PARTICLE-STABILIZED FOAMS USING SUSTAINABLE MATERIALS

Abstract

Described is a method of preparing foams, wherein a suspension comprising an aqueous liquid, particles and at least one surfactant is provided, wherein the at least one surfactant at least partially hydrophobizes a surface of the particles, and wherein the suspension comprising the particles having the at least partially hydrophobized surface is foamed. The at least one surfactant is selected from surfactants having a backbone chain comprising at least nine carbon atoms, the at least one surfactant preferably being an amphiphilic molecule consisting of a tail coupled to a head group, wherein the tail comprises the backbone chain comprising at least nine carbon atoms.

Claims

1. A method of preparing foams comprising the steps of: providing a suspension comprising an aqueous liquid, preferably water, particles, and at least one surfactant, wherein the at least one surfactant at least partially hydrophobizes a surface of the particles; and foaming the suspension comprising the particles having the at least partially hydrophobized surface, characterized in that the at least one surfactant is selected from surfactants having a backbone chain comprising at least nine carbon atoms, the at least one surfactant preferably being an amphiphilic molecule consisting of a tail coupled to a head group, wherein the tail comprises the backbone chain comprising at least nine carbon atoms.

2. The method according to claim 1, wherein the at least one surfactant is selected from the group consisting of polyelectrolytes, proteins, polysaccharides, glycerols, glycerides, fatty acids, ammonium compounds, alkyl compounds, or combinations thereof.

3. The method according to claim 2, wherein the polyelectrolytes and/or the proteins and/or the polysaccharides and/or the glycerols and/or the glycerides and/or the fatty acids and/or the ammonium compounds and/or the alkyl compounds have at least one group selected from bromides, amines, phosphates, phosphonates, sulfates, amides, carboxylic acids, pyrrolidines, betaines or gallates or corresponding salts.

4. The method according to any one of the preceding claims claim 1, wherein the at least one surfactant is a glycerol monostearate-based compound, cetrimonium bromide, tetradecyltrimethylammonium bromide, nonylamine, or combinations thereof.

5. The method according to claim 1, wherein the at least one surfactant is present in amounts of about 0.001% by weight up to about 5% by weight per total weight of the particles, preferably of about 0.01% by weight up to about 2% by weight per total weight of the particles.

6. The method according to claim 1, wherein one or more additives are added to the suspension, the additives being selected from: stabilizing agent, plasticizer, superplasticizer, retarder, accelerator, binding agent, wetting agent, gas generating agent, hardening agent, and rheology modifier.

7. The method according to claim 1, wherein the pH-value of the suspension is adjusted to about 3 to 14, preferably to about 8 to 14, prior to foaming the suspension or after foaming the suspension.

8. The method according to claim 1, wherein the particles are inorganic particles, preferably selected from aluminosilicates or calcium silicates, in particular inorganic particles obtained from mineral processing tailings, catalyst residues, coal bottom ash, rice husk ash, palm oil ash, waste glass, paper sludge ash, sludge from water treatments, mica, vermiculite, microsilica, ground granulated blast-furnace slags, pigments, perlite, or ceramic waste material.

9. The method according to claim 1, wherein the particles comprise fly ash particles and/or earth particles.

10. A foamable suspension comprising: an aqueous liquid, preferably water, particles having the at least partially hydrophobized surface as obtained in claim 1, and optionally one or more additives selected from stabilizing agents, plasticizers, superplasticizers, retarders, accelerators, binding agents, wetting agents, gas generating agents, hardening agents and rheology modifiers.

11. Use of the foamable suspension according to claim 10 for preparing a foam, wherein the foamable suspension is mechanically foamed, preferably by means of a mixer, and/or wherein the foamable suspension is in-situ foamed by adding a gas generating agent to the foamable suspension.

12. A foam comprising a foamed suspension, the foamed suspension comprising: an aqueous liquid, preferably water, particles having the at least partially hydrophobized surface as obtained in claim 1, and optionally one or more additives selected from stabilizing agents, plasticizers, superplasticizers, retarders, accelerators, binding agents, wetting agents, gas generating agents, hardening agents and rheology modifiers.

13. The foam according to claim 12, wherein the particles having the at least partially hydrophobized surface represent at least about 50% of the total solids part of the foam, preferably about 80% of the total solids part of the foam, and/or wherein the foam density is in the range of about 10 kg/m.sup.3 to about 1000 kg/m.sup.3, preferably in the range of about 30 kg/m.sup.3 to about 800 kg/m.sup.3, and/or wherein the foam has a porosity of about 20% by volume to about 99% by volume, preferably of about 50% by volume to about 98% by volume, and/or wherein the foam has a conductivity in the range of about 0.01 W/(mK) to about 0.3 W/(mK), preferably in the range of about 0.02 W/(mK) to about 0.2 W/(mK), and/or wherein the foam comprises bubbles of gas having a size in the range of about 1 μm to about 1 mm, preferably of about 10 μm to about 100 μm.

14. A method of preparing a porous article comprising the steps of: providing a foam according to claim 12, casting or extruding or additive manufacturing, in particular 3D-printing, said foam, and optionally setting, and/or optionally drying, and/or optionally sintering.

15. Use of the foam according to claim 12 to produce porous articles, wherein the foam is subjected to casting or extrusion or additive manufacturing and optionally setting and/or optionally drying and/or optionally sintering.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0086] Preferred embodiments of the invention are described in the following with reference to the drawings, which are for the purpose of illustrating the present preferred embodiments of the invention and not for the purpose of limiting the same. In the drawings,

[0087] FIG. 1 shows a schematic representation of a method of preparing foams and porous articles according to a first embodiment;

[0088] FIG. 2 shows a schematic representation of a method of preparing foams and porous articles according to a second embodiment;

[0089] FIG. 3 shows an SEM image of a foam comprising fly ash particles;

[0090] FIG. 4 shows an SEM image of a foam comprising earth particles;

[0091] FIG. 5 shows a further SEM image of a foam comprising fly ash particles;

[0092] FIG. 6 shows a further SEM image of a foam comprising earth particles;

[0093] FIG. 7 shows an image of a foam comprising earth particles;

[0094] FIG. 8 shows an image of a foam comprising fly ash particles;

[0095] FIG. 9a shows a schematic representation of a gas-liquid interface in a foam according to the invention;

[0096] FIG. 9b shows an enlarged view of the gas-liquid interface according to FIG. 9a, wherein the established percolating network is evident.

DESCRIPTION OF PREFERRED EMBODIMENTS

[0097] In FIGS. 1 and 2 different methods for preparing foams and porous articles are depicted. In fact, in FIG. 1 a particle-stabilized foam is prepared by producing, in a first step, a first suspension comprising an aqueous liquid and particles. For example, earth particles in an amount of about 40% by weight per total weight of the suspension or fly ash particles in an amount of about 50% by weight per total weight of the suspension can be added to water. In a second step, a surfactant having a backbone chain comprising at least nine carbon atoms is added to the suspension in order to at least partially hydrophobize the surface of the particles. For instance, Cremodan in an amount of about 0.72% by weight per total weight of the suspension can be added. Besides, additives to adjust or improve mechanical properties can be added to this suspension, too. For example, a stabilizing agent such as a cellulose compound against cracking, a network-stabilizing agent such as a protein or polysaccharide in order to improve the mechanical properties and a rheology modifier such as fumed silica can be added. In one embodiment hydroxypropyl cellulose in an amount of about 0.5% by weight per total weight of the particles, Xanthan in an amount of about 0.2% by weight per total weight of the particles and fumed silica in an amount of about 2% by weight per total weight of the particles are added. In a third step, this suspension is agitated with a mixer in order to get air into the suspension. Thereby, a particle-stabilized foam is formed. In case of this particular composition of the suspension, a particle-stabilized foam with a porosity of up to 98% is generated. In a fourth step, the particle-stabilized foam can be processed so as to create a desired porous article. For example, the particle-stabilized foam can be hardened by means of air drying only. To get a rapid hardening, it is conceivable to add a hardening agent to the particle-stabilized foam. For example, a dispenser comprising the particle-stabilized foam stored in a first cartridge and a hardening agent such as cement, for example in an amount of about 2% to 20% by weight per total weight of the suspension, stored in a second cartridge can be used, wherein the two components are then extruded through a dual static mixer. In this case, the cement starts reacting with water which initiates a rapid setting. However, it is likewise conceivable to sinter the particle stabilized-foams or to cast the particle-stabilized foams. In the former case, the particle-stabilized foams can be sintered at temperatures in the range of about 800° C. to 1400° C. during a period of time of about one hour to three hours, wherein the heating rate is between about 0.5° C./min to about 5° C./min, and whereas the cooling rate is set to about 10° C./min to 2° C./min. In the latter case, an additive such as sodium silicate, for example in an amount of about 10% or 20% by weight per total weight of the suspension, can be added to the particle-stabilized foam prior to casting the foam.

[0098] In FIG. 2 a particle-stabilized foam is prepared by in-situ foaming, wherein a gas-generating agent is used. According to this method, a first solution comprising an aqueous liquid such as water and surface-modified particles is mixed with a second solution comprising a gas-generating agent such as hydrogen peroxide. In the present example, the first solution additionally comprises a catalyst such as manganese oxide (MnO) and sodium hydroxide (NaOH) and the second solution additionally comprises a hardening agent such as sodium silicate or cement. Here, these two solutions are initially separately stored in two cartridges of a dispenser and then discharged through a dual static mixer, whereby a foam expansion is generated.

[0099] In FIGS. 3 and 4, particle-stabilized foams prepared with fly ash particles and earth particles are depicted, respectively. In particular, FIG. 3 depicts a foam comprising fly ash particles, wherein 58% per weight of fly ash per total weight of the suspension, 0.72% per weight of Cremodan per total weight of the suspension and 41.28% by weight of water per total weight of the suspension were initially used for preparing the foamable suspension. After foaming said suspension, the foamed suspension was subjected to sintering at 900° C., whereby the foam according to FIG. 3 was obtained.

[0100] FIG. 4 depicts a foam comprising earth particles, wherein 45% by weight of earth particles per total weight of the suspension, 0.04% by weight of TTAB per total weight of the suspension and 54.96% by weight of water per total weight of the suspension were initially used for preparing the foamable suspension. The suspension was then foamed by mechanically foaming for 5 minutes. The image depicted in FIG. 4 was taken after 3 days and after drying the foamed suspension at ambient temperature.

[0101] FIG. 5 depicts a foam comprising fly ash particles, wherein 58% by weight of fly ash particles per total weight of the suspension, 0.05% by weight of TTAB per total weight of the suspension, 34.75% by weight of water per total weight of the suspension, and 7.2% by weight of sodium silicate (Na.sub.2O.SiO.sub.2) per total weight of the suspension were initially used for preparing the foamable suspension. The suspension was then mechanically foamed and subsequently dried. Said drying process comprised the steps of i) drying at a temperature of 40° C. and at 100% humidity for 24 hours, ii) drying at a temperature of 40° C. and at 65% humidity for 6 days, and iii) drying at 25° C. for 20 days.

[0102] FIG. 6 depicts a foam comprising earth particles, wherein 45% by weight of earth particles per total weight of the suspension, 0.59% by weight of Cremodan™ per total weight of the suspension, 10% by weight of Portland Cement per total weight of the suspension, and 44.41% by weight of water per total weight of the suspension. The suspension was then mechanically foamed and subsequently dried.

[0103] FIG. 7 depicts a foam comprising earth particles, wherein 45% by weight of earth particles per total weight of the suspension, 0.04% by weight of nonylamine per total weight of the suspension, 10% by weight of Cement per total weight of the suspension, and 44.96% by weight of water per total weight of the suspension are mechanically foamed. Subsequently, the foam was dried in an oven at a temperature of 60° C. The foam of this example comprises pores of a small size, which is a result of a higher mixing speed applied in the foaming process, namely 800 revolutions per minute (rpm). The small pore size is further illustrated by the two Swiss Franc piece placed on top of the foam.

[0104] FIG. 8 depicts a foam comprising fly ash particles, which was generated from two solutions. Namely a first solution comprising 68% by weight of fly ash particles per total weight of the first solution, 0.4% by weight of Cremodan™ per total weight of the first solution, 0.1% by weight of manganese oxide per total weight of the first solution, and 31.5% by weight water per total weight of the first solution, as well as a second solution comprising 29.6% by weight of sodium silicate (Na.sub.2O.SiO.sub.2) per total weight of the second solution, 14.9% by weight of hydrogen peroxide (H.sub.2O.sub.2) per total weight of the second solution, and 55.5% by weight of water per total weight of the second solution. After mixing the two solutions with each other a foam was generated by in-situ gas generation and solidified by the sodium silicate. The thus created foam was then dried in a drying process comprising the steps of i) drying at a temperature of 40° C. and at 100% humidity for 24 hours, ii) drying at a temperature of 40° C. and at a humidity of 65% for 6 days, and iii) drying at a temperature of 25° C. for 20 days. The thereby dried foam comprises pores having a size of up to 1 to 2 millimeters. The large pore size is further illustrated by the one Swiss Franc piece placed on top of the foam.

[0105] It directly follows from these examples that significantly less amounts of long-chain surfactants are needed in order to achieve the same stability of a foam generated with a short-chain surfactant. In fact, more than one order of magnitude less long-chain surfactant is needed, see e.g. the foam prepared by 0.04% by weight of nonylamine (9 carbon atoms in the backbone chain) per total weight of the suspension according to FIG. 9 as compared to an equally-stable foam prepared by about 2% by weight of propyl gallate (3 carbon atoms in the backbone chain) per total weight of the suspension.

[0106] FIGS. 9a and 9b depict schematic illustrations of the particle stabilization of the gas bubbles established in foams according to the invention. As follows from these figures, the surface-modified particles are adsorbed on the surface of the gas bubbles generated upon the foaming of the foamable suspension by means of the surfactants. In addition, the particles partially hydrophobized by the long backbone-chain of the surfactants form a percolating network around the gas bubbles and throughout the continuous liquid medium, which additionally increase stability of the foam.

[0107] With respect to FIGS. 1, 2, 9a and 9b it should be noted that the compounds are not drawn to scale. In fact, the size of the particles is typically much larger than the size of the surfactants.

EXAMPLE I

Mechanical Foaming and Addition of Cement

[0108] Step 1: Optional milling of aluminosilicate particles obtained from secondary raw materials. [0109] Step 2: Dissolution of a long-chain surfactant in water. [0110] Step 3: Addition of the optionally milled aluminosilicate particles to the dissolved long-chain surfactant, wherein the particles are distributed by means of mixing at 200 revolutions per minute. [0111] Step 4: Adjusting the pH-value of the suspension of Step 3 to a pH-value of 9-10. [0112] Step 5: Foaming the suspension obtained in Step 4 by using a high shear mixer at 800 to 1000 revolutions per minute. [0113] Step 6: Dispersion of cement in water [0114] Step 7: Mixing of the foamed suspension of Step 4 with the dispersion of cement in water of Step 5 with a high shear mixer at 800 to 1000 revolutions per minute. [0115] Step 8: Casting or extruding or 3D-printing the mixed foamed suspension of Step 7. [0116] Step 9: Setting the casted or extruded or 3D-printed foamed suspension of Step 8 by covering or placing it in a humidity chamber at 70-100% of humidity at a temperature of 25° C. for 4-7 days. [0117] Step 10: Drying the porous article of Step 9.

EXAMPLE II

Mechanical Foaming and Addition of Alkaline Solution

[0118] Step 1: Optional milling of aluminosilicate particles obtained from secondary raw materials. [0119] Step 2: Dissolution of a long-chain surfactant in water. [0120] Step 3: Addition of the optionally milled aluminosilicate particles to the dissolved long-chain surfactant, wherein the particles are distributed by means of mixing at 200 revolutions per minute. [0121] Step 4: Adjusting the pH-value of the suspension of Step 3 to a pH-value of 9-10. [0122] Step 5: Foaming the suspension obtained in Step 4 by using a high shear mixer at 800 to 1000 revolutions per minute. [0123] Step 6: Addition of a sodium silicate solution (Na.sub.2O.SiO.sub.2) to the foamed suspension obtained in Step 5 either by mixing using a high shear mixer or by mixing using a dual static mixer with a single extruder, wherein the foamed suspension of Step 5 is comprised in one cartridge and the sodium silicate solution is comprised in another cartridge. [0124] Step 7: Casting or extruding or 3D-printing the mixed suspension of Step 6. [0125] Step 8: Setting the casted or extruded or 3D-printed mixed suspension of Step 7 by covering it at 40-80° C., preferably 60° C. for 24 h. [0126] Step 9: Drying the porous article of Step 8.

EXAMPLE III

In-Situ Foaming and Alkaline Solution

[0127] Step 1: Optional milling of aluminosilicate particles obtained from secondary raw materials. [0128] Step 2: Dissolution of a long-chain surfactant in water. [0129] Step 3: Addition of the optionally milled aluminosilicate particles to the dissolved long-chain surfactant. [0130] Step 4: Adjusting the pH-value of the suspension of Step 3 to a pH-value of 9-10 and addition of a catalyst, e.g. manganese oxide (MnO). [0131] Step 5: Preparation of an alkaline solution with water and hydrogen peroxide (H.sub.2O.sub.2). [0132] Step 6: Mixing of the suspension of Step 4 and the solution of Step 5 using a dual static mixer with a single screw extruder. [0133] Step 7: Casting or extruding or 3D-printing the mixed suspension of Step 6. [0134] Step 8: Setting the casted or extruded or 3D-printed mixed suspension of Step 7 by covering it at a temperature of 40-80° C., preferably 60° C. for 24 h. [0135] Step 9: Drying the porous article of Step 8.

EXAMPLE IV

Mechanical Foaming and Sintering

[0136] Step 1: Optional milling of aluminosilicate particles obtained from secondary raw materials. [0137] Step 2: Dissolution of a long-chain surfactant in water. [0138] Step 3: Addition of the optionally milled aluminosilicate particles to the dissolved long-chain surfactant, wherein the particles are distributed by means of mixing at 200 revolutions per minute. [0139] Step 4: Adjusting the pH-value of the suspension of Step 3 to a pH-value of 9 to 10. [0140] Step 5: Foaming the suspension obtained in Step 4 by using a high shear mixer at 800 to 1000 revolutions per minute. [0141] Step 6: Casting or extruding or 3D-printing the foamed suspension of Step 5. [0142] Step 7: Drying the porous article of Step 6. [0143] Step 8: Sintering the dried porous article of Step 7 at temperatures between about 600° C. to 1200° C. depending on the particle composition for about 2 h.