Claims:1. A process for the preparation of acrylonitrile based superabsorbent polymer, said process comprising the following steps:
(a) polymerizing acrylonitrile monomer along with at least one cross-linking agent in the presence of sulphuric acid and a pair of redox initiators, under continuous stirring for a predetermined time period, at a first predetermined temperature, to obtain a cross-linked polyacrylonitrile;
(b) hydrolyzing said cross-linked polyacrylonitrile under continuous stirring, at a second predetermined temperature, with at least one alkali in the presence of at least one alkali salt to obtain a hydrolyzed mass comprising hydrolyzed cross-linked polyacrylonitrile; and
(c) isolating said hydrolyzed cross-linked polyacrylonitrile from said hydrolyzed mass and drying the isolated hydrolyzed cross-linked polyacrylonitrile in an oven at a temperature in the range of 90° C to 110° C, to obtain said acrylonitrile based superabsorbent polymer.

2. The process as claimed in claim 1, wherein said cross-linking agent is at least one selected from the group consisting of N,N'-Methylene bisacrylamide, ethylene glycol diacrylate, polyethylene glycol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, glycerol triacrylate, glycerol trimethacrylate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate and triglycol diacrylate.

3. The process as claimed in claim 1, wherein said pair of redox initiators is a persulphate – bisulphite pair, wherein said persulphate is at least one selected from the group consisting of potassium persulphate, sodium persulphate and ammonium persulphate; and said bisulphite is at least one selected from the group consisting of sodium metabisulphite and potassium metabisulphite.

4. The process as claimed in claim 1, wherein said alkali is selected from the group consisting of sodium hydroxide and potassium hydroxide and said alkali salt is selected from the group consisting of sodium chloride and potassium chloride.

5. The process as claimed in claim 1, wherein said first predetermined temperature is in the range of 40° C to 60° C and said second predetermined temperature is in the range of 70° C to 95° C.

6. The process as claimed in claim 1, wherein said predetermined time period is in the range of 2 hours to 10 hours.

7. An acrylonitrile based superabsorbent polymer prepared by the process of claim 1, wherein said superabsorbent polymer possesses water absorbency in the ratio of 300 to 650 g/g-water. , Description:FIELD
The present disclosure relates to a process for the preparation of a superabsorbent polymer from acrylonitrile.
BACKGROUND
Preparation of acrylonitrile based superabsorbent polymer (SAP) is advantageous over an acrylic acid based one. Acrylic acid monomer, on storage for a long time, agitation and at increasing temperatures forms its dimer, due to which there is an extra step of purification of the monomer needed before polymerization and also after the final product is obtained. Unlike acrylic acid, acrylonitrile does not dimerize and hence the extra step of purification to remove the dimer is not required. Furthermore, handling and storage of acrylonitrile is easier and the monomer can even be stored at room temperatures for a long time.
Preparation of superabsorbents using acrylonitrile is a process known in the state of the art. However, none of the processes in the state-of-the-art provide a superabsorbent with a high water absorption capacity above 500 g/g-water. The reason is due to less conversion into hydrolysed material during hydrolysis.
Hence, there is felt need to provide a better and a more efficient process of preparing an acrylonitrile based superabsorbent polymer which has a high water absorption capacity.
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 provide a better and more efficient process of preparation of polyacrylonitrile superabsorbent polymer.
Another object of the present disclosure is to provide a superabsorbent polymer with a higher absorption capacity.
Still another object of the present disclosure is to provide a process to facilitate a higher conversion after hydrolysis.
Yet another object of the present disclosure is to provide a process for the preparation of polyacrylonitrile superabsorbent polymer in a shorter time.
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
A process for the preparation of acrylonitrile based superabsorbent polymer is disclosed.
Acrylonitrile monomer is polymerized along with a cross-linking agent (0.25 to 1 % with respect to monomer) in a reactor by providing the monomer in water, sulphuric acid, a pair of redox initiators and at least one cross-linking agent under continuous stirring for a time period in the range of 2 hours to 10 hours, at a temperature in the range of 40° C to 60° C to obtain cross-linked polyacrylonitrile.
The cross-linked polyacrylonitrile is further subject to hydrolysis using an alkali in the presence of at least one alkali salt under continuous stirring, at a temperature in the range of 70° C to 95° C to obtain a hydrolyzed mass comprising hydrolyzed cross-linked polyacrylonitrile.
Typically, the alkali is selected from the group consisting of potassium hydroxide and sodium hydroxide. The alkali salt is selected from the group consisting of KCl and NaCl.
The hydrolyzed cross-linked polyacrylonitrile is isolated from the hydrolyzed mass by neutralizing the excess alkali using acetic acid during precipitation in methanol followed by drying in an oven to obtain the superabsorbent polymer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING:
The present disclosure will now be described with the help of the accompanying drawing in which:
Figure 1 depicts an FTIR graph of a cross-linked PAN prepared by the process of the present disclosure.
Figure 2 depicts an FTIR graph of a hydrolyzed cross-linked PAN prepared by the process of the present disclosure.
Figure 3 depicts an FTIR graph of a hydrolyzed cross-linked PAN with 1% NaCl solution (NaCl as additive) prepared by the process of the present disclosure.
DETAILED DESCRIPTION
The disclosure will now be described with reference to the accompanying embodiments which do not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration.
The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
A process for the preparation of a superabsorbent polymer from acrylonitrile is described herein.
A reactor vessel equipped with openings for a stirrer, a thermocouple pocket, a condenser and nitrogen supply is provided. The reactor vessel is initially maintained at a temperature in the range of 20° C to 30° C under atmospheric pressure.
A predetermined quantity of acrylonitrile dissolved in a predetermined quantity of water is taken in the reactor. To this solution is added sulphuric acid (10 %) to maintain the pH of ~ 2-3. The solution is constantly bubbled with an inert gas, a non-limiting example of which is nitrogen, to free the solution from any dissolved oxygen. Bubbling is carried out for a time period in the range of 15 min to 2 hours.
While continuously stirring the solution, a pair of redox initiators in an amount in the range of 0.1 wt% to 2.0 wt% with respect to acrylonitrile in a suitable quantity of water and a cross-linking agent in an amount in the range of 0.1 wt% to 1.0 wt% with respect to the monomer-acrylonitrile, in a suitable quantity of water are added to the solution.
The pair of redox initiators is used to initiate the polymerization of acrylonitrile. The pair of redox initiators is selected : as a persulphate – bisulphite pair, wherein the persulphate is at least one selected from the group consisting of potassium persulphate, sodium persulphate and ammonium persulphate; and the bisulphite is at least one selected from the group consisting of sodium metabisulphite and potassium metabisulphite; and
In an embodiment, the pair of potassium persulphate and sodium bisulphite is used as a redox initiator.
The cross-linking agent is at least one selected from the group consisting of N,N'-methylene bisacrylamide, ethylene glycol diacrylate, polyethylene glycol diacrylate, ethylene glycol dimethacrylate, polyethylene glycol dimethacrylate, glycerol triacrylate, glycerol trimethacrylate, trimethylolpropane trimethacrylate, trimethylolpropane triacrylate and triglycol diacrylate. In an embodiment, 0.5 wt% of N,N’-methylene bisacrylamide is used.
With continuous stirring, the reaction temperature is raised to a temperature in the range of 40° C to 60° C. The reaction mixture is stirred continuously for a time period in the range of 1 hour to 2 hours for the polymerization reaction to complete.
The precipitated cross-linked polyacrylonitrile is then filtered and washed with plenty of water followed by washing with methanol. The washed cross-linked polyacrylonitrile is dried in an oven for a time period in the range of 2 hours to 10 hours at a temperature in the range of 90° C to 110° C.
The dried cross-linked polyacrylonitrile is hydrolyzed using an alkali in the presence of alkali salts in a reactor. The hydrolysis is carried out at a temperature in the range of 70° C to 95° C. The reaction is continued till complete hydrolysis is achieved to result in a hydrolyzed mass comprising hydrolyzed cross-linked polyacrylonitrile. This is indicated by a change in color of the reaction mixture from white to deep red to light brown to yellow and finally to light yellow.
The alkali is at least one selected from the group consisting of sodium hydroxide and potassium hydroxide and the alkali salt is at least one selected from the group consisting of sodium chloride and potassium chloride.
The hydrolyzed cross-linked polyacrylonitrile is isolated from the hydrolysis mixture by treating with acetic acid and methanol. The isolated hydrolyzed cross-linked polyacrylonitrile is further dried in an oven at a temperature in the range of 90° C to 110° C to obtain the acrylonitrile based superabsorbent polymer.
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 laboratory scale experiments can be scaled up to industrial/commercial scale:

EXPERIMENTS:
Experiment 1: Precipitation polymerization of acrylonitrile and hydrolysis with 5.6 g KOH for 4 hours (Comparative Example)
20 g of acrylonitrile was taken in 300 ml of water in a 1L reaction kettle equipped with a stirrer, condenser, thermometer pocket and nitrogen gas inlet.
The reaction mixture was taken in a water bath for temperature control. 0.545 g of 10 % sulfuric acid was introduced in the reaction kettle. Nitrogen was bubbled through this reaction mixture for about 30 minutes. 0.1 g of N,N'-methylene bisacrylamide in 10 ml water, 0.7 g of potassium persulphate in 10 ml of water and 0.5 g of sodium metabisulphite in 7 ml of water were added into the reaction kettle at around 20° C. The reaction mixture with continuous stirring was gradually heated. The reaction mass became slightly hazy and opaque when the temperature of the reaction mass was around 23 to 24° C.
With an increase in temperature, turbidity in the reaction kettle also increased. The reaction temperature was raised to 50° C and kept constant for 4 hours under continuous stirring to complete the reaction. The cross-linked polyacrylonitrile was precipitated using methanol as a non-solvent. The precipitated polymer was filtered and washed with plenty of water, followed by washing with methanol and dried in an oven at 105° C for 4 hours. The cross-linked polymer was characterized by FTIR the graph of which is shown in Figure 1. The IR spectra of XL-PAN showed a peak at 2244.8 cm-1, in the characteristic absorption band for –C=N (2210-2260 cm-1), confirming the presence of the nitrile group.
Hydrolysis of XL-PAN (H-XL-PAN)
10 g of cross-linked polyacrylonitrile and 200 ml of water were taken in a 1L reaction kettle equipped with a nitrogen inlet, stirrer, condenser and thermometer pocket.
5.6 g of KOH solution was added into the reaction kettle to form a hydrolysis mixture. The hydrolysis mixture was kept in an oil bath for temperature control and hydrolysis with the KOH solution was carried out at 95° C till complete hydrolysis was achieved. During the hydrolysis, the color of reaction mass changed from white to deep red to light brown to yellow to finally light yellow indicating the completion of hydrolysis. During hydrolysis, NH3 was observed to evolve from the reaction mixture as a by-product which was carried from the condenser to the scrubber (H2SO4 solution).
The reaction product obtained was a highly thick hydrolyzed mass comprising hydrolyzed cross-linked polyacrylonitrile. The hydrolyzed cross-linked polyacrylonitrile was isolated from the hydrolyzed mass by acetic acid and methanol. The hydrolyzed cross-linked polyacrylonitrile was further dried in an oven at 110° C to obtain the superabsorbent polymer.
Experiments 2-3: Precipitation polymerization of ACN and hydrolysis with increased quantity of KOH and/or hydrolysis time (Comparative Examples)
The hydrolyzed cross-linked polyacrylonitrile was prepared similar to Experiment 1 but the hydrolysis time was increased to 12 hours in Experiment 2 and both the quantity of KOH and hydrolysis time were increased in Experiment 3. In Experiment 3, the quantity of KOH used was 7 g and the hydrolysis time was 8 hours. The hydrolyzed cross-linked poly-acrylonitrile was characterized by FTIR the graph of which is shown in Figure 2. The IR spectra of XL-PAN showed a peak at 2244.8 cm-1, in the characteristic absorption band of –C=N (2210-2260 cm-1). The peak at 2244.8 cm-1 has almost disappeared for the IR spectra of saponified / hydrolyzed PAN, but two new peaks at 1564.2 cm-1 and 1672.9 cm-1 for –COOH and –CONH2 are seen. The peak at 1672.9 cm-1 is in the characteristic absorption band of the carboxamide-functional groups of the amide moiety (1640-1690 cm-1) of acrylamide. The very intense characteristic peak at 1564.2 cm-1 is due to the C=O asymmetric stretching in the carboxylate anion that is reconfirmed by another sharp peak at 1404 cm-1, which is related to the symmetric stretching of the carboxylate anion. This indicates that -C=N of PAN was saponified into –COOH and –CONH2 successfully.
Experiments 4-7: Precipitation polymerization of ACN and hydrolysis with increased quantity of KOH and/or hydrolysis time with KCl / NaCl additives
A hydrolyzed cross-linked polyacrylonitrile was prepared similar to Experiment 1. However, during hydrolysis along with varying amounts of KOH, alkali chloride salts KCl and NaCl were used as additives. The hydrolyzed cross-linked poly-acrylonitrile using 1% NaCl additive was characterized by FTIR, the graph of which is shown in Figure 3. The IR spectra of XL-PAN had showed a peak at 2244.8 cm-1, which is in the characteristic absorption band of –C=N (2210-2260 cm-1). This peak has almost disappeared for the IR spectra of saponified / hydrolyzed PAN, but two new peaks at 1571 cm-1 and 1667 cm-1 for –COOH and –CONH2 are seen. The peak at 1667 cm-1 is in the characteristic absorption band of the carboxamide-functional groups of the amide moiety (1640-1690 cm-1) of acrylamide. The very intense characteristic band at 1571 cm-1 is due to the C=O asymmetric stretching in the carboxylate anion that is reconfirmed by another sharp peak at 1405 cm-1, which is related to the symmetric stretching mode of the carboxylate anion. This indicates that -C=N of the PAN was saponified into –COOH and –CONH2 successfully.
Water absorbency of the samples prepared was measured by the following procedure:
Water Absorbency
The superabsorbent sample was weighed (~0.2 g) and poured in a large excess (300 ml) of distilled water to form a slurry and allowed to stand for 30 minutes at room temperature (30° C). The slurry was stirred occasionally with a glass rod. The amount of water absorbed by the superabsorbent sample was determined gravimetrically after the swelling equilibrium was reached after 30 minutes. Then the slurry was filtered through 100 micron mesh wire sieve. The water absorbency (Q) was determined by weighing the swollen superabsorbent polymer and calculated according to the following relationship:
Q (g / g) = (W2 – W1) / W1
W2 = Weight of the swollen gel
W1 = Weight of the dried superabsorbent.
The following Table 1 summarizes the amounts of cross-linked polyacrylonitrile (XL-PAN), water, KOH and additives taken, the hydrolysis time and the resultant water absorbency of the samples prepared.
Table 1: Water Absorbency of cross-linked PAN (XL-PAN) at various degrees of hydrolysis and hydrolysis times
Experiments XL-PAN (g) Water (ml) KOH (g) Time
(h) Additives
Absorbency
g/g [H2O]
1 10 200 5.6 4 No additive 375
2 10 200 5.6 12 483
3 10 200 7 8 480
4 10 200 7 4.5 1% NaCl 688
5 10 200 7 4 1% KCl 626
6 10 200 10 1.5 1% NaCl 658
7 10 200 10 1.25 1% NaCl 630

All the samples were characterized for FTIR by Nicolet FTIR spectrometer Model 6700.
The water absorbency values obtained from the samples which were hydrolyzed using KOH in the presence of 1% of alkali salts NaCl and KCl were more than those hydrolyzed in the absence of the alkali salts.
The process of the present disclosure, successfully, achieves a better superabsorbent with high water absorbency.
TECHNICAL ADVANCES AND ECONOMICAL SIGNIFICANCE
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of:
? a better and more efficient process of preparation of polyacrylonitrile superabsorbent polymer;
? a superabsorbent polymer useful for hygiene and non-hygiene applications with a higher absorption capacity;
? a process to facilitate a higher conversion after hydrolysis; and
? a process for the preparation of polyacrylonitrile superabsorbent polymer in a shorter time.
The foregoing description of the specific embodiments so fully reveals the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications 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.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, is 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.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the components and component parts of the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. These and other changes in the preferred embodiment as well as other embodiments of the disclosure 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.