CN113307904B - Preparation method of water-absorbent resin - Google Patents
Preparation method of water-absorbent resin Download PDFInfo
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- CN113307904B CN113307904B CN202110664159.3A CN202110664159A CN113307904B CN 113307904 B CN113307904 B CN 113307904B CN 202110664159 A CN202110664159 A CN 202110664159A CN 113307904 B CN113307904 B CN 113307904B
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F220/00—Copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical or a salt, anhydride ester, amide, imide or nitrile thereof
- C08F220/02—Monocarboxylic acids having less than ten carbon atoms; Derivatives thereof
- C08F220/04—Acids; Metal salts or ammonium salts thereof
- C08F220/06—Acrylic acid; Methacrylic acid; Metal salts or ammonium salts thereof
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F2/00—Processes of polymerisation
- C08F2/12—Polymerisation in non-solvents
- C08F2/14—Organic medium
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
- C08J3/00—Processes of treating or compounding macromolecular substances
- C08J3/24—Crosslinking, e.g. vulcanising, of macromolecules
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- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
- C08J2333/00—Characterised by the use of homopolymers or copolymers of compounds having one or more unsaturated aliphatic radicals, each having only one carbon-to-carbon double bond, and only one being terminated by only one carboxyl radical, or of salts, anhydrides, esters, amides, imides, or nitriles thereof; Derivatives of such polymers
- C08J2333/02—Homopolymers or copolymers of acids; Metal or ammonium salts thereof
Abstract
The invention provides a reverse suspension polymerization preparation method of a water-absorbent resin for reducing short-term extractables and improving short-term back-seepage amount, and more particularly relates to a method for obtaining a water-absorbent resin with high gel strength by removing part of petroleum hydrocarbon solvents in batches and then carrying out multiple surface cross-linking reactions while or after dehydration. Compared with a one-time surface crosslinking process, the method can prepare a water-absorbent resin product with a multilayer structure, and further contributes to reducing the dissolution rate of low-molecular-weight compounds after the product absorbs liquid for a short time, effectively improving the touch and reducing short-term back permeation.
Description
Technical Field
The present invention relates to a process for producing a water-absorbent resin having a low short-term extractables and a short-term back-bleeding amount by an improved surface cross-linking process.
Background
Super Absorbent Polymers (SAP) are slightly crosslinked polymer compounds, and are widely used in the field of sanitary materials such as paper diapers and sanitary napkins due to their strong water absorption and retention capacities, water-blocking materials for cables and optical cables, and water-retaining agents special for agriculture, forestry and horticulture. As water-absorbent resins for sanitary materials, partially neutralized products of polyacrylic acid, neutralized products of starch-acrylic acid graft polymers, hydrolyzed products of starch-acrylonitrile graft polymers, saponified products of vinyl acetate-acrylic ester copolymers, and the like are known.
Currently, the most widely used are the polyacrylic acids and their sodium salts in the series, of which about 95% are prepared by aqueous solution polymerization and about 5% by reversed phase suspension polymerization. The water-absorbent resin prepared by the reversed-phase suspension polymerization has a larger specific surface area due to its macroscopic morphology in the form of piled grape strings than the irregular block morphology obtained by aqueous solution polymerization through grinding, and therefore has a faster liquid-absorbing rate when in contact with water, which enables it to absorb a large amount of water in a short time. With the continuous expansion of the composite core market, higher requirements are put forward on the lightness and thinness degree of the core structure, and for the paper diaper, the possibility of reducing the red buttocks of the baby and the dryness degree after liquid absorption are required to be improved. This also stimulates the development of the water-absorbent resin to some extent toward improvement in extractables after liquid absorption, rewet content and gel touch.
As for the water-absorbent resin, the extractable matter and the back-bleeding amount are improved by: controlling the polymerization process, increasing molecular weight and reducing side reactions such as chain transfer and the like as much as possible; the cross-linking degree between molecular chains is improved through internal cross-linking; again, by surface crosslinking to form a suitable "core-shell structure". For example, norer (application publication nos. CN 106987075A and CN 102702656A) discloses a method for preparing a water-absorbent resin which is polymerized at a low temperature and subjected to surface crosslinking multiple times and to which inorganic particles are added, and can prepare a water-absorbent resin having low extractables; sumitomo refinement (CN104507565B and CN 103502287A) discloses a water-absorbent resin having excellent absorption performance, good shape retention and good gel strength by reversed phase suspension polymerization in a two-step process in the presence of a surfactant, sucrose fatty acid ester, a hydrophobic polymeric dispersant and a thickener, by reasonably adjusting the concentration gradient of an internal crosslinking agent and a surface crosslinking agent and by azeotropically removing water before surface crosslinking; subsequent Sumitomo refinement (US2008/0280154A1) further discloses a process for the preparation of water-absorbent resins with high water retention capacity and gel strength and low extractables by two azeotropic dehydrations and two surface crosslinkings. Wanhua chemical patent (CN 103910896A) discloses a method for improving the uniformity and reaction efficiency of surface crosslinking by adding a surfactant and a phase transfer catalyst during surface crosslinking, and can improve the saline back-osmosis of SAP.
Although researchers at home and abroad have made a lot of reports on how to reduce the long-term extractables of water-absorbent resins, there are few data on studies on short-term extractables. The long term extractables reflect the stability of the SAP against hydrolysis after imbibing the liquid, while the short term extractables can be referenced as an index reflecting the short term rewet ability and feel.
Disclosure of Invention
The invention provides a method for preparing water-absorbent resin with low short-term extractables and low back-seepage amount by creatively carrying out surface crosslinking operation after regulating the content of petroleum hydrocarbon solvent in a system to a proper value at the same time or after azeotropic dehydration process, carrying out surface crosslinking once every regulation and control and repeating for two to three times based on the prior reverse suspension polymerization preparation process of the water-absorbent resin.
In order to achieve the above purpose, the technical scheme of the invention is as follows:
a method for producing a water-absorbent resin, comprising the steps of:
1) dissolving a dispersant in a petroleum hydrocarbon solvent to form a so-called oil phase;
2) pouring acrylic acid monomer into sodium hydroxide solution with mass concentration of 20-40%, and after acid-base neutralization reaction and cooling to room temperature, sequentially adding aqueous solution containing initiator and internal cross-linking agent to form so-called water phase;
3) pouring the water phase obtained in the step 2) into the oil phase obtained in the step 1), stirring and mixing, introducing nitrogen to fully remove oxygen, heating, and carrying out one-section water-in-oil reversed-phase suspension polymerization to obtain a suspension of one-section polymerized hydrogel particles;
4) cooling the suspension obtained in the step 3), adding the water phase prepared in the step 2) again, stirring for 5-30min, and repeating the step 3), namely heating for second-stage polymerization to obtain the agglomerated second-stage polymerized hydrogel particles;
5) removing 70-90% of water in the hydrogel particles obtained in the step 4) by azeotropy, simultaneously or subsequently, removing 10-60% of petroleum hydrocarbon solvent based on the total amount of the petroleum hydrocarbon solvent in two to three stages, and then respectively adding aqueous solution of a surface cross-linking agent into each stage to carry out surface cross-linking for multiple times to obtain water-absorbent resin mixed solution; preferably, when the two stages are divided, the petroleum hydrocarbon removal amount A percent controlled in the first stage is more than or equal to the petroleum hydrocarbon removal amount B percent controlled in the second stage (B is the cumulative removal amount of two times), preferably, the A is more than or equal to 10 and less than or equal to 30, and the B is more than or equal to 30 and less than or equal to 60; when the method is divided into three stages, the petroleum hydrocarbon removal amount a percent controlled in the first stage is more than or equal to the petroleum hydrocarbon removal amount b percent controlled in the second stage is more than or equal to the petroleum hydrocarbon removal amount c percent controlled in the third stage (b is the twice cumulative removal amount, and c is the three times cumulative removal amount), preferably, a is more than or equal to 10 and less than or equal to 20, b is more than or equal to 20 and less than or equal to 40, and c is more than or equal to 40 and less than or equal to 60; A. b, a, B and c are all percentage contents based on the initial amount of the petroleum hydrocarbon solvent.
6) And filtering, drying and screening the water-absorbent resin mixed solution to obtain the target SAP particles in the shape of grape bunches.
In the invention, the dispersant is a surfactant with HLB value less than 7, preferably at least one of sucrose fatty acid ester, sorbitan monostearate, sorbitan monooleate, triglycerol monostearate and octadecyl monophosphate, and the dosage of the dispersant is 0.01-5% of the mass fraction of the acrylic acid monomer.
In the present invention, the petroleum hydrocarbon solvent is at least one of aliphatic hydrocarbon, alicyclic hydrocarbon or aromatic hydrocarbon, and suitable aliphatic hydrocarbon is selected from n-pentane, n-hexane, n-heptane, petroleum ether, etc.; suitable alicyclic hydrocarbons are selected from cyclopentane, methylcyclopentane, cyclohexane, methylcyclohexane, and the like; suitable aromatic hydrocarbons are benzene, toluene, xylene, etc.
In the invention, the initiator is selected from one or more of sodium persulfate, potassium persulfate, ammonium persulfate or 2,2' -azobisisobutylamidine dihydrochloride, and the dosage of the initiator is 0.005-5% of the mass fraction of the acrylic acid monomer.
In the invention, the internal crosslinking agent is selected from hydroxyl-containing ethylene glycol, propylene glycol, glycerol, pentaerythritol, polyglycerol, polyvinyl alcohol and tris (hydroxymethyl) aminomethane; ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polyglycerol glycidyl ether containing an epoxy group; one or more of ethylene glycol diacrylate, propylene glycol diacrylate, N' -methylene bisacrylamide, polyethylene glycol diacrylate, polypropylene glycol diacrylate, pentaerythritol triallyl ether, ethoxylated glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, triallylamine, pentaerythritol tetraacrylate and dipentaerythritol hexaacrylate containing reactive double bonds, wherein the amount of the used one or more is 0.005-1% of the mass fraction of the acrylic acid monomer.
In the invention, the water phase in the step 2), namely the neutralization degree (the reaction degree of acid-base neutralization) of the acrylic acid neutralization solution is 60-85%, and the mass concentration of acrylic acid monomer is 20-50%. The mass ratio of the oil phase to the water phase is 0.1 to 10, preferably 1 to 5. 4) The mass ratio of the water phase in the step (3) to the water phase in the step (b) is 0.5-10, preferably 1-2.
In the invention, the polymerization temperature in the steps 3) and 4) is 30-120 ℃, preferably 50-90 ℃; the polymerization time is from 0.1 to 5 hours, preferably from 0.5 to 2 hours. 4) The cooling temperature in the step is 0-50 ℃, preferably 5-30 ℃.
The surface cross-linking agent in the invention is a compound which can form covalent bonds or ionic bonds with carboxyl groups: the compounds capable of forming covalent bonds include polyol compounds, epoxy compounds, and polyvalent amines such as polyethyleneimine, and preferably one or more of ethylene glycol, propylene glycol, 1, 4-butanediol, ethylene carbonate, propylene carbonate, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, glycerol, tris (hydroxymethyl) aminomethane, and pentaerythritol; the compound capable of forming an ionic bond includes inorganic salts of polyvalent metals such as calcium, magnesium, aluminum, iron, copper, zinc, and the like. The amount of the surface cross-linking agent added is 0.01 to 2%, preferably 0.02 to 0.2% of the total mass of acrylic acid. When the materials are added in batches, the adding amount is 0.01-0.1% each time.
The mass concentration of the aqueous solution of the surface cross-linking agent in the present invention is 0.1 to 20%, preferably 1 to 10%.
In the present invention, the azeotropic dehydration in step 5) is carried out at a temperature of 80-150 ℃, preferably at a temperature of 100-130 ℃, for a period of 1-5h, preferably for a period of 1-3 h.
In the invention, the surface cross-linking temperature in the step 5) is 60-150 ℃, preferably 80-100 ℃, and the time is 0.5-5h, preferably 0.5-2 h.
In the present invention, the drying temperature in step 6) is 80-180 ℃, preferably 100-130 ℃, and the drying time is 0.5-5h, preferably 1-2 h.
The content of the petroleum hydrocarbon solvent in the system is adjusted while or after azeotropic dehydration is carried out, and the surface crosslinking procedure is carried out once per adjustment, so that the thickness and the composition of the formed core-shell structure are conveniently controlled by the type and the concentration of the surface crosslinking agent added each time and the amount of the removed petroleum hydrocarbon solvent.
Thus, on the one hand, as the oil-to-water ratio decreases, the aqueous solution of the surface cross-linking agent can more sufficiently and rapidly diffuse and penetrate into the surface layer of the water-absorbent resin particles; on the other hand, the size of the formed droplets of the surface cross-linking agent aqueous solution can be increased, and the depth of the surface cross-linking agent aqueous solution penetrating into the surface layer is increased; finally, the 'shell layer' structure with different compositions and concentration gradients can be formed by adjusting the concentration and the type of the surface cross-linking agent. The beneficial effects brought are that: the core-shell structure formed by multilayer distribution and composition and the cross-linking structure under multiple concentration gradients enable the protection effect of SAP on internal low-molecular compounds to be stronger after short-term imbibition, help to slow down the diffusion rate of SAP to the surface and the inside of solution, and improve the touch feeling of imbibed products, so that a feasible idea is provided for the development of low-short-term extractables and SAP products with low short-term rewet amount.
The basic performance index test method of the water-absorbent resin comprises the following steps:
(1) extractable matter content (refer to GB/T22875-2018 and ISO 17190-10-2001)
1. Measuring 200mL of 0.9% NaCl solution into a 250mL beaker by using a measuring cylinder, weighing 1.0g of SAP sample to be accurate to 0.005g, adding the SAP sample into the solution, sealing the opening of the beaker by using a sealing film, placing the beaker on a magnetic stirrer, stirring for 16h at 500 +/-50 rpm, and simultaneously preparing a blank sample (200mL of 0.9% NaCl solution);
2. stopping stirring, allowing the colloid to settle to the bottom, filtering the supernatant in the beaker with a Buchner funnel and filter paper, and collecting at least 50mL of filtrate;
3. accurately measuring 50mL of filtrate, carrying out titration test by using an automatic potentiometric titrator, firstly, carrying out titration on a blank solution (100mL of a 0.9% NaCl aqueous solution), and carrying out titration by using a 0.1mol/L NaOH solution until the pH value is 10; then titrated with 0.1mol/L hydrochloric acid solution until pH 2.7; obtaining blank titration quantities of [ bNaOH ] and [ bHCL ] mL respectively;
4. adding 0.9% NaCl solution to the sample extract to 100mL, and carrying out the same titration operation to obtain [ NaOH ], [ HCl ] (mL) titration amounts, respectively;
then:
note: the standard test time of extractables is 16h, the long-time hydrolysis resistance of the reaction SAP is guaranteed, and the short-term extractables of 1h and 4h can be used as index references for evaluating the short-term back-seepage amount and the touch to a certain extent.
(2) Return seepage of pure water in short term
1. 80g of deionized water was poured into a 9.5 cm-diameter petri dish, 1.0g of water-absorbent resin was rapidly added and timing was started, and the petri dish was shaken to uniformly disperse the SAP therein.
After 2.30min, 10 pieces of filter paper with accurately measured weight are placed on the gel, the 1200g weight is pressed on the center of the culture dish, and the time is measured for 5min again,
3. the weight gain of the filter paper is the back-seepage quantity of the water-absorbent resin.
Detailed Description
The present invention will be described in detail with reference to specific examples
Example 1:
260g of n-heptane was charged into a 1L four-necked round bottom flask equipped with a stirrer, a reflux condenser, a thermometer and a nitrogen inlet tube. 0.46g of sucrose fatty acid ester (Mitsubishi chemical, S-370) was added thereto, heated to 80 ℃ and uniformly dissolved and dispersed at a stirring speed of 350rpm, and then cooled to 30 ℃ to obtain an oil phase for later use. 120g of 32% w.t. sodium hydroxide aqueous solution was dropwise added to a 500mL Erlenmeyer flask containing 92g of acrylic acid monomer and 35g of deionized water while stirring under cooling, and when the temperature was lowered to 30 ℃ or below, 10g of an aqueous solution containing 0.14g of potassium persulfate and 0.092g of polyethylene glycol 400 diacrylate were added thereto and sufficiently dissolved and mixed to obtain a neutralized solution with acrylic acid, i.e., an aqueous phase for use.
Half of the prepared water phase was added to the oil phase, and replaced with nitrogen gas for 30min while stirring. Then the temperature is raised to 70 ℃, and the reaction is carried out for 1h under the condition for one-stage water-in-oil reversed-phase suspension polymerization, thus obtaining the suspension containing one-stage polymerized SAP particles. And continuously cooling the suspension to 23 ℃ to separate out part of the surfactant, adding the other half of the prepared water phase, and repeating the steps to perform two-stage water-in-oil reversed-phase suspension polymerization to obtain the suspension containing two-stage polymerization SAP particles with the agglomeration effect.
Continuously heating the suspension to 120 ℃ for azeotropic dehydration, removing 120g of water, simultaneously removing 52g (20% of initial amount) of n-heptane, then reducing the oil bath temperature to 83 ℃, adding 1.8g of aqueous solution of 2% ethylene glycol diglycidyl ether (669) in mass concentration, and carrying out primary surface crosslinking reaction for 30 min; then, 52g of n-heptane (20% of the initial amount, 40% of the initial amount was accumulated) was removed by filtration, and 2.7g of an aqueous solution of ethylene glycol diglycidyl ether having a mass concentration of 2% was added again to carry out a secondary surface crosslinking reaction for 1 hour. Finally the n-heptane solution containing the grape bunch-like SAP particles was filtered off and dried at 130 ℃ for 1 h. Finally, screen meshes with different meshes are used for screening and combining the SAP and the SAP product with the required grain diameter is further obtained.
Example 2:
the procedure of example 1 was repeated except that the first removal of n-heptane was changed to 78g (30% of the initial amount) and the second removal of n-heptane was changed to 78g (30% of the initial amount, totalizing 60% of the initial amount).
Example 3:
the procedure of example 1 was repeated except that the first removal of n-heptane was changed to 52g (20% of the initial amount) and the second removal of n-heptane was changed to 104g (40% of the initial amount, totalizing 60% of the initial amount).
Example 4:
the procedure of example 1 was repeated except that the amount of the aqueous solution of 2% ethylene glycol diglycidyl ether added in the first surface-crosslinking was changed to 0.9g, and the amount of the aqueous solution of 2% ethylene glycol diglycidyl ether added in the second surface-crosslinking was changed to 3.6 g.
Example 5:
except that 2.76g of 2% Al was used2(SO4)3·18H2The procedure of example 4 was repeated except that 0.9g of an aqueous solution of 2% ethylene glycol diglycidyl ether in place of the aqueous solution of O was once surface-crosslinked.
Example 6:
except that the removal amount of n-heptane at the first time was changed to 26g (10% of the initial amount), 0.9g of an aqueous solution of ethylene glycol diglycidyl ether having a mass concentration of 2% was added to carry out a surface crosslinking reaction for 30 min; the removal amount of n-heptane for the second time is changed to 56g (20% of initial amount, accumulated 30% of initial amount), 1.4g of aqueous solution with the mass concentration of 2% ethylene glycol diglycidyl ether is added, and secondary surface crosslinking reaction is carried out for 30 min; the removal amount of the n-heptane for the third time is changed into 56g (20% of initial amount, accumulated 50% of initial amount), 2.3g of aqueous solution with the mass concentration of 2% ethylene glycol diglycidyl ether is added, and three times of surface crosslinking reaction is carried out for 30 min; otherwise, the procedure of example 1 was followed.
Comparative example 1:
the procedure of example 1 was repeated except that n-heptane was not removed, and after azeotropic dehydration, temperature was lowered to 83 ℃ and 4.5g of a 2% aqueous solution of ethylene glycol diglycidyl ether was added at once at this temperature to conduct surface crosslinking.
Comparative example 2:
the procedure of example 1 was repeated except that n-heptane was not removed after azeotropic dehydration.
Comparative example 3:
the procedure of example 1 was repeated except that the amount of n-heptane removed for the first time was changed to 26g (10% of the initial amount) and the amount of n-heptane removed for the second time was changed to 26g (10% of the initial amount, totalizing 20% of the initial amount).
Comparative example 4:
the procedure of example 1 was repeated except that the second n-heptane removal was changed to 156g (60% of the initial amount, cumulatively 80% of the initial amount).
The basic properties of the SAPs prepared in the above-described exemplary working examples are shown in table 1:
TABLE 1 basic Properties of Water-absorbent resins obtained in representative working examples
As can be seen from the examples, both short-term extractables and short-term rewet amounts of the water-absorbent resin are reduced and the feeling of the gel surface after liquid absorption is also improved by multiple solvent adjustments and multiple surface crosslinkings, relative to comparative examples, and thus the present invention is useful for improving the dryness of the diaper after liquid absorption and reducing the infant's red buttocks which may be caused by the migration of low-molecular extractables. This indicates that the concentration gradient formed by multiple surface cross-linking is more effective and the "core-shell structure" is more uniformly distributed. And we can optimize this by adjusting the content of petroleum hydrocarbon, the type and concentration distribution of the surface cross-linking agent. No matter n-heptane is not removed, or the removal amount is not proper, or only one-time surface crosslinking is directly carried out, the short-term extractables and the back-seepage amount are reduced to a certain extent.
In conclusion, the above description is only for the purpose of illustrating the preferred embodiments of the present invention and not for the purpose of limiting the same, and any modifications of the present invention, including equivalent substitutions and additions of various materials, are within the scope of the present invention, as will be appreciated by those skilled in the art. The scope of the invention is defined by the appended claims.
Claims (23)
1. A method for producing a water-absorbent resin, comprising the steps of:
1) dissolving a dispersant in a petroleum hydrocarbon solvent to form an oil phase;
2) pouring an acrylic acid monomer into an alkali solution with the mass concentration of 20-40%, and after the acid-base neutralization reaction is carried out and the solution is cooled to room temperature, sequentially adding an aqueous solution containing an initiator and an internal crosslinking agent to form a water phase;
3) pouring the water phase obtained in the step 2) into the oil phase obtained in the step 1), stirring and mixing, introducing nitrogen to fully remove oxygen, heating, and carrying out one-section water-in-oil reversed-phase suspension polymerization to obtain a suspension of one-section polymerized hydrogel particles;
4) cooling the suspension obtained in the step 3), adding the water phase prepared in the step 2) again, stirring for 5-30min, heating, and carrying out two-stage water-in-oil reversed-phase suspension polymerization to obtain agglomerated two-stage polymerized hydrogel particles;
5) removing 70-90% of water in the hydrogel particles obtained in the step 4) by azeotropy, simultaneously or subsequently, removing 40-60% of petroleum hydrocarbon solvent based on the total amount of the petroleum hydrocarbon solvent in two to three stages, and then respectively adding aqueous solution of a surface cross-linking agent to carry out surface cross-linking for multiple times in each stage to obtain water-absorbent resin mixed solution;
6) and filtering, drying and screening the water-absorbent resin mixed solution to obtain the grape bunch-shaped SAP particles.
2. The method according to claim 1, wherein the alkali solution in step 2) is a sodium hydroxide solution.
3. The method of claim 1, wherein when divided into two stages, the amount of petroleum hydrocarbon removed controlled in the first stage is a% or less than the amount of petroleum hydrocarbon removed controlled in the second stage is B%; when the method is divided into three stages, the petroleum hydrocarbon removal amount a percent controlled in the first stage is more than or equal to the petroleum hydrocarbon removal amount B percent controlled in the second stage is more than or equal to the petroleum hydrocarbon removal amount c percent controlled in the third stage, and A, B, a, B and c are all percentage contents based on the initial amount of the petroleum hydrocarbon solvent.
4. The method according to claim 1, wherein the dispersant in step 1) is a surfactant having an HLB value of less than 7, and is used in an amount of 0.01 to 5% by mass based on the mass fraction of the acrylic acid monomer.
5. The method of claim 4, wherein the dispersant in step 1) is at least one of sucrose fatty acid ester, sorbitan monostearate, sorbitan monooleate, triglycerol monostearate, and stearyl monophosphate.
6. The method according to any one of claims 1 to 5, wherein the petroleum hydrocarbon solvent in step 1) is at least one of an aliphatic hydrocarbon, an alicyclic hydrocarbon or an aromatic hydrocarbon.
7. The method according to claim 6, wherein the aliphatic hydrocarbon is selected from one or more of n-pentane, n-hexane, n-heptane, petroleum ether; the alicyclic hydrocarbon is selected from one or more of cyclopentane, methyl cyclopentane, cyclohexane and methyl cyclohexane; the aromatic hydrocarbon is selected from one or more of benzene, toluene and xylene.
8. The method according to any one of claims 1 to 5, wherein the mass ratio of the oil phase to the aqueous phase is from 0.1 to 10.
9. The method of claim 8, wherein the mass ratio of the oil phase to the water phase is 1 to 5.
10. The method according to any one of claims 1 to 5, wherein the aqueous phase obtained in step 2), i.e., the neutralized solution of acrylic acid, has a neutralization degree of 60 to 85% and a mass concentration of acrylic acid monomer of 20 to 50%.
11. The method of any one of claims 1-5, wherein in step 2) the initiator is selected from one or more of sodium persulfate, potassium persulfate, ammonium persulfate, or 2,2' -azobisisobutylamidine dihydrochloride, in an amount of 0.005-5% by mass of the acrylic acid monomer; and/or the internal crosslinking agent is selected from hydroxyl-containing ethylene glycol, propylene glycol, glycerol, pentaerythritol, polyglycerol, polyvinyl alcohol and tris (hydroxymethyl) aminomethane; ethylene glycol diglycidyl ether, propylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, polypropylene glycol diglycidyl ether, and polyglycerol glycidyl ether containing an epoxy group; one or more of ethylene glycol diacrylate, propylene glycol diacrylate, N' -methylene bisacrylamide, polyethylene glycol diacrylate, polypropylene glycol diacrylate, pentaerythritol triallyl ether, ethoxylated glycerol triacrylate, trimethylolpropane triacrylate, pentaerythritol triacrylate, triallylamine, pentaerythritol tetraacrylate and dipentaerythritol hexaacrylate containing reactive double bonds, wherein the amount of the used one or more is 0.005-1% of the mass fraction of the acrylic acid monomer.
12. The method according to any one of claims 1 to 5, wherein the polymerization temperature in 3) step and 4) step is 30 to 120 ℃; the polymerization time is 0.1-5 h; and/or, 4) the cooling temperature in the step is 0-50 ℃.
13. The method of claim 12, wherein the polymerization temperature in steps 3) and 4) is 50-90 ℃; the polymerization time is 0.5-2 h; and/or, 4) the cooling temperature in the step is 5-30 ℃.
14. The process according to any one of claims 1 to 5, wherein the mass ratio of the aqueous phase in step 4) to that in step 3) is from 0.5 to 10.
15. The method of claim 14, wherein the mass ratio of the aqueous phase in step 4) to the aqueous phase in step 3) is 1-2.
16. The method according to any one of claims 1 to 5, wherein the surface cross-linking agent in step 5) is a compound capable of reacting with the carboxyl group to form a covalent bond or an ionic bond: the compound capable of forming a covalent bond includes one or more of polyol compounds, epoxy compounds and polyethyleneimine; and/or, the total addition amount of the surface cross-linking agent is 0.01-2% of the total mass of the acrylic acid, and when the surface cross-linking agent is added in batches, the addition amount is 0.01-0.1% each time; and/or, the mass concentration of the surface cross-linking agent aqueous solution in the step 5) is 0.1-20%.
17. The method of claim 16, wherein the compound capable of forming a covalent bond comprises one or more of ethylene glycol, propylene glycol, 1, 4-butanediol, ethylene carbonate, propylene carbonate, (poly) ethylene glycol diglycidyl ether, (poly) propylene glycol diglycidyl ether, glycerol, tris (hydroxymethyl) aminomethane, pentaerythritol; the compound capable of forming an ionic bond includes inorganic salts of polyvalent metals of calcium, magnesium, aluminum, iron, copper and zinc; and/or, the total addition amount of the surface cross-linking agent is 0.02-0.2% of the total mass of the acrylic acid; and/or, the mass concentration of the surface cross-linking agent aqueous solution in the step 5) is 1-10%.
18. The process according to any one of claims 1 to 5, wherein the azeotropic dehydration in step 5) is carried out at a temperature of from 80 to 150 ℃ for a period of from 1 to 5 hours.
19. The method as claimed in claim 18, wherein the azeotropic dehydration temperature in the step 5) is 100-130 ℃ and the time is 1-3 h.
20. The method according to any one of claims 1 to 5, wherein the surface cross-linking temperature in step 5) is 60 to 150 ℃ and the time is 0.5 to 5 hours.
21. The method of claim 20, wherein the surface cross-linking temperature in step 5) is 80-100 ℃ for 0.5-2 hours.
22. The method according to any one of claims 1 to 5, wherein the drying temperature in step 6) is 80 to 180 ℃ and the time is 0.5 to 5 hours.
23. The method as claimed in claim 22, wherein the drying temperature in step 6) is 100-130 ℃ and the drying time is 1-2 h.
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