Temperature-responsive water-absorbing microspheres, and preparation method and application thereof

Abstract

This invention belongs to the field of polymer water-absorbing microsphere technology, specifically relating to a temperature-responsive water-absorbing microsphere, its preparation method, and its applications. The invention employs reverse-phase suspension polymerization, using liquid paraffin as the oil phase, Span 80 as the emulsifier, and mixing SA-NVCL aqueous solution, CS aqueous solution, and neutralized AA in a specific ratio to form the aqueous phase. N,N'-methylenebisacrylamide is used as the crosslinking agent, and potassium persulfate as the initiator. Polymerization is carried out at 75°C to obtain microspheres. The resulting microspheres are regularly spherical with an average particle size of 53.39 μm and a maximum swelling ratio of 615.46 g / g. They exhibit both pH-responsive and temperature-sensitive characteristics, good salt tolerance and cycling stability, and can significantly improve soil water retention and water holding capacity. The raw materials of this invention are green and biodegradable, the process is simple, the conditions are mild, and it is suitable for industrial production, possessing significant application value in agricultural water conservation and soil improvement in arid and high-temperature regions.

Description

Technical Field

[0001] This invention belongs to the field of polymer water-absorbing microsphere technology, specifically relating to a temperature-responsive water-absorbing microsphere, its preparation method, and its application. Background Technology

[0002] Superabsorbent polymers (SAPs), as a class of functional polymer materials capable of rapidly absorbing and retaining large amounts of water, are widely used in agricultural drought resistance and soil improvement. Among them, water-absorbing microspheres have attracted much attention due to their uniform particle size, large specific surface area, good dispersibility, fast water absorption rate, and ease of uniform distribution in soil.

[0003] However, existing water-absorbing microspheres have the following technical drawbacks: First, the raw materials are highly dependent on petroleum-based monomers (such as acrylic acid, acrylamide, acrylonitrile, etc.), have poor biodegradability, and long-term application can easily lead to the accumulation of high molecular weight residues in the soil and ecological risks. Second, the environmental adaptability is weak: the swelling performance decreases sharply under complex conditions such as acid, alkali, and salt ions. Third, the function is single and lacks intelligent response characteristics, and the high-temperature water retention capacity is insufficient: ordinary water-absorbing materials do not have temperature or pH response capabilities, and their water retention capacity decreases rapidly under high-temperature and drought conditions. Fourth, the structural stability is poor and the recycling performance is weak: traditional materials have problems such as network collapse, chain segment breakage, and structural damage during multiple water absorption-release cycles, resulting in decreased water absorption and retention performance and short service life. Fifth, the preparation process is complex and the conditions are harsh, making it difficult to produce on a large scale.

[0004] To mitigate the environmental problems caused by petroleum-based monomers, researchers have attempted to introduce natural polymers such as sodium alginate (SA), chitosan, and starch for composite modification. However, they have yet to achieve green microspheres that simultaneously possess high water absorption, pH responsiveness, temperature responsiveness, salt tolerance, and cycling stability. Therefore, developing a multifunctional water-absorbing microsphere with natural polymers as the main component and petroleum-based monomers as a supplement, while also exhibiting intelligent responsiveness, is of great significance. Summary of the Invention

[0005] To address the aforementioned problems, the present invention aims to provide a temperature-responsive water-absorbing microsphere, its preparation method, and its applications. This invention uses natural polymers such as SA and N-vinylcaprolactam (NVCL) grafts and cassava starch (CS) as the main components, and constructs a three-dimensional cross-linked network with acrylic acid (AA). Composite water-absorbing microspheres are prepared using reverse-phase suspension polymerization. The material is green and biodegradable, has a wide pH response range, possesses temperature-sensitive water-locking properties, good salt tolerance, and strong recyclability. It can significantly improve soil water retention and is suitable for various complex agricultural environments.

[0006] Specifically, the present invention provides the following technical solution: In a first aspect, the present invention provides a temperature-responsive water-absorbing microsphere having a three-dimensional network structure formed by chemical crosslinking of sodium alginate-grafted poly(N-vinylcaprolactam) copolymer, cassava starch, and partially neutralized acrylic acid. In the three-dimensional network structure, the volume ratio of sodium alginate-grafted poly(N-vinylcaprolactam)-grafted copolymer, cassava starch, and partially neutralized polyacrylic acid is 3:1:5~9.

[0007] Preferably, in the three-dimensional network structure, the volume ratio of sodium alginate-grafted poly(N-vinylcaprolactam)-grafted copolymer, cassava starch, and partially neutralized acrylic acid is 3:1:7.

[0008] Preferably, the sodium alginate-grafted poly(N-vinylcaprolactam) graft copolymer has sodium alginate as the main chain and N-vinylcaprolactam as the side chain, wherein the N-vinylcaprolactam is covalently grafted onto the sodium alginate main chain.

[0009] Preferably, the water-absorbing microspheres are spherical with an average particle size of 53.39±1.91 μm.

[0010] Preferably, the water-absorbing microspheres maintain their swelling capacity within a pH range of 3 to 11, with a swelling ratio of 212.14 g / g at pH 3, 325.41 g / g at pH 5, 615.46 g / g at pH 7, 540.64 g / g at pH 9, and 248.65 g / g at pH 11; the retention rate of the swelling ratio of the water-absorbing microspheres at pH 7 after 4 cycles is 73.7%.

[0011] Preferably, the temperature-sensitive phase transition temperature of the water-absorbing microspheres is 45~46℃; the swelling ratio is 615.46 g / g at 20℃, 613.69 g / g at 30℃, 566.27 g / g at 40℃, and 461.97 g / g at 50℃.

[0012] Preferably, the water-absorbing microspheres possess water absorption capacity in 0.02 M to 0.1 M NaCl, Na2CO3, NaHCO3, MgCl2, or FeCl3 salt solutions; wherein, the swelling ratio in 0.1 M NaCl solution is 76.32 g / g.

[0013] Preferably, when the amount of the water-absorbing microspheres added to the soil is 0.05~0.20 g / 100 g of air-dried soil, the maximum water holding capacity of the soil increases from 39.47% to 55.90%~86.77%.

[0014] Preferably, the water-absorbing microspheres can achieve a soil water retention rate of over 57.78% under high temperature conditions of 46℃.

[0015] A second aspect of the present invention provides a method for preparing the above-mentioned temperature-responsive water-absorbing microspheres, comprising the following steps: A sodium alginate solution and N-vinylcaprolactam monomer were mixed, and then a crosslinking agent and an initiator were added sequentially to carry out a free radical grafting polymerization reaction to obtain a sodium alginate grafted poly(N-vinylcaprolactam) copolymer. Add the emulsifier to the organic solvent and stir until completely dissolved and homogeneous to obtain an oil phase solution; mix the sodium alginate-grafted polyN-vinylcaprolactam aqueous solution, the cassava starch aqueous solution, and the acrylic acid solution partially neutralized with sodium hydroxide solution to obtain an aqueous phase solution; An aqueous solution was added to an oil solution and stirred to disperse it into a stable water-in-oil system. Then, a crosslinking agent and an initiator were added sequentially to carry out a polymerization reaction. After the reaction was completed, the mixture was washed and vacuum dried to obtain temperature-responsive water-absorbing microspheres.

[0016] Preferably, the mass ratio of the sodium alginate solution, N-vinylcaprolactam monomer, crosslinking agent, and initiator is 2:0.8~2.4:0.007~0.011:0.021~0.033; the crosslinking agent is selected from one or more of N,N'-methylenebisacrylamide (MBA), polyethylene glycol diacrylate (PEGDA), ethylene glycol dimethacrylate (EGDMA), N,N'-bisacryloylcysteine ​​(BAC), calcium chloride, or calcium sulfate; and the initiator is selected from one or more of potassium persulfate (KPS), ammonium persulfate (APS), sodium persulfate (SPS), 2,2'-azobisisobutyramidine hydrochloride (V-50), and 4,4'-azobis(4-cyanopentanoic acid) (V-501).

[0017] Preferably, the free radical graft polymerization reaction is carried out at a temperature of 70-80°C for 1-1.5 h.

[0018] Preferably, the emulsifier is Span 80, the organic solvent is liquid paraffin, and the amount of emulsifier added is 0.5~0.6 g / 30 mL of liquid paraffin.

[0019] Preferably, the sodium alginate-grafted poly(N-vinylcaprolactam) aqueous solution has a mass fraction of 0.5-3 wt%, the cassava starch aqueous solution has a mass fraction of 0.5-3 wt%, and the degree of neutralization of the acrylic acid solution partially neutralized with sodium hydroxide solution is 50-80%, wherein the concentration of the sodium hydroxide solution is 3 mol / L and the concentration of the acrylic acid solution is 99%.

[0020] More preferably, the sodium alginate-grafted poly(N-vinylcaprolactam) aqueous solution has a mass fraction of 2 wt%, the cassava starch aqueous solution has a mass fraction of 2 wt%, and the degree of neutralization of the acrylic acid solution partially neutralized with sodium hydroxide solution is 70%.

[0021] Preferably, the mass of the crosslinking agent added in the polymerization reaction is 0.1 to 1 wt% of the total mass of the aqueous solution; the mass of the initiator added is 0.1 to 1 wt% of the total mass of the aqueous solution.

[0022] Preferably, the washing is performed by alternating washing with petroleum ether and anhydrous ethanol; the polymerization reaction temperature is 70~80℃, and the reaction time is 1~1.5 h; the vacuum drying temperature is 55~65℃.

[0023] A third aspect of the present invention provides an application of the temperature-responsive water-absorbing microspheres described in the first aspect in improving the soil environment and enhancing the soil's water-holding and water-retention capacity.

[0024] One or more embodiments of the present invention have at least the following beneficial effects: (1) This invention successfully prepared ASNC composite water-absorbing microspheres by reverse suspension polymerization. While improving water absorption performance, it introduces a large number of natural polymer components, reduces the proportion of petroleum-based monomers, enhances the biodegradability potential of the material, and is more environmentally friendly and suitable for green agricultural scenarios.

[0025] (2) The microspheres prepared by this invention maintain effective swelling capacity in the pH range of 3 to 11, and reach a maximum swelling ratio of 615.46 g / g at pH=7. They can adapt to acidic, neutral and alkaline soil environments, breaking through the limitations of traditional materials on pH conditions.

[0026] (3) The microspheres prepared by this invention have unique temperature-sensitive response and high-temperature water-locking ability. They undergo obvious phase transformation at 45~46℃, and the network shrinks moderately at high temperature, reducing rapid water loss and significantly improving water retention stability under high temperature environment. They are suitable for application in arid and high-temperature areas.

[0027] (4) The microspheres prepared by this invention have good salt resistance. They maintain a certain swelling capacity in NaCl, Na2CO3, NaHCO3, MgCl2 and FeCl3 solutions, and can resist the charge shielding effect of salt ions. They are suitable for moderately saline-alkali environments.

[0028] (5) The microspheres prepared by this invention have excellent recycling performance. After four water absorption-release cycles, the swelling ratio still remains at 73.7%. The network structure is stable, not easy to collapse or break, and can be reused for a long time, reducing application costs.

[0029] (6) The microspheres prepared by the present invention can significantly improve the soil water holding and water retention capacity. After adding 0.2 g of microspheres, the maximum water holding capacity of the soil increased from 39.47% to 86.77%. It can still effectively lock water under high temperature conditions, greatly delay water evaporation, and improve the utilization rate of agricultural water.

[0030] (7) The overall preparation process of the present invention is simple, the conditions are mild, no inert gas protection is required, the raw materials are cheap and readily available, the operation is stable and controllable, and it is easy to scale up industrial production. Attached Figure Description

[0031] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0032] Figure 1 (a) FTIR spectra of SA, NVCL and SA-NVCL for the water-absorbing microspheres prepared in Example 1 of the present invention; (b) FTIR spectra of SA-NVCL, CS, MBA and ASNC microspheres; (c) SEM morphology of ASNC microspheres; (d) particle size distribution and cumulative distribution curve of ASNC microspheres. Figure 2 The swelling ratio of the water-absorbing microspheres prepared in Example 1 of this invention was observed in (a) different pH values; (b) different temperatures; (c) different concentrations and types of salt solutions; and (d) changes in cyclic swelling ratio. Figure 3 Thermosensitive transmittance curves for NVCL, SA-NVCL, and ASNC; Figure 4 (a) Effect of different addition amounts of the water-absorbing microspheres prepared in Example 1 of the present invention on the maximum water holding capacity of the soil; (b) Changes in water retention rate at different temperatures; (d) Changes in water retention rate of soil and ASNC-modified soil at different temperatures. Detailed Implementation

[0033] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0034] The protection scheme of the present invention will be described below through specific embodiments. It should be noted that these embodiments are only used to facilitate understanding by those skilled in the art and should not be considered as limiting the scope of protection of the present invention. Unless otherwise stated, the reagents used in the embodiments are all commercially available.

[0035] Example 1: This example provides a temperature-responsive water-absorbing microsphere and its preparation method. The specific preparation method is as follows: (1) Preparation of SA-NVCL grafted product: Prepare 100 mL of 2 wt% SA aqueous solution, stir until transparent, then add 2 g NVCL and stir to dissolve; then add 0.25 wt% N,N'-methylenebisacrylamide (MBA) and 0.75 wt% potassium persulfate (KPS) with the sum of SA and NVCL monomers, react at 75℃ for 1 h to obtain SA-NVCL aqueous solution, freeze dry to obtain solid SA-NVCL product.

[0036] (2) Preparation of oil phase: Measure 30 mL of liquid paraffin into a 50 mL three-necked flask, add 0.506 g of Span 80, and stir at 600 rpm for 15 min until completely dissolved to obtain a stable oil phase solution.

[0037] (3) Preparation of mixed aqueous phase: Prepare 2 wt% SA-NVCL aqueous solution and 2 wt% CS aqueous solution respectively; neutralize 99% AA to 70% with 3 mol / L NaOH solution (to obtain a partially neutralized acrylic acid solution containing acrylic acid and sodium acrylate). Then, measure 3 mL of SA-NVCL aqueous solution, 1 mL of CS aqueous solution and 7 mL of neutralized AA solution, and mix them evenly to obtain a mixed aqueous phase.

[0038] (4) Reverse suspension polymerization: The mixed aqueous phase is slowly added to the oil phase solution and stirred at 75°C for 30 min to form a uniform and stable water-in-oil system; then 0.25 wt% of MBA crosslinking agent is added to the aqueous phase and stirred for another 30 min; then 0.75 wt% of KPS initiator is added to the aqueous phase and reacted at 75°C for 1 h.

[0039] (5) After the reaction is complete, the product is washed three or more times with petroleum ether and anhydrous ethanol; then dried in a vacuum oven at 60°C to constant weight to obtain ASNC water-absorbing microspheres.

[0040] Comparative Example 1: The difference between this comparative example and Example 1 is that sodium alginate was not grafted in step (1); the specific preparation method is as follows: (1) Preparation of oil phase: Measure 30 mL of liquid paraffin into a 50 mL three-necked flask, add 0.506 g of Span 80, and stir at 600 rpm for 15 min until completely dissolved to obtain a stable oil phase solution.

[0041] (2) Preparation of mixed aqueous phase: Prepare 2 wt% SA aqueous solution and 2 wt% CS aqueous solution respectively; neutralize AA with NaOH solution to 70%. Then, measure 3 mL of SA aqueous solution, 1 mL of CS aqueous solution and 7 mL of neutralized AA solution, and mix them evenly to obtain mixed aqueous phase.

[0042] (3) Reverse suspension polymerization: The mixed aqueous phase is slowly added to the oil phase solution and stirred at 75°C for 30 min to form a uniform and stable water-in-oil system; then 0.25 wt% of MBA crosslinking agent is added to the aqueous phase and stirred for another 30 min; then 0.75 wt% of KPS initiator is added to the aqueous phase and reacted at 75°C for 1 h.

[0043] (4) After the reaction is complete, the product is washed three or more times with petroleum ether and anhydrous ethanol; then dried in a vacuum oven at 60°C to constant weight to obtain ASC water-absorbing microspheres.

[0044] Comparative Example 2: The difference between this comparative example and Example 1 is that the NVCL monomer was directly added to the mixed aqueous phase (without pre-graft polymerization); the specific preparation method is as follows: (1) Preparation of oil phase: Measure 30 mL of liquid paraffin into a 50 mL three-necked flask, add 0.506 g of Span 80, and stir at 600 rpm for 15 min until completely dissolved to obtain a stable oil phase solution.

[0045] (2) Preparation of mixed aqueous phase: Prepare 2 wt% SA aqueous solution and 2 wt% CS aqueous solution respectively; neutralize AA with NaOH solution to 70%. Then, measure 3 mL of SA aqueous solution, 1 mL of CS aqueous solution, 7 mL of neutralized AA solution and 2 g of NVCL, and mix them evenly to obtain mixed aqueous phase.

[0046] (3) Reverse suspension polymerization: The mixed aqueous phase is slowly added to the oil phase solution and stirred at 75°C for 30 min to form a uniform and stable water-in-oil system; then 0.25 wt% of MBA crosslinking agent is added to the aqueous phase and stirred for another 30 min; then 0.75 wt% of KPS initiator is added to the aqueous phase and reacted at 75°C for 1 h.

[0047] (4) After the reaction is complete, the product is washed three or more times with petroleum ether and anhydrous ethanol; then dried in a vacuum oven at 60°C to constant weight to obtain ASC-N water-absorbing microspheres.

[0048] Comparative Example 3: The difference between this comparative example and Example 1 is that in step (3), SA-NVCL is not included, while the content of other components and the preparation method are completely consistent with Example 1.

[0049] Comparative Example 4: The difference between this comparative example and Example 1 is that step (3) does not contain CS, while the content of other components and the preparation method are completely consistent with Example 1.

[0050] Comparative Example 5: The difference between this comparative example and Example 1 is that in step (3), AA is not included, while the content of other components and the preparation method are completely consistent with Example 1.

[0051] Comparative Example 6: The difference between this comparative example and Example 1 is that in step (3), AA is added in excess, that is, the volume ratio of SA-NVCL aqueous solution, CS aqueous solution and AA solution is 3:1:10, and the content of other components and preparation methods are completely consistent with Example 1.

[0052] Comparative Example 7: The difference between this comparative example and Example 1 is that in step (3), a small amount of AA is added, that is, the volume ratio of SA-NVCL aqueous solution, CS aqueous solution and AA solution is 3:1:4, and the content of other components and preparation methods are completely consistent with Example 1.

[0053] Comparative Example 8: The difference between this comparative example and Example 1 is that in step (3) and step (4), the crosslinking agent is in excess, that is, the amount of MBA crosslinking agent added accounts for 1 wt% of the total mass of the aqueous phase, while the content of other components and the preparation method are completely consistent with Example 1.

[0054] Comparative Example 9: The difference between this comparative example and Example 1 is that in steps (3) and (4), the crosslinking agent is used in small amounts, specifically, the amount of MBA crosslinking agent added accounts for 0.1 wt% of the total mass of the aqueous phase. The content of other components and the preparation method are completely consistent with Example 1. In this comparative example, after reducing the concentration of MBA crosslinking agent to 0.1 wt%, the crosslinking density of the system decreased significantly. Due to insufficient construction of the polymer network skeleton, the structural stiffness and deformation recovery ability of the microspheres are weakened during the molding process, making them prone to breakage or deformation under shear field.

[0055] Experimental Example 1: This experimental example characterizes the morphology and structure of the temperature-responsive water-absorbing microspheres prepared in Example 1. Depend on Figure 1 As shown in a, Fourier transform infrared (FTIR) spectroscopy reveals that SA is at approximately 3400 cm⁻¹. -1A broad and strong absorption peak appears nearby, corresponding to the stretching vibration of -OH; NVCL reaches 2925 cm⁻¹. -1 and 1653 cm -1 Characteristic absorption peaks appeared in the vicinity, with the peak at 2925 cm⁻¹. -1 Belongs to -CH stretching vibration, 1653 cm -1 This is attributed to the stretching vibration of the C=O group in the amide group. Compared to SA and NVCL, the SA-NVCL sample exhibits a higher stretching vibration at 1733 cm⁻¹. -1 The presence of a distinct absorption peak nearby, along with the retention of characteristic absorption signals from some polysaccharide backbones and lactam groups, indicates that NVCL has been successfully grafted onto the SA molecular chain, forming an SA-NVCL graft copolymer.

[0056] Depend on Figure 1 Further analysis of the network structure of ASNC microspheres can be conducted using the method described in section b. SA-NVCL at 1735 cm⁻¹ -1 The vicinity exhibits a carbonyl absorption peak, with CS at 1076 cm⁻¹. -1 The characteristic peak of CO stretching vibration in the polysaccharide backbone appears nearby, with the MBA at 1673 cm⁻¹. -1 A characteristic absorption peak for amide carbonyl groups appears nearby. ASNC microspheres simultaneously retain some characteristic absorption peaks for SA-NVCL, CS, and MBA, with a peak at 1024 cm⁻¹. -1 The presence of prominent absorption peaks nearby indicates that all components jointly participated in constructing the cross-linked network structure. Compared with the raw materials, the positions and intensities of some peaks in ASNC changed to some extent, which may be related to the enhanced intermolecular hydrogen bonding, chain segment entanglement, and the formation of the cross-linked network, indicating that the composite microspheres have been successfully prepared.

[0057] Figure 1 Image c shows the SEM image of the ASNC microspheres prepared in Example 1. As can be seen from the image, the obtained microspheres are generally spherical with a smooth surface and clear particle boundaries, indicating good droplet sphericity during the reverse suspension polymerization process and a relatively stable polymerization and solidification process. The microspheres have a relatively uniform particle size distribution, with no obvious collapse or severe aggregation observed. Only a small number of fine particles adhere to the surface of the larger particles, which may be related to the small droplets formed during emulsification. The relatively complete spherical structure is beneficial for the microspheres to maintain a stable morphology during water absorption and swelling, and provides a good structural basis for their water retention and sustained-release properties.

[0058] Depend on Figure 1As shown in Figure d, the average particle size of the ASNC microspheres prepared in Example 1 is 53.39 ± 1.91 μm, of which 50% of the particles have a particle size close to 53.39 μm, 10% of the particles have a particle size less than 33.11 μm, and 90% of the particles have a particle size less than 78.11 μm. Combined with the SEM results, it can be seen that this system can prepare spherical particles with good dispersibility and a particle size in the micrometer scale.

[0059] Experimental Example 1: To investigate the water absorption and swelling properties of the microspheres, this experiment used a gravimetric method to determine their water absorption behavior in deionized water. First, the dried microspheres were placed in a pre-weighed centrifuge tube, and the total mass of the centrifuge tube and the dried microspheres was recorded as W0, with the mass of the dried microspheres denoted as W. d Subsequently, appropriate amounts of deionized water, solutions with different pH values ​​(3, 5, 7, 9, 11), and 0.1 mol / L solutions of NaCl, MgCl2, and FeCl3 were added to centrifuge tubes to completely submerge the microspheres. The tubes were then allowed to stand at room temperature to absorb the liquid. Centrifugation was performed at set time intervals to remove unabsorbed free liquid. The total mass of the centrifuge tubes and the swollen microspheres was then weighed and denoted as W. t When two consecutive measurements show a small change in mass, the microspheres are considered to have reached liquid absorption equilibrium, and the total mass at this point is recorded as W. eq Each sample was tested in triplicate, and the average value was taken.

[0060] The water absorption ratio SR of microspheres at time t t Calculate according to formula (1):

[0061] Balanced water absorption ratio SR eq Calculate according to formula (2)

[0062] (1) pH response performance Figure 2Figure a illustrates the swelling performance of the ASNC microspheres prepared in Example 1 of this invention under different pH conditions. As shown in the figure, the swelling ratio exhibits a significant non-linear trend with pH: at pH = 3, 5, 7, 9, and 11, the swelling ratios are 212.14, 325.41, 615.46, 540.64, and 248.65 g / g, respectively. The maximum value of 615.46 g / g is reached under neutral conditions (pH = 7). Under acidic conditions (pH = 3-5), the carboxyl groups (-COOH) in the system exist in an unionized form, resulting in strong hydrogen bonding between molecular chains and network shrinkage, leading to a low swelling ratio. When the pH increases to neutral, the carboxyl groups gradually ionize into -COO-, generating electrostatic repulsion between molecular chains. Simultaneously, the osmotic pressure difference increases, causing significant expansion of the network structure, thus rapidly increasing the swelling ratio. When the pH further increases to alkaline conditions (pH = 9-11), although -COO-... - As the concentration increases, the ionic strength in the solution also increases. External ions exert a shielding effect on the charges in the network, weakening the electrostatic repulsion and causing partial contraction of the network structure, resulting in a decrease in the swelling ratio. Therefore, this system exhibits typical pH-responsive characteristics, indicating that it has a certain degree of environmental responsiveness.

[0063] The specific data is shown in Table 1: Table 1

[0064] (2) Temperature response performance Figure 2 Figure b shows the swelling properties of the ASNC microspheres prepared in Example 1 of this invention under different temperature conditions. The results show that the swelling ratios at 20, 30, 40, and 50℃ are 615.46, 613.69, 566.27, and 461.97 g / g, respectively. It can be seen that the swelling ratio remains relatively stable within the 20-30℃ range (approximately 615 g / g), indicating that the material has good water absorption properties at room temperature. As the temperature increases to 40℃ and 50℃, the swelling ratio gradually decreases, to 566.27 g / g and 461.97 g / g, respectively. This phenomenon is mainly related to the NVCL structure introduced into the system. NVCL has certain temperature response characteristics. As the temperature increases, the hydrophobic interactions of the molecular chains strengthen, causing the network structure to shrink to a certain extent, leading to the release of water molecules and thus reducing the swelling ratio. Furthermore, increasing the temperature also accelerates the movement of water molecules, reducing the stable binding ability of water in the network. Therefore, this system exhibits certain temperature-responsive behavior. like Figure 3As shown in the temperature-sensitive transmittance curve, the ASNC microspheres prepared in Example 1 of this invention undergo a significant phase transition at 45~46℃. The network shrinks moderately at high temperatures, reducing rapid water loss and significantly improving water retention stability under high-temperature conditions, making it suitable for application in arid and high-temperature regions.

[0065] The specific data is shown in Table 2: Table 2

[0066] (3) Salt tolerance Figure 2 Figure c illustrates the effect of different salt solutions and concentrations on the swelling performance of the ASNC microspheres prepared in Example 1 of this invention. In NaCl, Na₂CO₃, NaHCO₃, MgCl₂, and FeCl₃ solutions, the swelling ratio decreased as the salt concentration increased from 0.02 M to 0.10 M. In NaCl solution, the swelling ratio decreased from 165.13 g / g at 0.02 M to 76.32 g / g at 0.10 M; in the Na₂CO₃ system, it decreased from 84.61 g / g to 44.03 g / g; and in NaHCO₃, it decreased from approximately 113.01 g / g to 48.43 g / g. In contrast, the effect was more significant in the multivalent ion systems, with the MgCl₂ and FeCl₃ systems consistently exhibiting lower swelling ratios, both below 10 g / g. This phenomenon is mainly due to the ion shielding effect and ion cross-linking: the monovalent ion Na… + By compressing the electrical bilayer and reducing the osmotic pressure difference, the -COO content is weakened. - Electrostatic repulsion between them reduces the swelling ratio; while multivalent ions such as Mg 2 + Fe 3+ It not only has a stronger charge shielding capability, but can also interact with -COO - Coordination occurs, forming secondary ionic crosslinking sites, significantly increasing the network crosslinking density and leading to more pronounced structural shrinkage, thus significantly reducing the swelling ratio. The results show that this material exhibits a typical decreasing trend in salt resistance in salt solutions, and is more sensitive to multivalent ions.

[0067] Specific data are shown in Tables 3-7: Table 3

[0068] Test Example 2: Recycling Performance Figure 2In Figure d, the cyclic water absorption performance of the ASNC microspheres prepared in Example 1 of this invention is shown. The results show that the swelling ratios in the first to fourth cycles were 615.46, 599.43, 531.31, and 453.92 g / g, respectively. It can be seen that the swelling ratio gradually decreases with increasing cycle number, but remains at approximately 453.92 g / g after the fourth cycle, with a retention rate approximately 73.7% of the first swelling, indicating that the material has good structural stability and reusability. The decrease in swelling performance is mainly due to irreversible shrinkage or slight damage to some network structures during repeated water absorption and dehydration, and the possible elution of some soluble low-molecular-weight substances, leading to a reduction in effective water absorption sites. However, the overall decrease is relatively slow, indicating that the three-dimensional cross-linked network of this system is relatively stable.

[0069] Experimental Example 3: Soil Water Holding Capacity Weigh 100 g of air-dried soil and mix it thoroughly with 1 g of water-absorbing microspheres. Place the mixture into an acrylic tube of known mass and record the total mass as m1. The blank control group only contains soil and no water-absorbing microspheres. Then, slowly immerse the acrylic tube containing the sample into deionized water, allowing the soil and microspheres to fully absorb water until saturated. After complete swelling, remove the tube and allow it to stand until no obvious free water drips. Weigh the tube again and record the total mass as m2. Next, dry the sample in an oven to constant weight, cool it to room temperature, and weigh the tube again. Record the total mass as m3.

[0070] The maximum water-holding capacity of soil is calculated using the following formula:

[0071] like Figure 4 As shown in Figure a, the maximum water holding capacity of pure soil is 39.47%. When 0.05 g, 0.10 g, and 0.20 g of the microspheres prepared in Example 1 are added, the maximum water holding capacity of the soil is 55.90%, 68.89%, and 86.77%, respectively, which greatly improves the water retention capacity.

[0072] like Figure 4 As shown in Figure b, after 22 days, the water retention rate of pure soil at 20℃ was 12.73%, and it almost completely lost water at 46℃; Figure 4 As shown in Figure c, the soil with the microspheres prepared in Example 1 had a water retention rate of 74.49% at 20℃ and still reached 57.78% at 46℃, demonstrating an extremely significant water retention effect.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. Temperature-responsive water-absorbing microspheres, characterized by, The water-absorbing microspheres have a three-dimensional network structure formed by chemical cross-linking of sodium alginate-grafted polyN-vinylcaprolactam graft copolymer, cassava starch and partially neutralized acrylic acid. In the three-dimensional network structure, the volume ratio of sodium alginate-grafted poly(N-vinylcaprolactam)-grafted copolymer, cassava starch, and partially neutralized polyacrylic acid is 3:1:5~9.

2. The temperature-responsive water-absorbing microspheres according to claim 1, wherein In the three-dimensional network structure, the volume ratio of sodium alginate-grafted poly(N-vinylcaprolactam)-grafted copolymer, cassava starch, and polyacrylic acid is 3:1:

7. Preferably, the sodium alginate-grafted poly(N-vinylcaprolactam) graft copolymer has sodium alginate as the main chain and N-vinylcaprolactam as the side chain, wherein the N-vinylcaprolactam is covalently grafted onto the sodium alginate main chain.

3. The temperature-responsive water-absorbing microspheres according to claim 1, wherein The water-absorbing microspheres are spherical with an average particle size of 53.39±1.91 μm.

4. The temperature-responsive water-absorbing microspheres according to claim 1, wherein The water-absorbing microspheres maintain their swelling capacity within a pH range of 3 to 11, with a swelling ratio of 212.14 g / g at pH 3, 325.41 g / g at pH 5, 615.46 g / g at pH 7, 540.64 g / g at pH 9, and 248.65 g / g at pH 11. The retention rate of the swelling ratio at pH 7 after four cycles is 73.7%. Preferably, the thermosensitive phase transition temperature of the water-absorbing microspheres is 45~46℃; the swelling ratio is 615.46 g / g at 20℃, 613.69 g / g at 30℃, 566.27 g / g at 40℃, and 461.97 g / g at 50℃. Preferably, the water-absorbing microspheres possess water-absorbing capacity in 0.02 M to 0.1 M NaCl, Na2CO3, NaHCO3, MgCl2, or FeCl3 salt solutions; The swelling ratio in 0.1 M NaCl solution was 76.32 g / g. Preferably, when the amount of the water-absorbing microspheres added to the soil is 0.05~0.20 g / 100 g of air-dried soil, the maximum water holding capacity of the soil increases from 39.47% to 55.90%~86.77%. Preferably, the water-absorbing microspheres can achieve a soil water retention rate of over 57.78% under high temperature conditions of 46℃.

5. A method for producing the temperature-responsive water-absorbing microspheres according to any one of claims 1 to 4, characterized by, Includes the following steps: A sodium alginate solution and N-vinylcaprolactam monomer were mixed, and then a crosslinking agent and an initiator were added sequentially to carry out a free radical grafting polymerization reaction to obtain a sodium alginate grafted poly(N-vinylcaprolactam) copolymer. Add the emulsifier to the organic solvent and stir until completely dissolved and homogeneous to obtain an oil phase solution; mix the sodium alginate-grafted polyN-vinylcaprolactam aqueous solution, the cassava starch aqueous solution, and the acrylic acid solution partially neutralized with sodium hydroxide solution to obtain an aqueous phase solution; An aqueous solution was added to an oil solution and stirred to disperse it into a stable water-in-oil system. Then, a crosslinking agent and an initiator were added sequentially to carry out a polymerization reaction. After the reaction was completed, the mixture was washed and vacuum dried to obtain temperature-responsive water-absorbing microspheres.

6. The production method according to claim 5, wherein The mass ratio of sodium alginate solution, N-vinylcaprolactam monomer, crosslinking agent, and initiator is 2:0.8~2.4:0.007~0.011:0.021~0.033; the crosslinking agent is selected from one or more of N,N'-methylenebisacrylamide, polyethylene glycol diacrylate, ethylene glycol dimethacrylate, N,N'-bisacryloylcysteine, calcium chloride, or calcium sulfate; the initiator is selected from one or more of potassium persulfate, ammonium persulfate, sodium persulfate, 2,2'-azobisisobutyramidine hydrochloride, and 4,4'-azobis(4-cyanopentanoic acid); preferably, the free radical graft polymerization reaction is carried out at a temperature of 70~80℃ for 1~1.5 h.

7. The production method according to claim 5, wherein The emulsifier is Span 80, the organic solvent is liquid paraffin, and the amount of emulsifier added is 0.5~0.6 g / 30 mL of liquid paraffin.

8. The production method according to claim 5, wherein The sodium alginate-grafted poly(N-vinylcaprolactam) aqueous solution has a mass fraction of 0.5-3 wt%, the cassava starch aqueous solution has a mass fraction of 0.5-3 wt%, and the acrylic acid solution partially neutralized with sodium hydroxide solution has a degree of neutralization of 50-80%, wherein the concentration of the sodium hydroxide solution is 3 mol / L and the concentration of the acrylic acid solution is 99%.

9. The production method according to claim 5, wherein The crosslinking agent added in the polymerization reaction is 0.1~1 wt% of the total mass of the aqueous solution; the initiator added is 0.1~1 wt% of the total mass of the aqueous solution. Preferably, the washing is performed by alternating washing with petroleum ether and anhydrous ethanol; the polymerization reaction temperature is 70~80℃, and the reaction time is 1~1.5 h; the vacuum drying temperature is 55~65℃.

10. The application of the temperature-responsive water-absorbing microspheres according to any one of claims 1 to 4 in improving the soil environment and enhancing the soil's water-holding and water-retention capacity.

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