Preparation method of acrylic acid-based super absorbent resin with low residual monomer

Abstract

The present invention provides a method for preparing an acrylic acid-based super absorbent resin with low residual monomers, which belongs to the field of water-absorbing materials and comprises the following steps: spraying a residual monomer reducing agent on polyacrylic acid / sodium polyacrylate hydrogel for mixing, granulation, drying and crushing to obtain original particles, wherein the residual monomer reducing agent is one or more of peroxide, persulfide, reductive chain transfer agent and alcohol amine; spraying a surface crosslinking agent on the original particles for surface crosslinking, and then spraying an anti-caking agent for anti-caking treatment to obtain an acrylic acid-based super absorbent resin with low residual monomers. The present invention does not need to add residual monomer reducing auxiliary agents such as alkyl peroxides and azo-based agents and residual monomer reducing photoinitiators, and thus does not need to be subject to the dependence of the agent half-life on the heat preservation and ultraviolet irradiation maturation period, can shorten the process cycle, improve production efficiency, and the residual monomers of the output super absorbent resin can be controlled below 100ppm.

Description

Technical Field

[0001] The invention belongs to the field of water-absorbing materials, and in particular relates to a method for preparing an acrylic acid-based highly water-absorbing resin with low residual monomers. Background Art

[0002] Based on the raw materials used in their manufacture, super absorbent resins can generally be divided into three categories: starch-based, cellulose-based, and synthetic resin-based. Among them, the unsaturated monomer acrylic acid can initiate polymerization reactions due to its olefinic bond structure in its molecules. Its product, acrylic acid-based super absorbent resin SAP, is polyacrylic acid (sodium) with a certain degree of neutralization. It is the most widely studied and applied due to its high water absorption rate, stable quality, non-corruption, simple synthesis process, and low cost advantages, and occupies a dominant position in the market.

[0003] Since the olefinic structure of acrylic acid is active, it is easy to undergo slow self-polymerization under the influence of light or the rise of storage environment temperature at room temperature, and violent explosion polymerization will occur at high temperature; therefore, a certain amount of molecular inhibitor hydroquinone (HQ) or methyl parahydroxybenzene (MEHQ) needs to be added to the commercial acrylic acid monomer during the manufacturing, transportation, storage, and use processes, and a certain amount of air is introduced to effectively inhibit the polymerization reaction by inhibiting the generation of the polymerization reaction by forming an effective inhibition effect on free radicals; therefore, when acrylic acid-based super absorbent resin is manufactured, the inhibitor will hinder the complete completion of the polymerization reaction, and a trace amount of acrylic acid monomer fails to participate in the polymerization reaction in time, and remains in the super absorbent resin in the form of acrylic acid monomer.

[0004] In view of the irritation of acrylic acid monomers to the skin, the super absorbent resin manufacturing industry has formulated and issued a national standard to limit the residual amount of acrylic acid monomers in super absorbent resins. The revised new standard GB / T 22875-2018 "Super absorbent resins for diapers and sanitary napkins" will be implemented on January 1, 2019, which stipulates that the residual monomer (acrylic acid) in baby diapers (sheets, pads) is ≤800mg / Kg; that is, ≤800ppm; the residual monomer (acrylic acid) in adult diapers (sheets, pads) is ≤1000mg / Kg, that is, ≤1000ppm; the residual monomer (acrylic acid) in sanitary napkins (pads) is ≤1000mg / Kg, that is, ≤1000ppm.

[0005] At present, the world's mature commercial acrylic acid-based superabsorbent resin manufacturing processes can be roughly divided into reverse suspension process and aqueous solution process.

[0006] The reverse suspension process uses Class A and Class B organic solvents as the continuous phase, which can obtain products with lower residual monomers. However, the manufacturing process safety, environmental protection control, and removal of product organic solvents are all very difficult and demanding.

[0007] The aqueous solution polymerization method is adopted by most manufacturers because there is no organic solvent involved in the polymerization system, so the process is safe and the environmental risk is low. However, the residual acrylic acid monomer in the product obtained by the aqueous solution polymerization method can also be removed by an organic solvent washing method. For example, patent CN 102702405B discloses a technology for reducing the residual acrylic acid monomer by washing with ethanol. The introduction of organic solvent ethanol will also increase the risk of organic solvent removal in production process safety, environmental protection and product quality.

[0008] In order to accelerate the participation of residual acrylic acid monomers in the polymerization during the aging period, some studies have adopted the method of adding additives such as alkyl peroxides and azo initiators during the polymerization reaction process. For example, patent CN 111116817B uses polymerization heat pyrolysis additives to generate free radicals to initiate the residual acrylic acid monomers to participate in the polymerization; there are also methods such as patent CN 108003270B that add photoinitiators to the polymerization liquid and continuously use ultraviolet irradiation during the aging stage of the polymerization reaction to reduce the residual acrylic acid monomers.

[0009] However, since the half-life of residual monomer reduction additives such as alkyl peroxides and azo compounds and photoinitiators under polymerization temperature rise conditions is 8-10 hours, reducing the efficiency of residual monomers is heavily dependent on excessive additives and heat preservation and aging time, as well as ultraviolet irradiation time during the aging period, resulting in low efficiency. Summary of the invention

[0010] Based on the above background problems, the present invention aims to provide a method for preparing an acrylic acid-based super absorbent resin with low residual monomers, wherein a residual monomer reducing agent is sprayed into a polyacrylic acid / sodium polyacrylate hydrogel for mixing and granulation, which can not only shorten the process cycle and improve production efficiency, but also control the residual monomers of the produced super absorbent resin to be below 100 ppm.

[0011] To achieve the above object, the technical solution provided by the embodiment of the present invention is:

[0012] The method for preparing an acrylic acid-based super absorbent resin with low residual monomers comprises the following steps:

[0013] Spraying a residue reducing agent on the polyacrylic acid / sodium polyacrylate hydrogel to perform mixing, granulation, drying, and crushing to obtain original particles, wherein the residue reducing agent is one or more of peroxide, persulfide, reductive chain transfer agent, and alcohol amine;

[0014] The original particles are sprayed with a surface crosslinking agent for surface crosslinking, and then an anti-caking agent is sprayed for anti-caking treatment to obtain an acrylic acid-based super absorbent resin with low residual monomers.

[0015] The peroxide and persulfide residual monomer reducing agent of the present invention can absorb the polymerization heat and neutralization heat stored in the hydrogel or be heated in the subsequent drying, and the initiator pyrolyzes to generate new free radicals, which trigger the residual acrylic acid monomer olefin bonds to participate in polymerization, thereby reducing the residual monomers.

[0016] The reducing chain transfer agent type residual monomer reduction agent of the present invention can carry out transfer reaction with macromolecular free radicals that have reached sufficient molecular weight but are still growing slowly, terminate the active chain, and terminate the inefficient and slow ripening chain growth; at the same time, the chain transfer agent itself generates new free radical active centers, which in turn continue to initiate the polymerization of acrylic acid monomers, thereby achieving the purpose of removing residual acrylic acid monomers.

[0017] The alcoholamine type residual monomer reducing agent of the present invention has a lone pair of electrons on the nitrogen atom, and the hydroxyl group has a certain alkalinity. The acrylic acid monomer and the alcoholamine are easy to undergo a condensation reaction to generate an amide polymer that is insoluble in water, and can also generate acrylic acid ethanolamine salt. The reaction consumes the acrylic acid monomer, thereby achieving the purpose of reducing residual monomers.

[0018] In one embodiment, the peroxide is hydrogen peroxide, and the persulfide is one or more of sodium persulfate, potassium persulfate, and ammonium persulfate.

[0019] In one embodiment, the added amount of the peroxide and persulfide is 0.01-0.2% of the mass of the reaction monomers.

[0020] In one embodiment, the reducing chain transfer agent is one or more of sodium hypophosphite, sodium formate, and sodium sulfite; the alcoholamine is one or more of ethanolamine, diethanolamine, and triethanolamine.

[0021] In one embodiment, the amount of the reducing chain transfer agent and the alcohol amine added is 0.1-3.0% of the mass of the reaction monomers.

[0022] The invention limits the addition amount of peroxide and persulfide to 0.01-0.2%, limits the addition amount of reducing chain transfer agent and alcohol amine to 0.1-3.0%, within the limited range, with the increase of the usage amount of the residue reducing agent, the removal rate of the monomer is improved; continuing to increase the usage amount will not only increase the cost, but also affect other properties of the water-absorbing resin.

[0023] In one embodiment, the surface crosslinking agent is a mixture of a propylene glycol aqueous solution and an ethylene glycol diglycidyl ether aqueous solution, the surface crosslinking temperature is 100-150° C., and the surface crosslinking time is 20-40 min.

[0024] In one embodiment, the anti-caking agent is a sodium aluminum sulfate aqueous solution and silicon dioxide powder, the anti-caking treatment temperature is 110-130° C., and the treatment time is 3-5 min.

[0025] In one embodiment, the polyacrylic acid / sodium polyacrylate hydrogel is synthesized by the following steps:

[0026] The acrylic acid monomer aqueous solution or the acrylic acid-sodium acrylate monomer aqueous solution is mixed with an internal crosslinking agent and an initiator, and a free radical polymerization reaction is carried out at 15-40° C., and the mixture is kept warm and aged for 10 minutes to 8 hours to obtain a polymer colloid;

[0027] The polymer colloid is added into alkaline solution and neutralized to a neutralization degree of 65-80% to obtain the polyacrylic acid / sodium polyacrylate hydrogel.

[0028] In one embodiment, the mass concentrations of the acrylic acid monomer aqueous solution and the acrylic acid-sodium acrylate monomer aqueous solution are both 25-35%, and the neutralization degree of the acrylic acid-sodium acrylate monomer aqueous solution is ≦20%.

[0029] In one embodiment, the internal crosslinking agent is one or more of acrylate, methacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triethyl, pentaerythritol triacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hexaacrylate, diethylene glycol dimethacrylate, polyethylene glycol diacrylate, and the amount of the internal crosslinking agent added is 0.25-0.35% of the mass of the reaction monomer;

[0030] The initiator is an oxidant-reducing agent system, the oxidant is one or more of hydrogen peroxide, sodium persulfate, potassium persulfate, and ammonium persulfate, and the added amount of the oxidant is 0.0015-0.15% of the mass of the reaction monomer; the reducing agent is one or both of VC and sodium VC, and the added amount of the reducing agent is 0.005-0.05% of the mass of the reaction monomer.

[0031] Compared with the prior art, the embodiments of the present invention have at least the following effects:

[0032] The present invention sprays a residual monomer reducing agent into a polyacrylic acid / sodium polyacrylate hydrogel for mixing and granulation. The residual monomer reducing agent is one or more of peroxide, persulfide, reductive chain transfer agent and alcohol amine. No residual monomer reducing auxiliary agents such as alkyl peroxide and azo and residual monomer reducing photoinitiator are added. Therefore, the half-life of the agent is not subject to the dependence of the aging insulation period and the ultraviolet irradiation aging period. The process cycle can be shortened, the production efficiency can be improved, and the residual monomer of the output super absorbent resin can be controlled below 100 ppm, and the residual monomer is far lower than the index of 800 ppm and 1000 ppm. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described in combination with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0034] In order to solve the problems of long process cycle and the like of existing methods of adding alkyl peroxides, azo-type residual monomer reducing agents and residual monomer reducing photoinitiators, the embodiment of the present invention provides a method for preparing an acrylic acid-based super absorbent resin with low residual monomers, wherein a residual monomer reducing agent is sprayed into a polyacrylic acid / sodium polyacrylate hydrogel for mixing and granulation, wherein the residual monomer reducing agent is one or more of peroxides, persulfides, reductive chain transfer agents, and alcohol amines, and there is no need to add alkyl peroxides, azo-type residual monomer reducing agents and residual monomer reducing photoinitiators, and thus there is no need to be restricted by the dependence of the agent half-life on the aging insulation and ultraviolet irradiation aging period, thereby shortening the process cycle and improving production efficiency.

[0035] Next, the technical solution of the present invention is described in detail through specific embodiments.

[0036] Example 1

[0037] The method for preparing an acrylic acid-based super absorbent resin with low residual monomers comprises the following steps:

[0038] (1) 20000 g of acrylic acid, 43060 g of deionized water, and 60 g of an internal crosslinking agent, pentaerythritol triethyl ester, were added to an enamel reactor with insulation, and high-purity nitrogen was introduced for deoxygenation and stirring for 20 min. The electronic thermometer was observed. When the material temperature was about 20° C., initiators were added under nitrogen stirring: 82.50 g of a 2.5% sodium VC aqueous solution and 160 g of a 2% sodium persulfate aqueous solution;

[0039] The temperature of the mixed liquid begins to rise, and the liquid gradually becomes viscous. Turn off the nitrogen, and wait until the temperature rises to the highest point (the peak value is 90-100°C, and the experimental observation is 98.1°C). After keeping the temperature for 1.5 hours, take out the polymerized colloid;

[0040] (2) using a small meat grinder to cut the polymer colloid into granular strips, spraying 16540 g of a 48.5% sodium hydroxide neutralizing solution onto the chopped granular strips; then mixing, extruding, and granulating to obtain a hydrogel with a total neutralization degree of about 72%;

[0041] (3) weighing 2000 g of the hydrogel prepared in step (2), spraying 10 g of a 1.5% sodium persulfate aqueous solution on the hydrogel for mixing and granulation, and placing the chopped rubber particles on a stainless steel screen, and then placing the stainless steel screen in a dehumidified hot air oven at a temperature set to 150-160° C. for dehumidification and drying, and after the rubber particles are dried and reduced to a water content of less than 3%, taking out the rubber particles and crushing them with a pulverizer to obtain original particles;

[0042] It should be noted that the amount of sodium persulfate added in this embodiment is 0.03% of the mass of the acrylic acid monomer. In this embodiment, 20,000 g of acrylic acid can be used to prepare about 79,900 g of hydrogel with a neutralization degree of 72%. On average, about 500 g of acrylic acid is used per 2,000 g of hydrogel. Therefore, the amount of sodium persulfate added is (10×1.5%) / 500=0.03%.

[0043] (4) taking the original particles prepared in step (3), spraying a surface crosslinking agent on the original particles, wherein the surface crosslinking agent is a mixture of 1.9 g of a 35% propylene glycol aqueous solution and 0.25 g of a 10% ethylene glycol diglycidyl ether aqueous solution, and then placing the particles in a constant temperature oven at 120° C. for heating for 30 min to perform surface crosslinking;

[0044] Then take it out and spray it with an anti-caking agent, which is formed by mixing 0.5g of fumed silica powder into 5g of 18% sodium aluminum sulfate aqueous solution, and put it into a constant temperature oven at 120°C for 3-5 minutes. After cooling, the finished product of the super absorbent resin can be obtained.

[0045] Example 2

[0046] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that, in step (3), 15 g of a 1.5% sodium persulfate aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, i.e., the amount of sodium persulfate added in this embodiment is 0.045% of the mass of the acrylic acid monomer.

[0047] Example 3

[0048] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that in step (3), 15 g of a 15% sodium hypophosphite aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, that is, the amount of sodium hypophosphite added in this embodiment is 0.45% of the mass of the acrylic acid monomer.

[0049] Example 4

[0050] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that in step (3), 30 g of a 15% sodium hypophosphite aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, that is, the amount of sodium hypophosphite added in this embodiment is 0.9% of the mass of the acrylic acid monomer.

[0051] Example 5

[0052] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that, in step (3), 20 g of a 20% mass concentration ethanolamine aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, that is, the amount of ethanolamine added in this embodiment is 0.8% of the mass of the acrylic acid monomer.

[0053] Example 6

[0054] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that, in step (3), 35 g of a 20% mass concentration ethanolamine aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, that is, the amount of ethanolamine added in this embodiment is 1.4% of the mass of the acrylic acid monomer.

[0055] Example 7

[0056] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that in step (3), 10 g of a 1.5% sodium persulfate aqueous solution and 20 g of a 15% sodium hypophosphite aqueous solution are sprayed onto 2000 g of the hydrogel for mixing and granulation.

[0057] Example 8

[0058] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 1 in that, in step (3), 6 g of a 1.5% sodium persulfate aqueous solution, 15 g of a 15% sodium sulfite aqueous solution, and 15 g of a 20% ethanolamine aqueous solution are sprayed onto 2000 g of the hydrogel for mixing and granulation.

[0059] Examples 1-8 are all preparation methods using a closed insulated enamel kettle as a reactor and a low-dose initiator deoxygenated with nitrogen to initiate a mild reaction. The residual acrylic acid monomer value, absorption rate, absorption amount, centrifugal water retention, and 0.3 PSI pressurized absorption of the super absorbent resin prepared in Examples 1-8 were tested, and the test methods of each parameter are as follows:

[0060] 1. Residual acrylic acid monomer value

[0061] According to the determination method in the Appendix (Normative Appendix) of GB / T 22875-2018 "Superabsorbent Resins for Diapers and Sanitary Napkins", the specific test method is as follows:

[0062] Weigh 1.000±0.005g of the sample and pour it into a beaker, then add 200mL of 0.9% saline, put in a stirring rotor and cover the beaker with a watch glass or paraffin film; then place the beaker on a magnetic stirrer and stir at a rate of 250±50r / min for 1h, then filter the solution with a membrane filter, save the filtrate as the test solution, and finally determine it with high performance liquid chromatography (HPLC). The test conditions for high performance liquid chromatography analysis are:

[0063] —Mobile phase: acetonitrile: phosphoric acid aqueous solution = 10:90

[0064] —Flow rate: 1.0mL / min

[0065] —Injection volume: 20μL

[0066] —Detector: UV wavelength 210nm

[0067] The test solution was injected into the HPLC for testing and the peak area was calculated. Two parallel tests were performed.

[0068] According to the peak area of ​​the test solution and the standard working curve of acrylic acid concentration, the residual monomer (acrylic acid) content in the sample is calculated according to formula (1), in milligrams per kilogram (mg / kg):

[0069] Residual monomer (acrylic acid) A = (c / m) × 200 Formula (1)

[0070] In formula (1), c is the concentration of acrylic acid in the sample solution, in milligrams per liter (mg / L), and m is the mass of the sample weighed, in grams (g).

[0071] The average of the two test results is taken as the test result, and the result is rounded to an integer.

[0072] 2. Absorption speed

[0073] According to the vortex method described in EDANA442.2-2002 adopted by the European and Japanese industries, the specific test method is as follows:

[0074] 50.0±1.0ml of 0.9% NaCl solution was measured and placed in a 100ml beaker, and a rotor was placed on a magnetic stirrer, and stirred at a speed of 600rpm; (2.000±0.001)g of the finished super absorbent resin was weighed and poured into the vortex all at once, and the timing was started after the addition; while the sample absorbed the physiological saline, the vortex in the middle began to disappear, and the liquid level reached the horizontal state after the vortex disappeared, which was taken as the end point, and the time required to reach the end point (unit: seconds) was measured, and each sample was tested twice in parallel, and the arithmetic mean was the measurement result.

[0075] 3. Absorption (absorption rate)

[0076] According to the determination method in the Appendix (Normative Appendix) of GB / T 22875-2018 "Superabsorbent Resins for Diapers and Sanitary Napkins", the specific test method is as follows:

[0077] Weigh (2.000±0.001) g of the finished product of super absorbent resin, pour into a test standard tea bag and drop it to the bottom, seal the tea bag and soak it in a 25±2°C, 0.9% NaCl aqueous solution to absorb liquid for 30 minutes, take out the tea bag and hang it dripping for 10 minutes before weighing it, subtract the weight of the blank group tea bag not filled with super absorbent resin from the obtained value, and then divide it by the weight of the super absorbent resin before soaking to get the absorption value.

[0078] 4. Centrifugal water retention

[0079] According to the determination method in the Appendix (Normative Appendix) of GB / T 22875-2018 "Superabsorbent Resins for Diapers and Sanitary Napkins", the specific test method is as follows:

[0080] Place the tea bag after the absorption amount is measured in a centrifuge with a centrifugal force of 250g, spin dry and remove liquid for 3 minutes, then take it out and weigh it. Subtract the mass of the wet empty tea bag after centrifugation from the obtained value, and then divide it by the weight of the super absorbent resin before soaking to get the centrifugal water retention value.

[0081] 5. Pressurized absorption capacity

[0082] Weigh (0.900 ± 0.001) g of the finished super absorbent resin and place it in a standard cylindrical test column with a sieve bottom. Place a weight on the powder and apply 0.3 PSI (i.e. 2068 Pa or 21.099 g / cm 2 ) Allow the super absorbent resin powder to absorb 0.9 wt % NaCl aqueous solution for 60 min, and divide the value obtained after measuring the liquid absorption weight by the weight of the super absorbent resin powder to obtain the compressive absorption weight value.

[0083] The test results are shown in Table 1:

[0084] Table 1 Test results of various indicators of super absorbent resins of Examples 1-8

[0085]

[0086]

[0087] By comparing Examples 1-8 and the control group, it can be seen that the residual amount of acrylic acid monomer can be significantly reduced by spraying the residual monomer reducing agent of the present invention, and the polymerization heat preservation and aging time can be shortened; in addition, by comparing the residual amount of acrylic acid monomer in the intermediate original particles and the finished super absorbent resin, it can be seen that the present invention can further reduce the residual acrylic acid monomer through surface cross-linking treatment and anti-caking treatment.

[0088] In addition, by comparing Examples 1 and 2, Examples 3 and 4, and Examples 5 and 6, it can be seen that the reduction range of residual monomers can be controlled by the dosage of the residual monomer reduction agent, which can reach below 100 ppm, and when other conditions remain unchanged, as the dosage of the residual monomer reduction agent increases, the effect of removing residual monomers is better.

[0089] In addition, it can be seen from Table 1 that the super absorbent resin prepared by the method of the present invention can meet and exceed the product technical index requirements of GB / T22875-2018 "Super absorbent resin for diapers and sanitary napkins".

[0090] Example 9

[0091] The method for preparing an acrylic acid-based super absorbent resin with low residual monomers comprises the following steps:

[0092] (1) Add 11132 g of deionized water to a batching tank with cooling, then start the stirrer, add 5000 g of acrylic acid and mix evenly, insert an electronic thermometer, and add 1145 g of a 48.5% NaOH aqueous solution dropwise while stirring, and control the liquid temperature to always be 15-25° C. to prepare a 20% neutralization degree of acrylic acid-sodium acrylate mixed monomer; add 13 g of a crosslinking agent, trimethylolpropane triacrylate, to the mixed monomer and stir evenly; put the prepared mixed solution into a reactor PP plate, and add initiators in a stirring state: 60 g of a 2.5% mass concentration of sodium VC aqueous solution and 400 g of a 2% mass concentration of sodium persulfate aqueous solution; the temperature of the mixed solution begins to rise, and the stirring is stopped. The liquid gradually becomes viscous. After the temperature rises to the highest point and shows a downward trend, it is observed for 10 minutes as a basis for judgment (the peak value is between 90 and 100° C., and the experimental observation is 95.3° C.); then a PAA-Na hydrogel with a neutralization degree of 20% is obtained;

[0093] (2) The PAA-Na hydrogel was chopped into pellets using a small meat grinder, and 2975 g of a 48.5% sodium hydroxide neutralizing solution was sprayed on the chopped pellets for neutralization and mixing, and then extruded and granulated to obtain a hydrogel with a total neutralization degree of about 72%;

[0094] (3) weighing 2000 g of the hydrogel prepared in step (2), spraying 10 g of a 1.5% sodium persulfate aqueous solution on the hydrogel for mixing and granulation, and placing the chopped rubber particles on a stainless steel screen, and then placing the stainless steel screen in a dehumidified hot air oven at a temperature set to 130-160° C. for dehumidification and drying, and when the rubber particles are dried and reduced to a water content of less than 3%, taking out the rubber particles and crushing them with a pulverizer to obtain original particles;

[0095] (4) taking the original particles prepared in step (3), spraying a surface crosslinking agent on the original particles, wherein the surface crosslinking agent is a mixture of 1.9 g of a 35% propylene glycol aqueous solution and 0.25 g of a 10% ethylene glycol diglycidyl ether aqueous solution, and then placing the particles in a constant temperature oven at 120° C. for heating for 30 min to perform surface crosslinking;

[0096] Then take it out and spray it with an anti-caking agent, which is formed by mixing 0.5g of fumed silica powder into 5g of 18% sodium aluminum sulfate aqueous solution, and put it into a constant temperature oven at 120°C for 3-5 minutes. After cooling, the finished product of the super absorbent resin can be obtained.

[0097] Example 10

[0098] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 9 in that in step (3), 20 g of a 15% sodium hypophosphite aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, that is, the amount of sodium hypophosphite added in this embodiment is 0.6% of the mass of the acrylic acid monomer.

[0099] Embodiment 11

[0100] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 9 in that in step (3), 25 g of a 20% mass concentration ethanolamine aqueous solution is sprayed onto 2000 g of the hydrogel for mixing and granulation, that is, the amount of ethanolamine added in this embodiment is 1% of the mass of the acrylic acid monomer.

[0101] Example 12

[0102] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 9 in that in step (3), 5 g of a 1.5% sodium persulfate aqueous solution and 15 g of a 15% sodium hypophosphite aqueous solution are sprayed onto 2000 g of the hydrogel for mixing and granulation.

[0103] Embodiment 13

[0104] The method for preparing a low residual monomer acrylic acid-based super absorbent resin is different from that of Example 9 in that in step (3), 4 g of a 1.5% sodium persulfate aqueous solution, 12 g of a 15% sodium sulfite aqueous solution, and 6 g of a 20% ethanolamine aqueous solution are sprayed onto 2000 g of the hydrogel for mixing and granulation.

[0105] Examples 9-13 all adopt a preparation method with a high-dose initiator initiation intensity without heat preservation and deoxidation by nitrogen intervention. The residual acrylic acid monomer value, absorption rate, absorption amount, centrifugal water retention, and 0.3PSI pressure absorption of the super absorbent resin prepared in Examples 9-13 were tested. The test methods of each parameter are shown above, and the test results are shown in Table 2:

[0106] Table 2 Test results of various indicators of super absorbent resin of Examples 9-13

[0107]

[0108]

[0109] By comparing Examples 9-13 and the control group, it can be seen that the residual amount of acrylic acid monomer can be significantly reduced by spraying the residual monomer reducing agent of the present invention. Similarly, the reduction range of residual monomer can be controlled by adjusting the addition amount of the residual monomer reducing agent.

[0110] It should be pointed out that a person skilled in the art can make several modifications and improvements without departing from the inventive concept of the present invention, and these modifications and improvements all fall within the protection scope of the present invention.

Claims

1. A method for preparing a low residual monomer acrylic acid-based super absorbent resin, characterized in that: The following steps are involved: Spraying a residue reducing agent on polyacrylic acid / sodium polyacrylate hydrogel to perform mixing, granulation, drying, and crushing to obtain original particles, wherein the residue reducing agent is a reducing chain transfer agent and / or an alcohol amine; The original particles are sprayed with a surface crosslinking agent for surface crosslinking, and then an anti-caking agent is sprayed for anti-caking treatment to obtain an acrylic acid-based super absorbent resin with low residual monomers; The reductive chain transfer agent is sodium hypophosphite or sodium formate; the alcoholamine is one or more of ethanolamine, diethanolamine, and triethanolamine; The amount of the reducing chain transfer agent and the alcohol amine added is 0.1-3.0% of the mass of the reaction monomers; The surface cross-linking agent is a mixture of a propylene glycol aqueous solution and an ethylene glycol diglycidyl ether aqueous solution; The anti-caking agent is sodium aluminum sulfate aqueous solution and silicon dioxide powder.

2. The method for preparing the acrylic acid-based super absorbent resin with low residual monomers according to claim 1, characterized in that: The surface cross-linking temperature is 100-150°C, and the surface cross-linking time is 20-40min.

3. The method for preparing the acrylic acid-based super absorbent resin with low residual monomers according to claim 1, characterized in that: The anti-knot treatment temperature is 110-130℃ and the treatment time is 3-5min.

4. The method for preparing the acrylic acid-based super absorbent resin with low residual monomers according to claim 1, characterized in that: The polyacrylic acid / sodium polyacrylate hydrogel is synthesized by the following steps: The acrylic acid monomer aqueous solution or the acrylic acid-sodium acrylate monomer aqueous solution is mixed with an internal crosslinking agent and an initiator, and a free radical polymerization reaction is carried out at 15-40° C., and the mixture is kept warm and aged for 10 minutes to 8 hours to obtain a polymer colloid; Adding the polymerized colloid into an alkali solution and neutralizing it to a neutralization degree of 65-80% to obtain a polyacrylic acid / sodium polyacrylate hydrogel; The internal crosslinking agent is one or more of acrylate, methacrylate, trimethylolpropane triacrylate, trimethylolpropane trimethacrylate, pentaerythritol triethyl, pentaerythritol triacrylate, pentaerythritol tetramethacrylate, dipentaerythritol hexaacrylate, diethylene glycol dimethacrylate, and polyethylene glycol diacrylate.

5. The method for preparing the acrylic acid-based super absorbent resin with low residual monomers according to claim 4, characterized in that: The mass concentrations of the acrylic acid monomer aqueous solution and the acrylic acid-sodium acrylate monomer aqueous solution are both 25-35%, and the neutralization degree of the acrylic acid-sodium acrylate monomer aqueous solution is ≤20%.

6. The method for preparing the acrylic acid-based super absorbent resin with low residual monomers according to claim 4, characterized in that: The amount of the internal crosslinking agent added is 0.25-0.35% of the mass of the reaction monomer; The initiator is an oxidant-reducing agent system, the oxidant is one or more of hydrogen peroxide, sodium persulfate, potassium persulfate, and ammonium persulfate, and the added amount of the oxidant is 0.0015-0.15% of the mass of the reaction monomer; the reducing agent is one or both of VC and sodium VC, and the added amount of the reducing agent is 0.005-0.05% of the mass of the reaction monomer.

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