EP1924374A2

EP1924374A2 - Inorganic foam producing method and a foam produced according said method

Info

Publication number: EP1924374A2
Application number: EP06775831A
Authority: EP
Inventors: Oksana Dr.-Ing. Lavrentyeva; Christian Dipl.-Ing. Soltmann; Michael Dr. Rer. Nat. Jeske; Georg Prof. Dr.-Ing. Gratwohl; Dietmar Dr.-Ing. Koch
Original assignee: Universitaet Bremen
Current assignee: Universitaet Bremen
Priority date: 2005-08-23
Filing date: 2006-08-10
Publication date: 2008-05-28
Also published as: DE102005039774A1; WO2007022750A2; WO2007022750A3

Abstract

The invention relates to an inorganic foam producing method consisting i) in producing a substantially homogeneous suspension containing water, an inorganic powder and a stabiliser, ii) in adding a substantially non-water-miscible and substantially non-polar liquid and an emulsifier into said suspension, iii) in homogenising and emulsifying said mixture in such a way that an emulsion is obtainable, iv) in foaming the emulsion obtained at the stage iii) by evaporating the substantially non-polar liquid during drying in such a way that an inorganic foam body is obtainable and v) in sintering said inorganic foam body. The use of polymer substitutes, complex foaming methods and a stabilisation/fixation of the foam structure by binding agents is superfluous during the production of the inorganic foams.

Description

Process for the preparation of inorganic foams, then produced foam and use thereof

The present invention relates to a process for producing inorganic foams, to an inorganic foam produced by the process, and to the use thereof.

Inorganic foams in themselves are already known. They are manufactured by a variety of processes. From US Pat. No. 3,090,094, for example, a process is known in which polymer foams are impregnated with an inorganic slip and are prepared in such a way that a layer of defined thickness remains on the polymer foam. The polymer foam is then burned out leaving a void in the webs of the inorganic foam after sintering.

In order to achieve a favorable structure of the webs or walls, a number of methods for the direct foaming of ceramic slips have been developed. The foaming can be realized by various methods, most often mechanical methods, such as intensive stirring with turbulent flow conditions or blowing gas streams are applied. However, blowing agents are also used. In the foamed state of the ceramic slurry is solidified, which in turn is to be realized via different mechanisms. This can be done, for example, by gelling monomers analogously to gel casting or else by denaturing or crosslinking biomolecules.

Another procedure is the introduction of pore formers in a green body. As a pore-forming agent may serve a solid second phase, which is removed during sintering. In addition to combustible solids and liquids can be used as a placeholder. This process is based on the production of an emulsion of a flowable inorganic system and a immiscible component. By solidification of the inorganic system, the structure formed is obtained, see, for example, WO03 / 076109. After sintering form usually closed-cell bodies, typically with porosities of> 40%, in exceptional cases, porosities of up to 70% are achieved.

In all of these methods, the high proportion of burn-out placeholders and / or binders requires a very careful process control in order to allow the escape of the decomposition gases. In particular, known processes have the disadvantages of costly preparation, for example due to the use of polymers as placeholders of the resulting foam structure, require the use of expensive binders, require the subsequent burning out of the polymeric placeholder or binder, resulting in a longer residence time in the kiln at higher temperatures. In addition, the resulting gases must be disposed of or cleaned. Finally, the variation of the foam morphology obtained is also limited, and shrinkage of the finished foam product may result in loss of volume.

From DE 196 12 985 Al a process for the production of open-cell, inorganic sintered foam products is known, wherein Schlickermaterial of sinterable inorganic powder, a fluidizing this slip material making fluid, evaporable material and a propellant gas forming material in a Schäumungsstufe with release of the propellant gas in a foamed product is converted. Then, the flowability of the slip material is substantially eliminated to form an open-pore intermediate body. Binders also used in the process are removed and the resulting green foam body is finally sintered.

From DE 199 28 997 Al a method for foaming of metals is known, wherein a metallic starting material containing a propellant, heated by a spatially limited energy supply, brought to a foaming temperature and at least partially foamed to metal foam.

DE 198 10 544 A1 describes a process for producing a metallic, porous product, in which initially a combustible porous foam with open pores is produced, a slurry of a mixture of skeletal metal particles and property modifying particles is applied to the combustible, porous foam, and finally the combustible, porous foam is burned and the metallic skeleton is sintered.

It is an object of the present invention to provide a process for the production of inorganic foams, which overcomes the disadvantages of the prior art, in particular to dispense with the use of polymeric Platzhalter, complicated foaming process and a stabilization / fixation of the foam structure by binder.

It is also an object to provide an inorganic foam produced by the process according to the invention and to enable its use.

The object is achieved by a process for producing inorganic foams, comprising the steps:

(i) preparing a substantially homogeneous suspension comprising water, inorganic powder and stabilizer;

(ii) adding a substantially immiscible, substantially non-polar liquid and an emulsifier to the suspension;

(iii) homogenizing and emulsifying to obtain an emulsion;

(iv) foaming the emulsion obtained in step (iii) by evaporating the substantially non-polar liquid during drying to obtain an inorganic foam body; and

(v) sintering the inorganic foam body. Furthermore, it is preferable that the inorganic powder is metal powder, mineral powder, ceramic powder, metal carbide powder, metal nitride powder or mixtures thereof.

In one embodiment, it is provided that the particle size of the inorganic powder is about 500 nm to about 5 microns.

In a further embodiment it is provided that the stabilizer is an electrosteric or electrostatic stabilizer, preferably a polyelectrolyte. The stabilizer is of course intended for the purpose of preventing agglomeration of the powder particles and lowering of the agglomerates in the suspension. Suitable stabilizers will be readily apparent to one skilled in the art.

Furthermore, it is proposed that the water-immiscible, substantially non-polar liquid is an alkane or alkene, preferably an alkane, having a boiling point of about 60 ° C to about 200 ° C.

A preferred embodiment is characterized in that the emulsifier is selected from the group consisting of anionic surfactants, nonionic surfactants, amphoteric surfactants or mixtures thereof.

Most preferably, no binder is added during the process.

In a likewise preferred embodiment it is provided that the steps (i) - (iv), in particular step (iv), are carried out at a temperature in the range of about 10 to about 40 ° C, preferably about 15 to about 25 ° C ,

It is further contemplated that steps (i) - (iv), particularly step (iv), be performed at a relative humidity of about 10 to about 70%. It is also proposed that steps (i) - (iv), in particular step (iv), be carried out at a pressure of about 900 to about 1200 hPa.

Further, it is preferably provided that step (iv) is carried out for a period of about 20 minutes to about 3 days.

It is further contemplated that the inorganic foam body may be functionalized during steps (i) - (iv) or after step (v).

A function carrier can already be introduced into the suspension. Subsequent coating or infiltration of the sintered foams is also possible.

It is preferably provided that the inorganic foam body is functionalized with catalytically active substances.

It is particularly preferred that about 70 to about 79 wt .-% of inorganic powder; from about 0.3 to about 0.6 weight percent stabilizer; from about 18% to about 25% by weight of water; from about 0.6% to about 5% by weight emulsifier; and from about 0.8 to about 3.3% by weight of substantially nonpolar liquid.

Furthermore, it is preferably provided that additional burnable, pore-forming substances, preferably polymer fibers or graphite, are added during production. These substances promote the formation of a defined porosity in the webs of the foam structure. Furthermore, it is preferably provided that additional inorganic, incombustible fibers are added during production.

According to the invention is also an inorganic foam, which is prepared by a process according to the invention.

The foam may be open-cell or closed-cell.

According to the invention is also the use of the inorganic foam as a carrier material, insulating material, filter material or lightweight structural element.

It has surprisingly been found that the process according to the invention can dispense with the use of polymeric spacers, complicated foaming processes and stabilization / fixation of the foam structure by binders. This eliminates the otherwise required expensive temperature control for burning out the polymeric placeholder and binder. In the process according to the invention, the foam formation and the foam growth after homogenization / emulsification occur automatically during the drying step. At the phase boundary of the emulsion of aqueous suspension and nonpolar liquid to the surrounding atmosphere, on the one hand, the evaporation of the water during drying causes an increase in the viscosity of the slurry and, on the other hand, an expansion during the evaporation of the nonpolar liquid in the emulsion. The non-polar liquid is enveloped by the emulsifier and therefore can not escape unhindered into the atmosphere. These two processes can proceed at different speeds and are affected by various factors. While the foam morphology can be set by varying the vapor pressure of the non-polar liquid, a specific adjustment of the pore size is possible, for example, by a variable adjustment of process parameters, such as temperature, relative humidity and pressure. Both foams with a network structure with different cell diameters and structures with directional channels can be produced here. The formation of foams with a gradient structure is also possible. The purity of the process produced by the process according to the invention is also particularly advantageous. foams. Since no auxiliary materials such as polymeric spacers or binders are used, the resulting foams are highly pure. The erfmdungsgemäße process can be carried out at room temperature and thus requires no special equipment. Furthermore, it is compatible with existing systems, so no new investments are required. Finally, the erfϊndungsgemäße process shows good environmental friendliness, since no toxic substances arise that would have to be disposed of consuming.

As inorganic powders, preference may be given to using ceramic or metallic powders. However, it is also possible to produce composite foams from ceramic and metallic powders. By incorporation of burn-out constituents, such as polymer fibers or graphite, the porosity of the foam webs can also be influenced.

Other features and advantages of the present invention will become apparent from the following detailed description of examples with reference to the accompanying drawings.

1 shows a scanning electron micrograph of a foam produced according to Example 1;

Fig. 2 is a scanning electron micrograph of a foam prepared according to Example 2;

Fig. 3 is a scanning electron micrograph of a foam prepared according to Example 3;

Fig. 4 is a scanning electron micrograph of a foam prepared according to Example 4; Fig. 5 is a scanning electron micrograph of a foam prepared according to Example 5; and

6 is a scanning electron micrograph of a foam prepared according to Example 6.

example 1

73.1% by weight Al ₂ O ₃ powder (700 nm average particle size), 0.6% by weight stabilizer (aliphatic, carboxylic acid-containing, alkali-free, electrosteriscous polyelectrolyte with pH 9, Dolapix CE64, Zschimmer and Black) and 23.9% by weight of water were dispersed by mechanical stirring. Then, propellant (1.8% by weight ml of octane) and emulsifier (0.6% by weight of nonionic surfactant Lutensol ON80, BASF) were added and emulsified. After emulsification, the emulsion obtained was poured into plastic Petri dishes (diameter 50 mm, height 13 mm). At an initial height of the cast emulsion of 4 mm, the foam grew at a relative humidity of 20% and at a temperature of 20 ° C to a height of 12 mm within 6 hours. The foam was then sintered (at 2 K / min to 1400 ⁰ C, 2 hours hold, cooling at 5 K / min). After sintering, the porosity of the inorganic foam was 93.6%. The foam structure of the resulting foam is shown in FIG.

Example 2

72.7% by weight Al ₂ O ₃ powder (700 nm average particle size), 0.6% by weight stabilizer (aliphatic, carboxylic acid-containing, alkali-free, electrophilic polyelectrolyte with pH 9, Dolapix CE64, Zschimmer and black) and 23.8% by weight of water were dispersed by mechanical stirring. Then blowing agent (1.7 wt .-% ml of hexane) and 1.2 wt .-% emulsifier (mixture of different surfactants (5.5 wt .-% sodium laureth sulfate, 2 wt .-% laureth-3, 1.6 wt % Disodium laurethsulfosuccinate, 1.5% by weight cocamidopropyl betaine, 0.6% by weight hydrolyzed collagen, water) was added and emulsified after emulsification. trischals made of plastic analogously to Example 1 poured. At an initial height of the poured emulsion of 4 mm, the foam grew at a relative humidity of 20% and a temperature of 20 ° C to a height of 15 mm within 1.5 hours. The obtained Schaurnkörper was sintered (at 2 K / min at 1400 ⁰ C, 2 hours holding time, cooling at 5 K / min), and the porosity of the foam after sintering was 97.7%. The corresponding foam structure is shown in FIG.

Example 3

72.6% by weight Al ₂ O ₃ powder (700 nm average particle size), 0.6% by weight stabilizer (aliphatic, carboxylic acid-containing, alkali-free, electrophilic polyelectrolyte with pH 9, Dolapix CE64, Zschimmer and black) and 23.8% by weight of water were dispersed by mechanical stirring. Then blowing agent (1.8 wt .-% octane) and 1.2 wt .-% emulsifier (mixture of different surfactants (5.5 wt .-% sodium laureth sulfate, 2 wt .-% laureth-3, 1.6 wt. % Dinetrelium laurethsulfosuccinate, 1.5% by weight of cocamidopropyl betaine, 0.6% by weight of hydrolyzed collagen, water) were added and emulsified After emulsification, the emulsion was poured into Petri dishes analogously to Example 1. At an initial level of When the emulsion was poured in, the foam grew to a height of 16.2 mm within 6 hours at a relative humidity of 20% and a temperature of 20 ^° C. The resulting foam body was sintered (at 2 K / min to 1400 ^° C.) , 2 hours holding time, cooling at 5 K / min), and the porosity of the resulting foam after sintering was 94.7% The corresponding foam structure is shown in FIG.

Example 4

76.9% by weight of aluminum powder (average particle size 2 μm), 0.5% by weight of stabilizer (aliphatic, carboxylic acid-containing, alkali-free, electrophilic polyelectrolyte with pH 9, Dolapix CE64, Zschimmer and black) and 19.1% by weight % Water were dispersed by mechanical stirring. Then blowing agent (1.5% by weight of decane) and 2% by weight of emulsifier (mixture of different surfactants (5.5% by weight of sodium laureth sulfate, 2% by weight of laureth-3, 1.6% by weight) were added. disodium trium laureth sulfosuccinate, 1.5% by weight cocamidopropyl betaine, 0.6% by weight hydrolyzed collagen, water) were added and emulsified. After emulsification, the emulsion was poured analogously to Example 1 in Petri dishes. At an initial height of the cast-emulsion of 4 mm of foam grew at a relative humidity of 45% and a temperature of 2O ⁰ C to a height of 5 mm within 3 days. The foam body obtained was sintered (5 K / min to 600 ° C, holding time 2 hours, cooling at 5 K / min, under argon atmosphere) and the porosity of the obtained foam after sintering was 53%. The corresponding foam structure is shown in FIG.

Example 5

78.3% by weight of aluminum powder (average particle size 2 μm), 0.5% by weight of stabilizer (aliphatic, carboxylic acid-containing, alkali-free, electrophilic polyelectrolyte with pH 9, Dolapix CE64, Zschimmer and black) and 18.8% by weight % Water were dispersed by mechanical stirring. Then blowing agent (1.4 wt .-% heptane) and 1 wt .-% emulsifier (mixture of different surfactants (5.5 wt .-% sodium laureth sulfate, 2 wt .-% laureth-3, 1.6 wt .-% Dinetrium laureth sulfosuccinate, 1.5% by weight cocamidopropyl betaine, 0.6% by weight hydrolyzed collagen, water) were added and emulsified After emulsification, the emulsion was poured into Petri dishes analogously to Example 1. At an initial height of the cast emulsion of 4 mm, the foam grew to a height of 25 mm within 2.5 hours at a relative humidity of 20% and a temperature of 20 ° C. The resulting foam body was sintered (5 K / min to 650 ° C., holding time 2 Hours, cooling at 5 K / min, under hydrogen atmosphere) and the porosity of the resulting foam after sintering was 95% The corresponding foam structure is shown in FIG.

Example 6

71.7% by weight Al ₂ O ₃ (average particle size 700 nm), 0.6% by weight stabilizer (aliphatic, carboxylic acid-containing, alkali-free, electrophilic polyelectrolyte with pH 9, Dolapix CE64, Zschimmer and black) and 23.4 % By weight of water were dispersed by mechanical stirring. yaws. 1.4% by weight of Al / Mg 5 powder was stirred into the suspension (average particle size 25 μm). Then blowing agent (1.7 wt .-% heptane) and 1.2 wt .-% emulsifier (mixture of different surfactants (5.5 wt .-% sodium laureth sulfate, 2 wt .-% laureth-3, 1.6 wt. % Dinetraline laurethsulfosuccinate, 1.5% by weight of cocamidopropyl betaine, 0.6% by weight of hydrolyzed collagen, water) were added and emulsified After emulsification, the emulsion was poured into Petri dishes analogously to Example 1. At an initial level of cast-emulsion of 4 mm of foam grew at a relative humidity of 58% and a temperature of 20 ° C to a height of 33 mm within 2 hours. the foam body obtained was sintered (3 K / min to 1500 ⁰ C, holding time 2 Hours, cooling at 5 K / min), and the porosity of the obtained foam after sintering was 98% The corresponding foam structure is shown in FIG.

The inorganic foams prepared according to the process of the invention can find many uses in the art since both open-cell and closed-cell foams can be made. While foams with a closed pore structure can be used for thermal and acoustic insulation as well as for lightweight structural elements, open-cell foam structures are particularly suitable as support materials for chemical and biological catalysts as well as for use as filter material and adsorber. Foams according to the invention can also be used as bone implants.

The features disclosed in the specification, the claims and the appended drawings may be essential both individually and in any combination for realizing the invention in its different embodiments.

Claims

claims

A process for producing inorganic foams comprising the steps of:

(iii) homogenizing and emulsifying to obtain an emulsion;

(v) sintering the inorganic foam body.

2. The method according to claim 1, characterized in that the inorganic powder is metal powder, mineral powder, ceramic powder, metal carbide powder, metal nitride powder or mixtures thereof.

3. The method according to claim 1 or 2, characterized in that the particle size of the inorganic powder is about 500 nm to about 5 microns.

4. The method according to any one of the preceding claims, characterized in that the stabilizer is an electrosteric or electrostatic stabilizer, preferably a polyelectrolyte.

5. The method according to any one of the preceding claims, characterized in that the substantially immiscible with water, substantially non-polar liquid is an alkane or alkene, preferably an alkane, having a boiling point of about 60 ⁰ C to about 200 ° C.

6. The method according to any one of the preceding claims, characterized in that the emulsifier is selected from the group consisting of anionic surfactants, nonionic surfactants, amphoteric surfactants or mixtures thereof.

7. The method according to any one of the preceding claims, characterized in that no binder is added during the process.

8. The method according to any one of the preceding claims, characterized in that the steps (i) - (iv), in particular step (iv), at a temperature in the range of about 10 to about 40 ⁰ C, preferably at about 15 to about 25 ° C.

9. The method according to any one of the preceding claims, characterized in that the steps (i) - (iv), in particular step (iv), are carried out at a relative humidity of about 10 to about 70%.

10. The method according to any one of the preceding claims, characterized in that the steps (i) - (iv), in particular step (iv) is carried out at a pressure of about 900 to about 1200 hPa.

11. The method according to any one of the preceding claims, characterized in that step (iv) is carried out for a duration of about 20 minutes to about 3 days.

12. The method according to any one of the preceding claims, characterized in that the inorganic foam body during the steps (i) - (iv) or after step (v) is functionalized.

13. The method according to claim 12, characterized in that the inorganic foam body is functionalized with catalytically active substances.

14. The method according to any one of the preceding claims, characterized in that about 70 to about 79 wt .-% of inorganic powder; from about 0.3 to about 0.6 weight percent stabilizer; from about 18% to about 25% by weight of water; from about 0.6% to about 5% by weight emulsifier; and from about 0.8 to about 3.3% by weight of substantially nonpolar liquid.

15. The method according to any one of the preceding claims, characterized in that additional burnable, pore-forming substances, preferably polymer fibers or graphite, are added during production.

16. The method according to any one of the preceding claims, characterized in that additional inorganic non-burnable fibers are added during the production.

17. An inorganic foam prepared by a process according to any one of claims 1 to 16.

18. Inorganic foam according to claim 17, characterized in that the foam is open-cell or closed-cell.

19. Use of the inorganic foam according to claim 17 or 18 as a carrier material, insulating material, filter material or lightweight structural element.