117] Figure 12Shows the water absorption swelling ratio, equilibrium swelling ratio and swelling equilibrium time of SNA gels with a molar ratio of St to monomer of 2:1 (A, B, C), 3:1 (D, E, F), 4:1 (G, H, I), 5:1 (J, K, L), 6:1 (M, N, O) at different temperatures. The SNA gel with a molar ratio of St to monomer of 2:1 reaches the swelling equilibrium fastest at 35 °C (93 h), and at the same time shows the lowest equilibrium swelling ratio (10.21 g / g); it shows the highest equilibrium swelling ratio (13.22 g / g) and the optimal swelling equilibrium time (240 h) at 45 °C, and also shows a relatively good equilibrium swelling ratio (13.08 g / g) and a relatively good swelling equilibrium time (198 h) at 25 °C. The SNA gel with a molar ratio of St to monomer of 3:1 reaches the swelling equilibrium fastest at 35 °C (81 h), and at the same time shows the lowest equilibrium swelling ratio (10.58 g / g); it shows the highest equilibrium swelling ratio (11.83 g / g) at 0 °C; it shows similar equilibrium swelling ratios (11.32 g / g, 11.44 g / g) and swelling equilibrium times (141 h, 144 h) at 10 °C and 25 °C. The SNA gel with a molar ratio of St to monomer of 4:1 reaches the swelling equilibrium fastest at 35 °C (72 h), and at the same time shows the lowest equilibrium swelling ratio (11.76 g / g); it shows the highest equilibrium swelling ratio (13.51 g / g) at 45 °C; it shows the optimal swelling equilibrium time (171 h) at 25 °C, and also shows a relatively good equilibrium swelling ratio (12.46 g / g). The SNA gel with a molar ratio of St to monomer of 5:1 reaches the swelling equilibrium fastest at 10 °C (78 h), and at the same time shows the lowest equilibrium swelling ratio (9.28 g / g); it shows the highest equilibrium swelling ratio (13.14 g / g) and the optimal swelling equilibrium time (123 h) at 0 °C. The SNA gel with a molar ratio of St to monomer of 6:1 reaches the swelling equilibrium fastest at 10 °C (54 h); it shows the lowest equilibrium swelling ratio (9.51 g / g) at 25 °C; it shows the lowest equilibrium swelling ratio (8.15 g / g) at 45 °C.

[0118] Figure 13 Shows the temperature responsiveness of SNA gels with a molar ratio of St to monomer of 2:1, 3:1, 4:1, 5:1 and 6:1. As Figure 13 shown in A, the temperature responsiveness change trends of the SNA gels with a molar ratio of St to monomer of 2:1 and 5:1 are completely opposite. As the temperature increases, the SNA gel with a molar ratio of St to monomer of 2:1 first increases, then decreases and then increases again, and the image shows an "N" - type change. The SNA gel with a molar ratio of St to monomer of 5:1 first decreases, then increases and then decreases. As Figure 13As shown in B, the temperature-responsive change trends of SNA gels with a molar ratio of St to monomer of 3:1, 4:1, and 6:1 are similar. As the temperature increases, they show a "W"-shaped change. Among them, the SNA gel with a molar ratio of t to monomer of 4:1 shows excellent equilibrium swelling ratio at all temperatures.

[0119] Combining the results of the structural characterization, water absorption and swelling properties, and temperature-responsive properties of SNA, considering the arid and cold climate in the northwest as the application environment, which requires good swelling properties at both low and normal temperatures, this patent selects the SNA gel with a molar ratio of NIPAM to AM of 1:1 and a molar ratio of St to monomer of 4:1 as the coating material for fertilizers.

[0120] 6. Study on the water retention and water holding properties of SNA-SRF in soils at different temperatures

[0121] (1) Water retention properties of SNA-SRF in soils

[0122] Figure 14 It shows the water retention rates (A) of 200 g of dry soil after adding 0 g and 1 g of SNA-SRF in a 25 °C soil environment, and the water retention rates (B) of 200 g of dry soil after adding 1 g of SNA-SRF in soil environments at 0 °C, 10 °C, 25 °C, 35 °C, and 45 °C. As Figure 14 shown in A, in a 25 °C environment, the water retention rates of 200 g of dry soil after adding 0 g and 1 g of SNA-SRF are 46.70% and 52.43% respectively. The water retention rate of the group adding 1 g of SNA-SRF is 5.73% higher than that of the blank group. As Figure 14 shown in B, in environments at 0 °C, 10 °C, 25 °C, 35 °C, and 45 °C, the water retention rates of 200 g of dry soil after adding 1 g of SNA-SRF are 52.57%, 52.17%, 52.43%, 52.03%, and 52.93% respectively. SNA-SRF shows good water retention ability.

[0123] (2) Water holding properties of SNA-SRF in soils

[0124] Figure 15 It shows the water holding properties (A) of 200 g of dry soil after adding 0 g and 1 g of SNA-SRF in a 25 °C soil environment, and the water holding properties (B) of 200 g of dry soil after adding 1 g of SNA-SRF in soil environments at 0 °C, 10 °C, 25 °C, 35 °C, and 45 °C. As Figure 15 shown in A, in a 25 °C soil environment, the water holding rates of 200 g of dry soil after adding 0 g and 1 g of SNA-SRF are 19.17% and 45.63% respectively. The water holding rate of the group adding 1 g of SNA-SRF is 26.46% higher than that of the blank group. As Figure 15As shown in Figure B, the rapid water loss stage occurs within 0 - 30 d. After 30 d, the water retention rate curves at each temperature gradually reach a plateau successively. In environments of 0℃, 10℃, 25℃, 35℃, and 45℃, after adding 1 g of SNA - SRF, the water retention rates of 200 g of dry soil are 52.40%, 48.10%, 45.63%, 30.92%, and 34.42% respectively, and the times to reach the plateau are 35 d, 40 d, 50 d, 30 d, and 45 d respectively. In the soil environments of 0℃ and 10℃, due to the relatively low environmental temperature, the evaporation rate of water is slow, and a small amount of water solidifies and remains in the soil, resulting in a relatively high water retention rate. In the environments of 25℃, 35℃, and 45℃, the water retention rates can still be maintained at 45.63%, 30.92%, and 34.42% respectively. This is mainly because the three - dimensional network structure of SNA improves the water - holding capacity and interception capacity of the soil, reduces water evaporation, and can be regarded as a "water storage reservoir", which retains and supplies more water for crops over time and effectively prevents water loss. Adding SNA - SRF can effectively slow down water evaporation and improve the water - retaining performance of the soil.

[0125] 7. Sustained - release and controlled - release performance of SNA - SRF in different temperature water environments and soils

[0126] (1) Sustained - release and controlled - release performance of SNA - SRF in water

[0127] Figure 16 Figure shows the nitrogen release rate of urea and SA - SRF in a 25℃ water environment (A), the nitrogen release rate of SNA - SRF in water environments of 0℃, 10℃, 25℃, 35℃, and 45℃ (B), the cumulative nitrogen balance release rate (C), and the nitrogen balance release time (D). As Figure 16 shown in Figure A, in a 25℃ water environment, pure urea as the blank group is rapidly and completely released within 24 h, and the cumulative nitrogen balance release rate is 100.00%. The nitrogen cumulative release of SNA - SRF reaches equilibrium at 204 h, and the cumulative nitrogen balance release rate is 88.95%. As Figure 16 shown in Figure B, the nitrogen release curves of SNA - SRF in water environments of 0℃, 10℃, 25℃, 35℃, and 45℃ show a rapid release stage within 0 - 60 h, a gradually decreasing nitrogen release rate within 61 - 90 h, and the nitrogen release curve gradually enters a plateau stage and reaches equilibrium after 90 h.

[0128] As Figure 16As shown in Figures C and D, the cumulative nitrogen balance release rates of SNA-SRF in water environments at 0°C, 10°C, 25°C, 35°C, and 45°C were 85.57%, 82.98%, 88.95%, 80.86%, and 91.76% respectively, and the required times were 120 h, 132 h, 204 h, 96 h, and 156 h respectively. Compared with 10°C and 35°C, SNA-SRF has better slow-release performance in water environments at 0°C, 25°C, and 45°C.

[0129] The slow-release behavior of coated fertilizers is an important means to evaluate slow / controlled-release fertilizers. In this patent, the nitrogen slow-release performance of SNA-SRF in water environments and soils at different temperatures was evaluated.

[0130] (2) Slow / controlled-release performance of SNA-SRF in soil

[0131] Figure 17 shows the nitrogen release rates of urea and SNA-SRF in a 25°C soil environment (A), the nitrogen release rates of SNA-SRF in soil environments at 0°C, 10°C, 25°C, 35°C, and 45°C (B), the cumulative nitrogen balance release rate (C), and the nitrogen balance release time (D). As Figure 17 shown in A, in the 25°C soil environment, pure urea as the blank group was rapidly and completely released within 5 days, and the cumulative nitrogen balance release rate was 100.00%. The nitrogen cumulative release of SNA-SRF reached equilibrium at 40 days, and the cumulative nitrogen balance release rate was 77.94%. As Figure 17 shown in B, the nitrogen release curves of SNA-SRF in soil environments at 0°C, 10°C, 25°C, 35°C, and 45°C were in a rapid release period from 0 to 15 days, the nitrogen release rate gradually decreased from 15 to 30 days, and the nitrogen release curve gradually entered a plateau period after 30 days to reach equilibrium. Figure 17 As shown in C and D, the cumulative nitrogen balance release rates of SNA-SRF in soil environments at 0°C, 10°C, 25°C, 35°C, and 45°C were 78.76%, 76.38%, 77.94%, 75.83%, and 81.07% respectively, and the required times were 30 days, 35 days, 40 days, 30 days, and 40 days respectively. Compared with the cumulative nitrogen balance release rate of SNA-SRF in water, the cumulative nitrogen balance release rate of SNA-SRF in soil was less, probably because the soil environment is more complex than the water environment, and the swelling performance of SNA and the slow-release performance of SNA-SRF are affected by more factors.

[0132] The initial nitrogen release rates, 25-day cumulative nitrogen release rates, and cumulative nitrogen balance release rates of SNA-SRF in soil environments at 0°C, 10°C, 25°C, 35°C, and 45°C are statistically shown in Figure 18 According to the test results of slow / controlled-release performance, SNA-SRF fully complies with the national standard for slow-release fertilizers.

[0133] According to the test results of FTIR and XRD characterizations, this patent successfully prepared a temperature-responsive starch-based hydrogel SNA with starch as the base material, AM as the monomer, and NIPAM as the temperature-responsive group.

[0134] Combined with the SEM characterization test results of SNA, as well as the water absorption and swelling performance and temperature-responsive performance results, for the application environment of the arid and cold climate in the northwest, this patent selected the SNA gel with a molar ratio of NIPAM to AM of 1:1 and a molar ratio of St to monomer of 4:1 as the coating material for fertilizers.

[0135] In environments of 0°C, 10°C, 25°C, 35°C, and 45°C, after adding 1 g of SNA-SRF, the water holding rates of 200 g of dry soil were 52.57%, 52.17%, 52.43%, 52.03%, and 52.93% respectively, and the water retention rates were 52.40%, 48.10%, 45.63%, 30.92%, and 34.42% respectively. SNA-SRF has good water holding and water retention properties. Adding SNA-SRF can effectively slow down water evaporation and improve the water retention performance of the soil.

[0136] The cumulative nitrogen balance release rates of SNA-SRF in water environments of 0°C, 10°C, and 25°C were 85.57%, 82.98%, and 88.95% respectively, and the cumulative nitrogen balance release rates in soil environments were 78.76%, 76.38%, and 77.94% respectively. SNA-SRF has excellent slow-release performance in low-temperature and normal-temperature environments, which meets the application environment of the arid and cold climate in the northwest. According to the slow-release performance test results, SNA-SRF fully meets the national standard for slow-release fertilizers.

[0137] The working principle of the embodiments of this invention is as follows: First, by adjusting the molar ratio of N-isopropylacrylamide (NIPAM) to acrylamide (AM), and the molar ratio of starch (St) to monomer (NIPAM + AM), a series of SNA hydrogels with different water absorption and swelling properties were successfully prepared. It was found that when the molar ratio of NIPAM to AM was 1:1, the SNA gel exhibited the optimal water absorption and swelling performance, which was mainly due to its appropriate gel skeleton thickness, width, and pore size. These characteristics enabled the gel to quickly and effectively bind water molecules while maintaining the structural stability. Similarly, when the molar ratio of St to monomer was 4:1, the SNA gel also showed excellent water absorption and swelling performance, which benefited from the combined action of hydrophilic monomers and starch gels.

[0138] Secondly, an in-depth study was conducted on the temperature-responsive properties of the SNA gel. The experimental results showed that SNA gels with different molar ratios of NIPAM to AM exhibited different swelling behaviors at different temperatures. In particular, when the molar ratio of NIPAM to AM was 1:1, the SNA gel could maintain a high equilibrium swelling ratio under multiple temperature conditions, indicating its good temperature adaptability. At the same time, the adjustment of the molar ratio of St to the monomer also significantly affected the temperature responsiveness of the SNA gel. Among them, the SNA gel with a molar ratio of St to the monomer of 4:1 also showed stable swelling properties at different temperatures.

[0139] When the SNA gel was applied to the soil environment, by measuring its water retention and water holding properties at different temperatures, it was found that SNA-SRF could significantly improve the water retention rate and water holding rate of the soil. Especially in a low-temperature environment, SNA-SRF could slow down the evaporation rate of water and provide a stable water supply for crops. This characteristic makes SNA-SRF particularly suitable for the arid and cold climate conditions in the northwest.

[0140] The controlled-release properties of SNA-SRF in water environments and soils at different temperatures were studied. The experimental results showed that SNA-SRF exhibited good nitrogen slow-release properties in both water and soil, and could continuously release nutrients for a long time to meet the growth needs of crops. Especially in low-temperature and normal-temperature environments, the slow-release properties of SNA-SRF were more excellent, meeting the national standards for slow-release fertilizers.

[0141] In this part of the study, by optimizing the preparation process conditions of the SNA gel, a SNA-SRF slow-release fertilizer with excellent water absorption and swelling properties, temperature-responsive properties, and controlled-release properties was successfully prepared. This fertilizer showed good water retention and water holding properties, as well as stable nutrient release properties under the arid and cold climate conditions in the northwest, providing strong support for the sustainable development of agricultural production.

[0142] Working principle: Through heat treatment and mechanical stirring in an ethanol solution, the crystalline structure of amorphous starch was broken and transformed into an amorphous form that was more easily processed and reacted. This transformation process significantly improved the solubility and reactivity of starch, laying a foundation for subsequent functional modification. Subsequently, N-isopropylacrylamide (NIPAM) and acrylamide (AM) were used as copolymerization monomers, and under the action of a cross-linking agent N,N'-methylenebisacrylamide (MBA), a temperature-responsive starch-based gel (SNA) was formed through a free radical polymerization reaction. Through its temperature-sensitive characteristics, this gel can undergo volume phase changes at different temperatures, adjusting the microporous structure, thereby achieving precise control of the nutrient release rate.

[0143] In the preparation of slow-release fertilizers, urea, as the core nutrient component, is first wetted with absolute ethanol to enhance its surface affinity, and then coated with multiple layers of SNA powder by the disk method to form a temperature-responsive coating layer. This coating layer not only effectively restricts the rapid release of nutrients, but also can automatically adjust the release rate according to the change of environmental temperature, realizing the intelligent supply of nutrients. This design greatly improves the utilization rate of fertilizers and reduces nutrient loss and environmental pollution.

[0144] Through analytical methods such as FTIR, XRD, and SEM, the change rules of the chemical structure, crystal morphology, and microscopic morphology of SNA gel are revealed. These changes not only verify the successful progress of the cross-linking reaction, but also reveal the influence of different monomer ratios on the gel properties. For example, an appropriate ratio of NIPAM and AM can form a stable cross-linked structure, enhancing the swelling performance and temperature responsiveness of the gel; while a moderate starch content helps to form a uniform three-dimensional network structure, improving the overall performance of the gel.

[0145] Applying the optimized SNA gel to the preparation of slow-release fertilizers, the SNA-SRF slow-release fertilizer with excellent water absorption and swelling performance, temperature-responsive performance, and slow and controlled release performance is successfully prepared. This fertilizer can significantly improve the water holding rate and water retention rate in the soil environment, providing a stable water supply for crops; at the same time, its temperature-responsive characteristics enable the nutrient release rate to be automatically adjusted according to the change of environmental temperature, meeting the nutrient requirements of crops at different growth stages. Especially in the arid and cold climate conditions in the northwest, SNA-SRF shows excellent adaptability and stability, providing strong support for the sustainable development of agricultural production.

[0146] Although the embodiments of the present invention have been shown and described, it will be understood by those of ordinary skill in the art that various changes, modifications, substitutions, and variations can be made to these embodiments without departing from the principles and spirit of the invention, and the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A temperature-responsive starch-based gel-coated slow-release fertilizer, characterized in that: Including the following ingredients: Amorphous starch: The starting material is ordinary corn starch; Cross-linking agent: N,N'-methylenebisacrylamide (MBA); N-isopropylacrylamide (NIPAM): used to impart temperature responsiveness to the gel; Acrylamide (AM): as a comonomer to improve gel properties; Urea: As the core component of slow-release fertilizer; Anhydrous ethanol: used to wet the urea surface and promote uniform coating of SNA powder.

2. A temperature-responsive starch-based gel-coated slow-release fertilizer according to claim 1, characterized in that: The method for preparing the amorphous starch comprises the following steps: Step S101, adding 25 g to 35 g of common corn starch into a three-necked round-bottom flask containing 100 ml to 150 ml of 45% to 55% ethanol by volume; Step S102, placing the flask in a water bath at 80°C to 90°C, and mechanically stirring at a speed of 140 rpm to 160 rpm for 1.5 hours to 2.5 hours; Step S103, filtering the mixed reaction liquid after the stirring treatment to separate the solid matter, and repeatedly washing it at least three times with 45% to 55% ethanol by volume to remove impurities and unreacted substances; Step S104, placing the washed solid matter in a blast drying oven and drying at 45° C. to 55° C. for 20 to 28 hours; Step S105, crushing the dried solid matter and sieving it through a sieve of 80 to 120 meshes to obtain the desired amorphous starch powder.

3. A temperature-responsive starch-based gel-coated slow-release fertilizer according to claim 1, characterized in that: The preparation method of temperature-responsive starch-based gel (SNA) comprises the following steps: Step S201, weighing 4 g to 6 g of the amorphous starch prepared according to claim 1 into a three-necked round-bottom flask, adding 90 ml to 100 ml of distilled water, and stirring to dissolve the starch evenly; Step S202, weighing N-isopropylacrylamide (NIPAM) and acrylamide (AM) at a specific molar ratio (such as NIPAM:AM=1:1 to 3:1), and adding a certain amount of N,N'-methylenebisacrylamide (MBA) as a cross-linking agent, wherein the amount of MBA added is 0.5% to 2% of the mass of the starch; Step S203, pre-crosslinking treatment is performed in a water bath at 50° C. to 60° C. with stirring at a speed of 140 rpm to 160 rpm for 25 minutes to 35 minutes; Step S204, potassium persulfate (KPS) is weighed as an initiator, the mass of which is 0.1% to 0.5% of the mass of starch, dissolved in 5 ml of water to form an initiator solution, and slowly added dropwise to the flask; Step S205, raising the temperature to a preset temperature (e.g., 60° C. to 70° C.) to allow the mixed solution to undergo a cross-linking reaction until the reaction is complete; Step S206, taking out the prepared gel, placing it in a vacuum drying oven at 45° C. to 55° C. and drying it for 44 to 52 hours, and grinding part of the gel to a desired particle size for later use.

4. The temperature-responsive starch-based gel-coated slow-release fertilizer according to claim 1, characterized in that: A method for preparing a temperature-responsive starch-based gel-coated slow-release fertilizer comprises the following steps: Step S301, spraying 5 ml to 15 ml of anhydrous ethanol on the surface of urea to uniformly wet the surface; Step S302, taking the SNA powder prepared according to claim 2 and ground to less than 100 mesh, and uniformly coating it on the surface of urea by a disc method, with the number of coating layers being 2 to 4; Step S303, drying at 45° C. to 55° C. for 44 to 52 hours to obtain a temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF).

5. A temperature-responsive starch-based gel-coated slow-release fertilizer according to claim 4, characterized in that: In the step S302, the number of coating layers of the SNA powder is preferably 3, and the particle size of the urea particles is controlled between 1 mm and 3 mm.

6. The temperature-responsive starch-based gel-coated slow-release fertilizer according to claim 1, characterized in that: The temperature-responsive starch-based gel-coated slow-release fertilizer (SNA-SRF) can adjust the release rate of nutrients according to changes in ambient temperature, and is particularly useful for improving fertilizer utilization, reducing nutrient loss and environmental pollution.

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