Process for preparing a porous material

Abstract

The present invention is directed to a process for preparing a porous material, at least comprising the steps of providing a gel comprising a solvent (S), wherein the solvent (S) has a volume (V1), pressurizing the gel with carbon dioxide at a temperature and a pressure at which carbon dioxide solubilizes in the solvent (S) forming gas-expanded liquid (EL), wherein the gas-expanded liquid (EL) has a volume (V2) and (V2) is greater than (V1); removing supernatant liquid, and drying the gel. The present invention further is directed to the porous material obtained or obtainable according to the process as such as the use of the porous material according to the invention in particular for medical, biomedical and pharmaceutical applications or for thermal insulation.

Claims

1. A process for preparing a porous material, the process comprising: a) providing a gel comprising a solvent (S), wherein the solvent (S) has a volume (V1), b) pressurizing the gel with carbon dioxide at a temperature and a pressure at which carbon dioxide solubilizes in the solvent (S) forming a gas-expanded liquid (EL), wherein the gas-expanded liquid (EL) has a volume (V2) and (V2) is greater than (V1), wherein in b), a volume (V3) which is a difference of (V2) and (V1) of a supernatant liquid is formed; c) removing supernatant liquid, and d) drying the gel, wherein b) further is carried out at a temperature in a range of from 31° C. to 90° C., wherein b) is carried out at a pressure in a range of from 0.1 Pc,mix to 0.99 Pc,mix, wherein Pc,mix is the pressure of the critical point of the solvent—carbon dioxide system at a temperature T, and wherein b) comprises a phase wherein the pressure is gradually increased from 1 to 150 bar over a time in a range of from 30 to 90 minutes.

2. The process of claim 1, wherein the gel is an organic gel.

3. The process of claim 2, wherein the gel is provided in the form of a monolithic block.

4. The process of claim 1, wherein the solvent (S) is selected from the group consisting of an alcohol, a ketone, an ester, an aldehyde, an alkyl alkanoate, an amide, a sulfoxide, an aliphatic halogenated hydrocarbon, a cycloaliphatic halogenated hydrocarbon, a halogenated aromatic compound, a dialkyl ether, a cyclic ether, a fluorine-comprising ether, and an acetal.

5. The process of claim 1, wherein b) is carried out at a temperature in a range between a critical point (T1) of carbon dioxide and a critical point (T2) of the solvent (S).

6. The process of claim 1, wherein b) is carried out at a temperature in a range of from 31° C. to 60° C.

7. The process of claim 1, wherein b) is carried out at a pressure in a range of from 50 to 100 bar.

8. The process of claim 1, wherein b) is carried out at a temperature in a range of from 31° C. to 60° C. and a pressure in a range of from 50 to 100 bar.

9. The process of claim 1, wherein during the phase wherein the pressure is gradually increased, the pressure is gradually increased from 50 to 150 bar over a time in a range of from 30 to 90 minutes.

10. The process of claim 1, wherein b) and c) are carried out simultaneously.

11. The process of claim 1, wherein the drying of d) is carried out by converting the liquid comprised in the gel into a gaseous state at a temperature and a pressure below the critical point of the liquid or the gas-expanded liquid (EL) comprised in the gel.

12. The process of claim 1, wherein the drying of d) is carried out under supercritical conditions.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1: shows pictures of solvent spillage of alginate gel recorded by camera in autoclave with two glass windows. The picture (a) was taken at t=0 min, P=0 bar; picture (b) was taken at t=10 min, P=99 bar; picture (c) was taken at t=30 min, P=99 bar; and picture (d) was taken at t=60 min; P=99 bar.

(2) FIG. 2: shows a diagram of the solvent spillage during pressurization of alginate gels (y axis: % solvent removed; x axis: time in minutes; 1: 1.5% Na-alginate, T=35° C., p=67 bar; 2: 3.0% Na-alginate, T=35° C., p=67 bar; 3: 1.5% Na-alginate, T=60° C., p=99 bar; 4: 3.0% Na-alginate, T=60° C., p=99 bar)

(3) FIG. 3: shows a diagram of the solvent spillage during pressurization of alginate gels (y axis: % solvent removed; x axis: time in minutes; 1: T=35° C., 2: T=60° C., A: 1.5% Na-alginate, B: 3.0% Na-alginate; the arrow indicates end of pressurization)

(4) FIG. 4: shows a diagram of the solvent spillage during pressurization of silica gel (y axis: % solvent removed; x axis: time in minutes; 1 g: T=35° C., P=65 bar)

(5) FIG. 5: shows a diagram of the solvent spillage during pressurization of polyurethane gel of thickness 15 mm (y axis: % solvent removed; x axis: time in minutes; 1d: T=60° C., P=90 bar; 2d: T=35° C., P=65 bar)

(6) FIG. 6: shows a diagram of the solvent spillage during pressurization of polyurethane gel of thickness 9 mm (y axis: % solvent removed; x axis: time in minutes; 1f: T=60° C., P=90 bar; 2f: T=35° C., P=65 bar)

(7) FIG. 7: shows the conditions different drying profiles for pressurization times of 10 (FIG. 7(a)), 30 (FIG. 7(b)), 60 (FIG. 7(c)) and 90 min (FIG. 7(d)) and a final pressure of 160 bar at a temperature of 60° C. Graph a indicates the pressure, graph b indicates the outflow rate (y axis: pressure in bar; x axis: time in minutes)

(8) FIG. 8 shows a diagram of the Removal of solvent at subcritical conditions (y axis: normalized extraction; x axis: pressure in bar). Graph a shows the results for a pressurization of 10 minutes, graph b for a pressurization of 30 minutes, graph c for a pressurization of 60 minutes, and graph d for a pressurization of 90 minutes.

(9) FIG. 9 shows a diagram of the comparison of slower pressurization vs. Faster pressurization (y axis: normalized extraction; x axis: time in minutes). Graph a shows the results for a pressurization of 10 minutes, graph b for a pressurization of 30 minutes, graph c for a pressurization of 60 minutes, and graph d for a pressurization of 90 minutes.

(10) FIG. 10 shows a diagram of the Process compression requirements (y axis: CO2 Inflow in g/min; x axis: pressure in bar). Graph a shows the results for a pressurization of 10 minutes, graph b for a pressurization of 30 minutes, graph c for a pressurization of 60 minutes, and graph d for a pressurization of 90 minutes.

(11) Examples will be used below to illustrate the invention.

EXAMPLES

(12) 1. Biopolymer Gel Based on Alginate:

(13) Suspensions of Na-alginate/water/CaCO3 were prepared for two different Na-alginate concentrations: 1.5 wt. % and 3.0 wt. %, with a CaCO3 content of 0.36 g CaCO3 for each gram of Na-alginate. The suspension was sheared using an Ultra Turrax T45 and was left to rest for 15 min to allow the air trapped in the suspension to escape.

(14) A fresh solution of GDL was prepared in water with a GDL amount of 0.64 g per gram of Na-alginate. The GDL solution was poured in the respective Na-alginate/water/CaCO3 suspension and sheared using the Ultra Turrax T45 mixer for 10 seconds followed by pouring 500 g of the suspension in polypropylene molds of 20 cm by 20 cm. The amount of water used for dissolving the GDL was chosen such as the final Na-alginate concentration was kept at 1.5 wt. % or 3.0 wt. %.

(15) The gelation time was around 40 seconds and the gels were aged for 24 hours in the closed container. After aging the gels, they were cut into three equal slabs for solvent exchange.

(16) The solvent exchange was done in increments of 20 wt. % ethanol after every 2 hours until a concentration of at least 99.0 wt. % (measured using Anton Paar DMA 4500 density meter) was obtained at equilibrium [5]. The solvent exchange was done in the same molds used for gelation and the slabs were turn over each solvent exchange cycle. The thickness of the slabs after solvent exchange was approximately 5 mm.

(17) 2. Organic Gel Based on Polyurethane:

(18) In a polypropylene container, 48 g Lupranat® M200 (oligomeric MDI with an NCO content of 30.9 g per 100 g in accordance with ASTM D-5155-96 A, functionality in the range of 3 and a viscosity of 2100 mPa.Math.s at 25° C. in accordance with DIN 53018) were dissolved under stirring in 200 g MEK at 20° C. leading to a clear solution. Similarly, 8 g MDEA (3,3′-5,5′-tetraethyl-4,4′-diaminophenylmethane), 4 g Ksorbate solution (20% potassium sorbate in monoethylene glycol) and 8 g n-butanol were dissolved in 220 g MEK to obtain a second solution. The solutions were combined in a rectangular container (20×20×5 cm height) by pouring one solution into the other, which led to a clear, homogeneous mixture of low viscosity. The container was closed with a lid and the mixture was gelled at room temperature for 24 h. The gel was cut into slabs of 9 mm and 15 mm thickness.

(19) 3. Inorganic Gel Based on Silica:

(20) In a polypropylene container, 11.79 g tetraethyl orthosilicate, 2.61 g ethanol, 0.46 g distilled water and 0.06 g 1M HCl were stirred for 30 min. Similarly, 1.33 g 28-30% aq. ammonium hydroxide, 0.28 g distilled water and 2.61 g ethanol were stirred for 5 min in a second container, then 25.88 g ethanol were further added. The solutions were combined by pouring one solution into the other, stirring for 1 min and pouring into a container. The container was closed with a lid and the mixture was gelled at room temperature for 16 h. The obtained gel had a thickness of 10 mm.

(21) 4. Measurements

(22) All the experiments were done using an autoclave equipped with two glass windows (FIG. 1). The capacity of the autoclave was 500 mL and it was positioned in a horizontal arrangement. The high-pressure carbon dioxide was liquefied using a refrigeration unit and supplied using an air operated pump, followed by heating the CO2. The temperature of the autoclave was controlled with an accuracy of ±3° C. using an external heating jacket connected to an on/off controller. The pressure inside the autoclave was controlled manually using a needle valve (±5 bar).

(23) The gel slabs were preheated in an ethanol or MEK bath to 35° C. or 60° C. After an equilibration time of one hour, the slabs were weighed and immediately positioned inside the preheated autoclave. The slabs were positioned over a mesh located at the middle of the autoclave so as to leave the bottom half of the autoclave free for the accumulation of the spilled liquid from the gel (FIG. 1).

(24) The autoclave was pressurized at a rate of 1 bar/s and the desired final pressure was kept constant. The amount of solvent mixture removed from the gel was measured using the digital camera (Logitech C920 HD Pro) and the white led fixed with a diffuser. Pictures every 5 seconds were taken and analyzed afterwards to determine the position of the liquid/gas interphase. The position of the interphase was used to calculate the volume of the spilled liquid using prior calibration data. After equilibrium of the system was reached (approximately 6 hours), the autoclave was depressurized and the obtained porous material was dried in an oven at 110° C. for 24 hours to obtain the weight of the polymer.

(25) 5. Effect of the Variation of the Drying Profiles

(26) Example 1 was repeated using different drying profiles shown in FIG. 7. The pressurization time was set at 10, 30, 60 and 90 min. For all examples, the final pressure was 160 bar. The temperature was 60° C. and the outlet valve opening: 15 min.

(27) As shown in FIG. 8, it was possible to remove considerable amounts of solvent out of the gel before reaching supercritical conditions. The results are summarized in table 1.

(28) TABLE-US-00001 TABLE 1 Pressurization Time (min) Subcritical Extraction (wt. %) 10 0 30 5.3 60 37.2 90 45.8

(29) It was found that a slower pressurization has no negative effect on the drying kinetics compared to a faster pressurization (FIG. 9, table 2).

(30) TABLE-US-00002 TABLE 2 Pressurization Time (min) Time for 98 wt. % of extraction* (min) 10 181.2 30 186.4 60 187.8 90 186.7

(31) The results summarized in FIG. 10 and table 3 show that a slower pressurization has a clear advantage on the reduction of compression requirements which translates in energy savings and lower CAPEX.

(32) TABLE-US-00003 TABLE 3 Pressurization Time (min) Max throughput difference, % 10 0 30 29 60 52 90 48

(33) The throughput difference is calculated according to formula (I):

(34) $\begin{array}{cc}\mathrm{diff}.=\frac{\mathrm{Curve},n-\mathrm{Curve},1}{\mathrm{Curve},1}& \left(I\right)\end{array}$

LITERATURE CITED

(35) EP 1199280 A1 US 2008/0152715 WO 2002/032462 A1 GB 2322326 US 2003/109421 WO 2007/013881 A2 WO 2009/016677 A2 Al-Hamimi et al, Anal. Chem. 2016, 4336-4345