Chitosan-based super absorbent resin as well as preparation method and application thereof

Abstract

The invention relates to the technical field of polymer hydrogel, in particular to chitosan-based super absorbent resin as well as a preparation method and application thereof. The method comprises the following steps: mixing shrimp and crab shell powder rich in chitin and calcium carbonate with an aqueous alkali solution, and performing freezing-unfreezing-heat preservation treatment to obtain freeze-thawed shrimp and crab shells; crushed unfrozen shrimp and crab shells and acrylic monomers are used as raw materials, graft copolymerization is performed in the presence of a cross-linking agent and a thermal initiator to form a graft polymer, under the cross-linking action of the cross-linking agent, the graft polymer is subjected to cross-linking reaction to form hydrogel, and the chitosan-based super absorbent resin is obtained. The preparation method provided by the invention not only overcomes the defects of large alkali consumption, tedious decalcification steps and equipment corrosion caused by acidic conditions required by chitosan dissolution in the traditional process, but also directly reserves calcium carbonate in the shrimp shells as a chitosan-based super absorbent resin component, omits the decalcification step, and reduces the production cost. The alkali aqueous solution is shared by the neutralization reaction and the deacetylation reaction of the acrylic acid, so that the efficient utilization of the alkali aqueous solution is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer hydrogels, and in particular to a chitosan-based super absorbent resin and a preparation method and application thereof. Background Art

[0002] Chitosan, the only naturally occurring alkaline polysaccharide discovered to date, has attracted considerable attention for its many unique physical, chemical, and biological properties. The abundance of amino groups in its molecular structure endows chitosan-based superabsorbent resins with superior performance compared to traditional starch- and cellulose-based biopolymers or purely chemical synthetic resins. Research has shown that these chitosan-based superabsorbent resins possess unique antibacterial activity, pH-sensitive responsiveness, ion-responsiveness, and drug-loading properties, offering broad and significant potential applications in medicine, hygiene, and nursing.

[0003] However, chitosan in nature mostly exists in the form of chitin. Its industrial production requires deacetylation by heating in a high-concentration (40%-50%) NaOH solution. The degree of deacetylation typically needs to be increased to over 70% for widespread commercial application. It's worth noting that chitin doesn't exist independently, but rather forms a complex structure with components such as proteins, calcium carbonate, and pigments. It's widely found in the exoskeletons of arthropods (shrimp, crabs, locusts, and silkworm pupae). Therefore, its industrial production requires a series of purification processes involving deproteinization, decalcification, and decolorization.

[0004] During the preparation of acrylic superabsorbent resins, numerous factors influence their performance. The degree of neutralization of acrylic acid, a key parameter, plays a crucial role in the final performance of acrylic superabsorbent resins. To precisely control the degree of neutralization, the amount of NaOH used in the system must be strictly controlled. Furthermore, the concentration and dosage of NaOH are crucial factors in promoting the deacetylation of chitin to chitosan.

[0005] As a major aquaculture country, my country produces a large amount of inedible shell waste every year. At present, only a small part of this waste is processed into chitin, while a considerable part is directly discarded. This treatment method not only leads to a huge waste of biomass resources, but also may cause environmental pollution problems. Patent 107056986A and Patent 112390910A respectively proposed a homogeneous deacetylation treatment method using chitin as raw material, by directly synthesizing chitosan-based super absorbent resin containing cross-linking agents or completely free of chemical cross-linking agents under homogeneous conditions, thereby realizing a homogeneous synthesis path from chitin to chitosan super absorbent resin. However, these methods have not yet broken through the technical bottleneck of directly synthesizing chitosan-based super absorbent resin from shrimp and crab shells. Patent 110078940A and the document "Preparation and Properties of Shrimp Shell Powder Modified Starch-Based Superabsorbent Resin" describe a process for preparing a bio-calcium composite superabsorbent resin. This technology utilizes shrimp and crab shell powder as a bio-calcium source, introducing bio-calcium powder into the synthesis of a starch-based superabsorbent resin to enhance its overall water absorption. However, in this system, starch remains the main reactant, with bio-calcium powder serving only as an additive. Summary of the Invention

[0006] In response to the problems existing in the prior art, the present invention provides a chitosan-based super absorbent resin and its preparation method and application. The present invention uses shrimp and crab shells as raw materials, directly mixes shrimp and crab shell powder with an alkaline aqueous solution, combines freeze-thaw-insulation treatment to force the alkaline aqueous solution to penetrate and destroy the dense structure of chitin, converts acid- and alkali-insoluble chitin into acid-soluble chitosan, and then copolymerizes and crosslinks the chitosan with acrylic monomers, simultaneously deacetylates and converts it into a super absorbent resin to obtain a chitosan-based super absorbent resin. The preparation method of the present invention is the first to achieve a one-pot method for preparing chitosan-based super absorbent resin from shrimp and crab shells. It not only overcomes the defects of traditional processes such as large alkali dosage, cumbersome decalcification steps, and the need for acidic conditions for chitosan dissolution, which leads to equipment corrosion, but also directly retains the calcium carbonate in the shrimp and crab shells as a component of the chitosan-based super absorbent resin, eliminating the decalcification step, and utilizing acrylic acid neutralization reaction and deacetylation reaction to share the alkaline aqueous solution, achieving efficient utilization of the alkaline aqueous solution.

[0007] In order to achieve the above object, the technical solution adopted by the present invention is: The first object of the present invention is to provide a method for preparing a chitosan-based super absorbent resin, comprising the following steps: S1. Pre-treating shrimp and crab shells to obtain shrimp and crab shell powder rich in chitosan and calcium carbonate.

[0008] S2. Mixing shrimp and crab shell powder rich in chitin and calcium carbonate with an alkaline aqueous solution and subjecting it to a freeze-thaw-insulation treatment to obtain frozen-thawed shrimp and crab shells. During the freezing process, the alkaline aqueous solution forms ice crystals, which assist in destroying the dense structure of the shrimp and crab shells and promote the penetration of alkaline molecules into the chitin particles. During the thawing-insulation process, the alkaline molecules that enter the chitin particles fully deacetylate the chitin from the inside out to convert it into chitosan, thereby obtaining frozen-thawed shrimp and crab shells. In contrast, the traditional preparation method directly mixes chitin with a NaOH solution. During heating, the chitin surface is deacetylated, while the interior is not completely deacetylated, resulting in a low degree of deacetylation of the obtained chitosan, which remains insoluble.

[0009] S3, the frozen-thawed shrimp and crab shells are crushed to obtain thawed shrimp and crab shells. Wherein, due to the freeze-thaw-insulation treatment with alkaline aqueous solution in S2, the obtained frozen-thawed shrimp and crab shells themselves are wet, so wet crushing is preferred. In addition, although alkaline aqueous solution is used in step S2, the amount used is small. After freezing-thaw-insulation treatment, the frozen-thawed shrimp and crab shells will shrink into a ball, and are relatively hard. Simply using stirring cannot fully disperse them. The purpose of wet crushing is to fully disperse the frozen-thawed shrimp and crab shells that form hard lumps in the aqueous solution to ensure that they can fully react with the acrylic monomers added subsequently, while also playing the role of diluting the system.

[0010] S4. Using thawed shrimp and crab shells and acrylic monomers as raw materials, a graft copolymerization reaction is carried out in the presence of a cross-linking agent and a thermal initiator. During the graft copolymerization reaction, the acrylic monomer undergoes free radical polymerization to generate polyacrylic acid chains. The initiator triggers the chitosan in the thawed shrimp and crab shells to form chitosan macromolecular free radicals. At the same time, the polyacrylic acid chains with shorter chain lengths are grafted onto the main chain of the chitosan macromolecular chain to form a graft polymer. Under the cross-linking action of the cross-linking agent, the graft polymer undergoes a cross-linking reaction to form a hydrogel. After drying, a chitosan-based super absorbent resin is obtained.

[0011] Preferably, the mass fraction of the alkali in the aqueous alkali solution is 25% to 40.5%; the mass ratio of the aqueous alkali solution to the shrimp and crab shell powder is 4.8 to 10:1; and the mass ratio of the alkali in the aqueous alkali solution to the dried shrimp and crab shell powder is 1.5 to 3.6:1. When the mass concentration of the aqueous alkali solution exceeds the above range, the solution lacks water, making it difficult for the shrimp and crab shell powder to fully soak and freeze to form ice crystals. Conversely, if the mass concentration of the aqueous alkali solution is below this range, the total amount of aqueous alkali solution used must be fixed, necessitating the introduction of more water, resulting in oversized ice crystals formed during freezing and failing to effectively promote the penetration of alkali molecules into the chitin particles.

[0012] Preferably, the freeze-thaw-insulation treatment conditions are: first freeze at -23°C to -18°C for at least 5 hours until completely frozen, then thaw at 95°C, and finally keep warm at 65°C to 95°C for 1.5 hours to 6 hours.

[0013] Preferably, the mass ratio of thawed shrimp and crab shells to acrylic acid monomers is 1:5.9-16. Meanwhile, the mass concentration of the monomers in the entire reaction system is controlled to be 10wt%-23.35wt%, and the neutralization degree of acrylic acid is 34%-55%.

[0014] Preferably, the acrylic monomer is selected from acrylic acid or a mixture of acrylic acid and acrylamide; in the mixture of acrylic acid and acrylamide, the mass percentage of acrylamide is 0.1 wt % to 50 wt %.

[0015] Preferably, the graft copolymerization reaction conditions are: reaction at 65° C. to 75° C. for 3 h to 7 h.

[0016] Preferably, the hydrogel obtained by the graft copolymerization reaction is further post-treated, and the post-treatment operation is as follows: The hydrogel was cut into pieces, immersed in a 40 vol% to 70 vol% ethanol aqueous solution, washed, dehydrated with anhydrous ethanol, and then dried.

[0017] Preferably, the mass concentration of shrimp and crab shell powder in the reaction system is 0.81wt%~2.42wt%.

[0018] Preferably, the alkali in the aqueous alkali solution is selected from NaOH or KOH.

[0019] Preferably, the shrimp and crab shells are fresh shrimp and crab shells or cooked shrimp and crab shells.

[0020] Preferably, the total amount of alkali used should be sufficient to achieve a neutralization degree of acrylic acid of 34.3% to 55%.

[0021] Preferably, the cross-linking agent is selected from N,N'-methylenebisacrylamide or disodium ethylenediaminetetraacetate; wherein the mass of the cross-linking agent accounts for 0.08‰~1.21‰ of the total mass of the reaction system.

[0022] Preferably, the initiator is a persulfate, and the persulfate is selected from ammonium persulfate, potassium persulfate or sodium persulfate; wherein the mass of the initiator accounts for 0.60‰~1.65‰ of the total mass of the reaction system.

[0023] Preferably, the specific operation of the pretreatment is: deproteinizing, defatting and crushing the shrimp and crab shells in sequence to obtain shrimp and crab shell powder rich in chitin and calcium carbonate; it can also be directly partially decalcified to obtain incompletely decalcified shrimp and crab shell powder. No decalcification or a small amount of decalcification is beneficial to the performance of the chitosan-based super absorbent resin.

[0024] Preferably, the specific operation of the deproteinization and defatting treatment is: boiling the shrimp and crab shells with a 5wt% to 10wt% NaOH solution.

[0025] The second object of the present invention is to provide a chitosan-based super absorbent resin obtained by the above preparation method.

[0026] Preferably, the chitosan-based super absorbent resin is in the form of a translucent hydrogel, is in the form of white particles after drying, and is in the form of a transparent elastic hydrogel after absorbing water and swelling again.

[0027] The third object of the present invention is to provide the use of the chitosan-based super absorbent resin in the preparation of water-absorbing materials.

[0028] Compared with the prior art, the present invention has the following beneficial effects: 1. The present invention provides a method for preparing a chitosan-based super absorbent resin, comprising the steps of pre-treating shrimp and crab shells to obtain shrimp and crab shell powder rich in chitosan and calcium carbonate; mixing the shrimp and crab shell powder rich in chitosan and calcium carbonate with an alkaline aqueous solution, and subjecting the mixture to a freeze-thaw-insulation treatment. During the freezing process, the alkaline molecules enter the chitosan particles with the assistance of ice crystals, and then during the thawing-insulation process, the chitosan is deacetylated and converted into chitosan to obtain frozen-thawed shrimp and crab shells; crushing the frozen-thawed shrimp and crab shells to obtain thawed shrimp and crab shells; and thawing the shrimp and crab shells to obtain thawed shrimp and crab shells. Shrimp and crab shells and acrylic monomers are used as raw materials, and a graft copolymerization reaction is carried out in the presence of a crosslinking agent and a thermal initiator. During the graft copolymerization reaction, the acrylic monomer undergoes free radical polymerization to generate polyacrylic acid chains, and the initiator triggers chitosan in the thawed shrimp and crab shells to form chitosan macromolecular free radicals. At the same time, polyacrylic acid chains with shorter chain lengths are grafted onto the main chain of the chitosan macromolecular chain to form a graft polymer. Under the crosslinking action of the crosslinking agent, the graft polymer undergoes a crosslinking reaction to form a hydrogel, and after drying, a chitosan-based super absorbent resin is obtained.

[0029] Compared to the existing multi-step, complex process of isolating chitin from shrimp and crab shells, followed by separate decalcification and deacetylation before resin synthesis, the present invention directly mixes shrimp and crab shell powder with a limited amount of aqueous alkaline solution, then combines a freeze-thaw (freeze-thaw-hold) process to force the aqueous alkaline solution to penetrate and destroy the chitin's dense structure. This converts the acid- and alkali-insoluble chitin into acid-soluble chitosan. Subsequently, the chitosan is grafted and polymerized with acrylic monomers, simultaneously deacetylated and converted into a superabsorbent resin, resulting in a chitosan-based superabsorbent resin. This process overcomes the drawbacks of conventional processes, such as the high alkali dosage (40%-50% concentration, more than 10 times the mass ratio), the cumbersome decalcification process, and the acidic conditions required for chitosan dissolution, which can lead to equipment corrosion. Furthermore, the calcium carbonate in the shrimp shells is directly retained as a component of the chitosan-based superabsorbent resin, eliminating the decalcification step. Furthermore, the acrylic acid neutralization reaction and the deacetylation reaction share NaOH, achieving efficient utilization of the aqueous alkaline solution.

[0030] 2. The present invention enhances the deacetylation effect (deacetylation degree reaches above 83.48%) through freeze-thaw cycles (freezing at -18°C and then thawing at 65°C-95°C), allowing chitin to be directly converted into soluble chitosan without complete purification. At the same time, calcium carbonate is retained to enhance the water retention performance of the chitosan-based super absorbent resin, significantly improving the utilization rate of shrimp shell resources and product performance. This breaks through the technical bottleneck of the traditional process of separating chitosan production from the preparation of chitosan-based super absorbent resin and generating large amounts of waste.

[0031] 3. The chitosan-based superabsorbent resin of the present invention not only has excellent water absorption properties but also significantly increases the direct utilization rate of shrimp and crab shells, resolving the waste of shrimp and crab shell biological resources and the resulting environmental pollution issues encountered in existing technologies. The chitosan-based superabsorbent resin of the present invention has a water absorption rate of up to 1715 (g / g) ± 14 (g / g) in pure water and 104.87 (g / g) ± 1.15 (g / g) in physiological saline.

[0032] 4. The preparation method of the present invention is simple and effective. Specifically, the shrimp and crab shell powder in the present invention does not require prior decalcification or the use of a large amount of strong alkali for deacetylation. The alkaline aqueous solution serves as both a deacetylation agent for the chitin in the shrimp and crab shells and a neutralizing agent for the grafting monomer acrylic acid, thus achieving an integrated production process for directly preparing chitosan-based superabsorbent resin from shrimp and crab shell powder. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 The infrared spectra of the insoluble substances precipitated on the surface of the hydrogel before or after the polymerization reaction of chitin, chitosan, 14#, 16# and 17# in experiment (1), among which 14# includes the precipitated before and after the reaction, and 16# and 17# are the precipitated after the reaction.

[0034] Figure 2 This is a diagram of the water retention performance of the chitosan-based superabsorbent resin in a 65°C oven after it fully absorbed water in the screening experiment (2).

[0035] Figure 3 This is a diagram showing the water retention performance of the chitosan-based superabsorbent resin at room temperature after fully absorbing water in the screening experiment (2).

[0036] Figure 4 The XPS full spectrum and O1s, N1s, Ca2p and C1s fine spectra of shrimp shell powder, alkali-treated shrimp shell powder and shrimp shell powder resin in the screening experiment (3) are shown.

[0037] Figure 5The XPS full spectrum and O1s and C1s fine spectra of starch-shrimp shell powder-acrylic acid graft copolymer and shrimp shell powder-acrylic acid graft copolymer in the screening experiment (3) are shown, where a is the XPS full spectrum of starch-shrimp shell powder-acrylic acid graft copolymer, a1 is O1s, a2 is C1s, and b is the XPS full spectrum of shrimp shell powder-acrylic acid graft copolymer, b1 is O1s, and b2 is C1s.

[0038] Figure 6 The thermogravimetric and differential thermogravimetric curves of the chitosan-based superabsorbent resin after direct grinding in the screening experiment (6).

[0039] Figure 7 The thermogravimetric and differential thermogravimetric curves of the chitosan-based superabsorbent resin after alkali-treated shrimp shell powder and 6# water absorption-and-drying treatment in the screening experiment (6), where a is HSh and b is HSh / SAP.

[0040] Figure 8 This is the infrared spectrum of chitosan-based superabsorbent resin synthesized under the same conditions of chitosan, chitosan + calcium carbonate and shrimp shell powder in the screening experiment (7). DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solution of the present invention in conjunction with the data in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0042] It should be noted that the professional terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the following embodiments of the present invention can be purchased from the market or prepared by existing methods.

[0043] The existing technology for preparing chitosan-based superabsorbent resins requires isolating chitin from shrimp and crab shells and subjecting it to a multi-step purification process involving decalcification, deproteinization, and decolorization. Subsequently, high-concentration NaOH solutions (typically 40% to 50% by weight) are used for high-temperature deacetylation to obtain chitosan with a high degree of deacetylation (≥70%). This process is complex and requires a large amount of alkali (typically more than 10 times the mass of the chitin). Furthermore, the synthesis of acrylic resins requires strict control of the degree of neutralization, and the amount of NaOH used in the deacetylation reaction and neutralization steps conflict (e.g., Patents 107056986A and 112390910A). This prevents the recycling of the aqueous alkali solution, increasing costs and pollution risks. Although existing technologies have attempted to use shrimp and crab shell powder as a bio-calcium additive (such as Patent 110078940A), it is only an auxiliary component of starch-based resin (the main component is still starch), and it fails to directly utilize the chitin and calcium carbonate in the shrimp shells as the core components of the resin. It still requires pre-decalcification treatment, resulting in low resource utilization and inability to achieve high-value utilization of all components of shrimp and crab shell waste.

[0044] In view of the problems existing in the above-mentioned prior art, the present invention provides a method for preparing a chitosan-based super absorbent resin, comprising the following steps: pre-treating shrimp and crab shells to obtain shrimp and crab shell powder rich in chitosan and calcium carbonate; mixing the shrimp and crab shell powder rich in chitosan and calcium carbonate with an alkaline aqueous solution, and subjecting the mixture to a freeze-thaw-insulation treatment, wherein during the freezing process, the alkaline molecules enter the chitosan particles with the assistance of ice crystals, and then during the thawing-insulation process, the chitosan is deacetylated and converted into chitosan to obtain frozen-thawed shrimp and crab shells; crushing the frozen-thawed shrimp and crab shells to obtain to thawed shrimp and crab shells; using thawed shrimp and crab shells and acrylic monomers as raw materials, a graft copolymerization reaction is carried out in the presence of a cross-linking agent and a thermal initiator. During the graft copolymerization reaction, the acrylic monomer undergoes free radical polymerization to generate polyacrylic acid chains, the initiator triggers the chitosan in the thawed shrimp and crab shells to form chitosan macromolecular free radicals, and at the same time, the polyacrylic acid chains with shorter chain lengths are grafted onto the main chain of the chitosan macromolecular to form grafted polymers. Under the cross-linking action of the cross-linking agent, the grafted polymer undergoes a cross-linking reaction to form a hydrogel, and after drying, a chitosan-based super absorbent resin is obtained.

[0045] The technical solution of the present invention is further studied below, and the specific research methods and results are as follows: The invention uses shrimp shell powder as a raw material for research. The shrimp shell powder is prepared according to the following steps: removing obvious impurities from shrimp shells, washing with tap water, adding a 5wt% NaOH solution of 5 times the weight of the shrimp shells, boiling for 1 hour, removing the shells, adding a 7.5wt% NaOH solution of 3 times the weight of the shrimp shells, and continuing to boil for 1 hour. After removing the shells, the shells are washed with tap water until neutral, then washed and soaked with deionized water for 3 times, exposed to sunlight for decolorization, crushed, and passed through a 120-mesh sieve to obtain shrimp shell powder rich in chitin and calcium carbonate, wherein the calcium carbonate content is 70.30%.

[0046] The following is a screening experiment: (1) Investigate the effect of shrimp shell powder pretreatment using existing technology on the yield and water absorption rate of bio-calcium powder composite super absorbent resin (hereinafter referred to as "super absorbent resin"): According to the protocol in Table 1, shrimp shell powder was mixed with a NaOH solution and insulated. Acrylic acid was then added and stirred evenly. Significant insoluble matter was observed, which remained insoluble even after prolonged storage. NaOH solution was added as needed to control the neutralization of the acrylic acid. The crosslinker N,N'-methylenebisacrylamide and the initiator ammonium persulfate were then added. Finally, pure water was added to control the concentrations of the reactants in the reaction system. The reaction mixture was sealed with plastic wrap and reacted at 65°C for 5 hours to produce a hydrogel. Significant insoluble matter precipitated on the surface of the hydrogel. The insoluble matter was separated, and the remaining hydrogel was weighed. The hydrogel yield was calculated as the percentage of the mass of the hydrogel after the reaction to the total mass of the reactants before the reaction. The hydrogel was soaked in a 40 vol% ethanol solution for over 2 hours, then chopped with scissors and soaked again in 70 vol% ethanol for at least 2 hours. The solution was then dehydrated by switching to anhydrous ethanol. The fully dehydrated hydrogel was dried in a 65°C forced air drying oven until its mass remained unchanged. This yielded a superabsorbent resin. The resin was then weighed and the resin yield was calculated as the percentage of the net mass of the dry superabsorbent resin divided by the sum of the masses of all reactants excluding water. The water absorption rate of each superabsorbent resin in pure water was determined using the "tea bag" method.

[0047] Table 1 Yield and water absorption rate of super absorbent resin prepared by simple replacement using existing technology Based on the data presented in Table 1, when a superabsorbent resin is directly synthesized using shrimp shell powder alone without the addition of a biopolymer, its water absorption performance remains insufficient, even after heat treatment with a NaOH solution. Some biocalcium powder-based superabsorbent resins even partially dissolve upon contact with water, failing to effectively form a complete hydrogel structure. This demonstrates that simply replacing shrimp shell powder with existing technologies is ineffective for preparing chitosan-based superabsorbent resins. To investigate the cause of insoluble matter precipitation on the hydrogel surface, the following investigation was conducted: shrimp shell powder was treated according to the screening experiment #14 in Table 1, followed by the addition of acrylic acid to fully dissolve it. The insoluble matter was collected by centrifugation and purified to obtain the insoluble matter before polymerization. Furthermore, the insoluble matter precipitated on the hydrogel surfaces after polymerization reactions #14, #16, and #17 was collected, washed multiple times with pure water, dehydrated with anhydrous ethanol, and air-dried to obtain the insoluble matter precipitated after polymerization (not superabsorbent resin).

[0048] Depend on Figure 1 The results show that the infrared spectrum of the insoluble matter collected before the reaction is almost the same as that of the commercial chitin, which is obviously different from the infrared spectrum of chitosan, indicating that the insoluble matter collected before the reaction is still chitin rather than acid-soluble chitosan; the insoluble matter precipitated after the reaction is at 1721cm -1 A characteristic absorption peak attributable to the C=O residue in polyacrylic acid appeared, but the peak intensity was weak, indicating that the insoluble material precipitated on the gel surface after the reaction was a chitin-grafted acrylic acid polymer. The amount of acrylic acid grafted onto the chitin molecule was relatively small, essentially retaining the typical characteristics of chitin. However, the degree of reaction with 17# was relatively higher than that with 14# and 16#. This indicates that directly treating shrimp and crab shell powder with existing technology is not sufficient to convert chitin into acid-soluble chitosan, nor is it sufficient to directly use shrimp and crab shell powder to prepare chitosan-based superabsorbent resins.

[0049] To further investigate the effect of pretreatment methods on the deacetylation degree of chitin in shrimp shell powder, shrimp shell powder-NaOH mixtures were prepared according to conditions 14# and 17#, left at room temperature overnight, then heated in a 95°C waterbath for 3 h. Decalcification was then performed by adding a 30% ethanol solution containing 2 mol / L hydrochloric acid. This process was repeated three times. The precipitate was collected by centrifugation and repeatedly washed with water to obtain alkaline-treated chitin or chitosan. The raw shrimp shell powder was treated with the same decalcification method, and the carbon and nitrogen content was determined by elemental analysis. The deacetylation degree of chitin in the raw shrimp shell powder was calculated to be 1.56±0.04%. Using the same mass of NaOH, at a 25wt% NaOH solution fraction, the deacetylation degree of 14# was only 27.09±0.09%, while at a 40wt% NaOH solution fraction, the deacetylation degree of 17# increased to 64.97±0.15%. Since the NaOH dosage of the entire system must be controlled during the preparation of superabsorbent resin to ensure that the neutralization degree of acrylic acid is within a reasonable range, it is obviously impossible to further increase the NaOH concentration. Generally, the deacetylation degree of commercial chitosan exceeds 70% to have good solubility, and chitosan has poor solubility in acrylic acid. This is why insoluble matter precipitates after the addition of acrylic acid.

[0050] Then, shrimp shell powder-NaOH mixtures were prepared according to the conditions of 14# and 17# respectively. After mixing, they were placed in a -18℃ refrigerator and frozen overnight. The next day, they were taken out of the refrigerator and found to be completely frozen. They were placed in a 95℃ water bath for heating for 3 hours. Decalcification, purification and drying were carried out in the same way as the shrimp shell powder-NaOH mixture placed at room temperature. The deacetylation degree of shrimp shell powder treated with 25wt% NaOH solution freeze-thaw treatment reached 75.78±0.17%, and the deacetylation degree of shrimp shell powder treated with 40wt% NaOH solution freeze-thaw treatment reached 83.48±0.18%, which was much higher than the two shrimp shell powders without freeze-thaw treatment. They can be completely dissolved in dilute acid solution, indicating that under the condition of limited alkali, the freeze-thaw treatment can significantly improve the deacetylation degree of chitosan obtained by converting chitin in shrimp shells, providing the possibility of directly preparing chitosan superabsorbent resin from shrimp and crab shells.

[0051] (2) Investigate the effect of calcium carbonate removal rate on the yield and water absorption rate of chitosan-based superabsorbent resin: Weigh 5 portions of shrimp shell powder, 15 g each, and calculate the required amount of 2 mol / L hydrochloric acid based on the preset calcium carbonate removal rate, i.e. 20%, 40%, 60%, 80% and 100%. Subsequently, add the hydrochloric acid solution to the shrimp shell powder at a slow rate and stir the mixture thoroughly until no bubbles are generated in the reaction system. Afterwards, let the mixture stand for 3 hours and collect the remaining solid matter by centrifugation. Wash the collected solid matter repeatedly with pure water until the washing liquid is free of Cl using AgNO3 solution. -The solid matter was then dried under a forced air conditioner at 65°C and pulverized to obtain partially decalcified shrimp shell powder. The yield, calcium carbonate content, and actual decalcification rate of the partially decalcified shrimp shell powder are shown in Table 2. Shrimp shell powder and shrimp shell powder with different degrees of decalcification were taken and placed in 100mL centrifuge tubes respectively. 25.51wt% NaOH solution (10 times the mass of shrimp shell powder) was added thereto. After vortex dispersion, it was transferred to a -18℃ refrigerator for freezing. After the shrimp shell powder was completely frozen, it was taken out and placed directly in a 95℃ water bath to thaw and then heated for 3h (the time was counted after the center temperature reached 95℃, the same below). The agglomerated shrimp shell powder was crushed with a glass rod, water was added and stirred evenly, and then acrylic acid (12 times the mass of dry shrimp shell powder) was added and stirred until the sample was completely dissolved. Then, the initiator ammonium persulfate and the cross-linker N,N'-methylenebisacrylamide were added respectively, and water was added so that the mass percentage of shrimp shell powder in the reaction system was 1.20wt%, the mass percentage of AA was 14.50wt%, the neutralization degree of AA was 38.50%, the amount of initiator was 1.00‰, and the amount of cross-linker was 0.20‰. After stirring evenly, the tube was sealed and placed in a 65℃ water bath for reaction for 5h. After the reaction is complete, remove the hydrogel, invert it, cool it to room temperature, wipe off any liquid adhering to the tube wall, and weigh it to calculate the hydrogel yield. Transfer the hydrogel to a beaker, soak it in a 40 vol% ethanol-water solution for over 2 hours, chop it into pieces with scissors, and soak it again in 70 vol% ethanol for at least 2 hours. Dehydrate it in anhydrous ethanol. Dry the fully dehydrated hydrogel in a 65°C forced air drying oven until its mass remains unchanged. Remove it, weigh it, and calculate the superabsorbent resin yield. Measure the water absorption rate of the superabsorbent resin in pure water and saline.

[0052] Table 2 Yield and water absorption rate of super absorbent resin prepared by using shrimp shell powder and partially decalcified shrimp shell powder From Table 2, it can be concluded that partial decalcification of shrimp shell powder can increase the yield of hydrogel after the reaction, but the improvement of the water absorption performance of super absorbent resin by this treatment is limited. It is worth noting that with the increase of decalcification rate, the water absorption performance of super absorbent resin shows a significant downward trend. The super absorbent resin that has fully absorbed water to the equilibrium state was placed in a 65°C oven for air drying. The results showed that the shrimp shell powder raw material and the super absorbent resin with a theoretical decalcification rate of 80% and 100% had similar water retention properties, and all of them basically completely lost water within 16 hours. The super absorbent resin with a theoretical decalcification rate of 40% showed better water retention performance, and it took 24 hours to completely lose water. The specific data are as follows. Figure 2As shown. Furthermore, the superabsorbent resin, which had been fully dried at 65°C, was placed back into excess pure water. After 12 hours of reabsorption, it was filtered and weighed, and then dried again at 65°C to constant weight. This cycle was repeated five times. The experimental results showed that, with the exception of the 100% decalcified superabsorbent resin, all other superabsorbent resins prepared from shrimp shell powder maintained a good water absorption rate.

[0053] Weigh 50g of super absorbent resin that has fully absorbed water and swelled, place it in three 100mL beakers, and record the initial mass. Then, place the beakers open at room temperature (the daily maximum and minimum temperature data for 20 days are as follows). Figure 3 As shown), a layer of filter paper was placed about 10 cm above the beaker to prevent dust from falling in. The beaker was weighed every 24 hours. Figure 3 As shown. The experimental results show that, consistent with the results of the oven water retention experiment, the superabsorbent resin with 100% decalcification had the worst water retention, while the superabsorbent resin with 40% decalcification exhibited the best water retention. After 21 days of storage at room temperature, the mass retention rate of the 100% decalcified superabsorbent resin was only 0.15%, while the mass retention rate of the 40% decalcified superabsorbent resin was as high as 15.47%. Furthermore, the superabsorbent resin prepared from pure shrimp shell powder also exhibited a water retention rate of 7.03%, demonstrating relatively good water retention.

[0054] (3) Investigate the differences between chitosan-based superabsorbent resin prepared directly from shrimp shell powder and the publicly available bio-calcium powder composite superabsorbent resin: To investigate whether chitosan-based superabsorbents prepared directly from shrimp shell powder are consistent with composite superabsorbents prepared from shrimp shell powder as described in Patent 110078940A and the literature "Preparation and Properties of Shrimp Shell Powder-Modified Starch-Based Superabsorbent Resins," 1.0 g (labeled 1#) and 0.6 g (labeled 2#) of shrimp shell powder were weighed and placed in 100 mL centrifuge tubes. Subsequently, 6.42 g and 3.85 g of a 25 wt% NaOH solution were added dropwise to each tube, respectively. After ensuring uniform mixing by vortexing, the tubes were frozen overnight at -18°C. The next day, the tubes were removed, thawed in a 95°C water bath, and then heated for 3 hours. After cooling, the tubes were weighed and the contents were crushed with a glass rod. Half of each weight was weighed and transferred to another 100 mL centrifuge tube. 10 mL of pure water was added and the tubes were sheared at 5000 rpm for 3 minutes using an internal homogenizer. Next, 0.2 g of acorn starch (laboratory-prepared, sourced from Pingtian Lake, Chizhou, Anhui) was added to the #2 shrimp shell powder. 1.29 g of 25 wt% NaOH solution was added and stirred thoroughly to dissolve the starch. Subsequently, 3 g of acrylic acid was added to each of the two centrifuge tubes. After thorough stirring, 37.5 mg of ammonium persulfate was added and the solution was rehydrated to a net mass of 30 g. The centrifuge tubes were incubated at 65°C for 5 h. Upon removal, a viscous, flowable colloid was obtained. 50 mL of pure water was added, the mixture was stirred thoroughly, and the mixture was allowed to soak overnight. The mixture was then centrifuged at 5000 rpm for 20 min, and the supernatant was collected. Anhydrous ethanol was added dropwise to the supernatant with stirring until no flocculent precipitate formed. After settling for 2 h, the precipitate was collected by centrifugation again, dissolved in pure water, and then precipitated with anhydrous ethanol. This process was repeated three times. Finally, the collected hydrogel was air-dried at 65°C and pulverized to obtain crosslinker-free resins, namely shrimp shell powder-acrylic acid graft copolymer and starch-shrimp shell powder-acrylic acid graft copolymer. Furthermore, the alkali-treated shrimp shell powder remaining in the centrifuge tube was repeatedly washed with pure water and dried to obtain alkali-treated shrimp shell powder, abbreviated as HSh.

[0055] Depend on Figure 4The results showed that the major elemental composition of shrimp shell powder did not change significantly before and after alkali treatment. However, a distinct characteristic peak for Na was observed in the elemental analysis of the superabsorbent resin. Narrow scan analysis of the major elements revealed a decrease in the nitrogen content of the shrimp shell powder after alkali treatment, while an increase in the calcium content. This suggests that during the alkali treatment, the chitin in the shrimp shell powder undergoes deacetylation, converting to chitosan, accompanied by a decrease in molecular weight and the loss of some components during the washing process. This result further confirms that freeze-thaw alkali treatment enhances the solubility of organic matter in shrimp shell powder. Furthermore, narrow scan analysis of the superabsorbent resin revealed the presence of nitrogen and calcium. Given that these two elements can only be derived from chitin and calcium carbonate in the shrimp shell powder, this suggests that chitin and calcium carbonate are involved in the formation of the superabsorbent resin.

[0056] Depend on Figure 5 It was found that for the two resins synthesized under exactly the same conditions, the Na and N elements of the super absorbent resin with added starch were higher than those of the super absorbent resin without added starch. The narrow scan results of the O and C elements showed significant differences between the two, indicating that the system with added starch was significantly different from the pure shrimp shell powder system, and starch was conducive to the reaction. This further shows that a simple substitution cannot be made based on the existing public technology.

[0057] (4) Investigate the effects of shrimp shell powder dosage, acrylic acid dosage, acrylic acid neutralization degree and initiator dosage on the yield and water absorption rate of super absorbent resin: Weigh the shrimp shell powder and place it in a 100mL centrifuge tube. Add 25.46wt% NaOH solution to it so that the mass of NaOH is 1.5 times the mass of the shrimp shell powder. After vortex dispersion, place it in a -18℃ refrigerator and freeze it overnight. Take out the completely frozen shrimp shell powder, thaw it in a 95℃ water bath and then heat it for 3h. After crushing, add appropriate amount of water, and use an internal cutting homogenizer to shear at 5000rpm (i.e. wet crushing) until the shrimp shell powder becomes a uniform suspension. Add acrylic acid and stir well. After the shrimp shell powder is completely dissolved, add NaOH to adjust the neutralization degree of acrylic acid according to the actual amount of acrylic acid, then add crosslinking agent and initiator in sequence, and finally add water to make the concentration of shrimp shell powder 0.93 wt%~1.88wt%, the concentration of AA is 12.58wt%~18.90wt%, the neutralization degree of AA is 35%~55%, the mass ratio of AA to shrimp shell powder is 8~16:1, the initiator concentration is 0.62‰~1.64‰, the cross-linker concentration is 0.20‰, and the detailed conditions are shown in Table 3. After the reaction solution is stirred evenly, it is sealed with plastic wrap and heated in a water bath at 65℃ for 5h. The obtained hydrogel is purified and dried according to the method in (1) to obtain a chitosan-based super absorbent resin directly prepared from shrimp shell powder. Under most conditions, the water absorption performance of the super absorbent resin obtained in pure water can reach more than 1000 times, and it also has good water absorption performance in physiological saline.

[0058] Table 3 Effects of shrimp shell powder dosage, acrylic acid dosage, acrylic acid neutralization degree and initiator dosage on the yield and water absorption rate of super absorbent resin Table 3 shows that, while keeping other conditions unchanged, varying the shrimp shell powder dosage (numbers 1-5) and increasing the mass ratio of acrylic acid to shrimp shell powder (decreasing shrimp shell powder dosage and decreasing concentration) slightly increases the yield of superabsorbent resin and the water absorption rate in pure water, but the changes are not significant. All resins achieve water absorption rates exceeding 1200 times. The water absorption rate in saline solution decreases significantly with decreasing shrimp shell powder dosage, indicating that increasing shrimp shell powder dosage not only conserves acrylic acid but also improves the salt tolerance of superabsorbent resins. Varying the acrylic acid dosage (numbers 6-9) shows a significant decrease in yield and water absorption rate with decreasing acrylic acid concentration. However, excessive acrylic acid dosage (number 6) leads to a slight decrease in water absorption rate in saline solution. Therefore, adding too much acrylic acid is not advisable from the perspective of conserving fossil energy. Changing the initiator dosage (numbers 10-14) shows a significant increase in sample yield with increasing APS dosage, indicating that water removal from the superabsorbent resin is more difficult, resulting in a significant decrease in water absorption. This is because excessive initiator reduces the average chain length of the grafted polymer, enhancing hydrophilicity and increasing water solubility. The superabsorbent resin yield exceeds 100% because, to ensure the resin's water absorption capacity, drying at excessively high temperatures is not possible. The drying conditions used in the present invention, at 65°C, were not able to completely remove all water from the hydrogel. This also indirectly reflects the superabsorbent resin's water retention capacity. Clearly, the more thoroughly the hydrogel is dehydrated during the treatment step, the higher the water absorption rate will be upon repeated water absorption. Changing the NaOH dosage, i.e., the degree of acrylic acid neutralization, (numbers 15-18), shows little effect on yield as the degree of neutralization decreases, but significantly increases water absorption. This is primarily because a higher degree of neutralization, i.e., a larger amount of NaOH, increases the content of more dissociated sodium carboxyl groups in the resin, leading to an increase in water-soluble products in the resin, resulting in a decrease in water absorption.

[0059] (5) The effects of the mass fraction of NaOH in the NaOH solution, the mass ratio of NaOH to shrimp shell powder, the treatment temperature and the amount of cross-linking agent on the gel yield and water absorption rate of chitosan-based super absorbent resin were investigated: Weigh the shrimp shell powder and place it in a 100mL centrifuge tube. Fix the amount of acrylic acid and shrimp shell powder. The mass ratio of acrylic acid to shrimp shell powder is 15:1. According to the conditions in Table 4, add NaOH solution to the shrimp shell powder, vortex disperse and place in a -18℃ refrigerator to freeze overnight. Take out the completely frozen shrimp shell powder, thaw it in a water bath and then heat it for 3h. After crushing, add appropriate amount of water, and use an internal cutting homogenizer to shear at 5000rpm (i.e., wet crushing) until the shrimp shell powder becomes a uniform suspension. Add acrylic acid and stir thoroughly. After the shrimp shell powder is completely dissolved, add NaOH as needed to reach the specified Neutralization degree, then add crosslinking agent and initiator in sequence, and finally add water to make the concentration of shrimp shell powder about 1.0wt%, the concentration of AA is 15.0wt%, the neutralization degree of AA is 42.5%, the initiator concentration is 1.0‰, and the crosslinker concentration is 0.17‰~0.30‰. Detailed conditions are shown in Table 4. After the reaction solution is stirred evenly, it is sealed with plastic wrap and heated in a water bath at 65℃ for 5h. The hydrogel is purified and dried according to the method in (1) to obtain chitosan-based super absorbent resin. The water absorption performance in pure water reaches 1060 times~1380 times, and it also has good water absorption performance in physiological saline.

[0060] Table 4 Effects of alkaline aqueous solution concentration, dosage, treatment temperature and cross-linking agent dosage on chitosan-based superabsorbent resin gel yield and water absorption rate during pretreatment Table 4 shows that with increasing crosslinker dosage (numbers 1-5), the gel yield and water absorption in pure water showed little change, while the water absorption in saline decreased slightly. This is primarily due to the low crosslinker dosage, low crosslinking density, and high soluble components, resulting in low water absorption. A high crosslinker dosage increases the crosslinking density, making it difficult for water to penetrate the chitosan-based superabsorbent resin, resulting in a lower water absorption rate and greater sensitivity in saline. Increasing the amount of pretreatment alkali (numbers 5-9) slightly increased the yield and water absorption in pure water, while significantly increased the water absorption in saline. Increasing the amount of alkali significantly increased the degree of chitin deacetylation, thereby increasing the solubility of chitosan in acrylic acid. Changing the pretreatment temperature (numbers 5, 10-12) had no significant effect on the yield or water absorption rate, but a slightly lower temperature was clearly beneficial (numbers 11 and 12). Changing the concentration of the pretreatment alkaline solution (numbers 5, 13-15) had no significant effect on the yield, but lower concentrations of alkaline treatment were more beneficial in increasing the water absorption rate of the chitosan-based superabsorbent resin (number 15). Higher alkaline treatment concentrations and temperatures clearly favored the conversion of chitin to highly deacetylated chitosan, increasing its acid solubility. However, this also reduced the molecular weight of chitosan, resulting in an increase in the soluble components in the final product and a decrease in water absorption rate.

[0061] (5) Investigate the effects of monomer dosage and acrylamide dosage in the monomer on the yield and water absorption properties of chitosan-based superabsorbent resin: The total monomer concentration was fixed at 14.5wt%, and shrimp shell powder was weighed and placed in a 100mL centrifuge tube. According to the conditions in Table 5, a 30.68wt% NaOH solution was added to the shrimp shell powder. After vortex dispersion, it was placed in a -18℃ refrigerator and frozen overnight. The completely frozen shrimp shell powder was taken out, thawed in a 95℃ water bath, and then heated for 3h. After crushing, an appropriate amount of water was added, and an internal cutting homogenizer was used to shear at 5000rpm (i.e., wet crushing) until the sample became a uniform suspension. Acrylic acid was added and stirred thoroughly. After the shrimp shell powder was completely dissolved, the mixture was sieved and the mixture was sieved. After the solution, NaOH was added as needed to achieve the predetermined neutralization degree of acrylic acid, and then acrylamide (AM), crosslinking agent and initiator were added in sequence. Finally, water was added to make the concentration of each reactant reach the conditions in Table 5. The reaction solution was stirred evenly and sealed with plastic wrap. It was heated in a water bath at 65℃ for 5 hours. The hydrogel was purified and dried according to the method in (1) to obtain a chitosan-based super absorbent resin. The ratio of acrylic acid to shrimp shell powder was 6 times and 12 times respectively. The water absorption rate in pure water was not much different, but the water absorption performance in physiological saline was significantly different. With the increase of AM dosage, the water absorption rate increased significantly. Further increase of AM dosage significantly decreased the water absorption performance, but it could still be maintained at more than 1300 times.

[0062] Table 5 Effect of monomer dosage and acrylamide dosage in monomer on yield and water absorption performance of chitosan-based super absorbent resin Table 5 shows that when the monomer mass is 6 times (number 1) and 12 times (number 2) the mass of shrimp shell powder, the hydrogel yield and water absorption rate in pure water are similar. However, when the monomer dosage is large, the water in the hydrogel is more difficult to remove, and the water absorption rate in physiological saline is significantly lower. Small increases in the amount of acrylamide in the monomer (numbers 2-4) significantly increase the water absorption rate, but further increases (numbers 5-8) lead to a gradual decrease in the water absorption rate.

[0063] (6) Investigate the effects of pretreatment time and cross-linking agent dosage on the yield and water absorption rate of chitosan-based superabsorbent resin: Weigh the shrimp shell powder and place it in a 100mL centrifuge tube. Add 35.78wt% NaOH solution, vortex disperse it and place it in a -18℃ freezer for 5h. Take out the preliminarily frozen shrimp shell powder, thaw it at room temperature and place it in the refrigerator again to freeze overnight. The next day, take out the completely frozen sample and place it in a 95℃ water bath to thaw and then heat it for 1.5h~6h. Take it out and mash it, add appropriate amount of pure water, and shear it at 5000rpm (i.e. wet crushing) until it becomes a uniform suspension. Add acrylic acid, crosslinker and initiator in turn. Add water as needed to make the concentration of each substance in the reaction system as shown in Table 6. Stir it evenly and seal it with a fresh-keeping bag and place it in a 65℃ water bath for 5h. The hydrogel is purified according to the method in (1). It was found that the hydrogel with a high crosslinker content was very difficult to dehydrate in ethanol. It was dried at 65℃ for 24h. The appearance of the chitosan-based super absorbent resin was not significantly different from that of the chitosan-based super absorbent resin prepared under other conditions, but its yield was much higher than the normal level. This is because when the cross-linking agent is high, the chitosan-based super absorbent resin has a higher cross-link density and contains more bound water. Conventional dehydration and drying methods cannot remove the bound water in the chitosan-based super absorbent resin. The water absorption performance results are shown in Table 6. After two freeze-thaw treatments, shrimp shell powder can be well dissolved even after heating for only 1.5 hours. The prepared chitosan-based super absorbent resin has excellent water absorption capacity, reaching up to 1715 times.

[0064] Table 6 Effect of pretreatment time and crosslinking agent dosage on chitosan-based superabsorbent resin yield and water absorption rate The data in Table 6 show that with increasing heat pretreatment time (numbers 1-4), the degree of deacetylation increases significantly, the water absorption rate significantly improves, and the yield remains at a high level. Further increasing the amount of crosslinker (numbers 5-9) results in significantly higher yields than normal, while the water absorption rate drops sharply. The increased amount of crosslinker increases the crosslink density of the resin, making it more difficult to remove water. However, even with a crosslinker dosage of 0.6‰, the water absorption rate still reaches 834 times. Existing technologies that add such high levels of crosslinker clearly cannot achieve such high water absorption rates, further demonstrating that the chitosan-based superabsorbent resin prepared from shrimp shell powder differs significantly from resins prepared using existing technologies.

[0065] The chitosan-based super absorbent resins directly prepared from shrimp shell powders numbered 6# and 9# in Table 6 were directly ground into powders. The TG curves of the chitosan-based super absorbent resins directly prepared from shrimp shell powders were tested at 10K / min under N2 protection conditions. The results are as follows: Figure 6As shown, it can be clearly seen that the chitosan-based super absorbent resin directly prepared from shrimp shell powder has two obvious mass loss stages before 300°C. The first stage is before 145°C, with a maximum mass loss of about 10%, which is the loss of free water in the chitosan-based super absorbent resin directly prepared from shrimp shell powder. The second stage is before 300°C, with a cumulative mass loss of nearly 40%, which is the loss of bound water in the chitosan-based super absorbent resin directly prepared from shrimp shell powder. 6# was immersed in pure water to fully absorb water, repeatedly immersed, and then pulverized after being blown dry at 65°C. The TG curve of the chitosan-based super absorbent resin directly prepared from shrimp shell powder was tested under the same conditions, as shown in the figure. Figure 7 As shown, the TG curve results are consistent with Figure 6 Compared with direct grinding, significant changes have occurred. The cumulative mass loss at around 300°C is about 30%, which is significantly lower than that of the direct grinding sample. The end temperature of the mass loss stage is obviously shifted towards high temperature. The mass loss rate in the first stage is less than 6%. Compared with the shrimp shell powder raw material after alkali treatment, the chitosan-based super absorbent resin has obvious calcium carbonate decomposition characteristics, indicating that the shrimp shell powder has successfully reacted with acrylic acid, and the calcium carbonate component in the shrimp shell powder is preserved in the chitosan resin.

[0066] (7) Verify the effect of shrimp shell powder replacing chitosan or chitin to prepare chitosan-based super absorbent resin: Weigh 2.41g of shrimp shell powder, 0.81g of chitin, and 1.61g of calcium carbonate (analytical grade) into a 100mL centrifuge tube. Add approximately 9.50g of a 35.94wt% NaOH solution to each tube. Vortex disperse the mixture and freeze overnight at -18°C. Thaw the mixture at 95°C and then heat for 3h. Add 10mL of pure water and shear at 5000rpm for 5min. Rinse the blade with pure water. Slowly add 14.5g of acrylic acid to the shrimp shell powder and stir to dissolve. Simultaneously, weigh two portions of chitosan (0.81g each), disperse them in 20mL of pure water, and add 14.5g of acrylic acid to each portion. Once the chitosan is completely dissolved, add 1.61g of calcium carbonate to one portion and stir until completely dissolved. Crosslinking agent and initiator were added to shrimp shell powder, chitin / calcium carbonate, chitosan / calcium carbonate and chitosan solution respectively, and water was added to make the concentration of crosslinking agent and initiator reach 0.4‰ and 1.0‰ respectively. After thorough stirring, the mixture was sealed with plastic wrap and placed at 65℃ for reaction for 5h. The hydrogel was purified and dried according to the method in (1). The water absorption performance results are shown in Table 7.

[0067] Table 7 Yield and water absorption rate of chitosan, chitosan / chitin simulated shrimp shell powder and chitosan-based super absorbent resin prepared from shrimp shell powder As shown in Table 7, the water absorption capacity of pure chitosan resin (No. 1, denoted as CTS / SAP) and chitosan-calcium carbonate composite resin (No. 2, denoted as CTS / CaCO3 / SAP) is significantly poor. The water absorption capacity of chitosan and calcium carbonate mixed to simulate shrimp shell powder resin (No. 3) and shrimp shell powder directly prepared resin (No. 4, denoted as HSh / SAP) is similar, but the shrimp shell powder resin is more effective. In addition, a series of operations such as decalcification of shrimp shell powder are required to obtain chitosan. The fully absorbed resin was repeatedly soaked in pure water, dried again, and ground into powder. The infrared spectrum was measured. The results are as follows Figure 8 As shown in FIG. 3 , the infrared spectra of the resin directly prepared from shrimp shell powder (HSh / SAP) are basically consistent with those of the resin prepared from chitosan and calcium carbonate, indicating that the chitosan-based super absorbent resin directly prepared from shrimp shell powder in the present invention is chitosan-based super absorbent resin.

[0068] (8) Verify the effect of shrimp shell powder source on the yield and water absorption properties of chitosan resin: Weigh NaOH separately and place it in a 100 mL centrifuge tube. Add pure water to it to make the concentration of NaOH 30 wt%. After cooling, add cooked shrimp shell powder (shrimp shell powder obtained by pre-treating the cooked shrimp shells, wherein the chitosan content is 28.29% and the calcium carbonate content is 66.86%) and the raw shrimp shell powder used in (1) to (7) respectively. After vortex dispersion, place it in a -18 ° C refrigerator and freeze it overnight. Take out the completely frozen shrimp shell powder, thaw it in a 95 ° C water bath, and then heat it for 3 h. Take it out and crush it, and add Dilute with an appropriate amount of pure water, then add acrylic acid 7 times the mass of shrimp shell powder and stir until fully dissolved, add crosslinking agent and initiator in sequence and add water to make the shrimp shell powder concentration 2.0wt%, acrylic acid concentration 14.5wt%, acrylic acid neutralization degree 40%, crosslinking agent 0.30‰, initiator 1.20‰, stir evenly and seal with plastic wrap, place at 65℃ for 5h, purify and dry according to the method in (1) to obtain chitosan-based super absorbent resin directly prepared from raw shrimp shell powder and cooked shrimp shell powder synthesized under the same conditions.

[0069] Table 8 Yield and water absorption properties of chitosan-based superabsorbent resin prepared from raw shrimp shell powder and cooked shrimp shell powder It can be seen from Table 9 that under the same conditions, the water absorption performance of the chitosan-based superabsorbent resin prepared from cooked shrimp shell powder is slightly better than that from raw shrimp shell powder. This is because the high-temperature cooking during cooking will destroy the dense structure of chitosan, making it easier to react with NaOH for deacetylation during the later alkali treatment, further improving the deacetylation degree of chitosan and its solubility in acrylic acid. At the same time, cooking treatment will also reduce the molecular weight of chitosan, which also has a beneficial effect on increasing the degree of reaction with acrylic acid.

[0070] Example 1 A method for preparing a chitosan-based super absorbent resin comprises the following steps: S1. Take 1.21g of shrimp and crab shell powder, mix it with 9.24g of 35.78wt% NaOH solution, vortex disperse it, and place it in a -18℃ refrigerator to freeze for 24h, then transfer it to a 95℃ water bath to thaw, and heat it for 3h to obtain frozen-thawed shrimp and crab shells; crush the frozen-thawed shrimp and crab shells, add an appropriate amount of pure water, and use an internal cutting homogenizer to shear (wet crushing) at 5000rpm to obtain a uniform suspension to obtain thawed shrimp and crab shells.

[0071] S2. Using thawed shrimp and crab shells and acrylic monomers as raw materials, with an acrylic acid content of 14.5 wt% relative to the total mass of the reaction system, a neutralization degree of 41.03%, a crosslinker of 0.241‰ N,N'-methylenebisacrylamide, an initiator of 1.00‰ ammonium persulfate, and a shrimp and crab shell powder concentration of 1.21 wt%, a graft copolymerization reaction was carried out in the presence of the crosslinker and a thermal initiator to obtain a chitosan-based superabsorbent resin.

[0072] It should be noted that when numerical ranges are mentioned in the present invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, the present invention describes preferred embodiments to avoid redundancy. Although preferred embodiments of the present invention have been described, those skilled in the art will be able to make additional changes and modifications to these embodiments once they understand the basic inventive concepts.

Claims

1. A method for preparing a chitosan-based super absorbent resin, characterized in that: The following steps are involved: The shrimp and crab shells are pretreated to obtain shrimp and crab shell powder rich in chitosan and calcium carbonate; Shrimp and crab shell powder rich in chitosan and calcium carbonate is mixed with an alkaline aqueous solution and subjected to a freeze-thaw-holding process. During the freezing process, the alkaline molecules enter the chitosan particles with the assistance of ice crystals. Subsequently, during the thawing-holding process, the chitosan is deacetylated and converted into chitosan to obtain frozen-thawed shrimp and crab shells. The frozen-thawed shrimp and crab shells are crushed to obtain thawed shrimp and crab shells. The method uses thawed shrimp and crab shells and acrylic monomers as raw materials, and carries out a graft copolymerization reaction in the presence of a crosslinking agent and a thermal initiator. During the graft copolymerization reaction, the acrylic monomer undergoes free radical polymerization to generate polyacrylic acid chains, and the initiator triggers chitosan in the thawed shrimp and crab shells to form chitosan macromolecular free radicals. At the same time, polyacrylic acid chains with shorter chain lengths are grafted onto the main chain of the chitosan macromolecular chain to form a graft polymer. Under the crosslinking action of the crosslinking agent, the graft polymer undergoes a crosslinking reaction to form a hydrogel, and after drying, a chitosan-based super absorbent resin is obtained.

2. The method for preparing a chitosan-based super absorbent resin according to claim 1, wherein: The mass fraction of alkali in the alkali aqueous solution is 25%~40.5%; the mass ratio of the alkali aqueous solution to the shrimp and crab shell powder is 4.8~10:1; and the mass ratio of the alkali in the alkali aqueous solution to the shrimp and crab shell powder is 1.5~3.6:

1.

3. The method for preparing a chitosan-based super absorbent resin according to claim 1, wherein: The conditions for the freeze-thaw-insulation treatment are: first freeze at -23°C~-18°C for at least 5 hours, then thaw at room temperature~95°C, and finally keep warm at 65°C~95°C for 1.5 hours~6 hours.

4. The method for preparing a chitosan-based super absorbent resin according to claim 1, wherein: The mass ratio of thawed shrimp and crab shells to acrylic acid monomers is 1:5.9~16.

5. The method for preparing a chitosan-based super absorbent resin according to claim 1, wherein: The acrylic monomer is selected from acrylic acid or a mixture of acrylic acid and acrylamide; in the mixture of acrylic acid and acrylamide, the mass percentage of acrylamide is 0.1wt% to 50wt%.

6. The method for preparing a chitosan-based super absorbent resin according to claim 1, wherein: The conditions for the graft copolymerization reaction are: reaction at 65°C~75°C for 3h~7h.

7. The method for preparing a chitosan-based super absorbent resin according to claim 1, wherein: The specific operation of the pretreatment is: deproteinizing, defatting and crushing the shrimp and crab shells in sequence to obtain shrimp and crab shell powder rich in chitosan and calcium carbonate; it can also be directly partially decalcified to obtain incompletely decalcified shrimp and crab shell powder.

8. A chitosan-based super absorbent resin obtained by the preparation method according to any one of claims 1 to 7.

9. The chitosan-based super absorbent resin according to claim 8, characterized in that: Chitosan-based super absorbent resin is in the form of translucent hydrogel, which becomes white granules after drying and becomes transparent elastic hydrogel after reabsorbing water and swelling.

10. Use of the chitosan-based super absorbent resin according to claim 8 in preparing a water-absorbing material.

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