DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides a method for production of polyacrylate SAP from inferior acrylic acid.

BACKGROUND
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] A superabsorbent polymer is a cross-linked partially neutralized polymer, including cross-linked polyacrylic acids, capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining the aqueous liquids under a certain pressure in accordance with the general definition of superabsorbent polymer. Superabsorbent polymer may be formed into particles, generally referred to as particulate superabsorbent polymer. The acronym SAP may be used in place of superabsorbent polymer, superabsorbent polymer composition, and particles hereof. A primary use of superabsorbent polymer and superabsorbent polymer compositions is in sanitary articles, such as babies' diapers, incontinence products, or sanitary towels. A comprehensive survey of superabsorbent polymers, and their use and manufacture, is given in F. L. Buchholz and A. T. Graham (editors) in “Modern Superabsorbent Polymer Technology,” Wiley-VCR, New York, 1998.
[0004] Typically, the polyacrylate superabsorbent polymers (SAPs) are made at an industrial scale by reacting vinyl carboxylic acid monomer or sodium salt of vinyl carboxylic acid with a cross-linker of different chain lengths to produce the polyacrylate superabsorbent polymers (SAPs).Vinyl carboxylic acid (acrylic acid) monomer, if not stored properly, tend to undergo Michael addition reaction to produce dimerized product, utilization of which for production of polyacrylate SAP results in SAP particles with increased residual monomer (RM) content, which may be disadvantageous for its utility in the end applications, such as in production of sanitary articles, like babies' diapers, incontinence products, or sanitary towels. To alleviate this technical problem, conventionally, only such acrylic acid monomeris used, in polymerization reaction, that has acrylic acid dimer content of less than 500 ppm, and in case the dimer content was found to be higher than 500 ppm, such batch is refused, resulting in significant economic loss.
[0005] Accordingly, there remained a long-felt need in the art to device a method of production of polyacrylate SAP that can make use of acrylic acid, irrespective of its dimer content, while keeping the residual monomer (RM) content of final/resultantpolyacrylate SAP within the acceptable limits.

OBJECTS OF THE INVENTION
[0006] An object of the present disclosure is to provide a method of production of polyacrylate SAP that can make use of acrylic acid, irrespective of its dimer content, while keeping the residual monomer (RM) content of final/resultantpolyacrylate SAP within the acceptable limits.
[0007] Another object of the present disclosure is to provide a method of production of polyacrylate SAP that aids in reduction of wastage, conserves energy and improves overall productivity.
[0008] Another object of the present disclosure is to provide a method of production of polyacrylate SAP that precludes/obviates, at least in part, requirement of stringent storage conditions of acrylic acid.
[0009] Further object of the present disclosure is to provide a method of production of polyacrylate SAP that is economical.
[0010] Still further object of the present disclosure is to provide a process that is technically and commercially feasible.
[0011] Other objects of the present invention will be apparent from the description of the invention herein below.

SUMMARY
[0012] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure providesa method for production of polyacrylate SAP from inferior acrylic acid.
[0013] The present disclosure is on a premise of surprising observations of inventors of the present application that when acrylic acid monomers are subjected to polymerization utilizing a combination of redox initiator, a first thermal polymerization initiator, and a second thermal polymerization initiator; and when controlled drying conditions are used, the method of production of polyacrylate superabsorbent polymer (SAP) is amenable to utilization of inferior acrylic acid (i.e. acrylic acid monomer that has dimer content as high as 5% by wt.) while keeping the residual monomer content of SAP below 500 ppm. Without wishing to be bound by the theory, it is believed that utilization of a second/additional thermal initiator having a decomposition temperature of more than about 120°C (apart from use of a redox initiator and a first thermal initiator) results in the reduction of the residual monomer content in the SAP. Typically, the first thermal initiator has a decomposition temperature of 50°C and above (but less than about 120°C), which generates free radicals when the polymerization reaction temperature reaches the decomposition temperature of the first thermal initiator. During the polymerization, the second thermal initiator remains intact. When the SAP is subjected to drying, wherein, typically, temperature of more than 150°C is utilized (for example, temperature ranging from about 170°C to about 200°C), the dimers break-down generating the monomers, the second thermal initiator gets decomposed generating the free radical aiding in further polymerization of the monomers (generated from the dimer breakage). Maintenance of low air flow rate during the initial drying phase may help in moisture retention in the SAP for a longer time period providing mobility to the monomers generated from the dimer breakage, which in turn accentuates the polymerization of monomers resulting in SAP with reduced content of residual monomers.
[0014] Accordingly, an aspect of the present disclosure provides a method for production of polyacrylate superabsorbent polymer (SAP) with residual monomer content of less than 500 ppm based on dry weight of the SAP, the method including the steps of: taking acrylic acid monomer; contacting acrylic acid with one or a combination of cross-linking agents; effecting addition of a redox initiator, a first thermal polymerization initiator, and a second thermal polymerization initiator to produce polyacrylate superabsorbent polymer; and drying the polyacrylate superabsorbent polymer, said step of drying comprising: (a) exposing the polyacrylate superabsorbent polymer (SAP) to a first temperature at a first air flow rate ranging from about 0.8 m/s to about 1.4 m/s for a first time period; and (b) exposing the polyacrylate superabsorbent polymer (SAP) from step (a) to a second temperature at a second air flow rate for a second time period, the second air flow rate being higher than the first air flow rate to produce the polyacrylate SAP with residual monomer content of less than 500 ppm.
[0015] In an embodiment, the first temperature ranges from about 170°C to about 200°C. In an embodiment, the first temperature ranges from about 175°C to about 185°C. In an embodiment, the first temperature is about 180°C. In an embodiment, the second temperature ranges from about 170°C to about 200°C. In an embodiment, the second temperature ranges from about 175°C to about 185°C. In an embodiment, the second temperature is about 180°C. In an embodiment, the first air flow rate ranges from about 1.0 m/s to about 1.4 m/s. In an embodiment, the first air flow rate is about 1.2 m/s. In an embodiment, the second air flow rate ranges from about 1.5 m/s to about 2.5 m/s. In an embodiment, the second air flow rate ranges from about 1.7 m/s to about 2.0 m/s. In an embodiment, the second air flow rate is about 1.9 m/s. In an embodiment, the first time period ranges from about 5 minutes to about 60 minutes. In an embodiment, the second time period ranges from about 5 minutes to about 60 minutes.
[0016] In an embodiment, the step of drying the polyacrylate superabsorbent polymer (SAP) comprises exposing the polyacrylate superabsorbent polymer (SAP) to a temperature ranging from 170°C to 200°C in a hot air dryer at a first air flow rate ranging from about 1.0 m/s to about 1.4 m/s for a first time period, followed by exposing the polyacrylate superabsorbent polymer (SAP) to a temperature ranging from 170°C to 200°C in the hot air dryer with air flow rate ranging from about 1.5 m/s to about 2.5 m/s for a second time period. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 400 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles. In an embodiment, the polyacrylate SAP particles are treated with a surface crosslinker. In an embodiment, the surface crosslinker is selected from cyclic carbonate esters, polyols, diglycidyl ethers and combination thereof.
[0017] In an embodiment, the acrylic acid monomer comprises any or a combination of acrylic acid, glacial acrylic acid, and salt of acrylic acid. In an embodiment, the acrylic acid monomer comprises glacial acrylic acid, wherein the glacial acrylic acid is neutralized before contacting with the one or a combination of cross-linking agents. In an embodiment, the one or a combination of cross-linking agents are selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinyliccross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent.
[0018] In an embodiment, the acrylic acid monomer comprises dimer content ranging from 500 ppm to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 1000 ppm to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 1% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 2% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 3% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 4% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content of about 3.5%. In an embodiment, the acrylic acid monomer comprises dimer content of about 4%. In an embodiment, the acrylic acid monomer comprises dimer content of about 3%. In an embodiment, the acrylic acid monomer comprises dimer content of about 2.5%. In an embodiment, the acrylic acid monomer comprises dimer content of about 5%.
[0019] In an embodiment, the redox initiator includes (a) a reducing agent, such as ascorbic acid, sodium ascorbate, sulfite or bisulfite of alkali metal, a sugar, an aldehyde or a primary or secondary alcohol, and (b) an oxidizing agent, such as hydrogen peroxide, an alkyl peroxide, like t-butyl hydroperoxide, benzoyl peroxide, 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; sodium peracetate; and other redox initiators as known to persons skilled in the art. In an embodiment, the first thermal polymerization initiator has a decomposition temperature ranging from about 30 °C to 80 °C. In an embodiment, the second thermal polymerization initiator has a decomposition temperature higher than about 120 °C. In an embodiment, thefirst thermal polymerization initiator is selected from class of inorganic peroxides. In an embodiment, thesecond thermal polymerization initiator is selected from class of t-alkyl peroxides.

DETAILED DESCRIPTION
[0020] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0021] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the “invention” may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the “invention” will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0022] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0023] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0024] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0025] The term “SAP” or “superabsorbent polymer” as used herein synonymously and interchangeably, throughout the present disclosure, denotes natural, semi-synthetic or synthetic polymeric materials that can absorb large amount of a liquid relative to its own mass. Such superabsorbent polymer (SAP) may be water-swellable, water-insoluble organic or inorganic materials including superabsorbent polymers and superabsorbent polymer compositions capable, under the most favorable conditions, of absorbing at least about 10 times their weight, or at least about 15 times their weight, or at least about 25 times their weight in an aqueous solution containing 0.9 weight percent sodium chloride.
[0026] The term “Absorption Under Load” or “AUL” as used herein synonymously and interchangeably, throughout the present disclosure, is a measure of the ability of a superabsorbent polymer to absorb fluid under an applied pressure and is stated as grams of liquid absorbed per gram weight of the sample (g/g) at an applied pressure. AUL is determined as follows: 0.9 g of weighed SAP sample is placed in a plastic cylinder having inner diameter of 6 cm and height of 5 cm with a nylon screen fabric (mesh size 400 mesh) at bottom. The SAP particles are uniformly dispersed and initial weight of the setup was measured (A). A Teflon made plastic plate with a metal piston is placed on test substance. Thereafter, the entire testing setup is placed on a ceramic filter plate (porosity=0) covered with a Whatman filter paper (porosity =25) and soaked with 0.9% NaCl solution upto top edge of filter plate and the SAP sample is allowed to absorb liquid for 60 minutes. Afterwards, the testing set up is slowly moved out of NaCl solution and gently remove the wet SAP sample which is stuck to the Teflon plate. The weight of the swollen testing setup without Teflon plate and metal piston was recorded (B). The gram amount of the NaCl solution that had been retained per gram of sample was calculated according to the following equation:
AUL=
Where AUL is in g/g at 0.3 psi and C is the actual weight of SAP sample in grams.
[0027] The term “Centrifuge Retention Capacity” or “CRC” as used herein synonymously and interchangeably, throughout the present disclosure, is the ability of the particulate superabsorbent polymer (SAP) to retain liquid therein after being saturated and subjected to centrifugation under controlled conditions and is stated as grams of liquid retained per gram weight of the sample (g/g). CRC testing may be conducted at an assigned testing temperature for an assigned period of testing time, noted as CRC(testing temperature, testing time). For example, CRC(23° C, 0.5 hr) refers to a CRC with a testing temperature of about 23°Cand a testing time of 0.5 hour. CRC of SAP samples is determined by standard method no. ISO 17190-6. 0.2 g of weighed SAP samples are placed in non-woven bags of dimension 6 cm X 8 cm and submerged in beaker containing 0.9% NaCl solution for half an hour. Empty bags are used as controls and treated in similar way. The bags are then placed in centrifuge basket and centrifuged at 250 g for 3 min. The bags are removed and weighed. CRC of the samples are measured by using following equation:
CRC = (w1 – w2 –w3) / w3
Where, w1 is the weight of wet sample and bag, w2 is the weight of empty bag and w3 is the weight of dry sample.
[0028] The term “acrylic acid” or “acrylic acid monomers” or “monomer” or “monomer composition” as used herein synonymously and interchangeably throughout the present disclosure denotes the acrylic acid monomers, glacial acrylic acid monomers, salt of acrylic acid monomers such as monovalent metal salts, divalent metal salts, ammonium salts or organic amine salts of acrylic acid and the like monomers with acrylate moiety that may find utility in preparation of the polymer, particularly, the polyacrylate polymers.
[0029] The term “inferior acrylic acid” or “inferior acrylic acid monomer” or “inferior acrylic acid monomer composition” as used herein synonymously and interchangeably throughout the present disclosure denotes the acrylic acid (or glacial acrylic acid or salt of acrylic acid) monomer that has a dimer content of more than 500 ppm, or more than about 1000 ppm, or more than about 1%, or more than about 2%, or more than about 3%, or more than about 4%, and generally falling with the range of about 1% to about 5%. A person skilled in the art would appreciate that commercially available solutions of acrylic acid monomer may contain dimer of acrylic acid due to Michael addition reaction of the acrylic acid monomers. The propensity of the Michael addition reaction increases with increase in the temperature to which the monomers are exposed, and hence, typically, the monomer solutions are stored at temperature below 25oC. This is particularly problematic for the temperate regions such as India, wherein either during the conveyance or during the storage, in case the stringent storage condition of temperature below 25oC is not maintained, the dimer content of the monomer increases. Utilization of such monomers (having increased dimer content) in the polymerization reaction for production of SAP results in SAP with increased residual acrylic acid monomer content (referred to as “RM content” herein).
[0030] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure providesa method for production of polyacrylate SAP from inferior acrylic acid.
[0031] The present disclosure is on a premise of surprising observations of inventors of the present application that when acrylic acid monomers are subjected to polymerization utilizing a combination of redox initiator, a first thermal polymerization initiator, and a second thermal polymerization initiator; and controlled drying conditions are used, the method of production of polyacrylate superabsorbent polymer (SAP) is amenable to utilization of inferior acrylic acid (i.e. acrylic acid monomer that has dimer content of more than 500 ppm) while keeping the residual monomer content of SAP below 1000 ppm, preferably below 500 ppm.
[0032] Without wishing to be bound by the theory, it is believed that utilization of a second/additional thermal initiator having a decomposition temperature of more than about 150°C (apart from use of a redox initiator and a first thermal initiator) results in the reduction of the residual monomer content in the SAP. Typically, the first thermal initiator has a decomposition temperature of 30°C and above (but less than about 80°C), which generates free radicals when the polymerization reaction temperature reaches the decomposition temperature of the first thermal initiator. During the polymerization, the second thermal initiator remains intact. When the SAP is subjected to drying, wherein, typically, temperature of more than 150°C is utilized (for example, temperature ranging from about 170°C to about 200°C), the dimers break-down generating the monomers, the second thermal initiator gets decomposed generating the free radical aiding in further polymerization of the monomers (generated from the dimer breakage). Maintenanceof low air flow rate during the initial drying phase may help in moisture retention in the SAP for a longer time period providing mobility to the monomers generated from the dimer breakage,which in turn accentuates the polymerization of monomers resulting in SAP with reduced content of residual monomers.
[0033] Accordingly, an aspect of the present disclosure provides a method for production of polyacrylate superabsorbent polymer (SAP) with residual monomer content of less than 1000 ppm, and preferably less than 500 ppm based on dry weight of the SAP, the method including the steps of: taking acrylic acid monomer; contacting acrylic acid with one or a combination of cross-linking agents; effecting addition of a redox initiator, a first thermal polymerization initiator, and a second thermal polymerization initiator to produce polyacrylate superabsorbent polymer; and drying the polyacrylate superabsorbent polymer to produce the polyacrylate SAP with residual monomer content of less than 500 ppm, said step of drying comprising: (a) exposing the polyacrylate superabsorbent polymer (SAP) to a first temperature at a first air flow rate ranging from about 0.8 m/s to about 1.4 m/s for a first time period, followed by exposing the polyacrylate superabsorbent polymer (SAP) from step (a) to a second temperature at a second air flow rate for a second time period, the second air flow rate being higher than the first air flow rate.
[0034] In an embodiment, the first temperature ranges from about 170°C to about 200°C.In an embodiment, the first temperature ranges from about 175°C to about 185°C. In an embodiment, the first temperature is about 180°C. In an embodiment, the second temperature ranges from about 170°C to about 200°C. In an embodiment, the second temperature ranges from about 175°C to about 185°C. In an embodiment, the second temperature is about 180°C. In an embodiment, the first air flow rate ranges from about 1.0 m/s to about 1.4 m/s. In an embodiment, the first air flow rate is about 1.2 m/s. In an embodiment, the second air flow rate ranges from about 1.5 m/s to about 2.5 m/s. In an embodiment, the second air flow rate ranges from about 1.7 m/s to about 2.0 m/s. In an embodiment, the second air flow rate is about 1.9 m/s. In an embodiment, the first time period ranges from about 5 minutes to about 60 minutes. In an embodiment, the second time period ranges from about 5 minutes to about 60 minutes.
[0035] In an embodiment, the step of drying the polyacrylate superabsorbent polymer (SAP) comprises exposing the polyacrylate superabsorbent polymer (SAP) to a temperature ranging from 170°C to 200°C in a hot air dryer at a first air flow rate ranging from about 1.0 m/s to about 1.4 m/s for a first time period, followed by exposing the polyacrylate superabsorbent polymer (SAP) to a temperature ranging from 170°C to 200°C in the hot air dryer with air flow rate ranging from about 1.5 m/s to about 2.5 m/s for a second time period.
[0036] In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 800 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 700 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual acrylic acid monomer content of less than 550 ppm based on dry weight of the SAP. In an embodiment, the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 400 ppm based on dry weight of the SAP.
[0037] In an embodiment, the polyacrylate superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles. In an embodiment, the polyacrylate SAP particles are treated with a surface crosslinker. In an embodiment, the surface crosslinker is selected from cyclic carbonate esters, polyols, diglycidyl ethers and combination thereof. In an embodiment, the surface crosslinker is selected from bi-functional or multi-functional molecules such as 1,3-dioxolan-2-one (ethylene carbonate), 4-methyl-1,3-dioxolan-2-one (propylene carbonate), 4,5-dimethyl-1,3-dioxolan-2-one, 4,4-dimethyl-1,3-dioxolan-2-one, 4-ethyl-1,3 dioxolan-2-one, 4-hydroxymethyl-1,3-dioxolan-2-one, 1,3 dioxan-2-one, 4-methyl-1,3-dioxan-2-one, 4,6-dimethyl-1, 3-dioxan-2-one and 1,3-dioxolan-2-one, propylene carbonate, diols or polyfunctional alcohols such as diethylene glycol, triethylene glycol, polyethylene glycol, glycerol, polyglycerol, propylene glycol, trimethylolpropane, pentaerythritol, polyvinyl alcohol, sorbitol, poly(glycidyl ethers) such as ethylene glycol diglycidyl ether, polyethylene glycol diglycidyl ether, glycerol diglycidyl ether, glycerol polyglycidyl ether, sorbitol polyglycidyl ether, polyglycerolpolyglycidyl ether, pentaerythritolpolyglycidyl ether, propylene glycol diglycidyl ether and polypropylene glycol diglycidyl ether, polyamines such as ethylenediamine, diethylenetriiamine, triethylenetetraiamine and polyethyleneimines, diethanolamine, triethanolamine, polyoxypropylene, oxyethylene-oxypropylene block copolymers, sorbitan fatty acid esters, polyoxyethylenesorbitan fatty acid esters or combination(s) thereof. Other suitable surface crosslinkers are polyvalent metal ions capable of forming ionic crosslinks. Examples are magnesium, calcium, barium and aluminium ions, added as hydroxides, carbonates or hydrogen carbonates.
[0038] In an embodiment, the acrylic acid monomer comprises any of acrylic acid, glacial acrylic acid, and salt of acrylic acid. In an embodiment, the acrylic acid monomer comprises glacial acrylic acid, wherein the glacial acrylic acid is neutralized before contacting with the one or a combination of cross-linking agents. In an embodiment, the one or a combination of cross-linking agents is selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinylic cross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent. Exemplary di-vinylic cross-linking agent includes 1,4-butanediol diacrylate, 1,4-butanediol, dimethacrylate, 1,3-butylene glycol diacrylate, 1,3-butylene glycol dimethacrylate, diethylene glycol diacrylate, diethylene glycol dimethacrylate, ethoxylatedbisphenol A diacrylate, ethoxylatedbisphenol A dimethacrylate, ethylene glycol dimethacrylate, 1,6 hexanedioldiacrylate, 1,6 hexanedioldimethacrylate, neopentylglycoldimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol diacrylate, polyethylene glycol dimethacrylate, triethylene glycol diacrylate, triethylene glycol dimethacrylate, tripropylene glycol diacrylate, tetraethylene glycol diacrylate, tetraethylene glycol dimethacrylate, N,N'-methylenebisacrylamide, N,N'-methylene bismethacrylamide, divinyladipate, and divinyl esters of a polycarboxylic acid, but not limited thereto. Exemplary tri-vinylic cross-linking agent includes pentaerythritol triacrylate, trimethylolpropanetriacrylate, Trimethylolpropane trimethacrylate, tris(2-hydroxyethyl) isocyanuratetriacrylate, and tris(2-hydroxyethyl) isocyanuratetrimethacrylate, but not limited thereto. Exemplary tetra-vinyliccross-linking agent includes pentaerythritoltetraacrylate and the likes. Exemplary penta-vinylic cross-linking agent includes dipentaerythritolpentaacrylate and the likes. Exemplary di or polyallylic cross-linking agent includes diallyl maleate, diallylfumarate, diallyl succinate, tetraallyl ammonium halides and the likes.
[0039] In an embodiment, the acrylic acid monomer comprises dimer content ranging from 500 ppm to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 1000 ppm to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 2% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 3% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content ranging from 4% to 5%. In an embodiment, the acrylic acid monomer comprises dimer content of about 3.5%. In an embodiment, the acrylic acid monomer comprises dimer content of about 4%.In an embodiment, the acrylic acid monomer comprises dimer content of about 3%. In an embodiment, the acrylic acid monomer comprises dimer content of about 2.5%. In an embodiment, the acrylic acid monomer comprises dimer content of about 5%.
[0040] In an embodiment, the redox initiator includes (a) a reducing agent, such as ascorbic acid, sodium ascorbate, isoascorbic acid, sulfinic acids, hydroxy alkyl sulfinic acids such as hydroxy methyl sulfinic acid, 2-hydroxy-2-sulfinacetic acid and its salts, formadine sulfinic acid and its salts, alkyl sulfinic acids such as propyl sulfinic acid and isopropyl sulfinic acid, aryl sulfinicacids such as phenyl sulfinic acid, a reducing sugar such as sorbose, fructose, glucose, lactose, mannose and derivatives thereof, ammonium or alkali metal hydrogensulfate, -sulfate, -thiosulfate, -hyposulfate or -sulfide, metal salts such as iron II ions or silver ions or sodium hydroxyl methyl sulfoxylate, reducing nitrogen compounds such as hydroxylamine, hydroxylamine hydrosulfate and hydroxylammonium salts, polyaminesand the likes, and (b) an oxidizing agent such as hydrogen peroxide, an alkyl peroxide, like t-butyl hydroperoxide, benzoyl peroxide, 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; sodium peracetate; and other redox initiators as known to persons skilled in the art.
[0041] In an embodiment, the first thermal polymerization initiator has a decomposition temperature ranging from about 30 °C to about 80 °C. In an embodiment, the second thermal polymerization initiator has a decomposition temperature higher than about 120°C. In an embodiment, the first thermal polymerization initiator is selected from class of inorganic peroxides such as sodium, potassium and ammonium peroxodisulfates. In an embodiment, the second thermal polymerization initiator is selected from class of t-alkyl peroxides such as tert-butyl hydroperoxide, di-tert-butyl peroxide, tert-butyl cumyl peroxide and tert-amyl hydroperoxide
[0042] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.
[0043] EXAMPLE 1
[0044] 180 g of glacial acrylic acid with dimer content of about 3% was taken in jacketed glass reactor and diluted with 313 g of water. For neutralization, 155.5 g of 50 wt% sodium hydroxide solution was added drop-wise and the temperature of the reaction mixture was maintained below 30°C. After completion of neutralization process, 1.6 g polyethylene diacrylate (Mn 500) (cross-linking agent) in 20 g of acrylic acid was added. The mixture was purged with nitrogen at 25°C for half an hour. Thereafter, 0.3 g sodium persulfate (first thermal polymerization initiator), 0.2 g t-butyl hydroperoxide (TBHP) (second thermal polymerization initiator), 0.29 g 30% H2O2 and 0.035 g sodium ascorbate (redox initiators) were added sequentially. The temperature of the reactor was raised to 70°C. The polyacrylate superabsorbent polymer (SAP) gel formation was observed immediately. It was kept for half an hour. Then gel was chopped to pieces of 1-3 cm in size. The chopped gel was dried in hot air dryer at 180°C at air flow rate of 1.2 m/s for 15 min followed by 1.9 m/s for 15 min. The dried SAP was ground and sieved and the particles in the range 150-800 micron were selected for analysis.
[0045] EXAMPLE 2
[0046] Superabsorbent polymer was prepared as in Example 1, except that the dimer content/ concentration in glacial acrylic acid was 1.4%.
[0047] EXAMPLE 3
[0048] Superabsorbent polymer was prepared as in Example 2, except that the amount of TBHP used was 0.4 g (instead of 0.2 g).
[0049] COMPARATIVE EXAMPLE 1
[0050] Superabsorbent polymer is prepared as in Example 1, except that TBHP was not added, and 0.5 g of sodium persulfate was used.
[0051] COMPARATIVE EXAMPLE 2
[0052] Superabsorbent polymer was prepared as in example 1, except utilization of different drying conditions, wherein the chopped gel was dried in hot air dryer at 180°C at an air flow rate of 1.9 m/s for 30 min.
[0053] COMPARATIVE EXAMPLE 3
[0054] Superabsorbent polymer was prepared as in example 1 except that TBHP was not added, and 0.5 g of sodium persulfate was used, and drying condition was same as in comparative example 2.
[0055] COMPARATIVE EXAMPLE 4
[0056] Superabsorbent polymer was prepared as in comparative example 1, except that the dimer content/concentration in glacial acrylic acid was 1.4%.
[0057] COMPARATIVE EXAMPLE 5
[0058] Superabsorbent polymer was prepared as in comparative example 3, except that dimer content/concentration in glacial acrylic acid was 1.4%.
[0059] Table 1 below provides details of the dimer content present in the glacial acrylic acid (monomer), the second thermal polymerization initiator, drying conditions and effect thereof on Centrifuge Retention Capacity (CRC) and residual monomer (RM) content of the resultant polyacrylate SAP.

Table 1: Effect of utilization of a pair of thermal polymerization initiators and drying conditions on the resultant polyacrylate SAP
Sample Dimer % TPI2* Heating condition CRC RM
Example 1 3 TBHP,
0.2 g 180°C, slow air flow rate followed by high 38.5 376
Comparative Example 1 3 Nil 180°C, slow air flow rate followed by high 37 1660
Comparative Example 2 3 TBHP,
0.2 g 180°C, high air flow rate of 1.9 m/s 43 1011
Comparative Example 3 3 Nil 180°C, high air flow rate of 1.9 m/s 38.6 2382
Example 2 1.4 TBHP,
0.2 g 180°C, slow air flow rate followed by high 39 227
Comparative Example 4 1.4 Nil 180°C, slow air flow rate followed by high 33 309
Comparative Example 5 1.4 Nil 180°C, high air flow rate 36 402
Example 3 1.4 TBHP,
0.4 g 180°C, slow air flow rate followed by high 34 182

*TPI2 denotes the second thermal polymerization initiator.
[0060] Based on the above table, it is crystal clear that when acrylic acid monomers are subjected to polymerization utilizing a combination of redox initiator, a first thermal polymerization initiator, and a second thermal polymerization initiator; and when controlled drying conditions are used, the method of production of polyacrylate superabsorbent polymer (SAP) is amenable to utilization of inferior acrylic acid (i.e. acrylic acid monomer having high dimer content, for example, 3% by wt.) while keeping the residual monomer content of SAP below 500 ppm.
[0061] Although the subject matter has been described herein with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein. Furthermore, precise and systematic details on all above aspects are currently being made. Work is still underway on this invention. It will be obvious to those skilled in the art to make various changes, modifications and alterations to the invention described herein. To the extent that these various changes, modifications and alterations do not depart from the scope of the present invention, they are intended to be encompassed therein.

ADVANTAGES
[0062] The present disclosure provides a method of production of polyacrylate SAP that can make use of acrylic acid, irrespective of its dimer content, while keeping the residual monomer (RM) content of final/resultantpolyacrylate SAP within the acceptable limits.
[0063] The present disclosure provides a method of production of polyacrylate SAP that aids in reduction of wastage, conserves energy and improves overall productivity.
[0064] The present disclosure provides a method of production of polyacrylate SAP that precludes/obviates, at least in part, requirement of stringent storage conditions of acrylic acid.
[0065] The present disclosure provides a method of production of polyacrylate SAP that is economical.
[0066] The present disclosure provides a method that is technically and commercially feasible.
,CLAIMS:1. A method for production of polyacrylate superabsorbent polymer (SAP) with residual monomer content of less than 500 ppm based on dry weight of the SAP, the method comprising the steps of:
taking acrylic acid monomer;
contacting the acrylic acid monomer with one or a combination of cross-linking agents;
effecting addition of a redox initiator, a first thermal polymerization initiator, and a second thermal polymerization initiator to produce polyacrylate superabsorbent polymer (SAP); and
drying the polyacrylate superabsorbent polymer to produce the polyacrylate SAP with residual monomer content of less than 500 ppm, the step of drying comprising:
(a) exposing the polyacrylate superabsorbent polymer (SAP) to a first temperature at a first air flow rate ranging from 0.8 m/s to 1.4 m/s for a first time period; and
(b) exposing the polyacrylate superabsorbent polymer (SAP) from step (a) to a second temperature at a second air flow rate for a second time period, the second air flow rate being higher than the first air flow rate.
2. The method as claimed in claim 1, wherein the acrylic acid monomer comprises any of: acrylic acid, glacial acrylic acid and a salt of acrylic acid.
3. The method as claimed in claim 1, wherein the acrylic acid monomer comprises glacial acrylic acid, and wherein the glacial acrylic acid is neutralized before contacting the glacial acrylic acid with the one or a combination of cross-linking agents, further wherein the glacial acrylic acid is neutralized from 50 mol% to 85 mol%.
4. The method as claimed in claim 1, wherein the one or a combination of cross-linking agents are selected from di-vinylic cross-linking agent, tri-vinylic cross-linking agent, tetra-vinylic cross-linking agent, di-allylic cross-linking agent, tri-allylic cross-linking agent and tetra-allylic cross-linking agent.
5. The method as claimed in claim 1, wherein the redox initiator comprises:
(a) a reducing agent selected from ascorbic acid, sodium ascorbate, sulfite of alkali metal, bisulfite of alkali metal, a sugar, an aldehyde, a primary alcohol and a secondary alcohol; and
(b) an oxidizing agent selected from hydrogen peroxide, an alkyl peroxide selected from t-butyl hydroperoxide, benzoyl peroxide, and 1,1-di-t-butylperoxy-3,3,5-trimethylcyclohexane; dicumyl peroxide; caprylyl peroxide; and sodium peracetate.
6. The method as claimed in claim 1, wherein the first thermal polymerization initiator has a decomposition temperature ranging from 30 °C to 80 °C, said first thermal polymerization initiator being one or a combination of inorganic peroxides, and wherein the second thermal polymerization initiator has a decomposition temperature higher than 120 °C, said second thermal polymerization initiator being one or a combination of t-alkyl peroxides.
7. The method as claimed in claim 1, wherein the first temperature ranges from 170°C to 200°C, the first air flow rate ranges from 1.0 m/s to 1.4 m/s and the first time period ranges from 5 minutes to 60 minutes, and wherein the second temperature ranges from 170°C to 200°C, the second air flow rate ranges from 1.5 m/s to 2.5 m/s and the second time period ranges from 5 minutes to 60 minutes.
8. The method as claimed in claim 1, wherein the first temperature ranges from 175°C to 185°C and the first air flow rate is about 1.2 m/s, and wherein the second temperature ranges from 175°C to 185°C and the second air flow rate is about 1.9 m/s.
9. The method as claimed in claim 1, wherein the acrylic acid monomer comprises dimer content ranging from 500 ppm to 5%, and wherein the polyacrylate superabsorbent polymer (SAP) has residual monomer content of less than 400 ppm based on dry weight of the SAP.
10. The method as claimed in claim 1, wherein the polyacrylate superabsorbent polymer (SAP) is milled to produce polyacrylate SAP particles, and wherein the polyacrylate SAP particles are treated with a surface crosslinker, said surface crosslinker selected from cyclic carbonate esters, polyols, diglycidyl ethers and combination thereof.