Preparation method of polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea slow-release fertilizer

Abstract

The invention relates to the field of slow-release fertilizers, in particular to preparation of a hydrogel slow-release fertilizer. The invention discloses a preparation method of a slow release fertilizer of polyaspartic acid, carboxymethyl cellulose hydrogel, stearic acid and urea. Comprising the following steps: (1) preparing polysuccinimide, (2) preparing a polyaspartic acid gel solution by using the polysuccinimide, and then preparing a polyaspartic acid / carboxymethyl cellulose hydrogel solution by using the polyaspartic acid gel solution; the method further comprises the following steps: (3) preparing stearic acid / urea particles; 4, the polyaspartic acid / carboxymethyl cellulose hydrogel solution and the stearic acid / urea granules are used for preparing and obtaining the polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea slow-release fertilizer.By means of the method, the slow-release fertilizer with the dual functions of water retention and slow release can be prepared and obtained.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of slow-release fertilizers, in particular to the preparation of hydrogel slow-release fertilizers. BACKGROUND

[0002] Fertilizers and water are important factors that restrict agricultural production, so it is very important to improve the utilization rate of water resources and fertilizer nutrients. Nitrogen is the most widely used plant nutrient, and is generally considered to be the limiting nutrient for yield [1] . Urea is the most commonly used nitrogen-based nutrient fertilizer due to its high nitrogen content and low cost, and is widely used. However, the nitrogen in urea can be metabolized by urease catalysis and nitrification in the soil to form nitrite and nitrate ions. These excess ions are leached from the soil by high water level irrigation or heavy rainfall, resulting in high concentration pollution in the ground and water surface, thereby causing harm to the environment and humans [2] . Therefore, it is necessary to control the use of the minimum but sufficient amount of fertilizer throughout the crop planting process [3] .

[0003] Reducing fertilizers while maintaining high utilization efficiency is to use slow-release fertilizers (CRF), which can reduce fertilizers by at least 20-30% while meeting the same productivity and not damaging the environment [4] . Coated slow-release fertilizers are widely used in CRF because they do not interact with the fertilizer itself to reduce the efficiency of fertilizer use, and are simple to operate. [5] However, traditional CRF usually coats a hydrophobic material on the outside of the fertilizer to achieve the purpose of slow release, and such materials generally lack water regulation ability. When there is a small amount of water in the environment, there is no water molecule penetration effect without water absorption material, and only when there is a large amount of water in the environment can the penetration be completed, making it difficult to achieve "on-demand release".

[0004] Hydrogel is composed of polymeric materials, and its hydrophilic structure enables it to accommodate a large amount of water in a three-dimensional network, which makes it widely used in industrial and agricultural fields [6]Hydrogels themselves have excellent swelling properties, and are usually used in combination with agricultural water-retaining agents to solve the problem of uneven distribution of water resources in agriculture. Polyaspartic acid hydrogel (PASP) is widely used in the field of agricultural water-retaining agents due to its biodegradability and abundant hydrophilic groups, but its weak mechanical properties are the main defect that hinders its wide application in agricultural production. Double network (DN) hydrogel is composed of two networks with different properties, one of which is highly cross-linked but brittle and prone to breakage during deformation, and the other is low in cross-linking degree but has good network flexibility. The brittle network is generally referred to as the "first network", and the elastic network is referred to as the "second network". The bonds that break within the brittle network are called sacrificial bonds, and studies have shown that the toughening mechanism of DN hydrogel is based on sacrificial bonds and the unfolding of ordered domains within the organic components of the gel-like structure [7,8] . In previous work, carboxymethyl cellulose (CMC) was introduced into P hydrogel as the first network of DN hydrogel, and P was used as the second network through its covalent cross-linking, to construct P-C DN hydrogel, solving the problem of weak mechanical properties of P [9] .

[0005] Although hydrogel can be used as a coating layer of CRF to control the release of fertilizer through water, the high swelling property of water-retaining agent as the outer layer of slow-release material may cause the rupture and burst release of CRF due to low cross-linking degree, water erosion, and diffusion barrier effect

[10] .

[0006] In previous studies, hydrophobic structures have also been used as the coating layer of CRF. Stearic acid (SA) is a widely used hydrophobic fatty acid, which is a unique material due to its high molecular weight, melting point and boiling point, and minimum water solubility (2.9 mg / L). Generally, SA has controllable hydrophobicity and biodegradability in soil, making it an ideal candidate for environmentally friendly and SRF preparation

[12] . However, the lack of water-retaining effect makes it difficult to solve the problem of uneven distribution of water. It is proved that both hydrophilic coating and hydrophobic coating of CRF still have major problems.

[0007] Current problems of nitrogen fertilizer: (1) Urea is easy to form nitrite and nitrate ions in soil. These excess ions are leached from the soil through high water level irrigation or heavy rainfall, causing high concentration pollution in the ground and water surface, thereby harming the environment and humans. Slow-release urea can effectively alleviate the problem of excessive release rate of urea. (2) Nitrogen fertilizers usually do not have water absorption and retention effect, while hydrogel slow-release fertilizer not only has slow-release effect but also has water retention effect.

[0008] The existing double-coated slow-release fertilizer is: polyacrylamide hydrogel as the outer layer, ethyl cellulose as the inner layer of double-coated slow-release fertilizer, however, polyacrylamide is not easy to be quickly degraded by the environment and has the defect of polluting the environment.

[0009] References:

[0010] [1] Ibrahim, A. A., & Jibril, B. Y. (2005). Controlled Release of Paraffin Wax / Rosin-Coated Fertilizers. Industrial & Engineering Chemistry Research, 44(7), 2288-2291.

[0011] [2] Messiga, A. J., Haak, D., & Dorais, M. (2018). Blueberry yield and soil properties response to long-term fertigation and broadcast nitrogen. Scientia Horticulturae, 230, 92-101.

[0012] [3] Krasilnikov, P.; Taboada, M. A.; Amanullah Fertilizer Use, Soil Health and Agricultural Sustainability. Agriculture 2022, 12, 462. DOI:10.3390 / agriculture12040462.

[0013] [4] Gil-Ortiz, R.; Naranjo, M.Á.; Ruiz-Navarro, A.; Caballero-Molada, M.; Atares, S.; García, C.; Vicente, O. New Eco-Friendly Polymeric-Coated Urea Fertilizers Enhanced Crop Yield in Wheat. Agronomy 2020, 10, 438

[0014] [5] Ramli, R. A. (2019). Slow-release fertilizer hydrogels: a review. Polymer Chemistry, 10(45), 6073-6090.

[0015] [6] Ahmed, E. M. (2015). Hydrogel: Preparation, characterization, and applications: A review. Journal of Advanced Research, 6(2), 105-121.

[0016] [7] Chang, L., Xu, L., Liu, Y., & Qiu, D. (2021). Superabsorbent polymers used for agricultural water retention. Polymer Testing, 94, 107021.

[0017] [8] Yang, F.; Tadepalli, V.; Wiley, B. J. 3D Printing of a Double Network Hydrogel with a Compression Strength and Elastic Modulus Greater than those of Cartilage. ACS Biomate rials Science & Engineering 2017, 3 (5), 863-869. DOI: https: / / doi.org / 10.1021 / acsbiomate rials.7b00094.

[0018] [9] Matsuda, T.; Kawakami, R. K.; Nakajima, T.; Gong, J. P. Crack Tip Field of a Double-Network Gel: Visualization of Covalent Bond Scission through Mechanoradical Polymerization. Macromolecules 2020, 53, 8787-8795.

[0019]

[10] Manghnani, P. N., Nelson, A. Z., Wong, K., Lee, Y. W., Khan, S.A., & Doyle, P. S. (2024). From burst to controlled release: using hydrogel crosslinking chemistry to tune release of micro-crystalline active pharmaceutical ingredients Electronic supplementary information (ESI) available. See DOI: https: / / doi.org / 10.1039 / d4pm00186a. RSC Pharmaceutics, 2(1), 94-101. SUMMARY

[0020] The technical problem to be solved by the present application is to provide a double-coated polyaspartic acid / carboxymethyl cellulose hydrogel / stearic acid / urea slow-release fertilizer.

[0021] To solve the above technical problems, the present application provides a preparation method of a polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea slow-release fertilizer (double-coated slow-release fertilizer), comprising the following steps: 1) preparation of poly(succinimide) (high molecular weight poly(succinimide)); 2) preparation of a polyaspartic acid gel solution using the poly(succinimide), and then preparation of a polyaspartic acid / carboxymethyl cellulose hydrogel solution using the polyaspartic acid gel solution; and the following steps:

[0022] 3) preparation of stearic acid / urea particles;

[0023] 4) preparation of a polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea (polyaspartic acid / carboxymethyl cellulose hydrogel / stearic acid / urea) slow-release fertilizer using the polyaspartic acid / carboxymethyl cellulose hydrogel solution and the stearic acid / urea particles.

[0024] Improvements of the preparation method of the present application:

[0025] The step 3) is:

[0026] Add 3.4-3.6 g of stearic acid to 100 mL of ethanol and stir (stir for 1.5-2.5 h) to obtain a stearic acid solution (concentration 3.4-3.6%); then add 9-11 mL of stearic acid solution dropwise to 34-36 g of urea for coating to obtain stearic acid / urea granules;

[0027] Step 4) involves: dripping 9-11 mL of a viscous gel mixture of polyaspartic acid and carboxymethyl cellulose (PC) into 34-36 g of stearic acid / urea granules for coating, thereby obtaining a slow-release fertilizer of polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea.

[0028] As a further improvement to the preparation method of the present invention:

[0029] The coating process in steps 3) and 4) is as follows: rotation speed 600±100r / min, temperature 30±5℃, and coating time 1±0.2h.

[0030] As a further improvement to the preparation method of the present invention, step 2) is as follows:

[0031] Add 3±0.3g of ethylene glycol diglycidyl ether and 3±0.3g of carboxymethyl cellulose to a polyaspartic acid gel solution prepared from 6g of polysuccinimide powder. Place the mixture at 50±5℃ and stir at 800±50 rpm for 1±0.2h (at which point a gel has formed). Then adjust the stirring speed to 300±50 rpm and stir for another 5±0.5h to obtain a polyaspartic acid / carboxymethyl cellulose hydrogel solution (PC, a viscous gel solution).

[0032] In this invention:

[0033] Stearic acid (preferably 3.5g) was added to ethanol (100mL) and stirred for 2 hours to obtain a 3.5% stearic acid solution. The stearic acid solution was then added dropwise to a coating machine containing urea, and after coating for 1 hour, a slow-release fertilizer of stearic acid / urea was obtained. The amount of stearic acid solution added was 10ml, and the amount of urea was 35g.

[0034] The polyaspartic acid hydrogel solution was poured into a coating machine containing urea. After coating for 1 hour, a slow-release fertilizer of polyaspartic acid / urea was obtained. The amount of polyaspartic acid solution added was 10 ml, and the amount of urea was 35 g.

[0035] The polyaspartic acid-carboxymethyl cellulose hydrogel solution was poured into a coating machine containing urea. After coating for 1 hour, a slow-release fertilizer of polyaspartic acid-carboxymethyl cellulose / urea was obtained. The amount of polyaspartic acid-carboxymethyl cellulose solution added was 10 ml, and the amount of urea was 35 g.

[0036] The stearic acid / urea slow-release fertilizer was poured back into the coating machine, followed by the polyaspartic acid hydrogel solution. After coating for 1 hour, a polyaspartic acid / stearic acid / urea slow-release fertilizer was obtained. The amount of polyaspartic acid solution added was 10 ml, and the amount of stearic acid / urea was 35 g.

[0037] The stearic acid / urea slow-release fertilizer was poured back into the coating machine, and then the polyaspartic acid-carboxymethyl cellulose hydrogel solution was poured into the coating machine for coating for 1 hour to obtain a polyaspartic acid-carboxymethyl cellulose / stearic acid / urea slow-release fertilizer. The amount of polyaspartic acid-carboxymethyl cellulose solution added was 10 ml, and the amount of stearic acid / urea was 35 g.

[0038] This invention introduces a polyaspartic acid-carboxymethyl cellulose dual-network hydrogel into a slow-release fertilizer. The outer layer of the double-coating provides slow release and water retention, while stearic acid serves as the inner layer, providing the same slow-release effect. This solves the key problems of low fertilizer utilization and lack of water retention, resulting in a slow-release fertilizer with excellent water retention, biodegradability, and slow-release properties.

[0039] This invention employs an expandable, water-retaining, and biodegradable PC / DN hydrogel as the outer layer for controlled-release fertilizer (CRF) applications, and an environmentally friendly hydrophobic S coating as the inner layer. This overcomes the limitations of low fertilizer utilization efficiency and water scarcity. Simultaneously, the water retention and mechanical properties of the constructed PC / S / U were explored, its controlled-release effect was analyzed, and the potential controlled-release mechanism was investigated. Subsequently, the application of PC / S / U in controlled-release rapeseed cultivation demonstrated its potential as a novel solution for plant growth under drought and nutrient-deficient conditions.

[0040] In other words, this invention first coats the surface of urea with a layer of stearic acid as a slow-release material, forming a first protective layer, and then coats the urea with a hydrogel material as a second water-retaining layer. Comparing their water retention and water holding effects, it can be seen that the slow-release fertilizers coated with the hydrogel layer all have a certain water-retaining effect. At the same time, the double-network hydrogel, due to its excellent mechanical properties, can effectively resist the soil pressure after swelling in the soil, allowing it to lock in more water. Specifically, urea releases all its nitrogen on the fifth day, while the slow-release fertilizer coated with only one layer of hydrogel and stearic acid releases all its nitrogen by the 20th day, and the double-coated slow-release fertilizer takes about 30 days to release almost all its nitrogen. This proves that the double-layer and double-network hydrogel PC / S / U is the optimal controlled-release water-retaining fertilizer. Comparing its application in plant cultivation shows that the excellent water retention and slow-release effects are more beneficial to plant growth.

[0041] In summary, this invention designs a slow-release fertilizer with dual functions of water retention and slow release by using hydrophilic PC hydrogel as the outer layer of the slow-release material and hydrophobic stearic acid as the inner layer.

[0042] Compared with the prior art, the present invention has the following advantages:

[0043] (1) The combination of biodegradable agricultural water-retaining agent (polyaspartic acid hydrogel) with slow-release fertilizer solves the problem that slow-release fertilizer does not have a water-retaining effect. At the same time, the constructed double network structure also effectively solves the poor mechanical properties of polyaspartic acid hydrogel.

[0044] (2) A unique double-coating layer structure was designed, with hydrophilic hydrogel as the outer layer of the slow-release fertilizer and hydrophobic stearic acid as the inner layer of the slow-release fertilizer, thus constructing a double-coating layer slow-release fertilizer with a hydrophilic outer layer and a hydrophobic inner layer.

[0045] Traditional slow-release fertilizers suffer from drawbacks such as non-biodegradability, lack of water retention, poor slow-release performance, and difficulty in delivering fertilizers to plants on demand. This invention addresses these weaknesses by designing an outer hydrophilic layer for water absorption and retention, while the inner layer utilizes its inherent hydrophobic properties to prevent water molecules from rapidly entering the core and causing release. This results in a dual-functional slow-release fertilizer that combines water retention and slow release.

[0046] The release of fertilizer requires moisture, but traditional slow-release fertilizers are unusable in drought conditions when plants are short of water (they cannot fully release fertilizer when there is only a small amount of water). This invention uses a swellable hydrogel as the outer layer, which effectively absorbs trace amounts of moisture from the environment and stores it within the gel network. When water molecules enter the gel, they diffuse to the stearic acid-encapsulated core. Through capillary action and osmosis, the fertilizer is slowly released. Simultaneously, after absorbing a certain amount of water, the outer hydrogel will release more water under drought conditions, providing some water resources for the plants. If stearic acid is placed in the outer layer and the hydrogel in the inner layer, the slow-release fertilizer cannot absorb moisture from the environment and cannot achieve continuous release. When there is too much water, due to osmosis, water diffuses into the inner gel layer, causing the gel to swell, expanding the outer hydrophobic layer, and the excessive water entry will lead to explosive release.

[0047] In summary, this invention prepared a double-network hydrogel using polyaspartic acid and carboxymethyl cellulose, and constructed a double-coated slow-release fertilizer with an inner hydrophobic layer and an outer hydrophilic layer by using stearic acid as the first coating layer and the gel as the second layer. This provides important technical parameters and references for the development of slow-release fertilizers with excellent water retention, biodegradability, and slow-release properties. Attached Figure Description

[0048] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0049] Figure 1Scanning electron microscope images of U, S / U, P / U, PC / U, P / S / U, PC / S / U (A, B, C, D, E, F are surface images of U, S / U, P / U, PC / U, P / S / U, PC / S / U, A) I B I C I D I E I F I (See the corresponding cross-sectional view).

[0050] Figure 2 Energy dispersive spectra of U, S / U, P / U, PC / U, P / S / U, and PC / S / U (denoted by A, S, P ... Ⅱ B Ⅱ C Ⅱ D ⅡI E Ⅱ F Ⅱ express).

[0051] Figure 3 A comparison chart of water contact angles for S, P, PC, P / S, and PC / S;

[0052] Figure 4 Comparison chart of water holding capacity of U, S / U, P / U, PC / U, P / S / U, and PC / S / U:

[0053] Figure 5 Comparison chart of water retention rates for U, S / U, P / U, PC / U, P / S / U, and PC / S / U:

[0054] Figure 6 Comparison of soil leaching column experimental results for U, S / U, P / U, PC / U, P / S / U, and PC / S / U:

[0055] Figure 7 This is a diagram showing the results of a plant cultivation experiment.

[0056] Figure 7 middle:

[0057] A is a photograph of the rapeseed taken 30 days after it began to grow;

[0058] B is a top view of all plants after 30 days;

[0059] C represents the plant height measured every five days.

[0060] After the rapeseed seeds germinate in the incubator, they are transferred to barren soil and placed in it. The plants are watered every five days, and their growth is monitored. Detailed Implementation

[0061] The present invention will be further described below with reference to specific embodiments, but the scope of protection of the present invention is not limited thereto:

[0062] In the performance testing method of the present invention,

[0063] Water contact angle test. See Tan, X., Lv, Q., Dong, G., Zhang, Z., Chai, D.-f., Zhao, M., Zhang, W., & Li, J. (2024). Insight into the preparation and improved properties of cellulose citrate ester hydrogel slow-release fertilizer. Industrial Crops and Products, 222, 119517.

[0064] For soil water retention, please refer to Wu, Y., Liu, J., Hu, R., Wang, J., Cai, X., Wu, W., Song, X., Zhuge, C., Huang, M., Xie, H., & Ren, G. (2025). From weak to strong: the construction of dynamic reversible hydrogen bond in polyasparticacid double network hydrogel. Separation and Purification Technology, 369,133023.

[0065] For soil water holding, please refer to Wu, Y., Liu, J., Hu, R., Wang, J., Cai, X., Wu, W., Song, X., Zhuge, C., Huang, M., Xie, H., & Ren, G. (2025). From weak to strong: the construction of dynamic reversible hydrogen bond in polyasparticacid double network hydrogel. Separation and Purification Technology, 369,133023.

[0066] For the determination of slow-release urea content, please refer to Li, Z.; Zhang, M. Progress in the preparation of stimulus-responsive cellulose hydrogels and their application in slow-release fertilizers. Polymers 2023, 15, 3643.

[0067] For plant cultivation, please refer to Xiang, Y., Ru, 7606-7615.

[0068] Example 1: Preparation method of polysuccinimide:

[0069] Refer to Example 1 of "Preparation Method of Polyaspartic Acid-Carboxymethyl Cellulose Dual Network Hydrogel Water Retention Agent" (202411858722.0).

[0070] Example 2: Preparation method of polyaspartic acid gel and polyaspartic acid-carboxymethyl cellulose hydrogel, comprising the following steps in sequence:

[0071] 1) Referring to step 1) of Example 2 of "Preparation Method of Polyaspartic Acid-Carboxymethyl Cellulose Dual Network Hydrogel Water Retention Agent" (202411858722.0), specifically:

[0072] The 6g polysuccinimide powder obtained in Example 1 was dissolved in 36mL of 2M sodium hydroxide solution. After the polysuccinimide powder was completely dissolved, it was magnetically stirred at 500 rpm for 1h (to achieve the purpose of opening the polysuccinimide ring). Then, the pH was adjusted to 4.8 with phosphoric acid to obtain polyaspartic acid gel (P).

[0073] 2) Add 3g of ethylene glycol diglycidyl ether and 3g of carboxymethyl cellulose to the polyaspartic acid gel obtained in step 1) and place it in a 50℃ water bath and stir at 800 rpm for 1 hour; at this time, a gel has been formed. Then adjust the speed to 300 rpm and stir for another 5 hours to obtain a viscous gel solution - polyaspartic acid / carboxymethyl cellulose hydrogel solution (PC).

[0074] Example 3: Preparation method of stearic acid / urea (S / U) slow-release fertilizer, comprising the following steps:

[0075] 1) Stearic acid (3.5g) was added to ethanol solution (100mL) and stirred for 2 hours to obtain a stearic acid solution with a concentration of 3.5% (g / 100ml).

[0076] 2) Weigh 35g of urea granules (U) and place them in a coating machine. Add 10mL of stearic acid solution from step 1) onto the urea and coat for 1 hour (600r / min, 30℃) to obtain stearic acid / urea granules as a slow-release fertilizer.

[0077] Example 4: Preparation method of polyaspartic acid / urea (P / U) slow-release fertilizer, comprising the following steps:

[0078] Weigh 35g of urea granules and place them in a coating machine. Add 10mL of polyaspartic acid gel solution from step 1) of Example 2 onto the urea. Coat for 1 hour (rotation speed 600r / min, temperature 30℃) to obtain polyaspartic acid / urea granules as a slow-release fertilizer.

[0079] Example 5: Preparation method of polyaspartic acid-carboxymethyl cellulose / urea (PC / U) slow-release fertilizer, comprising the following steps:

[0080] Weigh 35g of urea granules and place them in a coating machine. Add 10mL of the polyaspartic acid / carboxymethyl cellulose hydrogel solution (PC) obtained in step 2) of Example 2 to the urea. Coat for 1 hour (rotation speed 600r / min, temperature 30℃) to obtain polyaspartic acid-carboxymethyl cellulose / urea granules as a slow-release fertilizer.

[0081] Example 6: Preparation method of polyaspartic acid / stearic acid / urea (P / S / U) slow-release fertilizer, comprising the following steps:

[0082] Weigh 35g of the stearic acid / urea granules obtained in Example 3 and place them in a coating machine. Add 10mL of the polyaspartic acid gel solution (P) from step 1) of Example 2 onto the urea. Coat for 1 hour (rotation speed 600r / min, temperature 30℃) to obtain a slow-release fertilizer of polyaspartic acid / stearic acid / urea.

[0083] Example 7: Preparation method of polyaspartic acid-carboxymethyl cellulose / stearic acid / urea (PC / S / U) slow-release fertilizer, comprising the following steps:

[0084] Weigh 35g of the stearic acid / urea granules obtained in Example 3 and place them in a coating machine. Add 10mL of the polyaspartic acid / carboxymethyl cellulose hydrogel liquid (PC) obtained in step 2) of Example 2 to the urea. Coat for 1 hour (rotation speed 600r / min, temperature 30℃) to obtain a slow-release fertilizer of polyaspartic acid-carboxymethyl cellulose / stearic acid / urea.

[0085] The products obtained above were subjected to the following experiments.

[0086] Experiment 1, Scanning Electron Microscopy:

[0087] The above products (U, S / U, P / U, PC / U, P / S / U, PC / S / U) were examined using a scanning electron microscope (SEM), and the resulting SEM images are shown below. Figure 1 As stated above.

[0088] according to Figure 1 It can be seen that the slow-release fertilizer with coated stearic acid and hydrogel has a complete structure, and the coating layer adheres well to the surface of the slow-release fertilizer.

[0089] Experiment 2, Energy Dispersive Spectrometer:

[0090] The above products (U, S / U, P / U, PC / U, P / S / U, PC / S / U) were detected using energy-dispersive X-ray spectroscopy (EDS), and the resulting EDS chromatograms are shown below. Figure 2 As stated above.

[0091] according to Figure 2 It can be seen that the coating layer has a different elemental composition than urea, and EDS can clearly show how the coating layer adheres to the urea surface. At the same time, it can be observed that the thickness of the double-layer coating (P / S / U and PC / S / U) is much greater than that of the single-layer coating (S / U, P / U, PC / U), proving that the coating is intact.

[0092] Experiment 3, Water Contact Angle:

[0093] Referring to the above embodiments, various materials for coating the surface of urea particles were prepared:

[0094] Stearic acid solution was added dropwise onto a glass slide and lyophilized (using conventional lyophilization methods). The resulting solution was labeled as S.

[0095] Polyaspartic acid gel was dropped onto a glass slide, lyophilized, and the resulting gel was labeled P.

[0096] Polyaspartic acid / carboxymethyl cellulose hydrogel was dropped onto a glass slide, freeze-dried, and the resulting product was labeled PC.

[0097] First, stearic acid solution was dropped onto a glass slide and freeze-dried. Then, the same volume of polyaspartic acid gel was dropped onto the same position and freeze-dried again. The result was labeled as P / S.

[0098] First, stearic acid was dropped onto a glass slide and freeze-dried. Then, the same volume of polyaspartic acid / carboxymethyl cellulose hydrogel was dropped onto the same position and freeze-dried again. The resulting product was labeled PC / S.

[0099] The water contact angle (WCA) of the above materials S, P, PC, P / S, and PC / S was measured, and the resulting WCA diagram is shown below. Figure 3 The water contact angle data refers to the angle between the water droplet surface and the material when a water droplet lands on the membrane, used to determine the hydrophilicity or hydrophobicity of the material.

[0100] according to Figure 3 It can be seen that the hydrogel coating exhibits high hydrophilicity due to its rich content of hydrophilic hydroxyl and carboxyl groups, while stearic acid exhibits high hydrophobicity due to the presence of its hydrophobic chains.

[0101] Experiment 4, Water Holding Capacity:

[0102] Wrap one side of a vertical, two-sided transparent PVC pipe with gauze, and add a certain amount of dry sand (50g, dried in an oven at 0 ℃ for 8 h) to the other side. When the sand is 3 cm away from the top of the pipe, add 1 g of slow-release fertilizer (U, S / U, P / U, PC / U, P / S / U, PC / S / U). The control group is the group without slow-release fertilizer. Weigh the contents (W1). Drip a certain amount of aqueous solution into the opening and measure the weight after the water has passed through the gauze (W2).

[0103] Water holding capacity:

[0104] according to Figure 4 It can be seen that: U and S / U have water holding capacities similar to the control group because their outer coating layer is not a swellable hydrogel. P / U, PC / U, P / S / U, and PC / S / U all have hydrogel outer coating layers, which will swell during the test, increasing their water holding capacities.

[0105] Experiment 5, Water retention rate:

[0106] Place 1g of the above products (U, S / U, P / U, PC / U, P / S / U, PC / S / U) into a beaker containing 100g of soil (dry soil), and add 20ml of water. The total weight after adding the products is W1. The control group consists of dry soil without the products. Measure the remaining body weight (W2) daily to assess weight loss, and calculate W using the formula provided in the equation.

[0107] Water retention rate =

[0108] according to Figure 5 It can be seen that dual-network hydrogels have better water retention than single-network hydrogels due to their superior mechanical properties. The water retention capacity of PC hydrogel coatings is much higher than that of P hydrogels.

[0109] Experiment 6: Soil leaching column experiment:

[0110] 1g of the above products (U, S / U, P / U, PC / U, P / S / U, PC / S / U) and 100g of barren soil (with nutrients such as N, P, K, etc.) washed off were placed in a glass column. 50ml of water was added daily, and the nitrogen content in the solution eluted from the column was measured.

[0111] according to Figure 6 It can be seen that the slow-release effect of the double-coated layer is higher than that of the single-coated layer and pure urea, indicating that the slow-release urea of ​​the double-network hydrogel and stearic acid has a good effect as a controlled-release fertilizer.

[0112] Experiment 7: Plant Cultivation Experiment

[0113] After the rapeseed seeds germinated in the incubator, individual seedlings were transferred to infertile soil (100g, soil washed to remove nutrients, free of N, P, and K), and 1g of the product was placed in it. Water was applied every five days (30 ml), and the plant growth was monitored.

[0114] according to Figure 7 It can be seen that rapeseed needs water during the planting process, and water-retaining hydrogels can provide water, while the slow-release effect can provide the plant with a continuous supply of nutrients.

[0115] In summary, the slow-release fertilizers of Examples 3-7 all exhibited excellent slow-release effects. At the same time, the constructed dual-network hydrogel can better store water in the soil due to its excellent mechanical properties. That is, the present invention preferably uses a slow-release fertilizer PC / S / U of polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea.

[0116] It should be noted that using polyaspartic acid-carboxymethyl cellulose as the inner coating and stearic acid as the outer coating is not advisable. Firstly, the stearic acid coating on the outermost layer will prevent water molecules from entering the polyaspartic acid-carboxymethyl cellulose hydrogel, inhibiting hydrogel swelling and thus eliminating its water-retention effect. Secondly, after the hydrogel absorbs water, the water will quickly penetrate into the urea, accelerating the urea release rate and failing to achieve a sustained-release effect.

[0117] Finally, it should be noted that the above examples are merely some specific embodiments of the present invention. Obviously, the present invention is not limited to the above embodiments and many variations are possible. All variations that can be directly derived or conceived by those skilled in the art from the disclosure of this invention should be considered within the scope of protection of this invention.

Claims

1. A preparation method of a slow-release fertilizer of polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea, comprising the steps of 1) preparation of a polysuccinimide, 2) preparation of a polyaspartic acid gel solution using the polysuccinimide, and 3) preparation of a polyaspartic acid / carboxymethyl cellulose hydrogel solution using the polyaspartic acid gel solution; characterized in that Further comprising the following steps: 3) preparing stearic acid / urea particles; 4) using the polyaspartic acid / carboxymethyl cellulose hydrogel liquid and the stearic acid / urea particles to prepare a slow-release fertilizer of polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea.

2. The preparation method according to claim 1, characterized in that: the step 3) is: 3.4-3.6 g of stearic acid is added to 100 mL of ethanol for stirring to obtain a stearic acid solution; 9-11 mL of the stearic acid solution is then dropped into 34-36 g of urea for coating to obtain stearic acid / urea particles; the step 4) is: 9-11 mL of a viscous gel mixture of polyaspartic acid-carboxymethyl cellulose is dropped into 34-36 g of the stearic acid / urea particles for coating to obtain a slow-release fertilizer of polyaspartic acid-carboxymethyl cellulose hydrogel-stearic acid-urea.

3. The preparation method according to claim 1, characterized in that: the coating of the step 3) and the step 4) is all at a rotation speed of 600±100 r / min, a temperature of 30±5℃, and a coating time of 1±0.2 h.

4. The method of claim 1, wherein the step 2) is: 3±0.3 g of ethylene glycol diglycidyl ether and 3±0.3 g of carboxymethyl cellulose are added to 6 g of polyaspartic acid gel liquid prepared from poly succinimide powder, and stirring is performed at 50±5℃ at 800±50 r / min for 1±0.2 h, and then the rotation speed is adjusted to 300±50 r / min for further stirring for 5±0.5 h to obtain a polyaspartic acid / carboxymethyl cellulose hydrogel liquid.

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