WO2020008376A1

WO2020008376A1 - Super absorbent polymer and a process for preparing the same

Info

Publication number: WO2020008376A1
Application number: PCT/IB2019/055658
Authority: WO
Inventors: Pradip Munshi; Santosh Agrawal; Sukdeb SAHA; Kishan AGARWALA; Elango SUBRAMANYAM; Raksh Vir Jasra
Original assignee: Reliance Industries Limited
Priority date: 2018-07-05
Filing date: 2019-07-03
Publication date: 2020-01-09

Abstract

The present disclosure relates to process for preparing a super absorbent polymer. The process involves partially neutralizing aqueous acrylic acid with a neutralizing agent to obtain a partially neutralized aqueous acrylic acid, to which a cross-linking agent, a water-swellable polysaccharide and a polymerization initiator are added to obtain a blend. The blend is irradiated with microwaves, and then cooled to obtain a polymer gel comprising the super absorbent polymer and 30 wt% to 75 wt% water, which is dried by intermittently irradiating with microwaves to obtain partially dried super absorbent polymer comprising 1 wt% to 10 wt% water. The partially dried super absorbent polymer is ground to obtain the super absorbent polymer in a powdered form, having a particle size in the range of 50 microns to 900 microns and hardness in the range of 0.5 to 4 kgf.

Description

SUPER ABSORBENT POLYMER AND A PROCESS FOR PREPARING THE SAME

FIELD

The present disclosure relates to super absorbent polymers and a process for preparing the same.

DEFINITIONS

As used in the present disclosure, the following words and phrases are generally intended to have the meaning as set forth below, except to the extent that the context in which they are used to indicate otherwise.

Super Absorbent Polymers (SAPs): The term “Super absorbent polymers” refers to polymers having the ability to absorb and retain extremely large amounts of a liquid relative to their own mass. Partially neutralized acrylic acid: The term“partially neutralized acrylic acid” refers to the molar ratio of neutralized acrylic acid formed after neutralization to the total amount of acrylic acid prior to neutralization. In the present disclosure“partially neutralized acrylic acid” refers to 60 to 90 mole% neutralized aqueous acrylic acid with respect to the total amount of acrylic acid used prior to neutralization. Water absorbency: The term“Water absorbency” refers to a measurement of the amount of water which is absorbed by a material.

BACKGROUND

The background information herein below relates to the present disclosure but is not necessarily prior art. Super absorbent polymers (SAP) have a wide variety of applications such as agriculture, sanitary purposes, dehydration of moist substances or solvents or chemicals, desiccants, and medical applications.

Synthesis of SAP by solution polymerization method involves steps such as polymerization, cutting and/or shredding, drying and grinding. The step of grinding is crucial as grinding provides SAP particles with higher surface area, thereby allowing faster absorption of liquids. For general use, it is desired that the SAP has white color. Conventional processes for the preparation of SAP provides hard rubbery gel after the step of polymerization. The hard rubbery gel is cut and/or shredded into smaller pieces. These smaller pieces are dried and finally subjected to grinding.

The step of grinding is difficult and energy intensive due to hard and rubbery nature of the gel. Process steps of cutting and/or shredding are time consuming and energy intensive steps. The rubbery gel does not allow uniform drying. Thus, the conventional process is associated with drawbacks such as being time consuming and energy intensive process steps, and formation of hard rubbery gel that is difficult to process and grind.

There is, therefore, felt a need to provide a simple process for preparation of SAP that provide SAP in a form which is easy to process and grind. Further, it is desired that the SAP is white in color.

OBJECTS

Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows: It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.

It is another object of the present disclosure to provide a simple process for preparing SAP.

It is yet another object of the present disclosure to provide SAP in a form that is easy to process and grind, particularly having hardness less than 4 kgf. It is still another object of the present disclosure to provide SAP having white color.

Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.

SUMMARY

In a first aspect, the present disclosure provides a process for preparing super absorbent polymer. The process comprises partially neutralizing aqueous acrylic acid, by adding a predetermined amount of a neutralizing agent to obtain a mixture comprising partially neutralized acrylic acid. A cross-linking agent, a water-swellable polysaccharide and a polymerization initiator are added to the partially neutralized acrylic acid under agitation to obtain a blend. The blend is irradiated with microwaves, followed by cooling the irradiated mixture to obtain a polymer gel comprising the super absorbent polymer and 30 wt% to 75 wt% water. The polymer gel is partially dried by intermittently irradiating with microwaves till the water content is reduced to an amount in the range of 1 weight% to 10 weight% to obtain partially dried super absorbent polymer. The partially dried super absorbent polymer is ground to obtain the super absorbent polymer in a powdered form, having a particle size in the range of 50 microns to 900 microns and hardness in the range of 0.5 to 4 kgf.

In a second aspect, the super absorbent polymer comprises 60 wt% to 75 wt% of crosslinked metal salt of polyacrylic acid and 15 wt% to 25 wt% of crosslinked poly acrylic acid, with respect to the total weight of the super absorbent polymer. The super absorbent polymer further comprises 1 wt% to 5 wt% of a crosslinked water-swellable polysaccharide, 0.5 wt% to 5 wt% of a crosslinking agent, 0.1 wt% to 5 wt% of a polymerization initiator and 4 wt% to 8 wt% water, with respect to the total weight of the super absorbent polymer. The metal salt of polyacrylic acid, polyacrylic acid and the water-swellable polysaccharide, are crosslinked with each other. The super absorbent polymer obtained by the process of the present disclosure has white color, water absorbency in the range of 350 g to 450 g water per g of the SAP, moisture content in the range of 1 weight % to 10 weight %, and hardness in the range of 2 to 4 kgf.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING The present disclosure will now be described with the help of the accompanying drawing, in which

Figure-1 illustrates the particle size distribution of SAP obtained from Example 1 ;

Figure-2 depicts a photograph of SAP obtained in Example 1 ; and Figure-3 depicts a photograph of SAP obtained in Example 9 (prior art). DETAILED DESCRIPTION

Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details, are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.

The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising,"“including,” and“having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.

The conventional processes for the preparation of super absorbent polymer (SAP) by solution polymerization method involve steps such as polymerization, cutting and/or shredding, drying and grinding. The step of grinding is crucial as grinding provides SAP particles with higher surface area, thereby allowing faster absorption of liquids. Thus, conventional processes for the preparation SAP are associated with drawbacks such as being time consuming, involving energy intensive process steps, and formation of hard rubbery gel that is difficult to process and grind.

The present disclosure envisages a simple process that provides SAP in a form that is simple, efficient and economic. Further, the present disclosure envisages a process for preparing SAP that is white in color.

In a first aspect, the present disclosure provides a process for preparing super absorbent polymer. The process is described in detail herein below.

Aqueous acrylic acid is partially neutralized by adding a predetermined amount of a neutralizing agent to obtain a mixture comprising partially neutralized acrylic acid.

A cross-linking agent and a polymerization initiator are added to the partially neutralized mixture under agitation to obtain a blend. The blend is irradiated with microwaves, followed by cooling the irradiated mixture to obtain a polymer gel comprising the super absorbent polymer and 30 wt% to 75 wt% water.

The polymer gel is partially dried by intermittently irradiating with microwaves to obtain partially dried super absorbent polymer comprising 1 wt% to 10 wt% water. The partially dried super absorbent polymer is ground to obtain the super absorbent polymer in a powdered form, having a particle size in the range of 50 microns to 900 microns and hardness in the range of 0.5 to 4 kgf.

In accordance with one embodiment of the present disclosure, the super absorbent polymer is obtained in the form of a powder. The particle size of the SAP powder is in the range of 100 microns to 500 microns and hardness in the range of 1 to 3 kgf.

The weight ratio of acrylic acid to water in aqueous acrylic acid is in the range of 2: 1 to 1 :2. In accordance with one embodiment of the present disclosure, the weight ratio of acrylic acid to water in aqueous acrylic acid is 1 : 1.

The neutralizing agent is at least one alkali selected from the group consisting of sodium hydroxide, potassium hydroxide, calcium hydroxide and ammonium hydroxide. In accordance with one embodiment of the present disclosure, the alkali is potassium hydroxide.

The step of partially neutralizing involves neutralizing 60 to 90 mole % of aqueous acrylic acid, preferably neutralizing 70 to 80% aqueous acrylic acid. In accordance with one embodiment of the present disclosure, the step of partially neutralizing involves neutralizing 75 mole % of aqueous acrylic acid.

The amount of the cross-linking agent is in the range of 3.3 to 50 wt% with respect to the total weight of acrylic acid.

In accordance with one embodiment of the present disclosure, the amount of the cross-linking agent is 5.55 wt% with respect to the total weight of acrylic acid. The cross-linking agent is N,N'-methylenebisacrylamide.

The amount of the polymerization initiator is in the range of 1 to 5 wt% with respect to the total weight of acrylic acid. The polymerization initiator is at least one selected from the group consisting of sodium persulfate (Na₂S₂0g), potassium persulfate (K₂S₂0g), and ammonium persulfate ((NH₄)₂S₂0g).

In accordance with one embodiment of the present disclosure, the polymerization initiator is ammonium persulfate ((NH₄)₂S₂0g).

In accordance with the embodiments of the present disclosure, in step of adding, a water- swellable polysaccharide is added to the mixture under agitation.

In accordance with the embodiments of the present disclosure, the amount of water-swellable polysaccharide is in the range of 1 to 5 wt% with respect to the total weight of acrylic acid. In accordance with one embodiment of the present disclosure, the amount of water-swellable polysaccharide is 1.8 wt% with respect to the total weight of acrylic acid.

It is observed that when the preparation of SAP is carried out using 6 wt% water-swellable polysaccharide with respect to the total weight of acrylic acid, the SAP has brownish appearance. The water-swellable polysaccharide is at least one selected from the group consisting of carboxymethylcellulose, hydroxyethylcellulose and hydroxypropylcellulose.

In accordance with one embodiment of the present disclosure, the water-swellable polysaccharide is carboxymethylcellulose.

The water-swellable polysaccharide provides backbone that holds acrylic polymer as pendants in an ordered manner. The swollen polysaccharide provides water channel that allows passage of foreign materials such as water there through.

The SAP obtained by the process of the present disclosure is biodegradable due to the use of water swellable polysaccharide.

In the present disclosure, the microwaves have frequency of 2450 MHz. The microwaves are emitted by a microwave source having output in the range of 140 to 700 watt, preferably 240 to 300 watt. In accordance with the embodiments of the present disclosure, the step of polymerization involves irradiating the blend with microwaves for a time period in the range of 5 seconds to 300 seconds followed by cooling for a time period in the range of 30 seconds to 600 seconds.

Thus, the polymerization in the process of the present disclosure is simple, efficient and time saving.

In the present disclosure, the polymer gel is not subjected to intermediate steps of cutting, shredding or grinding before the step of partially drying.

In accordance with embodiments of the present disclosure, the step of drying involves the sub-steps of (a) irradiating the polymer gel with microwaves for a time period in the range of 2 seconds to 400 seconds to obtain a heated polymer gel, (b) allowing the heated polymer gel to cool for a time period in the range of 2 seconds to 400 seconds, and (c) repeating steps (a) and (b) till the water content of partially dried super absorbent polymer is reduced to an amount in the range of 1 wt% to 10 wt%.

The step of polymerization and the step of drying are carried out in single reactor. Since both these steps are carried out in the same reactor, the process of the present disclosure is a one pot process. Thus, the process of the present disclosure is simple and convenient.

In accordance with one embodiment of the present disclosure, the step of drying involves the following sub-steps. First sub-step involves irradiating the polymer gel with microwaves for 30 seconds and stopping irradiation to let the heated polymer gel to cool for 1 minute. Second sub-step involves irradiating the polymer gel with microwaves for 20 seconds and stopping irradiation to let the heated polymer gel to cool for 1 minute. Third step involves irradiating the polymer gel with microwaves for 10 seconds and stopping irradiation to let the heated polymer gel to cool for 5 minutes.

It was observed that carrying out the process of drying using intermittent irradiating helps in obtaining white colored SAP.

Brown or coloured SAP is formed as a result of degradation of the polymer chains caused by overheating of the SAP.

Thus, the step of drying in the process of the present disclosure is efficient, time saving and fast. In a second aspect, the super absorbent polymer comprises 60 wt % to 75 wt % of crosslinked metal salt of polyacrylic acid and 15 wt% to 25 wt% of crosslinked poly acrylic acid, with respect to the total weight of the super absorbent polymer. The super absorbent polymer further comprises 1 wt% to 5 wt% of a crosslinked water-swellable polysaccharide, 0.5 wt% to 5 wt% of a crosslinking agent, 0.1 wt% to 5 wt% of a polymerization initiator and 4 wt% to 8 wt% water, with respect to the total weight of the super absorbent polymer. The metal salt of polyacrylic acid, polyacrylic acid and the water-swellable polysaccharide are crosslinked. The super absorbent polymer obtained by the process of the present disclosure has white color, water absorbency in the range of 350 g to 450 g water per g of the SAP, moisture content in the range of 1 weight % to 10 weight %, and hardness in the range of 2 to 4 kgf.

In an exemplary embodiment of the present disclosure, the super absorbent polymer comprises 65 wt% to 70 wt% of crosslinked potassium salt of polyacrylic acid and 17.5 wt% to 22.5 wt% of crosslinked polyacrylic acid, with respect to the total weight of the super absorbent polymer. The super absorbent polymer further comprises 1 wt% to 3 wt% of N,N'- methylenebisacrylamide as crosslinking agent, 4 wt% to 8 wt% water, and 0.5 wt% to 5 wt% of N,N'-methylenebisacrylamide. The potassium salt of polyacrylic acid, polyacrylic acid and carboxymethylcellulose are crosslinked with each other.

In another exemplary embodiment of the present disclosure, the super absorbent polymer comprises 65 wt% to 70 wt% of crosslinked potassium salt of polyacrylic acid, 17.5 wt% to 22.5 wt% of crosslinked polyacrylic acid and 1 wt% to 3 wt% of crosslinked carboxymethylcellulose as water-swellable polysaccharide, with respect to the total weight of the super absorbent polymer. The potassium salt of polyacrylic acid, polyacrylic acid and carboxymethylcellulose are crosslinked with each other. The super absorbent polymer further comprises 1 wt% to 3 wt% of N,N'-methylenebisacrylamide as crosslinking agent, 4 wt% to 8 wt% water, and 0.5 wt% to 5 wt% of N,N'-methylenebisacrylamide.

In accordance with the embodiments of the present disclosure, the metal salt of polyacrylic acid is selected from the group consisting of potassium salt, sodium salt, ammonium salt and calcium salt.

In accordance with one embodiment of the present disclosure, the metal salt of polyacrylic acid is potassium salt. It is observed that, the step of drying by intermittently irradiating the polymer gel helps in obtaining SAP having desired properties. When the process of drying is carried out with continuous microwave irradiation instead of intermittently irradiation as mentioned herein above, the SAP obtained had brownish appearance. The brownish coloration of SAP appears due to overheating of the polymer gel during the step of drying. In the process of the present disclosure, the overheating of the polymer gel has been avoided by intermittent irradiation of the polymer gel by microwaves. Thus, the intermittent irradiation of the polymer gel leads to formation of SAP having white appearance.

The SAP obtained by the process of the present disclosure is crumbly in nature and has high grindability index. The crumbly nature of SAP is a result of use of microwave radiations for polymerization as well as use of intermittent radiations for drying step. Using microwave radiations during polymerization and for intermittent drying leads to the formation of structured voids in the SAP. Due to the formation of structured voids, the SAP obtained is highly porous and fluffy in nature. A photograph of the SAP produced by the process of the present disclosure is shown in Figure-2. Due to the fluffy and crumbly nature, the SAP can be ground easily.

Formation of voids during the step of drying is possible due to microwaves which induce heating by dielectric oscillation. Water and the compounds having polyol groups possess high dielectric constant and therefore interact efficiently with the microwaves. Microwaves penetrate into the polymerized matrix with ease and interact with water leading to internal heating and generation of water vapour that leads to the formation of voids. Furthermore, the polymerization step is rapid and leads to the formation of a cross-linked polymer framework before the water evaporates. The water vapour gets trapped and expands this cross-linked polymer framework leading to the formation of fluffy SAP.

It is observed that the volume of the SAP obtained by the process of the present application is at least 10 times more than the volume of SAP obtained by the conventional process.

This phenomenon of internal heating and formation of water vapour is not possible in the conventional method. SAP obtained by conventional processes is compact, hard and rubbery. The SAP needs to be cut and/or shredded into smaller pieces before the step of drying. A photograph of the SAP pieces produced by the conventional heating process is shown in Figure-3. These compact, hard and rubbery pieces are difficult to dry and/or grind. The step of drying in the process of the present disclosure can be completed in a time period in the range of 2 minutes to 60 minutes. Thus, the process of the present disclosure is efficient and time saving. Further, the process of the present disclosure leads to uniform drying of the super absorbent polymer. The conventional process requires a long time period, such as several hours, for the step of drying. Further, the drying is not uniform.

The SAP obtained by the process of the present disclosure does not need cutting. It is fluffy and can be directly taken as such to the step of grinding.

The process of the present disclosure is simple and efficient. The energy requirement of the process is low as compared to conventional heating process. Therefore, the process of the present disclosure is economic.

The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.

The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be tested to scale up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.

Experiments:

General procedure for determination of water absorbency of SAP

1 g of SAP sample was immersed in water at ambient temperature and the mixture so obtained was stirred for 30 minutes. The mixture was filtered through a strainer of 400 mesh. The water-absorbed SAP residue was collected and weighed. A saline solution can also be used in place of water.

Water absorbency (Q) of the SAP was reported as grams of water absorbed per gram of SAP, and was calculated as per the following formula: Q = [(W-Wo)AVo], where, Wo is the initial weight of SAP (before adding to water or saline solution); and W is the final weight of swollen water-absorbed SAP.

General procedure for determination of moisture content (MC) of SAP The moisture content (MC) of SAP was determined by drying 1 g SAP in a petridish under air circulating oven at 105 °C for 4 hours. The final weight of dry SAP was determined and the moisture content was calculated using the following formula:

% MC = [(Po - P)/Mo] x 100 where, P and Po are weights of petridish containing SAP before and after drying, respectively. Mo is mass of the SAP sample taken for the experiment.

General procedure for determination of hardness of SAP

Hardness was measured in accordance with ASTM D 4197-01 using Cadmech tablet hardness tester, Type CMTT, M/C R/1262/96. The SAP sample specimen was kept in the holder pot and compressed. The maximum reading of the force gauge was noted. Average of three consecutive readings was reported.

General procedure for determination of crushing strength of SAP

Crushing strength of SAP was measured in accordance with ASTM method D4179 and D6175. Grindability index

Grindability index relates to the ease with which the polymer gel comprising super absorbent polymer can be grinded to obtain its powdered form.

High grindability index indicates that the polymerized mass can be easily grinded to obtain powdered form. Low grindability index indicates that the polymerized mass is difficult to grind. Generally, the polymerized mass having low grindability index is rubbery in nature; and the rubbery polymerized mass is cut and shredded before it can be grinded.

Preparation of SAP

Experiment 1:

In a container equipped with a magnetic stirrer and kept in an ice bath, was added acrylic acid (5.5 g) and water (5.5 g) under constant stirring to obtain an aqueous solution of acrylic acid. KOH (7.5 g of 77 % aqueous solution) was slowly added to the aqueous solution of acrylic acid, under constant stirring, to obtain a mixture of partially neutralized acrylic acid, comprising 75 % neutralized acrylic acid and acrylic acid.

After neutralization, the mixture was allowed to cool to 25 °C, and 0.9 g of 0.33% aqueous solution of N,N'-methylenebisacrylamide, 0.9 g of 3.2% aqueous solution of ammonium persulphate and carboxymethylcellulose (CMC) (0.1 g) were added to the mixture to obtain a blend. The blend was transferred to a Teflon beaker and the Teflon beaker was introduced into a microwave chamber capable of emitting irradiation at 2450 MHz with maximum power of 700 W.

The blend was irradiated at 280 Watt for 20 seconds to obtain a polymer gel and then irradiation was stopped to allow the polymer gel to cool for 1 minute. Polymerization was complete in 1 minute to provide a polymer gel comprising super absorbent polymer. The polymer gel contained 50 wt% of water with respect to the amount of water in the blend.

The polymer gel was dried by intermittently irradiating the polymer gel with microwaves. In the first step, the polymer gel was irradiated by microwaves at 700 W for 30 seconds to obtain a heated polymer gel followed by cooling for 1 minute. In the second step, the polymer gel was irradiated by microwaves at for 20 seconds at 700 W followed by cooling for 1 minute. In the third step, the polymer gel was irradiated by microwaves at for 10 seconds at 700 W followed by cooling for 5 minutes. The partially dried SAP obtained after the third step showed presence of 6 weight% moisture.

The partially dried SAP containing 6 weight% moisture was ground in a mixer-blender to obtain SAP in the form of powder having particle size of 100 to 500 microns. Figure-1 illustrates the particle size distribution of SAP obtained from experiment 1. A photograph of SAP obtained in experiment 1 is illustrated in Figure-2.

Thus, the present disclosure provides a simple process for preparation of the super absorbent polymer. The process is simple and fast. The SAP is crumbly and has high grindability index. It was possible to grind the SAP with ease.

Experiment 2:

SAP was prepared using a procedure similar to that of experiment 1 , except that the step of polymerization was performed with microwaves emitted at a power of 700 Watt instead of power of 280 Watt in experiment 1. The results are summari ed in Table 1. Table 1: Effect of microwave output power used for the polymerization of blend during the preparation of SAP

It is observed that the SAP obtained in this experiment had a brownish appearance. Thus, it is clear that the use of high power of microwaves leads to brownish appearance. Experiment 3:

SAP was prepared using a procedure similar to that of experiment 1 , except that the step of drying was carried out using continuous microwave irradiation emitted at a power of 700 W for 1 minute, instead of performing the step of drying by intermittent microwave irradiations. The results are summari ed in Table 2. Table 2: Effect of microwave output power used for the drying of polymer gel during the preparation of SAP

It is observed that the SAP obtained in this experiment had a brownish appearance. Thus, it is clear that the use of drying by intermittently irradiating with microwaves is necessary for obtaining SAP with white appearance.

Experiment 4:

SAP was prepared using a procedure similar to that of experiment 1 , except that the step of polymerization was performed with microwaves emitted at a power of 200 Watt, instead of using microwave irradiation emitted at a power of 280 Watt. The results are summarized in Table 3.

Table 3: Effect of microwave output power used for the polymerization of blend during the preparation of SAP

It is observed that the SAP was white in appearance; however, the polymerization was sluggish.

Effect of amount of water-swellable polysaccharide Experiment 5:

SAP was prepared using a procedure similar to that of experiment 1, except that the blend was prepared without addition of carboxymethylcellulose (CMC).

Experiment 6:

SAP was prepared using a procedure similar to that of experiment 1, except that the blend was prepared with a concentration of 6 weight % CMC with respect to acrylic acid.

The results are summarized in Table 4.

Table 4: Effect of amount of water-swellable polysaccharide used for preparing SAP, on the properties of SAP

It is observed that the SAP obtained in absence of CMC had higher moisture content, and that the SAP obtained with 6 weight% had a brownish appearance and also had higher water absorbency. Experiment ?:

SAP was prepared using a procedure similar to that of experiment 1, except that the blend was prepared with a polymerization initiator concentration of 2 weight % with respect to acrylic acid. The results are summarized in Table 5.

Table 5

Conventional processes

Experiment 8: (Conventional process) Polymer gel containing 50 wt% of water was prepared using a procedure similar to that of experiment 1. The polymer gel was dried by conventional heating at 105 °C for 4 hours. The results are summarized in Table 6. It was observed that the SAP obtained by this process was not crumbly and had a low grindability index.

Experiment 9: (Conventional process)

In a container equipped with a magnetic stirrer and kept in an ice bath, was added acrylic acid (5.5 g) and water (5.5 g) under constant stirring to obtain a mixture. 77 % aqueous solution of

KOH (7.5 g) was slowly added under constant stirring to the mixture for partial neutralization of acrylic acid. 75 % neutralization of acrylic acid was achieved.

After neutralization, temperature of the neutralized mixture was allowed to reduce to 25 °C, and 0.9 g of 0.33% aqueous solution of N,N'-methylenebisacrylamide, 0.9 g of 3.2% aqueous solution of ammonium persulphate and CMC (0.1 g) were added to the mixture to obtain a blend.

The blend was subjected to polymerization by conventional heating at 65 °C. It was observed that the polymerization started with a rapid exotherm and a polymer gel containing SAP was obtained, wherein the SAP was highly elastic by nature. The rubbery SAP was cut into smaller pieces and the step of drying was performed by heating in an oven at 105 °C for 4 hours.

The dried pieces of SAP were rubbery in nature. These pieces could not be ground.

A photograph of SAP obtained in experiment 9 is illustrated Figure 3.

Table 6: Comparison of properties of SAP prepared by conventional processes with the SAP prepared by processes of the present disclosure

It is observed that the dried pieces of SAP obtained in experiment 9 had hardness of 2 Kgf, crushing strength of 2 Kgf/particle and was not crumbly and had low grindability index.

TECHNICAL ADVANCEMENTS The present disclosure described herein above has several technical advantages including, but not limited to, the realization of: a simple, economic, efficient and one pot process for preparation of SAP; a simple process which does not require intermediate steps of cutting, shredding or grinding of polymer before the step of drying; - a process for preparation of white colored SAP which can be ground with ease; and a time saving process for preparation of crumbly SAP.

The embodiments as described herein above, and various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Throughout this specification the word“comprise”, or variations such as“comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps. The use of the expression“at least” or“at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.

The foregoing description of specific embodiments so fully reveal the general nature of the embodiments herein, that others can, by applying current knowledge, readily modify and/or adapt for various applications of such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Further, it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Having described and illustrated the principles of the present disclosure with reference to the described embodiments, it will be recognized that the described embodiments can be modified in arrangement and detail without departing from the scope of such principles.

While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other modifications in the nature of the disclosure or the preferred embodiments will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.

Claims

CLAIMS:

1. A process for preparing a super absorbent polymer (SAP), said process comprising the following steps: i. partially neutralizing aqueous acrylic acid, by adding a predetermined amount of a neutralizing agent to obtain a mixture comprising partially neutralized acrylic acid; ii. adding a cross-linking agent and a polymerization initiator to said mixture under agitation to obtain a blend; iii. irradiating said blend with microwaves, followed by cooling to obtain a polymer gel comprising the super absorbent polymer and 30 wt% to 75 wt% water; iv. drying said polymer gel by intermittently irradiating the polymer gel with microwaves, to obtain partially dried super absorbent polymer comprising 1 wt% to 10 wt% water; v. grinding said partially dried super absorbent polymer to obtain super absorbent polymer in powdered form, having particle size in the range of 50 microns to 900 microns and hardness in the range of 0.5 to 4 kgf.

2. The process as claimed in claim 1, wherein the weight ratio of acrylic acid to water in said aqueous acrylic acid is in the range of 2: 1 to 1 :2.

3. The process as claimed in claim 1, wherein the neutralizing agent is at least one alkali selected from the group consisting of sodium hydroxide, potassium hydroxide, ammonium hydroxide and calcium hydroxide.

4. The process as claimed in claim 1 , wherein the step of partially neutralizing involves neutralizing 60 to 90 mole% of aqueous acrylic acid. 5. The process as claimed in claim 1, wherein the amount of the cross-linking agent is in the range of 0.

5 to 5.0 wt% with respect to the total weight of acrylic acid.

6. The process as claimed in claim 1, wherein the cross-linking agent is N,N'- methylenebisacrylamide.

7. The process as claimed in claim 1, wherein the amount of the polymerization initiator is in the range of 0.1 to 5 wt% with respect to the total weight of acrylic acid.

8. The process as claimed in claim 1, wherein the polymerization initiator is at least one selected from the group consisting of sodium persulfate (Na S Ox), potassium persulfate (K S O ), and ammonium persulfate ((NH₄)₂S₂0g).

9. The process as claimed in claim 1, wherein in step (ii) a water-swellable polysaccharide is added to said mixture under agitation.

10. The process as claimed in claim 9, wherein the amount of the water-swellable polysaccharide is in the range of 1 to 5 wt% with respect to the total weight of acrylic acid.

11. The process as claimed in claim 9, wherein the water-swellable polysaccharide is at least one selected from the group consisting of carboxymethylcellulose, hydroxyethylcellulose, and hydroxypropylcellulose.

12. The process as claimed in claim 1, wherein the microwaves have a frequency of 2450 MHz.

13. The process as claimed in claim 1, wherein the microwaves are emitted by a microwave source having output in the range of 140 to 700 watt, preferably 240 to 300 watt.

14. The process as claimed in claim 1, wherein the step of irradiating involves subjecting the blend to polymerization reaction using microwaves, for a time period in the range of 5 seconds to 300 seconds, followed by cooling for a time period in the range of 30 seconds to 600 seconds.

15. The process as claimed in claim 1, wherein the step of drying involves the following sub-steps: (a) irradiating the polymer gel with microwaves for a time period in the range of 2 seconds to 400 seconds to obtain a heated polymer gel;

(b) allowing the heated polymer gel to cool for a time period in the range of 2 seconds to 400 seconds; and (c) repeating sub-steps (a) and (b) till the water content of the partially dried super absorbent polymer is reduced to an amount in the range of 4 wt% to 8 wt%.

16. The process as claimed in claim 1, wherein the step of irradiating and the step of drying are carried out in a microwave reactor.

17. A super absorbent polymer comprising: i) crosslinked metal salt of polyacrylic acid in an amount in the range of 60 wt% to 75 wt%; and ii) crosslinked poly acrylic acid in an amount in the range of 15 wt% to 25 wt%; wherein said metal salt of polyacrylic acid and said polyacrylic acid are crosslinked with each other; and wherein the weight percentages (wt%) of the components are with respect to the total weight of the super absorbent polymer.

18. The super absorbent polymer as claimed in claim 17, further comprising: i) crosslinked water-swellable polysaccharide in an amount in the range of 1 wt% to 5 wt%; ii) a crosslinking agent in an amount in the range of 0.5 wt% to 5 wt%; and iii) water in an amount in the range of 4 wt% to 8 wt%; wherein said metal salt of polyacrylic acid and said polyacrylic acid said water- swellable polysaccharide are crosslinked with each other; and wherein the weight percentages (wt%) of the components are with respect to the total weight of the super absorbent polymer.

19. The super absorbent polymer as claimed in claim 17, further comprises a polymerization initiator in an amount in the range of 0.1 wt% to 5 wt% with respect to the total weight of the super absorbent polymer.

20. The super absorbent polymer as claimed in claim 17, wherein said metal salt of polyacrylic acid is selected from the group consisting of potassium salt, sodium salt, ammonium salt and calcium salt.

21. The super absorbent polymer as claimed in claim 17, comprising: i) crosslinked potassium salt of polyacrylic acid in an amount in the range of 65 wt% to 70 wt%; and ii) crosslinked poly acrylic acid in an amount in the range of 17.5 wt% to 22.5 wt%; wherein potassium salt of polyacrylic acid, polyacrylic acid and carboxymethylcellulose are cross! inked with each other; wherein the weight percentages (wt%) of components are with respect to the total weight of the super absorbent polymer.

22. The super absorbent polymer as claimed in claim 21, further comprising: i) crosslinked carboxymethylcellulose as water-swellable polysaccharide, in an amount in the range of 1 wt% to 3 wt%; ii) N,N'-methylenebisacrylamide as crosslinking agent, in an amount in the range of 1 wt% to 3 wt%; and iii) water in an amount in the range of 4 wt% to 8 wt%; wherein potassium salt of polyacrylic acid, polyacrylic acid and carboxymethylcellulose are crosslinked with each other; and wherein the weight percentages (wt%) of components are with respect to the total weight of the super absorbent polymer.

23. The super absorbent polymer as claimed in claim 21, further comprises ammonium persulfate as polymerization initiator, in an amount in the range of 0.5 wt% to 3 wt% with respect to the total weight of the super absorbent polymer;

24. The super absorbent polymer as claimed in claim 17, wherein said super absorbent polymer is characterized by: i. white color; ii. water absorbency in the range of 350 g to 450 g water per g of the SAP; iii. moisture content in the range of 5 wt % to 7 wt %; and iv. hardness in the range of 2 to 4 kgf.