|Title of Invention||
NOVEL SUPERABSORBENT HYDROGEL/S AND THE METHOD OF OBTAINING THE SAME
|Abstract||Novel superabsorbent cellulosic hydrogels obtained by a method which comprises simultaneous grafting and chemical crosslinking of ethylenically unsaturated monomers onto cellulose ether backbones in the presence of free radical initiator of chemical or non-chemical origin in a homogeneous phase in polar solvent/s at a temperature of 15-100° C, reaction time from instantaneous to 24 hours to achieve the gel point, and an inert or ambient environment. Ethylenically unsaturated monomers herein include acrylic monomers preferably containing carboxamide groups and the like; the cellulose ether backbone comprising any of the carboxyalkylcellulose, hydroxyalkylcellulose or the like; the chemical initiator such as water soluble persulfates of ammonium, potassium, sodium or other alkali metals, ammonium cerric nitrate, peroxides such as hydrogen peroxide or organic peroxides, water soluble azo compounds such as 2,2'-azobis-(2-amidinopropane) and the like, used either alone or in combination with co-initiator; the non-chemical initiator such as electromagnetic radiations; the chemical crosslinking being carried out in the presence of a bisacrylamide. The hydrogels obtained herein possess water absorption potential of at least 15000% on mass-by-mass basis, while retaining structure and fluid absorption properties at temperatures upto 90° C.|
|Full Text||This invention relates to novel superabsorbent hydrogels, particularly semi-Synthetic grafted crosslinked hydrogels. More particularly it relates to interpenetrated networks of derivalized celluloses, grafted and crosslinked with ethylenically unsaturated monomers in aqueous solution. It describes simple methods of making the hydrogel, which is obtained by grafting the backbone, i.e. ccllulosics and their derivatives with
water-soluble ethylenically unsaturated monomer in polar medium. It _ernplqys ..... a crosslinking agent in the presence of a free radical initiator of chemical or non-chemical
origin to promote crosslinking simultaneously. Background
Supcrabsorbenl poKmeis (SAPs). the hvdrogels or hydrocolloids are capable of absorbing many tunes then own weight of fluids such as water and retain it under moderate pressure. These materials aie of diverse chemical origins as recognized in the prior art. Owing to their ability to absorb fluids, they find extensive use in sanitary products including baby napkins, meant 10 absorb baby urine and faecal moisture, female sanitary pads meant to absortrmenstrual fluid and others.
These materials find important application in agriculture as water retaining soil conditioners, a use that is likely to catch up in future as water is being recognized to become the most valuable and scarce commodity in future. The technologies and products that conserve and promote its judicious and enhanced use are likely to be the most sought after in future. The term soil conditioner implies compounds, which favourably alter the physical and/or chemical properties of soil. The concept of using polymer materials as soil conditioners is not new. Natural polymers such as polyuronic acids, alginic acids, agar, gum. pectin, starch have been successfully used in the past for soil condilionmg. However, their eas> bio degradation and low water holding properties are serious bottlenecks in their practical use.
Purely synthetic SAPs include poly aery lates, sulfonatcd polystyrene, polyvinylalcohol, polyethylene oxides. polyvinylpyrollidone, polyacrylonitriles, polyacrylamide and the like. Sonic of these like poly aery lamide have been used for water retention purposes in agriculture. The synthetic superabsorbents used typically for this purpose are generally derived in a two-step process, making the preparation technically difficult and expensive. Another major disadvantage, which limits their wide scale use in agriculture, is their nonbiodegradability in soil, a major cause of concern from environment viewpoint.
There is, thus, a need for superabsorbents based on natural raw materials, which can be chemical entangled with hydrophilic units of synthetic superabsorbent polymers to yield products with superior water holding characteristics and the desired persistence.
A need also exists for a supcrabsorbent material which combines the advantage of liquid absorption potential of conventional SAPs and the advantageous liquid distribution properties of cellulose and their derivatives, by virtue of which the resultant hydrogels do not form soft gelatinous masses when hydrated, have good absorbent properties, gradual releasing potential and controlled biodegradability. Moreover, there is a need for a simple, convenient and inexpensive method for making such materials. Prior Art
Variety of superabsorbent polymers have been developed following different procedures and used under diverse use situations. US Patent No. 3, 669,103 discloses a process for acrylic acid and acrylamide based gelling polymers for use in personal care products. US patent no. 6,500,947 describes a method of making superabsorbent hydrogel from cellulose fibers obtained from wood pulp, by sulfonation of the fillers. However, the use of sulfuric acid renders the hydrogcl mechanically unstable resulting in soft gelatinous mass on exposure to water, making il difficult to handle in practice. In US Patent No. 4,244,880, hydrogels meant for temperature controlled solute delivery system in human body include erosslinked poly N-isopropylacrylamide and erosslinked cellulose ether gels. The method used involves exposure of the reaction mixture to nitrogen atmosphere containing less than 2% oxygen. Another example of such hydrogels is provided in US Patent 5,064,653, which describes hydrophilic foam compositions containing hydrogels belonging to the category of starch grafted on copolymcrs of acrylamide salts, acrylate salts and mixtures thereof. Hydrophilic properties of carboxymethyl cellulose have been utili/ed in the US Patent no. 3,586,648 in treatment of polyurcthane foams in such a manner so as to render the latter hydrophilic.
Use of erosslinked polyacrylamides in plant growing media is well established. US Patent no. 4,579,578 describes free radical polymeri/ation of acrylamide in the presence of N,N-methyIcne bisacrylamide resulting in a hydrogcl capable of absorbing 30 limes its own volume of water.
Similar plant growth compositions are described in US Patent no. 4,559,074 wherein erosslinked non-ionic polyaciylamidc has been incorporated into the porous growth medium. Yet another evidence of the versatile potential of carboxymethyl
cellulose is provided by US Patent no. 6,387,978 reporting preparation of crosslinked eurboxy methyl cellulose involving ionic crosslinking by unions or metal cations, non-ionic crosslinking by chemical crosslinking agent or high energy gamma radiations.
Similar type of polyacrylamide absorbent materials are known in the art and arc described in US Patent nos. 4102340; 3,229,769; 3,670,731. The polymerization techniques for the aforesaid materials include the use of anionic peroxide catalysts, photopolymerization with a riboflavin activator and the like.
The hydrogeis established in the aforesaid patents cither do not possess considerable mechanical strength to be used in agriculture or lack the requisite swelling potential. Some of these arc of complete synthetic origin, which limits their use in agriculture.
A need, therefore exists to develop a superabsorbent material which is not purely synthetic, which owing to the chemical crosslinks is not biodegradable, but in due course of time may degrade down, which is a smart hydrogel meaning that it responds to the changes in pH, temperature of the soil / media environment and can swell while remaining stable at high temperatures similar to those encountered in arid region soils. The present invention aims at satisfying these needs of agricultural usage. The description herein of certain advantages and disadvantages of known derivati/cd cellulose or cellulosic fibres and method of their preparation is not intended to limit the seope of the present invention. Indeed, the present invention may include some or all of the methods and chemical reagents described above in addition to the novel ones, without suffering from the same disadvantages. Statement of the invention
This invention relates to novel superabsorbenl cellulosic hydrogeis and method of obtaining the same, characterized by simultaneous grafting and chemical crosslinking of elhylenically unsaturaled monomers onto cellulose ether backbones in the presence of free radical initiator of chemical or non-chemical origin in a homogeneous phase in polar solvent/s at a temperature of 15-100° C, reaction time from instantaneous to 24 hours to achieve the gel point, and an inert or ambient reaction environment. In a preferred embodiment, the superabsorbent polymers are formed from ethylenically unsaturaled monomer/s comprising an acrylamide, an acrylonitrile, an acrylamido-
propanesulfonic acid, an acrylic acid or the like, used at a concentration ranging from 3-85% on mass-by-mass basis of weight of the backbone, preferably from 10 to 70%. grafted on to a cellulose ether backbone exemplified by cellulose derivatives such as earboxyalkylcellulosc, hydroxyalkylccllulose or the like and simultaneously erosslinking the same using a chemical crosslinker exemplified by a bisacrylamidc at a concentration of 0.05-10% on mass-by-mass basis of the weight of the backbone, preferably at 1 to 3.5 percent, all homogenized in a polar solvent such as water, methanol, ethanol, propanol and the like, either alone or in mixtures; more preferably water alone and healed at a preferable temperature of 25° to 60° C followed by addition of a free radical chemical initiator comprising any of the water soluble persulfatcs such as ammonium, potassium, sodium or other alkali metal persulfates, ammonium cerric nitrate, peroxides such as hydrogen peroxide or organic peroxides, water soluble a/.o compounds such as 2,2"-azobis-(2-amidinopropanc) and the like, either alone or in combination with co-initiator, used at a concentration of 0.005 to 6.5%, preferably from 0.01 to about 5% on mass by mass basis of the total weight of backbone polymer plus vinyl monomer, or exposure to the non-chemical initiator such as electromagnetic radiation exemplified by gamma rays, high energy ultraviolet rays and the like, in an inert or ambient reaction environment, preferably ambient environment to achieve the gel point in a time period of instantaneous to 24 hours. Alternatively, irradiation of the homogenized reaction mixture along with the chemical initiator in a microwave oven for a preferable period ranging from two seconds to five minutes also results in the superabsorbent polymer with desired characteristics.
The superabsorbent polymers of the present invention have exceptionally higher water absorptive properties relative to known superabsorbent polymers, of the order of at least 15000% on mass-by-mass basis. The hydrogels of the present invention remain intact even at temperatures above 50° C. These do not present any undesirable risks to soil environment owing to their controlled biodegradation over time unlike the known non-biodegradable hydrogels used in agriculture. Brief description of drawings
While the specification concludes with claims particularly pointing out and distinctly claiming the present invention, it is believed that features of the invention will be better understood from the following description of its preferred embodiments.
Figure 1 is a scanning electron micrograph of a representative backbone polymer. Figure 2 is scanning electron micrograph of a representative supcrabsorbenl polymer obtained by grafting an acrylamide on to the above mentioned backbone polymer in the presence of a bisaerylamide
Detailed description of the invention
The present invention relates to novel superabsorbent cellulosic hydrogels and method of
obtaining the same, characterized by simultaneous grafting and chemical crosslinking of
ethylenically unsaturated monomers onto cellulose ether backbones in the presence of
initiator of chemical or non-chemical origin, the chemical initiator being used with or
without co-initiator, in a homogeneous phase in polar solvent/s at a temperature of 15-
100 C, reaction time to achieve the gel point from instantaneous to 24 hours and an inert
or ambient reaction environment.
The hydrogels obtained by following the proccdurc/s described herein have improved
grafting efficiency preferably of the order of greater than 70%, crosslinking density of
less than 50%, a free swell capacity of greater than 15000% in distilled water and of
greater than 7000% in ionic solutions preferably those containing NH4+, NOj", N(V ions
and the like.
As used herein and in the claims, the singular forms 'a\ 'an* and 'the1 include plural
reference unless the context clearly dictates otherwise.
The hydrogels of the present invention preferably possess highly desirable water sorptivc
characteristics. For example, the hydrogels herein preferably have a grafting efficiency in
the range 5->95 percent. The grafting efficiency is a measure of how efficiently the
ethylencally unsaturated monomer is grafted onto the backbone. The superabsorbent
polymers of the present invention also preferably have a crosslinking density of less than
about 50 percent.
The superabsorbent polymers of the present invention have remarkably high water
absorption potential expressed as percent swelling, from about 2000% to 40,000%, this
being expressed in the pH range 4-9, preferably > 2000 % at pH 4.0, more preferably >
4000% at pll 9.0 and most preferably greater than 15000% at pi I 7.0. Percent swelling is
computed as follows:
Ps(Weight of swollen material weight of dry sample) x 100
Weight of dry sample
This may also be expressed as free swell. The free swell measures the ability of a polymer to absorb fluid, preferably water without being subjected to a confining or
restraining pressure. In the present invention, free swell is preferably determined by gravimetric method described in one of the examples provided herein.
The superabsorbent polymers of the present invention possess granular texture, thus making them potential candidates for use in diverse areas. These are insoluble in water but swell with a high rate of expansion. These polymers are preferably prepared by a process which may include but preferably docs not require any inert atmosphere, without affecting adversely the degree of swelling of the resulting product.The structure of the gel also remains intact.
The superabsorbent polymers of the present invention also preferably have a residual moisture retention potential of the order of>30% on mass-by-mass basis at 15 bars pressure. The residual moisture retained, as measured in the present invention by pressure plate membrane apparatus, points towards the ability of the material to retain fluid preferably water/ ionic solution against different pressure gradients as equivalent to the stress levels generated in plant-soil systems.
The process of the present invention has a number of advantages over known processes. For example, one of the preferred steps in the present invention involves the use of limited amount of water i.e. at high consistency, yet achieving high water absorbency. Another novelty lies in the fact that I he reaction can be accomplished in temperature range of 20°C to 40°C without the need to heat at temperature greater than 50°C.
As used herein, the term consistency refers lo the concentration of backbone polymer in the reaction mixture. As such, the consistency represents the weight of backbone polymer present in a mixture divided by total weight of the mixture multiplied by 100.
Cellulose ethers termed as backbone polymer herein for grafting and crosslink ing of cthylinically unsaturatcd monomers thereon include for example, carboxymethyl cellulose, hydroxypropyl cellulose, hydroxycthyl cellulose, hydroxypropylmclhyl cellulose, ethyl hydroxy ethyl cellulose, methyl cellulose and the like.
It is preferred in the present invention that the ethylenically unsaluraled monomer contains at least one carboxamide group. Suitable ethylenically unsaturated monomers are acrylic monomers. Particularly preferred monomers include such specific compounds as acrylamide, meihacrylamide, N-mcthaciylamide, N-ethacrylamide, N-
isopropylacrylamide, diacetoneacrylamide, 2-acryiamido-2-methyl-1 -propanesulfonic acid and its sails and the like. Acrylonilrilc, mclhacrylonilrilc and the like are also suitable for use as an ethylenically unsaturated monomers in the present invention.
The elhylenically unsalurated monomer may further contain an aeid moiety. Such monomers are also well known in the art and include such specific compounds as acrylic acid, melhacrylic acid, ethaerylic acid, a-chloroacrylie acid, u-cyanoacrylic aeid, vinyl sulfonic acid, acrylamidopropanesulfonic acid, crotonic acid, acryloxypropionic acid and the like.
The more preferred ethylenically unsaturated monomers include acrylamide, methacrylamide, aerylonilrile, acrylic acid, methacrylic aeid, alpha-cyanocrylic acid and aerylamide-2-methy 1-1 -propane sulfonic acid and its salts.
Specially preferred monomers are acrylamide, aerylouitrile and 2-acrylamido-2-methy 1-1-propane sulfonic acid and mixtures thereof.
The grafting may be carried out using monomer as such without rceryslallization and/or neutralization or they may be recrystallized and/or neutralized, completely or partially, prior to mixing with the backbone polymer. Preferably, the amide monomers are used as such without recrystallization and acid monomers without neutralization in the grading process. Grafting efficiency gels lowered if the aeid monomers are neutralized prior to grafting.
Compounds that are used to neutralize the acid group include those which do not have any adverse effect on the grafting process, yet they sufficiently neutralize the acid. The amount of monomers in the reaction mixture can be such that is suitable lo result in hydrogels having superabsorbent properties, as well as other desirable characteristics described herein. It is preferred that the amount of monomers used in the present invention be within the range from about 3% to 85% by weight based on the total weight of reaelants. Preferably, the amount is within the range of from about 10 lo 70% and more preferably from about 17 to 55 percent by weight, based on the total weight of the reactanls.
It is preferred in the present invention that the cellulose ether backbone is grafted with the ethylenieally unsalurated monomer in the presence of a free radical initiator which may or may not be of chemical origin. Suitable chemical free radical initiators
used in the present invention include for example ammonium cerric nitrate, water soluble persulfates such as ammonium per sulfate, potassium per sulfale, sodium per sulfate and other alkali metal per sulfates, hydrogen peroxide, organic peroxide, water soluble azo compounds such as 2,2'-a/obis-(2-amidinopropane) and the like. Some of these initiators may or may not be combined with the co-initiators. For example, persulfates, may be combined with tctramethyl cthylcne diamine (TEMED) or used as such. Hydrogen peroxide may be combined with iron, sulfites or amines to initiate the grafting reaction or may be used as such under mild alkaline conditions and reduced pressure. Persulfates with or without TEMED are particularly preferred initiators for use in the present invention. Non-chemical means of initiation as used in the present invention include electromagnetic radiation such as gamma rays, high-energy ultraviolet rays and the like.
The total amount of initiators used may range from about 0.005 to about 6.5%, preferably from 0.01 to about 5% and more preferably from 0.5 to 4.0% on mass-by-mass basis of the total weight of backbone polymer plus vinyl monomer.
In order to obtain hydrogels of very high absorbancy and to render them water insoluble, the grafting of monomer on to the cellulosic backbone is achieved in the presence of chemical crosslinker. Crosslinking agents used in the present invention include those having polyfunclional groups capable of creating inter and intracrosslinks between backbone and grafted chains.
The crosslinking agent is used in an amount that affords hydrogcl with desired crosslinking density. Preferably, the superabsorbent in the present invention has a crosslinking density in the range 0.01-50%, more preferably, 1-40% and most preferably 10-35 percent.
Preferably, the crosslinking agent is used in an amount ranging from 0.05-10% based on the total weight of the monomer and the backbone polymer. More preferably, the amount of crosslinking agent varies between 1 to 3.5 percent. It is observed in the present invention that use of more than 4% crosslinker on weight basis leads to products with lower absorbaney but equilibration swelling time is attained ai a faster rate. Polyfunctional crosslinking agents preferably used in the present invention are selected from the following: N, N-mclhylenebisacrylamide, N, N-cthy]cnebi;nicrylamide N, N-dihydroxyethylenobisacrylamide and the like and/or the mixtures thereof.
The crosslinking agents preferably used in the present invention are typically soluble in water at a temperature ranging from 5°-50°C, more preferably IVoml5°-40°C.
Another preferred embodiment describes the methods of preparation of the SLiperabsorbcnt materials of the present invention.
In one method, the backbone material is homogenized in distilled water to get lump free slurry. An aqueous solution of monomer and crosslinker is added to the former with continuous stirring. The consistency of the reaction mixture is kept below 20%, preferably below 17% and more preferably in the range 3% -15%.
Monomer/s and crosslinking agents along with backbone polymer are homogeni/ed in appropriate amount of the solvent. Preferably the reaction is carried out in the presence of water as solvent, more preferably distilled water although instead of distilled water other polar solvents like methanol, elhanol, propanol and the like either alone or in mixtures of different proportions with water are also used. It has been observed that the amount of water present in the reaction mixture influences the grafting efficiency. For instance, a very low amount of water ( This homogenized reaction mixture is then treated with free radical initiator/ initiator-accelerator system, while continuous stirring is maintained. The hydrogels so obtained are dried following appropriate workup procedures as known in the art.
The reaction temperature as well as the reaction period will depend on the amount of initiator used and the technique used to attain the gel point. For example, the reaction temperature generally employed in the present invention lies in the range 15 -100 C, preferably 20°-80° C and more preferably 25° to 60°C. In the present invention, the time required for completion of the reaction ranges from instantaneous to 24 hours. Instead of attaining the required temperature through conventional methods over a prolonged period, microwave assisted polymerization as used in the present invention yields hydrogels in a time period ranging from 2 seconds to 5 minutes, preferably from 5 seconds to 2 minutes whereas more preferable lime of exposure is 10 seconds to 1 minute for efficient grafting.
Additionally, synthesis of hydrogel by use of initiator without the help of co-initiator is effectively achieved as in ease of ammonium per sulfate without letramelhyl ethylenediamine by this mode of heating in a limited time interval of 35-50 seconds. The hydrogel containing mixture is treated with appropriate solvent to remove the homopolymer and the unreacted backbone, if left. Afterwards, the xerogel is obtained by dehydration using organic solvent like acetone, mcthunol, etc.
The grafting reaction is carried out in inert atmosphere or under ambient environment. Preferably, gel is prepared in ambient environment. Reaction vessels used ill the present invention include simple glass wares such as beakers or other similar containers.
Recovery of the xerogel in the present context means that all of the water present in the hydrogel is removed therefrom. Preferably, this recovery is done by continuous magnetic Stirring of the gel in a dehydrating solvent like acetone, methanol, ethanol, propanol and the like for a lime period ranging from 30-60 minutes. The xerogel then can be recovered by suction filtration.
The recovered xerogel may still contain traces of water, which may remain entrapped in the inner matrix of the polymer. Preferably, the amount of this water is --50% on weight basis of the grafted hydrogel, more preferably The invention will be illustrated but not limited by the following examples; those skilled in the art recognise that various modifications can be made to the invention without departing from the spirit and scope thereof.
Examples Test Methods
The following test methods were used to measure and optimize various hydo-physieal characteristics of the inventive superabsorbenl hydrogels.
Free Swell Test: Sieve method was used to measure the water absorption potential and the pattern as a (unction of monomer, erosslinker and/ or initiator concentration, quantity
of water in reaction mixture, equilibration temperature, time and pH of the swelling medium, under zero load or free swell of the superabsorbent of the present invention.
In this test, 0.1 g, on dry weight basis, of superabsorbent gel was immersed in 100 nil distilled water taken in a beaker. The swelling was allowed for 2 hours at 5 different temperatures viz. 20, 30, 40, 50 and 60°C. The gels were then filtered through plastic sieve, extra water wiped off, and finally weighed. The most optimum temperature at which gels of different compositions showed maximum swelling was used to study the swelling behavior for different time periods ranging from 2 hrs to 24 hours.
The results were used to calculate the amount of distilled water in gram absorbed per gram of superabsorbent material and expressed as Ps (% swelling).
Ps = (Weight of swollen material - weight of dry sample) x 100
Weight of dry sample
Residual Moisture Retention Test Method:
This method was used to determine the moisture retention by the superabsorbents of the present invention under different pressure gradients. This test was carried out in pressure plate membrane apparatus. Example 1
This example provides a representative method to prepare hydro gel of the present invention at an optimum consistency level.
Carboxymclhylccllulosc sodium salt (CMC-Na) of viscosity (1500 ePs) and degree of substitution not less than 0.4, was procured from S.D. fine - Chcm, Limited, Mumbai, INDIA and used after first immersing in acetone and then drying in oven at 50' C for 24 hours. A 1.0 g sample of CMC - Na salt was homogenized using magnetic stirrer of high speed in 15 ml distilled water (pi I 7.0). Aerylamide (0.75 g) and methylcnebis acrylamidc (0.05 g) were dissolved in 5 ml water and added to the slurry of CMC-Na salt taken in a 250 ml glass beaker. The reaction mixture was allowed to attain temperature of 40°C in a thermostat controlled water bath. Ammonium per sulfate(APS,0.05 g) and lelramethylethylene diamines (TEMED, 0.2 ml) were added with continuous stirring. Gel formed immediately with a sudden rise in temperature (80°C). The gel was kept overnight undisturbed. Excess water was added and stirred magnetically for 2 hours.
The gel mass was centrifuged and separated from liquid phase. Aeetone (250) ml was added in 3 installments to remove water from the gel. The super absorbent in the shrunken state was first dried in oven at 50°C for 24 hours and then preserved in CaCl2 desiceator under atmospheric pressure. The % add-on was 44.00% and grafting efficiency 61 percent. Example 2
A sample of CMC-Na procured and dried as described in example 1. Acrylamide (0.75g) and diliydroxyethylcnebisacrylamide (0.05 g) were dissolved in 15 ml water. CMC - Na salt (l.Og) was added continuously with manual stirring to achieve homogeneous slurry. The reaction mixture was irradiated in microwave oven for 15 seconds and immediately brought out. Without delay, ammonium per sulfate (0.05 g) and tctramcthylethylcnediamine (0.2 ml) were added successively with continuous stirring. The gel point reached immediately but was kept as such overnight. The gel mixture was stirred magnetically in excess water for 2 hours. The gel mass was centrifuged at 15000 rpm speed and separated from liquid phase. Acetone (300 ml) was added in 3 installments to remove water from the gel. The superabsorbenl in the shrunken state was first dried in oven at 50° C overnight and then preserved in CaC^ desiccator under atmospheric pressure. Add-on was 38 % and grafting efficiency was 54%.
CMC-Na salt (2.05 g), acrylamide (1.50 g), methylenebisacrylainide (O.lg), ammonium per sulfate (0.1 g), TEMED (0.4 ml) were added in the order described here in 15 ml water taken in a glass beaker. The reaction mixture was kept in a microwave oven and irradiated in installments of 10 seconds each for a total 30 seconds. The gel obtained was worked up as described in example 1. Example 4
Example 3 was repeated except that the reaction excluded the use of TEMED as initiator. The time of microwave exposure was 35 seconds. Example 5
Example 3 was repealed except that instead of water, mclhanol was used and time of microwave exposure was 45 seconds.
The hydrogcl (1.0 g) developed in llie present invenlion as described in example 1 was hydrolysed partially by immersing in 50 ml (0.5 N aqueous NaOH) and stirring magnetically at 25n C for 24 hours. The swollen gel so achieved was cenlrifugcd, washed many times alternately with water and methanol and finally dried as described in example 1. The xerogel was preserved in a CaCla dcscicalor under reduced pressure. The yield was 80 percent. Example 7
This example describes the free swell behaviour of a superabsorbent developed in the present invention as described in the example no. 6. The results are summari/ed in lables 1-2.
Table 1: Swelling behaviour of a representative hydrogcl at different temperatures
Table 2. Swelling behaviour as studied for different time periods
Spectral characterization ofhydrogels of present invention
Comparative FT-IR analysis of CMC-Na salt and the hydrogel obtained therefrom by
grafting Acrylamide by method described in example 1 proved that grafting has taken
place on the backbone polymer in the present invention.
Absorption bands at 3500-3300 cm"1 (O-H stretching) 2875 cm"1 (CH stretching) 1600
cm"' (CO stretching), 1019cm"1 (C-O-C stretching) are attributed lo Oil groups, C'-II
bonds, carboxylate groups and anhydroglucose rings respectively of derivatized
cellulose. In grafted hydrogel, addilional bands al 1670 cm"1, 1660 cm"1 (amide I), 1616
cm"1 (amide II bands) can be attributed to CONHa group and thus provide evidence of
uC-Solid slale NMR analysis of Ihe CMC-Na sample mentioned above described the structures and mode of linkage in ungrafted cellulose ether used in the present invention, more particularly CMC-Na salt and the grafted and crosslinked hydrogel obtained therefrom.
In UC NMR spectrum of pure CMC - Na salt sharp peak at 178.09ppm pertains to C-8 carbon, which is maximum downfield due lo carboxylulc union character. Peak at
104.83 ppm may be assigned to C-l, which is surrounded on two sides by O atoms leading to downfield shift. Broad peak in the range 80-83 ppm is due to C-4 and Cl b-C of carboxymethyl group. The prominent signal at 74.79 ppm is characteristic of C-2, C-3 and C-5 carbon whereas peak at 61.76 ppm pertains to C-6 carbon. In the C-l 3 NMR of Polyacrylamide (grafted) - methylenebisacrylamide (crosslinked)-CMC hydrogel of the present invention, in addition to the above mentioned peaks, a prominent peak at 1 79.640 ppm which approximately overlaps the parent 178.1 ppm peak of COO" carbon, pertains lo CONII2 carbon. A sharp peak at 42.08 ppm pertains to methane carbons in the network, which are directly attached to Cl 1-NH- group. A shoulder to this peak at 38.73 ppm is assigned lo methylcne carbons. A broad cluster of peaks in the range 58-64 ppm point towards chemical C-ll-O-CFb linkage between carboxylate oxygen of CMC-Na and vinyl Clk of acrylamide. Peak at 54 ppm points towards ether linkage between hydroxyl oxygen and vinyl Clh end of acrylamide.
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1) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, characterized by simultaneous grafting and chemical cross linking of ethylinically unsaturated monomers onto cellulose based backbones, either alone or in combination, in presence of free radical initiator of chemical or non-chemical origin and cross linker of chemical origin, in a homogeneous polar phase at a temperature of 15-100° C, an inert or ambient reaction environment, and reaction time from instantaneous to 24 hours to achieve the gel point, and hydrolysis of the product during or post reaction, to result in neutral or near neutral xerogel product with nearly 50% monomer grafting on the backbone, water absorption potential of at least 15000 to 60000 %, preferably at least 35000%, mass-by-mass of the xerogel, and a situation specific release of the absorbed fluid, for use as such or along with other inputs in diverse areas of applications.
2) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claim 1, wherein the ethylinically unsaturated monomer(s) used for grafting are exemplified by any one of an acrylamide, an acrylonitrile, an acrylamido-propane sulfonic acid, an acrylic acid or the like, used at a concentration ranging from 3-85% by. mass of the mass of the backbone, preferably from 10 to 70% on mass by mass basis.
3) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claim 1, wherein the cellulose based backbone is exemplified by cellulose and or its derivatives such as carboxyalkylcellulose, hydroxyalkylcellulose or the likes, either alone or in combinations.
4) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claim 1, wherein the chemical initiator used is any of water soluble persulfates such as ammonium, potassium, sodium or other alkali metal persulfates, ammonium cerric nitrate, peroxides such as hydrogen peroxide or organic peroxides, water soluble azo compounds such as 2,2'-azobis-(2-amidinopropane) and the like, used either alone or in combination with co-initiator exemplified by ammonium persulfate alone or in combination with
tetramethylethylenediamine and the like, and wherein the chemical initiator is used at a concentration of 0.005 to 6.5%, preferably 0.01 to about 5% on mass by mass basis of the total weight of backbone polymer plus vinyl monomer.
5) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claims 1 and 4, wherein the non-chemical initiator is electromagnetic radiation exemplified by gamma rays, high energy ultraviolet rays and the like.
6) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claim 1, wherein chemical crosslinking is carried out in the presence of bisacrylamide exemplified by N, N-dihydroxyethylenebisacrylamide, N, N-methylene bisacrylamide and the like at a concentration of 0.05-10% on mass-by-mass basis of the backbone plus monomer mass, preferably at 1 to 3.5 percent.
7) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claim 1, wherein homogenization and reaction of monomer, crosslinker, chemical initiator and backbone polymer are achieved in a polar solvent such as water, methanol, ethanol, propanol and the like, either alone or in mixtures; preferably water alone at preferable temperature of 25° to 60° C, with continuous stirring in an inert or ambient reaction environment, preferably ambient environment, and reaction time of instantaneous to 24 hours to achieve the gel point to yield the superabsorbent xerogel polymer of claim 1.
8) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claims 1 and 7, wherein the homogeneous mixture of claim 7 involving all the reactants including the initiator in an inert or ambient reaction environment, preferably ambient environment, may alternatively be irradiated in a microwave oven for a preferable period ranging from two seconds to five minutes to yield the superabsorbent xerogel polymer of claim 1.
9) Novel superabsorbent cellulosic hydrogels and method of obtaining the same, as claimed in claim 1, wherein hydrolysis of the reaction product during or post reaction is carried out employing the usual hydrolyzing reagents such as hydroxides of sodium, potassium or others, and freeing the hydrolyzed product of any excess of the reagent, to yield the superabsorbent xerogel polymer of claim 1.
10) The novel cellulosic superabsorbent xerpgel polymers as claimed in any of the above claims and substantiated in the body of the application, which retain structure and fluid absorption properties at temperatures upto 90° C, either alone or in combination with other inputs such as agrochemicals including fertilizers, pesticides and the like, for use in diverse areas of application such as agriculture particularly water stress agriculture, controlled environment hi-tech agriculture, post harvest conservation of agricultural produce; personal hygiene products and the others wherein the term agriculture includes floriculture, horticulture, forestry and other areas related to plant systems in any manner.
|Indian Patent Application Number||3462/DEL/2005|
|PG Journal Number||52/2011|
|Date of Filing||23-Dec-2005|
|Name of Patentee||INDIAN COUNCIL OF AGRICULTURAL RESEARCH|
|Applicant Address||INDIAN COUNCIL OF AGRICULTURAL RESEARCH, KRISHI BHAWAN, DR.RAJENDRA PRASAD ROAD, NEW DELHI-110001|
|PCT International Classification Number||A61K15/60|
|PCT International Application Number||N/A|
|PCT International Filing date|