Absorbent Soy Protein Isolate Base Hydrogels

Abstract

Absorbent hydrogels are formed from soy protein isolate. More specifically, an absorbent hydrogel is formed by contacting soy protein isolate with urea to produce solubilized soy protein isolate which is combined with 2-mercaptoethanol to form a first mixture. The first mixture is heated and combined with a polymerizable monomer and ammonium persulfate to form a second mixture. The second mixture is then heated, followed by removal of the hydrogel from the second mixture. Optionally, the as-formed hydrogel may be subjected to a washing process to extract non-reactant components from the gel and then dried.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This Application claims the benefit of U.S. Provisional Application No. 61/030,783 entitled “Absorbent Hydrogels” filed on Feb. 22, 2008.

FIELD OF THE INVENTION

The present invention pertains to the field of absorbent and superabsorbent hydrogels and, more specifically, to hydrogels derived from soy protein isolate.

BACKGROUND OF THE INVENTION

Absorbent and superabsorbent hydrogels are typically obtained from vegetable or animal proteins, mixtures of vegetable and animal proteins, cellulose, hemicellulose, saccharides (typically C5 and C6 sugars present in plant derived proteins), and polysaccharides. In general, hydrogels are utilized in industrial dewatering applications; maintaining moisture retention in soils, especially in regions experiencing low rainfall; remediation of heavy metal contaminated soil, based on complexation of heavy metal cations with polycarboxylic acids; in control of wildfires, based on application of water-saturated hydrogels; and in diaper and other water/urine absorbing applications.

The combined market for hydrogels is over one billion pounds per year in the U.S. and about 2.5 times that globally, with a growth rate in both markets of approximately 3% per year. Most water-soluble polymers and hydrogels are currently prepared from petroleum-based monomers. Petroleum-based feedstocks for hydrogels include polyvinyl alcohol, polyacrylic acid, polyacrylamide and maleic anhydride/butylene copolymers.

Existing hydrogels are characterized by incorporation of a relatively high percentage of water solubilizing groups such as carboxylic acid, amide and alcohol groups. High performance hydrogels are further characterized by appropriate levels of cross-linking as illustrated in polysuccinimide cross-linked with polyaspartic acid and cross-linked copolymers of maleic anhydride and maleimide.

In prior USDA-funded work, soluble soy protein from soy protein isolate has been converted to high performance hydrogels by using ethylenediaminetetraacetic dianhydride (EDTAD) as the key reagent to provide protein cross-linking and introduce pendant carboxylic acid groups by reaction with lysine amine groups. See D. C. Hwang and S. Damodaran, J. Appl. Polymer Sci. 1996, 62, 1285. See, also U.S. Pat. No. 5,847,089 to Damodaran & Hwang and U.S. Pat. No. 6,310,105 to Damodaran. However, this work has economic disadvantages in that EDTAD is priced at $1139/50 g (equivalent to $1262/pound) in the 2007-2008 Aldrich catalog. Currently, EDTAD is not available at bulk scale.

Work by Yang, et al. describes methods for grafting methacrylic acid to the protein in soy protein isolate when using persulfate radical initiators. See C. Yang, et al., Journal of Applied Polymer Science 2006, 102, 4023-4029. However, Yang et al., does not employ acrylate based cross-linkers that are necessary to convert soy protein to a hydrogel. Methods for grafting acrylic monomers to cellulose using ceric ammonium nitrate (Ce(IV)) initiated radical grafting have been described. Although the grafted cellulose products did have somewhat enhanced water absorption compared to non-modified cellulose, none of these products were indicated as having hydrogel properties. See E. Rezai and R. R. Warner, Journal of Applied Polymer Science 1997, 65, 1463-1469; and V. Jain, H. Xiao, and Y. Ni, Journal of Applied Polymer Science 2007, 105, 3195-3203. Hydrogel production using methods for grafting acrylic acid to artemesia seed gum (a natural high molecular weight polysaccharide) using a microwave oven have also been described. See J. Jhang, et al., Journal of Macromolecular Science 2007, 44, 881-885.

SUMMARY OF THE INVENTION

The present invention is directed to forming absorbent hydrogels from soy protein isolate. More specifically, in accordance with the invention, an absorbing hydrogel is formed by initially contacting soy protein isolate with urea at elevated temperature for about 0.5 hours to produce solubilized soy protein isolate. The solubilized soy protein isolate is then mixed with 2-mercaptoethanol, wherein the 2-mercaptoethanol is preferably in an amount of about 25% by weight of the soy protein isolate. The first mixture is then heated at a temperature of about 80° C. for approximately 1 hour. A polymerizable monomer in an amount of about 200% by weight of soy protein isolate and a radical initiator, preferably, ammonium persulfate, in an amount of about 20% by weight of soy protein isolate are combined with the first mixture to form a second mixture. After heating the second mixture at a temperature of about 80° C. for approximately 1-3 hours, the hydrogel is removed from the second mixture. Optionally, the as-formed hydrogel may be subjected to a washing process utilizing either water or an organic solvent, an initial water wash followed by washing with organic solvent, or using a mixture of water and organic solvent in order to extract non-reactant components from the gel, before subjecting the gel to a final drying process. The resultant dried absorbent and superabsorbent hydrogels have high water uptake ratios, and can be utilized for a variety of applications.

Additional objects, features and advantages of the invention will become readily apparent from the following description of preferred embodiments.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is generally directed to the formation of absorbent and superabsorbent hydrogels utilizing a soy protein isolate base. At this point it should be noted that the term absorbent hydrogel is meant be inclusive of all hydrogels herein, while the term superabsorbent hydrogel is an absorbent hydrogel (typically a polymer) that can absorb about 100 times or more its weight in water and does not easily release this liquid under pressure.

In accordance with the present invention, a solubilized protein isolate is combined at elevated temperatures with a disulfide cleavage reagent, a polymerizable monomer and a radical initiator, preferably ammonium persulfate (APS), to produce an absorbent hydrogel. More specifically, soy protein isolate and urea are heated at a temperature of approximately 80° C. for approximately 0.5 hours to produce a solubilized soy protein isolate. The solubilized soy protein isolate is then combined with a disulfide cleavage reagent of 2-mercaptoethanol in an amount of about 25% by weight of the soy protein isolate and heated at a temperature of about 80° C. for approximately 1 hour to produce a first mixture. The first mixture is combined with a polymerizable monomer of methacrylic acid in an amount of about 200% by weight of the soy protein isolate, and APS in an amount of about 20% by weight of the soy protein isolate to form a second mixture. Preferably, an acrylate cross-linking agent is also utilized in an amount of about 20% by weight of soy protein isolate. Acceptable acrylate cross-linking agents include difunctional polyethylene glycol diacrylate (PEGDA) and trimethylolpropane triacrylate. This second mixture is degassed and then heated at a temperature of about 80° C. for approximately 1.0 hour, and a hydrogel is extracted from the mixture. Optionally, additives may be included in the formation of the hydrogel. For example, clays, such as sepiolite, bentonite and/or hydrotalcite, may be utilized to vary the quantity and rate of water uptake. Additionally, mold inhibitors may be incorporated into the hydrogel compositions.

In a first method of extraction, the hydrogel is extracted by pouring the second mixture into a liquid in which the hydrogel is not readily soluble, such as acetone. In a second method of extraction, the hydrogel is extracted by adjusting the pH to precipitate the hydrogel, and the hydrogel is filtered out of the mixture and extracted with methanol using a Soxhlet extractor. Optionally, the as-formed hydrogel may be subjected to a washing process utilizing either water or an organic solvent, an initial water wash followed by washing with organic solvent, or using a mixture of water and organic solvent in order to extract non-reactant components from the gel, before subjecting the gel to a final drying process. The resultant dried absorbent and superabsorbent hydrogels have high water uptake ratios, and can be utilized for a variety of applications.

Advantageously, the above-described methods of the present invention can be utilized to produce a superabsorbent hydrogel. While not intending to limit the present invention, the invention can be readily understood by the example set forth below.

Experimental Approach

The soy protein isolate utilized in the examples discussed below was Pro-Cote 200 obtained from DuPont Soy Polymers, St. Louis, Mo. All acrylic monomers and cross-linking agents were supplied with phenolic radical inhibitors so all were pre-treated by passing through columns of Aldrich Inhibitor Remover (No. 311332) and then stored at freezer temperatures until use. However, it was found that removal of inhibitor was not necessary if the reactive solution was degassed to remove oxygen which is a strong free radical inhibitor. The oxygen can also be removed at the beginning of the process or at initial steps by bringing the liquids to a boil, or degassing by bubbling an inert gas through the liquid typically with heating. The inert gas can be nitrogen, argon or the like.

Hydrogel Preparation by Grafting Carboxylic Acid Functionality to Soy Protein Isolate

Soy protein isolate was prepared by solubilization of soy protein from soy meal at high pH, removing insoluble material, and then precipitating dissolved soy protein at its isoelectric point (pH ˜4.5) or by precipitation from solvents such as acetone in connection with grafting of methacrylic acid to the soy protein isolate. To follow grafting of carboxylic acid to soy protein isolate, a Fourier Transform Infrared. (FTIR) spectrometer-based analytical method was developed to determine the loading of grafted methacrylic acid on protein. This method involved construction of a calibration curve by preparing a series of analytical standards of soy protein isolate containing variable amounts of polyacrylic acid, finely grinding these mixtures, and measuring the ratio of infra-red (IR) absorbencies of carboxylic acid plus carboxylate ion (formed by zwitterion formation with basic amino acids such as lysine) to the soy protein isolate Amide 1 band in these mixtures. A plot of moles of carboxylic acid groups added per 10⁵ grams of total mixture versus this IR ratio gave a relatively straight line. Due to the close wave number proximities of carboxylic acid, carboxylate, and amide 1 bands, deconvolution programs were applied to IR spectra to obtain accurate absorbance data. Hwang and Damodaran (op. cit.) apparently achieved maximum loading of carboxylic acid functionality of 65 mole carboxylic acid groups per 10⁵ grams of soy protein isolate, whereas in the present invention carboxylic acid loading of over 350 mole carboxylic acid per 10⁵ grams of soy protein isolate was obtained.

The following is a description of the approach used to prepare grafted soy protein isolate samples that were evaluated for their water uptake values. Soy protein isolate was typically immersed in 8M urea at 80° C. for 0.5 hours to help solubilize protein components and this mixture was then mixed with 2-mercaptoethanol (25% of soy protein isolate) and held at 80° C. for 1.0 hour to cause cleavage of protein disulfide bridges to thiol groups. At this time, methacrylic acid (MAA) (200% of soy protein isolate), optionally an acrylate cross-linker (20% of soy protein isolate), and APS (20% of soy protein isolate) were added and the mixtures were heated at 80° C. for 2-3 hours. The cross-linkers employed were PEGDA or trimethylolpropane triacrylate. These mixtures were then either poured into acetone or adjusted to pH 4.5 to cause appreciable precipitation before being filtered and dried under vacuum. These materials were then extracted with methanol with a Soxhlet extractor to remove methacrylic acid homopolymer. Typical Grafting Percentages (GP) obtained were in the 60%-70% range and were calculated using the relationship:

GP=100×{(weight product−weight soy protein isolate)/weight soy protein isolate}.

When acrylate cross-linkers were not used, the extracted products were cross-linked with glutaraldhyde under basic conditions in 8M urea and the products were isolated and dried. Some methacrylic acid grafted soy protein isolate samples that were cross-linked with acrylate cross-linkers were also cross-linked with glutaraldehyde.

Numerous hydrogel candidates prepared by the method described above were subjected to water uptake measurements. Water uptake values (determined on a weight water uptake to weight dry hydrogel basis) observed was measured up to about 25.

Once a hydrogel is formed, the as-prepared hydrogel is preferably washed or subjected to an extraction process in order to improve the performance of the gel. More specifically, the as-prepared hydrogel may be washed multiple times with water, or may be washed with an organic solvent, such alcohols, ketones and ethers, although methanol is preferred. Organic solvent washing/extraction can be efficiently performed in a Soxhlet extractor. This additional washing/extraction process removes unreacted monomers and low molecular weight products, resulting in a final hydrogel or superabsorbent hydrogel product having faster and more efficient water uptake values compared to as-prepared hydrogels or once-swelled gels.

Although described with reference to preferred embodiments of the invention, it should be readily understood that various changes and/or modifications can be made to the invention without departing from the spirit thereof. Therefore, while the forms of the invention herein disclosed constitute presently preferred embodiments, many others are possible. It is not intended herein to mention all of the possible equivalent forms or ramifications of the invention. It is to be understood that the terms used herein are merely descriptive, rather than limiting, and that various changes may be made without departing from the spirit of the scope of the invention. In general, the invention is only intended to be limited by the scope of the following claims.

Claims

1. A method for making an absorbing hydrogel comprising:

a. solubilizing soy protein isolate to produce solubilized soy protein isolate;

b. combining to form a first mixture the solubilized soy protein isolate with a cleavage agent to cause cleavage of protein disulfide bridges to thiol groups in the soy protein isolate;

c. heating the first mixture at an elevated temperature;

d. combining the first mixture with a polymerizable monomer and a radical initiator to form a second mixture;

e. heating the second mixture at an elevated temperature; and

f. removing the hydrogel from the second mixture.

2. The method of claim 1, wherein the cleavage agent is 2-mercaptoethanol which is added in an amount of about 25% by weight of the soy protein isolate.

3. The method of claim 1, wherein the first mixture is heated at the elevated temperature of about 80° C. for approximately 1 hour.

4. The method of claim 1, wherein the polymerizable monomer is added in an amount of about 200% by weight of the soy protein isolate.

5. The method of claim 1, wherein the radical initiator is ammonium persulfate which is added in an amount of about 20% by weight of the soy protein isolate.

6. The method of claim 1, wherein the second mixture is heated at the elevated temperature of about 80° C. for approximately 1 hour.

7. The method of claim 1, wherein the polymerizable monomer is methacrylic acid.

8. The method of claim 1, wherein the second mixture further comprises an acrylate cross-linking agent in an amount of about 20% by weight of the soy protein isolate.

9. The method of claim 3, wherein the acrylate cross-linking agent is selected from the group consisting of polyethyleneglycol diacrylate and trimethylolpropane triacrylate.

10. The method of claim 1, wherein the soy protein isolate is solubilized by contact with urea at the elevated temperature of approximately 80° C.

11. The method of claim 1, wherein the hydrogel is removed by pouring the second mixture into a liquid in which the hydrogel is not readily soluble.

12. The method of claim 11, wherein the liquid is acetone.

13. The method of claim 1, wherein the hydrogel is removed by adjusting the pH to precipitate the hydrogel and filtering the hydrogel out of the second mixture.

14. The method of claim 1, wherein the hydrogel is extracted with methanol and a Soxhlet extractor to remove methacrylic acid homopolymer.

15. The method of claim 1, further comprising:

g. washing the absorbent hydrogel with an organic solvent; and

h. drying the washed hydrogel to form an absorbent hydrogel having a modified water uptake greater than a water uptake of the absorbent hydrogel before washing.

16. The method of claim 15, wherein washing of the absorbent hydrogel is conducted utilizing a Soxhlet extraction apparatus.

17. The method of claim 15, further comprising:

i) swelling the absorbent hydrogel with water prior to washing the absorbent hydrogel with the organic solvent.

18. The method of claim 15, wherein the organic solvent is methanol.

19. The method of claim 1, wherein the absorbent hydrogel is a superabsorbent hydrogel.

20. An absorbent hydrogel produced by the process of claim 1.