BIODEGRADABLE HYDROGEL

Abstract

A hydrogel comprises one or more hydrosoluble polysaccharides which are cross-linked by cross-linking agents, wherein the cross-linking agents form covalent bonds with the polysaccharides, and wherein the cross-linking agents comprises humic and/or fulvic acids.

Claims

1. A hydrogel comprising one or more hydrosoluble polysaccharides which are cross-linked by cross-linking agents, wherein said cross-linking agents form covalent bonds with said polysaccharides, and wherein said cross-linking agents comprise at least one of humic or fulvic acids.

2. The hydrogel according to claim 1, wherein said at least one of humic or fulvic acids are, at least in part, complexed to clay, so as to form an organo-mineral complex.

3. The hydrogel according to claim 2, wherein a weight ratio between said at least one of humic or fulvic acids and the clay is between 0.05 w/w to 2 w/w.

4. The hydrogel according to claim 1, wherein said at least one of: humic or fulvic acids form covalent bonds with said polysaccharides.

5. The hydrogel according to claim 1, wherein a weight ratio between said at least one of: humic or fulvic acids and said polysaccharides is between 0.02 w/w and 1 w/w.

6. The hydrogel according to claim 1, wherein said cross-linking agents comprise an auxiliary cross-linking element which is covalently bonded to both said at least one of: humic or fulvic acids and to said polysaccharides.

7. The hydrogel according to claim 6, wherein said auxiliary cross-linking element is a polycarboxylic acid.

8. The hydrogel according to claim 1, wherein the clay is selected from the group consisting of: smectites, attapulgite, vermiculite, allophane and mixture thereof.

9. The hydrogel according to claim 8, wherein the clay is Ca2+ Montmorillonite.

10. The hydrogel according to claim 1, wherein the polysaccharides are highly hydrophilic substituted polymers selected from the group consisting of: celluloses, dextrans and substituted dextrans, starches and substituted starches, natural gums, glycosaminoglycans, chitosan, alginates, pectins and mixtures thereof.

11. The hydrogel according to claim 1, wherein the polysaccharides are selected from the group consisting of: carboxymethyl celluloses, corn starches, potato starches and mixtures thereof.

12. A method for preparing a hydrogel according to claim 1, comprising the step of cross-linking one or more hydrosoluble polysaccharides by forming covalent bonds with a cross-linking agent, wherein the cross-linking agent comprises at least one of: humic or fulvic acids.

13. The method according to claim 12, wherein, before said cross-linking, said at least one of: humic or fulvic acids are contacted with clay, so as to form an organo-mineral complex.

14. The method according to claim 12, wherein said cross-linking is obtained by contacting said at least one of: humic or fulvic acids and said polysaccharides at a temperature and for a time suitable to form covalent bonds between said at least one of humic or fulvic acids and the polysaccharides.

15. The method according to claim 12, wherein said cross-linking is carried out at a temperature between 80 C. and 150 C.

16. The method according to claim 14, wherein said polysaccharides and said cross-linking agents are dehydrated before being heated to said temperature.

17. The method according to claim 12, wherein said cross-linking is obtained by contacting said at least one of: humic or fulvic acids and said polysaccharides in presence of a polycarboxylic acid.

18. Use of a hydrogel according to claim 1 as adsorbent material for manufacturing biodegradable diapers or as adsorbent material or carrier for environmentally dangerous molecule or substances of interest, or as coating for seeds or fertilizers in agriculture.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0088] FIG. 1DLS (Dynamic Light Scattering) particle size analysis of a generic colloidal clay suspension A (Ca-montmorillonite), which can be used for the preparation of a hydrogel according to the invention.

[0089] FIG. 2DLS particle size analysis of a colloidal clay suspension A (Ca-montmorillonite) mixed with humic acids, in a further step of the preparation of a hydrogel according to the invention.

[0090] FIG. 3Particle size analysis of colloidal clay suspension A (Ca-montmorillonite) mixed with humic acids with the subsequent addition of a complexing agent such as Al3+.

[0091] FIG. 4ATR (Attenuated Total Reflection) spectrum of a hydrogel of the invention having a weight ratio between polysaccharides and clay of 80:20.

[0092] FIGS. 5a to 5dPictures taken at subsequent times showing the unfolding and swelling of the hydrogel of the invention when immersed in water at room temperature.

[0093] FIG. 6ATR spectrum of a mixture of polysaccharide and humic acids after the esterification reaction.

[0094] FIG. 7representative diagram of the hydrogel of the invention according to one embodiment, showing the basic components and interactions among them.

EXPERIMENTAL SECTION

Example 1

[0095] Preparation of a cross-linked organic-mineral polymer composite hydrogel (weight ratio polysaccharide/clay: 80/20, with addition of citric acid as auxiliary cross-linking element)

[0096] Chemical Solutions Employed, Description:

[0097] [UM]. Humic acids solution (average molecular weight 50000 uma): 1 ml solution=0.065 g humic acids, concentration 1.28 10.sup.6 M. Extracted from common pot-soil. pH adjusted to 7 with KOH and HCl solutions.

[0098] [OM]. Suspension of organo-mineral complexes=20 ml solution [UM]+2 g Ca-Montmorillonite powder (common montmorillonite for enological uses). The clay is dispersed in [UM] with magnetic stirring (t 30), and sonication (30). The pH is adjusted to 9 with a KOH solution.

[0099] [CMA]. Carboxymethyl cellulose (CMC)/Corn starch solution: 40 ml H.sub.2O, +0.6 g CMC sodium salt, +0.3 g of waxy corn starch (amylopectin). Solubilised at T=90 C. using a magnetic stirrer to facilitate the starch gelatinisation. pH adjusted to 9 using a KOH solution.

[0100] [CA]. Citric acid solution: 0.525 g granular citric acid (for common enological uses) in 50 ml distilled water, concentration 0.05 M.

[0101] Synthesis Description:

[0102] 1. Mix 1.07 ml of [OM] suspension with 17.8 ml [CMA] solution, to obtain an 80/20 w/w CMA/clay suspension (0.4 g polysaccharide, 0.1 g clay)

[0103] 2. Thoroughly homogenise with magnetic stirring at T 90 C.

[0104] 3. Add 0.4 ml of 0.05 M citric acid solution [CA]

[0105] 4. Adjust pH to 5.5 with HCl solution

[0106] 5. Homogenise the mixture for 30 at 90 C. with magnetic stirring

[0107] 6. Dehydrate sample at T=50 C.

[0108] 7. Cook in oven at 120 C. for 6 hours

[0109] 8. Swelling: Hydration of the cooked sample with distilled water at room temperature

[0110] 9. Dehydrate sample in acetone bath and re-hydrate in distilled water for two times. The measured swelling degree (swollen weightdry weight)/(dry weight) is 74. Additional dehydration cycles increase the degree of swelling.

Example 2

[0111] Preparation of a Cross-Linked Organic-Mineral Polymer Composite Hydrogel (Weight Ratio Polysaccharide/Clay: 60/40, with Addition of Citric Acid as Auxiliary Cross-Linking Element)

[0112] Chemical Solutions Employed, Description:

[0113] [UM]. Humic acids solution (average molecular weight 50000 uma): 1 ml solution=0.065 g humic acids, concentration 1.28 10.sup.6 M. Extracted from common pot-soil. pH corrected to 7 with KOH and HCl solutions.

[0114] [OM]. Suspension of organo-mineral complexes=3.3 ml solution [UM]+0.66 g Ca-Montmorillonite powder (common montmorillonite for enological uses)+15 ml distilled H2O. The clay is dispersed in humic acids with magnetic stirring (t 30), and sonication (30). The pH is adjusted to 9 with a KOH solution.

[0115] [CMC]. Carboxymethyl cellulose (CMC) solution: 40 ml H.sub.2O, +1 g CMC sodium salt, solubilised at T=50 C. with magnetic stirring. pH adjusted to 9 using a KOH solution.

[0116] [CA]. Citric acid solution: 0.525 g granular citric acid (for common enological uses) in 50 ml distilled water, concentration 0.05 M.

[0117] Synthesis Description:

[0118] 1. Mix the [OM] suspension and [CMC] solution to obtain a 60/40 w/w CMC/clay (1 g polysaccharide, 0.66 g clay).

[0119] 2. Thoroughly homogenise with magnetic stirring (t 30)

[0120] 3. Add 2 ml 0.05 M citric acid [CA]

[0121] 4. Adjust pH to 5.5 with HCl solution

[0122] 5. Homogenise solution for 30 with magnetic stirring T 25 C.

[0123] 6. Dehydrate sample at T=50 C.

[0124] 7. Cook in oven at 136 C. for 6 hours

[0125] 8. Swelling: hydration of the cooked sample with distilled water at room temperature

[0126] 9. Dehydrate sample in acetone bath and re-hydrate in distilled water for two times. The measured swelling degree (swollen weightdry weight)/(dry weight) is 48. Additional dehydration cycles increase the degree of swelling.

Example 3

[0127] Preparation of a cross-linked organic-mineral polymer composite hydrogel with approximate composition: 71.4% montmorillonite clay, 21.4% natural polymer, 7% humic acid, 0.2% citric acid (weight ratio polysaccharide/clay: 25/75, cross-linking with citric acid, with addition of citric acid as auxiliary cross-linking element)

[0128] Chemical Solutions Employed, Description:

[0129] [UM]. 5% Humic acids solution: 100 ml distilled water+5 g humic acids (humic acids saltsSigma Aldrich).

[0130] [CL]. 8% Clay suspension: 100 ml distilled water+8 gr. clay powder (common montmorillonite for enological uses).

[0131] [CMC]. 2.5% Carboxymethyl cellulose solution: 100 ml distilled water, +2.5 g CMC (CMC sodium saltSigma Aldrich).

[0132] [CA] Citric acid solution 0.1 M (21.0 gr/l) (citric acid monoidrateSigma aldrich).

[0133] Synthesis Description:

[0134] 1. Mix the [UM] solution and [CL] suspension to obtain an organo-mineral suspension with a weight ratio humic acids/clay=0.10.

[0135] 2. Mix the obtained suspension and [CMC] solution to prepare a suspension with a weight ratio CMC/clay=25/75.sub.w/w.

[0136] 3. Add [CA] to obtain an 1% citric acid/[CMC] fraction.

[0137] 4. Thoroughly homogenise with magnetic stirring during 1 hour at T 50.

[0138] 5. Adjust pH to 4.1 with HCl solution.

[0139] 6. Homogenise solution for 2 hours with magnetic stirring T 50 C. and check pH until this appear stable.

[0140] 7. Dehydrate sample at room temperature.

[0141] 8. Cook in oven at 85 C. for 6 hours.

[0142] 9. Hydrate the cooked sample by submerging in distilled water at room temperature for 24 hours. The measured swelling degree is 29.

[0143] 10. Dehydrate sample in acetone bath or by ventilation at room temperature and re-swelling in distilled water for 4 hours two times. The measured swelling degree is 53.

Example 4

[0144] Preparation of a cross-linked organic-mineral polymer composite hydrogel with approximate composition: 42.9% montmorillonite clay, 42.9% natural polymer, 14.2% humic acid (weight ratio polysaccharide/clay: 50/50, cross-linking with humic acid only)

[0145] Chemical Solutions Employed, Description:

[0146] [UM]. 5% Humic acids solution: 100 ml distilled water+5 g humic acids (humic acids saltsSigma Aldrich).

[0147] [CL]. 8% Clay suspension: 100 ml distilled water+8 gr. clay powder (common montmorillonite for enological uses).

[0148] [CMC]. 2.5% Carboxymethyl cellulose solution: 100 ml distilled water, +2.5 g CMC (CMC sodium saltSigma Aldrich).

[0149] Synthesis Description:

[0150] 1. Mix the [UM] solution and [CL] suspension to obtain a organo-mineral suspension with a weight ratio humic acids/clay=0.33.

[0151] 2. Mix the obtained suspension and [CMC] solution to prepare a suspension with a weight ratio CMC/clay=50/50.sub.w/w.

[0152] 3. Thoroughly homogenise with magnetic stirring during hour at T 50

[0153] 4. Adjust pH to 4.75 with HCl solution.

[0154] 5. Homogenise solution for 2 hours with magnetic stirring T 50 C. and check pH until this appear stable.

[0155] 6. Dehydrate sample at room temperature

[0156] 7. Cook in oven at 110 C. for 4 hours

[0157] 8. Hydrate the cooked sample by submerging in distilled water at room temperature for 24 hours. The measured swelling degree is 23.

[0158] 9. Dehydrate sample in acetone bath or by ventilation at room temperature and re-swelling in distilled water for 4 hours two times. The measured swelling degree is 45.

Example 5

[0159] Preparation of a cross-linked organic-mineral polymer composite hydrogel with approximate composition: 48.8% montmorillonite clay, 48.8% natural polymer, 2.4% humic acid (weight ratio polysaccharide/clay: 50/50, cross-linking with humic acid only)

[0160] Chemical Solutions Employed, Description:

[0161] [UM]. 5% Humic acids solution: 100 ml distilled water+5 g humic acids (humic acids saltsSigma Aldrich).

[0162] [CL]. 8% Clay suspension: 100 ml distilled water+8 gr. clay powder (common montmorillonite for enological uses).

[0163] [CMC]. 2.5% Carboxymethyl cellulose solution: 100 ml distilled water, +2.5 g CMC (CMC sodium saltSigma Aldrich).

[0164] Synthesis Description:

[0165] 1. Mix the [UM] solution and [CL] suspension to obtain a organo-mineral suspension with a weight ratio humic acids/clay=0.05.

[0166] 2. Mix the obtained suspension and [CMC] solution to prepare a suspension with a weight ratio CMC/clay=50/50.sub.w/w.

[0167] 3. Thoroughly homogenise with magnetic stirring during hour at T 50.

[0168] 4. Adjust pH to 4.75 with HCl solution.

[0169] 5. Homogenise solution for 2 hours with magnetic stirring T 50 C. and check pH until this appear stable.

[0170] 6. Dehydrate sample at room temperature.

[0171] 7. Cook in oven at 110 C. for 4 hours.

[0172] 8. Hydrate the cooked sample by submerging in distilled water at room temperature for 24 hours. The measured swelling degree is 40.

[0173] 9. Dehydrate sample in acetone bath or by ventilation at room temperature and re-swelling in distilled water for 4 hours two times. The measured swelling degree is 101.

Example 6

[0174] Preparation of a cross-linked organic-mineral polymer composite hydrogel with approximate composition: 42.9% montmorillonite clay, 42.9% natural polymer, 14.2% humic acid (weight ratio polysaccharide/clay: 50/50, cross-linking with humic acid only)

[0175] Chemical Solutions Employed, Description:

[0176] [UM]. 5% Humic acids solution: 100 ml distilled water+5 g humic acids (humic acids saltsSigma Aldrich).

[0177] [CL]. 8% Clay suspension: 100 ml distilled water+8 gr. clay powder (common montmorillonite for enological uses).

[0178] [CMC]. 2.5% Carboxymethyl cellulose solution: 100 ml distilled water, +2.5 g CMC (CMC sodium saltSigma Aldrich).

[0179] Synthesis Description:

[0180] 1. Mix the [UM] solution and [CL] suspension to obtain a organo-mineral suspension with a weight ratio humic acids/clay=0.33;

[0181] 2. Mix the obtained suspension and [CMC] solution to prepare a suspension with a weight ratio CMC/clay=50/50.sub.w/w.

[0182] 3. Thoroughly homogenise with magnetic stirring during hour at T 50.

[0183] 4. Adjust pH to 4.75 with HCl solution.

[0184] 5. Homogenise solution for 2 hours with magnetic stirring T 50 C. and check pH until this appear stable.

[0185] 6. Dehydrate sample at room temperature.

[0186] 7. Cook in oven at 150 C. for 6 hours.

[0187] 8. Hydrate the cooked sample by submerging in distilled water at room temperature for 24 hours. The measured swelling degree is 6.

[0188] Results and Discussion:

[0189] 1. Examples 1 to 3 disclose the preparation of hydrogels wherein the polysaccharides are cross-linked by means of cross-linking agents formed by the organo-mineral complex and the polycarboxylic acid. A schematic representation of the hydrogel structure is shown in FIG. 7.

[0190] 2. Examples 4 to 6 disclose the preparation of hydrogels wherein the polysaccharides are cross-linked by means of cross-linking agents formed by the organo-mineral complex only.

[0191] 3. During the first swelling, some of the humic acids disperse into the solution (fraction that did not take part in the reaction), this release of humic acids in solution decreases drastically during subsequent re-swellings.

[0192] 4. The sample dry weight tends to decrease after each dehydration/absorption cycle, whereas the absorption capacity increases.

[0193] 5. The hydrogel displays uniform volume increase along the three dimensional axes during swelling therefore maintaining its original shape; in other words, the swelling of a thin slurry of dry hydrogel along a given dimensional axis, once immersed in water, will be proportional to its initial size along that axis, with a fast swelling rate (1 hour to reach full swelling).

[0194] 6. The sample dry weight goes through several decreases during the synthesis procedure. The loss in dry matter weight is initially due to the moisture present in the polymer matrix (10 wt % ascertained), which is removed during cooking. After each subsequent swelling and dehydration cycle, the sample dry weight decreases further first due to the impurities released and soluble fractions or unreacted fraction which are partially extracted during each hydration phase. After that the loss in dry matter weight is due to the depolymerisation process or natural degradation of the hydrogel.

[0195] 7. Comparing the DLS analysis performed on a suspension of humic acids and clay (FIG. 2), and a pure clay suspension (FIG. 1), it can be seen that the presence of humic acids makes possible to obtain a roughly monodimensional suspension, which results narrower compared to the one with only clay. The addition of complexing cations such as Ca.sup.2+, Fe.sup.3+, Al.sup.3+ can favour the formation of particles (complexes) with a relatively higher average hydrodynamic diameter, but displaying a lower standard deviation around the average (FIG. 3).

[0196] 8. In FIG. 4 the typical ATR spectrum of the hydrogel of the invention consisting of humic acids, Ca-Montmorillonite, carboxymethylcellulose and waxy corn starch is shown.

[0197] 9. In FIG. 5a is shown a sample of the hydrogel prepared according to Example 2, as soon as immersed in water at room temperature. FIGS. 5b to 5d shows the same sample while unfolding and swelling in the water. FIG. 5d is the hydrogel after 30 minutes of immersion.

[0198] 10. In a separate test, a mixture of polysaccharides and humic acids having the concentration of the hydrogel of the invention, has been prepared and subjected to reaction conditions used for the hydrogel. The mixture has been analysed before and after the reaction. In FIG. 6 is reported the ATR spectrum of the mixture after the reaction, wherein a significant peak at 1732 cm.sup.1, which is not present in the ATR spectrum of the mixture before reaction, may be noted. This peak corresponds to an carbonylic group belonging to an ester bond which proves that the esterification reaction between polysaccharides and humic acids took place.