DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides polyacrylate super absorbent polymer (SAP) particles with reduced extractables and residual monomers, and method of production thereof.

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 thereof. 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). These SAP particles, typically exhibit high extractable and residual monomers, which is disadvantageous when such SAP particles are used for production of articles.
[0005] Accordingly, there remains a need in the state of art for polyacrylate superabsorbent polymer (SAP) particles that exhibits reduced extractables and residual monomers. Need is also felt of method of production of polyacrylate superabsorbent polymer (SAP) particles that exhibits reduced extractables and residual monomers.

OBJECTS OF THE INVENTION
[0006] An object of the present disclosure is to provide polyacrylate superabsorbent polymer (SAP) particles that exhibit reduced extractables and residual monomers.
[0007] Another object of the present disclosure is to provide a method of production of polyacrylate superabsorbent polymer (SAP) particles that exhibit reduced extractables and residual monomers.
[0008] Yet another object of the present invention is to provide a process that is technically and commercially feasible.
[0009] Other objects of the present invention will be apparent from the description of the invention herein below.

SUMMARY
[0010] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides polyacrylate super absorbent polymer (SAP) particles with reduced extractables and residual monomers, and method of production thereof.
[0011] An aspect of the present disclosure provide polyacrylate super absorbent polymer (SAP) particles that exhibit (i) extractables below 4.5% by weight of the SAP particles, and (ii) residual monomers below 325 ppm. In an embodiment, the SAP particles exhibit extractables below 4.0% by weight of the SAP particles. In an embodiment, the SAP particles exhibit residual monomers (RM) below 300 ppm. In an embodiment, the SAP particles exhibit centrifuge retention capacity (CRC) of at least 24 g/g. In an embodiment, the SAP particles exhibit absorbency under a load (AUL) of at least 20 g/g (AUL0.7).
[0012] Another aspect of the present disclosure provides a method for production of polyacrylate superabsorbent polymer (SAP) particles, the method including the steps of: taking acrylic acid monomer; contacting acrylic acid with an internal cross-linking agent; effecting addition of a polymerization initiator to produce polyacrylate superabsorbent polymer; drying the polyacrylate superabsorbent polymer; and subjecting the dried polyacrylate superabsorbent polymer to size reduction to produce SAP particles, wherein the internal cross-linking agent comprises ethoxylated TMPTA with average molecular weight of 912, and wherein the method affords production of polyacrylate super absorbent polymer (SAP) particles that exhibit (i) extractables below 4.5% by weight of the SAP particles, and (ii) residual monomers below 325 ppm. In an embodiment, the acrylic acid monomer is neutralized before the step of contacting the acrylic acid monomer with an internal cross-linking agent. In an embodiment, concentration of said internal cross-linking agent ranges from 2000 ppm to 15000 ppm by weight of the acrylic acid monomer. In an embodiment, the SAP particles exhibit extractables below 4.0% by weight of the SAP particles. In an embodiment, the SAP particles exhibit residual monomers (RM) below 300 ppm. In an embodiment, the SAP particles exhibit centrifuge retention capacity (CRC) of at least 24 g/g. In an embodiment, the SAP particles exhibit absorbency under a load (AUL) of at least 20 g/g (AUL0.7). In an embodiment, the method further includes a step of surface crosslinking. In an embodiment, the step of effecting surface crosslinking of said SAP particles includes treating the SAP particles with a surface cross-linker composition. In an embodiment, the surface cross-linker composition comprises a surface cross-linker in a concentration ranging from 0.1% to 5% by dry weight of the SAP particles. In an embodiment, the surface cross-linker is selected from any or a combination of cyclic carbonate esters (for example, ethylene carbonate, propylene carbonate), di-functional alcohols (for example, ethylene glycol, propylene glycol), tri-functional alcohols (for example, glycerol) or combination thereof. In an embodiment, the surface cross-linker is an in-situ product generated by reaction between glycerol and ethylene carbonate at a temperature ranging from 150oC to 200oC. In an embodiment, the surface cross-linker composition includes less than 10% of water by dry weight of the SAP particles. In an embodiment, the surface cross-linker composition includes less than 5% of water by dry weight of the SAP particles.

DETAILED DESCRIPTION
[0013] 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.
[0014] 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.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] 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 (Centrifugal retention capacity) at least about 20 times their weight, or at least about 25 times their weight, or at least about 35 times their weight in an aqueous solution containing 0.9 weight percent sodium chloride.
[0019] The term “acrylic acid” or “acrylic acid monomers” 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.
[0020] The term “residual monomer content” or “RM content” as used herein synonymously and interchangeably throughout the present disclosure denotes the amount of monomers that are remained in the SAP. Specifically, it is an amount (in ppm) obtained by adding 1.000 g of SAP to 200 mL of 0.9% by weight sodium chloride aqueous solution, stirring for 1 hour, and then measuring the amount of residual monomer dissolved into the solution by high performance liquid chromatography.
[0021] The term “extractables” as used herein throughout the present disclosure denotes the extractables of the SAP particles as determined by the EDANA (European Disposables and Nonwovens Association) recommended test method No. 470.2-02 “Extractable”.
[0022] 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 (pore size <25µm) and soaked with 0.9% NaCl solution up to 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 or 0.7 psi depending upon the load used and C is the actual weight of SAP sample in grams.
[0023] 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 centrifugal acceleration of 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.
[0024] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure provides polyacrylate super absorbent polymer (SAP) particles with reduced extractables and residual monomers, and method of production thereof.
[0025] An aspect of the present disclosure provide polyacrylate super absorbent polymer (SAP) particles that exhibit (i) extractables below 4.5% by weight of the SAP particles, and (ii) residual monomers below 325 ppm. In an embodiment, the SAP particles exhibit extractables below 4.0% by weight of the SAP particles. In an embodiment, the SAP particles exhibit residual monomers (RM) below 300 ppm. In an embodiment, the SAP particles exhibit centrifuge retention capacity (CRC) of at least 24 g/g. In an embodiment, the SAP particles exhibit absorbency under a load (AUL) of at least 20 g/g (AUL0.7).
[0026] Another aspect of the present disclosure provides a method for production of polyacrylate superabsorbent polymer (SAP) particles, the method including the steps of: taking acrylic acid monomer; contacting acrylic acid with an internal cross-linking agent; effecting addition of a polymerization initiator to produce polyacrylate superabsorbent polymer; drying the polyacrylate superabsorbent polymer; and subjecting the dried polyacrylate superabsorbent polymer to size reduction to produce SAP particles, wherein the internal cross-linking agent comprises ethoxylated TMPTA with average molecular weight of 912, and wherein the method affords production of polyacrylate super absorbent polymer (SAP) particles that exhibit (i) extractables below 4.5% by weight of the SAP particles, and (ii) residual monomers below 325 ppm. In an embodiment, the acrylic acid monomer is neutralized before the step of contacting the acrylic acid monomer with an internal cross-linking agent. In an embodiment, concentration of said internal cross-linking agent ranges from 2000 ppm to 15000 ppm by weight of the acrylic acid monomer. In an embodiment, the SAP particles exhibit extractables below 4.0% by weight of the SAP particles. In an embodiment, the SAP particles exhibit residual monomers (RM) below 300 ppm. In an embodiment, the SAP particles exhibit centrifuge retention capacity (CRC) of at least 24 g/g. In an embodiment, the SAP particles exhibit absorbency under a load (AUL) of at least 20 g/g (AUL0.7). In an embodiment, the method further includes a step of surface crosslinking. In an embodiment, the step of effecting surface crosslinking of said SAP particles includes treating the SAP particles with a surface cross-linker composition. In an embodiment, the surface cross-linker composition comprises a surface cross-linker in a concentration ranging from 0.1% to 5% by dry weight of the SAP particles. In an embodiment, the surface cross-linker is selected from any or a combination of cyclic carbonate esters (for example, ethylene carbonate, propylene carbonate), di-functional alcohols (for example, ethylene glycol, propylene glycol), tri-functional alcohols (for example, glycerol) or combination thereof. In an embodiment, the surface cross-linker is an in-situ product generated by reaction between glycerol and ethylene carbonate at a temperature ranging from 150oC to 200oC. In an embodiment, the surface cross-linker composition includes less than 10% of water by dry weight of the SAP particles. In an embodiment, the surface cross-linker composition includes less than 5% of water by dry weight of the SAP particles.
[0027] Ethoxylated TMPTA (Ethoxylated trimethylolpropane triacrylate) is available commercially with different average molecular weights. Structure of Ethoxylated TMPTA is shown below:

[0028] In the above structure, n + m + o = 14 for Ethoxylated TMPTA with average molecular weight of 912. Any commercially available ethoxylated TMPTA with an average molecular weight of 912 can be used in the present invention.
[0029] 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 internal cross-linking agent.
[0030] In an embodiment, the polymerization initiator includes a combination of one or more redox initiators and one or more thermal polymerization initiators.
[0031] In an embodiment, the one or more 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 known to persons skilled in the art.
[0032] In an embodiment, the one or more thermal polymerization initiator is selected from any or a combination of a class of inorganic peroxides and a class of t-alkyl peroxides. In an embodiment, the thermal polymerization initiator includes a combination of thermal polymerization initiator, wherein at least one thermal initiator has a decomposition temperature ranging from about 30 to 80 °C.
[0033] In an embodiment, the method further includes a step of surface crosslinking. In an embodiment, the step of effecting surface crosslinking of said SAP particles includes treating the SAP particles with a surface cross-linker composition. In an embodiment, the surface cross-linker composition comprises a surface cross-linker in a concentration ranging from 0.1% to 5% by dry weight of the SAP particles. In an embodiment, the surface cross-linker composition comprises a surface cross-linker in a concentration ranging from 0.1% to 1% by dry weight of the SAP particles. In an embodiment, the surface cross-linker is selected from any or a combination of cyclic carbonate esters (for example, ethylene carbonate, propylene carbonate), di-functional alcohols (for example, ethylene glycol, propylene glycol), tri-functional alcohols (for example, glycerol) or combination thereof. In an embodiment, the surface cross-linker is an in-situ product generated by reaction between glycerol and ethylene carbonate at a temperature ranging from 150oC to 200oC. In an embodiment, the surface cross-linker composition includes less than 10% of water by dry weight of the SAP particles. In an embodiment, the surface cross-linker composition includes less than 5% of water by dry weight of the SAP particles.
[0034] 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.
[0035] Production of polyacrylate SAP particles (Solution Polymerization Reaction)
[0036] 2 kg acrylic acid was neutralized with 3.4 kg of 50 % sodium hydroxide solution in a jacketed vessel circulated with chiller for cooling. Vessel was also equipped with an overhead stirrer and temperature sensor and nitrogen sparger. First, water and acrylic acid was added to the vessel circulated with chilled water. Then, 50% sodium hydroxide solution was added slowly to neutralize acrylic acid keeping the temperature below 25 °C to avoid self-polymerization. Subsequently, 30 g of ethoxylated TMPTA cross-linker (with desired molecular weight, as shown in Table 1 below) was added to the neutralized solution and the mixture was purged with nitrogen/argon for 0.5-1h with flow rate of 2-20L/min. 60 g of 1% potassium persulfate solution, 45g of 0.5% H2O2 solution and 35 g of 0.1% ascorbic acid solution were prepared. Sodium persulfate solution and H2O2 solution were premixed before addition to the preheated kneader reactor and ascorbic acid was injected from a separate nozzle to preclude pre-polymerization and chocking of feeding/supply-lines thereby. The aqueous monomer and TMPTA mixture was continuously fed into the reactor. Polymerization starts in the kneader reactor immediately, as the reactor was preheated at desired temperature of 80oC. The reactor was kept under nitrogen blanket for the whole reaction period and the polymer gel obtained was few mm in size due to chopping effect of the twin shaft. The polymer gel was immediately dried in hot air dryer at about180oC. Brittle cake obtained therefrom was then broken down into few mm size by a shredder and then subjected to milling and sieving to obtain particles with size ranging from150-850µm.
[0037] Surface crosslinking of SAP particles (SXL)
[0038] In a further step of surface cross-linking (SXL), 200 g of SAP particles prepared above were cross-linked with 0.4 g of ethylene carbonate (EC) and 8 g of water in plough shear mixer under controlled spraying and continuous agitation conditions. In the step of surface treatment, the wet SAP mixture is heated to 180oC for 20-25 min in a forced convection oven (Esco Isotherm model). The dry SAP particles were sorted through Retsch AS 200 vibratory sieve shaker as per the ISO 17190-3 method. Mean Particle Size of the resultant SAP particles was found to be 400-500 µm.
[0039] Table 1 below depicts properties of SAP particles prepared using different molecular weight ethoxylated TMPTA as internal crosslinker, wherein values provided against “base polymer” indicates properties of the SAP particles that were not subjected to surface crosslinking, and values provided against “SXL” indicates properties of the SAP particles that were subjected to surface crosslinking.

Internal Cross-linker Sample CRC
(g/g) AUL (0.7)
psi RM
(ppm) Extractable (%)
TMPTA-428 Base Polymer 37 7.8 558 9.3
SXL 31 20.7 876 7.4
TMPTA-692 Base Polymer 32 8.0 334 8.1
SXL 29 21.2 709 7.5
TMPTA-912 Base Polymer 28 10.4 312 4.3
SXL 25 21.5 277 3.8

[0040] Inventors of the present application surprisingly found that when ethoxylated TMPTA with molecular weight of about 912 is used as an internal crosslinker in solution polymerization reaction, it affords SAP particles with superior properties, as can be seen from Table 1 above.
[0041] 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
[0042] The present disclosure provides polyacrylate superabsorbent polymer (SAP) particles that exhibit reduced extractables and residual monomers.
[0043] The present disclosure provides a method of production of polyacrylate superabsorbent polymer (SAP) particles that exhibit reduced extractables and residual monomers.
[0044] The present disclosure provides a method of production of polyacrylate SAP that is economical.
[0045] The present disclosure provides a method that is technically and commercially feasible.
,CLAIMS:1. Polyacrylate super absorbent polymer (SAP) particles, wherein the polyacrylate super absorbent polymer (SAP) particles exhibit (i) extractables below 4.5% by weight of the SAP particles, and (ii) residual monomers below 325 ppm.
2. The polyacrylate super absorbent polymer (SAP) particles as claimed in claim 1, wherein the SAP particles exhibit extractables below 4.0% by weight of the SAP particles.
3. The polyacrylate super absorbent polymer (SAP) particles as claimed in claim 1, wherein the SAP particles exhibit residual monomers (RM) below 300 ppm.
4. The polyacrylate super absorbent polymer (SAP) particles as claimed in claim 1, wherein the SAP particles exhibit centrifuge retention capacity (CRC) of at least 24 g/g.
5. The polyacrylate super absorbent polymer (SAP) particles as claimed in claim 1, wherein the SAP particles exhibit absorbency under a load (AUL) of at least 20 g/g (AUL0.7).
6. A method for production of polyacrylate superabsorbent polymer (SAP) particles, the method comprising the steps of:
taking acrylic acid monomer;
contacting the acrylic acid monomer with an internal cross-linking agent;
effecting addition of a polymerization initiator to produce polyacrylate superabsorbent polymer;
drying the polyacrylate superabsorbent polymer; and
subjecting the dried polyacrylate superabsorbent polymer to size reduction to produce SAP particles,
wherein the internal cross-linking agent comprises ethoxylated TMPTA with average molecular weight of 912, and wherein the method affords production of polyacrylate super absorbent polymer (SAP) particles that exhibit (i) extractables below 4.5% by weight of the SAP particles, and (ii) residual monomers below 325 ppm.
7. The process as claimed in claim 6, wherein the acrylic acid monomer is neutralized before the step of contacting the acrylic acid monomer with an internal cross-linking agent.
8. The process as claimed in claim 6, wherein concentration of said internal cross-linking agent ranges from 2000 ppm to 15000 ppm by weight of the acrylic acid monomer.
9. The process as claimed in claim 6, wherein the SAP particles exhibit extractables below 4.0% by weight of the SAP particles.
10. The process as claimed in claim 6, wherein the SAP particles exhibit residual monomers (RM) below 300 ppm.
11. The process as claimed in claim 6, wherein the SAP particles exhibit centrifuge retention capacity (CRC) of at least 24 g/g.
12. The process as claimed in claim 6, wherein the SAP particles exhibit absorbency under a load (AUL) of at least 20 g/g (AUL0.7).
13. The process as claimed in claim 6, wherein the method further comprises a step of surface crosslinking, said step of effecting surface crosslinking of said SAP particles comprising treating the SAP particles with a surface cross-linker composition, wherein said surface cross-linker composition comprises a surface cross-linker in a concentration ranging from 0.1% to 5% by dry weight of the SAP particles.
14. The process as claimed in claim 13, wherein the surface cross-linker composition comprises less than 5% of water by dry weight of the SAP particles, and wherein the surface cross-linker is selected from any or a combination of cyclic carbonate esters, di-functional alcohols, and tri-functional alcohols.