Claims:1. A method for the production of a superabsorbent polymer, said method comprising introducing a moist dough of a carbohydrate graft-copolymer into a twin screw reactor and washing said dough with a solvent to form free-flowing particles of the super absorbent polymer.

2. The method as claimed in claim 1, wherein the solvent is an alcohol solvent.

3. The method as claimed in claim 1 or claim 2, wherein said washing of the moist dough in the twin screw reactor is carried out under stirring in said reactor to form free-flowing particles.

4. The method as claimed in any one of the preceding claims, wherein said washing of the moist dough is carried out at room temperature.

5. The method as claimed in any one of the preceding claims, wherein the method for the production of a superabsorbent polymer comprises a subsequent step of drying of the free-flowing particles of the superabsorbent polymer, sieving and packaging.

6. The method as claimed in any one of the preceding claims, wherein the moist dough of a carbohydrate graft-copolymer is prepared by combining a carbohydrate substrate, a monomer comprising at least one a, ß- unsaturated nitrile or unsaturated carboxylic acid derivative and an initiator to isothermally initiate polymerization to form a carbohydrate graft-copolymer moist dough.

7. The method as claimed in any one of the preceding claims, comprising the steps of:
(i) Combining a carbohydrate substrate, a monomer comprising at least one a, ß- unsaturated nitrile or carboxylic acid derivative and an initiator to isothermally initiate polymerization to form a carbohydrate graft-copolymer, the said carbohydrate graft -copolymer forming a moist dough;
(ii) Introducing the said moist dough of step (i) into a twin screw reactor and washing the said dough with an alcohol solvent under stirring at room temperature in the said reactor to form free-flowing particles;
(iii) Drying of the free-flowing particles of step (ii), sieving and packaging to obtain the superabsorbent polymer.

8. The method as claimed in claim 6 or claim 7, wherein the carbohydrate substrate is selected from starch, cellulose or lignin.

9. The method as claimed in claim 8, wherein the starch is selected from native starches corn starch, waxy maize starch, wheat starch, potato starch, dextrin starches, dextran starches, corn meal, peeled yucca root, unpeeled yucca root, oat flour, banana flour and tapioca flour.

10. The method as claimed in claim 6 or claim 8, wherein the a, ß-unsaturated nitrile or carboxylic acid derivative is selected from acrylonitrile, acrylic acid, acrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylamide, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, potassium acrylate and derivatives and mixtures thereof, derivatives of maleic acid and itaconic acid and esters and amides(amidines) of a, ß-unsaturated acids.

11. The method as claimed in claim 8, wherein weight ratio of the carbohydrate substrate to the monomer is in the range from 1:1 to 1:10.

12. The method as claimed in claim 8, wherein the initiator is selected from ceric ammonium nitrate, ammonium persulfate, sodium persulfate, potassium persulfate, ferrous peroxide, ferrous ammonium sulfate-hydrogen peroxide, L-ascorbic acid and potassium permanganate-ascorbic acid.

13. The method as claimed in claim 8, wherein step (i) of the method after isothermal polymerization further comprises the step of saponifying the carbohydrate graft-copolymer formed in the polymerization and subsequently adjusting the pH between 6.0 to 8.0 to form the moist dough before washing the said dough in step (ii) of the method;
Wherein the monomer is at least acrylonitrile.

14. The method as claimed in claim 8, wherein step (i) of the method of polymerization further comprises the step of adding a cross-linking agent to form a cross-linked graft-copolymer, neutralizing the said cross-linked graft-copolymer and subsequently adjusting the pH between 6.0 to 8.0 to form the said moist dough before washing the said dough in step (ii) of the method;
Wherein the monomer is selected from acrylic acid, acrylamide, methacrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, potassium acrylate, and derivatives and mixtures thereof, derivatives of maleic acid and itaconic acid and esters and amides(amidines) of a, ß-unsaturated acids.

15. The method as claimed in claim 14, wherein the cross-linking agent is selected from glycerides, diepoxides, diglycidyls, cyclohexadiamide, methylene bis-acrylamide, bis-hydroxyalkylamides, bis-hydroxypropyl adipamide, formaldehydes, urea-formaldehyde, melamine-formaldehyde resins, isocyanates, di-isocyanates, tri-isocyanates, epoxy resins, self-cross-linking polymers, and derivatives and mixtures thereof.

16. The method as claimed in claim 8, wherein the alcohol solvent for washing in step (ii) of the method is selected from methanol, ethanol, propanol and isopropanol.

17. The method as claimed in claim 2, wherein the alcohol solvent for washing is methanol.

18. The method as claimed in claim 2, wherein the total amount of methanol used for washing per kg of dough ranges from 0.8.to 3.2.

19. The method as claimed in claim 1, wherein the room temperature at which said method is carried out ranges 15 to 450C.

20. The method as claimed in claim 1, wherein twin screw of the reactor of the method operates at a speed ranging from 5 to 35 rpm.

21. The method as claimed in claim 1, wherein moisture content of the dough ranges from 75 to 90% by weight.

22. The method as claimed in claim 1, wherein step (ii) of the method further comprises the step of subjecting the formed free-flowing particles to alcohol solvent washing in twin screw reactor for multiple times until the moisture content of the said particles is reduced to 15% by weight.

23. The method as claimed in claim 1, wherein final moisture content of the product after drying at step (iii) of the said method ranges from 8-12% by weight.

24. The method as claimed in claim 1, wherein step (ii) of the method is performed in a continuous or batch process.

25. A superabsorbent polymer produced by the method as claimed in any one of the preceding claims having a water absorbance capacity ranging from 500 g/g to 980g/g and a monomer impurity ranging from 2 ppm to 10 ppm.

26. A system for washing a superabsorbent polymer subsequent to forming a moist dough of a carbohydrate graft-copolymer, the system comprising:
A twin screw reactor (100) comprising:
(a) inlet (101) for receiving the moist dough of a carbohydrate graft-copolymer ;
(b) inlet (102) for receiving an alcohol solvent for washing;
(c) two screws (103, 103’) passing through the length of the reactor;
(d) an outlet for recovery of the used alcohol solvent (104);
(e) an outlet (105) comprising a die plate, the die plate configured to pass the moist dough after the said dough passes through the said reactor to form granulated free-flowing particles of the superabsorbent polymer;
Wherein the two screws of the said reactor rotates at a speed and the moist dough is washed with an alcohol solvent as it passes through the two screws of the said reactor at room temperature.

27. The system as claimed in claim 26, wherein the alcohol solvent for washing is selected from methanol, ethanol, propanol and isopropanol.

28. The system as claimed in claim 27, wherein the alcohol solvent for washing is methanol.

29. The system as claimed in claim 28, wherein the total amount of methanol used for washing per kg of dough ranges from 0.8.to 3.2.

30. The system as claimed in claim 26, wherein the room temperature ranges from 15 to 450C.

31. The system as claimed in claim 26, wherein twin screw (103, 103’) of the reactor operates at a speed ranging from 5 to 35 rpm.

32. The system as claimed in claim 26, wherein the said system is configured to reduce the moisture content of the formed free-flowing particles to 8-12% by weight or less by multiple alcohol solvent washing in twin screw reactor followed by drying.

33. The system as claimed in claim 26, wherein the said system is configured to operate in a continuous or batch process.
, Description:FIELD OF INVENTION
The present invention relates to an efficient method for production of a superabsorbent polymer. The superabsorbent polymer has a high water absorbing capacity and is highly pure due to reduced levels of monomer impurity.

BACKGROUND OF INVENTION
Polymer chemists have developed a class of materials referred to as superabsorbent polymers (SAPs). SAPs are materials that imbibe or absorb at least 10 times their own weight in aqueous fluid and that retain the imbibed or absorbed aqueous fluid under moderate pressure. The imbibed or absorbed aqueous fluid is taken into the molecular structure of the SAP rather than being contained in pores from which the fluid could be eliminated by squeezing. Some SAPs can absorb up to 1,000 times their weight in aqueous fluid.
SAPs are utilized in various applications that include personal care articles (diapers, napkins, etc.), agriculture, horticulture, transportation for perishable goods, fire fighting, communication cables, drug delivery, etc. Examples of liquids that could be absorbed are water, urine, blood, aqueous solutions of salts, fertilizers, pesticides, inks, etc.
"Totally synthetic copolymers," are a type of SAP that is made by copolymerizing acrylic acid and acrylamide in the presence of an initiator. Almost all totally synthetic copolymer SAPs are used in baby diapers, adult diapers, hospital bed pads, cable coating, and the like.

Another type of SAP, called starch graft copolymer, uses a natural polymer, such as a starch, to form an SAP product . In a graft copolymerization process, the starch is combined chemically with a polymerizable monomer in the presence of initiator.

US3640925A is directed to a process in which starch is simultaneously gelatinized and reacted with a monomer in a graft copolymerization reaction to produce a starch graft copolymer having combined properties of those of starch and those of a homopolymer obtained from the monomer.

Various processes have been developed for the preparation of SAP.

U.S. Pat. No. 7459501B2 is directed to a method for producing a SAP which involves graft polymerizing a monomer (acrylic acid or its ester, optionally including acrylamide) onto a starch in the presence of a cross-linker and an initiator under isothermal conditions. Embodiments include a batch method which involves combining the reactants in water, heating the mixture to about 170° F., and maintaining that temperature for about 15 minutes. The resulting viscous mass was neutralized with base and isolated after adding large volumes of methanol (about 1.5 gallons and about 2.0 gallons) to convert the viscous mass into a physical form that can be processed. It is noted that immersing the cross-linked starch graft copolymer in alcohol causes the alkali starch graft copolymer to precipitate into particles that are later screened to the desired size after drying. The alcohol removes the water and extraneous salts from the cross-linked starch graft copolymer. However this method uses several gallons of methanol for isolation by precipitation. Also this method of producing a SAP avoids a saponification process, handling acrylonitrile. Furthermore, SAPs produced by this method has a low water absorption capacity and low purity.

Indian Patent No. IN254607 (Patent Application 3400/DELNP/2006) is directed to a method and a product formed by entrapping a bioactive, growth-promoting additive in a starch matrix to form a starch-based, superabsorbent polymer product for use in agricultural applications. The method involves forming a starch graft copolymer having a starch matrix; (2) saponifying the mixture; (3) precipitating the saponified starch graft copolymer from the mixture to form particles of SAP product that are sized for use in agricultural applications; and (4) adding the-bioactive, growth-promoting additive so that it is entrapped in the starch matrix. Saponification produces a highly viscous mass of saponificate having a dough-like consistency. The saponificate (either with or without the bioactive, growth-promoting additive) is then precipitated into solid form using a water-miscible solvent such as an alcohol, e.g., methanol, ethanol, propanol, or isopropanol. The saponificate is immersed in alcohol, causing the alkali starch graft copolymer to precipitate, forming particles that may be dried and screened to the desired size. The alcohol removes water from, desalts, and granularizes the neutralized starch graft copolymer saponificate. However this method uses several gallons of methanol for isolation by precipitation. Furthermore, SAPs produced by this method has a low water absorption capacity and low purity.

The aforesaid conventional systems involve the isolation of product from the viscous polymerization dough by use of large amounts of alcohol solvents such as methanol. The washing being the critical step, is currently done at plant and faces problem of higher methanol use, obtaining a low quality product and low water absorption capacity. The use of large amounts of methanol leading to the environmental problems associated with handling toxic alcohols, such as methanol, are minimized. Furthermore, methanol (and other alcohols) is flammable and may pose a fire hazard. Moreover, methanol is costly to dispense and to purchase. The isolation of one pound of polymer can require as much as 3 gallons of methanol. As a result, a 10 million pound SAP/year plant can require as much as 30 million gallons/year of methanol.
Patent No. US7591974 B2 is directed to a method of producing a superabsorbent polymer product for use in agricultural applications, which involves forming a polymerization dough of starch graft copolymer and reducing the moisture content of the polymerization dough by drying the polymerization dough in a extruder heated at a temperature between about 50° C. and about 160° C. However at such high extruder temperatures, the polymer material may plasticize and loose its water absorption properties.
US patent US8507607B2 is directed to a process for the formation of a graft copolymer which involves: (1) combining water, a carbohydrate, at least one a,ß-unsaturated carboxylic acid derivative, and a catalyst to form a combination having initiation conditions; (2) introducing the combination to a reactor having a reaction zone providing initiation conditions, and (3) forming the graft copolymer therein under substantially adiabatic conditions to provide a free-flowing copolymer. The method disclosed does not utilize acrylonitrile, does not require saponification generating large volumes of ammonia, nor are large volumes of methanol or other lower alcohols required to effectuate work-up of a viscous mass of initially formed polymer. However the risk to meet the product specification under solvent less condition is more as it is prone to plasticizing of the resulting dough at elevated temperatures and making it useless for the desired application.

WO2004022608 A1 is directed to a continuous process for the production of dried Superabsorbent polymers (SAPs) the polymerization reaction is carried out either in an initially homogenous aqueous monomer solution (bulk aqueous solution polymerization) or in a heterogeneous water-in-oil reactant mixture (reverse phase suspension or emulsion polymerization) within a continuous closed polymerization reactor (1), then the resulting polymer gel is dried within a continuous moved bed in a closed dryer (2), avoiding the needs of standard intermediate maturity tank(s). The polymerization reaction is carried out in a continuous twin-shaft kneader reactor and the drying is done directly in a continuous moved-bed dryer 2 (e.g. rotary dryer, drum dryer, Discotherm dryer).

There is a long felt need to develop an efficient method for the production of a superabsorbent polymer which uses a low amount of alcohol solvent for washing, increase in production capacity, is simple, cheap, environmental friendly technique and results in a SAP having a high water absorbing capacity, which is highly pure and environment friendly product due to reduced levels of monomer impurity in the final product. The present inventors have surprisingly developed an efficient method for the preparation of superabsorbent polymer which ameliorates the aforesaid shortcomings of the prior art.
OBJECTS OF THE INVENTION
It is an object of the present invention to provide an efficient method for the production of a superabsorbent polymer.
It is another object of the present invention to provide an efficient method for the production of a superabsorbent polymer which uses low amounts of alcohol washing solvent.
It is another object of the present invention to provide an efficient method for the production of a superabsorbent polymer which helps in reducing the toxic levels of monomer impurity to environmentally permissible levels in the final SAP product.
It is another object of the present invention to provide an efficient method for the production of a superabsorbent polymer which shows increase in production capacity, is simple, cheap, environmental friendly technique.
It is another object of the present invention to provide a superabsorbent polymer having a high water absorbing capacity and which is highly pure due to reduced levels of monomer impurity content.
SUMMARY OF THE INVENTION
It is an aspect of the present invention to provide a method for production of a superabsorbent polymer comprising the steps of:
(i) Combining a carbohydrate substrate, a monomer comprising at least one a, ß- unsaturated nitrile or unsaturated carboxylic acid derivative and an initiator to initiate polymerization to form a carbohydrate graft-copolymer, the said carbohydrate graft -copolymer forming a moist dough;
(ii) Introducing the said moist dough of step (i) into a twin screw reactor and washing the said dough with an alcohol solvent under stirring at room temperature in the said reactor to form free-flowing particles;
(iii) Drying of the free-flowing particles of step (ii), sieving and packaging to obtain the superabsorbent polymer.

It is another aspect of the present invention to provide a superabsorbent polymer having a high water absorbing capacity and low levels of monomer impurity.

It is yet another aspect of the present invention to provide a system for washing a superabsorbent polymer subsequent to forming amoist dough of a carbohydrate graft-copolymer.

BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings wherein:
Figure-1: Process flow diagram of preparation of SAP (200) according to a conventional method employing a plant washing step.
Figure-2: Diagram of Twin screw reactor washing with alcohol solvent of the moist dough (100) according to an embodiment of present invention.
Figure-3: Process diagram of preparation of SAP (300) according to a method of the present invention employing a twin screw reactor washing with alcohol solvent.
Figure-4: Photographic image of the polymer product manufactured by conventional methods of direct water removal without methanol washing.

DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.

Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the scope of the invention as defined by the appended claims and their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, steps or components but does not preclude the presence or addition of one or more other features, steps, components or groups thereof.
The term “SAP” as used herein means superabsorbent polymer.
The term “moisture content” as used herein means the amount (percentage) of aqueous fluid by weight in the polymerization dough.
The term “water absorption” as used herein means property of absorbing water when superabsorbent polymer is exposed to water. Water absorption capacity (WAC) consists of adding water or an aqueous solution to material, followed by sieving and quantification of the water retained by the gelled material in the Sieve.
The term “monomer impurity” as used herein means the monomers which remain unreacted during polymerization and which also undergoes saponification to give further derivatised monomers and are present in the final product as residual monomers.
The present invention relates to an efficient method for the production of a superabsorbent polymer (SAP). SAPs as three-dimensional polymeric structures are capable of absorbing large quantities of water without disintegrating. The present invention relates to obtaining a graft polymer by combining a carbohydrate substrate, a monomer comprising at least one a, ß- unsaturated nitrile or unsaturated carboxylic acid derivative and an initiator to isothermally initiate polymerization to give a moist dough which is washed in a twin screw reactor to produce free flowing particles capable of retaining their absorption properties to give a resultant super absorbing product of desired properties.

The initial step of the process for formation of SAP involves forming a graft copolymer isothermally by combining a carbohydrate substrate, a monomer comprising at least one a, ß- unsaturated nitrile or unsaturated carboxylic acid derivative and an initiator. The moist polymerization dough is then subjected to washing. Washing of the moist polymerization dough is a critical step for removal of moisture content and removal of unreacted monomers and helps in achieving a SAP of high water absorption capacity and high purity. However washing in conventional processes for SAP production are performed in-plant which requires large amounts of methanol for washing thereby increasing the production cost of SAP. Also handling such large amount of methanol poses environmental hazards and raises serious safety concerns due to high flammability of alcohols. Furthermore, the product obtained by the said conventional processes has a low water absorption capacity and low purity.

Figure-1 illustrates a process flow diagram of preparation of SAP (200) according to a conventional method employing a plant washing step. In the conventional process, a starch slurry is prepared in water. To this ceric ammonium nitrate (CAN) (202) is added for activation (203). This is followed by addition of acrylonitrile (ACN) (204). The resulting mixture undergoes polymerization (205). This graft polymer is then transferred to a saponification chamber and treated with KOH (207). Steam is purged (206) to carry out the saponification and also to scrub out the ammonia formed in the saponification process. The dough thus obtained is neutralized using OPA (orthophosphoric acid) (208). The resultant dough is then subjected to a plant washing process (209) using several gallons of methanol solvent (210) for isolation.

In contrast, in the process of the present invention, the polymerization dough prepared in the plant is introduced into a twin screw reactor and subjected to alcohol washing in the reactor to form granulated particles. Due to washing in the twin screw reactor, a lower amount of alcohol solvent for washing is consumed. The lower amount of solvent thereby reduces the cost of production and exposure to the solvent. There is an increase in production capacity as the process becomes continuous. The inventors have surprisingly found that the low amount of alcohol solvent does not reduce the effectiveness of the process, in fact enhances its washing ability as in counter flow the particles always meet fresh solvent during its entire process. The process results in a high quality SAP product by reducing monomer impurity level to environmentally friendly levels and having a high water absorption capacity.

During the washing of the dough in the twin screw two things happen:
1) Methanol removes the water
2) The twin screw exposes the fresh surface of the dough due to its cutting process for the methanol to remove the moisture from the dough.
For this purpose methanol has to continuously remove the water inside the dough. The twin screw efficiently does this process as the dough is rotated within the twin screw the blades of the twin screw cuts the dough and exposes a fresh surface of the dough every time which mixes with methanol efficiently to remove the water..
3) This process also helps to remove residual monomers and other impurities trapped in the super absorbent polymer
Therefore, in an aspect, the present invention provides a method for the production of a superabsorbent polymer, said method comprising introducing a moist dough of a carbohydrate graft-copolymer into a twin screw reactor and washing said dough with an alcohol solvent to form free-flowing particles of the super absorbent polymer.

In an embodiment, said washing of the moist dough in the twin screw reactor is carried out under stirring in said reactor to form free-flowing particles.

In an embodiment, said washing of the moist dough is carried out at room temperature.

In an embodiment, the method for the production of a superabsorbent polymer comprises a subsequent step of drying of the free-flowing particles of the superabsorbent polymer, sieving and packaging.

In an embodiment, the moist dough of a carbohydrate graft-copolymer is prepared by combining a carbohydrate substrate, a monomer comprising at least one a, ß- unsaturated nitrile or unsaturated carboxylic acid derivative and an initiator to isothermally initiate polymerization to form a carbohydrate graft-copolymer moist dough.

In an embodiment, the method of the present invention comprises the steps of:
(i) Combining a carbohydrate substrate, a monomer comprising atleast one a, ß- unsaturated nitrile or unsaturated carboxylic acid derivative and an initiator to isothermally initiate polymerization to form a carbohydrate graft-copolymer, the said carbohydrate graft-copolymer forming a moist dough;
(ii) Introducing the said moist dough of step (i) into a twin screw reactor and washing the said dough with an alcohol solvent under stirring at room temperature in the said reactor to form free-flowing particles;
(iii) Drying of the free-flowing particles of step (ii), sieving and packaging to obtain the superabsorbent polymer.

Carbohydrate substrate is a major component of the graft polymer. Suitable carbohydrate substrate is selected from starch, cellulose or lignin. Preferably, starch is employed as the substrate because of its cost, availability and reactivity and because a range of physical properties can be obtained with starch-graft copolymer.
Suitable substrates include starches, flours, and meals. More specifically, exemplary starches include native starches corn starch, waxy maize starch, wheat starch, potato starch, dextrin starches, dextran starches, corn meal, peeled yucca root, unpeeled yucca root, oat flour, banana flour, and tapioca flour. The starch may be gelatinized to provide optimal absorbency. An exemplary starch is gelatinized cornstarch.
The monomer is graft polymerized onto a carbohydrate substrate in the presence of an initiator. In an embodiment, the monomer comprising a, ß-unsaturated nitriles and carboxylic acid derivatives are selected from acrylonitrile. acrylic acid, acrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylamide, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, derivatives thereof, and derivatives and mixtures thereof. Derivatives of maleic acid and itaconic acid can be used. In addition other esters and amides (amidines) of other a, ß- unsaturated acids can be used.
In an embodiment, the weight ratio of the carbohydrate substrate to the monomer is in the range from 1:1 to 1: 10.In a preferred embodiment the weight ratio is from 1:1 to 1:6.
Suitable initiator as used herein is selected from cerium (+4) salts, such as ceric ammonium nitrate, ammonium persulfate, sodium persulfate, potassium persulfate, ferrous peroxide, ferrous ammonium sulfate-hydrogen peroxide, L-ascorbic acid, and potassium permanganate-ascorbic acid. Alternatively persulfates and peroxides as well as vanadium and manganese etc may be used.
Step (i) of the method of present invention forming the carbohydrate graft-copolymer moist dough includes two preferred embodiments.
In a first preferred embodiment, step (i) of the method involves saponifying the carbohydrate graft-copolymer formed in polymerization to form the moist dough before washing the said dough in step (ii) of the method;
Wherein the monomer is at least acrylonitrile.
In a second preferred embodiment, step (i) of the method involves cross-linking the graft-copolymer by adding a cross-linking agent to form a cross-linked carbohydrate graft-copolymer and neutralizing the said cross-linked carbohydrate graft-copolymer to form the moist dough before washing the said dough in step (ii) of the method;
Wherein the monomer is selected from acrylic acid, acrylamide, methacrylamide, 2-acrylamido-2-methyl-propanesulfonic acid, methacrylic acid, vinyl sulfonic acid, ethyl acrylate, potassium acrylate, and derivatives and mixtures thereof. Derivatives of maleic acid and itaconic acid can be used. In addition other esters and amides (amidines) of other a, ß- unsaturated acids can be used
Suitable cross-linking agent as used herein is selected from glycerides, diepoxides, diglycidyls, cyclohexadiamide, methylene bis-acrylamide, bis-hydroxyalkylamides, bis-hydroxypropyl adipamide, formaldehydes, urea-formaldehyde, melamine-formaldehyde resins, isocyanates, di-isocyanates, tri-isocyanates, epoxy resins, self-cross-linking polymers, and derivatives and mixtures thereof.
The pH of the graft-copolymer may be adjusted to a desired value for the particular agricultural application. For example, the starch graft copolymer may be neutralized. Alternative pH values may be desirable depending upon the type of soil and the type of crop the resulting SAPs will be applied to. The resulting pH for most agricultural applications typically ranges from 6.0 to 8.0.
The moist dough of the grafted-copolymer formed in step (i) of the method is then introduced into a twin screw reactor and subjected to an alcohol solvent washing under stirring at room temperature in Step (ii).
Suitable alcohol solvents used for washing as used herein is selected from methanol, ethanol, propanol and isopropanol. In a preferred embodiment, methanol is used for washing.
In an embodiment, the total amount of methanol used for washing per kg of dough ranges from 0.8 to 3.2.
In an embodiment, the room temperature ranges from15 to 45 0C.In a preferred embodiment, the room temperature ranges from 22 to 35 0C.
In a twin screw reactor, the two co-rotating screws intermesh as they turn and the moist dough is worked into continuous dough thereby. Because the operating parameters and configuration of the twin screw reactor is selected to maximize SAP product production and performance in various settings, the configuration and operating parameters may vary greatly.
Suitable twin screw reactors from commercial sources may be employed having variously sized screw arms. The barrels within which the extrusion screw(s) mix the polymerization dough may also vary in length.
The speed of the twin screw effects the residence time the moist dough is spent in each barrel. In an embodiment, the twin screw of the reactor operates at a speed ranging from 5 to 35 rpm.
In an embodiment, initial moisture content of the dough formed at step (i) of the method ranges from 75 to 90% by weight which is then subjected to washing with alcohol solvent in a twin screw extruder at step (ii) of the method for multiple times until the moisture content of the formed granulated free flowing particles is reduced to 15 % by weight; and wherein the said free-flowing particles of step (ii) are further dried at step (iii) of the method to obtain a SAP product with final moisture content ranging from 8-12% by weight. In one exemplary embodiment, the polymerization dough having approximately 80% initial moisture content by weight may be dried to 12% moisture content or less by weight after twin screw reactor washing.
The step of washing of the polymerization dough in a twin screw reactor with alcohol solvent may be performed either in a continuous or batch process. In an embodiment, the washing is performed in a continuous process. In another embodiment, washing is done in a batch process.
In step (iii) of the method, the free-flowing particles formed at step (ii) are further dried by conventional methods. In an embodiment, the drying is done till final moisture content of the product ranges from 8-10%. The dried SAP particles made may then be passed through a particle separation system such as a screening system and packaged to obtain the desired SAP product. Depending on the agricultural application, the final SAP product may have a desired particle size.
The SAP produced by the process of the present invention has a high water absorbance capacity and having a high purity.
In an embodiment, water absorbance capacity of the SAP ranges from 500 g/g to 980g/g. In an embodiment, the final SAP product has a monomer impurity ranging from 2 to 15 ppm. In another embodiment, the SAP has a monomer impurity from 2 to 10 ppm.
The wash process is to achieve the removal of the water by methanol .The solvent also helps in removal of other organic impurities such as residual monomers and soluble salts. The process of the twin screw remains the same. It helps in methanol to penetrate its surface by constantly exposing the surfaces of the dough to methanol interaction and efficiently helps in reduction of the residual monomer content and other soluble salts.
The present invention also provides a system for washing a superabsorbent polymer subsequent to forming moist dough of a carbohydrate graft-copolymer, the system comprising:
A twin screw reactor comprising:
(a) inlet for receiving the moist dough of a carbohydrate graft-copolymer ;
(b) inlet for receiving an alcohol solvent for washing;
(c) two screws passing through the length of the reactor;
(d) an outlet for recovery of the used alcohol solvent;
(e) an outlet comprising a die plate, the die plate configured to pass the moist dough after the said dough passes through the said reactor to form granulated free-flowing particles of the superabsorbent polymer;
Wherein the two screws of the said reactor rotates at a speed and the moist dough is washed with an alcohol solvent as it passes through the two screws of the said reactor at room temperature.

Figure-2 illustrates diagram of a twin screw reactor washing with alcohol solvent according to an embodiment of the present invention. The twin screw reactor (100) comprises the inlets (101) for receiving the moist dough of a carbohydrate graft-copolymer, an inlet (102) for receiving an alcohol solvent for washing; two screws (103,103’) passing through the length of the reactor; an outlet (104) for recovery of the used alcohol solvent; an outlet (105) comprising a die plate, the die plate configured to pass the moist dough after the said dough passes through the said reactor to form granulated free-flowing particles of the superabsorbent polymer;

Figure-3 depicts a process diagram of preparation of SAP (300) according to an embodiment of the present invention employing a twin screw reactor washing with alcohol solvent.
• Step 301 (Starch hydration): In this step, starch (pre-gelatenized) (1) is hydrated with water (2).
• Step 302 (Starch activation): Ceric ammonium nitrate (CAN) dissolved in water (3) is added to the hydrated starch of step 301 for starch activation.
• Step 303 (Grafting): Acrylonitrile (ACN) (4) is added to the activated starch of step 302 for grafting.
• Step 304 (Saponification and Dough cooling): The resulting mixture of step 303 is then transferred to a saponification chamber and treated with KOH (5) solution. Steam (6) is purged to carry out the saponification and also to scrub out the ammonia formed in the saponification process. The dough thus obtained is neutralized to pH 6-8 using OPA (orthophosphoric acid) (7) and cooled to room temperature. Steam is released.
• Step 305 (Granulating and Methanol Wash): The resultant moist dough of step 304 is washed with methanol in a twin screw reactor to remove water.
• Step 306 (Methanol recovery): Methanol solvent employed in washing is recovered by passing through a distillation column (9) and the recovered methanol (8) is employed for multiple washings of the granulated particles in the twin screw reactor. The distillation column bottoms to effluent treatment plant (ETP) (10).
• Step 307: Wet granules are collected through the discharge screw at the right end of the reactor and methanol solvent absorbing the moisture from the dough elutes from the top left hand corner of the reactor and is taken for methanol recovery of step 306.
• Steps 308, 309 (Drying, Grading and Packing): The granulated product is further dried, sieved and packaged to obtain the final finished product of superabsorbent polymer (11).

Advantages of the present invention are:
• The amount of methanol used in the washings is substantially reduced to almost one-fifth of the conventional processes.
• The use of low amount of methanol reduces cost of production and exposure to the solvent. The lower amount of methanol does not reduce the effectiveness of the process; in fact washing ability is enhanced because in counter flow the particles always meet fresh solvent during its entire process.
• Increase in production capacity as the process becomes continuous.
• The benefit is retaining the desired water absorption properties without subjecting to plasticizing conditions and at the same time bringing down the consumption of the washing solvent to drastically reduced levels when used in a continuous manner. Thereby reducing the economic, environmental impact and also reducing exposure to solvents.
• SAP produced by method of present invention has a high water absorption capacity.
• SAP is highly pure due to reduction in monomer impurity levels.
• SAP is environmental friendly as monomer impurity is reduced to permissible levels and highly safe due to reduction in alcohol volume required for washing.
• SAPs produced by present invention may be used in agricultural or horticultural applications. Applying SAPs to soil in agricultural settings results in earlier seed germination and/or blooming, decreased irrigation requirements, increased propagation, increased crop growth and production, increased crop quality, decreased soil crusting, increased yield and decreased time of emergence.
EXAMPLES:
The following examples are meant to illustrate the present invention. The examples are presented to exemplify the invention and are not to be considered as limiting the scope of the invention

EXAMPLE -1:
Preparation of the SAP dough from acrylonitrile monomer and its washing as per present invention
53 g of starch (pre-gelatenized) was hydrated with 630 g of water at 30°C for 1 hr. To this was added 1.78 g ceric ammonium nitrate (CAN) dissolved in83.5g of water for activation. This was followed by addition of 61.62 g of acrylonitrile (ACN). The resulting mixture was maintained at 30-45° C with stirring for 3h. This graft polymer was then transferred to the saponification chamber and treated with 96.0 g KOH (45%) solution. The reaction mixture was heated to 95°C and maintained at this temperature for 2h. Steam was purged to carry out the saponification and also to scrub out the ammonia formed in the saponification process. The 986 g dough thus obtained was neutralized to pH 7 using 13.9 g of OPA (orthophosphoric acid) (45%) and cooled to room temperature.
The resultant 1000 g dough was washed with 1.2 kg of methanol in a twin screw reactor to remove water. Methanol was decanted from the reactor after allowing the solid to settle down. The process was repeated with 1.0kg and 0.7kg (total 2.9 kg) of Methanol and to yield 171 g of granulated product.
Results: Water absorption capacity: 550-600 g/g

EXAMPLE -2:
Washing of SAP dough prepared from acrylonitrile monomer in a Continuous process as per present invention
Dough was prepared on a large scale by following the process of Example-1. 20 kg of the dough was charged into a continuous twin screw reactor at a rate of 6.0 kg/hr as shown in Figure-2 and passing a counter current of methanol (7.2 kg/hr) with dough to methanol ratio of 1:1.2. Dry granulated particles were collected through the discharge screw at the right end of the reactor and methanol solvent absorbing the moisture from the dough elutes from the top left hand corner of the reactor and is taken for recovery. The granulated product was dried further to get ~3.8 kg of the final product with an effective RT (residence time) of 18 min.
Results: Water absorption capacity: 610g/g
Residual Monomer: acrylic acid: 10ppm
Acrylamide: Not detected
During the saponification process the acrylonitrile monomer is also converted (saponified or hydrolysed) to acrylamide and then further to acrylic acid .Therefore the monomer residues will also include acrylic acid and acrylamide.

EXAMPLE -3:
Preparation of SAP dough by using acrylic acid and acrylamide and washing in Batch twin screw reactor according to present invention
82 g of starch was hydrated with 700 g of water at 30°C and stirred for 1 hr at room temperature. After 1 h of hydration was added 82g of the acrylic acid under stirring. This was followed by addition of 41 g of acrylamide and 0.2 g of the methylene bisacrylamide dissolved in 90 g of water. The resulting mixture was heated to 83°C with slow stirring. 0.2 g of the ammonium persulfate in 5 ml water was added and the temperature was maintained till polymerization reaction was complete. The dough was cooled to 25°C and then neutralized with KOH (45%) solution till pH of the resultant dough is 7.5.
1kg dough obtained was washed with 1.2 kg of the methanol in the twin screw reactor for 0.5h and material was allowed to settle. Solvent was decanted and fresh 1kg of the solvent was added and the process was repeated. The granulated particles generated during the process were further washed with 1.0 kg of the methanol. Finally the 245g of the granulated particles were dried in the oven at 50°C till moisture content was ~8-10%.
Result: Water absorption capacity: 640g/g,
Residual Monomer: acrylic acid: 10ppm
Acrylamide: <2ppm

EXAMPLE -4:
Preparation of SAP dough using mixture of starch:
To 650g of water at 30°Cwas added 26.3 g of mixed potato and corn starch (1:1). The mass was stirred for 1 hr at 85°C temperature. After 1 h of hydration was added 57 g of the acrylic acid under stirring followed by addition of 24g of acrylamide and 0.25 g of the methylene bisacrylamide dissolved in 50 g of water. The resulting mixture was heated to 83°C with slow stirring. 0.25g of the ammonium persulfate in 5 ml water was added and the temperature was maintained till polymerization reaction was complete. The dough was cooled to 25°C and then neutralized with KOH (45%) solution till pH of the resultant dough is 7.5.
1kg dough obtained was washed with 1.2 kg of the methanol in the twin screw reactor for 0.5h and material was allowed to settle. Solvent was decanted and fresh 1kg of the solvent was added and the process was repeated. The granulated particles generated during the process were further washed with 1.0 kg of the methanol. Finally the wet granulated particles were dried in the oven at 50°C till moisture content was ~8-10% to get ~ 160g of the final super absorbent polymer
Result: Water absorption capacity: 980 g/g,
Residual Monomer: acrylic acid: 12 ppm
Acrylamide: < 2ppm

EXAMPLE -5:
ATI process: Comparative example as per ATI patent IN254607
Distilled water (1,400 ml) was placed in a 3-liter resin kettle and was subjected to constant agitation with a stirrer. Starch flour or meal (110 g) was slowly added to the kettle, and the resulting mixture was stirred for approximately five minutes. A slow stream of nitrogen gas was added to the mixture while the mixture was heated until it reached a temperature of approximately 95° C. Upon reaching this temperature, the mixture was maintained at this temperature and stirred for approximately 45 minutes to ensure that the starch was gelatinized. The heating mantle was then removed, and the resin kettle was placed in a cold-water bucket bath. The mixture was continuously stirred under nitrogen until the temperature reached 25° C. Acrylonitrile (122 g). The resulting mixture was continuously stirred under nitrogen for approximately 10 minutes. A catalyst solution including cerium ammonium nitrate (5.5 g) dissolved in 0.1 M nitric acid solution (50 ml) was added to the mixture while the mixture cooled. The mixture was continuously stirred under nitrogen while the resin kettle remained in the cold-water bucket for approximately 60 minutes. The temperature of the mixture at the end of the 60 minutes was approximately 40° C. A solution including potassium hydroxide flakes (90 g) dissolved in water (200 g) was added to the mixture during stirring and heating. The mixture was stirred and heated until a temperature of 95° C was achieved, after which the mixture was stirred for an additional 60 minutes. The mixture was then neutralized to a pH of 7.5 using a 10% solution of hydrochloric acid. The resulting dough was then cooled to a temperature of about 40° C.
The viscous dough of 2 kg was precipitated in 12 kg of methanol.
Result: Water absorption capacity: 450g/g
A comparative analysis of the conventional plant washing process and twin screw washing process according to present invention is provided in below Table-1
Analysis As per ATI patent IN254607 Twin Screw washing as in Example-2 of present invention
NO of washings 1 Counter flow Continuous
Methanol used per kg of Dough 6:1 1.2:1
Color and appearance of SAP product Dark VV Light
Monomers present (PPM) in SAP product
Acrylic Acid 125ppm 10ppm
Acrylamide 4 ppm ND
LOD (moisture content)
Methanol 8-9%

1-2 % 9.5 % (9.4)

0.1%
Water absorption capacity (g/g) 450 600+
Table-1
From the aforesaid Table-1, it is observed that the consumption of methanol is reduced to almost 1/5th by twin screw reactor washing of present invention. Furthermore, the SAP produced has a higher water absorption capacity and lower level of monomers than the SAP produced according to conventional process.
EXAMPLE-6
Comparative examples
Experimental data for the water absorption capacity (WAC) of the direct water removed from the dough is depicted in below Table-2:
Experiment WAC Ratio of Methanol: dough for washing
After 25% water removal from dough followed by methanol washing 298g/g
1.1:1
Total water removal without washing with methanol Poor absorption results in dark brown compound (as depicted at Figure-4) not swellable in water NA

Table-2
As observed from the aforesaid Table-2, methanol washing after 25% water removal from dough as in some conventional processes results in a product with very low WAC of 298g/g. In another experiment which involves direct water removal without washing with methanol as disclosed in conventional processes (eg. US7591974B2, US8507607B2, WO2004022608 A1), it is observed that the product obtained is a dark brown polymer (Figure 4) having poor water absorption capacity and not swellable in water. In contrast, the step of methanol washing according to the method of the present invention results in high WAC of 500 g/g to 980g/g as illustrated by the examples 1-4.
Furthermore, prior art US8507607B2 is directed to a process for the formation of a graft copolymer which involves under substantially adiabatic conditions to provide a free-flowing copolymer. It is observed that in the adiabatic process the polymerization proceeds exothermically and the heat generated during the polymerization is not absorbed by the cooling system .This heat can be used instead for removing the water from the dough. But this process can result into the dough becoming a hard mass unable to swell (such as in Figure 4) and finally can result into an SAP which does have water absorbancy or very poor water absorbancy. In contrast, the inventive process reaction of the present invention is carried out under isothermal conditions where the heat generated during polymerization is taken up by the cooling system and water is then removed from the dough using methanol. This process avoids the final SAP from losing its absorption properties

The advantage of present invention therefore lies is retaining the desired water absorption properties without subjecting to plasticizing conditions and at the same time bring down the consumption of the washing solvent to drastically reduced levels when used in a continuous manner. Thereby reducing the economic, environmental impact and also reduces the exposure to the solvents.
It is to be understood that the present invention is susceptible to modifications, changes and adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention.