METHOD OF PREPARING A NANO- AND/OR MICROSCALE CELLULOSE FOAM

Abstract

The present invention relates to a method for the preparation of a nano- and/or microscale cellulose-based foam. The method comprises the steps of (i) providing a suspension (1) comprising nano- and/or microscale cellulose in an aqueous medium, (ii) simultaneously cooling and agitating the suspension (1) in a mechanical step (2a; 2b) to obtain an at least partially frozen suspension. (iii) freezing the at least partially frozen suspension (5) to obtain a substantially frozen suspension, (iv) treating the suspension under solvent-exchange (7; 8) and (v) removing the solvent (10; 13) to obtain a substantially dry foam (40A) comprising nano- and/or microscale cellulose.

Claims

1. A method for the preparation of a porous nano- and/or microscale cellulose-based foam comprising the steps of: a) providing a suspension comprising nano- and/or microscale cellulose in an aqueous medium; b) simultaneously cooling and agitating the suspension in a mechanical step to obtain an at least partially frozen suspension; c) freezing the at least partially frozen suspension to obtain a substantially frozen suspension; d) treating the suspension under solvent-exchange; and e) removing the solvent to obtain a substantially dry foam comprising nano- and/or microscale cellulose.

2. The method according to claim 1, wherein the suspension comprises 0.3 to 3 wt. % of nano- and/or microscale cellulose.

3. The method according to claim 1, wherein the suspension additionally comprises urea.

4. The method according to claim 1, wherein the at least partially frozen suspension in step b) comprises 10-90 wt % of aqueous medium in solid state.

5. The method according to claim 1, wherein the at least partially frozen suspension obtained in step b) is poured into a mold.

6. The method according to claim 1, wherein the solvent is a water-miscible solvent with a boiling point below 100° C. at standard ambient conditions.

7. The method according to claim 1, wherein the solvent is recycled.

8. The method according to claim 1, wherein the method additionally comprises the step of hydrophobizing the foam comprising nano- and/or microscale cellulose.

9. The method according to claim 1, wherein the nano- and/or microscale cellulose is nano- and/or microfibrillated cellulose.

10. A nano- and/or microscale cellulose based foam obtainable by a method comprising the steps of: a) providing a suspension comprising nano- and/or microscale cellulose in an aqueous medium; b) simultaneously cooling and agitating the suspension in a mechanical step to obtain an at least partially frozen suspension; c) freezing the at least partially frozen suspension to obtain a substantially frozen suspension; d) treating the suspension under solvent-exchange; and removing the solvent to obtain a substantially dry foam comprising nano- and/or microscale cellulose.

11. The nano- and/or microscale cellulose based foam according to claim 10, wherein the foam has a density of 0.01-0.3 g/cm.sup.3.

12. The nano- and/or microscale cellulose based foam according to claim 10, wherein the foam has a specific BET surface area between 30 m.sup.2/g and 100 m.sup.2/g.

13. The nano- and/or microscale cellulose based foam according to claim 10, wherein the foam has an oil absorption property of more than 40 L.sub.oil/kg.sub.cell.

14. The method according to claim 1, wherein the nano- and/or microscale cellulose based foam produced in accordance to steps a)-e) is used to sorb fluids, in particular for the absorption of oil.

15. The method according to claim 1, wherein the nano- and/or microscale cellulose based foam produced in accordance to steps a)-e) is used as an electrical insulating material.

16. The method according to claim 2, wherein the suspension comprises 0.9 to 2.5 wt. % of nano- and/or microscale cellulose.

17. The method according to claim 3, wherein the suspension comprises between 0.5 and 3 wt % urea.

18. The method according to claim 4, wherein the at least partially frozen suspension in step b) comprises 30-70 wt % of aqueous medium in solid state.

19. The method according to claim 6, wherein the solvent is ethanol.

20. The nano- and/or microscale cellulose based foam according to claim 13, wherein the foam has an oil absorption property of more than 70 L.sub.oil/kg.sub.cell.

21. The method according to claim 14, wherein the nano- and/or microscale cellulose based foam produced in accordance to steps a)-e) is used to absorb oil.

Description

SHORT DESCRIPTION OF THE FIGURES

[0056] The invention will be described in more details by the following exemplary embodiments in FIGS. 1 to 9, without the invention being restricted to these embodiments. In particular, individual steps are exchangeable between the exemplary embodiments and can be equally combined. Same reference signs in the figures indicate same or similar objects. It shows:

[0057] FIG. 1: A first embodiment of the process according to the invention.

[0058] FIG. 2: A second embodiment of the process according to the invention.

[0059] FIG. 3: A third embodiment of the process according to the invention.

[0060] FIG. 4: Comparison between a foam obtained by a method according to the invention and a foam obtained without solvent exchange.

[0061] FIG. 5: Comparison between foams obtained by rapid freezing with and without solvent exchange.

[0062] FIG. 6: Comparison between foams obtained by unidirectional ice templating with and without solvent exchange.

[0063] FIG. 7: Pictures from cut foams from the foams of FIGS. 4, 5 and 6.

[0064] FIG. 8: SEM imaging of the cut foams of FIG. 7.

[0065] FIG. 9: Optical microscope of the cut foams of FIG. 7.

[0066] FIG. 11: Influence of the concentration of nano- and/or microfibrillated cellulose on porosity and wall thickness.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0067] FIG. 1 shows a first exemplary embodiment for the process of the preparation of a nano- and/or microscale cellulose-based foam. A suspension 1 of nano- and/or microscale cellulose in water mixed with urea is applied to a low-temperature extruder 2a, in which the suspension is partially frozen, conveyed and agitated simultaneously and continuously. The partially frozen suspension is extruded into molds 3. The molds 3 are conveyed into a freezer 5 by means of a conveyor 4. The molds with the frozen suspension 3′ are transported to a first solvent bath 7 by further conveying means 6 to exchange the water for ethanol. In order to complete solvent exchange and enhance thawing the molds are placed into a second solvent bath 8. The ethanol containing wet foams 3″ obtained after the solvent exchange bath 7 and 8 are placed into a convection dryer 10 for drying the foam. The ethanol of the solvent bathes 7 and 8 is recycled by conventional recycling means 9 and led back to the solvent baths 7 and 8.

[0068] FIG. 2 shows a second embodiment of the process according to the invention. Compared to FIG. 1, the suspension 1 is partially frozen simultaneously to an agitation step in a batch process 2b providing different batches 11 with partially frozen suspension. The batches 11 are placed on a conveyor 4 and transported into a freezer 5 releasing batches with substantially completely frozen suspensions 11′. In FIG. 2, the frozen suspensions 11′ are thawed in a defrosting device 12 before placement into the solvent baths 7 and 8. The ethanol containing wet foams 11″ are dried in a dryer 10. The ethanol of the solvent bathes 7 and 8 is recycled as previously described.

[0069] FIG. 3 shows a third embodiment of a process according to the invention. The first steps of the process are identical to the ones described in FIG. 2. However, in FIG. 3 the substantially frozen suspension 11′ is directly placed into a solvent bath 7 without an additional thawing step. The suspension is left in solvent bath 7 until the solvent exchange has reached equilibrium and the suspension is substantially thawed such that a wet foam is obtained. After the solvent bath, the wet foam is subjected to a drainage step 13. The solvent of the drainage step 13 is subjected to recycling means 9 and led back to solvent bath 7. The less wet foam 14 as compared to the examples given with regard to FIGS. 1 and 2 is conveyed to a convection dryer 10 to obtain a substantially dry foam. The solvent which was removed in the convection dryer 10 is fed to the recycling means 9 by conventional methods 15.

EXAMPLES

[0070] Materials

[0071] The microfibrillated cellulose used in the following examples has a specific BET surface area of 240 m.sup.2/g. According to Tappi T271 pm-91 the amount of cellulose particles with a fiber length of less than 200 μm is 80% or higher with regard to the dry content of cellulosic material used in the suspension.

[0072] Water was used with the grades according to ISO 3696 (1987) and ASTM (D1193-91). The ethanol used was ethanol absolutus with 5% isopropanol purchased from Alcosuisse, purity of >99%. Urea was purchased from Merck KGaA with a purity >99%.

Example 1

[0073] A suspension containing 0.9 wt % microfibrillated cellulose and 0.9 wt % urea in water has been provided. The suspension has been mechanically agitated and simultaneously cooled to a half-frozen suspension, meaning that approximately 50 wt % of the water was present in solid state. The half-frozen suspension was transferred into a casting mold of cubic shape with an edge length of 3.2 cm. The suspension was then completely frozen by means of a freezer with a temperature of −35° C. and for at least three hours, until the suspension was substantially frozen.

[0074] The obtained frozen cubes of microfibrillated cellulose-based foam have been subjected to a solvent exchange bath in ethanol until the foam was substantially solvent saturated. The ethanol has been removed by drying the foam in a convection oven at 65° C. resulting in a dry foam (FIG. 4, 40A).

Example 2

[0075] The frozen cellulose-based cubes have been prepared as described with regard to example 1, but the cubes have been thawed in water. The water has been removed by drying in a convection oven at 65° C. resulting in dry foam (FIG. 4, 40B).

[0076] FIG. 4 provides a comparison between the foam 40A obtained according to example 1 and the foam 40B obtained according to example 2. Without thawing the foam in ethanol (example 2), a severe shrinkage of the material and structural loss can be observed (FIG. 4, 40B), whereas the solvent exchange, respectively the solvent thawing step, supports the maintenance of the structure of the foam upon drying (FIG. 4, 40A). The edge length of the cubic microcellulose based foam 40A varied between 2.9 cm and 2.7 cm, corresponding to a partial shrinkage in volume between 25 and 40%. The shrinkage of foam 40B could not be measured due to partial collapse of the foam, what indicates that solvent exchange is a prerequisite for maintaining the structure of the foam.

Comparative Example 3

[0077] A suspension containing 0.9 wt % microfibrillated cellulose and 0.9 wt % urea in water has been provided. The suspension has been poured into casting molds of cubic shape with an edge length of 3.2 cm and subjected to rapid freezing with liquid nitrogen for 10 minutes. The resulting frozen microcellulose-based foams in the cubes have been thawed in ethanol (FIG. 5, 50A), followed by drying according to the conditions as mentioned with regard to example 1 and 2.

Comparative Example 4

[0078] A suspension containing 0.9 wt % microfibrillated cellulose and 0.9 wt % urea in water has been provided. The suspension has been poured into casting molds of cubic shape with an edge length of 3.2 cm and subjected to rapid freezing with liquid nitrogen for 10 minutes.

[0079] The resulting frozen microcellulose-based foams in the cubes have been thawed in water (FIG. 5, 50B), followed by drying according to the conditions as mentioned with regard to example 1 and 2.

[0080] In FIG. 5, both foams 50A and 50B show a severe shrinkage upon drying. The foam thawed in water 50B showed a much higher shrinkage than the foam thawed in ethanol 50A. Further, the cube shapes collapsed upon drying and are completely lost compared to foam 40A (example 1).

Comparative Example 5

[0081] A suspension containing 0.9 wt % microfibrillated cellulose and 0.9 wt % urea in water has been provided. The suspension has been poured into casting molds of cubic shape with an edge length of 3.2 cm and subjected unidirectional ice templating, meaning the casting molds with the suspension were placed in a freezer at −35° C. for 3 hours until the suspension was substantially frozen. The resulting frozen microcellulose-based foams in the cubes have been thawed in ethanol (FIG. 6, 60A), followed by drying according to the conditions as mentioned with regard to example 1 and 2.

Comparative Example 6

[0082] A suspension containing 0.9 wt % microfibrillated cellulose and 0.9 wt % urea in water has been provided. The suspension has been poured into casting molds of cubic shape with an edge length of 3.2 cm and subjected unidirectional ice templating, meaning the casting molds with the suspension were placed in a freezer at −35° C. for 3 hours until the suspension was substantially frozen. The resulting frozen microcellulose-based foams in the cubes have been thawed in water (FIG. 6, 60B), followed by drying according to the conditions as mentioned with regard to example 1 and 2.

[0083] In FIG. 6. the foam 60A maintains its cubic shape after drying. However, the edge length of the cubic microcellulose based foam 60A varied between 2.7 cm and 2.6 cm corresponding to a shrinkage between and 40% and 44.5% and shrinkage is thus higher compared to example 1 with foam 40A. The foam 60B showed a severe shrinkage after drying. The cubic shape is lost.

[0084] Cut Foam Picture

[0085] The foams 40A, 50A and 60A have been cut in the center and imaged with a conventional camera of 12 Megapixel. The images of the respective foams are shown in FIG. 7. The foam 40A obtained by a method according to the invention (example 1) shows a good homogeneity of the pores by visual inspection as can be seen in FIG. 7A. For foam 60A (comparative example 5) the freezing direction can be determined since crystals have been formed along the freezing direction as provided in FIG. 7C. Further, the visual inspection of foam 60A indicates channels rather than pores. Foam 50A (comparative example 3) as imaged in FIG. 7B and obtained by rapid freezing does not show pores by visual inspection and seems of rather compact structure.

[0086] Scanning Electron Microscope and Optical Microscope

[0087] The microscopic images of foams 40A, 50A and 60A are provided in FIG. 8. The images have been recorded with a FEI NanoSEM 230. FIGS. 8A, 8C and 8E were recorded with a resolution of 1 mm. FIG. 8B was recorded with a resolution of 300 μm FIGS. 8D and 8F were recorded with a resolution of 50 nm.

[0088] Optical microscope images of foam 40A, 50A and 60A were recorded with a Zeiss Axioplan and a magnification up to 40× and are shown in FIG. 9, FIGS. 9A, 9C, 9E and 9F show a resolution of 200 μm, FIG. 9B shows a resolution of 50 μm and FIG. 9F of 100 μm.

[0089] FIGS. 8A and 8B show the microscopic images of the foam 40A, obtained by a method according to the invention (example 1). In FIG. 8A the homogeneous pore distribution is clearly visible. FIG. 8B is a higher resolution of FIG. 8A showing that the pores are separated by walls of cellulose fibers. The pore structure was also confirmed by optical microscope imaging as shown in FIGS. 9A and 9B.

[0090] FIGS. 8C and 8D show the microscopic images of the foam 50A (comparative example 3), obtained by rapid freezing with nitrogen. FIG. 8C shows a resolution of 1 mm, suggesting a very compact structure of small pores. FIG. 8D is a higher resolution of FIG. 8C and shows a network of cellulose fibers without the presence of structured pores. The optical microscope images of foam 50A in FIGS. 9C and 9D also show a loose fiber network instead of pores.

[0091] FIGS. 8E and 8F display the microscopic images of the foam 60A obtained by unidirectional ice templating according to comparative example 5. While the visual inspection in FIGS. 4 and 6 suggested a similar structure of foam 40A and foam 60A, the microscopic images for the foam 60A show channels rather than pores. The formation of channels is also confirmed by optical microscope imaging of foam 60A, which images are shown in FIGS. 9E and 9F.

[0092] Bet Measurements

[0093] BET measurements were performed on a Tristar II plus and according to ISO 9277, 2.sup.nd edition, 2010-09. The foam was divided in the center with a scalpel and the sample was comminuted such that it could be filled into the respective BET tube. Two measurements have been performed for each foam, except for foam 50B due to its severe shrinkage and unsuitability for such measurements. The results of the measurements are provided in table 1.

TABLE-US-00001 TABLE 1 BET measurements BET Average BET foam surface in m.sup.2/g surface in m.sup.2/g 40A - 1 59.70 57.22 40A - 2 54.74 40B - 1 3.93 3.80 40B - 2 3.68 50A - 1 9.83 9.93 50A - 2 10.03 50B - 1 3.54 3.54 50B - 2 — 60A - 1 57.93 57.53 60A - 2 57.14 60B - 1 0.85 1.86 60B - 2 2.88

[0094] The BET surface area is similar for the foams 40A and 60A, both subjected to a solvent exchange step, while all foams lacking the solvent exchange step show a severely reduced BET surface.

[0095] Oil Absorption Measurements

[0096] Foams 40A and 60A have been subjected to oil absorption measurements. Oil absorption measurements were conducted as follows: Approximately 100 ml of a mixture of straight-chain paraffins and 1-methylnaphthalene with a density of ρ=820 kg/m.sup.3 and a surface tension σ=23.0±0.3 mN/m were used as oil. The foam was stored in the oven at 60° C. for at least one hour to make sure it was fully dry, then taken out, foam mass was measured and recorded. Then, a beaker was filled with approx. 100 mL of oil, the foam was immersed, kept in the oil for at least 30 s, then taken out using tweezers and the total mass of foam (m.sub.foam) and oil (m.sub.oil) was measured again. Oil absorption capacity was then calculated as C=m.sub.oil/ρ.sub.oil=(m.sub.tot−m.sub.foam)/ρ.sub.oil.

TABLE-US-00002 TABLE 2a absorption measurements according to initial microfibrillated cellulose concentration foam mass foam mass ratio ratio [g] before [g] after [L.sub.oil/kg.sub.cell] [L.sub.oil/kg.sub.cell] foam sample absorption absorption mean std. 40A 1 0.391 20.22 61.5 ± 0.33 40A 2 0.402 21.01 60A 1 0.306 18.26 62.7 ± 8.05 60A 2 0.311 14.41

[0097] Comparing these examples with regard to their absorption properties (table 2a), the absorption seems to be slightly better with the foam obtained by unidirectional ice templating (60A, comparative example 5) than those obtained with a foam according to example 1 (40A).

[0098] However, using the same initial concentration of microfibrillated cellulose to obtain foams 40A and 60A leads to different foam masses before absorption. The foam mass of foam 40A is higher compared to the foam mass of foam 60A. Without being bound by theory, it is expected that the different masses are a result of the different preparation methods. In the preparation method according to the invention (example 1), the cellulose-containing suspension is concentrated during the cooling-agitation step and thus resulting in a higher concentration of cellulosic material compared to the method of unidirectional ice templating (comparative example 5). In order to compare the results on the basis of the same foam mass before oil absorption, another foam has been prepared with a method according to the invention as presented in example 7.

Example 7

[0099] A foam 70A has been prepared according to example 1 with the exception that a suspension containing 0.6 wt % microfibrillated cellulose and 0.6 wt % urea in water has been provided. The foam 70A has a foam mass before absorption similar to those of foam 60A. When the absorption capacity is compared in relation to the foam mass after preparation of the foams and before absorption measurement, the absorption capacity of the foam 70A obtained according to a method of the invention significantly increases compared to a foam obtained by unidirectional ice templating 60A (table 2b).

TABLE-US-00003 TABLE 2b absorption measurement according to foam mass before absorption foam mass foam mass ratio ratio [g] before [g] after [L.sub.oil/kg.sub.cell] [L.sub.oil/kg.sub.cell] foam sample absorption absorption mean std. 40A 1 0.391 20.22 61.5 ± 0.33 40A 2 0.402 21.01 60A 1 0.306 18.26 62.7 ± 8.05 60A 2 0.311 14.41 70A 1 0.274 16.64 72.7 ± 0.6 70A 2 0.279 17.22

[0100] Mechanical Strength

[0101] Foams 40A and 60A obtained with the method described with respect to example 1 and comparative example 5, respectively, have been subjected to mechanical strength measurements.

[0102] Mechanical strength measurement were conducted as follows: The test specimens are loaded with a preload of 0.1 N. Then a force F.sub.Max of 2 N is applied at a test speed of 2 mm/min. The deformation of the test specimens is recorded. After reaching F.sub.Max, the test specimen is relieved and a relaxation time of one minute is maintained. A further measuring cycle is then carried out. A total of five measuring cycles are carried out per test specimen.

[0103] The open surface (i.e. the surface which is free during the freezing process) was oriented parallel to the test direction.

[0104] The measurements were performed on a tensile tester purchased from Zwick-Roell (Ulm, Germany, Type: BTI-FB005TN.D14)

[0105] FIG. 10 groups the deformation of the five measured cycles in relation to the preparation method of the foams. Graph 101 shows the formation with regard to foams 40A obtained by a method according to the invention. Graph 102 shows the deformation with regard to foams 60A obtained by unidirectional ice templating. The foams obtained by the method according to the invention are less deformed than the foams obtained by unidirectional ice templating.

[0106] The average deformation at a standard force of 2 Newton in relation to the preparation method of the foams is displayed in table 3. The test specimens produced by a method according to the invention (ICM) show an average deformation, which is 54.75% lower than that of the test specimens produced with the state of the art unidirectional ice templating (DF).

TABLE-US-00004 TABLE 3 Average deformation in relation to the preparation method. foam Preparation method Deformation in mm 40A ICM 0.48 60A DF 1.07

[0107] Wall Thickness

[0108] FIG. 11 shows SEM images, recorded with Fei Nova Nanosem 230 Instrument (Fei, USA), of cellulose foams according to the invention and produced via a agitation-cooling process according to the invention with different concentration of microfibrillated cellulose: (a) 0.69 wt. %, (b) 1.16 wt. %, (c) 2.03 wt. %, (d) 2.39 wt. %. Images on the left column display the characteristic porosity with pore sizes in the order of 100 μm. Images on the right column display the wall thickness and theses are in the ranges of 1 to 10 μm and increases for increasing concentrations of microfibrillated cellulose.