Process for producing porous alginate-based aerogels

Abstract

The present invention relates to a process for preparing a porous material, at least comprising the steps of providing a mixture (I) comprising a water soluble polysaccharide, at least one compound suitable to react as cross-linker for the polysaccharide or to release a cross-linker for the polysaccharide, and water, and preparing a gel (A) comprising exposing mixture (I) to carbon dioxide at a pressure in the range of from 20 to 100 bar for a time sufficient to form a gel (A), and depressurizing the gel (A). Gel (A) subsequently is exposed to a water miscible solvent (L) to obtain a gel (B), which is dried. The invention further relates to the porous materials which can be obtained in this way and the use of the porous materials as thermal insulation material, for cosmetic applications, for biomedical applications or for pharmaceutical applications.

Claims

1. A porous material obtained by a process comprising: a) providing a mixture (I) comprising (i) a water soluble polysaccharide, (ii) a compound which reacts as a cross-linker for the water soluble polysaccharide or which releases a cross-linker for the water soluble polysaccharide, and (iii) water; b) preparing a gel (A) by a process comprising b1) exposing the mixture (I) to carbon dioxide at a pressure of from 20 to 100 bar for a time sufficient to form a gel (A), and b2) depressurizing the gel (A); c) exposing the gel (A) to a water miscible solvent (L) to obtain a gel (B); and d) drying the gel (B), wherein the water miscible solvent (L) is at least one selected from the group consisting of a C1 to C6 alcohol a C1 to C6 ketone, and a mixture thereof, wherein the water soluble polysaccharide is an alginate, and wherein the compound which reacts as a cross-linker for the water soluble polysaccharide or which releases a cross-linker for the water soluble polysaccharide in the mixture (I) comprises calcium carbonate, wherein the porous material has a pore volume in the range of from 2.1 to 9.5 cm.sup.3/g for pore sizes <150 nm.

2. The porous material of claim 1, wherein the preparing in b) is carried out at a temperature of from 10 to 40° C.

3. The porous material of claim 1, wherein carbon dioxide in the exposing in b1) is gaseous.

4. The porous material of claim 1, wherein a water insoluble solid (S) is further added to the mixture (I).

5. The porous material of claim 1, wherein a compound (C) is further added to the mixture (I) in the providing in a), wherein the compound (C) is at least one selected from the group consisting of a natural hydrocolloid forming polymer, a synthetic hydrocolloid-forming polymer, and a mixture thereof.

6. The porous material of claim 1, wherein the exposing in c) is carried out at a pressure of from 10 to 100 bar.

7. The porous material according to claim 1, wherein the drying in d) is carried out by converting a liquid in the gel (B) into a gaseous state at a temperature and a pressure below a critical temperature and a critical pressure of the liquid in the gel (B).

8. The porous material according to claim 1, wherein the drying in d) is carried out under supercritical conditions.

9. A cosmetic, biomedical, or pharmaceutical product, comprising: a thermal insulation material comprising the porous material of claim 1.

Description

EXAMPLES

1. Example 1

(1) Sodium alginate solution was prepared by gentle stirring of sodium alginate powder (obtained from Sigma-Aldrich) with appropriate amount of water for 12 h. Calcium carbonate was suspended in water by vigorous mixing for 5 min. Keeping the agitation up, a certain part of the suspension was bled off and immediately transferred into the sodium alginate solution (0.25; 0.5; 1.0 wt %) to reach a target sodium alginate/CaCO.sub.3 ratio (Table 1). The mixture was again agitated until it became homogeneous. All prepared suspensions were transferred into a high pressure autoclave for subsequent gelation. Final concentrations of sodium alginate solutions are listed in Table 1. Sodium alginate/CaCO.sub.3 ratio of 1:0.1825 denoted as factor F=1.0.

(2) The autoclave was pressurized with gaseous carbon dioxide up to 50 bar at room temperature (25° C.). Pressure was maintained for 12 h and then slowly released (0.2 bar/min). Hydrogels formed were either transparent or translucent. The gels were washed with water and successively immersed in ethanol/water mixtures with concentrations of 30, 60, 90 and 100 wt % for 12 h in each. Step wise concentration is recommended as high concentration gradients during solvent exchange cause irreversible shrinkage to the hydrogel.

(3) Alcogels were packed into filter paper parcels, placed into preheated high pressure autoclave (40° C.) and filled with ethanol to prevent premature solvent evaporation. Supercritical drying was performed using the same autoclave as for gelation. The autoclave was sealed and CO.sub.2 was filled in by a membrane pump. Keeping the pressure constant around 120 bar, 6-7 residence volumes of CO.sub.2 was used to dry the gel. Then system was depressurized in 1 h followed by cooling down to room temperature. Properties of the resulting aerogels are summarized in Table 1.

(4) Alternative with high pressure solvent exchange—The gels were washed under increased pressure of CO.sub.2 of 50 bar at room temperature with water and successively immersed in ethanol/water mixtures with concentrations of 30, 60, 90 and 100 wt % for 12 h in each. Step wise concentration is recommended as high concentration gradients during solvent exchange cause irreversible shrinkage to the hydrogel

(5) TABLE-US-00001 TABLE 1 Properties of the aerogels from Example 1 Final sodium alginate Bulk BET sur- BJH pore Thermal concentration, density, face area, volume, conductivity, wt % F g/cm.sup.3 m.sup.2/g cm.sup.3/g mW/m .Math. K .sup.1) 0.49 0.5 0.037 473 ± 90 5.68 0.25 1.0 0.024 436 ± 96 4.34 0.49 1.0 0.028 479 ± 67 6.98 22 ± 2 0.97 1.0 0.042 487 ± 67 4.55 0.25 2.0 — — — 18 ± 2 0.50 2.0 — — — 19 ± 2 .sup.1) Thermal conductivity was determined by hot-wire measurements at ambient pressure and room temperature. Procedure as described by Reichenauer et. al, (Reichenauer, G., Heinemann, U., and Ebert, H.-P. (2007). Relationship between pore size and the gas pressure dependence of the gaseous thermal conductivity. Colloids and Surfaces A: Physicochemical and Engineering Aspects 300, 204-210).

(6) For characterization of the porous structure of aerogels a Nova 3000 Surface Area Analyzer from Quantachrome Instruments was used. It uses adsorption and desorption of nitrogen at a constant temperature of −196° C.

(7) According to the present invention, CaCO.sub.3 is directly dispersed in water, which is then slowly dissolved by decreasing the pH value. Solubility of carbon dioxide increases with rising pressure along with lowering of pH down to 3. Solubility of calcium carbonate also increases with pressure resulting in release of calcium ions. At conditions used in this study for gelation (50 bar, 25° C.), solubility of CaCO.sub.3 is much higher (2.9 g/L) than at ambient conditions (0.01 g/L). Concentration of CaCO.sub.3 in the final mixtures lies between 0.23 and 4.4 g/L. Hence more than half of introduced calcium carbonate is dissolved at equilibrium conditions and available to crosslink alginate chains. The hydrogels obtained using the disclosed CO.sub.2 induced gelation were stable at ambient conditions and can be stored in pure water for weeks without visible degradation (in presence of a preservative to avoid bacterial decomposition).

(8) The samples prepared by the process disclosed herein show extremely high BJH pore volumes of up to 5.68 and 6.98 cm.sup.3/g, respectively, displaying values close to silica aerogels (typically around 6 cm.sup.3/g).

2. Example 2

(9) Sodium alginate of 3 wt % was mixed with 3 wt % water solution of the second polymer (Table 2). In cases of base soluble mixtures such as Lignin and Eudragit L100, 1 M sodium hydroxide was used instead water to dissolve Eudragit L100 and lignin. To incorporate insoluble gel/aerogel matrices, hydrophobic silica alcogels using MTMS were first prepared using the recipe described in (Rao et al., 2006, Synthesis of flexible silica aerogels using methyltrimethoxysilane (MTMS) precursor. Journal of Colloid and Interface Science 300, 279-285). Then alcogels were washed with water to obtain hydrogel. The later was crushed and dispersed in sodium alginate solution. Mixture was diluted with water to reach the overall polymer concentration of 1.5 wt %. The subsequent procedure was the same as described in Example 1, with the only difference that solid calcium carbonate was dispersed in the mixture instead of using suspended CaCO.sub.3.

(10) TABLE-US-00002 TABLE 2 Properties of hybrid alginate-based aerogels Bulk BET sur- BJH pore density, face area, volume, Polymer F g/cm.sup.3 m.sup.2/g cm.sup.3/g Carboxymethyl 2.0 0.025 812 7.90 cellulose Gellan gum 2.0 0.033 346 2.98 Eudragit L100 1.0 0.039 — — Gelatin 1.0 0.043 208 3.24 Polyvinylalcohol 1.0 0.038 690 5.47 Pluronic 1.0 0.051 444 7.93 Starch .sup.1) 1.0 0.066 544 6.77 Starch .sup.1) 2.0 0.058 435 4.08 Polyethylene glycol 1.0 0.054 555 6.96 (Mw = 10 000) Polyethylene glycol 1.0 0.068 591 6.94 (Mw = 100 000) Methylcellulose 1.0 0.058 505 9.50 Lignin .sup.2) 2.0 0.062 456 2.6  Amidated Pectin .sup.3) 1.0 0.099 739 7.88 h-Carrageenan .sup.3) 1.0 0.049 593 4.77 Hydrophobic silica 1.0 0.201 436 2.11 dispersed in alginate .sup.3) .sup.1) alginate/starch ratio of 5.0 (g/g) was used, .sup.2) alginate/lignin ratio of 4.0 (g/g) was used .sup.3) mixture of alginate (3 wt %) and second biopolymer (3 wt %) not diluted further to 1.5%.

(11) The thermal conductivities of select biopolymer mixtures (overall composition: 1.5 wt %) were analyzed and are presented in Table 3. The method of measurement is by hot-wire measurements as described in Reichenauer et. al, (Reichenauer, G., Heinemann, U., and Ebert, H.-P. (2007). Relationship between pore size and the gas pressure dependence of the gaseous thermal conductivity. Colloids and Surfaces A: Physicochemical and Engineering Aspects 300, 204-210).

(12) TABLE-US-00003 TABLE 3 Thermal conductivity of hybrid alginate-based aerogels Weight Thermal ratio conduc- (Alginate/ Cross- tivity, Second second linking Bulk Dimensions mW/ Bio- Bio- factor density, (mm) m .Math. polymer polymer) (F) g/cm.sup.3 l w h K .sup.1) Starch 1.0 1.0 0.100 77.6 34.6 6.9 21.6 Starch 1.0 2.0 0.067 76.5 34.6 9.5 20.4 Lignin 3.0 1.0 0.048 51.3 34.7 7.3 19.4 .sup.1) Thermal conductivity was determined by hot-wire measurements at ambient pressure and room temperature

3. Example 3

(13) Powder of zinc hydroxy carbonate, nickel hydroxy carbonate or cobalt carbonate was added to sodium alginate solution of 3 wt %. The mixture was vortexed for 1 min. The subsequent procedure was the same as described in Example 1. Textural properties are listed in Table 4.

(14) TABLE-US-00004 TABLE 4 Properties of alginate-based aerogels with various cations Bulk BET sur- BJH pore Crosslinking density, face area, volume, cation Factor .sup.1) g/cm.sup.3 m.sup.2/g cm.sup.3/g Zinc (ZIN-D001) 1.0 0.093 553 5.23 Cobalt (COB-D001) 1.0 0.098 546 6.36 Nickel (NIC-D001) 1.0 0.100 668 6.40 .sup.1) molar amount of cations was the same as for calcium in example 1