Starch-based high-water-absorption gel slow-release urea particles and preparation method thereof

Abstract

The invention belongs to the field of bio-based macromolecules and agricultural water absorption and retention fertilizers, and discloses starch-based high-water-absorption gel slow-release urea granules and a preparation method thereof.The preparation method comprises the following steps that 1, starch and urea are evenly premixed and then conveyed to a melting and plasticizing area of a screw extruder; (2) injecting an aqueous solution of a hydrophilic vinyl monomer and a cross-linking agent into a melting and plasticizing area of an extruder, melting and gelatinizing starch, and homogenizing and mixing the starch, the monomer and the cross-linking agent to form a pre-reaction starch-based melt; (3) injecting an initiator into a reaction zone of the extruder in stages, regulating and controlling the reaction temperature, initiating generation of starch free radicals and carrying out graft copolymerization reaction, and simultaneously realizing synchronous loading of urea; and (4) after the reaction is completed, carrying out saponification treatment, and carrying out extrusion molding, drying and pelletizing to obtain the water-retaining slow-release urea particles. The urea particles have the high water absorption characteristic, the water absorption rate is larger than or equal to 100 g / g, and the urea particles have the advantages of being continuous, efficient, short in reaction time, free of solvent residues and the like.

Description

Technical Field

[0001] The present invention belongs to the field of bio-based polymers and agricultural water-absorbing and water-retaining fertilizers, and specifically relates to starch-based highly water-absorbing gel slow-release urea particles prepared based on a one-step reaction extrusion technology and a preparation method thereof. Background Art

[0002] Water conservation in agriculture is one of the core approaches to resolving the triangular conflict of "water resources, food, and ecology." While traditional chemical fertilizers have played an important role in increasing crop yields, their excessive application has led to a series of serious environmental problems, such as eutrophication of water bodies, soil acidification, and structural degradation caused by nitrogen loss. Furthermore, traditional fertilizers suffer from low utilization rates, insufficient release during droughts, and easy loss with water during heavy rainfall or inappropriate irrigation, exacerbating resource waste and environmental burdens. Therefore, developing efficient and water-saving agriculture has become a key strategic direction for ensuring food security and ecological sustainability. Superabsorbent hydrogels, due to their excellent water absorption and retention capacity, as well as their nutrient loading and slow-release properties, have shown great potential for agricultural water conservation, fertilizer reduction and efficiency improvement, and soil improvement. These hydrogels are typically composed of three-dimensional polymer networks formed by physical entanglement and chemical grafting of linear or branched polymer chains containing hydrophilic groups such as carboxyl, hydroxyl, or amino groups. However, traditional petroleum-based superabsorbent hydrogels are limited in their agricultural applications by their high cost, non-degradability, and environmental pollution. Starch is a natural polymer material that is widely available, low-cost, renewable and biodegradable. It has highly reactive hydroxyl sites and is easily grafted and polymerized with hydrophilic vinyl monomers. It is an ideal raw material for constructing environmentally friendly, highly absorbent gels for agricultural use.

[0003] In response to the above problems, a slow-release urea system is constructed using starch-based highly absorbent gel as a carrier to achieve the organic integration of water absorption, water retention and nutrient slow-release functions, which has become an important direction for the current transformation of agricultural green fertilizers. This type of material, relying on its continuously connected three-dimensional porous structure and hydrophilic groups, can quickly absorb water and store it in the soil. It can quickly absorb and store water in the soil, and at the same time slowly release the embedded urea nutrient factors and other fertilizer components through the osmotic pressure gradient and diffusion mechanism, thereby significantly improving the water and fertilizer utilization efficiency, and is expected to provide a solution for the integrated fertilizer and water precision management system. However, the current preparation of starch-based hydrogel slow-release urea is mainly divided into coating method and reaction mixing method, both of which have obvious technical bottlenecks. (1) The coating method forms a dense low-permeability membrane by coating the surface of urea particles with starch-based hydrogel to delay the release of urea. Although this method is relatively simple to operate, the membrane layer is easy to rupture under mechanical disturbance of the soil and microbial degradation, which easily causes sudden release of urea, making it difficult to achieve long-term slow-release control. (2) The reaction mixing method refers to embedding urea into the gel network structure during the hydrogel graft copolymerization process to achieve embedded release. However, the current mainstream reaction mixing method generally has the following technical problems: ① Batch intermittent dilute solution and suspension polymerization is usually not more than 5wt%, the reaction efficiency is low, the post-processing steps are cumbersome, the energy consumption is high, and it is easy to cause the three-dimensional structure of the gel to be destroyed; ② The starch graft copolymerization reaction efficiency is not high, the monomer conversion rate is less than 85%, the water absorption performance is poor, and a long time (>2h) of reaction is required to form a gel system; ③ The particle size distribution is difficult to control during the product crushing and granulation process, and the structure is easily destroyed, resulting in large performance fluctuations, resulting in unstable water absorption and sustained release performance of the final product.

[0004] Existing patents also show similar technical limitations. For example, Chinese patent CN 119350094 A discloses a pH-responsive starch-based hydrogel-coated slow-release fertilizer, which is prepared by a coating method. The key technology is to convert ordinary corn starch into amorphous starch, increase its reactivity and hydrophilicity, and provide a basis for subsequent cross-linking reactions. However, the water absorption performance of the slow-release fertilizer produced therefrom is still poor, and the maximum equilibrium swelling rate (water absorption rate) is only 6.91g / g, which is difficult to meet the high moisture absorption requirements of farmland. For another example, Chinese patent CN 106064998 A discloses a double-layer coated environmentally friendly slow-release fertilizer, the core of which is urea particles; the inner coating is formed by spraying an ethanol solution of ethyl cellulose and stearic acid with a high-pressure spray gun; the outer coating is formed by adhering and wrapping starch-based super absorbent resin powder on its outside. However, the maximum water absorption ratio of starch-based superabsorbent resin is 126.4g / g, and the water absorption ratio of its slow-release urea granules is only about 44.24g / g. It still has defects such as insufficient water absorption performance and fragile gel structure, and cannot adapt to the continuous nutrient release needs in complex farmland environments.

[0005] In summary, there is an urgent need to develop a starch-based highly absorbent gel slow-release urea fertilizer with excellent water absorption, water retention and slow-release properties to meet the dual needs of agricultural water conservation and yield increase and environmentally friendly fertilization. Summary of the Invention

[0006] The purpose of the present invention is to provide a starch-based highly absorbent gel sustained-release urea granule prepared based on a one-step reaction extrusion technology and a preparation method thereof.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A method for preparing starch-based highly absorbent gel-slow-release urea granules comprises the following steps:

[0009] (1) After starch and urea are premixed uniformly, they are transported to the melt plasticizing zone of a twin-screw extruder;

[0010] (2) injecting an aqueous solution of a hydrophilic vinyl monomer and a cross-linking agent into the melt plasticizing zone of a twin-screw extruder to melt and gelatinize the starch and homogeneously mix the monomer and the cross-linking agent to form a pre-reacted starch-based melt;

[0011] (3) Injecting the initiator aqueous solution into the reaction zone of the twin-screw extruder in stages, controlling the heating temperature of the reaction zone, initiating the generation of starch free radicals and the occurrence of graft copolymerization reaction, while urea is synchronously embedded in the three-dimensional network structure of the gel through hydrogen bonding and diffusion migration;

[0012] (4) After the starch graft copolymerization is completed, an alkali aqueous solution is injected into the end section of the twin-screw extruder to carry out a saponification reaction, and then the mixture is extruded, dried, and pelletized to obtain starch-based highly absorbent gel-released urea granules;

[0013] The raw material ratio is as follows: by mass, 30-90 parts of starch, 30-60 parts of hydrophilic vinyl monomer, 0.1-3 parts of initiator, 0.01-0.8 parts of cross-linking agent, 5-20 parts of alkali, the water content accounts for 30%-70% of the total mass of the raw materials, and the urea content is 20%-150% of the sum of the mass of starch and monomer.

[0014] Preferably, the screw speed of the twin-screw extruder is controlled at 30-120 rpm; the heating temperature of the melt plasticization zone in steps (1) and (2) is 50-80°C, the heating temperature of the grafting reaction zone in step (3) is 70-120°C; and the temperature of the saponification reaction in step (4) is 80-100°C.

[0015] Preferably, the screw speed of the twin-screw extruder is controlled at 40-80 rpm; the heating temperature of the melt plasticization zone in step (1) is 60-75°C, the heating temperature of the grafting reaction zone in step (3) is 80-90°C; and the temperature of the saponification reaction in step (4) is 90-100°C.

[0016] Preferably, the amylose content in the starch is not higher than 30%.

[0017] Preferably, the mass ratio of starch to monomer is 2:1-1:1, the content of cross-linking agent is 0.1%-0.5% of the mass of monomer, the content of initiator is 1%-3% of the mass of starch, the content of alkali is 32%-42% of the mass of monomer, the content of water accounts for 40%-60% of the total mass of raw materials, and the content of urea is 40%-100% of the sum of the mass of starch and monomer.

[0018] Preferably, the initiator is one or a mixture of two or more of ammonium persulfate, potassium persulfate, sodium persulfate and cerium ammonium nitrate.

[0019] Preferably, the hydrophilic vinyl monomer is a mixture of one or both of acrylamide and acrylic acid.

[0020] Preferably, the cross-linking agent is one or a mixture of two or more of N,N-methylenebisacrylamide, N-hydroxymethylacrylamide and tetraallyloxyethane.

[0021] Preferably, the urea is agricultural urea with a nitrogen content of ≥45% and a particle size of 0.1-2 mm.

[0022] Preferably, the initiator comprises a primary initiator, ammonium persulfate, and a secondary initiator, ceric ammonium nitrate. An aqueous ammonium persulfate solution is first added to initiate the starch graft copolymerization reaction, and then an aqueous ceric ammonium nitrate solution is added to further initiate the starch graft copolymerization reaction. The aqueous initiator solution is added to the twin-screw extruder reaction zone in a multi-stage injection manner (2-4 times). The initiators injected in each stage can be the same or different, thereby achieving staged initiation of the graft copolymerization reaction and dynamic and precise control of the reaction process.

[0023] Preferably, the feed rate of step (1) is 10-80 kg / h, the feed rate of step (2) is 10-60 L / h, and the feed rate of step (3) is 5-20 L / h.

[0024] Preferably, the mass ratio of the primary initiator to the secondary initiator is 1.5:1-4.5:1, and the aqueous alkali solution is a sodium hydroxide solution and / or a potassium hydroxide solution with a mass fraction of 50±10%.

[0025] Preferably, the parameters of the twin-screw extruder are as follows: aspect ratio (L / D) is 30-60, screw diameter (D) is 20-75 mm, die aperture is The extruder adopts a multi-section independent temperature control design, and corresponding solid and liquid feed ports are set in specific functional sections of the machine body.

[0026] Preferably, the starch is one or a mixture of two or more of ordinary corn starch, waxy corn starch, tapioca starch and potato starch.

[0027] The method of the present invention is to construct a multi-step coordinated control system for reactive extrusion, and utilize urea as a slow-release factor and starch plasticizer in the reaction system at the same time, to achieve dual control of starch gelatinization and graft copolymerization reaction behavior under high temperature, high pressure, and high shear field conditions, and establish an integrated and continuous preparation process for graft copolymerization of starch and hydrophilic vinyl monomers and synchronous loading of urea. During the reactive extrusion process, urea can form hydrogen bonds with starch molecules. This interaction helps to deconstruct the original strong hydrogen bond network within and between starch molecules, thereby promoting starch gelatinization at lower temperatures, reducing the viscosity of the system, improving the fluidity and processing properties of the starch-based melt, and thus reducing the dependence on external water added during the extrusion process. At the same time, adding urea can significantly improve the flexibility and deformation tolerance of starch-based hydrogels. Even under large external strain, the gel structure can still maintain a stable structure to a large extent. As a plasticizer, urea gives the gel greater plasticity by weakening the interaction between the molecular chains inside the gel, so that it can still maintain the integrity of the three-dimensional network structure within a large strain range. This structural flexibility not only helps to improve the water absorption and water retention capacity of the gel, but also enhances the sustained-release stability of the urea-loaded system, which plays a key role in achieving the high water absorption performance and sustained-release synergistic function of the urea granules of the present invention.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The present invention is based on a twin-screw composite reaction extrusion system. Through multi-step coordinated regulation, it realizes an integrated reaction process of starch melting phase change, homogeneous mixing of reactants, free radical initiation and graft copolymerization, and synchronous loading of urea. It successfully establishes a new process for the efficient preparation of starch-based highly absorbent gel-released urea granules by "one-step" continuous reaction extrusion, effectively overcoming the technical problems of low reaction efficiency and complex process of traditional intermittent dilute solution systems.

[0030] (2) The present invention can accurately match the mixing intensity, mass transfer and heat transfer requirements, and reaction kinetic equilibrium time required for starch melting phase change, homogeneous mixing of reactants, graft copolymerization reaction, and synchronous loading of urea during the reaction process by optimizing the screw configuration topology and dynamically controlling the shear force field. This method breaks through the difficulty of nonlinear coupling of mass transfer, phase change, and reaction in high-viscoelastic melts, and realizes a short-time (<15 min) efficient grafting reaction, with a monomer conversion rate greater than 90% and a grafting efficiency exceeding 70%.

[0031] (3) The urea granules of the present invention have excellent water absorption, water retention and sustained release properties. The urea loading capacity can reach more than 60%, the water absorption rate is ≥100g / g, and the urea sustained release period is ≥28 days. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a schematic diagram of the one-step reactive extrusion preparation process of the invention.

[0033] Figure 2 FTIR spectra of corn starch and starch-based superabsorbent gels for sustained-release urea prepared in Examples 1, 5, 7, and 9.

[0034] Figure 3 13C-NMR spectra of corn starch and starch-based highly absorbent gel sustained-release urea prepared in Examples 1, 5, 7, and 9.

[0035] Figure 4 The sustained-release curves of pure urea and the starch-based highly absorbent gel sustained-release urea prepared in Examples 1, 5, 7, and 9 are shown.

[0036] Figure 5 The following are scanning electron micrographs of the starch-based superabsorbent gels prepared in various examples and comparative examples after saturated water absorption and swelling.

[0037] Figure 6 The starch-based highly absorbent gel sustained-release urea particles prepared in the present invention (left picture) and their morphology after absorbing water and swelling (right picture) are shown. DETAILED DESCRIPTION

[0038] The present invention will be described in further detail below with reference to specific examples, but the embodiments of the present invention are not limited thereto. For process parameters not particularly noted, conventional techniques may be used.

[0039] The present invention provides a starch-based highly absorbent gel sustained-release urea prepared based on a one-step reaction extrusion technology and a preparation method thereof. The preparation raw materials include starch, urea, a hydrophilic vinyl monomer, an initiator, a cross-linking agent, an alkali solution and deionized water.

[0040] In the embodiments of the present invention, ordinary corn starch, waxy corn starch, tapioca starch and hydroxypropyl corn starch (food grade, COFCO Huanglong Food Industry Co., Ltd.), acrylamide (industrial grade, Zhejiang Xinyong Biochemical Co., Ltd.), N,N-methylenebisacrylamide (analytical grade, Shanghai Maclean Biochemical Technology Co., Ltd.), ammonium persulfate (analytical grade, Shanghai Maclean Biochemical Technology Co., Ltd.), ceric ammonium nitrate (analytical grade, Shanghai Maclean Biochemical Technology Co., Ltd.), sodium hydroxide (industrial grade, Tianjin Bohai Chemical Co., Ltd.), sodium nitrate (analytical grade, Shanghai Maclean Biochemical Technology Co., Ltd.),

[0041] Urea (for agricultural use, Henan Xinlianxin Chemical Industry Co., Ltd.), ethanol (analytical grade, Sinopharm Chemical Reagent Co., Ltd.), urease kit (analytical grade, Shanghai Yuanye Biotechnology Co., Ltd.), α-amylase (≥500 units / mL, Shanghai MacLean Biochemical Technology Co., Ltd.), and amyloglucosidase (100,000 u / mL, Shanghai MacLean Biochemical Technology Co., Ltd.).

[0042] Example 1

[0043] 1. Experimental equipment and process parameters

[0044] Equipment: A custom co-rotating, intermeshing twin-screw extruder (L / D ratio = 52, screw diameter D = 36mm) is equipped with 13 independently temperature-controlled sections, one solid powder side feed port, and four liquid injection ports. The die has a 2mm aperture. Extruder sections 1-4 are used for starch melt gelatinization and homogenous mixing of reactants; sections 5-10 are used for starch graft copolymerization and simultaneous loading of urea; sections 11 and 12 are used for saponification; section 13 is the die extrusion molding section; and section 14 is used for drying and pelletizing. The auxiliary equipment primarily includes a loss-in-weight solid feeder 1, which forcibly conveys the starch and urea mixture into the twin-screw extruder; high-pressure liquid metering pumps (2, 3, 4, and 5), which respectively inject the monomer-crosslinker solution, initiator (APS, CAN), and saponification alkali solution. High-pressure liquid metering pump 2 injects the reaction monomer and crosslinker into the twin-screw reaction extruder, thereby melting and plasticizing the starch and homogenously mixing it with the starch to form a starch-based melt. High-pressure liquid metering pumps 3 and 4 are used to inject the initiator in stages, triggering the generation of starch macromolecular free radicals and initiating the graft copolymerization reaction with the monomer. High-pressure liquid metering pump 5 injects the alkali solution to saponify the amide groups of the polyacrylamide into more hydrophilic carboxylates.

[0045] Process parameters:

[0046] Table 1 Temperature gradient settings (14 sections of the twin-screw extruder function are set in sequence):

[0047]

[0048] Screw speed: 50rpm (reaction time <15min)

[0049] 2. Raw material ratio and pretreatment

[0050] Table 2 Raw material formula (solid content in the reaction system>50%)

[0051]

[0052] Preprocessing:

[0053] 1. Solid materials: premix urea and starch evenly using a high-speed mixer (speed 1000 rpm, time 5 min);

[0054] 2. Liquid preparation:

[0055] Monomer solution: Dissolve AM in deionized water at a ratio of 1:1 (w / w) and add MBA;

[0056] Initiator solution: APS and CAN were prepared into 2.0 wt% aqueous solutions respectively.

[0057] 3. Process steps and function analysis

[0058] Step 1: Starch melting, plasticization and homogenization

[0059] Section 1 (feed): premixed starch and urea were continuously fed to the twin-screw extruder at a rate of 26.0 kg / h by a loss-in-weight feeder;

[0060] Sections 2-4 (homogeneous mixing of reactants and starch gelatinization): An aqueous solution of the hydrophilic vinyl monomer acrylamide (AM) and the crosslinker N,N'-methylenebisacrylamide (MBA) is injected into section 2 at a rate of 20.0 L / h using a high-pressure liquid pump. Under the action of a temperature gradient of 50-80°C and a high shear field, the starch achieves melt gelatinization, molecular chain expansion, and effective mixing with the monomer and crosslinker to form a homogeneous pre-reacted starch-based melt.

[0061] Step 2: Graft copolymerization with starch in stages

[0062] Section 5 (first initiation): an ammonium persulfate (APS) aqueous solution was injected at a rate of 15.0 L / h to induce the starch C6 hydroxyl group to form free radicals and start graft polymerization;

[0063] Section 8 (secondary initiation): An aqueous solution of ceric ammonium nitrate (CAN) was injected at a rate of 7.5 L / h to preferentially activate the starch C2 / C3 sites, promote multi-site grafting, and inhibit the homopolymerization of the monomers;

[0064] Section 5-10 (starch graft copolymerization and urea simultaneous loading): The temperature of the reaction zone is regulated at 80-90°C to start the starch macromolecular free radical initiation and graft copolymerization. At the same time, urea is simultaneously embedded in the starch grafted polyacrylamide gel network through hydrogen bonds.

[0065] Step 3: Saponification and extrusion

[0066] Section 11-12 (saponification reaction): Injection of 50.0 wt% NaOH solution (at a rate of 7.0 L / h) to convert the amide groups in the polyacrylamide side chains into more hydrophilic sodium carboxylates;

[0067] Section 13 (extrusion molding): The saponified starch-based super absorbent gel slowly releases urea. Die head aperture continuous extrusion molding;

[0068] Section 14 (drying and pelletizing): The material is conveyed by rollers and dried with hot air at 100°C for 30 minutes, and pelletized to produce water-retaining slow-release urea granules with uniform particle size and excellent performance.

[0069] Example 2

[0070] The difference between this embodiment and embodiment 1 is that the mass fraction of urea in the raw material ratio is 40 parts, and the feeding rate of the premixed starch and urea through the loss-in-weight feeder in step 1 is 32.0 kg / h.

[0071] Example 3

[0072] The difference between this embodiment and embodiment 1 is that the mass fraction of urea added is 60 parts, and the feeding rate of the premixed starch and urea in section 1 through the loss-in-weight feeder is 38.0 kg / h.

[0073] Example 4

[0074] The difference between this embodiment and embodiment 1 is that the mass fraction of urea in the raw material ratio is 80 parts, and the feeding rate of the premixed starch and urea through the loss-in-weight feeder in step 1 is 44.0 kg / h.

[0075] Example 5

[0076] The difference between this embodiment and embodiment 1 is that the mass fraction of urea in the raw material ratio is 100 parts, and the feeding rate of the premixed starch and urea through the loss-in-weight feeder in step 1 is 50.0 kg / h.

[0077] Example 6

[0078] The difference between this embodiment and embodiment 1 is that ordinary corn starch is replaced with waxy corn starch in the raw material ratio.

[0079] Example 7

[0080] The difference between this embodiment and embodiment 1 is that the ordinary corn starch in the raw material ratio is replaced by hydroxypropyl-modified ordinary corn starch.

[0081] Example 8

[0082] The difference between this embodiment and embodiment 1 is that ordinary corn starch is replaced by tapioca starch in the raw material ratio.

[0083] Example 9

[0084] The difference between this embodiment and embodiment 1 is that the rate of the high-pressure liquid metering pump in section 2 is adjusted to 40.0 L / h.

[0085] Example 10

[0086] The difference between this embodiment and embodiment 1 is that, in the raw material ratio, the mass fraction of the cross-linking agent MBA is 0.15 parts.

[0087] Example 11

[0088] The difference between this embodiment and embodiment 1 is that, in the raw material ratio, the mass fraction of the initiator APS is 0.15 parts.

[0089] Example 12

[0090] The difference between this embodiment and embodiment 1 is that, in the raw material ratio, the hydrophilic vinyl monomer is a mixture of acrylic acid and acrylamide in a mass ratio of 1:1.

[0091] Example 13

[0092] The difference between the embodiment and embodiment 1 is that, in the process parameters, the screw speed of the extruder is 100 rpm.

[0093] Comparative Example 1

[0094] The difference between this comparative example and Example 1 is that no urea is added and the starch feed rate in section 1 is adjusted to 20.0 kg / h; the other process steps are the same as those in Example 1, but no urea is involved in the starch melt plasticization and the starch melt graft copolymerization process.

[0095] Comparative Example 2

[0096] The difference between this comparative example and Example 1 is that only APS initiator is used for single initiation, the concentration of the APS aqueous solution is adjusted to about 1.34 wt %, and the injection rate into the extruder is 22.5 L / h. The other process is the same as that of Example 1.

[0097] Comparative Example 3

[0098] This comparative example employed a traditional dilute solution polymerization method, using a 10L glass reactor instead of an extruder. The formulation was the same as in Example 1, except that the solids content was adjusted to 10 wt%. Starch and urea were dispersed in water, followed by the addition of monomer, crosslinker, and initiator, followed by stirring for 2 hours. After completion of the reaction, alkaline solution was added for saponification for 30 minutes. The product was then dried at 100°C for 10 hours and then crushed and sieved.

[0099] The structural characterization and test results analysis of the starch-based highly absorbent gel sustained-release urea prepared in the embodiment are as follows:

[0100] FTIR analysis

[0101] FTIR technology can provide important information about the molecular structure and intermolecular interactions of substances and is one of the key technologies for studying the molecular properties of starch graft copolymers. The prepared starch-based superabsorbent gel was analyzed using a PerkinElmer FTIR Spectrum 3 equipped with a Zn-Se ATR (Attenuated Total Reflection) accessory with a resolution of 0.5 cm -1 , scanning range 4000cm -1 -600cm -1 , the number of scans was 128. Before testing, the sample was soaked in ethanol solution to remove unreacted monomers and in distilled water to remove loaded urea.

[0102] Figure 2 The FTIR spectra of corn starch and starch grafted polyacrylamide superabsorbent gels prepared in Examples 1, 5, 7, and 9 are shown. It can be seen that the typical infrared absorption characteristics of corn starch include: a strong absorption peak (3220 cm) caused by the stretching vibration of the OH bond. -1 ,ν1),; the absorption peak generated by CH bond stretching vibration (2927cm -1 , ν2); and the peak caused by the stretching vibration of the C-O-C bond (1148 cm -1 、1077cm -1 and 995cm -1 ,ν5,ν6,ν7). In addition, 1640cm -1 The absorption peak (ν4) at γ highlights the binding characteristics of water molecules in the amorphous region of starch. After the grafting reaction, the infrared spectra of the starch-grafted polyacrylamide copolymers prepared in Examples 1, 5, 7, and 9 not only retained the characteristic absorption peaks of the original starch, but also showed new absorption features, such as the stretching vibration of the NH group of the amide group (3201 cm -1 ,ν7), stretching vibration of C=O (1657cm -1 ,ν8), NH bending vibration (1608cm-1 ,ν9) and CN stretching vibration (1412cm -1 , ν 10 These newly added characteristic peaks indicate that acrylamide has been grafted onto starch molecules, and starch-grafted polyacrylamide highly absorbent gel for slow-release urea has been successfully prepared.

[0103] 13C-NMR analysis

[0104] A detailed characterization and analysis of the starch-based superabsorbent gel-based sustained-release urea was performed using a Bruker Avance Neo 500M NMR superconducting nuclear magnetic resonance spectrometer. The Bruker Avance Neo 500M NMR spectrometer offers high sensitivity and excellent frequency resolution, providing a frequency resolution of ≤0.005 Hz and a phase resolution of ≤0.006°. These characteristics are crucial for accurately analyzing the microstructure of the starch-based superabsorbent gel-based sustained-release urea. Prior to the experiment, the sample was dried, crushed, passed through a 200-mesh sieve, and fully swelled in heavy water. The sample was tested at a concentration of 30 mg / mL to ensure good flowability and uniformity.

[0105] Figure 3 Liquid showing corn starch and starch-based superabsorbent gels initiated by APS and CAN 13 C-NMR spectrum. In the spectrum of corn starch, the characteristic resonance peaks of C1, C4, and C6 reveal the presence of glucose residues, which form the basis of the starch molecular structure. The C1 resonance peak appears at 100.5 ppm and represents the aldehyde carbon of the glucose unit; the resonance peak at 77.18 ppm corresponds to the connecting C4 carbon atom of the glucose unit that forms the polysaccharide chain; and the resonance peak at 60.71 ppm reflects the C6 carbon atom of the secondary alcohol group of the glucose unit. The C2, C3, and C5 resonance peaks are in the range of 70.9 to 73.8 ppm, reflecting the specific structure and spatial arrangement of the other carbon atoms in the glucose ring.

[0106] The grafted polyacrylamide copolymer initiated by APS and CAN retains the resonance peaks of the native starch glucose residues while also displaying characteristic resonance peaks of the polyacrylamide side chains. Resonance peaks in the 179.95-179.22 ppm range are attributed to the amide functional groups (-CONH2) on the grafted polyacrylamide side chains, indicating the successful grafting of the acrylamide monomer. Resonance peaks at 41.08-42.50 ppm reveal carbon atoms (-CH2-CH-) on the polyacrylamide backbone, further confirming the formation of grafted polyacrylamide chains. These characteristic peaks further confirm that a graft copolymerization reaction has occurred between starch and acrylamide, rather than simply physical mixing. Compared to the resonance peak of C6 in native corn starch at 60.71 ppm, the resonance peak of C6 in the starch graft copolymer initiated only by APS shifts upfield to 60.62 ppm, showing a slight chemical shift, indicating that the grafting of acrylamide occurs primarily at some of the C6 hydroxyl oxygen groups. In contrast, the C2, C3, and C5 resonance peaks of the starch graft copolymer using only CAN initiator showed slight chemical shifts in the range of 70.5 to 73.8 ppm, revealing that the grafting of acrylamide occurred simultaneously at the C2 and C3 positions of the starch glucose ring.

[0107] Determination of monomer conversion and graft copolymerization efficiency

[0108] After all extruder parameters stabilized, approximately 100 g of starch-based superabsorbent gel-release urea was collected at the die head and immersed in 500 mL of 0.5% (w / v) hydroquinone ethanol solution to quench free radical reactions. The mixture was then immersed for 24 hours and then soaked in distilled water to remove the urea load. In addition to quenching the reaction, the ethanol solution also removed unreacted monomers. The soaked reaction product was dried in a 60°C vacuum oven for 12 hours and pulverized through a 200-mesh sieve. A 25 g sample, after screening, was dispersed in 500 mL of a 3 / 7 ethanol / water (v / v) mixture and stirred at room temperature for 24 hours to extract the polyacrylamide homopolymer. The insoluble solids were centrifuged and dried, and the soluble fraction was collected after solvent evaporation. The monomer conversion and graft copolymerization efficiency of the starch-based superabsorbent gel-release urea prepared in each example are listed in Table 3.

[0109] The monomer conversion rate and graft copolymerization efficiency of starch graft copolymerization, as well as the expressions of starch graft content are as follows:

[0110]

[0111] Among them, N q N is the nitrogen content of the reaction product after separation of unreacted monomers; t is the theoretical nitrogen content based on the reactive extrusion feed rate; N gis the nitrogen content of the grafted starch that is insoluble after separation of the homopolymer; f is the proportion of the insoluble portion in the composite solvent. The nitrogen content of the material was analyzed using a CHNS elemental analyzer.

[0112] Starch grafted polyacrylamide side chain molecular weight (M W ) determination

[0113] To determine the molecular weight of starch-grafted polyacrylamide side chains, the starch must first be removed from the extracted graft copolymer by enzymatic hydrolysis. The specific procedure is as follows: 100 mg of a starch-based superabsorbent gel sample, free of polyacrylamide homopolymer, is dispersed in 50 mL of deionized water. 0.2 mL of α-amylase and 0.01 mL of amyloglucosidase are then added, followed by enzymatic hydrolysis at 50°C for 6 h in a waterbath with shaking to ensure complete hydrolysis of the starch to glucose. The hydrolyzate is filtered through a 5.0 μm aqueous filter and then tested. The test conditions are as follows: a saline solution containing 0.1 M sodium nitrate is used as the mobile phase, filtered through a 0.2 μm organic filter. Shodex 804 and 806M gel chromatography columns are used in series. The injection volume is 30 μL, the flow rate is 0.5 mL / min, the column and detector temperatures are set at 45°C, the measurement wavelength is 622 nm, and the dn / dc value is 0.15 g / mL. The results of the side chain molecular weights of the starch grafted polyacrylamide prepared in each example are listed in Table 3.

[0114] Water absorption performance determination

[0115] The water absorption performance of the starch-based super absorbent gel sustained-release urea was measured using the tea bag method. The results of the water absorption performance of the starch-based super absorbent gel sustained-release urea prepared in each example are listed in Table 1. The specific steps are as follows: Accurately weigh approximately 0.3g of sample (initial dry basis mass is recorded as M1) and place it in a 100-mesh nylon mesh bag. Thereafter, the hydrogel is immersed in a beaker filled with 1500mL of distilled water. After it reaches swelling equilibrium, the mass of the hydrogel after water absorption is recorded (recorded as M2). To ensure the repeatability and accuracy of the experimental results, the measurement was repeated three times. The water absorption capacity is expressed as follows:

[0116]

[0117] Microscopic morphology observation of starch-based superabsorbent gel for sustained release of urea

[0118] A Phenom ProX G6 desktop scanning electron microscope (SEM) equipped with a freezing sample stage was used to perform microscopic characterization of the three-dimensional network structure of the starch-based highly absorbent gel for sustained-release urea at low temperatures. First, a sample with a diameter of approximately 5 mm was cut from the gel that had reached water absorption equilibrium and placed on the freezing sample stage. The sample stage temperature was set to -20°C to ensure that the sample was quickly and directionally frozen within 1 minute. After freezing, a blade was used to neatly cut off the ice layer on the surface to reveal the internal structure. The sample was then placed in the electron microscope sample chamber and evacuated under high vacuum conditions for 10 minutes. During observation, the accelerating voltage was adjusted to 10 kV, and the sample stage temperature was kept constant to ensure the structural stability of the sample under electron beam irradiation.

[0119] Figure 4 The following figure shows a scanning electron micrograph of a starch-based superabsorbent gel for sustained-release urea after saturated water absorption equilibrium and directional freezing treatment on a low-temperature sample stage. As can be seen from the figure, the micromorphology of all starch-based superabsorbent gels exhibits a typical three-dimensional honeycomb network, which is the key to their high water absorption, but there are differences in pore density and wall thickness. These morphological differences directly confirm the correlation between water absorption rate, grafted polyacrylamide side chain molecular weight, and sustained-release performance in Table 3, indicating that regulating urea loading, starch type, and process parameters are the core means of regulating the performance of the three-dimensional network structure. The three-dimensional network structures of the gels in Example 4 (80% urea) and Example 6 (waxy starch) are optimal, combining high water absorption, sustained-release performance, and mechanical stability.

[0120] Urea cumulative release rate determination

[0121] 300.0g dry sandy soil (100-200 mesh) is placed in a PVC cylindrical tube of 5cm in diameter and 50cm in height. In order to prevent soil loss and ensure the effective penetration of urea leachate, a cotton layer is laid in advance at the bottom of the container. Subsequently, 5.0g starch-based high-water gel slow-release urea is respectively packed into a non-woven bag, placed in the soil column of construction, and covered with 100.0g dry soil. According to established time intervals (1, 3, 5, 7, 10, 14, 21 and 28 days), 100.0g deionized water is gradually dripped into the soil column to simulate the slow-release process of urea in soil. By collecting whole leachates at the bottom of the soil column, and adopting urea test kit to measure urea content therein, thus the release of urea is quantitatively analyzed. The results of the cumulative release rate of the starch-based high-water-absorbing gel slow-release urea prepared by each embodiment are shown in Table 3, and the slow-release effect is shown in Table 4 and Figure 4 .

[0122] Table 3

[0123]

[0124]

[0125] Effect of urea content on performance

[0126] As can be seen from Examples 1-5, with the increase of urea content (20% to 100%), the monomer conversion rate and the graft copolymerization efficiency all show an upward trend (Examples 1-4), but when the urea content is 100% (Example 5), there is a slight decrease. This shows that urea can promote the reaction within a moderate range (20%-80%), but excessive amounts may affect the reaction equilibrium. The increase in urea content significantly improves water absorption (from 212.5g / g to 360.8g / g), indicating that urea as a plasticizer improves the three-dimensional network structure of the gel and enhances hydrophilicity. With the increase of urea content, the 28-day cumulative release rate gradually decreases (93.5% to 79.3%), indicating that high urea loading slows down the release rate, but the release rate of Example 5 (100% urea) rebounds, which may be related to the overload of the network structure.

[0127] As the urea content increases from 20% to 80% (Example 1-4 grafted polyacrylamide side chain molecular weight (M W ) from 5.31×10 5 Da increased significantly to 7.58×10 5 Da, but when the urea content is 100% (Example 5), the molecular weight decreases slightly (7.11×10 5 Urea as a plasticizer can promote starch melting and free radical grafting reaction within a moderate range (20%-80%), prolong the growth time of acrylamide chain, and thus increase molecular weight. Excessive urea (100%) may lead to low viscosity of the system, decreased initiator efficiency or increased chain termination reaction, which in turn inhibits molecular weight growth. High molecular weight (such as 7.58×10 5 Da) corresponds to a higher water absorption (310.8 g / g) because the long chains form a more stable three-dimensional network.

[0128] Effect of starch type

[0129] As can be seen from Examples 6-8, waxy corn starch (Example 6) exhibits the highest graft copolymerization efficiency (92.7%) and water absorption (404.2 g / g), and the grafted polyacrylamide side chain has the highest molecular weight (8.05×10 5 Da), because of its high branched starch content, it is easier to form a uniform network. Because its branched structure provides more grafting sites, the monomers are more easily polymerized into long chains. Hydroxypropyl modified starch (Example 7): The performance is poor (grafting efficiency 78.2%, water absorption rate 192.8g / g), and the grafted polyacrylamide side chain has the lowest molecular weight (4.56×10 5Da), which may be due to chemical modification interfering with the free radical grafting reaction, which may hinder the free radical transfer and lead to limited chain growth. The performance of cassava starch (Example 8) is close to that of waxy corn starch, indicating that starch with higher amylopectin content is more conducive to the reaction.

[0130] Influence of formulation and process parameters

[0131] Starch to monomer ratio (Example 9): When the starch to acrylamide ratio is 1:1, the grafting efficiency decreases (75.4%), the water absorption rate decreases (165.2 g / g), but the side chain molecular weight is the highest (9.54×10 5 Da), which may be due to the increase of homopolymer due to excessive monomer. Cross-linking agent dosage (Example 10): When the cross-linking agent is increased to 0.15%, the water absorption rate decreases (173.6g / g), because the high cross-linking density limits the network expansion. Initiator dosage (Example 11): When APS is reduced to 0.15%, the molecular weight decreases (4.97×10 5 Da) urea release rate increased (95.4%), and insufficient initiator led to a decrease in chain initiation efficiency. Screw speed (Example 13): When the speed was increased to 100 rpm, all properties decreased, and the molecular weight decreased significantly (4.58×10 5 Da). On the one hand, the chain growth is insufficient due to the shortened reaction time, and on the other hand, the high shear force may cause the side chain of the grafted polyacrylamide to break.

[0132] Comparative analysis

[0133] Comparative Example 1 (without urea): The monomer conversion rate and water absorption rate are significantly lower than those in Example 1, confirming the key role of urea in plasticization and network formation. Comparative Example 2 (single initiation): The grafting efficiency is lower than that of Example 1 of the double initiation system, indicating that staged initiation can improve the reaction efficiency. Comparative Example 3 (traditional dilute solution method): All properties are the worst, highlighting the advantages of one-step reactive extrusion technology in high-efficiency, high solid content systems. In addition, Comparative Example 2 (single initiation): The molecular weight (6.72×10 5 Da) is comparable to that of Example 3 using the dual-initiator system, but the grafting efficiency is lower (80.4% vs 89.5%), indicating that dual-initiator can optimize the distribution of grafting sites rather than simply increase the molecular weight. Comparative Example 3 (traditional dilute solution method): The molecular weight is the lowest (4.03×10 5 ), the chain collision probability is reduced due to the low solid content (10wt%), and the chain growth is limited.

[0134] Table 4

[0135]

[0136] The urea granules of the present invention have a porous fractal network structure, a water absorption rate of ≥100g / g, and a urea sustained-release period of ≥28 days, achieving synergistic optimization of water absorption and water retention and sustained-release functions. The present invention provides a preparation method based on a one-step reactive extrusion technology, which realizes the integrated continuous production of starch melt plasticization, graft copolymerization and urea loading through the synergistic effect of high viscoelastic shear and temperature of a twin-screw extruder. This method breaks through the bottleneck of low efficiency and complex process of traditional dilute solution polymerization, and has significant advantages such as short reaction time (<15min), high monomer conversion rate (>90%), and no solvent residue. It successfully constructs a three-dimensional gel network with both high water absorption and sustained-release properties, providing a green and efficient solution for the integrated management of agricultural fertilizer and water.

[0137] Those skilled in the art should understand that the above embodiments are only specific implementations of the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the technical concept and spirit of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing starch-based highly absorbent gel sustained-release urea granules, characterized in that: The steps include: (1) After starch and urea are premixed uniformly, they are transported to the melt plasticizing zone of a twin-screw extruder; (2) injecting an aqueous solution of a hydrophilic vinyl monomer and a cross-linking agent into the melt plasticizing zone of a twin-screw extruder to melt and gelatinize the starch and homogeneously mix the monomer and the cross-linking agent to form a pre-reacted starch-based melt; (3) injecting the initiator aqueous solution into the reaction zone of the twin-screw extruder in stages, controlling the heating temperature of the reaction zone, initiating the generation of starch free radicals and the graft copolymerization reaction with the hydrophilic vinyl monomer; (4) After the starch graft copolymerization is completed, an alkali aqueous solution is injected into the end section of the twin-screw extruder to carry out a saponification reaction, and then the mixture is extruded, dried, and pelletized to obtain starch-based highly absorbent gel-released urea granules; The raw material ratio is as follows: by mass, 30-90 parts of starch, 30-60 parts of hydrophilic vinyl monomer, 0.1-3 parts of initiator, 0.01-0.8 parts of cross-linking agent, 5-20 parts of alkali, the water content accounts for 30%-70% of the total mass of the raw materials, and the urea content is 20%-150% of the sum of the mass of starch and monomer.

2. The preparation method according to claim 1, characterized in that The screw speed of the twin-screw extruder is controlled at 30-120 rpm; the heating temperature of the melting and plasticizing zones in steps (1) and (2) is 50-80°C, the heating temperature of the reaction zone in step (3) is 70-120°C; and the temperature of the saponification reaction in step (4) is 80-100°C.

3. The preparation method according to claim 2, characterized in that The screw speed of the twin-screw extruder is controlled at 40-80 rpm; the heating temperature of the melting and plasticizing zone in step (1) is 60-75°C, the heating temperature of the reaction zone in step (3) is 80-90°C; and the temperature of the saponification reaction in step (4) is 90-100°C.

4. The preparation method according to claim 3, characterized in that The amylose content in the starch is not higher than 30%.

5. The preparation method according to any one of claims 1 to 4, characterized in that The mass ratio of starch to monomer is 2:1-1:1, the content of the crosslinking agent is 0.1%-0.5% of the mass of the monomer, the content of the initiator is 1%-3% of the mass of the starch, the content of the alkali is 32%-42% of the mass of the monomer, the content of water accounts for 40%-60% of the total mass of the raw materials, and the content of urea is 40%-100% of the sum of the mass of the starch and the monomer.

6. The preparation method according to claim 5, characterized in that The initiator is one or a mixture of two or more of ammonium persulfate, potassium persulfate, sodium persulfate and cerium ammonium nitrate; The hydrophilic vinyl monomer is a compound of one or both of acrylamide and acrylic acid; The cross-linking agent is one or a mixture of two or more of N,N-methylenebisacrylamide, N-hydroxymethylacrylamide and tetraallyloxyethane.

7. The preparation method according to claim 6, characterized in that The initiator includes a main initiator ammonium persulfate and an auxiliary initiator ammonium cerium nitrate. An ammonium persulfate aqueous solution is first added to initiate starch graft copolymerization reaction, and then an ammonium cerium nitrate aqueous solution is added to further initiate starch graft copolymerization reaction.

8. The preparation method according to claim 7, characterized in that The feed rate of step (1) is 10-80 kg / h, the feed rate of step (2) is 10-60 L / h, and the feed rate of step (3) is 5-20 L / h; the mass ratio of the main initiator to the auxiliary initiator is 1.5:1-4.5:1, and the aqueous alkali solution is a sodium hydroxide solution and / or a potassium hydroxide solution with a mass fraction of 50±10%.

9. The preparation method according to claim 8, characterized in that The parameters of the twin-screw extruder are as follows: aspect ratio (L / D) is 30-60, screw diameter (D) is 20-75 mm, and die aperture (φ) is 1-4 mm; the starch is one or a mixture of two or more of ordinary corn starch, waxy corn starch, tapioca starch and potato starch.

10. Starch-based highly absorbent gel sustained-release urea granules prepared by the method according to any one of claims 1 to 9.

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