DESC:TECHNICAL FIELD
[0001] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure relates to a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles. An aspect of the present disclosure also provides surface crosslinked polyacrylate super absorbent polymer (SAP) particles with superior extractable properties while maintaining high CRC and high AUL.

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. For polymerization, radical initiators such as persulfate salt, hydrogen peroxide and organic peroxide are generally used. The resultant polymer gel is micronized and dried by using one or more drying equipments such as belt dryer, drum dryer, fluidized bed dryer etc. The resultant dried polymer is then crushed to the particle size ranging from 150 to 850 micron to get the required water absorption capacity. The resultant SAP particles are then tested in terms of its Absorption Under Load (AUL), Centrifuge Retention Capacity (CRC) and other properties for further finishing stages.
[0005] Accordingly, there remained a long felt need in the state of art for a method of crosslinking of polyacrylate super absorbent polymer (SAP) particles. Need was also felt of improved polyacrylate super absorbent polymer (SAP) particles that exhibit superior absorption rate and extractable properties while maintaining high CRC.

OBJECTS
[0006] An object of the present disclosure is to provide a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles that exhibit low extractables.
[0007] Another object of the present disclosure is to provide a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles that has superior absorption rate, high CRC and high AUL.
[0008] Another object of the present disclosure is to provide a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles that is economical and industrially applicable.
[0009] Further object of the present disclosure is to provide surface crosslinked polyacrylate super absorbent polymer (SAP) particles that has low extractables, superior absorption rate, high CRC and high AUL.

SUMMARY
[0010] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure relates to a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles. An aspect of the present disclosure also provides surface crosslinked polyacrylate super absorbent polymer (SAP) particles with superior absorption rate and extractable properties while maintaining high CRC.
[0011] The present disclosure is on the premise of surprising observation by inventors of the present disclosure that when polyacrylate super absorbent polymer (SAP) particles are surface crosslinked using a combination of ethylene carbonate and polyethylene glycol (PEG), the resultant surface crosslinked polyacrylate super absorbent polymer (SAP) particles exhibits superior AUL and extractable properties while maintaining high CRC of the SAP. Also surprising was the observation that when polyethylene glycol (PEG) having molecular weight ranging from 600 to 4000 is used, the improvement in AUL and extractable properties was more prominent as compared to polyethylene glycol (PEG) of higher molecular weights.
[0012] In stark contrast to the conventional belief/knowledge, it could surprisingly be noted that subjecting the treated polyacrylate super absorbent polymer (SAP) particles at a temperature less than about 175°C (preferably, ranging from 155°C to 170°C) for a time period ranging from 20 minutes to 120 minutes (preferably, ranging from 20 minutes to 90 minutes) affords surface crosslinked polyacrylate SAP particles that exhibit remarkably low extractables (e.g. less than about 8%).
[0013] Accordingly, an aspect of the present disclosure relates to a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles, said method comprising the steps of: treating polyacrylate super absorbent polymer (SAP) particles with a crosslinker solution to obtain treated polyacrylate super absorbent polymer (SAP) particles; and exposing the treated polyacrylate super absorbent polymer (SAP) particles to a temperature ranging from 155°C to 170°C for a time period ranging from 20 minutes to 90 minutes to obtain surface crosslinked polyacrylate super absorbent polymer (SAP) particles.
[0014] In an embodiment, the step of exposing the treated polyacrylate SAP particles comprises exposing the treated polyacrylate SAP particles at a temperature ranging from 155°C to 165°C for a time period ranging from 30 minutes to 90 minutes to obtain surface crosslinked polyacrylate SAP particles.
[0015] In an embodiment, the crosslinker solution comprises ethylene carbonate, polyethylene glycol (PEG) and water. In an embodiment, the polyethylene glycol (PEG) has a number average molecular weight (Mn) ranging from 600 to 4000. In an embodiment, the polyethylene glycol (PEG) has a number average molecular weight (Mn) ranging from 600 to 1500.
[0016] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount ranging from 1% to 10% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) in an amount ranging from 0.1% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount ranging from 1% to 10% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0017] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount ranging from 2% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) having a number average molecular weight (Mn) ranging from 600 to 1500 in an amount ranging from 0.1% to 1% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount ranging from 2% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0018] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount of about 3% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) having a number average molecular weight (Mn) ranging from 600 to 1500 in an amount of about 0.5% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount of about 4% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0019] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 26 to 30 g/g at 0.3 psi. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 21 to 26 g/g at 0.7 psi. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit CRC ranging from 30 to 35 gm/gm.
[0020] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit extractables below 8%. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit extractables ranging from 5% to 8%.
[0021] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) ranging from 55 to 65. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) of about 56.
[0022] Another aspect of the present disclosure provides surface crosslinked polyacrylate super absorbent polymer (SAP) particles, said surface crosslinked SAP particles comprising from 1.0 to 5.0 % by wt. of a surface crosslinking agent based on the weight of SAP particles and exhibiting extractables ranging from 5% to 8%. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit AUL of more than 26 g/g at 0.3 psi. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit CRC of more than 30 gm/gm. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit AUL ranging from 26 to 30 g/g at 0.3 psi. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit CRC ranging from 30 to 35 gm/gm. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL of more than 21 g/g at 0.7 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 21 to 26 g/g at 0.7 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) of about 56.

DETAILED DESCRIPTION
[0023] 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.
[0024] 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.
[0025] 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.
[0026] 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.
[0027] 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.
[0028] 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.
[0029] 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.
[0030] 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.
[0031] Free swelling is the ability of the particulate superabsorbent polymer (SAP) to absorb the liquid and is expressed as grams of liquid absorbed per gram weight of the sample (g/g). The free swelling rate/capacity of SAP is measured by standard method no. ISO 17190-5. The procedure is same as CRC, except, instead of centrifugation step, excess solution is removed by hanging the bags for ten minutes.
[0032] The term “extractables” as used herein throughout the present disclosure, denotes soluble portion of SAP particles. Extractables of the SAP particles is determined by the EDANA (European Disposables and Nonwovens Association) recommended test method No. 470.2-02 “Extractable”.
[0033] The present disclosure relates generally to the field of super absorbent polymers (SAPs). More particularly, the present disclosure relates to a method of surface crosslinking of polyacrylate super absorbent polymer (SAP) particles. An aspect of the present disclosure also provides polyacrylate super absorbent polymer (SAP) particles with superior absorption rate and extractable properties while maintaining high CRC and high AUL.
[0034] The present disclosure is on the premise of surprising observation by inventors of the present disclosure that when polyacrylate super absorbent polymer (SAP) particles are surface crosslinked using a combination of ethylene carbonate and polyethylene glycol (PEG), the resultant polyacrylate super absorbent polymer (SAP) particles exhibits superior AUL and extractable properties while maintaining high CRC of the SAP. Also surprising was the observation that when polyethylene glycol (PEG) having molecular weight ranging from 600 to 4000 is used, the improvement in AUL and extractable properties was more prominent as compared to polyethylene glycol (PEG) of higher molecular weights.
[0035] In stark contrast to the conventional belief/knowledge, it could surprisingly be noted that subjecting the treated polyacrylate super absorbent polymer (SAP) particles at a temperature less than about 175°C (preferably, ranging from 155°C to 170°C) for a time period ranging from 20 minutes to 120 minutes (preferably, ranging from 20 minutes to 90 minutes) affords surface crosslinked polyacrylate SAP particles that exhibit remarkably low extractables (e.g. less than about 8%).
[0036] Accordingly, an aspect of the present disclosure relates to a method of surface crosslinking of polyacrylate super absorbent polymer (SAP) particles, said method comprising the steps of: treating the polyacrylate super absorbent polymer (SAP) particles with a crosslinker solution to obtain treated polyacrylate super absorbent polymer (SAP) particles; and exposing the treated polyacrylate super absorbent polymer (SAP) particles to a temperature ranging from 155°C to 170°C for a time period ranging from 20 minutes to 90 minutes to obtain crosslinked polyacrylate super absorbent polymer (SAP) particles.
[0037] In an embodiment, the step of exposing the treated polyacrylate SAP particles comprises exposing the treated polyacrylate SAP particles at a temperature ranging from 155°C to 165°C for a time period ranging from 30 minutes to 90 minutes to obtain surface crosslinked polyacrylate SAP particles. In a preferred embodiment, the step of exposing the treated polyacrylate SAP particles comprises: exposing the treated polyacrylate SAP particles at a temperature ranging from 157°C to 163°C for a time period ranging from 40 minutes to 80 minutes to obtain surface crosslinked polyacrylate SAP particles. Most preferably, the step comprises exposing the treated polyacrylate SAP particles at a temperature of about 160°C for a time period of about 60 minutes to obtain surface crosslinked polyacrylate SAP particles.
[0038] In an embodiment, the crosslinker solution comprises ethylene carbonate, polyethylene glycol (PEG) and water. In an embodiment, the polyethylene glycol (PEG) has a number average molecular weight (Mn) ranging from 600 to 4000. In an embodiment, the polyethylene glycol (PEG) has a number average molecular weight (Mn) ranging from 600 to 1500.
[0039] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount ranging from 1% to 10% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) in an amount ranging from 0.1% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount ranging from 1% to 10% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0040] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount ranging from 1% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) having a number average molecular weight (Mn) ranging from 600 to 1500 in an amount ranging from 0.1% to 3% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount ranging from 1% to 7% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0041] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount ranging from 2% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) having a number average molecular weight (Mn) ranging from 600 to 1500 in an amount ranging from 0.1% to 1% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount ranging from 2% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0042] In an embodiment, the crosslinker solution comprises ethylene carbonate in an amount of about 3% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) having a number average molecular weight (Mn) ranging from 600 to 1500 in an amount of about 0.5% by weight of the polyacrylate super absorbent polymer (SAP) particles and water in an amount of about 4% by weight of the polyacrylate super absorbent polymer (SAP) particles.
[0043] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL of more than 26 g/g at 0.3 psi. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 26 to 30 g/g at 0.3 psi.
[0044] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL of more than 21 g/g at 0.7 psi. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 21 to 26 g/g at 0.7 psi.
[0045] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit CRC of more than 30 gm/gm. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit CRC ranging from 30 to 35 gm/gm.
[0046] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit extractables below 8%. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit extractables ranging from 5% to 8%.
[0047] In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) ranging from 55 to 65. In an embodiment, the crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) of about 56.
[0048] Another aspect of the present disclosure provides surface crosslinked polyacrylate super absorbent polymer (SAP) particles, said surface crosslinked SAP particles comprising from 1.0 to 5.0 % by wt. of a surface crosslinking agent based on the weight of SAP particles and exhibiting extractables ranging from 5% to 8%.
[0049] In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit AUL of more than 26 g/g at 0.3 psi. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit AUL ranging from 26 to 30 g/g at 0.3 psi. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit CRC of more than 30 gm/gm. In an embodiment, the surface crosslinked polyacrylate SAP particles exhibit CRC ranging from 30 to 35 gm/gm. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL of more than 21 g/g at 0.7 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 21 to 26 g/g at 0.7 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) of about 56.
[0050] Another aspect of the present disclosure provides polyacrylate super absorbent polymer (SAP) particles exhibiting AUL of more than 26 g/g at 0.3 psi and extractables below 8%. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit CRC of more than 30 gm/gm. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 26 to 30 g/g at 0.3 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit CRC ranging from 30 to 35 gm/gm. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit extractables ranging from 5% to 8%. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL of more than 21 g/g at 0.7 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 21 to 26 g/g at 0.7 psi. In an embodiment, the polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) of about 56.
[0051] 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.

EXAMPLES
[0052] Production of polyacrylate SAP particles (Polymerization Reaction)
[0053] 2 kg acrylic acid was neutralized with 3.4 kg of 22 % sodium hydroxide solution in a jacketed vessel. Subsequently, 30 g of TMPTA cross-linker was added to the neutralization solution. 60 g of 1% potassium persulfate solution, 45g of 0.5% H2O2 solution and 35 g of 0.1% ascorbic acid solution were prepared and transferred to the preheated kneader reactor through the feeder. 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 35oC. The reactor was kept under nitrogen blanket for the whole reaction period and the gel obtained was few mm in size due to chopping effect of the twin shaft. This gel was immediately dried in hot air dryer at 150oC - 200oC with desired air flow rates. Brittle cake obtained were broken down into few cm size by a shredder and then subjected to milling and sieving to obtain desired particle size range (150-850µm).
[0054] Surface crosslinking of SAP particles
[0055] 200 g of polyacrylate SAP particles were cross-linked by spraying a crosslinker solution having ethylene carbonate (EC), polyethylene glycol (PEG) of different molecular weights and water in the specified amounts as provided in Table 1 below on polyacrylate SAP particles in a plough shear mixer under controlled spraying and continuous agitation conditions. The wet (treated) SAP particles were then exposed to higher temperatures for a defined time period in a forced convection oven to obtain the crosslinked polyacrylate SAP particles. Table 1 below illustrates treatment parameters and properties of resultant suface treated polyacrylate SAP particles.
Table 1: Treatment parameters and properties of surface crosslinked polyacrylate SAP particles
Sample
No. EC
(wt %) PEG
Mn
(0.5%) Water
(%) Temp
(oC) Time (min) AUL
(0.3 psi) AUL
(0.7
psi) CRC
(g/g) Extractables
(%) Free swelling
Base 3 0 4 - - 14 8.4 33 9.5 53
SXL1 3 0 4 150 60 22 17 32 9.0 52
SXL2 3 0 4 160 60 26 22 32 6.6 52
SXL3 3 0 4 170 60 21 17 28 7 49
SXL4 3 0 4 180 30 23 18 30 8.5 50
SXL5 3 600 4 160 60 28 23 33 5.9 56
SXL6 3 1500 4 160 60 28 22 31 8 56
SXL7 3 4000 4 160 60 28 21 32 6.4 56
SXL8 3 6000 4 160 60 27 19 32 8.9 56

[0056] As can be seen from Table 1 above, non-crosslinked polyacrylate SAP particles (denoted as “Base”) exhibited AUL of 14 at 0.3 psi and of 8.4 at 0.7 psi.
[0057] Surprisingly, it could be noted that if the treated SAP particles are exposed to temperatures below 155°C, the extractables remains above 9% i.e. very high, above the desired values (See, Entry marked as “SXL1” obtained by exposing treated SAP particles to 150°C for 60 minutes); further, if the treated SAP particles are exposed to temperatures above 175°C, the extractables remains above 8.5% i.e. very high, above the desired values despite of reducing the exposure time (See, Entry marked as “SXL4” obtained by exposing treated SAP particles to 180°C for 30 minutes). Notably, when the treated SAP particles are exposed to a temperature within the range of 155°C to 170°C, surface crosslinked SAP particles could be obtained that exhibit remarkably low extractables while maintaining high CRC (See, entries marked as “SXL2” and “SXL3”). These observation were quite surprising and in contrast to the conventional knowledge and practice.
[0058] As can also be seen from Table 1 above, polyacrylate SAP particles crosslinked with a crosslinker solution having only ethylene carbonate (EC) as a crosslinker exhibited AUL of 26 at 0.3 psi and of 22 at 0.7 psi (See, an entry marked as “SXL2”). Polyacrylate SAP particles crosslinked with a crosslinker solution having a combination of ethylene carbonate (EC) and polyethylene glycol (PEG) as crosslinkers exhibited superior AUL of 28 at 0.3 psi and of 23 at 0.7 psi (See, an entry marked as “SXL5”).
[0059] It could also be observed, albeit surprisingly, that when polyethylene glycol (PEG) of number average molecular weight (Mn) ranging from 600 to 4000 (and particularly, from 600 to 1500) was used (in combination with ethylene carbonate), the enhancement in AUL and extractable properties was more prominent as compared to the SAP particles corsslinked using polyethylene glycol (PEG) of molecular weight higher than 6000.
[0060] To further study the effect of varying the amounts of ethylene carbonate and polyethylene glycol (PEG-600), experiments were conducted keeping the exposure temperature at 160°C and exposure time of 60 minutes. Table 2 below illustrates properties of resultant suface crosslinked polyacrylate SAP particles.
Table 2: Properties of surface crosslinked polyacrylate SAP particles
Sample
No. EC
Wt (%) PEG-600
(wt. %) AUL
(g/g at 0.3 psi CRC
(g/g) Free swelling
Extractables
(%)
SXL-2 3 0 26 32 52 6.6
SXL-9 2.5 0.5 28 35 60 6.1
SXL-10 2 1 28 34 56 6.1
SXL-11 1.5 1.5 29 34 57 6.0
SXL-12 1 2 27 35 57 6.0
SXL-13 0.5 2.5 27 37 57 5.9
SXL-14 0 3 26 34 57 6.0

[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 for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles that exhibit low extractables.
[0063] The present disclosure provides a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles that has superior absorption rate, high CRC and high AUL.
[0064] The present disclosure provides a method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles that is economical and industrially applicable.
[0065] The present disclosure provides surface crosslinked polyacrylate super absorbent polymer (SAP) particles that exhibit low extractables, superior absorption rate, high CRC and high AUL.

,CLAIMS:1. A method for production of surface crosslinked polyacrylate super absorbent polymer (SAP) particles, said method comprising the steps of:
treating polyacrylate super absorbent polymer (SAP) particles with a crosslinker solution to obtain treated polyacrylate super absorbent polymer (SAP) particles; and
exposing the treated polyacrylate super absorbent polymer (SAP) particles to a temperature ranging from 155°C to 170°C for a time period ranging from 20 minutes to 90 minutes to obtain the surface crosslinked polyacrylate super absorbent polymer (SAP) particles.
2. The method as claimed in claim 1, wherein the crosslinker solution comprises ethylene carbonate, polyethylene glycol (PEG) and water.
3. The method as claimed in claim 2, wherein polyethylene glycol (PEG) has a number average molecular weight (Mn) ranging from 600 to 4000.
4. The method as claimed in claim 1, wherein the crosslinker solution comprises: ethylene carbonate in an amount ranging from 2% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles, polyethylene glycol (PEG) having a number average molecular weight (Mn) ranging from 600 to 1500 in an amount ranging from 0.1% to 1% by weight of the polyacrylate super absorbent polymer (SAP) particles; and water in an amount ranging from 2% to 5% by weight of the polyacrylate super absorbent polymer (SAP) particles.
5. The method as claimed in claim 1, wherein the surface crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 26 to 30 g/g when measured at 0.3 psi and exhibit AUL ranging from 21 to 26 g/g when measured at 0.7 psi.
6. The method as claimed in claim 1, wherein the surface crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit CRC ranging from 30 to 35 gm/gm and exhibit extractables ranging from 5% to 8%.
7. The method as claimed in claim 1, wherein the surface crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit free swelling (0.9%, NaCl) ranging from 55 to 65.
8. Surface crosslinked polyacrylate super absorbent polymer (SAP) particles, said surface crosslinked SAP particles comprising from 1.0 to 5.0 % by wt. of a surface crosslinking agent based on the weight of SAP particles and exhibiting extractables ranging from 5% to 8%.
9. The surface crosslinked polyacrylate SAP particles as claimed in claim 7, wherein said surface crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit extractables ranging from 5% to 7%.
10. The surface crosslinked polyacrylate SAP particles as claimed in claim 7, wherein said surface crosslinked polyacrylate super absorbent polymer (SAP) particles exhibit AUL ranging from 26 to 30 g/g when measured at 0.3 psi and exhibit AUL ranging from 21 to 26 g/g when measured at 0.7 psi; exhibit CRC ranging from 30 to 35 gm/gm free swelling (0.9%, NaCl) ranging from 55-65.